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Solid base catalyzed depolymerization of lignin
into low molecular weight products†
Richa Chaudharya,b and Paresh L. Dhepe*a,b
For the biorefinery concept to become commercially viable, it is essential to add value to lignin which is
the only naturally available aromatic polymer. A one-pot depolymerization of lignin into reactive substituted phenolic compounds ( platform chemicals and octane enhancers) with low molecular weight is of
paramount importance and for that, the development of an environmentally benign method is necessary.
Herein we report the depolymerization of high molecular weight lignin (60 000 Da) over various recyclable solid base catalysts at 250 °C over 1 h. Under these conditions, most of the zeolitic catalysts (NaX,
NaY, NaP) showed very high yields of low molecular weight products compared to other catalysts (MgO,
CaO, HT, HAP). But in particular, over NaX the maximum yield (51%) of low molecular weight products
was achieved. Identification and quantification of products was done by GC, GC-MS, HPLC, LC-MS,
Received 28th September 2016,
Accepted 29th November 2016
DOI: 10.1039/c6gc02701f
rsc.li/greenchem
1.
CHNS, NMR and FT-IR techniques. The revelation of retention of most of the functional groups on products present in lignin was confirmed by FT-IR studies. It is observed that the efficiencies of catalysts
were dependent on pH, the cation, the type and concentration of basic sites, etc. A unique study on the
product adsorption capacities on solids was done and it is recognized that as the strength of basic sites
increases, adsorption enhances.
Introduction
Lignocellulosic material (wood, forest residues, and agricultural residues) has huge potential to be used as a starting
material in the place of fossil feedstocks for the synthesis of
chemicals. This profusely available renewable material is made
up of three components, namely cellulose,1 hemicellulose2
and lignin. Under the biorefinery concept, although cellulose
and hemicelluloses have been thoroughly studied for their
conversions into bulk and fine chemicals,3–6 lignin has drawn
less attention in this aspect. It is estimated that in the biosphere ca. 3 × 1011 tons of lignin is available and additionally
ca. 2 × 1010 tons is generated annually, thus making it one of
the most abundant naturally available polymers.7 Lignin is
obtained as a major by-product during bio-ethanol production
and is also isolated as black liquor in paper and pulp industries. It is a complex 3-D amorphous polymer present in plants
(10–25%) comprising of three basic phenylpropanol alcohol
precursors, namely coumaryl alcohol, coniferyl alcohol and
sinapyl alcohol, which are collectively called monolignols. In
lignin, numerous aromatic units are linked via uC–O–Cu
a
Catalysis and Inorganic Chemistry Division, National Chemical Laboratory,
Dr. Homi Bhabha Road, Pune 411008, India. E-mail: pl.dhepe@ncl.res.in
b
Academy of Scientific and Innovative Research (AcSIR), New Delhi, India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c6gc02701f
778 | Green Chem., 2017, 19, 778–788
(60–70%) and uC–Cu bonds.8 Of the various types of uC–O–
Cu linkages present in lignin, the β-O-4 linkage is the most
abundant linkage as it comprises around 45–60% of all the
linkages found in lignin; other linkages such as dihydrobenzofuran, β–β, β-5 and 4-O-5 are present in minor quantities.9,10
Contrary to the well established structures of cellulose and
hemicelluloses, lignin does not have a definite structure
(Fig. S1, ESI†) since it varies depending on the local weather,
humidity, temperature, plant species, the age of the plant etc.
Nevertheless, as discussed, several building blocks of lignin
are randomly distributed and coupled via numerous linkages
to produce the world’s only naturally occurring aromatic
polymer.
Conventionally lignin is employed for the generation of
heat and power (cogeneration) but it is argued in many forums
that until valuable chemicals are synthesized from lignin
through a biorefinery concept, the cellulose to ethanol conversion process will not become economical.11
Since the polyphenolic aromatic structure of lignin is
ideally suitable to obtain value added low molecular weight
aromatic compounds, which can be used as building blocks or
platform chemicals for the subsequent synthesis of several
chemicals; in recent times efforts have been devoted to the
development of catalytic methods for the depolymerization of
lignin. Several literature reports suggest that depolymerization
of lignin is possible using solid acids,12–14 homogeneous
acids,15,16 ionic liquids,17,18 supported metal catalysts,19–22
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supercritical fluids23,24 etc. to yield low molecular weight aromatic products. However, most of these methods are associated with quite a few drawbacks such as the use of model compounds instead of lignin and also operation at either higher
temperatures (≥250 °C) or pressures (>3 MPa) or both.
Amongst all the methods known, the base catalyzed depolymerization (BCD) method is well studied. BCD of lignin using
sodium hydroxide (NaOH) in an aqueous media at high temperatures (240–340 °C) and high pressures (∼10 MPa) has been
shown to produce phenol and its derivatives such as catechol,
syringol, guaiacol, vanillin etc. in quantitative yields.25–28 It has
also been shown that organosolv olive tree pruning lignin can
be depolymerized with 4 wt% NaOH at 300 °C & 9 MPa to yield
20% catechol. Studies on the effect of base (NaOH, KOH, Ca
(OH)2, LiOH, or K2CO3) on the depolymerization have also
been carried out and it was observed that NaOH catalyst shows
the paramount depolymerization activity due to its strong
strength.28 In yet another report, the depolymerization of Kraft
lignin is reported with 5 wt% NaOH at 270–315 °C & 13 MPa to
yield monomeric products.29 Besides these, a few more reports
also discuss the depolymerization of lignin using homogeneous bases.28,30–34 Nevertheless, to surmount the apparent
drawbacks (corrosivity, separation of catalyst etc.) associated
with the use of a homogeneous base, solid bases along with
mostly metal catalysts are also studied for depolymerization.
A hydrotalcite supported Ni catalyst (Ni/HTC) was reported
to cleave the β-O-4 aryl–ether bond present in the lignin model
compound, 2-phenoxy-1-phenethanol (PE) at 270 °C to yield
phenol and acetophenone as products.2 The same catalyst was
also used for the depolymerization of organosolv and ballmilled lignin to yield a mixture of alkyl-aromatic products.2
The employment of a porous metal oxide supported copper
catalyst (Cu-PMO) for the depolymerization of organosolv
lignin into derivatives of catechol and some oligomers under
3–6 MPa H2 pressure at 140–220 °C is also reported.35 The Cudoped PMO catalyst was again studied for the conversion of
the lignin model compound dihydrobenzofuran (DHBF) under
supercritical conditions to achieve ether hydrogenolysis and
aromatic ring hydrogenation at 300 °C.36 Recently, at 250 °C
the depolymerization of pine lignin with MgO as a catalyst was
reported to yield ca. 13% of phenolic monomers at >90% conversion level.37 However, the use of low molecular weight
lignin (Mn = 892 Da & Mw = 1360 Da) and the formation of
solids of high molecular weight (Mn = 540–600 Da and Mw =
790–900 Da) are drawbacks of this method. Recently, over a
nitrate-intercalated hydrotalcite catalyst, cleavage of the β-O-4
linkage in the lignin model compound 2-phenoxyl-1-phenethanol (PE) to phenol, acetophenone and 1-phenethanol was
shown to be possible. Furthermore, the same catalyst was evaluated for the depolymerization of lignin-enriched substrates at
275 °C and ca. 8% yield for monomers was achieved.38 The use
of high temperature, difficulties in effective recycling of catalyst and low yields with lignin substrates (Mn = 890–1700 Da,
Mw = 4900–8600 Da) are a few shortcomings of this study.
Considering the drawbacks associated with these reports
(either the use of low molecular weight lignins or the use of
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model compounds and low yields of desired products), it is
indispensable to develop a solid base catalyzed methodology
for the efficient depolymerization of actual high molecular
weight lignin into low molecular weight aromatic products
under milder reaction conditions (T ≤ 250 °C) in a one-pot
process. For this purpose, diverse solid base catalysts were
evaluated for their depolymerization activities at low temperature (≤250 °C) and atmospheric pressure with lignin having a
molecular weight of ca. 60 000 Da.39 To carry out the depolymerization study, several types of solid base catalysts could be
used, e.g. alkali metal exchanged zeolites, clays, alkali and
alkaline earth metal oxides, and rare earth metal oxides
etc.2,35,37 Particularly, zeolites with a lower Si/Al ratio were used
in this work because they tend to be more stable under alkaline conditions (for detailed discussions please refer to section
2.3.9, ESI†).
2. Experimental section
2.1
Chemicals & materials
Alkaline lignin used in this study was obtained from TCI
chemicals, India.40 Basic zeolites, NaX (Si/Al = 1.2)41 NaP (Si/Al
= 1.69)42 and KLTL (Si/Al = 2.7)43 were synthesized using
known procedures. NaY (Si/Al = 3.6) zeolite was procured from
Zeolyst international, USA.44 Metal oxides such as calcium
oxide (99.99%) and magnesium oxide (98%) were purchased
from Sigma Aldrich. Hydrotalcite (Mg/Al = 3) and hydroxyapatite (Ca/P = 1.66) were purchased from Sigma Aldrich.
Aromatic monomers (guaiacol, 2-methoxy-4-methylphenol,
pyrocatechol, resorcinol, 2,6-dimethoxyphenol, 4-hydroxy
benzyl alcohol, homovanillic acid, hexadecane, diethylterephthalate,
2,4-di-tert-butylphenol,
acetoguaiacone,
4-hydroxy-3-methoxybenzyl alcohol, 1,2,4-trimethoxybenzene,
vanillin, eugenol, 3,4-dimethoxyphenol) used for GC calibration were purchased from Sigma Aldrich and Alfa Aesar. All
the chemicals were used as received. Prior to the catalytic runs
all the catalysts were activated at 150 °C for 2 h under vacuum
(10−4 MPa).
2.2
Base catalyzed depolymerization
Prior to the reactions, detailed characterization of the alkaline
lignin and catalyst was done (sections 1 and 2, ESI†). The synthesis procedures of NaX and KLTL are mentioned in sections
2.1 & 2.2, ESI.† Lignin depolymerization reactions were conducted in a batch mode autoclave (Parr, USA). EtOH : H2O (1 : 2
v/v) was used as the reaction media in all the reactions. In a
typical reaction, the lignin : solvent (ethanol : water) mass ratio
was 1 : 60 and the lignin : catalyst mass ratio was maintained at
1 : 1. Initially, the reactor was flushed with nitrogen and the
heating of reactor was started under slow stirring (100 rpm).
After attaining the desired reaction temperature (250 °C), the
stirring rate was increased to 1000 rpm and this was considered as the starting time of the reaction. After the reaction,
the reactor was cooled to room temperature under air and
water flow. The catalyst was separated from reaction mixture
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by centrifugation and washed thoroughly with EtOH : H2O in
order to remove any adsorbed lignin or products on the catalyst (section 3.1, ESI†).
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2.3
Extraction of products
After separation of the solid (catalyst and any other degradation products insoluble in solvent) from the solvent
(ethanol : water), acidification of the liquid layer was done with
2 N HCl solution until the pH reached 1–2. This treatment
helped precipitate out the high molecular weight products
(filter cake). After separation of the precipitate from the liquid
by centrifugation and filtration, the following workup procedure was applied. The liquid fraction was subjected to a
liquid–liquid extraction process with diethyl ether (DEE) and
ethyl acetate (EtOAc), since lignin is not soluble in these solvents (Table S1, ESI†). The solid (filter cake) was also subjected
to DEE and EtOAc treatments for the extraction of any products in these solvents. The soluble fraction was filtrated and
the undissolved solid (residual lignin or oligomers) was ovendried at 55 °C. The organic solutions obtained after treating
the liquid and solid were vacuum evaporated to obtain the oily
products. The percentage of organic solvent soluble products
was calculated based on the solid recovered after evaporating
the respective solvents (section 4, ESI†). The detailed methodology for the separation of products is presented in Fig. 1.
2.4
Analysis of depolymerized products
The organic solvent soluble products were analyzed by gas
chromatography (GC), GC-mass spectroscopy (GC-MS), high
performance liquid chromatography (HPLC) and LC-MS techniques for the identification and quantification of products
(section 5, ESI†). The GC was equipped with a flame ionization
detector (FID) and a capillary column, HP-5 (5% phenyl 95%
dimethylpolysiloxane, 30 m × 0.25 mm). The temperature
Fig. 1 Methodology for separation of products (*analysed using GC,
GC-MS and HPLC).
780 | Green Chem., 2017, 19, 778–788
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program used was: 100 °C (hold time of 4 min) → 280 °C
(ramp rate of 10 °C min−1 and held for 20 min). Nitrogen was
used as a carrier gas. To determine the molecular weight of
the products identified from GC, a Varian make 3800 GC-MS,
(Saturn 2000 MS) with a VF-5 capillary column (5% phenyl
95% dimethyl polysiloxane) (30 m × 0.25 mm) was used.
Helium was used as a carrier gas and the column oven
program used was same as that for GC analysis. From the
mass spectrum, the m/z value of the compounds was obtained
and the GC-MS peaks were matched with the mass spectra in
the NIST library for compound identification.
3. Results and discussion
3.1.
Base catalyzed lignin depolymerization
For base catalyzed depolymerization of lignin performed in
ethanol : water, a variety of solid base catalysts were used and
their physico-chemical properties are summarized in the ESI
(Fig. S2 and Table S2).† The reactions were performed in a
batch mode reactor and after the reaction, low molecular
weight products were extracted from the reaction mixture in
diethyl ether (DEE) and ethyl acetate (EtOAc) solvents (for
more details please refer to the Experimental section).
3.1.1. Evaluation of various solid base catalysts in lignin
depolymerization. For the exploration of an efficient catalytic
system to depolymerize lignin, various types of solid base catalysts were evaluated at 250 °C for 1 h. As can be seen from
Fig. 2, zeolites (NaX, NaY, NaP, KLTL), anionic clay
(Hydrotalcite (HT)), single metal oxides (MgO, CaO) and
hydroxyapatite (HAP) were active for the depolymerization of
lignin with varying yields (10–51%) of diethyl ether (DEE) and
ethyl acetate (EtOAc) soluble low molecular weight products.
Fig. 2 Depolymerization of lignin using solid base catalysts. Reaction
conditions: lignin : NaX = 1 : 1 wt/wt, EtOH : H2O (1 : 2 v/v) 30 mL,
250 °C, 1 h. DEE: diethyl ether, EtOAc: ethyl acetate, SR Mix. Sol.: products extracted from solid (catalyst + adsorbed products) using a
mixture of DEE and EtOAc (1 : 1 v/v). All the experiments were done
3 times and ±3% error in yields was observed. The results presented in
this figure are the average of three runs.
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Among all the catalysts, the Na exchanged X zeolite (NaX)
showed the best activity of 51%. All other zeolite catalysts
(NaY, NaP and KLTL) also showed better activity (30–38%)
compared to the other catalysts. Under similar reaction conditions, without catalyst, a 24% yield for low molecular weight
products was observed, which illustrates that lignin depolymerization is a catalytic reaction. To corroborate this fact; NaP
catalyst was added to a reaction without catalyst, and a 13%
increase in yield (37%) was observed (section 3.2, ESI†).
For the comparison of yields obtained with solid base catalysts, homogeneous base (Na2CO3) catalyzed depolymerization
of lignin was carried out under similar conditions (250 °C, 1 h)
(Fig. 2). The results reveal that with a homogeneous base, 34%
yield is obtained, which was comparable with most of the solid
base catalysts, yet it was lower than that obtained with the NaX
catalyst. Conventionally, homogeneous base catalyzed lignin
depolymerization reactions are carried out with strong bases
like NaOH or CsOH instead of a weak base like Na2CO3. But
under these reaction conditions, when the effect of pH on the
depolymerization was studied (for details please refer to
section 3.1.4.), it is established that a pH of 9 is the optimum
to achieve the best catalytic results and with a weak base like
Na2CO3 employed in this work, a pH of 9.2 under the reaction
conditions could be maintained.
3.1.2. Identification and quantification of depolymerized
products. The formation of low molecular weight products was
confirmed by GC, GC-MS, HPLC and LC-MS analysis. In the
GC-MS profiles (Fig. S3–S6, ESI†) of the DEE and EtOAc
soluble products obtained in NaX catalyzed reaction, the
majority of the peaks identified were for low molecular weight
products. Furthermore, with the aid of commercially procured
standard compounds, it was feasible to quantify these products (ca. 35% out of 51% products) and the details on their
structures and yields are summarized in Table 1. From the
identified products it is safe to comment that ethanol, which
is used as a reaction solvent, does not participate in the reaction as a substrate. This is because in most of the products,
–OCH3 (methoxy) groups are attached to the aromatic ring,
instead of the –OCH2CH3 groups to be expected if ethanol contributes towards product formation. This also reveals that
lignin does not undergo an alcoholysis reaction but rather is
depolymerized through a hydrolysis reaction (by yielding –OH
groups on aromatic rings. The presence of –OH groups is also
observed in almost all identified products). A careful look at
the GC-MS chromatographs of the DEE and EtOAc extracted
products from liquid reveals that the concentration of low
molecular weight products extracted in DEE is much higher
compared with EtOAc (Fig. S3 and S4, ESI†). This is understandable since from the liquid, products were first extracted
in DEE and then EtOAc was used to extract the products from
liquid. However, the weight of the products isolated in EtOAc
(Fig. S4, ESI†) is higher than those extracted in DEE (Fig. S3,
ESI,† NaX catalyzed reaction). Considering this, it is expected
that in EtOAc solvent, oligomers (dimers, trimers) are extracted
(which again are depolymerization products but not complete).
This phenomenon is also evident from the fact that a higher
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concentration of products (typically oligomers which are
precipitated out during acidification)27,29 from filter cake
(solid) were extracted in EtOAc (Fig. S6, ESI†) than DEE
(Fig. S5, ESI†).
3.1.3. Product adsorption study. An attentive look at Fig. 2
reveals that with MgO, CaO, HT and HAP catalysts, the contribution by DEE soluble products towards the overall yield is
higher than EtOAc extracted products. This trend is contradictory to the results obtained with NaX, NaY, NaP and KLTL
catalysts (Fig. 2) where more products were extracted in EtOAc
than extracted in DEE. But the total yield for DEE soluble products is still higher for the NaX, NaY and NaP catalysts than
other catalysts. It is astonishing to see (Fig. 2) that with HAP,
HT, MgO and CaO as catalysts, the overall yield of products
(10–18%) is lower than for the non-catalytic reaction (24%).
This is probably due to two factors: the adsorption of formed
products on these catalysts, or further (degradation) reactions
of the formed products on these catalysts. It is also reasonable
to believe that both of these reasons might be responsible for
observing this phenomenon. To understand these results
further, the weight of the separated solid (by centrifugation
before acidification, Fig. 1) was checked and it was noticed
that with these catalysts, only an increase in the weight of
solid (after recovery, the solid was dried in an oven at 55 °C for
16 h) was seen compared to any other catalyst such as NaX,
NaY, NaP and KLTL (Tables S3 and S4, ESI†).
Since lignin and depolymerized low molecular weight products are soluble in the reaction solvent (ethanol : water), the
solid recovered after the reactions is either only catalyst or catalyst with adsorbed products. To comprehend this possibility,
products adsorbed on these catalysts (HAP, HT, MgO and CaO)
were attempted to be extracted in the mixture of DEE and
EtOAc and as can be seen in Fig. 2 and Table S3 (ESI†), only
1–7% of products could be extracted from these solids. Still,
even after the extraction of adsorbed products in the mixture
of solvents, the weight of the solid (catalyst + adsorbed products) was higher than the charged catalyst (Table S3, ESI†).
This is indicative of the fact that products are strongly
adsorbed on the catalyst.
To check the adsorption of these products further, a set of
experiments was done with a mixture of five aromatic monomers ( phenol, p-cresol, guaiacol, eugenol and vanillin) which
resemble products identified in the depolymerization reactions
of lignin. The reactions were done under optimized reaction
conditions (250 °C, 1 h) with NaX and CaO as catalysts and it
was apparent from the GC chromatographs that while slight
decreases in the peak intensities of eugenol (5%) and vanillin
(41%) were seen with NaX as a catalyst (Fig. 3D and E); with
CaO as a catalyst, a huge decrease in the peak intensities for
3 monomers was observed, along with the disappearance of
the peaks for eugenol and vanillin (Fig. 3A–C). Moreover, the
recovered CaO catalyst was washed with organic solvents (DEE,
EtOAc) and it was observed that the separation of few products
was possible from the catalyst (Fig. S7, ESI†). These results
clearly indicate that on CaO, a large amount of products were
adsorbed, but on NaX, a minimal amount of products was
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Table 1 Quantification of identified lignin depolymerization products. Reaction conditions: lignin : NaX = 1 : 1 wt/wt, EtOH : H2O (1 : 2 v/v) 30 mL,
250 °C, 1 h
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Monomer
Structure
Yield (wt%)
Guaiacol
1
Pyrocatechol
0.3
2-Methoxy-4-methylphenol
3.7
Resorcinol
0.8
4-Hydroxy benzyl alcohol
1.4
2,6-Dimethoxy phenol
0.03
Eugenol
0.4
1,2,4-Trimethoxybenzene
0.03
Vanillin
3.6
3,4-Dimethoxyphenol
0.02
4-Hydroxy-3-methoxybenzylalcohol
0.3
Acetoguaiacone
4.1
2,4-Di-tert-butylphenol
0.5
Hexadecane
Homovanillic acid
0.5
0.3
Diethyl-terephthalate
0.6
adsorbed. Similar experiments were also performed with HAP,
HT & MgO, and analogous results to those obtained with CaO
were seen. Although it was obvious from the above study that
782 | Green Chem., 2017, 19, 778–788
products are adsorbed on CaO, there was a likelihood that the
products underwent some reaction under the reaction conditions employed for the adsorption study. Hence, to check
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pH of all the catalysts (section, 2.3.7, ESI†) was measured and
the following order of decreasing pH was seen:
CaO ðpH " 13:4Þ > HT ðpH 12:5Þ > MgO ðpH 10:5Þ
> KLTL ðpH 9:4Þ > NaX ðpH 9:2Þ
> NaY ðpH 8:7Þ ¼ NaP ðpH 8:7Þ
% HAP ðpH 8:5Þ > Millipore water ðpH 6:8Þ:
As can be seen from Fig. 4, the maximum product yield (51%)
was observed at pH 9.2, measured for the NaX catalyst. It is
anticipated that with an increase in pH, side reactions are possibly dominant and hence the yield for the desired products
would decrease. Since during the reactions with NaY (38%) and
NaP (34%), which have a pH of 8.7, good activities compared
with other catalysts were observed (Fig. 2), the reaction with NaX
was carried out at pH 8.7 (by decreasing the loading of catalyst)
but a decrease in yield (39%) was observed. This highlights the
fact that pH 9.2 might be better to achieve higher yields (Fig. 4).
Subsequently, reactions were done with CaO and MgO by
charging appropriate quantities of catalysts to maintain a pH
of 9.2 (Table S6, ESI†).
As can be seen from Fig. 5, when the pH was maintained at
9.2, the product yield was almost doubled with these catalysts
Fig. 3 GC chromatographs of a mixture of phenol, p-cresol, guaiacol,
eugenol and vanillin in 30 mL EtOH : H2O (1 : 2 v/v) (A) before reaction
with CaO, (B) after reaction with CaO at 30 °C, (C) after reaction with
CaO at 250 °C, (D) before reaction with NaX, (E) after reaction with NaX
at 250 °C. Reaction conditions: monomers (0.1 g each), catalyst (0.5 g),
1 h.
this, the adsorption study was subsequently done under
ambient conditions (30 °C, 1 h, nitrogen flushing) and the
results imply that the adsorption of formed products is even
possible under ambient conditions (Fig. 3). Similarly, it is also
indicative that these products are not degrading under the
reaction conditions (no prominent extra peaks were visible in
the GC chromatographs, Fig. 3).
Furthermore, adsorption studies were done with single
molecules. Vanillin and eugenol were chosen for the study as
these molecules adsorbed much more compared to others on
the catalyst. It was observed from the earlier studies that these
products adsorbed on catalyst under ambient conditions, so
further experiments were performed at 30 °C for 1 h. It is
understood from the study that with single molecules, a
similar type of adsorption phenomenon was observed
(Table S5, Fig. S8, ESI†).
3.1.4. Effect of basicity on the catalytic activity. To realize
the reason for the disparity observed in the activity of the
various catalysts employed in this work, it was important to
correlate the activity with the basicity of catalysts. Firstly, the
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Fig. 4
Effect of pH on the product yield.
Fig. 5 Effect of pH on lignin depolymerization using various solid base
catalysts. Reaction conditions: lignin (0.5 g), catalyst, 30 mL EtOH : H2O
(1 : 2 v/v), 250 °C, 1 h.
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(CaO, 39% yield; MgO, 38% yield) compared to reactions performed at a higher pH (Fig. 2 and 4). These results establish
the fact that pH is a vital factor in achieving higher yields of
products. It is also possible that with a decrease in catalyst
loading (CaO and MgO, Table S6, ESI†) to maintain the pH at
9.2, the amount of products adsorbed on the catalyst may
change. This in turn can show higher yields in the reaction. To
ascertain this fact, the weight of the solid obtained after the
reaction (catalyst + adsorbed products) was checked (Table S3,
ESI†). As can be seen from the data, an almost similar percentage (54% for 0.5 g catalyst and 45% for 0.0055 g catalyst) of
increase in the weight of solid was observed after recovering
the solid from the reaction mixture. This may suggest that due
to the decrease in loading of catalyst, while maintaining the
pH, yields might have improved. However, at the same time,
when the NaX loading was changed to maintain the pH at 8.7
instead of 9.2, a decrease in yield was seen. This means that
the pH is also playing a crucial role in achieving higher yields
of product. However, detailed study on this is necessary to conclude these findings. Although it is observed that pH plays an
important role in governing the activity of a catalyst, it is
related to the amount of basic sites available on the catalyst.
This is particularly important in this work since all the catalytic reactions were performed by charging a similar weight of
catalyst. Therefore, a TPD-CO2 study was performed to know
the concentration of basic sites on each catalyst (Fig. S2, ESI†)
and the following trend of basicity with respect to each catalyst
was seen:
CaO ð1:96 mmol g&1 Þ > NaX ð0:42 mmol g&1 Þ
% HT ð0:40 mmol g&1 Þ > NaY ð0:34 mmol g&1 Þ
> MgO ð0:24 mmol g&1 Þ > KLTL ð0:14 mmol g&1 Þ
% HAP ð0:12 mmol g&1 Þ > NaP ð0:09 mmol g&1 Þ:
From this data it was tricky to draw any conclusions since
on both NaX and HT catalysts, although almost the same
amount of basic sites are available they showed very different
depolymerization activities (NaX, 51%; HT, 10.2%) (Fig. 2).
Nonetheless, this difference in activity can partially be
explained on the basis of the dissimilarity between the adsorption of products on these catalysts (since compared to NaX,
the adsorption of products on HT was very high, for more
information, please refer to section 3.1.3; Fig. 5). Comparable
observations of differences in activity even with almost similar
basic site concentrations were also made with NaP (0.09 mmol
g−1, 34%) and HAP (0.12 mmol g−1, 18%) catalysts. So, it is
broadly suggested that irrespective of the concentration of
basic sites on the catalyst, adsorption also plays an important
role in governing the activities of catalysts (section 3.1.3). To
extend this further, when the pH was maintained at 9.2 with
CaO and MgO catalysts, improvements in activities were
achieved (Fig. 5) even though these catalysts are known to
adsorb products (section 3.1.3; Fig. 2 and 3). To explain this, it
is proposed that the adsorption of products might be a pH
dependent phenomenon, i.e. with an increase in pH, the possi-
784 | Green Chem., 2017, 19, 778–788
bility of adsorption of products increases. This is evident from
the fact that the product adsorption study with catalysts like
NaX ( pH, 9.2) and CaO ( pH, 13.4) was done with dissimilar
pH values (section 3.1.3). But when the reaction with CaO was
performed at a pH of 9.2, higher activity (39%, Fig. 5) was seen
compared to the reaction done with the same catalyst at a pH
of 13.4 (Fig. 2). This possibly may be because a lower amount
of products is adsorbed at a pH of 9.2. This explanation is
practical, considering that under the reaction conditions,
5 model compounds used in the adsorption study did not
show any degradation (Fig. 3, section 3.1.3).
Other interesting information that could be extracted from
the catalytic results and the data on the concentration of basic
sites on different catalysts is that even if similar catalytic
activity was achieved with NaP (34%) and NaY (38%) catalysts
(Fig. 2), the concentration of basic sites on these catalysts is
different (NaY = 0.34 mmol g−1; NaP = 0.09 mmol g−1). To
explain this fact, data on the strength of the basic sites was
helpful (Fig. S2, Table S2, ESI†). By considering that the desorption of CO2 at lower temperatures (<500 °C) represents the
presence of weak basic sites and the desorption of CO2 at
higher temperatures (>500 °C) represents the presence of
strong basic sites, the difference in activities is correlated with
the strength of the basic sites in the catalysts. From the
TPD-CO2 profiles of both NaY and NaP catalysts, it can be seen
that both these catalysts have only weakly basic sites and they
show similar pH (8.7), whereas from the TPD-CO2 profiles of
other catalysts (HT, MgO, CaO, HAP) which typically showed
lower product yields (Fig. 2) it could be observed that they have
either strong basic sites along with weak basic sites, or have
only strong basic sites (Fig. S2, Table S2, ESI†). These observations may suggest that the presence of strong basic sites
might be responsible for achieving lower yields. This also can
be correlated with the fact that due to the presence of strong
basic sites, the pH of these catalysts was higher ( pH > 10) even
when the total basicity was lower (MgO, 0.24 mmol g−1; HAP,
0.12 mmol g−1) in these catalysts than the best catalyst, NaX
(0.42 mmol g−1, pH 9.2). The observance of weak basic sites on
zeolites is expected since in zeolites basicity arises due to covalently bonded framework oxygen, and it is reported that if
basic zeolites are prepared by an ion-exchange method then
those show weak basicity compared to alkaline earth metal
oxide catalysts.
Even though efforts have been made to correlate the activity
with basicity of the catalysts, discussions on the strength of
basic sites cannot be established only on the basis of TPD-CO2
studies since basic catalysts invariantly gain their basicity after
treating them at very high temperatures (>550 °C) by removing
water and carbon dioxide which have poisoned the catalytically
active sites. Nevertheless, since in this work water is used as
one of the solvents, it is possible that basic sites on catalysts
may get poisoned and have different morphologies than
expected. Moreover, MgO and CaO, being almost insoluble in
water (reaction solvent), would form hydroxide species
(Mg(OH)2 and Ca(OH)2)45 which would have completely
different basic strengths under the reaction conditions. These
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Green Chemistry
hydroxide species are known to be sparingly soluble in water
and give turbid solutions (limewater). Particularly, at higher
temperatures, many hydroxides tend to form oxides again and
thus lose their ability to dissolve in water (solubility of
Ca(OH)2 in water: 0.159 g per 100 g at 25 °C to 0.071 g per
100 g at 100 °C; solubility of CaO in water: 0.120 g per 100 g at
25 °C to 0.054 g per 100 g at 100 °C).46 On the contrary, the
solubility of Mg(OH)2 in water follows the reverse trend and
higher solubility in water is found with an increase in temperature (0.0009 g per 100 g at 18 °C to 0.004 g per 100 g at 100 °C;47
solubility of MgO in water, 0.086 g per 100 g at 30 °C) (Table S7,
ESI†). The calculations suggest that >95% of catalyst still
remains in a heterogeneous form (Table S7, ESI†). Nonetheless,
typically when the solubility of M(OH)x is <0.01 g per 100 g of
water, it is considered as insoluble in water.48 Furthermore, to
check whether Ca(OH)2 could catalyse this reaction, an experiment with 0.016 g Ca(OH)2 was carried out, considering that
only 3.17% of CaO exists in Ca(OH)2 form (Table S7, ESI†). The
observance of 23% yield of products states that Ca(OH)2 does
not contribute to the reaction. This is because without catalyst,
only 24% yield was observed. This shows that the reaction is
mainly catalyzed by solid CaO and MgO.
Additionally, since Na exchanged zeolites showed good
activity, and from the TPD-CO2 and catalytic results it is
known that greater basic strength might not be favorable in
this reaction, other alkali ions (Cs, K etc. having higher basic
strength compared to Na) exchanged zeolites were not used
except KLTL in this study. For the same reason, other alkaline
earth oxides (BaO, SrO) having higher basic strengths compared to CaO and MgO were also not expected to show
enhancement in yields and thus were not evaluated.
Nonetheless, scope still remains to evaluate the catalytic
activity of these catalysts having higher basic strength by maintaining a pH of ca. 9.2.
Thus, all the above discussions establish the fact that catalytic activities are dependent on many factors like pH, the concentration of basic sites, the type of basic sites, the presence of
alkali metal, the adsorption of products, etc., and all these
factors are interconnected. Therefore, it is essential to optimize the catalyst properties to achieve higher activity and from
this work, it is proposed that to achieve higher yields the most
important factor is to maintain the pH of the reaction solution
around 9.2. Moreover, it is proposed that zeolitic catalysts, if
they are stable under reaction conditions and are completely
insoluble in water, would be better choices than metal oxides.
3.1.5. Effects of temperature and time on lignin depolymerization. By now, it is apparent that amongst all the catalysts evaluated, NaX shows the best yield (51%) within 1 h at
250 °C. Nevertheless, to enhance the yield of low molecular
weight products, the reaction was conducted at 240 °C for 1 h
with the NaX catalyst as it was expected that by lowering the
reaction temperature, degradation of the formed products (if
any) would be suppressed. However; with a decrease in temperature, a decline in the yield (33%) was observed (Fig. S9(A),
ESI†). A further decrease in temperature to 230 °C again
showed lower yields (14%). This decline in yield may be attri-
This journal is © The Royal Society of Chemistry 2017
Paper
buted to a simple temperature effect or the properties of
ethanol. Since ethanol has Pc of 6.06 MPa and Tc of 241 °C, it
is evident that at the reaction temperature (250 °C), ethanol is
present in a subcritical state (4.2 MPa pressure was observed at
250 °C). Though at 240 °C ethanol can also be under the subcritical state, the pressure was only 3.7 MPa. To comprehend
the effect of pressure, the reaction was carried out at 240 °C by
charging 0.3 MPa of nitrogen (at room temperature) and a
total pressure of 4.3 MPa was attained at 240 °C. Under these
conditions, 43% yield was obtained which is 10% higher than
was achieved when the reaction was carried out in the absence
of nitrogen pressure at 240 °C. This suggests that pressure
plays an important role in achieving higher yields. In our
earlier work on solid acid catalyzed depolymerization, the
effect of pressure on the reaction was studied in detail and it
was established that with an increase in pressure, the yield of
products increases.12,13 Subsequently, the reaction was performed at higher temperature (260 °C) believing that higher
pressures would benefit the reaction, but again, a slight
decrease in yield (46%) was seen, which was against expectation. Thus, 250 °C is an optimum temperature to carry out
the depolymerization of lignin using NaX catalyst. Since on the
NaX catalyst, unlike CaO and other catalysts, only a minimal
quantity of products was adsorbed under the reaction conditions, it is assumed that the decrease in yield at higher reaction temperatures is due to degradation and/or repolymerization reactions. Nevertheless, detailed study is required to
comment further on this matter.
The effect of reaction time on the product yield was investigated at 250 °C over NaX as a catalyst in a ethanol : water (1 : 2
v/v) solvent system (Fig. S9(B), ESI†). The product yield was
increased up to 51% for a reaction time of 1 h and beyond this
time the product yield started declining. However, a careful
look at the results implies that although the overall yield
decreased over 2 h, the contribution from DEE soluble products was increased in the 2 h reaction. This implies that
EtOAc soluble products like oligomers were converted into low
molecular weight products over this period of time.
Furthermore, to make sure that there is not a significant
effect from a series of degradation reactions, we performed
depolymerization reactions by normalizing the reaction time
to the mass of catalyst. Details are represented in Fig. S10.† As
can be seen from Fig. S10(A) (ESI†), when the reactions were
performed for 1 h, with a decrease in CaO loading, an increase
in yield was observed. This may be due to pH or a product
adsorption effect. Later, reactions were performed with varying
catalyst loading and accordingly the reaction time was altered.
For example, with 0.5 g CaO catalyst loading, the reaction was
performed for 1 h; however, when 0.1 g of CaO was charged in
the reactor, the reaction was carried out for 20 min
(i.e. 5 times decrease in both catalyst and time). Subsequently,
the reaction was carried out with 0.05 g CaO for 10 min.
The results (Fig. S10(B), ESI†) show that with a decrease in
time, according to the decrease in catalyst loading, a slight
improvement in the yields was possible. However, this increase
is marginal considering the results obtained at 1 h with
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Paper
Green Chemistry
0.005 g catalyst (Fig. S10(A), ESI†). This suggests that maintaining the pH at 9.2 along with time is important to achieve
better yields.
Mostly, lignin depolymerization reactions are carried out in
a alcohol : water mixture to achieve better yields due to the
solubility of lignin in the solvent system which makes the reaction system homogeneous (when water soluble catalysts are
used). Since the alkaline lignin used in this study is completely
soluble in water (Table S1, ESI†), the reaction was carried out
at 250 °C with NaX as a catalyst for 1 h in only water. However,
no product formation was seen in this reaction. This result
could not be explained on the basis of a pressure effect as with
water as a solvent at 250 °C, a total pressure of 4.0 MPa was
observed while when the reaction is carried out with an
ethanol : water (1 : 2 v/v) solvent system, 4.2 MPa pressure was
monitored yet 51% yield was achieved. So, it can be considered
that the presence of ethanol in subcritical conditions is
helpful in promoting the reaction. Subsequently, reactions
were performed by varying the ratio of ethanol–water (2 : 1,
1 : 1, 1 : 5 v/v) and the maximum yield was observed in 1 : 2
(v/v) solvent system. One of the reasons for reactions to
proceed in the presence of ethanol might be due to the formation of sodium ethoxide (reaction between Na+ on the catalyst and ethanol), which can act as a strong base. This scenario
is not possible when only water is used in this reaction. The
in situ formation of sodium ethoxide could not be confirmed
under reaction conditions. Yet, it is expected that when the
reaction is completed, Na+ will compensate the negative
charge on the zeolites. This is because during the spent catalyst characterization studies no loss of Na+ was observed (for
more details, please see section 2.3.9, ESI†).
3.1.6. Catalyst recyclability. In the catalyst recyclability
study, NaX was recovered from the reaction mixture by centrifugation and later was washed with ethanol : water (1 : 2 v/v),
and used in the next reaction (for details, see section 2.3.8,
ESI†). Prior to the catalytic runs, characterization of fresh and
spent NaX was done ( please refer to Table 2, Fig. 6 and S12,
ESI†). With fresh catalyst, a 51% yield was observed and later
in the 1st recycle run, a decrease in yield to 34% was seen. In
subsequent recycle runs similar yields were seen (35% in 2nd
run, 33% in 3rd run) (Fig. S11, ESI†). Nevertheless, it is interesting to note here that though in recycle runs the total yield
was decreased but the formation of DEE soluble products was
the same, which actually is important in this reaction. The
non-catalytic reaction under similar reaction conditions gave
24% yield which is still less than the recycle runs.
Table 2
Catalyst
Fig. 6
3.2.
TPD-CO2 profile of (a) fresh and (b) spent NaX.
Lignin and products characterization
In order to establish a correlation between the structure of the
starting material (lignin) and the depolymerization products
with different functional groups (Table 1), FT-IR analysis of
alkaline lignin, DEE and EtOAc soluble products was done.
As observed from Fig. 7, alkaline lignin shows a broad peak
at 3500–3100 cm−1 due to stretching vibrations of alcoholic
and phenolic hydroxyl groups (O–H stretching). The peak
observed at 1660 cm−1 was attributed to the presence of CvO
Fig. 7
FT-IR spectra of alkaline lignin.
TPD-CO2, N2 sorption and ICP-OES analysis of fresh & spent NaX catalyst
Nitrogen sorption
ICP-OES
NaX
TPD-CO2
Total basicity
(mmol g−1)
BET surface
area (m2 g−1)
Pore size
(nm)
Pore volume
(cm3 g−1)
Si/Al ratio
actual
Si/Al ratio
observed
Na content
(mg g−1)
Fresh
Spent
0.42
0.41
582
586
1.1
1.1
0.3
0.3
1.2
—
1.2
1.2
17.3
20.5
786 | Green Chem., 2017, 19, 778–788
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stretching in α,β-unsaturated aldehydes or ketones. The peak
can also be assigned to the presence of –CvC– in alkanes. In
the range of 1500–1585 cm−1, peaks due to aromatic ring
vibrations (C–C stretch (in ring)) are expected. The peak at
1362 cm−1 is characteristic of bend/rock vibration in alkanes.
In lignin chemistry, this wavelength is also assigned to the syringyl ring breathing mode with C–O stretching. The peaks at
1124 and 1196 cm−1 can be assigned to deformation vibrations
of C–H bonds in syringyl rings or else can be typically assigned
to the C–O stretch in alcohols or ethers. Again a strong peak
appearing at 1024 cm−1 is attributed to the C–O stretch in C–O
(alcohol, ether) or the in-plane deformation vibrations of C–H
bonds in aromatic rings. The observance of a peak at 799 cm−1
is due to deformation vibrations of C–H (oop) bonds associated to aromatic rings.
An observation of the IR spectrum for DEE and EtOAc
soluble products (Fig. 8a and b) shows peaks due to C–H
deformation at 620–646 cm−1. The presence of aromatic C–H
bending was confirmed with the peak at 700–865 cm−1. A
careful look indicates that the intensity of peaks at 1022 and
1068 cm−1 due to alkoxy groups has decreased, indicating that
the –C–O–C– bonds are breaking during depolymerization.
The retention of the peak at 1112/1128 cm−1, though with
lower intensity, indicates the presence of vibrations of C–H
bonds in the syringyl ring. The intense peak seen at 1276 cm−1
might be due to asymmetrical ether formation (Ar–O–CH3).
The peak that appeared at 1363–1378 cm−1 is due to bend/rock
vibrations in alkanes. The intensities of the peaks from 1450
to 1600 cm−1 emphasize that the aromaticity is intact in the
Paper
products. An intense peak that arises at 1716 and 1725 cm−1
can be assigned to α,β-unsaturated aldehydes or ketones in the
depolymerized products. The new peak appearing at 2849 and
2855 cm−1 is attributed to the presence of C–H stretching in
the aldehyde group. Moreover, another peak at 2927 and
2931 cm−1 can be assigned to C–H stretching in alkanes/alkyl
groups. Broad peaks appearing at 3394 and 3359 cm−1 confirm
the cleavage of the Ar(C)–O–Ar bond to form the Ar(C)–OH
bond. It can be clearly understood that most of the functionalities present in lignin are retained in the products. Moreover,
the appearance and increase in the intensities of peaks of
–OCH3, –CHO, and –CH3 groups validate that lignin undergoes
a depolymerization reaction and gives rise to aromatic monomers which have functional groups as mentioned above.
Characterizations of lignin and products were also done with
XRD, 1H, 13C NMR, HPLC, LC-MS, microanalysis, EDAX,
TGA-DTA, UV visible techniques (Table S8, Fig. S13–S21, ESI†).
Conclusion
In summary, the depolymerization efficiencies of various solid
base catalysts were evaluated and it was revealed that the solid
base catalyst NaX is capable of depolymerizing high molecular
weight lignin into low molecular weight aromatic products
(51% yield) in an ethanol : water (1 : 2 v/v) ratio solvent system.
The optimization of reaction conditions and detailed studies
on catalyst properties reveal that a pH of 9.2 is optimum to
catalyze depolymerization and a catalyst having strong basicity
is detrimental in yielding higher quantities of low molecular
weight products. Unique product adsorption studies indicate
that strong bases have more tendency to adsorb the products.
Moreover, it is shown with the help of various analytical tools
that most of the functional groups present on lignin are
retained in the products, thereby improving the atom
efficiency.
Acknowledgements
The author Richa Chaudhary thanks the University Grant
Commission (UGC), India for a Senior Research Fellowship
(SRF).
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Fig. 8
FT-IR spectra of DEE (a) and EtOAc (b) soluble products.
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Article
pubs.acs.org/EF
Cite This: Energy Fuels 2019, 33, 4369−4377
Depolymerization of Lignin Using a Solid Base Catalyst
Richa Chaudhary†,‡ and Paresh L. Dhepe*,†,‡
†
Catalysis and Inorganic Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
Academy of Scientific and Innovative Research (AcSIR), New Delhi 110 025, India
‡
Downloaded by NANYANG TECHNOLOGICAL UNIV at 18:32:48:733 on June 02, 2019
from https://pubs.acs.org/doi/10.1021/acs.energyfuels.9b00621.
S Supporting Information
*
ABSTRACT: Lignin extraction from lignocellulosic biomass has attracted considerable attention for an alternative production
of sustainable fuels and chemicals. We report the lignin isolation from coconut coir using Klason, organosolv, and soda methods
and the depolymerization of isolated lignin to value-added chemicals using a solid base catalyst. The yield of isolated lignin by
the Klason method was found to be about 4 to 6 times higher than that by other methods. The structure of isolated Klason
lignin (CC-KL), organosolv lignin (CC-ORGL), and soda lignin (CC-SL) was studied using attenuated total reflection (ATR),
NMR, microanalysis, and so forth. The monomer molecular formula derived from microanalysis suggested that coir lignin is rich
in guaiacyl units. ATR and 13C NMR clearly indicate that CC-ORGL contains more C−C bonds compared to CC-KL and CCSL. Subsequently, these isolated lignins were depolymerized over a solid base catalyst (NaX) under atmospheric pressure. CCSL shows a high yield of aromatic products (28%) compared to CC-ORGL and CC-KL. In order to develop a sustainable future
technology, one-pot depolymerization of coconut coir was performed which resulted in a high yield (64%) of aromatic
products.
1. INTRODUCTION
Cost-effective and efficient conversion of lignocellulosic
biomass, mainly carbohydrates and lignin, is fundamental to
realizing the full potential of the lignocellulose feedstock.
Lignin is especially hard to degrade and represents the major
hurdle for the efficient utilization of lignocellulosic biomass. In
the biorefinery concept, the foremost footstep is to isolate
lignin from lignocellulosic biomass. For the isolation of lignin,
different methods are known in the literature, for example,
Klason, kraft, soda, lignosulfonate, hydrolysis, ionic liquids,
enzymatic, microwave isolation, and so forth.1−10 Out of these
known isolation processes, mainly four processes are currently
used on industrial scale for isolation of lignin; the sulfite
(lignosulfonate), soda, Kraft, and organosolv processes.11
Because coconut is one of the oldest crops grown worldwide
and especially in India, where it presently covers ca. 1.5 million
hectares of land,12 it can be a potential source of the
lignocellulosic material. The approximate quantity of coconut
production per annum around the world is 62.4 × 106
tonnes.13 According to the official website of International
Year for Natural Fibres 2009, approximately, 500 000 tonnes of
coconut fibers (coir) are produced annually worldwide, mainly
in India and Sri Lanka.14 Its total value is estimated at $100
million. India and Sri Lanka are the main exporters, followed
by Thailand, Vietnam, Philippines, and Indonesia. Around half
of the coconut fibers produced are exported in the form of raw
fiber.15 Moreover, it is the most readily available material that
farmers have access to. Therefore, this abundantly available raw
material was chosen as a viable source for lignin isolation and
production of value-added chemicals. A typical cross section of
the coconut is shown in Figure 1 and as seen, coir is the middle
part of the coconut.
Coir is a biomass residue which is generated during the
extraction of coir fiber from the coconut husk. The available
© 2019 American Chemical Society
Figure 1. Cross section of the coconut.
literature for the chemical composition of coconut coir (CC) is
presented in Table 1.
As seen from Table 1, the main contents of CC are cellulose,
hemicellulose, and lignin. Cellulose and hemicellulose are
polysaccharide compounds, while lignin is a macromolecular
polyphenolic compound.23 Compared to other lignocellulosic
materials (wheat straw, rice husk, bagasse, etc.), wherein lignin
content is in the range of 10−-35%, coir contains a higher
amount of lignin (20−48%).23−25 Its accumulation on the
ground during rainy seasons can pollute the soil and water
through leaching of polyphenols which makes the coir pith
unfit for the normal landfill practices. Therefore, suitable waste
management strategies have to be employed to solve the
pollution risks arising due to this particular lignocellulosic
biomass. Recent research programs are focusing to convert
waste coir pith into useful products such as biomanure,26
biochar,27 and so forth. Thus, the lignocellulosic composition
of CC has a huge potential to be explored for its utilization as a
substrate for the production of value-added chemicals.
Received: March 1, 2019
Revised: April 10, 2019
Published: April 23, 2019
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a
Table 1. Chemical Composition of CC
water soluble (%)
pectin and related compounds (%)
5.25
Nd
Nd
Nd
Nd
Nd
Nd
3.30
nd
nd
nd
nd
nd
3.0
hemicellulose (%)
0.25
31.1
15−28
16.8
0.15−0.25
0.25
cellulose (%)
lignin (%)
ash (%)
refs.
43.44
33.2
35−60
68.9
43.0
36−43
43.44
45.84
20.5
20−48
32.1
45.0
41−45
45.84
2.22
nd
nd
nd
nd
nd
5.6
16
17
18
19
20
21
22
a
(nd): not determined.
Figure 2. Isolation of lignin by Klason, organosolv, and soda processes.
2. EXPERIMENTAL SECTION
Looking at the presence of the high lignin content in CC, it
was believed that coir can be converted into value-added
chemicals, and thus an alternate source of income can be
generated for the rural population. Several literature reports
suggest that depending upon the isolation procedures used,
lignin properties vary.28 In order to avoid the use of complex
lignin as a substrate, most of the reports in the literature
worked with lignin model compounds and tried to develop
systems for its valorization. Moreover, few of the researchers
have used real lignin as well as isolated lignins for
depolymerization with different catalytic systems such as
solid acids, solid bases, ionic liquids, and so forth.28−33 In the
present work, the initial phase of the research was to try to
understand the lignin content in CC. Further, focus was on the
isolation of lignin from CC by three methods, namely, K,
organosolv, and soda processes followed by its complete
characterization and depolymerization. Depolymerization of
isolated lignin was studied at the best optimized reaction
condition (250 °C, 1 h) described in our previous work.30,34
Furthermore, possibility of direct utilization of CC as a
substrate for the production of value-added chemicals was also
studied.
2.1. Chemicals and Materials. CC samples (free of the edible
part) were first dried in sunlight for two days and further oven-dried
at 55 °C for 16 h. The dried CC was crushed to the powder form and
again kept in oven at 55 °C for 16 h followed by vacuum drying at 120
°C for 4 h and stored in an air-tight lid container. Basic zeolite and
NaX (Si/Al = 1.2) was synthesized using the known procedure.35
Various aromatic monomers such as guaiacol (99%), 2-methoxy-4methylphenol (98%), pyrocatechol (99%), resorcinol (99%), 2,6dimethoxyphenol (99%), 4-hydroxy benzyl alcohol (99%), diethylterephthalate (99%), 2,4-di-tert-butylphenol (99%), acetoguaiacone
(98%), 4-hydroxy-3-methoxybenzyl alcohol (98%), 1,2,4-trimethoxybenzene (97%), vanillin (99%), eugenol (99%), and 3,4-dimethoxyphenol (99%) used for GC calibration were purchased from SigmaAldrich, Alfa Aesar, and TCI chemicals. All the chemicals were used as
received. Solvents such as ethanol (99%, Changshu Yangyuan
Chemical Co., Ltd, China), diethyl ether (DEE, 99.9%, LOBA
Chemie), and ethyl acetate (99.9%, LOBA Chemie) were purchased
and used as received. NaOH (98%, Loba Chemie), H2SO4 (98%,
Loba Chemie), HCl (37%, Merck), and HF (48%, Merck) were also
purchased and used as received.
2.2. Isolation of Lignin from CC. As known, depending upon the
isolation methods, the structural and chemical properties of isolated
lignin vary. It is well known from the literature that linkages and
functional groups present in lignin varies from plant to plant. Because
of these reasons, determination of the exact structure, bonding, and
functional groups present in lignin is still a big challenge for the
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Figure 3. Methodology for products extraction (* analyzed by GC, GC-MS, and HPLC).
3. RESULTS AND DISCUSSIONS
From the isolation studies, it was realized that the CC sample
used in this study contains 48% of lignin.
3.1. X-ray Diffraction. To understand the crystalline and
amorphous nature of CC and isolated lignin (Klason,
organosolv, and soda), X-ray diffraction (XRD) analysis was
carried out (Figure 4). In the CC sample, a peak at 21.7° is
observed for the crystalline phase of cellulose and 16.7° for the
amorphous phase of cellulose. These very low intensity peaks
are in contrast to other samples wherein peaks for cellulose
have higher intensity because the CC sample contains very
high concentration of lignin (48%), and thus concentration of
cellulose is very low compared to other lignocellulosic material
samples. After the isolation of lignin (Klason, organosolv, and
soda) from CC, no peaks are observed for the crystalline and
amorphous phase of cellulose. This confirms that isolated
samples do not contain cellulose impurities. A broad peak
pattern for the isolated samples confirms the amorphous
nature of lignin.
3.2. Microanalysis. To derive the monomer molecular
formula (MMF) for the samples, elemental analysis (C, H, and
O) for CC and isolated lignin was performed (Tables 2 and S2,
Supporting Information). It is well known that the
composition of polysaccharides and lignin varies as the origin
and type of the plant varies. Even for the same species
collected from different places, the variation is shown in its
composition as it is affected by the type of soil, climate, and so
forth. Here, four different coir samples were used, and the
results are compared with the isolated lignin samples (Tables 2
and S2, Supporting Information). It was observed that CC has
a higher O/C ratio (1.09) compared to isolated lignin (0.46−
0.56). This is very obvious as the oxygen content decreases
after the removal of polysaccharides (C5 and C6 sugars) from
CC. The higher heating value (HHV) calculated using the
Dulongs formula shows that coir has a less HHV (13.4 MJ/kg)
compared to isolated lignin (22.3−24.9) because of the
researchers. Among various procedures known for the isolation of
lignin, in this work, Klason, organosolv, and soda processes for the
delignification of CC were employed (for more details on isolation
procedures, please refer the Supporting Information). A brief on the
isolation methods is illustrated in Figure 2.
From the Klason method, 48% of the lignin yield was obtained.
Because this procedure is known for the quantification of lignin
present in the lignocellulosic biomass, it is estimated that the coir used
in this study has 48% lignin content. In order to avoid any
experimental error, all the isolation experiments were performed at
least three times to check the reproducibility. Based on the lignin
present in CC (considering the Klason method), 16 and 20% isolation
of lignin was possible using organosolv and soda processes,
respectively.
2.3. Catalytic Runs of CC and Isolated Lignin. Prior to the
catalytic runs, the best active alkali metal-substituted zeolite catalyst
(NaX) explored in our previous work was thoroughly characterized
(Table S1, Supporting Information).30 In a typical reaction, the
lignin/NaX or coir/NaX mass ratio was maintained at 1:1 (lignin/
NaX molar ratio = 13.3), and the lignin: solvent (EtOH/H2O, 1:2 v/
v) mass ratio was maintained at 1:60. Initially, the reactor was flushed
with nitrogen, and the heating of the reactor was started under slow
stirring (100 rpm). After attaining the desired reaction temperature
(200 °C/250 °C), the stirring rate was increased to 1000 rpm, and
this was considered as the starting time of the reaction. After the
reaction, the reactor was cooled to room temperature. The catalyst
was separated from the reaction mixture by centrifugation and washed
thoroughly with EtOH/H2O (1:2 v/v) in order to remove any
adsorbed lignin or products on the catalyst. After separation of the
catalyst from the reaction mixture, acidification of the liquid layer was
carried out with 2 N HCl solution until the pH reached to 2. The
acidification process helped precipitate out the high molecular weight
products (filter cake). Further, centrifugation and filtration processes
were carried out. The liquid and solid fractions were subjected to the
extraction process for the isolation of products. Organic solvents DEE
and ethyl acetate (EtOAc) were used for product isolation. Further,
the organic solvent (DEE and EtOAc) soluble products were analyzed
using GC and GC-MS. The methodology for the extraction of
products is represented in Figure 3.
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another case, low O/C and high H/C ratios are suitable for
using those as fuels. The high HHV and lower O/C ratio of
isolated lignin show that it is a good source for the production
of fuels and chemicals. Moreover, double-bond equivalence
(DBE) of all the samples (coir and isolated lignin) was also
calculated, and the data show the DBE between 5.6 and 5.8 for
all the lignin samples. This shows a good correlation with the
MMF and building blocks of lignin, that is, sinapyl, coniferyl,
and coumaryl alcohol. DBE 4 is considered for one benzene
ring and one for the exo carbon−carbon double bond.
Furthermore, a higher value of DBE for the CC-ORGL sample
is observed that reflects the fact that CC-ORGL has a highly
condensed more aromatic structure. During organosolv
pulping treatment, condensation reactions besides the
fragmentation of lignin might also occur which gives rise to
new carbon−carbon bonds in ORGL. According to a former
study, these carbon−carbon interunit bonds were more easily
formed by guaiacyl (G)-type lignin due to the presence of the
free C-5 position.36,37 It can be stated that CC-ORGL is rich in
the 5−5 biphenyl type of linkages. The MMF was also derived
using elemental analysis and based on that, it is suggested that
lignin is rich in guaiacyl units which very well matches with the
literature.38
3.3. Thermogravimetric Analysis−Differential Thermal Analysis (TGA−DTA). Thermal analysis of CC and
isolated lignins was performed under the nitrogen atmosphere
upto 1000 °C (Figures S2−S5, Table S3, Supporting
Information). It was observed that CC and lignin started
losing weight at around 150 °C, may be due to the removal of
moisture. Cleavage of α and β-aryl-alkyl-ether linkages was
observed in the range of 200−450 °C. Complete decomposition of organic moieties was observed at ∼600 °C. Klason
lignin shows ca. 35% of weight loss from 200 to 500 °C. It
might be due to the cleavage of the side chain or aryl-alkylether linkages followed by the cleavage of C−C linkages
present in lignin structural units. It can be stated from the
thermogravimetric analysis−differential thermal analysis
(TGA−DTA) of Klason lignin that it contains more number
of aryl-alkyl-ether linkages as it takes long time to decompose
at ∼200−300 °C. It is well known from the literature that
organosolv lignin is rich in carbon−carbon linkages between
lignin subunits. It is well matched from the TGA−DTA
analysis of organosolv lignin. Similar observation was made for
the soda lignin also as it shows weight loss in the range of
200−360 °C for ether linkages and until 400 °C for the
cleavage of C−C linkages. A careful observation suggests that
Figure 4. (A) XRD patterns of CC and isolated lignin {organosolv
(CC-ORGL), soda (CC-SL), and Klason (CC-KL) lignin}, (B) XRD
patterns of different samples of CC (CC-1-4).
presence of the higher oxygen content and the presence of the
polysaccharide in CC. This can be simply understood based on
the molecular formula of the polysaccharide (cellulose and
hemicellulose) and lignin monomer such as guaiacol. The
molecular formula of cellulose (C6 sugar) and hemicellulose
(C5 sugars) is C6H12O6 and C5H10O5, respectively. These will
give an O/C ratio of 1 which well resembles with the O/C
ratio of CC of an approximate molecular formula C7H9−11O6.
Coniferyl alcohol, a lignin building block unit, and guaiacol
(lignin monomer), having the molecular formulas C10H12O3
and C7H8O2, respectively, show a less oxygen content with a
low O/C ratio. Similarly, the H/C ratio was also calculated and
observed to be ca. 0.1. The low H/C and O/C ratios are
desirable for the use of coir for energy generation, while in
Table 2. Microanalysis of CC and Isolated Lignin
microanalysis
CC
CC-ORGL
CC-SL
CC-KL
C (%)
H (%)
O (%)
O/C
H/C
HHV (MJ/kg)a
DBEb
MMFc
pHd
45.35
4.86
49.79
1.09
0.11
13.4
2.18
C7.6H8.84O6.22
6.4
60.78
5.41
33.81
0.56
0.09
22.3
5.8
C10.1H10.7O4.2
6.3
63.58
6.08
29.34
0.46
0.10
24.8
5.6
C10.6H12.1O3.8
5.9
63.59
6.12
29.39
0.46
0.10
24.9
5.6
C10.6H12.1O3.8
2.95
a
Higher heat value = [0.3383 × C + 1.442 × [H − (O/8)] + 9.248 × S], where C, H, O, and S are wt % of carbon, hydrogen, oxygen, and sulfur.
DBE = [C − (H/2) + (N/2) + 1] where C, H, and N are number of carbon, hydrogen, and nitrogen atoms found from the MMF. cMMF = 100 −
(“C” wt % + “H” wt % + “O” wt %). dpH was measured by dissolving the 0.08 g sample in 5 mL water.
b
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for the presence of nonconjugated phenolic compounds, which
confirms the presence of hydroxyl groups. Moreover,
occurrence of a shoulder band at 230 nm confirms the
presence of mono- and disubstituted aromatic rings.
3.5. ATR Spectroscopy. The ATR spectroscopy analysis
of coir and isolated lignin (from CC) (Figure 5) confirms the
all the lignin samples have DTA peak maxima at different
positions. This is because each lignin has different−different
linkages with various functional groups (shown in attenuated
total reflection (ATR) and 13C NMR). Moreover, molecular
weight and degree of polymerization also reflect in TGA−DTA
graphs. Klason lignin shows peak maxima at 390 °C, having the
highest molecular weight and soda lignin (low molecular
weight compared to Klason lignin) shows peak maxima at 375
°C, which indicates the complete cleavage of carbon−carbon
linkages present in the lignin structure. However, organosolv
lignin has the least molecular weight among them, and it shows
the peak maxima at 335 °C, which indicates the splitting of the
aliphatic side chains and the C−C bond.
It was also observed that in extracted lignin, 22−25% of the
unburnt residue is obtained, while in the coir sample, only 6%
residue is observed. These studies suggest that carbon in the
sample will remain unburnt. This can be explained on the basis
of the MMF derived from elemental analysis. Coir has 7.6 “C”,
8.8 “H”, and 6 “O”. Considering this, almost 95% of carbon
can possibly burn in the form of CO and CH4. It is possible
that 6 “C” will be consumed in the form of 6 CO and another
1.2 carbon can be consumed as 1.2 CH4 molecules. Still, 0.4
“C” is remaining which will remain as the unburnt residue. A
quick calculation discloses that this remaining 0.4 “C” out of
7.6 “C” gives rise to ca. 5% of the residue. This percentage of
the unburnt residue matches well with the experimental data.
Moreover, it is well correlated with the ash analysis which is
also 3−4%. Similarly, for isolated lignin, it contains 10 “C”, 11
“H”, and 4 “O”. After the complete use of “H” and “O” present
in lignin in the form of 4CO + 3CH4, there will be remaining 3
“C” as the unburnt residue. This will give ca. 30% of the
residue. Likewise, the moisture present in the sample can be
correlated with the dryness analysis. For example, from dryness
analysis (Section 2.1, Supporting Information), it was
calculated that coir shows 94.4% dryness in the sample
which shows that the remaining is moisture (ca. 6−7%). This
result also matches well with the experimental data (8%
moisture) obtained from TGA−DTA.
Although all the lignins were isolated from same CC, still,
slight quantitative variation in the loss of the weight percentage
can be derived from TGA data. The decreasing order for the
percentage of weight loss of aryl-ether linkages (200−350 °C)
was as follows: CC-KL (17%) > CC-SL (18%) > CC-ORGL
(20%). The percent weight loss for the cleavage of C−C
linkages (350−400 °C) is 10% in CC-ORGL, while it was 8%
for both CC-KL and CC-SL. At the temperature range of 400−
600 °C, ca. 20% weight loss was observed for CC-ORGL, while
for CC-KL and CC-SL, 18% weight loss was seen. Moreover,
different DTA peak positions also confirm the variation in the
molecular weight of lignins. Hence, it can be clearly stated that
depending upon the selection of the isolation procedure,
lignins with different properties can be obtained.
3.4. UV−Visible Analysis. Contribution of color in lignin
is identified by using UV−vis spectroscopy. Samples were
prepared in EtOH/H2O (1:2 v/v) and were analyzed in the
range of λ = 200−800 nm (Section 2.6, Figure S6, Supporting
Information). UV−vis spectra show an adsorption band at 280
nm and a shoulder at 230 nm with a gradual decrease in
absorption extending toward the visible region, representing
the presence of several different chromophores. These bands
are common in all the samples which corresponds to π−π*
electronic transition in the aromatic ring of unconjugated
phenolic units. The peak appearing at 280 nm can be assigned
Figure 5. ATR analysis of coir (CC) and isolated lignin (CC-KL, CCORGL, and CC-SL).
presence of various functional groups. In the ATR spectrum of
the coir absorption band at the region near 1720 cm−1, a
carboxyl group of acetyl ester in cellulose and carboxyl
aldehyde in lignin is observed.39 Lignin present in the coir
gives characteristics peak at 1220, 1608, and 1720 cm−1 for the
aromatic skeletal vibrations and CO stretching in ketone,
carbonyl, and ester groups. Isolation of lignin reduces
hydrogen bonding due to the removal of the hydroxyl groups.
This results in the decrease of −OH group concentration, as
can be seen from the decreased intensity of the peak between
3650 and 3200 cm−1 compared to the coir. The absorption
band present at 2900−2800 cm−1 can be assigned for the
asymmetric stretching and symmetric stretching of the C−H
bond in methyl and methylene groups. Significant functional
groups in the range of 1700−1550 cm−1 are present in all the
isolated lignin samples. Peaks in the range of 1600−1400 cm−1
confirm the presence of aromaticity or the benzene ring.
Moreover, this band can be assigned for the presence of CC
attached to the aromatic rings. The ATR spectrum of isolated
lignin clearly indicates the presence of the characteristic band
of the C−O stretching of alkoxy groups or the presence of
ether linkages in the region of 1300−1000 cm−1. Peaks in the
range of 850−820 cm−1 correspond to substituted phenolics
and alkene groups. The observance of a peak at 780 cm−1 is
due to deformation vibrations of C−H (oop) bonds associated
to aromatic rings. All the absorption bands observed are
summarized in Table 3.
Furthermore, a careful observation at the ATR spectra of
isolated lignins shows the variation in the presence and
intensities of peaks. CC-ORGL and CC-SL shows the presence
of a more intense peak for the CC group attached to
aromatic rings at 1600−1400 cm−1 and for C−C stretching in
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Table 3. Summary of ATR Bands Present in Coir (CC) and Isolated Lignin (CC-SL, CC-ORGL, and CC-KL)
a
wavenumber (cm−1)
−1
Band (cm )
CC
CC-SL
CC-ORGL
CC-KL
3650−3200
2960−2910
1740−1680
1670−1620
alcoholic and phenolic O−H stretching (free and involved in hydrogen bonding)
C−H asymmetric stretching in the methyl and methylene group
CO stretching in unconjugated ketone, carbonyl, and ester groups
CO stretching in the conjugated substituted aryl group
type of vibration
3298
2916
1720
-
3280
2922
1690
-
3378
2949
1690
-
1615−1595
1520−1505
1470−1440
1370−1350
1300−1200
CO stretching with aromatic skeleton vibrations
aromatic skeleton vibrations
deformation vibrations of the C−H bond
aliphatic C−H stretching in methyl and phenolic OH
C−C, C−O, CO stretching in guaiacyl units
1608
1520
1440
1598
1508
1450
1355
1208
1120−1105
1195−1124
1027−1010
875−700
deformation vibrations of C−H bonds in aromatic rings
deformation vibrations of C−H bonds in syringyl rings
C−O stretching in alcohol, ether/in-plane deformation vibrations of C−H bonds in aromatic rings
substitution on the aromatic ring or substituted phenolics
1113
1027
-
1605
1510
1442
1366
1228
1283
1110
1020
804
3386
2889
1706
1677
1645
1518
1290
1200
1105
1172
1010
812
780
1145
1010
873
a
(-) Not observed.
guaiacyl units at 1300−1200 cm−1. Again, a more intense peak
for the deformation vibrations of C−H bonds in aromatic rings
was observed in CC-ORGL at 1110 cm−1. A peak at 1690
cm−1 was observed in CC-ORGL and CC-SL for the presence
of CO stretching in unconjugated ketone, carbonyl, and
ester groups which was not observed in CC-KL.
3.6. Solid-State 13C NMR. 13C NMR characterizations of
coir (CC) and isolated lignin were performed which reveals
mainly the presence of monolignols and end group
distributions. The 13C NMR spectra of coir and isolated lignin
(Klason, organosolv and soda lignin) are shown in Figures S7−
S10, Supporting Information. Peaks appearing in the range of
160−180 ppm represent the presence of the ester group. The
high intense peak as observed in the range of 110−150 ppm
which corresponds to the presence of sp2 carbon in aromatics
and alkenes confirms the aromatic nature of the samples. The
chemical shift for the aromatic regions for all the samples was
recorded in the range of 90−140 ppm. The presence of sp3
carbon attached to the oxygen atom is also observed in the
range of 55−90 ppm. An intense peak at 55−56 ppm common
in all the lignin samples shows the presence of the methoxyl
group attached to the aromatic ring. The appearance of
methoxy groups in the range of 20−50 ppm was observed in all
the samples. Detailed summary for the assigned peaks are
tabulated in Table 4.
The CC-KL shows the presence of C3 and C4 in the
etherified guaiacyl units at 147.13 ppm, while it was not
observed in CC-ORGL and CC-SL. Moreover, the intensity of
the peak at 130.85 ppm for C2,6 in p-hydroxyphenyl units is
more in CC-ORGL. A comparatively more intense peak for the
presence of C3,5 in p-hydroxyphenyl units is observed in CCORGL at 116.12 ppm, while the presence of C2 in the guaiacyl
unit is only present in CC-SL at 111.11 ppm. The presence of
C2,6 in tricin was observed in CC-ORGL and CC-SL at 106.82
ppm and 106.65 ppm, respectively, while C8 tricin is present in
CC-KL and CC-SL at 92.52 and 92.51 ppm, respectively.
Similarly, Cβ in β-O-4 substructures was observed in CCORGL and CC-SL at 82.28 and 84.76 ppm, respectively, while
Cγ in β-O-4 substructures is present in CC-ORGL at 67.06
ppm. Among all lignins, a highest intense peak at 56.05 ppm
was observed in CC-KL for the presence of the large amount
Table 4. Summary on 13C NMR of CC and Isolated Lignin
(CC-KL, CC-ORGL, CC-SL)a
chemical shift (ppm)
functional group
CC
CC-KL
CC-ORGL
CC-SL
CO group in Hibbert’s
ketone
C4 in the p-hydroxyphenyl
unit
C in β-O-4 substructures
linked with CO
C3 and C4 in the etherified
guaiacyl unit
C2,6 in p-hydroxyphenyl units
C3,5 in p-hydroxyphenyl units
C2 in guaiacyl units
C2,6 in tricin
C8 in tricin
Cβ in β-O-4 substructures
Cα in β-O-4 substructures
175.53
177.91
175.04
175.87
177.98
-
-
160.14
173.39
161.95
174.82
-
155.55
151.89
153.20
152.37
-
147.13
-
116.22
105.47
99.18
93.49
89.04
83.66
74.74
72.60
64.48
55.44
-
115.61
92.52
89.28
72.97
130.85
116.12
106.82
100.86
82.28
72.41
129.19
115.94
111.11
106.65
92.51
88.31
84.76
73.35
56.05
46.64
43.47
67.06
63.14
55.88
48.93
42.86
65.18
56.10
49.75
46.52
42.92
43.47
36.87
-
36.23
39.59
36.50
32.69
30.48
21.17
-
32.90
30.15
23.62
14.58
29.71
23.18
14.08
30.31
19.15
14.91
Cγ in β-O-4 substructures
C−H in the methoxyl group
α carbon CH2 with the
aliphatic substituted group
α carbon in the phenyl
propanol group
−CH2 alkyl group
terminal −CH3 group
a
4374
-
(-) not assigned.
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30
studies. A careful look at the GC-MS chromatographs of
the organic solvent soluble products reveals that DEE contains
higher concentration of low molecular weight products than
EtOAc. This might be because the products were extracted
consecutively in DEE and EtOAc. However, the weight of
products extracted in EtOAc is higher than in DEE. A similar
phenomenon was also observed in our previous work.30
Further, it was assumed that the variation in the properties of
lignin yields different types of products, for example, in the
case of CC-ORGL, it was expected to observe a higher C−C
type of products or dimeric/trimeric products because the C−
C bond is difficult to break under this reaction condition.
Looking at the GC-MS chromatographs of products, it is clear
that under this reaction condition, ether bonds break. More to
this, the catalytic results (product yields and products formed)
also show that the reaction is governed as per the molecular
weight of lignin which is explained above in this section. The
identified products are tabulated in Table S4, Supporting
Information.
As it is always preferable to use abundant and inexpensive
lignocellulose material (CC) directly for the synthesis of valueadded aromatic chemicals in order to develop a sustainable
future technology, here, we have also used it as a substrate.
Hydrolysis of CC was also carried out using NaX as a catalyst,
and it was compared with noncatalytic reaction (Figure 6B).
When reactions were carried out at 200 °C for 1 h, it was
observed that direct hydrolysis of CC shows a good product
yield of 64% compared to noncatalytic reaction (27%). The
product yield was calculated by considering 0.5 g of the
substrate charged. CC contains 48% of lignin; however, under
this reaction condition, we observed a peak for HMF from GCMS and small peaks for glucose and oligomers from HPLC.
Hence, the yield was calculated by considering the weight of
products obtained after the reaction/substrate charged.
Further, the formation of aromatic products was confirmed
by using GC, GC-MS, and HPLC techniques. The GC-MS
chromatogram of the products confirms the formation of
aromatic products (Figure S16, Supporting Information). The
quantification of aromatic products was carried out by
procured standard compounds, and product distribution is
given based on total detected products from GC-MS and
HPLC (Table S5, Supporting Information). Cellulose and
hemicellulose are also present in CC; therefore, in order to
check the formation of any sugar products, HPLC analysis was
also performed, and very low intensity peaks for glucose and
HMF were observed and quantified (Figure S17, Table S5,
Supporting Information). A meticulous look to the formation
of more aromatic products (mostly lignin depolymerized
products) in the case of coir can be understood as lignin
present in CC: the catalyst wt/wt ratio is almost double (0.24
g lignin present in coir). Hence, the product yield increases.
Further the variation observed in CC and isolated lignin
products is possible due to the presence of cellulose and
hemicellulose in coir.
of methoxyl groups attached to the aromatic rings. It was
clearly understood from ATR and 13C NMR that CC-ORGL
contains more C−C bonds compared to CC-KL and CC-SL
which correlates well with the literature also. Moreover,
although same coir is used for the isolation of lignin using
different isolation procedures, different structures of lignins are
observed.
Further on the basis of 13C NMR, analysis peaks were
correlated to the structures present in isolated lignin samples
(Figure S11, Supporting Information).
3.7. Depolymerization of Isolated Lignin Using a
Solid Base Catalyst (NaX). Depolymerization studies were
carried out for isolated Klason lignin (CC-KL), organosolv
lignin (CC-ORGL), soda lignin (CC-SL), and CC using NaX
as the basic catalyst.30 All the reactions were conducted in a
batch mode autoclave (Parr, USA). EtOH/H2O (1:2 v/v) was
used as the solvent system in all the reactions. After the
reaction, the extraction of products from the reaction mixture
was carried out as shown in Figure 3. When reactions with
different lignins were carried out at 250 °C for 1 h, variation in
the depolymerization product yield was observed from 5 to
28% (Figure 6A). It is known from the literature that the order
Figure 6. (A) Depolymerization of isolated lignin and coir using NaX.
Reaction conditions: lignin/coir (0.5 g), NaX (0.5 g), EtOH/H2O
(1:2 v/v, 30 mL), 250 °C, 1 h, (B) direct hydrolysis of CC using NaX.
Reaction conditions: coir (0.5 g), NaX (0.5 g), EtOH/H2O (1:2 v/v)
30 mL, 200 °C, 1 h.
of the molecular weight (Da) of isolated lignin is in the
following order: Klason lignin (CC-KL) > soda lignin (CCSL) > organosolv lignin (CC-ORGL).40,41 As seen from Figure
6, CC-KL showed the lowest depolymerization product yield
mostly due to higher molecular weight.41 Although, in the case
of CC-ORGL and CC-SL, lower molecular weight of lignin is
present but still not much improvement in the yield was seen.
The probable reason for the lower yield is that CC-ORGL is
known to have more C−C linkages compared to C−O−C
linkages (as proven by TGA, ATR, and NMR studies), and
very few C−C linkages can be broken at milder reaction
conditions and thus it may give a lower yield. It can be seen
from the ATR spectra of CC-ORGL and CC-SL, and a more
intense peak at 1600−1400 cm−1 was observed which belongs
to CC groups attached to aromatic groups and 1300−1200
cm−1 for C−C stretching in guaiacyl units. Moreover, 13C
NMR also validates this fact as the intensity of the peak at
150−110 ppm for the presence of sp2 carbon is more in CCORGL compared to CC-SL. Considering this, it was obvious
to see the maximum depolymerization product yield (28%)
with the CC-SL sample. Further, the formation of aromatic
products was confirmed using GC-MS (Figures S12−S15,
Supporting Information). Products formed with isolated lignin
samples were observed similar with commercial lignin
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.energyfuels.9b00621.
Isolation of lignin from CC by the Klason method,
organosolv method, and soda method; characterization
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DOI: 10.1021/acs.energyfuels.9b00621
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of CC and isolated lignins via dryness analysis, analysis
of ash, XRD analysis, elemental analysis, thermogravimetric analysis-differential thermal analysis (TGA−
DTA), ultraviolet−visible (UV−vis) spectroscopy,
ATR spectroscopy, and solid-state 13C NMR; analysis
of the reaction mixture by gas chromatography (GCFID), gas chromatography mass spectrometry (GC-MS)
and high-performance liquid chromatography (HPLC)
(PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: pl.dhepe@ncl.res.in. Phone: +91-20 2590-2024. Fax:
+91-20 2590-2633.
ORCID
Richa Chaudhary: 0000-0002-9960-5266
Paresh L. Dhepe: 0000-0001-6111-8916
Author Contributions
All the authors contributed equally to this work. All authors
discussed the results and implications and commented on the
manuscript at all stages.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
R.C. thanks the University Grants Commission (UGC), India,
for Research Fellowship.
■
ABBREVIATIONS
GC-FID = gas chromatography-flame ionization detector
GC-MS = gas chromatography mass spectrometry
HPLC = high-performance liquid chromatography
ATR = attenuated total reflection spectroscopy
NMR = nuclear magnetic resonance spectroscopy
TGA−DTA = thermogravimetric analysis−differential thermal analysis
XRD = X-ray diffraction
UV−vis = ultraviolet−visible
■
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4377
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Energy Fuels 2019, 33, 4369−4377
Magnetic Nanoparticles: Synthesis,
Functionalization, and Applications
Varun Chaudhary
School of Materials Science and Engineering, Nanyang Technological University,
Singapore 639798, Singapore; Interdisciplinary Graduate School, Nanyang Technological University,
Singapore 639798, Singapore
Richa Chaudhary
Catalysis and Inorganic Chemistry Division, National Chemical Laboratory, Pune 411008, India;
Academic of Science and Innovative Research, New Delhi 110020, India
CONTENTS
1. Introduction
2. Synthesis of Magnetic Nanoparticles
3. Particle Size and Shape Effect on the Magnetic
Properties
4. Functionalization and Surface Coating of
Magnetic Nanoparticles
5. Applications
6. Summary
References
1. INTRODUCTION
Magnetic nanoparticles (MNPs) are of high interest
because of their multidisciplinary applications, including
biomedicine [1–8], magnetic fluids [9, 10], catalysis [11, 12],
magnetic resonance imaging [13–16], data storage [17, 18],
thermal management [9, 10, 19–23], magneto-resistance sensors [24], spintronics [25], environmental remediation and
water treatment [26–28]. Magnetic nanoparticles can be
found almost everywhere, even inside us. The human brain
contains over 108 MNPs of iron oxide per gram of tissue [29].
Ferritin, an iron storage protein which contains magnetic
nanoparticles, is present in almost every living cell.
In principle, every material shows a response to an
applied magnetic field. On the base of their response to
ISBN: x-xxxxx-xxx-x/$xx.00
Copyright © xxxx by American Scientific Publishers
All rights of reproduction in any form reserved.
the magnetic field, they can be distinguished into (a) ferromagnetic, (b) paramagnetic, (c) diamagnetic, (d) antiferromagnetic, and (e) ferrimagnetic. Antiferromagnetic and
ferrimagnetic materials come under the broad category of
ferromagnetic materials. Ferromagnetic (therefore, antiferromagnetic and ferrimagnetic materials) materials show a
strong attraction to a permanent magnet. However, paramagnetic and diamagnetic materials show week attraction and
repulsion, respectively, under an applied magnetic field. The
magnetic susceptibility (!m " is generally used to describe the
category of magnetic materials. For diamagnetic materials,
the !m is negative and temperature independent. Paramagnetic materials exhibit positive !m , which decreases with
increasing temperature. For example, typical values of !m for
diamagnetic copper and paramagnetic aluminium at room
temperature are −0#96 × 10−5 and 2#07 × 10−5 , respectively.
The !m for ferromagnetic materials are very high compared
to those of para- and diamagnetic materials, reaching up
to 106 . Ferromagnetic materials show permanent magnetic
moments even in the absence of an external field. Figure 1
shows the schematic of the field dependence on magnetization for ferromagnetic, paramagnetic, and diamagnetic
materials.
The magnetic properties depend on the exchange coupling forces between adjacent atoms and the distance
between them. It is expected that temperature, which is
directly related to materials entropy, has a vital role in
determining the magnetic properties. When temperature
increases, the arrangement of atomic spin become randomized, i.e., the system entropy increases, yielding a decrease
in saturation magnetization (MS ". The critical temperature
Encyclopedia of Nanoscience and Nanotechnology
Edited by H. S. Nalwa
Volume xx: Pages (1–31)
2
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Figure 1. The magnetization (M! response with applied magnetic field
(H ! in ferromagnetic, paramagnetic, and diamagnetic materials.
where MS becomes zero is known as the Curie temperature
(TC ! where the state of the material becomes paramagnetic.
The TC of iron, cobalt and nickel is 770 ! C, 1121 ! C and
358 ! C, respectively [30].
2. SYNTHESIS OF MAGNETIC
NANOPARTICLES
Magnetic nanostructures with special properties are an exciting area in science and technology. The nanoparticles of
specific size and shape are essential since the magnetic
characteristics such as coercivity, blocking temperature, etc.
depend on size and shape [31]. Shape and size control,
and therefore the anisotropic magnetic structures, are vital
for most applications. The change in reaction temperature,
time, gas flow, and precursors to the solvent ratio and
amount of stabilizing ligands can affect the final characteristics and therefore the size, shape, and size distribution of the
nanoparticles. Hence, the optimum reaction parameters are
vital for the required product. The ratio of the surface area
to the volume of the particle increases as the particle size
decreases. Nanoparticles exhibit very high surface energy
especially if particle size is less than 10 nm. The surface of
each particle allows an additional product of its chemical
modification, which affect the nanomaterial properties significantly. Especially, for magnetic nanoparticles, this modified surface possesses different magnetic characteristics than
those of the particle core [32]. Many types of nanoparticles
with different sizes and shape are now being synthesized via
several physical and chemical methods and can be characterized using various techniques such as with atomic force
microscopy, scanning tunneling microscopy, X-ray diffraction, transition electron microscopy, etc.
The synthesis of these magnetic nanostructures can be
achieved either by bottom-up or top-down methods, similar to the fabrication of other kind of nanoparticles. When
the nanostructure is created from small building blocks, it is
called a bottom-up approach. Moreover, if the nanostructure is created by cutting a bigger piece, it is called a topdown approach. The top-down method has been explored
for several decades in the nanostructuring process but is
reaching its limits. The bottom-up approach, a self-assembly
or self-organization method, has great potential for creating
nanostructures.
There are a number of methods for synthesizing
the MNPs, including high-energy ball milling, microemulsion, co-precipitation, micelles, polyols, mechanochemical, etc. [2, 9, 15, 19, 21, 22, 33–63]. All these methods
have their own advantages and limitations. In the highenergy ball-milling technique, one has to use two or more
bulk metals in the required ratio for a particular time to
obtain a fine powder. However, there are some problems
with the ball-milling technique, the particle size distribution is comparatively broader than those that are chemically
synthesized. Mechanically synthesized nanoparticles have a
large hysteresis loss (area enclosed by the ascending and
descending branches of the magnetization curve) because
of mechanical strain during sample preparation. Chemical
methods allow the formation of nanoparticles with a narrower particle size distribution. In the chemical method, we
usually start from metal precursors and dissolve them in a
solvent. Magnetic nanoparticles of the oxides can be easily
synthesized using different chemical methods. Additionally,
chemical methods are not suitable for the preparation of
ternary or quaternary alloy nanoparticles due to the different potential energy of the starting precursors. High-energy
Table 1. Summary of synthesis methods used for magnetic nanoparticles [12, 64].
Method
Co-precipitation
Thermal decomposition
Micro-emulsion
Hydro- and
solvothermal
Polyol and sol–gel
Biosynthesis
Electrochemical
Aerogel vapour
Sonochemical
High energy
ball milling
Process and
reaction condition
Reaction
temperature ( ! C)
Reaction
time
Size
distribution
Shape
control
Yield
Very simple, ambient
Complicated, insert atmosphere
Complicated, ambient
Simple, high pressure
20–150
100–350
20–80
150–220
Minutes
Hours to days
Hours
Hours to days
Relatively narrow
Very narrow
Narrow
Very narrow
Not good
Very good
Good
Very good
High
High
Low
High
Complicated, ambient
Complicated, ambient
Complicated, ambient
Complicated, insert atmosphere
Very simple ambient
Very simple
25–200
Room temperature
Room temperature
More than 100
20–50
–
Hours
Hours to days
Hours to days
Minutes to hours
Minutes
Hours to days
Narrow
Broad
Medium
Relatively narrow
Narrow
Broad
Good
Bad
Medium
Medium
Bad
Bad
Medium
Low
Medium
High
Medium
High
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
ball milling is appropriate for these ternary or quaternary
alloy nanoparticles [19, 21].
Magnetic nanoparticles can be divided into transition or
rare earth metals (e.g., Fe, Ni, Co, Gd etc.), alloy nanoparticles (e.g., Fe–Co, Fe–Ni, Fe–Ni–Mn, Fe–Pt, etc.) and oxide
of metals (Fe3 O4 , Fe2 CoO4 , Fe2 Mnx Zn1−x Fe4 , etc.). Fe, Ni,
and Co are the most useful magnetic materials that have ferromagnetic properties at room temperature and paramagnetic at higher temperature. However, nanoparticles of these
elements have not been fully studied because of their active
surface in nanoscale. There are several studies on synthesis and magnetic properties of Fe, Ni, and Co nanoparticles
available in the literature [65–67].
In the next sections, we will discuss the synthesis and magnetic properties of Fe–Co, Fe–Ni and oxide nanoparticles.
2.1. Fe–Co Nanoparticles
The Fe–Co binary alloys have been extensively studied
because of their advantageous soft magnetic properties,
e.g., highest levels of saturation magnetization, high permeability, low magnetocrystalline anisotropy, high Curie temperature, and their catalytic activities [68, 69]. These alloys
in bulk form are attractive for elevated temperature applications e.g., turbine engine components, transformers, and
magnetic bearings. In last decade, FeCo nanoparticles have
been a subject of research for their exceptional physical,
chemical, and magnetic properties that differ from their
counter bulk materials. The novel properties are due to
small particle size, high surface-to-volume ratio, morphology, and surface coating of nanoparticles [60,70]. FeCo
nanoparticles are an important soft material; however to
synthesize monodisperse particles is a big challenge because
they are sensitive to oxidation and therefore chemically
unstable.
Chaubey et al., [60] synthesized monodisperse FeCo
nanoparticles by reductive decomposition of metal acetylacetonates in oleylamine and 1,2-hexadecanodol under a
hydrogen plus argon atmosphere at 300 " C. They have investigated the effect of particle size and composition on magnetization and proposed that the saturation magnetization
(MS ! is 207 emu/g for 20 nm particles while for 10 nm
3
it is only 129 emu/g. Figure 2 shows TEM image and the
change in magnetic properties with particles size for Fe–Co
nanoparticles.
The maximum MS for FeCo (with a Fe/Co molar ratio
of 1.5) was approximately equal to the bulk value. In addition, for the same particle size, the MS depends on the reaction temperature; decreasing the reaction temperature from
300 " C to 290 " C for 20 nm particles results in decrease of
the magnetization from 207 to 171 emu/g [60].
FeCo nanoparticles in the range of 5–10 nm size having high MS were synthesized using a modified chemical
reduction method [71]. Monodisperse FeCo nanoparticles
exhibited saturation magnetization of 220 emu/g. The magnetization was found to be dependent on the temperature at
which the reducing agent was introduced. FeCo MNPs were
synthesized by a surfactant-free hydrothermal method [72].
The particle size was controlled by reaction time and temperature. The MS and maximum coercivity (HC ! were found
to be ∼148.2 emu/g and ∼411.0 Oe at room temperature,
respectively. A quaternary microemulsion system was used
to synthesize size-controlled FeCo nanoparticles [73]. The
effects of process parameters such as the water-to-surfactant
molar ratio and molar concentration of metal salts on the
particle size were explored.
Poudyal et al., prepared monodisperse air-stable FeCo
nanoparticles with average sizes of 8, 12, and 20 nm by a
novel synthetic method [74]. First they synthesized different sizes of CoFe2 O4 nanoparticles by a chemical solution
method. The as-synthesized CoFe2 O4 nanoparticles were
then mixed with NaCl powder and heated to 400–500 " C
under argon plus hydrogen gas atmosphere. Monodisperse
FeCo nanoparticles were recovered by dissolving the NaCl
in water and subsequent wash by ethanol and acetone. FeCo
nanoparticles retained the same size of the starting CoFe2 O4
nanoparticles, and therefore the size of the FeCo nanoparticles can be well tuned by controlling the size of the CoFe2 O4
nanoparticles. The MS of FeCo nanoparticles increased with
increasing the particle size. FeCo alloy nanoparticles with
several Fe to Co ratios were synthesized using a modified
polyol process [75]. Fe-rich particles had a cubic shape with
a mean particle size of 100 nm, while Co-rich particles had a
spherical shape. A maximum MS of 212 emu/g was recorded
Figure 2. (A) TEM bright field image of FeCo nanoparticles with an average particle size of 20 nm, and (B) the variation of magnetization of FeCo
nanoparticles with particle size and molar ratio of Fe and Co (inset). Reprinted with permission from [60], G. S. Chaubey, et al., J. Amer. Chem.
Soc. 129, 7214 (2007). © 2007, American Chemical Society.
4
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
for both Fe60 Co40 and Fe75 Co25 MNPs. In a different study,
Fe100−x Cox nanoparticles with an average particle size of
∼30 nm were also synthesized using a modified polyol process [76]. The shape of Fe100−x Cox nanoparticles, which
was confirmed by transmission electron microscopy, was
found to be cubic and hexagonal for Fe57 Co43 and Fe34 Co66 ,
respectively. The magnetic moment of Fe57 Co43 nanoparticles (21.5 kG) was more than that of the magnetic moment
of Fe34 Co66 nanoparticles (18 kG). Both samples exhibited
the coercivity in the range of 150–165 Oe.
2.2. Fe–Ni Nanoparticles
Fe–Ni MNPs exhibit magnetization comparable to the bulk
and can be prepared by many techniques while their coercivity values become higher than their bulk value [59, 61].
Fe0!635Ni0!365 MNPs were prepared by the electrical explosion
of wire [77]. Increase of the energy injected into the wire
led to an increase of the specific surface of the nanoparticles from 4.6 to 13.5 m2 /g. The fabricated MNPs were
spherical and weakly aggregated with an average diameter
in the range of 54–160 nm depending on the specific surface. The phase composition of FeNi MNPs contained of
two solid solutions of Ni: "-phase (5–10 wt%) and #-phase
(90–95 wt%). The obtained high values of the MS are in
accordance with phase composition and average diameters
of the MNPs. Comparative analysis of structure, and magnetic and microwave properties showed that the obtained
MNPs are important magnetic materials with high MS and
significant zero-field microwave absorption.
Another composition, Fe45 Ni55 MNPs with average diameter of 69.5 nm were also synthesized by electrical explosion
of wire [78, 79]. FeNi MNPs/copolymer were prepared and
interaction between the MNPs and polymeric matrix was
studied. The MS for bared MNPs and MNPs in composite
was found to be 125 emu/g and 120 emu/g, respectively. XPS
analysis of the samples was conducted by varying the electron exit angle with respect to the surface [79]. The main
phase was the FeNi (fcc) core while the barrier layer was
NiFe2 O4 (spinel) with a thickness of 2–6 nm [79].
Structural and magnetic properties of thermal plasma synthesized Fe100−x Nix (x = 25$ 33$ 50$ 67, and 75) MNPs were
investigated [80]. High-temperature gas phase nucleation
and growth environment inside thermal plasma reactor facilitate stabilization of the disordered #-FeNi (fcc) phase for
x = 33$ 50$ 67, and 75. However, for composition x = 25,
∼8% of the disordered bcc phase along with ∼92% fcc phase
was obtained. The average crystallite size (in the range of
30–40 nm) as well as particle size distribution was almost the
same for all the composition. The MS for these nanoparticles were close to that for their corresponding bulk. Room
temperature Mössbauer spectroscopic data revealed that the
alloy nanoparticles contain two different sites for Fe corresponding to high moment/low moment states. The average magnetic moments/f.u. were found to be matched with
the values for bulk Fe1−x Nix alloys. The highest ordered
magnetic moment was found to be 1.4 %B /f.u. for Fe50 Ni50
nanoparticles.
Table 2 summarizes how magnetic properties of FeCo and
FeNi nanomaterials change with chemical synthesis method
and composition.
2.3. Iron Oxide-Based Nanoparticles
Out of eight iron oxides, hematite ("-Fe2 O3 &, magnetite
(Fe3 O4 &, and maghemite (#-Fe2 O3 & are promising candidates due to their polymorphism involving temperatureinduced phase transition [12, 64]. These three iron oxides
have unique biochemical, magnetic, and catalytic properties. Hematite is widely used in pigments, catalysts, and gas
sensors due to its low cost and its high corrosion resistance behavior. In general, hematite can be used as a starting material for the synthesis of magnetite and maghamite.
Hematite crystallizes in the rhombohedral lattice system
(R3c) with two thirds of the octahedral sites occupied by
Fe3+ . Magnetite has a face-centered cubic (Fd3m) spinel
structure based on 32 O2− , and both divalent and trivalent iron with stoichiometric ratio of half (FeII /FeIII = 1/2).
Divalent irons (FeII & can be partially or fully replaced by
other divalent ions, e.g., Mn, Zn, Co, etc. Maghemite is a
cubic structure with 32 O2− , 21-1/3 Fe3+ , and 21/3 vacancies. Maghemite can be synthesized by the oxidation of
magnetite.
CoFe2 O4 has an inverse spinal structure; the relative distribution of the Co2+ and Fe3+ cations in the tetrahedral and
octahedral sites depends on the production method and heat
treatment process [82, 83]. The magnetic moments of cations
in the A (tetrahedral coordination of four O2− ions) sites
and B (octahedral coordination of six O2− ions) sites are
aligned antiparallel with respect to one another, resulting in
a ferromagnetic ordering. The quantity of Co2+ in B-sites of
the sample, synthesized by the sol–gel method was less than
that of the synthesis by the ball-milling method. CoFe2 O4
prepared by ball milling exhibited smaller saturation magnetization than that synthesized by the sol–gel method. On
the other hand, the coercivity for the ball-milled sample was
larger. The magnetic moments of the slowly cooled and the
quenched CoFe2 O4 were 3.4 and 3.9 %B /fu, respectively [83].
Co-precipitation is the most conventional method for
obtaining oxide MNPs [64]. This method consists of the mixing of ferric and ferrous ions in basic solutions. The size and
shape of the MNPs can be controlled by the salt (e.g., chlorides, sulfates, nitrates, etc.) used in the reaction, the ferric and ferrous ions ratio, the reaction temperature, the pH
value, ionic strength of the media, stirring rate, and dropping
speed of the basic solution. Suspensions of Fe3 O4 MNPs in
an aqueous solution without surfactant were studied [84].
The Fe3 O4 nanoparticles were oxidized to #-Fe2 O3 by direct
aeration of the suspension at ∼100 # C. The nanoparticles
were nearly spherical with an average diameter of ∼10 nm
and narrow size distribution. However, the synthesis by the
co-precipitation method resulted in a wide particle size distribution of MNPs, which can result in non-ideal magnetic
behavior (e.g., a wide range of blocking temperature) for
many applications. Figure 3 shows the schematic representations of the synthesis of Fe3 O4 MNPs by co-precipitation,
reverse microemulsion, and thermal decomposition [85].
Highly crystalline Fe3 O4 nanoparticles were synthesized
by a one-step hydrothermal process without using surfactants [86]. Ferrous chloride (FeCl2 & and diamine hydrate
(H4 N2 H2 O) were used as starting materials. The Fe3 O4
MNPs with average particle size of 40 nm possessed a MS of
85.8 emu g−1 , lower than that of the MS (92 emu g−1 & of corresponding bulk. The well-crystallized Fe3 O4 grains formed
5
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Table 2. Change in magnetic properties with synthesis method and composition of materials.
Magnetic properties
Method
MNPs and size
Ms (emu/g)
Hc (Oe)
Comment
Ref.
231.46
214
800
32.5
–
–
[33]
[34]
–
[35]
–
[36]
–
–
[37]
[38]
–
[39]
–
[40]
–
[41]
Co-precipitation
Chemical solution mixing
and hydrogen reduction
Wet chemistry
Fe71 Co29 16.26–20.00 nm
Fe-50wt%-Co ∼50 nm
Chemical vapor
condensation
Polyol
Polyol
42.08 for 12 nm
271.3 for 12 nm
83.58 for 14 nm
991.2 for 14 nm
133.3 for 18 nm
811.2 for 18 nm
238
30
Fe40 Co60 10–40 nm
Fex Co1−x /Coy Rez Fe3−y−z O4 , 86 for Re = none
Re = none, La, Nd, Gd, 76 for Re = La
∼230–270 nm
82 for Re = Nd
63 for Re = Gd
Fe, Fe–Co, Al(III)
179 for Fe
450 for Fe
protected Fe–Co
217 for Fe–Co
650 for Fe–Co
131 for Fe–Co–Al
1070 for Fe–Co–Al
Fe–Co and Fe–Cr
Higher Hc for Fe–Co then
Fe–Cr
nanocluster wire,
Diameters∼10–30 nm
Fe–Co, surround by
111.6, 144.5 after
43, 70 after thermal
graphite shell of 1.2 to
thermal treatment
treatment
2 nm thickness, 8 nm
Fe–Co 35–200 nm
–
–
Fe–Co
–
–
Polyol
Fe–Co 50–300 nm
Polyol
FeCo 50–300 nm
–
–
Wet-chemical
Fe60 Co40 15 nm
–
–
Chemical reduction
FeCo 10 and 20 nm
Polyol
Fe 10–150 nm
Wet chemistry
Fe1−x Nix ,
x = 0!25" 0!50" 0!75
12–15 nm
Wet chemical
Fex Ni1−x " x = 0.15, 0.20,
0.22, 0.25, 0.30, 0.50 and
0.65 ∼10 nm for x = 0.50
to ∼20 nm for x = 0.15
Adsorption and reduction
processes
Fex Coy Ni100−x−y (x = 21,
24, 33, 37, 46 and
y = 60, 46, 48, 48, 35),
11.9–17.7 nm increase
with Fe content
Sonochemical synthesis
Solvothermal synthesis
Thermal reduction
Thermal decomposition
Thermal decomposition
Fe–Co attached with
carbon nanotubes
13–25 nm
Fe–Co-35wt% 12–18 nm
110 for 13 nm,
250 for 25 nm
225 for Fe70 Co30
207 for 20 nm
and 129 for 10 nm
210 for 150 nm
90 for 10 nm
120.76 for x = 0!25
39.34 for x = 0!50
47.64 for x = 0!75
530 for 13 nm
525 for 25 nm
–
Fe rich FeCo is cubical
while Co rich alloys are
nearly spherical
Shape change from
spherical to cubic when
concentration of Fe
increased from 40 to 60
at%, Ms decrease upto
20% because of
oxidization within 7 days
Size change by variation of
metal ion concentration
A carbon shell was
surrounded on each
annealed particle
–
–
436.7 for x = 0!25
267.0 for x = 0!50
261.4 for x = 0!75
–
–
114.7–132.0
693.8–618.2
[42]
[47]
[48]
[50]
[51]
[60]
Size change by varying the
molar concentration of
Pt nuclei
The coercivity decreases
with increasing Ni
concentration and with
decreasing the size, the
magnetization change
with Ni
For x = 0.15–0.50 two fcc
phases but for x = 0.65
the system condensed
into a mixture of the
FCC and BCC phases
The Hc of Fex Coy Ni100−x−y
alloy nanoparticles
attached on CNTs
decreases and Ms
increases with increasing
Fe content
[52]
[53]
[54]
[55]
6
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Table 2. Continued.
Magnetic properties
Method
MNPs and size
Ms (emu/g)
Hc (Oe)
Comment
Ref.
Particle sizes of the
produced Fe–Ni particles
can be controlled by the
change of the
concentration of an
initial solution
Monodisperse and soluble
in water
TC =82 " C for Fe70 Ni30
Addition of 1 wt% Mn
to Fe75 Ni25 reduced the
TC to 78 " C
Particle size, crystallite size,
and crystal phases varied
with the type of polyol
The magnetization of the
nanoparticles was much
lower than that of the
bulk material
The sphere size can be
tuned from 70 to 25 nm
in diameter by changing
the concentrations of
metal ions
[56]
Ultrasonic spray pyrolysis
Fe–Ni ∼28 by XRD ∼50
by TEM
–
–
High temperature pyrolysis
Fe–Ni 9 nm
40
–
Chemical reduction
Fe90 Ni10 to Fe70 Ni30
80 for Fe70 Ni30 ,
100 for Fe80 Ni20
117 for Fe88 Ni12
Polyol
Fe20 Ni80 19 nm 14.8 nm
73 emu/g and
83 emu/g,
Reverse micelle technique
Fe0!2 Ni0!8 4–12 nm
Solution phase reduction
Fe–Ni 70 nm 25 nm
98 for 25
99 for 70
246 for 25
143 for 70
Organometallic route
Fe–Ni
69 for Fe25 Ni75
89 for Fe35 Ni65
191 for Fe25 Ni75
103 for Fe35 Ni65
–
41 and 123
12
–
[15]
[81]
[61]
[57]
[58]
[59]
Figure 3. Synthesis of iron oxide nanoparticles by (a) co-precipitation, (b) reverse microemulsion, and (c) thermal decomposition. Reprinted with
permission from [85], J. Gautier, et al., Nanotechnology 24, 432002 (2013). © 2013, IOP Publishing.
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
under appropriate hydrothermal conditions were responsible for the increased MS in Fe3 O4 MNPs. Moreover, Zheng
et al. reported a hydrothermal route for preparing Fe3 O4
MNPs with an average particle size of 27 nm in the presence of a surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT) [87]. Highly crystalline magnetite nanoparticles
with an average particle size of 39 nm, a good monodispersity, and relatively enhanced magnetic properties were
prepared by the co-precipitation of ferrous Fe2+ and ferric
Fe3+ ions by (N(CH3 !4 OH), followed by a hydrothermal at
250 ! C [88].
Iron oxide nanoparticles with average particle sizes in
the range 4–28 nm were prepared by thermal decomposition of an iron precursor in high-boiling solvents [89].
The MS was found to increase by increasing the size of
the nanoparticle [89, 90]. MFe2 O4 (M = Mn, Fe, Co, Ni,
and Zn) MNPs was synthesized by a technique in which
a long-chain amine was used as a reducing and surfacefunctionalizing agent [91]. The particle sizes were tuned by
the manipulation of the amine to precursor molar ratio and
by seed-mediated growth. Figure 4 shows the TEM images
of MnFe2 O4 nanoparticles with average size of 2, 3, 4, 6, 9,
12, and 16 nm, and selected area diffraction pattern (SAED)
for Fe3 O4 nanoparticles.
The MS of the MNPs was changed by the composition and
nanoparticle size from 27 emu/g (ZnFe2 O4 with particle size
of 4 nm) to 86 emu/g (MnFe2 O4 with particle size of 16 nm).
The MS of MFe2 O4 MNPs was maximum for M = Mn, while
minimum for M = Zn. An increment in MS with increasing
the particle size was also observed (Fig. 5).
The decomposition of Fe(cup)3 (cup = N -nitrosophenylhydroxylamine), Fe(acac)3 (acac = acetylacetonate),
or Fe(CO)5 followed by oxidation can lead to high-quality
monodispersed iron oxide MNPs [64, 92]. However, high
7
temperatures and complicated operation are required
in thermal decomposition method. Monodisperse Fe3 O4
MNPs were synthesized by high-temperature solution-phase
reaction of Fe(acac)3 in phenyl ether with alcohol, oleic
acid, and oleylamine [92]. Seed-mediated growth was used
to control particle size from 3 to 20 nm. The as-synthesized
Fe3 O4 nanoparticle assemblies can be transformed easily
into either "-Fe2 O3 or #-Fe nanoparticle assemblies by
changing the annealing conditions.
Recently, the leaf extract of Cynometra ramiflora was used
to synthesize iron oxide nanoparticles [93]. The addition of
iron sulfate to the leaf extract results in the formation of
iron oxide nanoparticles. Crystal structure and shape of the
MNPs were confirmed by XRD and SEM, respectively. The
catalytic activity of these nanoparticles as Fenton-like catalyst was examined for the degradation of Rhodamine-B dye.
Magnetic core–shell hybrid nanoparticles with magnetic
poly lactide-co-glycolide nanoparticle (core), surface modified with folate-chitosan conjugate (shell) were studied for
anti-cancer therapeutic and magnetic resonance imaging
(MRI) contrast agents [16]. The folate-chitosan conjugate
was prepared using carbodiimide crosslinking chemistry at
an optimized folate to amine (chitosan) molar ratio for
the coating on poly lactide-co-glycolide nanoparticles loaded
with docetaxel and well-packed superparamagnetic iron
oxide nanoparticles. The biocompatible hybrid nanoparticles
provide receptor-targeted docetaxel and iron oxide nanoparticles delivery for anti-cancer therapy and MRI, respectively.
Enhancement in nanoparticle uptake by folate receptor
positive oral cancer cells caused a significant increase in
docetaxel-mediated cytotoxicity. However, polymeric encapsulation and folate-chitosan coating negatively affects the
magnetic property of iron oxide nanoparticles; their aggregation in the core shortened the overall relaxation time
Figure 4. TEM images of MnFe2 O4 MNPs of (a) 2 nm, (b) 3 nm, (c) 4 nm, (d) 6 nm, (e) 9 nm, (f) 12 nm, (g) 16 nm, and (h) SAED pattern
of Fe3 O4 nanoparticles. The corresponding inset shows the histogram of average particle size distribution. Reprinted with permission from [91],
J. Mohapatra, et al., Cryst. Eng. Comm. 15, 524 (2013). © 2013, Royal Society of Chemistry.
8
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Figure 5. Field-dependent magnetization loop of (a) MFe2 O4 nanoparticles for M = Mn, Fe, Co, Ni and Co, (b) MnFe2 O4 for particle sizes varying
from 2 nm to 16 nm. Reprinted with permission from [91], J. Mohapatra, et al., Cryst. Eng. Comm. 15, 524 (2013). © 2013, Royal Society of
Chemistry.
thereby enhancing the nanoparticle relaxivity to provide
better in vitro MR imaging. Iron oxide nanoparticles and
the anticancer drug, Methotrexate, were encapsulated into
polycaprolactone–polyethylene glycol (PCL–PEG) nanoparticles for local treatment [94]. The drug encapsulation efficiency achieved for Fe3 O4 MNPs modified with PCL–PEG
copolymer was 92.36%.
Magnetic interaction of the Fe3 O4 nanoparticles grown on
zeolite was studied [94]. The average particle size of Fe3 O4
can be controlled by zeolite content. The average hyperfine
field of Fe at octahedral and tetrahedral sites vary with the
size of nanoparticle. With the increase of zeolite from 0 to
100 mg, the saturation magnetization decreases from 63 to
53 emu/g. The nanoparticles in the pores of zeolite are connected with the surface particles in low zeolite content and
get isolated as the amount of zeolite increases.
Egg-shaped MNPs with a core–shell morphology were
synthesized by thermal decomposition [95]. Oxidation of
FeO to Fe3 O4 was found to be the mechanism for the shell
formation. As-made nanoparticles exhibited high values of
exchange bias at 2 K. Oxidation led to a decrease of the
exchange field from 2880 Oe to 330 Oe. At temperatures
higher than the Néel temperature of FeO (200 K) there
was no exchange bias. An interesting observation was made
showing the exchange field to be higher than the coercive
field at temperatures close to magnetite’s Verwey transition.
3. PARTICLE SIZE AND SHAPE EFFECT
ON THE MAGNETIC PROPERTIES
Magnetic properties can be altered if at least one dimension of the specimen is in the nanometer range [96].
Size-specific magnetic properties, for example, superparamagnetism, remanence enhancement and exchange averaging
of anisotropy are usual, when the small dimensions become
comparable to a magnetic domain scale. The transition from
superparamagnetic to single-domain to multi-domain for
MNPs depends on the size and/or geometry of the particles.
The influence of particle radius (r) on the magnetic properties can be expressed by
!
6kB T
(1)
r= 3
KU
where kB , T and KU are the Boltzmann constant, temperature, and anisotropy constant, respectively. Saturation
magnetization varies with particle size until it reaches a
threshold size, which is close to MS of the bulk value.
A model is presented to describe the magnetic properties of nanomaterials as a function of particle size [97]. The
model is applied to the decay rate of the magnetic after
the effect in nanomagnets and the variation of spontaneous
magnetization versus temperature. However, the results
showed a deviation from the behavior of bulk media above a
critical temperature. A modified Presiach-Arrhenius (MPA)
model has recently been presented, which predicts time
reliability of magnetization state in MNPs based on quasiequilibrium thermodynamics [98–100.] This model allows
the derivation of an analytical formula for the time variation
of the magnetization based on a Gaussian distribution.
The role of the surfactant and reducing agent on the
shape, size, and magnetic properties of iron oxide nanoparticles was studied [101]. The pseudospherical and faceted
particles (4–20 nm) with a Ms of 80 to 85 emu/g at 5 K
was obtained when using oleic acid as a surfactant. On the
other hand, decanoic acid results in larger pseudocubic particles of 45 nm and a larger Ms of 92 emu/g at 5 K, close to
the expected value for bulk magnetite. The use of reducing
agents 1,2-hexadecanediol and hydrazine results in uniform
and nonuniform oxidation of the nanoparticles, respectively.
The size and shape of MNPs can be changed by introducing the different reducing agent e.g., 1,2-hexadecanediol,
hydrazine, fecanoic acid, etc. [101, 102]. Figure 6 shows
the TEM images of MNPs that have a cube and spherical shape and corresponding magnetic properties [103]. The
field-dependent magnetization measurements showed that
the cube has a higher Ms of 165 emu/g (Fe + Zn) than that
of the sphere of 145 emu/g (Fe + Zn). The cubic iron oxide
MNPs exhibited a higher degree of crystallinity than their
spherical counterparts [104].
FePt nanoparticles with the shape of an octapod, cuboctahedron, and nanocube were prepared from cuboctahedral
seeds [105]. The formations of FePt nanostructures were
attributed to the differences in the growth rate between the
111 and 100 planes of cuboctahedral seeds. The order of
volume, V!cube" > V!octapod" > V!cuboctahedron" revealed the order
of Ms , Ms !cube" > Ms !octapod" > Ms !cuboctahedron" . Larumbe et al.
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
9
Figure 6. (a) TEM images of (a) cube (18 nm), (b) sphere nanoparticle (22 nm), (c) cube exhibiting lattice fringes of 100, and (d) M–H curves of
cube and sphere measured at 300 K. Ms of cube is 165 emu/g (Fe + Zn), and that of sphere is 145 emu/g (Fe + Zn). Simulated magnetic spin states
of (e) cube and (f) sphere. The color map indicates the degree of spin canting against external magnetic field where red indicates nondeviated
spins and blue indicates highly canted spins. Local spin states on the surfaces of nanoparticles are depicted on the right corners of parts (e) and (f).
Reprinted with permission from [103], S.-H. Noh, et al., Nano Lett. 12, 3716 (2012). © 2012, American Chemical Society.
studied the effect of Ni doping on the structural and magnetic properties of Fe3 O4 MNPs [106]. The oxidation of
the Ni atoms to the 2+ state and the Ni2+ cations in the
Fe2+ octahedral sites was observed. The estimated magnetic moment confirmed the partial substitution of the
Fe2+ cations by Ni2+ atoms in the octahedral sites of the
spinel structure. The properties of ferrite-based MNPs are
highly sensitive to the cation distribution, which can be controlled by reaction parameters and substitution of different
metals. Gopal et al. prepared Ni0!8−x Zn0!2 Mgx Fe2 O4 ferrites
with x = 0.0–0.8 using the egg-white method [107]. Magnetic properties measured at room temperature revealed
a decrease in Ms up to Mg content of 0.6. The unexpected increase in the Ms at Mg content of 0.8 was
attributed to the tendency of Mg2+ ions to occupy the tetrahedral site. The decrease in the value of coercivity with
increasing magnesium content can be explained based on the
magneto-crystalline anisotropy. Different magnetic properties (MS and HC " of concave cubic-and octahedron-shaped
Fe63 Ni37 nanoparticles were observed for these FeNi alloy
nanoparticles, which were correlated with surface anisotropy
and existence of two-phases (Fig. 7) [108].
We have summarized the dependence of MS and HC on
size and shape of the nanoparticles in Table 3.
The surface and interface effects become more important when particle size decreases, e.g., cobalt nanoparticle
with a diameter of ∼1.6 nm exhibits 60% of the total number of spins on the surface [12, 114]. Therefore, the surface spins play an important role in the resulting magnetic
properties of MNPs. The broken symmetry on the surface
forces changes in the lattice constant and atom coordination.
Under these conditions, surface atoms experience large
10
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Figure 7. SEM images of (a) concave cube (CC) and (b) cuboctahedron (COh) nanoparticles with their corresponding higher magnification images
and schematic diagrams. (c) Hysteresis loops for both CC and COh nanoparticles, with an expanded view, (b) zero-field cooled (dashed lines) and
field-cooled (solid lines) magnetization curves with an applied magnetic field of 100 Oe. Reprinted with permission from [108], N. Moghimi, et al.,
Journal of Materials Chemistry C 2, 6370 (2014). © 2014, Royal Society of Chemistry.
anisotropy, and core–surface exchange anisotropy can also
occur. Large anisotropy on the surface due to the broken symmetry of their surrounding is called Neel surface
anisotropy [21, 115].
With decreasing particle size, the magnetic anisotropy
(Keff ! exceeds the value obtained from the crystalline and
shape anisotropy, which is expected from the change in
surface anisotropy. The anisotropy energy of a spherical particle (Keff ! can be defined as the sum of the contribution from
the bulk (core) and from the surface: Keff = KV + #6/d!KS .
Where KV , KS , and d are bulk anisotropy, surface anisotropy
and particle diameter, respectively. Hence, the Keff can be
changed by modifying the surface of the nanoparticles [116].
Table 3. The dependence of magnetic properties on the size, shape and composition of MNPs.
MNPs
Fe3 O4
Fe3 O4
CoFe2 O4
MnFe2 O4
Zn0"4 Fe2"6 O4
CoFe2 O4
Particle
size (nm)
4.2
7.4
8.1
17
45
4.9
6.3
8.6
Magnetic properties
Shape
Pseudospherical
Pseudospherical
Pseudocubic
Ms (emu/g)
HC (Oe)
Comment
Ref.
75
70
65
82
92
60.4
64.8
58
318
270
70
364
340
–
The particles size and shape were tuned
by changing the reducing agents in the
reaction. The magnetic measurement
was done at 5 K
[101]
[109]
–
NaOH, alkanolamines isopropanolamine
(MIPA) and diisopropanolamine
(DIPA) were used as co-precipitation
agents. The magnetic properties were
performed at 300 K
NaOH, MIPA and DIPA were used as
co-precipitation agents. The magnetic
properties were performed at 300 K
NaOH, MIPA and DIPA were used as
co-precipitation agents. The magnetic
properties were performed at 300 K
Ms is measured in emu/g (Mn+Zn). The
magnetic measurement was done at
300 K. Volume of sphere = volume of
cube
Combining nonhydrolytic reaction with
seed-mediated growth results in a
high-quality and monodisperse spinel
cobalt ferrite, Ms and Hc were
measured at 5 K. Volume of
sphere = volume of cube
4.2
4.8
18.6
9.3
11.7
59.5
22
18
Sphere
Cube
30.6
46
48.8
57.1
54.6
35.2
145
165
10
8
Sphere
Cube
80
80
–
−60
16000
9500
[109]
[109]
[103]
[110]
11
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Table 3. Continued.
MNPs
Fe3 O4
Fe3 O4
!-Fe2 O3
FePt
Fe3 O4
Ni0$04 Fe2$96 O4
Ni0$06 Fe2$94 O4
Ni0$11 Fe2$89 O4
Ni0$8 Zn0$2 Fe2 O4
Ni0$6 Zn0$2 Mg0$2 Fe2 O4
Ni0$4 Zn0$2 Mg0$4 Fe2 O4
Ni0$2 Zn0$2 Mg0$6 Fe2 O4
Zn0$2 Mg0$8 Fe2 O4
Fe3 O4
MnFe2 O4
CoFe2 O4
NiFe2 O4
CoFe2 O4 /ball milled
CoFe2 O4 /Sol gel
Particle
size (nm)
8.5
8
12
12 (side)
12 (width)
12 (width)
14.5
12
11.8
12 (dia)
6.8 (dia)
8
8
10
8
36
41
45
35
59
12
12
12
12
Magnetic properties
Shape
Ms (emu/g)
Sphere
Cube
Sphere
Cube
Rod
Octahedron
Sphere
Cube
Cube
Octapod
Cuboctahedron
Sphere
Sphere
Sphere
Sphere
Sphere
200
200
HC (Oe)
31
40
80
40
18
80
75
75
2.5
2.0
0.1
0
0
0
0
∼55
0
300
330
164
1461
11
80.1
84.2
80.5
82.8
43.1
41.7
41.0
30.4
36.1
101
110
99
85
153
180
250
190
65.8
57.0
35.0
17.4
11.9
80.9
83.1
1750
500
The competition between both magnetic orders—surface
and core—defines the ground state of the particle, which
can differ from a single domain with the perfect magnetic ordering corresponding to the bulk material. High-field
irreversibility, high saturation fields, extra anisotropy contributions, and shifted loops after field cooling processes
have been observed and attributed to the core-to-surface
coupling [114].
4. FUNCTIONALIZATION AND
SURFACE COATING OF MAGNETIC
NANOPARTICLES
The functionalization of nanoparticles is important for
applications in catalysis, bio-labeling, and bioseparation.
Particularly in liquid-phase catalytic reactions, such small
and magnetically separable particles are useful as quasihomogeneous systems with the advantages of high dispersion, high reactivity, and easy separation [12]. Without a
surface coating, iron oxide MNPs have hydrophobic surfaces
with a large surface-area-to-volume ratio. Surface coating
of the MNPs is required for chemical and physical stability.
Figure 8 shows the schematic representation of the stabilization of MNPs by the surface coating of encapsulations.
Comment
Ref.
Ms and Hc were measured at 293 K.
Volume of sphere = volume of cube
Ms and Hc were measured at 300 K
[104]
Ms and Hc were measured at 10 K,
Volume of sphere = volume of cube.
The change in Hc was explained by their
shape differences. Ms and Hc were
measured at 5 K. V"cube# > V"octapod#
> V"cuboctahedron#
The composition was changed by
changing the precursors ratio.
Magnetic properties were measured at
300 K
Magnetic properties were measured at
300 K.
[112]
The magnetic susceptibility gradually
decreased as M2+ changed from Fe2+
to Co2+ to Ni2+ : magnetic spin
magnitude changed from ∼4 %B to
3 %B to 2 %B
In CoFe2 O4 , less Co2+ ions in the B-sites
results in smaller anisotropy. The
larger Hc values of ball milled samples
are due to the higher of Co2+ ions in
B-sites
[113]
[111]
[105]
[106]
[107]
[82]
The hydrophobic interactions between the nanoparticles results in particle agglomeration. These agglomerated
particles behave as clusters and exhibit strong magnetic
dipole–dipole attractions between them, resulting in a ferromagnetic interaction. When two large particle clusters
approach one another, each of them comes into the magnetic field of the neighbor. Therefore, the adherence of remnant MNPs results in a mutual magnetization, resulting in
increased aggregation properties.
A surface coating of MNPs is required to protect
the agglomeration of nanoparticles and provide functional
groups for the conjugation of surrounding materials such as
drugs, targeting ligands, proteins, and DNA sequences [64,
118, 119]. For these coating functions, a diverse group
of materials have been investigated including inorganic
materials (e.g., gold, silica, etc.) and organic materials
(e.g., dextran, chitosan, poly-ethylene glycol, etc.) [120]. The
functionalization and surface coating of MNPs affects the
magnetic properties. The unit of Ms is usually emu/g and
therefore, a non-magnetic coating on the surface will necessarily reduce the Ms value. If the coating is magnetic, the
interface coupling between the two different types of ordering (e.g., ferromagnetic and ferrimagnetic) can result in a
change in anisotropy and a shifting in the hysteresis loop.
The shift of the hysteresis loop is known as “exchange bias.”
The impact of various types of coatings on the magnetic
12
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Figure 8. Schematic representation of the stabilization of MNPs by surface coating with (a) inorganic materials, (b) organic materials (c) encapsulation into nanospheres (d) nanocapsules. Reprinted with permission from [117], L. H. Reddy, et al., Chem. Rev. 112, 5818 (2012). © 2012, American
Chemical Society.
Figure 9. (a) Schematic diagram showing the steps for synthesizing Fe3 O4 core and SiO2 –NH2 shell (Fe3 O4 @SiO2 -NH2 !. (b) TEM image of a
bare Fe3 O4 nanoparticle. (c) Evaluation of particle size and distribution by dynamic light scattering. (d) Magnetization properties of the Fe3 O4 ,
Fe3 O4 @SiO2 , and Fe3 O4 @SiO2 –NH2 (MCSNP) particles. Reprinted with permission from [123], A. K. Chaurasia, et al., Scientific Reports 6, 33662
(2016). © 2016, Nature Publishing Group.
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
properties of functionalized MNPs will be discussed in the
following section.
Patsula et al. studied the magnetic properties of differently sized poly ethylene glycol-coated iron oxide MNPs
synthesized by thermal decomposition and sucrose-coated
MNPs synthesized by co-precipitation in terms of their
potential application for hyperthermia [121].
Šulek et al. fabricated functionalized silica shell MNPs
for the covalent attachment of cholesterol oxidase [122].
The surface engineering of !-Fe2 O3 MNPs was performed
by the proper deposition of silica onto the nanoparticle
surface under strictly regulated reaction conditions. Further, the surface functionalization of silica-coated MNPs
was performed by coating the particles with organosilane followed by glutaraldehyde activation. The average
size of uncoated !-Fe2 O3 MNPs was found to be around
20 nm in diameter. Recently, Chaurasia et al. proposed
the exposure of multiple drug resistant bacteria (MDRB)
trapped by magnetic core shell nanoparticles (MCSNPs)
to a radio-frequency (RF) current as an alternative to
the usage of chemical antibiotics against bacteria [123].
MCSNPs used in this study consisted of an Fe3 O4 core
and SiO2 –NH2 shell (Fe3 O4 @SiO2 –NH2 " described in the
schematic diagram (Fig. 9(a)). Figures 9(b), (c) show TEM
images of a bare Fe3 O4 nanoparticle, and evaluation of
particle size and distribution by dynamic light scattering,
respectively. Saturation magnetization of the Fe3 O4 core
alone, Fe3 O4 core with silica coating (Fe3 O4 @SiO2 ", and
amine functionalized Fe3 O4 @SiO2 –NH2 (MCSNP) particles
were found to be 55.0, 40.9, and 38.8 emu/g, respectively
(Fig. 9(d)).
The bacteria can be effectively trapped using MCSNP,
and their biofilm formation can be efficiently inhibited
13
by MCSNPs. Furthermore, it was shown that MCSNPtrapped bacteria can be entirely eliminated within 30 min
when treated with MCSNPs under an RF current using
capacitive electric transfer (CET) devices. MCSNP-based
physical treatment can be an alternative antibacterial
strategy and can be used for environmental and therapeutic
applications.
Yang
et
al.
synthesized
urchin-like
Fe3 O4 @
SiO2 @ZnO/CdS core–shell microspheres, in which the multiple functional components were integrated into a single
microcomposite [124]. A magnetic metal-organic framework
composite was prepared by a novel and green strategy.
The nano Fe3 O4 @SiO2 core was first coated by a shell of
Cu(OH)2 as the self-template, followed by a conversion of
Cu(OH)2 into HKUST-1 in a water-ethanol mixture solvent,
and Fe3 O4 @SiO2 @HKUST-1 core–shell nanostructures
were obtained [125]. HKUST-1 is a copper-based MOF
with unsaturated metal sites and good stability. A Bi-Ifunctionalized-magnetic HKUST-1 composite was prepared
through a flexible coordination-based post-synthetic strategy. The synthesis process for Fe3 O4 @SiO2 @ZnO/CdS
core–shell microspheres and bi-I-functionalized magnetic HKUST-1 nanocomposites are illustrated in
Figure 10.
The core–shell morphology was confirmed by TEM
and the result of thermogravimetric analysis showed
that the magnetic composites had excellent thermal stability. FESEM images, XRD patterns, and
magnetic properties of the as-synthesized samples:
(a) Fe3 O4 (F), (b) Fe3 O4 @SiO2 (FS), (c) ZNO seeded
Fe3 O4 @SiO2 , (d) Fe3 O4 @SiO2 @ZnO (FSZ) and
(e) Fe3 O4 @SiO2 @ZnO/CdS (FSZC8 " are shown in
Figure 11.
Figure 10. Schematic representation of the preparation of (a) Fe3 O4 @SiO2 @ZnO/CdS core–shell microspheres. Reprinted with permission from
[124], J. Yang, et al., Catalysis Science and Technology 6, 4525 (2016). © 2016, Royal Society of Chemistry. (b) Bi-I-functionalized magnetic HKUST-1
composites. Reprinted with permission from Ref. [125], L. Huang, et al., Journal of Materials Chemistry A 3, 11587 (2015). © 2015, Royal Society
of Chemistry.
14
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Figure 11. (A) FESEM images of the as-synthesized samples: (a) Fe3 O4 (F), (b) Fe3 O4 @SiO2 (FS), (c) ZNO seeded Fe3 O4 @SiO2 ,
(d) Fe3 O4 @SiO2 @ZnO (FSZ), (e) Fe3 O4 @SiO2 @ZnO/CdS (FSZC8 !. (B) XRD patterns of the as-synthesized samples: (a) F, (b) FS, (c) ZNO
seeded Fe3O4@SiO2 , (d) FSZ, (e) FSZC8 . (C) Magnetic hysteresis loops of the products at room temperature (300 K) (inset image shows photographs of separation and re-dispersion processes of the FSZC8 microspheres using a magnet). Reprinted with permission from [124], J. Yang,
et al., Catalysis Science and Technology 6, 4525 (2016). © 2016, Royal Society of Chemistry.
5. APPLICATIONS
Magnetic nanoparticles have numerous applications in a
wide range of scientific fields. Some of the potential applications of MNPs are shown in Figure 12.
5.1. Medical Applications (Drug Delivery,
Magnetic Resonance Imaging,
Hyperthermia, and Bioseparation)
In the medical fields, the inspiration for integrating nanotechnology has been expanding exponentially, particularly in
the areas of drug delivery with the objective of directing
the drug to the disease site with minimal side effects and
reducing the dosage through more localized and efficient
targeting [126–128]. This involves magnetic drug targeting,
whereby an external magnetic field gradient is applied at
the target tissue to deliver the drug through active targeting
using high-affinity ligand attachment, as well as therapeutic
strategies. For these medical applications, the MNPs must
have good magnetic saturation, biocompatibility, and interactive functional groups at the surface. The surfaces of
MNPs could be modified through the creation of few atomic
layers of organic polymer or inorganic metallic or oxide
surfaces [120].
For biomedical applications, MNPs that present superparamagnetic behavior at room temperature is preferred [129–131]. MNPs should be stable in water and
in a physiological environment for biological applications.
Biomedical applications can be divided in (a) in vivo and
(b) in vitro applications. For in vivo applications, the MNPs
should be encapsulated by a biocompatible polymer to prevent the original structure. For in vitro applications, composites consisting of superpara MNPs dispersed in diamagnetic
particles with long sedimentation times in the absence of a
magnetic field can be used. Antioxidant enzymes coated iron
oxide nanoparticles stabilized using pluronic F-127 showed
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
15
Figure 12. Potential applications of magnetic nanoparticles.
considerable accumulation in aortic endothelial cells in vitro,
under the application of magnetic field [132].
Magnetic hyperthermia is a promising tool for multimodal
cancer treatment. The technique consists of targeting MNPs
to tumor tissue followed by application of an external alternating magnetic field that induces heat through Néel relaxation loss of the MNPs [1, 133, 134]. The temperature in
tumor tissue is increased to 43 ! C, which causes necrosis
of cancer cells, but does not damage surrounding normal
tissue. Therefore, to understand the heating efficiency of
MNPs in biological conditions is of high interest to research.
Therapies based on magnetic hyperthermia have already
been proven in several types of cancer e.g., brain tumor,
prostate cancer, or breast carcinoma [135–137].
MNPs such as iron oxide cube shaped nanoparticles [138], hybrid core–shell (e.g., Fe3 O4 @CaMoO4 :Eu,
Fe3 O4 @YPO4 :Eu etc.) [139, 140], iron oxide multigrain
structures (monocrystalline nanoflowers) [141] or magnetite
nanocrystals synthesized in magnetosomes by magnetotactic
bacteria [142, 143] have been recently investigated for magnetic hyperthermia, resulted in promising heating ability.
MNPs of magnetite (Fe3 O4 ! and related spinels with cobalt,
nickel, manganese, or other substitutions were also extensively studied for the use of hyperthermia [144–146]. The
experimental results for magnetic hyperthermia vary widely.
Hence, there is a strong need for the optimization of MNPs
for magnetic hyperthermia.
In magnetic hyperthermia, Néel relaxation, Brownian
relaxation, and hysteresis loss result in thermal energy upon
stimulation [147]. The relative contribution of each of them
is strongly dependent on size, shape, crystalline anisotropy,
and agglomeration of the nanoparticles [147, 148]. The heating produced by the MNPs under an AC field is usually
defined by the specific absorption rate (SAR) [146]. Large
SAR is essential for the therapeutic potential of this technique, while the AC field and frequency are limited for clinical attentions.
Nanoparticles extracellularly produced by the bacteria
Geobacter sulfurreducens were investigated that contain Co
or Zn dopants to tune the magnetic anisotropy, saturation magnetization, and particle sizes [146]. The heating
mechanisms specific to either Co or Zn doping are determined from frequency-dependent specific absorption rate
(SAR) measurements and innovative AC susceptometry simulations. While both particle types undergo magnetization
relaxation and show heating effects in water under low AC
frequency and field, only Zn-doped particles maintain relaxation combined with hysteresis losses even when immobilized. This magnetic heating process could prove important
in the biological environment where nanoparticle mobility
may not be possible. The biogenic Zn-doped particles can be
promising for combined diagnostics and cancer therapy. The
SAR dependence on magnetic field for doped and undoped
samples are shown in Figure 13.
Chains of magnetosomes extracted from AMB-1 magnetotactic bacteria were shown to be highly efficient for
cancer therapy under an AC field [137]. A suspension containing MDA-MB-231 breast cancer cells was exposed to an
alternative magnetic field of frequency 183 kHz and field
strengths of 20, 40, or 60 mT, resulting in destroying all
these cells. The antitumoral activity of the extracted chains
of magnetosomes was demonstrated further by showing that
they can be used to fully eradicate a tumor xenografted
under the skin of a mouse. The higher efficiency of the
extracted chains of magnetosomes compared with various
other materials (e.g., whole inactive magnetotactic bacteria,
individual magnetosomes not organized in chains, and two
different types of chemically synthesized superparamagnetic
iron oxide nanoparticles) was demonstrated.
Hugounenq et al. report a novel flower-shaped structure
of magnetic iron oxide synthesized by a modified “polyol”
protocol [141] (Fig. 14). These nanoflowers can be dispersed in water and exhibited interesting magnetic properties and a great capacity of heating. The value of the specific
loss power (SLP) of these nanoflowers was one order of
magnitude higher than the SLP reported for conventional
single-domain iron oxide nanoparticles at the same applied
AC field. Alphandery et al. report the magnetic properties
16
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Figure 13. SAR dependence on applied magnetic field for (a) Fe3 O4 with particle size 13.3 nm (MNB ! and (b) Co0"71 Fe2"58 O4 (Co0"7 ! in water
at different frequencies, (c) SAR versus applied magnetic field for the two samples showing the highest experimental SAR values (Co0"42 Fe2"58 O4
(Co0"4 ! and Zn0"16 Fe2"84 O4 (Zn0"2 ! in water at 87 kHz), and (d) SAR versus magnetic field dependence for Fe3 O4 with particle size of 16.1 nm (MNA !
and Zn0"2 samples suspended in glycerol. Reprinted with permission from the [146], E. Cespedes, et al., Nanoscale 6, 12958 (2014). © 2014, Royal
Society of Chemistry.
and heating efficiency of cobalt-doped chains of magnetosomes extracted from magnetotactic bacteria for applications in cancer therapy [143]. The SAR of was increased
from ∼400 W/g for the undoped chains of magnetosomes
to ∼500 W/g for the cobalt-doped chains of magnetosomes,
at an oscillating magnetic field of 80 mT and frequency of
183 kHz. Guibert et al. studied the role of aggregation of
iron oxide nanoparticles on their heating properties by the
coupling of dynamic light scattering (DLS) and hyperthermia experiments [148]. Bare and poly-acrylic acid-coated
nanoparticles were used for the study of aggregation effect.
The specific loss power of nanoparticles in small loose aggregates is similar to that of well-dispersed nanoparticles while
the formation of large and dense aggregates leads to a significant decrease of the specific loss power. However, for
intermediate aggregation states, DLS experiments alone do
not allow fully grasping the links between heating properties
and aggregation. The compactness of aggregates resulted in
a decrease in heating efficiency.
The separation and sorting of biological cells is critical for
biomedical applications including diagnostics, therapeutics
and fundamental cell biology. Magnetic sorting using magnetic particles has now become a routine technique for the
cell separation from biological suspensions [149, 150].
MNP-based sandwich assays were developed for the
detection of DNA samples by taking advantage of
magnetic separation and amplified fluorescence detection [117, 151]. In MNP-based sandwich assay, two oligonucleotide sequences—one is complementary to one end
of the target DNA and the other is a mismatched
sequence to DNA—were conjugated onto the gold nanoparticles. These magnetic and gold nanoparticles were then
hybridized with the target DNA to sandwich it. Further,
fluorescent (Cy3-labeled) DNA probes complementary to
the mismatched oligonucleotide sequence (known as barcode DNA) were immobilized onto the gold nanoparticles
(Fig. 15). The sandwich complex was then separated by the
application of magnetic field.
The nanoparticles can reach the tumor by passive extravasation, due to the tumor’s increased permeability. This effect
is known as the enhanced permeability and retention (EPR)
effect. On the other hand, MNPs transmit near the tumor
cells with the application of an external magnetic field of
suitable strength (Fig. 16). The nanoparticles should not
flow rapidly from the targeted tissue; therefore, the strength
of magnetic field should be higher than the hydrodynamic
blood flow [117]. The magnetic field should be ∼1 T for
efficient immobilization of the MNPs against the Brownian
and hydrodynamic forces in the blood flow. In addition, the
immobilization of MNPs also depends on particle size and
their distribution [152–154].
Detection of rare circulating tumor cells (CTCs) in the
peripheral blood of metastatic cancer patients is promising for improved diagnosis, staging, and prognosis of cancers [155]. The iron oxide MNPs with average particle size
of about 200 nm were functionalized with the recognition
peptide through biotin–streptavidin interaction for CTC isolation. A schematic of the isolation of CTCs with high
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
17
Figure 14. TEM micrographs and Fourier transformation of (A, B, and C) flowerlike nanoparticles, (D, E, and F) particles synthesized by polyol,
and (G–I) particles synthesized by co-precipitation. Reprinted with permission from [141], P. Hugounenq, et al., J. Phys. Chem. C 116, 15702 (2012).
© 2012, American Chemical Society.
capture efficiency by peptide functionalized MNPs from
spiked human blood under a magnetic field is shown in
Figure 17.
5.2. Cooling Applications
Room temperature magnetic cooling based on the magnetocaloric effect (MCE) has significant advantages compared with conventional gas-compression cooling technique,
e.g., no greenhouse gases as well as high energy efficiency [19, 22, 156–179]. Today’s research is focused on
finding the new magnetocaloric materials and an optimal
design of magnetic refrigerator for near room temperature applications [156, 174]. The search of new materials
with large MCE has gained large momentum in the last
decade. Scientists and researchers throughout the world
have devoted much attention to searching the new magnetocaloric materials. The world is warming in part because of
air conditioners and refrigeration, and we are trying to stay
cool in the warm world by using air conditioners!
Nowadays, the target to control global warming below
2 ! C is the main focus of the international climate
18
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Figure 15. Schematic representation of amplified fluorescence detection of DNA based on magnetic separation. Reprinted with permission
from [117], L. H. Reddy, et al., Chem. Rev. 112, 5818 (2012). © 2012, American Chemical Society.
debate [180, 181]. Companies are making more than
180 million cooling device every year by using 10 K tons
of environmentally harmful hydrofluorocarbons (HCFCs),
which may be equivalent to 28–45% of CO2 emission in
2050 [182].
Magnetic cooling, which is an environmentally friendly
and energy-efficient technology, may be a good alternative
Figure 16. Schematic representation of the accumulation of PEGylated
nanoparticles in tumor cells via the EPR effect and by the use of magnetic field. Reprinted with permission from [117], L. H. Reddy, et al.,
Chem. Rev. 112, 5818 (2012). © 2012, American Chemical Society.
for making an improvement, as it is more energetically
efficient than the current conventional cooling techniques.
Magnetic cooling can achieve 60% of Carnot (ideal)
efficiency in the laboratory, while the best gas compression
cycle can reach only 40% [156]. In addition, compressors are
Figure 17. Schematic representation of circulating tumor cells isolation with MNPs modified with the epithelial cell adhesion molecule
(EpCAM) targeting peptide. (a) Screening of EpCAM targeting peptides from de novo designed peptide pool (b) Isolation of tumor cells in
a magnetic field. Reprinted with permission from [155], L. Bai, et al.,
Journal of Materials Chemistry B 2, 4080 (2014). © 2014, Royal Society
of Chemistry.
19
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
noisy and vibrate a lot, whereas magnetic cooling devices
can be silent and vibration free [183, 184].
Magnetocaloric nanoparticles (MCNPs) have attracted
much interest because they can exhibit superior properties
compared to the bulk materials. The nanocomposites offer a
better match to the desired entropy-temperature curves than
that of bulk material and improve the performance of the
cycle. When the particle size and domain size (∼10–15 nm)
are similar, the behavior can be superparamagnetic or spin
glass at room temperature. Superparamagnetic particles possess large moment, high saturation magnetization, and a nonhysteretic M –H curve with zero remanence and coercivity.
There have been very few studies to explore the possibility of
cooling using superparamagnetic nanoparticles. Ferrite and
other nanoparticles show interesting magnetocaloric behavior. The maximum entropy change is usually concentrated
near the paramagnetic-ferromagnetic (PM-FM) transition
and decays sharply on both sides the transition temperature. On the other hand, Co–, Mn–Zn– and Mg-ferrite
nanoparticles show a broad temperature span of change in
magnetic entropy, resulting in large relative cooling power
capacity (RCP). Although theoretical studies have suggested
that magnetocaloric properties can be enhanced for superparamagnetic ferrite nanoparticles, there has not been much
focus on these materials. These nanoparticles can be suspended in a host matrix to enhance the surface area or in
a carrier fluid to develop self-pumping cooling devices. The
RCP of nanoparticles can be increased through a broad magnetic phase transition, which will be useful for low-grade
waste heat recovery and heat pumps. Ucar et al. reviewed
the RCP (in joule/$) of various magnetocaloric materials
and found that FeNi-based materials are very useful for
such applications [176]. For self-pumping cooling systems,
MNPs are suitable if the TC lies between room temperature
and the device operating temperature [9, 185–187]. Recently
developed, Fe-based MNPs for MCE applications are listed
in the Table 4.
R2 Fe17 (R = rare earth) intermetallic compounds show
moderate MCE near room temperature. Gorria et al.
discussed the potential for P2 Fe17 nanostructured material as room temperature magnetocaloric materials [172].
They highlighted the differences in the MCE of arc-melted
bulk and mechanically alloyed nanoparticles. The maximum change in entropy (!SM " was found to be less in
nanoparticles while the working temperature span increased
by a factor close to 2, resulting in increased relative cooling power (RCP) compared to the bulk alloy [172]. The
increase in RCP was attributed to the broadening in magnetic entropy in nanoparticles. The exchange interactions
in a nanostructured material usually have a distribution of
magnetic transitions, which results in broader magnetization
change with temperature and therefore large working temperature span. Álvarez et al. used a high-energy ball mill to
produce nanocrystalline Nd2 Fe17 powders [188]. A decrease
of mean crystalline size to ∼10 nm was observed, resulting
in considerable expansion between disordered grain boundaries. They observed that the nanocrystalline samples exhibit
a distribution in TC , decreased maximum value of !SM and
a high working temperature span; the magnetization versus
temperature curve reveals a slower decrease than that of the
bulk sample. The MCE difference between bulk and ballmilled samples and the increased working temperature span
with milling time is illustrated in Figure 18.
In another study, the correlation between the broadening of !SM and the TC distribution in nanostructured
Pr2 Fe17 and Nd2 Fe17 powder synthesized by high-energy
ball milling was studied [193]. In both cases, the local
environment of Fe atoms, and therefore the magnetic
interactions, change with increasing milling time, resulting in a broader TC distribution. Er2 Fe17 exhibits both
Table 4. Magnetocaloric properties for iron based magnetocaloric materials.
Sample
Size (nm)
Pr2 Fe17
Pr2 Fe17
Nd2 Fe17
Nd2 Fe17
NdPr Fe17
20
20
14
24
20
Fe72 Ni28
Fe70 Ni30
(Fe70 Ni30 "97 Mo3
(Fe70 Ni30 "96 Mo4
(Fe70 Ni30 "89 Zr7 B4
(Fe70 Ni30 "89 Zr7 B4
(Fe70 Ni30 "89 B11
(Fe70 Ni30 "89 B11
(Fe70 Ni30 "92 Mn8
(Fe70 Ni30 "95 Mn5
(Fe70 Ni30 "99 Cr1
(Fe70 Ni30 "97 Cr3
(Fe70 Ni30 "95 Cr5
(Fe70 Ni30 "94 Cr6
(Fe70 Ni30 "93 Cr7
–
–
–
–
20
20
12
12
15
14
12
10
13
12
11
TC (K)
!SM (J-kg−1 K−1 )
R2 Fe17 (R = Rare earth) magnetic nanoparticles
292
2.1
292
4.5
340
1.6
340
1.85
303
2.1
Fe–Ni–M (M = B, Mn, Cr, Mo) magnetic nanoparticles
333
0.5
333
0.32
320
1.69
300
1.67
353
0.7
353
∼2.8
381
0.73
381
2.1
340
1.67
338
1.45
398
1.58
323
1.49
258
1.45
245
1.22
215
1.11
!H (T)
RCP
Ref.
1.5
5
1.5
1.5
2
107
573
83
87
175
[172]
[172]
[188]
[188]
[189]
5
5
5
5
1.5
5
1.5
5
5
5
5
5
5
5
5
250
342
440
432
[190]
[190]
[191]
[191]
[167]
[167]
[21]
[21]
[19]
[192]
[10]
[10]
[10]
[10]
[10]
∼300
155
640
466
470
548
436
406
366
306
20
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Figure 18. Temperature dependence of !!SM ! values under the application of magnetic field of 1.5 T for bulk and milled Nd2 Fe17 samples.
Temperature span ("TFWHM # is shown by the horizontal lines for each
sample. The inset shows the magnetic field dependence of "TFWHM for
all bulk and powder samples. Reprinted with permission from [188],
Á. Pablo, et al., J. Phys.: Condens. Matter. 22, 216005 (2010). © 2010,
IOP Publishing.
direct and inverse MCE with moderate !SM and adiabatic
temperature (!Tad # change [194]. The inverse MCE was
due to the crystalline electric field-level crossover in the
Er sub-lattice and ferrimagnetic interactions between the
Er and Fe sub-lattice. The effect of demagnetizing factor on !SM and RCP in Er2 Fe17 prepared by arc melting was investigated [195]. NdPrFe17 ribbons composed of
nanocrystals with in amorphous phase shows two successive
phase transitions, giving rise to increased working temperature span and enhanced RCP [189]. The RCP values for
Fe17 NdPr ribbons were larger than those of Fe17 Pr2 bulk
crystals.
Chaudhary et al. studied the MCE of (Fe70 Ni30 #100−x Ax
nanocrystalline powders with A = Mn, Cr, and B, produced by high-energy ball milling. Boron, manganese, and
chromium were individually used to reduce TC to near room
temperature [10, 19–21, 192]. %-(Fe70 Ni30 #89 B11 nanoparticles were found to exhibit very high RCP up to 640 J-kg−1
for a field change !Hof 5 T, with TC ∼381 K. A broad
operating temperature range along with a moderate change
in entropy and very high RCP make these nanoparticles
potential candidates for magnetic cooling applications in
low grade waste heat recovery [21]. The %-(Fe70 Ni30 #95 Mn5
$TC ∼ 338 K) and %-(Fe70 Ni30#92 Mn8 $TC ∼ 317 K) nanoparticles possess good RCP up to 470 J-kg−1 and 415 J-kg−1 ,
respectively, for a field change of 5 T. Different compositions such as Fe70 Ni30 , (Fe70 Ni30 #99 Cr1 , (Fe70 Ni30 #97 Cr3 ,
(Fe70 Ni30 #95 Cr5 , (Fe70 Ni30 #94 Cr6 , and (Fe70 Ni30 #93 Cr7 were
synthesized by high-energy ball milling [10]. The effect of
alloying of Fe70 Ni30 with Cr on magnetic phase transition temperature (TC # was studied (Fig. 19). The TC for
Fe70 Ni30 , (Fe70 Ni30 #99 Cr1 , (Fe70 Ni30#97 Cr3 , (Fe70 Ni30 #95 Cr5 ,
(Fe70 Ni30 #94 Cr6 , and (Fe70 Ni30#93 Cr7 is 438 K, 398 K, 323 K,
258 K, 245 K, and 215 K, respectively.
Figure 19. Left axis shows the temperature dependence of magnetization M$T # for (a) Fe70 Ni30 , (b) (Fe70 Ni30 #99 Cr1 , (c) (Fe70 Ni30 #97 Cr3 ,
(d) (Fe70 Ni30 #95 Cr5 , (e) (Fe70 Ni30 #94 Cr6 , and (f) (Fe70 Ni30 #93 Cr7 while the right axis shows the corresponding derivative with respect to temperature.
Reprinted with permission from [10], V. Chaudhary and R. V. Ramanujan, Scientific Reports 6, 35156 (2016). © 2016, Nature Publishing Group.
21
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Figure 20. Temperature dependence of −!SM under magnetic field ranging from 0.5 T to 5 T for (a) (Fe70 Ni30 "99 Cr1 , (b) (Fe70 Ni30 "97 Cr3 ,
(c) (Fe70 Ni30 "95 Cr5 , (d) (Fe70 Ni30 "94 Cr6 , and (e) (Fe70 Ni30 "93 Cr7 . (f) Dependence of −!SM (left axis) and RCP (right axis) on chromium content
in (Fe70 Ni30 "100−x Crx nanoparticles. Reprinted with permission from [10], V. Chaudhary and R. V. Ramanujan, Scientific Reports 6, 35156 (2016).
© 2016, Nature Publishing Group.
For a 1 T applied magnetic field, !SM for (Fe70 Ni30"99 Cr1 ,
(Fe70 Ni30 "97 Cr3 , (Fe70 Ni30 "95 Cr5 , (Fe70 Ni30 "94 Cr6 , and
(Fe70 Ni30 "93 Cr7 at their TC was found to be 0.38 J-kg−1 K−1 ,
0.27 J-kg−1 K−1 , 0.37 J-kg−1 K−1 , 0.29 J-kg−1 K−1 , and
0.28 J-kg−1 K−1 , respectively [10] (Fig. 20). When the
field was increased to 5 T, !SM for (Fe70 Ni30"99 Cr1 ,
(Fe70 Ni30 "97 Cr3 , (Fe70 Ni30 "95 Cr5 , (Fe70 Ni30 "94 Cr6 , and
(Fe70 Ni30 "93 Cr7 was found to be 1.58 J-kg−1 K−1 ,
1.49 J-kg−1 K−1 , 1.45 J-kg−1 K−1 , 1.22 J-kg−1 K−1 , and
1.11 J-kg−1 K−1 , respectively.
Rosenweig described the principle of magnetocaloric
self-pumping using ferrohydrodynamic equations [186].
Figure 21 shows the schematic diagram of the magnetothermal self-pumping principle, having a tube filled by ferofluid,
constant magnetic field, and 4 regions represented by 1, 2,
3, and 4.
The Bernoulli equation for the fluid flow inside the magnetic field is
dH
dp
dv
dh
+ #v
+ #g
− $0 M
=0
ds
ds
ds
ds
Let us consider that the ferrofluid is an incompressible fluid
with a constant density. Equation (3) with the dimension per
unit volume can be written as [196]
! H2
v12
M dH
+ #gh1 − $0
2
0
! H2
v2
= p2 + # 2 + #gh2 − $0
M dH
2
0
p1 + #
(4)
In Figure 21, a tube filled with ferrofluid has two sections
represented by a cold and hot region. A constant magnetic
field was applied in the middle of the tube. All the gain
here is because of the change in magnetization of the ferrofluid in the cold and hot regions. The magnetization of
ferrofluid is higher in the cold region, which will be attracted
(2)
Where v is the fluid velocity along the distance s, and h is
height with the reference of ground level. The integration of
Eq. (2) from the section denoted by 1 to the section denoted
by 2 can be represented by
!2M
! 2 dp
v 2 − v12
+ 2
+ g%h2 − h2 " − $0
dH = 0
#
2
1
1 #
(3)
Figure 21. Schematic representation of magnetothermal self-pumping
principle.
22
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
by the magnetic field. When the ferrofluid reached the hot
region, it started to get hot and lost its magnetization. If we
apply the Bernoulli equation, neglecting gravitational potential energy and letting that kinetic energy for cross-section
of the tube is constant, the expression for regions 1 and 2
can be made as [186, 196]
p1 = p2 − !0 "M̄H #2
where
M̄ =
1 !H
M dH
H 0
(5)
(6)
Similarly, the application of the ferrohydrodynamics
Bernoulli equation between regions 3 and 4, i.e., the pressure difference between regions 3 and 4
p4 = p3 − !0 "M̄H #3
(7)
The ferrohydrodynamics Bernoulli equation is not applicable on the part of the tube where the magnetic field is
applied, since the assumption of an isothermal flow field is
inherent and its derivation is not involved here. By neglecting the acceleration, gravity, and friction, Rosenweig shows
the following governing equation inside the magnetic
field [186].
0 = −$p∗ + !0 $H
(8)
Where p is the composite pressure as magnetic field is
uniform, with results p2∗ = p2∗ . The change in the pressure
between regions 4 and 1 can now be defined as
∗
%p = p4 − p3 = !0 H&M̄"T1 # − M̄"T1 #' = !0 H%M̄
(9)
Actually this pressure difference "%p# can be considered
as the basis for the self-pumping of magnetocaloric fluid.
Self-pumping of the magnetothermal fluid may be used for
thermal management of electronic systems and in the development of heat driving pump systems.
By using the principle proposed by Rosenweig, in 2004,
Love et al. suggested a magnetocaloric pumping by having only thermal and magnetic fields [197, 198]. A uniform magnetic field coincident with a temperature gradient
yields a force in the magnetic fluid. They have provided the
basic description of the process and the limitations. They
developed a finite-element model with a series of experiments. In 2004 Ganguly et al. simulated the thermomagnetic convection and explained the origin of this kind of
convection [199]. A parametric study was shown to associate
the heat transfer, temperature difference, cavity dimension,
magnetic field, and fluid viscosity. Thermomagnetic convection is the result of both magnetic field and temperature gradients. Warmer fluid of lower magnetic susceptibility moves
away from the field, while colder fluid with a higher susceptibility moves toward the magnetic field.
In 2005, Mukhopadhyay et al. used scaling analysis to characterize thermomagnetic heat transfer in a
two-dimensional enclosure filled with a ferrofluid [200].
Their results excellently matched the detailed numerical
simulation.
In 2008, Li et al. established a prototype of a miniature
automatic cooling device without any moving mechanical
part. An experimental study to investigate the performance
matric of the cooling device and the effects the magnetic
field and temperature distribution of the fluid on the cooling
performance of the device was investigated. It was demonstrated that the magnetic field and the temperature gradient,
jointly is the source of the driving force and no additional
energy required to run the device.
In 2009, Lian et al. developed an automatic energy transport device by using a temperature sensitive magnetocaloric
ferrofluid as a coolant [201, 202]. The magnetic field gradient and the fluid temperature variation results the fluid
motion in a loop. They mentioned that these devices can be
applicable for energy transport systems including lab-on-achip, energy conversion devices, electronic cooling devices,
etc. since no mechanical moving part is required. They
stated that the changing magnetic field and temperature
variation of magnetic fluid can control the energy transport
process in such devices.
In 2011, Xuan and Lian presents a practical design of
thermomagnetic convection in electronic cooling [203]. An
earth magnet and the waste heat generated from hot source
(e.g., a chip) were used to maintain the flow of working
ferrofluid, which passages heat to a far end for dissipation.
This cooling device does not need any additional energy as
the waste heat dissipation is used for driving the ferrofluid
and therefore completely self-powered device. As the heat
load increases (i.e., initial temperature of chip increased),
a higher heat dissipation rate can be achieved due to a
more thermomagnetic convection. Therefore, the devices
based on thermomagnetic convection can be treated as selfregulating devices.
Later in 2011, Pal et al. invented a thermomagnetic pump
without any external pressure gradient [204]. The increased
temperature of the ferrofluid from the heat load and magnetic field gradients results the driving forces to move the
ferrofluid. This kind of self-pumping can have many applications in cooling especially microelectronic devices. Performance of the thermomagnetic pump was also studied
experimentally to characterize the pump pressure head and
discharge under different working conditions, i.e., the magnetic field strength, heating power, and ferrofluid properties. They reported that the pump offers precise flow control
and high flow rate compared to similar pumps in its category. In 2015 Rahman and Suslov explained the linear
stability of magneto-convection of a ferrofluid contained
between two heated plates under uniform applied magnetic
field [205]. They also explained that the thermomagnetic
convection arises due to the variation in magnetization with
temperature.
In 2016, we investigated a self-pumping magnetic cooling
device that does not require a mechanical pump or external
energy input. Cooling by 28 # C was achieved in less than
3 min by the application of 0.3 T magnetic field [185]. In this
invention, we have used Mn–Zn ferrite nanoparticles to prepare the ferrofluid. Mn0(4 Zn0(6 Fe2 O4 nanoparticles were synthesized via the hydrothermal method [206]. In a typical
synthesis, manganese(II) chloride tetrahydrate, zinc chloride
and iron(III) chloride hexahydrate were used as a starting precursor. Sodium hydroxide was used to adjust the pH
value. Each starting precursor was dissolved separately in
appropriate molar quantities of purified water. 5M–NaOH
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
23
Figure 22. Surface coating of Mn–Zn-ferrite nanoparticles to prepare ferrofluid for magnetic cooling applications.
was added to the iron chloride solution until the pH value
became 8. The precipitate was centrifuged and washed four
times with deionized_water. The salt solutions were then
added together and vigorously stirred while adding sodium
hydroxide drop-wise until the pH of the reaction mixture
reached a value of 11. The resulting slurry was decanted into
a pressure vessel and placed into an oven at 190 ! C for 4 h.
The nanoparticles obtained by this method were washed several times with deionized water followed by vacuum drying
overnight. For the surface coating, the dried nanoparticles
were added to a mixture of oleic acid and HNO3 in the
ratio of 1:10, followed by 2 h mechanical stirring at 80 ! C
(Fig. 22). After cooling, the nanoparticles were extracted
from the excess HNO3 and oleic acid by a permanent magnet. Finally, the coated particles were dispersed in deionized
water (Fig. 22).
The stability of nanoparticles in the fluid is crucial for
self-pumping magnetic cooling. The ratio of thermal energy
to magnetic energy results in an expression for particle size
d < !6kT /"MH#, suggesting that small particles can be suspended in a fluid, even in high magnetic fields [186, 207].
A self-pumping magnetic cooling prototype was built and
experiments were carried out to determine the effect of
Figure 23. Performance of the self-pumping magnetic cooling prototype; load temperature decreases ∼20 ! C with the application of 0.3 T
magnetic field.
magnetic field on cooling [185, 206]. Figure 23 shows the
temperature profile of heat load with and without a magnetic field. A reduction in temperature of ∼20 ! C can be
seen.
Based on the literature, the advantages of thermomagnetic convection include (a) no moving mechanical part,
(b) no external energy required to run the devices as it take
energy from the body which we wanted to cool, (c) no need
for dynamic seals, (d) the service temperature can be tuned
by choosing the ferrofluid having suitable TC , and (e) the
potential application can be enlarged from cooling of small
electronic devices to cooling of large space craft electronics.
5.3. Catalysis Applications
An environmentally friendly procedure with heterogeneous
and reusable catalyst for organic and inorganic reactions is
of high interest. The application of functionalized MNPs
results in better reaction process, including reaction time,
catalyst loading, reaction temperature, and reactant concentration, with reduced waste production and uninterrupted production. These magnetic supported catalysts show
high catalytic activity and a high degree of chemical stability [208]. Many functionalized MNPs has been designed
and widely used either as a catalyst or catalyst support
in traditional metal catalysis, organocatalysis, and enzyme
catalysis [119, 209].
MNPs were used as a core for –OSO3 H-functionalized
mesoporous MCM-41 shell to provide a core–shell solid acid
catalyst (Fe3 O4 @MCM-41-OSO3 H) [209]. The obtained
nano-magnetic catalyst was used in the three-component
reaction of indoles, aldehydes, and thiols to furnish the
desired 3-[(aryl)(arylthio)methyl]-1H-indoles in good yields
under very mild reaction conditions. In another study, an
Fe3 O4 @Silica sulfuric acid core–shell composite was prepared using Fe3 O4 and silica sulfuric acid nanoparticles as
the core and shell, respectively [210]. The catalytic activity of this solid acid nanocomposite was probed through
one-pot synthesis of 3,4-dihydropyrimidinones via threecomponent couplings of aldehydes, $-diketone, and urea
under solvent-free conditions. In this reaction, Fe3 O4 @Silica
sulfuric acid showed a highly catalytic nature, short reaction
time, and recycle exploitation. The nanomagnetic catalyst
24
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
can be easily separated from the solution using a small permanent magnet.
Sulfonic-acid-supported silica-coated MNPs (Fe3 O4 @
SiO2 @PrSO3 H) were prepared by a facile immobilization technique [211]. The final catalyst was found to be
an efficient and environmentally benign solid acid for
the Pechmann condensation of substituted phenols with
ethyl acetoacetate leading to the formation of coumarin
derivatives. After the reaction, the catalyst could be
effortlessly separated by external magnet and reused for
22 consecutive runs, without any significant loss in catalytic efficiency. Dezfoolinezhad et al. prepared core double shell Fe3 O4 @SiO2 @polyionene/Br−
3 nanoparticles for
the syntheses of imidazole derivatives under solvent-free
conditions [212]. The polyionene was prepared by reacting 1,4-diazabicyclo[2.2.2]octane (DABCO) and 1,4-dibromo
butane in dimethylformamide/methanol. It was then added
to the previously formed core–shell Fe3 O4 @SiO2 nanoparticles. Recyclability of this magnetic catalyst was studied in the
reaction between 4-chloro-benzaldehyde and orthophenylene diamine. The catalyst was found to be reusable six times
without significant loss (∼10%) of activity. A core-doubleshell Fe3 O4 @SiO2 @Zn(Por)OP magnetic particles were
designed and prepared by coating a core–shell Fe3 O4 @SiO2
nanoparticles with a zinc porphyrin-based organic polymer (Zn(Por)OP) [213]. Under the optimal conditions,
a propylene carbonate yield of 97% was achieved using
Fe3 O4 @SiO2 @Zn(Por)OP catalyst with potassium iodide as
co-catalyst.
In another study, a multifunctional Fe3 O4 @SiO2 @PEIAu/Ag@PDA catalyst with highly stabilized reactivity was
synthesized by a self-assembled method [214]. A thin coating of the SiO2 on Fe3 O4 MNPs resulted in a negatively charged surface. Then poly(ethyleneimine) polymer
(PEI) which has positively charge surface was self-assembled
onto the Fe3 O4 @SiO2 . Next, negatively charged glutathione
capped gold nanoparticles (GSH-AuNPs) were electrostatically self-assembled onto the Fe3 O4 @SiO2 @PEI. After that,
silver was grown on the surface of the nanocomposite due
to the reduction of the dopamine in the alkaline solution. The Fe3 O4 @SiO2 @PEI-Au/Ag@PDA nanocomposite
showed a high Ms of 48.9 emu/g. The Fe3 O4 @SiO2 @PEIAu/Ag@PDA nanocomposite was used to catalyze the
reduction of p-nitrophenol (4-NP) to p-aminophenol
(4-AP). These catalysts can be easily recycled from the
reaction mixture by magnetic separation and no significant loss of the activity was observed after a number of
cycles [212–214]. Table 5 shows the list of magnetic catalysts
for different chemical reactions.
Photocatalytic performances were evaluated by the
photocatalytic elimination of rhodamine B (RhB)
under visible light irradiation [124]. Compared with
the Fe3 O4 @SiO2 @ZnO microspheres, the Fe3 O4 @
SiO2 @ZnO/CdS microspheres showed enhanced visible
light photocatalytic activity, benefiting from the sensitization
of CdS, which can extend the visible light absorption
and facilitate the separation of photoinduced carriers.
Fe3 O4 @SiO2 @ZnO/CdS with 10.1 wt% CdS exhibited the
best photocatalytic ability. Furthermore, these multifunctional microspheres exhibit excellent magnetic response
Table 5. List of applications of modified MNPs catalysts for different
chemical reactions.
Magnetic catalyst
Sulfuric-acidfunctionalized
MCM-41-coated
magnetite
nanoparticles with
core shell structures
Sulfuric-acidfunctionalized
silica-coated
magnetite
Silica-coated magnetic
nanoparticle
Sulfonic-acidsupported silica
coated-magnetic
nanoparticles
(Fe3 O4 @SiO2 @
PrSO3 H)
Fe3 O4 @aminocellulose
hybrid
Fe3 O4 @SiO2 @
polyionene/Br−3
Fe3 O4 @SiO2 @
Zn(Por)OP
Fe3 O4 @SiO2 @PEIAu/Ag@PDA
Reaction
Ref.
Reaction of indoles, aldehyde and
thiols under mild reaction
conditions
[209]
Synthesis of
1-amidoalkyl-2-naphtols via the
one pot three component
reaction of naphthol, aldehydes
derivatives, urea under solvent
free conditions
Synthesis of phthalazine
derivatives by the treatment of
phthalhydrazide with aromatic
aldehydes and cyclis or acyclic
1,3-diketones under acidic
conditions
One-pot synthesis of N-substituted
pyrroles at room temperature in
aqueous phase
[210]
CuO-mediated controlled radical
polymerization of styrene
Syntheses of imidazole derivatives
under solvent-free conditions
The cycloaddition of CO2 to
propylene oxide
The reduction of p-nitrophenol
(4-NP) to p-aminophenol
(4-AP)
[216]
[215]
[211]
[212]
[213]
[214]
and high stability. The core–shell Fe3 O4 @SiO2 @ZnO/CdS
maintained high photocatalytic activity after multiple cycles.
Zhang et al. investigated a highly efficient and easily recoverable magnetic nanoparticle-supported palladium
catalyst in the Hiyama reaction of aryltrialkoxysilanes
with a variety of organic halides (Fig. 24(a)) [11]. The
reactions produced the corresponding products in good
yields. To evaluate the recyclability of the SiO2 @Fe3 O4 Pd catalyst in the Hiyama reaction, 4-iodoanisole and
phenyltrimethoxysilane were chosen as model substrates.
After the model Hiyama reaction was carried out under the
optimized reaction conditions, the catalyst was recovered by
an external magnetic separation. More than 99% of the catalyst could simply be recovered by fixing a magnet near the
reaction vessel. For the model Hiyama reaction, the supported palladium catalyst SiO2 @Fe3 O4 –Pd exhibited high
stability and recovered and reused at least 10 times without
significant loss of catalytic activity.
A
magnetic
silica
supported
copper
catalyst
(Fe3 O4 @SiO2 @Cu) was prepared by one-pot synthesis
methodology (Fig. 24(b)) [217]. The catalyst was used for
the efficacious amination of aryl halides in aqueous medium
under microwave irradiation. Shelke et al. reported an
efficient protocol for the synthesis of medicinally important
pyrazole derivatives, 4-methoxyaniline, and Ullmann-type
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
25
Figure 24. (a) Stepwise preparation of the catalyst and its application in the Hiyama reaction. Reprinted with permission from [11], L. Zhang,
et al., Catalysis Science and Technology 2, 1859 (2012). © 2012, Royal Society of Chemistry. (b) Synthesis of the magnetic silica supported copper
catalyst followed by amination of 4-nitro bromobenzene using Fe3 O4 @SiO2 @Cu. Reprinted with permission from [217], R. B. Nasir Baig and
R. S. Varma, RSC Advances 4, 6568 (2014). © 2014, Royal Society of Chemistry. (c) Nanocat-Fe–CuO catalyzed Ullmann condensation reaction of
4-methoxyphenol with iodobenzene. Reprinted with permission from [218], S. N. Shelke, et al., ACS Sustainable Chemistry and Engineering 2, 1699
(2014). © 2014, American Chemical Society. (d) Synthetic route toward nanocatalysts and their application in the Suzuki Miyaura coupling reaction.
Reprinted with permission from [219], Y. Long, et al., New J. Chem. 39, 2988 (2015). © 2015, Royal Society of Chemistry. (e) Preparation of the
nanocatalyst Fe–Pd and its application in the C–N coupling reaction. Reprinted with permission from [220], S. Sa, et al., Green Chemistry 16, 3494
(2014). © 2014, Royal Society of Chemistry.
condensation reaction using magnetically separable and
reusable magnetite-supported copper (nanocat-Fe–CuO)
nanoparticles (Fig. 24(c)) [218]. Nanoparticles with an
average size of 20–30 nm were synthesized using wet
impregnation techniques from readily available inexpensive starting materials and were recycled six times
without a loss in catalytic activity. Long et al. prepared
Fe3 O4 MNPs functionalized with amino groups by a facile
one-pot template-free technique to immobilize PdII and
Pd0 using a metal adsorption and reduction procedure
(Fig. 24(d)) [219].
The PdII catalyst exhibited better catalytic activity for carbonylative cross-coupling reactions than the Pd0 catalyst.
The proposed reaction mechanism of Suzuki carbonylative
cross-coupling reactions using the Pd catalyst was inferred.
The agglomeration of Pd0 nanoparticles, which was confirmed by TEM, was the reason for different catalytic
activities of the reactions catalyzed by immobilized PdII
and Pd0 catalysts. The PdII catalyst revealed high efficiency
and stability during recycling stages. Sa et al. established
the immobilization of Pd on the magnetite surface to form
Nanocat-Fe–Pd using inexpensive precursors and investigated its catalytic role in the Buchwald-Hartwig reaction for
arylation of amines and amides (Fig. 24(e)) [220]. C–N bond
formation was achieved in moderate to excellent yields and
the catalyst was separated and recycled up to five cycles with
a miner loss in yield [220, 221].
5.4. Environmental Applications
Magnetic nanoparticles, such as nano zero-valent iron
(nZVI), magnetite (Fe3 O4 !, and maghemite ("-Fe2 O3 !
nanoparticles have potential applications for treatment of
polluted water or subsurface environments [26, 28, 222–224].
26
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
Figure 25. The effect of humic acid in aqueous solutions on the removal of heavy metals by (a) Fe3 O4 /HA and (b) Fe3 O4 . Influence of pH on the
removal of heavy metals by (c) Fe3 O4 /HA and (d) Fe3 O4 . 100 ml of aqueous solution containing 0.1 mg/L heavy metals (pH 6.0) was added 10 mg
of Fe3 O4 /HA or Fe3 O4 . Reprinted with permission from [226], J.-F. Liu, et al., Environ. Sci. Technol. 42, 6949 (2008). © 2008, American Chemical
Society.
These MNPs have a large removal capacity, fast kinetics,
and high reactivity for removal of contaminant due to their
extremely small particle size and magnetic behavior. The
contaminant removal mechanism is different for different
MNPs and significantly depends on the surface properties
of the MNPs. The strong reducing property of iron is supportive to degrade the toxic organic and inorganic pollutants
to nontoxic elements. Wanna et al. reported hybrid MNPs
based on poly methyl methacrylate and super-paramagnetic
iron oxide nanoparticles with selective surface modification
for heavy metal removal by application of external magnetic
fields [225]. The efficiency of these hybrid nanoparticles for
heavy metal removal were studied on Pb(II), Hg(II), Cu(II),
and Co(II) in aqueous solutions. The removal efficiency of
the hybrid nanoparticles depends on the cation radius and
the pairing of metal ions by the amine group.
Humic-acid-coated Fe3 O4 nanoparticles (Fe3 O4 /HA) were
synthesized by co-precipitation for the removal of toxic
Hg(II), Pb(II), Cd(II) and Cu(II) from the water [226].
Figure 25 shows the impact of humic acid and pH on the
removal of metals by Fe3 O4 and Fe3 O4 /HA . With the addition of humic acid in the range of 0–20 mg/L dissolved
organic carbon (DOC), larger metal removals were observed
for Fe3 O4 /HA in comparison to Fe3 O4 .
The adsorption behavior of perfluorooctane sulfonate
(PFS) on Al2 O3 , Fe2 O3 , SiO2 , and TiO2 nanoparticles was
studied in terms of adsorption isotherms and influences
of pH, ionic strength, and heavy metallic cations [227].
The PFS adsorption exhibited strong pH dependence on
nano-oxides because of different species of surface hydroxyl
groups. Electrostatic interaction and sulfonic group of PFS
allow the hydrogen bonds on the surface of nano-oxides.
In addition, the coexisting PFS and heavy metallic cations
greatly enhanced their adsorption onto the nano-oxides. The
removal mechanism of Cr(VI) and As(V) pollutants from
water by !-Fe2 O3 nanoparticles was attributed to electrostatic interactions with the polarized oxygen atoms on the
iron oxide surface at low pH values [228–233]. The arsenic
adsorption on varying adsorbents including iron oxides,
aluminium hydroxides, alumina, and carbon as a arsenic
removal in drinking water treatments was studied [228]. The
adsorption capacities for arsenite and arsenate increased
with decreasing size of the magnetite particles [228]. The
Fe3 O4 @SiO2 /GO adsorbent was developed by modification of graphene oxide with SiO2 -coated Fe3 O4 nanoparticles. The Fe3 O4 @SiO2 /GO adsorbent was used for the
removal of As(III) and As(V) from environmental waters.
Fe3 O4 @SiO2 /GO provided high adsorption capacities, i.e.,
7.51 and 11.46 mg-g−1 for As(III) and As(V), respectively, at
pH 4.0. The adsorbent removal for As(III) was better than
As(V) [233].
Lakshmanan et al. used microemulsion-prepared iron
oxide MNPs and protein-functionalized nanoparticles in
water treatment [234]. More than a 90% reduction of microbial content compared to untreated samples was seen. The
mineral ion composition was not significantly affected by
the treatment, which indicates their potential in the treatment process. The adsorption of Hg2+ on Fe3 O4 @SiO2 ,
Fe3 O4 @SiO2 @HKUST-1, and Bi-I-functionalized magnetic
MOF composites was investigated [125]. A good adsorption selectivity of Bi-I-functionalized magnetic MOF
composites towards Hg2+ was demonstrated, with high
Magnetic Nanoparticles: Synthesis, Functionalization, and Applications
adsorption capacity and fast adsorption dynamics. The BiI-functionalized magnetic HKUST-1 composite was then
employed for the selective removal of Hg2+ from water,
demonstrating great potential application of the prepared
magnetic MOF composite as a fascinating adsorbent in environmental monitoring [125].
Magnetic nanostructures also have promising applications
in the field of spintronic devices, sensors, high-density data
storage media, and electrodes for batteries [25, 235, 236].
!-Fe2 O3 and Fe3 O4 possess low cost, are environmental
friendly, and have promising high theoretical capacities
and therefore are promising anode materials for lithium
ion batteries [237, 238]. The theoretical capacities for
!-Fe2 O3 and Fe3 O4 were found to be 1005 mAhg−1 and
928 mAhg−1 [238]. Fe3 O4 MNPs with an average size of
∼20 nm were homogeneously attached on a 3D graphene for
a anode of lithium-ions batteries [235]. The porous feature
of Fe3 O4 /porous graphene can help to enhance the electrolyte/material interfacial reactivity. One-pot hydrothermal
method for in situ filling of carbon nanotubes (CNT) with
Fe3 O4 nanoparticles was demonstrated [238]. The synthesized Fe3 O4 @CNT showed a mesoporous texture with a
surface area of 109.4 m2 g−1 . When evaluated for lithium
storage at 1.0 C (1 C = 928 mAg−1 ", the mesoporous
Fe3 O4 @CNT can retain at 358.9 mAhg−1 after 60 cycles.
The Fe2 O3 -graphene nanosheet composite synthesized by
a microwave-assisted hydrothermal technique exhibited a
large reversible capacity of 1184 mAh-g−1 at a small current of 100 mA-g−1 [239]. Uniform distribution of Fe3 O4
nanoparticles in the carbon nanotube was helpful for
enhancing the electrical conductivity, cyclic stability, and
rate capability.
6. SUMMARY
The synthesis of MNPs with a wide range of compositions and tunable sizes and shapes has made considerable progress. However improving the existing synthesis
routes and/or finding a novel method that can produce reliable MNPs with the characteristics required for numerous
applications, such as biomedical, catalyst, magnetic cooling, and environmental, is a continuous interest of scientific
research. In this chapter, we summarized recent synthesis
routes and surface functionalization of MNPs. The effect
of the shape and size of the nanoparticles on the specific properties was also discussed. Metallic nanoparticles
(e.g., Fe–Co, Fe–Ni) have a higher magnetization than those
of oxide (e.g., CoFe2 O4 , NiFe2 O4 " counterparts. However,
high reactivity and toxicity of metallic nanoparticles make
them inappropriate for many applications. Therefore metallic nanoparticles are necessary to protect with a coating of
a polymer or a silica layer against the surroundings. Surface
functionalization and modification of MNPs with designed
active sites, including catalysts, ligands, enzymes, drugs, and
other species are promising for a variety of applications.
Acknowledgments
The authors thank Professor Raju V. Ramanujan for the discussion and continuous support during the writing of this
chapter.
27
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http://172.16.14.200/ipms04/details.aspx?id=2015122157450
IP GROUP>IPMS >Record detail
Level 1:Core information
Record no
2015122157450
Activity status
Active
Working Title
SOLID BASE CATALYZED DEPOLYMERIZATION OF LIGNIN
Working Inventors
PARESH LAXMIKANT DHEPE;RICHA KUMARI;
Email address of corresponding
inventor
pl.dhepe@ncl.res.in
Contributing institutions
NCL
Administered by
CSIR
Ownership
Categories > Subjects
Hybrid and inorganic materials
Categories > Portfolio
Value addition of bioresources, renewable polymers
Disclosure number
2015-INV-0130
Disclosure date
12/17/2015
Level 2:Decision and
sending to IPMD
NCL No
2016-NCL-0003
Date sent to IPMD
1/22/2016
Filing recommendation
Provisional filing
PCT recommendation
Yes
Notes on recommendation
Level 3:Action by CSIR
CSIR No
2016-NF-0020
CSIR case manager
ID
Priority date
3/4/2016
Level 4:Patent applications
filed
Country
IN
Divisional?
Note on divisional
Designated country
Application number
201611007650
Provisional filing date
3/4/2016
Complete filing date
3/3/2017
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Not published
Publication date
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Granted number
Granted date
Final title
AN IMPROVED HETEROGENEOUS BASE CATALYZED PROCESS FOR DEPOLYMERIZATION OF LIGNIN
Final Inventors
PARESH LAXMIKANT DHEPE;RICHA KUMARI;
Abstract
Level 5:Status and history
Case status
Complete specification filed
Notes(correspondence)
Level 6:Licensing
Licensing status
Notes(licensing)
Licensing case no
License date
09-04-2018, 05:44 PM