Microporous and Mesoporous Materials 126 (2009) 65–71
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials
journal homepage: www.elsevier.com/locate/micromeso
Synthesis and characterization of hybrid MCM-41 materials for heavy
metal adsorption
Kostas Dimos a,*, Panagiota Stathi b, Michael A. Karakassides a, Yiannis Deligiannakis b
a
b
Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece
Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, Agrinio 30100, Greece
a r t i c l e
i n f o
Article history:
Received 20 March 2009
Received in revised form 7 May 2009
Accepted 12 May 2009
Available online 21 May 2009
Keywords:
Hybrid
MCM-41
Dithiocarbamate
Mesoporous
Heavy metal adsorption
a b s t r a c t
A small dithiocarbamate molecule, N-(2-Aminoethyl)dithiocarbamate, was synthesized, characterized
and afterwards used to modify and activate the surfaces of MCM-41 materials. The structure and the surface charge properties of the starting and the novel organic–inorganic hybrid mesoporous materials were
studied by means of powder X-ray diffraction, Fourier transform infrared spectroscopy, DTA/TG thermal
analyses, surface area measurements and potentiometric acid–base titrations. The hybrid materials
retained the regular hexagonal arrangement of cylindrical pores which is the characteristic of the
MCM-41 solids, while a high content (2.57 mmol/g) of the organic molecules in the final products was
achieved. Despite the high concentration of the dithiocarbamate molecules in the pores of the hybrid
MCM-41 materials, final solids retained high specific surface areas (632 m2/g) indicating a homogenous
incorporation of the small organic molecules in the pores. A surface complexation model was developed
to explain the results of the potentiometric titrations and to describe the surface charge and H-binding
properties of the starting and final hybrid materials. These materials are promising heavy metal adsorbents due to the presence of the effective dithiocarbamate groups and the low pH value (3.2) of the point
of zero charge.
Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction
The discovery of the mesoporous materials family M41S at Mobil’s laboratories in 1992 [1,2] offered a new category of porous
materials with many applications as catalysts [3,4], adsorbents
[5,6], sensors [7] separators [8], etc. The most interesting M41S
member is the MCM-41. The MCM-41 solids exhibit high specific
surface areas (1000 m2/g), high crystallinity, high thermal stability, uniformity of hexagonal cylindrical pores, narrow pore distribution and regulated pore diameter from 15 to 100 Å. These
characteristics render these solids as ideal heavy metal adsorbents
[9–11]. The main disadvantage of MCM-41 is the lack of effective
binding groups and permanent negative charge as in the case of
clays. For this reason, in the last decades huge effort has been taken
in the modification of the MCM-41 surface with thiol groups.
In most cases, for the modification of mesoporous silicas,
mercaptopropyltrimethoxysilane is used as thiol source. The final
adsorbents are produced either by co-condensation [12–20] or by
post-grafting [8–11,21–27], while the same molecules have been
successfully used for the modification of clays [28–30]. Although
the modification of porous materials with dithiocarbamate groups
* Corresponding author. Tel.: +30 26510 97367; fax: +30 26510 97074.
E-mail addresses: kdimos@cc.uoi.gr (K. Dimos), me01791@cc.uoi.gr (P. Stathi),
mkarakas@cc.uoi.gr (M.A. Karakassides), ideligia@cc.uoi.gr (Y. Deligiannakis).
1387-1811/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2009.05.021
is of high importance, these groups are effective chelating agents
for metal complexing and there has been an effort in the modification of clays [31], activated carbon [32] and mesoporous silicas
[33–35].
In this aspect we report in this work the synthesis of a novel
hybrid mesoporous material based on MCM-41. The MCM-41
material was modified by post-grafting of N-(2-Aminoethyl)dithiocarbamate (AEDTC) molecules on the pores surfaces of the mesoporous solid. The organic molecules of this substance were in a
zwitterionic form and thereby their grafting was feasible by the silanol groups of the surfaces. AEDTC molecules were chosen in order
not to block the MCM-41 pores. The final hybrid adsorbents were
characterized with regard to their structural and surface charge
properties.
2. Experimental
2.1. Reagents
All materials were of reagent or analytical grade and were used
as purchased without further purification. Tetraethylorthosilicate
(TEOS) 98% was purchased from Sigma-Aldrich (131903), aqueous
ammonia solution (NH3) 25%wt. from Fluka (09860), cetyltrimethylammonium bromide (CTAB) 95% from Sigma-Aldrich (855820),
66
K. Dimos et al. / Microporous and Mesoporous Materials 126 (2009) 65–71
ethanol (EtOH) 99.5% from Panreac (121086.1212), ammonium nitrate (NY4NJ3) 98+% from Sigma-Aldrich (221244), carbon disulfide (CS2) 99.5+% from Merck (1.02211.1000), ethylenediamine
(H2N(CH2)2NH2) 99+% from Sigma-Aldrich (240729), diethyl ether
(Et2O) 99+% from Fluka (31700), hydrochloric acid (HCl) 1N from
Riedel de Haën (35328), sodium hydroxide pellets (NaOH) 99+%
from Merck (1.06498.1000) and nitric acid (HNJ3) 65% from Riedel de Haën (30709). The solutions for the potentiometric titrations were prepared with ultra pure water (Milli-Q) by Milli-Q
academic system with conductivity of demineralised water of
18.2 lS cm1.
2.2. Synthesis
The MCM-41 sample was synthesized by hydrolyzing 50 g tetraethylorthosilicate (TEOS), added in one litre polyethylene bottle
containing 417.5 g H2O, 268.5 g NH3 (25%wt) and 10.5 g cetyltrimethylammonium bromide (CTAB). Each of the previous additions
was stirred for 30 min. The product was retrieved after heat treatment at 80 °C for 96 h, which can be slightly considered as a hydrothermal treatment. It was filtered, rinsed with cold ethanol (EtOH)
and finally placed on a plate for air-drying (sample: MCM-41).
Afterwards it was treated with NH4NO3 in EtOH for 30 min at
70 °C for the removal of the surfactant molecules, filtered, rinsed
with cold EtOH and the same procedure was repeated. Finally
the end product was placed on a plate for air-drying (sample:
MCM-41NH4) [36].
A solution of CS2 (0.95 g, 0.0125 mol) in EtOH (50 g) was added
dropwise to a solution of H2N–CH2–CH2–NH2 (0.75 g, 0.0125 mol)
in EtOH (50 g) at 5 °C to synthesize N-(2-Aminoethyl)dithiocarbamate. After stirring for one hour the retrieved white powder was
filtered, rinsed with cold EtOH and ether (Et2O) and dried under
vacuum. The retrieved powder was dissolved in hot H2O and
recrystallized at RT. The white crystals were rinsed with EtOH
and Et2O, dried under vacuum and finally ground so that it was acquired as powder [37]. The synthesis of the zwitterionic form (inner salt) of the N-(2-Aminoethyl)dithiocarbamic acid (AEDTC) was
achieved via the following chemical reaction:
H2 NACH2 ACH2 ANH2 þ CS2 ! H3 Nþ ACH2 ACH2 ANðHÞACS2
ð1Þ
The synthesis of the hybrid material was carried out with the
heat treatment of 400 mg MCM-41NH4 in 75 ml EtOH at 50 °C
for 2 h with 300 mg AEDTC and addition of 5 ml HCl 1 N for the dissolution of the organic molecule in EtOH. The final product was
isolated by filtration, rinsed with EtOH and dried at RT (sample:
MCM-41AEDTC).
KBr pellets and were used for recording the spectra, which were
the average of 64 scans at 2 cm1 resolution.
X-ray powder diffraction data were collected on a D8 Advance
Bruker diffractometer using Cu Ka (40 kV, 40 mA, k = 1.54178 Å)
radiation and a secondary beam graphite monochromator. Diffraction patterns were collected in the 2h range from 2 to 80 degrees,
in steps of 0.02 degrees and 2 s counting time per step.
The nitrogen adsorption–desorption isotherms were measured
at 77 K on a Sorptomatic 1990, thermo Finnigan porosimeter. Specific surface areas SBET were determined by the Brunauer–Emmett–Teller (BET) method using adsorption data points in the
relative pressure P/Po range 0.01–0.30. Surface areas St were also
determined from t-plots which were constructed using nitrogen
adsorption data on a nonporous-hydroxylated silica standard.
The desorption branches of the isotherms were used for the pore
size calculations according to the Kelvin equation rk 4.146/log Po/P (Å), where Po is the saturated vapour pressure in equilibrium
with the adsorbate condensed in a capillary or a pore, P is the vapour pressure of liquid contained in a cylindrical capillary, and rk
is the Kelvin radius of the capillary or pore. The Kelvin equation
was used according to BJH method for the calculation of core radii
from the pressure values of the isotherm, the pore radius
combining the last with the t-values from the standard isotherm
and finally the pore size distribution (PSD) of the samples. All
samples used for the surface analyses were outgassed at
250 °C for 10 h under high vacuum (105 mbar) before the
measurements.
The surface charge properties of the starting and the hybrid
materials (MCM-41NH4 and MCM-41AEDTC) were evaluated
by potentiometric acid–base titrations. Acid–base potentiometric
titrations were used to measure the surface proton adsorption as
described earlier [38,39]. A 12.5 mg portion of material was suspended in a titration cell containing 12.5 mL of Milli-Q water to
yield material concentration 1 g/L. The suspension allowed to
equilibrate for 30 min under continuous stirring and purged with
N2 prior to the titration. Then it was divided into two equal volume
portions, one each for the alkalimetric and acidimetric titrations.
The alkalimetric titrations were performed with 12.5 mM NaOH
and the acidimetric titrations with 12.5 mM HNO3. In all titrations
the Metrohm 794 Basic Titrino burette was used and the pH was
measured with Metrohm Pt-glass electrode type (6.0239.100).
The titration experiments were repeated in triplicate. A key parameter in this type of experiments is the achievement of thermodynamic equilibrium after each acid–base addition. In our setup a
stability of ±0.01 mV in the electrode reading was required, corresponding to a stability of ±0.05 at pH values. Typically in the present experiment, pH equilibrium was attained within 20 min after
each acid–base addition.
2.3. Characterization
Single-crystal X-rays diffraction study of the organic substance
(AEDTC) was performed with a Bruker P4 diffractometer using Mo
Ka (50 kV, 40 mA, k = 0.71073 Å) radiation with a suitable single
crystal (0.45 0.30 0.15 mm). AEDTC was also studied by means
of NMR spectroscopy, elemental analysis, thermal analyses and
infrared spectroscopy. Nuclear Magnetic Resonance spectra were
recorded with a Bruker AC at 250 MHz, using D2O + NaOH as solvent. Elemental analysis was performed with a CHNS Perkin Elmer
2400 II Elemental Analyzer. Thermal analyses were studied with a
Perkin Elmer Pyris Diamond TG/DTA analyser in atmosphere with a
3 °C/min heating rate up to 800 °C while the melting point of the
substance was determined with a Stuart Scientific melting point
apparatus.
Infrared spectra were measured on a Perkin Elmer GX, Fourier
transform spectrometer in the frequency range of 400–
4000 cm1. Samples were dispersed, pulverized in the form of
3. Results and discussion
3.1. AEDTC characterization
Single crystal X-rays diffraction for AEDTC gave the following
data: Monoclinic, P21/c, a = 6.990 (3) Å, b = 10.160 (5) Å, c = 8.688
(4) Å, a = 90.04 (3)o, b = 92.51 (2)o, c = 90.00 (2)° and V = 616.08
(9) Å3. These data come in full agreement with those reported by
Yamin et al. about the crystallography of N-(2-Aminoethyl)dithiocarbamic acid (AEDTC) [37], confirming the synthesis of the zwitterionic form of AEDTC. Powder X-ray diffraction pattern of the
substance is available as supplementary material. 1H NMR spectra,
also available as supplementary material, resulted: dH(250 MHz;
D2O + NaOH; Me4Si) 2.70 (2 H, t, J 6.25, N(D2)CH2) and 3.49 (2 H,
1
t, J 6.25, CH2N(D)CS
2 ). H NMR data come also in full agreement
with those reported in Ref. [40].
K. Dimos et al. / Microporous and Mesoporous Materials 126 (2009) 65–71
67
The melting point of the organic substance AEDTC was found
193 °C (from H2O) which is very reasonable compared to the reported AEDTC sodium salt mp of 201 °C [41,42]. The results above
are confirmed by the elemental analysis of the sample (Found: C,
26.58; H, 5.80; N, 20.43; S, 47.19. Calc. for C3H8N2S2: C, 26.45; H,
5.92; N, 20.56; S, 47.07%).
FT-Infrared spectrum of AEDTC in Fig. 1 showed main peaks
at mmax(KBr pellets)/cm1: 3310w m(NH), 3231vs m(NH3+), 2940br
masym.(CH2), 2851br msym.(CH2), 1585s d(NH), 1508vs m(N CS2 ),
1478vs m(CN), 1442s d(CH2) and 1005vs m(C S) [43,44]. The shift
of the N CS
2 band at higher frequency and respectively of the
C S band at lower frequency indicate that the molecule has three
resonance forms [44].
3.2. Structural study of starting and hybrid materials
FT-Infrared spectra (a) and (b) of MCM-41NH4 and MCM41AEDTC, respectively, are shown in Fig. 1. The two spectra have
common peaks at mmax(KBr pellets)/cm1: 3473br m(H2O), 1639w
d(H2O), 1235w masym.(Si–O–Si) longitudinal-optical mode, 1087vs
masym.(Si–O–Si) transverse-optical mode, 960 m masym.(Si–OY),
800 m d(Si–O–Si) and 463s q(Si–O–Si) [45,46]. Spectrum (b) of
MCM-41AEDTC has additional peaks assigned to the organic
molecules AEDTC. In particular, the spectrum of the hybrid material has a shoulder in its main peak at 1005 cm1 which is
attributed to m(C S). Moreover distinct peaks at 1505 and
1590 cm1 are assigned as m(N CS
2 ) and d(NH), respectively.
Spectrum (b) has common peaks with spectrum (c) also in the
region 2400–3250 cm1. Spectral pattern in this region has a fingerprint character of the AEDTC presence in the hybrid material.
The above indicate the successful importation of the organic
molecules in the MCM-41 pores.
X-ray powder diffraction patterns of the samples MCM-41,
MCM-41NH4 and MCM-41AEDTC within the 2h range 1.5°–7°
are shown in Fig. 2. Those are typical of MCM-41 samples with
the characteristic strong reflection at low scattering angles 2h
(2o) corresponding to a d100 spacing at about 39.9, 41.5 and
41.6 Å using Bragg’s law, respectively. The fact that all three samples show the reflections which correspond to d110, d200 and even
d210 spacings indicates that the MCM-41 structure is maintained
in both (NH4 and AEDTC) cases providing good quality and high
range hexagonal uniform pores.
Thermal analysis of the final product (MCM-41AEDTC) indicates the presence of dithiocarbamate groups inside MCM-41
pores as suggested by the decrease of %TG signal (35%wt) in the
temperature range of 200 to 300 °C. Also, in the DTA signal an exo-
Fig. 1. FT-Infrared spectra of MCM-41NH4 (a), MCM-41AEDTC (b) and AEDTC
(c).
Fig. 2. X-ray diffraction patterns in the low angle region for the MCM-41 matrix for
samples MCM-41 (a), MCM-41NH4 (b) and MCM-41AEDTC (c).
thermic peak appears at 292 °C, which originates from the combustion of the organic substance. On the other hand, the small
inset in Fig. 3 shows the %TG signal of the MCM-41NH4 sample
in which the corresponding loss of weight at temperatures 200–
300 °C is only 2%. This is due to the low gram-equivalent weight
(18 g/greq) of NH4+ in comparison with the high AEDTC molecular
weight (136.24 g/mol) and thus the NH4+ including compound is
giving smaller %TG signal losses. Moreover, NH4+ is decomposed
in an extended thermal region in contrary to the combustion of
AEDTC molecules which is observed in the range of 200–300 °C.
Thus the above suggests that the 35%wt loss of the hybrid material
is due to the presence of the organic molecules. The small endothermic peak at 163 °C in the DTA signal is believed to originate
from molecule deformation as no respective weight loss is
Fig. 3. DTA/TG diagrams of the MCM-41AEDTC sample. Inset: %TG diagram of the
MCM-41NH4 sample.
68
K. Dimos et al. / Microporous and Mesoporous Materials 126 (2009) 65–71
Table 2
Reactions and stability constants (pK) used to fit the experimental data.
pKa
Reaction
(De-)protonation reactions of MCM-41NH4
3
{JY + H + M XOHþ
2
4
{JY M {J + Y+
+
+
9
NY4 M NY3 + Y
Deprotonation reaction of MCM-41AEDTC
10
AEDTC-H M AEDTC + H+
a
Fig. 4. Nitrogen adsorption–desorption isotherms for MCM-41NH4 (a) and MCM41AEDTC (b). Inset: pore distributions calculated from N2 desorption branches.
observed. The hybrid’s material high loading with AEDTC is due to
the small carbon chain length of the organic molecules which prevents the pore blocking and thus fills in a high percentage.
Moreover, it has been shown that MCM-41 materials have a
varying hydroxyl group concentration on the pores surface and
authors have estimated at aJY: 3 lmol/m2 [47], while others at
0
aJY : 4.5 lmol/m2 [48]. Considering that these materials have
specific surface area at about 1000 m2/g, these values can be converted to aJY: 3 mmol/g and aJY’: 4.5 mmol/g, respectively. Given that the hybrid material has a 35%wt. organic content and
that the AEDTC molecular weight is 136.24 g/mol, it is concluded
that the AEDTC content in the hybrid material is 2.57 mmol/g.
Comparison of this value with the previous values of the hydroxyl
groups concentration on the pores surface, which are responsible
for the AEDTC grafting, shows that the hybrid’s material completeness in AEDTC may have reached even 85%, leaving intact the pore
system structure as observed by other characterization techniques.
The completeness percentage would have been even bigger if the
ion exchange reactions with the AEDTC molecules in the synthesis
of the hybrid material were repeated. However this treatment was
not repeated in order to retain the high crystallinity in the final hybrid material.
The nitrogen adsorption–desorption isotherms of the samples
MCM-41NH4 and MCM-41AEDTC are shown in Fig. 4. Both
the samples have type IV classification isotherms [49], which is
the characteristic of adsorption of mesoporous materials MCM41. The presence of a sharp sorption step in adsorption curves, near
to a 0.3 value of P/Po indicates that both solids possess a well-defined array of regular mesopores. Specific surface area was calculated using BET equation and was found to be 948 m2/g for the
MCM-41NH4 and 632 m2/g for the MCM-41AEDTC sample and
from t-plots 937 m2/g and 647 m2/g respectively [50,51]. This
33% decrease of the specific surface area provides evidence that
the pores are filled with the organic molecule which has a significant bigger volume from the NHþ
4 cations without blocking the
pores. The fact that the isotherm for the MCM-41AEDTC sample
is similar in shape to that of the parent sample, suggests that the
organic molecules should be dispersed uniformly throughout the
pores. The desorption branch was used to calculate mesopore size
1.6
5.0
9.3
6.9
Errors, pK: ±0.2.
distributions by means of the Barrett-Joyner-Halenda (BJH) method [52]. From the PDS curve of the MCM-41NH4 sample its mean
pore diameter was calculated at about 27.1 Å, while the pores of
the MCM-41AEDTC sample show a size distribution with an average pore diameter of 26.4 Å (Fig. 4 inset). Nevertheless it is a fact
that the BJH method tends to underestimate pore diameter [53],
so the Kruk–Jaroniec–Sayari (KJS) geometrical model was also used
for the pore diameter calculation [54,55], though this model
slightly overestimates the diameter. The KJS geometrical model is
based on the following formula:
W KJS ¼ c d100 ½q V p =ð1 þ q V p Þ1=2
ð2Þ
½
½
where wKJS is the pore diameter, c = [8 / (3 p)] = 1.213 is a constant, d100 is the (1 0 0) interplanar spacing, q is the density of pore
walls (taken as the density of amorphous silica, 2.2 g/cm3) and Vp is
the pore volume, equal to 0.80 cm3/g and 0.54 cm3/g for MCM41NH4 and MCM-41AEDTC samples, respectively, as estimated
from the nitrogen adsorption–desorption isotherms (Table 1). The
formula gives a diameter equal to 40.2 Å for MCM-41NH4 and to
37.2 Å for MCM-41AEDTC. These values highly differ from the
BJH method calculated, which in reality means that actual pore
diameter is somewhere between the two calculated values. The
most significant pore parameters derived from X-ray diffraction
patterns and N2 adsorption data for the two samples are listed in
Table 1.
3.3. Surface charge properties of starting and hybrid materials
The H-binding properties of the surface groups of the MCM
materials used in this work were described by the Diffuse Layer
Model (DLM) which is a surface complexation model. Surface Complexation Models (SCMs) can model successfully the adsorption of
ions on charged surfaces by assuming that adsorption involves
both a coordination reaction at specific surface sites and an interaction between the adsorbed ions and the organic ligand on the
modified material [56].
FITEQL 4.0 [57] was used to determine the best fit of various
surface complexation reactions or combinations of reactions to
the experimental adsorption data. The Davies equation was used
to calculate the activity coefficient [56]. Relative errors of 5% in
the concentration of surface sites and pH were allowed in FITEQL
input values. All the pertinent reactions at the solid solution interface and their pK values, used for the fit of the experimental
H-binding data are listed in Table 2.
Štamberg et al. have successfully used SCM to describe the Hbinding properties of an MCM-41 material [58]. In our modelling
Table 1
Pore parameters derived from X-ray diffraction patterns and N2 adsorption data.
Samples
SBET (m2/g)
St (m2/g)
Vpore (cm3/g)
d100 (Å)
ao (Å)
dBJH (Å)
wKJS (Å)
p (Å)
MCM-41NY4
MCM-41AEDTC
948
632
937
647
0.80
0.54
41.5
41.6
47.9
48.0
27.1
26.4
40.2
37.2
20.8/7.7
21.6/10.8
K. Dimos et al. / Microporous and Mesoporous Materials 126 (2009) 65–71
69
we have assumed three different populations of H-binding sites on
the starting and the hybrid materials, XOH sites (a), NHþ
4 (b) and
AEDTC groups (c).
XOH sites represent the amphoteric hydroxyl groups on the
MCM-41 surface. H-exchange at the surface hydroxyl sites of
MCM-41 can be modelled according to reactions (3) and (4):
Kþ
int
XOH þ HþS $ XOHþ2
ð3Þ
K
int
XOH $ XO þ HþS
ð4Þ
Hþ
s
where
describes the protons near the charged solid surface [56].
According to this model, at any pH value the total concentration of
surface groups XOH is given by Eq. (5):
Rð XOHÞ ¼ ½ XOH þ ½ XO þ ½ XOHþ2 ð5Þ
Depending on pH of the solution, a surface site can be at neutral,
protonated,
or
deprotonated
form.
The
pH
where
[{JY2+] = [{J] is the Point of Zero Charge [56]. The equilibrium constants for the surface reactions are given by Eqs. (6) and
(7):
F W ½ XOHþ2 0
RT
e
½ XOHðHþS Þ
½ XO ðHþS Þ FRTW0
¼
e
½ XOH
Kþint ¼
ð6Þ
K int
ð7Þ
where W0 is the electrostatic surface potential [56], F is the Faraday
constant, R is the gas constant and T is the absolute temperature.
The relation of the surface charge r with W0 is given by the Gouy
–Chapman equation (8):
r ¼ 0:1174 I1=2 sin h
W0 F
2RT
ð8Þ
where I is the ionic strength.
The H-binding reaction of the NHþ
4 counterions is shown in the
following (9):
NHþ4
þ
$ NH3 þ H
ð9Þ
The protonation of AEDTC groups (third binding population
sites) on the hybrid material was described by the following reaction (10):
AEDTC H $ AEDTC þ H
ð10Þ
where the symbol () has been added to notify that the AEDTC moieties are considered as part of the solid matrix. Of importance is to
notice that the pK for the protonation of the AEDTC in reaction (10)
(Table 2) has to be determined by the fit to the acid–base titration
data. This implies that we consider that the pK of the AEDTC attached on the solid matrix might differ from the, known [59], pK
in solution.
Fig. 5A, top, shows the acid–base potentiometric titration
experimental data and the theoretical fit for sample MCM41NH4. CA and CB in the vertical axis of Fig. 5A represent the acid
or base addition to the two solution portions used for the acidimetric and alkalimetric titrations, respectively. The term CA–CB has no
physical meaning but is used as we combine the data from the two
titrations in one common diagram. At pH 3–4 we observe a rapid
loss of protons from the surface groups resisting the pH increase
with base addition. Those protons derive from the deprotonation
reaction (4) of the XOH sites. In the pH region 4–9 the slightest
base addition launches the pH value, indicating that all XOH sites
have converted to {J. Eventually at pH > 9 the sample MCM41NH4 resists again at the pH increase due to deprotonation reaction (9) of the NHþ
4 counterions. The theoretical fit anticipates the
pK values of the three participating reactions (3), (4), and (9). Those
Fig. 5. Potentiometric acid–base titration (symbols) and theoretical fit (solid line)
for sample MCM-41NH4 (A) and speciation analysis of the surface species derived
by using the parameters in Table 2 (B).
values are: pKint+ = 1.6 ± 0.2 for reaction (3), pKint = 5.0 ± 0.2 for
reaction (4) and pK = 9.3 ± 0.2 for reaction (9), also listed in Table 2.
The estimated pK
int value is 3.7 less than analogous pK values for
MCM-41 material [58], a fact with great physical meaning which
will be discussed later.
The pH of the Point of Zero Charge can be estimated from the
following Eq. (11):
pHpzc ¼ 1=2ðjpKþint j þ jpKint jÞ ¼ 3:2 0:2
ð11Þ
This value is among the lowest PZC reported for oxides bearing
protonable surface groups [60]. This implies that in aqueous solution at any pH value above 3.2 the {J species will dominate and
the MCM-41 surface will bear a progressively increasing negative
charge, e.g. which is counterbalanced by the NHþ
4 cations. The
physical meaning of the low pHPZC value is that the material is a
very promising adsorbent in a wide pH range as it bears negative
charge at pH > 3.2. The above can be visualized by the speciation
analysis, displayed in Fig. 5B.
Fig. 6A, top, shows the acid–base potentiometric titration
experimental data and the theoretical fit for sample MCM41AEDTC. By comparing the curves in Figs. 5A and 6A we notice
that the incorporation of the AEDTC in the MCM-41 pores has two
significant effects on the H-binding properties of the final hybrid
material. First, the NHþ
4 ions are largely exchanged by the zwitterions H3N+–CH2–CH2–N(H)–CS
2 as evidenced by their loss in the Hbinding data for MCM-41AEDTC. Second and the main difference,
is the loss of protons at pH 5–7. As we show by the theoretical fit to
the data, solid line in Fig. 6A, this is due to the deprotonation of the
R–CS2 groups, reaction (10) in Table 2, with a pK value of 6.9. This
pK value is considerably lower than the 3.3 reported previously
70
K. Dimos et al. / Microporous and Mesoporous Materials 126 (2009) 65–71
surface area (632 m2/g) while the AEDTC content reached 35%wt.
Moreover the study of the surface charge properties of the hybrid
material resulted a significant low pH of the point of zero charge
(pHPZC = 3.2). These results render the hybrid material as a promising heavy metal adsorbent regarding also the AEDTC presence in
the pores of the MCM-41 solid. Heavy metal adsorption experiments by the hybrid material are currently carried out in our
laboratory.
Acknowledgments
This research was co-funded by the European Union in the
framework of the program ‘‘Pythagoras I” of the ‘‘Operational Program for Education and Initial Vocational Training” of the 3rd Community Support Framework of the Hellenic Ministry of Education,
funded by 25% from national sources and by 75% from the European Social Fund (ESF). We would like to thank the X-ray Laboratory and the NMR Centre of the University of Ioannina for
measurements. Helpful and stimulating discussions with Ass. Prof.
M. Siskos and Lecturer Dr. I. Koutselas are gratefully acknowledged.
Appendix A. Supplementary data
250 Hz 1H NMR spectra of AEDTC on Figs. S1 and S2, experimental and simulated X-ray powder diffraction patterns of AEDTC on
Fig. S3. Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.micromeso.2009.
05.021.
References
Fig. 6. Potentiometric acid–base titration (symbols) and theoretical fit (solid line)
for sample MCM-41AEDTC (A) and speciation analysis of the surface species
derived by using the parameters in Table 2 (B).
[31] for a dithiocarbamate in clay. The difference in the pK values is
not trivial to explain. We may speculate that local electrostatic
interactions of the zwitterionic form H3N+–CH2–CH2–N(H)–CS
2 inside the MCM-41 tubes are responsible for the stabilisation of the
H – binding on the –CS
2 moiety as evidenced by the low pK.
The resulting speciation, Fig. 6B, helps to visualize all the protonation events. At pH > 3 the surface sites are in the form XO.
Accordingly, at pH < 8 a fraction of 10–15% of NH3 is formed. The
speciation in Fig. 6B shows that in the hybrid material MCM41AEDTC, nearly 30% of the initial NHþ
4 ions are still present,
i.e. non-exchanged by the AEDTC, while the other 70+% of the
{J sites are occupied by the AEDTC molecules. From the concentration of the hybrid material (1 g/L) and the AEDTC groups
(2.12 mM) in the titration solutions, the hybrid’s material content
in AEDTC can be estimated, which results in a 2.12 mmol/g content. This value is considered underestimated as from the thermogravimetric analysis the content was estimated at 2.57 mmol/g, a
value, most likely, closer to reality. Of primary importance, however is the role of the R–CS
2 groups of the AEDTC moieties which
at pH > 6 are gradually becoming dominant. These groups are
effective chelating agents for metal complexing, making the hybrid
material an ideal heavy metal adsorbent.
4. Conclusions
In the present work a novel hybrid mesoporous material was
synthesized based on MCM-41 and the organic molecule N-(2Aminoethyl)dithiocarbamate (AEDTC). The hybrid material
retained its structure properties, i.e. the high crystallinity and high
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