UEMK4363 Catalysts and Catalytic Processes
Analysis of Physical and Chemical Properties of Vanadium Phosphorus Oxide (VPO)
Catalyst
Assignment 1
Group No: 3
Student
ID
Name
1900234
Kee You Wei
1704052
Lee Ler Ping
1701489
Ong Loke Boon
1704752
Tew Jia Yee
Table of Content
Contents
Abstract .......................................................................................................................... 3
1.0
Morphology .......................................................................................................... 4
2.0
Surface Area ......................................................................................................... 6
3.0
Pore Size ............................................................................................................... 8
4.0
Particle Size ........................................................................................................ 10
5.0
Structure............................................................................................................. 11
6.0
Conclusion .......................................................................................................... 13
7.0
Reference ............................................................................................................ 14
Abstract
The physical and chemical properties of a catalyst is an influencing factor in affecting its
performance in catalyzing chemical reactions. In this study, catalysts produced using the Sol gel
method has been investigated in terms of its physical characteristics including morphology,
surface area, pore size/volume, particle size as well as overall structure. Investigation showed that
the physical properties of the catalyst is greatly affected by the parameters involved during the Sol
gel fabrication stage such as the amount of solvent used, drying temperature and duration. The
resulting physical properties of the catalyst in turn affects the performance in catalyzing chemical
reactions in terms of conversion, selectivity and yield. Amount of solvent used in the sol gel
method has shown to be one of the primary factors in determining various catalyst physical
property including catalyst morphology, surface area and pore size. With increased amount of
solvent used, the morphology of the catalyst product changes, increasing the surface area and
decreases pore size.
3
1.0
Morphology
Maleic anhydride, or in short, MA, is industrially produced where it is primarily applied in
synthesizing polymers like unsaturated polyester resins. MA can also be used as lubricating oil
additives. According to a study done by Fernández, Vega and Díez (2010), a large portion of MA
is manufactured via selective oxidation of n-butane over vanadium phosphorus oxide (VPO)
catalyst. This is because the synthesis of MA using butane as feedstock is economically
advantageous compared to benzene due to the increase in benzene market price. There are several
methods that can be applied to synthesize the VPO catalyst used in this MA production process,
for instance sol-gel method, hydrothermal synthesis, ultrasound technique, microwave irradiation
and ball-milling process. Among these methods, the sol-gel method is chosen as the VPO catalyst
synthesis method in this assignment as it has a significantly higher selectivity compared to
traditionally prepared catalysts, in which the data will be shown in the subsequent sections.
As stated in Salazar and Hohn (2013), the principle of sol-gel technique to produce VPO
catalyst involves reacting vanadium (V) triisopropoxide oxide with ortho-phosphoric acid in an
aprotic solvent. After that, the products undergo drying in an autoclave at high pressure with a
controlled excess of solvent. This step forms a VOPO4 gel with molecules that are interlayer
entrapped. The alkoxide hydrolysis forms alcohol which in turn reduces the vanadium during
drying, therefore converting the VOPO4 to the precursor, which is VOHPO4·0.5H2O. Nonagglomerated platelets are formed during this stage, which will then undergo dehydration and
subsequent evaluation in a butane-air mixture.
Morphology is an important catalyst property that can impact the VPO catalytic
performance in terms of active phase catalytic activity. The sol-gel method is often utilized to
manufacture metal oxide catalysts as this method allows regulation of the nanostructure and
composition of the final material during the early synthesis stages. Among the many sol-gel
procedures, the metal alkoxide method, or in short, MAM is very commonly used. In this method,
the metal alkoxides are hydrolysed and condensed to obtain colloidal gels where the structural
characteristics can be manipulated. When MAM is applied to vanadium phosphorous oxides, it is
found that reactions that take place in aprotic solvents and without water will form compact and
small particles.
4
Thus, characterization of the crystallite morphology of the VPO catalyst can determine the
influence of the sol-gel synthesis method on the catalytic activity. In an experiment conducted by
Salazar and Hohn (2013), it is observed that the particle morphology is largely affected by the
drying type during the sol-gel method. For atmospheric pressure drying, spherical particles were
yielded while platelets were obtained when high pressure drying was utilized. Besides that,
morphology is also largely impacted by the solvent amount. Figure 1 illustrates the SEM
micrographs of VPO precursors prepared with different solvent amount.
Figure 1: SEM Micrographs of VPO Precursors Prepared Via Alkoxide Method and (a)
Autoclave Drying, Low Amount of Solvent (LS); (b) Autoclave Drying, Medium Amount of
Solvent (MS); (c) Autoclave Drying, High Amount of Solvent (HS); (d) AD (Salazar and
Hohn, 2013).
As shown in Figure 1, among high pressure treatments, larger particles were formed when
low amount of solvent (LS) was applied while for medium amount (MS) and high amount (HS),
the researchers observed not much significant discrepancies in terms of morphology. The LS
morphology is typical of the precursor with a close to one phosphorous/vanadium ratio. Hence, in
autoclave drying, the added solvent amount affects the conditions where complete vaporization
occurs and thus affects the final morphology.
5
2.0
Surface Area
According to Salazar and Hohn (2013), the sol-gel procedure to synthesis VPO catalyst involves
a crucial part which is the procedure utilized to remove solvent from the gel. This drying process
can be performed via two methods, which are: i) evaporation; and ii) supercritical drying. During
the evaporation process, the high surface tension at the liquid-gas interface will influence the gel
structure. Initially, liquid fills the very small gel pores. However, the pore walls are not able to
resist the force applied during liquid vaporization and eventually the pore walls will collapse. This
will result in a significant damage to the gel structure, an increase in particle size and a reduction
in the surface area. Nevertheless, this issue can be solved if evaporation is performed close to the
solvent’s critical point where no surface tension is available. An autoclave is used to carry out the
process and a suitable excess of solvent is added to achieve the critical point before the solvent is
lost.
Salazar and Hohn (2013) has conducted an experiment where different solvent amounts
were added and the surface areas of all the autoclave-prepared precursors were obtained and
tabulated in Table 1. It is observed that the amount of solvent added has an influence on the
surface area. The variance analysis showed that the surface areas were similar for MS and HS but
different for LS. To make comparison between the means, the Bonfferoni t test was carried out.
The results showed that solvent vaporization during drying before the critical point caused a huge
surface area loss.
Table 1: BET Surface Areas for Products Dried After Adding Different Solvent Amounts
(Salazar and Hohn, 2013).
Precursor
Average Surface Area
(m2/g)
Low Amount of Solvent (LS)
65
Medium Amount of Solvent (MS)
102
High Amount of Solvent (HS)
121
Salazar and Hohn (2013) also obtained the infrared spectra patterns for the dried materials
when different solvent amounts were used. These spectra and the reference spectra are shown in
Figure 2. Figures 2b to 2d depicts the spectra evolution from VOPO4 towards the precursor when
the added solvent amount was reduced.
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Figure 2: DRIFTS Spectra of Different VPO (a) Slurry Dried on Nitrogen; (b) High Amount
of Solvent; (c) Medium Amount of Solvent; (d) Low Amount of Solvent; (e)
VOHPO4·0.5H2O. NA: Non-Assigned Band (Salazar and Hohn, 2013).
Figure 2b shows the appearance of bands at 1054 cm-1, 977 cm-1 and 1196 cm-1. Then, in
Figure 2c, 1104 cm-1 and 932 cm-1 are added and eventually 1133 cm-1 band appears in Figure
2d. This marks the completion of the six characteristic bands of the precursor’s fingerprint spectra.
This shows that the final materials’ chemical composition is affected by the added solvent amount,
indicating that chemical transformations occur during autoclave drying in addition to the physical
changes. It is worth to note that although MS and HS have similar surface area, there are
discrepancies in their phase compositions. This indicates that the surface area difference is not
solely due to phase composition discrepancies, the autoclave drying procedure also plays a part in
manufacturing vanadium phosphates with high surface area.
To summarize it, the solvent amount added to the slurry drastically impacts the surface
areas as it affects the autoclave’s final pressure. A double in the amount of solvent will result in a
double in the surface area.
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3.0
Pore Size
Synthesis of vanadium phosphorus oxide via the sol gel method for selective oxidation of n-butane
has been thoroughly explored and investigated in various studies, each with different approach in
terms of methodology and selection of catalyst precursors. Different synthesis methodologies are
expected to affect the resulting catalyst properties and catalytic performance. To determine the
effect of synthesis methodology on the resulting catalyst property, mainly concerning the catalyst
pore size and pore volume, proposed sol gel methodology from various studies had been
investigated and correlated to their resulting catalytic performance.
In a study conducted by Salazar and Hohn (2013), Vanadium (V) triisopropoxide oxide
was reacted with ortho-phosphoric acid in an aprotic solvent to allow gelation and formation of a
colloidal structure, followed by drying at high temperature in an autoclave to form a gel of
vanadium phosphorus oxide. In this technique, the amount of solvent used greatly affects the
morphology and therefore the pore size or volume of the resulting catalytic structure. Three levels
of solvent (100 mL, 50 mL, 20 mL) are used in similar experimental set up which resulted in
activated catalysts of different pore sizes, as shown from SEM analysis in Figure 3.
Figure 3: (a) Traditional synthetic route, (b) Sol gel method, 20 mL solvent, (c) Sol gel
method (50 mL), (d) Sol gel method (100 mL) (Salazar and Hohn, 2013).
Investigation through sol gel method reveals that while then platelet morphology of the
catalyst produced through sol gel method are similar (Figure 3(b), (c) and (d)) and differ from
catalyst made using traditional impregnation method, as shown in Figure 3(a), varying degrees of
solvent added during the colloid formation stage would result in catalyst with different pore sizes.
As more solvent are being added, the size of the pores would decrease, with higher number of
pores and total pore volume. Due to the increased pore volume, Salazar and Hohn (2013) stated
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that the surface area of the catalyst has increased dramatically, doubling for when the amount of
solvent is doubled, based on BET surface area measurement.
The effect of changing pore size and pore volume due to the use of different amount of
solvent in the colloid formation stage is further investigated through investigating the performance
of each catalyst individually, in the reaction of n-butane partial oxidation to maleic anhydride.
Using similar experimental set up (similar reactant concentration, catalyst weight and reaction
condition) with the synthesized catalysts with different pore sizes, the resulting butane conversion
and selectivity are listed in Table 2.
Table 2: Resulting butane conversion and MA selectivity from catalysts with different pore
sizes (Salazar and Hohn, 2013).
Solvent amount (mL)
Butane Conversion (%)
MA selectivity (%)
20
31.1
41.8
50
20.6
28.0
100
28.4
12.1
As the amount of solvent used increases, the pore size decreases and pore volume of the
resulting catalyst increases. Higher pore volume should allow more active sites within a given
catalyst weight, which should correspond to a higher conversion and selectivity. However, from
Table 2, as overall pore volume increases, the butane conversion and selectivity decreases, apart
from similar butane conversion resulting from catalyst formed using 20 mL and 100 mL solvent.
Salazar and Hohn (2013) stated that although the pore volume and thus surface area of the catalyst
can be increased through the addition of more solvent, this would also result in a decreased active
species precursor concentration in the colloid formation stage, causing the resulting catalyst to
have a lower active site concentration on its surface area. In this case, the benefit from increasing
pore volume and surface area through the addition of more solvent had been outweighed by the
accompanying lower active species concentration on the resulting catalyst surface.
In summary, based on the study conducted by Salazar and Hohn (2013), the amount of
solvent used in sol gel method is one of the predominant factors in affecting the pore volume and
pore size of the resulting catalyst. Increasing amount of solvent used would result in a catalyst
with smaller pore size and higher pore volume. However, increased solvent amount would also
lead to a decreased active species concentration, leading to lower catalytic performance.
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4.0
Particle Size
Salazar and Hohn (2013) also stated that particle size is a significant factor in the catalytic
materials' activity because small crystallites that are exposed to the plane are thought to be more
selective. Thus, in this chapter, the impact of sol-gel method on crystallite particle size is discussed.
Based on the studies done by Kamiya and his team (mentioned in Chapter 2.0), the reduced
precursor HS, MS, and LS all have crystallite sizes that are determined by SEM to be between
400 nm and 500 nm in length. The thickness of crystallite that determined by XRD analysis for
LS is 17 nm, while the thickness of crystallite for HS and MS are unable to determine due to low
concentration of VOHPO4•0.5H2O. Isopropanol was introduced as an additional solvent and
allowed to age for 15 hours prior to the drying step in order to boost the concentration of
hemihydrate in the precursors. As the result, the length of crystallite that estimated by SEM to be
between 700 nm and 1100 nm, while the thickness of crystallite that estimated by XRD analysis
are to be between 12 nm and 18 nm. As activated catalysts from LS precursor, small nonagglomerated crystallites have length around 600 nm. The thickness of crystallite estimated by
XRD analysis is 14 nm. To conclude, the precursors prepared via sol-gel method leads to selective
small crystallites of vanadyl pyrophosphate (Salazar and Hohn, 2013).
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5.0
Structure
To properly analyse the structure of the VPO catalyst, there are 3 distinct characteristics of the
structure to be considered which are the availability and accessibility of the active site along the
pore channel and ultimately the ability for the reacted product maleic anhydride to be diffused out
of the catalyst pore channel. The n-butane molecular width is found to be 2.9 nm and the product
maleic anhydride is 100 nm as the larger molecule by a factor of 34.48 using an instrument called
atomic force microscopy (Zhang, et al., 2020). Common redox reactions include the oxidation of
n-butane to maleic anhydride while the catalyst's surface acidity has a significant impact on the
first step, which is the dehydrogenation of n-butane into olefin. Therefore, a specific quantity of
lattice oxygen is required for the epoxidation of alkene to create MA. The synthesis of maleic
anhydride from n-butane clearly involves a specific number of dehydrogenation sites and lattice
oxygen (Li, et al., 2022).
VPO catalyst in this study exhibits distinct physiochemical properties, such as surface
acidity, lattice oxygen, surface oxygen, valance state, P/V ratio, V+4/V+5 ratio, etc., that are
highly dependent on the precursor preparation and catalyst activation conditions (Faizan, et al.,
2022). They also each have distinctive structural morphology and chemical composition. The
precursor performance in this case will depend on the activation condition. For instance,
triisopropoxide oxide reacts with ortho-phosphoric acid in an aprotic solvent solution before
drying the materials with autoclave. The 𝑉𝑂𝑃𝑂4 gel is porous and can easily hold the incoming
molecules as it is. The alcoholic solution was then included to produce the VPO precursor. The
catalyst as it was used made use of the partial oxidation of n-butane. Therefore, during synthesis,
alternative metal dopants, structure-directing groups, or template agents are added to change the
physical and chemical properties of VPO are usually Bi, Ni, Zn, Ce, Cu, Nb, Cr, Co, Zr and is
denoted as Y metal species in the research paper.
The well-known supercritical drying method or evaporation can be used to carry out this
drying process. The high surface tension present at the liquid-gas interface has an impact on the
gel structure during the evaporation process. The gel has tiny liquid-filled holes that are originally
filled with gel. The walls of the pores collapse as the liquid is vaporised because they are unable
to withstand the force being applied to them (Salazar and Hohn, 2013). If the evaporation process
is carried out close to the solvent's critical point, where there is no surface tension, this issue can
be avoided.
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An X-Ray Diffusion (XRD) analysis is done to determine the crystalline phase of the VPO
precursor and activated catalyst respectively. The primary phase of a VPO catalyst is thought to
be a combination of the phases (𝑉𝑂)2 𝑃2 𝑂7 and 𝑉𝑂𝑃𝑂4 . For Y-modified VPO catalysts,
additional peaks at 19.6, 22.3, and 24.4 °C, corresponding to the - 𝑉𝑂𝑃𝑂4 phase, occurred in
comparison to the activated bulk VPO catalyst. And for Y-modified VPO catalysts, the peaks
show up at 25.1° and 29.2° corresponding to the II- 𝑉𝑂𝑃𝑂4 phase (Li, et al., 2022). No peaks
corresponding to Y species are discovered, indicating that Y species may be widely disseminated
on the catalyst's surface or may enter the VPO catalyst's lattice in the form of Y3+. The
phenomenon suggests that the addition of Y might alter the VPO catalyst's phase structure.
Figure 3: XRD spectra for all VPO precursors (A) and activated catalyst (Li, et al., 2022).
The addition of Y has essentially no influence on the precursor's phase, indicating that the
crystal phase of the VPO precursor is unaffected by the addition of Y. The (0 0 1) surface of the
𝑉𝑂𝐻𝑃𝑂4 • 0.5𝐻2 𝑂 phase is topologically changed into the (𝑉𝑂)2 𝑃2 𝑂7 (2 0 0) surface. And a
higher fraction of active surface (𝑉𝑂)2 𝑃2 𝑂7 is indicated by a higher ratio of I (0 0 1)/I (2 2 0). (2
0 0). The catalyst with 0.01Y-VPO obtains the maximum value. This result indicates that the
modification of Y significantly improves the information of the active (𝑉𝑂)2 𝑃2 𝑂7 phase.
12
6.0
Conclusion
In a nutshell, sol-gel method can be employed to control the properties of precursor
(VOHPO4•0.5H2O) such as morphology, surface area, pore size, particle size and structure are
candidates to yield an improved catalyst. In consequent, the improved properties will increase the
catalytic activity of VOP and further enhance the selectivity to produce MA. Besides, the drying
method of precursor (VOHPO4•0.5H2O) also bring a significant impact on the properties of
catalyst (VOP).
13
7.0
Reference
Faizan, M., Zhang, R. and Liu, R. (2022) “Vanadium Phosphorus Oxide Catalyst: Progress,
development and applications,” Journal of Industrial and Engineering Chemistry, 110, pp. 27–67.
Available at: https://doi.org/10.1016/j.jiec.2022.02.049.
Fernández, J.R., Vega, A. and Díez, F.V., 2010. Partial oxidation of n-butane to maleic anhydride
over VPO in a simulated circulating fluidized bed reactor. Applied Catalysis A: General, [ejournal]
376(1-2),
pp.76-82.
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[Accessed
<
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November 2022].
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by exfoliation–reduction of VOPO4·2H2O in primary alcohol,” Catalysis Today, 78(1-4), pp.
281–290. Available at: https://doi.org/10.1016/s0920-5861(02)00319-x.
Li, W. et al. (2022) “Increasing maleic anhydride selectivity for n-butane oxidation by Y-modified
VPO catalysts,” Fuel, 333, p. 126214. Available at: https://doi.org/10.1016/j.fuel.2022.126214.
Salazar, J. and Hohn, K. (2013) “Partial oxidation of n-butane over a sol-gel prepared vanadium
phosphorous
oxide,”
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3(1),
pp.
11–26.
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at:
https://doi.org/10.3390/catal3010011.
Zhang, X. et al. (2020) Structure sensitivity of N-butane hydrogenolysis on supported IR catalysts,
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(Accessed: November 14, 2022).
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