UNIVERSITI TUNKU ABDUL RAHMAN
Faculty
: Lee Kong Chian Faculty of Engineering & Science
Programme
: BEng (Honours) Chemical Engineering
Course
: UEMK4363 Catalysts and Catalytic Processes
Session
: Oct 2022
Due Date
Part 1: 22nd November 2022
Lecturer
: Mr. Prakas Palanychamy
Expected Duration to
: 4 weeks
Complete Assignment
ASSIGNMENT: PART 1
GROUP MEMBERS:
Name
Programme
Student ID
1. CHAI XIN HUI
CL
1803105
2. NG JIAN MIN
CL
1803240
3. TONG SHI HUI
CL
1801752
ABSTRACT
Microwave irradiation (MW) is a technique used at the last stage of catalyst synthesis. It was
used to perform drying and calcination towards the wetted catalyst. Compared to other heating
methods such as conventional oven heating and ultrasound, it provides more comprehensive
heating due to its heating mechanism. It was proven by research studies that the MW heating
method can enhance the activity, selectivity and stability of the heterogeneous catalyst.
Vanadium Phosphorus Oxide (VPO) is a catalyst used widely in n-butane catalytic selective
oxidation. Among several synthesis techniques of VPO, MW is found to be an effective
technique to improve the catalyst performance. MW of VPO will affect catalyst properties such
as morphology, surface area, pore size, pore volume, particle size and the structure of the
catalyst. To investigate the transformation of VPO on each catalyst’s properties, several
instrumental analyses are applied to perform catalyst characterization. Scanning Electron
Microscopy (SEM) is applied to observe the morphology of VPO and it was found to be in a
thin rosette-type structure with uniform crystal size. The different catalyst structures shown by
MW will lead to smaller platelets size and larger surface area which have a positive impact
towards the selectivity and n-butane conversion. Brunauer-Emmer-Teller (BET) method is
applied to observe the surface area, pore size and pore volume of the VPO. BET applied the
adsorption theory of nitrogen gas onto the surface of the catalyst. Calculations through the BET
equation and Barrett, Joyner, and Halenda (BJH) method can be used to determine surface area
and pore size or volume respectively. The surface area, pore size and pore volume of VPO
synthesized through MW is larger than from conventional heating due to the effectiveness of
volatile compounds removal and prevention of the closure of pore. Particle size is observed
through X-ray diffraction (XRD). The full width at half maximum (FWHM) can be determined
from the XRD pattern and it could be used to determine crystallite sizes at different planes with
the aid of the Debye-Scherrer equation. The particle size observed through XRD is smaller and
it contributes to a higher surface area which will yield higher selectivity. Lastly, catalyst
chemical structure observed the availability of V5+ and V4+ ions on the VPO catalyst using the
Temperature Programme Reduction (TPR) analyser with hydrogen gas and equipped with a
thermal conductivity detector (TCD). More V5+ ions are observed on the CPO catalyst surface
synthesized with MW. Therefore, higher n-butane conversion and selectivity can be yielded as
more side is offered for effective oxidation. In short, MW can produce VPO catalyst with better
catalyst properties and they are observed to yield higher selectivity and higher conversion.
i
TABLE OF CONTENT
CHAPTER 1 .............................................................................................................................. 1
MICROWAVE IRRADIATION ............................................................................................... 1
CHAPTER 2 .............................................................................................................................. 2
CATALYSTS PROPERTY....................................................................................................... 2
2.1
Morphology ............................................................................................................... 2
2.2
Surface Area .............................................................................................................. 4
2.3
Pore Size / Volume .................................................................................................... 6
2.4
Particle Size ............................................................................................................... 8
2.5
Chemical Structure .................................................................................................. 10
CONCLUSION ....................................................................................................................... 12
REFERENCES ........................................................................................................................ 13
ii
CHAPTER 1
MICROWAVE IRRADIATION
In the last stage of catalyst synthesis, the drying and calcination processes are involved. This
is a crucial step for Vanadium Phosphorus Oxide (VPO) synthesis as the synthesis process
involved reflux with distilled water (Rownaghi, Hin, and Wang, 2009). Catalyst properties can
be varied when different drying or calcination approach is applied. Heating techniques
available in the industry include conventional heating, microwave irradiation and ultrasound.
One of the promising methods is microwave irradiation, which can perform rapid direct heating
with minimal heat loss to consolidate the particles due to the high heating rates at around 1000
o
C/min facilitated by the microwave irradiation effect (Chaves et al., 2016).
Microwave irradiation is capable to generate power of 140 to 300W within a period of
2 to 15 minutes under the frequency of 915 MHz to 2450 MHz. The mechanism of microwave
irradiation is explained by the polarization phenomena in which when the material is exposed
to the high-frequency electromagnetic field, the charge of the material will be polarized and
generate friction and heat. Microwave irradiation is well-established in many industries
including methane conversion, carbon dioxide separation, and ammonia production. It can also
be applied in homogeneous and heterogeneous catalytic reactions as microwaves can
selectively heat the catalyst or active sites on the catalyst (Muley et al., 2021; Taufiq-Yap et
al., 2010).
Current researches show microwave irradiation can enhance the activity, selectivity and
stability of heterogeneous catalysts. Catalyst activity refers to the strength of the chemical bond
between the reactants, intermediates product and the catalyst surface. When the polar reactant
molecules exhibit higher rotational velocities than conventional heating, it will lead to faster
entry into transition states and improved reaction kinetics. Increased product selectivity can
also be achieved by microwave irradiation due to its faster heating rates and higher
temperatures at certain sites (such as metal sites on the catalyst surface). Then it will
consequently leads to the rapid activation of reactant. Catalyst stability can be affected by
particle size variation, poisoning and deactivation due to coking. Microwave irradiation is
capable to slow down the catalyst deactivation process and extend catalyst life in some cases
(Chaves et al., 2016).
1
CHAPTER 2
CATALYSTS PROPERTY
2.1
Morphology
In heterogeneous catalysis, the distribution of catalytic active material onto the catalyst surface
and the total surface area available are vital, especially in evaluating the selectivity and
conversion of the catalyst. These aspects could be evaluated by observing the catalyst
morphology as it could aid to observe the surface structure. As conforming to Parvez (2019),
scanning electron microscopy (SEM) is widely implemented to characterise the surface
morphology of solid materials (eg. solid catalyst particles), thus in this section the working
principle, sample preparation, and data analysis of SEM will be discussed to characterise the
morphology of the catalyst synthesised from the microwave irradiation.
First and foremost, SEM operates on the premise of focusing a primary electron beam
with high kinetic energy in the range of 1-40 keV to the atomic electrons of the specimen
whereby the electron scanning is performed with a raster pattern (Kang, 2014). As the beam
interacts with the sample, the electrons are scattered and these secondary electrons are slowed
down as a result of hitting the specimen surface. In this instance, numerous signals have been
generated such that the signals produced are highly localised in the space right beneath the
beam. Eventually, an image is generated on the screen by collecting the secondary electrons
emitted from each point of the specimen, also the signals are used to change the brightness of
a cathode ray tube that is rastering in time with the electron beam, thereby illustrating the
morphology of the specimen. Moreover, it is essential to operate SEM in a vacuum to prevent
interactions between electrons and gas molecules so that a high-resolution image can be
acquired (Sharma, 2018).
Also, conducting a proper sample preparation prior to the analysis is crucial in order to
obtain a clear image. First of all, a two-sided carbon tap should be fixed on the stub before
applying the microwave irradiation synthesised VPO catalyst to the two-sided carbon tap,
which decreases the charging issues and aids in obtaining a good image (Sharma, 2018).
Additionally, in order to prevent surface overcharging when using SEM, the samples must also
be electrically conductive as the overcharging could result in high brightness and subpar photos.
However, the VPO catalyst is a non-conductive material therefore in this instance the VPO
catalyst are sputter coated with a thin coating of metal, such as gold or platinum, that could
quickly reflect electrons using the Sputter Coater machine in order to provide an electronconductive surface (Kang, 2014). The sample covering stub will then be attached to the sample
2
holder and placed in the specimen chamber or sample holding chamber. Subsequently, the
analysis can be conducted.
(b)
(a)
Figure 2.1: SEM Micrograph of VPO Catalyst Synthesise with (a) Microwave Irradiation (b)
Conventional Oven-Dried (Kang, 2014).
The catalyst morphology is usually analysed by observing the micrograph generated
from the SEM as the alteration of surface structure is able to be perceived, which potentially
can be the active site for the reaction. Hence, it is possible to envision the selectivity and
conversion of that particular catalyst by just evaluating the micrograph. Figure 2.1 illustrates
the micrograph of the VPO catalyst that has been generated by the SEM such that the VPO
catalyst has been synthesised using the microwave irradiation method with a sesquihydrate
precursor route. A previous study declares that the VPO catalyst’s structure is made up of platelike crystals, which are arranged in clusters that have a rosette shape (Rajan et al., 2013). From
Figure 2.1(a), it depicts the structure of catalysts dried by microwave irradiation, which has
crystal-like clusters stacked up into rosette shapes, in contrast to a catalyst prepared by normal
oven drying, which has the characteristic rosette-type structure as shown in Figure 2.1(b).
According to Kang (2014), the catalyst prepared using the microwave irradiation method tends
to show a reduction in the folded-edges structure, which was considered to be the distinctive
properties of catalysts generated using the sesquihydrate precursor method, as well as a
decrement in the size of platelets, this draws the difference in the surface morphology for both
the catalysts synthesised from microwave irradiation and conventional drying method as shown
in Figure 2.1. It also mentioned in a peer study that the catalyst synthesised with microwave
irradiation showed a thin rosette-type structure with uniform crystal size, and the smaller
platelets that were found in these catalysts led to a higher surface area and, consequently,
increased catalyst performance such that a higher maleic anhydride selectivity and n-butane
conversion can be acquired (Leong, Kang, and Yap, 2018).
3
2.2
Surface Area
The surface area of a catalyst is another factor that contributes to the efficiency of the catalyst.
As discussed in Chapter 2.1, the surface morphology of VPO has changed when a different
heating method is applied. Surface morphology has a strong relationship with the surface area
of the catalyst. According to Faizan, Zhang and Liu (2022), surface area of VPO has a linear
relationship with the conversion of n-butane in catalytic selective oxidation. When surface area
of a catalyst is larger, there are more active sites available for the binding of reactants and more
reactions can occur at the same time. Common method applies to measure the total surface area
of the catalyst is Brunauer-Emmer-Teller (BET) method. This method applies the
physisorption of nitrogen onto the catalyst surface at a low temperature and determines the
amount of nitrogen needed to form a complete monolayer on the surface of the catalyst. As
reported by Rownaghi, Hin, and Wang (2009), they conducted a surface area analysis of VPO
that was synthesized via microwave irradiation method with BET method at 77 K.
As aforementioned, the BET method applies the theory of adsorption. There are several
factors that affect the amount of gas being absorbed on a surface: (i) temperature; (ii) pressure;
and (iii) the interaction between solid and gas. Nitrogen gas is used commonly as the adsorbate
in the BET analyser. Other than nitrogen gas, noble gas such as Argon gas, and carbon dioxide
could also use as adsorbates in the BET analyser. However, nitrogen gas is preferable due to
its availability in high purity, lower price, inert properties which can prevent chemisorption
from happening and the most important factor: it has strong interaction with most solids
(including VPO catalyst in this case). The temperature of the analysis is kept constant at 77 K
to ensure the interaction between nitrogen gas and surface of solid catalyst could be strong in
order to obtain the measurable amount of nitrogen gas being adsorbed. Factor (ii) will be
varying throughout the analysis in order to plot the adsorption isotherm (amount of nitrogen
gas adsorbed at different pressure). By determining the amount of nitrogen gas absorbed, the
surface area of the sample can be estimated.
Micromeritics Instrument Corporation (2022) presented the gas adsorption theory in 4
stages. In the first stage, nitrogen is purged towards the surface of catalyst, and isolate sites on
the surface of catalyst begin to adsorb nitrogen molecules at low pressure. When the pressure
goes higher in the second stage, more nitrogen molecules will be adsorbed to form a fullcoverage monolayer on the surface. Monolayer referred to one molecule thick layer on the
surface of the sample. Further increase of pressure at the third stage will cause multilayers to
begin to form and the small pore will be filling. At this stage, information is enough to calculate
the internal surface area of catalyst. BET equation as shown in Equation 2.1 below is used to
4
relate the volume of gas adsorbed on the surface with a given partial pressure. With Equations
2.2 and 2.3, total surface area and specific surface area of catalyst can be calculated respectively
given the calculated monolayer absorbed gas volume, 𝑉! .
!
"(!! $!)
="
&
'$& !
"'
+"
"'
!!
"
𝑆 = 𝜎 × #! × 𝑁$
!
(
𝑆%&' = )
(2.1)
(2.2)
(2.3)
Rownaghi, Hin, and Wang (2009) compared VPO catalyst prepared by conventional
heating method and microwave irradiation method. They found the specific surface area, 𝑆%&'
of microwave irradiation synthesized VPO is higher than the other conventional heating
method: 46 m2 g-1. The result is proven by SEM morphology within the same study whereby
the crystal size obtained from the study is smaller and a smaller crystal size will contribute to
a higher surface area. A study done by Pillai, Sahle-Demessie and Varma (2003) compared the
catalyst from ultrasound irradiation, microwave irradiation and conventional heating. Their
results also proved that microwave irradiation will give the highest 𝑆%&' , 16.8 m2 g-1 cat. Lastly,
Taufiq-Yap et al. (2007) show similar results in their research. Microwave irradiation is better
in preparation for VPO catalyst as it could produce catalyst with larger 𝑆%&' , 34 m2 g-1 cat
which is the highest in their study. The results are proven with the aid of X-ray diffraction
(XRD), whereby smaller particle size yields higher specific surface area.
In general, microwave heating will give a higher specific surface area as compared with
conventional heating due to the different heating mechanisms. In terms of selectivity and
conversion, microwave irradiation achieved a better result compared to conventional heating.
Hamzehlouia et al. (2018) obtained a significantly higher selectivity of Maleic Anhydride for
the microwave irradiation method (nearly 100%) than for the conventional heating method.
Rownaghi, Hin, and Wang (2009) reported the conversion of n-butane is 81 % for microwave
irradiation and 52% for conventional heating. Selectivity for microwave irradiation is also
higher for Maleic Anhydride and lower for side products (carbon dioxide and carbon
monoxide). They tested the catalytic activity of n-butane oxidation in a fixed bed reactor with
a gas space hourly velocity of 2,400 h-1 and temperature of 673K. Finally, they claimed that
catalyst with a larger surface area will exhibit higher activity. This claim also agreed with the
study done by Hutchings (2004). When the specific surface area of catalyst is larger, it provides
more routes for oxidation and at the same time yield more Maleic Anhydride.
5
2.3
Pore Size / Volume
As mentioned in Chapter 2.2, pores size and pore volume will be affected by the heating
approaches. They are also the factors contributing to the performance of catalyst. Theoretically,
larger pore size and larger pore volume are preferable. Larger pore size enables particles to
diffuse in and only chemisorption can happen for reaction. Whereas larger pore volume offered
a larger surface area as well as more vacancies as the active sites. Small pore size in a catalyst
will block reactants from entering the pore. Then, the pore could only be left empty, or it will
allow smaller particles to diffuse into it which might lower the selectivity of the desired product.
The most common instrument to measure the pore size and pore volume is the same as
the equipment used to measure the surface area of the catalyst – BET analyzer. There are 2
reported instruments used for BET analysis: Sorptomatic 1990 series (Rownaghi, Hin, and
Wang, 2009; Taufiq-Yap et al., 2007) and Micromeritics AutoChem II Instrument (Model 2920)
(Pillai, Sahle-Demessie and Varma, 2003). Chapter 2.2 explained 3 stages of the 4-stageadsorption theory and the 4th stage is where the data will generate for the pore diameter, volume
and distribution calculation. In the 4th stage, gas pressure will be further increased until the
sample is completely covered by nitrogen gas. The method of Barrett, Joyner, and Halenda
(BJH) is used to determine pore diameter, volume and distribution (Micromeritics Instrument
Corporation, 2022). The BJH method requires information from experimental isotherms and
uses the Kelvin model of pore filling for calculations. However, this method is only available
to the pore size of mesopore and small macropore. The largest pore radius, 𝑟* and average pore
size, 𝑟)+ can be calculated using Equation 2.4 and 2.5 respectively (Fu et al., 2020). Pore volume,
𝑉+ can be estimated based on the amount of nitrogen being adsorbed and the calculated 𝑟)+ .
Theory of the modified Kelvin equation of pore filling (Equation 2.4) is based on adsorption
theory as well (Mitropoulos, 2007). Condensation of nitrogen gas will occur first in small pores
follow by large pores along with the increase of pressure. With this nature, pore volume with
respective size ranges could be determined from the isotherm generated from BET analysis.
𝑟* = −
.
,-"
"
3
"#
/'012
,"$
𝑟)+ = (
%&'
(2.4)
(2.5)
Prior to any BET analysis, the sample must be degassed in order to remove unwanted
contaminants that might affect the results of analysis such as water, carbon dioxide, dirt etc.
Catalyst preparation method by Rownaghi, Hin, and Wang (2009) involved the reflux of
catalyst precursors with distilled water. These contaminants will block the surface area and
6
prevent the nitrogen gas to be adsorbed on the surface of catalyst which can cause the
inaccuracy of data. The sample will be degassed in a vacuum at a high temperature of 120 °C
for 1 hour as reported by Al-Otaibi (2010). Whereby the temperature is the highest temperature
that will not damage the structure of the sample and optimize the degassing time. After the
degassing, the sample will move to the analysis port and liquid nitrogen will be introduced to
the sample for cooling purposes. When the sample is maintained at the low temperature of 77K,
nitrogen gas is injected into the sample cell for the adsorption to begin.
As outlined by Rownaghi, Hin, and Wang (2009), conventional heating diffuses the
heat from the outermost surface of the catalyst towards the inner surface. This heating
mechanism results in a slow heating process and a slower rate of evaporation. Whereas
microwave irradiation transfers the heat from the inside of the catalyst as the heat is produced
by the interaction between irradiation and the polar bond inside the catalyst body. When a wet
catalyst is exposed to microwave irradiation, the wettest part will absorb more energy and turns
into the hottest part. This ensures the highest rate of evaporation could occur at the wettest part
and the water could be removed in a faster and more effective manner. When the contaminants
are removed more effectively, it lowered the chances of the pores being blocked or occupied.
Thus, there will be more active sites available for the reaction. In short, microwave irradiation
will affect the morphology of the catalyst, specific surface area, pore volume, pore size and
pore distribution of the catalyst.
Other than that, conventional heating might cause the pore of the catalyst to shrink.
Taufiq-Yap et al. (2007) and Zeng, Jiang and Niu (2005) provided detail explanations with
regard to the statement. Before the catalyst being heated, gas and liquids will occupy the pores
of the catalyst and form a meniscus. The meniscus is present with surface tension and is hard
to be removed. The surface tension of the gas and liquids will cause it holds the framework of
catalyst. When the heat is exerted on the catalyst surface, capillary force will draw the liquid
from the pores which cause the breakdown of the framework. Upon heating, the pore will close
due to the actions mentioned. On the other hand, when microwave irradiation is applied,
heating simultaneously occurs on both the inner and outer surfaces of catalyst. Surface tension
and capillary force will be disappeared. Thus, the collapse of the 3D network structure
framework could be prevented, and the closure of pores can be reduced. In short, conventional
heating will result in the closure of pores but microwave irradiation can preserve the available
pores. As a result, higher pore volume and larger pore size are available which gives a higher
selectivity of Maleic Anhydride and higher conversion of n-butane.
7
2.4
Particle Size
Apart from that, the catalytic performance may also be impacted by particle size by considering
that the surface area of the catalyst could be significantly affected by the particle size, which
the increase in particle size will result in a lower surface. The interaction of molecules with one
another depends on the surface area where the interactions, such as collisions, are inversely
correlated with surface area. Thence, smaller particle size will lead to higher surface area and
eventually arouse a higher catalytic performance. According to Bergeret and Gallezot (2008),
X-ray diffraction (XRD) is widely used for characterising the particle size of the catalysts, thus,
in this instance, X-ray diffraction (XRD) is implemented to evaluate the catalyst particle size,
thus the XRD will be further discussed in the following.
Fundamentally, when penetrating radiation X-rays are focused on a sample fixed on the
axis of the spectrometer it will penetrate the crystalline solid and be dispersed, and then the
phenomenon of diffraction takes place such that the diffraction is the process by which
scattered X-rays are subjected to constructive and destructive interference (Palanychamy, 2012;
Kang, 2014). These X-rays are produced by a cathode ray tube which is then filtered to produce
monochromatic radiation, focused by collimation, and pointed at the sample. When a crystal is
exposed to a monochromatic x-ray incident, vibrational energy is applied to the atomic
electrons in the crystal with the same frequency as the incident ray's frequency and is
accelerated. These accelerated electrons subsequently radiate in all directions with radiation
that has the same frequency as the incident x-rays. The radiation emitted by the electrons is out
of phase with each other such that these radiations may interfere constructively or destructively
producing a diffraction pattern (i.e. maxima and minima) in certain directions. During the
analysis, the sample will be rotated in order to minimise sample heating. Subsequently, the
variations in the diffracted X-ray intensity are quantified, recorded, and plotted against the
sample rotation angles, then the particle size could be determined.
Besides, it is of utmost importance to prepare the XRD sample properly as improper
sample preparation (eg. over-grinding and under-grinding) will lead to some consequences
such as absorption contrast, poor signal to noise ratio, amorphization, and so on. According to
Jumali (2016), the XRD sample should be crushed first before sending it for analysis if the
sizable sample solid is implemented. In this instance, since the VPO catalyst exists in the
powdery form then grinding process can be omitted (Shan, 2009). The VPO catalyst is first
used to fill an empty sample holder, then the sample is compressed on a sample holder using a
glass slide to ensure a smooth surface of the powder. Eventually, the extra powder is removed
8
from the sample holder’s edges, then carefully insert the sample into the corresponding XRD
slot. Lastly, the analysis can be conducted.
Figure 2.2: XRD Pattern of VPO Catalyst (Kang, 2014).
First and foremost, the main characteristic peaks are observed from the XRD pattern
corresponding to the reflection of planes which will then be used to compute the crystallite
sizes using the Debye-Scherrer equation, such that T is the crystallite size for (h k l) phase, λ
is the X-Ray wavelength of radiation for Cu Kα, FWHM is the full width at half maximum at
the peak and θ is the diffraction angle for the phase.
𝑇(Å) = 𝛽
0.89𝜆
ℎ 𝑘 𝑙 𝑐𝑜𝑠𝜃ℎ 𝑘 𝑙
(2.6)
The XRD peak width from the graph is used to calculate the crystallite size of the
catalyst at distinct planes. For instance, the crystallite size is determined for the (0 2 0) plane
using the FWHM shown in the graph as the (0 2 0) plane is the surface that favours the synthesis
of maleic anhydride (Palanychamy, 2012). Hence, the catalytic performance of the VPO
catalyst would greatly benefit from the exposure of the (0 2 0) plane, thus a smaller crystallite
size in the (0 2 0) plane is more favourable. Theoretically, in comparison to conventional
preparation methods, microwave irradiation results in a catalyst with a thinner layer of particles
at the (0 2 0) plane. This scenario could possibly cause by the utilization of a distinct reducing
agent, such that isobutanol is used to prepare the catalyst via the dihydrate route in the
conventional method, whereas 1-butanol is used to synthesize the catalyst via sesquihydrate
route in microwave irradiation. Long alkyl chains (eg. 1-butanol) significantly increase the
hydrophobicity, which causes ions to diffuse more slowly through the solvent and eventually
results in the production of smaller particles (Rownaghi, 2010). In short, the VPO catalyst with
a smaller particle size tends to have higher selectivity and conversion since it has a higher
surface area by which more active phases are exposed to the reactants.
9
2.5
Chemical Structure
Catalyst structure can be classified into two categories which are physical structure: surface
area, porosity, and particle size which have been discussed in Section 2.1 to Section 2.4; and
chemical structure: the types and arrangement of chemical compounds that are available on the
catalyst. In this case, catalyst structure is studied based on the chemical structure which to
investigate how microwave irradiation affects the availability of V5+ and V4+ on the VPO
catalyst by using the Temperature Programme Reduction (TPR) analyser.
TPR in Hydrogen gas is carried out using a Thermo Electron TPDRO 1100 instrument
equipped with a thermal conductivity detector (TCD). It investigates the number of reducible
metal species and infers the amount and nature of the oxygen species of the catalysts. This
technique is done by measuring the composition of the outlet gas by using the TCD in order to
determine the amount of the reducing gas (in this case, 5% H2 in N2 gas is used) that has been
reacted with the catalyst at different temperatures. As the catalyst is being reduced, hydrogen
is consumed and bulk oxygen that presents in the catalyst is removed simultaneously (Pirola et
al., 2018; Kang, 2014).
In order to avoid interfering with the signal output of the TPR analyses, pre-treatment
is carried out before the analysis to eliminate the impurities such as water, carbon monoxide or
carbon dioxide where the catalysts are cleaned by heating them from room temperature to 473
K in a purified nitrogen flow (25 cm3/min) and holding them to that flow at 473 K for 30
minutes before cooling them to ambient temperature. Then only the catalyst will be heated at
5 K/min up to 1173 K under 25 cm3/min flow of 5% mixture of H2 in N2 and carried out the
TPR analysis. TPR profile is a plot of the hydrogen consumption of a catalyst as a function of
time, which will be converted to a function of temperature. The area of peaks observed in TPR
provides information on the composition of reducible compounds mixture (Kang, 2014).
Table 2.5.1: Performance of VPOs Catalyst under Two Different Synthesis Methods (Kang,
2014).
Catalyst Condition
Percentage of
Vanadium Ion
V4+
V5+
n-Butane
Conversion
(%)
MA
CO
CO2
Selectivity (%)
Microwave Irradiation
(MW)
53.54
46.46
25
91
3
6
Conventional Oven
Heating (CO)
55.81
44.19
18
87
6
7
10
(a)
(b)
Figure 2.5.1: (a) TPR Analysis of VPO Catalyst within the Specified Temperature Range; (b)
Amount of O2 Removed from the VPO Catalyst in TPR (Kang, 2014).
As shown in Table 2.5.1, catalysts prepared via microwave irradiation (MW) showed
an increment in both activity and selectivity as compared to catalyst produced via the
conventional oven heating method (CO). This is attributed to the amount of V5+ and V4+
induced via microwave irradiation and this can be proved by the TPR analysis. Where in Figure
2.5.1 (a), MW have two reduction peaks at 793 K and 1007 K whereas CO showed two
reduction peaks at 793K and 981K. The first and second peak indicates the reduction of V5+
and V4+ respectively. The second reduction peak observed for MW was slightly higher
temperature than the CO which indicates through microwave irradiation, VPO catalyst
becomes physically stronger pellets, their vanadium valence has been induced as well as the
bonding between the oxygen species and catalyst surface also has been strengthened up (Pirola
et al., 2018; Kang, 2014).
Besides, Taufiq-Yap et al. (2007) claimed that a good correlation was reported between
the number of oxygen species removed from both phases (V4+and V5+) via reduction with the
n-butane conversion and MA selectivity, respectively. As such, when a higher amount of
oxygen was removed from the microwave-heated catalyst revealed that microwave irradiation
is capable to produce VPO catalyst that is able to release the highly reactive and selective
oxygen species for n-butane oxidation to MA and subsequently enhance the conversion and
selectivity of the selective oxidation of n-butane (Taufiq-Yap et al., 2010). This can be proved
by the result shown in Table 2.5.1 and Figure 2.5.1 (b), in which under microwave irradiation,
a higher amount of oxygen are removed from VPO catalyst having the better conversion of nbutane and MA selectivity compared to conventional oven heating.
11
CONCLUSION
In conclusion, the catalyst characterisations obviously show that the microwave irradiation
technique can help to enhance the catalytic performances by increasing the activity and
selectivity of the catalysts. In this instance, the activity and selectivity of the catalysts can be
improved by varying the surface area and particle sizes of the catalyst such that smaller particle
sizes will give a higher surface area which eventually can boost the catalytic activity as more
active species are available to the reactants, n-butane in this case. The discussion reveals that
there are several techniques that can be implemented to discern the catalytic properties, for
example, SEM is used to observe catalyst morphology, BET is utilised to evaluate surface area
together with pore size or pore volume, XRD is implemented to determine the particle size,
and TPR is employed to assess the chemical structure of the catalyst. From the micrograph
acquired using the SEM, it shows a significant change in the catalyst morphology forming a
crystal-like cluster stacked up into rosette shapes which have subsequently induced higher
surface area as observed using the BET. Also, in terms of pore size or pore volume, microwave
irradiation could provide a larger pore size due to the fact that it provides a higher rate of
evaporation where it reduced the likelihood of the pores getting clogged or occupied since the
water and contaminants were eliminated more efficiently. Besides, the catalyst produces from
microwave irradiation tends to have a smaller particle size owing to the fact that the 1-butanol
used in synthesizing the catalyst could greatly enhance the hydrophobicity, which allows ions
to pass through the solvent more slowly and ultimately leads to the production of smaller
particles. Finally, it was discovered that higher reduction in oxygen from the microwaveirradiated catalyst allowed the highly reactive and selective oxygen species for n-butane
oxidation to MA to be released, which improved the conversion and selectivity of the selective
oxidation of n-butane. In short, a catalyst with a smaller particle size, and higher surface area
with oxygen species removal synthesized from microwave irradiation is able to augment the
activity and selectivity of the catalyst, eventually acquiring a higher yield of product which is
maleic anhydride in this instance.
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