Methanol assisted methane conversion for higher hydrocarbon over bifunctional Zn-modified Mo/HZSM-5 catalyst

Article abstract of DOI:10.1016/j.molcata.2014.12.019

The co-conversion of methane with methanol was studied over Mo/H-ZSM-5 and Zn-Mo/H-ZSM-5 catalysts prepared by the wet impregnation method. The catalysts were calcined at 550 °C followed by reduction at 450 °C for 5 h. The activity tests were carried out at three different temperature viz. 550, 600, and 650 °C. BET, BJH pore volume distribution, H₂-TPR, NH₃-TPD, FTIR, SEM-EDX, TEM, and XRD techniques were used for the characterization of the pre-reduced catalyst. Incorporation of 2% Zn on to 5% Mo/H-ZSM-5 catalyst significantly improved the activity of the catalyst for methane conversion. Thermodynamic studies revealed that temperature below 600 °C, methane aromatization is not possible, however, in the presence of methanol over bifunctional catalyst it becomes thermodynamic feasible (ΔG = -8.8, -15.5, and -22.2 kJ/mol at 400, 500, and 600 °C). The main products of the reaction were ethylene, ethane, propane, butane butylenes and aromatics such as benzene, toluene, ethyl benzene and xylene. Selectivity of alkylated aromatic product enhance in the presence of methanol.

Full text of DOI:10.1016/j.molcata.2014.12.019

Accepted Manuscript
Title: Methanol assisted methane conversion for higher
hydrocarbon over bifunctional Zn-modified Mo/HZSM-5
catalyst
Author: Sachchit Majhi Ajay. K Dalai Kamal K Pant
PII:
S1381-1169(14)00579-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.12.019
MOLCAA 9381
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
16-9-2012
24-12-2014
27-12-2014
Please cite this article as: Sachchit Majhi, Ajay.K Dalai, Kamal K Pant,
Methanol assisted methane conversion for higher hydrocarbon over bifunctional
Zn-modified Mo/HZSM-5 catalyst, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2014.12.019
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Methanol assisted methane conversion for higher hydrocarbon over bifunctional
Zn-modified Mo/HZSM-5 catalyst
Sachchit Majhi ab, Ajay. K Dalai b, Kamal K Pant a*
aDepartment of Chemical Engineering, Indian Institute of Technology Delhi, Hauz Khas,
New Delhi 110016, India
bDepartment of Chemical and Biological Engineering, University of Saskatchewan,
Saskatoon, SK S7N 5C5, Canada
Highlights


Novel Zn-Mo/HZSM-5 catalyst was prepared for methane conversion with methanol.
The change in textural properties, and product selectivity, were investigated with Zn
addition.

Incorporation of Zn increased the conversion and aromatic selectivity.
Abstract
The co-conversion of methane with methanol was studied over Mo/H-ZSM-5 and Zn-
Mo/H-ZSM-5 catalysts prepared by the wet impregnation method. The catalysts were
calcined at 550 oC followed by reduction at 450 oC for 5h. The activity tests were carried
out at three different temperature viz. 550, 600, and 650oC. BET, BJH pore volume
distribution, H2-TPR, NH3-TPD, FTIR, SEM-EDX, TEM, and XRD techniques were used
for the characterization of the pre-reduced catalyst. Incorporation of 2%Zn on to
5%Mo/H-ZSM-5 catalyst significantly improved the activity of the catalyst for methane
o
conversion. Thermodynamic studies revealed that temperature below 600 C, methane
aromatization is not possible, however in the presence of methanol over bifunctional
catalyst it becomes thermodynamic feasible (∆G= -8.8, -15.5, and -22.2 kJ/mol at 400,
o
500,and 600 C). The main products of the reaction were ethylene, ethane, propane,
1

butane butylenes and aromatics such as benzene, toluene, ethyl benzene and xylene.
Selectivity of alkylated aromatic product enhance in the presence of methanol.
Keywords: Co-conversion; HZSM-5; Bifunctional catalyst; Additives; Aromatics;
Reaction mechanism
*Corresponding author
Email: kkpant@chemical.iitd.ac.in; Tel.:+91 11 26596177; Fax: +91 11 26596172
1.0
Introduction:
With the increased demand for energy worldwide and the limited worldwide
availabilities of crude oil reserves have led to exorbitantly high crude oil prices. The
abundance of natural gas worldwide has drawn much attention toward its effective
utilization as fuel. The reserves of natural gas are located in remote areas where high
transport costs affect exploitation; however, by adopting procedures for the conversion
of natural gas to higher hydrocarbons these costs can be reduced [1]. Efforts are being
made to convert methane into high-density liquid fuel by various researchers in industry
and academia [2]. Methane can be converted to higher hydrocarbon by mainly two
routes: indirect and direct methods. The indirect route for methane conversion (e.g.,
Fischer-Tropsch, Mobil Process, and Shell Middle Distillate Synthesis) requires the
production of synthesis gas (CO and H2) from methane by the costly and inefficient
process of steam reforming [3]. The direct route of methane conversion to higher
hydrocarbons has attracted researchers, due to its potential for utilizing natural gas as
industrial feedstock. However, the usefulness of this process is still limited as it has not
been possible to achieve large-scale conversion of methane directly to higher
hydrocarbons. Owing to its stable configuration of four symmetric C-H bonds having
2

bond energy of 435 kJ/mol it is not feasible at low temperatures whereas at high
temperature the tendency of coke deposition on the active surface also increases. In
order to suppress coking with reasonably high conversion, additives such as alkanes or
alcohols are required in the reactant stream to activate the methane molecule at low
temperatures [4-7].Thermodynamically; methane is unstable only above 530 ᴼC,
becoming more unstable than benzene above 1030 ᴼC [3]. The equilibrium methane
conversion at atmospheric pressure is low at moderate temperatures (about 5% at 600
ᴼC, 11.4% at 700 ᴼC and 16.2% at 750 ᴼC) [5]. For the case of methane methanol
simultaneous conversion, ΔG= -8.8, -15.5, and -22.2 kJmol-1 at 400, 500, and 600 ᴼC,
respectively, for one mole of methane converted per mole of methanol (Co-conversion
of methane with methanol to benzene) [7]. The methanol to gasoline (MTG) is a highly
exothermic reaction (∆Hr= −1670 kJ/kg); whereas, methane conversion is an
endothermic process. The present work examines a combination of the exothermic
conversion of methanol with the highly endothermic methane cracking reaction to obtain
relatively thermo neutral reaction.
The probable reaction mechanisms of the methane-methanol reaction can be
understand by the following mechanism:
+
-


Z CH3OH2


-
+
Z-H+ +CH OH
Z CH +H O

(1)

3
3
2
+
Z-CH3  Z-H+ +CH2
(2)
(3)
(4)
(5)
2CH 2  C 2 H 4  Oliogomerization  Aliphatics+Aromatics+H 2
Aromatization
Mn+O2-
CH4 Z-[CH3]+ +Mn+O2- +H2
Z-H+
+
Z-CH3  H+Z- +CH2
3

+
-
n+O2-
2CH2  C2H4 HZ+ MAliphatics+Aromatics+H2
(6)
Oliomerization
Aromatization
Where H+Z− =HZSM-5, Mn+= Mo or Zn oxide
Among the several catalysts tested, Mo/HZSM-5 has been reported the most
promising catalyst [8, 9, 10, 11]. This could be due to its framework of HZSM-5 and
active sites located in cavities or channels of the zeolite pore system, which is
predetermined by their structural nature, particularly the shape and size of pores. Thus,
the accessibility of sites and nature of the products formed during the reaction is
controlled by zeolite [12, 13]. Having a two-dimensional porous structure and pore
diameter close to the dynamic diameter of the benzene molecule, HZSM-5 as support
for the aromatization of methane inhibition effectively ensures the formation of other
products [14]. To modify the activity of the catalyst, the addition of a metal promoter was
implemented, enhancing the catalytic activity and selectivity to benzene and other
hydrocarbons [15].
In this work, a relatively thermo-neutral reaction by means of methanol as a co-
reactant over the catalysts, namely 5%Mo/HZSM-5 and 5%Mo-2%Zn/HZSM-5,
prepared by the impregnation method was studied. These catalysts were characterized
by BET surface area, pore volume, BJH adsorption-desorption, and temperature
programmed reaction (TPR) method. The acidity measurements of the calcined catalyst
were conducted by FTIR and temperature programmed desorption (TPD) using NH3.
The surface morphology was analyzed by SEM-EDX and TEM analyses. The
percentage coke formed during the reaction was determined by TGA. The activity tests
4

of these catalysts were performed in a quartz reactor over the temperature range of 550
to 650°C at atmospheric pressure.
2.0
2.1
Experimental
Catalyst Preparation
The chemicals used were (NH4)6Mo7O24.4H2O (Merck, Germany) and
Zn(NO3)2.6H2O (Merck, Inida) as the metal precursor. Commercial HZSM-5 was
supplied by Sud-chemie, having a silica alumina ratio of 55. Two catalysts were
prepared by impregnation over an HZSM-5 support. The first catalyst, 5.0% Mo/HZSM-5
(ascribed as CAT-1), was prepared by impregnation with an aqueous solution of
ammonium heptamolybdate [(NH4)6Mo7O24·H2O)] over HZSM-5 zeolite. After
impregnation, the catalyst was dried at 110±5°C overnight and then calcined in air at
550±5°C for 5 h. The second catalyst, 2% Zn-5% Mo/HZSM-5 (ascribed as CAT-2), was
prepared by impregnation on HZSM-5 with ammonium heptmolybdate and a
Zn(NO3)2.6H2O solution. The subsequent steps were the same as for CAT-1.
2.2
Catalyst Characterization
The textural properties of the catalyst and adsorption-desorption isotherms were
determined using nitrogen and a Micromeritics ASAP 2010 apparatus. For the cross-
sectional area of N2, a value of 0.162 nm2 was used. Prior to the experiments, the
samples were degassed at 150°C in the vacuum condition for 3 h to remove moisture
and gases. Micropore distributions were derived from the Horvath-Kawazoe (HK)
method; whereas, mesopore distributions were derived from the Barrett-Joyner-Halenda
5

(BJH) method. Surface areas and pore volumes of the catalyst samples were measured
with the BET method.
The temperature program reduction (TPR) study was done to determine the
reduction temperature using a Micrometrics Pulse Chemisorb 2720 apparatus.
Approximately 0.1 g of the sample was taken in a U-type quartz tube and then reduced
from ambient to 950°C at a ramp rate of 10°C/min, under a hydrogen-argon mixture
(10% hydrogen), at a flow rate of 20 ml/min. The hydrogen consumption during the TPR
was recorded by means of a thermal conductivity detector (TCD). The acidity of the
HZSM-5 and metal loaded HZSM-5 catalysts were measured by temperature
programmed desorption (TPD) of ammonia using the same equipment. For this 0.2 g of
catalyst sample was pretreated in a flow of helium at 500°C for 1 h to remove moisture
present in the sample and then cooled down to room temperature. To complete
saturation, an ammonia-helium mixture (5:95) was passed to the sample for 1 h.
Physisorbed ammonia was removed by flowing helium for 30 min. After that, TPD was
carried out in a stream of helium with a flow rate of 20 ml/min (from room temperature to
650°C), at a ramp rate of 10°C/min. NH3 desorption was measured using a thermal
conductivity detector (TCD) and the total desorbed NH3 was obtained from the
integrated peak area of the TPD profiles relative to the calibration curve.
The surface morphology and localized metal concentration of the catalysts were
determined by SEM-EDX analysis using a Carl Zeiss SMT EVO series scanning
electron microscope. The sample preparation was done by mounting wafer form
samples on the sticky carbon tape and electrically conducting by plating the sample with
silver (20 nm thick) using a Bio-Rad Polaran sputter coater. Particle size of the calcined
catalyst were analysed by Philips CM12 model Transmission Electron Microscopy
6

(TEM). The samples were prepared by ultrasonic dispersion of the calcined catalyst in
methanol for 2 h and the sample was then dispersed onto a carbon copper grid. The X-
ray diffraction patterns of all three calcined catalysts were examined using a Phillips X
Pert Diffractometer PW 1390 at 40 kV and 30 mA on Cu Kα radiation. The
diffractograms were collected at 2θ between 10 to 80°. FT-IR spectra of HZSM-5, CAT-
1, and CAT-2 catalysts were recorded in the vibrational region between 400 and 2000
cm-1 with a PerkinElmer spectrum 100 infrared spectrometer using the conventional KBr
disk technique. The catalyst sample was first dried at 350° for 2 h and the wafers were
prepared with a ratio of catalyst : KBr = 1:100 under a pressure of 5 kg/cm3.
2.3
Catalytic Activity Test
The catalytic activity test was performed in a fixed bed reactor made up of quartz
having internal diameter of 10 mm. The schematic diagram of the experimental set-up is
shown in Figure1. Approximately 3g of catalyst (0.5 mm size) mixed with silicon carbide
to avoid channeling was kept in the middle zone of the reactor. A K-type of
thermocouple was placed near the catalyst bed to measure the temperature of the bed
controlled by a PID controller.
7

Figure 1. Experimental set-up for methane conversion
Prior to the catalytic test, the catalysts were first reduced in a stream of hydrogen and
methane mixture (3:1) for 5 h, at a temperature of 460°C, which was predetermined
through the TPR study. After reduction, the reactor was heated under nitrogen stream to
the desired reaction temperature. Methane was introduced into the reactor through a
calibrated mass flow controller. The gas hourly space velocity of methane was varied
between 400 and 600 cm3/gh. The flow rate of methanol was 0.5 cm3/h for all cases.
The flow rate of the product gases were measured by a soap bubble flow meter and the
mixture components were analyzed by gas chromatography. The amounts of aliphatic
and aromatic hydrocarbons were determined using FID-based gas chromatography
equipped with Porapak-Q column. The methane conversion and selectivity were
calculated on the carbon number basis. The conversion and selectivity are calculated by
the following formula:
8

(moles of CH fed - moles of CH out +








4
4
moles of CH4 produced from methanol)
molesof CH4 fed
Conversion(%)=
×100




(Methane produced from methanol under the same condition without methane addition
in the feed)








Amount of product formed
Selectivity =
×100
(moles of CH4 fed - moles of CH4 out +
moles of CH4 produced from methanol)






3.0
3.1
Results and Discussion
Surface Area and Pore Volume of Catalysts
The BET surface area, pore volume, average pore diameter of the parent zeolite
(HZSM-5, Si/Al=55), and the other two bimetallic catalysts are given in Table 1. The
data show that the surface area of catalyst decreased due to metal loading. The
decrease in surface area with the increased metal concentration may be due to partial
pore blockage during the impregnation of the Mo/Zn on the HZSM-5 which was also
supported by the decrease in pore volume with the increase of metal concentration.
9

Table1. BET surface area, pore volume, average pore diameter, and percentage metal
dispersion of the support and catalysts.
Pore
Volume
(cm3/g)
0.36
Avg. Pore
Diameter
(A)
Percent
Metal
BET SA
(m2/g)
Composition
Nomenclature
Dispersion
-
H-ZSM-5
5%Mo/HZSM-5
5%Mo-
H-ZSM-5
CAT-1
262.14
250.49
55.89
0.34
55.01
1.28
CAT-2
236.01
0.32
724.39
0.72
2%Zn/HZSM-5
0.05
450
0.04
HZSM-5
400
0.03
0.02
350
0.01
300
0.00
0
10
20
30
40
50
60
250
Pore Diameter (nm)
200
150
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Relative Pressure (P/P )
o
0.025
0.005
200
400
CAT-1
0.020
CAT-2
0.004
180
0.015
0.003
350
160
0.010
0.002
300
0.005
140
0.001
0.000
250
0.000
120
0
10
20
30
40
50
60
70
0
10
20
30
40
50
Pore Diameter (nm)
Pore Diameter (nm)
100
200
150
100
80
60
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Relative Pressure (P/P )
Relative Pressure (P/P )
o
o
Figure 2. N2 adsorption-desorption isotherm and BJH pore size distribution (inset) of the
two catalysts.
10

Figure 2 shows the N2 adsorption/desorption isotherms and pore size distribution of
HZSM-5 and other two metal impregnated catalysts. Both isotherm showing a typical
Type IV isotherm, with a capillary condensation at P/Po of 0.7–0.9 and a sharp rise in
the gas uptake at higher relative pressure, signifying the presence of large mesopores.
The BJH isotherm and pore volume distribution analyze indicate the presence of
distinctive mesopores. For all of the catalyst samples, the effective diameters were
almost equivalent to those of the original HZSM-5, which suggest that the metal has not
much effect on pore size distribution. This could be the exchange of metal ions with
protons of the H-ZSM-5 as a result of which metal particles are present at the outer
surface as well as outside the channels and no pores were blocked. On the other hand,
due to non-interference of the metal particles to the channels and pore openings of the
H-ZSM-5, no significance difference has occurred in the pore size distribution of the
base material and both the catalysts. According to the BJH calculation, the most of
these mesopores and in the range of 2–21 nm, with a sharp maxima at about 11.5 nm
(Figure 2, inset), indicating that the catalysts exist in the combined form of micro and
meso porosity which is favoured from the mass transport point of view for methane
conversion process.
3.2. TPR, Chemisorption, and TPD Analysis
To determine the reducibility of the calcined catalysts, temperature-programmed
reductions were performed from ambient temperature to 950°C. Figure 3 shows the H2-
TPR profile for the two catalysts, with two distinct peaks observed for each CAT-1 and
CAT-2. The first peak appeared at the temperature range between 430-450°C,
corresponding to the reduction of molybdenum species. The second peak significantly
shifted to a higher temperature (~950°C), this is attributed either to the reduction of the
11

MoO nanoparticles being reduced to active Mo, or due to a strong interaction between
MoO in the HZSM-5 framework reduced at higher temperatures [16]. There was no
peak found at this temperature range for zinc oxide indicating that zinc oxide was not
reduced within this temperature range which is in agreement with the literature [17, 18]
Figure 3. H2-TPR profiles of CAT-1 and CAT-2 catalyst.
The acidity of calcined catalysts were determined by NH3-TPD analysis. As
shown in Figure 4, the NH3-TPD spectra of CAT-1 and CAT-2 resemble moderate acid
sites. It has been reported that acid sites are classified as weak (<300°C), moderate
(300~450°C), and strong (450~550°C), depending upon the desorption temperature
[19]. Results indicate that presence of molybdenum into HZSM-5 decreases the
strength of stronger acidic sites .Ma et al. reported that reported that ammonia adsorbs
12

on both the weak (Lewis) and strong (Brönsted) acid sites. Incorporation of Mo and zinc
in HZSM-5 slightly changed the acidic properties of the catalysts due to which catalyst
loses some of Brönsted acid sites and, ultimately, forming Lewis acid sites. Two
reasons behind the Lewis acid formation are: firstly, the migration of Mo into the zeolite
channel and, secondly, the pore occupied by the access metal concentration inside the
zeolite [20, 21].
Figure 4. NH3-TPD profiles of CAT-1 and CAT-2 catalyst.
3.3
Catalyst Morphology
3.3.1 XRD analysis
13

Figure 5.XRD analysis of HZSM-5, CAT-1, and CAT-2 calcined catalysts.
XRD technique was used for the identification of the phase of molybdenum and
zinc oxide on the HZM-5 support and their interaction. The diffraction peaks for HZSM-
5, CAT-1, and CAT-2 (Figure 5) did not show any appreciable change in 2θ position,
implying that there was no any phase change in the catalysts after the impregnation of
Mo and Zn on HZSM-5. Also, no new XRD peaks were observed for the molybdenum
and zinc species, indicating that Mo and Zn are highly dispersed on the HZSM-5
surface and/or in its framework. Xu et al.[22] explained that Mo species on HZSM-5
prepared by the impregnation method are generally well dispersed on the zeolite
14

surface and the crystal size of MoO3 and Al2(MoO4)3 below 4 nm cannot be detected by
the XRD technique.
3.4
SEM-EDX and TEM analyses
Figure 6: SEM-EDX analysis of CAT-1 and CAT-2.
The surface morphology of CAT-1 and CAT-2 was studied by SEM analysis. As
shown in the SEM micrographs (Figure 6 inset), the particles are irregular in shape with
uneven distribution and their size varied from 200–257 µm. The average concentrations
of Mo and Zn in the calcined CAT-1 and CAT-2 catalysts were analyzed by EDX
analysis. Molybdenum was found to be less than of 4% whereas zinc for CAT-2, was
1.3%. These results are shown in Table 2 and the EDX elemental mapping for both
catalysts is shown in Figure 6. From the data, it can be perceived that the Mo
concentration for both catalysts is less that 4% and the Zn concentration is about 1.3%
for CAT-2.
15

Table2. Elemental analysis of CAT-1 and CAT-2.
Catalyst
CAT-1
CAT-2
Si (Wt %)
18.8
Al (Wt %)
19.2
O (Wt %)
58.4
Mo (Wt %)
3.6
Zn (Wt %)
-
17.6
16.8
60.9
3.4
1.3
TEM analysis was also used to determine the size and shape of the catalyst
particles. From Figure 7, it can be seen that Mo particles are represented by the dark
spots in the micrograph. The TEM images also showed clusters of metal particles with
support, having an uneven distribution and being heterogeneously dispersed in an
average cluster size of 20–100 nm.
Figure 7. TEM analysis of CAT-1 and CAT-2 calcined catalysts.
16

3.5
FT-IR Analysis
Figure 8. FTIR analysis of HZSM-5, CAT-1 and CAT-2 calcined catalysts.
The framework structure of HZSM-5 and the two catalysts were characterized by
FT-IR analysis. The position of all the structure-sensitive bands in the IR spectra of the
HZSM-5 zeolite and the two calcined catalysts are observed at the same position
(Figure 8), which suggests that the basic zeolite structures are not affected much by the
incorporation of Mo and Zn species. However, as indicated by the results of NH3 TPD
(Figure 4), where NH3 is desorbed at temperatures in the range of 110-400°C (weak site
range), it is interesting to note how the strong sites are converted into weak sites. The
17

absorption bands at 1222, 1100, 795, 547, and 453 cm-1 are characteristic of the HZSM-
5 framework [20]. Although both catalysts show similar bands at the same position,
there is a difference in the decrease in intensity, suggesting that the loading of the metal
species affect the bonding behavior in the zeolite framework.
3.6
Synthesis and Product Selectivity
It is widely accepted that the most promising catalyst for non-oxidative methane
conversion is Mo/H-ZSM-5 [23]. By the incorporation of zinc on HZSM-5, some of the
protons are replaced by zinc atom, which increases reaction rate. Iglesia et al.
explained that the activation of C-H bond occur on protonic acid sites, but they
recombine again with abundant surface hydrogen species, because protonic acid sites
are not able to remove hydrogen via recombinative desorption. Zn cations can prevent
these recombination reactions by providing a function for recombinative desorption and
removing the hydrogen atoms formed during C-H activation [13]. Keeping those in mind
effects of activation procedures on methane-methanol co-conversion were compared
over the CAT-1 and CAT-2 catalysts. In our previous work, the effect of copper and zinc
loading on HZSM-5 were studied for methanol to higher hydrocarbon conversion [23,
24]. It was found that the aromatics selectivity varied from 66 to 77%, rest were aliphatic
hydrocarbon. In the same way in the present investigation methanol conversion was
tested over CAT-1 and CAT-2 catalysts under the same reaction conditions as that of
methane methanol co-conversion at 550 ºC. Nitrogen was used to maintain methane
partial pressure. Methanol was found to be 100% conversion for both the catalyst. The
product selectivity are shown in Table 3 for methanol conversion on water free basis.
18

Table 3. The hydrocarbons product selectivity for the catalytic conversion of methanol
over CAT-1 and CAT-2 at 550 ᴼC. The data were taken at 1 h of time on stream.
Selectivity
CAT-1
3.2
CAT-2
2.4
Methane
Ethylene
Ethane
8.6
9.2
1.6
1.8
Propylene
Propane
Butylenes
Butane
3.4
3.5
1.8
1.1
4.2
3.8
5.2
5.7
Benzene
Toluene
9.2
7.8
6.8
6.4
C8 aromatics
24.7
31.3
25.2
33.1
aromatics
C
The catalytic performances of co-conversion of methane with methanol over CAT-1 and
CAT-2 catalysts at two GHSV were studied at three different temperatures for 5 h of
reaction. Figure 8 shows the methane conversion with the time on stream. The results
indicated that the methane conversion over CAT-2 is higher than CAT-1 where as
methanol was completely reacted. The decrease in methane conversion with time on
stream may be due to the carbonaceous deposition of methane/methanol over the
catalyst. This is supported by the TGA analysis of the catalyst after the reaction (Figure
9). Zn has a significant contribution on the methane conversion. Table 4 and 5 shows
the conversion results of effect of Zn on the reactivity of methane over catalysts CAT-1
and CAT-2.
19

Figure 9. Catalytic methane conversion vs time on-stream on (A) CAT-1 at GHSV 400
ml/h.gcat, (B) CAT-2 at GHSV 400 ml/h.gcat, (C) CAT-1 at GHSV 600 ml/h.gcat, (D) CAT-
2 at GHSV 600 ml/h.gcat.
From Figure 9, it can be revealed that there is a significant improvement in the catalytic
performance after the incorporation of Zn. Table 4 and 5 shows the product selectivity
distribution for CAT-1 and CAT-2. The main gaseous products of the reaction were
ethylene, ethane, propylene, propane, butylenes and butane. The liquid product mainly
contained benzene, toluene, ethyl benzene and xylene. Incorporation of zinc enhanced
the selectivity towards aromatic range products. It can be seen that the as the
temperature increases the methane conversion and ethylene selectivity increased
20

remarkably. This may be due to the dominancy of cracking reaction. At a lower
o
temperature 550 C, the selectivity of toluene and xylene is higher as compared higher
o
temperature (600 and 650 C). The alkylation reaction favored at lower temperature. At
relatively higher temperature, the contribution of oligomerization was favored because
of which coke reposition occurred on the catalyst. The high selectivity of aromatics
indicates a high activity of HZSM-5 and high concentration on its surface of strong acid
centers responsible for aromatization of methane and methanol. The shift of the
reaction toward low selectivity when temperature increases may be due to plugging of
zeolite channels and voids with coke deposits, which deteriorate the catalyst activity.
Table 4. The effect of temperature on product selectivity over CAT-1 and CAT-2 at a
methane flow rate 400 cm3/g h for methane conversion. The data were taken at 1 h of
time on stream.
Selectivity
CAT-1
600 ᴼC
11.64
4.72
CAT-2
600 ᴼC
8.79
550 ᴼC
7.95
650 ᴼC
12.83
1.61
550 ᴼC
5.32
650 ᴼC
8.45
Ethylene
Ethane
2.75
6.21
2.94
4.67
1.26
0.97
1.37
1.14
1.30
0.53
Propylene
Propane
Butylenes
Butane
1.42
0.83
0.62
1.01
2.00
3.93
0.56
1.87
2.60
0.77
0.45
0.71
1.87
0.38
0.00
0.20
0.50
0.46
29.44
23.68
20.71
10.36
37.92
10.16
16.96
14.56
38.33
5.00
12.17
29.65
37.62
5.89
15.20
19.92
28.00
20.90
10.20
10.71
21.74
38.60
Benzene
Toluene
21.22
16.42
C8 aromatics
aromatics
C
9
21

Table 5. The effect of temperature on product selectivity over CAT-1 and CAT-2 at a
methane flow rate 600 cm3/g h for methane conversion. The data were taken at 1 h of
time on stream
Selectivity
CAT-1
600 ᴼC
14.75
1.73
CAT-2
600 ᴼC
12.70
3.83
550 ᴼC
8.01
650 ᴼC
12.51
1.52
550 ᴼC
6.59
650 ᴼC
11.88
1.89
Ethylene
Ethane
3.93
3.33
1.31
1.59
2.32
8.16
1.91
1.54
Propylene
Propane
Butylenes
Butane
0.83
0.45
1.15
1.28
1.22
0.37
0.38
1.46
1.07
0.93
0.43
0.36
1.16
0.38
1.01
3.77
0.15
0.00
32.12
18.75
21.11
12.40
23.73
24.24
17.54
14.13
21.36
12.40
16.69
29.96
27.97
12.08
29.16
6.72
25.50
18.20
27.57
8.49
21.88
14.35
24.90
22.83
Benzene
Toluene
C8 aromatics
aromatics
C
9
3.7 TGA analysis of spent catalyst
Thermo gravimetric analysis (TGA) of spent catalyst (CAT-2) was done in a SDT Q600
thermal analyzer. TGA results are shown in Figure 9. The weight loss was about 7% for
550 ᴼC, 8.5% for 600 ºC and 11% for 650 ᴼC. The reduction pattern indicates that
between ambient and 150 ᴼC variation in wt. percentage was approximately 5%, which
indicates it might be due to the presence of moisture and volatiles matter deposited in
the porous structure during the reaction. Above 150 ᴼC, a rise in the peak indicates the
different carbonaceous compounds decomposed with some accumulation of mass due
to reaction in presence of oxygen.
22

Figure 10. TGA profile of the spent CAT-2 catalyst (GHSV-400 ml/h.gcat)
4.0
Conclusion
5%Mo/HZSM-5 (CAT-1) and 2%Zn-5% Mo/HZSM-5(CAT-2) catalysts were
prepared by co-impregnation and tested for the co-conversion of methane with
methanol at the condition of atmospheric pressure and three temperature (550, 600,
and 650°C). It was found that the catalytic activity and selectivity of 5% Mo/HZSM-5 can
be enhanced by the addition of Zn. A high methane conversion over the zinc-loaded
Mo/HZSM-5 catalyst was found at a reaction temperature of 650°C; whereas, high
selectivity was achieved at a reaction temperature of 600°C. BET surface area and pore
volume were found to decrease in order when metal impregnation took place in HZSM-
23

5. The BJH isotherm and pore volume distribution analysis revealed the mesoporous
nature of the catalyst. FTIR and XRD analyses suggested that the original framework
remained the same, even after metal impregnation; whereas, the TPD results defined
the moderate acidic nature of the catalyst.
24

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