(322g) Effect of Catalyst Composition and Reaction Mechanism Study on Non-Oxidative Methane Conversion into Higher Hydrocarbons

Direct non-oxidative
methane conversion to higher hydrocarbons is a potential approach to convert
natural gas into valuable hydrocarbons.¹ Zeolite supported molybdenum
(Mo) catalyst is the most effective catalyst reported
for direct non-oxidative methane conversion into higher hydrocarbons in which HZSM-5
& HMCM-22 are the two well-known zeolites as support.² Mo/HMCM-22
catalyst was reported for effective catalytic
performance due to unique pore structure and framework of
HMCM-22 zeolite support.³ Different factors such as reaction
parameters (T, P and GHSV), catalyst formulation, such as Mo loading over
HMCM-22 zeolite support, and SiO₂/Al₂O₃ ratio
(SAR) of HMCM-22 zeolite support significantly affects the reaction. In this regard
we have studied here the effect of metal (Mo) loading over HMCM-22 (SAR-55)
support and space velocity (GHSV) for non-oxidative methane conversion
reaction. Firstly, effect of different Mo loading (2, 5 and 10 wt%) was
analyzed and observed that 5 wt% Mo loading is suitable for benzene formation,
major product of the reaction as shown in Figure 1(a). Effect of GHSV for the
reaction has been shown in Figure 1 (b) for 5wt% Mo loaded HMCM-22 (SAR-55)
catalyst, and it was observed that lower space velocity (720 ml/gm-hr) is
effective for aromatics (benzene) formation compared to higher space velocity
(1200 ml/gm-hr). Higher space velocity gives more ethylene compared to benzene
due to lower residence time. Theoretical calculations
were performed using density functional theory (DFT) for the Mo/Zeolite catalyst.
Molybdenum carbide formed during the reaction has been proposed to be the active
phase for C-H activation of methane, C₂ coupling and aromatic
formation reactions.⁴ Mo_xC_y⁵ type
nanocluster were supposed to be suitable form of molybdenum carbide species for
methane activation and C₂ coupling. In this reference, Mo₄C₂
nanocluster grafted over Al and Si site of zeolite channel was reported for C-H
bond activation and a 112 kJ/mol energy barrier was observed when grafted over Al
site.⁶ In the study we have investigated the C-H bond activation and
ethylene (primary intermediate of benzene) formation via C₂ coupling
over Mo₄C₂ nanocluster. Schematic reaction paths for methane
activation, ethane and ethylene formation over the Mo₄C₂
cluster and also the potential energy diagram for all these steps have been shown
in Figure 2(a and b). Methane molecule gets activated over the Mo₄C₂
nanocluster with an activation barrier 112.8 kJ/mol to form CH₃
and H adsorbed on the cluster. In the next step the second CH₄ molecule
dissociated over the same site with 115.5 kJ/mol activation barrier to form two
CH₃ groups attached to the same Mo atom, which then undergo C-C
coupling reaction with an activation barrier of 150 kJ/mole which is
comparatively high to form ethane. The energy barrier for C-H bond activation
of ethane (89 and 94 kJ/mol) is relatively low and forms ethylene (C₂H₄),
which is the major primary unit of benzene formation, over the catalyst as also
observed experimentally, in Figure 1(a) and (b).  

Figure
1. (a) Product distribution for different
Mo loading over HMCM-22 (SAR-55) catalyst (b) Product distribution for
5wt%Mo/HMCM-22 (SAR-55) catalyst at two different GHSV (720 & 1200
ml/gm-hr)

Figure 2. (a) Schematic
paths for methane activation and C-C coupling over Mo₄C₂
nanocluster (b) Potential energy diagram for C-H activation and C₂
coupling over Mo₄C₂ nanocluster

References

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Zhou, S. Zuo, S. Xing, J. Phys. Chem. C 2012, 116, 4060-4070.

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Acknowledgement

We
would like to acknowledge Gas Authority of India Limited (GAIL) for financial
support