(743h) Experimental and Theoretical Investigation on Methane Dehydroaromatization Under Non-Oxidative Conditions

Advances
in hydraulic fracturing technology has generated huge industrial attention
towards catalytic conversion of low cost natural gas to valuable hydrocarbons. Developing
molecular level understanding of catalyst active sites and its deactivation
mechanism will help in the process of developing an effective commercial non-oxidative
methane dehydroaromatization (MDA) catalyst. Mo_xC_y/H-ZSM-5
is one of the suitable catalyst for the MDA process. Mo/HZSM-5
contains both metal site (Mo species) and acid site (H-ZSM-5) hence act as a
bi-functional catalyst to efficiently activate methane on Mo species to produce
C₂H₄ which than oligomerize and aromatize to benzene and
its derivatives on acidic sites of shape selective zeolite¹-² . Nature of the
active site for methane is suggested to be carbide or oxy-carbide but active phase
structure is strongly debated. Lezcano-Gonzàlez and co-workers³  have carried
our operando X-ray method and asserted MoC₃ as possible active Mo speciation
responsible for methane activation and formation of aromatics. Density
functional theory calculations have been employed to study the effect of charge
on catalytically active molybdenum carbide nano-cluster and its stability which
will help in designing better MDA catalyst with suitable promoter species.  In
the study we have presented a detail investigation of effect of residual
charge, Figure 1(a) showing a linear correlation between charges on MoC₃
cluster on methane activation underlying the effect of reducibility of Mo_xC_y
cluster. Further, reaction mechanism insights have been studied for the methane activation and formation of ethylene,
through formation of C₂H₆ and further C-H activation. Potential
energy diagram for methane following scheme, Figure
1(b) shows ethylene will be formed easily on the
catalyst surface, which eventually lead to formation for benzene with an intrinsic
methane activation barrier of ~1.23 eV. The two resultant CH₃
groups attached to the Mo₂C₆ cluster undergo
C-C coupling reaction with relatively lower barrier of +0.35eV. Following these
theoretical insights, H-ZSM-5 impregnated with Mo were tested experimentally
for MDA under non-oxidative conditions. In order to minimize coke deposition two
different approach a) carbon dioxide (CO₂) as co-feed with methane⁴ and b) in-situ
topotactic transformation of MoC form hexagonally close packing to face
centered cubic⁵ by pre-treating
catalyst with inflow of H2 at 2400ml/hr-gm GHSV for 6 hours at 350°C and then
increasing the temperature to desired reaction condition with constant inflow
of 30ml/min during carburization period carried out on a fixed bed reactor.
Addition of CO₂ to methane feed reduces not only inert coke but also
reactive coke as elucidated by TGA analysis, Figure 1(c). In the current
study, the effect of variation in CO₂ concentration was studied. Results
depict that addition of CO₂
enhances benzene percentage upto 35% on mole basis, Figure 1(d) in
product stream as higher oxygen donor ability of CO₂ eliminate
surface carbon by forming CO. Phase transformation of MoC hinders the migration
of external surface Mo species to BAS present in zeolite channels resulting in
retardation of inert coke deposition at pore entrance which results in enormous
catalyst active with high selectivity towards aromatics, Figure 1(e) and less
coke deposition resulting in high catalyst stability which is validated through
coke gasification kinetics using Kissinger plot.

Figure 1. (a)  Effect of residual
charges on methane C-H bond activation, (b)Reaction diagram for methane
activation and formation of ethylene on Mo₂C₆
nanocluster, (c) TGA
analysis for Mo/HZSM-5, (d)-(e) Product
selectivity during MDA process for Mo/HZSM-5.

Acknowledgement

Financial
support provided by Gas Authority of India limited (GAIL) for carrying out this
research work is gratefully acknowledge.

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