Oxygenate-based routes regulate syngas conversion over oxide–zeolite bifunctional catalysts

Abstract

The emerging oxide–zeolite bifunctional catalysis for direct syngas conversion has drawn extensive interest, both academically and industrially, with further exploration urging a clear mechanistic understanding of this complex catalytic network. Herein, using a specially designed quasi-in situ, solid-state nuclear magnetic resonance-gas chromatography/gas chromatography-mass spectrometry analysis strategy, this reaction is fully monitored from the very early induction period to steady-state conversion under high-pressure flow-reaction conditions, using ZnAlOx/H-ZSM-5 composites as model catalysts. We identify abundant critical and/or transient intermediates in dynamic evolution, including carboxylates, alkoxyls, acid-bounded methyl-cyclopentenones and methyl-cyclopentenyl carbocations, providing direct evidence of vigorous regulation by unique, oxygenate-based pathways of the reaction network. This proposed mechanism overturns the general cognition of oxide–zeolite reactions as simple tandem catalysis, and highlights the many roles (both positive and negative) of CO and H2 molecules via oxygenate-based routes, thus dictating the final product. The current characterization technology and its mechanistic understanding would benefit further exploration in bifunctional catalysis. Syngas conversion to hydrocarbons on oxide–zeolite bifunctional catalysts is a promising process, yet a complete mechanistic understanding is still lacking. Now the reaction network has been comprehensively evaluated on ZnAlOx/H-ZSM-5 using a combination of NMR- and gas chromatography-based techniques.