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

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 ZnAIO(x)/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 H-2 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.