A chemist’s guide to catalysis

A team of researchers delineated the most important characteristics of ideal catalysts in the conversion of methanol to propylene, a process needed to make plastics, fibers and important chemicals.

Zeolites – porous compounds of Aluminum Silicate – have long been used as catalysts for this reaction. In particular, a certain topology called ZSM-5 showed great promise. 

The team, lead by researchers at KAUST in Saudi Arabia and TUDelft in the Netherlands, set out to characterize what parameters most affect catalytic activity, to provide a rational and systematic basis for catalyst design.

To do that, they synthesized an array of modified ZSM-5 catalysts, by either altering the aluminum to silicon ratio during the synthesis, or by modifying the catalysts post-synthesis by removal of metal ions, or by incorporation of alkaline earth metals. 

The rationale was to alter the distribution and composition of acid sites in the catalysts. 

Two types of acid sites are present in Zeolites; Brønsted acid sites (proton donors), and Lewis acid sites (electron pair acceptors). Reducing aluminium or incorporation of alkaline earth metals typically neutralizes the acidity of Brønsted acid sites.

The team demonstrated the efficiency of Brønsted acid site isolation, which involves preventing secondary reactions that lead to undesired products, publishing their results in Nature Chemistry¹. More importantly, they showed that the presence of Lewis acid sites increased the lifetime of the catalysts by making them more resistant to coking, a thermal process used in petroleum refining. 

“Our results on the synergistic effects of Brondsted and Lewis acid sites will help design more efficient zeolite catalysts for the transformation of fossil fuels or even CO₂ into valuable chemicals," says Jorge Gascon, a professor at KAUST's catalysis center and the principal investigator on the project.

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