The heart of darkness (in heterogeneous catalysis) - Aditya Bhan

Heterogeneous catalysts enable functionalization and derivatization of molecules for use as energy carriers, polymer precursors, and fine chemicals and mitigate the environmental consequences engendered in their production and consumption. We illustrate recent advances in our understanding of the mechanisms and site requirements in catalysis by surfaces, with emphasis on concepts that tackle ubiquitous selectivity and lifetime challenges, based on case studies of two catalytic systems deployed in industrial practice.

The first part of this presentation will describe the mechanistic origins of over-oxidation and C-C bond scission products in the partial oxidation of propylene to acrolein on mixed metal oxide catalysts. We combine transient kinetic studies, co-feed experiments of aldehydes and carboxylic acids formed as byproducts in propylene oxidation, and isotopic-labeling studies to elucidate the reaction mechanisms, identify the existence and involvement of relevant surface intermediates, and develop an extensive reaction network describing the formation of all C2 – C6 products (> 20 C2-C6 products are formed in this chemistry at carbon selectivity as low as 0.001%), and illustrate the underlying mechanisms for C-C bond cleavage and formation reactions. These mechanistic insights provide guidance for process conditions and catalyst development to minimize the formation of undesired products.

The second part of this presentation will discuss parametric investigations of catalyst lifetime pursuant to changes in methanol space velocity and inlet methanol pressure and interpret trends of cumulative and transient selectivities to implicate formaldehyde, formed in transfer dehydrogenation reactions of methanol, as the key intermediate in transforming active olefin- and aromatic- chain carriers to inactive polycyclic intermediates in methanol-to-olefins catalysis. We also demonstrate efficacy of a bifunctional strategy via physical addition of rare earth oxides in improving the lifetime of methanol-to-olefins chabazite-type zeolite catalysts without disrupting the high selectivity to ethylene and propylene.