Methanol-to-hydrocarbons catalysis occurs via a complex reaction network; the species include oxygenates and acyclic and cyclic and aliphatic and aromatic hydrocarbons, and the reactions include methylation, oligomerization, hydrogen transfer, cyclization, β-scission, and dealkylation. The use of small-pore zeolites, e.g., HSAPO-34 in methanol-to-olefins catalysis, adds further complexity because (i) species remain entrained within zeolite cavities during catalysis and (ii) rapid catalyst deactivation condemns continuous flow reactors with persistent temporal gradients. Here, we report the preparation of HSAPO-34 formulations diverse in morphology and composition (crystallite size, H+ density, and Si speciation) and trends of lifetime and selectivity in methanol-to-olefins catalysis with these material properties. Reaction-transport formalisms show that effects of crystallite size, H+ density, and Si speciation on lifetime and selectivity manifest together as consequences of diffusional constraints. Interpretation of trends in lifetime and selectivity within the context of this reaction-transport formalism reveals that specific classes of reactions, within the complex reaction network, experience stronger diffusional constraints than others.Total turnovers decrease with increasing diffusional constraints because dehydrocyclization reactions producing entrained hydrocarbons experience stronger diffusional constraints than reactions producing light olefins. Methane, ethane, and propane selectivities increase with increasing diffusional constraints because transfer hydrogenation reactions of methanol, ethylene, and propylene experience stronger diffusional constraints that reactions producing light olefins. These results provide guidance, founded on principles of transport phenomena and reaction engineering, on material properties necessary to maximize lifetime and light olefins selectivity in methanol-to-olefins catalysis [1]. More broadly, the methodology, results, and discussion herein represent a case-study on the assessment of physical and chemical origins of lifetime and selectivity trends in complex reaction networks in heterogeneous catalytic processes with both temporal and spatial gradients.<br/><br/>

Effect of diffusional constraints on lifetime and selectivity in methanol-to-olefins catalysis

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.