Effects of diffusional constraints on lifetime and selectivity in methanol-to-olefins catalysis on HSAPO-34

Reaction-transport formalisms show that the effects of crystallite size, H⁺ density, and Si speciation of HSAPO-34 on catalyst lifetime and selectivity in methanol-to-olefins catalysis are all manifestations of diffusional constraints. Both structural catalyst properties—i.e., crystallite size and H⁺ density—and intrinsic kinetic constants (per H⁺)—regulated, in silicoaluminophosphates, by Si speciation—affect the severity of these diffusional restrictions. Methanol-to-olefins catalysis on HSAPO-34 occurs by a complex network of autocatalytic reactions in which temporal gradients persist along with spatial gradients inherent to continuous-flow, fixed-bed reactors. Invocation of reaction rates in the interpretation of lifetime and selectivity trends in such systems requires defined quantities averaged in both time and space because measured observables conflate instantaneous reaction rates with spatial and temporal gradients. Quantities defined herein, i.e., total turnovers and cumulative selectivity, provide such rigorous assessments of lifetime and selectivity that permit causative correlation between rates of reactions within the complex network of autocatalytic reactions and material properties of HSAPO-34. Total turnovers decreases with increasing diffusional constraints because dehydrocyclization reactions experience stronger diffusional constraints than olefins methylation, aromatics dealkylation, and methanol transfer hydrogenation. Cumulative selectivity to paraffins increases with increasing diffusional constraints because transfer hydrogenation reactions of methanol, ethylene, and propylene experience stronger diffusional constraints than all other reactions within the complex reaction network. The approaches detailed herein codify the chemical and physical origins of trends in process outcomes with system variables for reaction systems characterized by complex reaction networks and prevailing spatial and temporal concentration gradients.