Simultaneous Evaluation of Reaction and Diffusion over Molecular Sieves for Shape-Selective Catalysis

Abstract

Shape selectivity is the critical issue of molecular sieve catalysis. As an important aspect of shape-selective catalysis, the significance of diffusion in conversion, deactivation, and product selectivity over molecular sieve catalysts has been recognized but still has not been interpreted quantitatively and explicitly. In the present work, direct tracking molecular diffusion in the SAPO-34 catalyst during the methanol-to-olefins (MTO) reaction is successfully realized by combining the MTO reaction with the diffusivity evaluation by a pseudo-gas chromatography method which was operated in one catalyst bed under the real reaction condition. Diffusion behavior over the working catalyst can be measured along with the reaction to reflect the evolution in shape-selectivity catalysis in real time. For the first time, the accessibility of the catalyst microporous surface for the reactant was quantified during the reaction, which provided a rational understanding of the reaction and deactivation of MTO over the catalyst with continuous coke deposition. The evaluation of configurational diffusion hindrance in SAPO-34, because of the accumulation of organic species retained in the catalyst, corresponds well to the evolution of methanol conversion and product distribution and provides the scientific basis for manipulating the MTO reaction from the point of shape-selective catalysis.