Strategy to enhance selectivity in a two-step reaction using the R1-R2-D reactions-diffusion phenomena

Abstract

An analytically derived criterion for R1-R2 phenomenon is combined with the Weisz criterion for R1-D phenomenon to allow the development of assignment criteria for the overall controlling factors in R1-R2-D phenomena. The overall assignment criteria are found to be dependent on the internal effectiveness factors η_1 and η_2, as well as the rate of the individual reactions at the surface of the catalyst particle. Applying the developed criteria to a two-step methanol oxidation reaction suggests that the selectivity of the formaldehyde intermediate species can be enhanced at high reaction temperature when catalysts are specifically designed to enhance the rate of formaldehyde formation. However, CO formation needs to be suppressed to enhance selectivity towards formaldehyde at moderately low temperature. This reaction-diffusion theoretical framework provides guidance for the development of highly selective catalyst for two-reactions-in-series systems and can be extended for higher-number multiple reactions in series and in parallel.