Manipulating the architecture of zeolite catalysts for enhanced mass transfer

Abstract

Zeolites belong to microporous solids that are widely used as adsorbents, catalysts and catalyst supports. As diffusion in the microspores is slow, it is highly desirable to develop approaches to enhance mass transfer or to avoid the negative effect from mass transfer resistance. In this perspective, we underline pore architecture control for hierarchical zeolite, and spatial locations of metal nanoparticles on zeolite. For solid acid catalysts, the construction of auxiliary porosity in crystalline zeolites leading to the formation of hierarchical zeolite is a promising solution. Most successful pore-architecture construction strategies are based on templating approaches within the classic LaMer crystallization framework, whereas our strategies are based on the non-classic orientated attachment growth mechanism in dry gel crystallization. Organic additives, such as organosilanes, can be incorporated with protozeolite particles to produce hybrid mesocrystals. Combustion of organics in the mesocrystal affords hierarchical beta zeolites, the pore-architecture of which is tunable with respect to additive selection. Such a crystallization based design of pore-architecture merits small primary crystal size and improved pore-connectivity, which have been verified to be the key factors that affect diffusion. On the other hand, for zeolite supported metal catalysts that suffer from deactivation caused by pore blocking, we propose a strategy to avoid the deactivation problem by depositing metal nanoparticles exclusively on the external surfaces of support. This can be achieved by simply leaving the structure directing agents in the pores before supporting metal particles. Side reactions caused by diffusion resistance are minimized, and moreover, the involatile byproducts, if any, do not produce additional diffusion resistance. Such a simple yet effective architecture control significantly prolongs the lifetime of Au/TS-1 catalyst in direct propene epoxidation with H2 and O2. From these demonstrations, it is important to design purpose-orientated zeolite synthesis to enhance mass transfer, for which the architecture plays a key role.