Fundamental understanding and catalytic applications of hollow MFI-type zeolites

In this review, we summarize the fundamental understanding and recent progresses on hollow MFI-type zeolite synthesis, hollow cavity formation mechanism, and mass transport/catalytic performance intensification, especially from the viewpoints of industrial application. Obviously, introduction of hollow cavities into the micropores of MFI-type zeolites significantly facilitates both internal mass transport and catalytic performance in many Brönsted or Lewis acid catalyzed reactions, i.e. catalytic oxidation, methanol to hydrocarbons and catalytic cracking reaction. It is majorly assigned to the shortened diffusion path length within zeolite crystal, while there is no change of external surface, thus leading to lower mass transport energy barrier. In general, hollow zeolites are synthesized by two strategies, including selective desilication method and in-situ dissolution-recrystallization (also remarked as crystalline rearrangement) method, ascribing to the reassembly of dissolved species with low aggregation under the effect of organic templates in hydrothermal conditions in latter process. Notably, selective dissolution is the driving force of these two synthesis routes, thus it is key to tune the composition and dispersion of framework elements and intracrystalline defects for controlling the size and distribution of internal hollow cavities. Moreover, through rearrangement, previously impregnated metal nanoparticles with tunable composition and size can be encapsulated within zeolite crystal, thus showing relatively high thermal stability and catalytic activity in catalytic oxidation reactions, selective hydrogenation, cross-coupling, esterification, and C1 transformation.