Unravelling the Crucial of Spatial Al Distribution to Realize Precise Alkali-Treatment for Target Acid-Catalyzed...

Constructing mesoporous structure within zeolites by alkali-treatment is an effective protocol to improve their diffusion properties. However, undesirable changes in Brönsted acid site (BAS) densities always offset this advantage in acid-catalyzed reactions. In this context, the crucial roles of spatial aluminum (Al) distribution were unraveled during alkali-treatment of MFI zeolite and the desirable BAS density was achieved in obtained hierarchical samples for the target reactions. Various characterization methods, particularly the multiple one- and two-dimensional magic-angle spinning (MAS) NMR techniques, were performed to track the alkali-treatment processes. For the sample with a more uniform spatial Al distribution, more tetrahedral Al sites would fall off and migrate around the Si-OH in zeolite as Al(OH)₄ ^-. Those re-deposited Al(OH)₄ ^- sites were easily transformed into NMR-invisible Al sites during the calcination process, which contributed negligibly to both Brönsted and Lewis acidities, thus being referred to "acid-free" Al species. While most tetrahedral Al sites were preserved after the alkali-treatment of sample with non-uniform Al distribution and the BAS density gradually increased with treatment time. According to the requirements of typical acid-catalyzed reactions, such as catalytic cracking of 1,3,5-triisopropylbenzene and methanol-to-olefins, the desired hierarchical zeolite catalysts were developed by matching the amounts of extracted Si and generated "acid-free" Al during the precise alkali-treatment.