Sn-Mfi Zeolite Nanosponge Having High Content of Lewis Acid Sites on External Surfaces, Exhibiting High Activity in Baeyer-Villiger Oxidation of Bulky Ketone

Abstract

We developed a three-step synthesis strategy to obtain Sn-incorporated MFI-type zeolite nanosponge (Sn-MFI-ns) assembled by ultrathin (~2.5 nm) zeolite frameworks possessing uniform-sized (~4 nm) mesopores: 1) synthesis of MFI-type borosilicate nanosponge using a zeolite structure-directing-surfactant, 2) deboronation of the borosilicate using HNO3, and 3) gas-phase incorporation of Sn into the boron-vacant sites via silanol groups using (CH3)2SnCl2 as a precursor. The Sn-MFI-ns shows high crystallinity, consequently high thermal stability, and highly porous structure. The Sn content can be systematically controlled by the amount of the precursor, with Si/Sn ratios ranging from 30 to 200. The Sn species is highly dispersed over entire range of the zeolite surfaces, acting as Lewis acid sites. Due to the highly mesoporous structure, the Sn-MFI-ns has a significant number of Lewis acid sites on the external surfaces and mesopore walls, which are easily accessible to bulky molecules, compared to solely microporous zeolite (Sn-bulk-MFI). Consequently, the Sn-MFI-ns exhibits much higher catalytic activity than the Sn-bulk-MFI with high product selectivity in Baeyer-Villiger oxidation of 2-adamantanone, a molecule larger than the micropore apertures of MFI-type zeolite. The activity of Sn-MFI-ns is comparable to Sn-MCM-41 exposing almost all Sn to mesopore walls, advantageous to the bulky molecules’ reaction.

Keywords: Sn-MFI nanosponge, External Lewis acids, Baeyer-Villiger oxidation, 2-adamantanone