Tracking the solid-state incorporation of Sn into the framework of dealuminated zeolite beta, and consequences for catalyst design

Sn-Beta has emerged as a state-of-the-art catalyst for a range of sustainable chemical transformations. Conventionally prepared by bottom-up hydrothermal synthesis methods, recent research has demonstrated the efficiency of several top-down methods of preparation. One attractive top-down approach is Solid-State Incorporation, where a dealuminated Beta zeolite is physically mixed with a solid Sn precursor, in particular Sn(ii) acetate, prior to heat treatment at 550 °C. This procedure is fast and benign, and metal incorporation requires no solvents and hence produces no aqueous Sn-containing waste streams. Although the performances of these catalysts have been well explored in recent years, the mechanism of heteroatom incorporation remains unknown, and hence, opportunities to improve the synthetic procedure via a molecular approach remain. Herein, we use a range of in situ spectroscopic techniques, alongside kinetic and computational methods, to elucidate the mechanisms that occur during preparation of the catalyst, and then improve the efficacy of the synthetic protocol. Specifically, we find that successful incorporation of Sn into the lattice occurs in several distinct steps, including (i) preliminary coordination of the metal ion to the vacant lattice sites of the zeolite during physical grinding; (ii) partial incorporation of the metal ion into the zeolite framework upon selective decomposition of the acetate ligands, which occurs upon heating the physical mixture in an inert gas flow from room temperature to 550 °C; and (iii) full isomorphous substitution of Sn into the lattice alongside its simultaneous oxidation to Lewis acidic Sn(iv), when the physically mixed material is exposed to air during a short (