Simultaneous isomorphous incorporation of boron and germanium in MFI zeolites

Abstract

The synthesis and characterization of MFI zeolites containing boron and germanium simultaneously incorporated in the framework are reported. By preparing materials using a wide range of compositions and synthesis pH, it is shown that germanium uptake is primarily controlled by the pH of the synthesis gel, with increasing germanium uptake with decreasing pH. Incorporation of boron is affected by pH and boron content in the synthesis gels. Larger boron uptakes are seen when both parameters decrease. A linear decrease in unit cell volume with boron content indicates framework incorporation of boron in the samples. 11B MAS NMR studies show that boron is tetrahedrally incorporated in the framework and reveal the possibility of a site comprised of boron atoms surrounded by three silicons and one germanium as nearest neighbors, apart from the boron site with an all-silica environment. This work shows how highly crystalline, phase pure B–Ge-MFI zeolites can be made with tunable compositions that can be used in applications of boron-containing zeolites where they could benefit from the presence of germanium.