Synthesis of Sn-Beta with Exclusive and High Framework Sn Content

Sn-Beta zeolite was prepared by acid dealumination of Beta zeolite, followed by dehydration and impregnation with anhydrous SnCl₄. The formation of extraframework Sn (EFSn) species was prevented by the removal of unreacted SnCl₄ in a methanol washing step prior to calcination. The resulting Sn-Beta zeolites were characterized by X-ray diffraction, Ar physisorption, NMR, UV/Vis, and FTIR spectroscopy. These well-defined Lewis acid zeolites exhibit good catalytic activity and selectivity in the conversion of 1,3-dihydroxyacetone to methyl lactate. Their performance is similar to a reference Sn-Beta zeolite prepared by hydrothermal synthesis. Sn-BEA zeolites that contain EFSn species exhibit lower catalytic activity; the EFSn species also catalyze formation of byproducts. DFT calculations show that partially hydrolyzed framework Sn-OH species (open sites), rather than the tetrahedral framework Sn sites (closed sites), are the most likely candidate active sites for methyl lactate formation.