Cooperative Catalysis by Surface Lewis Acid/Silanol for Selective Fructose Etherification on Sn-SPP Zeolite

While Lewis-acid zeolites, such as Sn-Beta, catalyze glucose isomerization in an alcoholic medium, mesoporous Sn-SPP catalyzes both glucose isomerization to fructose and fructose etherification (formally ketalization) to ethyl fructoside, enabling fructose yields in excess of the glucose/fructose equilibrium. Using periodic density functional theory calculations and force-field-based Monte Carlo simulations, the ketalization reaction mechanism and adsorption behavior were examined. The silanols on the Sn-SPP mesopore surface facilitate the ketalization reaction through hydrogen bonding interactions at the transition state, only possible via a Sn–O–Si−OH moiety, present in Sn-SPP but not in Sn-Beta. Fructose ketalization is favored over glucose acetalization due to differences in stability of the oxonium intermediates, which are stabilized by the Sn-SPP active site. The open site of hydrophobic Sn-Beta cannot perform these reactions because its active site does not contain an adjacent silanol of the right ...