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 geometry. In addition
to the more favorable activation barrier of the catalytic process,
the adsorption at the catalytic site in Sn-SPP is also found to be
more favorable than for Sn-Beta, in spite of competitive adsorption
between fructose and ethanol in the ethanol-saturated zeolites.