(437b) Effects of Hydrophilicity on Aldol Condensation and Esterification Reactions of Acetaldehyde and Ethanol over Ti-BEA Catalysts

Aldol condensation and esterification reactions can be used to upgrade ethanol and acetaldehyde to produce higher-value intermediates for the synthesis of polymers, jet fuels, and other compounds. Transition metal-substituted *BEA zeolites (M-BEA) give greater rates and exhibit higher stability compared to bulk metal oxides (e.g., TiO₂) for these reactions due to their higher tolerance to poisons and facile regeneration by heat treatments; however, the mechanism for aldol condensation reactions within M-BEA catalysts and the effects of silanol (SiOH) groups proximate to active sites are not known. Here, we seek to answer these questions by combining kinetic and spectroscopic measurements for a series of Ti-BEA catalysts with controlled densities of (SiOH)₄ nests.

Turnover rates for aldol condensation and esterification (1 - 10 kPa C₂H₅OH, 0.1 - 1 kPa C₂H₄O, 15 kPa H₂, 503 K) increase by a factor of 10 from hydrophobic Ti-BEA ([(SiOH)₄] ~ 0 (unit cell)^-1) to hydrophilic ones ([(SiOH)₄] 0.5 - 5 (unit cell)^-1), yet do not change monotonically with [(SiOH)₄]_. These differences are not due to changes in the mechanism for aldol condensation or esterification, which proceed by kinetically relevant aldol addition of enolate or nucleophilic attack of ethanol respectively to an adsorbed acetaldehyde. Infrared spectroscopy shows SiOH can irreversibly react with C₂H₆O to produce ethoxysilane (Si-OC₂H₅) that might influence reaction rates because activation enthalpies of aldol condensation and esterification increase with silanol density. Rates measured as functions of reactant pressures (r_ester ~ [C₂H₄O]¹[C₂H₆O]⁰; r_aldol ~ [C₂H₄O]^{1 to 2} [C₂H₆O]^{-0.7 to -0.3}) imply the most abundant reactive intermediates (MARI) differ between these two reactions, which further suggests aldol condensation and esterification predominantly occur on distinct sets of active sites since reaction conditions are the same. Ongoing measurements seek a clearer relationship between activation enthalpy, entropy, and silanol densities for aldol condensation and esterification on Ti-BEA catalysts.