(616g) Understanding the Influence of Metal Composition on Aldol Condensation Reactions over MgxAlOy-Type Materials Via Combined Experimental and Computational Investigations

An important chemistry in the toolkit for the efficient conversion of biomass derivatives to fuels and chemicals is aldolization, a C-C bond forming reaction often carried out over solid acids or bases. Mixed metal oxides derived from layered double hydroxides (LDHs) are among the most catalytically active and tunable solid bases which can enable these reactions. Thermal decomposition yields particularly active materials which have properties that can be adjusted in a variety of ways such as the metal composition. While these materials have been studied for some time, much of the work toward understanding C-C bond forming reactions on these materials lacks clear connections between structural properties, intrinsic catalytic performance, and quantum chemical theories.

In this work we synthesized Mg-Al mixed oxides with varying Mg:Al molar ratios via the thermal decomposition of alkali-free layered double hydroxide precursors and examined them in the liquid-phase aldolization of acetone as a model reaction. Thermal and structural characterizations demonstrate the requirements for LDH decomposition without causing structural collapse. Reactivity trends and identification of active sites are examined via data-fitted reaction kinetics models and discussed in connection with ex-situ and in-situ site titration experiments as well as infrared spectroscopy and reversible poisoning experiments. Additional details are gathered via density functional theory (DFT) simulations of acetone aldolization over model Mg_xAlO_y surfaces. These simulations suggest that the doping of Al creates metal vacancies which act as strong base sites. Consistent with experiment, these strongly basic sites are shown to be crucial to aldolization and help to explain the high sensitivity of aldolization rates to Mg:Al molar ratios in these mixed oxides. These studies guide future work toward developing materials comprising more exotic metal combinations in order to improve the selectivity and reactivity of heterogeneous aldolization reactions.