(476c) Controlling Aldolization over Mg-Al Mixed Oxides Derived from Alkali-Free Layered Double Hydroxides

The efficient conversion of biomass derivatives to fuels and chemicals involves exploiting their oxygen-based functionalities, notably absent in conventional fossil feedstocks. One key chemistry for C-C bond formation between oxygenates is aldolization, 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. These LDH materials consist of two or more differently-charged metals (usually 2+/3+) which form uniform bimetallic hydroxide phases in charge balance with compensating anion layers. Their catalytic performances are highly sensitive to the identities and ratios of the metals as well as treatment conditions, with thermal decomposition yielding highly active, high surface area mixed oxides. However, much of the work on these materials to date lacks clear connections between structural properties, intrinsic catalytic performance, and quantum chemical theories, especially with respect to bimolecular aldolization reactions.

In this work we synthesized Mg-Al mixed oxides 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. Avoiding alkali-based syntheses limits the impact played by non-structural impurities in the final oxide framework. Thermal and structural characterizations demonstrate the requirements for LDH decomposition without causing structural collapse. Reactivity trends in acetone coupling 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. Additional details are gathered by comparison with quantum chemistry simulations of acetone aldolization over model Mg-Al surfaces. These studies lay the intellectual groundwork required to rationally develop new mixed oxide catalysts for the selective aldolization of biomass-derivable oxygenates.