Catalysis of aldol condensations and hydrogenations in flow conditions using (Cu:)Mg:Al oxides

Abstract

Aldol condensation and subsequent hydrogenation is a reaction pathway relevant for transforming biomass feedstock into usable fuel. Hydrotalcites are versatile solid bases, whose catalytic properties can be fine-tuned by changing elemental composition, calcination into mixed metal oxides and rehydration. Herein, the viability of hydrotalcite-derived catalysts was researched for aldol condensation and reduction with H2 using a model system of benzaldehyde and acetone. Preliminary tests were done in a batch reactor testing the catalytic performance of different hydroxide and oxide catalysts. It was found that while being effective in producing benzylideneacetone, even more so when previously rehydrated (increasing both conversion and yield of benzylideneacetone), mixed metal oxide catalysts deactivate < 24 h in flow conditions. Flushing acetone or calcination at 450 ◦C did not prove effective in restoring the activity of the catalysts. The use as hydrogenation catalysts was studied using 1:1:1 Cu:Mg:Al and 0.2:1.8:1 Cu:Mg:Al mixed metal oxides. Using a model system of benzylideneacetone in isopropanol, complete and selective conversion to 4-phenyl-2-butanol was achieved already at 50 ◦C. Furfurylideneacetone showed similar results, but minimally less selectivity. It was shown that pre-treatment of those catalysts (rehydration, reduction) can be performed in-situ inside a flow reactor. Those findings indicate promising use cases for hydrotalcite-based materials, especially for the use as hydrogenation catalyst. Further studies can use the results as a starting point for optimizing the reaction and elucidate the mechanism of catalyst interaction and deactivation which can deepen our general understanding of heterogeneous catalysis.