Designing Porous Boroaluminate Catalysts via Non-Hydrolytic Ether Elimination

Abstract

Boroaluminates are mixed oxides of boria and alumina that show promise for various catalytic applications due to their mild acidity. In this work, boroaluminate xerogels were successfully synthesized via alternative non-hydrolytic sol-gel (NHSG) ether elimination pathways, offering a controlled, template-free strategy for their design. We investigated the synthesis using four distinct alkoxide precursor combinations, utilizing aluminum isopropoxide and sec-butoxide as aluminum sources, alongside boron methoxide and isopropoxide as boron sources. This ether elimination approach provides an excellent alternative to traditional alkyl halide elimination routes, allowing the final porosity, coordination states, and surface acidity to be targeted more easily solely through precursor selection and precursor-to-solvent ratios. The synthesized boroaluminates exhibit a highly homogeneous bulk dispersion of aluminum and boron throughout the xerogel networks (as confirmed by XPS, ICP-OES, and EDS mapping), though calcination at 600°C appears to have led to a surface depletion of boron species. Three of the four synthetic pathways yielded high-surface-area materials (590–772 m² g⁻¹) without the use of structure-directing agents. Analysis by ¹¹B and ²⁷Al MAS NMR, combined with NH₃-TPD, revealed that precursor choice directly affects the local atomic coordination, yielding unique catalytic centers—specifically 5-coordinated aluminum alongside 3- and 4-coordinated boron—and a heterogeneous distribution of weak and medium acid sites. Additionally, these materials were evaluated as heterogeneous catalysts in the dehydration of ethanol. The boroaluminates exhibited highly temperature-dependent selectivity, favoring diethyl ether at lower temperatures and shifting predominantly to ethylene at higher temperatures. Four of the five prepared samples showed promising catalytic activity, consistently achieving ethanol conversions between 79% and 93% at 275°C.