Structured hydroxyapatite composites as efficient solid base catalysts for condensation reactions

Herein, we report the use of structured hydroxyapatite composite (SHCs) as highly efficient and recyclable solid base catalysts for various condensation reactions. Catalyst performance as function of catalyst loading, reaction time and reaction temperature were studied in the solventless self-aldol condensation reaction of butyraldehyde to 2-ethylhexenal under mild reaction conditions. SHC catalysts were found to outperform benchmark solid base catalysts such as MgO, TiO2, calcium carbonate and hydroxyapatites. Characterization of the synthesized SHC catalysts by a range of surface analysis, spectroscopic and electron microscopy techniques, showed that a moderate acid/base ratio and high BET surface area to be key to their high efficiency. Furthermore, recycling experiments showed the catalyst to be stable over multiple runs. Moreover, the most active SHC catalyst was investigated in other prototypical condensation reactions such as the Knoevenagel condensation, Claisen-Schmidt condensation and Henry reaction, again showing excellent performance. These results highlight the versatility of these SHC materials and their potential for industrial employment as solid base catalysts.