Hydroxyapatite‐Based Catalysts: Influence of the Molar Ratio of Ca to P

There are hundreds of publications dealing with heterogeneous catalysis by catalysts based on calcium hydroxyapatite (HA, Ca 10 − x (HPO 4 ) x (PO 4 ) 6 − x (OH) 2 − x ). HA, with high thermal stability and tunable acid/base properties, can be used directly as heterogeneous catalysts. Alternatively, HA can serve as a starting material for making metal ion-exchanged HA catalysts, supported catalysts, and grafted catalysts. Some composite catalysts also contain HA as a component. Even though HA-based catalysts are very useful in catalyzing many organic and inorganic reactions, most of the research in this field used catalysts based on stoichiometric HA, but the influence of Ca/P ratio on the physicochemical properties and catalytic performance of HA-based catalysts has been studied less frequently. That may constitute an interesting research topic because in converting organic chemicals such as biomass or biomass-derived chemicals, high selectivity to desirable products is highly desirable, and HA-based catalysts with tunable acid/base properties may help boost selectivity. In this chapter, we summarize key examples on the influence of Ca/P ratio on the performance of HA-based catalysts in organic and inorganic reactions and propose several ideas for future research.