Synthesis of hydroxyapatite – containing catalysts from bone waste – conventional route versus microwaves

The use of the hydroxyapatite (HA) contained in animal bone residues for the synthesis of active catalysts is an innovative solution to manage large quantities of bone waste (BW). In this study, two methods of preparing HA - containing catalysts were explored: i) conventional route using bone char (BC) from BW calcined at 500 - 900 o C, and ii) microwaves route using aqueous phase extraction from BW by a microwave (MW) -assisted process at 95 o C. BC mainly consisted of B-type HA, in which carbonate groups replace phosphate in the structure; however, its low surface area limited its catalytic performance. Modification with silica, MW treatment and surfactant charge had a critical effect on the formation of the porous structure, and consequently, on increasing the surface area of the composite catalysts. Cationic surfactants promoted silica network formation and resulted in improved porous structures due to their weaker interaction with the positively charged BC. The obtained HA-containing catalysts were demonstrated to be effective platforms for immobilising metallic species. Incorporating Ti and Zn by direct synthesis generated highly dispersed active sites, as confirmed by physicochemical characterization. Notably, an increase in sea buckthorn pyrolysis temperature (from 250 °C to 600 °C) was observed after just 5 min of MW irradiation using catalysts produced via the MW-assisted method, indicating enhanced heat transfer within the reactor. Meanwhile, catalysts produced via the conventional method significantly impacted the composition of the pyrolysis gas phase by promoting the formation of CO 2 and hydrocarbons ≥C 2 , such as C 2 H 2 , C 2 H 6 , C 3 H 8 , C 4 H 10 , C 5 H 12 , and C 6 H 14 . • Hydroxyapatite-containing catalysts obtained from bone waste using conventional and microwave methods • Hydroxyapatite as a platform for immobilization of catalytic active sites • Formation of hierarchical structure of composite catalysts using surfactants • New methods for catalyst production - conventional vs. microwave