Acetilação do glicerol sobre hidroxiapatitas contendo Cu, Co ou Ni: efeitos das condições reacionais

Abstract

The glycerol acetylation reaction, with a view to producing glycerol esters, was studied on hydroxyapatite-based catalysts. The acid-base catalysts used in the reaction, especially the acidic solids, deactivate in the reaction medium due to the high acidity and the presence of water. In this work, hydroxyapatites are presented as an alternative to acid-base catalysts due to their resistance to water poisoning and their modulable acid-base property. The catalysts were prepared through the simultaneous co-precipitation method, in which Cu, Co or Ni precursors, with different metal contents, were dispersed in the hydroxyapatites. The solids were calcined at 550°C for 6 h and named MeHAPX, (Me = Co, Cu or Ni); X represents the metal content. The catalysts were evaluated in the glycerol acetylation reaction, initially at a molar ratio of glycerol and acetic acid of 0.25, 25 mg of catalyst, 6 hours of reaction at 80°C and stirring at 1000 rpm. Different temperature conditions and molar ratios were tested. The solids were characterized, after the reaction, by X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM-EDS), Electron Paramagnetic Resonance (EPR) and X-Ray Excited Photoelectron Spectroscopy (XPS). The results show that the solid CuHAP17 showed 50% glycerol conversion and 10.8% selectivity for triacetin. On the other hand, CoHAP9 obtained greater selectivity to triacetin reaching the value of 21.6%. The solids containing copper had a better performance compared to those containing nickel and cobalt, due to the higher acidity observed in the former. This fact is due to the presence of other phases in the structure, such as CuO, libetenite and monetite, in addition to the hydroxyapatite phase. The solids were not deactivated in the presence of water, although the Cu active site leached from the structure. On the other hand, samples containing Co or Ni deactivated in continuous catalytic cycles. Additionally, solids containing low levels of Cu form Cu2O phases on the catalyst surface and adsorption of species from possible oligomers was observed. Co and Ni-based materials had lower catalytic performance due to leaching of active sites and adsorption of organic compounds from reactants and reaction products.