Characterization and catalytic performances of copper and cobalt-exchanged hydroxyapatite in glycerol conversion for 1-hydroxyacetone production

Abstract

Calcium hydroxyapatites (HAP) containing cobalt or copper were synthesized and catalytically evaluated in the dehydration of glycerol for valuable chemical intermediates production. The solids were characterized by chemical analysis, X-ray diffraction (XRD), Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy (SEM-EDS), acid-base measurements, Raman spectroscopy and nitrogen adsorption–desorption isotherms. Cobalt-containing HAPs had the characteristic hydroxyapatite (Ca10−x(HPO4)(PO4)6−x(OH)2−xx(H2O)x) and monetite Ca(HPO4) phases, independently of the cobalt contents, being mesoporous materials. This was proven to be due to Ca2+ ions replaced by Co2+ ones in HAP structure; since the cobalt amount increased, a dispersion of nanosized cobalt species was suggested. Copper-containing hydroxyapatites showed the HAP and monetite phases. Increased copper contents provided the ionic exchange of Ca2+ by Cu2+ and resulted in the libethenite (Cu2(OH)(PO4)) phase formation as well as the deposition of CuO on solid surface. These solids possessing distinct morphologies had micro and mesopores and preserved the HAP and monetite phases, even increasing the copper content. The catalytic evaluation in the dehydration of glycerol was dependent on the content of the metals as well as the nature of the active phase. Generally, copper-containing samples were more active than those possessing cobalt, probably due to the either stable libethenite phase presence or great acidity of the formers. The catalytic performance of the solids with low copper contents showed the elevated selectivity to 1-hydroxyacetone and products from its dimerization–cyclysation reactions whereas samples possessing cobalt were selective to 1-hydroxyacetone and light products.