Portal de Periódicos da CAPES

Oxidation processes on stoichiometric and nonstoichiometric hydroxyapatites

1997; Elsevier BV; Linguagem: Inglês

10.1016/s0167-2991(97)81029-4

2542-6613

H. Hayashi, Hiroyoshi Kanai, Yukihiko Matsumura, Shigeru Sugiyama, J.B. Moffat,

Catalysis and Oxidation Reactions

Hydroxyapatite has a hexagonal structure constructed from columns of Ca and O atoms that are parallel to the hexagonal axis. Three oxygen atoms of each PO4 tetrahedron are shared by one column, with the fourth oxygen atom attached to a neighboring column. The hexagonal unit cell of hydroxyapatite contains 10 cations located on two sets of nonequivalent sites, 4 on site (1) and 6 on site (2) . The calcium ions on site (1) are aligned in columns, while those on site (2) are in equilateral triangles centered on the screw axes. The site-cations are co-ordinated to six oxygen atoms, belonging to different PO4 tetrahedra, and also to three oxygen atoms at a large distance. Calcium hydroxyapatite [Ca10(PO4)6(OH)2 ] is an inorganic solid that exhibits these characteristics. It does not readily lose hydroxyl groups and the lattice is believed to be stable up to 1000°C. Calcium hydroxyapatites (CaHAp) were prepared from Ca(NO3)2.4H2O (BDH AnalaR) and (NH4)2HPO4 (BDH AnalaR) according to the method previously described. Strontium hydroxyapatites (SrHAp) were similarly prepared from the corresponding strontium compound (Wako Pure Chemical). Lead cation was ion-exchanged into the hydroxyapatites following a specific procedure. Calcium hydroxyapatites of various stoichiometries catalyze the partial oxidation of methane with oxygen at 600°C to carbon monoxide, carbon dioxide, formaldehyde, hydrogen, and water. While the stoichiometric composition (Ca/P=l.65) produced predominantly carbon dioxide the molar ratio of H2/COx in the products was as high as 1.3. With a decrease in the Ca/P ratio, the H2/COx ratio decreased, while the selectivities to CO2 and CO decreased and increased, respectively, the latter at least down to a Ca/P value of 1.61, after which it remained approximately unchanged.