The oxidation of methane on PO 4 -deficient barium hydroxyapatite (BaHAp) has been investigated in the presence and absence of tetrachloromethane (TCM). In the absence of TCM, the principal product was CO 2 with less than 20% selectivity to partial oxidation and oxidativecoupling products. On introduction of a small partial pressure of TCM to the feedstream and with higher times-on-stream, the formation of CO 2 was virtually eliminated while CO and the C 2 hydrocarbon became the dominant products. The observations with TCM can be attributed to the chlorination of the catalyst and, in particular, the formation of the corresponding chlorapatite.
Oxidative coupling of methane to ethylene and ethane has been investigated on binary hydroxyapatite catalysts; calcium-lead hydroxyapatite (CaHAp-PbHAp) and barium-lead hydroxyapatite (BaHAp-PbHAp) at 973 K. Although BaHAp, CaHAp and PbHAp were inactive for the oxidative coupling of methane, those binary hydroxyapatites afforded C2 compounds rather selectively. During the oxidation on CaHAp-PbHAp catalysts, those catalysts were mainly converted to Ca5 .5Pb4.5(PO4)6(OH)2 to afford great activities while an unfavorable conversion of CaHAp to Ca3(PO4)2, which was inactive for the oxidation of alkanes, was rather suppressed. In contrast, BaHAp in the BaHAp-PbHAp catalysts was extensively converted to inactive Ba3(PO4)2, resulted in the complete deactivation on BaHAp-PbHAp, particularly on PbHAp-rich catalysts.
Binary hydroxyapatite, calcium-lead hydroxyapatite, was prepared by three different procedures; (A) the reaction with calcium and lead acetates with disodium hydrogenphosphate, (B) the surface-treatment of calcium hydroxyapatite with aqueous lead cation, and (C) kneading of the component hydroxyapatites. It was revealed that the thermal stability, the nature of the phosphate groups and the catalytic activities for the oxidative coupling of methane of the binary hydroxyapatite were strongly dependent on the preparation procedures.
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