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Calcium–Lead Hydroxyapatites: Thermal and Structural Properties and the Oxidation of Methane

“…Thus, for example, in the oxidative coupling of methane the conversion and the selectivity to C > hydrocarbons have been observed to increase while recent results reveal increased selectivities to carbon monoxide in the partial oxidation of methane on stoichiometric and nonstoichiometric calcium (41)(42)(43)47) and strontium (45)(46)(47) hydroxyapatites. In contrast, with strontium hydroxyapatites ion-exchanged with lead, methyl chloride with selectivities as high as 70% was obtained with TCM in the feedstream (48,49). The effects of the addition of TCM have tentatively been attributed to the formation of chlorapatites resulting from the exchange of hydroxyl groups by chlorine in the hydroxyapatites (41-43, 45, 46, 48, 49).…”

Facile Formation of the Partial Oxidation and Oxidative-Coupling Products from the Oxidation of Methane on Barium Hydroxyapatites with Tetrachloromethane

“…7 However, the selectivity to CO 2 (77%) on BaHAp was evidently greater than that on CaHAp (less than 40%), 7 while the selectivity to C 2 compounds (ethane and ethylene) on BaHAp (less than 7%) was smaller than that on CaHAp (20-30%). 7 Although it is impossible to compare the activities on BaHAp at 973 K with those on SrHAp due to the thermal instability of the latter at 973 K, SrHAp produces CO in high selectivity at 873 K. 10 Upon addition of TCM at 0.085 kPa into the feedstream, the conversion of methane and selectivity to C 2 compounds increased slightly at 0.5 h on-stream. In contrast, at 6 h on-stream on introduction of TCM (0.085 kPa) the conversion and selectivity to CO 2 decreased markedly, as observed on other hydroxyapatites, [5][6][7][9][10][11][12]16,17 while the selectivity to C 2 compounds increased 7-fold, as observed on a wide variety of catalysts other than hydroxyapatites.…”

“…[3][4][5][6][7] On introduction of small partial pressures of tetrachloromethane (TCM) to the feedstream, the conversion of methane decreased while the selectivity to CO increased with increasing time-on-stream. [5][6][7] However, carbon monoxide was produced selectively regardless of the addition of TCM on strontium hydroxyapatites (SrHAp), [8][9][10] and in the presence of TCM, methyl chloride also formed with selectivities exceeding 50%. 11,12 In contrast, significant selectivities to C 2 compounds were observed on CaHAp [13][14][15][16][17] and SrHAp, 11,12 both doped by lead.…”

“…[5][6][7] However, carbon monoxide was produced selectively regardless of the addition of TCM on strontium hydroxyapatites (SrHAp), [8][9][10] and in the presence of TCM, methyl chloride also formed with selectivities exceeding 50%. 11,12 In contrast, significant selectivities to C 2 compounds were observed on CaHAp [13][14][15][16][17] and SrHAp, 11,12 both doped by lead. Upon addition of TCM, the deep oxidation to CO 2 was depressed on both lead-doped catalysts and concomitantly the selectivities to CH 3 Cl and/or CO increased.…”

“…Upon addition of TCM, the deep oxidation to CO 2 was depressed on both lead-doped catalysts and concomitantly the selectivities to CH 3 Cl and/or CO increased. 11,12,16,17 The apatite structure [M 10 (PO 4 ) 6 (OH) 2 ], where M is a divalent cation, X is P, As, Si, or S, for example, and Z is OH or one of the halogens, is a solid of considerable flexibility in its elementary compositions. 18 One of the more common forms, CaHAp [stoichiometric form, Ca 10 (PO 4 ) 6 (OH) 2 ], has received considerable attention, 19 and earlier studies from a number of laboratories have shown that CaHAp is a bifunctional catalyst, with acidic and basic properties, depending on their composition [Ca 10-x (HPO 4 ) x (PO 4 ) 6-x (OH) 2-x ; 0 e x e 1].…”