Effects of the thermal stability and the fine structure changes of strontium hydroxyapatites ion-exchanged with lead on methane oxidation in the presence and absence of tetrachloromethane

The oxidation of methane has been investigated in the presence and absence of tetrachloromethane (TCM) on strontium hydroxyapatite with (SrPbHAp) and without (SrHAp) ion-exchanged lead. In the absence of TCM at 873 K, the introduction of lead to strontium hydroxyapatite resulted in a substantial increase in the selectivity to C-2 hydrocarbons and an increase in the conversion of methane and specific activities. In the presence of TCM, the conversion of methane and the selectivity to C-2 compounds decreased with increasing time-on-stream while that to carbon monoxide increased. However, the change in the specific activities with time-on-stream is dependent upon the composition of the catalysts. Some SrPbHAp catalysts showed evidence of transformation from apatites to the corresponding phosphates at temperatures greater than 873 K. At 773 K, at which temperature the aforementioned hydroxyapatites are relatively stable, no formation of C-2 compounds was observed in the absence of TCM, and the conversion of methane and the specific activities (with the exception of SrPb26HAp at 0.5 h on-stream) increased with the lead content of the catalysts. On introduction of TCM, the selectivities to CH3Cl and CO2 increased and decreased, respectively, on both SrHAp and SrPbHAp catalysts. The results of extended X-ray absorption fine structure (EXAFS) spectroscopic measurements suggest that the catalytic properties of SrPbHAp at 773 K in the absence of TCM are related to the nearest-neighbor distances of the Pb-O bond rather than those of the Sr-O bond. (C) 1998 Academic Press.