Strontium phosphate hydroxyapatites (Sr-HAP) with various Sr/P molar ratios were synthesized under hydrothermal conditions. X-ray photoelectron spectroscopy revealed a close correlation between the near-surface and bulk Sr/P molar ratios of catalyst samples. Sr-HAP catalysts show high 1-butanol selectivity in the gas-phase conversion of ethanol. Sr-HAP catalysts with higher Sr/P molar ratios showed higher catalytic activity and 1-butanol selectivity. The density of both relatively strong basic sites and acidic sites was seen to increase with increasing Sr/P molar ratios, though the basic site density was significantly higher than the acidic site density. The rate-determining step in ethanol conversion over Sr-HAP catalysts was considered to be the dimerization process, as aldol condensation is mainly accelerated by base catalysis, which would explain why the Sr-HAP catalysts with higher basic site density showed higher catalytic activity and 1-butanol selectivity.
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