Selective synthesis of 1-butanol from ethanol over strontium phosphate hydroxyapatite catalysts

Abstract

We previously reported the ability of strontium hydroxyapatite to catalyze the conversion of ethanol into 1-butanol with higher selectivity than that reported for calcium hydroxyapatite catalysts. In the present study, we investigated the catalytic conversions of ethanol over substituted hydroxyapatites, such as Sr 10(PO 4) 6(OH) 2, Ca 10(VO 4) 6(OH) 2, Sr 10(VO 4) 6(OH) 2, and Ca 10(PO 4) 6(OH) 2, and their solid solutions, such as Ca 10 − z Sr z (PO 4) 6(OH) 2, Ca 10(PO 4) 6 − x (VO 4) 6 − x (OH) 2, and Sr 10(PO 4) 6 − x (VO 4) 6 − x (OH) 2, were investigated. The strontium phosphate hydroxyapatite [Sr 10(PO 4) 6(OH) 2: Sr–P] exhibited the highest 1-butanol selectivity among the tested catalysts in the region of the ethanol conversions between 1 and 24%. The reaction mechanism of 1-butanol formation over the Sr–P hydroxyapatite catalyst includes the dehydrogenation of ethanol into acetaldehyde, the aldol condensation of acetaldehyde into crotonaldehyde, and the hydrogenations of crotonaldehyde, 2-buten-1-ol, and/or butyraldehyde into 1-butanol. The Sr–P hydroxyapatite catalyst showed high selectivity into crotonaldehyde in the aldol condensation of acetaldehyde and inhibited the coking in the hydrogen transfer reaction of 2-buten-1-ol into 1-butanol, which might be reasons why the Sr–P hydroxyapatite catalyst showed the high 1-butanol selectivity in the catalytic conversion of ethanol.