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.