Characterisation of the active sites in butan-2-ol dehydrogenation over calcium–copper and calcium–sodium–copper phosphates Original Research Article

Different compositions of calcium–copper Ca10.5−xCux(PO4)7 (0 ≤ x ≤ 1) and calcium–sodium–copper phosphates Ca10−x/2NaxCu0.5(PO4)7 (0 ≤ x ≤ 1) belonging to the whitlockite-type structure were synthesised and characterised. They crystallise in the rhombohedral space group R3c (Z = 6) and are isostructural with the tricalcium phosphate β-Ca3(PO4)2. The catalytic behaviour of these series of phosphates was studied in butan-2-ol conversion. Pure tricalcium phosphate β-Ca3(PO4)2 was found to be inactive while the samples containing Cu2+ ions exhibited mainly a dehydrogenation activity producing methyl ethyl ketone. An increase in Cu2+ concentration in the catalysts improved the dehydrogenation activity. Structural characteristics of these Cu2+ ions were investigated by spectroscopic techniques and correlated to the catalytic behaviour. UV–VIS and EPR showed that the Cu2+ ions are distributed in the phosphate lattice between two different positions: the Ca(4) and the Ca(5) sites. Progressive substitution of Cu2+ located in the Ca(4) sites by sodium resulted in a decrease in the activity. The Ca9.5NaCu0.5(PO4)7 that contains cupric ions only in the Ca(5) sites is inactive. 31P MAS NMR investigations of Ca10.5(PO4)7 and Ca10Na(PO4)7 showed that the occupancy level of the Ca(4) sites does not modify notably the symmetry of the (PO4)3− groups. UV–VIS and XPS analysis performed on the samples after a catalytic test showed that the dehydrogenation reaction lowers the amount of Cu2+ ions in the phosphate by reducing them to Cu+. It was proven that the active sites are the Cu2+ ions hosted by the Ca(4) sites.