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

Different compositions of calcium-copper Ca10.5-xCux(PO4)(7) (0 less than or equal to x less than or equal to 1) and calcium-sodium-copper phosphates Ca10-x/2NaxCu0.5 (PO4)(7) (0 less than or equal to x less than or equal to 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 beta-Ca-3(PO4)(2) The catalytic behaviour of these series of phosphates was studied in butan-2-ol conversion. Purl tricalcium phosphate beta-Ca-3(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. P-31 MAS NMR investigations of Ca-10.5(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. (C) 2000 Elsevier Science B.V. All rights reserved.