The dehydrogenation of ethane over chromium catalysts

Abstract

The nature of the active sites for ethane dehydrogenation over chromium supported on silica gel and the effect of the support on the properties of chromium catalysts for the dehydrogenation of ethane were investigated. Low nuclearity A-type chromium(II) ions are the active sites for ethane dehydrogenation on Cr/CiO(,2) catalysts. Infrared and electron paramagnetic resonance spectroscopies were used to determine the nature of the active sites. The presence of chromium(II) ions was confirmed by chemiluminescence and hydrogen evolution studies. No evidence was found to distinguish precisely between isolated or paired chromium(II) as the correct nuclearity of the active sites on Cr/SiO(,2) catalysts. However, the higher activity of (alpha)-chromia with respect to the supported chromium catalysts suggests that pairs of chromium ions may be needed for ethane dehydrogenation. Higher dehydration temperatures of the Cr/SiO(,2) samples increased the concentration of active A-type chromium(II) ions. Also, the apparent activation energies were the same for different dehydration temperatures, suggesting that the number but not the type of site changes upon dehydration at elevated temperatures. The apparent activation energy was 28 (+OR-) 1 Kcal/mole and the ethane pressure dependency was 0.70 ((+OR-) 0.05). The active A-type chromium(II) sites were selectively poisoned by carbonaceous deposits during the dehydrogenation of ethane. The poisoning was reversible for samples with lower activities and only partially reversible for samples with higher activities. The catalytic activity for ethane dehydrogenation decreased in the order (alpha)-Cr(,2)O(,3) > Cr/Al(,2)O(,3) > Cr/SiO(,2) >> CrY. The higher activity of (alpha)-Cr(,2)O(,3) may be related to a higher concentration of pairs of chromium ions on the surface. Very good selectivities ((TURN)95%) were observed for the ethylene formation.