Active sites in sol–gel prepared silica-alumina for photoinduced non-oxidative methane coupling

The photoinduced non-oxidative methane coupling at room temperature was examined by using a series of silica-alumina samples prepared by a sol–gel method, in order to clarify the photoactive sites of silica-alumina. The maximum yield of ethane, the main product in this reaction condition, was obtained on the sample containing around 20 mol% of Al. The variation of ethane yield was consistent with the variation of UV-absorption intensity. Ethene as the minor product, which is obtained by thermal desorption after photoreaction, was produced on the samples of larger Al content. The aluminium sites in the low Al content samples exhibited higher specific activity and higher ethane selectivity than those in high Al content samples. The silica-alumina sample was characterized by using N2-adsorption, XRD, XPS and Al K-edge XANES. In addition to UV spectroscopy, phosphorescence emission spectroscopy was also applied to clarify the photoactivation properties of silica-alumina. It is concluded that the highly dispersed and isolated tetrahedral AlO4 species in silica matrix, which exhibit the fine structural phosphorescence spectra centred at 520 nm, would be the highly active sites for ethane formation. On the other hand, the aggregated Al oxide species would be the active sites for producing both ethane and ethene with low activity.