Ethane dehydrogenation over pore-expanded mesoporous silica-supported chromium oxide: 2. Catalytic properties and nature of active sites

Abstract

Pore-expanded mesoporous MCM-41 silica-supported chromium oxide catalysts with different chromia loadings were synthesized using three different methods. The catalysts were thoroughly characterized and used for ethane dehydrogenation (DH). Reaction studies revealed that the ethane conversion and coke formation on the catalyst depend on the chromia loading and the catalyst preparation method. All catalysts were highly selective toward ethylene. However, among the three catalysts containing 5 wt% chromia, the 5 wt% Cr/PE-MCM-41 catalyst exhibited the best performance with an ethane conversion of 23% and ∼99% ethylene selectivity at 650 °C (C 2H 6/N 2 = 0.176, total contact time = 0.0032 g min/mL). The chromia catalysts prepared using as-synthesized pore-expanded mesoporous silica (Cr/PE-MCM-41) also exhibited the highest stability. XPS analysis revealed the presence of three different chromium, namely Cr(II), Cr(III) and Cr(VI), whose overall surface concentration as well as relative content varied with the reaction time-on-stream (TOS). Furthermore, the nature and relative amounts of surface chromium species were monitored by in situ XPS measurements as a function of TOS. Correlation of the XPS findings with ethane DH data showed clearly that the Cr(III) species are the main active sites.