Photo-assisted reforming and cracking reactions of hydrocarbons using high-flux solar-simulated radiation

A xenon arc lamp with an ellipsoidal mirror, capable of producing flux densities of 132 watts/cm{dollar}sp2{dollar}, is used to investigate photoassisted catalystic reforming and cracking reactions of normal alkanes (C{dollar}sb2{dollar}-C{dollar}sb6{dollar}). Vanadium pentoxide (V{dollar}sb2{dollar}O{dollar}sb5{dollar}) on various forms of silica has been used as the catalyst. V{dollar}sb2{dollar}O{dollar}sb5{dollar} over silica gel is an effective catalyst for the reforming of hexane to benzene for both the photoassisted and thermal reactions. This form of silica contains 712 ppm of sodium and {dollar}<{dollar}5 ppm calcium. Other forms of silica which contain significant amounts of calcium ({dollar}sim{dollar}50-200 ppm) are less effective in reforming hexane to benzene. The selectivity (percent of products) of benzene for the photoassisted reaction is a function of the radiant power and the carrier gas. Using argon as the carrier gas with a radiant power of 380 watts in a 6 mm diameter focus, the benzene selectivity was 63%.; When V{dollar}sb2{dollar}O{dollar}sb5{dollar} is impregnated on silica which contains high amounts of calcium, such as a quartz frit or powdered quartz, the photoassisted reaction of hexane leads primarily to cracking into ethene. The photoassisted reaction is much more effective in this cracking reaction than the thermal reaction. Using argon as the carrier gas and a radiant power of 381 watts in a 6 mm diameter focus, the selectivity for ethene was 58% and only 3% for benzene for the photoassisted reaction. In contrast the thermal reaction using V{dollar}sb2{dollar}O{dollar}sb5{dollar}/quartz powder at 635{dollar}spcirc{dollar}C gave a selectivity for ethene of only 21% and 37% for benzene. The photoassisted reaction did not show any selective dependence on UV radiation. Apparently the radiation activates the catalyst for the cracking reaction by attaining a high surface temperature on the solid catalyst particles. This mode of activation could utilize radiation of all wavelengths and may even use the infrared region more effectively.; A mechanism is proposed for the photocatalytic cracking reaction which accounts for the unique production of ethene under irradiation.