Improvement of the stability of aluminum oxide support in the presence of sulfur-contaminated volatile organic compounds

Abstract

Methanol (CH3OH) and methyl mercaptan (CH3SH), considered as volatile organic compound (VOC) and sulfur containing volatile organic compound (S-VOC) respectively, have health and environmental effects that can range from local irritating odorous gases to global effects such as climate change. An efficient method for VOC removal is the catalytic oxidation technique. However, sulfur present in the S-VOC can effectively poison the commonly used noble metal catalysts during the oxidation reaction. Hence, finding more active and stable catalysts for the environmental application of the S-VOC oxidation is highly demanded. The work in this thesis concentrates on the improvement of the stability of alumina support for vanadia catalysts used in the oxidation of methanol and methyl mercaptan. First, vanadia catalysts supported by the promoted supports were prepared to obtain a weight composition of 80% (support), 20% (promoter) and 1.5% (active phase). The alumina supports were promoted by silica, gadolinia, hafnia and prepared by two methods: conventional impregnation and sol-gel. The active phase vanadia was added on those supports by wet impregnation method. The prepared fresh catalysts were characterized by XRay Fluorescence, X-Ray Diffraction, N2 Physisorption, Raman Spectroscopy, Temperature Programmed Reduction and NH3-TPD. Furthermore, these catalysts were tested for oxidation activity. It was found that all the fresh catalysts achieved similar results in terms of selectivity and activity. Second, the study of the sulfur effect on the catalytic performance was done on catalysts supported by the sol-gel promoted alumina. The catalysts were poisoned through an accelerated aging procedure and tested in the oxidation reaction. It was found that the poisoned catalysts achieved a higher activity and selectivity at lower operating temperatures than in their fresh states. The poisoned silica promoted catalyst showed the highest total conversion of 92% and selectivity of 61% at 445℃. This is due to the absorbed state of SO2 in the form of SO4 containing the lattice oxygen responsible for the oxidation reaction by vanadia. Furthermore, the resistance towards sulfur impurities is based on the reaction mechanism of the catalyst and its high BET surface area and high surface acidity.