Modifying Effect Of Boron On Alkali Metal Ion-Containing FeO_x/SBA-15 Catalysts For Epoxidation Of Propylene

This dissertation contributes to the studies on modifying effect of boron on alkali metal ion-containing FeO_x/SBA-15 catalysts for epoxidation of propylene with N2O. The structures and catalytic performances of the catalysts with and without boron have been investigated in detail. The stability of catalyst and inhibition of catalyst deactivation have also been discussed.The investigation of the effect of a series of additives showed that boron significantly enhanced the PO formation activity of the K~+-FeO_x/SBA-15 catalysts. The presence of boron remarkably increased the PO selectivities as well as C3H6 conversions for the K~+-, Rb+- and Cs+-modified FeO_x/SBA-15 catalysts although the catalytic performances of the Li+ and Na+-FeO_x/SBA-15 catalysts were not obviously altered by boron. The B- and K~+-doubly modified catalyst exhibited the best catalytic performance. For the K~+-FeO_x/SBA-15 catalysts with low K/Fe ratios, the modifying effect of boron was not evident. However, for the K~+-FeO_x/SBA-15 catalysts with K/Fe ratios of≥2.5, boron increased PO formation activity significantly. The optimization revealed that the B-K~+-1 wt%FeO_x/SBA-15 catalyst with B/K ratio of 0.5 and K/Fe ratio of 2.5 gives the best PO formation activity. Over this catalyst, C3H6 conversions were 4.8 and 8.9% at 350 and 375 oC, and PO selectivities were 68 and 79%, respectively. These values are even better than those reported for the KCl-FeO_x/SBA-15 catalyst.Characterizations using XRD, N2 sorption, TEM, NH3-TPD, H2-TPR, UV-Vis, UV-Raman, EPR and XPS spectroscopy suggest that the mesoporous structure is destroyed as K/Fe ratio exceeds 5, and the collapse of mesopores can change the coordination structure of iron. The presence of boron suppresses the strong interactions between K~+ and the silica wall of SBA-15, which keeps the ordered mesoporous structure and the tetrahedral coordination structure of iron species, and thus enhances the catalytic performances. We speculate that the interaction among boron, potassium ion and iron species are crucial for obtaining high catalytic performances. This interaction also inhibit the reducibility of iron species, suppressing the reactivity of lattice oxygen. Boron can regulate the acid-base property of the catalyst. Boron may also improve the electrophilicity of the oxygen species because of its electron-deficient nature. These may all contribute to the promotion for PO formation.Studies on catalyst stability reveal that similar to other catalysts, the activity of the B-K~+-1 wt%FeO_x/SBA-15 catalyst also decreases with time on stream, but it can be recovered by regenerating the catalyst. It is clarified that the catalyst deactivation is due to carbon deposition on catalyst surface. We found the addition of a small amount of O2 into the feed gas or the modification of catalyst with Ce could inhibit the carbon deposition and suppress the catalyst deactivation.