| WO3/ZrO2type solid superacids have attracted much attention recently, because they are noncorrosive, environment-friendly and reusable at high temperature. The typical feature of these solid superacids is their high acidity and stablity. Compared with those traditional solid acid catalysts, they make up the deficiency in acid strength, and meet the requirement for the strong acid-catalyzed reactions and catalyze the reactions at relatively low temperatures. As a result, energy can be saved and side reactions are reduced. Compared with SO42-/ZrO2type solid superacids, despite their weaker acid strength, WO3/ZrO2type solid superacids present much better stability under H2, O2and H2O atmosphere in the course of the heterogeneous catalysis. Therefore, they are recognized as a kind of novel green catalytic materials having potential applications.In order to improve the quality of gasoline to meet the need of environment protection requirements, the demand for processes to produce clean fuels with high octane numbers and non-aromatic hydrocarbons has increased enormously. The hydroisomerization of C7+n-alkanes (C7and higher paraffins) to their branched isomers with high octane number is among the most promising routes, in which the isomers can act as octane-enhancing components when added into gasoline. Pt supported WO3/ZrO2has recently emerged as the most promising catalyst of a new generation in C7+hydroisomerization due to its high catalytic efficiency. Compared with the traditional Pt/zeolite catalysts, Pt/WO3/ZrO2exhibits higher activity at lower temperature due to its stronger acidity, and higher selectivity and better stability comparing with Pt/SO42-/ZrO2catalysts.Although it has been reported that the addition of small amounts of Al2O3and Ga2O3to Pt/WO3/ZrO2enhanced the catalytic activity and selectivity for n-heptane isomerization, further research is still required to figure out the mechanism of promoting effect of those promoters. In addition, there has been no report on influences of preparative conditions and preparative methods on the catalytic behavior. In the present study, Al2O3or Ga2O3-doped Pt/WO4/ZrO2catalysts were prepared using constant pH coprecipititation method. We observed catalytic activity and selectivity in n-heptane isomerization and elucidated the promotion reasons of additives, also compared the influences of preparative conditions and preparative methods on the catalytic behavior. The main contents of this thesis are as follows.Constant pH coprecipititation method was used to synthesis Al2O3-doped Pt/WO3/ZrO2(Al/Zr molar ratio0.005-0.08, WO3content9-24wt%) catalysts, several techniques including XRD, N2-adsoption, SEM, TEM, XPS, CO chemsoption, cumene cracking and etc. were applied to characterize the structure, physicochemical properties and acidity of the catalysts, and their catalytic behavior for n-heptane hydroisomerization was examined. The results indicate that the incorporation of small amounts of Al2O3into Pt/WO3/ZrO2would improve the catalyst activity remarkably. When Al/Zr molar ratio is0.02and WO3content is15wt%, the hydroisomerization of n-heptane reaches the highest on Pt/WO3/Al2O3-ZrO2(denoted by Pt/15WAZ0.02), under the reaction temperature of200℃and WHSV of0.9h-1. The conversion of n-heptane on Pt/15WAZ0.02is about80%, which is significantly greater than Pt/15WZ (40%), meanwhile the catalyst shows no obvious deactivation over an extended80hours reaction. Furthermore, the selectivity of C7isomers increases on Al2O3-doped Pt/WO3ZrO2. especially when the conversion is above60%. For example, when the hydroisomerization of n-heptane reaches84.8%, the selectivity of C7isomers is91.5%on Pt/15WAZ0.02, but only77.3%on Pt/15WZ. In view of the characterization results, on the one hand, the incorporation of small amounts of Al2O3into Pt/WO3/ZrO2increases the dispersion of Pt and L acid sites on the support, which are beneficial for improving B acid concentration on the catalyst surface. At the same time, the enhancements of Pt dispersion and W6+reductibility, are also beneficial to increase the concentration of carbenum ions on the catalyst surface, and thus greatly improve the isomerization activity. On the other hand, the introduction of Al2O3contributes to forming more H-in H2, accelerates the hydride transfer reaction of branched-C7+with H-, and consequently enhances the selectivity of C7isomers. The preparative method had a significant influence on its activity for n-heptane hydroisomerization. Alumina-doped Pt/WO3/ZrO2catalysts were prepared by the constant and variable pH methods, respectively. The catalyst prepared by the constant pH method has higher Pt dispersion and its support has more L acid sites, leading to generation of more B acid sites on this catalyst in H2, which is the reason of higher activity for n-heptane hydroisomerization observed on the Pt/WO3/Al2O3-ZrO2catalyst prepared by the constant pH method than variable pH method. The same method was used to synthesis Ga2O3-doped Pt/WO3/ZrO2(Ga/Zr molar ratio0.005-0.045, WO3content9-24wt%) catalysts. The structure, physicochemical properties and acidity of the catalysts were characterized, and their catalytic performace for n-heptane hydroisomerization was examined. The results indicate that the incorporation of small amounts of Ga2O3into Pt/WO3/ZrO2would improve the catalyst activity remarkably. The hydroisomerization of n-heptane reaches the highest on the Pt/WO3/Ga2O3-ZrO2catalyst when the Ga/Zr molar ratio is0.02-0.04and WO3content is15-18wt%, under the reaction temperature of180℃and WHSV of0.9h"1. The conversion of n-heptane on the Pt/15WGZ0.02is about82%, which was significantly greater than Pt/15WZ (29%), meanwhile the catalyst shows no obvious deactivation over an extended80hours reaction. Furthermore, the selectivity of C7isomers increases on Ga2O3-doped Pt/WO3/ZrO2, especially when the conversion is above60%. For example, when the hydroisomerization of n-heptane reaches81.9%, the selectivity of C7isomers is92.2%on Pt/15WGZ0.02, which is higher than on Pt/15WZ (80.5%). The reasons of higher catalytic activity on Pt/WO3/Ga2O3-ZrO2catalyst are as follows, on the one hand, the incorporation of small amounts of Ga2O3into Pt/WO3/ZrO2increases the dispersion of Pt and L acid sites on the support, which are beneficial for improving B acid concentration on the catalyst surface. On the other hand, the enhancements of Pt dispersion and W6+reductibility, are also beneficial to increase the concentration of carbenum ions on the catalyst surface, and thus greatly improve the isomerization activity. The reason of higher selectivity of C7isomers on Pt/WO3/Ga2O3-ZrO2catalyst is owing to the introduction of Ga2O3, which contributes to forming more H" in H2and accelerates the hydride transfer reaction of branched-C7+with H-. |
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