Hydroisomerization Of N-heptane Over Pt/WO₃/ZrO₂Catalyst Doped With In₂O₃and Fe₂O₃

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 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, WO3ZrO2type 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 WO3ZrO2has 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/WO3ZrO2exhibits 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 Al2O3、Ga2O3and ZnO to Pt/WO3ZrO2enhanced 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 the present study, we focus on exploring new additives to promote catalytic activity and selectivity of Pt/WO3ZrO2for n-heptane isomerization and elucidating the promotion reason of additives, we also compare the influence of precipitation methods on the catalytic behavior. The main contents of this thesis are as follows.Constant pH coprecipititation method was used to synthesis In2O3-doped Pt/WO3/ZrO2(In/Zr molar ratio0.01-0.04, WO3content9-24wt%) catalysts, several techniques including XRD, N2-adsoption, SEM, TEM, XPS, CO chemsoption, H2-TPD, 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 In2O3into Pt/WO3/ZrO2makes the catalyst form more Pt0and W5+in H2and increases the dispersion of Pt on the support, which is beneficial for improving B acid concentration on the catalyst surface. At the same time, the enhancement of Pt0concentration and Pt dispersion is also beneficial to form more heptene, thus increase the concentration of carbenum ions on the catalyst surface, and greatly improve the isomerization activity. On the other hand, the introduction of In2O3contributes to forming more H-in H2, accelerates the hydride transfer reaction of branched-C7+with H-, thus improves the selectivity of C7isomers at the same conversion level, especially when the conversion is above70%. The hydroisomerization of n-heptane reaches the highest on the Pt/WO3/In2O3-ZrO2catalyst when the In/Zr molar ratio is0.02and WO3content is15-24wt%, for example, under the condition of WHSV of0.9h"1and reaction temperature of200℃, when the WO3content of is18wt%, the conversion of n-heptane on the catalyst is about72%and the selectivity of C7isomers is about96%, and the catalyst shows no obvious deactivation over an extended80hours reaction.The same method was used to synthesis Fe2O3-doped Pt/WO3/ZrO2(Fe/Zr molar ratio0.002-0.04, 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 Fe2O3into Pt/WO3/ZrO2makes the catalyst form more Pt0and W5+in H2and increases the dispersion of Pt on the support, which is beneficial for improving B acid concentration on the catalyst surface. At the same time, the improvement of Pt0concentration and Pt dispersion is also beneficial to form more heptene, thus increase concentration of carbenium ions on the catalyst surface, and thus improve the isomerization activity significantly. On the other hand, the introduction of Fe2O3contributes to forming more H-in H2, thus accelerates the hydride transfer reaction of branched-C7+with H-, and consequently enhances the selectivity of C7isomers at the same conversion level, especially when the conversion is above70%. The hydroisomerization of n-heptane reaches the highest on the Pt/WO3/In2O3-ZrO2catalyst when the In/Zr molar ratio is0.02and WO3content is15-24wt%, for example, under the condition of WHSV of0.9h-1and reaction temperature of200℃, when the WO3content of is18wt%, the conversion of n-heptane on the catalyst is about75%and the isomerization selectivity is about95%. The catalyst shows no obvious deactivation over an extended80hours reaction. Preparative method has enormous influence on the reaction activity. The catalytic activity on Pt/WO3/Fe2O3-ZrO2catalyst synthesized through constant pH method is much better than the one through non-constant pH method, because the constant pH method leads to a higher surface area and B/L ratio as well as more B acid sites in the presence of H2.