| In order to improve the quality of gasoline and enable it to better meet environmental protection regulations, there has been an increasing demand for processes that can produce clean fuels with non-aromatic hydrocarbons and high octane numbers. One potential solution is to isomerize n-C7 and higher n-paraffins, which are a major constituent of some crude oil fractions, to their high-octane-number branched isomers as octane-enhancing components. Research on n-heptane isomerization has focused on Pt or Pd catalysts loaded on zeolites, SAPO molecular sieves, WO3/ZrO2 and heteropolyacids, because these kinds of catalysts have good iso-C7 selectivity.Sulfated zirconia (SZ) has received a lot of interest in the last few decades due to its strong acidity, and thus high activity to catalyze many reactions (e.g. n-alkanes isomerization) even under mild conditions. The Pt/SZ catalyst has been commercialized in the isomerization process of C5-C6 paraffins, giving a higher RON of products than the zeolite catalyst because of lower reaction temperature. In contrast to skeletal isomerization of C4-C6 n-alkanes,n-C7 isomerization prones to cracking, leading a low iso-C7 selectivity especially when the conversion is high. Over the Pt/SZ catalyst, there are few reports on its catalytic performance in n-C7 isomerization because of poor iso-C7 selectivity, which is attributed to weak metal function of Pt in Pt/SZ. Hence, achieving high n-C7 isomerization selectivity at high conversion is a big challenge for Pt/SZ.Although it has been reported that the addition of small amounts of Al2O3 to Pt/SO42-/ZrO2 enhanced the catalytic activity and selectivity for n-heptane isomerization, further research is still required to figure out the mechanism of promoting effect of Al2O3. In the present study, we focus on exploring new additives to promote catalytic activity and selectivity of Pt/SO42-/ZrO2 for n-heptane isomerization and elucidating the promoting reasons of additives. The main contents of this thesis are as follows.Constant pH co-precipitation method was used to synthesis Fe2O3-doped Pt/SZ catalysts (Fe2O3 content 1-5 wt%, SO42- loading 1.5-9 wt%). We investigated the influences of SO42- loading and Fe2O3 content on their catalytic activity and selectivity in n-heptane isomerization. The results indicate that the incorporation of small amounts of Fe2O3 into Pt/SZ improves its activity remarkably, and under comparable conversion, the selectivity of Fe2O3-doped catalysts is significantly higher than that of Pt/SZ catalyst, especially when the conversion is higher than 55%. When SO42-loading is 4.5% and Fe2O3 content is 3.5%(labelled as Pt/4.5SZF3.5), the catalytic performance is best, giving significantly higher conversion and selectivity than Pt/SO42-/ZrO2 catalyst. Under the WHSV of 0.9 h"1 and the reaction temperature of 190℃, the conversion of n-heptane on Pt/4.5SZF3.5 declines slightly at the beginning 1.5 h, but in the subsequent reaction time (from 1.5 h to 80 h), the activity of the catalyst remains stable, whose conversion is about 77% and iso-C7 selectivity is about 85%. We discussed the promoting reasons of Fe2O3 from the aspects of crystal structure, acidity, metal function of Pt and etc., which were characterized by XRD, cumene cracking, toluene hydrogenation and other methods. The results indicate that the incorporation of small amounts of Fe2O3 into Pt/SZ improves its activity in H-heptane isomerization because of two reasons. Firstly, the incorporation of Fe2O3 increases the amount of B acid sites on the catalyst surface, which should also participate in the skeletal isomerization of n-alkanes. Secondly, the transformation of ZrO2 from the metastable tetragonal phase to the monoclinic phase is retarded upon the incorporation of Fe2O3, thus improves the intensity of tetragonal ZrO2. The reason for an increase in C7 isomers selectivity is as follows. The incorporation of Fe2O3 improves the metal function of Pt on the catalysts, thus improves the hydrogenation-dehydrogenation function of the catalysts, reaching a good balance with acidity.The same method was used to synthesis Co2O3-doped Pt/SO42-/ZrO2 catalysts (CO2O3 content 1.5-6 wt%, SO42- loading 1.5-9 wt%). We investigated the influences of SO4- loading and Co2O3 content on their catalytic activity and selectivity in n-heptane isomerization. The results indicate that the incorporation of small amounts of CO2O3 into Pt/SZ improves its activity remarkably, and under comparable conversion, the selectivity of Co2O3-doped catalysts is significantly higher than that of Pt/SZ catalyst, especially when the conversion is higher than 60%. When SO42-loading is 3% and Co2O3 content is 4.5%(labelled as Pt/3SZC4.5), the catalytic performance is best, giving significantly higher conversion and selectivity than Pt/SO42-/ZrO2 catalyst. Under the WHSV of 0.9 h-1 and reaction temperature of 205℃, the conversion of n-heptane on Pt/3SZC4.5 declines slightly at the beginning 5 h, but in the subsequent reaction time (from 5 h to 80 h), the activity of the catalyst remains stable, whose conversion is about 72% and iso-C7 selectivity is about 90%. We discussed the promoting reasons of Co2O3 from the aspects of crystal structure, acidity, metal function of Pt and etc., which were characterized by XRD, cumene cracking, toluene hydrogenation and other methods. The results indicate that the incorporation of small amounts of Co2O3 into Pt/SZ improves its activity in n-heptane isomerization because of two reasons. Firstly, the incorporation of Co2O3 increases the amount of B acid sites on the catalyst surface, which should also participate in the skeletal isomerization of n-alkanes. Secondly, the transformation of ZrO2 from the metastable tetragonal phase to the monoclinic phase is retarded upon the incorporation of Co2O3, thus improves the intensity of tetragonal ZrO2. The reason for an increase in C7 isomers selectivity is as follows. The incorporation of Co2O3 improves the metal function of Pt on the catalysts, thus improves the hydrogenation-dehydrogenation function of the catalysts, reaching a good balance with acidity. |
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