| As a kind of high octane gasoline components without olefin,aromatics sulfur and other impurities,the paraffin isomerized gasoline is in line with the development trend of future gasoline standards and the environmental requirements,so it has been paid attention to and developed vigorously in recent years.As a kind of new solid-acid catalysts,sulfated zirconia showed excellent reactivity in the low temperature isomerization of alkanes,which was considered as a kind of isomerization catalysts with industrial application prospect.Sulfated zirconia was characterized by super acid properties,good thermal stability,renewable,simple preparation process,however,there were some problems when it was used in the isomerization.Based on it,this paper planned to prepare a series of solid superacid catalyst from three aspects:carrier modification,active component modification and active metal component modification.Then the n-hexane isomerization reactivities of the catalysts were investigated in a fixed-bed micro reactor,and the physical and chemical properties of catalysts would be analyzed systematically by XRD,BET,SEM,TEM,TG-DSC,FT-IR,Py-IR,NH3-TPD,H2-TPR,XPS,MAS NMR et al.Later,the structure and activity of the catalyst were correlated and on this basis,the deactivation and regeneration behaviors were studied,and the mechanism of isomerization on catalyst was explored.In this paper,the main research contents are as follows:Firstly,the Pd loaded sulfated zirconia was prepared by traditional precipitation-impregnation method in this thesis,and the optimum preactivation/prereduction conditions and mechanisms were studied.The results showed that,the pretreatment could influence the sulfates,acid centers and metal centers on the surface of catalyst,and too high reduction temperature could lead to the decomposition of sulfates and the decrease of reactivity.Activity results displayed that,the sulfated zirconia showed the best reactivity after it was activated in air at 450 ? then reduced at 200 ?,the initial conversion rate was up to 91%,and the initial selectivity was up to 30%.Compared with the superacid and mordenite in the structure/reactivity results,it can be seen that the strong acid center was the key to the high isomerization activity of sulfated zirconia at low temperature.Next,Al(NO3)3 and pseudohyalite were used as the source of alumina,and coprecipitation and kneading methods were used to prepare series of alumina modified sulfated zirconia.Combined with the characterization results,it could be seen that the amount and method of adding alumina would influence the physical structure,microstructure,acid property,reduction property and catalytic activity of the catalyst:when the alumina content was low,more tetragonal zirconia proportions could be observed for the catalyst prepared by coprecipitation,and its reactivity was better than the ones prepared by kneading;when the alumina content was high,the catalysts prepared by kneading had advantages in structure and reactivity.When the catalysts were treated by washing,the loss of surface free sulfates could lead to a decrease in the acid sites,especially the Bronsted acid sites,this in turn effected the reactivity,while the presence of alumina could inhibit the loss of sulfate species and the decline of acid centers.The isomerization reactivity results showed that the high aluminum contents catalyst prepared by kneading method and washing showed a better activity and stability.The hydrothermal treatment of zirconia hydrogel would influence the hydroxyl group structure on the surface of zirconia,and then the high-temperature crystallization behavior of zirconia and the load of sulfates may be changed.As a result,the number of strong acid centers on the catalyst decreased especially the strong Bronsted acid centers,and the isomerization reactivity showed a less initial cracking rates and better stability.Later,based on the catalytic activity of superacid in the isomerization,the deactivation and regeneration of the catalyst were studied.Combined with the TG,XPS and acid characterization results,it could be seen that the change of sulfates on the catalyst and the formation of carbonaceous substances were the two main causes for deactivation.During the reaction,some sulfates were lost with the reactants,and some others were reduced to S4+and S2-because of the oxidative dehydrogenation(ODH),leading to an irreversible deactivation.During the reaction,under the action of the strong Bronsted acid sites of the catalyst,more carbon deposits were likely to be formed,and it could cover the acid sites and block the catalyst channel structure,finally the reactivity was affected.When the deactivated catalysts were regenerated under air or nitrogen,the reactivity of regenerated catalyst was greatly depended on the regeneration atmosphere and temperature,and the regeneration mechanisms were different under different regeneration atmospheres.In addition,the deactivation and regeneration of superacid catalyst could be effectively improved by adding alumina to the catalyst and by washing the catalyst with water.Finally,nickel was used as the second metal and Pd-Ni bimetallic catalyst was prepared by incipient-wetness impregnation method.The results showed that both hydrogen spillover and bimetallic synergistic effect were existed in the catalyst during reaction.Combined with the XPS and H2-TPR results,it could be seen that the introduce of trace Pd could greatly promote the reduction of Ni,and under the effect of Pd-Ni bimetal,the sulfates on the catalyst surface were easier to be reduced.At the same time,the nickel loaded could also influence the structure and other properties of the catalyst,and led to a decrease of the number of sulfates on the catalyst and a change of the acidity.The reactivity results displayed that,the catalyst loaded 0.1 wt%Pd and 1.0wt%Ni showed the best isomerization reactivity under suitable reduction conditions and reaction temperature,the initial conversion rate was up to 95%,and the stability of the catalyst was improved. |
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