| In recent years,light hydrocarbon aromatization,as a novel comprehensive utilization technology of low carbon hydrocarbon,has become one of the research focus in the field of industrial catalysis.The development and improvement of the catalyst is the essential issue.At present the light hydrocarbon aromatization catalyst mainly concentrate in the two series of acid and alkali.Acidic catalysts is mainly HZSM-5 and metal modified ZSM-5,which usually has higher activity,with the advantages that in addition to carbon deposition deactivation,other ways of inactivation was not obvious,but the liquid yield and selectivity of aromatics should be improved.Alkaline catalysts mainly include Pt/KL and its modified series,which has higher liquid yield and selectivity of aromatics than acid series,but the regeneration performance is poor,lack of anti carbon and sulfur resistance etc.In view of the above,the research on improvement the shortcomings of existing catalyst performance,moreover,acceleration of the development of new catalysts with high efficiency has important strategic and practical significance.In this paper,based on the existing problems and defects of light hydrocarbon aromatization catalyst,with the purpose of research and development of new and efficient L-based light hydrocarbon aromatization catalyst,lots of valuable research results are carried out through positive research and exploration work.Points are as follows:Firstly,Nano L zeolite was successfully synthesized by using the method of hydrothermal synthesis.Through systematically investigated the effect of crystallization and aging conditions on the synthesized L,the optimal synthesis conditions are optimized for L zeolite synthesis,i.e.with the crystallization temperature of 170 ?,the crystallization time of 24 h,the aging temperature of 30 ?,the aging time of 6 h.It was verified by orthogonal test as well.SEM and N2 adsorption-desorption analysis results show that L zeolite prepared by the optimal conditions was nanoscale.Catalytic conversion of n-pentane probe reaction results show that L zeolite merely has a certain activity of n-pentane isomerization,the aromatization activity is weak which depends on its pore characteristics and acid-base properties.To modify L zeolite with single metal Pd through impregnation tech.Catalytic conversion of n-pentane probe reaction results showed that the single metal Pd modified catalyst has certain advantages in terms of isomerization performance of low carbon alkane,but the aromatization performance is still poor.When reaction temperature was 480 ?,Pd/L catalyst has the best isomerization performance,which conversion of raw material was 95.6%,isomerization rate was 70.62%;When reaction temperature was 500 ?,it achieved the best aromatization performance,which conversion of raw material was 92.32%,aromatization rate was up to 12.41%.The reaction temperature and the loading amount of Pd are the sensitive factors affecting the catalyst performance.To improve the light hydrocarbon aromatization performance of modified L zeolite,Pd and Pt bimetal-modified experiment was carried out.The effects of the introduction of Pd method(impregnation and hydrothermal method)on the physicochemical properties of catalyst were studied.The results show that the introduction method of Pd had little effect on the crystal properties of Pt/KL,but more influence on its acid-base properties.The Pd introduction of hydrothermal method can reduce alkaline and increase acidic of the catalysts.Both the introduction method and doping amount of the second metal Pd have important effect on aromatization reaction of n-hexane and resistance to sulfur poisoning performance.The reaction performance of Pt-Pd/KL prepared by impregnation method was better than that of Pt/PdKL catalyst prepared by hydrothermal method,and the aromatization was the best when the loading amount of Pd is 0.2%(mass fraction).For the single function of catalyst Pt/KL,either enhance alkaline or strengthen the enrichment ability of electron of Pt can enhance the aromatization performance.The ZSM-5-L composite zeolite was successful synthesis by in-situ hydrothermal technique.Base on ZSM-5 zeolite,ZSM-5-L zeolite was firstly synthesized by single-stage crystallization.Through investigating the effect of the ratio of raw materials and pH value and other factors on the composite,the optimum synthesis conditions was confirmed,that is,n(L)/n(ZSM-5)=1:1,L phase single-stage crystallization temperature is 170 oC,reaction time is 48 h,system pH=10.5-11.5.The structure of composite molecular sieve which was synthesized according to the conditions was overgrowth crystal.In order to improve the crystal morphology of the composite molecular sieves,adopted synthesis method of L two-stage crystallization.The results showed that two-stage crystallization could regulate growth and morphology of composite zeolite.Through the organic regulation on crystallization temperature and frequency,core-shell-structured composite molecular sieve was formed.The best conditions for synthesis were confirmed,i.e.first temperature is 100 oC,crystallization time is 8 h,second temperature is 175 oC,crystallization time is 28 h.The aromatization of different morphologies and their mechanical mixing samples of ZSM-5-L composite molecular sieve were evaluated by N-pentane aromatization reaction.The experimental data show that ZSM-5-L core-shell composite molecular sieve has the optimal comprehensive performance,on account of its hierarchical pore distribution optimization and also the appropriate acidity.In the probe reaction of n-pentane aromatization,the activity and BTX yield reached the maximum when the reaction temperature was 520 oC,the optimal space velocity is 1.5~2.0 h-1.To improve the light hydrocarbon aromatization of ZSM-5/L composite zeolite,experiment of Zn metal modified was carried out.The effects of modification method(the core modification,the shell modification and the core-shell modification)on the catalyst performance were studied.The results show that the modification method has significant influence on the catalytic performance.Compared the activity from the perspective of aromatization,the aromatization rate from high to low is: Zn-ZSM-5/Zn-L>Zn-ZSM-5/L>ZSM-5/Zn-L>ZSM-5-L,among which the core-shell modified Zn-ZSM-5/Zn-L has the best aromatization performance.The optimum reaction temperature is 520 oC.Compared the activity from the perspective of isomerization,the isomerization rate from high to low is: Zn-ZSM-5/L>ZSM-5/Zn-L>ZSM-5-L>Zn-ZSM-5/Zn-L,among which Zn-ZSM-5/Zn-L has the best isomerization performance.The optimum reaction temperature is 480 oC.The single life of Zn-ZSM-5/Zn-L is about 36 hours in n-pentane aromatization reaction.Experiment of Ga and Pt bi-metal modified to ZSM-5/L composite zeolite was bring in the research.The effects of modified method,and metal loading factors influence on the aromatization performance of catalyst were investigated.The results show that bimetal grading-modified catalyst has optimal performance,furthermore,the bimetal modified Ga1.0-Z/Pt0.6-L(Ga,1.0wt.%;Pt,0.6wt.%)catalyst has the best comprehensive performance.The reason is that the grading modification could relatively continuously regulate acidity,acid type and L/B acid ratio of the catalyst,combined with the modulation of pore properties,therefore,the reaction performance of light hydrocarbon aromatization of the catalyst improved.By means of investigating the effects of reaction conditions on the catalyst performance,the optimum reaction conditions were confirmed,reaction temperature is 520 °C,WHSV is 1.5 h-1,the single reaction life of the catalyst is 144 h. |
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