Effect Of Al-modified KIT-6 Supported Tungsten Oxide On The Self-disproportionation Reaction Of 1-butene

At present,the main production processes of propylene include catalytic cracking,steam cracking,methanol to olefins and propane dehydrogenation.However,these process technologies have problems such as the production of a large number of C4 olefin by-products and the low yield of propylene.Therefore,this paper selected the 1-butene selfdisproportionation reaction to prepare propylene,which can effectively solve the problems existing in the traditional production process.Because the raw material of the reaction was 1-butene,1-butene was first required to undergo isomerization reaction to generate 2-butene,and then 1-butene and 2-butene undergo disproportionation reaction to generate propylene,therefore,a bifunctional catalyst with both acidic and disproportionate activity was required.(1)In this paper,the thermodynamic analysis of the self-disproportionation reaction of 1-butene was carried out using calculation software.It was found that when the reaction temperature increased,the conversion rate of 1-butene and the yield of propylene gradually increased.When the temperature was raised to 350℃,the theoretical yield of propylene can reach a maximum of 28.9 mol/%.But the propylene yield decreased when the temperature continued to increase,because the high temperature would produce more isomeric products and C5,C6 olefins.Therefore,the reaction temperature of 350℃ was most favorable for the formation of propylene.When the reaction pressure was changed,the 1-butene conversion rate and the yield of each component change slightly,therefore,the 1-butene self-disproportionation reaction was selected to be carried out at atmospheric pressure.(2)A series of Al-KIT-6 carriers were synthesized by doping different amounts of aluminum,and the disproportionation performance test was carried out with 1-butene as the reactant with ammonium metatungstate as the precursor,and the disproportionation performance test was carried out with excessive impregnation method supporting different amounts of tungsten oxide as the catalyst.The properties of the catalyst and support were characterized by XRD,N2 adsorption-desorption,Py-IR,H2-TPR and NH3-TPD,the result showed that increasing the doping amount of aluminum could not only increase the acidity of the support and provided a large number of acid sites for the catalyst,but also the active tungsten oxide species could be highly dispersed on the surface of the carrier and it could have a four-coordinate structure.When the Si/Al ratio was 5,the molecular sieve channel structure collapsed due to excessive aluminum doping,which reduced the dispersion of tungsten oxide on the surface of the support was reduced,and tungsten oxide crystal particles were formed.When the Si/Al ratio was 10,the molecular sieve still retained a good pore structure,the acidity and amount of acid of the carrier were moderate,and the dispersion degree of tungsten oxide on the surface of the carrier was increased.Tungsten oxide formed a large number of tetracopid tungsten oxide species.By changing the amount of aluminum doping and the amount of tungsten oxide load,the results found that the surface tungsten oxide species of 10%W/10A1-KIT-6 catalyst were the most dispersed.(3)In a self-assembled fixed-bed reactor,the effect of the above supported tungsten-based catalyst on the self-disproportionation reaction of 1-butene was investigated.The results showed that the yield of propylene gradually increased with the increase of the doping amount of aluminum.When the Si/Al ratio was 10,the yield of propylene could reach 14.4 mol%;the yield of propylene gradually decreased with the increase of the doping amount of aluminum.Combined with the results of catalyst acidity and structure characterization,it can be seen that when 10Al-KIT-6 was used as the support and the tungsten oxide load was 10%as the catalyst,the acidity and acid content of the catalyst were moderate,and the dispersion degree of tungsten oxide was the best and presents a large number of tetracoordinates structure,which was beneficial to improve the disproportionation reaction activity.(4)The 10%W/10A1-KIT-6 catalyst was applied in the self-disproportionation reaction of 1-butene,and the effects of different space velocities of the raw materials and reaction temperature on the reaction were investigated.The results showed that with the increase of the reaction temperature,the conversion rate of 1-butene and the production of propylene decreased after the first increase.When the temperature reached 350℃,the conversion rate of 1-butene and the yield of propylene reached the maximum values,58.2 mol%and 14.4 mol%,respectively.The experimental results are consistent with the thermodynamic results analysis.With the increased of the airspeed of 1-butene,the conversion rate of 1-butene decreased,and the yield of propylene showed a trend of first increasing and then decreasing,because when the airspeed increased,carbon deposits could be reduced,which was conducive to the increase of propylene yield,but continue to increase the airspeed,the catalyst and the reactants contact time was too short,the propylene yield decreased,and the disproportionation activity decreased.