| Cyclohexene,which has an active double bond,is widely used as an intermediate for the production of medicine,food,dyes and polyester materials etc.It can be used to prepare chemicals with high additional values such as cyclohexanol,adipic acid and caprolactam.The route via selective hydrogenation of benzene to prepare cyclohexene possesses cheap feedstock,safe process,atomic economy and clean production.Thus it has a great application prospect.However,the hydrogenation of benzene to prepare cyclohexane is thermodynamically much more favorable.Development of catalyst is the key to improve the selectivity to cyclohexene for selective hydrogenation of benzene in the four-phase reaction system.In this work,we developed Ru-based catalysts with Zn promoter and investigated the effects of preparation methods on catalyst structures and their catalytic performance.The catalysts were characterized by electron microscope,N2 adsorption-desorption isotherms,H2 temperature programmed reduction and X-ray diffraction etc.We explored the relationship between the properties of catalysts and their catalytic performance.(1)Ru-Zn catalysts prepared by co-precipitation at low alkalinity As reported in literatures,most of Ru-Zn catalysts were prepared by co-precipitation at high alkalinity,which needed repeated washing with alkaline solution and produced a lot of wastewater.Therefore,we proposed the method to prepare Ru-Zn catalyst by co-precipitation at low alkalinity.We investigated the effects of the ZnCl2 amounts and controlled the Zn content of the catalyst by controlling the ZnCI2 amount.The optimized Zn content was 16.7%by weight.Under the condition of 1200 r/min,150?,5 MPa H2,selectivity of cyclohexene of the catalyst reached 80%at a benzene conversion of 47%.The activity index?50(mass of converted benzene per hour over 1 g Ru at a benzene conversion of 50%)was 204g/(g·h)and the catalytic performance maintained the same after it was recycled for five times.The reaction condition was optimized.We found that the stirring speed should be in the range between 1100 r/min and 1300 r/min.The increase of the concentration of ZnSO4 in the reaction system resulted in improved activity as well as depressed selectivity.When the concentration of ZnSO4 was about 0.4 M,the selectivity of cyclohexene reached 80%.Also,we studied the effects of the NaOH amounts in the co-precipitation process and found that it had a great influence on the amount and morphology of ZnO species.With increased alkalinity,the ZnO crystals transformed from a few cones with large size to a large amount of spindly needle-like crystals.The Zn content first increased then decreased.When the pH of the solution after reduction was about 10,the catalyst got highest Zn content(22.1%)and strongest interaction between ZnO crystals and Ru particles,thus its activity was lowest.We proposed that the activity of Ru-Zn catalyst prepared by co-precipitation was determined by Zn content,the interaction between ZnO crystals and Ru particles,and the Ru particle size.Meanwhile the selectivity to cyclohexene was determined by Zn content.Increasing Zn content and the interaction between ZnO crystals and Ru particles and the Ru particle size,the surface area and activity of catalyst decreased while the selectivity of cyclohexene increased.When the pH of reduction solution was controlled as 6,the Zn content of the Ru-Zn catalyst was about 19%.ZnO crystals were stable and the selectivity index S40 reached 80%.Using ZnSO4 as precusors to prepare Ru-Zn catalyst instead of ZnCl2,the Zn species were different.Increasing the ZnSO4 amount with fixed RuCl3 and NaOH amounts,the Zn content of the catalyst first increased then decreased and the Zn species changed from needle-like ZnO to hexagonal ZnO prisms and finally flake zinc sulfate hydroxide hydrate.When the Zn/Ru ratio by weight reached 0.23,Zn species were all Zn(OH)3·ZnSO4·H2O salts.The catalytic performance of above catalysts was similar with catalysts prepared by ZnCl2 precusors.That is,the selectivity of cyclohexene reached 80.7%at a benzene convertion of 45.5%when the reaction time was 40 min.(2)Ru-Zn catalyst prepared by H2 reduction in Zn2+ solution We also investigated the preparation method which introduced Zn species into Ru-based catalysts by H2 reduction in Zn2+ solution.The influences of RuCl3 and Ru(OH)3 precusors on the catalyst structures and its catalytic performance were studied.Ru-Zn-A series catalysts prepared by direct reduction of RuCl3 in ZnSO4aqueous solution had low surface area(<8 m2/g)and low Zn content(Zn/Ru<0.03wt/wt).The reduction time influenced Zn content and the valence states of Ru and Zn metals.The Ru-Zn-A3 catalyst which was reduced for 12 h had highest Ru0/Ru + and Zn/ZnO ratio,as a result,it had the highest activity and selectivity to cyclohexene for selective hydrogenation of benzene.Its activity index?50 was 169.2 g/(g·h)and the selectivity to cyclohexene reached 82.4%with a benzene conversion of 42.9%.On the other hand,Ru-Zn-B series catalysts were prepared by reducing Ru(OH)3 in different types of Zn2+ solution.Ru-Zn-B series catalysts got relatively high surface area(70?110 m2/g)and Zn content(Zn/Ru>0.11 wt/wt),in which Zn species existed as ZnO crystals.Increasing the alkalinity of the solution before reduction improved the Zn content of Ru-Zn-B catalysts.Zn/Ru ratio by weight of Ru-Zn-B4 catalyst was0.23 and its selectivity to cyclohexene reached 76.1%at a benzene conversion of45.4%when reacted for 20 min.(3)Ru-Zn/ZrO2 catalysts prepared by reaction-adsorption method In order to increase the utilization of Ru metal and make Ru and Zn species more uniform,we proposed the reaction-adsorption method with the assist of intermediate metal to prepare Ru-Zn/ZrO2 catalysts.First,La(OH)3 was introduced into ZrO2support.Then Ru3+ reacted with La(OH)3 and was loaded.The reduction in ZnSO4solution introduced Zn2+ and finally we got Ru-Zn/ZrO2 catalyst.By monitoring the preparation process and characterizing samples in the preparation process,we confirmed the preparation process and controlled Ru and Zn contents by adjusting the La content.With fixed amount of RuCl3,part of RuCl3 could not be loaded on support when the La/Ru molar ratio was only 0.5.When the La/Ru molar ratio changed from0.8 to 1.7,Ru contents of all the catalysts were the same(11.5%),which meant all Ru3+ were loaded on the support,and the Zn contents increased from 1.83%to 7.74%.Ru and Zn species of Ru-Zn/ZrO2 catalysts prepared by reaction-adsorption method were uniformly dispersed and the Ru metal was about 2?3 nm.There were interaction between Ru and the support.With increasing the Zn content,the activity decreased and selectivity to cyclyhexene increased.The selectivity S40 reached 78%on the condition that La/Ru molar ratio was 1.4?1.7 and Zn/Ru mass ratio was 0.31?0.55.The activity index ?40 of the catalyst with the La/Ru molar ratio of 1.4 was 1670g/(g·h)and this catalyst showed similar catalytic performance during five tests.The effects of loading amounts of Ru were also investigated.When the Ru loading amount was 5%and Zn/Ru mass ratio was 0.55,the activity index ?40 of the catalyst was 1233g/(g h)and the selectivity to cyclohexene was 78.8%.The preparation and reaction conditions were also inspected.Optimized conditions were as follows:concentration of ZnSO4 solution in the reduction process was 0.75 M,reduction time was 10?30 h,the amount of catalyst was 0.3 g,concentration of ZnSO4 solution in the reaction system was 0.4 M.The activity of catalysts prepared by reaction-adsorption method was as eight times as that of catalysts by co-precipitation method,but their selectivity was slightly lower than the latter. |
PDF Full Text Request