A Study On Catalytic Performance Of Complete Liquid-Phase Catalysts In A Fixed Bed Reactor

Catalysis is an improtant field in chemistry,as some 90%of chemical processes involve catalysts in at least one of their steps.At present because of fastly developing science and technology,the invention of new materials,and stricter environmental regulations,new catalytic materials and new technology for preparation of catalysts are required.Traditional preparation methods of catalyst include precipitation,sol-gel method,microemulsion,impregnation and ion-exchange etc.The first three methods relate only to the catalyst of high content of active components.For low content of active component,supported catalysts can be prepared by the last two methods to load active component onto support.Various methods of preparing catalyst have respective advantages,disadvantages and applications.In recent years much attention has been dedicated to the development of slurry phase reactor for the advantages such as high efficiency of heat transfer,elimination of hot point of the bed,and easy realization of constant temperature operation.One-step synthesis of dimethyl ether(DME)is a strongly exothermic reaction,suitable to be carried out in slurry bed reactor.The catalyst is a combination of various dehydration catalyst such as ?-Al2O3 or HZSM-5 and industrial methanol catalyst,but its fast deactivation in slurry reactor hinders further industrialization.Aimed to solve the rapid deactivation of the catalyst in slurry reactor for one-step synthesis of DME from syngas,a novel method for preparing catalyst,named complete liquid-phase technique,was invented by our group according to the characteristics of catalysts used in slurry bed reactor.Preliminary studies have shown that the catalyst displayed good stability in slurry reactor,with activity comparable to the catalysts prepared by traditional methods.Our further quantum chemistry calculation indicates that the complete liquid-phase technology greatly influenced the structure and the surface property of the catalyst,and consequently influenced reaction mechanism.In order to explore if the complete liquid-phase method can be developed as a general purpose technology,this paper investigates the application of complete-liquid phase catalyst in fixed bed reactor,based on the previous research work of our group.First,an Al-based solid acid catalyst and composite catalyst Cu-Zn-Al were prepared by liquid-phase technology.Then,the catalysts were systematically evaluated with the synthesis of DME by methanol dehydration and CO hydrogenation in fixed bed reactor as model reactions and compared with the catalyst prepared by conventional co-precipitation method.catalyst in fixed bed and slurry bed reactors were explored.In the end,an improved scheme of the application of completeliquid-phase catalyst in fixed bed reactor was proposed,and experimental verification was also carried out.The main conclusions are as follows:1.The catalyst prepared by complete liquid-phase technology had different structural characteristics and surface property from tradiational catalysts.The catalyst showed good stability of methanol dehydration and CO hydrogenation in fixed bed reactor and effectiveness of ethanol synthesis.It is concluded that the complete liquid-phase technology can be developed into a universal method so as to expand the application of the catalyst in fixed bed reactor by changing the heat treatment medium while maintaining its struture and catalytic property.2.The stucture of the A1 based solid acid catalysts prepared by complete liquid-phase is AlOOH.Though the activity of the catalyst for methanol dehydration in a fixed bed reactor was low at the initial stage,it increased gradually to the activity of the catalysts prepared by traditional method and heat-treated at the same temperature.The catalyst showed excellent stability during the reaction period of 580 h in fixed bed reactor.3.The reason for the low initial activity of Al-based catalyst is that the catalyst was covered by a layer of organics,which covered the active sites partly.With the progress of reaction,the organics was desorbed,the active sites were exposed,thus the activity of methanol dehydration increased gradually.4.When the Cu-Zn-Al composite catalyst prepared by complete liquid-phase method was used for CO hydrogenation,the covering of active sites by surface carbonaceous species,along with the enrichment of Zn and Al on surface,caused low content of Cu species on surdace,and low activity of the catalyst.Research shows that,in complete liquid-phase process,the interaction of heat treatment medium with Cu was strong,which caused reduction of Cu2+,and even covering of Cu with carbonaceous layer.The carbonaceous covering of Cu was the main reason for low activity.5.The surface carbonaceous species of complete liquid-phase catalyst,which included hydrocarbons,carboxylic acids,aldehydes and ketones,existed on the surface and inside the pore of the catalysts.The binding capacity of carbonaceous species was:Cu>Zn>Al.6.Coke burning-off was an effective way for removing the surface organics of catalyst prepared by complete liquid-phase technology and resuming the activity to that of the catalyst prepared by traditional method.7.Replacing the liquid paraffin with glycerin reduced the heat treatment temperature and carbon content on the surface of complete liquid-phase catalysts.Because carbon on the surface was easy to remove,the activity of the catalyst was similar with the that of the catalysts prepared by traditional method,meanwhile its own characteristics were maintained.8.In the process of preparing complete liquid-phase catalyst,adding the surfactant PVP improved its specific surface area.Because the PVP could not be removed,the active sites were isolated,the conversion of CO decreased.