The Study Of Alumina-Supported Solid Base Catalysts In Vapor-Phase Aldol Condensation Of Methyl Acetate With Formaldehyde

Methyl acrylate?MA?is the most versatile series of fundamental industrial monomer widely used in the manufacture of paint additives,chemical fiber,textile,paper making,and leather treating agents.There are many traditional synthesis methods of methyl acrylate,such as acetylene process,acrylonitrile hydrolysis process and propylene oxidation esterification process.These traditional methods in the production of methyl acrylate process more or less have serious pollution,large energy consumption,intermediate toxicity and low yield.In addition,the production process above mainly relies on petrochemical raw materials,and with the increasing shortage of petroleum resources,the cost of producing acrylate through the petrochemical route is also increasingly high.China is a large coal country,with a large number of coal resources.With the rapid development of modern coal chemical industry,the production capacity of methanol and acetic acid appears serious surplus.Therefore,the vapor phase aldol condensation of methyl acetate?Ma?with formaldehyde?FA?to methyl acrylate has great potential.Compared with traditional methods,vapor phase aldol condensation of methyl acetate with formaldehyde is really a kind of resource-saving and environmentally-friendly green synthesis process,which not only get rid of the dependence on petroleum chemical raw materials,but also reduces the cost of production of methyl acrylate.The key to realizing this technique is acquiring a highly efficient catalyst.In this paper,a series of Al₂O₃-supported alkali metal,alkaline earth metal and rare earth metal catalysts were prepared by the incipient wetness impregnation method for the vapor-phase condensation reaction of methyl acetate with formaldehyde.The structure and physicochemical properties of the catalyst were studied by various characterization methods,which were related to the catalytic activity of the catalyst.The main work are as follows:1.The Ba/Al₂O₃ catalysts for the vapor-phase condensation of methyl acetate with formaldehydeAlumina was prepared by precipitation method with aluminum nitrate as the source and ammonia as the precipitant.The obtained alumina was used as a carrier to prepare a series of solid base catalysts by the method of incipient wetness impregnation.The experimental results show that the Al₂O₃ with a precipitation pH of5.5 and the calcination temperature of 500?showed the best catalytic activity,which the conversion of Ma is 33.4%and the selectivity of MA is 85.8%.For the supported catalysts,the addition of alkali metal,alkaline earth metal and rare earth metal can obviously improve the selectivity of the catalyst to the methyl acrylate.Among them,the best catalytic performance was obtained with the Ba/Al₂O₃ catalyst,which the initial conversion to Ma achieved 37.6%and the selectivity to Ma reached 94.1%under the condition of raw Ma/FA/CH₃OH=1:2:2 and the reaction temperature of390?.Although,as the reaction progressed,the activity decreased slowly due to the carbon deposition on the surface of the catalyst.The used catalyst can be regenerated by calcination at 550°C under an air atmosphere.A series of characterization results showed that the acid sites on the surface of the catalyst are the main factors affecting catalyst activity,this could be due to acid sites mainly affect the adsorption process of gaseous reactants on the catalyst surface.Among them,the acid sites of medium strength play a key role,because the sharp decrease of the number of weak acid sites does not significantly change the activity of the catalyst,but the number of middle strength acid sites basically remains unchanged,which is consistent with the change trend of activity.The weak basic sites on the surface of the catalysts mainly affect the selectivity to the methyl acrylate,which is not only reflected in the activity of alumina at different roasting temperatures,but also proved by the increase in the number of basic sites of the catalyst caused by the addition of Ba species.Combining the catalytic activity and selectivity with the result of characterization,it can be concluded that the vapor-phase aldol condensation of methyl acetate with formaldehyde is a kind of acid-base bifunctional catalytic reaction,and the moderate Lewis acid and weak base sites on the surface of catalysts may be the main active center of this reaction.2.The Ba/?-Ti–Al₂O₃ catalysts for the vapor-phase condensation of methyl acetate with formaldehydePrevious work indicated that the vapor-phase condensation of methyl acetate with formaldehyde was an acid-base bifunctional catalytic reaction,and the moderate Lewis acid and weak base sites might be the main active centers of the reaction.Based on this,this part of our work would start from improving the number and strength of acid sites on the catalyst surface.For titanium oxides,it has been reported that although they have relatively low specific surface areas,the surface Lewis acid sites of TiO₂ are multivalent and stronger than those of alumina.The more surface Lewis acid sites may be obtained by complexes of aluminum and titanium,which provide certain possibilities for the catalytic activity of titanium-doped mesoporous Al₂O₃ towards the target reaction.Therefore,we selected TiO₂ as the doped species and introduced it into alumina,hoping to change the surface acid environment of alumina and improve the catalytic activity of the catalyst to the target reaction.In this work,mesoporous?-Ti-Al₂O₃ was successfully prepared by an evaporation-induced self-assembly method using aluminum isopropoxide as the aluminum source,butyl titanate as the titanium source and glucose as the additive.Then we used it as a carrier of Ba/?-Ti-Al2O3 catalyst to catalyze the vapor-phase condensation of methyl acetate with formaldehyde to methyl acrylate in a fixed-bed reactor.The physicochemical properties of the catalysts were surveyed by XRD,N₂adsorption-desorption and XPS.The acid-base properties of the catalysts were characterized by TPD of NH₃-CO₂ and FTIR spectra of pyridine adsorption.The characterization results showed that titanium was successfully incorporated into the framework of?-Al₂O₃.In addition,the doping of the titanium species had a significant influence on the acidic properties of the catalyst,increasing the total amount of acid centers?especially medium acid centers and strong acid centers?,which was attributed to the formation of Al-O-Ti linkages in the framework of?-Ti-Al₂O₃.The experimental results showed that the Ba/?-Ti-Al₂O₃ catalyst exhibited the best catalytic activity and selectivity on the vapor-phase condensation of methyl acetate with formaldehyde,and the conversion of Ma achieved 50.0%with a selectivity of Ma of 90.2%,which was likely due to the more Lewis acid sites on the surface of Ba/?-Ti-Al₂O₃ catalyst,especially the medium Lewis acid sites.After a total reaction time of 300 h,the activity of the Ba/?-Ti-Al₂O₃ catalyst still retained its initial activity after regeneration,indicating a good reusability.Compared with the Ba/Al₂O₃ catalyst,the Ba/?-Ti-Al₂O₃ catalyst had better catalytic activity,stability and potential for practical application.3.The Ba-La/Al₂O₃ catalysts for the vapor-phase condensation of methyl acetate with formaldehydeAlthough the Ba/Al₂O₃ catalyst showed good catalytic activity and selectivity to the target product,it still had disadvantages such as fast deactivation rate and serious carbon deposition in the reaction process.In order to reduce the rate of carbon deposition in the reaction and stabilize the activity of the catalyst,we studied this part.Based on the Ba/Al₂O₃ catalyst,a series of modfied catalysts of the third component,such as Li,Cs,La,Ce and Zr,were prepared by co-impregnation method to adjust the acidity and basicity of the catalyst surface,so as to obtain catalysts with higher catalytic activity and stability.Furthermore,the properties of the catalysts and the carbon deposition performance on the catalyst surface were surveyed by XRD,N₂adsorption–desorption,TG-DTA,and SEM.The acid–base properties of the catalysts were characterized by TPD of NH₃-CO₂ and FTIR spectra of pyridine adsorption.The results showed that the addition of appropriate amount of La could significantly stabilize the catalytic activity of Ba/Al₂O₃ catalyst.The initial catalytic activity of the optimal catalyst 5Ba-0.5La/Al₂O₃ to methyl acetate was up to 43.5%,and the selectivity to the target product methyl acrylate was up to 93.2mol%.Moreover,the activity and selectivity of the catalyst after a total reaction time of 10 h were still better than that of the catalyst without adding La species.Combined with the characterization results,we found that although the activity of the5Ba-0.5La/Al₂O₃ catalyst was not obviously increased compared with 5Ba/Al₂O₃catalyst,the carbon deposition was greatly suppressed in the target reaction due to the alkaline function of La₂O₃.The IR results indicated that the surface of the Al₂O₃ and metal modified Al₂O₃ catalysts mainly consisted of Lewis acidic sites.Furthermore,the relative amounts of these sites declined when adding metal to the Al₂O₃ supports.The results of the NH₃-TPD showed that the addition of lanthanum species can cause an increase in weak acid sites and a decrease in strong acid sites,making the surface acidity of the catalyst more suitable for the condensation reaction.The CO₂-TPD analysis showed that adding lanthanum species could also increase total CO₂ uptake,indicating that some new base sites formed on the catalyst surface.Furthermore,the quantitative results further demonstrated the changes in the acid–base environment on the catalyst surface.Combining the catalytic activity and selectivity with the characterization results,it can be concluded that the addition of lanthanum species significantly reduced the number of strong acid sites on the catalyst surface,inhibited the generation of carbon species in the reaction process,and stabilized the catalytic activity of the catalyst.4.The study of Ba-Al catalysts on carbon deposition in the vapor-phase condensation of methyl acetate with formaldehyde.Although deactivated catalysts can regain catalytic activity by roasting at high temperature in air atmosphere,the service life and anti-carbon deposition ability of industrial catalysts will directly affect the economic benefits of catalysts.The formation of carbon deposition species is not only the main reason affecting the decline of catalyst activity,but also one of the main obstacles restricting the industrial application of this reaction.Carbonaceous species have a very complex nature and are generally considered to be composed of polycyclic aromatic hydrocarbons and hydrogen-poor carbonaceous sediments,which formed gradually in the process of catalytic reation,reducing the available surface area,and resulting in the decline of catalyst activity.In order to better understand the complex nature of carbon species,and further study the catalytic mechanism and deactivation mechanism of the condensation reaction,in this part,a series of Ba-La-modified alumina have been characterized and tested as catalysts for vapor-phase aldol condensation of methyl acetate with formaldehyde to methyl acrylate.The carbon formation,acid-base property and deactivation/reactivation behavior on spent catalysts were investigated by a series of characterization techniques.Characterization results showed that the number of weak and strong acid on the surface of the catalysts changes a little with the increase of the formaldehyde proportion,while the amount of medium-strong acidity decreased obviously corresponding to the temperature range of NH₃ desorption peak in the range of 200³00?.This indicates that the increase of formaldehyde content in the raw material mainly affects the medium strong acid sites on the catalyst surface.On the other words,the medium-strength Lewis acid site may play a key role in the activation of formaldehyde.Excessive formaldehyde ratio can activate more formaldehyde but also accelerate the catalyst deactivation caused by carbon deposition covering the acid sites.Therefore,it is consistent with the previous results,further proving that the vapor-phase aldol condensation of methyl acetate with formaldehyde belongs to acid-base bifunctional catalytic reaction,and the moderate Lewis acid site and weak base site are the active centers of condensation reaction.