Study On Gold Catalyst For Oxidative Esterification Of Methacrolein To Methyl Methacrylate

Methyl methacrylate(MMA)is used in many fields widely and it is an important chemical raw material.The acetone cyanohydrin method(ACH method)is one of the traditional production processes,but it has disadvantages,such as high pollution,strong corrosiveness to equipment,and relatively low atomic utilization rate,etc.With the development of technology,Japan has developed a route with a shorter path and less environmental impact.This method uses isobutene as raw material,and then methacrylaldehyde(MAL)could be obtained by oxidation reaction.Finally,MAL is oxidized and esterified to MMA.In this route,the one-step oxidative esterification of MAL to prepare MMA is the key.The preparation of catalysts is very important to improve the reaction performance and economic benefits.This paper chooses a supported gold catalyst which uses gold as the main active center,and its activity is much higher than other precious metals(palladium,platinum,etc.),which has greater advantages in this reaction system.Besides that,the support is selected from silica with relatively large specific surface area and porosity.Moreover,different types of additives can be added in order to modify the support.In order to further improve the stability of the gold catalyst,adding a base metal nickel to the gold catalyst can not only improve the reactivity by utilizing the synergistic effect of the two metals,but also reduce the input cost to some degree,which is more conducive for industrial scale-up applications.The main research contents and results are as follows:Firstly,the effect of different additives of the catalyst support is investigated,and the content of the additives is optimized.When CaO and ZnO are added,the BET specific surface area of the support is significantly reduced,and the dispersibility of metal particles becomes poor,and thus the catalytic performance is also reduced.When adding the basic additive(MgO)and the acidic additive(Al2O3)at the same time,the catalytic activity of the catalyst can be significantly improved.Therefore,on this basis,the content of MgO and Al2O3 is optimized.TEM characterization shows that when the content of MgO is higher,the particle size of the prepared metal particles is larger,and the particle size directly affects the activity of the catalyst.By optimization,adding 10 wt%MgO and 10 wt%Al2O3 into the SiO2 support can obtain the best catalytic performance,this is,the conversion of MAL is 80.1%,and the selectivity of MMA is 90.7%.According to the optimal formula of support,a SiO2-Al2O3-MgO(SAM)support is prepared by spray drying,and gold and nickel are loaded to obtain an Au-NiOx/SAM catalyst.The experimental conditions(temperature,concentration and time,etc.)of the impregnation process may affect the structure and performance of catalysts.The TEM characterization shows that the average particle size of the metal particles increases with the increase of calcination temperature.When the temperature is controlled at 450℃,the average particle size is 3.2 nm,and the yield of MMA is the best.In addition,the contents of gold and nickel are explored.The catalytic activity of the catalyst increases with the increase of the gold content,and it increases first and then decreases with the increase of the nickel content.This shows that when the nickel content is too high,it may affect the exposure of gold active sites.And there is an optimal ratio of gold and nickel contents(1.1 wt%Au and 1.1 wt%Ni).The five cycles of reuse experiments for the catalyst also show that the catalyst has good stability.Finally,the effect of the reaction conditionson the performance of catalysts in the reaction of MAL to MMA was investigated.When the reaction temperature is 80℃,the reaction time is 2 h,and the oxygen pressure is 0.4 MPa(O2+ N2 total pressure is 3.2 MPa),the conversion rate of MAL can reach 82.5%and the selectivity to MMA can reach 91.0%.