Study On Preparation Of Gold-based Catalyst For Oxidative Esterification Of Methacrolein

In this paper,a composite oxide carrier was obtained by high-temperature calcination using a hydrotalcite structure as the precursor.The supported gold catalyst was prepared by the deposition precipitation method.By changing carrier composition,reduction method,and calcination temperature,a series of influencing factors such as carrier acidity and alkalinity,gold particle size and carrier additives were investigated.The prepared Au catalysts showed high performance in the one-step oxidation and esterification reaction of MAL to MMA.The specific research contents and results are as follows:First,the magnesium-aluminum composite oxide carrier was prepared from a magnesium-aluminum hydrotalcite precursor and compared with the activity of silica gel carrier reported by Asahi-Kasei Corporation.In order to explore the influence of carrier acidity and alkalinity,the ratio of magnesium nitrate and aluminum nitrate was changed.A series of composite oxide carriers with different magnesium-aluminum ratio(Mg/Al molar ratio)were prepared.According to the reaction results,it is found that if there are no acid sites in the carrier,the conversion rate of the catalyst is low,and the oxidation ability is lacking.If there is no basic site in the carrier,the conversion rate and selectivity of the catalyst are very low,and the esterification activity is insufficient.Through TEM characterization,it is found that when the ratio of magnesium to aluminum is high,the gold particles in the catalyst are severely aggregated and the activity is significantly reduced.The reason is that the higher the magnesium-aluminum ratio,it is easier to form magnesium-aluminum spinel,and the dispersion of gold nanoparticles is weakened.By changing the calcination temperature of Au/MgO-Al2O3-0.2 to investigate the influence of the gold particle size,it is found that the optimal gold particle size is about 3 nm and the conversion of MAL is higher than 99%and MMA selectivity is higher than 95%over the catalyst.When the gold particle size is greater than 5 nm,the catalyst is severely deactivated.In addition,the difference of air calcination and hydrogen reduction catalysts was compared,it was found that when the ratio of magnesium to aluminum was low,the catalyst by air calcination was better,and when the ratio of magnesium to aluminum was high,the performance of catalyst by hydrogen reduction was better.The stability of Au/MgO-Al2O3-0.2 was also tested through a cyclic reaction experiment.An attempt was made to introduce the nickel component during the gold loading process,and the influence of the nickel component on the stability of the catalyst was investigated.According to the research of magnesium-aluminum carrier,we expanded to other acid-base carriers and prepared zinc-based carriers and cerium-based carriers.The conversion rate and selectivity of Au/ZnO-Al2O3-1 are both above 90%,indicating that high basic amount in the Zn-based carrier will not reduce the activity of the catalyst.Under the condition of hydrogen reduction,the more zinc content in the zinc-aluminum carrier,the better the catalyst activity is.Because copper has a preferential adsorption effect on gold,a zinc-copper carrier was also tried,but the reaction activity of the gold particles supported on the zinc-copper carrier was not good,indicating that the interaction between gold and copper is not conducive to the oxidative esterification reaction.There is still a gap for the conversion of MAL between the prepared cerium-based support(ZnO-CeO2,MgO-CeO2)and the magnesium-aluminum support,indicating that the oxygen vacancies in the cerium oxide cannot play a decisive role in the oxidative esterification activity of the catalyst.Finally,on the basis of the optimized acid-base carrier MgO-Al2O3-0.2,the carrier was further modified,and a variety of carrier additives were added to improve the activity.Firstly,the effect of alkaline support additives on the activity was investigated.It was found that when alkaline oxides such as ZnO,Fe2O3 or La2O3 were added to the magnesium-aluminum support,the activity of the gold catalyst was consistent with that of the magnesium-aluminum support.But when adding variable-valence metal support additives,such as Co3O4,the selectivity of the catalyst was increased by 2%to 97.7%,and the conversion rate is also maintained at 97.3%.TEM images showed that the average particle size of the gold particles of the catalyst was 3.14 nm.The deviation of the Au peak position found in the XPS spectrum indicates that the electronic state of the gold nanoparticles in the catalyst has changed,the electron cloud density has increased,and the Co3O4 in the carrier has electronic interaction with the gold particles,which improves the selectivity of the catalyst.However,the addition of Bi2O3,PbO,ZrO2 and other valence metals will reduce the catalyst activity.Finally,the influence of the support additives Co3O4 and La2O3 on the stability of the catalyst was investigated.