Study Of Mg-Al Composite Oxides For Catalyzing Acetone Condensation Reaction

As a popular raw material to develop fine chemical products,acetone has significant market values.Isophorone,one of the aldol condensation products of acetone,is a good solvent for polymer materials,widely used in plastic,pesticide,pharmaceutical,paint and other industries.In this paper,a series of Mg-Al oxides and M-Al composite oxides modified by different rare earth elements?M?were prepared by coprecipitation method,and their properties and surface acid-base properties were characterized by XRD,BET,EDS,TPD and In-situ FTIR methods.Their activities were evaluated on fixed bed microreactor,and then the relationship of acetone condensation reaction catalytic properties and its surface acid-base properties were analyzed.The adsorption states of acetone and its condensation products on the surface acid-base center of Al₂O₃,MgO,Mg-Al oxides were detected online by In-situ FTIR technology,which can also track acetone condensation reaction process,and infer acetone condensation reaction mechanism initially.The results showed that the Mg-Al catalyst included complete MgO crystal structure,and there are only weak acid position?L-acid sites?,weak basic site?OH^-?,middle strong basic site(Mg²⁺-O^2-)and strong basic site(O^2-)on the surface of the catalyst.There were complete MgO and MO₂ crystal structure on the surface of M-Al oxides modified by different rare earth elements?M?,and modified catalyst has weaker acid position,stronger strength and density of middle strong basic site than pure Mg-Al catalyst.The fixed-bed reaction results showed that,among the different composite oxides catalysts,MA3^#catalyst exhibited better catalytic performances for acetone condensation(X_Ac=38.12%?S_IP=76.62%)and MAC1^#modified catalyst displayed the best catalytic activity(X_Ac=42.59%?S_IP=80.91%).The relationship of the reaction activity and acid-base properties of surface inferred that the larger ratio of of middle strong basic site density to weak acid site density is favor for acetone condensation reaction,and suitable middle strong basic density is beneficial to the synthesis of isophorone.In-situ FTIR spectra showed that acetone was absorbed on the L-acidic sites where its condensation reaction occurred quickly,and mesityl oxide was absorbed through C=O bond on the L-acid center?main absorption mode?and middle strong basic center of catalysts,which inferred that the dimerization reaction of acetone occurred on the weak acid site at low temperature.As the temperature increased,the acetone absorbed on the L-acidic sites faded away,but the mesityl oxide adsorbed on the basic sites increased.At high temperature,acetone further reacted with mesityl oxide on the middle strong basic center of catalysts to form isophorone.However,the catalytic selectivity to isophorone is closely related to the density of the middle strong basic sites on the catalysts.