Preparation Of Hydrotalcite-based Solid Base And Its Catalytic Performance For Aldol Condensation

Biomass is an important renewable resource.It is an effective way to realize the utilization of biomass to convert it into organic intermediates with higher value through depolymerization and catalytic transformation.Aldol condensation is an important reaction to increase the length of carbon chain in organic synthesis reactions.Alkalis and acidic solutions are often used as catalysts for the traditional aldol condensation reaction,which is easy to cause environmental pollution and equipment corrosion.Heterogeneous catalysts have the advantages of being environmentally friendly and recyclable,and have gradually become research hotspots.Hydrotalcite（LDHs）and its calcined products（LDO）are a kind of heterogeneous catalysts with research value and potential,which are often used as solid base catalysts in catalytic industry due to their unique structure and properties and their own alkalinity.In this paper,three hydrotalcite solid base catalysts were prepared for the aldol condensation reaction between acetone and benzaldehyde,and their catalytic performance was investigated.The main research contents are as follows:（1）CaMgAl hydrotalcite was synthesized by co-precipitation method.It was characterized by thermogravimetric（TG）,Fourier infrared spectroscopy（FT-IR）,X-ray diffraction（XRD）,CO₂ temperature programmed desorption（CO₂-TPD）,scanning electron microscope（SEM）.The results showed that the prepared CaMgAl hydrotalcite had a better layered structure,and the elements of Ca,Mg and Al were evenly dispersed in the sample.Compared with Ca Al and Mg Al hydrotalcite,the CaMgAl hydrotalcite had weak and medium strong alkaline sites.The samples with the molar ratio of CaMgAl of 2:1:1 showed higher activity.The mass ratio of catalyst dosage to reaction liquid was 1:14,and the molar ratio of benzaldehyde/acetone was1:9.At 130°C,the reaction was carried out for 4 hours,the conversion rate of benzaldehyde reached 96.6%,the yield of benzylidene acetone was 87.8%,and the yield of dibenzylidene acetone was 6.7%.（2）Synthesize reconstructed CaZnAl hydrotalcite by rehydration.It was characterized by TG,FT-IR,XRD,CO₂-TPD,SEM,N₂-adsorption desorption and other characterization methods.The results showed that by adjusting the Zn/Al molar ratio in the metal cation composition of the laminate,the catalyst samples had more weak and medium strong alkaline sites,and the purpose of adjusting the acid-base sites on the catalyst surface was achieved.The sample with the molar ratio of Ca/Zn/Al of 4:0.3:0.7 had higher activity.The molar ratio of benzaldehyde/acetone was 1:10,the ratio of the amount of catalyst to the mass of the reaction liquid was1:25,and the reaction was carried out at 130°C for 4 hours.The conversion rate of benzaldehyde was 98.7%.The yield of benzylidene acetone was 88.5%,and the yield of dibenzylidene acetone was 7.0%.（3）Preparation of Ca O-La₂O₃ composite metal oxide.It was characterized by TG,CO₂-TPD,SEM,XRD and other characterization methods.The results showed that the addition of La element increaseed the number of weak and moderately basic sites in the Ca O-La₂O₃ sample,thereby increasing the catalytic activity.The sample with Ca/La molar ratio of 4:1 had higher catalytic activity.The molar ratio of benzaldehyde to acetone was 1:9,the mass ratio of catalyst to reaction liquid was 1:28,at 130°C,the reaction was carried out for 3 hours,the conversion of benzaldehyde reached97.9%,the yield of benzylidene acetone was 84.0%,and the yield of dibenzylidene acetone was 11.7%.