Preparation And Catalytic Performance Of Core-shell Layered Double Metal Hydroxide Composites

The development of suitable catalysts that have high activity,abundant efficiency,and easy recovery as well as lesser ecological contamination suitable for various industrial processes is the rising area concerning green chemical processes.The solid base catalyst has the characteristics of simple synthesis process,uniform pore size distribution,large specific surface area,high alkalinity,etc.,and has a wide range of industrial applications.Layered double hydroxide(LDH)is a typical 2D anionic layered material.Due to the special structure,core-shell structure composite material based on LDH is one of the hotspots in recent years.How to construct LDH-based composite materials with high catalytic performance and strong stability has a positive and far-reaching impact on the catalytic process and the synthesis of fine chemicals.To solve the above problems and develop highly active catalyst with good stability,the synthesis of propylene glycol monomethyl ether(PGME)was chosen as model reaction.The formation of active sites and reaction mechanism were studied particularly.The separation process of PGME was simulated.Finally,the application of core-shell LDH composite in the synthesis of polyether polyols was investigated.The main work and results are as follows:(1)Multi-element metal composites based on MgAl-LDH were prepared by the nucleation-crystallization isolation method.It was found that CeMgAl-LDO(P123)appeared high catalytic activity in the synthesis of propylene glycol monomethyl ether,with conversion of 97.6%for propylene oxide and selectivity of 89.8%for PGME.According to the literature and experimental results,the synthesis mechanism of PGME was explored,that the reaction followed the reverse-Markovnikov rule and went through three reaction stages.The strength of the medium strong base is the key factor of the reaction,and too strong base will affect the selectivity of PGME.(2)Three SiO₂@LDH nanocomposites of different shell morphologies were synthesized successfully by various methods(Self-assembled process;Co-precipitation process;Sol-gel process).The results indicate that LDH shell oriented vertically on the core could be generated by the sol-gel method(SG-SiO₂@LDH)and displays the highest catalytic activity and stability among three materials.The horizontally oriented shell morphology tends to be formed by the self-assembly method(SA-SiO₂@LDH),which maintains the original LDH structure but unstable.The formation of a third morphology called mixed platelet is contributed to the coexistence of horizontally and vertically platelets,which has been seen in the case of co-precipitation(CP-SiO₂@LDH).Core-shell materials prepared by in-situ coprecipitation method are more suitable for organic synthesis because of the simple preparation process and high repeatability.(3)Silicon@P123-Templated Ce-doped Layered double hydroxide(SiO₂@CeMgAl-LDH(P123))composites were synthesized via a facile in-situ co-precipitation method.Meanwhile,the calcined powder(SiO₂@CeMgAl-LDO(P123))possessed an excellent core-shell structure and a high surface area inherited from the LDH structure,which led to an outstanding catalytic activity under mild reaction conditions.Cerium oxide provides a large amount of oxygen vacancy and significantly improves the medium basic strength of the material,which facilitates the selective ring-opening of PO.Furthermore,the introduction and removal of P123 make the cerium oxide uniformly dispersed on the LDH layers,providing more reaction sites for the reaction of methanol and propylene oxide.The core-shell structure prepared by the in-situ co-precipitation method could solve the shortcomings of agglomeration of layered double hydroxide and prolong the catalytic life evidently.(4)Aspen Plus simulation software was used to study the refining of propylene glycol monomethyl ether and the recovery of reaction solvent.A two-column sequential distillation process was designed.By optimizing the solvent recovery tower T101,the methanol content at the top of the tower reached 99.81%,and the methanol could be recycled.The process conditions of the product separation tower T102 were optimized,and the effects of factors such as reflux ratio,distillate feed ratio,number of trays,and feed position of raw materials on the content and yield of PGME were investigated.Using the central combined experimental design(CCD),the model was constructed through response surface analysis and verified,and the optimal design parameters were obtained by combining the model calculation and fitting.The content of PGME in the top distillate of T102 was 98.64%,and the total yield was 99.14%.(5)SiO₂@CeMgAl-LDO was used in the reaction process of the ring-opening polymerization of propylene oxide.The results showed that the catalyst appeared a good catalytic activity for the synthesis of low molecular weight polypropylene glycol and polypropylene glycol monomethyl ether.