Synthesis And Catalytic Property Of Novel MOFs Consisted Of Multi-metal Ions By Different Routes

MOFs have good structural tunability,and can be controllably synthesized with different properties and porous structures by changing metals and ligands,which have good application prospects,especially in the field of heterogeneous catalysis.Compared with single-metal MOF materials,multi-metal MOF materials can contain multiple metal active sites,and can be designed by adjusting the ratio of metals to control the structure and function,beneficial to improve the catalytic performance of the material and the stability of the framework structure,which can be better used in heterogeneous catalytic reactions,especially the reactions related to the catalytic conversion of olefins studied in this system.The catalytic conversion of olefins includes the epoxidation of olefins and the isomerization of epoxy products.Their products are important reaction intermediates in industrial production processes,but the activation of molecular oxygen during the reaction is a challenge,so that the epoxidation reaction is difficult to become efficient and clean.In this dissertation,based on the characteristics of MOF materials and the requirements of the target reactions,multi-metal MOF materials with synergistic effects are designed and synthesized,and the performance of the materials for catalyzing the green and efficient conversion of olefins is systematically studied.The specific research contents are as follows:（1）Except the researches that CHP or TBHP is generally used as the initiator to achieve high-efficiency epoxidation in the air epoxidation system,it is reported for the first time that potassium persulfate is used as an initiator to efficiently carry out the epoxidation of olefins under milder conditions.Co-MOF is used as the catalyst in the epoxidation reaction because of its simple synthesis,stable structure and excellent ability of molecular oxygen activation.When potassium persulfate is used as the initiator and the reaction temperature is 80 ^oC,with cheap and clean molecular oxygen as the oxygen source,the Co-MOF material can convert all olefin substrateα-pinene（3 mmol）into epoxy compound in 5 h.Furthermore,when increasing the amount of substrate to 10 mmol,a good conversion can be still achieved,in which the yield of epoxy compound can reach 61.3%,far more than the efficiency of other initiators such as aldehydes or peroxy compounds.Different Co-based heterogeneous catalytic materials can also perform highly efficient epoxidation reactions under the initiation of potassium persulfate,and other cyclic olefins also have a good epoxidation effect when potassium persulfate is used as the initiator.（2）To date,it has been difficult to develop catalytic systems for activating O₂in the absence of initiator and sacrificial co-reductant for high-efficiency epoxidation of olefins under mild conditions.In this work,using terephthalic acid as the ligand,different compositions of Zn Co-MOF have been prepared via a dry-gel crystallization method,which is used to catalyze the epoxidation ofα-pinene with air at 90 ^oC.Zn_0.1Co₁-MOF-D2-24performs the best catalysis activity to give 95.5%ofα-pinene conversion and 96.7%of epoxide selectivity in the absence of any initiator.The epoxidation results show that Zn and Co can effectively activate air O₂to epoxidizeα-pinene in high efficiency.This bimetal catalytic effect is assigned to the contribution of interacted lattice Zn and Co ions embedded into crystalline MOF framework with similar electronic structure and ion radius,which avoids the use of organic peroxide initiators and other co-reducing agents.Under anhydrous conditions,Zn_0.1Co₁-MOF-D2-24 shows stable catalytic activity in the epoxidation ofα-pinene with air by at least six recycles.（3）To catalyze efficient epoxidation of macrocyclic olefin cyclooctene with air O₂,we use trimesic acid as the ligand and introduce the rare earth metal La to synthesize La Co-MOF materials.Through various characterizations,we prove that Co and La are not physically mixed in the bimetallic La₂Co₈-MOF-BTC-H material,but co-exist in the coordination structure with obvious interactions.The acidity of the bimetallic material has been significantly enhanced.We also compare the effects of different metal ratios,different metal combinations,different crystallization temperatures and times on the material structure and reactivity,and determine the synthesis conditions of the La₂Co₈-MOF-BTC-H material with the optimal catalytic performance.Under the conditions of 80 ^oC and 30 ml/min air flow rate,the conversion of cyclooctene can reach 96.2%and the selectivity of epoxy compounds can reach 98.7%after 5 h.The La₂Co₈-MOF-BTC-H material shows good stability,and its activity does not decrease significantly after 6 recycles.（4）One of the synthetic methods of campholenic aldehyde as an important intermediate of sandalwood series perfumes and essences is obtained by the isomerization reaction ofα-epoxypinane（the epoxy product ofα-pinene）.We have for the first time reported the construction of strong Lewis acidity through one route of pre-calcining octahedral Zr-MOF material,which is synthesized by the rotating hydrothermal crystallization method with regular octahedral structure,large specific surface area and very uniform particle size.After low-temperature calcination,the resulting material Zr-MOF-R-X-Cal can basically retain an intact structure of Zr-MOF-R precursor,as proven by various characterizations.More importantly,the surface Lewis acidity and Lewis acid ratio on the calcined materials are significantly enhanced.Thus-calcined materials have exhibited a high catalytic activity for selective isomerization ofα-epoxypinane to campholenic aldehyde.Zr-MOF-R-300-Cal presents the best catalytic effect on this isomerization to obtain 95.9%conversion ofα-epoxypinane and 82.4%selectivity of campholenic aldehyde,much superior to the Zr-MOF-R precursor and those catalytic materials reported previously in the literature.Notably,the catalytic activity of the Zr-MOF-R-300-Cal material is not decreased a lot after repeated-use 6 times,indicative of its excellent stability.（5）The tandem reaction can realize two or more steps reactions on the same material,which can not only reduce the number of reaction steps but also greatly improve the reaction efficiency.To achieve greener and more efficient conversion of olefins,we design and synthesize different forms of multi-metal MOF materials,such as bimetallic,trimetallic and MOF@MOF,in order to combine the epoxidation reaction and the isomerization reaction into a tandem reaction.A variety of bimetallic and trimetallic MOF materials have good catalytic activity for epoxidation reaction,but the catalytic activity for the isomerization process in the tandem reaction is poor,indicating that the active sites catalyzing the epoxidation reaction are not suitable for isomerization reaction.The Zr_0.2Co₁-MOF-BTC-H@Zr-MOF-H that belongs to MOF@MOF-type material with good catalytic activity for the tandem reaction,can not only convert all alkene substrates,but also obtain 27.8%selectivity to campholenic aldehyde.Compared with the selectivity of campholenic aldehyde in the previous epoxidation system,the Zr_0.2Co₁-MOF-BTC-H@Zr-MOF-H material can really catalyze the tandem reaction of alkenes,but the selectivity of campholenic aldehyde as the target product of isomerization,needs to be further improved.