Bi-functionalized MOFs Serving As Self-supported Catalysts For CO₂ Cycloaddition Reaction

CO₂ concerned with environmental problems was also an abundant carbon source for organic synthesis. One of the most promising reaction schemes in this area currently seemed to be the synthesis of cyclic carbonates from CO₂ and epoxides, which were useful as raw materials. As a new class of porous material, various MOFs have been suggested in this reaction. Several cases were generally claimed to be effective but lack of active sites need co-catalysts, while some cases overcame the separation problem but suffered from the harsh reaction conditions. In this work, with the special properties of MOF materials, such as large surface areas, controllable pore structures and versatile chemical compositions etc. We synthesized and employed series of bi-functional MOF materials combining active sites for the CO₂ cycloaddition reaction based on the catalytic mechanism, and discussed the synergistic effect from the sites decorated in the framework.（1） Many organic functional groups played crucial roles for reactions such as amine group, carboxylic group, hydroxyl group and acylamide group. CO₂ could be adsorbed and activated by the amine groups, which might be beneficial for the insertion of CO₂ into the C–O bond of epoxides. Then acylamide group, hydroxyl group and carboxylic group could activate the epoxide ring via hydrogen bonds. We demonstrated the catalysis capability of a series of isomorphic MOFs functionalized on the pore wall by both acylamide and-H,-NO2, NH2,-OH,-COOH groups in the aspects of CO₂ cycloaddition reaction, which were named 1-H, 1-NO2, 1-NH2, 1-OH and 1-COOH respectively. Without any co-catalyst, with the reaction condition of 3 MPa and 100 oC, these MOFs exhibited excellent catalysis ability but different catalytic activities dependent on the organic groups, revealing the best one for MOF functionalized by-NH2 and acylamide groups. The yields of the cyclic carbonates were higher than 90% with the selectivity of 99%, indicated the perfect synergistic effect between-NH2 and acylamide groups. The employment of aniline and ligand one as catalysts revealed that the arrangement of the groups decorated in the framework was benefitial for their performance of synergistic effect. Compared with other catalysts, 1-OH and 1-COOH despite having both acid and basic active sites, showed rather low yield, even less than that of catalyst 1-H. An inactive organic group-NO2 was chosen to replace-OH/-COOH fixing on the hole wall. 1-NO2 gained almost the same conversation with 1-H. It proved there was no space effect. We supposed maybe-OH/-COOH surely had participated in the reaction by activating epoxides as reported, then possibly the acid site asorbed the products strongly impeding the desorption. Furthermore, we also estimated the recycle of 1-NH2. The results indicated that it had good stability of being reused for at least three times without significant decrease of its catalytic activity.（2） Quaternary ammonium salt was a kind of homogeneous catalyst with high activity for CO₂ cycloaddition reactions. Postsynthetic Modification（PSM） has been drawn great attention as an efficient method which could lead to directional modification of improving the physical and chemical properties of MOFs. We chose 1-NH2 as mother framework. Taking advantage of the reaction between-NH2 group and CH3 I gaining ammonium salts group, 1-NH2 was successfully modified by the PSM method. The modified materials were named CH3I-1-NH2-2, CH3I-1-NH2-4 and CH3I-1-NH2-6. FT-IR test revealed that ammonium salt groups were successfully introduced into the framework. Powder X-ray diffractions studies indicated a singlecrystal-to-single-crystal transformation nature of the PSM process with CH3I-1-NH2-2 and CH3I-1-NH2-4. However the structure of CH3I-1-NH2-6 was destroyed. It was found that the modification led to the decline of CO₂ adsorption capacity. On the contrary, the catalytic activity of the MOFs could be significantly improved by the modification. With CH3I-1-NH2-4 as catalyst, under the reaction condition of 100 oC and 2 MPa, the yield of cyclic carbonates was increased to 85% with selectivity of 99% from 15%（selectivity of 82%） in the absence of co-catalyst. The synergistic effect from amino, acylamide and ammonium salt groups decorated in the framework lead to the high yield. The Postsynthetic Modification method offered suggestion for improving the catalytic activity of MOF materials.（3） Many MOFs which introduced into the synthesis of cyclic carbonate from epoxides and CO₂ including the MOFs we employed showed high activities. However, the high CO₂（> 2 MPa） pressure was needed due to the gas-liquid-solid heterogeneous system in nature that the MOFs could not contact with gaseous reactant fully. We chose the MOF material UTSA-16（UTSA = University of Texas at San Antonio） for the reaction which was reported of high porous material for CO₂ capture. UTSA-16 was subjected to post-synthetic modification with Li+ and Na+ according to an ion-exchange method which resulted in the formation of two isomorphous frameworks, UTSA-16-Li and UTSA-16-Na. All of the exchanged compounds retained the same framework structure with the parent framework, which revealed by their powder X-ray diffraction patterns. TGA and N2 adsorption tests indicated that the thermal stability and BET surface area of the materials were not changed. Their CO₂ adsorption capacities were in the order of UTSA-16, UTSA-16-Na and UTSA-16-Li. During the reaction, UTSA-16 was evaluated as regard its potential for CO₂ conversion to cyclic carbonates. The coordinated unsaturated Co（1） ion in the framework acted as Lewis acid for activating the epoxides by bonding to the oxygen atom. K species located in the pores played a crucial role in adsorbing and activating CO₂ thus decreased the reaction pressure. Therefore, without co-catalyst, UTSA-16 worked as self-supported catalyst showed activities to many epoxides under relatively mild condition（120 oC and 1 MPa） for producing corresponding cyclic carbonates, with the synergistic effect between K species and the coordinated unsaturated Co（1） ions. The employment of K+ and Co2+ as homogenous catalysts revealed that the arrangement of the groups decorated in the framework was benefit for their performance of synergistic effect. UTSA-16 was further tested for its reusability, the result showed that it resulted in virtually constant yield for the fifth run.