Supercritical Carbon Dioxide Catalytic Hydrogenation To Formic Acid On The Immobilized Ruthenium Catalyst

Carbon dioxide which causes the greenhouse effect is the most abundant carbon resource and the fixation and utilization of carbon dioxide effectively is meaningful both for environmental protection and energy shortage problems. The exploitation of the low chemical potential energy resource is also one of the important way for the sustainable development strategy.In addition, carbon dioxide is a very inert molecular, so its activity and conversion is of important field for C1 chemistry and is of challenged basic theory problem. This dissertation investigated the supercritical carbon dioxide hydrogenation to formic acid using the immobilized ruthenium catalyst. (Supported by the National Natural Science Foundation of China (No. 20173048) and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20030335068)).The synthesis of formic acid from carbon dioxide hydrogenation is not only an atom economic reaction, but also following the development trend of green chemistry. To overcome the disadvantage of high active homogeneous catalyst, such as the difficulty of separation and recycle, high cost and so on. The inorganic oxide and polymer resin, witch has higher mechanical strength and chemical stability, were functionalized as catalyst supports to anchor the homogeneous ruthenium complex with high activity and selectivity. These immobilized Ru catalyst exhibited good catalytic performance in hydrogenation reaction.Many results were obtained by investigating catalyst itself and reaction conditions in detail, which could effect the catalytic performance greatly, and this is help to the development of promising catalysts. More than one technique (such as element analysis, BET, FT-IR, UV-Vis, XRD, TEM and quantum chemistry theoretic calculation) is adopted to analysis and characterize the process of functionalization, ruthenium complex immobilization and the differences of the catalyst activity, and discussed the possible structure ofimmobilized ruthenium catalyst, the way of the carbon dioxide activity and the possible reaction mechanism. The results we obtained are as the following:1. Varieties of immobilized ruthenium catalysts were synthesized and they were used to the reaction of formic acid synthesis from SCCO2 hydrogenation. The immobilized ruthenium catalyst makes it easy to separate and recycle the catalyst from reaction system. The result of element analysis, FT-IR, and atom absorption indicated that the process of supports functionalization and ruthenium immobilization went well. The analysis of XRD and TEM showed that the structure of MCM-41 is complete after functionalization and immobilization.2. For the three type of immobilized ruthenium catalyst supported on y-Al2O3> S1O2 and MCM-41, both catalysts supported on SiC>2 and molecular sieve had high activity, and the TON number reached as high as 792 and 969 respectively, far higher than the activity of the one supported on Y-AI2O3. Besides, the catalyst supported on sieve was more stable, and its activity still preserved about 90 % after it reused 3 times.3. The immobilized ruthenium catalyst supported on PS resin also successfully catalyzed the synthesis of formic acid form supercritical carbon dioxide hydrogenation. And the catalyst with lower crosslinking degree and smaller size resin support has higher reaction activity and stability. The immobilized ruthenium catalyst could adjust and alter greatly, and exhibited more helpful application outlook.4. The effect of deferent ligands on the activity of immobilized ruthenium catalyst anchored on MCM-41 support was firstly investigated. As for the phosphine ligands with the same coordinate atom, the activity changed as: PMe3>PPh3>Pli3P=O;and for the ligands wih similar structure and different coordinate atom, the activity changed as the following: PPb.3>AsPh3>NPli3;and for the bidentate phosphine ligands, the activity changed as dppe>BINAP>dppbo The quantum theory calculation agreed with this result. It indicated that catalyst with bidentate phosphine ligands had higher activity than that with PPI13 ligand, but the stability was the opposite.The highest activity was obtained from catalyst MCM-(CH2)3NHCH3-RuCl2(PPh3)3, and the TON reached 1090. The activity of catalyst wih -SH functional group is lower than that with -NH2 group, but the former one was more stable.5. When the functional group has special structure or different coordinate atoms, the catalyst exhibited very different activity. Supported on the resin with higher chelate ability, the catalyst was more stable, but the activity could decrease greatly. The cage structure and four coordinate N atoms of methenamine could make higher activity and stability. Catalyst supported on phosphine functionalized carrier also had high activity, because it coordinated with ruthenium atom to form complex and provide an environment which was closer to the homogenous calatysis system. But this kind of carrier was more difficult to prepare and its activity would loss when exposured in the atmosphere.6. The reaction mechanism of supercritical carbon dioxide hydrogenation to formic acid on immobilized ruthenium catalyst was discussed, and proposed that the activation of carbon dioxide was via carbon dioxide abnormal insertion into an M-H bond to give metallo carboxylic acid under the action of ruthenium complexes during the hydrogenation of carbon dioxide to formic acid.In this mechanism, a phosphine ligand in the ruthenium hydride complexes is replaced by protonic solvent ROH to generate the chain carrier. Subsequent insertion of carbon dioxide in to the Ru-H bond occurs to give the formato complex. Then hydrogenolyis of the RU-O2CH bond by molecular hydrogen forms formic acid, regenerating the catalytic species, finishing the catalytic cycle.7. The optimization of reaction conditions was carried out in supercritical carbon dioxide hydrogenation to formic acid reaction with immobilized catalyst. The effect of pressure, temperature, CO2 supercritical state, stirring rate, solvent and base were investigated in detail, and many experiential rules were obtained. This provides firmed ground for the further research.