Research On Rhodium Complex Catalyst Based On Methanol Carbonylation Reaction

The industry mainly adopts the Monsanto process,which uses methanol and carbon monoxide as raw materials for the carbonylation reaction to produce acetic acid.The catalyst used in the Monsanto process([Rh(CO)2I2]-)easily forms dimers during the reaction and then decomposes to form RhI3 precipitation,and the noble metal rhodium catalyst is finally deactivated.Therefore,it is a research hotspot in this field to prevent Rh precipitation deactivation and improve Rh catalysts’ stability under the condition of maintaining catalyst activity.Based on the previous research,this paper explores the coordination of different ligands with Rh(Ⅰ)and uses steric hindrance to avoid the formation of Rh(Ⅲ)dimers effectively.The Rh(Ⅰ)compound is the precursor and the electron-donating group compound with a specific structure selected as the ligand.Using the method of B3LYP,the base group Lanl2dz was selected to explore the reaction mechanism of the rhodium catalyst catalyzed by the carbonylation reaction.Simultaneously,the theoretical research results are used to explore the critical relationship between the catalyst structure and the reaction performance and finally guide the laboratory to synthesize highperformance catalysts with higher activity and stability.The catalytic mechanism was corrected through the experimental characterization results,and the reaction mechanism of the rhodium-based catalyst catalyzed by the carbonylation reaction to acetic acid was obtained.Obtain the catalytic mechanism and intermediate structure characteristics through theoretical research,and guide the laboratory to synthesize the target catalyst.Simultaneously,the target catalyst’s structure is confirmed through infrared,electron microscopy,nuclear magnetic,XPS,and other means.Also,the catalyst evaluation experiment was carried out in the micro-reactor,and the product was analyzed by gas chromatography.The specific research results are as follows:1.For the Rh(Ⅰ)-o-aminophenol catalyst,according to the theoretical calculation results,the catalytic mechanism is similar to that of the industrial rhodium iodine catalyst.When the rate-determining step CH3I addition reaction reaches the transition state,the Rh-N bond is slightly elongated,and the Rh-O bond is elongated and broken to adapt to the change of configuration in the catalyst catalysis process.The use of steric hindrance effectively avoids the formation of Rh(Ⅲ)precipitation.The nitrogen and oxygen atoms in the ligand increase the electron density of the active center Rh(Ⅰ),which is conducive to the nucleophilic addition reaction,and the activity of the catalyst is improved.Through theoretical calculations,the rate-determining reaction step barrier in the catalytic process of Rh(Ⅰ)-o-aminophenol catalyst is reduced by 33.31 kJ/mol coMPared with the rate-determining reaction step barrier in the catalytic process of Monsanto catalyst.Combining the results of infrared and XPS,it concluded that the nitrogen and oxygen elements in o-aminophenol are combined with the central metal rhodium of Rh2(CO)4Cl2.By investigating the optimal reaction conditions of Rh(Ⅰ)-o-aminophenol catalyzed carbonylation,it was found that under the reaction conditions of 190℃,3.5 MPa,6 wt%water and 49 wt%acetic acid solvent,the Rh(Ⅰ)-o-aminophenol catalyst has the best catalytic performance in catalyzing the carbonylation with methanol as the raw material,without the production of by-products such as methyl acetate,and the selectivity of acetic acid is 100%.After determining ICP-MS,in the Rh(Ⅰ)-o-aminophenol catalytic system,the loss of precious metal rhodium was only 0.01 g/L before and after the catalytic reaction.It can be seen that the catalyst has better stability.2.Rh(Ⅰ)-8-hydroxyquinoline catalyst,the characteristic peak of the terminal rhodium carbonyl peak appears in the infrared spectrum.Combined with the results of the XPS characterization,Rh(Ⅰ)binds to the nitrogen and oxygen atoms in the ligand 8-hydroxyquinoline.Combining the two characterization methods of AAS and ICP-MS,it is found that the molar ratio of rhodium to nitrogen is about 1.21.It confirmed once again that the ligand 8-hydroxyquinoline successfully coordinated with Rh2(CO)4Cl2.The Rh(Ⅰ)-8-hydroxyquinoline catalyst molecule is a four-coordinated structure with an asymmetric structure.Due to the asymmetric molecular configuration during the catalysis process,it is beneficial to coordinate with the flexibility of the molecular configuration during the rate-determining reaction of methyl iodide Change.The reaction is easier to proceed and the formation of rhodium precipitation is not easy.ICP-MS was used to determine the rhodium content in the catalytic system before and after the catalytic reaction of Monsanto catalyst and Rh(Ⅰ)-8-hydroxyquinoline catalyst.The performance coMParison under the same conditions shows that the acetic acid yield and acetic acid selectivity of the Rh(Ⅰ)-8hydroxyquinoline catalytic system are higher than those of the Rh2(CO)4Cl2 catalytic system.The Rh(Ⅰ)8-hydroxyquinoline catalyst is a catalytic system with good performance.Simultaneously,in the Rh(Ⅰ)-8hydroxyquinoline catalytic system,the loss of precious metal rhodium before and after the catalytic reaction is reduced by 0.05 g/L coMPared to the Rh2(CO)4Cl2 catalytic system.Therefore,coMPared with the rhodium iodine catalyst,the Rh(Ⅰ)-8-hydroxyquinoline catalyst has higher stability.3.The Rh(Ⅰ)complex connected to the N and O in the 3-hydroxy-2-nitropyridine complex.According to the results of infrared and XPS characterization,it is preliminarily concluded that the nitrogen and oxygen elements in 3-hydroxy-2-nitropyridine combine with the central metal rhodium of Rh2(CO)4Cl2 to form Rh-N coordination bonds and Rh-O coordination bonds.The asymmetric chelating structure of the catalyst helps to catalyze the deformation of the molecular configuration during the reaction.In the process of catalytic reaction,the force between Rh-N bonds is more robust,and the force between Rh-O bonds is weaker and it is obviously elongated and broken.This combination of strength and weakness facilitates the synergistic effect of the catalyst during catalysis and avoids the formation of dimers.According to the theoretical calculation of the energy of each molecular configuration in the catalytic process,after coMParing with the barriers of each elementary reaction in the catalytic process of Monsanto catalyst,it is found that the catalytic activity of the Rh(Ⅰ)-3-hydroxy-2-nitropyridine catalyst is higher in the catalytic process.By investigating the optimized conditions for the catalytic carbonylation of Rh(Ⅰ)-3-hydroxy-2-nitropyridine catalyst,at 190℃,3.5 MPa,6 wt%water and 54 wt%solvent content,Rh(Ⅰ)-3-hydroxy-2-nitropyridine catalyst has the best catalytic performance for the carbonylation of methanol,and the selectivity of acetic acid reaches 100%.ICP-MS was used to measure the content of rhodium in the catalyst.CoMPared with the catalytic system of Rh2(CO)4Cl2,in the catalytic system of Rh(Ⅰ)-3-hydroxy-2-nitropyridine catalyst,the loss of the noble metal is reduced by 0.07 g/L.