| The hydroformylation of alkenes is a major commercial process used for the production of oxygenated organic compounds. The homogeneous catalyst and organic or aqueous solvents are commonly employed for the hydroformylation of light alkenes. However, the hydroformylation of long-chain olefins in aqueous-organic system is noneffective due to the low-solubility of long-chain olefins in water. Moreover, the homogeneous hydroformylation meets the deactivation of rhodium catalysts at high temperature distillation in separation process of catalysts. Up to now, most of the hydroformylation of long-chain olefins applied the high-pressure-cobalt technique, but it has some disadvantages, such as complicated process of catalyst recycle, a great number of byproducts, high energy consumption and so on. In order to solve these problems, the heterogenization of homogeneous catalysis for hydroformylation of high olefins has been studied in recent years, but there still have been some disadvantages, such as low activity and high loss of catalysts. So, it is great significant to develop new supported catalysts.The production of isononyl-aldehydes from mixed octenes (2,4,4-trimethyl-1-pentene and 2,4,4- trimethyl-2-pentene) by hydroformylation is of important industrial application. Firstly, thermodynamic analysis were done, and Benson and Constantinou-Gani groups contribution methods were used to calculate the absent thermodynamic data, on the basis of molecular structure of 2,4,4-trimethyl-1-pentene, 2,4,4-trimethyl-2-pentene, isononyl- aldehydes and the organic synthetic principle. It was found that the hydroformylation of isooctene was strongly exothermic reaction and its equilibrium constant was very large under reaction conditions, therefore, the reaction was governed by kinetic process.The supported rhodium catalysts modified with phosphoric acid were prepared by incipient impregnation using activated carbon as support and rhodium trichloride trihydrate as precursor. N2 adsorption, Boehm titration and FT-IR spectra were used to investigate the influence of support, pretreatment and catalyst preparation conditions on catalyst performance. The results showed that the surface properties of support had great influence on catalyst. The alkali-heat-treatment of activated carbon was beneficial to improve catalytic performance, and was better than acid treatment and alkali washing treatment. Furthermore, the basic groups such as hydroxyl and carbonyl on activated carbon surface, as well as distribution of mesopores both got increased after alkali-heat-treatment. Orthogonal experiment was designed to optimize the process of support pretreatment and catalyst preparation. The optimized pretreatment conditions were alkali/carbon mass ratio of 1.0, activation temperature of 790?C, activation time of 75 min and pH of 9.0. The optimized catalyst preparation conditions were rhodium loading of 0.8%, phosphoric acid loading 5.0% and impregnation temperature of 45?C. The yield of isononyl-aldehydes could be up to 50.5% and the selectivity approach to 100% under the optimized conditions.The effects of reaction conditions on the hydroformylation of mixed octenes over Rh-P/AC were studied. The results showed that Rh-P/AC catalyst exhibited a high activity and selectivity to aldehydes. The average TOF (turnover frequency) within 5.0 h for the hydroformylation of mixed octenes over Rh-P/AC catalyst was up to 539.4 h-1. The problems of recycle and low activity of some other catalysts were well solved in this system. It was found that Rh-P/AC can be recycled 4 times without significant loss of rhodium, demonstrating that catalyst had high stability. Also, the catalytic activity of Rh-P/AC could be increased when toluene was used as reaction solvent.From the characterization of N2 adsorption, FT-IR, XPS and XRD, it was found that micropore surface area and pore volume of the activated carbon were nearly unchanged after loading with RhCl3. However, the total surface area and pore volume decreased, indicated that the active component dispersed on the external surface of mesopores and macropores. Simultaneously, it was observed that the -OH, -C=O and basic groups acted as the adsorption center to bond metal species by dipole-dipole (Ï€-d) interaction and redox reaction mechanism. After loading, the rhodium species exists in Rh2O3, Rh? and C-O-Rh complexes. There was the coordination interaction between Rh species and phosphoric acid species in the phosphoric acid modified catalysts. The XRD results showed that rhodium species dispersed monolayer on support.The experimental results revealed that alkali metals and some transition metals (including V, Zn, Zr, Cu, Pt, Mo and Co) could promote hydroformylation activity and selectivity of the Rh-P/AC catalysts. Among them, the combination of KOH and V exhibited the best promoting effect. The impregnation sequence had also influence the activity of the catalyst for hydroformylation. The catalyst impregnated RhCl3 after impregnation of promoters had the higher activity. The optimal loading amount of V species was 1.0% (mass fraction). The results of XRD and XPS indicated that V species increased the dispersity of Rh species and the binding energy of Rh3d on the support, as well as inhibited the formation of Rh? species.Hydroformylation reaction kinetics of mixed octenes on Rh-P/AC catalyst was studied in a high-pressure batch reactor under conditions free from the influence of external and internal diffusions. And the kinetics parameters were determined by fitting the changes of octenes concentrations with time. Based on the Langmuir-Hinshelwood mechanism, the intrinsic kinetic models were established, and model parameters were fitted by using mathematical method. It can be confirmed that kinetic model LHHW IV with desorption of aldehydes from catalyst as rate-control step could depict the experimental data well. The kinetic model may provide a reference for the scaling-up of isononyl-aldehydes synthesis from hydroformylation of mixed octenes.
|
PDF Full Text Request