Study On The Preparation Of Palladium Chloride Catalyst Supported On Organic-inorganic Hybrid Materials And Its Catalytic Property In The Oxidative Carbonylation

Homogeneous catalysts show high catalytic activity and selectivity, but the catalysts are very difficult to recovery and recycle as both reagents and catalysts are generally in the same phase. This drawback has become the"bottleneck"to develop homogeneous catalytic reaction to commercialization. In recent years, the development of highly effective, environmentally friendly and recyclable"green"catalysts has been attracted significant interests. Organic-inorganic hybrid materials (OIH), in which the organic groups are covalently attached to porous inorganic solids, can be used as catalyst support. The goal in this paper is to utilize the organic moiety as the active site and the inorganic solid to provide avenues for recovering the active site. This new kind of catalyst system is expected to contribute to the development of commercialization of homogeneous catalytic reactions.OIH was prepared by Sol-gel technique usingγ-chloropropyltriethoxysilane (CPTES) as spacer, 1,2-diaminocyclohexane as organic ligand, and tetraethoxysilane (TEOS) as precursor, then the OIH reacted with Pd(PhCN)2Cl₂ to make heterogeneous catalyst PdCl₂/OIH (A). The structure of support and heterogeneous catalyst PdCl₂/OIH (A) were characterized by EA, BET, FT-IR, XPS, TGA, DTA, XRD and 1HNMR. The palladium loading and leaching in the reaction solution were detected by atomic absorption (AA).The catalytic property of PdCl₂/OIH (A) in the oxidative carbonylation of phenol to diphenyl carbonate (DPC) was studied in this paper. The effects of different reaction parameters such as the molar ratios of organic moiety to TEOS, inorganic cocatalysts, solvents, palladium loading and the molar ratios of phenol to palladium on the catalytic activity and palladium leaching were studied in details. Cu2O and tetrahydrofuran (THF) were found to be the best additives in these heterogeneous catalyst systems in terms of higher yield of DPC and lower palladium leaching in the reaction solution, and the heterogeneous catalyst revealed higher catalytic activity comparing with the homogeneous catalyst PdCl₂. In the presence of 3A molecular sieves as dehydrating agent, and the reaction temperature of 110℃, reaction time of 5 h, and pressure of 4.4 MPa with the molar ratio of organic moiety to TEOS was 1:9, n(Pd²⁺):n(Cu2O):n(BQ):n(Bu4NBr):n(PhOH) =1:6:30:30:1000, the yield of DPC based on phenol and TON were 6.2% and 31 mol DPC/mol Pd using PdCl₂ as main catalyst in the homogenous catalytic system. However, the heterogeneous PdCl₂/OIH (A) revealed excellent catalytic performance in terms of the yield of DPC and TON reached 12.0% and 60 mol DPC/mol Pd, and palladium leaching was only 3.0wt% in the reaction solution.The products in the oxidative carbonylation of phenol to DPC with different catalytic systems were identified by GC-MS. The results revealed that the distribution of products was related to compounets in the catalytic systems. PdCl₂/OIH (A)-Cu2O-THF was not only revealing higher catalytic activity, but also giving lower palladium leaching in the reaction solution, and fewer byproducts comparing with PdCl₂/OIH (A)-Co(OAc)2-CH2Cl₂. The existence of Pd(0) in the recovered catalyst after reaction was confirmed by XRD, and a possible mechanism in the oxidation carnonylation of phenol to DPC in the presence of Cu2O and heterogeneous catalyst was also studied, which follows the"multi-electronic transfer"mechanism.Palladium chloride catalysts PdCl₂/OIH (B) anchored on silica-based support were synthesized by Sol-gel technique usingγ-aminopropyltriethoxysilane (APTES) as spacer, 2-pyridinecarboxaldehyde and 2-acetylpyridine as organic ligands and TEOS as precursor. It is also used as catalyst in the oxidative carbonylation of phenol to DPC. The support and the heterogeneous catalyst were characterized by EA, BET, FT-IR, TGA and XPS. The effects of different reaction parameters such as reaction temperature, reaction pressure, reaction time, inorganic cocatalysts, solvents and organic moieties on the yield of DPC and palladium leaching were also studied. The heterogeneous catalyst prepared from 2-acetylpyridine revealed the highest catalytic activity. The yield of DPC and TON reached 13.7% and 68.5 mol DPC/mol Pd, and the palladium leaching was 4.0wt% in the reaction solution.The recycle performance of PdCl₂/OIH (B) prepared from 2-acetylpyridine was studied without employing 3A molecular sieves. The heterogeneous catalyst revealed good recycle catalytic performance and stability for the yield of DPC and palladium leaching remained almost unchanged during 4 recycles.Hybrid mesoporous silica MCM-41 (O-MCM-41) was prepared using APTES as spacer, TEOS as precursor, and 2-acetylpyridine as organic moiety by anchored method and co-condensation method. The heterogeneous palladium catalysts (PdCl₂/O-MCM-41) were then prepared by the reaction of the hybrid O-MCM-41 with Pd(PhCN)2Cl₂. The products were characterized by means of FT-IR, XRD, XPS and N2 adsorption-desorption isotherms. The results suggest that the hexagonal mesoporous framework of the supported catalyst was kept perfectly by anchored method, and the structure was destroyed badly by co-condensation method.The catalytic property of the heterogeneouse catalyst was affected by preparation method and silica based support, which increased with the disorder of the support. The catalyst prepared by anchored method reveals poor catalytic activity in the oxidative carbonylation of phenol to DPC. The yield of DPC and TON were 5.8% and 29 mol DPC/mol Pd, respectively.The heterogeneous catalysts supported on OIH were more active and more stable comparing with traditional catalysts supported on inorganic and polymer supports under the same reaction conditions in terms of higher yield of DPC and lower palladium leaching in the reaction solution.