| As a kind of typicalδ-donor ligands, N-heterocyclic carbenes (NHC) become a class of important ligands for their versatilities in the formation of transition metal complexes, which can substitute classical 2e- donor ligands such as amines, ethers and phosphanes in metal coordination chemistry. The properties of NHC ligands are similar to that of organophosphines, and its have been described as alternatives to phosphines in terms of bonding and reactivity. In general, NHCs are more stable than phosphines, and have some advantages on forming strong bonds to metal centers, with little tendency of dissociation. A range of homogeneous NHC complexes, appeared during the past few years, have showed the unique catalytic properties for a variety of reactions such as Suzuki and Heck Coupling reactions, olefin metathesis reactions, etc. They show excellent catalytic activity due to the strong abilities of NHCs in stabilizing and activating active metal centers. In recent years, the development of NHC-metal complexes is fast, and many researchers have been exploring the applications of this kind of typical ligands. However, as a homogeneous catalyst system, NHC-metal complexes are difficult to be separated from the products, thus restricting the practical applications of such ligands.Recently, more attention has been drawn on the synthesis of organic-inorganic hybrid materials containing metal complexes. Especially, mesoporous molecular sieves with high surface area and uniform pore sizes have been widely used as support for the preparation of the hybrid materials. Organic-inorganic hybrid materials are a kind of heterogeneous catalytic materials and they offer several advantages in catalytic fields, e.g. the high activity (originating from homogeneous catalysts) and easy separation. Besides, the catalytic performance may also be improved by changing the structure and the surface properties of the solid supports. In recent decades, there were some reports concerning on the synthesis and catalytic property of organic-inorganic hybrid materials containing NHC-metal complexes, and it was found that this kind of hybrid materials show excellent catalytic performances and potential applications in a variety of reactions such as Suzuki and Heck reactions, etc. In order to achieve more catalytic applications, we tried to prepare some novel organic-inorganic hybrid mesoporous materials containing NHC-metal complexes by the post-grafting method (using SBA-15 or MCM-41 as supports). Their catalytic properties were also investigated in the selective oxidation of benzyl alcohol, the epoxidation of cyclooctene, and the C-C coupling reactions. In addition, the nature of active sites of the catalysts was also discussed by combining different characterization means. The main research contents are as follows:We synthesized a kind of organic-inorganic hybrid mesoporous materials (1-methyl imidazole as a ligand and Pd as active site) by post-grafting and co-condensation methods. The imidazole ligand was introduced onto the amino-functionalized SBA-15 and this obtained organic-inorganic hybrid materials containing NHC ligands (named as 1c), and then the imidazole-functionalized materials reacted with Pd complexes to obtain Pd-NHC complexes (named as 1d). The successful synthesis of the resultant catalysts were confirmed by combining different characterization means. Moreover, 1d is active and stable in the selective oxidation of benzyl alcohol. Also, 1d showed good catalytic performance in Suzuki and Heck Coupling reactions. In addition, hybrid materials containing other kinds of transition metals-NHC complexes were also prepared, and it was found that the hybrid materials (containing Mo speies) showed good activity and stability for the epoxidation of cyclooctene. We prepared the hybrid material with N-methyl imidazole as a ligand and Pd cation as active site (denoted as 1e) by using the treatment of H2 reduction. First, mesoporous SBA-15 grafted by 3-aminopropyl groups reacted with NHC-Pd complexes (named as NHC-Pd-SBA-15), and the NHC-Pd-SBA-15 material was reduced with H2 to obtain Pd nanoparticles. 1e kept the long-range mesopore ordering of the SBA-15 material and the structure of organic ligand wasn't destroyed, which are demonstrated by various characterizations. As shown by TEM images, a small part of Pd nanoparticles with average size ca. 3.5 nm existed on the surface of mesoporous SBA-15. The catalytic performance of 1e was investigated for the aerobic oxidation of benzyl alcohol, and this material showed excellent activity and stability, and no obvious leaching of Pd species could be detected during the reaction process. The size of Pd nanoparticles was still kept after reused several times, indicating the excellent stability of Pd particles against sintering. We proposed here that the combination of NHCs ligand and the size-restricted mesopores of SBA-15 might be benefit for the formation of active and stable palladium nanoparticles. The effects of the types of external bases, reaction temperature, supports and loading amount of Pd species were also investigated and then the reaction conditions were optimized. By comparing with the catalytic performance of 1d and 1e in the oxidation of alcohol, we confirmed that the presence of palladium nanoparticles should be responsible for the higher activity of 1e. It's worthy to mention here that previous studies clearly showed that the supported Pd nanoparticles were efficient catalysts for the oxidation of alcohol. However, the leaching and/or the agglomeration of coordinated palladium nanoparticles to form huge particles of palladium resulted in the considerable decrease of catalytic activity. Our current results indicated that the existence of NHCs ligand and the size-restricted mesopores of SBA-15 might be benefit for the formation of active and stable palladium nanoparticles. In order to open up the applications of 1e, this material was also used for catalyzing Suzuki and Heck Coupling reactions and showed relatively high activity. Besides, in the same way, Mo-containing hybrid material was also prepared by introducing oxodiperoxo molybdenum species onto the NHCs functionalized SBA-15 material (the obtained material was denoted as SBA-I-Mo-nano). And it's found that this material is an active catalyst for the liquid-phase epoxidation of cyclooctene and show relatively high stability after used in this reaction for several times.Another kind of NHC-Pd functionalized SBA-15 material (named as 7t) was synthesized by a post-grafting method, in which the N-substituted imidazole with 2,6-diisopropylphenyl group was used as the ligand. The catalytic performance of this catalyst was also investigated for the aerobic oxidation of benzyl alcohol. It was found that 7t is active and stable catalyst for the reaction, and it could be reused for three times without deactivation, and no obvious leaching of active Pd species could be observed during the reaction process. These results confirm further that NHC is a kind of efficient ligand for stabilizing palladium species. Meanwhile, it also suggest that the catalytic performance of NHCs functionalized hybrid materials could be adjusted efficiently by the substitution of methyl group with other groups (e.g., 2,6-diisopropylphenyl).Besides, the NHC-functionalized SBA-15 hybrid materials (e.g. 1c) were further used for catalyzing transesterification of phenol with dimethyl oxalate. It is found that this kind of metal-free hybrid material shows good catalytic performance for the transesterificaiton reacion. We suppose that N-methyl imidazole (as Lewis base) might be mainly responsible for the good activity of the hybrid catalyst. Meanwhile, it is possible that the transesterification reaction could also be improved by the presence of the mesoporous SBA-15. |
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