The 2V-P,N polymer supported palladium catalyst for methoxycarbonylation of acetylene
Graphical abstract
Introduction
Acrylic esters are important bulk chemicals which are widely used in textile, leather finishes, polymer synthesis etc. [1], [2], [3]. Since Reppe and co-works reported alkoxycarbonylation of terminal alkynes firstly in 1939, carbonylation of acetylene with CO and alcohol, a non-petroleum approach, has been extensively studied as potential route for direct synthesis of acrylic esters [4]. In addition, as a highly versatile process of high atom economy (Scheme 1), acrylic acid and their derivatives could be synthesized in one step [5], [6], [7].
So far, the homogeneous palladium catalyst system consisting of 2-pyridyl-diphenylphosphine (2-PyPPh2), palladium acetate (Pd(OAc)2) and a sulfonic acid have received great attention due to their high activity and selectivity in the carbonylation of alkynes under mild conditions [8], [9], [10], [11], [12], [13]. Drent et al. employed the complex catalysts for methoxycarbonylaiton of propyne giving turn over numbers as high as 40,000 molproduct molcat−1 h−1 [14], [15]. Tang and co-workers reported that the catalyst exhibited high activity for hydrocarbonylation of acetylene [16], [17]. It is especially worth mentioning that the organic ligand 2-PyPPh2 is of critical importance in the reaction that replacement of it by a phenyl group or a heteroaromatic ring without N atoms results in a dramatic decrease in catalytic activity [14], [15], [18]. This is attributed to that 2-PyPPh2 functions as a bidentate ligand, and this type of P-coordinated N-promoted 2-PyPPh2 ligand facilities rapid proton transfer in the rate determining protonlysis step. In addition, the N atom of the pyridyl ring can also work as acceptor of the alcoholic proton, thus favouring the cleavage [18].
Such kind of above homogeneous catalytic system play an extremely important role in the field of modern catalysis due to their high activity and selectivity. However, the difficulties of recovery and recycling restrict their more extensive application [19]. A series of approaches have been developed to solve this problem [20], the main one is to heterogenize the metal complex, where silica-based materials and magnetic nanoparticles are the most commonly used materials for such supports [21], [22], [23], [24], [25]. However, their chemical nature limits their potential for chemical modification process. Moreover, the catalytic moieties are usually in homogeneously distributed and pendent into the pore volume thus deactivating the catalytic system [26]. Recently, porous organic polymers (POPs) have emerged as a versatile platform for the development of highly efficient catalysts due to their porous nature and high surface area [27], [28], [29]. Recently, we developed a porous organic ligand (POL–PPh3) supported Rh catalysts with high activities and stabilities for hydroformylation of olefins [30], [31]. Sun et al. reported a hierarchical porous ionic organic polymer as a new platform for heterogeneous phase transfer catalysis [32]. Therefore, the POPs supported catalysts have great potential to be developed using in industrial fields. Doherty et al. immobilized 2-PyPPh2 to an insoluble solid support, but the process they employed for copolymerization is difficult to operate [33]. Inspired by the POPs material, we have successfully synthesized a POL-2V-P,N porous organic polymer under solvothermal conditions and prepared the insoluble POPs supported palladium complex catalyst. The synthesized heterogenized homogeneous catalyst showed high activity for the methoxycarbonylation of acetylene with CO and CH3OH to produce methyl acrylate.
Section snippets
Materials
All materials (A.R. grade) were obtained from commercial sources and used without further purification unless otherwise stated. Tetrahydrofuran (THF) was distilled from sodium and pyridine (py) from calcium hydride under argon. Acetone was refluxed with 4A zeolite and methanol with magnesium and iodine under argon.
Preparation of POL-2V-P,N
All operations were carried out under anaerobic anhydrous conditions. The synthesis of POL-2V-P,N is shown in Scheme 2. [2-C5H4NZnCl·C5H5N]2 was prepared according to the method of
Characterization of POL-2V-P,N
Fig. 1 shows the TG curve of the POL-2V-P,N sample, N2 sorption isotherms, SEM and TEM images. TG analysis was utilized to examine the thermal stability of the POL-2V-P,N. The TG curve showed a minute weight loss at about 200 °C owing to the removal of physically absorbed solvent in Fig. 1(A). The biggest weight loss was observed in the range of 400–500 °C due to the decomposition of the polymer. It indicated that the POL-2V-P,N was a highly thermally stable material. As shown in Fig. 1(B), the N2
Conclusions
In summary, POL-2V-P,N was successfully synthesized by polymerizing 2-vinyl-functional diphenyl-2-pyridylphosphine. And the heterogenized homogeneous Pd/POL-2V-P,N catalyst was prepared by immobilizing Pd(OAc)2 on the POL-2V-P,N support. The results of 31P NMR, XPS, EXAFS and FT-IR of the Pd/POL-2V-P,N catalyst indicated that Pd was coordinated with the exposed P and N atoms of the 2V-P,N polymer. The heterogenized Pd/POL-2V-P,N catalyst showed higher activity for methoxycarbonylation of
Acknowledgements
We thank Prof. Xiangping Hu and Mr. Xianchun Liu for useful discussions and contributions. We are also grateful for Mr. Yimin Li, Miss. Huimin Gong and Mr. Jike Jiang for technical assistance. This work was supported by the National Natural Science Foundation of China (21273227).
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