Palladium covalently anchored imidazolium functionalized SBA-15: An efficient recoverable heterogeneous catalyst for the hydrocarboxylation of styrene

An efficient Pd/Imi-SBA-15 heterogeneous catalyst was synthesized by a covalent anchoring strategy for the hydrocarboxylation of styrene. Various characterization methods (e.g. FT-IR, XRD, 29Si MAS NMR, BET and ICP) demonstrated that PdCl2 was successfully anchored in the channels of SBA-15, and the mesoporous structure of the SBA-15 substrate was maintained throughout the grafting process. The Pd/Imi-SBA-15 catalyst was proven to be highly efficacious, persistent and reusable in the catalytic system for the hydrocarboxylation of styrene due to an outstanding synergistic effect between H+/Cl--Pd. The presence of H+/Cl- could promote the formation of the H-Pd complex, and accelerate the reaction process. The DFT calculations indicated that a higher orbital energy level of the adsorbed styrene in the [HPdLCl] complex (L = PPh3) existed than that of free styrene, realizing that the [HPdLCl] complex promoted the electron activity of styrene and thus facilitated the styrene carbonylation process. Furthermore, PPh3 was more likely to induce the transformation of styrene into branched products. In the end, a practicable catalytic mechanism of Pd/Imi-SBA-15 for hydrocarboxylation was proposed.