This study is led by Prof. Yingwei Li (School of Chemistry and Chemical Engineering, South China University of Technology) and Prof. Kui Shen (School of Chemistry and Chemical Engineering, South China University of Technology). In this contribution, they report the encapsulation of N-doped carbon into metal-organic frameworks (MOFs) with controllable nanoarchitectures and porosities as multifunctional catalysts for highly efficient CO2 fixation. They assembled MIL-101 embedded ionic liquids (i.e., BmimBr@MIL) that is subsequently used as a precursor to construct composite materials with mesopore pores in sizes of 3.5 to 20 nm. During the pyrolysis, the crystallinity of MIL-101 can be well preserved, while its structure achieved a dense-to-porous transformation with N-doped nanocarbon (CN) formed in the framework of MIL-101. The obtained CN@MIL can efficiently facilitate the mass diffusion of substrates, exhibiting excellent catalytic performance in the synthesis of cyclic carbonates from epoxides and CO2 under mild and co-catalyst-free conditions (90 °C and ambient pressure of CO2).
Li et al. drive away at developing efficient heterogeneous catalysts for CO2 cycloaddition under mild conditions without co-catalysts. Their team proposed a facile one-step pyrolysis strategy to in-situ encapsulate N-doped nanocarbons into the pores of MOFs and thus achieve multi-functional catalytic sites in the obtained MOF-based composites. They have found that the enhanced thermal stability of MIL-101 by the space-occupying effect of BmimBr is the key to prevent the porous structure of MIL-101 from collapsing. Therefore, during the pyrolysis, the crystallinity of MIL-101 can be mostly preserved, while its structure achieves a dense-to-porous transformation with N-doped nanocarbon (CN) being formed in the framework.
The researchers also investigated the dense-to-porous transformation mechanism by TG-MS. The obtained results unequivocally indicate that the ligands of BmimBr@MIL(0.67) can be decomposed via a different mechanism from those of its parent MIL-101, which further confirms that the enhanced stability of BmimBr@MIL(0.67) can efficiently prevent the porous structure of MIL-101 from collapsing.
To simultaneously investigate the advantages of hierarchical pores and multi-active sites of CN@MIL(400, 0.67, 30), they evaluated the catalytic performances of the as-synthesized materials for the CO2 cycloaddition with large-size epoxides. They found that the optimized CN@MIL (400, 0.67, 30) exhibits excellent catalytic activity and can achieve 96% yield towards the cycloaddition of 4-(2,3-epoxypropoxy)carbazole (4-EPC) to 4-(((9H-carbazol-4-yl)oxy)methyl)-1,3-dioxolan-2-one (4-CDO) at 1 bar CO2 in the absence of co-catalysts.
This controlled pyrolysis strategy can not only provide new insight into the pyrolysis process of ILs encapsulated inside MOF frameworks, but also offer an alternative and efficient way to integrate MOFs with many other functional large molecules for advanced applications.
See the article:
N-doped nanocarbon embedded in hierarchically porous metal-organic frameworks for highly efficient CO2 fixation
Science China Chemistry