Highly Efficient Pd@CN Catalysts with Quasi-Ordered Mesopores Synthesized from Recycled Mother Liquid of Zeolitic Imidazolate Frameworks

Zeolitic imidazolate frameworks (ZIFs) are promising porous materials; however, the mother liquid after crystallization was often thrown away, causing the waste of resources and environmental pollution. Therefore, it is urgent to develop methods for realizing the recycling utilization of the mother liquid of ZIFs. Herein, unreacted Co ions, imidazole linkers, and possibly intermediate precursor species in the mother liquid of ZIF-67 were recovered by rotary evaporation (marked as ZIF(Re)) and employed to synthesize porous carbon materials via one-step calcination. After pickling and Pd loading, the hierarchically porous Pd@CN catalysts were obtained, and the porous structures can be tuned by modulating the calcination temperature and pickling time. The as-fabricated Pd@CN-800-60 catalyst demonstrates comparable properties to the ZIF-67-derived catalyst Pd@CN(67)-800-60 not only in porous structures but also in hydrogenation ability (a turnover frequency (TOF) of 109.56 h^-1for the phenol hydrogenation to cyclohexanone). Moreover, the porous structures of Pd@CN-800-60 are relatively more ordered than Pd@CN(67)-800-60, and the quasi-ordered mesoporous structures strengthen the permeation of substrates and the accessibility of active components. More importantly, the productivity of Pd@CN-800-60 is 3.4 times higher than that of Pd@CN(67)-800-60. From a sustainable perspective, this study provides not only an efficient approach for recycling the mother liquid of ZIFs but also a low-cost procedure for preparing efficient Pd@CN catalysts with quasi-ordered mesoporous structures.