Controllable synthesis of ultrasmall Pd nanocatalysts templated by supramolecular coordination cages for highly efficient reductive dehalogenation

Supported ultrasmall noble metal nanocatalysts (UNMNs) are one of the most important classes of solid materials for heterogeneous catalysis. Herein, a simple and efficient supramolecular coordination cage (SCC) template-strategy has been developed to synthesize UNMNs with controllable size and size distribution. A series of SCCs, including M2L4, M4L2, M6L4 and M12L24, with well-defined sizes and shapes as well as different numbers of Pd ions were designed and synthesized as templates. Subsequently, the corresponding Pd nanocatalysts M2@CMC, M4@CMC, M6@CMC and M12@CMC were prepared by an impregnation-reduction method on carboxymethylcellulose (CMC) hydrogel supports. It was found that the employment of SCCs as templates could not only significantly reduce the aggregation tendency of Pd nanoparticles but also play an important role in regulating their size and size distribution. For example, the analysis of the catalyst size distribution indicated that the greater the number of Pd ions the cage possesses, the bigger the size of the catalyst. Moreover, with the decrease of the concentration of the template, the size of the Pd nanocatalyst also decreased obviously. Particularly, the resultant catalyst with a nano-Pd loading as high as 12.63% could still maintain a narrow size distribution. Furthermore, the as-prepared Pd nanocatalyst could serve as a highly efficient polychlorinated biphenyl (PCB) degrader both in a stirred vessel and a continuous flow reactor because of its excellent catalytic efficiency in the reductive dehalogenation reaction under mild conditions. In a word, the SCC template-strategy employed in this study provides new guidelines for the preparation of size-controllable UNMNs on a variety of supports, along with high noble metal loading and catalytic activity.