Abstrakt
2D-to-3D transformation of layered zeolite precursors (e.g., MCM-22 P, IPC-1 P, pre-FER) into three-dimensional frameworks in combination with metal encapsulation has been recently shown as efficient synthetic strategy to stabilize metal clusters in a zeolite matrix. Even though, tailoring the number of acid sites in a zeolite support and its effect on the properties of thus prepared metal@zeolite materials (e.g., metal dispersion and catalytic activity) remained unrevealed.
Herein, we report on the synthesis of Pd@MCM-22 catalysts via modified 2D-to-3D transformation method, which allows to achieve close to nominal loading of noble metal (e.g., 0.1, 0.3, 0.8 wt%) by time-separation of swelling of MCM-22 P layered zeolite precursor and subsequent incorporation with metal. In turn, alternating Si/Al ratio of MCM-22 P (e.g., SiAl = 15, 20, 30) enabled the preparation of Pd@MCM22 catalysts with variable concentration of acid sites (0.28 - 0.55 mmol/g), while not affecting the Pd dispersion.
Decrease in Pd dispersion with increasing metal loading was accompanied with declining catalytic activity in a model reaction of selective hydrogenation of nitro-group in 3-nitrotoluene: 0.1 wt% Pd {dispersion = 52.6%, reaction rate = 21.2 mol.g(-1)Pd.h(-1)} > 0.3 wt% Pd {9.6 mol.g(-1)Pd.h(-1)} > 0.8 wt% Pd {23.2%, 3.7 mol.g(-1)Pd.h(-1)}. The results of this study reveal that the reported synthesis strategy can be successfully used to confine small metal nanoparticles in a zeolite matrix while tuning its acidic characteristics for designing bifunctional metal acidic catalysts.