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2D-to-3D transformation of layered zeolite precursors (e.g., MCM-22P, IPC-1P, 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-22P layered zeolite precursor and subsequent incorporation with metal. In turn, alternating Si/Al ratio of MCM-22P (e.g., Si/Al = 15, 20, 30) enabled the preparation of Pd@MCM-22 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-1Pd·h-1) > 0.3 wt. % Pd (9.6 mol·g-1Pd·h-1) > 0.8 wt. % Pd (23.2%, 3.7 mol·g-1Pd·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. |
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