The thermodynamically stable metal organic framework UiO-66-NH2 has experimentally been demonstrated as an ideal platform to isolate metal clusters within its nanocages; however, the electronic structures and the dynamics of the encapsulated metal clusters are still unclear. Ab initio molecular dynamics simulations combined with density functional theory-based methods were employed to search the stable structures of Pd n @UiO-66-NH2 composites, and their electronic properties were analyzed in detail. We found that the thermodynamics of the composites are highly correlated with charge transfer between the Pd n cluster and the UiO-66-NH2 framework, as well as the deformation energy of the framework. In addition, both ab initio molecular dynamics simulations and density functional theory calculations show that the small Pd clusters can easily diffuse into the tetrahedral cage of UiO-66-NH2 from the octahedral cage through the window connecting these two types of cages, with a small energy barrier.

The confinement effect has been proposed to explain the enhanced catalytic activity for chemical reactions inside the metal organic framework-based composites; however, the exact mechanism remains unclear. In this study, the chemoselective cinnamaldehyde (CAL) hydrogenation was theoretically investigated using Pt cluster-confined UiO-66-NH2. Thermodynamically stable structures of Pt n @UiO-66-NH2 composites with n up to 32 were found using ab initio molecular dynamics (AIMD) simulations combined with the density functional theory-based method. The AIMD simulations indicate that the O-tail adsorption mode of CAL was easily formed during its diffusion from the tetrahedral cage into the octahedral cage, as a result of the confinement effect of the framework. Intriguingly, it was found that the O-tail adsorption structure of CAL in Pt28@UiO-66-NH2 has a small free energy barrier for hydrogenation, less than 31.8 kJ/mol, giving rise to the product of cinnamal alcohol that is industrially desired. These barriers are much lower than those of the other potential reaction pathways starting from the CO and CC adsorption modes, although both of which are thermodynamically more stable. The computations support the fact that the limited space of the UiO-66-NH2 cages plays a key role in the chemoselective hydrogenation of CAL.

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