Theoretical Evidence on the Confinement Effect of Pt@UiO-66-NH₂ for Cinnamaldehyde Hydrogenation

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 Ptn@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.