Enabling long-distance hydrogen spillover in nonreducible metal-organic frameworks for catalytic reaction

Hydrogen spillover, a phenomenon where hydrogen atoms activated on a metal surface migrate onto a support material, plays a significant role in heterogeneous catalysis and hydrogen storage. While extensively studied in reducible metal oxides, where long-distance proton-electron coupled migration is established, its behavior on nonreducible supports remains less understood. Hydrogen migration on nonreducible supports is often limited by defects and short distances. This is a significant limitation because many industrial catalysts utilize nonreducible supports due to their superior thermal and structural stability, as well as tunable acidity. Metal-organic frameworks (MOFs), known for their porous crystalline structure, precise molecular arrangement, and synergistic catalytic effects, offer a promising alternative as catalyst supports. Previous studies on hydrogen spillover in MOFs have shown short-range hydrogenation processes, often limited by thermodynamic barriers or requiring metal vacancy defects. Some recent work suggests long-range hydrogen spillover, but the mechanism and extent of migration remain unclear, particularly in the presence of structural changes in the MOF. This research aims to investigate the mechanism of hydrogen spillover in both reducible and nonreducible MOFs, focusing on how to promote long-distance migration in the latter while maintaining structural stability, and further leveraging this controlled spillover for selective catalytic reactions.

The literature on hydrogen spillover reveals a rich history primarily focused on reducible metal oxides like MoO3 and TiO2. In these systems, the mechanism involves coupled proton and electron migration facilitated by the material's electronic structure. However, the occurrence and extent of hydrogen spillover on nonreducible oxides such as aluminosilicates and Al2O3 have been debated, with recent consensus indicating defect-dependent, short-range mobility. MOFs have emerged as a promising class of support materials, offering advantages like tunable porosity and precise structural control. Early work on hydrogen spillover in MOFs often focused on improving hydrogen adsorption through metal doping. Studies on carboxylate-based MOFs like MOF-5 showed short-range hydrogenation, with metal vacancy defects playing a crucial role in overcoming migration barriers. Investigations using ZIF-8 showed limited spatial scope for hydrogen spillover. While a recent study reported long-range hydrogen spillover in Pt@MOF-801 facilitated by water, the accompanying MOF structural collapse complicated the interpretation of the spillover mechanism and migration distance. A significant gap in the literature remains concerning the precise control of hydrogen spillover distance in MOFs and its subsequent application in catalysis.

The researchers employed a multi-pronged approach involving synthesis, characterization, and catalytic testing. They investigated the reduction behavior of various MOFs (reducible and nonreducible) using thermogravimetric analysis (TGA) in flowing H2 gas. This revealed the impact of the MOF's reduction potential on its stability under hydrogen spillover conditions, with reducible MOFs decomposing more readily due to hydrogenolysis. To overcome this limitation, they focused on nonreducible MOFs, particularly Zn-ZIF-8, known for its high thermal stability and functional group flexibility. A sandwich nanostructured MOFs@Pt@MOFs catalyst model was developed to control hydrogen spillover distance. This involved synthesizing Zn-ZIF-8@Pt@Zn-ZIF-8 nanocubes with varying outer shell thicknesses (15-50 nm). A solvent-assisted ligand exchange/reduction strategy was utilized to introduce different functional groups (CHO, OH, NO2, NH2) into the outer Zn-ZIF-8 shell. The morphology, structure, and functional group incorporation were confirmed by transmission electron microscopy (TEM), X-ray diffraction (XRD), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and ¹H nuclear magnetic resonance (¹H NMR) spectroscopy. X-ray photoelectron spectroscopy (XPS) depth profiles verified the complete encapsulation of Pt nanoparticles. The hydrogen spillover efficiency was assessed using cyclooctene hydrogenation as a model reaction. The reaction kinetics were studied to determine activation energies. Isotope labeling experiments (using D2O and D2) were performed to track the movement of hydrogen atoms and deuterium atoms. In situ X-ray absorption spectroscopy (XAS) and XRD were used to monitor the structural changes and oxidation states of metal atoms during hydrogen exposure. In situ XPS further explored H-binding sites. First-principles atomistic simulations were conducted to calculate hydrogen migration energy barriers. Finally, the controlled hydrogen spillover was applied to the selective hydrogenation of 5-chloroquinoline.

The study demonstrated that hydrogen spillover in reducible MOFs (e.g., Cu-MOF-2) leads to metal node reduction and framework damage. In contrast, hydrogen spillover in nonreducible Zn-ZIF-8 is significantly enhanced by the presence of water molecules or specific functional groups in the MOF structure. Water molecules facilitate proton hopping with a low energy barrier (0.2 eV), enabling long-distance hydrogen spillover (exceeding 50 nm) in Zn-ZIF-8@Pt@Zn-ZIF-8 (H2O). The introduction of functional groups like CHO also enhances spillover, but with higher activation energies (0.47 eV for CHO, compared to 0.2 eV for water-assisted proton hopping). Different functional groups exhibited varying levels of hydrogen spillover enhancement, with OH showing higher activity than CHO or NH2. The spillover distance was inversely correlated with shell thickness. The catalysts showed zero-order reaction kinetics, enabling activation energy determination. Isotope labeling experiments confirmed hydrogen atom diffusion through the MOF via water-assisted pathways or interaction with functional groups. In situ XAS and XRD revealed that hydrogen spillover in Zn-ZIF-8 does not involve changes in the oxidation state or coordination environment of Zn atoms, indicating that the spillover process is not dependent on metal redox reactions. The controlled spillover hydrogenation of 5-chloroquinoline using Zn-ZIFs@Pt@Zn-ZIFs catalysts achieved high selectivity (>99%) to the desired primary product, surpassing many reported heterogeneous catalysts. This highlights the potential of controlled hydrogen spillover for selective catalytic reactions.

This research successfully demonstrates that hydrogen spillover in nonreducible MOFs can be effectively controlled and extended over long distances (over 50 nm) by leveraging water molecules or strategically incorporated functional groups. This contrasts sharply with the previously observed short-range limitations in nonreducible materials. The findings highlight the crucial role of water and functional groups in mediating proton transfer, and the importance of careful design to promote efficient hydrogen spillover. The high selectivity achieved in the hydrogenation of 5-chloroquinoline exemplifies the practical applications of this controlled spillover, opening possibilities for many other challenging reactions. The work clarifies the underlying mechanism, showing that spillover does not require metal redox reactions, providing a new strategy for designing highly efficient and selective catalysts.

This study provides a comprehensive understanding of hydrogen spillover in MOFs, demonstrating the crucial role of water and functional groups in promoting long-distance hydrogen migration in nonreducible MOFs. The controlled spillover achieved enabled highly selective catalytic hydrogenation reactions. Future research could explore a wider range of functional groups, MOF structures, and reaction types to further optimize the catalyst design and extend the applicability of this controlled hydrogen spillover strategy. Exploring other nonreducible supports could also broaden this methodology’s applicability.

The study focused on a specific type of MOF (Zn-ZIF-8) and a limited set of functional groups. The generalizability of the findings to other MOFs and functional groups requires further investigation. While the model reaction (cyclooctene hydrogenation) provides insights into spillover, its relevance to all catalytic reactions needs to be explored further. The computational methods employed are approximations, and more sophisticated methods may provide further insights into the hydrogen migration mechanisms.