Selective ligand removal to improve accessibility of active sites in hierarchical MOFs for heterogeneous photocatalysis

Shaghayegh Naghdi,Maytal Caspary Toroker,Santu Biswas,Rémy Guillet-Nicolas,Tushar Gupta,Freddy Kleitz,Bernhard Christian Bayer,Ariane Giesriegl,Jia Wang,Dominik Eder,Thomas Haunold,Günther Rupprechter,Alexey Cherevan

doi:10.1038/s41467-021-27775-7

Abstract

Metal-organic frameworks (MOFs) are commended as photocatalysts for H2 evolution and CO2 reduction as they combine light-harvesting and catalytic functions with excellent reactant adsorption capabilities. For dynamic processes in liquid phase, the accessibility of active sites becomes a critical parameter as reactant diffusion is limited by the inherently small micropores. Our strategy is to introduce additional mesopores by selectively removing one ligand in mixed-ligand MOFs via thermolysis. Here we report photoactive MOFs of the MIL-125-Ti family with two distinct mesopore architectures resembling either large cavities or branching fractures. The ligand removal is highly selective and follows a 2-step process tunable by temperature and time. The introduction of mesopores and the associated formation of new active sites have improved the HER rates of the MOFs by up to 500%. We envision that this strategy will allow the purposeful engineering of hierarchical MOFs and advance their applicability in environmental and energy technologies.

Highlights

Metal-organic frameworks (MOFs) are commended as photocatalysts for H2 evolution and CO2 reduction as they combine light-harvesting and catalytic functions with excellent reactant adsorption capabilities
MOFs can be seen as model systems for exploring advanced concepts, such as bridging homogeneous and heterogeneous photocatalysis as well as singlesite photocatalysis[9]
We envision the greatest benefits for energy conversion technologies, involving electrocatalysis and photocatalysis, where - in addition to the enhanced porosity - the partial ligand removal is expected to create unsaturated metal sites that can serve as adsorption sites for reactants and cocatalysts and as potential catalytic centers

Summary

Introduction

Metal-organic frameworks (MOFs) are commended as photocatalysts for H2 evolution and CO2 reduction as they combine light-harvesting and catalytic functions with excellent reactant adsorption capabilities. The combined results of XRD, DRIFTS, TGA/differential scanning calorimetry (DSC), Raman, XPS, and TEM/SAED confirm that the heat treatment at 300 °C allows for selective removal of BDC-NH2 from the mixed-ligand MOFs, while preserving the characteristic structure.

Results

Conclusion