Leveraging Conformational Dynamics of MOFs to Overcome Conversion-Selectivity Trade-Off in CO2 Hydrogenation to Ethanol.

In this study, we exploit the structural dynamics of MIL-53(Al) metal-organic framework (MOF) to develop a highly efficient and selective aqueous-phase heterogeneous catalyst, [MIL-53-Co(OH)], featuring atomically dispersed CoII(OH) active species at the MOF's nodes for the hydrogenation of CO2 into ethanol. Under mild conditions (130°C, 20 bar, H2/CO2: 3), MIL-53-Co(OH) achieves 92% CO2 conversion with an ethanol productivity of 10 133 µmol gcat -1 h-1 and 93% selectivity. Comparisons with the rigid MIL-68(Al) and breathing-suppressed variants of MIL-53 MOFs revealed that the lattice flexibility of MIL-53-Co(OH) enhances ethanol productivity by at least 3.5-fold while suppressing the formation of CH3OH. Experimental, structural and computational analysis suggest that the reversible narrow-pore (np)↔large-pore (lp) interconversion of MIL-53-Co(OH) periodically optimizes the Al2[μ3-O-Co(OH)] active-site geometry at MOF's node, which synchronizes in situ generated CH3OH activation followed by CO insertion to promote C‒C coupling. During the np→lp transition, the transient lattice expansion relaxes the Al-[μ3-O-Co(OH)]-Al hinge, which reduces the activation barrier of σ-bond metathesis between the Co-H bond and C-O bond of CH3OH, a key step in the catalytic cycle. This strategy of leveraging conformational dynamics of MOFs for active-site engineering opens new avenues in designing highly active earth-abundant metal catalysts for challenging chemical transformations.