Computational design of a metal-based frustrated Lewis pair on defective UiO-66 for CO2 hydrogenation to methanol

Abstract

Metal–organic framework (MOF)-based catalysts have shown enormous potential in CO₂ conversion to high-value added chemicals. Herein, we report a density functional theory (DFT) study on CO₂ hydrogenation to methanol (CH₃OH) on defective UiO-66, in which a frustrated Lewis pair (FLP) is created by removing one organic linker. The hydrogenation is considered a three-stage transformation: (1) CO₂ is hydrogenated into formic acid (HCOOH); (2) HCOOH is converted to formaldehyde (HCHO) via hydrogenation and dehydration; (3) HCHO is hydrogenated into CH₃OH. The reaction mechanisms are investigated in detail by studying the optimized structures and the corresponding Gibbs energies for the elementary processes involved in the transformation. For CO₂ hydrogenation to HCOOH, three pathways are computed comparatively. In Pathway I, adsorbed CO₂ reacts with H₂ to form HCOOH directly and this step features a high free energy barrier comparable to that in the gas phase. In the other two pathways, adsorbed H₂ is first split into a proton (H⁺) and a hydride (H⁻) on the FLP, and then CO₂ is hydrogenated into HCOOH via a stepwise mechanism (Pathway II) or a concerted mechanism (Pathway III). The DFT calculations reveal that the energy barriers in Pathway II and III are reduced significantly compared to that in Pathway I, and Pathway III is the most favourable for CO₂ hydrogenation to HCOOH. Subsequent calculations suggest that the conversion of HCOOH to HCHO and further to CH₃OH is also facile via H₂ dissociation and the concerted transfer of H⁺ and H⁻ to HCOOH and HCHO. These results highlight the importance of FLP-assisted heterolytic dissociation of H₂ in promoting CO₂ hydrogenation. This study suggests that the defective UiO-66 with a FLP might be a potential catalyst for CO₂ hydrogenation and it could facilitate the bottom-up design of efficient MOF-based catalysts for CO₂ utilization.