Multifunctionalized zirconium-based MOF as a novel support for dispersed copper: application in CO2 adsorption and catalytic conversion

Abstract

CO₂ conversion and utilization for global sustainability is an integral part of greenhouse gases management, typically for the production of fuels and specialty chemicals. Added value products, such as methanol, methane or formate, can be obtained by electrocatalysis and thermocatalysis, the two techniques addressed in this study. The main motivation of this study is to develop a copper based catalyst active in both processes, confronting the main concerns regarding typical metal catalysts related to nanoparticles aggregation and concomitant deactivation. For this, modified NU-1000, a water-stable mesoporous MOF, is used as a platform for the simultaneous coordination–stabilization of copper single atoms and CO₂ adsorption. NU-1000 is synthetized with primary amino groups (–NH₂ with affinity for CO₂) by modifying the ligand prior to MOF synthesis, while post-synthetic solvent-assisted ligand incorporation is applied to insert thiol functionalities (–SH with affinity for copper) within the framework. To make the functionalized MOF catalytically active, a Cu²⁺ salt is impregnated into the MOF channels, which is further reduced with H₂ to Cu⁺/Cu⁰ before performance assessment in CO₂ conversion processes. The as-synthetized and spent catalysts were analysed regarding the structure (X-ray diffraction, infrared), bulk (mass spectrometry) and surface (X-ray photoelectron spectroscopy) composition, morphology (electronic microscopy and energy dispersive spectroscopy) and textural properties (N₂ physisorption). The electrocatalytic reduction of CO₂ was performed in the potential range of −0.8 to −1.8 V, indicating the formation of formic acid. Thermocatalytic experiments were carried out in an economically and energetically sustainable low-pressure (1 MPa) hydrogenation process. Methanol was obtained with 100% selectivity at temperatures up to 280 °C, and a space-time yield of ca. 100 mg_MeOH g_cat⁻¹ h⁻¹ which overcomes that of commercial CuZnO NPs designed for this purpose.