Green Supercritical CO₂ Synthesis of [Copper Clusters@FeBTC]@rGO Catalyst for Highly Efficient Hydrogenation of CO₂ to Methanol

Abstract

In the vast majority of studies, methanol production via CO2 hydrogenation is realized using bifunctional catalysts, commonly CuZnO phases. The basic motivation of this study is to confront main concerns regarding typical copper-based catalysts, which are the aggregation of the active metal nanoparticles (NPs) and its concomitant deactivation. For this, an extensive research on composites of mesoporous iron trimesate metal–organic frameworks (MOFs) [crystalline MIL-100­(Fe) and semiamorphous FeBTC], loaded with copper clusters (CuCs) into the pores, is performed, because it is a pioneer in the area. The growth and aggregation of CuCs are hindered by the confinement of the active entities into the MOF mesocages. From the two studied MOFs, only the copper loaded semiamorphous FeBTC behaves as a catalyst. To further enhance the catalytic activity, CuCs@FeBTC NPs are dispersed on the plates constituting a reduced graphene-oxide aerogel. Main steps in the preparation of these complex catalysts involve the use of sustainable technology based on supercritical CO2, used either as a solvent or drying agent. The reduction strategy designed to obtain the active catalyst (Cu2+ to Cu+/Cu0 in CuCs) was crucial to upgrade the end product performance. A double reduction route, applying ascorbic acid preceding thermal H2 reduction, turns out to be necessary to attain a significant catalytic activity of the confined Cu2+ species. The as-synthesized and spent catalysts were analyzed in regard of the structure (X-ray diffraction, infrared), bulk (mass spectrometry) and surface (X-ray photoelectron spectroscopy) composition, morphology (microscopy and energy dispersive spectroscopy) and textural properties (N2 physisorption). The catalytic performance of reduced CuCs@FeBTC and [CuCs@FeBTC]@rGO composites was tested in the CO2 hydrogenation reaction at 10 bar. Methane and methanol were obtained as valuable reaction products. When using the [CuCs@FeBTC]@rGO aerogel, methanol yield was noticeably high, in comparison to reported data, in the order of 86 mgMeOHgCu –1 h–1; while the amount of side products (methane, CO) was almost totally hindered up to 260 °C.