HKUST-1 Metal–Organic Framework Nanoparticle/Graphene Oxide Nanocomposite Aerogels for CO2 and CH4 Adsorption and Separation

The development of nanostructured composites made of metal-organic frameworks (MOFs) and graphene-based components, including exfoliated nanoplates of graphene oxide (GO) or reduced (rGO) graphene oxide, is an area of great interest in gas storage and separation. To improve the industrial viability, it is commonly demanded to build these nanocomposites with the shape of compact units, such as monoliths, foams, pellets, or films. Methods to generate those 3D nanocomposites involving rGO are abundant; however, they become scarce when GO is the desired support due to the difficulty in maintaining the carbon matrix oxidized during the structuration process. In this work, a methodology based on the use of supercritical CO2 (scCO2) is described for the synthesis of nanocomposites involving a discontinuous MOF phase, e.g. nanoparticles (NPs) of HKUST-1, decorating the surface of a continuous GO matrix, with surface oxygen groups favoring MOF attachment. The use of this new supercritical methodology allows the nanostructuration of the composite in the form of 3D aerogels while avoiding the reduction of GO. Enhanced values of textural properties, determined by low-temperature N2 adsorption-desorption, were observed for the nanocomposites in comparison to the values calculated for similar physical mixtures, highlighting an increase of 40-45% in the value of the surface area for samples with a high percentage of HKUST-1. Moreover, the composite aerogels, displaying a type II isotherm, outperform pristine HKUST-1 in regard to the CH4 practical working capacity at high pressure. Particularly, a composite exhibiting more than 2-fold the working capacity of net HKUST-1 NPs was obtained. Columns involving the composite aerogel as the stationary phase were used to study the separation of N2/CO2 and CH4/CO2 gas mixtures. The results showed a high selectivity of the nanostructured HKUST-1@GO composites for CO2, with breakthrough times of ca. 20 min g-1 and stable cyclic operations.