HKUST-1@ACM hybrids for adsorption applications: A systematic study of the synthesis conditions

This work constitutes a guide towards the preparation of metal-organic framework materials (MOF) supported and/or confined in activated carbon monoliths (ACM). The resulting hybrid porous materials exhibit improved physico-chemical properties as compared with their parent constituents. The different hybrids were obtained exploring several experimental approaches, which were thoroughly discussed. All hybrids were characterized by N₂ adsorption isotherms, SEM, TGA and XRD techniques. The characterization studies pointed out that the preparation conditions are of paramount importance in defining the nucleation and growth of HKUST-1, either outside of the carbon grains (surface coating), and/or in the internal pores (bulk confinement). While the surface coating was achieved by directly synthesizing the MOF in presence of the ACM (i.e. in-situ synthesis), bulk confinement is favored after applying an additional step that involves a pre-nucleation at low temperatures (5 °C). The hybrid material with the best performance, sample A5(1), shows enhanced mechanical properties compared to its parent counterparts, combined with high apparent surface area (up to 1300 m²/g), an improved crushing strength (about 8 times superior to ACM) and a geometrical density of around 0.45 cm³/g, which almost duplicates that of ACM. Last but not least, the adsorption behavior of the hybrid was tested for CO₂ and CH₄ adsorption. Application of IAST equation to the single component adsorption isotherms at room temperature gives rise to a CO₂/CH₄ selectivity factor of 5.5 in the hybrid material, larger than that on ACM (S = 3.5).