AbstractsChemistry

Adsorption-based methods, such as pressure swing adsorption (PSA) or vacuum swing adsorption (VSA), are promising for capturing CO₂ from natural gas or flue gas. CO₂ adsorbents take a variety of forms, but one approach is the use of metal-organic frameworks (MOFs). These have attracted tremendous attention over the past decade due their porosity, high surface area, high pore volume, tuneable pore sizes and topologies. Previous studies on adsorbents of this type, such as CPO-27(Mg), HKUST-1, MOF-177 or MIL-101, have reported good CO₂ adsorption capacities. Moreover, through introducing specific polar groups onto the organic linker or by grafting components onto coordinatively unsaturated sites (CUS) of specific MOFs, increases in the CO₂ affinity have been observed, particularly at low pressure. This project investigated the potential of MOFs for post-combustion carbon capture and high pressure separation processes. Two classes of MOFs were chosen: 1) MOFs containing CUS, which allow further postsynthetic modification (PSM) by grafting/impregnating these materials with amines. 2) Flexible MOFs, due to their good selectivities and high CO₂ capacities. Enhanced CO₂ capacities were sought by two approaches or a combination of both: (i) prefunctionalisation of MIL-53 and MIL-101 (where substituent groups are incorporated into the linker unit before MOF construction) and (ii) postsynthetic modification (PSM) of MIL-100 and MIL-101 (where substituents like ethylenediamine (ED), diethylenetriamine (DETA), 2nd generation polypropylenimine actamine dendrimer (DAB-AM-8) and polyethyleneimine (PEI) are added after MOF construction). In the first part of this study, MIL-100/MIL-110(Al), MIL-100(Fe), MIL-101(Cr)-NH₂, MIL-101(Al)-NH₂, MIL-53(Al)-NH₂ and STA-16(Co) were synthesised and characterised by Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (PXRD), elemental analysis, N₂ sorption at -196 °C, helium density, scanning electron microscopy (SEM) and thermal decomposition. Despite the highly acidic environment during the preparation of MIL-100(Al), a 60:40 mixture with MIL-110(Al) was obtained in all cases. The formation of STA-16 was found to be strongly dependent on the pH. Slight changes in the pH resulted in the formation of a mixture with or pure phase of a new polymorphous MOF, denoted here as CoMOF. In contrast to the microporous character of STA-16(Co), CoMOF is non-porous and therefore not applicable for CO₂ capture. For this reason, no PSM on the CUS of MIL-100/MIL-110(Al) and STA-16(Co) were conducted. In the second part of this study, all MOFs (except CoMOF) were evaluated for their ability to capture CO₂ by measuring adsorption equilibria in the temperature range of 25 to 75/105 °C and a pressure range of 0 to 0.5 bar which is appropriate to the VSA process for post-combustion capture. Among them, MIL-53(Al)-NH₂ was found to have the highest CO₂ adsorption capacity in the studied pressure and temperature range, while the MOFs containing CUS showed lower adsorption capacities. In order to…