Amine functionalised metal-organic frameworks for carbon dioxide capture

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 improve the capacities of MOFs containing CUS, PSM of MIL-100(Fe) and MIL-101(Cr/Al)-NH2 were performed. Despite slower kinetics after the modification of the MIL-101-NH2 frameworks, higher adsorption capacities were observed at pressures below 0.15 bar over the entire temperature range evaluated. A smaller impact of ED and DETA relative to PEI and DAB-AM-8 was expected due to the lower amine densities. However in comparison with the MOFs prior to PSM, the adsorption capacities were mostly lower or similar, suggesting that most of the amine groups inside the pores are not accessible for CO2. In other words, the results suggest that most of the pores were blocked. The maximum loadings of the amines used in this work were 150 wt% and 100 wt% for MIL-101(Cr)-NH2 and MIL-101(Al)-NH2, respectively. The best performing materials were MIL-101(Cr)-NH2-DAB-AM-8-145 and MIL-101(Al)-NH2-PEI-96. Despite the promising results for MIL-101(Al)-NH2 containing 96 wt% PEI, the similar loadings of DAB-AM-8 (94 wt%) and DETA (88 wt%) resulted in complete pore filling and consequent reductions in CO2 adsorption capacities at low pressures. While PSM of MIL-101 frameworks showed promising results, the increase in CO2 adsorption capacity in PSM MIL-100(Fe) was relatively limited. One reason for this is the lower pore volume of the latter MOF, which decreases the amount of amine that can be incorporated. It was also found that the ED modified MIL-100(Fe) performed better than the PEI modified analogue; this result is probably due to the smaller size of ED in comparison with PEI. The comparison with the adsorption capacities of DAB-AM-8, DETA and PEI revealed that only MIL-101(Cr)-NH2-DAB-AM-8-145 and MIL-101(Al)-NH2-PEI-96 showed higher CO2 capacities than those of the neat amines and the parent MOF. In the third part of this study, the stability of these MOFs under cycling conditions and in the presence of moisture (1 vol% H2O) was tested, since this would be essential for industrial applications. Instability in the presence of H2O is a known problem for many MOFs, since phase transformations and/or decomposition of the framework structure can occur. Due to the slower CO2 adsorption/desorption kinetics at lower temperatures on MIL-101(Cr)-NH2-PEI-68/88/130, MIL-101(Cr)-NH2-DAB-AM-8-145 and amine modified MIL-101(Al)-NH2, the cycling experiments were performed at 75 or 105 °C. The cycling temperature of DETA and ED modified MIL-101(Cr)-NH2 was set to 45 °C. Other materials were cycled at 25 °C. The working capacities in each cycle of the PSM materials were lower or similar to those of the parent materials. One exception was MIL-101(Cr)-NH2-DAB-AM-8-145 which outperformed MIL-101(Cr)-NH2. The improvement in working capacity and cycling at somewhat elevated temperature can be beneficial for post-combustion capture, since the temperature of the flue gas to be treated is often elevated also. The presence of amines resulted in decreased H2O capacities due to the lower pore volumes. Significant decreases in the performance of MIL-100(Fe) were found during the wet cycling processes due the progressive filling of the pores by H2O. PXRD studies showed that MIL-101(Cr)-NH2, MIL-100(Fe), MIL-100(Fe)-ED and MIL-100(Fe)-PEI remained stable in the presence of H2O. In contrast to this, MIL-101(Al)-NH2 remained stable during the wet cycling experiments, but exhibited partial decomposition when H2O desorbed upon heating at 110 °C in Ar atmosphere. While MIL-53(Al)-NH2 exhibited a working capacity of 2.88 wt% under dry conditions, H2O was found to almost completely disable the CO2 adsorption/desorption during cycling. Improvements were sought by PSM with benzoic anhydride. Due to the size of the anhydride, the working capacity was reduced to 0.37 wt% under dry conditions. The presence of H2O resulted in further reduction of the working capacity to 0.08 wt% in the PSM material. Despite the decrease in working capacities both neat and PSM materials were stable during the wet cycling process. In the third part of this study, high pressure CO2 and N2 adsorption/desorption experiments were performed on MIL-100/MIL-110(Al), MIL-100(Fe), MIL-101(Cr/Al)-NH2 and STA-16 frameworks in the temperature range of 25 to 105 °C and pressure range of 0 to 40 bar. All materials were found to selectively adsorb CO2, with relatively high CO2 adsorption capacities observable in the MIL-101 frameworks. The hydrothermal stability of selected MOFs was evaluated by measuring CO2 isotherms before and after the treatment with H2O vapour (30, 60 and 90 % RH). While MIL-100/MIL-110(Al) and MIL-101(Al)-NH2 showed decreases in the CO2 uptake after the treatment with H2O at 30 % RH, the phosphonate framework STA-16(Co) remained stable up to 60 % RH. PXRD studies revealed that the lower CO2 adsorption capacities are due to the collapse of the MIL-110(Al) and MIL-101(Al)-NH2 frameworks. In contrast to MIL-110, the diffraction pattern of MIL-100(Al) remained unchanged. Structural changes were observed in STA-16(Co) after the adsorption of H2O at 90 % RH; and this also resulted in a small decrease in CO2 capacity from 13.2 wt% to 12.5 wt% at 25 °C and 5 bar. In the last part of this study, the effect of flue gas impurities on the stability and the CO2 uptake of MIL-101(Cr), MIL-101(Cr)-NH2, MIL-101(Al)-NH2, MIL-100(Fe), MIL-53(Al)-NH2 and STA-16(Co) were evaluated by performing PXRD and FTIR studies, screening tests and a series of cycling experiments using NO, NO2 and SO2. The MOFs were chosen in order to determine the effects of the CUS, flexible vs rigid frameworks, the type of metal cation and the nature of the organic linker molecule. In order to determine the effect of NO, NO2 and SO2 on the stability of the MOFs, PXRD studies were conducted after the materials had been subjected to 10000 ppm contaminant at 25 °C and at elevated temperatures (50, 80 °C). Decreased crystallinities and therefore partial loss in long-range crystallographic ordering or partially collapsed structures were observed in all MOFs. This suggests that the frameworks might ultimately collapse upon prolonged exposure to all contaminants examined. MIL-53(Al)-NH2 was found to be flexible upon NO2 adsorption, with a transformation from narrow pore (np) to large pore (lp) form taking place. STA-16(Co) was partially transformed into CoMOF after NO and SO2 exposure, while NO2 resulted in a complete collapse of the framework. FTIR studies of the samples after the exposure indicated that NO2 is strongly interacting with the CUS in MIL-100(Fe), MIL-101(Cr), MIL-101(Cr)-NH2, MIL-101(Al)-NH2 and STA-16(Co). During the screening tests, the MOFs were subjected to 5000 ppm of contaminant over a heat ramp from 20 to 110 °C with a heating rate of 2 °C min-1.