New Materials and Process Development for Energy-Efficient Carbon Capture in the Presence of Water Vapor

Using quantum chemical methods, we have screened a variety of functional groups for their ability to bind CO2 and water. Most functional groups bind water more strongly than CO2, making them ineffective for incorporation into materials for removing CO2 from humid flue gas. Preliminary calculations suggest that adding perfluorinated functional groups strongly suppresses water binding, while having little effect on CO2 binding. Preliminary process-level modeling has shown that if materials are not available for capturing CO2 from humid flue gas, it may be economically feasible to remove much of the water from the flue gas prior to CO2 removal. This opens up a wider range of material and process combinations for cost-effective CO2 capture. Introduction Adsorption processes based on porous solids are a promising technology for removing CO2 from power plant flue gas. The premise of this proposal is that gamechanging improvement of adsorption separation processes for carbon capture and storage (CCS) will require simultaneous development of new materials and specially designed processes that take advantage of these new materials. Competitive adsorption of water is regarded as the single greatest technical hurdle to adsorption-based CCS, but there is very little research on new water-tolerant CCS adsorbents. One goal of this project is to develop new metal-organic framework (MOF) materials with extraordinarily high selectivity for CO2 over nitrogen and water, together with suitably high absolute capacity for CO2. Several related strategies are being explored, and molecular modeling is being used to guide the synthetic effort. In addition, we are using state-of-the-art process-level modeling to optimize adsorption processes around the new class of sorbents and explore the real efficiency limits of MOF-based adsorption processes that meet desired CCS technical and economic criteria, while minimizing the life-cycle environmental impact. Adsorption processes are already used in large-scale applications, such as air separation, with high reliability. Discovery of new, water-stable and water-tolerant adsorbents for CO2 capture and new, optimal process configurations for these sorbents would be a stepout development in CCS. Background Research on MOFs continues to explode. Many research groups are investigating MOFs for separation of CO2/N2 mixtures, motivated by carbon capture. However, very few papers report results that take water into account. A small number of papers have appeared recently on the stability of MOFs, including their stability in water or humid gas streams. Results Molecular-level Modeling We are using molecular-level modeling to 1) screen new candidate materials and 2) obtain insights into the structures that provide the best performance. One focus this year was to screen functional groups that could be incorporated into existing MOF materials for selective adsorption of CO2 over N2 in the presence of water vapor. ZIF-8 was chosen as a candidate MOF for functionalization due to its stability in the presence of water (even under boiling water conditions). In addition, this material has been shown to be very hydrophobic and to adsorb almost no water. This has been demonstrated experimentally [1-2] and by simulations from our group [3]. A variety of different functional groups were tested for their ability to enhance the target properties of ZIF-8. The binding energies for water and CO2 were calculated as an initial measure of the ability to adsorb CO2 in the presence of water. The goal was to find functional groups that would increase the adsorption of CO2 compared to the unfunctionalized material but that would not adsorb too much water. Quantum mechanical calculations were performed to obtain the binding energies. Calculations were performed on a small and a large cluster extracted from the ZIF-8 structure, as shown in Figures 1. For the small clusters, we used a high level of theory, namely second order Møller–Plesset perturbation theory (MP2) with large basis sets such as 6311++G(2d,p). For the large clusters, we used less expensive density functional methods using the wB97xD functional and a 6-31G(d,p) basis set. Figure 1. Small and large clusters of ZIF-8 used for the quantum chemical calculations. For the large cluster, the different functional groups were added not only on the central ring of the cluster but on all of the six peripheral rings as well. This was done to observe any possible synergetic effect between the functional groups of adjacent rings, and indeed we did observe such effects for some functional groups, where the interaction of guest molecules with the cluster was significantly enhanced when groups were present on every ring of the cluster rather than only on the central ring. As expected from our choice of functional groups, we observed an increase in the interaction energy between the ZIF cluster and the CO2 molecules upon functionalization with all of the functional groups tested. In some cases, the enhancement was quite pronounced, for example an increase in the magnitude of the binding energy from -13 kJ/mol to -94 kJ/mol. At the same time, when these groups were tested for their ability to interact with water, the same trend was observed: in every case, the functional group increased the binding of water as well. There was no functional group among the ones tested that could bind CO2more strongly than H2O, supporting our initial idea that simple functionalization was unlikely to be a successful strategy. Functionalization with perfluoro group was given special attention. In a similar set of calculations, the ZIF-8 methyl groups on the imidazole ring (Figure 1) were replaced by perfluorinated methyl groups. The perfluorination of the methyl groups resulted in a significant decrease in the interaction with water (from -36 kJ/mol to -24 kJ/mol), making the material more hydrophobic. At the same time, this change had almost no effect on the interaction with CO2, with binding energies of-13 kJ/mol for the original cluster and -12 kJ/mol for the cluster with the perfluorinated methyl groups. 1 Thus, the material with perfluoromethyl groups is predicted to be significantly more hydrophobic than the parent material but to adsorb essentially the same amount of CO2. While the parent ZIF-8 material is not particularly good for CO2 capture, this finding may allow us to find other materials that can be made hydrophobic while retaining their ability to adsorb CO2. Process-level Modeling There are two main objectives of the process-level modeling in this project: first, to design a system that utilizes materials discovered in the project and, second, to provide feedback to the materials discovery effort on the desired material properties. While our primary focus is the development of materials that can adsorb CO2 in the presence of water, we thought it worthwhile to first investigate how much it would cost to remove water vapor from the flue gas prior to the CO2/N2 separation. A new graduate student was recruited for this effort, and during the past several months, we designed several different processes for water removal and determined the corresponding cost under different requirements and operating conditions. The total cost of the water removal system was estimated using a framework proposed by Hasan et al. [4]. The total annualized cost (TAC) was divided into the annualized investment cost (AIC) and the annual operating cost (AOC). The detailed scheme for calculating the AIC is shown in Table 1. The AOC was determined by the usage of utilities including process utilities (cooling water, steam, refrigerant) and electricity. All equipment costs and utility costs were estimated using the Aspen Process Economic Analyzer. 1 It should be noted that with the simple cluster models, these energies are not expected to reflect the adsorption enthalpies measured in the real materials. Rather, we are seeking trends with these calculations. Table 1. Parameters for the economic analysis