Rapid economic growth and continued industrial development have led to an increase of carbon dioxide in the atmosphere. Hence, mitigation strategies such as Carbon Capture, Utilization, and Storage (CCUS) play an important role to limit the contribution of these emissions to the global climate change [1]. Absorption with aqueous amine solutions (e.g., monoethanolamine, MEA) is the most mature technology and the one conventionally used to capture CO2 at large scale [2]. However, it presents several disadvantages such as high-energy consumption required for regeneration, degradation in the presence of oxygenated compounds, loss of amines by evaporation, and corrosion [3]. Alternatively, solid adsorbents are becoming popular aimed at improving the shortcomings of amine solutions. Among them, Metal-Organic frameworks (MOFs) are a class of porous crystalline materials consisting of metal nodes connected by organic linkers, bearing high surface areas and highly tunable pore characteristics [4]. In specific, MOFs with open metal uncoordinated sites have demonstrated excellent qualities for CO2/N2 separations at low CO2 partial pressures [5]. However, in some cases, because CO2 typically adsorbs via weak physisorption interactions, most of these synthesized structures cannot satisfy industrial requirements [6].
Therefore, amine-functionalized MOFs are attracting great attention, as they are more effective than the corresponding original solid adsorbents. Studies have shown that amine-functionalized MOFs have potential for greater adsorption capacity, higher selectivity, faster CO2 adsorption kinetics, and lower regeneration temperatures, making them good candidates for process adsorbents [7][8][9]. In addition, formation of ammonium carbamate chains takes place for different diamine molecules on the adsorbent, leading to isotherms exhibiting step-like shapes at given temperatures [7][10][11].
However, the optimal (i) MOF/amine combinations, and (ii) number of amine functional groups required for optimizing the performance of such materials for real-world CO2 capture applications, remain elusive [12]. Therefore, molecular simulations (Monte Carlo) were used in this contribution to explore the relationship between the structure of MOFs and their CO2 adsorption performance when amino-functionalized. A series of amine-grafted MOF-74, and the expanded version M/DOBPDC, were screened, establishing the most promising amines for adsorbing low-concentration CO2, while considering their regeneration performance.
Primary, secondary and tertiary diamine-appended molecules of different chain lengths were explicitly grafted to the unsaturated metal sites [13]. Their behavior was evaluated in terms of adsorption isotherms, selectivity, cyclic working capacity and regenerability. Good agreement between simulation results and available experimental data was obtained. Moreover, two potential structures were found with high cyclic working capacities if used for Temperature Swing Adsorption processes: dimethylethylenediamine/Mg/DOBPDC and methane diamine-Zn/DOBPDC [14]. Furthermore, it was found that more amine functional groups grafted on the MOFs and/or full functionalization of the metal centers do not lead to better CO2 separation capabilities due to steric hindrances. In addition, multiple alkyl groups bonded to the amino group yield the characteristic shift in the step-like adsorption isotherms in the larger pore structures, at a given temperature, due to the carbamic acid formation within the pore.
Our calculations shed light on how functionalization can enhance gas adsorption via the cooperative chemi-physisorption mechanism of these materials, and how the materials can be tuned for desired adsorption characteristics.
Financial support for this work has been provided by Khalifa University (projects CIRA 2018-103 and RC2-2019-007).
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Figure. Comparison of simulated (filled symbols) adsorption isotherms of carbon dioxide with available experimental data (open symbols) for selected functionalized MOF structures. (Lines are to guide the eyes). Three-dimensional structure representation of carbamic acid functionalization for mmen-Zn/DOBPDC (Color code: C, H, O, N, Zn in gray, white, red, blue, and purple, respectively]
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