(461e) Identifying Optimal Kinetic Pathways of a Functionalized Solid Sorbent for CO2 Capture through Mathematical Programming

Recent research has been directed into finding viable solid sorbents for post-combustion CO₂ capture that can be alternatives to aqueous solvents which are the current leading commercial technology. For these sorbents to be attractive on the commercial scale, they must exhibit lower regeneration energy, faster kinetics, higher CO₂ selectivity, and higher working capacities. To realize the full potential of these systems, accurate predictive models that can be used for system design is absolutely necessary. Specifically, accurate modeling of the adsorption equilibrium for CO₂ as well as the other components present in a flue gas source is a very important and crucial step in developing these full system models. Current modeling for most sorbents uses classical isotherm models, such as a Langmuir isotherm model, to predict the adsorption equilibrium. However, when they are functionalized and complicated chemisorption interactions arise, these simple models fail to accurately represent the equilibrium. Traditional isotherm models have been developed for physical adsorption. These models fail to represent the complex equilibrium behavior of reactive adsorbents. While there has been attempts in the literature on developing weighted isotherm models, these models are highly parameterized. These models are neither predictive, nor provide any insight into the reaction pathways and products formed. A fundamental understanding of the reaction pathways can be instrumental in designing the functionalized MOFs for higher loading, better kinetics and lower energetics.

The sorbent studied in this work is a novel functionalized metal-organic framework (MOF) that has been shown to exhibit many of the important characteristics for CO₂ capture^1,2,3. The MOF, Mg₂(dobpdc) (dobpdc^4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), contains one-dimensional hexagonal channels with unsaturated Mg²⁺ sites and is functionalized by binding the diamine 2,2-dimethyl-1,3-diaminopropane (dmpn) to these sites which then chemisorb CO₂ in cooperative mechanism to form carbamate and carbamic acid species. This cooperative adsorption results in step shaped isotherms which are very difficult to predict using traditional isotherm models. Also, experimental work has been done to investigate which products are formed in the presence of CO₂ but the dominating reactions and pathways that give these products are still relatively unknown³. Since the reactions take place at functionalized sites in the MOF and exhibit the step-shaped behavior, it is impractical to experimentally determine reaction products and reaction pathways as a function of partial pressure of CO₂ and temperature. To determine which set of reactions can best represent the system chemistry, first a set of plausible reactions including chain formation reactions and numerous ways that these chains can interact are proposed based on the information from the sparse NMR data. A mixed integer non-linear programming (MINLP) problem is solved to identify the optimal kinetic pathway as well as model parameters considering both physisorption and chemisorption. Since growing chain lengths can lead to a highly parameterized system, an information-theoretic criterion is considered as the optimization objective. For thermodynamic consistency, energetics of the candidate reactions are considered as constraints. The results of this model development are then compared to previous attempts at modeling the adsorption equilibrium using traditional methods.

References:

[1] – Milner, P.J., Siegelman, R.L., Forse, A.C., Gonzalez, M.I., Runcevski, T., Martell, J.D., Reimer, J.A., Long, J.R. A Diaminopropane-Appended Metal-Organic Framework Enabling Efficient CO₂ Capture from Coal Flue Gas via a Mixed Adsorption Mechanism. Journal of the American Chemical Society. 2017; 139 (38), 13541-13553

[2] Mcdonald, T.M., et al., Cooperative insertion of CO₂ in diamine-appended metal-organic frameworks. Nature. 2015; 519, 303-308

[3] Forse, A.C., Milner, P.J., Lee, J., Redfearn, H.N., Oktawiec, J., Siegelman, R.L., Martell, J.D., Dinakar, B., Porter-Zasada, L.B., Gonzalez, M.I., Neaton, J.B., Long, J.R., Reimer, J.A., Elucidating CO₂ Chemisorption in Diamine-Appended Metal-Organic Frameworks. J. Am. Chem. Soc. 2018; 140(51), 18016-18031