(58c) Process Modeling and Experimental Studies of a Diamine-Appended Metal–Organic Framework for CO2 Capture

Recent focus on climate change has driven research in the area of carbon capture, specifically in the area of post-combustion capture from power generation systems. Current commercial technologies for post-combustion CO₂ capture are mainly based on amine-based solvents, but they suffer from high energy penalty, degradation and low working capacity. Functionalized sorbents offer a potential alternative technology. However, viable sorbents should have lower regeneration energy, faster kinetics, higher CO₂ selectivity, and higher working capacity under the low partial pressure of CO₂ for 90% CO₂ capture from the post-combustion flue gas. A novel class of diamine-appended metal–organic frameworks (MOFs) offers a number of these features. These MOFs exhibit step-shaped isotherms under low partial pressures of CO₂ and lead to higher working capacities under similar temperature and pressure intervals when compared to traditional sorbents. The MOF studied in this work, 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 a cooperative mechanism to form carbamate and carbamic acid species. Along with high working capacities, this functionalized MOF also shows excellent long-term stability and maintains performance under humid conditions, both desirable attributes for flue gas CO₂ capture.¹

For minimizing the penalty due to CO₂ capture, contactors for these functionalized MOFs should be optimally designed. The step-shaped isotherms exhibited by these MOFs cannot be adequately represented by traditional isotherms. It is suspected that significant physical adsorption and multiple chemical reactions occur simultaneously, and that multiple assumptions underlying well-studied traditional isotherms are violated in the actual mechanism of CO₂ uptake in this MOF class. A new isotherm model that captures the unique mechanism of CO₂ uptake is developed in this work. Data from thermogravimetric analysis (TGA) are used to develop the kinetic model. Using these newly developed models, a temperature swing adsorption system is optimally synthesized. This system targets a thermal management strategy that effectively manages the rapid heat transfer required to fully exploit the step isotherm behavior and realize the projected working capacity increases. Due to the cyclic nature of the process and due to spatial variation in the loading profile along the contactor, a multi-objective optimization problem is solved to minimize the amount of sorbent and energy required for post-combustion CO₂ capture.

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