Abstract
Carbon capture and sequestration is a key element of global initiatives to minimize anthropogenic greenhouse gas emissions. Although many investigations of new candidate CO2 capture materials focus on equilibrium adsorption properties, it is also critical to consider adsorption/desorption kinetics when evaluating adsorbent performance. Diamine-appended variants of the metal–organic framework Mg2(dobpdc) (dobpdc4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) are promising materials for CO2 capture because of their cooperative chemisorption mechanism and associated step-shaped equilibrium isotherms, which enable large working capacities to be accessed with small temperature swings. However, the adsorption/desorption kinetics of these unique materials remain understudied. More generally, despite the necessity of kinetics characterization to advance adsorbents toward commercial separations, detailed kinetic studies of metal–organic framework-based gas separations remain rare. Here, we systematically investigate the CO2 adsorption kinetics of diamine-appended Mg2(dobpdc) variants using a thermogravimetric analysis (TGA) assay. In particular, we examine the effects of diamine structure, temperature, and partial pressure on CO2 adsorption and desorption kinetics. Importantly, most diamine-appended Mg2(dobpdc) variants exhibit an induction period prior to reaching the maximum rate of CO2 adsorption, which we attribute to their unique cooperative chemisorption mechanism. In addition, these materials exhibit inverse Arrhenius behavior, displaying faster adsorption kinetics and shorter induction periods at lower temperatures. Using the Avrami model for nucleation and growth kinetics, we determine rate constants for CO2 adsorption and quantitatively compare rate constants among different diamine-appended variants. Overall, these results provide guidelines for optimizing adsorbent design to facilitate CO2 capture from diverse target streams and highlight kinetic phenomena relevant for other materials in which cooperative chemisorption mechanisms are operative.
Highlights
Rising atmospheric CO2 levels and the associated increase in average global temperatures have created an urgent need to curb anthropogenic CO2 emissions.[1]
To elucidate whether the observed induction period is intrinsic to the CO2 adsorption kinetics of m-2-m–Mg2(dobpdc) or is an artifact of the experimental setup, we investigated CO2 adsorption in the bare framework material with no appended diamines, Mg2(dobpdc) (Fig. 3e and f)
The foregoing results describe a systematic investigation into the CO2 adsorption kinetics of diamine-appended variants of the metal–organic framework Mg2(dobpdc) using a thermogravimetric analysis (TGA)-based assay
Summary
Need to curb anthropogenic CO2 emissions.[1]. While long-term solutions to this challenge necessitate a shi to renewable energy sources, fossil fuels will continue to supply a major portion of global energy in the near future.[2]. As a result of this unique capture mechanism, diamine-appended variants of Mg2(dobpdc) exhibit step-shaped CO2 adsorption pro les, which give rise to large CO2 cycling capacities that are accessible with relatively small temperature swings.[25] Importantly, by varying the metal cation,[25] diamine,[26,27,28] or organic linker,[28] the adsorption step position can be tuned in pressure by over ve orders of magnitude (from $10À5 to $1 bar at 40 C) to enable the precise targeting of speci c CO2 separation conditions These materials have been shown to maintain high CO2 working capacities a er 1000 adsorption/desorption cycles under humid gas streams.[27]. On the basis of these correlations, we conclude with guidelines for the optimization of adsorbent structure and process parameters in CO2 capture applications
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