Strong binding site molarity of MOFs and its effect on CO2 adsorption

Abstract

a b s t r a c tMetal organic frameworks (MOFs) with strong binding sites usually have remarkable CO 2 adsorption capacity at ambient conditions. A new material parameter, the strong binding site molarity, defined as the number of the strong binding sites divided by the molecular weight of the unit cell of MOF, is introduced to understand CO 2 adsorption on these MOFs. The CO 2 adsorption capacity can be correlated to the strong binding site molarity in 1:1 ratio. The strong binding site coverage by CO 2 molecules can also be correlated to the isosteric heat of CO 2 adsorption at a given pressure. The strong binding site molarity is used to predict CO 2 adsorption amount of several new types of MOFs to show its uses in selection or design of MOFs for CO 2 capture. IntroductionMetal organic frameworks (MOFs) are known for their extraordinarily high surface area, tunable pore size and adjustable internal surface properties, and have shown good potential for the utilization in CO 2 capture [1]. Among all types of MOF materials, MOFs with unsaturated metal sites (or called open metal sites) are reported to exhibit remarkable CO 2 adsorption capacity at low pressures [2e4]. The unsaturated metal sites are usually obtained following desolvation of the MOF material, where one of the solvent molecules in the coordination sphere of the metal center is removed in vacuo at elevated temperatures [1]. Thus the unsaturated metal sites serve as charge-dense binding sites for CO 2 , which is adsorbed more strongly on these sites owning to its greater quadrupole moment and polarizability compared with N 2 [1]. Neutron diffraction measurements and density functional theory (DFT) calculations have confirmed that the unsaturated metal sites are primary binding sites for CO 2 [5,6]. Recent studies have also shown that amine modification on the unsaturated metal sites of MOFs can enhance the CO 2 adsorption capacity for the application of CO 2 capture from air or from stationary combustion sources [7e9]. A hydrogen-bonded complex involving two carbamic acid moieties was formed which results in 2:2 amine:CO 2 stoichiometry with higher capacity and weaker binding energy than the 2:1 stoichiometry observed in most amine-functionalized adsorbents [10]. Overall, both unsaturated metal cations or alklyamines inside the pores of MOFs can be considered as strong binding sites for CO 2 adsorption, resulting in high CO 2 adsorption capacity at low pressure range [1,8,9].Many studies have investigated the nature of binding site and its effect on CO 2 adsorption at low pressure for a specific MOF [5e12]. However, there lacks effort to compare CO 2 adsorption on various MOF with strong binding sites. We analyzed CO 2 adsorption on various MOFs with strong binding sites and found that the CO 2 adsorption data can be correlated by one parameter related to the amount of the strong binding sites per unit mass of MOF. Here we report analysis of the CO 2 adsorption on various MOFs by this parameter. Results and discussionWe define binding site molarity S...

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