Abstract
Adsorption-based processes using metal-organic frameworks (MOFs) are a promising option for carbon dioxide (CO2) capture from flue gases and biogas upgrading to biomethane. Here, the adsorption of CO2, methane (CH4), and nitrogen (N2) on Zn(dcpa) MOF (dcpa (2,6-dichlorophenylacetate)) is reported. The characterization of the MOF by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and N2 physisorption at 77 K shows that it is stable up to 650 K, and confirms previous observations suggesting framework flexibility upon exposure to guest molecules. The adsorption equilibrium isotherms of the pure components (CO2, CH4, and N2), measured at 273–323 K, and up to 35 bar, are Langmuirian, except for that of CO2 at 273 K, which exhibits a stepwise shape with hysteresis. The latter is accurately interpreted in terms of the osmotic thermodynamic theory, with further refinement by assuming that the free energy difference between the two metastable structures of Zn(dcpa) is a normally distributed variable due to the existence of different crystal sizes and defects in a real sample. The ideal selectivities of the equimolar mixtures of CO2/N2 and CO2/CH4 at 1 bar and 303 K are 12.8 and 2.9, respectively, which are large enough for Zn(dcpa) to be usable in pressure swing adsorption.
Highlights
Metal-organic frameworks (MOFs) are being touted as the generation materials for several adsorptive separation and purification processes [1,2]
MOFs are porous crystalline materials consisting of metal centers connected by organic moieties [3]
An unlimited amount of MOF structures can be envisioned and perhaps synthesized; the materials can be tailored for specific applications through pore size tuning and functionalization [4]
Summary
Metal-organic frameworks (MOFs) are being touted as the generation materials for several adsorptive separation and purification processes [1,2]. An unlimited amount of MOF structures can be envisioned and perhaps synthesized; the materials can be tailored for specific applications through pore size tuning and functionalization [4]. These features place MOFs as a very diverse class of materials with potential applications in most fields of chemical engineering [5,6,7,8,9,10,11,12]. Framework flexibility generally manifest itself through breathing or gate-opening effects [13]. The most well-known cases of MOF flexibility are the breathing behavior of MIL-53 [15,16,17] and the gate-opening of ZIF-8 [18,19]
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