Understanding the Thermodynamic and Kinetic Behavior of the CO₂/CH₄ Gas Mixture within the Porous Zirconium Terephthalate UiO-66(Zr): A Joint Experimental and Modeling Approach

Abstract

International audienceA combination of experimental (gravimetry, microcalorimetry, and quasi-elastic neutron scattering) measurements and molecular modeling was employed to understand the coadsorption of CO2 and CH4 in the zirconium terephthalate UiO-66(Zr) material from both the thermodynamic and kinetic points of view. It was shown that each type of molecules adsorb preferentially in two different porosities of the material, that is, while CO2 occupy the tetrahedral cages, CH4 are pushed to the octahedral cages. Further, a very unusual dynamic behavior was also pointed out with the slower molecule, that is, CO2, enhancing the mobility of the fast one, that is, CH4, that contrasts with those usually observed so far for the CO2/CH4 mixture in narrow window zeolites where the molecules are most commonly diffusing independently or slowing-down the partner species. Such behavior was interpreted in light of molecular simulations that evidenced a jump type mechanism involving a tetrahedral cages-octahedral cages-tetrahedral cages sequence that occurs more frequently for CH4 when in presence of CO2. The consequences in terms of CO2/CH4 selectivity and the possible use of this MOF-type material in a PSA process are then discussed. It is thus clearly emphasized that this MOF material combines several favorable features including a good selectivity, high working capacity, and potential easy regenerability that make it as a good alternative candidate of the conventional NaX Faujasite used in pressure swing adsorption

The right isotherms for the right reasons? Validation of generic force fields for prediction of methane adsorption in metal-organic frameworks

“…Yang and Zhong [50] reported an over-estimation in the cases of CH4 on IRMOF-1 and Cu-BTC using OPLS-AA and suggested a re-parameterisation of the C and O atoms of the carboxylate groups, reducing εii by as much as 30%. A similar reduction in εii across all framework atoms was suggested for UFF by Fairen-Jimenez et al [51] and Pérez-Peritello et al [23] (who scaled εUFF by 0.59 and 0.69 respectively) based on simulations of methane adsorption in ZIF-8 and ZIF-69, while a similarly large over-estimation (~50%) has been reported in UiO-66(Zr) using UFF [22]. We would suggest that while UFF and OPLS-AA are suitable for qualitative prediction of adsorption isotherms in MOFs, both are likely to over-estimate the amount adsorbed, as well as low-coverage properties such as heats of adsorption or Henry's constants, by a significant amount.…”

“…Since then, GCMC has been employed to predict CH4 adsorption isotherms in a wide range of MOFs and related structures [14,15,16,17,18,19,20,21,22,23], with reasonable agreement with experiment reported in many cases.…”

“…Although DREIDING has been shown to significantly over-estimate methane adsorption when compared to experiment in ZIF-8 [23], very good agreement was observed in the case of UiO-66(Zr) [22] (~9-20% over-estimation for θ < 0.5) and in the present work, while simulations in IRMOF-1 and IRMOF-6 [12] and in IRMOF-8 [48] found excellent quantitative agreement with experiment (5-10% difference when averaged over the full isotherm). While there are too many variables to fully rationalise these apparent differences in quantitative agreement (e.g.…”

Structural Origin of Unusual CO2Adsorption Behavior of a Small-Pore Aluminum Bisphosphonate MOF

“…Flexible MOFs show adsorption isotherms that sometimes deviate strongly from the standard convex shape of the expected I-type (at 77 K) or Langmuir-shaped (at room temperature) isotherm 17 and often exhibit inflection points 18 or steps. 19 We have previously made tremendous effort to understand the guest-induced flexible behavior of intermediate and large pore MOFs, [11][12][13][14]20,21 mostly based on polycarboxylate ligands. To complement this, we are currently investigating structural changes of small-pore-type MOFs with pore dimensions below 5 Å and the resulting effect on the adsorption of guest molecules with comparable size.…”

Enhanced selectivity of CO2 over CH4 in sulphonate-, carboxylate- and iodo-functionalized UiO-66 frameworks

“…37,38 Its cubic 3D framework consists of a centric octahedral cage connected to eight corner tetrahedral cages through triangular windows (Fig. 1).…”