Xylene isomers are precursors in many important chemical processes, yet their separation via crystallization or distillation is energy intensive. Adsorption presents an attractive, lower-energy ...alternative and the discovery of adsorbents which outperform the current state-of-the-art zeolitic materials represents one of the key challenges in materials design, with metal–organic frameworks receiving particular attention. One of the most well-studied systems in this context is UiO-66(Zr), which selectively adsorbs o-xylene over the other C8 alkylaromatics. The mechanism behind this separation has remained unclear, however. In this work, we employ a wide range of computational techniques to explore both the equilibrium and dynamic behavior of the xylene isomers in UiO-66(Zr). In addition to correctly predicting the experimentally observed ortho-selectivity, we demonstrate that the equilibrium selectivity is based upon the complete encapsulation of o-xylene within the pores of the framework. Furthermore, the flexible nature of the adsorbent is crucial in facilitating xylene diffusion and our simulations reveal for the first time significant differences between the intracrystalline diffusion mechanisms of the three isomers resulting in a kinetic contribution to the selectivity. Consequently, it is important to include both equilibrium and kinetic effects when screening MOFs for xylene separations.
As a class of porous materials, metal–organic frameworks (MOFs) show promise for the adsorption-based separation of mixtures of gases. The design of any process involving selective adsorption ...requires knowledge of mixture adsorption isotherms. Ideal adsorbed solution theory (IAST) predicts mixture adsorption equilibria using only single-component data, thereby minimizing the need for experimental adsorption data. In this work we perform a systematic study of the applicability of IAST to MOFs by using grand canonical Monte Carlo (GCMC) simulations to investigate the suitability of IAST for the prediction of the adsorption of mixtures of molecules of differing sizes, asphericities, and polarities in a range of structurally different MOFs. We show that IAST is generally accurate for MOFs. Where we find IAST is less accurate, deviations result from both mixture effects, in the form of nonidealities in the adsorbed phase, and characteristics of the adsorbent structures. In terms of the MOF structure, departures from IAST are a consequence of heterogeneities both on the scale of the unit cell and on shorter length scales, whereby competition for adsorption sites has a strong influence.
The incorporation of coordinatively unsaturated metal sites (cus’s), also known as open metal sites, into metal–organic frameworks (MOFs), significantly enhances the uptake of certain gases, such as ...CO2 and CH4, especially at low loadings when fluid–framework interactions play the predominant role. However, due to the considerably enhanced, localized guest interactions with the cus’s, it remains a challenge to predict correctly adsorption isotherms and mechanisms in MOFs with cus’s using grand-canonical Monte Carlo (GCMC) simulations based on generic classical force fields. To address this problem, we carefully investigated several well-established semiempirical model potentials and used a multiobjective genetic algorithm to parametrize them using accurate ab initio data as reference. The Carra–Konowalow potential, a modified Buckingham potential, in combination with the MMSV potential for the cus’s gives not only adsorption isotherms in very good agreement with experiments but also correctly captures the adsorption mechanisms, including adsorption on the cus’s, for CO2 in CPO-27-Mg and CH4 in CuBTC. Moreover, the parameters obtained also give quantitative predictions of CH4 adsorption in PCN-14, another MOF with Cu cus’s, which is an important step for developing transferable force fields that reliably predict adsorption in MOFs with cus’s.
Grand canonical Monte Carlo simulations were performed to predict adsorption isotherms for hydrogen in a series of 10 isoreticular metal−organic frameworks (IRMOFs). The results show acceptable ...agreement with the limited experimental results from the literature. The effects of surface area, free volume, and heat of adsorption on hydrogen uptake were investigated by performing simulations over a wide range of pressures on this set of materials, which all have the same framework topology and surface chemistry but varying pore sizes. The results reveal the existence of three adsorption regimes: at low pressure (loading), hydrogen uptake correlates with the heat of adsorption; at intermediate pressure, uptake correlates with the surface area; and at the highest pressures, uptake correlates with the free volume. The accessible surface area and free volume, calculated from the crystal structures, were also used to estimate the potential of these materials to meet gravimetric and volumetric targets for hydrogen storage in IRMOFs.
Whereas grand-canonical Monte Carlo (GCMC) simulations based on generic force fields provide good predictions of adsorption isotherms in metal–organic frameworks (MOFs), especially at higher ...temperature, they fail to correctly describe the adsorption mechanism in MOFs with coordinatively unsaturated sites (cus's) at low temperatures, even for nonpolar fluids such as methane. To address this problem, we directly implemented the potential energy surface calculated by a hybrid DFT/ab inito method in the GCMC simulations using the adsorption of methane on CuBTC as an example. A comparison with previously published in situ experiments shows that our approach not only quantitatively predicts adsorption isotherms for a wide range of temperatures and pressures but also provides the correct description of the adsorption mechanism, including adsorption on the cus's. We also show that care must be taken when selecting the ab initio method to be coupled with GCMC simulations to obtain accurate predictions.
Metal−organic frameworks (MOFs) synthesized in a building-block approach from organic linkers and metal corner units offer the opportunity to design materials with high surface areas for adsorption ...applications by assembling the appropriate building blocks. In this paper, we show that the surface area calculated in a geometric fashion from the crystal structure is a useful tool for characterizing MOFs. We argue that the accessible surface area rather than the widely used Connolly surface area is the appropriate surface area to characterize crystalline solids for adsorption applications. The accessible surface area calculated with a probe diameter corresponding to the adsorbate of interest provides a simple way to screen and compare adsorbents. We investigate the effects of the probe molecule diameter on the accessible surface area and discuss the implications for increasing the surface area of metal−organic frameworks by the use of catenated structures. We also demonstrate that the accessible surface area provides a useful tool for judging the quality of a synthesized sample. Experimental surface areas can be adversely affected by incomplete solvent removal during activation, crystal collapse, or interpenetration. The easily calculated accessible surface area provides a benchmark for the theoretical upper limit for a perfect crystal.
Ab initio molecular dynamics (AIMD) simulations have been used to predict structural transitions of the breathing metal–organic framework (MOF) MIL-53(Sc) in response to changes in temperature over ...the range 100–623 K and adsorption of CO2 at 0–0.9 bar at 196 K. The method has for the first time been shown to predict successfully both temperature-dependent structural changes and the structural response to variable sorbate uptake of a flexible MOF. AIMD employing dispersion-corrected density functional theory accurately simulated the experimentally observed closure of MIL-53(Sc) upon solvent removal and the transition of the empty MOF from the closed-pore phase to the very-narrow-pore phase (symmetry change from P21/c to C2/c) with increasing temperature, indicating that it can directly take into account entropic as well as enthalpic effects. We also used AIMD simulations to mimic the CO2 adsorption of MIL-53(Sc) in silico by allowing the MIL-53(Sc) framework to evolve freely in response to CO2 loadings corresponding to the two steps in the experimental adsorption isotherm. The resulting structures enabled the structure determination of the two CO2-containing intermediate and large-pore phases observed by experimental synchrotron X-ray diffraction studies with increasing CO2 pressure; this would not have been possible for the intermediate structure via conventional methods because of diffraction peak broadening. Furthermore, the strong and anisotropic peak broadening observed for the intermediate structure could be explained in terms of fluctuations of the framework predicted by the AIMD simulations. Fundamental insights from the molecular-level interactions further revealed the origin of the breathing of MIL-53(Sc) upon temperature variation and CO2 adsorption. These simulations illustrate the power of the AIMD method for the prediction and understanding of the behavior of flexible microporous solids.
Metal−organic frameworks (MOFs) have shown adsorption behavior that is not observed in other microporous materials such as zeolites or activated carbons. This study used grand canonical Monte Carlo ...simulation to evaluate a particular form of behavior, which corresponds to the presence of unusual type V adsorption isotherms. Study of a series of MOFs in the IRMOF family, containing chemically similar linkers of different length, showed that the presence of type V adsorption depends on a fine balance between the strength of the fluid−fluid and fluid−solid interactions, which in turn is a strong function of the length of the linker and therefore the pore size. A transition from type V behavior to the more common type I behavior is observed as the temperature increases. The temperature at which this transition occurs increases, and the transition becomes more diffuse, as the length of the linker increases. This type V behavior leads to an interesting possibility in the design of MOF adsorbents for use in gas separation and gas storage applications.
Para-disubstituted alkylaromatics such as p-xylene are preferentially adsorbed from an isomer mixture on three isostructural metal–organic frameworks: MIL-125(Ti) (Ti8O8(OH)4(BDC)6), MIL-125(Ti)-NH2 ...(Ti8O8(OH)4(BDC-NH2)6), and CAU-1(Al)-NH2 (Al8(OH)4(OCH3)8(BDC-NH2)6) (BDC = 1,4-benzenedicarboxylate). Their unique structure contains octahedral cages, which can separate molecules on the basis of differences in packing and interaction with the pore walls, as well as smaller tetrahedral cages, which are capable of separating molecules by molecular sieving. These experimental data are in line with predictions by molecular simulations. Additional adsorption and microcalorimetric experiments provide insight in the complementary role of the two cage types in providing the para selectivity.
One of the strategic goals of the modern automobile manufacturing industry is to replace gasoline and diesel with alternative fuels such as natural gas. In this report, we elucidate the desired ...characteristics of an optimal adsorbent for gas storage. The U.S. Department of Energy has outlined several requirements that adsorbents must fulfill for natural gas to become economically viable, with a key criterion being the amount adsorbed at 35 bar. We explore the adsorption characteristics of novel metal−organic materials (IRMOFs and molecular squares) and contrast them with the characteristics of two zeolites, MCM-41, and different carbon nanotubes. Using molecular simulations, we uncover the complex interplay of the factors influencing methane adsorption, especially the surface area, the capacity or free volume, the strength of the energetic interaction, and the pore size distribution. We also explain the extraordinary adsorption properties of IRMOF materials and propose new, not yet synthesized IRMOF structures with adsorption characteristics that are predicted to exceed the best experimental results to date by up to 36%.
PDF
