The suitability of zeolites for a certain application strongly depends on their structural features. Among the types of shape selectivity, there is the still quite unexplored "cage or window effect" consisting of an unusual nonmonotonic increase of the Henry coefficient with chain length in cagelike zeolites when the guest hydrocarbon becomes too long to fit comfortably inside the wider part of the cages. This phenomenon has been addressed for alkanes in various zeolites, but a study dealing with alkenes is lacking. Because of both scientific interest and the impact on the petrochemical industry, we aimed at assessing window effects for a variety of alkenes regarding the position and number of the double bond. We used advanced molecular simulation techniques and considered the rigid all-silica channel-like OFF and cagelike ERI, CHA, and ITQ-29 zeolites. Our study reveals results similar to those of alkanes when the double bond is located at the chain extremes. Conversely, less molecular flexibility induced by intermediate positions of the double bond or the presence of more than one bond lead to a weakness of the window effect, except for the ITQ-29 because of its considerably larger cage. These findings result in significant values of this type of selectivity for separations of saturated and unsaturated hydrocarbons with chain lengths commensurate with the zeolite cages.

Storage and separation of carbon dioxide and methane and their mixtures are important processes for environmental and energetic reasons. We study these processes using hydrated nanoporous materials and explore the use of solvents as alternative to improve the performance of these materials. We used boronate ester covalent organic frameworks (COF-5, -6, -10, and -102) because of their stability upon water. The best separation for hydrated structure is obtained with COF-102. However, the improvement on the separation performance requires a high percentage of hydration, reducing the capacity of the structure. To overcome this limitation, we suggest to introduce room-temperature ionic liquid as a solvent. Our simulations show that the use of small amounts of ionic liquids in the structure leads to higher values of adsorption selectivity than the use of hydrated structures.

Competitive adsorption of water is an important issue in the adsorption-based industrial processes of bio-and flue gases separation. The dehumidification of gases prior to separation would increase process complexity and lower its economic interest. In this work, large-scale computational screening was applied to identify Metal-Organic Frameworks (MOFs) structures which exhibit high CO 2 /CH 4 selectivity and total loading higher than 0.5 mol/kg (in the presence of water). High-throughput Grand Canonical Monte Carlo (GCMC) screening of nearly 3000 existing MOF materials was carried out. Initial selection assumed fixed values of pore limiting diameter (PLD) and Henry's constant for water and allowed one to preselect 764 structures. After GCMC simulations carried for 50/50 CO 2 /CH 4 mixture, at ambient conditions (p = 1 bar, T = 298 K), and variable gas humidity (0%, 5%, 30% and 40%) the final selection revealed 13 most promising MOFs structures. We focused on analysis of the correlations between the properties of the selected MOFs and the separation selectivity. We show that the selectivity is a complex function of the porous materials characteristics and finding selective sorbent, performing well in dry and wet conditions requires careful analysis of available MOFs.

The separation and purification of light hydrocarbons is challenging in the industry. Recently, a ZJNU-30 metal-organic framework (MOF) has been found to have the potential for adsorption-based separation of olefins and diolefins with four carbon atoms [H. M. Liu et al. Chem.-Eur. J. 2016, 22, 14988-14997]. Our study corroborates this finding but reveals Fe-MOF-74 as a more efficient candidate for the separation because of the open metal sites. We performed adsorption-based separation, transient breakthrough curves, and density functional theory calculations. This combination of techniques provides an extensive understanding of the studied system. Using this MOF, we propose a separation scheme to obtain a high-purity product.

Efficient separation and storage of gas streams involving light hydrocarbons is essential for industrial applications. These hydrocarbons are widely used as energy resources and/or chemical raw materials in various chemical reactions. Here, we focus on the separation of acetylene from methane and carbon dioxide. The separation of acetylene from carbon dioxide is, in particular, challenging due to the similar kinetic diameters and boiling points of the molecules. In recent years, considerable progress has been made in adsorption-based separations using porous metal−organic frameworks (MOFs). Most reported studies are experimental. We present a computational study on these gas separations using a variety of MOFs. This allows investigation of the competitive gas adsorption, which is experimentally challenging, as well as understanding the adsorption mechanisms at the molecular level, which in turn allows further experimental MOF design for this application. MOFs with open metal sites, and particularly Fe-MOF-74, seem to be good for this separation, with a trade-off between physical adsorption capacity and selectivity. Based on experimental single-adsorption isotherms at various temperatures, we developed and validated a specific parameterization to account for the interactions of the olefin with the open metal sites. In addition to volumetric and calorimetric adsorption, we comprehensively investigate the characteristics of the interaction between the MOFs and the guest molecules in terms of binding sites and density profiles. The overall agreement of our simulated results with experimental data for pure components points to the reliability of the models and methods to successfully predict the separation of mixtures.

According to the European Commission, in 2016 the residential sector represented 25.4% of the final energy consumption. Heating and cooling in EU households account for 69.1 % of the total energy consumption. The fraction of 84% for heating and cooling is still generated from fossil fuels, and only 16% is generated from renewable energy. To decrease carbon dioxide emissions of fossil fuel consumption, it is crucial to find alternatives to supply the heating and cooling demand. Alternatives such as adsorption-based heat pumps and desiccant cooling systems are receiving much attention because of their moderate energy consumption. These systems are based on reversible exothermic adsorption and endothermic desorption of working fluids. In this work, we combined experiments and simulations to evaluate the viability of several zeolites and MOFs with water for cooling systems applications. We combined the study of adsorption mechanisms and the dynamics of water inside the pores of the structures, thereby obtaining an overall understanding of the working-pair. We found that the Al content in FAU-topology zeolites is a key factor for an efficient process. We also identify ZJNU-30 metal-organic framework as a suitable candidate for cooling applications because of its outstanding water capacity, cooling capacity and coefficient of performance.

Adsorption in metal–organic frameworks with coordinatively unsaturated sites (CU’s) offers a cost-effective alternative to cryogenic processes for olefin/paraffin separation. Since experiments on adsorption equilibrium of gas mixtures are challenging, molecular simulation methods can be exploited to characterize the adsorption at CU’s. This work computationally addresses the adsorptive olefin/paraffin separation using Cu-BTC. To this end, we developed a parametrization of specific interactions between hydrocarbon molecules and the CU’s, which is of great interest for the modeling community. We obtain the host–guest interacting parameters for linear hydrocarbons by fitting to experimental single-component adsorption isotherms of ethane, ethene, propane, and propene and examine their transferability to larger hydrocarbons. In addition, we propose force field parameters for branched isobutane and isobutene. We then predict the adsorption selectivity of the binary alkane/alkene mixtures for chains from two to five carbon atoms. Cu-BTC was found potential candidate for the separation, especially for isobutane/isobutene (iC4) mixtures. Besides, our results allowed the rationalization of the difference in the uptakes experimentally observed between both iC4 hydrocarbons.

The product of catalytic isomerization is a mixture of linear and branched hydrocarbons that are in thermodynamic equilibrium, and their separation becomes necessary in the petrochemical industry. Zeolite 5A is usually industrially used to sieve alkane isomers, but its pore size allows only the separation of linear alkanes from the monobranched and dibranched alkanes by a kinetic mechanism. A more efficient approach to improve the average research octane number would be to adsorptively separate the di-methyl alkanes as products and recycle both the linear and mono-methyl alkanes to the isomerization reactor. Since the microscopic processes of adsorbates in zeolites are generally difficult or impossible to determine by experiments, especially in the case of mixtures, molecular simulation represents an attractive alternative. In this computational study, we propose a conceptual separation process for hexane isomers consisting of several adsorptive steps. Different zeolite topologies were examined for their ability to conduct this separation based on adsorption equilibrium and kinetics.