Computer-Assisted Screening of Ordered Crystalline Nanoporous Adsorbents for Separation of Alkane Isomers

Phase behavior of n-hexane confined in unconsolidated nanoporous media: an experimental investigation at varying pore sizes and temperatures

We investigated the effect of confinement on the phase behavior of hexane in nanopores of mesoporous silica at varying pore diameters and temperatures using a patented gravimetric apparatus. The adsorption and desorption isotherms were experimentally measured, and the capillary condensation and evaporation pressures were calculated from the isotherms. The results show that, for all pore sizes and temperatures utilized here, the confinement of fluids significantly lowers the vapor-liquid phase transition pressures. However, its evaporation, i.e., liquid-vapor phase transition, occurs at a lower pressure than its capillary condensation counterpart. The experimental findings demonstrate that the confinement effect becomes weaker in wider nanopores due to the reduced solid-fluid interactions in larger spaces. Furthermore, it is evident from isotherms that hexane rapidly approaches a supercritical-like state at high temperatures when confined in smaller pores, resulting in an ambiguous vapor-liquid phase transition. In contrast, this behavior disappears in larger pores at similar temperatures. Moreover, the present study compares the fully gravimetric adsorption method against the thermogravimetric approach. The results show that the fully gravimetric method, which directly measures the mass of the adsorbed or condensed fluids, provides significant advantages over the thermogravimetric counterpart. The findings of this study are expected to be of fundamental interest to a wide range of science and engineering communities concerned about the behavior of heavier hydrocarbons in various industrial applications, and modeling the confined phase behavior of fluids and developing robust equations of state (EOS).

Adsorption and dynamics of linear and mono-branched hexane isomers in MIL-140 metal-organic frameworks

Recent breakthrough experiments revealed the iso-reticular Zr-MOFs, MIL-140B and MIL-140C, as promising sorbents for the separation of C6 isomers. Interestingly while the ultra-small pore MIL-140B exhibited hexane isomer sorption hierarchy according to the normal boiling point order (n-C6 > 3MP (3-methyl pentane)), an uncommon shift in the elution order was observed in the larger pore MIL-140C. It was only speculated that the flexibility of the MOFs might be the origin of this intriguing behavior. Herein, flexible force field hybrid osmotic Monte Carlo combined with molecular dynamics simulations were carried out to unravel the microscopic mechanism of the adsorption and dynamics of both C6 isomers in MIL140B and MIL140C. Thermodynamically preferred adsorption of n-C6 over 3MP was predicted for MIL-140B and to a slightly less extent for MIL-140C. Interestingly while the mobility of n-C6 was found to remain higher than that of 3MP in the whole range of loading for MIL-140B, 3MP becomes more mobile than n-C6 at saturation in MIL-140C. This suggests that this kinetics order is most probably the origin of the inversion of the elution order observed experimentally for MIL-140C. The translational and rotational dynamics of the two guests in MIL-140B and MIL-140C was further understood in-depth.

Zeolites in Adsorption Processes: State of the Art and Future Prospects

Zeolites have been widely used as catalysts, ion exchangers, and adsorbents since their industrial breakthrough in the 1950s and continue to be state-of the-art adsorbents in many separation processes. Furthermore, their properties make them materials of choice for developing and emerging separation applications. The aim of this review is to put into context the relevance of zeolites and their use and prospects in adsorption technology. It has been divided into three different sections, i.e., zeolites, adsorption on nanoporous materials, and chemical separations by zeolites. In the first section, zeolites are explained in terms of their structure, composition, preparation, and properties, and a brief review of their applications is given. In the second section, the fundamentals of adsorption science are presented, with special attention to its industrial application and our case of interest, which is adsorption on zeolites. Finally, the state-of-the-art relevant separations related to chemical and energy production, in which zeolites have a practical or potential applicability, are presented. The replacement of some of the current separation methods by optimized adsorption processes using zeolites could mean an improvement in terms of sustainability and energy savings. Different separation mechanisms and the underlying adsorption properties that make zeolites interesting for these applications are discussed.

In Silico Evolution of High-Performing Metal Organic Frameworks for Methane Adsorption

The increased use of transition fuels, such as natural gas, and the resulting increase in methane emissions have resulted in a need for novel methane storage materials. Metal-organic frameworks (MOFs) have shown promise as efficient storage materials. A virtually limitless number of potential MOFs can be hypothesized, which exhibit a wide variety of different structural and chemical characteristics. Because of the numerous possibilities, identification of the best MOF for methane storage can be a potentially challenging problem. In this work, determination of the best such MOF was cast as an inverse function problem. The function, a random forest (RF) model using 12 structural and chemical descriptors, was trained on 10% of a data set consisting of 130 398 hypothetical MOFs (hMOFs) to predict simulated methane uptake. The RF model was tested on the remaining 90% of the data. After validation, a genetic algorithm (GA) was used to evolve in silico the best MOFs for methane adsorption. The RF model was imbedded into the GA as the fitness function to predict the methane uptake of the evolved MOFs (eMOFs). The best 15 eMOFs matched hMOFs found in the top 1% of the database. Nine of the 15 eMOFs were found in the top 0.1%. More impressively, two of the eMOFs matched the top two hypothetical MOFs with the highest methane uptake values out of the entire database of 130 398 MOFs. Further, by leveraging the ensemble nature of the GA, it was possible to characterize the importance of the different material properties for methane adsorption, providing fundamental insight for future material design strategies.

How Reliable Is the Ideal Adsorbed Solution Theory for the Estimation of Mixture Separation Selectivities in Microporous Crystalline Adsorbents?

Microporous crystalline adsorbents such as zeolites and metal-organic frameworks (MOFs) have potential use in a wide variety of separation applications. The adsorption selectivity S _ads is a key metric that quantifies the efficacy of any microporous adsorbent in mixture separations. The Ideal Adsorbed Solution Theory (IAST) is commonly used for estimating the value of S _ads, with unary isotherms of the constituent guests as data inputs. There are two basic tenets underlying the development of the IAST. The first tenet mandates a homogeneous distribution of adsorbates within the pore landscape. The second tenet requires the surface area occupied by a guest molecule in the mixture to be the same as that for the corresponding pure component. Configurational-bias Monte Carlo (CBMC) simulations are employed in this article to highlight several scenarios in which the IAST fails to provide a quantitatively correct description of mixture adsorption equilibrium due to a failure to conform to either of the two tenets underpinning the IAST. For CO₂ capture with cation-exchanged zeolites and MOFs with open metal sites, there is congregation of CO₂ around the cations and unsaturated metal atoms, resulting in failure of the IAST due to an inhomogeneous distribution of adsorbates in the pore space. Thermodynamic non-idealities also arise due to the preferential location of CO₂ molecules at the window regions of 8-ring zeolites such as DDR and CHA or within pockets of MOR and AFX zeolites. Thermodynamic non-idealities are evidenced for water/alcohol mixtures due to molecular clustering engendered by hydrogen bonding. It is also demonstrated that thermodynamic non-idealities can be strong enough to cause selectivity reversals, which are not anticipated by the IAST.

Tuning the hexane isomer separation performances of Zeolitic Imidazole Framework-8 using mechanical pressure

Hybrid osmotic Monte Carlo simulations were performed to anticipate the tunability of the separation performance of the flexible Zeolitic Imidazole Framework-8 (ZIF-8) via the application of an external mechanical pressure. This synergistic combination of mechanical control of the pore aperture/cage dimension and guest adsorption was applied to the challenging hexane isomers separation processes of vital importance in the field of petrochemical industry. The application of a mechanical pressure above 1 GPa was predicted to boost the linear hexane/2-methylpentane and 2-methylpentane/2,3-dimethylbutane selectivity by 40% and 17%, respectively, as compared to the pristine ZIF-8. We further unraveled the microscopic origin of this optimized performance with an in-depth analysis of the critical interplay between the structural changes of the ZIF-8 framework and the conformational rearrangements of C6 isomers under mechanical pressure.

In-silico identification of adsorbent for separation of ethane/ethylene mixture

We present here a high-throughput computational screening of 4,821 real metal-organic framework (MOF) structures that do not contain any open metal sites to isolate the best performing candidate for separation of ethane/ethylene mixture at ambient conditions. The MOF structures were assessed on the basis of several adsorption-based separation performance metrics. Some of these metrics were found to correlate strongly among themselves. We have presented various structures-property correlations which unfold useful insights. MOF ATAGEJ is found to be the top performing MOF with highest adsorbent performance score 12.38 mol/kg and regenerability 93.88%. Several other MOFs OTOLIU (MIL-167), UMUMOG (UBMOF-8), and TOVGES (PCN-230) containing tetravalent metal cations such as Zr⁴⁺ and Ti⁴⁺ are found to be potential structures that are thermally, mechanically, and chemically stable and performs better than zeolites. Adsorption selectivity shows exponential correlation with difference of heat of adsorption of ethane and ethene at 0.1 bar and 298 K. We have also presented how various performance metrics correlate among themselves. These correlations unfold useful insights. Graphical abstract.

Synthetic Macrocycle-Based Nonporous Adaptive Crystals for Molecular Separation

The exploitation of new materials for adsorptive separation of industrially important hydrocarbons is of great importance in both scientific research and petrochemical industry. Nonporous adaptive crystals (NACs) as a robust class of synthetic materials have drawn much attention during the past five years for their superior performance in adsorption and separation. Pillararenes are the main family of macrocyclic arenes used for NACs construction, where the structure-function relationship has been intensively studied. In the past two years, some emerging types of synthetic macrocyclic arenes have been successfully brought into this research field, showing the gradual enrichment and diversification of NACs materials. This Minireview summarizes the recent advances of synthetic macrocycle-based NACs, which are categorized by various practical applications in molecular separation. Besides, NACs-based vapochromic supramolecular systems are also discussed. Finally, future perspectives and challenges of NACs are given. We envisage that this Minireview will be a useful and timely reference for those who are interested in new molecular and supramolecular crystals for storage and separation applications.

Metrics for Evaluation and Screening of Metal-Organic Frameworks for Applications in Mixture Separations

For mixture separations, metal-organic frameworks (MOFs) are of practical interest. Such separations are carried out in fixed bed adsorption devices that are commonly operated in a transient mode, utilizing the pressure swing adsorption (PSA) technology, consisting of adsorption and desorption cycles. The primary objective of this article is to provide an assessment of the variety of metrics that are appropriate for screening and ranking MOFs for use in fixed bed adsorbers. By detailed analysis of several mixture separations of industrial significance, it is demonstrated that besides the adsorption selectivity, the performance of a specific MOF in PSA separation technologies is also dictated by a number of factors that include uptake capacities, intracrystalline diffusion influences, and regenerability. Low uptake capacities often reduce the efficacy of separations of MOFs with high selectivities. A combined selectivity-capacity metric, Δq, termed as the separation potential and calculable from ideal adsorbed solution theory, quantifies the maximum productivity of a component that can be recovered in either the adsorption or desorption cycle of transient fixed bed operations. As a result of intracrystalline diffusion limitations, the transient breakthroughs have distended characteristics, leading to diminished productivities in a number of cases. This article also highlights the possibility of harnessing intracrystalline diffusion limitations to reverse the adsorption selectivity; this strategy is useful for selective capture of nitrogen from natural gas.

Energy-efficient separation alternatives: metal–organic frameworks and membranes for hydrocarbon separation

The diversity of metal–organic frameworks enables the design of highly efficient adsorbents and membranes towards hydrocarbon separations for energy consumption mitigation.

Highlighting the Influence of Thermodynamic Coupling on Kinetic Separations with Microporous Crystalline Materials

The main focus of this article is on mixture separations that are driven by differences in intracrystalline diffusivities of guest molecules in microporous crystalline adsorbent materials. Such "kinetic" separations serve to over-ride, and reverse, the selectivities dictated by mixture adsorption equilibrium. The Maxwell-Stefan formulation for the description of intracrystalline fluxes shows that the flux of each species is coupled with that of the partner species. For n-component mixtures, the coupling is quantified by a n × n dimensional matrix of thermodynamic correction factors with elements Γ _ij ; these elements can be determined from the model used to describe the mixture adsorption equilibrium. If the thermodynamic coupling effects are essentially ignored, i.e., the Γ _ij is assumed to be equal to δ _ij , the Kronecker delta, the Maxwell-Stefan formulation degenerates to yield uncoupled flux relations. The significance of thermodynamic coupling is highlighted by detailed analysis of separations of five different mixtures: N₂/CH₄, CO₂/C₂H₆, O₂/N₂, C₃H₆/C₃H₈, and hexane isomers. In all cases, the productivity of the purified raffinate, containing the tardier species, is found to be significantly larger than that anticipated if the simplification Γ _ij = δ _ij is assumed. The reason for the strong influence of Γ _ij on transient breakthroughs is traceable to the phenomenon of uphill intracrystalline diffusion of more mobile species. The major conclusion to emerge from this study is that modeling of kinetic separations needs to properly account for the thermodynamic coupling effects.

High-throughput computational screening of metal–organic frameworks with topological diversity for hexane isomer separations

A large-scale computational screening of 13 512 MOFs with topological diversity was carried out to search the optimal candidates for the simultaneous separation of two dimethyl butanes from the quinary equimolar mixture of hexane isomers.

Adsorption of Cyclohexane in Pure Silica Zeolites: High-Throughput Computational Screening Validated by Experimental Data

Adsorption of cyclohexane in pure silica zeolites was studied experimentally and by molecular simulations. Based on the adsorption isobars obtained from the quasi-equilibrated temperature adsorption and desorption (QE-TPDA) measurements and reported adsorption isotherms for high-silica zeolites Y, ZSM-5, and ZSM-11 we refined Lennard-Jones parameters for guest-host interactions available in the literature. Adsorption of cyclohexane from equimolar mixture of twisted-boat and chair conformations has been screened in 171 pure silica zeolitic structures using grand canonical Monte Carlo simulations. Almost 20 frameworks showing extraordinary preference for adsorption of the chair conformation over the twisted boat one or vice versa were found. This selectivity was attributed to the geometry of channels and cavities present in the pore structures, as all t-boat selective structures possess channels or cavities of 8.3-9.1 Å. We also differentiated ways of chair-selectivity depending on the size and shape of the channels or cavities and also on the arrangement of the guest molecules in the pores.

Advances in Organic Solvent Nanofiltration Rely on Physical Chemistry and Polymer Chemistry

The vast majority of industrial chemical synthesis occurs in organic solution. Solute concentration and solvent recovery consume ~50% of the energy required to produce chemicals and pose problems that are as relevant as the synthesis process itself. Separation and purification processes often involve a phase change and, as such, they are highly energy-intensive. However, novel, energy-efficient technologies based on polymer membranes are emerging as a viable alternative to thermal processes. Despite organic solvent nanofiltration (OSN) could revolutionize the chemical, petrochemical, food and pharmaceutical industry, its development is still in its infancy for two reasons: (i) the lack of fundamental knowledge of elemental transport phenomena in OSN membranes, and (ii) the instability of traditional polymer materials in chemically challenging environments. While the latter issue has been partially solved, the former was not addressed at all. Moreover, the few data available about solute and solvent transport in OSN membranes are often interpreted using inappropriate theoretical tools, which contributes to the spread of misleading conclusions in the literature. In this review we provide the state of the art of organic solvent nanofiltration using polymeric membranes. First, theoretical models useful to interpret experimental data are discussed and some misleading conclusions commonly reported in the literature are highlighted. Then, currently available materials are reviewed. Finally, materials that could revolutionize OSN in the future are identified. Among the possible applications of OSN, isomers separation could open a new era in chemical engineering and polymer science in the years to come.

Water-Stable Metal-Organic Framework with Three Hydrogen-Bond Acceptors: Versatile Theoretical and Experimental Insights into Adsorption Ability and Thermo-Hydrolytic Stability

A new microporous cadmium metal-organic framework was synthesized both mechanochemically and in solution by using a sulfonyl-functionalized dicarboxylate linker and an acylhydrazone colinker. The three-dimensional framework is highly stable upon heating to 300 °C as well as in aqueous solutions at elevated temperatures or acidic conditions. The thermally activated material exhibits steep water vapor uptake at low relative pressures at 298 K and excellent recyclability up to 260 °C as confirmed by both quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) method as well as adsorption isotherm measurements. Reversible isotherms and hysteretic isobars recorded for the desorption-adsorption cycles indicate the maximum uptake of 0.19 g/g (at 298 K, up to p/p₀ = 1) or 0.18 g/g (at 1 bar, within 295-375 K range), respectively. The experimental isosteric heat of adsorption (48.9 kJ/mol) indicates noncoordinative interactions of water molecules with the framework. Exchange of the solvent molecules in the as-made material with water, performed in the single-crystal to single-crystal manner, allows direct comparison of both X-ray crystal structures. The single-crystal X-ray diffraction for the water-loaded framework demonstrates the orientation of water clusters in the framework cavities and reveals their strong hydrogen bonding with sulfonyl, acyl, and carboxylate groups of the two linkers. The grand canonical Monte Carlo (GCMC) simulations of H₂O adsorption corroborate the experimental findings and reveal preferable locations of guest molecules in the framework voids at various pressures. Additionally, both experimental and GCMC simulation insights into the adsorption of CO₂ (at 195 K) on the activated framework are presented.

Iron-Based Metal-Organic Framework with Hydrophobic Quadrilateral Channels for Highly Selective Separation of Hexane Isomers

A novel iron-based microporous metal-organic framework built of trinuclear iron clusters [Fe₃(μ₃-O)(COO)₆] and 2,2-bis(4-carboxyphenyl)-hexafluoropropane (6FDCA) has been prepared by solvothermal synthesis. It exhibits excellent chemical stability and strong hydrophobic character. More importantly, this material is capable of separating hexane isomers with good separation performance on the basis of a kinetically controlled process, making it a promising candidate for improving the research octane number of gasoline.

Commensurate-incommensurate adsorption and diffusion in ordered crystalline microporous materials

For homologous series of linear chain molecules, there could be either a match, or mismatch, between the characteristic periodicity of the host structure and the characteristic length of the guest molecules. The major objective of this article is to highlight the influence of commensurateness, or incommensurateness, on both the adsorption and diffusion characteristics. Published experimental data, backed by molecular simulation results, are used to highlight the attendant non-monotonicity in adsorption strengths and diffusivities. We demonstrate the possibility of separating mixtures of n-alkanes, n-alcohols, and hydrocarbon isomers by appropriate and judicious choice of the dimensions, topology, and connectivity of the crystalline host material. Of particular practical interest are entropy-based separations that manifest at pore saturation conditions, relying on differences in the saturation capacities of the constituent species; the exploiting of such entropy effects is discussed with the aid of several examples.

Machine Learning Using Combined Structural and Chemical Descriptors for Prediction of Methane Adsorption Performance of Metal Organic Frameworks (MOFs)

Using molecular simulation for adsorbent screening is computationally expensive and thus prohibitive to materials discovery. Machine learning (ML) algorithms trained on fundamental material properties can potentially provide quick and accurate methods for screening purposes. Prior efforts have focused on structural descriptors for use with ML. In this work, the use of chemical descriptors, in addition to structural descriptors, was introduced for adsorption analysis. Evaluation of structural and chemical descriptors coupled with various ML algorithms, including decision tree, Poisson regression, support vector machine and random forest, were carried out to predict methane uptake on hypothetical metal organic frameworks. To highlight their predictive capabilities, ML models were trained on 8% of a data set consisting of 130,398 MOFs and then tested on the remaining 92% to predict methane adsorption capacities. When structural and chemical descriptors were jointly used as ML input, the random forest model with 10-fold cross validation proved to be superior to the other ML approaches, with an R² of 0.98 and a mean absolute percent error of about 7%. The training and prediction using the random forest algorithm for adsorption capacity estimation of all 130,398 MOFs took approximately 2 h on a single personal computer, several orders of magnitude faster than actual molecular simulations on high-performance computing clusters.

Materials Genome in Action: Identifying the Performance Limits of Physical Hydrogen Storage

The Materials Genome is in action: the molecular codes for millions of materials have been sequenced, predictive models have been developed, and now the challenge of hydrogen storage is targeted. Renewably generated hydrogen is an attractive transportation fuel with zero carbon emissions, but its storage remains a significant challenge. Nanoporous adsorbents have shown promising physical adsorption of hydrogen approaching targeted capacities, but the scope of studies has remained limited. Here the Nanoporous Materials Genome, containing over 850 000 materials, is analyzed with a variety of computational tools to explore the limits of hydrogen storage. Optimal features that maximize net capacity at room temperature include pore sizes of around 6 Å and void fractions of 0.1, while at cryogenic temperatures pore sizes of 10 Å and void fractions of 0.5 are optimal. Our top candidates are found to be commercially attractive as "cryo-adsorbents", with promising storage capacities at 77 K and 100 bar with 30% enhancement to 40 g/L, a promising alternative to liquefaction at 20 K and compression at 700 bar.

Cadmium-BINOL Metal-Organic Framework for the Separation of Alcohol Isomers

The large‐scale isolation of specific isomers of amyl alcohols for applications in the chemical, pharmaceutical, and biochemical industries represents a challenging task due to the physicochemical similarities of these structural isomers. The homochiral metal–organic framework cadmium–BINOL (BINOL=1,1′‐bi‐2‐naphthol) is suitable for the separation of pentanol isomers, combining adsorption selectivities above 5 with adsorption capacities of around 4.5 mol kg⁻¹. Additionally, a slight ability for separation of racemic mixtures of 2‐pentanol is also detected. This behavior is explained based on matching shapes, strength of host–guest interactions, and on the network of hydrogen bonds. The last of these explains both the relative success and shortfalls of prediction methods at high loadings (ideal adsorbed solution theory) or at low coverage (separation factors), which are therefore useful here at a qualitative level, but not accurate in quantitative terms. Finally, the high selectivity of cadmium–BINOL for 1‐pentanol over its isomers offers prospects for practical applications and some room for optimizing conditions.

Screening metal–organic frameworks for mixture separations in fixed-bed adsorbers using a combined selectivity/capacity metric

For screening purposes, mixture separations with MOFs are evaluated on the basis of a combined selectivity/capacity metric.

Screening metal–organic frameworks for separation of pentane isomers

Pentane isomers can be fractionated using Fe₂(BDP)₃ to yield three different fractions depending on the degree of branching.

In silico prediction and screening of modular crystal structures via a high-throughput genomic approach

High-throughput computational methods capable of predicting, evaluating and identifying promising synthetic candidates with desired properties are highly appealing to today's scientists. Despite some successes, in silico design of crystalline materials with complex three-dimensionally extended structures remains challenging. Here we demonstrate the application of a new genomic approach to ABC-6 zeolites, a family of industrially important catalysts whose structures are built from the stacking of modular six-ring layers. The sequences of layer stacking, which we deem the genes of this family, determine the structures and the properties of ABC-6 zeolites. By enumerating these gene-like stacking sequences, we have identified 1,127 most realizable new ABC-6 structures out of 78 groups of 84,292 theoretical ones, and experimentally realized 2 of them. Our genomic approach can extract crucial structural information directly from these gene-like stacking sequences, enabling high-throughput identification of synthetic targets with desired properties among a large number of candidate structures.

High-throughput computation aids design of new functional materials. Here, Yu et al. develop a high-throughput screening method for a group of zeolites with crystalline modular structures which are viewed as having gene-like stacking codes, and identify the most promising structures with desired properties.

On the performance of FAU and MFI zeolites for the adsorptive removal of a series of volatile organic compounds from air using molecular simulation

Volatile organic compound (VOC) emissions can cause serious risk to human health and the environment. In this work, we used Monte Carlo simulations to assess the performance of industrially important zeolites for the adsorption-based removal of a number of common air pollutants, particularly small saturated and unsaturated hydrocarbons: propane, butane, propene, and 1-butene. We focused on the cage-like FAU and channel-like MFI zeolites. The adsorption isotherms of the multicomponent N2/O2/Ar/VOC mixtures at real concentrations and room temperature reveal a considerable influence of the host topology and pore dimensions. While the adsorption of the VOCs from the mixture in FAU is almost negligible, it is remarkable in MFI. The adsorption selectivity of each VOC over the air compounds exhibits a maximum at about 10(6)-10(7) Pa, and then decreases to virtually zero due to entropic effects. This behaviour for selectivity is maintained regardless of the chain length and the presence of double bonds in the VOC, but the values are indeed affected. Also, we examined the selectivity at 10(7) Pa for a number of other widely used zeolites, with pore features ensuring the diffusion of the adsorbates. Apart from MFI, we also found the channel-like MEL and MTW zeolite candidates for the targeted air decontamination.

Separating mixtures by exploiting molecular packing effects in microporous materials

We examine mixture separations with microporous adsorbents such as zeolites, metal-organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs), operating under conditions close to pore saturation. Pore saturation is realized, for example, when separating bulk liquid phase mixtures of polar compounds such as water, alcohols and ketones. For the operating conditions used in industrial practice, pore saturation is also attained in separations of hydrocarbon mixtures such as xylene isomers and hexane isomers. Separations under pore saturation conditions are strongly influenced by differences in the saturation capacities of the constituent species; the adsorption is often in favor of the component with the higher saturation capacity. Effective separations are achieved by exploiting differences in the efficiency with which molecules pack within the ordered crystalline porous materials. For mixtures of chain alcohols, the shorter alcohol can be preferentially adsorbed because of its higher saturation capacity. With hydrophilic adsorbents, water can be selectively adsorbed from water-alcohol mixtures. For separations of o-xylene-m-xylene-p-xylene mixtures, the pore dimensions of MOFs can be tailored in such a manner as to allow optimal packing of the isomer that needs to be adsorbed preferentially. Subtle configurational differences between linear and branched alkane isomers result in significantly different packing efficiencies within the pore topology of MFI, AFI, ATS, and CFI zeolites. A common characteristic feature of most separations that are reliant on molecular packing effects is that adsorption and intra-crystalline diffusion are synergistic; this enhances the separation efficiencies in fixed bed adsorbers.

Crystalline capsules: metal-organic frameworks locked by size-matching ligand bolts

Metal-organic frameworks (MOFs) are shown to be good examples of a new class of crystalline porous materials for guest encapsulation. Since the encapsulation/release of guest molecules in MOF hosts is a reversible process in nature, how to prevent the leaching of guests from the open pores with minimal and nondestructive modifications of the structure is a critical issue. To address this issue, we herein propose a novel strategy of encapsulating guests by introducing size-matching organic ligands as bolts to lock the pores of the MOFs through deliberately anchoring onto the open metal sites in the pores. Our proposed strategy provides a mechanical way to prevent the leaching of guests and thereby has less dependence on the specific chemical environment of the hosts, thus making it applicable for a wide variety of existing MOFs once the size-matching ligands are employed.

Discovery of optimal zeolites for challenging separations and chemical transformations using predictive materials modeling

Zeolites play numerous important roles in modern petroleum refineries and have the potential to advance the production of fuels and chemical feedstocks from renewable resources. The performance of a zeolite as separation medium and catalyst depends on its framework structure. To date, 213 framework types have been synthesized and >330,000 thermodynamically accessible zeolite structures have been predicted. Hence, identification of optimal zeolites for a given application from the large pool of candidate structures is attractive for accelerating the pace of materials discovery. Here we identify, through a large-scale, multi-step computational screening process, promising zeolite structures for two energy-related applications: the purification of ethanol from fermentation broths and the hydroisomerization of alkanes with 18-30 carbon atoms encountered in petroleum refining. These results demonstrate that predictive modelling and data-driven science can now be applied to solve some of the most challenging separation problems involving highly non-ideal mixtures and highly articulated compounds.

Towards computational design of zeolite catalysts for CO2 reduction

Computational search of structure database for CO₂ reduction catalysts using molecular simulation and machine learning.