Special issue on metal-organic frameworks for emerging chemical technologies

Nanoporous materials such as zeolites have an extraordinary capability for selective adsorption, transport, and catalytic conversion of molecules. They are at the heart of industrial processes that generate refined fuels, petrochemicals, fine chemicals, and other products. However, an expanded range of such materials is necessary to develop new technologies for efficient production of cleaner fuels and petrochemicals, water purification, greenhouse gas capture and biorefining, among others. Metal-Organic Frameworks (MOFs) are porous crystalline materials assembled by coordinative bonding between metal centers and organic linker molecules, and have recently allowed a very large expansion of the available suite of nanoporous materials. More than 7000 different MOF materials have already been synthesized, and there is an essentially infinite variety of possible MOF structures accessible with current synthesis techniques.

The abundance of nanoporous MOF structures has created many opportunities for obtaining novel adsorption, catalytic, and transport properties. A number of initial demonstrations of such properties have created great expectations for the role of MOFs in new chemical technologies. However, fulfilling these expectations will require the combined use of chemical engineering and materials chemistry principles to identify promising MOFs, understand and control their properties, and scalably fabricate robust MOF adsorbents, catalysts, and membranes. This Special Issue brings together 19 invited articles from experts worldwide. These articles thematically address key issues in MOF science and engineering including their predictive screening and selection, fundamentals of adsorption and diffusion in MOFs, evaluation of their catalytic and separation properties, membrane fabrication strategies, and scaled-up synthesis.

The Special Issue opens with a review article by Venna and Carreon discussing the state of the art and emerging challenges in MOF membranes for gas separation. MOFs are particularly promising materials for membrane-based separation of CO₂ from N₂, CH₄, or H₂. All three gas separations have significant implications for climate change mitigation and energy production. The authors review key advances in the synthesis of highly defect-free MOF membranes. Emerging challenges such as reliable MOF crystal and thin film growth methods, membrane reproducibility issues, and cost are identified and discussed. Next, Kwon and Jeong demonstrate an example of MOF membrane engineering and defect elimination through careful control of counter-diffusion membrane synthesis techniques. Their article highlights the excellent performance that can be obtained from engineered MOF membranes for the industrially important separation of propylene from propane. Caro et al. show how the development of effective substrate seeding methods allows the growth of low-defect MOF membranes. Furthermore, they demonstrate how post-synthetic membrane modification strategies allow the manipulation of gas adsorption and diffusion transport properties, thereby producing a membrane with high H₂/CO₂ separation performance. Coronas et al. illustrate the fabrication and use of MOF membranes in combined separation-reaction processes, particularly the intensification of equilibrium-limited chemical reactions. They show that a MOF membrane reactor significantly increases the conversion achievable in the esterification of acetic acid, by means of their selective removal of water from a complex mixture of reactants and products.

MOF-based catalysis is a rapidly growing area of interest, due to the many catalytic functionalities accessible through MOFs that are difficult to achieve with other nanoporous catalysts. Huang et al. show how a stable MOF photocatalyst material can be assembled from zirconium ions and halogenated organic linkers. They demonstrate that good visible-light photocatalytic performance can be obtained for reactions such as the oxidation of aromatic alcohols to aldehydes, thereby suggesting MOF photocatalysts as a viable alternative to conventional inorganic (e.g., TiO₂) photocatalysts. Using a similar zirconium-based MOF containing aminated linkers, Llabrés i Xamena et al. show that MOFs can be highly selective catalysts for conversion of biomass-derived alcohols into higher-value products, for example the production of levulinate esters by levulinic acid esterification under mild conditions. Their investigation highlights the catalytic mechanisms possible in MOFs, such as a dual acid-base mechanism involving both the zirconium centers and the amine functional groups on the linkers.

MOF materials have also generated great interest for the development of new CO₂ adsorption technologies. They have some of the highest internal surface areas known in nanoporous materials, and simultaneously offer the possibility of selective gas adsorption by appropriate selection of linkers and metal sites. Zhao et al. synthesize zirconium-based MOFs containing carboxylic acid functional groups, and ion exchange these groups to introduce metal ions into the pores of the MOF. This post-synthesis modification allows them to demonstrate one of the highest binary CO₂/N₂ selectivities known in MOFs, thus highlighting ion exchange as an effective way of introducing selective adsorption sites into MOFs. In a related study, Jiang et al. demonstrate a comprehensive modeling approach to evaluate the use of ion-exchanged MOFs for CO₂ capture applications. They calculate the fundamental adsorption properties of candidate MOFs through detailed molecular simulations, and then use this data to optimize a process model of a four-stage vacuum swing adsorption (VSA) unit for CO₂ capture from flue gas. Their optimized process model suggests that MOFs yield lower energy penalties per ton of CO₂ captured than current benchmark zeolite adsorbents, and highlights the value of multiscale modeling in the development of MOF-based adsorption systems.

A related approach is taken by Ferreira et al., who consider the purification of natural gas by CO₂-selective aluminum-based MOFs. They obtain detailed experimental CO₂ and CH₄ adsorption data over a range of operating conditions, and use this information in conjunction with a VSA process model to optimize a process for pipeline-quality methane production. Serra-Crespo, Gascon, Kapteijn et al. study in detail the CO₂ and CH₄ adsorption properties of a similar aluminum-based MOF containing amine functional groups, using experimental adsorption measurements and their interpretation by fixed-bed breakthrough adsorption simulations. Their investigation reveals very high CO₂ selectivities even at high operating pressures, as well as the importance of non-isothermal effects in the accurate simulation of MOF-based adsorptive processes. Krishna, Zhu et al. combine experimental breakthrough data and simulations to study the promising CO₂/CH₄ and propylene/propane separation properties of a nickel-based MOF, which contains chemically unsaturated ‘open metal’ sites for selective olefin binding. This article shows that the inclusion of intracrystalline diffusion effects – which are not usually considered in breakthrough curve simulations – can significantly improve the agreement between experimental breakthrough data and predictive simulations. Apart from CO₂, gases like NH₃ are important targets of ongoing efforts to develop effective adsorbents for filtering toxic industrial chemicals (TICs). To this end, Walton et al. present a systematic study of NH₃ removal from air by zirconium-based MOFs functionalized with different types of acidic groups. A trade-off is found between the strength of the acidic groups and the reduction in available porosity of the MOF due to the presence of the acidic groups. This finding is then applied to the selection of an optimal MOF structure, which is shown to closely approach the industrial NH₃ removal target.

The adsorptive separation and storage of hydrocarbons by MOFs is a topic of high importance for applications such as onboard methane storage in the automotive industry and olefin/paraffin separations in the petrochemical industry. Three articles in this Special Issue highlight the key role of computational approaches in identifying promising MOFs from a very large pool of possible MOF structures and understanding their interactions with hydrocarbons. Keskin et al. demonstrate a quantitative structure-property relationship (QSPR) approach to discover promising methane storage MOFs. The results of detailed molecular simulations using a large set of MOFs are used to parameterize a multivariable correlation (QSPR) for methane adsorption, which in turn allows the identification of structural parameter ranges required for a MOF to satisfy the desired performance criteria. A predicted list of top-performing MOFs is suggested for synthesis and experimental investigation. Borah, Snurr et al. demonstrate a computational approach to answer the practically relevant question of whether the presence of higher-alkane impurities significantly hinders methane uptake into onboard-storage MOF adsorbents. Through a molecular simulation study of loading-dependent diffusion of methane and higher alkanes in six MOFs of interest, the authors conclude that methane diffusivities are unlikely to be affected significantly by the higher alkanes. Computational predictions of this nature are valuable in reducing the considerable experimental effort of measuring multicomponent diffusion in MOFs. Xi et al. conduct a detailed molecular simulation study of the effect of variations in crystal structure of a set of zinc-based MOFs on their ethane/ethylene separation behavior. This work highlights the effects of cooperative and competitive binary adsorption on the ethane selectivity observed in this class of MOFs, and suggests guidelines for selecting MOFs with structural features that enhance alkane selectivity.

The scaled-up production and characterization of MOF adsorbents in the form of pelletized extrudates is a critical step in their pilot-scale and (eventually) industrial applications. Grande et al. present two articles addressing these important issues. In their first article, Grande et al. describe a detailed formulation for the production of extrudates of a cobalt-based MOF material, and investigate the role of other extrudate components such as the binder material and the plasticizer. An important finding is that the extrusion process can preserve the high crystallinity, porosity, and internal surface of the MOF. In their second article, Grande et al. study in detail the application of the above MOF adsorbent extrudates in a pressure swing adsorption (PSA) process. The fundamental adsorption and diffusion properties of several gases are measured, and then used to design a PSA cycle for hydrogen purification from steam-methane reforming off-gas. A process is developed to yield pure hydrogen with high recovery and productivity.

The last two articles in the Special Issue highlight the promise of MOFs for liquid-phase separations and chiral separations, both of which are emerging areas in comparison to the more widely studied small-molecule gas separation processes. Yang et al. investigate the separation of 5-hydroxymethylfurfural (HMF) from fructose in aqueous solutions. HMF is an important biobased chemical and can be obtained by dehydration of fructose. Using liquid-phase adsorption breakthrough measurements, it is shown that zinc-based MOFs can selectively remove HMF with good adsorption-desorption kinetics, high regenerability, and high stability in multiple cycles. Denayer and Duerinck conclude this Special Issue with a review article highlighting the progress and opportunities in the separation of chiral molecules by MOFs. These challenging enantiomer-selective separations are of great interest for pharmaceutical and analytical applications. The authors illustrate the versatile routes available to modify achiral MOFs or synthesize intrinsically chiral MOFs with specific pore architectures, and discuss current advances in performing chiral separations with such MOFs. The potential for synthesis of enantioselective MOF membranes is also discussed. The authors identify unresolved challenges such as maximizing the long-term stability of MOF chiral phases and their fabrication into monodisperse spheres needed to achieve high chromatographic resolution.

It is now evident that Metal-Organic Frameworks have evolved beyond the realm of pure synthetic chemistry, and have emerged as an extensive class of materials of high worldwide interest in technological areas central to modern chemical engineering. The chemical engineering community is playing a major role in addressing critical scientific and engineering hurdles in order to realize the widespread application of MOFs as advanced adsorbents, catalysts, and membranes. This Special Issue captures the key dimensions of our rapid progress to date in this challenging yet fast-expanding field of MOFs for new chemical technologies, and highlights its potential impact on the energy, petrochemical, water purification, environmental remediation, biorefining, pharmaceutical, and other industries in the coming years.