Noncovalent chemistry of xenon opens the door for anesthetic xenon recovery using Bio-MOFs

Designing an inexpensive and highly efficient recovery process for xenon (Xe) is gaining importance in the development of sustainable applications. Using metal organic frameworks (MOFs) for separating Xe from anesthetic gas mixtures has been a recent topic studied rarely and superficially in the literature. We theoretically investigated Xe recovery performances of 43 biological MOFs (Bio-MOFs) formed by biocompatible metal cations and biological endogenous linkers. Xe uptakes and Xe permeabilities in its binary mixtures with CO2, O2, and N2 were investigated by applying Grand Canonical Monte Carlo and Molecular Dynamics simulations. Materials with metalloporphyrin, hexacarboxylate, triazine, or pyrazole ligands, dimetallic paddlewheel units, relatively large pore sizes (PLD > 5 Å and LCD > 10 Å), large void fractions (≈0.8), and large surface areas (>2900 m2 g-1) have been determined as top performing Bio-MOFs for Xe recovery. By applying Density Functional Theory simulations and generating electron density difference maps, we determined that Xe-host interactions in the top performing Bio-MOFs are maximized mainly due to noncovalent interactions of Xe, such as charge-induced dipole and aerogen-π interactions. Polarized Xe atoms in the vicinity of cations/anions as well as π systems are fingerprints of enhanced guest-host interactions. Our results show examples of rarely studied aerogen interactions that play a critical role in selective adsorption of Xe in nanoporous materials.

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.

Quadrupolar Interaction with Zeolite and Pressure Swing Adsorption in Portable Medical Oxygen Concentrators for Breathing of Covid-19 and COPD Patients

Pressure swing adsorption (PSA) technology is used in medical oxygen concentrators for selective adsorption of nitrogen under pressure from air to produce oxygen-enriched air for breathing. The functional material in oxygen concentrators is aluminosilicate zeolite like Li-13X, 5A (LTA) with a relatively high Al-content. It selectively adsorbs nitrogen from the air into their molecular sieves due to the strong quadrupolar interaction between the adsorbed nitrogen and the electrical field of the charge balancing non-framework exchangeable cations of aluminosilicate zeolites. In an oxygen concentrator, two adsorbent vessels packed with the zeolite working alternately in a cyclic process are used for a near-continuous production of the oxygen-enriched air.

Striking dual functionality of a novel Pd@Eu-MOF nanocatalyst in C(sp2)-C(sp2) bond-forming and CO2 fixation reactions

Pd nanoparticles were immobilized on a highly porous, hydrothermally stable Eu-MOF via solution impregnation and H2 reduction to yield a novel Pd@Eu-MOF nanocatalyst. This composite was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), inductively coupled plasma optical emission spectroscopy (ICP-OES), powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopy (XPS). Unprecedentedly, the Pd@Eu-MOF nanocatalyst could be applied with excellent results in two strikingly different, mechanistically distinct, reactions i.e., Suzuki-Miyaura cross-coupling and cycloaddition of CO2 to a range of epoxides. Under the best reaction conditions, 98-99% yields have been attained in both catalytic processes. Moreover, in either case the heterogeneous catalyst was easily recovered and efficiently reused for more than four cycles, indicating its high stability and reproducibility. PXRD, TEM and XPS measurements on the recycled catalyst confirmed that it maintained its original structure and morphology; no Pd NP agglomeration was observed.

Liesegang Banding for Controlled Size and Growth of Zeolitic-Imidazolate Frameworks

Here, the formation of periodic precipitation (Liesegang bands) from hybrid (organic-inorganic) components is reported, namely ZIF-8, ZIF-67, and their mixed metal derivatives. The spacing and width laws that characterize the Liesegang system are determined and the resulting pattern is exploited to crystal engineer various sizes and doping of the ZIF material. Several key parameters that govern the crystallization process of ZIF-8 are investigated in each band and the growth of the particles in each band is found to follow an Ostwald ripening mechanism.

CH4/N2 Adsorptive Separation on Zeolite X/AC Composites

A series of zeolite X/activated carbon (AC) composites were prepared from the same starting materials at various activation time. The corresponding modified samples were obtained by being treated with diluted NH4Cl solution. The relationship between porosity development, surface properties, and CH4/N2 adsorption performance was investigated. The increase of micropore volume is beneficial to the improvement of CH4 and N2 adsorption capacity, but more sensitive for CH4. In addition, the polar functional groups of zeolite X/AC composites may enhance CH4 adsorption capacity. More importantly, both developing micropore structure and surface modification contributed to enhance the adsorption selectivity αCH4/N2. As the optimum sample of these studies, HZAC(24) showed CH4 adsorption capacity of 17.3 cm3/g and the highest adsorption selectivity αCH4/N2 of 3.4. The CH4 and N2 adsorption isotherms of all samples can be well fitted by the Langmuir–Freundlich model. HZAC(24) showed an excellent cyclability of adsorption/desorption of CH4 with a neglectable capacity loss after subsequent cycles. Moreover, HZAC(24) displayed relatively rapid adsorption kinetics. These properties of zeolite X/AC composites are essential for the adsorptive separation of CH4 from N2 in the pressure swing adsorption (PSA) process.

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.

Adsorptive process design for the separation of hexane isomers using zeolites

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.

Computational evaluation of aluminophosphate zeotypes for CO2/N2 separation

Zeolites and structurally related materials (zeotypes) have received considerable attention as potential adsorbents for selective carbon dioxide adsorption. Within this group, zeotypes with aluminophosphate composition (AlPOs) could be an interesting alternative to the more frequently studied aluminosilicate zeolites. So far, however, only a few AlPOs have been characterised experimentally in terms of their CO₂ adsorption properties. In this study, force-field based grand-canonical Monte Carlo (GCMC) simulations were used to evaluate the potential of AlPOs for CO₂/N₂ separation, a binary mixture that constitutes a suitable model system for the removal of carbon dioxide from flue gases. A total of 51 frameworks were considered, all of which have been reported either as pure AlPOs or as heteroatom-containing AlPO derivatives. Prior to the GCMC simulations, all structures were optimised using dispersion-corrected density-functional theory calculations. The potential of these 51 systems for CO₂/N₂ separation was assessed in preliminary calculations (Henry constants and CO₂ uptake at selected pressures). On the basis of these calculations, 21 AlPOs of particular interest were selected, for which 15 : 85 CO₂/N₂ mixture adsorption isotherms were calculated up to 10 bar. For adsorption-based separations using an adsorption pressure of 1 bar (vacuum-swing adsorption), AlPOs with GIS, ATN, ATT, and SIV topologies were predicted to be most attractive, as they combine high CO₂/N₂ selectivities (75 to 140) and reasonable CO₂ working capacities (1 to 1.7 mmol g^-1). Under pressure-swing adsorption conditions, there is a tradeoff between selectivity and working capacity: while highly selective AlPOs like GIS reach only moderate working capacities, the frameworks with the highest working capacities above 2 mmol g^-1, AFY, KFI, and SAV, have lower selectivities between 25 and 35. To gain atomic-level insights into the host-guest interactions, interaction energy maps were computed for selected AlPOs. The computational assessment presented here can guide future experimental efforts in the development and optimisation of AlPO-based adsorbents for selective CO₂ adsorption.

CO2 Capture and Separations Using MOFs: Computational and Experimental Studies

This Review focuses on research oriented toward elucidation of the various aspects that determine adsorption of CO₂ in metal-organic frameworks and its separation from gas mixtures found in industrial processes. It includes theoretical, experimental, and combined approaches able to characterize the materials, investigate the adsorption/desorption/reaction properties of the adsorbates inside such environments, screen and design new materials, and analyze additional factors such as material regenerability, stability, effects of impurities, and cost among several factors that influence the effectiveness of the separations. CO₂ adsorption, separations, and membranes are reviewed followed by an analysis of the effects of stability, impurities, and process operation conditions on practical applications.