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.

Multiscale exploration of hydrocarbon adsorption and hopping through ZSM-5 channels - from Monte Carlo modelling to experiment.

In this article the results of statistical MC modelling corroborated by the FT-IR spectroscopy and gravimetric adsorption studies of low aliphatic hydrocarbons in ZSM-5 (Si/Al = 28 or silicalite) are presented. The extension of the existing Dubbeldam's forcefield towards inclusion of the finite aluminium-containing zeolites is proposed and its applicability is verified experimentally for the sorption of linear and branched hydrocarbons. The FT-IR spectroscopy applicability to follow the kinetics of small hydrocarbon adsorption has been successfully verified by the application of the Crank solution for diffusion to spectroscopy derived results.

Zeolitic Microporous Materials and Their Applications.

Research in the field of zeolites is a very active and relevant area, since these materials are still widely used as catalysts and adsorbents in many industrial applications, despite the appearance of other fascinating microporous materials with excellent properties [...].

ITQ-69: A Germanium-Containing Zeolite and its Synthesis, Structure Determination, and Adsorption Properties.

In this work, a new zeolite named as ITQ-69, has been synthesized, characterized and its application as selective adsorbent for industrially relevant light olefins/paraffins separations has been assessed. This material has been obtained as pure germania as well as silica-germania zeolites with different Si/Ge ratios using a diquaternary ammonium cation as organic structure directing agent. Its structure was determined by single-crystal X-Ray diffraction showing a triclinic unit cell forming a tridirectional small pore channel system (8×8×8R). Also, it has been found that Si preferentially occupies some special T sites of the structure as deduced from Rietveld analysis of the powder X-ray diffraction patterns. In addition, the new zeolite ITQ-69 has been found to be stable upon calcination and thus, its adsorption properties were evaluated, showing a promising kinetic selectivity for light olefin separations in the C3 fraction.

Preparation of Continuous Highly Hydrophobic Pure Silica ITQ-29 Zeolite Layers on Alumina Supports.

The preparation of continuous layers of highly hydrophobic pure silica ITQ-29 zeolite, potentially applicable as hydrophobic membranes for separation of molecules based on their polarity, has been investigated. Continuous layers of intergrown ITQ-29 zeolite crystals were successfully grown on porous alumina supports by optimization of the synthesis conditions, such as the appropriate selection of the seeds, the procedure for the gel preparation, and the calcination conditions. This resulted in the formation of all silica ITQ-29 zeolite layers without the presence of germanium required in previously reported ITQ-29 membranes, with the subsequent improvement in quality and stability, as verified by the absence of cracks after calcination. We have proved that the incorporation of aluminum from the support into the zeolite layer does not occur, neither during the secondary growth nor through migration of aluminum species during calcination.

Solvent-Free Synthesis of ZIFs: A Route toward the Elusive Fe(II) Analogue of ZIF-8.

Herein we report the synthesis of an elusive metal-organic framework, the iron(II) analogue of ZIF-8 with the formula Fe(2-methylimidazolate)2, here denoted as MUV-3. The preparation of this highly interesting porous material, inaccessible by common synthetic procedures, occurs in a solvent-free reaction upon addition of an easily detachable template molecule, yielding single crystals of MUV-3. This methodology can be extended to other metals and imidazolate derivatives, allowing the preparation of ZIF-8, ZIF-67, and the unprecedented iron(II) ZIFs Fe(2-ethylimidazolate)2 and Fe(2-methylbenzimidazolate)2. The different performance of MUV-3 toward NO sorption, in comparison to ZIF-8, results from the chemisorption of NO molecules, which also causes a gate-opening behavior. Finally, the controlled pyrolysis of MUV-3 results in a N-doped graphitic nanocomposite that exhibits extraordinary performance for the oxygen evolution reaction (OER), with low overpotential at different current densities (316 mV at 10 mA cm-2), low Tafel slope (37 mV per decade), high maximum current density (710 mA cm-2 at 2.0 V vs RHE), and great durability (15 h).

Study of Short-Chain Alcohol and Alcohol-Water Adsorption in MEL and MFI Zeolites.

In this paper, we present a comparative study of the adsorption behavior of short chain alcohols (pure and in aqueous solution) into silicalite-1 (MFI-type zeolite) and silicalite-2 (MEL-type zeolite). For quite some time, silicalite-1 has been the reference material to address the problem of adsorptive-based separation, mostly for hydrocarbon mixtures. Interestingly, being structurally close to silicalite-1, adsorption studies using silicalite-2 are scarce and to the best of our knowledge, a comparative study of their behavior for alcohol-water mixtures has not been published to date. We have here resorted to molecular simulation techniques to analyze the adsorption and diffusion phenomena in both zeolites at 25 and 50 °C for pure methanol, ethanol, 1-butanol, and water, and for some relevant compositions of alcohol/water mixtures. In addition to the dilute regime in the mixture, our study ranges from intermediate alcohol concentrations to alcohol-rich phases, relevant to alcohol purification processes. Besides, we have performed volumetric and calorimetric measurements of single-component adsorption of alcohols in pure silica MEL zeolite, which were used to validate the model potentials used in the simulations. We observe that the zigzag channels of MFI zeolite are most likely responsible for its somewhat higher affinity for alcohols. This leads to higher adsorption selectivities when compared to those of MEL zeolite. We have also found that the choice of water model strongly conditions water coadsorption into the zeolites and subsequently the predictions of the adsorbent's selectivity in alcohol/water systems. Despite considerable differences for adsorbed pure components, diffusivities of alcohol and water adsorbed from mixtures are relatively similar, as a consequence of the strong hydrogen bonds between hydroxyl groups and water.

Critical Role of Dynamic Flexibility in Ge-Containing Zeolites: Impact on Diffusion.

Incorporation of germanium in zeolites is well known to confer static flexibility to their framework, by stabilizing the formation of small rings. In this work, we show that the flexibility associated to Ge atoms in zeolites goes beyond this static effect, manifesting also a clear dynamic nature, in the sense that it leads to enhanced molecular diffusion. Our study combines experimental and theoretical methods providing evidence for this effect, which has not been described previously, as well as a rationalization for it, based on atomistic grounds. We have used both pure-silica and silico-germanate ITQ-29 (LTA topology) zeolites as a case study. Based on our simulations, we identify the flexibility associated to the pore breathing-like behavior induced by the Ge atoms, as the key factor leading to the enhanced diffusion observed experimentally in Ge-containing zeolites.

Sugar-based synthesis of an enantiomorphically pure zeolite.

Zeolites, well-known by their high selectivities in catalytic and separation processes due to their porous nature, play a crucial role in various applications. One significant long-term objective is the synthesis of enantiopure zeolites, potentially enabling enantioselective processes. Earlier attempts result in partial success, yielding some enantiomorphically enriched zeolites. In this study, we introduce a zeolite synthesis approach utilizing chiral organic structure directing agents (ch-OSDAs) derived from sugars, guiding the crystallization process toward achieving enantiomorphically pure S-STW zeolite. The purity of the zeolite is confirmed through extensive analyses of individual crystals using single-crystal X-ray diffraction, extracting Flack parameters and space groups. Theoretical and structural investigations confirm that the sugar-derived ch-OSDA perfectly fits the characteristic helicoidal channel of the zeolite structure, featuring its efficacy in achieving enantiopure zeolites.

Synthesis of a novel zeolite through a pressure-induced reconstructive phase transition process.

The first pressure-induced solid-phase synthesis of a zeolite has been found through compression of a common zeolite, ITQ-29 (see scheme, Si yellow, O red). The new microporous structure, ITQ-50, has a unique structure and improved performance for propene/propane separation with respect the parent material ITQ-29.

New insights on CO2-methane separation using LTA zeolites with different Si/Al ratios and a first comparison with MOFs.

LTA zeolites can be synthesized with tailored adsorption properties by controlling the Al content in the framework. In this work, we have demonstrated that it is possible to adjust the polarity of the zeolitic adsorbent to optimize its thermodynamic adsorption properties for the energetically relevant CO(2)/CH(4) separation process. The thermodynamic study has been made from the corresponding adsorption isotherms of the pure gases carried out at different pressures and temperatures, as well as breakthrough separation experiments of CO(2)/CH(4) mixtures and the results were compared to those reported on MOFs. The separation values obtained allow us to conclude that LTA zeolites offer unique possibilities for CH(4) upgrading from natural gas.

Supramolecular self-assembled molecules as organic directing agent for synthesis of zeolites.

Solid materials with uniform micropores, such as zeolites, can act as selective catalysts and adsorbents for molecular mixtures by separating those molecules small enough to enter their pores while leaving the larger molecules behind. Zeolite A is a microporous material with a high void volume. Despite its widespread industrial use in, for example, molecular separations and in detergency, its capability as a petroleum-refining material is limited owing to its poor acid-catalytic activity and hydrothermal stability, and its low hydrophobicity. These characteristics are ultimately a consequence of the low framework Si/Al ratio (normally around one) and the resulting high cationic fraction within the pores and cavities. Researchers have modified the properties of type-A zeolites by increasing the Si/Al compositions up to a ratio of three. Here we describe the synthesis of zeolite A structures exhibiting high Si/Al ratios up to infinity (pure silica). We synthesize these materials, named ITQ-29, using a supramolecular organic structure-directing agent obtained by the self-assembly, through pi-pi type interactions, of two identical organic cationic moieties. The highly hydrophobic pure-silica zeolite A can be used for hydrocarbon separations that avoid oligomerization reactions, whereas materials with high Si/Al ratios give excellent shape-selective cracking additives for increasing propylene yield in fluid catalytic cracking operations. We have also extended the use of our supramolecular structure-directing agents to the synthesis of a range of other zeolites.

Unusually Low Heat of Adsorption of CO2 on AlPO and SAPO Molecular Sieves.

The capture of CO2 from post-combustion streams or from other mixtures, such as natural gas, is an effective way of reducing CO2 emissions, which contribute to the greenhouse effect in the atmosphere. One of the developing technologies for this purpose is physisorption on selective solid adsorbents. The ideal adsorbents are selective toward CO2, have a large adsorption capacity at atmospheric pressure and are easily regenerated, resulting in high working capacity. Therefore, adsorbents combining molecular sieving properties and low heats of adsorption of CO2 are of clear interest as they will provide high selectivities and regenerabilities in CO2 separation process. Here we report that some aluminophosphate (AlPO) and silicoaluminophosphate (SAPO) materials with LTA, CHA and AFI structures present lower heats of adsorption of CO2 (13-25 kJ/mol) than their structurally analogous zeolites at comparable framework charges. In some cases, their heats of adsorption are even lower than those of pure silica composition (20-25 kJ/mol). This could mean a great improvement in the regeneration process compared to the most frequently used zeolitic adsorbents for this application while maintaining most of their adsorption capacity, if materials with the right stability and pore size and topology are found.

Estimating CO2/N2 Permselectivity through Si/Al = 5 Small-Pore Zeolites/PTMSP Mixed Matrix Membranes: Influence of Temperature and Topology.

In the present work, the effect of zeolite type and topology on CO2 and N2 permeability using zeolites of different topology (CHA, RHO, and LTA) in the same Si/Al = 5, embedded in poly(trimethylsilyl-1-propyne) (PTMSP) is evaluated with temperature. Several models are compared on the prediction of CO2/N2 separation performance and then the modified Maxwell models are selected. The CO2 and N2 permeabilities through these membranes are predicted with an average absolute relative error (AARE) lower than 0.6% taking into account the temperature and zeolite loading and topology on non-idealities such as membrane rigidification, zeolite⁻polymer compatibility and sieve pore blockage. The evolution of this structure⁻performance relationship with temperature has also been predicted.

Synthesis and structure determination via ultra-fast electron diffraction of the new microporous zeolitic germanosilicate ITQ-62.

Here, we present the synthesis and structure determination of the new zeolite ITQ-62. Its structure was determined via ultra-fast electron diffraction tomography and refined using powder XRD data of the calcined material. This new zeolite contains a tridirectional channel system of highly distorted 8-rings, as well as a monodirectional 12-ring channel system.

A highly stable and hierarchical tetrathiafulvalene-based metal-organic framework with improved performance as a solid catalyst.

Herein we report the synthesis of a tetrathiafulvalene (TTF)-based MOF, namely MUV-2, which shows a non-interpenetrated hierarchical crystal structure with mesoporous one-dimensional channels of ca. 3 nm and orthogonal microporous channels of ca. 1 nm. This highly stable MOF (aqueous solution with pH values ranging from 2 to 11 and different organic solvents), which possesses the well-known [Fe3(µ3-O)(COO)6] secondary building unit, has proven to be an efficient catalyst for the aerobic oxidation of dibenzothiophenes.

Pure silica ITQ-32 zeolite allows separation of linear olefins from paraffins.

ITQ-32 is able to separate propene from propane and represents a clear improvement with respect to previous zeolites in achieving the separation of trans-2-butene and 1-butene from the C(4) fraction using only one zeolite.

Gas confinement in compartmentalized coordination polymers for highly selective sorption.

Discrimination between different gases is an essential aspect for industrial and environmental applications involving sensing and separation. Several classes of porous materials have been used in this context, including zeolites and more recently MOFs. However, to reach high selectivities for the separation of gas mixtures is a challenging task that often requires the understanding of the specific interactions established between the porous framework and the gases. Here we propose an approach to obtain an enhanced selectivity based on the use of compartmentalized coordination polymers, named CCP-1 and CCP-2, which are crystalline materials comprising isolated discrete cavities. These compartmentalized materials are excellent candidates for the selective separation of CO2 from methane and nitrogen. A complete understanding of the sorption process is accomplished with the use of complementary experimental techniques including X-ray diffraction, adsorption studies, inelastic- and quasi-elastic neutron scattering, magnetic measurements and molecular dynamics calculations.

Control of zeolite framework flexibility and pore topology for separation of ethane and ethylene.

The discovery of new materials for separating ethylene from ethane by adsorption, instead of using cryogenic distillation, is a key milestone for molecular separations because of the multiple and widely extended uses of these molecules in industry. This technique has the potential to provide tremendous energy savings when compared with the currently used cryogenic distillation process for ethylene produced through steam cracking. Here we describe the synthesis and structural determination of a flexible pure silica zeolite (ITQ-55). This material can kinetically separate ethylene from ethane with an unprecedented selectivity of ~100, owing to its distinctive pore topology with large heart-shaped cages and framework flexibility. Control of such properties extends the boundaries for applicability of zeolites to challenging separations.