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

Toward the development of sensors for lung cancer: The adsorption of 1-propanol on hydrophobic zeolites

A healthy breath is mainly composed of water, carbon dioxide, molecular nitrogen, and oxygen and it contains many species, in small quantities, which are related to the ambient atmosphere and the metabolism. The breath of a person affected by lung cancer presents a concentration of 1-propanol higher than usual. In this context, the development of specific sensors to detect 1-propanol from breath is of high interest. The amount of propanol usually detected on the breath is of few ppb; this small quantity is a handicap for a reliable diagnostic. This limitation can be overcome if the sensor is equipped with a pre-concentrator. Our studies aim to provide an efficient material playing this role. This will contribute to the development of reliable and easy to use lung cancer detectors. For this, we investigate the properties of a few hydrophobic porous materials (chabazite, silicalite-1, and dealuminated faujasite). Hydrophobic structures are used to avoid saturation of materials by the water present in the exhaled breath. Our experimental and simulation results suggest that silicalite -1 (MFI) is the most suitable structure to be used as a pre-concentrator.

Hydrogen bonding to oxygen in siloxane bonds drives liquid phase adsorption of primary alcohols in high-silica zeolites

Upon liquid phase adsorption of C1-C5 primary alcohols on high silica MFI zeolites (Si/Al = 11.5-140), the concentration of adsorbed molecules largely exceeds the concentration of traditional adsorption sites: Brønsted acid and defect sites. Combining quantitative in situ1H MAS NMR, qualitative multinuclear NMR and IR spectroscopy, hydrogen bonding of the alcohol function to oxygen atoms of the zeolite siloxane bridges (Si-O-Si) was shown to drive the additional adsorption. This mechanism co-exists with chemi- and physi-sorption on Brønsted acid and defect sites and does not exclude cooperative effects from dispersive interactions.

Screening of Alkali Metal-Exchanged Zeolites for Nitrogen/Methane Separation

Methane (CH4) is the primary component of natural gas and must be purified to a certain level before it can be used as pipeline gas or liquified natural gas (LNG). In particular, nitrogen (N2), a common contaminant in natural gas needs to be rejected to increase the heating value of the gas and meet the LNG product specifications. The development of energy-efficient N2 removal technologies is hampered by N2's inertness and its resemblance to CH4 in terms of kinetic size and polarizability. N2-selective materials are so rare. Here, for the first time, we screened 1425 alkali metal cation exchange zeolites to identify the candidates with the best potential for the separation of N2 from CH4. We discovered a few extraordinary zeolite frameworks capable of achieving equilibrium selectivity toward N2. Particularly, Li+-RRO-3 zeolite with a specific two-dimensional structure demonstrated a selective N2 adsorption capacity of 2.94 mmol/g at 283 K and 1 bar, outperforming the capacity of all known zeolites. Through an ab initio density functional theory study, we found that the five-membered ring of the RRO framework is the most stable cationic site for Li+, and this Li+ can interact with multiple N2 molecules but only one CH4, revealing the mechanism for the high capacity and selectivity of N2. This work suggests promising adsorbents to enable N2 rejection from CH4 in the gas industry without going for energy-intensive cryogenic distillations.

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.

Molecular dynamic simulation studies of adsorption and diffusion behaviors of methanol and ethanol through ZSM-5 zeolite

Due to the importance of synthesis gas's entire conversion to methanol, the separation of methanol from unconverted synthesis gas is an industrial challenge. In this work, the influence of temperature, guest molecules concentrations (methanol and ethanol), and acid site density (Si/Al) of zeolites on the diffusion of methanol and ethanol, pure and binary mixture (80% methanol and 20% ethanol) in silicalite-1 and HZSM-5 (Si/Al = 47 and 23) were studied by using of the COMPASS force-field molecular dynamics method. Also, the adsorption of pure methanol and ethanol and binary mixture through these zeolites has been studied by using the Grand Canonical Monte Carlo (GCMC) method. The calculated adsorption rate and isosteric heat of adsorption for ethanol are lower and higher than methanol, respectively. The results of the binary mixture show that HZSM-5 (Si/Al = 23) has the lowest adsorption selectivity and most diffusion selectivity. The calculated diffusion coefficients of methanol and ethanol guest molecules decreased with rising guest molecule concentration and Si/Al-ratios. The effect of both agents was investigated by analysis of mean square displacement (MSD) and RDF diagram.

Using Molecular Simulations to Unravel the Benefits of Characterizing Mixture Permeation in Microporous Membranes in Terms of the Spreading Pressure

The separation performance of microporous crystalline materials in membrane constructs is dictated by a combination of mixture adsorption and intracrystalline diffusion characteristics; the permeation selectivity S _perm is a product of the adsorption selectivity S _ads and the diffusion selectivity, S _diff. The primary objective of this article is to gain fundamental insights into S _ads and S _diff by use of molecular simulations. We performed configurational-bias Monte Carlo (CBMC) simulations of mixture adsorption equilibrium and molecular dynamics (MD) simulations of guest self-diffusivities of a number of binary mixtures of light gaseous molecules (CO₂, CH₄, N₂, H₂, and C₂H₆) in a variety of microporous hosts of different pore dimensions and topologies. Irrespective of the bulk gas compositions and bulk gas fugacities, the adsorption selectivity, S _ads, is found to be uniquely determined by the adsorption potential, Φ, a convenient and practical proxy for the spreading pressure π that is calculable using the ideal adsorbed solution theory for mixture adsorption equilibrium. The adsorption potential Φ is also a proxy for the pore occupancy and is the thermodynamically appropriate yardstick to determine the loading and composition dependences of intracrystalline diffusivities and diffusion selectivities, S _diff. When compared at the same Φ, the component permeabilities, Π _i for CO₂, CH₄, and N₂, determinable from CBMC/MD data, are found to be independent of the partners in the various mixtures investigated and have practically the same values as the values for the corresponding unary permeabilities. In all investigated systems, the H₂ permeability in a mixture is significantly lower than the corresponding unary value. These reported results have important practical consequences in process development and are also useful for screening of materials for use as membrane devices.

Water/Alcohol Mixture Adsorption in Hydrophobic Materials: Enhanced Water Ingress Caused by Hydrogen Bonding

Microporous crystalline porous materials such as zeolites, metal-organic frameworks, and zeolitic imidazolate frameworks (ZIFs) have potential use for separating water/alcohol mixtures in fixed bed adsorbers and membrane permeation devices. For recovery of alcohols present in dilute aqueous solutions, the adsorbent materials need to be hydrophobic in order to prevent the ingress of water. The primary objective of this article is to investigate the accuracy of ideal adsorbed solution theory (IAST) for prediction of water/alcohol mixture adsorption in hydrophobic adsorbents. For this purpose, configurational bias Monte Carlo (CBMC) simulations are used to determine the component loadings for adsorption equilibrium of water/methanol and water/ethanol mixtures in all-silica zeolites (CHA, DDR, and FAU) and ZIF-8. Due to the occurrence of strong hydrogen bonding between water and alcohol molecules and attendant clustering, IAST fails to provide quantitative estimates of the component loadings and the adsorption selectivity. For a range of operating conditions, the water loading in the adsorbed phase may exceed that of pure water by one to two orders of magnitude. Furthermore, the occurrence of water-alcohol clusters moderates size entropy effects that prevail under pore saturation conditions. For quantitative modeling of the CBMC, simulated data requires the application of real adsorbed solution theory by incorporation of activity coefficients, suitably parameterized by the Margules model for the excess Gibbs free energy of adsorption.

Adsorption of water, methanol, and their mixtures in slit graphite pores

The behavior of water, methanol, and water-methanol mixtures confined in narrow slit graphite pores as a function of pore size was investigated by Monte Carlo, hybrid Monte Carlo, and Molecular Dynamics simulations. Interactions were described using TIP4P/2005 for water, OPLS/2016 for methanol, and cross interactions fitted to excess water/methanol properties over the whole range of concentrations, which provide a rather accurate description of water-methanol mixtures. As expected for hydrophobic pores, whereas pure methanol is adsorbed already from the gas phase, pure water only enters the pore at pressures well beyond bulk saturation for all pore sizes considered. When adsorbed from a mixture, however, water adsorbs at much lower pressures due to the formation of hydrogen bonds with previously adsorbed methanol molecules. For all studied compositions and pore sizes, methanol adsorbs preferentially over water at liquid-vapor equilibrium conditions. In pure components, both water and methanol are microscopically structured in layers, the number of layers increasing with pore size. This is also the case in adsorbed mixtures, in which methanol has a higher affinity for the walls. This becomes more evident as the pore widens. Diffusion of pure water is higher than that of pure methanol for all pore sizes due to the larger size of the methyl group. In mixtures, both components present similar diffusivities at all pore sizes, which is explained in terms of the coupling of molecular movements due to strong hydrogen bonding between methanol and water molecules. This is particularly evident in very narrow pores, in which pure methanol diffusion is completely impeded on the time scale of our simulations, but the presence of a small amount of water molecules facilitates alcohol diffusion following a single-file mechanism. Additionally, our results indicate that pure water diffusivities display a non-monotonous dependence of pore size, due to effects of confinement (proximity to a fluid-solid-fluid transition induced by confinement as reported in previous work) and the dynamic anomalies of water.