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.

Methodologies for evaluation of metal–organic frameworks in separation applications

The separation performance of fixed-bed adsorbers is governed by a number of factors that include (a) adsorption selectivity, (b) uptake capacity, and (c) intra-crystalline diffusion limitations.

n- and Isoalkane Adsorption Mechanisms on Zeolite MCM-22

Low-coverage adsorption properties (Henry constants, adsorption enthalpy, and entropy) of linear and branched alkanes (C3-C8) on zeolite MCM-22 were determined using the chromatographic technique at temperatures between 420 and 540 K. It was found that adsorption enthalpy and entropy of linear alkanes vary in a nonmonotonic way with carbon number. The adsorption behavior of alkanes was rationalized on the basis of the pore geometry. Short molecules prefer to reside in the pockets of the MCM-22 supercage, where they maximize energetic interaction with the zeolite. Longer molecules reside in the larger central part of the supercage. For carbon numbers up to six, singly branched alkanes are selectively adsorbed over their linear counterparts. This preference originates from the entropic advantage of singly branched molecules inside MCM-22 supercages, where these species have high rotational freedom because of their small length.

A Unified Single-Event Microkinetic Model for Alkane Hydroconversion in Different Aggregation States on Pt/H−USY-Zeolites

A single-event microkinetic model for the catalytic hydroconversion of hydrocarbons on Pt/H-US-Y bifunctional zeolite catalysts developed for low-pressure vapor phase conditions was extended to cover high-pressure vapor phase and liquid phase conditions. The effect of the density of the bulk hydrocarbon phase on the physisorption as well as on the protonation steps of the reaction network was accounted for explicitly and can be interpreted in terms of "compression" of the hydrocarbon sorbate inside the zeolite pores and "solvation" of the catalyst framework by the dense bulk hydrocarbon phase. The bulk phase density effect on the physisorbed state is described via a single excess free enthalpy of physisorption. A dense bulk hydrocarbon phase destabilizes the sorbate molecules inside the catalyst pores. An expression of the excess free enthalpy of physisorption involving the fugacity coefficient and a zeolite dependent factor allows description of physisorption data. Typical excess free enthalpy values are in the range 1.5-5.1 kJ mol(-1) increasing with carbon number in the series of C5-C16 alkanes. At high-pressure vapor phase and liquid phase conditions, the excess standard protonation enthalpy is estimated at -7.8 kJ mol(-1) leading to relatively more stable carbenium ions at dense bulk phase conditions. As a result of the excess physisorption and protonation properties, the lightest hydrocarbons in mixtures are more competitive at dense phase conditions and their conversion is enhanced compared to low-density conditions.

Adsorption of liquid mixtures on silicalite-1 zeolite: a density-bottle method

A technique that measures the effective density of a zeolite after adsorption from the liquid phase was developed to measure the absolute amounts of liquid mixtures adsorbed on zeolites without using a nonadsorbing solvent. Since the fugacities of the adsorbing components in solution can be dramatically different with or without the addition of a nonadsorbing solvent, this technique measures mixture isotherms that can be used for analyzing pervaporation through zeolite membranes. A nonideal solution, methanol/acetone, was used as an example to show that its adsorption isotherms on silicalite-1 zeolite at 294 K differ dramatically from those measured with the nonadsorbing solvent method. The methanol/acetone fugacity ratio is different for the two methods because of different concentrations in the liquid phase. Methanol preferentially adsorbs on silicalite-1 at low methanol concentrations and acetone preferentially adsorbs at high methanol concentrations. The density bottle method was used to show that n-hexane preferentially adsorbs from n-hexane/3-methylpentane liquid mixtures, and at high n-hexane concentrations, essentially no 3-methylpentane adsorbs, as has been predicted previously by simulations. A larger molecule, 2,2-dimethylbutane, adsorbed so slowly at 294 K that silicalite had only 16% of saturation coverage after 370 h, but it was saturated after 1650 h; at 423 K, saturation was obtained in less than 24 h.