Modeling the Effect of Structural Changes during Dynamic Separation Processes on MOFs

A model able to describe the effect of structural changes in the adsorbent or adsorbed phase during the dynamic (breakthrough) separation of mixtures on metal–organic frameworks (MOFs) is presented. The methodology is exemplified for a few pertinent case studies: the separation of xylene isomers and ethylbenzene on the flexible MOF MIL-53 and the rigid MOF MIL-47. At low pressures, no preferential adsorption of any component occurs on both MOFs. Contrarily, at higher pressures separation of ethylbenzene (EB) from <i>o</i>-xylene (oX) occurs on MIL-53 as a result of the breathing phenomenon within the MIL-53 structure. The increase in selectivity, starting from the gate-opening pressure, could be modeled by using a pressure-dependent saturation capacity for the most strongly adsorbed component oX. In the separation of <i>m</i>-xylene (mX) from <i>p</i>-xylene (pX) on the rigid MOF MIL-47, separation at higher pressures is a result of preferential stacking of pX. Here, the selectivity increases once the adsorption of pX switches from a single to a double file adsorption. By implementing a loading dependent adsorption constant for pX, the different unconventional breakthrough profiles and the observed selectivity profile on MIL-47 can be simulated. A similar methodology was used for the separation of EB from pX on MIL-47, where the separation is a result from steric constraints imposed onto the adsorption of EB

Nonuniform Chain-Length-Dependent Diffusion of Short 1‑Alcohols in SAPO-34 in Liquid Phase

Liquid-phase diffusion of 1-alcohols in SAPO-34 was explored by batch experimentation. The uptake of pure and binary mixtures of 1-alcohols, dissolved in <i>tert</i>-butanol, was obtained for C1–C8 1-alcohols at temperatures between 25 and 80 °C, concentrations varying between 0.5 and 10 wt %, and crystal sizes between 7.5 and 20 μm. The experimental uptake data were fitted with an intracrystalline diffusion model and a linear driving force model. The intracrystalline diffusion coefficient showed a nonuniform stepwise decrease with chain length, ranging from 10^–12 m²/s for methanol to 10^–20 m²/s for 1-pentanol. No effect of the external concentration on the intracrystalline diffusion coefficient was observed. Variation of the crystal size showed that the intracrystalline diffusion is the rate-limiting step. On the basis of the Arrhenius equation, the activation energies of diffusion of ethanol, 1-propanol, and 1-butanol were determined, being, respectively, 27.8, 47.8, and 47.2 kJ/mol. Co-diffusion occurred in the uptake of binary mixtures of methanol/ethanol, methanol/1-propanol, and ethanol/1-propanol, where mutual effects could be noticed. From this experimental work, it could be concluded that the small dimensions of the SAPO-34 framework generate a very sterically hindered diffusion of 1-alcohols into the crystals, resulting in a chain-length-dependent behavior, interesting to obtain efficient kinetic-based separations

Nonideality in the Adsorption of Ethanol/Ethyl Acetate/Water Mixtures On ZIF‑8 Metal Organic Framework

Fisher esterification of acetic acid (or acetic anhydride) with ethanol (EtOH) is the main industrial process for the synthesis of ethyl acetate (EA). Nonetheless, the separation of the produced ester from ethanol is challenging since these volatile organic compounds (VOCs) form an azeotropic mixture. In this work, the adsorption and separation EtOH/EA/water mixtures on the ZIF-8 metal–organic framework is studied. The present study aims to characterize the adsorptive behavior of ZIF-8 with non-ideal EtOH/EA/water mixtures. Single and multicomponent adsorption isotherms, obtained by gravimetry and breakthrough experiments, show high adsorption capacity and selectivity toward ethyl acetate. Pulse chromatography experiments confirm the higher interaction strength between EA and ZIF-8 in the low-pressure range at a low degree of pore filling (Henry’s region). The breakthrough profiles show development of intermediate plateaus in specific concentration ranges, with complex breakthrough evolution that has been modeled using a combined ideal and real adsorption solution theory model to accounts for deviations from ideality

Competitive Adsorption of C20−C36 Linear Paraffins on the Amorphous Microporous Silica−Alumina ERS-8 in Vapor Phase and Liquid Phase

Adsorption of C20−C36 linear paraffins on the amorphous microporous silica−alumina ERS-8 was studied at vapor phase and liquid phase conditions. Henry adsorption constants and low coverage adsorption enthalpies were determined using the pulse chromatographic method at temperatures between 90 and 370 °C in gas phase. The low coverage adsorption enthalpy increases linearly with carbon number with 5.5 kJ/mol per additional methyl group. Competitive adsorption in liquid and dense vapor phase conditions was studied by performing column breakthrough experiments with various binary C20−C36 n-paraffin mixtures diluted in short chain length alkane solvents or undiluted as bulk mixture, at temperatures ranging from 25 to 300 °C and pressures from 3 to 110 bar. Both the adsorption capacity and the selectivity are strongly temperature and pressure dependent. At low temperature and high pressure, all n-paraffins are adsorbed equally and no separation is possible. With increasing temperature and decreasing pressure, the density of the bulk phase decreases, and a transition from a pure liquid paraffin stream to a dense vapor stream occurs. In such conditions, longer n-paraffins are adsorbed preferentially compared to the shorter n-paraffins. The selectivity increases with increasing difference in chain length between the adsorbing n-paraffins

Parallel Tempering Simulations of Liquid-Phase Adsorption of <i>n</i>-Alkane Mixtures in Zeolite LTA-5A

Adsorption of n-alkane mixtures in the zeolite LTA-5A under liquid-phase conditions has been studied using grand canonical Monte Carlo (GCMC) simulations combined with parallel tempering. Normal GCMC techniques fail for some of these systems due to the preference of linear molecules to coil within a single cage in the zeolite. The narrow zeolite windows severerly restrict interactions of the molecules, making it difficult to simulate cooperative rearrangements necessary to explore configuration space. Because of these reasons, normal GCMC simulations results show poor reproducibility in some cases. These problems were overcome with parallel tempering techniques. Even with parallel tempering, these are very challenging systems for molecular simulation. Similar problems may arise for other zeolites such as CHA, AFX, ERI, KFI, and RHO having cages connected by narrow windows. The simulations capture the complex selectivity behavior observed in experiments such as selectivity inversion and azeotrope formation

Modeling of Toluene Acetylation with Acetic Anhydride on H-USY Zeolite

The liquid-phase acetylation of toluene with acetic anhydride was carried out in a continuous-flow reactor over H-USY zeolites with different Si/Al ratios at 180 °C, at different contact times and feed compositions. H-USY is an active catalyst for this reaction because the main reaction products at all times on stream are the desired methylacetophenone (MAP) and its reaction byproduct acetic acid. Within the different MAP isomers, the selectivity toward 4-MAP equals 85%. Although the initial acetic anhydride conversion is 100%, the zeolite is subject to deactivation. Small amounts of side products such as methylbenzoic acid and isopropenyltoluene were also identified and their formation explained. The data and insights obtained during these experiments were used to obtain models describing the formation of MAP and the other components present in the reactor effluent. The most plausible model, obtained via model discrimination, was validated at different reaction conditions and takes into account adsorption of the chemical compounds, the catalytic reactions, and deactivation of the catalyst. It also includes hydrolysis of acetic anhydride and the formation of side products originating from MAP. According to this model, catalyst deactivation starts from MAP and acetic anhydride, whereby acetic acid is liberated. Fitting of the model to the experimental data shows that the kinetic constant for the formation of 4-MAP is comparable to that of the deactivation reaction

An Amine-Functionalized MIL-53 Metal−Organic Framework with Large Separation Power for CO₂ and CH₄

An Amine-Functionalized MIL-53 Metal−Organic Framework with Large Separation Power for CO2 and CH4</sub

Vapor-Phase Adsorption and Separation of Ethylbenzene and Styrene on the Metal–Organic Frameworks MIL-47 and MIL-53(Al)

Separation of styrene (ST) from ethylbenzene (EB) remains an industrially relevant challenge in the production of polystyrene. Adsorptive separation with metal–organic frameworks (MOFs) is a potential alternative for the conventional vacuum distillation process. Adsorption and separation of ST and EB on the MOFs MIL-47 and MIL-53­(Al) were studied under vapor-phase conditions. ST and EB show traditional type I isotherms on MIL-47. Contrarily, ST adsorption isotherms show steep steps on MIL-53­(Al) as a result of the breathing of the flexible MOF upon increased adsorbate pressure. The separation potential of both MOFs was investigated by performing vapor-phase breakthrough experiments at total hydrocarbon partial pressures between 1.14 and 16.4 mbar and temperatures between 35 and 90 °C. ST is preferentially adsorbed on both MOFs. Although the MOFs are isostructural, the evolution of selectivity with temperature and pressure is different for both materials due to the different interaction and separation mechanisms

Prediction of Molecular Separation of Polar–Apolar Mixtures on Heterogeneous Metal–Organic Frameworks: HKUST‑1

Due to the combination of metal ions and organic linkers and the presence of different types of cages and channels, metal–organic frameworks often possess a large structural and chemical heterogeneity, complicating their adsorption behavior, especially for polar–apolar adsorbate mixtures. By allocating isotherms to individual subunits in the structure, the ideal adsorbed solution theory (IAST) can be adjusted to cope with this heterogeneity. The binary adsorption of methanol and <i>n</i>-hexane on HKUST-1 is analyzed using this segregated IAST (SIAST) approach and offers a significant improvement over the standard IAST model predictions. It identifies the various HKUST-1 cages to have a pronounced polar or apolar adsorptive behavior

Pulse Gas Chromatographic Study of Adsorption of Substituted Aromatics and Heterocyclic Molecules on MIL-47 at Zero Coverage

The low coverage adsorptive properties of the MIL-47 metal organic framework toward aromatic and heterocyclic molecules are reported in this paper. The effect of molecular functionality and size on Henry adsorption constants and adsorption enthalpies of alkyl and heteroatom functionalized benzene derivates and heterocyclic molecules was studied using pulse gas chromatography. By means of statistical analysis, experimental data was analyzed and modeled using principal component analysis and partial least-squares regression. Structure–property relationships were established, revealing and confirming several trends. Among the molecular properties governing the adsorption process, vapor pressure, mean polarizability, and dipole moment play a determining role