Sorption Of N-hexane Research Articles

Nanoporous triclinic δ modification of syndiotactic polystyrene

For the δ form of syndiotactic polystyrene, a nanoporous triclinic modification, different from the well-established and widely used nanoporous monoclinic modification, is presented. Detailed routes to triclinic or monoclinic modifications, which involve different guest removal procedures from different δ-clathrates, are described and rationalized. Surprisingly, both triclinic and monoclinic δ modifications can be obtained by both triclinic and monoclinic δ-clathrates. The triclinic modification, with respect to the monoclinic one, presents the advantage of higher uptake of pollutants having molecular volume in the range 0.13–0.22 nm3, mainly when present only as traces in water or in air. In particular, the different molecular selectivity of triclinic and monoclinic δ modifications, favoring n-decane and n-hexane sorption, respectively, is described. This behavior is attributed to the crystalline cavities of the triclinic δ modification, which are halved in number and increased in volume with respect to those of monoclinic δ modification.

Sorption of volatile organic compounds and their mixtures on montmorillonite at different humidity

The vapor-phase sorption of volatile organic compounds (VOCs), i.e. n-hexane, benzene and methanol, along with sorption of their binary mixtures, i.e. benzene/n-hexane and benzene-methanol, on montmorilllonite with different water content was studied. The absolutely dry mineral did not exhibit selectivity towards the studied VOCs sorbed separately. The hydration inhibited sorption of hydrocarbons and promoted that of methanol because of intercalation of its molecules to the interlayer space of the swelling mineral and dissolution in the water films on the external mineral surface. Unlike separate sorption of benzene and n-hexane, sorption of their binary mixture on the montmorillonite, even equal by volume, was selective. The components shared the same sorption sites with benzene being more active due to its ability to form the donor–acceptor complexes with the mineral surface in addition to Van-der-Waals interactions. Opposite, in the benzene/methanol mixture the undoubted predominance of hydrophilic methanol over benzene was revealed, which increased with increasing humidity and was significantly stronger compared to the differences in sorption of methanol and benzene sorbed separately. In the binary mixtures unequal by volume, preadsorption played an important role in VOCs competition with sorption of aliphatic n-hexane being suppressed stronger than that of aromatic benzene. In the benzene/methanol mixture, methanol was predominantly sorbed at all the studied volume ratios and hydration degrees, occupying its specific sorption sites. Hydration of the montmorillonite caused the reverse impact on the sorption of benzene and methanol. As a result, at the full hydration state methanol sorption in the mixture reached that of the pure methanol.

Synthesis of MFI/EMT zeolite bi-layer films for molecular decontamination

A bi-layer film composed of a thick layer of ZSM-5 zeolite (MFI-structure type) and a thin layer of EMC-2 zeolite (EMT-structure type) was synthesized on aluminum alloys that are used widely in aerospace industry. The bottom MFI layer was obtained by direct hydrothermal synthesis while secondary growth method was used for the top EMC-2 layer. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results revealed that both layers are highly crystallized and uniform with a total film thickness around 22μm. Nitrogen sorption measurements indicated that ZSM-5 and EMC-1 layers present an accessible porosity. The high micropore volumes lead to considerably good adsorption properties towards pollutants. Such a result was confirmed by the n-hexane sorption capacity of the bi-layer film which is identical to what is expected for a corresponding powder mixture. Adhesion of zeolite films on aluminum substrate was investigated by scratch tests which highlighted a good adhesion between the bottom layer and the support as well as between the two zeolite layers. This bi-layer film is promising for technological uses in molecular decontamination.

Sorption and breathing properties of difluorinated MIL-47 and Al-MIL-53 frameworks

Synthesis of two difluorinated metal hydroxo terephthalates [MIII(OH)(BDC-F2)]·n(guests) (MIII=V, MIL-47-F2-AS or 1-AS; Al, Al-MIL-53-F2-AS or2-AS) (BDC-F2=2,5-difluoro-1,4-benzenedicarboxylate; AS=as-synthesized) has been accomplished by a hydrothermal method using microwave irradiation (1-AS) or electric heating (2-AS), respectively. The empty-pore forms of the title compounds [VIV(O)(BDC-F2] (MIL-47-F2, 1) and [AlIII(OH)(BDC-F2)] (Al-MIL-53-F2, 2) have been prepared after removal of the unreacted or guest H2BDC-F2 molecules under vacuum by direct thermal treatment or solvent-exchange of occluded molecules followed by thermal activation. The high thermal stability of the compounds (340°C, 1; 480°C, 2) has been revealed by thermogravimetric analysis. N2, CO2 and n-hexane sorption analyses confirm that the thermally activated compounds possess noticeable microporosity. An ideal CO2/N2 selectivity value of 11 has been deduced for 2 from the single component adsorption isotherms. The breathing behavior of 2 upon adsorption of CO2 and n-hexane are similar to monofluorinated Al-MIL-53, but different from parent Al-MIL-53. H2O sorption analyses demonstrate that the fluorination induce a certain extent of hydrophobicity in the frameworks, similar to their monofluorinated counterparts. Remarkably, the n-hexane adsorption capacity of both mono- and difluorinated MIL-47 at 50°C and p/p0=0.5 exceeds that of parent MIL-47 owing to the enhanced hydrophobicity achieved through fluorination of their frameworks.

Zeolite hybrid films for space decontamination

A bi-layer film composed of a thick layer of ZSM-5 zeolite (MFI-structure type) and a thin layer of EMC-1 zeolite (FAU-structure type) was synthesized on aluminum alloys widely used in aerospace industry. The bottom MFI layer was obtained by direct hydrothermal synthesis while secondary growth method was used for the top FAU layer. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results revealed that both layers are highly crystallized, uniform and well-intergrown with a total film thickness around 10μm. Nitrogen sorption measurements indicated that ZSM-5 and EMC-1 layers present an accessible porosity. The high micropore volumes lead to considerably good adsorption properties towards pollutants. Such a result was confirmed by the n-hexane sorption capacity of the hybrid film which is identical to what is expected for a corresponding powder mixture. Adhesion of zeolite films on aluminum substrate was investigated by scratch tests which highlighted a good adhesion between the bottom layer and the support as well as between the two zeolite layers. The process described in this paper for the formation of a MFI/FAU bi-layer film can be easily extended to other hybrid coating film.

Natural gas sweetening—the effect on CO2–CH4 separation after exposing a facilitated transport membrane to hydrogen sulfide and higher hydrocarbons

The PVAm/PVA blend composite membrane was exposed to synthetic natural gas mixtures containing aggressive gases like hydrogen sulfide (H2S) at a concentration of 1mol% and H2S in combination with the condensable hydrocarbons n-hexane and propane. The effect on the performance of the membrane was studied under different relative humidity conditions and the possible interactions between the membrane and impurities were analyzed. The performance of the membrane was found to be reduced at low humidity conditions due to H2S induced conditioning and sorption of n-hexane both in the polysulfone support and PVAm/PVA selective layer, while at high relative humidity the CO2 facilitated transport helps the membrane to retain its performance to a value very close to that of a fresh membrane. The CO2 facilitated transport is closely related to the membrane swelling. The combined effects of H2S and hydrocarbons are comparable to the effects of H2S alone. Under high humidified conditions, the maximum CO2 permeance loss is 18% and the maximum CO2/CH4 selectivity loss is 16% after two weeks exposure at this harsh and humidified conditions. Our conclusion based on the current investigations, is that the aggressive environment does not introduce permanent damage to the material and the PVAm/PVA blend membrane is keeping its separation performance quite well after the exposure to H2S and hydrocarbons, and may have a potential for being used in natural gas sweetening.

Heating and sorption effects on silicalite-1 unit cell size and geometry

The temperature response of the silicalite-1 unit cell is investigated via X-ray diffraction experiments and lattice dynamics simulations, with the use of a newly proposed force-field. The numerical results are in very good agreement with present and literature data. Further analysis of the present data shows that the Rigid Unit Mode (RUM) mechanism that is proposed in the literature is the dominant mechanism of thermal contraction of the zeolite. In addition, the expansion of the silicalite-1 unit cell upon n-hexane sorption is investigated. Simulation results show that even though the discernible changes of the crystal dimensions upon heating or sorption may affect membrane-substrate mechanical interactions, they have a minimal effect on the overall sorption capacity.

Key steps influencing the formation of ZSM-5 films on aluminum substrates

Zeolite ZSM-5 films were synthesized on various supports, including aluminum alloys widely used in aerospace industry. The different key steps influencing the formation of the MFI films, from substrates cleaning to in situ crystallization time were investigated. A complete characterization of the hybrid material is proposed using various techniques, such as X-ray diffraction, scanning electron microscopy, X-ray fluorescence, scratch test and nitrogen/ n-hexane sorption measurements. A minimum 9 μm thick, dense, continuous and highly crystallized film of ZSM-5 zeolite has been obtained. Depending on the substrate nature, the ZSM-5 film molar Si/Al ratio can vary between 15 and 30. The microporous coating is well adhered to the aluminum substrates. Sorption capacities were identical on ZSM-5 films compared to corresponding powders, revealing the absence of additional diffusion barriers. The mechanical stability and sorption capacity of the ZSM-5 films are promising for technological uses in space decontamination and corrosion protection of aluminum substrates.

Liquid sensing properties of melt processed polypropylene/poly(ε-caprolactone) blends containing multiwalled carbon nanotubes

The sensing properties of polypropylene (PP)/poly(ε-caprolactone) (PCL) blends containing multiwalled carbon nanotubes (MWNT) were studied in terms of their electrical resistance change in presence of liquids (solvents). The preparation of co-continuous blends based on the double percolation concept was done by melt mixing of electrically conductive PCL composites containing 3 wt.% MWNT and neat PP in ratios of 30:70, 40:60, and 50:50. The electrical resistance change of the PCL–MWNT composites and blends was monitored in a solvent immersion/drying cycle. Various solvents, such as n-hexane, ethanol, methanol, water, toluene, chloroform, and tetrahydrofuran were successfully detected, yielding different responses and reversibility of the resistance changes. PP and PCL were tested separately for solvent sorption using ethanol and n-hexane, both showing a low sorption of n-hexane. Ethanol sorption was large for PCL and almost absent for PP. The 50/50 blend composites with 3 wt.% MWNT in the PCL phase presented larger resistance changes for n-hexane, showing larger sensing ability for this solvent compared to PCL composites with 1 and 3 wt.% loadings. The opposite response was observed for immersion in ethanol where the PCL–MWNT composites showed larger changes than the blends. As the ratio of the conductive PCL phase over PP in the blend composition (i.e. the overall MWNT content) decreased, larger resistance changes were observed. The liquid sensing properties of compression-moulded discs and melt-drawn filaments were compared indicating higher responses for the discs.

ChemInform Abstract: On the Role of the Pore Size and Tortuosity for Sorption of Alkanes in Molecular Sieves

Adsorption of light alkanes on the acidic zeolites theta-1 (H-TON), ferrierite (H-FER), and ZK-5 (H-KFI) was studied by gravimetry, calorimetry, and infrared spectroscopy. Sorption of alkanes on the Bronsted acid sites is energetically favored, and the uptake at low alkane partial pressures is directly correlated to the concentration of acid sites. With H-FER, the accessibility of the two intersecting channel systems depends on the chain length of the alkanes. Although propane and n-butane sorb in both channels, the sorption of n-pentane is hindered in the 8-ring channels and sorption of n-hexane only occurs in the 10-ring channels. In the presence of cavities (such as found in H-KFI) pronounced intermolecular interactions between sorbed alkanes were monitored. In general, the heat of adsorption of alkanes increases with decreasing pore size. At pore sizes smaller than 0.5 nm repulsion forces start to be important, their impact being most prominent for larger isoalkanes.

N-Hexane sorption in poly(ethylene- co-1-octene)s: Effect on phase composition and character

Diffusion of small-molecule penetrants in semi-crystalline polymers is retarded by two factors: penetrant detour bypassing impenetrable crystals and the constraining effect of the crystals on the amorphous component. Previous experiments have shown that the latter factor becomes much less important at higher penetrant concentration in the polymer. Structural changes in a series of poly(ethylene- co-1-octene)s occurring on saturation in n-hexane at 296 K, covering a wide range of crystallinity (17–75 wt.%), were studied by wide-angle X-ray scattering, Raman spectroscopy and NMR spectroscopy. Densification of the crystal unit cell and partial dissolution of the interfacial component on n-hexane sorption are the main experimental findings. The conclusion is that the penetrant molecules increase the mobility of the polymer chain segments adjacent to the crystal interface, enabling better packing of the crystal stems and importantly also causes a reduction in the constraining factor ( β) for diffusion.

Equilibrium Isotherms of Volatile Alkanes, Alkenes, and Ketones on Activated Carbon

The sorption of volatile organic compounds (VOCs) on activated carbon was measured. Equilibrium isotherms at 298 K were obtained for n-hexane, cyclohexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, cyclohexane, 1-hexene, acetone, 2-butanone, and 2-pentanone. The effect of temperature on n-hexane sorption was studied by varying the temperature from (288 to 318) K. Data in this study were collected using a gravimetric apparatus. Some data were replicated with a volumetric apparatus for comparison using an error analysis. For the gravimetric data, it was found that the Langmuir equation (mean deviation 0.020 to 0.025) and the Freundlich equation (mean deviation 0.005 to 0.010) gave good fits of the sorption. Nonpolar or weakly polar sorbates, the sorbates with small steric profiles, or low molecular weight sorbates were more readily adsorbed on the microporous activated carbon. Polar sorbates were more readily adsorbed at high pressure, and low temperature improved the adsorption.

Kinetic studies on sorption of n-hexane in vanadium substituted MFI type zeolites of various crystal morphologies

The MFI type materials isomorphously substituted with vanadium form crystals of two morphology types. Investigations of sorption kinetics for n-hexane indicated for both morphologies a non-typical increase in the value of corrected transport diffusion coefficient with the crystals dimensions. An increase in the D 0 values with the vanadium content of the crystals has also been found, although it is not so well expressed as that with the dimensions. The increase in the D 0 values is from 1.1 × 10 −11 to 1.1 × 10 −10 m 2/s and may be a consequence of an additional system of larger pores, which is not reflected in the adsorption isotherms due to common occurrence of these pores in all crystals. It is also possible that vanadium causes a superior structure ordering and a decrease in/weakening of diffusion barriers.

Influence of Pore Dimension and Sorption Configuration on the Heat of Sorption of Hexane on Monodimensional Siliceous Zeolites

Sorption of n-hexane on monodimensional pure silica SSZ-35, CIT-5, ZSM-12, and ZSM-22 zeolites with different pore dimension and on recently synthesized ITQ-29 was studied by IR spectroscopic and computational chemistry methods. Heats of sorption of n-hexane on these zeolites was determined experimentally from the temperature dependence of the intensity of IR bands of sorbed hexane as well as from theoretical calculations. Calculations have shown the different orientations of sorbed hexane molecules inside zeolite channels, which depend on the type of zeolite and loading. At high loadings, ordering of hexane inside the channels is observed due to optimization of sorbate-sorbate and sorbate-zeolite interaction energies. Such ordering is responsible for the increase of the sorption energy. A decrease of the sorption energy upon increasing the pore dimension of zeolite was observed, in agreement with results previously published in the literature. Effects of pore diameter of zeolites and ordering of molecules inside zeolite channels on the sorption energy of hexane are discussed.

A study of the adsorption thermodynamics of n-hexane on ion-exchanged X zeolites

In this work, the free energy changes and entropy changes of adsorption of n-hexane on zeolites of the MxNa87-2xX (M=Co,Ni,Zn,Cd) type were determined using their isosteric sorption heats. It was found that the exchange of Na ions in NaX with bivalent cations (Co, Ni, Zn, Cd) significantly alters the adsorption characteristics of NaX zeolite. The free energy changes and entropy changes of n-hexane adsorption were dependent on the surface coverage and on the nature of the charge-balancing cation. The specific influence of the exchanged cations on the free energy changes and entropy changes of sorption of n-hexane originates from the size, location and electronic configuration of the cation.

Molecular simulations of the adsorption of cycloalkanes in MFI-type silica

A new force field for the simulation of the adsorption of cycloalkanes in nanoporous silica affords a significant improvement over any previously employed force field. The simulated isotherms reproduce the most salient features in the experimental isotherms extremely well. The study of cyclo-pentane, -hexane, and -heptane adsorption in MFI-type silica indicates an inflection for cyclopentane but not for cyclohexane at intermediate pressure. If corroborated by experiments, such an inflection point would afford an excellent calibration point for further force field developments. At low pressures, mixture isotherms of cyclohexane and n-hexane show a temperature dependence on the selectivity in accordance with recent results by J. P. Fox and S. P. Bates, J. Phys. Chem., 2004, 108, 17136. This dependence is caused by a difference in temperature dependence of the Henry coefficient for both molecules. At high pressures entropy effects due to packing always favor the sorption of n-hexane. Furthermore, the influence of the flexibility of the zeolite framework on the adsorption of these rather bulky molecules is investigated. It is found that this influence of the flexibility on the adsorption of cyclohexane is as small as with n-alkanes.

Effect of surface etching of ZSM-5 zeolite crystals on the rate of n-hexane sorption

Samples of the as-synthesized zeolite ZSM-5 were purified by etching the surface of crystals with an aqueous-acetone solution of hydrogen fluoride. The solution appeared to be very useful for removing by-products from the samples without affecting morphology or size of the crystals. Measurements of n-hexane uptake with a gravimetric method were performed to verify the effect of the sample treatment. The rates of the n-hexane sorption for the etched samples were up to two orders of magnitude higher than that for the non-etched one. The corrected diffusion coefficient of n-hexane was found to be only several times lower than those obtained with the methods of PFG NMR and FR as reported in the literature.

Application of Thermal Analysis for Explaining the Sorption of Benzene and N-Hexane on Silicalite

The origin of kinks (steps) on sorption isotherms was examined for the sorption of benzene and n-hexane on silicalite-1. In both cases sorption revealed the existence of two different binding sites. There was no equilibrium (or a very slow one), between molecules bound at different sites. Sorption energies within particular centres display more or less wide overlapping distributions leading to a single resultant isotherm. Depending on differences in binding energies and degree of overlapping, the resulting isotherms exhibit steps (benzene) or no steps (n-hexane). In fact, the sorption isotherm of benzene being a sum of two elemental isotherms (Ω1 and Ω2) of different shapes is characterized by a ‘kink’, in contrast to n-hexane the elemental isotherms of which are of the same shape.

On the Role of the Pore Size and Tortuosity for Sorption of Alkanes in Molecular Sieves

Adsorption of light alkanes on the acidic zeolites theta-1 (H-TON), ferrierite (H-FER), and ZK-5 (H-KFI) was studied by gravimetry, calorimetry, and infrared spectroscopy. Sorption of alkanes on the Brønsted acid sites is energetically favored, and the uptake at low alkane partial pressures is directly correlated to the concentration of acid sites. With H-FER, the accessibility of the two intersecting channel systems depends on the chain length of the alkanes. Although propane and n-butane sorb in both channels, the sorption of n-pentane is hindered in the 8-ring channels and sorption of n-hexane only occurs in the 10-ring channels. In the presence of cavities (such as found in H-KFI) pronounced intermolecular interactions between sorbed alkanes were monitored. In general, the heat of adsorption of alkanes increases with decreasing pore size. At pore sizes smaller than 0.5 nm repulsion forces start to be important, their impact being most prominent for larger isoalkanes.

Studies on the synthesis of ETS-10 I. Influence of synthesis parameters and seed content

The influence of various parameters such as temperature, concentration of the ingredients and amount of seed used on the kinetics of the synthesis of the large-pore molecular sieve ETS-10 is reported in the paper. Detailed physicochemical characterizations of the samples have been carried out with instrumental techniques such as X-ray diffraction (XRD), scanning-electron microscopy (SEM), ultraviolet-visible (UV-VIS) spectroscopy, infrared (IR) spectroscopy nuclear magnetic resonance (NMR) and sorption of water, n-hexane and mesitylene.