Adsorption Of Isomers Research Articles

Impacts of controlled microwave field irradiation on o-cresol and p-cresol adsorption capability of activated carbon

ABSTRACT To access the feasibility of microwave in promoting adsorbability of carbonaceous adsorbents, microwave irradiation on activated carbon (AC) was conducted at powers of 400–800 W and duration of 10 min. Accordingly, the temperature rising of AC under microwave field were studied. Moreover, the alterations in physicochemical properties of AC and impacts on cresol isomer adsorption were investigated. Results indicated that the heating curve of AC displays the initial fast temperature rising stage and the final slow stage. Additionally, the bulk temperature at irradiation terminal increases with microwave power. The temperature rising further increases the pores with a diameter range of 1.00–6.00 nm of AC; it also increases the oxygenic functional groups of AC after irradiation at 400 W and 800 W, but decreases that of AC after irradiation at 640 W. The saturation adsorption capacity of o-cresol and p-cresol on the irradiated AC rises with elevated temperature. Additionally, the cresol isomer adsorption kinetics on the irradiated AC follows the Elovich model. The above-mentioned equilibrium and kinetics suggest that the cresol isomer adsorption on the irradiated AC is dominant by chemisorption. Finally, the optimum irradiation power for o-cresol and p-cresol adsorption is 800 W and 400 W, respectively, thereby fabricating AC with developed pores and abundant oxygenic functional groups. Accordingly, the saturation adsorption capacity of o-cresol and p-cresol reaches up to 111.11 mg·g−1 and 95.97 mg·g−1, respectively. Overall, microwave irradiation is a viable option to promote cresol isomer adsorption on AC.

Synthesis of silicalite-1 zeolite membranes for vapor-permeation separation of dichlorobenzene Isomers

Silicalite-1 zeolitic membranes have been successfully fabricated on the porous α-Al2O3 support by templated and template-free protocol, respectively, for vapor permeation separation of dichlorobenzene (DCB) isomers. After proving the high quality of the membranes by single gas permeation (He and SF6) performance, the vapor-permeation of DCB isomers over two types of the silicalite-1 membrane was then investigated. The separation results clearly indicated that under the lower partial pressure and higher temperature, the effect of DCB isomer adsorption on the permeance could be kept at a sufficiently low level and sharp selectivity become more important. Thus, high p-DCB selectivity could be achieved. Comparatively, the template-free silicalite-1 zeolite has a much higher p-DCB selectivity due to the relatively fewer inter-crystalline gaps. Under certain separation conditions, the highest selectivity of p-DCB for p-/m-DCB and p-/o-DCB binary systems could reach 165 and 113, respectively.

Efficient Separation of Xylene Isomers by a Pillar-Layer Metal-Organic Framework.

The separation of xylene isomers is one of the most challenging issues in the chemical industry because of the similarity of their boiling points and kinetic diameters. This study focuses on the use of pillar-layer MOF-Co(aip)(bpy)0.5 for adsorption and separation of xylene isomers. It was found that Co(aip)(bpy)0.5 exhibited a significant para-selectivity in liquid-phase competitive adsorption of xylene isomers, and the competitive separation factors reached as high as 30 for p-xylene versus m-xylene and 16 for p-xylene versus o-xylene. Desorption experiments further confirmed the preferential adsorption of p-xylene on the adsorbent. Molecular simulations and calculations revealed that the order of interaction strengths for xylene molecules and the adsorbent framework was p-xylene ≫ o-xylene ≈ m-xylene, which illustrated the selective adsorption phenomena arising from the mechanism for microscopic interactions. This work broadens the application of pillar-layer MOF materials in the field of xylene isomer adsorption and separation.

A Comparative Study of Adsorption of Isomeric Molecules on Carbon Sorbents from a Gas and a Liquid

A comparative study of adsorption of isomeric alkylbenzenes, alkylphenols, and alkanes on carbon sorbents from a gas and a liquid phase using an extended variant of molecular statistics calculations is carried out. In simulation, conformational non-rigidity of molecules is taken into account. This method is shown to satisfactorily estimate proportions between the adsorption characteristics for isomers of hydrocarbons adsorbed on graphitized thermal carbon black from a gas phase. In the case of adsorption from a liquid phase on porous graphitized carbon, additionally, the effect of a solvent and polarizability of the sorbent surface by charged sites of a sorbate molecule are considered. It is demonstrated that, for correct prediction of the ratio between the Henry constants for adsorption of isomers of alkylphenols and alkylbenzenes from solutions, it is necessary to consider both these factors.

Azobenzene Adsorption on the MoS2(0001) Surface: A Density Functional Investigation within van der Waals Corrections

The interest in light-sensitive organic molecules, such as azobenzene, has increased because of their ability to functionalize two-dimensional layered systems and engineer their electronic structure. In this work, we explore the azobenzene trans and cis isomer adsorption on a molybdenum disulfide MoS2(0001) layer employing the density functional theory (DFT) within van der Waals (vdW) corrections to the semilocal exchange–correlation functional. We found that the aromatic rings of azobenzene lay parallel to the surface (two in the trans isomer and one in the cis isomer), which contributes to increasing the configuration stability by vdW interactions. Furthermore, we found a relatively large work function change upon adsorption because of the electron density rearrangement, and hence, it might affect the electronic transport properties within the single MoS2(0001) layer. We observed an increase in the relative DFT + vdW total energy among the azobenzene isomers (trans and cis) from 0.51 eV in the gas-phase...

Liquid-Phase Multicomponent Adsorption and Separation of Xylene Mixtures by Flexible MIL-53 Adsorbents

The MIL-53 class of metal-organic frameworks (MOFs) has recently generated interest as potential adsorbents for xylene mixture separations. Cost-effective separation of xylene isomers is challenging owing to the similarity in their molecular structures, kinetic diameters and boiling points. Here we report a systematic experimental and computational study of xylene isomer adsorption in MIL-53 adsorbents, focusing particularly on the effects of different metal centers, determination of separation properties with industrially relevant quaternary liquid-phase C8 aromatic feeds, and a predictive molecular simulation methodology that accounts for all relevant modes of MIL-53 framework flexibility. Significant scale-up of MIL-53 synthesis was carried out to produce high-quality materials in sufficient quantities (300-500 g each) for detailed measurements. Single-component adsorption simulations incorporating the MIL-53 ‘breathing’ and linker flexibility effects showed good agreement with experimental isotherms. ...

Removal of linear and monobranched alkane from aviation gasoline by 5A zeolite adsorption for octane number enhancement

AbstractDue to continued concern about effects of tetraethyl lead (TEL) emission on the environment and human health, the complete removal of TEL from aviation gasoline (avgas) is a desirable pursuit. In order to maintain high antiknock performance of avgas, an advanced adsorption method was developed using 5A zeolite to remove linear and monobranched alkane from avgas for octane number enhancement. An experimental study of single, binary, and quaternary adsorption of octane isomers on a fixed bed reactor was performed in this work. The effects of partial pressure, operating temperature, and initial mixture composition on breakthrough curves and sorption selectivity were thoroughly studied. At fixed partial pressure, normal octane was the strongest‐adsorbed component, followed by 2‐methylheptane, 2, 5‐dimethylhexane, and 2, 2, 4‐trimethylpentane. Adsorption capacity of all components increased with increased partial pressure and decreased with increased operating temperature. However, for all binary mixtures studied in this work, the sorption selectivity decreased as total pressure increased. At equimolar concentrations the normal and monobranched isomers were preferentially adsorbed, while when the volume percentages of the isomers were different, the adsorption of the isomer with the higher volume ratio was favoured. The quaternary breakthrough curves for octane isomers at 473 K with each component having partial pressure of 0.15 kPa also indicated the feasibility of simultaneous removal of normal and monobranched isomers from a multiple‐component mixture by this 5A zeolite.

The effect of pore shape on hydrocarbon selectivity on UiO-66(Zr), HKUST-1 and MIL-125(Ti) metal organic frameworks: Insights from molecular simulations and chromatography

Configurational Bias Grand Canonical Monte Carlo simulations have been used to show that the alkane isomer adsorption selectivity of porous MOF materials containing two pore types depends on the orientation of organic linkers’ phenyl groups. These simulations were performed at low pressure (0.1kPa) using mixtures of n-hexane and its branched isomers (2,2-dimethylbutane, 2,3-dimethylbutane and 2-methylpentane). Where possible, we compared the results with our gas chromatography results. In typical 1D narrow pore materials, the linear isomer is usually preferentially adsorbed over its branched isomers. In MOF materials exhibiting a 3D pore system with two pore types, a large one interconnected by smaller pores, the selectivity order is the inverse. Here, we show that this depends on the degree of opening of the access windows, which can allow it to be either “closed”, or to mimic a small pore channel. The consequence of this is the possibility (in the linear/branched mixture case) for the linear alkane to remain linear and thus maximize its interactions with the pore. The linear/aromatic mixture case considers a mixture of benzene and n-hexane, to show that a more favorable packing efficiency pushes the selectivity towards the aromatic molecule, regardless of the degree of the pore opening, although n-hexane can increase its competitiveness for the adsorption sites in materials where it can remain in mostly linear conformations.

Adsorption and Separation of Xylene Isomers: CPO-27-Ni vs HKUST-1 vs NaY

The separation of xylene isomers is one of the most difficult separations in petrochemistry. Adsorbents have to preferentially adsorb para or meta isomers, but there are only a few zeolites which fulfill the criteria of selectivity and adsorption capacity. In this study, we evaluate two metal–organic frameworks (MOF) with coordinatively unsaturated metal sites (cus), i.e., CPO-27-Ni and HKUST-1, in the adsorption and separation of p-xylene, m-xylene, and o-xylene in the gas phase. The results are compared with those of the zeolite NaY. CPO-27-Ni and HKUST-1 are both ortho-selective adsorbents, but molecular simulations indicate that the reasons for the ortho selectivity are quite different. The CPO-27-Ni structure is intrinsically ortho selective, and the ortho selectivity is further enhanced by electrostatic effects. HKUST-1, on the other hand, is intrinsically (weakly) para selective but the para selectivity is overcompensated by electrostatic effects, which overall leads to a slight preference for the ortho isomer adsorption. The insights in the adsorption behavior of xylenes that are provided by this study provide useful guidelines in the quest for para-selective MOFs materials.

Selectivity of Adsorption of Thiophene and its Derivatives on Titania Anatase Surfaces: A Density Functional Theory Study

We used density functional theory calculations to examine the adsorption of thiophene and its derivatives (methyl and dimethyl thiophenes) on the (001), (101) and (100) surfaces of titania anatase, of relevance to adsorptive desulfurization of liquid fuels. The adsorption of isomers of methyl and dimethyl thiophenes allows evaluation of electronic, steric hindrance and dispersive attraction impacts on adsorption energies. Adsorption of all species is found to preferentially form a sulfone-like species, where the sulfur containing molecule is partially oxidized on the surface. Oxygen molecules trapped in O-vacancies may serve as the preferred adsorption site for sulfoxide formation. Comparison is made between DFT and DFT-D results, which include a semi-empirical dispersion correction. DFT-D gives a more accurate representation of the interactions between the adsorbate (thiophene) molecules and the adsorbent (anatase) surface. The selectivity of different surfaces towards sulfur-containing molecules is estimated by comparing the adsorption energies of the thiophenes and their corresponding hydrocarbons.

Characterization of aminophenol isomer adsorption on silver nanostructures

Surface-enhanced infrared absorption (SEIRA), temperature-programmed desorption, and density functional theory calculations were used to explore the adsorption of aminophenol isomers on vacuum deposited silver films and silver powder. Salts of the aminophenolate ions were synthesized to aid in vibrational spectral interpretation. No evidence for dissociation of any of the aminophenols into aminophenolate ions in the monolayer or multilayer was found when adsorbed on silver nanostructures (SNS). SEIRA demonstrated additional hydrogen-bonding interactions in a multilayer of 2-aminophenol and 3-aminophenol adsorbed on SNS beyond what is present in the aminophenol powders. Surface-enhanced Raman spectroscopy (SERS) experiments also showed that a 2-aminophenol monolayer adsorbed on SNS is photolytically oxidized to either 3-aminophenoxazone or 2,2′-dihydroxyazobenzene, while neither 3-aminophenol nor 4-aminophenol showed any SERS activity. This work is expected to have a significant impact in a range of environmental, biochemical, and industrial applications where phenol chemistry is important.

Adsorption of xylene isomers on ordered hexagonal mesoporous FDU-15 polymer and carbon materials

The oil industry has been facing the challenges of separation of xylene isomers, o-xylene, m-xylene and p-xylene or removing them from the environment. In our present work, we investigated the adsorption of the three isomers on two mesoporous materials, FDU-15-350 polymer and FDU-15-900 carbon materials. The isomer adsorption capacities are well correlated with their physical pore properties. It is found that the micropores are very crucial for the adsorption of these three isomers. The more micropore volume the adsorbent has, the better the adsorption capacity is. Henry’s constants were also calculated for the three isomers on the two adsorbents. Both on FDU-15-350 polymer and FDU-15-900, the Henry’s constants for the three isomers show the same trend o>m>p xylene which is coincidently in accordance with their polarity trend, indicating more polar adsorbate is preferred for adsorption on the two adsorbents. The isosteric heats of adsorption are correlated with the microporosity and the size of the adsorbate molecule. More microporosity and smaller molecules give higher heats of adsorption. Extracted information on pore properties of adsorbents by using the three isomers has very similar results as that resolved from nitrogen adsorption, indicating the feasibility of using the three isomers as adsorbates to extract pore information.

Transient permeation of butanes through ZSM‐5 and ZSM‐11 zeolite membranes

AbstractButane isomer adsorption isotherms and diffusivities in the transport pathways through Al‐ZSM‐11 (MEL), Al‐ZSM‐5 (MFI), and B‐ZSM‐5 zeolite membranes were measured by a transient permeation technique. The permeate responses to feed step changes were measured between 383 and 473 K, and the diffusion was modeled as Maxwell–Stefan surface diffusion with dual‐site Langmuir adsorption. The effects of adsorption and diffusion on the n‐C4/i‐C4 ideal selectivities were quantified, and the selectivity differences between the membranes were explained based on the adsorption properties and the diffusivities. The resistance of the porous tubular supports affected the coverage in the zeolite at the permeate boundary, and the ratio of the n‐C4/i‐C4 diffusion driving forces consequently increased with increasing temperature. The n‐C4 diffusivities were almost the same in the three membranes, but the i‐C4 diffusivities in the MEL membrane were an order of magnitude higher than those in the MFI membranes. The bulkier i‐C4 molecules are apparently significantly less constrained in the straight, circular pores and slightly larger intersections of MEL zeolite, although the structural differences do not affect the mobility of n‐C4. Higher i‐C4 coverages in the Al‐ZSM‐11 and B‐ZSM‐5 membranes compared to the Al‐ZSM‐5 membrane were attributed to a smaller change in entropy upon adsorption in the channels of Al‐ZSM‐11 and to a higher heat of adsorption in the intersections of the more substituted B‐ZSM‐5. Initial transient flux overshoots and undershoots after i‐C4 feed concentration changes were apparently caused by a small amount of pressure‐driven flow (<10% of the total i‐C4 flux and <0.1% of the total n‐C4 flux) through parallel pathways. The effective membrane thicknesses were also estimated from the transient measurements. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2816–2834, 2004

Adsorption of isomers of butanol on bismuth single crystal plane electrodes

Cyclic voltammetry, impedance and chronocoulometry have been employed for quantitative study of normal butanol ( n-BA), iso-butanol ( iso-BA), sec-butanol ( sec-BA) and tert-butanol ( tert-BA) adsorption at the bismuth single crystal plane¦aqueous Na 2SO 4 solution interface. The adsorption isotherms, Gibbs energies of adsorption, the surface excess and other adsorption parameters have been determined. As found, the adsorption characteristics of the isomers of butanol depend on the crystallographic structure of the electrodes, as well as on the geometrical structure of hydrocarbon chains of adsorbate. The adsorption characteristics obtained from the impedance and chronocoulometric measurements are in good agreement within the limits of surface charge densities — 24 < σ < 3 μC cm −2. The adsorption of n-BA, sec-BA, iso-BA and tert-BA on Bi single crystal planes is physical and is limited by the rate of diffusion of organic molecules to the electrode surface. The adsorption activity of adsorbates at the bismuth¦solution interface increases in the sequence tert-BA < sec-BA < iso-BA < n-BA as the adsorption at the air¦solution interface increases. In the case of all compounds studied, the adsorption activity of planes increases in the sequence (̄10̄1) ≤ (001) < (2̄1̄1) < (01̄1) as the superficial density of atoms increases. According to the values of the standard Gibbs energy of adsorption, it was established that the hydrophilicity of electrodes increases in the sequence Sb(111) < Bi(01̄1) < Bi(211¯) < Bi(̄10̄1) ≤ Hg < Bi(001) < Bi(111) < Cd(0001) < Zn(0001) < Ag(111) < Ag(100) < Ga.

A computational study of the adsorption of the isomers of butanol on silicalite and H-ZSM-5

We have studied the adsorption of the four isomers of butanol on silicalite and on H-ZSM-5 using an energy minimization procedure supplemented by a Monte Carlo/molecular dynamics algorithm to assist in the location of minima. The energetics and the geometries of adsorption of the butanol isomers in the pores of silicalite and H-ZSM-5 are reported. The effect of the relaxation of both the adsorbent framework and of the adsorbate molecule is investigated. Significant changes in the direction of the surface hydroxyl groups at certain crystallographic positions are induced by alcohol physisorption. For both silicalite and H-ZSM-5, similar energy values were obtained for each butanol isomer sorbed at a number of different crystallographic positions. We therefore predict that there are a range of physisorbed states for all butanols at ambient temperatures. The small variations in the adsorption energetics and sites between isomers may be explained in the terms of pore-confinement effects on the adsorption of molecules with dimensions similar to those of pentasil channels.

Dihydrogen carrier gases in gas-solid chromatography. Study of hydrogen weight and nuclear spin isomer adsorption

A new version of gas-solid chromatography (GSC) of hydrogen isotopes and spin isomers which makes use of the adsorbable dihydrogen carrier gases (H2, HD and D2) is described. Major advantages of the new technique in comparison with conventional GSC with inert carrier gases are discussed. The new version of GSC was found to be a powerful tool for acquisition of the data necessary to design adsorption systems to separate hydrogen isotopes.

Reversible and resistant components of PCB adsorption-desorption: isotherms

Results from laboratory equilibration studies indicate that sediment-adsorbed 2,4,5,2',4',5'-hexachlorobiphenyl (HCBP) fractions may be comprised of both reversibly and strongly bound or resistant components. This evidence suggests that for many environmental modeling efforts it may be inappropriate to treat this and other PCB isomer adsorption reactions as either completely reversible or completely irreversible. The initial refinement of such models requires a means of estimating the relative magnitudes of the respective sorbed PCB fractions. To this end, a computational method has been derived to allow prediction of the magnitude of the reversible and more strongly adsorbed HCBP fractions from conventional isotherm data. The derivation, which is based upon experimentally observed linear HCBP isotherms, has also been applied to other organic molecules of similar isotherm character. This methodology provides an initial quantitative approximation of the strongly bound, resistant, sediment fractions while utilizing relatively simple experimental adsorption-desorption data. Results suggest that for montmorillonite clay, kaolinite clay, and natural lake sediment (Saginaw Bay, MI) linear isotherms effectively describe the relationships between both adsorbed HCBP fractions and the aqueous HCBP concentration. The implications of these and other model predictions for the cycling of PCB's in natural waters are discussed.

Adsorption of oleic acid soap in styrene-butadiene latex

Abstract A study has been made of adsorption of isomers in soaps made from commercially available oleic acids to rubber by separating and analyzing the serum from styrene-butadiene (S-BR) latex. Both the cis and trans forms of potassium 9-octadecenoate (potassium oleate and elaidate) adsorb strongly to the rubber particles. The rubber phase was separated from the serum without disturbing the soap distribution by successive filtration through Millipore filters of diminishing pore size. These filters had been thoroughly washed to remove surface-active wetting and plasticizing agents and treated with methylene blue to block active sites which would otherwise adsorb emulsifier. Potassium erucate (C22−) was added as an internal standard to the rubber-free serum, which was then acidified and extracted with ethyl ether. The extraction glassware was rinsed with cyclohexane to remove adsorbed fatty acid. Methyl esters of the recovered emulsifier-erucic acid mixture were quantitatively analyzed by gas liquid chromatography. The results show selective adsorption of certain isomers on the rubber particles. In cases where only enough emulsifier was present to cover the rubber surface, the octadecenoic isomers were greatly depleted in the serum indicating preferential adsorption. Addition of emulsifier to the latex increases the quantities of selectively adsorbed isomers in the serum until with large excesses, the serum soap composition approaches that of the original emulsifier.