Xylene Isomer Mixtures Research Articles

High p-xylene selectivity in aluminum-based metal–organic framework with 1-D channels

The separation of highly pure p-xylene (pX) from xylene isomers is an industrially important and challenging issue. Although simulated moving bed (SMB) processes using faujasite zeolites are currently used for pX separation, developing novel adsorbents with improved pX separation performances is strongly needed. In this study, an aluminum-based metal–organic framework (MOF), MIL-120(Al) with 1-D channels of approximately 7 Å, exhibited considerably high pX selectivities compared to xylene isomers (αpX/oX: ∼31; αpX/mX: ∼17; αpX/EB: ∼7.5; αpX/OME: ∼11), which are superior to reported values for other MOFs and zeolites under similar conditions. Such high selectivities may originate from the proper pore shape and size of MIL-120(Al). MIL-120(Al) also showed good cyclic adsorption properties as well as superior hydrothermal and chemical stabilities. Finally, a dynamic simulation showed that MIL-120(Al) could achieve approximately complete separation of pX from a xylene isomer mixture using an SMB process.

Selected Tricyclic Fused Systems: Host Behavior in the Presence of Mixed Xylenes and Ethylbenzene

Two tricyclic fused host systems, namely, N,N′-bis(5-phenyl-5-dibenzo[a,d]cycloheptenyl)ethylenediamine H1 and N,N′-bis(5-phenyl-10,11-dihydro-5-dibenzo[a,d]cycloheptenyl)ethylenediamine H2, were investigated for their selectivity behavior in xylene isomer (o-Xy, m-Xy, p-Xy) and ethylbenzene (EB) mixtures. H1 only formed a complex with p-Xy, and H2 only with o-Xy, in single solvent recrystallization experiments. When H1 and H2 were recrystallized from equimolar mixtures of every combination of the four organic solvents, contrasting host selectivities were observed: H1 favored p-Xy and H2 favored o-Xy. High selectivities for p-Xy (87.2–88.7%) were noted in equimolar binary experiments with H1, while H2 furnished crystals with 91.3% o-Xy from an o-Xy/p-Xy mixture. Remarkably, only the apohost was recovered when p-Xy or o-Xy (for H1 or H2) was absent from the mixtures. Constructed selectivity profiles affirmed observations from these equimolar experiments. Thermal analyses demonstrated that H1·p-Xy was significantly less stable than H2·o-Xy, and SCXRD analyses explained these observations: p-Xy, in its complex with H1, was accommodated in wide open channels and did not experience any noncovalent interactions with the host molecule; on the other hand, o-Xy in H2 was bound tightly in near-discrete cages and, furthermore, interacted with H2. Both host compounds demonstrated the ability to separate various of the mixtures reported in this work.

A gating ultramicroporous metal-organic framework showing high adsorption selectivity, capacity and rate for xylene separation

Adsorptive separation of p-xylene (pX) from xylene isomers is a key process in chemical industry, but known adsorbents cannot simultaneously achieve high adsorption selectivity, capacity, and rate. Here, we demonstrate gating ultramicropore as a solution for this challenge. Slight modification of the synthetic condition gives rise to isomeric metal-organic frameworks α-[Zn(pba)] (MAF-88, H2pba = 4-(1H-pyrazol-4-yl)benzoic acid) and β-[Zn(pba)] (MAF-89) possessing similar pillared-column structures, porosities, and high pX capacities of 2.0 mmol g−1, but very different framework/pore topologies, pore sizes, and pX selectivities. For binary and ternary mixtures of liquid xylene isomers, MAF-88 with narrow one-dimensional (1D) channels shows pX selectivity of 11 and 1.6, while MAF-89 with 3D-connected quasi-discrete pores shows pX selectivity up to 221 and 46, respectively. Thermogravimetry, differential scanning calorimetry, and time-dependent separation experiments reveal that the kinetic effects of the gating pores play more important roles than the thermodynamic effects, which is further confirmed by single-crystal X-ray diffraction and computational simulations.

VOC Mixture Sensing with a MOF Film Sensor Array: Detection and Discrimination of Xylene Isomers and Their Ternary Blends.

Detection and recognition of volatile organic compounds (VOCs) are crucial in many applications. While pure VOCs can be detected by various sensors, the discrimination of VOCs in mixtures, especially of similar molecules, is hindered by cross-sensitivities. Isomer identification in mixtures is even harder. Metal-organic frameworks (MOFs) with their well-defined, nanoporous, and versatile structures have the potential to improve the VOC sensing performance by tailoring the adsorption affinities. Here, we detect and identify ternary xylene isomer mixtures by using an array of six gravimetric, quartz crystal microbalance (QCM)-based sensors coated with selected MOF films with different isomer affinities. We use classical molecular simulations to provide insights into the sensing mechanism. In addition to the attractive interaction between the analytes and the MOF film, the isomer discrimination is caused by the rigid crystalline framework sterically controlling the access of the isomers to different adsorption sites in the MOFs. The sensor array has a very low limit of detection of 1 ppm for each pure isomer and allows the isomer discrimination in mixtures. At 100 ppm, 16 different ternary o-p-m-xylene mixtures were identified with high classification accuracy (96.5%). This work shows the unprecedented performance of MOF-sensor arrays, also referred to as MOF-electronic nose (MOF-e-nose), for sensing VOC mixtures. Based on the study, guidelines for detecting and discriminating complex mixtures of volatile molecules are also provided.

Selective inclusion of p-xylene by bis-hydrazone compounds

p-Xylene can be separated from a mixture of xylene isomers by the selective inclusion property of three novel bis-hydrazone compounds. Crystallization of these compounds in solvents containing p-XY generate voids which are complementary to only p-XY.

Molecular dynamics simulation of adsorption and separation of xylene isomers by Cu-HKUST-1.

Metal-organic frameworks (MOFs) are widely used in the adsorption separation of various gases. A fundamental understanding of the effective separation of xylene isomers helps improve aromatic products' separation efficiency and reduce industrial separation costs. Grand Canonical Monte Carlo (GCMC) simulations combined with Molecular Science is widely used to predict gas adsorption and diffusion in single crystals with metal-organic frameworks. We performed a GCMC + MD combined approach to study xylene isomers' adsorption and separation in Cu-HKUST-1 to predict the permeability and selectivity of the ternary gas mixture in the MOF with the adsorption and diffusion usage data. Most current studies take into account the computational cost and difficulty. Most recent research models are limited to the adsorption of a single or specific molecule, such as hydrogen, methane, carbon dioxide, etc. For this reason, we report an attempt to study the adsorption separation of aromatic gases (p-xylene/o-xylene/m-xylene) based on Cu-HKUST-1 single-crystal materials based on some previous research methods with an appropriate increase in computational cost. To predict the adsorption selectivity and permeability of the ternary mixture of xylene isomers on the MOF surface, the model simulation calculates key parameters of gas adsorption, including gas adsorption volume (N), the heat of adsorption (Q st), Henry coefficient (K), and diffusion coefficient (D).

Formation Process of Columnar Grown (101)-Oriented Silicalite-1 Membrane and Its Separation Property for Xylene Isomer

Silicalite-1 membrane was prepared on an outer surface of a tubular α-alumina support by a secondary growth method. Changes of defect amount and crystallinity during secondary growth were carefully observed. The defect-less well-crystallized silicalite-1 membrane was obtained after 7-days crystallization at 373 K. The silicalite-1 membrane became (h0l)-orientation with increasing membrane thickness, possibly because the orientation was attributable to “evolutionally selection”. The (h0l)-oriented silicalite-1 membrane showed high p-xylene separation performance for a xylene isomer mixture (o-/m-/p-xylene = 0.4/0.4/0.4 kPa). The p-xylene permeance through the membrane was 6.52 × 10−8 mol m−2 s−1 Pa−1 with separation factors αp/o, αp/m of more than 100 at 373 K. As a result of microscopic analysis, it was suggested that a very thin part in the vicinity of surface played as effective separation layer and could contribute to high permeation performance.

Separation principle of xylene isomers and ethylbenzene with hydrogen-bonded host frameworks via first-principles calculation

Separation of molecular isomers, which have similar physical properties, is hardly achieved with conventional separation methods based on phase equilibria. However, using selective inclusion of target molecules into dismantlable molecular framework allows molecular isomers to be effectively separated from one another. For that purpose, we consider the hydrogen-bonded organic framework (HOF), which can undergo solvent-mediated crystallization. Herein, we theoretically elucidated the separation mechanism of the mixture of xylene isomers (i.e., o-, m-, and p-xylene) and ethyl benzene (EB) using guanidinium (G) cation and organosulfonate anion (S) host systems (i.e., 2(G)+4,4′-biphenyldisulfonate (G2BPDS) and 2(G)+2,6-naphthalenedisulfonate (G2NDS) GS-host systems). Density functional theory (DFT) calculations were carried out to investigate separation mechanisms in terms of thermodynamics (i.e., formation energy, interaction energies of guest-host and guest-guest, and vacancy formation energy) and kinetics (i.e., surface energy) considering the solvent-mediated crystallization process. We theoretically predicted that G2BPDS system could effectively separate EB from xylene isomers, and G2NDS system could separate each xylene isomer by sequential separation process.

Catalytic Cracking of Biomass-Derived Hydrocarbon Tars or Model Compounds To Form Biobased Benzene, Toluene, and Xylene Isomer Mixtures

The gasification of biomass is one of the most prominent technologies for the conversion of the raw material feedstock to polymers, useful chemical substances, and energy. The main engineering challenge during the processing of wastes is the presence of tars in gaseous reaction products, which could make this operation methodology unsuccessfully due to the blockage of separating particle filters, fuel line flow, and substantial transfer losses. Catalytic hydrocarbon cracking appears to be a promising developing approach for their optimal removal. However, it is still highly desirable to enhance the catalysts’ activity kinetics, selectivity, stability, resistance to (ir)reversible coke deposition, and regeneration solutions. The purpose of this Review is to provide a comparative systematic evaluation of the various natural, synthetic, and hybrid ways to convert the model molecular compounds into benzene, toluene, xylene, (poly)aromatics, syngas, and others. The recent scientific progress, including calcite...

Elucidation and characterization of entropy effects in mixture separations with micro-porous crystalline adsorbents

Separations of mixtures of xylene isomers, alkane isomers, linear alkanes, and linear alcohols using micro-porous crystalline adsorbents such as zeolites and MOFs are often carried out under conditions in which the pores are nearly saturated with guest molecules. Such separations are often dominated by factors other than the relative binding strengths of the constituents; the component that is preferentially adsorbed under pore saturation conditions is the guest molecule that has the higher saturation capacity, and packs more efficiently within the microporous channels. Higher saturation capacities, and packing efficiencies, arise from a wide variety of factors such as (a) smaller size, (b) shorter length, (c) smaller footprint, (d) commensurateness of molecular configuration with channel geometry. A common characteristic of all these separations is that at low pore occupancies, θmix<0.5, the separations are dominated by differences in binding strengths, and at pore occupancies θmix>0.5, the separations become increasing influenced by differences in saturation capacities. Remarkably, at pore saturation, i.e. θmix≈1, the component that packs more poorly is virtually excluded. Statistical thermodynamics, and the Boltzmann expression S=kBlnW, applied to lattice models, are used to quantify and rationalize the influence of packing efficiencies, and the phenomena of selectivity reversals.

Luminescent Metal-Organic Frameworks for the Detection and Discrimination of o-Xylene from Xylene Isomers.

Differentiation of xylene isomers remains as one of the most important challenges in the chemical industry, mainly due to the similar molecular sizes and boiling points of the three xylene isomers. Fluorescence-based chemical sensors have attracted wide attention due to their high sensitivity and versatile applications. Here, we report a novel fluorescent metal-organic framework named NUS-40, which is able to selectively detect and discriminate o-xylene from other xylene isomers. Suspension of NUS-40 in o-xylene produces a distinct red shift in the fluorescence emission compared to that in either m-xylene or p-xylene. Moreover, the extent of peak shift is dependent on the concentration of o-xylene in xylene isomer mixtures, and the observed linear correlation between fluorescence intensity and o-xylene concentration is beneficial for quantitative detection. The possible mechanism of such responsive fluorescence behavior was investigated by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance, and vapor sorption experiments. In addition, facile metalation of the porphyrin centers with metal ions provides an additional route to fine-tune the sensing properties.

Core-shell silica particles with dendritic pore channels impregnated with zeolite imidazolate framework-8 for high performance liquid chromatography separation

Metal-organic frameworks (MOFs) have been emerged as promising stationary phases for separations. However, the irregular shapes and wide size distribution of MOF particles have led to the high column backpressure and low column efficiency. We described here a kinetic controlling method to deposit zeolitic imidazolate framework (ZIF-8) onto the pore surface using core-shell silica spheres as support. By varying the volume ratio of N,N-dimethylformamide and methanol, the formation speed of ZIF-8 crystals were greatly suppressed, resulting in the growth of very thin layer (∼3.4nm) of ZIF-8 nanocrystals on the pore surface of the spheres instead of freedom growth of micron sized crystals in the solution. Thus prepared hybrid particles combined the merits of high selectivity of MOFs and the high separation performance of core-shell silica spheres. As a result, separation performance as high as 210000 plates/m for a mixture of xylene isomers was achieved.

Synthesis of large-pore face-centered-cubic periodic mesoporous organosilicas with unsaturated bridging groups

Periodic mesoporous organosilicas (PMOs) with ethenylene (–CHCH–) bridging groups in the framework and large spherical mesopores of tunable size arranged in face-centered-cubic structures (Fm3m symmetry) were synthesized using Pluronic F127 (EO106PO70EO106) as a surfactant and xylene (isomer mixture) as a swelling agent at low temperature (7 °C) under moderately acidic conditions (0.1 M HCl). The resulting PMOs were characterized by small-angle X-ray scattering, nitrogen adsorption, transmission electron microscopy and solid-state 29Si NMR. The unit-cell parameters were enlarged from 31 to 42 nm and the pore diameters increased from 14 nm to as much as 22 nm while increasing the amount of the swelling agent in the reaction mixture, until these two structural parameters reached a plateau. The enlargement was not accompanied by a loss of structural ordering, which is exceptional for not only PMOs, but also other surfactant-templated materials. While the extraction of the surfactant was ineffective in generating readily accessible mesopores for samples prepared with low relative quantities of the swelling agent, extracted samples prepared with higher proportions of the swelling agent exhibited good mesopore accessibility. On the other hand, the calcination under argon at 350 °C afforded well accessible mesopores in all cases, even though the unit-cell shrinkage was significant and a substantial fraction of Si–C bonds was cleaved.

Selectivities at infinite dilution of xylene isomers in ionic liquids

A database on activity coefficients of organic solutes at infinite dilution in ILs was collected from literature sources. The activity coefficient model with the combinatorial term represented by the Kikic et al. was used as a modification to Flory's equation. The activity coefficients had been used to correlate the solubility parameter of ionic liquids. The obtained solubility parameters of ionic liquids had been further correlated based on a concept of the group contribution method. Through the analysis of the database and the prediction results of selectivities at infinite dilution of xylene isomers, we showed that higher selectivity can be achieved by using ILs as working solvents for separation of xylene isomer mixtures.

Xylene-Induced Auditory Dysfunction in Humans

Animal data indicate that xylene induces cochlear dysfunction, characterized by the loss of outer hair cells. There is little evidence regarding xylene-induced ototoxicity in humans. The aim of the study was to investigate the possible adverse effects of xylene on the peripheral and central auditory system in humans. A total of 30 medical laboratory workers who had been exposed to a mixture of xylene isomers, together with 30 nonexposed control participants matched for gender, age, and educational level were selected. Participants of both groups were not exposed to noise levels above 85 dBA time-weighted average. All participants were evaluated with a comprehensive audiological test battery, which included measures of peripheral and central auditory function. Peripheral auditory measures included pure-tone audiometry and distortion product otoacoustic emissions. Behavioral measures of central auditory function included a pitch pattern sequence test, an adaptive test of temporal resolution, a dichotic digit test, and a masking level difference test. The auditory brainstem response was used to objectively evaluate the function of the auditory pathways at the brainstem level. Speech perception in quiet and in noise was evaluated using the Hearing In Noise Test (HINT). The xylene-exposed participants were extensively evaluated with regard to their exposure to both noise and xylene. Noise dosimetry was conducted over an 8-hr work shift to obtain noise-exposure levels for each xylene-exposed worker. Airborne xylene concentrations were obtained at 11 different workstations throughout the medical laboratories, and methyl hippuric acid levels per gram of creatinine in urine were obtained for each xylene-exposed subject. Finally, a detailed interview exploring current and past solvent and noise exposure was conducted. The xylene-exposed participants showed significantly worse pure-tone thresholds in comparison with the nonexposed participants. The xylene-exposed participants demonstrated significantly worse results than the control group participants for the pitch pattern sequence test, dichotic digit test, HINT, and the auditory brainstem response (absolute and interpeak latencies). No significant differences between the xylene-exposed and nonexposed participants were observed for distortion product otoacoustic emissions, adaptive test of temporal resolution, or the masking level difference test. A significant correlation between the concentrations of methyl hippuric acid in urine and pure-tone thresholds (2 to 8 kHz) was found in xylene-exposed workers. Also, participants with high cumulative dose of xylene exposure presented with poorer test results than participants with low cumulative dose of xylene exposure. The results of the present research suggest that xylene is associated with adverse central auditory effects and poorer sound detection abilities in humans. A major limitation of the study is that the results found among xylene-exposed participants cannot be proved to be permanent, and thus further research should be conducted to clarify this limitation. Workers exposed to this chemical should be routinely evaluated with a comprehensive audiological test battery, to detect early signs of auditory dysfunction.

Preparation of a MOF membrane with 3-aminopropyltriethoxysilane as covalent linker for xylene isomers separation

In this study, a continuous-growth Zn2(BDC)2DABCO membrane was synthesized on the NH2-terminated SAMs modified porous SiO2 substrates by using the MOF nanocrystals as seed, which was prepared by an easily scalable method with acetic acid as capping reagent. The seed crystals and membranes were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The thickness of the membranes can be varied by adjusting the repetitions of growing process. The xylene isomer mixture separation performance tests of MOF membrane were taken out.

Pervaporation behavior of PVA membrane containing β‐cyclodextrin for separating xylene isomeric mixtures

To evaluate molecular recognition function of β‐cyclodextrin to xylene isomers, β‐CD polymer of branching chain extension (β‐CD‐EGDE) was synthesized by crosslinking β‐CD with ethylene glycol diglycidyl ether (EGDE). The pervaporation blend membranes of β‐CD‐EGDE/PVA were prepared by casting an aqueous solution of PVA and β‐CD polymer mixture, and the membranes were used for separation of p‐/m‐ and p‐/o‐xylene mixtures. It was observed that the pristine PVA membrane almost had no selectivity for xylene isomer mixtures. The PVA membrane incorporating β‐CD polymer had molecular recognition function, which selectively facilitated the transport of the xylene isomers. To ascertain pervaporation behavior, the sorption and desorption processes of the membrane in xylenes were investigated. The sorption result showed that the complex formation constant between β‐CDs and xylenes played a key role in swelling behavior. There was a significant difference between diffusion coefficients D and D0, calculated from the sorption and desorption measurements, respectively, indicating that the diffusivity selectivity in desorption stage may have remarkable effect on the total selectivity during pervaporation process. © 2012 American Institute of Chemical Engineers AIChE J, 59: 604–612, 2013

Evaluation of photoionization detector performance in photocatalytic studies for removing volatile organic compounds

Aims: The aim of this study was to evaluate the performance of photoionization detector (PID) system as a substitution for gas chromatography in the measurement of a 3 xylene isomer mixtures as a representative of the volatile organic compounds in photocatalytic studies. Materials and Methods: This study has been carried out by using test setup for generating known concentrations from equal ratio of 3 xylene isomers. The concentration values to be evaluated were classified into 4 concentration ranges from 0.1 of threshold limit values (TLV) to 2 of TLV to evaluate the PID system appliance compared with that in the reference method. The test was done 4 times for each evaluation concentration in 3 relative humidity levels (0%, 20%, and 80%). Results: The correlation between the PID results and the National Institue of Occupational Safety and Health (NIOSH) reference method results in an atmosphere with relative humidity of 0%, 20%, and 80% were good and, respectively, were 0.993, 0.992, and 0.991 and total correlation was 0.989. The paired t test indicates a significant difference between actual concentrations in reference method and the extracted concentration from PID. Conclusions: Although the results presented by PID in the present study are different from those extracted from the reference method (from 10 to 260 ppm), the equipment response is linear. So, the results are acceptable in photocatalytic studies in case the contaminant concentration is measured by the same equipment either before or after the reactor for calculation of the removal efficiency. PID calibration with the test material(s) is recommended.

Nanocomposite MFI-alumina hollow fibre membranes prepared via pore-plugging synthesis: Influence of the porous structure of hollow fibres on the gas/vapour separation performance

One major drawback in the synthesis of zeolite membranes is the lack of reproducibility ascribed to the support quality. In this paper, a gas–liquid displacement technique was used prior to pore-plugging hydrothermal synthesis to evaluate the quality of hollow fibre supports in terms of pore size distribution of surface pores. Using these supports, four nanocomposite MFI-hollow fibre membrane families were identified, showing different gas/vapour separation performance. The effective MFI thickness of these membranes (0.6–1.2 μm) was computed from pure N 2 and CO 2 permeance using Maxwell–Stefan modelling, while their separation performance was evaluated in the separation of n-butane/H 2, CO 2/N 2 and ternary xylene isomer mixtures. The MFI-alumina hollow fibres displaying the best quality corresponded to those synthesized using hollow fibres with surface pore size distributions centred at 0.2 μm, suggesting more efficient pore plugging. Grain boundaries between nearby zeolite crystals only appear to play a role in the case of xylene isomer separation.

Pervaporation Separation of Xylene Isomers by Hybrid Membranes of PAAS Filled with Silane-Modified Zeolite

Silicalite-1 zeolite was modified with 3-aminopropyltriethoxysilane (APTES) and then loaded into poly(acrylic acid) sodium (PAAS) to prepare PAAS/silicalite-1 hybrid pervaporation membranes for the separation of xylene isomer mixtures. Characterization by Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopies confirmed that chemical modification on the zeolite surface had taken place. Based on sorption equilibrium experiments, the diffusion coefficients of permeates in the membrane were obtained, and the order of the diffusion coefficients of xylene isomers was found to be Do < Dm < Dp. An investigation of the effects of the original versus modified zeolite loading on the pervaporation performance was performed. With the original zeolite, the permeation flux of the binary xylene mixtures showed little change, whereas the selectivity of the hybrid membrane exhibited some enhancement. After modification of the zeolite surface, the selectivity of the hybrid membrane was clearl...