Xylene Isomers Research Articles

Separation of Xylene Isomers in the Anion-Pillared Square Grid Material SIFSIX-1-Cu.

Xylene isomer separation is considered one of the seven separation challenges that changed the world. In addition, the high-energy demand of xylene separation highlights the need for efficient novel adsorbents. Herein, the liquid-phase separation potential of the anion-pillared hybrid material SIFSIX-1-Cu was studied for preferential adsorption of o-xylene and m-xylene over p-xylene, which was inspired by a previous complexation crystallization method for separating m-xylene. We report detailed experimental liquid-phase adsorption experiments, yielding selectivities of 3.0 for o-xylene versus p-xylene and 2.6 for m-xylene versus p-xylene. Our theoretical calculations thus provide a reasonable explanation that the xylene adsorption selectivity is attributed to the C-H⋅⋅⋅F interaction, and the host-guest interaction order agrees with the adsorption priority: o-xylene > m-xylene > p-xylene.

Photochemically Activated Dimagnesium(I) Compounds: Reagents for the Reduction and Selective C−H Bond Activation of Inert Arenes

AbstractThe photochemical activation of dimagnesium(I) compounds, and subsequent high yielding, regioselective reactions with inert arenes are reported. Irradiating benzene solutions of [{(ArNacnac)Mg}2] (ArNacnac=[HC(MeCNAr)2]−; Ar=2,6‐diisopropylphenyl (Dip) or 2,4,6‐tricyclohexylphenyl (TCHP)) with blue or UV light, leads to double reduction of benzene and formation of the “Birch‐like” cyclohexadienediyl bridged compounds, [{(ArNacnac)Mg}2(μ‐C6H6)]. Irradiation of [{(DipNacnac)Mg}2] in toluene, and each of the three isomers of xylene, promoted completely regio‐ and chemo‐selective C−H bond activations, and formation of [(DipNacnac)Mg(Ar′)] (Ar′=meta‐tolyl; 2,3‐, 3,5‐ or 2,5‐dimethylphenyl), and [{(DipNacnac)Mg(μ‐H)}2]. Fluorobenzene was cleanly defluorinated by photoactivated [{(DipNacnac)Mg}2], leading to biphenyl and [{(DipNacnac)Mg(μ‐F)}2]. Computational studies suggest all reactions proceed via photochemically generated magnesium(I) radicals, which reduce the arene substrate, before C−H or C−F bond activation processes occur.

Multi-objective Optimization of the TEG Dehydration Process for BTEX Emission Mitigation Using Machine-Learning and Metaheuristic Algorithms

Natural gas processing involves the removal of acidic gases, followed by dehydration. The dehydration process is primarily carried out through absorption in triethylene glycol (TEG). During the dehydration process, volatile organic compounds (VOCs) including benzene, toluene, ethylbenzene, and isomers of xylene together known as BTEX present in natural gas are absorbed into the glycol solvent. During the thermal regeneration process of TEG, a substantial amount of BTEX and VOCs are emitted resulting in adverse environmental and health impacts, leading to the need for strict regulations. Effective mitigation of emission is essential for the natural gas industry. There are several process parameters associated with major equipment that may impact BTEX emission. Regulation of some of these parameters is found to have an adverse impact on the dehydration process as well. In this work, a multi-objective optimization is performed in order to find the optimal operating conditions related to the process parameters to mitigate BTEX emission. It is also essential to select the most important parameters from a larger set so that a reliable analysis can be performed using the reduced set of parameters. The analysis is performed using data-driven modeling using machine-learning algorithms, followed by optimization with a probabilistic technique. The least absolute shrinkage and selection operator (lasso) method is used for variable selection; support vector regression (SVR) is used for metamodeling; and a novel metaheuristic optimization algorithm, efficient ant colony optimization (EACO), is used for optimization. The surrogate or metamodel is generated with data obtained from process simulation carried out using ProMax. Using the lasso–SVR–EACO strategy, different optimized sets of operating conditions that minimize the BTEX emission for a stipulated dry gas water content, were obtained. The results show that BTEX released from the dehydration process can be mitigated by optimally choosing the TEG circulation rate, reboiler temperature, stripping gas flow rate, and absorber pressure.

First-principles investigation of dimethyl-functionalized MIL-53(Al) metal–organic framework for adsorption and separation of xylene isomers

Functionalization of organic linker in metal–organic frameworks (MOFs) has been a popular method to utilize the porous material for a wide range of applications. In xylene separation, dimethyl functionalization of MOFs had demonstrated improved adsorptions properties. However, the effect of different xylene isomers on the organic linker is still not fully understood. In this work, the MIL-53(Al) MOF was functionalized with two methyl groups in ortho-, meta- and para-positions. The energy and thermochemical properties of xylene isomers when complexed dimethyl functionalized MIL-53(Al) MOFs were investigated using semiempirical quantum chemical PM6 method. The adsorption model of xylene isomers over meta-dimethyl-MIL-53(Al) (MDM) produced the lowest energy and the most thermochemically stable complex. The electronic properties, natural charge, nuclear magnetic resonance, aromaticity and molecular electrostatic potential of MDM complexes were further characterized using density functional theory (DFT) calculations at B3LYP/6-31G(d) level. DFT calculations revealed that the complex of MDM with m-xylene had the lowest adsorption energy and the highest reactivity, caused by the oxygen atoms surrounding the Al metal ion. In contrast, the complex of MDM with o-xylene had the highest energy of adsorption and the lowest reactivity, mostly governed by weak π interactions. Based on the results, we propose the use of MDM MOF for adsorption and separation of xylene isomers.

Application of gas chromatography separation based on metal-organic framework material as stationary phase

金属有机骨架材料(MOFs)是一类由有机配体和金属离子(或金属簇)自组装形成的新型多功能材料。MOFs具有孔隙度高、比表面积大、孔径可调、化学和热稳定性高等特点,被广泛应用于吸附、分离、催化等多个领域。近年来,MOFs作为新型气相色谱固定相用于分离异构体受到了广泛关注。与传统无机多孔材料相比,MOFs在结构和功能上展现出高度的可调性,通过合理地选择配体和金属中心,可以设计合成具有不同孔道大小和孔道环境的MOFs,从而分别从热力学和动力学角度优化色谱分离效果,有效提高分离选择性。该文结合MOFs的结构,讨论了MOFs气相色谱固定相分离不同类型分析物的分离机理。分离机理主要包括MOFs孔道的分子筛效应或形状选择性,MOFs不饱和的金属位点与分析物中不同的官能团之间产生的相互作用,分析物与MOFs孔道之间产生的不同范德华力、π-π相互作用和氢键相互作用。此外,MOFs的手性分离可能主要依赖于外消旋体与手性MOFs中手性活性位点之间的相互作用。该文也对不同分析目标物进行了归类,综述了多种MOFs气相色谱固定相对烷烃、二甲苯异构体和乙基甲苯、外消旋体、含氧有机物、环境有机污染物的气相色谱分离效果。最后,该文还对MOFs在该领域的应用进行了总结与展望,旨在为MOFs气相色谱高效分离的研究提供参考。

FTIR spectral studies of the binary solutions of acetone with xylene isomers

FTIR spectra of neat acetone (ACT), xylene isomers (o-xylene (OXY), m-xylene (MXY), and p-xylene (PXY)) and their binary solutions at various ACT molar concentrations are recorded in the range of 4000—400 cm–1. Although neat ACT is a mixture of monomers, dimers, and trimers, the number of dimers is larger than that of monomers/trimers, as suggested by a relatively higher intensity of the dimmer ν(C=O) band with respect to that of the monomers/tri­mers. The shifts suffered by some of the fundamental absorption bands of xylene isomers/ACT confirm the presence of (ACT)C=O⋯H(OXY/MXY/PXY aromatic C—H or methyl) and (ACT methyl)H⋯π(OXY/MXY/PXY) H-bonds in all the solutions but with different strengths, as suggested by different magnitudes of the vibrational band shifts. The asymmetric and symmetric stretching bands of either C—H or CH3 of OXY/PXY experience unequal force constants in ACTOXY and ACTPXY binary solutions with a 0.2 mole fraction of ACT. The νas(C—H) doublet due to the intramolecular coupling appears only in ACTOXY and ACTPXY binary solutions.

Mo-Modified ZSM-5 zeolite with intergrowth crystals for high-efficiency catalytic xylene isomerization

Mo/ZSM-5 catalysts of xylene isomerization were prepared on the intergrowth ZSM-5 support by an impregnation–calcination–reduction procedure.

Efficient separation of xylene isomers by using a robust calcium-based metal–organic framework through a synergetic thermodynamically and kinetically controlled mechanism

A microporous metal–organic framework with a robust structure has been developed. The compound exhibited highly efficient separation of xylene isomers which is a result of a synergetic thermodynamic and kinetic mechanism.

Unusual porous crystals via catenation of 1D ladders: unprecedented mixture effects on the adsorption of xylene isomers in a SCSC mode

The pseudo 2D open-channel crystals formed via catenation of 1D ladders show unusual mixture effects on the adsorption of o-, m-, and p-xylene isomers in a SCSC mode, and are a practical template for structural determination of small hydrocarbons.

Systematic Analysis Reveals Thermal Separations Are Not Necessarily Most Energy Intensive

Summary At the industrial level, separation of a variety of mixtures is predominantly carried out by thermally driven processes such as distillation. Nevertheless, the current literature seems to suggest that much is to be gained by replacing these processes with alternative non-thermal methods, which will potentially require a fraction of the energy used by distillation. This is primarily due to the widespread perception that thermal separation processes, particularly those that involve vaporization, are highly energy intensive. However, this belief is not well founded, because it is based on extrapolation from comparisons in the literature, many of which make ad-hoc assumptions and result in incorrect conclusions. Our analysis shows that this confusion arises because the processes utilize different types of energy: heat and electricity. Here, we present a consistent framework that enables a fair comparison between different processes that consume different forms of energies. Using this framework, we refute the general perception that thermal separation processes are inherently the most energy intensive and conclusively show through the examples of two challenging mixtures constituting components with low relative volatilities, that distillation processes, when run properly, can consume remarkably lower fuel than non-thermal membrane alternatives, which have often been touted as more energy efficient.

The Effects of Functional Groups and Missing Linkers on the Adsorption Capacity of Aromatic Hydrocarbons in UiO-66 Thin Films

The adsorption of benzene, toluene, ethylbenzene, and xylene isomers, also known as BTEX, from the gas phase into porous thin films of the metal–organic framework UiO-66-X, where X = H, NH2, and NO2, was measured to quantify adsorption capacity. The thin films were grown by a vapor-conversion method onto Au-coated quartz microbalance crystals. The MOF thin films were characterized by IR and Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The thin films were activated by heating under high vacuum and exposed to each gas to calculate the Henry’s constant. The results demonstrate that the functional groups in the organic linker and missing-linkers both play important roles in the adsorption capacity. Several trends can be observed in the data. First, all the compounds in the BTEX family have lower Henry’s constants in the UiO-66-H films compared to the UiO-66-NH2 and UiO-66-NO2 films, which can largely be attributed to the absence of a functional group on the linker. Second, at 25 °C, the Henry’s constants for all the BTEX compounds in UiO-66-NO2 films are larger than UiO-66-NH2 films. Third, the role of missing linkers is addressed by comparing the measured adsorption capacity to ideal pore filling. The results show that the UiO-66-H films are the most defect-free and the UiO-66-NO2 films have the most missing linker defects.

Sorption behaviour of xylene isomers on biochar from a range of feedstock

Inland oil spillage is one of the widespread sources of crude oil volatile organic compound emissions (CVEs) for which the long-term remedial solutions are often complex and expensive. This paper investigates the potential of a low-cost containment solution for contaminated solids by volatile organic compounds (VOCs) using biochar. The results of an extensive experimental investigation are presented on the sorption kinetics of xylene isomers (one type of the most frequently detected CVEs) on commercial biochar produced by prevalent feedstocks (wheat, corn, rice and rape straw as well as hardwood) at affordable temperatures (300–500°C). Chemical and physical properties of biochar were analysed in terms of elemental composition, scanning electron microscopy, specific surface area, ATR-FTIR spectra and Raman spectrometry. We show that for high-temperature biochar with similar surface chemistry, the sorption efficiency is mainly controlled by porous structure and pore size distribution. Biochar samples with higher specific surface area and higher volume of mesopores showed the highest sorption capacity (45.37–50.88 mg/g) since the sorbate molecules have more access to active sites under a greater intra-particle diffusion and elevated pore-filling. P-xylene showed a slightly higher sorption affinity to biochar compared to other isomers, especially in mesoporous biochar, which can be related to its lower kinetic diameter and simpler molecular shape. The sorption capacity of biochar produced at higher pyrolysis temperatures was found to be more sensitive to changes in ambient temperature due to dominant physical adsorption. Elovich kinetic model was found to be the best model to describe xylenes’ sorption on biochar which indirectly indicates π–π stacking and hydrogen bonding as the main mechanism of xylene sorption on these types of biochar.

Synthesis of highly active Cu(I)-Y(III)-Y zeolite and its selective adsorption desulfurization performance in presence of xylene isomers

A bimetal-exchanged NaY zeolite (Cu(I)-Y(III)-Y) with a desirable adsorptive desulfurization (ADS) performance was prepared and characterized by means of X-ray diffraction, specific surface area measurements, X-ray fluorescence spectrometer, X-ray photoelectron spectroscopy, thermal gravity analysis and Fourier transform infrared spectroscopy. The effect of Y(III) ions on ADS in the presence of the xylenes was investigated. Results indicated that the ADS performance of Y(III)-Y is higher than that of most reported CeY. The Y(III)-based Cu(I)-Y(III)-Y demonstrated the higher breakthrough loading than those of reported Ce(III)/Ce(IV)-based transition metal Y zeolites, showing that Y(III) ions play a promoting role in improving the ADS selectivity. For Cu(I)-Y(III)-Y, a new strong S-M interaction (S stands for sulfur, while M stands for metal) active site was formed, which might be caused by the synergistic effect between Cu(I) and Y(III). The Cu(I)-Y(III)-Y, which combined the advantages of Cu(I)-Y and Y(III)-Y, is a kind of promising adsorbent. The breakthrough loading decreased in the order of Cu(I)-Y(III)-Y > Y(III)-Y > Cu(I)-Y, and the effect of xylene isomers on the sulfur removal was in the order of ortho-xylene > meta-xylene > para-xylene, which exhibited the same trend with the bond order of xylenes.

Chemical Composition of the Essential Oils from the Leaf, Stem-Bark and Twig of Terminalia mantaly H. Perrier (Combretaceae) from Nigeria

The essential oils from the leaf, stem-bark and twig of Terminalia mantaly were isolated by hydrodistillation. The volatile oils analysed on Gas Chromatography (GC) and Gas Chromatography – Mass Spectrometer (GC - MS). The identified constituents were twelve, twenty-three and seventeen for the leaf, stem-bark and twig and accounted for 89.57%, 95.77%, 95.92% respectively. Hexahydrofarnesylacetone (30.05%) and Z-pinane (16.71%) were the main constituents in the leaf oil and nonanal (21.16%) and heptanal (10.57%) were principal component in the stem-bark volatile oil. Xylene isomers namely, meta and para (21.98%-23.56%) were the major components of the twig with substantial amount of nonanal (13.64%).

Efficient separation of xylene isomers by a guest-responsive metal\u2013organic framework with rotational anionic sites

The separation of xylene isomers (para-, meta-, orth-) remains a great challenge in the petrochemical industry due to their similar molecular structure and physical properties. Porous materials with sensitive nanospace and selective binding sites for discriminating the subtle structural difference of isomers are urgently needed. Here, we demonstrate the adaptively molecular discrimination of xylene isomers by employing a NbOF52−-pillared metal–organic framework (NbOFFIVE-bpy-Ni, also referred to as ZU-61) with rotational anionic sites. Single crystal X-ray diffraction studies indicate that ZU-61 with guest-responsive nanospace/sites can adapt the shape of specific isomers through geometric deformation and/or the rotation of fluorine atoms in anionic sites, thereby enabling ZU-61 to effectively differentiate xylene isomers through multiple C–H···F interactions. ZU-61 exhibited both high meta-xylene uptake capacity (3.4 mmol g−1) and meta-xylene/para-xylene separation selectivity (2.9, obtained from breakthrough curves), as well as a favorable separation sequence as confirmed by breakthrough experiments: para-xylene elute first with high-purity (≥99.9%), then meta-xylene, and orth-xylene. Such a remarkable performance of ZU-61 can be attributed to the type anionic binding sites together with its guest-response properties.

Precisely regulating Brønsted acid sites to promote the synthesis of light aromatics via CO2 hydrogenation

Significant progresses have recently been made in the direct conversion of CO2 to aromatics, whereas it remains a big challenge to selectively synthesize light (C6–8) aromatics because of the perplexing role of Brønsted acid sites (BAS) in aromatic synthesis. Herein, a series of composite catalysts comprising Fe-based component and ZSM-5 zeolites with distinct Brønsted acidities were studied to explore the influence of BAS on the light aromatic synthesis and coke formation in CO2 hydrogenation. The results indicated that BAS of ZSM-5 are the main active sites for aromatization, and increasing Brønsted acidity significantly promotes the synthesis of aromatics, especially light aromatics. Further passivation of the external BAS of HZ(25) zeolite by silylation process could inhibit the alkylation of light aromatics and the isomerization of xylene. Consequently, light aromatics account for up to 75 % of aromatics, which is the highest value reported in CO2 hydrogenation, and p-xylene could make up as high as 72 % of xylene. Moreover, a larger density of BAS (>154 μmol g−1), which promote the formation of highly condensed, carbon-rich, and hard-to-oxidize coke, would accelerate the coke formation, degrade their physico-chemical properties, and shorten the catalyst lifetime.

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.

A study on the discrimination of xylene isomers vapors by quartz crystal microbalance sensors

A series of quartz crystal microbalance (QCM) gas sensors were developed for discrimination of the vapor of xylene isomers (ortho-xylene, meta-xylene and para-xylene). The investigated QCM sensors included an Au-bare sensor and those modified by self-assemble monolayer (SAM) formation by 1-butanethiol and 3-methyl-1-butanethiol in the presence of xylene isomers, as reaction solvent. The presence of the investigated thiols made a slight difference on the polarity of the sensors surface, estimated by measuring the contact angles. Although the modified sensors with 1-butanethiol were selective toward ortho-xylene vapors as that presented by Au-bare sensor, the sensitivity of the 1-butanethiol-modified sensors was significantly improved with respect to that demonstrated by the unmodified sensor. The sensors modified with 3-methyl-1-butanethiol presented different demeanor for the xylene isomers vapor. This attitude varied with the solvent used through SAM formation process. It was suggested that the observed attitude of the sensors is a reflection of the solvent effect on the structure and decoration of the formed SAM on the sensor surface and the polarity of the sensors’ surface. The results allowed suggesting the fabrication of sensitive sensor arrays for the discrimination of xylene isomers vapors.

Arene C−H Activation at Aluminium(I): meta Selectivity Driven by the Electronics of SNAr Chemistry

AbstractThe reactivity of the electron‐rich anionic AlI aluminyl compound K2[(NON)Al]2 (NON=4,5‐bis(2,6‐diisopropylanilido)‐2,7‐di‐tert‐butyl‐9,9‐dimethylxanthene) towards mono‐ and disubstituted arenes is reported. C−H activation chemistry with n‐butylbenzene gives exclusively the product of activation at the arene meta position. Mechanistically, this transformation proceeds in a single step via a concerted Meisenheimer‐type transition state. Selectivity is therefore based on similar electronic factors to classical SNAr chemistry, which implies the destabilisation of transition states featuring electron‐donating groups in either ortho or para positions. In the cases of toluene and the three isomers of xylene, benzylic C−H activation is also possible, with the product(s) formed reflecting the feasibility (or otherwise) of competing arene C−H activation at a site which is neither ortho nor para to a methyl substituent.

Arene C-H Activation at Aluminium(I): meta Selectivity Driven by the Electronics of SN Ar Chemistry.

The reactivity of the electron‐rich anionic AlI aluminyl compound K2[(NON)Al]2 (NON=4,5‐bis(2,6‐diisopropylanilido)‐2,7‐di‐tert‐butyl‐9,9‐dimethylxanthene) towards mono‐ and disubstituted arenes is reported. C−H activation chemistry with n‐butylbenzene gives exclusively the product of activation at the arene meta position. Mechanistically, this transformation proceeds in a single step via a concerted Meisenheimer‐type transition state. Selectivity is therefore based on similar electronic factors to classical SNAr chemistry, which implies the destabilisation of transition states featuring electron‐donating groups in either ortho or para positions. In the cases of toluene and the three isomers of xylene, benzylic C−H activation is also possible, with the product(s) formed reflecting the feasibility (or otherwise) of competing arene C−H activation at a site which is neither ortho nor para to a methyl substituent.