Xylene Molecules Research Articles

Highly Efficient Separation of Intermediate-Size m-Xylene from Xylenes via a Length-Matched Metal-Organic Framework with Optimal Oxygen Sites Distribution.

Xylene separation is crucial but challenging, especially for the preferential separation of the intermediate-size m-xylene from xylene mixtures. Herein, exploiting the differences in molecular length and alkyl distribution among xylenes, we present a length-matched metal-organic framework, formulated as Al(OH)[O2C-C4H2O-CO2], featuring an effective pore size corresponding to m-xylene molecular length combined with multiple negative O hydrogen bond donors distribution, can serve as a molecular trap for efficient preferential separation of the intermediate-size m-xylene. Benchmark separation performance was achieved for separating m-xylene from a ternary mixture of m-xylene/o-xylene/p-xylene, with simultaneous record-high m-xylene uptake (1.3 mmol g-1) and m-xylene/p-xylene selectivity (5.3) in the liquid-phase competitive adsorption. Both vapor- and liquid-phase fixed-bed tests confirmed its practical separation capability with benchmark dynamic m-xylene/p-xylene and m-xylene/o-xylene selectivities, as well as excellent regenerability. The selective and strong m-xylene binding affinity among xylene molecules was further elucidated by simulations, validating the effectiveness of such a pore environment for the separation of intermediate-size molecules.

Highly Efficient Separation of Intermediate‐Size m‐Xylene from Xylenes via a Length‐Matched Metal–Organic Framework with Optimal Oxygen Sites Distribution

AbstractXylene separation is crucial but challenging, especially for the preferential separation of the intermediate‐size m‐xylene from xylene mixtures. Herein, exploiting the differences in molecular length and alkyl distribution among xylenes, we present a length‐matched metal–organic framework, formulated as Al(OH)[O2C−C4H2O−CO2], featuring an effective pore size corresponding to m‐xylene molecular length combined with multiple negative O hydrogen bond donors distribution, can serve as a molecular trap for efficient preferential separation of the intermediate‐size m‐xylene. Benchmark separation performance was achieved for separating m‐xylene from a ternary mixture of m‐xylene/o‐xylene/p‐xylene, with simultaneous record‐high m‐xylene uptake (1.3 mmol g−1) and m‐xylene/p‐xylene selectivity (5.3) in the liquid‐phase competitive adsorption. Both vapor‐ and liquid‐phase fixed‐bed tests confirmed its practical separation capability with benchmark dynamic m‐xylene/p‐xylene and m‐xylene/o‐xylene selectivities, as well as excellent regenerability. The selective and strong m‐xylene binding affinity among xylene molecules was further elucidated by simulations, validating the effectiveness of such a pore environment for the separation of intermediate‐size molecules.

Effect of pore size distribution of biomass activated carbon adsorbents on the adsorption capacity

AbstractBACKGROUNDIn order to investigate the correlation between the pore size distribution of biomass activated carbon adsorbents (BACAs) and VOCs (volatile organic compounds) adsorption/desorption performance. Four BACAs with same specific surface area but different pore size distribution were prepared under different experimental conditions and processes.RESULTSThe impact of the pore size distribution of BACAs on the adsorption/desorption performance of benzene, toluene and xylene was investigated. The results indicated that the adsorption ability of the prepared BACAs for benzene, toluene and xylene was mostly affected by the pore sizes distributed in the 2.60 ~ 3.25, 2.68 ~ 3.35and 4.20 ~ 4.90 nm ranges, respectively, when the studied BACAs had similar specific surface area (SBET ≈ 1080 m2 g−1). However, the desorption amount of adsorbed benzene molecules mainly relied on the pore structures of BACAs having pore sizes in the 3.95 ~ 4.60 nm range.CONCLUSIONThe pore structures of BACAs distributed in different pore size ranges have various effects on the phenyl VOCs adsorption capacity. Benzene adsorption on the BACAs was mainly affected by the microporous structures. The pore structure with larger pore size was more favorable for the desorption of the adsorbed toluene and xylene molecules compared to the adsorbed benzene molecules. Benzene, toluene and xylene had low residual rates in the studied activated carbon adsorbents attesting to their superior regenerative properties. This work could provide an important reference for the design, preparation, and selection of activated carbon adsorbents for the adsorption capacity of benzene, toluene and xylene. © 2024 Society of Chemical Industry (SCI).

Surface-Modified Wrinkled Mesoporous Nanosilica as an Effective Adsorbent for Benzene, Toluene, Ethylbenzene, and Xylene in Indoor Air

Surfactant-extracted spherical porous silica nanoparticles with wrinkled structures were synthesized, and their adsorption performance was altered by grafting three organosilanes: n-octyltriethoxysilane, hexadecyltrimethoxysilane, and triethoxyphenylsilane onto their surface. The surface-modified silica nanoparticles were used to capture frequently detected hazardous indoor air chemicals. The physical and chemical properties of the samples were characterized using thermogravimetric analysis, Fourier transform infrared spectroscopy, N2 adsorption–desorption experiments, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy. Although the organosilane surface modification did not significantly change the surface areas and pore structures of porous silica nanoparticles, the capacities of the surface-modified porous silica nanoparticles for capturing benzene, toluene, ethylbenzene, and xylene (BTEX) molecules from air were considerably higher than those of pristine porous silica nanoparticles. The dispersion forces between adsorbates and adsorbents were the primary factor that affected the absorption capacity of the surface-modified porous silica nanoparticles. Consequently, the BTEX capturing potential of surface-modified mesoporous silica featuring a long alkyl chain was high because of the relatively high dispersion force between adsorbates and the adsorbent.

Reasonable construction of 2D porous NiO/Co3O4 nanosheets for efficient detection of xylene

P-type semiconductor materials with multivalent characteristics and high catalytic activity have attracted considerable interest for efficient detection of low reactive gases. In this paper, the NiO/Co3O4 nanocomposites with 2D porous sheet-like structure are synthesized through a facile hydrothermal route and following annealing treatment. The gas sensing test shows that the 6 wt% NiO/Co3O4 based sensor exhibits highest response (S = 12.27) to 100 ppm xylene at a relatively low temperature (140 ℃), which is about 3.74 times higher than that of Co3O4. Furthermore, the sensor also manifests a good selectivity and long-term stability. The enhanced sensing performance is attributed to the high specific surface area and the formation of p-p heterojuntions. The charge density difference images directly show that the electrons are mainly transferred from the interface of the NiO/Co3O4 heterostructure to the hydrogen ions in the xylene molecule, and the charge density near the adsorption region is significantly larger than that of the pure Co3O4 model. The research results further clarified the response mechanism of xylene.

Novel CoMn2O4 as a highly efficient catalyst for the oxidation of o-, m-, p-xylene: Preparation and kinetic study

In this research, a novel CoMn2O4 catalyst was prepared for the removal of o-xylene, m-xylene, and p-xylene. The catalyst showed better activity than that of MnO2 and Co3O4. The better activity of CoMn2O4 was attributed to the abundant concentration of Co2+, Mn3+, and lattice oxygen. Kinetic studies were carried out for the catalytic oxidation of three xylenes over the CoMn2O4 catalyst based on the Power-Low model, the Mars-van Krevelen model, the Eley-Rideal model, and the Langmuir-Hinshelwood model. All the three reactions could be most accurately described by the Mars-van Krevelen model, which suggested that the isomers of xylene had very little effect on the mechanism of xylene oxidation over the CoMn2O4 catalyst. The activation energies of the oxidation processes of o-xylene, m-xylene, and p-xylene were 94.64 kJ/mol, 78.46 kJ/mol, and 57.13 kJ/mol, respectively. The proposed mechanism based on the Mars-van Krevelen model suggested that xylene molecules reacted with the Co3+ and Mn4+/Mn3+ on the CoMn2O4 catalyst and participated in the cycle of adsorption, redox, and desorption.

ToTEM: A software for fast TEM image simulation.

ToTEM, a multislice-based image simulation software is developed for transmission electron microscope (TEM). This software implements the following major features: (i) capability of assigning three-dimensional potentials of atom into multiple slices and precise introduction of phase shift caused by the sub-pixel atomic position, (ii) employing CUDA coding and graphical processing units (GPU) with multithreading parallel algorithm based on the powerful batch (inverse) fast Fourier transform (FFT), which is especially beneficial for image simulation of scanning transmission electron microscopy (STEM) or (integrated) differential phase contrast (I)DPC, (iii) design for efficiently generating large batch of data set of high-resolution transmission electron microscopy (HRTEM) images. Image simulation acceleration for STEM has been verified by simulating a large-scale SrTiO3 . Additionally, iDPC image of MFI-type zeolites with xylene molecules encapsulated in straight channels demonstrates the advantage of iDPC in detecting light molecules.

Self-seeded growth of very large open-structured zeolite nanosheet assemblies with extraordinary micropore accessibility

Zeolite nanosheets (ZNs) offer improved micropore accessibilities and transport properties for enhanced molecular catalysis and separations. However, practical application of the ZN materials is hampered by the lack of efficient synthesis methods. Here, a ZN self-seeded method is demonstrated for single-step reproduction of flower-like assemblies of very large MFI ZN plates. The ZN plates are ∼60 nm-thick stacks of 4-nm-thick single-crystal sheets. The ZN flower growth involves terrace nucleation on the seed surfaces and subsequent ZN epitaxial growth in [010] orientation directed by a diquaternary agent. The open architecture of the assemblies prevented collapse and agglomeration of ZNs during thermal activation that effectively preserved the interconnected intra-sheet and inter-sheet micropore system. Thus, the ZN assemblies exhibited markedly enhanced molecular adsorption capacity and transport diffusivity for the probing xylene molecules as compared to the conventional crystals. The ZN assembly and its harvestable very large-sized ZNs have the potential for developing high-performance ZN adsorbents, catalysts, and molecular-sieve membranes. A facile self-seeded growth method reproduces zeolite nanosheet assemblies allowing nondestructive activation for high-performance adsorbents, catalysts, and molecular sieve membranes. • Established a new self-seeded method for reproducing MFI zeolite nanosheet (ZN) assemblies. • Revealed mechanisms of orthogonal ZN growth by defect-induced nucleation and epitaxial growth directed by diquaternary agent. • Demonstrated nondestructive activation of ZNs in the flower-like structure to preserve extraordinary micropore accessibility. • Characterized the interconnected multiscale micropore system that provides exceptional adsorption and transport properties.

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.

Molecular dynamics simulation of weak bonds in carbon fiber reinforced plastic adhesive joints

In carbon fiber-reinforced plastic (CFRP) adhesive joints, poor adhesion because of weak bonds may occur due to contamination at the bond line. We reproduced weak bonds by using a molecular dynamics simulation (MD) approach to investigate the mechanisms of weak bonds. Weak-bond models were created by inserting water (H2O), xylene (C8H10), and silicone molecules as contaminants in the two epoxy models. Tensile analysis was performed to evaluate the mechanical properties of the weak-bond models. The results revealed that the insertion of contaminants reduced the strength of these models. Stress concentration occurred due to the concentration of inserted molecules at the bond line. In addition, the yielding behavior was slower in the H2O model than in the C8H10 and silicone models, and the strength of the H2O model decreased significantly even at a small insertion rate compared with the C8H10 and silicone models. The reason for the difference in yielding behavior may be due to the high diffusion of H2O molecules into the whole epoxy resin. The weak-bond model was reproduced using MD, and the mechanisms of weak bonds were clarified by an analysis of van der Waals potential energy.

Molecular interaction studies of isopropyl acetate-xylene mixture using dielectric relaxation approach

Dielectric relaxation parameters of Isopropyl acetate (IPA)-xylene mixtures with different concentrations and temperatures have been measured in the frequency range of 10 MHz to 30 GHz using time domain reflectometry technique. Static dielectric constant, relaxation time, excess permittivity, excess relaxation time, Bruggeman factor, Kirkwood correlation factor and thermodynamic parameters have been determined to understand molecular association between the IPA and xylene molecules. For entire concentrations of IPA-xylene mixture the Kirkwood correlation factor is less than one which shows the antiparallel nature of electric dipole orientation. The experimental values of dielectric constant obtained from time domain reflectometry are in well agreement with theoretical values of dielectric constant obtained by Luzar model. Positive values of enthalpy and entropy indicating that the system is endothermic and less ordered while Gibbs free energy decreases with increase of IPA in xylene.

A Polymorphic Azobenzene Cage for Energy‐Efficient and Highly Selective p‐Xylene Separation

AbstractDeveloping the competence of molecular sorbents for energy‐saving applications, such as C8 separations, requires efficient, stable, scalable, and easily recyclable materials that can readily transition to commercial implementation. Herein, we report an azobenzene‐based cage for the selective separation of p‐xylene isomer across a range of C8 isomers in both vapor and liquid states with selectivity that is higher than the reported all‐organic sorbents. The crystal structure shows non‐porous cages that are separated by p‐xylene molecules through selective CH–π interactions between the azo bonds and the methyl hydrogen atoms of the xylene molecules. This cage is stable in solution and can be regenerated directly under vacuum to be used in multiple cycles. We envisage that this work will promote the investigation of the azo bond as well as guest‐induced crystal‐to‐crystal phase transition in non‐porous organic solids for energy‐intensive separations.

A Polymorphic Azobenzene Cage for Energy-Efficient and Highly Selective p-Xylene Separation.

Developing the competence of molecular sorbents for energy-saving applications, such as C8 separations, requires efficient, stable, scalable, and easily recyclable materials that can readily transition to commercial implementation. Herein, we report an azobenzene-based cage for the selective separation of p-xylene isomer across a range of C8 isomers in both vapor and liquid states with selectivity that is higher than the reported all-organic sorbents. The crystal structure shows non-porous cages that are separated by p-xylene molecules through selective CH-π interactions between the azo bonds and the methyl hydrogen atoms of the xylene molecules. This cage is stable in solution and can be regenerated directly under vacuum to be used in multiple cycles. We envisage that this work will promote the investigation of the azo bond as well as guest-induced crystal-to-crystal phase transition in non-porous organic solids for energy-intensive separations.

Long-lasting direct capture of xylene from air using covalent-triazine frameworks through multiple C-H…π weak interactions

Xylene molecules are a class of volatile organic compounds (VOCs) in the air that are harmful to human health. It is difficult to selectively capture xylenes from the air using existing porous sorbents, due to unspecific and insufficient sorbent/sorbate interactions. We herein report a series of covalent-triazine frameworks (CTFs) that enable selectively capturing of xylene compounds with high adsorption capacities at low pressure. The best-performing CTF material (CTF-2-400), which is comprised of biphenyl and triazine motifs, can adsorb a remarkable amount of o-xylene (358 mg g−1), m-xylene (392 mg g−1) and p-xylene (523 mg g−1) at 25 °C, outperforming most of emerging porous adsorbents. Its ability to capture low-concentration xylenes from the air has been demonstrated using a column breakthrough measurement. The column packed with CTF-2-400 (1 g) continuously captured xylene from the flowing air (~1300 ppm; 10 mL min−1) for 192 h; by contrast, the commercial activated carbon lost its xylene capture capability after only 35 h under the same conditions. We find that the xylene adsorption capacities of the evaluated CTF adsorbents are unrelated to their pore widths or surface areas, and infer that the multiple C-H…π interactions between the CTF framework and xylene molecules account for the observed excellent xylene capture performance.

From Quorum Sensing to Positional Sensing

With Quorum Sensing (QS) bacteria adapt their physiology and metabolism when they reach a certain density and activate genes relevant for their nutrient metabolism, survival and virulence. Genes controlled by QS have relevance for the pathogenic behavior of bacteria including adherence, secretion, toxin production, immune subversion, biofilm formation and resistance to both antimicrobial and biological/chemical stressors. Most studies assess QS by growing bacteria to various densities in liquid culture. Here, we report that when bacteria are grown on agar plates at various densities, in addition to QS, positional sensing (PS) comes into play. Genes controlled by PS can be studied by cloning their promoter in front of an autonomous bioluminescent cassette (luxCDABE) and measuring their time‐ and position‐dependent expression in biofilms by using an in vivo imaging system (IVIS). The Pseudomonas putida XylR/XylS intergenic region controlling genes relevant for the metabolism and degradation of xylene and related organic molecules was cloned in front of the autonomous luciferase cassette from Photorhabdus luminescence, with the sigma 54 controlled Ps1 promoter regulating the a luxCDABE genes and the Pr promoter regulating the XylR gene. Interestingly, when E. coli carrying this construct were grown to various density on agar plates, bioluminescence was not only expressed in a density‐ and time‐dependent, but also position‐dependent manner. Such positional cues may be mediated by signaling molecules in response to cell density, growth stage, and nutrient availability. Overexpression of the sigma factor 54 (NtrA) from Pseudomonas putida modified the density‐ and time‐dependent response suggesting a regulatory role in QS and PS. The use of bioluminescence in QS and PS promises to reveal important regulatory circuits and communication systems relevant for bacterial normal physiology, pathogenicity and virulence.

Dynamics of xylene isomers in MIL-53 (Al) MOF probed by solid state 2H NMR

Abstract With regard to understanding the effect of the dynamics of xylenes confined in the MOFs pores on the origin of xylene isomers separation, the mobility of para and ortho isomers of xylene in MIL-53(Al) MOF has been studied by solid-state 2H NMR. It is inferred that both isomers perform rotational and librational motions of the aromatic ring plane. The geometry, rates and energy barriers of the detected rotational modes for the molecules confined in the channels have been determined. The rotation around the axis perpendicular to the plane of aromatic ring is obliged with pairwise arrangement of xylene molecules along 1D channels of the MOF. The evolution of motional patterns with temperature shows that the pairwise arrangement of ortho isomer in the channels is held till 300 K, whereas the pairs are stable up to 405 K for the para isomer. This was rationalized by stronger interaction of o-xylene with the channel walls, while π–π intermolecular interaction is stronger for p-xylene. Due to stronger interaction between the unpaired molecules and MOF walls o-xylene exhibits notably higher temperature of the molecules desorption from the channels (494 K). In contrast, the p-xylene is capable to leave the channels of MIL-53 already at 405 K, moving in the channels predominantly in pairs. The different motional behavior of xylene isomers provides the conditions for a faster translational mobility along the MOF channel of para compared to the ortho isomer, thus accounting for the molecular mechanism behind the separation selectivity for xylene isomers.

Liquid-Phase Xylene Adsorption in Unary, Binary, and Ternary Solute Systems Using Raw and Ni2+ Ion-Exchanged Clinoptilolite: Experimental Study and Thermodynamic Assessment

AbstractThe adsorptive behavior of clinoptilolite (Cpt) zeolite in its raw and nickel-exchanged form towards m-xylene/p-xylene/ethylbenzene unary, binary, and ternary mixtures was investigated. The motivation behind the research was to elucidate whether Cpt in its raw or ion-exchanged form could exhibit distinctive selective adsorption behavior. The natural Cpt (Si/Al atomic ratio = 4.59) was ion-exchanged twice with 0.5 M Ni(NO3)2 solution at 80°C for 6 h. Adsorption experiments were done at 40°C, using a 4 vol.% solution of the aromatic materials in iso-octane. The maximum amount of the total specific adsorption capacity for binary solute systems was ~0.8 and ~2.0 mmol g–1 for the raw and ion-exchanged Cpt, respectively. For the ternary solute systems, unexpectedly, this capacity increased to ~2.0 and ~3.0 mmol g–1, respectively. For binary mixtures, both forms of Cpt were selective for p-xylene. For ternary mixtures, both forms exhibited a clear selectivity for m-xylene but the Ni-exchanged Cpt was significantly higher over the concentration range studied. The unexpected increase in the adsorption capacity in ternary systems was attributed to the expulsion of tightly bound trace water molecules hindering the access of xylene molecules to the 10-member ring channels of the zeolite framework. Substitution of Mg2+ by Ni2+ in the 10-member ring channels enhanced the adsorption capacity by providing more space and stronger electrostatic interactions between the new cation and the polar m-xylene molecule.

Self-Amplified Fluorescent Nanoparticles for Rapid and Visual Detection of Xylene in Aqueous Media.

Pollutant detection is of great importance for quality control of drinking water and environmental protection. The common methods of pollutant detection suffer from time-consuming procedures, bulky and expensive instruments, and complicated sample pretreatment. Herein, a type of conceptually new self-amplified fluorescent nanoparticle (SAFN) is constructed based on aggregation-induced emission (AIE) luminogens for rapid and visual detection of xylene in aqueous media. AIE luminogens are self-assembled into SAFNs in aqueous media, which emit efficiently due to the aggregation of luminogen molecules. The SAFNs of AIE luminogens stick xylene molecules from aqueous media through multiple interactions including hydrophobic and π-π interactions. Upon capturing xylene, SAFNs swell, which quench the fluorescence of the whole SAFNs, showing the self-amplification effect. Such a self-amplification effect is entirely different from that of conjugated polymers in the literature. Importantly, fluorescence quenching of SAFNs by xylene can be readily observed by the naked eye, which enables visual xylene sensing. The SAFNs enable rapid and visual detection of xylene in aqueous media with a low detection limit (5 μg/L) in the order of seconds. Given high sensitivity, rapid response, simple and easy operation, and low cost, SAFNs of AIE luminogens present a promising platform for visual detection of organic pollutants in aqueous media.

Facile synthesis of Ag-CuO/SBA-15 for aerobic epoxidation of olefins with high activity

An efficient ‘structure-property’ catalyst Ag-CuO/SBA-15 for aerobic epoxidation of olefins was designed and prepared via a simple grinding method, which provided a dynamic assembly driving force to push the precursors into the channels step by step. Partial melting on the precursor particles surface is necessary for synthesizing the Ag-CuO/SBA-15 catalysts. During the thermal grinding process, the partial melting Cu(NO3)2 first adhered on AgNO3 particles surface because the N of AgNO3 possess stronger interaction with Cu(NO3)2 than OH- of SBA-15, then the mixture of melting precursors on AgNO3 particles surface were sucked into the channels of SBA-15 step by step. Finally, high dispersion of both Ag and CuO nanoparticles were formed in the SBA-15 channels after calcination. The interaction between Ag and CuO, which was characterized by high-resolution transmission electron microscopy, UV–vis and x-ray powder diffraction, enhances the catalytic activity of the catalysts, giving high conversion and selectivity to the epoxides. The reaction mechanism is as follows, the solvent xylene molecule was first oxidized on the Ag surface to form methyl-benzaldehyde (R1), and then moved to the CuO surface to form the peroxidic radical (R2), the peroxidic radical attacked the trans-stilbene C=C bonds on the CuO surface to obtain the epoxides. This synergistic effect shows high activity toward the epoxidation of trans-stilbene.

Density Functional Study on Benzene, Toluene, Ethylbenzene and Xylene Adsorptions on ZnO(100) Surface

We study the interaction between benzene, toluene, ethylbenzene and xylene (BTEX) molecules with ZnO(100) surface by means of density functional theory-based calculations. We find that these interactions result in the physical adsorptions of BTEX gases with adsorption distances larger than 2 Å. These adsorptions are governed by the van der Waals interaction instead of the covalent interaction. We also find that the trend of the strength of BTX adsorptions on ZnO(100) surface is in line with the experimental trend of sensitivity of ZnO material towards BTX gases (benzene < tolune < xylene). We explain this relation by using one of the sensing mechanism within the ionosorption model. By using this relation, we also predict that the response of ZnO towards ethylbenzene will be similar to the response towards toluene since these two molecules have similar adsorption energies on ZnO(100) surface.