Xylene Isomers Research Articles

Experimental and theoretical study of the influence of solvent on asphaltene-aggregates thermo-oxidation through high-pressure thermogravimetric analysis

This study is focused on the understanding of n-C7 asphaltene-aggregates oxidative behavior under high-pressure conditions. For this aim, different asphaltene model solutions were prepared using three asphaltene concentrations (0.1, 1.0, and 10 wt%) on three xylene isomers. The experimental results revealed that the asphaltene aggregate size increase in the order o-xylene < m-xylene < p-xylene. Also, molecular dynamics (MD) simulations were carried out to explain the aggregation differences. It was found that the –CH3 location in the xylene has a substantial impact on the aggregation state. The o-xylene molecules surrounding the asphaltene aggregates oriented them –CH3 groups towards the aggregates, increasing their interaction energy and reducing the aggregation size. Further, the oxidative experiments of the aggregates showed that the oxygen chemisorption decreased as the asphaltene aggregate size increased, reducing their reactivity. Contrasting the xylene isomers, for concentrations >1.0 wt%, the mass gained by oxygen chemisorption increased in the order: p-xylene < m-xylene < o-xylene. This study improves our understanding of the relationship between the aggregation and thermal oxidation of crude oils, characterizing the physical behavior to propose alternatives that reduce asphaltene aggregation and improve the oxidative kinetic.

A Porous Polyaromatic Solid for Vapor Adsorption of Xylene with High Efficiency, Selectivity, and Reusability.

Development of porous materials capable of capturing volatile organic compounds (VOCs), such as benzene and its derivatives, with high efficiency, selectivity, and reusability is highly demanded. Here we report unusual vapor adsorption behavior toward VOCs by a new porous solid, composed of a polyaromatic capsule bearing a spherical nanocavity with subnano-sized windows. Without prior crystallization and high-temperature vacuum drying, the porous polyaromatic solid exhibits the following five features: vapor adsorption of benzene over cyclohexane with 90 % selectivity, high affinity toward o-xylene over benzene and toluene with >80 % selectivity, ortho-selective adsorption ability (>50 %) from mixed xylene isomers, tight VOCs storage even under high temperature and vacuum conditions, and at least 5 times reusability for xylene adsorption. The observed adsorption abilities are accomplished at ambient temperature and pressure within 1 h, which has not been demonstrated by organic/inorganic porous materials reported previously.

Kinetic modeling and thermodynamic analysis of toluene disproportionation reaction over ZSM-5 based catalysts

The aim of this work was to carry out an experimental study along with kinetic and thermodynamic analysis for the toluene disproportionation reaction under actual industrial operating conditions. The toluene disproportionation reaction was carried out under pressurized hydrogen and over three different ZSM-5 based catalysts, namely, ZSM-5, 3%Mo on ZSM-5, and 3%Mo on ZSM-5/Mordenite. The influence of reaction temperature and catalyst composition on conversion and product distribution was investigated. A network of reversible reactions and elementary rate laws reflecting the product distribution, including xylene isomers and other products, were proposed and validated using the obtained experimental data. The thermodynamic parameters were estimated using a chemical process simulation package (Aspen HYSYS), while numerical and statistical methods were used to estimate the kinetic parameters. A notable agreement between the model predictions and the experimental data was obtained. The Mo-impregnated, mordenite-containing catalyst (3%Mo on ZSM-5/Mordenite) was found to be the most active catalyst, presumably due to the additional metallic-active sites that were readily accessible through the mordenite's large pores compared to the plain ZSM-5 catalyst. In addition, the most active catalyst was found to have the lowest activation energies and lead to conversions that were limited thermodynamically.

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.

Microfluidic-assisted toxicity studies of jet fuels on environmental microorganisms – Towards new lab-on-a-chip sensing applications

In this paper, a lab-on-a-chip (LOC) for cellular studies with software-enhanced metrological parametrisation of toxin-to-cell effects on-chip is shown. A special design of LOC is proposed herein whose performance prior to use was confirmed utilising numerical simulations based on a laminar exhaust microflow module. Model microorganisms – Euglena gracilis – were used as a bioindicator to evaluate the toxicity of jet fuels, i.e. Jet A-1 and Jet A-1 + HEFA (Hydroprocessed Esters and Fatty Acids). Apart from the biological results that show notable cell degradation for both of the fuels after 20 min of exhaust exposure (e.g. mobility decrease by 80 % and 100 % for Jet A-1 and Jet A-1 + HEFA), interesting outcomes were also obtained in an exhaust gas chromatography analysis, being in strong correlation with the observed euglena colony behaviour. The major focus was put herein on the concentration of VOCs (Volatile Organic Compounds) and BTEX (Benzene, Toluene, Ethylbenzene and Xylene isomers). Although the sum of volatile hydrocarbon compounds in the exhaust gas decreased due to the addition of HEFA-type hydrocarbons to the fuel, the concentration of the most toxic, BTEX group, was increased about two times compared to the standard Jet A-1. On that basis, our preliminary tests show that further research in this regard is required, in the context of both the toxin-to-cell impact and its direct correlation with the fuel composition.

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.

C-H Activation of Inert Arenes using a Photochemically Activated Guanidinato-Magnesium(I) Compound.

UV irradiation of solutions of a guanidinate coordinated dimagnesium(I) compound, [{(Priso)Mg}2 ] 3 (Priso=[(DipN)2 CNPri 2 ]- , Dip=2,6-diisopropylphenyl), in either benzene, toluene, the three isomers of xylene, or mesitylene, leads to facile activation of an aromatic C-H bond of the solvent in all cases, and formation of aryl/hydride bridged magnesium(II) products, [{(Priso)Mg}2 (μ-H)(μ-Ar)] 4-9. In contrast to similar reactions reported for β-diketiminate coordinated counterparts of 3, these C-H activations proceed with little regioselectivity, though they are considerably faster. Reaction of 3 with an excess of the pyridine, p-NC5 H4 But (pyBut ), gave [(Priso)Mg(pyBut H)(pyBut )2 ] 10, presumably via reduction of the pyridine to yield a radical intermediate, [(Priso)Mg(pyBut ⋅)(pyBut )2 ] 11, which then abstracts a proton from the reaction solvent or a reactant. DFT calculations suggest two possible pathways to the observed arene C-H activations. One of these involves photochemical cleavage of the Mg-Mg bond of 3, generating magnesium(I) doublet radicals, (Priso)Mg⋅. These then doubly reduce the arene substrate to give "Birch-like" products, which subsequently rearrange via C-H activation of the arene. Circumstantial evidence for the photochemical generation of transient magnesium radical species includes the fact that irradiation of a cyclohexane solution of 3 leads to an intramolecular aliphatic C-H activation process and formation of an alkyl-bridged magnesium(II) species, [{Mg(μ-Priso-H )}2 ] 12. Furthermore, irradiation of a 1 : 1 mixture of 3 and the β-diketiminato dimagnesium(I) compound, [{(Dip Nacnac)Mg}2 ] (Dip Nacnac=[HC(MeCNDip)2 ]- ), effects a "scrambling" reaction, and the near quantitative formation of an unsymmetrical dimagnesium(I) compound, [(Priso)Mg-Mg(Dip Nacnac)] 13. Finally, the EPR spectrum (77 K) of a glassed solution of UV irradiated 3 is dominated by a broad featureless signal, indicating the presence of a doublet radical species.

Photodissociation dynamics of xylene isomers C6H4(CH3)2 at 157 nm using an ultracompact velocity map imaging spectrometer – The C7H7 channel

We investigated the photodissociation dynamics of three xylene isomers C6H4(CH3)2 at 157 nm. The center-of-mass translational energy distributions of C7H7 radicals were found to peak at 26 kJ mol−1. Although the ionization energy of the C7H7 tolyl fragment exceeds the energy of a 157 nm photon, C7H7+ was observed as a result of the photoionization of vibrationally ‘hot’ tolyl (C7H7) radicals and/or two-photon ionization. The formation of rovibrationally excited tolyl fragments was discussed. Our experiments suggest the presence of tolyl radicals in the interstellar medium as a precursor to methylated polycyclic aromatic hydrocarbons upon reaction with vinylacetylene (C4H4).

Water-Soluble Self-Assembled Cage with Triangular Metal-Metal-Bonded Units Enabling the Sequential Selective Separation of Alkanes and Isomeric Molecules.

The selective separation of structurally similar aliphatic/aromatic hydrocarbons is an essential goal in industrial processes. In this study, we report the synthesis of a water-soluble (Tr2M3)4L4 (Tr = cycloheptatrienyl ring; M = metal; L = organosulfur ligand) molecular cage (1) via self-assembly of the water-soluble acceptor tripalladium sandwich species [(Tr2Pd3)(CH3CN)][NO3]2 and the attachment onto L of solubilizing methoxyethoxy appendants to be utilized in an energy-friendly alternative approach to the separation of structurally similar molecules under ambient conditions. Cage 1, comprising a hydrophobic inner cavity, exhibited good solubility and stability in aqueous media. It also demonstrated excellent performance in the sequential separation of alkanes (C6-C9), xylene, and other disubstituted benzene isomers and cis/trans-decalin.

Efficient adsorption separation of xylene isomers on Cu-BTC@Fe3O4 by appropriate activation methods

Metal-organic frameworks (MOFs) are a class of nanoporous materials. The development of efficient and environmentally friendly methods for activating MOFs is of great significance for its industrial application. In this work, nanoscale magnetic core-shell Cu-BTC@Fe3O4 was prepared through a secondary growth strategy. The effects of different activation solvents (dichloromethane and ethanol, respectively) and temperatures on the surface area, pore structures and the adsorption of xylene isomers on Cu-BTC@Fe3O4 were investigated. Ethanol as an activation solvent could more easily displace the precursors remaining in the channels of Cu-BTC@Fe3O4 than dichloromethane, while the temperature change has no significant influence on the pore structure. The adsorption separation of xylene isomers on Cu-BTC@Fe3O4 composite materials was studied using the batch vapor-phase adsorption, and liquid-phase breakthrough experiments. The activated Cu-BTC@Fe3O4 (CH2Cl2-200 ​°C ​− ​C2H5OH) material has the highest adsorption capacity of 210.26 ​mg ​g−1 for p-xylene (pX). Cu-BTC@Fe3O4 (C2H5OH) and Cu-BTC@Fe3O4 materials have excellent adsorption capacity for m-xylene (mX), with the values of 200.19 ​mg ​g−1 and 219.63 ​mg ​g−1, respectively, which is about 4 times higher than that of Cu-BTC@Fe3O4 (CH2Cl2). The adsorption capacity sequence of xylene on Cu-BTC@Fe3O4 (C2H5OH) and Cu-BTC@Fe3O4 materials is m-xylene (mX) ​> ​p-xylene (pX) ​> ​o-xylene (oX), and that of other materials is pX ​> ​mX ​> ​oX.

Vapor-Phase Adsorption of Xylene Isomers and Ethylbenzene in MOF-74 Thin Films.

Thin films of Co-MOF-74 and Ni-MOF-74 were synthesized on Au-coated quartz crystal microbalance substrates by a vapor-assisted conversion (VAC) method that precludes the need for activation via postsynthetic solvent exchange. All thin films were structurally characterized by powder X-ray diffraction, reflection-absorption infrared spectroscopy, and Raman spectroscopy. Scanning electron microscopy (SEM) images reveal that the Ni-MOF-74 films exists as a dense base layer with hemispherical protrusions on the surface. In contrast, the scanning electron microscopy images of the Co-MOF-74 thin films show a rough surface with spherical deposits. The thin film morphologies were different than the powders resulting from the bulk synthesis. Gravimetric vapor-phase adsorption measurements for xylene isomers and ethylbenzene within Co-MOF-74 and Ni-MOF-74 thin films were conducted, and the results were compared with those reported for the corresponding bulk powders. Despite different morphologies, the saturation capacities of Ni-MOF-74 and Co-MOF-74 thin films were found to be nearly equivalent to those reported for the bulk powders. The results demonstrate that the VAC method can produce MOF-74 thin films that retain the intrinsic properties that are observed in bulk powders.

Identification and Quantitation of Aqueous Single- and Multianalyte Solutions of the Isomers Ethylbenzene, m-, p-, and o-Xylene Using a Single Specifically Tailored Sensor Coating and Estimation Theory-Based Signal Processing.

The isomer-specific detection and quantitation of m-, p-, and o-xylene and ethylbenzene, dissolved singly and as mixtures in aqueous solutions at concentrations from 100 to 1200 ppb by volume, is reported for a specifically designed polymer-plasticizer coating on a shear-horizontal surface acoustic wave (SH-SAW) device. The polystyrene-ditridecyl phthalate-blend coating was designed utilizing Hansen solubility parameters and considering the dipole moment and polarizability of the analytical targets and coating components to optimize the affinity of the sensor coating for the four chemical isomers. The two key coating sorption properties, sensitivity and response time constant, are determined by the (slightly different) dipole moments and polarizabilities of the four target analytes: as analyte dipole moment decreases, coating sensitivity increases; as analyte polarizability decreases, coating response time lengthens. Using the measured sensitivities and time constants for the targets, sensor signals were processed with exponentially weighted recursive-least-squares estimation (EW-RLSE) to identify (with near 100% accuracy) and quantify (with ± 5-7% accuracy) the isomers. This impressive performance was achieved by combining the specifically tailored, high-sensitivity coating and an SH-SAW platform (yielding a detection limit of 5 ppb for the analytes) and using the EW-RLS estimator, which estimates unknown parameters accurately even in the presence of measurement noise and for analytes with only minor differences in response. Identification of the xylene isomers is important for applications including environmental monitoring and chemical manufacturing.

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.

Selective toluene methylation to p-xylene: current status & future perspective

ABSTRACT Industrially, petroleum naphtha reforming and steam cracking are two major multistep and energy-intensive sources for p-xylene production. Additionally, p-xylene is also produced by selective toluene disproportionation, heavy (C9) aromatics trans-alkylation and isomerization of xylenes. Rapidly changing market dynamics from fuels to petrochemicals coupled with growing petrochemicals demand has led to the further exploration of economical and highly p-xylene selective processes by using surplus low-value aromatics. Methylation of toluene with methanol has been envisaged as one such potential opportunity to produce p-xylene. For the purpose, significant efforts have been made in the past forty years to develop suitable shape selective zeolite catalysts and technologies to make toluene methylation a highly p-xylene selective process. Current paper briefly highlights various advancements in the toluene methylation process development and aims to provide an extensive review of several works put up in the past to design suitable catalysts, catalyst’s limitations and developmental challenges for toluene methylation. Further, this paper provides an useful insight for two competing chemistries of toluene alkylation and side-by-side methanol reactions. Finally, motivated by competitive chemistry of methanol conversion reactions, existing catalytic limitations and novel synthesis opportunities in engineering shape selective zeolite catalysis, a way forward is proposed to arrive at a logical and futuristic catalyst design strategy for p-xylene production via toluene methylation. The proposed strategy is envisaged to offer low-to-moderate catalyst acidity, improved toluene methylation activity, high p-xylene selectivity and improved catalyst life which, otherwise, up till now have been the bottlenecks to develop distinctly economical toluene alkylation to p-xylene process.

Discrimination of xylene isomers in a stacked coordination polymer.

The separation and purification of xylene isomers is an industrially important but challenging process. Developing highly efficient adsorbents is crucial for the implementation of simulated moving bed technology for industrial separation of these isomers. Herein, we report a stacked one-dimensional coordination polymer {[Mn(dhbq)(H2O)2], H2dhbq = 2,5-dihydroxy-1,4-benzoquinone} that exhibits an ideal molecular recognition and sieving of xylene isomers. Its distinct temperature-adsorbate-dependent adsorption behavior enables full separation of p-, m-, and o-xylene isomers in both vapor and liquid phases. The delicate stimuli-responsive swelling of the structure imparts this porous material with exceptionally high flexibility and stability, well-balanced adsorption capacity, high selectivity, and fast kinetics at conditions mimicking industrial settings. This study may offer an alternative approach for energy-efficient and adsorption-based industrial xylene separation and purification processes.

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.

Separation of Aromatic Hydrocarbons in Porous Materials.

Industrial-scale thermal separation processes have contributed greatly to the rise in carbon dioxide emissions. Porous materials, such as metal-organic frameworks (MOFs), can potentially reduce these emissions by achieving nonthermal chemical separations through the physical adsorption of targeted species with high selectivity. Here, we report the synthesis of the channel-based MOFs NU-2000 and NU-2001, which are constructed from three-dimensional (3D) linkers, to separate the industrially relevant xylene isomers under ambient conditions by leveraging sub-Ångstrom differences in the sizes of each isomer. While the rotation of two-dimensional (2D) linkers in MOFs often affords changes in pore apertures and pore sizes that are substantial enough to hinder separation efficiency, increasing the linker dimensionality from 2D to three-dimensional (3D) enables precise control of the MOF pore size and aperture regardless of the linker orientation, establishing this design principle as a broadly applicable strategy.

Separation of Volatile Organic Compounds in TAMOF-1.

Separation of volatile organic compounds is one of the most studied processes in industry. TAMOF-1 is a homochiral metal-organic framework with a crystalline network of interconnected ≈1 nm channels and has high thermal and chemical stability. Thanks to these features, it can resolve racemic mixtures of chiral drugs as a chiral stationary phase in chromatography. Interestingly, the particular shape and size of its channels, along with the presence of metallic centers and functional groups, allow establishing weak but significant interactions with guest molecules. This opens interesting possibilities not only to resolve racemates but also to separate other organic mixtures, such as saturated/unsaturated and/or linear/branched molecules. In search of these applications, we have studied the separation of volatile organic compounds in TAMOF-1. Monte Carlo simulations in the grand-canonical ensemble have been carried out to evaluate the separation of the selected molecules. Our results predict that TAMOF-1 is able to separate xylene isomers, hexane isomers, and benzene-cyclohexane mixtures. Experimental breakthrough analysis in the gas phase and also in the liquid phase confirms these predictions. Beds of TAMOF-1 are able to recognize the substitution in xylenes and the branching in hexanes, yielding excellent separation and reproducibility, thanks to the chemical and mechanical features of this material.

Unraveling the Unique Role of Methyl Position on the Ring-Opening Barrier in Photocatalytic Decomposition of Xylene Isomers

Unraveling the Unique Role of Methyl Position on the Ring-Opening Barrier in Photocatalytic Decomposition of Xylene Isomers

Insights into pyrolysis kinetics of xylene isomers behind reflected shock waves

This work reports a kinetic study on the pyrolysis of three xylene isomers based on single-pulse shock tube experiments and detailed kinetic modeling. Speciation measurements for the post-shock gas mixtures are obtained via sampling and gas-chromatography-mass spectrometry over a temperature range of 1150−1650 K at a nominal pressure of 20 bar. A sub-mechanism incorporating consumption reactions of xylene isomers as well as subsequent pathways leading to polycyclic aromatic hydrocarbons (PAHs) is developed and integrated into our on-going PAH formation kinetic model. The model can satisfactorily predict the qualitative measurements and accurately characterize both the similarities and fuel-specific features in the pyrolysis of three xylenes. The three isomers decompose at similar rates, though m-xylene exhibits a slightly weaker reactivity. This arises from the fact that m-xylyl, different from its o- and p- counterparts, cannot undergo hydrogen loss to form m-xylylene diradical. Distinct consumption schemes of the three xylyl radicals also result in the different species pools observed at temperatures below 1400 K. A large amount of styrene results from a stepwise isomerization of o-xylylene following dehydrogenation of o-xylyl. An early formation of anthracene is noted as a unique phenomenon in o-xylene pyrolysis, which is attributed to specific reactions of o-xylyl. Due to a relatively high abundance of m-xylyl, the self-recombination product 3,3′-dimethylbibenzyl is among the major species formed at the initial stage of m-xylene pyrolysis. p-Xylyl predominantly converts to p-xylylene which is largely consumed via polymerization processes. Besides, mutual conversions among the three xylyl radicals are found to play an essential role in the pyrolysis of three xylenes. Toluene is measured of significant concentrations in the pyrolysis of all three isomers, which rationalizes that PAH speciation in the pyrolysis of three xylenes and toluene is highly similar at elevated temperatures. Modeling analyses show that apart from the benzyl/toluene chemistry, reactions involving C8 species also have important contributions to PAH formation in xylenes pyrolysis.