Showing papers on "Xylene published in 2013"

Ultraselective and sensitive detection of xylene and toluene for monitoring indoor air pollution using Cr-doped NiO hierarchical nanostructures

Abstract: Ultraselective and sensitive detection of xylene and toluene with minimum interferences of other indoor air pollutants such as benzene, ethanol, and formaldehyde is achieved using NiO hierarchical nanostructures doped with Cr. Pure and 1.15–2.56 at% Cr-doped NiO flower-like hierarchical nanostructures assembled from nanosheets are prepared by a simple solvothermal reaction and their gas sensing characteristics toward o-xylene and toluene gases are investigated. The 1.15 at% Cr-doped NiO hierarchical nanostructures show high responses to 5 ppm of o-xylene and toluene (ratio of resistance to gas and air = 11.61 and 7.81, respectively) and negligible cross-responses to 5 ppm of benzene, formaldehyde, ethanol, hydrogen, and carbon monoxide. However, pure NiO nanostructures show low responses to 5 ppm of o-xylene and toluene (ratio of resistance to gas and air = 2.01 and 1.14, respectively) and no selectivity toward any specific gas is observed. Significant enhancement of the response and selectivity to o-xylene and toluene is attributed to the decrease in the hole concentration in NiO and the catalytic oxidation of methyl groups by Cr doping.

Production of aromatic compounds from catalytic fast pyrolysis of jatropha residues using metal/hzsm-5 prepared by ion-exchange and impregnation methods

Abstract: Metal based-zeolite catalysts were successfully prepared by two different methods including ion-exchange and wet impregnation. HZSM-5 synthesized by hydrothermal method at 160 °C was used as a support for loading metals including Co, Ni, Mo, Ga and Pd. The metal/HZSM-5 had surface area and pore size of 530–677 m 2 /g and 22.9-26.0 A. Non- and catalytic fast pyrolysis of Jatropha residues using metal/HZSM-5 were studied using an analytical pyrolysis-GC/MS at 500 °C. Non-catalytic pyrolysis vapors contained primarily high levels acid (50.7%), N-containing compounds (20.3%), other oxygenated compounds including ketones, alcohols, esters, ethers, phenols and sugars (25.0%), while generated small amount of aromatic and aliphatic hydrocarbons of 3.0% and 1.0%. The addition of synthesized metal/HZSM-5 improved the aromatic selectivity up to 91–97% and decreased the undesirable oxygenated (0.6–4.0%) and N-containing compounds (1.8–4.6%). The aromatic selectivity produced by metal-ion exchanged catalysts was slightly higher than that produced by impregnated ones. At high catalyst content (biomass to catalyst ratio of 1:10), Mo/HZSM-5 showed the highest aromatic selectivity of 97% for ion-exchanged catalysts and Ga/HZSM-5 revealed the highest aromatics of 95% for impregnated catalysts. The formation of aromatic compounds could be beneficial to improve calorific values of bio-oils. The presence of metal/HZSM-5 from both preparation methods greatly enhanced MAHs selectivity including benzene, toluene, and xylene (BTX), while substantially reduced unfavorable PAHs such as napthalenes.

Gel‐Free Secondary Growth of Uniformly Oriented Silica MFI Zeolite Films and Application for Xylene Separation

Abstract: Gel-Free Secondary Growth of Uniformly Oriented Silica MFI Zeolite Films and Application for Xylene Separation Zeolite membranes : A promising method is reported for the fabrication of oriented silica MFI zeolite films (see picture; TPAOH = tetrapropylammonium hydroxide). The films synthesized using this method exhibit an outstanding performance for the separation of pand oxylene. Angewandte Chemie

Hydrodeoxygenation of Guaiacol, A Surrogate of Lignin Pyrolysis Vapors, Over Iron Based Catalysts: Kinetics and Modeling of the Lignin to Aromatics Integrated Process

Abstract: Biosourced aromatics (BTX (benzene, toluene, xylene) and phenols) could be produced by lignin pyrolysis coupled with catalytic hydrodeoxygenation (HDO) of uncondensed pyrolysis vapors. Guaiacol is used as a model compound to study the catalytic HDO over Fe/SiO2 catalyst. Experiments were conducted in a fixed bed reactor operated at 673 K (1 atm) with a gas mixture (guaiacol, H2, H2O, CO, CO2) that mimics the real gas composition from lignin pyrolysis. Fe/SiO2 catalyst was shown to be selective for guaiacol HDO into benzene and phenols because it does not catalyze the aromatic ring hydrogenation. Major and minor products are modeled by a semidetailed kinetic mechanism. A deactivation law is also determined. The kinetic model is then included in an Aspen Plus model of lignin to BTX process. Aspen Plus model handles (1) pyrolysis of lignin, including char, oligomers, gases and aromatic yields, (2) catalytic conversion of aromatics by the kinetic model, (3) heat exchangers, and (4) BTX vapors recovery by scru...

Improved para‐Xylene Selectivity in meta‐Xylene Isomerization Over ZSM‐5 Crystals with Relatively Long b‐Axis Length

Abstract: ZSM‐5 zeolite crystals with controllable b‐axis length (sheet‐like, S‐ZSM‐5; chain‐like, C‐ZSM‐5) have been synthesized by using urea and starch as additives in the starting aluminosilicate gels. X‐ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images show that these zeolite samples have good crystallinity. Transmission electron microscopy (TEM) images show that there is a strong chemical interaction between the stacked crystals in C‐ZSM‐5 samples. N2 sorption isotherms indicate that C‐ZSM‐5 crystals are mesoporous. Catalytic tests for the formation of p‐xylene from m‐xylene isomerization show that, compared with other zeolite catalysts, C‐ZSM‐5 catalysts give both high conversion and improved p‐xylene selectivity, which are attributed to the combination of relatively long b‐axis length and the present mesoporosity in the crystals. The improvement of p‐xylene selectivity in catalytic m‐xylene isomerisation is of great importance for selective industrial production of p‐xylene in the future.

Phytoremediation of BTEX from Indoor Air by Zamioculcas zamiifolia

Abstract: Zamioculcas zamiifolia has the potential to reduce the concentration of benzene, toluene, ethylbenzene, and xylene (BTEX) from contaminated indoor air. It can remove all four pollutant gases. Benzene, toluene, ethylbenzene, and xylene uptake per unit area of Z. zamiifolia leaf were about 0.96 ± 0.01, 0.93 ± 0.02, 0.92 ± 0.02, and 0.86 ± 0.07 mmol m−2 at 72 h of exposure, respectively. The physicochemical properties of each BTEX may affect its removal. Benzene, a smaller molecule, is taken up by plants faster than toluene, ethylbenzene, and xylene. The toxicity of BTEX on plant leaves and roots was not found. The chlorophyll fluorescence measurement (Fv/Fm) showed no significantly difference between controlled and treated plants, indicating that a concentration of 20 ppm of each gas is not high enough to affect the photosynthesis of the plants. The ratio of stomata and cuticles showed that 80 % of benzene, 76 % of toluene, 75 % of ethylbenzene, and 73 % of xylene were removed by stomata pathways, while 20, 23, 25, and 26 % of them were removed by cuticles. The BTEX removal efficiency by well-watered Z. zamiifolia was involved with day stomata opening and night closing, while the BTEX removal efficiency by water-stressed Z. zamiifolia can occur both day and night at a slightly lower rate than well-watered plants.

A highly efficient process for transforming methyl mercaptan into hydrocarbons and H2S on solid acid catalysts

Abstract: The catalytic conversion of CH 3 SH and CH 3 SCH 3 (DMS) on protonic zeolites H-ZSM-5, H-Y and H-ferrierite was studied in a gas flow reactor from 423 to 823 K. Below 700 K, CH 3 SH is converted at equilibrium into DMS and H 2 S. Above 700 K, light alkanes (C1–C3), benzene, toluene and xylene appear alongside H 2 S in the gas phase, and a carbonaceous deposit builds up on the catalyst. DMS is assumed to be the intermediate in the CH 3 SH transformation into H 2 S and hydrocarbon species. At 823 K, the CH 3 SH conversion is total on H-ZSM-5, and only partial on H-Y and H-ferrierite. These are selective to alkanes, and produce large quantities of coke. In contrast, much less coke builds up on H-ZSM-5, which is also more selective to aromatics. After calcination in air flow at 823 K, the spent H-ZSM-5 sample recovers the properties of the fresh catalyst. Similarities and differences with the methanol-to-hydrocarbons process are discussed.

Solubilization of Benzene, Toluene, and Xylene (BTX) in Aqueous Micellar Solutions of Amphiphilic Imidazolium Ionic Liquids

Abstract: Water-soluble ionic liquids may be considered analogues to cationic surfactants with a corresponding surface activity and ability to create organized structures in aqueous solutions. For the first time, the enhanced solubility of the aromatic hydrocarbons, benzene, toluene, and xylene, in aqueous micellar systems of 1-alkyl-3-methylimidazolium chlorides was investigated. Above a critical micelle concentration, a gradual increase in the concentration of aromatic hydrocarbons in the miceller solution was observed. This phenomenon was followed by means of the molar solubilization ratio, the micellar/water partition coefficient, and the number of solubilizate molecules per IL micelle. The molar solubilization ratio for ionic liquid micelles was found to be significantly higher when compared to that of ionic surfactants of similar chain length. The incorporation of the hydrocarbon into the micelle affects also an increase of the aggregation number.

Synthesis, physicochemical characterizations and catalytic performance of Pd/carbon-zeolite and Pd/carbon-CeO 2 nanocatalysts used for total oxidation of xylene at low temperatures

Abstract: In this work, xylene removal from waste gas streams was investigated via catalytic oxidation over Pd/carbon-zeolite and Pd/carbon-CeO2 nanocatalysts. Activated carbon was obtained from pine cone chemically activated using ZnCl2 and modified by H3PO4. Natural zeolite of clinoptilolite was modified by acid treatment with HCl, while nano-ceria was synthesized via redox method. Mixed supports of carbon-zeolite and carbonceria were prepared and palladium was dispersed over them via impregnation method. The prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Brunauer-Emmett-Teller surface area (BET), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric (TG) techniques. Characterization of nanocatalysts revealed a good morphology with an average particle size in a nano range, and confirmed the formation of nano-ceria with an average crystallite size below 60 nm. BET analysis indicated a considerable surface area for catalysts (∼1000 m2·g−1). FTIR patterns demonstrated that the surface groups of synthesized catalysts are in good agreement with the patterns of materials applied in catalyst synthesis. The performance of catalysts was assessed in a low-pressure catalytic oxidation pilot in the temperature range of 100° C-250°C. According to the reaction data, the synthesized catalysts have been shown to be so advantageous in the removal of volatile organic compounds (VOCs), representing high catalytic performance of 98% for the abatement of xylene at 250°C. Furthermore, a reaction network is proposed for catalytic oxidation of xylene over nanocatalysts.

Kinetics and Mechanism of Benzene, Toluene, and Xylene Methylation over H-MFI

Abstract: The methylation of benzene, toluene, para-xylene, and ortho-xylene over MFI structured H-ZSM-5 and mesoporous self-pillared pentasil (H-SPP) with dimethyl ether (DME) at low conversions ( 30:1) showed linear rate dependencies on aromatic pressure and zero dependence on DME pressure for benzene and toluene. These results are consistent with studies performed for olefin methylation, and are indicative of a zeolite surface covered in DME-derived species reacting with benzene or toluene in the rate-determining step. Saturation in the reaction rate was observed in xylene pressure dependence experiments (at 473 K, <5 kPa xylene); however, enhancement in the reaction rate was not observed when comparing ∼1 μm crystallite H-ZSM-5 and 2–7 nm mesopore H-SPP, indicating that xylene methylation proceeds in the absence of diffusion limitations. Simultaneous zero-order rate dependencies on xylene and DME pressures are described by a model based on adsorption of xylene onto a surface...

Synthesis and physicochemical characterization of nanostructured Pd/ceria‐clinoptilolite catalyst used for p‐xylene abatement from waste gas streams at low temperature

Abstract: BACKGROUND: The effect of Pd loading, xylene concentration and GHSV on xylene oxidation was tested over Pd/CeO2(30%)-clinoptilolite nanocatalysts at low temperatures. The catalysts were prepared by acid treatment of clinoptilolite, followed by the incipient wetness method of synthesized ceria and modified clinoptilolite in PdCl2 solution. The synthesized nanocatalysts were characterized by XRD, FESEM, EDAX, TEM, BET, FTIR and TG-DTG analysis. RESULTS: The XRD patterns confirmed the formation of crystalline ceria with an average crystallite size of 11.8 nm. FESEM images showed nanostructures in cavities of natural zeolite, brought about by ceria incorporation and acid activation. TEM analysis showed high dispersion of Pd with a size distribution between 6.6 and 36.7 nm. The quantitative analysis showed that the specific surface area of Pd(1%)/CeO2(30%)-clinoptilolite was 77 m2 g−1. The results showed that Pd(1%)/CeO2(30%)-clinoptilolite is the most appropriate catalyst, with the conversion more than 90% at 275 °C. CONCLUSIONS: Experimental results established effective performance and durability for the catalysts. As a result, clinoptilolite modification and ceria incorporation significantly altered the samples' morphology at nanoscale, improving the structure of composites and distribution of noble metals. A reaction path was suggested based on the adsorption-migration of species to reveal the mechanism of p-xylene oxidation over nanocatalysts. © 2012 Society of Chemical Industry

Synthesis and physicochemical characterization of nanostructured CeO2/clinoptilolite for catalytic total oxidation of xylene at low temperature

Abstract: A series of nanostructured CeO2/Clinoptilolite catalysts with different loadings of ceria were prepared by redox reaction followed by wet impregnation method and tested for oxidation of xylene The catalysts were characterized by XRD, FESEM, BET, FTIR, and TG-DTG analysis XRD data confirmed the formation of CeO2 as the crystalline phase with an average crystallite size of about 116 nm for three ceria loading of 10, 20, and 30% FESEM and size distribution analyses showed that nanocatalysts have nanometric particles with an average size of 3746 nm Specific surface analysis revealed that the synthesized nanocatalysts had large enough surface area for catalytic oxidation of p-xylene Furthermore, the results showed that the catalytic performance of the supported CeO2 catalysts was much higher than that of treated clinoptilolite, in particular, CeO2 (30%)/Clinoptilolite exhibited the highest conversion, 98% at 350°C It is observed that increasing both the xylene concentration and GHSV results in decreasing of the xylene conversion; however, even at higher concentrations of xylene (3000 ppm), the nanostructured catalyst has still enough destruction ability to reduce the pollutant A simplified reaction mechanism was proposed with respect to the behavior of adsorbed species on the catalyst surface to clarify the path through which the reaction components interact with each other © 2012 American Institute of Chemical Engineers Environ Prog, 32: 587–597, 2013

Multi-component adsorption of benzene, toluene, ethylbenzene, and xylene from aqueous solutions by montmorillonite modified with tetradecyl trimethyl ammonium bromide

Abstract: Multicomponent adsorption of benzene, toluene, ethylbenzene, and xylene (BTEX) was assessed in aqueous solutions by montmorillonite modified with tetradecyl trimethyl ammonium bromide (TTAB-Mt). Batch experiments were conducted to determine the influences of parameters including loading rates of surfactant, contact time, pH, adsorbate concentration, and temperature on the adsorption efficiency. Scanning electron microscope (SEM) and X-ray diffractometer (XRD) were used to determine the adsorbent properties. Results showed that the modification of the adsorbent via the surfactant causes structural changes of the adsorbent. It was found that the optimum adsorption condition achieves with the surfactant loading rate of 200% of the cation exchange capacity (CEC) of the adsorbent for a period of 24 h. The sorption of BTEX by TTAB-Mt was in the order of B T E X . The experimental data were fitted by many kinetic and isotherm models. The results also showed that the pseudo-second-order kinetic model and Freundlich isotherm model could, respectively, be fitted to the experimental data better than other available kinetic and isotherm models. The thermodynamic study indicated that the sorption of BTEX with TTAB-Mt was achieved spontaneously and the adsorption process was endothermic as well as physical in nature. The regeneration results of the adsorbent also showed that the adsorption capacity of adsorbent after one use was 51% to 70% of original TTAB-Mt.

Production of hybrid diesel fuel precursors from carbohydrates and petrochemicals using formic acid as a reactive solvent.

Abstract: We report the one-pot alkylation of mesitylene with carbohydrate-derived 5-(hydroxymethyl)furfural (HMF) as a step toward diesel-range liquids. Using FeCl(3) as a catalyst, HMF is shown to alkylate toluene, xylene, and mesitylene in high yields in CH(2)Cl(2) and MeNO(2) solvents. Efforts to extend this reaction to greener or safer solvents showed that most ether-based solvents are unsatisfactory. Acid catalysts (e.g, p-TsOH) also proved to be ineffective. Using formic acid as a reactive solvent, mesitylene could be alkylated to give mesitylmethylfurfural (MMF) starting from fructose with yields up to approximately 70 %. The reaction of fructose with formic acid in the absence of mesitylene gave rise to low yields of the formate ester of HMF, which indicates the stabilizing effect of replacing the hydroxyl substituent with mesityl. The arene also serves as a second phase into which the product is extracted. Even by using formic acid, the mesitylation of less expensive precursors such as glucose and cellulose proceeded only in modest yields (ca. 20 %). These simpler substrates were found to undergo mesitylation by using hydrogen chloride/formic acid via the intermediate chloromethylfurfural.

Diffusion of xylene isomers in the MIL-47(V) MOF material: A synergic combination of computational and experimental tools

Abstract: The dynamics of xylene isomers in the metal–organic framework MIL-47(V) has been investigated by combining molecular dynamics (MD) simulations and experimental tools including quasi-elastic neutron scattering (QENS) and deuterium nuclear magnetic resonance (2H NMR).The experimental and simulated self-diffusion coefficients (Ds) values for each single component isomer are in reasonable agreement in the whole range of temperatures. More interestingly, the simulations predict a nonmonotonous evolution of Ds with the temperature for all xylenes. Such an unusual trend is experimentally confirmed for p-xylene. Two distinct diffusion regimes are elucidated at the microscopic level: a low-temperature regime where the xylene molecules are close to the MIL-47(V) pore wall with a high activation energy barrier for the diffusion and a high-temperature regime where the xylene molecules are mainly located in the center of the channel associated with a lower activation energy for the diffusion. This dynamic behavior rem...

Xylene sensor using double-layered thin film and Ni-deposited porous alumina

Abstract: A xylene sensor using a double-layered metal-oxide thin film and Ni-catalyst deposited on a porous alumina is presented. The sensing film has a double-layered structure; the first layer is Fe2O3 + TiO2 (5 mol%) + MgO (4 mol%), and the second layer is WO3 + TiO2 (10 mol%). The thicknesses of the first and second layers are 200 nm and 70 nm, respectively. Both response and stability are improved by adopting a double-layered structure. The Ni-deposited porous alumina is placed over the sensing film. The pores in the porous alumina are cylindrical. The inner diameter and the height of the pores are about 200 nm and 60 μm, respectively. Xylene gas goes through the Ni-deposited pores in the porous alumina to reach the sensing film. The catalytic action of the deposited Ni enhances the chemical activity of xylene gas and the amount of its absorption on the sensing film. Thus the sensor is highly sensitive to xylene gas. The sensor is capable of detecting xylene at a concentration as low as 0.07 ppm. Moreover, the selectivity to xylene gas is satisfactory. The response and recovery times are approximately 20 s and 300 s, respectively, at an operating temperature of 340 °C. All films and Ni are deposited by rf sputtering while the porous alumina is formed by anodic oxidation of aluminum.

Introducing nanocrystalline CeO2 as heterogeneous environmental friendly catalyst for the aerobic oxidation of para-xylene to terephthalic acid in water

Abstract: CeO2 nanoparticles exposed in (100) and (111) surfaces have been synthesized and explored as a heterogeneous catalyst for the first time in the oxidation of para-xylene to terephthalic acid. The synthesis and catalysis reaction was environmental friendly, where water was used as the solvent. Ceria nanoparticles were synthesized with controlled size of 15 nm and high surface area of 268 m2 g−1 magnitude. These particles were exploited as a novel heterogeneous catalyst for aqueous phase oxidation of para-xylene to bypass all the hazardous steps involve in the manufacture of industrially important terephthalic acid. The result shows the formation of 30–40% terephthalic acid under mild reaction condition, i.e. at 70 °C in water, by avoiding the corrosive bromide promoter and acetic acid solvent. The recyclability studies reveal that the recovered ceria catalyst retained its activity in para-xylene conversion without the change in the fluorite crystal structure, crystallite size and morphology of CeO2. At last, a radical mechanism for this particular catalytic activity of the catalysis reaction has been proposed based on the high surface area and the corresponding available exposed active (100) and (111) surfaces.

Benzene, toluene and xylenes levels in new and used vehicles of the same model

Abstract: The aim of this work was to determine the level of benzene, toluene, o -xylene and m, p -xylene (BTX) in air samples collected from the cabins of new and used vehicles of the same model. Ten new vehicles were examined in order to check interior emission from materials used to equip the passenger compartment. In order to compare and define the impact of exhaust gases, air samples were also collected from two used cars, at different mileages (up to 20,000 km). All vehicles tested were of the same type. Samples were collected onto Carbograph 1TD sorbent, thermally desorbed and examined with the use of gas chromatography with flame ionisation and mass spectrometry detectors. All results obtained were referred to Polish and German requirements for indoor air quality (both in public buildings and in workspace environments). Average benzene, toluene, o -xylene and m, p -xylene concentrations in new cars were determined at the level of 11.8 μg/m 3 , 82.7 μg/m 3 , 21.2 μg/m 3 and 89.5 μg/m 3 , respectively. In the used cars, BTX concentration increased with increasing vehicle mileage. The most significant increase of BTX concentration was observed above 11,000 km mileage.

Kinetics of dealkylation–transalkylation of C9 alkyl-aromatics over zeolites of different structures

Abstract: Dealkylation of methylethylbenzenes as well as the conversion of mixtures trimethylbenzenes (TMBs) and methylethylbenzenes (MEBs) over medium- and large-pore zeolites with different topologies and acid-site concentrations were investigated in a fluidized-bed reactor. MEB conversion, its dealkylation selectivity and transalkylation selectivity were profoundly influenced by the topology of the zeolites. Zeolite beta shows a highest MEB conversion (70–80%) while ZSM-5 exhibited very high dealkylation selectivity. The results of kinetic study indicate that MEB conversion is influenced by zeolite type and by SiO2/Al2O3 ratio. The extent of TMB and MEB conversions over different zeolites as well as xylene yields indicate that while ZSM-5 could substantially convert MEBs (mainly by dealkylation), it could not effectively catalyze TMB conversion. On the other hand, the conversions of MEBs as well as TMBs were quite high over mordenite and zeolite beta, resulting in much higher xylene yields. These results indicate that while the dealkylation of MEBs is a necessary condition, it is not a sufficient condition to obtain higher xylene yield. Kinetic modeling results show that the transalkylation reaction was significantly faster than the disproportionation reaction indicating that the transfer of methyl group from TMB to toluene is a preferred route under the reaction conditions studied.

Adsorption and diffusion properties of xylene isomers and ethylbenzene in metal–organic framework MIL-53(Al)

Abstract: Adsorption and diffusion behaviour of C8 aromatic isomers, including para-xylene (PX), ortho-xylene (OX), meta-xylene (MX), and ethylbenzene (EB), in metal–organic framework Mil-53(Al) has been systematically investigated by using intelligent gravimetric analyzer, thermogravimetric analysis and molecular simulation respectively The results indicate that adsorptions of xylene isomers and ethylbenzene molecules in Mil-53(Al) at 303 K present type-I isotherms The deviation from normal Langmuir model of the isotherms at higher temperature can be, however, found because of the breathing effect of Mil-53(Al) framework In order to well understand the selective adsorption process of above adsorbates, the diffusion behaviour has been determined and the diffusion coefficient is in the order: OX > PX > MX > EB The adsorption thermodynamics have been determined by the isotherms at different temperature A sharp increase of the heat of sorption Q st suggests that a strong interaction between sorbates molecules and between sorbates and the framework appears as the increase of the loading Two desorption peaks in DTG curves suggests that two sorption locations exist in Mil-53(Al) to C8 alkylaromatics The molecular simulation results have been used to successfully explain the experimental phenomena and to well understand the underlying adsorption and diffusion features of the systems

Effect of Torrefaction on Bio-oil Upgrading over HZSM-5. Part 1: Product Yield, Product Quality, and Catalyst Effectiveness for Benzene, Toluene, Ethylbenzene, and Xylene Production

Abstract: A three-step bio-oil production process involving torrefaction pretreatment (at 225, 250, or 275 °C with a 20 min hold time), pyrolysis (at 500 °C with two heating rates), and secondary catalytic processing over HZSM-5 (at 400, 450, or 500 °C) was studied to determine process effects, particularly of torrefaction, on the yield of liquid and aromatic hydrocarbons and the quality of upgraded bio-oil. When bio-oils derived from torrefied biomass were catalytically cracked, average yields (%, w/w of feed) of reactor char (100% reduction), catalyst coke (21.4%), and catalyst tar (8.1%) were significantly reduced relative to the best-case conditions using non-torrefied feedstock. The reduction in coke (%, w/w of feed) as a result of torrefaction was 28.5% relative to the respective control for slow-pyrolysis bio-oil upgrading and 34.9% for fast-pyrolysis bio-oil upgrading. The concentration of aromatic product (benzene, toluene, ethylbenzene, and xylene, i.e., BTEX) was highest at 95 g L–1 for upgraded (at 500 ...

Isomerization kinetics of benzylic and methylphenyl type radicals in single-ring aromatics

Abstract: Mutual isomerization through H-atom shifts in benzylic and methylphenyl type radicals are examined for toluene, and o-, m-, and p-xylene isomers. In the high temperature pyrolysis and oxidation of toluene, three possible methylphenyl radical isomers can be formed (2-, 3-, and 4-methylphenyl). The 2-methylphenyl radical may undergo a facile isomerization to benzyl through a four-membered ring critical structure – a pathway not accessible to 3- and 4-methylphenyl. Electronic structure calculations show that 2-methylphenyl isomerization to benzyl is preferred energetically by at least 20 kcal/mol over other possibilities. Monte Carlo RRKM/master equation simulations illustrate that, at temperatures ⩾1200 K, and for almost all pressures, the 2-methylphenyl radical has a lifetime of only several microseconds, whereas 3- and 4-methylphenyl radicals have a substantially longer lifetime, allowing them to react with other species, including molecular oxygen, during toluene oxidation. This is also found to be the case for all dimethylphenyl radicals in the three xylene isomers. As a result, the structure of the xylene isomers and specifically the number of H atoms immediately adjacent to the methyl groups can have a direct impact on their high-temperature oxidation and appear to explain the observed differences in xylene oxidation behind reflected shock waves and in laminar premixed flames.