Benzene Formation Research Articles

Insight into an invisible active site presented in the low temperature pyrolysis of toluene by CoMn spinel catalyst

In this work, CoMn spinel (MnCo2O4+δ) was prepared from CoMn LDH and was applied as a catalyst toward toluene pyrolysis. Spinel samples with different degrees of crystallinity were evaluated by isothermal and stepwise heating experiments, as well as feeding toluene with different concentrations. Especially for the spinel with high degree of crystallinity, results exhibited a relatively low pyrolysis temperature of 400 °C, a high toluene conversion rate of ~ 100%, a large H2 production and an acceptable benzene formation. More importantly, by various characterization techniques, an invisible active site was detected and believed to be formed by the reaction of CoMn spinel with toluene, and was designated as manganosite-cobalt nanoparticles supported on carbon nanotubes (MnO-Co/CNTs), which significantly promoted the degradation and carbonization reactions in the low temperature pyrolysis of toluene.

Effect of recycled content and rPET quality on the properties of PET bottles, part II: Migration

Polyethylene terephthalate (PET) bottles were produced from three types of recycled PET (rPET) with four levels of recycled content. The migration of substances from these bottles to water was studied. Several migrated substances were detected. The migrated amounts of acetaldehyde and ethylene glycol complied with the limits given in the food contact material (FCM) legislation. Migration of 2‐methyl‐1,3‐dioxolane was below the limit of 10 μg·L−1, which is conventionally applied for non‐intentionally added substances (NIAS) not classified as ‘carcinogenic’, ‘mutagenic’ or ‘toxic to reproduction’ (CMR). Limonene, acetone, butanone and furan were also detected as migrants, of which limonene is a natural fragrant, and the other three are probably residues from solvents used to clean and protect the mould at the small‐scale production facility. Finally, benzene and styrene were also found as migrants from rPET. These migrants appear to originate from heat‐induced reactions within the PET matrix, which involve contaminants. The formation of benzene in rPET is attributed to polyvinylchloride as contaminant. The migrated amounts of benzene from the PET bottles with recycled content to the water simulant are relatively small (0.03–0.44 μg·L−1) after 10 days at 40°C. Consequently, the margin of exposure is 3.105–8.106. Hence, the level of concern for the public health is low, and the migrated amount represents a low priority for risk management. The FCM legislation demands a risk assessment for migrating NIAS. Depending on the underlying data and exposure scenario, different threshold limits in the food can be derived which can still be considered as safe.

Ayurvedic formulations containing benzoic and ascorbic acids as additives: benzene formation during storage and impact of additives on quality parameters.

Ayurvedic formulations are becoming the prior choice of people as health care supplements. The increasing demand for these formulations has led to extensive development of Ayurvedic pharmaceutical industries worldwide. The reaction between the preservatives (sodium benzoates and ascorbic acid) used in these formulations could generate benzene. Benzene is classified as class-1 human carcinogen and responsible for various short and long term health effects. In this study, 25 formulations (containing ascorbic acid and sodium benzoate) of various manufacturers available as over the counter products were obtained and their benzene content were determined using gas chromatograph with flame ionization detector. The result showed that 64% of the formulations were free from benzene contamination whereas 36% of formulations were found to be contaminated with benzene. A simple, less time-consuming, economic, and validated gas chromatographic method for estimation of benzene in Ayurvedic formulations was also developed successfully in present study. The data revealed that the level of benzene was within permissible limits, yet the presence of a carcinogen in the marketed formulations intended for internal use is an alarming situation.

A single pulse shock tube study of pentene isomer pyrolysis

A single-pulse shock tube study of the four pentene isomers is carried out at 2 ± 0.16 bar and 900–1600 K. C1 to C6 species profiles were recorded using gas chromatography mass spectrometry analyses. The species are identified using mass spectrometry and quantified by flame ionization detection. High-pressure limiting and pressure-dependent rate constants for 2M1B, 2M2B and 3M1B + Ḣ were calculated using RRKM theory with a Master Equation (ME) analysis using the Master Equation System Solver, MESS. A mechanism was formulated based on rate rules and theoretical calculations. Comparisons between experimental results and model simulations are provided for all of the five pentene isomers investigated with satisfactory agreement. Furthermore, an insight is provided into the influence of molecular structure on the reactivity of pyrolysis chemistry. Interestingly, it is found that the HACA mechanism is much less prominent for benzene formation compared to the role of cyclopentadienyl radical recombination with methyl radicals and also the recombination of propargyl radicals.

Density functional theory study on the catalytic degradation mechanism of polystyrene

The density functional theory method of B3LYP/6-311G(d) was used to study two catalytic degradation (acid-catalyzed and alkali-catalyzed) reaction mechanisms of polystyrene (PS). The geometric structure optimization and frequency calculations of all the molecules involved in the catalytic degradation were performed, and the standard thermodynamic parameters of each catalytic cracking path were obtained. The calculation results show that the energy barrier of the optimal reaction path’s rate control step to form styrene monomer is 68.2 kJ/mol in the alkali-catalyzed degradation reaction paths. In the acid-catalyzed cracking paths, the energy barrier of the optimal path’s rate control step to form styrene is 151.9 kJ/mol. The energy barriers of rate control steps for the formation of styrene monomer in both types of these catalytic cracking reactions are lower than those for other products, so the main degradation product in the two types of catalytic degradation is styrene monomer. Compared with pure thermal degradation, an obvious feature of acid-catalyzed degradation is the formation of benzene, indene, and derivatives of indene. The formation of benzene reduces the phenyl content of the PS main chain, which results in a reduction in the yield of styrene monomer. However, an alkali catalyst shows a positive catalytic effect, which increases the yield of styrene monomer.

Effect of surface structure and Pd doping of Fe catalysts on the selective hydrodeoxygenation of phenol

Density functional theory (DFT) calculations were performed to investigate the effects of surface structure and Pd doping of Fe catalysts on the activity and selectivity of phenol hydrodeoxygenation (HDO). Phenol adsorption via binding with the aromatic ring was found to be energetically more stable than adsorbing through solely the hydroxyl group on Fe(110), Fe(111) and Fe(211) surfaces. The adsorption strength on monometallic Fe surfaces increased in the sequence of Fe(211) < Fe(110) < Fe(111). Pd doping did not have a significant influence on the adsorption stability of phenol. The (211) facet of Fe featured particular steps showed good activity toward aromatics production, as evidenced by the lower barriers associated with the dehydroxylation followed by H addition to form benzene than the direct hydrogenation on the aromatic ring. In contrast, the ring saturation products would be dominant on Fe(110) and Fe(111) surfaces due to lower hydrogenation barriers on the aromatic ring of phenol as compared to the CAr-O bond cleavage. The kinetically preferred pathway for phenol conversion on Pd doped Fe(110) and Fe(111) surfaces was the direct hydrogenation on the aromatic ring, same to that identified on monometallic (110) and (111) surfaces. In comparison, on Fe(211), the difference in kinetic barrier between dehydroxylation and ring hydrogenation pathways became larger in the presence of Pd, promoting the selectivity to benzene formation. In the presence of Pd dopant, the H2 molecule was readily dissociated and the surface H* coverage was increased, leading to a lowering of the adsorption enthalpy of key surface species and ultimately enhancing the rates of HDO.

Thermochemistry of the lignin broken bits

The hydroxy- and methoxy substituted benzaldehydes are the simplest broken bits from the lignin valorization technologies. Comprehensive thermochemical study of these compounds was performed including vapour pressure measurements, combustion and differential scanning calorimetry. A large number of primary experimental results on vapour pressures, phase transitions, and enthalpies of formation have been collected from the literature and evaluated with help of empirical and quantum-chemical methods. The consistent sets of thermodynamic data were utilised for developing the simple GA method for prediction of enthalpies of vaporization and enthalpies of formation of di-substituted benzenes in the gas phase. Parameters of GA procedures are expected to be transferable for prediction of thermochemical properties of poly-hydroxy and poly-methoxy substituted benzenes appearing in reaction products during of the lignin valorization.

Studies on Benzene Formation in Beverages

Gas chromatography with a purge and trap apparatus was used to study the effects of benzene formation in mixtures of ascorbic acid and sodium benzoate. Ascorbic acid, sodium benzoate, citric acid, free radical scavenger,metal chelator, metal ion and hydroxyl radical were found to have inductive or inhibitive effects on benzene formation individually. On the eighth storage day, with one component's concentration constant, the benzene content in the mixture increased as the concentration of the other component increased but, after reaching a certain concentration, the benzene content decreased gradually. The results showed that citric acid, free radical scavenger and metal chelator had inhibitory effects on benzene formatlon. However, at the beginning, the hydroxy radical and metal ion had an inductive effect on benzene formation as their concentrations increased but, after concentration levels reached to a certain point, the effect on benzene formation reversed. Samples of oragne juice, apple juice and pizza sauce mixed with sodium benzoate all showed no significant effect on benzene formation, and the sample of apple juice plus citric acid also showed no significant effect on benzene formation.

Catalytic conversion of toluene over a biochar bed under an inert atmosphere – The comparison of chars from different types of wood and the role of selected metals

In this paper, toluene, a tar-representing compound, was used in pyrolytic conversion tests with a wood-derived char catalyst. The decomposition mainly occurred via two competing heterogeneous reactions; the favoured reaction led to coke formation on the catalyst bed. With char saturation of the carbon deposit, a second demethylation reaction gained significance, until selectivity towards benzene formation reached 12–16%. By-products of the secondary reaction (xylenes, ethylbenzene and styrene) were also detected; however, no gas-phase toluene decomposition occurred. The principles of toluene conversion were the same for all of the examined chars from the different tree species. However, a quantitative difference was observed between the chars from the wood from coniferous and deciduous trees. Pine yielded char with less mesopores and active sites and thus demonstrated a lower efficiency as a catalyst for toluene decomposition. An assessment of alkali and alkaline earth metals importance revealed that, while their presence in the char strongly enhances its oxidation rate, they did not affect the toluene pyrolysis.

Study on Products from Fuel-rich Methane Combustion near Sooting Limit Temperature Region and Importance of Methyl Radicals for the Formation of First Aromatic Rings

ABSTRACT Productions of mono-/di-cyclic aromatic hydrocarbons as well as smaller stable species from extremely fuel-rich CH4/air mixtures (equivalence ratio of 1.7–6.0 and fuel-to-mixture ratio of 15–38 mol.%) near sooting limit in terms of temperature, were investigated using a micro flow reactor with a controlled temperature profile at maximum wall temperature of 1300 K. Species measurements of O2, H2, CO, CO2, CH4, C2H2, C2H4, C2H6, benzene, toluene, styrene and naphthalene were performed with GC and GC/MS analysis. One-dimensional computations were also conducted with several detailed chemical kinetics. Most of the mechanisms comparably well predicted the smaller species except C2H2 (acetylene), which was overestimated by all the mechanism especially at moderate equivalence ratio (Ø 3.0). There were large discrepancies between measured and computed mole fractions of benzene and naphthalene at high equivalence ratio (Ø 4.0). Reaction path analysis indicated that reaction pathway branched from C2H3 reacting with methyl radical, which competes with C2H2 production, showed relatively low contribution to benzene formation at moderate equivalence ratio. Therefore, improvements of chemical kinetics with further consideration of reactions with methyl radical are necessary for precise prediction of products where abundant amounts of methyl radical exist.

Carbidic Mo is the sole kinetically-relevant active site for catalytic methane dehydroaromatization on Mo/H-ZSM-5

Systematic variation of Mo/Al = {0.10, 0.25, 0.35} in Mo/H-ZSM-5 catalysts effects ~20× change of Mo-to-H+ ratio during methane dehydroaromatization (DHA) reactions at steady state and on deactivating catalysts. Mo and Brønsted-acid site count, respectively enumerated by ethane hydrogenolysis probe reaction and dimethyl ether chemical titration, evolve disparately during DHA catalysis, indicating the two sites reduce in number by distinct mechanisms. The deactivation of Mo/H-ZSM-5 catalytic activity is uniquely attributed to the loss of active, or accessible, Mo sites, evinced by (i) the non-selective nature of deactivation (i.e. that deactivation occurs without modification of residual active sites), (ii) invariance in product distribution from Mo/H+ = 0.10–2.1, and (iii) linear correspondence between Mo-catalyzed ethane hydrogenolysis rate and active site count during DHA on deactivating samples. DHA forward rate – the intrinsic kinetic descriptor of catalyst activity – normalizes by Mo content at steady state and on deactivating catalysts with Mo/Al = 0.10–0.35 for Mo-based contact times in the range τMo = 0.79–44 molMo s mol−1C, unequivocally establishing Mo aggregates as the sole kinetically-relevant active site on Mo/H-ZSM-5. Brønsted-acid site content over a ~40× range in τH+ = 1.4–54 molH+ s mol−1C has no discernable correlation with or effect on DHA rate, methane conversion, or product distribution – demonstrating H+ does not catalyze any rate- or selectivity-determining steps in the benzene formation pathway and suggesting that the benefits of zeolitic acid sites are only to disperse Mo during catalyst synthesis and, conceivably, to catalyze equilibrated reaction steps.

Local structure elucidation and reaction mechanism of light naphtha aromatization over Ga embedded H-ZSM-5 zeolite: Combined DFT and experimental study

Abstract Local structures and mechanisms for n-pentane aromatization on Ga embedded H-ZSM-5 zeolite (Ga/ZSM-5) were elucidated using Synchrotron-based X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations to understand the role of Ga/ZSM-5 zeolite in aromatics synthesis. XAS data suggests that Ga ligates with four oxygen or four hydrogen atoms. Catalytic tests results suggest that conversion by Ga/ZSM-5 catalyst cannot occur via C6–C8 non-aromatic intermediates, while the availability of Ga metal sites promotes the aromatization of C2 and C3 species. Therefore, conversion of n-pentane to benzene or toluene comprises four steps, i) cracking, ii) GaH2 activation, iii) cyclization, and iv) dehydrogenation. Our model predicts the key intermediate in n-pentane aromatization on Ga/ZSM-5 zeolite to be a five-membered Ga-C4 ring structure. The ring undergoes expansion to form a seven-membered Ga-C6 ring. Moreover, we discuss thermodynamics and kinetic results for the benzene and toluene formation pathways. Our results provide new finding for the role of Ga/ZSM-5 zeolites in n-pentane aromatization processes.

The Application of Attenuated Total Reflection Infrared Spectroscopy to Investigate the Liquid Phase Hydrogenation of Benzaldehyde Over an Alumina-Supported Palladium Catalyst

The hydrogenation of benzaldehyde in cyclohexane over a 5 wt% Pd/Al2O3 catalyst at 313 K is firstly investigated at ambient pressure in a stirred batch reactor. The formation of benzyl alcohol is a facile process and a small mass imbalance is indirectly attributed to the formation of benzene as a by-product. No hydrogenolysis reaction to form toluene is observed. Secondly, examination of this reaction system by attenuated total reflection infrared (ATR-IR) spectroscopy enables the chemistry at the liquid/solid interface to be probed. Specifically, the ν(C=O) modes of solvated and adsorbed benzaldehyde are evident at 1712 and 1691 cm−1 respectively, providing information on how the reagent is partitioning within the reaction medium. Spectral acquisition on initiation of hydrogenation then enables the benzaldehyde → benzyl alcohol transition to be tracked. The additional presence of a broad CO stretching band of chemisorbed carbon monoxide (1852–1929 cm−1) is attributed to the hydrogen-assisted decarbonylation pathway that forms the benzene by-product.

Mechanism and Kinetics of Ethane Aromatization According to the Chemical Transient Analysis

The need for on-purpose techniques for the conversion of cheaper and abundant light alkanes to petrochemical products has revitalized research interests on light alkanes aromatization. Here ethane/propane aromatization and ethylene oligomerization over the representative Zn-HZSM-5 and Pt/HZSM-5 catalysts have been studied by the step-perturbation transients to provide insight into the kinetics and mechanisms leading to higher olefins and aromatics formation from ethane aromatization. The time-dependent catalytic behavior during the build-up and back-transient between ethane and inert, ethane and propane, as well as ethylene and inert, has been extensively discussed. We suggested that the hydrocarbon-pool mechanism be involved once ethylene was produced from the dehydrogenation of ethane. The oligomerization/cracking, cyclization, and dehydrogenation/hydride transfer reactions involved with the hydrocarbon-pool species reach the thermodynamic equilibrium quickly. The initial ethane dehydrogenation and the final formation of aromatics from their corresponding intermediates are slow surface-reactions. The rate constants k for benzene, toluene, and xylene formation from the “lumped hydrocarbon-pool” have been evaluated based on the first-order kinetic model of the back-transient. The rate constants k for aromatics over the Pt0 clusters/particles in the Pt/HZSM-5 are ⁓ 20–30% higher than that over the Zn (II) cations in the Zn-HZSM-5 catalyst.

Medium-Pressure Reactivity of Acetylene on Pd-Cu Alloy Nanoparticles Supported on Thin Silica Films.

The ability to correlate industrial high-pressure catalysis with high-vacuum research has been of great interest for decades. We employed a double-chamber vacuum system to study the self-hydrogenation of acetylene to ethylene and its trimerization to benzene at medium pressures to compare the reactivity in this pressure range to the known model catalytic acetylene reactivity in ultrahigh vacuum (UHV). We measured the reactivity of Pd-Cu bimetallic alloy nanoparticles (ANPs) with different elemental compositions deposited on top of native SiO2/Si(100) and on bilayer SiO2/Ru(0001) surfaces, where the latter was shown to contribute to ANP stability. Following exposure to 0.5 mbar of acetylene, ANPs on both surfaces catalyze the formation of ethylene and benzene, with ethylene as the more probable product. The ANPs on bilayer SiO2/Ru(0001) were highly selective toward ethylene formation, with an ethylene/benzene ratio of more than 2 orders of magnitude, whereas on the native SiO2/Si(100) there was a significantly lower selectivity (about 5) at the same temperature range and catalyst elemental composition. Interestingly, these selectivity values are similar to those found under UHV conditions. In addition, ANPs grown on native SiO2/Si(100), unlike SiO2/Ru(0001), revealed an optimal temperature for ethylene and benzene formation due to the limited stability of the particles.

Role of the metal-support interface in the hydrodeoxygenation reaction of phenol

In this work, the effect of interfacial sites between Pd particles and Nb2O5 species is investigated by testing a series of Pd-Nb2O5/SiO2 catalysts with different niobium loadings for the HDO reaction of phenol in the gas phase. Important differences in the selectivity to deoxygenated product were observed depending on the presence of niobium oxide close to Pd particles, which reveals the key role of the type of active phase in the control of reaction steps. It was found that Pd/SiO2 catalyst promotes hydrogenation pathways, producing cyclohexanone as the major product. For Pd-Nb2O5/SiO2 catalyst containing a Nb/Pd molar ratio of 0.5, a sharp increase in the selectivity to benzene is observed (7.5-fold). Increasing the Nb/Pd molar ratio, the formation of benzene is enhanced. The results showed that the Pd-Nb2O5 interface, composed by an oxophilic oxide in the perimeter of the metal particle, is responsible for the activation of the CO bond, promoting the deoxygenation reaction.

Promoting Aromatic Hydrocarbon Formation via Catalytic Pyrolysis of Polycarbonate Wastes over Fe- and Ce-Loaded Aluminum Oxide Catalysts.

Converting polycarbonate (PC) plastic waste into value-added chemicals and/or fuel additives by catalytic pyrolysis is a promising approach to dispose of solid wastes. In this study, a series of Fe-Ce@Al2O3 metal oxides were prepared by coprecipitation, impregnation, and a direct mixing method. The synthesized catalysts were then employed to investigate the catalytic conversion of PC wastes to produce aromatic hydrocarbons. Experimental results indicated that Fe-Ce@Al2O3 prepared by coprecipitation possessed superior catalytic activity because of its high content of weak acid sites, large pore volume, high surface area, and well dispersion of Fe and Ce active species, leading to an ∼3-fold increase in targeted monocyclic aromatic hydrocarbons compared to that achieved noncatalytically. Moreover, an increase in the catalyst to feedstock (C/F) mass ratio was beneficial to the production of aromatic hydrocarbons at the expense of phenolic products, and elevating the C/F ratio from 1:1 to 3:1 considerably increased the benzene formation as the enhancement factor was increased from 2.3 to 8.8.

The Chemistry of Cosmic Dust Analogues from C, C2, and C2H2 in C-Rich Circumstellar Envelopes.

Interstellar carbonaceous dust is mainly formed in the innermost regions of circumstellar envelopes around carbon-rich asymptotic giant branch (AGB) stars. In these highly chemically stratified regions, atomic and diatomic carbon, along with acetylene are the most abundant species after H2 and CO. In a previous study, we addressed the chemistry of carbon (C and C2) with H2 showing that acetylene and aliphatic species form efficiently in the dust formation region of carbon-rich AGBs whereas aromatics do not. Still, acetylene is known to be a key ingredient in the formation of linear polyacetylenic chains, benzene and polycyclic aromatic hydrocarbons (PAHs), as shown by previous experiments. However, these experiments have not considered the chemistry of carbon (C and C2) with C2H2. In this work, by employing a sufficient amount of acetylene, we investigate its gas-phase interaction with atomic and diatomic carbon. We show that the chemistry involved produces linear polyacetylenic chains, benzene and other PAHs, which are observed with high abundances in the early evolutionary phase of planetary nebulae. More importantly, we have found a non-negligible amount of pure and hydrogenated carbon clusters as well as aromatics with aliphatic substitutions, both being a direct consequence of the addition of atomic carbon. The incorporation of alkyl substituents into aromatics can be rationalized by a mechanism involving hydrogen abstraction followed by methyl addition. All the species detected in gas phase are incorporated into the nanometric sized dust analogues, which consist of a complex mixture of sp, sp2 and sp3 hydrocarbons with amorphous morphology.

Renewable Aromatics from Tree-Borne Oils over Zeolite Catalysts Promoted by Transition Metals.

Despite the ever-growing demand for benzene-toluene-xylene (BTX), the alternative route of production from tree-borne oils is rarely investigated and poorly understood. Here, we have synthesized a Zn-loaded Y-zeolite catalyst for the continuous production of bio-BTX from tree-borne oils (nonedible seed oil), e.g., neem oil. Our approach involves low-temperature selective cracking-dehydrogenation-aromatization of neem oil over metal-supported catalysts to xylene-rich aromatics. The physicochemical properties of the prepared catalyst were characterized using powder XRD, N2 physisorption, TEM, NH3-TPD, XPS, Py-FTIR, solid-NMR, and TG analyses. Mesoporous Y-zeolites with a pore diameter of 7.4 Å showed better selectivity toward aromatics and were found to be the most effective catalyst for the aromatization process, especially for BTX. The aromatic yield was found to increase with the addition of Zn, and the highest conversion of 90-94% with an ∼75% BTX yield was achieved with the ZnY catalyst. During aromatization, a sizable number of short alkanes and olefins were also obtained on acidic Y-zeolites. The off-gas composition shows the presence of ∼45% C2-C4 olefins with 8.9% H2. The incorporation of Zn species can promote the dehydrogenation activity, and the subsequent aromatization required a suitable pore network. The optimized ZnY catalyst inspires the formation of toluene and xylenes, inhibiting the formation of benzene and gaseous alkanes.

Direct synthesis of aromatics from syngas over Mo-modified Fe/HZSM-5 bifunctional catalyst

Direct synthesis of aromatics from syngas was investigated over a Mo-modified Fe/HZSM-5 bifunctional catalyst. The strong interaction of Fe-Mo promoted the dispersion of Fe species, strengthening the synergistic effect between iron carbides and acid sites in this reaction and promoting the formation of aromatics. Compared with the Fe/HZSM-5 catalysts modified by Zn, Mn, Pd, Na and K, MoFe/HZSM-5 displayed the optimal catalytic performance with a high aromatics productivity of 62.5 mmolaromatics/(gFe h. Especially, the increase of Mo loading decreased the external Brönsted acidity of catalyst, promoting the formation of benzene, toluene and xylene in aromatics, whereas suppressing the fraction of heavy aromatics.