Formation Of Toluene Research Articles

Palladium Nanoparticles Supported on Titanate Nanobelts for Solvent‐Free Aerobic Oxidation of Alcohols

AbstractThe solvent‐free selective oxidation of alcohols by O2 without any additives has been studied using palladium nanoparticles supported on a titanate nanobelt. The catalysts were prepared by polyol reduction and impregnation with different kinds of supports. They were evaluated in oxidation reactions of benzyl alcohol. The calcined titanate nanobelt was found to be the best support for the solvent‐free Pd‐catalyzed oxidation of alcohols and excellent catalytic activity as well as high selectivity to the corresponding benzaldehyde could be obtained. Moreover, palladium nanoparticles immobilized on the calcined titanate nanobelt could completely suppress the formation of toluene and benzoic acid, an effect that might be caused by the enhanced basic nature and morphology of the support. The catalyst was not only active and selective towards carbonyl compounds, but also recyclable under the oxidation conditions. This catalyst was also applicable for the oxidation of other alcohols.

Diels–Alder cycloaddition of 2-methylfuran and ethylene for renewable toluene

Diels–Alder cycloaddition of biomass-derived 2-methylfuran and ethylene provides a thermochemical pathway to renewable toluene. In this work, the kinetics and reaction pathways of toluene formation have been evaluated with H-BEA and Sn-BEA catalysts. Kinetic analysis of the main reaction chemistries reveals the existence of two rate-controlling reactions: (i) Diels–Alder cycloaddition of 2-methylfuran and ethylene where the production rate is independent of the Brønsted acid site concentration, and (ii) dehydration of the Diels–Alder cycloadduct where the production rate is dependent on the Brønsted acid site concentration. Application of a reduced kinetic model supports the interplay of these two regimes with the highest concentration of toluene measured at a catalyst loading equal to the transition region between the two kinetic regimes. Selectivity to toluene never exceeded 46%, as 2-methylfuran was consumed by several newly identified reactions to side products, including dimerization of 2-methylfuran, the formation of a trimer following hydrolysis and ring-opening of 2-methylfuran, and the incomplete dehydration of the Diels–Alder cycloadduct of 2-methylfuran and ethylene.

Dehydrogenation of methylcyclohexane to toluene over partially reduced Mo–SiO2 catalysts

Dehydrogenation of methylcyclohexane (MCH) has been carried out on a variety of partially reduced Mo(x)–SiO2 (x: Mo/Si molar ratio×100) catalysts differing by the molybdenum loading and which were prepared by sol–gel method. The objective was to maximize hydrogen production by monitoring the selectivity toward toluene formation. The MCH dehydrogenation was carried out in a down-flow reactor at 673K and 2.2MPa of total hydrogen pressure. The effect of Mo molar loading on the structure of the catalysts was studied by N2 physisorption, TPR, XRD, UV–vis DRS, NH3-TPD, XPS and TEM techniques. From the activity tests, it was concluded that Mo10–SiO2 catalyst, with Mo molar ratio of 10, exhibited the highest activity and the largest yield of toluene thanks to the existence of a good compromise between reduced Mo species and bulk MoO3. An increase of the molybdenum loading from 10 to 15 (molar ratio) led to: (i), the formation of non-active MoO3 phase; (ii), a low stable molybdenum species during on-stream operation; and (iii), a large coke formation due to an increase of the catalyst acidity.

Upgrading Process of 4-Methylanisole as a Lignin-Derived Bio-Oil Catalyzed by Pt/γ-Al2O3: Kinetic Investigation and Reaction Network Development

This study investigated the catalytic upgrading of 4-methylanisole using Pt/γ-Al2O3 in the presence of hydrogen with a fixed-bed tubular flow reactor at 573–673 K, 8–20 bar, and space velocities in the range of 3–120 (g of 4-methylanisole)/(g of catalyst × h). Selectivity-conversion data were used to determine an approximate reaction network. It is observed that the scission of the Cmethyl–O bond is faster that the scission of the Caromatic–O bond. So 4-methylphenol forms as a primary intermediate product via scission of the Cmethyl–O bond, and then hydrogenolysis of the Caromatic–O occurs giving toluene. Also another possible deoxygenation route involves a direct Caromatic–O bond-breaking reaction, leading to toluene. Multiple transalkylation reactions including intramolecular and bimolecular rearrangement account for methylphenols with multiple methyl substituents. The major products were 4-methylphenol, 2,4-dimethylphenol, and 2,4,6-trimethylphenol. So we inferred that transalkylation involving methyl in methoxy groups is one of the kinetically significant reaction classes catalyzed by the acidic support. A pseudo-first-order kinetic model was found to describe well the observed product distribution and to estimate kinetic parameters such as kinetic constants and activation energy of reactions. Kinetic studies revealed that the overall conversion improves with increasing temperature, pressure, and inverse space velocity. The formation of 4-methylphenol by the hydrogenolysis reaction was about two times faster than the formation of toluene by hydrodeoxygenation. The apparent activation energy for the formation of 4-methylphenol as the main product was approximately 15.5 kJ/mol.

Catalytic hydrodechlorination of benzyl chloride promoted by Rh-N-heterocyclic carbene catalysts.

The rhodium(I) complexes [Rh(Cl)(COD)(R-NHC-(CH2 )3 Si(OiPr)3 )] [COD=cyclooctadiene; R=2,6-diisopropylphenyl (1 a); n-butyl (1 b)] are effective catalyst precursors for the homogeneous hydrodechlorination of benzyl chloride using HSiEt3 as hydrogen source. This reaction is selective to the formation of toluene. However, in presence of a stoichiometric amount of potassium tert-butoxide (KtBuO) the formation of mixtures containing toluene together with 17-19 mol % of the C--C homocoupling product, namely PhCH2 CH2 Ph, is observed. A mechanism proposal based on experimental insights and theoretical calculations at the DFT level that allows explanation of the experimental findings is included. Moreover, the heterogeneous catalytic system based on catalyst 1 a supported on MCM-41 has been demonstrated to be effective for the solvent-free hydrodechlorination of benzyl chloride using HSiEt3 and HSiMe(OSiMe3 )2 .

Kinetic modeling of methane dehydroaromatization chemistry on Mo/Zeolite catalysts in packed-bed reactors

This paper develops a detailed chemical kinetics reaction mechanism to represent methane dehydroaromatization (MDA) chemistry on bi-functional Mo/zeolite catalysts. The model is validated using a range of previously published results from packed-bed experiments. The reaction mechanism consists of four methane activation reactions on Mo2C sites. The resulting gas-phase hydrogen and ethylene continue to react on Brønsted acid sites within the zeolite structure using 46 reaction steps. In addition to the desired benzene formation, the model also predicts the formation of toluene, naphthalene, and other side products. In addition to reaction kinetics on the catalyst surfaces, the packed-bed model incorporates a Dusty-Gas model that accommodates ordinary and Knudsen diffusion as well as pressure-driven advection. Because the model represents essentially all published reports of MDA performance, it is reasonably expected that the model can be applied as a predictive tool to support reactor and process development.

Destruction of the Tar Present in Syngas by Combustion in Porous Media

The cleaning of syngas is one of the most important challenges in the development of technologies based on gasification of biomass. Tar is an undesired byproduct because, once condensed, it can cause fouling and plugging and damage the downstream equipment. Thermochemical methods for tar destruction, which include catalytic cracking and thermal cracking, are intrinsically attractive because they are energetically efficient and no movable parts are required nor byproducts are produced. The main difficulty with these methods is the tendency for tar to polymerize at high temperatures. An alternative to tar removal is the complete combustion of the syngas in a porous burner directly as it leaves the particle capture system. In this context, the main aim of this study is to evaluate the destruction of the tar present in the syngas from biomass gasification by combustion in porous media. A gas mixture was used to emulate the syngas, which included toluene as a tar surrogate. Initially, CHEMKIN was used to assess the potential of the proposed solution. The calculations revealed the complete destruction of the tar surrogate for a wide range of operating conditions and indicated that the most important reactions in the toluene conversion are C6H5CH3 + OH ↔ C6H5CH2 + H2O, C6H5CH3 + OH ↔ C6H4CH3 + H2O, and C6H5CH3 + O ↔ OC6H4CH3 + H and that the formation of toluene can occur through C6H5CH2 + H ↔ C6H5CH3. Subsequently, experimental tests were performed in a porous burner fired with pure methane and syngas for two equivalence ratios and three flow velocities. In these tests, the toluene concentration in the syngas varied from 50 to 200 g/Nm3. In line with the CHEMKIN calculations, the results revealed that toluene was almost completely destroyed for all tested conditions and that the process did not affect the performance of the porous burner regarding the emissions of CO, hydrocarbons, and NOx.

Catalytic conversion of dimethyl ether to toluene over the molecular sieves modified with tungsten oxide

The catalytic performance of various zeolites modified with tungsten oxide in the conversion of dimethyl ether (DME) to toluene in the presence of oxygen was investigated in a continuous flow fixed-bed reactor. The results indicated that WO3/HZSM-5 as a catalyst is highly selective in the conversion of DME to toluene; under the optimized conditions, i.e. atmospheric pressure, 290°C, and with a DME/O2 mol ratio of 2:1, the conversion of DME is 98.97%, with the selectivity of 39.71% to toluene. The characterization results about the catalyst structure and acidity illustrate that the porous structure of ZSM-5 is suitable for the formation of toluene from DME. The doping of WO3 adjusts the distribution of the surface acid sites and then inhibits the formation of side-products in the presence of oxygen, which is able to enhance the selectivity to toluene.

Selective hydrogenation of benzoic acid over Au supported on CeO2 and Ce0.62Zr0.38O2: Formation of benzyl alcohol

The gas-phase hydrogenation of benzoic acid was studied over Au supported on CeO2 and Ce0.62Zr0.38O2 (CZ). HAADF-STEM has established formation of nanoscale (mean=1.5–2nm) Au particles, which is consistent with CO adsorption measurements. Incorporation of Au facilitated partial support reduction during TPR to 573K where the presence of Zr increased oxygen mobility, resulting in a greater degree of Ce4+ reduction (to Ce3+) in Au/CZ, as demonstrated by oxygen storage capacity and XPS measurements. Hydrogenation of an aqueous benzoic acid feed generated benzaldehyde and benzyl alcohol with a higher rate over Au/CZ that is attributed to the action of oxygen vacancies, which activate the carboxyl function for hydrogen attack. A parallel/consecutive kinetic model has been applied to quantify catalytic selectivity. A concerted (single step) conversion is proposed for Au/CeO2 that involves bridging interaction of the benzoate with Ce cations and Au nanoparticles with hydrogen addition. A stepwise conversion on Au/CZ is achieved via a Mars and van Krevelen mechanism with benzoic acid activation at an oxygen vacancy and reaction with surface hydrogen to generate benzaldehyde as a reactive intermediate that is converted to benzyl alcohol via nucleophilic CO attack. Switching from an aqueous to ethanolic feed increased rate due to greater oxygen vacancy availability with higher selectivity to benzaldehyde and appreciable toluene formation over Au/CZ.

Selective oxidation using supported gold bimetallic and trimetallic nanoparticles

Supported gold nanoparticles are highly effective for a range of redox reactions. In these reactions the activity is often enhanced by the addition of a second or indeed a third metallic component. A model reaction that is often investigated is the selective oxidation of benzyl alcohol using molecular oxygen as terminal oxidant. In the presence of a solvent the complexity of this reaction can often be missed. However, in the solvent-free oxidation of benzyl alcohol to benzaldehyde using supported gold palladium nanoparticles as catalysts, there are two pathways to the principal product, benzaldehyde. One is the direct catalytic oxidation of benzyl alcohol to benzaldehyde by O2, while the second is the disproportionation of two molecules of benzyl alcohol to give equal amounts of benzaldehyde and toluene. The formation of toluene is an unwanted side reaction. In this paper the research on this reaction will be reviewed and two strategies described that can be used to switch off the formation of the non desired toluene. The first involves the use of basic supports for the gold palladium nanoparticles, which is highly effective in suppressing the formation of toluene and this may be related to the morphology of the gold palladium nanoparticles and their interaction with the support. The second involves the introduction of platinum to the gold palladium nanoparticles which also switches off toluene formation on supports that permit toluene formation in the absence of platinum. This effect may be related to the relative stability of platinum hydride.

Toluene in sewage and sludge in wastewater treatment plants

Toluene is a compound that often occurs in municipal wastewater ranging from detectable levels up to 237 μg/L. Before the year 2000, the presence of the aromatic hydrocarbons was assigned only to external sources. The Enhanced Biological Nutrients Removal Processes (EBNRP) work according to many different schemes and technologies. For high-efficiency biological denitrification and dephosphatation processes, the presence of volatile fatty acids (VFAs) in sewage is required. VFAs are the main product of organic matter hydrolysis from sewage sludge. However, no attention has been given to other products of the process. It has been found that in parallel to VFA production, toluene formation occurred. The formation of toluene in municipal anaerobic sludge digestion processes was investigated. Experiments were performed on a laboratory scale using sludge from primary and secondary settling tanks of municipal treatment plants. The concentration of toluene in the digested sludge from primary settling tanks was found to be about 42,000 μg/L. The digested sludge supernatant liquor returned to the biological dephosphatation and denitrification processes for sewage enrichment can contain up to 16,500 μg/L of toluene.

Selective conversion of m-cresol to toluene over bimetallic Ni–Fe catalysts

The catalytic conversion of m-cresol in the presence of H2 has been investigated on SiO2-supported Ni, Fe, and bimetallic Ni–Fe catalysts at 300°C and atmospheric pressure. Over the monometallic Ni catalyst, the dominant product is 3-methylcyclohexanone while 3-methylcyclohexanol and toluene appear in smaller amounts, even at high conversions. By contrast, on Fe and Ni–Fe bimetallic catalysts, the dominant product is toluene while the hydrogenation products (3-methylcyclohexanone and 3-methylcyclohexanol) are practically negligible in the entire range of conversions.To explain these differences, we have proposed a deoxygenation path that starts with the tautomerization of m-cresol to an unstable ketone intermediate (3-methyl-3,5-cyclohexadienone). The fate of this intermediate is determined by the ability of the catalyst to either hydrogenate the carbonyl group or the ring. The former would mostly occur on Fe and Ni–Fe catalysts that contain an oxophilic metal (Fe), while the latter would occur on Ni, which has a higher affinity for the aromatic ring.Hydrogenation of the carbonyl group produces a very reactive unsaturated alcohol (3-methyl-3,5-cyclohexadienol), which can be easily dehydrated to toluene. This would explain the high selectivity of Fe and Ni–Fe to toluene. By contrast, hydrogenation of the ring would result in 3-methylcyclohexanone, which can be further hydrogenated to 3-methylcyclohexanol. On supports that contain acid sites, which are active for dehydration, the formation of toluene would occur via dehydration of the alcohol and subsequent dehydrogenation. On the catalysts investigated in this work, dehydration of the corresponding alcohol does not occur, so the only path to toluene is via hydrogenation of the carbonyl of the unstable ketone intermediate.In addition, to the products mentioned above, xylenol is also observed in significant yields, which indicate that transalkylation of m-cresol is another reaction path occurring on these catalysts.

Deoxygenation of m-cresol on Pt/γ-Al2O3 catalysts

In this work the deoxygenation of m-cresol, used as a bio-oil model compound, was studied at atmospheric pressure, using Pt/γ-Al2O3 catalysts. The main products obtained in this system were toluene, benzene and methylcyclohexane. Possible reaction routes for cresol deoxygenation were proposed. The influence of the metal loading, H2/cresol ratio and temperature on products distribution was analyzed. Catalyst deactivation was studied by TPO. The catalysts were characterized by XPS, XRD, BET, TPR, and TEM. Product distribution is explained based on the relative rates of cresol dehydroxylation and demethylation, which leads to toluene and phenol formation, respectively. High selectivity to toluene was obtained with the catalyst with the higher metal loading (1.7wt.%), while in the case of low metal loading (0.05wt.%) high yields of light hydrocarbons were obtained. Coke deposition took place preferentially on the metal particles and could be removed by treatment at low temperatures, e.g. 350°C. This catalyst displays a very good activity and selectivity towards toluene formation at mild temperature (300°C) and atmospheric pressure, which is a better performance than that already reported for other catalysts.

Alkylation of Benzene with Methane over ZnZSM-5 Zeolites Studied with Solid-State NMR Spectroscopy

Alkylation of benzene with methane was studied under oxidization condition over ZnZSM-5 zeolites by using in situ solid-state NMR spectroscopy and GC-MS analysis. The experimental results indicate that the alkylation reaction occurs with selective formation of toluene at ternperatures of 523-623 K using O-2 or N2O as the oxidant. Using C-13 isotope labeled reactants, the conversions of methane and benzene were independently monitored, and their respective role in the reaction was determined. It was found by NMR spectroscopy that methane was first activated into methoxy species and zinc methyl intermediates. As an electrophilic agent, the methyl group of methoxy species could directly attack phenyl ring to produce toluene via electrophilic substitution reaction, while the zinc methyl species was not directly involved in the alkylation reaction. However, the similar nature of zinc methyl species (Zn-CH3) to organozinc compounds allowed the facile oxidization of zinc methyl species (Zn-CH3) into methoxy species. As confirmed by GC-MS experiments, methane exclusively provided the methyl group of toluene product while benzene afforded the phenyl ring. Experimental results also indicated that neither methane nor benzene alone could generate toluene.

Switching-off toluene formation in the solvent-free oxidation of benzyl alcohol using supported trimetallic Au–Pd–Pt nanoparticles

Trimetallic Au-Pd-Pt nanoparticles have been supported on activated carbon by the sol-immobilisation method. They are found to be highly active and selective catalysts for the solvent-free aerobic oxidation of benzyl alcohol. The addition of Pt promotes the selectivity to the desired product benzaldehyde at the expense of toluene formation. Detailed aberration corrected STEM-XEDS analysis confirmed that the supported particles are indeed Au-Pd-Pt ternary alloys, but also identified composition fluctuations from particle-to-particle which vary systematically with nanoparticle size.

The central role of ketones in reversible and irreversible hydrothermal organic functional group transformations

Studies of hydrothermal reactions involving organic compounds suggest complex, possibly reversible, reaction pathways that link functional groups from reduced alkanes all the way to oxidized carboxylic acids. Ketones represent a critical functional group because they occupy a central position in the reaction pathway, at the point where CC bond cleavage is required for the formation of the more oxidized carboxylic acids. The mechanisms for the critical bond cleavage reactions in ketones, and how they compete with other reactions are the focus of this experimental study. We studied a model ketone, dibenzylketone (DBK), in H2O at 300°C and 70MPa for up to 528h. Product analysis was performed as a function of time at low DBK conversions to reveal the primary reaction pathways. Reversible interconversion between ketone, alcohol, alkene and alkane functional groups is observed in addition to formation of radical coupling products derived from irreversible CC and CH homolytic bond cleavage. The product distributions are time-dependent but the bond cleavage products dominate. The major products that accumulate at longer reaction times are toluene and larger, dehydrogenated structures that are initially formed by radical coupling. The hydrogen atoms generated by dehydrogenation of the coupling products are predominantly consumed in the formation of toluene. Even though bond cleavage products dominate, no carboxylic acids were observed on the timescale of the reactions under the chosen experimental conditions.

Selective suppression of disproportionation reaction in solvent-less benzyl alcohol oxidation catalysed by supported Au–Pd nanoparticles

Disproportionation of benzyl alcohol has been identified as the source of toluene formation in the solvent free oxidation of benzyl alcohol using supported gold palladium catalysts. There is a slight increase in the disproportionation reaction, and hence the toluene selectivity, when this reaction is performed in a continuous mode using a micro-packed bed reactor when compared to the same reaction performed in a conventional glass stirred batch reactor. Oxidation and disproportionation reactions respond slightly differently to the changes in reaction parameters, like oxygen concentration and pressure, when a micro packed bed reactor was used instead of a conventional glass stirred reactor. When MgO supported gold–palladium catalysts were used for this reaction, the toluene selectivity reduced substantially at the cost of conversion.

Selectivity of the Formation of the Ring-Closed Products and Methylcyclohexenes in the Dehydrogenation of Methylcyclohexane to Toluene

In our previous publication (Usman et al., 2011), the by-products formation during the dehydrogenation of methylcyclohexane over 1.0 wt% Pt/γ-Al2O3 was studied under wide range of operating conditions. The formation and yield patterns of benzene, cyclohexane, and isomers of xylene were studied. In the present contribution, the formation and yield patterns of the ring-closed products (sum of ethylcyclopentane and dimethylcyclopentanes) and methylcyclohexenes are studied. The yields of both of the ring-closed products (RCPs) and methylcyclohexenes (MCHes) were observed to increase with an increase in pressure. However, the effect of hydrogen concentration was found affecting differently at the low and at the high pressures. The yield patterns of methylcyclohexenes suggested methylcyclohexenes as the reaction intermediate in the formation of toluene.

Catalytic Hydroprocessing of p-Cresol: Metal, Solvent and Mass-Transfer Effects

A systematic study of the comparative performances of supported Pt, Pd, Ru and conventional CoMo/Al2O3, NiMo/Al2O3, NiW/Al2O3 catalysts as well as the effects of solvent, H2 pressure and temperature on the hydroprocessing activity of a representative model bio-oil compound (e.g., p-cresol) is presented. With water as solvent, Pt/C catalyst shows the highest activity and selectivity towards hydrocarbons (toluene and methylcyclohexane), followed by Pt/Al2O3, Pd and Ru catalysts. Calculations indicate that the reactions in aqueous phase are hindered by mass-transfer limitations at the investigated conditions. In contrast, with supercritical n-heptane as solvent at identical pressure and temperature, the reactant and H2 are completely miscible and calculations indicate that mass-transfer limitations are eliminated. All the noble metal catalysts (Pt, Pd and Ru) show nearly total conversion but low selectivity to toluene in supercritical n-heptane. Further, conventional CoMo/Al2O3, NiMo/Al2O3 and NiW/Al2O3 catalysts do not show any hydrodeoxygenation activity in water, but in supercritical n-heptane, CoMo/Al2O3 shows the highest activity among the tested conventional catalysts with 97 % selectivity to toluene. Systematic parametric investigations with Pt/C and Pt/Al2O3 catalysts indicate that with water as the solvent, the reaction occurs in a liquid phase with low H2 availability (i.e., low H2 surface coverage) and toluene formation is favored. In supercritical n-heptane with high H2 availability (i.e., high H2 surface coverage), the ring hydrogenation pathway is favored leading to the high selectivity to 4-methylcyclohexanol. In addition to differences in H2 surface coverage, the starkly different selectivities between the two solvents may also be due to the influence of solvent polarity on p-cresol adsorption characteristics.

Structure Formation of Toluene around C60: Implementation of the Adaptive Resolution Scheme (AdResS) into GROMACS.

For the example of C60 solutes in toluene, we present the implementation of the adaptive resolutions scheme (AdResS) for molecular simulations into GROMACS. In AdResS a local, typically all-atom cavity is coupled to a surrounding of coarse-grained, simplified molecules. This methodology can not only be used to reduce the CPU time demand of atomistic simulations but also to systematically investigate the relative influence of different interactions on structure formation. For this, we vary the thickness of the all atom layer of toluene around the C60 and analyze the first toluene layers in comparison to a full bulk simulation.