Partial Oxidation Of Toluene Research Articles

Experimental and kinetic modeling study of tar partial oxidative reforming by dielectric barrier discharge plasma using toluene as a model compound

In this study, partial oxidation of toluene as a biomass tar surrogate was carried out in a dielectric barrier discharge (DBD) plasma reactor. In order to understand the reaction characteristics and thus potential of the approach, the effect of reaction temperature, O2/toluene molar ratio (OTR) and discharge power on performances was investigated by experiments and kinetic modeling. The results showed that the three factors could change the E/n and hence the G-values of species, as well as alter O2 concentration in background gas and input energy, leading to the variation in active species production and thus, the performance. Appropriate high temperatures, as well as higher OTRs and discharge power obtained better performances by promoting toluene destruction and gas product production. Excited N2 species play a crucial role in the process, by means of participating in reactions directly and strongly influencing on the production of secondary active species. At 300 °C, the highest toluene conversion of 100.0 % with an energy efficiency of 25.7 g/kWh was achieved without catalysts usage, a comparable performance as compared to the steam reforming by plasma catalysis, demonstrating the potential of this method for the purification of gasification gases. Furthermore, based on the rate of production (ROP) and sensitivity analyses of the model developed, as well as experimental results, the reaction mechanism was proposed.

Boosting the performance of Co-doped CeO2 catalyst in the catalytic oxidation of toluene by modulating oxygen vacancies

Metal doping is one of the most effective strategies to enhance redox capacity and modulate the oxygen vacancies in Ce-based catalysts. This manuscript reports a Co-doped CeO2 catalyst (Co0.2Ce0.8 (W/E = 1/1)) for the catalytic oxidation of toluene fabricated by a facile co-precipitation method. The appropriate ratio of deionized (DI) water/ethanol in the reaction solvent contributed to the mesocrystallization of the CeO2 phase and the highly uniform dispersion of the Co-O-Ce mixed bond in the catalyst. The feasible lattice distortion induced the generation of richer oxygen vacancies in both the surface and bulk of the catalyst. Meanwhile, the efficient cycle of Co2+ + Ce4+ ↔ Co3+ + Ce3+ enhanced the redox property of the catalyst. As revealed by in situ DRIFTS studies, the as-formed surface oxygen vacancies promoted the adsorption and partial oxidation of toluene. The oxygen vacancies in the bulk of the catalyst improved the migration of lattice oxygen, accelerating the mineralization of the aromatic ring. Thus, the excellent low-temperature oxidation of toluene by Co0.2Ce0.8 (W/E = 1/1), reducing T90 to 192 °C, was attributed to the synergism between the catalyst’s surface and bulk oxygen vacancies.

Chemical looping partial oxidation (CLPO) of toluene on LaFeO3 perovskites for tunable syngas production

Tar is the by-product of coal and biomass gasification which decreases the purity of the primary syngas and causes blockages in the pipes. Chemical looping partial oxidation (CLPO) is a promising technology to convert tar to high-purity syngas in a sustainable way. Here, we synthesized LaFeO3 perovskites as oxygen carriers with five different wet chemistry synthesis methods and tested their performance for toluene CLPO in a fixed-bed reactor. The template synthesis method presents the highest purity, the most oxygen species adjacent to oxygen vacancy, the best resistance to the carbon deposition, and exhibits the best performance in CLPO, with up to ∼100 % toluene conversion efficiency, up to ∼100 % CO selectivity, and the best recyclability. The H2/CO ratio in the syngas products varies between 0.86 and 2.2 in different stages of the toluene CLPO process. By adjusting the reaction time of toluene catalytic cracking, the H2/CO ratio is tunable within this range. Based on these results, we designed a new in-situ CLPO process for tar-containing syngas purification that can achieve tunable H2/CO ratio for syngas. In the process, toluene cracking occurs in front of toluene partial oxidation by using reverse flow design to simultaneously achieve ∼100 % toluene conversion ratio and high conversion rate. This work shows that LaFeO3 is highly efficient in converting toluene to high-purity syngas, and it has the potential to convert tar-containing syngas in-situ to achieve energy- and cost-effective manufacture for high-value-added syngas products.

Roles of Oxygen Vacancies in the Bulk and Surface of CeO2 for Toluene Catalytic Combustion.

Catalytic combustion technology is one of the effective methods to remove VOCs such as toluene from industrial emissions. The decomposition of an aromatic ring via catalyst oxygen vacancies is usually the rate-determining step of toluene oxidation into CO2. Series of CeO2 probe models were synthesized with different ratios of surface-to-bulk oxygen vacancies. Besides the devotion of the surface vacancies, a part of the bulk vacancies promotes the redox property of CeO2 in toluene catalytic combustion: surface vacancies tend to adsorb and activate gaseous O2 to form adsorbed oxygen species, whereas bulk vacancies improve the mobility and activity of lattice oxygen species via their transmission effect. Adsorbed oxygen mainly participates in the chemical adsorption and partial oxidation of toluene (mostly to phenolate). With the elevated temperatures, lattice oxygen of the catalysts facilitates the decomposition of aromatic rings and further improves the oxidation of toluene to CO2.

Direct Benzyl Alcohol and Benzaldehyde Synthesis from Toluene over Keggin-Type Polyoxometalates Catalysts: Kinetic and Mechanistic Studies

The catalytic activity of various Keggin polyoxometalate catalysts has been investigated in the gas-phase partial oxidation of toluene to produce benzyl alcohol and benzaldehyde. The catalyst systems HPMo12O40, HPMo11VO40, FePMo12O40, and PMo11FeO39were prepared and characterized by FT-IR, UV-visible, SEM, XRD, TGA, and cyclic voltammetry. The acid/base properties were evaluated using the decomposition of isopropanol. Catalytic studies were carried under atmospheric pressure and over the temperature range 200°C–350°C, using carbon dioxide as a mild oxidant. Toluene conversion and product distribution depend mainly on the catalyst composition and operating conditions. In addition to benzaldehyde, benzyl alcohol is obtained with a high selectivity on the PMo11FeO39catalyst. The kinetic data show that the reoxidation of the reduced catalyst is the rate-limiting step for the partial oxidation reaction of toluene.

Direct ring C[sbnd]H bond activation to produce cresols from toluene and hydrogen peroxide catalyzed by framework titanium in TS-1

A series of TS-1 samples with varying Ti content was prepared and applied in the partial oxidation of toluene with H2O2 as oxidant. The TS-1 samples and relevant systems were characterized by XRD, FT-IR, in situ FT-IR, nitrogen adsorption/desorption, DR UV–vis, XPS, ICP-AES, EPR, SEM, TEM and TG. The results showed that the framework Ti species was responsible for the selective ring CH bond activation, resulting in toluene hydroxylation; while the extra-framework Ti species was advantageous to the formation of side chain oxidation products and to the deep oxidation of both kinds of the products formed. Under the optimized reaction conditions, the toluene conversion reached 38.9% with 84.3% selectivity to cresols (TOF, 2.66 h−1), while, the selectivity of para-cresol was 66.8%, which might be caused by the pore structure and steric hindrance of methyl group. The catalyst regenerated by calcination exhibited stable activity and selectivity after the second run.

Mesoporous V-AlPO – New Partial Oxidation Catalyst

<p>Vanadium incorporated hexagonal mesoporous aluminophosphate characteristic of MCM-41 has been synthesized. Characterization of the catalyst has been done by using low angle XRD, N<sub>2</sub> adsorption, UVVIS DRS, thermal analysis, XPS and ESR spectroscopy. These techniques confirm the presence of vanadium in both +4 and +5 oxidation states in the calcined material. Liquid phase partial oxidation of toluene has been carried out on V-AlPO using 70% TBHP and 30% H<sub>2</sub>O<sub>2</sub> as oxidants. When 70% TBHP is used as an oxidant, it resulted in side chain oxidation giving rise to aldehyde/acid where as with 30% H<sub>2</sub>O<sub>2</sub>, side chain as well as ring oxidation takes place. Catalytic activity of V-AlPO has been compared with that of VMCM-48 and other vanadium containing catalytic systems. It has been observed that mesoporous V-AlPO shows higher conversion and is also selective towards side chain oxidation products. The higher conversion of V-AlPO over V-MCM-48 has been attributed to the stabilization of the active V<sup>+5/+4</sup> species in AlPO framework as compared to silicate analogue.</p>

Partial oxidation of toluene with nitrous oxide under supercritical conditions

The partial oxidation of toluene with nitrous oxide over the H-ZSM-5 catalyst under supercritical conditions at temperatures of 370–420°C and pressures of 70–160 atm has been investigated for the first time. The maximum cresol selectivity under these conditions is 32%. The amounts of the resulting cresol isomers form the following decreasing sequence: m-cresol > o-cresol > p-cresol. The partial oxidation of toluene is accompanied by disproportionation and biphenyl formation.

Partial Oxidation of Toluene in CH3COOH by H2O2 in the Presence of VO (acac)2 Catalyst

The partial oxidation of toluene by hydrogen peroxide with catalytic amount of bis (acetylacetonato) oxovanadium (VO(acac)2) in acetic acid is studied in detail. Benzaldehyde is found to be the main product in the reaction, and V51NMR and ESI-MS are used to detect the possible intermediate species. The effect of reaction parameters, such as the amounts of catalyst and hydrogen peroxide used, the reaction temperature, and reaction time on the reaction are investigated. The mechanism of toluene partial oxidation is also studied, and the hydroxyl radical is found to be the key active species. The solvent acetic acid coordinates with vanadium atom, and the toluene partial oxidation passes through cage reaction to form the C6H5CH·(OOH) radical, which promptly decomposes to benzaldehyde and hydroxyl radical. C6H6CH2OOCH2C6H6 is also found to be one possible intermediate.

Effect of Fe doped over V–Sb oxide catalyst in toluene selective oxidation

Mixed oxides Fe x V 1− x SbO 4 with x = 0–1.0 were prepared by slurry method and characterized by X-ray diffraction (XRD), Mössbauer spectroscopy, and temperature-programmed reduction (TPR). The samples with x = 0, 0.4 and 1.0 were evaluated as catalyst in vapor phase toluene partial oxidation reaction. It was found by XRD that Fe, as doping, was introduced in the rutile structure of VSbO 4. The BET surface area of partially substituted solid with x = 0.4 was higher than those obtained for VSbO 4 and FeSbO 4 samples. It was shown that V is responsible of the hydrocarbon molecule activation. The catalyst with Fe x = 0.4 presented lower specific rate reaction and lower selectivity to total oxidation products than undoped solid (VSbO 4), which it would be related to a partial replace of V by Fe in the V–O–V chains. In conclusion, the solid activity and selectivity to total oxidation decrease due to a higher isolation of V sites, when V 3+ is substituted by Fe 3+ in the rutile structure.

Partial oxidation of toluene to benzaldehyde and benzoic acid over model vanadia/titania catalysts: role of vanadia species

Pure and K-doped vanadia/titania prepared by different methods have been studied in order to elucidate the role of vanadia species (monomeric, polymeric, bulk) in catalytic toluene partial oxidation. The ratio of different vanadia species was controlled by treating the catalysts in diluted HNO 3, which removes bulk vanadia and polymeric vanadia species, but not the monomeric ones, as was shown by FT-Raman spectroscopy and TPR in H 2. Monolayer vanadia species (monomeric and polymeric) are responsible for the catalytic activity and selectivity to benzaldehyde and benzoic acid independently on the catalyst preparation method. Bulk V 2O 5 and TiO 2 are considerably less active. Therefore, an increase of the vanadium concentration in the samples above the monolayer coverage results in a decrease of the specific rate in toluene oxidation due to the partial blockage of active monolayer species by bulk crystalline V 2O 5. Potassium diminishes the catalyst acidity resulting in a decrease of the total rate of toluene oxidation and suppression of deactivation. Deactivation due to coking is probably related to the Brønsted acid sites associated with the bridging oxygen in the polymeric species and bulk V 2O 5. Doping by K diminishes the amount of active monolayer vanadia leading to the formation of non-active K-doped monomeric vanadia species and KVO 3.

Partial Gas-Phase Oxidation of Toluene by the Heteropoly Acid H4PMo11VO40Supported on ShAS-2 Aluminosilicate

The effects of reaction conditions and concentration of the heteropoly acid H4PMo11VO40 supported onto ShAS-2 bead aluminosilicate on the conversion of toluene into benzoic acid in the partial oxidation of toluene by atmospheric oxygen were studied. The results demonstrated that the conversion of toluene was an extremal function of temperature, space velocity (v), and toluene concentration (C0) in the initial air mixture. An increase in the heteropoly acid concentration from 2 to 30% increased the conversion of toluene into benzoic acid in the partial oxidation of toluene from 1.5 to 12.6% at optimum process parameters: T = 300°C, v = 2000 h-1, and C0 = 13.72 g/m3.

Structural and catalytic properties of vanadia-based fibrous catalysts in toluene partial oxidation

Structured vanadia-based catalysts in the form of woven fabrics were successfully synthesised by incipient-wetness impregnation and sol–gel methods. The catalysts were investigated by Raman, X-ray photo-electron spectroscopies as well as high-resolution transmission electron microscopy (HRTEM). The spectroscopic results revealed that the elemental fibres consisted of a silica core covered by vanadia/titania. The modification of fibre silica surface by alumina resulted in a support with an increased dispersion of active vanadia/titania layer. Crystalline anatase was not found to be necessary for the formation of active surface vanadia species. Drying of titania layer without calcination led to a higher vanadia dispersion on titania. Structured fibrous vanadia-based catalysts demonstrated comparable activity and identical selectivity towards the products of toluene partial oxidation (benzaldehyde and benzoic acid) as granulated vanadia/titania with the same vanadium coverage. The transient response technique was applied to elucidate the role of the support on the active species. Thus, the V 2O 5/TiO 2/SiO 2 woven fibre fabrics show promise for hydrocarbon partial oxidation, providing advantages due to its open macro-structure suitable for design of structured catalytic beds.

DRIFTS and transient-response study of vanadia/titania catalysts during toluene partial oxidation

The work was aimed on the determination of the role of bulk V2O5 present in vanadia/titania catalysts for toluene partial oxidation. Two catalysts with 1.8 and 11.1 wt% of V were studied by in situ DRIFTS and transient-response methods. Bulk V2O5 was found by FT-Raman spectroscopy only in the 11.1 wt% V/TiO2 catalyst while monolayer vanadia species (monomeric and polymeric) in both samples. Toluene interaction in the presence of gaseous oxygen showed that the selectivity to benzaldehyde and benzoic acid was similar for the two catalysts. The nature of adsorbed species was also similar. Toluene interaction with the oxidised samples in the absence of gaseous oxygen showed rapid formation of coordinatively adsorbed benzaldehyde (∼1625 cm−1) and benzoate/carboxylate species (1600–1400 cm−1), accompanied by the disappearance of the monomeric vanadia species (∼2037 cm−1). The catalyst containing V2O5 could convert toluene more deeply than the catalyst without V2O5, namely to adsorbed benzoic acid (∼1670 cm−1). Additionally, large quantities of gaseous products (COx, benzaldehyde, H2O) were obtained with the former catalyst, while almost none with the latter. Thus, bulk V2O5 could supply oxygen for the monolayer vanadia species easily reducible by toluene. This oxygen is required for the product formation and desorption.

Einfluss von Pyridin auf die heterogen katalysierte Patialoxidation von substituierten Methylaromaten zu ihren Aldehyden

Vanadium-containing catalysts used in the partial oxidation of toluene and substituted toluenes, resp., to their aldehydes need two different catalyst functions: (1) acidic surface sites (Lewis-acid sites) for reactant chemisorption and (2) basic sites for an easier desorption of products. Therefore, the simultaneous addition of nonoxidizable pyridine to the feed leads to an increased aldehyde selectivity at similar conversion compared to runs without co-feeding of pyridine. This result is mainly due to a blockade of Bronsted-acid sites of the catalyst surface. Additionally, the desorption of the desired products could be sped up by an increased basicity of the catalyst surface. Beside catalyst properties, also different substituent effects play an important role for chemisorption. For example, the addition of pyridine to a p-chlorotoluene-containing reaction feed does not enhance aldehyde selectivity, otherwise, pyridine addition to p-methoxytoluene-containing feed results in drastically increased aldehyde selectivity.

Effect of Pyridine Admixture on Heterogeneously Catalyzed Partial Oxidation of Substituted Methyl Aromatics to their Corresponding Aldehydes

Vanadium-containing catalysts used in the partial oxidation of toluene and substituted toluenes, resp., to their aldehydes need two different catalyst functions: (1) acidic surface sites (Lewis-acid sites) for reactant chemisorption and (2) basic sites for an easier desorption of products. Therefore, the simultaneous addition of nonoxidizable pyridine to the feed leads to an increased aldehyde selectivity at similar conversion compared to runs without co-feeding of pyridine. This result is mainly due to a blockade of Bronsted-acid sites of the catalyst surface. Additionally, the desorption of the desired products could be sped up by an increased basicity of the catalyst surface. Beside catalyst properties, also different substituent effects play an important role for chemisorption. For example, the addition of pyridine to a p-chlorotoluene-containing reaction feed does not enhance aldehyde selectivity, otherwise, pyridine addition to p-methoxytoluene-containing feed results in drastically increased aldehyde selectivity.

Implication of the acid–base properties of V/Ti-oxide catalyst in toluene partial oxidation

The work presents the effect of K-doping on V/Ti-oxides taking into account: the surface acid–base properties and the structure of surface vanadia species in respect to the catalyst performance and deactivation. The structure of active surface species determines redox properties, which are related to the catalytic performance by the Mars–van Krevelen mechanism. The reducibility of surface vanadia is studied by temperature-programmed reduction (TPR) in H 2. The molecular structure of surface vanadia is determined by FT-Raman spectroscopy in a controlled atmosphere. Surface acid–base properties are characterised via temperature-programmed desorption (TPD) of pyridine with mass spectrometric analysis of the products. Transient response techniques with continuous monitoring of the composition of gaseous phase are applied to follow the catalyst surface transformations. Evolution of benzaldehyde (BA) formed during interaction of toluene with the pre-oxidised catalyst (without gaseous oxygen) gives information about the nucleophilicity of surface oxygen. Addition of potassium to surface vanadia leads to an increased oxygen nucleophilicity, resulting in a higher selectivity towards BA formation. In general, increase in surface basicity decreases catalytic activity, but at the same time the catalyst deactivation due to coking is suppressed. This allows catalyst optimisation in view of a better control of the partial oxidation process.

Deactivation kinetics of V/Ti-oxide in toluene partial oxidation

Deactivation kinetics of a V/Ti-oxide catalyst was studied in partial oxidation of toluene to benzaldehyde (BA) and benzoic acid (BAc) at 523–573 K. The catalyst consisted of 0.37 monolayer of VO x species and after oxidative pre-treatment contained isolated monomeric and polymeric metavanadate-like vanadia species under dehydrated conditions as was shown by FT Raman spectroscopy. Under the reaction conditions via in situ DRIFTS fast formation of adsorbed carboxylate and benzoate species was observed accompanied by disappearance of the band of the monomeric species (2038 cm −1) (polymeric species were not controlled). Slow accumulation of maleic anhydride, coupling products and/or BAc on the surface caused deactivation of the catalyst during the reaction. Temperature-programmed oxidation (TPO) after the reaction showed formation of high amounts of CO, CO 2 and water. Rate constants for the steps of the toluene oxidation were derived via mathematical modelling of reaction kinetics at low conversion and constant oxygen/toluene ratio of 20:1. The model allows predicting deactivation dynamics, steady-state rates and selectivity. The highest rate constant was found for the transformation of BA into BAc explaining a low BA yield in the reaction.

Transient kinetics of toluene partial oxidation over V/Ti oxide catalysts

Transient kinetics in the toluene oxidation over V/Ti oxide catalysts prepared by grafting and impregnation have been compared. V 4+ cations are supposed to be the sites for the formation of electrophilic oxygen species participating in deep oxidation. Another oxygen species (probably nucleophilic) present on the oxidised catalyst surface are responsible for benzaldehyde formation. Selectivity of 80–100% can be obtained during the initial period of the reaction on the grafted catalysts in the presence of gaseous oxygen and during the interaction of toluene (without O 2 in the mixture) with partially reduced catalysts.

Formation of Active Sites for Selective Toluene Oxidation during Catalyst Synthesis via Solid-State Reaction of V2O5 with TiO2

Interaction of V2O5 with TiO2 during the preparation of V/Ti-oxide catalysts via solid-state reaction has been studied by means of in situ FT-Raman spectroscopy, HRTEM and XPS. This interaction results in the formation of monomeric vanadia species with vanadium in tetrahedral coordination. The bridging oxygen in the V–O–Ti bond is suggested to be responsible for the catalytic activity during the partial oxidation of toluene. The formation of the monomeric vanadia species correlates with the improved catalyst performance, characterized by reaction rate and selectivity to benzaldehyde and benzoic acid. Mechanical activation by intensive grinding of V2O5/TiO2 mixture via ball milling was necessary for the interaction of the oxides during the calcination. The monomeric species formation was observed at a temperature as low as 523 K. The dynamics of V2O5/TiO2 interaction strongly depends on the presence of moisture during the calcination. In dry oxidative atmosphere, a part of the monomeric species is rapidly formed. Then, the process slows down and becomes diffusion-controlled. During the calcination in humid oxidative atmosphere, quick amorphization of bulk crystalline V2O5 was observed followed by slow formation of the monomeric vanadia species.