Oxidation Of O-xylene Research Articles

Fe-doping induced textural and morphological evolution of MnO2 and its enhanced performance for o-xylene complete oxidation

Fe-MnO2 nanocomposite was prepared via a redox–precipitation method combined with a subsequent aging treatment at low-temperature. Various characterizations showed that Fe was incorporated into MnO2 lattice that changed the relative intensity of MnO2 crystal planes. Thus, the structural defect was formed and the anisotropic growth of the MnO2 crystallite phase was suppressed. As a result, the morphology of MnO2 was transformed from nanowires to nanoparticles. In addition, the catalytic property of Fe-MnO2 was significantly enhanced than that of pristine MnO2 for the complete oxidation of o-xylene. This work provides a simple and effective strategy for regulating MnO2 texture and activity.

Elucidating the low temperature chemistry of o-xylene enhanced by the reaction pool of dimethyl ether

The branched aromatic hydrocarbon with short side chains, such as o-xylene, has weak low temperature oxidation reactivity, while it could proceed low temperature chain recycling reactions by blending a fuel component with higher oxidation reactivity. However, the study of low temperature oxidation kinetics of o-xylene is very scarce. Therefore, this work investigated the low temperature oxidation process of o-xylene in an atmospheric jet stirred reactor by adding a biofuel with higher low temperature oxidation reactivity, i.e. dimethyl ether (DME). A series of characteristic low temperature intermediates, such as phenyl hydroperoxides and ketohydroperoxide from the oxidation of o-xylene, were detected by using the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The chemical structures of representative low temperature intermediates were determined by comparing the ionization thresholds from measured photoionization efficiency curves with calculated ionization energies. A kinetic model for o-xylene and DME mixture was developed and verified by literature and present experimental data. Rate of production (ROP) and sensitivity analyses were performed, which indicate that the low temperature oxidation chemistry of o-xylene, such as the negative temperature coefficient behavior, is enhanced by the reaction pool of DME. According to the classical low temperature oxidation mechanism of n-alkanes, the low temperature reactions of o-xylene were proposed. In particular, phenyl hydroperoxides and ketohydroperoxides were revealed to be key indicators for the occurrence of first and second O2 addition reactions during the oxidation of o-xylene, respectively. In addition, the sensitivity analysis uncovers the interaction kinetics between the reaction pools of o-xylene and DME, as well as their effects on fuel consumption and products formation. In summary, present evidence elucidates the low temperature chemistry of a representative branched aromatic, which could contribute to the development of future engines and control of combustion pollutants.

Performance of an Aliovalent-Doping MnCeOx/Cordierite Monolithic Catalyst Derived from MOFs for o-Xylene Oxidation

Performance of an Aliovalent-Doping MnCeOx/Cordierite Monolithic Catalyst Derived from MOFs for o-Xylene Oxidation

Promotion of catalytic performance of Pd/Al2O3 for o-xylene oxidation by morphological control

Pd/Al2O3 catalysts with different support morphologies (nanoflake, nanorod, nanobeam) were prepared and tested for o-xylene oxidation, and characterized by BET, XRD, TEM, XPS, NMR, CO-TPR-MS, H2-TPR and in-situ DRIFTS. The results indicated that the morphology has a strong effect on the dispersion of Pd species and the oxidation capacity of the catalysts. Compared with nanorod and nanobeam, the nanoflake morphology provides more surface defect sites, penta-coordinated Al3+ and surface OH species. Such sites (defect sites, AlP sites and OH species) enabled the immobilization of Pd to form Pd metal particles of small size, resulting in high Pd dispersion with primary exposure of Pd(100) facets possessing more defect sites for the adsorption. The OH species were also the adsorption sites. Both of them are conducive to the reactivity of adsorbed o-xylene. Moreover, the OH species on Pd/Al2O3 represented activated oxygen, indicating that o-xylene can be oxidized by OH directly. It means that these small Pd particles with more active surface OH species are responsible for the excellent catalytic performance of Pd/Al2O3-NF. Its o-xylene oxidation activity is much better than that of other Pd-based catalyst previously reported. Consequently, morphology control can manipulate Pd particle size and surface OH species to effectively improve the catalytic performance of Pd/Al2O3.

Room-temperature degradation of o-xylene in simulated air using an online-regenerable plasma-catalysis reactor with low amounts of nanosized noble metals on Co3O4

The plasma catalytic degradation of o-xylene in simulated air was improved by loading low amounts of Pt, Pd, or Au onto Co3O4. At room temperature, o-xylene conversion and CO x selectivity using a 0.1 wt% Pt/Co3O4 catalyst reached 98.9% and 80%, and the energy efficiency was at the top level in comparison with values in the literature. A stable o-xylene degradation performance could be obtained by online regenerating the heat-insulated reactor with a high energy density. After characterization, it was found that the loading of nanosized Pt not only increased the Co3+/Co2+ ratio, where the Co3+ benefitted the formation of reactive oxygen species, but also conduced Pt0 to oxygen activation, resulting in effective promotion of complete o-xylene oxidation. Operando plasma diffuse reflectance infrared Fourier transform spectroscopy demonstrated the complete o-xylene oxidation and proved that Pt played a key role in the complete oxidation of o-xylene.

Low temperature oxidation of o-xylene by Mn-based catalysts prepared by hydrothermal calcination: Influence mechanism of complicated flue gas components and intermediate product

Catalytic oxidation technology has proven to be an effective method for purifying flue gas of CFPPs. However, conventional catalysts face limitations such as limited activity and vulnerability to contamination from SO2 and H2O. This study aims to assess the efficiency of a newly synthesized Mn-based catalyst, prepared via a hydrothermal-calcination technique, in removing VOCs from simulated coal combustion flue gas. The results demonstrate that the catalyst produced through hydrothermal synthesis, followed by calcination at 400 °C, exhibits a catalytic oxidation efficiency of 96.2% for VOCs at 350 ℃. Furthermore, the catalyst demonstrates anti-poisoning properties. To gain a deeper understanding, we utilized DRIFTS to investigate the mechanism of o-xylene adsorption on the catalyst surface, reaction conversion intermediates, and the influence of complex flue gas components (NH3, NO, and SO2) on the catalytic reaction process. Our investigation provides insight in the improvement of Mn-based catalysts for the removal of VOCs from coal flue gas.

KINETIC EQUATIONS FOR RADICAL-CHAIN OXIDATION INVOLVING PROCESS-INHIBITING ALKYL (OR HYDRO)TETRAOXYL FREE RADICAL

The derivation of kinetic equations for the oxidation processes by the free-radical nonbranched-chain mechanism is shown. This derivation is based on the proposed reaction scheme for the initiated addition of free radicals to the multiple bond of the molecular oxygen includes the addition reaction of the peroxyl free radical to the oxygen molecule to form the tetraoxyl free radical. This reaction competes with chain propagation reactions through a reactive free radical. The chain evolution stage in this scheme involves a few of free radicals, one of which – alkyl(or hydro)tetraoxyl – is relatively low-reactive and inhibits the chain process by shortening of the kinetic chain length. The rate equations (containing one to three parameters to be determined directly) are deduced using the quasi-steady-state treatment. These kinetic equations were used to describe the γ-induced nonbranched-chain processes of free-radical oxidation of liquid o-xylene at 373 K and hydrogen dissolved in water containing various amounts of oxygen at 296 K. The ratios of rate constants of competing reactions and rate constants of addition reactions to the molecular oxygen are defined. In these processes the oxygen with the increase of its concentration begins to act as an oxidation autoinhibitor (or an antioxidant), and the rate of peroxide formation as a function of the dissolved oxygen concentration has a maximum. It is shown that a maximum in these curves arises from the competition between hydrocarbon (or hydrogen) molecules and dioxygen for reacting with the emerging peroxyl 1:1 adduct radical. From the energetic standpoint possible nonchain pathways of the free-radical oxidation of hydrogen and the routes of ozone decay via the reaction with the hydroxyl free radical in the upper atmosphere (including the addition yielding the hydrotetraoxyl free radical, which can be an intermediate in the sequence of conversions of biologically hazardous UV radiation energy) were examined. The energetics of the key radical-molecule gas-phase reactions is considered.

Construction of Pt-MnO2 interface with strong electron coupling effect for plasma catalytic oxidation of aromatic VOCs

The design of efficient catalysts with desirable active sites is an important direction for the plasma catalytic oxidation of VOCs. In this research, a combination of hydrothermal and impregnation methods was used to load Pt nanoparticles on MnO2 with different crystalline phases. The effect of Pt/MnO2 catalysts with different interactions between Pt and MnO2 on the plasma catalytic oxidation of o-xylene under ambient conditions was investigated. The plasma catalytic performance of different Pt/MnO2 varied greatly due to the different physicochemical properties of MnO2 with different crystalline phases. Among them, Pt/α-MnO2 showed the best catalytic performance with 100% o-xylene conversion and 93% CO2 selectivity at room temperature and 100 J/L energy density. The characterization results showed that the most electron transfer from Mn to Pt at the Pt/α-MnO2 interface produced the most oxygen vacancies and afforded the highest oxygen migration ability. The construction of Pt-MnO2 interface with strong electron coupling effect led to the excellent plasma catalytic efficiency as well as stability towards VOCs oxidation. This work provides inspiration for the design of efficient catalyst for the plasma catalytic oxidation of aromatic VOCs.

Roles of Oxygen Species in Low-Temperature Catalytic o-Xylene Oxidation on MOF-Derived Bouquetlike CeO2

To realize efficient low-temperature catalytic o-xylene oxidation, MOF-derived CeO2-X catalysts were prepared via the pyrolysis of MOF precursors with different ratios of cerium nitrate to trimesic acid. Among the synthesized catalysts, the bouquet like CeO2-1 exhibited the best activity with T50 and T90 of 156 and 198 °C and the lowest activation energy of 60.67 kJ·mol-1 (WHSV= 48 000 mL·g-1·h-1, o-xylene concentration = 500 ppm). o-Xylene was completely mineralized, and no change in conversion efficiency or CO2 yield was found at 5 vol % H2O for over 50 h. The rich active oxygen species (XPS: Osur/Olatt = 0.69) and abundant oxygen vacancies (Raman: ID/IF2g = 0.036) of CeO2-1 made crucial contribution to its superior catalytic activity. The O2-TPD and H2-TPR results confirmed that CeO2-1 had more surface active oxygen and better mobility of bulk oxygen. Moreover, the reaction routes under different atmospheres were probed through in situ DRIFTS, in which oxygen vacancy played a key role in promoting the adsorption and activation of molecular oxygen and facilitating the migration of the bulk lattice oxygen.

Improved and Reduced Performance of Cu- and Ni-Substituted Co3O4 Catalysts with Varying CoOh/CoTd and Co3+/Co2+ Ratios for the Complete Catalytic Oxidation of VOCs.

The Co3O4 spinel is one of the most promising transition metal oxide (TMO) catalysts for volatile organic compound (VOC) treatment. Substituting effects have usually been utilized to improve the catalytic performance of the Co3O4 spinel. In this study, Cu- and Ni-substituted Co3O4 catalysts derived from mixed metal-organic frameworks (MMOFs) retained similar spinel structures but exhibited improved and reduced performance for o-xylene oxidation, respectively. Physicochemical characterization and DFT calculations revealed that Cu and Ni substitution into the Co3O4 spinel varied the valence (Co3+/Co2+) and geometry (CoOh/CoTd) distributions of Co cations through different partial electron transfer and substitution sites. The higher Co3+/Co2+ and CoOh/CoTd ratios of the CuCo2O4 catalyst contributed to the superior reducibility and oxygen mobility, which facilitated the oxidation of intermediates at lower temperatures in the catalytic oxidation of o-xylene. Meanwhile, the NiCo2O4 catalyst with lower Co3+/Co2+ and CoOh/CoTd ratios could not completely oxidize intermediates under the same conditions due to inferior redox properties. Therefore, the CuCo2O4 catalyst showed superior catalytic activity and stability to the NiCo2O4 catalyst for the catalytic oxidation of o-xylene. This work provides insights into the synthesis of substituted Co3O4 catalysts from MMOFs and mechanism of substituting effects, which might guide the design of efficient TMO catalysts for VOC treatment.

Engineering Platinum Catalysts via a Site-Isolation Strategy with Enhanced Chlorine Resistance for the Elimination of Multicomponent VOCs

Pt-based catalysts can be poisoned by the chlorine formed during the oxidation of multicomponent volatile organic compounds (VOCs) containing chlorinated VOCs. Improving the low-temperature chlorine resistance of catalysts is important for industrial applications, although it is yet challenging. We hereby demonstrate the essential catalytic roles of a bifunctional catalyst with an atomic-scale metal/oxide interface constructed by an intermetallic compound nanocrystal. Introducing trichloroethylene (TCE) exhibits a less negative effect on the catalytic activity of the bimetallic catalyst for o-xylene oxidation, and the partial deactivation caused by TCE addition is reversible, suggesting that the bimetallic, HCl-etched Pt3Sn(E)/CeO2 catalyst possesses much stronger chlorine resistance than the conventional Pt/CeO2 catalyst. On the site-isolated Pt-Sn catalyst, the presence of aromatic hydrocarbon significantly inhibits the adsorption strength of TCE, resulting in excellent catalytic stability in the oxidation of the VOC mixture. Furthermore, the large amount of surface-adsorbed oxygen species generated on the electronegative Pt is highly effective for low-temperature C-Cl bond dissociation. The adjacent promoter (Sn-O) possesses the functionality of acid sites to provide sufficient protons for HCl formation over the bifunctional catalyst, which is considered critical to maintaining the reactivity of Pt by removing Cl and decreasing the polychlorinated byproducts.

N-doped carbon-modified palladium catalysts with superior water resistant performance for the oxidative removal of toxic aromatics

The supported palladium catalysts perform well in the oxidative removal of hazardous aromatic hydrocarbons. However, water vapor can seriously deactivate the catalysts especially in the low-temperature regime. Hence, improving moisture resistance of the Pd-based catalysts is full of challenge in the removal of aromatics. Herein, we report a new type of Pd@NC/BN catalysts featured with nitrogen-doped carbon layers modified Pd supported on hexagonal boron nitride (h-BN), and the relationship between structure and water resistance of the catalysts. The results show that in the presence of 10 vol% H2O in the feedstock, the Pd@NC/BN catalyst could effectively oxidize o-xylene (with an almost 87% removal efficiency), whereas o-xylene conversion declined from 69% to 20% over the conventional Pd/Al2O3 at a reaction temperature of 210 °C and a space velocity of 40,000 mL/(g h). The adsorption of H2O was significantly inhibited on the nitrogen-doped carbon layers due to the hydrophobic nature. Meanwhile, the oxygen species active for o-xylene oxidation were not only from the adsorbed gas-phase oxygen but also from the new active oxygen (*OOH and *OH) species that were generated via the interaction of O2 and H2O in the presence of water in the feedstock. It is concluded that the reactive oxygen species that accelerated the activation and cleavage of C−H bonds significantly facilitated the conversion of key intermediate species (from benzaldehyde to benzoic acid), thus playing a decisive role in o-xylene oxidation. The present work provides a direction for developing the superior water resistance catalysts with hydrophobic nature and good water activation ability in the oxidative removal of volatile organic compounds.

Siloxane-modified MnOx catalyst for oxidation of coal-related o-xylene in presence of water vapor

In coal-combustion energy production, presence of water vapor in flue gas causes catalyst deactivation and leads to the release of large quantities of volatile organic compounds (VOCs). In this study, design of a low-temperature, hydrophobic catalyst for flue gas purification was achieved by modifying support material with inorganic siloxane. Introduction of 5% water vapor into simulated flue gas at 300 °C reduced oxidation efficiency for o-xylene removal by 26% with unmodified MnOx/γ-Al2O3 catalyst, whereas with modified catalyst MnOx-Si0.9/γ-Al2O3 oxidation efficiency was reduced by only 5%. MnOx-Si0.9/γ-Al2O3 exhibited stable catalytic efficiency for o-xylene gas oxidation containing water vapor for over 200 min. Water-resistance of the catalyst was effective for removal of multi-coal combustion pollutants (Hg0 and NO) and moreover, hydrophobicity of the catalyst led to a reduction in surface sulfate deposition, thereby lowering toxicity of SO2 from simulated flue gas. DRIFTS analysis showed that the hydrophobic catalyst surface not only reduces water adsorption, but also promotes water volatilization. Based on molecular adsorption energies, catalyst support modification with siloxane inhibits water adsorption and promotes organic adsorption and thus provides a new strategy for preparing water-resistant catalysts for flue gas purification.

Simultaneous oxidation of aromatic compounds using Sr‐doped lanthanum manganites as catalysts

Strontium-doped lanthanum manganites, La1-xSrxMnO3 (LSMO), are promising and affordable catalysts for oxidative degradation of volatile organic compounds. LSMO catalysts (x = 0, .1, .2, and .3) were prepared by the citrate-nitrate autocombustion (CNA) and coprecipitation synthesis. The phase composition was confirmed by X-ray diffraction and Rietveld refinement analysis, while the oxygen content was determined by Mohr's salt permanganate titration. Morphology and porosity of prepared catalysts was correlated to catalytic oxidation of benzene, toluene, ethylbenzene and o-xylene. It was observed that both synthesis methods yielded catalysts of similar average pore size diameter and specific surface area, but the pore size distribution differed: CNA-prepared catalysts had a multimodal pore size distribution, while the coprecipitated ones had a single maximum at 4 nm. Catalysts prepared by the CNA method have shown a higher catalytic activity in the temperature range 373–723 K, as the presence of Mn3+/Mn4+ mixed valences increased their reducibility.

Engineering Pt@MnOx/γ-Al2O3 catalyst with enhanced Pt-MnOx interface to boost plasma catalytic oxidation of o-xylene

Plasma catalysis technology is suitable for the emission control of low concentration volatile organic compounds (VOCs), but the energy cost and complete oxidation are the bottleneck for its application. Herein, MnOx/γ-Al2O3 catalysts incorporated with small amounts of Pt nano particles were designed for plasma catalytic oxidation of o-xylene which is one typical kind of VOCs from paint industry. A dielectric barrier discharge reactor installed with Pt0.1Mn6/γ-Al2O3 catalyst can decompose 90% o-xylene oxidation with 90% CO2 selectivity at an energy density of 211 J/L and a reaction temperature of 150 °C, while only 29% o-xylene conversion and 5% CO2 selectivity were obtained without discharges under the same reaction condition. The energy cost is as high as 12.3 g/kWh which is reached top level at the same VOCs concentration. The excellent performance of Pt@MnOx/γ-Al2O3 catalyst is due to the synergistic effect of Pt nano particles and MnOx, which promoted the activation of O2 and the complete oxidation to CO2 of by-products.

Utilization of Hematite Particles for Economical Removal of o-xylene in a High-Temperature Gas-Solid Reactor.

To establish a novel approach for VOCs resource utilization, coupled o-xylene oxidation and hematite reduction was investigated in this study in a high-temperature gas-solid reactor in the temperature range 300–700 °C. As the o-xylene-containing inert gas (N2) stream traveled through the hematite particle bed, its reaction behavior was determined in programmed heating and constant temperature modes. Consequently, the effect of bed temperature, flow rate and o-xylene inlet concentration on both o-xylene removal performance and degree of hematite reduction was studied. The raw hematite and solid products were analyzed by TGA, XRF, XRD and SEM-EDS. The results showed that a temperature above 300 °C was required to completely eliminate o-xylene by hematite, and both o-xylene removal capacity and degree of hematite reduction at 5% breakthrough points enhanced on increasing the temperature and decreasing the flow rate. The increment in temperature from 300 °C to 700 °C led to a gradual reduction of Fe2O3 to Fe3O4, FeO and metallic iron. Thus, this study provides a novel, economic and promising technology for treating the VOC pollutants.

Effect of different reduction methods on Pd/Al2O3 for o-xylene oxidation at low temperature

Pd/Al2O3 was pretreated by CO, H2 and NaBH4 reduction, respectively. The reduced catalysts were tested for o-xylene oxidation and characterized by power X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and temperature-programmed decomposition of palladium hydride (TPDH). The characterizations indicate the pretreatments lead to distinct Pd particle sizes and amount of surface activated oxygen species, which are responsible for the catalytic performance. Compared with H2 and NaBH4 reduction methods, CO reduction shows a strong interaction between Pd and Al2O3 with smaller Pd particle size and more surface activated oxygen. It exhibited excellent catalytic performance, complete oxidation of 50 ppmV o-xylene at 85°C with a WHSV of 60,000 mL/(g∙hr).

"Derivation of Kinetic Equations for Free-Radical Nonbranched-Chain Processes of Hydrocarbon and Hydrogen Oxidation"

The derivation of kinetic equations for the oxidation processes by the free-radical nonbranched-chain mechanism is shown. This derivation is based on the proposed reaction scheme for the initiated addition of free radicals to the multiple bond of the molecular oxygen includes the addition reaction of the peroxyl free radical to the oxygen molecule to form the tetraoxyl free radical. This reaction competes with chain propagation reactions through a reactive free radical. The chain evolution stage in this scheme involves a few of free radicals, one of which (tetraoxyl) is relatively low-reactive and inhibits the chain process by shortening of the kinetic chain length. The rate equations (containing one to three parameters to be determined directly) are deduced using the quasi-steady-state treatment. The kinetic description with use the obtained rate equations is applied to the γ-induced nonbranched-chain processes of the free-radical oxidation of liquid o-xylene at 373 K and hydrogen dissolved in water containing different amounts of dioxygen at 296 K. The ratios of rate constants of competing reactions and rate constants of addition reactions to the molecular oxygen are defined. In these processes the oxygen with the increase of its concentration begins to act as an oxidation autoinhibitor (or an antioxidant), and the rate of peroxide formation as a function of the dissolved oxygen concentration has a maximum. It is shown that a maximum in these curves arises from the competition between hydrocarbon (or hydrogen) molecules and dioxygen for reacting with the emerging peroxyl 1:1 adduct radical. From the energetic standpoint possible nonchain pathways of the free-radical oxidation of hydrogen and the routes of ozone decay via the reaction with the hydroxyl free radical in the upper atmosphere (including the addition yielding the hydrotetraoxyl free radical, which can be an intermediate in the sequence of conversions of biologically hazardous UV radiation energy) were examined. The energetics of the key radical-molecule gas-phase reactions is considered.

Effect of promoters on o-xylene oxidation pathways reveals nature of selective sites on TiO2 supported vanadia

The o-xylene oxidation pathways on TiO2 supported vanadium oxide catalysts are affected by Cs+ and Sb4+ promoters. Reaction kinetics change remarkably at 360 °C: while at lower temperatures the rate determining step is the C-H bond cleavage in xylene methyl groups, the catalyst reoxidation becomes kinetically limiting at higher temperatures. Promotion by Sb4+ is related to an increase in the availability of oxygen at high temperatures for the reoxidation of V sites. The presence of Cs+ enhances the reoxidation of V sites and increases the ratio of nucleophilic to electrophilic oxygen on the surface. Catalytic tests of benzene oxidation confirm the lack of electrophilic sites on Cs-promoted VOx/TiO2 catalyst. Combined evidence points to a key role of the bridging lattice oxygen in V-O-V species as active site for the activation of o-xylene and its selective transformation to phthalic anhydride.

Combined experimental and theoretical study of o-xylene elimination on Fe–Mn oxides catalysts

The development of low-cost and easily accessible catalysts to realize the practical applications of catalytic combustion of volatile organic compounds remains a challenge. In this work, a series of Fe–Mn oxides catalysts were prepared via a facile redox-precipitation route for the elimination of o-xylene. Among the synthesized catalysts, Fe3Mn1-RP exhibited excellent activity for o-xylene elimination with a T50 and T90 of 223 °C and 236 °C, respectively (o-xylene concentration = 500 ppm, WHSV = 36,000 mL g−1 h−1). Characterization results demonstrated that superior catalytic activity could be achieved from large specific surface area, good reducibility and high proportion of Mn4+. Besides, high Fe contents proved beneficial in generating additional oxygen vacancies, thereby improving the performance of the catalyst. The stable crystal structures and surface electron density distributions of the catalysts, and adsorption sites of o-xylene on the catalyst surface, were also determined through density functional theory (DFT) calculations to provide an in-depth mechanism on how the o-xylene oxidation occurred. Moreover, analysis of the energy barrier during the oxidation process proved that the ring-opening reaction on the surface of Fe3Mn1-RP with an activation energy as low as 2.46 eV would more likely occur via oxygen vacancies.