Oxidation Of P-xylene Research Articles

Isothermal Seeded Semicontinuous Reactive Crystallization of Crude Terephthalic Acid Crystals

In the reactive crystallization of terephthalic acid (TA) by liquid-phase oxidation of p-xylene (PX), four simultaneous physicochemical processes occurred, including the oxidation of PX to TA, the nucleation of TA from solution, the growth of TA crystals, and the incorporation of impurity 4-carboxybenzaldehyde (4-CBA). In this work, by using the experimental technique of seeded semicontinuous reactive crystallization, the growth of TA crystals and the incorporation of impurity 4-CBA were studied. The effects of supersaturation and temperatures on the crystallization kinetics were experimentally examined. A detailed semicontinuous reactor model including chemical reactions, TA growth, and 4-CBA incorporation was developed. The model was solved numerically to optimize the model parameters for crystallization kinetics. The effect of temperatures on the growth rate and the incorporation rate was studied and correlated. The correlated results agreed with the experimental results satisfactory. The obtained crystallization kinetics can be used as essential data to predict the growth of TA crystals and the incorporation of impurity 4-CBA.

Simple and effective synthesis of methoxy-dimethylbenzene from electrochemical oxidation of p-xylene in methanol solvent catalyzed by SO 42−/ZrO 2-M xO y

A series of composite solid superacid catalysts SO 4 2−/ZrO 2-M x O y (M = Ti, Ni, Si) were prepared and investigated by means of Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). All catalysts exhibit good catalytic activity in the electrochemical reaction of p-xylene with methanol assisted with a pair of porous graphite plane electrodes and the selectivity of main products higher than 90% were observed. In particular, SO 4 2−/ZrO 2-SiO 2 catalyst exhibited excellent catalytic activity. According to the experimental results, a possible free radical reaction mechanism confirmed by XPS and ultraviolet–visible (UV–vis) spectra was proposed. It may be concluded that a simple and feasible electrochemical catalytic oxidation reaction at room temperature and standard atmosphere may be possible.

Synthesis and Study of Unsymmetrical Schiff Base Mn(III) Complexes with Pendant Aza-crown or Morpholino Groups as Catalyst in Aerobic Oxidation for p-Xylene to p-Toluic Acid

A series of novel unsymmetrical Schiff base Mn(III) complexes with pendant aza-crown or morpholino groups have been synthesized and studied as catalysts in aerobic oxidation of `p-xylene to p-toluic acid (PTA). The oxidation of p-xylene to p-toluic acid with air at 120°C under normal atmospheric pressure occurred efficiently in the presence of aza-crown ether substituted unsymmetrical Schiff base Mn(III) complexes. Significant selectivity (up to ∼90%) and conversion levels (up to ∼40%) were obtained. The effect of the aza-crown ring appended in Mn(III) Schiff base complexes on the oxidation of p-xylene were also investigated by comparison with the morpholino group pendant analogues. The addition of alkali metal ions accelerates the rate of conversion of p-xylene to p-toluic acid.

CoBr2-MnBr2 containing catalysts for catalytic oxidation of p-xylene to terephthalic acid

Catalytic oxidation of p -xylene to terephthalic acid was studied with catalysts containing CoBr 2 and MnBr 2 . The catalysts contain neither highly corrosive hydrogen bromide nor other metal ions, and have the advantage of easy catalyst recovery. The maximum product yield obtained was 93.5% at 100 °C, which was much lower than the reaction temperature used for the commercial processes (175–225 °C). Catalytic oxidation of p -xylene (PX) to terephthalic acid (TA) was studied with catalysts containing cobalt acetate, manganese acetate, CoBr 2 and MnBr 2 . The catalysts contain neither highly corrosive hydrogen bromide nor other metal ions, and have the advantage of easy catalyst recovery. The effects of Br/Co atomic ratio, reaction time and temperature, PX concentration, oxygen pressure, and catalyst concentration on PX conversion and product/intermediate yields were investigated. The catalyst system had a suitable reaction temperature of 100 °C, which was much lower than the commercial process temperature (175–225 °C). The maximum product (TA) yield was 93.5%, obtained at a Br/Co atomic ratio of three. Higher Br concentration resulted in the lower TA yield, which was ascribed to the benzylic bromide formation. The synthesis of TA could be adequately described as four reaction steps in series (PX → p -tolualdehyde → p -toluic acid → 4-carboxybenzaldehyde → TA), with a pseudo-first-order rate equation for each step, and the third step was rate-limiting. The rate constant ratios ( k j / k 3 , j = 1 → 4) obtained at 100 °C were similar to the k j / k 3 values reported earlier for cobalt acetate/manganese acetate/HBr catalysts in a range of 185–191 °C.

Aerobic oxidation of p-xylene over metalloporphyrin and cobalt acetate: Their synergy and mechanism

The aerobic liquid-phase oxidation of p-xylene (PX) over metalloporphyrin and Co(OAc) 2 was studied, and the co-catalysis between metalloporphyrin and Co(OAc) 2 for the oxidation of PX was discovered for the first time. The results showed that both the PX conversion and terephthalic acid yield could be increased significantly despite the fact that only a minute amount of metalloporphyrin was added to the reaction mixture in addition to the Co(OAc) 2 catalyst. The effects of the structure of metalloporphyrin and reaction conditions such as temperature, air pressure and catalyst concentration on the overall reaction performance were also studied. A possible mechanism for the observed synergy between metalloporphyrin and Co(OAc) 2 as co-catalysts for the aerobic liquid-phase oxidation of PX was proposed based on some experimental observations. The results suggested that the PX oxidation was improved because of the acceleration of the chain initiation of PX oxidation by metalloporphyrin, and the acceleration of the chain initiation itself was due to the ease of peroxide formation over metalloporphyrin.

Semicontinuous Studies on the Reaction Mechanism and Kinetics for the Liquid-Phase Oxidation of p-Xylene to Terephthalic Acid

The reaction mechanism and kinetics for the liquid-phase catalytic oxidation of p-xylene to terephthalic acid are briefly reviewed, which indicates that the available literature is not complete when one looks for the industrially applicable mechanism and kinetics. In this work, a detailed radical chain reaction mechanism for the liquid-phase catalytic oxidation of p-xylene to terephthalic acid is proposed. Using several assumptions, a simple fractional-like kinetic model is derived from the assumed reaction mechanism. Several semicontinuous oxidation experiments are carried out at different reactant feed rates and temperatures. The experimental results show that the measured concentrations of liquid-phase reactants increase approximately linearly with time, which indicates that the reaction rate for the liquid-phase reactants is approximately independent from its concentration. The derived kinetic model can explain and correlate these experimental results satisfactorily. By combining the results determined in batch oxidation experiments with the semicontinuous experimental results, the kinetic model parameters are determined by data fitting. The correlated results generally agree with both the semicontinuous and the batch experimental results satisfactorily.

Synthesis, Structure, Spectral and Electrochemical Properties, and Catalytic Use of Cobalt(III)−Oxo Cubane Clusters

Olive-green cobalt(III) complexes having the general formula Co4O4(O2CMe)4L4 (1) where L = py (1a), 4-Mepy (1b), 4-Etpy (1c), and 4-CNpy (1d) have been prepared by the H2O2 oxidation of a mixture of Co2+, MeCO2-, and pyridine or 4-substituted pyridines in a 1:2:1 molar ratio in methanol. Spectroscopic and X-ray crystallographic studies show that these complexes contain a tetrameric cubane-like core [Co4(mu3-O)4]4+ where the four cobalt atoms form an approximate tetrahedron with edge lengths of approximately 2.75 A. Each cobalt in the crystallographically determined structure of Co4(mu3-O)4(mu-O2CMe)4L4 in 1a.NaClO4.3.5H2O and 1b.3H2O is hexacoordinate. Infrared spectra of the complexes show characteristic bands near 700, 635, and 580 cm-1 due to the central cubane-like core. 1H NMR spectra of the complexes show that the dissolved species are essentially diamagnetic and also that the complexes maintain their integrity in solution. UV-vis spectra of the green solutions have been interpreted in terms of ligand-field and charge-transfer bands. The electrochemical behavior of the complexes studied by cyclic and differential pulse voltammetric techniques indicates that the [(CoIII)4(mu3-O)4]4+ core present in the complexes undergoes a reversible one-electron oxidation to the [(CoIII)3CoIV(mu3-O)4]5+ core with an E1/2 value below 1 V. This suggests that these complexes of cobalt may be suitable as catalysts for the oxidation of organic compounds. Preliminary investigations indicate that 1a has a role to play in the cobalt-catalyzed aerobic oxidation of neat ethylbenzene and p-xylene.

An efficient aerobic oxidation for p-xylene to p-toluic acid catalyzed by cobalt (II) hydroxamates with benzo-15-crown-5

Cobalt (II) hydroxamates with the benzo-15-crown-5 (B15C5) pendant have been synthesized and characterized. These complexes were successfully employed to the oxidation of p-xylene to p-toluic acid with air at 110°C under normal atmospheric pressure. The effects of the B15C5 pendant, the length of chain bonding of B15C5 in these complexes and the addition of alkali metal ions on the oxidation for p-xylene are also investigated by the comparison with the crown-free analogue.

Kinetic Study of the Thermal Oxidation of p-Xylene to Terephthaldehyde

Thermal oxidation of p-xylene to terephthaldehyde (TPAL) was investigated in a tubular reactor at 1 bar with various reaction variables, that is, temperature (803−863K), space time (10620−46980 L·s/mol), and feed concentration of p-xylene (2500 and 7500 ppm). A kinetic modeling was performed on the basis of a power-law rate equation using a proposed simplified molecular reaction scheme. The reaction scheme consists of three reaction pathways: (1) p-xylene to p-tolualdehyde (PTAL), (2) p-xylene to TPAL, and (3) p-xylene to COx (CO and CO2). Model parameters were estimated from the experimental data through minimization of the objective function. The activation energy for the formation of TPAL from p-xylene was 49.2 kJ/mol. The order of reaction was approximated to zero with respect to partial pressure of p-xylene. The kinetic model yields an excellent fit of the experimental data.

Aerobic liquid-phase oxidation of p-xylene over metalloporphyrins

The five metalloporphyrins ( T (p- Cl ) PPM , M = Fe , Mn , Co , Cu , Zn ) with different metal nuclei were synthesized, and their catalytic aerobic liquid-phase oxidations of p-xylene into p-toluic acid, p-toluic aldehyde and terephthalic acid using a low concentration of acetic acid as solvent without any halide additives, were studied. The p-xylene conversions and the oxidation product distributions were found to be affected by the structures and concentration of the metalloporphyrins as well as the reaction parameters such as time, temperature and air pressure. The formation of some intermediate oxidation products in the oxidation process also influenced the reaction conversions and the product distribution. Among the metalloporphyrins used, tetrakis(p-chlorophenylporphinato)manganese chloride ( T (p- Cl ) PPMnCl ) was the most efficient catalyst for the oxidation of p-xylene. Under the conditions of 180 °C and 2.0 MPa, 44% conversion of p-xylene and 85% selectivity of p-toluic acid were obtained. Based on the results obtained, a preliminary mechanism of the oxidation of p-xylene over metalloporphyrins was proposed.

Data mining macrokinetic approach based on ANN and its application to model industrial oxidation of p-xylene to terephthalic acid

Considering that kinetics and thermodynamics are coupled with heat and mass transfer effects being present in the liquid-phase catalytic oxidation of p-xylene to terephthalic acid (OXTA) in an industrial type of continuous stirred tank reactor (CSTR) and that the time evolution of the concentration of all the interest intermediate and final products of OXTA in the industrial CSTR cannot be obtained to estimate macrokinetic parameters, a novel data mining macrokinetic approach based on artificial neural network (ANN) was proposed to develop the macrokinetic model of OXTA in the industrial CSTR, which mines intrinsic kinetics and transport phenomena information from the sample data collected from OXTA in the industrial CSTR. Firstly, the reaction orders of OXTA are estimated by the mass transfer-free experiment data in the laboratory semi-batch stirred tank reactor. The kinetics of OXTA in the industrial CSTR is assumed to be zeroth-order with respect to gaseous reactants, 0.65-order with respect to p-xylene, and first-order with respect to the other liquid reactants, respectively. Secondly, ANN is employed to model the influence of the reaction parameters on the rate constants of OXTA in the industrial CSTR. Based on the sample data collected from OXTA in the industrial CSTR, heuristic differential evolution algorithm is employed to adjust the weights and thresholds of the rate constant ANN in such a way that it minimizes the prediction error of the macrokinetic model, and thus the optimal weights and thresholds are obtained and the macrokinetic model of OXTA in the industrial CSTR is developed. The reliability of the macrokinetic model was investigated and the satisfactory results were obtained. Further, a generalized macrokinetic approach for multi-phase reaction in industrial reactor was suggested too.

A comparison of substrate dynamics in human CYP2E1 and CYP2A6

Considering the dynamic nature of CYPs, methods that reveal information about substrate and enzyme dynamics are necessary to generate predictive models. To compare substrate dynamics in CYP2E1 and CYP2A6, intramolecular isotope effect experiments were conducted, using deuterium labeled substrates: o-xylene, m-xylene, p-xylene, 2,6-dimethylnaphthalene, and 4,4′-dimethylbiphenyl. Competitive intermolecular experiments were also conducted using d 0- and d 6-labeled p-xylene. Both CYP2E1 and CYP2A6 displayed full isotope effect expression for o-xylene oxidation and almost complete suppression for dimethylbiphenyl. Interestingly, ( k H/ k D) obs for d 3- p-xylene oxidation (( k H/ k D) obs = 6.04 and ( k H/ k D) obs = 5.53 for CYP2E1 and CYP2A6, respectively) was only slightly higher than ( k H/ k D) obs for d 3-dimethylnaphthalene (( k H/ k D) obs = 5.50 and ( k H/ k D) obs = 4.96, respectively). One explanation is that in some instances ( k H/ k D) obs values are generated by the presence of two substrates-bound simultaneously to the CYP. Speculatively, if this explanation is valid, then intramolecular isotope effect experiments should be useful in the mechanistic investigation of P450 cooperativity.

Catalytic oxidation of p-xylene in water by cobalt(III) cubane cluster

A tetrameric cubane complex Co 4 III ( μ 3 -O ) 4 ( μ - O 2 CCH 3 ) 4 ( 4 -CNpy ) 4 ( I ) has been prepared in good yield by H 2O 2 oxidation of Co II in presence of CH 3 CO 2 2 - and 4-cyanopyridine (4-CNpy) in methanol. The complex has been characterized by various physicochemical techniques including elemental analysis, UV–Vis, infrared and 1H NMR spectroscopy and cyclic voltammetry. Oxidation of p-xylene under mild pressure with dioxygen in aqueous media has been studied using the tetrameric cubane complex I as the catalyst. The main oxidation product in the catalytic reaction is p-toluic acid is, with terephthalic acid forming in traces. The reaction rate and product yield (maximum p-toluic acid yield ∼36% at 20 bar oxygen pressure at 130 °C) are linearly dependent upon the applied pressure and reaction temperature.

Atmospheric Oxidation Mechanism ofp-Xylene: A Density Functional Theory Study

We report an investigation of the mechanistic features of OH-initiated oxidation reactions of p-xylene using density function theory (DFT). Reaction energies for the formation of the aromatic intermediate radicals have been obtained to determine their relative stability and reversibility, and their activation barriers have been analyzed to assess the energetically favorable pathways to propagate the p-xylene oxidation. OH addition is predicted to occur dominantly at the ortho position, with branching ratios of 0.8 and 0.2 for ortho and ipso additions, respectively, and the calculated overall rate constant is in agreement with available experimental studies. Under atmospheric conditions, the p-xylene peroxy radicals arising from initial OH and subsequent O(2) additions to the ring are shown to cyclize to form bicyclic radicals, rather than to react with NO to lead to ozone formation. With relatively low barriers, isomerization of the p-xylene bicyclic radicals to more stable epoxide radicals likely occurs, competing with O(2) addition to form bicyclic peroxy radicals. The study provides thermochemical and kinetic data for assessment of the photochemical production potential of ozone and formation of toxic products and secondary organic aerosol from p-xylene oxidation.

Electrochemical Oxidation of p-Xylene in Methanol Solvent Catalyzed by the Monometal and Multimetal Compounds

The electrochemical oxidation of p-xylene in a methanol solvent catalyzed by monometal and multimetal compounds to methoxyl p-xylene derivatives on a pair of porous graphite plane electrodes with a chemical conversion of >80% was described and investigated. From the results of ultraviolet-visible (UV−Vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), powder X-ray diffractometry (XRD), gas chromatography−mass spectrometry (GC-MS), and fluorescence spectroscopy, a possible free-radical reaction mechanism was proposed.

Optimum Ratio of Co/Mn in the Liquid-Phase Catalytic Oxidation of p-Xylene to Terephthalic Acid

Liquid-phase catalytic oxidation of p-xylene to terephthalic acid was performed at 150−210 °C over Co−Mn−Br catalyst system. There was an interesting synergistic effect of cobalt and manganese catalyst. In the previous papers (Ind. Eng. Chem. Res. 2005,44, 261−266; 4518−4522; 7756−7760), a lumped kinetic scheme and a fractional kinetic model for the liquid-phase oxidation of p-xylene were proposed and tested, and the effects of catalyst concentration, water content, and guanidine catalyst additive were investigated. In this paper, the synergistic effect of cobalt and manganese on the oxidation kinetics was studied. Experiments of several levels of Co/Mn ratio and temperature were carried out in a semibatch oxidation reactor where the gas and liquid phases were well-mixed. The results showed that the variation of the Co/Mn ratio in the catalyst did affect the activity and selectivity of the catalyst system. For the main reaction, there existed an optimum Co/Mn ratio at which the rates of the latter two slo...

Synthesis and Study of Mono-Schiff Base MnIII Complexes with Aza-crown or Morpholino Pendant as Catalyst in Aerobic Oxidation of p-xylene to p-toluic Acid

Novel mono-Schiff base MnIII complexes with pendant aza-crown or morpholino groups have been synthesized and studied as catalysts in aerobic oxidation of p-xylene to p-toluic acid. The oxidation of p-xylene to p-toluic acid with air at 120 °C under normal atmospheric pressure occured efficiently in the presence of aza-crown ether substituted mono-Schiff base MnIII complexes. Significant selectivity (up to ~90%) and conversion levels (up to ~38%) were obtained. The effect of the aza-crown ether pendant in MnIII Schiff base complexes on the oxidation of p-xylene was also investigated by comparison with the morpholino pendant analogues. The addition of alkali metal ions accelerated the rate of conversion of p-xylene to p-toluic acid.

The catalytic oxidation of organic compounds in superacids

Transformations of aromatic and aliphatic organic substances in anhydrous trifluoroacetic acid (TFA) at room temperature were studied. TFA was found to be capable of activating molecular oxygen dissolved in it. This activated oxygen endows the acid with strong oxidizer properties. Transformations of organic substances in TFA involved the formation of peroxo compounds, and alkylaromatic molecules were only oxidized at the benzene ring, side chains remaining intact. The transformations of toluene and p-xylene in trifluoroacetic acid were similar in a way to the biochemical oxidation of these substrates by mono-and dioxygenases. Mechanisms were suggested for the oxidation of toluene, p-xylene, and propylene by active oxygen in superacids.

Development of a tubular high-density plasma reactor for water treatment

Experiments have yielded a number of important insights into the energy distribution, sparging and oxidation of methyl tert-butyl ether (MTBE), benzene, ethylbenzene, toluene, m- and p-xylene, and o-xylene (BTEX) in a dense medium plasma reactor (DMPR). It has been found that the DMPR transferred a relatively small amount of electrical energy, approximately 4% in the form of sensible heat, to the surrounding bulk liquid. Rate constants associated with plasma initiated oxidation, interphase mass transfer and photolysis were determined using a combination of non-linear least squares analysis and M atlab ® optimization for each species. The rate constants developed for the DMPR, in conjunction with a species mass balance on a prototype tubular high-density plasma reactor, have been applied to determine the removal rates of MTBE and the BTEXs when operating in batch and continuous flow configurations. The dependence of contaminant concentration on parameters such as treatment time, the number of pin electrodes, electrode gap, and volumetric flow rate has been determined. It was found that, under various design specifications and operating conditions, the tubular high-density plasma reactor may be an effective tool for the removal of volatile organic compounds from aqueous solutions.

Effects of Guanidine on the Liquid-Phase Catalytic Oxidation of p-Xylene to Terephthalic Acid

In previous papers [Wang, Q.; Li, X.; Wang, L.; Cheng, Y.; Xie, G. Ind. Eng. Chem. Res. 2005, 44 (2), 261−266; 2005, 44 (13), 4518−4522], a lumped kinetic scheme and a fractional kinetic model for the liquid-phase oxidation of p-xylene catalyzed by Co−Mn−Br was proposed and tested and the effect of water on the oxidation reaction kinetics was investigated. In this paper the effect of guanidine catalyst additive on the liquid-phase oxidation of p-xylene to terephthalic acid over Co−Mn−Br catalyst system was studied. Experiments on four levels of guanidine composition were carried out in a semibatch oxidation reactor where the gas and liquid phases were well mixed. The results show that the addition of guanidine apparently accelerates the oxidation of the second methyl group. However, for the oxidation of the first methyl group there is an optimal guanidine composition w0. When the composition of guanidine is lower than w0, the oxidation rates for the first methyl group increase with the increase of guanidi...