A periodic DFT study of intramolecular isomerization reactions of toluene and xylenes catalyzed by acidic mordenite

A periodic density functional theory (DFT) study of the isomerization reactions of toluene and xylene catalyzed by acidic mordenite is reported. Monomolecular isomerization reactions have been considered and analyzed. The different reaction pathways have been discussed in detail. The use of periodic structure calculations allows consideration and analysis of zeolite electrostatic contributions and steric constraints that occur within zeolite micropores. Major differences in the details of protonation reaction pathways are found when periodic structures are used rather than small cluster models of the Br?nsted acidic site. Complex relationships are found between zeolite topology and reaction pathways.     

Toluene combustion: reaction paths, thermochemical properties, and kinetic analysis for the methylphenyl radical + O2 reaction

Aromatic compounds such as toluene and xylene are major components of many fuels. Accurate kinetic mechanisms for the combustion of toluene are, however, incomplete, as they do not accurately model experimental results such as strain rates and ignition times and consistently underpredict conversion. Current kinetic mechanisms for toluene combustion neglect the reactions of the methylphenyl radicals, and we believe that this is responsible, in part, for the shortcomings of these models. We also demonstrate how methylphenyl radical formation is important in the combustion and pyrolysis of other alkyl-substituted aromatic compounds such as xylene and trimethylbenzene. We have studied the oxidation reactions of the methylphenyl radicals with O2 using computational ab initio and density functional theory methods. A detailed reaction submechanism is presented for the 2-methylphenyl radical + O2 system, with 16 intermediates and products. For each species, enthalpies of formation are calculated using the computational methods G3 and G3B3, with isodesmic work reactions used to minimize computational errors. Transition states are calculated at the G3B3 level, yielding high-pressure limit elementary rate constants as a function of temperature. For the barrierless methylphenyl + O2 and methylphenoxy + O association reactions, rate constants are determined from variational transition state theory. Multichannel, multifrequency quantum Rice-Ramsperger-Kassel (qRRK) theory, with master equation analysis for falloff, provides rate constants as a function of temperature and pressure from 800 to 2400 K and 1 x 10(-4) to 1 x 10(3) atm. Analysis of our results shows that the dominant pathways for reaction of the three isomeric methylphenyl radicals is formation of methyloxepinoxy radicals and subsequent ring opening to methyl-dioxo-hexadienyl radicals. The next most important reaction pathway involves formation of methylphenoxy radicals + O in a chain branching process. At lower temperatures, the formation of stabilized methylphenylperoxy radicals becomes significant. A further important reaction channel is available only to the 2-methylphenyl isomer, where 6-methylene-2,4-cyclohexadiene-1-one (ortho-quinone methide, o-QM) is produced via an intramolecular hydrogen transfer from the methyl group to the peroxy radical in 2-methylphenylperoxy, with subsequent loss of OH. The decomposition of o-QM to benzene + CO reveals a potentially important new pathway for the conversion of toluene to benzene during combustion. A number of the important products of toluene combustion proposed in this study are known to be precursors of polyaromatic hydrocarbons that are involved in soot formation. Reactions leading to the important unsaturated oxygenated intermediates identified in this study, and the further reactions of these intermediates, are not included in current aromatic oxidation mechanisms.     

DFC-1氢型丝光沸石的吸附热与甲苯歧化催化活性的研究

在工业生产条件下对催化剂和催化作用的动态研究已经引起重视。吸附热能反映催化剂表面能量状况和吸附分子间的相互作用,是表征催化剂表面性质的重要参量。对于国产甲苯歧化催化剂DFC-1氢型丝光沸石(简称HMDFC-1)的吸附热和催化活性之间关系的报道还不多。在用吸附差热方法考察HMDFC-1的表面酸性时发现,在动态条件下HMDFC-1存在着对NH_3可逆和不可逆两种吸附,而对吡啶则只有不可逆吸附。本文利用脉冲色谱法分别测定了苯、甲苯、二甲苯在HMDFC-1上以及在被NH_3中毒的HM吸附热,和甲苯在被吡啶中毒的HMDFC-1上的吸附热,并以吸附热的差别区别了两类DFC-1上的酸中心,找出了催化活性随甲苯吸附热的变化曲线。     

Adhesion of a pair of polyethylene moldings by polyethylene gels

It was found that polyethylene gels in solvents such as benzene, toluene, xylene, decalin, tetralin, tetrachloroethylene, 1,1,2,2-tetrachloroethane, and chlorobenzene are effective for adhesion of a pair of polyethylene plates. In particular, the adhesion strength of polyethylene gels in decalin, tetralin and tetrachloroethylene was strong enough for practical use.Adhesive effect appears due to local dissolution of the surface of polyethylene plate in contact with the gel with increasing temperature, and subsequent recrystallization.     

Effect of Preparation Methods of Bi2O3 Nanoparticles on their Photocatalytic Activity

Bi2O3 nanoparticles were prepared by means of ammonia precipitation, polyol mediated methods and microemulsion chemical method. The structure and properties of the as-prepared nanoparticles, having been submitted to a heat-treatment test at 750℃, were characterized by means of XRD. BET, XPS and UV-Vis absorption techniques. The photocatalytic oxidation reactions of benzene, toluene and xylene were used as the model reaction to measure the photocatalytic activity of Bi2O3 nanoparticles, respectively. The results show that the crystallite size of Bi2O3 prepared with different methods and calcined at 750℃ were 50. 6, 38.5 and 31.5 nm, respectively. The photocatalytic activity of Bi2O3 nanoparticles prepared with the microemulsion chemical method was higher than that of the particles prepared with the polyol mediated method; and that of the particles prepared with the micromulsion chemical method was the highest among the three. The degradation rates of the three pollutants xylene, toluene and benzene decreased in sequence.     

Quantitative analysis of trace‐level benzene,toluene, ethylbenzene,and xylene in cellulose acetate tow using headspace heart‐cutting multidimensional gas chromatography with mass spectrometry

This study describes a method for the quantification of trace‐level benzene, toluene, ethylbenzene, and xylene in cellulose acetate tow by heart‐cutting multidimensional gas chromatography with mass spectrometry in selected ion monitoring mode. As the major volatile component in cellulose acetate tow samples, acetone would be overloaded when attempting to perform a high‐resolution separation to analyze trace benzene, toluene, ethylbenzene, and xylene. With heart‐cutting technology, a larger volume injection was achieved and acetone was easily cut off by employing a capillary column with inner diameter of 0.32 mm in the primary gas chromatography. Only benzene, toluene, ethylbenzene, and xylene were directed to the secondary column to result in an effective separation. The matrix interference was minimized and the peak shapes were greatly improved. Finally, quantitative analysis of benzene, toluene, ethylbenzene, and xylene was performed using an isotopically labeled internal standard. The headspace multidimensional gas chromatography mass spectrometry system was proved to be a powerful tool for analyzing trace volatile organic compounds in complex samples.     

气相色谱法分离和测定溶剂型木器涂料中正丁醇与苯系物

在气相色谱法测定溶剂型木器涂料中正丁醇及苯系物(包括苯、甲苯及二甲苯)时采用了VF-200MS型号的毛细管色谱柱。由于此型号的毛细管柱具有较强极性,不仅能达到正丁醇与苯系物之间的有效而满意的分离,而且还能使苯系物(包括苯、甲苯、乙苯、二甲苯)之间和二甲苯的同分异构体之间的有效分离。测定中采用正戊烷作内标和用火焰离子检测器检测。此方法的检出限(2S/N)在1.7~3.5 ng之间。用一含有正丁醇的硝基漆样品按所提出的方法进行多次重复测定,并在此样品的基础上加入各被测化合物的标准后进行测定,从而对方法的回收率和精密度作了测试,测得其回收率在87.8%~100.0%之间,测定值的相对标准偏差(n=6)均小于2.5%。     

Energy transfer from cis- and trans-decalin to benzene and toluene in their irradiated mixtures

The fluorescence from β^- particle irradiation of cis- and trans-decalin containing benzene or toluene has been studied as a function of aromatic concentration from ⋍ 0.002 to 0.1 M and over a spectral range that encompasses both the solvent and aromatic fluorescence. By comparisons with the fluorescence obtained using sub-ionization excitation of the decalin, the effect of benzene and toluene to intrude into the geminate ion-pair decay process has been extracted and rate constants for their scavenging action obtained via fitting to the standard diffusion model. The rate constants are compared to those reported from microwave conductivity studies on the “escaped” mobile hole in these liquids. For the reaction between trans-decalin⁺ + toluene, the rates are in good agreement. However, for the reactions of either cis- or trans-decalin⁺ with benzene, the rate constants extracted from the fluorescence analysis are about an order of magnitude larger. The discrepancies suggest the existence of differences in the internal energies and structures of the decalin positive ions when observed on the very short time scale of geminate recombination (probed in the fluorescence measurements) and that which is observed on the much longer time scales that are probed in the microwave experiments.An analysis of the development of aromatic fluorescence permits extraction of the fraction of aromatic fluorescence that derives from ionic recombination (as opposed to energy transfer) and the averaged efficiency of this recombination. In all of the systems studied here the ionic fraction remains high (i.e. >;20%) even at millimolar concentrations of the aromatic.     

Recognition of benzene, toluene and xylene using TGS array integrated with linear and non-linear classifier

Three volatile organic compounds (VOCs): benzene, toluene and xylene were measured with an array of six Taguchi gas sensors in the air with variable humidity content. The recognition of single compounds was performed, based on measurement results. The principal component analysis (PCA) pointed at humidity as the main classification factor in the measurement data set. The linear discriminant analysis (LDA) was applied to overcome this drawback and enforce classification with respect to benzene, toluene or xylene. It was shown that discriminant function analysis (DFA), which is an LDA method allowed for 100% success rate in test samples recognition of benzene. It did not allow for accurate recognition of test samples of toluene or xylene. Following, the non-linear classifier, radial basis function neural network (RBFNN) was applied. A specific configuration of input ‘s was found, which provided for successful recognition of each single compound: benzene, toluene or xylene in air with variable humidity content.     

Molar excess enthalpies of cis-decalin + benzene, + toluene, +isooctane and +heptane at 298.15 K

Molar excess enthalpies H^E of cis-decalin + benzene, +toluene, +isooctane and +heptane mixtures have been measured by an LKB flow microcalorimeter at 298.15 K. The experimental results are analyzed using the Flory-Patterson-Prigogine theory. The isomer effect of decalin molecule and the effect of the molecular size and shape of the component molecules are discussed.     

Arene Selectivity by a Flexible Coordination Polymer Host

The coordination polymers [Ag₄(O₂CCF₃)₄(phen)₃] ? phen ? arene ( 1? phen ? arene) (phen=phenazine; arene=toluene, p‐xylene or benzene) have been synthesised from the solution phase in a series of arene solvents and crystallographically characterised. By contrast, analogous syntheses from o‐xylene and m‐xylene as the solvent yield the solvent‐free coordination polymer [Ag₄(O₂CCF₃)₄(phen)₂] ( 2 ). Toluene, p‐xylene and benzene have been successfully used in mixed‐arene syntheses to template the formation of coordination polymers 1? phen ? arene, which incorporate o‐ or m‐xylene. The selectivity of 1? phen ? arene for the arene guests was determined, through pairwise competition experiments, to be p‐xylene>toluene≈benzene>o‐xylene>m‐xylene. The largest selectivity coefficient was determined as 14.2 for p‐xylene:m‐xylene and the smallest was 1.0 for toluene:benzene.     

Factors affecting the sorption of Eu(III) on modified silica gel</p> </p>

Summary 8-Hydroxyquinoline in benzene, xylene, chloroform and toluene diluents was used to modify silica gel as a solid phase extractant (SPE) for the sorption of Eu(III) in batch extraction techniques. Influences of solid/liquid ratio, pH, metal ion concentration, particle size and temperature were studied. The optimum initial pH is 4.2, while the maximum sorption capacities for the prepared impregnated resins in benzene, xylene, chloroform and toluene diluents are 18.52, 14.98, 14.79 and 5.94 mg ^. g^-1, respectively. The sorption process is found to be affected by both metal ion concentration and particle size of the impregnated resin. Thermodynamic parameters for the sorption of Eu(III) were determined and the reaction is found to be exothermic and spontaneous with enthalpy-14.23 and-23.71 kJ ^. mol^-1 for benzene and xylene as diluents. Release of the element from the loaded solid particles into 0.01M HNO₃ is@85% and@53% from 8-HQ/benzene/silica gel and 8-HQ/xylene/silica gel.</p> </p>     

Adsorption studies of acetic acid dimers on activated charcoal from organic solvents

Acetic acid exists as dimers in organic solvents like benzene, toluene and xylene. Adsorption of dimeric acetic acid on activated charcoal (AC) at various temperatures from benzene, toluene and xylene solutions have been studied. The system obeys Langmuir isotherm, thus signifying a monolayer adsorption of dimers. Corrections on AC-solvent pore volume fillings, molecular cross sectional surface area of acetic acid dimers, the adsorption equilibrium constants, the free energy change and the enthalpy change values are computed at different temperatures for the three solvents. The adsorption process has been found to be physisorption type. The FTIR measurements show that the adsorbed acetic acid dimer seems to retain the cyclic structure against the open chain non-cyclic structure.     

Study on adsorption kinetic of aromatic hydrocarbons onto activated carbon in gaseous flow method

The adsorption behavior of benzene, toluene, o-xylene, m-xylene, and p-xylene onto activated carbon was investigated using the flow method. The removal efficiency of aromatic hydrocarbons in the gaseous phase was estimated based on the adsorption kinetic constants and the saturated amount of aromatic hydrocarbons adsorbed on the activated carbon. The saturated amount of benzene and toluene adsorbed was greater than that of xylene adsorbed because the molecular sizes of benzene and toluene are smaller than that of xylene. The adsorption kinetic constant increased in the order of xylene, toluene, and benzene. Those of the three xylene isomers were similar. These results indicated that the adsorption rate of benzene by the activated carbon was the fastest and the kinetic constant depended upon the different between the boiling point and the melting point and the molecular size of the aromatic hydrocarbons.     

苯酚、马尿酸和甲基马尿酸的反相高效液相色谱分析

本文提出了人体内苯、甲苯和二甲苯的代谢产物苯酚、马尿酸和甲基马尿酸的反相高效液相色谱分析法,讨论了其保留机制和样品预处理技术.以ODS为固定相,甲醇-水-醋酸为流动相时可实现良好分离.方法回收率为97.3%,相对标准偏差为1.02%.提出的方法可用于尿样分析,适用于临床与职业病防治的监测分析.     

High benzene selectivity of mesoporous silicate for BTX gas sensing microfluidic devices

The gas selectivities of highly ordered mesoporous silicates and commercially-obtained porous silicates with respect to benzene, toluene and xylene were studied. After studying the porosities, pore uniformities, and surface silanol structures of the silicates and their relationships to gas selectivity in detail, we found that we could achieve high benzene selectivity by controlling the micropore size (less than 1 nm). Concluding that mesoporous silicate has a suitable micropore size and structure for benzene selectivity, we also observed that mesoporous silicate SBA-16 exhibited a high (>6) benzene selectivity from toluene and xylene even in a pseudo-atmospheric environment. A benzene detection limit of about 100 ppb was achieved by introducing SBA-16 into a microfluidic device originally developed for the separate detection of benzene, toluene, and xylene gases.     

Zn/HZSM-5分子筛催化剂对丙烷芳构化反应中芳烃分布的影响

采用浸渍法,制备了Ｚｎ／ＨＺＳＭ－５分子筛催化剂,考察了锌含量、反应温度对丙烷芳构化反应的影响,根据产物分布情况,讨论了锌物种在丙烷芳构化反应中的作用,并以氮气作载气,带入烷基芳烃乙苯、二甲苯、甲苯,考察了温度对催化剂脱烷基反应的影响。结果表明,锌物种不公能够催化丙烷的脱氢活化,提高其转化率,而且能促进随后进行的齐聚、脱氢芳构化第一系列反应,提高芳烃的选择性,并进一步催化烷基芳烃的脱烷基反应,导致     

十氢萘气相氧化裂解制低碳烯烃的研究

十氢萘气相氧化裂解（GOC）与传统的热裂解工艺相比,O2的存在降低了十氢萘GOC反应的活化能,使反应在较低温度下具有高的反应性能;O2同时起到消除积炭的作用,提高体系的抗积炭能力。十氢萘GOC反应在较低温度下即可获得较高的液体收率;高温下由于十氢萘裂解深度较高,低碳烯烃收率可高于液体收率,在所得的液体中,芳烃,尤其是BTX（苯、甲苯和二甲苯）占主要部分。十氢萘GOC反应制备低碳烯烃的适宜反应条件为,700℃~800℃,停留<0.4s,烷氧摩尔比0.3~0.5,空气可代替纯氧进料。800℃,烷氧摩尔比0.5,停留0.4s,可获得37％左右的低碳烯烃收率和50%左右的液体收率（BTX收率为29％）。     

Theoretical study of the formation of naphthalene from the radical/π-bond addition between single-ring aromatic hydrocarbons

The experimental investigations performed in the 1960s on the o-benzyne + benzene reaction as well as the more recent studies on reactions involving π-electrons highlight the importance of π-bonding for different combustion processes related to PAH's and soot formation. In the present investigation radical/π-bond addition reactions between single-ring aromatic compounds have been proposed and computationally investigated as possible pathways for the formation of two-ring fused compounds, such as naphthalene, which serve as precursors to soot formation. The computationally generated optimized structures for the stationary points were obtained with uB3LYP/6-311+G(d,p) calculations, while the energies of the optimized complexes were refined using the uCCSD(T) method and the cc-pVDZ basis set. The computations have addressed the relevance of a number of radical/π-bond addition reactions including the singlet benzene + o-benzyne reaction, which leads to formation of naphthalene and acetylene through fragmentation of the benzobicyclo[2,2,2]octatriene intermediate. For this reaction, the high-pressure limit rate constants for the individual elementary reactions involved in the overall process were evaluated using transition state theory analysis. Other radical/π-bond addition reactions studied were between benzene and triplet o-benzyne, between benzene and phenyl radical, and between phenyl radicals, for all of which potential energy surfaces were produced. On the basis of the results of these reaction studies, it was found necessary to propose and subsequently confirm additional, alternative pathways for the formation of the types of PAH compounds found in combustion systems. The potential energy surface for one reaction in particular, the phenyl + phenyl addition, is shown to contain a low-energy channel leading to formation of naphthalene that is energetically comparable to the other examined conventional pathways leading to formation of biphenyl compounds. This channel is the first evidence of a reaction which involves an aromatic radical adding to the nonradical π-bond site of another aromatic radical which leads directly to a fused ring structure.     

HZSM-5催化甲苯和甲醇烷基化反应机理的密度泛函理论研究

对二甲苯(PX)是重要的有机化工原料,主要用于生产对苯二甲酸(PTA)和对苯二甲酸二甲酯(DMT), PTA和 DMT可经缩聚生产化纤、合成树脂和塑料等聚酯产品. PX主要通过甲苯歧化、二甲苯异构化或甲苯与 C9芳烃烷基转移等方式生产.由于三种二甲苯和乙苯的沸点接近,需要经过吸附分离或深冷分离才能得到高纯度的 PX,传统工艺物料循环量大,设备庞大,操作费用高.而通过甲苯和甲醇烷基化反应直接高选择性生成 PX,可大大降低成本,具有非常高的经济效益和研究价值.自1970年代以来,国内外众多科研院所对甲苯和甲醇烷基化催化剂进行了广泛研究,但催化剂选择性和稳定性仍需进一步提高.为了加深对甲苯和甲醇烷基化反应的认识,指导催化剂开发,有必要对甲苯和甲醇烷基化生成二甲苯的反应机理进行深入研究.当前甲苯和甲醇烷基化机理研究主要存在以下问题：(1)计算得到的能量多为电子能,而非自由能;(2)所采用的模型多为团簇模型,使用 ONIOM方法,对长程作用力描述不充分;(3)认为甲苯只有一种吸附状态;(4)没有考虑偕烷基化反应.本文采用周期性模型,通过密度泛函理论研究了 HZSM-5分子筛上甲苯和甲醇烷基化反应机理,通过计算熵得到了反应自由能,并考虑了偕烷基化反应.由于甲基的存在,在甲苯的吸附态中,甲基会伸向孔道的不同方向,因此我们认为甲苯有多种吸附态,而不同的吸附态会生成不同的二甲苯.结果表明,甲苯可以在对位、间位、邻位和偕位上通过协同机理或分步机理发生烷基化反应.在协同机理中,甲苯在对位、间位、邻位和偕位发生烷基化反应的自由能垒分别为167,138,139和183 kJ/mol.在分步机理中,甲醇脱水生成甲氧基的自由能垒为145 kJ/mol,是决速步骤;而甲苯和甲氧基对位、间位、邻位和偕位烷基化的自由能垒分别为127,105,106和114 kJ/mol.两种机理中 PX的生成能垒均比 MX和 OX高,与文献报道的结果不同.文献均认为, PX的生成能垒最低.一方面这可能是由于所采用模型的不同,本文采用周期性模型,能更充分考虑长程作用力的影响;另一方面可能是由于对甲苯吸附态的不同处理,我们认为甲苯有多种吸附态,不同的吸附态会生成不同的二甲苯,而文献均只考虑了一种甲苯吸附态.但是,在实验中, PX选择性最高.这可能是由于：(1) PX在 HZSM-5孔道的扩散速率比 MX和 OX高2–3个数量级;(2)甲苯和甲醇烷基化生成的 MX和OX迅速发生异构化反应生成 PX,异构化反应速率高于甲苯烷基化速率.两种机理中, C8H11+都是重要的中间物种,它可以反馈一个质子给分子筛骨架,生成二甲苯;也可以脱烷基生成甲烷和乙烯等气相产物.研究发现,甲烷的生成是由于 C8H11+物种中的一个 H质子从苯环上的碳原子转移到甲基上的碳原子造成的,计算得到的对位、间位和邻位 C8H11+生成甲烷的能垒分别为136,132和134 kJ/mol.由于十元环孔道的限制, HZSM-5孔道中很难通过甲苯歧化反应生成苯;偕烷基化生成的碳正离子有可能脱烷基生成乙烯和乙烷等产物,进而生成苯.碳正离子脱烷基反应生成了大量气相产物,造成反应液收降低.碳正离子脱烷基反应与甲醇制烯烃过程的烃池机理相一致,因此甲苯和甲醇烷基化反应也遵循烃池机理.