Influence of exchanged cations (Na, Cs, Sr and Ba) on xylene permeation through ZSM-5/SS tubular membranes

Na-ZSM-5 membranes were synthesized by secondary growth on the outer surface of stainless steel porous tubes. The membranes were ion-exchanged with Cs⁺, Ba²⁺ and Sr²⁺ to investigate their effect upon the separation of p-xylene from m-xylene and o-xylene. The permeation through the membranes was measured between 150 and 400 °C using each xylene isomer separately and a ternary mixture. All the membranes were selective to p-xylene in the temperature range studied. N₂ and xylene permeation measurements together with SEM observations were used to determine whether or not cracks and/or pinholes developed after exposure to the xylene isomers at high temperature (400 °C). Neither pore blockage nor extra-zeolitic pores developed after the ion exchange procedure and subsequent calcination. Furthermore, duplicate synthesized membranes of each cation form had similar separation factors and permeances. The duplicate values differ much less than the measurement error. The p-xylene permeation flux decreased in the order: Na-ZSM-5 > Ba-ZSM-5 > Sr-ZSM-5  Cs-ZSM-5 while the permeation flux of the m- and o-xylene decreased in the order Na-ZSM-5 > Sr-ZSM-5 > Ba-ZSM-5 > Cs-ZSM-5. The membrane that exhibited the best performance was Ba-ZSM-5, with a maximum p/o separation factor of 8.4 and a p-xylene permeance of 0.54 × 10⁻⁷ mol s⁻¹ m⁻² Pa⁻¹ at 400 °C.     

Study of intermolecular interactions in binary mixtures of methyl acrylate with benzene and methyl substituted benzenes at different temperatures: An experimental and theoretical approach

The ultrasonic speeds, u and viscosities, η of the binary mixtures of methyl acrylate with benzene, toluene, o-xylene, m-xylene, p-xylene, and mesitylene over the whole mole fraction range were measured at six different temperatures and at atmospheric pressure. From the experimental data, the excess isentropic compressibility, K_S^E, excess ultrasonic speed, u^E, excess molar isentropic compressibility, K_s,m^E, excess specific impedance, Z^Eand deviations in viscosity, Δηhave been calculated. The partial molar isentropic compressions, K_s,m,1 and K_s,m,2, and excess partial molar isentropic compressions, ${\mathop K\limits^{ - E}}$_s,m,1 and ${\mathop K\limits^{ - E}}$_s,m,2 over the whole composition range, partial molar isentropic compressions, and , and excess partial molar isentropic compressions, $\mathop K\limits^{ - \hat E} $_s,m,1 and $\mathop K\limits^{ - \hat E} $_s,m,2 of the components at infinite dilution have also been calculated. The results specified the existence of weak interactions between unlike molecules, and these interactions follow the order: benzene > toluene > p-xylene > m-xylene > o-xylene > mesitylene. The magnitude of interactions was found to be dependent on the number and position of the methyl groups in these aromatic hydrocarbons.     

Kinetics of sorption of aromatics in zeolite ZSM-5

The kinetics of sorption of benzene, p-xylene and o-xylene into crystals of zeolite ZSM-5 in the hydrogen form were investigated gravimetrically for well-defined constant boundary conditions and constant temperature (298 K) of the sorbent. Zeolite crystals were mounted on aluminium foil separated in space, the metal foil acting as sample holder and heat sink in the exothermic sorption process. The influence of crystal size on the rate of sorption was studied by comparing two samples of the zeolite differing only in crystal size.

Sorption of benzene into ZSM-5 follows a pattern expected on the basis of the diffusion equation with constant diffusivity. p-Xylene is sorbed initially faster than benzene; its rate of sorption decreases at higher degrees of loading where a hysteresis loop in the isotherm is observed. Sorption of o-xylene proceeds much more slowly than that of p-xylene and does not seem to be controlled by diffusion.     

Gas phase catalytic hydrodechlorination of chlorophenols using a supported nickel catalyst

The gas phase hydrodechlorination of single component and mixtures of o-, m- and p-chlorophenol was studied over the temperature range 423 K≤T≤573 K using a 1.5% (w/w) Ni/SiO₂ catalyst. The variation of catalyst activity with time-on-stream for each isomer is illustrated and the role of reaction temperature and thermodynamic limitations are addressed. The catalyst exhibits both short term and irreversible long term deactivation which is accounted for in terms of competitive adsorption and electronic effects. The hydrogen treatment of both phenol and chlorobenzene under the same reaction conditions are also considered for comparative purposes. The presence of the hydroxyl function enhances the rate of hydrodechlorination via an inductive effect. The relationship between rate and isomer structure is discussed on the basis of reactant adsorption where steric hindrance, in the case of the ortho-form, appreciably restricts dechlorination to such an extent that o-chlorophenol remains unreacted in equimolar o-/p- and o-/m-chlorophenol mixtures. Catalytic hydrodechlorination is viewed as a non-destructive low energy methodology for handling concentrated chlorine gas streams and relationships that describe the dependence of dechlorination rate on chlorine concentration are provided and can be used to evaluate the productive lifetime of the catalyst.     

Effect of temperature on the product selectivity and aromatics distribution in aromatization of propane over H-GaAlMFI zeolite

The influence of temperature on the product selectivity, aromatics distribution, p-X/o-X and p-X/m-X product ratios, aromatization/cracking and aromatization/dehydrogenation ratios have been exhaustively investigated on H-GaAlMFI at different iso-conversions of propane. These studies show a profound influence of temperature on the product selectivity/distribution of the propane aromatization reaction. The aromatization activity of the H-GaAlMFI zeolite decreases at higher temperatures relative to the cracking and dehydrogenation activities. The benzene selectivity increases whereas the toluene and C₈ selectivity decreases with increasing reaction temperature. The product selectivity and the formation of xylene isomers are kinetically controlled.     

SEPARATION OF m-XYLENE FROM o-XYLENE BY EXTRACTIVE DISTILLATION

m-Xylene is difficult to separate from o-xylene by rectification because the relative volatility is only 1.12. m-Xylene can be separated from o-xylene using extractive distillation in which the extractive agent is composed of the proper higher boiling organic compounds. Typical effective agents are benzoates, DMSO, DMFA, adiponitrile, maleic anhydride, phthalic anhydride, polychloroaromatics and sulfolane.     

Selectivity in heterogeneous catalytic processes

The selectivity of several catalytic systems was studied. Shape selectivity of Pt on carbon-fiber catalysts was demonstrated in the competitive hydrogenation of 1-hexene and cyclohexene and in the parallel dehydrogenation of cyclohexanol to cyclohexanone and phenol. Both reactions were carried out in a gas-phase fixed-bed reactor. Catalysts prepared on carbon fibers, containing pores with small constrictions (5 Å) yielded significantly higher rates of hydrogenation of 1-hexene compared to those of cyclohexene and selectively produced cyclohexanone from cyclohexanol. Other catalysts, supported on carbon fibers with large constrictions (7 Å) or activated carbon, displayed comparable rates of hydrogenation for both reactants and yielded cyclohexanone as well as phenol from cyclohexanol. Nitration of o-xylene with nitrogen dioxide was carried out in the gas phase over a series of solid acid catalysts packed in a fixed bed. Several zeolites, supported sulfuric acid, and sulfated zirconia were tested. Zeolite H-β was found to be the most active and selective catalyst for the production of 4-nitro-o-xylene. A preliminary kinetic model indicated that the selectivity to 4-nitro-o-xylene increased with decreasing concentration of nitrogen dioxide. Alkylation of phenol with methanol was performed on zeolites, supported sulfuric and phosphoric acids, and sulfated zirconia packed in a fixed-bed. The ratio of o- to c-alkylation, measured at 180°C and methanol to phenol feed molar ratio of unity, ranged from 4 with the supported acids to 2 with zeolite H-β. This ratio decreased with temperature. The ratio of o- to p-cresol changed from about 2 in zeolites in supported sulfuric acid and to 0.5 in phosphoric acid supported on carbon fibers.     

COMPOSITION AND QUALITY OF THE GASOLINE OBTAINED FROM SYNGAS ON Cr2O3-ZnO/ZSM5 CATALYSTS

The kinetic equations for the formation of the lumps of the gasoline produced (C₅⁺ fraction), paraffins (C₅-C₈), xylenes (o-xylene, m-xylene and p-xylene) and aromatics (benzene, toluene, C₉-C₁₁) in the transformation of syngas on Cr₂O₃-ZnO/ZSM5 bifunctional catalyst have been established as a function of the concentration of both reactants (CO and H₂) and CO₂ byproduct. The effect of the operating conditions on the RON and on the molecular weight of the gasoline has been studied by experimentation in an integral fixed bed reactor in the range between 10 and 50atm and between 300 and 425°C. The octane index increases with Cr/Zn atomic ratio of the Cr₂O₃-ZnO metallic function, with Si/Al ratio of the HZSM5 zeolite, with space time and with the CO/H₂ molar ratio in the feed, in that order, whereas it passes through a maximum with pressure (at 20 atm) and with temperature (at 375°C). On the other hand, the molecular weight of the gasoline increases with the Cr/Zn atomic ratio, pressure, CO/H₂ molar ratio and space time, and decreases with the Si/Al ratio and temperature.     

Phase-transfer catalysed synthesis of disubstituted poly(phenylene vinylene)

An attempt to synthesize a series of phenyl ring disubstituted poly(o-, m- and p-phenylene vinylene) is reported. In this ‘one-pot’ reaction, a phase-transfer catalyst was used to promote the extraction of strong base deprotonated bischloromethyl aromatic anions from the aqueous to the organic phase to sustain a nucleophilic substitution reaction with the remaining bischloromethyl monomers. The polymers obtained were characterized for their chemical compositions and physical properties by elemental, i.r. and u.v. spectroscopic, and thermal analyses. The o-substituted polymers were found to have low melting temperatures, good solubilities and high molecular weights, but low yields. The m-substituted monomers, however, produced only non-conjugated poly-m-xylylenes. The p-substituted polymers offered the best combination of yield and physical properties. Films cast from soluble poly(2,5-dibutoxy-p-phenylene vinylene) have electrical volume resistivity in the range 300–500 ohm cm when doped with concentrated sulfuric acid.     

Heterogeneous photocatalytic decomposition of gas-phase chlorobenzene

Bench-scale laboratory experiments were conducted using broad spectrum UV-A to induce TiO₂-based photocatalytic treatment of mono-chlorobenzene under aerobic and anaerobic conditions. Previous photolytic-based studies conducted on chlorinated aromatics support the decomposition pathway of de-halogenation followed by cleavage of the aromatic ring due to attack by OH radicals. However, in this study of anaerobic degradation, GC–MS and HPLC analyses of intermediate by-products accumulated on the catalyst surface indicate that chlorophenols (o-, m-, p-) are the predominant species rather than benzene or phenol, and that dechlorination proceeds after ring cleavage.     

Hydrocarbon sorption properties of pure silica MCM-22 type zeolite

The adsorption properties of the MWW compositional end member, ITQ-1, have been explored empirically and via computer simulation using n-hexane, 3-methylpentane, 2,3-dimethylbutane, p-xylene and ethylbenzene as adsorbate probes. n-Hexane and 3-methlypentane diffuse rapidly and both have a sorption limit of four molecules per unit cell, three of which are in the large channel cavities. 2,3-dimethylbutane sorbs more slowly through the 10-ring windows; the limiting three molecules per unit cell are all in the large cavities. The two aromatics adsorb in both channel systems, with p-xylene diffusing rapidly and ethylbenzene more slowly but much faster than 2,3-dimethylbutane. No clear distinction between intra- and extra-crystalline adsorption was observed. Computer modeling indicated distinctions between the two channel systems.     

Electrochemical and spectroscopic properties of 1:2 Ni complexes of 1,3-substitued (CH3, OCH3) phenyl-5-phenylformazans

In this study, new 1:2 nickel complexes of 1-[o-, m-, p-(methyl, metoxyphenyl)]-3-(p-metoxyphenyl)-5-phenylformazans were synthesized. Their structures were elucidated and spectral behaviors were investigated with the use of elemental analysis, GC–mass, ¹H NMR, ¹³C NMR, IR, and UV–vis spectra. The redox characteristics of these compounds have been investigated in nonaqueous dimethylsulfoxide at platinum and ultramicro platinum (10 μm) electrodes. Through controlled potential electrolysis, the oxidation products of each class of compounds can be separated and identified. The oxidation mechanism is suggested and it is proved. It was observed the oxidation mechanism take place in a single step two-electron or one-electron transfer to a disproportionation or dimerization reactions following the radical formation step. Eventually the relation between their absorption properties and electrochemical properties was examined.     

Selective gas-phase oxidation at oxide nanoparticles on microporous materials

Catalytic oxidation over single metal or bimetallic oxides in nano-size generated on various matrices including zeolite via the chemical vapor deposition (CVD) technique and other methods are reviewed along with other interesting catalytic reactions. Among the various matrices studied, the partially deboronated borosilicate (DBH) played a unique role for the para-selective oxidation of alkylaromatics. The CVD Fe/Mo/DBH activated by the proper calcination procedure exhibited higher catalytic activity, better product selectivity, and superior stability to form aldehydes, in particular, terephthaldehyde and p-tolualdehyde, by the gas-phase O₂ oxidation of p-xylene than its impregnated counterpart and bulk catalytic material. Current mechanistic understanding on the unusual promoting role of CO₂ over the same catalyst was postulated for the O₂ oxidation reaction.

The silica overlayer in nano-size deposited on zeolite by the CVD of silicon alkoxides not only increased the para-selective property, but also provided other useful functions such as control of pore opening and thermal stability of the zeolite catalyst. Other oxidation reactions such as oxidative dehydrogenation of alkylaromatics and alkanes were also reviewed along with the selective N₂O oxidation of benzene to phenol.

CVD Fe/Mo/ZSM-5 oxidized p-xylene to produce aldehydes, but disproportionation and dehydrocoupling of p-xylene to toluene and pseudocuemen, and trimethylbiphenyl-methane, respectively, remained the predominant reactions.     

Direct NO decomposition over La2−xBaxNiO4 catalysts containing BaCO3 phase

Catalytic and in situ Fourier Transform Infrared (FTIR) spectroscopic studies were conducted to investigate the adsorption and oxidation of o- and p-chlorophenol over a 3.6 wt.% V₂O₅/TiO₂ catalyst. At a space velocity of approximately 53,000 cm³ g⁻¹ h⁻¹, this catalyst was found to be active for the oxidation of o- and p-chlorophenol at temperatures as low as 200 °C, yielding CO₂ and HCl as the main products. Trace amounts of higher molecular weight products were also detected at the reactor outlet indicating the operation of additional condensation, coupling and chlorination/dechlorination side reactions in parallel to the main complete oxidation scheme. The in situ FTIR studies revealed that different phenols adsorb on the V₂O₅/TiO₂ catalyst through their hydroxyl group. Furthermore, the formation of similar surface species (i.e., maleates, acetates, formates and an aldehyde-type species) was observed. The results were compared with those of previous studies on the oxidation of m-dichlorobenzene (m-DCB) and benzene and suggest that a similar reaction mechanism is operating in all cases, although the relative kinetic significance of the different steps varies with the presence and the position of the hydroxyl and chlorine groups on the aromatic ring.     

Catalytic oxidation of volatile organic compounds (VOCs) mixture (isopropanol/o-xylene) on zeolite catalysts

The catalytic oxidation of isopropanol and o-xylene alone and in mixture was investigated over basic zeolites (CsX and NaX) and an acidic zeolite (HY). For a given temperature, the conversion of VOCs mixture into CO₂ increases with the basicity of the zeolite. Results show that VOCs are oxidized through a basic mechanism over NaX, and through an acidic mechanism over HY. Over HY, the presence of isopropanol increases the oxidation of o-xylene, probably because of the formation of isopropyldimethylbenzenes obtained via an acidic mechanism. The addition of platinum over zeolites increases the rate of the VOCs oxidation, this behavior being more pronounced over PtHY maybe because of a higher Pt dispersion.     

甲醇甲苯烷基化流化床反应器的数值模拟

利用甲醇甲苯烷基化工艺生产对二甲苯具有良好的应用前景。甲醇甲苯烷基化催化剂较易积炭失活，且反应存在明显热效应。流化床因传热传质性能好、易实现催化剂连续再生，适合用作甲醇甲苯烷基化反应器。本文采用离散颗粒模型，对甲醇甲苯烷基化流化床反应器进行了数值模拟研究，重点考察了进料比、反应压力、分段进料对反应特性的影响。结果表明，当甲苯进料量给定时：降低反应物中甲苯甲醇比可有效提升对二甲苯产率和选择性，但产物中对二甲苯和烯烃摩尔比值较低；提高反应压力可显著提升甲醇和甲苯转化率，但会降低对二甲苯选择性；在低苯醇比基础上采用甲醇分段进料方式不仅可有效提高甲苯利用率，还可灵活调节产物中对二甲苯和烯烃比率；流化床反应器气体返混不利于获得高对二甲苯选择性，且操作条件变化会造成流化床反应器内气固流动改变，导致气固接触效率或反应物局部分压发生改变，这亦将对反应转化特性造成显著影响。这些结果对于流化床反应器优化和放大具有一定的指导意义。     

Hybridization of metal–organic framework and monodisperse spherical silica for chromatographic separation of xylene isomers

Metal–organic frameworks(MOFs) packed in the column have been a promising candidate as the stationary phase for high performance liquid chromatography(HPLC). However, the direct packing of irregular MOF powder could raise some problems like high back pressure and low column efficiency in the HPLC separation. In this work, UiO-66 capable of separating xylenes was supported effectively on the surface of the monodisperse spherical silica microspheres by one-pot method. The hybridization of Ui O-66 and silica microspheres(termed UiO-66@SiO_2 shell–core composite) was prepared by stirring the suspension of the precursors of Ui O-66 and\\COOH terminated silica in the N,N-dimethylformamide with heating. The shell–core composite material UiO66@SiO_2 was characterized by SEM, TEM, PXRD and FTIR. Then, it was used as a packing material for the chromatographic separation of xylene isomers. Xylene isomers including o-xylene, m-xylene and p-xylene were efficiently separated on the column with high resolution and good reproducibility. Moreover, the Ui O-66@SiO_2 shell–core composites packed column still remained reverse shape selectivity as Ui O-66 possessed, and the retention of xylenes was probably ascribed to the hydrophobic effect between analytes and the aromatic rings of the Ui O-66 shell. The Ui O-66@SiO_2 shell–core composites obtained in this study have some potential for the separation of structural isomers in HPLC.     

Kinetics of phase transfer catalyzed reduction of nitrochlorobenzenes by aqueous ammonium sulfide: Utilization of hydrotreater off-gas for the production of value-added chemicals

The reduction of nitrochlorobenzenes (NCBs) was carried out in an organic solvent, toluene, under liquid–liquid mode with phase transfer catalyst, tetrabutylammonium bromide (TBAB). The selectivity of chloroanilines (CANs) was found to be 100%. The reaction rate of m-nitrochlorobenzene (MNCB) was found to be highest among the three NCBs followed by o- and p-nitrochlorobenzene (ONCB and PNCB). The reactions were found to be kinetically controlled with apparent activation energies of 22.8, 19.6 and 9.4 kcal/mol for ONCB, PNCB and MNCB, respectively. The effects of different parameters such as TBAB concentration, NCB concentration, sulfide concentration, ammonia concentration, and elemental sulfur loading on the conversion and reaction rate of NCBs were studied to establish the mechanism of the reaction. The rate of reaction of NCBs was found to be proportional to the concentrations of the catalyst and NCBs and to the cube of the concentration of sulfide. A generalized empirical kinetic model was developed to correlate the experimentally obtained conversion versus time data for the three NCBs.     

Sorption kinetics study of the diethylbenzene isomers in MFI-type zeolites

The uptake rates of the three diethylbenzene isomers in MFI-type zeolites were studied with a sorption kinetics apparatus of the barometric type. It was observed that the uptake rates depend crucially on the position of the substituted ethyl groups. While p-diethylbenzene was adsorbed with uptake rates of the same order of magnitude as benzene and the monosubstituted benzenes, diffusion of m-diethylbenzene was approximately four orders of magnitude slower. No sorption uptake was observed for o-diethylbenzene. In the experiments with H-ZSM-5, sorption kinetics were partly determined by a secondary process additional to Fickian diffusion. Comparison to the results obtained with silicalite-1 and earlier results proved that the secondary process is correlated with the existence of acidic sites inside the zeolite and can be tentatively ascribed to an isomerization reaction.     

Influence of Pt particle size on catalytic combustion of xylenes on carbon aerogel-supported Pt catalysts

Pt catalysts supported on a carbon aerogel with different Pt particle sizes were studied in the combustion of o-xylene and m-xylene. Results found show that the activity of the catalysts increased with larger Pt particle size. In addition, the catalysts were activated during consecutive combustion runs and during time on stream. This activation depends on the Pt particle size and type of xylene isomer. Activation was due to the increase in Pt particle size during reaction. The lower activity of catalysts with smaller Pt particle size was due to the stronger PtO bonds formed during xylene combustion by the smaller Pt particles.