Deep oxidation in propane oxidative dehydrogenation to propene over V2O5/γ-Al2O3 studied by in-situ DRIFTS

In-situ DRIFTS was used to study the deep oxidation of propane, a side reaction during propane oxidative dehydrogenation to propene. Strong adsorption of propene was supposed to be the main reason for the deep oxidation. It was found that gaseous oxygen in the feed and the reaction temperature had great influence on the reaction. To obtain a relative high selectivity to propene, the reaction temperature should be maintained at 150250 °C with a proper content of gaseous oxygen in the feed for a certain catalyst and some modifiers which could weaken the adsorption of propene on the catalyst surface would be favorable.     

Selective alkylation of xylenes by alcohols on zeolite catalysts

The peculiarities of catalytic performance of crystalline aluminosilicates of different types and compositions (X, Y including dealuminated Y, mordenite, pentasil ZSM-5), as well as of amorphous aluminosilicate catalyst in conversion of xylene + alcohol mixtures were studied. New data were obtained for alkylation ofo-xylene withtert-butyl alcohol, concerning controlling the selectivity and stability of the zeolite catalysts in reactions proceeding with the participation of water, including the water evolved during the reaction, in particular by controlling the acidic properties and hydrophobycity of the zeolites. A catalyst ensuring production of 1,2-dimethyl-4-tert-butylbenzene (DMTBB) with a 94% yield and selectivity of alcohol conversion to the target product of 94–97% was developed. The catalyst can be used as the basis for a high-performance and environmentally safe method for the synthesis of DMTBB. The catalysts developed can be also used for selective alkylation ofo-xylene by C₃-C₅ alcohols and for alkylation ofm-xylene bytert-butyl alcohol.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No, 12, pp, 2912–2917, December, 1996.     

Pervaporation separation of aromatic/aliphatic hydrocarbons by crosslinked poly(methyl acrylate-co-acrylic acid) membranes

Copolymers of methyl acrylate and acrylic acid were synthesized to fabricate membranes ionically crosslinked using aluminum acetylacetonate for the separation of toluene/i-octane mixtures by pervaporation at high temperatures. The formation of the ionic crosslinking via bare aluminum cations was characterized by UV–VIS spectroscopy and solubility tests. Reproducibility and the reliability of the methodology for membrane formation and crosslinking were confirmed. The effects of acrylic acid content, crosslinking conditions, pervaporation temperature, and feed composition on the normalized flux and the selectivity for toluene/i-octane mixtures were determined. A typical crosslinked membrane showed a normalized flux of 26 kg μm m⁻² h⁻¹ and a selectivity of 13 for a 50/50 wt.% feed mixture at 100°C. The pervaporation properties including solubility selectivity and diffusivity selectivity are discussed in terms of swelling behavior. The performance of the current membranes were benchmarked against other membrane materials reported in the literature.     

Separation of p-xylene from multicomponent vapor mixtures using tubular MFI zeolite mmbranes

MFI zeolite membranes have been synthesized on tubular -alumina substrates to investigate the separation of p-xylene (PX) from m-xylene (MX) and o-xylene (OX) in binary, ternary, and simulated multicomponent mixtures in wide ranges of feed pressure and operating temperature. The results demonstrate that separation of PX from MX and OX through the MFI membranes relies primarily on shape-selectivity when the xylene sorption level in the zeolite is sufficiently low. For an eight-component mixture containing hydrogen, methane, benzene, toluene, ethylbenzene, PX, MX, and OX, a PX/(MX + OX) selectivity of 7.71 with a PX flux of 6.8 × 10⁻⁶ mol/(m² s) was obtained at 250 °C and atmospheric feed pressure. The addition of a small quantity of nonane to the multicomponent mixture caused drastic decreases in the fluxes of aromatic components and the PX separation factor because of the preferential adsorption of nonane in the zeolite channels. The nanoscale intercrystalline pores also caused serious decline in the PX separation factor. A new method of online membrane modification by carbonization of 1,3,5-triisopropylbenzene in the feed stream was found to be effective for reducing the intercrystalline pores and improving the PX separation.     

Maleic Anhydride Crosslinked Alginate-Chitosan Blend Membranes for Pervaporation of Ethylene Glycol-Water Mixtures

Calcium alginate-chitosan (CA/CS) blended membranes were prepared and crosslinked with maleic anhydride (MA) for the pervaporation (PV) separation of ethylene glycol (EG)/water mixtures at 30°C. The structure and properties of blend membranes were studied with the aid of FTIR, XRD, TGA, and SEM. The effect of experimental parameters such as feed composition, membrane thickness, and permeate pressure on separation performance of the MA crosslinked membranes were determined in terms of flux, selectivity, and pervaporation separation index. Sorption studies were carried out to evaluate the extent of interaction and degree of swelling of the blend membranes in pure, as well as in binary mixtures. The experimental results suggested that the crosslinked membrane (M-CA/CS) exhibited a good selectivity of 302 at a normalized flux of 0.38 kg.m^{? 2}.h^{? 1}.10 μ m at 30°C for 96.88 wt% EG aqueous solution.     

Separation of xylenes by extractive crystallization with calixarenes

Several calixarenes1–5 and benzopinacolone6 were recrystallized from 1:1:1 mixtures of the three xylene isomers.p-Iso-propylcalix[4]arene 1 was shown to extractp-xylene with 86% selectivity.p-Iso-propylbishomooxacalix[4]arene5 extractso-xylene with 84% selectivity.     

MFI zeolite membranes from a- and randomly oriented monolayers

c-Oriented columnar MFI films made by secondary growth of randomly oriented seed monolayers, deposited using sonication-assisted covalent attachment, exhibit n-hexane/2,2-dimethylbutane separation factor of up to 10⁴, n-/i-butane separation factor of up to 50, and p-/o-xylene separation factor of up to 2. A MFI film from a-oriented seed layer shows lower separation factors for the linear vs. branched isomers but higher separation factor for p-/o-xylene.     

Derivative Enthalpies of Adsorption of p-Xylene and m-xylene onto NaY and BaY Zeolites at 150°C: Contribution to the prediction of adsorption selectivity

The derivative enthalpies of adsorption of m-xylene and p-xylene onto the NaY and BaY zeolites were measured at 150°C, then compared with those obtained at 25°C, and finally used to predict the selectivity of adsorption of xylene mixtures. Significant differences were observed as the temperature was elevated: for the NaY zeolite, the adsorbate-adsorbate interactions became prevalent, in contrast with the BaY zeolite, between zeolite and derivative interactions were stronger. The difference between the adsorption derivative enthalpies of the two xylenes displayed an abrupt variation from 2 molec. ^–1 for both zeolites, the filling from which selectivity towards m-xylene for the NaY zeolite and towards p-xylene for the BaY zeolite appeared. The preferentially adsorbed xylene was closely connected with the sense of this difference, which changed with the zeolite.This revised version was published online in November 2005 with corrections to the Cover Date.     

Pervaporation dehydration of ethylene glycol with chitosan–poly(vinyl alcohol) blend membranes: Effect of CS–PVA blending ratios

Chitosan–poly(vinyl alcohol), CS–PVA, blended membranes were prepared by solution casting of varying proportions of CS and PVA. The blend membranes were then crosslinked interfacially with trimesoyl chloride (TMC)/hexane. The physiochemical properties of the blend membranes were determined using Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), tensile test and contact angle measurements. Results from ATR-FTIR show that TMC has crosslinked the blend membranes successfully, and results of XRD and DSC show a corresponding decrease in crystallinity and increase in melting point, respectively. The crosslinked CS–PVA blend membranes also show improved mechanical strength but lower flexibility in tensile testing as compared to uncrosslinked membranes. Contact angle results show that crosslinking has decreased the surface hydrophilicity of the blend membranes. The blend membrane properties, including contact angle, melting point and tensile strength, change with a variation in the blending ratio. They appear to reach a maximum when the CS content is at 75 wt%. In general, the crosslinked blend membranes show excellent stability during the pervaporation (PV) dehydration of ethylene glycol–water mixtures (10–90 wt% EG) at different temperatures (25–70 °C). At 70 °C, for 90 wt% EG in the feed mixture, the crosslinked blend membrane with 75 wt% CS shows the highest total flux of 0.46 kg/(m² h) and best selectivity of 986. The blending ratio of 75 wt% CS is recommended as the optimized ratio in the preparation of CS–PVA blend membranes for pervaporation dehydration of ethylene glycol.     

Dehydration of acetic acid by pervaporation using SPEK-C/PVA blend membranes

Sulfonated cardo polyetherketone (SPEK-C) and poly(vinyl alcohol) (PVA) blend membranes were prepared by solution casting method and used in pervaporation (PV) dehydration of acetic acid. The membranes were characterized by Fourier transform infrared (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and contact angle meter. The results show that thermal crosslinking occurred to the membrane under high temperature annealing. The effective d-spacing (inter-segmental spacing) decreased with PVA content decreasing. The hydrophilicity of the blend membrane increased with SPEK-C content increasing. Swelling and sorption experiments show that the swelling degree of the blend membrane increased, however both the sorption and diffusion selectivities decreased with increasing PVA content. The diffusion selectivity is higher than the sorption selectivity. This suggests that PV dehydration of acetic acid is dominated by the diffusion process. The pervaporation separation index (PSI) of the membrane increases with increasing PVA content and arrives at a maximum when the SPEK-C/PVA ratio is 3/2, then decreases with further addition of PVA. The membrane has an encouraging separation performance with a flux of 492 g m⁻² h⁻¹ and separation factor of 59.3 at 50 °C at the feed water content 10 wt%.     

m-Xylene Isomerization in SAPO-11/HZSM-5 Mixed Catalyst

The catalytic isomerization of m-xylene was studied over a solid acid silicoaluminophosphate type SAPO-11, mixed to HZSM-5 zeolite. The reaction was processed varying the temperature and weight hourly space velocity, using a fixed bed continuous flow reactor. The m-xylene suffers isomerization to p-xylene and o-xylene by molecular displacement of methyl groups. The mixed catalyst was selective to p-xylene at 623 K and 2.5 h⁻¹ with a maximum p/o ratio of 2.05. The ethylbenzene formation was not observed in the products. In this process an apparent activation energy of the order of 13.9 kJ mol⁻¹ was obtained. This revised version was published online in June 2006 with corrections to the Cover Date.     

Electrochemical properties of polyaniline in p-toluene sulfonic acid solution

Polyaniline was deposited potentiodynamically on a stainless steel substrate in the presence of an inorganic acids (sulfuric acid). The electrochemical characterization of the electrode was carried out by means of cyclic voltammetry and electrochemical impedance spectroscopy in the organic acids (p-toluene sulfonic acid) solution. The results show that polyaniline has a high specific capacitance of 431.8 F g⁻¹ at 1 mV s⁻¹, high coulombic efficiency of 95.6% at 20 mV s⁻¹, and exhibits a high reversibility. This indicates the promising feasibility of the polyaniline used as an electrochemical capacitor material in the electrolyte of p-toluene sulfonic acid solution especially at high charge–discharge process.     

Development of a membrane-based vapor-phase bioreactor

A vapor-phase bioreactor has been developed utilizing porous metal membranes in a cylindrical design employing radial flow as opposed to traditional axial flow for the vapor stream. The system was evaluated for the biodegradation ofp-xylene (p-xylene) from a water-saturated air stream byPseudomonas putida ATCC 23973 immobilized onto sand. The biocatalyst was placed in the annular space between two cylindrical, porous stainless-steel membranes. Details of the reactor system are presented along with biological data verifying system performance. The feed flow rate andp-xylene concentration were varied between 60 and 130 cm³/min and 15–150 ppm, respectively. Continuous reactor operation was maintained for 80–200 h with removal efficiencies (based onp-xylene disappearance) between 80 and 95%. The effluent concentration histories were compared to determine the operating range of the bioreactor.     

Disproportionation of ethylbenzene in the presence of C8 aromatics

The selective synthesis of p-diethylbenzene (p-DEB) by disproportionation of ethylbenzene (EB) in the presence of aromatics like m-and p-xylene isomers has been studied over a pore size regulated HZSM-5 catalyst. The industrial feed having different compositions of ethylbenzene and xylene isomers was used for the experimentation. Hence, they were expected to hinder the movement of reactant molecules both on the external surface and within the zeolite channels. It was observed that irrespective of the different feed compositions the concentration of the xylene isomers was intact in the product. There is no other byproducts formation like para-ethylmethyl benzene. The effects of varying the concentration of aromatic compounds in the feed on ethylbenzene conversion and product distribution over the parent and modified H-ZSM-5 catalyst have been discussed. Ethylbenzene disproportionation reaction follows the pseudo first order reaction with an activation energy of 8.6 kcal/mol.     

Chemical-Modified Nylon 4 Membrane for Pervaporation

Abstract

A hydrophilic polymer membrane was synthesized with 2-hydroxyethyl methacrylate (HEMA) onto a Nylon 4 polymer backbone, PHEMA-g-N4. The membranes were water permselective because of the hydrophilicity, and the water permselectivity increased with increasing the degree of grafting. Permseparation of water was investigated with respect to the feed aqueous alcohol concentration, feed temperature, size of the alcohols, and degree of grafting. The separation factors of this PHEMA-g-N4 membrane were higher than those of the unmodified Nylon 4 membrane for pervaporation of aqueous ethanol solution, while the permeation rate was slightly lower. A separation factor of 98 and a 194 g/m²·h permeation rate could be obtained. Compared with an unmodified Nylon 4 membrane, the PHEMA-g-N4 membrane effectively increased the pervaporation separation index for the water-ethanol mixtures on pervaporation separation.     

Mechanisms of Xylene Isomer Oxidation by Non-thermal Plasma via Paired Experiments and Simulations

Xylene is a widely used solvent and industrial chemical, but it is also considered to be a volatile organic compound (VOC) pollutant. Meanwhile, non-thermal plasma (NTP) is a potential method for remediating VOC contaminants, especially aromatic hydrocarbons. During NTP degradation of xylene, the different oxidation mechanisms of three isomers, p-xylene, o-xylene and m-xylene, have attracted lots of attention but not been studied at the molecular level. In this study, the individual degradation rates of xylene isomers in a NTP system are measured. The results show the oxidation degradation rates have the following order: o-xylene?>?p-xylene?≈?m-xylene. Molecular dynamics simulations with an applied external electric field were adopted to examine the oxidation process of xylene isomers, as well. The oxidation rates from the simulations were calculated, the order of which is in a good agreement with the experimental results. The oxidation pathways of xylene isomers were analyzed more thoroughly to explain the rate differences. The external electrical field is found to have two effects: one is to speed up the oxidation rate of xylene isomers overall, and the other is to alter the oxidation pathways to increase the probability of the faster ring cleavage pathways of o-xylene.

     

N.m.r. characterisation of linkage isomers

Summary Halogenation of Ln^III, Ce^IV and ZrO^II -diketone/-ketoester derivatives and their mixed ligand complexes of the types Ln(AA)₂(Sal) and Ln(AA)(Sal)₂ by NCS, NBS and PyHBr₃ yield different isomeric products depending on the nature of the solvent medium, the reagent and the reaction time. The halogenation, if carried out in glacial acetic acid, irrespective of the reagent, yields the stable S-hall, product wherein the three chelate rings remain imperturbed in respect of metal coordination. When the reaction is carried out in 5% v/v DMF-CHCl₃ employingN-halosuccinimide and maintaining correct reaction times, it is possible to isolate individually three other isomeric products. The isomers prepared are Ln(OO)₃, Ln(OO)₂(OX), Ln(OO)(OX)₂ and Ln(OX)₃ where (OO) represents diketone oxygen linkage and (OX) represents diketone oxygen and substituted halogen linkage to the central metal ion. The four linkage isomers have been identified by a comparison of the number of observed³H n.m.r. or¹³C n.m.r. signals with those expected for a given isomer on the basis of symmetry considerations in the tris-chelated octahedral structures.     

Preferential sorption and permeation of binary liquid mixtures in poly (vinyl chloride) membrane by pervaporation

Preferential sorptions and pervaporation selectivities in poly (vinyl chloride) (PVC) membrane for various binary liquid mixtures were investigated. Methanol/n-propanol, benzene/n-hexane, and ethanol/water mixtures were selected as the binary liquid mixture. In the methanol/n-propanol mixture, methanol was preferentially sorbed in the PVC membrane and predominantly permeated. In the benzene/n-hexane mixture, benzene was incorporated and permeated preferentially. In the ethanol/water mixture, ethanol was preferentially sorbed in the PVC membrane and water was preferentially permeated. The preferential sorptions were analyzed according to Mulder's model derived from Flory-Huggins thermodynamics. The pervaporation selectivity in these systems were discussed using a sorption selectivity and diffusion selectivity. © 1995 John Wiley & Sons, Inc.     

Synthesis and characterization of an ultrathin polyion complex membrane containing β-cyclodextrin for separation of organic isomers

An ultrathin polyion complex (PIC) layer containing β-cyclodextrin (β-CD) was formed on the surface of a charged base membrane. Thus, a positively charged copolymer containing β-CD was synthesized by the radical copolymerization of β-CD monomer and allylamine, and was used to modify the surface of a Nafion membrane containing negatively charged fixed ions. Electron spectroscopy for chemical analysis (ESCA) confirmed the presence of the copolymer on the surface of the Nafion membrane due to the formation of a PIC layer. The ability of the membrane to separate organic isomer mixtures was tested by the pervaporation technique, using butanol isomer mixtures as the feed model. The membrane exhibited a good selectivity toward butanol isomers, indicating the effectiveness of β-CD as a selective fixed carrier for the isomers.     

The Al3+ Sensitivity of Chitosan-Silk Fibroin Complex Membrane on Swelling and Its Application on Chemical Valve for the Separation of Isopropanol-Water Mixture

Abstract

A novel natural biopolymer complex membrane, namely, a crosslinked chitosan-silk fibroin complex membrane (semi-IPN membrane) was prepared. The complex membranes manifested a good ion sensitivity when swelled in AlCl₃ aqueous solutions with different concentrations. According to its different swelling values with different Al³⁺ concentrations, the complex membrane could act as a chemical valve to control the flux of isopropanol-water mixture when separated by pervaporation. The pervaporation results indicated that the flux also showed an ion sensitivity when the Al³⁺ content in the feed was changed. In the meantime, the ion sensitivity of the flux expressed the same tendency as that of the swelling ratio. In addition, the flux of isopropanol-water mixture was significantly improved while the high selectivity was maintained when suitable amount of AlCl₃ was added to the feed.