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Journal ArticleDOI

Alkylation of toluene with methanol on microporous MxH3−XPW12O40 [M=K+, Cs+, NH4 + and Tl+]

01 Mar 1998-Reaction Kinetics and Catalysis Letters (Springer Science and Business Media LLC)-Vol. 63, Iss: 2, pp 365-370

AbstractAlkylation of toluene was carried out on microporous salts of phosphotungstic acid. Among the systems studied, M2.5H0.5PW12O40 showed maximum catalytic activity.p-Xylene selectivity was evaluated as a function of x. A linear correlation was observed betweenpara-selectivity and number of strong acid sites with strength of H0≤−5.6. more

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Journal ArticleDOI
Abstract: Acidic cesium salts of dodecatungstophosphoric acid (H 3 PW) supported on silica and MCM-41 were characterized by multinuclear MAS NMR, FTIR, and catalytic test reactions. 31 P-NMR data indicate that the supported samples contain salts of various stoichiometries (Cs 3 PW and Cs 2 HPW) and even H 3 PO 4 . According to 29 Si-NMR data, deposition of the Cs salt does not change the relative amount of various SiOH groups in the case of the silica support. In contrast, when the support is MCM-41, the relative amount of the SiOH groups increases significantly, indicating the breaking of the siloxane bonds resulting from their interaction with H 3 PW. The 133 Cs MAS-NMR spectra also suggest that the CsPW salt interacts more strongly with the MCM-41 support than with SiO 2 . The results of catalytic studies show that cesium salts of dodecatungstophosphoric acid with low Cs content supported on silica or MCM-41 (Cs 1.7 H 1.3 [PW 12 O 40 ]-on-SiO 2 and Cs 1.8 H 1.2 [PW 12 O 40 ]-on-MCM41) are active, selective, and recyclable catalysts in Friedel–Crafts alkylations (isopropylation, transalkylation), the aromatic ortho -Claisen rearrangement, and the dehydration of pinacol. The specific activity of these catalyst preparations are comparable with that of neat Cs 2.6 H 0.4 [PW 12 O 40 ]. This indicates that the active species are in a well-dispersed form.

28 citations

Journal ArticleDOI
Abstract: The aim of present work was to evaluate a role of the secondary structures of the K2HPW12O40 and K2.5H0.5PW12O40 salts in their catalytic activity for vapour-phase dehydration of ethanol. Particular attention was directed to the role of H3PW12O40 (HPW) existing as a self-organized surface layer, which covers partially or entirely the K3PW12O40 core. The results showed that both salts are much more active than bulk HPW. It turned out that the dehydrated protons present in the studied salts transform ethanol to ethylene much easier than the hydrated protons existing in the HPW. The ammonia sorption measurements demonstrated that the [N2H7]+ adducts were formed easier on the surface of the K2HPW12O40 salt whereas the formation of the N H 4 + cations dominated in the bulk HPW. The structures of potassium tungstophosphates after catalytic reaction were investigated by the FT-IR, XRD and N2-sorption methods. A self-organized surface layer of HPW in the K2HPW12O40 salt was unstable because it transformed to the crystalline HPW, due to the reaction temperature as well as to the presence of ethanol or water (dehydration product). However, the structure of the K2HPW12O40 salt can be partially restored after ageing for a month at ambient temperature in air with the relative humidity of 25%. In contrast, the secondary structure of the K2.5H0.5PW12O40 salt remained practically unchanged after catalytic reaction. It makes this salt a very promising catalyst for the ethanol dehydration, with the conversion of 99.4% and the selectivity to ethylene attaining ca. 100% at 448 K.

14 citations

Journal ArticleDOI
Abstract: A combination of montmorillonite (MT) clay and catalysts was used to explore possible synergistic effects in reducing the flammability of poly(styrene-co-acrylonitrile) (SAN). Several catalysts, including ammonium phosphomolybdate hydrate (NHPMo), melamine phosphomolybdate hydrate (MEPMo), zinc phosphomolybdate hydrate (ZnPMo) and sodium phosphomolybdate hydrate (NaPMo), were used. The combination of MT (Cloisite 20A) with NHPMo showed synergistic enhancement in improving the char formation and reducing the peak mass loss rate of SAN40 as compared with SAN40 composites containing MT or NHPMo alone, while similar synergistic performance was not obtained between MT and NaPMo or ZnPMo or MEPMo. The better flame retardancy with this combination is probably due to the two primary aspects. One is the increased catalytic activity of the combination of Cloisite 20A and NHPMo probably due to the overlapping degradation temperature range of the combination of 20A/NHPMo and SAN40, which probably results in more char formation; the other is more NHPMo being around or on the clay stacks while less is in the galleries of the clay; this probably bridges the clay stacks and strengthens the clay network with char formed during the degradation process. High temperature rheological data confirmed the formation of a stronger network structure in SAN40/20/NHPMo; this benefits flame retardancy by allowing fewer cracks to form in the outer char layer on the polymer mass. The more continuous char layer suppresses bubbles transport of fuel vapors and heat transfer through the char layer, thereby reducing the mass loss rate.

13 citations

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Journal ArticleDOI
Abstract: Heteropolyanions are polymeric oxoanions which are formed by the condensation of more than two different oxoanions [Eq. (1)l. Polyanions consisting of one kind of oxoanion are called isopolyanions [Eq. (2)]. Acidic elements such as Mo, W, V, Nb, and Ta are present as oxoanions in aqueous solutions and polymerize to form polyanions at low pH. Free acids (or acid forms) of these species are called heteropoly and isopoly acids, respectively. Here, the term “heteropoly compounds” is used for heteropoly acids and their salts.

893 citations

Journal ArticleDOI
Warren W. Kaeding1, C. Chu1, Lewis B. Young1, B. Weinstein1, Stephen A. Butter1 
Abstract: Toluene has been alkylated with methanol over ZSM-5-class zeolite catalysts to produce xylenes and water. A near equilibrium mixture containing 24% of the para isomer was usually observed. Production of over 90% para isomer in the xylene product was achieved when the catalyst was modified by impregnation with phosphorus and with boron compounds. para-Xylene selectivity was also enhanced by coking and coating the catalyst surface with heat-stable polymers. A controlled reduction in the effective dimensions of the catalyst pore openings, and/or channels, along with deactivation of acidic surface sites, has been proposed as a mechanism for the selective production of p-xylene.

491 citations

Journal ArticleDOI
Abstract: Liquid-phase alkylation of m -xylene or trimethylbenzene with cyclohexene was studied using heteropoly compounds and other typical solid acids as the catalysts; the results are discussed in relation to the surface properties of the catalysts. Cs 2.5 H 0.5 PW 12 O 40 (abbreviated as Cs 2.5 -salt) was remarkably more active than H 3 PW 12 O 40 , zeolites, Nafion-H, SO 4 2− /ZrO 2 , H 2 SO 4 , and AlCl 3 HCl. Solid-state 31 P NMR revealed that Cs 2.5 -salt mainly consists of polyanions with one proton (Cs 2 HPW 12 O 40 ) and those without a proton (Cs 3 PW 12 O 40 ). The quantity of acid sites of Cs 2.5 -salt was determined to be 161 μmol g −1 by temperature-programmed desorption (TPD) of NH 3 , which was much greater than the estimated quantity of surface acid sites of H 3 PW 12 O 40 , and less than that of zeolites and SO 4 2− /ZrO 2 . The acid strength of Cs 2.5 -salt measured by Hammett indicators and TPD of NH 3 was smaller than that of SO 4 2− /ZrO 2 . It is presumed that the high activity of Cs 2.5 -salt is brought about by its large number of surface acid sites and its acid—base bifunctional nature, in which a caesium ion possibly increases the basicity of the heteropoly anion.

315 citations

Book ChapterDOI
Abstract: Publisher Summary This chapter focuses on organic catalysis over crystalline alumiosilicates. Zeolites are crystalline aluminosilicates composed of SiO4 and AlO4 tetrahedra arranged in various geometric patterns. The chapter discusses reactions over a wide variety of crystalline aluminosilicates and emphasizes on catalysis over X- and Y-type faujasites. Unusual opportunities for organic catalysis were found to exist when these were base exchanged to substantially eliminate their alkali metal content. It is shown that catalytic activity in ion-exchanged faujasites is influenced by cation type, cation location in the lattice, zeolite Si/Al ratio, and the presence of proton donors. The existence of several crystallographically distinct cation locations provides the basis for a potential heterogeneity of sites in catalytic reactions. The Linde workers made calculations for the electrostatic field strengths of surface cations near SII and SIII in an X-type zeolite of Si/Al ratio 1.0 and in a Y-type zeolite of Si/Al ratio 2.0. In both cases, the fields has been significantly larger near SIII than SII at a distance of 2 Ǻ from the center of the cation for the univalent cation-exchanged systems, provided both sites are occupied.

217 citations

Journal ArticleDOI
Abstract: A study has been made of the catalytic activity and properties peculiar to zeolite Y in various cationic and decationized forms during the alkylation reaction of toluene with methanol. It has been shown that the activity of the cation exchanged zeolite Y is in the following order: REY > HY > divalent > monovalent. It has been found that the composition of the xylene mixture obtained deviates greatly from the thermodynamic equilibrium values. At thermodynamic equilibrium, the xylene mixture contains about 22% p-xylene, while in this investigation p-xylene comprises about 50% of the mixture. This might be due to the fact that, given our experimental conditions, the secondary isomerization reaction of the initially formed xylene was depressed in the super cage of the zeolite catalysts. It has been established that p-xylene can be obtained selectively using highly active zeolite catalysts under appropriate reaction conditions.

108 citations