Heteropolyacid Catalysts Research Articles

Recent (1987‐1993) developments in heteropolyacid catalysts in acid catalyzed reactions and oxidation catalysis

Recent (1987‐1993) developments in heteropolyacid catalysts in acid catalyzed reactions and oxidation catalysis

Vapour phase carbonylation of methanol or dimethyl ether with metal-ion exchanged heteropoly acid catalysts

Metal-ion exchanged heteropoly acids of the general formula M[W12PO40](M = a group VIII metal) supported on SiO2 have been found to be excellent catalysts for the vapour phase carbonylation of methanol or dimethyl ether to methyl acetate at 225 °C and 1 atm total operating pressure.

Dispersion and Fixation of 12-Tungstophosphate Anion on a Silica Surface Modified with Silane Agents Having an Amine Group and Their Catalytic Properties

Abstract Silica gel (SiO2) was modified by a dip-cure method with silane coupling agents having an amine-group: (3-anilinopropyl), (3-aminopropyl), and N-[3-(2-aminoethyl)aminopropyl] trimethoxysilanes. 12-tungstophosphate (PW12) was deposited on the carriers as a test catalyst material. The amounts and the dispersed states of the silane agents and PW12 on the modified SiO2 were successfully analysed by gravimetry as well as UV and FT-IR spectrometries. The amount of PW12 deposited increased in proportion to the amount of the silane agents on SiO2 in the case of deposition under a constant concentration of PW12-solution, although only a little PW12 was deposited directly on SiO2. The acidic site of PW12 was confirmed to interact directly with the amine-group on the modified SiO2, and the strength of the interaction was correlated with the basicity of the amine-group. The activity per gram of catalyst for the dehydration of 2-propanol as a test reaction is generally higher than that on a directly deposited one, although the activity per PW12 unit certainly decreases with the SiO2-modification. The activity was confirmed to increase by a pretreatment at high temperatures, such that the amine-group was completely decomposed. It was clearly shown that the present modification method could be effectively applied for preparing a highly dispersed heteropoly acid catalyst on carriers.

Catalytic Activity of H 3PMo 12O 40-Blended Polysulfone Film in the Oxidation of Ethanol to Acetaldehyde

A heteropoly compound has acidic and redox catalytic properties. It also shows a characteristic sorption behavior depending on the properties of the adsorbates. Most nonpolar substances such as olefins are adsorbed only on the surface (surface area is less than 10 m[sup 2]/g), while polar compounds such as alcohols, water, pyridines, and ethers mostly penetrate into the bulk to form a pseudo-liquid phase. Thus, some reactions occur only on the surface, whereas others occur in the bulk of the heteropoly acid. A heteropoly compound is highly soluble in some organic solvents. Taking advantage of this property, heteropoly acid can be blended with a polymer to form a porous film by using a common solvent which dissolves both the heteropoly compound and the polymer. Since polymer materials are thermally unstable at high temperatures, their utilization has been restricted within the lower temperature ranges. Therefore, if the heteropoly acid and blended polymer film is used as a catalyst, it must be applied in low-temperature reactions. However, if the heteropoly acid catalyst is well dispersed in the bulk of the film, a high catalytic activity can be expected for certain reactions even at low reaction temperatures. In this study H[sub 3]PMo[sub 12]O[sub 40]-blendedmore » polysulfone (abbreviated as PMo-PSF hereafter) film was prepared and its catalytic activity for the ethanol conversion reaction was investigated. 5 refs., 4 figs., 1 tab.« less

Methyl t-butyl ether decomposition in an inert membrane reactor composed of 12-tungstophosphoric acid catalyst and polyphenylene oxide membrane

The methyl t-buyl ether (MTBE) decomposition reaction was carried out in an inert membrane reactor and in a conventional fixed bed reactor to compare the reactor performance of the two reactors. The inert membrane reactor was composed of 12-tungstophosphoric acid (H 3PW 12O 40) catalyst and polyphenylene oxide membrane. The maximum attainable conversion of MTBE in the inert membrane reactor was always greater than that in the fixed bed reactor at temperatures ranging from 80 to 110°C. The selective permeation of methanol through the membrane caused chemical equilibrium shift toward the favourable direction. It was also revealed that the adsorption characteristics of MTBE into the bulk of the heteropoly acid catalyst also played an important role in the increase in MTBE conversion.

Methyl t-butyl ether decomposition in an inert membrane reactor composed of 12-tungstophosphoric acid catalyst and polyphenylene oxide membrane

The methyl t-butyl ether (MTBE) decomposition reaction was carried out in an inert membrane reactor and in a conventional fixed bed reactor to compare the reactor performance of the two reactors. The inert membrane reactor was composed of 12-tungstophosphoric acid (H 3PW 12O 40) catalyst and polyphenylene oxide membrane. The maximum attainable conversion of MTBE in the inert membrane reactor was always greater than that in the fixed bed reactor at temperatures ranging from 80 to 110° C. The selective permeation of methanol through the membrane caused chemical equilibrium shift toward the favourable direction. It was also revealed that the adsorption characteristics of MTBE into the bulk of the heteropoly acid catalyst also played an an important role in the increase in MTBE conversion.

Liquid-phase synthesis of methyl tert-butyl ether on heterogeneous heteropoly acid catalyst.

The liquid-phase synthesis of methyl tert-butyl ether (MTBE) by the reaction between methanol and tert-butyl alcohol with several heteropoly acid catalysts was studied. Among the catalysts tested, acidic cesium salts of 12-tungstophosphoric acid and 12-tungstosilicic acid showed high activities and selectivities for the production of MTBE. The origin of high activity and selectivity of cesium heteropoly acids was discussed from kinetic viewpoints.

Single crystal and powder diffraction studies of the structure of heteropolymolybdophosphoric acid catalysts

Single-crystal structures were obtained of several vanadium-substituted forms (with coexistent Cu ions) of heteropolyacid catalysts prepared via hydrothermal synthesis. All V-containing species are of the inverse Keggin type; excessive addition of Cu reverts the structure back to the normal Keggin form. Structural details of the two poly-anions indicate qualitatively different bonding of the bridging oxygen atoms active in catalysis. From the single-crystal data reference powder patterns of the two base structures were simulated and experimentally verified. Failure to observe these patterns in clear form in conventional diffraction experiments is traced back to rapid changes in the state of hydration of the materials, with time constants comparable to acquisition times of powder diffraction data. Thermal dehydration leads to intermediate structures different from those subjected to dehydration in air. The anhydride of PVMo11 is stable up to 670 K.

A heteropoly acid catalyst and its convenient, recyclable application to liquid-phase cyclotrimerization of propionaldehyde

Heteropoly acids catalyse the cyclotrimerization of propionaldehyde to 2,4,6-triethyl-1,3,5-trioxane; catalyst and product characteristically separate into two phases so the recovered catalyst can be used repeatedly in the reaction.

Evaluation of diffusivity and solubility of alcohol and water in heteropoly acid catalyst by use of isotope-replacement technique.

A transient response methode using the isotope replacement technique was used to verify pseudo-liquid phase behavior of 12-tungstophosphoric acid and to investigate the solubility and mobility of polar substances such as alcohols and ethers. The relationship between the concentration of labeled molecules in gas phase and that in bulk phase can reasonably be assumed as linear in this technique, regardless of intrinsic non-linear relationships of diffusion and absorption. It was found that the polar molecules can be absorbed into the bulk phase of catalyst under these experimental conditions. The small values of effective diffusivities, which are on the order of 10 -10 m 2 s -1 , are observed to be closed to that in liquid-filled pores. The amount of absorbed molecules of alcohols, determined by polarity, tend to have a unique stoichiometriy corresponding to the number of protons in H 3 PW 12 O 40 , and the mobility depends on either polarity or molecular size.

Application of heteropoly acid catalyst in an inert polymer membrane catalytic reactor in ethanol dehydration

The ethanol dehydration reaction was carried out in an inert membrane catalytic reactor. 12-Tungstophosporic acid as a catalyst and polysulfone as an inert membrane were used in this study. Ethanol conversion and ethylene selectivity were remarkably enhanced in comparison with those in a fixed-bed reactor under the same reaction condition.

水晶振動子を利用した微小重量測定技術によるヘテロポリ酸触媒内の拡散係数の測定

A piezoelectric quartz crystal microbalance was used to measure the diffusion coefficient of water in 12-tungstophosphoric acid catalyst. The catalyst was coated on the electrode surface of the quartz resonator and the time course of weight change caused by sorption/desorption could be followed in the nanogram range by measuring the frequency change. It was found that the diffusion rate increased with concentration of water absorbed and the response curve was successfully analyzed by assuming a diffusion coefficient as a function of concentration. This concentration dependency indicates that the diffusion process of water in a heteropoly acid is affected by the interaction with active sites. Isotope replacement studies using the same detection method revealed that the concentration dependency of diffusion coefficient disappeared. The obtained values of diffusion coefficients showed the same order as that by the unsteady method and lay in a reasonable order of magnitude.

Oxydehydrogenation of isobutyric acid with heteropolyacid catalysts: Experimental observations of deactivation

the heteropolyacid catalysts H 5Mo 10V 2PO 40 and H 6Mo 9V 3PO 40 have been used for producing methacrylic acid via the oxydehydrogenation of isobutyric acid. This reaction proceeds via a redox process involving lattice oxygen from the Keggin structure of the catalyst molecules. Changes in the oxidation state of the catalyst produce a deactivation effect which is reversible by fully reoxidizing the catalyst. An irreversible, long-term type of deactivation related to a loss of molybdenum from the catalyst has also been observed. This occurs through the formation of a volatile Mocontaining gas-phase species. Passage of the IBA feed over a bed of molybdenum trioxide prior to entering the reactor eliminates both types of deactivation.

New synthetic route of polyoxytetramethyleneglycol by use of heteropolyacids as catalyst

AbstractPolyoxytetramethyleneglycol (PTMG) can be directly prepared from tetrahydrofuran (THF) and water in the presence of heteropolyacids (HPA), without the hydrolysis step. The reaction is carried out by mixing two liquid phases: the THF phase and a heteropolyacid catalyst phase. In this reaction the molar ratio of water to a heteropolyacid (H2O/HPA) in the catalyst phase plays an important role in the THF polymerization activity and the molecular weight of PTMG. IR spectrometric studies revealed that THF is coordinated to a heteropolyanion through either a water molecule or a proton in the catalyst phase. The latter type of coordination bings about the activation of THF capable of initiating the ring‐opening polymerization even in the presence of water at a lower acid strength. The PTMG prepared by this method has a narrow molecular weight distribution (M̄w/M̄n = 1.5 or less) and a number average molecular weight of 500–2000 which are requisite for the production of polyurethane elastomers. A new polymerization mechanism named “Phase Transfer Polymerization” is proposed for elucidating a narrow molecular weight distribution.

Heteropolysalt-supported heteropolyacids as a new class of acid-base and redox catalysts

Heteropolyacid catalysts of composition H3+n PV n Mo12−n O40 (n = 0,1,2,3) when supported on K3PMo12O40 desplay new types of acid-base and redox properties and have enhanced thermal stability.

Catalysis by heteropoly compounds: IV. Oxidation of methacrolein to methacrylic acid over 12-molybdophosphoric acid

Oxidation of methacrolein over heteropoly acid catalysts (mainly 12-molybdophosphoric acid) at 300 °C was investigated by use of a flow method. Primary (Keggin) and secondary structures of the catalyst were rather stable upon heat treatment at 250–350 °C, and reproducible data of the oxidation reaction were obtained by the pretreatment at 350 °C in a N 2 + O 2 + H 2O stream. Effects of oxygen in the feed gas demonstrated that oxygen atoms of polyanions or steam were directly involved in the reaction and the reduced polyanions were reoxidized by gaseous oxygen. Presence of steam had a remarkable effect both on the rate and selectivity. Rate equation, ESR study, and the comparison between the catalytic oxidation (flow method) and noncatalytic oxidation (pulse method) indicated that the reduction of catalysts was rate-determining in a redox mechanism and the catalysts were in a highly oxidized state under the present reaction conditions. Effects of metal salt formation and silica support were also examined. On the basis of these results, a possible reaction mechanism with an ester- or diol-type intermediate has been proposed.

Liquid phase oxidation of cyclo-olefins by a PdSO4–heteropolyacid catalyst system

By a PdSO4–heteropolyacid(H3PMo6W6O40)catalyst system in the liquide phase cyclo-olefins, particularly cyclopentene and cyclohexene, are oxidized to the corresponding ketones.

Mixed Heteropoly Acid Catalysts for the Vapor Phase Air Oxidation of Naphthalene1

Mixed Heteropoly Acid Catalysts for the Vapor Phase Air Oxidation of Naphthalene¹

Anhydrous acetic acid

In order to investigate the effect of Lewis acidic metals on the acid properties of Keggin heteropolyacids catalysts, a set of metal-exchanged HPAs (i.e. M3/nPW12O40, M3/nPMo12O40 and M4/nSiW12O40 (M = Cu, Co and Mn, n = 2; M = Fe, n = 3) were synthesized and their activity on glycerol esterification with acetic acid was evaluated. This procedure avoids the use of corrosive Brønsted acid catalysts. Regardless of the heteropoly anion, the activity of catalysts in terms of metal cations followed the sequence: Fe3+> Cu2+> Mn2+> Co2+. It has been found that among the HPAs, only H4SiW12O40 has the acidic protons that can be successfully exchanged with Fe3+ cations, resulting in Fe4(SiW12O40)3 salt, the most active and selective catalyst. The highest conversion (ca.99.9%) was achieved in Fe4(SiW12O40)3-catalyzed esterification reactions, along with the highest selectivity for di and triacetyl glycerol (ca.55 and 42%, respectively). Effects of temperature, stoichiometry of reactants, concentration and nature of catalysts were assessed. A kinetic study of Fe4(SiW12O40)3 or H4SiW12O40-catalyzed reactions was conducted and the activation energy determined.