Cesium Salts Of 12-tungstophosphoric Acid Research Articles

Carbonylation of Dimethyl Ether on Ag/ Cs1.5H1.5PW12O40: In-Situ FTIR Spectroscopy Study of the Ag-Carbonyls

AbstractHalide-free carbonylation of dimethyl ether (DME) to methyl acetate over heterogeneous catalysts offers a potentially green route to the production of important industrial chemicals. Previously we have shown that the addition of 1% of silver to Cs1.5H1.5PW12O40led to an increase in activity by a factor of two. Formation of silver carbonyl clusters on the surface of the catalyst was demonstrated by in situ FTIR spectroscopy. The v(CO) of silver carbonyl was observed at 2188 cm-1 and may be explained by the formation of cationic silver carbonyl Ag(CO)+on the surface of the acidic cesium salt of 12-tungstophosphoric acid. In view of the increased efficiency, the novel bifunctional catalyst Ag/ Cs1.5H1.5PW12O40is expected to hold significant promise for practical use in new processes of basic organic chemistry.

Solid-State NMR Characterization of Brønsted Acid Sites of Cesium Salt of 12-Tungstophosphoric Acid

Solid cesium salts of 12-tungstophosphoric acid show high surface area and strong acidity, which stimulate their applications as the catalysts for many reactions. Characterization of acid sites of the cesium salts is crucial since the information obtained could provide the details of mechanism of catalysis by these solid porous materials. By using 1H magic angle spinning nuclear magnetic resonance (1H MAS NMR), two types of acidic protons with the signals at 9.0 and 5.0 ppm have been identified for cesium salt of 12-tungstophosphoric acid, Cs2HPW12O40. A two-dimensional (2D) 1H MAS exchange NMR experiment has shown that the magnetization transfer occurs between different protonic sites at 293–473 K. Weak temperature dependence of the rate of magnetization transfer with apparent activation energy of 6.4 ± 0.3 kJ mol–1 is indicative of the spin-diffusion process rather than the chemical exchange between the acid sites. The 2D CP/MAS 1H–31P heteronuclear correlation NMR experiment reveals that the 1H NMR res...

Solid-state NMR study of the kinetics and mechanism of dimethyl ether carbonylation on cesium salt of 12-tungstophosphoric acid modified with Ag, Pt, and Rh

By using 13C magic angle spinning (MAS) NMR, the mechanism of dimethyl ether (DME) carbonylation with carbon monoxide has been studied on solid metal-containing acidic cesium salt of 12-tungstophosphoric acid, M/Cs2HPW12O40 (M/HPA, M=Ag, Pt, and Rh). The kinetics of the reaction has been monitored with 1H MAS NMR in situ. Activation of DME occurs on acidic OH groups of M/HPA and gives rise to the surface methoxy groups at 293–473K. Carbon monoxide forms the surface metal–carbonyl complexes on M/HPA at 293–473K. The insertion of CO from the carbonyl into the CH3O bond of surface methoxide results in the acetate group attached to the Keggin anion, from which the target product, methyl acetate, is produced under the interaction with dimethyl ether. The reaction rate decreases in the following order: Rh/Cs2HPW12O40≫Pt/Cs2HPW12O40>Ag/Cs2HPW12O40, which correlates well with concentration of the reaction intermediates, methoxy groups, and metal–carbonyls. The higher concentration of these intermediates detected on Rh/Cs2HPW12O40 is in charge of the higher carbonylation rate on this catalyst with respect to M/HPA (M=Ag, Pt) catalysts.

ChemInform Abstract: A Green Approach to the Synthesis of Chalcones via Claisen—Schmidt Condensation Reaction Using Cesium Salts of 12‐Tungstophosphoric Acid as a Reusable Nanocatalyst.

AbstractCesium salts of 12‐tungstophosphoric acid are shown to be efficient nanocatalysts for the Claisen—Schmidt condensation.

An aluminum promoted cesium salt of 12-tungstophosphoric acid: a catalyst for butane isomerization

Aluminum doped cesium salts of tungstophosphoric acid, AlxCs2.5–3xH0.5PW12O40 (x = 0.05, 0.1 and 0.15), were prepared via a solid state synthesis method and their catalytic activity in n-butane isomerization reaction was tested using the 13C MAS NMR technique and 1-13C-n-butane as the reactant. All AlxCs2.5–3xH0.5PW12O40 catalysts show higher catalytic activity than Cs2.5H0.5PW12O40. Among them, Al0.1Cs2.2H0.5PW12O40 exhibits the best performance. The MAS NMR technique was further employed to characterize the acidity of the catalysts using trimethylphosphine and 2-13C-acetone as the basic probe molecules. Al0.1Cs2.2H0.5PW12O40 possesses higher Bronsted acid strength than Cs2.5H0.5PW12O40, which parallels the activity of the catalysts. For a catalyst used in n-butane isomerization reaction, to enhance its Bronsted acid strength rather than to increase its acid density is more important for improving the activity of the catalyst.

A green approach to the synthesis of chalcones via Claisen-Schmidt condensation reaction using cesium salts of 12-tungstophosphoric acid as a reusable nanocatalyst

The catalytic efficacy of different cesium salts of 12-tungstophosphoric acid as nanocatalysts was checked in Claisen-Schmidt condensation of aldehydes and ketones under solvent-free conditions to produce chalcones. The reaction is fast and environmentally benign with excellent selectivity for the synthesis of chalcones. Results of potentiometric titration and catalytic reactivity of the catalysts indicated that the catalytic reactivity should be directly proportional to the strength and number of the surface acid sites, which are the ones accessible to the reactants. Reusability of the catalysts and acidic properties of reused catalysts were investigated. Coking could cause a loss in catalyst acidity, leading to a decrease in the product yield. But the amount of coke remains constant in successive runs. Furthermore, the use of this catalyst is feasible because of its easy preparation and handling, and stability. Large-scale synthesis is also beneficial in this method. Two new compounds are synthesized and fully characterized. .

Carbonylation of dimethyl ether on solid Rh-promoted Cs-salt of Keggin 12-H 3PW 12O 40: A solid-state NMR study of the reaction mechanism

The carbonylation of dimethyl ether (DME) with carbon monoxide on Rh-promoted cesium salt of 12-tungstophosphoric acid, Rh/Cs 2HPW 12O 40 (HPA), has been studied with 13C solid-state NMR. The bi-functional character of Rh/Cs 2HPW 12O 40 catalyst in halide-free carbonylation of DME has been directly demonstrated. The activation of the C–O bond of DME proceeds on Brønsted acid sites of HPA with the formation of the methyl group attached to the surface of HPA (methoxy species), whereas the role of rhodium consists in trapping carbon monoxide from gaseous phase and a transfer of CO to the center of DME activation, acidic OH-group of the catalyst, in the form of rhodium carbonyls. The lattice of Cs 2HPW 12O 40 makes it possible to locate these two different active centers in close proximity to each other, e.g., on two adjacent oxygen atoms, terminal and bridging, of one Keggin anion, thus facilitating the insertion of carbon monoxide from rhodium carbonyl into the C–O bond of methoxy-group to produce the acetate group bound to the Keggin anion. The latter offers finally methyl acetate under the interaction with DME, the intermediate surface methoxy-groups being restored.

Effects of acidity and microstructure on the catalytic behavior of cesium salts of 12-tungstophosphoric acid for oxidative dehydrogenation of propane

A series of Cs x H 3− x PW 12O 40 ( x = 0.9–3.0) samples with regularly dependent acidity values and gradually transformed microstructures was synthesized by a precipitation method. Along with the increase in Cs content in the Cs x H 3− x PW 12O 40 samples, the concentration of acid sites decreased monotonically, and their morphologies changed gradually from compact large lumps to nano-sized particles. Mesoporous channels were formed probably because of the stacking of nanoparticles in the samples with x > 2.1. When the Cs x H 3− x PW 12O 40 samples were used as catalysts for oxidative dehydrogenation (ODH) of propane, higher conversions of propane were achieved over the catalysts with higher acidity. The formation of mesoporous structure improved propane conversion evidently. The best yield of propylene was achieved over the Cs 1.5H 1.5PW 12O 40 catalyst, which possessed a high concentration of acid sites and non-porous structure. Our studies demonstrate that not only the acidity but also the porous structure of the Cs x H 3− x PW 12O 40 play key roles in the ODH of propane.

Methanolysis of Castor Oil Catalysed by Solid Potassium and Cesium Salts of 12-Tungstophosphoric Acid

Methanolysis of castor oil to methyl esters, a key step in Biodiesel production, was studied with the use of KOH, H2SO4 and 12-tungstophosphoric acid (H3PW12O40, HPW) as the homogeneous catalysts. Reaction was also performed in the presence of solid salts of HPW, namely M x H3−x PW12O40, where M = K or Cs and x = 2, 2.5 and 3 (abbreviated as K2, K2.5, K3 and Cs2, Cs2.5, Cs3, respectively). The HPW salts were precipitated by K2CO3 and Cs2CO3 or CsCl. Their properties were characterized by BET, electron microscopy (SEM, EDS) and colloidal particles size distribution (laser diffraction technique). The potassium doped HPW samples, K2 and K2.5, prove to be much more active catalysts (ca. 3 times) than their Cs-containing analogues. Among the K, Cs salts, K2 salt was the most active catalyst. The activity of catalysts was found to depend on preparation stages such as the temperature of drying or annealing and aging of the samples. Microscopic studies evidenced colloidal form of Cs and K salts particles under the catalytic reaction. The size of colloidal particles was found to depend upon the type of cation, Cs+ or K+, as well as the “history” of catalysts preparation (temperature of drying). Based on the results obtained in this work, we concluded that activity was determined by the accessibility of the reactants to acid sites which is facilitated by the high surface area and open structure of the colloidal form. This may lead to better utilization of acid sites and higher activity of samples with lower content of K+ or Cs+ cations in the HPW.

Asymmetric Diels-Alder Reactions on Supported Bis(oxazoline) Catalysts

Abstract Periodic mesoporous ethane-silicas functionalized with sulfonic acid, cesium salts of 12-tungstophosphoric acid, activated silica, and SBA-15 have been used as solid supports for the immobilization of the bis(oxazoline) copper (II) catalyst via non-covalent interactions. These heterogeneous catalysts have been evaluated for the Diels-Alder reaction between 3-(( E )-2-butenoyl)-1,3-oxazolin-2- one and cyclopentadiene. The performance of the catalyst relies on the nature of both the support and the anion on the surface of the support. The small number of the silanol groups on the support leads to poor enantioselectivity when SBA-15 is used.

Acidic cesium salts of 12-tungstophosphoric acid as catalysts for the dehydration of xylose into furfural

Cesium salts of 12-tungstophosphoric acid, Cs x H 3− x PW 12O 40 (Cs x PW), in the bulk form or supported on medium-pore MCM-41 (3.7 nm) or large-pore (9.6 nm) micelle-templated silicas are active solid acid catalysts for the cyclodehydration of xylose into furfural, in a toluene/water solvent system (T/W) or in dimethyl sulfoxide (DMSO). The catalytic results are comparable to those obtained using sulfuric acid, under similar reaction conditions. The initial activities increase in the order H 3PW 12O 40 < Cs 2.0PW < Cs 2.5PW < silica-supported CsPW catalysts (15, 34 wt % PW). Higher HPA loadings, larger pore diameter of the parent silica support, and higher reaction temperatures lead to higher furfural yields. The stability and reusability of the MCM-41-supported CsPW is higher in DMSO than in T/W.

Pore Structure and Shape Selectivity of Platinum-Promoted Cesium Salts of 12-Tungstophosphoric Acid

Pore-widths and pore-size distributions of 0.5 wt% Pt-CsxH3-xPW12O40 have been studied by means of adsorption of various molecules. For the distributions of micropore and mesopore, isotherms of Ar and N2 adsorption were analyzed, respectively. Pt-Cs2.1H0.9PW12O40 possessed only ultramicropores. On the other hand, the pores of Pt-CsxH3-xPW12O40 (x = 2.3, 2.5, 2.8 and 3.0) showed bimodal distributions in the range from micropore to mesopore, and the widths of both pores tended to increase as the Cs content increased. From the amounts and rates of adsorption for n-butane and isobutane, the pore width of Pt-Cs2.1H0.9PW12O40 was determined to be close to the molecular size of n-butane, that is, 0.43 nm. The fraction of external surface area in the total surface area of Pt-Cs2.1H0.9PW12O40 was estimated to be only 0.06 from the adsorption of 1,3,5-trimethylbenzene and t-plot of N2 adsorption. Pt- Cs2.1H0.9PW12O40 exhibited a shape selectivity due to the uniform ultramicropores and small external surface area; it catalyzed the oxidation of n-butane but not that of isobutane. SEM and TEM measurements revealed the primary crystallites and their aggregated states.

Heterogeneous Catalysts for Halide-Free Carbonylation of Dimethyl Ether

Acidic cesium salts of 12-tungstophosphoric acid promoted with rhodium, Rh/CsxH3-xPW12O40 (1.5≤x≤2 and Rh≥0.1%), were shown to be novel and effective catalysts for halide-free carbonylation of dimethyl ether (DME) to methyl acetate. The catalysts were characterized by chemical analysis, nitrogen adsorption, XRD, SEM, TGA-DTA and IR spectroscopy. Their behavior may be explained by assuming that (1) activation of the C—O bond in the DME molecule and formation of a metal-alkyl bond occurs in the presence of the strong acid sites and (2) these strong acid sites act in conjunction with Rh carbonyl complexes, which are responsible for CO insertion and acetate formation.

Catalysis by Heteropoly Compounds

Skeletal isomerization ofn-butane has been studied over bifunctional catalysts consisting of noble metals and an acidic cesium salt of12-tungstophosphoric acid, Cs2.5H0.5PW12O40. At 473 K in the presence of 0.05 atm of H2, 1 wt% Pt-Cs2.5H0.5PW12O40(1% in weight of Pt impregnated to Cs2.5H0.5PW12O40) showed activity and selectivity comparable to 1 wt% Pt-sulfated zirconia, while at 573 K and 0.5 atm of H2, 1 wt% Pt- and Pd-Cs2.5H0.5PW12O40became muchmore active and selective for the formation of isobutane than 1 wt% Pt-sulfated zirconia and 1 wt% Pt-exchanged ZSM-5. Negative dependence of the initial reaction rate on H2pressure and little dependence on Pt loading level for Pt-Cs2.5H0.5PW12O40indicate that the reaction proceeds through a bifunctional mechanism in whichn-butane is rapidly dehydrogenated ton-butenes on Pt,n-butenes isomerize to isobutylene on acid sites, and then isobutylene is hydrogenated to isobutane on Pt. A remarkable effect of the presence of hydrogen was the repression of the catalyst deactivation. Thisis probably caused by the hydrogenation of coke and/or its precursor and a decrease in the butene partial pressure. Very interesting was the unique function of protons; protons present in proximity of Pt or Pd particles suppress the hydrogenolysis that takes place on Pt or Pd and greatly increase the selectivity to isobutane.

Molecular design of solid acid catalysts. Isomerization of n-butane catalyzed by acidic cesium salts of 12-tungstophosphoric acid combined with platinum

Our recent efforts made for the molecular design of solid acid catalysts utilizing heteropoly compounds are described first and, then, as an example, the isomerization of n-butane in the presence of hydrogen catalyzed by a bifunctional catalyst consisting of Pt and Cs 2.5H 0.5PW 12O 40 was examined mostly at 573 K. Presence of Pt suppressed substantially the catalyst deactivation and gave much higher stationary activity, although the initial activity was lowered a little due to a decrease in the concentration of butenes produced. It was further demonstrated that the activity of Pt for hydrogenolysis was nearly completely suppressed by the protonic acid sites present near Pt, giving a very high selectivity for isomerization.

Skeletal Isomerization of n-Butane Catalyzed by an Acidic Cesium Salt of 12-Tungstophosphoric Acid

Abstract A salt of H3PW12O40, Cs2.5H0.5PW12O40, catalyzed efficiently the skeletal isomerization of n-butane to isobutane at 300 °C. The activity and selectivity were much higher than those of SO22−/ZrO2. H-ZSM-5 was more active than Cs2.5H0.5PW12O40, but the selectivity was very low.

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.

A Pronounced Catalytic Activity of an Acidic Cesium Salt of 12-Tungstophosphoric Acid for Ester Decomposition in Solid–liquid System

Abstract The acidic Cs salt of 12-tungstophosphoric acid (Cs/polyanion = 2.5) was much more active for liquid phase decomposition of cyclohexyl acetate than Nafion, HY-zeolite, H-ZSM-5, Al3+-exchanged montmorillonite, and SO42−/ZrO2.