http://www1.udel.edu/chem/polenova/PDF/Polyoxometalates/POMs_Catalysis_NMR_ChemRev1998.pdf

Catalysis by Heteropoly Acids and Multicomponent Polyoxometalates in Liquid-Phase Reactions

Recent Advances in Polyoxometalate-Catalyzed Reactions

Water-tolerant solid acid catalysts.

Publisher Summary This chapter describes the essence of the catalytic chemistry of heteropoly compounds in solution and in the solid state. The catalytic properties of heteropoly compounds have drawn wide attention, owing to the versatility of these compounds as catalysts, which has been demonstrated both by successhl large-scale applications and by promising laboratory results. Heteropolyanions are polymeric oxoanions formed by condensation of more than two different mononuclear oxoanions. Heteropolyanions formed from one kind of polyanion are called isopolyanions. Acidic elements such as Mo, W, V, Nb and Ta, which are present as oxoanions in aqueous solution, tend to polymerize by dehydration at low pH, forming polyanions and water.Heteropoly catalysts can be applied in various ways. They are used as acid as well as oxidation catalysts. They are used in various phases, as homogeneous liquids, in two-phase liquids (in phase-transfer catalysis), and in liquid-solid and in gas-solid combinations, etc.

Catalytic Chemistry of Heteropoly Compounds

Catalysis by heteropoly acids (HPAs) and related compounds is a field of growing importance, attracting increasing attention worldwide, in which many new and exciting developments are taking place in both research and technology [1–111, HPAs are polyoxometalates incorporating anions (heteropolyanions) having metal-oxygen octahedra as the basic structural units [ll-141. Among a wide variety of HPAs those belonging to the so-called Keggin series are the most importance for catalysis. They include heteropolyanions (HPANs)

Heteropoly Acids and Related Compounds as Catalysts for Fine Chemical Synthesis

Catalysis by Heteropoly Compounds

Skeletal isomerization of n-butane to isobutane has been studied with a flow reactor, mainly at 573 K, over caesium hydrogen salts of 12-tungstophosphoric acid (trihydrogen phosphododecatungstate), CsxH3–xPW12O40. The activity was highly dependent on the caesium content, being a maximum for x= 2.5 where the number of acid sites on the surface was greatest. The initial rate was first order with respect to the butane pressure (ca. 0.05–0.5 atm) at both 423 and 573 K on Cs2.5H0.5PW12O40. Cs2.5H0.5PW12O40 was found to be much more active and selective (83%cf. 60%) than SO42–/ZrO2 at the steady state at 573 K. H-ZSM-5 showed a higher activity than Cs2.5H0.5PW12O40 and SO42–/ZrO2 but the selectivity was only 14%. Cs2.5H0.5PW12O40 is a promising catalyst for this reaction and the low deactivation of Cs2.5H0.5PW12O40 is probably responsible for the high catalytic activity.

Skeletal isomerization of n-butane over caesium hydrogen salts of 12-tungstophosphoric acid

Acid-catalyzed reactions by acidic cesium salts of 12-tungstophosphoric acid (H3PW12O40) have been studied in relation to the acidities and microstructure of the particles. Liquid-phase alkylation of 1,3,5-trimethylbenzene with cyclohexene, decomposition of cyclohexylacetate and gas-phase skeletal isomerization of n-butane were performed over heteropoly compounds and other typical solid acids such as zeolites, Nafion-H, SO42-/ZrO2, etc. It was found that Cs2.5H0.5PW12O40 showed very high activities for these reactions and the excellent selectivity for the skeletal isomerization. XRD, TEM and N2 adsorption revealed that Cs2.5H0.5PW12O40 consists of ultrafine primary particles (∼8 nm) and therefore has a high surface area. High resolution solid-state NMR demonstrated that the chemical shift of 31P (central atom of the polyanion) is determined by the number of proton which is directly attached to the polyanion and all protons are distributed randomly through the whole bulk of the particles. Owing to this, Cs2.5H0.5PW12O40 possesses a high surface protonic acidity, which is a reason for its high activity. The quantity of acid sites of Cs2.5H0.5PW12O40 was less than those of zeolites and SO42−/ZrO2 and the acid strength was lower than that of SO42−/ZrO2. It is presumed, therefore, that the acid-base bifunctional action together with the high surface acidity brought about the high activity of Cs2.5H0.5PW12O40.

4.8 Novel Catalysis of Cesium Salt of Heteropoly Acid and its Characterization by Solid-state NMR

A solid superacid, Cs2.5H0.5PW12O40, catalyzed the alkylation of isobutane with butenes at room temperature. Cs2.5H0.5PW12O40 was much more active than SO42−/ZrO2, a well-known solid superacid, and was highly selective.

Alkylation of Isobutane with Butenes Catalyzed by a Cesium Hydrogen Salt of 12-Tungstophosphoric Acid

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 Cs2.5H0.5PW12O40 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.

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

Kyutae Na

Papers

Catalysis by Heteropoly Compounds

Skeletal isomerization of n-butane over caesium hydrogen salts of 12-tungstophosphoric acid

4.8 Novel Catalysis of Cesium Salt of Heteropoly Acid and its Characterization by Solid-state NMR

Alkylation of Isobutane with Butenes Catalyzed by a Cesium Hydrogen Salt of 12-Tungstophosphoric Acid

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