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

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.

Heterogeneous catalysis by heteropoly compounds of molybdenum and tungsten

The goal of this review is to present some aspects of the use of a test reaction, i.e., methanol oxidation, as a tool to characterize oxidation catalysts. The selectivity pattern and the formation rates of the reaction products are used to characterize both structural (dispersion) as well as chemical properties (acid-base and redox) on supported oxide catalysts, especially for molybdenum-based systems supported on silica and vanadia on titanium oxide. This highly sensitive technique which gives information on the catalytically active sites at the molecular level characterizes a catalyst at work and is particularly well-adapted to the study of supported catalysts.

Methanol oxidation as a catalytic surface probe

Dehydration of glycerol in gas phase using heteropolyacid catalysts as active compounds

A freshly precipitated zirconia was impregnated with an ethanol solution of H3[W12PO40]6H2O (TPA) in order to obtain 0‐25 wt.% TPA/ZrO2. The solids, heat-treated from 373 to 1073 K, were examined by means of differential thermal analysis, X-ray diffraction, surface area, infrared, Raman, 31 P nuclear magnetic resonance analyses, and used as catalysts in the decomposition of isopropanol. Below 673 K, TPA on ZrO2 exist as distorted intact Keggin species interacting with Zr‐OH or Zr C groups. However, at 773 K, bulk-like intact TPA species were detected. Above 773 K, the Keggin structure of TPA collapse and transforms into WO3 and phosphorous oxides. The characteristic diffraction peaks of TPA cannot be observed even with 20 wt.% TPA loading, indicating a high dispersion or a very small crystal size of TPA, while 20 wt.% TPA loaded on SiO2 clearly shows the presence of TPA particles.The heat-treatment temperature affects the catalytic activity of TPA on Zirconia. The reaction rate was 3.3 times higher in a TPA/ZrO2 calcined at 773 than that at 473 K. ©2000 Elsevier Science B.V. All rights reserved.

B.K. Hodnett

Papers

Application of temperature-programmed desorption to the study of heteropoly compounds: Desorption of water and pyridine

Study of heteropoly compounds with the Keggin structure by temperature-programmed reduction and HD exchange