The structure of molybdenum-heteropoly acids under conditions of gas-phase selective oxidation catalysis: a multi-method in situ study

Abstract

The present study focuses on the evidence about the existence of Keggin ions under various reactive conditions. The stability of the hydrated parent heteropoly acid (HPA) phases is probed in water, by thermal methods in the gas phase, by in situ X-ray diffraction and in situ EXAFS. An extensive analysis of the in situ optical spectra as UV-Vis-NIR in diffuse reflectance yields detailed information about the activated species that are clearly different from Keggin ions but are also clearly no fragments of binary oxides in crystalline or amorphous form. Infrared spectroscopy with CO as probe molecule is used to investigate active sites for their acidity. Besides -OH groups evidence for electron-rich Lewis acid sites was found in activated HPA. All information fit into a picture of a metastable defective polyoxometallate anion that is oligomerised to prevent crystallisation of binary oxides as the true nature of the "active HPA" catalyst. The as-synthesized HPA crystal is thus a pre catalyst and the precursor oxide mixture is the final deactivated state of the catalyst.

One-step synthesis of methylmethacrylate from methacrolein over Keggin-type heteropoly compounds

“…Note that two minor peaks at about 1,079 cm −1 and 766 cm −1 were observed over CsPMo 11 VO 40 and KPMo 11 VO 40 , which are characteristic of V atoms substituting for Mo in the Keggin anions [10,22,26]. However, the two peaks disappeared over LaPMo 11 VO 40 , and a small peak at about 1,020 cm −1 occurred over LaPMo 11 VO 40 , which was ascribed to the band υ(V=O) in free V 2 O 5 [26].…”

“…Usually, changes of the secondary structure of HPCs would affect its catalytic performance. For example, HPCs used in the commercial process for the oxidation of MAL to MAA were presumably Cs x H 3+y−x PV y Mo 12−X O 40 (x=0-2, y=1-2), in which formation of Cs acidic salts increased the surface area and the thermal stability of the catalyst remarkably [6][7][8][9][10][11][12][13]. In other studies [14], the presence of Cu 2+ played an important role in supplementing lattice oxygen from gas oxygen, which was believed to be a rate-determining step.…”

Effect of cesium content on the structure and catalytic performances of heteropoly compounds in one-step synthesis of methylmethacrylate from methacrolein

“…3 shows infrared spectra of the HPCs with different Cs content after pretreatment. The catalysts exhibited four main characteristic FT-IR bands at 1,054-1,064 cm −1 , 954-961 cm −1 , 860-880 cm −1 and 740-790 cm −1 , attributable to υas (P-Oi), υs (Mo= [11,[17][18]. Meanwhile, the vibration frequency of υs (Mo=O) shifted down when Cs substituted for hydration proton.…”

The Oxidation of Isobutane to Methacrylic Acid: An Alternative Technology for MMA Production

“…47,48 More generally, it is currently believed that for exothermic reactions, and specifically for oxidations, the true working state of the POM, does not correspond to its crystalline form. 49 The presence of steam and the large amount of heat released provoke an incipient surface decomposition, which leads to the expulsion of the Mo species from the anion as a metastable defective compound is developed. The latter then evolves into an "oligomerized" surface form, which prevents the segregation of binary oxides.…”