Bismuth molybdate catalyst reduction

Fig. 12. Dependence of ESR signal strength and change of crystal voltage on degree of reduction of bismuth molybdate catalysts during propene oxidation. ---, AV ...

Some remarks must be made about the role of oxygen coordination. Several authors have remarked that the coordination in catalytic oxides is of major importance. Mitchell and Trifiro (e.g. ref. 219) concluded that a bismuth molybdate catalyst is most active if the amount of tetrahedrally coordinated molybdenum is large in comparison with octahedrally coordinated molybdenum. However, V205 and Sb2Os are structures with specific octahedral coordination [142] and often the coordination is changed by reduction of the catalyst or by the support [203]. In a- and /3-cobalt molybdates the coordination differs, but the catalytic behaviour is really the same. The low temperature Bi2Mo06 (7 phase) has an octahedral coordination but is an effective catalyst. 

Aykan (35) reported that ammoxidation of propylene occurred over a silica-supported bismuth molybdate catalyst in the absence of gas-phase oxygen, although the catalytic activity decreased rapidly with increasing catalyst reduction. The reduction process was followed by X-ray and it was found that phase changes which occurred in the catalyst and the decrease in catalytic activity corresponded quantitatively to the depletion of lattice oxygen. 

Peacock et al. (37) showed that a bismuth molybdate catalyst can be reduced with propylene and that the oxygen appearing in the gaseous products (acrolein, carbon dioxide, and water) can be quantitatively replaced in the lattice. The amount of oxygen removed during reduction corresponds to the participation of many sublayers of oxide ions. 

Matsuura (130) measured the ESR spectra of bismuth molybdate catalysts both before and after reduction of butene. A board signal of high intensity was observed for the y phase of bismuth molybdate. The authors proposed that in the layered Bi2MoOs structure, a (Mo042 ) layer shifts with respect to the nearest layers and causes the formation of Mo-Bi-Mo sites. The results supported the earlier proposed reaction site model, based on adsorption measurements, which consisted of an A site (Bi) and two B sites (Mo). 

Application of Raman spectroscopy to a study of catalyst surfaces is increasing. Until recently, this technique had been limited to observing distortions in adsorbed organic molecules by the appearance of forbidden Raman bands and giant Raman effects of silver surfaces with chemisorbed species. However, the development of laser Raman instrumentation and modern computerization techniques for control and data reduction have expanded these applications to studies of acid sites and oxide structures. For example The oxidation-reduction cycle occurring in bismuth molybdate catalysts for oxidation of ammonia and propylene to acrylonitrile has been studied in situ by this technique. And new and valuable information on the interaction of oxides, such as tungsten oxide and cerium oxide, with the surface of an alumina support, has been obtained. 

Pulse experiments with a Sn/Sb = 2/1 catalyst in the absence of oxygen have been carried out by Barannik et al. [38,39]. The activity rapidly decreases with increasing reduction, while the selectivity strongly increases. This is in contrast with bismuth molybdates, which demonstrate a similarly decreasing activity, but a constant (high) selectivity level. 

Dealkylation may also become the predominant reaction with bismuth molybdate. This was shown by van der Wiele [347] for the progressive reduction of the catalyst with toluene pulses. While initially the product spectrum is similar to that obtained in presence of air, a shift to benzene formation occurs at increased reduction. 

The occurrence of an almost constant, albeit rather low, activity level, which is reached after a number of pulses, signifies that a certain quasiequilibrium concentration of active sites is maintained by transport of bulk oxygen anions to the surface. Such a mobility of oxygen is particularly observed for bismuth molybdates and some related catalysts (see below). Typical examples of catalysts which completely loose their activity at a low degree of reduction are the antimonates this is primarily caused by the absence of anion mobility. 

Haber (59, 113) has reported data supporting the concept that the low-valence cation is responsible for propylene activation. He studied the kinetics of catalyst reduction for a-, /)-, and y-bismuth molybdates using both hydrogen and propylene as reductants. Because the coordination of molybdenum varies among the three phases (tetrahedral in a phase, tetrahedral and octahedral in /3 phase, and octahedral in y phase), the... 

As discussed previously, the relative rates of reduction of several bismuth molybdate-based catalyst systems using propylene decrease in the order multicomponent system > Bi2Mo2O9(j6) Bi2Mo3O,2(a) > Bi3FeMo2Oj2... 

Fig. 3. Proposed structure of active site of bismuth molybdate propylene ammoxidation catalyst. 0 , oxygen responsible for a-H abstraction O, oxygen associated with Mo, responsible for oxygen insertion into the allylic intermediate and , proposed center for O2 reduction and dissociative chemosorption.

In early patents, it was claimed that various phosphate catalysts, including calcium/nickel phosphate and bismuth/molybdate, were active. A variety of other catalysts was also introduced, including the zinc and magnesium ferrites developed by Petrotex." The best ferrite catalyst included chromium to prevent excessive catalyst reduction." PhilUps described tin oxide catalysts that were promoted with 4% phosphate and 1% lithium as well as the bismuth/phosphate catalysts that were promoted with boron or lithium compounds. 

Introduction of a support (i.e. Ti02) to these systems can induce positive effects on the active phase (FeMo), as to avoid an excessive sintering of the particles during the thermal treatment and/or modification of the reduction capacity and the acid properties. The synergy of iron molybdate and the support is therefore another way for improving the catalytic performance of these solids. Such benefitial effects have been detected in bismuth-molybdenum-titania mixed oxides prepared via sol-gel, in addition these solids resulted to be amorphous materials with a unique morphology and extraordinary dispersion of the active phase [12]. These results encouraged us to extend this field to iron molybdenum oxide catalysts. 

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