Keggin structure heteropoly

Recently, Corma et al. have patented a process of oxidizing cycloalkane with molecular oxygen to produce cycloalkanol and/or cycloalkanone in the presence of hydrotalcite-intercalated heteropoly anion [Co MnCo (H20)039] (M = W or Mo), which comprised one cobalt as a central atom and another as a substitute of a W=0 fragment in the Keggin structure [98]. At 130 °C and 0.5 MPa, 64 and 24% selectivity to cyclohexanone and cyclohexanol, respectively, was achieved at cyclohexane conversion about 5%. This catalytic system could be of practical importance provided a true heterogeneous nature of catalysis and good catalyst recyclability had been proved. Unfortunately, this information was lacking in [98]. 

Catalytic oxidation of propylene to acrolein was first discovered by the Shell group in 1948 on Cu20 catalyst (/). Both oxidation and ammoxidation were industrialized by the epoch-making discovery of bismuth molybdate catalyst by SOHIO (2-4). The bismuth molybdate catalyst was first reported in the form of a heteropoly compound supported on Si02, Bi P,Mo,2052/Si02 having Keggin structure but it was not the sole active species for the reactions. Several kinds of binary oxides between molybdenum trioxide and bismuth oxide have been known, as shown in the phase... 

Early attempts to use heteropoly compounds as catalysts are summarized in reviews published in 1952 (//) and 1978 (7). The first industrial process using a heteropoly catalyst was started up in 1972 for the hydration of propylene in the liquid phase. The essential role of the Keggin structure in a solid heteropoly catalyst was explicitly shown in 1975 in a patent concerning catalytic oxidation of methacrolein. Systematic research in heterogeneous catalysis with these materials started in the mid-1970s and led to the recognition of quantitative relationships between the acid or redox properties and catalytic performance... 

Fig. 2. Primary, secondary, and tertiary structures of heteropoly compounds, (a) Primary structure (Keggin structure, XM 204o) (b) secondary structure (H3PW 204o 6H20) (c) secondary structure (Cs3PW 204o) (d) tertiary structure [Cs25Ho5PW 204o, cubic structure as in (c)].

Keggin-type heteropoly compounds having Mo and V as addenda atoms are usually used for such oxidations. The catalysts reported in patents often contain several elements other than Mo, V, and P. An excess amount of P is added to stabilize the structure, and the presence of additional transition elements like Cu improves redox reversibility. Supported heteropoly catalysts are also important for industrial applications and have been characterized (69, 325, 326). 

In the Keggin structure of the 12-heteropolymolybdates and 12-heteropoly-tungstates, it is possible to substitute atoms other than vanadium or niobium for tungsten or molybdenum. 

Lin S., Zheng Y., Xu L.-M. and Wang S.-M., Synthesis, characterization and catalytic activity for phenol hydroxylation of rare earth molybdovanadophosphate tetrabasic heteropoly complexes with Keggin structure., Chem. J. Chinese Univ., 21 (2000) pp. 1248 (in Chinese). 

Mo-heteropolyanions with a Keggin structure are readily reduced to give heteropoly blue in solution. A trend exists toward anions with X of higher charge (small value of n) being more easily reduced, while the reduction potential decreases in the order V > Mo > W. In the case of XMnWn04oH2", the Mn + in the anion is oxidized by O2. 

NMR has also been widely used for structural elucidation of vanadium-containing mixed addenda heteropoly compounds, owing to the large natural abundance of nuclei. The number of structural isomers increases with the increase in the number of addenda atoms and their position in the Keggin structure is confirmed by NMR spectroscopy [138]. 

But if the epoxidation conditions used is not suitable for this catalytic system, such as no additive added, part of the used catalyst will convert to a heteropoly-phosphotungstate with the Keggin structure [42], and then the recycle catalytic activity will decrease. 

In this laboratory the conversion of methanol (refs. 12-16) and the partial oxidation of methane (ref. 17) have been studied on heteropoly oxometalates, particularly those with Keggin structure. These are ionic solids with high molecular weight, cagelike anions and cations which may be chosen from a wide variety of possibilities, ranging from the proton to charged organic species. 

The infrared spectra also demonstrate that the supported species retain the Keggin structure for temperatures as high as 700°C, in contrast to the observation that bulk HPMo has apparently lost a substantial portion of its Keggin structure by 450°C. The Si02 support is evidently playing a dual role in the present experiments, that of increasing the operative surface area of the HPMo as well as providing an enhancement of the thermal stability of the supported heteropoly oxometalate. 

A single sharp pak at -3.2 ppm (Fig. la) was observed for H3PM012O40, on addition of hydrogen peroxide this peak completely disappeared and a new peak at 4.1 ppm (Fig.lb) was observed. This peak was shown to be due to the formation of molybdenum peroxospecies [5] whose structure was different from the Keggin structure of the heteropoly compound. 

Figure 1. Keggin structure for heteropoly anions showing arrangement of ZOr, octahedra around the central MO tetrahedron...

Spectra of Reduced Anions. IR spectra of the PV2W10O40" anions are very similar to those reported for other heteropoly tungstates with the Keggin structure. Of particular interest however is the T2 mode of the central PO4 group which occurs as an intense sharp absorption at 1080... 

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