Keggin type structures

The development of catalysts obtained by means of heteropolyacids (HPA) and related compounds is a very important growing field. Since HPA are less corrosive and produce lower amount of wastes than conventional acid catalysts, they can be used as replacement in ecofriendly processes. Within the HPA, there is a special interest in those which present a Keggin type structure. 

In recent years heteropolycompounds have been studied for the oxidation of propane to acrylic acid and of isobutane to methacrylie aeid. Rohm Haas Company was the first in 1981 to claim the one-step oxidation of isobutane to methacrolein and methacrylie acid (55). Even though no reference is made to heteropolycompounds, the claimed catalyst compositions correspond to Keggin-type structures. In the patents later issued by Sumitomo (56,57) an important role was claimed to be played by vanadium (in an anionic position), by cesium (in a cationic position), as well as by an excess of phosphorus with respect to the stoichiometric composition. These catalysts gave selectivities to methacrylie acid plus methacrolein close to 70 %, with isobutane conversions in the 10 to 13 % range. Besides carbon oxides, acetic acid was the main by-product. 

Some new structural polyoxometalates were obtained, which are listed in the Table 1. In Table 1, the compound [Bu4N]6[H30]2[Moi304o]2 synthesized by Professor K.L. Tang was a new Keggin-type isopolyoxomolybdate, in which, the [BuaN] cations, [H3O] cations, [Moi304o] anions have different symmetry. Both anions have Keggin-type structures. The structures are based on a central Mo(A)04 slightly distorted tetrahedron surrounded by 12 Mo(B)Oe octahedra. 

Heteropoly Anions with Keggin-Type Structures... 

Keggin-type structure [17]. The infrared signals at 569, 871, and 994 cirr indicate that the heteropoly anion collapses completely towards M0O3 at 873 K. 

In the present work we describe the preparation and properties in ethanol conversion of conjugated polymer supported heteropolyanions of Keggin-type structure. Thus 12-molybdophosphoric acid (H3PM012O40) has been supported on polyacetylene (PAc)... 

A variety of methods to study the acidity of heteropoly compounds are reported in the literature in line with the study of the catalytic applications of the HPAs. These methods, mainly applied on Keggin type structures, evaluate the acid strength and the amount of molecules adsorbed with probe molecules (ammonia, pyridine) different from actual reactants. The studies that used actual reactant molecules, such as alcohols, determine the total amount of adsorption sites rather than the number of active acid sites at a certain temperature of reaction. In fact, the studies reported in the literature focused on the intermediate adsorbed species rather than on the in situ identification and quantification of the products of... 

It is generally accepted that the oligomer responible for the layer separation of conventional Al-pillared smectites is Ali30(f(0H)2 f (ref. <1). The structure of this cation is a central four-coordinated aluminum atom surrounded by twelve AlOg-octahedra joined by common edges to form a Keggin-type structure. 

Fig. 15. Crystal structure of the Keggin-type polyoxometallate ion [PMo427W7.73O40]6. Adapted from (608).

Herve et al. (57) investigated the thermal changes of structures by means of XRD and TG-DTA for Keggin-type heteropolyacids and proposed Scheme 2. Infrared spectroscopy of H4PMo, VO40 showed the release of vanadium atoms to form H3PM012O40 and vanadium phosphate species (55). Exposure to water vapor induces the decomposition of the latter (indicated by the disappearance of a band at ca. 1037-1030 cm -1) (58). 

Table XVII is a comparison of the catalytic activities for liquid-phase MTBE synthesis from isobutylene and methanol (179). The catalyst structure and composition have a strong effect on the activity. The highest activity per proton was obtained with a Dawson-type heteropolyacid, H6P2W 8062, although the acid strength of H WigO is lower than that of the Keggin-type H3PW12O40 (Section HI). Water added to the mixture has little effect on the reaction rate at water concentrations less that 2 wt%, but at 5 wt% the rate is less by a factor of 2.5. At the same time the selectivity is less due to the formation of (erf-butyl alcohol.

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). 

The y-isomer Keggin-type silicodecatungstate [y-SiWio036]8- has been used as a structural motif for di-metal-substituted POMs with bis- i-oxo(or hydroxo)-dimetal cores [97-102]. The di-iron-substituted silicotungstate [y-SiWi0 Fe(OH2) 2O3g]6 was synthesized by the reaction of [y-SiWi0O36]8- with Fe(N03)3 in acidic aqueous... 

Figure 5.28 The two-dimensional supramolecular network in [RE(NMP)4(H20)4][H cGeMoi204o] [79]. (Reprinted with permission from H. Zhang, et al., Synthesis, crystal structure, and photochromism of novel two-dimensional supramolecular networks based on Keggin-type polyoxoanion and lanthanide coordination cations, Inorganic Chemistry, 42, no. 24, 8053-8058 (Figure 3), 2003. 2003 American Chemical Society.)...

Niu, J., Zhao, J. Wang, J. et al. (2004) Synthesis, property and crystal structure of a novel two-dimensional network organic-inorganic hybrid compound based on the neodymium center and Keggin-type heteropolyanion of [a- N Journal of Molecular Structure, 699, 85-92. 

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