Vanadate catalyst

Rearrangement of dehydrolinalool (4) using vanadate catalysts produces citral (5), an intermediate for Vitamin A synthesis as well as an important flavor and fragrance material (37). Isomerization of the dehydrolinalyl acetate (6) in the presence of copper salts in acetic acid followed by saponification of the acetate also gives citral (38,39). Further improvement in the catalyst system has greatly improved the yield to 85—90% (40,41). 

Another important use of a-pinene is the hydrogenation to i j -pinane (21). One use of the i j -pinane is based on oxidation to cis- and /n j -pinane hydroperoxide and their subsequent catalytic reduction to cis- and /n j -pinanol (22 and 23) in about an 80 20 ratio (53,54). Pyrolysis of the i j -pinanol is an important route to linalool overall the yield of linalool (3) from a-pinene is about 30%. Linalool can be readily isomerized to nerol and geraniol using an ortho vanadate catalyst (55). Because the isomerization is an equiUbrium process, use of borate esters in the process improves the yield of nerol and geraniol to as high as 90% (56). 

The production of acetic acid from n-butene mixture is a vapor-phase catalytic process. The oxidation reaction occurs at approximately 270°C over a titanium vanadate catalyst. A 70% acetic acid yield has been reported. The major by-products are carbon oxides (25%) and maleic anhydride (3%) ... 

This paper is a summary of our current understanding of this system. In particular, we will be discussing the observations in terms of selectivity with respect to the availability of reactive lattice oxygen. The organization of the paper is as follows. First, the general features of the reaction scheme for alkane oxidation on vanadate catalysts will be presented. This is followed by a discussion of results on the effect of ease of removal of oxygen from the lattice on the selectivity, and then a discussion on the importance of the atomic arrangement of the active sites. 

Because of the rapid developments in the field of heterogeneous catalysis, the material reviewed here is exclusively dedicated to selective oxidations. No attention is given to total oxidations or combustion processes (including the problem of automotive exhaust gases). There is one exception, however the oxidation of sulphur dioxide to trioxide. Work on vanadate catalysts for this reaction is close to research on selective catalysts and therefore included. 

Anthraquinone is the primary product of the oxidation of anthracene over V2Os-based catalysts. The reaction is very selective and high yields of anthraquinone are possible due to its relatively high stability. An iron vanadate catalyst is used in the industrial process and yields of 80—90 mol. % are reported at 320—370°C. Phthalic anhydride, maleic anhydride and carbon oxides are the by-products. 

Yoshida et alP2 observed that oxygen within a layer of 20 A thickness of the surface of copper vanadate catalysts takes part both in reduction by CO and in CO oxidation. The Cu ions are also very mobile in this layer and surface enrichment of these ions occurs upon 02 treatment after reduction at 200°C. 

Propane Butane Hexane Alkenes Vanadate catalysts 50 50 38... 

These results are to be compared with those for toluene in which the same tin vanadate catalyst was used at a temperature of 290° C. Benzoic acid was the only product condensed. 

The fact that both space time and per cent yields of phthalic anhydride show decreases with temperature increases above 280° C. shows that the lowest temperature for efficient operation was not found. The abnormally low temperature at which the tin vanadate catalyst becomes active should make it applicable to the oxidation of a number of substances that ordinarily would decompose at the temperatures that are required for the activation of most of the oxidation catalysts. 

Tian, H., Wachs, I. and Briand, L. (2005). Comparison of UV and Visible Raman Spectroscopy of Bulk Metal Molybdate and Metal Vanadate Catalysts, J. Phys. Chem. B., 109, pp. 23491-23499. 

Surface Composition of Bulk Metal Molybdate and Vanadate Catalysts... 

The TOF values of bulk metal molybdates were extrapolated to 300 C in order to compare their values with the corresponding bulk metal vanadate catalysts. Figure 11.11 shows that, in general, bulk metal vanadates possess one order of magnitude higher TOF values ( 2 to 14 sec ) than their corresponding bulk metal molybdates ( 0.1 sec ) for methanol selective oxidation. The TOF values of pure... 

Figure 11.10 Comparison of the TOFs towards methanol selective oxidation products of monolayer supported vanadium (A symbols) and bulk metal vanadate catalysts ( ) at 300°C (From Briand, L.E., Jehng, J.-M., ComagUa, L.M., Hirt, A.M., and Wachs, l.E. Catal. Today 2003, 78, 257-268. With permission.)...

Sugiyama, S., Hashimoto, T., Shigemoto, N., and Hayashi, H. Redox behaviors of magnesium vanadate catalysts during the oxidative dehydrogenation of propane. Catal Lett. 2003, 89, 229. 

Citral can also be produced from dehydrolinalool via a Meyer-Schuster rearrangement [32, 33], rearrangement using a vanadate catalyst (34), or by rearrangement of its acetate in the presence of copper salts [35, 36], trisilylorthovanadates (37), or vanadium catalysts in the presence of silanols (38) and yields of up to 90% can be obtained [39,40]. 

The correlation between smface V M ratios and the catalytic properties of different metal vanadate catalysts is illustrated in Fig. 7.6. These investigations clearly demonstrated that the phase composition and eiuichment of vanadium in the near- smface- region (investigated by X-ray photoelectron spectroscopy (XPS)) strongly depend on the nature of the metal used in the metal vanadates. Fmtiiermore, these two properties play an important role on the activity and selectivity behavior of the catalysts [99]. It was observed that higher surface V M ratios enhanced the selectivity of CPy but decreased the conversion of 2MP. 

FIGURE 7.6 Correlation between the maximum conversion at given temperature and the near-surface-region V M molar ratio of various metal vanadate catalysts (reaction conditions catalyst wt. = 1 g reactant molar ratio MP H O NH3 air = 1 13 7 26 22 total flow = 9506 mL/h GHSV = 9500 h ) [96,98],... 

DhachapaUy, N., Kalevaru, V.N., Brtlckner, A., and Martin, A. Metal vanadate catalysts for the ammoxidation of 2-methylpyrazine to 2-cyanopyrazine. Appl Catal A 443-444, 111-118(2012). 

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