Molybdenum-based catalysts

Al203-supported M0O3 catalysts are rather prone to cause side reactions above 120°C, e.g. hydrogenation, isomerization (Ichinose 1978), polymerization, and deposition of carbon (Rodriguez-Ramos 1995), but these can be suppressed by the inclusion of small amounts of KOH or NaOH in the catalyst (Bradshaw 1967 Alkema 1968). As the temperature is raised, the metathesis rate passes through a 

Support Pretreatment temperature of support/°C Catalyst activation temperature/ C Initial rate of propene metathesis / % min m  

Si02-supported Mo catalysts show somewhat similar behaviour to those supported on AI2O3. Table 5.3 indicates relative activities of some catalysts 

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CHjiCH-CN. Volatile liquid b.p. 78"C. Manufactured by the catalytic dehydration of ethylene cyanhydrin, by the addition of hydrogen cyanide to ethyne in the presence of CuCI or the reaction of propene, ammonia and air in the presence of a molybdenum-based catalyst. 

The oxidative dehydration of isobutyric acid [79-31-2] to methacrylic acid is most often carried out over iron—phosphoms or molybdenum—phosphoms based catalysts similar to those used in the oxidation of methacrolein to methacrylic acid. Conversions in excess of 95% and selectivity to methacrylic acid of 75—85% have been attained, resulting in single-pass yields of nearly 80%. The use of cesium-, copper-, and vanadium-doped catalysts are reported to be beneficial (96), as is the use of cesium in conjunction with quinoline (97). Generally the iron—phosphoms catalysts require temperatures in the vicinity of 400°C, in contrast to the molybdenum-based catalysts that exhibit comparable reactivity at 300°C (98). 

Ring-Opening Polymerization. Ring-opening polymerization of cycloolefins in the presence of tungsten- or molybdenum-based catalysts proceeds by a metathesis mechanism (67,68). 

AROM)-RCM- and -CM sequences initiated by chiral molybdenum-based catalysts [194] or, more recently, also by ruthenium-based [195] catalysts. 

For a chiral molybdenum-based catalyst available in situ from commercial components, see (a) Aeilts SL, Cefalo DR, Bonitatebus PJ, Houser JH, Hoveyda AH, Schrock RR (2001) Angew Chem Int Ed 40 1452 (b) For the first enantiomerically pure solid-sup-ported Mo catalyst, see Hultzsch KC, Jernelius JA, Hoveyda AH, Schrock RR (2002) Angew Chem Int Ed 41 589 (c) For a chiral Mo catalyst, allowing RCM to small- and medium-ring cyclic amines, see Dolman SJ, Sattely ES, Hoveyda AH, Schrock RR (2002) J Am Chem Soc 124 6991 (d) For a novel adamantyl imido-molybdenum complex with advanced selectivity profiles, see Tsang WCP, Jernelius JA, Cortez GA, Weatherhead GS, Schrock RR, Hoveyda AH (2003) J Am Chem Soc 125 2591... 

Propylene, ammonia, steam and air are fed to a vapour-phase catalytic reactor (item A). The feedstream composition (molar per cent) is propylene 7 ammonia 8 steam 20 air 65. A fixed-bed reactor is employed using a molybdenum-based catalyst at a temperature of 450°C, a pressure of 3 bar absolute, and a residence time of 4 seconds. Based upon a pure propylene feed, the carbon distribution by weight in the product from the reactor is ... 

Simple alkenes can give turnover numbers in the order of several 100,000 with tungsten or molybdenum based catalysts, including the in situ prepared catalysts (e.g. WC16, PhOH, SnBu4), provided that the alkene is thoroughly purified. A convenient purification method is percolation of the alkene over neutral alumina to remove peroxides. 

Ruthenium complexes B also undergo fast reaction with terminal alkenes, but only slow or no reaction with internal alkenes. Sterically demanding olefins such as, e.g., 3,3-dimethyl-l-butene, or conjugated or cumulated dienes cannot be metathesized with complexes B. These catalysts generally have a higher tendency to form cyclic oligomers from dienes than do molybdenum-based catalysts. With enol ethers and enamines irreversible formation of catalytically inactive complexes occurs [582] (see Section 2.1.9). Isomerization of allyl ethers to enol ethers has been observed with complexes B [582]. 

Sulfided and alkalinizeS molybdenum based catalysts have recently been claimed (14,15) mixed Mo(Co)K sulfides yield similar alcohol distributions under substoichiometric syngas and H S (0.05-0.1 vol %)y (16,17). ... 

Temperature Programmed Reaction. Examination of another redox system, propylene oxidation on M0O3, provides further insight. It is well accepted that propylene oxidation on molybdenum-based catalysts proceeds through formation of allylic intermediates. From isotopic studies it has been demonstrated that formation of the allylic intermediate is rate-determining (H/D effect), and that a symmetric allylic species is formed ( C labelling). 

An area, in which catalytic olefin metathesis could have a significant impact on future natural product directed work, would be the desymmetrization of achiral molecules through asymmetric RCM (ARCM) with chiral molybde-num-(for a chiral molybdenum-based catalyst available in situ from commercial components, see Refs 156 and 156a-156c) or ruthenium-based catalysts. 

Molybdenum-based catalysts are highly active initiators, however, monomers with functionalities with acid hydrogen, such as alcohols, acids, or thiols jeopardize the activity. In contrast, ruthenium-based systems exhibit a higher stability towards these functionalities (19). An example for a molybdenum-based catalyst is (20) MoOCl2(t-BuO)2, where t-BuO is the tert-butyl oxide radical. The complex can be prepared by reacting M0OCI4 with potassium tert-butoxide, i.e., the potassium salt of terf-butanol. 

The molybdenum-based catalyst MoOCl2(t-BuO)2 has been used to copolymerize norbornene and dicyclopentadiene (20). The polymeric product exhibits a single peak in gel permeation chromatography. 

In the same way, dipyridyl amide-functionalized supports suitable for the SPE of metal ions from aqueous solutions can be prepared. The resins are synthesized via the copolymerization of the functional monomer endo-norbornene-5-yl-N,N-di-2-pyridyl carboxylic amide with a molybdenum-based catalyst (42). Essentially no loss of performance was observed after extensive use over more than twenty cycles. After exposure to air for at least 2 months, a change in color from bright white to yellow was observed. However, this change in color did not influence the characteristic properties of the resins. 

Hydrogenation and hydrocracking activity of iron catalysts has been extensively investigated using coal and model compounds (93 -95). Iron catalysts can hydrogenerate olefinic unsaturated bonds, while they are known to be less active for the hydrogenation of aromatic rings compared with molybdenum-based catalysts. 

The promotional effect of first-row transition metal on the activity of molybdenum-based catalysts has also been correlated with a similar activity... 

The selectivity of the acrylonitrile formation with respect to ammonia is very low (<10%) for the molybdenum-based catalysts (mainly due to N2 formation) but very high (100%) for the Sn—Sb—(Fe) catalyst. This is in agreement with the results of the separate oxidation of ammonia, which only in the case of Sn—Sb—(Fe) demands a temperature above that of the propene ammoxidation. 

Selectivities and activities with molybdenum-based catalysts for toluene ammoxidation... 

Before World War II, the use of molybdenum based catalysts was popular, with molybdenum oxide (and sometimes sulphide) used for a number of isomerization processes.4 These types of catalyst were less sensitive to sulphur in the feed but were phased out owing to inferior selectivity.5... 

As already mentioned, with time the mid-molecule cleavage typical of a bifunctional catalyst decreases over the molybdenum based catalyst and the demethylation reaction becomes dominant. Demethylation also increases with increasing pressure. Amir-Ebrahimi and Rooney proposed that the metallacyclobutane isomerization mechanism should have a significant methanation and homologation contribution.34 Homologation products were not analysed in this study but have been observed in studies of the C4 and C5 reactions 35 however, methane was an important component of the cracking products over the molybdenum catalysts. 

Olefin metathesis is a unique reaction and is only possible by transition metal catalysis. In fact only complexes of Mo, W, Re, and Ru are known to catalyze olefin metathesis. Once it was known that metallocarbenes were the actual catalytic species, a variety of metal carbene complexes were prepared and evaluated as catalysts. Two types of catalysts have emerged as the most useful overall. The molybdenum-based catalysts developed by Schrock and ruthenium-based catalysts developed by Grubbs. 

As part of a program exploring functionalized bicyclic /3-lactams, the ring-closing metathesis of the /3-lactam diene 46 in the presence of a molybdenum-based catalyst (5 mol%) was shown to afford access to the fused 1,3-thiazepine 47 in 78% yield (Equation 7) <1997CC155>. 

It is naturally outside the scope of this article to cover the vast amount of literature dealing with hydrotreatment catalysts, even although an up to date review on this subject is not available. The latest reviews on molybdenum-based catalysts were written by Massoth in 1978,43 Grange,44 Furimsky 45 and Ratnasamy46 in 1980, but some articles partly covering this field can be usefully consulted 47-490,6... 

Chromium- and molybdenum-based catalysts yield predominantly 1,2-poly-butadiene. Several soluble chromium compounds such as Cr(Acac)3 give apparently hydrocarbon-soluble catalysts when activated with AIR3. Also, soluble catalysts are formed from soluble molybdenum compounds such as Mo(Acac)3 or Mo02(Acac)2 on reaction with AlEt3 in the presence of ethyl acetate or sulphides. ESR studies of Mo(OR)xCl4 x-based catalysts showed the Mo(III) species to be present in the catalysts. It should be mentioned, however, that catalysts based on group 6 metals have not been extensively studied [7],... 

Schrock, Gibson et al. [52d] found that styrene and 1,3-pentadiene could be used as chain transfer reagents for the living ring-opening olefin metathesis polymerization of norbornene with molybdenum based catalyst 35a. Renewed norbornene addition to a polymerization mixture containing initiator 35a and 30 equivalents of styrene resulted in the formation of polynorbomene with a low polydispersity and a molecular weight controlled by the number of norbornene equivalents in each of the individual monomer solutions, Eq. (38). This method allows a more efficient use of the catalyst. 

In 2005, Piers et al. prepared the 14-electron (14e) phosphonium alkyh-dene ruthenium complex 24. This catalyst displays higher activity in the RCM of diethyl diallylmalonate at 0 °C when compared to the second generation catalyst 3 (> 90% conversion after 2 h for 24 versus 25% conversion after 4 h for 3 and > 90% after 5 h for the Schrock molybdenum-based catalyst) (Eq. 27). RCM reactions of trisubstituted, six-membered ring, or seven-membered ring substrates are catalyzed at room temperature affording good... 

Hydrotreating catalysts are usually alumina supported molybdenum based catalysts with cobalt or nickel promotors. By 1990, the demand for hydrotreating catalysts is expected to reach 80,000,000 pounds annually (1). The increased demand for these catalysts and the limitations on the availability and supply of the active metals increase the urgency to develop effective catalyst regeneration techniques. 

The cyclization of 8c to 9c created a trisubsti-tuted double bond, thus preparing new ground for the application of RCM in complex systems. Actually the substrate failed to cyclize with the ruthenium-based Grubb s catalyst, but 20 mol% of a molybdenum-based catalyst described by Schrock led to the cyclized product 9c in 86 % yield (benzene, 55 "C), unfortunately again with a 1.0 1.0 ratio of Z to isomers [11]. 

Several procedures for this chemoselective oxidation utilize molybdenum-based catalysts, with either hydrogen peroxide or r-butyl hydroperoxide as the stoichiometric oxidant. These include ammonium molybdate in the presence of a ph e transfer reagent and hydrogen peroxide, which with pH control (potassium carbonate) will selectively oxidize a secondary alcohol in the presence of a primary alcohol without oxidizing alkenes. In addition hindered alcohols are oxidized in preference to less hindered ones (Scheme 18). 

Schrock and Hoveyda have reported the synthesis and activity of a number of chiral molybdenum-based catalysts for enantioselective olefin metathesis (for example, 97a, b) (Scheme 14) to date, the majority of successful... 

Scheme 14 Chiral molybdenum-based catalysts for asymmetric olefin metathesis...

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