Tungsten complexes oxidation catalysts

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

Epoxidation systems based on molybdenum and tungsten catalysts have been extensively studied for more than 40 years. The typical catalysts - MoVI-oxo or WVI-oxo species - do, however, behave rather differently, depending on whether anionic or neutral complexes are employed. Whereas the anionic catalysts, especially the use of tungstates under phase-transfer conditions, are able to activate aqueous hydrogen peroxide efficiently for the formation of epoxides, neutral molybdenum or tungsten complexes do react with hydrogen peroxide, but better selectivities are often achieved with organic hydroperoxides (e.g., TBHP) as terminal oxidants [44, 45],... 

Since the epoxidation step involves no formal change in the oxidation state of the metal catalyst, there is no reason why catalytic activity should be restricted to transition metal complexes. Compounds of nontransition elements which are Lewis acids should also be capable of catalyzing epoxidations. In fact, Se02, which is roughly as acidic as Mo03, catalyzes these reactions.433 It is, however, significantly less active than molybdenum, tungsten, and titanium catalysts. Similarly, boron compounds catalyze these reactions but they are much less effective than molybdenum catalysts 437,438 The low activity of other metal catalysts, such as Th(IV) and Zr(IV) (which are weak oxidants) is attributable to their weak Lewis acidity. 

Rouchaud and co-workers492 494 studied the liquid phase oxidation of propylene in the presence of insoluble silver, molybdenum, tungsten, and vanadium catalysts. Moderate yields of propylene oxide were obtained in the presence of molybdenum catalysts. These reactions almost certainly proceed via the initial formation of alkyl hydroperoxides, followed by epoxidation of the propylene by a Mo(VI)-hydroperoxide complex (see preceding section). 

It is worthwhile to comment on the catalytic species. As opposed to oxometal species, which convert amines to imines, hydroperoxymetal complexes (MOOH) convert amines to nitrones. Thus the oxidation of amines is a convenient way of distinguishing the active species. The reactivity of oxometal versus peroxometal species is illustrated in Fig. 22. In practice, tungsten is the catalyst of choice to convert amines to nitrones [130]. 

Bulk Mixed Oxide Catalysts. - Raman spectroscopy of bulk transition metal oxides encompasses a vast and well-established area of knowledge. Hie fundamental vibrational modes for many of the transitional metal oxide complexes have already been assigned and tabulated for systems in the solid and solution phases. Perhaps the most well-known and established of the metal oxides are the tungsten and molybdenum oxides because of their excellent Raman signals and applications in hydrotreating and oxidation catalysis. Examples of these two very important metal-oxide systems are presented below for bulk bismuth molybdate catalysts, in this section, and surface (two-dimensional) tungstate species in a later section. 

The same acidic chloroaluminate ionic liquids have been used as solvent for tungsten aryl oxide complexes for the metathesis of alkenes [24]. Slightly acidic chloroaluminates also dissolve the [Cl2W=NPh(PMe3)3] complex which catalyze ethene oligomerization without the addition of co-catalysts [25]. In a similar way, Ni-catalyzed 1-butene dimerization into linear octenes was carried out in acidic chloroaluminates buffered with small amount of weak bases [26]. Neutral chloroaluminates (l-ethyl-3-methylimidazolium chloride/AlCl3 = 1) were employed to immobilize ruthenium carbene complexes for biphasic ADMET (acyclic diene metathesis) polymerization of an acyclic diene ester [27]. 

The first-stage catalysts for the oxidation to methacrolein are based on complex mixed metal oxides of molybdenum, bismuth, and iron, often with the addition of cobalt, nickel, antimony, tungsten, and an alkaU metal. Process optimization continues to be in the form of incremental improvements in catalyst yield and lifetime. Typically, a dilute stream, 5—10% of isobutylene tert-huty alcohol) in steam (10%) and air, is passed over the catalyst at 300—420°C. Conversion is often nearly quantitative, with selectivities to methacrolein ranging from 85% to better than 95% (114—118). Often there is accompanying selectivity to methacrylic acid of an additional 2—5%. A patent by Mitsui Toatsu Chemicals reports selectivity to methacrolein of better than 97% at conversions of 98.7% for a yield of methacrolein of nearly 96% (119). 

This reaction is cataly2ed in solution by complexes of tungsten, molybdenum, or rhenium in high oxidation states, eg, Re ". Examples of active catalysts are... 

Liquid-Phase Epoxidation with Hydroperoxides. Molybdenum, vanadium, and tungsten have been proposed as Hquid-phase catalysts for the oxidation of the ethylene by hydroperoxides to ethylene oxide (205). tert- uty hydroperoxide is the preferred oxidant. The process is similar to the arsenic-catalyzed route, and iacludes the use of organometaUic complexes. 

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