Propylene allylic oxidation

With higher alkenes, three kinds of products, namely alkenyl acetates, allylic acetates and dioxygenated products are obtained[142]. The reaction of propylene gives two propenyl acetates (119 and 120) and allyl acetate (121) by the nucleophilic substitution and allylic oxidation. The chemoselective formation of allyl acetate takes place by the gas-phase reaction with the supported Pd(II) and Cu(II) catalyst. Allyl acetate (121) is produced commercially by this method[143]. Methallyl acetate (122) and 2-methylene-1,3-diacetoxypropane (123) are obtained in good yields by the gas-phase oxidation of isobutylene with the supported Pd catalyst[144]. 

Reaction of the environment with the starting material The commonest example of this type of interaction is the protonation of the substrate by acids in the electrolysis medium, but pH effects will be dealt with in a later section. There are, however, other chemical interactions which can occur. For example, the mechanism and products of the oxidation of olefins are changed by the addition of mercuric ion to the electrolysis medium. In its absence, propylene is oxidized to the allyl cation (Clark et al., 1972),... 

Heterogeneous palladium catalysts proved to be active in the conversion of simple alkenes to the corresponding allylic acetates, carbonyl compounds, and carboxylic acids.694 704 Allyl acetate or acrylic acid from propylene was selectively produced on a palladium on charcoal catalyst depending on catalyst pretreatment and reaction conditions.694 Allylic oxidation with singlet oxygen to yield allylic hydroperoxides is discussed in Section 9.2.2. 

During the history of a half century from the first discovery of the reaction (/) and 35 years after the industrialization (2-4), these catalytic reactions, so-called allylic oxidations of lower olefins (Table I), have been improved year by year. Drastic changes have been introduced to the catalyst composition and preparation as well as to the reaction process. As a result, the total yield of acrylic acid from propylene reaches more than 90% under industrial conditions and the single pass yield of acrylonitrile also exceeds 80% in the commercial plants. The practical catalysts employed in the commercial plants consist of complicated multicomponent metal oxide systems including bismuth molybdate or iron antimonate as the main component. These modern catalyst systems show much higher activity and selectivity... 

Investigations into the scheelite-type catalyst gave much valuable information on the reaction mechanisms of the allylic oxidations of olefin and catalyst design. However, in spite of their high specific activity and selectivity, catalyst systems with scheelite structure have disappeared from the commercial plants for the oxidation and ammoxidation of propylene. This may be attributable to their moderate catalytic activity owing to lower specific surface area compared to the multicomponent bismuth molybdate catalyst having multiphase structure. 

To test this hypothesis, propylene was oxidized on a deuterated catalyst surface. The ir-allyl route would yield acrolein containing no deuter-... 

Under the conditions of stoichiometric (eq. (4)) or catalytic (Scheme 2) reactions, propylene is oxidized to isopropenyl acetate as the main reaction product, along with allyl and cis- and trans -n-propenyl acetates. Higher acyclic alkenes C4-C10 are converted to mixtures of allyl and vinyl esters [5]. Cyclic alkenes also produce homoallylic esters [6, 7]. 

In allylic oxidation, an olefin (usually propylene) is activated by the abstraction of a hydrogen a to the double bond to produce an allylic intermediate in the rate-determining step (Scheme 1). This intermediate can be intercepted by catalyst lattice oxygen to form acrolein or acrylic acid, lattice oxygen in the presence of ammonia to form acrylonitrile, HX to form an allyl-substituted olefin, or it can dimerize to form 1,5-hexadiene. If an olefin containing a jS-hydrogen is used, loss of H from the allylic intermediate occurs faster than O insertion, to form a diene with the same number of carbons. For example, butadiene is fonned from butene. 

The most industrially significant and well studied allylic oxidations center around the formation of acrylonitrile from ammoxidation (Eq. 7), and acrolein from oxidation (Eq. 8) of propylene ... 

The rate of conversion of propane is practically the same in the presence and in the absence of ammonia. The oxidation yields propylene and carbon oxides, which are the prevailing products. However, when ammonia is added to the feedstock, the yield to propylene remains unchanged, while the yield to carbon oxides is remarkably decreased in favour of the formation of acrylonitrile. This suggests that in the absence of ammonia the propylene is oxidized to a compound or to an intermediate which under these conditions is burnt to carbon oxides. The addition of ammonia allows this intermediate compound (which might be acrolein or an allyl radical species) to be quickly transformed to the stable... 

The data in Table 1 summarize catalytic activities for epoxidation of a variety of olefins over an unpromoted 5%Ag/Al203 catalyst. These data illustrate the preferential reactivity at the allylic position relative to addition of oxygen across the C=C bond. While the selectivity to ethylene oxide is typical for an unpromoted catalyst, the selectivities to propylene oxide and butylene oxides are non-existent for propylene, 1-butene, and 2-butene, respectively. In addition to small amounts of the selective allylic oxidation products (acrolein in the case of propylene and butadiene in the case of 1-butene), the only products are those of combustion. However, the results for butadiene reveal it is possible to epoxidize this non-allylic olefin at moderate selectivity and activity. What is not obvious from Table 1 is the short-lived nature of this activity. After 2-3 hours of reaction time, activity and selectivity typically decreased to approximately <1% conversion of C4H6 and approximately 50-75% selectivity to epoxybutene. A typical chromatogram of the activity of an... 

The mechanism of allylic oxidation of propylene has been investigated... 

Scheme 5.10. A proposed mechanistic pathway for the allylic oxidation of propylene over bismuth molybdate catalyst.

Vapor-phase aerobic oxidations of lower olefins, e. g. propylene to acrolein or acrylic acid and isobutene to methacrolein or methacrylic acid, are well-established bulk chemical processes [1,2], They are usually performed over oxidic catalysts, such as bismuth molybdate or heteropoly compounds, although the scope of these allylic oxidations is limited to olefins that cannot form 1,3-dienes via oxidative dehydrogenation. Thus 1- and 2-butene are converted to butadiene, and methylbutenes to isoprene, and with higher olefins complex mixtures result from further oxidation. Hence, such methodologies are not relevant in the context of fine chemicals. 

Moiseev and coworkers showed [10,13] that giant palladium clusters with an idealized formula Pd56iL5o(OAc)igo (L = phenanthroline or bipyridine) are highly active catalysts for allylic oxidation of olefins. The catalytically active solution was prepared by reduction of Pd(OAc)2, e. g. with H2, in the presence of the ligand, L, followed by oxidation with O2. The giant palladium cluster catalyzed the oxidation of propylene to allyl acetate under mild conditions. Even in 10% aqueous acetic acid, allyl acetate selectivity was 95-98 % [10]. Oxidation catalyzed by Pd-561 in water afforded a mixture of allylic alcohol (14%), acrolein (2%), and acrylic acid (60%), and only 5% acetone [10]. 

The most industrially significant and well-studied allylic oxidation reaction is the ammoxidation of propylene ( eq. 8 ) which accounts for virtually all of the 8 billion pounds of acrylonitrile produced annually world-wide. The related oxidation reaction produces acrolein ( eq. 9 ), another important monomer. Although ammoxidation requires high temperatures, the catalysts are, in general the same fof both processes and include bismuth molybdates, uranium antimonates (USb30j Q), iron antimonates, and bismuth molybdate based multicomponent systems. The latter category includes many of todays highly selective and active commercial catalyst systems. 

Surface Reaction Mechanism. The mechanism of catalytic alkene ammoxidation is invariably linked to allylic oxidation chemistry. Allylic oxidation is the selective oxidation of an alkene at the allylic carbon position. Selective allylic oxidation and ammoxidation proceed by abstraction of the hydrogen from the carbon positioned a to the carbon-carbon double bond. This produces an allylic intermediate in the rate-determining step. In the case where propylene is the hydrocarbon, the reaction is as follows ... 

When propylene was used for a fuel instead of ethylene, partial oxidation of propylene to acrolein was performed with a 96 % selectivity under short circuit conditions. Acrolein is a Jt-allyl oxidation product at a Pd° catalyst and is not a Wacker oxidation product at a Pd catalyst. When the oxidation rate of propylene was accelerated by an apphed voltage, acetone was produced with 90 % selectivity [5]. The anode potentials in operation were lower than a redox potential of Pd (+0.74 V (Ag/AgCl)) under short circuit conditions. On the other hand, the potentials were higher than the redox potential under applying voltage conditions [6, 7]. The oxidation state of Pd at the anode was Pd° under the former conditions and was Pd " under the latter conditions. The product selectivities to acrolein and acetone were able to control in the propylene oxidation by tuning anode potentials in operation. 

Direct oxidation of propylene with air or pure oxygen (equivalent to ethylene oxide manufacturing) is not efficient, since the silver catalysts used in the direct ethylene oxidation are not suitable for the reaction of alkenes with allylic hydrogen atoms (like propylene). Direct oxidation of propylene results mainly in acrolein formation and total oxidation. Some 3% of the world capacity of PO is produced by very recently developed processes, for example, hydroperoxidation of cumene and propylene and catalytic epoxidation of propylene using H2O2. 

Valuable products are produced from the oxidation of both ethylene and propylene (Figs. 1 and 2). Ethylene is epoxidized with oxygen in the vapor phase over a silver catalyst, and propylene is epoxidized with an alkyl hydroperoxide in the liquid phase using a molybdeniim catalyst system. Vinylic oxidation products or their stable isomers, including acetaldehyde, acetone, and vinyl acetate, have been manufactured by a series of related catalytic reactions. These reactions occur either in solutions of palladium complexes or on the surfaces of supported palladium catalysts. Bismuth molybdate is an effective catalyst for allylic oxidations of propylene, which are of paramount importance to the chemical industry. Propylene is oxidized in the vapor phase to give acrolein for acrylic acid manufacture or, in the presence of ammonia, to give acrylonitrile. Second- and third-generation catalysts,... 

Allylic Oxidation. The Wacker reaction and related palladium-catalyzed oxidations which proceed via nucleophilic attack on coordinated alkene have been widely practiced in industry to produce acetaldehyde, acetone, and vinyl acetate. An alternative pathway is available to alkenes in the coordination sphere of palladium(ll) complexes, which could lead to another important family of oxidation products. Insertion into the allylic C-H bond of 1-alkenes gives TT-allyl complexes which, on attack by external nucleophiles, would produce a family of allylic oxidation products including a,fi-unsaturated alcohols, carbonyl compounds, and carboxylic acids. Electron- withdrawing anionic ligands such as trifluoroacetate enhance the ability of the palladium center to insert into C-H bonds in this manner [26] (Fig. lA). Catalytic conversion of propylene to allyl acetate has been achieved in high selectivity in the presence of catalytic quantities of palladium(ll) trifluoroacetate [27]. 

An example of this t3T)e of reaction which does not produce a byproduct is the production of allyl alcohol from propylene oxide ... 

Propylene oxide-based glycerol can be produced by rearrangement of propylene oxide [75-56-9] (qv) to allyl alcohol over triUthium phosphate catalyst at 200—250°C (yield 80—85%) (4), followed by any of the appropriate steps shown in Figure 1. The specific route commercially employed is peracetic acid epoxidation of allyl alcohol to glycidol followed by hydrolysis to glycerol (5). The newest international synthesis plants employ this basic scheme. 

Allyl Glycidyl Ether. This ether is used mainly as a raw material for silane coupling agents and epichlorohydrin mbber. Epichlorohydrin mbber is synthesized by polymerizing the epoxy group of epichlorohydrin, ethylene oxide, propylene oxide, and aHyl glycidyl ether, AGE, with an aluminum alkyl catalyst (36). This mbber has high cold-resistance. 

Crystallinity is low the pendent allyl group contributes to the amorphous state of these polymers. Propylene oxide homopolymer itself has not been developed commercially because it cannot be cross-baked by current methods (18). The copolymerization of PO with unsaturated epoxide monomers gives vulcanizable products (19,20). In ECH—PO—AGE, poly(ptopylene oxide- o-epichlorohydrin- o-abyl glycidyl ether) [25213-15-4] (5), and PO—AGE, poly(propylene oxide-i o-abyl glycidyl ether) [25104-27-2] (6), the molar composition of PO ranges from approximately 65 to 90%. 

---