1- Phenylethanol oxidation

The correlation between the coverage of surface platinum atoms by bismuth adatoms (Ggi) and the measured rate of 1-phenylethanol oxidation was studied on unsupported platinum catalysts. An electrochemical method (cyclic voltammetry) was applied to determine G i and a good electric conductivity of the sample was necessary for the measurements. The usual chemisorption measurements have the disadvantage of possible surface restructuring of the bimetallic system at the pretreatment temperature. Another advantage of the electrochemical polarization method is that the same aqueous alkaline solution may be applied for the study of the surface structure of the catalyst and for the liquid phase oxidation of the alcohol substrate. 

Bi does not adsorb hydrogen, thus a Bi/Pt coverage can be calculated from the hydrogen chemisorption data. It is seen in Figure 2 that there is an excellent correlation between the Bi-coverage of Pt and the rate of 1-phenylethanol oxidation. It seems that the hydrogen chemisorption ability of Pt or the size of active sites ensembles has to be minimized to avoid deactivation. There are indications in the literature that the suppression of hydrogen sorption on a Pt electrode can eliminate the poison formation (20). 

Figure 2. Bi coverage of unsupported Pt and the rate of 1-phenylethanol oxidation as a function of the overall Bi tj ratio.

Propylene oxide is a colorless, low hoiling (34.2°C) liquid. Table 1 lists general physical properties Table 2 provides equations for temperature variation on some thermodynamic functions. Vapor—liquid equilibrium data for binary mixtures of propylene oxide and other chemicals of commercial importance ate available. References for binary mixtures include 1,2-propanediol (14), water (7,8,15), 1,2-dichloropropane [78-87-5] (16), 2-propanol [67-63-0] (17), 2-methyl-2-pentene [625-27-4] (18), methyl formate [107-31-3] (19), acetaldehyde [75-07-0] (17), methanol [67-56-1] (20), ptopanal [123-38-6] (16), 1-phenylethanol [60-12-8] (21), and / /f-butanol [75-65-0] (22,23). 

The hydroperoxide process involves oxidation of propjiene (qv) to propylene oxide by an organic hydroperoxide. An alcohol is produced as a coproduct. Two different hydroperoxides are used commercially that result in / fZ-butanol or 1-phenylethanol as the coproduct. The / fZ-butanol (TBA) has been used as a gasoline additive, dehydrated to isobutjiene, and used as feedstock to produce methyl tert-huty ether (MTBE), a gasoline additive. The 1-phenyl ethanol is dehydrated to styrene. ARCO Chemical has plants producing the TBA coproduct in the United States, Erance, and the Netherlands. Texaco has a TBA coproduct plant in the United States. Styrene coproduct plants are operated by ARCO Chemical in the United States and Japan, Shell in the Netherlands, Repsol in Spain, and Yukong in South Korea. 

Ethylbenzene Hydroperoxide Process. Figure 4 shows the process flow sheet for production of propylene oxide and styrene via the use of ethylbenzene hydroperoxide (EBHP). Liquid-phase oxidation of ethylbenzene with air or oxygen occurs at 206—275 kPa (30—40 psia) and 140—150°C, and 2—2.5 h are required for a 10—15% conversion to the hydroperoxide. Recycle of an inert gas, such as nitrogen, is used to control reactor temperature. Impurities ia the ethylbenzene, such as water, are controlled to minimize decomposition of the hydroperoxide product and are sometimes added to enhance product formation. Selectivity to by-products include 8—10% acetophenone, 5—7% 1-phenylethanol, and <1% organic acids. EBHP is concentrated to 30—35% by distillation. The overhead ethylbenzene is recycled back to the oxidation reactor (170—172). 

After epoxidation, propylene oxide, excess propylene, and propane are distilled overhead. Propane is purged from the process propylene is recycled to the epoxidation reactor. The bottoms Hquid is treated with a base, such as sodium hydroxide, to neutralize the acids. Acids in this stream cause dehydration of the 1-phenylethanol to styrene. The styrene readily polymerizes under these conditions (177—179). Neutralization, along with water washing, allows phase separation such that the salts and molybdenum catalyst remain in the aqueous phase (179). Dissolved organics in the aqueous phase ate further recovered by treatment with sulfuric acid and phase separation. The organic phase is then distilled to recover 1-phenylethanol overhead. The heavy bottoms are burned for fuel (180,181). 

PO—SM Coproduction. The copioduction of propylene oxide and styrene (40—49) includes three reaction steps (/) oxidation of ethylbenzene to ethylbenzene hydroperoxide, (2) epoxidation of ethylbenzene hydroperoxide with propylene to form a-phenylethanol and propylene oxide, and (3) dehydration of a-phenylethanol to styrene. 

The oxidation step is similar to the oxidation of cumene to cumene hydroperoxide that was developed earlier and is widely used in the production of phenol and acetone. It is carried out with air bubbling through the Hquid reaction mixture in a series of reactors with decreasing temperatures from 150 to 130°C, approximately. The epoxidation of ethylbenzene hydroperoxide to a-phenylethanol and propylene oxide is the key development in the process. 

It is carried out in the Hquid phase at 100—130°C and catalyzed by a soluble molybdenum naphthenate catalyst, also in a series of reactors with interreactor coolers. The dehydration of a-phenylethanol to styrene takes place over an acidic catalyst at about 225°C. A commercial plant (50,51) was commissioned in Spain in 1973 by Halcon International in a joint venture with Enpetrol based on these reactions, in a process that became known as the Oxirane process, owned by Oxirane Corporation, a joint venture of ARCO and Halcon International. Oxirane Corporation merged into ARCO in 1980 and this process is now generally known as the ARCO process. It is used by ARCO at its Channelview, Texas, plant and in Japan and Korea in joint ventures with local companies. A similar process was developed by Shell (52—55) and commercialized in 1979 at its Moerdijk plant in the Netherlands. The Shell process uses a heterogeneous catalyst of titanium oxide on siHca support in the epoxidation step. Another plant by Shell is under constmction in Singapore (ca 1996). 

A three-step process involving the oxidation of acetophenone, hydrogenation of the ketone to a-phenylethanol, and dehydration of the alcohol to styrene was practiced commercially by Union Carbide (59) until the early 1960s. Other technologies considered during the infancy of the styrene industry include side-chain chlorination of ethylbenzene followed by dehydrochlotination or followed by hydrolysis and dehydration. 

Styrene oxide [96-09-3] M 120.2, b 84-86 /16.5mm, d 1.053, n 1.535. Fractional distn at reduced pressure does not remove phenylacetaldehyde. If this material is present, the styrene oxide is treated with hydrogen under 3 atmospheres pressure in the presence of platinum oxide. The aldehyde, but not the oxide, is reduced to 6-phenylethanol) and separation is now readily achieved by fractional distn. [Schenck and Kaizermen J Am Chem Soc 75 1636 1953.]... 

Hydrogenation of styrene oxide over palladium in methanol 66 gives exclusively 2-phenylethanol, but in buffered alkaline methanol the product is l-phenylelhanol. If alcoholysis of the epoxide by the product is troublesome, the problem can be eliminated by portion-wise addition of the epoxide to the reaction, so as always to maintain a high catalyst-to-substrate ratio. The technique is general for reactions in which the product can attack the starting material in competition with the hydrogenation. 

Sheldon et al. have combined a KR catalyzed by CALB with a racemization catalyzed by a Ru(II) complex in combination with TEMPO (2,2,6,6-tetramethylpi-peridine 1-oxyl free radical) [28]. They proposed that racemization involved initial ruthenium-catalyzed oxidation of the alcohol to the corresponding ketone, with TEMPO acting as a stoichiometric oxidant. The ketone was then reduced to racemic alcohol by ruthenium hydrides, which were proposed to be formed under the reaction conditions. Under these conditions, they obtained 76% yield of enantiopure 1-phenylethanol acetate at 70° after 48 hours. 

The above-described reverse reaction (viz. the Fe-catalyzed dehydrogenation of alcohols to ketones/aldehydes) has been reported by Williams in 2009 (Table 9) [58]. In this reaction, the bicyclic complex 16 shows a sluggish activity, whereas the dehydrogenation of l-(4-methoxyphenyl)ethanol catalyzed by the phenylated complex 17 affords the corresponding ketone in 79% yield when 1 equiv. (relative to 17) of D2O as an additive was used. For this oxidation reaction, l-(4-methoxyphenyl) ethanol is more suitable than 1-phenylethanol and the reaction rate and the yield of product are higher. 

A mononuclear diastereopure high-spin Fe alkylperoxo complex with a pen-tadentate N,N,N,0,0-ligand 33 (Scheme 17) was reported by Klein Gebbink and coworkers [109, 110]. The complex is characterized by unusual seven-coordinate geometry. However, in the oxidation of ethylbenzene the iron complex with 33 and TBHP yielded with large excess of substrate only low TON s (4) and low ee (6.5%) of 1-phenylethanol. 

The oxidation of a series of meta- and para-substituted a-phenylethanols shows that electron-donating substituents facilitate reaction (p = —1.01) . A similar study of primary aliphatic alcohols confirmed this trend p = —1.06 + 0.06). 

The presence of radical-stabilising groups can lead to C-C fission, for example in the V(V) oxidations of 2-phenylethanol and a-/ert-butylbenzyl alcohol, ... 

Hartmans S, JP Smits, MJ van der Werf, F Volkering, JAM de Bont (1989) Metabolism of styrene oxide and 2-phenylethanol in the styrene-degrading Xanthobacter strain 124X. Appl Environ Microbiol 55 2850-2855. 

Control experiments do not provide evidence for oxidation of the secondary alcohol groups in the glycoside or for degradation of the ligand backbone. A similar regioselectivity was also observed in a benzyl alcohol/1-phenylethanol model system that showed no proof for the oxidation of the secondary alcohol by formation of acetophenone (18, 23,26). 

Figure 6.16. Aerobic oxidation of 1-phenylethanol catalysed by palladium complexes of a fluorous pyridine...

The complex [Rh(COD)L L2]+, where L1 = PPh3 and L2 = pyridine, and a neutral benzoate complex, Rh(COD)(PPh3)(OCOPh), also effect highly selective hydrogenation of 1-alkynes to 1-alkenes as well as reduction of 1-alkenes and ketones to alcohols (139) the one equivalent of base required may be related to monohydride formation [Eq. (25)]. The bisphosphine complexes also catalyze reduction of styrene oxide to 2-phenylethanol and phenylacetaldehyde (140) ... 

Use of ferrocenylmonophosphine (fU-(A)-PPFA 5a for the same reaction improved the enantioselectivity.24,25,26 Here, the hydrosilylation product was oxidized into ( y)-l-phenylethanol 3 with 52% ee (entry 3). The ferrocenylmonophosphine 6 supported on Merrifield polystyrene resin has been also used for the hydrosilylation of styrene, though the enantioselectivity was lower (15% ee) (entry 4).27 Several chiral (/ -/V-sulfonylaminoalkyl)phosphines 7 were prepared from (A)-valinol and used for the asymmetric hydrosilylation of styrene.28 For styrene, phosphine 7a which contains methanesulfonyl group was most effective giving (asymmetric hydrosilylation (entries 6-9).29,29a... 

Oxiranes undergo ring-opening with cerium(VI) ammonium nitrate and an excess of a quaternary ammonium halide to yield haloethanols [34], The reaction occurs with high regio- and stereo-selectivity, for example, / (+)-styrene oxide produces S(+)-2-chloro-2-phenylethanol in 85% yield with 96% ee. 

K(,SiW 02 )Mn I I) Manganese ion-substituted silicon polyoxotungstate can be used as a mediator for alcohol oxidation. Constant potential electrolysis of 1-phenylethanol at 1.25 V in the presence of 5 mol% of the catalyst gave acetophenone in 61% yield [33]. 

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