Cycle 1-butene oxidation

The dehydrogenation of 2-butanol is conducted in a multitube vapor-phase reactor over a zinc oxide (20—23), copper (24—27), or brass (28) catalyst, at temperatures of 250—400°C, and pressures slightly above atmospheric. The reaction is endothermic and heat is suppHed from a heat-transfer fluid on the shell side of the reactor. A typical process flow sheet is shown in Figure 1 (29). Catalyst life is three to five years operating in three to six month cycles between oxidative reactivations (30). Catalyst life is impaired by exposure to water, butene oligomers, and di-j -butyl ether (27). 

The initial rapid decrease in activity (stage 1) is not caused by deactivation processes but is a result of the reduction of the catalyst under reaction conditions. The reduction of the catalyst follows from the reaction equations, which represent the catalytic cycle during 1-butene oxidation ... 

Cutlip and Kenney (44) have observed isothermal limit cycles in the oxidation of CO over 0.5% Pt/Al203 in a gradientless reactor only in the presence of added 1-butene. Without butene there were no oscillations although regions of multiple steady states exist. Dwyer (22) has followed the surface CO infrared adsorption band and found that it was in phase with the gas-phase concentration. Kurtanjek et al. (45) have studied hydrogen oxidation over Ni and have also taken the logical step of following the surface concentration. Contact potential difference was used to follow the oxidation state of the nickel surface. Under some conditions, oscillations were observed on the surface when none were detected in the gas phase. Recently, Sheintuch (46) has made additional studies of CO oxidation over Pt foil. 

FIGURE 1.30. Reductive cycle voltammetry of lrcm.y-2,3-dinitro-2-butene in 83-17 acetonitrile-water (+0.1 M Bu4PF6) at room temperature at 0.5 V/s and structural characteristics of the three oxidation states. Dotted curve, one-electron reversible wave centered at the same °. Adapted from Figure 4 in reference 73, with permission from Elsevier. 

The [Os3(CO)io( t-H)( t-OSi)]surface catalyzes the isomerization and hydrogenation of olefins. When the hydrogenation of ethylene is carried out at 90 °C the trinuclear framework of the initial cluster remains intact in all the proposed elementary steps of the catalytic cycle [133]. However, at higher reaction temperatures the stability of the [Os3(CO)io( t-H)( t-OSi)]sujface depends on the nature of the reactant molecule. It is moderately active in the isomerization of 1-butene at 115 °C but decomposes under reaction conditions to form surface oxidized osmium species that have a higher activity [134]. 

The in situ regeneration of Pd(II) from Pd(0) should not be counted as being an easy process, and the appropriate solvents, reaction conditions, and oxidants should be selected to carry out smooth catalytic reactions. In many cases, an efficient catalytic cycle is not easy to achieve, and stoichiometric reactions are tolerable only for the synthesis of rather expensive organic compounds in limited quantities. This is a serious limitation of synthetic applications of oxidation reactions involving Pd(II). However it should be pointed out that some Pd(II)-promoted reactions have been developed as commercial processes, in which supported Pd catalysts are used. For example, vinyl acetate, allyl acetate and 1,4-diacetoxy-2-butene are commercially produced by oxidative acetoxylation of ethylene, propylene and butadiene in gas or liquid phases using Pd supported on silica. It is likely that Pd(OAc)2 is generated on the surface of the catalyst by the oxidation of Pd with AcOH and 02, and reacts with alkenes. 

The basic mechanism of hydrogenation is shown by the catalytic cycle in Fig. 7.3. This cycle is simplified, and some reactions are not shown. Intermediate 7.9 is a 14-electron complex (see Section 2.1). Phosphine dissociation of Wilkinson s complex leads to its formation. Conversion of 7.9 to 7.10 is a simple oxidative addition of H2 to the former. Coordination by the alkene, for example, 1-butene, generates 7.11. Subsequent insertion of the alkene into the metal-hydrogen bond gives the metal alkyl species 7.12. The latter undergoes reductive elimination of butane and regenerates 7.9. 

Based on Fig. 8.2, suggest tentative catalytic cycles for the oxidation of (a) ethylene to vinylacetate (VA) and (b) 2-butene to methyl ethyl ketone (MEK). 

The catalytic cycle presumably involves the oxidative addition with coupling of two molecules of ethylene to a Ti" species, generating a titanacyclopentane. A 1,3-hydrogen shift across the ring generates 1-butene. Because it does not involve a hydride intermediate that might catalyze a double-bond shift, 1 -butene is stable... 

Consider the reaction of 2,3-dimethyl-2-butene with H2 catalyzed by Pd/C. Two new C-H bonds are made, and a C=C 7r bond breaks. The fact that the addition is stereospecifically syn suggests that an insertion reaction is occurring. The Pd metal is in the (0) oxidation state (i.e., d10), so it can react with H2 by an oxidative addition to give two Pd-H bonds. At this point Pd is in the (II) oxidation state. Coordination and insertion of the alkene into one of the Pd-H bonds gives Pd-C and C-H bonds. Finally, reductive elimination gives the product and regenerates Pd°, which begins the catalytic cycle anew. 

As the next example of soft catalysis, we shall discuss the dimerization of ethylene to 1-butene, which is catalyzed by rhodium complexes in a redox cycle (Scheme 2-6). The active Rh catalyst A undergoes oxidative addition of HCl and insertion of ethylene into the Rh-H bond to give the Rh alkyl complex B. The following ethylene insertion reaction is the rate-determining step and is favored by the medium-hard Rh center. The resulting Rh butyl complex C has a hard-soft dis-... 

Oxi dationT Subsequent rate measurements were made for a mixture of 2% CO, 3% O2 and 1% 1-butene in argon. As this mixture was introduced to the reactor at temperatures above the previously determined multiple steady state region, very dramatic concentration oscillations were obtained as shown in, Figure 3 for a temperature of 150°C and a feed rate of 100 cmv min at NTP. This limit cycle was characterized by a periodic rapid rate of oxidation of CO and 1-butene with a period of 31 minutes yielding minima in the reactant concentrations and a corresponding maximum in product CO concentration. The spontaneous oscillations once established were quite reproducible with only minor variations attributed to flow rate and temperature fluctuations. Gas phase temperature variations within the reactor were not detected. 

About 65% butene conversion is achieved at more than 95% selectivity." Phil-hps also developed the 0-X-D process, which operated under similar conditions. Both processes use fixed beds of catalyst. Oxidative dehydrogenation with metal oxide catalysts involves a redox mechanism and the catalyst used should not be reduced irreversibly dining the cycle. 

---