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Scheme 2. pressure drop and separation step is not needed (a green advantage). 2.8.1. Oxidations in CO2: experimental results Clearly, the oxidation of cyclohexane (first to cyclo- hexanone/cyclohexanol, subsequently to adipic acid) is one of the more commercially important oxidations performed industrially (Scheme 2) [13]. Cyclohexane is oxidized in the liquid phase using air (at temperatures of 395–435 K and pressures in the 10–20 bar range) to a mixture of cyclohexanone and cyclohexanol. Magnesium or cobalt salts are employed to catalyze the reaction. Srinivas and Mukhopadhyay [113] examined the oxidation of cyclohexane in CO2 with oxygen at temperatures between 430 and 470 K and pressures up to ≈200 bar. Interestingly, a catalyst is not mentioned by the authors, despite the fact that one is employed industrially. The authors found that the condition of the feed (one-phase, two-phase, prox- imity to a phase boundary) exhibited a strong effect on the product profile and the rate of product forma- tion. Not surprisingly, given the discussion above, the highest rates (for both cyclohexane and cyclohexanol formation) were observed in the single phase system where CO2 was the minor component; i.e. CO2 was employed to homogenize the mixture of cyclohexane and oxygen, leading to high concentrations of each reactant and hence high rates. Another oxidation process of great import industri- ally is the formation of epoxides from alkenes. Most important is probably the generation of propylene ox- ide from propylene. Currently, propylene oxide is pro- duced via one of three processes (primarily). First, chlorohydrin (from chlorine and propylene) can be re- acted with base to generate propylene oxide and salt (Scheme 3); a very large volume of wash water is re- quired to work up the product. Scheme 3. Scheme 4. E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 149 One can also produce propylene oxide via a co- product process where an intermediate is peroxidized with oxygen, and the oxygen transferred to propy- lene, creating propylene oxide and a byproduct alco- hol (which is then transformed to a co-product) [13]. The most widely used co-product processes for PO production also create styrene or methyl tertiary butyl ether (Scheme 4). There is significant interest in designing a process which only produces PO from propylene and oxygen, as MTBE is now environmentally suspect and the de- mand for styrene tends to fluctuate while that for PO remains consistently strong. As such, propylene oxide production is more energy intensive and wasteful than desired because a co-product must currently be pro- duced along with PO. Consequently, Baiker et al. [114] investigated the oxidation of propylene with an oxy- gen/hydrogen mixture using a Pt/Pd on TS-1 (titanium silicate) catalyst in a two-phase system (methanol was employed as the primary solvent). The reaction pro- ceeds via formation of hydrogen peroxide from H2 and O2 over the Pd, followed by oxidation of propy- lene to PO. Both nitrogen and CO2 were employed as solvents for the H2/O2 mixture. Baiker found that the yield of PO increased markedly upon switching from nitrogen to CO2 in the upper phase of the mixture and that increasing pressure enhanced the yield still fur- ther. As in previous cases, these results may derive from the simple fact that use of CO2 as the solvent for the reactant gases allows for greatly enhanced concen- trations of these gases in the lower (or liquid) phase, enhancing rates. Eckert et al. as well as Beckman et al. have inves- tigated an interesting route to alkylene oxides [115]. As shown originally by Richardson et al. [116], hy- drogen peroxide will react with a bicarbonate salt un- der basic conditions to form the percarbonate ion, which will then react with alkenes to form the epox- ide. This reaction is an analogy to epoxidation usingPDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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