Conversion cyclohexane

BASF. In the Badische process, cyclohexanone is produced by Hquid-phase catalytic air oxidation of cyclohexane to KA oil, which is a mixture of cyclohexanone and cyclohexanol, and is followed by vapor-phase catalytic dehydrogenation of the cyclohexanol in the mixture. Overall yields range from 75% at 10% cyclohexane conversion to 80% at 5% cyclohexane conversion. 

Interestingly, Tanaka (ref. 27) has reported the cooxidation of cyclohexane with acetaldehyde in the presence of a Co(OAc)2 catalyst in acetic acid at 90 °C (3). Adipic acid was obtained in 73% selectivity at 88% cyclohexane conversion. 

Sample Time (h) Si/M ratio Cyclohexane Conversion (%) TBHP Conversion (%) Mono- oxygenated selectivity (%) Ketone/ Alcohol ratio... 

Recently, Corma et al. have patented a process of oxidizing cycloalkane with molecular oxygen to produce cycloalkanol and/or cycloalkanone in the presence of hydrotalcite-intercalated heteropoly anion [Co MnCo (H20)039] (M = W or Mo), which comprised one cobalt as a central atom and another as a substitute of a W=0 fragment in the Keggin structure [98]. At 130 °C and 0.5 MPa, 64 and 24% selectivity to cyclohexanone and cyclohexanol, respectively, was achieved at cyclohexane conversion about 5%. This catalytic system could be of practical importance provided a true heterogeneous nature of catalysis and good catalyst recyclability had been proved. Unfortunately, this information was lacking in [98]. 

A reaction that appears to have potential for carbohydrate to substituted cyclohexane conversions, and has not been applied to carbohydrate-derived starting materials, involves the intramolecular bonding of the electrophilic carbon atoms of aldehydes and the nucleo-... 

It is clear that the cyclohexane conversion increases with an increasing sweep ratio y- that is, with increasing driving forces for mass transfer through the membrane. In addition, the introduction of a more diluted feed leads to an enhanced conversion. Proof of an effect of the realized product removal via the Vycor glass membrane was the fact that the achieved conversions exceeded the equilibrium conversions (shown as dotted fines in Fig. 12.10). 

Fig. 12.12. Calculated dependence of cyclohexane conversion (Eq. (37)) as a function of the Damkohler number (Eq. (41)) for a) the conventional fixed-bed reactor b) the diluted fixed-bed reactor and c) the membrane reactor with an optimized thickness ofVycor glass membrane (the dashed line corresponds to a hypothetically higher membrane selectivity, Sm). The range in which the membrane reactor experiments were performed is also indicated. Parameters T = 473 K, x r H =...

Similarly, cyclohexane is readily oxidized by cobalt(III) acetate in acetic acid at moderate temperatures.29Sa d In the absence of oxygen at 80°C the main products were 2-acetoxycyclohexanone and cyclohexyl acetate. Cyclohexane was about half as reactive as toluene under these conditions. Oxidation with Co(III) acetate in the presence of oxygen gave adipic acid as the main product. This reaction has been developed into a process for the single-stage oxidation of cyclohexane to adipic acid.296,297 Selectivities of approximately 75% have been claimed at roughly 80% cyclohexane conversion. 

Figure 4. The temperature dependences of cyclohexane conversion (curve 1) and benzene yield (curve 2) for the membrane catalyst, obtained by IR-pyrolysis of PAN and ammonium perrhenate, containing 5 mass % of Re.

Figure 4 shows the temperature dependences of cyclohexane conversion (curve 1) and benzene yield (curve 2). The maximal benzene productivity was 1.76 mol/m2h at 673 K with the catalyst containing 5 % Re. The usual Re/C catalysts require Re loading as much as 30% for achievement the similar activity at such operation conditions [8], This may be explained by the membrane form of catalyst, used in this work, in spite of the absence of absolute permselectivity of the membrane. 

In the cyclohexane oxidation route cyclohexane is oxidized with air at 125-126°C and 8-15 bar in the liquid phase using Co or Mn naphthanates as the catalyst. This affords a mixture of cyclohexanol and cyclohexanone via a classical free radical autoxidation mechanism. Cyclohexane conversion is limited to 10-12% in order to minimize by-product formation via further oxidation. The selectivity to cyclohexanol/cyclohexanone is 80-85%. 

Catalytic tests were run in a pulse reactor, at 400 °C, with a cyclohexane/ oxygen/ammonia feed composition in mol% of 3/6/4 (the balance being He). The main products obtained were adiponitrile (ADN) and benzene, with an overall selectivity of more than 90% (the cyclohexane conversion is not reported). The rates of benzene and ADN formation are plotted in Figure 20.10 as functions of the Sb20s content of catalysts. It is shown that the overall formation of benzene considerably decreased on increasing the amount of Sb in catalysts. The formation of ADN decreased, but the decrease was less pronounced than that of... 

Simon and Germain [128] investigated several catalytic systems and found that cyclohexane conversion is high (70%) even in the absence of catalyst at 460 °C (feed cyclohexane/oxygen/ammonia 1/3.6/1.5) yielding mainly cyclohexene, benzene and CO with minor amounts of other compounds. 

Fig. 11.13. Cyclohexane conversion vs. the (permeation/reaction rate) ratio. Curves 1 and 2 for mesoporous membranes with Knudsen separation factors. Curves 3 and 4 for microporous membranes with a separation factor of 100. Curves 5 and 6 for membranes permeable only to hydrogen. Odd (even) numbered curves correspond to an inert sweep gas rate of 1 (10) times the cyclohexane flow. The temperature is 477 K, Pfeed= 100 kPa. From Flarold et al. [130] with permission.

Figure 2. Product distribution for cyclohexane conversion to either benzene or hydrogenolysis (>90% n-hexane) products over (a) pure nickel on alumina and (b) the same catalyst after treatment with hexamethyldisilane in H2. Reaction conditions are discussed in references (6) and (8).

The addition of an alkyl nitrite (e.g., isoamyl nitrite) as a promoter in the aerial oxidation of cyclohexane in the presenceofaCo/Mncatalystled to an improvement in the conversion and in the selectivity to Ol/One and AA, compared to the unpromoted reaction [7nj. For instance, at 120 °C and 9 atm oxygen pressure, 9.7% cyclohexane conversion was obtained in 3 h reaction time, with selectivity to 01/Oneof65%andto AA of 35%. In the absence of the nitrite, the conversion was less than 4%. 

Many companies have studied the optimization of catalyst composition and process conditions in order to improve the performance of the reaction and the economics ofthe process. In the Gulf process, the reaction is carried out at 90-100 °C, with a Co(III) acetate catalyst and acetic acid as the solvent [17]. The molar selectivity is around 70-75%, for a cyclohexane conversion that can be as high as 80-85%. The high concentration of Co(III) acetate used also favors the direct reaction ofthe cation with cyclohexane, generating the cydohexyl radical. In fact, in Gulf patents the reaction is reported to occur in a critical amount of Co(III) (25-150 mmoles per mole of cyclohexane). The catalyst is activated during the initial induction period, and water is also added in the initial stage to enhance the selectivity to AA, but the rate of production decreases because the induction period increases. 

In Amoco patents [18b], the addition of controlled amounts of water after the initiation ofthe oxidation reaction is claimed to be a tool to obtain a better yield to AA. The best yield achieved was 88% (based on the identifiable compounds) at 98% cyclohexane conversion, with a Co(II) acetate catalyst, an acetic add solvent, at 95 °C and 70 atm air pressure. It is reported that water, if present during the induction period, depletes the concentration of free radicals in the absence of water, the yield was remarkably lower. These results are comparable to those attained by the air /nitric acid two-step oxidation of cydohexane. 

Copper or iron phthalocyanines encapsulated inX or Y zeolites [25g], which catalyze the oxidation of cyclohexane to Ol/One and to AA with oxygen (in the presence of small amounts of t-BuOOH) at near-ambient conditions. The catalyst remains in the solid phase throughout the reaction, and can be easily filtered off. Moreover, the solvent type affects performance best selectivity to AA (41%) is achieved with methanol [25g], at 12.7% cyclohexane conversion, with a halogen-substituted phthalocyanine of Fe encapsulated in an X zeolite. Cyclohexanone and cyclohex-andione are hypothesized to be the intermediate compounds of the reaction. Incorporation of the zeolite-encapsulated Fe phthalocyanine inside a polymer matrix can serve to enhance catalyst stability and limit leaching phenomena [25h[. 

In many cases, small amounts of AA are formed (selectivity less than 10%) together with Ol/One, especially at high cyclohexane conversion. 

The best result reported in the open literature is of 73% conversion with 73% selectivity to AA, obtained at normal O2 pressure, in acetic acid with 1 mol% Mn (acac)2, the by-products being glutaric acid (9%), succinic acid (6%), cyclohexyl acetate (2%) and cyclohexanol (1%) [30bj. The generation of the PINO from NHPI (Scheme 7.7) with oxygen is assisted by the Co(II) species therefore, the addition of a small amount of Co (O Ac) 2 enhances the oxidation process. In contrast, if the reaction is performed in an acetonitrile solvent, with a Co(OAc)2 catalyst at 75 °C, the main product is cyclohexanone (78% selectivity at 13% cyclohexane conversion). 

Co-catalyst TfC) Time (h) Cyclohexane conversion (%) AA selectivity (%) Cyclohexanone selectivity (%)... 

Other examples of nitroxyl radicals such as TEMPO [3Id, j] have been used successfully in several examples of environmentally friendly liquid-phase oxidations with. oxygen. Sheldon et al. have reported on the use of N-hydroxysaccharin as an alternative to NHPI in the oxidation of cycloalkanes to dicarboxylic acids [31h]. Other examples include the aerobial oxidation in the presence of NHPI, o-phenanthroline and bromine, in an acetonitrile/CCU solvent and in the absence of metals, at 100 °C. The selectivity was 75% to AA and 22% to cyclohexanone, at 48% cyclohexane conversion [31i]. 

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