Processes cyclohexane

Nitric acid is used for nitrating numerous other compounds to produce nitrates. Nitric acid is used to produce adipic acid (C6H4O10), which is used in the production of nylon (see Nylon). In this process, cyclohexane is oxidized to a cyclohexanol-cyclohexanone mixture. Cyclohexanol and cyclohexanone are then oxidized with nitric acid to adipic acid. 

In the Dupont process, cyclohexane is reacted with air at 150 °C and 10 atm pressure in the presence of a soluble cobalt(II) salt (naphthenate or stearate). The conversion is limited to 8-10% in order to prevent consecutive oxidation of the ol-one mixture. Nonconverted cyclohexane is recycled to the oxidation reactor. Combined yields of ol-one mixture are 70-80%.83,84,555 The ol-one mixture is sent to another oxidation reactor where oxidation by nitric acid is performed at 70-80 °C by nitric acid (45-50%) in the presence of a mixture of Cu(N03)2 and NH4V03 catalysts, which increase the selectivity of the reaction. The reaction is complete in a few minutes and adipic acid precipitates from the reaction medium. The adipic acid yield is about 90%. Nitric acid oxidation produces gaseous products, mainly nitric oxides, which are recycled to a nitric acid synthesis unit. Some nitric acid is lost to products such as N2 and N20 which are not recovered. 

In the Dupont process, cyclohexane is reacted with air at 150 °C and 10 atm pressure in the presence of a soluble cobalt(ll) salt (naphthenate or stearate). The conversion is limited to 8-10% in order to prevent consecutive oxidation of the ol-one mixture. Nonconverted cyclohexane is... 

In another example of an industrial process, cyclohexane photonitrosylation, leads to a nitrosocyclohexane, which is converted to caprolactam, a precursor to nylon-6 polymerization (Scheme 6.226).155... 

The mechanism of benzene hydrogenation has been studied with the aid of [14-C]labelled hydrocarbons.The reverse process, cyclohexane dehydrogenation, has been the subject of twelve papers by Tetenyi et al., published between 1961 and 1974. The results of these have been summarized in a review by Tetenyi.In both systems the point of interest was whether the intermediates cyclohexene and cyclohexadiene were formed during the reaction. From experiments in which a 1 1 mixture of labelled [14-C]cyclohexane and inactive cyclohexene was allowed to react in the presence of Ni, Pt, and Rh catalysts, it was deduced that [14-C]-cyclohexene was produced from cyclohexane. Thus, a stepwise mechanism was proposed, involving both cyclohexene and cyclohexadiene as intermediates. 

Cydohexane is the most important cyclic alkane in industrial organic chemistry and plays a major role in the industrial production of important monomers, such as, for example, adipinic add, adipodinitrile, hexamethylenediamine, hexamethy-lene diisocyanate, and Eproduction processes cyclohexane is oxidized in the first step in a liquid-phase reaction to a mixture of cydohexanol and cyclohexanone. Details about this process are given in Sedion 5.3.3.1. 

Benzene was first isolated by Faraday in 1825 from the liquid condensed by compressing oil gas. It is the lightest fraction obtained from the distillation of the coal-tar hydrocarbons, but most benzene is now manufactured from suitable petroleum fractions by dehydrogenation (54%) and dealkylation processes. Its principal industrial use is as a starting point for other chemicals, particularly ethylbenzene, cumene, cyclohexane, styrene (45%), phenol (20%), and Nylon (17%) precursors. U.S. production 1979 2-6 B gals. 

For the refiner, the reduction in benzene concentration to 3% is not a major problem it is achieved by adjusting the initial point of the feed to the catalytic reformers and thereby limiting the amount of benzene precursors such as cyclohexane and Cg paraffins. Further than 3% benzene, the constraints become very severe and can even imply using specific processes alkylation of benzene to substituted aromatics, separation, etc. 

From stochastic molecnlar dynamics calcnlations on the same system, in the viscosity regime covered by the experiment, it appears that intra- and intennolecnlar energy flow occur on comparable time scales, which leads to the conclnsion that cyclohexane isomerization in liquid CS2 is an activated process [99]. Classical molecnlar dynamics calcnlations [104] also reprodnce the observed non-monotonic viscosity dependence of ic. Furthennore, they also yield a solvent contribntion to the free energy of activation for tlie isomerization reaction which in liquid CS, increases by abont 0.4 kJ moC when the solvent density is increased from 1.3 to 1.5 g cm T Tims the molecnlar dynamics calcnlations support the conclnsion that the high-pressure limit of this unimolecular reaction is not attained in liquid solntion at ambient pressure. It has to be remembered, though, that the analysis of the measnred isomerization rates depends critically on the estimated valne of... 

The following acid-catalyzed cyclizations leading to steroid hormone precursors exemplify some important facts an acetylenic bond is less nucleophilic than an olelinic bond acetylenic bonds tend to form cyclopentane rather than cyclohexane derivatives, if there is a choice in proton-catalyzed olefin cyclizations the thermodynamically most stable Irons connection of cyclohexane rings is obtained selectively electroneutral nucleophilic agents such as ethylene carbonate can be used to terminate the cationic cyclization process forming stable enol derivatives which can be hydrolyzed to carbonyl compounds without this nucleophile and with trifluoroacetic acid the corresponding enol ester may be obtained (M.B. Gravestock, 1978, A,B P.E. Peterson, 1969). 

We have seen that alkanes are not locked into a single conformation Rotation around the central carbon-carbon bond m butane occurs rapidly mterconvertmg anti and gauche conformations Cyclohexane too is conformationally mobile Through a process known as ring inversion, chair-chair mterconversion, or more simply ring flipping, one chair conformation is converted to another chair... 

The activation energy for cyclohexane ring inversion is 45 kJ/mol (10 8 kcal/mol) It IS a very rapid process with a half life of about 10 s at 25°C... 

A potential energy diagram for nng inversion m cyclohexane is shown m Figure 3 18 In the first step the chair conformation is converted to a skew boat which then proceeds to the inverted chair m the second step The skew boat conformation is an inter mediate in the process of ring inversion Unlike a transition state an intermediate is not a potential energy maximum but is a local minimum on the potential energy profile... 

Conformational inversion (ring flipping) is rapid in cyclohexane and causes all axial bonds to become equatorial and vice versa As a result a monosubstituted derivative of cyclohexane adopts the chair conforma tion in which the substituent is equatorial (see next section) No bonds are made or broken in this process... 

Among all the isomers as 1 2 dichlorocycio hexane is unique in that the ring flipping process typ ical of cyclohexane derivatives (Section 3 9) converts it to its enantiomer... 

Section 13 13 Many processes such as conformational changes take place faster than they can be detected by NMR Consequently NMR provides information about the average environment of a proton For example cyclohexane gives a single peak for its 12 protons even though at any instant 6 are axial and 6 are equatorial... 

Ring inversion (Section 3 9) Process by which a chair conforma tion of cyclohexane is converted to a mirror image chair All of the equatonal substituents become axial and vice versa Also called ring flipping or chair-chair interconversion... 

Catalyst recovery is a major operational problem because rhodium is a cosdy noble metal and every trace must be recovered for an economic process. Several methods have been patented (44—46). The catalyst is often reactivated by heating in the presence of an alcohol. In another technique, water is added to the homogeneous catalyst solution so that the rhodium compounds precipitate. Another way to separate rhodium involves a two-phase Hquid such as the immiscible mixture of octane or cyclohexane and aliphatic alcohols having 4—8 carbon atoms. In a typical instance, the carbonylation reactor is operated so the desired products and other low boiling materials are flash-distilled. The reacting mixture itself may be boiled, or a sidestream can be distilled, returning the heavy ends to the reactor. In either case, the heavier materials tend to accumulate. A part of these materials is separated, then concentrated to leave only the heaviest residues, and treated with the immiscible Hquid pair. The rhodium precipitates and is taken up in anhydride for recycling. 

Since adipic acid has been produced in commercial quantities for almost 50 years, it is not surprising that many variations and improvements have been made to the basic cyclohexane process. In general, however, the commercially important processes stiU employ two major reaction stages. The first reaction stage is the production of the intermediates cyclohexanone [108-94-1] and cyclohexanol [108-93-0], usuaHy abbreviated as KA, KA oil, ol-one, or anone-anol. The KA (ketone, alcohol), after separation from unreacted cyclohexane (which is recycled) and reaction by-products, is then converted to adipic acid by oxidation with nitric acid. An important alternative to this use of KA is its use as an intermediate in the manufacture of caprolactam, the monomer for production of nylon-6 [25038-54-4]. The latter use of KA predominates by a substantial margin on a worldwide basis, but not in the United States. 

Process and company Temperature, Pressure, Catalyst or Reactor Cyclohexane KA yield. Reference... 

Regardless of the techniques used to purify the KA oil, several waste streams are generated during the overall oxidation—separation processes and must be disposed of. The spent oxidation gas stream must be scmbbed to remove residual cyclohexane, but afterwards will stiU contain CO, CO2, and volatile hydrocarbons (especially propane, butane, and pentane). This gas stream is either burned and the energy recovered, or it is catalyticaHy abated. 

High Peroxide Process. An alternative to maximizing selectivity to KA in the cyclohexane oxidation step is a process which seeks to maximize cyclohexyUiydroperoxide, also called P or CHHP. This peroxide is one of the first intermediates produced in the oxidation of cyclohexane. It is produced when a cyclohexyl radical reacts with an oxygen molecule (78) to form the cyclohexyUiydroperoxy radical. This radical can extract a hydrogen atom from a cyclohexane molecule, to produce CHHP and another cyclohexyl radical, which extends the free-radical reaction chain. 

Cyclohexane, produced from the partial hydrogenation of benzene [71-43-2] also can be used as the feedstock for A manufacture. Such a process involves selective hydrogenation of benzene to cyclohexene, separation of the cyclohexene from unreacted benzene and cyclohexane (produced from over-hydrogenation of the benzene), and hydration of the cyclohexane to A. Asahi has obtained numerous patents on such a process and is in the process of commercialization (85,86). Indicated reaction conditions for the partial hydrogenation are 100—200°C and 1—10 kPa (0.1—1.5 psi) with a Ru or zinc-promoted Ru catalyst (87—90). The hydration reaction uses zeotites as catalyst in a two-phase system. Cyclohexene diffuses into an aqueous phase containing the zeotites and there is hydrated to A. The A then is extracted back into the organic phase. Reaction temperature is 90—150°C and reactor residence time is 30 min (91—94). 

ARCO has developed a coproduct process which produces KA along with propylene oxide [75-56-9] (95—97). Cyclohexane is oxidized as in the high peroxide process to maximize the quantity of CHHP. The reactor effluent then is concentrated to about 20% CHHP by distilling off unreacted cyclohexane and cosolvent tert-huty alcohol [75-65-0]. This concentrate then is contacted with propylene [115-07-1] in another reactor in which the propylene is epoxidized with CHHP to form propylene oxide and KA. A molybdenum catalyst is employed. The product ratio is about 2.5 kg of KA pet kilogram of propylene oxide. 

Other processes explored, but not commercialized, include the direct nitric acid oxidation of cyclohexane to adipic acid (140—143), carbonylation of 1,4-butanediol [110-63-4] (144), and oxidation of cyclohexane with ozone [10028-15-5] (145—148) or hydrogen peroxide [7722-84-1] (149—150). Production of adipic acid as a by-product of biological reactions has been explored in recent years (151—156). 

Isomerization. Isomerization is a catalytic process which converts normal paraffins to isoparaffins. The feed is usually light virgin naphtha and the catalyst platinum on an alumina or zeoflte base. Octanes may be increased by over 30 numbers when normal pentane and normal hexane are isomerized. Another beneficial reaction that occurs is that any benzene in the feed is converted to cyclohexane. Although isomerization produces high quahty blendstocks, it is also used to produce feeds for alkylation and etherification processes. Normal butane, which is generally in excess in the refinery slate because of RVP concerns, can be isomerized and then converted to alkylate or to methyl tert-huty ether (MTBE) with a small increase in octane and a large decrease in RVP. 

Cyclohexane. The LPO of cyclohexane [110-82-7] suppUes much of the raw materials needed for nylon-6 and nylon-6,6 production. Cyclohexanol (A) and cyclohexanone (K) maybe produced selectively by using alow conversion process with multiple stages (228—232). The reasons for low conversion and multiple stages (an approach to plug-flow operation) are apparent from Eigure 2. Several catalysts have been reported. The selectivity to A as well as the overall process efficiency can be improved by using boric acid (2,232,233). K/A mixtures are usually oxidized by nitric acid in a second step to adipic acid (233) (see Cyclohexanol and cyclohexanone). 

A one-step LPO of cyclohexane directly to adipic acid (qv) has received a lot of attention (233—238) but has not been implemented on a large scale. The various versions of this process use a high concentration cobalt catalyst in acetic acid solvent and a promoter (acetaldehyde, methyl ethyl ketone, water). 

---