Cyclohexanes selectivity

The influence of various physical parameters was studied in the case of the oxidation of cyclohexane selected as a model reaction. Since the main product is cyclohexanone, its rate of formation was chosen as representative of the photocatalytic activity of the system. 

Cyclohexane Selectivity, 100 um -Benzene Selectivity, 100 um Cyclohexane Selectivity, 5 um Benzene Selectivity, 5 um... 

The per-pass conversion is 5-7%, to limit the consecutive reactions in fact, alcohol and ketone are more reactive towards oxygen than cyclohexane. Selectivity to the KA Oil is around 75-80% (product weight ratio Ol/One 60/40) by-products are carboxylic acids (6-hydroxyhexanoic, n-butyric, n-valeric, succinic, glutaric and adipic among others), and cyclohexyl hydroperoxide. Unconverted cyclohexane is recycled [2a, cj ... 

Fig. 12. Comparison of temporal cyclohexene conversion and cyclohexane selectivity profiles with 180 ppm and <6 ppm of organic peroxides in feed T = 343 K, P = 13.6 MPa, OWHSV = 20h-i (49).

Takahashi and Yang (2002) studied adsorption of benzene and cyclohexane on various Y-zeolites. AgY showed superior benzene/cyclohexane selectivities to NaY, PdY, and H-USY. Separation factors as high as lO were obtained with AgY at low concentrations of benzene. More details will be given in Chapter 10 (10.8). 

Figure 4-23. Calculated and experimental selectivities and distribution coefficients for the type-I ternaries in the 2,2,4-trimethyl pentane-cyclohexane-furfural-benzene system.

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). 

The hydrogenation of 2 methyl(methylene)cyclohexane is an example of a stereo selective reaction meaning one m which stereoisomeric products are formed m unequal amounts from a single starting material (Section 5 11)... 

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). 

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). 

Cyclohexene can also be oxidized in cyclohexene-2-one which is hydrated into cyclohexan-l-ol-3-one. Dehydrogenation of this compound gives resorcinol selectively (57). 

Benzene, toluene, and xylene are made mosdy from catalytic reforming of naphthas with units similar to those already discussed. As a gross mixture, these aromatics are the backbone of gasoline blending for high octane numbers. However, there are many chemicals derived from these same aromatics thus many aromatic petrochemicals have their beginning by selective extraction from naphtha or gas—oil reformate. Benzene and cyclohexane are responsible for products such as nylon and polyester fibers, polystyrene, epoxy resins (qv), phenolic resins (qv), and polyurethanes (see Fibers Styrene plastics Urethane POLYiffiRs). 

Perhaps the greatest utility of (36-39) (and many of their precursors, such as 41, 42 and 48) is as starting points for the synthesis of highly functionalized cyclohexanes. For example, (38) has been converted in five steps (overall yield 70-75%) to streptamine (57) the key step is selective ring-opening with hydrazine to give intermediate (56) (75AG(E)630). 

Recendy, Darzens reaction was investigated for its synthetic applicability to the condensation of substituted cyclohexanes and optically active a-chloroesters (derived from (-)-phenylmenthol). In this report, it was found that reaction between chloroester 44 and cyclohexanone 43 provided an 84% yield with 78 22 selectivity for the axial glycidic ester 45 over equatorial glycidic ester 46 both having the R configuration at the epoxide stereocenter. 

Solvents influence rate as well as selectivity. The effect on rate can be very great, and a number of factors contribute to it. In closely related solvents, the rate may be directly proportional to the solubility of hydrogen in the solvent, as was shown to be the case for the hydrogenation of cyclohexene over platinum-on-alumina in cyclohexane, methylcyclohexane, and octane 48). Solvents can compete for catalyst sites with the reacting substrates, change viscosity and surface tension (108), and alter hydrogen availability at the catalyst surface. 

Diketones can be reduced usually in high selectivity to either an intermediate ketol or thediol (72). Selectivity to the ketol depends in large measure on both catalyst and solvent. In cyclohexane solvent, the maximal yield of ketol obtained on partial hydrogenation of biacetyl fell in the order 5% Pd-on-C (99%), 5% Rh-on-C (92%), 5% Pt-on-C (88%), 5% Ru-on-C (63%) from acetylacelone the descending order was 5% Pd-on-C (86%), 5% Rh-on-C (60%), 5% Ru-on-C(35%), 5% Pt-on-C (27%)(56) from 1,4-cyclohexanedione in isopropanol initial selectivity to the ketol fell in the sequence 5% Pd-on-SiO, (96%), 5% Ir-on-C (95%), 5% Ru-on-C (92%), 5% Pt-on-C (67%) (73). Generalizing from these data, it appears palladium is a good first choice to achieve maximal selectivity. 

This result reveals that exciplex formation plays a principal role in the initiation of polymerization. Since the absorption band is broadened toward longer wavelengths as the result of formation of CTC between AN and aniline, a certain concentration of aniline can be chosen so that 365-nm light is absorbed only by the CTC but not by the aniline molecule. Therefore, in this case the photopolymerization may be ascribed to the CTC excitation selected. For example, a 5 x 10 mol/L aniline solution in AN could absorb light of 365 nm, while solutions in DMF or cyclohexane with the same concentration will show no absorption. Obviously, in this case the polymerization of AN is caused by CTC excitation. The rates of polymerization for different amines were found to be in the following order (Table 12) ... 

This is also an endothermic reaction, and the equilibrium production of aromatics is favored at higher temperatures and lower pressures. However, the relative rate of this reaction is much lower than the dehydrogenation of cyclohexanes. Table 3-6 shows the effect of temperature on the selectivity to benzene when reforming n-hexane using a platinum catalyst. 

In the petrochemical field, hydrogen is used to hydrogenate benzene to cyclohexane and benzoic acid to cyclohexane carboxylic acid. These compounds are precursors for nylon production (Chapter 10). It is also used to selectively hydrogenate acetylene from C4 olefin mixture. 

Selected Properties of Hydrogen, Important Ci-Cio Paraffins, Methylcyclopentane and Cyclohexane ... 

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