Cyclohexane, oxidative dehydrogenation product selectivity

Minachev et al. (41, 42) have recently examined alkali metal ion forms of various zeolites (A, X, Y, L, chabazite, erionite, and mordenite) for cyclohexane oxidative dehydrogenation. Not surprisingly these alkali metal ion forms are considerably less active than those containing transition metal ions (reaction temperatures of approximately 300° and 450°C, respectively). Further, cyclohexene rather than benzene is the predominant product (selectivity to cyclohexane 67-84%), particularly with small-pore zeolites. In fact, NaA was the most active zeolite tested (42), which strongly suggests that the reaction is simply occurring on the outer surface of the zeolite crystallites. 

Minachev et al. [76] studied oxidative dehydrogenation of cyclohexane on zeolite cationic forms at 300-475 °C, the main reaction product of which is cyclohexene. Cyclohexadiene and C02 are also formed, and at long-term contacts benzene is detected. Cyclohexene yield and selectivity of the reaction depend on zeolite structure and composition, reaction temperature and oxygen cyclohexane ratio in the reaction mixture. Among alkaline cationic forms of zeolite, the highest cyclohexene yield (21%) is observed for NaA zeolite (66% selectivity). 

By far the largest outlet for benzene (approx. 60%) is styrene (phenyl-ethene), produced by the reaction of benzene with ethylene a variety of liquid and gas phase processes, with mineral or Lewis acid catalysts, are used. The ethylbenzene is then dehydrogenated to styrene at 600-650°C over iron or other metal oxide catalysts in over 90% selectivity. Co-production with propylene oxide (section 12.8.2) also requires ethylbenzene, but a route involving the cyclodimerization of 1,3-butadiene to 4-vinyl-(ethenyl-) cyclohexene, for (oxidative) dehydrogenation to styrene, is being developed by both DSM (in Holland) and Dow. 60-70% of all styrene is used for homopolymers, the remainder for co-polymer resins. Other major uses of benzene are cumene (20%, see phenol), cyclohexane (13%) and nitrobenzene (5%). Major outlets for toluene (over 2 5 Mt per annum) are for solvent use and conversion to dinitrotoluene. 

The best olefin yields were observed over Pt-coated monoliths. In the case of ethane/02 mixtures, selectivities to ethylene up to 65% at 70% ethane conversion and complete O2 conversion were reported." The oxidative dehydrogenation of propane and -butane produced total olefin select vies of about 60% (mixtures of ethylene and propylene) with high paraffin conversions." " Mixtures of ethylene, propylene and 1-butene were observed by the partial oxidation of -pentane and n-hexane ethylene, cyclohexene, butadiene and propylene were the most abundant products of the partial oxidation of cyclohexane." ... 

Caprolactam (world production of which is about 5 million tons) is mostly produced from benzene through three intermediates cyclohexane, cyclohexanone and cyclohexanone oxime. Cyclohexanone is mainly produced by oxidation of cyclohexane with air, but a small part of it is obtained by hydrogenation of phenol. It can be also produced through selective hydrogenation of benzene to cyclohexene, subsequent hydration of cyclohexene and dehydrogenation of cyclohexanol. The route via cyclohexene has been commercialized by the Asahi Chemical Company in Japan for adipic acid manufacturing, but the process has not yet been applied for caprolactam production. 

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