Tuning millisecond chemical reactors for the catalytic partial oxidation of cyclohexane Original Research Article

It is demonstrated that millisecond partial oxidation of cyclohexane can be tuned by varying the catalyst and operating conditions to generate product distributions that favor (1) oxygenates, (2) olefins, or (3) syngas (H2 and CO). High selectivities to parent oxygenates require low conversions using low-temperature catalysts, such as Ag or Co. Olefins are favored by Pt or Pt-Sn and H2 addition eliminates the production of CO and CO2, thereby increasing olefin selectivities. For syngas, Rh is the catalyst of choice. Finally, a Pt-10% Rh single gauze gives high selectivities to both oxygenates and olefins. Conventional methods for the partial oxidation of cyclohexane are liquid-phase processes that are plagued by poor conversions, high recycle costs, long residence times (minutes to hours), and expensive catalysts. In contrast, with a cyclohexane–oxygen feed at C6H12/O2=2, a Pt-10% Rh single gauze catalyst can give total selectivities exceeding 80% to oxygenates and olefins at 25% cyclohexane conversion and complete oxygen conversion. The products consist of nearly 60% selectivity to the C6 products, cyclohexene and 5-hexenal. The temperature profile attained in the single-gauze reactor allows the preservation of these highly non-equilibrium products. Alternative catalysts for cyclohexane oxidation to oxygenates and olefins include α-alumina monoliths coated with Pt, Rh, Pt-Rh, Pt-Sn, Co, Mo or Ag. The Co, Mo and Ag catalysts give very high selectivities to C6 oxygenates but are hindered by poor conversions (<5%) of both cyclohexane and oxygen at these millisecond contact times. H2 addition to cyclohexane oxidation feed mixtures over Pt and Pt-Sn is shown to significantly increase the selectivities to C6 olefins while reducing the formation of CO and CO2. Cyclohexane oxidation in air over Rh monoliths enables the production of high yields (>95%) of syngas. This process could find applications in the automotive industry as the production of hydrogen from liquid fuels becomes important.