Oxygenates and Olefins from Catalytic Partial Oxidation of Cyclohexane and n-Hexane in Single-Gauze Chemical Reactors

Abstract The single-gauze millisecond reactor has shown promise for the production of oxygenates and olefins from n-butane [1–3]. In this work, experiments are performed by reacting cyclohexane or n-hexane with oxygen over Pt–10%Rh single-gauze catalysts with ~ 100-μm wire diameter and 20, 40, or 80 mesh (wires per inch). The 40-mesh catalyst provides the optimal amount of heterogeneous chemistry to initiate the necessary gas-phase reactions. Using the 40-mesh single gauze, we vary alkane/oxygen molar ratios (C6/O2) from 0.5 to 5.0, N2 dilution from 10% to 75%, flow rates from 1.0 to 3.5 standard liters per minute (superficial catalyst contact times from ~1.0 to ~ 0.3 ms, respectively), preheat temperatures from 100°C to 300°C, and pressures from 1.2 to 2 atm. The alkane/oxygen ratio is the most important reactor operating variable because temperatures, reactant conversions, and product selectivities all change significantly as C6/O2 is varied. Parent oxygenates and olefins are maximized at C6/O2 ≈ 2. Low dilution favors olefin formation while an optimum in N2 dilution is observed for oxygenate selectivity in our experiments. Lower flow rates (longer contact times) promote higher COx selectivities. Lower feed temperatures favor the production of oxygenates over olefins. Higher reactor pressures (up to 2 atm) increase the yield of key products and allow complete oxygen conversion. Cyclohexane partial oxidation in the single-gauze reactor can produce ~ 85% selectivity to olefins and oxygenates at 25% cyclohexane conversion and 100% oxygen conversion, with cyclohexene and 5-hexenal as the dominant products [4]. Cyclohexene is more valuable than cyclohexane, and 5-hexenal is a 1,6-difunctional olefinic aldehyde which is potentially useful for the manufacture of polymers. Partial oxidation of n-hexane in the single-gauze reactor can produce 70% selectivity to oxygenated hydrocarbons, including > 50% selectivity to C6 oxygenates with ~ 35% selectivity to 2,5-dimethyltetrahydrofuran at ~ 20% n-hexane conversion and 100% oxygen conversion [5]. Synthesis of polymers with special properties from the monomer 2,5-dimethyltetrahydrofuran is possible. Density-Functional Theory (DFT) with the B3LYP/6-31 + G(d) method for geometry optimizations has been employed for the prediction of reaction enthalpies and rate-constant parameters for the partial oxidation of cyclohexane in single-gauze chemical reactors. The model includes 29 species and 46 irreversible (23 reversible) gas-phase reactions. The energetics of major gas-phase reaction channels are probed by locating stable reactants, products, and transition-state intermediates. One-dimensional reactor simulations are carried out using the DFT mechanism, and qualitative agreement with experimental data is obtained. Understanding the favored reaction pathways suggests ways to adjust reactor operation for desired product distributions. Detailed numerical simulations of the surface-assisted gas-phase process, including surface chemistry and fluid dynamics, will allow the investigation of experiments which are costly or potentially dangerous to carry out.