Partial oxidation phases

At times it might be necessary to employ gases other than oxygen (e.g. argon or air) to aid in the deposition of less noble metals or partially oxidized phases. Changing to gases other than oxygen will affect the flame, and process variables will have to be adjusted. 

In 1967, Piccinin and Toussaint (337) reported the nitric acid oxidation of platinum metal, presumably in the presence of cyanide ions, yielding crystals of an unusual platinum(III) complex, K2Pt(CN)5(H20)3. Only the crystallographic and chemical analysis have been reported for this complex. Recent x-ray powder data has shown K2Pt(CN)s(H20)3 to be 4b (515). Cocrystallization of K2Pt(CN)4 and K2Pt(CN)e (571) is reported not to give evidence of the formation of a partially oxidized phase (337). 

Since 1960, the Hquid-phase oxidation of ethylene has been the process of choice for the manufacture of acetaldehyde. There is, however, stiU some commercial production by the partial oxidation of ethyl alcohol and hydration of acetylene. The economics of the various processes are strongly dependent on the prices of the feedstocks. Acetaldehyde is also formed as a coproduct in the high temperature oxidation of butane. A more recently developed rhodium catalyzed process produces acetaldehyde from synthesis gas as a coproduct with ethyl alcohol and acetic acid (83—94). 

Isopropyl alcohol can be partially oxidized by a noncatalytic, liquid-phase process at low temperatures and pressure to produce hydrogen peroxide [7722-84-1] and acetone (24—26). 

Partial oxidations over complex mixed metal oxides are far from ideal for singlecrystal like studies of catalyst structure and reaction mechanisms, although several detailed (and by no means unreasonable) catalytic cycles have been postulated. Successful catalysts are believed to have surfaces that react selectively vith adsorbed organic reactants at positions where oxygen of only limited reactivity is present. This results in the desired partially oxidized products and a reduced catalytic site, exposing oxygen deficiencies. Such sites are reoxidized by oxygen from the bulk that is supplied by gas-phase O2 activated at remote sites. 

Quantitative analysis can be carried out by chromatography (in gas or liquid phase) during prolonged electrolysis of methanol. The main product is carbon dioxide,which is the only desirable oxidation product in the DMFC. However, small amounts of formic acid and formaldehyde have been detected, mainly on pure platinum electrodes. The concentrations of partially oxidized products can be lowered by using platinum-based alloy electrocatalysts for instance, the concentration of carbon dioxide increases significantly with R-Ru and Pt-Ru-Sn electrodes, which thus shows a more complete reaction with alloy electrocatalysts. 

Oxidations and partial oxidations constitute one of the most important classes of gas-phase reactions. For this reason, the drivers for and benefits when undergoing micro-channel investigations of these reactions are discussed here in advance of the class as whole. In addition, the specific drivers and benefits are discussed for the individual oxidation reactions as throughout all chapters of this book with regard to reactions. 

Propene is an intermediate utilized in the chemical and pharmaceutical industries. The partial oxidation of propene on cuprous oxide (CU2O) yields acrolein as a thermodynamically imstable intermediate, and hence has to be performed under kinetically controlled conditions [37]. Thus in principle it is a good test reaction for micro reactors. The aim is to maximize acrolein selectivity while reducing the other by-products CO, CO2 and H2O. Propene may also react directly to give these products. The key to promoting the partial oxidation at the expense of the total oxidation is to use the CU2O phase and avoid having the CuO phase. 

Gas-phase reaction 8 [CP 8] Syngas generation by partial oxidation of methane... 

Spencer and Pereira (1987) studied the kinetics of the gas-phase partial oxidation of CH4 over a Mo03-Si02 catalyst in a differential PFR. The products were HCHO (formaldehyde), CO, C02, and H20. 

Fig. 1. Relationship between catalyst temperature and reaction time in methane partial oxidation catalyzed by Ni/Si02 (temperature of the gas phase (a) 1019 K, (b) 899 K, (c) 809 K, (d) 625 K). The reaction was carried out in a fixed-bed reactor (a quartz tube of 2 mm inside diameter) at atmospheric pressure. Before reaction, the feed gas was allowed to flow through the catalyst undergoing heating of the reactor from room temperature to 1073 K at a rate of 25 K min-1 to ignite the reaction, and then the reactant gas temperature was decreased to the selected value. Reaction conditions pressure, 1 atm catalyst mass, 0.04 g feed gas molar ratio, CH4/O2 = 2/1 GHSV, 90,000 mL (g catalyst)-1 h-1) (25).

Fig. 10 Thermodynamic equilibrium compositions on dry basis for the partial oxidation of ethanol. All species are in gas phase. Initial concentrations of CO, CH4, CH3CHO are taken as zero in the calculation.

Gas chromatography is especially useful for gas-phase analysis of partial oxidation, hydrogenation, or hydroconversion products as in many cases a full carbon balance (educts, products, and all side products), in order to evaluate sample performance. As the detection and quantification of permanent gases such as N2, 02, CO, and C02 and also of higher boiling compounds are standard separation problems for gas chromatography, it is wise to employ the method regarding this problem. 

A parallel reactor system for liquid-liquid phase reactions such as oxidation reactions with H202 at ambient pressure was reported from hte Aktiengesellschaft. If compared with other chemistries, rather mild-reaction conditions (ambient pressure, moderate temperature) are often applied in liquid-phase oxidation for fine chemical production with terminal oxidants that can be dosed as liquids (e.g., aqueous H202 or organic peroxides). The reaction that was investigated was the partial oxidation of... 

Why are partial gas-phase oxidations so important for the chemical industry ... 

E. Yu. Batyan, S. V. Matveichuk, and G. A. Branitskii, Structural phase transformations in silver-ceramic systems and their relation to catalytic properties in the process of methanol partial oxidation, Kinet. Catal. 136, 816-820 (1995). 

---