Promoting Partial Oxidation Chemistries

Anthony F. Sammells, James H. White, and Richard Mackay 

The principles behind this membrane technology originate from solid-state electrochemistry. Conventional electrochemical halfceU reactions can be written for chemical processes occurring on each respective membrane surface. Since the general chemistry under discussion here is thermodynamically downhill, one might view these devices as short-circuited solid oxide fuel cells (SOFCs), although the ceramics used for oxygen transport are often quite different. SOFCs most frequently use fluorite-based solid electrolytes - often yttria stabUized zirco-nia (YSZ) and sometimes ceria. In comparison, dense ceramics for membrane applications most often possess a perovskite-related lattice. The key fundamental 

Following a brief description of some selected technical criteria which may be used for the rational selection of oxygen mediating membranes, a discussion follows on some partial oxidation chemistries which we hope will illustrate the potential of this OTM technology. 

---

I 7 Membranes for Promoting Partial Oxidation Chemistries At the reducing membrane surface... 

Figure 7.1 Principles behind dense oxygen transport membranes for promoting spontaneous value added partial oxidation chemistry.

This process is used to treat gas streams containing high concentrations of H2S. The chemistry of the units involves partial oxidation of hydrogen sulfide to sulfur dioxide and the catalytically promoted reaction of hh.S and SO2 to produce elemental sulfur. The reactions are staged and arc. is lollows ... 

Chemistries such as gasification, carboxylation, carbonylation, partial oxidation, and salt splitting may see much greater emphasis in manufacturing. These chemistries will need concurrent development of more selective catalytic and biocatalytic systems and promoters, as well as processes requiring much less ex-... 

Rare earth oxides are useful for partial oxidation of natural gas to ethane and ethylene. Samarium oxide doped with alkali metal halides is the most effective catalyst for producing predominantly ethylene. In syngas chemistry, addition of rare earths has proven to be useful to catalyst activity and selectivity. Formerly thorium oxide was used in the Fisher-Tropsch process. Recently ruthenium supported on rare earth oxides was found selective for lower olefin production. Also praseodymium-iron/alumina catalysts produce hydrocarbons in the middle distillate range. Further unusual catalytic properties have been found for lanthanide intermetallics like CeCo2, CeNi2, ThNis- Rare earth compounds (Ce, La) are effective promoters in alcohol synthesis, steam reforming of hydrocarbons, alcohol carbonylation and selective oxidation of olefins. 

Mallat, T., Seyler, L., Mir Alai, M., et al. (1998). Aerobic Oxidation of a-Substituted Alcohols over Promoted Platinum Metal Catalysts, in B. Hodnett, J. Clark and K. Smith (eds). Supported Reagents and Catalysts in Chemistry, 216, Royal Society of Chemistry, Cambridge, pp. 66-71, cit. in Jenzer, G., Sueur, D., Mallat, T, etal. (2000). Partial Oxidation of Alcohols in Supercritical Carbon Dioxide, Chem. Commun., 22, pp. 2247-2248. 

Protection of aldoses at the non-anomeric positions makes it possible to use many of the common procedures in organic chemistry for oxidizing lactols as shown with mannofura-nose 1 and glucopyranose 3 (O Table 1). The reactions can be divided into three main categories oxidations mediated by activated dimethyl sulfoxide (DMSO), oxidations with chromi-um(VI) oxides, and oxidations catalyzed by ruthenium oxides. The DMSO-mediated oxidations of alcohols can be promoted by several activators [27]. With the partially protected aldoses the activation has mainly been achieved with acetic anhydride and oxalyl chloride. Competing /3-elimination does usually not occur unless the eliminating group is an ester, e. g., an acetate or a benzoate [27]. 

Instead of ion conductivity, another class of oxides providing electronic conductivity can also be used for gas detection. Promoted by various dopants, the adsorption of the measured gas components or the partial pressure of oxygen modifies the defect chemistry in the material, leading to a variation of bulk or surface conductivity. 

---