Molecular sieves, oxygen production

Cobalt salts are used as activators for catalysts, fuel cells (qv), and batteries. Thermal decomposition of cobalt oxalate is used in the production of cobalt powder. Cobalt compounds have been used as selective absorbers for oxygen, in electrostatographic toners, as fluoridating agents, and in molecular sieves. Cobalt ethyUiexanoate and cobalt naphthenate are used as accelerators with methyl ethyl ketone peroxide for the room temperature cure of polyester resins. 

In this work we present results obtained with the YSZ reactor operated in the hatch mode with electrochemical oxygen addition, and with the quartz plug flow reactor operated in the continuous-flow steady-state mode. In the case of continuous flow operation, the molecular sieve trap comprised two packed bed units in parallel in a swing-bed arrangement (Fig. 1), that is, one unit was maintained at low temperature (<70°C) to continuously trap the reactor products while the other was heated for -30 min to 300°C to release the products in a slow stream of He. 

On-line GC analysis (Shimadzu GC 14A) was used to measure product selectivity and methane conversion. Details on the analysis procedure used for batch and continuous-flow operation are given elsewhere [12]. The molecular sieve trap was found to trap practically all ethylene, COj and HjO produced a significant, and controllable via the adsorbent mass, percentage of ethane and practically no methane, oxygen or CO, for temperatures 50-70 C. The trap was heated to -300°C in order to release all trapped products into the recirculating gas phase (in the case of batch operation), or in a slow He stream (in the case of continuous flow operation). 

Conclusive evidence has been presented that surface-catalyzed coupling of alcohols to ethers proceeds predominantly the S 2 pathway, in which product composition, oxygen retention, and chiral inversion is controlled 1 "competitive double parkir of reactant alcohols or by transition state shape selectivity. These two features afforded by the use of solid add catalysts result in selectivities that are superior to solution reactions. High resolution XPS data demonstrate that Brpnsted add centers activate the alcohols for ether synthesis over sulfonic add resins, and the reaction conditions in zeolites indicate that Brpnsted adds are active centers therein, too. Two different shape-selectivity effects on the alcohol coupling pathway were observed herein transition-state constraint in HZSM-5 and reactant approach constraint in H-mordenite. None of these effects is a molecular sieving of the reactant molecules in the main zeolite channels, as both methanol and isobutanol have dimensions smaller than the main channel diameters in ZSM-S and mordenite. 

Maruoka and co-workers recently reported an example of a Zr-catalyzed cyanide addition to an aldehyde [64]. As is also illustrated in Scheme 6.20, the reaction does not proceed at all if 4 A molecular sieves are omitted from the reaction mixture. It has been proposed that the catalytic addition proceeds through a Meerwein—Ponndorf—Verley-type process (cf. the transition structure drawn) and that the crucial role of molecular sieves is related to facilitating the exchange of the product cyanohydrin oxygen with that of a reagent acetone cyanohydrin. The example shown is the only catalytic example reported to date the other reported transformations require stoichiometric amounts of the chiral ligand and Zr alkoxide. 

This determines the size of molecules that can be admitted and the rate at which different molecules diffuse towards the surface. Molecular sieves, with their precise pore sizes, are uniquely capable of separating on the basis of molecular size. In addition, it is sometimes possible to exploit the different rates of diffusion of molecules to bring about their separation. A particularly important example referred to earlier, concerns the production of oxygen and nitrogen from air. 

Several review articles [37-40] have been written discussing the use of molecular sieves for the isomerization of light olefins, especially butene. The major driving force was the requirement in the 1990 amendments to the Clean Air Act in the United States that required the addition of oxygenates to gasoline in amounts up to 2.7wt% oxygen in the final gasoUne product [37]. The primary additive chosen to meet these requirements was MTBE, with lesser amounts of the ethyl ether (ETBE) or tert-amyl methyl ether (TAME) as supplements. One major possible route to meet these requirements was the isomerization of linear butenes (1-butene, cis-2-butene and trans-2-butene) into isobutene (2-methyl-l-propene). 

Chemicals. Synthesis gas, as an equimolar mixture of carbon monoxide and hydrogen, was purchased from either Air Products Ltd or British Oxygen Company Ltd. Ru(acac)3 was purchased from Johnson Matthey Chemicals Ltd and used without further purification Ru3(C0)j2 Rh6(C0)i6 and Rh(C0)2acac were prepared according to literature procedures (10-12). Glacial acetic acid and the various additives/promoters were purchased from BDH Ltd and used without further purification. Tetraglyme (ex-Aldrich Chemical Company Ltd) was dried over activated 3A molecular sieves before use. 

The catalyst samples were prepared by pelletizing mixtures of powdered carbides and inert materials (for instance, BaS04). Oxygen or nitrous oxide were used as oxidants. Experiments were run in a quartz flow reactor at atmospheric pressure at 973-1023 K utilizing 0.2-0.5 g of carbide at flow rate of 30-100 cm3/min. The reactants and reaction products were separated on CaA molecular sieves and l,2,3-tn. v-/ -cyanoethoxypropane/ polysorb A columns. 

Figure 9 shows a modern air-separation plant with front-end cleanup and product liquefaction. Production of such plants can exceed 2800 tons per day of liquid oxygen with an overall efficiency of about 15 to 20% of the theoretical optimum. The recent introduction of molecular sieve technology has provided an arrangement that increases the product to about 85% of the air input to the compressor. Thus, there has been a strong tendency over the past decade to retrofit older air-separation plants with this new arrangement to improve the process. 

The remaining gas mixture now has the composition nitrogen 3.7 mol%, oxygen 1.0 mol%, methane 47.9 mol%, C02 47.4 mol%. The third heuristic in Table 3.1 applies try to match the products. The appropriate selector is sharp split . Table 3.8 present lists of characteristic properties. Potential methods are absorption, cryogenic distillation, molecular sieving, membranes and equilibrium adsorption. 

Nitrogen production via PSA (pressure swing absorption) is based on the principle that nitrogen and oxygen have different absorption rates on carbon molecular sieves (CMS). Some of the nitrogen production processes that use this technology are described in References 1 and 3-6. 

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