Direct Single-Step Process Routes

The gas-solid carbonation of calcium-based materials is being investigated for the separation and concentration of C02 using calcium oxide/calcium carbonate car-bonation/calcination cycles (e.g., Ref. [43]), but not for the production of valuable carbonate materials. 

Similarly, the wet, aqueous processes can also be used to improve the stability of ashes from waste or solid fuel combustion. In some cases, such as Estonian oil shale the ashes bind significant amounts of C02, often allowing for simple and cheap processing. On the other hand, the amounts of solid material will not be such that an effect noticeable from a CCS point of view is achieved, while at the same time the produced carbonate cannot be qualified as a valuable product. 

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Direct (Single-Step) Process Routes 14.4.2.1 Gas-Solid Processes... 

The current two-step industrial route for the synthesis of methanol, from coal or methane to synthesis gas and then from synthesis gas to methanol, has certain drawbacks. The economic viability of the whole process depends on the first step, which is highly endothermic. Thus a substantial amount of the carbon source is burned to provide the heat for the reaction. It would be highly desirable, therefore, to replace this technology with a technically simpler, single-step process. This could be the direct partial oxidation of methane to methanol, allowing an excellent way to utilize the vast natural-gas resources. Although various catalysts, some with reasonable selectivity, have been found to catalyze this reaction (see Sections 9.1.1 and 9.6.1), the very low methane conversion does not make this process economically feasible at present. 

It is important for the discussion below to distinguish between direct and indirect process routes. Direct carbonation is the simplest approach to carbonate production (or mineral carbonation see Section 14.4) and the principal approach is that a suitable feedstock-for example, serpentine or a Ca/Mg-rich solid residue-is carbonated in a single process step. For an aqueous process this means that both the extraction of metals from the feedstock and the subsequent reaction with the dissolved C02 to form carbonates takes place in the same reactor. 

The most direct route towards functionalized aliphatic polyesters is based on the functionalization of polyester chains. This approach is a very appealing because a wide range of functionalized aliphatic polyesters could then be made available from a single precursor. This approach was implemented by Vert and coworkers using a two-step process. Eirst, PCL was metallated by lithium diisopropylamide with formation of a poly(enolate). Second, the poly(enolate) was reacted with an electrophile such as naphthoyl chloride [101], benzylchloroformate [101] acetophenone [101], benzaldehyde [101], carbon dioxide [102] tritiated water [103], ot-bromoacetoxy-co-methoxy-poly(ethylene oxide) [104], or iodine [105] (Fig. 26). The implementation of this strategy is, however, difficult because of a severe competition between chain metallation and chain degradation. Moreover, the content of functionalization is quite low (<30%), even under optimized conditions. 

The 2-butanol feedstock is conventionally obtained by the sulfuric acid-catalyzed addition of water to -butenes. This is a two-step reaction involving sulfation and hydrolysis in which the conversion of -butenes is 90% and selectivity to 2-butanol is 95% (15). During operation the sulfuric acid becomes diluted and must be reconcentrated before reuse. In 1983 Deutsche Texaco commercialized a single-step route in which 2-butanol is formed by the hydration of -butenes in the presence of a strongly acidic ion-exchange resin containing sulfonic acid groups (16—18). The direct reaction is carried out at 150—160°C and 7 MPa. Virtually anhydrous 2-butanol is recovered in this process (19). Direct hydration requires lower utilities and investment costs, operates at 99% selectivity to 2-butanol, but is hindered by low (5—15%) -butene conversion per pass. 

A direct one-step route to 1,1,1,2,-tetrafluoroethane (HFC-134a CF3CH2F) can be written as shown in Eq. (6). Currently, no commercially viable process has been described for this approach involving high-conversion single-pass yields. This has to do with equilibrium limitations, as explained below. Because of this, decoupling of the overall process into... 

This chemistry has provided one of the most direct routes to the indolo[2,3-a]carbazole alkaloid ring system (Scheme 11), a common functionality of several biologically active molecules such as the potent antitumor agent rebeccamycin and arcyriaflavin A. The indolo[2,3-fl]carbazole derivative has been obtained in satisfactory yield through the reaction of l,4-di((9-trifluoroacetamidophenyl)-l,3-butadiyne with 3,4-dibromomaleimide in the presence of tetrakis(triphenylphosphine)palladium(0) and potassium carbonate. The proposed mechanism of this polyannulation process, which generates four new bonds in a single step, is outlined in Scheme 12. 

Improved efficiency in the synthesis of 24 was ultimately achieved via an enzymatic desymmetrization approach (Scheme 7). In the key step of this route, an asymmetric oxidation of achiral amine 41 promoted by monoamine oxidase (MAON) under an oxygen atmosphere afforded intermediate 42. In this streamlined process, sodium bisulfite was included in the enzymatic oxidation mixture to effect direct conversion to sulfonate 44. Treatment of 44 with sodium cyanide provided the trans-nitrile 43 as a single diastereomer in approximately 90% yield from pyrrolidine 41. As in the second-generation synthesis, the nitrile is hydrolyzed to the methyl ester under Pinner conditions (HCI, methanol). In the manufacturing process, the product was converted to its free base using NaOH, then crystallized as the HCI salt from i-propanol and methyl t-... 

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