Fuel Production by Supercritical Water
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10.3 rOle OF sCW in ChemiCal synthesis
Due to the unique properties possessed by SCW in which numerous types of organic
reactions can be carried out with ease, this medium has been widely exploited for a
variety of chemical synthesis [11–13,15–28]. Parsons [16], Katritzky et al. [17], An
et al. [18], Leif and Simoneit [19], and Savage [11] provide good reviews of the types
of synthetic organic chemistry that can be carried out in SCW. Savage [11] provides
a brief account of the types of chemical synthesis that are possible in SCW. These
include the following:
1. Hydrogenation/dehydrogenation reactions using transition metal complexes
2. C–C bond formation reaction such as Friedel–Crafts alkylation reactions
3. Rearrangement reactions such as formation of ketones by rearrangement of
pinacol and two different bicyclic diols
4. Hydration and dehydration reactions such as conversion of alcohols to
olefins (e.g., conversion of tert-butyl alcohol to isobutylene)
5. Elimination reaction such as facile decarboxylation of carboxylic acid
6. Hydrolysis such as conversion of esters to carboxylic acids and alcohols
7. Partial oxidation such as conversion of methane to oxygenates or higher
hydrocarbons
8. H–D exchange such as substitute of H by D in alpha positions of ketone
carboxyl groups
Savage [11] gives numerous examples of these different types of chemical synthesis.
He also points out that future research should be more focused on the use of SCW to
carry out these and other novel chemical synthesis. While not all chemical synthesis
are targeted toward synthetic fuels, many such as hydrogenation/dehydrogenation,
C–C bond formation, hydration/dehydration, hydrolysis, and partial oxidation play
important role in the generations of synthetic fuels or various important additives to
the synthetic fuels. This subject will be under intense future research investigation.
Some details of the specific examples quoted by Savage [11] as they relate to
fuels are worth noting. As an example of C–C bond formation, both phenol and
p-cresol can be successfully alkylated with tert-butyl alcohol and 2-propanol at
275°C in the absence of any added acid catalyst to produce sterically hindered
phenols [18]. Water in these alkylation reactions serves as both catalyst and reactant. xu and Antal [21,22] were successful in converting tert-butyl alcohol to
isobutylene by dehydration reaction. In the absence of an added acid, hydronium
ions formed by the dissociation of water are the primary catalytic agents. The
dehydration of other alcohols such as cyclohexanol, 2-methylcyclohexanol, and
2-phenylethanol to form alkenes is also reported [23–25]. Esters can undergo
an autocatalytic hydrolysis to form carboxylic acids and alcohols [17,18]. Partial
oxidation of methane in SCW at 400°C to form methanol has been explored with
both homogeneous free radical reactions [26,27] and heterogeneous catalytic
reactions [28]. High selectivities for oxygenates, but very low methane conversions, have been obtained. More research to synthesize fuel components or fuel
additives in SCW continues to be pursued.