Supercritical water ethanol dehydration

As mentioned earlier, at 500° C and 34.5 MPa supercritical water has a small dielectric constant, a very low ion product, and behaves as a high temperature gas. These properties would be expected to minimize the role of heterolysis in the dehydration chemistry. As shown in Table 1, the conversion of ethanol to ethylene at 500° C is small, even in the presence of 0.01M sulfuric acid catalyst. The appearance of the byproducts CO, C02) CH i+ and C2H6 points to the onset of nonselective, free radical reactions in the decomposition chemistry, as would be expected in the high temperature gas phase thermolysis of ethanol. 

Early work (7) in this laboratory established the heterolytic nature of ethanol dehydration in supercritical water. Trace (0.001 to 0.01 M) concentrations of strong mineral acids (such as H2SO4 and HCl) were found to catalyze significant conversions of ethanol to ethene in water at 385 C, 34.5 MPa after a few... 

Alcohols undergo dehydration in supercritical and hot water (41). Tertiary alcohols require no catalyst, but secondary and primary alcohols require an acid catalyst. With 0.01 MH2SO4 as a catalyst, ethanol eliminates water at 385°C and 34.5 MPa to form ethene. Reaction occurs in tens of seconds. Only a small amount of diethyl ether forms as a side reaction. 

The reaction chemistry of simple organic molecules in supercritical (SC) water can be described by heterolytic (ionic) mechanisms when the ion product 1 of the SC water exceeds 10" and by homolytic (free radical) mechanisms when <<10 1 . For example, in SC water with Kw>10-11 ethanol undergoes rapid dehydration to ethylene in the presence of dilute Arrhenius acids, such as 0.01M sulfuric acid and 1.0M acetic acid. Similarly, 1,3 dioxolane undergoes very rapid and selective hydration in SC water, producing ethylene glycol and formaldehyde without catalysts. In SC methanol the decomposition of 1,3 dioxolane yields 2 methoxyethanol, il lustrating the role of the solvent medium in the heterolytic reaction mechanism. Under conditions where K klO"11 the dehydration of ethanol to ethylene is not catalyzed by Arrhenius acids. Instead, the decomposition products include a variety of hydrocarbons and carbon oxides. 

Metal alkoxides can by decomposed in supercritical alcohols to produce various oxides, such as Ti02 (73-75). Metal alkoxides also react readily with water to form hydroxide species that are easily dehydrated into oxides upon heating. As a solution of the alkoxides and alcohol is heated, some of the alcohol decomposes into water and aldehyde. The metal alkoxides react with the water formed and then undergo dehydration at the elevated temperatures. Compared with samples made via conventional sol-gel synthesis, powders synthesized in supercritical ethanol exhibit a higher degree of crystallinity and contain less hydroxide. 

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