Alcohols, supercritical

Selective oxidation of ethylene to acetaldehyde was carried out over carbon-supported Pd and Pt membrane catalysts.1322 The concept of supported liquid-phase catalysis was also successfully applied in the Wacker oxidation.1323 The Wacker reaction can be performed in alcohol-supercritical C02.1324 C02 as cosolvent accelerates reaction rates and remarkably affects the selectivity towards methyl ketone in the presence of an alcohol. 

Promising results were observed in Friedel-Crafts alkylation77 and epoxidation.78 Higher rates or better selectivities were found for hydroformylations in supercritical C02.79-84 Simple trialkyl phosphines, for examples, were shown to provide highly active Rh catalysts.81 Hydroboration showed enhanced regioselec-tivity.85 The Wacker reaction performed in alcohol-supercritical C02 exhibits high reaction rates and markedly increased selectivity toward methyl ketone.86... 

While hexane is widely accepted as the most effective solvent used today, there are concerns about its flammability, exposure, and environmental impacts. Research has focused on various alternative solvents in the hopes of finding one with acceptable performance while providing greater safety. Alternative solvents that have received some attention include isopropyl alcohol, supercritical carbon dioxide, and other fluids. However, no economical alternative to n-hexane has been accepted at this point, and the best available control technology emphasizes containment and limiting fugitive hexane emissions. 

Kocon L, Despetis E, Phalippou J (1998) Ultralow density silica aerogels by alcohol supercritical dryiitg. J Non-Cryst Solids 225 96-100... 

High-temperature alcohol supercritical extraction techniques (ASCE) bring the wet gel to the supercritical state of the solvent (usually methanol or ethanol) in an autoclave or other pressure vessel. This involves high pressures (above 8 MPa) and temperatures (above 260° C). A number of studies have been performed to examine the effects of solvent fill volume, pre-pressure, and other processing parameters (see for example Phalippou et al. [23], Danilyuk et al. [24], Pajonk et al. [25]). The low-temperature extraction techniques (CSCE) are based on supercritical extraction of CO2, which has a lower critical-point temperature than the alcohol mixture that remains in the sol-gel pores after polymerization. The CSCE methods... 

CP co-precursor, OM organic modifier, S Silylotion, M methoxylation, abbreviations used in the Precursor column are defined in Table 3.1 abbreviations used in the Additional Reagent(s) Employed column and in foomotes, below, are defined in Table 3.2 AMB ambient pressure extraction, ASCE alcohol supercritical extraction, CSCE CO2 supercritical extraction, RSCE rapid supercritical extraction. The temperature indicated in the Hydrophobicity column indicates the maximum temperature at which the aerogels were hydrophobic. 

Kocon, L. Despetis, F. Phalippou, J., J., Ultralow density siUca aerogels by alcohol supercritical drying,... 

Yoda S, Tasaka Y, Uchida K, Kawai A, Oshima S, Ikazaki F (1998) Ti02-impregnated Si02 aerogels by alcohol supercritical drying with zeolite. J Non-Cryst Solids 225 105-110... 

Biodiesel can also be produced from noncatalytictransesterification with supercritical alcohol. Supercritical alcohol transesterification is a process that has several advantages, such as reduced reaction time and easy separation and purification of the products, besides allowing the use of raw material with high contents of free fatty acids. However, this technique requires relatively severe operating conditions, therefore requiring special equipment that makes the costs associated with the process very expensive (Shimoyama et al., 2009 Tan et al., 2010 Atadashi et al., 2011). 

Hexane extraction of stabilized rice bran has been the most commonly used method for obtaining rice bran oil. Hexane has the advantage of high oil recovery, but safety concerns regarding hexane have led to a search for alternative extraction methods. Less hazardous approaches such as the use of isopropyl alcohol, supercritical fluid extraction, or enzyme-assisted extraction are potential alternative processing methods. 

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