Ind. Eng. Chem. Res. 2004, 43, 2605-2609
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APPLIED CHEMISTRY CO2-Catalyzed Acetal Formation in CO2-Expanded Methanol and Ethylene Glycol Xiaofeng Xie, Charles L. Liotta, and Charles A. Eckert* Schools of Chemical Engineering and Chemistry and Biochemistry and Specialty Separations Center, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332-0100
Gas-expanded liquids are tunable media for reactions and separations. We report for the first time that CO2-expanded alcohols are catalytic media for cyclohexanone acetal formation. This process offers the opportunity to replace some environmentally harmful acid catalysts, as the CO2-based acid catalysts are self-neutralizing and result in no solid waste. We propose that the alkylcarbonic acids formed in situ from CO2 and alcohols are the acid catalysts and that the catalytic capacity can be tuned with CO2 pressure and temperature. The kinetics of cyclohexanone acetal formation in CO2-expanded methanol was investigated under various CO2 pressures over a temperature range of 25-50 °C without the addition of acid. Pseudo-first- and second-order rate constants were evaluated. These rate constants go through a maximum and the superficial activation energies go through a minimum between 10 and 40 bar of CO2. Introduction Acids are the most used catalysts in industry, and they produce more than 1 × 108 t/year of products.1 The most commonly used conventional homogeneous acid catalysts include liquid acids (HF, H2SO4, HClO4, H3PO4) and nonregenerable Lewis acids (BF3, AlCl3), which are still used in many well-known industrial processes, such as the synthesis of ibuprofen and the production of high-octane fuels.2 Although high reaction activities and selectivities are common properties of homogeneous acid catalysts, environmental problems are associated with their use, including high toxicity, corrosion, catalyst waste, use of large amounts of catalyst, and difficulty of separation and recovery. Since the 1940s, the tendency has been to replace, when possible, such homogeneous acid catalysts by solid acids, which present clear advantages over the former. However, the disposal of many solid acid catalysts is still an environmental issue. For reactions in which water participates as a reactant or product, such as hydrolysis, hydration, and esterification, only a few solid acids are acceptable in terms of activity, stability, and insolubility.3 Recently, extensive research has been performed on catalytic solvents such as near-critical water4,5 and ionic liquids6-9 as possible replacements for acid catalysts. These catalytic solvents offer opportunities to catalyze reactions that traditionally require the addition and postreaction neutralization of acid with subsequent salt disposal. For example, for Friedel-Crafts acylations, the waste salt generally amounts to 5-10 kg/kg of product. In this work, CO2-expanded alcohols are shown to act as acid catalysts that do not require neutralization, regeneration, or waste disposal. * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: 1 404 894 7070. Fax: 1 404 894 9085.
Scheme 1. Two Equilibria in the Formation and Dissociation of an Alkylcarbonic Acid
CO2-expanded liquids are alternative media for separations10-13 and reactions.14-18 Different reactions have been run in CO2-expanded liquids to make use of the various advantages of these media, such as increased solubility of gases,19 enhanced mass transport, safety, and facile product separation. It is known that CO2 can react with alcohols to form alkylcarbonic acids (Scheme 120) (e.g., methylcarbonic exists at low temperatures and decomposes upon melting at -36 °C21), which have been used as reactants in some Ugi reactions.22,23 The acid strength of some alkylcarbonic acids has been examined with diazodiphenylmethane and Reichardt’s dye in CO2-expanded alcohols.24 However, the catalytic role of the alkylcarbonic acids that form in situ in CO2-expanded alcohols has not been investigated. In this work, the acetal formation reactions of cyclohexanone in methanol and ethylene glycol are shown to be catalyzed under CO2 pressure, possibly by the alkylcarbonic acids formed between CO2 and alcohols. Acetals are commonly used to protect the carbonyl groups of aldehydes and ketones from basic nucleophilic reagents. They are typically formed by reacting aldehydes and ketones with a large excess of an alcohol in the presence of a trace of strong acid (Scheme 2). Because the formation of acetals is reversible, the high yield of acetals is accomplished by either the use of
10.1021/ie034103c CCC: $27.50 © 2004 American Chemical Society Published on Web 04/28/2004
2606 Ind. Eng. Chem. Res., Vol. 43, No. 11, 2004 Scheme 2. Acetal and Hemiacetal Formation from Aldehydes and Ketones with Alcohols in the Presence of Acid
spectrometry (MS) and flame ionization (FI) detectors. The MS detector was used for compound identification, whereas the FI detector was used for quantification. External standards of known concentration of all products and reactants were used to calibrate the FI detector. Results and Discussion
excess alcohol as the solvent, the removal of the water byproduct, or both. Not only are acetals widely used protective groups for carbonyl compounds, but they have also been employed increasingly as efficient chiral auxiliaries for the synthesis of enantiomerically pure compounds.25 Ethylene glycol is another common protection agent of carbonyl groups. One equivalent of 1,2-diol reacts to form a cyclic acetal, in which the acetal group is part of a five-membered ring. Cyclohexanone is chosen as a substrate here because it is more active than linear ketones as addition to the double bond relieves both angle strain in the ring and the small interference of the R-hydrogen atoms with the keto group.26 Experimental Section Chemicals. The chemicals used in these investigations were obtained and used without further purification, except for carbon dioxide, which was passed through gas purifiers from the cylinder to other parts of the experimental system. They include methanol (Aldrich, HPLC grade, anhydrous, >99.93%,
