Ind. Eng. Chem. Res. 2000, 39, 2843-2848
2843
Production of Hydrogen Peroxide in Liquid CO2. 3. Oxidation of CO2-Philic Anthrahydroquinones Dan Haˆ ncu and Eric J. Beckman*
Ind. Eng. Chem. Res. 2000.39:2843-2848. Downloaded from pubs.acs.org by MIDWESTERN UNIV on 01/23/19. For personal use only.
Chemical Engineering Department, University of Pittsburgh, 1249 Benedum Hall, Pittsburgh, Pennsylvania, 15261
Hydrogen peroxide is currently produced via the sequential hydrogenation and oxidation of a 2-alkylanthraquinone. Use of liquid CO2 as the process solvent could ameliorate several environmental and engineering problems inherent to the conventional process. Oxidation reactions of perfluoroether-functionalized anthrahydroquinones (FAQH2s) generated in situ from Pd-catalyzed hydrogenation of functionalized anthraquinones (FAQs) were conducted in a highpressure batch reactor in liquid CO2. The reaction was found to be first order with respect to both O2 and FAQH2. We have also found that the reactivity of FAQH2 in the oxidation is not affected by the size of the “CO2-philic” tail or the nature of the linker between the aromatic rings and the CO2-philic tail. These results are correlated with our previous conclusions on the phase behavior of FAQs in CO2 and kinetic studies of hydrogenation of FAQs. Finally, an optimum structure of FAQ to be used in the process is proposed. Introduction Hydrogen peroxide has been generated by the alkylanthraquinone/anthrahydroquinone process since the late 1930s.1 Here, H2O2 is generated by sequential hydrogenation and oxidation of an alkyl-anthraquinone dissolved in a mixture of organic solvents, followed by the recovery of the product in a liquid-liquid extraction against water. This process suffers from innate inefficiencies owing to transport limitations in both reactions2 and also organic contamination of the product during recovery by liquid-liquid extraction. This contamination plus a nonoptimal partition coefficient mandate use of distillation to both concentrate and purify H2O2. Finally, overhydrogenation of the anthraquinone and the solvent during each process cycle requires constant disposal of nonreactive byproducts and anthraquinone (AQ) makeup. The oxidation of the anthrahydroquinone (AQH2) is
a major contributor to the total cost of H2O2 production due to high-energy consumption and high investment costs. Kinetic studies of the oxidation of 2-ethyltetrahydroanthrahydroquinone (eH4AQH2) in organic solvents by atmospheric pressure O2 have shown that the rate is governed by the transport of oxygen through the gasliquid interface. The reaction is first order with respect to both the organic substrate and oxygen, a result that has been explained using a radical mechanism.3 * To whom correspondence should be addressed. Tel.: (412) 624-9630.Fax: (412)624-9639.E-mail:
[email protected].
The number of the aromatic rings connected directly to the hydroquinone strongly affects its reactivity during oxidation. Early studies by Weissberger et al.4 showed that benzohydroquinone (BQH2) could react with oxygen only in a basic solution (pH ) 7.2-8.2). The benzoquinone (BQ) formed initially in this reaction is further oxidized by hydrogen peroxide to hydroxybenzoquinone and humic acids. These side reactions can be avoided and hydrogen peroxide isolated if all labile hydrogens are substituted by alkyl groups, as shown by the autoxidation of durohydroquinone ((CH3)4BQH2). Later studies indicated that the direct oxidation of BQH2 with dioxygen could be conducted in the presence of a variety of catalysts: silver carbonate,5 alumina-supported copper(II) sulfate,6 and vanadyl acetylacetonate (VO(acac)2),7 although hydrogen peroxide could be isolated only when using VO(acac)2. In the case of anthraquinones, AQH2 is 5 times more reactive than eH4AQH2 during the oxidation because of greater aromaticity.1,8 Production of Hydrogen Peroxide in CO2. Production of hydrogen peroxide based on the anthraquinone-anthrahydroquinone process is an ideal target for CO2 technology.9 First, the replacement of the organic solvent with CO2 will eliminate the organic contamination of the aqueous product during the extraction stage. Second, because hydrogen and oxygen are miscible in all proportions with CO2 above its critical temperature,10,11 the resistance to mass transfer owing to the diffusion of the reactants across the gas-liquid boundary vanishes. Under these conditions, both hydrogenation and oxidation reactions can be conducted under kinetic control, thus reducing the probability for byproduct formation. Because oxidation will likely be rapid, oxidation and product extraction could be conducted in a single column, eliminating an entire unit operation.12 In a fortuitous added benefit, the pH of water in contact with liquid CO2 drops to 2.85,13 the same point to which current H2O2 manufacturers lower the pH to stabilize the product.14 Previous work has shown that the solubility of 2-alkylanthraquinones in CO2 is
