Ind. Eng. Chem. Res. 2008, 47, 8025–8031
8025
Improved Procedure for Hydrogen Peroxide Production by Oxidation of Secondary Alcohols and Oxygenated Cosolvents† ´ lvaro Garriga Meco, and Ana Padilla Polo* Pilar de Frutos Escrig, A Centro de Tecnologı´a de Repsol YPF, Carretera de Extremadura, km 18, 28931, Mo´stoles, Madrid, Spain
An improved procedure for the production of hydrogen peroxide via oxidation, with molecular oxygen, of secondary alcohols admixed with oxygenated co-solvents as primary alcohols and/or ether has been studied. The selectivity, conversion, and rates have been increased, in comparison to the data obtained in the absence of co-solvents. The increase observed in the oxidation rate of 1-phenylethanol in the presence of co-solvents has been interpreted using the chain co-oxidation reaction theory. The relative values of the termination constants of 1-phenylethanol and the tested co-solvents have been estimated. The lowest values of the termination constant of the peroxy radicals derived from the co-solvents explain the increase in the steady concentration of free radicals in the reaction media and, consequently, the enhancement observed in the co-oxidation rates. Introduction Hydrogen peroxide (H2O2) is a commercial chemical that is produced in large scale for its use in numerous applications.1 This compound actually is very interesting, because it is considered a green oxidant owing to the fact that the only by product produced in the reaction using hydrogen peroxide as oxidant is water.2 The most commercially used processes for the production of hydrogen peroxide includes antrahydroquinone oxidation, hydrogen peroxide extraction, and reduction of the resulting antraquinone to yield antrahydroquinone.3,4 Considerable efforts for developing processes that imply the direct reaction of hydrogen and oxygen have been made;5–9 however, until now, this type of process has not achieved important commercial acceptance, because of some limitations, such as as explosion risks, the need to work at high pressures, and the high reaction heat that is produced, which makes very high refrigeration necessary. It is known that hydrogen peroxide is also formed via secondary alcohol oxidation; for example, the oxidation of isopropanol yields mixtures of organic products and hydrogen peroxide.10 Other secondary alcohols that have been mentioned as possible starting materials for the production of hydrogen peroxide include 1-phenylethanol and cyclohexanol.11–16 The oxidation of secondary alcohols with molecular oxygen, in combination, to produce hydrogen peroxide with the corresponding ketone, under suitable pressure and temperature conditions, from the industrial point of view, is accompanied by secondary reactions that reduce the selectivity to the desired hydrogen peroxide. Hence, a desirable objective for the improvement of this type of process is to increase the selectivity to hydrogen peroxide under common reaction conditions, avoiding its decomposition (or reaction) with the reactants (i.e., with the secondary alcohols that are used as starting materials or with the ketones that are produced in the reaction). Another desirable objective is to enhance the reaction rate, because that may cause a decrease in the reaction equipment size and, thereby, a decrease in the investment necessary to operate the process on a commercial scale. Increasing the reaction rate may be achieved by increasing the reaction temperature, but at the * To whom correspondence should be addressed. Tel.: +34 91 348 77 25. Fax: +34 91 348 86 13. E-mail address: apadillap@ repsolypf.com. † In memory of Dr. Juan Antonio Delgado Oyagu¨e.
expense of a decrease in the selectivity to hydrogen peroxide and an increase in the operation risk. Accordingly, the real technique feels a need to develop secure procedures that simultaneously allow one to increase the oxidation rate of the secondary alcohols with molecular oxygen and the selectivity to hydrogen peroxide of this reaction. Here, we wish to present that these two objectives may be achieved simultaneously when liquid-phase secondary alcohol oxidation with molecular oxygen is conducted, in combination with primary alcohols and/or ethers. These solvents act as co-oxidants and produce an increase in the selectivity and oxidation rate without significant oxidation of the same. A similar effect can be observed when a hydrocarbon product RAH is added17–19 to another hydrocarbon product RBH; a relative decrease in the oxidation reaction rate can be produced, which implies that the termination constant of the RAO2• peroxidic radical is greater than that of the RBO2• radicals and that the value of the reaction reaction constant ktAB is between ktAA and ktBB. For the same reason, one can conclude that the addition of RBH to RAH can cause an increase in the reaction rate. Therefore, the co-oxidation study of the mixture of two substances RAH and RBH allows the establishment of a classification of the relative values of the termination constants ktAA and ktBB. Experimental Section Materials. The reactants that were used;1-phenylethanol (98% purity), 1-octanol (99% purity), 2-(2-ethoxyethoxy)ethanol (99+% purity), 2-phenylethanol (99% purity), 1-hexanol (98% purity), 1,6-hexanodiol (97% purity), and bis(2-methoxyethyl)ether (99% purity);were provided by Aldrich and used without previous purification. Synthetic air N50 (maximum impurity levels: H2O, e4 ppm; CO, e0.01 ppm; CO2, e1 ppm; and hydrocarbons, e0.1 ppm) was used as the oxidant. Oxidation Experiments. The reaction was conducted in a Pyrex three-neck round-bottom flask that was equipped with a temperature control and register, a mechanical stirring system, a refrigeration system, and an oxygen analyzer to register the oxygen concentration in the exit gas (usually, the oxygen concentration varies between 21 vol % at the beginning of the reaction, and 16 vol % at the end). Air was bubbled in the system using a mass flow controller, to measure the amount of gas that was introduced. The procedure was as follows: 50 g of reaction mixture (secondary alcohol or the mixture secondary alcohol/cosolvent)
10.1021/ie800159u CCC: $40.75 2008 American Chemical Society Published on Web 07/29/2008
8026 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008
were introduced in the reaction flask. The mixture was heated to the selected temperature (usually 115 °C) and, when that temperature was attained, air was introduced in the system (this is considered to be reaction time t ) 0). The reaction was maintained for 7 h, and, during this time reaction, aliquots were obtained to analyze the hydrogen peroxide concentration (via iodometric titration) and the concentration of organic compounds (via gas chromatography). The selectivity to hydrogen peroxide was calculated by considering the number of moles of hydrogen peroxide formed, relative to the number of moles of secondary alcohol reacted; the secondary alcohol conversion was calculated by dividing the number of moles of secondary alcohol reacted by the number of moles of initial secondary alcohol; the yield to hydrogen peroxide was determined by multiplying the hydrogen peroxide selectivity by the secondary alcohol conversion; and the reaction rate constant was considered to be the number of moles of hydrogen peroxide formed during the first hour of reaction. The experimental data were adjusted using Statgraphics software. Results and Discussion In the current case, the production of hydrogen peroxide occurs via the oxidation of a secondary alcohol (in particular, 1-phenylethanol) with molecular oxygen (see Scheme 1), in the absence and presence of different substrates that have been chosen to be tested as co-solvents (these are listed in Table 1). First, the oxidation of 1-phenylethanol was performed at 115 °C under a partial pressure of oxygen (pO2) of 0.21 atm and in an air flow of 15.5 L/h. The reaction time was 7 h. These conditions were used to establish the behavior of the selected co-solvents. The results obtained are shown in Table 2. At the temperature selected, both the oxidation rate of the secondary alcohol (1-phenylethanol) and the yield of hydrogen peroxide are substantially enhanced, when one compares the results obtained in the absence and in the presence of cosolvents. This observable fact is surprising and totally unexpected, because the concentration of 1-phenylethanol in the mixtures is less than that of the pure alcohol, but, nevertheless, the oxidation rates of the 1-phenylethanol in the mixtures are, in most of the experiments, clearly greater than those of pure 1-phenylethanol. This phenomenon has not been described in the previous literature, except for the co-oxidation of hydrocarbons and mixtures of alcohols and hydrocarbons, although the increase in the co-oxidation rate values is considerably less that those in our study.20–22 Those co-solvents that contain ether groups in their molecules provide higher yields to hydrogen peroxide and higher oxidation rates. Although it is not shown in Table 2, in all the experiments, the selectivity toward the corresponding ketone (that is, acetophenone) is >99%. This is consistent with the small amount of secondary reaction products that are detected via gas chromatography (GC), such as phenol and di(1-phenylethyl)ether (
