The present invention relates to a method of producing hydrogen peroxide according to the cyclic anthraquinone process. The working solution to be used contains as the reaction carrier at least two differently substituted 2-alkylanthraquinones and/or the corresponding 2-alkyltetrahydroanthraquinones. In another aspect, the present invention relates to a novel reaction carrier.
In the so-called cyclic anthraquinone process for producing hydrogen peroxide, 2-alkylanthraquinones and/or their nuclear-hydrogenated 2-alkyl-.alpha.- and/or .beta.-tetrahydroanthraquinones, functioning as reaction carriers, are hydrogenated in an organic solvent system in the presence of a hydrogenating catalyst with hydrogen or a gas containing hydrogen, whereby the reaction carriers are converted at least partially into the hydroquinone form. The solution, containing one or more reaction carriers in the hydrogenated or oxidized form and the organic solvent system, is generally designated as the working solution. After the hydrogenation stage, the working solution is freed from the hydrogenating catalyst and is then treated in the oxidation stage with an oxygen-containing gas, during which the quinone form of the reaction carriers re-forms together with the formation of hydrogen peroxide. After separation of the resulting hydrogen peroxide from the oxidized working solution, customarily done by extraction with water and/or with an aqueous solution containing hydrogen peroxide, the working solution is fed back to the hydrogenation stage. Aside from the cited stages, the process can also include a regeneration of the working solution, in which case anthraquinone derivatives such as anthraquinone epoxides formed in the cyclic process and which are inactive as reaction carriers are re-activated and/or 2-alkyltetrahydroanthraquinones are dehydrogenated to the corresponding 2-alkylanthraquinone derivatives and, as required, even losses of reaction carriers are replaced by the addition of the corresponding 2-substituted anthraquinones and/or their tetrahydro derivatives. A further stage relates to the regeneration of the catalyst in order to maintain a high activity. A survey of the cyclical anthraquinone process is contained in Ullmann's Encyclopedia of Industrial Chemistry, 5.sup.th ed. (1989), vol. A13, 447-457 which is relied on and incorporated herein by reference.
High requirements are placed on the reaction carriers in order to assure the highest possible system output in large-scale plants with the lowest possible susceptibility to interruptions and the lowest possible loss of reaction carriers. One of the requirements concerns in particular the highest possible solubility of the reaction carriers in the solvent system both in the quinone form as well as in the hydroquinone form. The solubility of the hydroquinone form is decisive for the maximal H.sub.2 O.sub.2 equivalent obtainable in constant operation (=g H.sub.2 O.sub.2 per liter working solution). Further requirements concern the kinetics of hydrogenation and of oxidation; both reactions should take place as rapidly as possible. Since the hydrogenation and the oxidation are often influenced in an opposite manner by a change in the structure of a reaction carrier even a good reaction carrier system consisting of two or more components often represents only a compromise. Important too are the highest possible chemical stability of the reaction carrier in the catalytic hydrogenation, a high oxidation stability with respect to oxygen and hydrogen peroxide, and furthermore, a high stability with respect to acids and/or alkalis as are used in the regeneration. Finally, the reaction carrier should be as insoluble as possible in water, toxicologically harmless and economically available.
According to GB patent 1,252,822 one or more 2-alkylanthraquinones with 2 to 6 carbon atoms in the alkyl group, especially 2-ethyl-, 2-tert.-butyl- and 2-amylanthraquinone can be used in the anthraquinone process for producing hydrogen peroxide. The 2-alkyl-tetrahydroanthraquinones which form in the hydrogenation stage are also effective.
In the GB patent previously cited, not a single 2-alkylanthraquinone reaction carrier with 6 C atoms in the alkyl group is mentioned by way of example or even emphasized. In EP-A documents 0286610 and 0778085 2-hexenylanthraquinone is cited as a reaction carrier along with other 2-alkylanthraquinones and mixtures. Which of the possible hexenyl isomers is meant and whether or which advantages can be achieved therewith can not be gathered from these EP documents. It is known that as the chain length of the alkyl substituent increases in 2-alkylanthraquinones the quinone solubility increases; however, at the same time, and this is probably more important for practicability, the rate of hydrogenation drops off sharply. Thus, it was not obvious to seriously consider the use of a 2-C.sub.6 -alkylanthraquinone as a reaction carriers.
It follows from JP-A 58 180452 and JP-A 59 051235 that 2-(4-methyl-3-pentenyl)-1,4-dihydroanthraquinone as well as 2-(4-methyl-3-pentenyl)anthraquinone obtainable therefrom and 2-(4-methylpentyl)anthraquinone can be used as reaction carriers for the production of hydrogen peroxide. The production of the cited compounds--the parent compound is obtained by Diels-Alder reaction from 1,4-naphthoquinone and myrcene--can be gathered from these documents. Concerning the use of these compounds in the cyclical anthraquinone process for the production of hydrogen peroxide, it is solely mentioned that the same results can be obtained as with known 2-alkylanthraquinones.
The requirements placed on a good reaction carrier are occasionally only partly fulfilled when using a single 2-alkylanthraquinone and/or the corresponding 2-alkyltetrahydroanthraquinone, formed in situ, as a function of the operating conditions. Great efforts were therefore undertaken by those in this field of technology to improve the reaction carrier by using at least two different 2-alkylanthraquinones and/or their tetrahydro derivatives. However, advantages regarding the one or the other requirement on a good system of reaction carriers often oppose disadvantages regarding other criteria.
It is possible according to DE-AS 11 95 279 to raise the yield of hydrogen peroxide and/or to minimize the creation of byproducts during the hydrogenation if, instead of using a single 2-alkylanthraquinone such as 2-ethyl-, 2-isopropyl-, 2sec.-butyl- or 2-tert.-butylanthraquinone, an almost eutectic mixture of at least two 2-alkylanthraquinones such as preferably 2-ethyl and 2.-sec.-butylanthraquinones is used in a weight ratio of 27 to 73 and if the degree of hydrogenation is held below 40%. A disadvantage of this method is the necessity of having to limit the degree of hydrogenation. An even more significant disadvantage is the unsatisfactory hydrogenation kinetics of these eutectic mixtures. Similar mixtures of two C.sub.1 to C.sub.4 alkylanthraquinones, which can be present in the so-called "anthra" system but also the tetra system, are known from U.S. Pat. No. 2,966,397.
U.S. Pat. No. 4,374,820 suggests using a mixture of 2-tert.-butylanthraquinone and 2-sec.-amylanthraquinone including their tetrahydro compounds. This system does have good oxidation kinetics but unsatisfactory hydrogenation kinetics. On the other hand, DE-OS 11 12 051 and 11 06 737 recommend using a mixture of isomeric 2-amylanthraquinones, especially a mixture of 2.-sec.-amyl- and 2-tert.-amylanthraquinone and their tetrahydro derivatives as reaction carriers. A high H.sub.2 O.sub.2 equivalent can be obtained with such systems on account of their good quinone and hydroquinone solubility; however, the unsatisfactory hydrogenation kinetics are also disadvantageous here, the consequence of which is a poor space-time yield.
Even the use of a reaction carrier system based on 2-ethylanthraquinone (EAQ) and 2-amylanthraquinone (AAQ) and their tetrahydro derivatives (THEAQ and THAAQ) is known--see EP-A 0453949 and Chemical Economics Handbook--SRI International, June 1992, CEH Product Review Hydrogen Peroxide. A reaction carrier system on this basis (EAQ/THEAQ and AAQ/THAAQ) results in comparison to a reaction carrier system based on 2-ethylanthraquinone and 2-ethyl-tetrahydroanthraquinone in an increased H.sub.2 O.sub.2 equivalent which can also be maintained under operating circulatory conditions. A disadvantage of the reaction carrier system based on EAQ/THEAQ and AAQ/THAAQ is its susceptibility to interruptions in the hydrogenation stage which makes itself noticeable in a reduced take-up of hydrogen. When using a suspension catalyst such as Pd-black this behavior forces the need for a relatively high circulation of hydrogenation catalyst and to further elevate it upon interruptions; however, this lowers the economy of the process.
It is therefore an object of the present invention to produce hydrogen peroxide using a working solution containing at least two differently substituted 2-alkylanthraquinones and/or their tetrahydro compounds which method exhibits to a lesser extent the disadvantages of the methods using the previously known 2-alkylanthraquinone combinations, especially those based on ethyl- and amylanthraquinone and their tetrahydro derivatives.
A further object of the system of the invention is to obtain, given good hydrogenation kinetics, a higher H.sub.2 O.sub.2 equivalent which can be reliably controlled operationally and it should be less problem-prone.