Pregeneration of tetrahydroanthraquinones in a make-up solution to be added to a hydrogen peroxide working solution

The invention provides a process to increase the mol ratio of nuclearly hydrogenated working compound to the total working compound in a make-up solution to be added to the working solution of an operating plant in producing hydrogen peroxide by the cyclic reduction, oxidation, and extraction of an alkylated anthraquinone. The process produces the nuclearly hydrogenated working compound in the make-up solution without producing substantial quantities of undesired by-products.

This invention is directed to an economical process for hydrogenating a 
2-alkylanthraquinone to form the 5,6,7,8-tetrahydro derivative in a 
working solution without simultaneously producing substantial quantities 
of undesired by-products and to a process for producing hydrogen peroxide 
using said working solution. 
U.S. Pat. No. 2,158,525 to Riedl et al teaches that on oxidation 
polynuclear organic hydroquinones are capable of forming hydrogen peroxide 
and the corresponding quinones. At the present time almost all of the 
commercial hydrogen peroxide is produced by the anthraquinone process 
which includes the steps of cyclically hydrogenating a "working" solution 
containing a 2-alkylanthraquinone to form the corresponding 
2-anthrahydroquinone and oxidizing the latter to form the 
2-alkylanthraquinone and hydrogen peroxide. The polynuclear hydroquinone 
and quinone compounds capable of forming hydrogen peroxide in such a 
process are called "working compounds." The working compounds presently 
believed to be in use commercially are 2-ethylanthraquinone, 
2-t-butylanthraquinone, 2-amylanthraquinone (usually the mixed sec and 
tert amyl compounds) and derivatives thereof. During the hydrogenation 
step some of the anthraquinone is nuclearly hydrogenated to form the 
5,6,7,8-tetrahydro derivative, commonly abbreviated to "tetrahydro" or 
"tetra"; the tetra derivatives are also capable of producing hydrogen 
peroxide and are also considered to be working compounds. 
Many other by-products are also formed in the working solution; these 
by-products are undesirable as they do not contribute to the production of 
hydrogen peroxide. Therefore, the presence of such other by-products is 
economically undesirable as they represent a loss of the expensive working 
compound, moreover, it is frequently necessary to purify the working 
solution by contacting it with a compound such as an activated alumina, to 
convert some of these by-products to working compounds. However, such a 
treatment is usually expensive and results in an overall loss of expensive 
working compound and expensive solvent. In addition, some undesired 
by-products are extracted into the aqueous hydrogen peroxide, increasing 
the dissolved carbon content of the hydrogen peroxide, or even causing an 
undesired color or odor to develop in the hydrogen peroxide. 
The optimum operating conditions for an anthraquinone process hydrogen 
peroxide plant are related to the quantity of hydrogen peroxide produced 
per cycle and the time required to complete such a cycle. The presence of 
a tetrahydro working compound in the working solution is one of the 
important factors to be considered in establishing the optimum operating 
conditions for a plant. 
U.S. Pat. No. 3,073,680 to Jenney et al teaches that it is desirable if at 
least 85% of the working compound is in the tetrahydro form and its 
teachings are incorporated herein by reference. More specifically, the 
Jenney et al patent teaches that 
2-ethyl-5,6,7,8-tetrahydroanthrahydroquinone can be produced by the 
hydrogenation of the 2-ethylanthraquinone at 50.degree. C. and 340 kPa. 
The working solution described in the referenced patent initially 
contained 8.4 g/l of a working compound, of which 64% was 
2-ethylanthraquinone and 36% was 2-ethyl-5,6,7,8-tetrahydroanthraquinone. 
However, after 7 hours of hydrogenation, 4.7 g/l working compound 
remained, and all was in the tetrahydro form, representing approximately a 
40% yield based on the original working compound. 
U.S. Pat. No. 2,495,229 to Dawsey et al teaches a process to produce a 
tetrahydroanthrahydroquinone in glass apparatus in approximately 85% 
yield. However, the Dawsey et al process requires a special porous nickel 
catalyst not suitable for use in a peroxygen process. 
British Pat. No. 1,390,408 teaches a process for manufacturing hydrogen 
peroxide using a working solution containing 
2-tert-amyltetrahydroanthraquinone and 
2-sec-isoamyltetrahydroanthraquinone. The tetrahydroanthraquinones are 
first prepared by the process of U.S. Pat. No. 2,495,229 using a porous 
nickel catalyst at atmospheric pressure and elevated temperatures. The 
process of British Pat. No. 1,390,408 has the disadvantage of requiring a 
preliminary hydrogenation step to prepare the tetrahydroanthraquinone. 
Subsequently, the tetrahydroanthraquinones are isolated and a second, 
different hydrogenation step is necessary to form the 
2-amyltetrahydroanthrahydroquinone in the working solution. It is more 
desirable economically to synthesize the tetrahydroanthrahydroquinone in 
the solvent to be used in the working solution. 
During operation it is necessary to add or make up both solvent and working 
compound to the working solution to compensate for losses. Usually the 
make-up working compound is added as the anthraquinone form rather than 
the tetrahydro form or mixture of anthraquinone and its tetrahydro 
derivative of the working compound. 
It is known that the tetrahydro component of a working compound increases 
gradually with use. However, during this period other of the by-products 
are formed which degrade the working compound in the working solution. 
Thus, even when a relatively small proportion of the anthraquinone form of 
make-up working compound is added to the working solution the relatively 
severe hydrogenation conditions result in undersirable by-product 
formation. In addition, the productivity of a plant is depressed during 
the period in which the tetrahydro component is forming in the working 
solution. This reduced productivity represents a significant economic 
loss. 
It is desirable to develop a rapid process to increase the ratio of the 
tetrahydro component of the working compound to the total working compound 
to be added to the working solution without producing substantial 
quantities of undesired by-products, and without depressing the 
productivity of the catalyst or working solution of the operating plant. 
This objective has been achieved by the process of the present invention 
which comprises preparing a first solution containing a non-nuclearly 
hydrogenated alkylanthraquinone in an inert solvent or solvents and 
adjusting the concentration of the solution so that the concentration of 
the non-nuclearly alkylanthraquinone of the working compound is no greater 
than the saturation concentration of the corresponding anthrahydroquinone 
formed in the subsequent hydrogenation step. This first solution is then 
hydrogenated at 50.degree. C. or less in the presence of a palladium 
catalyst, whereby the ratio of tetrahydrogenated working compound to the 
total working compound is increased without substantial formation of 
undesired by-products, and is then added as a make-up solution to the 
plant working solution. 
While the working compound may be based on any 2-alkylanthraquinone 
compound suitable for use in producing hydrogen peroxide by a cyclic 
hydrogenation and oxidation process, it is desirable that the 
2-alkylanthraquinone working compound be selected from the group 
consisting of, 2-ethylanthraquinone, 2-t-butylanthraquinone, and 
2-amylanthraquinone. 
The working compound can be dissolved in any inert solvent to form the 
first solution. However, it is preferred that the inert solvent employed 
be a component of a working solution to be used in the hydrogen peroxide 
process so that the solution of the working compound produced by this 
invention is suitable for addition directly to a process stream of a plant 
producing hydrogen peroxide by the cyclic hydrogenation and oxidation of 
an alkylated anthraquinone. The solvent may be either fresh solvent or 
reclaimed solvent. For example, it is well known that a substantial 
quantity of solvent is evaporated from the working solution in the 
oxidizer and can be recovered from the effluent air or oxygen either by 
condensation or adsorption. Alternatively, if it is desired only to 
increase the concentration of working compound in the working solution 
without addition of solvent, the solvent employed may be that which is 
contained in at least a portion of the plant working solution, optionally 
after a purification step, with the additional working compound simply 
being added thereto to form the first solution. 
It is well known that in the manufacture of hydrogen peroxide, a palladium 
catalyst is preferred over a nickel catalyst for hydrogenating the working 
compound to the anthrahydroquinone form because it is less likely to cause 
nuclear hydrogenation. However, it is surprising to find that a palladium 
catalyst is suitable to catalyze the hydrogenation of the non-nuclearly 
hydrogenated form of the working compound to the tetrahydro form of the 
working compound and is critical for the economics of the present 
invention in that it is not necessary to use a first catalyst to prepare 
the tetrahydro form of the working compound and a second catalyst for the 
reduction of the quinone form to the hydrogenating form of the working 
compound. 
The hydrogenation step of the present invention may take place in a plant 
hydrogenator operating under the critical conditions claimed herein. 
However, in the absence of excess hydrogenation capacity it is more 
economical to avoid reducing the plant productive capacity and, instead to 
hydrogenate the make-up working solution in a separate hydrogenator. 
Frequently, an operating hydrogen peroxide plant may utilize a sidestream 
hydrogenator or a pilot unit with a pilot scale hydrogenator to evaluate 
catalysts, solvents, and working compounds and either such a sidestream 
hydrogenator or the hydrogenator of a pilot unit may serve well for use in 
the practice of this invention. 
Although the hydrogenator may be operated on 100% recycle of the working 
solution until the desired ratio of the tetrahydro working compound to the 
total working compound is obtained, it is also possible when preparing the 
make-up solution in a pilot scale hydrogenator to operate with only part 
of the working solution passing through the oxidizer and extractor cycle. 
For safety reasons and economics when practicing the present invention in 
a pilot unit, it may be undesirable to circulate the hydrogenated solution 
through the rest of the pilot unit without the oxidizer and extractor 
cycles in operation. By maintaining a recycle of at least 90% of the 
solution through the hydrogenator the depth of hydrogenation is increased 
to permit substantial formation of the tetrahydro working compound within 
the hydrogenator. At the same time, the remaining 10% or less of the 
working solution can be oxidized, the hydrogen peroxide extracted, and the 
working solution recycled back to the hydrogenator thereby eliminating the 
need to blanket the rest of pilot unit with an inert gas as a safety 
precaution. 
When practicing the present invention in a sidestream hydrogenator it may 
be desirable to gradually increase the concentration of the working 
compound in the working solution by adding up to 10% of the first solution 
from the sidestream hydrogenator as a make-up solution to the plant 
working solution and recycling at least 90% of the first solution to the 
sidestream hydrogenator and concomitantly replacing the portion of the 
first solution added as a make-up solution to the working solution with 
additional solvent and working compound. 
It is critical to this process that at least 90% and, more preferably at 
least 95%, of the hydrogenated working solution is recycled to the 
hydrogenator until the desired ratio is reached so as to minimize the 
production of undesired by-products. It is not critical whether the 
hydrogenator is either a fixed bed hydrogenator or a fluid bed 
hydrogenator. 
It is also critical for the practice of this invention that the temperature 
of the hydrogenator be maintained at less than about 50.degree. C. to 
prevent substantial formation of undesired by-products. Although 
temperatures of less than 40.degree. C. are not objectionable with regard 
to the formation of undesired by-products, it is clear that such lower 
temperatures reduce both the hydrogenation rate and the solubility of the 
working compound. Therefore, the preferred operating temperature is 
between about 40.degree. C. and less than about 50.degree. C. 
The pressure of the hydrogen gas in the hydrogenator is not critical over 
the range of about 50 to about 400 kPa. One skilled in the art will 
recognize that increasing the pressure will increase the rate of 
hydrogenation. However, within the above range there is little effect of 
pressure on the rate of formation of undesired impurities. Preferably, the 
hydrogen pressure will range between 100 and 200 kPa to minimize safety 
hazards and optimize equipment cost. 
Additional non-nuclearly hydrogenated working compound may be added to the 
solution undergoing hydrogenation to increase the concentration of the 
working compound to a desired or design level. The additional working 
compound may be added in one or several increments providing the total 
concentration of the non-nuclearly hydrogenated working compound does not 
exceed the saturation concentration of the hydroquinone form in the 
hydrogenator.

The following non-limiting examples illustrate to one skilled in the art 
the best mode for practicing the claimed invention. 
EXAMPLES 
A hydrogenation pilot was designed to permit control of flow rate of 
solution, solution temperature, pressure, and gas flow rate. The palladium 
catalyst taught in U.S. Pat. No. 3,635,841 was used in a fixed bed 
configuration to eliminate catalyst attrition with time and to eliminate 
filtration problems. Suitable sampling ports were provided. The quinone 
form of the working compound was dissolved in a suitable solvent and the 
system was purged of air using nitrogen. When the desired temperature was 
attained, the nitrogen was vented and replaced with hydrogen which was fed 
to the system at a preset pressure. Flow rates of the quinone solution and 
gas purge were adjusted and the hydrogenation allowed to proceed. 
Periodically, samples were withdrawn from the solution reservoir through a 
sampling valve and filter, oxidized, extracted with 10% H.sub.2 SO.sub.4, 
and analyzed by gas chromatograph to determine content of the 
2-alkyl-5,6,7,8-tetrahydroanthraquinone and its parent quinone. 
COMATIVE EXAMPLE 
A working solution was prepared consisting of 25% by weight 
2-amylanthraquinone (AAQ) in a mixed solvent, 67% diisobutylcarbinol 
(DIBC) and 33% C.sub.9 -C.sub.12 aromatic hydrocarbon solvent (Shell Sol 
or merely SSol). The hydrogenator was charged with 324 g of the catalyst 
disclosed in U.S. Pat. No. 3,635,841 and operated at 50.degree. C. 
hydrogenator inlet temperature to remove inherent poisons from the working 
solution. A rapid decline in the titers (78 to 53) was noticed during the 
first two days of operation. The catalyst bed was replaced with 324 g of 
fresh catalyst and the operation restarted. A similar decline in the 
titers was again observed. Heating the catalyst bed under a high purge of 
N.sub.2 did not improve the titers. After 24 hours of operation, the 
catalyst was replaced with 534 g of fresh catalyst and operations were 
resumed. At the end of the third day, an additional 116 g of fresh 
catalyst was charged, bringing the total catalyst weight to 650 g. The 
operation was resumed at about 45.degree. C. at the hydrogenator inlet 
temperature. A rapid decline was observed in the titers from 173 to 118. 
The catalyst bed was subsequently heated to about 100.degree. C. with a 
purge of nitrogen through the bed to eliminate water from the catalyst 
bed. At this point, the working solution was assumed to be free from 
poisons and suitable for use to obtain meaningful data. Continuous 
operation was begun at a 45.degree. C. hydrogenator inlet temperature. The 
titers were observed to drop from 126 to 106, in approximately 40 hours. 
The catalyst bed was regenerated with nitrogen for two hours and the 
operation restarted. During 52 hours of operation the titers gradually 
dropped from 128 to 101. This repeated decline in the titers was 
attributed to an impurity coating the catalyst bed. The operation was 
interrupted and the catalyst bed was washed with the mixed solvent in an 
effort to remove the suspected impurities from the catalyst bed. Following 
the washing of the catalyst bed, round-the-clock operation at 45.degree. 
C. hydrogenator inlet temperature was restarted, the downward trend of the 
titers continued during the subsequent 70 hours of continuous operation. 
EXAMPLE 1 
The hydrogenator pilot was operated with a solution containing 1,600 g of a 
6% solution of 2-ethylanthraquinone in a solvent containing 27% trioctyl 
phosphate and 73% SSol solvent system. After the system was thoroughly 
purged with nitrogen, hydrogenation was begun at 45.degree. C. and 375 kPa 
of hydrogen. After 20 hours of hydrogenation, the concentration of 
5,6,7,8-tetrahydro-2-ethylanthrahydroquinone was observed to be 4 weight 
percent, at which point additional 2-ethyl-anthraquinone was added to 
bring the total working compound concentration up to 9%. After an 
additional 8 hours of hydrogenation, the concentration of the tetrahydro 
derivative had reached 6.25%. No unwanted by-products were observed. 
EXAMPLE 2 
The equipment of Example 1 was operated under the same conditions as in 
Example 1, but instead contained a 15% solution of 2-amylanthraquinone in 
a C.sub.9 aliphatic alcohol SSol solvent mixture. After 11 hours, the 
concentration of the tetrahydro derivative had increased to 4.6%. 
Additional 2-amylanthraquinone was added to bring the solids concentration 
up to 20% percent. Hydrogenation was continued for an additional 16 hours 
at which point the concentration of the tetrahydro working compound 
(H.sub.4 AAQ) was 8.2%. An additional portion of 2-amylanthraquinone was 
added to bring the concentration of solids to 25%, and after 19 additional 
hours of hydrogenation, the level of tetrahydro working compound was 
12.3%. Under the mild conditions employed, no unwanted by-product 
formation was detected. 
EXAMPLE 3 
Example 1 was repeated with 210 g of catalyst and 15 liters of a 16% AAQ 
solution in the DIBC-SSol solvent system. The hydrogenation inlet 
temperature ranged from 40.degree. C. and the pressure ranged from 375 kPa 
to 410 kPa. Portions of the reaction mixture were analyzed by gas 
chromatography at various time intervals. The formation rate of the 
5,6,7,8-tetrahydro working compound (H.sub.4 AAQ) is shown in Table 1. 
Liquid chromatographic analysis of the final product showed no other 
quinone related by-products. 
EXAMPLE 4 
H.sub.4 AAQ was also prepared in a conventional process pilot employing 
90-95% recycle to the fixed-bed hydrogenator to simulate a plant start-up. 
The initial solution consisted of 22 liters of 15% 2-amylanthraquinone 
(AAQ) and in the DIBC-SSol mixed solvent. The solution was subjected to 
hydrogenation (15 cm fixed-bed catalyst), oxidation, and extraction in the 
process pilot. The following pilot conditions were observed: 
______________________________________ 
Hydrogenator Inlet Temperature 
40-45.degree. C. 
Hydrogenator Inlet Pressure 
375-410 kPa 
Oxidizer Temperature 40-42.degree. C. 
Oxidizer Pressure 210-245 kPa 
Extractor Temperature 25-30.degree. C. 
Total Flow to Hydrogenator 
600 ml/minute 
(Including Recycle) 
Recycle Flow to Hydrogenator 
540 ml/minute 
Flow to Oxidizer 60 ml/minute 
______________________________________ 
The formation rate of H.sub.4 AAQ is shown in Table 2. Gas and liquid 
chromatography analyses showed no other quinone by-products. 
EXAMPLE 5 
Example 1 was repeated with a solution containing approximately 4.5% EAQ 
and operated at 40.degree. C. and 375 kPa. The results are presented in 
Table 3. 
EXAMPLE 6 
Example 5 was repeated with a solution containing approximately 15% AAQ in 
a DIBC-SSol solvent using 200 g catalyst. The results are presented in 
Table 4. 
Pursuant to the requirements of the patent statutes, the principle of this 
invention has been explained and exemplified in a manner so that it can be 
readily practiced by those skilled in the art, such exemplification 
includes what is considered to represent the best embodiment of the 
invention. However, it should be clearly understood that, within the scope 
of the appended claims, the invention may be practiced by those skilled in 
the art, and having the benefit of this disclosure, otherwise than as 
specifically described and exemplified herein. 
TABLE 1 
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(Example 3) 
Rate of Formation of the Tetrahydro Compound 
Time % % 
Hours AAQ H.sub.4 AAQ 
______________________________________ 
0 16.0 0.0 
2 1.2 
4 3.0 
6 4.5 
8 5.8 
10 6.7 
14 10.5 
20 14.1 
22 15.0 
24 15.7 
26 15.8 
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TABLE 2 
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(Example 4) 
Rate of Formation of the Tetrahydro Compound 
Time % % 
Hours AAQ H.sub.4 AAQ 
______________________________________ 
0 15.0 0.0 
8 3.0 
24 3.8 
32 4.8 
52 7.4 
56 7.8 
60 6.7 8.2 
______________________________________ 
TABLE 3 
______________________________________ 
(Example 5) 
Rate of Formation of the Tetrahydro Compound 
Time % % 
Hours EAQ H.sub.4 EAQ 
______________________________________ 
0 4.5,4.3 0.0 
5 3.19 1.15 
8 2.62 1.79 
12 1.99 2.44 
16 1.41 2.91 
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TABLE 4 
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(Example 6) 
Rate of Formation of the Tetrahydro Compound 
Time % % 
Hours AAQ H.sub.4 AAQ 
______________________________________ 
0 14.6,14.6 
0.0 
4 12.8 1.8 
11 9.7 4.6 
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