Anthraquinone process hydrogenation catalyst

The chemical yield of hydrogen peroxide and the anthraquinone per process cycle is very high, but other secondary reactions necessitate regeneration of the working solution and hydrogenation catalyst, and the removal of organic material from the extracted hydrogen peroxide. 

The alkylated anthraquinone process accounts for over 95% of the world production of H202, mainly because the it operates under mild conditions and direct contact of 02 and H2 is avoided. In this process, 2-alkylanthraquinone (the alkyl group is typically an ethyl, terf-butyl or amyl group) is dissolved in a mixture of a non-polar solvent (C9-Cn alkylbenzene) and a polar solvent [Trioctyl phosphate (TOP), or tetrabutyl urea (TBU) or diisobutyl carbinol (DIBC)] and then hydrogenated over a precious metal (Pd or Ni) catalyst in a three-phase reactor (trickle bed or slurry bubble column) under mild reaction conditions (<5bar, <80 °C) to generate 2-alkylanthrahydroquinone [1-3, 5], The latter is then auto-oxidized with air in a... 

In the first step of the process the anthraquinone is hydrogenated to the hydroquinone with palladium as the preferred catalyst on carriers, such as gauze, or in suspension. The reaction is carried out at about 40°C and at pressures up to ca. 5 bar with cooling and only to ca. 50% hydrogenation to suppress side reactions (see below). 

While the chemical yield of hydrogen peroxide in the AO process is very high, the loss of quinone/hydroquinone via the formation of these by-products necessitates the regeneration of the reaction mediators, hydrogenation catalyst, and removal of organic by-products. Periodically, fresh anthraquinone and solvent are added to compensate for losses. 

Catalyst decomposition depends heavily on the specific process conditions employed. Producers operate under two different regimes the all-tetra system, in which no specific actions are taken to either suppress the formation of tetra or to dehydrogenate it back to anthraquinone, and the anthra system, in which efforts are made to minimize the tetra content. Tetra formation can be reduced by the use of selective hydrogenation catalysts and specific operating conditions (i.e., solvent choice and specialized quinones). In addition, tetra can be dehydrogenated in the presence of activated alumina (AI2O3) in the catalyst regenerator (Eq. (14.12)). 

Pure hydrogen and oxygen mixtures are highly explosive. Reactions that involve such mixtures are carried out safely in microchannel reactors. For example, the direct preparation of hydrogen peroxide is obtained with a special catalyst, avoiding the circuitous anthraquinone process, used at the industrial scale. Calculations of explosion limits clearly demonstrate that there is a considerable shift when explosive reactions are carried out in microchannels. The safety of the process is not only due to the avoidance of thermal runaway (because of large surface-area-to-volume ratio), but also due to the fact that radical chains are broken down due to the increased wall collision in the small channels of the reactor. 

The above method has now been largely replaced by a newer process, in which the substance 2-ethylanthraquinone is reduced by hydrogen in presence of a catalyst to 2-ethylanthraquinol when this substance is oxidised by air, hydrogen peroxide is formed and the original anthraquinone is recovered ... 

Stretford A process for removing hydrogen sulfide and organic sulfur compounds from coal gas and general refinery streams by air oxidation to elementary sulfur, using a cyclic process involving an aqueous solution of a vanadium catalyst and anthraquinone disulfonic acid. Developed in the late 1950s by the North West Gas Board (later British Gas) and the Clayton Aniline Company, in Stretford, near Manchester. It is the principle process used today, with over 150 plants licensed in Western countries and at least 100 in China. 

Stretford plants have been in operation for 30 years. There are hundreds of such plants worldwide, used in a variety of sulfur removal operations (Dalrymple 1989). In a Stretford process, the hydrogen sulfide in the feed gas stream is absorbed and oxidized to elemental sulfur in aqueous phase, using pentavalent vanadium which is subsequently reduced from a pentavalent form to a tetravalent form. Later in the process, the vanadium is re-oxidized back again, using anthraquinone disulfonic acid (ADA) as a catalyst, and the elemental sulfur is floated to the surface of the solution and removed. 

Arco have developed an integrated process for the production of industrially important epoxides via an adapted AO process (Figure 1.13).33 34 A sulfonic acid substituted alkylhydroanthraquinone alkylammonium salt is reacted with molecular oxygen to form the alkylanthraquinone and hydrogen peroxide. The hydrogen peroxide is then reacted with an alkene in the presence of a titanium zeolite catalyst (TS-1 see Chapter 4). The epoxide product is then separated, and the anthraquinone salt recycled to a hydrogenator for reaction with... 

The advantage of the above two methods are high yields of epoxides, and the titanium silicalite catalyst is not deactivated or poisoned by the contaminants in the crude oxidation mixture. Hence, the processes are commercially attractive. The in situ hydrogen peroxide generation based on the AO process from either the anthraquinone/anthrahydroquinone or ketone/alcohol redox couples has also been used for the following synthetic reactions ... 

The direct oxidation of benzene into phenol constitutes one of the challenges in chemistry to substitute the cumene process at the industrial level. Such oxidation has also been achieved with several TpfCu complexes as catalysts, leading to moderate yields and high selectivity toward phenol, in a transformation using hydrogen peroxide as the oxidant and at moderate temperatures. The same catalytic system has been employed for the selective oxidation of anthracenes into anthraquinones (Scheme 24). 

Schemes 9-3 and 9-4 are sequences of two substitutions, first a metallo-de-hydrogenation, followed by a halogeno-de-metallation. Scheme 9-3 is analogous to the well known electrophilic aromatic sulfonation of anthraquinone in position 1. This isomer is obtained only if the reaction is run in the presence of catalytic amounts of mercury (ii) salts. Nowadays, however, larger effort is devoted to either replace mercury by other catalysts, or in the search for processes leading to (practically) complete recovery of the mercury. This case raises two questions with respect to the reaction sequence (9-3) first, whether it is possible to apply a one-pot process with catalytic amounts of a mercury compound (not necessarily HgO) to the synthesis of compounds 9.5, and second, whether mercury can be completely recycled in processes using either stoichiometric or catalytic amounts of the element.

Hydrogen peroxide in combination with catalysts such as TS-1 acts as a good, "clean" epoxidation system. T e reactions that could be carried with this catalyst include ammoximation of cyclohexanone, epoxidation of propene and other small alkenes, and hydroxylation of aromatics and linear alkanes (Chapter 4). The system produces little waste, avoids the use of hazardous chemicals such as alkyl hydroperoxide, and reduces process complexity. However, the key parameter for industrial development is the cost of H2O2. H2O2 is produced by only a few companies, and very large capital expenditure is required, because H2O2 synthesis (by alkyl-anthraquinone route) is economical only when large quantities are produced. 

In the chemical industry, more than a megaton of hydrogen peroxide is produced yearly in a biphasic reaction scheme known as the anthraquinone auto-oxidation process, where reduced anthraquinone is used to reduce oxygen to H2O2 and where the anthraquinone is reduced again by hydrogen on a palladium catalyst. 

The process has been briefly described by Turunen (1997) as an example of process intensification activities. Most of the hydrogen peroxide production is nowadays based on the anthraquinone method. The differences between the technologies include mainly differences in solvents, catalysts and equipment types and details. The process has less than ten main unit operations including two multiphase reactors, liquid-liquid extraction, gas desorption, distillation and filtration. The process conditions do not include high temperatures or pressures. The necessary properties are not readily available from literature because of the large number of components in the process liquid. However, the measurement of the most of the properties is relatively easy because of the mild conditions. The number of components which take part in the main production reactions and separation steps is small. Therefore it was possible to develop reliable models for most of the unit operations and to base the design on these nnodels. However, the side reactions and by-products involve complicated chemistry and... 

The catalyst used in this process is Raney Ni, or Pd/Al203. Anthraquinone dissolved in a solvent mixture was hydrogenated by the catalyst, then oxidized with air. By this method, H2O2 can be produced and the starting material, anthraquinone, recovered [31], In the process, anthraquinone degradation is a problem and results in decreased efficiency. Researchers are therefore trying to replace the chemical reduction step of anthraquinone with electrochemical reduction [30, 32, 33],... 

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