Anthraquinone hydrogenation process

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 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 ... 

Although considered an active participant in the process cycle, the tetrahydroaLkylanthraquinone (10) may not be a significant part of the catalytic hydrogenation because, dependent on the concentration in the working solution, these could all be converted to the hydroquinone by the labile shift per equation 17 and not be available to participate. None of the other first- or second-generation anthraquinone derivatives produce hydrogen peroxide, but most are susceptible to further reaction by oxidative or reductive mechanisms. 

Working Solution Composition. The working solution in an anthraquinone process is composed of the anthraquinones, the by-products from the hydrogenation and oxidation steps, and solvents. The solvent fraction usually is a blend of polar and aromatic solvents which together provide the needed solubiUties and physical properties. Once the solution has been defined, its composition and physical properties must be maintained within prescribed limits for achieving optimum operation. 

Interest has continued in on-site manufacture of hydrogen peroxide from the elements, particularly for remote sites located considerable distances from wodd-scale anthraquinone processes. However, no commercial-scale direct combination plants have been constmcted as of this writing. 

High pressure Hquid chromatography (qv) (138) and coulometry can be used to detect and quantify anthraquinones and thek derivatives in a hydrogen peroxide process working solution. 

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. 

Takahax A variation of the Stretford process for removing hydrogen sulfide from gas streams, in which naphthaquinone sulfonic acid is used in place of anthraquinone disulfonic acid. Four variants have been devised types A and B use ammonia as the alkali, types C and D use sodium hydroxide or carbonate. Developed by the Tokyo Gas Company and licensed in the United States by Ford Baken and Davis, Dallas, TX. Many plants are operating in Japan. 

At present, its synthesis requires a sequential hydrogenation and subsequent oxidation of alkyl anthraquinone [248]. One of the main problems of this process is the high cost of the quinine solvent and the need for continuous anthraquinone replacement. The manufacturing process is also hindered by frequent storage and transport problems. In view of these obstacles, the development of a new synthetic method is of great interest from a commercial standpoint. 

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. 

The quantum yield of the primary hydrogen abstraction process is unity, and is independent of irradiation wavelength, light intensity and temperature. Its wavelength range follows of course the absorption spectrum of anthraquinone so that this actinometer is particularly well suited for the UV region. 

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... 

The hydrogenation step in the anthraquinone process of AKZO-Nobel is an industrial realization of a monolithic reactor and includes a lot of pioneering work from the Anderson group (59-63). More examples of the use of monoliths can be found in Refs. 5 and 64. 

Compound 18 showed a remarkable color change from orange to brown (Amax=670 nm) in DMSO upon adding F". Color changes are most probably due to a charge-transfer process and electron-rich formation of hydrogen bonds between thiourea-bound F and the electron-deficient anthraquinone moiety. The anion was believed to form a 2 1 anion-to-ligand ratio as shown in Fig. 4. 

---