The Institute for Materials and Chemical Processes of the National Institute of Advanced Industrial Science and Technology (AIST), in the framework of NEDO international government and private-sector joint research with Maruzen Petrochemical (Maruzen) and the NOK Corporation (NOK), developed a new method for synthesizing phenol that consolidates the conventional three-step reaction process into just one step.
The unique feature of the new method is its use of a small-diameter tube coated with a thin palladium layer located within the reaction vessel. The method uses much less energy and solvents, and produces far less waste than the conventional process. Furthermore, it represents a step away from massive chemical plants toward much smaller desktop models. Applicable to more than just phenol synthesis, the method can be effectively used in adding hydroxyl groups to specialized chemicals and other organics as detailed in Science (Vol.295, p.105, 4 January 2002).

The conventional indirect, three-step method of phenol synthesis is consolidated into a one-step process.
Most phenol today is synthesized under an indirect three-step process (called the cumene method) in which benzene and propylene are first reacted at 200 &Mac251;C to 250 &Mac251;C to yield cumene (step one), which is oxidized with oxygen at 80 &Mac251;C to 130 &Mac251;C to generate cumene hydroperoxide (step two). This substance is then decomposed with sulfuric acid to obtain phenol along with acetone (step three). Step one calls for the use of a large quantity of benzene, much of which must be recovered and reused. As the main product produced in step two is an explosive peroxide, only 25% of the cumene is allowed to react in order to keep the concentration of the former low. The unreacted cumene must consequently be recovered and reused. Step three yields equal quantities of phenol and acetone through decomposition by sulfuric acid, but the demand for the former far outpaces that for the latter, so phenol manufacturers are left with the task of finding ways to dispose or otherwise get rid of massive amounts of acetone. In addition, the large volumes of solvents used as diluent for decomposition and the various trace byproducts generated in step three make purifying the phenol difficult. In contrast, the new method introduces oxygen to benzene to directly produce phenol in one step, bypassing the problems of the conventional method and using far less energy.
The new method employs within the reaction vessel a porous, small-diameter tube made of alumina and coated with a palladium layer. Hydrogen is passed over either the inner or outer surface of these tubes (the inner surface in the example shown) at 150 &Mac251;C to 250 &Mac251;C to obtain phenol. (See accompanying figure.) Oxygen is introduced to the benzene ring when hydrogen adsorbed onto the surface of the palladium layer attaches and becomes activated, permeates through to the other side, and joins with an oxygen molecule, at which time activated oxygen is released to double bond with the benzene ring. (Benzene epoxide first forms and then isomerizes into phenol.)

Experts have long known that palladium adsorbs and activates hydrogen, which then permeates through the palladium. Knowing that this property of palladium could be easily used to purify, detach, and attach hydrogen, researchers studied the metal at great lengths. However, no one had used a hydrogen-permeable palladium layer to catalyze oxidation, which makes the use of the layer in this method all the more revolutionary.

As noted above, the conventional method requires the unreacted reactants (benzene and cumene) to be recovered for subsequent use. In addition, the products must be purified by distillation, and the acetophenone and a-methylstyrene that appear as byproducts must be collected, reacted, and reused. This complicated manufacturing process consumes huge amounts of energy. Under the new method, however, the reaction takes place in the gas phase. Phenol is produced directly from benzene. The large disparity in the melting and boiling points of benzene (which melts at 5.5 &Mac251;C and boils at 80 &Mac251;C) and phenol (which melts at 41 &Mac251;C and boils at 182 &Mac251;C) means the phenol can be easily purified without the complex procedures and equipment associated with the separation and distillation in the conventional method.

The method does not require distillation or large equipment, and it can be used to produce larger batches through the addition of more tubes, which are 2 mm in diameter, 30 cm long, and coated with a 10-cm-long palladium layer. The method could be used to make 100,000 metric tons of phenol per year on a desktop, whereas a conventional facility with a similar capacity would be massive.

The new method can be used to add oxygen to naphthalene, pyridine, and other organic substances in addition to benzene. Of course, it can be used to produce phenol, cresol, and other such heavily used substances, but this very valuable technology can also be adopted to make naphthol and other specialized chemicals used in the production of dyes, Vitamin K3, drugs, and agricultural products.

In order to make ultra-small synthesizing facilities a reality, the method will be fine-tuned to make a wide variety of specialized chemicals, and modular reaction vessels containing large bundles of tubes will be developed.