Catalyst application Activated Carbon div. Japan EnviroChemicals, Ltd.

Catalysts are necessary for the organic synthesis and the chemical industry. Various kinds of catalysts are developed according to the type of reactions. They can be categorized into two groups by state. One is a homogeneous catalyst, which is used in the solution state. The other is a heterogeneous catalyst, which is used in the solid state.

In a heterogeneous catalyst, it is easy to recover product materials and to maintain the performance of catalyst. Therefore, it is widely used for mass production of basic chemical substances in the chemical industry. As a heterogeneous catalyst, simple substances such as platinum, palladium, and ferric oxide, inorganic compounds with complicated structures like zeolite, and immobilized metallic complexes are used.

A heterogeneous catalyst consists of main catalyst, catalyst support, and sub-catalyst. The optimal catalyst is prepared by combination of these three items according to the type of reactions. Activated carbons are frequently used as catalyst supports because of their outstanding performance. Also, activated carbons are used as catalysts themselves, since they are catalytically active due to their unique surface characteristic and surface oxide.

The main purpose to use a support is to increase the effective surface area of the catalyst. Also, it is used to improve aeration, heat resistance, and anti-poisoning. The summaries are explained below.

Increase of activity

In a heterogeneous catalyst, many reactions proceed on the surface of catalyst. To increase the catalyst efficiency, therefore, it is essential to make the surface area larger. When precious metals like platinum, palladium, and so on are used as catalysts, they are pulverized from 1 to 100nm to be distributed and carried on the porous supports with a pore structure and a large surface area. This enlarges the surface area for reaction, and increases the catalyst activity per unit weight.

Improvement of aeration

When catalytically active substances are uniformly distributed on the surface of support, the aeration is improved. Therefore, the heat of reaction can be easily removed to prevent from local over heat

Improvement of heat resistance

The catalyst is generally used for high temperature reactions. When only catalytically active substances are used as a catalyst, the catalyst activity of the substances are deteriorated by sinter due to their thermal instability. In case that the substances are put on the supports, the heat resistance can be improved at a high temperature of 200 to 500 degree Celsius.

Improvement of anti-poisoning

In case that the reaction system contains the catalyst poisons that decrease the catalyst activity, it is better that catalytically active substances are distributed on the supports. This enables the carriers to adsorb or decompose the catalyst poisons. As a result, it is possible to protect the catalysts from poisoning.

Previously, porous minerals such as acid clay, diatomite, and pumice are used as supports. Since they are unstable in the quality and lack of reproducibility, however, each substance was replaced by silica-alumina, silica, and alumina, respectively.

The principal supports are activated carbon, alumina, silica, zeolite, titania, and magnesia. The feature of each support is shown in Table-1.

Originally, activated carbon has the catalyst activity due to its surface oxides and unique surface features. Therefore, it is used as a catalyst without general catalytic components such as platinum and palladium, and so forth. Typical reactions are shown in Table 2.

Activated carbon as catalysts or catalyst supports is used in the chemical reaction process in a variety of chemical industries. This fact diversifies requirements for the characteristics of activated carbon. Therefore, most of the activated carbon used for catalysts and catalyst supports are developed by repeating adjustments of properties based on the standard grade to meet requirements of each customer. Main requirements are about pores like surface area, impurities like sulfur and metal content, raw materials, and shapes. The most suitable product is decided by the combination of these factors.