¹Kushkarina S.M., ²Shaimardan M.

¹A.B.Bekturov Institute of chemical sciences Almaty, Kazakhstan

²Kazakh- British Technical University, Almaty, Kazakhstan

The effect of promoter and activated carbon carriers on rhodium catalyst in the process of hydrogenation of benzene

Development of new active and selective catalysts of removing a small amount of benzene from gasoline is an issue of current importance, as during incomplete combustion of benzene containing gasoline in exhaust gases very strong carcinogen, namely benzopyrene is formed. According to existing Euro-2 standards the amounts of benzene in gasoline must not exceed 5% and Euro-3 restricts the amounts of benzene up to 1%. The most effective way of removing benzene from gasoline is its catalytic hydrogenation into more ecologically safe cyclohexane. The present report discusses rhodium catalyst supported on sibunit (Siberian activated carbon) modified with molybdenum salt and its influence on the reaction of hydrogenation of benzene and toluene. In the presence of rhodium catalyst [2] the factors, like reaction medium, type of carrier, temperature, and hydrogen pressure can greatly effect on the process of hydrogenation of benzene . In addition to these, modification of catalyst can change the selectivity and activity of benzene hydrogenation.

Rhodium is the most active catalyst for hydrogenation of benzene [1, 2]. It has been observed that modifier reduces the reaction rate of hydrogenation of benzene and has no effect on the rate of hydrogenation of toluene. 3%Rh/sibunit catalyst were investigated by scanning electron microscopy. The effect of molybdenum on the catalyst surface has been discussed and compared with experimental data.

3%Rh/sibunit catalyst has been prepared by impregnation method and modified with ammonium molybdate, further 3%Rh/sibunit+0,03%Mo has been used for the reaction of hydrogenation of benzene and toluene. Activated carbon carrier was heated in quartz furnace during four hours in nitrogen atmosphere at the temperature of 400°C. Modification of catalyst was also prepared by impregnation. Hydrogenation of benzene and toluene has been carried out in the kinetic hydrogenation autoclave at the temperature of 40°C and hydrogen pressure 40atm.

Early it has been determined that the rate of hydrogenation of xylene and cumene is lower than that of benzene and toluene [2]. That is why in this experiment the selectivity of benzene hydrogenation has been compared to the selectivity of toluene. Picture 1 shows hydrogenation rates of benzene and toluene catalyzed by modified and non- modified catalysts.

Picture 1. Hydrogenation of benzene and toluene on 3%Rh/sibunit and 3%Rh/sibunit+0,03%Mo catalysts.

It can be seen that initial hydrogenation rate of benzene on 3%Rh/sibunit catalyst is 110ml/min and 3%Rh/sibunit+0,03%Mo is lower, it is 70ml/min. And hydrogenation rate of toluene on 3%Rh/sibunit and 3%Rh/sibunit+0.03%Mo is almost the same. In early publications [3] it has been observed that molybdenum salts block the surface of rhodium and activated carbon, so it will cause resistance for adsorption of hydrogen. The lower hydrogenation rate of benzene on modified catalyst can be explained with the same reason.

Picture 2 illustrates how rhodium salt has been equally covered on a surface of catalyst. After modification of catalyst the elements distribution on the catalyst surface is different.

Picture 3. Elements assignment in atomic and mass percentage.

As sibunit is graphite structured activated carbon, its pour size is almost the same, so molybdenum oxide covers surface area of rhodium and activated carbon thereby worsening catalyst activity. Compared to fruit- stone activated carbon that possesses with different pore size, molybdenum oxides cover only the biggest pore and the smallest pore can be adsorbed by benzene.

Reference

1. Сокольский Д.В. Гидрирование в растворах. КазССР. – Алматы: Наука, 1979, 364с.

2. Конуспаев С.Р. Шаймардан М. // Хим. журнал Казахстана, 2006, № 1. С.154-174.

3. Reyes P., Fernández J., Concha I., Pecchi G., Granados M. L., Fierro J. L. G. // Catal. Lett. 1995. – 34, №3 – 4. C.331 – 341.