Activation of hydrogen over sulfate-promoted iron oxide

The mechanism for activation of hydrogen over sulfate-promoted iron oxide has been investigated by means of hydrogen TPD and FTIR spectrometry of adsorbed pyridine. The catalyst activated isomerization of 1-butene to 2-butene and hydrogenation of 1,3-butadiene to form 1-butene, trans- and cis-2-butene, and small amounts of n-butane, but the isomerization activity decreased when the catalyst was prereduced and disappeared when the catalyst was reduced at 723 K although the hydrogenation activity still remained. In the section of the isomerization of 1-butene when the reaction was carried out in the absence of hydrogen, it is clear that the sulfate itself in the catalyst catalyzed the isomerization of 1-butene in the absence of hydrogen. However, in the presence of hydrogen, the bifunctional mechanism was observed. It should be, therefore, considered that the isomerization was promoted by the protonic acids formed on the Fe-sulfate catalyst by the dissociation of hydrogen molecules. In TPD experiments, the hydrogen TPD spectra appeared in a wide range of temperatures between 373 and 623 K or above. In FTIR spectrometry of the adsorbed pyridine on the catalyst, an increase in intensity of protonic acid sites as well as a decrease of Lewis acid sites was observed by heating the pyridine-covered catalyst in the presence of hydrogen at 423 K or over. This indicates the conversion of hydrogen molecules into protonic acids. Also, we tried to find hydride-like species on the surface of the catalyst but could not find them. In conclusion, the presence of sulfate species in the iron oxide catalyst is essential for the activation of hydrogen, promoting the bifunctional activity for the hydrogenation and the isomerization of olefins. The overall expression for the mechanism of the activation of hydrogen over the Fe-sulfate catalyst is explained by the following equation (Fe2+ + H-2 = Fe-0 + 2H(+)) followed by another equation ([Fe2+ - O2-] + H+ + H- = Fe+ - H + H+ - O-).