Dehydration of methanol to dimethyl ether catalyzed by oxidized activated carbons with varying surface acidic character

摘要

Several different activated carbons were oxidized with H_2Ov (NH_4)_2S_20_8 and HNO_3 in order to introduce different oxygen surface complexes. The changes in the surface chemistry of the activated carbons after their oxidizing treatments were studied by different techniques such as temperature-programme desorption, X-ray photoelectron spectroscopy, titration with HCl and NaOH and measurements of the pH at the point of zero charge. The activity to produce dimethyl ether (DME) was related to the concentration of H+ ions at the point of zero charge of the samples, from which the turnover frequency of the reaction was obtained. Activated carbons oxidized with (NH_4)_2S_20_8 had the strongest acid groups and were the most active in the dehydration reaction of methanol to DME. The high activity of these samples was only due to a small fraction of carboxyl acid groups, i.e., those that were decomposed at the lower temperatures, at the start of the CO_2 desorption profiles.