Studies of the Molybdenum Catalysts. Part I. Changes in the State of the Hydrated Molybdenum Oxide Catalyst during a Reaction between Thiophene and Hydrogen

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Abstract
The reaction between thiophene and hydrogen on hydrated molybdenum oxide was studied. Structural and compositional changes in the catalyst were found during the hydrodesulfurization of thiophene. The main products of the reaction were n-butane, butenes, and hydrogen sulfide. The conversion of thiophene increased with the progress of the reaction and reached a steady value after passing through a maximum. This reaction process was distinguished into two stages. One is the “aging stage,” and the other is the “stationary stage.” At the aging stage, three reactions, that is: i) dehydration, ii) reduction, and iii) sulfurization occur. The catalyst reacts with the reactants and changes in structure and composition. Hydrated molybdenum oxide was converted to MoO3 by liberating the structural water, and MoO3 was reduced to MoO2. A part of the MoO3 was also sulfurized directly with thiophene to MoS2 during the aging stage, but MoO2 could not easily be sulfurized. At the stationary stage, no further change in the catalyst was found, and only the catalytic hydrodesulfurization of thiophene occurred. The conversion at the stationary stage, when the aging temperature was low, was larger than the conversion at a high temperature. The difference between the conversions at different reaction temperatures was assumed to result from the structural and compositional changes in the catalyst caused by the different temperatures of the aging processes. In this reaction, the dehydration at the aging stage controls the activity of the catalyst.

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