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Direct dimethyl ether (DME) synthesis from synthesis gas is studied with regard to potential effects of methanol dehydration on methanol formation and copperbased catalyst performance. For this, the influence of the operating conditions (space velocity, temperature, pressure, time-on-stream and syngas composition) on activity, selectivity and stability of the catalyst was studied and compared for methanol synthesis and direct DME synthesis. The advantage of the direct over the two-step DME synthesis is apparent at conditions where syngas conversion to methanol is thermodynamically limited. However, under the applied operating conditions, results suggest that combining methanol synthesis and dehydration has a negative effect on the methanol formation kinetics. The origin of the observed phenomena is investigated by varying dehydration catalyst and by introducing dehydration products (DME and water) into the methanol synthesis feed. Choice of the solid acid catalyst does not seem to affect methanol formation, and DME is also found to be practically inert over the methanol synthesis catalysts. Water injection, on the other hand, led to a significant decrease in the methanol synthesis rate. Thus, formation of an additional amount of water through methanol dehydration might be an explanation for the lower methanol formation rate in the direct DME synthesis.

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This paper addresses the effect of catalyst particle size distributions on Fischer−Tropsch catalytic performance for cobalt catalysts promoted by rhenium on alumina or modified alumina as catalyst support. The catalyst particle size will affect diffusion of reactants as well as of products. Some key findings are that the C5+ selectivity is rather constant for particle sizes up to ca. 400 μm and that the selectivity reduction thereafter is largely explained by an increase in the methane yield as the effective H2/CO ratio increases. In the whole range of particle sizes investigated, the olefin to paraffin ratio of the light products decreases linearly as the diffusion length becomes longer. Moreover, this ratio is the same for all catalysts in spite of a large variation in selectivity to higher hydrocarbons. It follows that readsorption of olefins and continued chain growth only plays a minor role. The roles of added water and conversion level are also discussed.