Role of dehydration catalyst acid properties on one-step DME synthesis over physical mixtures

The direct synthesis of dimethyl ether (DME) was studied in a continuous high-pressure unit composed basically of a Berty reactor and on-line gas chromatograph. A commercial methanol synthesis catalyst and some solid-acid catalysts (alumina, HZSM-5, tungsten–zirconia and sulfated-zirconia) were used as physical mixtures. The dehydration catalysts were characterised by pyridine adsorption followed by IR spectroscopy and tested in the methanol dehydration reaction itself. All samples were active regardless of the differences in their acidity and a relationship between catalytic activity for methanol-to-DME and acidity could be envisaged; the activity of a solid acid on this reaction was found to be determined mainly by the number of its more acidic sites. On the one-step DME synthesis from syngas the addition of an acid catalyst to the catalytic system strongly shifted the methanol synthesis reaction, increasing significantly the pass conversion of CO. As a general rule, it could be concluded that the determining rate of DME direct synthesis is determined by the acid properties of the dehydrating catalyst, i.e., its acid strength and number of acid sites.