Effects of Acidities of H\|ZSM\|5 Zeolites on Dimethyl Ether Synthesis from Syngas over Composite Catalysts

The H\|ZSM\|5 zeolites as dehydration component of composite catalyst for dimethyl ether (DME) synthesis were treated by steam at different temperatures, and the effects of different acid sites of H\|ZSM\|5 zeolites on DME formation have been investigated. The profiles of NH 3\|TPD show that there were two desorption peaks of a mmonia over H\|ZSM\|5 zeolites. The low\|temperature desorption peak is usually acribed to the weak acidic OH group, and the high\|temperature one to the strong Brnsted\|type OH group. CO 2\|TPD also gives two different basic sites. The amount of acid sites of H\|ZSM\|5 zeolites , especially that of strong acid sites decreased more drastically with the increasing hydrothermal treatment temperature. The experimental results indicate that DME was formed mainly on the weak acidic site of H\|ZSM\|5 zeolite. The H\|ZSM\|5 zeolites treated by steam at 600 ℃ exhibited high activity for DME formation. When it was used as dehydration component, CO conversion is more than 90%, selectivities to DME and MeOH are higher than 96%, DME selectivity to all organic products is more than 80%, and the content of DME in products is more than 20%(%) under the reaction conditions: H 2/CO≈2, 4~6 MPa, 2 000~3 000 h -1 , 260 ℃. Enhancement of reaction pressure increases the CO conversion and DME content in products, but has no significant effect on thd selectivity to DME.