Hierarchical zeolites for dimethyl ether dehydration into light olefins

Dimethyl ether (DME) conversion into light olefins (DTO) is a process that can lead to the sustainable production of molecules like ethylene, propylene, and butenes, which are key building blocks in the chemical industry. Acid catalysts involved in the conversion of methanol and DME into hydrocarbons (MTO and DTO) are usually affected by fast deactivation due to coke generation. In the present study, four hierarchical zeolites were synthesized with different Si/Al ratios according to two procedures: a post-treatment (named “etching”) of a HZSM-5 microporous zeolite with a solution of ammonium fluoride and hydrofluoric acid, and a one-pot bottom-down approach involving an organosilane to induce mesoporosity during the hydrothermal synthesis. The samples were characterized from a physico-chemical standpoint to assess crystallinity, textural properties, and acidity. All the synthesized zeolites were then tested for about 14 h in the DTO process at a temperature range of 300–375 °C. Results showed that hierarchical zeolites with mild acidity have a very good stability, even when they are tested at the highest temperature, at which conventional microporous samples deactivate quickly. Conversion of 80–90 % is achieved at 375 °C and a space velocity of 1 gcat∙h∙molC−1, with propylene as the most abundant product. Samples prepared via one-pot synthesis resulted in a greater propylene-to-ethylene ratio, partly due to shape selectivity related to the pore size distribution.