Neat dimethyl ether conversion to olefins (DTO) over HZSM-5: Effect of SiO2/Al2O3 on porosity, surface chemistry, and reactivity

Abstract

This study investigates the unique effect of the SiO2/Al2O3 ratio (30, 80, and 280) on the physicochemical and reactivity properties of the HZSM-5. This catalyst can be used for light olefin production using neat dimethyl ether (DTO). Crystallinity and porosity of the HZSM-5 samples as measured by XRD and N2 isotherms, show minimal effect of the SiO2/Al2O3 ratio. Acidity measurements by NH3-TPD, on the other hand, exhibit weak and strong acid sites, with both their ratio and total acidity being reduced with increasing SiO2/Al2O3. Experiments in the Berty reactor allow analyzing the changes of dimethyl ether (DME) conversion, light olefin selectivity and catalyst stability with SiO2/Al2O3 ratio. Moreover, these studies show that from the three HZSM-5 catalysts considered, the one with the SiO2/Al2O3=280 gives the highest light olefin selectivity. A maximum performance of 59% DME conversion at 1.16% coke load, and 50% light olefins selectivity (5% ethylene, 21% propylene, 24% butane) are observed at 450°C. In addition, this thermal level leads not only to find a promising catalyst performance for the SiO2/Al2O3=280 ZSM5 catalyst but to establish also its unchanged activity with time-on-stream. On the basis of the experimental data obtained it is postulated that an “in series” reaction network is adequate to describe the chemical changes with formed olefins undergoing eventually further transformation to paraffins and heavier aromatic hydrocarbons.