Production of Light Olefins from Methanol and Ethanol Using ZSM-5 Type Zeolite Catalysts

Various ZSM-5 type zeolites containing alkaline earth metals (AE-ZSM-5) were synthesized and catalytic performances in the conversion of methanol to light olefins were investigated. The AE-ZSM-5 showed increased selectivity for light olefins at temperatures above 500 degrees C. High (C2H4 + C3H6) yields (>50 C%) were obtained over Mg-ZSM-5, Ca-ZSM-5, and Sr-ZSM-5 with SiO2/Al2O3 ratios of 200-800, 100-800, and 80-300, respectively. Maximum (C2H4+ G(3)H(6)) yield of ca. 65 C% was achieved over Ca-ZSM-5 with SiO2/Al2O3 ratios of 100-300. The strong acid sites of ZSM-5 became weakly acidic after incorporation of alkaline earth metals, which suppressed coking and dealumination, resulting in improved catalyst lifetime. However, part of the incorporated alkaline earth metals underwent leaching from the weak acid sites due to H2O and CO2 produced during the reaction, which resulted in the regeneration of the strong acid sites of the zeolite. These strong acid sites promote the formation of carbonaceous deposits that are ultimately responsible for the deactivation of the zeolite catalysts. Investigation of the stabilization of weak acid sites found that the zeolite catalyst lifespan could be greatly extended by modification with alkaline earth metal carbonates. SrCO3/Sr:ZSM-5 (SiO2/Al2O3 = 100) maintained a (C2H4 + C3H6) yield of more than 50 C% for 2000 h at 550 degrees C with only three regeneration cycles. The high catalytic performance of CaCO3/Ca-ZSM-5 was evaluated by a bench-scale test. Total C2H4 and C3H6 production was 11.52 kg from a methanol feed of 42.347 kg after 266 h. The conversion of ethanol to light olefins over various ZSM-5 zeolites with different acidity, especially zeolites with isomorphous framework-substitution, was also investigated. The highest propylene yield of ca. 29 C% was obtained over co-modified La/P/ZSM-5(Ga) with P/Ga and La/Ga ratios of 1 and 0.4, respectively. The introduced lanthanum reacts with the pre-introduced phosphorus to regenerate some of the Bronsted acid sites (Si(OH)Ga).