Role of Shape Selectivity and Catalyst Acidity in the Transformation of Chloromethane into Light Olefins

The transformation of chloromethane into light olefins (C2–C4) has been studied both on HZSM-5 catalysts with different SiO2/Al2O3 ratios (30, 80, and 280) and on SAPO-n catalysts (SAPO-34 and SAPO-18) in order to analyze the role of shape selectivity and acidity in the kinetic behavior. The catalysts have been prepared by agglomerating these acid functions with bentonite and α-Al2O3, and the kinetic runs have been performed in a fixed bed reactor under the following operating conditions: 350 and 450 °C; space time, 2.35, 5.89, and 14.99 gcat h (molCH2)−1; and time on stream, 255 min. A comparison of the reaction indices (conversion of chloromethane, selectivity to light olefins, and propylene fraction) at zero time and throughout time on stream using the different catalysts has allowed establishing, on the one hand, the significance of the shape selectivity and acidity of these catalysts (which are more influential in this reaction than in the transformation of methanol), and on the other, the need for a compromise between these properties. A HZSM-5 zeolite catalyst with moderate acidity (SiO2/Al2O3 = 80) has a good kinetic behavior at 350 °C and recovers its activity by coke combustion with air. However, all the catalysts studied undergo irreversible deactivation at 450 °C by dealumination of the acid function to form AlCl3.