Synthesis, characterization and catalytic performance of SAPO-34 molecular sieves for methanol-to-olefin (MTO) reaction

The methanol-to-olefin (MTO) process is important because it offers an alternative to the conventional oil route for the conversion of natural gas or coal to light olefins. The most promising catalysts for the MTO reaction are solid acids with a zeolite structure, where reactants and products such as ethylene and propylene may freely diffuse through active catalysts, but products with larger kinetic diameters are trapped within the cages. In this work, SAPO-34 molecular sieves were synthesized by hydrothermal crystallization, using triethylamine (TEA) as the template. These samples were characterized by X-ray diffraction, scanning electron microscopy, thermo-gravimetric analysis-differential scanning calorimetry, energy dispersive spectroscopy, Fourier transform-infrared (FT-IR) and N(2) adsorption-desorption. The effects of template concentration and crystallization time on the physicochemical properties and catalytic performance were investigated. The crystallinity and morphology of SAPO-34 were influenced by the concentration of the template. Pure SAPO-34 was obtained when the TEA/Al(2)O(3) molar ratio was higher than 2.0. With increased crystallization time, the Si content in SAPO-34 crystals increased, which influenced the surface acidity. FT-IR spectra indicated that all samples were dominated by Lewis acid sites and the sample crystallized for 11 h exhibited the lowest number of Lewis acid sites. The catalytic performance of SAPO-34 was tested with the MTO reaction. High selectivity to olefins (C(2)H(4) + C(3)H(6)) was obtained over the catalyst synthesized with a TEA/Al(2)O(3) ratio of 3.0 and crystallization time of 11 h. At the reaction temperature of 450 degrees C, the methanol conversion approached 100% and the yield of C(2)-C(4) olefins was more than 80%. Hence, SAPO-34 molecular sieves were shown to be excellent catalysts for the MTO reaction. (C) 2010 Curtin University of Technology and John Wiley & Sons, Ltd.