Effect of silica/alumina ratio and structure-directing agent on the physical and chemical properties of SAPO-34

This work describes how the silica/alumina ratio (SAR) affects the framework, texture, chemical composition, and acidity of SAPO-34 molecular sieves synthesized via hydrothermal crystallization using morpholine (MOR) or triethylamine (TEA) as structure-directing agents (SDA’s). The solids were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N 2 adsorption-desorption isotherms, thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FT-IR), X-ray fluorescence spectrometry (XRF), silicon-29 MAS NMR spectroscopy ( 29 Si MAS NMR), aluminum-27 MAS NMR spectroscopy ( 27 Al MAS NMR), and temperature-programmed desorption of ammonia (NH 3 -TPD). The solids synthesized in this work had crystalline phases, vibrational modes, and morphology characteristics of silicoaluminophosphate (SAPO) materials. Pure SAPO-34 was obtained only with morpholine, while SAPO-18/34 was crystallized with TEA. The SAPO-34 synthesized with MOR had the higher relative crystallinity and incorporation of Si into the framework. The SAPO-18/34 prepared with TEA had the larger surface areas. The solids prepared with SAR 0.3 had only Si(4Al) coordination, independently of the SDA used. The SAPO-34 synthesized with MOR and SAR 0.6 had the highest total acidity (9.75 μmol NH 3 m −2 ). SAPO molecular sieves were synthesized hydrothermally with different structure-directing agents and silica/alumina ratios. The effects of both variables on the physical and chemical properties of the samples obtained were evaluated. Pure SAPO-34 was obtained with morpholine, while SAPO-18/34 was crystallized with TEA. The SP34T-0.6 sample prepared with triethylamine and SAR 0.6 showed promising characteristics for use as catalyst in MTO reactions: AEI/CHA intergrowths, the lowest acidity, the smallest crystallite size, and a large BET surface area.