Preparation and characterization of the silicoaluminophosphate SAPO-39

Abstract

SAPO-39, a silicon-containing aluminophosphate molecular sieve analogous to AlPO₄-39 and MAPO-39 (ATN topology, 4.0-Å wide pore opening and tetragonal symmetry), was synthesized as a pure phase by hydrothermal crystallization from a gel with molar composition: 1.3 di-n- propylamine: 0.3 SiO₂: 1.0 Al₂O₃: 1.0 P₂O₅: 50 H₂O at 200°C in 48 h. Investigations on the crystal structure, morphology, thermal conversion, adsorption, acidity, and catalytic properties were carried out using XRD, SEM, t.g., FT i.r. methods and methanol-to-olefins conversion (MTO) reaction. The product of maximum crystallinity was obtained in 48 h, beyond which a new phase started forming and after around 56 h a mixture of SAPO-31 and SAPO-41 was produced. The crystals of SAPO-39 were found to be irregular aggregates of thin and elongated stick-like particles. The nature of the surface hydroxyl groups in SAPO-39 was studied by FT i.r. spectroscopy of adsorbed NH₃. The studies reveal the presence of silicon in the framework. Further, the conversion of methanol to hydrocarbons gives a selectivity for C₂ and C₃ hydrocarbons of about 43%, which corroborateøs the presence of considerable Brønsted acidity in the synthesized SAPO-39. The morphology, thermal decomposition, sorption properties, acidity, and catalytic properties have been compared with those of MAPO-39 and AlPO₄-39.