Synthesis Optimization of SAPO-34 in the Presence of Mixed Template for MTO Process

Abstract

A series of SAPO-34 materials were synthesized hydrothermally in the presence of mixed templating agent (named TEDE) of tetraethyl ammonium hydroxide (TEAOH) and diethylamine (DEA). The effects of template amount, SiO2/Al2O3 and P2O5/Al2O3 molar ratios and the crystallization temperature on the structure and acidity were studied by XRD, nitrogen adsorption and NH3-TPD techniques. The results showed that SAPO-34 and SAPO-5 are competing phases at low template concentration. Pure SAPO-34 with high crystallinity and large BET surface area were obtained at a TEDE/Al2O3 ratio of 2.0. Varying the SiO2/Al2O3 ratio at 0.1~1.0 did not change the purity of SAPO-34, but affected its crystallinity and acidity. An optimal synthesis procedure giving a homogeneous SAPO-34 framework with high crystallinity, consists of heating a gel of composition Al2O3:P2O5:0.6SiO2:1.0TEAOH:1.0DEA:53H2O at 100 °C for 24h firstly and 200 °C for 72h later. Such a reproducible procedure gave rise to an acidic catalyst active in the MTO reactions with 100% methanol conversion, 85.2% ethylene and propylene selectivity and a longest lifetime.