Characterization and hydroisomerization performance of SAPO-11 molecular sieves synthesized by dry gel conversion

Abstract

SAPO-11 molecular sieves were synthesized by two types of dry gel conversion (DGC) methods, namely, steam-assisted conversion (SAC) and vapor phase transport (VPT), and by conventional hydrothermal (CHT) method. The properties of these samples were characterized by XRD, XRF, SEM, N 2-adsorption, Pyridine-IR and 29Si MAS NMR methods. The results show that di- n-propylamine (DPA) is a suitable structure-directing agent (SDA) for the preparation of impurity-free SAPO-11. SAPO-11 molecular sieves synthesized by SAC and VPT exhibit higher crystallinity, higher Si content and more strongly acidic sites than that synthesized by CHT. 29Si MAS NMR results demonstrated that the two samples obtained by the DGC methods contain more Si(nAl)(0 < n < 4) species than that obtained by CHT. These facts suggest that the SAC and VPT methods contribute either to better Si incorporation into the AlPO 4 framework or to better Si dispersal, thus decreasing the size of Si islands and increasing the number and strength of acidic sites. The results from the hydroisomerization of n-dodecane indicate that the Pt/SAPO-11 synthesized by SAC possesses the highest hydroconversion activity and isomer yield among those three catalysts, due to the high acidity of the molecular sieve supporter.