Characterization and hydroisomerization performance of SAPO-11 molecular sieves synthesized in different media

摘要

Silicoaluminophosphates SAPO-11 molecular sieves were synthesized from a conventional aqueous medium,a two-liquid phase medium and a HF medium,respectively,and were characterized by various methods such as XRD,SEM,pyridine-FT-IR,NH3-TPD,29Si MAS-NMR.The morphologies of the SAPO-11 molecular sieves obtained from the conventional aqueous medium and the two liquid-phase media exhibit pseudo-spherical aggregates ranging from 7 to 10 mu m assembled from cubic plate microcrystallites,whereas the SAPO-11 molecular sieves synthesized from the HF medium exhibit hexagonal prismatic aggregates of about 1-3 mu m length.The synthesis media used highly influenced the number and the strength of SAPO-11 acid sites.Compared to SAPO-11 obtained from conventional aqueous medium,SAPO-11 prepared from two-liquid phase media possessed a similar number of Br0nsted acid sites but a much larger number of Lewis acid sites,whereas the SAPO-11 synthesized from HF medium had a similar number of Lewis acid sites but a much lower number of Br0nsted acid sites.The 29Si MAS-NMR spectra showed that the HF could change the manner of Si incorporation into the S APO-11 framework.The hydroisomerization of n-hexadecane on Pt/S APO-11 samples showed that Pt/S APO-11 obtained from the two-liquid phase media had the largest yield of isomers.The conversion activity of n-hexadecane was dependent on the amount of BrOnsted acid of S APO-11 molecular sieves,while its hydrocracking activity was related to the strong acid sites of SAPO-11.