This
work involves methanol-to-propylene (MTP) conversion over
aluminosilicate MFI (ZSM-5), borosilicate MFI (B-ZSM-5), and all-silica
MFI (Silicate-1). Both B-ZSM-5 and Silicate-1 were inactive in MTP
reaction. However, extruded Silicate-1 sample prepared by extrusion
with Al2O3 binder, followed by acid washing
treatment, displayed a rapid increase in methanol conversion in the
initial 30 h time on stream (TOS), and then retained 99% methanol
conversion, higher propylene selectivity (52.2%), and higher propylene/ethylene
ratio (11.3) for 400 h TOS. Silicate-1 modified with AlCl3 or Al(NO3)3 showed similar catalytic performance
as the extruded samples. Several AlCl3-modified Silicate-1
samples after various TOS values were regenerated and characterized
by NH3-termperature-programmed desorption (TPD), Fourier
transform infrared (FT-IR) spectroscopy, and 27Al magic
angle spinning nuclear magnetic responance (MAS NMR). The results
point to continuous aluminum insertion into Silicate-1 framework during
reaction in situ, which rationalizes the superior catalytic performance.
By analyzing catalytic performances of AlCl3-modified samples
with different amounts of defect sites, we concluded that Al migration
is related to the defect sites. Finally, a catalyst with a much longer
lifetime of 960 h and 53.2% propylene selectivity was developed by
incorporating Al migration into hierarchical samples.