A COMPARISON OF SILICEOUS FAUJASITIC ZEOLITES PRODUCED BY DIRECT SYNTHESIS OR BY SECONDARY SYNTHESIS

Synthetic cubic faujasites (Si/Al = 4) are synthesized hydrothermally and compared with (i) materials of similar framework composition produced by secondary synthesis from zeolite Y using silicon hexafluoride and (ii) zeolite ZSM-20. Solid-state NMR (Si-29 and Al-27), XRD, FTIR, and surface analysis are used to characterize the materials, which are also evaluated as catalysts by using n-hexane conversion as a test reaction. Si-29 NMR results suggest that the materials produced by primary synthesis have aluminum ordering differing from that observed in materials with similar framework composition (Si/Al) but produced by secondary synthesis. The difference evident in the Si-29 NMR may account for the higher catalytic activity observed with the products of direct synthesis. Surface analysis using depth profiling shows that the siliceous frameworks produced by direct synthesis are more homogeneous than materials produced by secondary synthesis.