THE EFFECTS OF THE SILICA SOURCE ON THE CRYSTALLIZATION OF ZEOLITE NAX

The effects of varying the starting silica source on the synthesis of molecular sieve zeolite NaX were investigated, while all other reactio n parameters were kept constant. The silica sources were all powders o f varying types and purity. Characterizations of the silica sources, t he silicate solutions, and the synthesis products were completed by po wder X-ray diffraction, electron microscopy, particle-size analysis, n .m.r. spectroscopy, laser light scattering, and elemental analysis. Th e use of different silica sources significantly influenced the outcome of the synthesis experiments. There were large differences in the res ults from the various batch synthesis mixtures, even though the batch compositions were all the same (neglecting impurities). The experiment s, all conducted at 115 degrees C in sealed 6 ml autoclaves, yielded p roducts of different particle sizes, different amounts of impurity zeo lite phases, and different conversion rates to zeolite NaX. The n.m.r. spectra of the dissolved silica sources were all effectively the same ; however, the levels of trace impurities were very different. The ext ent to which nuclei formed in these experiments was shown to correlate to the impurity level of any one of several elements in the silica so urces, but not to the turbidity of the filtered solutions as noted by a simple light-scattering measurement.