Abstract:

The crystallization process of SAPO-11 was studied using a combination of X-ray diffraction, scanning electron microscopy, X-ray fluorescence, nuclear magnetic resonance, and X-ray photoelectron spectroscopy. In the initial stage of crystallization, SAPO-11 was formed along with an unknown crystalline material composed of Si-P-Al. As crystallization evolved, the crystalline material dissolved. The SAPO-11 formation rate increased greatly, which is characteristic of fast crystallization. After 2.33 h, the relative crystallinity of SAPO-11 reached ~100% and remained at a high level until crystallization was complete. Si was incorporated into the SAPO-11 framework from the initial stage of crystallization. The Si content of the solid samples increased with crystallization time. Most of the Si atoms existed as Si islands in the SAPO-11 framework, resulting in the presence of multiple coordination environments, i.e., Si(nAl, (4-n)Si), n=0-4. X-ray photoelectron spectroscopy analysis revealed Si enrichment on the external surfaces of the SAPO-11 crystals. Based on the experimental results, the distribution of Si in the crystals is not uniform, showing an increasing trend from the core to the surface.

Key words: SAPO-11 molecular sieve, Characterization, Crystallization process, Silicon distribution