Study of crystallization process of SAPO-11 molecular sieve

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 similar to 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. (C) 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.