期刊
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
卷 127, 期 41, 页码 14454-14465出版社
AMER CHEMICAL SOC
DOI: 10.1021/ja054014m
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The hydrothermal crystallization of CoAPO-5 molecular sieves has been studied using time-resolved in-situ SAXS/WAXS, UV-vis, Raman, and XAS. Data collected during heating to 180 degrees C allowed the observation of different steps occurring during the transformation of the amorphous gel into a crystalline material from a macroscopic and atomic perspective. Raman spectroscopy detected the initial formation of Al-O-P bonds, whereas SAXS showed that these gel particles had a broad size distribution ranging from ca. 7 to 20 nm before crystallization began. WAXS showed that this crystallization was sharp and occurred at around 160 degrees C. Analysis of the crystallization kinetics suggested a one-dimensional growth process. XAS showed that Co2+ transformed via a two-stage process during heating involving (i) a gradual transformation of octahedral coordination into tetrahedral coordination before the appearance of Bragg peaks corresponding to AFI, suggesting progressive incorporation of Co2+ into the poorly ordered Al-O-P network up to ca. 150 degrees C, and (ii) a rapid transformation of remaining octahedral Co2+ at the onset of crystallization. Co2+ was observed to retard crystallization of AFI but provided valuable information regarding the synthesis process by acting as an internal probe. A three-stage, one-dimensional crystallization mechanism is proposed: (i) an initial reaction between aluminum and phosphate units forming a primary amorphous phase, (ii) progressive condensation of linear Al-O-P chains forming a poorly ordered structure separated by template molecules up to ca. 155 degrees C, and (iii) rapid internal reorganization of the aluminophosphate network leading to crystallization of the AFI crystal structure.
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