Grandjean Didier, Beale Andrew M, Petukhov Andrei V, Weckhuysen Bert M
Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands.
J Am Chem Soc. 2005 Oct 19;127(41):14454-65. doi: 10.1021/ja054014m.
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 Co(2+) 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 Co(2+) into the poorly ordered Al-O-P network up to ca. 150 degrees C, and (ii) a rapid transformation of remaining octahedral Co(2+) at the onset of crystallization. Co(2+) 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.
采用时间分辨原位小角X射线散射/广角X射线散射、紫外可见光谱、拉曼光谱和X射线吸收光谱研究了CoAPO-5分子筛的水热结晶过程。在加热至180℃的过程中收集的数据，使得我们能够从宏观和原子层面观察到无定形凝胶转变为晶体材料过程中发生的不同步骤。拉曼光谱检测到Al-O-P键的初始形成，而小角X射线散射表明，在结晶开始前，这些凝胶颗粒具有约7至20nm的宽尺寸分布。广角X射线散射表明，这种结晶过程很尖锐，发生在约160℃。结晶动力学分析表明这是一个一维生长过程。X射线吸收光谱表明，Co(2+)在加热过程中通过两步过程发生转变：(i) 在对应于AFI的布拉格峰出现之前，八面体配位逐渐转变为四面体配位，这表明Co(2+)逐渐掺入到无序的Al-O-P网络中，直至约150℃；(ii) 在结晶开始时，剩余的八面体Co(2+)迅速转变。观察到Co(2+)会延缓AFI的结晶，但通过作为内部探针，提供了有关合成过程的有价值信息。提出了一种三阶段一维结晶机理：(i) 铝和磷酸盐单元之间的初始反应形成初级无定形相；(ii) 线性Al-O-P链逐渐缩合，形成由模板分子隔开的无序结构，直至约155℃；(iii) 磷酸铝网络的快速内部重组导致AFI晶体结构的结晶。
