Time-resolved in-situ energy and angular dispersive X-ray diffraction studies of the formation of the microporous gallophosphate ULM-5 under hydrothermal conditions
Time-resolved in-situ energy and angular dispersive X-ray diffraction studies of the formation of the microporous gallophosphate ULM-5 under hydrothermal conditions
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DOI：
10.1021/ja982441c
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发表时间：
1999-02-10
影响因子：
15
通讯作者：
Férey, G
中科院分区：
文献类型：
--
作者：
Francis, RJ;O'Brien, S;Férey, G
使用时间分辨能量色散和角色散 X 射线衍射原位水热合成了大孔氧氟化没食子磷酸盐 ULM-5。研究了各种合成参数，如温度、试剂化学计量、酸味剂材料和 pH 值，并确定了它们对结晶的影响。发现所用磷源的性质（正磷酸或五氧化二磷）对反应途径具有深远的影响。使用正磷酸，ULM-5 被发现在短诱导期后非常迅速地形成。在等温水热条件下使用正磷酸对 ULM-5 结晶进行了动力学分析。将实验确定的反应程度 (α) 与时间数据与通过各种理论模型预测的数据进行比较表明，在较宽的温度和 pH 范围内，可以通过三维扩散控制过程对结晶进行建模。该过程发生的速率基本上与温度和 pH 值无关。相比之下，使用五氧化二磷，发现 ULM-5 的形成是通过形成两种不同的结晶中间相中的任一种来进行的，随后它们以强烈依赖于温度的速率反应形成 ULM-5 最终产物。形成的每个中间相的相对量主要取决于所使用的精确试剂化学计量。已经确定了 ULM-5 只能通过任一中间相形成的条件。中间相向产物相转变的机制似乎是直接的固-固转变，或者是通过中间微晶表面仅少量材料的溶解或非晶化。
The hydrothermal synthesis of the large-pore oxy-fluorinated gallophosphate ULM-5 has been followed in situ using time-resolved energy dispersive and angular dispersive X-ray diffraction. A variety of synthetic parameters such as temperature, reagent stoichiometry, sourer materials, and pH have been studied, and their effect on the crystallization determined. The nature of the phosphorus source used, either orthophosphoric acid or phosphorus pentoxide, is found to have a profound influence on the reaction pathway. Using orthophosphoric acid, ULM-5 is found to form very rapidly following a short induction period. A kinetic analysis of the crystallization of ULM-5 using orthophosphoric acid under isothermal hydrothermal conditions has been performed. Comparison of the experimentally determined extent of reaction (alpha) versus time data with those predicted by various theoretical models indicates that over a wide range of temperatures and pH the crystallization can be modeled by a three-dimensional diffusion-controlled process. This process occurs at a rate essentially independent of temperature and pH. In contrast, using phosphorus pentoxide, the formation of ULM-5 is found to proceed via the formation of either of two distinct crystalline intermediate phases, which subsequently react to form the ULM-5 final product at a rate which is strongly dependent on temperature. The relative quantities of each intermediate phase formed depend critically on the precise reagent stoichiometry used. Conditions have been identified in which ULM-5 can be formed exclusively via either intermediate phase. The mechanism of transformation of intermediate to product phases appears to be either a direct solid-solid transformation, or via the dissolution or amorphorization of only a small quantity of material at the surface of the intermediate crystallites.