Water content and particle texture of synthetic hydrotalcite-like layered double hydroxides
Water content and particle texture of synthetic hydrotalcite-like layered double hydroxides
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DOI：
10.1021/cm00050a017
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发表时间：
1995-02
影响因子：
8.6
通讯作者：
S. Yun;T. Pinnavaia
中科院分区：
文献类型：
--
作者：
S. Yun;T. Pinnavaia
X)/x~2、x~3和x~4分别采用可变和恒定pH共沉淀法进行了制备。在常用的可变pH法中，LDH的沉淀在起始碳酸盐溶液的pH值升高时开始，在pH 10时结束。相反，恒pH法允许整个沉淀过程在pH 10时进行。这两种合成方法都得到了风干LDH碳酸盐，其中含有两种类型的水，即颗粒间孔隙水和地表水，其中水凝聚在聚集的晶片之间，地表水结合到走廊内和外部基面上。加热到60℃很容易去除颗粒间的孔隙水，但随着Al3+含量的增加，完全去除表面水的温度从240℃增加到280℃(5℃/min)。表面水的双峰损失与晶内表面结合的“内在”表面水和吸附在微晶外表面的“外在”表面水的存在是一致的。LDH反应产物的颗粒织构，如晶体形态、比表面积和颗粒间孔径分布所反映的，高度依赖于制备方法和层电荷密度。可变pH法可获得表面粗糙、比表面积较大的细晶，而恒定pH法可获得较大、结构良好的六方晶体。变pH法制备的产物均为中孔，半径在50-300A之间，而恒定pH法制备的镁铝和镁铝LDH碳酸盐晶体的介孔分布较窄，半径在20A附近。透射电子显微镜图像为颗粒间孔隙水在边面晶体聚集形成的空洞中的容纳提供了证据。界面层堆积无序也被观察到，这既有助于外部表面水的结合，也有助于中孔的形成。
x)/x~ 2, 3, and 4, were prepared by complementary variable-and constant-pH coprecipitation methods. In the commonly used variable pH process, LDH precipitation was initiated at the elevated pH value of the starting carbonate solution and terminated at pH 10. In contrast, the constant pH method allowed the entire precipitation process to be carried out at pH 10. Both synthesis methods yielded air-dried LDH carbonates containing two typesof water, namely, interparticle pore water, wherein water is condensed between aggregated crystal platelets, and surface water, which is bound to intragallery and external basal planes. Interparticle pore water was readily removed by heating to 60 C, but the temperature for complete removalof surfaces water increased from 240 to 280 C (5 C/min) with increasing Al3+ content. A bimodal loss of surface water was consistent with the presence of “intrinsic” surface water bound to intracrystal gallery surfaces and “extrinsic” surface water adsorbed at theexternal surfaces of the crystallites. The particle textures of the LDH reaction products, as reflected in crystal morphologies, surface areas, and interparticle pore size distributions, were highly dependent on the preparation method and thelayer charge density. Fine grained crystals with rough surfaces and relatively high surface areas were obtained by the variable pH method, whereas the constant pH method afforded larger, well-formed hexagonal crystals. All of the products prepared by the variable pH method exhibited mesopores with radii in the range 50—300 A. In contrast, the constant pH method gave MgsAl—and Mg4Al—LDH carbonatecrystals with narrow mesopore distributions near 20 A radius. TEM images provided evidence for the accommodation of interparticle pore water in voids formed by edge-face crystal aggregation. Cofacial layer stacking disorders also were observed that contribute both to the binding of extrinsic surface water and to the formation of mesopores.