University of Konstanz , Department of Chemistry, Universitaetstrasse 10, 78457 Konstanz, Germany.
University of Siegen , Department of Chemistry and Biology, Adolf-Reichwein-Strasse 2, 57076 Siegen, Germany.
ACS Appl Mater Interfaces. 2017 Apr 5;9(13):11599-11608. doi: 10.1021/acsami.6b16721. Epub 2017 Mar 22.
Heterogeneous catalysis can be understood as a phenomenon which strongly relies on the occurrence of thermodynamically less favorable surface motifs like defects or high-energy planes. Because it is very difficult to control such parameters, an interesting approach is to explore metastable polymorphs of the respective solids. The latter is not an easy task as well because the emergence of polymorphs is dictated by kinetic control and materials with high surface area are required. Further, an inherent problem is that high temperatures required for many catalytic reactions can also induce the transformation to the thermodynamically stable modification. Alumina (AlO) was selected for the current study as it exists not only in the stable α-form but also as the metastable γ-polymorph. Kinetic control was realized by combining an aerosol-based synthesis approach and a highly reactive, volatile precursor (AlMe). Monolithic flakes of AlO with a highly porous, hierarchical structure (micro-, meso-, and macropores connected to each other) resemble so-called aerogels, which are normally known only from wet sol-gel routes. Monolothic aerogel flakes can be separated from the gas phase without supercritical drying, which in principle allows for a continuous preparation of the materials. Process parameters can be adjusted so the material is composed exclusively of the desired γ-modification. The γ-AlO aerogels were much more stable than they should be, and even after extended (80 h) high-temperature (1200 °C) treatment only an insignificant part has converted to the thermodynamically stable α-phase. The latter phenomenon was assigned to the extraordinary thermal insulation properties of aerogels. Finally, the material was tested concerning the catalytic dehydration of 1-hexanol. Comparison to other AlO materials with the same surface area demonstrates that the γ-AlO are superior in activity and selectivity regarding the formation of the desired product 1-hexene.
多相催化可以被理解为一种强烈依赖于热力学上不利的表面图案（如缺陷或高能面）出现的现象。由于很难控制这些参数，因此探索相应固体的亚稳同素异形变体是一种有趣的方法。后者也不是一件容易的事情，因为同素异形变体的出现取决于动力学控制，并且需要具有高表面积的材料。此外，一个固有的问题是，许多催化反应所需的高温也可能导致向热力学稳定的变体转变。氧化铝（AlO）被选为本研究的对象，因为它不仅存在于稳定的α-形式，而且还存在于亚稳的γ-多晶型体中。动力学控制是通过结合气溶胶合成方法和高反应性、挥发性前体（AlMe）来实现的。AlO 的整体薄片具有高度多孔的分级结构（相互连接的微、介和大孔），类似于所谓的气凝胶，通常仅从湿溶胶-凝胶途径中了解到。气凝胶薄片可以从气相中分离出来，而无需超临界干燥，这在原则上允许连续制备材料。可以调整工艺参数，使材料仅由所需的γ-变体组成。γ-AlO 气凝胶比它们应该更稳定，即使经过长时间（80 h）的高温（1200°C）处理，只有一小部分转化为热力学稳定的α-相。后一种现象归因于气凝胶的非凡隔热性能。最后，该材料在 1-己醇的催化脱水方面进行了测试。与具有相同表面积的其他 AlO 材料相比，γ-AlO 在形成所需产物 1-己烯的活性和选择性方面表现优异。
