Walton Krista S, Snurr Randall Q
Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA.
J Am Chem Soc. 2007 Jul 11;129(27):8552-6. doi: 10.1021/ja071174k. Epub 2007 Jun 20.
The surface area is one of the most important quantities for characterizing novel porous materials. The BET analysis is the standard method for determining surface areas from nitrogen adsorption isotherms and was originally derived for multilayer gas adsorption onto flat surfaces. Metal-organic frameworks (MOFs) are a relatively new class of crystalline, porous materials that have been shown to exhibit very large BET surface areas. These materials are microporous and possess surfaces that are far from flat. In some MOFs, adsorption occurs through a pore-filling mechanism rather than by layer formation. Thus, it is unclear whether BET surface area numbers reported for these materials are truly meaningful. Given the standard practice of reporting BET surface areas for novel porous materials, a critical test of the BET method is much needed. In this work, grand canonical Monte Carlo simulations were used to predict adsorption isotherms for nitrogen in a series of MOFs. The predicted isotherms were used as pseudoexperimental data to test the applicability of the BET theory for obtaining surface areas of microporous MOFs. BET surface areas calculated from the simulated isotherms agree very well with the accessible surface areas calculated directly from the crystal structures in a geometric fashion. In addition, the surface areas agree well with experimental reports in the literature. These results provide a strong validation that the BET theory can be used to obtain surface areas of MOFs.
表面积是表征新型多孔材料的最重要的量之一。BET分析是根据氮气吸附等温线测定表面积的标准方法，最初是为多层气体在平面上的吸附而推导出来的。金属有机框架（MOF）是一类相对较新的晶体多孔材料，已被证明具有非常大的BET表面积。这些材料是微孔的，其表面远非平坦。在一些MOF中，吸附是通过孔填充机制发生的，而不是通过层形成。因此，尚不清楚报道的这些材料的BET表面积数字是否真的有意义。鉴于报道新型多孔材料BET表面积的标准做法，非常需要对BET方法进行关键测试。在这项工作中，使用巨正则蒙特卡罗模拟来预测一系列MOF中氮气的吸附等温线。预测的等温线用作伪实验数据，以测试BET理论用于获得微孔MOF表面积的适用性。从模拟等温线计算得到的BET表面积与以几何方式直接从晶体结构计算得到的可及表面积非常吻合。此外，这些表面积与文献中的实验报道也非常吻合。这些结果有力地验证了BET理论可用于获得MOF的表面积。
