A computational method to characterize framework aluminum in aluminosilicates.

Zeolites are aluminosilicates that are used in many applications involving catalysis, separation, and gas storage. Unlike the Si/Al ratio of their framework, the location of the aluminum atoms is not well described. It has proved difficult to establish the local structure surrounding the aluminum sites by diffraction methods, since aluminum and silicon are very close in X-ray scattering power and thus XRD gives only the weighted average of the Si O and Al O distances. Extended X-ray absorption fine structure (EXAFS) spectroscopy can be used to probe the local structure around aluminum in zeolites. This information is less accurate than the average from XRD, but the ability of EXAFS to probe the local structure around a selected element allows easy in situ measurements. Aluminum Xray adsorption near edge structure (XANES) spectroscopy can also be used to extract such information, although it does not yield interatomic distances. Several attempts have been made to probe structural variations by MAS-NMR spectroscopy, but as soon as even small amounts of aluminum are present in the framework, the resolution is degraded and the spectra provide insufficient information about aluminum site preferences. These experimental challenges, combined with the importance of aluminum site distributions, have created a significant opportunity for molecular modeling, and important additional information about the local structure surrounding aluminum sites has come from simulations. The aluminum distribution on the crystal level, as well as the distribution on the level of a single unit cell, remains a subject of debate. Herein, we present an alternative theoretical approach in which we identify those experimentally accessible properties that are crucially dependent on the aluminum distribution and associated cation distribution. Once these properties have been identified, we compute the most likely positions of aluminum in zeolites by matching simulation results with available experimental data. This crucially depends on the quality of the force field. Recently, two force fields were developed for sodium and protons in aluminosilicates that gave results that were not only qualitatively but also quantitatively in good agreement with experiments. We have performed molecular simulations to obtain the adsorption and diffusion properties of alkanes in a variety of zeolitic structures of industrial importance by varying the positions of the aluminum atoms. Figure 1 shows the unit cells