Understanding Aluminum Location and Non-framework Ions Effects on Alkane Adsorption in Aluminosilicates:  A Molecular Simulation Study

Abstract

In our previous work, a computational method to characterize framework aluminum in aluminosilicates was proposed (García-Pérez, E., et al. Angew. Chem., Int. Ed. 2007, 46, 276). In this work, the method was adopted to identify the most likely positions of aluminum in TON, FER, and MOR zeolites and to understand their different adsorption behaviors in detail. The simulations show that the location of aluminum affects the positions of the ions, and thus influences the adsorption. With the determined structures, the effects of non-framework ions on the adsorption behaviors of alkanes in these zeolites were studied systematically and the relations of the macroscopic adsorption behaviors of alkanes to their microscopic structures were elucidated. The results provided a better understanding of the influences of the position and density of aluminum on adsorption in zeolites from a microscopic level that may guide the future rational synthesis of new structures.

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

“…One could argue that such an empirical model may have very limited values if extended to other zeolites. In this respect it is encouraging that GarciaPerez and co-workers 122,123 showed that these potentials also work quite well for other zeolites (FER and TON).…”

“…This suggestion in ref 206 was followed up by Liu and co-workers. 122,123,207 Liu et al showed that, with the united-atom force field of Dubbeldam et al, 94,97 which uses a σ CH 2 O ) 3.58 Å, an excellent agreement with the experimental data could be obtained. Interestingly, this agreement could only be obtained if the model included the effect of the acid sites in FER.…”

Zeolite microporosity studied by molecular simulation

“…The framework of MOR consists of two distinct cavities, which are twelve membered main channels (7.0 × 6.5 A) in the direction of the z axis and eight membered side pockets (3.4 × 4.8Å) in y direction [38]. In N a-MOR, the adsorption of guest molecules is very sensitive to the distribution of aluminum atoms [59,60]. In our present study, we randomly distribute Al atoms over all tetrahedral sites of the N a-MOR supercell, in such a way that the Löwenstein rule as well as the guidelines outlined by Alberti et al [61] are satisfied.…”

Modeling of Transport and Accessibility in Zeolites

“…Zeolites with a Si/Al ratio higher than 1 can be obtained from random substitution of aluminum by silicon, either ignoring [12,13] or taking into account distribution rules [14][15][16][17][18]. The aluminum atoms can also be assigned to energetic and entropic preferential positions [19][20][21][22][23][24], or using theoretical approaches that identifies experimentally accessible properties dependent on the aluminum distribution and associated cation distribution [25,26]. Substitution of silicon for aluminum generates a negative net charge in the zeolite framework that needs to be compensated by either nonframework protons or cations in order to make the zeolite charge neutral.…”

“…Each substitution creates a negative charge in the framework that is compensated by a counterion or a proton. The location of these counterions influences the adsorption and the catalytic properties for these materials [25,26]. In addition, zeolites act as shape-selective catalysts with a high selectivity and reaction yield.…”