The behavior of aluminum sites in H-[Al]-RUB-18 catalysts: A theoretical-experimental investigation

Abstract

Functional acidic sites can be generated in the structure of layered silicates by the isomorphic substitution of silicon by aluminum atoms, leading to suitable heterogeneous catalysts. H-[Al]-RUB-18 samples were synthesized with Si/Al molar ratios of 15, 30, and 60. Experimental characterizations showed that two different aluminum sites were obtained: one tetrahedrally coordinated (framework-Al), very similar to those found in microporous zeolites; and the other octahedrally coordinated, analogous to silanol groups (hydrated aluminol sites). Computational simulations based on Density Functional Theory (DFT) were used to test the atomic positions of these aluminum sites and confirmed, in terms of electronic energies, the tetrahedral and octahedral positions already predicted in the experimental characterizations. In addition, ethanol dehydration reaction was used to probe the acidic sites and evaluate the performance of the different [Al]-RUB-18 samples. Although a widely known reaction for zeolites and other porous solids, the chemical analysis in this work indicated that the Al-substituted layered silicates are promising candidates to be used in heterogeneous catalysis. The experimental catalytic data showed that the presence of aluminum atoms improved the reaction conversion. Moreover, MEP calculations also presented, in each step of ethanol dehydration, Alframework sites with the lowest electronic energy barriers.