An embedded QM/MM study for different SiO2/Al2O3 ratios of the HZSM-5 zeolite and for their interaction with N-heptane

We carried out a theoretical study of the HZSM-5 zeolite, for different SiO2/Al2O3, ratios, that interacts with the n-heptane molecule. The study was performed using a QM/MM (quantum mechanics/molecular mechanics) methodology. For the QM part, we have chosen a hybrid Hartree-Fock density functional theory (DFT). The hybrid ACM/DZP approach, as implemented in Turbomole, was used for the treatment of the QM cluster containing 84 atoms that represents a ring structure model of the zeolite-n-heptane interacting system. The MM part was represented by means of an electrostatic forcefield (ESFF), which assesses the electronic embedding. The chosen QM/MM silicalite base model contains 3862 atoms. The studied SiO2/Al2O3 ratios were 2300, 573.5, 287.7, and 189.83, containing 1, 4, 8, and 12 Al atoms, respectively. For the first ratio, the site for the substitution of Al for Si was that of minimum QM total energy value, because this replacement was done in the QM region. For the other SiO2/Al2O3, ratios, the Al atoms were randomly spread through the MM region in accordance with the Lowenstein substitution rule. These results show the importance of the environment on the electronic properties in the QM region, where the active site lies, and their effects on the earlier steps on the activation experienced by the n-heptane moiety. A minimal content of 12 Al atoms produces significant effects of the environment on the electronic structure of the QM region. Moreover, the carbocationic character of n-heptane increases with the aluminum content. (C) 2002 Wiley Periodicals, Inc.