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

Abstract

ABSTRACT:We carried out a theoretical study of the HZSM-5 zeolite, for different SiO 2 /Al 2 O 3 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 SiO 2 /Al 2 O 3 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 SiO 2 / Al 2 O 3 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

DFT study of the interaction of the HZSM-5 zeolite with the benzene molecule

“…Also, it is consistent with the concept that Al 3+ should be substituted at the most electropositive site, and the acid hydrogen should be bonded to the most electronegative oxygen, as previously shown in Ref. [18]. In order to further discriminate among chemically active sites, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were calculated.…”

“…Correspondingly, in the complex system the C5 site has −0.07, whereas the remaining carbon charges range from 0.00 to 0.05 electrons, except C1, which has a small nucleophilic character with −0.02 electrons. Then, from the two different population analysis we find the same tendency, although it has been emphasized that the RD scheme could be more useful than the Mulliken scheme in analyzing the behavior of charges in zeolites [18].…”

Quantum, classical, and multi-scale simulation of silica–water interaction: molecules, clusters, and extended systems

“…Compared to the effort on bio-molecules, [65][66][67][68][69][70][71][72][73][74][75][76][77][78], rather fewer groups have reported applications of such methods in materials research [30,56,79,80]. The approach is described below in terms of SiO 2 systems but is general.…”

“…In the last 15 years, researchers have modernized the idea and rejuvenated the subject [65][66][67][68][69][70][71][72][73][74][75][76][77][78], inspired by modern computer technologies and advancements in computational methods and capabilities and motivated by the need for insight into complicated material systems.…”

Transparent interface between classical molecular dynamics and first‐principles molecular dynamics

“…Ideas of embedding quantum mechanical treatment in a classical environment in computational physics and chemistry have been realized in scientific research for many years 1–3. Although pioneering work can be traced back as early as the 1970s 1, 2, many research groups have been making a substantial effort since the late 1990s to rejuvenate the subject 4–16. These recent activities are greatly inspired by modern computer technologies and advancements in computational methods and capabilities.…”