Effect of temperature and branching on the nature and stability of alkene cracking intermediates in H-ZSM-5

Abstract

Catalytic cracking of alkenes takes place at elevated temperatures in the order of 773–833 K. In this work, the nature of the reactive intermediates at typical reaction conditions is studied in H-ZSM-5 using a complementary set of modeling tools. Ab initio static and molecular dynamics simulations are performed on different C4single bond C5 alkene cracking intermediates to identify the reactive species in terms of temperature. At 323 K, the prevalent intermediates are linear alkoxides, alkene π-complexes and tertiary carbenium ions. At a typical cracking temperature of 773 K, however, both secondary and tertiary alkoxides are unlikely to exist in the zeolite channels. Instead, more stable carbenium ion intermediates are found. Branched tertiary carbenium ions are very stable, while linear carbenium ions are predicted to be metastable at high temperature. Our findings confirm that carbenium ions, rather than alkoxides, are reactive intermediates in catalytic alkene cracking at 773 K

Reaction Mechanism for Direct Cyclization of Linear C5, C6, and C7 Alkenes over H‐ITQ‐13 Zeolite Investigated Using Density Functional Theory

“…Sauer and co‐workers reported that tert ‐butyl cation was more favorable than isobutene and surface alkoxides in H‐FER zeolite . However, with respect to linear alkenes such as pentene, it was found that linear alkoxides and alkene π‐complexes were prevalent intermediates in H‐ZSM‐5 at 323 K, and an increase in the temperature to 573 K, which is near to practically operational methanol‐to‐hydrocarbons temperature, caused alkoxides to be metastable as a result of enormous entropic loss, while π‐complexes were still highly stable . This shows that the exact state (π‐complex, carbenium ion or alkoxide) of alkene adsorbing on Brønsted acidic zeolites depends on the zeolite framework, isomers structure and reaction temperature.…”

Insight into the Role of Water on the Methylation of Hexamethylbenzene in H‐SAPO‐34 from First Principle Molecular Dynamics Simulations

“…The static periodic DFT calculations are performed using the Vienna Ab Initio Simulation Package (VASP 5.3) using a methodology frequently applied in literature . We here opted to use the revPBE functional due to its improved performance for solid‐state calculations compared to the commonly used PBE functional .…”

“…As earlier work showed that a partial Hessian vibrational analysis (PHVA) is able to predict adsorption entropies within a 10–15 J/(mol K) error of computationally more demanding full Hessian calculations (FHVA) and experimental data, it has become a frequently applied and attractive methodology . For this reason, it is also used for the normal mode analysis (NMA) in this work using TAMkin .…”

Progress in Developing a Structure‐Activity Relationship for the Direct Aromatization of Methane

“…Ethylene aromatization easily occurs on the BAS of a zeolite. Four different possibilities for alkene activation are mentioned in the literature (Figure ) . But most papers assume that ethylene is activated by forming a carbocation (C 2 H 5 + ) with the help of the proton from the acid site .…”

“… Four different modes of activation for an alkene – 1: Van der Waals complex, 2: π ‐complex, 3: carbenium ion, 4: alkoxide …”