n‐Butane Monomolecular Cracking on Acidic Zeolites: A Density Functional Study

Abstract

Density functional theory was used to study monomolecular cracking reaction of n-butane on acidic zeolite. Geometry optimizations of the local structures of the species reacting with the zeolite were carried out by using the 5T cluster model at the B3LYP/6-311＋G(3df,2p)//B3LYP/6-31G(d) level. In this work, the deprotonation modes of two different order C atoms in n-butane cracking on H-ZSM-5 were explored in detail by the density functional theory. The calculated activation barriers at the B3LYP/6-311＋G(3df,2p)//B3LYP/6-31G(d) level are 238 kJ/mol for the primary C-C cracking, 217 kJ/mol for the secondary C-C cracking, 296 kJ/mol for the primary dehydrogenation, and 242 kJ/mol for the secondary dehydrogenation, respectively. The difference of the activation barrier is 54 kJ/mol for the deprotonation reactions of two different order C atoms in n-butane. For the dehydrogenation reaction, the results indicate that the reaction is expected to be preferred at the secondary C atom. Additionally, the relation between the acidity and structure of the cluster shows that the acidity effect of the zeolite can be simulated by modifying the peripheral bonds of the cluster for the 5T cluster. Finally, the zeolite acidity effects on the reaction barriers were investigated by changing the cluster terminal Si-H bond length.

Theoretical Analysis of the Influence of Pore Geometry on Monomolecular Cracking and Dehydrogenation of n-Butane in Brønsted Acidic Zeolites

“…Several studies have also reported free energies or entropies of activation. 3,5,6,11,23,24,32 Good agreement between theory and experiment has been reported for ΔS ⧧ int for alkane cracking in MFI using a composite hindered rotor approach adapted from that reported by Grimme 33 to account for internal rotations. 11 However, no previous studies have attempted to describe the activation entropy for dehydrogenation.…”

“…We also note that theoretical investigations of the effects of active site environment on kinetics have been very limited. While a few studies have investigated the effects of induced changes in acidity, 14,24,29 only three of the aforementioned studies have investigated the effects of structural confinement. 5,20,22 Sharada et al have reported that activation energies for n-butane cracking and dehydrogenation differ for intersection and sinusoidal channel sites of MFI.…”

Understanding Brønsted‐Acid Catalyzed Monomolecular Reactions of Alkanes in Zeolite Pores by Combining Insights from Experiment and Theory

“…However, these authors attributed such differences to differences in acidity among zeolites. It is noted that differences in acidity, if present, would be expected to influence the kinetics of monomolecular activation of alkanes based on a number of experimental and theoretical studies, a subject that has recently been reviewed by Boronat and Corma and by Derouane et al . This raises the issue of whether the acidity of protons is correlated with zeolite structure and confinement, since this Minireview is focused primarily on the effects of structure and confinement on alkane cracking and dehydrogenation kinetics.…”

“…[60] However,t hese authors attributed such differencest od ifferences in acidity among zeolites. It is noted that differences in acidity,i fp resent, would be expected to influence the kinetics of monomolecular activation of alkanes based on an umber of experimental [5,6,61] and theoretical [5,7,[62][63][64][65] studies, as ubject that has recently been reviewed by Boronat and Corma [66] and by Derouane et al [49] This raises the issue of whether the acidity of protons is correlated with zeolite structure and confinement, since this Minireview is focused primarily on the effectso fs tructure and confinement on alkane cracking and dehydrogenation kinetics. Several studies have concluded that the acidity is independento ft he zeolite structurea nd the locationo ft he acid sites, [5][6][7] buti sd ependent on the heteroatom (Al, B, Ti) [5] and on the concentration of framework Al atoms for lowv alues of the Si/Al ratio [67][68][69] for which the acidity is lower as ar esult of the presence of Al next-nearest-neighbor pairs.…”

“…[71,72] However, despite the ever-growing power of computers, the quantum chemicalt reatment of large systems such as zeolites still carries ah igh computational cost. While early studies of zeolitecatalyzed reactions relied on small cluster representations to reduce the overall number of atoms, [23,62,65] these do not permitproper captureofthe effects of the zeoliteenvironment on the Brønsted acid sites. [73,74] To correctly describe the crucial long-range interactions encountered by guest molecules entering the zeolite pores, am ore comprehensive representation of the zeolite structure is required.…”