β-Scission of Olefins on Acidic Zeolites: A Periodic PBE-D Study in H-ZSM-5

Abstract

Eight β-scission modes involving C 6 and C 8 olefin isomers are investigated using dispersion-corrected density functional theory (i.e., PBE-D) calculations. Potential energy surfaces are evaluated within an acidic H-ZSM-5 supercell containing a single, isolated active site. Minimum energy pathways are localized using the nudged elastic band method. The relative enthalpic barriers of β-scission steps can be described by the substitution order of the carbocationic carbon atom present in the reactant and transition states. Specifically, the total charge on the hydrocarbon fragment containing the β C atom increases going from the physi-or chemisorbed reactant state to the β-scission transition state; the magnitude of this change (+0.37e−0.97e) is found to correlate nearly monotonically with the activation energy (89−233 kJ mol −1 ). A comparison of 1°to 3°(E 1 ) and 3°to 1°(E 2 ) β-scission modes as well as 2°to 3°(B 1 ) and 3°to 2°(B 2 ) β-scission modes reveals that the barrier heights depend on the substitution order of the β C, indicating that a subcategorization of βscission modes is required based on the substitution order of the β C atom. Isomerization reactions, which are fast with respect to β-scission, enable reactant hydrocarbons to explore and find low-barrier β-scission pathways. Selectivities predicted on the basis of the relative barrier heights of β-scission modes accessible to C 6 and C 8 species indicate agreement with experimental observations.

Mechanism of n-butane skeletal isomerization on H-mordenite and Pt/H-mordenite

“…Using an enthalpy of adsorption of a C8 alkene of À160 kJ mol À1 , calculated using the method of Fogash et al [23], a true activation energy around 140 kJ mol À1 is calculated. This value is in reasonable agreement with a reported value of 153 kJ mol À1 obtained computationally by Mazar et al [54] for beta-scission of 3,3-dimethylhexoxide on H-ZSM-5.…”

“…An argument had been put forth that 2,2,4-trimethylpentane is selectively formed from n-butane and cracks to eventually make two isobutane molecules [57]. In light of the findings of de Lucas et al [55] and an equilibrium content of 2,2,4-trimethylpentane at 600 K of only 1.5 mol% [54], this preferential pathway is not a likely scenario.…”

Reaction pathway and kinetics of C3–C7 olefin transformation over high-silicon HZSM-5 zeolite at 400–490°C

“…Compared to the previous studies on olefin transformation [2,8,10,14,15,31,32], the temperature investigated in this work is in moderate range. The temperature dependence indicates that, under the high temperature, the bimolecular cracking will be insignificant and the monomolecular cracking becomes dominant.…”

“…Besides, the 6 th and 10 th reactions involve the β-scission of tertiary carbenium ion, therefore their activation energies are smaller than those of the 3 rd and 7 th reactions [8,32]. The predicted product distribution is shown in Figure 4-7, in which the reaction rate constants were calculated from the Arrhenius equation using the parameters presented in Table 3.…”

Revisiting carbenium chemistry on amorphous silica-alumina: Unraveling their milder acidity as compared to zeolites

“…In the literature, energy barriers for b-scission steps in zeolites were evaluated mainly from alkoxides, with values as high as 150-200 kJ mol À1 [108][109][110][111][112]. Earlier studies performed on small clusters (neglecting confinement effect) even concluded to barriers above 200 kJ mol À1 [62][63][64][105][106][107].…”