How Chain Length and Branching Influence the Alkene Cracking Reactivity on H-ZSM-5

Abstract

Catalytic alkene cracking on H-ZSM-5 involves a complex reaction network with many possible reaction routes and often elusive intermediates. Herein, advanced molecular dynamics simulations at 773 K, a typical cracking temperature, are performed to clarify the nature of the intermediates and to elucidate dominant cracking pathways at operating conditions. A series of C4–C8 alkene intermediates are investigated to evaluate the influence of chain length and degree of branching on their stability. Our simulations reveal that linear, secondary carbenium ions are relatively unstable, although their lifetime increases with carbon number. Tertiary carbenium ions, on the other hand, are shown to be very stable, irrespective of the chain length. Highly branched carbenium ions, though, tend to rapidly rearrange into more stable cationic species, either via cracking or isomerization reactions. Dominant cracking pathways were determined by combining these insights on carbenium ion stability with intrinsic free energy barriers for various octene β-scission reactions, determined via umbrella sampling simulations at operating temperature (773 K). Cracking modes A (3° → 3°) and B2 (3° → 2°) are expected to be dominant at operating conditions, whereas modes B1 (2° → 3°), C (2° → 2°), D2 (2° → 1°), and E2 (3° → 1°) are expected to be less important. All β-scission modes in which a transition state with primary carbocation character is involved have high intrinsic free energy barriers. Reactions starting from secondary carbenium ions will contribute less as these intermediates are short living at the high cracking temperature. Our results show the importance of simulations at operating conditions to properly evaluate the carbenium ion stability for β-scission reactions and to assess the mobility of all species in the pores of the zeolite.

Dynamic Features of Transition States for β‐Scission Reactions of Alkenes over Acid Zeolites Revealed by AIMD Simulations

“…As the forthcoming analysis will illustrate, our AIMD method does include unexpected structural behavior of the TS impacting entropic contributions. On the other hand, our free energy barriers are consistent with the ones obtained in biased MD for type A (53 kJ mol −1 ) and B 2 (around 70 kJ mol −1 ) β‐scissions by Cnudde et al …”

“…Cnudde et al. found a free energy barrier for B 2 cracking of dibranched C 8 alkenes close to 70 kJ mol −1 at 773 K, lower than by static calculations (90 to 197 kJ mol −1 ) . The B 1 reaction was assumed to play a minor role compared to B 2 , and it was not considered in their study.…”

“…For acid‐catalyzed reactions in zeolites, severe limitations of static calculations were identified recently, both in terms of chemical nature of intermediates and of free energy estimations. Advanced ab initio molecular dynamics (AIMD) is a relevant option, although computationally highly demanding, that brought some unprecedented level of knowledge for alkene isomerization and cracking . Cnudde et al.…”

“…Again, the free energy barrier obtained for the type B 2 cracking (84.4±5.3 kJ mol −1 ) is lower than the barriers reported previously. In static DFT, a wide range of large barriers were found (from ≈90 to more than 170 kJ mol −1 ,). In kinetic modeling an energy barrier of 118 kJ mol −1 for C 12 cracking in H‐BEA was proposed .…”

Kinetic Modeling of Catalytic Olefin Cracking and Methanol-to-Olefins (MTO) over Zeolites: A Review

“…Furthermore, general mechanistic details [39,44] and the relationship between catalyst properties, the morphology of the catalyst and product compositions can be elucidated [45]. Similar investigations exist for the cracking of paraffins [46,47] and olefins [48][49][50] using different zeolites.…”