Predicting the effect of framework and hydrocarbon structure on the zeolite-catalyzed beta-scission

Abstract

Developing improved zeolites is essential in novel sustainable processes such as the catalytic pyrolysis of plastic waste. This study used density functional theory to investigate how alkyl chain length, unsaturated bonds, and branching affect β-scission kinetics in four zeolite frameworks, a key reaction in hydrocarbon cracking. The activation enthalpy was evaluated for a wide variety of 23 hydrocarbons, with 6 to 12 carbon atoms, in FAU, MFI, MOR, and TON. The consideration of both branched and linear olefin and diolefin reactants for the β-scission indicates how the reactant structure influences the intrinsic cracking kinetics, which is especially relevant for the catalytic cracking of plastic waste feedstocks. Intrinsic chemical effects, such as resonance stabilization, the inductive effect, and pore stabilization were found to provide an essential contribution to the activation enthalpy. Additionally, a predictive group additive model incorporating a novel so-called "pore confinement descriptor" was developed for fast prediction of the β-scission activation barrier of a wide range of molecules in the four zeolites. The obtained model can serve as an input for detailed kinetic models in zeolite-catalyzed cracking reactions. The acquired fundamental insights in the cracking of hydrocarbons, relevant for renewable feedstocks, correspond well with experimental observations and will facilitate an improved rational zeolite design.

Fig. 1. Overview of the considered β-scission reactions in this work. The green background denotes hydrocarbons stemming from polyethylene thermal pyrolysis, the blue background from polypropylene thermal pyrolysis.

Fig. 2. Schematic representation of the general β-scission reactants investigated in this work (left) with three proposed group additive-based modeling approaches (right). The vector R₁–R₄ corresponding to the representation of the side-chains concatenated with the double bond value (DB) and in one case a pore confinement descriptor Δ(∑d⁻¹). The illustrative values of the vector represent the β-scission of 2-methyloct-2-ylium.

Fig. 3. The variation of activation enthalpy (Δ^‡H) with increasing chain length of linear components (left) investigated in FAU, MFI, MOR, and TON for the depicted reaction template (right).

Fig. 4. Correlation between the activation enthalpy (Δ^‡H) and the pore confinement descriptor Δ(∑d⁻¹) for linear carbenium ions, together with the R²-value. The arrow indicating increasing carbon chain length.

Fig. 5. The variation of activation enthalpy (Δ^‡H) with increasing chain length of branched components (left) for the depicted reaction template (right).

Fig. 6. Correlation between the activation enthalpy (Δ^‡H) and the pore confinement descriptor Δ(∑d⁻¹) for branched carbenium ions, together with the R²-value. The arrow indicating the direction of increasing carbon chain length.

Fig. 7. Reaction template of the β-scission of the studied unsaturated hydrocarbons with increasing chain length R (R = 1–7).

Fig. 8. The variation of activation enthalpy (Δ^‡H) with increasing chain length of saturated (blue) and unsaturated linear carbenium-ions (orange).

Fig. 9. The activation enthalpy of (a) 2,4-dimethylpent-2-ylium, (b) 2,4-dimethylhex-2-ylium, (c) 2,4-dimethylhept-2-ylium, (d) 2,3,4-trimethylpent-2-ylium, (e) 2,3,4-trimethylhex-2-ylium, (f) 2,3,4-trimethylhept-2-ylium in FAU, MFI, MOR, and TON.

Fig. 10. The activation enthalpy of (c) 2,4-dimethylhept-2-ylium, (g) 2,4-dimethylnon-2-ylium, (h) 2,4,6-trimethylhept-2-ylium, (i) 2,4,6-trimethylnon-2-ylium in FAU, MFI, MOR, and TON.

Fig. 11. Parity plots of the predicted and calculated activation enthalpy by the modified group additive model with pore confinement descriptor (mGAV + Δ(∑d⁻¹)_MM) for the four different zeolites. The model trained and evaluated on all reactions included in this work.