Parallel Alkane Dehydrogenation Routes on Brønsted Acid and Reaction-Derived Carbonaceous Active Sites in Zeolites

Abstract

Alkane dehydrogenation rates on acidic zeolites measured in the presence of co-fed H2 during initial contact with reactants solely reflect protolytic reactions at Brønsted acid sites, while rates measured without co-fed H2 and at later reaction times reflect additional contributions from an extrinsic dehydrogenation function derived from reactants and products. This extrinsic function consists of unsaturated organic residues that catalyze dehydrogenation turnovers by accepting H-atoms from alkanes and recombining them as H2. Such hydrogen transfer routes are inhibited by alkenes and H2 products and proceed with activation barriers much lower than for protolytic dehydrogenation at H+ sites, causing them to become more prevalent at lower temperatures and for zeolites with lower H+ densities. The number, composition, and reactivity of these extrinsic carbonaceous active sites depend on the local concentrations of reactants and products, which vary with alkane and H2 pressure, bed residence time, and axial mixing. These extrinsic catalytic moieties form within H2-deficient regions of catalyst beds but can be removed by thermal treatments in H2, which fully restore zeolite catalysts to their initial state. Carbonaceous deposits do not catalyze alkane cracking reactions; thus, cracking rate constants serve as a reporter of the state of proton sites, and their invariance with product pressure, residence time, and axial mixing confirms that protons remain unoccupied and undisturbed as extrinsic organic residues change in number, composition, and reactivity. The rates of the reverse reaction (alkene hydrogenation) under H2-rich conditions inhibit the formation and the reactivity of these organic residues, and taken together with formalisms based on nonequilibrium thermodynamics, they confirm that alkane dehydrogenation occurs solely via protolytic routes only at the earliest stages of reaction in the presence of added H2. These findings provide a coherent retrospective view of the root causes of the literature discord about alkane dehydrogenation turnover rates and activation barriers on acidic zeolites, variously attributed to extraframework Al or radical active sites and to turnovers limited by alkene desorption instead of protolytic steps. Importantly, these findings also prescribe experimental protocols that isolate the kinetic contributions of protolytic dehydrogenation routes, thus ensuring their replication, while suggesting strategies to deposit or remove extrinsic organocatalytic functions that mediate hydrogen transfer reactions.

Acid Catalysis over Low-Silica Faujasite Zeolites

“…70% of pristine uptake capacity at p/p 0 = 0.01, as confirmed by Ar-adsorption isotherms (Figure S4 insets). 29 Si solid-state NMR spectra (Figure S6 and Table S2) were used to measure the distribution of Q 4 (nAl) Si species on H-FAU zeolites. The existence of Q 2 Si species (a shoulder peak at ∼−88 ppm) 13 in these spectra suggests that mild dealumination occurs upon moderate ion exchange; consequently, proton-form materials exhibit enhanced framework Si/Al ratios (Table 1, column 4) compared to bulk Si/Al ratios (Table 1, column 2).…”

“…18,19 For low-silica H-FAU zeolites reported here (Figure 1b−g), Figure S8, we observed only the asymmetric band centered at From ICP analysis for H-FAU zeolites. b Framework Si/Al ratios are determined from 29 Si MAS NMR data in accordance with "Loewenstein's rule". 14 c Calculated via a combination of bulk Si/Al ratio (column 2), Na/Al ratio (column 3), and framework Si/Al ratio (column 4), considering that the total number of T atoms per unit cell is 192 and each Na + cation is coordinated with four water molecules.…”

“…We compared proton reactivities of H 3630 and H 3650 on FAU zeolites in high-temperature reactions by molecular dehydrogenation and cracking of propane (eqs 2 and 3) probe reactions. Kester et al 29 reported that carbonaceous deposits contribute extrinsic active sites to promote alkane dehydrogenation, but intrinsic H + -catalyzed propylene formation rates can be assessed when samples are pretreated in H 2 to remove any remnant reactive carbon species, and H 2 is present in the inlet stream to mitigate on-stream deposition of organic species. We pretreated samples using H 2 /He mixtures (p H 2 = 35 kPa, H 2 / He = 1:2) and cofed H 2 (p H 2 = 5.3 kPa, H 2 /C 3 H 8 /Ar/He = 3/ 3/1.5/60) in the inlet stream at different temperatures (818, 833, 848, 863, 878, and 893 K).…”

“…Details consist of the synthesis recipes, characterization results (XRD patterns, Ar-adsorption isotherms, SEM images, 29 Si solid-state NMR, infrared Spectra), and the reactivity analysis. (PDF)…”

Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites

“…Concerning the ranking of activation energy (or enthalpy) between cracking and dehydrogenation, the literature reports contradicting results, ,− probably because the values depend on the properties of the zeolite and maybe also to experimental difficulties in determing precise dehydrogenation kinetics (carbonaceous deposits may be formed). − Surprisingly (considering that the previously mentioned experimental difficulties should also impact the determination of activation entropies), a larger consensus however exists regarding a higher measured intrinsic activation entropy for dehydrogenation with respect to cracking. ,,, …”