Which Activation Energy Do We Measure? Analysis of the Kinetics of Propene-3-¹³C Double-Bond-Shift Reaction on Silicalite-1 by ¹H MAS NMR In Situ

Abstract

Correct assignment of the experimental activation energy of chemical reactions on solid catalysts to either apparent (E app ) or intrinsic (E int ) activation barriers represents a long-standing problem in the kinetic measurements performed with the use of magic-angle spinning (MAS) NMR spectroscopy in a closed microreactor. Here, the transformation of propene on silicalite-1 has been investigated. It is established that a double-bond-shift reaction represents the main route for propene transformation at 296− 333 K with the involvement of hydrogen-bonded silanol groups as active sites. The kinetics of a double-bond-shift reaction was followed with 1 H MAS NMR spectroscopy in situ by monitoring the 13 C-label transfer in adsorbed propene-3-13 C from the CH 3 group to the CH 2 group. The experimentally measured activation energy E exp for this reaction is discussed in terms of assigning E exp to either apparent (E app ) or intrinsic (E int ) activation barrier. For the correct assignment of E exp , an approach that is based on accounting the adsorption equilibrium established between the reactant in the gas phase and the adsorbed state on the catalyst for the reaction carried out in the batch microreactor is developed. This approach allows the estimation of E int and E app from E exp , provided that the adsorption constant K ads and the adsorption enthalpy H ads are known. On the basis of the developed approach, the experimentally measured E exp of 71 kJ mol −1 has been concluded to represent an intrinsic activation barrier E int for the studied reaction.

Propane Transformation on Zn-Modified Zeolite. Effect of the Nature of Zn Species on Alkane Aromatization and Hydrogenolysis

“…Activation energies estimated on the basis of Scheme 1 are shown in Table S3 and referred to intrinsic activation barriers. 48 The values obtained for the Zn 2+ /H-BEA sample coincide with those ones determined previously for the Znmodified BEA zeolite prepared by the impregnation and containing zinc species of various types. 24 Thus, the activity of that sample appears to be referred to the presence of Zn 2+ cations.…”

Propylene Transformation on Zn-Modified Zeolite: Is There Any Difference in the Effect of Zn2+ Cations or ZnO Species on the Reaction Occurrence?

“…Being adsorbed on the zeolite samples, propylene undergoes slow oligomerization at 298−523 K. This is not typical behavior for such systems since oligomerization proceeds very fast on acid-form zeolites. 30,57,58 Moreover, only minor 13 Clabel scrambling between C-3 and C-1 positions in propylene hydrocarbon skeleton and the formed oligomeric olefins is detected, starting at 298 K. This is again not typical for olefin interaction with BAS, 70,74 which provides protonation of the olefin. Carbenium ion intermediate formed at olefin protonation affords oligomeric species by the ion interaction with the other olefin molecule.…”

Unraveling the Effect of Silanol Defects on the Insertion of Single-Site Mo in the MFI Zeolite Framework

“…The 1 H NMR spectra are the mirror image of the IR spectra and also display non-H-bonded silanols (∼1–3 ppm) and H-bonded silanols (∼3–6 ppm), which according to the literature coincides with the zone for isolated/vicinal and H-bonded nest silanols, respectievely . Non-H-bonded silanol contributions increase in the zeolite samples with smaller crystal sizes.…”