TY - JOUR

T1 - 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

AU - Arzumanov, Sergei S.

AU - Stepanov, Alexander G.

PY - 2018/10/18

Y1 - 2018/10/18

N2 - Correct assignment of the experimental activation energy of chemical reactions on solid catalysts to either apparent (Eapp) or intrinsic (Eint) 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 1H MAS NMR spectroscopy in situ by monitoring the 13C-label transfer in adsorbed propene-3-13C from the CH3 group to the -CH2 group. The experimentally measured activation energy Eexp for this reaction is discussed in terms of assigning Eexp to either apparent (Eapp) or intrinsic (Eint) activation barrier. For the correct assignment of Eexp, 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 Eint and Eapp from Eexp, provided that the adsorption constant Kads and the adsorption enthalpy Hads are known. On the basis of the developed approach, the experimentally measured Eexp of 71 kJ mol-1 has been concluded to represent an intrinsic activation barrier Eint for the studied reaction.

AB - Correct assignment of the experimental activation energy of chemical reactions on solid catalysts to either apparent (Eapp) or intrinsic (Eint) 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 1H MAS NMR spectroscopy in situ by monitoring the 13C-label transfer in adsorbed propene-3-13C from the CH3 group to the -CH2 group. The experimentally measured activation energy Eexp for this reaction is discussed in terms of assigning Eexp to either apparent (Eapp) or intrinsic (Eint) activation barrier. For the correct assignment of Eexp, 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 Eint and Eapp from Eexp, provided that the adsorption constant Kads and the adsorption enthalpy Hads are known. On the basis of the developed approach, the experimentally measured Eexp of 71 kJ mol-1 has been concluded to represent an intrinsic activation barrier Eint for the studied reaction.

KW - SOLID-STATE NMR

KW - N-BUTANE CONVERSION

KW - H/D EXCHANGE

KW - ZEOLITE H-ZSM-5

KW - BECKMANN REARRANGEMENT

KW - C-H

KW - CYCLOHEXANONE OXIME

KW - BUTENE CONVERSION

KW - HYDROGEN-EXCHANGE

KW - LOW-TEMPERATURE

UR - http://www.scopus.com/inward/record.url?scp=85055258949&partnerID=8YFLogxK

U2 - 10.1021/acs.jpcc.8b06044

DO - 10.1021/acs.jpcc.8b06044

M3 - Article

AN - SCOPUS:85055258949

VL - 122

SP - 23432

EP - 23440

JO - Journal of Physical Chemistry C

JF - Journal of Physical Chemistry C

SN - 1932-7447

IS - 41

ER -