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 (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...