Journal
JOURNAL OF CATALYSIS
Volume 344, Issue -, Pages 817-830Publisher
ACADEMIC PRESS INC ELSEVIER SCIENCE
DOI: 10.1016/j.jcat.2016.08.007
Keywords
Keggin heteropolyacids; Methylcyclohexane ring-contraction; Bifunctional catalysis; Site proximity; Acid strength; Deprotonation energy; Diffusional enhancements of secondary reactions
Categories
Funding
- Chemical Sciences, Geosciences, Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy [DE-AC05-76RL0-1830]
- Chevron Fellowship
Ask authors/readers for more resources
Methylcyclohexane ring contraction is used here to assess the effects of acid strength and metal-acid site proximity on turnover rates and selectivities for bifunctional catalysts consisting of Keggin type polyoxometalates (POM) with different central atoms that act as Bronsted acids. Bifunctional catalysts with metal sites that fully equilibrate cycloalkanes and cycloalkenes give methylcyclohexene conversion rate constants that decreased exponentially with increasing deprotonation energy, a rigorous descriptor of acid strength, consistent with the ion-pair character of transition states that mediate the kinetically relevant ring contraction of bound methylcyclohexoxide intermediates. The measured rates of formation of each allcylcyclopentane isomer, however, do not reflect their intrinsic formation kinetics, because of fast diffusion-enhanced interconversions within acid domains; thus, isomer selectivities cannot be used to infer, even indirectly, the strength of acid sites, as often proposed in previous studies. Selectivities reflect instead diffusional effects that become more severe as the number and strength of acid sites and the size and diffusive resistances increase within these acid domains, which shift, in turn, the products formed from relative abundances dictated by kinetics to those prescribed by thermodynamics. A rigorous accounting of these diffusional effects using a kinetic-transport model leads to ratios of intrinsic rate constants for the formation of the different alkylcyclopentane isomers that do not depend on acid strength, because all isomerization routes are mediated by bicyclo[3.1.0]hexyl cation transition states similar in the amount and location of charge and that therefore benefit to the same extent from the more stable conjugate anions characteristic of stronger acids. (C) 2016 Elsevier Inc. All rights reserved.
Authors
I am an author on this paper
Click your name to claim this paper and add it to your profile.
Reviews
Recommended
No Data Available