TY - JOUR
T1 - Catalysis on solid acids
T2 - Mechanism and catalyst descriptors in oligomerization reactions of light alkenes
AU - Sarazen, Michele L.
AU - Doskocil, Eric
AU - Iglesia, Enrique
N1 - Funding Information:
We thank Professor Matthew Neurock (University of Minnesota) and Dr. David Hibbitts (UC-Berkeley, now at University of Florida) for their technical advice on theoretical matters related to this manuscript. We also acknowledge and thank Dr. David Law (BP) for the Tri-cat MFI sample, Dr. Neng Guo (BP) for the synthesis of the Ga and B MFI materials, and Dr. Stacey Zones (Chevron) for the Fe-MFI sample, as well as general guidance about zeolite materials and their physicochemical properties. MLS acknowledges a Graduate Research Fellowship from the National Science Foundation and computational resources from National Science Foundation’s Extreme Science and Engineering Discovery Environment (XSEDE; ACI-1053575 ( CTS140040 Liand TG-CHE140066 )). The financial support for this research from BP p.l.c. as part of the XC 2 Program is gratefully acknowledged.
Publisher Copyright:
© 2016 Elsevier Inc.
PY - 2016/12/1
Y1 - 2016/12/1
N2 - This study addresses fundamental descriptions of confinement and acid strength effects on stability for transition states and intermediates involved in alkene oligomerization on solid acids. Kinetic and infrared data and theoretical treatments that account for dispersive interactions show that turnover rates (per H+) on aluminosilicates and heterosilicates with microporous voids (TON, MFI, BEA, FAU) and on mesoporous acids (amorphous silica-alumina, dispersed polyoxometalates) reflect the free energy of C[sbnd]C bond formation transition states referenced to gaseous alkenes and bound alkene-derived precursors present at saturation coverages. These free energy barriers decrease as the size of confining voids decreases in aluminosilicates containing acid sites of similar acid strength and approaches bimolecular transition state (TS) sizes derived from density functional theory (DFT) for propene and isobutene reactants. Such TS structures are preferentially stabilized over smaller bound precursors via contacts with the confining framework. These effects of size, typically based on heuristic geometric analogies, are described here instead by the dispersive component of DFT-derived energies for TS and intermediates, which bring together the effects of size and the shape, for different framework voids and TS and precursor structures derived from alkenes of different size; these organic moieties differ in “fit” within voids but also in their proton affinity, as a result of the ion-pair character of TS structures. The larger charge in TS structures relative to their alkene-derived precursors causes free energy barriers to decrease as conjugate anions become more stable in stronger acids. Consequently, oligomerization rate constants decrease exponentially with increasing deprotonation energy on unconfined acid sites in polyoxometalates and silica-alumina and on confined sites within MFI frameworks with Al, Ga, Fe, or B heteroatoms. Reactivity descriptions based on geometry or acid strength are replaced by their more relevant energetic descriptors–van der Waals confinement energies, proton affinities of organic molecules, and deprotonation energies–to account for reactivity, here for different reactants on diverse solid acids, but in general for acid catalysis.
AB - This study addresses fundamental descriptions of confinement and acid strength effects on stability for transition states and intermediates involved in alkene oligomerization on solid acids. Kinetic and infrared data and theoretical treatments that account for dispersive interactions show that turnover rates (per H+) on aluminosilicates and heterosilicates with microporous voids (TON, MFI, BEA, FAU) and on mesoporous acids (amorphous silica-alumina, dispersed polyoxometalates) reflect the free energy of C[sbnd]C bond formation transition states referenced to gaseous alkenes and bound alkene-derived precursors present at saturation coverages. These free energy barriers decrease as the size of confining voids decreases in aluminosilicates containing acid sites of similar acid strength and approaches bimolecular transition state (TS) sizes derived from density functional theory (DFT) for propene and isobutene reactants. Such TS structures are preferentially stabilized over smaller bound precursors via contacts with the confining framework. These effects of size, typically based on heuristic geometric analogies, are described here instead by the dispersive component of DFT-derived energies for TS and intermediates, which bring together the effects of size and the shape, for different framework voids and TS and precursor structures derived from alkenes of different size; these organic moieties differ in “fit” within voids but also in their proton affinity, as a result of the ion-pair character of TS structures. The larger charge in TS structures relative to their alkene-derived precursors causes free energy barriers to decrease as conjugate anions become more stable in stronger acids. Consequently, oligomerization rate constants decrease exponentially with increasing deprotonation energy on unconfined acid sites in polyoxometalates and silica-alumina and on confined sites within MFI frameworks with Al, Ga, Fe, or B heteroatoms. Reactivity descriptions based on geometry or acid strength are replaced by their more relevant energetic descriptors–van der Waals confinement energies, proton affinities of organic molecules, and deprotonation energies–to account for reactivity, here for different reactants on diverse solid acids, but in general for acid catalysis.
KW - Acid strength
KW - Alkene
KW - Confinement
KW - Heteropolyacids
KW - Oligomerization
KW - Zeolites
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U2 - 10.1016/j.jcat.2016.10.010
DO - 10.1016/j.jcat.2016.10.010
M3 - Article
AN - SCOPUS:84995695234
SN - 0021-9517
VL - 344
SP - 553
EP - 569
JO - Journal of Catalysis
JF - Journal of Catalysis
ER -