TY - JOUR
T1 - Stability of bound species during alkene reactions on solid acids
AU - Sarazen, Michele L.
AU - Iglesia, Enrique
N1 - Funding Information:
We also acknowledge with thanks the financial support from the National Science Foundation for a Graduate Research Fellowship (to M.L.S.) and from the BP XC2 Program for all research activities leading to the results reported in this article. Computational resources were provided by BP High Performance Computing (BP, p.l.c.) and by the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation (Grant CHE-140066).
PY - 2017/5/16
Y1 - 2017/5/16
N2 - This study reports the thermodynamics of bound species derived from ethene, propene, n-butene, and isobutene on solid acids with diverse strength and confining voids. Density functional theory (DFT) and kinetic data indicate that covalently bound alkoxides form C-C bonds in the kinetically relevant step for dimerization turnovers on protons within TON (0.57 nm) and MOR (0.67 nm) zeolitic channels and on stronger acids HPW (polyoxometalate clusters on silica). Turnover rates for mixed alkenes give relative alkoxide stabilities; the respective adsorption constants are obtained from in situ infrared spectra. Tertiary alkoxides (from isobutene) within larger voids (MOR, HPW) are more stable than less substituted isomers but are destabilized within smaller concave environments (TON) because framework distortions are required to avoid steric repulsion. Adsorption constants are similar on MOR and HPW for each alkoxide, indicating that binding is insensitive to acid strength for covalently bound species. DFT-derived formation free energies for alkoxides with different framework attachments and backbone length/structure agree with measurements when dispersion forces, which mediate stabilization by confinement in host-guest systems, are considered. Theory reveals previously unrecognized framework distortions that balance the C-O bond lengths required for covalency with host-guest distances that maximize van der Waals contacts. These distortions, reported here as changes in O-atom locations and dihedral angles, become stronger for larger, more substituted alkoxides. The thermodynamic properties reported here for alkoxides and acid hosts differing in size and conjugate-anion stability are benchmarked against DFT-derived free energies; their details are essential to design host- guest pairs that direct alkoxide species toward specific products.
AB - This study reports the thermodynamics of bound species derived from ethene, propene, n-butene, and isobutene on solid acids with diverse strength and confining voids. Density functional theory (DFT) and kinetic data indicate that covalently bound alkoxides form C-C bonds in the kinetically relevant step for dimerization turnovers on protons within TON (0.57 nm) and MOR (0.67 nm) zeolitic channels and on stronger acids HPW (polyoxometalate clusters on silica). Turnover rates for mixed alkenes give relative alkoxide stabilities; the respective adsorption constants are obtained from in situ infrared spectra. Tertiary alkoxides (from isobutene) within larger voids (MOR, HPW) are more stable than less substituted isomers but are destabilized within smaller concave environments (TON) because framework distortions are required to avoid steric repulsion. Adsorption constants are similar on MOR and HPW for each alkoxide, indicating that binding is insensitive to acid strength for covalently bound species. DFT-derived formation free energies for alkoxides with different framework attachments and backbone length/structure agree with measurements when dispersion forces, which mediate stabilization by confinement in host-guest systems, are considered. Theory reveals previously unrecognized framework distortions that balance the C-O bond lengths required for covalency with host-guest distances that maximize van der Waals contacts. These distortions, reported here as changes in O-atom locations and dihedral angles, become stronger for larger, more substituted alkoxides. The thermodynamic properties reported here for alkoxides and acid hosts differing in size and conjugate-anion stability are benchmarked against DFT-derived free energies; their details are essential to design host- guest pairs that direct alkoxide species toward specific products.
KW - Alkene adsorption
KW - Density functional theory
KW - Heterogeneous catalysis
KW - Zeolites
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U2 - 10.1073/pnas.1619557114
DO - 10.1073/pnas.1619557114
M3 - Article
C2 - 28461504
AN - SCOPUS:85019952627
SN - 0027-8424
VL - 114
SP - E3900-E3908
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 20
ER -