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The acidity of anatase titania before and after KOH doping was probed by pyridine adsorption in a pulse microreactor and modeled by DFT optimization of the geometry of CO and pyridine adsorption on a periodic slab of (101) and (100) surfaces using a GGA/PBE functional and verified by an example of a single-point calculation of the optimized geometry using an HSE-06 hybrid functional. The anatase (101) surface was slightly more acidic compared to the (100) surface. Both experimental and computational methods show that the acidity of anatase surfaces decreased after KOH doping and increased after the dissociative adsorption of water. Higher acidity of Ti metal centers was indicated by the shortening of the Ti-N, Ti-C, and C-O bond lengths, increasing the IR frequency of CO and pyridine ring vibrations and energy of adsorption. The DFT calculated energy of pyridine adsorption was analyzed in terms of binding energy and the energy of lattice distortion. The latter was used to construct Hammett plots for the adsorption of 4-substituted pyridines with electron-donating and -withdrawing substituents. The Hammett rho constant was obtained and used to characterize the acidity of various metal centers of −1.51 vs. −1.46 on pristine (101) and (100) surfaces, which were lowered to −1.07 and −1.19 values on KOH-doped (101) and (100) surfaces, respectively. The mechanism of lowering surface acidity via KOH doping proceeds through the stabilization of the atomic structure of Lewis acid centers. When an alkaline metal cation binds to several lattice oxygen atoms, the surface structure becomes more rigid. The ability of Ti atoms to move toward the adsorbate is restricted. Consequently, the lattice distortion energy and binding energy are decreased. In contrast, higher flexibility of the outermost layer of Ti atoms as a result of electron density redistribution, for example, in the presence of water on the surface, allows them to move farther outward, make shorter contacts with the adsorbate, and attain higher energies of binding and lattice distortion.
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Kinetics of Carboxylic Acid Enolization on Metal Oxides in Vapor Phase
An invisible, reversible catalytic reaction called enolization occurs consistently when carboxylic acid vapors contact metal oxide surfaces, a pathway widely invoked in mechanistic proposals for decarboxylative ketonization but not sufficiently examined experimentally. While the CO₂H group responsible for adsorption readily forms surface carboxylates, the weaker α-C–H acidity becomes evident only through reversible H/D exchange. The formation of an enolized surface carboxylate enables its subsequent condensation with a second carboxylate, a transformation widely regarded as the rate-determining step in the decarboxylative ketonization mechanism relevant to oxygen removal in biofuel upgrading. In our kinetic study, the rate of approaching equilibrium was measured for H/D isotopic exchange on alpha-carbon of isobutyric acid used in various concentrations in a vapor phase mixture with D2O as well as for reversed D/H exchange between alpha-deuterated isobutyric acid and H2O upon contact with monoclinic zirconia and anatase titania catalysts. Faster rate for H/D vs. D/H exchange points to alpha-deprotonation, i.e., enolization, as the rate determining step of the exchange mechanism. The intrinsic rate of enolization was deduced using McKay equation for equilibrium reactions. Kinetic activation parameters were obtained through temperature dependence of the rate constant for both exchange directions, H/D and D/H. KOH doping on ZrO2 changes the geometry of the transition state leading to higher rates of enolization and increasing H/D kinetic isotope effect from 1.4 to 5.8. The opposite effect of KOH doping is observed on anatase TiO2 – enolization rates are slightly decreased, kH/kD remains relatively constant at 2.6–2.8 indicating that the nature of basic centers on TiO2 is unaffected. These results confirm C–C coupling, not enolization, being the rate limiting step of the decarboxylative ketonization mechanism.
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- Award ID(s):
- 1955130
- PAR ID:
- 10658140
- Publisher / Repository:
- Springer
- Date Published:
- Journal Name:
- Catalysis Letters
- Volume:
- 156
- Issue:
- 1
- ISSN:
- 1011-372X
- Subject(s) / Keyword(s):
- Isobutyric acid Isotopic exchange McKay method Monoclinic zirconia Anatase titania
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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