(48d) Aldol Condensation and Esterification of Oxygenates On Bifunctional Metal-TiO2 Catalysts

Aldol condensation of oxygenates via sequential C-C bond coupling and oxygen removal provides a promising way to convert biomass-derived feedstocks into useful fuels and chemicals [1,2]. In this presentation, we elucidate parallel condensation and esterification pathways of C₂-C₅ oxygenates on acid-base site pairs of TiO₂ with the aid of hydrogenation-dehydrogenation sites on metal catalysts (Cu, Pt), and the consequences of reactant structure and strength of acid-base site pairs in such cascade reactions.

Aldol condensation of alkanals and alkanones catalyzed by TiO₂ is equilibrium-limited and suffers rapid deactivation, because active sites on TiO₂ are readily blocked by unreactive residues derived from α,β-unsaturated carbonyl products. Hydrogenation of these unsaturated products to less reactive alkanals and alkanones on Cu or Pt co-catalysts drives the condensation forward and leads to stable condensation rates. In addition, alkanols can be directly feed as reactants and equilibrated with their carbonyl analogs on Cu surfaces (but not on Pt), which are then regarded as a reactant pool.

Kinetic studies showed that pool conversion rates (sum of condensation and esterification) are proportional to the amount of TiO₂ in the physical mixtures and to alkanal/alkanone pressures, whereas esterification/condensation rate ratios depend on alkanol/alkanal ratios in the equilibrated reactants. These data, taken together with a normal kinetic isotope effect (2.4, ethanol-D₆, 523 K) and the identity of the condensation products formed, indicate that enolates serve as common intermediates in condensation and esterification reactions, and their formations via α-H abstraction of alkanals/alkanones are kinetically relevant. Density functional theory (DFT) simulation on model TiO₂ oligomers supported that the enolate formations have higher activation free energy barriers than subsequent bimolecular C-C and C-O formation steps, in which calculated activation free energy barriers for C₂-C₅ oxygenates and the corresponding kinetic isotope effect were consistent with the experiments. The optimized structure of the enolate transition state and the independence of oxygenate reactivity with deprotonation energies for the α-carbon in reactants revealed the concerted nature of the α-C-H bond activation on Ti-O site pairs with O-atoms acting as the Lewis base and Ti centers as the Lewis acid, where the transition state is neither too early nor late.

The formed enolates then react via either nucleophilic attack with another alkanal/alkanone (to form α, β-unsaturated carbonyls) or with alkanols (to form unstable hemiacetals), followed by fast hydrogenations to saturated carbonyls or dehydrogenations to esters in the presence of a metal catalyst, respectively. Esterification/condensation rate ratios increase with the size of reactant and do not show primary kinetic isotope effects (1.2, ethanol-D₆, 523 K), which also agreed well with the DFT simulations. Consistent with the kinetic branching pathways of the enolate intermediates, the esterification turnovers on the bifunctional metal-TiO₂ catalysts were decreased apparently by using Pt as admixture instead of Cu, because of the low reactivity of Pt in hydrogenation of C=O bonds, although the condensation rates were still maintained by the hydrogenation of C=C bonds on Pt surfaces.

Catalytic activity of TiO₂ in such oxygenate conversions is determined by the strength of acid and base sites on the surface. Acetone condensation rate on anatase TiO₂ was approximately ten times higher per surface Ti⁴⁺ cation than on rutile TiO₂. DFT calculations of model TiO₂ oligomers showed that basic sites on rutile were much stronger than anatase, which causes desorption of condensation products to be much harder from rutile than from anatase and is reflected by lower rates on rutile when compared to anatase. Analogous mechanisms to mediate oxygenate reactions by acid-base site pairs were also found on monoclinicZrO₂, tetragonal ZrO₂ and CeO₂ surfaces. The relative reactivity of these oxide surfaces and the consistent theoretical predictions unveil the presence of site pairs of intermediate acid-base strength [3], which stabilize the transition states involving concerted interactions of the C and H atoms in C-H bonds with the Ti and O atoms of the Ti-O pairs on TiO₂ surface, respectively.

The authors acknowledge the financial support of BP through the XC² program and useful discussions with Drs. Glenn Sunley and John Shabaker (BP), Dr. Elif Gurbuz and Mr. Stanley Herrmann (UC-Berkeley), and Prof. Matthew Neurock (University of Virginia).

References

1. Sad, M., Neurock, M., Iglesia, E. J. Am. Chem. Soc. 133, 20384 (2011).
2. Rekoske, J. E., Barteau, M. A. Ind. Eng. Chem. Res. 50, 41 (2011).
3. Tanabe, K. in “Acidity and Basicity of Solids” (Fraissard, J. and  Petrakis, L. Eds.) p. 353. Kluwer Academic Publishers, Dordrecht, 1994.