Experimental and theoretical assessment of the mechanism and site requirements for ketonization of carboxylic acids on oxides

Abstract

Ketonization of carboxylic acids removes O-atoms and forms new CC bonds, thus providing routes from sustainable carbon feedstocks to fuels and chemicals. The elementary steps involved and their kinetic relevance, as well as the number and nature of the active sites on active TiO 2 and ZrO 2 catalysts, remain matters of active discourse. Here, site titrations demonstrate the requirement for coordinatively-unsaturated M-O-M sites (M=Ti, Zr) with specific geometry and intermediate acid-base strength. The measured site densities allow rigorous reactivity comparisons among catalysts based on turnover rates and activation free energies, as well as the benchmarking of mechanistic proposals against theoretical assessments. Kinetic, isotopic, spectroscopic, and theoretical methods show that C 2-C 4 acids react on anatase TiO 2 via kinetically-relevant CC coupling between 1-hydroxy enolate species and coadsorbed acids bound at vicinal acidbase pairs saturated with active monodentate carboxylates. Smaller Ti-Ti distances on rutile TiO 2 lead to the prevalence of unreactive bidentate carboxylates and lead to its much lower ketonization reactivity than anatase. The prevalent dense monolayers of chemisorbed acid reactants reflect their strong binding at acid-base pairs and their stabilization by H-bonding interactions with surface OH groups derived from the dissociation of the carboxylic acids or the formation of 1-hydroxy enolates; these interactions also stabilize CC coupling transition states preferentially over their carboxylate precursors; high coverages favor sequential dehydration routes of the αhydroxy-γ-carboxy-alkoxide CC coupling products over previously unrecognized concerted six-membered-ring transition states. Infrared spectra show that ubiquitous deactivation, which has precluded broader deployment of ketonization in practice and unequivocal mechanistic inquiries, reflects the gradual formation of inactive bidentate carboxylates. Their dehydration to ketene-like gaseous species is faster on anatase TiO 2 than on ZrO 2 and allows the effective scavenging of bidentate carboxylates via ketene hydrogenation to alkanals/alkanols on a Cu function present within diffusion distances. These strategies make anatase TiO 2 , a more effective catalyst than ZrO 2 , in spite of its slightly lower initial turnover rates. This study provides details about the mechanism of ketonization of C 2-C 4 carboxylic acids on TiO 2 and a rigorous analysis of the sites required and of active and inactive bound species on TiO 2 and ZrO 2. The preference for specific distances and for intermediate acid-base strength in M-O-M species is consistent with the structure and energy of the proposed transition states and intermediates; their relative stabilities illustrate how densely-covered surfaces, prevalent during ketonization catalysis, represent an essential requirement for the achievement of practical turnover rates.

Kinetics of Valeric Acid Ketonization and Ketenization in Catalytic Pyrolysis on Nanosized SiO2, γ‐Al2O3, CeO2/SiO2, Al2O3/SiO2 and TiO2/SiO2

“…Whereas the majority of current experimental results support the β‐keto acid mechanism, we think that it is still too early to completely disregard the concerted mechanism in favor of the β‐keto acid route because we believe the current results still do not conclusively confirm it either. In particular, a recently published isotope labeling study failed to confirm the β‐keto acid mechanism . Specifically, the article reported by Gumidyala et al.…”

“…Taking into account the high surface stability of carboxylate species (for acetates, =1590–1532 cm −1 , =1470–1420 cm −1 ), which remain present in IR spectra even at 400 °C, we assume that it is the weakly bound acid form that undergoes a transformation on the surface to yield a ketone. In fact, the participation of bidentate carboxylates in the formation of ketones was refuted in the recent work by Wang et al …”

Ketone Formation from Carboxylic Acids by Ketonic Decarboxylation: The Exceptional Case of the Tertiary Carboxylic Acids

“…[16] These results were clearly incompatible with the widely accepted b-keto acid mechanism and the authors proposedac arbon-centered anion as intermediate instead (Scheme2). [22][23][24][25][26][27][28][29] For the theoretical examination it was mandatory to "fix" the reactionc enters on ac rystalline surface to get reliable information and to avoid excessive freedom for reactants and catalysts/catalytic sites. Scheme 2, alternative pathway), which are not observed in general, this mechanism was modifieda nd the decarboxylation and carbon-carbon bond formationw ere proposed to occur in ac oncerted fashion.…”

“…[27] Essential reactions teps such as carbon-carbon bond formation to the b-keto-acid followed by decarboxylation are identical. Then, the resulting dianionic species attacks the acylium fragment nucleophilically,f orming ac arbon-carbon bond and therewith the bketo-acid.…”

Reduction of Propanoic Acid over Pd‐Promoted Supported WOx Catalysts

“…A splitting of 80 cm −1 between the symmetrical and asymmetrical C−O=O modes is in the range of bidentate and bidentate bridging species. These species have been observed on TiO 2 in the absence of any active metal, and calculated binding modes of acetic acid adsorption on TiO 2 suggest that this species is a bidentate bridging species . In contrast, a splitting of 230 cm −1 between the two modes observed in the presence of WO x is closer to the expected values found in unidentate or asymmetrical bidentate carboxylate species, indicative of weaker binding of the propanoate species on WO x…”