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.

a b s t r a c tRates and selectivity of TiO 2 -catalyzed condensation of C 3 oxygenates (propanal, acetone) are limited by ubiquitous effects of side reactions, deactivation, and thermodynamic bottlenecks. H 2 together with a Cu function, present as physical mixtures with TiO 2 , circumvents such hurdles by scavenging unsaturated intermediates. They also render alkanols and alkanals/alkanones equivalent as reactants through rapid interconversion, while allowing esterification turnovers by dehydrogenating unstable hemiacetals. Oxygenates form molecules with new CAC and CAO bonds and fewer O-atoms at nearly complete conversions with stable rates and selectivities. Kinetic, isotopic, and theoretical methods showed that rates are limited by a-CAH cleavage from carbonyl reactants to form enolate intermediates, which undergo CAC coupling with another carbonyl species to form a,b-unsaturated oxygenates or with alkanols to form hemiacetals with new CAO bonds, via an intervening H-shift that forms alkoxide-alkanal pairs. Titrations with 2,6-di-tert-butylpyridine, pyridine, CO 2 , and propanoic acid during catalysis showed that Lewis acid-base site pairs of moderate strength mediate enolate formation steps via concerted interactions with the a-H atom and the enolate moiety at transition states. The resulting site-counts allow rigorous comparisons between theory and experiments and among catalysts on the basis of turnover rates and activation free energies. Theoretical treatments give barriers, kinetic isotope effects, and esterification/-condensation ratios in excellent agreement with experiments and confirm the strong effects of reactant substituents at the a-C-atom and of surface structure on reactivity. Surfaces with TiAOATi sites exhibiting intermediate acid-base strength and TiAO distances, prevalent on anatase but not rutile TiO 2 , are required for facile a-CAH activation in reactants and reprotonation of the adsorbed intermediates that mediate condensation and esterification turnovers.

A green approach to efficient conversion of cellulose into hexitols and other lighter alcohols through two steps is reported. In this process, cellulose is hydrolyzed to glucose by acids formed reversibly in situ from hot water, and the glucose is hydrogenated on Ru/C. Understanding of the green aqueous catalytic systems should lead to more efficient conversion of cellulose into fuels and chemicals.

Hydrogenolysis of biomass-derived glycerol is an alternative route to sustainable production of propylene glycol. Cu-ZnO catalysts were prepared by coprecipitation with a range of Cu/Zn atomic ratio (0.6-2.0) and examined in glycerol hydrogenolysis to propylene glycol at 453-513 K and 4.2 MPa H 2 . These catalysts possess acid and hydrogenation sites required for bifunctional glycerol reaction pathways, most likely involving glycerol dehydration to acetol and glycidol intermediates on acidic ZnO surfaces, and their subsequent hydrogenation on Cu surfaces. Glycerol hydrogenolysis conversions and selectivities depend on Cu and ZnO particle sizes. Smaller ZnO and Cu domains led to higher conversions and propylene glycol selectivities, respectively. A high propylene glycol selectivity (83.6%), with a 94.3% combined selectivity to propylene glycol and ethylene glycol (also a valuable product) was achieved at 22.5% glycerol conversion at 473 K on Cu-ZnO (Cu/Zn = 1.0) with relatively small Cu particles. Reaction temperature effects showed that optimal temperatures (e.g. 493 K) are required for high propylene glycol selectivities, probably as a result of optimized adsorption and transformation of the reaction intermediates on the catalyst surfaces. These preliminary results provide guidance for the synthesis of more efficient Cu-ZnO catalysts and for the optimization of reaction parameters for selective glycerol hydrogenolysis to produce propylene glycol.