Mechanistic Insights into Metal-Catalyzed Hydrogenations and the Role of Metal Oxide Co-catalysts

The mechanistic origins of bifunctional synergies between metal nanoclusters and insulating oxide materials, present as a support or volumetric diluent, for catalytic hydrogenation reactions remains controversial despite decades of research. These bifunctional routes are often attributed to reaction pathways mediated by hydrogen spillover whereby H atoms migrate from metal surfaces, where they form from dissociated H2, to oxide surfaces and react with unsaturated molecules; however, such routes are unlikely to occur with insulating oxides which do not interact favorably with H atoms except at crystallographic defects. Herein, alternative explanations for such effects are investigated by exploring the mechanistic underpinnings of toluene hydrogenation on Pt surfaces and extending these insights to bifunctional reactions with γ-Al2O3 as a co-catalyst as an example of such phenomena.Turnover rates of methylcyclohexane formation from toluene and H2 reactants were measured on SiO2-supported Pt nanoparticles (3.6 mean Pt diameter) across a wide range of temperatures (333-533 K) and reactant pressures (20-80 kPa H2, 0.3-3 kPa toluene). Gaseous pressures of 1-methylcyclohexene and 4-methylcyclohexene intermediates are also reported. These methylcyclohexene species form as reactive intermediates during kinetically-relevant toluene hydrogenation routes; their gas-phase concentrations reflect the pseudo-steady state balance of their rates of formation (from toluene-H2 reactants) and consumption (by reaction with H2 to form methylcyclohexane) at lower temperatures (393-493 K), and their approach to thermodynamic equilibrium with toluene and H2 reactants at higher temperatures (493-533 K). These insights are leveraged to reduce the mechanistic complexity of toluene hydrogenation into two half reactions allowing for the kinetically-relevant steps and most abundant surface intermediates to be identified through kinetic modeling coupled with density functional theory calculations that probe binding energies and geometries of surface intermediates. Methylcyclohexane formation is shown to be kinetically limited primarily by the addition of the second H atom to toluene at low temperatures (333-493 K) and by the addition of the second H atom to methylcyclohexene intermediates at higher temperatures (493-533 K). These kinetic interpretations along with density functional theory calculations also show that the Pt surfaces are saturated with molecularly bound forms of toluene at all temperatures considered. Such a shift in the kinetically relevant steps towards later H additions along the toluene hydrogenation reaction coordinate as the temperature increases occurs because each H-addition step incurs a large entropic penalty due to the loss of highly entropic translation degrees of freedom of H2 molecules. The toluene-derived intermediates that participate in these later H-addition reactions are formed exothermically from stoichiometric amounts of toluene and H2, resulting in a negative apparent activation enthalpy when their hydrogenation becomes rate limiting. These behaviors account for the decrease in methylcyclohexane formation rates with increasing temperature that was observed at higher temperatures (>453 K). The ubiquitous presence of gaseous methylcyclohexene intermediates during toluene-H2 reactions on Pt surfaces informs plausible bifunctional reactions on Pt/SiO2+γ-Al2O3 mixtures. Two novel mechanistic interpretations are proposed and compared to relevant experimental and theoretical observations. One route involves the formation of partially hydrogenated toluene-derived intermediates (THn*) from toluene-H2 reactions on Pt surfaces. These species can desorb and hydrogenate at nearby γ-Al2O3 surfaces, without requiring atomic contact between Pt and γ-Al2O3, thus circumventing the monofunctional bottlenecks at Pt surfaces and increasing methylcyclohexane formation rates. Another route involves the formation of THn* species that undergo slow H-addition reactions to form methylcyclohexane, but which compete effectively for binding sites on crowded Pt surfaces with the predominant reactive intermediates involved in monofunctional routes. These stranded, less reactive THn* species can desorb to give very low equilibrium concentrations of their gaseous analogs; their migration to and hydrogenation at γ-Al2O3 surfaces present beyond atomic distances leads to their scavenging from the fluid phase, thus decreasing their equilibrium surface coverages and increasing the binding spaces available for the competitive reactions of intermediates that mediate monofunctional hydrogenation turnovers at Pt surfaces. Toluene hydrogenation was carried out over Pt/SiO2+γ-Al2O3 mixtures with varying separation between the Pt/SiO2 and γ-Al2O3 domains (via variation in the aggregate domain size of the respective catalytic components contained in physical mixtures). Methylcyclohexane formation rates were largest for Pt/SiO2+γ-Al2O3 mixtures with the smallest separation (and therefore greatest site proximity) and decreased asymptotically towards the rate observed on Al2O3-free Pt/SiO2 as the domain sizes of the respective components increased. Such trends resemble characteristic features of bifunctional reactions mediated by mobile and highly reactive gaseous intermediates. 1,3-Cyclohexadiene and 4-methylcyclohexene reactions with H2 were carried out over Pt-free γ-Al2O3 surfaces (30-300 Pa hydrocarbon, 0-90 kPa H2, 393 K). These unsaturated molecules are shown to hydrogenate predominantly via reaction with molecular H2 at rates that are catalytically significant in the context of Al2O3-catalyzed rates in bifunctional Pt/SiO2+γ-Al2O3 mixtures necessary to account for rate enhancements. Reaction-transport modelling based on the proposed bifunctional mechanisms and measured reactivity trends for independent Pt/SiO2 and γ-Al2O3 functions show that bifunctional synergies are best attributed to γ-Al2O3-mediated scavenging of methylcyclohexadiene inhibitors at low temperatures (393 K), prevalent only on Pt/SiO2 catalysts with small Pt clusters (0.7 nm mean diameter). At higher temperatures (493-533 K), in which methylcyclohexene hydrogenation becomes rate-limiting on Pt surfaces, an additive methylcyclohexene hydrogenation route at nearby γ-Al2O3 is likely to become more prevalent. These results thus provide novel mechanistic interpretations of bifunctional hydrogenation catalyzed by Pt/SiO2+γ-Al2O3 mixtures involving mobile, highly reactive gaseous molecular shuttles rather than H-atom spillover. Insights gained from this work advance the understanding of bifunctional synergies between metals and their nearby supports or diluent materials in composite catalytic materials.

Advisor: Iglesia, Enrique