RESUMO
Identifying individual reactive intermediates within the "zoo" of organometallic species that form on catalytic surfaces during reactions is a long-standing challenge in heterogeneous catalysis. Here, we identify distinct reactive intermediates, all of which exist at low coverages, that lead to distinguishable reaction pathways during the hydrogenolysis of 2-methyltetrahydrofuran (MTHF) on Ni, Ni12P5, and Ni2P catalysts by combining advanced spectroscopic methods with quantum chemical calculations. Each of these reactive complexes cleaves specific C-O bonds, gives rise to unique products, and exhibits different apparent activation barriers for ring opening. The spectral features of the reactive intermediates are extracted by collecting in situ infrared spectra while sinusoidally modulating the H2 pressure during MTHF hydrogenolysis and applying phase-sensitive detection (PSD), which suppresses the features of inactive surface species. The combined spectra of all reactive species are deconvoluted using singular-value decomposition techniques that yield spectra and changes in surface coverage for each set of kinetically differentiable species. These deconvoluted spectra are consistent with predicted spectral features for the reactive surface intermediates implicated by detailed kinetic measurements and DFT calculations. Notably, these methods give direct evidence for several anticipated differences in the coordination and composition of reactive MTHF-derived species. The compositions of the most abundant reactive intermediate (MARI) on Ni, Ni12P5, and Ni2P nanoparticles during the C-O bond rupture of MTHF are identical; however, MARI changes orientation from Ni3(µ3-C5H10O) to Ni3(η5-C5H10O) (i.e., lies more parallel with the catalyst surface) with increasing phosphorus content. The shift in binding configuration with phosphorus content suggests that the decrease in steric hindrance to rupture the 3C-O bond is the fundamental cause of increased selectivity toward 3C-O bond rupture. Previous kinetic measurements and DFT calculations indicate that C-O bond rupture occurs on Ni ensembles on Ni, Ni12P5, and Ni2P catalysts; however, the addition of more electronegative phosphorus atoms that withdraw a small charge from Ni ensembles results in differences in the binding configuration, activation enthalpy, and selectivity. The results from this in situ spectroscopic methodology support previous proposals that the manipulation of the electronic structure of metal ensembles by the introduction of phosphorus provides strategies for designing catalysts for the selective cleavage of hindered C-X bonds and demonstrate the utility of this approach in identifying individual reactive species within the zoo.
RESUMO
Lewis acid sites in zeolites catalyze aqueous-phase sugar isomerization at higher turnover rates when confined within hydrophobic rather than within hydrophilic micropores; however, relative contributions of competitive water adsorption at active sites and preferential stabilization of isomerization transition states have remained unclear. Here, we employ a suite of experimental and theoretical techniques to elucidate the effects of coadsorbed water on glucose isomerization reaction coordinate free energy landscapes. Transmission IR spectra provide evidence that water forms extended hydrogen-bonding networks within hydrophilic but not hydrophobic micropores of Beta zeolites. Aqueous-phase glucose isomerization turnover rates measured on Ti-Beta zeolites transition from first-order to zero-order dependence on glucose thermodynamic activity, as Lewis acidic Ti sites transition from water-covered to glucose-covered, consistent with intermediates identified from modulation excitation spectroscopy during in situ attenuated total reflectance IR experiments. First-order and zero-order isomerization rate constants are systematically higher (by 3-12×, 368-383 K) when Ti sites are confined within hydrophobic micropores. Apparent activation enthalpies and entropies reveal that glucose and water competitive adsorption at Ti sites depend weakly on confining environment polarity, while Gibbs free energies of hydride-shift isomerization transition states are lower when confined within hydrophobic micropores. DFT calculations suggest that interactions between intraporous water and isomerization transition states increase effective transition state sizes through second-shell solvation spheres, reducing primary solvation sphere flexibility. These findings clarify the effects of hydrophobic pockets on the stability of coadsorbed water and isomerization transition states and suggest design strategies that modify micropore polarity to influence turnover rates in liquid water.
