Ceria has recently shown intriguing hydrogenation reactivity in catalyzing alkyne selectively to alkenes. However, the mechanism of the hydrogenation reaction, especially the activation of H, remains experimentally elusive. In this work, we report the first direct spectroscopy evidence for the presence of both surface and bulk Ce-H species upon H dissociation over ceria via in situ inelastic neutron scattering spectroscopy. Combined with in situ ambient-pressure X-ray photoelectron spectroscopy, IR, and Raman spectroscopic studies, the results together point to a heterolytic dissociation mechanism of H over ceria, leading to either homolytic products (surface OHs) on a close-to-stoichiometric ceria surface or heterolytic products (Ce-H and OH) with the presence of induced oxygen vacancies in ceria. The finding of this work has significant implications for understanding catalysis by ceria in both hydrogenation and redox reactions where hydrogen is involved.

Lignin is an abundant natural resource that is a potential source of valuable chemicals. Improved understanding of the pyrolysis of lignin occurs through the study of model compounds for which phenethyl phenyl ether (PhCH(2)CH(2)OPh, PPE) is the simplest example representing the dominant beta-O-4 ether linkage. The initial step in the thermal decomposition of PPE is the homolytic cleavage of the oxygen-carbon bond. The rate of this key step will depend on the bond dissociation enthalpy, which in turn will depend on the nature and location of relevant substituents. We used modern density functional methods to calculate the oxygen-carbon bond dissociation enthalpies for PPE and several oxygen-substituted derivatives. Since carbon-carbon bond cleavage in PPE could be a competitive initial reaction under high-temperature pyrolysis conditions, we also calculated substituent effects on these bond dissociation enthalpies. We found that the oxygen-carbon bond dissociation enthalpy is substantially lowered by oxygen substituents situated at the phenyl ring adjacent to the ether oxygen. On the other hand, the carbon-carbon bond dissociation enthalpy shows little variation with different substitution patterns on either phenyl ring.

We calculated an overall alpha/beta-selectivity for the pyrolysis of phenethyl phenyl ether as a composite of the alpha/beta-selectivities in the hydrogen abstraction reactions by the phenoxyl and by the benzyl radical that is in excellent agreement with experiment. The difference between the individual selectivities for these radicals is explained by analyzing the electronic structure of the transition states. Spin delocalization of the single electron favors the alpha-pathways. An opposing effect occurs for polarized transition states, such as the transition states for the hydrogen abstraction by the electrophilic phenoxyl radical, where the adjacent ether oxygen in phenethyl phenyl ether stabilizes the beta-transition states. These results indicate that theory will be able to provide excellent predictions of alpha/beta-product selectivities for more complicated lignin model compounds bearing multiple substituents. We have developed a scheme to predict alpha/beta-product selectivities in the pyrolysis of model compounds for the beta-ether linkage in lignin. The approach is based on computation of the relative rate constant, which profits from error cancellation in the individual rate constants. The Arrhenius prefactors depend strongly on the description of the low-frequency modes for which anharmonic contributions are important. We use density functional theory in combination with transition-state theory in this analysis. Diagonal anharmonic effects for individual low-frequency modes are included by employing a second-order Wigner-Kirkwood expansion in a semiclassical expression for the vibrational partition function. The composite alpha/beta-product selectivity is obtained by applying quasi-steady-state kinetic analysis for the intermediate radicals.

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The biopolymer lignin is a potential source of valuable chemicals. Phenethyl phenyl ether (PPE) is representative of the dominant β-O-4 ether linkage. DFT is used to calculate the Boltzmann-weighted carbon-oxygen and carbon-carbon bond dissociation enthalpies (BDEs) of substituted PPE. These values are important for understanding lignin decomposition. Exclusion of all conformers that have distributions of less than 5% at 298 K impacts the BDE by less than 1 kcal mol(-1). We find that aliphatic hydroxyl/methylhydroxyl substituents introduce only small changes to the BDEs (0-3 kcal mol(-1)). Substitution on the phenyl ring at the ortho position substantially lowers the C-O BDE, except in combination with the hydroxyl/methylhydroxyl substituents, for which the effect of methoxy substitution is reduced by hydrogen bonding. Hydrogen bonding between the aliphatic substituents and the ether oxygen in the PPE derivatives has a significant influence on the BDE. CCSD(T)-calculated BDEs and hydrogen-bond strengths of ortho-substituted anisoles, when compared with M06-2X values, confirm that the latter method is sufficient to describe the molecules studied and provide an important benchmark for lignin model compounds.

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Lignin is an underused but major component of biomass. One possible area of utilization is the production of carbon fiber. A necessary processing step is the stabilization of lignin fiber (typically in an oxygen environment) before high temperature treatment. We investigate oxidative, thermal conversion of lignin using computational methods. Dilignol model compounds for the most common (seven) linkages in softwood are chosen to represent the diverse structure of lignin. We perform molecular dynamics simulation where the potential energy surface is described by a reactive force field (ReaxFF). We calculate overall activation energies for model conversion and reveal initial mechanisms of formaldehyde formation. We record fragmentation patterns and average carbon oxidation numbers at various temperatures. Most importantly, we identify mechanisms for stabilizing reactions that result in cyclic and rigid connections in softwood lignin fibers that are necessary for further processing into carbon fibers.

Studies on the pyrolysis mechanisms of lignin model compounds have largely focused on initial homolytic cleavage reactions. It has been noted, however, that concerted mechanisms may also account for observed product formation. In the current work, the latter processes are examined and compared to the former, by the application of density functional theory calculations to fully substituted lignin models. Results show that activation energies for the concerted reactions are somewhat lower than the bond dissociation energies of the homolysis reactions. Kinetic analysis revealed that the concerted pathway is the retro-ene fragmentation mechanism.