Identities and Catalytic Functionalities of Reactive Hydrogen Species during Hydrogen Addition and Oxygen Removal Reactions of Carboxylic Acids, Carbonyls, and Phenols at Transition Metal-solvent Interfaces

At transition metal and protic solvent interfaces, H-adatoms (H*) derived from H2 (10-60 bar) dissociative adsorption, and protons (H+) derived from either H* ionization or solvent self-ionization are catalytically reactive H species. These H species participate in or assist with hydrodeoxygenation of phenolics and carbonyls to produce alkanes and alkanols; yet their specific identities and catalytic functions remain unclear. This thesis interrogates the origin, chemical identities, and electronic charge of reactive H species and connects these properties to their catalytic roles within the complex reaction network. These roles depend on the solvent and reactant identity and lead to rate equations that depend on the number of catalytically active sites during the hydrodeoxygenation of C3-C6 carbonyls, acetic acid, and phenols on Ru, Pt, and Pd clusters in protic solvents. Across all three catalytic systems of carbonyl hydrogenation, acetic acid hydrodeoxygenation, and phenolic hydrodeoxygenation, H+ and H* both play their respective roles as intermediates in the overall catalytic system. It is proposed that H+, derived from H* ionization, adds onto the carbonyl oxygen followed by H* addition onto the carbonyl carbon, thus leading to C=O hydrogenation. Proton transfer steps cause the rate constants to increase as the proton affinity of the carbonyl group increases or as the solvent dielectric permittivity increases. Acetic acid undergoes rate limiting C-O bond rupture on a vacant Ru site to form a surface acyl group. The acyl undergoes H* and H+ addition to form a valuable ethanol product. The selectivity of ethanol is found to increase with H* and H+ concentrations that are in turn determined by the square root of the H2 pressure and acetic acid concentration, respectively. Quasi-equilibrated H* addition to the aromatic ring of guaiacol leads to the formation of partially saturated enol intermediates. H+ from the solvent catalyzes the tautomerization of the partially saturated enol, leading to the formation of its keto tautomer that either undergoes C-OCH3 bond cleavage or another H* addition to form hydrodeoxygenation or hydrogenation products.

Department dc:contributor.department

Chemical Engineering Applied Chemistry

Year dc:date.issued

2019

Author dc:creator

• Shangguan, Junnan

Advisor dc:contributor.advisor

• Chin, Ya-Huei (Cathy)

• Heterogeneous catalysis
• Hydrogenation
• Isotopic labeling
• Kinetic measurements
• Solvent effect
• Transition metal

Handle dc:identifier.uri

http://hdl.handle.net/1807/107962

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/107962