(198f) Molecular Reaction Networks of Photocatalytic and Photochemical Reactions of Platform Phenolic Compounds From Lignin

Lignin biomass can be converted to the high-value low molecular weight organic compounds. Certain phenolic compounds obtained from lignin find applications as food natural flavors, taste enhancers, antioxidants, fragrances, and fine chemicals for synthesis of sustainable bio-degradable polymers. Photochemical and photocatalytic reactions rely upon the sustainable, abundant and free light of the Sun to convert photon energy to the energy of chemical bonds or to accelerate chemical reactions.

The goal of our research is to investigate molecular reaction networks and mechanisms of heterogeneous photocatalytic and photochemical reactions of few major phenolic compounds from lignin with C=C and C=O bonds in the molecule, such as isoeugenol, eugenol and vanillin. For photooxidation, we use oxygen from air under the "green" ambient conditions, near-UV light, and nanocrystalline TiO₂ as photocatalyst in the non-aqueous solvent. For photolysis and photo-isomerization, we use the UV light vs. near-UV light in the non-aqueous solvent. Molecular products of photoreactions of isoeugenol, eugenol and vanillin were determined qualitatively and quantitatively using thin layer chromatography (TLC), high performance liquid chromatography HPLC with optical detection (HPLC-UV), and with dual UV- and mass-spectrometric detection via electrospray ionization (HPLC-MS-ESI).

Photocatalytic oxidation of isoeugenol with oxygen from air produces dehydrodiisoeugenol at the high (up 80 %) yields, and eugenol and vanillin as minor products. Photooxidation or photoisomerization of eugenol does not proceed under the same conditions. Photooxidation of vanillin leads to vanillic acid and dimeric product. Using photoexcitation with light of the longer wavelengths leads to fewer molecular products of photoreactions; photocatalytic vs. photochemical oxidation (without TiO₂ photocatalyst) leads to the higher yields.