(544b) Group IB Metal Activated Anatase TiO2 for Selective Catalytic Deoxygenation of Lignin Fragments

Lignin is a major component of lignocellulosic biomass. The primary chemical structure of lignin is composed of propyl benzene units linked by a complex network of aromatic C-O and aliphatic C-O bonds. Catalytic hydrodeoxygenation (HDO) has been recognized as an important process strategy to convert lignin fragments to useful products. Supported metal catalysts for HDO in the literature typically showed the following performance characteristics: (1) production of fully saturated products as alkanes, and (2) formation of mixed aromatics and phenolic products, often with saturated hydrocarbons. Catalyst deactivation due to coke formation on supports or metal sintering was often encountered. The supported noble metals or transition metals provide the active sites.

As an abundant and representative component of lignin, guaiacol as a model compound has been widely studied in evaluating the effectiveness of hydrodeoxygenation catalysts. We recently discovered that Group IB metals such as Au and Ag, known for poor hydrogenation activity are capable of activating anatase TiO₂ (TiO2-A) to generate unconventional hydrogenation sites for selective HDO of guaiacol by producing phenolics as the main products. While Au and Ag particles on several supporting materials, including rutile TiO₂, ZrO₂, Al₂O₃, SiO₂, and activated carbon, did not show distinguishable performance compared to that of the supports alone in the conversion of guaiacol, these metals on anatase TiO₂ support exhibited high activity with excellent stability and remarkable selectivity to phenolics in guaiacol hydrodeoxygenation. For example, at guaiacol conversion of 43% over a 0.4 wt% Au/TiO₂-A-40 nm catalyst, the selectivity to phenolics reached 87%. No aromatics and saturated hydrocarbons such as cyclohexane were formed. Gold particle size ranging from 2.7 to 41 nm was found to affect the catalyst activity but not the product selectivity. The activity of the catalysts did not markedly decrease in accordance with the increase of Au particle size or the decrease of total surface area of the gold particles in the catalysts, suggesting that activated TiO₂-A surface away from the interface between Au and the TiO₂-A also contributed to the HDO performance. The reaction rates of 0.26 and 0.91 (min^−1 g-cat^−1 cm³) were determined for guaiacol hydrogenation and catechol hydrogenation, respectively. Bimolecular methylation was established as the dominant mechanism for methyl group transfer from guaiacol to phenolics. While silver and copper showed similar selectivity at subnanometer particle sizes, considerably less Ag and Cu were effective to achieve similar guaiacol conversion levels as with Au/ TiO₂-A. Temperature programmed reduction profiles of Ag/ TiO₂-A and Cu/ TiO₂-A showed that partial reduction of TiO₂-A was enhanced by the presence of Ag and Cu. Only when TiO₂-A reduction took place, the Group IB became active for the HDO, with phenolics as the only deoxygenation products.

Two major pathways of guaiacol hydrogenation to phenolics over anatase TiO₂supported Group IB metals catalysts are proposed: (1) direct hydrogenation of guaiacol to form phenol and methanol, (2) hydrodehydroxylation of catechol intermediate formed from transmethylation between guaiacol and phenolics.

The pyrolytic product of a virgin lignin extracted from corn-stalk was studied with the catalysts. The results are consistent with the proposed mechanistic pathways in producing phenolic compounds.