(720h) Fundamentals of Mechanocatalysis for Lignin Valorization

Converting lignin to value-added chemicals will be essential as the world shifts away from non-renewable sources of carbon. Often, solvent recovery can be the most costly step in catalytic processes for biomass upgrading. Alternatively, mechanocatalytic reactions can be performed under solvent free conditions. Mechanocatalysis uses mechanical energy, typically in the form of collisions or impacts, to drive catalytic reactions in place of direct thermal energy. A basic phenomenon of mechanocatalysis is the creation of hot spots, but the exact conditions at the point of collision have not been well characterized. Here, the chemistry of mechanocatalytic systems is studied through the catalytic reactivity of benzyl phenyl ether (BPE), a lignin model ether, and insight into the nature of hotspot is provided studying the decomposition of carbonates.

Simultaneous hydrogenolysis and hydrodeoxygenation is seen as one of the more promising approaches for lignin conversion to chemicals. Combining the depolymerization reaction and product upgrading into one system allows for efficient processing through easier separation of volatile products. The hydrogenolysis of BPE over nickel catalysts was investigated as a model system for lignin depolymerization. Reactivity studies show a BPE conversion of 55% after 1 h, but a carbon balance of only 76%, with toluene (37% yield) and phenol (20% yield) as the major products. Thermogravimetric analysis shows that approximately 30 wt% of the products were strongly adsorbed on the catalyst surface and not removed during the washing step. Additional investigations will focus on the hydrodeoxygenation of phenol, determining the role of the support, characterizing the effect milling has on the catalyst, and the catalyst recyclability. Finally, the applicability of this system for lignin valorization will be discussed.

To better describe catalytic activity in ball mill reactors, hot spot temperatures and durations need to be determined. The mechanochemical decomposition of calcium carbonate was investigated as a model reaction because one of the main products is gaseous (CO₂), and the thermal kinetics have been well documented. The milling of CaCO₃ produced CO₂ at rates of between 9.0^.10^-3 µmol_CO2 min^-1 g^-1_reagent at 22.5 Hz and 1.4^.10^-1 µmol_CO2 min^-1 g^-1_reagent at 30 Hz (Figure 2), which correspond to apparent reactor temperatures of at least 790 K and 860 K, respectively. These values represent lower bounds for the collision temperature as only a small fraction of the CaCO₃ is present at rapidly decaying hot spots at a given time. The combination of this data with audio analysis of the collision frequencies allows for the calculation of a “per collision” rate. Complimentarily, the collision conditions can be determined by modeling the energy dissipation of the collisions from friction and translating that to a thermal profile in the powder bed. The preliminary results from the model show peak hot spot temperatures of 1128 K with durations of 15 ms. Results of applying the model to the carbonate decomposition are compared to the experimental results. Validation of this model will allow for its application in the mechanocatalytic hydrogenolysis of BPE, depolymerization of lignin, and many other reactions to better understand catalyst performance and the system energy balance.

These two studies focus on elucidating the underlying chemical mechanisms and reaction conditions present in mechanocatalytic reactors. These insights will aid in directed research and optimization of catalysts and reactor scale-up for direct biomass valorization (e.g. lignin depolymerization) and product up-grading via mechanocatalysis