Andrew W. Tricker

(Advisors: Prof. Carsten Sievers, Prof. Matthew Realff, Prof. Valerie Thomas)

will defend a doctoral thesis entitled,

Fundamentals of Mechanocatalysis for Lignin Valorization

on

Wednesday, June 02 at 3:00 p.m.

https://bluejeans.com/242330355

Abstract:

Lignin, the largest natural source of aromatics, is an appealing sustainable feedstock for many chemicals and materials. The depolymerization of lignin to mono-aromatics has remained challenging and industrial applications have remained elusive. A promising approach to biomass valorization and deconstruction has been mechanocatalysis. This approach uses mechanical energy, often supplied in ball mill reactors, to drive reaction under solvent free and ambient conditions. However, fundamental understanding of mechanocatalysis remains enigmatic, presenting its own set of challenges. The aim of this thesis is to lay fundamental groundwork to better understand mechanocatalytic systems and how these systems can be applied for depolymerizing and valorizing lignin.

Characterizing the structure of lignin isolated from traditional and alternative industrial processes can be used to assess their viability as feedstock for depolymerization processes. The lignin samples are characterized by elemental analysis, nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), and thermogravimetric analysis (TGA). Quantification of the β‑O‑4 ether bond content shows partial depolymerization, with all samples having less than 12 bonds per 100 aromatics. This results in a theoretical monomer yields less than 5%, strongly suggesting the alternative fractionation processes generate highly condensed lignin structures that are no more suitable for catalytic depolymerization than kraft lignin.

To better understand the environments in mechanochemical reactors, a three-part modeling approach to describe the reactive conditions created during collisions is presented.  The approach is focused on the creation of hot spots as the mechanism of enhanced reaction. Here, energy dissipated during a collision is converted to heat in the milling media, and the reaction proceeds thermochemically. The final result of the model is the extent of reaction over a single collision. To verify the approach, the mechanochemical decomposition of calcium carbonate is studied. The real-time CO2 production under varying milling frequencies is measured using an in-line mass spectrometer. The model describes hot spots with temperatures exceeding 1000 K that persist for tens of milliseconds.

Novel behavior of catalysts under mechanocatalytic conditions is explored by introducing a new approach for ammonia production at nominally ambient conditions. As proof of concept, ammonia is synthesized mechanocatalytically by ball milling titanium in a continuous gas flow. The ammonia synthesis reaction is proposed to follow a transient Mars-van Krevelen mechanism under mechanically activated conditions, where molecular nitrogen incorporation into the titanium lattice and titanium nitride hydrogenation occur in thermodynamically distinct environments. The reactivity of nitrided titanium supports that lattice nitrogen plays a role in ammonia formation. The in situ formed titanium nitride is catalytically active, and the nitride regeneration reaction is determined to be the rate-limiting step.

Finally, the mechanocatalytic hydrogenolysis of benzyl phenyl ether (BPE), a model lignin ether, is demonstrated over supported nickel catalysts at room temperature and atmospheric hydrogen pressure. The hydrogenolysis reaction network closely follows those of solution-based reactions. The mechanical energy during milling not only drives the chemical reactions, but also activates the nickel and exposes fresh metallic surfaces. Recycle experiments shows continual deactivation over three reaction cycles and the formation of polyaromatic coke species. The formation of the carbon deposits is expected to be the primary cause of deactivation. Varying support properties shows that the hydrogenolysis rate is largely independent of the support properties, but the enhanced reactivity of the oxide supports during milling contributes to the carbon loss.

Committee