(600n) Fundamental Understanding of Lignin Deconstruction – Quantum Chemical Investigation and Microkinetic Modeling of Prevalent Reactions

Lignin, a heterogeneous polymer, which accounts for 20-33% of wood, has attracted much attention lately due to the potential in deriving valuable intermediates for fuels and chemicals. There is a significant initiative to understand the deconstruction chemistry of lignin into useful compounds with lower carbon number, but very little at the mechanistic level is known thus far due to its complex structure. Lignin is composed of three types of phenyl propanoid monomer units linked together by various carbon-carbon and carbon-oxygen bonds. Basic understanding gained from experiments defines lignin according to several types of bonds between C₉ monomers with β-O-4 being the most common bond type, which accounts for c.a. 50% of all the bonds.  Although many model compounds containing the β-O-4 bond type have been used to understand the underlying lignin depolymerization chemistry during acid-catalyzed hydrolysis, it is not sufficient to consider lignin as being represented by one bond type. Quantum chemical calculations allow the mechanism underlying the conversion of many possible lignin model compounds with different bond types to be explored, which could further aid in unraveling the reaction pathways leading to small molecule derivatives. In this study, we use quantum chemical calculations with transition state theory to understand reaction mechanisms, rate parameters, and solvent effects during the acidolysis of lignin model compounds. The kinetic rate parameters of the reaction of model compounds can be provided as input to a microkinetic model to understand the prevalent reactions and product composition during the deconstruction of actual lignin under various reaction conditions. In this presentation, we will discuss our efforts in generating a realistic structure of lignin based on known monomer and bond compositions, and kinetic modeling of reactions unraveled by quantum chemistry calculations.