Atomistic Origins of Biomass Recalcitrance in Organosolv Pretreatment

Secondary plant cell walls are made from three common biopolymers, cellulose, lignin, and hemicellulose, representing a critical feedstock for sustainable biomaterial production. Separating lignocellulosic biomass components for use in tailored sustainable energy and materials applications is challenging, as the biopolymers are in close proximity within the plant secondary cell wall. Organic solvents are used to pretreat recalcitrant biomass and separate the interacting polymers, solubilizing the lignin fraction for lignin-first valorization approaches. However, no single organosolv pretreatment approach has proven superior for heterogeneous biomass samples. Simulation offers a complementary atomic view into interactions between biomass components, resolving mechanistic hypotheses for how biomass composition influences separations processes. Using molecular dynamics simulations, we quantify lignin-cellulose interactions through binding free energies determined from 300 lignin polymer models in nine solvent environments, across four crystalline cellulose faces, with an aggregate simulation time of nearly 154 microseconds. The binding free energy determined from simulation categorizes the solvents. For poor lignin solvents, all lignin polymers bind strongly to cellulose. By contrast, polar protic solvents such as methanol and ethanol favor the unbinding between lignin and cellulose in all conditions, regardless of charge for the lignin monomer tested. Aprotic organic solvents separate lignin from cellulose only for uncharged lignin monomers, with charged lignin monomers associating to cellulose. While polar protic solvents are most effective at breaking apart lignin-cellulose interactions for charged lignin species, solvent dynamics highlight that there is no single optimal solvent to facilitate lignin-cellulose separation, particularly as some solvents demonstrate greater effectiveness for skewed S:G ratios. Instead, the optimal solvent for a given lignin sample will depend on the lignin compound and the net charge for the lignin polymers.