Biomass recalcitrance is a fundamental bottleneck to producing fuels from renewable sources. To understand its molecular origin, we characterize the interaction network and solvation structures of cellulose microfibrils via all-atom molecular dynamics simulations. The network is divided into three components: intrachain, interchain, and intersheet interactions. Analysis of their spatial dependence and interaction energetics indicate that intersheet interactions are the most robust and strongest component and do not display a noticeable dependence on solvent exposure. Conversely, the strength of surface-exposed intrachain and interchain hydrogen bonds is significantly reduced. Comparing the interaction networks of I(β) and I(α) cellulose also shows that the number of intersheet interactions is a clear descriptor that distinguishes the two allomorphs and is consistent with the observation that I(β) is the more stable form. These results highlight the dominant role of the often-overlooked intersheet interactions in giving rise to biomass recalcitrance. We also analyze the solvation structures around the surfaces of microfibrils and show that the structural and chemical features at cellulose surfaces constrict water molecules into specific density profiles and pair correlation functions. Calculations of water density and compressibility in the hydration shell show noticeable but not drastic differences. Therefore, specific solvation structures are more prominent signatures of different surfaces.

Pretreatment for deconstructing the multifaceted interaction network in crystalline cellulose is a limiting step in making fuels from lignocellulosic biomass. Not soluble in water and most organic solvents, cellulose was found to dissolve in certain classes of ionic liquids (ILs). To elucidate the underlying mechanisms, we simulated cellulose deconstruction by peeling off an 11-residue glucan chain from a cellulose microfibril and computed the free-energy profile in water and in 1-butyl-3-methylimidazolium chloride (BmimCl) IL. For this deconstruction process, the calculated free-energy cost/reduction in water/BmimCl is ∼2 kcal/mol per glucose residue, respectively. To unravel the molecular origin of solvent-induced differences, we devised a coarse graining scheme to dissect force interactions in simulation models by a force-matching method. The results establish that solvent-glucan interactions are dependent on the deconstruction state of cellulose. Water couples to the hydroxyl and side-chain groups of glucose residues more strongly in the peeled-off state but lacks driving forces to interact with sugar rings and linker oxygens. Conversely, BmimCl demonstrates versatility in targeting glucose residues in cellulose. Anions strongly interact with hydroxyl groups, and the coupling of cations to side chains and linker oxygens is stronger in the peeled-off state. Other than enhancing anion-hydroxyl group coupling, coarse-grain analysis of force interactions identifies configuring cations to target side chains and linker oxygens as a useful design strategy for pretreatment ILs. Furthermore, the state dependence of solvent-glucan interactions highlights specific stabilization and/or frustration of the different structure states of cellulose as important design parameters for pretreatment solvents.

Cellulose is present in biomass as crystalline microfibrils held together by a complex network of intermolecular interactions making it difficult to initiate its hydrolysis and conversion to fuels. While cellulose is insoluble in water and most organic solvents, complete dissolution of cellulose can be achieved in certain classes of ionic liquids (ILs). The present study was undertaken to analyze the thermodynamic driving forces of this process and to understand how the anions and cations comprising an IL interact with the different moieties of glucose residues to cause dissolution. All-atom molecular dynamics (MD) simulations were performed at two extreme states of cellulose dissolution: a crystalline microfibril and a dissociated state in which all the glucan chains of the microfibril are fully separated from each other by at least four solvation shells. MD simulations of the two states were carried out in water and in the IL 1-butyl-3-methylimidazolium chloride (BmimCl) to provide a comprehensive analysis of solvent effects on cellulose dissolution. The results reveal two important molecular aspects of the mechanism of cellulose dissolution. The first is that the perturbation of solvent structures by the dissolved glucan chains can be a crucial factor in determining solubility, particularly for the insolubility of cellulose in water at 300 K. Second, both the Cl(-) and the Bmim(+) ions of BmimCl interact with the moieties of glucan residues that form intersheet contacts, the most robust component of the interaction network of crystalline cellulose. Cl(-) anions can form hydrogen bonds (HBs) with the hydroxyl groups of glucan chains from either the equatorial or the axial directions. For Bmim(+) cations, the calculated density profiles reveal that the contacts with glucan chains along the axial directions are closer than those along the equatorial directions. On the basis of the results of atomistic MD simulations, we propose that interacting with glucan chains along axial directions and disrupting the intersheet contacts of cellulose is an important ability of cellulose pretreatment solvents.

The entropic driving forces of cellulose dissolution in water and in the ionic liquid 1-butyl-3-methylimidazolium chloride (BmimCl) are investigated via molecular dynamics simulations and the two-phase thermodynamic model. An atomistic model of cellulose was simulated at a dissociated state and a microfibril state to represent dissolution. The calculated values of entropy and internal energy changes between the two states inform the interplay of energetic and entropic driving forces in cellulose dissolution. In both water and BmimCl, we found that the entropy associated with the solvent degrees of freedom (DOF) decreases upon cellulose dissolution. However, solvent entropy reduction in BmimCl is much smaller than that in water and counteracts the entropy gain from the solute DOF to a much lesser extent. Solvent entropy reduction in water also plays a major role in making the free energy change of cellulose dissolution unfavorable at room temperature. In BmimCl, the interaction energies between solvent molecules and glucan chains and the total entropy change both contribute favorably to the dissolution free energy of cellulose. Calculations at different temperatures in the two solvents indicate that changes in total internal energy are a good indicator of the sign of the free energy change of cellulose dissolution.