Solid-state NMR studies of solvent-mediated, acid-catalyzed woody biomass pre-treatment for enzymatic conversion of residual cellulose.

Enzymes selectively hydrolyze the carbohydrate fractions of lignocellulosic biomass into corresponding sugars, but these processes are limited by low yields and slow catalytic turnovers. Under certain conditions, the rates and yields of enzymatic sugar production can be increased by pretreating biomass using solvents, heat and dilute acid catalysts. However, the mechanistic details underlying this behavior are not fully elucidated, and designing effective pretreatment strategies remains an empirical challenge. Herein, using a combination of solid-state and high-resolution magic-angle-spinning NMR, infrared spectroscopy and X-ray diffractometry, we show that the extent to which cellulase enzymes are able to hydrolyze solvent-pretreated biomass can be understood in terms of the ability of the solvent to break the chemical linkages between cellulose and non-cellulosic materials in the cell wall. This finding is of general significance to enzymatic biomass conversion research, and implications for designing improved biomass conversion strategies are discussed. These findings demonstrate the utility of solid-state NMR as a tool to elucidate the key chemical and physical changes that occur during the liquid-phase conversion of real biomass.

Recycling of multilayer plastic packaging materials by solvent-targeted recovery and precipitation.

Many plastic packaging materials manufactured today are composites made of distinct polymer layers (i.e., multilayer films). Billions of pounds of these multilayer films are produced annually, but manufacturing inefficiencies result in large, corresponding postindustrial waste streams. Although relatively clean (as opposed to municipal wastes) and of near-constant composition, no commercially practiced technologies exist to fully deconstruct postindustrial multilayer film wastes into pure, recyclable polymers. Here, we demonstrate a unique strategy we call solvent-targeted recovery and precipitation (STRAP) to deconstruct multilayer films into their constituent resins using a series of solvent washes that are guided by thermodynamic calculations of polymer solubility. We show that the STRAP process is able to separate three representative polymers (polyethylene, ethylene vinyl alcohol, and polyethylene terephthalate) from a commercially available multilayer film with nearly 100% material efficiency, affording recyclable resins that are cost-competitive with the corresponding virgin materials.

Effects of chloride ions in acid-catalyzed biomass dehydration reactions in polar aprotic solvents.

The use of polar aprotic solvents in acid-catalyzed biomass conversion reactions can lead to improved reaction rates and selectivities. We show that further increases in catalyst performance in polar aprotic solvents can be achieved through the addition of inorganic salts, specifically chlorides. Reaction kinetics studies of the Brønsted acid-catalyzed dehydration of fructose to hydroxymethylfurfural (HMF) show that the use of catalytic concentrations of chloride salts leads to a 10-fold increase in reactivity. Furthermore, increased HMF yields can be achieved using polar aprotic solvents mixed with chlorides. Ab initio molecular dynamics simulations (AIMD) show that highly localized negative charge on Cl- allows the chloride anion to more readily approach and stabilize the oxocarbenium ion that forms and the deprotonation transition state. High concentrations of polar aprotic solvents form local hydrophilic environments near the reactive hydroxyl group which stabilize both the proton and chloride anions and promote the dehydration of fructose.

Kinetics of Levoglucosenone Isomerization.

We studied the acid-catalyzed isomerization of levoglucosenone (LGO) to 5-hydroxymethylfurfural (HMF) and developed a reaction kinetics model that describes the experimental data across a range of conditions (100-150 °C, 50-100 mm H2 SO4 , 50-150 mm LGO). LGO and its hydrated derivative exist in equilibrium under these reaction conditions. Thermal and catalytic degradation of HMF are the major sources of carbon loss. Within the range of conditions studied, higher temperatures and shorter reaction times favor the production of HMF. The yields of HMF and levulinic acid decrease monotonically as tetrahydrofuran is added to the aqueous solvent system, indicating that water plays a role in the LGO isomerization reaction. Initial-rate analyses show that HMF is produced solely from LGO rather than from the hydrated derivative of LGO. The results of this study are consistent with a mechanism for LGO isomerization that proceeds through hydration of the anhydro bridge, followed by ring rearrangement analogous to the isomerization of glucose to fructose.