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Epoxide ring opening by alcohols generates molecules that are used in a wide range of applications, including as polyols that are components of polyurethanes. The effects of nucleophile structure on regioselectivity and initial reaction rates were compared for base-catalyzed and coordination-catalyzed 1,2-epoxyoctane ring opening by 1-propanol (1PrOH) and tert-butanol (tBuOH) using potassium tert-butoxide and tris(pentafluorophenyl)borane as case studies. Experimentally measured initial rates of the tris(pentafluorophenyl)borane-catalyzed system were deconvoluted using a microkinetic model built upon a triple-pathway (Lewis acid, water-mediated, and alcohol-mediated) reaction scheme we previously developed. Benchmark, base-catalyzed systems consistently showed slow rates and low selectivities to the primary alcohol regioisomer (P1), especially for the bulkier nucleophile. In contrast, aryl boranes achieved high P1 selectivity and relatively fast epoxide ring-opening rates. Interestingly, low water levels led to tBuOH being the faster nucleophile, while at high water levels, 1PrOH had faster rates. Analyses of reactive flux and catalyst speciation using the model showed that the unexpectedly high relative rate of tBuOH at low water levels was due to higher concentrations of catalytically active species from faster pathways and lower flux through the slow, low-selectivity alcohol-mediated pathway for tBuOH.

The economic viability of the biofuel industry has been plagued in part by the incomplete valorization of lignin, which is currently being burned for process heat. One of the roadblocks to effectively converting lignin into usable fuels and chemicals is that the structure of lignin has yet to be entirely understood owing to its polydispersity, complexity, and hyper-branched topology. Libraries of structural representations of lignin accounting for these facets have recently been proposed for wheatstraw, an herbaceous biomass, based on a stochastic generation method that creates lignin molecules that collectively conform to properties measured experimentally. We have extended this stochastic method to accommodate more complexity and any type of biomass, i.e., softwood, hardwood, or herbaceous. The unique mechanistic details for several of the new lignin bond types are essential in deciding rules for bond formation in the algorithm. Further, we present two successful methods of decreasing the degrees of freedom during optimization of crucial parameters. Apart from generating libraries of lignin structures, the added complexity allows for the exploration of “lignin space”, which we coin here to represent all possible structures of lignin given the experimental characteristics of monomer distribution, bond distribution, molecular weight distribution, and branching coefficient. Using our overall approach, lignin libraries for any biomass source with reliable and consistent experimental data can be generated for future kinetic modeling studies or molecular simulations, and guidance can be provided to experimentalists to design and characterize lignin.

Thin films (∼115 μm thick) of milled wood lignin from hybrid poplar and acid-washed hybrid poplar were pyrolyzed at 500 °C and ∼55 °C/s at five pressures (4, 250, 500, 750, and 1000 mbar) to determine the impact of secondary liquid intermediate reactions on the product distribution. For both milled wood lignin extracted from poplar and acid-washed hybrid poplar wood, pressure had a significant effect on the product distribution for thin film pyrolysis between 4 and 1000 mbar. For lignin, lowering the pressure from 1000 mbar to 4 mbar reduced the char yield from 36 to 23% and enhanced production of large cluster pyrolytic lignin. However, the pressure did not dramatically impact the gas yield (CO2, CO, methane, H2, ethane, propane, and butane), nor did it significantly impact the release of monomeric phenolic compounds. ICR-MS shows limited changes in the composition of lignin oligomers. The increase in the production of large lignin oligomers observed by UV fluorescence and the reduction of char yield with vacuum confirm the importance of oligomeric combination reactions to form large polyaromatic structures in the liquid intermediate. For hybrid poplar, lowering the pressure from 1000 mbar to 4 mbar decreased the char yield from 19 to 7% and enhanced production of heavy sugars (cellobiosan and cellotriosan). ICR-MS results clearly show the importance of dehydration reactions in the liquid intermediate. Lowering the pressure also enhanced production of CO, CO2, and methane due to heterogeneous catalysis by residual alkali and alkaline earth metals in the solid wood matrix. However, it also decreased production of levoglucosan from 10 to 6.1 wt %. The yields of levoglucosan and cellobiosan obtained for hybrid poplar were higher and lower, respectively, compared with those expected if the pyrolysis products were the result of the additive contribution of hybrid poplar constituents. This result could be explained by the tendency of lignin liquid intermediate to bubble vigorously, contributing in this way to the removal of cellulose oligomers from the liquid intermediate.