Aqueous oxidative depolymerization of low-density polyethylene (LDPE) can convert highly recalcitrant, water-insoluble, and biologically persistent polyethylenes to value-added products. In this study, aqueous oxidation reaction conditions including temperature, time, KMnO4 loading, and initial O2 pressure were screened for their impact on the yields of water-soluble and water-insoluble products, water-soluble product distributions, and water-insoluble product properties. Additionally, the potential for biological utilization of the water-soluble fraction was demonstrated. It was shown that increasing oxygen consumption during the oxidation reaction is correlated to increased water-soluble product yields and that the primary water-soluble products are C4 to C9 saturated linear diacids and hydroxy- and oxo-substituted diacids as determined by LC-QTOF. Water-insoluble products are expected to comprise >C10 oxidized waxes that can be recovered by ethanol solubilization. Characterization of physical properties of the water-insoluble products shows the trends that increasing oxygen, carbonyl, and/or carboxylate contents correlate to decreasing thermal transition temperatures (melting and crystallization temperatures as determined by DSC), decreasing crystallinity, and increasing ethanol solubility. Finally, it was demonstrated that Thermus thermophilus HB8 could grow on water-soluble products of LDPE oxidation as the sole carbon source and accumulate polyhydroxyalkanoates (PHAs) from growth on these substrates.

Feedstock heterogeneity is a fundamental obstacle to cost-competitive biobased products. Agricultural products like corn stover have anatomical components that vary in their chemical composition, mechanical properties, structure, and response to chemical and biological treatments. A technique that can enrich streams in select anatomical fractions would allow a tailored deconstruction approach to increase overall process efficiency. Air classification can be leveraged for such refining; however, fundamental characterization and understanding of the particle properties that underly the physics of air classification are only modestly documented. Here, we determine fundamental particle properties including mass-to-area ratio, drag coefficient, and partition velocity that describe how anatomical tissues of corn stover behave during air classification. Mass-to-area ratios of anatomical tissues vary by nearly two orders of magnitude from 2.3 mg/mm2 for cob to 0.04 mg/mm2 for leaf. Drag coefficients of longer, fibrous materials (i.e., rind, husk, and sheath) are shown to correlate with particle area (p-value < 0.001) whereas granular tissues (i.e., cob, pith, and leaf) correlate better with mass-to-area ratio (p-values < 0.001). When compared to experimental observations, a simulated two-stage air classification and size reduction scenario predicts the overall partitioning of anatomical tissues within 15% for pith, husk, rind, and cob tissues. The model predicts an air-classified fraction preferentially enriched in cob (purity = 20%), rind (purity = 74%), and pith (purity = 4.5%) with a mass yield of 47%. Empirical relations for these properties can be used to predict the partitioning of corn stover during air classification based on anatomical type and size.

Corn stover was subjected to dilute sulfuric acid pretreatment to assess the impact of pretreatment conditions on lignin extractability, properties, and utility as a phenol replacement in wood phenol-formaldehyde (PF) adhesives. It was identified that both formic acid and NaOH could extract and recover 60-70% of the lignin remaining after pretreatment and enzymatic hydrolysis under the mildest pretreatment conditions while simultaneously achieving reasonable enzymatic hydrolysis yields (> 60%). The availability of reaction sites for the incorporation of lignins into the PF polymer matrix (i.e., unsubstituted phenolic hydroxyl groups) was shown to be strongly impacted by the pretreatment time and the recovery. Finally, a lignin-based wood adhesive was formulated by replacing 100% of the phenol with formic-acid-extracted lignin, which exhibited a dry shear strength exceeding a conventional PF adhesive. These findings suggest that both pretreatment and lignin extraction conditions can be tailored to yield lignins with properties targeted for this co-product application.

Nuclear magnetic resonance (NMR) relaxometry is applied to provide direct measurement of water adsorption in anatomical fractions of corn stover. NMR transverse T2 relaxation time distribution measurements indicate multiple water populations, which vary with anatomical fraction and water adsorption. Measured T2 data are used to calculate thermodynamic properties of Brunauer-Emmet-Teller adsorption theory using a model to estimate mono and bilayer relaxation. T2 data are used directly to determine rotational diffusion correlation times indicating adsorption interaction strength. T1-T2 longitudinal-transverse relaxation time correlation measurements quantify differences in the molecular level structural order of the adsorbate surface water as a function of water activity, i.e. relative humidity or water vapor partial pressure. The T1/T2 ratio provides a measure of the surface energy related to the adsorption strength and surface diffusive mobility of the water adsorbate and differentiates the anatomical fractions. The results indicate that direct measurement of NMR relaxation times can be used to characterize corn stover biomass water adsorption, data relevant to processing and handling considerations.

Biomass feedstock heterogeneity is a principal roadblock to implementation of the biorefinery concept. Even within an identical cultivar of corn stover, different bales contain not only varying abundance moisture, ash, glucan, and other chemical compounds, but also varying abundance of tissue anatomies (e.g., leaf, husk, cob, or stalk). These different anatomical components not only differ in their response to pretreatment and enzymatic hydrolysis to glucose, but also vary in their mechanical and conveyance properties. Although this heterogeneous nature of corn stover feedstock has been identified as a challenge, a fundamental knowledge gap of how these tissues behave during biorefining processing remains. In this work, we demonstrate the use of a commercial fiber image analyzer typically used for wood fiber characterization to monitor the particle size and shapes of non-woody feedstock during milling, pretreatment, and hydrolysis. Additionally, we present novel use of Gaussian process classification to distinguish bundle, parenchyma, and fiber particles to an accuracy of 96.4%. Quantitative probability distribution plots for characteristics such as length and roundness allow elucidation of particle morphology as pretreatment and enzymatic hydrolysis progress. In both stalk pith and stalk rind, particles peel into individual cells whose walls are subsequently fragmented during enzymatic hydrolysis.

Lignin has an adverse impact on the deconstruction of plant cell wall biopolymers in biorefining processes and its reduction and/or alteration during biosynthesis is one target for decreasing plant cell wall recalcitrance. In this work, the impact of two brown midrib mutations (bmr6 and bmr12) in two sorghum background lines (the commercial hybrid Atlas and near-isogenic BTx623) on lignin properties and the plants’ response to cell wall deconstruction to monomeric sugars via alkaline pretreatment and enzymatic hydrolysis is investigated with the goal of assessing how differences in lignin content and properties impact the plant’s response to pretreatment. We identify that both bmr sorghum lines show significantly lower abundance of water-extractable sugars (glucose, sucrose, and fructose) and alkali-saponifiable p-coumarate. Furthermore, both these properties exhibited identical trends across both background lines. Next, both untreated and mild alkali-pretreated bmr sorghums were shown to exhibit higher glucose hydrolysis yields following enzymatic hydrolysis than the control lines. Following pretreatment, the Atlas bmr sorghums exhibited more lignin solubilization and the solubilized lignin was of lower molar mass than the background control line suggesting that differences in the lignin response to pretreatment resulted these differences. Finally, significant differences were observed in the lignin content, lignin monomer distribution, and inter-unit linkages in the Atlas bmr line relative to the control line with key differences including lower syringyl monomer content in both bmr lines, higher relative abundance of β-O-4 linkages in the bmr6 line, and the presence of 5-hydroxy guaiacyl monomers and benzodioxane (α-O-5/β-O-4) linkages in the bmr12 line.

In this study, a biobased phenolic adhesive was successfully developed by entirely substituting both petroleum-based phenol and formaldehyde with an unmodified corn stover biorefinery lignin and glyoxal (a biobased dialdehyde), respectively. Lignin-glyoxal resins were synthesized using an alkaline catalyst with a molar ratio of lignin to glyoxal of 1:2. Chemical, thermal, and mechanical properties of the lignin, lignin-based resins, and final adhesives were assessed following appropriate standard test methods. The analysis of lignins and lignin-based resin molar mass was performed using gel permeation chromatography. The lignin-glyoxal resin was found to have a 3-fold higher average molecular weight than the starting lignin, demonstrating the successful integration of lignin into the polymeric resin network. The curing of the formulated adhesives was studied using differential scanning calorimetry and dynamic mechanical analysis. Although the lignin-glyoxal resin had a higher curing temperature (167 °C) than a conventional phenol-formaldehyde resin (142 °C) and the formulated lignin-formaldehyde resin (146 °C), the rate and degree of cure were similar or better than the other two resins. The adhesion strengths of the formulated adhesives were determined using single-lap-joint veneer samples cured according to recommended press parameters for commercial adhesives. The lignin-glyoxal adhesive had a relatively high dry adhesion strength (3.9 MPa), with over 90% wood failure, but failed the wet adhesion test (boiling water test). Although the formulated lignin-glyoxal adhesive failed the boiling water test, it had excellent stability at room temperature water, remaining intact after 1 week during the water immersion test. The high dry adhesion strength makes this class of lignin-based formaldehyde-free adhesives a unique biobased glue for the production of interior grade plywood and oriented strand boards.