Enhancing Lignin Renewable Carbon Fiber and Plastic Via Lignin Biosynthesis and Its Chemistry

Abstract

This research aims to address challenges in lignin renewable materials and advance the understanding of the lignin structure-properties relationship. Lignin, the second most abundant biopolymer on Earth, is a major byproduct in pulping and biorefining industries. Lignin is a phenolic polymer with a high carbon content (up to 60%), making it a promising precursor for carbon fiber and could significantly reduce the carbon fiber price by replacing the petroleum-based precursor polyacrylonitrile (PAN), which accounts for approximately 50% production cost. We have advanced understanding on how lignin structure influences carbon fiber through two chapters: (II) Enhancing the multi-functional properties of renewable lignin carbon fibers via defining the structure-property relationship using different biomass feedstocks. (III) Discovering biomass structural determinants defining the properties of plant-derived renewable carbon fiber. In Chapter 2, the lignin from different biomass, including hardwood (sugar maple), softwood (loblolly pine and red cedar), and herbaceous plants (switchgrass and corn stover), has large variations in different monolignol proportions and chemical linkage profiles. The differences provide a platform to study on how lignin properties could impact on lignin-based carbon fiber performance. Linear regression models were established to define the relationship between carbon fiber mechanical properties and lignin structural characteristics. The results highlighted that the content of β-O-4 linkages correlates significantly with the tensile strength and elastic modulus of lignin carbon fibers, indicating that more linear β-O-4 linkages could promote the carbon fiber performance. This concept is further invested using a biomass collection of 12 overexpression genes in the monolignol biosynthesis pathway. Lignin from Comt and F5H overexpression genes had an increased β-O-4 linkages, had the best two carbon fiber mechanical and electroconductive performances. Meanwhile, the C3H with the lowest β-O-4 percentage and higher molecular weight than the wildtype control had the lowest mechanical and electrical properties. The new principle was therefore used successfully guide the feedstock design to derive lignin for improved carbon fiber performance. Besides the feedstock design, we also carry out the lignin chemical design to derive the structure for better carbon fiber based on our discovery. A new type of lignin high-molecular weight esterified linkage lignin (HiMWELL) was designed with more esterified linkage structures and higher molecular weight. The HiMWELL-based carbon fiber achieved a record tensile strength and better mechanical properties than the control polyacrylonitrile carbon fiber. In Chapter 3, Lignin from eight sorghum samples with diverse characteristics was fabricated into carbon fibers to study structure-function relationship. lignin uniformity was found to correlate with CF mechanical performance. Lignin content and composition did not impact on carbon material properties. And this finding also informs new possibility for feedstock genetic engineering for quality carbon fibers. Plastic waste also poses a significant environmental challenge due to low recycling rates and environmental persistence. The unique photodegradation characteristic of lignin may enable the manufacturing of new recyclable polymer. We have carried out the design in Chapter V: Phototunable lignin plastics to enable recyclability. We first demonstrated the highly effective phototunable lignin depolymerization in the complex polymer blend matrix and explored the molecular mechanisms. A new blend design polymer was designed which had lignin cores grafted with PMMA and had a significantly improved mechanical properties when compared to PMMA. More importantly, the mechanical properties of the UV-treated blend decreased drastically and enabled a recycle process integrating UV and alkaline treatments to recycle PMMA for plastics and fractionated lignin for bioconversion or other applications.