(2dw) Advanced Materials from Renewable and Refinable Polymers

As the backbone of most plastics, the essence of combustible energy, the secret ingredient in many Space Age ceramics, and even the foundation of life, carbon has been intertwined with human technology almost from its inception. Especially as we push for a more sustainable and resilient chemical economy, the role of carbon cannot be overlooked. From closing out dependency on finite petroleum reserves to enabling the next generation of aerospace vehicles, carbon provides us with countless opportunities, still waiting to be discovered.

Research Experience

Renewables. In the category of sustainability, biocarbon primarily originates from one of two molecules: polysaccharides (chiefly cellulose) or lignin. Although cellulose has been the frontrunner for biorefinery products due to its longstanding value in paper-making, or more recently, the energy value of cellulosic ethanol, lignin is the second most abundant biopolymer on the planet. Due to its complex structure, and respectable heat of combustion, lignin has historically been used to more than cover the energy needs of papermills by fueling recovery boilers. While this fit the design paradigm of the late 1800’s when the kraft process was developed, pigeonholing such a bountiful resource to simply boiling water is troubling. In fact, modern advancements in the field have revealed a litany of potential value-added uses for this renewable byproduct of cellulose isolation. Notably, the high carbon content (>55%) and accessible hydroxyl groups of lignin make it a prime candidate for a range of higher-value materials, such as activated carbon, carbon fibers, and engineered polymers such as polyurethane foams.

However, many veterans of the biomass sector are likely aware of the adage “You can make anything you want from lignin, except money”. Largely, this is due to the generally poor properties that many lignin-based materials suffer from. But it should come as no surprise that byproduct lignin from a cellulose-centric process struggles to perform. One would not expect a standard automobile to operate on crude oil, so how much can we expect of similarly impure and poorly defined lignin? Between the inherent impurities, wide range of functionalities, and broad distribution of molecular weights present in lignin, numerous problems can arise from substituting lignin directly into a process that was originally designed for well-defined petrochemical feedstocks. As with crude oil, fractionation is a path to realizing the potential of lignin as a natural resource. However, unlike early crude oil upgrading, it is imperative that lignin refining strategies maintain sustainability as a core principle. To that end, ALPHA (Aqueous Lignin Purification using Hot Agents) presents a means of simultaneously purifying and fractionating lignin through the use of renewable solvents such as aqueous acetic acid or ethanol at only mildly elevated temperatures.

Originating as internally led phase behavior studies and culminating in collaborative work with other teams at Clemson University, Montana State University, and Michigan State University, the ALPHA process has proven capable of converting a single lignin stream into several concurrent fractions of controlled molecular weight and purity. These fractions have shown a significant improvement in key performance metrics in lignin-based carbon fibers, activated carbons, and polyurethane foams.

Refinables. While the field of bio-renewable carbon prioritizes sustainability and accessible cost of manufacturing, applications in the defense and aerospace sectors place performance above nearly all else. Well-ordered carbon offers a valuable combination of mechanical strength/modulus, chemical stability, thermal/electrical conductivity, and temperature resilience. Unfortunately, well-ordered carbon is not a particularly simple material to fabricate, shape, or otherwise process.

Because carbon does not melt, carbonaceous precursors are used to prepare virtually all advanced carbon materials. A quality precursor should be primarily comprised of carbon, as heteroatoms are prone to vaporizing or otherwise inducing defects during processing. Mesophase (also called liquid crystalline) pitches are ideal precursor candidates due to their extraordinary carbon content and two-dimensional ordering. Mesophase pitches are generally composed of varying aromatic molecules (such as naphthalene oligomers) and can be produced from naturally occurring petroleum bottoms or intentionally synthesized from specific monomer units. Due to the large, planar structure of these molecules, molten mesophase pitches retain a degree of order, yet still can be melt processed as if liquid. This allows for the production of anisotropic pitch fibers, which can be carbonized into fibers or matrices possessing a highly crystalline nanostructure.

While the production of mesophase pitch fibers is a studied field, the synthesis and mechanisms that render a pitch mesophase versus isotropic are not well-understood. As with lignin, the research question is complicated by the complex nature of most mesophase pitches. These pitches typically begin as a petroleum “heavies” and are subsequently exposed to varying degrees of heat soaking. This results in an innumerable quantity of individual chemical constituents and obscures the underlying phenomena that governs the anisotropic–isotropic phase transition.

To combat the above issue, in collaboration with industrial, academic, and DoD partners, we are studying both the phase behavior, and composition–property relations of a model mesophase pitch system. When properly synthesized, pyrene can be oligomerized into a pitch consisting of almost solely oligomers (i.e., monomer, dimer, trimer, etc.) with only trace levels of ring fragmentation/alkylation. When combined with existing supercritical fractionation techniques, this allows for the isolation of analytically pure samples of pyrene dimer and trimer. The pyrene trimer is of particular interest because it is the smallest known aromatic compound to exhibit thermotropic mesophase (MW = 598 Da). This allows for a systematic exploration of phase behavior, where each “designed mixture” is characterized by the mesophase content (as a function of temperature), softening point, and char yield. Because smaller oligomers depress and larger oligomers elevate the softening point of a mixture, understanding their impact is critical for successfully optimizing the properties and mesogenicity of a pyrene pitch precursor.

Research Interests

While my prior experience with lignin focuses primarily on the production of materials and polymers, I want to expand the scope of this work into energy and fuels. Generally, cellulose or oil crops are thought of as the primary biofuel sources, and I want to change that paradigm. Lignin has a remarkable heating value and high carbon content, making it perhaps the closest thing we have to growing gasoline on trees. With the mounting push for electrification of vehicular powerplants, we still do not have a solution for aviation. The jet engine simply cannot be electrified as we know it. Furthermore, the transition from combustible fuels presents an unprecedented logistical challenge for not only aviation, but also maritime shipping and road transport. An intermediate to long-term solution for green liquid fuel is more than a convenience—it is a necessity. Reductive fractionation of lignin has the potential to fill this niche.

Specifically, I am interested in combining my previous experience in liquifying lignins with green solvents, and my previous experience with high-pressure (supercritical if necessary) systems, to investigate the use of non-conventional reductive fractionation schemes. Currently, much of the existing lignin depolymerization field is focused on using hydrogen gas and various exotic catalysts. Instead, I would investigate carbon monoxide and ammonia as alternative reductive depolymerization agents. In conjunction with ALPHA fractionation, these renewably sourceable gasses could allow for depolymerization and separation to occur simultaneously, thereby producing both material precursors and petro-similar fuels. In addition to this focus on reactive separations of lignin, there is a tremendous opportunity for integrating existing lignin-based plastics (and even depolymerized lignin oils from above) with my prior involvement in lignin-based carbon fibers to make completely green composites.

Lastly, I am interested in continuing research efforts towards optimizing mesophase pitches for advanced carbon applications. Currently, processing of mesophase pitch into final products is a laborious and expensive process owing to difficulties is oxidative stabilization and densification. Better understanding of the chemical structure–property relations of the precursors in conjunction with appropriate synthesis and/or fractionation of these pitches has the potential to alleviate this bottleneck.

Teaching Interests

I entered my program with two years of undergraduate research experience in my group, a passion for knowledge, and investment in the success of my pears, so I naturally transitioned into a mentoring role for the newer students of my lab. To date, I have played a major role in mentoring 7 undergraduate and 3 graduate researchers in my group. I am currently supported through a Department of Education Fellowship (GAANN) focused on fostering the development of academic leaders in areas of national need. Through this program, I have taken multiple courses to further my aptitude as an instructor. Additionally, I have voluntarily given approximately 15% of lectures for an undergraduate Thermodynamics II class in a mentored teaching experience in Fall ’21 and plan on the same for Fall ‘22. Also, due to my passion for interacting with the Unit Ops Lab at Clemson, I have assisted in teaching of both UO I and II twice each. I particularly enjoy teaching courses with hands-on components because of the nature of my hardware-heavy research experiences. I am also at home teaching any courses in the broader domains of thermodynamics, fluid mechanics, materials, and polymer science.