Chemistry and Properties of Carbon Fiber Feedstocks from Bitumen Asphaltenes

Carbon fibers are materials of paramount impor-tance for composites applied in fields such as aerospace engineering, medicine, and renewable energy. Currently, most of the production of carbon fibers uses polyacrylonitrile, which incurs significant greenhouse gas emissions and high production costs. Therefore, carbon fiber manufacturing from asphaltene-enriched feedstocks is attractive as it could add value to extra-heavy fossil fuels and cut down precursor costs by similar to 90%. Recent studies indicate that some asphaltene-rich samples feature rheological properties that make them optimal for melt-spinning and carbon fiber production; in other cases, the spun asphaltene feedstocks are unsuitable for such applications. In this work, bitumen asphaltenes were subjected to upgrading processes under three distinctive conditions to tweak their rheological properties in order to produce carbon fibers. The treated samples were comprehensively studied by separations based on solubility and extrography, and subsequently characterized by ultrahigh-resolution mass spectrometry, gas-phase fragmentation, and thermogravimetric analysis. The results indicate that upon thermal processing, asphaltene-rich feedstocks produced a mixture with diverse solubility, i.e., maltenes, asphaltenes, and toluene-insoluble material. The molecular composition of remnant asphaltenes suggests that thermal treatment dramatically decreased molecular polydispersity in terms of the content of heteroatoms, alkyl chains, and structural motifs, i.e., single-core vs multicore, also known as island vs archipelago. The processed asphaltenes revealed high abundances of alkyl-depleted island species. Such thermally treated samples produced no stable carbon fibers. Conversely, a feedstock treated with molten sodium in a proprietary process designed to remove sulfur, revealed increased content of alkyl-side chains and archipelago structural motifs. This sample produced stable carbon fibers. Furthermore, thermal analysis coupled with mass spectrometry (TGA-HRMS) was conducted to understand thermal desorption and pyrolysis profiles for the samples, as well as the presence of occluded compounds and carbon residue formation. The TGA-HRMS results are consistent with extrography and IRMPD FT-ICR MS studies and confirmed the ultrahigh abundance of multicore compounds in the desulfurized sample. Although the sample size is limited, and thus, correlations between molecular composition and rheology properties are not achieved, this is the first study that aims to understand the role of feed composition in the ability to generate carbon fibers from asphaltene-enriched feedstocks. Collectively, the results indicate that samples comprised of abundant highly aromatic asphaltenes, dominant in alkyl-depleted single-core structures, are unlikely optimal for carbon fiber applications. Conversely, a sample with a marked increase in H/C, a significant decrease in S content, and abundant multicore species could generate stable carbon fibers. More studies are underway to find correlations between molecular features and carbon fiber production.