LEO and LiMO Fuels: Structural and Rheological Characterization of Solvolytically Fractionated Lignin Dispersed in Alcohols
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The mild thermal solvolysis of lignin in alcohols is a promising technology for obtaining carbon-neutral fuels. Incorporating lignin in ethanol and methanol leads to a rise in volumetric energy density in concentrated dispersions. The deployment of these fuels and their concentrated formulations depends on their structural and rheological properties. Here, we investigate mildly depolymerized Protobind 1000 lignin dispersed in ethanol (LEO) and methanol (LiMO) at different percentages of solids. Small-angle X-ray scattering (SAXS) data of diluted and concentrated LEO and LiMO dispersions show that, in these formulations, lignin exists in two states: individual lignin coil structures and their aggregates. The lignin coil structures are a few nanometers in size (3–5 nm) in both solvents. Direct observation of the lignin coil structures was achieved by cryo-TEM images and supports the findings of the SAXS measurements. The radius of gyration of the lignin coil structures in dilute dispersions increases with increasing lignin content, while in concentrated dispersions, the opposite trend is observed. It is hypothesized that in the concentrated regime, the lignin structures are more compressed by adjacent coils, resulting in the formation of a network-like arrangement. Furthermore, the aggregates of lignin coil structures can be detected by SAXS in the concentrated dispersions as they exhibit an upturn at low Q. Rheology measurements also indicate the presence of very fragile aggregate networks. The concentrated dispersions exhibit shear-thinning behavior in the shear rate range of 10–3–100 s–1 and Newtonian behavior at higher shear rates (100–103 s–1). Moreover, the viscosities at high shear for ethanol and methanol samples are almost identical at the same solid percentages despite having slightly different water contents, suggesting that the interactions between lignin and the solvents do not determine the rheological behavior but rather the formation of the network structures formed by coils. Such polymeric networks are responsible for the stability of the dispersions at high solid percentages, while at low solid loads, the Brownian motion is sufficient to maintain the nanometer-sized lignin coils in the solvent.
|Journal||ACS Sustainable Chemistry & Engineering|
|Number of pages||9|
|Publication status||Published - 2022|