(717c) Implications of Physicochemical Structure on Efficient Valorization of Candlenut Shells

Opponents to biomass valorization cite the “food vs. fuel” conflict¹ to avoid using the 140 Gigatons of wasted biomass generated around the world annually.² The utilization of the non-edible fraction of biomass waste therefore represents an abundant, renewable feedstock to address the growing fuel and chemical demands of society. Utilization of these non-edible biomass wastes has the potential to produce value-added products while simultaneously diverting waste from landfills as well as its subsequent decomposition emissions.

Biomass is primarily composed of cellulose, hemicellulose, and lignin, wherein lignin serves as the largest natural sources of aromatic compounds on Earth, typically accounting for 15-30 wt% of plant matter.³ Nut shells, on the other hand, have reported up to 40% lignin⁴ content. In 2012, vanilla seed coats and other nut shells were found to contain a novel homopolymer of caffeyl alcohol, coined C-lignin.⁵ C-lignin is unique as it is often found as a homopolymer bound by benzodioxane linkages and potentially depolymerizes to the single product catechol⁵ which is a useful precursor molecule for proteins⁶ and biomimetic materials.⁷ Candlenut shells (CNS) from the Euphorbiaceae family are one such source of C-lignin that further represents a centralized feedstock due to current processing of the internal seed. Compared to traditional woody biomass, the physiological role and physicochemical properties of the candlenut shell introduce significant challenges to its effective valorization.

This work aims to understand the role of CNS physicochemical structure on its valorization potential through a combined biological and chemical lens. Candlenut shells are comprised of specialized macrosclereid cells containing a lignified secondary cell wall and designed to restrict water uptake and protect the seed. This cell type (Figure 1a) gives CNS a distinct non-porous structure that first, significantly differs from traditional biomass, and second, severely inhibits effective extraction. Due to its nonporous structure, lignin extraction has been found to directly correlate with particle surface area. By increasing surface area from 0.36 m²/g to 1.32 m²/g, lignin extraction yield increases 98%. Furthermore, increased extraction time indicates an asymptote (Figure 1b) wherein increasing the solvent-biomass ratio does not result in additional lignin yield. Relating this finding back to CNS structure reveals the presence of cutin and suberin moieties that effectively block further lignin recovery due to the lack of internal pores. This work utilizes multiple organic solvents in sequential extraction steps along with a range of characterization techniques to uncover the role of candlenut shell physicochemical structure on their efficient valorization through a quantitative analysis of both lignin and suberin/cutin extraction components and the effect of solvent properties.

References:

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