Ternary ionic liquid-water pretreatment systems of an agave bagasse and municipal solid waste blend

Abstract

Background: Pretreatment is necessary to reduce biomass recalcitrance and enhance the efficiency of enzymatic saccharification for biofuel production. Ionic liquid (IL) pretreatment has gained a significant interest as a pretreatment process that can reduce cellulose crystallinity and remove lignin, key factors that govern enzyme accessibility. There are several challenges that need to be addressed for IL pretreatment to become viable for commercialization, including IL cost and recyclability. In addition, it is unclear whether ILs can maintain process performance when utilizing low-cost, low-quality biomass feedstocks such as the paper fraction of municipal solid waste (MSW), which are readily available in high quantities. One approach to potentially reduce IL cost is to use a blend of ILs at different concentrations in aqueous mixtures. Herein, we describe 14 IL-water systems with mixtures of 1-ethyl-3-ethylimidazolium acetate ([C₂C₁Im][OAc]), 1-butyl-3-ethylimidazolium acetate ([C₄C₁Im][OAc]), and water that were used to pretreat MSW blended with agave bagasse (AGB). The detailed analysis of IL recycling in terms of sugar yields of pretreated biomass and IL stability was examined.

Results: Both biomass types (AGB and MSW) were efficiently disrupted by IL pretreatment. The pretreatment efficiency of [C₂C₁Im][OAc] and [C₄C₁Im][OAc] decreased when mixed with water above 40%. The AGB/MSW (1:1) blend demonstrated a glucan conversion of 94.1 and 83.0% using IL systems with ~10 and ~40% water content, respectively. Chemical structures of fresh ILs and recycle ILs presented strong similarities observed by FTIR and ¹H-NMR spectroscopy. The glucan and xylan hydrolysis yields obtained from recycled IL exhibited a slight decrease in pretreatment efficiency (less than 10% in terms of hydrolysis yields compared to that of fresh IL), and a decrease in cellulose crystallinity was observed.

Conclusions: Our results demonstrated that mixing ILs such as [C₂C₁Im][OAc] and [C₄C₁Im][OAc] and blending the paper fraction of MSW with agricultural residues, such as AGB, may contribute to lower the production costs while maintaining high sugar yields. Recycled IL-water mixtures provided comparable results to that of fresh ILs. Both of these results offer the potential of reducing the production costs of sugars and biofuels at biorefineries as compared to more conventional IL conversion technologies.Graphical abstractSchematic of ionic liquid (IL) pretreatment of agave bagasse (AB) and paper-rich fraction of municipal solid waste (MSW).

Keywords: Agave bagasse; Biomass blend; Biomass pretreatment; IL recycling; Ionic liquid; Municipal solid waste; Ternary system.

Graphical abstract

Schematic of ionic liquid (IL) pretreatment of agave bagasse (AB) and paper-rich fraction of municipal solid waste (MSW)

Fig. 1

Aqueous ionic liquid systems employed in the pretreatment of agave bagasse (AGB), municipal solid waste (MSW), and an AGB/MSW (1:1) blend

Fig. 2

Process flowsheet of the IL-water pretreatment systems on agave bagasse (AGB), municipal solid waste (MSW), and an AGB/MSW (1:1) blend

Fig. 3

Compositional analysis of untreated and pretreated biomass under different aqueous ionic liquid systems

Fig. 4

Glucan (A) and xylan (B) conversion contour plots for ternary ionic liquid systems of [C₂C₁Im][OAc], [C₄C₁Im][OAc] and water. (I) Agave bagasse, (II) municipal solid waste, and (III) AGB/MSW (1:1) blend

Fig. 5

Work flow of ionic liquid recycling of [C₂C₁Im][OAc] and [C₄C₁Im][OAc] in agave bagasse

Fig. 6

¹H-NMR analysis of 3 series of recycled [C₂C₁Im][OAc] and [C₄C₁Im][OAc] in agave bagasse. 0 Fresh ionic liquid, 1 1st recycle, 2 2nd recycle, and 3 3rd recycle

Fig. 7

Glucan and xylan conversion of pretreated agave bagasse by recycled [C₂C₁Im][OAc] and [C₄C₁Im][OAc] on 72-h saccharification time