Interplay of Acid–Base Ratio and Recycling on the Pretreatment Performance of the Protic Ionic Liquid Monoethanolammonium Acetate

Abstract

The use of protic ammonium ionic liquids (PILs) in the pretreatment of lignocellulosic biomass is a promising alternative to using expensive aprotic ionic liquids such as 1-ethyl-3-methylimidazolium acetate, [Emim]­[OAc]. In this work, the PIL monoethanolammonium acetate, [MEA]­[OAc], was used for the pretreatment of sugar cane bagasse. The study investigated changing the acid base ratio (ABR) from 0.1 to 10 and recycling of the solvent. We determined the lignin extraction, lignin recovery, solvent recovery, and enzymatic saccharification yield and compared the performance to that of the pure amine base, monoethanolamine. We found that lignin extraction and glucose release during enzymatic saccharification increased with base (amine) content, reaching up to 84% and 96%, respectively, after 72 h of saccharification for the 0.1 ABR. Up to 97% of the solvent was recovered for the 1.0 ABR. A higher acid content led to increased hemicellulose extraction into the liquid phase and reduced ionic liquid recovery. A partial conversion of the PIL into N-(hydroxyethyl) acetamide was observed after pretreatment, with up to 86% of conversion after the sixth use for the 1.0 ABR. A negative correlation (R 2 = 0.96) was found between the acetamide content in the solvent and the saccharification yield. The drop in pretreatment performance was also correlated with a decrease in accumulated lignin recovery and the molecular weight of the isolated lignins. Acetamide formation was reduced when excess base was present. Recycling of the mixture with 0.5 ABR showed that the performance was unchanged (97% saccharification yield) after three uses, although 28% of the mixture was converted into the acetamide. The study shows that protic acetate ILs made from primary amines form an equilibrium with their amide and a low ABR is required in order to maintain high pretreatment efficiency when the PIL is reused.

Effect of Protic Ionic Liquids in Sugar Cane Bagasse Pretreatment for Lignin Valorization and Ethanol Production

“…Saha et al (2017) reported lignin removal of 90.1% from bagasse using [EMIM]­OAc at the optimal pretreatment conditions: 2 h at 140 °C and ionic liquid to bagasse ratio equal to 20:1 (w/w). Although the studies use a high amount of IL in the pretreatment step, PILs can be recycled and reused at least three or four times, which shows the process is potentially economically feasible. ,, …”

“…Another aspect that must be considered is the amount of water used to wash the biomass after pretreatment, as it influences the recovery cost of the PIL, which needs to be recycled to make the process viable . Studies in the literature have shown that the PIL can be recycled and reused in the pretreatment at least four times (>98%) without showing a decrease in lignin removal. ,,, Barcelos et al. (2021) using [Ch]­[Lys] PIL in pretreatment to produce ethanol reached an optimal design resulting in a predictable ethanol selling price of $3/gge and carbon footprint of 16.4 gCO 2e /MJ, an 82% reduction compared to gasoline.…”

“…Immidazolium acetate (AIL) was considered the best IL for lignin dissolution; however their results show that the AIL does have certain drawbacks, such as high viscosity and operating temperature, as well as recoverability. Besides that, extended dissolution times are required for the processing (above 12 h), and the harsh pretreatment conditions degrade the cellulose structure and lead to the destruction of the fermentable sugars. , Based on the disadvantages of the available AILs for delignification and regeneration of lignin, PILs offer an attractive alternative for the selective dissolution of lignin. , …”

Biocatalysis in ionic liquids: state-of-the-union

“…189,190 In the ionoSolv process, 191 deconstruction of lignocellulose is conducted in low-cost (ca. $ 1-2 / kg) PILs, exemplified by 1-methylimidazolium chloride, [hmim][Cl], 192 [Et 3 NH][HSO 4 ], 193,194,195,196 198 at elevated temperatures, affording a cellulose rich pulp. In contrast, when low-cost tetramethylguanidinium (TMG) hydrogen sulfate was used, the biomass completely dissolved and the cellulose was precipitated with ethanol.…”

Operational and biochemical aspects of co-digestion (co-AD) from sugarcane vinasse, filter cake and deacetylation liquor

“…Numerous types of pre-treatments of lignocellulosic biomass have been developing for the release of sugars (e.g., hexoses and pentoses) for the production of 2G ethanol (Moraes et al 2015a). Alkaline pretreatments are common for the delignification of biomass, having additional effects on the silica removal (ash insoluble component) or the partial removal of hemicelluloses (including acetyl and uronic acid groups) and the swelling of cellulose, resulting in a substantial increase in the fiber surface (Carvalho et al 2016).The residues generated are potential sources for AD (Rabelo et al 2011), although little has been studied about the anaerobic co-AD, especially for the recent and innovative pre-treatment of biomass and hydrolysis, e.g., deacetylation process, pre-treatment with ionic liquids, hydrolysis using genetically modified yeast, among others (Nakasu et al 2020). The complexity of such substrates for AD may be one of the factors driving the integration of the 1G2G ethanol process by co-AD of its residues, e.g., with 1G vinasse that is already recognized as a substrate for biogas production (Ferraz Júnior et al, 2016).…”