Ionic Liquid Pretreatment of Lignocellulosic Biomass with Ionic Liquid Water Mixtures

Ionic liquid pretreatment of lignocellulosic biomass with ionic liquid water mixtures

Agnieszka Brandt,[a],[b] Michael J. Ray,b,c Trang Q. To,a David J. Leak,b,[c] Richard J. Murphyb,c and Tom Weltona*

Abstract

Ground lignocellulosic biomass (Miscanthus giganteus, Pine (Pinus sylvestris) or Willow (Salix viminalis)) was pretreated with ionic liquid-water mixtures of 1-butyl-3-methylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium hydrogen sulfate. A solid fraction enriched in cellulose was recovered, which was subjected to enzymatic hydrolysis. Up to 90% of the glucose and 25% of the hemicellulose contained in the original biomass were released by the combined ionic liquid pretreatment and the enzymatic hydrolysis. After the pretreatment, the ionic liquid liquor contained the majority of the lignin and the hemicellulose. The lignin portion was partially precipitated from the liquor upon dilution with water. The amount of hemicellulose monomers and their conversion into furfurals was also examined. The performance of ionic liquid water mixtures containing 1,3-dialkylimidazolium ionic liquids with acetate, methanesulfonate, trifluoromethanesulfonate and chloride anions was investigated. The applicability of the ionic liquid 1-butylimidazolium hydrogensulfate for lignocellulose pretreatment was also examined. It was found that ionic liquid liquors containing methyl sulfate, hydrogen sulfate and methanesulfonate anions were most effective in terms of lignin/cellulose fractionation and enhancement of cellulose digestibility.

Introduction

The rising demand for liquid transportation fuels is placing increasing demands on finite oil reserves, raising prices and encouraging the search for oil in more remote locations, often in fragile ecosystems. In addition, the planet’s climate is affected by carbon dioxide emitted from the use of fossilised carbon as an energy source. The production of many chemicals and materials is also reliant on fossil fuel resources.

Lignocellulose, essentially the cell wall material of woody plants, is a porous micro-structured composite mainly consisting of cellulose, hemicellulose and lignin. It has been projected that lignocellulosic biomass has the potential to be a large-scale, low-cost and sustainable feedstock for renewable fuels and chemicals.1 Compared to starch or vegetable oil substrates that are currently used as biofuel and biomaterial feedstocks, significantly higher biomass yields per unit area of land are expected, while requiring less energy and material input for its production.2 Various plant species have been suggested as being suitable dedicated biofuel crops, with properties such as rapid growth, low fertiliser input, and short harvest cycles. Currently favoured crops include grasses (miscanthus, switchgrass), hardwoods (willow, poplar, eucalyptus) and softwoods (pine, fir, spruce). Careful implementation of lignocellulose based technology as a substitute for fossil resources could help reduce man-made carbon dioxide emissions in a sustainable way.3

Proposed routes for transforming lignocellulosic biomass into useful products are the microbial fermentation of the glucose and other carbohydrates contained in the biomass and the thermo-chemical conversion of the lignocellulose via pyrolysis or gasification. For the fermentation route, deconstruction of the lignocellulosic matrix is necessary before the carbohydrates are released. A typical deconstruction sequence producing fermentable carbohydrates is: size reduction to chips, a pretreatment that solubilises the hemicellulose and alters/removes lignin,4 followed by detoxification and neutralisation. The pretreated biomass is subsequently processed using hydrolytic enzymes (saccharification) to produce sugar monomers.

The pretreatment step is responsible for a significant portion of the energy consumption and cost of the biofuel production process and improvements are required.5 A large number of pretreatment options are defined in the literature, such as dilute acid, concentrated acid, ammonia fibre expansion (AFEX), lime and organolv pretreatment. Different plant groups exhibit distinct tissue structures and varying cell wall composition, which leads to a variable resistance to deconstruction.

This paper explores the potential of certain ionic liquids as pretreatment solvents, in particular their mixtures with water. Ionic liquids are a diverse group of salts that are liquid at ambient temperatures or melt at slightly elevated temperatures. In the last two decades, ionic liquids containing organic cations with quaternised ammonium, phosphonium and sulfonium cores have enjoyed increasing popularity in many fields of research.6

Many ionic liquids have negligible vapour pressures under process-relevant conditions and the nature and combination of cation and anion can be tuned to suit a particular application. Cellulose and lignocellulose processing are only two out of many recently explored applications for these alternative solvents.7 Ionic liquids are polar solvents with varying degrees of hydrogen-bonding ability.8 It has been found that the ionic liquid needs to contain anions with high hydrogen-bond basicity such as chloride, phosphates, phosphonates and carboxylates in order to be able solubilise cellulose.9 The hydrogen-bond acidity also plays a role. If a hydrogen-bond acidic functionality is incorporated into the ionic liquid structure, it will compete for the hydrogen-bond basic site on the anion and reduce cellulose solubilisation.10, 11 Water also decreases the solubility of cellulose,12 probably for a similar reason. The empirical Kamlet-Taft solvent descriptors can be used to predict cellulose solubility.13 Cellulose can be reconstituted by adding a protic antisolvent, such as water or alcohols, and spun into fibres or films. A variety of homogenous derivatisations of cellulose dissolved in ionic liquids can be accomplished.14

Ionic liquids were initially used in cellulose processing15 before their application was extended to lignocellulose processing. The solubility of lignocellulose in ionic liquids has been reported in various hydrogen-bond basic ionic liquids, suggesting that ionic liquids which are cellulose solvents are also suitable for lignocellulose processing.16, 17 Reduced crystallinity of the cellulose contained in lignocellulose was observed upon precipitation with an antisolvent.18 A correlation between the hydrogen-bond basicity of the anion and the ionic liquid’s ability to swell and partially dissolve wood chips has been observed.19 The solubility of lignin in ionic liquids also seems to depend on the anion.17, 20 It has been shown that Kraft pulp lignin has a very high solubility in the ionic liquids 1,3-dimethylimidazolium methyl sulfate, [C1C1im][MeSO4], and 1-butyl-3-methylimidazolium methyl sulfate, [C4C1im][MeSO4].20

Enhanced glucose release from ionic liquid pretreated wood has also been observed, mainly with dialkylimidazolium ionic liquids containing acetate, chloride and dimethyl phosphate anions.21-23 However, the sugar release by hydrolytic enzymes was often less than 80-90% (which is expected for an effective pretreatment operation). Recently, the impact of ionic liquid pretreatment on biomass composition has received attention. It was noted that lignin and hemicellulose are partially removed during pretreatment with 1-ethyl-3-methylimidazolium acetate, [C2C1im][MeCO2].17, 24-26 A correlation between lignin removal and cellulose digestibility was suggested.17, 21

Various publications concluded that application of methyl sulfate containing ionic liquids in lignocellulose pretreatment did not enhance cellulose digestibility,17, 21, 24, 27 despite their ability to dissolve large amounts of lignin.

Water reduces not only cellulose solubility in ionic liquids,12 but also the effectiveness of ionic liquid pretreatment with [C2C1im][MeCO2].21, 28 Biomass contains significant quantities of water, 2-300% relative to the oven-dried weight. In addition, ionic liquids are hygroscopic and will absorb significant quantities of moisture when exposed to air.29 The drying of ionic liquids requires heat and vacuum, particularly when the ionic liquids are strongly hydrogen bond basic, like [C2C1im][MeCO2]. Therefore, an ionic liquid pretreatment that tolerates moisture would be beneficial for the over-all energy and cost balance of a lignocellulose processing system using ionic liquids.

An advantage of ionic liquid pretreatment could be the recovery of a separate lignin fraction which could be converted to aromatic, value-added chemicals. Lignin recovery from ionic liquids has been achieved after treatment of sugar cane bagasse with 1-butyl-3-methylimidazolium alkylbenzenesulfonate, [C2C1im][ABS], an ionic liquid mixture containing aromatic sulfonate anions, mainly xylenesulfonate.30 Lignin recovery has also been observed after pretreatment with [C2C1im][MeCO2], when the regeneration solvent was a mixture of water and acetone.20, 26

This study investigates the influence of water on the effectiveness of ionic liquid pretreatment. We have devised a notation to indicate the amount of the ionic liquid contained in the pretreatment solvent/liquor. This involves a subscript being added to the usual ionic liquid notation indicating the ionic liquid content in volume percent (vol%), with the remainder being water. An example is [C4C1im][MeSO4]80%, which is a mixture of 80 vol% [C4C1im][MeSO4] and 20 vol% water. Conversions of vol% into weight percent (wt%) and mole percent (mol%) were calculated and are listed in Table 1. When allowing [C4C1im][MeSO4] to equilibrate with the moisture in the laboratory air a water content of 70,400 ppm or 7.0 wt% was measured (last entry of Table 1). Although the moisture content of air is variable, the measurement demonstrates the highly hygroscopic nature of this ionic liquid.

Table 1: Ionic liquid concentration in aqueous pretreatment liquors.

*These ionic liquids are solid at room temperature. Therefore vol% and wt% were calculated using the density at 80°C.

The aim of this work is to investigate the effect of the composition of the ionic liquid liquor on the pretreatment. Solid recovery, pulp composition, its enzymatic digestibility, the precipitation of a lignin-containing fraction and the production of furfurals in the liquor were investigated. The application of an ionic liquid with a monoalkylated imidazolium cation was also examined. Pretreatment of different feedstocks was carried out to assess their recalcitrance towards pretreatment with ionic liquid water mixtures.

Results and Discussion

Tissue softening of Miscanthus chips

In preliminary experiments, we observed substantial disintegration of Miscanthus cross sections immersed in the ionic liquid 1-butyl-3-methylimidazolium methyl sulfate,[C4C1im][MeSO4], when heated above 80°C. This encouraged us to investigate the application of this ionic liquid for biomass pretreatment. The use of [C4C1im][MeSO4], dried to a water content below 0.3 wt%, resulted in formation of a degraded biomass-ionic liquid composite that was not enzymatically digestible. In contrast, using a mixture of 80 vol% ionic liquid and 20 vol% water yielded a solid fraction that was separable from the (intensely coloured) ionic liquid fraction and highly digestible. It was concluded that a certain amount of water was necessary for successful pretreatment with [C4C1im][MeSO4]. In the “dry” sample, 0.3wt% water was contained in the ionic liquid as residual moisture and 0.7 wt% was introduced with the air-dried biomass containing 8 wt% moisture, supplying 1.1 wt% or 15 mol% water in total. This was apparently not sufficient to obtain an enzymatically digestible pulp.

Influence of the water content on the saccharification yield after ionic liquid pretreatment with [C4C1im][MeSO4]

A range of ionic liquid water mixtures were used for pretreatment of Miscanthus to explore the effect of the water content in more detail. The effect of water on the enzymatic release of glucose and hemicellulose is shown in Figure 1. The yields are calculated based on the glucose and hemicellulose content found in the untreated Miscanthus feedstock (on an oven-dry basis), which were 43.6 wt% and 24.3 wt%, respectively. In preliminary experiments, it was shown that the only detectable hemicellulose sugar released during saccharification was xylose.

Figure 1: Sugar yields obtained from Miscanthus pulp after pretreatment with [C4C1im][MeSO4] or [C4C1im][HSO4] water mixtures at 120°C. The [C4C1im][MeSO4] pretreatment was carried out for 22 h, while [C4C1im][HSO4] pretreatment lasted 13 h, and the saccharification 96 h.

The best saccharification yields were obtained after pretreatment with mixtures containing 60-90vol% ionic liquid. Pretreatment with [C4C1im][MeSO4]90%, resulted in the release of 92% of the glucose originally contained in the biomass. Pretreatment with [C4C1im][MeSO4]80% and [C4C1im][MeSO4]60%,resulted in the release of 89% and 87% based on the original glucan content. Glucose yields decreased when the ionic liquid content was higher or lower. The hemicellulose yield was significantly lower than the glucose yield, regardless of the mixture composition; 24% of the hemicellulose sugars (based on the initial hemicellulose content) were released after [C4C1im][MeSO4]60% pretreatment. Similar yields were obtained with mixtures containing 40-90 vol% [C4C1im][MeSO4].

Water sensitivity of [C4C1im][MeSO4]

When attempting to recycle [C4C1im][MeSO4], we found that the ionic liquid anion was partially hydrolysed. After recording a mass spectrum of the recovered ionic liquid, a high abundance of a negatively charged species at m/z=97 was detected, which was ascribed to the hydrogen sulfate, [HSO4]-, anion. This led to the conclusion that the ester bonds in methyl sulfate anions are hydrolytically unstable under the conditions of the pretreatment and mixtures of the ester and the hydrolysed form are produced.

The extent of anion hydrolysis depended on the water content of the liquor (Figure 2). The more water was present in the mixture, the greater the anion hydrolysis, with exception of mixtures where the water content was higher than 90 mol%. These results suggest that without extreme precautions to protect [MeSO4]- containing ionic liquids, [HSO4]- will be present and other studies using these ionic liquids should be interpreted in this light.20

Figure 2: Ratio of [MeSO4]- anions to ionic liquid cations in the recycled ionic liquid after pretreatment of Miscanthus (detected by 1H-NMR), the remaining anions being [HSO4] -.

Influence of the water content on the enzymatic saccharification of [C4C1im][HSO4] treated miscanthus

With the knowledge that the binary 1-butyl-3-methylimidazolium methyl sulfate water mixtures turned into quaternary mixtures of two ionic liquids plus two molecular solvents (water and methanol) we set out to identify the active component(s). Miscanthus was pretreated with aqueous mixtures of [C4C1im][HSO4], which allowed us to exclude methyl sulfate and methanol. The saccharification yields obtained from the pulps pretreated with various [C4C1im][HSO4] water mixtures are shown in Figure 1. The glucose yields were almost identical to the glucose yields obtained with the quaternary mixtures. The pattern of hemicellulose release was also similar, however, after [C4C1im][HSO4]40%-80% pretreatment, less hemicellulose was recovered than after treatment with the equivalent methyl sulfate containing mixtures.