Utilization of Ionic Liquids in Lignocellulose Biorefineries as Agents for

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Utilization of ionic liquids in lignocellulose biorefineries as agents for separation, derivatization, fractionation or pretreatment Susana Peleteiro, Sandra Rivas, Jose L. Alonso, Valentín Santos, and Juan C. Parajo J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b03461 • Publication Date (Web): 03 Sep 2015 Downloaded from http://pubs.acs.org on September 8, 2015

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Journal of Agricultural and Food Chemistry

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Utilization of ionic liquids in lignocellulose biorefineries as agents for

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separation, derivatization, fractionation or pretreatment

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Susana Peleteiro,†,‡ Sandra Rivas,

†,‡

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Parajó*,†,‡

6

†

7

Science. Polytechnical Building, As Lagoas, 32004 Ourense, Spain.

8

‡

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San Cibrao das Viñas, Ourense, Spain.

José L. Alonso,

†,‡

Valentín Santos†,‡ and Juan C.

Chemical Engineering Department. University of Vigo (Campus Ourense). Faculty of

CITI (Centro de Investigación, Transferencia e Innovación), University of Vigo, Tecnopole,

10 11

*Author to whom correspondence should be addressed (Phone: +34988387033, Fax.:

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+34988387001; e-mail: [email protected])

13

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ABSTRACT: Ionic liquids (ILs) can play multiple roles in lignocellulose biorefineries,

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including utilization as agents for the separation of selected compounds, or as reaction media

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for processing lignocellulosic materials (LCM). Imidazolium-based ILs have been proposed

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for separating target components from LCM biorefinery streams, for example the dehydration

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of ethanol-water mixtures, or the extractive separation of biofuels (ethanol, butanol) or lactic

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acid from the respective fermentation broths. As in other industries, ILs are potentially

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suitable for removing Volatile Organic Compounds or carbon dioxide from gaseous

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biorefinery effluents. On the other hand, cellulose dissolution in ILs allows to carry out

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homogeneous derivatization reactions, opening new ways for product design and/or for

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improving the quality of the products. Imidazolium-based ILs are also suitable for processing

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native LCM, allowing the integral benefit of the feedstocks via separation of polysaccharides 1

ACS Paragon Plus Environment


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and lignin. Even strongly lignified materials can yield celullose-enriched substrates highly

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susceptible to enzymatic hydrolysis upon ILs processing. Recent developments in enzymatic

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hydrolysis include the identification of ILs causing limited enzyme inhibition and the

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utilization of enzymes with improved performance in the presence of ILs.

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Keywords: Ionic liquids, lignocellulosic materials, biorefineries, pretreatment, fractionation

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Introduction: lignocellulosic materials as feedstocks for the industry

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The mankind is facing major challenges such as the increased pressure on supplies

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caused by the growing global population, the anthropogenic climate change, the fast depletion

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of fossil resources, the volatility of oil prices, and the geopolitical risks affecting the safe

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supply of raw materials. In this context, the development of cost- and energy-efficient

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processes for manufacturing renewable transportation fuels and chemicals to supplement or

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replace those derived from petroleum is imperative.1 It has been suggested that the future

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chemical industry should be based on novel routes based on renewable raw materials and

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providing chemicals with similar or more advanced properties than the ones currently

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produced.2

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Environmental sustainability would be favoured by using widespread, renewable

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resources as raw materials, following one of the guiding principles of Green Chemistry.3 In

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quantitative terms, biomass offers the only sustainable alternative to fossil fuels as a source of

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carbon for our chemical and material needs.4 Lignocellulosic materials (denoted LCM) stand

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for the many types of vegetal biomass mainly made up of carbohydrate polymers (cellulose

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and hemicelluloses) and polyphenol-based lignin.5 The plant cell walls found in 2


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lignocellulosic biomass are complex structures difficult to break down, or deconstruct, into

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their component polymers and monomers.6

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Typical LCM suitable as feedstocks for chemical processing include softwoods,

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hardwoods, dedicated energy crops, industrial byproducts (such as bran or bagasse),

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agricultural residues and byproducts (such as straw, grasses, husks or shells), and some

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municipal solid wastes (such as cardboard, waste paper and gardening residues). LCM present

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favourable characteristics as sustainable and environmentally friendly sources of chemicals

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and fuels,7 including:

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low purchase cost,8

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large availability and huge generation rate. Nature produces about 180 billion metric

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tons of biomass/year, of which about 75% is in the form of carbohydrates.9 Based on

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the global annual production of biomass (1 × 1011 tons), and on the specific energy

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contents of biomass and crude oil, it has been concluded that in only one decade,

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Earth’s plants can renew in the form of cellulose, hemicellulose, and lignin all of the

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energy stored as conventional crude oil,10

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secure supply, as LCM can be produced locally,11

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low nitrogen and sulfur contents,

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CO2- neutrality, since vegetal biomass comes from photosynthesis, and the CO2 from

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biomass does not represent a net input to the amount of carbon already making part of

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the carbon cycle,

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no competition with the food chain.12

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However, the use of sustainable feedstocks is not enough for sustainability, and must be

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completed with the protection of the environment using greener methodologies.4

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Strategies for the utilization of lignocellulosic materials: biorefineries

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In general terms, the benefit of LCM can be accomplished according to two different

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philosophies:

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•

gasification), or

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direct utilization as a whole (for example, by combustion, pyrolysis, liquefaction or

•

LCM “fractionation” (in which the major components of the feedstock are separated

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into “fractions” made up of compounds with related properties) followed by separate

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processing of each fraction for specific purposes.

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LCM fractionation is the conceptual basis of the “lignocellulose biorefinery”, which has

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been defined by the National Renewable Energy Laboratory (NREL) as ‘‘a facility that

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integrates conversion processes and equipment to produce fuels, power and chemicals from

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biomass’’. According to this definition, biorefineries are expected to work under a general

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idea analogous to today’s petroleum refineries.13 The IEA Bioenergy Task 42 broadened the

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biorefinery concept, proposing that biorefinery stands for “the sustainable processing of

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biomass into a spectrum of bio-based products (food, feed, chemicals and/or materials) and

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bioenergy (biofuels, power and/or heat)”, in a way that a biorefinery can be a concept, a

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facility, a process, a plant, or even a cluster of facilities.14 Integrated biorefineries,

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characterized by a reduced carbon footprint of the final products, can provide a sustainable

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approach to valuable products that can also improve biomass processing economics as well as

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environmental issues,14 and have been considered crucial for extracting the maximum value

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from biomass.15 In the medium term, relatively small-scale biorefineries making use of local

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or regional resources have been considered as the most favored method to introduce more

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advanced (green, whole-crop, and lignocellulosic) biorefinery processes into the market.14

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The main issue in LCM processing lies within its complex structure and chemical

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composition, an important issue when envisioning new processes.16 4


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Usually, LCM fractionation intends the selective separation of the structural

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components (lignin, cellulose and hemicelluloses), which have to be recovered and further

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processed to yield an array of tailored commercial products. A recent analysis of the potential

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demand, the biomass availability and the energy efficiency led to the conclusion that the

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production of chemicals from biomass is more beneficial than the production of transportation

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fuels or electricity.17

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Desirably, operation in biorefineries should follow the basic principles of Green

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Chemistry, for example regarding an efficient utilization of raw materials, and avoiding the

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utilization of wastes instead of performing end-of-pipe waste remediation,3 enabling the

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production of end-products fulfilling the societal needs, and shortening the dependence on

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fossil resources.18 In this way, a sustainable industrial and societal development can be

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achieved by meeting the needs of the present generation without compromising the needs of

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future generations to meet their own needs.19

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On the other hand, the implementation of biorefineries has been identified as the most

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promising route to the creation of a new domestic biobased industry (NREL, 2014), paving

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the way for a future low carbon bioeconomy,20,21 which would be driven by clean, sustainable

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environmental development, economic growth and green politics,22 and result in a highly

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efficient and cost-effective processing of biological feedstocks to a range of bio-based

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products.23 On this basis, the implementation of biorefineries could also contribute to the

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revitalization of rural areas.24 There is a long way to go, since just about 5% of the European

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economy was bio-based in 2010; whereas in the USA, about 12% of all products (excluding

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energy use) in 2010 was originated from biomass.25

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Ionic liquids in lignocellulose biorefineries

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Increasing attention is being paid to the utilization of ionic liquids (ILs) in a variety of fields.

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ILs are salts composed of large organic cations and inorganic or organic anions, with melting

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points below 100 ºC, which can be designed to be liquid at room temperature.26 Their low

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volatility (many ILs can be distilled at 200–300 ºC just under significantly reduced pressure

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and at very low distillation rate)27 limits the losses in vapour phase. This is an important

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advantage over the conventional solvents employed in industries, which can be responsible for

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emissions of Volatile Organic Compounds (VOC), identified as a major source of waste,28,29

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and show hazards related to inhalation and explosion.30 Additionally, some ILs present

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catalytic activity. In this case, they show many of the advantages of both homogenous and

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heterogeneous catalysts, such as high acid density, uniform catalytic active centers, easy

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separation and recyclability.31

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The most common ILs can be classified in four groups according to their cations:

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quaternary ammonium ILs, N-alkylpyridinium ILs, N-alkyl-isoquinolinium ILs, and

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imidazolium-based ILs.32 Among them, the latter group has received special attention, and is

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the focus of this study. Figure 1 presents the general formula of 3-methylimidazolium-based

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ILs ([mim]) employed for biomass processing, as well as the nomenclature employed in this

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work for the various ILs.

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ILs are considered as green chemicals,33,34 and offer a unique environment for

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chemistry, biocatalysts, separation, material synthesis, and electrochemistry.35 However, ILs

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can be used as more than just alternative green solvents, since they differ from molecular

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solvents by their unique ionic character and their structure and organization, which can lead to

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specific effects.27 Non-flammability, lack of toxicity and thermal and chemical stability are

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usual ILs properties of special importance for application in biorefineries. An ideal ionic

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liquid should be inexpensive, non-toxic, biodegradable and recyclable.3

6


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Thermally stable ILs can be employed as reaction media or catalysts to carry out

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reactions at high temperatures using conventional equipment, in a way that their negligible

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vapour pressure would facilitate the recovery of volatile reaction products. Due to the unique

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solvent properties of ILs, these solvents promise advantages with regard to conversion and

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selectivity, leading to energy savings when compared to conventional solvents.26 Additionally,

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their properties can be tuned for specific chemical tasks36 by selecting the type of the

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constituent cation and anion:37 for example, a number of ionic liquids show unique solvating

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properties.38 The increasing knowledge on the chemical, physical and technical properties of

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ILs is expected to allow a wider and more efficient utilization.

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Potential applications of ILs in biorefineries

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Based on the above information on the nature of LCM and properties of ILs, a number of

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potential applications of ILs in lignocellulose biorefineries have been identified, including

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their utilization as agents for separation, cellulose dissolution and derivatization, LCM

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fractionation or LCM pretreatment (see next sections). The utilization of ILs as reaction media

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and/or catalyst for the hydrolysis-dehydration of polysaccharides into furans is also an

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interesting research subject, but it falls out of the scope of this article.

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ILs as separation agents for compounds present in biorefinery streams

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ILs find a number of applications in biorefineries, for example in processes dealing with the

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production of second-generation biofuels (such as bioethanol or biobutanol), or short-chain

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organic acids. In this kind of technologies, LCM are processed to hydrolyze polysaccharides

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(cellulose and/or hemicelluloses) into sugars, which are fermented to yield the target products.

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One of the major techno-economic challenges in biorefineries manufacturing second-

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generation ethanol from LCM is the comparatively low ethanol content of the fermentation 7



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broth. The ethanol concentration can be increased by ordinary distillation up to concentrations

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near the azeotrope (95.5 wt% ethanol), but the separation becomes increasingly difficult when

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approaching this threshold. On the other hand, fuel applications require almost anhydrous

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ethanol, making the utilization of alternative separation technologies (typically, azeotropic or

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extractive distillation) necessary. Both azeotropic and extractive distillations involve the

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addition of a third component (entrainer) able to break the azeotrope. Imidazolium-based ionic

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liquids have been successfully employed as separation agents for the dehydration of ethanol-

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water mixtures,26,39,40 as they can greatly enhance the relative volatility of ethanol over water,

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showing advantages derived from low viscosity, thermal stability, good solubility and lower

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corrosiveness respect to the high melting salts suitable for the same purpose.40 In a

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comparative evaluation of [bmim]BF4, [emim]BF4 and [bmim]Cl (see Figure 1 for

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nomenclature), the ability to improve the separation was found to follow the order [bmim]Cl >

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[emim][BF4] > [bmim][BF4], in a way that the two best ILs performed better than the

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reference compound.39,41 Figueroa et al.41 simulated the ethanol recovery using a number of

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ILs ([bmim]Cl, [emim]Cl, [emim]BF4, [bmim]Ac, [bmim]BF4, [bmim]N(CN)2, [hmim]Cl and

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[bmim]mSO4), and concluded that, in the best cases, high purity ethanol could be obtained at

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96 wt% recovery. In a related study, Meinsderma et al.42 proposed an energy-efficient process

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for ethanol dehydration based on extractive distillation with [emim]N(CN)2 enabling the

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production of 99.91% ethanol in facilities at the pilot scale. Other reported advantages of ILs

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as separation agents include:40

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ILs do not pollute the distillate due to their non-volatile character,

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reduced heat duties are required because of their non-volatility, high selectivities and heat capacities,

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•

ILs properties such as solubility, capacity, selectivity, viscosity and thermal stability) can be tailored, and 8


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only one distillation column is required.

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•

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Alternatively, ILs can be used for ethanol extraction from aqueous solutions. Arlt et al.43

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proposed a number of ILs (including imidazolium- and phosphonium- based ones) as potential

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solvents for the extractive separation of close-boiling or azeotropic mixtures, including water–

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ethanol. This approach was claimed to be superior to conventional extractive rectification in

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terms of cost-effectiveness and exergetic aspects, as a result of the selectivity and the unusual

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characteristic profile of the ionic liquids. Solubility data for ternary mixtures of ethanol, water

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and a hydrophobic IL ([bmim]PF6, [hmim]PF6 or [omim]PF6) were reported by Swatloski et

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al.,44 whereas [hmim]Tf2N was found to be suitable for separating ethanol from water, but

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acceptable recovery rates could be only achieved using unreasonably high solvent/feed

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ratios.45

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tetradecyltrihexylphosphonium-based ILs obtained by combination with seven different

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anions have been recently proposed as candidates for ethanol recovery from aqueous solutions

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by liquid–liquid extraction.46 On the basis of equilibrium calculations, the authors suggested

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the use of a single liquid–liquid extraction stage coupled to extractive fermentation as a

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favourable solution to recover ethanol from dilute aqueous solutions, a possibility favored by

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the comparatively low toxicity of some of the ILs assayed.

Based

on

ternary

liquid–liquid

equilibrium

studies,

a

number

of

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Biobutanol (n-butanol of biological origin) is an attractive biofuel,47,48 with higher energy

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density and lower volatility as compared to ethanol. Biobutanol production from LCM

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presents a number of challenging technological problems, some of them related to the

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complex composition of culture media, which contain large amounts of water, microbial cells

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(usually, bacteria belonging to the Clostridium genus), non-polysaccharide byproducts

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resulting from LCM hydrolysis, residual sugars and volatile fermentation co-products (such as

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acetone and ethanol). Additionally, the fermentation products are toxic to cells, a fact limiting

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the maximum achievable butanol concentration (which is typically within the 1-2 wt% range). 9



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In a screening study, Knoshaug and Zhang49 found just two Lactobacillus strains able to

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tolerate and grow in up to 3 wt% butanol, the same level achieved by a mutant Clostridium

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biejerinckii strain.30 Distillation, the technology traditionally employed to separate the volatile

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components present in this type of fermentation broths,50 involves a high steam

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consumption.51 Alternative, energy-efficient separation methods have been proposed,

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including adsorption-desorption, extraction and membrane-based technologies. ILs may find

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application in these two latter technologies.

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Very high boiling extractants have been recommended as extraction solvents for butanol

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fermentation products, a purpose for which ILs are suitable,30 either by direct contact with the

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broth inside the fermenter52 or by downstream processing. ILs containing anions such as PF6−

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or Tf2N− are water immiscible and enable the formation of biphasic systems suitable for

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extraction applications.53 In particular, ILs such as [hmim]PF6 or [bmim]Tf2N, together with

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others bearing a tetracyanoborate anion (such as [dmim]TCB) have been considered for n-

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butanol extraction;30,51,54 whereas the same purpose has been achieved using imidazolium-

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based ILs with alkyl chains of varying length in combination with tetrafluoroborate or

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trifluoromethanesulfonate anions.30 The hydrophobicity was correlated to the butanol

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distribution coefficient between ILs and water, whereas the extraction efficiency and

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selectivity were directly related to the polarity.30 Ha et al.55 studied eleven imidazolium-based

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ionic liquids as extraction agents to recover butanol from aqueous media, and also found an

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interrelationship between extraction efficiency, selectivity and polarity. The best results were

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achieved with [omim]Tf2N, which showed high extraction efficiency and allowed more than

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74% butanol recovery in a single extraction stage. Fadeev and Meagher56 employed

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[bmim]PF6 and [omim]PF6 to extract butanol from aqueous solutions, and subjected the ionic

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liquid-rich phase (simulating the composition in equilibrium with 1 wt% butanol – water

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solution) to pervaporation through a commercial polydimethylsiloxane membrane, obtaining 10


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an IL - free permeate. Pervaporation with IL-supported membranes 51,57 has been employed to

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reduce the IL demand and to avoid its direct contact with the production organisms, avoiding

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possible toxic effects.58

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Imidazolium-based ILs have also been proposed as separation agents for short-chain

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organic acids, including lactic acid.59–62 The biotechnological production of lactic acid in

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biorefineries by hydrolysis-fermentation of LCM is an interesting possibility, since this

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compound is an important commodity chemical for the production of specialty chemicals such

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as 2,3-pentanedione, acrylic acid, propionic acid, pyruvic acid and polylactic acid. Lactic acid

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can be obtained at high yield by fermentation of LCM-derived pentoses or hexoses.63

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Depending on the operational conditions, the lactic fermentation can be stereospecific, or it

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may lead mixtures of the two isomers. Interestingly, polylactic acid stereocomplexes with

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improved properties can be produced from pure optical isomers. Several 1-alkyl-3-

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methylimidazolium hexafluorophosphates (including [bmim]PF6, [hmim]PF6 and [omim]PF6)

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have been employed for the extractive fermentation of lactic acid, acting as a water-

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immiscible phase (instead of conventional organic solvents) to which lactic acid can be

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transferred in situ from the fermentation broth, avoiding cell inhibition.59,60 Although the

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extractability of the organic acid in the pure ILs was poor, the addition of tri-n-butylphosphate

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increased the extraction ability to a level similar to those of conventional organic solvents.

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The relatively low toxicity of the considered ILs allowed the growth of the fermenting cells

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(Lactobacillus), which were able to consume glucose and to produce lactate. A comparative

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evaluation of the various ILs showed that the length of the alkyl substituent in the

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imidazolium cation had little influence on cell survival.60

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In biorefineries, ILs can play a number of general roles common to other industries, for

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example in the removal of Volatile Organic Compounds (VOC) and hazardous pollutants

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from effluents.64–66 ILs such as [bmim]PF6, [bmim]Tf2N, or [bmim]BF4 can be directly 11



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employed in bioreactors, either for performing enzyme-mediated biocatalytic reactions67 or in

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the presence of whole-cells, for example in VOC absorption-biodegradation systems,67,68

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enabling the biotransformation of toxic substrates in the presence of an immiscible liquid

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phase acting as a substrate reservoir.

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The ability of ILs to selectively dissolve gases, together with their non-volatility, makes

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them potentially for gas separations69 (for example, CO2), acting as conventional liquid phases

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in absorbers or as components of supported liquid membranes.53,70,71 CO2 has relatively high

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solubility in a number of imidazolium ILs, particularly in those containing the

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bis(trifluoromethylsulfonyl)imide anion, whereas there is little difference in CO2 solubility

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between ionic liquids possessing the tetrafluoroborate or hexafluorophosphate anion.69

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Amine-functionalized imidazolium ionic liquids have been specifically designed for CO2

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capture.72

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ILs as agents for cellulose dissolution - regeneration and homogeneous derivatization

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Cellulose is hardly soluble in many conventional solvents because of its strong intermolecular

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hydrogen bonding. Traditional cellulose solvents, such as carbon disulfide, N-

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methylmorpholine-N-oxide (NMMO), and mixtures of N, N- dimethylacetamide and lithium

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chloride (DMAC/LiCl), show a number of practical drawbacks, including limited dissolving

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capability, toxicity, high cost, difficult solvent recovery, uncontrollable side reaction, and

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instability during cellulose processing,73 and/or the need for multi-step pretreatments followed

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by prolonged stirring.74

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The ability of some molten ILs (N-alkylpyridinium or N- arylpyridinium chloride

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salts) for dissolving cellulose (in the presence of a nitrogen-containing base) is known since

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the mid 1930s,75 but the interest in this topic was renewed when Swatloski et al.76 claimed the

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ability of ILs made of a number cations (including alkyl-imidazolium-based ones) and anions 12


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(including Cl−, I−, PF6−, BF4−, acetate and trifluoroacetate) for achieving cellulose dissolution.

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Cellulose dissolution in ILs involves the interaction of hydroxyl groups in cellulose with both

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the cation and anion in IL.77 The oxygen atoms of OH groups and IL anions act as electron

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donors along the dissolution process, in a way that anions acting as strong electron donors (for

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example, halogen and pseudohalogen ions) gave good cellulose dissolution results; whereas

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hydrogen atoms of hydroxyl groups and IL cations act as electron acceptors. The review by

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Wang et al.78 summarizes extensive literature information on ILs able to dissolve cellulose,

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the respective cellulose solubilities and the suitable operational conditions.

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After dissolution, cellulose can be regenerated by adding a miscible anti-solvent such

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as water, alcohols, ethers or ketones. When an anti-solvent such as water is added to the

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homogeneous IL-cellulose system, the IL ions form hydrogen bonds with water molecules,

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and are displaced into the aqueous phase; whereas cellulose (which previously interacted with

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IL) is expelled and rebuilt its intra- and intermolecular hydrogen bonds, and is then

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precipitated.79

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Cellulose dissolution/ regeneration in ILs is affected by many factors, including the

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type of anion and cation, the basicity and H-bonding capability of the anion, and the position

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and length of side chain in cation.80 As representative examples, [bmim]Cl,76 [amim]Cl;73,81

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and [emim]Ac82can dissolve cellulose at comparatively high concentrations. Based on their

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physicochemical properties, ILs with strongly basic anions (such as formate, acetate, or

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phosphate) have been cited as favorable candidates for cellulose dissolution under mild

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operational conditions.73,74,83 The dissolved cellulose could be readily regenerated by anti-

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solvent addition: for example, water, ethanol, or acetone have been employed to regenerate

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cellulose from [bmim]Cl;84 whereas Zhang et al.81 employed water to regenerate cellulose

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from [amim]Cl.

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The regenerated cellulose can be different from the native cellulose in terms of macro

320

and microstructure, crystallinity and even degree of polymerization, depending on the

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processing conditions.79,85 In fact, cellulose regenerated from ILs was found to be essentially

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amorphous and porous.38 Additionally, it can be noted that complete cellulose dissolution in

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ILs provides the opportunity of working with new functional groups, opening new ways for

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product design or enabling a better control of the reaction (for example, regarding the degree

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of substitution).86 The manufacture of cellulose derivatives in homogenous systems by

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esterification, etherification,73,86 acylation, carbanilation,87 carboxymethylation, succination,

327

phthalation,88 alkylation, silylation, and halogenation86 has been reported. Other interesting

328

examples of cellulose derivatization in ILs have been reported in related studies.89–94 On the

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other hand, cellulose dissolution -regeneration enables the recovery of cellulose nanofibers, or

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its utilization as films, powder, gels, or capsules.95

331 332

ILs as agents for LCM fractionation

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The complicated chemical composition and morphology of native LCM, characterized by the

334

presence of both non-structural components and a crosslinked, tridimensional matrix of

335

polysaccharides and lignin, together with the crystalline nature of cellulose, are major

336

hindrances for processing. Considered as feedstocks for chemical and/or biotechnological

337

processess, LCM are recalcitrant substrates that must be deconstructed (or fractionated) to

338

achieve the separation of the their major components. This is a key step in the conversion of

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lignocellulosic biomass into fuels and valuable chemicals and materials.96 LCM dissolution

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may imply the hydrolysis of carbohydrate polymers, or the chemical disruption of the

341

lignocellulose composite.97 ILs or ILs mixtures (eventually in the presence of catalysts, water

342

and/or other components) have demonstrated great promise as efficient solvents for biomass

343

fractionation, via total or partial dissolution.98 The interaction of ILs with lignocellulose may 14


Page 14 of 35

Page 15 of 35


344

involve ionic, π–π, and hydrogen bonding interactions. LCM dissolution in ILs depends on a

345

number of factors, including the type of constituent cation and anion, the nature and previous

346

processing of biomass (for example, milling or drying), the characteristics of the substrate

347

(including composition, particle size distribution, moisture), the relative amount of LCM,

348

other operational conditions (including temperature heating profile, maximum temperature

349

and processing time), and the possible presence of additional components in the medium (for

350

example, water, catalysts or co-solvents). For example, DMSO has been used in combination

351

with [bmim]Cl or [amim]Cl to achieve the partial dissolution of lignified raw materials,96,99

352

whereas the utilization of two ILs ([emim]Ac in combination with [amim]Cl or [bmim]Cl) has

353

been proposed for the same purpose.100

354

Complete dissolution has been reported for LCM of low lignin content (such as

355

bagasse, straws, grasses, or husks), but also for recalcitrant substrates such as hardwoods or

356

softwoods. Concerning wood dissolution, [bmim]Cl and [emim]Ac have been identified as the

357

two most promising ionic liquids.101 Representative studies and reviews on LCM dissolution

358

using a number of imidazolium-based ionic liquids have been reported in the past few

359

years.37,38,88,96,97,99,102–112 Once biomass is dissolved, the various structural components can be

360

recovered using a number of operational strategies, many of them based on the precipitation of

361

cellulose (or cellulose-enriched solids) upon anti-solvent addition.97

362

An alternative to completely dissolving biomass is to selectively dissolve just a single

363

biomass component.12 This approach may involve hemicellulose hydrolysis or lignin extraction.

364

Delignification-based processes usually enable the recovery of polysaccharides (at least,

365

cellulose) as insoluble solids.26,113,114 Representative information on the delignification of

366

multiple LCM in media containing imidazolium-based ionic liquids (including [emim]Ac,

367

[emim]Ace, [emim]ABS, [emim][Cl], [bmim]Ac, [bmim]Ace, [bmim]mSO4, [bmim]Br,

368

[bmim]BF4, [bmim]PF6, [amim]Cl, and [hmim]Cl) has been summarized in recent 15



369

studies.37,97,100,113,115-118

370

The ideal technology for LCM biorefineries would be the one where the three major

371

components of the biomass were obtained at high yields and purities.119 However, the

372

recovery of hemicelluloses (the most easily hydrolyzable of the three main lignocellulosic

373

polymers) is challenging,37 as this fraction can be considerably depolymerized and dissolved

374

in ILs, and eventually decomposed (as it has been found in the processing of pine wood with

375

[Hmim]Cl),113 resulting in losses that increase the cost of downstream separation.110 While

376

delignification increases with high temperatures, such conditions promote hemicellulose

377

losses.120 On the other hand, treatments allowing delignification and hemicellulose

378

solubilization simultaneously may entail losses of cellulose as soluble hydrolysis products.114

379

In some dissolution-regeneration approaches, hemicellulose and cellulose can be obtained as

380

separated fractions from the regenerated material, using specific solvents to maximize the

381

fractionation process.37,121 The selective dissolution of hemicelluloses (up to 39% of the total)

382

from extracted or native spruce wood has been addressed in literature using switchable ionic

383

liquids.101 Starting from bleached paper-grade pulps, separate fractions of hemicelluloses and

384

cellulose (both of high purity) were recovered upon processing with [emim]Ac containing 15-

385

20% water.122 Under severe conditions (for example, in high temperature treatments with

386

strongly acidic ILs), the formation of carbohydrate degradation products able to react with

387

lignin (leading to the formation of pseudolignin) has been reported.114 Oppositely, in

388

treatments with ILs possessing strongly basic anions, hemicelluloses are converted into

389

oligosaccharides, generating only trace amounts of monomeric sugars (xylose and glucose).123

390

In some operational schemes, cellulose and lignin are precipitated sequentially, and the

391

hemicellulose-derived saccharides left in the ILs phase are subjected to further chemical

392

modification.124

16


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Page 17 of 35


393

The integral benefit of the various LCM components can be achieved by coupling

394

conventional separation treatments with IL processing. Following this idea, dissolution in

395

[bmim]Cl followed by NaOH extraction has been reported for bagasse utilization,121 whereas

396

the implementation of a first stage of hemicellulose removal (for example, hydrothermolysis

397

or prehydrolysis) before treatment with ILs (aiming at the separation of cellulose and lignin)

398

has also been proposed.97 This latter approach follows the philosophy underlying the

399

commercial prehydrolysis-kraft process, and is being studied as an alternative to increase the

400

added-value from hemicelluloses in the conventional kraft pulping technology.

401 402

ILs as pretreatment agents for the enzymatic hydrolysis of cellulose

403

The enzymatic hydrolysis of cellulose leads to glucose solutions suitable as fermentation

404

media for manufacturing a wide range of biofuels, solvents, polymers and other marketable

405

compounds. However, native LCM is recalcitrant to the action of cellulases, a fact attributed

406

to a number of factors, such as substrate accessibility, cellulose degree of polymerization,

407

crystallinity, particle size, and porosity, as well as to the presence of hemicellulose and

408

lignin.111 In particular, cellulose is embedded in a matrix of lignin and hemicelluloses, which

409

are crosslinked by ester and ether linkages in the plant cell wall, hindering the access of

410

enzymes to the cellulose glycosidic bonds. To disrupt the cell wall structure, some type of

411

pretreatment (by mechanical, biological, physical and/or chemical methods, individually or in

412

combination) is necessary. This type of processing is referred in literature as “pretreatment”,

413

and is been extensively reviewed owing to its economic and technological importance.

414

Chemical pretreatments modify the composition of substrates, and some of them can be

415

considered as true fractionation treatments. This dual behavior (as pretreatment and/or

416

fractionation agents) defines well the potential of ILs in the field: in fact, a number of

417

references included in this work (particularly, some of the ones employed to discuss the LCM 17



418

fractionation by ILs) are directly oriented to the production of substrates suitable for

419

enzymatic hydrolysis, since ILs show ability for removing lignin, disrupting the crystalline

420

structure of cellulose, and increasing the accessibility of enzymes to cellulose.32 The ILs most

421

frequently utilized for LCM pretreatment are the ones suitable for cellulose dissolution, for

422

example [emim]Ac, [amim]Cl, [bmim]Cl, and dialkylimidazolium dialkylphosphates, which

423

have been applied to a number of substrates.104,112,125–129 Depending on the IL employed, the

424

effects caused by processing may resemble those of aqueous stages performed under alkaline

425

or acidic conditions, as it happens for acetate- and chloride- based ILs, respectively.130 Data

426

on the comparative saccharification efficiency achievable with selected ILs have been

427

reported recently.129

428

In typical processing schemes, LCM are treated with ILs to cause dissolution-

429

regeneration or delignification, yielding cellulose-enriched substrates of decreased

430

crystallinity and improved enzyme accessibility, which are extensively washed to remove the

431

residual IL and further hydrolyzed with enzymes at high yield with fast kinetics. It can be

432

noted that ILs are comparatively expensive, and that efficient and cost-effective recovery

433

methods have to be implemented for economic feasibility. The best IL depend on the substrate

434

considered;

435

different substrates.109 In terms of saccharification yields, material recovery and

436

delignification, [emim]Ac ranks as the most suitable IL for biomass pretreatment;130 but

437

[bmim]Cl performed very well for simple cellulosic substrates, and [bmim]HSO4 has been

438

recommended for processing hybrid aspen.112 Acetate-based ILs present advantages over

439

chloride-based ILs derived from their lower melting point, lower viscosity, lower corrosive

440

character and ability for working with higher loadings.131 Strongly acidic ionic liquids may

441

cause partial polysaccharide depolymerization, hydrolysis and dehydration, leading to the

442

formation of furans as reaction byproducts.

112

and for a given IL, the optimal processing conditions must be optimized for

18


Page 18 of 35

Page 19 of 35


443

It must be considered that, in integrated processes, ILs may be present in the aqueous

444

hydrolysis media as a result of recycling streams coming from IL recovery stages. This is

445

important since the presence of ILs in the hydrolysis media (even in trace amounts) can inhibit

446

or denature enzymes, depending on the type and concentration of the considered IL.

447

According to Turner et al.132, Trichoderma reesei cellulases are subjected to ionic strength-

448

induced inactivation and unfolding in media containing as little as 22 mM [bmim]Cl.

449

Refolding of denatured cellulase was possible only when the enzyme was contacted with

450

solutions containing 0–5% [bmim]Cl. The Cl− ion was, in part, responsible for the inactivation

451

of the cellulase, since the IL produces a dehydrating and denaturing environment.

452

In most cases, the enzyme activity drops quickly when increasing the ionic liquid

453

concentration, but no general conclusions on this subject can be drawn owing to the diversity

454

of conditions assayed. In integrated biorefinery processes, keeping a very low IL

455

concentration in the hydrolysis media would require extensive processing and clean up of the

456

cellulosic substrates. Because of this, the cellulases should be preferably able to perform

457

optimally in solutions containing up to 15 wt% IL, which correspond with the amount retained

458

by the hydrolysis substrates upon processing.133 To deal with this problem, two possible

459

approaches can be followed (individually or simultaneously): utilization of ILs causing

460

minimal inhibition and/or using cellulases with increased tolerance to the IL.

461

The IL [emim]Ac shows minimal inhibitory effects on enzymes compared to ionic

462

liquids containing Cl− or Br− anions;96,133,134 whereas favorable compatibility has been also

463

reported for [emim]DEP,135 [mmim]DEP and [mmim]DMP.136 Concerning the role of

464

enzymes, Wahlström and Suurnäkki125 summarized literature on the action of cellulases and

465

other glycosyl hydrolases in IL solutions. Interesting possibilities lay in the utilization of

466

enzymes from extremophilic strains able to perform at high temperature and salt

467

concentration, as the utilization of enzymes capable of withstanding higher concentrations of 19



468

salt and ionic liquids and higher pH would make the washing steps less cumbersome,

469

facilitating the industrial biomass conversion process.133,137 Datta et al.133 investigated the

470

stability of recombinant, extremophilic enzymes, as a function of IL concentration, and

471

reported that (unlike the reference Trichoderma viride cellulase), the tested enzymes retained

472

between 44% and 79% of their activity in the presence of 15% [emim]Ac. In this field, a GH5

473

family cellulase able to keep activity in the presence 30% (v/v) IL has been identified;138

474

whereas an hyperthermostable archaeal endo-glucanase was reported to retain a considerable

475

activity (30–50%) in the presence of 25% (v/v) [mmim]DMP.139

476

In conclusion, ILs are potentially suitable for a number of duties in LCM biorefineries.

477

Considered as agents for physicochemical processing of liquid process streams, ILs are

478

suitable as entrainers for the dehydration of ethanol-water mixtures, as well as agents for the

479

in situ separation of microbial metabolites (including selected alcohols and acids) from the

480

fermentation media. ILs are also suitable for preventing pollution from gaseous waste streams,

481

for example by removing Volatile Organic Compounds or carbon dioxide from them.

482

Concerning the chemical modification of native LCM, operational schemes based on

483

dissolution-regeneration or delignification provide interesting alternatives for developing

484

alternative processing technologies. One of the most suited schemes for LCM biorefineries

485

(pretreatment/fractionation followed by enzymatic hydrolysis and fermentation) could be

486

conveniently perfomed by using selected ILS (causing limited losses of enzymatic activity

487

and/or showing enhanced compatibility with microorganisms), as well as by using tolerant

488

enzymes produced by extremophilic microorganisms.

489 490

Acknowledgement

20


Page 20 of 35

Page 21 of 35


491

The authors are grateful to the Spanish “Ministry of Economy and Competitivity” for

492

supporting this study, in the framework of the research Project “Advanced processing

493

technonologies for biorefineries” (reference CTQ2014-53461-R), partially funded by the

494

FEDER program of the European Union. Ms. Sandra Rivas thanks the Ministry for her

495

predoctoral grant.

496 497

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868


FIGURE LEGEND

869 870 871

Figure 1. General nomenclature employed for imidazolium-based ILs and representative examples of ILs employed for biomass processing.

872

33



873

Page 34 of 35

TOC GRAPHIC

874 875 876 877 878

Ionic liquids R1

+

N

N

CH3

─

A

Products

879 880 881

LIGNOCELLULOSE BIOREFINERY .Physical separation .Chemical modification

34


Page 35 of 35


GENERAL FORMULA

R1

N

N+

CH3

GENERAL NOMENCLATURE

Anion Substituent Methyl imidazolium cation (mim)

Representative ILs cited in this study [Hmim]Cl [mmim]DEP [mmim]DMP [emim]ABS [emim]Ac [emim]Ace [emim]BF4 [emim]Cl [emim]DEP

[emim]N(CN)2 [bmim]Ac [bmim]Ace [bmim]BF4 [bmim]Br [bmim]Cl [bmim]HSO4 [bmim]mSO4 [bmim]N(CN)2

[R1mim]A

A─

[bmim]N(CN)2 [bmim]PF6 [bmim]Tf2N [hmim]Cl [hmim]PF6 [hmim]Tf2N [omim]PF6 [omim]Tf2N [dmim]TCB [amim]Cl

Typical R1 and nomenclature Hydrogen, H Methyl, m Ethyl, e Butyl, b Hexyl, h Octyl, o Decyl, d Allyl, a

Typical A− and nomenclature Chloride, ClBromide, BrTetrafluoroborate, BF4− Hexafluorophosphate, PF6 − Dicyanamide, N(CN)2 − Hydrogen Sulfate, HSO4 − Acetate, Ac − Tetracyanoborate, TCB − Methylsulfate, mSO4 − Dimethylphosphate, DMP − Diethylphosphate, DEP − Acesulfamate, Ace − Bis(trifluoromethylsulfonyl)imide,Tf2N − Alkylbenzenesulfonate, ABS −

Figure 1


Utilization of Ionic Liquids in Lignocellulose Biorefineries as Agents for

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