Conversion of Biomass and Its Derivatives to Levulinic Acid and

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Conversion of biomass and its derivatives to levulinic acid and levulinate esters via ionic liquids Yong Wei Tiong, Chiew Lin Yap, Suyin Gan, and Winnie Soo Ping Yap Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00273 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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Industrial & Engineering Chemistry Research

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Conversion of biomass and its derivatives to levulinic

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acid and levulinate esters via ionic liquids

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Yong Wei Tiong†, Chiew Lin Yap†, Suyin Gan‡,*, Winnie Soo Ping Yap†

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†

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Selangor, Malaysia

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‡

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Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor, Malaysia

Faculty of Science, University of Nottingham Malaysia Campus, Jalan Broga, 43500 Semenyih,

Department of Chemical and Environmental Engineering, Faculty of Engineering, University of

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*Corresponding Author: Suyin Gan

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Tel: +6 (03) 8924 8162; Fax: +6 (03) 8924 8001

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Email: [email protected]

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ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Biomass has emerged as an abundant and relatively low cost carbon resource alternative to

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fossil fuel resources in the sustainable production of specialty chemicals and biofuel. Levulinic

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acid is an attractive platform chemical. Upgrading of levulinic acid produces levulinate esters,

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which serve as a transportation fuel and fuel additive. The present review focuses on the

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development of sustainable conversion of biomass into levulinic acid and levulinate esters via

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ionic liquids dual solvent-catalysts. The synthesis routes of levulinic acid and levulinate esters, and

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the corresponding ionic liquids are introduced. The biomass pretreament, as well as the

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conversions of lignocellulosic biomass and their derivatives into levulinic acid and levulinate

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esters, are detailed in relation to the catalytic role, properties and performance of acidic ionic

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liquids. Finally, the operating conditions affecting the ionic liquids catalytic conversions are

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discussed as part of a comprehensive review of this topic.

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Keywords: Biomass; biofuels; ionic liquids; levulinic acid; levulinate esters


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1. Introduction

The huge consumption of conventional fossil fuels has led to a swift decline in the fuel

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reserves and a rapid increase of global energy consumption, alongside with growing concerns over

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environmental pollution and carbon emissions.1 Although a shift to shale gas has recently occurred,

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the adverse environmental impacts associated to the use of this energy source remain unresolved.

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Hence, international efforts have persisted in developing renewable energy such as biomass, solar,

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thermal, tidal, wind, hydro, etc.2 Among these, biomass has been recognised as the only carbon-

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based renewable resource, which affords an environmentally beneficial reduction in the carbon

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footprint of chemicals and liquid fuels production, in a green and sustainable perspective.3 Nature produces a vast amount of biomass annually estimated at 170 billion metric tonnes,

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75% of which can be assigned to the category of carbohydrates, but only 3-4% of these compounds

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are consumed by humans for food and non-food purposes.4 According to a report by the Imperial

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College Centre for Energy Policy and Technology, the available biologically productive land area

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is approximately 13 Gha (1Gha = 109 ha) whereby an estimated 1.5 Gha are used to grow arable

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crops and 4 Gha are occupied by forests.5 Hence, this suggests that a total area of 5.5 Gha has the

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potential of suppling biomass feedstock. This abundance and wide availability makes biomass as

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one of the most promising alternative energy source worldwide. In the past 50 years, a substantial

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amount of research has been conducted to produce biodiesel and other biofuels to meet growing

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energy demands.6 For instance, agricultural residues such as oil palm fronds,7 wheat straw,8

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bamboo9 as well as paper pulp, wood chips and switchgrass10 were reportedly utilised to produce

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levulinate esters (LE), which is a high potential biofuel.11

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The National Renewable Energy Laboratory (NREL), US has identified levulinic acid (LA)

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as one of the most promising building blocks for organic synthesis.12 Levulinic is a member of

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gamma-keto acids which can be synthesised through deep hydrolysis of biomass.13 The importance

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of LA also lies in its dual functional groups of ketone and carboxylic acid, which can undergo

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various reactions. It serves as an ideal platform chemical for the production of a wide range of

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value-added products such as textile dye, animal feed, coating material, resins, polymers,

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herbicides, pharmaceuticals, food flavouring agents, solvents, plasticisers and anti-freeze agents.14-

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alcohols produces LE17 while hydrogenation of LA produces 2-methyltetrahydrofuran (MTHF),18

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γ-valerolactone (GVL)19 and valeric acid (VA).20 VA can be further esterified to produce valerate

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esters.21 Furthermore, dehydration of LA produces angelica lactone which can be further

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hydrodeoxygenated to form compounds with C7-C10 gasoline-like hydrocarbon branches.22

LA also acts as a precursor in the production of biofuel. For example, esterification of LA with

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LE are produced by esterification of LA with alcohol. The alkyl component from alcohol

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dictates the variability in the physical properties of LE. Methyl levulinate which has the shortest

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alkyl chain is a potential gasoline additive, whereas ethyl and higher alkylated levulinates which

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have better solubility in aromatics-rich diesel range fuels,23 are potential diesel blend components

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and biodiesel.24 Ethyl and higher alkylated levulinates have similar boiling points to those of heavy

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gasoline compounds (over 475 K) or of the middle diesel fuel boiling range.25 Thus, their blending

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with diesel would neither significantly alter the volatility of diesel fuel nor require any

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modification to existing engine design.24 LE also serve as solvents, plasticizers or precursors in the

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production of various synthetic materials.17 The keto ester functional groups of LE allow various

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condensation and addition reactions in the synthesis pathways.26,27 In summary, LE offer a

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promising prospect as an important platform chemical in the portfolio of a futuristic biorefinery. 4 ACS Paragon Plus Environment

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Although the utilisation of renewable biomass feedstock allows both LA and LE to be

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produced at a lower cost, improvements in the use of more environmentally benign catalysts and in

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reducing the process energy requirements are essential to develop a more sustainable production

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route. LA and LE are conventionally produced with the aid of mineral acid catalysts which are

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corrosive and thus pollute the environment.14 The used acid are difficult to separate and recover

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from final reaction mixture upon completion.5 Hence, replacement of mineral acid catalysts with

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heterogeneous acid type catalysts which are more easily separable and reusable could be of an

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advantage.28 However, deactivation of heterogeneous catalysts upon recycling was usually

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observed if they were used without re-treating, due to the adsorption of formed carbonaceous

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products on the solid acid surface and the gradual loss of acid sites.17 These challenges have

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prompted researchers to seek alternative, environmentally benign catalysts for biomass processing.

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Recent LA and LE productions via green chemistry have focussed on the utilisation of

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novel and promising ionic liquids (ILs). ILs are typically composed of an organic cation and an

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inorganic anion, which could act as dual solvent-catalysts that offer numerous advantageous over

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conventionally used organic solvents.29 It possesses several remarkable properties such as

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negligible vapour pressure, non-flammable, high thermal stability, recyclability, broad liquid range

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and excellent dissolving capacity,29–31 all of which support sustainable production. The potential of

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ILs is further emphasised by the fact that their structural functionalities on the cationic or anionic

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part can be finely tuned which has made it possible to design new ILs with targeted properties.32

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Meanwhile, these ILs could introduce specific features such as dual hydrophobicity-hydrophilicity

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ends, control of solute solubility and additional functional groups for catalysis or reactivity

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purposes. This flexibility offers a broad room for further exploration and improvements.33



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Lignocellulose consists of three main compositions, i.e. cellulose, hemicellulose and lignin.

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These made up its complex structure with distinctive chemical and physical properties that resists

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in various chemical and biological microbial attacks.34 Considerable efforts have been made to

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reduce the structural complexity of the lignocellulose without destroying its fermentable sugar

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content, prior to further conversion into various bio-based materials, such as LA and LE. There are

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four main pretreatment strategies developed for lignocellulose, which include physical, chemical,

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physiochemical and biological methods.31 However, all these pretreatments exhibit varying degree

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of drawbacks, such as the need for specialised equipment and extreme operating conditions or the

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release of toxic pollutants.31 To address these problems, recent progress has suggested to apply ILs

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dual solvent-catalysts system which allows several reactions to be conducted concurrently (i.e.

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biomass pretreatment, catalytic chemical conversion and purification steps) in one-pot.35

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In this context, ILs are viewed as prospective dual solvent-catalysts for one-pot production

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approach (i.e. biomass pretreatment and various bio-based production steps) where the technical,

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economic and environmental aspects were addressed through their inherent characteristics of being

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less corrosive, easily separable, recyclable, applicability to continuous process and lesser waste

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water production. Among the different types of ILs, considerable efforts have been made to design

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acidic ionic liquids (AILs) as potential and alternative catalyst for the processing of energy

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conversion materials.36 The acidic nature of AILs makes it a more efficient catalyst in various

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catalytic chemical conversions. Specifically, cations of the AILs influence the accessibility of the

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acidic active sites, whilst anions of that dictate the intensity of the acidity.37 Therefore, the

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development of technical feasible and environmentally friendly ion pairs of AILs for biomass

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pretreatment and catalytic chemical conversion into LA and LE is highly demanded.


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Over the last decade, the utilisation of ILs for the production of biofuel and biodiesel from

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biomass has expanded significantly.31,37–41 However, both LA and LE productions via ILs are still

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limited to bench scale. Several reviews on the production of LA platform chemical from biomass

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and derivatives have been published.2,15,16,42 These reviews broadly discussed various catalytic

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systems applied in the production, i.e. homogeneous mineral acid, fluorinated solvents/acid,

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heterogeneous solid acid, biphasic media, solvolysis, supercritical fluids and ILs. Additionally,

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there were only two reported reviews on LE synthesis from biomass and derivatives, in which the

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catalytic systems of mineral acids, heterogeneous catalysts and ILs were widely discussed.13,43 To

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date, a critical evaluation of the synthesis of LA and LE from biomass and derivatives, specifically

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by ILs and its coupling methods, has yet to be conducted. In light of this, this paper aims to address

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this knowledge gap by comprehensively reviewing various ILs and its coupling methods for the

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conversions of biomass and derivatives to LA and LE. The theoretical background, detailed

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mechanistic insights, catalytic properties of the ILs as well as reaction operating conditions, are

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discussed alongside with the performance data for both sequential and one-pot production. The

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environmental impacts of the process and the recyclability of the catalysts are also discussed as part

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of the sustainability evaluation of the production.

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2. Synthesis route of LA and LE from biomass

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Lignocellulose primarily contains cellulose, hemicellulose and lignin. Cellulose and

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hemicellulose are highly functionalised polysaccharides that constitute a major portion of plant

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biomass,44 and play a decisive role in depolymerisation and esterification routes for the production

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of LA and LE, respectively. Upon hydrolysis, they are macro sugar polymers that depolymerise

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into C5 and C6 sugar monomers and/ or oligomers, prior further converted to intermediate products



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(i.e. furfural, 5-hydroxymethylfurfural (5-HMF)), and finally to LA in the presence of an acid

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catalyst.45,46

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Cellulose is the major substrate conversion to LA and LE where it breaks down into C6

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sugars (glucose, galactose and mannose). To a lesser extent, hemicellulose also breaks down into

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C6 sugars, apart from the major components of C5 sugars (xylose and arabinose). Aside from that,

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lignin is a phenolic polymer that may be dissolved in the reaction solution as an acid soluble

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lignin.47 Figure 1 shows a schematic diagram for the conversions of each component of

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lignocellulosic biomass to LA and LE.


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5-HMF Glucose

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Fructose

C6-sugars

Cellulose 4 Glucose

Xylan

5

Galactose

6

Mannose

7

C5-sugars

Decomposition products (humins)

LA ROH/ H+

Mannan 8 Xylose

Arabinose

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Furfural LE Arabinan

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Ethanoic acid

Galactoglucomannan

Hemicellulose Lignin

Acid soluble lignin + Insoluble lignin

Formic acid 4-O-methyl glucuronic acid

+ Decomposition products

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Figure 1. Conversion of lignocellulosic biomass components to LA and LE. Reproduced with permission from Girisuta et al.48 Copyright 2013 Elsevier. 9 ACS Paragon Plus Environment


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Specifically, LA and LE are synthesised via two major routes, i.e. cellulose (hexose) and

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hemicellulose (pentose) routes.49 The hexose sugars route is more common and is related to the

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degradation of cellulosic carbohydrate of lignocellulosic biomass50 in which 5-HMF is the key

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intermediate compound. The hexose route involves five major steps as follows: (1) hydrolysis of

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polymeric cellulose into monosaccharides, (2) isomerisation of aldose-type sugars (glucose) to

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ketose-type sugars (fructose) by allowing five-membered ring formation, (3) dehydration of

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fructose to generate 5-HMF, (4) rehydration of 5-HMF to produce LA, then (5) esterification of

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LA with alcohol to produce LE.47,49 Meanwhile, the pentose route is resulted from the

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degradation of hemicellulosic carbohydrate of lignocellulosic biomass50 in which furfural and

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furfuryl alcohol are intermediate products. Due to the branched and amorphous structure,

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hemicellulose is easily hydrolysed and converted into end products as follows: (1) hydrolysis of

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hemicellulose into C5 sugars (mainly xylose), (2) dehydration of xylose to furfural, (3) separation

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and transformation of furfural into furfuryl alcohol via a gas-phase hydrogenation step, (4) acid

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catalysed ring-opening of furfuryl alcohol in water to produce LA, then (5) esterification of LA

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with alcohol to produce LE.51 In certain circumstances, the ring opening of furfuryl alcohol can

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also be achieved in the presence of alcohols where the alcoholysis subsequently affords LE and

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the hydrolysis process is skipped to furnish LA,52 as shown in Figure 2. However, under the

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severe reaction conditions, the intermediate products (sugars, 5-HMF and furfural) may further

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react with sugars to form dark-brownish decomposition products (humins),47 as shown in Figure

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1. The pentose route is an alternative to the hexose route in the conversions of cellulose-derived

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substrates. The details of the reaction pathways of hexose and pentose routes are shown in

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Figure 2.


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Lignocellulosic Biomass

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Pentose Route

Hexose Route

Hemicellulose

Cellulose

3

4

H+

H+

Hydrolysis

Hydrolysis

5

Isomerisation 6

7

8

9

H+

-H2O

Dehydration

H+

Furfural

-H2O

Dehydration

5-HMF

10

11

H2 12

Hydrogenation H2O/ H+

Furfuryl alcohol H2O/ H+

Hydration

LA

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Hydrolysis 14

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ROH/ H+

ROH/ H+ Alcoholysis

Esterification

LE 16

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Figure 2. Reaction pathways of hexose and pentose route for production of LA and LE. Reproduced with permission from Neves et al.53 Copyright 2013 Elsevier.

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3. Comparison of ILs with other catalysts The conversion can be catalysed by various catalysts which include mineral acids,

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heterogeneous catalysts and ILs. Mineral acids such as H2SO4 and HCl are the conventional

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catalysts for the production of LA and LE. In fact, the first biorefinery for the production of LA

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from biomass via mineral acid catalyst has been practiced industrially in Caserta, Italy.54 However,

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these catalysts are unrecyclable and can cause equipment corrosion. The production catalysed by

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mineral acid requires harsh reaction conditions, and the waste generated needs a large volume of

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base for neutralisation. Thus, replacement of such mineral acids by heterogeneous acid type

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catalyst that are separable and reusable is highly desirable.28 Nonetheless, heterogeneous catalysts

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are easily deactivated due to the adsorption of carbonaceous products on the solid acid surface and

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the gradual loss of acid sites,55 hence, re-treatment is often needed in recycling runs. With regard

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to this, efforts have been made to develop environmentally benign processes for conversion of

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biomass and derivatives to LA and LE using ILs as a greener and sustainable option. Additionally,

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since ILs play the dual role of solvent-catalysts, a biphasic system is readily formed at the end of

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the production (refer to Section 6.5), thereby the catalyst separation becomes favourable with no

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organic solvent needed. Nevertheless, the high cost of ILs remain a challenging issue. This can be

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overcome by synthesising the ILs from low cost raw materials and recycling the ILs for

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subsequent catalytic runs.56-58 A comparison of the advantages and disadvantages of ILs with

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mineral acids and heterogeneous catalysts is listed in Table 1.

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Table 1. Comparison of ILs with mineral acid and heterogeneous catalysts. Catalyst Mineral acid

Advantages

•

Disadvantages

High process effectiveness59

• • • •

Heterogeneous catalysts

• • •

No corrosion and erosion63 Easy to handle64 Recyclable64

• •

• • ILs

• • • • •

Greener66 Milder reaction conditions66 Higher process effectiveness66 Recyclable66 Recoverable67

•

Corrosive60 Used acid is difficult to recover60 Contaminate end-products61 Large amount of acidic wastes generated62 Leaching of acidic groups of the catalyst63 Extreme reaction conditions are required which results in deactivation due to coking 62 Restricted accessibility of the matrix-bound acid sites65 High molecular weight to activesite ratios65 High material cost66,68

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4. Acidic ionic liquids Conversions of biomass and derivatives to LA and LE have been achieved via acidic ionic

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liquids (AILs). The acidic nature of ILs is the main criteria for the catalytic dehydration of

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carbohydrates to a variety of bio-based compounds which includes furan derivatives (5-HMF,

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furfural), LA, LE and other related compounds.69 AILs are a group of functionalised task specific

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ionic liquids (TSILs) which possess a functional group covalently tethered to the cation or anion

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(or both) of the ILs. AILs are low melting ionic salts with acidic characteristics. Their acidic

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character can be categorised into three major groups:69,70 (1) Bronsted, (2) Lewis, or (3) a



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combination of Bronsted and Lewis acids. The acidic function(s) or group(s) can be either cation,

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anion or both. Lewis acidic ionic liquids (LAILs) possess a Lewis acid site which can accept lone

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pair electrons from a Lewis base, whilst, Bronsted acidic ionic liquids (BAILs) possess a Bronsted

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acid site which can transfer a proton to a Bronsted base. BAILs are further divided into sole acid

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site and multi-acid sites. Basically, there are two types of BAILs with sole acid site, as follows: (1)

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BAILs with acidic cation such as sulfonic acid (–SO3H), carboxyl acid (–COOH) or hydroxyl acid

7

(–H), which are covalently bonded to the anion, and (2) BAILs with monoacid anion such as

8

hydrogen sulphate [HSO4], dihydrogen phosphate [H2PO4], etc. BAILs with multi acid sites can

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also be separated into two types, as follows: (1) BAILs with two acidic sites, of which the acidity

10

is contributed by the same or different type of functional groups, either solely from cation such as -

11

SO3H or -COOH or -H groups, or solely from anion, such as [HSO4], [COOH] and [H2PO4], (2)

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BAILs with three or more acidic sites, of which the acidity is contributed by various combinations

13

of functional groups, such as -SO3H, [HSO4], [COOH], and [H2PO4].71 Bronsted–Lewis acidic

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ionic liquids (BLAILs) possess both Bronsted and Lewis acidic sites. The addition of Lewis acidic

15

site to the catalyst gives a better catalytic performance than solely Bronsted acid site,72 presumably

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caused by the synergistic effect between both acidic sites in promoting the catalytic chemical

17

reactions. Moreover, the organic units of LAILs and BAILs can couple with various supporters to

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form heterogeneous catalysts with solid carriers, which are known as supported LAILs and

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supported BAILs, respectively. A detailed categorisation of AILs is illustrated in Figure 3.


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1

AILs

LAILs (i) [Bmim][Cl]/ZnCl2 (ii) [Hmim][Cl]/ZnCl2 (iii) [Emim][Cl] / CrCl3/HY zeolite

BAILs with sole acid site

BAILs with multiacid sites

Supported BAILs (i) [C4H6N2(CH2)3SO3H] [H2PW12O40] (HPA/ILs) (ii) [BSO3Hmim][Cl] immobilised it on microball silica gel

BAILs with acidic cation (i) [C3SO3Hmim][Cl] (ii) [BSmim][Br]

BAILs with two acid sites (i) [C3SO3HPPh3][HSO4] (ii) [C3SO3Hmim][H2PO4] (iii)[C3SO3Hmim][CH3SO3]

BAILs with monoacid anion (i) [Bmim][HSO4] (ii)[Emim][MeSO4]

BAILs with three or more acid sites (i) [C4(Mim)2][2HSO4] (ii) [C4(Mim)2][2CH3SO3]

Supported LAILs (i) ILs-H3[PW12O40] (ILs-POM salt) (ii) [Bmim][Cl]-FeCl4 onto mesoporous sieves

BLAILs (i) [HO3S-(CH2)3mim]Cl-ZnCl2 (ii) [Smim][FeCl4]

2 3

Figure 3. Main categories of AILs with examples given in each category.



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5. Biomass pretreatment with ILs Lignocellulosic biomass has been recognised as one and the only fixed carbon alternative

3

source to the first generation feedstock such as starch and edible triglyceride.73 The majority of

4

carbon content of lignocellulosic biomass are stored within the structures of cellulose and

5

hemicellulose which are enclosed by lignin, by strong covalent, and intra- and inter-molecular

6

hydrogen bonding.74 They form a network of fibres that provide mechanical tensile strength to the

7

plant cell wall that could hinder the digestibility of the structures and physically barrier the surface

8

accessibility.75-78

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Pretreatment is key to unlocking the recalcitrance of lignocellulosic biomass towards

10

various chemical processes in the production of platform chemicals. ILs acts as a dual solvent-

11

catalysts to dissolve the lignocellulosic matrix by breaking apart its matrix structure, thereby

12

enabling an easier extraction of cellulose components, as shown in Figure 4. An example of the

13

dissolution mechanism of cellulose by imidazolium based ILs is illustrated in Figure 5.

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15


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Figure 4. Schematic representation of pretreatment effect on lignocellulosic biomass. Reproduced with permission from Tadesse and Luque.79 Copyright 2011 Royal Society of Chemistry.

4

5 6 7 8 9

Figure 5. Proposed dissolution mechanisms of cellulose by imidazolium based ILs: (A) interactions between the basic anion and the cellulose hydroxyl groups break up the hydrogen bond network between the cellulose chains. (B) Hydrophobic interactions take place between the hydrophobic face of cellulose and the imidazolium ring. (C) The acidic proton at the C-2 position 17 ACS Paragon Plus Environment


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on the imidazolium ring interacts with the cellulose hydroxyl groups. Originally published in Wahlstrom and Suurnakki80 under CC-BY-NC-SA 3.0 licence.

3 4

The degree of dissolution in ILs is positively related to the hydrogen bond accepting ability

5

of the anion.81 The most prevalent ILs used in pretreatment processes are those of imidazolium

6

cation based attached with various anions, which fall into three groups: (1) halide based ILs such

7

as 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]), 1-ethyl-3-methylimidazolium chloride

8

([Emim][Cl]) and 1-octyl-3-methylimidazolium chloride ([Omim][Cl]), (2) acetate based ILs such

9

as 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) and 1-butyl-3-methylimidazolium acetate

10

([Bmim][OAc]), and (3) the large coordinating anion based ILs such as 1-butyl-3-

11

methylimidazolium tetrafluoroborate ([Bmim][BF4]) and 1-butyl-3-methylimidazolium

12

hexafluorophophate ([Bmim][PF6]).80,82 Among these, [Cl] anion imidazolium based ILs are an

13

excellent combination that exhibits excellent solubility for both biomass and cellulose, causing a

14

powerful breaking down of the intramolecular hydrogen bonds of the cellulose structure network

15

without further derivatisation.83 The non-derivatising nature of ILs produces no fermentation

16

inhibitors and thus the products are easily recovered.40 Additionally, their solubility is further

17

enhanced by the formation of hydrogen bonds between [Cl] anions of ILs and the hydroxyl groups

18

of sugars in biomass.84 For instance, [Bmim][Cl] gave an Avicel cellulose dissolution up to 25%

19

under microwave heating, whilst, 10% solubility was achieved at 100°C.84 Pyridinium based ILs,

20

3-methyl-N-butylpyridinium chloride ([BmPy][Cl]), were more efficient than those of

21

imidazolium based, in which up to 39% Avicel cellulose dissolution could be achieved.85 In

22

contrast, [BF4] and [PF6] anions are large and noncoordinating85 with weak hydrogen-bonding

23

basicity,38 leading to a poorer dissolving ability for biomass.79 Furthermore, smaller size anions of

24

the ILs favour the cellulose dissolution due to its flexible dipoles arrangement in combination with

25

cellulose.86 18 ACS Paragon Plus Environment

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1

In this context, ILs was recently extended to dissolve natural lignocelluloses and to

2

separate its constituents. Kilpelainen et al.87 reported that both hardwoods and softwoods such as

3

Norway spruce, Southern pine, and their thermomechanical pulp fibres were readily soluble in

4

various [Cl] anion imidazolium based ILs, which were functionalised by strong hydrogen bond

5

acceptors such as aromatic rings and allyl groups. The authors found that the solubility of biomass

6

was highly dependent on the particle size and moisture of the wood sample, in which the former

7

served as the dominant factor. To date, ILs pretreatment has been studied on various biomass such

8

as giant silver grass, pine and willow,74 oil palm such as empty fruit bunch and oil palm

9

fronds,68,88-91 switchgrass92-94 and sugarcane bagasse.95-98 They have been proven to be superior to

10

conventional methods, particularly in altering the physiochemical properties of the biomass

11

macromolecular components such as cellulose, to ease their extraction. Sugars released from ILs

12

pre-treated wood has also been discovered, particularly with dialkylimidazolium based ILs

13

containing [Cl], [MeCO2] and [MeSO4] anions.99 Brandt et al.74 reported that ILs with strong

14

hydrogen acceptance ability such as those of [MeSO4], [HSO4] and [CH3SO3] anions based, could

15

disrupt the intra and inter-molecular hydrogen bonds in the covalent structure of cellulose or

16

biomass, thereby enhancing biomass dissolution. In the biomass dissolution process, lignin and

17

hemicellulose were partially removed from the pretreatment solution. Hence, the lignin fraction

18

could be recovered and converted to value-added aromatic chemicals. Pretreatment with 1-butyl-3-

19

methylimidazolium alkylbenzene sulfonate ([Emim][ABS]), an ILs mixture containing aromatic

20

sulfonate anions, mainly xylenesulfonate,100 was able to dissolve lignin of sugarcane bagasse.

21

Lignin solubilisation has also been reported after pretreatment with acetate based ILs,

22

[Emim][OAc], when the solvent was a mixture of water and acetone.101 In addition, the cellulose

23

dissolving ILs ([Emim][OAc]) was used for the solubilisation of paper-grade kraft pulp into a

24

separated cellulose and hemicellulose fraction. The products were of high purity, and no yield loss 19 ACS Paragon Plus Environment


1

was observed as it might, resulting from depolymerisation.102 Overall, ILs enable the selective

2

separation of biomass fractions, i.e. cellulose, hemicellulose and lignin, with high yields.

3

Page 20 of 61

Furthermore, biodegradable ILs was recently developed for biomass pretreatment.103

4

Among these, amino acid based bio-ILs, 1-ethyl-3-methylimidazolium glycinate ([Emim][Gly]),

5

could effectively dissolve the biomass completely by converting the type I cellulose (native

6

cellulose) to type ΙΙ (crystalline and amorphous cellulose).104,105 Cholinium based ILs, could

7

effectively treat lignocellulose as observed from their high total reducing sugars yields in the

8

subsequent hydrolysis. Examples of cholinium based ILs used in biomass pretreatment include

9

cholinium argininate ([Ch][Arg]) in rice straw,106 choline acetate ([Ch][OAc]) in bamboo107 and

10

bagasse67 as well as cholinium taurate ([Ch][Tau]) in wheat straw.103 All these studies reported

11

significant improvement in the subsequent hydrolysis of biomass after the ILs pretreatment

12

presumably caused by the extensive removal of lignin. Additionally, renewable ILs which were

13

synthesised from biomass “wastes”, i.e. lignin monomers and hemicellulose are alternatives to

14

conventional ILs for the pretreatment process. For instance, Socha et al.108 synthesised a series of

15

tertiary amine-based ILs from aromatic aldehydes derived from lignin and hemicellulose. These

16

ILs were used to pretreat switchgrass biomass. High yields of total reducing sugars were generated,

17

which were comparable to those of conventional ILs, [Emim][OAc]. These concepts of using low

18

environmental impact bio-ILs and biomass-derived ILs highlight the significant potential for

19

future lignocellulosic biorefineries development.

20

However, biomass pretreatment has been recognised as the second largest cost contributor

21

in sugar-based biofuel production.109 This is primary caused by the ILs water-wash step in the

22

pretreatment process which generates large volumes of wastewater. To address this problem, Xu et

23

al.35 have recently developed a high gravity biomass conversion in which all catalytic conversions 20 ACS Paragon Plus Environment

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1

are included in one-pot production approach. The process allows ILs pretreatment and subsequent

2

chemical conversion to occur in one-pot for the production of concentrated cellulosic bioethanol.

3

The optimised one-pot configuration managed to reduce the need of ILs amount by ~90% and

4

pretreatment related water inputs and wastewater generation by ~85%. For multi-ton ILs

5

production, the use of protic ILs (ILs with a protonated amine for cation) would be an option, as

6

the synthesis method involved is solely a simple neutralisation that does not require any

7

purification step.110 Hence, protic ILs is of lower cost than traditional dialkylimidazolium based

8

ILs.109 For example, the bulk production of triethylammonium hydrogen sulfate ([HNEt3][HSO4])

9

is estimated as $1.24 kg−1. This compares favourably with organic solvents such as acetone or

10

ethyl acetate, which sell for $1.30–$1.40 kg−1.111 The cost of protic ILs is predominated by the

11

type of amine attached, since the price of sulphuric acid used is less costly and the synthesis

12

process of the ILs is relatively simpler. For instance, the synthesis step of protic ILs, e.g.

13

[HNEt3][HSO4], only involves 7 steps, whilst, acetate based ILs, e.g. [Emim][OAc], consists as

14

much as 30 steps.109 Overall, protic ILs appear to be a more eco-friendly ILs, alongside with a

15

reduction of unwanted waste by-products, solvent losses, energy usage and carbon dioxide

16

generation.

17

Recent studies showed the possibly of one-pot production of LA and LE from various

18

lignocellulosic biomasses via various catalytic methods, including wheat straw,8,112,113

19

bamboo,9,113,115 oil palm biomass such as empty fruit bunch, fronds and kenaf,59,68,77,116–118

20

sorghum grain,78 marine biomass,14,119 bagasse,120,121 rice husk,122 rice straw,123 switchgrass, wood

21

chips and paper pulp10 as well as paper waste.124 Nevertheless, the use of ILs for the one-pot

22

production of LA and LE from various lignocellulosic biomasses is still at the developing stage,

23

with limited research reported,7,114,115,117,118 which will be critically discussed in Section 6.3. As



Page 22 of 61

1

shown in Figure 6, the ILs-catalysed strategy allows the discarding of biomass pretreatment step,

2

thereby giving a shorter process compared to the conventional strategy. It reduces the time, cost

3

and energy consumption of the conversions, signifying a technological breakthrough in the organic

4

synthesis of LA and LE.


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1


Lignocellulosic biomass


2

Biomass pretreatment 3

with ILs

4

5

One-pot reaction with Cellulose regeneration

ILs, includes biomass

and ILs removal

pretreatment, hydrolysis,

6

Catalytic chemical 7

process with ILs, includes hydrolysis,

8

dehydration, rehydration and esterification

dehydration, rehydration and

9

esterification

10 11

12

Chemicals:

Chemicals: •

LA

•

LA

•

LE

•

LE

13

Conventional strategy

Recently developed one-pot strategy

14 15 16 17

Figure 6. Comparison of conventional strategy and recently developed one-pot strategy, for the production of LA and LE from biomass feedstock. Reproduced with permission from Xu et al.35 Copyright 2016 Royal Society of Chemistry.

18 19



1 2

Page 24 of 61

6. Conversions from biomass and its derivatives to LA and LE In general, the preparations of both LA and LE from biomass or its derivatives require the

3

presence of an acid catalyst. LE is typically synthesised using a conventional esterification method,

4

i.e. by reacting the biomass or its derivatives with alcohol under reflux condition. The feedstock

5

of biomass and its derivatives include sugars, cellulose and furfuryl alcohol. Monosaccharide,

6

disaccharide and polysaccharide present as building blocks of the complex carbohydrate network.

7

In the case of lignocellulosic materials, the recalcitrance and insolubility of the reactants in the

8

reaction medium add to the necessity of having depolymerisation step which consists of hydrolysis,

9

dehydration and rehydration, entailing a series of complex reactions and drastic reaction

10 11

conditions. The following discussion begins with the conversions from the simple structure of sugar,

12

followed by that of cellulose, and the last from the complex structure of biomass, for the both

13

productions of LA and LE. Then, the synthesis of LE from furfuryl alcohol and LA conversions to

14

LE was further discussed. Furfuryl alcohol is formed by the reduction of furfural, obtained from

15

the xylan hemicellulose component of lignocellulosic biomass (refer to Figure 2). The conversion

16

from furfuryl alcohol to LA and LE via pentose pathway is therefore an alternative method to

17

those conversions from cellulose-derived feedstock via hexose pathway. The mechanistic insights

18

on both pentose and hexose routes for the syntheses of LA and EL have been widely discussed in

19

Section 2. In the final process, LA can be upgraded to LE by undergoing a conventional

20

esterification with alcohol under reflux condition.

21


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1 2


6.1. Sugars to LA and LE Figure 7 shows how these compounds are oxidised into LA and LE. To date, there are

3

limited studies reported on the sugar conversion to LA and LE using ILs.125–128 As this conversion

4

is a simple process, cheaper options such as heterogeneous catalysts129–131 are generally preferable

5

over ILs. Since ILs are of higher cost, they cater more for complicated conversions such as those

6

from biomass feedstock. In addition, sugar is not renewable and sustainable, so it is less preferred

7

as compared to other feedstock. The utilisation of ILs for the conversion of sugars into LA125,126

8

and LE127,128 are shown in Table S1 of Supporting Information. Glucose and fructose are the two

9

common monosaccharides that have been used to produce LA and its esters.125–128 In addition, a

10

disaccharide of sucrose and a polysaccharide of inulin have also been investigated for the

11

conversions to LE.127,128



Page 26 of 61

Isomerisation

Glucose

Fructose Dehydration

1

Dehydration 2

3

Dehydration Sucrose

4

5-HMF 5

Dehydration

6

7

Rehydration 8

9

Inulin 10

11

Esterification 12

LA LE

13

14

Figure 7. Conversion of sugars to LA and LE.


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1


Among the monosaccharides, fructose gives significant LE yield, which makes it the most

2

desirable feedstock for the conversion. Hu et al.132 claimed that fructose consistently gave better

3

yields for LA and LE compared to glucose during acid-catalysed conversion of sugars.

4

Saravanamurugan et al.128 studied the reactivity of different carbohydrates (i.e. fructose, glucose

5

and sucrose) in a series of acidic anion based SFILs. Fructose gave the highest yields of 67-77% of

6

LE, whereas glucose gave limited yields of only 3-13%. Almost all glucose stopped at the alkyl

7

glucoside step in the reaction, thereby was unlikely to be isomerised into fructose. The acidity of

8

ILs thereby could not be fully unleashed for the successive dehydration and rehydration of the

9

substance into LA and further esterification into LE. Moreover, glucose reacted with alcohol to

10

form other by-products such as pyranoside esters. However, Ramli and Amin125 reported that

11

glucose conversion could achieve 67.8% LA yield via [Smim][FeCl4]. Both counterparts of the ILs

12

structures contain acidic properties (Bronsted acid and Lewis acid sites), and were thus able to

13

fulfil the acidity requirements of the successive conversion reactions. This work was in agreement

14

with the recent study conducted by Kumar et al.126 The synergistic effect of the Lewis acid NiCl2

15

and NiSO4 assisted in improving the LA yield by 3% and 9.7%, respectively, as compared to

16

solely Bronsted acidic ILs. These observations further indicated that solely Bronsted acidity was

17

not sufficient to achieve optimum efficiency for the conversion of glucose to LA. Among these

18

two Lewis acid metal salts, the sulfate salt co-ordinated with the -OH groups of the α-anomers of

19

glucose, forming stronger hydrogen bonds than the chloride salt. Hence, this result suggested that

20

the extent of hydrogen bonding ability of the Cl-atom was not high enough to alter the process to a

21

great extent. Moreover, the higher activity of the sulfates could also be attributed to reduced side

22

product formation.126 The synergistic catalytic effect of using mixed acids catalytic system will be

23

extensively discussed in Section 6.2 and 6.3.



1

Page 28 of 61

Apart from the use of glucose and fructose as feedstock, Chen et al.127 attempted to use

2

inulin as feedstock for the LE synthesis via IL-based polyoxometalate salts (IL-POMs), IL-

3

H3[PW12O40], and obtained a high yield of 67%.127 However, fructose was still superior to inulin

4

with the highest yield of 82% of LE production, which was in agreement with Hu et al.132 The

5

modified ILs (IL-POMs) is not considered as conventional ILs due to their melting points of over

6

100oC.133 The extended hydrogen bonding networks between cations and anions of the ILs could

7

reason for its existence in solid state. POMs is a heterogenous acidic catalyst that act as a support

8

for ILs. The coupling provides flexible adaptability of both inorganic and organic groups. The

9

functional propyl sulfonic acid group in the organic cation of IL-POMs also provides active acid

10

sites which favours the esterification reaction.133 Furthermore, tethering organic groups to POMs

11

enhanced heterogenisation of heteropolyacid (HPA)-promoted reactions, which favours catalyst

12

recycling.127 This catalytic system also simplify a multi-stage process into one-pot production by

13

omitting isolation and purification steps, in addition to providing promising production yields.127

14

To sum up, the ILs deserve further exploration in this application.

15

6.2. Cellulose to LA and LE

16

Table S2 of Supporting Information summarise the studies on the ILs catalysed one-pot

17

conversion of cellulose to LA56–58,117,134–136 and LE.56,136,137 Ren et al.57,58 conducted a study for the

18

cellulose conversion to LA via ILs composed of -SO3H functionalised group cation and acidic

19

anion. These ILs are made up of BAILs with multi-acid sites, of which both counterparts contains

20

acidic properties, or known as acidic anion based -SO3H functionalised ionic liquids (SFILs). The

21

introduction of -SO3H functionalised groups evidently enhanced the acidity of ILs, providing a

22

promising alternative to mineral acid catalyst (i.e. HCl, H2SO4, etc.). Hence, the reaction catalysed

23

by acidic anion based SFILs exhibited higher LA yield than acidic anion based ILs, as the former


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1

ILs have multi-acid sites in both cation and anion. Among the acidic anions tested, [HSO4] gave

2

the highest catalytic performance. [HSO4] anion has high viscosity property attributed to the strong

3

cation-anion interactions arising from the absence of alkyl chain in the structure.138 The strong

4

forces between cation and anion trigger the molecules to interact intensively and thus increase the

5

ILs capability in promoting a higher turnover synthesis. Moreover, the acidic anions could

6

possibly undergo glycosidic oxygen protonation during the catalytic cleavage of cellulose.80 With

7

reference to the relative acidity order of acidic anion based SFILs, i.e. [HSO4] > [CH3SO3] > [1-

8

NS] > [H2PO4], higher acidity of anions significantly favours the production of LA.79 This is in the

9

agreement with the conversion of cellulose to LE.137 Apart from that, the cation of the ILs also

10

contribute to its acidity strength although to a lesser extent. Specifically, the strength of Bronsted

11

acidity of an ILs is dependent on the nitrogen group and the carbon chain length of the cations.139

12

Increasing Bronsted acid strength would subsequently improve the ILs catalytic performance. In

13

the context of the cations of ILs, the LA yields decrease in the order of imidazolium > pyridinium >

14

phosphonium > ammonium.57,58 The elongating alkyl carbon chain linked with the -SO3H

15

functionalised group from methyl to butyl could also reduce the acidity strength of the ILs. In any

16

case, the acidity of an ILs is most dependent on its anions, rather than its cations.58 In terms of

17

methods applied, the hydrothermal method58 gave a higher LA yields than the microwave-assisted

18

synthesis method.57 Different operating conditions, particularly the reaction temperature and time

19

(refer to Section 7), also affect the catalytic performance of LA production.117

20

An exceptional case was observed on halide based SFILs, [C3SO3Hmim][Cl], in which it

21

gave the highest LA yield (66.2%)58 irrespective of its lower acidity relative to [HSO4] anion

22

based ILs. The formation of hydrogen bonding between [Cl] counterpart and hydroxyl protons of

23

the carbohydrates could have promoted the breaking of the extensive hydrogen bonding networks



Page 30 of 61

1

of cellulose and hence facilitated the cellulose dissolution.140 In this regards, stronger hydrogen

2

bond acceptor on ILs such as [Cl] may promote the catalytic performance of LA production by

3

increasing the accessibility of the acid sites rather than the usual acidity strength. However, [Cl]

4

based SFILs was unsuitable for the conversion into LE, as indicated by an extremely low yield of

5

1.1%.137 This could perhaps be linked to the inefficiency of [Cl] based SFILs in esterification

6

reaction137 and their role in only favouring the prior depolymerisation reaction.57,58,136 Furthermore,

7

ILs bearing highly coordinating anions such as [Cl], [BF4] and [PF6] might impede the catalytic

8

function of the cation of ILs on esterification, even in the presence of sulfonic acid functionalised

9

group. These ILs are hygroscopic, hence, in the production of LE, the anions of ILs tend to interact

10

with the water molecules in the water-ethanol medium to form hydrogen bonds.141 This strong

11

interaction not only stabilises the absorbed water molecules, but also leads to structural changes on

12

ILs,141 thereby possibly reducing the catalytic ability of ILs in the esterification. In contrast,

13

Amarasekara and Wiredu56 and Wiredu et al.136 achieved low LA yields despite using the same [Cl]

14

based SFILs ([C3SO3Hmim][Cl]). This was presumably attributed to the high loadings of cellulose

15

and ILs used in the catalytic systems that caused undesired cross polymerisation resulting in by-

16

products.

17

Apart from that, supported BAILs (heterogeneous catalyst coupled with BAILs) have also

18

been used to catalyse the conversions. Sun et al.134 conducted a cellulose conversion via

19

heteropolyacid (HPA) ILs in a water-methyl isobutyl ketone (MIBK) catalytic system and

20

obtained a significant LA yield of 63.1%. The high performance was mainly due to (1) HPA ILs

21

dissolve in water, forming a homogeneous catalytic system that favours the contact of the catalyst

22

and cellulose, (2) HPA molecule being sterically constrained, thereby having less interaction with

23

the anions of the ILs, but becomes more available for hydrogen bonding with cellulose and hence


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1

increases its solubility, and (3) HPA offering a Bronsted acid property which enhances the

2

catalytic performance. In general, HPA ILs exhibited higher catalytic activity than sole HPA

3

catalyst, as the former has strong Bronsted acidity with double acid sites in the cation. Furthermore,

4

the recycled catalyst performed efficiently of up to six recycling runs, without appreciable loss of

5

production yield.

6

Aside from that, Wiredu et al.136 utilised another supported BAILs (i.e. ZSM-5 coupled

7

with halide based SFILs ([C3SO3Hmim][Cl])) to catalyse the conversion and managed to improve

8

the LA (+6%) and LE (+13.2%) yields significantly. This enhancement of product yields might be

9

due to the co-existence of both Lewis and Bronsted acidic sites that leads to the synergistic

10

catalytic effect of the catalysts, and thus facilitates the isomerisation of glucose to fructose (refer to

11

Section 6.3) which is an essential step in the LA production. A proposed mechanism of this

12

catalytic conversion is as follows: (1) the breakdown to glucose from cellulose is completely

13

dissolved in [C3SO3Hmim][Cl] to form a homogeneous solution, (2) Cl in [C3SO3Hmim][Cl]

14

favours the Lewis acid sites of ZSM-5 to isomerise the glucose into fructose, (3) the ion-exchange

15

of [C3SO3Hmim][Cl] with Bronsted acid sites of ZSM-5 promotes the release of H+ , which could

16

readily contact fructose, and finally (4) [C3SO3Hmim]+ in [C3SO3Hmim][Cl] facilitates the

17

stabilisation of 5-HMF and LA, thereby preventing them from further decomposition into waste

18

by-products.142 In another study, Shen et al.135 utilised BLAILs (i.e. Lewis acid InCl3 coupled with

19

BAILs with multi-acid sites ([BSmim][HSO4])), to further enhance the LA yield (+3.3%). The

20

addition of Lewis acid InCl3 facilitated the hydrolysis of glucose, in which the metal ions-ILs

21

system favours the isomerisation of glucose to fructose. Specifially, the complexes

22

[InClm(SO4)n]2n- enhance the isomerisation of the α-anomers to the β-anomers through the



Page 32 of 61

1

formation of hydrogen bonding between oxygen atom in [SO4]2- or [Cl]- in metal chlorides and the

2

hydroxyl groups in glucose.143

3

6.3. Lignocellulosic biomass to LA and LE

4

Table S3 of Supporting Information summarise the pertinent references on the direct

5

conversions of biomass to LA7,114,115,117,118 and LE7 via ILs. The theoretical yield of LA/LE is

6

determined based on a given cellulose content of biomass, whereas the process efficiency refers to

7

the efficiency of biomass conversion to LA/LE based on the theoretical yield. ILs can act as dual

8

solvent-catalysts that enable all the catalytic conversions, i.e. biomass pretreatment,

9

depolymerisation and esterification, in one-pot.35 Nonetheless, due to the structural complexity and

10

recalcitrance of lignocellulose, the functionality of catalysts will be reduced somewhat.13 Thereby,

11

the production with a separate biomass pretreatment process in any case would still give a better

12

turnover synthesis of LA compared to without it.117

13

Among the available ILs, the AILs which possess both Lewis and Bronsted acidities

14

(BLAILs) have been reported to demonstrate promising results in polymerisation,

15

transesterification and oxidation reactions.144–146 With regard to this, a recent study introduced 1-

16

sulfonic acid-3-methylimidazolium tetrachloroferrate ([Smim][FeCl4]), a BLAILs, for the

17

conversion of oil palm frond to LA and LE.7 The findings showed that BLAILs in general

18

performed better than other types of AILs. The co-existence of both Bronsted and Lewis groups on

19

BLAILs presumably led to a stronger and more controllable acidity of LAILs which overcame the

20

weaker acidity of BAILs.70 This was in agreement with Antonetti et al.72 who reported that

21

although Bronsted acid sites played a greater role than Lewis acids in a catalytic reaction, solely

22

BAILs in the reaction proceeded very slow under the Bronsted acid conditions. In the conversion,

23

the biomass was first broken into glucose by the Bronsted acid sites ([Smim]), followed by the 32 ACS Paragon Plus Environment

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1

isomerisation into fructose by the Lewis acid sites (FeCl4). The fructose was then simultaneously

2

dehydrated to 5-HMF, and rehydrated again into LA by Bronsted acid sites ([Smim]).7

3

Nonetheless, excess Lewis acid sites (high Lewis/Bronsted ratio) can lead to the transformation of

4

glucose to undesirable waste by-products, the so-called humins.147 These dark, tarry to solid,

5

insoluble polymeric structures are favoured by the aqueous acidic media of most lignocellulosic

6

biomass transformations.148 Humin formation results in tedious separation of catalyst and

7

product.149 Hence, appropriate Lewis/Bronsted ratio of ILs is vital to achieve a full turnover of

8

organic synthesis.

9

However, Zhou et al.115 showed that LAILs (i.e. Lewis acid ZnCl2 coupled with

10

[Bmim][Cl]) exhibited slightly better catalytic performance than BLAILs (e.g. [Smim][FeCl4]7)

11

for the production of LA. This was presumably caused by the presence of Lewis acid ZnCl2 on the

12

ILs. Lewis acid metal salt is well known for the breakdown of cellulosic biomass,150 mainly due to

13

the chelation of metal ion on the glycosidic oxygen of the cellulosic biomass structure. Williams

14

and Horne151 revealed that Zn2+ is among the most effective metal chlorides in cellulose

15

degradation for biochar production through pyrolysis. This suggested that the coupling of Lewis

16

acid metal salt with neutral halide based ILs, especially Lewis acid ZnCl2, could presumably

17

improve its catalytic performance in the synthesis of LA.

18

Lewis acid metal salt also has an added advantage of being easy to separate from the

19

reaction products.152 Some types of Lewis acid metal salts (i.e. CrCl3 and FeCl3) have been

20

coupled with HY zeolite to make a new hybrid catalyst, which are then coupled with neutral halide

21

based ILs.117,118 With respect to this, lower LA yield was reported for the system catalysed by

22

Lewis acid FeCl3/HY zeolite coupled with [Bmim][Br]117 than Lewis acid CrCl3/HY zeolite

23

coupled with [Emim][Cl].118 However, the results might be affected by the discrepancies in 33 ACS Paragon Plus Environment


Page 34 of 61

1

reaction temperature and reaction time adopted (refer to Section 7). In any case, the purpose of

2

implementing a combination of catalysts (i.e. zeolite and metal salts) is to enhance the

3

performance of the organic synthesis. The use of solely zeolite or metal salt was proven to give

4

rise to low LA and LE yields,64,152–154 presumably due to the formation of intermediate compound,

5

5-HMF, which allows the omission of ring cleavage process to form linear-type LA and formic

6

acid.64 In addition, the low acidity and porosity of the catalysts could also partially account for the

7

poor catalytic performance on LA yield. In this regard, the hybrid catalysts which formed from the

8

combination of porous zeolite catalyst and acidic metal salt have shown to enhance the catalytic

9

performance of LA yield.117,118 Earlier researchers mainly used Lewis acid CrCl2, as a part or a

10

whole catalyst for biomass hydrolysis.64,155,156 However, CrCl2 would pollute the environment due

11

to it is toxicity. Current research has moved towards the use of another commercially available

12

metal salt, Lewis acid FeCl3, which is eco-friendly.54 The biomass conversion catalysed by the

13

common hybrid catalyst i.e. Fe-zeolite based, coupled with neutral halide based ILs such as

14

[Bmim][Br], exhibited a moderate LA yield.117 Impregnating the metal on a zeolite as a support

15

could possibly improve their separation from the reaction products.117 However, the eco-friendly

16

Lewis acid FeCl3/HY zeolite coupled with neutral halide based ILs117 gave a slight lower LA yield

17

than Lewis acid CrCl3/HY zeolite coupled with neutral halide based ILs.118 These works are in

18

agreement with Peng et al.152 whereby Lewis acid CrCl3 alone could catalyse efficiently to

19

produce 67 mol% of LA, which was better than Lewis acid FeCl3. In general, the efficiencies of

20

LA yield with various types of metal salts could be summarised as: ZnCl2 > CrCl3 > FeCl3. This is

21

in accordance with Adnan et al.157 who reported that among the LAILs tested (i.e. ZnCl2, FeCl3,

22

SnCl2 and CuCl2 coupled with choline chloride (ChCl) based ILs), ChCl-ZnCl2 exhibited the

23

highest activity in oleic acid conversion. Similarly, Sunitha et al.158 obtained a high product yield


Page 35 of 61 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

when the same LAILs (i.e. ChCl-ZnCl2) was applied for esterification of long chain carboxylic

2

acids.

3

Apart from that, Zhou et al.114 reported that BAILs with sole acid site, [Bmim][HSO4], was

4

less efficient with a moderate LA yield of 17.9%. This further suggested that Bronsted acidic site

5

(e.g. HSO4) alone did not fulfil the full turnover synthesis as compared to BLAILs7 and LAILs.115

6

This also confirmed that the absence of Lewis acid site on ILs may reduce the catalytic

7

isomerisation of glucose to fructose.152 Reduction in fructose formation reduces the overall

8

reaction process.

9

To sum up, Lewis acid sites on ILs are essential in the catalytic conversions of biomass to

10

LA and LE. They play the roles of (1) chelating and weakening the glycosidic bonds of

11

polysaccharides of biomass, (2) enhancing the hydrolysis of polysaccharides into monosaccharides,

12

and (3) isomerising glucose to fructose via enediol form.159 From a mechanistic aspect, couplings

13

of Lewis acid metal salt-ILs complex would promote the rapid conversion of α-anomer to β-

14

anomer of glucose through hydrogen bonding between hydroxyl groups of ILs.142 The β-anomer of

15

glucose is a cyclic aldose, which would revert to acyclic form by combining with the Lewis acid

16

complex to form an enolate structure. This allows the conversion of the aldoses to ketoses,

17

followed by isomerisation into fructose.142

18

By far, the most successful families of ILs for biomass dissolution are alkyl imidazolium

19

cations based, such as [Emim][Cl]118 and [Bmim][Br],117 owing to the unusually low melting

20

points of many imidazolium salts.86 The alkyl side chain of ILs does not directly affect the

21

biomass dissolution to a large extent, but it alters the dissolving ability of ILs towards it.160

22

Biomass solubility is decreased with the increase of the carbon chain length.161,162 As the carbon

23

chain length increases, the hydrophilicity of ILs decreases and the affinity between the ILs and 35 ACS Paragon Plus Environment


Page 36 of 61

1

biomass is weakened. Moreover, the larger size of the ILs leads to higher melting temperature, as

2

the van der Waals interactions between the alkyl side chains start outweighing the symmetry

3

effect.163 As the results, smaller alkyl groups (e.g. [Emim][Cl]118) performed better than larger

4

alkyl group (e.g. [Bmim][Br]117) in LA production.

5

6.4. Furfuryl alcohol to LE

6

The production of LA from furfuryl alcohol conversion via ILs is still absence, and only

7

two studies have reported this for LE production,149,164 as shown in Table S4 of Supporting

8

Information. In accordance with the conversions from cellulose57,58 and sugars,128 LE yield also

9

increases with the acidity of the anions of ILs in furfuryl alcohol conversion. The efficiencies of

10

ILs for LE production are in accordance with the increase of the acidity strength of their anions, as

11

follows: [CF3COO] < [PTSA] < [ClSO3H] < [HSO4]. ILs with weak acidity anion, [CF3COO],

12

performed the poorest in LE production, whereas, ILs with strong acidity anion, [HSO4],

13

performed the best, in agreement with other feedstock conversions.57,58,128,137 Apart from that,

14

[MIM] cations based ILs performed poorer than those of [NMP] cations. The mono-alkyl CH3

15

structure of [NMP] cation highly promotes the formation of LE intermediates.165 [NMP] cation

16

possesses superior catalytic activity, is easier to synthesise and purify and is more economical

17

compared to pyrrolidine and imidazolium cations based ILs.166 In addition, extending the cation

18

alkyl chain with 1,4-butane sultone increases the stability and enhances the Bronsted acidity of ILs,

19

especially for [HSO4] anion based SFILs, [Bmim-SH][HSO4], in which a high LE yield has been

20

observed (95%).149 [Bmim-SH][HSO4] offers the advantage of higher recyclability than

21

[NMP][HSO4], in which the efficiency of the latter fell to 50% upon reuse although it gave slightly

22

higher LE yield.149 In fact, an increase in both reaction temperature and catalyst concentration

23

improved the furfuryl alcohol conversion and selectivity towards LE.167 In contrast, as the 36 ACS Paragon Plus Environment

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1

substrate concentration increases from 5-15%, the conversion and selectivity towards LE was

2

greatly reduced which could be due to the accumulation of ether intermediates of furfuryl

3

alcohol.149

4

Wang et al.164 utilised multi-alkyl -SO3H functionalised groups on the cation of ILs to

5

promote their Bronsted acidity for higher catalytic activity in LE production. In this catalytic

6

activity, intermediates products (i.e. 2-alkoxymethylfuran and 4,5,5-trialkoxypentan-2-one) were

7

produced from furfuryl alcohol conversion and thus led to a high LE yield of 95%. An advantage

8

of this catalytic system was that undesired diethyl ether (DEE) formed by the side reaction of

9

intermolecular dehydration with ethanol. The Hammett method acidity test of the ILs indicated

10

that the acidity and the molecular structure have strong effects on the catalytic activity of ILs in

11

LE production. Briefly, the acidity of ILs acids decreases in the order: [(HSO3-p)2im][HSO4] >

12

[BsMim]- [HSO4] ≈ [BsTmG][HSO4] ≈ [BsPy][HSO4] > [BMim][HSO4] > [BsTmG][CF3COO],

13

and the yield of LE follows the same trend.

14

6.5 LA to LE

15

In contrast to numerous studies utilising mineral acid and heterogeneous catalysts,168–172

16

there is currently no reported literature on the utilisation of ILs for the conversion of LA to LE.

17

Based on the literature,166,173–175 AILs could be a potential catalyst as they provide sufficient acidic

18

catalytic power for esterification reaction. In the early stage, LAILs (i.e. coupling of aluminium

19

(III) chloride and 1-butylpyridinium chloride) have been tested in the esterification of carboxylic

20

acid,173 but the direction of AILs rapidly moved towards the more manageable BAILs with multi-

21

acid sites (i.e. [HSO4] anion based SFILs).176 Cole et al.65 first synthesised a series of [HSO4]

22

anion based SFILs for the esterification of acetic acid with ethanol. Furthermore, Gui et al.177 have

23

further illustrated the merits of using [HSO4] anion based SFILs for esterification as an efficient 37 ACS Paragon Plus Environment


Page 38 of 61

1

and recyclable catalytic system. Despite the excellent conversion and selectivity, the results were

2

not entirely satisfactory. [HSO4] anion based SFILs are costly and require complex preparation.

3

High concentration (i.e. 100-300% (w/w) carboxylic acids) is necessary in order to achieve a

4

considerable conversion.60,178 Furthermore, the ILs being similar in nature to H2SO4 in terms of

5

extreme acidity, can cause serious equipment corrosion.60, 179,180

6

To develop new classes of “greener” ILs with simple preparation procedures, BAILs with

7

sole acid site such as the noncorrosive and environmentally benign [HSO4] anion based ILs could

8

be an option.181,182 Briefly, the Bronsted acidic salts are prepared by addition of an equimolar

9

amount of strong inorganic acid (i.e. HCl, H2SO4, HNO3, CF3COOH and H3PO4) to a commercial

10

N-base, making up the Bronsted acidic salts, for instance, imidazolium, pyrrolidine, piperidine,

11

morpholine and betaine.176 However, sole acid site BAILs with anion other than [HSO4] resulted

12

in significantly lower ester conversions.7,57,58,128,149,164,176 This suggested that sufficient acidic

13

protons on the anionic component (i.e. [HSO4]) of ILs is essential in the activation of the reaction,

14

since strong acidity would promote the reaction efficiently. With regard to this, some of the [HSO4]

15

anion based ILs have been applied in common organic synthesis reaction, for instance, 1-

16

methylimidazolium hydrogen sulfate ([Hmim][HSO4]), 1-ethyl-3-butylimidazolium hydrogen

17

sulfate ([Emim][HSO4]), 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO4]),

18

pyridinium hydrogen sulfae ([Hpy][HSO4]), 2-methylpyridine hydrogen sulfate ([Hmpy][HSO4]),

19

triethylammonium hydrogen sulfate ([Et3NH][HSO4]), triethylammonium dihydrogen phosphate

20

([Et3NH][H2PO4]) and N-methylpyrrolidinium hydrogen sulfate ([HPyrr][HSO4]) have been

21

proven to catalyse the esterification of carboxylic acid reactions,60,175,176,183 through greener and

22

sustainable chemical processes. In particular, certain BAILs with sole acid site have been tested in

23

the esterification of acetic acid with butanol, octanol and methyl β-D-glucopyranoside,176 and thus


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1

these [HSO4] anion based ILs are presumed to be feasible in the esterification reaction of LA to

2

LE. In any case, [HSO4] anion based ILs have been reported as feasible ILs in the production of

3

LA and LE from biomass114 and its derivatives,125,149 which have been discussed (refer to Section

4

6.1-6.4). They also offer the advantages of having low environmental impacts and being nearly

5

noncorrosive and recyclable.60 Moreover, the characteristic [HSO4] acidic anion allows one end of

6

the ILs to function as dual solvent-catalysts and another end to be immiscible with esters, forming

7

a biphasic solution even at room temperature (Figure 8).175,183 This allows the esters to be

8

constituted at the upper layer, while the ILs residues and substrate are retained at the lower layer,

9

favouring the shift of the reaction towards products, and also the esters recovery.70,176 The removal

10

of water during the reactions is unnecessary as it is miscible with ILs residues.177 Hence, a solvent-

11

free separation is applicable in this case. Moreover, this biphasic system is also applicable for the

12

conversions of biomass181,182 and its derivatives (sugars, cellulose)56,125,184 to both LA and LE via

13

one-pot production. 14 Reactants (alcohols and carboxylic acids)

15 Reactants

16 17

18

BAILs

Esters

Reactants Reactants Water

BAILs BAILs

19 20 21 22

Figure 8. Esterification of alcohols and carboxylic acid catalysed by BAILs. Reproduced with permission from Yue et al.70 Copyright 2011 Elsevier.

23



1 2

Page 40 of 61

7. Overview of the current ILs catalytic status and associated factors Overall, LA yield, followed by its ester yield in the catalytic conversions can be improved

3

by using ILs with (1) high acidity strength, (2) strong molecular interactions between cation and

4

anion, (3) heterogeneous or hybrid catalyst supports, (4) the addition of Lewis acid co-catalysts, or

5

(5) their combinations. In general, BAILs with multi-acid sites (i.e. acidic anion based SFILs) and

6

BLAILs catalyse the conversions to both LA and LE more effectively as compared to the BAILs

7

with sole acid site (i.e. acidic anion based ILs). However, BAILs with multi-acid sites could be too

8

corrosive for the reactor.60,179 Hence, BAILs with sole acid site, particularly [HSO4] anion based

9

ILs, offer a better option for an environmentally benign one-pot reaction conversion. Contrastingly,

10

providing support to LAILs/ BAILs or mixed acid catalytic system (BLAILs) could be an

11

alternative technique to enhance the LA and LE yield, since the pore structure of the support could

12

exert significant influence on the reaction.185,186

13

In terms of feedstock, sugars (i.e. fructose) and furfuryl alcohol gave highest production

14

yield at relatively mild temperatures (i.e. 110-150oC). However, lignocellulosic biomass is highly

15

recommended for commercial-scale production from sustainability and economical viewpoints.

16

Direct conversion from lignocellulosic biomass gave relatively lesser yields due to the

17

recalcitrance of the biomass substrate. To address this problem, suitable ILs hold a promising

18

dissolution ability that enable both biomass pretreatment and catalytic chemical conversion into

19

LA and EL to occur in a one-pot system187 (refer to Section 5). Although multi-stage processes

20

can be more complex, continuous processes in one-pot reaction with optimised operating

21

conditions can be very effective in minimising labour, maintenance and energy costs associated

22

with comparable batch operations.


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1

The chemical conversion processes via ILs are versatile because of the wide selection of

2

reaction conditions and range of ILs catalysts. ILs with long chain alkyl possess higher viscosity

3

and thus requires higher reaction temperature and vice-versa.187 High temperature reduces

4

viscosity by destabilising the H-bonding network between ILs, thereby make ILs molecules more

5

readily to solvate and dissolve the cellulosic molecules of biomass.30 Zakrzewska et al.188 reported

6

that a 20-30oC increase in temperature in dehydration of carbohydrates with ILs would double the

7

intermediate yield (i.e. 5-HMF), which subsequently enhanced the LA and LE yield. However,

8

elevated temperature produces side reactions, and give rise to undesired by-products. Conversely,

9

low temperature lengthens the reaction time and reduces the catalyst activity considerably.6 The

10

contact times of the starting material and its surrounding environment (solvent, catalyst) would

11

also alter the product distribution profile.6 Additionally, high loadings of substrate (i.e. biomass or

12

derivatives) would increase the probability of the reactive compounds colliding with each other

13

and cause cross polymerisation via self-condensation of furans in the substrate which lead to

14

undesired products formation such as polyketones and humins,117 thereby reducing the conversions

15

into LA and LE.6 Apart from that, water acts as a base in the synthesis process which reduces the

16

acidity strength as a catalyst in esterification, but it favours depolymerisation (i.e. hydrolysis,

17

dehydration and rehydration) reaction for the formation of LA.188 Precisely, when the β-1,4-

18

glycosidic bonds of the dissolved cellulose are first attacked by ILs, the hydrolysis process is then

19

initiated.188 Hence, insufficient water in the catalytic system impedes proper integration of

20

cellulose in the reaction,56 whilst, excessive water can precipitate the dissolved cellulose from the

21

ILs which make the homogenous hydrolysis of cellulose in ILs nearly impossible.117 In any case,

22

water favours the biphasic reactions as it is miscible with ILs at the aqueous phase (bottom layer),

23

separated with the organic phase (upper layer)189 (refer to Section 6.5: Figure 8).



1 2

Page 42 of 61

8. Challenges To date, the conversion of biomass to LA and LE via ILs have not proceeded beyond

3

bench-scale. The cost of ILs is the most pronounced challenge and has been highly recognised as

4

the major process cost driver. Seeking cheaper raw materials for ILs synthesis is vital for long-

5

term production. For instance, inexpensive feedstock such as sulphuric acid, simple amines and

6

imidazolium were combined into a range of [HSO4] anion based ILs.109,111 In addition, for a

7

feasible operation, the ILs recovery for the recycling purposes has to be as efficient as possible.

8

Nonetheless, the development of effective methods for the recovery of ILs remains another main

9

hurdle for the large scale process production. Specifically, the ILs recovery from the biomass

10

hydrolysis step is complicated because the sugars produced have high affinity towards ILs.

11

Additionally, both components of sugars and ILs have low volatility, hence, they are difficult to be

12

separated.190 Liquid-liquid extraction and distillation have been proposed for product recovery.191

13

Since most of the by-products such as intermediate furanic compounds (i.e. furfural, 2-

14

furancarboxyaldehyde and 5-HMF), fatty acid methyl esters (FAME) and phenolic compounds

15

have close boiling points, distillation would not be an appropriate technique in extracting the target

16

compounds (i.e. LA and LE).9,192 This problem can be tackled by using biphasic catalytic systems

17

(refer to Section 6.5: Figure 8). Nonetheless, some LA and LE still remain in the ILs phase after

18

reaction and further separation and purification processes might still be a necessary task for total

19

separation. Conversely, liquid-liquid extraction which is based on the concept of immiscibility of

20

ILs with traditional solvents has facilitated the purification and extraction of the desired organic

21

products. This could be the most appropriate technique up-to-date.193 Hu et al.192 investigated the

22

extraction of both LA and LE from reaction mixture with twelve different solvents. Ethyl acetate

23

gave the best extraction efficiency, followed by 2-octanol, 1-butanol, ethyl butyrate, toluene,

24

methyl acetate and ethyl ether. Some studies used methyl isobutyl ketone (MIBK) to extract LA2, 42 ACS Paragon Plus Environment

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1

57,58,194

and hexane to extract LE.57 However, ethyl acetate is the most commonly used solvent in

2

extracting these two compounds (i.e. LA and LE).56,125,136,184,195 Aside from that, recyclability of

3

ILs is an important factor contributing to the cost of production. Most of the studies suggested that

4

the ILs are capable of recycling up to a maximum of seven consecutive runs, signifying its

5

potential recycling ability.60,134,137,177,183,188,196 The stability of the ILs can be examined by 1H-

6

NMR spectra analysis.56 However, although ILs is recyclable, the purification of recycled ILs

7

remains problematic. In this regard, the ILs chemostability, thermostability, catalytic performance

8

and recovery are still in the assessing stage for long term chemical industrial processing conditions,

9

particularly the challenges faced by the presence of the contaminants derived from biomass

10

processing are critical to be resolved. In any case, the information on the reusability of the ILs is

11

still scarce.197 Further research is also necessary to develop efficient processes that are able to

12

manage the increase of substrate loading in order to improve the concentrations of the target

13

products.

14

Finally, there is a growing concern over the environmental impacts of ILs, although ILs has

15

been tagged as “green reaction media” in the catalytic processes. The ideal ILs are cited as

16

nontoxic, biodegradable and recyclable. However, some [Cl] based ILs are toxic, corrosive and

17

very hygroscopic,198 although they have been reported to favour the conversion of biomass to LA

18

and LE (refer to Section 6). El-Harbawi199 reported the estimated LC50 toward aquatic organisms

19

for some immidazolium based ILs ([BMIM][HSO4], [BMIM][TFSI] and [HeMIM][NTf2]) were

20

199.98 mgL-1-374.11 mgL-1, which were practically non-toxic. Wu et al.200 claimed that the length

21

of alkyl side chain has a more substantial influence than the methyl group in imidazolium ring on

22

the toxicity of ILs. Longer alkyl chains impart higher toxicity. A number of non-toxic and

23

generally recognised as safe (GRAS) anions with acidic nature are potential candidates for



Page 44 of 61

1

exploration in the production of LA and LE.201 ILs with [SO4]2- anion are highly suggested as an

2

alternative environmental benign ILs with the benefit of an acidic nature,202 which favours the

3

catalytic conversion of biomass to LA and LE (refer to Section 6). Moreover, the [SO4]2- anion

4

based ILs with combination of alkyl group are stable under mild condition, have no toxic by-

5

product formation and form a biphasic layer with the organic layer.203 Therefore, the alkyl sulphate

6

group such as [HSO4], [MeSO4] and [EtSO4] are newly developed ILs with the advantage of an

7

acidic nature. In between, [HSO4] anion based ILs have been studied for the conversion of biomass

8

and derivatives, i.e. cellulose, sugars and furfuryl alcohol (refer to Section 6).

9

9. Conclusion

10

LA and LE are promising biorefinery chemicals which can be synthesised from biomass

11

and its derivatives via ILs with minimal environmental footprint. The development of more

12

selective ILs remains a key technical barrier to improve the production efficiency and reduce by-

13

products. Although ILs are traditionally recognised as “green reaction media”, the synthesis of

14

more environmentally benign ILs using sustainable processes and simpler synthesis routes is

15

highly required. To achieve process feasibility in terms of cost, the ILs catalysed conversions

16

studies should focus on using inedible biomass or biomass waste as feedstock, recyclability of ILs

17

catalysts, deployment of one-pot reaction approach and optimisation of production conditions.

18

Nonetheless, the entailed knowledge in these areas for the production of LA and LE is scarce and

19

is at the developing stage. Hence, this provides avenues for further exploration, which are

20

projected to expand considerably in the coming years. To conclude, biomass conversions into LA

21

and LE via ILs have the potential to be developed as green and sustainable routes to generating

22

future bio-based fuels and chemicals.


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1

Associated contents

2

Supporting information:

3

Table S1-S3 compile the literature works on the conversions of sugars, cellulose and various

4

biomass feedstock to LA and LE, respectively, using ILs at various reaction conditions. Table S4

5

compiles the literature works on the conversion of furfuryl alcohol to LE using ILs at various

6

reaction conditions.

7

Author information

8

Corresponding Author

9

* Email: [email protected]

10

Tel: +6 (03) 8924 8162; Fax: +6 (03) 8924 8001

11

Notes

12

The authors declare no competing financial interest.

13

Acknowledgements

14

This work was financially supported by Ministry of Higher Education (MOHE), Malaysia, under

15

the Fundamental Research Grant Scheme (FRGS/2/2014/SG01/UNIM/02/1). The University of

16

Nottingham Malaysia Campus is acknowledged for its support towards this project.



Page 46 of 61

1

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1 2


Table of Contents/Abstract Graphics Lignin

3

Cellulose

4 5

Hemicellulose

6 7 8


Bio-based chemicals

9 10

Levulinic acid Biofuels

11 12 13

Fuel additives

Carbohydrates 14 15

Levulinate esters

Furfuryl alcohol


61

Conversion of Biomass and Its Derivatives to Levulinic Acid and

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