Task-specific ionic liquid for the depolymerisation of starch-based industrial waste into high reducing sugars

Development of a simple route for the catalytic conversion of starch-based industrial waste (potato peels) and potato starch into reducing sugars was investigated in two ionic liquids for comparison – 1-allyl-3methylimidazolium chloride [AMIM]Cl and 1-(4-sulfobutyl)-3-methylimidazolium chloride [SBMIM]Cl. Over a two hour period, a 20 wt % solution containing up to 43% and 98% of reducing sugars at low temperature in aqueous [SBMIM]Cl were achieved for the starch-based waste and the potato starch, respectively. In addition, the use of microwave and low frequency ultrasound to perform the depolymerisation of the raw starch-based material was explored and compared with conventional heating processes.


Introduction
A growing concern in environmental sustainability in our society has become an important aspect for both ecosystem health and economic development. The intensive consumption of fossil fuels that will eventually run out renders renewable resources as an attractive proposition. Some by-products can be considered as sustainable energy for the synthesis of chemicals [1]. Currently, a Finnish company, which produces pre-cooked vacuum potatoes, generates several tons of waste from potato peels daily. In our previous study [2], a weight percentage of sugars was performed on by a total hydrolysis of the by-product, which is mainly composed of glucose (80.2%), mannose (4.9%) and galactose (3.2%). More than 88% can be subsequently considered as the total sugar potential. This by-product is mainly composed of starch, the principal constituent of potatoes. Starch is basely composed of two macromolecules, amylose and amylopectin, trapped into granules. Its depolymerisation into reducing sugars is mainly performed under concentrated strong acidic conditions and/or high temperature, for long reaction time [3], [4]. However, starch molecules are not prone to accept water dissolution, notably due to the strong intra and intermolecular hydrogen bonds. These latters can be generally broken down under high temperature, shear and acidic conditions, yielding both free macromolecules [5]. The depolymerisation process in a water medium is therefore of a heterogeneous nature and suffers some inevitable limitations (existence of diffusion layers, limitation of the mass transfer, lack of efficient mixing, etc.) whereas homogeneous media will certainly bring a higher reactivity [6], [7]. One possibility for the dissolution of starch is to use ionic liquids [8].
Known as salt with a melting point below 100 °C, ionic liquids possess attracted properties as new generation of solvents, negligible vapour pressure, wide liquid ranges (up to 400 °C) and the ability to dissolve carbohydrate [9]. Dissolution of carbohydrates up to 20-wt % in ionic liquids has been reported previously [10]. In 2006, Remsing et al. investigated the solvation of cellulose in an imidazolium-based ionic liquid bearing a chloride counter-anion [11]. Due to their high nucleophilic capacity, chloride ions are enabled to interact with the hydroxyl protons of carbohydrates and to break down the hydrogen-bonding network to promote dissolution. In our experiments, the first selected ionic liquid was 1-allyl-3-methylimidazolium chloride [AMIM]Cl, which has an excellent ability to dissolve carbohydrates [12] and depolymerise them in the presence of solid catalysts [13] or acid [14]. Brønsted acidic ionic liquids (BAILs) possess simultaneously a proton acidity with the Brønsted function and properties of ionic liquids -non-volatile, recyclable [7], [15], [16]. A wide range of moieties can be classified in the Brønsted framework: mineral acids, sulfonates, phosphonates, and carboxylic acids. Johnson et al. [17] published a detailed review about fundamentals of BAILs and their use in various organic reactions with different location of the Brønsted acid function (anion or/and cation). The strength of the acidity depends on the position of the acidic function; -COOH or -SO 3 H function on cation possess strong intrinsic acidity [17]. SO 3 Hfunctionalised ionic liquids are strong Brønsted acids [6], [15], [18] and possess great potential as dual catalyst/solvent system and non-volatile acidic materials [19]. 1-(4-sulfobutyl)-3-methylimidazolium chloride [SBMIM]Cl possesses Brønsted-acidic sulfonic group on the cation to play the role of both solvent and catalyst.
The chloride anion was preserved to enable the primary target, i.e. the solubilisation of the solute [20].
Both ionic liquids (see Fig. 1 for structures and abbreviations) are already well known in literature as they have been previously employed mainly for the dissolution and hydrolysis of cellulose into reducing sugars [7], [21], [22], [ [26].    Table 1 and spectra in Fig. 2 and second generations imidazolium-based ionic liquids possess a weak acidity often tied to the nature of counter-anion, making it reasonable to reach a low 6% of depolymerisation [39], [40].

Effect of temperature on the depolymerisation of potato starch
Temperature also plays an important role in the efficiency of depolymerisation of starch. In order to compare the results with our previous study performed in an aqueous acidic medium [2], the depolymerisation of the three starch-based starting materials was performed in an ionic liquid medium ranging between 60-90 °C.
Temperature has an effect on the viscosity of the ionic liquids by decreasing it [44]. The use of an ionic liquid allows work to be conducted at higher operating temperatures than those used in aqueous sulphuric acid. Indeed, in the latter, the starch easily undergoes gelatinisation at around 65 °C, making any further transformation difficult. Reactions were performed in 20 % (w/w) of water on a 10 wt % solution of all three starch-based materials in [SBMIM]Cl. Temperature effect on the depolymerisation is shown in Fig. 3. It has been previously shown that [SBMIM]Cl possess a higher ability to dissolve cellulose than neutral ionic liquids at 100 °C [21].
Whatever the nature and composition of the starch-based material, the highest TRS yield was obtained at 80 °C.  Microwave and ultrasound irradiations may enhance the hydrolysis of carbohydrates into sugars due to their own specific effects. With microwave irradiation, a reaction media is heated from the inner to the outer layer and can reduce the reaction time from hours to minutes [2]. Low frequency ultrasound irradiation generates shock waves, which allow an efficient stirring of the reaction medium and increase the total reducing sugar content [2]. were reached. An ultrasonic bath may not be powerful enough to allow the mixing of a highly heterogeneous and viscous system that would require the use of an ultrasonic probe, directly immersed in the solution for a direct irradiative mode. Our previous research performed using a sulphuric acid medium provided similar results [2].
The depolymerisation under microwave irradiation offered the highest TRS content within 60 min regardless of the starting material. Due to their strong polar character, ionic liquids are a very suitable medium for microwave irradiation. This is confirmed by the fact that for potato starch, a temperature of 60 °C was high enough to generate engaging amounts of reducing sugars. However, the brown aspect of the solution after microwave irradiation of the two other starting materials could be explained by the caramelisation reaction. Caramelisation of short-chain or monomeric sugars is known as the Maillard reaction. This was also observed by Lajunen et al. [45] for the depolymerisation of barley starch in imidazolium-based ionic liquids under microwave irradiation.
An appropriate Plexiglas helix-ended rod was introduced into the microwave reactor to limit the effect of thermal gradient and local hot spots, but this remained inefficient and could not attenuate caramelisation.
However, the combination of rapid heating in an ionic liquid medium increased the yield of reducing sugars whilst reducing the reaction time. A set temperature can be reached in a really short time through consecutive rotation of the ionic molecules. This renders the combination of microwave heating/ionic liquid as very attractive. For all raw materials, the total reducing sugars reached 3 to 10 fold under microwave irradiation than with conventional heating in similar conditions. Microwave technology has previously been employed for the conversion of cellulose into reducing sugars or 5-HMF in ionic liquids [46], [47], [48], [49] or for the production of furfural from sugars with Brønsted-acidic ionic liquids [50]; no reports exist for starch in ionic liquids conditions. The conversion of cellulose into reducing sugars reached 48% in only 8 min of irradiation with a HY zeolite catalyst at 180 °C [46]. In this study, 61 % of potato starch was converted into reducing sugars under microwave irradiation with the Brønsted-acidic ionic liquid, and only 4 % using conventional heating. therefore water is required for the hydrolysis. The results confirm that water can improve the hydrolysis reaction of starch into sugars in a Brønsted-acidic ionic liquid, which corroborates with previous reports [53]. The authors suggested that aqueous Brønsted-acidic ionic liquids promoted the attack of the glycosidic bonds of cellulose for its conversion into α-glucose. A total hydrolysis of potato starch was achieved probably because α-glycosidic bonds are easily cleaved compare to β-glycosidic bonds (cellulose). The aqueous ionic liquid was able to dissolve potato starch, whilst the key to the hydrolysis of the Brønsted-acidic function is in the form of a superacid and may be considered as a simple hydrolysis.

Conclusion
In this study, we optimised the parameters to dissolve and depolymerise a starch-based industrial waste in ionic liquids.
[AMIM]Cl appeared to be more suitable for the dissolution of potato starch due to the imidazolium ring disrupts the dissolution process of carbohydrate in ionic liquids, but the method described herein generated the greatest yield of reducing sugars. The addition of water overcame the high viscosity of a 20 wt % solution.
Finally, microwaves only appear to reduce the reaction time by reaching the required temperature in a short time period.