Dissolution and hydrolysis of fibre sludge using hydroxyalk y limidazolium 1 hydrogensulphate ionic liquids

11 The dissolution and hydrolysis of wet fibre sludge in ionic liquids (ILs) with different reaction 12 conditions are performed in this study. Novel types of hydroxyalkylimidazolium hydrogensulphate 13 ILs, [glymim]HSO 4 , [hemim]HSO 4 and [hpmim]HSO 4 , are especially designed and aimed to 14 combine the dissolution and hydrolysis of wet fibre sludge in a one-step pretreatment. The results 15 were obtained based on the analysis of total reducing sugars (TRS) with the DNS method. The 16 dissolution and hydrolysis of fresh wild horse chestnut seed (Aesculus hippocastanum) with the 17 same ILs were also carried out as a comparison reference. Since fibre sludge is cellulose based and 18 horse chestnut seed is starch based, a direct comparison of the difference in functions between these 19 ILs in one-step dissolution and hydrolysis were analysed based on the results. 20

reduce resource usage and recover the waste, in order to promote sustainability. A significant 28 amount of research, therefore, has been conducted to convert solid residual materials towards other 29 useful materials [1] [2]. As a result, there is a growing tendency in the Finnish pulp and paper 30 industry to look for options for the reuse or recycling of solid residuals. 31 Fibre sludge is a by-product from a pulp mill in which the residuals from the chemical pulping 32 process in Finland totals approximately 300,000 tonnes as dry material annually [3] [4]. Depending 33 on the pulp mill, fibre sludge is either combusted for energy production and/or disposed at landfills. 34 However, fibre sludge is rich in cellulose and it is possible to be used as a raw material in the 35 production of more valuable products, such as biofuel [3]. 36 Ionic liquids (ILs) have been viewed as remarkable environmental friendly solvents, compared to 37 other volatile organic compounds for biomass pretreatment due to their broad liquid region, high 38 thermal stability, negligible vapour pressure and that no formed toxic or explosive gases are 39 released during utilisation [5] [6] [7]. 40 41 Most ILs consist of organic cation and inorganic anion. By varying the nature of the anion or cation 42 present in the IL, the resulting physical and chemical properties of the IL such as melting point, 43 viscosity, hydrophobicity and hydrolysis stability can be directly affected [8].This means, ILs can 44 be tailored for a specific application. Due to this enormous potential, ILs have been put forward to 45 be used in various fields. For example, ILs can be used in biocatalysis, batteries, waste recycling 46 and cellulose processing [9]. 47 48 In our previous study [3], a task-specific ionic liquid 1-(4-sulfobutyl)-3-methylimidazolium 49 chloride ([SBMIM]Cl) with a Brønsted -acid function has been used in a one-step dissolution and 50 hydrolysis of fibre sludge [3]. The study proved that it is possible to combine both dissolution and 51 hydrolysis in the pretreatment of lignocellulosic materials, but the levels of total reducing sugars 52 were not high enough. Therefore, a one-step pretreatment of fibre sludge with three novel types of 53 hydroxyalkylimidazolium hydrogensulphate ILs is of particular interest in this study. The 54 commonly used ILs in pretreatment of lignocellulosic materials often contain chloride anions and 55 since chloride has so many negative impacts on human health and the environment, the use of other 56 anions that are also effective in pretreatment should be considered. 57

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The aim of this study is to combine the dissolution and acidic hydrolysis stages into one single fibre 59 sludge pretreatment step using hydroxyalkylimidazolium hydrogensulphate ILs. At the end of 60 pretreatment, a certain amount of fermentable sugars is expected to be produced. 61

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In addition to fibre sludge, horse chestnut seeds (Aesculus hippocastanum, collected from 63 Germany) are also pretreated with the same method in this study. Since horse chestnut seed is a 64 starch based biomass compared to fibre sludge which is cellulose based, by the end of this study, 65 obtained results of these two types of feedstock in pretreatment can be directly compared. Fibre sludge, also known as a primary sludge, was provided by a Finnish pulp mill (UPM 70 Pietarsaari pulp mill) as a by-product from its chemical pulping process of Scots pine (Pinus 71 sylvestris), Norway spruce (Picea abies), downy birch (Betula pubescens) and silver birch (Betula 72 pendula). According to chemical analysis, fibre sludge contains mass fraction of approximate 93-73 94% cellulose and 6-7% hemicelluloses of total carbon materials [3]. The fibre sludge that is 74 supplied by a pulp mill is dried by a roll press to obtain a high solid concentration, which has a 75 mass fraction of around 47% (See Figure 1). 76

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The official standard method T 203cm-99 was used to determine α-, β-and γ-cellulose contents of 79 the dried fibre sludge sample. Undergraded, higher-molecular-weight cellulose is indicated by α-80 cellulose, β-cellulose indicates degraded cellulose while γ-cellulose mainly consists of 81 hemicellulose [4]. In addition, the total carbon content (TC) of the dry sample was determined by 82 elementary analysis with a Perkin Elmer CHNS analyser [3] [4]. 83 shows the characterization of fibre sludge that was used in the experiments and also some 84 literature values are given as a reference. 85 Horse chestnut seed is low in protein (<5%) and fat but contain complex carbohydrates. As such, 90 starch is the main component of horse chestnut seed. In this investigation fresh wild edible horse 91 chestnut seeds were collect from a street in northern Germany due to the huge amount of this type 92 of chestnut seeds dropping beside the street each year. The characterization of horse chestnut seed is 93 presented in 94 [hpmim]HSO 4 that were used in this study, their chemical structures are illustrated in Figure 2. 99 These ILs are particularly designed for the combined application of dissolution and hydrolysis of 100 lignocellulosic materials in a one-step reaction. 101 102

Ionic liquids preparation 103
Hydroxyalkylimidazolium hydrogensulphate ILs were obtained from a two steps process in which 104 hydroxyalkylimidazolium chlorides were prepared by the following reported method described in 105 literature [12]. 3-Chloro-1-propanol 28,1 g (0,29 mol), 2-chloro-1-ethanol 23,7 g (0,29 mol) and 3- ILs were analysed by FTIR and the resulting spectra are presented in Figure 3. The results clearly 133 show that a small amount of water is present in these ILs, which is either due to the fact that the 134 original ILs contain a small amount of water or the ILs possibly absorb water from the air, or both. 135 Therefore, ILs with a water content of 9-13 dm 3 m -3 were determined by Karl Fischer titration and 136 further detailed analysis on their chemical structures by nuclear magnetic resonance (NMR) were 137 performed. 138  During dissolution and hydrolysis for every reaction observations showed that a gel was formed, 170 however, the amount of formed gel is related to the amount of fibre sludge in ILs. The more fibre 171 sludge presented, the more gel was formed. Due to the low amount of IL, fibre sludge was not 172 totally dissolved. 173

Determination of total reducing sugars (TRS) 174
The total amount of reducing sugars including the content of glucose, xylose and cellobiose were 175 measured according to the DNS method after one-step dissolution and hydrolysis. This method 176 Methylcellulose has thermo-sensitive properties that allow it to be dissolved in water at low 216 temperature (approximately under 40°C) and it displays reversible gelation at a particular 217 temperature [17] [18]. The gelation of methylcellulose depends on the concentration of 218 methylcellulose, temperature and the type of salt solution used [19] [20]. Therefore, under the same 219 reaction conditions used in this study, the amount of gel formed can be affected by the type of IL 220 used. Figure 5 shows that the TRS yield of the fibre sludge sample S4, with weight ratio of 20% in 221 [glymim]HSO 4 is the lowest. A possible explanation for this could be that the [glymim]HSO 4 had a 222 greater effect on the gelation process than for other ILs and as a result, the formed gel blocked any 223 further reactions. 224

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The results obtained from horse chestnut seed samples (S1, S2 and S3) are exactly the opposite to 226 fibre sludge of which horse chestnut seed sample S1 in [glymim]HSO 4 returned the lowest 227 production of TRS. 228

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The major difference between cellulose and starch is that starch has its α-1,4 and α-1,6 glycosidic 230 bonds linked whereas for cellulose is β-1,4 glycosidic bonds linked. By having different glucose 231 molecule linkages, there are vast differences in properties. As can be seen from Figure 5

239
From Figure 6, sample S19 is of particular interest to the authors since it can be directly compared 240 to sample 16 ( Figure 5) which was performed with similar reaction conditions. Both utilising wet 241 fibre sludge with a weight ratio of 5%, in [glymim]HSO 4 , the major difference between the two 242 samples was that S19 was exposed in air for 2 hours, followed by pretreatment. The results show 243 that there was a higher TRS yield in S19 than in S16 which can be explained by the reaction that 244 took place in the pretreatment process. In both samples cellulose was dissolved and further 245 hydrolysed in the IL, but the mixture of S16 and IL did not contain much water. The small amount 246 of water that was present caused the uncompleted hydrolysis of S16 where on the other hand, the 247 mixture of S19 and IL was first exposed in the air which allowed moisture to be absorbed from the 248 air. Therefore, more water was available for the hydrolysis process and so more TRS was produced 249 in the pretreatment of S19. The optimal wet fibre sludge weight ratio is around 10% while a greater weight ratio of fibre sludge 265 in an IL will cause gelation during the pretreatment resulting in the gel blocking any further 266 reactions. To achieve higher TRS yields, further optimisation of IL structures and dissolution 267 conditions is required. The amount of water (contained in ILs or samples) involved in the reaction 268 should be also considered and optimised. In the next study, we will test [glymim]OH in the 269 dissolution and hydrolysis of lignocellulosic materials and also the recycling of ILs.        35,00% S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18