One-pot selective production of levulinic acid and formic acid from spent coffee grounds in a catalyst-free biphasic system

Journal Pre-proofs One-pot selective production of levulinic acid and formic acid from spent coffee grounds in a catalyst-free biphasic system Bora Kim, Jeongwoo Yang, Minji Kim, Jae W. Lee PII: DOI: Reference:

S0960-8524(20)30167-X https://doi.org/10.1016/j.biortech.2020.122898 BITE 122898

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Bioresource Technology

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29 November 2019 24 January 2020 24 January 2020

Please cite this article as: Kim, B., Yang, J., Kim, M., Lee, J.W., One-pot selective production of levulinic acid and formic acid from spent coffee grounds in a catalyst-free biphasic system, Bioresource Technology (2020), doi: https://doi.org/10.1016/j.biortech.2020.122898

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One-pot selective production of levulinic acid and formic acid from spent coffee grounds in a catalyst-free biphasic system Bora Kim, Jeongwoo Yang, Minji Kim and Jae W. Lee* Department of Chemical and Biomolecular Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon, Republic of Korea, 34141 *

Corresponding author

Tel: +82-42-350-3940 Fax: +82-42-350-3910 E-mail: [email protected]

Keywords Spent coffee grounds (SCGs); platform chemical; levulinic acid; biphasic system; catalyst-free

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Abstract This study introduces the catalyst-free production of levulinic acid (LA) and formic acid (FA) from spent coffee grounds (SCGs) as a starting material in a biphasic system of 1,2dichloroethane (DCE)-water at temperatures above 160 ℃. In addition to the advantage of using the biphasic system attributed to the product equilibrium, DCE served as a source of hydrogen induced by subcritical water (SCW). The effect of temperature, the amount of DIW and DCE, and the pretreatment on SCG (raw or lipid extracted SCG (LE-SCG)) on the overall reaction and humin formation were studied. The maximum conversion of LA and FA was 47 and 29 w/w% of the total convertible monosaccharides in raw SCGs while 43 and 28 w/w% of the conversion were obtained at 180 ℃ when LE-SCG was used. The solvothermal effects of two media provides a non-catalytic route to utilize undried SCG for the production of LA and FA.

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1.

Introduction The world is experiencing an environmental crisis caused by the excessive use of

petroleum-derived energy. In response, researchers have focused on finding alternatives to petroleum-derived chemicals, including so called green chemicals derived from biomass such as secondary biomass, food waste, etc. Recently, spent coffee grounds (SCGs) have attracted attention as a green alternative to petroleum instead of being dumped into landfill since they have shown possibility as a source of bio-oil based on a SCG derived lipid (Park et al., 2016; Park et al., 2018; Son et al., 2018). After the lipid extraction, a considerable amount of residual SCG is still left as waste and is buried in landfills, and only a part of the solid can be utilized further as fertilizers, and in other applications. Indeed, the cellulose and hemicellulose composition in SCG accounts for more than 40 % of the dry weight of SCG. In that light, an approach to utilize the carbohydrate in SCG could potentially replace the chemicals produced from the petroleum-based industry rather than being dumped (Kim et al., 2019b; Mata et al., 2018; Vardon et al., 2013).

Levulinic acid (LA) has been selected as a promising platform chemical by the US Department of Energy on the basis of its high reactivity and a wide range of functionality based on a ketone and a carboxylic functional group (Werpy & Petersen, 2004). This water soluble organic compound is a versatile building block for a number of pharmaceuticals, plasticizers, and other additives such as γ-valerolactone, succinic acid, alkyl levulinates, 1,4-pentanediol, etc. (Galletti et al., 2012b; Mascal & Nikitin, 2010; Pileidis & Titirici, 2016). In addition to LA, formic acid (FA) also has received attention as a promising hydrogen carrier due to its relatively high hydrogen content (Ando et al., 2000). However, to date, FA is exclusively being produced 3

from petroleum-based materials. Having remarkable potential, LA and FA and their esterified forms have been researched intensively, particularly their production from non-edible biomass such as rice straw, corn stover, microalgae, etc. (Chen et al., 2017; Dussan et al., 2013; Elumalai et al., 2016; Im et al., 2015; Kang et al., 2018; Kim et al., 2019a; Kim et al., 2017b; Yang et al., 2018). However, the biomass usually requires pretreatment in order to disrupt or delignify the structure to improve the subsequent chemical conversion reaction by breaking the cellulosic polymer into smaller blocks. During the pretreatment process, the use of catalysts and solvents is inevitable, which leads to low economic viability.

The conventional method for the production of LA and FA is a chemical conversion that employs mineral acid catalysts such as hydrochloric acid, phosphoric acid, sulfuric acid, etc. owing to their high catalytic activity and low cost (Elumalai et al., 2016; Muranaka et al., 2014). However, the use of these acids also delivers undesirable side reactions that lower the yields of LA and FA. In addition, the issues of corrosion and recycling have motivated research towards a new route for the production of LA in a more sustainable manner. Heterogeneous catalysts (zeolite, graphene oxide, Amberlyst, etc.), being selective and non-corrosive, have been studied for the conversion of LA (Chen et al., 2017; Ozsel et al., 2019). However, the catalyst deactivation impedes further utilization of heterogeneous catalysts for the conversion of biomass to LA. Ionic liquids and supercritical fluids may be considered attractive for LA production given their good performance, but the large amount of chemicals and associated high costs may limit their use commercially (Badgujar et al., 2019; Chen et al., 2019; Kumar et al., 2018).

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The production of LA and FA from raw SCGs has rarely been dealt with in depth despite that SCG contains a considerable amount of carbohydrates that can be converted to LA and a coproduct, formic acid (FA). This work applied a non-catalytic hydrothermal process for producing LA and FA by exploiting the advantages of subcritical water, the properties of which such as ion product and dielectric constant change in its liquid state above its normal atmospheric boiling point (Kim et al., 2017a; Park et al., 2018). In this work, two methods were compared to produce LA and FA: a single-pot conversion of raw SCGs and a two-step production by lipid separation followed by the conversion of LA and FA in order to maximize the applicability of lipids and cellulosic components, respectively. This work elucidates the mechanism of LA and FA production directly from raw SCGs with no catalyst, and investigates the effects of the reaction variables of pretreatment conditions, reaction time and temperature, solvent amount, and moisture content on the yield by focusing on the maximum LA and FA production. Further analyses of humin products verify the effect of temperature on the production of LA and FA. By utilizing carbohydrate rich SCGs in a catalyst-free biphasic system, this approach enables sustainable production of LA and FA, valuable platform chemicals, with high productivity.

2.

Materials and methods 2.1 Materials The experiments were conducted using SCGs locally provided by Starbucks and they

were stored in an as-received state without any further treatment. The initial moisture of the SCGs was measured in quadruple by a freeze-drying method and had a value of 57.98 ± 0.02 w/w%. For distilled water (DIW), Ultrapure Milli-Q water with a resistivity of 18.2 MΩ·cm was used throughout the experiment. 1,2-dichloroethane (DCE) obtained from Junsei Chemical 5

(guaranteed reagent grade, Junsei Chemical, Japan). Levulinic acid (LA) and formic acid (FA) were used to quantify the amount derived from SCGs as standard chemicals, where both were provided by Tokyo Chemical Industry Co., Ltd., Japan. Glucose, fructose, galactose, and xylose (ACS reagent grade, Sigma Aldrich) are all used to quantify the amount of carbohydrate in the SCGs and used in bioLC and HPLC. To quantify the monosaccharides in the SGCs, the NREL method (Sluiter et al., 2008) was employed, where guaranteed grade H2SO4 was obtained from Junsei Chemical, Japan.

2.2 Single-pot conversion of levulinic acid from raw spent coffee grounds (SCGs) In order to produce LA from raw SCGs, the simple method suggested in previous studies was modified in this study (Kim et al., 2017a; Park et al., 2018). Raw SCGs (untreated) were loaded with distilled water (DIW) and soaked for 2 h. Additional DIW and DCE were then added in a 100 mL SUS360 reactor containing a Teflon liner. The reactor was heated to a desired temperature using a thermostat bath for 3 h total including the heating-up time. The reaction variables are as follows: soaking condition (soaking at room temperature (RT) or 100 ℃ for 2 h before reaction), the amounts of DCE (4.17 and 8.33 mL per gram dried SCGs) and DIW (8.33 and 16.67 mL per gram dried SCGs), and the reaction temperature (160, 180, and 200 ℃). After the reaction was done, the post-reaction medium, which consists of an upper aqueous phase and an organic phase at the bottom, was collected. The concentration of the aqueous phase was normalized using additional DIW. Before the analytical work, the aqueous phase of each sample wherein the monosaccharides, LA and formic acid (FA), and intermediates were dissolved was filtered using a 0.2 μm filter to be analyzed. LA and FA were calculated based on their respective calibration curves using known amounts ranging from 0.1 mg/mL to a maximum of 50 mg/mL. 6

2.3 Two-step conversion of levulinic acid from SCGs In the long-term view of biomass valorization, SCGs have the advantage of having considerable amounts of both lipids and cellulosic components since each can be utilized as biooil and cellulose-derived chemicals, respectively. It is important to focus on the cellulosic proportion of SCG in order to convert it into LA at the highest possible yield. Thus, a two-step conversion process was employed to maximize the valorization of lipid-extracted residual SCGs (LE-SCG) by separating lipids first for bio-oil production and the residual solid was subjected to LA conversion.

Based on the preliminary test, 8.33 mL DIW per gram dried SCGs was added to untreated SCGs and the mixture was subjected to constant temperature conditions of room temperature (25 ℃), 100 ℃, and 180 ℃ for 2 h. When the pretreatment was completed, lipid was separated by adding 10 mL chloroform and vortexing for more than 5 min followed by separation with a syringe. This lipid extraction step was performed at least four times until the lipid layer turned transparent. The filtered lipid was then dried in an oven set at 60 ℃ overnight in order to evaporate the chloroform and weighed to calculate the amount of extracted lipid based on the dry weight of the initial SCGs. The residual LE-SCG was freeze-dried, and the completely dried LE-SCG was subjected to the same conversion method described in section 2.2 for the production of LA. The reaction variables are the temperature of the pretreatment condition (at room temperature, 100 ℃, and 180 ℃ for 2 h before reaction), the amounts of DCE (4.17 and 8.33 mL per gram dried SCGs) and DIW (8.33 and 16.67 mL per gram dried SCGs),

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and the reaction temperature (160, 180, and 200 ℃). The method for the quantification of the products is the same as described in section 2.2.

2.4 Analytical methods The main analysis tool is a high performance liquid chromatograph (HPLC, Dionex Ultimate 3000, Thermo Scientific) equipped with a RI detector Aminex HPX-87H ion exclusion column (300 × 7.8 mm) operated at 65 ℃. As a mobile solution, a 0.01 N H2SO4 solution was used at a flow rate of 0.6 mL/min. The percent yield of LA and FA was calculated based on the weight of dried mass of loaded SCGs throughout the work. All experiments were conducted in quadruple and the reported data is the average of the runs.

In order to acquire thermogravimetric data, a thermogravimetric analysis (TGA) was carried out on a TG209 F1 Libra (NETZSCH) from room temperature to 900 °C with a ramping rate of 10 °C min−1 under a N2 condition. Before the measurement, raw SCGs containing moisture were oven-dried at 60 ℃ in order to evaporate the bound moisture.

For measuring the amount of convertible monosaccharides from SCGs, Bio-liquid chromatography (BioLC, Dionex ICS-5000, Thermo Scientific) was used. And to determine the degree of hydrolysis of DCE at high temperature of 160–200 °C, the amount of chloride ions was measured by ion chromatography (Metrohm, 881 Compact IC pro, Swiss), with a high pressure pump (run flow rate of 0.7 mL/min and pressure of 7.14 MPa), eluents in tanks (eluent A: water containing 3.2 mM Na2CO3 and 1 mM NaHCO3/eluent B: ultra-pure water), a conductivity detector, and chromatographic columns, Metrosep A Supp5 150 (150 × 4.0 mm I.D.). The 8

column temperature was kept at 30 °C throughout the analysis and the recording time was 20 min. The chromatographic system is completely controlled by MagIC Net version 2.4. The ion concentration was then obtained in mg kg−1. For humin analysis, the morphology of the solid samples after the conversion reaction was characterized using scanning electron microscopy (SEM, SU8230, Hitachi) and a compositional analysis of C, H, O, N, and S was conducted via an elemental analyzer (Flash 2000 series, Thermo Scientific).

3.

Results and discussion 3.1 Characteristics of SCGs The amount of monosaccharides in raw SCGs based on the NREL method was confirmed

as galactose 8.04 ± 0.31, glucose 10.12 ± 0.42, xylose 0.02 ± 0.00, mannose 19.70 ± 0.80 w/w% of dried SCG. These results are consistent with the TGA data in Fig.1(a), where the cellulose and hemicellulose account for about 40 w/w% of dried raw SCGs. Not only from the cellulose, it is also revealed that hemicellulose also can be hydrolyzed to release hexoses, which follows the same reaction pathway as that of glucose in the production of LA and FA (Dussan et al., 2015; Girisuta et al., 2008). Xylose, which ends up as furfural and FA if the same acidic condition applied, was found to be negligible in raw SCGs.

Since the proportion of hexoses from hemicellulose takes a large part of the total convertible sugars in SCGs, it is important to confirm the conversion rate of each hexose towards LA and FA. In the process of LA production, an important rate determining step is the decomposition of cellulosic polymers into hexoses followed by the conversion steps of hexoses to HMF with fructose as an intermediate (Shen & Wyman, 2012). In the isomerization of 9

hexoses into fructose, the equilibrium constant for the isomerization of mannose to fructose is greater than that of glucose to fructose, indicating mannose is more easily isomerized in a subcritical (SCW) condition than glucose and galactose (Gao et al., 2015; Khajavi et al., 2005). The results of the component analysis support that SCGs can be a useful source for LA production because mannose consists of more than half of the convertible monosaccharides in SCGs.

3.2 Conversion of dried and raw SCGs via catalyst-free single-pot conversion process Prior to investigating of the effect of variables on the LA and FA conversion, one-pot conversion of untreated SCGs to LA was conducted via the single-pot conversion process at respective temperatures of 160, 180, and 200 ℃. Both dried and raw SCGs were compared and the amounts of DCE and DIW were fixed at 4.17 and 8.33 mL per gram dried SCGs, respectively. The yield was calculated based on the solid mass of dried SCGs, and therefore in the case of wet SCGs, the yields of LA and FA were calculated after subtracting the moisture content in wet SCGs.

As shown in Fig.2(a), the yield of LA and FA showed an overall increase when wet SCGs were used and the maximum yield was obtained when the reaction was conducted at 180 ℃. The probable reason for the difference in the yield is that wet SCGs contain more water than dried SCGs, where the water can be utilized as a catalyst in this particular reaction of high temperature. The increased reaction temperature induces drastic changes in SCW and it is thereby a strong source of hydrogen and hydroxide ions that can act as a catalyst. Thus, the ion product of SCW triggers the hydrolysis of DCE, synergistically creating not only a biphasic 10

system but also an acidic environment by releasing H+ and Cl- in the form of hydrochloric acid (Kim et al., 2017a; Park et al., 2018). In addition, the experimental results showed higher Clconcentrations when wet SCGs were used at each temperature condition. Furthermore, the effect of capillary water in wet SCGs led to high yields of LA and FA compared to the dried SCGs as the water in wet SCGs facilitates the diffusion of H+ within the cellulosic microstructure, where dried SCGs require re-moisturizing prior to the acid-catalyzed decomposition of the structure (Tukacs et al., 2017). The synergetic effect of water on LA and FA yields was confirmed by the differences in pH between dried and wet condition (Fig.2(a)), showing an increase in acidity when the wet condition was applied at the same temperature condition. When a dried condition was applied, pH ranged from 0.29 to 1.23, whereas wet SCG showed a range from 0.03 to 1.11 at the respective temperature condition.

This increased acidity in the reaction medium initiates the cellulose and hemicellulose decomposition from the beginning of the conversion process and is involved the dehydration of monosaccharides and in the production of LA and FA. Many studies have investigated the effect of temperature (the changes in the properties of SCW) on the conversion of hexoses to LA and FA (Asghari & Yoshida, 2007; Li et al., 2009; Shen & Wyman, 2012). Based on the results in Fig.2(a), the reaction temperature of 160 ℃ was not sufficient for SCW to trigger the decomposition of DCE that renders the reaction medium more acidic (pH≈1.23). As the rate of reaction increases with temperature, the yields of LA and FA substantially increased. At the reaction temperature of 200 ℃, however, the exposure of DIW and DCE at high temperature induced decomposition of monosaccharides and HMF and further decomposition of LA and FA in the SCW environment, resulting in the lower yields of LA and FA compared to the results 11

obtained at 180 ℃. Shen and Wyman (2012) explained that the side reaction of HMF to byproducts accelerates as the reaction temperature increases, and therefore a decrease in LA yield was observed in their experiment where hydrochloric acid catalyzed levulinic acid formation from cellulose at 200 ℃ (Shen & Wyman, 2012).

Furthermore, it was also observed that the rate of decomposition of LA was accelerated with an increase in the acid concentration. Having DCE as a source of catalyst at higher temperature thus accelerates the side reactions of byproduct formation such as water-soluble product or humin, leading to decreased in LA and FA. In order to confirm the effect of temperature and the acidic environment on the byproduct formation, an elemental analysis of the solid after the reaction was conducted (Fig.2(b)). An increase in humin formation with increasing temperature was indirectly observed as the ratio of H/C and O/C in the van Krevelen diagram decreased accordingly as the temperature increased. The same effect of temperature and acid concentration on the humin formation was also confirmed in a previous work (van Zandvoort et al., 2013).

Another noteworthy feature in Fig.2(a) is the increase in the ratio of LA to FA as the reaction temperature increased from 180 to 200 ℃. When dried SCGs were used, the ratio increased from 2.13 to 3.43, and the ratio of wet SCGs was 1.94 and 3.08 at 180 and 200 ℃, respectively. The reason for the increase in LA content compared to FA is related to the higher reactivity of FA relative to that of LA at high temperature, and furthermore FA may be involved in further decomposition reaction as a catalyst of the hydrogen source (Asghari & Yoshida, 2007; Kupiainen et al., 2011). Therefore, as the reaction temperature increases, the rate of FA 12

decomposition exceeds the decomposition rate of LA, which leads to increase the ratio of LA to FA as shown in Fig.2(a).

3.3 Single-pot conversion of LA and FA from raw SCGs 3.3.1

Effect of temperature

Fig.3. presents the results of one-pot conversion of wet SCGs with various conditions of DCE and DIW amounts at different temperatures of 160, 180, and 200 ℃, where (a) and (b) show the product yield when wet SCGs were soaked at RT and 100 ℃ before the high temperature experiment, respectively. Two conditions of DIW and DCE soaking experiments were applied: for condition 1, 4.17 mL DCE and 8.33 mL of DIW per gram dried SCGs were added whereas for condition 2, 8.33 mL DCE and 16.67 mL DIW per gram dried SCGs were used.

Obvious differences confirm that the temperature is a primary factor affecting LA and FA production. The reaction temperature of 180 ℃ yielded the highest yield overall when the same conditions were applied regardless of the amount of DCE and DIW. The same trend was also observed in Fig.2 and Fig.4. An increase in the reaction temperature is known to reduce the interaction of hydrogen bonding between monomers of cellulose (β-(1→4)-glycosidic bonds) and hemicellulose with a variety of glycosidic bonds by offering accessibility of acidic protons to the reaction site (Victor et al., 2014). As revealed in previous studies, the role of DCE and DIW as catalysts in this biphasic system is substantial, as subcritical water (SCW) induces the decomposition of DCE, providing an acidic environment by releasing hydrogen and chloride ions that can act as Lewis/Brønsted acids (Kim et al., 2017a; Park et al., 2018). In fact, 13

hydrochloric acid has been employed as a more effective catalyst for LA production compared to other homogeneous mineral catalysts in several works (Bevilaqua et al., 2013; Tabasso et al., 2014). Acidic protons derived from the SCW and decomposed DCE can easily access the reaction site and facilitate the conversion process of cellulosic polymer to monosaccharides.

As explained in section 3.2, in this single-pot conversion of LA from raw SCGs, the reaction temperature of 160 ℃ was not high enough to initiate the changes in SCW properties and consecutive hydrolysis of DCE. As a consequence, the number of available protons that can initiate the polymer decomposition is insufficient. Therefore, only a small amount of soluble sugar and the biphasic environment produced the LA and FA to a certain extent. At 180 ℃, however, about 80 w/w% of convertible sugar was converted into the targeted products, LA and FA, when condition 1 was applied (Fig 3(a)). This conversion yield of LA without any external catalyst was comparable to previous research regarding LA production from biomass using hydrochloric acid at 150 – 220 ℃ (Bevilaqua et al., 2013; Galletti et al., 2012a; Kumar et al., 2018; Pulidindi & Kim, 2018; Victor et al., 2014; Yan et al., 2008).

When the reaction was conducted at 200 ℃, the decomposition of hexoses was accelerated compared to the reaction of hexoses to HMF, and the yields of LA and FA decrease accordingly. Khajavi et al. (2005) revealed that the degree of decomposition of hexoses to byproduct is in the order of fructose, glucose, mannose, and galactose rather than conversion to HMF (Khajavi et al., 2005). Also, the activation energy of the decomposition of hexoses to unwanted byproduct could be lowered further if the reaction occurs in an acidic environment. Lastly, both LA and FA are rather exposed to the further hydrolysis at high temperature, as 14

explained in section 3.2. The tendency of decreased LA and FA yields as the reaction temperature increased was also observed in a prior study (Weingarten et al., 2012). FA acts as a hydrogen source and unwanted side reaction leads to the formation of soluble polymer and mainly humin, which hinders the dehydration reaction, lowering the LA yield. van Zandvoort et al. (2013) also confirmed that the increase in humin formation mainly depends on the reaction temperature in acid-catalyzed biomass conversion since HMF is prone to polymerization (van Zandvoort et al., 2013). Overall, the reaction temperature of 180 ℃ was suitable for LA conversion from raw SCGs.

3.3.2

The effect of solvent and distilled water (DIW): Raw SCGs

The advantages of having a biphasic system using dichloromethane (DCM), γvalerolactone, 2-butanol, methyl isobutyl ketone, etc. for biomass conversion into LA have been demonstrated in various studies and are attributed to the impact of product distribution equilibrium of intermediates between organic and aqueous phases (Kumar et al., 2018; Torres et al., 2010; Wettstein et al., 2012). In our DCE-DIW biphasic system, HMF, an intermediate of LA, could be continuously extracted into the organic phase, and thereby the forward reaction of LA formation can be promoted by preventing the product saturation. At the temperature of 160 ℃, the effect of the increase in the amount of DCE or DIW is negligible due to the insufficient supply of protons to initiate the conversion reaction even though the DCE-DIW biphasic system exists (Fig.3). However, as the temperature increases to 200 ℃, the yields of LA and FA decreased as the amount of DCE and DIW is doubled. This result is related to the increased acidity (attributed to both SCW as a catalyst and protons from degradation of DCE) and the biphasic environmental effect when the amounts of DCE and DIW were increased. 15

Although the increase in the acidity may cause cleavage of cellulosic decomposition, the excessive protons also cause decomposition of HMF. Therefore, excessive amounts of DIW and DCE bring undesirable side reactions (for example, polymerization to humin), resulting in lowered yields of LA and FA, as discussed in section 3.2.

3.4 Two-step conversion of LA from lipid extracted SCG (LE-SCG) 3.4.1

Effect of pretreatment temperature

Since the effect of reaction temperature was discussed in section 3.3.1, this section focuses on the effect of pretreatment temperature on the conversion ratio of LE-SCG to LA and FA. In Fig.3, raw SCGs were soaked in DIW for 2 h at different temperature of RT and 100 ℃ before the soaked biomass is subjected to the experiment. Decreased overall yields of LA and FA were observed when the soaking temperature was increased from RT to 100 ℃. The reason for applying the pretreatment was to help the release of sugar by breaking the linkage of cellulose and hemicellulose in raw SCGs using hot water. To test this hypothesis, the pretreatment of raw SCGs was executed at different temperatures of RT, 100, and 180 ℃ followed by lipid extraction and the different pretreatment temperature showed interesting results, as shown in Fig.4. As the treatment temperature increases from RT to 100 ℃, the yields of LA and FA slightly increase when the reaction temperature was 160 ℃, but remained nearly the same for most conditions.

Further increase in the pretreatment temperature to 180 ℃ resulted in a significant drop in overall yields of LA and FA (Fig.4(c)). The reason for the low yields of LA and FA is the increase in the soluble sugars during the pretreatment. The solubilized sugars were washed out during the lipid extraction and washing step after the pretreatment. Therefore, as the pretreatment 16

temperature was increased from RT to 180 ℃, the amount of soluble sugars that were washed out increases and a smaller amount of convertible sugars remains available in LE-SCG. Indeed, hot compressed water (HCW) of over 180 ℃ has been dealt with in previous works as a method of biomass treatment (Ando et al., 2000; Kobayashi et al., 2009; Kumar et al., 2009; Yu et al., 2008). The decomposition of cellulose requires higher temperature while hot compressed water of 180 ℃ is sufficient for hemicellulose to be decomposed. Thus, as shown in Fig.4(a) and (c), a drastic decrease in the LA yield is observed (about 52 % total when condition 3 applied at 180 ℃).

In addition, interesting results were observed in comparison of wet SCGs and LE-SCG conversion at the same conditions (Fig.3 and Fig.4). Specifically, at 180 ℃, unlike the tendency of decreased yield in Fig.3 when the amounts of DIW and DCE were doubled, the yields of LA and FA were enhanced, as shown in Fig.4. A plausible reason for this result is that during the process of pretreatment and lipid extraction, water soluble and solvent soluble impurities of raw SCGs were washed out, lowering the side reactions of byproduct formation. Not only is the absolute amount of convertible sugars increased via the pre-extraction of lipid, but LE-SCG has much lower likelihood to have impurities (proteins, metals, etc.) involving side reactions that could hinder the LA and FA formation.

The amount of SCG derived lipid (where the chloroform of the organic phase was completely dried) was roughly the same (20 w/w% of untreated dried SCG) regardless of the treatment temperature. The representative TGA data of the extracted lipids (lipid extracted after

17

pretreatment at RT) is shown in Fig.1(b). SCG-derived lipid contains 15 % free fatty acid, 10 % impurities, and the rest is triglycerides.

3.4.2

The effect of solvent and distilled water (DIW): LE-SCG

The yields of LA and FA were maximized when 8.33 mL DCE and 16.67 mL DIW were used per gram dried SCGs at 180 ℃ in Fig.4. At the reaction temperature of 160 ℃, the effect of SCW induced catalyst production may be weak in the biphasic system resulting in lower yields of LA and FA. The amount of water increases in condition 2 merely dilute the acid concentration, and therefore condition 2 showed the lowest yields of LA and FA. At the temperature of 160 ℃, the yields of LA and FA are raised again as the DCE amount is increased in condition 3 (Fig.4(a) and (b)). On the other hand, an opposite tendency was shown at 200 ℃ where the effect of temperature is substantial. The excess DIW or DCE actively causes side reactions. The overall LA yield then declined compared to the result obtained at the temperature of 180 ℃. Compared to conditions 1 and 2 at 200 ℃, the excess DIW leads to increased LA and FA yields, but the additional increase in DCE (condition 3) instead lowers the yields due to the excessive catalytic activity and causes undesirable byproduct formation such as humin or water soluble polymer (van Zandvoort et al., 2013). The same trend of an increase in the Clconcentration with temperature was already observed in previous research (Kim et al., 2017a; Park et al., 2018).

In this work, as higher reaction temperature is applied, a higher acidity was measured. This also affects the FA yield, which has higher reactivity compared to the LA, and therefore the yield of FA at condition 3 at 200 ℃ is far lower than the theoretical mass of LA to FA. As the 18

same trend is observed in Fig.2(b), the van Krevelen diagram of LE-SCG sample of condition 3 at 200 ℃ (Fig.4(d)) displayed a transition into humin by showing a decrease in the H/C and O/C ratio as the reaction temperature increases from 160 to 200 ℃. However, the amount of Cl- ion concentration involved on the surface of humin appears to be independent of the reaction temperature. Instead, a structural difference by temperature was observed; the solid structure at 160 ℃ was similar to the raw SCGs, and at higher magnification, a development of roundish humin formation was observed. At 180 ℃, the overall size reduction of SCG particles was observed compared to the raw SCGs and a few independent spherical shaped humin particles were observed. The overall particle size reduction is clearly differentiated from the raw SCGs when the LE-SCG was treated at 200 ℃, and the humin particles formed in spherical shape were distributed all over the observed area. The humin formation via the dehydration pathway and its morphological difference also corresponded to the results of a previous study (van Zandvoort et al., 2013).

3.4.3

Effect of reaction time on the conversion

A few studies have also dealt with the reaction time as an important variable in LA conversion along with temperature (Asghari & Yoshida, 2007; Girisuta et al., 2008; Weingarten et al., 2012). In Fig.5, 8.33 mL DCE and 16.67 mL DIW were used per gram dried LE-SCG (pretreated at RT) at 180 and 200 ℃ for different reaction times of 1.5, 3, and 4.5 h. First, at 200 ℃, the reduced reaction time causes slight increases in the overall yields of LA and FA. However, as the reaction time was extended to 4.5 h, the FA and LA nearly disappeared. When the same experiment was conducted at 180 ℃, the overall yield was maximized at 3 h experiment. 19

Between the yields conducted at 180 and 200 ℃ for 1.5 h, the total conversion yield is higher at 200 ℃. Higher temperature can accelerate the reaction of cellulosic decomposition, one of the main rate determining steps; however, the humin formation from HMF is known to be very sensitive to temperature (Shen & Wyman, 2012). Therefore, drastic decreases in the LA and FA yields were observed with increased time at 200 ℃, indicating that the variable of reaction temperature is more significant with regard to the LA and FA conversion compared to the reaction time. Compared to the previous work using SCGs, the approach of LA production in this biphasic system at high temperature without external aid of a catalyst or a physical assistant was superior in terms of the amount of solvent, reaction time, and yield (14.5 vs.18.0 w/w% based on the raw SCGs weight and 23.0 w/w% based on the weight of LE-SCG) (Tukacs et al., 2017).

4.

Conclusions The biphasic system of DCE-DIW has been investigated using SCGs without a catalyst to

produce LA and FA. The yields of LA and FA were maximized when raw SCGs was used with a solid to water ratio of 8.33 at 180 ℃ for 3 h. The maximum yield of LA and FA were 47 and 29 w/w%, respectively, based on the convertible monosaccharides in raw SCGs. By exploiting the solvothermal effect of DCE, the catalyst-free conversion of LA and FA in the biphasic systems verifies the simple and sustainable production compared to the LA and FA production using a catalyst.

E-supplementary data of this work can be found in online version of the paper. 20

5.

Acknowledgement

This work was supported by the National Research Foundation (NRF) funded by the Ministry of Science and ICT (NRF-2019M3F2A1072237 & NRF-2019M2A7A1001773), South Korea.

6.

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(a)

(b)

Figure 1. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of (a) oven dried SCG and (b) SCG derived lipid, free fatty acid (FFA, oleic acid) and soybean oil (TAG).

28

30

1.5

Formic acid Levulinic acid pH 1.0 20

0.5

pH

Yield, loaded solid mass based %

(a)

10 0.0

0

160℃

180℃

200℃

Dried SCG

160℃

180℃

200℃

-0.5

Wet SCG

(b)

Figure 2. (a) LA and FA yields of wet SCG and oven dried SCG via single-pot conversion using 4.17 mL DCE and 8.33 mL DIW per gram dried SCG at different temperatures of 160, 180, and 200 ℃. (b) A van Krevelen plot of solid residue after the conversion (Single pot conversion at 160 ℃: triangles, single pot conversion at 180 ℃: reverse triangles, and single pot conversion at 200 ℃: diamonds (dried: open, wet: filled)).

29

30

Yield, loaded solid mass based %

(a)

Formic acid Levulinic acid

20

10

0

RT Cond_1

Cond_2

Cond_1

160℃

Cond_2

180℃

Cond_1

Cond_2

200℃

30

Yield, loaded solid mass based %

(b)

Formic acid Levulinic acid

20

10

0

100℃ Cond_1

Cond_2

Cond_1

160℃

Cond_2

180℃

Cond_1

Cond_2

200℃

Figure 3. LA and FA yields of untreated wet SCG via single-pot conversion after soaking at different temperatures of (a) room temperature (RT) and (b)100 ℃. The reaction condition is 1) 4.17 mL DCE and 8.33 mL of DIW per gram dried SCG and 2) 8.33 mL DCE and 16.67 mL of DIW per gram dried SCG at different temperature of 160, 180, and 200 ℃.

30

40

40

(b)

Formic acid Levulinic acid

Yield, loaded solid mass based %

Yield, loaded solid mass based %

(a)

30

20

10

20

10

Pretreated at room temperature

Pretreated at 100℃

Cond_1 Cond_2 Cond_3 Cond_1 Cond_2 Cond_3 Cond_1 Cond_2 Cond_3

Cond_1 Cond_2 Cond_3 Cond_1 Cond_2 Cond_3 Cond_1 Cond_2 Cond_3

160℃

Yield, loaded solid mass based %

30

0

0

(c)

Formic acid Levulinic acid

180℃

200℃

160℃

180℃

200℃

40

(d)

Formic acid Levulinic acid 30

20

10

0

Pretreated at 180℃ Cond_1

Cond_2 180℃

Cond_3

Cond_1

Cond_2

Cond_3

200℃

Figure 4. LA and FA yields of LE-SCG after the lipid-solid separation pretreated at different temperatures of (a) room temperature, (b) 100, and (c) 180 ℃. The reaction conditions after the pretrement are as follows: condition 1) 4.17 mL DCE and 8.33 mL of DIW, 2) 4.17 mL DCE and 16.67 mL of DIW and 3) 8.33 mL DCE and 16.67 mL of DIW per gram dried SCG at different temperature of 160, 180, and 200 ℃. (d) A van Krevelen plot of solid residue after the conversion of LE-SCG (Conversion at 160 ℃: triangle, conversion at 180 ℃: reverse triangle, and conversion at 200 ℃: diamond). The LE-SCGs were produced via conversion with condition 3 at different temperatures of 160, 180, and 200 ℃ after lipid-solid separation pretreated at room temperature.

31

Yield, loaded solid mass based %

40

Formic acid Levulinic acid 30

20

10

0

180°C 1.5 h

3h

200°C 4.5 h

1.5 h

3h

4.5 h

Figure 5. LA and FA yields at different reaction times of 1.5, 3, and 4.5 h with the fixed amount of 8.33 mL DCE and 16.67 mL DIW per gram dried LE-SCG (pretreated at RT) at temperatures of 180 and 200 ℃.

Graphical abstract

32

Highlights 

Spent coffee grounds are utilized to produce levulinic and formic acid (LA and FA).



A catalyst-free biphasic system is introduced to obtain the highest LA and FA yields.



The LA and FA yields were maximized when LE-SCG was used at 180 ℃.



Solvothermal effect of DCE-DIW system is crucial to obtaining the high yields.

Bora Kim: Methodology, Validation, Investigation and Writing the Article Jeongwoo Yang: Formal analysis, Writing – Review & Editing. Minji Kim: Data curation, Writing – Review & Editing Jae W. Lee: Supervision, Funding, Writing – Drafting, Review & Editing.

33