Production of xylose, levulinic acid, and lignin from spent aromatic biomass with a recyclable Brønsted acid synthesized from d-limonene as renewable feedstock from citrus waste

Bioresource Technology 293 (2019) 122105

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Production of xylose, levulinic acid, and lignin from spent aromatic biomass with a recyclable Brønsted acid synthesized from d-limonene as renewable feedstock from citrus waste Mangat Singh, Nishant Pandey, Pratibha Dwivedi, Vinod Kumar, Bhuwan B. Mishra

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Center of Innovative and Applied Bioprocessing (CIAB), Sector 81 (Knowledge City), S.A.S. Nagar, PO Manauli, Mohali 140306, Punjab, India

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Spent aromatic waste p-Cymene-2-sulphonic acid Xylose Levulinic acid Lignin

This work aimed to develop a green protocol for chemical processing of spent aromatic biomass to obtain xylose, levulinic acid, and lignin in good yields via treatment with p-cymene-2-sulphonic acid (p-CSA), a Brønsted acid synthesised from d-limonene as a renewable feedstock from citrus waste. Chemical processing of palmarosa biomass with p-CSA under heating in an autoclave resulted in hydrolysate containing xylose (~16% yield). Further processing of pre-treated biomass with p-CSA in presence of aq. HCl under refluxing caused a selective degradation of cellulose to levulinic acid (~22% yield with respect to biomass). The residual biomass was used to afford lignin in good yields.

1. Introduction Production of ethanol or furfural from lignocelluloses in the single product plants have been found commercially nonviable either due to unprofitable process or requires a high selling price with lack of target beneficiary for the products, which may be linked to their reliance on a single component of biomass (Caes et al., 2015; Joyce et al., 2015; Ji et al., 2019). Therefore, it has been prospected that the processing cost can be greatly reduced when all the components of the biomass are

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converted to value-added products (Singh et al., 1984; Ramos et al., 2017) such as selective hydrolysis of hemicellulose to xylose for production of furfural or xylitol, degradation of cellulose to levulinic acid by hydrothermal treatment, and utilization of residual biomass to obtain lignin. Such an integrated plant utilizing all the components of biomass for production of xylose, levulinic acid, and lignin would be commercially more acceptable. Identification of xylitol as a high value polyalcohol produced by the reduction of D-xylose and its increasing use in food and pharmaceutical

Corresponding author. E-mail address: [email protected] (B.B. Mishra).

https://doi.org/10.1016/j.biortech.2019.122105 Received 16 July 2019; Received in revised form 30 August 2019; Accepted 1 September 2019 Available online 04 September 2019 0960-8524/ © 2019 Elsevier Ltd. All rights reserved.

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amount of cellulose, hemicellulose, and lignin. Cellulose is the predominant component (35–40%), followed by hemicellulose (25–30%) and lignin (15–20%) (Rout et al., 2013). Earlier, spent aromatic biomass has been studied for supply of nutrients to the other crops, and in nutrient recycling for fertilizer economy (Mukherjee et al., 2015). Synthesis of hydroxymethyl furfural (HMF) by degradation of cellulose isolated from aromatic waste is prospected (Peralta-Yahya and Keasling, 2010). The essential oils obtained from such aromatic crops have been studied as potential, drop-in ready advanced biofuel chemicals (Pinzi et al., 2009). However, antimicrobial activities of citral and geraniol occurring in these aroma oils against Saccharomyces cerevisiae may inhibit microbial fermentation in biorefineries (Helal et al., 2006). Earlier, we have reported an efficacious protocol for production of p-cymene sulphonic acid dihydrate (p-CSA), a strong, biodegradable, non-foaming, and recyclable Brønsted acid synthesized from d-limonene as renewable feedstock from citrus (Citrus reticulata) fruit waste (Dwivedi et al., 2018). This manuscript describes an integrated and efficacious methodology for chemical processing of spent aromatic biomass with p-CSA to afford value added products under organic solvent free condition. The reagent p-CSA used in the study causes fewer environmental problems than the strong inorganic acids, such as H2SO4 and H3PO4. The reagent p-CSA was earlier applied as an interesting alternative to inorganic acids in some reactions, such as esterification, hydrolysis, alkylation etc. (Clark et al., 2012). However, there has been no study on the conversion of biomass into platform chemicals such as xylose and LA using p-CSA. Also, fewer attempts have been made on valorisations of spent aromatic waste, and hence, the method described herein, appears promising for the waste to wealth value added recovery of chemicals such as xylose, LA, and lignin from spent aromatic biomass which does not currently have high-value applications, instead the majority is disposed of or used for burning purposes. Easy and efficient process for reagent recycling and recovery of products are the other noticeable advantages of this methodology.

industries has established D-xylose, a platform chemical from lignocelluloses. Earlier, extensive researches based on acid-base chemistry have been carried out to produce xylose through the hydrolysis of xylan, a polysaccharide belonging to hemicellulosic fraction of plant biomass (Liu et al., 2016; Hilpmann et al., 2019). Acid hydrolysis using mineral acids (e.g. HCl, H2SO4, HNO3, etc.), organic acids (CH3COOH, oxalic acid and citric acid, etc.) and Lewis acids (AlCl3, SnCl4, BF3, SO2, etc.) have been considered as a promising pre-treatment method resulting in high yields of the hemicellulosic sugars in the hydrolysate (Xue et al., 2018; Li et al., 2016; Stein et al., 2011; Qing et al., 2018). However, acid pre-treatment has many drawbacks, for example, nonrecyclability and corrosion effects of acids, high cost of reactors, lack of product electivity, formation of gypsum and inhibitory by-products (Jonsson and Martin, 2016; Hu and Ragauskas, 2012; Rabemanolontsoa and Saka, 2016) etc. Also, the purification of xylose by removal of proteins, colour, metal ions and other impurities from hydrolysate pose substantial challenges. Alkaline treatment (KOH, NaOH, Na2CO3 etc.) although increases the cellulose digestibility by removal of lignin from biomass, however, it causes less solubilisation of the hemicelluloses (Figueiredo et al., 2017). Also, high processing cost and the toxicity of processed waste water are the serious limitations (Yang and Wyman, 2008). Apart from these, acid-base hydrolysis of non-xylan polymers results in the hydrolysates containing monosaccharides as impurities, for example glucose, arabinose, galactose, mannose, etc. Many times aliphatic carboxylic acids, phenylic compounds, furans, etc. are also obtained as by-products in hydrolysates (Klinke et al., 2002). The sustainability and process economics are the main concerns in levulinic acid (LA) production from lignocellulosic feedstock. Earlier, significant amount of researches for LA production from lignocelluloses using solid catalysts (for example, noble metal, supported metals, molecular sieves, transition metal oxide etc.) have been reported (Yan et al., 2017; Chen et al., 2019). The processing of biomass at high pressure and temperature in presence of solid catalysts such as H-form zeolites, strong acid cation exchange resins, and solid metal phosphates (e.g. Zr(HPO4)2·H2O and Zr(PO4)(H2PO4)∙2H2O) often suffers from low product yields, long reaction times, and difficulty in the recovery of catalyst from solid residues (De et al., 2016; Kawashima et al., 2008; Parshetti et al., 2015). Also, due to the solid-solid mass transfer limitation, solid catalysts demonstrate poor efficacy in LA production, especially from cellulose and raw biomasses as feedstock. Therefore, homogeneous acids, such as HCl, H2SO4, H3PO4 and ionic liquids, have been the more popular catalysts for high LA yields (Brouwer et al., 2017; Liu et al., 2018; Shen et al., 2015). However, the processing of biomass with the mineral acids have several drawbacks, for example, low product selectivity, difficulty in handling, non-recyclability, corrosion effects, etc. Many times, they require co-catalysts, costly organic solvents, and electrolytes for the process efficacy (Li et al., 2014; Jeong et al., 2017). Lemongrass [Cymbopogon flexuosus (Steud.) Wats, (syn. Andropogon nardus var. flexuosus Hack; A. flexuosus Nees)], citronella grass [Cymbopogon winterianus Jowitt ex Bor (C. nardus var. mahapangiri auct.)] and palmarosa [Cymbopogon martini (Roxb.) Wats. var martinii (syn. C. martini Sapg var. motia)] are subtropical plants grown for their commercial essential oils in Guatemala, Brazil, China, India, Indonesia, Haiti, Madagascar, and other Eastern African countries. Extraction of volatile oil from these aromatic crops results in generation of spent aromatic waste. Approximately 30,000,000 tons per annum aromatic waste are generated worldwide from industrial processing of lemongrass alone (Kaur et al., 2010). India is a leading cultivator of aromatic crops (for example, lemongrass, citronella, palmarosa, patchouli, etc.), and generates approximately 6.0 million tons per annum of spent aromatic waste. Majority of this waste is used for burning purposes that adversely affects the human health (Dwivedi et al., 2018; Singh et al., 2017). Spent aromatic biomass (lemongrass, citronella grass, and palmarosa fibres) consists of lignocellulosic materials comprising of a high

2. Material and methods 2.1. Chemicals All the reactions were carried out in one-hour oven dried glassware at 100 °C. All reagents and solvents including 5-hydroxymethylfurfural (HMF), levulinic acid (LA), formic acid (FA), D-xylose, D-glycose, and hydrochloric acid (HCl) were purchased from Sigma Aldrich India as analytical grade and used as received without any modifications. 2.2. General experimental Thin layer chromatography (TLC) was performed on 60 F254 silica gel, pre-coated on aluminum plates and revealed with either a UV lamp (λmax = 254 nm) or a specific colour reagent (Draggendorff reagent or iodine vapours) or by spraying with methanolic-H2SO4 solution and subsequent charring by heating at 100 °C. Infrared spectra recorded as Nujol mulls in KBr plates. Monosaccharides, carboxylic acids, HMF, and levulinic acid were detected and quantified on HPLC (Agilent, Model 1260) using Agilent Hi-Plex H analytical column (300 mm length; 8 mm porosity). Nature of lignin was investigated using XRD (Panalytical X’PERT PRO) at scans of scattering angle (2 Theta) versus intensity from 10 to 90 Theta (step size = 0.001°) with a working voltage of 45 kV using a Cu Kα Ni-filtered radiation (λ = 1.5406 Å). Elements/ components in isolated lignin was analysed with EDS analyser (Model XFlash-6130, Bruker), with a 0–5 keV. To enhance the conductivity, a fine film of gold was sprayed on the samples prior to analysis. 2.3. Feedstock preparation and determination of fractional composition Spent aromatic biomass was collected from the hydro-distillation 2

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facility installed at agricultural farm, Center of Innovative and Applied Bioprocessing (CIAB), Mohali, Punjab, India. It was shade dried to a final moisture content of < 7% wt followed by grinding to a particle size ranging between 5 and 4 mm using a commercial grinder machine (Kinematica PX-MFC 90 D) and sieved to 0.5 mm (30 mesh) particle size. Standard National Renewable Energy Laboratory (NREL) protocol was followed to determine the primary composition of spent aromatic biomass, such as carbohydrate sugars, lignin, and ash.

phase of 5 mM H2SO4, flow rate of 0.7 mL per min, refractive index (RI) as detector at 55 °C, run time of 60 min. Quantitative estimations of xylose, glucose, arabinose, acetic acid, HMF, formic acid and levulinic acid in hydrolysate was made using a calibration curve drawn by plotting concentration of known standards versus peak area values from the RI chromatograms.

2.4. Production of p-cymene sulphonic acid (p-CSA) from citrus waste

The residual biomass was boiled in 2% aq. NaOH in a RB flask for 2 h at 100 °C. After time elapsed, the reaction mixture was cooled to room temperature followed by centrifugation at 8000 RPM for 20 min. The black liquid was separated and was subsequently neutralized with 0.5 N H2SO4 solution, wherein, lignin precipitated at pH 3.0. The precipitate was collected and washed with water followed by overnight drying at 50 °C in a hot air oven to afford lignin (7.6 mg, ~8% yield). Total lignin yield obtained was calculated using the following formulae.

2.8. Production of lignin from residual biomass

The reagent p-CSA was prepared in 10.0 g scale by processing of solid citrus (Citrus reticulate) fruit waste in accordance to the method reported by Dwivedi et al., 2018. 2.5. Pre-treatment of spent aromatic waste with p-CSA A glass media bottle (50 mL) was charged with a powdered spent aromatic biomass (1.0 g, moisture content < 7%), water (20 mL) and pCSA (0.6 g). The reaction mixture was placed in an autoclave and heated to 121 °C for 90 min. After time elapsed, the reaction mixture was cooled to room temperature followed by filtration. Residual biomass was collected and kept for drying in a hot air over at 50 °C for further use. Concentration of xylose in the filtrate (hydrolysate) was determine by HPLC analysis. The p-CSA from hydrolysate was recovered by washing with ethanol after concentration under reduced pressure. Dxylose was obtained as precipitate in excellent yields.

Lignin yield (wt%) =

Amount of isolated lignin × 100 Initial spent aromatic biomass (g)

3. Results and discussion This study has been carried out to explore the possibility for production of xylose, levulinic acid, and lignin from spent aromatic waste (lemongrass, citronella grass, and palmarosa fibres) by the application of p-CSA under an integrated chemical approach. The methodology begins with the extraction of citrus oil (~96% d-limonene) from citrus fruit processing waste, which as such creates environmental pollution, but can be effectively utilized as a renewable feed stock for the production of p-CSA (~88% yield based on d-limonene content of citrus oil). The conversion of d-limonene occurring in citrus oil to afford pcymene was carried out by chemical reduction over a pre-heated K-10 montmorillonite clay in the presence of Pd/C catalyst. The product pcymene was recovered from reaction mixture simply by distillation. Earlier, Dwivedi et al. (2018) reported a straight forward method to produce p-CSA by sulphonation of p-cymene with conc. H2SO4 at ambient temperature, wherein, p-CSA (~90% yield based on p-cymene) could be directly recovered from reaction mixture through the crystallization after adding water followed by cooling 4 h at ~5 °C. However, the approach recruited for the application of p-CSA on spent aromatic waste includes: (a) compositional analysis of spent aromatic biomass; (b) production of xylose from aromatic biomass; (c) production of levulinic acid from pre-treated biomass; (d) production of lignin from residual biomass via extraction under basic condition.

2.6. One pot production of levulinic acid from pre-treated spent aromatic biomass Pre-treated spent aromatic biomass (1.0 g), 2 N HCl (20.0 mL) and pCSA (1.0 g) were loaded in a thick-walled high-pressure glass reactor (Ace Glass, USA) of 120 mL capacity sealed (back) with silicone rubber. The glass reactor was set to an oil bath for 2 h heating at 180 °C with increments of 10 °C/min. After time elapsed, the sealed glass tube was removed from oil bath and subsequently cooled to room temperature by application of cold water. The reaction mixture was centrifuged at 8000 rpm for 20 min. Pellet was collected and stored for further use. Aqueous phase after dilution with ethanol was subjected to HPLC analysis for detection and quantification of levulinic acid. Product was purified from reaction mixture by simple distillation. 2.7. Quantitative HPLC analysis of spent aromatic biomass hydrolysate Xylose, glucose, arabinose, acetic acid, HMF, formic acid and levulinic acid in hydrolysate of spent aromatic biomass was quantified by High-performance liquid chromatography (HPLC, Agilent Technologies 1200 infinity series) using analytical standards of D-xylose, D-glucose, Darabinose, acetic acid, HMF, formic acid and levulinic acid under chromatographic conditions stated as: Agilent Hi-Plex H column (300 mm length; 8 µm porosity), RI detector operated at 60 °C, mobile

3.1. Compositional analysis of spent aromatic biomass The fractional composition of spent aromatic waste (lemongrass, citronella grass, and palmarosa fibres) before and after the pre-treatment with p-CSA was determined following the Standard National Renewable Energy Laboratory (NREL) protocol, the results are

Table 1 Fractional composition of spent aromatic biomass. SN

1 2 3

Spent aromatic biomass

Palmarosa Lemmongrass Citronella

Before pre-treatment

After pre-treatment **

Biomass (g)

Glucan (wt %)

Xylan* (wt %)

Lignin (wt %)

Ash (wt %)

Extractives (wt %)

Glucan (wt %)

Xylan* (wt %)

Lignin# (wt %)

Ash (wt %)

Yield (wt %)

1.0 1.0 1.0

34.79 35.86 33.65

21.77 18.84 18.71

20.58 19.48 22.94

7.30 7.21 6.42

13.06 14.53 14.80

44.29 43.31 40.81

< 1% < 2% < 2%

26.97 23.13 28.68

7.10 6.96 6.32

76.30 80.70 80.00

* Includes Xylose, Arabinose, Acetyl groups etc. ** Include both acid soluble and Klason lignin. # Acid insoluble lignin. 3

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the resulting hydrolysate may be used as substrate for various fermentative processes. Therefore, by minimizing the toxic compounds formed during hydrolysis, one can probably minimize the need for detoxification and at the same time make the process more economic. Thus, under the optimized reaction condition, a conical flask (50 mL) charged with dried and powdered spent aromatic waste (1.0 g), p-CSA (600 mg, 60% W/W) and water (20 mL) was autoclaved at 121 °C for 90 min followed by cooling to room temperature and filtration to afford a hydrolysate containing xylose (164 mg, ~76% yield with respect to xylan in spent aromatic waste; ~16% yield with respect to initial biomass). Concentration of hydrolysate under reduced pressure afforded a brown syrup, wherefrom, p-CSA could be recovered with a high efficacy via dissolving in ethanol, meanwhile, xylose precipitated. Repeated washing of precipitate with ethanol followed by a subsequent drying under hot air oven resulted in good yields of xylose. The reagent p-CSA was recovered and reused for > 3 times from hydrolysate without the significant loss in the activity. Once the selective hydrolysis of xylan polymer in palmarosa biomass using p-CSA was established, the methodology was further extended over to other spent aromatic waste e.g. lemongrass, citronella, etc. containing a similar gross primary composition, such as carbohydrate sugars, lignin, and ash. The results demonstrated an efficacious hydrolysis of xylan polymer in lemongrass and citronella biomass using p-CSA as a reagent (Fig. 1).

summarized in Table 1.

3.2. Production of xylose from aromatic biomass In general, the pre-treatment of biomass is done to alter hemicellulose and/or lignin, to increase the pore volume and the internal surface area, and decrease the degree of polymerization and crystallinity of lignocelluloses. Toluene sulfonic acid (p-TSA), an aromatic organic acid with eSO3H function exhibits a higher catalytic activity for hydrolysis of cellulosic materials in water. However, p-TSA is obtained by the sulphonation of toluene as an intermediate in the production of p-cresol, hence, it is petrochemical in origin. Due to prices and demand for limited oil resources continue to rise worldwide, pcymene sulphonic acid (p-CSA) synthesised from d-limonene as renewable feedstock from citrus waste appears a promising and sustainable alternative for the hydrolysis of lignocelluloses (Clark et al., 2012, 2013). Therefore, in a model reaction 1.0 g of dried and powdered palmarosa waste was treated with p-CSA (200 mg, 20% W/W) in water as solvent under autoclave condition. A pink colour hydrolysate obtained after filtration was subjected to HPLC analysis which demonstrated a selective hydrolysis of xylan polysaccharide occurring in biomass to xylose (74 mg, ~7% yield) while glucose, arabinose, acetic acid, etc. could be observed only in traces. In order to access the yield further, the hydrolysis of palmarosa waste was investigated with respect to the variation of reaction time and p-CSA loadings. Xylose concentration in hydrolysate showed a direct dependence on reaction time and loading of p-CSA. With the increase of p-CSA loading, the xylose yield was enhanced significantly. Similarly, an increase in reaction time from 60 to 90 min exerted a profound impact on xylose yields. The highest concentration of xylose from aromatic biomass was obtained with a condition of 121 °C temperature, 90 min reaction time at 60% W/W loading of p-CSA (Fig. 1). As evident from Table 2, p-CSA caused a selective and near complete (> 90%) hydrolysis of xylan polymer in palmarosa waste. The levels of the other side products (glucose, arabinose and CH3COOH) were greatly lower (< 1%) than those reported in the hydrolysis by using mineral acids (Xue et al., 2018). No lignin by-products could be observed in the hydrolysate, and the methodology ruled out the need of organic solvent pre-treatment prior to acid hydrolysis of biomass. Since, it has been prospected that the toxicity of the hemicellulosic hydrolysate is caused by aggregate effect of numerous compounds rather than a single toxic component,

3.3. Production of levulinic acid from pre-treated biomass Production of LA from biomass using mineral acids is generally carried out by refluxing under elevated temperatures (140–200 °C) because of the fact that a high temperature contributes to the acceleration of the reaction rate and conversion efficacy due to an ease in donation or acceptance of electrons during the process. Therefore, in a typical run, the pre-treated biomass (after extraction of xylose) was reacted with p-CSA and aq. HCl in a glass pressure reactor under stirring condition at 140 °C. After usual workup, the reaction mixture was centrifuged, and the aqueous phase was separated from pellet by decantation. HPLC analysis of aqueous phase established the formation of LA as product (6.4 mg, < 1% yield). In order to access the yield further, the reaction was studied with respect to the effect of temperature, HCl concentrations, and p-CSA loadings. It was observed that the liquid products of pre-treated biomass in presence of aq. HCl alone under the hydrothermal condition were many, with low yield and poor selectivity. These products included the monomers such as glucose, and the derivatives of monomers formed by the conversion of cellulose. Upon addition of p-CSA, a large quantity LA was formed with a lower amount of formic acid, and HMF after 2 h reaction time (Table 2). This indicated that the p-CSA promotes both the yield of, and selectivity to, the liquid products. The decrease in 5-HMF yields with increasing p-CSA loadings was attributed to its further degradation into levulinic acid and formic acid. Thus, the yield of levulinic acid, and formic acid all increased with increasing the p-CSA loadings. The amount of p-CSA notably influenced the yield of liquid products from cellulose conversion (Table 2). Yields of LA increased with increasing the amount of p-CSA until maximum to 1.0 equivalent (0.47 mol% with respect to biomass). However, a further increase in the amount of p-CSA, did not cause an increase in the LA concentration due to near complete degradation of 5-HMF to LA and formic acid. The concentration of HCl dramatically influenced the yield of LA. Increasing p-CSA loading at constant acid concentration resulted in a gradual increase of LA yields which soon raised to a maximum beyond which it could not be enhanced significantly unless the HCl concentration was increased. Yield of LA increased linearly with an increasing HCl concentration at constant p-CSA loading. However, formation of LA as product could not be observed significantly when HCl and p-CSA were used alone in the hydrothermal processing. Thus, a high yield of LA may be linked to the synergistic effect p-CSA and

Fig. 1. Comparative xylose yield from different spent aromatic biomass via processing with p-CSA as a reagent. Reaction conditions: Spent aromatic biomass (1.0 g), p-CSA (600 mg, 60% W/W) and water (20 mL) at 121 °C temperature for 90 min in an autoclave. 4

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Table 2 Reaction optimization for LA production from pre-treated palmarosa biomass via processing with p-CSA and aq. HCl under refluxing at 180 °C. Entrya

p-CSAb

HCl Concentrations 2 N HCl

1 N HCl

0.5 N HCl

Cellulose degradation products (wt%)

1 2 3 4 a b

0.125 (0.05) 0.25 (0.12) 0.5 (0.23) 1.0 (0.47)

LA

Gluc

FA

HMF

LA

Gluc

FA

HMF

LA

Gluc

FA

HMF

14.10 16.56 20.15 22.69

12.11 8.59 5.59 1.34

4.88 5.76 6.67 9.12

0.24 0.21 0.10 ND

4.79 7.64 9.75 12.42

15.39 17.31 16.63 13.86

1.99 3.09 4.04 4.25

ND 0.32 0.27 0.18

0.64 1.0 5.39 6.86

6.40 12.56 16.11 16.48

0.35 0.49 2.17 2.75

0.15 0.20 0.31 0.28

Solid-liquid ratio, 1:20 in all sets of reaction. Loading of p-CSA in grams (mol%); ND, not detected; LA, levulinic acid; Gluc, Glucose; FA, formic acid; HMF, 5-hydroxymethylfurfural.

generated by the interaction of hydrophobic p-cymene group with water molecules, a property that is mostly lacking in case of mineral acids, such as HCl and H2SO4, pushes p-CSA in close proximity to cellulose, resulting in a rapid degradation of cellulose to glucose. A subsequent isomerization of glucose generates fructose which on dehydration results into the formation of 5-hydroxymethylfurfural (5-HMF) as intermediate product that undergoes rehydration-induced ring cleavage to yield LA along with one equivalent of formic acid at a molecular level.

aqueous HCl. The effect of temperature on LA yield was also studied at constant pCSA loading and HCl concentration. Lowering of the processing temperature caused a rapid decrease in the LA yields. Maximum yield of LA from the biomass could be obtained by heating the reaction mixture at 180 °C temperature. A further increase in reaction temperature beyond 180 °C lowered the LA yield due to decomposition of HMF (in situ generated) to humins and unidentified soluble products. Moreover, no substantial increase in LA yield could be observed by increasing the reaction time beyond 2 h. Since, designed methodology shows high selectivity towards LA, the reaction predominantly produced LA while formic acid was obtained in a small quantity. These two products were soluble in the reaction media obtained after the reaction, and both have been shown as weight percentage to the starting material in Table 2. No black tar formation was established by dissolving the residue (solid left after the reaction) in methanol. Hence, neither black tar formation nor gasification could be observed in processing of the biomass using p-CSA and aq. HCl under the optimized reaction conditions. The highest concentration of LA from aromatic biomass was obtained with a condition of 180 °C temperature, 2 h reaction time, 1.0 equivalent loading of p-CSA, and 2 N HCl. The special hydrophobic and hydrophilic moieties in p-CSA may be responsible for a high LA yields. Once the methodology for production of LA from pre-treated palmarosa biomass was established, it was further extended over to the pre-treated biomass of lemongrass and citronella. The results demonstrated a selective and efficacious degradation of cellulose to LA in presence of p-CSA and aq. HCl (Fig. 2). A plausible mechanism involves the acid hydrolysis of polymeric cellulose into glucose. Strong repulsion

3.4. Production of lignin from residual biomass via extraction under basic condition The residual biomass was first washed with plenty of water to remove the traces of acids followed by drying at 50 °C in a hot air oven. The dried material was further boiled in aq. NaOH solution for 2 h. The reaction mixture was cooled to room temperature and centrifuged. The aqueous phase (brown liquid) was separated and neutralized with 0.5 N H2SO4 solution to pH 3.0 for precipitation of lignin. The precipitate was collected and washed with water followed by overnight drying at 50 °C in a hot air oven to afford lignin (7.6 mg, ~8% yield). The isolated lignin was further characterized by EDS, FT-IR, and XRD analysis. Elemental composition of palmarosa isolated lignin is determined by Energy-dispersive X-ray spectroscopy (EDS) analysis which shows elemental composition: C, 74.59%; O, 25.28%. FT-IR spectrum of the lignin exhibits similarity of absorption bands characteristic to standard lignin samples (Sigma Aldrich). Absorption bands at 3500 cm−1 indicates the phenolic and alcoholic groups. Absorption bands observed between 2800 and 2900 cm−1 show the presence of eCH2eH and > CeH stretching while bands observed between 1600 and 1780 cm−1 indicate the presence of carbonyl function. Bands at around 1400 cm−1 show the presence of aromatic ring. Absorption bands between 1300 and 1100 cm−1 show the presence of syringyl and guaiacyl residues while absorption peaks at around 800 cm−1 is established for CeH bending vibrations of aromatic rings. X-ray diffraction pattern of lignin shows a diffraction peak centred at 21.53°, a typical for standard pure lignin isolated from other biomass residues. The mass balance as well as carbon balance of product distribution from aromatic spent biomasses was significantly calculated based on the experiments performed in this paper. The material balance and carbon balance of each step for the designed protocol is shown in Table 3. The main products were xylose, levulinic acid, formic acid, glucose, and lignin. In this process, the considerable amount of lignin in the residues was degraded, and about 7–11 wt% of which was recovered effectively when the spent biomass conversion was approximately 80–90 wt%. This result also suggests that most of the lignin (about 14–18 wt%) was decomposed in the reaction process. About ~ 20–25% weight loss was attributed to the presence of solvent extractives as well as base soluble lignin degradation products. It is noticeable that the mass balance closures were not close to 100%

Fig. 2. Comparative yield of cellulose degradation products from spent aromatic biomass via processing with p-CSA/aq. HCl under refluxing at 180 °C. 5

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Table 3 Mass balance of products produced from spent aromatic biomass. Spent biomass

Input

Out put a

Palmarosa Lemmon grass Citronella grass a b c d e

Biomass (g)

Biomass conversion (wt %)

Liquefied products (wt%)

Residues (wt %)b

Lignin (wt %)

Residuec (wt%)

Carbon balance (wt %)d

Carbon balance (wt %)e

1.0 1.0 1.0

60.34 56.67 52.34

33.16 35.14 32.33

37.5 41.33 45.66

7.6 10.70 11.43

< 6.0 < 10.0 < 15.0

90.80 85.46 78.60

93.22 93.82 95.77

Biomass loading at 1.0 g p-CSA and 20 mL 2 N HCl. Residues obtained by degradation of cellulose. Residue remaining after extraction of lignin. Carbon balance after xylan hydrolysis. Carbon balance after degradation of cellulose.

because some products decomposed from the hemicelluloses and cellulose were not detected in HPLC analysis. In addition, some solids were adsorbed in the filter paper and funnel wall during filtration, therefore, could not be collected.

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4. Conclusions This study clearly demonstrated that the spent aromatic biomass, which as such have no high value applications and creates environmental pollution, can be effectively utilized as a renewable feedstock for production of valuable chemicals such as xylose, levulinic acid, and lignin. On sustainability point of view, the reaction system shows noticeable advantages such as non-corrosive, by using p-CSA rather than mineral acids; green reagent, by using p-CSA synthesised from d-limonene; recyclability of reaction system; and use of water as a solvent instead of the organic solvents. Thus, this approach would be promising and commercially more acceptable method for the production of xylose, levulinic acid and lignin from spent aromatic residues. Acknowledgment Support of Center of Innovative and Applied Bioprocessing (CIAB), Mohali (India) for carrying out this research work is sincerely acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.122105. References Brouwer, T., Blahusiak, M., Babic, K., Schuur, B., 2017. Reactive extraction and recovery of levulinic acid, formic acid and furfural from aqueous solutions containing sulphuric acid. Sep. Purif. Technol. 185, 186–195. Caes, B.R., Teixeira, R.E., Knapp, K.G., Raines, R.T., 2015. Biomass to furanics: renewable routes to chemicals and fuels. ACS Sustain. Chem. Eng. 3, 2591–2605. Chen, Y., Cao, X., Zhu, S., Tian, F., Xu, Y., Zhu, C., Dong, L., 2019. Synergistic hydrothermal liquefaction of wheat stalk with homogeneous and heterogeneous catalyst at low temperature. Bioresour. Technol. 278, 92–98. Clark, J.H., Farmer, T., Macquarrie, D.J., Sherwood, J., 2013. Using metrics and sustainability considerations to evaluate the use of bio-based and non-renewable Brønsted acidic ionic liquids to catalyse Fischer esterification reactions. Sustain. Chem. Process. 1, 1–13. Clark, J.H., Fitzpatrick, E.M., Macquarrie, D.J., Pfaltzgraff, L.A., Sherwood, J., 2012. pCymenesulphonic acid: an organic acid synthesised from citrus waste. Catal. Today 190, 144–149. De, S., Dutta, S., Saha, B., 2016. Critical design of heterogeneous catalysts for biomass valorization: current thrust and emerging prospects. Catal. Sci. Technol. 6, 7364–7385. Dwivedi, P., Singh, M., Sehra, N., Pandey, N., Sangwan, R.S., Mishra, B.B., 2018a. Processing of wet Kinnow mandarin (Citrus reticulata) fruit waste into novel Brønsted acidic ionic liquids and their application in hydrolysis of sucrose. Bioresour. Technol. 250, 621–624. Dwivedi, P., Singh, U., Jatav, S., Sangwan, R.S., Mishra, B.B., 2018b. Iodosylbenzene (PhIO) mediated synthesis of rose oxide from β-citronellol and its application for in

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