Fuel Processing Technology

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1 Fuel Processing Technology 128 (214) Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: Catalytic hydrolysis of cellulose and oil palm biomass in ionic liquid to reducing sugar for levulinic acid production Nur Aainaa Syahirah Ramli, Nor Aishah Saidina Amin Chemical Reaction Engineering Group (CREG), Energy Research Alliance, Faculty of Chemical Engineering, Universiti Teknologi Malaysia (UTM), Johor Bahru, 8131 Johor, Malaysia article info abstract Article history: Received 21 March 214 Received in revised form 8 August 214 Accepted 8 August 214 Available online 28 August 214 Keywords: Cellulose Oil palm biomass Ionic liquid Reducing sugar Fe/HY catalyst Levulinic acid Biomass is now regarded as a potential feedstock to produce renewable valuable chemicals that can be derived from sugar. In this study, it was demonstrated that Fe/HY catalyst was able to promote the hydrolysis in ionic liquid of oil palm biomass to reducing sugar without a prior pretreatment step. Initially, cellulose was utilized as a model compound and the effects of several variables including temperature, time, catalyst loading, cellulose loading, and 1-butyl-3-methylimidazolium bromide (BMIMBr) purity on the hydrolysis process were evaluated. A total reducing sugar (TRS) yield of.8% was obtained using Fe/HY catalyst in BMIMBr at 12 C within 3 h. Next, the same catalyst was applied for direct hydrolysis of oil palm frond (OPF) and empty fruit bunch (EFB). The TRS yields obtained were 27.4% and 24.8%, respectively, while the efficiencies were 54.6% and 58.5% for OPF and EFB, respectively. The catalyst, tested for five runs, exhibited a minimal loss in the catalytic activity signifying its potential recyclability ability. Further conversion of the cellulosic hydrolysate led to promising levulinic acid yield and process efficiency. The experimental results confirmed that Fe/HY catalyst and BMIMBr have the potential to be used in a lignocellulosic biorefinery at mild process conditions for processing renewable feedstocks. 214 Elsevier B.V. All rights reserved. 1. Introduction Concerns about increasing petroleum oil prices compel the chemical industry to explore alternatives to fossil resources for basic chemical productions [1]. Biomass is more favorable as compared to other resources since biomass does not compete with food chain [2]. The hydrolysis of cellulose, major component of biomass, to produce reducing sugars is important in the utilization of biomass in the chemical industrial processes. The reducing sugars can be converted into a wide range of valuable chemicals such as ethanol, 5-hydroxymethylfurfural, and levulinic acid [3 5]. Cellulose is a crystalline polymer of D-anhydroglucopyranose units bonded together in long chains by β-1,4-glycosidic bonds [5]. The extensive networks of inter- and intramolecular hydrogen bonding and Van der Waals interactions make cellulose insoluble in water and most conventional organic solvents. The insoluble property is the cause of biomass recalcitrant in the hydrolysis process. The cellulose dissolution is important to make cellulose more accessible for catalytic processes [6]. Biomass feedstocks have been used extensively in the productions of biofuels and other bio-based chemicals. Oil palm residues are the most abundant biomass wastes in Malaysia. Malaysia is one of the largest palm oil producers in the world with a large area of plantation around 5 million ha [7]. The oil palm industries in Malaysia alone generate Corresponding author. Tel.: ; fax: address: noraishah@cheme.utm.my (N.A.S. Amin). 3 million tons per year of palm oil residues in the form of empty fruit bunch (EFB), trunks and oil palm fronds (OPF). These biomasses contain a large amount of cellulose, which can be fractionated to release sugars that can be further converted to biobased products [8,9].Biobased products are produced in biorefinery that incorporates biomass conversion processes [1]. There are many ways to extract sugars from biomass materials including by the use of ionic liquids [11,12]. In recent times, efforts have been made to hydrolyse cellulose and biomass in ionic liquids [5, 13 15]. Among various types of ionic liquids, imidazolium ion based ionic liquids such as 1-butyl-3-methylimidazolium chloride, 1-butyl- 3-methylimidazolium bromide and 1-allyl-3-methylimidazolium chloride are well acknowledged for their cellulose dissolution competency [5,14,16]. In order to further enhance the hydrolysis of cellulosic biomass, homogeneous acid catalyst was utilized [17 2]. The system was effective but caused many problems such as equipment corrosion, environmental pollution, and acid recovery. Heterogeneous catalyst could overcome the problems associated with the hydrolysis of cellulosic biomass in the homogeneous system. Early studies on the hydrolysis of cellulose over heterogeneous catalysts have been reported in 28 [21 23] followed by more recent extensive studies [24 27]. Product separation is more convenient in a heterogeneous catalytic system and the catalyst can be recycled for repeated use [25 27]. Besides, thorough reviews on cellulose hydrolysis using both homogeneous and heterogeneous catalysts can be found elsewhere [28,29] / 214 Elsevier B.V. All rights reserved.

2 N.A.S. Ramli, N.A.S. Amin / Fuel Processing Technology 128 (214) Several studies have reported that metal salts can catalytically hydrolyse carbohydrates effectively into useful feedstock chemicals. Peng et al. have applied metal chlorides as catalyst for cellulose hydrolysis [3]. Earlier researchers have mostly used chromium chlorides as catalyst for the cellulose and biomass hydrolysis [1,31 33]. However, considering the toxicity and environmental pollution caused by chromium chlorides, we envisage that commercially available Lewis acid FeCl 3 is able to act as an eco-friendly catalyst for the catalytic hydrolysis of cellulose. Fe-zeolite based catalyst is expected to demonstrate a high catalytic activity for the hydrolysis of cellulose and biomass to sugars, with the additional possibility of being easy to be separated from the reaction products by impregnating the metal on a zeolite as support. Currently, extensive study on the conversion of sugars into biofuels and other feedstock chemicals [34] is being carried out. Among these explorations, one attractive approach is the production of levulinic acid. Levulinic acid is a versatile building block containing a ketone carbonyl group and an acidic carboxyl group, which can be used for the preparation of various high-value added organic chemicals, polymers, resin, flavor substances, and fuel additives with numerous potential industrial applications [35]. Some levulinic acid derivatives can be used as fuel additives. Esterification of levulinic acid with alcohols produces levulinic esters which can be used as diesel additives [36]. Levulinic acid can also undergo hydrogenation to produce γ- valerolactone (GVL), with a potential to be blended with gasoline as well as to serve as a precursor of polymers and fine chemicals [37]. In this study, the production of total reducing sugars (TRSs) through the cellulose hydrolysis process using Fe/HY as the solid catalyst has been demonstrated in 1-butyl-3-methylimidazolium bromide (BMIMBr) ionic liquid. The Fe/HY catalyst and BMIMBr are combined to afford a good performance for cellulose hydrolysis since Fe/HY is a solid catalyst with acidic sites and BMIMBr favors the cellulose dissolution. Fe/HY and parent HY zeolite were characterized to evaluate the effect of physicochemical properties of the catalysts on the hydrolysis process. The factors influencing the cellulose hydrolysis involving temperature, time, catalyst loading, cellulose loading, and BMIMBr purity were examined. Next, the potential of oil palm frond (OPF) and empty fruit bunch (EFB) for TRS production using the same catalyst were evaluated. The reusability and leaching of Fe/HY catalyst were also investigated. Subsequently, the production of levulinic acid was inspected from the cellulosic hydrolysate containing reducing sugars generated from cellulose, OPF, and EFB hydrolysis. Direct conversion of the feedstocks to levulinic acid was also tested for comparing and determining the role of ionic liquid in the hydrolysis process. The findings from this study are expected to improve the knowledge of catalytic transformation to renewable feedstocks. 2. Methods 2.1. Materials 1-Butyl-3-methylimidazolium bromide (BMIMBr), iron chloride (FeCl 3 ), levulinic acid, 5-hydroxymethylfurfural (5-HMF), formic acid, and furfural were purchased from Merck, Germany. NaY zeolite (SiO 2 /Al 2 O 3 = 5) was obtained from Zeolyst International. Cellulose was supplied by Sigma Aldrich, USA while NH 4 Cl was from Qrec, New Zealand. Meanwhile, OPF and EFB were supplied by Malaysia Palm Oil Board (MPOB), Kuala Lumpur, Malaysia Catalyst preparation The acidic form of NaY zeolite was prepared by ion exchanging the Na + from NaY zeolite with NH 4 Cl. NaY zeolite was stirred with 2 M of NH 4 Cl at room temperature for 2 h. The solution was washed with distilled water before being dried overnight at 12 C. The material was calcined at 5 C for 5 h resulting in the acidic form of zeolite, HY zeolite. The Fe/HY catalyst was prepared according to the wetness impregnation method. FeCl 3 aqueous solution (1 w/v%) and HY zeolite powder (1 g) were mixed and stirred at room temperature for 2 h and the mixture was dried overnight at 12 C. Finally, the Fe/HY catalyst was calcined at 5 C for 5 h Catalyst characterization As for catalyst characterization, the FTIR spectra were recorded using a Perkin Elmer equipment in the range of 4 4 cm 1. The morphology of the catalyst was evaluated using field emission scanning electron microscopy (FESEM, Hitachi SU82). Meanwhile, the catalyst crystallinity was analyzed by using a Bruker D8 advance diffractometer system (Cu Kα radiation, 4 kv, 3 ma) and the 2θ angle between 5 and 8. N 2 physisorption method was applied to determine the surface area and pore volume of the catalysts using a Micromeritics ASAP22 instrument. The acid properties of the catalyst samples were determined using temperature-programmed desorption (TPDRO 11 series, Thermo Finnigan) with ammonia as the probe molecule (NH 3 -TPD). Initially, a catalyst sample (5 mg) was degassed at 15 C for.5 h under constant nitrogen flow (2 ml/min). Then, the sample was cooled to 5 C and ammonia was adsorbed for 1 h. After saturation, nitrogen was purged for.5 h at 2 ml/min to remove excess ammonia on the catalyst surface. Finally, the catalyst sample was heated from 5 C to 8 C under constant helium flow (2 ml/min). The concentration of desorbed ammonia was quantified by a thermal conductivity detector (TCD) Procedure for hydrolysis An amount of.1 g of cellulose was dissolved in 2 g of BMIMBr by heating and stirring at 1 C until a transparent solution was formed. Next, distilled water and catalyst,.1 g each, were added to the solution. The reaction mixture was heated and stirred at a specified reaction temperature and time in a closed batch reactor in the oil bath. Product samples were diluted with 1 ml distilled water and filtrated before being subjected to TRS analysis. Randomly selected experimental runs were repeated to test the reproducibility of the data. The same steps were further applied to the hydrolysis of OPF and EFB Analysis of cellulose hydrolysis product The amount of TRS in the product sample was measured using the DNS method. 1 ml of supernatant from the product sample was mixed with 4 ml of DNS reagent. The resulting solution was heated in boiling water for 5 min. The absorbance of the mixture was measured at 54 nm using a UV vis spectrophotometer. The TRS concentration was calculated by using a calibration curve of the standard glucose solution. Meanwhile, the TRS yield, theoretical TRS yield and efficiency were calculated according to Eqs. (1), (2), and (3), respectively. TRS yield ð% amount of TRS amount of feedstock 1% cellulose content in biomass 1:11 Theoretical TRS yield ð% ð2þ amount of biomass where the value of 1.11 is the molecular weight of sugar divided by the molecular weight of cellulose (MW of sugar, C 6 H 12 O 6 = 18, MW of cellulose, C 6 H 1 O 5 =162). TRS efficiency ð% TRS yield theoretical TRS yield 1% ð1þ ð3þ

3 492 N.A.S. Ramli, N.A.S. Amin / Fuel Processing Technology 128 (214) where efficiency refers to the efficiency of biomass conversion to TRS based on the cellulose content. The byproducts from cellulose hydrolysis (levulinic acid, 5-HMF, formic acid, furfural) were detected using HPLC (Perkin Elmer Series 2) under the following conditions: column, Hi Plex H; flow rate,.6 ml/min; mobile phase, 5 mm H 2 SO 4 ; detector, UV 21 nm; retention time, min; column temperature, C; and injection volume, 2 μl. The concentration of the product was determined using the standard calibration curves of respective component. The product yields were calculated according to the following equation (Eq. (4)): Product yield ð% 2.6. Biomass feedstock characterization amount of product amount of initial cellulose 1%: The chemical compositions of OPF and EFB were determined by thermal gravimetric analysis (TGA) using a NETZSCH STA 449F3 instrument. The biomass samples were heated with a heating rate of 1 C/min under N 2 flow, from 3 to 7 C. TGA method has been proven as a method to determine biomass compositions [38,39]. The characterization of sugars and ash content in OPF and EFB was in accordance with the standard laboratory analytical procedures provided by the National Renewable Energy Laboratory (NREL) [4] Levulinic acid production The purified hydrolysate from cellulose, OPF, and EFB hydrolysis proceeded to be investigated for levulinic acid production by mixing each sample with.1 g catalyst, heated to 12 C for 3 h, and stirred at 2 rpm. The purified hydrolysate was obtained from the separation of residues and catalyst from the hydrolysis product. For comparison, direct conversion of the feedstocks to levulinic acid was conducted in an aqueous solution. The feedstock and catalyst at 1:1 ratio were mixed in 2 ml water, heated to 12 C for 6 h, and stirred at 2 rpm. The concentration of levulinic acid in the liquid product was determined by using HPLC (Perkin Elmer Series 2) under the following conditions: column, Hi Plex H; flow rate,.6 ml/min; mobile phase, 5 mm H 2 SO 4 ; detector, UV 21 nm; retention time, 25 min; column temperature, C; and injection volume, 2 μl. The concentration of the product was determined using the standard levulinic acid calibration curve. The levulinic acid yield, theoretical levulinic acid yield, and efficiency are calculated according to the following equations (Eqs. (5) (7)): Levulinic acid yield ð% amount of levulinic acid 1% amount of feedstock where the amount of feedstock refers to the amount of TRS from the hydrolysis or the amount of initial feedstock. Theoretical levulinic acid yield ð% Þ cellulose content in biomass :71 ¼ amount of biomass where the value of.71 is the molecular weight of levulinic acid divided by the molecular weight of cellulose (MW of levulinic acid, C 5 H 8 O 3 = 116, MW of cellulose, C 6 H 1 O 5 = 162). Levulinic acid efficiency ð% levulinic acid yield theoretical levulinic acid yield 1% where efficiency refers to the efficiency of biomass conversion to levulinic acid based on the cellulose content. ð4þ ð5þ ð6þ ð7þ 3. Results and discussion 3.1. Fe/HY catalyst characterization The FTIR spectra of the catalysts confirmed the vibration of lattice structure and zeolite framework in the range of and 4 3 cm 1 due to the external linkage of the HY catalyst (Fig. S1). The morphology of Fe/HY and the influence of Fe species impregnation on the HY zeolite structure can be evaluated by the FESEM images (Fig. S2). The FESEM images exhibited uniformly distributed small crystallites. It is demonstrated that the morphology of catalyst did not change significantly with Fe impregnation, indicating that no significant crystalline transformation occurred during the modification. The XRD patterns of HY and Fe/HY catalysts are shown in Fig. S3. The peaks observed from the Fe/HY catalyst XRD pattern matched with the parent HY, which indicates that the impregnation of Fe has no obvious effect on the zeolite structure. Thus, the FAU structure remained intact for the Fe/HY catalyst. The crystallite sizes of HY and Fe/HY catalysts, estimated at the highest diffraction peaks, are 43.6 nm and 21.6 nm, respectively. The crystallite size decreased after the impregnation of Fe species on HY zeolite. Similar phenomena were reported when different metals were introduced on porous materials [41 43]. The less intense Fe/HY XRD peak compared to HY zeolite gave an indication of low crystallinity. Impregnation of Fe on Na-A zeolite also led to a decrease in zeolite crystallinity [44]. It is reported that low crystallinity reduced the crystallite size [45]. Partial interaction between Fe species and zeolite framework from the incorporation of Fe to zeolite lattice structure which occurred during the impregnation could probably cause the slight decrease of Fe/HY crystallinity, thus decreasing the crystallite size. Further characterizations were conducted to determine the porosity and acidity of the catalysts (Table 1). The surface area and pore volume of Fe/HY catalyst were smaller than HY zeolite due to the existence of a larger constituent inside the pores after the impregnation process. The surface area and pore volume of Fe/HY catalyst are m 2 /g and.15 cm 3 /g respectively, while the surface area and pore volume of HY zeolite are m 2 /g and.37 cm 3 /g respectively. These effects might be attributed to the presence of iron oxide and extra framework aluminum species in the zeolite pores and channels. Y type zeolite consisted of microporous and mesoporous structures. Although the Fe species could have blocked the micropores, the average pore diameter (4 V/A) of Fe/HY catalyst was slightly larger since there were still excess mesopores available [46]. The t-plot method (Fig. S4) exhibits the reduction of the micropore volume after impregnation. The average pore diameter (4 V/A) of HY zeolite and Fe/HY catalyst was sufficient for the reactant to penetrate into the active sites since the ionic liquid media have dissolved the cellulose in order to enhance the hydrolysis process. On the other hand, the strength of the catalyst acidity increased from 1.58 mmol/g to 4.99 mmol/g, after FeCl 3 was impregnated into the HY zeolite. The NH 3 -TPD profiles of HY zeolite and Fe/HY catalysts (Fig. 1) exhibit two desorption peaks that existed at low and moderate temperatures. The desorption peak at higher temperature indicated the stronger acidic sites in the Fe/HY zeolite catalyst sample. Besides, the acid site density of the catalyst also increased from 4.27 mmol/cm 3 to mmol/cm 3 after metal impregnation signifying that the interaction between the two compounds has remarkably increased the catalyst Table 1 Physico-chemical properties of Fe/HY and parent HY zeolite catalyst. Properties HY zeolite Fe/HY zeolite BET surface area, m 2 /g Total pore volume, cm 3 /g Average pore diameter (4 V/A), nm Acidity, mmol/g Acid sites density, mmol/cm

4 N.A.S. Ramli, N.A.S. Amin / Fuel Processing Technology 128 (214) Intensity (a.u) HY zeolite Fe/HY zeolite Temperature ( o C) Fig. 1. NH 3 -TPD profiles of HY zeolite and Fe/HY zeolite catalysts. acidic sites. The combination of acidic FeCl 3 with high surface area HY zeolite would synergize the cellulose hydrolysis Effect of catalyst properties on cellulose hydrolysis Table 2 compares the performances of various solid catalysts in ionic liquids for cellulose hydrolysis in producing reducing sugar. Other than Fe/HY catalyst, FeCl 3 and HY zeolite catalysts were also tested to evaluate the effect of physico-chemical properties of the catalysts on cellulose hydrolysis. In the observation, higher TRS yield was produced by using Fe/HY catalyst (.8%) compared to HY zeolite (3.1%). On the other hand, FeCl 3 gave a slightly higher TRS yield (64.1%) than Fe/HY catalyst (.8%). Although the highest TRS yield was observed over FeCl 3, but being homogeneous, separation and recycling of this catalyst face many challenges. Impregnation of FeCl 3 with HY zeolite, with a larger surface area, may overcome the drawback. From NH 3 -TPD analysis, Fe/HY has a higher acidity compared to HY zeolite. The increase in acidity is attributed to the presence of Fe species [44]. Fe/HY catalyst performed comparably with FeCl 3 as catalyst with respect to TRS yield. Meanwhile, as acidity increased, the TRS yield doubled over Fe/HY compared to HY zeolite. From the results, it is surmised that cellulose hydrolysis over Fe/HY in BMIMBr is predominantly influenced by the catalyst acidity while the surface area derived from HY zeolite provided the catalytic sites. Previously, Cai and co-workers have focused on the application of few types of zeolites on cellulose depolymerization in BMIMCl [47]. It is found that HY zeolite with the largest pore size favored cellulose hydrolysis compared to the other zeolites (HZSM, Hβ, and SAPO 34) [47]. Besides, it has been verified that HY zeolite demonstrated a significant enhanced catalytic activity in BMIMCl compared to its activity in water [47]. The other solid catalyst, Amberlyst 15, previously used for cellulose hydrolysis in BMIMCl, reported 28% of TRS yield [48]. Amberlyst 15 works as a homogeneous catalyst in BMIMCl by releasing H + into the aqueous solution due to the ion exchange process [43].Asa consequence, the low activity exhibited by Amberlyst 15 for the TRS production was related to the slow release of H 3 O + which affected the cellulose dissolution in BMIMCl [49]. Hsu et al. have reported a production of 24% TRS yield from cellulose using EMIMCl ionic liquid as both catalyst and solvent [5]. Their results supported the notion that the catalyst used in this present study enhanced the cellulose hydrolysis in ionic liquid. The effect of different types of ionic liquids should be evaluated since different ionic liquids may have different interactions with the catalyst used. Rinaldi et al. reported that the hydrolysis of glycosidic linkages is catalyzed by the solid acid surfaces [48]. Thus, it is suggested that the solid catalyst for cellulose hydrolysis in ionic liquids should be an acidic material with a large surface area, since the viscosity of cellulose in ionic liquid solution makes the transport of cellulose chains to the catalytic sites a challenging process [48]. In another development, Cai et al. proposed that the performance of cellulose hydrolysis using HY zeolite catalyst in ionic liquid was due to the ion exchange between ionic liquid and the catalyst to release H + [47]. In this study, the high efficiency of Fe/HY BMIMBr system for cellulose hydrolysis was attributed to the synergistic effect of the catalyst and ionic liquid. The complete dissolution of cellulose using BMIMBr left the cellulose chains more accessible to catalytic transformation. The transport of the dissolved cellulose to the catalytic active sites was facilitated by the large surface area and pore diameter of the Fe/HY solid acid catalyst. Besides, the release of H + could also have occurred from the ion exchange of ionic liquid and zeolite. All the three steps, cellulose dissolution by ionic liquid, transportation of dissolved cellulose to catalytic sites, and generation of H + from zeolite effectively enhanced the overall hydrolysis process Effect of parameters Effect of reaction temperature and reaction time Reaction temperature and reaction time are the important factors that affected the destruction of hydrogen bond in cellulose network. The hydrolysis of cellulose catalyzed by Fe/HY catalyst in BMIMBr was examined at different reaction temperatures and times (Fig. 2a). At 8 C, the hydrolysis rate was slow and TRS yield was kept nearly constant at 17%, suggesting that this reaction temperature was not suitable for the cellulose hydrolysis. The TRS yield amplified with increasing reaction temperature from 8 to 12 C. The highest TRS yield was obtained at 12 C for 3 h of hydrolysis with a.8% yield. It is suggested that the cellulose hydrolysis reaction was extended at a higher temperature to overcome the activation energy of the reaction [51]. After reaching the maximum yield, the TRS yield slowly reduced from the fourth to the fifth hour. The decrease of the TRS yield with prolonged reaction time seemed to be related to the conversion of the formed sugars, which was promoted by the catalyst in the ionic liquid [24]. The TRS yield also declined at 14 C indicating that the TRS product formed was not stable at higher temperature and degraded into other byproducts. From the results, it can be seen that reaction temperature has a significant effect on cellulose hydrolysis. On the other hand, reaction time did not give a momentous effect on the TRS yield, since some of the sugars were converted to other byproduct compounds, such as levulinic acid, 5-hydroxymethylfurfural (5-HMF), formic acid, and furfural. The distribution of by-products from the transformation of Table 2 Cellulose hydrolysis for reducing sugar production in the presence of various solid catalysts in ionic liquids. Catalyst Reaction temperature ( C) Reaction time (h) TRS yield (%) Reference Fe/HY in BMIMBr This study FeCl 3 in BMIMBr This study HY zeolite in BMIMBr This study HY zeolite in BMIMCl [47] HZSM-5 zeolite in BMIMCl [47] H-Beta zeolite in BMIMCl [47] SAPO 34 in BMIMCl [47] Amberlyst 15 in BMIMCl [48] EMIMCl [5]

5 494 N.A.S. Ramli, N.A.S. Amin / Fuel Processing Technology 128 (214) TRS, detected using HPLC, is depicted in Fig. 2b for 12 and 14 C reaction temperatures from 1 to 5 h reaction times. Only the byproduct yields corresponding to the two temperatures are provided since the TRS yields (Fig. 2a) are higher compared to 8 and 1 C. Nevertheless, the same trends in the byproduct distribution could still be observed at the lower temperatures. The same phenomenon was also reported by Cai and co-workers for the catalytic conversion of cellulose by zeolites in BMIMCl where other soluble products, such as fructose, formic acid and levulinic acid were found in the reaction mixture [47]. Elevated reaction temperature not only enhanced the reaction rate of cellulose conversion but also appeared to facilitate the side reactions. These reaction products were in accordance with the cellulose hydrolysis path. Pertaining to the reaction rate and TRS yield from cellulose hydrolysis, the following experiments were set at 12 C for 3 h Catalyst loading (g) Effect of catalyst loading Fig. 3 shows the effect of catalyst loading, varied from to.2 g on the TRS yield. It is obvious that catalyst loading significantly affected the cellulose hydrolysis. Few factors could have attributed to this condition. In the absence of catalyst, a TRS yield of 2.8% was obtained at 12 C for 3 h. Guo et al. have suggested that [H + ], from the dissociation of water, might have catalyzed cellulose hydrolysis to a certain extent [14]. Besides, acid derived from the ionic liquid used might also influenced the reaction, as has been discussed by Gazit and Katz [52]. The anion of ionic liquid such as halides, can assist the proton abstraction from the acidic spot of the imidazolium part, thus forming an acid which catalyze the hydrolysis process [52,53]. Product yield (wt%) Reaction time (h) Furfural Formic acid 5-HMF Levulinic acid (a) 8 C 1 C 12 C 14 C 1h 2h 3h 4h 5h 1h 2h 3h 4h 5h 12 C 14 C Reaction temperature ( C) - time (h) (b) Fig. 2. (a) Effect of reaction temperature and reaction time on TRS yield from cellulose hydrolysis catalyzed with Fe/HY catalyst in BMIMBr (.1 g cellulose, 2 g BMIMBr,.1 g water,.1 g catalyst); (b) yield of byproducts from cellulose hydrolysis at reaction temperatures 12 and 14 C for 1 to 5 h reaction times. Fig. 3. Effect of catalyst loading on TRS yield from cellulose hydrolysis catalyzed with Fe/HY catalyst in BMIMBr (.1 g cellulose, 2 g BMIMBr,.1 g water, 12 C, 3 h). After the addition of.5 g and.1 g of Fe/HY catalyst, the TRS yield increased to 5.8% and.8%, respectively, which implied that there were sufficient catalytic active sites available for the hydrolysis of cellulose substrate in the system under the experimental conditions. However, additional increment in catalyst dosage to.15 g and.2 g resulted in decreasing TRS yield to 54.6% and 43.4%, respectively. This is perhaps due to the accelerated degradation of the reducing sugar to byproducts caused by the excessive active sites in the system. The reducing sugar formed may have further converted into 5 hydroxymethylfurfural, furfural, levulinic acid, formic acid, and humin as has been similarly reported [54]. Obviously, there is an optimum catalyst loading to achieve the maximum TRS yield. The effect of catalyst loading on cellulose hydrolysis infers that total acidity derived from catalyst is one of the main factors influencing the reaction rate. The same trend has also been reported previously by using HY zeolite in BMIMCl ionic liquid. The higher loading of solid acid affected the cellulose hydrolysis by enhancing the reaction rate [47] Effect of cellulose loading The profile of TRS yield with cellulose loading is presented in Fig. 4. The effect of this parameter was investigated because process economics and environmental friendliness can be improved if higher cellulose dosage was used with a constant amount of ionic liquid and catalyst. From the result, increasing cellulose content from.1 g to.15 g did not decrease the TRS yield significantly. This shows that there is an excess reactive catalyst left from the hydrolysis of.1 g cellulose. On the other hand, the TRS yield decreased from.8% to 52.4% with increasing cellulose dosage from.1 g to.3 g. The higher cellulose loading would increase the probability of the reactive compounds colliding Cellulose dosage (g) Fig. 4. Effect of cellulose dosage on TRS yield from cellulose hydrolysis catalyzed with Fe/HY catalyst in BMIMBr (2 g BMIMBr,.1 g water,.1 g catalyst, 12 C, 3 h).

6 N.A.S. Ramli, N.A.S. Amin / Fuel Processing Technology 128 (214) with each other and caused cross polymerization to produce undesired products. The other reason might be due to insufficient catalyst for the additional cellulose loading. Hence, the optimal catalyst and cellulose loading for maximum TRS yield have to be scrutinized from the economical aspect. Other than cellulose loading, cellulose crystallinity might also affect the hydrolysis process. Shimizu et al. have reported the effect of cellulose crystallinity on the catalytic activity of H 3 PW 12 O 4 as a representative catalyst [55]. The cellulose crystallinity decreased after two days of ball milling treatment, thus increasing the TRS yield from 5% to 17%. In the present study, it is proved that no cellulose pretreatment is needed for the hydrolysis process using Fe/HY catalyst in BMIMBr ionic liquid Effect of water percentage in ionic liquid After completion of cellulose dissolution in BMIMBr, the β-1,4- glycosidic bonds of the cellulose dissolved can be easily attacked by the active sites of Fe/HY catalyst, which promotes the hydrolysis process. The effect of BMIMBr impurities on cellulose hydrolysis was examined in the presence of water. Liu et al. have verified that water can weaken the dissolution capability of the ionic liquid [56].However, Guo and team demonstrated that water is a necessary reactant in cellulose hydrolysis in ionic liquid, but an excess amount will also affect the ionic liquid dissolution properties [14]. In this study, it was found that the water addition in BMIMBr had impacted the hydrolysis process. As shown in Fig. 5, with the addition of water (1 wt.%) in BMIMBr, the TRS yield slightly increased from.8% to 61.2%. The explanation of this behavior is that water lowered the viscosity of ionic liquid and at the same time promoted the hydrolysis reaction and the mass transfer [14]. However, the TRS yield reduced significantly when 1 wt.% of water was added to BMIMBr. This is probably due to the opposition between ionic liquid and water to form hydrogen bonding with the cellulose, which imposed limit on the possible reaction [56]. Theother reason for the decrease in TRS yield with water addition is that excessive water can precipitate the dissolved cellulose from the ionic liquids, thus making the homogeneous hydrolysis of cellulose in ionic liquid nearly impossible [47]. Meanwhile, water was gradually added to cellulose depolymerization using HY zeolite in BMIMCl by Cai et al. [47]. This method proved that the amount of water added and the timing were crucial for cellulose hydrolysis in ionic liquid Determination of oil palm biomass composition Table 3 Composition of oil palm biomass. Components Composition (wt.%) Oil palm frond Empty fruit bunch Moisture content Cellulose Hemicellulose Lignin Ash content stages of weight losses. Initial weight losses were observed between 3 and 1 C, which can be regarded as the evaporation of residual water in the biomass samples. The second stage at a temperature range of C is the decomposition of holocellulose. Meanwhile, the degradation of lignin occurred at C. The residue is defined as the ash content. The temperature ranges are aligned with a previous TGA study of biomass composition [4,57]. The cellulose and hemicellulose contents in OPF and EFB were analyzed using an acid hydrolysis procedure according to the NREL method [4]. In this method, cellulose and hemicellulose in biomass were degraded to glucose and xylose subunits, respectively. From that basis, it was assumed that the glucose and xylose compositions represented the cellulose and hemicellulose in the biomass sample. From Table 3, OPF contains 45.2% and 2.3% of cellulose and hemicellulose, respectively, while EFB contains 38.2% and 23.4% of cellulose and hemicellulose, respectively. From these analyses, OPF contains higher holocellulose content and lower lignin content compared to EFB Utilization of lignocellulosic biomass The potential of OPF and EFB to produce TRS at mild conditions in BMIMBr is shown in Table 4. With Fe/HY as catalyst, the cellulosic hydrolysate containing 27.4% and 24.8% of TRS was produced from the hydrolysis of OPF and EFB, respectively. Higher TRS yield can be obtained from biomass with higher cellulose content. The TRS yield from biomass was less than that from cellulose since cellulose in the biomass sample is less accessible to the catalyst. Higher lignin content in the biomass also contributed to lower TRS yield, as lignin is naturally recalcitrant to chemical processes. Lignin is linked by covalent bonds and cross-linked with hemicellulose and cellulose, respectively, which The composition of OPF and EFB was determined using TGA and acid hydrolysis process (Table 3). Fig. 6 illustrates the thermal degradation curves of holocellulose and lignin in the biomass with three distinct Water percentage in ionic liquid (wt%) Fig. 5. Effect of water percentage in ionic liquid on TRS yield from cellulose hydrolysis catalyzed with Fe/HY catalyst in BMIMBr (.1 g cellulose, 2 g BMIMBr,.1 g water,.1 g catalyst, 12 C, 3 h). Weight (wt%) H 2 O Cellulose + hemicellulose Lignin Ash EFB H 2 O Cellulose + hemicellulose OPF Lignin Ash Temperature ( o C) Fig. 6. TG analysis of oil palm biomass.

7 496 N.A.S. Ramli, N.A.S. Amin / Fuel Processing Technology 128 (214) Table 4 Yield of TRS from various biomass feedstocks and catalysts. Biomass Cellulose content (wt.%) Catalyst/enzyme Reaction temperature ( C) Reaction time (h) TRS yield (%) Oil palm fronds 45.2 Fe/HY Empty fruit bunches 38.2 Fe/HY Cryptomeria japonica (wood) a 43. Dowex 5WX Spruce wood b n.a. Amberlyst 15DRY Palm stem c 42.6 Amberlyst IR Chlorella (algae) d 35.3 HCl Sugarcane bagasse e 62.6 Cellulase a Hydrolysis in BMIMCl, [5]. b Hydrolysis in BMIMCl, [48]. c Hydrolysis in BMIMCl, [58]. d Pretreated in EMIMCl for 3 h, [59]. e Biomass was delignified and pretreated in EMIM-DEP for 3 h, []. hindered the hydrolysis process. The TRS theoretical yields from OPF and EFB can be calculated based on the cellulose content. Theoretically, 5.2% and 42.4% TRS yield could be obtained from the hydrolysis of OPF and EFB, respectively. Meanwhile in comparison with the theoretical value, the efficiencies have been accounted as 54.6% for OPF and 58.5% for EFB. Similarly with cellulose, other parallel reactions could have also occurred in the system producing other by-products such as levulinic acid, formic acid, and 5-hydroxymethylfurfural. Thus, 1% efficiency of the hydrolysis process to sugars could not be obtained in this study. From Table 4, it is obvious the TRS yields obtained in this study were higher and comparable with the yield reported using other biomass resources and solid catalysts in ionic liquid [5,48,58]. Some of the biomass hydrolysis processes have also been carried out using mineral acid and enzyme [59,]. Although TRS yield was up to 8% in the presence of acid or enzyme, the technology needed biomass pretreatment [,61]. In addition, the longer reaction time and higher cost for the enzymatic hydrolysis restricted large scale application. Moreover, the use of mineral acid in biomass hydrolysis raised drawbacks since the catalyst cannot be recycled for repeated use and the mineral acid can cause serious problem to the environment. Considering all these aspects, solid acid hydrolysis is preferred for the production of reducing sugars Catalyst recycle Catalyst recycle is always an important aspect in the solid acid catalytic system. To evaluate the performance of the former, cellulose, OPF, and EFB were hydrolyzed for five successive runs at 12 C for 3 h, using.1 g feedstock, and 2 g BMIMBr, and.1 g Fe/HY (for the first cycle). After each catalytic cycle, the Fe/HY catalyst was recovered and washed Cellulose OPF EFB Recycle times Fig. 7. Recycle of Fe/HY catalyst for cellulose, OPF, and EFB hydrolysis to TRS (.1 g feedstocks, 2 g BMIMBr, 12 C, 3 h). with water. The catalyst was dried overnight at 12 C, calcined at 5 C for 5 h to remove adsorbed by-products, and returned to the subsequent cycles. For the subsequent cycles, the reaction was conducted at 12 C for 3 h, using.1 g feedstock, 2 g BMIMBr, and the remaining catalyst from the previous cycle. The recycling performance of the Fe/HY catalyst was conducted for five times to study its activity (Fig. 7). The trend indicated that Fe/HY catalyst was still active in the next recycle run, although the TRS yield gradually decreased after the 5th cycle, from 61% to 46% for cellulose, from 27% to 21% for OPF, and from 24% to 19% for EFB conversion, respectively. The decrease in the TRS yield with the number of recycling is due to the amount of remaining catalyst after each recycle process. As the amount of catalyst receded during the filtration steps, the number of active sites available for the hydrolysis process also reduced. It has been demonstrated that ionic liquid entering the zeolite pores could be removed by simple calcination [47]. Since the Fe/HY catalyst was calcined after each run, pores blocked by ionic liquid, humins and other compounds should not be the reason accounting for the activity loss. The separation of sugars from ionic liquid is still a challenge. After the catalyst was separated from the ionic liquid, the produced monosaccharides can be further converted into other value-added products such as 5-hydroxymethylfurfural and levulinic acid. These products can be separated from the ionic liquid system by liquid liquid extraction [62]. To study the possibility of Fe leaching from the catalyst, a few samples were subjected to AAS analysis. The amount of Fe ions was found to be less than 1% from the initial Fe. This indicates that the cellulose hydrolysis is mainly due to Fe present on the catalyst surface rather than the trace amount of leached Fe ions Levulinic acid production The hydrolysates of cellulose, OPF and EFB hydrolysis were further utilized for levulinic acid production in the presence of Fe/HY catalyst (Table 5). Levulinic acid is considered as one of the valuable chemicals due to its potential as an important basic chemical. At mild reaction temperature, the conversion of cellulosic hydrolysate from cellulose to levulinic acid gives a promising yield, which is up to 72%. The cellulosic hydrolysate from OPF and EFB also dehydrated to a high and comparable levulinic acid yield. Small difference between the levulinic acid yields from the cellulosic hydrolysates infers that BMIMBr and Fe/HY catalyst give the same effect on the dehydration reaction. Besides, the high yield of levulinic acid obtained from the dehydration of cellulosic hydrolysates may be attributed to the presence of levulinic acid that may have been produced from the hydrolysis of the feedstocks before. It is also suggested in previous study that monosaccharides formed during the ionic liquid hydrolysis are consumed in subsequent reactions, such as dehydration reaction to form HMF and levulinic acid [63],indicating that the reaction proceeds by a cellulose sugars HMF levulinic acid sequence. For comparison, cellulose, OPF, and EFB were directly converted to levulinic acid using the same Fe/HY catalyst but in an aqueous solution. The levulinic acid yield from OPF and EFB

8 N.A.S. Ramli, N.A.S. Amin / Fuel Processing Technology 128 (214) Table 5 Levulinic acid production from TRS, cellulose, OPF, and EFB. Feedstock Reaction conditions c Levulinic acid yield (%) Levulinic acid efficiency (%) f Temperature ( C) Time (h) From hydrolysate d From feedstock d Theoretical e Cellulose a OPF a EFB a Cellulose b OPF b EFB b a Feedstocks were hydrolyzed to produce TRS before subjected to levulinic acid production. b Feedstocks were directly subjected to levulinic acid production in an aqueous solution. c Reaction condition for levulinic acid production. d Calculated using Eq. (4), cellulosic hydrolysate with reducing sugar yields of.8%, 27.4% and 24.8%, for cellulose, OPF, and EFB, respectively. e Calculated using Eq. (5). f Calculated using Eq. (6). using Fe/HY catalyst in an aqueous solution was much less. The recalcitrance of lignocellulosic biomass has hindered the hydrolysis process to convert biomass to levulinic acid. The levulinic acid yield was doubled in a two-step reaction using ionic liquid compared to a one step reaction. The same trend has also been reported previously by Yang et al., for a two-step reaction of cotton straw [64]. Products from the first OPF and EFB hydrolysis step contained monosaccharides and residues, e.g., lignin and extractives. The residues may limit efficient utilization of the hydrolysates for levulinic acid production by the dehydration reaction. Therefore, in the two- step process, the second step was conducted after the separation of residues from the hydrolysate containing reducing sugars. The theoretical levulinic acid yield from OPF and EFB can be calculated based on the cellulose content. The yield and the efficiency of the process are tabulated in Table 5. The use of BMIMBr in the hydrolysis has increased the overall process efficiency by 23% and 3% for OPF and EFB, respectively. Moreover, less time was needed for the reaction in the presence of ionic liquid. From the results, it can be deduced that the application of BMIMBr ionic liquid has assisted the overall reaction by weakening the glycosidic bonds in the cellulose structure and dissolving the cellulose compound at the same time. 4. Conclusions The combination of Fe/HY zeolites with BMIMBr were exhibited as efficient systems for the hydrolysis of cellulose under mild condition without a pretreatment step. Experimental results demonstrated the physico-chemical properties of Fe/HY catalyst and process variables including reaction temperature, reaction time, catalyst loading, cellulose loading, and BMIMBr purity affected the hydrolysis reaction for TRS formation. With cellulose, OPF and EFB, the TRS yields are.8%, 27.4% and 24.8%, respectively, at 12 C for 3 h reaction time. The solid acid catalyst used can be recycled up to five runs, with gradual decrease in the TRS yield. The leaching effect of the catalyst was also low. The levulinic acid yield calculated based on the cellulosic hydrolysate containing reducing sugar from the hydrolysis of cellulose, OPF and EFB are 72%, 68.2% and 71.5%, respectively. In comparison to the conversion of OPF and EFB in an aqueous solution, the use of BMIMBr has increased the efficiency of the biomass feedstock conversion to levulinic acid by 2 to 3%. The overall results succinctly indicate that these largely available renewable agricultural residues are susceptible to be hydrolyzed over Fe/HY catalyst and BMIMBr for further conversion to a wide range of valuable chemicals. Acknowledgment The authors would like to express their sincere gratitude for the financial support received from Universiti Teknologi Malaysia under the Research University Grant (RUG) vote number 2H75 as well as to the Ministry of Higher Education (MOHE) for sponsoring one of the authors, (N.A.S.R.) under MyBrain15. Appendix A. 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