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Production of Liquefied Oil Palm Empty Fruit Bunch (EFB) based Polyols via Microwave Heating Umar Adli Amran, Sarani Zakaria, Chin Hua Chia, Zhen Fang, and Mohamad Zulfahdli Masli Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02098 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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Production of Liquefied Oil Palm Empty Fruit Bunch (EFB) based Polyols via Microwave Heating Umar Adli Amran†, Sarani Zakaria*†, Chin Hua Chia†, Zhen Fang*‡, & Mohamad Zulfahdli Masli† †

Bioresources and Biorefinery Laboratory, School of Applied Physics, Faculty of Science and

Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia ‡

Biomass Group, College of Engineering, Nanjing Agricultural University, 40 Dianjingtai Road,

Nanjing, Jiangsu 210031, China

ABSTRACT: Optimization of microwave-assisted liquefaction of oil palm empty fruit bunch fiber (EFB) and EFB cellulose (EFBC) in ethylene glycol (EG) were carried out to produce polyols. The liquefaction residues and hydroxyl numbers of resultant polyols from respective sources were studied and compared. EFB produced minimum residue of 3.22% at the optimal parameters of 160°C and 15 min. Meanwhile optimum liquefaction of EFBC produced 1.03% residue at 175°C and 40 min. The maximum hydroxyl numbers of both EFB (749.22 mgKOH/g) and EFBC (639.91 mgKOH/g) polyols were obtained at optimum conditions. FTIR analysis revealed the degradation mechanism of cellulose and lignin in EFB at different temperatures. Lignin was found to be liquefied easily at lower temperatures (130 and 145°C). However most of the cellulose began to be liquefied at the optimum temperature (160°C) and severely degraded at higher temperatures (175 and 190°C).


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KEYWORDS: Polyhydric alcohol; hydroxyl and acid numbers; residue; cellulose; lignin

1. INTRODUCTION Continuous depletion of fossil fuel as well as increased in ecological awareness, draws remarkable interest in the utilization of sustainable alternative resources for the production of biofuels and value added chemicals. Lignocellulose biomass comprises mainly cellulose, hemicellulose, and lignin components, emerged as one of the best candidates to replace or substitute petroleum based chemicals. This biomass is renewable, abundant, sustainable, and cheap source raw materials for biochemical, bio-composite, biofuel and etc. Lignocellulosic materials have been converted to diverse of chemicals via processes such as solvolysis liquefaction, pyrolysis, and gasification. These processes effectively convert solid biomass into liquid products that can be used as biofuels or value-added polymers. From the processes mentioned, direct solvolysis liquefaction is mostly practiced for the synthesis of bio-based polymer precursors. Malaysia is the world second largest producer of palm oil (32.14%) after Indonesia (53.04%)1. Palm oil extraction and refinery produced various by-products and wastes that have economical potentials. Oil palm empty fruit bunch (EFB) is one of the lignocellulosic wastes generated in enormous amount from oil palm fresh fruit bunches (FFB). It was estimated that EFB wet weight was around 22-23% from total weight of FFB. In 2015, there were 104.28 million tons of FFB produced from Malaysian oil palm industry thus, it was calculated that about 23.98 million tons of EFB produced2. Presently, EFB is used back as mulch (organic fertilizer) in plantation sites and some of it is burnt onsite to produce steam and electricity. However this tremendous amount


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of waste still has other high value added potential to be harnessed. This biomass waste can be converted into bio-based chemicals via thermochemical process such as liquefaction process. Liquefaction has been one of the popular processes to convert lignocellulose biomass into various value added bio-based fine chemicals by using different liquefying solvents. This process was used by researchers to produce polymer precursors such as polyols (multiple hydroxyl compound) and phenolic derivatives for the synthesis of polyurethane3 and phenolic resins4 respectively. Microwave irradiation has been used as an alternative method to conventional heating in the organic preparation and synthesis. The application of microwave energy has been proven to significantly reduce the liquefaction time due to its rapid and homogeneous heating. This profound advantage allows an efficient and cost effective process of converting wastes into wealth. Microwave-assisted liquefaction were carried out by several researchers on woods5, lignin6, cellulose7, bamboo8, and corn stover9 to produce polyols. Polyols have been widely studied for the synthesis of adhesives10, coatings11, foams12 and films13, 14. Each lignocellulosic biomass has complex and different component compositions which resulted in different optimized liquefaction parameters and products quality. Jasiukaitytė et al. (2009) has studied the liquefaction of cellulose in ethylene glycol (EG) to elucidate its liquefied products with different type of catalysts15. However the study did not report on the hydroxyl number of resultant liquefied cellulose which is important for polyols. Recently, oil palm EFB fiber was liquefied using phenol as solvent and has been used for the preparation of phenolic resin in composite application16. EFB comprises of cellulose, hemicellulose and lignin as the main components besides other extractives. Compared to lignin, cellulose contains more hydroxyl (OH) functional groups on its


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backbone. In addition, crystalline cellulose is the rate determining factor in liquefaction process as it gives higher resistance to liquefaction reaction than amorphous hemicellulose and lignin. Thus, liquefaction of EFB-based cellulose (EFBC) will provide a clear understanding on the role of cellulose in the liquefaction of EFB, as well as its contribution on the polyols’ OH numbers. In this work, liquefaction of EFB fiber and EFBC in EG was conducted via microwave heating to obtain polyols. This study was mainly focusing on the optimization of liquefaction parameters, such as temperature and time, on the OH and acid numbers of the polyols. 2. MATERIALS AND METHODS 2.1. Materials Oil palm empty fruit bunch fiber (EFB) and EFB pulp sheets were supplied by Malaysian Palm Oil Board and Eko Pulp and Paper Sdn. Bhd. Malaysia, respectively. EFB pulp sheets were bleached by using chlorite bleaching method as in Section 2.2 to produce EFB cellulose (EFBC). The EFB and EFBC were ball milled with a SHQM planetary ball miller (Chun Long instrument, Lianyungang City, Jiangsu Province China) with two ceramic milling cylinders (50 mL). Five grams of EFB or EFBC were placed into each cylinder with 10 (∅= 10 mm) and 40 (∅= 6 mm) ceramic balls. The mill rotated horizontally at a constant speed of 230 rpm for 12 hours at room temperature. After ball milled, the samples were sieved through 80-120 mesh sieve screen followed by drying at 105°C for 24 hours then placed in a dessicator before further used. Moisture content of EFB and EFBC were measured using moisture analyzer (AND MX-50, New York). Reagent grade, ethylene glycol (EG), sulfuric acid (H2SO4, purity of 95-98%) used in this study were purchased from Xilong Chemical Co. Ltd. (Guangzhou, China). Magnesium oxide (MgO), sodium hydroxide (NaOH, purity of ≥96%), phthalic anhydride, pyridine (99%


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analytical reagent), imidazole (99%) and 1,4-Dioxane (ACS reagent 99.0%) were purchased from Aladdin chemical reagent company (Shanghai). 2.2. Bleaching process of EFB pulp Bleaching process as reported earlier17 was employed with some modification to remove lignin and hemicellulose from EFB pulp sheets. The pulp sheets were cut into small pieces, soaked in distilled water for 24 hours and followed by disintegration. The pulp were bleached using four stages of bleaching method (DEED). Where D is composed of 1.7% sodium chlorite bleaching at 80°C for 4 hours and process E is an alkaline treatment on EFB pulp sheets with 4–6% NaOH solution at 80°C for 3 hours. After finished, the sample was rinsed with ultrapure water to remove bleaching chemicals, lignin and hemicellulose until it becomes neutral. The bleached EFB pulp was then dried at 105°C for 24 hours to yield EFB cellulose (EFBC). 2.3. Composition analysis The ash, glucan, lignin, water and ethanol extractives contents of EFB were analyzed by using NREL’s laboratory Methods18-20. Sugars (glucose, xylose and mannose) were measured by HPLC (20A, Shimadzu, Kyoto) with Aminex HPX-87P column (300x7.8mm, BioRad, CA) equipped with a refractive index detector (RID-10A, Shimadzu) operated at 80°C and 0.6 ml/min flowrate using ultra-pure water as mobile phase. Standard sugar solutions with different concentrations were used for calibration. Elemental analysis (CHNS) was conducted by using an organic elemental-analyzer (Elementar Analysensysteme GmbHD-63452 Hanau, Germany) via combustion under oxidized condition at temperature of 950°C. All component analyses of the samples were carried out in duplicates.


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2.4. Microwave liquefaction process Microwave assisted liquefaction was carried out using a well-controlled Anton Parr microwave reactor (Monowave 300, Graz, Austria) equipped with a stirrer, temperature-pressure regulator and magnetic stirrer. Built-in infrared (IR) sensor was used to measure the liquefaction temperature throughout this experiment. Biomass sample (EFB or EFBC) (2.0g), ethylene glycol (EG) (6.0 g, 8.0 g, and 10.0 g) and sulfuric acid, H2SO4 (3wt% based on EG used) were discharged into a 30-mL borosilicate glass vial containing a magnetic bar. The vial was sealed with a polytetrafluoroethylene (PTFE)-coated silicon septum and closed with a polyether ether ketone (PEEK) snap cap. The mixture was irradiated to reach a designated temperature (130, 145, 160, 175 and 190°C) with a constant time (2 min) and constant power of 850 Watt. The holding time was varied (5-30 min for EFB and 5-45 min for EFBC) with stirring at constant speed of 600 rpm. After the reaction completed, the temperature was rapidly reduced to 70°C by compression air flushing. The vial containing liquefied products were immediately quenched in cold water (3-5°C) after the reactor’s swivel was opened. The liquefied product was diluted with dioxane-water (4/1 v/v) solution, stirred for 30 minutes and filtered using polytetrafluoroethylene (PTFE) membrane filter to remove the residues. Residue produced was washed and dried at 105°C for 12 hours in an oven and then weighed to determine residue percentage. Residue percentage was calculated by using equation (1): Residue (%) = ( Wr / Wi ) × 100%

(1)

where Wr and Wi are weight of residue and initial weight of biomass sample respectively. After the removal of residues, the diluted liquefied product was neutralized with magnesium oxide (MgO) and filtered again. Neutralized liquefied product was then evaporated to remove


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diluent at 60°C under reduced pressure. Evaporated liquefied EFB and EFBC were labeled as EFB polyol and EFBC polyol respectively. All liquefaction reactions were carried out in duplicates. The polyols and residues were used for analyses. For the reference sample, mixture of ethylene glycol (10.0 g) with sulfuric acid (3 wt%) heated at 160°C in 15 min was prepared. This reference sample was denoted as treated EG. 2.5. Determination of hydroxyl and acid numbers of polyols The hydroxyl number of polyols was determined by the standard esterification method using phthalic anhydride according to the ASTM D427421 (Test Method D). Polyols (1g) and the 25 mL of phthalation reagent were heated at 100°C for 15 min, then cooled to room temperature, 50 mL pyridine was added followed by addition of 10 mL of ultrapure water. The mixture was then titrated with 0.5 M NaOH to its equivalent point. The phthalation reagent was a solution of phthalic anhydride (112 g) and imidazole (17 g) in pyridine (700 mL). All of the samples subjected to hydroxyl number determination were measured in duplicate. Hydroxyl number (OH no.) of polyols was calculated by using equation (2): Hydroxyl number = (B1 − S)(56.1)(N)/W

(2)

where S and B1 are the volume (mL) at the equivalence point of the sample and blank (no polyols), respectively; N is normality of NaOH; and W is the weight of the sample. The acid number of polyols were determined according to the ASTM D466222 standard method. Four grams of polyol in a 50 mL dioxane−water (4/1 v/v) solution were titrated with 0.1 N NaOH to the equivalent point. All of the samples subjected to acid no. determination were duplicated. Acid number (acid no.) was calculated by using equation (3):


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Acid number. = (S − B1)(56.1)(N)/W

(3)

where S and B1 are the milliliters at the equivalence point of the sample and blank (no polyol; N is normality of NaOH; and W is the weight of the sample. 2.6. X-ray diffraction (XRD) X-ray diffractometer (Bruker AXS D8 Advance. Karlsruhe, Germany) was used to obtain powder X-ray diffraction of the liquefaction residues. The residues were scanned in the range of 5-60°. The crystallinity index (CrI%) of the EFB and EFB residues were determine by using Segal’s equation23 (4): CrI% = [(I002-IAM)/I002] × 100%

(4)

where I002 represent the maximum intensity at 2θ = 22.5°; and IAM is the minimum intensity at 2θ = 18°. 2.7. Fourier transform infrared (FTIR) spectroscopy Infrared spectra of EFB and EFBC polyols were investigated by using Fourier Transform Infrared (FTIR) Spectrophotometer (Bruker Alpha instrument, Bruker Optics GmbH, Ettlingen, Germany) with a diamond ATR (attenuated total reflectance) single reflection sampling module cell. Samples were held and clamped against the ATR crystal with an integrated clamping mechanism to ensure constant pressure were applied on all samples. The spectrum of each sample was acquired by the average of 64 scans with 2 cm-1 resolution ranged between 4000 to 650 cm-1. 2.8. Scanning electron microscopy (SEM)


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Morphology of the raw EFB and EFB residues at different temperature was observed using a scanning electron microscope (SEM-Philips XL-30) 3. RESULTS AND DISCUSSION 3.1. Composition and elemental analysis. Carbohydrates and lignin are major compositions of lignocellulosic materials. These components must be analyzed to understand the degradation and conversion products of lignocellulosic materials in liquefaction study. Table 1 shows the lignocellulosic composition of EFB fibers. The main components of the EFB are glucan, lignin and xylan. Glucan mass fraction (wt%) represents cellulose content whereby xylan and mannan represent hemicellulose components in the lignocellulosic materials. These components will later affect the liquefaction reaction of EFB. Table 1. Compositions of oil palm empty fruit bunch fiber (EFB) EFB components

Mass Fraction (wt%)

Ash%

0.80 (±0.02)

Total extractives%

13.91 (±0.84)

Lignin (acid soluble)%

5.72 (±0.92)

Lignin (acid insoluble)%

18.46 (±0.95)

Glucan%

37.41 (±0.65)

Xylan%

20.81 (±0.80)

Mannan%

1.65 (±0.15)

3.2. Effects of liquefaction conditions on liquefaction residue


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The optimization of microwave-assisted liquefaction of oil palm empty fruit bunch fiber (EFB) and cellulose (EFBC) were successfully carried out. The effects of liquefaction temperature and time on the liquefaction products were discussed in this paper. The liquefaction yield was presented in the form of residual percentage (%) for accurate representation of the resultant liquefaction reaction. Results on the conversion efficiencies for EFB and EFBC can also be found in the Table S1 and S2 respectively, at the supplementary information. The effects of temperature and time on the liquefied EFB and EFBC residues are illustrated in Figure 1a and 1b, respectively. The lowest EFB residue produced (3.22%) was at 160°C and 15 min which was later denoted as the optimum temperature and time for EFB liquefaction. Meanwhile, the optimum temperature for liquefaction of EFBC was at 175°C with the minimum residue of 1.03% formed at 40 min. However, approximately similar amount of residue (1.29%) was observed at 190°C with 25 min liquefaction time. Temperature plays an important role to enhance solvent and catalytic attack onto glycosidic linkages that leads to the dehydration, decarboxylation and cleavage of macromolecular cellulose into smaller fragments24.


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Figure 1. The effects of temperature and time on the (a) EFB and (b) EFBC liquefaction residue. (EG/EFB and EG/EFBC ratio: 5.0; H2SO4 concentration: 3 wt%) Two major factors which influence the formation of residues during the liquefaction of lignocellulosic biomass are; biomass degradation and recondensation reaction of resultant liquefied products or self-polymerized lignin25. It was reported that recondensation tend to occur if mixture of liquefied cellulose and lignin products coexisted26. In recondensation reaction, cellulose degradation products such as ethyl levulinate and hydroxymethylfurfural (HMF) derivatives react with phenolic derivatives from the degradation of lignin into polymerized residue27. Apart from that, lignin also self-polymerized or reacted with xylan in acidic medium to produce insoluble residue28. Categorized as grass29, EFB lignin contains all three lignin monolignols such as phydroxyphenyl (H), guaiacyl (G) and syringyl (S) with H/G/S ratio of 2/30/6830. Syringyl is more prone to thermal softening and easier to dissolve in alcohols compared to guaiacyl. Meanwhile, guaiacyl is more susceptible to self-condensation and recondensation reactions30. Hence, the delignification or liquefaction of lignin is depending on lignin content and syringyl/guaiacyl (S/G) ratio in plants31. Since EFB has high S/G ratio (68/30), most of its lignin liquefied easily at the pre-optimum temperature and time. During the liquefaction from lower to the optimum temperatures (130-160°C), most extractives, lignin and some accessible holocellulose (hemicellulose and amorphous cellulose) of EFB were liquefied resulted in the minimum residue produced32. However, higher temperatures (175 and 190°C) have promoted recondensation of EFB liquefied products thus increased the residue. In contra, EFBC residue showed decrement over time when temperature increased to 175°C and


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190°C. EFBC contains no lignin which eliminated the effect of recondensation. The remaining EFBC residue might be due to a very small amount of carbonized residue. It can be seen that the effect of time on liquefaction residues for both EFB and EFBC was different. In EFB liquefaction, the lowest residue formation was achieved in the range of 15-20 min for all temperatures studied. Extension of liquefaction time showed increment of liquefaction residue due to the increasing recondensation reaction. Therefore, obtaining optimum time for EFB liquefaction is important. However recondensation in EFBC liquefaction is insignificant, in which its residue continued to decrease when liquefaction is prolonged. Minimum EFBC residue was obtained at 40-45 min of liquefaction. Hence, the liquefaction time of EFBC can be continued until least residue formed or it can be stopped if suitable properties of polyols have been obtained. 3.3. Effects of liquefaction parameters on polyol hydroxyl and acid numbers The liquefaction and degradation of EFB into bio-based EFB polyol are temperature dependent. The effect of temperature and time on EFB polyols hydroxyl and acid numbers are presented in Figure 2a and 2b respectively. In comparison to EG (936.89 mgKOH/g), all obtained EFB polyols have lower hydroxyl numbers. EFB polyols at 130 and 145°C showed a significantly reduced hydroxyl numbers, which were 427.71 and 538.73 mgKOH/g respectively. When the optimized temperature of 160°C was reached, the hydroxyl number was increased to the maximum value (749.22 mgKOH/g). However, when temperatures of 175 and 190°C were used, the hydroxyl number gradually decreased to 681.56 and 589.39 mgKOH/g respectively. The hydroxyl number of treated EG was found to be the lowest (332.07 mgKOH/g). This significant reduction of hydroxyl number portrays the detrimental effect of sulfuric acid catalyst used on


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liquefied solvent itself in liquefaction reaction. However, the substitution of the loss hydroxyl number by the liquefied EFB was proved as all EFB polyols possessed higher hydroxyl number than treated EG.


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Fig. 2. Effects of (a) temperature and (b) time on EFB polyol hydroxyl and acid numbers (EG/EFB ratio: 5.0 ; H2SO4 concentration: 3 wt%) Great reduction of hydroxyl number from EG to 130 and 145°C polyols was due to the liquefaction of lignin and hemicellulose at lower temperatures. Compared to cellulose, lignin and hemicellulose yield lower hydroxyl (OH) group, but higher in methoxy (O−CH3) and carbonyl (C=O) groups respectively33, resulting in reduced hydroxyl number. Maximum hydroxyl number of EFB polyol obtained at 160°C was due to the degradation of most celluloses into –OH containing compounds such as EG glucosides and hydroxyethyl levulinates34. Cellulose yields more OH groups which gave higher hydroxyl number than lignin phenolic hydroxyl groups7. Further decrease of hydroxyl numbers at 175°C and 190°C were caused by the OH groups consumption in recondensation and dissociation into organic acids such as formic acid, levulinic acid, acetic acid, oxalic acid and 2-hydroxy-butyric acid contributed to the increase in acid number35. The conversion of liquefied product compounds into acids might be the cause of the gradual increase in acid number (acid no.). In Figure 2b, the maximum hydroxyl number (749.22 mgKOH/g) for EFB polyol obtained was at 15 min liquefaction time. Increase in both hydroxyl and acid numbers from 5 to 15 minutes indicated more lignocellulosic components were converted into hydroxyl compounds and further converted into acids35. As the liquefaction continued, OH number began to reduce due to recondensation has taken place, consuming the hydroxyl groups. Figure 3 displays the OH and acid numbers of EFBC polyols. The maximum hydroxyl number obtained (639.91 mgKOH/g) was at 175°C for 40 min liquefaction time. Acid number for EFBC polyols at all temperatures increased over time indicating the intensive conversion of liquefied EFBC products into acids without being consumed in any recondensation reaction.


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Figure 3a and 3b illustrate the effects of temperature and time on the hydroxyl and acid numbers of EFBC polyols respectively. Only samples at temperature of 160-190°C and time of 15-45 min were evaluated for hydroxyl and acid numbers due to the considerable amount of EFBC residues produced. In Figure 3a, the maximum hydroxyl number of EFBC polyol (639.91 mgKOH/g) was obtained at 175°C and 40 min. All EFBC polyols possessed lower hydroxyl number as compared to the optimum EFB polyol. This was an unexpected result since cellulose provides more OH compounds than EFB which is a lignocellulosic material36. Higher temperature and prolonged time were needed to reduce EFBC residue had consequently caused OH-containing compounds in EFBC polyols to be severely degraded into organic acids which showed the increased in acid number (Figure 3b).


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Fig. 3. Effect of temperature and time on the EFBC polyol (a) hydroxyl number and (b) acid number (EG/EFBC ratio: 5.0 ; H2SO4 concentration: 3 wt%) 3.4. Effects of liquefaction conditions on the crystallinity of EFB residues


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XRD analysis was used to elucidate the changes in crystallinity of EFB residues during liquefaction reaction. XRD analysis was not carried out for EFBC residues due to insufficient amount of residue required for analysis. Figure 4a and 4b show the effects of temperature and time on the crystallinity of EFB residues respectively. The diffraction patterns of all EFB residues show typical cellulose structure with a sharp peak at 22.2° and broad peaks at 14.7° and 16.3°17. Cellulose is the rate determining factor in a lignocellulosic liquefaction. Thus, it is important to investigate the changes of cellulose crystallinity in each liquefaction stages. In Figure 4a, results illustrate that cellulose peaks intensify for raw EFB up to 130°C of liquefaction temperature. At 130°C most lignin in EFB was dissolved which exposed the cellulose structure and resulted in the peak intensification. When temperature was increased to 145 and 160°C, the cellulose peaks began to reduce. However, cellulose peak was still clearly observed at 160°C with substantial amount of silica crystalline peaks emerged. Cellulose which was detected in residue at 145 and 160°C were probably in the form of microfibrillated cellulose37. Further increased of temperature to 175 and 190°C has portrayed the severe disruption of cellulose peak. At these temperatures, most cellulose might be liquefied and degraded.


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Fig. 4. XRD diffractograms of EFB residues with different liquefaction (a) temperature and (b) time. Table 2. Crystallinity index of EFB residues at different temperature Temp (°C)

CrI (%)

EFB

43.8 (± 5.1)

130

68.6 (± 3.9)

145

64.4 (± 5.7)

160

61.6 (± 4.3)

175

40.6 (± 2.8)

190

24.4 (± 3.0)

(EG/EFB ratio: 5.0; Time: 15 min)


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Table 3. Crystallinity index of EFB residues at different time Time (min)

CrI (%)

EFB

43.8 (± 5.1)

5

65.3 (± 3.9)

10

69.2 (± 3.2)

15

71.7 (± 4.1)

20

68.5 (± 3.6)

25

64.6 (± 3.5)

30

62.2 (± 4.7)

(EG/EFB ratio: 5.0; Temp: 160°C) Increase in reaction time did not significantly disrupted the crystallinity of cellulose in EFB residues as shown in Figure 4b. The intensity of cellulose peaks increased for raw EFB up to 15 min liquefaction time due to the delignification process. The peaks began to decrease in EFB residues from 15 to 30 min liquefaction time. Tables 2 and 3 present the crystallinity index (CrI%) of the EFB residues at different temperatures and times respectively. Table 2 shows that crystallinity index increased from 43.8 to 64.4 % for EFB after the liquefaction process at 145°C. It was due to the degradation of lignin, which leaving cellulose as the main composition of the residues. The CrI% began to decrease significantly when temperature increased from 145°C (64.4 CrI%) to 190°C (24.4 CrI%), suggesting most degradation of cellulose occurred at temperature greater than 160°C. As can be seen in Table 3, liquefaction time has no significant effect on CrI% of the residues obtained at 160°C. The remaining of high crystalline cellulose in the liquefaction residue has contributed to the high crystallinity index. Besides the role of temperature, polymerized products from


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recondensation reaction might have protected the crystalline cellulose part from being further liquefied. 3.5. FTIR analysis of EFB polyols Figure 5a and 5b shows the infrared spectra of EFB and EFBC polyols at different liquefaction temperatures respectively. Compared to EG, broadening of O−H vibration peak at 3300-3400 cm-1 in all EFB polyols indicated the presence of additional alcohol and phenolic hydroxyl from the degradation of EFB. C−H bending (880-1300cm-1) and C−O stretching vibration (1200-1300 cm-1) peaks of alcohols and esters in all spectra belong to EG. In Figure 5a, both peaks at 1125 cm-1 (C-H in-plane deformation of syringyl unit)38 and 1650 cm-1 (C=C stretching)39 represent the degradation products of lignin. These peaks began to appear in 130°C polyol and present in all EFB polyols spectra. This confirmed that the liquefaction and degradation of lignin has already occurred at 130°C. Shoulder peak appeared at 1720 cm-1 (C=O stretching of aldehyde or ketone) in 160°C polyol, later intensified as observed in 190°C polyol. This peak belongs to the degradation products of cellulose such as formaldehyde, hydroxymethylfurfural (HMF), furfural, levulinic acid etc.5. It demonstrated that, cellulose began massively degraded at 160°C and even higher degradation products of cellulose were obtained at higher temperatures. For EG, 882cm-1 peak belongs to CH2 rocking40. This peak intensity decrement for EG and polyol at 145°C can be explained by increasing in EFB liquefied components which reduced EG peaks (864 cm-1, 882 cm-1, 1040 cm-1, 1085 cm-1, 1410 cm-1, 1462 cm-1). However, the 882cm-1 peak intensity has increased further for polyols at 145°C to 190°C probably due to out of plane deforming vibration of O−H in carboxylic groups (882 cm-1) which overlapped at the same peak. This carboxylic O−H belongs to organic acids produced via severe degradation of lignocellulosic components35.


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Fig. 5. FTIR spectra of (a) EFB and (b) EFBC polyols at different temperature (EG/Biomass ratio: 5.0 ; H2SO4 concentration: 3 wt% ; Time: 15 min (EFB), 40 min (EFBC))


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Similar peaks at 1125 and 1650 cm-1 as for EFB polyols (Figure 7a) were also appeared for EFBC polyols (Figure 5b) showing that there were peaks overlapped between lignin and cellulose degradation products. For EFBC, peak at 1125 cm-1 is belongs to ether (C−O−C) groups from cellulose derivatives i.e. EG-glucosides and 2-ethyl levulinate34,

41

while peak at

1650 cm-1 represents carbonyl (C=O) groups of carboxylic acids produced by extensive degradation of cellulose42. For EFBC polyol at 175°C, it can be noticed that both 1720 cm-1 (C=O of aldehydes and ketones) and 1125 cm-1 (C−O−C of cellulose derivatives) intensified at 160°C, suggesting more intermediate cellulose degradation products formed within the temperature range. At 190°C, higher intensity peaks were at 1125 and 1650 cm-1, indicating the continuation of intermediate cellulose derivatives formation and extensive conversion of the intermediates into acids, respectively. A small shoulder peak presented at 920 cm-1 in EFBC polyols corresponds to C−O−C stretching of β-glucopyranoses of carbohydrates from cellulose degradation43. 3.6. Morphology of liquefaction residues The morphologies of EFB residues are shown in Figure 6a-f. Figure 6a, shows the surface of EFB fiber was smooth and rigid. After liquefaction at 130°C and 145°C, the size of EFB residues was progressively reduced, greater amount of smaller EFB particles could be seen (Figure 6b-c). At this stage, cell wall was degraded while lignin and hemicellulose were dissolved and decomposed exposing cellulose microfibrils to the liquefying reagent37. Cellulose remained as the major composition of the residues and the effect of recondensation was insignificant. However, the residue produced at the optimum liquefaction temperature (160°C) showed a smooth surface of polymerized residue (Figure 6d). At this temperature, the exposed cellulose microfibrils began to degrade thus enhanced the effect of recondensation via reaction with lignin


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degradation products. At higher temperatures (175 and 190°C), the residue produced gradually became rougher with small fibrous material observed (Figure 6e-f). The rough surfaces were formed due to the rapid polymerization of liquefied products during liquefaction reaction resulted in insoluble residues. Fibrous material presented in the residues might be from the remaining of cellulose microfibrils. Silica also clearly observed and identified on the residue surface in Figure 6f.


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Fig. 6. SEM micrographs of liquefaction residues at different temperature (a) EFB (b) 130°C (c) 145°C (d) 160°C (e) 175°C and (f) 190°C 4. CONCLUSION


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EFB and EFB cellulose (EFBC) were liquefied in acidified ethylene glycol with 3.22% and 1.03% residues formed at the optimal conditions, respectively. Temperature had greatly influenced the residue formation, hydroxyl and acid numbers. For EFB, lignin degraded at lower temperature (130°C) meanwhile the degradation of most cellulose occurred at 160°C with some portion only degraded at higher temperatures (175 and 190°C). Recondensation reaction imposed detrimental effects by increasing residue formation conjointly reducing hydroxyl number of obtained EFB polyols. Conversion of EFBC to obtain the maximum yield required higher temperature than EFB has jeopardized the hydroxyl number of EFBC polyols. AUTHOR INFORMATION Corresponding Author *Tel.: +603-89213261.

Fax: +603-89213777. E-mail: [email protected]

*Tel.: +86-86-25-58606657. Fax: +86-86-25-58606637.

E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. U.A.A., S.Z. and C.H.C. conceived and designed all the experiments. U.A.A carried out the experiments. Z.F. and M.Z.M. participated in the discussion. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENTS


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The authors would like to thank Universiti Kebangsaan Malaysia (UKM) and Malaysian Ministry of Education (MOE) for the financial support via research grant (PRGS/1/2015/SG06/UKM/01/1). Special thanks to the Center for Research and Instrumentation Management (CRIM), UKM for the instrument services. ABBREVIATIONS EFB, Empty Fruit Bunch; EFBC, Empty fruit bunch cellulose; EG; ethylene glycol. REFERENCES 1.

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Production of Liquefied Oil Palm Empty Fruit Bunch ... - ACS Publications

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