Catalytic Performance of Sulfonated Carbon Catalysts for Hydrolysis of Palm Oil Empty Fruit Bunch

Received: 21st February 2020 Revised: 17 May 2020 Accepted: 31 May 2020 Online: 30 June 2020 Utilizing lignocellulosic biomass into valuable products, such as chemicals and fuels, has attracted global interest. One of lignocellulosic biomass, palm oil empty fruit bunch (EFB), has major content of cellulose (30-40%), which is highly potential to be a raw material for fermentable sugar production. In this research, a series of sulfonated carbon catalysts with various concentrations of sulfuric acid (H2SO4, 10-30 v/v%) solutions have been successfully prepared and applied for a single stage of heterogeneous acid-catalyzed hydrolysis over microcrystalline cellulose and EFB under moderate temperature condition and ambient pressure. The catalysts' physical and chemical properties were characterized by using a Thermogravimetric Analyzer (TGA), X-ray diffractometer, surface area analyzer, and Fourier-transform infrared spectrophotometer. The characterization results showed that sulfonated carbon had relatively similar physical properties with the parent of active carbon. The hydrolysis activity of sulfonated carbon catalysts gave various Total Reducing Sugar (TRS). The effects of sulfate loading amount in catalyst samples and various ionic liquids were investigated. The hydrolysis of pure microcrystalline cellulose powder (Avicel) using 30%-sulfonated carbon (30-SC) catalyst in i-butyl-3-methylimidazolium chloride ([BMIMjCl) ionic liquid at 150°C yielded the highest TRS of 16.11%. Subsequently, the catalyst of 30-SC was also tested for hydrolysis of EFB and produced the highest TRS of 40.76% in [BMIMjCl ionic liquid at 150°C for 4 h. The obtained results highlight the potential of sulfonated carbon catalysts for hydrolysis of EFB into fermentable sugar as an intermediate product for ethanol production.


Introduction
Lignocellulose is a relatively low-cost carbon resource to produce essential platform chemicals and fuels [1,2]. It consists mainly of cellulose ( 30 -55 %), hemicellulose ( 25 -30 %), and lignin ( 25 -30 %) [ 3 ]. Cellulose is a carbon-neutral feedstock because the C02 generated is recaptured at the end of their use. The monomer of cellulose i.e., glucose, can be applied as an intermediate compound for various useful chemicals, such as polymers, medicines, surfactants, and fuels [1, 2, 3 , 4 ]. This utilization of cellulose as an alternative feedstock will reduce the fossil fuel demand and contribute toward the mitigation of C02 driven climate change [ 5 ]. Many types of research have explored hydrolysis of cellulose using enzymes [6], dilute acids [ 7 ], and sub-or super-critical water [8]. On the other hand, these methods have many drawbacks due to technical and cost issues. The high cost of enzymes, difficulty in separation, corrosion of reactors, undesirable waste and extreme reaction conditions become serious problems. Therefore, a breakthrough in the cellulose hydrolysis method is required. α The investigation of more environmentally sustainable chemical processes has stimulated the use of heterogeneous solid acids as replacements for the homogeneous liquid acid catalysts, such as H2S 04 [ 9 ].
The heterogeneous solid acid catalyst is potentially applicable for the efficient cellulose hydrolysis due to the easily separated from the product, recyclable, and low energy consumption [ 3 , 10,11,12]. The hydrolysis of cellulose has been reported using various heterogeneous solid catalysts [10,11,12], such as magnetic solid acid catalyst [ 13 ], sulfonated zirconia [ 14 ], sulfonated activated-carbon [ 15 ], hetero poly acids [16], Amberlyst® 15 [ 17 ], H-form zeolite, montmorillonite, and acid-activated montmorillonite [18]. Among various types of heterogeneous solid catalysts, carbon-based solid acid exhibited superior catalytic activities. The excellent recyclability, inexpensive, and naturally existing raw materials of carbon-based solid acid catalyst shows its potential application in the production of intermediate products from cellulose.
The sulfonated carbon catalyst with lower H2S 04 concentration for hydrolysis reaction of EFB under moderate temperature conditions and ambient pressure has not been reported elsewhere. In this research, single step hydrolysis of cellulose using solid acid catalysts, sulfonated carbons prepared using a low concentration of H2S 04 (10-30 v / v%), was investigated for the potential application in fermentable sugar production from EFB. The physical and chemical properties of sulfonated carbon catalysts were characterized before tested for cellulose hydrolysis. The effects of sulfate loading amount in catalyst samples and various ionic liquid were investigated. Subsequently, the catalyst sample with the best hydrolysis activity was used in the hydrolysis of lignocellulosic biomass, EFB.

Preparation of Sulfonated Carbon Catalysts
The active carbon powder (specific surface area, 799.07 m 2 / g; pore volume, 0.41 cm 3 / g; and pore size, 2.04 nm) was refluxed in various concentration of H2S 04 (10-30 v / v%) at 120°C for 24 h, followed by cooling to room temperature and filtration to obtain a black precipitate. The black precipitate was then washed repeatedly with hot distilled water until impurities such as sulfate ions were no longer detected in the washed water.

Catalyst Characterization
The weight loss of catalyst samples was examined using the TGA LINSEIS STA instrument under air with a heating rate of io°C / min to understand thermal behavior decomposition occurred by increasing activation temperatures. Crystallographic phase identification of catalyst samples was analyzed using X-ray Diffraction (XRD) analysis, Phillip PW 1710 diffractometer, with Cult radiation at 40 kV and 30 mA, and secondary graphite monochromatic. The specific surface area and porosity of catalysts were measured by Tristar II 3020 Micromeritics Instrument through nitrogen adsorptiondesorption isotherms, at 77.3 K on liquid nitrogen. Prior to the analysis, the samples were degassed at 350°C for 3 h. The surface acidity of catalyst samples was determined by the irreversible adsorption of organic base pyridine. The amount of base adsorbed was considered as the acidity of catalyst samples in mmol / g. The types of groups and acid sites in catalyst samples were characterized by FT-IR spectroscopy in the KBr phase Cellulose is known to be insoluble in most solvents commonly used in chemical reactions. Therefore, the use of ionic liquids develops a breakthrough. Ionic liquids with high thermal stability, low vapor pressure, wide liquid temperature range, and good solubility, have gained high interest for various chemical reactions. Swatloski et al. [ 19 ] reported the application of ionic liquids for hydrolysis of cellulose at low temperatures, which became an important stage for breaking the chain between the sugar ethers. Some ionic liquids reported chloride acetate  26, 27 , 28, 29 , 30 ] and even more than 60% [ 29 , 31 ]. Concentrated H2S 04 (above 50 %) had an apparent swelling effect on cellulose [ 32 ]. Yoon etal. [ 33 ] reported H2S 04 concentration above 62% had promoted the total solubilization of microcrystalline cellulose. Although hydrolysis of cellulose using concentrated acids can be operated at low temperatures and atmospheric pressure, its water content leads to severe corrosion [ 34 ]. On the other hand, a higher temperature is usually required in the hydrolysis of cellulose under dilute acids [ 35 ]. Harris and Beglinger [ 36 ] used 0.5 wt% H2S 04 in a continuous reaction with a short residence time to minimize the degradation of wood products. Thompson and Grethlein [ 37 ] used 1 wt% H2S 04 in a continuous process at 240°C with a short residence time of 0.22 min to produce glucose of 50 %. θ θ θ using a Shimadzu, Prestige-21 FT-IR spectrometer after treated adsorption of organic base pyridine.

3 . Catalytic Performance Test
The catalytic performance of prepared sulfonated carbon catalysts was tested for the hydrolysis reaction of Avicel in a stainless-steel batch reactor. In this study, the effect of sulfate loading amount in catalyst samples and various ionic liquids of ([BMIM]C1 and [EMIM]OAc) were investigated. Sulfonated carbon catalyst powder sample (0.1 g) was dispersed by magnetically stirring in ionic liquid ( 3.8 g) containing Avicel (0.2 g) and distilled water ( 0.25 mL). The hydrolysis reaction was carried out at 150°C for 3 h. To terminate the reaction, distilled water (10 mL) was added into the reaction mixture, followed by filtration to separate reaction liquid and the used catalyst. The total reducing sugar (TRS) of liquid product was analyzed using DNS ( 3 , 5 -dinitrosalicylic acid) assays [18]. The catalyst sample, which showed the best performance in the hydrolysis of Avicel, was then applied in the hydrolysis of biomass feedstock, EFB. Furthermore, the effect of various hydrolysis time was also studied.

4 . Product Analysis [ 19 ]
A mixture of DNS reagent ( 0.25 mL) and liquid product ( 0.25 mL) was heated in a water bath at 5 -ioo°C for 5 min, cooled to room temperature, and then diluted by adding water (2 mL). The color intensity of the resulting mixtures was measured using UV-Vis Spectroscopy at a wavelength of 540 nm. The concentration of TRS was calculated based on the standard curve of glucose. The final solution was filtered and analyzed by high-performance liquid chromatography using a Prevail Carbohydrate ES column ( 4.6 x 250 mm) at 358 K equipped with a PL-ELS 1000 ELSD detector, with mobile phase was a mixture of water and acetonitrile ( 25 : 75 v / v) at a flow rate of 1.0 mL / min.
The products detected in HPLC chromatograms were glucose and sucrose. However, the concentration of these products was deficient (not shown here).

. Characterization of Catalysts
The thermal stability of active carbon and sulfonated carbon catalysts was studied by TG analysis under air condition, and the results are summarized in Figure 1. Sulfonated carbon catalysts were denoted by a number representing the concentration of H2SO 4 used in catalyst preparation, followed by -SC.  [ 23 , 24 , 25 ]. Compared to the active carbon (a), 30 -SC catalyst (b) presented more apparent decreasing tendencies, indicating that the 30 -SC catalyst has hydrophilic properties [26].  ight be attributed to amorphous carbon composed of aromatic carbon sheets oriented in a considerably unordered form. The weak and broad C (101) at 2 about 40 -50°w as possibly caused by the graphite structure [ 27 , 28]. The small sharp peak at 2 of 280 was observed for a 20-SC catalyst, which might reveal the structure of intermediate between graphite and amorphous carbon. It also has a disorderly layer or random layer lattice structure [ 25 ]. There was no noticeable difference in the XRD patterns between active carbon and prepared sulfonated carbon catalysts, suggesting that the sulfonation did not affect the microstructure of carbon. The specific surface area, pore-volume, and pore diameter of active carbon and sulfonated carbon catalysts are shown in Table 1. Sulfonation treatment resulted in an increase in the specific surface area from 789.34 to 846.59 m 2 / g, as can be seen in active carbon and 15 -SC. The different concentrations of H2SO 4 to prepare sulfonated carbon catalysts did not significantly affect the specific surface area, pore-volume, and pore size. This might be caused by the leaching of -S O3 H groups during washing and neutralizing. Li et al. [ 29 ] have also reported that there was no significant change round of physical properties by increasing of H2S 04 concentration from o.i to l M. The specific surface area of sulfonated carbon catalysts increased at sulfate loading was about 10-15 %, but decreased at sulfate loading was about 20-30 %. FTIR spectra of the active carbon and sulfonated carbon catalysts are shown in Figure 3 . The sulfonated carbons (Fig. 3 , spectrum (b-f )) showed peaks corresponding to -S 03 H groups. While the peak a 669 -680 cm -1 is associated with C-S stretching. The peaks in the range of 748 -883 cm -1 correspond to the S-0 stretching, while peaks at 1051 and 1265 cm -1 correspond to the symmetric and asymmetric stretching vibrations of S = 0, respectively. The peaks at 1571 and 1687 cm 1 were associated with the C-0 and -COOH stretching, respectively. There was no -S 03 H group in the active carbon (Fig. 3 , spectrum (a)). These results confirmed that the sulfonic acid groups were successfully attached to the surface of the sulfonated carbon catalysts.  The chemisorption of pyridine, followed by IR studies, is a valuable method to determine the nature of surface acid sites [ 38 , 39 ]. The peak of pyridinium ring vibrations was observed at 1545 and 1639 cm -1 due to the proton transfer from Brpnsted acid sites to pyridine [ 40 ]. As shown in Figure 3 (b-f ), sulfonated carbon catalysts exhibited prominent bands attributed to pyridine adsorbed on the Brpnsted acid sites at about 1631 cm 1 . Therefore, it can be assumed that sulfonated carbon catalysts contain only Brpnsted acid sites with higher intensity than active carbon due to the presence of the -S 03 H groups on the surface of the catalyst [ 41 ].  The effect of sulfate loading in catalyst samples and various ionic liquids were investigated through cellulose hydrolysis. The effect of ionic liquid was investigated using two ionic liquids, i.e., [ Figure 5 . The combination of different anions and cations in these ionic liquids contributes to cellulose ' s dissolution [22,31 ]. The anion in ionic liquids attacked the free hydroxyl group on cellulose and deprotonated it. At the same time, the hydroxide on the cellulose chain acted as an electron donor and interacted with the larger size cation in the imidazolium. Cellulose dissolution occurred once the hydroxyl groups on cellulose were separated, and the hydrogen bonds between cellulose were disrupted. Upon the addition of anti-solvent, regenerated cellulose was formed when ionic liquids were bonded to the antisolvent through preferential solute displacement. The action of ionic liquids might be substrate-specific in the pretreatment of lignocellulosic biomass. For example, [EMIM]OAc showed the best-performing pretreatment medium for sugarcane bagasse, as reported by Yoon et al. [ 33 ]. In this work, [BMIM]C1 showed better performance in the cellulose of EFB. It means that an ionic liquid suitable for pretreating one substrate might not be effective for different substrates. The utilization of acetate-based ionic liquid, [EMIM]OAc is limited due to less activation of the glycosidic bonds by weakly basic anions-containing ionic liquids [ 34 ]. Therefore, the chloride-based ionic liquids, [  The catalytic hydrolysis performance reported in this work was compared to that of the previous works based on sulfonated carbon catalysts [ 27 , 28, 29 ]. Onda et al. [ 27 ] prepared sulfonated carbon catalysts by a high concentration of H2SO 4 heated under argon flow at 150°C for 16 h. The catalysts gave cellulose conversion of 43 % with the glucose yield of 40.5 %. Li et al. [ 29 ] developed a new sulfonation process from carbon materials through a plasma process under dilute H2SO 4 and produced a catalyst that gave 40.1 % cellulose conversion with a yield of 34.6 %. Qin et al. [28] reported a green and universal process to sulfonate various carbon materials via innovative gas-liquid interfacial plasma (GLIP) under 1 M H2SO 4 at room temperature. Although the catalyst resulted in low glucose selectivity, the catalytic performance increased significantly by applying the green sulfonation process using a low concentration of H2SO 4 combined with the plasma method. In this work, sulfonated carbon catalysts prepared using a low concentration of H2S 04 (10-30 wt%) gave the highest TRS yield from EFB, 40.76 %. These results highlight the potential of sulfonated carbon catalysts prepared using a low concentration of H2SO 4 to be applied for the hydrolysis reaction of lignocellulosic materials into fermentable sugars. The 30 -SC catalyst was tested for hydrolysis of EFB, which has cellulose content about 30.41 %, the hydrolysis method is described in Figure 6. The TRS yield increased by increasing hydrolysis time from 2 h to 4 h but decreased at further prolonged hydrolysis time at 8 h ( Figure 7 ). The highest TRS yield of 40.76 % was obtained over 30 -SC catalyst in [BMIM]C1 ionic liquid at 150°C for 4 h. The decrease of TRS yield at 8 h reaction time might be caused by the degradation of sugar product, as reported by Yamaguchi et al. [ 35 ]. It was reported that hydrolysis reaction rates decreased after 6 hours due to a shortage of the water required for the reaction and blocking of acid sites by the reaction products.