Synthesis and characterization of activated carbon derived from rubberwood sawdust by using KOH/KMnO4 as multiple function activation agents

This research is focused on the effects of impregnation ratios using KOH/KMnO4 as multiple function activation agents on microporous of activated carbon. Rubberwood sawdust, the waste from wood processing industry, was used as a raw material in synthesis of activated carbon by chemical activation. The rubberwood sawdust was carbonized at 400 °C for 1 hour under an inert atmosphere to produce char. Chars were impregnated with different proportions of KOH and KMnO4. Impregnation time for each mixture proportion (Char: KOH: KMnO4) was 24 hours before activation for 3 hours at 800 °C. The surface area, pore volume, micropore volume, pore size distribution, porous structure, and adsorption isotherm were examined and considered as the properties of activated carbon. The results showed that the activated carbon with an impregnation ratio of 10:10:5, 10:30:0, and 10:30:5 have the surface area 750.89 m 2/g, 1574.39 m 2/g, and 1346.10 m 2/g, respectively. The pore volume and micropore volume were in the range between 0.3289-0.6405 cm 3/g and 0.1920-0.3533 cm 3/g. The average pore sizes were 1.75 nm, 1.63 nm, and 1.65 nm, respectively. These synthesized activated carbons were characterized as type I isotherm and microporous solids.


Introduction
Activated carbon is a carbonaceous material containing a pore structure with a high specific surface area and pore size distribution [1]. According to it has large surface area and high pore volume, it can be used in many applications such as an outstanding adsorbent [2] and an electrode material for supercapacitor [3]. To prepare the activated carbon, the raw material is carbonized at a temperature between 200 o C to 400 o C to eliminate volatile compounds and tar and produce char [4]. After that, the activation process is applied. This activation can be divided into two methods. The first method is physical activation, carried out at a high temperature range of 700-1100 o C by using CO2, steam, or a mixture of these gases. However, activated carbon obtained from this method has a low surface area. The second method is chemical activation involved the use of chemical agent to impregnate and carbonize raw material at a temperature range of 300 o C to 800 o C under an inert atmosphere. Therefore, chemical activation has benefits over physical activation, such as lower activation temperature, higher yield of activated carbon, IOP Publishing doi: 10.1088/1757-899X/1163/1/012019 2 and higher surface area. A widely chemical agent including the acidic group; sulfuric acid (H2SO4), phosphoric acid (H3PO4) [5,6], the alkaline group; potassium hydroxide (KOH), sodium hydroxide (NaOH) [7,8], and the metallic salt; zinc chloride (ZnCl2) [9] are used as a dehydrating agent in chemical activation. Many researches shown that the alkaline group generate more micropore surface area than acidic group [2,10]. Therefore, among the alkaline group, KOH is the most effectively used for chemical activation since It can produce micropores, it can easily use for various applications.
Biomass is an attractive raw material to produce activated carbon because it contains high cellulose, hemicellulose, and lignin [11]. The elemental compositions of biomass are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and slightly sulphur (S) [12]. Many studied related to the chemical activation of selected biomass are summarized in table 1. All activated carbons using KOH as a chemical activation agent are carried out at a temperature of 800 o C. Because the activation temperature greater than 800 o C has not shown a significant increase in the surface area [13]. Impregnation ratio is an essential parameter for chemical activation. It also affects active carbon porosity. The activated carbon impregnated at the ratio of 1: 3 has a higher surface area. In Thailand, Rubberwood is a high potential biomass. However, a little number of researches using rubberwood sawdust as a raw material for physical activation [14] or chemical activation with ZnCl2 presented in table 1, without studied the effect of KOH to the characteristic of activated carbon derived from this raw material.
Activated carbon can be used as an electrode material for electrostatic double layer capacitors (EDLCs). To improve the electrochemical properties of activated carbon, surface functional groups or metal oxide can be used. Manganese monoxide (MnO) has higher specific capacitance (1,350 F/g) compared with manganese oxides [15,16]. However, It has low conductivity and cycle stability. Therefore, MnO was used as an additive to improve the specific capacitance of electrode materials. Potassium permanganate (KMnO4) and KOH also have similar activation mechanisms that generate micropores [17]. KMnO4 was decomposed into K2O, MnO, and O2 at an activation temperature of 750 o C, and MnO was added to activated carbon [18]. Moreover, KMnO4 is low cost, environment friendly, and water-soluble. Thus, KMnO4 is useful to improve the specific capacitance of activated carbon.
In this research, the activated carbon derived from rubberwood sawdust by using KOH and KMnO4 as an chemical activation agent at the activation temperature of 800 o C were characterized. Surface area (SA), average pore diameter (DP), total pore volume (VTotal), pore size distribution, micropore fraction (MF), and surface morphology were determined. In addition, the effect of KMnO4 on porosity textural properties were discussed.

The precursor and characteristics
Rubberwood sawdust collected from Southern Biomass Fuel Company, Songkhla province, Thailand, was used as the precursor. The particle sizes of sawdust are around 0.2-1 mm. The rubberwood sawdust was cleaned and dried at 105 o C for 24 hours. Then, it was collected into a sample bag and kept in a dehumidifying box. The thermogravimetric analyzer, model TGA 701, LECO, USA (based on ASTM D7582-15) from the office of Scientific Instrument and Testing, Prince of Songkla University, Thailand was used to examined the proximate analysis of the raw material, carbonized char, and activated carbons. The characteristics are presented in Table 3.

Activated carbon preparation
The activated carbon was prepared from the Rubberwood sawdust (RS) by a two-step process. The first step was carbonization process. Rubberwood sawdust was dried and placed in a 4-inch diameter tube to produce char. This reactor was loaded into the tube furnace and heated up to 400 o C at a heating rate of 6 o C/min and maintained for 1 hour, and cooled down in the furnace for 12 hours. The second step, the chemical activation process. Using KOH and KMnO4 as the activating agents. The mixture of char (10 g), KOH (10 g, 30 g), and KMnO4 (0g, 5g) was stirred manually. The well-mixed samples were left for 24 hours and then loaded into the electric tube furnace and heated up to 800 o C for 3 hours. Nitrogen gas was introduced at 500 cm 3 /min to create inert atmosphere during the carbonization and activation process. The activated carbon derived from rubberwood sawdust was soaked with 0.1 Molar Hydrochloric acid (HCL) for 3 hours in room temperature and washed with distilled water until the pH 7 was reached to remove impurities in activated carbon. Finally, the sample was dried in an oven at 105 o C for 24 hours, collected into a sample zip-lock bag, and kept in a silica gel box to prevent moisture absorption. The activated carbon samples were labeled as 10:10:5 (Char: KOH: KMnO4), 10:30:0, and 10:30:5. The schematic preparation of activated carbon was shown in figure 1.

Textural characterization
The textural properties of activated carbons such as surface area (SA), total pore volume (Vtotal), micropore volume (VMic), average pore diameter (Dp), pore size distribution, micropore fraction (MF), and adsorption isotherm were analyzed by using the high throughput surface area and porosity analyzer, Micromeritics (ASAP2460) from the office of Scientific Instrument and Testing, Prince of Songkla University, Thailand. Field Emission Scanning Electron Microscope (FESEM) images were used to examine the morphological properties of activated carbon by Merlin compact, ZEISS Microscopy, Jena, Germany from the Center for Scientific and Technological Equipment, Walailak University, Thailand.

The yield of rubberwood sa1wdust char and activated carbon
The rubberwood sawdust char and activated carbon yield and activated carbon were presented in figure  2. The yield of char was considered as equation (1)

Pore size distribution
Relative pressure (P/P o )

Field Emission Scanning Electron Microscope analysis (FESEM) and EDX analysis
The images of rubberwood sawdust char, ratio 10:10:5, ratio 10:30:0, and ratio 10:30:5 are shown in figure 6, (a)-(d), respectively. The Field Emission Scanning Electron Microscope (FESEM) was used to observe the textural surface of activated carbons with 500x magnification. Char are macroporous (>50 nm) structures undeveloped into mesopore or micropore range. Therefore macropores are unnecessary for the process of adsorption compared with mesopore and micropore structure. Ratio 10:10:5 has less porous structure than 10:30:0, and 10:30:5, but all of those were micropores (< 2 nm), which is related to average pore diameter, as explained in table 3. Thus, chemical activation by using KOH/KMnO4 as a multiple function activation agent can produce activated carbon with a well-formed porous structure from rubberwood sawdust char to increase its adsorption capability. Figure 7

Conclusions
In this study, the rubberwood sawdust char with low surface area, pore volume, and large pore size can improve its properties after the chemical activation process. The yield of product with a ratio of 10:30:5 has the highest value of 18.96% by weight. Activated carbon without KMnO4 (ratio 10:30:0) contains the highest surface area, total pore volume, and micropore volume by 1,574.39 m 2 /g, 0.6405 cm 3 /g, and 0.3533 cm 3 /g, respectively. The ratio 10:30:5 has lower surface area (14.48%), total pore volume, and micropore volume than the ratio 10:30:0. MnO introduced into activated carbon, was blocked the pores of activated carbon, thus decreasing the surface area. The micropore fraction of activated carbon was around 55. 15-59.19%. Adding KMnO4 has not significant effect in micropore fractions. The average pore diameters at ratios 10:10:5, 10:30:0, and 10:30:5 were 1.75 nm, 1.63 nm, and 1.65 nm, which were in a range of micropores size. The adsorption isotherm can be characterized as type I isotherm that can be implied that the activated carbon was microporous solid. FESEM images of the product exposed the development of pore and porous structure from rubberwood sawdust char. Thus, the micropore of rubberwood sawdust char can be generated by using KOH/KMnO4 as an activation agent. Furthermore, the properties of activated carbon derived from rubberwood sawdust are suitable to be used as electrode material in supercapacitor application, and the next study will prove the effectiveness of KMnO4 on the specific capacitance.