Chen060214.qxd The Journal of Engineering Research Vol. 5, No.1 (2008) 47-54 1. Introduction Activated carbon is used in many applications due to its adsorption capacity and large internal surface area. These applications are partly facilitated by the fact that the carbon precursor or the preparation conditions could be chosen to achieve a unique set of desirable properties (Luehrs, et al. 1998; Wartelle, et al. 2000 and Byrne, et al. 1997). Both naturally occurring and synthetic carbona- ceous solid have been used (Gergova, et al. 1997; Jagtoyen, et al. 1992; Benaddi, et al. 2005; Yalcm, et al. 2000 and Nagano, et al. 2000). Though new synthetic precursors such as polymeric fibers and phenolic-resin have been reported to produce high surface area products (Cai, et al. 2004), agricultural wastes are increasingly being investigated as precursors (Wartlle, et al. 2004; Byrne, et al. 1997; Gergova, et al. 1996; Yalcm, 2000; Liou, 2004 and Carvalho, et al. 2003). Wood is the most favored source of activated carbon production (35%) fol- lowed by coal (28%), lignite (14%), coconut shell (10%), peat (10%) and others (3%) (Byrne, et al. 1977). _____________________________________ *Corresponding author’s e-mail: baba@squ.edu.om Other lignocellulosic materials are also used which include nutshell (Wartelle, et al. 2000), apricot stones (Gergova, et al. 1996), rice husks (Yalcm, et al. 2000), cork waste (Carvalho, et al. 2003) and Dates pits (Caturla, et al. 1991; Molina-Sabio, et al. 1995; Wigmans, et al. 1989; Pelekani, et al. 2000 and Daifullah, et al. 2003). The activating agents for preparing activated carbons can be used before or after carbonizing the raw materials. Some of the activating chemicals used in this process are boric acid, phosphoric acid, nitric acid and zinc chloride (Girgis, et al. 2002 and El-Hendawy, 2006). Once car- bonization is achieved, the agents most widely used to activate the char are: steam, carbon dioxide, air or their combinations. For example, when phosphoric acid is used as an acti- vation chemical, the acid react with the lignocellulosics thermal decomposition products. This leads to bond weak- ening and formation of cross-linked structure. This reduces the release of volatile materials, restricts tar for- mation and loss of porosity (Teng, et al. 1998). In addi- tion, the effect of the chemical agent leads to lower acti- vation temperature and higher carbon yields (El-Akkad, et al. 19978). Study of Date Palm Stem as Raw Material in Preparation of Activated Carbon O. Houache1, R. Al-Maamari2, B. Al-Rashidi2 and B. Jibril*2 1Oman Polypropylene LLC, Sohar Industrial Port Complex, P.O. Box 277, PC 322, Falaj, Al-Qabail, Sohar, Oman *2Department of Petroleum and Chemical Engineering, Sultan Qaboos University, P.O. Box 33, Al Khod, PC 123, Muscat, Oman Received 14 February 2006; accepted 10 March 2007 Abstract: Activated carbon adsorbent was prepared using Omani date palm tree stem as a precursor. Precursor samples were subjected to thermal treatment (at 400, 500 and 600 oC) before or after impregnation with either H3PO4 (85 wt %) or KOH (3 wt %). The activated carbon obtained was characterized by BET (surface area and porosity), Gas Pycnometry (true den- sity) and SEM (texture). Sample subjected to carbonization, without chemical activation, exhibited low surface areas ~ 1.0 m2/g at 400 and 500 oC and 124 m2/g at 600 oC. Further treatment of such samples with either the acid or the base did not show improvement in surface area or other properties. Impregnations of the precursor with acid before carbonization signif- icantly improved the surface area to as high as 1,100 m2/g at a carbonization temperature of 500 oC. Thus, activated carbon with a moderate surface area could be produced from date palm stem using low carbonization temperature. Keywords: Activated carbon, Chemical activation, BET surface area, Porosity, SEM, Date palm stem §°ûæŸG ¿ƒHôµdG Ò° – ‘ ΩÉN IOɪc πîædG ´hòL ΩG~îà°SG á°SGQO ¢TGƒM ôªY1…ôª©ŸG ~°TGQ ,2…~°TGôdG Q~H ,22*πjÈL ÉHÉH , áá°°UUÓÓÿÿGG,500 ,400 IQGôM äÉLQO ≈∏Y ájQGô◊G á÷É©ª∏d â©° NCG πîædG ´hòL äÉæ«Y ,§°ûæŸG ¿ƒHôµdG êÉàfE’ ΩÉN IOɪc »∏ÙG πîædG ´hòL ΩG~îà°SG ” á°SGQ~dG √òg ‘ : á«ë£°ùdG áMÉ°ùŸG ¢SÉ«b ≥jôW øY §°ûæŸG ¿ƒHôµdG IOƒL º««≤J ” .…~YÉb hG »° ªM ∫ƒ∏fi ‘ π«ªëàdG h ôª¨dG ∫ÓN øe »FÉ«ª«µdG §«°ûæàdG ~©H hCG πÑb ájƒÄe áLQO 600 h â©° NCG »àdG iôNC’G äÉæ«©dÉH áfQÉ≤e á«ë£°ùdG áMÉ°ùŸG å«M øe I~«L èFÉàf â£YCG ájQGô◊G á÷É©ŸG πÑb ¢ ª◊ÉH É¡à÷É©e ” »àdG äÉæ«©dG .á«æÑdG h áaÉãµdG h á«eÉ°ùŸG h .ájQGô◊G á÷É©ŸG ~©H É«FÉ«ª«c É¡£«°ûæJ ” »àdG ∂∏J hG ≥Ñ°ùe »FÉ«ª«c §«°ûæJ ¿h~H ájQGô◊G á÷É©ª∏d áá««MMÉÉààØØŸŸGG ääGGOOôôØØŸŸGG.πîædG ´hòL ,Êhεd’G ô¡ÛÉH í°ùŸG ,á«eÉ°ùŸG ,á«ë£°ùdG áMÉ°ùŸG ,»FÉ«ª«µdG §«°ûæàdG ,§°ûæŸG ¿ƒHôµdG : 48 The Journal of Engineering Research Vol. 5, No.1 (2008) 47-54 Based on the foregoing, we found it of interest to study different lignocellulosics materials as sources of activated carbon. Here, we report phosphoric acid and potassium hydroxide treatment of an Omani Date palm stem for preparing activated carbon. Oman and neighboring areas have millions of Date-palm-trees. Every year tons of Date palm tree trunk and stem are disposed off. This gives a free or low-cost, abundantly supplied precursor. As stated above, many workers have reported different aspects of production of activated carbon from Date pits, but to our knowledge, there are no reports on the use of Date-palm- tree stem for such endeavor. The aim of this work was to prepare and characterize activated carbons from Date palm stems using chemical activation method which will contribute to further understanding the effects of activat- ing chemical on the precursor's surface and other proper- ties of the final products. 2. Experimental Setup Activated carbon was prepared and characterized at the Petroleum and Chemical Engineering laboratory, Sultan Qaboos University, Oman. 2.1 Sample Preparation Date palm stems were obtained from Nizwa region of Oman. They consist of a major outer part having soft- mesh and channel-like structure and inner hard part hav- ing longitudinal fibrous structure, as shown in Fig. 1. Samples were prepared by cutting slices of 1 cm thick from different parts of the stem before drying them in a muffle heater at 105 oC for 24 hrs. Longer drying time did not show observable change in the samples weight. The dried samples were then used as precursors to obtain char and activated carbon. Before or after activation (impreg- nation) with either H3PO4 (85 wt %) or potassium hydrox- ide KOH (3 wt %) the weighed samples were carbonized in a muffle furnace under nitrogen atmosphere. The chemical activation was accomplished by having 30g of each dried sample immersed in 100g of activating agents [either H3PO4 (85 wt%) or KOH (3 wt%)] at room temperature then increasing the heating temperature to 85 oC at a heating rate of 15 oC/min. The sample was exposed to this temperature for 3 hrs before cooling to ambient temperature and left at rest for 16 hrs. These treated sam- ples were then dried at 105oC. In all cases, carbonization was done under flow of nitrogen (250 ml/min). It took place by heating the sam- ples at 100 oC for 20 min before raising the temperature in steps of 50 oC up to the desired temperature (400, 500 or 600 oC). The final temperature was held constant for two hours before cooling the sample to ambient temperature. Density and surface analysis took place after ground- ing and sieving the samples to sizes between 212 and 250 microns. In the test, , the samples are represented based on the activation procedure as follows: S-untreated = Date palm stem; S-C = Carbonized Date palm stem; S-C-H3PO4 = Carbonization of Date palm stem followed by acid activa- tion; S-C-KOH = Carbonization of Date palm stem fol- lowed by base activation; S-H3PO4-C = Acid activation followed by carbonization of Date palm stem and S-KOH- C = Base activation followed by carbonization of Date palm stem. 2.2 Characterization of Samples Samples of the activated carbon were characterized by physical adsorptions of N2 at 77.4 K, using a Quantachrome Autosorb 1C analyzer. The properties obtained are: (i) BET surface area (SBET, m2/g), (ii) total pore volume from nitrogen held as liquid at P/Po = 0.95 (VP, cm3/g) and (iii) average pore diameter. Prior to the analysis, samples were cleaned by heating them, under flow of helium, at 300 oC for 3 hrs. Densities of the sam- ples were measured by displacement of helium using a Quantachrome UltraPycnometer 1000. The textures of samples of the products were observed using SEM - scan- ning electron microscope (JEOL JSM 840A). It was equipped with an energy dispersive X-ray analyzer that allowed semi-quantitative analysis of solids. 3. Results and Discussion The precursor was subjected to different carbonization times and activations and the decreases in sample weight were recorded. The char yield of Date palm stem was compared with those obtained from other cellulose mate- rials such as almond shell and coconut shell, in Fig. 2. It could be observed that the char yielded from Date palm stem is substantially higher than that from other lignocel- luloic sources (Al-Rashidi 2004). Carbonization without chemical impregnation of the precursor yielded char that exhibited surface areas of about 1.0 m2/g at 400 and 500 oC. The area increased to 124 m2/g at 600 oC. The yields were improved by impregnations of the precursor with either H3PO4 or KOH before carbonization. The surface area also increased. The base treatment increased the area Figure 1. Sample of the date palm stem 49 The Journal of Engineering Research Vol. 5, No.1 (2008) 47-54 by one order of magnitude at 400 or 500 oC, but led to an inferior product at 600 oC, as shown in Table 1. On the other hand, acid treatment improved the surface areas at all temperatures: 690.9, 1101.0 and 632.2 m2/g at 400, 500 and 600 oC respectively. This suggests that by chemical treatment of Date palm stem, the chemistry of the car- bonization was changed. Due to dehydration, different macromolecular networks were formed which resulted in promotion of cross linking of structure and suppression of tar formation during the carbonization (Attia, 1997). Furthermore, the increase and decrease in surface areas with the chemical treatment, indicate the complex and interacting effects of different changes in the sample. Due to different degrees of interaction, some volatiles compo- nents may react and link to carbon structure, or other com- pounds could be formed thereby changing the product porosity and the extent of exposed surface area. The characteristic of the product was further explored by measuring the true densities. Figure 3 shows increase in densities with increase in carbonization temperature. When the char formed was subjected to post-treatment with either acid or base, different behaviors were observed. For the char produced at 400 oC, impregnation with KOH exhibited a little decrease in density. However, at higher temperatures, there was an increase of the densi- ty by 21.2 and 5.8% at 500 and 600 oC respectively. On the other hand, samples treated with acid, show a decrease in the densities; 33.5, 33.3 and 8.8% at 400, 500 and 600 oC respectively. In all cases, the samples were treated to adjust the pH of the water in which the char was suspend- ed. Depending on the surface morphology of the char, the KOH may be binded with surface functional groups (Shih, et al. 2005). Such interactions would lead to corss- linking reactions with some of the volatiles thereby retain- ing them in the solid matrix. This would result into increase in the net mass of the sample, thereby increasing the density. Addition of H3PO4 on the other hand, may lead to dehydrogenation and release of gases, thereby decreasing the net mass and density of the char. Impregnation of the precursor prior to carbonization lead to partly physical diffusion of the chemical to the internal structures of the samples and partly chemical reaction with compound in the structure. Carbonization resulted into chars of different characteristics. Their differences decrease with an increase in the temperature (Fig. 3). All samples appear to converge to constant similar densities. This implies that the final products obtained may have similar true densities (Shih, et al. 2005). The SEM micrographs of (a) Date palm stem and chars produced at (b) 400, (c) 500 and (d) 600 oC are shown in Fig. 4. The micrograph of the precursor exhibited a dense 0 10 20 30 40 50 400 500 600 Carbonization Temperature, oC C ha r Y ie ld (% ) Date stem Coconut shell Regenerated Cellulose Figure 2. Comparison of char yields for different pre- cursors at different carbonization tempera- ture Temperature ( oC) Sample 400 500 600 S-C 0.0936 0.7700 119.0 S-C-H3PO4 1.2600 0.3610 31.50 S-H3PO4-C 691.0 1100.0 632.0 (SBET, m2/g) S-KOH-C 27.80 29.40 94.70 S-C 539.0 295.0 28.20 S-C-H3PO4 104.0 67.00 41.40 S-H3PO4-C 36.90 41.60 37.30 Avg. pore diameter (Å) S-KOH-C 45.90 50.70 36.00 S-C 0.0012 6 0.00568 0.0839 S-C-H3PO4 0.0033 0.00061 0.0326 S-H3PO4-C 0.6370 1.1500 0.5890 (VP, cc/g) S-KOH-C 0.0320 0.0372 0.0852 Table 1. Surface area, acverage pore diameter, and total pore volume of samples prepared at different heat treatment 1 1.5 2 2.5 350 450 550 650 Carbonization Temperature, oC D en si ty , ( cm 3 / g) S-C S-H3PO4-C S-KOH-C S-C-H3PO4 S-C-KOH Figure 3. Density of different samples after carbon- ization 50 The Journal of Engineering Research Vol. 5, No.1 (2008) 47-54 structure and smooth surface. The chars show irregular and flaky texture. Interconnecting large pores that increase with the temperature could be observed. This suggests the increase in the release of the volatile com- pounds from the samples, in line with the increase in the true density. The surface textures were changed when the chars were subjected to H3PO4 and KOH (Fig. 5). The tex- ture became denser (higher for acid treated - Fig. 5(a-c), than base-treated samples - Fig. 5(d-f)) and exhibited pores of different sizes and shapes. This indicates the chemical effects on the samples or physical washing of organic matter from the surface. The base-treated samples exhibited grey particles covering the surface. Acid and base impregnation before carbonization resulted into more observable porous surface (Fig. 6 a-c (acid) and d-f (base)). Again, pores of different sizes and shapes could be observed. The higher surface area of samples (Fig. 6b) than that of (Fig. 6c) could be related to the fact that heat- ing above 500 oC destroyes the tiny pores structure and therefore reduces the available surface area. The smaller pore perimeters of samples in Fig. 6d-f than that of Fig. 6a-c suggest that the chemical washing done by the acid is superior to the one done by the base which might have led to higher surface area. The acid-impregnated samples exhibited surface area an order of magnitude higher than the base treated ones. The base treatment resulted into interaction that produced porous surface, with shallow pores, as indicated by the SEM images. Surface areas and porosities for the treated carbons are summarized in Table 1. The surface area increases and the average pore diameter decreases with temperature. Carbonized Date palm stems are characterized by poorly developed porous structures of low surface area (0 - 119 m2/g), low pore volumes (0.00126 - 0.0839 cm3/g) and medium to large average pore diameter (28.2 - 539.0 Å). Subsequent acid treatment of chars reduced porosity development with a decreased surface area (0.36-31.5 m2/g), decrease in the pore volume (0.0006-0.0326 cc/g) and consequent increase in the mean pore dimension (41.4-104 Å) compared to carbonization. Perhaps inor- ganic materials started to react at lower temperature but on heat treatment to higher temperature when the pores were being reorganized; inorganic materials might have filled or blocked some portion of the macropores (Lua, et al. 2004). Activation with phosphoric acid and then car- bonization at 500 oC led to the best porous product with surface area of 1100 m2/g, internal porosity of 1.15 cm3/g and a mean pore dimension of 41.6 Å. It can be observed a b c d Figure 4. SEM micrographs of Date palm stem chars produced at different temperatures: (a) precursor (as is), (b) 400oC (c) 500oC, (d) 600oC 51 The Journal of Engineering Research Vol. 5, No.1 (2008) 47-54 a b c d e f Figure 5. SEM micrographs of chars produced at different temperature and subjected for either H3PO4 or KOH: (a) S-400-acid, (b) S-500-acid, (c) S-600-acid (d) S-400 base, (e) S-500-base, (f) S-600-base 52 The Journal of Engineering Research Vol. 5, No.1 (2008) 47-54 a b c d e f Figure 6. SEM micrographs of chars produced by subjecting the precursor to either H3PO4 or KOH and then carbonized at different temperature: (a) S-acid-400, (b) S-acid-500, (c) S-acid-600, (d) S-base-400, (e) S-base-500, (f) S-base-600 53 The Journal of Engineering Research Vol. 5, No.1 (2008) 47-54 that activation with phosphoric acid and then carboniza- tion showed superiority under all treatments. The improvement in surface characteristics when comparing phosphoric acid activation to carbonization were very highly significant; also phosphoric acid activation result- ed in a highly significant improvement over potassium hydroxide activation. The results showed that the Date palm stem chars con- sist mainly of macropores and mesopores at low treatment temperature. This result may be further discussed based on the fact that the proportion of a particular porosity generated in the active carbon depends on the preparation conditions, nat- ural and chemical composition of the precursor (Plante, et al. 1988). The conversion of cellulose material to solid carbon yields porous carbon. The porosity depends on the original anatomy of the precursor. The activation treat- ment given; either by heat treatment or by chemicals results in the formation of surface oxygen complexes. This also results in the increase in the aromaticity of car- bon surface due to cross-linking and polymerization of organic molecules, removal of volatiles and opening of blocked pores as the heat treatment temperature is raised (Kidena, et al. 2004). During the process of activation, the spaces between the elementary crystallites become cleared of various carbonaceous compounds. This com- plex and different degrees of physical and chemical changes in the sample led to similarly irregular surface area, porosity and texture of the samples. 4. Conclusions Porous activated carbon adsorbent has been prepared from Date palm trees. High carbon yield was observed. The products exhibited dominance of meso and macro- pores, especially at low carbonization temperature (400 - 500oC). Generally, phosphoric acid treated precursor yielded higher porosity. A surface area of 1100 m2/g was obtained from the precursor impregnated with H3PO4 and carbonized at 500 oC. 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