Thermal Treatments and Activation Procedures Used in the Preparation of Activated Carbons

conditions on porous structures of olive stone activated by H3PO4. Fuel Processing Technology, Vol. 91, No. 1, (January 2010), pp. (80–87), ISSN 0378-3820. [103] Zhang, H., Yan, Y. & Yang, L. (2010). Preparation of activated carbon from sawdust by zinc chloride activation. Adsorption, Vol. 16, No. 3, (August 2010), pp. (161–166). [104] Zuo, S., Yang, J. & Liu, J. (2010). Effects of the heating history of impregnated lignocellulosic material on pore development during phosphoric acid activation. Carbon, Vol. 48, No. 11, (September 2010), pp. (3293–3295), ISSN 0008-6223. 2


Introduction
The preparation of activated carbons (ACs) generally comprises two steps, the first is the carbonization of a raw material or precursor and the second is the carbon activation.The carbonization consists of a thermal decomposition of raw materials, eliminating non-carbon species and producing a fixed carbon mass with a rudimentary pore structure (very small and closed pores are created during this step).On the other hand, the purpose of activation is to enlarge the diameters of the small pores and to create new pores and it can be carried out by chemical or physical means.During chemical activation, carbonization and activation are accomplished in a single step by carrying out thermal decomposition of the raw material impregnated with certain chemical agents such as H 3 PO 4 , H 2 SO 4 , HNO 3 , NaOH, KOH and ZnCl 2 (Hu et al., 2001;Mohamed et al., 2010).Physical or thermal activation uses an oxidizing gas (CO 2 , steam, air, etc.) for the activation of carbons after carbonization, in the temperature range from 800 to 1100 ºC.The carbonization can be carried out using tubular furnaces, reactors, muffle furnace and, more recently, in glass reactor placed in a modified microwave oven (Foo & Hameed, 2011;Tongpoothorn et al., 2011;Vargas et al., 2010).
Nowadays, the raw materials more used in the preparation of carbons are of lignocellulosic origin.Wood and coconut shells are the major precursors and responsible for the world production of more than 300, 000 tons/year of ACs (Mouräo et al., 2011).However, the precursor selection depends of their availability, cost and purity, but the manufacturing process and the application of the product are also important considerations (Yavuz et al., 2010).Figure 1 shows the number of publications studied in this chapter, related with the preparation of activated carbons from lignocellulosic materials in last two decades.A clear trend can be observed: the number of works increased in the years from 2000 to 2010.The obtained carbons were mainly employed in the removal of water pollutants.
In the present chapter the principal methods used in the preparation of activated carbons from lignocellulosic materials by chemical and physical procedures are discussed.An analysis of the experimental conditions used in the synthesis of ACs has been made attending to the carbon specific surface area.Also the advantages and disadvantages of each method are discussed.

Preparation of activated carbons
The preparation of ACs from lignocellulosic materials involved two processes, the carbonization and the activation, which can be performed in one or two steps depending on the activation method (physical or chemical, respectively).Specifically, when the carbonization is carried out in an inert atmosphere the process is called pyrolysis.According to the literature, the pyrolysis of lignocellulosic materials as coconut shells, olive stones, walnut shells, etc., gives rise to three phases: the char, oils (tars) and gases.The relative amount of each phase is a function of parameters such as temperature of pyrolysis, nitrogen flow rate and heating rate.For example, slow heating rates promote high yields of the carbon residue while flash pyrolysis is recommended for high liquid (oil) ratios (Mohamed et al., 2010).
During the pyrolysis of lignocellulosic precursors, a rudimentary porosity is obtained on the char fraction as a consequence of the release of most of the non-carbon elements such as hydrogen, oxygen and nitrogen in form of gases and tars, leaving a rigid carbon skeleton formed by aromatic structures.
There are two conventional methods for activating carbons: physical (or thermal) and chemical activation.During the chemical activation, the precursor is first impregnated or physically mixed with a chemical compound, generally a dehydrating agent.The impregnated carbon or the mixture is then heated in an inert atmosphere (Moreno-Castilla et al., 2001).On the other hand, during a physical activation process the lignocellulosic precursor is carbonized under an inert atmosphere, and the resulting carbon is subjected to a partial and controlled gasification at high temperature (Rodriguez-Reinoso & Molina-Sabio, 1992).
In the following sections the principal characteristics of the procedures used in the preparation of activated carbons from lignocellulosic precursors by physical and chemical methods are described.

Chemical activation
The carbonization step and the activation step simultaneously progress in the chemical activation (Hayashi et al., 2002a).In this case, the lignocellulosic precursor is treated primarily with a chemical agent, such as H 3 PO 4 , H 2 SO 4 , HNO 3 , NaOH, KOH or ZnCl 2 by impregnation or physical mixture and the resulting precursor is carbonized at temperatures between 400 and 800 ºC under a controlled atmosphere.The function of the dehydrating agents is to inhibit the formation of tar and other undesired products during the carbonization process.Also, the pore size distribution and surface area are determined by the ratio between the mass of the chemical agent and the raw material.Besides, activation time, carbonization temperature and heating rate are important preparation variables for obtaining ACs with specific characteristics (Mohamed et al., 2010).The effects of all these parameters in the textural characteristics of ACs employing different activating agents are discussed in the following sections.

Phosphoric acid (H 3 PO 4 )
In the last 20 years, the activation of lignocellulosic materials with H 3 PO 4 has become an increasingly used method for the large-scale manufacture of ACs because the use of this reagent has some environmental advantages such as ease of recovery, low energy cost and high carbon yield.H 3 PO 4 plays two roles during the preparation of ACs: i) H 3 PO 4 acts as an acid catalyst to promote bond cleavage, hydrolysis, dehydration and condensation, accompanied by cross-linking reactions between phosphoric acid and biopolymers; ii) H 3 PO 4 may function as a template because the volume occupied by phosphoric acid in the interior of the activated precursor is coincident with the micropore volume of the activated carbon obtained (Zuo et al., 2009).
The chemical and physical properties of ACs obtained by chemical activation with H 3 PO 4 are affected by the experimental conditions of preparation such as acid concentration, time of activation, impregnation ratio, carbonization temperature and heating rate.Also some recent works have shown that the atmosphere used in the carbonization process has an obvious effect on the physicochemical properties of ACs (Zuo et al., 2009).1. Experimental conditions of activated carbons obtained by chemical activation with H 3 PO 4 using different lignocellulosic precursors In most of the cited papers, the concentration of acid is greater than 50% (w/w) and the activation temperature for 75 % of these studies is between 350 and 600 ºC (see Table 1).Figure 2 shows the specific surface area calculated by the Brunauer, Emmett and Teller method (S BET ) of the ACs prepared in the contributions collected in Table 1.Carbons obtained with the highest phosphoric impregnation ratio (China Fir and avocado kernel seeds) are the materials with the largest S BET (1785 and 1802 m 2 g -1 ) .Additionally, the carbon obtained from Oil palm shell and activated using a rather low impregnation ratio (0.09) was one of the materials with a lower specific surface area (356 m 2 g -1 ). www.intechopen.com

Zinc Chloride (ZnCl 2 )
Chemical activation of lignocellulosic materials with ZnCl 2 leads to the production of activated carbons with good yield a well-developed porosity in only one step.Impregnation with ZnCl 2 first results in degradation of the material and, on carbonization, produces dehydration that results in charring and aromatization of the carbon skeleton and creation of the pore structure (Caturla et al., 1991).In this case, the precursor is impregnated with a concentrated ZnCl 2 solution during a given contact time, followed by evaporation of the solution and, finally, the precursor is carbonized in an inert atmosphere and thoroughly washed to extract the excess of ZnCl 2 .The amount of ZnCl 2 incorporated in the precursor and the temperature of heat treatment are the two variables with a direct incidence in the development of the porosity.Table 2 shows the experimental conditions used in the preparation of ACs by chemical activation with ZnCl 2 using N 2 as activation atmosphere.
The specific surface areas of the carbons reported in the papers of Table 2 are shown in Figure 3. Carbons obtained using the highest impregnation ratios (2 and 2.5) and an activation temperature of 800 ºC are the materials with the largest S BET (Caturla et al., 1991;Hu et al., 2001) .The carbon obtained from coconout shells reaches an S BET value of 2400 m 2 g - 1 , whereas for the carbon prepared from peach stones the S BET was 2000 m 2 g -1 .Other carbons prepared from coconut shells using an impregnation ratio of 1 and an activation temperature of 500 ºC show lower specific surface areas (1200 m 2 g -1 ).In any case, all the carbons prepared by chemical activation with ZnCl 2 attain S BET greater than 750 m 2 g -1 (Azevedo et al., 2007).The principal disadvantage of this activation is the environmental risks related to zinc compounds.www.intechopen.comIn general, the preparation of ACs by chemical activation with KOH and NaOH allows to obtain carbons with high specific surface areas (>1000 m 2 g -1 ).However, KOH and NaOH are corrosive and deleterious chemicals (Hayashi et al., 2002a).For this reason, recent studies have proposed the preparation of activated carbons by chemical activation with K 2 CO 3 in one step, in which the lignocellulosic materials is impregnated with a K 2 CO 3 solution and finally the impregnated precursor is thermally treated.K 2 CO 3 is a not deleterious reagent and it is broadly used for food additives (Hayashi et al., 2002a).
Table 3 summarizes the experimental conditions used in the preparation of ACs from lignocellulosic materials by chemical activation with NaOH, KOH and K 2 CO 3 .Carbons obtained by activation with NaOH are the materials showing higher S BET (see Figure 4), for example, the carbon obtained from flamboyant exhibiting a S BET near to 2500 m 2 g -1 .Also, the activation with K 2 CO 3 renders carbons with a competitive S BET (between 1200 and 1800 m 2 g -1 ) compared with those obtained by activation with KOH or NaOH.
Other interesting observation is that the specific surface areas of two ACs obtained from pistachio nut shells activated with KOH and treated in two different thermal configurations (a conventional electric oven and a modified microwave oven), were very similar (700 and 796 m 2 g -1 ), thus suggesting that the two methods (conventional and non-conventional) are effective for the preparation of ACs.

Physical or thermal activation
In a physical activation process, the lignocellulosic precursor is carbonized under an inert atmosphere, and the resulting carbon is subjected to a partial and controlled gasification at high temperature with steam, carbon dioxide, air or a mixture of these (Rodriguez-Reinoso & Molina-Sabio, 1992).Steam and CO 2 are the two activating gases more used in the physical activation of carbons.According to the literature, steam or CO 2 react with the carbon structures to produce CO, CO 2 , H 2 or CH 4 .The degree of activation is normally referred to as "burn-off" and it is defined as the weight difference between the carbon and the activated carbon divided by the weight of the original carbon on dry basis according with the following equation, where W 0 is the weight of the original carbon and W 1 refers to the mass of the activated carbon.The use of CO 2 during the activation process of a carbon material develops narrow micropores, while steam widens the initial micropores of the carbon.At high degrees of burn-off, steam generates activated carbons with larger meso and macropore volumes than those prepared by CO 2 .Consequently, CO 2 creates activated carbons with larger micropore volumes and narrower micropore size distributions than those activated by steam (Mohamed et al., 2010) Tables 4 and 5 show the experimental conditions used in the preparation of activated carbons from lignocellulosic materials by physical activation with CO 2 , steam and steam-N 2 admixtures.Normally, in these experiments the lignocellulosic precursor is carbonized in an inert atmosphere (N 2 ) at temperatures ranging from 400 to 950 ºC to produce carbons with rudimentary pore structures.These carbons are then activated with the selected gasification agent at temperatures around 800-1000 ºC to produce the final activated carbons.
Some additional studies combine the thermal or physical activation with chemical activation (also known as physicochemical activation, Table 6).Normally, physicochemical activation is performed by changing the activation atmosphere of the chemical activation by a gasification atmosphere (i.e., steam) at higher temperatures.In other cases, the chemical activation is carried out directly under the presence of a gasifying agent.The combination of both types of carbon activation renders ACs with textural and chemical properties which are different from those obtained by any of the activations alone.For example, steam reduces the occurrence of heteroatoms into the carbon structures.Also, combination of oxidizing reagents in the liquid phase (i.e., nitric or sulfuric acids) with gasification agents improves the development of porosity on the final carbons.
Figure 5 shows the specific surface area of activated carbons obtained by physical and physiochemical activation according with the experimental conditions cited in Tables 4, 5 and 6.In general, the ACs obtained by physical activation with CO2 show a higher specific surface area that those obtained by activation with steam.Additionally, the ACs obtained by physical activation with CO 2 using high heating rates (20 ºC min -1 ) are the adsorbents showing lower S BET (Corncob, Bagasse bottom ash and Sawdust fly ash).

Analysis of the methods use
The advantages and drawbacks of the following points.

Analysis of the methods used in the preparation of ACs
The advantages and drawbacks of the different types of carbon activation are discussed in the following points.

Conclusions
Attending to the works considered in this chapter, chemical activation is the most used method for the preparation of ACs (~60 %) from lignocellulosic precursors.Physical activation methods is used in 28% of the studies and a low quantity of studies combine both methods (i.e., physicochemical) to produce ACs.H 3 PO 4 and ZnCl 2 are the two more employed activating agents in the impregnation of lignocellulosic materials (30% and 24 %, respectively), whereas alkaline reagents such as KOH, NaOH and K 2 CO 3 have been considered because ACs with high specific surface can be obtained (1500-2500 m 2 g -1 ).Physical activation of lignocellulosic precursors normally renders carbons with lower specific surface area.However, when compared with chemical activation, this method is not corrosive and does not require a washing step.

Figure 1 .
Figure 1.Number of publications related with the preparation of activated carbons from lignocellulosic precursors in the last two decades

Figure 2 .
Figure 2. Specific surface area of activated carbons obtained by chemical activation of lignocellulosic materials with H 3 PO 4 (black bars: ACs with greater S BET )

F
r u i t s t o n e s J u t e C o c o n u t F i b e r s O l i v e -m i l l w a s t e w a t e r T e a p l a n t P i n e w o o d P e c a n s h e l l S t e m o f d a t e p a l m J a c k f r u i t p e e l w a s t e A l m o n d s h e l l O i l p a l m s h e l l P i s t a c h i o -n u t s h e l l s L i c o r i c e r e s i d u e s S e a -b u c k t h o r n s t o n e s C h i n a f i r O l i v e s t o n e A v o c a d o k e r n e l s e e

Figure 4 . 3 O
Figure 4. Specific surface area of activated carbons obtained by chemical activation of lignocellulosic materials with KOH, NaOH and K 2 CO 3 O l i v e -m i l l w a s t e w a t e r S o y b e a n o i l c a k e C a s s a v a p e e l C o f f e e e n d o c a r p S t e m o f d a t e p a l m P i s t a c h i o n u t s h e l l * C o r n c o b s P i s t a c h i o n u t s h e l l

3 . 1 32 Disadvantages
Chemical method Advantages  Activated carbons are obtained in one step  Shorter activation times  Lower temperatures of pyrolysis (600 an 800 ºC)  Better control of textural properties  High yield  High surface area of the ACs  Well-developed microporosity  Narrow micropore size distributions  Reduction of the mineral matter content n u t F i b e r s E u c a l y p t u s k r a f t l i g n i n O l i v em i l l w a s t e P i s t a c h i on u t s h e l l s C o f f e e e n d o c a r p P i s t a c h i on u t s h e l l www.intechopen.comLignocellulosic Precursors Used in the Synthesis of Activated Carbon-Characterization Techniques and Applications in the Wastewater Treatment Corrosiveness of the process  Requires a washing stage  Inorganic impurities  More expensive 3.2 Physical method Advantages  Avoids the incorporation of impurities coming from the activating agent  The process is not corrosive  A washing stage is not required  Cheaper Disadvantages  The activated carbons are obtained in two steps  Higher temperatures of activation (800-1000 ºC)  Poorer control of the porosity

Table 2 .
(Foo & Hameed, 2011)09)Moreno-Castilla et al., 2001)Activated Carbon-Characterization Techniques and Applications in the Wastewater Treatment 24 Experimental conditions of activated carbons obtained by chemical activation with ZnCl 2 using different lignocellulosic precursors Alkaline hidroxides (KOH, NaOH) and carbonates (K 2 CO 3 , Na 2 CO 3 ) have been used as activation reagents in the preparation of activated carbons with high specific surface.In general terms, chemical activation by KOH and NaOH consists in a solid-solid or solidliquid reaction involving the hydroxide reduction and carbon oxidation to generate porosity(Adinata et al., 2007).The activation with KOH was first reported in the late 1970s by AMOCO Corporation; since then many studies have been devoted to the preparation of ACs by chemical activation with KOH(Lua & Yang, 2004).In this context, two procedures have been used.The carbon precursor can be mixed with powder of KOH or impregnated with a concentrated solution of KOH and then the solid mixture or impregnated precursor is thermally treated under nitrogen(Bagheri & Abedi, 2009;Moreno-Castilla et al., 2001).Alternatively, the preparation of ACs by alkaline activation is made in two steps, in which the precursor is first pyrolyzed and the obtained carbon is activated with a solution of KOH(Bagheri & Abedi, 2009)or with pellets of KOH and finally thermally treated again.The activation step can be conducted in a glass reactor placed in a modified micro wave oven with a frequency of 2.45 GHz(Foo & Hameed, 2011).
www.intechopen.comFigure 3. Specific surface area of activated carbons obtained by chemical activation of lignocellulosic materials with ZnCl 2 (black bars: ACs with greater S BET ) 2.1.3Alkalis

Table 3 .
Experimental conditions of activated carbons obtained by chemical activation with NaOH and KOH using different lignocellulosic precursors www.intechopen.com

Table 4 .
Experimental conditions of activated carbons obtained from different lignocellulosic precursors by physical activation with CO

Table 5 .
Experimental conditions of activated carbons obtained from various lignocellulosic precursors by

Table 6 .
Experimental conditions of activated carbons obtained from different lignocellulosic precurso