Due to their high surface area, large pore volume, and favorable pore sizes, activated carbons (AC) (essentially, porous carbons) are being used in many applications, ranging from gas adsorption, storage and separation, to water cleaning, and as catalysts in various chemical reactions [1
]. The surface functional groups on activated carbons determine their adsorption behavior and surface reactivity, among other properties [4
]. In general, the final surface functionalities and properties are determined by the activation process and by the nature of charred precursors [6
]. Out of the different types of precursors, lignocellulosic biomass derived from agricultural wastes is a promising raw material, in part because of its abundant availability [2
]. Apart from the economic benefits, using these feedstocks for producing AC closes the carbon cycle and helps in solving air pollution and water pollution problems [7
]. Numerous lignocellulosic precursors—such as wood, almond shells, coconut shells, apple pulp, cotton stalks, rice husk, plum stones, and others—are being successfully used to produce activated carbons [7
]. This is basically a two-step process, of which the first one is carbonization in a high-temperature (300–900 °C) pyrolysis process, where moisture, volatiles, and most of the non-carbon hetero-elements in biomass (oxygen, hydrogen, nitrogen, and sulfur) are removed. The resultant charred material (biochar) has much higher carbon content. The second step is the activation process, intended to develop high internal porosity and surface area for enabling and enhancing the adsorption function. The main constituents of lignocellulosic biomass are cellulose, hemicellulose, and lignin. During the carbonization step, these macromolecules decompose at different rates and at different temperatures [6
]. Of all three, lignin is the most thermally resilient and ends up being the most abundant component of charred materials. Hence the respective proportions of the three components in the initial raw material and at the end of the carbonization step play a big role in determining the properties of charred materials, their behavior during activation, and eventually the yield and adsorptive properties of end products—activated carbons [1
Conversion of biomass to biochar is one of the newest approaches to carbon sequestration in terrestrial ecosystems. In general, biochar is a byproduct from pyrolysis of cellulosic biomass for production of biofuels and hydrogen. The char materials can act as a soil conditioner, enhancing plant growth by retaining humidity and supplying nutrients, and eventually improving soil physical and biological properties [16
]. Another potential use of biochar is a fuel additive in ruminant diets [17
]. However, a higher value product could be obtained by converting biochar to activated carbons. There are two main routes for this conversion: physical activation and chemical activation. Physical activation (sometimes also called thermal activation) is the process of slow oxidation of the char with mild oxidizing agents—such as steam and carbon dioxide—at temperatures usually between 600 °C and 900 °C. The slow oxidation process creates new porosity or enlarges the existing pores. Carbon dioxide is usually preferred over steam because of its low reactivity that permits controlling oxidation rates such that uniform porosity is developed. Chemical activation is based on char impregnation with chemicals (ZnCl2
, KOH, H3
) which promote dehydration, polycondensation, and gasification reactions at lower temperatures than those needed for physical activation. The yield is higher, but the process requires extensive washing of final product for elimination of chemicals, which is perceived as an environmental penalty.
This study is focused on physical activation by CO2 of biochar materials obtained from different lignocellulosic precursors, and characterization of their surface properties and porosity using nitrogen adsorption measurements. An attempt was made to relate the properties of activated carbons, such as surface area and pore volumes, to the activation conditions and the nature of precursor materials.
Using charred materials from various raw materials—pyrolyzed at identical conditions and physically activated to weight losses in a narrow range (30–35%)—made it possible to seek correlations between the nature of raw lignocellulose materials and the properties of char intermediates, on one hand, and function-enabling properties of activated carbons, on the other hand. It is generally agreed that the properties of activated carbons depend largely on the nature of lignocellulosic precursors, carbonization (pyrolysis) conditions, and activation procedures. Of the three components of lignocellulosic precursors—hemicellulose, cellulose, and lignin—lignin is more resistant to the thermal process and ends as the main component of chars and activated carbons [6
]. Cellulose and hemicellulose are thermally decomposed in larger proportion than lignin and end as being a minor component in charred materials and in physically activated carbons. Thus, the ratio of cellulose, hemicelluloses, and lignin in the charred precursor determine the properties of the activated carbon, but in different ways and in proportion to their initial weight content: thermally labile cellulose and hemicelluloses contribute to the pore system, while the more resistant lignin is retrieved in the solid carbon matrix.
Since the steps of biomass pyrolysis and physical activation of chars were done (as close as possible) in similar conditions, a closer look into the results may possibly reveal correlations, if any, between the nature of raw lignocellulosic materials, properties of intermediate chars, process conditions during activation, and useful properties of the end products—activated carbons. Searching for correlations between the raw material (wood, grass) and the product (activated carbon) is difficult because the sequence of physical and chemical transformations along the process chain may have weakened or even hidden any causality.
summarizes available information on raw biomass, charred materials, and activated carbons, including porosity, and surface area. The raw biomass contains significant amounts of moisture, and about half of its weight is elemental carbon. Analysis of dried wood crumbles before pyrolysis (Table 1
) shows that volatile products (to be collected as biofuel and gas products in the fast pyrolysis process) represent about 90% of weight, fixed carbon (to be retrieved as biochar) represents 4–8 wt %, and mineral ash is less than 2 wt %. The biochars contain much lower amounts of moisture and volatile products, and their carbon yield after TGA analysis in N2
at 1000 °C (Figure 3
) is 80–90%. No attempts were made to determine the ash content in this carbon solid. Activation in CO2
at 800 °C yields roughly 50–72% activated carbon, based on the initial char weight. The weight loss in the gasification reaction with CO2
is 20–45%, and the difference is made by moisture and volatiles.
Rutherford et al. [36
] studied chemical processes during pyrolysis of pine bark, pine wood, and poplar wood. They reported that thermal degradation of cellulose and hemicellulose in the early stages (below ~250 °C) is accompanied by an increase of aromatic structures, a decrease of aliphatic structures, and release of hydrogen and oxygen at a faster rate than carbon oxides. These conclusions were supported by Zhao et al. [24
] who performed TGA runs up to 900 °C in nitrogen using pure chemical components. They found that the decomposition of cellulose occurs in one sharp step (335 °C) and is almost complete at 900 °C; decomposition of hemicellulose occurs in two sharp steps (295 and ~750 °C) with little residue (~15%) at 900 °C; and thermolysis of lignin is a continuous process (~300–600 °C) with a larger residue (35%) at 900 °C.
The original charred materials used in this work have already porous structure (Table 1
), with BET surface areas between 150 m2
/g (HW) and 530 m2
/g (YP). Interestingly, the chars that produced activated carbons with the highest BET surface area after oxidation by CO2
have had modest surface areas before activation: 250 m2
/g (CO) and 340 m2
/g (WP). This proves that the activation step plays an important role in determining the final properties of activated carbons. As reiterated by Cha et al. [37
] in a recent review, porosity is generated during activation by selective oxidation of the weakest, unstructured regions in the charred material, where fine pores are developed. The kinetics of the oxidation process appears to be a key factor in controlling porosity.
The compensation relationship between Eact
) in oxidation kinetics by CO2
) has direct effects on the rate of activation, as illustrated in Figure 7
. The rate of activation increases with the increase of both parameters, but in different ways. The activated carbons with the largest BET area, WP, and CO, were obtained with the slowest oxidation reaction rates, corresponding to the lowest Eact
). They have been exposed the longest times at 800 °C before reaching the target weight loss. The slow rates of activation allowed extensive development of surface area and porosity. It is also observed from Table 1
that both WP and CO chars have similar moisture and volatile contents, but WP lost more weight in the gasification step at 800 °C (35%) than CO (21%). As a result, WP developed larger surface area and pore volume (1050 m2
/g; 1.44 cm3
/g) than CO (915 m2
/g; 1.15 cm3
/g). However, in relative terms, the fraction of mesopores (2–50 nm) in the total pore volume is equal and large (73%) for both these activated carbons. The dominant mesoporous structure of activated WP and CO carbons (Figure 6
) is an advantage for solution applications. It enables good penetration of the electric double layer at carbon/solution interface, and enhances adsorption properties not only for hydrated ions but also for large organic molecules (dyes, antibiotics) and micelles in colloidal solutions (oil in water, proteins) etc.
In contrast, the activated carbon derived from HW has moderate BET surface area (730 m2
/g) and total pore volume (0.40 cm3
/g) and contains a large proportion (67%) of micropores (<2 nm). The HW char had the faster oxidation reaction in CO2
, supported by the largest kinetic parameters (Figure 7
). A high proportion of micropores is desirable for gas phase applications based on adsorption in molecular-sized micropores, such as gas cleaning, storage, and separation.
Since the rate of activation in CO2
appears as an important factor for the development of porosity in the activated carbon, it is interesting to examine which char properties determine the rate of oxidation by CO2
. Figure 8
shows correlations between Eact
) parameters, on one hand, and the carbon yield in TGA tests at 1000 °C in dry nitrogen (Table 1
) on the other hand. Three of the four best ranking activated carbons based on the BET surface area (WP, CO, YP) were obtained from chars with the largest carbon yield in nitrogen. Apparently, high carbon yields at 1000 °C characterize more robust chars with slow activation rates (consistent with low Eact
)) which generate high surface areas. However, this rule is not obeyed by HW, an oak-hickory mixture ranked third on the BET scale. Although the charred HW gave the lowest carbon yield at 1000 °C, its reaction rate in CO2
was the fastest (Figure 7
) and the kinetic parameters were the largest (Figure 4
The above correlations gain substance when analyzed in the context of chemical processes that occur during thermal treatments. It is known that the major transformation occurring during pyrolysis of lignocellulose materials above 400–500 °C is the development of aromatic structures. Lignin already contains aromatic structures and is more resistant to pyrolysis and oxidation than cellulose and hemicellulose [38
]. At 800 °C, some biochars may still contain small amounts of aliphatic carbon from untransformed cellulose, but the major content is lignin carbon with fused aromatic rings. Dehydrogenation and condensation of aromatic structures intensify above 800–900 °C and continue at 1000 °C. The lignin-derived carbon provides the matrix for the development of microporosity. Low weight losses (of hydrogen) during dwelling for 30 min at 1000 °C and high carbon yields at the end of the thermal process in nitrogen signify the presence of robust, well-structured aromatic carbon in the materials derived from WP, CO, and YP chars. They will be converted to activated carbons with large BET surface area.
Oxidation by CO2
is endothermic and becomes thermodynamically possible only after about 700 °C; the rates increase significantly from 900 °C or 950 °C [39
]. Porosity is generated in the reaction of CO2
with organized aromatic structures of lignin-derived carbon, as found in the solid residue left from TGA runs in nitrogen at 1000 °C. If disordered hemicellulose-derived carbon structures were still present in biochar at 800 °C, they would be oxidized by CO2
but would not contribute much to the development of porosity and increase in surface area. Indeed, materials with the lowest carbon yields (HW, SG, RW) produced activated carbons with low surface areas and a large proportion of micropores.
A,B illustrate these conclusions. They compare BET values of activated carbons and the corresponding micropore/mesopore volumes ratios against the final carbon yield at 1000 °C in nitrogen (Figure 3
). The six activated carbons form two groups. Those chars with high carbon yield (YP, CO, WP) generate activated carbons with high BET surface area and more mesopores in their total porosity. Indeed, robust chars with slow activation rates have spent longer times in the activation step and develop larger pores and higher BET areas. Less robust chars, with low carbon yield in dry nitrogen (HW, RW, SG) have faster reaction rates during activation, which leads to primary development of micropores but limits the growth of BET surface area. A paradoxical situation is observed: in the group of activated carbon studied here, large surface area carbons are mostly mesoporous, while microporous carbons have quite low surface area. This contrasts with the known trends observed during physical activation of any given char, where initial development of micropores expands BET area, and further collapse of micropores into mesopores drops it. Evidently, comparing activated carbons obtained from different charred sources is not expected to reproduce familiar trends encountered in activation of uniquely sourced chars.