**2. Properties of nanoporous carbons**

Describing nanoporous carbon materials as carbon structures with enclosed porosity is correct but it is somewhat incomplete since properties, as for example, surface chemistry, chemical composition, morphology or electrical conductivity are also essential to understand their potential and performance in a given application.

Regarding the physical properties of nanoporous carbon materials, porosity is undoubtedly the most important one. IUPAC defines *nanopore*s as pores with apertures up to around 100 nm and, according to their size, *nanopores* are classified as follows [18]:


Due to their relevance for the adsorption process, micropores can be further distinguished between *narrow micropores* (i.e. *ultramicropores*) with width <0.7 nm and *wide micropores* (i.e. *supermicropores*) with width between 0.7 and 2 nm [18].

The limits for this pore classification were suggested by nitrogen adsorption-desorption data at −196°C [18] and rely on the fact that each pore size range corresponds to different pore filling mechanisms disclosed by the isotherm profile [19]. Micropore filling is regarded as a primary physisorption process and occurs at low relative pressures. Firstly, at very low relative pressure, adsorption occurs in the ultramicropores where due to the close vicinity of pore walls there is an overlapping of the adsorption forces favoring the enhancement of the adsorbent-adsorbate interaction, this process is termed "primary micropore filling." After this initial filling in the most energetic sites, the cooperative adsorbate-adsorbate interactions inside the supermicropores become more relevant (secondary process) occurring up to relative pressures ≈ 0.15 [18]. In mesopores, the physisorption comprises three stages: monolayer adsorption (i.e. all adsorbate molecules are in contact with the surface of the adsorbent) followed by consecutive adsorbate layers (multilayer adsorption) and finishing with capillary condensation [18]. The adsorption onto mesopores commonly presents a hysteresis loop [19]. The macropores are so wide that do not allow capillary condensation since behave as an open space (cannot be characterized by nitrogen adsorption whose maximum relative pressure occurs near the unity) [19].

the PSD, there are various mathematic models available, being those using the Non-Local Density Functional Theory (NLDFT) the most advanced ones. The use of nitrogen as adsorptive at −196°C has however diffusional limitations in ultamicroporous carbon materials; thus, for an accurate characterization of the micropore network, carbon dioxide adsorption at 0°C has been repeatedly proposed as a better alternative to nitrogen [20–22]. Besides the limitation of nitrogen for the characterization of ultramicropores, it is known that, due to its quadrupole moment, nitrogen molecule orientation during adsorption will be dependent on the surface chemistry of the adsorbent [18, 22]. To overcome this issue, other adsorptive gases like argon

**Figure 1.** Schematic representation of pore structure (bottom rectangle) and surface chemistry—oxygen, nitrogen, and

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Carbon atoms are the major element of nanoporous carbon skeleton but the elemental composition of these materials also includes hydrogen and oxygen and, depending on the precursor, preparation route and post-synthesis functionalization may also have nitrogen,

or krypton can be used.

sulfur groups (top circles)—in nanoporous carbons.

In nanoporous carbons, the porosity results from the voids between the randomly cross-linked graphite-like crystallites that constitute the structure of the carbon skeleton, from the consumption of less organized carbonaceous matter and also from the removal of reactive carbon atoms in the crystallite during activation process (**Figure 1**). Surface area and pore size distribution (PSD) are the physical properties that will greatly influence the performance of nanoporous carbons in a given process. These parameters are commonly obtained from the analysis of nitrogen adsorption isotherm at −196°C. The surface area (apparent surface area for microporous materials) is commonly obtained applying the Brunauer-Emmett-Teller (BET) equation. Regarding Nanoporous Carbon Synthesis: An Old Story with Exciting New Chapters http://dx.doi.org/10.5772/intechopen.72476 41

**2. Properties of nanoporous carbons**

40 Porosity - Process, Technologies and Applications

and performance in a given application.

occurs near the unity) [19].

• *Micropores*—pores with width less than 2 nm

• *Mesopores*—pores with width between 2 and 50 nm • *Macropores*—pores with width greater than 50 nm

*supermicropores*) with width between 0.7 and 2 nm [18].

Describing nanoporous carbon materials as carbon structures with enclosed porosity is correct but it is somewhat incomplete since properties, as for example, surface chemistry, chemical composition, morphology or electrical conductivity are also essential to understand their potential

Regarding the physical properties of nanoporous carbon materials, porosity is undoubtedly the most important one. IUPAC defines *nanopore*s as pores with apertures up to around

Due to their relevance for the adsorption process, micropores can be further distinguished between *narrow micropores* (i.e. *ultramicropores*) with width <0.7 nm and *wide micropores* (i.e.

The limits for this pore classification were suggested by nitrogen adsorption-desorption data at −196°C [18] and rely on the fact that each pore size range corresponds to different pore filling mechanisms disclosed by the isotherm profile [19]. Micropore filling is regarded as a primary physisorption process and occurs at low relative pressures. Firstly, at very low relative pressure, adsorption occurs in the ultramicropores where due to the close vicinity of pore walls there is an overlapping of the adsorption forces favoring the enhancement of the adsorbent-adsorbate interaction, this process is termed "primary micropore filling." After this initial filling in the most energetic sites, the cooperative adsorbate-adsorbate interactions inside the supermicropores become more relevant (secondary process) occurring up to relative pressures ≈ 0.15 [18]. In mesopores, the physisorption comprises three stages: monolayer adsorption (i.e. all adsorbate molecules are in contact with the surface of the adsorbent) followed by consecutive adsorbate layers (multilayer adsorption) and finishing with capillary condensation [18]. The adsorption onto mesopores commonly presents a hysteresis loop [19]. The macropores are so wide that do not allow capillary condensation since behave as an open space (cannot be characterized by nitrogen adsorption whose maximum relative pressure

In nanoporous carbons, the porosity results from the voids between the randomly cross-linked graphite-like crystallites that constitute the structure of the carbon skeleton, from the consumption of less organized carbonaceous matter and also from the removal of reactive carbon atoms in the crystallite during activation process (**Figure 1**). Surface area and pore size distribution (PSD) are the physical properties that will greatly influence the performance of nanoporous carbons in a given process. These parameters are commonly obtained from the analysis of nitrogen adsorption isotherm at −196°C. The surface area (apparent surface area for microporous materials) is commonly obtained applying the Brunauer-Emmett-Teller (BET) equation. Regarding

100 nm and, according to their size, *nanopores* are classified as follows [18]:

**Figure 1.** Schematic representation of pore structure (bottom rectangle) and surface chemistry—oxygen, nitrogen, and sulfur groups (top circles)—in nanoporous carbons.

the PSD, there are various mathematic models available, being those using the Non-Local Density Functional Theory (NLDFT) the most advanced ones. The use of nitrogen as adsorptive at −196°C has however diffusional limitations in ultamicroporous carbon materials; thus, for an accurate characterization of the micropore network, carbon dioxide adsorption at 0°C has been repeatedly proposed as a better alternative to nitrogen [20–22]. Besides the limitation of nitrogen for the characterization of ultramicropores, it is known that, due to its quadrupole moment, nitrogen molecule orientation during adsorption will be dependent on the surface chemistry of the adsorbent [18, 22]. To overcome this issue, other adsorptive gases like argon or krypton can be used.

Carbon atoms are the major element of nanoporous carbon skeleton but the elemental composition of these materials also includes hydrogen and oxygen and, depending on the precursor, preparation route and post-synthesis functionalization may also have nitrogen, sulfur or phosphorus containing groups. These heteroatoms are mainly located at the edges of the basal planes due to the presence of unsaturated carbon atoms that are highly reactive. **Figure 1** summarizes the most relevant oxygen, nitrogen, and sulfur-containing surface groups that may be present in the surface of nanoporous carbon materials, being important to mention that oxygen surface groups are, by far, the most common ones. These materials may also contain inorganic matter, sometimes attaining 20% (wt.), being this ash content mainly inherited from the carbon precursor [12].

requests the use of raw materials that gather high availability and constancy with high density and hardness, low inorganic content and, last but not the least, low cost. Thus, commercially available activated carbons are mainly produced from coal (i.e. lignite, sub-bituminous, bituminous and anthracite), coconut shell charcoal and wood charcoal. The raw material is a key issue for activated carbon producers since the development of the porous network occurs by consumption of the (char)coal precursor and the preparation yields can reach values as low as 10% (wt.). Consequently, assuring the supply of high quality precursors at a cost-effective price is of paramount importance to control the quality of the final material and the production cost. In 2015, the global activated carbon market was valued by Markets and Markets at USD 4.74 billion corresponding to 2743.7 kton being expected to continue to growth, mainly driven by more stringent government regulations to assure human and environmental protection. As already mentioned, (char)coal is the most important raw material for activated carbon producers but they are not the major (char)coal consumer. In fact, activated carbons are only a "by-product" of (char)coal production, which have the energy sector as major market (mainly for metallurgical, industrial and cooking fuel) that actually controls demand and prices. This reality, allied to the fact that most of raw materials came from the Asian continent, and that external factors (e.g. natural disasters) may compromise the quality, availability and cost of (char)coal, continues to drive the research for finding new, and more sustainable, raw

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Conventional methods for activated carbon production request the use of high temperature kilns where, under a controlled atmosphere, the carbon-rich raw materials are transformed into nanoporous carbons. There are numerous procedures for the production of activated carbons, although the great majority of synthetic routes published and patents registered worldwide, including those allowing nowadays industrial production, fall in two major categories: the selective gasification of raw materials' carbon atoms (physical or thermal activation), and the co-carbonization of the precursor with oxidizing and/or dehydrating agents (chemical activation). The number of steps requested for the production of activated carbons depends on the characteristics of the raw material, morphological specifications of the final product,

and on the type of activation. A general process flowsheet is presented in **Figure 2**.

**Figure 2.** General flowsheet for conventional physical (thermal) and chemical activation processes.

The most commonly used **pre-treatments** are those aiming to obtain a given particle size or shape, washing steps for dirt removal and/or for the reduction of the inorganic content (i.e. acid washing) and pre-oxidation to prevent fluidization of coking coal during carbonization [8, 12]. The raw material can be crushed and sieved to obtain powdered or granular particles with specific dimensions. Milling, binder addition and briquetting are other common options when working with precursors presenting low volatile content (e.g. medium-high rank coals,

materials and preparation routes for activated carbon production.

The elemental composition and type of surface groups of a nanoporous carbon influence its performance in both gaseous and liquid phase processes, due to specific interactions with the adsorptive and also the solvent in the case of adsorption from solution. Properties like hydrophobicity/hydrophilicity or acid/base behavior are highly dependent on the surface chemistry of these materials. Carbons are in general hydrophobic, but the presence of increasing amounts of oxygen surface functionalities favors the adsorption of water molecules due to the formation of hydrogen bonds and, consequently, its wettability. This ability of porous carbon materials may be advantageous, or not, depending on the desired application. For example, in the impregnation of carbon supports with catalysts in aqueous phase, a high wettability will increase the impregnation degree; but in the adsorption of organic compounds from diluted aqueous solutions, higher wettability may led to the formation of water molecule clusters that hinder the diffusion of the target compounds towards the adsorption active sites (the same being valid for gas steams purification).

Regarding acid/base character, nanoporous carbons are considered amphoteric materials due to the presence of both acid and basic sites in their surface. Thus, depending on the amount and strength of all the surface groups, the materials may present net acid, basic or neutral surfaces. The surface chemistry of nanoporous carbons can be assessed through numerous techniques, and the best way to get a good characterization is to employ complementary techniques and combine the analysis of the results. For example, Boehm and potentiometric titrations provide qualitative and quantitative information on the surface of the nanoporous carbons, while temperature programmed desorption (TPD) detects more oxygen groups than Boehm titration, although with less quantitative information. On the other hand, X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared spectroscopy (DRIFT) provide qualitative information about the surface of the nanoporous carbons [23].

Other properties of nanoporous carbon materials that may play important roles in specific applications are the morphology (powdered (PAC) and granular (GAC) forms are the most common ones), electrical conductivity, hardness and density.

For more specific information regarding properties of nanoporous carbon materials and characterization methods, several reference texts are available [8, 21, 23–27].
