Exploring Activated Carbons for Sustainable Biogas Upgrading: A Comprehensive Review

Abstract

Global energy supply remains, to this day, mainly dominated by fossil fuels, aggravating climate change. To increase and diversify the share of renewable energy sources, there is an urgent need to expand the use of biofuels that could help in decarbonizing the energy mix. Biomethane, obtained by upgrading biogas, simultaneously allows the local production of clean energy, waste valorization, and greenhouse gas emissions mitigation. Among various upgrading technologies, the use of activated carbons in adsorption-based separation systems has attracted significant attention due to their versatility, cost-effectiveness, and sustainability potential. The present review offers a comprehensive analysis of the factors that influence the efficiency of activated carbons on carbon dioxide adsorption and separation for biogas upgrading. The influence of activation methods, activation conditions, and precursors on the biogas adsorption performance of activated carbons is revised. Additionally, the role of adsorbent textural and chemical properties on gas adsorption behavior is highlighted. By synthesizing current knowledge and perspectives, this work provides guidance for future research that could help in developing more efficient, cost-effective, and sustainable adsorbents for biogas upgrading.

1. Introduction

In the race to meet the United Nations (UN) 2030 Agenda and its Sustainable Development Goals [1], there is an urgent need for alternative energy sources and technologies that can help signatory countries “increase substantially the share of renewable energy in the global energy mix” by 2030 (Goal 7, Target 7.2). Progress on this target could also positively impact other key objectives—for example, achieving universal access to affordable, modern energy services (Target 7.1) and reducing annual national greenhouse gas emissions (Indicator 13.2.2). Moreover, since access to clean energy underpins the achievement of many other goals, special attention must be paid to Goal 7. In this context, the International Energy Agency (IEA) forecasts an overall increase in approximately 60% in renewable energy consumption (including electricity, heat, and transport) by 2030, raising the share of renewables in final energy use from 13% in 2023 to nearly 20%. Renewable biofuels, hydrogen, and e-fuels are expected to contribute around 15% of that increase [2].

In particular, biogas production from various organic waste streams—such as municipal solid waste, wastewater, agricultural residues, livestock manure, fish farms, and food waste—simultaneously reduces greenhouse gas emissions in two ways: by replacing conventional energy sources and by avoiding emissions from the natural decomposition of organic matter. In addition to these climate benefits, biogas production offers a long list of co-benefits, including improved waste management, support for circular economy strategies, enhanced energy security, availability of a versatile local energy source, provision of clean cooking fuel, and synergies with agricultural systems [3,4]. It is therefore not surprising that global demand for biogases is expected to increase by 30% between 2024 and 2030 [2], helping to decarbonize energy systems where other renewable sources face technological or availability limitations, while also contributing to reductions in global methane emissions.

While the exact composition of biogas varies among authors, it is well established that its two main components are methane (40–80% v/v) and carbon dioxide (20–55% v/v). In addition to these, biogas may also contain other components in smaller proportions, including hydrogen sulfide, water vapor, nitrogen, oxygen, volatile organic compounds, ammonia, carbon monoxide, hydrocarbons, hydrogen, particulate matter, halogenated compounds, volatile siloxanes, and other trace gases. The specific composition depends on several factors, such as the type of feedstock, the anaerobic digestion process, and the operational conditions of the system [3,5,6,7,8,9,10].

In order to improve the usability of biogas, it must be upgraded to biomethane through the removal of carbon dioxide and other impurities. This process increases its heating value and energy density from approximately 21–24 MJ/m³ (raw biogas) to approximately 36 MJ/m³ (biomethane) [9,11]; enhances its applicability as a substitute for natural gas in sectors such as transport, industry, power generation, and heating, thus providing lower-carbon energy options for multiple sectors; ensures compatibility with existing natural gas infrastructure; and facilitates renewable energy integration. It also eliminates harmful components and can lead to economic benefits. Some studies also highlight that the high-purity CO₂ stream obtained during upgrading could be captured and permanently stored or reused in long-living products, contributing to carbon dioxide removal strategies [12]. In effect, biogas upgrading transforms a gas with limited applications into a versatile, high-value fuel that could help in climate change mitigation and even result in net negative greenhouse gas emissions when coupled with CO₂ capture and storage [13,14,15,16,17,18].

Several upgrading technologies are commercially available. Among adsorption-based methods, Pressure Swing Adsorption (PSA) and Vacuum Swing Adsorption (VSA) are the most established, typically using activated carbon or zeolite materials such as SAPO-34 and LTA [19,20,21]. Other commercial approaches include Water Scrubbing (WS) [22,23,24], Amine Scrubbing [25,26,27], Membrane Separation [28,29,30], and Cryogenic Separation [31,32,33].

Emerging technologies focus on increasing selectivity, lowering energy use, or enabling circularity. These include electrochemical CO₂ separation [8,34,35], biological upgrading processes such as hydrogenotrophic methanation [36,37] and algal biofixation [38,39], hydrate-based separation [40,41], and inorganic solvent scrubbing using alkaline solutions [42,43].

The selection of a biogas upgrading strategy depends on a combination of factors, including biogas composition, target biomethane quality, economic and environmental considerations, scalability, and material availability [7,8,10,44,45]. Current trends emphasize decentralized, small-scale upgrading systems; the integration of upgrading into biorefineries; and alignment with policy drivers [7]. Novel approaches and materials continue to emerge, particularly for low-cost, modular applications based on waste-derived adsorbents [46,47,48].

In particular, adsorption-based methods are widely regarded as among the most versatile and effective technologies for biogas upgrading [49]. Among these, Pressure Swing Adsorption (PSA) has been particularly established for CH₄/CO₂ separation [8]. The main advantages of this technology are the lack of chemical issues and its dry nature, which eliminates the concern of contaminated wastewater (amine and water scrubbing) [45].

Adsorbents play a critical role in CH₄/CO₂ separation. The choice of the adsorbent material is key for the optimization of a Pressure Swing Adsorption plant, as well as for other adsorption-based gas separation industrial applications such as Carbon Capture and Storage (CCS) [50], Adsorbed Natural Gas (ANG) [51], and hydrogen purification [52]. To achieve effective separation, an adsorbent must exhibit high selectivity, which depends on carefully designed properties [15,49,53,54]. Key characteristics include pore structure, surface chemistry, and interaction affinity with CO₂ molecules [49,55,56,57,58,59,60,61]. For instance, Gao et al. emphasize the importance of pore size distribution in enhancing CO₂ selectivity in carbon-based adsorbents [62].

In this context, material development plays a crucial role in adsorption-based upgrading. A wide range of adsorbents have been investigated for biogas upgrading systems, including carbonaceous materials such as activated carbons and Carbon Molecular Sieves with particular emphasis on biomass-derived adsorbents [63,64,65], zeolites [66,67], and metal–organic frameworks (MOFs) [68,69]. These materials aim to improve CO₂ selectivity, lower regeneration costs, and align with sustainability goals.

Activated carbons have attracted significant interest due to their developed porous structure, efficiency, adaptability, and relatively low cost, even more so when produced using renewable biomass as a source. Its performance can be further improved through surface modification and functionalization [50,70,71]. Emerging carbonaceous materials include modified activated carbons and biomass-derived adsorbents, each aiming to enhance selectivity, reduce cost, and improve scalability [49].

This review aims to provide a comprehensive and critical overview of current and emerging carbonaceous adsorbents for CH₄/CO₂ separation in the context of biogas upgrading. Building on the growing recognition of biogas as a key contributor to renewable energy systems and carbon mitigation strategies, the paper focuses on adsorption-based technologies, with an emphasis on the role, performance, and development potential of activated carbons. The review also highlights recent advances in material design strategies aiming to improve the adsorption behavior of these adsorbents for the optimization of biogas upgrading. This work intends to support future research efforts and inform material selection and process design for more efficient and climate-relevant biogas upgrading systems. In doing so, this review provides guidance for future research and innovation around cost-effective, climate-relevant upgrading materials, processes, and strategies.

2. Carbonaceous Adsorbents for Biogas Upgrading

Carbon-based adsorbent materials have general properties, such as high thermal and chemical stabilities, that make them suitable materials for different gas adsorption and storage applications [72], high specific surface area [52], large pore volumes, and tunable surface chemistry [73]. In addition, the required energy for the adsorption/desorption phases is low since they can be regenerated usually with temperatures lower than 373 K, and can work at atmospheric pressure [74]. Carbonaceous adsorbents include crystalline arrangements (allotropic carbon forms) such as graphite, graphene, and carbon nanotubes (CNT) or short-range ordered carbon atoms without a crystalline structure (amorphous forms), including Carbon Molecular Sieves (CMS) and Activated Carbons (AC).

In particular, understanding and optimizing the adsorption properties of activated carbons has attracted a lot of research in recent years. This type of adsorbent is known for having commendable textural and physicochemical properties, including high surface area (>1000 m² g⁻¹), tunable porosity, cycling stability, and low energy desorption requirements [75]. Their adsorption mechanism is based on equilibrium conditions, i.e., the amount of adsorbate equals the amount desorbed, resulting in their ability to retain adsorption over long periods of time, making them suitable for long-term storage and removal applications. They can be produced from a variety of different raw materials such as biomass, coal, wood, and polymers, which makes them an affordable alternative. These characteristics have resulted in ACs being widely used for several industrial and commercial adsorption applications, including wastewater treatment [76,77,78,79], food industry [80,81], capacitors [82,83,84,85], and gas storage and purification.

2.1. Activated Carbon Carbon Dioxide Adsorption Mechanism

Carbon dioxide adsorption by activated carbons is carried out by a physisorption process mainly based on weak intermolecular forces known as van der Waals forces, as indicated by low adsorption enthalpies ΔH° < 20 kJ/mol (chemical adsorption ΔH° values are within 80–200 kJ/mol) [86]. The forces behind the physisorption process are explained by temporary fluctuations in the electronic distribution around molecules, which results in instantaneous dipole moments [87]. When carbon dioxide molecules are in close vicinity to the surface of activated carbons, the pi electrons of the C-O bonds in CO₂ interact with the unhybridized orbitals of carbon surface atoms, resulting in dipole-induced quadrupole short-range attraction forces as schematized in Figure 1 [88]. Methane molecules are also attracted due to London Forces; however, the quadrupole moment of CO₂ results in preferential adsorption over the non-polar CH₄ molecules.

London forces are highly sensitive to the distance between the adsorbate molecules and the adsorbent surface groups. They are considered to be negligible at distances greater than two molecular layers, and thus the adsorption of carbon dioxide molecules happens in pores within the carbon structure, not greater than four or five molecular layers [89].

Activated carbons are prepared by a two-step method: First, a heat treatment, which raises the specific surface area, and, second, a physical or chemical activation in which porosity is developed and opened [20]. The resulting material is highly porous with a wide range of pore sizes (see Figure 2), with larger pores acting as transport pores, allowing the diffusion of the adsorbate molecules into the adsorption sites in the smaller pores (micropores) in which the gap between the layers of carbon plates are small enough for sufficient intermolecular forces for adsorption to occur. The diffusion into smaller pores is governed by pressure gradients between the outside of the carbonaceous material and the inside of the pores.

The CO₂ adsorption of activated carbons at 273 K was studied by Grand Canonical Monte Carlo (GCMC) simulations depicting the adsorption mechanism by micropores and mesopores as shown in Figure 3 [86]. The snapshots of pores of 6.3 and 6.5 Å widths show the formation of a single layer, also known as a monolayer, of carbon dioxide molecules with the molecules lying horizontally parallel to the surface. When pressure is increased over 100,000 Pa, CO₂ starts to penetrate voids in the carbonaceous surface, becoming fully packed at saturation pressures. On the other hand, in the mesopore range, a first layer is completed at low pressure, with pressures higher than 500,000 Pa, a second layer is formed, and voids in the AC surface are filled with CO₂ molecules. When the second layer is filled, the short-range intermolecular forces between the surface and adsorbate are negligible, and pore filling is carried out by capillary condensation.

2.2. Influence of Textural Properties

Textural properties, comprising the physical characteristics of the adsorbent that determine its microstructure and morphology, are therefore crucial to defining the adsorption behavior of activated carbons. They are the most influential parameters that define the material capacity to capture and retain carbon dioxide during the biogas upgrading process. They also govern the maximum adsorption capacity, the number of available surface sites for adsorption, as well as the size distribution of pores, which are relevant factors for carbon dioxide adsorption and separation.

Activated carbons’ textural properties are highly dependent on the substrate composition and preparation method, in such a way that they can be tuned to enhance the adsorption properties. Carbon dioxide adsorption capacities of activated carbons can reach values of 5.24 mol CO₂/g with good removal efficiencies from biogas, allowing biogas enrichment from 61 to up to 91% methane purity [91]. Understanding the role that each parameter has on the adsorption behavior of ACs is of great importance for the development of optimized adsorbents with industrial potential applicability.

2.2.1. Specific Surface Area (SSA)

This measures the total surface area of the adsorbent per unit of mass, and it is typically reported in square meters per gram (m² g⁻¹). A higher value of SSA means more available accessible area for adsorbate–adsorbent interactions, which translates into more active sites for the adsorption of molecules. Large surface areas are related to enhanced adsorption capacities [92]. Activated carbons can present surface areas of up to 3000 m² g⁻¹, which makes them suitable materials for adsorption-based industrial processes.

SSA is commonly measured by the Brunauer–Emmett–Teller (BET) method, which consists in obtaining the nitrogen adsorption isotherm at 77 K of the adsorbent, and the surface area is then calculated from the gas adsorbed volume. The obtained value is known as BET surface area (S_BET) [93]. Textural properties of different activated carbons reported in the literature, including S_BET, are shown in

Table 1. Effect of textural properties on the carbon dioxide uptake capacity of activated carbons as reported in the literature.

Sample (Substrate)	SBET (m2 g−1)	VTot (cm3 g−1)	Vmicro (cm3 g−1)	V<0.7 nm (cm3 g−1)	CO2 Uptake (mmol g−1) at 298 K	References
AC-1 (anthracite)	1004	0.48	0.23 a		3.16	[94]
AC-4 (anthracite)	1394	0.67	0.32 a		3.49	[94]
AC-2 (anthracite)	1615	0.80	0.22 a		2.77	[94]
CC2-800 (cotton fiber)	1371	0.609		0.3361	4.43	[95]
CC3-800 (cotton fiber)	1343	0.600		0.2568	3.75	[95]
CHPA-2-750-f Chestnut shell)	1792	0.729	0.293	0.297	13.36	[96]
Bamboo-1-1073	1273	0.51	0.31		5.9 b	[58]
Bamboo3-973	2332	1.00	0.37		7.0 b	[97]
Bamboo-5-973	2980	1.41	0.28		5.3 b	[97]
MCC	648	0.299	0.266	0.218 c	2.28	[98]
MCC-K1	916	0.432	0.367	0.25 c	2.42	[98]
MCC-K2	1057	0.581	0.474	0.271 c	2.77	[98]

^a Volume of pores under 0.1 nm. ^b Measured at 273 K and 1 bar. ^c Pore volume under 0.8 nm.

.

The influence of the surface area on the adsorption capacity was studied by Cui et al. [99] by comparing the carbon dioxide uptake by nine activated carbons produced from glucosamine hydrochloride with different textural properties. The ACs showing BET surface areas higher than 2000 m² g⁻¹ showed superior carbon dioxide adsorption capacities with values ranging from 17.08 to 18.63 mmol g⁻¹ at 2 MPa and 298 K. Under the reported adsorption conditions, a correlation of R² = 0.773 was found between the BET surface area and CO₂ uptake, signaling a high influence between surface area and adsorption capacity at high adsorption pressures.

Similar results were found by B. Ruiz et al. by analyzing eight activated carbons produced from chestnut shells [96]. S_BET values were calculated to be in the range between 1117 and 1792 m² g⁻¹. A correlation factor of R² = 0.956 was found between BET specific surface area and the CO₂ uptake at 3 MPa and 273 K. In the low-pressure range (P < 0.5 MPa), the presence of higher micropore volumes was reported to enhance the CO₂ adsorption capacity. Other textural parameters found to influence the CO₂ adsorption behavior were total pore volume, medium micropore volume, and mean micropore size.

By studying a set of three activated carbons produced from anthracite under different activation conditions, Li et al. depicted the influence of BET surface area, pore volume, and narrow micropore volume (>1 nm) on carbon dioxide adsorption for biogas upgrading [94]. Their results indicate that the maximum carbon dioxide adsorption capacity is obtained when surface area and narrow micropore volume are increased to values of 1394 m² g⁻¹ and 0.32 cm³ g⁻¹, respectively. A further increase in surface area while maintaining a narrow micropore volume indicates the creation of wider pores inside the carbonaceous structure. This is because, due to the weak character of the interactions involved in gas adsorption by activated carbons, the adsorbate molecules cannot be retained on the surface.

Wei et al. analyzed the influence of BET surface area on carbon dioxide adsorption for a series of fifteen ACs produced using bamboo as precursor [97]. Their results show that adsorption capacity at 0.1 MPa increased with increasing SSA of up to 2000 m² g⁻¹; however, further increases in SSA proved to be detrimental to adsorption capacity, with activated carbons with surface areas between 2000 and 3000 m² g⁻¹ showing progressively lower adsorption capacities with higher surface areas. High correlation factors (R² > 0.8) were found between CO₂ adsorption at low pressure and the volume of micropores in the range of 0.33–0.90 nm in the work.

2.2.2. Pore Size Distribution (PSD) and Pore Volume

PSD describes the range and proportions of pore sizes within the adsorbent, thus providing insights into the accessibility of the porous structure for the adsorbate molecules. This parameter describes not only the average pore size but also the distribution shape, indicating if the material has a narrow distribution, similar pore sizes, or a wide distribution, varying range of pore sizes. Its measurement is highly relevant for the study of adsorption properties, since, as already discussed, different pore sizes are related to mass transfer and molecular sieving effects and thus highly influence the adsorption properties of the adsorbate–adsorbent pair. Pore sizes are categorized into three main categories: micropores (pore diameter less than 2 nm), mesopores (pore diameters from 2 nm to 50 nm), and macropores (pores with diameters larger than 50 nm).

Activated carbons tend to have a developed microporous structure as depicted in Figure 4 for four activated carbons prepared from avocado stones by chemical activation using sulfuric acid [100]. The PSD profiles obtained for the samples show two main peak regions, indicating the predominant presence of pores with sizes under 1 nm and a lower presence of pores from 1.0 to 1.5 nm. The presented figure also highlights the effect of activation conditions upon the development of the porous structure of AC, with the activation temperature varying from 700 to 750 and 800 for C_H₂SO₄_1.5_700, C_H₂SO₄_1.5_750 and C_H₂SO₄_1.5_800, respectively, while C_H₂SO₄_1.5_700 and C_H₂SO₄_1_700 were both prepared under the same activation temperature but with different mass ratio avocado stone: H₂SO₄ for the first sample it was 1.5 and 1.0 for the second AC. The effect of activation conditions for a given method on the adsorption properties of carbon dioxide by ACs is further discussed in Section 3.

By integrating the pore size distribution curve obtained over the entire pore size range for each adsorbent, the total pore volume (V_TOT) can be calculated. Total pore volume can also be measured by intrusion of mercury under controlled pressure (MIP) [101], water displacement, and gas pycnometry with helium or nitrogen [102]. Furthermore, the volume of pores with a particular size range, i.e., the volume of micropores (V_micro), can be extracted from the N₂ adsorption isotherms and the corresponding PSD profiles. Both V_Tot and V_micro are measured in cm³ g⁻¹.

The adsorption of CO₂ by ACs driven by non-specific short-range interactions is carried out by a pore-filling process. When pore sizes are in the micropore range, the adsorption potential of each parallel adsorption plaque overlaps, enhancing the pore filling. Therefore, the adsorption process is highly dependent on the volume of pores in each size range [103]. In particular, the presence of narrow micropores (<0.8 nm) has been reported to be the dominant factor for CO₂ adsorption at low pressure.

Hong et al. [98] found a very close correlation between the volume of the narrow micropores (<0.8 nm) and CO₂ adsorption capacities at different temperatures for a set of six Wheat-Derived Activated Microporous Carbons obtained by chemical activation with different activation agent ratios (see Figure 5). Total pore volume and micropore volume were not well related to CO₂ adsorption capacities, especially when an increase in these properties meant a reduction in narrow micropore volume by a pore enlargement mechanism. In a similar manner, their results show an enhancement of CO₂ adsorption with increasing BET surface areas of up to 1500 m² g⁻¹, followed by a decrease in the CO₂ adsorption for higher BET surface areas linked to a smaller volume of pores under 0.8 nm.

Taking into consideration pore sizes in the range of 0.3–1.4 nm, Serafin et al. characterized the effect of narrow micropore size distribution of activated carbons derived from different biomass on CO₂ adsorption at 273 and 298 K and 0.1 MPa [104]. The volume of micropores in the range of 0.30–0.86 nm was crucial for CO₂ adsorption at 273 K; however, when the adsorption temperature increased to 298 K, the best linear relationship was found for pore volumes in the range 0.30–0.33 nm, close to the kinetic diameter of CO₂.

At higher pressures, adsorption follows capillary condensation and liquefaction of gas molecules inside wider pores, thus resulting in broader pores contributing to an enhancement of the adsorption capacity at high adsorption pressures. CO₂ adsorption by AC at pressures over 0.1 MPa has been shown to correlate with surface area, total pore volume, and volume of pores of a wider range. Ruiz et al. found a correlation value of 0.9563 between medium micropore volume (pore volume with pores of 0.7–2 nm) and CO₂ adsorption capacity at 298 K and 3 MPa for a set of eight AC [96]. On the same work, R² values of 0.9938 and 0.9668 were found for the linear relation between S_BET and V_TOT with CO₂ adsorption capacity, respectively.

Peredo et al. [105] also reported good correlation values between carbon dioxide maximum adsorption capacity and BET surface area (r² = 0.959) for a set of five commercial activated carbons; micropore volume also showed to have an important influence on the adsorption capacity at high pressure (r² = 0.883).

By analyzing 1300 data points from the literature via deep neural networks, Mehrmohammadi and Ghaemi investigated the impact of textural properties, particularly micropores, on CO₂ adsorption capacity on porous carbon materials [106]. Their results show a high influence of micropore volume in the range of carbon dioxide adsorption at low pressure (0.1–0.5 MPa) (see Figure 6). On the other hand, at pressures of 1.5 and 2.0 MPa, adsorption on mesopores is depicted by higher CO₂ uptakes within the mesopore volume range of 4–8 cm³ g⁻¹, attributed to the saturation of micropores at higher pressures, necessitating the contribution of mesopores to achieve higher CO₂ uptake.

In addition, some authors have signaled the importance of also measuring the ultramicropore volume (pore diameter lower than 0.7 nm) as it has been reported to enhance the CO₂ adsorption and selective separation [107,108,109]. Common methods to measure the PSD include Barret–Joyner–Halenda (BJH) method applied to BET adsorption isotherms, Mercury Intrusion Porosimetry (MIP) [110], Nuclear Magnetic Resonance (NMR) [111], Small Angle X-ray Scattering (SAXS) [112], and Molecular Simulations, such as Density Functional Theory (DFT) [113] and Grand Canonical Monte Carlo (GCMC) [114].

2.3. Influence of Chemical Composition and Surface Chemistry

Gas adsorption requires the adhesion or binding of the adsorbate molecules on the surface of the adsorbent. Depending on the surface chemistry, the interaction forces will be stronger, such as chemical bonding (chemisorption) or weaker, involving van der Waals forces (physisorption) [101]. When the adsorbate is constituted by more than one type of molecule, the ability to selectively adsorb one probe over another can be influenced by the strength of the attraction/repulsion forces between the surface and each of them. Furthermore, the reversibility of the adsorption process during the adsorbent regeneration is also affected by the surface chemistry.

To produce a highly purified methane stream from biogas by adsortpion-based separation methods such as Pressure Swing Adsorption (PSA) and Vacuum Swing Adsorption (VSA), the adsorbent material has to have a large working capacity, as well as a high CH₄/CO₂ selectivity factor (S_CH4/CO2), defined by Equation (1):

$${\mathrm{S}}_{{\mathrm{C}\mathrm{H}}_4/{\mathrm{C}\mathrm{O}}_2}={\displaystyle \frac{{\mathrm{X}}_{{\mathrm{C}\mathrm{O}}_2}{·\mathrm{Y}}_{{\mathrm{C}\mathrm{H}}_4}}{{\mathrm{X}}_{{\mathrm{C}\mathrm{H}}_4}{·\mathrm{Y}}_{{\mathrm{C}\mathrm{O}}_2}}}$$

(1)

where X refers to the molar fraction of the compound in the adsorbed state, and Y is the molar fraction of this compound in the gas phase. For biogas upgrading, methane, a non-polar molecule, can only be absorbed by physisorption, while the quadrupole moment of carbon dioxide results in its ability to interact through electrostatic interactions [102]. Adsorbent materials with surface electron donor groups can enhance the CO₂ adsorption through stronger hydrogen bonding and acid–base interaction, which translates into a higher CH₄/CO₂selectivity factor [115]. Some primary techniques used for chemical surface characterization of adsorbents include X-ray Photoelectron Spectroscopy (XPS) [116], Energy Dispersive X-ray Spectroscopy (EDX) [117], Fourier Transform Infrared Spectroscopy (FTIR) [118], and Thermal Programmed Desorption Mass Spectrometry (TPD-MS) [119].

To enhance the adsorption selectivity of AC, several authors have studied the introduction of heteroatoms such as nitrogen, sulfur, and phosphorus through post-synthesis treatment of ACs. This subject is discussed below in Section 4.

2.4. Section Summary

Activated carbons have textural and chemical properties that make them attractive candidates for biogas upgrading. ACs adsorb carbon dioxide molecules by short-range weak interactions that arise from temporary fluctuations in the electronic distribution around molecules, producing instantaneous dipoles. At low adsorption pressures, the presence of ultramicropores (pores under 0.7 nm) as well as surface basic functionalities has a positive impact on the carbon dioxide adsorption capacity and CH₄/CO₂ selectivity. On the other hand, at pressures over 1 MPa, the total pore and BET surface area also play an important role in the adsorption behavior. Identifying the impact of these properties is crucial for the optimization of ACs.

3. Factors Influencing the Properties of Activated Carbons Relevant for Biogas Upgrading

Both the adsorption capacity and adsorption selectivity of the activated carbons are dependent on the substrate and the activation process (activating agent and activation conditions). Specific activation methods, the activation conditions, and the activator agent define the development of porosity inside the adsorbent structure. The present section revises the effect of each of these factors on the adsorption behavior of ACs, as discussed in the literature.

3.1. Substrate

For biogas upgrading, ACs have been shown to selectively adsorb CO₂ from CH₄/CO₂ mixtures, as well as having important advantages that make them suitable materials for Biogas Upgrading [120]. Most common precursor sources on a commercial scale of ACs include wood, anthracite, bitumen charcoal, lignite, peat shell, and coconut; all these materials show a carbon content higher than 40%, low inorganic content, and good percentage yields, as shown in Figure 7 [121].

The selection of the substrate depends on the desired final characteristics of the ACs, as well as economic factors, and more recently, sustainability considerations. In this context, environmental concerns and the search for cost-effective alternatives have motivated the development of new alternative precursors that can reduce the environmental impact of AC while achieving competitive CO₂ adsorption performances. Biomass-derived activated carbons (BDAC) have had a growing interest as a cheap alternative with additional environmental advantages and high yield rates. Furthermore, BDAC obtained from organic residues allows the recovery of organic wastes from different sources, simultaneously promoting sustainability in biogas systems while mitigating waste management impacts, further enhancing the circular economy [122,123].

Table 2. Biomass-based activated carbons reported textural properties and CO₂ adsorption capacities.

Precursor	SBET (m2 g−1)	VTot (cm3 g−1)	Vmicro (cm3 g−1)	Yield (%)	CO2 Max. Uptake (mol kg−1) at 298 K	SCH5/CO2	References
Bamboo	1846	0.78	0.36	21.7	7.0	8.5	[97]
Coconut shell	1105	0.35	0.34		7.59	6.5	[120]
Olive stones	1178	0.49	0.45	-	10.873	-	[124]
Avocado stones	538	0.217	0.175	21	4.9142 (293 K)	-	[100]
Chestnut shell	1792	0.729	0.295	34.14	13.36	-	[96]
Defatted grape seeds	1604	0.672	0.250	-	11.48	-	[125]
Date seeds	798.38	-	0.28	7.25	2.9	-	[126]
Babassu coconut	809	0.39	0.32	14.6	10.49	4.2	[127]
Almond shell	326	0.24	0.23	24	2.18	-	[122]
Cherry stone	1045	0.48	0.40	-	10.88	3.61	[123]
Peanut shell	956	0.77	-	-	4.03	-	[128]
Sunflower seed	1790	0.43	-	-	4.61	-	[128]

presents different biomass-based ACs and their reported CO₂ adsorption capacity and CH₄/CO₂ adsorption selectivity.

The selection of the precursor material significantly influences the properties and performance of the final product, this was evidenced by Serafin et al. by comparing ACs produced from different biomass precursors (Pomegranate peels, tinder fungus: Piptoporus betulinus, Trametes versicolor (L.) Lloyd, mistletoe branches and leaves, carrot peels, kiwi fruit peels, fern leaves, sugar beet pulp) using the same activation method and activation conditions (chemical activation with KOH by impregnation at a mass ratio of 1:1 KOH/dry substrate followed by carbonization at 973 K) [104]. The ACs showed BET surface areas that ranged from 585 m² g⁻¹ to up to 1699 m² g⁻¹; total pore volume and micropore volume were also highly impacted, ranging from 0.28 cm³/g and 0.20 cm³/g to 0.80 cm³ g⁻¹ and 0.54 cm³ g⁻¹, respectively.

Table 3. Textural properties and CO₂ uptake at 1 bar and 298 K for activated carbons produced from biomass as reported by [104].

Substrate	SBET (m2 g−1)	Vtot (cm3 g−1)	Vmic (N2) (cm3 g−1)	CO2 Uptake
PP	585	0.28	0.2	4.11
CP	1379	0.58	0.51	4.18
FL	1593	0.74	0.54	4.12

shows the obtained textural properties and CO₂ uptake at 1 bar and 298 K of the three ACs that showed better CO₂ adsorption capacities. The work reports the cumulative micropore volume in the range 0.30–0.33 nm to show the best linear relationship between textural parameters and CO₂ adsorption capacity. In the work, total pore volume and BET surface area did not correlate with the adsorption capacity of the studied samples.

Similarly, using the same activation method and conditions, Vilella et al. showed the impact of the substrate on defining the adsorption properties of the resulting activated carbon. The authors obtained two AC by CO₂ physical activation at 1173 K using coconut shells and babassu coconut as carbonaceous precursors. The ACs showed significant differences in the textural properties: surface area of 1452 m² g⁻¹ and 809 m² g⁻¹, and total pore volume of 0.65 cm³ g⁻¹ and 0.39 cm³ g⁻¹ for coconut shell and babassu coconut, respectively [127].

3.2. Influence of the Activation Method

Two main methods are distinguished for AC production (see Figure 8): (a) chemical activation, the precursor is impregnated with an activated agent and then carbonized at low temperatures, activated agents include KOH, H₃PO₄ and ZnCl₂; (b) physical activation, the precursor is carbonized at temperatures ranging from 773 to 1073 K to remove non-carbonaceous atoms, followed by activation with an oxidizing agent, such as steam or carbon dioxide, at a high temperature (973 to 1273 K) [129].

Chemical activation usually requires lower activation temperatures and is related to higher yields. Another advantage of chemical activation is that it allows a very precise control and tailoring of the resulting textural properties by controlling the activation parameters. However, the main drawback of this technique is the corrosiveness and environmental risk of the chemical agents involved and the byproducts obtained during the washing of the adsorbent to remove any remains of the activation agent [131].

Activation parameters can have a major impact on the definition of the textural properties of the adsorbent obtained with a given activation method and activating agent. Factors such as the activation temperature and the activating agent/biomass weight ratio need to be carefully evaluated in order to optimize the adsorption behavior of AC.

Table 4. Effect of activation method and activation conditions on CO₂ uptake by activated carbons as presented in the literature. Some of the reported values were derived from graphical data; slight inaccuracies compared with actual values may exist.

Sample (Precursor)	Activation Method/Conditions	CO2 Uptake (mol kg−1)	Refs.
Olive stones	1. Physical: Carbonization at 873 K and H2O activation at 1023 K	(T = 303 K) 7.968	[124]
	2. Physical: Carbonization at 873 K and H2O activation at 1023 K	5.878
	3. Chemical: H3PO4 impregnation at a weight ratio of 1:3 at 383 K.	10.873
Post-consumer plastic waste	Chemical: Pyrolysis at 450 or 500 °C and activation at 1:1 mass ratio with:	(T = 298 K)	[57]
	KOH 450 and 500 °C,	41.3/84.8 mg g−1
	NaOH 450 and 500 °C,	29.2/29.1 mg g−1
	K2CO3 450 and 500 °C,	67.6/82.1 mg g−1
	Na2CO3 450 and 500 °C,	64.1/58.3 mg g−1
	FeCl3 450 and 500 °C,	33.7/43.5 mg g−1
	ZnCl2 450 and 500 °C.	34.6/29.8 mg g−1
Olive stones	Chemical: Carbonization at 500 °C and impregnation at a mass ratio of 1:1 with:	(T = 303 K)	[132]
	saturated KOH,	4.33
	85% H3PO4,	4.00
	KOH solution from banana peels extract	3.58
Pomegranate peel	Chemical: One-step activation with K2CO3 activating agent in proportions of 0.5:1 and 1:1 at varying temperatures:	(T = 298 K)	[133]
	0.5:1 at 850 °C,	11.8
	0.5:1 at 900 °C,	13.0
	0.5:1 at 950 °C,	15.1
	1:1 at 850 °C,	12.4
	1:1 at 900 °C,	13.7
	1:1 at 950 °C.	16.3
Chestnuts shells	Chemical: Carbonization at 750 °C followed by KOH impregnation in weight ratios of 1:1. and 2:1 and different temperatures:	% mass fraction	[96]
	1:1 at 750 °C,	35%
	1:1 at 800 °C,	42%
	1:1 at 900 °C,	51%
	2:1 at 750 °C.	58%
Coconut shell	Physical: Carbonization at 850 °C followed by CO2 activation at 750 °C until different burn-offs:		[134]
	25%	4.8/2.3 mmol cm−3
	35%	5.0/2.3 mmol cm−3
	52%	4.9/1.9 mmol cm−3
	79%	4.8/1.5 mmol cm−3
	94%	4.9/1.3 mmol cm−3
Almond Shell	Physical: Activation with air at two temperatures and different reaction times:	(298 K)	[135]
	400 °C and 28 min,	1.14
	400 °C and 40 min,	1.29
	400 °C and 42 min,	1.37
	450 °C and 26 min,	1.51
	450 °C and 32 min,	1.57
	450 °C and 36 min,	1.53
Cellulose	1. Hydrothermal activation with amines at 240 °C:	(273 K)	[136]
	diethylenetriamine (DETA),	4.38
	1,2-bis(3-aminopropylamino)ethane (BAPE),	5.33
	melamine (MELA).	7.34
	2. Hydrothermal activation with amines, KOH activation and carbonization at 800 °C:
	diethylenetriamine (DETA),	1.76
	1,2-bis(3-aminopropylamino)ethane (BAPE),	2.19
	melamine (MELA).	4.16
Cherry stones	Physical: single step activation with:	(303 K)	[137]
	CO2	5.14
	H2O vapor	4.48
Sugarcane	Physical: carbonization at 750 °C and activation at carbonized at 850 °C with:	(298 K)	[138]
	Air	1.6
	CO2	2.6
	Chemical: direct pyrolysis and activation of the precursor impregnated with:
	H3PO4	2.7
	NaOH	4.3

highlights the effect that these parameters can have on CO₂ uptake for Biogas Upgrading.

The effect of the activation method and activation agent (chemical: H₃PO₄, ZnCl₂ and physical: CO₂) on the textural properties of the activated carbon was tested by Arami et al., their results show that samples obtained by chemical activation with phosphoric acid followed by activation with carbon dioxide, showed overall higher surface areas and micropore volumes that those for zinc chloride followed by carbon dioxide [139]. Furthermore, physically activated carbons had a comparable surface area to that of the carbons obtained using ZnCl₂ chemical activation. On the other hand, physical activation resulted in a higher degree of heterogeneity of the pore size distribution.

Peredo et al. also analyzed the effect of the activation method on CH₄ and CO₂ adsorption for BDAC obtained from olive stones by physical activation (CO₂ and H₂O) and chemical activation (H₃PO₄) [124]. Their results show that chemical activation resulted in more developed textural properties (surface area, total pore volume, and micropore volume), which allowed higher intake of carbon dioxide (max. adsorption capacity: 10.873 mol g⁻¹. The main parameter for methane adsorption was found to be the presence of mesopores, which facilitated the diffusion of the gas molecules. All the samples showed higher adsorption capacities of carbon dioxide than those of methane. Furthermore, their findings demonstrated that even if the two physically activated ACs had similar textural properties, their gas uptake differ (7.968 mol g⁻¹ for H₂O activation and 5.878 mol g⁻¹ for CO₂ activation), explained by the effect of chemical surface groups that also showed to influence the adsorption behavior of the adsorbents, i.e., activation with H₂O resulted in highest content of oxygen surface groups which enhanced the CO₂ adsorption, proving than not only textural properties are relevant for gas adsorption and separation processes, chemical properties of the adsorbent surface are also a relevant parameter that can be optimized to enhance the wanted behavior of the adsorbent.

Krupšová et al. also explored the effect of surface groups on CO₂ adsorption properties. They prepared cellulose-based activated carbons modified with amines: diethylenetriamine (DETA), 1,2-bis(3-aminopropylamino)ethane (BAPE), and melamine (MELA), by (a) hydrothermal and (b) hydrothermal activation followed by chemical activation with KOH and pyrolysis [137]. The samples obtained by the thermal activation method showed overall better textural properties than their counterparts obtained by hydrothermal activation alone, explained by the KOH chemical activation step, which promotes the formation of micro- and mesopores. Furthermore, for the same activation protocol, more enhanced degrees of porosity and BET surface areas were observed on melamine-containing material, followed by samples doped with 1,2-bis(3-aminopropylamino)ethane and finally diethylenetriamine (as shown in Figure 9a). However, carbon dioxide adsorption as measured by the volumetric method at 0 °C showed higher CO₂ adsorption capacities for the samples activated by hydrothermal treatment. The adsorption capacity was found to increase with the increasing number of amine groups (nitrogen atoms) in the formula of amine used as an in situ N-modifying agent, indicating that the CO₂ uptake is dependent on the basicity of the materials in the measured pressure range and temperature.

Solís et al. explored the effect of diverse chemical activation agents, K₂CO₃, KOH, Na₂CO₃, NaOH, ZnCl_2, and FeCl_3, on the CH₄, CO_2, and CH₄/CO₂ mixture’s adsorption by activated carbons prepared from post-consumer plastic from the rejection fraction of a municipal solid waste treatment plant [57]. The use of K₂CO₃ displayed better carbon dioxide uptakes attributed to higher ultramicropore volume and oxygenated surface functional groups (see Figure 10). The carried-out dynamic CH₄/CO₂ 50/50 (v/v) adsorption tests depicted close behavior to commercial materials such as zeolites, and a methane yield of up to 95% was achieved. The work opens a new research line for valorizing non-recyclable plastic waste to produce microporous adsorbents for biogas upgrading.

The effect of the activation temperature on the resulting activated carbon’s textural properties was highlighted by Serafin et al. In the work, activated carbons were synthesized from cashew nut shells by chemical activation with KOH at different temperatures (1073, 1123, and 1173 K) [140]. Their findings showed that upon increasing activation temperature from 1073 to 1123, the resulting activated carbon had more developed porosity (total pore volume 0.847 vs. 0.448 cm³ g⁻¹) and surface area (1972 vs. 1051 m² g⁻¹), however, further activation temperature increase to 1173 resulted in loss of surface area and porosity attributed to some pores collapsing due to excessive temperature. The authors estimated the CH₄/CO₂ selectivity factor for the sample with better textural properties (activation at 1123) of up to 8.4, depicting a pronounced preference for CO₂ adsorption over CH₄.

Chen et al. [58] studied the effect of the activated agent/biomass impregnation ratio for a set of bamboo charcoal-based activated carbons. In the study, they prepared five ACs by chemical activation with KOH using KOH/biomass mass ratios of 0.5:1, 1:1, 2:1, 3:1, and 4:1 at 423.2 K. Textural properties (surface area and total pore volume) increased with higher mass ratios from 0.5:1 to 3:1. The 4:1 ACs showed a decreased in the textural properties. The five AC were almost completely microporous with pores under 2 nm. However, the two ACs produced with the lowest impregnation ratios had more pores under 0.55 nm, indicating that higher impregnation ratios result in the erosion of pores, resulting in wider pores. The study also analyzes the performance of the AC/water hybrid sorbents for Biogas Upgrading, composed of liquid and porous materials, by mixing the obtained AC with H₂O at mass ratios of 1.0, 3.0, and 7.0 wt%, respectively. The best adsorption and separation performance were depicted for the AC with an impregnation ratio of 2:1 and 7 wt%, showing a predicted selectivity of 16, explained by the pore size distribution.

Siemak and Michalkiewicz analyzed the effect of the precursor mass: activating agent ratio and the activation temperature by producing four Avocado stones derived AC using mass ratios of 1 and 1.5 by chemical activation with H₂SO₄ at 700, 750, and 800 °C [100]. The highest values for specific surface area, pore volume, and micropores were obtained for the 750 °C carbonization combined with activation at a mass ratio of 1.5. This sample also showed the highest CO₂ uptake at 0.1 MPa (3.78 mmol g⁻¹), explained by the developed porosity and higher surface area. The activation at 700 °C was found to be insufficient for the development of porosity, while activation at 800 °C resulted in the degradation of the porous structure. On the other hand, reducing the activating agent amount resulted in lower porosity.

Saadi et al. prepared a set of pomegranate peel waste-based ACs by one-step K₂CO₃ chemical activation in proportions of 0.5:1 and 1:1 and activation temperatures of 1123 K, 1173 K, and 1223 K [134]. The resulting adsorbents showed significant textural differences: higher activation temperatures resulted in higher surface areas and total pore volume for both activation ratios, which translated into higher CO₂ uptakes at pressures higher than 1 MPa. Upon correlation of the adsorption isotherms with the textural parameters of the adsorbents (Figure 11), authors were able to conclude that at low pressures (P < 0.1 MPa) the adsorption mechanism is ultramicropore filling; thus, the ACs with higher narrow micropore volume (under 0.7 nm) showed better uptake at low pressures. On the other hand, at pressures over 0.7 MPa BET surface area, total pore volume, micropore volume, mesopore volume showed a direct relationship with CO₂ capacity, among these properties, mesopore volume and surface area showed higher correlation factors (0.98 and 0.96, respectively) indicating that at high pressure both large and small pores are relevant for gas diffusion and adsorption. Similar findings were depicted for methane adsorption. Nevertheless, all the samples showed a significantly higher CO₂ pure gas adsorption capacity than that for methane.

Ruiz et al. give a comprehensive analysis of how textural properties of ACs can be tuned by choosing the right activation parameters (i.e., activating agent/precursor weight ratio, activation temperature and inert flow gas on the activation process) to meet the requirements for a specific application such as CH₄/CO₂ separation [96]. In order to do so, they obtained sustainable porous carbons from lignocellulosic wastes from the chestnut shell by means of KOH chemical activation. Their results show that increasing the activation temperature from 973 K to 1073 K results in pore widening, in agreement with the previously mentioned works. The ACs obtained at 973 were composed mainly of ultramicropores, while higher activation temperatures resulted in an increase in micropores and mesopores. Similarly, a higher activation agent/precursor weight ratio resulted in wider pores. On the other hand, the inert gas flow rate did not have a particular impact on the textural properties. The carbon dioxide adsorption capacity at 3 MPa was also found to be influenced by the BET surface area, total pore volume, and micropore volume.

Prauchner et al. extended the previous studies from the literature by exploring the effect of the activation conditions during chemical activation with dehydrating agents (H₃PO₄ or ZnCl₂) and physical activation with CO₂ systematically [135]. A set of thirty ACs was prepared to this end, and for each activation agent, the effects of temperature, chemical loading, and burn-off were studied. Their findings show that higher burn-offs (physical activation), as well as chemical loadings (chemical activation), resulted in higher total pore volumes due to the creation of mesopores and macropores; however, ultramicropore volume remains almost unchanged. Overall, their results show that physical activation at 1123 K resulted in higher CO₂ gravimetric uptakes, mainly attributed to higher ultramicropore volumes in comparison with chemically activated ACs. Physical activation also resulted in better volumetric uptakes, meaning higher adsorption density. Their systematic scanning of the resulting textural properties correlated with the activating procedures gives light into the mechanisms involved in the development of pores and CO₂ adsorption.

3.3. Section Summary

The adsorption behavior of activated carbons (ACs) in biogas upgrading depends on multiple factors that significantly determine the textural and chemical properties of the adsorbent, such as the substrate, activation method, and activation conditions. Biomass-derived ACs (BDACs) have emerged as sustainable alternatives to conventional precursors, offering environmental and cost advantages. Chemical activation generally results in higher surface areas and micropore volumes than physical activation. Nevertheless, activation conditions such as temperature, activation time, and activating agent ratio also strongly influence the adsorbent performance. Experimental studies reviewed in this section show that minor changes in these parameters can lead to significant variations in the textural and chemical properties of the resulting AC that can translate into an important enhancement of the CO₂ uptake (e.g., from 2.2 to 5.7 mol kg⁻¹).

4. Post-Synthesis Methods for Improving Activated Carbon CO₂ Adsorption and Separation for Biogas Upgrading

Activated carbons can be subjected to post-synthesis treatments to modify and enhance their adsorption capacity and selectivity. In particular, due to the Lewis Acid behavior of carbon dioxide, introducing basic surface functionalities on the adsorbent has been reported to favor CO₂ adsorption [141]. Different modification methods have been proposed in the literature (see

Table 5. Post-treatments for activated carbons derived from various precursors.

Precursor	Post-Treatment	References
Palm shell	Immersion for 3 days in chitosan solutions with initial concentrations of 0.1–2.0 g/L	[142]
Waste tea	Immersion in 5 mL of 3% w/v of EDA in methanol	[143]
Petroleum pitch	Treatment with H2S at elevated temperatures (600 °C and 800 °C) Pre-oxidation treatment with plasma followed by H2S treatment at elevated temperatures (600 °C and 800 °C)	[144]
Commercial AC	Air oxidation at 400 °C for 2 h, Pre-oxidation treatment followed by immersion in NH4OH for 36 h	[145]
Polyacrylonitrile	Mixing with sulfur in a 1:1 weight ratio for 1 h at 553 K	[146]
Commercial AC	I. Activation in HNO3, sequentially activated in tin chloride (SnCl2) and palladium chloride (PdCl2) and nickel dipping into a nickel bath II. Activation in HNO3, sequentially activated in tin chloride (SnCl2) and palladium chloride (PdCl2) and nickel dipping into a nickel bath, followed by oxidation at 573 K in an air stream	[147]
Commercial AC	I. Mixing with HNO3 M II. Mixing with HNO3 M followed by treatment with urea solution 1 M III. Mixing with HNO3 M followed by treatment with urea solution 1 M and thermal treatment	[92]
Commercial AC	Mixing with a saturated solution of ammonium sulfate salt at different AC/ammonium sulfate mass ratios (w(NH4)2SO4 = 4.76, 6.98 and 9.1%)	[59]

), including N, S, and Ni doping.

Rattanaphan et al. produced an activated carbon from waste tea from a local tea shop by chemical activation with KOH [143]. The sorbent was then modified by immersion in ethylene diamine (EDA). The resulting modified carbon showed a higher concentration of basic nitrogen functionalities, which resulted in a loss of surface area due to pore filling by entrapment of EDA molecules. Nevertheless, their results show an increase in CO₂ pure gas adsorption capacity, as well as a higher CO₂ adsorption capacity from a real biogas stream. To explain the behavior, the team carried out Density Functional Theory (DFT) analysis, from which stronger adsorption energies between carbon dioxide and the modified surface structures than those obtained for the unmodified carbon were calculated.

In this context, the application of molecular simulations to predict or explain specific interactions between surface groups and the adsorbate molecules has become a powerful tool to support and facilitate gas adsorption and separation experimental analysis. Using a combination of DFT and Grand Canonical Monte Carlo (GCMC) simulations, Lu et al. investigated the effect of edge-functionalization on nanoporous carbons’ CH₄/CO₂ competitive adsorption [148]. Their results show that adsorption selectivity towards carbon dioxide is enhanced by adding surface groups in the order of NH₂ > COOH > OH. The increased selectivity is attributed to electrostatic interactions, stronger adsorption energies, and high electronegativity/electropositivity. The improvement in selectivity was shown to be unaffected by temperature increases in the gas phase; however, it does rapidly decrease when the working pressure is increased.

Phalakornkule et al. studied the effect of chitosan surface modification of a palm-shell-based AC. To this end, native carbon was impregnated with chitosan at a 0.76% weight. Their results show an important improvement in biogas upgrading dynamic tests in comparison with the results obtained for the native carbon. Mainly, the MAC was able to maintain CH₄ breakthrough purity of 100% during the consecutive adsorption cycles, while the native ACs showed a sooner breakthrough of carbon dioxide, which lowered the purity of the methane stream after the second cycle [143].

Reljic et al. analyzed the performance of a petroleum pitch-based activated carbon produced by chemical activation with KOH before and after a post-treatment with H₂S [144]. In the work, two modification methods were proposed: (a) direct treatment with H₂S at elevated temperatures (600 °C and 800 °C), and (b) pre-oxidation treatment with plasma followed by H₂S treatment at elevated temperatures (600 °C and 800 °C). Carbon dioxide and methane uptake at atmospheric (0.1 MPa) and high pressure (4 MPa) adsorption results depicted the microporous structure as the main structural parameter defining the adsorption performance. Nevertheless, the surface chemistry also influenced the adsorption properties. Figure 12 shows the normalized uptake (mg/cm³) by the micropore volume (most influential textural parameter) for the different samples evaluated as a function of the atomic percentage of sulfur estimated from the XPS data. In this figure, a certain positive effect of the surface chemistry is observed, for oxidized samples, most probably due to the synergic effect between sulfur and oxygen functionalities. Overall, the cumulative effect of improved textural and chemical properties resulted in up to 27% higher CO₂ uptake at 4 MPa (oxidation + H₂S treatment at 800 °C).

In order to increase CH₄/CO₂ selectivity, Peredo et al. carried out the surface modification of a commercial AC by a two-step process: oxidative thermal treatment followed by ammonia modification [145]. Their results show that the modification compromised the textural properties of the AC, which significantly lowered the adsorption capacity of the modified AC. Nevertheless, a remarkable increase in the experimental selectivity was found upon surface modification, achieving a selectivity of 9.1 at 1 MPa and 129 at 2.17 MPa; these values are comparable to highly selective materials such as MOFs and zeolites. The governing factor behind the increased selectivity was found to be the presence of basic functionalities. The modification was proven to have cyclic stability over ten consecutive adsorption/desorption cycles.

Textural properties can also be modified by treatment of produced activated carbons, i.e., the pyrolysis of activated carbons at temperatures ranging from 773 to 1273 K results in the removal of heterogeneous atoms, leaving a stiff carbon matrix with a narrower pore size distribution consisting of micropores and ultramicropores. The resulting adsorbent, named Carbon Molecular Sieves (CMS), is considered a subrange of AC with interesting gas selectivity properties based on kinetic adsorption [149].

Carbon Molecular Sieves can effectively separate different gas molecules through a sieving process based on the size and kinetic diameter of each probe. They can be tailored to have a narrow pore size distribution that results in kinetically preferential adsorption of CO₂ from CH₄ due to its smaller kinetic diameter (3.3 Å for CO₂ vs. 3.8 Å for CH₄) [150]. Their separation performance is highly influenced by contact time, intraparticle diffusion, mass transfer rates, and adsorbate–adsorbent surface group interaction forces.

Yu et al. studied the effect of fluorination of a commercial CMS on CH₄/CO₂ mixture adsorption at various fluorine (F₂) partial pressures; the added fluorine groups resulted in an overall increase in pore sizes that became more pronounced at higher partial pressures. This in turn enhanced the carbon dioxide adsorption capacity from 1.61 to 2.04 mmol g⁻¹ at 298 K, while maintaining a selective adsorption towards this gas, evidenced by a longer breakthrough times during CH₄/CO₂ dynamic adsorption tests [151].

Commercial CMS usually shows lower adsorption capacities than activated carbons (ACs) and zeolites [152]. Nevertheless, their CH₄/CO₂ selectivity is higher, resulting in higher methane yields, i.e., Rainone et al. compared the adsorption properties of the CMS Xintao against two commercial ACs (Carbotech and Desotec). Their findings show that the CMS shows the best compromise between maximum equilibrium CO₂ adsorption capacity, CO₂ breakpoint adsorption capacity (Xintao: 0.37 mmol g⁻¹, Carbotech: 0.31 mmol g⁻¹, and Desotec: 0.95 mmol g⁻¹), kinetic parameters, and CH₄/CO₂ selectivity (Xintao: 12.71, Carbotech: 15.07, and Desotec: 2.89). The study also highlights the complete regenerability capacity of the CMS after 10 consecutive adsorption/desorption cycles. CMS has also been studied as an interesting alternative for membrane-based biogas upgrading due to its high thermal and chemical stability [153].

Section Summary

The post-synthesis modification of activated carbons is a promising approach to enhance the CO₂ adsorption performance for biogas upgrading. These techniques aim to improve key properties such as selectivity and adsorption capacity by introducing or modifying surface functional groups after the initial activation process. Among the most common methods, amine impregnation—particularly with TEPA and MEA—has shown significant improvements in CO₂ capture, especially under low-pressure conditions. Other modifications proposed in the literature include oxidation treatments and functionalization with metal oxides or salts and sulfur doping. Nevertheless, the post-synthesis modification often involves a loss of textural parameters or decreased regeneration performance. Despite these limitations, post-synthesis modifications offer a practical pathway to tailor activated carbons for specific biogas compositions and operating conditions, improving their applicability in upgrading processes.

5. Additional Factors That Influence the Adsorption Performance of ACs

Gil et al. also emphasized the importance of volumetric uptake experimental determinations for gas separation processes such as biogas upgrading by comparing two ACs produced from Novolac phenol formaldehyde resins, an AC obtained from the same residue mixed with olive stone, and two commercial ACs [154]. The authors determined important deviations between the CO₂ adsorption capacity from the CH₄/CO₂ gas mixtures on a mass basis than the tendencies observed for volumetric uptakes under the same conditions for the set of adsorbents; thus, the AC that showed higher carbon dioxide capacity was not determined to be the overall better option when plant optimization parameters are added into consideration. This assertion has important implications in the optimization of adsorption-based systems, as analyzing volumetric uptake becomes highly relevant for reducing equipment sizes in an industrial application and lowering the cost of the adsorbent inventory. In addition, among all the adsorbents, the activated carbon that showed better potential for biogas upgrading industrial application was found to be the one obtained from the mixture of olive stones and phenol formaldehyde resins. This result is also relevant in terms of new research on the production of ACs from mixtures of locally available organic residues.

Prauchner et al. also highlighted the importance of measuring CO₂ uptake both on a gravimetric and volumetric basis [135]. Their results show that upon comparison of ACs produced from the same precursor by different activation methods and conditions, important deviations from the CO₂ uptake can be found when calculated on a gravimetric versus volumetric basis (see Figure 13). In particular, higher gravimetric uptakes were related to higher ultramicropore volume, while volumetric adsorption was found to also be influenced by the AC skeleton density.

As Rios et al. discussed in their work, another factor to take into consideration for the design and optimization of industrial gas separation processes is the experimental CH₄/CO₂ adsorption equilibrium behavior [155]. As the authors discuss, the study of the CH₄/CO₂ mixture adsorption allows for the determination of the role of the adsorbent properties on the adsorption process when competition factors are involved. By studying the adsorption of carbon dioxide and methane from CH₄/CO₂ mixtures with different compositions by a commercial activated carbon. The total adsorbed amount of carbon dioxide increased along with CO₂ composition, indicating competition for adsorption sites that favored carbon dioxide adsorption due to its higher critical temperature of 304 K vs. 190 K for methane, as well as by its higher polarizability. The best selectivity values were obtained for low pressure (0.1 MPa) for a nearly equimolar mixture, indicating that the working pressure also highly impacts the adsorption behavior of mixtures.

In this context, Zhang et al. characterized the pure gas and CH₄/CO₂ mixture adsorption on a granular activated carbon (GAC) via air oxidation, carbonization, and physical activation using anthracite as the raw material [156]. The effect of the adsorption temperature (278, 298, and 318 K), as well as the CO₂ initial concentration in the gas phase (30%, 50%, and 70%), was analyzed. As the adsorption temperature was increased, the pure gas adsorption capacity of both methane and carbon dioxide was lowered, and the CO₂/CH₄ separation coefficient, an indicator of the adsorption selectivity, was also slightly lowered but remained around 3, which is suitable for industrial applications. The obtained binary gas mixture adsorption isotherms showed that carbon dioxide adsorption capacity is always greater than that of methane even when methane content of the inlet gas was significantly higher (CH₄/CO₂ 70/30% v/v) and in accordance with the previously reviewed work, higher CO₂ content in the initial gas mixture resulted in higher carbon dioxide uptake.

Similarly, Balys et al. [52] studied the gas separation process with the purpose of enriching biogas in biomethane by a biomass-based AC produced from coconut shell, a byproduct of waste rubber tire pyrolysis char (RPC), and a commercial carbon molecular sieve (CMS). To this end, they conducted breakthrough experiments at 293 K using two mixtures: (a) 75% CH₄ and 25% CO₂ and (b) 25% CH₄ and 75% CO₂. From the obtained breakthrough curves (see Figure 14), a higher CO₂ adsorption is noted for the carbon dioxide-rich mixture for all of the samples, explained by a higher partial pressure of CO₂ in the gas phase. The CO₂ adsorption capacity for the methane-rich mixture (75% CH₄) followed the order CMS > AC > RPC, while for the carbon-dioxide-rich mixture (75% CO₂), the uptake trend was AC > CMS > RPC. Furthermore, for the methane-rich, the three samples allowed a purity of CH₄ in the outlet stream above 95 vol%, which is the typical minimum purity level requested to meet natural gas grid quality. On the other hand, only the ACs produced a methane stream with the required purity for the CO₂-rich inlet, evidencing the importance of measuring not only the adsorption capacities of the adsorbent material but also its dynamic performance under different operating conditions.

CO₂ and CH₄ pure gas adsorption tests provide valuable information in terms of the adsorption capacity of the adsorbent. The use of predictive models to determine the gas mixture adsorption behavior dominates over real mixture adsorption experiments. In order to obtain more reliable data on the competitive factors and the influence of both textural and chemical properties of the adsorbent to selectively adsorb one component, it is highly recommended to evaluate the adsorption performance under biogas upgrading industrial-like conditions, both on an equilibrium and dynamic basis [146]. To this day, CH₄/CO₂ multicomponent adsorption isotherms over different pressures, temperatures, and gas feed compositions remain limited, and as the articles reviewed in the present section demonstrate, the conditions at which adsorption takes place are highly influential on the adsorption behavior.

Furthermore, the ability of the adsorbent material to be easily regenerated and to withstand several adsorption–desorption cycles without compromising its chemical/structural stability is key for the economic feasibility of an adsorption-based system. Additionally, materials with low energy regeneration costs are preferred. The stability of an adsorbent can be assessed by evaluating the adsorption isotherms through several adsorption–desorption cycles mimicking industrial temperature and pressure conditions [152].

Activated carbons tend to have lower heats of adsorption than those of other adsorbent materials such as MOF and zeolites. Typical isosteric heats of adsorption of ACs range from 15 to 25 KJ per mol; thus, regeneration during the desorption phase of ACs has low energy requirements. Chinea et al. evaluated the methane enrichment of biogas by adsorption of gas molecules in a pilot-scale biogas upgrading system [157]. By comparing a commercial activated carbon with calcium hydroxide, they concluded that the AC allows lower recovered gas purity (87% vs. 99% methane purity). Nevertheless, due to the ease of regeneration and the low cost of the adsorbent, AC adsorption-based biogas upgrading showed to have comparable estimated costs to other technologies available on the market, while calcium hydroxide showed to not be cost-effective due to the complexity of the regenerative process.

In order to evaluate and compare adsorbents based on available adsorption data for an initial selection of adsorbents for further tests, some authors have proposed the use of Adsorption Performance Indicators (API) that encompasses several relevant properties, thus allowing a comparison method. In this context, Álvarez-Gutiérrez et al. [123] proposed an API for biomass-based ACs that balances selectivity, S_1/2, working capacity, WC, and isosteric heat of adsorption, Q_st, (Equation (2)):

$$\mathrm{A}\mathrm{P}\mathrm{I}={\displaystyle \frac{({{\mathrm{S}}_{1/2}-1)}^{\mathrm{A}}\mathrm{W}{\mathrm{C}}_1^{\mathrm{B}}}{{\left|{\mathrm{Q}}_{\mathrm{s}\mathrm{t},1}\right|}^{\mathrm{C}}}}$$

(2)

In this equation, subscript 1 indicates CO₂. The isosteric heat of adsorption of the strong adsorbate, Q_st,1 is in the denominator because the heat generated during adsorption is detrimental to the performance of the process. A, B, and C have a default value of 1, and they can be redefined according to the parameter considered more relevant to the study. In the work, API was evaluated for two ACs produced by physical activation with CO₂ (CS-CO₂) and H₂O (CS-H₂O), respectively. The obtained results, shown in

Table 6. Selectivity, CO₂ working capacity, average CO₂ adsorption enthalpy, API indicator, and selection parameter S, for the CS adsorbents (Source: [123]).

Sample	SCO2/CH4	WC, CO2 (mol kg−1)	Qst, CO2 (KJ mol−1)	API
CS-CO2	4.35	2.83	21.15	1.27
CS-H2O	4.39	2.60	23.03	1.00

, show that according to the API values, the CO₂ activated carbon shows overall performance upon consideration of the energy needed for regeneration (Q_st), purity of the outlet and methane slip (S), and productivity (WC).

Abou-Alfa et al. [11] employed a novel Approximate Adsorption Performance Indicator (AAPI) for evaluating the adsorbent performance of one activated biochar produced at 900 °C (Bamboo-A-900) and two pyrolyzed biochars at 500 °C (Bamboo-500) and 700 °C (Bamboo-700), all of them prepared from bamboo. AAPI is based on pure gas adsorption isotherms and takes into account the approximate selectivity (AS), adsorption capacity of CO₂ (${\mathrm{n}}_{\mathrm{a}\mathrm{d}\mathrm{s}{\mathrm{C}\mathrm{O}}_2}^{\mathrm{B}}$), and heat of adsorption of CO₂ (Q_st,1), expressed as follows (Equation (3)):

$$\mathrm{A}\mathrm{A}\mathrm{P}\mathrm{I}={\displaystyle \frac{({{\mathrm{A}\mathrm{S}}_{1/2}-1)}^{\mathrm{A}}{\mathrm{n}}_{\mathrm{a}\mathrm{d}\mathrm{s}{\mathrm{C}\mathrm{O}}_2}^{\mathrm{B}}}{{\left|{\mathrm{Q}}_{\mathrm{s}\mathrm{t},1}\right|}^{\mathrm{C}}}}$$

(3)

The obtained AAPI results show that the choice of the adsorbent varies with the pressure (see Figure 15), i.e., at low pressure, bamboo-500 showed the highest AAPI values due to a its high AS, the lowest heat of CO₂ adsorption, and the lowest CH₄ adsorption capacity. However, at pressures above 0.12 MPa, AAPI values depict Bamboo-A-900 as the most effective adsorbent, attributed to more developed textural properties and the presence of oxygen surface functionalities.

The reviewed literature on the use of performance indicators for the evaluation of ACs highlights the importance of incorporating gas interactions and competitive adsorption factors when comparing adsorbent materials for CH₄/CO₂ adsorption and separation since assessing the adsorption behavior purely on the adsorption of pure gases does not take into consideration all the parameters involved for the industrial application of the adsorbent. Furthermore, for a particular AC, its performance is highly dependent on the working conditions (temperature and pressure).

5.1. Comparison of ACs with Other Adsorbent Materials for Biogas Upgrading

Several porous materials have been evaluated for biogas upgrading, including zeolites, metal organic frameworks (MOF), and carbonaceous adsorbents such as ACs and CMS. One of the main challenges for the implementation of activated carbons for biogas upgrading is their low selectivity compared with zeolites and Metal Organic Frameworks (

Table 7. Comparison of CH₄/CO₂ Selectivity (1 MPa) and Maximum Selectivity of Different Adsorbents. * Ideal selectivity values.

Sample	Type	Selectivity (1 MPa, T = 303 K)	Max. Selectivity	Mol (%) CO2	T (K)	Refs.
NaX *	Zeolite	76 (0.1 MPa)		50	303	[21]
CaX *	Zeolite	22 (0.1 MPa)		50	303	[21]
CaA *	Zeolite	44 (0.1 MPa)		50	303	[21]
4A *	Zeolite		200	50	293	[158]
5A	Zeolite		256.47	50		[159]
CMS CT-350 CarboTech	CMS	28.4		20		[154]
CMS-240 (Xintao)	CMS	34.9		10		[154]
P-AC	AC	38		50		[20]
CNR-115-ox-am	AC		129	50	303	[146]
CNR	AC	1.8		50	303	[160]
GAC 1240	AC	2.7		50	303	[160]
AC WV1050 *	AC	5.22	8.7	47	293	[156]
Norit R1 *	AC	2.7		42	298	[161]
HKUST-1-MOF *	MOF		6	50	293	[158]
MOF-5 *	MOF		15.53	50	298	[159]
MOF-177 *	MOF		4.43	50	298	[159]

). In this context, surface modification of ACs has become an active research field that has shown promising results for significantly increasing selectivity [146].

On the other hand, as discussed throughout the present review, ACs have properties that make them excellent candidates for CO₂ capture and separation, such as tunable textural properties, cyclic and thermal stability, low regeneration energy, and high adsorption capacities (see

Table 8. Main properties of adsorbents for CO₂/CH₄ separation (Source: [20]).

	AC	MOF	Zeolite
CO2 adsorption capacity	High	Medium	High
CO2/CH4 selectivity	Low	Medium	High
PSA regeneration	Feasible	Not documented	Feasible (P < Patm)
Cost (USD kg−1)	0.6–2.4	10.0–70.0	0.6–5.0
Other considerations	Sustainable precursors	Low thermal and hydrolytic stability	High CO2 adsorption heat

). In contrast, Zeolites are recognized for having high CH₄/CO₂ selectivity values; their main drawback is their high sensitivity to the presence of moisture. Finally, MOFs have poor thermal and hydrolytic stability as well as high production costs [162,163].

5.2. Section Summary

Additional factors significantly affect the performance of activated carbons (ACs) in biogas upgrading processes, beyond their textural and chemical properties. Operating temperature, pressure, and gas inlet composition greatly influence the adsorption capacity and selectivity of the adsorbent. The literature review shows a gap in equilibrium and dynamic adsorption experiments under real biogas-like conditions. Other relevant factors for the industrial transferability of the adsorbent are the carbon dioxide volumetric uptake, ease of adsorbent regeneration, and thermal stability. The use of Adsorption Performance Indicators (APIs) allows the comparison of different adsorbents on a multifactor basis. ACs remain competitive due to their availability, scalability, and lower environmental and economic costs.

6. Implementation Barriers and Challenges

While activated carbons offer important advantages in biogas upgrading, several challenges must still be addressed. For instance, the cost and availability of raw materials for producing activated carbons could play a critical role [164]. In this context, the use of locally sourced waste from other processes can be highly advantageous [165]. Additionally, the capital cost required for the installation and adaptation of biogas production and upgrading systems could also represent a significant hurdle for projects, especially if they are of smaller scales [166]. Similarly, of utmost importance are any economic incentives that could be available locally, as they can considerably improve the financial viability of upgrading operations [164]. Furthermore, costs associated with production could be considerable (for example, those associated with chemical activation), while regeneration of biowaste-derived activated carbons used in continuous flow systems is under-researched.

Two of the main environmental benefits of using activated carbons in biogas upgrading are CO₂ separation and capture, as well as their potential contribution to circular economy schemes when produced from locally sourced biowastes. Nevertheless, they also face challenges that must be improved on. As mentioned earlier, both physical and chemical activation processes involve significant amounts of energy, which, if sourced from non-renewable sources, will contribute to climate change and air quality problems. Likewise, if the substrates used are obtained from non-renewable sources, their environmental footprint may be of concern [5]. Figure 16 summarizes the main environmental impacts and challenges of activated carbons both during production and in use for biogas upgrading.

Finally, it is clear that activated carbons show many promises in biogas upgrading and that they exhibit great relevance within adsorption-based technologies. Their eco-friendliness, low energy needs, safe operation, removal efficiency, and flexible design make them very attractive. Nevertheless, there are also key areas where these materials must be further explored and improved. These include enhancing CO₂ adsorption capacity and selectivity, (particularly through chemical activation methods), addressing the energy requirements of regeneration through the study of its kinetics and thermodynamics, improving their performance in real biogas conditions taking into account impurities (i.e., H₂S, water vapor), and shaping the powders obtained into useful presentations with low pressure drops when used for instance in packed columns. Furthermore, improving production methods without negative impacts on the sustainability of biowaste-derived ACs must also be further researched. Likewise, life cycle assessments including all substances involved as well as all operational parameters require further consideration so as to reduce capital and operational costs [5,11,49,62].

7. Conclusions

The present work explores the potential of activated carbons for biogas upgrading, depicting several techno-economic advantages, including a high surface area of up to 3000 m² g⁻¹ for commercial activated carbons and up to 2000 m² g⁻¹ for biomass-based AC. In addition, their porosity can be tuned in order to obtain higher CO₂ adsorption capacities. In this context, maximum adsorption values of up to 10.9 mol kg⁻¹ are reported in the literature. Furthermore, due to their low CO₂ adsorption heat, typically ranging from 20 to 30 kJ mol⁻¹, they have low regeneration energy requirements. CH₄/CO₂ breakthrough adsorption experiments indicate that methane purity levels of up to 95% can be obtained using ACs as the adsorbent material through several adsorption–desorption cycles. Another important advantage is that they can be produced from sustainable sources such as agricultural and food wastes, and more recently, from plastic waste. The adsorption properties are shown to be highly influenced by the substrate choice and the activation process. The suitability of ACs requires the systematic exploration of how activation parameters (such as activation agent, activation agent ratio, flow rate, temperature) influence both the textural and chemical properties of the resulting adsorbent and, in turn, their effect on the adsorption behavior. Overall, the presence of developed ultramicroporosity has been found to be the most influential parameter for carbon dioxide uptake on ACs; nevertheless, the presence of wider pores is also required to allow the diffusion of the gas molecules into the adsorbent structure to reach the narrower pores. Additionally, the presence of basic surface groups can help enhance the selectivity towards carbon dioxide; selectivity values of up to 129 for modified AC are described in the literature. The addition of surface basic functionalities can result from choosing the right activation agent, or alternatively, can be produced by post-synthesis treatment of AC. Finally, although many promising carbonaceous adsorbents demonstrate favorable performance under laboratory conditions, they still face significant challenges in real-world biogas upgrading applications. Much of the ongoing research is based on equilibrium conditions and pure gas adsorption predictions. The presented literature also highlights the key relevance of taking into consideration industrial operation parameters such as dynamic performance, cyclability, volumetric volume uptake, adsorption, and regeneration behavior under pressure- or temperature-swing conditions and variable inlet gas compositions.