Over the past 10 y, governments worldwide have been establishing policies to provide renewable and clean energy. Their positions, which have been established from both increasing recognition of environmental problems and from economic demand, have led scientific and industrial communities to study and exploit fresh and useful alternatives to decrease the environmental effects from the use of fossil fuels. For these reasons, the increasing demand for electrical energy and transport fuel has motivated the United States [1] and European Union [2] governments, as well as scientific communities [3], to find new and green energy-carrier resources. Until recently, renewable and clean energy accounted for only a small section of the energy market; however, recent demand has led to growth in this area. Moreover, the abundance of natural gas deposits in some parts of the world will provide a massive resource of energy in the upcoming decades [4].

Natural gas is considered an alternative fuel because of its very low price as an efficient resource [5]. In addition, many researchers and industrial administrators are interested in using universal applications that can be used as alternative fuels for vehicles [6].

The greenhouse effect, which is widely believed to cause global warming, is a result of industrialization and the generation of gases such as methane (CH₄), carbon dioxide (CO₂), nitrous oxide (N₂O), and so on. Because of the rise in the production of greenhouse gases over the past century, the temperature of the earth has increased by 0.76% and is gradually increasing [7,8]. Many researchers have raised concerns over greenhouse gases and have studied methods directed at their elimination. In particular, CH₄, which is generated by organic waste, cow manure, and industrial waste, is being studied for use as a clean energy. CH₄ is an eco-friendly and generally lower risk energy source that could serve as an alternative, green energy in the international markets for years to come [9-11].

A large fraction of natural gas is CH₄, which has a low volumetric energy density (VED); however, its use is limited because of storage and transportation issues. For this reason, its compression and storage are of great interest to industrial and scientific communities.

The main challenge is the ability to store natural gas under atmospheric conditions (temperature and pressure) in order to reduce transportation limitations and facilitate its use [12]. In view of this important goal, it is necessary to take into account evaluation of the CH₄ storage capacity, which should reach a target of 150 v/v for the year 2015 [13]. This figure was recently revised to 180 v/v [14-16] (the volume of gas adsorbed at standard temperature and pressure: 298 K, 0.1 MPa, per volume of the storage vessel) or equivalently 35 wt% [17] at 3.5 MPa according to the U.S. Department of Energy.

Currently, CH₄ storage methods involve high-pressure compression in tanks (compressed natural gas [CNG]) [18,19] (the tank mass contributes largely to the storage system mass, significantly decreasing the CH₄ wt% capacity of the entire system) and/or storage in liquid form (liquid natural gas [LNG]) at temperatures down to 120 K (mainly used for intercontinental transportation). Together with the energy density requirements, CH₄ portability has strict requirements in terms of safety, refilling, and delivery. From this perspective, studies on adsorbed natural gas (ANG) and natural gas hydrate (NGH) have been carried out with the aim of representing an efficient alternative solution for CH₄ portability [20].

NGH is solid and has a theoretical volumetric capacity of 164 or 174 v/v, assuming complete filling of the water cages within the clathrate structure, but the formation conditions of NGH are rigorous and its formation rate is low [21]. In addition, the stored gas cannot be released from hydrates by reducing the pressure. Hence, NGH is not suitable as a commercial technique for CH₄ storage.

Compared to pressurization, cryogenic liquefaction, or any other storage method on the market, CH₄ storage through solid materials (ANG) presents advantages according to gravimetric and VED, safety, and energy efficiency. The classification of natural gas adsorption technologies is presented in Fig. 1. It is important to note that the critical issues related to commercialization of ANG storage technology not only include the storage capability of the adsorbent but also the facility costs and the cost of manufacturing. The most important evaluation of the performance of an ANG storage system is the volume of usable gas [22]. This is what scientists frequently call delivery, and it is defined as the volume of gas obtained from the storage vessel when the pressure is reduced from a storage pressure of 35 to 1 bar at room temperature [23]. Despite these issues, ANG technology, where natural gas is adsorbed by a porous adsorbent material at relatively low pressures (35/40 bar), is considered a promising solution in energy storage, especially in the natural-gas fuelled vehicles sector [24]. Within this global energy scenario, it is evident that one of the most important goals for the extensive use of CH₄ is the synthesis of CH₄ storage materials with good adsorption properties under convenient temperature and pressure, but particularly with on-board operating storage capabilities for vehicular applications [25,26].

Until now, all available materials present different limitations, especially for on-board storage applications. These limitations include (1) relatively high thermal stability, (2) slow desorption/absorption kinetics, (3) unstable structures [27], (4) high weight, (5) irreversibility upon cycling [28], and (6) high production costs. For these reasons, researchers have focused on the development and analysis of meso- and nano-structured systems with high specific surface areas to exploit CH₄ physisorption.

Much of the previous work in this field has involved the application of adsorbents for CH₄ storage (gas adsorption). Many researchers have studied the use of carbonaceous materials for gas adsorption. Carbonaceous materials have been widely used in industrial applications such as adsorbents [29-31], supercapacitors [32], and batteries. They can be produced from natural materials, including wood, charcoal, petroleum coke, sawdust, and coconut shell. Because of their advantages, carbonaceous materials have been of interest for decades. In particular, we have studied carbonaceous materials as gas adsorbents according to various surface treatments, such as ozone [33], plasma [34], thermal [35], and electrochemical treatment [36].

Park et al. [36] reported the pore structure and surface properties of activated carbons (ACs) for Cr(VI) adsorption enhanced by chemical modifications using acids and bases. Im et al. [37] reported that for hydrogen adsorption, the pore size of electrospun carbon nanofibers was controlled by activation.

The process of storing CH₄ through porous adsorbents utilizes the selective adsorption found in gas-solid interactions [38,39]. Typically, common porous adsorbents such as metal-organic frameworks (MOFs), zeolites, ACs, and activated carbon fibers (ACFs) are used in storage tanks. The significant parameters in the adsorption process are (1) the temperature and pressure during the adsorption and (2) the pore size distribution of the adsorbent. The cycle of adsorption and desorption is involved in the CH₄ storage process [40].

The different storage methods are as follows: (1) compression to 200-300 bar at room temperature and (2) liquefaction at low temperature. In these methods, the VED of liquefied CH₄ gas achieves ~64% of the VED of gasoline (34.2 MJ L^–1) but it must be stored in high-cost cryogenic vessels and suffers boil-off losses. While CNG necessitates the use of heavy, thick-walled, cylindrical storage tanks and multi-stage compressors to achieve a reasonable VED (9.2 MJ L^–1), it achieves only 27 % of the VED of gasoline. In spite of the improvements to cylinders and compressors, the majority of natural gas is stored in CNC tanks [41].

It is necessary to improve the CH₄ storage capacity and adsorption selectivity of porous adsorbents to avoid large amounts of combustion emissions from various sources. CH₄ storage can be stabilized and improved using the pore structure [42-45] and high specific surface area [46] of adsorbents.

In order to achieve a high storage capacity, the establishment of various methods used as CH₄ storage adsorbents is expected to enhance adsorption efficiency for CH₄ storage. Many adsorbents have advantages such as low cost implementation for reproduction and perdu elimination of loss problems.

The process of adsorption is reversible and modification of the adsorbent structure could improve the adsorption efficiency. Moreover, the selection of a suitable adsorbent could improve the CH₄ storage efficiency. At present, the universal adsorbent materials are zeolites [47], porous silicas [48], ACs [49], regular fibers, and so on. These materials have various pore sizes, structures, and specific surface areas, as well as many available fields. Some porous adsorbents for CH₄ storage are listed in Fig. 2 and Table 1 and will be explained in the following section.

Many MOFs have been prepared by combining organic (ligands) with inorganic units (metal clusters) through strong combination (reticular bond). The functionalities of the components can be varied and the flexibility in size and geometry has progressed toward more than 21,000 various MOFs being studied and announced, with their specific surface areas ranging widely from 1000 to 10,000 m²/g [50,51]. They have popularized many previous studies because of their excellent adsorption capacities, their specificity for the adsorption of CO₂ [52-54], H₂ [55-57], or hazardous gases, and their great selectivities [58-61]. In addition, they are widely used as chemical catalysts for synthesis. The adsorbents structures of MOF for CH₄ storage are the useful position at the core because of entanglement of molecules. Several structures have a huge adsorbent volume because the pores interact with metallic ions and organic molecules. MOFs are used in gas storage applications because their pore diameters are easily controlled.

Furthermore, conjugated microporous polymers (CMPs) have been of interest because of their porous structures with organic functionalities, for such applications as gas storage, capture, and separation [61-64]. In particular, the committee of CMPs offers novel materials capable of CH₄ storage and their properties can be reduced effectively. Jiang et al. reported that the CH₄ storage efficiencies of Co-CMP and Al-CMP were partially high, although the specific surface areas were lower than those of other MOF materials.

MOFs have excellent CH₄ adsorption capacity at high pressure and low temperature. However, many MOFs have low CH₄ adsorption performance compared to other adsorbents. In order to increase the CH₄ adsorption efficiency, MOFs should be further developed. In addition, the process of preparing MOFs from organic ligands and metal complexes is very expensive and complicated. MOFs accompany many problems because of hindrance during CH₄ adsorption. The use of MOFs in the field of CH₄ adsorption must therefore overcome these limitations.

Kaskel et al. [65] synthesized an MOF (HKUST-1) for CH₄ storage at low pressure. They confirmed that HKUST-1 showed a CH₄ adsorption capacity of 16.5 wt% at 298 K and 35 bar. In addition, the obtained specific surface area was higher than the values of different MOF samples.

Senkovska and Kaskel. [66] prepared an MOF using a porous coordination polymer and evaluated the CH₄ storage capacity. From the results, they confirmed that Cu₃(btc)₂ (HKUST-1) (btc = benzene-1,3,5-tricarboxylate) showed the highest CH₄ adsorption capacity at 303 K and 35 bar (15.7 wt%).

Zeolites exist in natural microporous crystalline silicate framework foams and they can be produced by artificial methods in the lab. They are classified by their pore size, which ranges from 0.5 to 1.2 nm, for the adsorption of gas molecules [67]. They also have been widely applied to gas separation and purification [68-70]. Many researchers have investigated CH₄ storage using zeolites because of the microporous structures between their porosity and CH₄ molecules in the zeolite frameworks. The metal cations in zeolites (i.e., Li, Na, and Al) affect the CH₄ adsorption. CH₄ storage using zeolites has been studied in previous works, mainly with zeolite 5A and zeolite 13X, which show CH₄ storage performance of 2-27 wt% at room temperature and high CH₄ gas pressure [71-74].

Grande and Blom. [75] prepared commercial zeolite 4A and zeolite 13X for efficient adsorption of CH₄ gas. This study was performed with a wide range of pressures (1-10 bar) and flow rates. They reported that zeolite 13X presented an efficiency of 16 wt% at 279 K and 1 bar for CH₄ storage.

Sethia et al. [76] reported that cesium-exchanged zeolite-X could adsorb a natural gas mixture. From the experiments, they reported that the cesium-exchanged zeolite showed a CH₄ adsorption capacity of 14 wt% at 303 K and 1 bar.

The carbonaceous adsorbent materials consist of carbon and show good properties such as high strength, heat and electrical conductivities, thermal and chemical stabilities, and eco-affinities [77-82]. They are especially superior for applications in gas storage or adsorption because of their high specific surface areas, large pore volumes, and light weight. Moreover, they have the following advantages for CH₄ storage: (1) carbonaceous materials are not sensitive to wet conditions; (2) the cost of carbonaceous materials is reasonable; (3) the adsorption-desorption temperatures are not particularly good under 373 K; (4) the energy consumption is efficiently low; and (5) they can be applied under atmospheric pressure. All of these conditions affect the studies presently discussed.

3.3.1. Activated carbons

The CH₄ storage capacity is highly influenced by the textural properties. The pore sizes of ACs are classified from micropore to macropore; thus, ACs are not suitable for selective adsorption of a specific gas. Carbonaceous materials as adsorbents generally have low adsorption performance for CH₄, with adsorption heats of less than 45 kJ/mol. The CH₄ storage capacity of an AC is ~14 wt% at 298 K and 35 bar [83]. The textural properties of ACs can be controlled by various activation factors and preparation [84-88]. In particular, the adsorption capacity of ACs as adsorbents for gas storage is influenced by the pore structures.

Sreńscek-Nazzal et al. [140] prepared KOH-AC from sugarcane molasses, which gave a CH₄ adsorption capacity of 19 wt% at 50 bar and 293 K. They emphasized that its remarkable CH₄ storage ability was a result of its preparation involving chemical activation with KOH and carbonization, which led to a good pore structure and thus good CH₄ storage ability. In addition, when prepared by carbonization at 1053 K with a KOH weight ratio of 2.8, the AC demonstrated a high specific surface area (2202 m²/g) and optimal pore structures.

Bastos-Neto et al. [89] reported ten AC samples prepared using different raw materials. They claimed that the good CH₄ adsorption ability was due to an optimal micropore volume (8-15 Å) and specific surface area (2336 m²/g). Moreover, they determined the CH₄ adsorption capacity, which was 19 wt% at 303 K and 35 bar. They observed that the AC samples prepared using different raw materials had different textural properties and CH₄ storage capabilities.

Policicchio et al. [114] synthesized high surface activated carbon (HSAC) samples with a pyrolysis temperature and time of 1073 K and 360 min, respectively. The HSAC samples were formed with specific surface areas of up to 1784.36 m²/g and narrow micropore sizes. The samples were then analyzed by the BET method for surface area and micropore volume measurements. The CH₄ storage capacities of HSAC samples were measured using a Sievert type (f-PcT) apparatus at 298 K and 35 bar. They verified that the CH₄ adsorption capacity was 16 wt% and was strongly influenced by the micropore sizes of the adsorbent materials.

Zhou et al. [92] reported the preparation of wet carbon from corncob with tailored pore sizes using a KOH activation agent. The wet carbon had an optimum narrow pore size distribution from 1.6 to 2.8 nm and presented a very high CH₄ storage capacity of 65 wt% at 275 K and 33.5 bar. They emphasized that the high CH₄ storage capacity was attributable to the presence of a tailored pore size distribution, together with a high percentage of optimal pore sizes.

3.3.2. Ordered porous carbons

Ordered porous carbon materials have attracted much research attention due to their wide applications in gas storage, and as electrode materials, supports, catalysts, etc. [93-98]. Many synthetic methods for ordered porous carbon materials have been reported including (1) direct synthesis by organic–organic self-assembly, involving a combination of carbon precursors and block copolymers as soft templates, and (2) nanocasting using silica materials as structural directing hard templates [99-102].

Cao et al. [102] reported that an ordered mesoporous carbon synthesized using a soft-templating method had a CH₄ adsorption capacity of 21 wt% at 300 K and 40 bar. Guan et al. [103] reported that template carbons synthesized using a pyrolysis method had a high specific surface area of ~1,500 m²/g. Zeolite-template carbon fabricated by carbonization of sucrose had a CH₄ adsorption capacity of 12 wt% at 300 K and 35 bar.

3.3.3. Activated carbon fibers

ACFs are attractive adsorbent materials because they have high specific surface areas and narrow pore size distributions [105-108]. In addition, the fibrous structure of ACFs is easier to handle than granular and powdered carbonaceous materials [109-110].

Shao et al. [111] prepared ACFs by a physical activation method using pitch-based carbon fibers as a raw material at 1173 K under a flow of H₂O/N₂. The obtained samples showed a CH₄ adsorption capacity of 11 wt% at 298 K and 15 bar. It was found that the pore structure of ACFs could be tuned by controlling the ratio of H₂O/N₂ as the active agent. In addition, the CH₄ adsorption capacity was highly influenced by the narrow micropore size distribution.

Alcañiz-Monge et al. [112] prepared ACFs by CO₂ and steam activation methods using petroleum-pitch based carbon fiber as a raw material. The prepared samples showed a CH₄ storage capacity of 16 wt% at 298 K and 40 bar. It was shown that the CH₄ storage capacity of the ACFs could be enhanced by increasing the micropore sizes.

Im et al. [137] prepared ACFs by fluorination modification using electrospun carbon fibers as a raw material at 1023 K for 3 h in an argon atmosphere. The obtained samples showed a CH₄ adsorption capacity of 17.5 wt% at 298 K and 35 bar. It was found that the pore structure of electrospun ACFs could be tuned by controlling the fluorination surface modification. In addition, the CH₄ adsorption capacity was highly influenced by the synergetic effects of the developed micropore structure and the guiding of methane to carbon pores by fluorine. Fig. 3 shows this data in comparison with other data. The F-EsACF-5 sample showed high methane storage capacity of about 17.5 wt%. Furthermore, the APCF-120M sample showed 17.1 wt% methane storage capacity. The APCF-120M sample was studied by the present authors and prepared by KOH activation using pitch based carbon fibers as a raw material. According to Fig. 3, carbonaceous adsorbent materials, such as AC and ACFs, are suitable for methane storage materials.

Useful adsorbents for CH₄ storage technologies should be improved in terms of their storage capacity and adsorption/desorption cycle durability. The study of vastly superior adsorbents for CH₄ storage requires the implementation techno-economical systems. Thus, it is significant that a large body of data has been collected using adsorption reactors and gas storage systems. In addition, because of the progress of nanoporous adsorbents for CH₄ storage, it is expected that improved techno-economical systems will enhance the adsorption performance of CH₄ gas. Furthermore, nanoporous carbonaceous materials as CH₄ storage adsorbents will be extensively developed by researchers and applied to alternative vehicle fuels.