Methane and Hydrogen Storage in Metal Organic Frameworks: A Review

1.0  INTRODUCTION

The presence of CO₂ in the atmosphere is considered to be one of the leading causes of global warming. This can be attributed to the fact that concentration of CO₂, a major contributor to carbon footprint is continuously increasing. NOAA headquarters recorded in 2018 that the concentration of CO₂ in the atmosphere averaged 405.0 ± 0.1 ppm in 2017 compared to 1750 (pre-industrial era) of 277 ppm in 1750 (Joos and Spahni, 2008).  A frontier factor to increased amount of global CO₂ emission increase is the high demand and use of fossil fuels for various uses such as energy generation, feedstock for petrochemicals and others. In order to minimize this increase, cleaner energy sources that would release lesser or no amount of CO₂ are being integrated into the energy mix.

One of the viable energy solutions that can significantly reduce the carbon footprint in the atmosphere is natural gas (NG) and hydrogen. NG is considered as a bridging fuel that can facilitate the transformation of the world’s energy mix to a carbon free one. NG whose main component is methane (CH₄) with a high Research Octane Number (RON=107) has been reported by different authors (Chen et al., 2014) as a promising candidate to replace “dirty” fossil fuels during this phase of energy transition. Müller et al. (2013) highlighted in their work that NG offers advantages such as low fuel costs due to a higher energy content when compared to gasoline, reduced carbon emission (20 % less CO₂ production) and lower pollution (reduces NOx emissions) over gasoline and its counterparts.

Hydrogen, another clean energy source (rather the cleanest energy source whose combustion products are only heat and water) is also a reliable innovative energy source that can totally eliminate the emission of green house gases in the atmosphere. Suh et al. (2011) reported that hydrogen’s energy content three times (123 MJ/kg) exceed that of gasoline (47.2 MJ/kg). Coupled with its environmental non-polluting nature, hydrogen possesses great potential as a fuel of the future that seeks to be carbon free. A key challenge to the use of hydrogen for practical and industrial application is the difficulty in storing the gas at low pressures and ambient temperatures. Unfortunately, natural gas faces similar challenges. In cases where hydrogen is to be used as a fuel source in automobiles, its low energy density requires that 13kg of hydrogen is consumed daily for 300 miles driven, requiring a very large pressurized storage onboard tank for storage and transportation of the gas; this also would invariably translate to frequent refilling (Suh et al., 2011).

These conditions of storage at very high pressures (which invariably results in high cost due to the design of special storage tanks) of NG (CNG=250 bar) and hydrogen (70 MPa) (Yamashita et al., 2015) create safety concerns and incur very high cost of storage. This has necessitated intense research into advanced technologies that can store these gases at low pressures and ambient temperatures where applicable. This review therefore evaluates the practical and industrial use of a novel adsorption technology, metal organic framework for methane and hydrogen storage at lower pressures and temperatures closer to ambient conditions than what is obtainable presently.

2.0  LITERATURE REVIEW

This section focuses on the advanced methods used in storing methane and hydrogen at low pressures and temperature conditions close to ambient conditions than what is obtainable in industrial technologies presently. Also, the determining factors that affect the deployment of this technology for industrial and commercial applications are also reviewed in this section.

2.1 METHANE AND HYDROGEN STORAGE

Different methods have been proposed and used for NG storage. Chen et al. (2014) itemized them as a.) liquefied natural gas (LNG) b.) compressed natural gas (CNG) c.) adsorbed natural gas (ANG) d.) natural gas hydrates (NGH). Amongst these storage systems, ANG has stood as the only option that can be applied at conditions close to atmospheric conditions (low pressures and room temperature). For LNG, process conditions are cryogenic (temp. of -161^oC and 100KPa), CNG is stored at very high pressures of 200-300 bar as a supercritical fluid at room temperature while NGH involves difficult-to-attain conditions for its formation and slow formation rate. The challenges faced by LNG, CNG and NGH have therefore propelled ANG as the viable storage medium for NG.

Hydrogen likewise boasts different storage technologies that have found difficulty in competing with gasoline for industrial applications at low pressures and room temperatures. Yaghi et al. (2016) highlighted these storage technologies with their various challenges in their comprehensive review work. The technologies are i.) compressed gaseous storage of hydrogen at pressures of 350 – 700 bar; giving an energy content of 4.4 MJ/l which is still way below that of gasoline (31.6 MJ/l). A drawback to this technology is the cost which they stated to be 100 times higher than that of gasoline ii.) Liquid storage of hydrogen at a temperature of -253^oC giving an energy content of 8.4 MJ/l.  Though its energy content is higher than that of compressed gas storage, the cost of liquefaction and the compulsory venting process in FCVs referred to as Dormancy (Schüth et al. 2009) poses serious challenge to its practical applications iii.) Solid-state storage involves using metal hydrides and complex hydride metals (borohydride) for hydrogen storage. Drawbacks to this technology include the high cost of high purity metals used in the production of hydrides and the non-reversibility of hydrogen uptake by these compounds. As such, adsorption still poses a better solution to the commercial and safe storage of hydrogen at low pressures and room temperature as it is obtained for methane storage. The aim of this study is to therefore review advances in adsorption technologies used that show promising potential for methane and hydrogen storage.

2.2 Adsorption Technology

Adsorption storage of NG and hydrogen have exhibited promising potential applicability in energy systems and processes due to the advancements recorded in the synthesis and functionalization of a Porous Coordination Network (PCN) with improved storage. One of such PCNs is Metal Organic Framework (MOF). Metal–organic frameworks (MOFs) are an emerging class of porous materials constructed from metal-containing nodes and organic linkers (Zhou et al., 2012), whose basic synthetic process is shown in Fig. 1. Due to the strong bonds that exist between the metal-containing nodes [also known as secondary building units (SBUs)] and organic linkers, MOFs usually boast a structure with permanent porosity and open crystalline frameworks. The organic spacers or the metallic SBUs can be altered to control the pore environment of the MOF, which controls its interactions with adsorbates (in this case NG and hydrogen).

Fig. 1. Synthesis of a MOF from metal clusters and organic linker forming a supramolecular building unit which then forms a 3-D framework with pores, (Zhou et al., 2011)

MOFs exhibit impressive characteristics for gas storage and separation such as extremely high surface area (up to 10000 m²/g), high porosity (up to 90%) and tunable pore sizes. The ease with which SBUs and organic linkers are altered, selected and functionalized has led to the synthesis of various types and structures of MOFs. As a result, the structures and properties of MOFs can be easily designed and systematically tuned by the judicious choice of building blocks (Zhou et al., 2011) unlike other porous compounds such as zeolites, activated carbon, polymers etc. MOFs therefore outperformed other traditional porous materials in terms of tunability, ease of functionalization and synthesis. Lozano-Castelló (2002) corroborated this by stating that traditional zeolites exhibit methane uptake below 100 cm³ (STP) cm^-3 which way below the uptake capacity of some MOFs such as HKUST-1 with a methane uptake of 267 cm³/cm³ at 298K and 35 bar (Peng et al., 2013). On a general note, most porous carbon materials exhibit low methane uptake capacity which according to Menon and Komarneni (1998) are in the ranges of 50 – 160 cm³/cm³.

2.2.1 Future target of Adsorption Technology for Methane and Hydrogen storage

For these clean energy sources to compete with conventional energy sources such as gasoline, a gravimetric target of for methane on-board storage systems have been set at 0.5 g_(methane) g⁻¹_(sorbent) or 700 cm³_(methane) g⁻¹_(sorbent) at 298 K and 65 bar by Advanced Research Projects Agency-Energy (ARPA-E) of the US DOE (ARPA-E, 2012). This gravimetric capacity translates to a volumetric capacity of 263 cm³ (STP) cm⁻³ when the density of methane (ρ = 0.188 g/cm³ at 250 bar) is used as a reference; considering a packing loss of 25% due to pelletization of MOF powder, the initial uptake capacity for methane based on the set target will then have to be 330 cm³ (STP) cm⁻³ (Yaghi et al. 2016).

In order for hydrogen to likewise compete with gasoline and other energy sources for practical applications, Fuel Cells Technology Office of the US Department of Energy (DOE) has set targets for storage and transportation capacities of adsorbents. The targets are 1.8 kWh/kg (5.5 wt% hydrogen uptake) on a mass basis and 1.3 kWh/l (0.040 kg H₂/L) on a volume basis. This is in addition to other targets such as reasonable driving range, reasonable refuelling time and minimum driving range of 300 miles (Yaghi et al., 2016). In terms of cost, the target is US$ 10 per kWh (US$ 333/kg of hydrogen storage capacity). This translates to a volumetric capacity of 40 g/l which is already achievable in second generation vehicles at a pressure of 10,000 psi. The operating conditions at which this target is to be met are -40 – 60^oC for temperature and pressures below 100 atm (1500psi) (Zoellter, 2015).

Meeting these targets both for methane and hydrogen storage will rather be difficult unless with the use of metal organic frameworks. For methane storage at the target set by DOE to be achieved, metal organic framework is likely the only possible option to be used. Though the process conditions of MOFs uptake of hydrogen [especially temperature of 77K (-196^oC)] at which they store hydrogen is still below the target of DOE, MOFs still remain one of the viable options in meeting the DOE’s target. In face of these challenges, MOF present itself a promising PCN for hydrogen storage in the near future at ambient temperature and pressure as intensive research continues in finding the optimal MOF structure that can meet DOE’s target (Suh et al., 2011).

2.3  Metal Organic Frameworks for Methane and Hydrogen Storage

Metal organic frameworks have been investigated for methane and hydrogen storage by different researchers (Suh et al., 2011; Chen et al., 2014; Yaghi et al., 2016; Hirscher et al., 2017; Kalidindi et al., 2018) and they have been revealed to be promising candidates for gas storage at low pressures and temperature of interest. The chemistry of metal organic frameworks synthesis is key to the utilization of these structures for gas storage. Several methods such as open metal dense sites (OMSs) (Gao et al., 2017), post synthetic modification [functional groups such as amino groups (Costa, 2008), pendant aldehyde (Burrows, 2008) and azides (Goto, 2008) are incorporated into the MOF structure], post synthetic metalation (Doonan et al., 2014) and post synthetic metal ion and ligand exchange (Kim et al. (2012) have been reported as synthetic pathways through which the adsorption capacity of MOFs can be increased.

Some MOFs structures have been reported to be good candidates for methane storage. They are M₂(dicarboxylate)₂dabco frameworks (Senkovska and Kaskel, 2008), Zn₄O-based MOFs (Eddoaudi et al., 2002), copper carboxylates frameworks (Peng et al., 2013), MIL-series (Rallapalli et al., 2010) and Zr-based frameworks (reported to be stable in the presence of water even though they do not exhibit storage capacity as high as benchmark MOFs in methane storage) (Chen et al., 2014). Certain MOFs have also been reported to be good candidates for hydrogen storage though not at the target of low pressures and temperatures of interest. They are MOFs designed with a.) open metal sites with low coordination number b.) extra framework cations c.) ultrahigh surface area.

3.0 INDUSTRIAL APPLICATIONS OF METAL ORGANIC FRAMEWORKS FOR METHANE AND HYDROGEN STORAGE

A lot of work has been done on the chemistry of MOFs for methane and hydrogen storage. Literature with regards to designing new MOFs and optimizing pristine structures for effective and efficient methane and hydrogen storage are copious (Wu et al., 2018; Liu et al., 2019; Koizumi et al., 2019; Broom et al., 2019 etc). The focus of this review is therefore to critically investigate the potential applicability of MOFs for practical and industrial storage of methane and hydrogen. Key performance indicators such as storage capacity, cost of production, stability and reusability/regeneration are used in this work to evaluate the readiness of MOF for industrial applications (Kim and Deep et al., 2016).

The gas storage capacity of MOF is a very important factor for its industrial usage. For methane and hydrogen storage at practical conditions, the delivery capacity of the adsorbent is as important as its storage capacity (Chen et al., 2014). The delivery capacity is the amount of gas stored in the adsorbent between the upper working pressure and the lower working pressure at constant temperature as depicted in Fig. 2. For methane storage in natural gas vehicles, the engine working pressure is 5 bar as such, this is considered as the lower working pressure for adsorbent storage (Mason et al., 2014). The upper working pressure is considered to be 65 or 35 bar as it can be actualized by using a single or two stage compressor, which is cost effective as the case maybe. The upper working pressure is kept at higher pressures (65 bar) because this translates to the storage of a higher amount of methane at a particular temperature. For high pressure cryogenic hydrogen storage, its delivery pressure is set at 100 atm based on DOE’s target. This is in a bid to bridge the associated cost gap between hydrogen storage and utilization and conventional fossil fuels as current hydrogen storage technologies store H₂ at pressures between 5000 to 10000psi (350 to 700 bar).

Some MOFs have stood out and still hold the record for methane and hydrogen storage presently. For methane storage, HKUST-1 is reported to have the highest volumetric uptake while MOF-519 has the highest volumetric delivery; Al-soc-MOF-1 is also reported to have the highest gravimetric uptake and delivery of methane (uptake pressures of 65 bar and delivery pressure of 5-65 bar, temperature of 298K) (Yaghi et al. 2016). Hydrogen storage using MOF presently occurs at cryogenic conditions and high pressures (~ 100 bars) which are setbacks for their practical applications. As such, there is still remaining at a serious challenge to the storage of hydrogen at room temperatures and low pressures particularly. This can be attributed to the decreasing strength of van der Waals force as temperature increases from cryogenic conditions. In the section, the key performance indicators will be discussed as it affects the industrial application of metal organic frameworks for methane and hydrogen storage.

3.1 Storage and Delivery capacity of metal organic frameworks for methane and hydrogen storage

Some authors have highlighted the storage and delivery capacity of methane using MOFs at different pressures and temperature in their works. The works of Chen et al. (2014) and Wu et al. (2018) tabulated the various uptake and delivery capacities of MOFs at different pressures and temperatures for methane as such, that information will not be repeated in this work. The practical and industrial use of MOFs for energy storage [methane and hydrogen (which is yet to be achieved)] at ambient temperatures and low pressures is currently faced with mature technologies that has been commercialized. In automobiles for instance, gasoline still remains a preferred option in terms of energy density which is a key determinant of the amount of energy the tank can store at ambient temperature and low pressures. In terms of volumetric capacity, gasoline exhibits higher energy density capacity (32 MJ/L) than hydrogen (8 MJ/L) (Suh et al., 2011) and 3.52 MJ/L for methane (Chen et al., 2014). This implies that hydrogen has to contain 4 times more volumetric energy density per litre to match gasoline likewise methane must contain 9.1 times more in volumetric energy density to match gasoline on competitive basis. To these setbacks, MOFs have been shown to provide viable solutions as they exhibit promising properties in storing and delivering these energy sources at low pressures and ambient temperatures at competitive volumetric and energy density, thereby challenging conventional means of energy storage for market place.

For methane, some MOFs have shown great promise for storage at pressures of 65 bar and 35 bar respectively and delivery pressure at 5 bar. It is the important to note that these MOFs are the closest adsorbents to meeting storage target set by DOE. In Table 1, the highest performing MOFs presently with their respective storage and delivery capacities are highlighted.

Table 1: MOFs for Methane storage and delivery at 298 K (room temperature)

MOF	Volumetric uptake (cm3/cm3)		Volumetric delivery (cm3/cm3)		Gravimetric uptake (g/g)		Gravimetric delivery (g/g)		REF
	65 bar	35 bar	65 – 5 bar	35 – 5 bar	65 bar	35 bar	65 -5 bar	35 – 5 bar
HKUST-1	267	211	190	150	0.216	-	0.154	-	Peng et al. (2013)
MOF-519	259	200	210	151	0.194	-	0.157	-	Gándara et al. (2014)
Al-soc-MOF-1	197	127	176	106	0.415	-	0.371	-	Alezi et al. (2015)
PCN-14	230	195	157	125	-	-	-	-	Peng et al. (2013)
UTSA-88a	248	204	185	141	-	-	-	-	Chang et al. (2015)
Ni-MOF-74	251	228	129	115	-	-	-	-	Peng et al. (2013)
MAF-38	263	226	187	150	-	-	-	-	Lin et al. (2016)
NU-800	232	174	197	139	-	-	-	-	Snurr et al. (2014)

As earlier stated, due to the packing loss resulting from pelletization in MOFs, the initial volumetric uptake for methane by MOFs have to be 330 cm³/cm³ while its working capacity at delivery pressure of 5 bar will be 315 cm³/cm³ to meet DOE’s target. As such, there is still a lot of work to be done in synthesizing a MOF structure that would meet such storage capacity and most importantly deliver methane at such designated capacity as most of the MOFs obtainable presently are below the target storage and delivery capacity.

 

Fig. 2. Uptake and delivery capacities of MOF for methane storage at 298K

Though methane storage using MOFs have shown promising potential at low pressures and atmospheric conditions, same is not obtainable for hydrogen storage. A major setback to the practical and industrial use of MOFs for hydrogen storage is the cryogenic conditions [cryogenic temperatures (-196^oC) and high pressure (~100 bar)] at which it is stored. This can be attributed to van der Waals force that becomes weaker at temperatures close to ambient and low pressures. If hydrogen is to be stored under such cryogenic conditions, it will necessitate designing specific tanks with special materials that can handle such cryogenic conditions. This cumulates to high cost of storage making it highly unattractive for practical applications. This is the main reason why adsorption storage of hydrogen is still very difficult today.

Like in the case of methane storage, some MOFs have shown promising adsorption capacity at cryogenic conditions (77K) and high pressures (70 – 110 bar) as shown in Table 2. It is important to state at this point that a fundamental approach to synthesizing a MOF with high hydrogen adsorption capacity is to ensure that the framework possesses high surface area (Yaghi et al., 2016). This is evident in the fact that most of the highest adsorption MOFs are all structures with high surface area as evident in Table 2.

Table 2: Metal Organic Framework for Hydrogen storage at -196^oC and high pressures (source: Yaghi et al., 2016)

MOF	BET Surface Area (m2/g)	Capacity (wt %)	Pressure (bar)	Ref.
MOF-210	6240	15.0	80	Furukawa et al. (2010)
DUT-32	6411	14.2	80	Grunker et al. (2014)
NU-100	6143	14.1	70	Farha et al. (2010)
MOF-200	4530	14.0	80	Furukawa et al. (2010)
NU-111	5000	11.9	110	Farha et al. (2012)
MOF-205	4460	10.7	80	Furukawa et al. (2010)
MOF-177	4500	9.9	70	Furukawa et al. (2007)
SNU-77H	3670	9.9	30	Park et al. (2011)

3.2 Stability, Regeneration and Reusability

A key determining factor is the ability of the compound to be re-used multiple times after several cycles of adsorption and desorption. Also its thermal and chemical stability at different process conditions is key as different conditions of application can affect the storage capacity of the framework. Various MOF synthesized for process applications have shown promising re-usability and regenerability by retaining adsorption capacity after several cycles (Constantino and Taddei et al., 2018 and Janiak et al., 2019). In terms of stability, MOFs used to face thermal and humid stability issues during its early synthesis days few decades ago but advanced methods of synthesis have helped alleviate the challenge to some extent. Use of high valence metals, N-donor and hydrophobic ligands for MOF synthesis has resulted in the formation of stable MOF structure in the presence of high temperatures. Though most researchers focus on thermal stability and stability in the presence of humidity, literature search revealed that some MOFs exhibit stability in the presence of different chemical conditions (Janiak et al., 2019). In this work therefore, the MOFs that have shown promising adsorption capacity for methane and hydrogen storage will be evaluated in terms of reusability, thermal and chemical stability.

MOFs considered in this case for methane and hydrogen storage have been shown to exhibit some level of reusability and thermal stability. This is evident especially for methane that is stored at atmospheric conditions and as such, MOF used for this purpose is not subjected to high temperatures though Alezi et al. (2015) reported the stability Al-soc-MOF-1 for CH₄ storage up to the temperatures of 340^oC. MOFs reviewed for hydrogen storage as highlighted in Table 2 at cryogenic conditions (77K) reported stable. In terms of regenerability, Farha et al. (2010) reported MOF NU-100 retained its adsorption capacity after five (5) sorption cycles and three (3) high pressure sorption cycles. Though literature for other high performing MOFs for methane and hydrogen storage reviewed in this work did not highlight their regenerability and re-usability, several studies from other authors of MOFs synthesized for process applications such as CO₂ capture (Ibarra et al., 2019) stated that MOFs can be regenerated and re-used multiple time without losing their pristine adsorption capacity. As such it can be inferred that MOFs possess the potential to be used for methane and hydrogen storage multiple times.

3.3 Cost of production

Cost is a very important parameter that determines the applicability of a technological process on commercial scale. It drives its consumption and use in the face of existing competitors. This therefore necessitates a review of the cost of production of MOFs that can be used for methane and hydrogen storage. DOE target for cost of production of MOF is $10/kg and $18/kg for NG storage and hydrogen storage in vehicles respectively as shown in Table 3. From the table, it is obvious the use of MOF for methane and hydrogen storage is still far from practical applications as its cost ($53.75/kg) is highly unattractive compared to DOE target and prevailing gas storage options such as compressed natural gas (CNG) ($10.46/MMBTU) for methane storage (Tractebel Engineering, 2015). This therefore prompts the need to drive down the cost of production of metal organic framework to ensure its commercial and industrial application.  

In this regard, the work of DeSantis et al. (2017) carried out a techno-economic analysis of metal organic frameworks for natural gas and hydrogen storage and highlighted ways through which the cost of production can be reduced. Their work recorded a reduced production cost of $35.73/kg of HKUST-1 as compared to the original cost of $53.75/kg. Similar works will have to be done so that the cost of producing metal organic frameworks can be driven to a competitive level at which the technology can compete with other conventional storage media for industrial and commercial application. This is very important as regards MOF use as a storage medium for methane and hydrogen in full scale industrial use and commercialization.

Table 3: Production cost of MOF for methane and hydrogen storage

Technology/Compound	Quantity	Cost	Ref
HKUST-1	1kg	$53.75	DeSantis et al. (2017)
Target cost (NG MOF)	1kg	$10	Service (2014)
Target cost (H2 MOF)	1kg	$18	DeSantis et al. (2017)

4.0 OUTLOOK AND CONCLUSION

Metal organic framework has been shown to present itself as a promising candidate for methane and hydrogen storage at low pressures and temperatures closer to ambient conditions as compared to what is obtainable today making them better storage media than conventional means of storing methane and hydrogen such as CNG, compressed hydrogen gas and liquid hydrogen. Though MOFs boast impressive adsorption/storage properties at conditions of interest, they have not been shown to exhibit exceptional properties in terms of regeneration and cost of production. This therefore poses challenge to their use in industrial and commercial applications as regeneration and cost of production are driving factors that can determine their competitiveness. In this regard, it is therefore necessary to synthesize MOFs that can retain their adsorption capacity after several hundred sorption cycles as this is what is obtainable in industrial and commercial processes. Also, novel methods of synthesizing MOFs that would drive down cost of production should be developed so that MOFs can have market prices that are low and at competitive levels with other conventional means of storing methane and hydrogen.

REFERENCES

Alezi, D., Belmabkhout Y., Suyetin M., Bhatt P. M., Weselinski L. J., Solovyeva V. A., Adil K., Spanopoulos L., Trikalitis P. N., Emwas A. & Edddaoudi M. (2015). MOF crystal chemistry paving the way to gas storage needs: aluminum-based soc-MOF for CH₄, O₂, and CO₂ storage. J. Am. Chem. Soc. 137, 13308–13318.

ARPA-E. (2012). MOVE Program Overview. http://arpae.energy.gov/sites/default/files/documents/files/MOVE_ProgramOver
view.pdf

Broom D. P., Hirscher M ., Webb C. J., Fanourgakis G. S., Froudakis G. E. & Trikalitis P. N. (2019). Concepts for improving hydrogen storage in nanoporous materials. Int. J. Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.224

Burrows A. D., Frost C. G., Mahon M. F. & Richardson C. (2008). Post-synthetic modification of tagged metal-organic frameworks. Angew Chem Int Ed, 47:8482–6.

Chen B., He Y., Zhou W. & Qiand G. (2014). Methane storage in metal–organic frameworks. Chem. Soc. Rev., 43, 5657—5678.

Constantino F., Taddei M., Vivani R., Tiana D., Sangregorio C., Carta M., Donnadio A., D’Amato R. (2018). Water-Based Synthesis and Enhanced CO2 capture Performance of Perfluorinated Cerium-Based Metal-Organic Frameworks with UiO-66 and MIL-140 topology. ACS Sus. Chem. Engr., 1 (7), 392-402

Costa J. S., Gamez P., Black C. A., Roubeau O., Teat S. J. & Reedijk J. (2008). Chemical modification of a bridging ligand inside a metal-organic framework while maintaining
the 3D structure. Eur J Inorg Chem,1551–4

DeSantis D., Mason J. A., James B. D., Houchins C., Long J. R. & and Veenstra M. (2017). Techno-economic analysis of metal-organic frameworks for hydro-gen and natural gas storage. Energy Fuels, DOI: 10.1021/acs.energyfuels.6b02510

Doonan J. C., Sumby C. J. & Evans J. D. (2014). Post-synthetic metalation of metal–organic
frameworks. Chem. Soc. Rev., 43, 5933

Eddaoudi M., Kim J., Rosi N., Vodak D., Wachter J., O’Keeffe M. & Yaghi O. M. (2002). Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage. Science, 295, 469–472

Farha, O. K., Yazaydin A. O., Eryazici I., Malliakas C. D., Hauser B. G., Kanatzidis M. G., Nguyen S. T. Snurr R. Q. & Hupp J. T. (2010). De novo synthesis of a metal–organic framework material featuring ultrahigh surface area and gas storage capacities. Nature Chem. 2, 944–948.

Farha, O. K., Wilmer C. E., Eryacizi I., Hauser B. G., Parilla P. A., O’Neill P. A., Sarjeant A. A., Nguyen S. T., Snurr R. Q. & Hupp J. T. (2012). Designing higher surface area metal–organic frameworks: are triple bonds better than phenyls? J. Am. Chem. Soc. 134, 9860–9863.

Gándara F., Furukawa H., Lee S. & Yaghi, O. M. (2014). High methane storage capacity in aluminum metal–organic frameworks. J. Am. Chem. Soc. 136, 5271–5274.

Gao F., Li Y., Ye Y., & Zhao L. (2017). A robust microporous ytterbium metal-organic framework with open metal sites for highly selective adsorption of CO₂ over CH_4. Inorg. Chem. Comm., 86, 137–139

Goto Y., Sato H., Shinkai S. & Sada K. (2008). “Clickable” metal−organic framework. J Am
Chem Soc, 130:14354–5.

Grunker R., Bon V., Muller P., Stoeck U., Krause S., Mueller U., SenkovskaI. & Kaskel S. (2014). A new metal–organic framework with ultra-high surface area. Chem. Commun. 50, 3450–3452.

Hirscher M., Balderas-Xicohténcatl R. & Schlichtenmayer M. (2017). Volumetric hydrogen storage capacity in metal-organic frameworks. Energy Technology, 10.1002/ente.201700636

Ibarra I. A., Maurin G., Gutierrez-Alejandre A., González-Zamora E., Castillo I., Zárate J. A., Sánchez-González E., & Juado-Vázquez T. (2019). Outstanding reversible H2S capture by an Al(III)-based MOF. Chem. Commun., DOI: 10.1039/C8CC09379B.

Janiak C., Weingart O., Mollmer J., Lange M., Nuhmen A. & Brandt P. (2019). Metal-Organic Frameworks with Potential Application for SO2 Separation and Flue Gas Desulfurization. ACS Appl. Mater. Interfaces, 11, 17350-17358

Joos F. & Spahni R. (2008). Rates of change in natural and anthropogenic radiative forcing over the past 20,000 years. P. Natl. Acad. Sci. USA, 105, 1425–1430,
https://doi.org/10.1073/pnas.0707386105

Kalidindi S. B., DMello M. E. & Bakuru V. R. (2018). Metal–Organic Frameworks for Hydrogen Energy Applications: Advances and Challenges. Eur. J. ChemPhyChem, 10.1002/cphc.201801147

Kim K. & Deep A. (2016). Metal−Organic Frameworks as a Potential Platform for Selective Treatment of Gaseous Sulfur Compounds. ACS Applied Materials and Interfaces, 8, 29835−29857

Lozano-Castello ´D., Alcaniz-Monge J., Casa-Lillo M. A. d. l., Cazorla-Amoros D. & A. Linares-Solano (2002). Fuel, 2002, 81, 1777–1803.

Mason J. A., Veenstra M. & Long J. R. (2014). Evaluating metal-organic frameworks for natural gas storage. Chem. Sci., 5, 32–51.

Menon V. C. & Komarneni S. (1998). Porous adsorbents for Vehicular Natural Gas Storage. J. Porous Mater., 1998, 5, 43–58.

Park H. J., Lim D.-W., Yang W. S., Oh T.-R. & Suh M. P. (2011). A highly porous metal–organic framework: structural transformations of a guest-free MOF depending on activation method and temperature. Chem. Eur. J. 17, 7251–7260.

Peng Y., Krungleviciute V., Eryazici I., Hupp J. T., Farha O. K. & Yildirim T. (2013). Methane storage in metal–organic frameworks: current records, surprise findings, and challenges. J. Am. Chem. Soc. 135, 11887–11894.

Rallapalli P., Patil D., Prasanth K. P., Somani R. S, Jasra R. V. and Bajaj H. C. (2010). Sorption studies of CO₂, CH₄, N₂, CO, O₂ and Ar on nanoporous aluminium terephthalate [MIL-53(Al)]. J. Porous Mater., 17, 523–528.

Schuth F., Felderhoff M. & Eberle U. (2009). Chemical and Physical Solutions for Hydrogen Storage. Angew. Chem. Int. Ed., 48, 6608 – 6630

Senkovska I. & Kaskel S. (2008). Microporous Mesoporous Mater., 112, 108–115.

Service, R. F. (2014). Stepping on the Gas. Science Magazine, 538– 541. doi:10.1126/science.346.6209.538

Snurr R Q., Hupp J. T., Farha O. K., Wilmer C. E. & Gomez-Gualdron D. A. (2014). Exploring the Limits of Methane Storage and Delivery in Nanoporous Materials. J. Phys. Chem. C., 118 (13), 6941-6951

Suh M. P., Park H. J., Prasad T. K. & Lim D. W. (2011). Hydrogen Storage in Metal Organic Frameworks. Chemical Reviews, 112, 782–835..

Tractebel Engineering S.A. (2015). CNG for commercialization of small volumes of associated gas.

Wu X., Peng L., Xiang S. & Cai W. (2018). Computational design of tetrazolate-based
metal–organic frameworks for CH₄ storage. Phys.Chem.Chem.Phys., 20, 30150

Yaghi O. M., Schoedel A. & Ji Z. (2016). The role of metal–organic frameworks in a
carbon-neutral energy cycle. Nature energy, 1, 1 – 13

Yamashita A., Kondo M., Goto S. & Ogami N. (2015). Development of High-Pressure Hydrogen Storage System for the Toyota “Mirai”. SAE International, http://papers.sae.org/2015-01-1169

Zhou H., Li J., Ma Y., McCarthy M. C., Sculley J., Yu J., Jeong H., & Balbuena P. B. (2011). Carbon dioxide capture-related gas adsorption and separation in metal-organic frameworks. Coordination Chem. Reviews 255, 1791–1823

Zhou H., Long J. R. & Yaghi O. M. (2012). Introduction to Metal-Organic Frameworks. Chem. Rev. 112 (2), 673-674

Zoellter, J. (2015). Mercedes-Benz F125 concept: Mercedes’ dream of a 2025 S-class
takes flight. Car and Driver; http://go.nature.com/kFaC3e