Porous Metal-Organic Frameworks: Promising Materials for Methane Storage

Abstract

Porous metal-organic frameworks (MOFs) have received extensive attention as an emerging class of adsorbents for methane storage. Although the MOF methane or natural gas fuel tank is already on board, methane storage capacities of MOFs under 65 bar and 298 K are still quite far from the new DOE targets, which certainly hampers further implementation of MOFs for such an important application. We speculate and further confirm that adjusting the storage temperature to 270 K (another feasible temperature) can notably improve methane storage capacities of MOFs at 65 bar. Our discovery of two unique MOFs with an extremely high deliverable capacity of ∼240 cm3 (STP) cm−3 and gravimetric capacity of ∼0.5 g/g will allow our industrial partners to fully explore the feasibility of MOFs for methane storage at 270 K. This review not only provides the updated status of MOFs for methane storage but also directs research endeavors to pursue better MOFs for practical applications. Metal-organic frameworks (MOFs) are the most promising porous adsorbents for methane storage; however, the highest reported storage capacities of about 270 cm3 (STP) cm−3 at 298 K and 65 bar are still much lower than the new US Department of Energy (DOE) target of 350 cm3 (STP) cm−3. Furthermore, it is very difficult to reach the DOE targets for volumetric (350 cm3 [STP] cm−3) and gravimetric (0.5 g [CH4]/g) storage capacities simultaneously for a single MOF. This review systematically evaluates and compares the methane storage capacities of reported MOFs at both 298 and 270 K. We found that slightly reducing the storage temperature to 270 K can significantly improve both the volumetric and gravimetric uptake. Our discoveries highlight that two unique MOFs, NU-111 and MOF-177 (which have high pore volumes of 2.09 and 1.89 cm3 g−1, respectively), not only have very high gravimetric capacities of 0.5 and 0.43 g/g, respectively, but also exhibit the highest working capacities ever reported: 239 and 230 cm3 (STP) cm−3, respectively. In addition, a usable empirical equation for predicting methane storage capacities (at 270 K and 65 bar) and some engineering strategies for thermal management are discussed. Metal-organic frameworks (MOFs) are the most promising porous adsorbents for methane storage; however, the highest reported storage capacities of about 270 cm3 (STP) cm−3 at 298 K and 65 bar are still much lower than the new US Department of Energy (DOE) target of 350 cm3 (STP) cm−3. Furthermore, it is very difficult to reach the DOE targets for volumetric (350 cm3 [STP] cm−3) and gravimetric (0.5 g [CH4]/g) storage capacities simultaneously for a single MOF. This review systematically evaluates and compares the methane storage capacities of reported MOFs at both 298 and 270 K. We found that slightly reducing the storage temperature to 270 K can significantly improve both the volumetric and gravimetric uptake. Our discoveries highlight that two unique MOFs, NU-111 and MOF-177 (which have high pore volumes of 2.09 and 1.89 cm3 g−1, respectively), not only have very high gravimetric capacities of 0.5 and 0.43 g/g, respectively, but also exhibit the highest working capacities ever reported: 239 and 230 cm3 (STP) cm−3, respectively. In addition, a usable empirical equation for predicting methane storage capacities (at 270 K and 65 bar) and some engineering strategies for thermal management are discussed. 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In order to increase the storage density of natural gas, one of the current technologies is compressed natural gas (CNG), stored as supercritical fluid at room temperature and 200–300 bar in an oversized fuel tank. To handle this high pressure, onboard tanks must be made either from thick metal, which makes them heavy and large, or from lighter but expensive carbon fiber, which is associated with cost, space, and safety issues for use in passenger vehicles. Recently, adsorbed natural gas (ANG) has been considered as a promising strategy for overcoming these issues, which involves filling storage tanks with porous materials that store methane at modest pressures. The relatively lower pressure would make tanks lighter, cheaper, and smaller and thus reduce the cost and space requirements. Therefore, ANG technology would be better suited for use in passenger cars. To fully implement natural gas fuel systems for passenger vehicles, it is important to find porous materials that can store and deliver large amounts of methane under a relatively low storage pressure (typically 35 or 65 bar). Recently, in order to guide research on adsorbent-based methane storage systems, the Advanced Research Projects Agency-Energy (ARPA-E) of the US Department of Energy (DOE) has initiated new methane storage targets with the ambitious goal of a volumetric storage capacity of 350 cm3 (STP) cm−3 and gravimetric storage capacity of 0.5 g (CH4) g−1 at room temperature.4See the DOE MOVE program at http://arpa-e.energy.gov/?q =arpa-e-programs/move.Google Scholar Compared with traditional porous materials, including zeolites and activated carbons, metal-organic frameworks (MOFs), emerging as a new class of solid crystalline materials, are particularly promising for high-density methane storage because of their high porosity, tunable pores, and versatile chemistry.5Furukawa H. Cordova K.E. O’Keeffe M. Yaghi O.M. The chemistry and applications of metal−organic frameworks.Science. 2013; 341: 974-986Crossref Scopus (909) Google Scholar, 6Furukawa S. Reboul J. Diring S. Sumida K. Kitagawa S. 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The powder or crystallite samples of MOF materials need to be compacted into different shapes (such as wafers or pellets) to minimize the void fraction in the storage tank. The actual packing density of MOFs is usually much lower than the crystallographic density (the upper limit of the packing density) because of their mechanical properties and the voids that exist between the crystallites in the shapes and those between the shapes, resulting in a significantly decreased volumetric uptake. Therefore, assuming a 25% loss in volumetric capacity as a result of packing an adsorbent inside a storage tank, the DOE set the target to 25% higher than the energy density of CNG at 250 bar and 298 K (equivalent to 263 cm3 [STP] cm−3). The volumetric capacities of MOFs mentioned in this review are calculated with crystallographic densities, which are certainly overestimates of actual packing densities; however, they are still useful for preliminary evaluations and comparisons of the intrinsic storage properties of different adsorbents. In the past two decades, great endeavors have been dedicated to developing new MOF materials for high-capacity methane storage.16Mason J.A. Veenstra M. Long J.R. Evaluating metal–organic frameworks for natural gas storage.Chem. Sci. 2014; 5: 32-51Crossref Google Scholar, 17Makal T.A. Li J.R. Lu W. Zhou H.C. Methane storage in advanced porous materials.Chem. Soc. Rev. 2012; 41: 7761-7779Crossref Scopus (282) Google Scholar, 18He Y. Zhou W. Qian G. Chen B. Methane storage in metal–organic frameworks.Chem. Soc. Rev. 2014; 43: 5657-5678Crossref Google Scholar, 19Konstas K. Osl T. Yang Y. Batten M. Burke N. Hill A.J. Hill M.R. Methane storage in metal organic frameworks.J. Mater. 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Snurr R.Q. Exploring the limits of methane storage and delivery in nanoporous materials.J. Phys. Chem. C. 2014; 118: 6941-6951Crossref Scopus (25) Google Scholar Our work recently demonstrated that incorporation of Lewis basic nitrogen sites into MOFs can significantly improve their methane storage and working capacities.30Li B. Wen H. Wang H. Wu H. Yildirim T. Zhou W. Chen B. Porous metal–organic frameworks with Lewis basic nitrogen sites for high-capacity methane storage.Energy Environ. Sci. 2015; 8: 2504-2511Crossref Google Scholar However, the gap between the current record storage capacities and the target (350 cm3 [STP] cm−3) is so large that it might be impossible to find a new MOF with suitable functional sites that can meet the targets. Therefore, conventional strategies may no longer work for further improving the storage capacity of MOF materials to reach the DOE targets under the currently proposed storage conditions, which has certainly hampered further implementation of MOF materials for such an important application. There is thus an urgent demand to develop a completely new strategy and/or concept to boost the methane storage capacities of MOFs. One of the most effective and direct strategies is to slightly reduce the methane storage temperature (e.g., 270 K, a feasible temperature for methane storage). This might be a feasible and convenient method to forcefully improve methane storage capacities without exploring new MOF materials. To give an overall understanding of the current situation for methane storage at 270 K, it is useful to summarize the methane storage data at 270 K for a large number of MOFs and systematically compare their storage performance with those at 298 K. In this review, we first discuss the most important fundamental knowledge for high-pressure methane storage and provide a brief overview of the recent progress on traditional materials (zeolites and carbons) and other emerging advanced materials (such as MOFs). Second, detailed data analysis indicates that a slightly decreased storage temperature can significantly enhance both total methane storage uptake and working capacities of MOFs. The highest volumetric and gravimetric uptakes can reach 301 cm3 (STP) cm−3 and 0.5 g/g at 270 K and 65 bar, respectively, getting closer to or meeting the new DOE targets. Importantly, the working capacities of MOFs at 270 K can systematically increase with pore volume, whereby those with high Brunauer-Emmett-Teller (BET) surface areas or functional sites (such as Lewis basic nitrogen sites) have better working capacities at 270 K. The most important finding is that NU-111 has the highest working capacity of 239 cm3 (STP) cm−3 reported so far, which is 34% higher than the 179 cm3 (STP) cm−3 at 298 K. Third, we build an empirical equation that can be used to predict and screen MOFs for methane storage at 270 K after their pore volumes and framework densities have been established experimentally. By making use of this equation together with our experiments, we speculate and further confirm another very promising material, MOF-177 (with a high BET surface area of 4,740 m2/g), which exhibits an exceptionally high working capacity of 230 cm3 (STP) cm−3 and gravimetric capacity of 0.43 g (CH4)/g under 65 bar and 270 K. Finally, we discuss the technical feasibility of ANG storage at 270 K in vehicle applications and propose a general solution for thermal management in ANG systems by leveraging existing vehicle thermal management and the trend of vehicle electrification. A basic step involved in the evaluation of methane storage capacities of porous materials is the collection of adsorption isotherms. Currently, commercial high-pressure adsorption instruments mostly utilize two types of experimental techniques to measure the amount of methane adsorbed: (1) gravimetric uptake analysis using a microbalance to record the change in weight of a sample at different methane pressures and (2) a volumetric method using a Sieverts apparatus to record the pressure change when dosing methane into a cell containing porous materials. Compared with gas-sorption measurements at low pressure, accurate high-pressure experiments are inherently more difficult and complicated. Large measurement errors may come from the sources of error in volume calibrations, pressure readings, sample mass, temperature measurements, and so on.25Peng Y. Krungleviciute V. Eryazici I. Hupp J.T. Farha O.K. Yildirim T. 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C. 2008; 112: 13047-13052Crossref Scopus (30) Google Scholar which can be calculated according to the following equation: ntot = nex + ρbulk(P,T) × Vp, where ρbulk is obtained from the NIST Refprop database34See the NIST Refprop database at http://webbook.nist.gov/chemistry/fluid/.Google Scholar and Vp is usually determined from an N2 adsorption isotherm at 77 K. Two types of expressions have been widely utilized to describe methane adsorption capacity: gravimetric and volumetric uptake. The gravimetric uptake represents the mass of methane adsorbed per unit mass of adsorbents, whereas the volumetric uptake is expressed as the volume of methane adsorbed under standard temperature and pressure divided by the volume of adsorbents. To date, most gas-adsorption measurements give gravimetric uptake based on per unit mass of adsorbent. However, for vehicular applications, the volumetric capacity is more important than the gravimetric capacity given that passenger vehicles have limited space for gas tanks. Therefore, the volumetric uptake needs to be calculated with the density of adsorbent. As mentioned previously, for most reported MOF materials, the ideal crystallographic density has been widely utilized for calculating the volumetric uptake.16Mason J.A. Veenstra M. Long J.R. Evaluating metal–organic frameworks for natural gas storage.Chem. Sci. 2014; 5: 32-51Crossref Google Scholar This calculation method represents the maximum possible volumetric uptake. The actual packing density of MOFs would be much lower than the crystallographic density.35Senkovska I. Kaskel S. High pressure methane adsorption in the metal-organic frameworks Cu3(btc)2, Zn2(bdc)2dabco, and Cr3F(H2O)2O(bdc)3.Microporous Mesoporous Mater. 2008; 112: 108-115Crossref Scopus (124) Google Scholar, 36Seki K. 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The working capacity is more important than the methane storage capacity in practical applications, because it determines the driving range of natural gas vehicles (NGVs). The choice of the upper adsorption pressure and low limiting working pressure are two key factors that determine the working capacity. Generally, two kinds of pressure (35 and 65 bar) are typically assumed to be the adsorption pressure because they are the maximum achievable pressure of inexpensive single-stage and two-stage compressors, respectively.18He Y. Zhou W. Qian G. Chen B. Methane storage in metal–organic frameworks.Chem. Soc. Rev. 2014; 43: 5657-5678Crossref Google Scholar In addition, two higher pressures of 80 and 100 bar are also considered the upper adsorption pressure by several groups.16Mason J.A. Veenstra M. Long J.R. Evaluating metal–organic frameworks for natural gas storage.Chem. Sci. 2014; 5: 32-51Crossref Google Scholar, 37Gándara F. Furukawa H. Lee S. Yaghi O.M. 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The working capacity of adsorbents in vehicular applications is also largely related to intrinsic thermal effects in the ANG storage system, as well as the adsorbents themselves, because adsorption is exothermic and desorption is endothermic.16Mason J.A. Veenstra M. Long J.R. Evaluating metal–organic frameworks for natural gas storage.Chem. Sci. 2014; 5: 32-51Crossref Google Scholar For example, if the heat released during refueling is not efficiently dissipated or offset, the adsorbent bed heats up resulting in less methane stored at the adsorption pressure. Similarly, if the heat absorbed during discharge cannot be resupplied, the temperature of the adsorbent bed drops, thus leading to more CH4 retained in the tank at low pressure. All the above factors significantly reduce the working capacity of adsorbents. To minimize losses in working capacity, it is thus necessary to operate under feasible temperature-management strategies in ANG storage systems. In addition, adsorption at a lower temperature (such as 270 K) or increasing the final releasing temperature for methane is also considered a strategy to improve the working capacity. Evaluation of porous materials has currently focused on adsorption of pure methane. Although methane is the main component (95%), commercial natural gas is a mixture that contains many other impurities, such as ethane (3.2%), propane (0.2%), and carbon dioxide (0.5%).41Liss W.E. Thrasher W.H. Steinmetz G.F. Chowdiah P. Attari A. Variability of Natural Gas Composition in Select Major Metropolitan Areas of the United States. GRI, 1992Google Scholar These impurities certainly have harmful effects on the storage capacity of an adsorbent. For example, despite small fractions in natural gas, heavier hydrocarbons and carbon dioxide can be preferentially adsorbed by the adsorbent upon charging42He Y. Krishna R. Chen B. Metal–organic frameworks with potential for energy-efficient adsorptive separation of light hydrocarbons.Energy Environ. Sci. 2012; 5: 9107-9120Crossref Scopus (161) Google Scholar and then accumulate inside the adsorbent after multiple operating (filling and emptying) cycles, thus resulting in dramatically decreased storage capacity. In addition, trace amount of other impurities, such as H2S, H2O, O2, and C4 hydrocarbons, may slowly poison methane adsorption sites or degrade the framework over a long time period. Obviously, natural gas storage is not the same as methane storage, which introduces more complexities. A recent study found that some outstanding MOFs for methane storage, such as HKUST-1, are not suitable for real natural gas storage because of the poisoning effect of heavier hydrocarbons.43Zhang H. Deria P. Farha O.K. Hupp J.T. Snurr R.Q. A thermodynamic tank model for studying the effect of higher hydrocarbons on natural gas storage in metal–organic frameworks.Energy Environ. Sci. 2015; 8: 1501-1510Crossref Google Scholar To minimize the effects of these impurities, a pre-adsorption system (such as guard beds) must be placed in the inlet of the ANG tank to