The Impact of Post-Synthetic Linker Functionalization of MOFs on Methane Storage: The Role of Defects

Introduction

Natural gas or methane is a viable alternative to gasoline, owing to its competitive cost, purity, and existing distribution infrastructure. In addition, it is environmentally less harmful than gasoline; while the combustion of methane results in the generation of 0.89 MJ per mole CO₂ emitted, the combustion of gasoline only produces 0.63 MJ per mole CO₂, resulting in some 30% lower carbon emissions. Despite its appealing properties, methane displays a number of drawbacks for vehicular applications, such as its low density, apolarity, and the fact that it forms an explosive mixture with air. For its application as an energy carrier, particularly for vehicular transport, it therefore needs to be (i) safely stored and (ii) densified to obtain a competitive volumetric energy density. The current technology for on-board application makes use of compressed methane at room temperature and ca. 250 bar. Such high pressures raise safety issues and result in higher costs, poorer volume effectiveness, and a limited driving range. Liquefied natural gas is an alternative allowing for longer driving range, but it still raises concerns associated with safety hazards and costs (Yeh, 2007). On the other hand, adsorbed natural gas may be safe and cost-effective if a low-cost high-capacity adsorbent is developed. Particularly, the Advanced Research projects Agency-Energy of the US Department of Energy set a target that 315 volumes of methane at standard pressure and temperature should be adsorbed by 1 volume of adsorbent at max 65 bar pressure and at ambient temperature. Metal–organic frameworks (MOFs) (Kitagawa et al., 2004; Rowsell and Yaghi, 2004) have been highlighted as promising materials for methane storage owing to their high surface areas, adequate pore volumes, as well as chemical and topological tunability (Getman et al., 2012; Peng et al., 2013). However, owing to the weak interaction of methane molecules with the framework, to increase methane capacities, it is essential that both the surface area and the framework–methane interaction strength are increased.

It has been reported that lattice defects of MOFs may alter their uptake of CO₂ and H₂ (Yang et al., 2012; Bueken et al., 2014; Ren et al., 2015). It is very likely that this phenomenon will have an impact on methane storage too. In particular, coordinatively unsaturated metal sites (Chen et al., 2011; Getman et al., 2012; Peng et al., 2013; Seung Koh et al., 2015) and pore diameters (Matranga et al., 1992; Cracknell et al., 1993; Gándara et al., 2014) have been highlighted to determine MOF methane capacities. Lattice defects would influence both of these by cation vacancies and linker defects. In this work, we demonstrate that lattice defects may be introduced into the MIL-101 (Férey et al., 2005) framework using solvent-assisted linker exchange (SALE) and measure its impact on methane uptake.

Materials and Methods

MIL-101(Cr) was synthesized, hydrothermally, according to Férey’s method (Férey et al., 2005). Synthesis of NH₂-MIL-101(Cr) was carried out by the chemical reduction of NO₂-MIL-101(Cr), as described by Stock using a longer reaction time (16 h) (Bernt et al., 2011; Juan-Alcañiz et al., 2013). SALE was carried out by infusing an aqueous solution of 2-amino-1,4-benzendicarboxylic acid (NH₂-BDC) into MIL-101(Cr), described in Szilágyi et al. (2014). UV–vis spectra were measured with a Perkin–Elmer Lambda 900 spectrophotometer equipped with an integrating sphere (“Labsphere”) in the 200–800 nm range. DRIFTS spectra were acquired of the samples pretreated at 453 K in a 20 cm³ min⁻¹. The flow, on a Nicolet model 8700 spectrometer, equipped with a high-temperature cell, and a DTGS-TEC detector, with 256 scans at 4 cm⁻¹ resolution from 4000 to 500 cm⁻¹ and using KBr as reference. Thermal analysis was carried out on samples pretreated under air flow at 323 K under air flow of 60 cm³ min⁻¹ at a temperature ramp rate of 5 K min⁻¹ in the 323–1073 K range, using a TGA/SDTA851e instrument by Mettler Toledo. Methane uptake was determined at 273 K by volumetric technique using a BEL Japan (Belsorp HP) apparatus. Isotherms were measured on typically 0.2 g sample. Before every measurement, the adsorbent was pretreated by increasing the temperature to 473 K at a rate of 10 K min⁻¹ under vacuum and maintaining the temperature overnight.

Results and Discussion

In materials science, it is generally acknowledged that point, line, surface, and volume defects have tremendous impact on the properties of condensed matter. As MOFs are a relatively new type of material, their defect structures and types are largely unknown (Cheetham et al., 2016). In fact, MOFs were practically viewed as ideal crystalline solids, and gas adsorption has been discussed in terms of fairly rigid and perfectly ordered crystal lattices (Simon et al., 2015; Smit, 2015). Most defects reported are related to the heterogeneity of building blocks, such as mixed cationic units (Brozek and Dincă, 2013; Szilágyi et al., 2013) or linkers (Deng et al., 2010; Karagiaridi et al., 2012; Szilágyi et al., 2014). It has, however, recently emerged that, similar to other materials classes, MOFs display a range of lattice defects (Fang et al., 2015), such as vacancies (Vermoortele et al., 2012a; Wu et al., 2013), surface defects (Choi et al., 2008; Ameloot et al., 2013), dislocations (Shöâeè et al., 2008; Walker and Slater, 2008), and defect voids (Whittington et al., 2014). More importantly, the physical and chemical impact of MOF defects has begun to be recognized (Bueken et al., 2014; Cliffe et al., 2014; Fang et al., 2014; Whittington et al., 2014; Ren et al., 2015).

There are various ways of introducing lattice defects into a MOF, among which post-synthetic (PS) linker exchange is prominent (Vermoortele et al., 2012b; Shearer et al., 2014; Tu et al., 2014). SALE should thus be an ideal tool to introduce defects in a MOF. MIL-101(Cr) has been chosen as the material under scrutiny, as it has been demonstrated that its linkers may be exchanged post-synthetically (Szilágyi et al., 2014); it will thus allow the verification of (i) the engineering of defect sites through SALE and (ii) its impact on methane adsorption. It has been reported that −NH₂ groups (Wood et al., 2012), and in general N sites (Li et al., 2015), may enhance methane storage on porous materials. To take advantage of this improvement, the PS exchange of the BDC linker to its NH₂-BDC has been chosen and SALE exchange was carried out, resulting in the post-synthetically linker-exchanged sample, PSM (Szilágyi et al., 2014). Furthermore, in order to distinguish between the effect of functional groups and that of defects the pristine NH₂-MIL-101 was also synthesized.

Defects may be categorized as external or internal defects, where the former ones only occur on particle surfaces. Internal surface defects may be static or dynamic (Cairns and Goodwin, 2013). Dynamic effects, cf., ligand rotation, will be disregarded as they are not formed upon or affected by SALE. Furthermore, synchrotron X-ray diffraction patterns and BET surface areas (MIL-101: 2136 m² g⁻¹, NH₂-MIL-101: 2001 m² g⁻¹, and PSM: 2059 m² g⁻¹) (Szilágyi et al., 2014) reveal an identical crystallographic structure in these samples, highlighting the lack of volume defects. As N₂, H₂, and CH₄ molecules are of similar size and polarity, surface and line defects on MOFs should affect their adsorption similarly. It has been previously shown that this is not the case for the hydrogen and nitrogen uptake of SALE-treated MIL-101 (Szilágyi et al., 2014), which suggests that potential surface and line defects do not play a key role in these systems. Therefore, only point defects will be further discussed. Three types of point defects may occur in MOFs, such as metal node vacancy, linker vacancy, and linker dislocation. Metal node vacancy may only occur upon SALE if it is accompanied by linker defects as the linker-metal bonds need to be first broken for the metal to be able to leach out. Linker vacancy and linker dislocation (dangling) are depicted in Figures 1A–C.

FIGURE 1

Figure 1. MIL-101 supercluster (A) defect-free, (B) with dangling linker, and (C) with linker vacancy; the orange polyhedra represent the cationic units, whereas C and O atoms are depicted in black and blue, respectively.

In order to verify the effect of MOF defects on methane adsorption, it first has to be established whether they have been introduced in framework. UV–vis spectroscopy gives information on metal-linker bonds, which could be affected by linker vacancies or dislocations. The metal–ligand charge-transfer peak undergoes a shift to higher wavelengths upon the addition of amino ligands (Supplementary Material), similar to what was observed for UiO-66 (Hendrickx et al., 2015). This shift appears to be proportional to the amino substitution as the peak maximum is at 514 nm for the pristine MIL-101, 528 nm for the pure NH₂-MIL-101, and 518 nm for PSM, containing ca. 20% NH₂-BDC (Szilágyi et al., 2014). It cannot thus be directly attributed to defects. Infrared spectroscopy, on the other hand, reveals some vibrational modes only present in the PSM sample (Figure 2). In the IR spectrum of PSM, an O–H stretching mode belonging to a carboxylate appears at 3301 cm⁻¹, a free C=O stretching mode appears at 1685 cm⁻¹ [which is a value very close to what was calculated for the NH₂-BDC (Karabacak et al., 2010)] and a CH out-of-plane deformation mode appears at 848 cm⁻¹. These additional modes only occur in the post-synthetically modified sample, which suggests that defects have been introduced in the pristine MIL-101 upon SALE. Furthermore, as only two types of linker defects could be introduced – either linker vacancy or linker dislocation – it can be deduced that the defects formed are dangling linkers. The additional −OH and C=O stretching modes reveal the presence of uncoordinated carboxylate groups. Correspondingly, the coordination sphere of the Cr³⁺ in the cationic units should be saturated with H₂O, as observed in the appearance of additional Cr–O stretching modes for the PSM sample (Figure 2). In addition, from previous PXRD and BET results, it had been deduced that this cannot be due to unreacted linkers (Szilágyi et al., 2014). Finally, our thermal gravimetric analysis (TGA) highlights that there is no substantial difference in the linker content of PSM and the pristine MIL-101 (Figure 3).

FIGURE 2

Figure 2. DRIFTS spectra of MIL-101 (black), NH₂-MIL-101 (red), and PSM (blue) in the (A) O–H stretching, (B) C=O stretching, and (C) C–H bending regions.

FIGURE 3

Figure 3. Normalized TGA curves for MIL-101 (black), NH₂-MIL-101 (red), and PSM (blue), highlighting the additional decomposition step at ca. 430°C in the insert. One hundred percent of relative weight loss corresponds to the perfectly pure, crystalline, and completely dehydrated MOF → Cr₂O₃ reaction, normalized to the molar mass of the samples. The first degradation step of the functionalized MOFs occurs at lower temperatures than that of the pristine MIL-101, which corresponds to the breaking off of the −NH₂ functional group.

The presence of dangling linker defects has been also confirmed by TGA as an additional decomposition step at around 430°C can be observed for the PSM sample (Figure 3). This step corresponds well with what was previously observed in the literature for the loss of carboxylate groups (Gadipelli and Guo, 2014). It is possible that the formation of the linker-cationic unit bond upon MOF self-assembly differs greatly from that upon linker exchange, and it is mechanistically hindered and/or energetically unfavorable for the linkers to bridge two cationic units post-synthetically, whence the presence of dangling linkers.

As displayed in Figure 1B, dangling linker defects alter the topology of potential methane adsorption pockets. To probe this, methane adsorption isotherms were measured, and a number of methane molecules adsorbed per formula unit (Cr₃(OH)O[linker]₃) and volumetric adsorption (cm³ CH₄ STP cm⁻³ sample) were compared for the three samples for the practically relevant pressure values of 5, 35, and 65 bar (Table 1).

TABLE 1

Table 1. Adsorbed CH₄ at 5, 35, and 65 bar.

For hydrogen adsorption, it was shown that the amino functional group acts as an adsorption site and indeed from the UV–vis spectra one expects that it will have an effect on the methane adsorption (Szilágyi et al., 2014). The addition of amino groups in the pristine MIL-101 increases the absorbed photon wavelength for both the pure NH₂-MIL-101 and PSM samples (Supplementary Material), which corresponds to a lower photon energy absorbed, i.e., a smaller band gap (Hendrickx et al., 2015). A smaller band gap in turn is indicative of weaker bonding (Ouyang et al., 2009). In this case, the Cr–O bonds are less strong, which should modify the interaction strength of methane on the cationic unit. By comparing the number of adsorbed CH₄ molecules per formula unit on MIL-101 and NH₂-MIL-101, it is clear that more methane molecules are adsorbed on the functionalized MOF in the low-loading region. This is related to the adsorption of the stronger sites, such as the coordinatively unsaturated metal site (Dietzel et al., 2009). However, at higher loading, or higher pressures, corresponding to pore filling, this effect no longer plays a key role. Therefore, it is possible to separate the effect of linker functionalization from that of linker defects at higher loadings.

Most importantly, the PSM sample has higher methane uptake than both pure MOFs, MIL-101 and NH₂-MIL-101, both gravimetrically (Figure 4) and per formula unit throughout the studied pressure range, including the practically relevant 5–35 bar. Furthermore, it should be noted that the said increase is reproducible and is of similar magnitude, by one-third throughout the isotherm.

FIGURE 4

Figure 4. Excess methane uptake on MIL-101 (black), NH₂-MIL-101 (red), and PSM (blue); isotherm at low methane loading in insert.

It has to be emphasized that the presence of new amino adsorption sites would only affect the methane uptake at the low-loading region, up to 5 bar, this is in good agreement with what was observed for the pure NH₂-MIL-101. In addition, the extent of the observed increase of methane adsorption on PSM in the low-loading range surpasses that observed for the pure NH₂-MIL-101, which has a higher proportion of amino groups, highlighting that additional effects need to be taken into consideration to explain the enhanced methane capacity of the post-synthetically modified MIL-101(Cr). In general, this enhancement cannot be explained by increased surface area as (i) MIL-101 (2136 m² g⁻¹) has a higher surface area than PSM (2059 m² g⁻¹) (Szilágyi et al., 2014) and (ii) the increase is observed throughout the isotherm and not just at high loadings, where the surface area or pore filling would play a key role.

These considerations lead to the explanation that the increase is a consequence of the defects introduced, i.e., dangling linkers upon SALE. It is stipulated that the dislocated linkers opened new pockets of adsorption, increasing the methane uptake throughout the isotherm. Such adsorption enhancement corresponds to a larger number of various, already existing, adsorption sites, or to an increased surface area available for the adsorption of methane. By analyzing the different pores and clusters in the MIL-101 structure, it may be possible that the superclusters become available for the/more CH₄ molecules. The diameter of the superclusters is 6 Å (Shin et al., 2015), so it is not able to accommodate two methane molecules (kinetic diameter of 3.8 Å) in its closed form. The opening of supercluster would increase its effective volume, approaching the ideal 7.5 Å value that has been observed for HKUST-1 (Rowsell and Yaghi, 2006). It should be noted that this is in agreement with our previous findings on the hydrogen uptake in these systems that did not seem to be influenced on the high loading (low interaction strength) regime by PSM or defects as the kinetic diameter of H₂ is 2.9 Å, and thus the opening of the supercluster will not effectively increase the number of H₂ molecules contained within (Szilágyi et al., 2014). We would like to point out that the observed CH₄ uptake for MIL-101(Cr) is lower than what was published previously (Hamon et al., 2009), this is also underpinned by a higher than 100% relative weight loss upon thermal treatment (Figure 3), which may be indicative of either metal vacancies or impurities whose decomposition results in the formation of volatile products. In the case of the pristine MIL-101 and PSM samples, it is likely to be a consequence of the latter one, which is also supported by the observed surface area being smaller than published elsewhere (Kim et al., 2013). On the other hand, lower than 100% relative weight loss is indicative of either linker vacancies or the presence of impurities, resulting in non-volatile thermal decomposition products. The lower than 100% relative weight loss observed for NH₂-MIL-101 is a consequence of some residual reagent applied in its synthesis (Bernt et al., 2011; Juan-Alcañiz et al., 2013).

Conclusion

We showed that it is possible to introduce defects in the robust MIL-101 with SALE. Although the formation of various types of defects may be possible, they are overwhelmingly dangling linkers, which allow the adsorption of an increased amount of CH₄ molecules on the framework. Given the high proportion of increased adsorption, over 33%, we are confident that the ultimate methane capacity of better performing MOFs could be significantly increased, and these result may open new possibilities in adsorbed natural gas technologies.

Author Contributions

Dr. PAS carried out the syntheses, IR and UV–vis measurements, Dr. PS-C measured the methane adsorption isotherms and carried out the thermal gravimetric analysis. All authors contributed to discussions and the interpretation of the experimental data.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The handling Editor declared a past coauthorship with the authors Petra Ágota Szilágyi and Hans Geerlings and states that the process nevertheless met the standards of a fair and objective review.

Funding

The authors would like to acknowledge financial support by the Agentschap.nl (#EOSLT07052).

Supplementary Material

The Supplementary Material for this article can be found online at https://www.frontiersin.org/article/10.3389/fenrg.2016.00009

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