Effect of Lithium Doping into MIL-53(Al) through Thermal

Article pubs.acs.org/JPCC

Effect of Lithium Doping into MIL-53(Al) through Thermal Decomposition of Anion Species on Hydrogen Adsorption Masaru Kubo, Atsushi Shimojima, and Tatsuya Okubo* Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *

ABSTRACT: Lithium-doped MIL-53(Al) (Li-MIL-53(Al)) is prepared by impregnating MIL-53(Al) with an ethanol solution of LiNO3, followed by heat treatment in vacuum. The nitrate anion is thermally decomposed and removed in the form of NO and N2O at 573 K. This temperature is much lower than the decomposition temperature of bulk LiNO3, which can be attributed to the smaller size of LiNO3 in the pores as well as to the high charge density of aluminum in the MIL-53(Al) skeleton. The doped amount can be varied by changing the concentration of the LiNO3 solution. The lithium doping enhances the hydrogen uptake from 1.66 to 1.84 wt % at 77 K and 1 atm when the doped amount is Li/Al = 0.036. This enhancement suggests that one lithium cation can adsorb two hydrogen molecules. However, the isosteric heat of hydrogen adsorption is not enhanced, possibly due to the interaction of the doped lithium cations with carboxyl groups, as suggested by 13C CP/MAS NMR. Electron-withdrawing oxygen atoms of the carboxyl group should weaken the affinity of the doped lithium cation to hydrogen molecules. Thus, the lithium cations only act as the additional adsorption sites with an affinity to hydrogen molecules similar to that of the internal surface of MIL-53(Al). Similarly, other alkaline/alkaline earth metal cations, such as Na+, Mg2+, and Ca2+, can also be doped into MIL-53(Al), resulting in the increase in the hydrogen uptakes to 1.76, 1.76, and 1.69 wt % for Na+, Mg2+, and Ca2+, respectively.

1. INTRODUCTION Hydrogen storage is a key technology to achieve a hydrogenbased economy that uses hydrogen as an energy carrier to power fuel cells and other systems. Although many materials have been explored for hydrogen storage, no material can achieve the target values of the U.S. Department of Energy (DOE), specifically, 5.5 wt % of gravimetric capacity and 40 g/ L of volumetric capacity at ambient condition.1 Metal−organic frameworks (MOFs) and covalent−organic frameworks (COFs) are desired for suitable hydrogen storage materials because of their high surface area and high porosity. MOFs and COFs can store hydrogen by physisorption utilizing weak interactions that enable fast and reversible charge and discharge. However, the weak interactions, such as van der Waals interactions, cannot achieve the storage of a large amount of hydrogen at ambient condition. Several MOFs can store hydrogen over 5.5 wt % at cryogenic temperature, whereas their capacity of hydrogen at room temperature cannot reach 2.0 wt %.2−8 Recently, introduction of alkaline cations into the nanospace of MOFs has attracted attention to overcome their low © 2012 American Chemical Society

hydrogen capacity at ambient temperature. In particular, a lithium cation is promising because of its low atomic weight and high affinity to hydrogen molecules due to charge-induced dipole interaction.9 It has been proposed by theoretical investigations that lithium-doped MOFs and COFs can achieve hydrogen uptake over 6.0 wt % at ambient condition.10,11 Indeed, several groups experimentally demonstrated that lithium doping into MOFs contributed to the increase of gas storage capacity at 77 K.12−20 These studies utilized specific MOFs containing peculiar functional groups, for example, hydroxyl groups that can form lithium alkoxide by the substitution of a proton with a lithium cation.15,17 MOFs with such functional groups are still limited; therefore, a new lithium-doping method that can be adapted to diverse MOFs should be developed. In 2005, we succeeded in lithium doping into mesoporous silica by heat treatment at 773 K after impregnation of the Received: November 16, 2011 Revised: April 24, 2012 Published: April 25, 2012 10260

dx.doi.org/10.1021/jp211029y | J. Phys. Chem. C 2012, 116, 10260−10265

The Journal of Physical Chemistry C

Article

mesopores with an ethanol solution of LiCl.21 The chloride anion was released in the form of ethyl chloride as a result of the reaction with ethoxy groups on silica.22 The hydrogen uptake of the lithium-doped mesoporous silica at 77 K and room temperature increased with the enhancement of the isosteric heat of adsorption (Qst).21,22 The removal of anion species by heat treatment can be an alternative approach for lithium doping into MOFs. However, the removal of chloride anions required ethoxy groups on the surface and heat treatment at high temperature above 773 K where MOFs are collapsed. Thus, other anions that can be thermally decomposed at relatively low temperatures should be explored for the lithium doping into MOFs. We chose LiNO3 as a lithium source, because it is well known that the nitrate anion can be thermally decomposed into NOx (NO, NO2, etc.) and O2.23 As a host MOF, MIL-53(Al) is chosen because it has high thermal stability up to 700 K and a high surface area around 1000 m2/g, as well as high stability against water.24 In this study, lithium-doped MIL-53(Al) (Li-MIL-53(Al)) is prepared through the decomposition of nitrate anions by heat treatment at 573 K after impregnation with an ethanol solution of LiNO3. The state of lithium is characterized by solid-state NMR. The hydrogen adsorption properties at 77 and 87 K of Li-MIL-53(Al) are examined. Furthermore, doping of other alkaline metal cations into MIL-53(Al) is tested, and its effect on the hydrogen uptake is investigated.

where P represents pressure, N is the amount adsorbed, T is the absolute temperature, and m and n are the number of coefficients required to adequately describe the isotherms. From these results, Qst is calculated as follows m

Q st = −R ∑ aiN i i=0

where R is the universal gas constant. Inductively coupled plasma atomic emission spectrometry (ICP-AES) measurement was performed using a P4010 (Hitachi) for the quantitative analysis of Li and Al species. CHN elemental analysis was performed using a CE-440 (Systems Engineering Inc.) for quantitative analysis of N species. Solid-state magic-angle spinning (MAS) NMR spectra were recorded using a JEOL CMX-300. 13C CP/MAS NMR spectra were recorded at a resonance frequency of 75.57 MHz with a contact time of 1.5 ms and a recycle delay of 5 s. 7Li MAS NMR spectra were recorded at a resonance frequency of 116.79 MHz with a pulse width of 1.0 μs and a recycle delay of 5 s. Adamantane and 1.0 M LiCl solution were used as references (at 37.85 and 0 ppm, respectively, relative to tetramethylsilane) for 13C CP/MAS NMR and 7Li MAS NMR, respectively. The thermogravimetry−mass spectrometry (TGMS) measurements were performed to analyze the gaseous species released from the samples during the heat treatment using a thermogravimetric analyzer, Thermoplus TG8120 (Rigaku), equipped with a mass spectrometer (Anelva). Fourier transform infrared (FT-IR) spectra of the samples in KBr pellets were obtained using a Magna 560 (Nicolet) equipped with an MCT (mercury cadmium telluride) detector at a nominal resolution of 2 cm−1.

2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were used as supplied without further purification. MIL-53(Al) was obtained from SigmaAldrich Co., and others, such as ethanol and LiNO3, were from Wako Chemical Co. 2.2. Preparation of Li-MIL-53(Al). MIL-53(Al) was activated at 473 K in vacuum for 4 h. The activated MIL53(Al) was immersed in a 0.5 M LiNO3/ethanol solution with stirring at room temperature for 24 h. After filtration, LiNO3doped MIL-53(Al) (LiNO3-MIL-53(Al)) was obtained by drying at 333 K for 1 day. For the removal of nitrate anions, LiNO3-MIL-53(Al) was heated at 573 K for 2 h in vacuum to give Li-doped MIL-53 (Li-MIL-53(Al)). To vary the amount of doped lithium, the concentrations of the LiNO3/ethanol solution were changed from 0.01 to 3.0 M. 2.3. Characterization. Powder X-ray diffraction (XRD) measurements were performed using an M03X-HF22 (Mac Science) equipped with a Cu Kα radiation source (wavelength of 0.15406 nm). Nitrogen adsorption−desorption measurement at 77 K and hydrogen adsorption measurements at 77 and 87 K were performed using an AUTOSORB-1 MP (Quantachrome Instrument). Prior to the measurements, the samples were evacuated at 573 K for 8 h under vacuum. The specific surface area of samples was calculated from the adsorption isotherm of nitrogen by the Brunauer−Emmett− Teller (BET) method in the range of 0.01 < P/P0 < 0.05. The micropore volumes were calculated by the Dubinin− Raduskevich (DR) method. The isosteric heat of hydrogen adsorption (Qst) was calculated from both hydrogen adsorption isotherms at 77 and 87 K via a virial equation, which gives reliable Qst.25−27 The following virial-type expansion with temperature-independent parameters ai and bi was applied ln P = ln N +

1 T

m

3. RESULTS AND DISCUSSION 3.1. Structural Characterizations. The XRD patterns of MIL-53(Al), LiNO3-MIL-53(Al), and Li-MIL-53(Al) are shown in Figure 1. The patterns of MIL-53(Al) and Li-MIL-

Figure 1. XRD patterns of (a) MIL-53(Al), (b) LiNO3-MIL-53(Al), and (c) Li-MIL-53(Al).

53(Al) suggest that they are a mixture of high-temperature with empty channels and low-temperature forms in which a water molecule is located at the center of the channels.24 The relatively small peak at 2θ = 12.2° is unambiguously assigned to the high-temperature form. The coexistence of both forms was also reported in the literature;28 therefore, it appears to be intrinsic to commercial MIL-53(Al). On the other hand, LiNO3-MIL-53(Al) exhibits a different pattern. MIL-53(Al) is reported to show a breathing effect that is caused by the structure shrinkage upon insertion of guest molecules, such as water.24 It is probable that the residual ethanol in LiNO3-MIL-

n

∑ aiN i+ ∑ biN i i=0

i=0

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53(Al) is responsible for the change in the XRD pattern. Actually, after the residual ethanol is completely removed from the pores by heat treatment at 573 K in vacuum, the XRD pattern of Li-MIL-53(Al) becomes identical to that for MIL53(Al). It indicates that the heat treatment at 573 K did not affect the crystal structure of MIL-53(Al). Note that the peaks ascribed to LiNO3 are not observed in both LiNO3-MIL-53(Al) and Li-MIL-53(Al). Pore structural parameters determined by nitrogen adsorption−desorption isotherms of MIL-53(Al) and Li-MIL-53(Al) (see the Supporting Information, Figure S1) are listed in Table 1. BET surface area and DR micropore volumes of the two samples are nearly identical, suggesting that the lithium doping does not deteriorate the framework.

Figure 3. MS profiles of the gaseous compounds generated from LiNO3-MIL-53(Al) upon heating.

Table 1. Pore Parameters of MIL-53(Al) and Li-MIL-53(Al) sample

BET surface area [m2/g]

total pore volume [cc/g]

micropore volume [cc/g]

MIL-53(Al) Li-MIL-53(Al)

976 958

1.53 1.34

0.34 0.33

high temperature over 700 K. The release of these species (m/z = 44) over 700 K was also observed for nondoped MIL-53(Al) (data not shown), indicating the destruction of the MIL-53(Al) framework to generate CO2. On the other hand, lithium (m/z = 7) is not detected in our temperature range. To further investigate the details of the released gaseous compounds, FT-IR spectroscopy was employed. LiNO3-MIL53(Al) was set in the quartz cell that was closed with CaF2 windows; then the cell was heated and evacuated. At first, the sample was pretreated at 473 K for 4 h in vacuum to evacuate residual ethanol. The cell was then heated to 573 K for 2 h. Finally, the cell was cooled to room temperature and evacuated. IR spectra were obtained every 10 K rise during heating, after cooling, and after evacuation. Figure 4 shows the IR spectrum

3.2. Reaction during Heat Treatment. Figure 2 shows the IR spectra of MIL-53(Al), LiNO3-MIL-53(Al), and Li-MIL-

Figure 2. FT-IR spectra of (a) MIL-53(Al), (b) LiNO3-MIL-53(Al), and (c) Li-MIL-53(Al).

Figure 4. FT-IR spectrum of the gas species released from LiNO3MIL-53(Al) by heat treatment at 573 K.

53(Al). LiNO3-MIL-53(Al) exhibits the peaks at 1385 and 1050 cm−1, which can be attributed to the nitrate anion degenerate vibration and bending vibration, respectively. After the heat treatment, these two peaks disappear in the spectrum of LiMIL-53(Al), suggesting that the nitrate anion was removed during the heat treatment. Quantitative analyses also confirm the removal of the nitrate anions. The amount of impregnated LiNO3 in LiNO3-MIL-53(Al) is calculated to be 1.17 wt %. After the heat treatment, the N content is decreased by 95%, while the lithium content remains constant. The molar ratio of loaded lithium to framework aluminum is Li/Al = 0.036, indicating that one Li is introduced per seven unit cells of MIL53(Al). TG-MS measurement was performed to investigate the gaseous compounds released from LiNO3-MIL-53(Al) at elevated temperatures. Figure 3 shows the profile of mass spectra of the gaseous compounds released during the heat treatment. The intensity of m/z = 30 (NO) increases from 523 K. The profile for m/z = 44, corresponding to N2O and CO2, exhibits two peaks at low temperature around 400 K and at

of released gaseous compounds after cooling subtracted with the spectrum after evacuation. The spectrum exhibits absorption bands of some gas species, including NO, N2O, CO, and CO2. Specifically, the peaks of NO and NO2, indicative of decomposition of nitrate anions, are observed from 523 K (see the Supporting Information, Figure S2). After the heat treatment at 573 K for 2 h, the intensities of these bands become constant, indicating that most of the nitrate anions are decomposed. For comparison, MIL-53(Al) was also subjected to the above measurements. However, no release of NO and N2O was detected. From these results, it is clear that the nitrate anion is decomposed by heat treatment. Yuvaraj et al.23 reported that the decomposition temperature of metal nitrates was inversely correlated with the charge density of the metal ions. The lithium cation has a low charge density and is, therefore, ineffective for polarizing the nitrate anion, resulting in a high decomposition temperature of 913 K. The decrease of the decomposition temperature (523−573 K) in LiNO3-MIL-53(Al) should be, in part, due to the increase of 10262



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shift anisotropy. Unfortunately, the 7Li MAS NMR spectrum cannot provide further information on the state of lithium. 3.4. Hydrogen Adsorption Properties. Hydrogen adsorption isotherms of MIL-53(Al) and Li-MIL-53(Al) at 77 and 87 K are shown in Figure 6. Hydrogen uptake at 77 K and

surface area, that is, a smaller particle size of LiNO3 in the pores. It has been reported that KNO3 in zeolite NaY, which has a similar pore size to MIL-53(Al), decomposes at a lower temperature (953 K) than bulk KNO3 (1113 K).29 However, a much larger reduction of the decomposition temperature (∼340 K) was observed for LiNO3-MIL-53(Al). We suppose that, in addition to the small particle size of LiNO3 doped in MIL-53, an aluminum cation that is rich in the pore of MIL53(Al) would contribute to the decrease of the decomposition temperature due to its high charge density.23 3.3. State of Lithium in Li-MIL-53(Al). To investigate the state of lithium species in Li-MIL-53(Al), solid-state NMR measurements were carried out. Figure 5, spectra a−c, shows

Figure 6. Hydrogen adsorption isotherms at 77 K (filled) and 87 K (open) of MIL-53(Al) (circle) and Li-MIL-53(Al) (triangle).

1 atm are 1.66 and 1.84 wt % for MIL-53(Al) and Li-MIL53(Al), respectively, confirming the enhancement of hydrogen uptake by lithium doping. To compare the effect of lithium doping on the hydrogen adsorption properties, the amount of adsorbed hydrogen was normalized by the surface area. The estimated densities are 5.23 and 5.90 molecule/nm2 for MIL53(Al) and Li-MIL-53(Al), respectively. The quantitative analysis reveals that one lithium cation is introduced per seven unit cells of MIL-53(Al), as mentioned above. The Connolly surface area30 for seven unit cells of MIL-53(Al) is about 3.0 nm2, as estimated from its crystal structure using Materials Studio 4.4 (Accelrys Inc.). These results, together with the Al/Li ratio (Li/Al = 0.036) obtained from the elemental analysis, show that one lithium cation introduced in Li-MIL-53(Al) adsorbs two hydrogen molecules at 77 K and 1 atm. The effect of the amount of lithium on hydrogen uptake was examined by changing the concentration of the LiNO3/ethanol solution used for impregnation. The numbers of adsorbed hydrogen molecules per unit area and the H2/Li ratios for the samples with different Li/Al ratios are plotted in Figure 7. In the low region of Li/Al < 0.04, the number of hydrogen per unit area increases linearly along with the Li/Al ratio,

Figure 5. 13C CP/MAS NMR spectra of (a) MIL-53(Al), (b) LiNO3MIL-53(Al), (c) Li-MIL-53(Al), (d) terephthalic acid, and (e) terephthalic lithium.

the 13C CP/MAS NMR spectra of MIL-53(Al), LiNO3-MIL53(Al), and Li-MIL-53(Al). The signals between δ = 128 and 136 ppm are assigned to the aromatic carbons. The signal at around δ = 170 ppm corresponds to the carboxyl carbon. The additional signals at δ = 173 ppm observed for MIL-53(Al) and Li-MIL-53(Al) are attributed to the carboxyl group under the influence of a water molecule.24 It is noteworthy that a new signal appears at δ = 175 ppm in the spectrum of Li-MIL53(Al). To investigate this new chemical shift, terephthalic lithium was prepared by neutralization of terephthalic acid with lithium hydroxide and was analyzed by 13C CP/MAS NMR. In the spectrum of terephthalic acid (Figure 5, spectrum d), the carboxyl carbon is observed at δ = 173.5 ppm. On the other hand, terephthalic lithium exhibits a downfield signal at δ = 177.8 ppm (Figure 5, spectrum e). This shift can be explained by the shielding effect. The carbon atom of the carboxyl group is deshielded by electron-withdrawing two adjacent oxygen atoms. The further downfield shift from 173.5 to 177.8 ppm for terephthalic lithium can be attributed to a greater deshielding effect induced by the enhancement of the electron-withdrawing capability of the oxygen atom coordinating with lithium cations. Therefore, the new signal at δ = 175 ppm observed for Li-MIL53(Al) strongly suggests that the doped lithium cations are coordinated with the oxygen atom of carboxyl groups. 7 Li MAS NMR measurement was also carried out (Figure S3, Supporting Information). The spectrum of Li-MIL-53(Al) shows one broad signal at δ = −0.45 ppm, which is much broader than that for LiNO3-MIL-53(Al). The broad signal is generally observed by solid-state NMR due to the chemical

Figure 7. Plots of the numbers of adsorbed hydrogen molecules per unit area (circle) and per lithium cation (square) as a function of the lithium/Al ratio of Li-MIL-53(Al). 10263



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suggesting that the doped lithium are effective adsorption sites. One doped lithium cation adsorbs 1.3−2.9 hydrogen molecules, which is in agreement with the previous theoretical study that the doped lithium cation into MOFs adsorbed three hydrogen molecules.31 On the other hand, in the high region of Li/Al > 0.04, the number of hydrogen per unit area does not change and the H2/Li ratio decreases with increasing the amount of doped lithium. This indicates that some of the doped lithium does not act as adsorption sites for hydrogen molecules. The previous studies also showed that a large amount of lithium caused a decrease of hydrogen uptake due to the aggregation of lithium.14,19 Excessively incorporated lithium may form lithium oxide that may not be adsorption sites for hydrogen molecules. In the present work, 0.5 M is the optimum concentration of the LiNO3/ethanol solution for the enhancement of hydrogen uptake of MIL-53(Al). The isosteric heat of hydrogen adsorption (Qst) of MIL53(Al) and Li-MIL-53(Al) is shown in Figure 8. In both cases,

Table 2. Hydrogen Adsorption Properties of MIL-53(Al) and Metal-Doped MIL-53(Al) sample MIL53(Al) Li-MIL53(Al) Na-MIL53(Al) Mg-MIL53(Al) Ca-MIL53(Al)

BET surface area [m2/g]

H2 uptake [wt %]

number of H2 per unit area [molecule/nm2]

Qst range [kJ/mol]

976

1.66

5.23

6.4−5.1

958

1.84

5.90

6.3−4.9

928

1.76

5.81

6.2−5.3

903

1.76

5.98

6.3−5.6

882

1.69

5.85

6.3−5.5

adsorption sites and their affinities to hydrogen atoms are similar to that of lithium cations. Since the atomic weight of lithium is the smallest among these cations, Li-MIL-53(Al) exhibits the highest hydrogen uptake at 77 K.

4. CONCLUSIONS We have demonstrated a facile method for lithium doping into MIL-53(Al) by the thermal decomposition of nitrate anions without the deterioration of the framework, which was owing to the unexpectedly low decomposition temperature of LiNO3 in the MIL-53(Al) skeleton. A suitable amount of doped lithium could enhance the amount of adsorbed hydrogen from 1.66 to 1.84 wt % at 77 K and 1 atm. In addition, this method enabled us to introduce other alkaline metal cations, such as Na+, Mg2+, and Ca2+, into MIL-53(Al), resulting in enhanced hydrogen uptakes. However, the isosteric heat of adsorption (Qst) was not enhanced, which was possibly due to the interaction between doped lithium cations and carboxyl groups of organic ligands that weakened the affinity of lithium to hydrogen molecules. We expect that our lithium-doping method based on the thermal decomposition of nitrate anions can be adapted to other MOFs to enhance their hydrogen adsorption capacity.

Figure 8. Isosteric H2 heat of adsorption of MIL-53(Al) (circle) and Li-MIL-53(Al) (triangle).

Qst decreases with increasing the hydrogen loading, because adsorption sites with higher hydrogen affinity are filled first and then other sites with lower hydrogen affinity are filled. Although lithium doping was expected to enhance Qst, LiMIL-53(Al) gave a similar Qst value to that of MIL-53(Al). In contrast, Himsl et al.17 reported that the lithium-doped, hydroxyl-modified MIL-53(Al) exhibited a more drastic increase of Qst (from 5.8 to 11.6 kJ/mol). In this case, the doped lithium cation binding with one oxygen atom possessed high charge that resulted in the strong charge-induced dipole interaction with hydrogen molecules. In contrast, in the present study, lithium cations are coordinated with carboxyl groups at the corner of the pore, as suggested by 13C CP/MAS NMR (Figure 5). The doped lithium cation was surrounded by electron-withdrawing oxygen atoms of carboxyl groups; therefore, the charge-induced dipole interaction between doped lithium and hydrogen molecules is considered to be weakened. Consequently, the doped lithium in Li-MIL-53(Al) only acts as additional adsorption sites for hydrogen molecules with a moderate affinity to hydrogen molecules. Finally, we examined the effect of other alkaline/alkaline earth metal cations, Na+, Mg2+, and Ca2+. Table 2 summarizes the results. Hydrogen uptakes of all doped MIL-53(Al) at 77 K and 1 atm are higher than that of MIL-53(Al), but lower than that of Li-MIL-53(Al). However, the numbers of adsorbed hydrogen molecules per unit area and Qst are almost the same as those of Li-MIL-53(Al). These results indicate that other alkaline/alkaline earth metal cations can also act as additional

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ASSOCIATED CONTENT

S Supporting Information *

FT-IR spectra of released gas species from LiNO3-MIL-53(Al) during heating, 7Li MAS NMR spectra of all samples, details of fitting and calculation of isosteric heat of hydrogen adsorption, and hydrogen adsorption isotherms of other alkaline-doped MIL-53(Al) at 77 K. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-3-5841-7348. Fax: +81-3-5800-3806. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported, in part, by the ENEOS Hydrogen Trust Fund and by a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS). M.K. acknowledges the support by a Grant-inAid for JSPS Fellows and Fellowship. 10264



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REFERENCES

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on May 2, 2012, with an error to the caption of Figure 6. The corrected version was reposted on May 10, 2012.

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Effect of Lithium Doping into MIL-53(Al) through Thermal

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