Effects of Metal Ions and Ligand Functionalization on Hydrogen

ARTICLE pubs.acs.org/JPCC

Effects of Metal Ions and Ligand Functionalization on Hydrogen Storage in Metal
Organic Frameworks by Spillover Wenxiu Cao, Yingwei Li,* Liming Wang,* and Shijun Liao Key Lab for Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China

bS Supporting Information ABSTRACT: Hydrogen storage by spillover is a promising technique to enhance the hydrogen uptakes in metal
organic frameworks (MOFs) at room temperatures. However, to date, little is known on the structure
property relationships of MOFs for spillover storage. In this work, the effects of chemical composition of MOFs on hydrogen storage by spillover were studied systematically. Two series of MOFs with similar surface areas and formula units but different metal ions (M) or organic linkers (L), M(OH)BDC (BDC = terephthalate) or Zn4OL3, were prepared and employed as the receptors for spiltover hydrogen atoms. It was found that the M(OH)BDC series with various metal ions exhibited very close hydrogen capacities at room temperature. However, the functionalization of the BDC ligand in IRMOF-1 with various groups affected the storage capacity by spillover significantly. The decorations of functional groups with strong electrophilicity (i.e., electronwithdrawing ability) on the BDC linkers remarkably enhanced the hydrogen uptakes by spillover. The experimental results were in good agreement with the density functional theory (DFT) calculations, which showed that the hydrogenations of the ligands with electron-withdrawing groups were thermodynamically more favored than those with electron-donating ones on the MOF structures. The new findings could provide a potential way to fabricate new metal
organic frameworks with high hydrogen storage capacities by spillover at room temperature.

1. INTRODUCTION Hydrogen (H2) as a clean and efficient energy carrier is considered as an alternative energy source for fossil fuels.1 However, to date, a safe, economically, and technically viable solution for onboard hydrogen storage remains one of the major challenges. There are currently several candidate storage systems for hydrogen including liquid or high-pressure H2 gas, chemical hydride, metal hydride, and porous adsorbents. Among these methods, adsorption on solid porous surfaces is an attractive option for efficient and relatively safe hydrogen storage. Large efforts have been devoted in recent years to developing nanostructured and porous materials for this purpose, such as carbons, zeolites, and metal
organic frameworks (MOFs),2
6 however, to date none of which is capable of satisfying the DOE criteria for use in transportation. MOFs are a new class of porous materials assembled with metal ions and organic linkers. Because of their low densities and high surface areas, MOFs exhibited exceptional H2 storage capacities by mass at 77 K.7
10 For example, a metal
organic framework (NU-100) based on a hexatopic carboxylate ligand showed an excess H2 uptake up to 99.5 mg g
1 at 77 K and 56 bar,10 representing the highest H2 adsorption capacity reported to date. However, no significant amounts of hydrogen could be adsorbed on the MOF materials at room temperature.11 The low hydrogen uptakes at ambient temperature may be due to the low heats of adsorption (normally ∼5 kJ mol
1) of H2 on the MOFs. Therefore, it is very important to increase the interaction between r 2011 American Chemical Society

hydrogen and the MOF surfaces in order to achieve a high capacity for H2 at near room temperatures. Hydrogen spillover, a well-documented phenomenon in catalysis,12 has been demonstrated as a promising approach to enhance the hydrogen storage capacities in nanostructured materials including carbons, zeolites, and MOFs at ambient temperature.13
32 Hydrogen spillover is defined as the dissociative chemisorption of hydrogen molecules on metal nanoparticles, followed by migration of hydrogen atoms onto the nearby surfaces of a receptor via spillover and surface diffusion. It is understandable that the binding of spiltover hydrogen atom and the receptor surface is much stronger than that in the physical adsorption system, where hydrogen molecule is adsorbed by van der Waals interaction.22,33,34 Consequently, the hydrogen uptake in porous materials (e.g., MOFs) can be enhanced substantially by hydrogen spillover. Although several thousand MOFs have already been reported and many of them have been claimed as promising materials for hydrogen storage at low temperatures (mostly at 77 K), very few of them have been studied for hydrogen storage by spillover at ambient temperature.15,24,28,29,35,36 In most of the reports on spillover storage, enhanced storage capacities on MOFs have been observed at near room temperatures. It is also noted that the spillover enhancements are remarkably different on various Received: April 18, 2011 Revised: June 10, 2011 Published: June 28, 2011 13829

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MOF samples.19,35 The difference in storage capacity by spillover could be largely related to the structural and surface characteristics of the MOFs, such as particle sizes, metal ions, or organic linking units. To understand the key factors that affect storage capacity by spillover, a fundamental examination of the structure
property relationships of MOFs is needed. However, to date, such a systematic study on the effects of structural and surface properties of MOFs on spillover storage is scarce.35 Here we attempt to disclose the correlation between the chemical composition of MOFs (e.g., metal ion and organic linker) and the H2 adsorption capacity by spillover. To achieve this, we chose two series of terephthalate-based MOFs (MILs and IRMOFs) that had very close surface areas and chemical formula units, respectively, as the secondary receptors for spiltover hydrogen atoms. Four MILtype MOFs, with similar formula unit (M(OH)BDC; M = Al, Cr, Fe, and V, BDC = terephthalate), were investigated to elucidate the influence of constituent metal ions on spillover storage, and five IRMOFs, with a similar formula unit Zn4OL3 (L = BDC, Br-BDC, NH2-BDC, NO2-BDC, and (CH3)2-BDC) were used to examine the impacts of organic ligands on H2 uptakes. The hydrogen adsorption isotherms of the pristine MOFs and Pt/C-MOF mixtures were measured at 298 K and up to 7.3 MPa. In comparing their dihydrogen uptakes and net storage capacities by spillover, correlations with their chemical compositions were discussed. Through this undertaking, the surface requirements of MOFs for a high hydrogen storage capacity by spillover may be formulated, and such understanding will facilitate a more rational design of new MOF materials for hydrogen storage by spillover.

2. EXPERIMENTAL METHODS 2.1. Preparation of MOFs. MIL-53 (M(OH)BDC, M = Al, Cr, or Fe). The MIL-53 (Al, Cr, or Fe) material was synthesized hydrothermally under autogenous pressure according to the reported procedures.37
39 In a typical synthesis, the metal salt of Cr(NO3)3 3 9H2O, Al(NO3)3 3 9H2O, or FeCl3 3 6H2O was heated with terephthalic acid (H2BDC) in H2O (for MIL-53 (Cr, or Al)) or DMF (for MIL-53 (Fe)) with the molar ratio reported in the literature.37
39 The obtained MIL-53 (Al, Cr, and Fe) solids were dried overnight under vacuum at 573, 523, and 573 K, respectively. MIL-68 (V(OH)BDC). MIL-68 (V) was synthesized based on the method developed by Ferey et al.40 VOSO4 3 xH2O (x = 3
5) was mixed with H2BDC in DMF, and the pH value of the solution was adjusted close to 1 with nitric acid. The resultant solution was introduced to a Teflon-lined stainless steel autoclave and then heated to 473 K for 72 h. A yellowish brown powder was isolated by filtration, washed with DMF, then dried in air at 323 K. The solid was degassed in vacuum at 423 K for 24 h before measurements. IRMOFs (Zn4OL3). IRMOFs (IRMOF-1, -2, and -3, and MTVMOF-5-AE and -AF) were synthesized by a solvothermal method according to the reported procedures with slight changes.41,42 In a typical synthesis of IRMOFs, the chemically functionalized benzene dicarboxylic acid (e.g., BDC (for IRMOF-1), Br-BDC (for IRMOF-2), or NH2
BDC (for IRMOF-3); 0.5 mmol) and zinc nitrate hexahydrate (Zn(NO3)2 3 6H2O; 1.5 mmol) were dissolved in 25 mL of DMF. The mixture was introduced in the hydrothermal bomb that was heated to 383 K. The temperature was held at 383 K for 24 h (or 48 h), and then cooled to room temperature in 24 h. It should be noted that it is difficult to synthesize an IRMOF-type structure when using NO2-BDC or

Figure 1. Powder XRD patterns of (a) MIL-53 (Cr, Fe, Al), (b) MIL-68 (V), and (c) IRMOFs.

2,5-(CH3)2-BDC as the ligand alone,42 and thus mixed organic linkers of BDC with NO2-BDC or (CH3)2-BDC (at a molar ratio of 1:1) were used to isolate the IRMOF-type structure of Zn4O(BDC)2.1(NO2-BDC)0.9 (MTV-MOF-5-AE) or Zn4O(BDC)1.3((CH3)2-BDC)1.7 (MTV-MOF-5-AF), respectively. As named by Yaghi et al.,42 MTV is the abbreviation of multivariate, and the symbols of “A”, “E”, and “F” denote BDC, NO2-BDC, and (CH3)2BDC ligand, respectively. The link molar ratios in the MTVMOFs were determined by 1H NMR. The block-shaped crystals were immersed with DMF and then sequentially in chloroform for three 24 h periods. Finally, the solvent was fully removed by degassing in vacuum at 120 °C for 24 h, yielding the porous materials. 2.2. Preparation of Pt/C
MOF Mixtures. In our spillover experiments, a catalyst containing 10 wt % Pt supported on carbon (SBET = 746 m2 g
1) was used for dissociation of H2. Active carbon can be considered to be the primary receptor for hydrogen atoms, and an MOF was the secondary spillover receptor. The catalyst and the secondary spillover receptor (at a weight ratio of 1:9) were ground together (with mortar and pestle) under N2 atmosphere for 30 min to produce the physical mixture. The mixture was immediately loaded into the sample holder for adsorption measurements and degassed under vacuum. 13830

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2.3. Characterization of Materials. Powder X-ray diffrac-

tion (XRD) patterns of the samples were obtained on a Rigaku diffractometer (D/MAX-IIIA, 3 kW) using Cu KR radiation (40 kV, 30 mA, λ = 0.1543 nm). The BET surface area measurements were performed with N2 adsorption/desorption isotherms at 77 K on a Micromeritics 2020 M instrument. The samples were outgassed under vacuum at 423 K for 24 h prior to measurements. 1H NMR measurements of digested MTV-MOFs in dilute DCl solution were carried out on a Bruker-Ultrashield 400 plus spectrometer. 2.4. Hydrogen Adsorption Isotherm Measurements. Highpressure hydrogen isotherm at 298 K and pressures up to 7.3 MPa were measured using a static volumetric technique with an automatic adsorption apparatus (Belsorp-HP). The hydrogen pressure was measured using a high-precision transmitter (0
2000 psi (absolute)) with enhanced static accuracy to 0.08% full-scale output. The tubing and valves for adsorption measurement were loaded in a thermostat with a constant temperature of 298 K ((0.1 K), and the sample holder was immersed in a temperature-controlled water bath at 298 K ((0.1 K). Typically, ∼200 mg of sample was used for each high-pressure adsorption measurement. Prior to the introduction of hydrogen (99.999%), the samples were degassed in vacuum (at 10
4 Torr) at 473 K for at least 12 h to remove any residual guest molecules to obtain the highest gas adsorption capacity. 2.5. Density Functional Theory Calculations. DFT calculations were carried out to examine the thermodynamics of the adsorption process, using Becke’s three-parameter hybrid functional together with LYP correlation functional (B3LYP),43
45 together with 6-311+G(2df, p) basis set. Scale factor of 0.9889 is

Figure 2. Nitrogen adsorption isotherms for MIL type MOFs measured at 77 K. Symbols: 2, MIL-53 (Al); [, MIL-53 (Cr); b, MIL-53(Fe); and 9, MIL-68 (V). Open symbols indicate desorption branches.

used for scaling the zero-point energies.46 The DFT calculations were carried out by using Gaussian 03 program.47

3. RESULTS AND DISCUSSION 3.1. Characterization of MOF Materials. Powder XRD patterns for MIL-53 (Al, Cr, or Fe), MIL-68 (V), and IRMOFs are shown in Figure 1. All diffraction peaks matched well with the simulated and the already published XRD patterns on the same MIL or IRMOF structure. The XRD patterns of the MOFs remained essentially unchanged after mixing with catalyst and subsequent H2 exposure (Supporting Information, Figure S1). The specific surface areas of the MILs (MIL-53 or MIL-68) were determined by N2 physisorption measurements at 77 K (Figure 2). The BET surface areas of the samples are summarized in Table 1. MIL-53 (Al) and MIL-53 (Cr) exhibit very close BET surface areas (∼1180 m2 g
1), which are in agreement with the literature data.37,48 No previous information is available on the surface area of MIL-53 (Fe). Llewellyn et al. indeed found that MIL-53 (Fe), after degassing at 473 K, exhibited closed pore structure, being inaccessible to most gases.39 Neither did we observe any nitrogen adsorption at 77 K for our MIL-53 (Fe) sample degassed at 473 K. However, it was interesting to note that when the activation temperature was increased to 573 K the as-synthesized MIL-53 (Fe) sample could adsorb over 200 cm3 g
1 N2 at 77 K up to 1 atm (Figure 2), rendering the BET surface area of 815 m2 g
1 for the MIL-53 (Fe). The much higher surface area obtained on our MIL-53 (Fe) sample than the reported one could be due to the higher activation temperature used in this work because the two samples exhibited the same crystal structure. A temperature of 473 K might not be high enough to evacuate the pores of the MIL-53 (Fe). The BET surface area of MIL-68 (V) was 909 m2 g
1, being slightly higher than the value (ca. 600 m2 g
1) obtained by Ferey and coworkers on the material.40 Figure 3 shows the N2 adsorption isotherms of the IRMOFs at 77 K. The BET surface areas of the samples are summarized in Table 2. All IRMOFs exhibited similar N2 isotherms with very close BET surface areas in the range of 1900
2600 m2 g
1. The surface areas are all in line with the literature reports on the IRMOF materials by using the same hydrothermal synthesis methods.42,49
52 After grinding with catalyst, no significant decrease in surface area was observed on the IRMOF sample, indicating the well maintenance of the MOF structure (Supporting Information, Figure S2). 3.2. High-Pressure Hydrogen Isotherms at 298 K. Figure 4 shows high-pressure hydrogen adsorption isotherms at 298 K for the MIL-53 (Al, Cr, or Fe) and MIL-68 (V) samples. The H2 storage capacities of the materials at 7.3 MPa are summarized in Table 1. It can be seen that there is no significant difference in H2 uptake for the pristine MIL type MOFs (ca. 0.15 to 0.2 wt %),

Table 1. BET Surface Areas and H2 Uptakes at 298 K and 7.3 MPa for the MIL-53 (Al, Cr, or Fe) and MIL-68 (V) Materials H2 uptake (wt %) 2


1

Hs/formula unitb

MOF

formula unit

SBET (m g )

pristine MOF

MIL-53 (Al)

Al(OH)(BDC)

1186

0.18

0.41

MIL-53 (Cr)

Cr(OH)(BDC)

1183

0.18

0.38

0.51

MIL-53 (Fe)

Fe(OH)(BDC)

815

0.15

0.35

0.51

MIL-68 (V)

V(OH)(BDC)

909

0.20

0.40

0.52

Pt/C
MOFa

0.53

a

Physical mixture of Pt/C catalyst with MOF at a weight ratio of 1:9. b Average spiltover hydrogen atoms (Hs) per formula unit. Hs/formula unit = (qn/100)/(0.9/Mf); qn is the net hydrogen uptake by spillover (wt %); Mf is the weight of the formula unit. 13831

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calculations. They did not find any chemisorption binding sites on the Zn4O clusters, and thus they concluded that the Zn4O corners will not play a significant role in H2 uptakes in the MOF materials by spillover. Figure 5 shows the hydrogen adsorption isotherms for the IRMOFs at 298 K up to 7.3 MPa. It is shown that the unmodified IRMOF materials exhibited slightly different storage capacities, in accordance with their surface areas featuring a physisorption mode. With the addition of a small quantity of Pt/C catalyst, the hydrogen uptakes of all IRMOFs were remarkably enhanced at all pressures up to 7.3 MPa. It is noted that a hydrogen storage capacity of 0.55 wt % was obtained on the Pt/C and IRMOF-1 (i.e., MOF-5) mixture at 7.3 MPa, which represented an enhancement factor of 1.96 (Table 2 and Supporting Information, Table S2) compared with the parent IRMOF-1 material. The enhancement by spillover is a little lower than that reported in a previous study, in which an enhancement of 3.3 was achieved on a similar IRMOF-1 material.15 The difference in spillover enhancement between the two studies might be explained by some possible factors. First, the particle size of the Pt nanoparticles on the 10 wt % Pt/C used here is speculated to be larger than that of the Pt/C employed in the literature with a much lower Pt loading of 5 wt %.15 As described in recent reports, larger Pt particles normally resulted in lower adsorption capacities by spillover.55,56 Second, the presence of lattice defects in the IRMOF-1 structure synthesized by using a direct mixing method (with H2O2 addition) developed by Huang et al.15,54 can significantly facilitate the spillover and thus enhance the hydrogen uptake.29,57 Moreover, the big crystal sizes (millimeter size) of the IRMOF-1 sample that were prepared by a solvothermal method in this work would lead to poor contacts with the catalyst and thus lower the spillover effect.19 In contrast, the previous work used a direct mixing method,15,54 which resulted in much smaller IRMOF-1 crystals. The hydrogen uptakes of the other four IRMOFs were also enhanced apparently by mixing the MOF crystals with the Pt/C catalyst (Figure 5 and Supporting Information, Figure S3). The enhancement factors of the MOFs were in the range of 1.6 to 3.0 (Supporting Information, Table S2). As shown in Table 2, Pt/ C
IRMOF-2 physical mixture had the highest H2 adsorption capacity among the IRMOF structures. The hydrogen uptake was 0.76 wt % at 7.3 MPa, corresponding to an enhancement factor of 3.0 compared with the parent IRMOF-2 (Zn4O(BrBDC)3). Note that the physical mixture by grinding is reproducible (Supporting Information, Figure S4). Furthermore, the adsorption was totally reversible. It can be seen from Figure S5 (Supporting Information) that the desorption branch almost followed the adsorption branch, although there appeared to be a slight hysteresis. The material was then degassed at a pressure of

which is in accord with their very close surface areas of the MOF samples featuring physical adsorption. The Pt/C catalyst employed in this study showed a hydrogen capacity of ca. 0.2 wt % at 7.3 MPa (Figure 4). By simply mixing the MIL-53 (Al, Cr, or Fe) or MIL-68(V) with a small amount of the Pt/C catalyst (at 9:1 mass ratio), the hydrogen uptake has been apparently enhanced at all pressures up to 7.3 MPa. In comparison with the parent MOF material, it can be seen that the hydrogen storage capacity has been increased by a factor of ca. 2.0 (Supporting Information, Table S1). On the basis of the weighted average of the catalystMOF mixture (assuming additivity), the expected H2 capacity of the mixture could be calculated (Figure 4 and Supporting Information, Table S1). Thus the net hydrogen uptake (qn) by spillover can be estimated for each MOF sample (Supporting Information, Table S1). Giving the net H2 capacity by spillover and the weight of the formula unit (M(OH)BDC, M = Al, Cr, Fe, or V), it is possible to calculate the average value of spiltover hydrogen atoms (Hs) per formula unit of the MOF. The calculated results are shown in Table 1. It can be seen that a similar mean number (∼0.5) of Hs per formula unit was obtained for all four MIL-type MOFs. Considering the close formula unit of the MOFs, the results suggest that the constituent metal ions in MOFs may not play an important role in determining the hydrogen uptake of the MOF material by spillover. Our experimental results are in good agreement with the theoretical data reported by Suri et al.,53 who investigated the chemisorption of spiltover hydrogen atom on the Zn4O corners of the MOF materials by using ab initio quantum chemistry

Figure 3. Nitrogen adsorption isotherms for IRMOFs measured at 77 K. Symbols: b, IRMOF-1; 1, IRMOF-3; 2, MTV-MOF-5-AE; [, IRMOF-2; 9, MTV-MOF-5-AF. Open symbols indicate desorption branches.

Table 2. BET Surface Areas and H2 Uptakes at 298 K and 7.3 MPa for the IRMOFs H2 uptake (wt%) MOF

formula unit

2


1

SBET (m g )

pristine MOF

Pt/C
MOFa

Hs/formula unitb

IRMOF-1

Zn4O(BDC)3

2586

0.29

0.55

2.07

IRMOF-2

Zn4O(Br-BDC)3

2079

0.26

0.76

5.66

MTV-MOF-5-AE

Zn4O(BDC)2.1 (NO2
BDC)0.9

2006

0.25

0.58

2.71

IRMOF-3

Zn4O(NH2
BDC)3

2200

0.3

0.46

1.54

MTV-MOF-5-AF

Zn4O(BDC)1.3 ((CH3)2-BDC)1.7

1908

0.23

0.38

1.39

a

Physical mixture of Pt/C catalyst with MOF at a weight ratio of 1:9. b Average spiltover hydrogen atoms (Hs) per formula unit. Hs/formula unit = (qn/100)/(0.9/Mf); qn is the net hydrogen uptake by spillover (wt %), Mf is the weight of the formula unit. 13832

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Figure 4. High-pressure hydrogen adsorption at 298 K for the MIL-53 (Al (a), Cr (b), Fe (c)) and MIL-68 (V) (d). Dotted line is predicted based on the weighted average of the mixture. Symbols: 9, pristine MOF; 2, 10 wt % Pt/C catalyst; b, Pt/C and MOF mixture.

Figure 5. High-pressure hydrogen adsorption at 298 K for the IRMOF samples: (a) IRMOF-1, (b) IRMOF-3, (c) IRMOF-2, and (d) MTV-MOF-5AE. Dotted line is prediction based on the weighted average of the mixture. Symbols: 9, pristine MOF; 2, 10 wt % Pt/C catalyst; and b, Pt/C and MOF mixture.

10
4 Torr overnight at 298 K. The second adsorption branch was in good agreement with the first adsorption branch (Supporting Information, Figure S5). The average amount of the spiltover hydrogen atoms (Hs) per formula unit of each IRMOF structure was also calculated based on the net H2 capacity by spillover and the weight of the formula unit (Zn4OL3). The results are shown in Table 2. Remarkable differences in the mean Hs value per formula unit were observed

between the five IRMOFs. The average amount of Hs adsorbed on one formula unit (Zn4O(BDC)3) of IRMOF-1 was 2.07. The number increased significantly when the BDC linkers were substituted with
Br groups. As can be seen from Table 2, IRMOF-2 exhibited the most mean Hs adsorption amount on the formula unit (Zn4O(Br-BDC)3). It was also interesting to note that the average amount of Hs per formula unit was enhanced apparently even though only 30% of the BDC ligands was replaced with 13833

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The Journal of Physical Chemistry C NO2-BDC with a formula unit of Zn4O(BDC)2.1(NO2-BDC)0.9 (Table 2). However, the substitution of BDC linker with
NH2, or
(CH3)2 group resulted in slightly less spiltover hydrogen atoms per formula unit for IRMOF-3 (Zn4O(NH2-BDC)3) or MTV-MOF-5-AF (Zn4O(BDC)1.3((CH3)2-BDC)1.7), respectively. The results imply the importance of ligand functionalization in constructing high-uptake MOF materials by hydrogen spillover because of the similar formula unit of the IRMOF structures. The marked difference observed in hydrogen storage capacities by spillover over the IRMOFs could be related to the electrophilicity of various substituted groups on the BDC ligands. Figure 6 shows the Hammett constants of the functional groups versus the average numbers of spiltover hydrogen atoms (Hs) per substituted BDC linker for the MOF samples. The mean Hs number for each BDC, Br-BDC, or NH2-BDC may be easily obtained by dividing the Hs number per formula unit (Zn4OL3) of the corresponding MOF (shown in Table 2) by 3 (i.e., the number of ligand (L)) because the Zn4O corners were shown to have no binding sites for hydrogen atom.53 The average amount of Hs per BDC linker substituted with
CH3 or
NO2 could be estimated by assuming the same mean amount of hydrogen atoms were adsorbed on one unsubstituted BDC linker for the formula unit of MTV-MOF-5-AF or MTV-MOF-5-AE as that for IRMOF-1 (Supporting Information, Table S2). A roughly exponential correlation between the Hammett constants of

Figure 6. Average number of spiltover H atom per substituted BDC linker in MOFs as a function of Hammett constant of functional group.

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functional groups and the average numbers of Hs for each substituted BDC linker was indeed observed. The value of Hs per NO2-BDC was slightly lower than expected. Nevertheless, the simple analysis is only an approximation. The amount of spiltover hydrogen atoms adsorbed on one unsubstituted BDC ligand in the MTV-MOF-5-AE should be much less than that in the IRMOF-1 because of a possible preferential adsorption of hydrogen atoms on the NO2-BDC. The results suggest that the functionalization of organic linkers with electron-withdrawing groups could largely enhance the hydrogen adsorption capacity of the MOF material by spillover. 3.3. Binding Energy of Hydrogen Atom with BDC Ligand. The remarkable enhancement in hydrogen adsorption on the BDC ligands substituted with electron-withdrawing groups (e.g.,
Br or
NO2) could be related to the increased hydrogenation energy of the linker. The hydrogenation energies of the BDC linkers with various substituted groups were calculated at the DFT level. Typical structures for BDCLi2-2-Br were displayed in Figure 7. The average binding energies and Gibbs free energies for one to three hydrogen molecules added to Li2-BDC and substituted Li2-BDC are listed in Table 3. The calculations on BDCLi2 + H2 agree qualitatively with the previous MP2/DGDZVP results by Suri et al.;53 that is, additions of hydrogen to aromatic carbons are favored thermodynamically over the additions to carboxylic carbons and oxygen atoms. Note that the calculations by Suri et al. did not include the zero-point energy corrections, and the MP2 calculations greatly underestimated the bond dissociation energy for H
H (389 vs 434 kJ/mol by B3LYP and 432 kJ/mol by experiment). Thereafter, Suri et al. predicted that addition of two H2 molecules to BDCLi2 being endothermic by 35 kJ/mol per H-atom compared with our slight exothermicity of 6.4 kJ/mol. The Gibbs free energy changes for the addition of hydrogen molecules to BDC linkers are in line with the enhanced adsorption for the Br- and NO2-substituted linkers and the reduced adsorption capability for NH2-BDC, where the ΔrG298K for the addition of three H2 changed from
58.0 and
54.1 kJ/mol for Br- and NO2-BDCs to +12.9 kJ/mol for NH2-BDCs (Table 3). Recently, several experimental and theoretical studies reported that modifying the surfaces of carbon nanostructures with oxygen groups, for example, semiquinone group, improved the reversible hydrogen storage capacity significantly.58
61 As suggested, the

Figure 7. Structures BDCLi2-2-Br (a), with two H added at C2 and C3 (b), 4 H added at C1, C2, C5, and C6 (c), and 6 H added at benzene ring (d). 13834

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Table 3. Changes of Energy and Gibbs Free Energy (ΔE0K and ΔG298K, kJ/mol) for Hydrogenation of BDCs BDCLi2 sites +H2

+2 H2

ΔE

ΔG

ΔE

ΔG

2-NH2
BDCLi2 ΔE

2-NO2
BDCLi2

ΔG

ΔE

ΔG

Dimethyl-BDCLi2 ΔE

ΔG

C2C3

43.1

70.9

3.2

32.3

87.3

115.0

28.5

55.4

49.1

77.6

C2C5

45.9

73.1

10.6

38.9

95.0

121.4

25.1

51.4

47.2

78.1

C1C2

66.5

32.4

58.7

103.4

130.0

43.5

68.3

68.4

93.2

C3C6

925

38.1

64.4

27.6

54.8

30.2

59.8

43.2

68.4

C5C6

37.2

64.9

29.4

58.0

30.9

60.3

C4C5

60.6

87.7

84.3

109.5

69.4

92.4

C1C2C3C4


2.6

50.2


31.1

25.6

35.0

87.4


31.7

28.2


9.4

47.5

C1C2C3C6 C1C4C5C6


25.6

29.4


59.2
30.9


1.8 24.5

14.4 8.3

69.2 61.7


47.9
35.6

6.6 18.5


28.0

30.7

C1C2C5C6


59.0


2.0

14.8

14.8


43.8

9.7

C2C3C4C5


49.0

6.0

14.5

69.4


70.5


7.8


35.1

18.5


41.4

14.1


35.6

18.5


140.3


58.0


70.6

12.9


136.6


54.1


111.7


25.1

C3C4C5C6 +3 H2

2-Br-BDCLi2

C1C2C3C4C5C6


102.9


25.5

possible explanations for the enhancement effects could be categorized as follows: (I) binding energy and surface coverage58
60 and (II) diffusion path.61 Our present results seemed to support hypothesis (I) because more spiltover hydrogen atoms were adsorbed on the MOF materials with ligands decorated with electron-withdrawing groups to have higher hydrogenation energies. The oxygen groups, such as semiquinone and carboxyl groups, formed on the carbon surfaces are known as electron-withdrawing groups as well.59 The functional group could provide diffusion paths for spiltover hydrogen atoms, like “carbon bridges” developed by Yang’s group.16,62 As reported in the literature, adsorbed small components such as hydrogen as well as water molecules may also serve as “bridges” for spillover.33,63 However, this alone could not explain the marked differences in storage capacities of the IRMOF structures in this study because it is apparent that all functional groups should have such bridging effects if existing.

4. CONCLUSIONS The effects of constituent units (e.g., metal ion and organic ligand) of MOFs on the hydrogen uptakes of the materials by spillover have been elucidated for the first time. Two series of MOFs, M(OH)BDC or Zn4OL, with similar surface areas and formula units but different metal ions (M) or organic linkers (L) were prepared and employed as the receptors for spiltover hydrogen atoms. It was found that the MOFs with different metal ions (but having the same BDC ligand) exhibited very close hydrogen capacities, suggesting that metal ions may not play an important role in the hydrogen uptakes of MOF materials by spillover. However, ligand functionalization was demonstrated to affect the storage capacities of MOFs by spillover significantly. The decoration of functional groups with strong electrophilicity (i.e., electron-withdrawing ability) on the organic linkers in MOFs could facilitate the adsorption of spiltover hydrogen atoms on the ligands and thus enhance the hydrogen uptakes by spillover. The experimental results were in line with the DFT calculations, which showed that the hydrogenation energies of the ligands with electron-withdrawing groups were much higher than those with electron-donating ones on the MOF structures. Our findings could provide a potential way to fabricate new metal
organic frameworks with high hydrogen storage capacities by spillover.

’ ASSOCIATED CONTENT

bS

Supporting Information. High-pressure hydrogen isotherms, powder XRD patterns, nitrogen adsorption isotherms, calculations of net hydrogen capacity by spillover, and average amount of Hs per BDC linker. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by NSF of China (20803024, 20936001, and 21073065), Doctoral Fund of Ministry of Education of China (200805611045), Guangdong Natural Science Foundation (8151064101000094 and 10351064101000000), the Fundamental Research Funds for the Central Universities (2009ZZ0023, 2011ZG0009), and the program for New Century Excellent Talents in Universities (NCET-08-0203). ’ REFERENCES (1) Schlapbach, L.; Z€uttel, A. Nature 2001, 414, 353. (2) Dillon, A. C.; Heben, M. J. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 133. (3) Yang, Z.; Xia, Y.; Mokaya, R. J. Am. Chem. Soc. 2007, 129, 1673. (4) Wang, L.; Yang, R. T. Energy Environ. Sci. 2008, 1, 268. (5) Zhao, D.; Yuan, D.; Zhou, H.-C. Energy Environ. Sci. 2008, 1, 222. (6) Murray, L. J.; Dinca, M.; Long, J. R. Chem. Soc. Rev. 2009, 38, 1294. (7) Rowsell, J. L. C.; Millward, A. R.; Park, K. S.; Yaghi, O. M. J. Am. Chem. Soc. 2004, 126, 5666. (8) Dailly, A.; Vajo, J. J.; Ahn, C. C. J. Phys. Chem. B 2006, 110, 1099. (9) Furukawa, H.; Ko, N.; Go, Y. B.; Aratani, N.; Choi, S. B.; Choi, E.; Yazaydin, A.; Snurr, R. Q.; O’Keefee, M.; Kim, J.; Yaghi, O. M. Science 2010, 329, 424. € Eryazici, I.; Malliakas, C. D.; (10) Farha, O. K.; Yazaydin, A. O.; Hauser, B. G.; Kanatzidis, M. G.; Nguyen, S. T.; Snurr, R. Q.; Hupp, J. T. Nat. Chem. 2010, 2, 944. 13835

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