Hydrogen and Methane Adsorption in Metal−Organic Frameworks: A

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J. Phys. Chem. C 2007, 111, 16131-16137

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ARTICLES Hydrogen and Methane Adsorption in Metal-Organic Frameworks: A High-Pressure Volumetric Study Wei Zhou,†,‡ Hui Wu,†,§ Michael R. Hartman,†,| and Taner Yildirim*,†,‡ NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, Department of Materials Science and Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104, Department of Materials Science and Engineering, UniVersity of Maryland, College Park, Maryland 20742, and Department of Nuclear Engineering and Radiation Health Physics, Oregon State UniVersity, CorVallis, Oregon 97331 ReceiVed: June 22, 2007; In Final Form: August 9, 2007

We report hydrogen and methane adsorption isotherms in two prototypical metal-organic framework compounds (i.e., MOF5 and ZIF8) over a large temperature (30-300 K) and pressure (up to 65 bar) range using a fully computer-controlled Sieverts apparatus. We find that, in a volumetric method, a proper choice of real gas equation of state is critical for obtaining reliable isotherm data. The widely used van der Waals equation of state (EOS) is not adequate to describe H2 and CH4, while the modified Benedict-Webb-Rubin (MBWR) EOS works well, even at very low temperatures and high pressures. With the known sample mass and bulk density, the skeleton density and the specific pore volume of MOF5 and ZIF8 were also measured. In addition to excess and absolute adsorption isotherms, we also introduce an “effective adsorption” which compares the amounts of gas adsorbed in a container with and without the adsorbent. At low temperatures, the maximal excess adsorption capacities of H2 and CH4 in MOF5 are found to be 10.3 wt % and 51.7 wt %, respectively, while they are only 4.4 wt % and 22.4 wt % in ZIF8. From the temperature-dependent isotherm data, the isosteric heat of adsorption (Qst) was also estimated. The excess Qst’s for the initial H2 and CH4 adsorption in MOF5 are ∼4.8 kJ/mol and ∼12.2 kJ/mol, respectively. We obtained similar Qst’s for ZIF8. We hope that the detailed isotherm curves reported here over a large temperature and pressure range will be a critical test for future grand canonical Monte Carlo simulations and force-field models.

I. Introduction High-capacity storage of gas fuels (such as H2 and CH4) is a crucial requirement for any successful fuel-cell related technologies. Physisorption on large surface-area materials is one of many approaches to solve this storage problem. Metal-organic framework (MOF) compounds, which consist of metal oxide/ nitride clusters connected by organic linkers, are promising materials that provide very high surface area and controllable pore sizes for energy storage, gas separation applications, or both.1-7 Accurate evaluation and rational development of the MOF materials require reliable adsorption data covering a large temperature and pressure range. Although gas adsorption data have been reported for quite a few MOF compounds,2,5,7-9 most of the measurements were done at liquid nitrogen temperature, room temperature (RT), or both. We believe that adsorption isotherm measurements at temperatures lower than 77 K are also indispensable because they can provide important information such as the maximal gas storage capacity of a material. For example, in this paper, we report isotherm curves at * Corresponding author: Dr. Taner Yildirim, email: [email protected]. † National Institute of Standards and Technology. ‡ University of Pennsylvania. § University of Maryland. | Oregon State University.

temperatures as low as 30 K, which effectively corresponds to the upper limit for the amount of gas that can be adsorbed under very large external pressures at ambient temperature. Similarly, high pressure isotherms make sure that we are also accessing pores that may be connected by narrow channels. To the best of our knowledge, we are not aware of any isotherm data for MOF materials at a temperature lower than 77 K. We also note that having high-pressure isotherms at multiple temperatures enable us to obtain an accurate isosteric heat of adsorption (Qst) as a function of gas coverage. Currently, almost all reported Qst’s are obtained from dual-temperature data, namely, at the normal boiling points of N2 and Ar (i.e., 77 and 87 K). Finally, we also note that, because of the well-defined crystal structure of the MOF materials, in addition to the standard Gibbs excess adsorption, one can also directly measure the “absolute adsorption” isotherm, which is not possible for amorphous materials such as activated carbon. Being able to measure the absolute isotherm is important for a direct comparison, validation, or both of computer simulations based on force-field models where the calculated quantity is actually the absolute adsorption. Besides excess and absolute adsorption, in this paper, we also introduce a third isotherm, the so-called “effective isotherm”, which compares the amount of H2 stored in a gas tank with and without the adsorbent. Hence, the “effective

10.1021/jp074889i CCC: $37.00 © 2007 American Chemical Society Published on Web 10/05/2007

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Figure 1. Crystal structures of (a) MOF5 and (b) ZIF8. MOF5 consists of ZnO4 clusters linked by BDC while ZIF8 consists of ZnN4 clusters linked by MeIM. For clarity, H atoms are not shown.

adsorption” provides a way to determine whether having the adsorbent in the container is beneficial at a given pressure and temperature. As we shall see below, for ZIF8, we find that the effective adsorption of hydrogen at room temperature is actually negative, indicating that having the adsorbent is actually detrimental (because of its own volume, which takes up extra space in the container). Here, we report a systematic study of the H2 and CH4 adsorption in MOF materials over a large temperature (30300 K) and pressure (up to 65 bar) range. We take MOF5 and ZIF8 as examples to demonstrate some characteristic parameters important for evaluating these crystalline porous materials. MOF5 is the most widely studied MOF material, which consists of ZnO4 clusters linked by 1,4-benzenedicarboxylate (BDC). ZIF8 is a newly discovered MOF compound, which consists of ZnN4 clusters linked by 2-methylimidazole (MeIM) and has an interesting zeolite-type structure with excellent chemical stability.6,7 The structures of these two MOF crystals are schematically shown in Figure 1. II. Experimental Methods A. Materials. MOF5 and ZIF8 samples were synthesized using the methods described in refs 10 and 7, respectively. These are the same samples that we used in our earlier study where we report the direct observation of hydrogen binding sites in these materials from Fourier-difference neutron powder diffraction.11,12 Before the isotherm measurements, the samples were degassed in vacuum at 423 K for 24 h to remove guest molecules. The typical weight loss after degassing is ∼4 wt % of the initial mass for MOF5 and ∼10 wt % for ZIF8. We used about 200 mg of the degassed samples in our isotherm measurements. B. Volumetric Method and Real Gas Equation of State. On the basis of the widely used volumetric method, we developed a fully computer-controlled Sieverts apparatus as schematically shown in Figure 2, operating in a sample temperature range of 4 to 1500 K and a pressure range of 0 to 100 bar. In the volumetric method, gas is admitted from a dosing cell with known volume to the sample cell; the gas pressure and temperature are controlled and recorded. Some of unique features of our setup are as follows. We use four pressure gauges13 with four different pressure ranges (20, 100, 500, and 3000 psi, respectively) to precisely measure the pressure.14 For isotherm measurements below room temperature, the sample temperature is controlled using a closed cycle refrigerator (CCR).15 Using a CCR has many advantages over a liquidnitrogen bath. In particular, the cold volume in the CCR (∼0.7 cm3 in our setup) does not change with time. It also allows us to choose any temperature from 12 to 300 K. The difference

Figure 2. Schematic view of the fully computer-controlled Sieverts apparatus that is used in this study. V0-V7 are computer-controlled high-pressure valves. N0-N3 are high-pressure needle valves which allow us to load/pump the dosing volume in a controlled manner to any target pressure. PG20-PG3000 are precision-pressure explosionproof (PPEP) transducers with 20, 100, 500, and 3000 psi range,13 respectively. They also have temperature sensors which allow us to monitor the temperature of the dosing volume. The sample cell can be placed in a high-temperature furnace for chemisorption studies or in a displex for low-temperature physisorption studies. The dosing volume of our apparatus is about 13 cm3 (including volumes of gauges). Most of the connections are welded to avoid any possible leak. The pipe connections are all VCR type with 1 µm filter gasket.

between the real sample temperature and the control setpoint is within 1 K over the whole operating temperature range. The sample cell and the dose cell are connected via 1/8′′ capillary tubing, which provides a sharp temperature interface between the sample temperature and the dose temperature (i.e., room temperature). The cold volumes for the empty cell were determined using He as a function of pressure at every temperature used for the real sample measurements and were used to calculate the sample adsorption. Since the adsorbed amount is deduced from the raw P-V-T data using a real gas equation of state, one critically important issue is the accuracy of the chosen equation of state (EOS) in terms of describing the real gas behavior within the desired temperature and pressure range. We found the widely used van der Waals (vdW) EOS16 works well at only ambient pressure and temperature, while it cannot satisfactorily describe the real gas behavior at low temperature and high pressure. Alternatively, for small gas molecules, the modified Benedict-Webb-Rubin (MBWR) EOS17 seems to work well over a wide temperature and pressure ranges. Figure 3 shows the n (mmol/cc) versus P plots for the ideal gas law, the vdW EOS and the NIST MBWR EOS at both RT and 77 K. At low T and high P, the difference between the three EOS’s is dramatic. Using an empty cell as a reference, we found the MBWR EOS best describes the real gas behavior of He, H2, and CH4. Using the vdW EOS, the empty cell appears to “adsorb” a large amount of H2 and CH4 (see Figure 3c). When the MBWR EOS is used, the nominal “empty-cell adsorption” turns out to be negligible (compared with the amount adsorbed by a real sample in our experiment). Therefore, in all our isotherm data reductions, the NIST MBWR EOS is used. The reason why the MBWR EOS works much better than other EOS’s can be understood as follows. The real gas behavior is relatively simple under ambient condition but gets rather complicated at high pressure and low temperature, because of the enhanced interaction between gas molecules. In the vdW

Hydrogen and Methane Adsorption

Figure 3. n vs P plots for (a) He and (b) H2 based on the ideal gas law, the widely used vdW EOS, and the most accurate NIST MBWR EOS at 250 K and 77 K, respectively. At low T and high P, the difference between the three EOS’s is dramatic. (c) H2 “adsorption” measurement on an empty stainless-steel cell at 77 K. Using the ideal gas law or the van der Waals equation, the empty cell appears to “adsorb” a large amount of H2. When the MBWR EOS is used, the nominal adsorption amount is negligible, as it should be. Hence, the MBWR EOS best describes the real gas behavior of H2.

EOS, only two coefficients are used to describe the P-V-T relationship for a real gas. In the ideal gas law, the interactions between molecules are totally ignored, and therefore there is no free parameter. Because of the simplistic assumptions in these two types of EOS’s, it is not possible to accurately describe the real gas behavior over a large P, T range. This is particularly the case for hydrogen because of its quantum nature, which becomes very important at low temperatures. In contrast, the MBWR EOS uses 33 parameters, which were derived by fitting the experimental P-V-T phase diagram data. The significantly larger number of parameters enables it to describe the real gas behavior over a much broader P, T range. In our apparatus and data analysis, we have effectively minimized the sources of error in a volumetric method18 (i.e., the errors in volume, pressure, sample mass, and temperature measurements, and errors due to the equation of state). On the basis of a comparison of the isotherm data from our instrument and those from a commercial one, we estimate the error bar of our isotherm data to be less than (2% and reproducibility within 0.5%. C. Definition of Excess, Absolute, and Effective Adsorption Capacity. The majority of the reported experimental adsorption data in the literature are excess adsorption isotherms. The Gibbs surface excess is the absolute amount of gas contained in the pores minus the amount of gas that would be present in the pores in the absence of gas-solid intermolecular forces. In contrast, computer simulations often directly yield the absolute amount adsorbed. Fortunately, for systems with known crystal structure, it is possible to directly measure the absolute adsorption in addition to excess adsorption. Hence, here, we report both isotherms. In addition to excess and absolute adsorptions, we also introduce another useful quantity

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Figure 4. Schematic view of “absolute”, “excess”, and “effective” adsorptions. The absolute adsorption is the total amount of gas introduced to the sample cell minus the amount outside the sample in the gas phase; thus, it accounts for the total amount of adsorbate molecules residing in pores. The excess adsorption is the absolute amount of gas contained in the sample pores less the amount of gas that would be present in the pores in the absence of gas-solid intermolecular forces. The effective adsorption is the amount of gas stored in a container with the adsorbent minus the amount of gas that can be stored in the same container without any adsorbent.

of adsorption, “effective adsorption”, which is the net gain of the amount of gas stored in a container with the help of an adsorbent material. It is basically the difference in the amount of gas in the sample cell with and without the adsorbent present. Hence, the effective adsorption can be negative, which indicates that having the adsorbent in the container makes the adsorption capacity worse because the adsorbent’s volume takes up more space than the volume of the gas adsorbed by the adsorbent. These three definitions of adsorptions (absolute, excess, and effective) are schematically shown in Figure 4. With sample mass and sample bulk density known, the skeleton density, and pore fraction of a porous material can be measured. Therefore, all three types of adsorption isotherms can be experimentally obtained provided that the crystal structure of the materials are known (which is the case for MOF5 and ZIF8). To sum up, the amount of gas adsorbed at the kth point of an isotherm, nkabs(T), is given by k

nkabs(T) )

{EOS(Pidose,Vdose,TRT) - EOS(Pif,Vdose,TRT)} ∑ i)1 EOS(Pkf ,VLine,TRT) - EOS(Pkf ,VCold,T) (1)

where Pidose is the dosing pressure at ith isotherm point, which is reduced to Pif after the sample valve (i.e., V1 in Figure 2) is opened. Vdose and TRT are the dosing volume and room temperature at which the measurement is taken, respectively. VLine is the volume of the high-pressure tubing from the sample valve V1 (see Figure 2) to the sample cell. EOS is the real gas equation of state that we used to obtain the amount of gas at a given pressure, volume, and temperature. The first line in eq 1 gives the total amount of gas that is transferred from the dosing volume to the sample cell and line. Subtracting the amount of gas in the line volume and dead volume (second line in eq 1) yields the net amount of gas that is adsorbed in the sample. VCold is the cold volume of the sample cell minus the sample dead volume. For the three different isotherms defined above, the cold volume is taken as follows:

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Vcell

for effective M VCold ) Vcell for absolute F Vcell - VSkeleton for excess

(2)

where Vcell is the cell volume, VSkeleton is the sample skeleton volume measured by He at room temperature, and M/F is the bulk volume of the sample obtained from its mass (M) and bulk density (F). III. Results and Discussion We first determined the skeleton densities of MOF5 and ZIF8. The volumes of the sample cell before and after loading the sample into the cell were measured at room temperature using He as the calibration gas. Assuming the excess adsorption of helium is negligible at room temperature and low pressure, the volume difference is the sample skeleton volume.19 The skeleton density of MOF5 is found to be ∼1.8 g/cm3. Since the bulk density of MOF5 is 0.59 g/cm3, the He calibrated specific pore volume is 66%. These compare reasonably well with the theoretical skeleton densities and pore fraction of 1.9 g/cm3 and 59%, calculated using a He probe assuming van der Waals interactions. Similarly, the skeleton density of ZIF8 is determined to be ∼1.4 g/cm3, and the He calibrated specific pore volume is 35%, in reasonable agreement with the theoretical skeleton densities and pore fraction of 1.3 g/cm3 and 30%. After having determined the skeleton densities of the samples, we are able to measure the three types of isotherms as defined above over a large temperature and pressure range. Figures 5 and 6 show the absolute, excess, and effective adsorption isotherms for H2 and CH4 in MOF5 and ZIF8, respectively. The desorption data were also collected, yet are not shown here since the adsorption and desorption processes for both MOF5 and ZIF8 were completely reversible with no hysteresis observed. This indicates that the adsorption/desorption process is very fast and the system reaches equilibrium within the measurement time for each data point (3 min in our case). Such fast adsorption/ desorption is typical for physisorption in porous materials with large surface area, and it is one of the big advantages of physisorption materials compared with hydride materials, where the adsorption/desorption kinetics are often very slow. Comparing the three types of adsorption isotherms shown in Figures 5 and 6, we note that, at low pressure and high temperature, the difference between the three adsorptions is negligible, while at high pressure or low temperature the difference becomes substantial. With increasing pressure, the absolute adsorption approaches a saturation value but the excess and effective adsorption isotherms reach a maximum and then start to decrease. This significant difference is proportional to the pore volume and the pressure, and it originates from the different definitions of adsorption (see Figure 4). The maximum in the excess adsorption isotherm occurs at the point where the gas densities in the sample pore and the bulk gas are increasing at the same rate with respect to pressure, so that an increase in pressure has no effect upon the amount adsorbed. At higher pressure, the gas density in the sample pore starts to saturate while the bulk gas density keeps increasing, resulting in a negative gain in the excess amount adsorbed. In addition to the pore volume considered in the excess adsorption, the effective adsorption also takes into account the sample skeleton volume in the gas container. Therefore, it clearly tells which pressure range is optimal at a certain temperature and is a very useful characteristic for a material in practical gas storage applications. For example, at RT, the H2 effective adsorptions in MOF5 and ZIF8 are nearly zero and negative, respectively, indicating that

Figure 5. Absolute, excess, and effective adsorption isotherms of H2 (left) and CH4 (right) in MOF5 over a broad pressure and temperature range.

these materials have no positive contribution to gas storage at this temperature compared with the pressurized gas tank. Note that this information is not readily available from the absolute or excess isotherm data plot since both of them show a positive quantity of adsorption. Next, we discuss the temperature variation of the isotherm curves, which provide important information about the possible maximum adsorption capacity and the strength of the binding potential of a given adsorbent. For hydrogen in MOF5 at 30 K, where hydrogen is still in the gas phase (up to ∼4 bar), we obtain a record high value of 10.3 wt %. Such a high value of excess adsorption at low T indicates that MOF5 has enough room to adsorb a large number of hydrogen molecules. Recently, we have undertaken a neutron diffraction Fourier difference study11 to identify where all of these hydrogen molecules are adsorbed. Interestingly, due to hydrogen-MOF lattice interactions, H2 molecules form small nanocages trapped in the interstitial sites of the MOF5 lattice. From this neutron study, we have identified that approximately 4 wt % out of the 10.3 wt % adsorbed hydrogen molecules are bonded near the ZnO4-

Hydrogen and Methane Adsorption

Figure 6. Absolute, excess, and effective adsorption isotherms of H2 (left) and CH4 (right) in ZIF8 over a broad pressure and temperature range.

metal clusters and the rest are located near or on top of the benzene linkers and the ZnO2 edges. With increasing temperature, we observe that the adsorption amount decreases very rapidly, reaching 1.35 wt % at 1 bar, 77 K for MOF5. The maximum hydrogen adsorption at 77 K occurs around 35 bar in the excess adsorption, and it is about 5.75 wt %. At 300 K, the adsorption isotherm is almost linear and therefore shows no saturation with increasing pressure. It is around 0.3 wt % at 60 bar. However, we note that the effective adsorption for H2 in MOF5 at 300 K is almost zero, indicating that having the MOF5 adsorbent in a gas tank at room temperature does not provide any advantage compared with an empty hydrogen tank. For methane in MOF5 (see the right panel in Figure 5), we obtained isotherms similar to those for H2, except with higher adsorption/desorption temperatures. Since the methane molecule is larger and has more atoms than hydrogen, the gas-host interaction is also larger, and therefore it is expected to be adsorbed at higher temperatures. For MOF5, we obtained an uptake of 13.5 wt % CH4 at 300 K and 36 bar, which corresponds to ∼110 cm3(STP)/cc in terms of volume capacity.

J. Phys. Chem. C, Vol. 111, No. 44, 2007 16135 This is not too far away from the DOE’s target of 180 cm3(STP)/cc set for practical methane storage.20 The adsorption values that we report for MOF5 at liquid nitrogen and room temperature generally agree with previously reported results, that is, 1.32 wt % H2 at 1 bar5 and 4.3 wt % H2 under 30 bar8 at 77 K and 16.2 wt % CH4 at 36 atm and 298 K2. The ∼20% discrepancy is likely due to the difference in sample quality and accuracy of measurement method. We emphasize that our low temperature and high pressure isotherms on MOF5 provide valuable information on the maximal excess adsorption capacities of H2 and CH4, which are 10.3 wt % and 51.7 wt %, respectively. The absolute adsorbed amounts are 10.6 wt % and 52.0 wt %, approximately corresponding to 41 H2 and 24 CH4 per MOF5 formula unit21 (i.e., 4 Zn). The maximal H2 and CH4 uptakes agree with previous neutron diffraction experiments.11,22 The temperature dependence of the H2 and CH4 adsorptions in ZIF8 (shown in Figure 6) are quite similar to MOF5. However, the maximum hydrogen adsorption capacity at 30 K is much smaller, only 4.4 wt %. At 77 K, this maximum adsorption capacity rapidly goes down to 1.3 wt % H2 at 1 bar and 3.3 wt % H2 at 30 bar. At room temperature and 60 bar, we observed a little excess adsorption (0.13 wt %) and negative effective adsorption, indicating that ZIF8 does not help to store more hydrogen compared with an empty H2 tank. ZIF8 can also adsorb CH4 up to 7.0 wt % at 300 K and 36 bar. From the isotherms at low temperature and high pressure, the maximal excess adsorption capacity of CH4 in ZIF8 was determined to be 22.4 wt %. The much smaller maximal gas storage capacity of ZIF8 is mainly due to its much smaller specific pore volume compared with that of MOF5. As mentioned earlier, the surface excess is the number of molecules in the pores (absolute adsorption) minus the number that would be present in the pore without adsorption at the density of the bulk gas. Similarly, the excess adsorption heat (Qst) is the actual binding energy plus the entropy of the molecules in the pores minus the energy and the entropy of adsorbate gas occupying nonadsorbing pores of the same volume. Therefore, it reflects the magnitudes of the binding energy of the adsorption sites in the porous materials. From the isotherm data of MOF5 and ZIF8, the excess isosteric heat of adsorption for both H2 and CH4 can be calculated using the modified Clausius-Clapeyron equation.23 We found that the excess Qst’s for the initial H2 and CH4 adsorption are ∼4.8 kJ/ mol and ∼12.2 kJ/mol for MOF5 and ∼4.5 kJ/mol and ∼12.0 kJ/mol for ZIF8, respectively. Note that the CH4 heat of adsorption is much larger than that of H2, simply because CH4, as a larger molecule, is expected to have more interaction (or stronger binding) with the adsorbent. In fact, assuming that the same atom-atom van der Waals potential works for both CH4 and H2, we expect that the Qst for CH4 to be at least twice that for H2 since CH4 have twice the number of hydrogen atoms. This is indeed the case for our reported Qst’s for both MOF5 and ZIF8. Furthermore, assuming that the H and C interactions are roughly the same, we expect that Qst for CH4 should be 2.5 times larger than that for H2. This crude estimate gives a Qst ) 12 kJ/mol for CH4 in MOF5 (compared with the measured 12.2 kJ/mol) and Qst ) 11.25 kJ/mol for CH4 in ZIF8 (compared with an observed value of 12.0 kJ/mol). Finally, we note that our reported initial Qst’s have two major sources of uncertainty: (1) the error bar of the isotherm data, particularly in the low P range and (2) the error induced by extrapolating the isotherm data to the limit of zero gas adsorption. From a trial and error analysis, we estimate the error

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Figure 7. (a) Excess adsorption isotherm data of H2 in MOF5, used in the heat of adsorption calculation. Dotted lines indicate P-T data at fixed amounts of adsorption. (b) ln P vs 1/T plot of H2 in MOF5 at various wt %. According to the Clausius-Claperyron equation, the isoteric heat of adsorption Qst ) -slope × R. (c) Qst plot for H2 adsorption in MOF5, as derived from b. (d) Qst plot for CH4 adsorption in MOF5.

bar of our initial Qst to be ∼ (5%. For H2 in MOF5, several groups have previously reported the initial heat of adsorption. Our number of ∼4.8 kJ/mol compares reasonably well with previously reported 4.1 kJ/mol8 and 4.7-5.2 kJ/mol,24 both obtained from volumetric measurements. Interestingly, an adsorption energy of ∼3.5 kJ/mol was found in an IR spectroscopy measurement.25 The relatively small value could be due to fact that what is reported in the IR experiment is not the actual “isosteric” heat of adsorption since the adsorbed amount of hydrogen decreases with increasing temperature in such an experiment. For the isosteric heat of adsoption, one has to determine the pressure-temperature (P,T) pairs at a given constant coverage and then apply the van’t Hoff equation to these (P,T) points in order to get the Qst at that coverage. This could be one possible reason for the ∼25% disagreement between Qst’s obtained by IR and the volumetric technique. Our isotherm data at a series of temperatures also enable us to calculate the heat of adsorption as a function of the adsorbed amount as shown in Figure 7, while usually only the initial heat of adsorption is reported in the literature. We found that the heat of adsorption decreases with increasing gas adsorption at the initial stage, and then increases at a very high adsorption amount. From our previous neutron diffraction study,11,12 we know that there exist various well-defined binding sites in these crystalline porous materials. Therefore, the variation of Qst at the initial stage indicates that the binding sites with strongest binding energies are occupied first, followed by gas adsorption on the less favorable binding sites. Interestingly, for methane, Qst first slowly decreases and then increases rapidly with methane loading. This indicates that, at high-concentration loading, methane-methane interactions in addition to methanehost lattice interactions become important. In fact, according to our recent neutron scattering study,22 we have observed that

both MOF5 and ZIF8 host lattices undergo a phase transition to a lower symmetry structure with increasing methane loading, probably to optimize both the gas-gas and gas-host interactions simultaneously. The observed increase of Qst with increasing coverage is consistent with this recent neutron study.22 From our isotherm measurements, the heats of adsorption of H2 in MOF5 and ZIF8 are relatively low, comparable to that of porous carbon materials. This means that the nature of gas adsorption in these materials is mainly physisorption. For practical H2 storage, a much higher heat of adsorption (∼20 kJ/mol) is desired. Thus, bare MOF materials might not be good candidates for hydrogen storage. Various functionalizations (e.g., metal doping) should be actively pursued in the future to enhance the H2 binding energy in these materials. IV. Conclusions In summary, we reported three types of adsorption isotherms for H2 and CH4 in MOF5 and ZIF8 over a large temperature and pressure range. We found that the widely used vdW EOS is inaccurate at high pressure and low temperature. The MBWR EOS is found to best describe the real gas behavior of He, H2, and CH4 and should be used in the data reduction. These results show that, in order to extract reliable isotherm data, choosing a proper real gas equation of state is crucial in a volumetric method. The maximal excess adsorption capacities of H2 and CH4 in MOF5 are measured to be 10.3 wt % and 51.7 wt %, respectively. ZIF8 has much less pore fraction and thus much more modest maximal H2 and CH4 capacities of 4.4 wt % and 22.4 wt %, respectively. Qst’s for the initial H2 and CH4 adsorption in MOF5 are ∼4.8 kJ/mol and ∼12.2 kJ/mol, respectively. The values for ZIF8 are comparable. All of these results provide a direct clue about the potential of using MOF

Hydrogen and Methane Adsorption materials for various gas storage applications. Also, our isotherms over a broad temperature range provide a complete data set for the theorists to test theoretical models (molecular dynamics or Monte Carlo methods) used for adsorption simulation in these materials. We propose that various functionalizations are essential to make metal-organic framework materials practical for energy storage. Acknowledgment. We acknowledge partial DOE support from EERE Grant DE-FC36-04GO14282 (W.Z. and T.Y.) and BES Grant DE-FG02-98ER45701 (T.Y.). References and Notes (1) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276. (2) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469. (3) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705. (4) Chae, H. K.; Siberio-Perez, D. Y.; Kim, J.; Go, Y. B.; Eddaoudi, M. A.; Matzger, J.; O’Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523. (5) Rowsell, J.; Millward, A.; Park, K.; Yaghi, O. M. J. Am. Chem. Soc. 2004, 126, 5666. (6) Huang, X. C.; Lin, Y. Y.; Zhang, J. P.; Chen, X. M. Angew. Chem., Int. Ed. 2006, 45, 1557. (7) Park, K. S.; Ni, Z.; Cote, A. P.; Choi, J. Y.; Huang, R.; UribeRomo, F. J.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10186. (8) Dailly, A.; Vajo, J. J.; Ahn, C. C. J. Phys. Chem. B 2006, 110, 1099. (9) Poirier, E.; Chahine, R.; Be´nard, P.; Lafi, L.; Dorval-Douville, G.; Chandonia, P. A. Langmuir 2006, 22, 8784.

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