J. Phys. Chem. C 2008, 112, 7079-7083
7079
Hydrogen Storage Properties of Rigid Three-Dimensional Hofmann Clathrate Derivatives: The Effects of Pore Size Jeffrey T. Culp,*,†,‡ Sittichai Natesakhawat,† Milton R. Smith,† Edward Bittner,† Christopher Matranga,† and Bradley Bockrath† National Energy Technology Laboratory, United States Department of Energy, P.O. Box 10940, Pittsburgh, PennsylVania 15236, and Parsons, P.O. Box 618, South Park, PennsylVania 15129 ReceiVed: NoVember 19, 2007; In Final Form: February 22, 2008
The effects of pore size on the hydrogen storage properties of a series of pillared layered solids based on the M(L)[M′(CN)4] structural motif, where M ) Co or Ni, L ) pyrazine (pyz), 4,4′-bipyridine (bpy), or 4,4′dipyridylacetylene (dpac), and M′ ) Ni, Pd, or Pt, has been investigated. The compounds all possess slitlike pores with constant in-plane dimensions and similar organic functionality. The pore heights vary as a function of L and provide a means for a systematic investigation of the effects of pore dimension on hydrogen storage properties in porous materials. Hydrogen isotherms were measured at 77 and 87 K up to a pressure of 1 atm. The pyz pillared materials with the smallest pore dimensions store hydrogen at a pore density similar to that of liquid hydrogen. The adsorbed hydrogen density drops by a factor of 2 as the relative pore size is tripled in the dpac material. The decreased storage efficiency diminishes the expected gravimetric gain in capacity for the larger pore materials. The heats of adsorption were found to range from 6 to 8 kJ/mol in the series and weakly correlate with pore size.
Introduction The hydrogen storage properties for a series of pillared layered metal cyanide derivatives have been investigated at cryogenic temperatures in an effort to determine the effects of pore dimensions and metal. The materials examined are a series of layered M(L)[M′(CN)4] compounds where L is a rigid dipyridyl derivative of varying length. The structures of the materials are described in Figure 1.1-3 Each member in the M(L)[M′(CN)4] series possesses a narrow slit pore whose x,ydimension is fixed at ∼5 Å × 5 Å by the in-plane structure of the M[M′(CN)4] network. By substitution of the interplane bridging ligand L, a series of porous solids results in which only the pore height is varied. As such, these materials provide a rare opportunity to determine the effects of increasing a pore dimension in a uniaxial fashion while keeping the in-plane dimensions small and constant. Additionally, the identity of the metals may be changed while preserving the framework structure. These variations allowed for a systematic investigation of the influence of both metal and pore size. Much of the experimental and theoretical investigations concerning the effect of pore size on hydrogen adsorption has been focused on carbon-based materials.4-11 For hydrogen in particular, an ideal slit pore width of 5.6 Å has been calculated to be the optimum geometry for the most efficient storage since this dimension provides the opportunity to adsorb two layers of H2 molecules.10 Experimental results on a series of porous carbons have supported this prediction.5 Recently, the interest in hydrogen storage has experienced a shift toward porous coordination polymers and metal organic frameworks since these materials provide a wide range of pore * Corresponding author. E-mail:
[email protected]. Phone: 412-386-5393. Fax: 412-385-4542. † National Energy Technology Laboratory. ‡ Parsons.
Figure 1. Idealized space-filling models of the structures of (A) M(pyz)[M′(CN)4], (B) M(bpy)[M′(CN)4], and (C) M(dpac)[M′(CN)4]. The depictions are approximations based on the reported crystal structure of Fe(pyz)[Pt(CN)4] (ref 2). The pyz ligand in Fe(pyz)[Pt(CN)4] is disordered about a C4V axis, and the depictions shown in the figure assume a fixed orientation of the pyz, bpy, and dpac ligands to highlight the presence of slit pores of varied dimensions. In (D), the M(L)[M′(CN)4] unit cell is shown for L ) pyrazine. For clarity, a “pore” in the present context refers to the free volume in a unit cell.
geometries and pore functionalities.12-15 The high level of synthetic control in these systems has led to a wide range of materials from those with very small pores which show selective adsorption of gases to very large pore materials with exceptional surface areas and pore volumes.16-22 When taken in total, the numerous reports of hydrogen adsorption in these porous coordination polymers have covered a very wide spectrum of pore shapes and sizes; however, systematic studies on the effects of pore dimension are rarer,23 since the investigations require a series of materials with similar motifs whose unit cell dimension can readily be varied in a systematic fashion.24,25 One particular synthetic strategy that is well-suited for the development of a series of materials with a systematic variation in pore geometry is through the use of pillared layered solids. These materials are rigid three-dimensional solids formed by the extended bridging of planar inorganic networks, typically accomplished with a rigid, linear diaza ligand such as pyrazine or 4,4′-bipyridine.1,25-30 We have chosen to take advantage of the structural control offered by the family of M(L)[M′(CN)4] polymeric metal
10.1021/jp710996y CCC: $40.75 © 2008 American Chemical Society Published on Web 04/08/2008
7080 J. Phys. Chem. C, Vol. 112, No. 17, 2008 cyanides shown in Figure 1. The structure of the Fe(pyrazine)[Pt(CN)4] analogue was reported by Niel et al.2 It was also reported that the substitution of Fe2+ with Co2+ or Ni2+ and/or Pt2+ by Ni2+ or Pd2+ led to an isostructural series of materials which have been studied for their interesting spin-crossover behavior.3 These materials are, in essence, rigid threedimensional pyrazine-bridged derivatives of the well-known Hofmann clathrates. Prior to the reports on M(pyrazine)[M′(CN)4], Mathey et al. reported in a brief communication that pillared Ni(L)[Ni(CN)4] (L ) pyrazine, 4,4′-bipyridine) materials could be formed by reaction of L with anhydrous Ni(CN)2.1 The structures and host-guest behaviors toward small organic molecules for these materials were qualitatively described; however, the gas storage behaviors of these pillared materials have yet to be reported. The potential to systematically vary the pore dimensions in these materials in a uniaxial fashion, i.e., increase z, while keeping the x,y-dimensions constant, provides a rare opportunity to investigate the hydrogen storage properties as a function of pore size within a family of materials with very similar ligand functionalities. Of particular interest are the ∼5 Å × 5 Å pore dimensions along the x,y-axes in these materials which are predicted to be ideal for significant interaction with adsorbed hydrogen.10,31 With these goals in mind, a series of M(L)[M′(CN)4] compounds where M is Ni2+ or Co2+, M′ is Ni2+, Pd2+, or Pt2+, and L is either pyrazine, 4,4′-bipyridine, or 4,4′-dipyridylacetylene has been synthesized. These materials thus provide a series of porous solids containing slitlike pores with nearly constant x,y-dimensions of approximately 5 Å × 5 Å and varying z-dimensions of approximately 5, 9, and 12 Å, respectively. The hydrogen uptake in these materials has been measured at 77 and 87 K up to a pressure of 1 atm. Experimental Section Synthesis. All chemicals were purchased from Sigma-Aldrich and used as received. The Co(pyrazine)[M(CN)4]‚2.5H2O compounds (M ) Ni, Pd, Pt) were prepared as previously described.3 The 4,4′-dipyridylacetylene was synthesized by a previously reported procedure.32 The Ni(H2O)2[M(CN)4]‚4H2O compounds (M ) Ni, Pd) were prepared by slowly mixing 0.1 M aqueous solutions of K2[M(CN)4](hydrate) and Ni(NO3)2‚ 6H2O and refluxing the resulting mixture overnight. The products were subsequently filtered, washed well with water, and air-dried. Purity was verified by determination of weight percent of metal oxides by thermogravimetric analysis (TGA) in dry air at 500 °C. The Ni(L)[M(CN)4] compounds, L ) pyrazine (pyz), 4,4′-bipyridine (bpy), and 4,4′-dipyridylacetylene (dpac), M ) Ni, or Pd, were prepared by a modification of a previously reported procedure for the preparation of Ni(bpy)[Ni(CN)4].1 Specifically, a 50 mL flask was charged with 3 mmol of Ni(H2O)2[M(CN)4]‚4H2O and the solid dehydrated at 140 °C under a N2 purge for 4 h. The sample was then cooled to room temperature, and 3 mmol of L dissolved in 30 mL of xylenes (L ) bpy, dpac) or acetone (L ) pyz) was added, and the mixture refluxed overnight. The resulting solids were then filtered and washed with excess solvent. The yields were quantitative. Prior to gas adsorption measurements, the highboiling xylene guests in Ni(L)[M(CN)4]‚(xylene), L ) bpy, dpac were exchanged with acetone via Soxhlet extraction overnight. Because the materials slowly lose solvent guests in air, X-ray diffraction (XRD) patterns were collected on freshly prepared materials with minimal guest loss. The weight percent metal in each sample was determined as the mass of metal oxide by TGA
Culp et al. in dry air at 500 °C. The structural integrity of the materials after the gas adsorption measurements was verified by powder XRD. Instrumentation. Gas adsorption measurements were performed using a Quantachrome Autosorb 1C volumetric isotherm apparatus on samples (∼100 mg) degassed under dynamic vacuum for 18-24 h at 90 °C. A solid quartz filler rod was used to reduce the void volume in the sample cell. All isotherms reported are in terms of excess H2 adsorption. Powder XRD measurements were performed on a PANalytical X’Pert Pro MPD powder diffractometer having a Θ-Θ configuration, a Cu X-ray source operated at 45 kV and 40 mA, and an X’Celerator detector with a monochromator. Patterns were recorded over a 2Θ range of 5-50° using a step size of 0.02 deg 2Θ and a scan step time of 50 s/deg 2Θ. Thermogravimetric analyses were performed on ∼5 mg samples using a PerkinElmer TGA7 thermogravimetric analyzer under a dry air purge of 20 mL/min. The samples were ramped to 500 °C at a rate of 5 °C/min. Isotherm Fits and Calculation of H2 Isosteric Heats of Adsorption. Isotherms were fit to the Langmuir-Freundlich (L-F) equation. The L-F equation allowed H2 storage values at saturation coverage to be estimated. It also allowed for a smooth interpolation between experimental data points for isosteric heat calculations. The coverage as a function of pressure in the L-F equation is given by
B(P(1/t) ) Q ) Qm (1 + B(P(1/t))) where Q ) amount of gas adsorbed, Qm ) amount of gas adsorbed at saturation coverage, P ) pressure, and B and t are constants. The L-F equation easily rearranges to yield the pressure as a function of Q/Qm at a constant temperature. From this equation and the fits obtained from the experimental data, plots of ln(P) as a function of Q could be generated from the experimental data at 77 and 87 K. Isosteric heats (qst) at a given coverage were then calculated using the differential equation qst ) -R[∂ ln(P)/∂(1/T)]n (see the Supporting Information). Results By appropriate substitution of the metal ions and pillar ligands, a series of pillared nickel cyanides has been prepared. Structural models of the M(L)[M′(CN)4] samples based on the reported structure of Fe(pyz)[Pt(CN)4] are given in Figure 1.2 The obtained XRD patterns for the M(L)[M′(CN)4] series agree very well with those calculated for a series of materials based on the Fe(pyz)[Pt(CN)4] framework, wherein the a- and b-axes remain constant and the c-axis varies according to the length of the pillar ligand L (see the Supporting Information). The interlayer spacings of 11.2 Å for the bpy derivative and 13.7 Å for the dpac derivative agree well with other reported structures containing these ligands. Due to the rotational degeneracy of the pillar ligands, the open channel views shown for the samples in Figure 1 are an idealized depiction of the slit-pore structure in these materials. A “pore”, in the usual sense, is not well defined in these materials because of the disorder in the orientation of the pillar ligands. In order to allow a convenient normalization of the adsorption over the entire series of samples, a pore is simply defined as the free volume within a unit cell volume. The unit cell for pyrazine is shown in Figure 1. By this definition, the pore dimensions are estimated to be 5 Å × 5 Å × 5 Å based on the unit cell parameters and taking into account approximate
Hydrogen Storage Properties of Hofmann Clathrates
J. Phys. Chem. C, Vol. 112, No. 17, 2008 7081
TABLE 1: Pore Volumes and H2 Adsorption Data for M(L)[M′(CN)4]
a
compound
d(001) (Å)
est pore vol, (cm3/mol)a
N2 pore vol (cm3/mol)
H2 per pore at 1 atm
H2 per pore at sat.
H2 heat of ads kJ/mol
Co(pyz)[Ni(CN)4] Co(pyz)[Pd(CN)4] Co(pyz)[Pt(CN)4] Ni(pyz)[Ni(CN)4] Ni(bpy)[Ni(CN)4] Ni(bpy)[Pd(CN)4] Ni(dpac)[Ni(CN)4]
7.0 7.0 7.0 7.0 11.2 11.2 13.7
65 65 65 65 150 150 215
73 70 78 70 136 131 237
2.4 2.5 2.4 2.2 3.0 3.0 3.2
2.8 2.8 2.7 2.7 3.5 3.5 4.5
7.2 7.8 7.6 7.2 7.5 7.0 6.0
Estimated with the software Platon using a probe radius of 1.5 Å.
carbon and nitrogen van der Waals radii. The free volume estimated by the crystallographic program Platon, using the CALC VOID function with a 1.5 Å probe radius, is 29% of the unit cell volume.33 This same correlation of the free volume per unit cell volume to a “pore” volume is applied to the bpy and dpac samples as well. Estimations of the porosities in these materials by the same technique as used in the pyz derivative gives values of 42% and 49% of the unit cell volumes for the bpy and dpac samples, respectively (see the Supporting Information for details). The pore volumes of the samples as calculated from the N2 adsorption isotherms at 77 K are compared in Table 1 with the pore volumes estimated with Platon using a 1.5 Å probe radius. The estimated volumes agree well with the calculated N2 pore volumes for the all of the samples. The excess H2 adsorption isotherms at 77 K for the series of Co(pyz)[M′(CN)4] compounds are shown in Figure 2A. The isotherms all show a relatively rapid increase at low pressure as expected for a small pore material. The slightly steeper slopes in the low-pressure regions of the isotherms for the Pd and Pt materials indicate a small enhancement in H2 uptake over that of the Ni analogue. This enhancement may be due to some interaction of the adsorbed H2 with the square-planar metal ions; however, a similar enhancement is not observed in the bpy pillared analogues of the Ni and Pd materials. Taking also into consideration the relative inertness of the square-planar dz2 orbitals, the variations in the isotherms are more likely the result of subtle differences in crystallite size and coherence. In Figure 2B, the excess H2 uptake has been plotted as mol of H2 per mol of Co(pyz)[M′(CN)4] in order to normalize the uptakes to the sample densities. From the data organized in Table 1, it can be seen that the total uptake of H2 for each sample is nearly identical, verifying the structural integrity of the series. Fitting of the isotherms with the L-F equation yields saturation coverages for all three samples of ∼2.7 H2/Co(pyz)[M′(CN)4]. The excess H2 adsorption isotherms at 77 K for the Ni(L)[Ni(CN)4] series are plotted in Figure 3. All three materials show similar gravimetric hydrogen uptakes at 1 atm; however, the shapes of the isotherms are significantly different. The pyz and bpy materials show a more rapid uptake at low pressure and a shallow slope at 1 atm, whereas the hydrogen uptake in the dpac material rises more slowly. Fits to the isotherms with the L-F equation yield saturation coverages of 2.7, 3.5, and 4.5 mol of H2 per mol of Ni(L)[Ni(CN)4] for L ) pyz, bpy, and dpac, respectively. Converted to weight percent, the saturation coverages are 1.76, 1.84, and 2.24 wt %, respectively. Isotherms were also measured at 87 K for the three Ni(L)[Ni(CN)4] samples, and their calculated isosteric heats of adsorption are plotted in Figure 4. The heats of adsorption for the pyz and bpy samples are nearly identical despite the nearly 2-fold difference in pore height. The dpac sample shows a heat of adsorption ∼1 kJ/mol lower than the pyz and bpy samples.
Discussion The reaction of a heterogeneous mixture of anhydrous Ni[Ni(CN)4] with pyz, bpy, or dpac in acetone or xylenes is an effective method for generating high-quality materials of M(L)[M′(CN)4], with L ) pyz, bpy, or dpac with finely tuned pore dimensions. It is also worth noting that a similar synthetic strategy has been used for the preparation of MO3(L)0.5 where M ) Mo, W and L ) pyz, bpy, which are a similar family of pillared layered solids.28,30,34 The series of M(L)[M′(CN)4] materials can be thought of as rigid three-dimensional derivatives of the well-known Hofmann clathrates Ni(NH3)2[Ni(CN)4]‚(guest). The Hofmann clathrate and its analogues have been widely studied for their host-guest behavior toward small aromatic molecules.35 Pillaring the structures with the linear diaza bridging ligands greatly enhances
Figure 2. (A) Hydrogen adsorption isotherms measured at 77 K plotted as excess H2 adsorbed vs pressure for the series Co(pyz)[M(CN)4], M ) Ni2+, Pd2+, Pt2+. The solid line is the L-F fit to the data. (B) Hydrogen adsorption at 77 K normalized as excess mol of H2 adsorbed per mol of Co(pyz)[M(CN)4] demonstrating the reproducibility of the hydrogen uptake between the samples. Key: blue diamonds, M ) Ni; pink squares, M ) Pd; yellow triangles, M ) Pt.
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Culp et al.
Figure 3. Hydrogen adsorption isotherms measured at 77 K plotted as excess H2 adsorbed vs pressure for the series Ni(L)[Ni(CN)4]. The solid line is the L-F fit to the data. Key: blue diamonds, L ) pyz; pink squares, L ) bpy; red circles, L ) dpac.
Figure 4. Isosteric heats of adsorption as a function of coverage for Ni(L)[M(CN)4]. Key: pink squares, L ) bpy, M ) Ni; blue diamonds, L ) pyz, M ) Ni; green circles, L ) bpy, M ) Pd; yellow triangles, L ) dpac, M ) Ni.
TABLE 2: Hydrogen Storage Densities and Surface Areas in M(L)[M′(CN)4] Compounds
compound
H2 pore density (g/L)a,b
N2 BET S.A. (m2/mmol)
H2 S.A. (m2/mmol)b
ratio H2 to N2 S.A.
Co(pyz)[Ni(CN)4] Co(pyz)[Pd(CN)4] Co(pyz)[Pt(CN)4] Ni(pyz)[Ni(CN)4] Ni(bpy)[Ni(CN)4] Ni(bpy)[Pd(CN)4] Ni(dpac)[Ni(CN)4]
80 74 72 76 50 54 38
127 122 138 124 234 220 398
215 210 204 204 267 266 346
1.7 1.7 1.5 1.6 1.1 1.2 0.9
a Using N2 pore volumes at 77 K. b Calculated from the saturation coverage of H2 at 77 K from the L-F fit and assuming a 12.3 Å2 crosssectional area for H2.
the structural robustness of the materials while still leaving an available pore system for host-guest interactions. In order to determine an accurate correlation between the pore size and hydrogen adsorption properties within a series of similar materials, it is important that the pore dimensions be known. Pore volume is often determined from the N2 adsorption isotherms at 77 K and the assumption that the density of N2 in the pores is equivalent to liquid N2. The pore volumes determined from the N2 adsorption data were compared to the pore volumes determined by the crystallographic program Platon using a probe radius of 1.5 Å on structure models derived from the Fe(pyz)[Pt(CN)4] structure using typical bond lengths and
angles (Table 1). Even though the structures could not be completely solved due to the quality of the diffraction patterns, calculations on the qualitative models with a 1.5 Å probe radius gave an estimated porosity that was reasonably close to the experimental N2 data over all the samples. This agreement strengthens the validity of the models assumed for this series. Calculations of the density of adsorbed hydrogen at saturation using the N2 pore volumes are listed in Table 2. In the case of the pyz samples, the adsorbed pore density of hydrogen is comparable to that of liquid hydrogen which is ∼70 g/L at 1 bar and 20 K. Similar high pore densities of adsorbed hydrogen have been reported in other materials with similar pore sizes.5,8,18,23 A correlation between the packing density of H2 and available pore volume can be deduced by following the trend over the three samples in the Ni(L)[Ni(CN)4] series. The powder diffraction pattern for the three samples show that the in-plane spacings for the three materials vary by
