8940
J. Phys. Chem. B 2000, 104, 8940-8945
Molecular Geometry-Sensitive Filling in Semi-Rectangular Micropores of Organic-Inorganic Hybrid Crystals Di Li†,‡ and Katsumi Kaneko*,† Department of Chemistry, Faulty of Science, Chiba UniVersity, 1-33 Yayoi, Inage, Chiba 263-8522, Japan, and New Energy and Industrial Technology DeVelopment Organization (NEDO), 3-1-1 Higashi Ikebukuro, Toshima-ku, Tokyo 170-6028, Japan ReceiVed: February 18, 2000; In Final Form: May 17, 2000
The combination of copper perchlorate, 2,3-pyrazinedicarboxylate (pzdc) and one of pyrazine (pyz), 4,4′bipyridine (bpy), or trans-1,2-bis(4-pyridyl)ethylene (bpe) yields a series of Cu complex-assembled compounds [Cu2(pzdc)2(PL)‚xH2O]n (PL ) pyz, bpy, or bpe). These compounds were characterized by elemental microanalysis, X-ray diffraction, thermogravimetric analysis, and gas adsorption. The XRD data show that the Cu complex-assembled solids possess a three-dimensional structure with one-dimensional semi-rectangular channels; the fixed width of the rectangular micropores is 6 Å and the other width varies from 4 to 11 Å with the length of ligand. The microporosity of the compounds was confirmed by N2, Ar, and CO2 adsorption. Dudinin-Radushkevich plots of the adsorption isotherms provided that the micropore volume for each compound changes with the different adsorbate, indicating the presence of the molecular geometry-sensitive micropore filling. Also we observed that the rectangular micropore has a deeper potential well than the slit-shaped pores of activated carbon fibers.
Introduction Vapor molecules are adsorbed in micropores from an extremely low pressure due to the overlapped molecular potential from opposite pore-walls.1,2 This adsorption is called micropore filling. In particular, the enhanced molecular potential of micropores is noticeable as the pore width is less than the trilayer thickness of adsorbed molecules, inducing an intense confinement effect for molecules. Kaneko et al.3-6 showed that CCl4 and alcohol molecules form partially ordered structures in slit-shaped micropores of carbon even at room temperature. Also these smaller micropores have been widely applied to gas separation and removal of pollutants.7 Consequently, the relationship between the molecular geometry and adsorption behavior should be elucidated more clearly. As to slit-shaped micropores of activated carbon, the two-stage mechanism of micropore filling containing monolayer adsorption on the pore wall and filling in the residual space after the monolayer adsorption was confirmed by high-resolution N2 adsorption and molecular simulation.8-10 However, studies on micropore filling are mainly limited on activated carbon and zeolite. Activated carbon and zeolite have basically slit- and cylindrical-shaped micropores, respectively. Micropore filling of pores with other geometries should be studied. Steele and Bojan11 showed that corners can work as strong sites for micropore filling in rectangular micropores with the aid of GCMC simulation. However, we cannot confirm their prediction experimentally because there is no good microporous solid whose micropore shape is rectangular. Development of silica of regular mesopore structures such as MCM-4112 or FSM13 have accelerated the combination of organic and inorganic chemistry using the * Author to whom correspondence should be addressed. † Chiba University. ‡ New Energy and Industrial Technology Development Organization (NEDO).
molecular templating and architecture techniques.14-16 These new researches are going to provide a variety of microporous solids. Kitagawa et al.17,18 and Yaghi et al.19-21 reported that the micropore size of metal-organic complexes can be controlled by organic ligands. Especially, Kitagawa et al. observed that crystalline Cu complex-assembled compounds can adsorb abundantly supercritical methane. These compounds have rectangular micropores according to X-ray diffraction examinations. Although these Cu complex-assembled compounds are not necessarily well characterized yet, a comparative study on micropore filling of different gases in the rectangular micropores is quite helpful to design a high-performance adsorbent. In this work, we synthesized a series of the Cu complexassembled compounds [Cu2(pzdc)2(PL)]n with stable pillaredlayer and noninterpenetrated structure and measured the adsorption isotherms of N2, Ar, and CO2 on these compounds. Here, pzdc is the symbol for 2,3-pyrazinedicarboxylate and PL is the designation of the pillar ligand. Pyrazine (pyz), 4,4′bipyridine (bpy), or trans-1,2-bis(4-pyridyl)ethylene (bpe) was selected as a pillar ligand. Experimental Section Synthesis of Cu Complex-Assembled Crystals. [Cu2(pzdc)2(pyz)‚2H2O]n (Cu-4 × 6). A 0.02 M Na2(pzdc) aqueous solution (50 mL) was added dropwise to a refluxing aqueous solution (50 mL) containing 0.02 M Cu(ClO4)2 and 0.05 M pyz. The blue microcrystals were collected by filtration and dried under reduced pressure (P < 10-3 Pa) for 4 h. Elemental microanalysis calculated for C16H12Cu2N6O10 (%): C 33.49, H 2.09, N 14.61; found: C 32.85, H 2.09, N 14.22. [Cu2(pzdc)2(bpy)‚4H2O]n (Cu-9 × 6). A mixture solution (50 mL) of 0.02 M Na2(pzdc) and 0.01 M bpy dissolved in EtOH/ H2O (1/1) was added slowly to a stirring aqueous solution (50
10.1021/jp000660q CCC: $19.00 © 2000 American Chemical Society Published on Web 09/01/2000
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J. Phys. Chem. B, Vol. 104, No. 38, 2000 8941
Figure 1. Schematic of atomic structure of Cu complexes.
mL) of 0.02 M Cu(ClO4)2 at room temperature. The blue microcrystals were obtained by filtration and dried under reduced pressure for 4 h. Elemental microanalysis calculated for C22H20Cu2N6O12 (%): C 38.43, H 2.91, N 12.23; found: C 38.39, H 3.27, N 12.00. [Cu2(pzdc)2(bpe)‚5H2O]n (Cu-11 × 6). Cu-11 × 6 was prepared in the same way as described for Cu-9 × 6. Elemental microanalysis calculated for C24H24Cu2N6O13 (%): C 39.40, H 3.28, N 11.49; found: C 39.79, H 3.70, N 11.45. Adsorption Measurements. The adsorption isotherms of N2 at 77 K, Ar at 77 K, and CO2 at 273 K were carried out volumetrically on Autosorb-1, Quantachrom. N2, Ar, and CO2 gases of high purity (99.99%) were used. Prior to the adsorption isotherm measurements, the samples (around 50 mg) were outgassed under vacuum (P < 10-4 Pa) at 373 K for 2 h. X-ray Diffraction (XRD) and Thermogravimetric Analysis (TGA). The powder X-ray diffraction (XRD) was measured using an angle-dispersive diffractometer (MXP3 system, MAC Science) with a monochromated Mo KR (λ ) 0.7093 Å) radiation at 15 kV. The thermogravimetric analysis (TGA) was performed on Thermal Analyzer System 001 (MAC Science) under a flow of argon gas. The sample was heated from room temperature to 773 K at 5 K/min. Results and Discussion Crystal Structure and Thermal Stability of Cu Complexes. The schematic representation of [Cu2(pzdc)2(PL)]n complex structure is shown in Figure 1. The complex consists of 2-D sheets of [Cu(pzdc)]n and pillar ligands that bridge each sheet. XRD structure determination18 confirms that Cu-4 × 6 microcrystals belong to monoclinic space group P21/c with a ) 4.693, b ) 19.849, c ) 11.096 Å, β ) 96.90°, V ) 1026.1 Å3, Z ) 2, and F ) 1.862 g/cm3. Cu-9 × 6 and Cu-11 × 6 have similar networks with Cu-4 × 6, but they have an elongated b parameter to 29.0 Å for Cu-9 × 6 and 32.9 Å for Cu-11 × 6. These values are in agreement with the expected magnitude of the corresponding pillar ligands. The detailed structure of Cu-4 × 6 complex crystals is exhibited in Figure 2, showing the presence of 1-dimensional micropores with approximately rectangular shape. By the XRD results and the van der Waals radii of constituent atoms, the cross-sectional dimensions (Å) of the channels of Cu complexes are estimated and have been included in the nomenclature of each complex. Thermogravimetric analysis indicates a weight loss in wt % of 6.46 for Cu-4 × 6, 10.67 for Cu-9 × 6, and 12.49 for
Cu-11 × 6 between 333 and 373 K. These weight losses are equivalent to the loss of two water molecules for Cu-4 × 6, four for Cu-9 × 6, and five for Cu-11 × 6. This result coincides with the elemental analysis. The complexes began to release the ligands at 491 K for Cu-4 × 6, at 478 K for Cu-9 × 6, and Cu-11 × 6. No weight loss was observed before the chemical decomposition of each complex. Micropore Filling and Microporosity of Cu ComplexAssembled Crystals. In case of molecular filling in rectangular micropores, it is more difficult to fill sufficiently the pore space with molecules than the slit-shaped micropores. As the fixed width of the rectangular micropores of Cu complex-assembled solids is only 6 Å and the other width is in the range of 4 to 11 Å, the filling ratio of the micropore space with molecules should sensitively depend on the geometry and size of an adsorbate molecule. Here, linear N2, CO2, and spherical Ar were used as the probe molecules to determine the filling state of the micropores of Cu complex-assembled solids. Nitrogen Adsorption. The N2 adsorption isotherms measured at 77 K on Cu complexes are given in Figure 3. All samples show typical isotherms of type I, confirming the presence of micropores without mesopores. The rectangular rising of the N2 adsorption isotherms at low relative pressures indicates that the size of micropores is extremely uniform, being a main characteristic of metal-organic complex solids. The complex solid with larger micropores has a larger N2 adsorption capacity. The N2 adsorption isotherms on a logarithmic relative pressure are shown in Figure 4. A pronounced difference is observed. The isotherm of Cu-4 × 6 is characterized by a large step located at P/P0 ) 10-4, which should be associated with the entrance blocking effect. Such a pore blocking phenomenon can be found on microporous AlPO4 samples or molecular sieve carbons.22-24 However, it has not been reported on a metal-organic complex solid. Hence, this should stem from the intrinsic reason which is associated with the smaller micropore size in Cu-4 × 6, since this width of 4 Å is about 1.3 times that of the single layer of nitrogen molecules. There is no such sharp step in the adsorption isotherms of Cu-9 × 6 and Cu-11 × 6, because the moleculepore wall interaction in Cu-9 × 6 and Cu-11 × 6 is much weaker than that in Cu-4 × 6. The Rs-method has been shown to be very effective for analysis of microporous solids, while the current BET evaluation includes great errors for microporous systems.1 Kaneko et al.25 extended this analysis to a smaller Rs region and proposed a subtracting pore effect (SPE) method for accurate determination of the specific surface area of microporous adsorbents from the high-resolution Rs-plots. The effectiveness of the SPE method
8942 J. Phys. Chem. B, Vol. 104, No. 38, 2000
Li and Kaneko
Figure 3. Adsorption isotherms of nitrogen on Cu complexes at 77 K.
Figure 4. The logarithmic relative pressure expression of adsorption isotherms of nitrogen on Cu complexes. Figure 2. Space-filling views of Cu-4 × 6 crystal structure. The hydrogen atoms are omitted for clarity.
was evidenced by GCMC-simulation for carbonaceous adsorbents.26,27 We applied the SPE method to analyze the N2 adsorption isotherms of Cu complex-assembled crystals. Here we used a nonporous Cu complex solid (Figure 3) as the reference sample which was synthesized by similar conditions to Cu-4 × 6. The adsorption isotherm of the reference sample is of representative Type II and the surface area is 13.2 m2/g. The constructed Rs-plots for the samples are shown in Figure 5. A steep rise in adsorption amount at Rs < 0.5 shows an explicit filling swing due to the enhanced molecular potential. All samples have a remarkable filling swing compared with those of slit-shaped microporous systems. Hence, the rectangular micropores of Cu complex-assembled crystals should have an intense potential overlapping effect.
The micropore parameters of Cu complexes from the Rsanalysis are summarized in Table 1. The micropore volume (Vm) and external surface area (aext) are calculated from the intercept and slope of the linear part in the high Rs region, respectively. The total surface area (atot) is estimated from the slope of a line passing the origin of coordinate axis and a point on Rs-plot at Rs ) 0.5. It is noteworthy that the surface area of Cu-11 × 6 goes over 1000 m2/g regardless of the presence of the filling swing. The micropore filling of vapors is well described by the Dudinin-Radushkevich (DR) equation:
ln W ) ln W0 + (A/βE0)2; A ) RT ln(P0/P)
(1)
Here, W and W0 are the amount of adsorption at P/P0 and the pore volume. A is the adsorption potential,and β and E0 are the
Micropore Filling in Organic-Inorganic Crystals
J. Phys. Chem. B, Vol. 104, No. 38, 2000 8943
Figure 5. Rs-plots of nitrogen adsorption isotherms for Cu complexes.
Figure 6. Adsorption isotherms of argon on Cu complexes at 77 K.
TABLE 1: Micropore Parameters of Cu Complexes by N2 Adsorption Isotherms complex Cu-4 × 6 Cu-9 × 6 Cu-11 × 6
DR RS DR RS DR RS
W0 mg/g
Vm mL/g
123.7 126.2 181.2 180.0 226.3 217.8
0.15 0.16 0.22 0.22 0.28 0.27
atot m2/g
aext m2/g
571.1
14.4
845.7
23.0
1012.9
21.1
βE0 kJ/mol
qst,φ)1/e kJ/mol
11.8
17.4
7.9
13.5
8.0
13.6
affinity coefficient and characteristic adsorption energy, respectively. All DR plots were almost linear in the higher P/P0 region, giving the micropore volume and βE0. The micropore volume has a good coincidence with that by Rs-plot for all samples (Table 1). Furthermore, the βE0 leads to the isosteric heat of adsorption qst,φ)1/e at the fractional filling of 1/e by the equation
qst,φ)1/e ) ∆HV + βE0
(2)
where ∆HV is the heat of vaporization of bulk liquid. The qst,φ)1/e values of the three samples are from 14 to 17 kJ/mol, being greater than that of activated carbon fibers with the slit-shaped micropores by 1-4 kJ/mol regardless of low density of the pore wall of the Cu complexes.28 These data strongly support that rectangular micropores have deeper potential wells than slitshaped pores for N2. Argon Adsorption. The Ar adsorption isotherms measured at 77 K on Cu complexes are given in Figure 6. The shape of each isotherm is very similar to that of N2 adsorption. Figure 7 shows that the pore-blocking phenomenon also occurs in Ar adsorption on Cu-4 × 6. The micropore volumes by DR analysis of Ar adsorption isotherms are 0.12, 0.19, and 0.28 mL/g for Cu-4 × 6, Cu-9 × 6, and Cu-11 × 6, respectively. As the bulk phase of Ar at 77 K is solid, we did not use the liquid density of Ar (1.374 g/mL at 90 K) for the calculation. The solid density of Ar (1.451 g/mL at 77 K) was estimated from observed values at 40 K (1.650 mL/g) and 85 K (1.4079 g/mL). Carbon Dioxide Adsorption. CO2 adsorption at 273 K at subatmospheric pressures can provide the correct filling state of narrow micropores without blocking effect by preadsorbed molecules for further adsorption.29-31 Therefore, CO2 adsorption is indispensable to understand the micropore filling phenomena.
Figure 7. The logarithmic relative pressure expression of adsorption isotherms of argon on Cu complexes.
Figure 8 gives the CO2 adsorption isotherms on the Cu complexassembled crystals. Cu-4 × 6 and Cu-9 × 6 with smaller pores have an enhanced adsorption compared with Cu-11 × 6 with larger pores at the low relative pressures. However, we cannot obtain explicit information about their maximum adsorption amounts since adsorption was measured only at a low relative pressure range of 0-0.029. Hence, DR analysis was carried out, as shown in Figure 9. It shows that the adsorption behavior of each complex is qualitatively different at low and high relative pressures or each DR plot is linear at low or high P/P0 region and shows a steep increase in adsorption amount at medium P/P0 range. Steele and Bojan11 suggest that two types of “sites” of corners and walls are present in the pore with the rectangular cross-section and an adsorbate is strongly adsorbed on the corner site at first, followed by multilayer adsorption or pore filling on the walls. At low relative pressures, the CO2 molecules occupy the energetically most favorable positions. There is a Cu atom at the corner of the rectangular pore of these Cu complex crystals
8944 J. Phys. Chem. B, Vol. 104, No. 38, 2000
Li and Kaneko TABLE 2: Micropore Parameters of Cu Complexes by CO2 Adsorption Isotherms complex Cu-4 × 6 Cu-9 × 6 Cu-11 × 6
Cu Wc βE0,c Wf βE0,f Vm mmol/g mg/g (mmol/g) kJ mol mg/g kJ/mol mL/g 3.7 3.3 3.1
7.32 (0.17) 6.27(0.14) 6.18(0.14)
11.1 12.1 10.2
186.3 349.5 399.9
10.8 8.6 7.0
0.18 0.34 0.39
TABLE 3: Micropore Volume of Cu Complexes and Space-Filling Ratios (SFR) of Micropores with Different Molecules Vm (mL/g) complex calcd N2
Ar CO2
observed SFR (calculated SFR) N2
Ar
CO2
Cu-4 × 6 0.39 0.15 0.12 0.18 0.39 (0.44) 0.31 (0.38) 0.46 (0.48) Cu-9 × 6 0.52 0.22 0.19 0.28 0.42 (0.50) 0.37 (0.42) 0.65 (0.68) Cu-11 × 6 0.56 0.28 0.28 0.39 0.50 (0.55) 0.50 (0.52) 0.70 (0.66)
Figure 8. Adsorption isotherms of carbon dioxide on Cu complexes at 273 K.
Figure 9. DR plots of carbon dioxide adsorption isotherms for Cu complexes.
(Figure 1). The great quadropole moment of a CO2 molecule can interact with the Cu atom and thereby the corner site should be the strongest for the CO2 molecule. Therefore the CO2 molecules must be adsorbed on the corners of rectangular channels at the low-pressure region. We tried to determine the possible adsorption amount on the corners using the DR plot. Since molecules are distributed according to the molecule-pore interaction potential energy with increase of pressure, the predominate adsorption transforms from the high-energy sites to the low-energy sites at the pressure corresponding to the bending point of the DR plot (Figure 9). Accordingly we can estimate the effective adsorption amount Wc on the high-energy sites from the bending point. The Wc value should indicate the effective adsorption amount on the strong corners rectangular channels and it is listed in Table 2. We find that Wc is about half of the content of Cu atoms for all samples. This is because the distance between the neighbor Cu atoms is about 4.7 Å along the channel direction which is less than the length of CO2
molecules (5.3 Å), a CO2 molecule is adsorbed on the corner site having two Cu atoms. Thus, CO2 molecules are adsorbed at first on the corners at an interval of one Cu atom. At high relative pressures, CO2 molecules fill the pores. The large pores can accommodate more adsorptive molecules. The order of the maximum amount adsorbed on the micropores (Wf) agrees with the order of the pore sizes, as shown in Table 2. Here the Wf was calculated by the intercept of the DR plot at high P/P0 region. The similar analysis using the DR plot was done by other researchers.8,32,33 Moreover, the steep increase in adsorption amount at medium P/P0 range is contributed to the rearrangement of adsorbed CO2 molecules. By DR analysis, we obtained the βE0 values corresponding to the low and high P/P0 regions, representing as βE0,c and βE0,f, respectively, and find that the βE0,c is larger than βE0,f for all samples, indicating that CO2 molecules initially adsorb on the strong sites located on the corners. The micropore volumes obtained by CO2 adsorption are larger than these by N2 and Ar adsorption. As CO2 adsorption was measured at 273 K, fortunately the presence of two sites should be observed. In case of N2 and Ar adsorption at 77 K, the interaction energy difference of two sites must be too small to give rise to stepwise adsorption. Micropore Filling of Molecules with Different Geometry. As Cu complex-assembled crystals have rectangular micropores of the molecular order, molecules cannot fill the micropore space perfectly. The space filling ratio (SFR) of micropores should be sensitive to the geometry of adsorbate molecules. Here, we calculated the SFRs with the different adsorbates under the assumption of rectangular pores, as listed in Table 3. The SFR values are obtained from the ratio of the void volume calculated from the XRD crystallographic data to the micropore volume measured by gas adsorption. These results indicate that (i) the spherical Ar molecules cannot fill the rectangular micropores effectively and have the lowest SFR, (ii) CO2 molecules of an ellipse shape are the most suitable for filling the rectangular micropores and have the largest SFR, and (iii) larger rectangular micropores have a larger SFR than smaller micropores. Since the CO2 molecule has a longer and thinner shape than the N2 molecule, the SFR values of CO2 are larger than those of N2. Figure 10 shows the schematic of the filling states of the rectangular micropores with N2, Ar, and CO2 molecules on Cu-9 × 6. The 12-6 Lennard-Jones size parameter σXX for a X2 molecule was used to describe the effective diameter of an adsorbate atom, because these parameters are often used in simulation studies on molecular adsorption which provide reasonable results.34-36 σNN is 3.32 Å for N234 and the N-N distance is 1.09 Å. σOO ) 3.03 Å and σCC ) 2.82 Å for CO2;35,36
Micropore Filling in Organic-Inorganic Crystals
J. Phys. Chem. B, Vol. 104, No. 38, 2000 8945 Acknowledgment. This study was supported by the ProposalBased New Industry Creative Type Technology R&D Promotion Program (99E10-009-1) from the New Energy and Industrial Technology Development Organization of Japan (NEDO). References and Notes
Figure 10. Schematic of filling state of the rectangular micropores with N2, Ar, and CO2 molecules on Cu-9 × 6 complex.
the O-O separation is 2.32 Å and the C-O separation is 1.16 Å. σArAr ) 3.40 Å for Ar.11 The volume of a single molecule is 28.2 Å3 for N2, 31.3 Å3 for CO2, and 20.6 Å3 for Ar. If adsorbate molecules are considered as a rigid body and fill the rectangular micropores with the largest density and the unit cell dimension does not change with temperature, we can calculate the SFR of the rectangular micropore with an arbitrary length by geometric principles. We also assumed that the linear N2 or CO2 molecules fill the rectangular micropores along the channel direction. This is consistent with the real adsorption of molecules, e.g., the CO2 molecule is energetically favored to lie flat on the wall at any pore width in order to make the three atoms of molecule be in the potential minimum.37 Also the molecular axis of the N2 molecule adsorbed on the graphite surface is parallel to the surface.38 The in situ XRD studies on methanol and ethanol in carbon micropores showed the oriented structure of these molecules along the micropore walls.5,6 The calculated SFRs are also given in Table 3. The comparison of the calculated and observed SFRs shows that the calculated SFRs are close to the observed SFRs for CO2, but, calculated SFRs are larger than observed ones for N2 and Ar. This fact indicates that the adsorption conditions of N2 and Ar at 77 K do not necessarily allow a complete adsorption due to the insufficient intrapore diffusion. Therefore, the observed SFR values of N2 and Ar should be underestimated. Nevertheless, both SFR values are similar to each other, supporting the presence of the molecular geometry-sensitive micropore filling.
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