A New Strategy for the Improvement of the N2 Adsorption Properties

J. Phys. Chem. C 2007, 111, 12011-12023

12011

A New Strategy for the Improvement of the N2 Adsorption Properties of Copper-Ion-Exchanged MFI-Type Zeolites at Room Temperature Atsushi Itadani,† Masashi Tanaka,† Toshinori Mori,† Mahiko Nagao,† Hisayoshi Kobayashi,‡ and Yasushige Kuroda*,† Department of Fundamental Material Science, Graduate School of Natural Science and Technology, Okayama UniVersity, Tsushima, Okayama 700-8530, Japan, and Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan ReceiVed: April 7, 2007; In Final Form: May 16, 2007

A copper-ion-exchanged MFI-type zeolite, CuMFI, which exhibits an extremely efficient adsorption of the dinitrogen (N2) molecule at room temperature, has been successfully prepared by ion exchange in an aqueous solution of Cu(CH3COO)2 containing a component of NH4CH3COO. This ion-exchange method has unique characteristics as follows; first, the copper ion exchange takes place exclusively on the ion-exchangeable site that acts as the active site for N2 adsorption, and second, the extent of the reduction of the divalent copper ion exchanged attains a high value of 88%. In addition, the CuMFI sample thus prepared exhibits a high efficiency for N2 adsorption at 298 K where the monovalent copper ions act as adsorption sites; approximately 86% of them are effective for N2 adsorption. In this case, the heats of adsorption of N2 were large, 87-60 kJ mol-1. These observations were rationalized as showing that the site-selective ion exchange with copper ions occurs, accompanying acetate ligands by which the effect of the size is brought, and that the divalent copper ions exchanged in this way can be reduced easily and efficiently to the monovalent ions; on that occasion the organic ligands incorporated in the ion-exchange process are decomposed, promoting the reduction of the copper ions. From the observations of photoluminescence spectra and X-ray absorption near-edge structure spectra in the processes both before and after N2 adsorption, it was deduced that the three-coordinated monovalent copper ions are active sites for N2 adsorption at room temperature, which was also supported by the results of both diffuse reflectance spectra and density functional theory calculations. The results obtained are expected to provide significant information for the development of materials that function efficiently as N2-fixation or N2-activation catalysts as well as NOx decomposition catalysts.

1. Introduction Recent years have seen a renewal of interest in the fields of fundamental research on adsorption, activation, and fixation of small molecules, in particular dinitrogen (N2), which shows a relatively less reactive nature. Modern research in such fields is dominantly focused onto four areas: research on N2 fixation in biological systems, on surface science for understanding catalytic activation of N2, on synthetic chemistry of metal complexes having the N2 ligand, and on gas-phase studies to understand the bonding properties and mechanisms of N2 activation.1 In addition, especially over the last several years we have longed for the development of new important materials that adsorb an N2 molecule rapidly and strongly even at room temperature. Actually, with the development of an informationoriented society, the manufacturing process of semiconductors has undeniably advanced by leaps and bounds. Therefore, it is necessary for the preparation of semiconductor materials to have a low impact on the environment; for example, the reuse of the rare gases that are used in the preparation of semiconductors will be recommended. Related to such processes, there is currently great interest in the design and preparation of materials * Author to whom correspondence should be addressed. Phone: +8186-251-7844. Fax: +81-86-251-7853. E-mail: [email protected]. † Okayama University. ‡ Kyoto Institute of Technology.

that can easily remove N2 from the semiconductor production process. Moreover, such materials are also expected for use as heat insulators. However, there are few materials that can adsorb N2 molecules easily, safely, and rapidly at temperatures around room temperature. We first discovered that a copper-ion-exchanged mordenite sample, which has an ion-exchange capacity beyond 100% (i.e., a nonstoichiometrically ion-exchanged material), exhibited a prominent adsorption feature for N2 molecules at room temperature.2-4 Both our group5-7 and Zecchina’s group8,9 found independently that the copper-ion-exchanged MFI-type zeolites (CuMFI) also have interesting and specific properties for N2 adsorption; these materials strongly adsorb N2 molecules at room temperature, while the N2 molecule is recognized as being an inactive supercritical gas at temperatures around 300 K, and ordinarily, it is adsorbed on a solid surface only at low temperatures, such as 77 K, or under high pressures.10-14 It is also known that CuMFI exhibits high catalytic activity in the decomposition reaction of NOx.15-32 The analysis of the active sites responsible for N2 adsorption onto CuMFI will be helpful in evaluating such a catalytic decomposition of NOx. So far, our group5 and others9,33-36 have reported that there are some exchangeable sites for the copper ions in MFI and that the extent of N2 adsorption on CuMFI depends on the ionexchange method used; the latter fact suggests selective occupancy of the copper ions at exchangeable sites in the MFI.37,38

10.1021/jp0727300 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/20/2007

12012 J. Phys. Chem. C, Vol. 111, No. 32, 2007

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TABLE 1: Amounts of Adsorption of N2 and CO and the Ratio of the Number of Copper Ions N2 adsorption

CO adsorption

ratio

sample

ion exchange level/%

Vm1a

Vm2a

Vcb

Vm1a

Vm2a

Vcb

nCu+c/nCu(T)e

nCu+(N)d/nCu+c

CuMFI(F) CuMFI(A) CuMFI(P) CuMFI(AAA) CuMFI(NM) CuMFI(C)

103 110 107 109 108 111

6.20 6.55 7.90 10.32 7.40 6.40

5.44 5.46 6.10 9.20 5.74 5.80

0.76 1.09 1.80 1.12 1.66 0.60

15.81 17.43 19.80 24.05 15.76 20.30

9.84 10.43 11.50 12.06 9.84 11.70

5.97 7.00 8.30 11.99 5.92 8.60

0.46 0.50 0.61 0.88 0.41 0.62

0.13 0.16 0.21 0.09 0.28 0.07

a Vm1 and Vm2: monolayer capacities (cm3 g-1 at standard temperature and pressure) obtained by applying the Langmuir equation to the first and second adsorption isotherms, respectively. b Vc: amount of chemisorption (Vc ) Vm1 - Vm2). c nCu+: the number of Cu+ ions. d nCu+(N): the number of Cu+ ions that interact strongly with N2 molecules. e nCu(T): total number of copper ions (Cu2+, Cu+).

An adsorbent of N2 effective at room temperature is expected to be develped as an N2-fixation material as part of an energy saving process, and an efficient material for N2 activation is expected to be discovered as well. The effect of the ionexchange method on the N2 adsorption properties of samples obtained remains to be clarified sufficiently. With such a background it is important to develop a more efficient preparation methods for CuMFI and to elucidate the states of the active sites for the decomposition of NOx and for N2 adsorption. The present work is intended to develop a preparation method for CuMFI endowed with an activity against N2 molecules at room temperature and also to obtain further information on the states of the active centers for N2 adsorption. 2. Experimental Section 2.1. Sample Preparation Methods. Approximately 5 g of a sodium-type MFI zeolite (NaMFI) having a silicon/aluminum ratio of 11.9, which had been kindly supplied by Tosoh Co., was dispersed in an aqueous solution of Cu(HCOO)2, Cu(CH3COO)2, or Cu(C2H5COO)2 with stirring at 300 K for 1 h.37,38 This process was repeated several times to obtain the CuMFI samples with the desired exchange level. The sample obtained was centrifuged and washed several times with distilled water, followed by drying at 300 K. These samples were abbreviated as CuMFI(F), CuMFI(A), and CuMFI(P), where F, A, and P represent, respectively, the formate, acetate, and propionate ions involved in the exchanging solution as counterions. In addition, by using a mixed aqueous solution of Cu(CH3COO)2 and NH4CH3COO (i.e., containing an excess of acetate ions) as the ion-exchange solution, the CuMFI sample was also prepared; this sample is abbreviated as CuMFI(AAA). For comparison, the copper ion exchange was also carried out by the usual method using an aqueous solution of CuCl2 or by applying the microwave-assisted method using an aqueous solution of Cu(NO3)2; the samples obtained are abbreviated as CuMFI(C) and CuMFI(NM), respectively.39 The extent of exchanged copper ions was determined by chelometric titration. The ionexchange level of the sample was evaluated by assuming that one divalent copper ion is exchanged with two monovalent sodium ions.40 The resultant ion-exchange levels were in the range of 103-111% for all the CuMFI samples, as given in Table 1. The N2 gas as the adsorbate was obtained by vaporizing liquid N2, and CO gas was purchased from GL Science Co. 2.2. X-ray Absorption Fine Structure Spectra. The measurement of X-ray absorption spectra, both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), was carried out in situ using beam line 10B of the Photon Factory (PF) at the Institute of Materials Structure Science (High-Energy Accelerator Organization, KEK, Tsukuba, Japan). Prior to the measurements, the samples were

heat-treated at various temperatures under a reduced pressure of 1 mPa to promote a reduction of Cu2+ to Cu+.40,41 2.2.1. EXAFS Data Analysis. Analysis of the data was performed with the aid of Maeda’s program.42 The outline of the analysis procedure was as follows. The extended X-ray absorption spectrum is derived from the absorption spectrum above the Cu K-edge. The background absorption other than that for the K-edge of an absorbing atom of interest was evaluated by a least-squares fitting procedure with the Victoreen formula, including a constant term to the preedge and was subtracted from the total absorption by extrapolation. The smooth K-shell absorption µ0 due to an isolated atom was estimated by using a cubic spline function. The modulation χ(k) was extracted and normalized by means of the following equation

χ(k) )

[µ(k) - µ0(k)] µ0(k)

where k is the photoelectron wave vector of the ejected electron defined as x8π2m(hν-E0)/h2, hν denoted the energy of the incident X-rays, and E0 is the threshold energy for liberation of the photoelectron wave. The resulting EXAFS modulation weighted by k3 was Fourier transformed to give a radialdistribution-like function. 2.2.2. Calculations by the Least-Squares Method. The structural parameters (interatomic distance, rj, mean-square displacement, σj2, and coordination number, Nj) were obtained by comparing the observed EXAFS spectra and the model function given by the single scattering theory as43,44

χ(k)calc )

∑{Nj/krj2} exp(-2σj2k2)

exp(-2rj/λj(k))Fj(π,k) sin(2krj + δj(k))

where Fj(π,k) is the back-scattering amplitude from each of the Nj scatterers j at distance rj from the X-ray absorbing atom. An amplitude reduction related to inelastic losses at the absorbing atom due to the many-body effect is introduced by the mean free path, λj(k), while δj(k) is the total scattering phase shift experienced by the photoelectron. The functions Fj(π,k) and δj(k) were from the paper reported by McKale et al.45 The observed spectra can be compared with the theoretical ones by a least-squares fitting procedure in either r- or k-space. In the present study, we performed least-squares calculations in the k-space, which were applied to the Fourier filtered, k3weighted χobs(k) values to minimize the error-square sum ∆: ∆ ) ∑m kmn{χobs(km) - χcalc(km)}2. The parameters Nj, rj, and σj were varied in a nonlinear least-squares curve fitting. In this experiment, least-squares fitting was applied to obtain the values of λj(k) by using the reference samples of Cu2O, CuO, or Cu(OH)2.

N2 Adsorption of MFI-Type Zeolites 2.3 Adsorption Measurements. The measurement of adsorption heats was carried out by using an adiabatic-type calorimeter that was made in our laboratory, which can be applicable in the pressure region between 0 and 13.3 kPa.46 The procedure for the measurement was as follows: First, simultaneous measurements of the heat of adsorption (calorimetrically) and the adsorption isotherm (volumetrically) were performed under in situ conditions at 298 K on the sample evacuated at 873 K under a pressure of 1.3 mPa for 4 h. After these measurements, the sample was degassed at 298 K under a pressure of 1.3 mPa for 4 h to remove weakly adsorbed molecules. Then, the second adsorption measurement was carried out again at 298 K. The equilibrium pressure was monitored by using an MKS Baratron (type 690) pressure sensor. 2.4. Photoluminescence Spectra. Photoluminescence (PL) spectra were recorded on a Hitachi F-2000 photoluminescence spectrophotometer in reflection geometry at room temperature with an excitation wavelength of 300 nm. All samples were evacuated at 873 K. 2.5. Thermogravimetric Analysis. Thermogravimetric (TG) analysis was performed using a ULVAC TGD-9600. The measurements were carried out at a heating rate of 5 K min-1 from room temperature to 1173 K under ambient conditions. 2.6. Infrared Spectra. A Digilab infrared spectrophotometer (FTS-4000MXK) equipped with a TGS detector was used in single-beam mode to collect IR spectra. Self-supported wafers were degassed at various temperatures under a reduced pressure of 1 mPa for 4 h. All spectra collected were averages of 256 scans at a nominal resolution of 2 cm-1. The in situ cell was used for the heat treatment and adsorption (i.e., adsorption of N2 or CO).41 Every spectrum of the zeolite samples was acquired as the inverse log of the single-beam emissivities of the sample divided by the emissivities recorded at room temperature for the cell including adsorptive gas. 2.7. Diffuse Reflectance Spectra. UV-vis-near-IR diffuse reflectance (DR) spectra were recorded at 300 K in the wave number range of 25 000-5000 cm-1 using a spectrophotometer (JASCO-550) equipped with an integral sphere. The powdered sample was placed in a reflectance cell made of fused silica. Spectralon (Labsphere, USA) was used as the reference material. 2.8. Temperature Programmed Desorption. The temperature programmed desorption (TPD) method developed by Cvetanovic and Amenomiya was applied to analyze the desorption process for CuMFI.47 Helium gas was used as a carrier at a rate of 60 cm3 min-1, and the heating rate was 5 K min-1. After the sample was evacuated at room temperature for 4 h, the thermal desorption run was started. The desorbed gas was detected by a thermal conductivity detector (TCD) and analyzed by mass spectrograph (Hiden Halo-100). 3. Results and Discussion 3.1. Properties of the CuMFI Samples Prepared by Using Aqueous Solutions Containing Copper Ions and Different Types of Carboxylate Anions. 3.1.1. Adsorption Features of the Samples. Figure 1 shows the first and second adsorption isotherms of N2 for CuMFI samples prepared by using aqueous solutions containing copper ions (Cu2+) and different carboxylate anions. For every sample, the amount of adsorption increases steeply in the lower pressure region and then gradually toward a saturation value. Such a Langmuir type isotherm is indicative of strong interaction between the copper ions and the N2 molecules even at 298 K. Furthermore, the amount of adsorption varies from sample to sample with regard to the kind of counterions involved in the ion-exchanging solution; it is in

J. Phys. Chem. C, Vol. 111, No. 32, 2007 12013

Figure 1. Adsorption isotherms of N2 on CuMFI at 298 K: CuMFI(P) (circles); CuMFI(A) (squares); CuMFI(F) (diamonds). Filled and open symbols represent the first and second adsorptions, respectively. The first adsorption was measured for the sample evacuated at 873 K, and then the sample was evacuated at 298 K, followed by the measurement of the second adsorption.

the order: propionate > acetate > formate ions. The monolayer capacities for these samples were evaluated by applying the Langmuir equation to the adsorption isotherms, which are given in Table 1. Here, Vm1 and Vm2 are monolayer capacities obtained by using the first and second adsorption isotherms, respectively, and Vc, which represents the amount of chemisorption, is obtained by subtracting Vm2 from Vm1. It is worth noting that the degree of strongly adsorbed N2 molecules, corresponding to the Vc value, also increased in the same order. CO molecules are often utilized as probe molecules to evaluate the monovalent copper ions by their specific interactions. The adsorption isotherms of CO on different CuMFI samples were measured. (The isotherms are given in the Figure 1S of the Supporting Information.) These isotherms of CO are similar to those of N2 shown in Figure 1, and the monolayer capacities for CO adsorption obtained are also listed in Table 1. The degree of reduction of CuMFI(P) evaluated from the irreversibly adsorbed amount was estimated to be 61% of the total amount of exchanged copper ions, and its value was the highest among the three samples studied. 3.1.2. Features of the Ion Exchange. With the data shown in Table 1, we could develop a material for the efficient adsorption of N2 at room temperature, which was prepared using the ionexchange method employing an aqueous solution of copper propionate. As mentioned in the previous paper,38 however, the preparation of a highly copper-ion-exchanged sample (>100%) is not easy when copper propionate is used, in comparison with the copper acetate or formate or CuCl2. This may be due to the difference in pKa values of the corresponding acids; the pKa values of formic, acetic, and propionic acids are 3.55, 4.56, and 4.67, respectively.48 Therefore, the concentration of Cu(C2H5COO)+ in the aqueous solution is greater than that of either Cu(CH3COO)+ or Cu(HCOO)+. The fact that propionic acid has a larger pKa value leads it to resist deprotonation, which is responsible for the tendency to form a complex with copper ions, in comparison with the systems with relatively small pKa values. This interpretation explains well the electron paramagnetic resonance data observed in our previous paper in that the electron paramagnetic resonance spectrum at 77 K for the


Itadani et al.

Figure 2. IR spectra of (a and b) CuMFI(P) and (c and d) CuMFI(F). The number indicates the temperature of evacuation: (1) 300; (2) 373; (3) 473; (4) 573; (5) 673; (6) 773; (7) 873 K.

sample prepared from copper propionate is different from that for the sample prepared from the copper formate.38 In the case of using Cu(C2H5COO)2 as the ion-exchanging solution, the Cu(C2H5COO)+ species formed leads to difficulties in the ionexchange procedure, because of the relatively large size in the associated Cu(C2H5COO)+ species and the existence of a higher concentration of Cu(C2H5COO)+ in solution. Such species resist ion exchange into the zeolite above ca. 100% (see below). The infrared absorption spectra of CuMFI(P) and CuMFI(F) that were evacuated at different temperatures are shown in Figure 2 as typical examples. The difference spectra between the samples evacuated at respective temperatures are also given in the Supporting Information (Figure 2S). The most prominent feature of the IR spectrum for CuMFI(P) is the appearance of the bands at around 2950 and 1600 cm-1, while for the CuMFI(F) sample no such bands were discernible. These are assigned to the ν(CH) and ν(COO) vibrations, respectively, which indicates the presence of carboxylate anion in the sample. The >CO stretching bands could be a favorable spectroscopic fingerprint for a better understanding of the carboxylated copper coordination structures. It is known that free carboxylate anions exhibit two dominant >CO asymmetric (νa(COO)) and symmetric (νs(COO)) vibration bands at 1570 and ∼1400 cm-1, respectively.49 In the case of complex formation, the carboxylate anion mainly coordinates to a metal by one of three modes: monodentate, bidentate (chelating), and bridging (between two metal ions). Here, we can distinguish among these three modes by taking advantage of the following information about the complexes coordinated by carboxylate anions: (i) Unidentate complexes have larger ∆ν () νa - νs) values, compared with free carboxylate anions, (ii) chelating complexes give ∆ν values significantly less than those of free carboxylate anions, and (iii) ∆ν values of bridging complexes are larger than those of chelating complexes and close to those of free carboxylate anions. Taking into account both the Si/Al ratio of the sample and the structure of the MFI-type zeolite, the last type of complex formation can be precluded. As shown in Figure 2, four absorption bands appear at 1595, 1587, 1415, and 1403 cm-1; the bands at 1595 and 1403 cm-1 are due to the >CO stretching vibration, as in the case of Cu(acetate)(PPh3)2, where a carboxylate anion coordinates to the copper ions in a bidentate form50 and the remaining two bands at 1587 and 1415 cm-1

Figure 3. DR spectra of CuMFI(P). The number indicates the temperature of evacuation: (1) 300; (2) 373; (3) 473; (4) 573; (5) 673; (6) 773; (7) 873 K.

are also ascribable to a bidentate species. The other two bands observed at 1670 and 1308 cm-1 can be seen and may be assigned to a unidentate species. The band at 1645 cm-1 and a shoulder band at 1625 cm-1 are assignable to the bending vibration of H2O, and the other bands at 1489, 1469, 1365, and 1340 cm-1 are assigned to the deformation modes of -CH3 as well as CH2 groups. After evacuating the sample at 673 K, these four bands vanished, which corresponds to the disappearance of the bands observed in the CH stretching vibration region (in the vicinity of 2950 cm-1). Thus, it should be noted that there are many associated species, such as [Cu(C2H5COO)]+, when the exchanging solution involves propionate anions instead of formate anions. From these results, it may be concluded that the carboxylate species with copper ions in MFI take part in the reduction of exchanged copper ions as well as in the siteselective ion exchange. 3.1.3. Spectroscopic Features of the Samples. The DR spectra (in the UV-vis-near-IR region) of CuMFI(P) evacuated at different temperatures are shown in Figure 3. After evacuation of the sample at 300 K, a broad and weak band around 12 500 cm-1 is observed, which is mainly ascribed to the d-d transition in the aqua complex of the divalent copper ion exchanged in MFI. This band shifted to the slightly higher wave number after evacuation at 373 K, indicating that some water molecules were

N2 Adsorption of MFI-Type Zeolites

Figure 4. PL spectra of CuMFI after evacuation at 873 K: (1) CuMFI(F); (2) CuMFI(A); (3) CuMFI(P).

desorbed and that the copper ion was partly coordinated to the lattice oxygen atoms, whereby the copper ion experienced a slightly stronger ligand field. Treatment at 573 K increased the intensity of the band, accompanying a shoulder at around 20 000 cm-1. This may be due to the formation of an oxygen-bridged species between copper ions to compensate for the change in the charges of the ion-exchanged species.51-56 Such behavior did not change, even after evacuation at 873 K. It should be noted that the band due to the Cu2+ species did not disappear completely, even after evacuation at 873 K. The extent of reduction of the Cu2+ species was approximately 60% for this sample (Table 1). The PL spectra of CuMFI(F), CuMFI(A), and CuMFI(P) are shown in Figure 4. These spectra are obviously caused by the electron transition from the 3d94s1 to 3d10 levels of the Cu+ species formed in the MFI after evacuation at higher temperatures.57-62 For CuMFI(P) a clear and strong band at 18 500 cm-1, together with a less strong band at 21 000 cm-1, is observed. Similar bands, not so distinguished, are also seen in the spectra for the other two samples. These spectra were resolved into three components by using the data obtained so far,6 and it was suggested that there are three types of Cu+ species that give the luminescence bands at 21 000, 18 500, and 19 500 cm-1. The former two bands are ascribed to the Cu+ species with a two- and three-coordination environment involving the lattice oxygen atoms, respectively, and the last one is due to the Cu+ species deposited on the surface of the siliceous part of the MFI.63 It is interesting to note that for CuMFI(P) the appearance of a distinct band at 18 500 cm-1 indicates the preferential formation of three-coordinated Cu+ species, compared to the cases of CuMFI(A), CuMFI(F), and also CuMFI(C) (not shown in Figure 4).6,7 Furthermore, it should be noted that the sample giving a strong band at 18 500 cm-1 exhibits a high adsorbability for N2 molecules at room temperature. Thus, the three-coordination site can be regarded as the active site for N2 adsorption. 3.1.4. Features of Ion Exchange in Samples Prepared by Employing Different Carboxylates as the Counterions. On the basis of the data given in Table 1 and Figures 2 and 4, we can propose a preparation method for an efficient material for N2

J. Phys. Chem. C, Vol. 111, No. 32, 2007 12015 adsorption at room temperature, where an aqueous solution of copper propionate is employed for the copper ion exchange. However, we were aware that when such an aqueous solution of copper propionate was used the copper-ion-exchange level could not exceed 110%.38 This may be due to the large concentration and size of the Cu(C2H5COO)+ ion. Here, it should be remembered that the pores in an MFI-type zeolite are approximately 0.55 nm in diameter and that the Cu(C2H5COO)+ species associated with water molecules, [Cu(H2O)n(C2H5COO)]+, are exchanged in the zeolite pores. Contrary to these species, the associated [Cu(H2O)n(CH3COO)]+ species are relatively smaller in size, and hence, their repulsion against the zeolite lattice is slight. The relative sizes of exchanged ions in MFI are given in Figure 3S of the Supporting Information. As can be deduced from this figure, the steric hindrance can make it easy to exchange in the site that acts as an active site for N2 adsorption; the site-selective ion exchange occurs in the case where the larger ion was used as the counterion. 3.2. Development of a New Ion-Exchange Method and the Adsorption and Reduction Features. 3.2.1. A New IonExchange Method. On the basis of the discussion mentioned above, it is expected that the sample having a higher exchange capacity can be prepared by employing an aqueous solution containing an excess amount of acetate ions that have an appropriate ion size and a suitable coordination ability as a ligand, and the exchanged site for the copper ion is controllable to some extent by utilizing these advantages successfully. In addition, the extent of reduction of the exchanged copper ions must be increased by taking advantage of the reducing power of the acetate ligands incorporated simultaneously. To increase the amount of the associated copper-ion species (including the acetate ion) in the ion-exchange solution, we planned to shift the equilibrium in the following equation to the right-hand side by adding NH4CH3COO as an additional component +NH4CH3COO

Cu2+ + CH3COO- 98 NH4+ + [CuCH3COO]+ + CH3COOBy using such an aqueous solution as the ion-exchange solution, the ion-exchange procedure described earlier was repeated, and samples having a higher exchange capacity of about 150% were easily prepared. In this experiment, we adjusted the ionexchange capacities of the samples to approximately 109% to enable a comparison among the samples. The sample prepared in such a manner is denoted as CuMFI(AAA). 3.2.2. Prominent Adsorption Properties of CuMFI(AAA). Figure 5 shows the first and second adsorption isotherms of N2 on CuMFI(AAA) at 298 K together with their heat-of-adsorption curves. The adsorbed amount increased remarkably in the lower pressure region, just as in the cases of other samples shown in Figure 1, which indicates a strong interaction between the N2 molecules and the monovalent copper ions, even at 298 K. The monolayer capacities estimated from these isotherms were 10.32 and 9.20 cm3 g-1 for the first and second adsorption, respectively (also shown in Table 1). For comparison, the first adsorption isotherm of N2 on CuMFI(P) is also included in Figure 5. The adsorption behavior of N2 on CuMFI(AAA) is particularly surprising in view of the fact that the adsorbed amount is greater for this sample than that for CuMFI(P) by about 1.3 times. In addition, the heat of adsorption in the initial stage of the first adsorption is 87 kJ mol-1, being a much higher value for N2 adsorption compared to the value obtained for Rh/SiO2.64 Moreover, it is interesting to find that CuMFI(AAA) yields a plateau on the heat-of-adsorption curve at ca. 60 kJ mol-1, which


Itadani et al.

Figure 6. IR spectra of CuMFI(AAA). The number indicates the temperature of evacuation: (1) 300; (2) 373; (3) 473; (4) 573; (5) 673; (6) 773; (7) 873 K.

Figure 5. (a) Adsorption isotherms and (b) differential heats of adsorption of N2 on CuMFI(AAA) and CuMFI(P) at 298 K: CuMFI(AAA) (circles); CuMFI(P) (squares). The meaning of the symbols is the same as in Figure 1.

corresponds to the adsorption region, 1.9-8.2 cm3 g-1 (i.e., physisorption region). This is evidence of the presence of energetically homogeneous sites (areas), that is, a specific feature for N2 adsorption on CuMFI(AAA). Another feature of this system is a distinct difference in the amounts of N2 adsorption between the first and the second adsorptions, 1.12 cm3 g-1. In other words, some N2 molecules are adsorbed irreversibly on CuMFI(AAA), even at 298 K, which is much higher than the critical temperature of N2 (126.2 K). The heats of adsorption give high values of 87-70 kJ mol-1 in the initial adsorption region up to ca. 1.3 cm3 g-1, corresponding to the irreversible adsorption (i.e., chemisorption, 1.12 cm3 g-1) just mentioned above. However, we only had limited information relating to the behavior of the irreversibly adsorbed species in this work and so did not consider this any further. 3.2.3. Features of the Exchanged State. Figure 6 shows the IR spectra of CuMFI(AAA) evacuated at selected temperatures. The features of these spectra are in agreement with those reported by Zecchina et al.65 and Niwa et al.66 The N-H stretching vibration (3400-3100 cm-1) and the bending vibration (1460 cm-1) of the NH4+ species, which are hydrogenbonded to the zeolite lattice in the bidentate and tridentate forms, are modified by the Fermi resonance to result a complicated spectrum. The band around 2900 cm-1 is assigned to νC-H of the organic component. In addition, a few small shoulder bands were observed around 1400 cm-1 in the spectrum of the sample treated at 373 or 473 K. These spectra are similar to those for CuMFI(P) shown in Figures 2a and 2b but are quite different from the spectra for CuMFI(A) prepared without the addition of NH4CH3COO. After evacuation of the sample at 873 K, no bands were observed except those due to SiOH and OH groups on the Brønsted acid sites and bands related to the zeolite lattice modes, which indicate a complete removal of the ammonium ions and any carbonaceous species arising from the acetate ions.

Figure 7. (a) XANES and (b) EXAFS spectra of CuMFI(AAA). The number indicates the temperature of evacuation: (1) 300; (2) 373; (3) 473; (4) 573; (5) 673; (6) 773; (7) 873 K.

We shall focus on the weak vibration bands from the carboxylate anions simultaneously incorporated with the copper ions. The >CO stretching pair bands at 1590 and 1403 cm-1 were assigned to the antisymmetric and symmetric bands of the bidentate ligand, respectively. These bands are due to species in which one carboxylate anion coordinates to one copper ion in the bidentate state. The monodentate species bands were observed at 1676 and 1360 cm-1.67 These four types of bands completely disappear after evacuation at 673 K, concomitant with the disappearance of the bands observed in the region associated with C-H vibrations. To understand these points more easily, the difference in the spectra are given in Figure 4S of the Supporting Information. From the above, it should be noted that there was more of the associated species, [Cu(CH3COO)]+, exchanged in the MFI in the present case, due to the addition of ammonium acetate to the solution. Taking this into consideration, the existence of the acetate species in the MFI is correlated to phenomena related to both the reduction behavior and the site-selective ion exchange of CuMFI(AAA), as described later. 3.2.4. Reduction of the DiValent Copper Ions. The XANES and EXAFS spectra of CuMFI(AAA) evacuated at different temperatures are shown in Figure 7. For the samples evacuated at 300 and 373 K, both the band due to the 1s-3d transition and the split band due to the 1s-4p transition are observed


Figure 8. DR spectra of CuMFI(AAA). The number indicates the temperature of evacuation: (1) 300; (2) 373; (3) 473; (4) 573; (5) 673; (6) 773; (7) 873 K. The DR spectrum of CuMFI(AAA)-74 evacuated at 300 K is also shown (spectrum number 8).

(Figure 7a).9,40,68-72 Corresponding to these bands, the EXAFS spectra have bands at 1.9 and 3.3 Å (after phase correction),73-75 which may be due to the formation of complexes, such as Cu(H2O)n(CH3COO)+. These results seem to be in accordance with the results obtained from IR spectra indicating the presence of carbonaceous impurities in the samples. However, a detailed assignment of the observed bands (at 1.9 and 3.3 Å) was not made here.76 The divalent copper ions commence to be reduced at a temperature of 473 K (the appearance of the strong band at 8.983 keV), and it completes at 673 K, as can be seen from the XANES data without contradiction with the IR data shown above and TPD (Figure 5S in the Supporting Information) data. By evacuation of the ion-exchanged sample at 873 K, the divalent copper ions were reduced to monovalent copper ions, of which the coordination number was estimated to be 2.8 by curve-fitting analysis. This indicates that there exist two- and three-coordinated copper ions in the samples and that the latter type of ions, which gives a luminescence band at 18 500 cm-1 in the PL spectrum, is dominant in number.6,7,72 Figure 8 shows the DR spectra of CuMFI(AAA) evacuated at different temperatures. For comparison, the DR spectrum of another sample of CuMFI(AAA) (CuMFI(AAA)-74; ionexchange level, 74%) after evacuation at 300 K is also included. After evacuation at 300 K, a broad band was observed in the 18 000-8000 cm-1 region. Judging from the band position, it can be ascribed to the d-d transition of Cu2+ forming a complex with oxygen atoms as a ligand. By referring to the IR spectral data, it is reasonable to suppose that such oxygen atoms originate in the acetate anions and/or H2O molecules. In the literature on Cu(H2O)42+ and Cu2(Ac)4, their DR spectra exhibit bands at 12 200 and 14 286 cm-1, respectively.51,77 The aqueous solutions of Cu(NO3)2 and Cu(OAc)2 employed in the present study gave DR bands at 12 200 and 13 080 cm-1, respectively. The spectra of CuMFI(AAA)-74 and CuMFI(AAA) exhibit bands centered at 12 600 and 13 000 cm-1, respectively. Taking into account the above data, this difference in the band position can be explained in terms of an increase in the strength of the ligand field in CuMFI(AAA), in which an acetate ligand is coordinated to some of the copper ions exchanged in the MFI. In CuMFI(AAA)-74, H2O was the major ligand, while in

J. Phys. Chem. C, Vol. 111, No. 32, 2007 12017 CuMFI(AAA) some acetate ions were coordinated along with some H2O molecules. These positions of the band are clearly different from those observed for [Cu(NH3)4]2+,77 which excludes a possibility of the coordination of NH3 to the copper ions in the exchange process, consistent with the IR data shown above. Ammonium ions are only exchanged with sodium ions existing in the starting material. After evacuation at 373 K, the position of the absorption maximum slightly shifts to the higher wave number side. This is due to an increase in the interaction of the divalent copper ions with the lattice oxygen atoms, resulting from the desorption of water molecules. Even after evacuation at 473 K, no change was observed in both the intensity and the position of the band. When evacuated at 573 K, the band intensity decreased remarkably, and a new shoulder band appeared at around 16 500 cm-1 (spectrum 4). By deconvoluting this spectrum, two absorption bands can be observed at 16 500 and 13 400 cm-1, as shown by dashed lines. On the basis of the experimental data using CO as a probe molecule, we have so far insisted that there are at least two types of copper ions exchanged in the MFI-type zeolites.78-80 Thus, we tentatively assigned these two bands to the copper ions exchanged at different sites: the two-coordinated copper ions responsible for the 16 500 cm-1 band and the threecoordinated copper ions responsible for the 13 400 cm-1 band. These bands eventually disappeared after evacuating the sample at 873 K. No contradictions can be found in the reduction features of copper ions among different experiments such as DR, XANES, IR, and TPD spectra. The organic residue derived from the acetate ligands (by decomposition) acts as a reductant in the reduction of copper ions from the divalent to the monovalent species. Incidentally, the ratio of the band areas for these two bands (A16500/A13400 ) 28/72) is close to that for the luminescence bands (E21000/E18500 ) 25/65), the latter being shown later. To further strengthen our interpretation of the DR spectra, we compared the excitation energy calculated using the hybrid density functional theory (DFT), B3LYP method,81 and the Gaussian03 program package82 with those obtained from the DR spectra in the d-d transition region for the respective bands of the two components discussed above. In the calculation, the 1s through 2p core electrons for Al, Si, and Cu atoms were replaced by the Los Alamos model core potentials.83 The Dunning-Huzinaga full double-ζ (D95) basis set is used for H and O atoms as well as the valence electrons for Al, Si, and Cu atoms.84 In these calculations, we adopted the pentameric models7 with the atomic composition of AlCuH13O17Si4 to represent the two- and three-coordinated Cu2+ cation in the MFI framework, where the Al atom is coordinated by four OSi(OH)3’s and the copper ion is anchored near the aluminum atoms in the zeolite framework. The formal charge assigned to the divalent copper ion compensated for any excess negative charge of the respective cluster together with the existence of the coordinated OH- ion; the charge of the clusters is set to neutral. For the geometry optimization, the four peripheral Si(OH)3 groups are fixed, and only the HOCuO4Al moiety is relaxed. Optimized structures are shown in Scheme 1. With the optimized structures, the time-dependent DFT calculation is carried out to evaluate the (singlet) d-d excitation energy. Among the excitations for the bidentate model, the lowest five energies are 6210, 12 340, 14 599, 14 760, and 20 083 cm-1, and their oscillation strengths are 0.0003, 0.0002, 0.0003, 0.0013, and 0.0001, respectively. We adopted the fourth one, which has the largest oscillation strength and a value close to the experimental excitation energy of 16 500 cm-1. The oc-


Itadani et al.

SCHEME 1: Optimized Structures for AlCuH13O17Si4 Clusters: (a) Bidentate Model and (b) Tridentate Model

Figure 9. MO contour maps relating to the d-d excitation of the bidentate model: (a) occupied MO (#66), which is one of the leading MOs, and (b) LUMO (#94). Both MOs have β-spins.

cupied manifold consists of several molecular orbitals (MOs), and the leading MOs are #66 (0.37), #67 (0.37), #71 (0.32), and #73 (0.31) with β-spin, whereas the destination of excitation is solely the lowest unoccupied molecular orbital (LUMO) (#94) with β-spin. The contour maps are drawn for #66 and #94 MOs in Figure 9. In MO #66, the magnitude of the 3d orbital on the Cu atom seems to be small compared to the contribution of the peripheral Si(OH)3 moiety. However, in MO #67 and #71, the atomic orbital (AO) component on the Cu atom is same as MO #66, and the AO components on the Si(OH)3 moiety are mutually cancelled. However, the LUMO has the clear shape of the Cu-OH antibonding phase. For the tridentate model, calculated excitation energies are relatively small compared to an experimental value of 13 400 cm-1. The lowest 10 energies are 5323, 7743, 7824, 10 808,

11 453, 11 614, 12 179, 12 663, 13 060, and 14 115 cm-1, and their oscillation strengths are 0.0004, 0.0021, 0.0003, 0.0036, 0.0020, 0.0052, 0.0012, 0.0008, 0.0036, and 0.0002, respectively. Considering both the energy and the oscillation strength, the ninth one is adopted. Among the source MOs of excitation, the leading MOs are #64 (0.42), #90 (0.41), and #87 (0.37) with β-spin, and the destination of excitation is solely the LUMO (#94) with β-spin again. The contour maps are drawn for #64 and #94 MOs in Figure 10. The characteristic feature of orbital phases is similar to that for the bidentate model except that the Cu 3d AO is more localized in MO #64. In addition, it can be said that the transition energy of three-coordination site is lower than that of the two-coordination site. Taking into account the present results using DFT, it is apparent that the position bound to three lattice oxygen atoms in a distorted trigonal structure (with the three lattice oxygen atoms) is expected to be the dominant one for the exchanged



Figure 11. (a) Adsorption isotherms and (b) differential heats of adsorption of CO on CuMFI(AAA) at 298 K. Filled and open symbols represent the first and second adsorptions, respectively. The first adsorption was measured for the sample evacuated at 873 K, and then the sample was evacuated at 298 K, followed by the measurement of the second adsorption.

Figure 10. MO contour maps relating to the d-d excitation of the tridentate model: (a) occupied MO (#64), which has the largest component, and (b) LUMO (#94). Both MOs have β-spins.

copper ion in the sample prepared from an aqueous solution of copper acetate and ammonium acetate. To obtain more detailed information on the active sites for N2 adsorption, we evaluated the number of Cu+ ions (reduced amount of Cu2+) in CuMFI(AAA) from the CO adsorption data. Figure 11 shows the adsorption isotherms and heats of adsorption of CO on CuMFI(AAA). Both isotherms of the first and second adsorption are of the Langmuir type, and the saturated value for the first adsorption is estimated to be 24.05 cm3 g-1 (at standard temperature and pressure). The difference in the adsorbed amount between the first and the second adsorption corresponds to the chemisorbed amount, Vc ) Vm1 - Vm2, and then Vc can be converted into the number of Cu+ species.6,78 The ratio of the number of Cu+ species (nCu+) to that of the total number of copper ions (nCu(T)), which is assumed to be a measure of reduction, is given in Table 1 together with the values for other samples. It can be seen that the extent of reduction is the greatest (88%) for CuMFI(AAA) among CuMFI samples. The heat of adsorption is 130 kJ mol-1 at the initial

adsorption stage and then decreases to yield a plateau of about 100 kJ mol-1 in the region of the adsorbed amount, 2.5-12.5 cm3 g-1. Such a plateau in the heat curve is indicative of the presence of an energetically homogeneous area to which threecoordinated sites correspond. Moreover, it is interesting to note that in the second adsorption where a reversible physisorption takes place the energetic homogeneity of the adsorption sites on CuMFI(AAA) is found out, in contrast to the other samples such as CuMFI(P), CuMFI(A), and CuMFI(F).30 3.2.5. EValuation of the ActiVe Site for N2 Adsorption. The PL spectrum of CuMFI(AAA) evacuated at 873 K was taken at 300 K, and it is shown in Figure 12. This spectrum is due to the electronic transition from the 3d94s1 to 3d10 orbital of the Cu+ species formed in the MFI after evacuation at 873 K.57-63,80 By resolving the observed spectrum, the luminescence band was found to be composed of three components: the luminescence bands centered at 21 000, 19 500, and 18 500 cm-1. Here, it is important to note that a large amount of N2 was adsorbed at 298 K on CuMFI(AAA), compared to the cases of adsorption on other CuMFI samples. In addition, the intensity of the 18 500 cm-1 band increased in the same manner as N2 adsorption on CuMFI samples: CuMFI(F) < CuMFI(A) < CuMFI(P) < CuMFI(AAA). From these results, it can be concluded that the site giving a luminescence band at 18 500 cm-1 acts as an active site for N2 adsorption at room temperature, in accordance with the previous description based on other experimental data.6,7 The adsorbed amount of N2 on CuMFI(AAA) corresponds to 77% of the total amount of Cu+ species, which was obtained from the second adsorption isotherm of N2; this value is in fairly good agreement with the value (65%) estimated by using the PL spectral bands on the assumption that all of the Cu+ species could be detected in the PL spectra and that the respective luminescence bands all had the same luminescence probability. This datum substantiates the validity of our assumption that the site-selective ion exchange occurs in the sample; namely, the


Figure 12. PL spectrum of CuMFI(AAA) after evacuation at 873 K. Dashed lines represent the component bands.

3-fold Cu+ species, which gives a luminescence band at 18 500 cm-1, is preferentially formed in CuMFI(AAA) prepared by using an aqueous solution containing an excess of acetate ions. Another reason for the increase in the amount of N2 adsorbed on CuMFI(AAA) is also possible; this increase corresponds to the increase in the amount of chemisorbed CO, resulting from the increase in the number of monovalent copper ions (Table 1). The latter increase bears on the presence of CH3COO- ions acting as a reductant; these anions were incorporated into the zeolite pore as an associated species of the copper ion. The detailed reduction mechanism, which is not understood at present, will be reported in a future publication. Figure 13 shows EXAFS, XANES, and DXANES (the first derivative of XANES) spectra of CuMFI(AAA) evacuated at 873 K before and after N2 adsorption. Taking into account the EXAFS data mentioned above (in section 3.2.4), the coordination number of the Cu+ ions in CuMFI(AAA) was evaluated to be 2.8 by curve-fitting analysis. It is reasonable to consider that the adsorption sites formed in the CuMFI are mainly in the twoor three-coordination structure situated at the intersection of two channels or on the main channel of the MFI. Such a consideration also has been described by many research groups: Zecchina’s group based on the experimental data,9 Nachtigall’s group proposed theoretically,33-36 Anpo’s group,80 and ours.7,68 Here, the copper ions are assumed to take a distorted linear or a trigonal pyramidal structure. In addition, we have so far claimed that the monovalent copper ions triply coordinated to the lattice oxygen atoms act as the active adsorption center for N2 molecules at room temperature, based on the results of EXAFS, PL, and IR experiments and DFT calculations.6,7,19 This is also supported by the present results. A pseudo-tetrahedral coordination environment around a copper ion will be established if N2 is adsorbed on the monovalent three-coordinated copper ions, as shown in Scheme 2. In such a process, the pristine two-coordination site remains unchanged, as it is even in an atmosphere of N2 under the same conditions. What should be noticed in our experimental data is that the first band of the EXAFS spectrum loses its intensity and simultaneously becomes broader after N2 adsorption, owing to the back-scattering from the nearest lattice oxygen atoms, and its distribution becomes wider, especially in the short distance direction (Figure 13a). This can be explained in terms of the increase in the Debye-

Itadani et al. Waller factor due to both Cu-N and Cu-O pairs resulting from the interactions of both Cu+-N2 (adsorbed species) and Cu+O2- (lattice oxygen). The protruding copper ion is displaced from its original position together with the appearance of an additional component due to Cu-N pair formation. With regards to the behavior over longer distances, the broadening of the spectrum may be attributed to the interaction of Cu+-(N)tN pairs. Such a view is supported by the fact that the original spectrum was almost recovered on evacuation of an N2-adsorbed sample at 300 K (spectrum 3). However, we have not succeeded in obtaining the structural parameters of the N2-adsorbed species from a detailed analysis of the EXAFS spectrum. On the basis of the above-mentioned structural models, the change in the XANES spectra can be discussed. The Cu+ ion has a d10 configuration, and thus the lowest-energy preedge features on the Cu K-edge will be 1s-4s and 1s-4p transitions, where only the transition to the 4p orbital is an electronic dipoleallowed transition, resulting in an intense feature in the preedge region. Figure 13b shows the XANES data for CuMFI(AAA). A pronounced change in the XANES spectra was observed after N2 adsorption. These spectra illustrate the fact that the preedge feature is very sensitive to the ion geometry and reflect the contribution of the 4p orbitals and the ligand-field splitting of the 4p orbitals through the interaction of N2 with 4p orbitals of the copper ions.85-95 Hence, the change in the XANES spectra observed in the present system allowed us to evaluate the change in the coordination environment around the Cu+ ion acting as the site for N2 adsorption. Important information on the coordination environment of the monovalent copper ion can be obtained from the preedge features of representative members of a series of Cu+ complexes with different coordination numbers, as reported by Solomon’s group.85,86 Linear, twocoordinate Cu+ sites will exhibit a large repulsive interaction along the z-axis, which is assumed to be a bonding axis, destabilizing the 4pz orbital. This results in the 1s-4px,y transition on the lower-energy side, yielding an intense preedge peak at 8.983 keV. Similarly, a split pattern is also expected from a three-coordination Cu+ environment, although the definition of the x-, y-, and z-axes may be different; that is, the plane formed from the x- and y-axes is the coordination plane (Scheme 3). Tetrahedral, four-coordinate Cu+ ions have all their 4p orbitals raised in energy due to a repulsive interaction with the ligands and show broad absorption features at energies >8.985 keV.86 The feature present in the data from the 873 K-treated sample is the appearance of a strong and sharp split band at 8.983 keV and around 8.995 keV. Another striking feature of the N2-adsorbed sample can be seen in the 1s-4p transitions, in the disappearance of the strong band at 8.983 keV, and in a slight increase in the intensity of the band at 8.986 keV resulting from an interaction with N2 at 300 K under a pressure of 13.3 kPa. Evacuation at 300 K almost recovered the original pattern. It is useful to remember the EXAFS and DR spectrum results; the active site for N2 adsorption forms a three-coordination structure. By considering the model constructed in the MFItype zeolite, the three-coordinate site of the 873 K-treated sample takes an arrangement close to a distorted trigonal pyramidal arrangement. In such a case with a distorted trigonal planar symmetry, the 4p orbital of the 873 K-treated sample would be split into two components: one band derived from a 1s-4px,y transition that is situated on the higher-energy side (8.995 keV) owing to a strong repulsion and the other from a 1s-4pz transition of the monovalent copper ion on the lower-energy side (8.983 keV). The N2 adsorption on this site results in a



Figure 13. (a) EXAFS, (b) XANES, and (c and d) DXANES spectra of CuMFI(AAA) at different stages: (1) evacuated at 873 K; (2) after N2 adsorption; (3) evacuated at 298 K after N2 adsorption. DXAFS spectra c and d correspond to the processes of 1 and 2.

SCHEME 2: Adsorption Model of N2 on the Monovalent Copper Ion

four-coordinated arrangement, namely, relatively close in structure to a distorted tetrahedral arrangement. This change brings about a variation in the spectral bands; especially, the 4pz level band changes through a mixing of the 4s orbital, and the newly observed band is slightly shifted to the higher-energy side (at 8.986 keV) through an interaction with N2.86 This situation is clearly seen in the first derivative of the XANES spectra, DXANES, shown in Figures 13c and 13d. Conclusively, it is fair to say that the monovalent copper ions exchanged at the three-coordination sites in MFI are the active centers for N2 adsorption by which a distorted tetrahedral arrangement can be established. However, Moretti’s, Kazansky’s, and Serykh’s groups have proposed other N2-adsorption models for copperion-exchanged MFI in their recent papers.96-101 At present, it is undecided which model among these is right.

SCHEME 3: Energy Diagrams of Two- and Three-Coordination Environment Evaluated from the XANES Data

4. Conclusions We have succeeded in preparing a copper-ion-exchanged MFI-type zeolite (CuMFI) having a very efficient adsorption capability (in both amount and energy) for N2 molecules at room temperature by the ion-exchange method employing an aqueous solution of Cu(CH3COO)2 containing NH4CH3COO species as

12022 J. Phys. Chem. C, Vol. 111, No. 32, 2007 auxiliaries. The adsorption properties toward N2 and CO molecules were examined and compared to those of samples prepared by other ion-exchange methods in which an aqueous solution of copper ions contains counterions such as Cl-, NO3-, HCOO-, CH3COO-, or C2H5COO-. For CuMFI(AAA), this sample being prepared by using a mixed aqueous solution of Cu(CH3COO)2 and NH4CH3COO, the characteristic features of N2 adsorption were observed. The differential heat of adsorption (qdiff) of N2 molecules gave a value of 87 kJ mol-1 in the initial adsorption stage, which is the largest value obtained so far. The qdiff curve for N2 adsorption yielded a plateau in the 1.9-8.2 cm3 g-1 region after chemisorption (1.3 cm3 g-1), reflecting an energetically homogeneous surface of the CuMFI(AAA) sample. Even in the reversible adsorption, the qdiff values were relatively large, 70-60 kJ mol-1. As for CO adsorption, the energetically homogeneous region corresponded to ca. 45% of the total CO adsorption, being consistent with the monolayer capacity for N2 adsorption. The present data obtained have been rationalized as being indicative of a site-selective ion exchange of copper ions occurring with the formation of the complex with acetate ligand. The copper ions thus exchanged were easily and efficiently reduced to the monovalent state owing to the decomposition of the organic ligands incorporated simultaneously with the copper ions in the ion-exchange process. Taking account of the photoluminescence spectra, the three-coordinate copper ions giving an luminescence band at 18 500 cm-1 are active sites for N2 adsorption at room temperature, which is consistent with the results obtained from DFT calculations. Moreover, these copper ions have a distorted tetrahedral arrangement after N2 adsorption, as is also supported by the XAFS data, showing a significant decrease in the intensity of the band at 8.983 keV for Cu+ formed in the MFI through N2 adsorption. The present study will be helpful in developing more efficient materials for the separation, fixation, and activation of N2 molecules. Acknowledgment. This work was supported by Grants-in Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (Grant Nos. 15350082, 17034046, 17655061, and 17036043). The measurements of XAFS spectra were performed under Proposal Nos. 2004G092 and 2004G329 of the Photon Factory Program Advisory Committee. We thank Professor M. Nomura and Drs. Y. Inada, A. Koyama, and N. Usami of KEK in Tsukuba for their kind assistance in measuring the XAFS spectra. A.I. acknowledges Okayama University for a postdoctoral fellowship. Supporting Information Available: Additional supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Himmel, H.-J.; Reiher, M. Angew. Chem., Int. Ed. 2006, 45, 6264. (2) Kuroda, Y.; Konno, S.; Morimoto, K.; Yoshikawa, Y. J. Chem. Soc., Chem. Commun. 1993, 18. (3) Kuroda, Y.; Yoshikawa, Y.; Konno, S.; Hamano, H.; Maeda, H.; Kumashiro, R.; Nagao, M. J. Phys. Chem. 1995, 99, 10621. (4) Kuroda, Y.; Maeda, H.; Yoshikawa, Y.; Kumashiro, R.; Nagao, M. J. Phys. Chem. 1997, 101, 1312. (5) Kuroda, Y.; Yagi, K.; Yoshikawa, Y.; Kumashiro, R.; Nagao, M. Chem. Commun. 1997, 2241. (6) Kuroda, Y.; Yoshikawa, Y.; Emura, S.; Kumashiro, R.; Nagao, M. J. Phys. Chem. B 1999, 103, 2155. (7) Kuroda, Y.; Kumashiro, R.; Itadani, A.; Nagao, M.; Kobayashi, H. Phys. Chem. Chem. Phys. 2001, 3, 1383.

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A New Strategy for the Improvement of the N2 Adsorption Properties

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