J. Phys. Chem. 1995, 99, 10019-10023
10019
Electron Spin Resonance and Electron Spin Echo Modulation Studies of Cupric Ion Ion-Exchanged into Siliceous MCM-41 Andreas Pdppl, Mike Newhouse, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: February 24, 1995; In Final Form: April 17, 1995'
Cupric ions have been introduced into siliceous MCM-41 by liquid and solid state ion exchange. Electron spin resonance and electron spin echo modulation techniques were used to study the coordination of Cu(I1) in dehydrated Cu-MCM-41 and its interaction with polar adsorbates such as D20 and NH3. This study reveals major differences in the location of cupric ions introduced by liquid or solid state ion exchange. Cupric ions exchanged by liquid state reaction are in a distorted tetrahedral geometry coordinated presumably to four framework oxygens in dehydrated Cu-MCM-41. After dehydration some copper ion pairs are observed. Polar adsorbates like water and ammonia remove the Cu(I1) from the ion exchange site to form [Cu(H20)6I2' or [CU(NH~)~]*+ complexes, indicating only a weak interaction of the cupric ion in the ion exchange site with the MCM-41 framework. Cu(I1) ions introduced by solid state ion exchange do not show any interaction with the MCM-41 framework. After dehydration the cupric ions are replaced by copper clusters which indicates that the cupric ions introduced by solid state ion exchange occupy different sites than by liquid phase ion exchange. However, similar interactions with adsorbates are observed in both solid state and liquid state ion-exchanged Cu-MCM-4 1.
Introduction
Experimental Section
Recently, a new class of mesoporous silica tubelike materials designated as MCM-41 has been discovered.'.2 These mesoporous materials are composed of a regular hexagonal array of silica tubes with internal diameters ranging between 20 and 100 A depending on the template and synthesis conditions used.3 These materials can be synthesized in a large range of framework SUA1ratios. The channel diameters are substantially greater than the channels and cages in aluminosilicate (zeolite) and aluminophosphate microporous materials which have maximum diameters of about 15 A. Thus, these new mesoporous materials are promising for new adsorptive and catalytic applications involving large molecules. A major challenge in the development of MCM-41 materials is the incorporation of transition metal ions to generate specific inorganic catalysts. Metal ion sites offer the potential of specifically tailored catalytic applications of microporous or mesoporous oxide material^.^ In this study we use electron spin resonance (ESR) and electron spin echo modulation (ESEM) to explore the incorporation of Cu(I1) into siliceous MCM-41 material via ion exchange. Nuclear magnetic resonance (NMR) investigations have suggested that about 20% of the silicon atoms are silanols in calcined siliceous MCM-41,z indicating some ion exchange capability for silanol protons by metal ions. Here we focus on the comparison of Cu(I1) ion exchange by two different methods, liquid state and solid state ion exchange. The geometry of adsorbed water and ammonia molecules with respect to the Cu(I1) ion in MCM-41 has been investigated. The results of Cu(I1) incorporation in siliceous MCM-41 has been contrasted to a former investigation of Cu(I1) ion exchange in silica g e l - ' Like in siliceous MCM-41 metal ion exchange is only caused by hydroxyl proton exchange in silica gel materials. Therefore, we expect similarities between Cu(I1) ion exchange in siliceous MCM-41 and silica gel.
The MCM-41 synthesis procedure used was similar to that reported by Beck et al.* Tetrabutylammonium silicate (TBAS) was prepared in a 10:1 ratio from tetrabutylammonium hydroxide (40 wt %, Aldrich) and fumed silica (Sigma). Cetyltrimethylammonium chloride (CTAC) (25 wt %, Aldrich) was used as the surfactant. Thus, 20.3 g of CTAC and 12.21 g of TBAS were mixed together with 5.94 g of H20. Finally, 5.91 g of fumed silica was dissolved in the mixture. The resulting reaction gel was placed in a Teflon bottle and heated for 3 days at 95 "C. The resulting reaction mixture was cooled to room temperature, filtered and washed with distilled water, and finally dried in air. The remaining template was removed by calcination as follows. The samples were heated in a nitrogen stream with a heating ramp 2 Wmin from room temperature to 540 "C. Then the material was heated in flowing oxygen at 540 "C for 16 h and cooled in flowing oxygen. Powder X-ray diffraction (XRD) pattems were collected before and after calcination to confirm that the product was MCM-41. The XRD spectra were recorded with a Philips PW 1840 diffractometer. Both spectra before and after calcination are in accordance with published results2 and show an intense peak at 28 = 2.2", indicating the existence of the hexagonal MCM-41 phase with a spacing of about d l = ~ 40 A. Liquid state Cu(I1) ion exchange was performed by adding 1 M NH40H solution dropwise to a 1 mM solution of Cu(N03)~ until the precipitation of Cu(0H)z was observed. Then a 0.1 M solution of NH4NO3 was added until the precipitate just dissolved.6 This Cu(I1) solution (60 mL) at pH = 8 was added dropwise to 0.3 g of calcined MCM-41 and stirred for 6 h. Finally, the MCM-41 was washed and filtered. The sample showed a light blue color and will be called "fresh". The amount of Cu(I1) added initially to the MCM-41 in the liquid state ion exchange corresponds to 1.5 x mol g-'. Solid state ion exchange was performed by mixing 0.2 g of calcined MCM-41 with 5 mg of Cu(II)C12, corresponding to a Cu(I1) concentration of 1.5 x mol g-I. The mixture was pressed in a stainless steel dye with a force of 2 t. The resulting
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Abstract published in Advance ACS Abstracts, June I , 1995.
0022-365419512099-10019$09.00/0
0 1995 American Chemical Society
Poppl et al.
10020 J. Pkys. Ckem., Vol. 99, No. 24, 1995
400G
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magnetic field, indicating a distribution of g values and hyperfine coupling parameterslo in a fresh (L)Cu-MCM-41 sample. The Cu(I1) ESR spectrum in a sample evacuated at 573 K changes drastically (Figure lb). The line width increased strongly, and the Cu(I1) parallel hyperfine coupling was no longer resolved. The Cu(I1) spectrum can then be described by an orthorhombic g tensor with ,g, = 2.07, g, = 2.13, and gzz= 2.49. We are not able to decide whether.the decrease in the Cu(I1) signal intensity is only caused by the increase in the line width or by partial reduction of the Cu(I1) ions as the signal-to-noise ratio of the Cu(I1) spectrum does not allow a reliable double integration of the spectrum to compare the intensity with the spectrum of a fresh sample within reasonable errors. Furthermore, a sharp signal at g = 2.004 was observed that is ascribed to an 0- lattice defect formed on evacuation close to the Cu(I1) ion.” Besides the orthorhombic Cu(I1) spectrum and the sharp signal of the 0- species, the spectrum of an oxidized (L)Cu-MCM-41 sample showed a new signal at H = 1700 G (Figure IC). This signal is located at half of the magnetic field of the Cu(I1) ESR spectrum. Such a signal near 1700 G was not observed in a dehydrated and oxidized reference MCM-41 sample. Therefore, we can exclude that this signal is caused by Fe(II1) impurities near g 4. We attribute this signal to Cu(I1) pairs that are formed during the oxidation. The two Cu(I1) ions involved in a Cu(I1) pair are coupled by dipolar interaction D and form a S = 1 system. We then expect two transitions due to Ams = f l at the magnetic field (H)positions:
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Figure 1. ESR spectra at 77 K of (L)Cu-MCM-41: (a) fresh sample, (b) evacuated at 573 K, (c) oxidized at 573 K. Note the different magnetic field scale in (c).
pellets were then heated for 15 h at 420 “C in air. After the heating process the pellets were ground to a fine powder. The sample showed a light yellow color and will also be called “fresh’. In the following the liquid state ion-exchanged sample is designated as (L)Cu-MCM-41 while the solid state ionexchanged material is designated as (S)Cu-MCM-41. “Activated” samples were prepared by evacuation ( < Torr) at 297 K, heating at 373 K for 15 h, and heating at 573 K for 3 h in vacuum to dehydrate the samples. Then the samples were oxidized at 573 K for 3 h and again evacuated at 297 K. Adsorbed samples were prepared from activated samples by adsorbing DzO at its vapor pressure at 297 K or I5NH3 (Cambridge Isotope Laboratories) at 6 Torr for 12 h. ESR spectra were recorded at 77 K on an ESP 300 Bruker spectrometer. ESEM measurements were performed on an ESP 380 Bruker FT ESR spectrometer at 4 K. The three-pulse sequence 90”-r-90”-T-90”-echo8 was used with t = 240 ns to enhance deuterium modulation. Field-swept ESE spectra were recorded using the two-pulse sequence 90”-z- 180”-zecho8 with r = 88 ns. Simulations of the time-domain deuterium ESEM patterns were performed using the analytical expressions derived by Dikanov et aL9 The modulation function is simulated and fitted to the experimental data by a least-squares procedure to determine the number of interacting nuclei N , the distance R between the paramagnetic center and the interacting nuclei, and their isotropic hyperfine interaction A,,,. Results The Cu(I1) ion ESR spectrum of a fresh (L)Cu-MCM-41 sample is illustrated in Figure la. The powder spectrum can be described by the following set of spin Hamiltonian parameters: an axial g tensor with gll = 2.325, g l = 2.064, and an axial metal ion hyperfine coupling with All = 0.0168 cm-I. The Cu(I1) hyperfine coupling in the perpendicular part of the powder spectrum was not resolved. The line widths of the Cu(I1) hyperfine signals in the parallel part increase with increasing
H = hv/g,@f Dig,@
(1)
where H points along the vector joining the two Cu(I1) ions in the pair. Also, three transitions arise for H directed perpendicular to the Cu(I1)-Cu(I1) direction. Two of these transitions are for Ams = f l at
and one is for Ams = f 2 at
H = [(hv)’ - D2]1/2(2gJ)-1
(2b)
which is approximately half the field of the other two lines.I2 All quantities have their usual meaning. In the spectrum illustrated in Figure IC only the Ams = f 2 transition is observed. In reported results on Cu(I1) pairs,’* the Am, = f l transitions are usually more intense than the Am, = 5 2 transition. But in the case of a distribution of the parameter D, the Ams = f l transitions are smeared out while the Am, = f 2 transition is less affected by a distribution of D. This is apparent by inspection of eqs 1 and 2. D enters the positions of the Ams = f l transitions to first order whereas D enters the position of the Am.7 = k 2 transition only to second order under the reasonable assumption D
