J. Phys. Chem. 1996, 100, 1793-1800
1793
Electron Paramagnetic Resonance and Electron Spin Echo Modulation Study of Surface Sites of the Porous Aluminosilicate MCM-41 Using Transition Metal Ion Probes Vittorio Luca,*,† Dugald J. MacLachlan,*,† Richard Bramley,† and Keith Morgan‡ Research School of Chemistry, The Australian National UniVersity, Canberra, ACT 0200 Australia, and Industrial Research Limited, P.O. Box 31-310, Lower Hutt, New Zealand ReceiVed: July 25, 1995X
The paramagnetic cations, Cu(II), Mo(V), and Cr(V) have been adsorbed or grafted onto the surfaces of the mesoporous aluminosilicate MCM-41 containing 5 mol % Al. Electron paramagnetic resonance and electron spin echo modulation spectroscopies were used to probe for a possible interaction between the adsorbed paramagnetic cations and 27Al nuclear spins contained within the MCM-41 wall structure. Limited evidence has been found for the binding of the probe cations to tetrahedral Al sites in the MCM-41 structure, suggesting that the incorporation of Al in MCM-41 does not produce genuine exchange sites similar to those observed in ion exchangers such as zeolites and clay minerals. It is suggested that adsorbed cations are mostly bound to SiO4 tetrahdra that are not adjacent to surface AlO4 tetrahedra. Models are put forward for the configuration of the adsorption sites and the location of the aluminum in the framework walls.
Introduction The property of organic molecules to organize the formation of inorganic structural elements such as oligomers of Si, Al, and P in the synthesis of porous silica/alumina gels, molecular sieves, and zeolites has been exploited to good effect for many years. Short-chain quaternary amines and alcohols of various types have been the usual choice as structure directors. The synthesis of high surface area silicas using surfactant micelles and reverse micelles is well documented. However, purposeful manipulation of macromolecular organic assemblies for the generation of ordered inorganic structures has been demonstrated only recently. Silicates with extremely high surface areas and a narrow distribution of pore sizes around 40 Å occur as products of the reaction of cetyltrimethylammonium (CTA) bromide with the layered silicic acid kanemite.1 Related materials consisting of hexagonal arrays of pores have been synthesized by workers at Mobil Corp. via tetraalkylammonium surfactant micellar templates.2 It was shown that by deliberate choice of surfactant chain length or the addition of organic “spacers” the size of the organic micelles and the resulting silica network could be controlled precisely.3 Mechanisms for the formation of this family of tubular silicates and aluminosilicates dubbed M41S by the Mobil group have resulted in much scientific interest, and a large body of literature has been generated in a relatively short space of time. This novel family of porous oxides have large uniform pores, but the walls appear to lack a well-defined structure, spectroscopic evidence suggesting similarity to amorphous silica. In the pure silica form these materials lack high surface acidities and cation exchange capacities. The Mobil workers did attempt to synthesize versions of their MCM-41 materials containing limited amounts of Al, and subsequently others have attempted to incorporate greater amounts of Al as well as other metals.4-6 However, without the benefit of a crystal structure for these materials it is often very difficult to deduce whether the hetero atom actually substitutes for Si in the framework or resides on the surface * Authors to whom correspondence should be addressed. Current address: School of Chemistry, The University of New South Wales, Sydney 2052, Australia. † The Australian National University. ‡ Industrial Research Limited. X Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-1793$12.00/0
either in monomeric form, as an amorphous oxide coating, or as small crystalline particles. A significant increase in acidity and cation exchange capacity has not been observed when a portion of the framework Si is replaced by a hetero atom of lower valence, creating a negative charge at the point of substitution. In contrast hetero atom substitution in zeolites and clay minerals results in pronounced acidity and an increase in cation exchange capacity. In the case of Al the limit of substitution appears to be around 10-20%.7 Questions still remain regarding the exact siting of Al and the reactivity of any surface Al sites. Moreover, little is known about the interaction of possible active sites introduced as a result of cation substitution with adsorbed cationic and molecular species. As with zeolites and clay minerals the replacement of Si by a cation of lower valence in the MCM-41 walls should produce a negative charge deficit, which, depending on the manner in which the charge is distributed around the point of substitution, will be compensated by a counterion, which will be relatively weakly bound, or will result in protonation of a Si-O-Al bridge to give an OH group. The proton of this OH group will in turn have a certain acidity dependent on the local charge distribution at the substitution site and if weakly bound will be exchangeable by cations such as Cu(II). Many workers have alluded to the possible generation of exchange sites in MCM-41 by Al for Si substitution in MCM-41, but no evidence of their existence has so far been forthcoming. In this study, we wish to determine if the introduction of Al into the MCM-41 structure produces exchangeable cation sites to which simple cations can bind or reactive species can graft. The possible interaction of cations such as Cu(II), Mo(V), and Cr(V) with structural Al in the MCM-41 is probed by the complementary techniques of electron paramagnetic resonance (EPR) and electron spin echo modulation (ESEM) spectroscopy in an attempt to characterize the adsorption sites on the surfaces of an aluminosilicate MCM-41 sample with Si/Al ) 19. EPR and ESEM spectroscopies are well suited to this task because of their ability to monitor the strong (EPR) and weak (ESEM) hyperfine interaction between paramagnetic cations and magnetic nuclei (e.g. 27Al) contained within the framework of porous and nonporous oxides.8 The metal species are introduced either © 1996 American Chemical Society
1794 J. Phys. Chem., Vol. 100, No. 5, 1996 by treatment with aqueous solutions or through grafting reactions in which volatile metal chlorides and oxides such as MoCl5 and CrO3 react with OH groups on the oxide surfaces.9 Experimental Section Syntheses. MCM-41 was synthesized using a method similar to that of Monnier et al.10 with the following molar ratios of reactants: 1 M SiO2:0.025 M Al2O3:0.115 M Na2O:0.233 M CTA-chloride:0.089 M tetramethylammonium hydroxide:125 M H2O. Tetraethyl orthosilicate solution (Aldrich) in 20 mL of ethanol was added to an aqueous solution containing CTAchloride (Fluka) and AlCl3·6H2O (Aldrich) in water. The synthesis was carried out at 60 °C under constant stirring for a period of 26 h, after which the product was filtered, thoroughly washed with distilled water, and finally calcined in air at 540 °C for 24 h. Copper Exchange of MCM-41 was carried out by overnight stirring of 150 mg of the MCM-41 in 100 mL of 0.050 M CuCl2 solution, the pH of which had been adjusted to 4.5 with HCl. The MCM-41 was then filtered and washed. Deuterium exchange of Cu(II)-MCM-41 was achieved by repeatedly evacuating and exposing the sample to a D2O-saturated atmosphere and finally soaking in D2O. The FTIR spectrum of the deuterated MCM-41 resembles that of amorphous silica and aluminosilicate gels.11 FTIR spectra show that evacuation at 25 °C does not completely remove physisorbed water from the MCM-41 pores and surfaces, as judged from the persistence of the HOH bending vibration at 1630 cm-1. However evacuation at 150 °C effectively eliminates this vibration, indicating the removal of surface water. Exchange of the protons for deuterons of surface hydroxyl groups was accomplished by first evacuation of the sample at 25 °C followed by a 24 h exposure to a saturated vapor pressure of D2O for 24 h at room temperature. This was repeated three times, followed by evacuation at 450 °C and reexposure to a saturated vapor pressure of D2O for 48 h at room temperature. The sample was reevacuated at 150 °C and exposed to a saturated D2O atmosphere one last time. Metal Grafting. Cr-grafting was carried out using the method of Kucherov and Slinkin.9,12 This entailed first hydrogenexchanging the calcined MCM-41 by treating with 1 M HCl for 15 min, followed by thorough washing until the washings were free of Cl- and then drying at 80 °C. The XRD pattern of the acid-treated calcined material was identical to that of an untreated sample, and there was no perceptible loss of Al as measured by energy dispersive X-ray (EDX) analysis. CrO3, 16-18 mg, was added to 180 mg of the dried acid-treated MCM-41 sample, and the mixture was ground and subsequently heated in air at temperatures between 300 and 500 °C. Mo-grafting was carried out using the method described by Che et al.13 MoCl5 (100 mg) was added to 100 mL of chloroform. To 27.5 mg of the calcined MCM-41, which had been dried under vacuum in a quartz EPR tube at 200 °C for 2 h, was added 0.5 mL of the MoCl5/chloroform solution. The EPR tube containing the chloroform suspension of MCM-41 was sealed and transferred to a vacuum line, where the solvent was removed under reduced pressure. Finally the sample was evacuated to 5 × 10-5 Torr. Electron Paramagnetic Resonance. X-band EPR spectra were collected on a Varian V-4502 spectrometer interfaced to a PC data acquisition system and equipped with an Oxford Instruments flow cryostat. For 35 GHz (Q-band) spectra a Varian microwave bridge was used. Spectrometer frequency was determined with a HP4254L frequency counter. Fields were measured with a Resonance Technology Ltd Model CX86 teslameter. Simulations of EPR spectra were achieved using
Luca et al. a second-order perturbation theory fitting program, SIMER, which allows for multiple species and the incorporation of a Gaussian background function. Electron Spin Echo Modulation. ESEM data was recorded on a home-built spectrometer using microwave frequencies of 9-10 GHz. Two-pulse echo decay envelopes were recorded using a π/2-τ-π pulse sequence and measuring the echo intensity as a function of τ, the interpulse separation. Threepulse-stimulated echo decays were recorded using a phasecycled π/2-τ-π/2-T-π/2 pulse sequence with fixed τ with the echo detected as a function of T. In the case of the threepulse sequence it is possible by choice of τ to enhance or suppress different frequencies. The echo decay envelopes are modulated by precessional frequencies of nearby nuclei and can be analyzed in terms of the number of interacting nuclei, the electron spin nuclear distance, and an isotropic hyperfine coupling. For interacting nuclei with I > 1/2 quadrupole interactions should also be considered. In cases where the hyperfine interaction is essentially dipolar and the quadrupole interaction is small compared to the hyperfine interaction the observed modulation frequencies are not shifted from their free precession values. General expressions for an arbitrary nuclear spin interacting with an electron spin S ) 1/2 have been derived,14 which consider the dipolar and hyperfine interactions in the limiting case of zero quadrupole interaction and hyperfine interaction smaller than dipolar interactions. Simulations of three-pulse ESEM spectra were performed using both simplex, ESFT, and nonlinear least-squares, FITESE, refinement programs, both based on the theory of Dikanov et al.14 As the nuclei studied here have small quadrupole moments and no shift of the nuclear frequencies from their Larmor frequencies is detected, the model used is justified. The fitting routine employs three parameterssn, the number of equivalent interacting nuclei at a distance r and having an isotropic hyperfine coupling aisosand employs the spherical approximation to account for the powder spectra.15 The errors quoted are those derived from least-squares fitting of the data and are a guide to the uniqueness of the models. Inadequacies in the theory employed result in estimated errors of n ( 1, r ( 0.1 Å, and aiso ( 0.05 MHz. MAS-NMR. Solid-state 27Al NMR spectra were acquired on a Varian Unity 500 spectrometer. Samples were packed into 5 mm rotors and spun at speeds of 11 kHz in a supersonic, single-tuned probe from Doty Scientific. Spectra were recorded using a single-pulse experiment, at 130 MHz and a spectral window of 100 kHz. Pulse repetition times were 0.2 s, the rf field was about 60 kHz, and pulse widths were 4 s. Peaks were referenced with respect to 1 M aluminum nitrate. X-ray Diffraction. X-ray diffraction patterns were recorded on a Siemens D5000 X-ray powder diffractometer using Cu KR radiation. Results The X-ray powder diffraction pattern of the synthesized material after calcination at 540 °C in air (Figure 1) is characteristic of MCM-41 having moderate ordering of the mesopores16,17 and is comparable to other published data of samples with similar Al contents. The existence of mesoporosity in the synthesized sample was confirmed by the N2 adsorption isotherm which was of type IV,18 BET surface area 920 m2 g-1. Transmission electron microscope lattice fringe images were also similar to those previously published. To establish that the Al is located predominantly within the MCM41 structure, a 27Al MAS-NMR spectrum was recorded of the material calcined to 540 °C in air. The presence of a main resonance at about 50 ppm indicates that most of the Al is in
Surface Sites of the Porous Aluminosilicate MCM-41
J. Phys. Chem., Vol. 100, No. 5, 1996 1795
Figure 1. Powder XRD pattern of MCM-41. Numbers above peaks are the d-spacings in angstroms. Figure 3. Three-pulse ESEM spectra of Cu(II)-grafted MCM-41 after (a) 1 h in a water-saturated nitrogen stream, ν ) 9.2467 GHz, H ) 3179 G, τ ) 142 ns, and (b) 1 h in a dry nitrogen stream, ν ) 9.3214 GHz, H ) 3198 G, τ ) 142 ns.
Figure 2. 298 K X-band EPR spectra of (a) Cu(II)-exchanged MCM41 exposed to a water-saturated nitrogen stream for 1 h, ν ) 9.2465 GHz, and (b) Cu(II)-exchanged MCM-41 after 1 h in a dry nitrogen stream, ν ) 9.2467 GHz.
tetrahedral coordination and therefore probably resides within the MCM-41 wall structure. A feature at 0 ppm, consisting of about 20% of the total aluminum, indicates octahedral coordination and therefore either extraframework or structural Al exposed and hydrated at the surface. Note that no special precautions were taken to exclude atmospheric moisture from the sample prior to recording the NMR spectra. This result is similar to that obtained by other workers who, as in the present study, used monomeric sources of Al.7,19 To probe the nature of the pore surfaces of the MCM-41 and the location of the aluminum, we attempted to exchange Cu(II) onto putative exchange sites from aqueous solution. The EPR spectrum of a Cu(II)-exchanged sample exposed to a watersaturated nitrogen stream for 1 h or more contains two Cu species (Figure 2a). Species A is identified by the g ) 2.07 feature, which is the characteristic g⊥ of a rigid limit Cu(II) spectrum, while species B with g ) 2.14 is characteristic of motionally averaged Cu(II) as occurs in solution or as has been observed in water-swelled clays20-22 and in some hydrated zeolites.23 Even in the completely wet state the relative intensities of the species A and B signal remained similar to that of the damp sample shown in Figure 2a. This indicates that on the EPR time scale a certain proportion of the Cu(II) remains bound to the MCM-41 walls even when the pores are filled with water. Modulation depth in the three-pulse ESEM experiment depends on the value of τ and is least when τ is equal to the modulation period of the nucleus and greatest when τ is equal to half this period.8b It is therefore possible to set
the τ value so as to enhance any possible 27Al modulation. The ESEM data of Figure 3a recorded on the hydrated Cu(II)exchanged MCM-41 with τ chosen to enhance 27Al modulation show extensive proton modulation, ca. 14 MHz, but no 27Al modulation, ca. 3.9 MHz. A Cu(II)-exchanged MCM-41 sample that was dehydrated by passing dry nitrogen over the sample gave the EPR spectrum of Figure 2b. This treatment ensures that sufficiently little water was present within the pores and that all the Cu(II) is immobile and probably anchored to the surface since a rigid limit spectrum is obtained. The EPR spectrum does not show any superhyperfine structure that might be associated with the adsorbed copper ions interacting strongly with any 27Al on the surface. Extremely weak aluminum modulation is detected in the corresponding three-pulse ESEM spectrum of Figure 3b together with some proton modulation. The Fourier transform of this data (not shown) indicates a very weak peak at about the 27Al Larmor frequency that is only just discernible from the background noise. To assess the significance of the aluminum modulation, and for comparison, EPR and ESEM spectra were recorded of three Al-containing oxide systems exchanged with copper. These are zeolite A, which has an Si/Al ratio of 1 and a correspondingly high cation exchange capacity; boehmite, which is a layered poorly crystalline Al oxide/hydroxide AlOOH; and a precipitated silica-alumina gel, Si/Al ) 19. Each of these three materials gives a rigid limit Cu(II) EPR spectrum (Figure 4) and no resolved aluminum superhyperfine structure. Recording the spectra at the Q-band did not further assist in identifying the species present, possibly as a result of g-strain. The zeolite A sample contains, in addition to Cu(II) resonances, EPR signals from some Mn2+ and iron oxide impurities. The axial Hamiltonian parameters of these spectra are similar and generally typical of tetragonally distorted octahedral Cu(II) adsorbed on hydrated oxide surfaces. The three-pulse ESEM data corresponding to these EPR spectra are given in Figure 5. For each of these samples the τ value was chosen to enhance the 27Al modulation, i.e. τ ) 140 ns at 3300 G. Both the zeolite and the gel were in the air-dried state, while the boehmite sample was examined as an aqueous suspension. As expected, the ESEM spectrum of the zeolite sample (Figure 5a) shows extensive 27Al modulation at the 27Al Larmor frequency, indicating that the hyperfine and quadrupole couplings are relatively weak and that the spectrum is dominated by the
1796 J. Phys. Chem., Vol. 100, No. 5, 1996
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Figure 4. 298 K X-band EPR spectra of (a) Cu(II)-exchanged zeolite A powder at ambient humidity, ν ) 9.2452 GHz, (b) Cu(II)-exchanged boehmite suspension, ν ) 9.2391 GHz, and (c) Cu(II)-exchanged aluminosilicate gel containing 5 mol % Al powder at ambient humidity, ν ) 9.2471 GHz.
Figure 6. Three-pulse ESEM spectrum of (a) D2O suspension of Cu(II)-exchanged MCM-41, ν ) 9.2108 GHz, H ) 3163 G, τ ) 290 ns, (b) sample from (a) dehydrated under vacuum, ν ) 9.2100 GHz, H ) 3188 G, τ ) 290 ns, and (c) sample from (b) evacuated at 150 °C, ν ) 9.1880 GHz, H ) 3164 G, τ ) 242 ns.
Figure 5. Three-pulse ESEM spectra of (a) Cu(II)-exchanged zeolite 4A powder at ambient humidity, ν ) 9.2797 GHz, H ) 3207 G, τ ) 142 ns, (b) Cu(II)-exchanged boehmite suspension, ν ) 9.3007 GHz, H ) 3205 G, τ ) 142 ns, and (c) Cu(II)-exchanged aluminosilicate gel at ambient humidity, ν ) 9.7100 GHz, H ) 3358 G, τ ) 134 ns.
Zeeman interaction.24,25 This in turn suggests that the encapsulated Cu(II) is relatively weakly and indirectly bound to Al oxygens. The ESEM spectrum of the Cu(II)-doped boehmite suspension (Figure 5b) is similar to that of the zeolite sample except that some proton modulation at 14 MHz is observed. The boehmite sample was recorded as an aqueous suspension, and it is therefore not surprising that the ESEM spectrum shows proton modulation. In this sample the copper appears to be surrounded
by water molecules, as both axial and equatorial waters are detected in the two-pulse experiment.26 The ESEM spectrum of the aluminosilicate gel sample containing a comparable amount of Al as in the MCM-41 sample (Figure 5c) shows weak but clearly discernible 27Al modulation. The Fourier transform (FT) spectrum confirms a single resonance at the 27Al Larmor frequency. The environment of the adsorbed Cu(II) ions in MCM-41 can be monitored by determining the degree of coordinative unsaturation of the Cu(II) by exchange with D2O. To assess the efficacy of deuterium exchange, we monitored the FTIR spectrum of the exchanged material as a function of D2O exchange and outgassing temperature and estimate >90% H-D exchange. The ESEM pattern of the D2O-soaked Cu(II)-exchanged MCM-41 sample is shown in Figure 6a and could be best simulated using a two-shell model comprising eight deuterons in the first shell at r ) 2.9 ( 0.1 Å with aiso ) 0.23 ( 0.13 MHz and four in the second, more distant shell at r ) 3.34 ( 0.06 Å and aiso ) 0.01 ( 0.06 MHz. When the MCM-41 is dehydrated by evacuation at 25 °C to 5 × 10-5 Torr, there is a
Surface Sites of the Porous Aluminosilicate MCM-41
J. Phys. Chem., Vol. 100, No. 5, 1996 1797 TABLE 1: EPR Parameters for Transition Metal Grafted MCM-41 sample Mo(V)-MCM-41 Mo(V)-MCM-41+ND3 Cr(V)-MCM-41, 300 °C Cr(V)-MCM-41, 500 °C Cr(V)-MCM-41+ND3 Mo(V)-SiO2a
a
species
g⊥
g|
%
1.919 1.963 1.893 1.898 1.950 1.947 1.950 1.947 1.928 1.951 1.892 1.866 1.755 1.968
80 20 70 30 92 8 85 15 90 10
Mo5+O6 Mo5+O5 Mo5+O4 tSiOMoCl4
1.940 1.947 1.941 1.925 1.974 1.991 1.974 1.982 1.993 1.973 1.944 1.957 1.926 1.952
A B A B A B A B A
Taken from ref 26.
Figure 7. Experimental (solid line) and simulated (dashed line) 298 K X-band EPR spectra of (a) Cr(V)-grafted MCM-41 at 300 °C, ν ) 9.2473 GHz, and (b) Mo(V)-grafted MCM-41 samples, ν ) 9.2479 GHz.
dramatic change in the ESEM pattern (Figure 6b) which could be fit with a model consisting of n ) 3.2 ( 0.1, r ) 2.65 ( 0.01 Å, and aiso ) 0.07 ( 0.01 MHz. The ESEM spectrum and simulation obtained after evacuation at 150 °C is shown in Figure 6c. This treatment totally removes adsorbed water since in the FTIR spectrum the HOH bending vibration at 1630 cm-1 is absent. Only minor surface dehydroxylation is indicated by the OH stretching region. Simulation of the ESEM spectrum was best accomplished using a model consisting of n ) 2.25 ( 0.05 deuterons at r ) 2.57 ( 0.02 Å and aiso ) 0.28 ( 0.01 MHz. The fact that the FTIR of this sample shows no physisorbed water and that the ESEM pattern of this sample continues to indicate deuterons proximal to Cu(II) indicates that these deuterons belong to OD groups formed by D-exchange of surface silanols or D2O/OD ligands. It is stressed that the progressive dehydration of the sample did not result in an increase in the 27Al modulation depth observed in the stimulated ESEM spectrum even when τ is chosen to enhance this frequency. The reaction of volatile oxides and chlorides with zeolite surface sites presents an alternative means to cation exchange from aqueous solution for introducing metal cations.9,12 When hydrogen-exchanged Al-containing zeolites and CrO3(s) are mixed, ground, and then heated to between 300 and 500 °C, an interaction takes place between the zeolite surfaces and Cr that results in the binding of Cr(V) to a zeolite oxygen of an AlO4 tetrahedron. This binding manifests itself as an intense resolved 27Al superhyperfine structure on the Cr(V) EPR spectrum. We use this reaction to graft Cr(V) EPR probes to acidic sites on the MCM-41 surfaces and then probe the local environment of the paramagnetic ion by EPR and ESEM. The EPR spectrum of Cr-grafted MCM-41 (Figure 7a) has apparent orthorhombic symmetry, and the spectral width is similar to that observed in zeolites, although the spectrum does not show the same 27Al superhyperfine structure that has been observed in zeolites.12 However, simulation of the spectrum in terms of a single orthorhombic Cr(V) species was not possible. A good fit was achieved (dashed line in Figure 7a) using two axial Cr(V) species whose parameters are given in Table 1. The parameters of the two species are very similar and may indicate a distribution of g values around the mean values rather than two distinct sites. The present EPR spectrum closely resembles that published by Kucherov and Slinkin12 for Cr(V) grafted onto
Figure 8. Two-pulse ESEM spectra of (a) Cr(V)-grafted MCM-41, ν ) 9.3230 GHz, H ) 3388 G, and (b) Mo(V)-grafted MCM-41, ν ) 9.21100 GHz, H ) 3390 G, and three-pulse ESEM of (c) Mo(V)-grafted sample from (b), ν ) 9.2110 GHz, H ) 3390 G, τ ) 145 ns.
silica-alumina gel using CrO3. The parameters obtained agree with those reported for monomeric square pyramidal Cr(V) on SiO2, g⊥ ) 1.975, g| ) 1.955-1.964.27 The two-pulse ESEM spectrum of the Cr-grafted sample is shown in Figure 8a and shows no modulation. The absence of modulation suggests that the sites to which Cr is grafted do not have magnetic nuclei (1H and 27Al) in their immediate environment. It is well-known that Mo is able to be grafted to silica surfaces using MoCl5 either from the vapor phase under anhydrous conditions or from chloroform solution.13,28 The grafting reaction can be described as follows.
MoCl5(g) + H-O-surface f Mo-O-surface + HCl(g) and the Mo-O-surface species have been thoroughly characterized by EPR spectroscopy.13,28-31 Analysis of spin Hamiltonian parameters for Mo(V) grafted to silica surfaces can
1798 J. Phys. Chem., Vol. 100, No. 5, 1996 provide the coordination number of the grafted cation. The EPR spectrum of Mo grafted to MCM-41 from chloroform solution is shown in Figure 7b. Two Mo(V)-grafted species A and B can be discerned in this spectrum, and spectral simulation gives the following parameters: g⊥A ) 1.940, g|A ) 1.919 (80%) and g⊥B ) 1.947, g|B ) 1.963 (20%). These parameters are consistent with assignment of species A and B to partially hydrolyzed and oxidized Mo2Cl10, which leads to the formation of molybdenum blues and monomeric Mo(V) species with mixed O/Cl coordination, respectively, as established by previous work.28 The two-pulse ESEM spectrum of this sample is given in Figure 8b and shows extensive proton modulation, ca. 14 MHz. However, neither the time domain nor the frequency domain spectra (not shown) show any evidence of 27Al modulation. To confirm this, we recorded the three-pulse ESEM spectrum of the same sample choosing the τ value so as to enhance the Larmor frequency of 27Al at the field corresponding to g⊥ (Figure 8c). Although shallow proton modulation is still observed at this τ value, there is once again no evidence for 27Al modulation. This result together with that using Cr(V)grafting reinforces the conclusion that these cations do not preferentially bind to sites near Al. In the case of the Cr(V)grafted sample in high temperatures used for the grafting reaction ensured the removal of surface hydroxyls, and so it is not surprising that little or no proton modulation is observed in the two-pulse ESEM spectrum of this material. The proton modulation that is observed in the ESEM spectra of the Mo(V)-grafted sample could be from a small amount of residual water present in the grafting solution and/or from proximal silanols that were not removed at the temperature used to pretreat the sample. To ascertain the degree of coordinative unsaturation of the Cr(V)- and Mo(V)-grafted samples, we used dry ND3 as an adsorbate. This enables us to select only those Mo(V) or Cr(V) species that have accessible binding sites. The X-band EPR spectrum of the Cr(V)-grafted MCM-41 sample prior to adsorption of ND3 and for which grafting was performed at 500 °C (Figure 9a) is similar in overall appearance to that of the sample grafted at 300 °C (Figure 7a). Spectral simulation (Figure 9a) gave parameters (Table 1) similar to the material reacted at 300 °C: two axial species with g⊥A ) 1.974 and g|A ) 1.950 and g⊥B ) 1.982 and g|B ) 1.947. Recording the spectrum of this sample at the Q-band frequency did not resolve any underlying subspectra. Adsorption of ND3 on the dehydrated Cr(V)-grafted MCM41 sample results in a strong interaction with Cr(V), as evidenced by marked changes in g values and considerable broadening of the initial spectrum (Figure 9b). An additional feature at g ) 1.967 suggests that at least two Cr(V) species may be present, and this may be part of the Cr(V) that has not reacted with ND3. Indeed, this spectrum can be well simulated (Figure 9b) with two axial Cr(V) species with g⊥A ) 1.993 and g|A ) 1.928 and g⊥B ) 1.973 and g|B ) 1.951, the latter representing less than 10% of the Cr(V) detected. Species A has parameters that agree with studies of ammonia coordinated to Cr(V) supported on SiO2,32 namely, g⊥ ) 1.992 and g| ) 1.925, while the parameters of species B correspond to those of water coordinated to Cr(V) supported on SiO2, namely, g⊥ ) 1.972 and g| ) 1.952.32 The ESEM spectrum of the Cr-grafted MCM-41 sample that had been outgassed and exposed to 25 Torr of ND3 for 24 h and again outgassed is shown in Figure 10. This spectrum was recorded at a field corresponding to the maximum EPR intensity. The τ value was set to enhance deuterium modulation, which does not completely suppress the 14N modulation and therefore
Luca et al.
Figure 9. Experimental (solid line) and simulated (dashed line) 298 K X-band EPR spectra of (a) Cr(V)-grafted MCM-41, ν ) 9.2469 GHz, (b) Cr(V)-grafted MCM-41 after exposure to ND3, ν ) 9.2486, and (c) Mo(V)-grafted MCM-41 after exposure to ND3, ν ) 9.2470 GHz.
Figure 10. Three-pulse ESEM spectra of (a) Cr(V)-grafted MCM-41 and (b) Mo(V)-grafted MCM-41 exposed to ND3.
complicates analysis of the data. Nonetheless, meaningful deductions could still be made by fitting the ESEM data with
Surface Sites of the Porous Aluminosilicate MCM-41 the deuterium frequency and ignoring the 14N modulation. The best fit to this ESEM data was with n ) 2.41 ( 0.05, r ) 2.54 ( 0.01 Å, and aiso ) 0.46 ( 0.01 MHz. In this case the coordination number will be a lower limit, as the modulation depth, which determines the modeled coordination number, is compromised by the 14N frequency. Bearing this in mind and that the EPR spectrum indicated 90% Cr(V) species with ND3 coordination, the simulation of 2.4 deuterons per Cr(V) suggests that each Cr(V) reacted with ammonia has one ND3 ligand. In the case of the Mo(V)-grafted sample no high-temperature thermal treatment was performed. The X-band (Figure 9c) and Q-band EPR spectrum (not shown) of the Mo(V)-grafted MCM41 sample exposed to ND3 vapor have parameters g⊥A ) 1.941 and g|A ) 1.893 (70%) and g⊥B ) 1.925 and g|B ) 1.898 (30%), consistent with assignment to 6-fold-coordinated Mo(V).13,28 Species A has parameters similar to those reported for Mo(V) in oxygen coordination grafted onto the surface of SiO2 interacting with H2O. We were unable to adequately simulate the spectra with rhombic parameters reported for Mo(V) on SiO2 interacting with NH3, g1 ) 1.956, g2 ) 1.900, and g3 ) 1.928. ESEM data recorded at the EPR signal maximum of the sample exposed to ND3 (Figure 10b) show fairly intense and persistent D modulation out to more than 3 µs. In this case the interference from the 14N frequency appears to be limited to less than about 0.5 µs. Therefore, it was possible to fit this data adequately by ignoring the first modulation. The best fit could be obtained with n ) 0.74 ( 0.03, r ) 2.21 ( 0.01 Å, and aiso ) 0.75 ( 0.03 MHz. If 30% of the EPR intensity comes from Mo(V) coordinated to one ND3, then we would expect a simulation coordination number of 0.9, which is close to the value obtained. Discussion The adsorption of ions at oxide surfaces is of fundamental and practical importance in the areas of catalysis and environmental aquatic chemistry. Because of the disordered nature of the porous oxides of Si, Al, Mg, Zn, Fe, etc., the exact nature of adsorbed metal and molecular entities more often than not defies simple elucidation. Recourse to a variety of sophisticated spectroscopic techniques is usually necessary to arrive at a relatively simple model for the configuration of the adsorbed species.26,33 While Stumm34 and others have contributed much to our knowledge of the chemistry occurring during ion adsorption at oxide surfaces, structural models of the adsorption sites remain poorly defined. The unique properties of ESEM to detect weak hyperfine interactions between a paramagnetic species and magnetic nuclei in the immediate surroundings of the paramagnet have been used to good effect in the studies of ion adsorption at oxide,26,33,35 zeolite and molecular sieve,8a and clay mineral surfaces.22 Although the crystallinity of zeolites is relatively poor, it is nonetheless far superior to that of simple oxides and clay minerals. Therefore, numerous studies have successfully defined ion adsorption sites on zeolite surfaces by the ESEM technique.8a MCM-41 has crystallinity in between that of clay minerals and simple amorphous oxides. The emerging picture of the structure of this novel material is one in which a relatively ordered arrangement of uniform pores is embedded in an essentially amorphous aluminosilicate network.36,37 The walls between pores in more crystalline samples have a thickness greater than 8 Å, and the ordering of the pores can be variable. Both of these properties are sensitive to a large number of synthesis parameters. The MCM-41 material studied here was deliberately synthesized with low Al/Si ratio to minimize the possibility of extraframework Al being present. Studies to data indicate that if a monomeric source of Al is used, then most of the Al is incorporated within the silicon oxide
J. Phys. Chem., Vol. 100, No. 5, 1996 1799 framework of the MCM-41 wall structure.7,19 Characterization data (XRD, N2 adsorption, MAS-NMR, TEM) obtained for the material under study confirm that a genuine MCM-41 material is formed. Al incorporation is evidenced by the existence of a tetrahedral Al peak in the 27Al MAS-NMR spectrum accounting for about 80% of the total Al. As stated earlier, this small proportion of octahedral Al may be from extraframework Al or alternatively from structural Al exposed and hydrated at the surface. Sass and Kevan38 have shown that the modulation of Cu(II) electron spin echoes from Al in the framework of ZSM-5 can be observed down to an Si/Al ratio of 70, which represents an Al concentration far smaller than in the present sample. In the case of the zeolites studied, Cu(II) interacts directly with the Al substitution site (an AlO4- tetrahedron) in the ZSM-5 framework, the charge of which it is balancing. Similar modulations of the Cu(II) electron spin echo by Al nuclei have been observed for Cu(II) bound to exchange sites created by the substitution of 27Al in the tetrahedral sheet of the clay mineral beidellite.22 In this mineral approximately one in seven silicon atoms is replaced by Al, giving rise to negative charge density localized on the basal oxygen atoms of the Al-containing tetrahedron. Therefore, if the Al for Si substitution occurring in MCM-41 is giving rise to classical exchange sites, we would certainly have expected to observe 27Al modulation in the ESEM spectrum of the Cu(II)-treated MCM-41. Since only feeble modulation has been observed which is considerably weaker than for an aluminosilicate gel with Al content comparable to the MCM-41 sample, we conclude that most of the Cu(II) does not interact directly with oxide ions of the Al substitution site but must be located at least at a distance beyond which dipolar coupling of the 27Al nuclear spin to the Cu(II) electron can occur. This minimum distance has been put at about 4.5 Å.38 This suggests that Cu(II) must be interacting with silanol groups in the MCM-41 pore walls. ESEM data of the deuterated samples that had been evacuated at 298 K and were shown by FTIR to contain no adsorbed water indicates that Cu(II) is still close to deuterons. These deuterons must be associated with OD groups of the MCM-41 walls. For the sample evacuated at 25 °C, our simulations indicate that each Cu(II) on average interacts with three deuterons at a distance of 2.7 Å, consistent with coordination of Cu(II) to the oxygens of two or three geminal OD groups or more likely the direct coordination to a single water molecule and an Si-OD group. ESEM shows that further dehydration at 150 °C under vacuum reduces the number of deuterons to which Cu(II) is bound, implying that some loss of coordinated water may be occurring at this temperature or that a low-temperature dehydroxylation occurs. Indeed, the FTIR data suggest that even after evacuation at 150 °C a significant population of interacting surface OD groups persist, as evidenced by an intense shoulder at 2500 cm-1. Although the incorporation of Al is expected to result in the formation of at least one OH group, it is apparent that OH groups OH-AlO3 tetrahedra, if present, are not favorable adsorption sites, at least in the case of Cu(II) adsorption from aqueous solution. To see if these OH groups can be attacked by volatile oxides and chlorides such as CrO3 and MoCl5, we attempted to graft these ions to MCM-41 surfaces using well-defined methods. As neither the ESE of Cr(V) or Mo(V) grafted to the surfaces of MCM-41 revealed any 27Al modulation, this implies that the OH groups with which these compounds have reacted are also not those of Al-containing tetrahedra. This is in agreement with the results using the Cu(II) probe ion. Subsequent adsorption of ND3 on the Cr(V)- and Mo(V)-grafted
1800 J. Phys. Chem., Vol. 100, No. 5, 1996 samples enabled a determination of the degree of coordinative unsaturation of these ions after the grafting reactions. In each case the metal ions interact with one ammonia ligand. Similarly, low degrees of coordinative unsaturation have also been observed using ESEM for Mo(V) on silica by the impregnation method,39 where the Mo(V) interacts with only one CD3OH molecule. The apparent lack of exchange cation-structural Al interaction can be rationalized in terms of the proposed mechanisms for the formation of M41S materials. Crucial to the formation of these mesostructured materials is the formation of an inorganic/organic interface between anionic silicate/aluminosilicate oligomers and the cationic surfactant head groups. Higher charge densities/polarizabilities of the anion should result in an increased probability of its being located at the inorganic/ organic interface. We propose that the higher charge of oligomeric silica anions compared to aluminosilicate or aluminate anions favors their binding at the surfactant interface, while most of the aluminum species occur in the “cement” between silica-coated surfactant rods. This means that the internal pore surfaces are effectively coated with a silica layer, and it is at this surface that adsorbed paramagnetic probe ions are bound. Hence, we would not observe much 27Al hyperfine interaction by EPR or ESEM, as for most of the aluminum the distance from transition metal ion probe to the aluminum would be greater than 4.5 Å, and the hyperfine interaction too weak to detect. This suggestion has significant implications for the synthesis of “active” M41S materials in that by careful choice of the hetero atom substitution and synthesis conditions it should be possible to selectively direct the hetero atom substitution to the surface of the framework walls, to the interior, or to be evenly distributed within the wall structure. An alternative explanation for the apparent lack of exchange cation-structural Al interaction could be that the exchange cations selectively bind to silanol OH groups rather than OH groups associated with substitutional Al sites. Theoretical studies of aluminosilicate gels and zeolites suggest that the strength of the OH bond in Si-OH-Si or Si-OH-Al groups correlates with the bond angles, and these, being smaller in the case of gels, result in lower acidity for gels compared with zeolites.40 Other factors can also play a part in determining the acidity of hydroxyls in Al-OH-Si groups in both gels and zeolites such as the flexibility of the structure. Additionally, NMR evidence suggests a further distinction between the dehydration of aluminosilicate gels and H-Y zeolite. Complete removal of water causes rupture of the Si-O linkage of the Si-OH-Al groups of gels but not zeolites, and this results in a strongly covalent Al-OH unit and therefore lower acidity for the gel.41 We favor the first of the above hypotheses on the strength of the observation that when Cu(II) is adsorbed onto the aluminosilicate gel containing an amount of Al comparable to the MCM-41 sample under study, 27Al modulation is readily observed. There is a pronounced difference in the accessibility of Al sites in these two materials to adsorbed transition transition metal ions even though evidence suggests that the wall structure of MCM-41 is essentially gel-like. Although our data point to only a feeble interaction between adsorbed transition metal ions and OH groups associated with Al substitution sites, they do not allow a definitive distinction to be made between the two hypotheses given above. Experiments are in progress on MCM-41 containing higher Al contents in an attempt to further clarify the binding sites of adsorbed cations.
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