ARTICLE pubs.acs.org/JPCC
Solid-State NMR Characterization of the Insertion of Cobalt into Aluminosilicate Materials Stefano Caldarelli,*,† Helene Pizzala,‡ Laura Arrighi,§,|| Fabio Ziarelli,^ and Guido Busca|| †
ISm2 UMR 6263, Aix Marseille Universite, Campus de Saint Jer^ome, Service 512 F-13013 Marseille, France Spectrometries Appliquees a la Chimie Structurale, UMR 6264 Laboratoire Chimie Provence, Universites Aix-Marseille I, II et IIICNRS, F-13397 Marseille, France § Dipartimento di Chimica e Chimica Industriale, Universita di Genova, Via Dodecaneso 31, I-16146 Genova, Italy Dipartimento di Ingegneria Chimica e di Processo, Universita di Genova, P.le J. F. Kennedy, I-16129 Genova, Italy ^ Spectrop^ole Federation FR1739, Universite d’Aix Marseille Universite, Campus de Saint Jer^ome, Service 511 F-13013 Marseille, France
)
‡
ABSTRACT: The location of cobalt in two porous metalsubstituted zeolites, mordenite and ZSM5, and in a reference amorphous silicoalumina material, was investigated by solidstate NMR spectroscopy. The hyperfine shift and pseudoshift effects due to the paramagnetic metal on the NMR spectra of silicon and aluminum were used to estimate the number of aluminum and silicon framework sites in proximity of the cobalt. It was possible to confirm previous IR findings that only a fraction of the SiOHAl sites were involved directly in the bonding of the metal. A semiquantitative solid-state NMR analysis proved that siloxane bridges, and preferentially those close to aluminum locations, played a concurrent marked binding role in zeolites. It was found that for both zeolites the metal penetrates deeply into the structure. Furthermore, analysis of the NMR spectra implies that the cobalt position must be crystallographically well determined.
’ INTRODUCTION Cobalt-containing microporous and mesoporous oxide materials are relevant from different points of view. In reducing conditions cobalt-containing oxides convert to metal catalysts which are active e.g. in FischerTropsch synthesis1 and in other hydrogenation reactions.2,3 The possibility of the wide use of hydrogen as the propellant of hydrogen-fueled fuel-cell-based vehicles forces the development of new materials for catalytic hydrogen production as well as for hydrogen purification and hydrogen storage. Cobalt catalysts are active for the production of hydrogen from (bio)ethanol via steam reforming,46 and from methane via dry reforming.7 Cobalt-containing intermetallics like ZrCo8 and Mg2Co9 may also be useful for hydrogen isotope storage. Oxidized cobalt catalyst may improve the performance of LSM SOFC cathodes.10 In the oxidized form cobalt-containing zeolites, such as Co-FER, Co-MFI and Co-MOR, are active catalysts for reducing NO with methane in oxidizing atmosphere,11 so allowing the denitrification (DeNOxing) of waste gases. Co2þ ions exchanging the protons of the protonic zeolites are considered to be the active sites in the reaction,12 while other authors suggested the presence of “extraframework” CoOx oxide clusters to represent the catalytically active species.13 In a series of recent papers the UVvis spectra of Co-FER, Co-MFI and Co-MOR have been studied in detail and assigned to Co2þ ions in different locations in the zeolite cavities.1417 The possible cooperation of residual protonic sites and Co cationic sites in partially exchanged Co-zeolites for the CH4SCR reaction has also been r 2011 American Chemical Society
discussed.18 Cosilicaalumina has also been investigated for comparison and shown to be active in NO oxidation to NO2,19 being possibly useful for a step previous to low temperature NH3SCR (fast SCR) catalysis,20 similar solids being also active catalysts for fast NH3SCR.21 Bulk and supported Co oxides are active in catalytic combustion22 and for oxidations also in the liquid phase.23 In this work we will present spectroscopic studies on cobaltzeolites and cobalt amorphous silicoalumina (ASA) materials aimed at revealing the localization of the metal sites. Previous characterization of these materials pointed out an interesting issue: the survival of a surprisingly large fraction of acid hydroxyls even for stoichiometric excess substitution of cobalt24 pointed out toward possible incomplete substitution of the internal sites, largely above the expected fraction of segregated and thus inaccessible OH sites. While UV and the use of IR molecular probes confirmed the presence of cobalt on the outer surface of the substituted zeolites,24,25 a reduced access of the metal to the interior of the zeolites could not be conclusively stated on the basis of these spectroscopies. Our strategy to locate the cobalt in the three materials studied here has been to detect the changes induced in the silicon and aluminum NMR spectra upon introduction of the metal ions. The paramagnetic contact shift to cobalt, which relocates the Received: April 22, 2011 Published: May 09, 2011 10569
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The Journal of Physical Chemistry C signal outside the usual observation window, is used to estimate the number of cobalt first neighbors. The dipolar coupling based pseudocontact shift is effective only for nuclei in the proximity of the metal, so that it is useful to assess T atoms close but not bound to cobalt.
’ EXPERIMENTAL SECTION The synthesis of the materials was described elsewhere.11 Characterization Techniques. Solid-state NMR spectra were recorded with a Bruker Avance DSX-400 spectrometer. 29Si MAS NMR spectra were recorded at 79.54 MHz, on a 4 mm CPMAS probehead, with a spinning rate of 10 kHz. The π/2 pulse length was 4.5 μs, and 4000 transients made of 2k data points were collected with a recycle delay of 30 s for MFI and MOR samples, and 250 s, for ASA samples. The recycle delays were chosen to ensure complete relaxation. CP-MAS spectra were obtained using the following parameters: a π/2 pulse length of 4.8 μs, a contact time of 5 ms for the parent samples and of 1.5 ms for cobalt-exchanged materials and a recycle delay of 3 s. The ppm scale was referenced to Si(CH3)4. 27Al MAS spectra were recorded at 104.34 MHz, on 2.5 mm CPMAS probehead, at a spinning rate of 30 kHz. The π/2 pulse length was 1.2 μs, and 2k transients were collected with a recycle delay of 1 s. The scale was referenced to Al(H2O)63þ. 27Al 5Q MAS NMR spectra26,27 were obtained with a two-pulse sequence: p1t1p2acquisition. The pulse lengths p1 and p2 were optimized to 3.4 and 1.1 μs, respectively. The delay t1 between the two pulses was incremented regularly, using the TPPI method to achieve pure phase spectra. For each of the 256 acquired t1 increments, between 100 and 12000 transients of 1024 data points were collected. Semiquantitative MAS NMR Analysis. The protocol proposed by Ziarelli et al.28 for quantitative solid-state NMR analysis was followed. This consists of carefully limiting the region of the MAS rotor used to that that provides a homogeneous response. A Rototec rotor was used here, open on both ends, in which the appropriate measurement volume had been previously precalibrated26 using a standard and limited using KelF 1 mm thick spacers. The mass of the material to analyze was measured on a balance providing a precision of 0.1 mg. The molecular weight of each material was estimated on the basis of the elemental analysis and of the SAR calculated through the NMR spectra. External referencing was performed using the Eretic protocol.29 The Eretic signal was obtained by an exponential decreased shaped pulse defined of 1000 points and with a full length equal to the acquisition time (10 ms). The frequency, the amplitude and the phase of this pulse were chosen to obtain an Eretic signal out of the populated spectral region and with intensity comparable to the sample signals. To inject the Eretic pulse during the acquisition time, the X channel of the probe was used. This procedure provided an estimation of the number of moles introduced in the rotor, to which the NMR signal was normalized. ’ RESULTS Chemical Analysis. The chemical analysis gave the following compositions for the solids under study: Co-ZSM5 = 1.7% Co (w/w), Co/Al atomic ratio 0.55; Co-MOR = 4.92% Co (w/w), Co/Al atomic ratio 0.68, Co-SA = 11.4% Co (w/w), Co/Al atomic ratio 0.76. From these data it is clear that both zeolites are a little overexchanged. The Co content of silicaalumina is
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Table 1. Residual νOH Intensity in the IR Spectrum in Metal Exchanged Zeolites, from Ref 24 silanol OH (%)
Bridging OH (%)
Co-H-MFI
57
50
Co-H-MOR
89
33
Figure 1. 29Si MAS NMR spectra of NH4-MFI (a), NH4-MOR (b), ASA (c) and 27Al MAS NMR spectra of NH4-MFI (d), NH4-MOR (e), and ASA (f).
definitely the highest, but the Co/Al atomic ratio is not much higher than for zeolites. The modification of the OH groups of the starting materials due to introduction of the metal has been the object of previous studies.24 In spite of the apparent overexchange deduced by chemical analysis, the real exchange of internal OHs by cobalt is only partial, as also the external silanols are significantly partially exchanged (Table 1). On the other hand, in the case of ASA, no reduction of the OH stretching band is observed upon exchange. This suggests that Co species must interact mostly with oxygen atoms involved in siloxane bridging.
’ SOLID STATE NMR Si and 27Al MAS spectra of the starting materials, before addition of cobalt, are shown in Figure 1. For the zeolites, they correspond to the known profiles.30 All aluminum is found to be essentially on one site, at 5060 ppm, corresponding to tetrahedral coordination. Extraframework aluminum, located at around 0 ppm, is present in a very minor portion only for the case of NH4-MFI. Since 27Al central transition NMR spectra are affected by second-order quadrupolar broadening and line shift, we performed a 5Q-MAS spectrum, which can enhance the spectral resolution by refocusing these nuisances (Figure 2). At the same time, from this kind of spectrum it is possible to estimate the actual chemical shift as well as the quadrupolar coupling coupling, CQ, these latter being linked to the local symmetry. The presence of a single tetrahedral site was confirmed for aluminum this way for the two zeolites (not shown). The associated composite quadrupolar parameter (CQ(1 þ η/3)1/2) is rather small, about 1 MHz, as expected for symmetrical sites (Table 2). In the 29Si spectrum of NH4-MOR two Q4 sites are detected at 107.3 (3Si 1Al) and at 114.5 ppm (4Si 0Al). For NH4-MFI, these two species are also present in the 29Si spectrum, with peaks at 108 and as two signals at 114 and 115 ppm, associated with two different Q4 environments. Starting Materials. The
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Figure 3. 29Si MAS NMR spectra of Co-H-MFI (a), Co-H-MOR (b), and Co-ASA (c) and 27Al MAS NMR spectra of Co-H-MFI (d), Co-HMOR (e), and Co-ASA (f).
Figure 2. 27Al 5Q-MQMAS spectra of SA (a), NH4-MFI (b), and NH4-MOR (c).
Additionally, a weak peak corresponding to a small amount of Q4 (2Si 2Al) species is observed for NH4-MOR and NH4-MFI. As this region of the spectrum may correspond to silanols as well, CPMAS spectra were performed, as in these latter the intensity of silanol signals is enhanced relatively to those of other sites remote from protons. The CPMAS spectra demonstrated indeed the presence of silanols but in a minor concentration, with peaks at 103 and 104 ppm for NH4-MOR and NH4-MFI respectively. Thus the peak detected in the MAS spectra is mostly due to nonsilanol species. The spectrum decomposition (Table 3) can thus be used to calculate a nSi/Al of 10 for NH4-MOR and of 18 for NH4-MFI. The ASA sample shows broader signals, as a consequence of the local structural disorder. The unique silicon-29 peak falls in the Q4 range, while the aluminum-27 signal is a superposition of three broad resonances corresponding to Al(VI), Al(V) and Al(IV), centered at 49, 24, and 0 ppm respectively, as previously observed in ASA of similar compositions.31 In this case, the 5QMAS spectrum allowed only better analysis of the tetrahedral and octahedral sites, the Al(V) not being excited above detection, probably due to a large quadrupolar coupling associated with a distorted geometry. In fact, already the tetra- and hexacoordinated sites possess large quadrupolar parameters, of the same order of magnitude for the two kinds of sites, which is a confirmation of the lower level of order of this family of samples.32 Further proof of the local disorder can be found in the shape of the MQMAS 2D diagram, where the signal aligns rather along a direction with a slope of 25/4, typical of a continuous variation of the quadrupolar parameter. An even larger quadrupolar constant can be expected for the distorted Al(V) groups, in view of the difficulty of excitation of its MQMAS signal. Metal-Exchanged Materials. The addition of cobalt induces severe changes in the NMR spectra of all studied materials (Figure 3). The first apparent effect is that all lines broaden,
Figure 4. 27Al MAS NMR spectra of NH4-MOR (a) and Co-H-MOR (b) using an Eretic signal as a visual intensity reference.
due to facilitated spin relaxation induced by the proximity of the unpaired electrons. A second effect is an apparent reduction of the observed signal upon exchange with the metal. This can be best visually appreciated by using an electronic external intensity reference (the so-called ERETIC method), which provides an artificial peak of constant intensity. Figure 4 shows the decrease in intensity for 27Al MAS NMR in the mordenite samples using this method, upon addition of cobalt. In fact, the missing intensity is localized in nuclei whose resonance is shifted by cobalt through the hyperfine contact term33 and possibly broadened among detection by relaxation effects. The decrease in intensity of the signal corresponding to the residual diamagnetic environment (i.e., located approximately at the same position as in the spectra of the non substituted materials) is thus a direct measurement of the number of spins of the observed isotope in close proximity to the metal. A third effect is the induction of an anisotropic line broadening on the residual NMR MAS spectrum of metal exchanged materials,3436 equivalent to a larger CSA and thus to a spectrum with an increased number of spinning sidebands with respect to the MAS spectrum of nonexchanged materials recorded at the same spinning rate (see Figure 5 for an example). The latter two effects will be analyzed in more detail in the following, since they can provide both direct and indirect leads as to the cobalt localization. In the MQMAS spectra, a part of the signal, corresponding to the narrow components, can still be excited for Co-H-MOR and Co-H-MFI. The very similar quadrupolar parameters obtained for metal-substituted and starting materials confirm the expected low impact on the overall structure of metal addition. Conversely, the metal-loaded ASA MQMAS spectrum contains only broadened signals in both dimensions. It can be inferred that the sample is randomly and homogeneously substituted and no parts 10571
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Table 2. NMR Parameters from MQMAS Experiments on 27 Al for Starting and Cobalt-Substituted Materials
Figure 5. 29Si MAS NMR spectra of Co-MOR, recorded at a spinning rate of 5 kHz. Spinning sidebands are marked with a star.
of it are spared by the metal perturbation, being either induced structural disorder or facilitated relaxation. Note that broadened components should be present in the MQMAS spectrum of ZSM5 and MOR as well, but are likely blanked by the presence of the more intense and narrow signals. Although specific NMR methods exist to promote broad signals in MQMAS by filtering out narrow components,37,38 such experiments provide only qualitative information on the site populations, to be used in conjunction with the MAS spectra. Thus, they were not pursued here and an alternative approach, described in the following, was employed to better quantify the effects of the metal substitution. Analysis of the MAS Spectra. A detailed analysis of the 29Si and 27 Al MAS spectra of the metal-substituted zeolites reveals the presence of two contributions (Table 3). Comparison of the MAS spectra of the materials with and without cobalt shows that no shift of the isotropic resonance frequency occurs for both components. On the other hand, the two components differ severely according to their chemical shift anisotropy. The broadened components in the case of Co-H-MOR constitute 78% and 72% of the 29Si and 27Al MAS spectrum, respectively. For Co-H-MFI, these values become 30% and 65%. In the case of ASA, it has been possible to assess the broad component (60%) only in the 29Si spectrum, due to overlap of the various Al signal. The source of an increased anisotropic broadening of a resonance, without a significant associated shift, arises from a pseudocontact interaction with paramagnetic cobalt, through the dipolar interaction between the electron spin density and the observed nucleus. The nuclei suffering this kind of broadening are located in proximity in space to the metal, but receive no direct contribution from the electron polarization, which is the case for nuclei just a few bonds away from the site of covalent binding of the cobalt. The presence of an essentially unaffected spectral component in the MAS spectrum certifies that a fraction of the sample silicon and aluminum atoms lies completely outside the sphere of influence of the transition metal (Table 3). It is interesting to note that the size of the metal-induced broadening in 29Si and 27 Al spectra is roughly proportional to the ratio of the respective gyromagnetic ratio, which suggests very similar distances for both nuclei from the metal. Following the same reasoning, this distance is essentially the same in all materials. Moreover, structural disorder due to introduction of the metal is to be excluded, due to the very clear series of detectable sidebands for the broadened site. Quantification of the NMR “Invisible Nuclei”. The NMR signal of nuclei in proximity to a paramagnetic center can be severely shifted by the effect of the spin-density of the unpaired electrons propagating through covalent bonds, the so-called hyperfine shift.39 If the NMR frequency observation range is kept the same as for a diamagnetic compound, these nuclei may become “invisible” in a regular MAS spectrum. On the other hand, the number of sites
δG2
δG1
δiso
PQa (MHz)
ASAb
49
350
58.3
3.7
Co-ASA
0 52
112 400
10.0 64.4
3.8 4.3
NH4-MFI
54
278
54.7
1.1
Co-H-MFI
53
284
55.8
1.1
NH4-MOR
54
281
55.0
1.1
Co-H-MOR
51
275
52.8
1.6
PQ= CQ(1 þ η/3)1/2. b Two distinguishable sites observed for this material. a
Table 3. Parameters of the Components in the MAS Spectra of the Cobalt-Exchanged Materials nucleus δiso/ppm Δcsa/ppm ηcsa width (Hz) rel int (%) Co-H-MOR
119
170
119
2.2
53
242
52
2.3
Si
118 119
160
Al
54
233
Si Al
Co-H-MFI
0.8 0.8
Si
112 113
165
78 22
21
72
20
28
0.8
12 6
30 70
0.5
15
65
9
35
54 Co-ASA
16 13
0.8
28
60
20
40
involved in a hyperfine coupling can be estimated by accurately evaluating the associated intensity loss in the MAS NMR spectrum. Quantitative measurements in MAS following an optimized protocol can indeed produce an error on the measure within 1%.28 In these conditions, the error on the measurement is determined mainly by the signal-to-noise ratio of the sample. We applied this procedure, adapted as detailed in the Experimental Section, to estimate the number of tetrahedral sites in the framework affected by a large paramagnetic shift, thus neighboring the oxygen sites covalently bound to paramagnetic cobalt. Note that cobalt aggregates and other diamagnetic forms of the metal would not affect the intensity in the MAS spectrum. Once the fraction of invisible nuclei per material has been estimated, the remaining intensity of the MAS spectra (Table 2) can be renormalized (Table 4) to provide an overall description of the paramagnetic-born interactions affecting (or not) the nuclei in the framework. We performed this analysis on both aluminum-27 and silicon29 MAS NMR spectra. For aluminum, the variation of the signal is largest for MOR (58%) and modest for ASA (13%) and ZSM5 (12%). In all cases, the aluminum sites are not completely blanked out upon cobalt insertion, consistently with the IR findings. All materials experience a sizable drop of the silicon29 signal intensity upon introduction of cobalt, of 4%, 9% and 19% for Co-H-MFI, Co-H-MOR and Co-ASA, respectively. These values well reflect the aluminum and thus the cobalt content. In light of the only partial involvement of AlOH in cobalt binding, the role of silanols or siloxanes in securing the metal must be thus significant. 1 H29Si CPMAS. To further explore the spectral effects due to the introduction of cobalt, we performed 1H to 29Si CPMAS 10572
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Table 4. Fraction of Nuclei in the MAS NMR Spectra Affected by Paramagnetic Interactions with Cobalt fraction of nuclei affected (%) materials
hyperfine 27
pseudocontact
unaffected
Al
Co-H-MFI
12
57
31
Co-H-MOR Co-ASA
58 13
30 n/a
12 n/a
Co-H-MFI
4
29
67
Co-H-MOR
9
71
20
19
48
33
29
Co-ASA
Si
Figure 6. 1H29Si CPMAS NMR spectra of NH4-MFI (a), NH4-MOR (b), ASA (c), Co-H-MFI (d), Co-H-MOR (e), and Co-SA (f).
experiments in the starting and metal-exchanged materials (Figure 6). Since this measurement relies on the initial polarization of protons, it can provide concurrent indirect information on the influence of cobalt on the hydrogen magnetization. A first glance at the two series of spectra confirm an intensity reduction in the metal substituted materials, but a quantitative analysis is complicated for this type of experiment, because the polarization transfer is influenced by hard-to-assess relaxation effects. Moreover, the Co-exchanged materials spectra were recorded with shorter contact time to palliate against the loss of the proton polarization during the transfer arising from shorter T1rho. Thus, we limited ourselves to a qualitative analysis of the CPMAS spectra. The ASA CPMAS spectrum profile is broad, and remains unchanged after cobalt introduction. Consistently with the results of the preceding sections, the MOR CPMAS spectrum is the most severely affected by the metal addition. The starting material shows the SiOHAl signal at 100 ppm already noticed in the regular MAS spectrum, with the addition of a SiOH resonance, at a slightly different position with respect to the MAS Q4 (Al2) signal. This contribution was hidden in the MAS spectrum, meaning that it constitutes a very small percentage of the silicon sites. Note that this peak could lead to a mild overestimation of the SAR for MOR. The CPMAS of the Co-HMOR is barely detectable, and has lost all of the fine structure to a broad lump. The ZSM5 CPMAS intensity is reduced in the metalsubstituted material, but the overall line shape structure is maintained, testifying again the existence of a section of the framework well isolated from the cobalt.
’ DISCUSSION Previous spectroscopic studies on cobalt-substituted aluminosilicates hinted toward a number of deviations from simple
coordination of the metal to the acid sites. As seen above, striking evidence in this respect comes from IR spectroscopy of all materials studied here, which suggests that a large fraction of the available AlOHSi bonds are spared by the metal substitution. Particularly, even though elemental analysis confirms metal loadings in slight excess of the stoichiometry required for charge balance, both Co-H-MOR and Co-H-MFI samples have not fully employed their acid sites to capture the metal (Table 1), although the creation of CoO bonds is demonstrated by UV spectroscopy. NMR results (Table 3) confirm these findings and add some details. Particularly, Co-ZSM5 employs a very small fraction of its SiOHAl centers to capture the cobalt (50% according to IR, 12% to MAS NMR), while Co-H-MOR engages a larger fraction of the acid sites in the cobalt binding (67% for IR and 58% for NMR). It is natural to ask whether this inactiveness of the Al centers toward coordination could be explained by segregation of the cobalt at the mouth of the channels. Of particular interest to this respect is the analysis of 27Al MAS NMR spectra for both zeolites, as the randomly distributed Al provides a faithful representation of the framework. The fraction of the framework nuclei exposed to the cobalt influence in the NMR spectrum is a good indicator of the degree of penetration of the metal inside the porous volume. This value can be estimated by dividing the fraction of T sites whose NMR spectrum is perturbed by cobalt by the percentage of cobalt atoms (Table 5, column IV), which can be calculated by adding up the values indicated in Table 3, scaled by the abundance of Si and Al from the respective SAR. Since the aluminum-27 MAS spectrum of Co-ASA could not be analyzed quantitatively, the reported values for this material are a lower and upper limit, corresponding to none or all to the remaining Al being dipolarly broadened by cobalt. The appreciable magnitude of the cobalt hyperfine effects on the silicon MAS spectra and, foremost, the large number of aluminum sites suffering cobalt-induced dipolar broadening suggest that a large fraction of zeolite framework sites is in proximity of cobalt(II) moieties and only about 1030% of the structure is off reach of paramagnetic influences. In fact, the decrease of the 27Al MAS signal upon insertion of cobalt points out that Co-H-MOR involves 57% of its SiOHAl sites in metal bonding, while 30% more Al sites are shown to be in proximity of the metal because of the evidence of a non-negligible throughspace dipolar effect on their NMR spectrum, leaving only 12% of the aluminum sites off reach to cobalt magnetic interactions. Similarly, a total of 69% of the aluminum sites are affected by cobalt insertion in ZSM5, consistently with the exclusion from the interaction of the T sites of the small cage. Thus, unless the aluminum is distributed specifically near the surface, these data already rule out the hypothesis of coordination limited at the surface. The interpretation of the 29Si NMR data for both zeolites requires more caution since the intensity loss caused by the paramagnetic shift is moderate and thus its estimation more affected by errors on the intensity measurements. At any rate, a considerable fraction of the silicon atoms (35%) is found to be influenced by Co in the MAS spectrum of ZSM5, which, adding the equivalent results for aluminum, yields an overall 36% of the T sites found to be neighboring metal atoms (Table 5) for this material. 1H29Si CPMAS experiments are in perfect agreement with this analysis, as they show that a significant part of the protons of the Co-H-MFI sample is not perturbed by the cobalt introduction and can transfer polarization to the Si. The CPMAS NMR spectrum indeed does not evolve upon addition of the 10573
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Table 5. Summary of Influence of the Cobalt Addition on the MAS NMR Signal of the Tetrahedral (T) Sites of the Studied Materials
a
I: % intensity loss of the T sites
II: I/% Co
III: % of T sites affected by Co
IV: III/% Co
Co-H-MFI
5.2
1.9
35
12.6
Co-H-MOR Co-ASA
12.6 19.4
2.0 1.1
75 5372a
12.2 3.04.1a
Limit cases estimation, using 0% and 100% for the values of dipolarly broadened aluminum.
metal, with the exception of line broadening and an intensity loss. The case of MOR can be analyzed in a similar manner showing that a much larger percentage of the Si sites are linked to a CoO site (75% overall, comparable to the figures calculated for 27Al NMR (71%)), a result mirrored by the 1H29Si CPMAS large intensity drop in going from NH4-MOR to Co-H-MOR. A first qualitative conclusion can be drawn at this point that cobalt in both zeolites penetrates deep inside the framework, as exemplified by the thirteen T sites estimated to be affected by the metal insertion. Hints toward the location of the CoO formation can be provided by further analysis of the NMR results. In the case of the zeolite materials, if the metal were bound to the acid site oxygen, it would produce paramagnetic shifts in the spectrum of both flanking Si and Al atoms. While in the case of MOR a comparable number of Co-shifted T sites can be found in Al and Si, for ZSM5 the number of Si T sites of Co-induced signal loss is more than 7-fold the aluminum ones. The nature of these excess siliconflanking binding moieties, either silanols or siloxane bridging oxygens, should be assessed next, although CPMAS experiments already demonstrated that silanols are present in a very limited fraction in both starting zeolites, and that IR shows furthermore that these species are not being completely involved in the capture of cobalt. If this latter binds to an oxygen atom shared by two T sites, these both will be affected by a paramagnetic shift and thus be accounted in the measured signal loss. Binding to silanols, on the other hand, would contribute to the disappearance of the signal of a single signal T site. Accordingly, the ratio between the observed NMR signal loss and the transition metal content in the material can provide a good indicator of the mode of coordination of cobalt (2 = shared oxygen, 1 = single site coordination, >2 = excess of loading). The analysis of the NMR data shows that both zeolites (Table 5, column II) are involving siloxane bridging oxygen rather than silanols to bind the metal. Surprisingly, the former are thus shown here to play a role equivalent, or even superior in the case of CoMFI, to SiOHAl groups in metal binding in zeolites. The reason for this peculiar behavior could lie in steric unavailability of the SiOHAl sites, which raises the question whether the metal can access freely the interior of both zeolites. A further interesting aspect to highlight is that, even if not all acid sites are capturing cobalt, a nonrandom localization of the metal bound in the framework of Co-MFI must be deduced by the larger fraction of aluminum MAS signal broadened with respect to the Si ones. It appears that the metal, while remaining bound to siloxanes, seeks to locate in portions of the zeolite neighboring the Al substitution, probably driven by electrostatic forces. This somewhat unexpected result is a further indication of a role of steric constraints in determining the cobalt binding sites in the extremely fit MFI channel. The validity of this analysis is reinforced by the results calculated for the Co-ASA, in which only about three/four
neighbors to cobalt-carrying oxygens are calculated, as expected for an interaction limited to the surface. The structure of the acid sites in ASA is a matter of debate, as the lack of bridging hydroxyl group signals in the IR spectrum suggests rather a localized OH and a free charge on the Si/Al neighbors.31 If this situation should persist after Co introduction, it would correspond to a ratio of 1 for the cobalt-shifted T site to cobalt ratio, in agreement with the observed value of 1.1. As a final comment on the information extracted by the NMR spectra, it is noteworthy that the well-resolved nature of the lowspinning rate MAS spectra implies a unique and relatively welldefined distance between all dipolarly affected T sites and the metal. It is thus reasonable to suggest that cobalt introduction is dominated by the overall steric constraints, and that the site of covalent binding is chosen accordingly. Finally, the analysis of Table 5 reveals a good agreement of the estimated loading of cobalt with the charge of the zeolitic framework. Thus, the excess cobalt measured by elemental analysis is in a form that does not involve covalent bonding to the zeolite framework, consistently with UVvis analysis suggesting the presence of aggregates.
’ CONCLUSIONS The introduction by chemical exchange of cobalt into the structure of three different materials, two zeolites and amorphous silicoalumina, has been shown, by IR and solid-state NMR, to follow specific schemes for each case, dominated by steric interactions in the case of the porous materials. The presence of a stoichiometric excess of cobalt has been assessed in all cases, suggesting the presence of nonframework forms of the metal. Both zeolites studied, mordenite and ZSM5, welcome the metal inside their framework, although a detectable amount of cobalt is located at the surface. While acid OHs are largely, but not completely, involved in the bonding of the metal the case of Co-MOR, they are mostly not accessed in Co-MFI. However, for this latter case, the cobalt is still preferentially located in proximity of the nonbonding aluminum sites, probably driven by Coulomb forces. Silanols, which are in low concentration in both zeolites, are not very relevant for metal capturing in these materials. Conversely, our analysis suggests that siloxane bridges can play a major role in the loading of the metal, especially in the case of Co-MFI. ’ AUTHOR INFORMATION Corresponding Author
*E-mail: s.caldarelli@univ-cezanne.
’ ACKNOWLEDGMENT The authors acknowledge C. Resini (now at Intitute Charle Gerhardt, Universite Montpellier II, Montpellier, France) for the 10574
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dx.doi.org/10.1021/jp2037679 |J. Phys. Chem. C 2011, 115, 10569–10575
