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Investigation of paramagnetic catalysts by solid state NMR spectroscopy
Colloids and Surfaces A: Physicochemical and Engineering Aspects 158 (1999) 97 – 106 www.elsevier.nl/locate/colsurfa
Investigation of paramagnetic catalysts by solid state NMR spectroscopy A. Tuel *, L. Canesson, J.C. Volta Institut de Recherches sur la Catalyse, CNRS, 2, a6enue A. Einstein 69626 Villeurbanne Cedex, France
Abstract A spin echo mapping (SEM) technique has been used to record the total 31P-NMR information in paramagnetic phosphate-based catalysts. For these materials, conventional NMR techniques give poor information and a great proportion of 31P nuclei cannot be detected due to broad and shifted signals. By contrast to conventional sequences, all 31P nuclei are detected using a mapping technique. Under specific conditions, the method is quantitative and gives direct information on the nature, the amount and the oxidation state of neighboring paramagnetic species. For vanadium phosphorus oxide catalysts (VPO), the technique is particularly interesting for characterizing intermediates formed during catalytic reactions, especially when the latter are amorphous and cannot be detected by XRD. Moreover, 31P-NMR spectra recorded at various temperatures give precious information on the magnetic properties of these materials. In the case of cobalt-substituted aluminophosphate molecular sieves, P/Co ratios as well as the various populations P(nCo, 4-nAl) can be derived from 31P-NMR spectra. Such parameters, which cannot be obtained by other spectroscopic techniques, are of particular importance for the understanding of the acidic and redox properties of these catalysts. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Paramagnetism; Spin echo mapping; NMR; VPO catalysts; CoAPO4-n
1. Introduction A great number of oxidation reactions in both the liquid and gas phases are now performed industrially in the presence of heterogeneous catalysts, which generally contain transition metals dispersed on the surface or in the bulk of inorganic supports. The performance of these catalysts strongly depends on the local environment * Corresponding author. Tel.: +33-4-72445395; fax: + 334-72445399. E-mail address: [email protected] (A. Tuel)
around active sites, as well as on their oxidation state and dispersion. Many spectroscopic techniques have been used to access the coordination and oxidation state of catalytic species but the simultaneous presence of dispersed and aggregated species as well as of highly crystalline and amorphous phases make that interpretation of the data is often difficult. Solid state NMR has sometimes been successfully applied to probe the catalytic sites at atomic scale in these compounds but the technique is limited to a few numbers of transition metals with appropriate oxidation states. When direct obser-
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vation of active species is not possible, an alternative consists in analyzing the influence of the latter on the NMR spectra of neighboring atoms. Nevertheless, a small amount of paramagnetic species like Fe3 + , V4 + or Co2 + , in the catalysts is generally enough to considerably broaden NMR lines and the information that can be derived from the spectra is poor [1]. Actually signals are not only broadened, but they are also shifted, which makes that a high proportion of the NMR signal cannot be detected using conventional sequences. For paramagnetic catalysts, we have previously reported that a spin echo mapping (SEM) technique could be used to obtain the total NMR information from a series of conventional spectra recorded at various irradiation frequencies [2]. The present paper reports on the potentiality of this technique to characterize two classes of transition metal — containing phosphate — based catalysts, namely vanadium phosphorus oxide catalysts (VPO) and cobalt-substituted aluminophosphate molecular sieves (CoAPO4 − n). The originality of the method, as well as its specificity with respect to other spectroscopic techniques are clearly illustrated.
2. Experimental All catalysts were prepared following recipes of the literature [3 – 5] and preliminary characterized by conventional techniques, principally X-ray diffraction (Philips PW 1710 diffractometer, Cu–Ka radiation). 31 P SEM-NMR spectra were acquired on static samples with a Bru¨ker DSX 400 spectrometer equipped with a standard variable temperature 7 mm probe head. Briefly, the SEM technique consists in recording a series of Hahn echo spectra (with t=20 ms) by incrementing the irradiation frequency below and above that of H3PO4 (d=0). Typically 1000 scans are accumulated for each spectrum with a pulse length of 3 ms (p/2) and a recycle delay of 0.5 s. The frequency increment Dv was 75 kHz and satisfied the relation Dv = 0.5v1, where v1 =gH1 is the irradiation frequency [6]. The number of spectra necessary to cover the whole spectral region is dictated by the frequency
limits beyond which no signal is detected. After Fourier transformation each spectrum is filtered following a method recently reported by Tong [6] to eliminate oscillations of the base line and increase the signal-to-noise ratio of the final SEM spectrum. For each irradiation frequency, the T2 relaxation time is measured and the intensity of the corresponding spectrum is corrected to account for the loss of magnetization during acquisition of the signal. This correction is necessary to obtain quantitative data, particularly in materials where diamagnetic and paramagnetic species coexist.
3. Results and discussion
3.1. VPO catalysts The method was first applied to the characterization of VPO catalysts, used at industrial scale for the production of maleic anhydride from nbutane. VPO catalysts consist of vanadium octahedra (VO6) connected to phosphorus tetrahedra (PO4). The numerous possibilities to connect these polyhedra generate a family of materials with different structures and vanadium oxidation states (from V3 + to V5 + ) [7]. Whilst V5 + phases can be easily characterized by conventional 31P and 51V MAS-NMR [8], this is not the case for materials with lower oxidation states. Fig. 1 shows that some reference compounds with vanadium oxidation states from +3 to + 5 can be immediately identified by their 31P SEM-NMR spectra. Indeed, the signal is shifted from ca. 0 ppm for VOPO4 materials (V5 + ) to 1600–2600 ppm for V4 + phases and approximately 4650 ppm for VPO4 [9]. The possibility to discriminate between these phases, and particularly those containing V4 + species is of great interest and will considerably help for the interpretation of NMR spectra of real catalysts. The following example illustrates the advantages of NMR as compared to long-range characterization techniques like XRD to provide information on the various phases that can be present in a catalyst and their evolution in course of reaction. (VO)2P2O7 (d=2600 ppm), which is
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recognized to be the active phase in the catalytic oxidation of n-butane to maleic anhydride can be prepared by activation of the hemihydrate VOHPO4 −0.5H2O (d =1625 ppm) in a butane/ air atmosphere. However, (VO)2P2O7 is rarely obtained as a pure phase and the catalyst is often contaminated by poorly crystallized materials containing V centers with higher oxidation states. The proportion of contaminating phases, which depends on the activation procedure and the mor-
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phology of the starting precursor, strongly affects the activity of the catalyst [10]. For specific morphologies of the precursor, whilst the material obtained after activation is completely amorphous by XRD [11], the 31P SEM-NMR spectrum can help to identify the nature of the various phases and estimate their relative amount in the catalyst (Fig. 2). The intense peak around 0 ppm is characteristic of phosphorus atoms in a local environment of V5 + species, similar to that encountered
Fig. 1. 31P SEM-NMR spectra of reference phases with vanadium oxidation state of +5 (a -VOPO4), +4 (P =(VO)2P2O7, H =VOHPO4 −0.5H2O, E= VO(PO3)2) and + 3 (VPO4).
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Indeed, the 31P SEM-NMR spectrum of a catalyst activated for a long period shows not only a signal around 2600 ppm but also a weak peak at 0 ppm and a broad line ranging from ca. 500 to 1500 ppm (Fig. 3a). Due to the value of the shift, which is intermediate between those of pure V5 + and V4 + phases, we have attributed the broad line to phases with a mixed valence V4 + /V5 + [13]. The presence of V5 + species in a V4 + matrix has often been invoked to explain the catalytic activity of these materials. The V4 + /V5 + phases are amorphous and more likely result from a partial reduction of V5 + phases formed during the activation of the precursor. The relative amount of these phases, which is estimated by integration of the signal between 500 and 1500 ppm in the 31P SEM-NMR spectra of the corresponding catalysts, can be increased by doping materials with
Fig. 2. XRD pattern (a) and 31P SEM-NMR spectrum (b) of VOHPO4 − 0.5H2O activated for 75 h in butane/air. (c) NMR spectra recorded after 0.1 h (1), 8 h (2), 84 h (3) and 132 h (4) of activation. Intensities were normalized with respect to the line at 0 ppm.
in VOPO4 phases. In a similar way, the signal around 2600 ppm can be assigned to an amorphous V4 + phase, which is a disordered precursor of (VO)2P2O7. Actually, increasing the activation time results in the progressive decrease of the line at 0 ppm and the simultaneous increase of the signal around 2600 ppm [12]. In the same time, we observe that the activity of the catalyst and the selectivity in maleic anhydride also increase. These observations, which are almost impossible to obtain by other spectroscopic techniques, allowed us to propose that the transformation VOHPO4 − 0.5H2O [ (VO)2P2O7 is not purely topotactic as generally claimed in the literature [12]. Indeed, NMR shows that VOHPO4 −0.5H2O is also partially transformed to amorphous V5 + phases, which are further reduced and crystallized to form (VO)2P2O7 in the presence of n-butane. Nevertheless, the crystallization of (VO)2P2O7 is never complete, even for very long activation times.
Fig. 3. 31P SEM-NMR spectrum of a catalyst activated for 720 h (a) and intrinsic activity of starting materials (·) and catalysts doped with Fe ( ) or Co( ) with the amount of V4 + /V5 + phases measured on NMR spectra (b).
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Fig. 4. 31P SEM-NMR spectra of VO(PO3)2 at different temperatures (a) and evolution of the inverse of the line shift with temperature for VOHPO4 − 0.5H2O (H), VO(PO3)2 (E) and (VO)2P2O7 (P).
transition metals like Fe or Co. An excellent correlation is observed between the intrinsic activity of the solids in the oxidation of n-butane and the relative amount of V4 + /V5 + species deduced from NMR spectra, which confirms that these species play a key role in the catalytic process [13] (Fig. 3b). This correlation, which is of prime importance to understand and improve the catalysts, would not have been possible without the help of solid state NMR since all these disordered phases cannot be observed by X-ray diffraction. This underlines the importance of the spin echo mapping technique, whose major interest results from the possibility to detect and characterize small amounts of amorphous or poorly crystalline materials in VPO catalysts. 31 P SEM-NMR spectra can also provide direct information on the magnetic properties of VPO catalysts [14,15]. The NMR shift observed for V3 + and V4 + containing materials results essentially from a contact term between the unpaired electrons of V centers and 31P nuclei, which ex-
plains why it increases from V5 + (configuration 3d0) to V3 + (3d2) phases. The shift is proportional to the molar magnetic susceptibility xm of the material: d= (Heff/b·N)xm·f
(1)
where Heff is the atomic hyperfine field for phosphorus (4.7.102 T), b the Bohr magneton, N is the Avogadro’s number and f is a fractional contribution of the unpaired electrons to 31P nuclei. Indeed, V and P centers are separated by a structural oxygen bridge and the contribution of the electronic density of vanadium to the contact term is low (typically f= 10 − 3) [16]. Since xm = C/(T − U) where C is the Curie constant and U is the Weiss temperature, the inverse of the line shift is proportional to temperature: 1/da(T–U)
(2)
In VPO catalysts, U" 0 is a consequence of the presence of V4 + dimers in the structure [17]. By recording NMR spectra at temperatures between
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150 and 400 K, we observe that 1/d varies linearly with T (Fig. 4a). Extrapolation to T = 0 gives U values of − 25, 0 and − 75 K for VOHPO4 − 0.5H2O (H), VO(PO3)2 (E) and (VO)2P2O7 (P), respectively, in excellent agreement with those obtained by direct measurement of the magnetic susceptibility (Fig. 4b). In particular, variable temperature NMR gives U =0 for VO(PO3)2, which is expected theoretically since the structure contains only V4 + monomers [7]. At 150 K, the SEM NMR spectrum of (VO)2P2O7 shows four distinct signals at ca. 3200, 3650, 4200 and 4850 ppm that have been attributed to the existence of four different oxidation states for V atoms in the unit cell, ranging from 3.69 to 4.44 (Fig. 5). This had been previously predicted by calculation of the bond strengths [18] but never observed experimentally. Therefore, this experiment constitutes the first direct evidence of the presence of various oxidation states in the (VO)2P2O7 structure.
served experimentally for various Mg or Mn-substituted aluminophosphates [21]. As an example, the 31P MAS spectrum of a zinc-substituted material ZnAPO4 − 50 with the composition Zn3Al5P8O64 clearly shows five lines attributed to the various P(nZn, 4-nAl) (05n5 4) phosphorus environments in the structure. Unfortunately, such information cannot be obtained on CoAPO4 − n materials since NMR signals are drastically broadened and show numerous and intense sidebands under MAS conditions. A quantitative examination of 31P Hahn-echo spectra of various CoAPO4 − n molecular sieves
3.2. CoAPO4 −n molecular sie6es Incorporation of cobalt at aluminum sites in the framework of aluminophosphate molecular sieves provides materials with potential applications in redox and acid catalyzed reactions. As an example, CoAPO4-5 has been used for the selective oxidation of p-cresol to p-hydroxybenzoic acid with molecular oxygen in the liquid phase [19] or for the formation of cyclohexanone from cyclohexane in the gas phase [20]. The catalytic activity of these materials depends on the location of the heteroatom in the structure. Co2 + cations can indeed either substitute for framework aluminum or be located inside the channels in the form of charge compensating cations or dispersed oxide species. Rietveld refinement of X-ray diffraction patterns of these materials can sometimes bring information about the substitution but the method necessitates highly crystalline materials. Moreover, the distribution of Co species in the framework is difficult to estimate, except in particular cases where Co occupies specific sites in the structure. As for 29Si-NMR lines in zeolites, it is expected that the presence of an heteroatom in the coordination sphere of phosphorus modify the NMR chemical shift of the latter. This was ob-
Fig. 5. 31P SEM-NMR spectrum of (VO)2P2O7 recorded at 150 K (a) and structure showing the vanadium atoms with four different oxidation states (b).
A. Tuel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 158 (1999) 97–106 Table 1 Percentage of cobalt in the various samples and NMR intensities obtained using a simple Hahn-echo sequence (IHE) and a spin echo mapping sequence (ISEM) No
Sample
Co (wt. %)
IHE
ISEM
1 2 3 4 5 6 7 8 9 10
AlPO4−5 CoAPO4−5 CoAPO4−39 CoAPO4−20 CoAPO4−46 CoAPO4−50 CoZnAPO4−50 CoZnAPO4−50 CoZnAPO4−50 DAF−2
0 5.6 4.7 15.4 15.2 16.8 11 5.2 2.6 37.3
1 0.58 0.65 0.25 0.11 0.02 0.16 0.53 0.76 0.03
0.98 1.03 1.04 0.96 0.97 0.98 1.01 1.06 0.99 0.95
shows that the amount of phosphorus atoms detected rapidly decreases with the framework Co content (Table 1). This was first observed by Peeters et al. [22] for materials with relatively low Co contents, and the authors concluded that phosphorus atoms with at least one Co in the first
Fig. 6.
31
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coordination sphere were invisible by NMR. For Co contents above 10 wt. %, only a few percent of nuclei are detected. This suggests that, as for VPO catalysts, the 31P-NMR lines are probably broadened and shifted in the presence of cobalt and that the SEM technique is necessary to observe the whole spectrum. Table 1 shows that all 31P nuclei are effectively detected by SEM, at least within experimental error. The estimation of the proportion of phosphorus nuclei observed by NMR was made by comparison of the intensity of 31 P signals with that of pure AlPO4-5, for which the SEM technique and a simple Hahn echo sequence give similar results. The difference between intensities observed by SEM NMR and those using a simple Hahn echo without incrementing the irradiation frequency is particularly pronounced for samples with high Co contents like CoAPO4-50 or DAF-2. Typical 31P SEMNMR spectra of CoAPO4 − n materials are shown in Fig. 6. NMR lines are usually broad and range between approximately 0 and 10 000 ppm.
P SEM-NMR spectra of various CoAPO4 −n molecular sieves.
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reasonable to assume that f, and thus d, are proportional to the number of Co atoms in the first coordination shell around 31P nuclei. Since the line shift is approximately 9100 ppm for P(4Co) species in DAF-2, the shift corresponding to P(nCo) species with 0 5 n54 is expected to be: dP(nCo)= (9000/4)n =2250n (ppm)
Fig. 7. 31P SEM-NMR spectra of AlPO4 − 5 (a), Co-impregnated AlPO4 −5 (b), the same after magnification by a factor of 20 (c) and CoAPO4 − 5 (d).
For DAF-2, a pure cobalt phosphate with Co/ P = 1 [23], only one signal is observed around 9100 ppm, characteristic of phosphorus with four cobalt atoms in the first coordination sphere: P(4Co). Information about the nature of the observed signals can be obtained by comparing the spectrum of a CoAPO4-5 sample with that of a Co-impregnated AlPO4-5, in which all Co species are inside the channels and not incorporated in the framework. Whilst the 31P SEM spectrum of CoAPO4-5 shows two major lines at approximately 0 and 2500 ppm, that of the impregnated material shows only the signal at 0 ppm (Fig. 7). This result is particularly important since it demonstrates that the presence of low field 31PNMR signals is a direct indication of the presence of framework Co species. Therefore, by contrast to many other spectroscopic techniques, 31PNMR is capable of discriminating easily and rapidly between framework and extraframework Co species in CoAPO4 −n molecular sieves [24]. Eq. (1) shows that the NMR line shift is proportional to a fraction f of the electronic density on the paramagnetic center. As a consequence, it is
This value is observed experimentally on samples with low Co contents for which signals corresponding to P(nCo) populations with n= 1 are weak (Fig. 7). For n= 1, NMR lines are broad and overlap, and attribution of the signals to various phosphorus environments is difficult. Nevertheless, the corresponding P(nCo) populations can be estimated by difference between the whole experimental spectrum and the contributions due to P(0Co) and P(1Co) species. Assignment of 31P SEM-NMR signals makes possible the calculation of the framework P/Co ratio: 4
P = Co
% P(nCo) n=0 4
% 0.25nP(nCo) n=0
Comparison of this ratio with that obtained by chemical analysis gives a direct estimation of the amount of extra framework Co species, since the latter cannot be detected by the technique. The distribution of Co species in the framework can be estimated by comparing P(nCo) populations obtained from NMR spectra with those calculated by a binomial theorem, assuming that Co randomly substitutes for aluminum. The agreement is very good for samples with low Co contents. However, for two of the studied samples, namely CoAPO4 − 50 and CoAPO4 − 20, the observed populations strongly differ from calculated ones (Table 2). For a series of CoZnAPO4 − 50 materials, populations are similar for low Co contents but the difference increases at high Co loading. By Rietveld refinement of the X-ray pattern of CoAPO4 − 50, Bennett et al. suggested that two of the Co atoms of the unit cell occupy specific positions in the framework whilst the third one is randomly distributed on Al sites [5]. Starting from this partially ordered unit cell, calculations of the
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Table 2 Comparison of P(nCo) populations deduced from NMR spectra (INMR) with those obtained using a binomial theorem (Icalc) No
4 6 7 8 9
Sample
CoAPO4−20 CoAPO4−50 CoZnAPO4−50 CoZnAPO4−50 CoZnAPO4−50 ZnAPO4−50 a
INMR
Icalc
P(0Co)
P(1Co)
P(nCo)a
P(0Co)
P(1Co)
P(nCo)a
0.35 0.03 0.25 0.57 0.77 1
0.65 0.57 0.49 0.35 0.21 0
0 0.40 0.26 0.08 0.02 0
0.2 0.15 0.316 0.60 0.78 1
0.4 0.36 0.42 0.33 0.20 0
0.4 0.49 0.26 0.07 0.02 0
Populations corresponding to n= 1.
various P(nCo) populations gives P(0Co)=0, P(1Co)= 0.58, P(2Co)=0.34, P(3Co)= 0.07 and P(4Co)=0.005. These populations, particularly P(0Co) and P(1Co) are in good agreement with those obtained from NMR spectra. Therefore, 31P SEM-NMR shows that the distribution of Co species in CoZnAPO4 −50 molecular sieves changes with the framework substitution level. Whilst the distribution is statistical at low Co loading, a partial ordering is observed for materials with more than two Co atoms per unit cell, in particular for the pure CoAPO4 −50 material. CoAPO4 − 20 is a cobalt aluminophosphate molecular sieve with the sodalite-type structure. The chemical composition shows Co substitutes for 33% of aluminum in the unit cell. The 31P SEM-NMR spectrum of the material is composed of two signals at approximately 0 and 4100 ppm, with relative intensities of 1:2 (Fig. 6), suggesting that Co occupies preferential positions in the lattice. The signals have been assigned to P(0Co)
Fig. 8. Sodalite cage of CoAPO4 − 20 showing the arrangement of Co atoms in the four-membered rings.
and P(2Co) species, respectively. These populations correspond to sodalite cages in which the two Co atoms share a four-membered ring (Fig. 8). This example illustrates once more the efficiency of the method to provide information on the environment and distribution of paramagnetic species in a crystalline framework.
4. Conclusion In this paper, we have shown that a special NMR technique could be successfully applied to the characterization of paramagnetic catalysts. Information concerning the nature, location and oxidation state of paramagnetic centers is indirectly obtained by observing the NMR spectrum of neighboring atoms, namely phosphorus atoms in the present case. Spectra are usually very broad, which makes that the proportion of 31P nuclei detected by conventional sequences is low, particularly for samples containing high amounts of paramagnetic species. It is thus necessary to use a mapping technique in order to observe signals within the whole spectral region. The presence of transition metals bonded to phosphorus shifts the corresponding 31P-NMR line and the shift is directly proportional to the density of unpaired electrons on the paramagnetic center. Thus, the NMR line shift gives direct indication on the number of paramagnetic species in the first coordination sphere of phosphorus as well as on their oxidation state. This information is very important to analyze VPO catalysts, particularly when
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they contain small amounts of poorly crystalline phases which cannot be detected by X-ray diffraction. The location and distribution of cobalt species in aluminophosphate molecular sieves can also be obtained from 31P SEM-NMR spectra of CoAPO4 − n molecular sieves. The presence of signals above 2000 ppm is a direct evidence for the presence of framework tetrahedrally coordinated Co2 + species. From the spectra, it is possible to calculate the framework P/Co ratio and to estimate the ordering of Co species in the structure.
References [1] J.C. Vedrine, J.M.M. Millet, J.C. Volta, J. Chem. Soc. Faraday Discuss. 87 (1989) 207. [2] A. Tuel, M.T. Sananes-Schulz, J.C. Volta, Catal. Today 37 (1997) 59. [3] J.W. Johnson, D.C. Johnston, A.J. Jacobson, J.F. Brody, J. Am. Chem. Soc. 106 (1984) 8123. [4] D.B. Akolekar, Catal. Lett. 28 (1994) 249. [5] J.M. Bennett, B.K. Marcus, Stud. Surf. Sci. Catal. 37 (1988) 269. [6] Y. Tong, J. Magn. Res. 119A (1996) 22. [7] E. Bordes, Catal. Today 1 (1987) 499. [8] F. Ben Abdelhouahab, R. Olier, N. Guillaume, F. Lefevre, J.C. Volta, J. Catal. 134 (1992) 151.
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[9] M.T. Sananes, A. Tuel, J.C. Volta, J. Catal. 145 (1994) 251. [10] M.T. Sananes, A. Tuel, G.J. Hutchings, J.C. Volta, J. Catal. 148 (1994) 395. [11] C.J. Kiely, A. Burrows, S. Sajip, G.J. Hutchings, M.T. Sananes, A. Tuel, J.C. Volta, J. Catal. 162 (1996) 31. [12] M. Abon, K. Bere, A. Tuel, P. Delichere, J. Catal. 156 (1995) 28. [13] M.T. Sananes-Schulz, A. Tuel, G.J. Hutchings, J.C. Volta, J. Catal. 166 (1997) 388. [14] M.T. Sananes, A. Tuel, J. Chem. Soc. Chem. Commun. 13 (1995) 1323. [15] M.T. Sananes, A. Tuel, Solid State NMR 6 (1996) 157. [16] J. Li, M.E. Lashier, G.L. Schraeder, B.C. Gerstein, Appl. Catal. 73 (1991) 83. [17] D.C. Johnston, J.W. Johnson, J. Chem. Soc. Chem. Commun. 23 (1985) 1720. [18] I.D. Brown, K.K. Wu, Acta Cryst. B32 (1976) 1957. [19] I.W.C.E. Arends, R.A. Sheldon, M. Wallau, U. Schuchardt, Angew. Chem. Int. Ed. Engl. 36 (1997) 1144. [20] P. Conception, A. Corma, J.M. Lopez Nieto, J. PerezPariente, Appl. Catal. A Gen. 143 (1996) 17. [21] A. Tuel, I. Arcon, N. Novak-Tusar, A. Meden, V. Kaucic, Microporous Mater. 7 (1996) 271. [22] M.P.J. Peeters, L.J.M. Van de Ven, J.W. de Haan, J.H.C. van Hooff, Colloids Surf. A Physicochem. Eng. Asp. 72 (1993) 87. [23] J. Chen, R.H. Jones, S. Natarajan, M.B. Hursthouse, J.M. Thomas, Angew. Chem. Int. Ed. Engl. 33 (1994) 639. [24] L. Canesson, Y. Boudeville, A. Tuel, J. Am. Chem. Soc. 119 (1997) 10754.
Investigation of paramagnetic catalysts by solid state NMR spectroscopy A. Tuel *, L. Canesson, J.C. Volta Institut de Recherches sur la Catalyse, CNRS, 2, a6enue A. Einstein 69626 Villeurbanne Cedex, France
Abstract A spin echo mapping (SEM) technique has been used to record the total 31P-NMR information in paramagnetic phosphate-based catalysts. For these materials, conventional NMR techniques give poor information and a great proportion of 31P nuclei cannot be detected due to broad and shifted signals. By contrast to conventional sequences, all 31P nuclei are detected using a mapping technique. Under specific conditions, the method is quantitative and gives direct information on the nature, the amount and the oxidation state of neighboring paramagnetic species. For vanadium phosphorus oxide catalysts (VPO), the technique is particularly interesting for characterizing intermediates formed during catalytic reactions, especially when the latter are amorphous and cannot be detected by XRD. Moreover, 31P-NMR spectra recorded at various temperatures give precious information on the magnetic properties of these materials. In the case of cobalt-substituted aluminophosphate molecular sieves, P/Co ratios as well as the various populations P(nCo, 4-nAl) can be derived from 31P-NMR spectra. Such parameters, which cannot be obtained by other spectroscopic techniques, are of particular importance for the understanding of the acidic and redox properties of these catalysts. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Paramagnetism; Spin echo mapping; NMR; VPO catalysts; CoAPO4-n
1. Introduction A great number of oxidation reactions in both the liquid and gas phases are now performed industrially in the presence of heterogeneous catalysts, which generally contain transition metals dispersed on the surface or in the bulk of inorganic supports. The performance of these catalysts strongly depends on the local environment * Corresponding author. Tel.: +33-4-72445395; fax: + 334-72445399. E-mail address: [email protected] (A. Tuel)
around active sites, as well as on their oxidation state and dispersion. Many spectroscopic techniques have been used to access the coordination and oxidation state of catalytic species but the simultaneous presence of dispersed and aggregated species as well as of highly crystalline and amorphous phases make that interpretation of the data is often difficult. Solid state NMR has sometimes been successfully applied to probe the catalytic sites at atomic scale in these compounds but the technique is limited to a few numbers of transition metals with appropriate oxidation states. When direct obser-
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vation of active species is not possible, an alternative consists in analyzing the influence of the latter on the NMR spectra of neighboring atoms. Nevertheless, a small amount of paramagnetic species like Fe3 + , V4 + or Co2 + , in the catalysts is generally enough to considerably broaden NMR lines and the information that can be derived from the spectra is poor [1]. Actually signals are not only broadened, but they are also shifted, which makes that a high proportion of the NMR signal cannot be detected using conventional sequences. For paramagnetic catalysts, we have previously reported that a spin echo mapping (SEM) technique could be used to obtain the total NMR information from a series of conventional spectra recorded at various irradiation frequencies [2]. The present paper reports on the potentiality of this technique to characterize two classes of transition metal — containing phosphate — based catalysts, namely vanadium phosphorus oxide catalysts (VPO) and cobalt-substituted aluminophosphate molecular sieves (CoAPO4 − n). The originality of the method, as well as its specificity with respect to other spectroscopic techniques are clearly illustrated.
2. Experimental All catalysts were prepared following recipes of the literature [3 – 5] and preliminary characterized by conventional techniques, principally X-ray diffraction (Philips PW 1710 diffractometer, Cu–Ka radiation). 31 P SEM-NMR spectra were acquired on static samples with a Bru¨ker DSX 400 spectrometer equipped with a standard variable temperature 7 mm probe head. Briefly, the SEM technique consists in recording a series of Hahn echo spectra (with t=20 ms) by incrementing the irradiation frequency below and above that of H3PO4 (d=0). Typically 1000 scans are accumulated for each spectrum with a pulse length of 3 ms (p/2) and a recycle delay of 0.5 s. The frequency increment Dv was 75 kHz and satisfied the relation Dv = 0.5v1, where v1 =gH1 is the irradiation frequency [6]. The number of spectra necessary to cover the whole spectral region is dictated by the frequency
limits beyond which no signal is detected. After Fourier transformation each spectrum is filtered following a method recently reported by Tong [6] to eliminate oscillations of the base line and increase the signal-to-noise ratio of the final SEM spectrum. For each irradiation frequency, the T2 relaxation time is measured and the intensity of the corresponding spectrum is corrected to account for the loss of magnetization during acquisition of the signal. This correction is necessary to obtain quantitative data, particularly in materials where diamagnetic and paramagnetic species coexist.
3. Results and discussion
3.1. VPO catalysts The method was first applied to the characterization of VPO catalysts, used at industrial scale for the production of maleic anhydride from nbutane. VPO catalysts consist of vanadium octahedra (VO6) connected to phosphorus tetrahedra (PO4). The numerous possibilities to connect these polyhedra generate a family of materials with different structures and vanadium oxidation states (from V3 + to V5 + ) [7]. Whilst V5 + phases can be easily characterized by conventional 31P and 51V MAS-NMR [8], this is not the case for materials with lower oxidation states. Fig. 1 shows that some reference compounds with vanadium oxidation states from +3 to + 5 can be immediately identified by their 31P SEM-NMR spectra. Indeed, the signal is shifted from ca. 0 ppm for VOPO4 materials (V5 + ) to 1600–2600 ppm for V4 + phases and approximately 4650 ppm for VPO4 [9]. The possibility to discriminate between these phases, and particularly those containing V4 + species is of great interest and will considerably help for the interpretation of NMR spectra of real catalysts. The following example illustrates the advantages of NMR as compared to long-range characterization techniques like XRD to provide information on the various phases that can be present in a catalyst and their evolution in course of reaction. (VO)2P2O7 (d=2600 ppm), which is
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recognized to be the active phase in the catalytic oxidation of n-butane to maleic anhydride can be prepared by activation of the hemihydrate VOHPO4 −0.5H2O (d =1625 ppm) in a butane/ air atmosphere. However, (VO)2P2O7 is rarely obtained as a pure phase and the catalyst is often contaminated by poorly crystallized materials containing V centers with higher oxidation states. The proportion of contaminating phases, which depends on the activation procedure and the mor-
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phology of the starting precursor, strongly affects the activity of the catalyst [10]. For specific morphologies of the precursor, whilst the material obtained after activation is completely amorphous by XRD [11], the 31P SEM-NMR spectrum can help to identify the nature of the various phases and estimate their relative amount in the catalyst (Fig. 2). The intense peak around 0 ppm is characteristic of phosphorus atoms in a local environment of V5 + species, similar to that encountered
Fig. 1. 31P SEM-NMR spectra of reference phases with vanadium oxidation state of +5 (a -VOPO4), +4 (P =(VO)2P2O7, H =VOHPO4 −0.5H2O, E= VO(PO3)2) and + 3 (VPO4).
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Indeed, the 31P SEM-NMR spectrum of a catalyst activated for a long period shows not only a signal around 2600 ppm but also a weak peak at 0 ppm and a broad line ranging from ca. 500 to 1500 ppm (Fig. 3a). Due to the value of the shift, which is intermediate between those of pure V5 + and V4 + phases, we have attributed the broad line to phases with a mixed valence V4 + /V5 + [13]. The presence of V5 + species in a V4 + matrix has often been invoked to explain the catalytic activity of these materials. The V4 + /V5 + phases are amorphous and more likely result from a partial reduction of V5 + phases formed during the activation of the precursor. The relative amount of these phases, which is estimated by integration of the signal between 500 and 1500 ppm in the 31P SEM-NMR spectra of the corresponding catalysts, can be increased by doping materials with
Fig. 2. XRD pattern (a) and 31P SEM-NMR spectrum (b) of VOHPO4 − 0.5H2O activated for 75 h in butane/air. (c) NMR spectra recorded after 0.1 h (1), 8 h (2), 84 h (3) and 132 h (4) of activation. Intensities were normalized with respect to the line at 0 ppm.
in VOPO4 phases. In a similar way, the signal around 2600 ppm can be assigned to an amorphous V4 + phase, which is a disordered precursor of (VO)2P2O7. Actually, increasing the activation time results in the progressive decrease of the line at 0 ppm and the simultaneous increase of the signal around 2600 ppm [12]. In the same time, we observe that the activity of the catalyst and the selectivity in maleic anhydride also increase. These observations, which are almost impossible to obtain by other spectroscopic techniques, allowed us to propose that the transformation VOHPO4 − 0.5H2O [ (VO)2P2O7 is not purely topotactic as generally claimed in the literature [12]. Indeed, NMR shows that VOHPO4 −0.5H2O is also partially transformed to amorphous V5 + phases, which are further reduced and crystallized to form (VO)2P2O7 in the presence of n-butane. Nevertheless, the crystallization of (VO)2P2O7 is never complete, even for very long activation times.
Fig. 3. 31P SEM-NMR spectrum of a catalyst activated for 720 h (a) and intrinsic activity of starting materials (·) and catalysts doped with Fe ( ) or Co( ) with the amount of V4 + /V5 + phases measured on NMR spectra (b).
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Fig. 4. 31P SEM-NMR spectra of VO(PO3)2 at different temperatures (a) and evolution of the inverse of the line shift with temperature for VOHPO4 − 0.5H2O (H), VO(PO3)2 (E) and (VO)2P2O7 (P).
transition metals like Fe or Co. An excellent correlation is observed between the intrinsic activity of the solids in the oxidation of n-butane and the relative amount of V4 + /V5 + species deduced from NMR spectra, which confirms that these species play a key role in the catalytic process [13] (Fig. 3b). This correlation, which is of prime importance to understand and improve the catalysts, would not have been possible without the help of solid state NMR since all these disordered phases cannot be observed by X-ray diffraction. This underlines the importance of the spin echo mapping technique, whose major interest results from the possibility to detect and characterize small amounts of amorphous or poorly crystalline materials in VPO catalysts. 31 P SEM-NMR spectra can also provide direct information on the magnetic properties of VPO catalysts [14,15]. The NMR shift observed for V3 + and V4 + containing materials results essentially from a contact term between the unpaired electrons of V centers and 31P nuclei, which ex-
plains why it increases from V5 + (configuration 3d0) to V3 + (3d2) phases. The shift is proportional to the molar magnetic susceptibility xm of the material: d= (Heff/b·N)xm·f
(1)
where Heff is the atomic hyperfine field for phosphorus (4.7.102 T), b the Bohr magneton, N is the Avogadro’s number and f is a fractional contribution of the unpaired electrons to 31P nuclei. Indeed, V and P centers are separated by a structural oxygen bridge and the contribution of the electronic density of vanadium to the contact term is low (typically f= 10 − 3) [16]. Since xm = C/(T − U) where C is the Curie constant and U is the Weiss temperature, the inverse of the line shift is proportional to temperature: 1/da(T–U)
(2)
In VPO catalysts, U" 0 is a consequence of the presence of V4 + dimers in the structure [17]. By recording NMR spectra at temperatures between
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150 and 400 K, we observe that 1/d varies linearly with T (Fig. 4a). Extrapolation to T = 0 gives U values of − 25, 0 and − 75 K for VOHPO4 − 0.5H2O (H), VO(PO3)2 (E) and (VO)2P2O7 (P), respectively, in excellent agreement with those obtained by direct measurement of the magnetic susceptibility (Fig. 4b). In particular, variable temperature NMR gives U =0 for VO(PO3)2, which is expected theoretically since the structure contains only V4 + monomers [7]. At 150 K, the SEM NMR spectrum of (VO)2P2O7 shows four distinct signals at ca. 3200, 3650, 4200 and 4850 ppm that have been attributed to the existence of four different oxidation states for V atoms in the unit cell, ranging from 3.69 to 4.44 (Fig. 5). This had been previously predicted by calculation of the bond strengths [18] but never observed experimentally. Therefore, this experiment constitutes the first direct evidence of the presence of various oxidation states in the (VO)2P2O7 structure.
served experimentally for various Mg or Mn-substituted aluminophosphates [21]. As an example, the 31P MAS spectrum of a zinc-substituted material ZnAPO4 − 50 with the composition Zn3Al5P8O64 clearly shows five lines attributed to the various P(nZn, 4-nAl) (05n5 4) phosphorus environments in the structure. Unfortunately, such information cannot be obtained on CoAPO4 − n materials since NMR signals are drastically broadened and show numerous and intense sidebands under MAS conditions. A quantitative examination of 31P Hahn-echo spectra of various CoAPO4 − n molecular sieves
3.2. CoAPO4 −n molecular sie6es Incorporation of cobalt at aluminum sites in the framework of aluminophosphate molecular sieves provides materials with potential applications in redox and acid catalyzed reactions. As an example, CoAPO4-5 has been used for the selective oxidation of p-cresol to p-hydroxybenzoic acid with molecular oxygen in the liquid phase [19] or for the formation of cyclohexanone from cyclohexane in the gas phase [20]. The catalytic activity of these materials depends on the location of the heteroatom in the structure. Co2 + cations can indeed either substitute for framework aluminum or be located inside the channels in the form of charge compensating cations or dispersed oxide species. Rietveld refinement of X-ray diffraction patterns of these materials can sometimes bring information about the substitution but the method necessitates highly crystalline materials. Moreover, the distribution of Co species in the framework is difficult to estimate, except in particular cases where Co occupies specific sites in the structure. As for 29Si-NMR lines in zeolites, it is expected that the presence of an heteroatom in the coordination sphere of phosphorus modify the NMR chemical shift of the latter. This was ob-
Fig. 5. 31P SEM-NMR spectrum of (VO)2P2O7 recorded at 150 K (a) and structure showing the vanadium atoms with four different oxidation states (b).
A. Tuel et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 158 (1999) 97–106 Table 1 Percentage of cobalt in the various samples and NMR intensities obtained using a simple Hahn-echo sequence (IHE) and a spin echo mapping sequence (ISEM) No
Sample
Co (wt. %)
IHE
ISEM
1 2 3 4 5 6 7 8 9 10
AlPO4−5 CoAPO4−5 CoAPO4−39 CoAPO4−20 CoAPO4−46 CoAPO4−50 CoZnAPO4−50 CoZnAPO4−50 CoZnAPO4−50 DAF−2
0 5.6 4.7 15.4 15.2 16.8 11 5.2 2.6 37.3
1 0.58 0.65 0.25 0.11 0.02 0.16 0.53 0.76 0.03
0.98 1.03 1.04 0.96 0.97 0.98 1.01 1.06 0.99 0.95
shows that the amount of phosphorus atoms detected rapidly decreases with the framework Co content (Table 1). This was first observed by Peeters et al. [22] for materials with relatively low Co contents, and the authors concluded that phosphorus atoms with at least one Co in the first
Fig. 6.
31
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coordination sphere were invisible by NMR. For Co contents above 10 wt. %, only a few percent of nuclei are detected. This suggests that, as for VPO catalysts, the 31P-NMR lines are probably broadened and shifted in the presence of cobalt and that the SEM technique is necessary to observe the whole spectrum. Table 1 shows that all 31P nuclei are effectively detected by SEM, at least within experimental error. The estimation of the proportion of phosphorus nuclei observed by NMR was made by comparison of the intensity of 31 P signals with that of pure AlPO4-5, for which the SEM technique and a simple Hahn echo sequence give similar results. The difference between intensities observed by SEM NMR and those using a simple Hahn echo without incrementing the irradiation frequency is particularly pronounced for samples with high Co contents like CoAPO4-50 or DAF-2. Typical 31P SEMNMR spectra of CoAPO4 − n materials are shown in Fig. 6. NMR lines are usually broad and range between approximately 0 and 10 000 ppm.
P SEM-NMR spectra of various CoAPO4 −n molecular sieves.
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reasonable to assume that f, and thus d, are proportional to the number of Co atoms in the first coordination shell around 31P nuclei. Since the line shift is approximately 9100 ppm for P(4Co) species in DAF-2, the shift corresponding to P(nCo) species with 0 5 n54 is expected to be: dP(nCo)= (9000/4)n =2250n (ppm)
Fig. 7. 31P SEM-NMR spectra of AlPO4 − 5 (a), Co-impregnated AlPO4 −5 (b), the same after magnification by a factor of 20 (c) and CoAPO4 − 5 (d).
For DAF-2, a pure cobalt phosphate with Co/ P = 1 [23], only one signal is observed around 9100 ppm, characteristic of phosphorus with four cobalt atoms in the first coordination sphere: P(4Co). Information about the nature of the observed signals can be obtained by comparing the spectrum of a CoAPO4-5 sample with that of a Co-impregnated AlPO4-5, in which all Co species are inside the channels and not incorporated in the framework. Whilst the 31P SEM spectrum of CoAPO4-5 shows two major lines at approximately 0 and 2500 ppm, that of the impregnated material shows only the signal at 0 ppm (Fig. 7). This result is particularly important since it demonstrates that the presence of low field 31PNMR signals is a direct indication of the presence of framework Co species. Therefore, by contrast to many other spectroscopic techniques, 31PNMR is capable of discriminating easily and rapidly between framework and extraframework Co species in CoAPO4 −n molecular sieves [24]. Eq. (1) shows that the NMR line shift is proportional to a fraction f of the electronic density on the paramagnetic center. As a consequence, it is
This value is observed experimentally on samples with low Co contents for which signals corresponding to P(nCo) populations with n= 1 are weak (Fig. 7). For n= 1, NMR lines are broad and overlap, and attribution of the signals to various phosphorus environments is difficult. Nevertheless, the corresponding P(nCo) populations can be estimated by difference between the whole experimental spectrum and the contributions due to P(0Co) and P(1Co) species. Assignment of 31P SEM-NMR signals makes possible the calculation of the framework P/Co ratio: 4
P = Co
% P(nCo) n=0 4
% 0.25nP(nCo) n=0
Comparison of this ratio with that obtained by chemical analysis gives a direct estimation of the amount of extra framework Co species, since the latter cannot be detected by the technique. The distribution of Co species in the framework can be estimated by comparing P(nCo) populations obtained from NMR spectra with those calculated by a binomial theorem, assuming that Co randomly substitutes for aluminum. The agreement is very good for samples with low Co contents. However, for two of the studied samples, namely CoAPO4 − 50 and CoAPO4 − 20, the observed populations strongly differ from calculated ones (Table 2). For a series of CoZnAPO4 − 50 materials, populations are similar for low Co contents but the difference increases at high Co loading. By Rietveld refinement of the X-ray pattern of CoAPO4 − 50, Bennett et al. suggested that two of the Co atoms of the unit cell occupy specific positions in the framework whilst the third one is randomly distributed on Al sites [5]. Starting from this partially ordered unit cell, calculations of the
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Table 2 Comparison of P(nCo) populations deduced from NMR spectra (INMR) with those obtained using a binomial theorem (Icalc) No
4 6 7 8 9
Sample
CoAPO4−20 CoAPO4−50 CoZnAPO4−50 CoZnAPO4−50 CoZnAPO4−50 ZnAPO4−50 a
INMR
Icalc
P(0Co)
P(1Co)
P(nCo)a
P(0Co)
P(1Co)
P(nCo)a
0.35 0.03 0.25 0.57 0.77 1
0.65 0.57 0.49 0.35 0.21 0
0 0.40 0.26 0.08 0.02 0
0.2 0.15 0.316 0.60 0.78 1
0.4 0.36 0.42 0.33 0.20 0
0.4 0.49 0.26 0.07 0.02 0
Populations corresponding to n= 1.
various P(nCo) populations gives P(0Co)=0, P(1Co)= 0.58, P(2Co)=0.34, P(3Co)= 0.07 and P(4Co)=0.005. These populations, particularly P(0Co) and P(1Co) are in good agreement with those obtained from NMR spectra. Therefore, 31P SEM-NMR shows that the distribution of Co species in CoZnAPO4 −50 molecular sieves changes with the framework substitution level. Whilst the distribution is statistical at low Co loading, a partial ordering is observed for materials with more than two Co atoms per unit cell, in particular for the pure CoAPO4 −50 material. CoAPO4 − 20 is a cobalt aluminophosphate molecular sieve with the sodalite-type structure. The chemical composition shows Co substitutes for 33% of aluminum in the unit cell. The 31P SEM-NMR spectrum of the material is composed of two signals at approximately 0 and 4100 ppm, with relative intensities of 1:2 (Fig. 6), suggesting that Co occupies preferential positions in the lattice. The signals have been assigned to P(0Co)
Fig. 8. Sodalite cage of CoAPO4 − 20 showing the arrangement of Co atoms in the four-membered rings.
and P(2Co) species, respectively. These populations correspond to sodalite cages in which the two Co atoms share a four-membered ring (Fig. 8). This example illustrates once more the efficiency of the method to provide information on the environment and distribution of paramagnetic species in a crystalline framework.
4. Conclusion In this paper, we have shown that a special NMR technique could be successfully applied to the characterization of paramagnetic catalysts. Information concerning the nature, location and oxidation state of paramagnetic centers is indirectly obtained by observing the NMR spectrum of neighboring atoms, namely phosphorus atoms in the present case. Spectra are usually very broad, which makes that the proportion of 31P nuclei detected by conventional sequences is low, particularly for samples containing high amounts of paramagnetic species. It is thus necessary to use a mapping technique in order to observe signals within the whole spectral region. The presence of transition metals bonded to phosphorus shifts the corresponding 31P-NMR line and the shift is directly proportional to the density of unpaired electrons on the paramagnetic center. Thus, the NMR line shift gives direct indication on the number of paramagnetic species in the first coordination sphere of phosphorus as well as on their oxidation state. This information is very important to analyze VPO catalysts, particularly when
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they contain small amounts of poorly crystalline phases which cannot be detected by X-ray diffraction. The location and distribution of cobalt species in aluminophosphate molecular sieves can also be obtained from 31P SEM-NMR spectra of CoAPO4 − n molecular sieves. The presence of signals above 2000 ppm is a direct evidence for the presence of framework tetrahedrally coordinated Co2 + species. From the spectra, it is possible to calculate the framework P/Co ratio and to estimate the ordering of Co species in the structure.
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