J. Phys. Chem. 1996, 100, 8977-8985
8977
X-ray Absorption Spectroscopic Study of Brønsted, Lewis, and Redox Centers in Cobalt-Substituted Aluminum Phosphate Catalysts Philip A. Barrett, Gopinathan Sankar,* C. Richard A. Catlow,* and John Meurig Thomas DaVy Faraday Research Laboratory, The Royal Institution of GB, 21 Albemarle Street, London, W1X 4BS U.K. ReceiVed: October 13, 1995; In Final Form: February 13, 1996X
The structure of acid sites in cobalt-substituted aluminophosphates (AlPOs) catalysts has been investigated, with EXAFS spectroscopy. Data obtained, using in situ methods, for the Co K-edge spectra of cobalt-substituted AlPO-5, AlPO-18, AlPO-36, and APSO-44 in as-prepared, calcined, and reduced states yield the local structure of the Co ions. Whereas the as-prepared materials clearly contain Co(II) ions in regular four-coordinated sites, complex behavior is exhibited by their calcined analogues, with essentially complete oxidation of Co(II) to Co(III) in CoAlPO-18, with the local coordination of the high-spin Co(III) being distorted. By contrast incomplete oxidation of the Co(II) is observed for CoAlPO-5 and CoAlPO-36. A combination of EXAFS data analysis with the results of computer modeling studies suggests that this behavior is interpretable in terms of the formation of oxygen vacancies by dehydroxylation which lead to undercoordinated Lewis acid, Co(II) sites. IR data of the reduced samples reveal the presence of Brønsted acid sites which, from the EXAFS analysis, are seen to correspond to Co(II) species in a distorted environment owing to the presence of a neighboring protonated oxygen ion. The implications of the results for the catalytic properties of the systems are considered.
Introduction When cobalt ions are incorporated by direct synthesis into the tetrahedral framework sites of microporous aluminum phosphates (AlPOs) or silicoaluminum phosphates (SAPOs), powerful and selective heterogeneous catalysts are produced.1 Many of these are potentially viable in industrial contexts in such processes as the autooxidation of cyclohexane (to cyclohexanol and cyclohexanone),2,3 and the selective conversion of methanol light alkenes.4-8 The full scope of these potentially useful catalysts has not yet been explored. In particular, the fundamental factors governing the generation of the acid sites by replacing Al by Co as a function of the precise nature of the microporous framework have yet to be understood. We have, however, proposed9 that the ease of interconversion of Co(II) and Co(III) ions in the framework site is critically dependent upon the particular AlPO framework within which the cobalt is accommodated. Moreover, in some AlPO structures, it seems10-12 that substitution by Co(II) leads to a predominance of Lewis centers (as in the case of CoAlPO-5 or CoAlPO-36), whereas in others (e.g., CoAlPO-18 and CoAPSO-44) Brønsted centers predominate.13,14 The correlation between catalytic performance and solid-state structure is more firmly established for MeAlPO (Me ) Co, Zn, Mg, etc.)1,11,13 and aluminosilicate zeolitic microporous solids than for many other catalysts. There is, therefore, a powerful incentive to define the relationship between the structures of the host AlPO and the ease of reducibility or oxidizability of the framework guest species as well as the overall facility with which Brønsted and Lewis centers are generated. These are the questions we address in this paper using X-ray absorption spectroscopy (XAS).15,16 Infrared spectroscopy is also used as a supplementary tool. In addition we employ computer simulation techniques to investigate the stabilities of certain of the models proposed for Lewis acid centers. X
Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(95)03034-6 CCC: $12.00
Figure 1. Framework structures of CoAlPOs and CoAPSO-44. The different framework topologies are generated in each case by the strict alternation of the Al(III) and P(V) T sites except in the case of CoAPSO-44 where Si (IV) is also present substituting for P(V). (a) AlPO-5; (b) AlPO-36; (c) AlPO-18; (d) AlPO-44.
We have chosen four distinct structure types (see Figure 1) derived from the AlPO-5, AlPO-36, AlPO-18, and AlPO-44 structures. X-ray absorption measurements, yielding detailed pre-edge, near-edge (XANES), and extended X-ray absorption fine structure (EXAFS) results, were made on the as-synthesized (templated), on the calcined (in O2 and hence oxidized), and on the reduced (in hydrogen) states. The thermal stability of CoAlPO-44 is such that during calcination in O2 (for the purpose of detemplation) the microporus structure collapses.17 Accordingly, we used the more stable CoAPSO-44 for comparative purposes.17 Our investigations shed light on the relative proportions of Brønsted and Lewis acid (catalytically active) sites that may be incorporated into a particular parent AlPO structure by © 1996 American Chemical Society
8978 J. Phys. Chem., Vol. 100, No. 21, 1996
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Figure 2. X-ray diffraction patterns of (a) CoAlPO-5, (b) CoAlPO36, (c) CoAlPO-18, and (d) CoAPSO-44.
framework substitution of Co ions. This should help guide future design and choice of a microporous catalyst with the required balance between activity (and active site concentration) on one hand and selectivity, especially shape-selectivity, which is governed by the precise structure of the microporous solid, on the other. The XAS data reported here afford valuable information about the different types of acid sites present in the four distinct kinds of solid catalysts under investigation. Experimental Section The Co-containing AlPOs (CoAlPO-5,7 CoAlPO-18,13 CoAlPO-3618) and CoAPSO-4419 were prepared using wellestablished procedures that entail the use of an appropriate organic template molecule for crystallization from the nutrient gel. The degrees of crystallinity and phase-purity of each of the as-prepared Co-containing materials were determined by X-ray diffraction (XRD): the XRD patterns for the three Cosubstituted AlPOs (-5, -18, and -36) and APSO-44 (Figure 2) were readily indexed to the respective structures and there were no extra reflections arising from other phases. In situ XAS measurements were carried out on station 7.1 of the CLRC, Daresbury Synchrotron Radiation source. This station, equipped with a Si(111) double crystal monochromator and ion chambers, was used for measuring incident (Io) and transmitted (It) beam intensities. Self-supporting wafers of the sample (typically 100 mg) were mounted on a specially designed quartz in situ cell provided with an inlet and a outlet for the particular gas flowing over the sample, as shown in Figure 3. Samples were calcined in O2 at 550 °C for 2 h and then subsequently reduced in an H2/N2 (10% H2) mixture at 400 °C for 1 h prior to the EXAFS measurement at room temperature (RT). The raw data were processed using the EXAFS analysis suite of programs available at the Daresbury Laboratory comprising, EXCALIB (for converting the recorded data to energy vs absorption coefficient), EXBROOK (for pre-edge and post-edge background subtraction) and EXCURV92 (for curve-fitting analysis using a least-squares procedure). A normal spinel (CoAl2O4 in which the high-spin Co(II) is known20 to be overwhelmingly in tetrahedral sites) was used as the model compound so as to yield the nonstructural parameter associated with the EXAFS data analysis. Values of nonstructural parameters obtained from the above analysis were further verified using an as-prepared sample of a recently reported open-
Figure 3. In situ cell employed for carrying out XAS measurements under a controlled atmosphere. The cell made of quartz was glued to an aluminum base which was mounted on the optical bench. Typically 120 mg of the sample was pressed into a pellet and held firmly in the stainless steel sample holder which enable us to move the sample into the furnace area for pretreatments under desired conditions and into the window region for XAS data recording at room temperature. The inlet and outlet permits the desired gas flow during pretreatment. This cell also has the advantage of being able to be attached to a conventional vacuum line (sustainable vacuum of 10-4 Torr) allowing other necessary controlled-atmosphere treatments.
framework cobalt phosphate known as DAF-2 (Davy Faraday2).21 Typical errors involved in the determination of coordination number and bond distances are (10% and (0.02 Å, respectively. Self-supporting wafers of the samples (typically 13 mg) were also loaded in a specially designed in situ quartz cell (similar to Figure 3) attached to a conventional vacuum system and placed within the infrared (Perkin-Elmer (1725X) FTIR) spectrometer. The samples were heated in vacuo initially to remove any adsorbed water and calcined at 550 °C with 60 Torr of oxygen and reduced at 400 °C in 60 Torr of hydrogen. The cell was evacuated and cooled to room temperature prior to recording the IR data. On average 128 scans were recorded for each experiment (2 cm-1 resolution). Calculations on the energetics of Brønsted acid and vacancy species in the materials studied are also reported. We employed the standard and widely used Mott-Littleton procedure22 available in the CASCADE code,23 the reliability of which for modeling defects in complex materials has been previously demonstrated.24,25 The method requires the specification of interatomic potentials; the recently developed parametrization for AlPO materials of Henson and Gale26 was employed. Results We discuss first the results for the as-prepared samples, model compounds, and then the calcined and reduced samples. Analysis of the EXAFS data allows us to track the key structural changes and differences.
Co-Substituted Aluminum Phosphate Catalysts
J. Phys. Chem., Vol. 100, No. 21, 1996 8979
Figure 5. Co K-edge EXAFS (on the left) and associated FTs (on the right) of the as-prepared (a) CoAlPO-5, (b) CoAlPO-36, (c) CoAlPO18, and (d) CoAPSO-44. The solid line shows the experimental data, and the dotted line represents the calculated EXAFS and FTs using a single Co-O shell.
Figure 4. Co K-edge XANES patterns of CoAlPOs, CoAPSO-44, and model compounds CoAl2O4 and as-prepared DAF-2. The pre-edge peak is denoted as 1s f 3d.
As-Prepared Catalysts. The Co K-edge XANES spectra of the as-prepared Co-substituted AlPOs, CoAl2O4, and CoPO4 0.5C2H10N2 (DAF-2) are given in Figure 4. The pre-edge feature (marked 1s f 3d) and the edge position of all the CoAlPOs match that of both CoAl2O4 and DAF-2, showing that the framework Co is in oxidation state II with a similar local environment in all samples. The Co K-edge EXAFS, together with the associated Fourier transforms (FTs) and the best fit to the first shell are shown in Figure 5, and the results are summarized in Table 1. It is clear that the Co in all the AlPOs has four oxygen neighbors each at an average distance of 1.93 ( 0.02 Å as in DAF-2 and CoAl2O4 where the Co(II) is, in each case, in the high-spin state. Since the values of the Debye-Waller factors (Table 1) are similar in all the materials, we conclude that there is no significant static disorder. Calcined Catalysts. Upon calcination in oxygen to remove the structure-directing organic template, the local environment around Co in the four materials, CoAlPO-5, -18, -36 and -44, undergoes dramatic changes. The Co K-edge EXAFS together with the associated FTs are given in Figure 6, and the best fit to the first shell, assuming a single oxygen shell, is summarized in Table 1. It is rather surprising that both the Co-O bond distance and coordination number have decreased to different values in the four materials, the changes being most pronounced in CoAlPO-18 and CoAPSO-44. The overall order for the changes in coordination number, distance, and Debye-Waller factor is CoAlPO-18 < CoAPSO-44 < CoAlPO-36 < CoAlPO5. The detailed aspects of the results are as follows. First, regarding CoAlPO-18, the decrease in the first-shell coordination number from nearly four to less than three is difficult to
rationalize without supposing that the Co environment after calcination is distorted. Distorted environments have been known to produce lower EXAFS amplitudes (in turn lower coordination numbers) arising from the interference between the EXAFS oscillations of the short and long metal-oxygen bonds;27 in certain instances, depending on the coordination number and bond distances, one would expect to see a clear beating effect.28 Furthermore, the refined coordination number of close to three and the Co-O distance of 1.83 Å suggests that most of the atoms in the coordination shell would be at the shorter distance for a distorted model. We have therefore adopted, for further analysis, a simple model of three short and one long Co-O distances. To overcome the correlation effects due to the two shells with same atom type (oxygen neighbor) in this model the following “constrained fitting” procedure was used. In the initial stages of the refinement, the Debye-Waller factors for both shells were constrained to be the same value and the bond distances refined to obtain the best fit to the experimental EXAFS data. Next, we removed the constraints and refined the data further, thereby yielding a reasonable set of structural parameters (given in Table 1). The calculated and experimental data for the best fit are shown in Figure 7a,b. The low coordination number obtained from the single-shell analysis of CoAlPO-18 (Table 1) is explicable in terms of the additive nature of the two EXAFS oscillations (as mentioned above) arising from three short and one long oxygen distances. This effect is demonstrated in Figure 7c,d where the individual contributions from the short and long oxygen shells are shown along with the sum of these two calculated EXAFS oscillations. It is instructive to take the summed EXAFS oscillation as the “experimental data” and analyze them using a single oxygen shell. The parameters N, R, and σ2 so obtained are similar to those of CoAlPO-18 (Table 1). The separation between the two shells (and their respective coordination numbers) is significant enough not to yield an average coordination number and bond distance from the data analysis; instead irrational structural parameters are obtained.
8980 J. Phys. Chem., Vol. 100, No. 21, 1996
Barrett et al.
TABLE 1: Structural Parameters Obtained from the Analysis of the EXAFS Data for the Model Compounds, the As-Prepared, Calcined, and Reduced Catalysts as-prepared
calcined 2
(Å2)
N
R (Å)
σ
CoAl2O4 DAF-2 CoAlPO-5 CoAlPO-36 CoAlPO-18 CoAlPO-18a
4.0 4.0 3.9 4.0 3.8
1.94 1.95 1.94 1.92 1.93
0.0035 0.0035 0.0037 0.0045 0.0035
CoAPSO-44
3.8
a
0.0045
reduced
N
R (Å)
σ2 (Å2)
3.6 3.25 2.6 3.0 1.0 2.8
1.92 1.87 1.82 1.82 2.04 1.85
0.009 0.007 0.002 0.003 0.007 0.007
N
R (Å)
σ2 (Å2)
3.9 4.1 3.6 3.0 1.0 3.6
1.94 1.93 1.90 1.90 2.04 1.93
0.0085 0.009 0.0075 0.004 0.0075 0.008
Results of the analysis based on the distorted model.
Figure 6. Co K-edge EXAFS (on the left) of the calcined samples and associated FT’s (on the right) of (a) CoAlPO-5, (b) CoAlPO-36, (c) CoAlPO-18, and (d) CoAPSO-44. The solid line shows the experimental data and the dotted line represents the calculated EXAFS and FTs using a single Co-O shell.
The dominant Co-O bond length so obtained for CoAlPO18 of 1.83 Å is far lower than the Co(II)-O distance of 1.93 Å for the as-prepared material. From an earlier study in this laboratory using the combined QuEXAFS/XRD technique29 on CoAlPO-18 recorded under operating conditions it was inferred (based on the shift in the Co K-edge position (1.2 eV) and decrease in Co-O distance) that Co(II) oxidises to Co(III) during in situ calcination in oxygen. Although there are few compounds containing cobalt in different oxidation states and with the same type of ligand, comparison between the X-ray absorption edge-position of Co(II) and Co(III) in oxides and cobaloximes suggest that the shift is of the order of 1.5-3 eV.30,32 Thus a positive edge shift in calcined CoAlPO-18, in combination with the shortened Co-O distance,31,32 strongly support our argument that there is a change in the oxidation state. However, a direct estimation of the Co(III)-O distance is not possible, since values for the ionic radius for a tetrahedral Co(III) are not available. (Although the Co(III)-O distance of 1.88 Å is proposed for a hetero-polycobalt(III) tungstate,33 it is not clear from the available structure whether this is the true distance, since some of the W-O bonds are found to be very short (ca. 1.4 Å)). However, it is possible to deduce, working on a reasonable assumption that the ionic radius of Co(III) is close to that of Fe(III) (where there is a known
Figure 7. (a) Experimental data for the Co K-edge EXAFS oscillations of the calcined CoAlPO-18 (solid line) and the best fit obtained using three short and one long Co-O distances (dashed line). The structural parameters obtained from this best fit are described in Table 2. (b) FT of the EXAFS data shown in (a). (c) EXAFS oscillations of the three oxygen neighbors at a distance of 1.83 Å (dashed); one oxygen neighbor at a distance of 2.04 Å (dotted); the sum of the three short and one long oxygen shells (solid line). (d) FTs of the EXAFS oscillations described in (c).
Fe(III)-O bond distance of 1.84 Å in FePO434), for a fourcoordinated, high-spin state, that the majority of the Co ions in the calcined CoAlPO-18 sample are in the high-spin oxidation state (III). In CoAlPO-5 and CoAlPO-36, the observed Co-O bond distances (see Table 1) are longer than the value of 1.83 Å found in CoAlPO-18. This is likely to be the result of incomplete oxidation of all the Co(II) producing an average Co-O distance intermediate between that in the as-prepared material (which contains only Co(II)) and that in the calcined CoAlPO-18 which, as argued above, contains predominantly Co(III). The presence of such Co(II) (which we refer to hereafter as “unoxidized species”) should lead to the formation of Brønsted acidity (in the absence of charge-compensating organic template molecules) through the protonation of one of the surrounding oxygens to conserve electroneutrality. However, IR spectra of the calcined CoAlPO-5, CoAlPO-18, and CoAlPO-36 (see Figure 8) were found to be similar without there being any OH stretching bands assignable to bridging hydroxyls (Brønsted acid sites), normally expected in the region ca. 3640-3580 cm-1 (see Figure 8).
Co-Substituted Aluminum Phosphate Catalysts
Figure 8. FTIR spectra of calcined (a) CoAlPO-5 (b) CoAlPO-18 (c) CoAlPO-36, and (d) CoAPSO-44, showing the stretching frequency of the hydroxyl region. The assignments marked in the figure are discussed in refs 37 and 57.
Figure 9. Co K-edge EXAFS and associated FTs of the reduced samples (a) CoAlPO-5, (b) CoAlPO-36, (c) CoAlPO-18, and (d) CoAPSO-44. The solid line shows the experimental data, and the dotted line represents the calculated EXAFS and FTs using a single Co-O shell.
AlternatiVely, charge compensation could occur by the generation of an oxygen ion Vacancy adjacent to the unoxidized Co(II).35 We may rule out the possibility that the results are interpretable in terms of extraframework Co(II) ions because independent experiments on Co(II)-cation-exchanged zeolites
J. Phys. Chem., Vol. 100, No. 21, 1996 8981 and SAPOs yielded a Co(II)-O distance of ca. 2.06 Å.36 Nonframework Co(II) species resulting from the collapse of the local structure can also be discounted on the basis of our QuEXAFS/XRD investigation9,17 of DAF-2: upon removal of the template from DAF-2 the structure collapses to form first an amorphous phase and then a dense structure. And the results obtained from the analysis of the associated EXAFS patterns of both the amorphous and dense phases are quite distinct17 from those of CoAlPOs and CoAPSO investigated here. We therefore conclude that unoxidized Co framework-substituted species are present which are dissimilar from the framework Co(II) which normally produces Brønsted acidity. The model referred to above involving generation of oxygen vacancies is considered further in the discussion section. The Debye-Waller factors listed in Table 1 are consistent with the presence of mixed Co environments in CoAlPO-5 and CoAlPO-36. In CoAPSO-44 , the presence of Si (substituted in place of P) gives rise to Brønsted acidity (see Figure 8) upon calcination owing to protonation of Al(III)-O-Si(IV) bridges. It would therefore be difficult to monitor the change in the Co oxidation state by IR spectroscopy as in CoAlPO-18. But from the closeness of the Co-O distance obtained from the EXAFS data analysis of CoAlPO-18 and CoAPSO-44 as well as the results from our earlier dynamic QuEXAFS experiments,17 examining the Co K-edge shift, we infer that the Co(II) undergoes oxidation to Co(III) in CoAPSO-44. Reduced Samples. The Co K-edge EXAFS data and the associated FTs along with the best fit using a single oxygen shell of the reduced CoAlPOs and CoAPSO-44 are shown in Figure 9 and the results obtained are given in Table 1. It is interesting to note that the Co-O distances are different from those in the as-prepared materials, although reduction should lead to the regeneration of Co(II). The Co-O interatomic distances in all four catalysts are found to equal or be above 1.90 Å. We can however be confident for the following reason, that Co(II) is being generated as from the earlier dynamic in situ QuEXAFS/XRD measurement29 carried out during reduction of CoAlPO-18, the shift in the Co K-edge again by 1.2 eV back to the position that was found for the as-synthesized materials and CoAl2O4 clearly shows the formation of this valence state. The IR spectra of the reduced CoAlPOs (given in Figure 10) show a band related to the OH stretching frequency (ca. 3600 cm-1)37 associated with the Brønsted acid sites confirming the formation of Co(II). However, the intensity of the OH band is different, following the order CoAlPO-5 < CoAlPO-36 < CoAlPO-18. Although direct comparisons of the intensities of the OH vibration in different structures cannot be interpreted quantitatively, they provide useful guidance when used in conjunction with the reported IR studies using probe molecules such as pyridine, and catalytic results (of various divalent metal ions as well as Si(IV) substituted in AlPO-5,5-8,10 AlPO-36,11 AlPO-18,13,37-39 and AlPO-44.14,19 These studies have indicated the presence of both Lewis and Brønsted acid sites with CoAlPO-5 having a higher proportion of Lewis sites while CoAlPO-18 has predominantly Brønsted acid sites. IR studies, with acetonitrile as a probe, clearly show that extraframework Co(II) (if present) does not contribute to the Lewis acidity.40 On the basis of these observations, we see that the degree of static disorder (see Table 1) obtained from the EXAFS analysis, especially in CoAlPO-18 which has predominantly Brønsted acid sites, is attributable to a distorted environment around Co(II). Further analysis of CoAlPO-18 using constrained refinement, as discussed above, indeed showed the presence of
8982 J. Phys. Chem., Vol. 100, No. 21, 1996
Barrett et al. TABLE 2: Fraction of Oxidized Co(III) (x) Estimated Using Eq 1 Employing the EXAFS-Derived Co-O Distance CoAlPO-5 CoAlPO-36 CoAlPO18 CoAPSO-44
R (Å)
x
1.92 1.87 1.83 1.85
0.20 0.45 1.00 0.80
a For a direct comparison we have also given the Co-O distances which are as given in Table 1.
Figure 10. IR spectra of reduced (a) CoAlPO-5, (b) CoAlPO-36, and (c) CoAlPO-18 and showing the hydroxyl stretching region. The assignments have been discussed elsewhere.37,57
three short (1.90 Å) and one long (2.04 Å) Co-O distances (see Table 1). The increase in the static disorder (DebyeWaller factor) when one oxygen shell was used in the analysis is due to the closeness of the three short and one long distance (a separation of 0.14 Å as opposed to 0.21 Å in the calcined material). This model is similar to that for the local structure of the acid-bearing Al(III) sites in aluminosilicates where the longer bond is shown to be associated with the protonated oxygen.41-45 We also note that the mixture of Co sites in the other three catalysts (except CoAlPO-18) is also reflected in the DebyeWaller factor. From the results given in Table 1 we see that this is a maximum for CoAlPO-5 and CoAlPO-36. However, a similar constrained EXAFS analysis to that performed for reduced CoAlPO-18 is not possible for the other three catalysts due to the presence of multiple Co(II) sites arising from the unoxidized Co species as well as the Brønsted acid site generated upon reduction. Caution must be exercised in interpreting these results, however, since the Debye-Waller factor is highly correlated with the coordination number in EXAFS analyses and in some instances may lead to overestimation of these parameters. This aspect will be considered in detail in the next section. Discussion The first unambiguous conclusion from our results is that cobalt in the as-prepared samples is present as high-spin Co(II) in a four-coordination site. Comparison of the pre-edge features and Co-O bond lengths in the Co-substituted AlPOs and APSO44 with those obtained for CoAl2O4 and DAF-2 clearly reveal a very similar environment in all these materials. We now turn our attention to the calcined and subsequently reduced catalysts discussing first the extent of Co(II) to Co(III) oxidation in the four materials studied. Subsequently we
examine the local environment of the oxidized Co(III) generated after calcination and then the local structure of the acid sites generated after reduction of this oxidized Co(III). Finally we consider the unoxidized species which have also been observed following calcination in oxygen. Extent of Oxidation of Co(II) to Co(III) upon Calcination. Upon calcination, a more complex structure emerges: the changes in the local environment of the CoAlPOs and CoAPSO44 can be rationalized in terms of an oxidized and an “unoxidized” component. Most of the UV-vis studies of CoAlPO-55,6, and CoAlPO-3646 indicated incomplete oxidation of Co(II) to Co(III) interpreting a band (350 nm) which appears upon calcination as being due to Co(III). Combined UV-vis and temperature-dependent ESR measurements8 of calcined CoAlPO-5, CoAlPO-34, and CoAPSO-34 also agree that Co(III) is formed in part; the decrease in ESR signal intensity (particularly at low temperature) is attributed to about 80% oxidation. However, a recent UV-vis and temperature-dependent ESR study47 on calcined CoAlPO-5 has cast some doubt on the previous interpretation.8 These authors have explained the appearance of the 350 nm band in the UV-vis and decrease in the temperature-dependent ESR signal as due to Co(II) in a distorted coordination environment and not to Co(III) formation. Clearly the extent of Co(III) formation in the calcined CoAlPOs and CoAPSOs cannot at present be unambiguously determined using these techniques. However, the reliability in the determination of bond distances from EXAFS data ((0.02 Å) allows us to use the derived Co-O distance as a guide for the extent of oxidation. In the case of the calcined CoAlPO-18 the dominant Co-O distance of 1.83 Å (which we recall is similar to Fe(III)-O distance) suggests that we have predominantly oxidized Co(III). Thus the fraction of oxidized Co(III) in the different materials studied here can be estimated from the different average Co-O distances obtained by EXAFS analysis, using a single oxygen shell approach (see Table 2). Here we indeed assume that, in CoAlPO-18, there is essentially complete oxidation of Co(II) to Co(III) and the dominant Co-O distance of 1.83 Å is representative of the oxidized state. We must also assume that the bond length of the “unoxidized” cobalt is the same as in the as-prepared catalysts which then allows us to use the Vegard relationship:
R1x + R2(1 - x) ) R
(1)
where R1 is the Co(III)-O distance of 1.83 Å, R2 represents the Co(II)-O distance (of 1.94 Å) obtained from the respective as-prepared materials (see Table 1), R is the average distance obtained from the EXAFS analysis using a single oxygen shell, and x is the fraction of oxidized Co(III) in the material. The estimated values of the fraction of oxidized Co(III) from this analysis are summarized in Table 2. It is clear that varying relative amounts of oxidized Co(III) are generated upon calcination in the various cobalt-substituted aluminophosphates. The results will serve as a useful guide for catalysis involving redox reactions. Furthermore, a systematic temperature-dependent
Co-Substituted Aluminum Phosphate Catalysts
J. Phys. Chem., Vol. 100, No. 21, 1996 8983
SCHEME 1
SCHEME 2
ESR study of various CoAlPOs of the type presented here will not only provide insight into the understanding of the extent of Co(III) formation but also assist interpretation of ESR results. We infer from the IR results showing the absence of the OH stretching frequency related to the Brønsted acid sites35,47 that the unoxidized part is not a conventional, tetrahedrally coordinated Co(II) species. As noted earlier, such Co(II) species may be present in the framework with an adjacent oxygen anion vacancy a crucial point to which we return to later. Local Structure of Co(III)-Oxidized Part. We have noted that the presence of high-spin tetrahedral Co(III) in any inorganic system is rare. Co(III) in a garnet48 is proposed to be in a distorted tetrahedral environment whereas in the heteropolytungstate it is present in regular tetrahedral geometry.33 Our EXAFS results on oxidized CoAlPO-18 and CoAPSO-44 can be best rationalized by a distorted four-coordinated model with three short and one long Co-O distances. On one hand this distorted environment could be an inherent property of highspin Co(III) in a tetrahedral environment; or on the other it could be due to other perturbations. The adoption of the distorted tetrahedral structure remains, however, somewhat of a puzzle and must be attributed to structural or chemical constraints. In contrast CoAlPO-5 and CoAlPO-36 have, as noted before, a mixture of both oxidized Co(III) and “unoxidized Co(II)”. This unoxidized part, in principle, should result in charge imbalance due to the low valence of the Co(II). Conventionally charge compensation (in absence of template) is achieved by the formation of a bridging hydroxyl, Brønsted acid site. However, as noted IR studies did not show any vibrational modes related to bridging hydroxyls, which suggests that the unoxidized species are not conventional, tetrahedrally coordinated Co(II) ions. Thus the effective charge of the lower valence cobalt must be compensated by some other means. The most plausible model involves formation of an oxygen vacancy. We note that one oxygen vacancy will be formed for every two Co(II) species and that we would expect on the basis of electrostatic factors that the vacancy would occupy a site adjacent to a cobalt ion. We therefore propose, the oxidized and “unoxidized” local structures of Co(III) and Co(II), as illustrated in Scheme 1.
Reduced Catalysts: Brønsted Acid Centers. Turning now to the samples reduced by hydrogen treatment, Co(III) is reduced as expected to Co(II) with the IR studies showing bands associated with the OH stretching frequency of the bridged hydroxyl; these have different intensities in the four materials (although we should stress that in CoAPSO-44 the higher concentration of Si(IV) substituting for P(V) is responsible for a larger proportion of the Brønsted acidity than the Co(II) that is formed after reduction). However, caution must be exercised in interpreting the intensities of the IR bands as a representative of the concentration of Brønsted acid centers. Also, both temperature-programmed desorption and IR studies using probe molecules indicated that the concentration of Bronsted acid centers is less in CoAlPO-5 and CoAlPO-36.6,10,11 A complex range of sites is clearly present in these materials as the bond lengths and coordination numbers obtained from the EXAFS data show considerable variation. Let us now consider the bond lengths in the reduced samples in greater detail, starting with CoAlPO-18. The large static disorder shows this site is likely to be distorted with three short and one long oxygen, but with a smaller separation compared to the calcined material. It is known that a distortion of this kind will occur upon protonation of one of the surrounding oxygens41-45 in many of the solid acid aluminosilicate materials. We have been able to identify the distortion as three short oxygens at 1.90 Å and one long oxygen at ca. 2.04 Åsthe local structure shown in Scheme 2. The other three materials have Co-O distances greater than 1.90 Å with CoAlPO-5 having the maximum value. Once again we attribute the large average Co-O distance and Debye-Waller factor to the presence of both Brønsted acid centres as well as the “unoxidized species” that did not generate Brønsted acidity, as discussed earlier. Unoxidized Co(II) Species: Lewis Acid Centers. The TPD and IR studies referred to above show that CoAlPO-56,10,40 and CoAlPO-3611 contain a higher proportion of Lewis acid sites compared to CoAlPO-18 or CoAPSO-44. We may identify these sites as the unoxidized species. As we recall, the trend in the estimated concentration (see Table 2) of unoxidized species from EXAFS (CoAlPO-5 > CoAlPO-36 > CoAPSO44 > CoAlPO-18) is close to that of the Lewis sites determined from TPD of ammonia and IR studies. Having established the geometry of the Brønsted acid site to be a distorted four-coordinated environment using the data from CoAlPO-18, we turn our attention to understanding the nature of Lewis acidity based on our EXAFS results. Focusing on the results obtained for CoAlPO-5 and CoAlPO-36, since these two materials possess the highest amount of Lewis acidity among the four materials studied here, we propose that the local environment associated with Lewis acid sites has a low “unsaturated” coordination number. It is known in the EXAFS analysis that large Debye-Waller factors result in higher values of the coordination number due to correlation effects, which in many instances leads to an overestimation, especially, of the coordination number. It is therefore very likely that, owing to the high Debye-Waller factors for CoAlPO-5 and CoAlPO-36 the refined coordination number of close to four is higher than
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Barrett et al.
TABLE 3: Defect Energies for Oxygen Vacancies in the Four Structures AlPO-5 AlPO-36 AlPO-18 AlPO-44
defect energya (eV)
difference (eV)
27.97 28.45 29.09 29.33
0.48 1.12 1.36
a This energy refers to the process in which an O2- anion is taken from the lattice to infinity.
TABLE 4: Energies (EDH) of the Dehydroxylation Reaction in the Four AlPO Frameworks EDH (eV) AlPO-5 AlPO-36 AlPO-18 AlPO-44
1.01 2.09 2.81 2.97
the real value of this quantity. Such a lower coordination number could be the result of an oxygen vacancy neighboring a Co(II) as discussed above. As argued earlier, such models imply an equal concentration of Co(II) in a four-coordinated environment, as illustrated in Scheme 1, part B in the previous section. We consider that such structures are the most likely models for the Lewis acid sites; they are, moreover, consistent with the refined coordination number of close to four in CoAlPO-5 after calcination. The mixture of both a three-coordinated as well as a four-coordinated Co(II) and the correlation effects discussed earlier explain why the average coordination number of 3.5 is not obtained. We further tested the viability of the oxygen vacancy model discussed earlier by performing computer simulations of the energetics of vacancy formation. We use the standard MottLittleton procedure22 with previously derived interatomic potentials.26 Our results reveal that it is more favourable by ca. 1 eV to generate an oxygen ion vacancy for AlPO-5 than for AlPO-18; AlPO-36 is found to fall in the intermediate energy range between these two (see Table 3). We may also use these techniques to investigate oxygen vacancy formation, which we propose occurs through severe dehydroxylation during the course of the calcination process. The possible reaction scheme is shown below:
2OH• f V0 + H2O
(2)
i.e., proton transfer followed by loss of water results in oxygen vacancy formation. In a series of calculations, which are summarized in Table 4 (the details of which will be discussed in a separate publication), we have estimated the energetics (EDH) of the reaction. The low values calculated are fully consistent with the formation of vacancies at high temperatures. It has been proposed earlier for many zeolitic systems that coordinatively unsaturated sites are responsible for Lewis acidity,49 but later NMR and other studies showed that extraframework Al(III) ions are the source of Lewis acidity in many aluminosilicates,50-52 although such extraframework sites are found not to be responsible for Lewis acidity in ZSM-5.53-55 Furthermore, extraframework cobalt generated under severe dehydroxylation of CoAPSO-44 has been shown not to be the source of Lewis acidity again using acetonitrile as an IR probe molecule.40 Coordinatively unsaturated sites are also proposed to be generated in SAPO-37 when subjected to high-temperature treatments above the normal calcination temperature.56 Considering the results of our EXAFS and the energy minimization calculations in conjunction with the models proposed in the
literature we suggest that framework Co(II) having a coordinatively unsaturated environment is the source for Lewis acidity. The presence of such centres indicates the unusual nature of the stereochemistry which can be created in the microporous environment. Also the creation of different types of acid centres is likely to be dependent on the microporous structure, especially the bond angles (Co-O-P) associated with cobalt substitution. Further experimental and theoretical studies to achieve a more detailed characterisation of these centres are in progress. Finally we note that our results, in particular the extent of redox process, have considerable implications for the catalytic application of these materials. It has been recently shown that CoAlPO-5 is able to oxidize cyclohexane to hexanol and hexanone under oxygen with a presure of ca. 12 bar at ca 120 °C.3 Although the activity is reasonable with CoAlPO-5, our results predict that CoAlPO-36 which has a pore size similar to that of CoAlPO-5 but with higher amounts of Co(III) generated upon calcination would introduce higher activity. On the other hand CoAlPO-18 and CoAPSO-44 which undergo oxidation to a greater extent than CoAlPO-5 may not have the same activity due to the steric constraints imposed by the framework as these two AlPOs have small pore sizes ca. 4 Å. Summary The results discussed above establish the precise and differing extents of oxidation of Co(II) to Co(III) in the four CoAlPOs studied. These differences are strongly correlated with the presence of Lewis acid sites in the materials, such sites being present in significant concentrations in CoAlPO-5 and CoAlPO36, which also show incomplete oxidation on calcination. A major factor in understanding these systems lies in consideration of the varying extent of the dehydroxylation reaction with oxygen vacancy formation. Our calculations imply that this reaction takes place most readily in CoAlPO-5 and -36sa result which provides strong support for a model which identifies the Lewis acid sites with vacancies adjacent to the Co(II) leading to an “undercoordinated” species. The presence of a neighboring vacancy seems to stabilise the Co(II) states, thus conferring upon it resistance to oxidation to Co(III). So far as Brønsted acid sites are concerned we have established that there is a distorted four-coordinated geometry around Co(II). The improved understanding that we have gained concerning acid sites and the extent of oxidation in these CoAlPOs and of the extent of oxidation have substantial implications for the catalytic applications of these materials. Acknowledgment. The authors thank EPSRC for financial support and CLRC for providing access to SRS facilities. The authors also thank Drs. R. H. Jones, J. Chen, P. A. Wright, and M. A. Roberts and Professor G. N. Greaves for useful discussions. The use of ICSD data base at Daresbury Laboratory is gratefully acknowledged. References and Notes (1) Thomas, J. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 913. (2) Lin, S.-S.; Weng, H.-S. Appl. Catal. A: General 1993, 105, 289. (3) Vanoppen, D. L.; De Vos, D. E.; Genet, M. J.; Rouxhet, P. G.; Jacobs, P. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 560. (4) Thomas, J. M.; Xu, Y.; Catlow, C. R. A.; Couves, J. W. Chem. Mater. 1991, 3, 667. (5) Montes, C.; Davis, M. E.; Murray, B.; Narayana, M. J. Phys Chem. 1990, 94, 6425. (6) Kraushaar-Czarnetzki, B.; Hoogervorst, W. G. M.; Andrea, R. R.; Emeis, C. A.; Stork, W. H. J. J. Chem. Soc., Faraday Trans. 1991, 87, 891. (7) Chen, J.; Sankar, G.; Thomas, J.M.; Xu, R.; Greaves, G. N.; Waller, D. Chem. Mater. 1992, 4, 1373.
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