Bronsted, Lewis, and Redox Centers on CoAPO-18 Catalysts. 1

Bronsted, Lewis, and Redox Centers on CoAPO-18 Catalysts. 1. .... Gavin Muncaster, Gopinathan Sankar, C. Richard A. Catlow, John Meurig Thomas, Simon ...
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J. Phys. Chem. 1994,98, 13350-13356

13350

Bronsted, Lewis, and Redox Centers on CoAPO-18 Catalysts. 1. Vibrational Modes of Adsorbed Water Leonard0 Marchese,*J Jiesheng Chen, and John Meurig Thomas* Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London WIX 4BS, U.K.

Salvatore Coluccia and Adriano Zecchina Dipartimento di Chimica I.F.M., Universith di Torino, via P. Giuria 7, 10125 Torino, Italy Received: June 7, 1994; In Final Form:August 31, 1994@

The surface acidic properties of calcined and reduced CoAPO-18 catalysts are reported. FTIR spectroscopy has revealed that the reduced template-free catalyst has bands at 3571 and 905 cm-' which have been assigned, respectively, to stretching and in-plane bending vibrations of hydroxyl groups bridging Co(II) and P [YOH and &HI. It is proved that Co(I1) substitutes for A1 in the lattice of the ALPO-18 structure. Bridged hydroxyls are the locus of Bronsted acidity, the strength of which has been tested by adsorption of H2O. It is revealed that some bands produced by interaction between bridged hydroxyls and water are sensitive to the acidic strength of the Bronsted centers; this strength is found to exceed that of the structurally similar HSAPO-34. Calcined CoAPO-18 has Co(II1) centers and the Bronsted acidity disappears, and it is deduced that redox Co(II)/Co(III) couples are intimately associated with the presence of Bronsted acid sites. Adsorption of HzO has revealed that Lewis acid centers are also present on both calcined and reduced CoAPO-18 catalysts. Both the spectroscopic features and the acidity of OH and OD groups are sensitive to the presence of Co in the structure of aluminophosphate molecular sieves, and an analysis of the OH and OD vibrations (both stretching and bending) in SAPO-34 and CoAPO-18 is presented.

Introduction Aluminophosphate molecular sieves (ALPOs) and their metalsubstituted analogues (MeAPOs) are of growing interest owing to their potential use as Isomorphous substitution of A13+ with bi- or trivalent transition-metal ions can, in principle, generate Bronsted and/or Lewis centers, as well as the redox centers Me3+/Me2+(Me = Co, Mn, Fe, etc.). It has been recognized, mainly by means of W-Vis spectro~copy~-'~ but also from EXAFS measurement^,'^ that Co2+ substitutes isomorphously for A13+in several ALPO-n(n = 5 , 11, 16, 21, 34, 36). But, hitherto, clear-cut experimental evidence that the negative charge of the framework is indeed balanced by protons (as shown in structure A of Scheme l), thus producing Bronsted acidity, has been lacking. The only direct evidence for the presence of Bronsted hydroxyls groups is reported in ref 5 where the formation of bridged hydroxyls after reduction in H2 of CoAPO-34 and CoSAPO-34 is observed. Peeters et al.l0 proposed that Lewis acid centers, such as anionic vacancies deriving from missing lattice oxygens (Scheme 2), act as the balancing charge on CoAPO-n (n = 5 , 11) because they did not observe any vibrational mode related to bridged hydroxyls. The vibrational properties of deuterated CoAPO-18 are compared here with those of HSAPO-34 and DSAPO-34 so as to seek clear-cut evidence that Bronsted hydroxyls do indeed exist on reduced CoAPO-18. Further and extensive evidence for the presence of Bronsted acidity in CoAPO-18 has been forthcoming using H2O and DzO as test molecules. It has often4-12been proposed that part of the structural Co2+ can be oxidized to structural Co3+ by thermal treatments in 0 2

* To whom correspondence should be addressed. Permanent address: Dipartimento di Chimica I.F.M., Universita di Torino, 10125 Torino, Italy. @Abstractpublished in Advance ACS Absrracrs, November 15, 1994.

0022-3654/94/2098-13350$04.50/0

SCHEME 1

I!

Structure A

Smture B

SCHEME 2

thereby generating redox centers CoZ+/Co3+.We report here the first direct evidence, using FTIR spectroscopy, that redox Co2+/Co3+centers in the CoAPO-18 structure are intimately associated with the presence of Bronsted acid centers. It is also inferred that Lewis acid sites exist both on reduced and on calcined samples.

Experimental Section CoAPO- 18 was prepared using Nfl-diisopropylethylamine (DPE) as the template and slightly modifying the procedure reported in ref 14. Cobalt acetate was added to the gel phase so as to attain an empirical composition: O.O8CoO:A1203:P205: 0.16HAc: 1.70DPE:SOH20. ICP-AES elemental analysis indicated that the molar ratio A1:Co:P is 0.95:0.049:1.00. SAPO34 was prepared as described in ref 17, and it was found that the molar ratio Al:Si:P is 1.0:0.19:0.77. The organic template was removed from the as-synthesized catalysts with the following procedure: (a) heating in vacuo with a slow increase of temperature up to 150 "C; (b) admitting 80-100 Torr of 02 and raising the temperature (10 "Urnin) from 150 to 550 "C; (c) calcining 3-4 h at 550 "C and changing the oxygen 3 times; and (d) evacuating at 550 "C for 20 min (final pressure (2-4) x Torr). 0 1994 American Chemical Society

Centers on CoAPO-18 Catalysts

J. Phys. Chem., Vol: 98, No. 50. 1994 13351

I

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3700

.

I

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'

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3500

'

I

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Wavenumbers [cm-'1

All thermal treatments were performed in situ on pelletized samples in an appropriately designed quartz cell for infrared measurements permanently connected to a vacuum rig Torr). The as-synthesized CoAPO-18 sample has a blue color typical of Co2+ in tetrahedral coordination, suggesting that Co ions substitute for A1 in framework positions!-12 The calcined CoAPO-18 pellet is green, from which fact the presence of Co3+ centers is inferred.4-7s9-12 When the sample was reduced in H2 (at 350 "C followed by an evacuation at 550 "C), the blue color reappeared, indicating that cobalt ions are now in a bivalent state and in tetrahedral coordination. Conversely, a reduction in CO of the calcined sample did not produce further color change, indicating that this treatment does not modify the oxidation state of the structural Co3+ (a detailed analysis of the UV-Vis spectra will be presented else~here'~). DCoAPO- 18- and DSAPO-34-deuterated samples were prepared by using the following HzO/D~O-exchangeprocedure: reduced CoAPO-18 and calcined SAPO-34 were evacuated at 550 "C for 30 min, left in contact with vapors of D20 (the vapor pressure at room temperature) at 45-65 "C for 20 min, and then evacuated at 250 "C for 20-30 min. The cycle was repeated 3 times, and as the final step, the pellet was evacuated at 550 "C. IR spectra were recorded on Perkin-Elmer (1725X) and Bruker (IFS88) FTIR spectrometers at a resolution of 4 cm-'.

Results and Discussion Figure 1 shows infrared spectra (in transmittance) of CoAPO18 after calcination (curve a) and reduction (curve b) and also the difference spectrum (in absorbance, curve c) obtained by subtracting curve a from b. Strong bands due to the fundamental vibrations of the framework To4 (T = P, Al) groups are visible below 1500 cm-' together with weaker combination and overtones in the 2500-1500-cm-' range. We do not discuss in detail the lattice vibrations: we note that they are only slightly modified on passing from the calcined to the reduced state-see the 2500-950-cm-' region in the difference spectrum. This effect is probably related to a different degree of distortion in the ALPO-18 structure produced by Co(I1) and Co(II1). The difference spectrum clearly indicates that, by reducing in H2, two bands are formed at 3571 and 905 cm-'. The shape

Figure 2. IR spectra of calcined (curve a, dashed line) and reduced (curve b) CoAPO-18, SAPO-34 (curve c), and AlF'O-18 (curve d) in the 3750-3350-cm-' (A) and 1020-780-cm-' (B) ranges. All the samples were outgassed at 550 "C.

of these bands is not symmetric, so suggesting a composite structure. Figure 2 shows expanded views of the 3750-3350-cm-' (section A) and 1020-780-cm-' (section B) regions of the spectra reported in Figure 1 (curves a and b). In the same figure, spectra of samples of HSAPO-34 (curve c) and ALPO- 18 (curve d) are included for the sake of comparison. Because both SAPO-34 and ALPO-18, like CoAPO-18, each have a chabazitelike structure?O a comparison between these materials is reliable. We notice the following: (i) Peaks at 3680 and 955 cm-' are present both in CoAPO18 (calcined and reduced) and in ALPO-18, whereas they are absent in HSAPO-34. The former has been observed on many aluminophosphate molecular sieves and has been assigned to the stretching vibration of POH groups in lattice defects (internal and external surface defects).16-19 Absorptions in the 1000750-cm-' peaks have never hitherto been reported; we have noticed, while studying many aluminophosphates with chabazite-like structures, that the intensity of the 955-cm-' peak correlates with the intensity of that at 3680 cm-' and that is particularly strong in CoAPO-18. This last observation suggests that the presence of Co is associated with lattice distortions a n d or lattice defects. (ii) A very small absorption at 943 cm-' is present both in calcined and reduced CoAPO-18 which is not seen in ALPO18 and HSAPO-34. The nature of this band, which indeed is due to the presence of cobalt in the ALPO-18 system, will be examined in an appropriate paragraph (vide infra). (iii) Bands at 3626 and 3599 cm-' in HSAPO-34 (Figure 2A, curve c), as well as in HSAPO-18, have been firmly attributed16-19 to Bronsted hydroxyls bridging Si and A1 pairs located in different positions of the framework. Bronsted hydroxyls with similar stretching frequencies have been found on many others SAPOS and aluminosilicate zeolites, and this strongly suggests that the band at 3571 cm-' found in reduced CoAPO-18 is also due to Bronsted OH bridging P and Co. In structure A of Scheme 1, Co2+ is hypothesized to occupy a substitutional tetrahedral position in the framework so a hydrogen is needed to balance the charge. The P-Co pairs play the same role as the Si-A1 pairs in HZSM-5, HY, and SAPOs. However, there is an important difference; in fact,

Marchese et al.

13352 J. Phys. Chem., Vol. 98, No. 50, 1994 TABLE 1: Spectroscopic Features of Briinsted OH and OD in Protonated and Deuterated CoAPO-18 and SAPO-34 Catalysts Si or Co content, atoms/g no. of OWg vOH, cm-'

VOD, cm-I &HI

cm-'

&D,

cm-'

area VOH? cm-' mm2)/g E - VOH: cdpmol area VOD, (cm-' mm2)/g area vodAarea vod area 600 exchange degree, %

COAPO-18

SAPO-34

2.3 x lozo 1.8 x lozob 3571,3460 (sh) 2637 905,915 (sh)

9.5 x 1020a 10.0 x 1020" 3626,3599 2674,2656 1055,1070' 865,889 6.0 x los 4.0 3.2 x 105

1.1 x 105 0.23 x 105 0.3 69

0.64 10.6 x 105 84

Determined in ref 17. Estimated using the extinction coefficient of the VOH of the SAPO-34 sample. Taken from ref 18. Integrated optical density divided for the weight (g/mmz) of the pellet used for the IR measurements. e Extinction coefficient obtained using the Lambert-Beer law. f AArea VOH: difference between the integrated optical density of OH before and after the DzO exchange.

It

3500

3000

2500

2000

B

(E)

while in the latter ions we are dealing with 111-IV pairs, in the former 11-V couples are involved. It has been reported in the literature2' that the A1-0 bond is not fully equivalent to the Si-0 (in fact, the length is slightly longer in the first case). On this basis, it is expected that in the CoAPO-18, the difference between the length of the Co-0 and P-0 bonds is even larger and that only the transition character of the Co2+(with its ligandfield stability in tetrahedral coordination) is conferring a sufficient stability to this structure. The stretching vibration of the bridged hydroxyls in CoAPO18 has lower frequency, larger width, and lower intensity compared with those in HSAPO-34. The lower intensity is easily understood because the concentration of Co in CoAPO18 is lower than the concentration of Si in SAPO-34 (see Table 1). The lower stretching vibration is understood if a higher Bronsted acidity is involved (we shall demonstrate in the next section by adsorbing H2O that such is the case). The presence of only one band (although complex, broad, and asymmetric) in COAPO-18 (instead of two peaks observed on HSAPO-34) can be explained on the grounds that Co(II) substitutes preferentially for one kind of A1 site in the ALPO18 structure. While SAPO-34 has a readily measurable SUOH ratio of 1:1, as demonstrated by combining FTIR,thermogravimetric, and elemental analysis,17 and NMR and elemental analysis,16 for CoAPO-18 determining the ratio is not easy since the amount of Co (and hence of OH) is very low. The concentration of OH in CoAPO-18 may nevertheless be estimated by means of the IR spectroscopy, if the extinction coefficientof the associated absorptions is known. The extinction coefficient of OH in SAPO-34 has been determined by combining accurate thermogravimetric, optical density, and elemental analysis data (see Table 1). We propose that this value is valid also for other MeAPOs and SAPOShaving OH absorptions in the same range (3650-3450 cm-'). Following this approximate hypothesis, the OH concentration in CoAPO-18 is estimated to be 1.8 x lozo atoms/g, quite close to the concentration of Co. The difference between the concentration of OH and Co (about 20%) should be attributed to the presence either of Lewis acid centers as proposed by Peeters et al.'O (Scheme 2) or of extraframework Co2+ acting as counterions. We have been investigating the reliability of these two hypotheses by studying samples with different amounts of cobalt using UV-Vis and FTIR spectroscopies.

1500

P)

i

-e 3

n

4

0.0 ,

I, 3500

3000

2500

2000

1500

Wavenumbers [cm-'1

Figure 3. (A) Infrared spectra in the transmittance scale of HzO adsorbed on reduced CoALPO-18 (increasing doses from curve 2 to curve 7); curve 1 is the bare reduced sample. (B) Difference spectra obtained by subtracting from each of the curves of A that of the bare sample. Having discussed the attribution of the 3571-cm-' band, we now move toward the discussion of the second prominent fingerprint feature of the reduced sample: the 905-cm-' band. Unlike the 3571-cm-l band, the assignment of the band at 905 cm-' is not straightforward because it does not have close analogues in other zeolites. Plausible assignments are as follows: (a) the 905-cm-' band is the nearly pure 6 bending mode of the Bronsted sites (in this respect, it is noticeable that in some hydroxy-bridged metal compounds,22the bending mode is at RZ 950 cm-'); (b) the 905-cm-' band is a stretching PO mode of [PO41 units perturbed by the proximity of Co2+. This hypothesis finds supportz3 in the fingerprint band at 960 cm-' of Ti-silicalite and at 1015 cm-' in Fe-silicalite. In order to distinguish between the two hypotheses, the behavior of this band upon interaction with water probe molecule is described and discussed in detail in the following. It will be shown that the interaction with water also provides direct information on the acidic strength of the Bronsted sites. H20D20 experiments will also be described. Interaction of HzO with Briinsted Acid Sites. Figure 3 shows infrared spectra in transmittance (3A) and in absorbance (3B) in the 3850-1300-~m-~range of increasing doses of H20 (from curve 2 to curve 7) on reduced CoAPO-18, and Figure 4 shows the 1000-750-cm-l range recorded under the same conditions as those of Figure 3. Bands at 3571 and 905 cm-' are progressively eroded in a strictly parallel way upon water dosage, and simultaneously, several bands in the 3900- 1300cm-' (listed in Table 2) and 1000-750-cm-' ranges are formed.

Centers on CoAPO-18 Catalysts 7

J. Phys. Chem., Vol. 98, No. 50, 1994 13353 0.2,

I t

0

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1000

800 Wavenumbers [cm"] 900

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800 Wavenumbers [cm"]

Figure 4. IR spectra in the 1000-750-cm-' region of HzO adsorbed on reduced CoAPO-18. (A) Spectra in the transmittance of different doses of H20. (B) Difference spectra. The downward and upward arrows indicate, respectively, increasing negative (eroded bands) and increasing positive (produced bands) values of optical densities (The arrows in Figure 4A are inverted because they represent the behavior of bands in transmittance.)

TABLE 2: IR Bands Formed in the Region 3800-1300 cm-' by Interaction of €I20with Reduced CoAPO-18, ALPO-18, and HSAPO-Ub COAPO-18 3768 w 3667 3800-2600 vb 2890 b, sh 2328 b 1800-1700 1625 1610 sh x1400"

ALPO- 18

2475

13618

HSAPO-34

3768 vw 3678 3800-2600 vb 2890 2475 1800-1700 1625 1610 x1350"

Position and shape of absorptions in this region cannot be detected with precision because of the presence of strong lattice vibrations. w = weak; vw = very weak, vb = very broad; sh = shoulder. Difference spectra reported in Figures 3 and 4 illustrate these effects more effectively. The 1000-750-~m-~Region. Beside the disappearance of the 905-cm-' band, other modifications occur in the 1000750-cm-' region when water is admitted onto the reduced CoAPO-18. These modifications are (i) a decrease of peaks of 943, 955, and 786 cm-' and (ii) an increase of complex absorptions in the 1000-950- and 850-800-cm-' ranges. We shall discuss the disappearance of the 943- and 955-cm-' peaks in a specific paragraph, whereas the other modifications, though significant and well reproducible from one experiment to another, will not be discussed here because they are still not extensively understood. As already mentioned, the main effect is the disappearance of the 905-cm-' band: this effect will be examined in light of the two previous assignments (&OH) or v(P0)). The following will be examined, in particular: (a) If the 905-cm-' band is a nearly pure OH bending mode (6), the interaction with H20 (either via hydrogen bonding or via proton transfer and subsequent H30+ formation) is expected to lead to the total disappearance of the absorption because it shifts upward (by hydrogen bonding) or it disappears (by proton transfer). (b) If the 905-cm-' band is a v(P0) mode perturbed by the presence of Co, than its disappearance upon H20 contact is only consistent with a proton transfer (because only in this case there is sufficiently strong perturbation to justify the total disappearance of the band).

3500

3000

2500

2000

1500

Wavenumbers [cm-'1 Figure 5. IR difference spectra of H20 adsorbed on (a) reduced CoAPO-18, (b) ALPO-18, and (c) HSAPO-34. An exploded view of the 3670-3780-cm-' region clearly shows that by adsorbing H20 on CoAPO-18 and on ALPO-18, AlOH hydroxyls are formed, whereas POH hydroxyls interact via H bonds. However, as the 905-cm-' band shifts upward upon contact15 with CO and Nh>whichdefinitely interact via hydrogen bonding only, we have to conclude that the 905-cm-' band is essentially a 6 mode of the OH Bronsted sites, even i f some mixing with PO stretching is not excluded. The 3900-1300-cm-' Region. Spectra of H20 adsorbed on HSAPO-34 and on ALPO-18 (Figure 5 ) are helpful in the assignment of the various components of the complex spectra of Figure 3. Starting from the simplest ALPO-18 system, we notice that the adsorption of H20 (curve b) generates a very broad absorption in the region 3800-2500 cm-' and a sharp peak at 1625 cm-': these spectral features are readily interpreted in terms of water physisorbed in the microcavities (Le., with H2O in a liquid-like ~ t a t e ~ ~A, fraction ~ ~ ) . of the water molecules is H bonded to POH, as is demonstrated by the negative peak at 3678 cm-' in curve b of Figure 5. On the other hand, H20 molecules adsorbed on HSAPO-34 generate the characteristic spectrum shown in curve c which has been interpreted17to be arising from oxonium H30+ ions produced by a proton transfer from bridged hydroxyls (Bronsteed acid groups) to water molecules. Comparison of spectrum a with spectra b and c readily suggests that the spectrum of H20 adsorbed on reduced CoAPO-18 is due to a mixture of two type of species: one similar to that formed on HSAPO-34 (Le., H20 interacting with protons) and the other similar to that formed on ALPO-18 (physisorbed water). The band at 3571 cm-' has spectral characteristics with respect to the adsoq~tion'~ of H20 similar to those of hydroxyls groups bridging A1 and Si in HSAPO-34 and HSAPO-18, and this strengthens the view that it is attributable to the stretching mode of Bronsted OH groups bridging Co and P as reported in Scheme 1 (structure A). The interaction between water and bridged hydroxyls in acidic zeolites has been recently reviewed by Pelmenschikov and van simulating the experimental infrared and by spectra by means of computational programs. Pelmenschikov and van Santen suggest that a trio of bands at 2900, 2450, and 1700 is ascribable to resonance interactions between the v(0H) f v(0H-O) combination modes and the &OH) and y(0H) overtones of the bridging OH groups perturbed by H-bonded molecular water. Their assignment is based on the well-known phenomena24 occurring in H-bond complexes where Fermi resonance effects ostensibly produce such a trio. Our own

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13354 J. Phys. Chem., Vol. 98, No. 50, 1994 SCHEME 3

w-

Ti

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- 0 3 - 7 4

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Structure A

Structure B

computational data reveal that all six bands which are characteristic of the water adsorbed on acidic zeolites and MeAPOs (see Figure 5 and Table 2), can be attributed to molecular water physisorbed via two H bonds to the acid catalyst surface (structure A of Scheme 3), without invoking any resonance effect. These arguments tend to discount the hypothesis that a real proton transfer occurs from the acidic molecular sieve to the water forming H30+ and/or H2n+102n+ions. However, due to the great and compelling similarity of the infrared spectrum of HzO in our zeolites with that of protonated water in strong mineral acids (see literature quoted in ref 17), we think that the proton-transfer theory cannot be discarded. In addition, it is worth mentioning that some recent NMR data2’ of HzO adsorbed on HZSM-5 indicate that a mixture of H30+ and H2O is produced. The evident conflict in interpreting theoretical and experimental data devolves upon the fact that the two species of Scheme 3 could have spectroscopic features very similar to and not distinguishable (at least with the approaches employed until now) from each other. Neutron diffraction studies, currently underway, should help clarify matters. Moreover, we have to mention that in the a and c spectra reported in Figure 5 , complex absorption bands centered at ~ 1 4 0 and 0 -1350 cm-’, respectively, are clearly observed. If hydrogen-bonded water (structure A of Scheme 3) is the only species formed upon water adsorption on CoAPO-18, the band at 1400 cm-’ has to be attributed to the 905-cm-’ bending mode shifted upward because of the hydrogen bonding. Conversely, if H30+ is the only product, the 1400-cm-’ band is one of the bending modes of the protonated species. For the time being, it is difficult to choose between the two hypotheses. However, we notice that following the first hypothesis, an upward shift of Av 1 500 cm-I must be accepted, which is indeed a very large figure and (to our knowledge) does not find precedent in literature. (On SAPO-34, the upward shift would be about 250300 cm-’ because the OH bending modes are in the 11001050-cm-I region: see the paragraph where we illustrate the DzO/H20 experiment.) In our opinion, this observation is more in favor of the protonated structure, even if a fully unambiguous proof is still lacking. In view of the small energetic difference between structures A and B (Scheme 3), the hypothesises of the simultaneous presence of both hydrogen-bonded and protonic species appears very reasonable.28 It is to be noted that on CoAPO-18, the bands at 3667 and 2328 cm-’ occur at lower frequencies than those on HSAPO34 (respectively at 3678 and 2475 cm-’). The former absorption is certainly related to the stretching vibration of the “free” OH groups in the H+/H20 complexs (03-H6 in both of the structures of Scheme 3), while the latter is related to the stretching vibrations of H-bonded Bronsted hydroxylsz6 ( O ’ - H W 3 in structure A) or, alternatively, to the stretching vibrations of OH groups in H30’ H bonded to the zeolite surface (01-H2-03H4,05 in structure B).

The lower frequencies of these vibrations indicate a stronger interaction between HzO (or H30’) and the catalyst surface and that CoALPO-18 has a stronger Bronsted acidity than HSAPO34. Temperature-programmed desorption (TPD)of NH3 and IR spectra of CO adsorbed15 at 77 K confirms this result. Interaction of H2O with Lattice Defects. When H20 is adsorbed on reduced and on calcined CoALPO-18, a peak at 3768 cm-’ is gradually formed (Figures 3 and 5 ) . Bands in similar positions have been detected on many aluminophosphate16-19 and aluminosilicatez9 zeolites and have been assigned to AlOH groups in internal and external defects or to micro particle^^^ of A1203. A dehydration-hydration mechanism involving the Si-O(H)-A1 bridgez9 in HZSM-5 and the Si0-Si distorted bridge30 in silicalites has been proposed. This process produces (partially) extraframework AlOH (peak at 3670-3665 cm-l; see ref 29) or SiOH (bands at 3740-3720 cm-’; see ref 30). An analogous mechanism could occur on particularly strained AI-0-P bridges, giving AlOH (band at 3768 cm-’) and POH species. The latter would not be easily detectable because they would overlap with the strong and broad vibrations of stretching OH of molecular water which, as reported above, is present both in calcined and reduced COALPO-18. In ALPO-18, the concentration of AlOH, formed by interaction with water, is much less than in CoAPO-18 (Figure 5 ) , and this suggests that the presence of Co ions in the framework induces the formation of defects (e.g., distorted A1-0-P bridges) in the ALPO-18 structure. A weak band is detected at 943 cm-’ both on calcined and on reduced CoALPO-18, whereas it is absent on ALPO- 18 (see Figure 4A). Figure 4B shows that this band is progressively shifted to 935 cm-’ when H20 is admitted on the reduced sample (the same effect is revealed on the calcined one). The precise nature of this absorption is not yet clear, but we argue that since it is affected by the presence of both strong and weak15 adsorbates like HzO and CO, respectively, it is a “surface” mode related to the presence of Lewis acid centers. For instance, tricoordinated Co2+ions (Scheme 2) could well have a stretching vibration at 943 cm-I shifting to lower frequency upon H2O (or CO) dosage when the Co2+ relaxes outward because of the coordination of another ligand. Relaxations of surface modes induced by CO coordination on coordinatively unsaturated A13+ and Mg2+ sites have been experimentally observed on various oxides such as y-AlzO3, MgO, etc. (see literature quoted in ref 31). Though this assignment is rather tentative, it has to be underlined that whatever is its precise nature, the vibration at 943 cm-’ certainly belongs to a mode of a sulface structure where Co ions are involved. H2OD20 Exchange on SAPO-34and on Reduced CoAPO18. Figure 6 shows infrared spectra of HSAPO-34 and DSAPO34 as well as spectra of the protonated and deuterated form of the CoAPO-18 sample. By exchanging HSAPO-34 catalysts with DzO (curve b), two pairs of peaks at 2674 and 2656 cm-’ and at 865 and 889 cm-I are formed. A small fraction of OH still remains in the DSAPO-34 structure. From the overall area of the 3626- and 3599-cm-I peaks before and after the D20 treatment, we estimate that the degree of isotope exchange is about 84%. The integrated area of the 865- and 889-cm-’ peaks is about 3 times as high as those at 2676 and 2656 cm-’. The latter peaks can be easily assigned to the stretching vibrations of the OD groups, the ratio YOH/YOD being 1.35 as expected by isotope effect^.^^^^^ The assignment of the 865- and 889-cm-’ peaks is also straightforward. Since these absorptions are very intense and

J. Phys. Chem., Vol. 98, No. 50, 1994 13355

Centers on CoAPO-18 Catalysts

3800

3600

3400

"

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2600

Wavenumbers [cm-'1

Wavenumbers [cm"] Figure 6. IR spectra of protonated (curves a) and deuterated (curves b) SAPO-34 and protonated (curves c) and deuterated (curves d) CoAPO-18. The absorbance scale of the SAPO-34 spectra is 4 time more intense than the CoAPO-18 spectra.

strictly related to those at 2674 and 2656 cm-', they certainly belong to bending vibrations (in-plane modes, 6 0 ~of ) the OD groups.32 Bending modes of the OH groups in aluminophosphate molecular sieves and zeolites are not directly observable by means of IR spectroscopy because they absorb in a region 1000- 1100 cm-' where the intense vibrations of the framework occur. However, they can be estimated if combination modes (i.e., stretching VOH deformation BOH) are detected. By means of diffuse reflectance infrared spectroscopy, it was found that HSAPO-34 has two bands at 4680 and 4670 cm-' in the region of the combination modes. Therefore, by subtracting the value of the fundamental stretching OH vibrations (3625 and 3600 cm-'), it could be estimated that the in-plane bending modes are respectively at 1055 and 1070 cm-I. Interestingly, the higher the stretching frequency of the Bronsted hydroxyls, the lower that of the deformation mode, and as was suggested in a combined FTIR and inelastic neutron scattering study of aluminosilicate H Y , this rule is also followed by Bronsted OH in zeolite.33 This study also reported direct evidence that Bronsted hydroxyls, whose stretching vibrations are at 3637 and 3548 cm-', have in-plane deformation modes at around 1089 cm-'. By exchanging HSAPO-34 with D20, we have observed directly the deformation modes of OD groups which, according to mass effects, are shifted to lower frequency and fall in a region where the vibrations of the framework are absent (Figure 6B, curve a). The two bending modes at 865 and 889 cm-'

+

are related to OD groups whose stretchings are at 2674 and 2656 cm-', re~pectively,~~ because (i) the ratios of the intensities of the two cofnponents of the bending and the stretching doublets are reversed as expected and (ii) the &H/&D ratios are 1.22 and 1.20 as calculated by considering the &H modes at 1055 and 1070 cm-', respectively.'* The latter data are considered reliable because the 8 0 H l d O D ratio for many homogeneous compoundsZZfalls in a range of values of 1.1-1.4. By exchanging reduced CoAPO-18 samples with D20, the two main bands at 3680 and 3571 cm-' are shifted downward to 2720 and 2637 cm-'; the V O H h O D ratio is 1.35, similar to that found for SAPO-34. The degree of exchange is about 69%, a value lower than that found for SAPO-34 (see Table 1). This difference can be justified by considering that CoAPO-18 has bridged hyroxyls strongly bonded to the structure or located in less accessible regions. It is indeed evident that a small fraction of OH of the reduced CoAPO-18 catalyst absorbs at very low frequency: see the shoulder on the low-frequency side of the 3571-cm-' band. After the isotopic exchange, the shoulder becomes a more definite, though very large, band with a maximum at about 3460 cm-'. The spectroscopic features of the 3460-cm-' band, very low frequency and large width, and its resistence to removal by D20 exchange suggest that this fraction of bridged OH should be H bonded to some oxygens of the framework in regions where the isotopic exchange cannot be very effective (e.g., hexagonal prisms?). It is noteworthy that the intensity of the OD bands in the DCoAPO-18 sample is much lower than expected. In fact, calculating the ratio AodAAoH, where AODis the integrated area of the OD bands and AAOH is the difference between the integrated area of the OH groups before and after the isotopic exchange, a value of 0.30 is found for CoAPO-18 and 0.64 for HSAPO-34. In other words, the extinction coefficient of the OD groups is much lower in CoAPO-18 than in SAPO-34. For the time being, this peculiar result can be considered as not fully understood. Further considerations will not be attempted because they would constitute excessive speculations. The following modifications occur in the low-frequency region of CoAPO- 18 by isotopic exchange: Upon deuteration, the absorption at 905 cm-I is deeply affected, as it is substituted by a doublet (maximum at 920 cm-' and shoulder at 907 cm-'). We interpret this modification as follows: (i) the band at 905 cm-' is shifted upon deuteration to frequencies lower than 800 cm-' (as expected for a OH bending mode), in agreement with the previous assignment; (ii) the band originally observed at 955 cm-' is shifted to 920 cm-'. As the vg55Ivg20 ratio is the expected one for the effect of deuteration on the stretching mode of a MOH group (vHOH/vHOD 1.03), the attribution of the 955-cm-' peak to a POH stretching vibration associated with partially extralattice P (for instance, at framework defects where the A1 has left the structure) is suggested. However, it must always be kept in mind that in these low-frequency ranges, the IR peaks have mixed character and that a mixture with bending POH modes cannot be excluded. The 920-cm-' peak is superimposed to the part of the 905-cm-' band which has resisted the deuteration procedure (vide supra).

Conclusions Reduced CoAPO- 18 catalysts have Bronsted acidity arising from hydroxyls bridging structural Co(I1) and P. When the catalysts are submitted to a calcination procedure ( 0 2 at 550 "C), the structural Co(II) are oxidized to Co(III) and the Bronsted acidity is lost. Co ions exert a strong influence both on the acidic and on the spectroscopic properties of the bridged

13356 J. Phys. Chem., Vol. 98, No. 50, 1994 hydroxyls, which are different from those of hydroxyls in HSAPO-34. Bridged OH'S in CoAPO-18 are more acidic and have lower stretching (3571-cm-') and bending (905-cm-') vibrations than in SAPO-34, whose stretching modes are at 3626 and 3599 cm-' and bending modes are in the 1100-1000-cm-' region. The acidic strength has been tested by adsorbing H20 on the dehydrated samples: HzO molecules interact with the bridged hydroxyls, giving either H20-H+ complexes and/or H30f strongly H bonded to the molecular sieve structure. Some IR modes of these species are sensitive to the acidic strength of the hydroxyls. The presence of Co in the aluminophosphate structure induces a local distortion on some A1-0-P bridges.

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