Langmuir 1999, 15, 5079-5087
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Fourier Transform Infrared Spectroscopy Study of CD3CN Adsorbed on Pure and Doped γ-Alumina E. Escalona Platero, M. Pen˜arroya Mentruit,* and C. Morterra† Departamento de Quı´mica, Universitat de las Illes Balears, Ctra. Valldemossa, km 7,5, 07071 Palma de Mallorca, Spain Received November 30, 1998. In Final Form: April 26, 1999 The ambient temperature adsorption of acetonitrile (in its d3 deuterated form, to avoid Fermi resonance band splitting in the analytical νCN spectral region) was studied on pure γ-alumina, γ-alumina doped with two different amounts of sulfates, γ-alumina doped with Ca, and a Ca-doped alumina that was thermally transformed into the δ,θ-alumina form. Acetonitrile adsorption readily reveals the different Lewis acidity due to different surface cationic species (e.g., Al3+ and Ca2+) but is only partly sensitive to the acid strength heterogeneity exhibited by Lewis sites due to the same cationic species (Al3+ ions, in the present case). Acetonitrile uptake perturbs all surface hydroxyl species that are free from H-bonding of the OH‚‚‚OH type by either ligand insertion (the OH species at ∼3780 cm-1) or by (weak) H-bonding (the OH species at ∼3740 cm-1). When a plain H-bonding is formed, the relevant νCN mode is hardly distinguishable from that of a liquidlike physisorbed phase. The presence of large amounts of surface sulfates induces weak Brønsted acidity in some OH species, whose H-bonding interaction with acetonitrile yields a νCN band upward shifted by some 10 cm-1 with respect to liquid nitrile. Acetonitrile adsorption reveals that the presence of sulfates increases the surface Lewis acidity of alumina, mainly by increasing the relative amount of the stronger family of Lewis acid sites. On undoped alumina, acetonitrile undergoes a slow hydrolysis reaction (leading to acetamide species) and a slow polymerization reaction (starting from a CD2CN- anion precursor). When abundant surface sulfates are present on alumina, nitrile reactions of both types are totally hindered, whereas the presence of Ca enhances the surface reactivity of acetonitrile, especially when the thermal transformation of γ-alumina into δ,θ-alumina increases the surface concentration of Ca dopant species.
Introduction Acetonitrile has been used as a molecular probe for both basic and acid centers on the surface of metal oxides and zeolites:1-7 • In the presence of basic O2- anions, acetonitrile molecule acts as a Brønsted acid. An H atom of the CH3 group is eliminated as a proton with formation of an OHgroup and of a CH2CN- carbanion (or its polymerization product(s)), which is stabilized on a coordinatively unsaturated (cus) cationic position of the surface. This process can be followed by IR spectroscopy: an intensity increase of the OH stretching bands and a wavenumber decrease of the stretching ν(CN) mode(s) (with respect to the liquid phase) are observed.8 • Acetonitrile molecules interact with Lewis and Brønsted acid sites through the electron lone pair located on the nitrogen atom, causing a wavenumber increase of the stretching ν(CN) mode(s) (with respect to the liquid phase). * To whom all enquiries should be addressed. † On leave from Department of Chemistry IFM, University of Turin, Italy. (1) Kno¨zinger, H.; Krietenbrink, H. J. Chem. Soc., Faraday Trans. 1 1975, 71, 2421. (2) Kno¨zinger, H.; Krietenbrink, H.; Mu¨ller, H. D.; Schulz, W. Proceedings of the International Congress on Catalysis; London, 1976; paper A.10. (3) Sempels, R. E.; Rouxhet, P. G. J. Colloid Interface Sci. 1976, 55, 263. (4) Scokart, P. O.; Declerck, F. D.; Sempels, R. E.; Rouxhet, P. G. J. Chem. Soc., Faraday Trans. 1 1977, 73, 359. (5) Pelmenschikov, A. G.; van Santen, R. A.; Ja¨nchen, J.; Meijer, E. J. Phys. Chem. 1993, 97, 11071. (6) Chen, J.; Thomas, J. M.; Sankar, G. J. Chem. Soc., Faraday Trans. 1994, 90, 3455. (7) Aboulayt, A.; Binet, C.; Lavalley, J.-C. J. Chem. Soc., Faraday Trans. 1995, 91, 2913. (8) Juchnovski, I.; Dimitrova, J.; Binev, I.; Kaneti, J. Tetrahedron 1978, 34, 779.
This increase is brought about by a strengthening of the CtN bond caused by rehybridization of the nitrogen orbitals, leading to an increase of the strength of the σ-bond component.9 According to Pelmenshikov et al.5 and Thomas et al.,6 who used the acetonitrile molecule as a specific probe for Lewis and Brønsted acidity at the surface of zeolites and zeolite-like systems, the upward frequency shift of the ν(CN) mode allows the distinction between Lewis coordinated nitrile species (∆νCN is in general of the order of some 40-60 cm-1) and nitrile species interacting with medium-weak Brønsted acid sites (∆νCN is normally of the order of some 20-30 cm-1). For both types of acid centers, ∆νCN is larger the stronger the acidity of the centers involved. • Acetonitrile can undergo an hydrolytic process catalyzed by either acid or basic sites, giving rise first to acetamide and then to acetic acid and ammonia.10 In general, the occurrence of the hydrolysis reaction does not prevent the use of acetonitrile as a molecular probe for surface acidity because the hydrolytic process, when present, takes place slowly and at the highest nitrile coverages. The aim of the present investigation is to check the sensitivity of acetonitrile as a probe molecule for the characterization of acid centers present at the surface of pure and doped transition aluminas. Related work on pure alumina has been reported by several authors.1-4 Kno¨zinger and co-workers1,2 first observed that acetonitrile interacts with the alumina surface in two different ways: (i) In the first one, a nitrile molecule is adsorbed through the N atom lone pair at surface Lewis acid sites and/or at surface hydroxyls groups. The two adsorbed species are (9) Purcell, K. F.; Drago, R. S. J. Am. Chem. Soc. 1966, 88, 919. (10) Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry, 2nd ed.; W. H. Freeman and Co.: New York, 1994.
10.1021/la981654c CCC: $18.00 © 1999 American Chemical Society Published on Web 06/18/1999
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distinguished by the IR spectral position of the ν(CN) mode, as the spectral shift is of the order of some +60 cm-1 in the former case and is virtually zero in the latter case. (ii) Alternatively, the nitrile molecules undergo a hydrolytic process, leading to the formation of acetamide. The hydrolytic process requires the presence of coordinated nitrile species as a precursor, on which a nearby basic OH group would move a nucleophilic attack. Approximately in the same years, the adsorption of acetonitrile was used by Rouxhet et al.3,4 to study the surface properties of a series of silica-alumina gels, and at the extremes of the series also the properties of pure silica gel and of pure alumina were considered. The authors concluded that, on silica gels, nitrile molecules can only H-bond to surface hydroxyls, yielding a spectral shift of the ν(CN) mode of some +15 cm-1, whereas on alumina nitrile molecules either coordinate at Lewis acid sites (∆νCN ≈ +60 cm-1) or are physically adsorbed (∆νCN ≈ 0 cm-1). Both Kno¨zinger et al.1,2 and Rouxhet et al.3,4 reported only one ν(CtN) band assignable to acetonitrile-d3 coordinated at the different types of surface Lewis acid sites present on transition aluminas. (It is known that at least three families of Lewis acid sites of rather different acid strength can be singled out on aluminas by the use of other probe molecules).11 As a consequence, Kno¨zinger et al.1 concluded that “the CtN stretching mode of nitriles does not seem to be very sensitive to the energetic heterogeneity of the alumina surface”. In light of previous work reported on the subject, the use of acetonitrile as a molecular probe to study pure and doped aluminas has seemed to us to be of some interest for the following reasons: (i) The gas/solid interaction of CD3CN with pure alumina isolated in different activation stages (a procedure very seldom adopted by other authors) should be useful to check the selectivity of this molecule as a probe for different types of surface Lewis acid centers present on pure transition aluminas. (ii) The investigation by nitrile adsorption of some selected surface-doped aluminas already characterized and described in the literature by other techniques should allow us to check the sensitivity of this probe molecule either to the presence of Lewis acid sites of different chemical nature (e.g., the addition of Ca2+ ions) or to the slight modifications of Lewis acidity caused by inductive effects produced by the presence of charge-withdrawing groups (e.g., the addition of acidic sulfate groups). (iii) It is known that, if checked by the use of strongly basic probes (but, perhaps, ammonia), pure aluminas do not exhibit protonic Brønsted acidity,11,12 whereas alumina doped with strongly acidic species may develop a weak surface protonic acidity.13 The use of acetonitrile as a surface probe for surface H-bonding interactions on pure and anion-doped aluminas should allow us to check if the analytical spectral features of adsorbed nitrile are actually sensitive to the strength of surface protonic acidity and if the results obtained with one homogeneous family of oxides are transferable to different oxidic systems. In the present investigation, deuterated acetonitrile was used in order to avoid the spectroscopic problems caused, with regular acetonitrile, by the Fermi resonance occurring between the ν(CN) mode and the combination mode δs(CH3) + ν(CC), as pointed out by Kno¨zinger et al.1 (11) Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497. (12) Kno¨zinger, H. In Elementary Reaction Steps in Heterogeneous Catalysis; Joyner, R. W., van Santen, R. A., Eds.; NATO ASI Ser., Ser. C 398; Kluwer Academic Publishers: Dordrecht, 1993; pp 267-285. (13) Waqif, M.; Bachelier, J.; Saur, O.; Lavalley, J.-C. J. Mol. Catal. 1992, 722, 127.
Escalona Platero et al.
Figure 1. Transmittance background spectra of various alumina systems, vacuum activated at ambient temperature: a, A300; b, AS300; c, ASS300; d, ACA300. The vertical arrows indicate the spectral position of the deformation mode (δHOH) of undissociated molecular water.
Experimental Section Materials. (1) γ-Al2O3 was selected as the reference “pure alumina” and will hereafter referred to as the A sample. It was prepared by calcination, at 873 K, of boehmite synthesized following the method described by McIver et al.14 It showed a BET surface area of 160 m2 g-1 and a mesoporous texture (although a small contribution of microporosity was also present) with a most frequent pore radius of 2 nm.15 The background IR spectrum of an A sample vacuum activated at ambient temperature is reported in Figure 1 (curve a) and is characteristic of highly scattering pure γ-Al2O3 preparations. Note, in particular, that the surface of the starting A preparation is heavily contaminated by highly resistant surface carbonate-like species (strong bands in the 1600-1250 cm-1 range, typical of bidentate surface carbonates), whereas a band at ∼1650 cm-1, indicated in the spectrum by an arrow and due to the δHOH bending mode of undissociated coordinated water, is relatively weak and severely overlapped by the absorptions of surface carbonate contaminants. (2) A slightly sulfated γ-Al2O3 specimen (hereafter referred to as the AS sample) was prepared by thermolysis, at 1173 K, of ammonium alum (Koch-Light 99.97%). Chemical analysis showed a residual sulfur content of 0.5 wt %, presumably due to tenaciously held SOn surface species. The BET surface area was 170 m2 g-1, and the most frequent pore radius was 10 nm.16 The background IR spectrum of AS outgassed at ambient temperature is reported in Figure 1 (curve b) and is typical of low scattering (i.e., highly transparent) γ-Al2O3 preparations. This preparation is almost free from surface carbonate-like contaminants, its δHOH mode at ∼1650 cm-1 presents a medium-high intensity (i.e., more coordinated water seems to be present on AS than on A sample), and the low-frequency cutoff is somewhat higher than on A sample due to a strong band at ∼1150 cm-1 ascribed to surface sulfates. (14) McIver, D. S.; Tobin, H. H.; Barth, R. T. J. Catal. 1963, 2, 485. (15) Escalona Platero, E.; Ruiz de Peralta, F.; Otero Area´n, C. Catal. Lett. 1995, 34, 65. (16) Escalona Platero, E.; Rubio Gonza´lez, J. M.; Otero Area´n, C. Thermochim. Acta 1986, 102, 303.
CD3CN Adsorbed on γ-Alumina After vacuum activation at high temperatures, the band of surface sulfates will move to frequencies as high as ∼1380 cm-1.17 (3) A highly sulfated alumina specimen (hereafter referred to as the ASS preparation) was also considered. To prepare it, the AS material was further sulfated by impregnation with dosed amounts of a 0.5 M solution of sulfuric acid. The residual sulfur content was 4 wt % (corresponding to ca. 4 SO42- groups per nm2, a surface loading close to a complete statistical monolayer).18 The background IR spectrum of ASS activated in vacuo at ambient temperature is shown in Figure 1 (curve c) and indicates that (i) the high transparency of the starting material is maintained, (ii) surface contamination by carbonate-like species is totally absent, (iii) a high intensity at ∼1650 cm-1 (the δHOH mode) indicates that the surface layer contains a large population of undissociated water molecules coordinated to surface cationic centers, and (iv) the low-frequency cutoff moved to ∼1250 cm-1, due to the very high intensity of the vibrations of surface sufates. After activation at high temperatures, the spectrum of ASS will show a very intense band of surface sulfates at ∼1410 cm-1 and a broad band centered at 3603 cm-1 due to OH groups different from those typical of pure γ-Al2O3. The presence on ASS of an OH band of appreciable intensity indicates that a complete monolayer of surface sulfates was not achieved and that the SO42- monolayer capacity given above has a merely statistical meaning. (4) A calcium-doped γ-Al2O3 specimen was prepared by impregnation of pure commercial (pseudo)-boehmite with a Ca nitrate solution, so as to obtain a CaO:Al2O3 ratio of 3 wt % in the final calcined material. The suspension was dried at 353 K and then fired at either 773 K (this material is still γ-Al2O3 and will be referred to as the ACA preparation) or 1300 K (the material is transformed into δ,θ-Al2O3 and will be referred to as the ACA(1300) preparation). ACA and ACA(1300) have a BET surface area of 192 and 87 m2 g-1, respectively. The background IR spectrum of an ACA sample activated in vacuo at ambient temperature is reported in Figure 1 (curve d) and shows that (i) the scattering loss profile is severe, (ii) the surface contamination with carbonate-like species is far heavier than that on the A sample (mainly due to highly ionic carbonates of quasi-D3h symmetry, consistent with the introduction of a basic dopant), and (iii) the δHOH band of coordinated water is a strong shoulder on the high-ν side of the carbonate band. After activation at higher temperatures, the OH stretching spectral profile of ACA samples is marginally modified with respect to pure γ-Al2O3 and δ,θ-Al2O3, as reported elsewhere.19 Each sample symbol (A, AS, ASS, ACA and ACA(1300)) may be followed in the text and figures by a subscript numeral T, representing the temperature (K) of vacuum thermal activation/ oxidation undergone by the sample prior to nitrile adsorption and IR investigation. IR Spectra. For IR studies, thin self-supporting wafers (about 0.03 g cm-2) were prepared and activated during 1 h at different temperatures, under a dynamic vacuum (residual pressure e 10-4 Torr). The homemade IR cell20 allowed high-temperature treatments, gas dosage, and room temperature spectroscopic measurements to be performed in a strictly in-situ configuration, so that background spectral subtractions and bands rationing (differential spectra) could be carried out routinely and reliably. Spectra were collected, at 3 cm-1 resolution, on an FTIR Bruker 66 spectrometer. For each sample the spectrum taken before the dosage of acetonitrile was used as a background. Unless otherwise stated, all the spectra corresponding to the adsorption of increasing doses of acetonitrile were collected in a maximum period of ∼20 min, to use the acetonitrile mainly as a molecular probe for surface acidity and to minimize the occurrence of hydrolytic processes. Acetonitrile-d3 (Aldrich, 99.95 atom % D) (17) Zecchina, A.; Escalona Platero, E.; Otero Area´n, C. J. Catal. 1987, 107, 244. (18) Nascimento, P.; Akratopoulou, C.; Oszagyan, M.; Coudurier, G.; Travers, C.; Joly, J. F.; Vedrine, J. C. In New Frontiers in Catalysis, 10th International Congress on Catalysis; Guczi, L., Solymosi, F., Tetenyi, P., Eds.; Akad. Kiado: Budapest, 1993; p 1185. (19) Morterra, C.; Magnacca, G.; Cerrato, G.; Del Favero, N.; Filippi, F.; Folonari, C. V. J. Chem. Soc., Faraday Trans. 1993, 89, 135. (20) Boccuzzi, F.; Coluccia, S.; Ghiotti, G.; Zecchina, A. J. Phys. Chem. 1978, 82, 1298.
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Figure 2. Differential absorbance spectra of CD3CN adsorbed at increasing pressure (up to saturated vapor pressure) onto A300 (a), A373 (b), and A873 (c). The broken-line curves are the spectra obtained after 20 min of evacuation at ambient temperature. Insets: differential spectra obtained, in the OH stretching region, upon nitrile adsorption at increasing pressure (solid lines) and after 20 min of evacuation (broken line). Curve 1 in inset c is the absorbance spectrum obtained before nitrile adsorption (background). was distilled in vacuo and turned gas-free by several freezepump-thaw cycles. Unsmoothed segments of some selected spectra were bandresolved by using an iterative program by Bruker (FIT), which fixes the number of components to be resolved and the desired accuracy, whereas all other spectral parameters can be either fixed or allowed to float.
Results and Discussion Pure Alumina: A Samples. Figure 2 reports the spectral pattern obtained in the 2400-2000 cm-1 interval upon dosage of CD3CN on A300 (section a), A373 (section b), and A873 (section c), respectively. Figure 2a mainly shows two bands of different intensity centered at 2262 and 2112 cm-1. Bands with virtually identical features are present also in the spectrum of liquid CD3CN21 and are thus assigned to the ν(CN) and νs(CD3) modes of a liquidlike phase physisorbed on the hydrated surface of A300. The presence of acetonitrile molecules H-bonded to surface hydroxyls should also yield ν(CN) and νs(CD3) bands at (21) Pouchert, C. J. The Aldrich Library of FT-IR Spectra, 1st ed.; 1985; Vol 2, 1083A.
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approximately the same wavenumber.1,2 In fact, the inset to Figure 2a shows that, in the high frequency ν(OH) region (where an overwhelming broad absorption due to OH species strongly interacting by H-bonding prevents an accurate observation of spectral changes), differential spectra indicate the presence of a weak OH component centered at ∼3740 cm-1, due to surface OH groups free from H-bonding. The free OH component is reversibly eroded by H-bonding upon nitrile adsorption and is slowly restored upon nitrile desorption. No interaction with Lewis or Brønsted acid sites takes place, as no ν(CN) bands at a wavenumber higher than that observed for the liquidlike phase are observed. The final spectrum of the adsorption run does not change with time (i.e., no new bands appear in any spectral range), and all bands decrease by outgassing, indicating a plain desorption of the weakly adsorbed species. Figure 2b (CD3CN on A373) shows, besides the bands already observed in Figure 2a, a new weak component centered at ∼2315 cm-1. The band (∆νCN ) +53 cm-1) is readily assigned to the ν(CN) mode of CD3CN interacting, through the N lone-pair, with Lewis acid sites (surface cus Al3+ ions).1,5,6 The corresponding νs(CD3) mode (2112 cm-1) is not affected by the coordination. In the inset to Figure 2b, the presence of nitrile molecules H-bonded to surface OH groups is more evident than on A300, as the selective erosion of the “free” OH component at ∼3740 cm-1 reaches now some 0.3 absorbance units. The absence at the surface of pure γ-alumina of Brønsted acid centers of medium-low strength, already established by pyridine adsorption,11 is here confirmed by the absence of ν(CN) bands upward shifted by some 10-35 cm-1 with respect to the liquid phase. Also on A373 the spectrum of the last nitrile dose was not altered with contact time (no surface reactions occur). On increasing the activation temperature of A samples (up to 873 K), several significant modifications are observed upon nitrile adsorption: (i) The relative intensity of the νCN component at ν > 2310 cm-1 increased (Figure 2c), due to the increased amount of Lewis acid centers caused by elimination of coordinated water and by surface dehydroxylation. This band is now very broad (∆ν1/2 ≈ 42 cm-1), definitely asymmetric on the high wavenumbers side, and its apparent maximum is observed at ∼2325 cm-1 when the nitrile pressure is low and at ∼2315 cm-1 when the pressure is high. A computer spectral resolution in terms of two spectral components was obtained for the ν(CN) band due to nitrile Lewis coordinated on A873 at an intermediate nitrile pressure. The resolution was carried out by fixing the spectral features of the low-frequency component at the values observed on the still highly hydrated A373 sample (νmax ) 2315.2 cm-1; ∆ν1/2 ) 25.6 cm-1) and allowing the spectral features of the other component to float freely. A symmetrical high-ν component was so obtained, with νmax ) 2330.0 cm-1 and ∆ν1/2 ) 23.1 cm-1. Some spectral features of nitrile Lewis coordinated at medium coverage on this and other pure and doped alumina systems are summarized in Table 1. The presence on highly dehydrated A samples of two closely overlapped ν(CN) spectral components in the 23502300 cm-1 interval suggests the existence of two families of Lewis acid centers (Al3+cus sites). The low-ν component (∼2315 cm-1) should be assigned to weak Lewis acid sites that start being formed already at low activation temperatures, when most of the surface dehydration is due to the elimination of coordinated water molecules, whereas the high-ν component (∼2330 cm-1) corresponds to Lewis acid sites with stronger electron-acceptor character, which
Escalona Platero et al. Table 1. Spectral Features of Acetonitrile Lewis Coordinated at Medium Coverage on Various Alumina Systems spectral features sample A300 A373 A873 AS300 AS523 AS723 AS873 ASS300 ASS523 ASS723 ASS873
νmax
∆ν1/2
relative intensity
2315.2
25.6
2315.2 2330.0 2313
25.6 23.1 27.7
2313 2328.4 2313 2330.0 2313 2330.4 2320.5
27.7 21.8 27.7 22.9 27.7 23.2 25.2
2320.5 2333.2 2320.5 2332.8 2320.5 2332.4
25.2 22.5 25.2 22.6 25.2 22.3
1 0 0.68 0.32 1 0 0.45 0.55 0.45 0.54 0.48 0.52 1 0 0.48 0.52 0.48 0.52 0.53 0.47
develop at higher temperature by water elimination from surface OH group pairs. The different composition of the surface hydrated layer and the different origin of Lewis acid sites, typical of ionic oxides, is confirmed for alumina by the observation that the spectral component eliminated from the background spectrum in the earliest stages of dehydration contains a fair spectral component centered at ∼1620 cm-1 (i.e., the δHOH mode of undissociated water). The data of nitrile uptake so confirm the heterogeneity of Lewis acid centers at the surface of γ-Al2O3 already established by the use of other probe molecules (e.g., CO and Py).11 Other Lewis base probes often led to the identification of up to three families of Lewis acid centers, whereas the use of acetonitrile singles out, here for the first time, the probable existence of two families of Lewis sites. This discrepancy is due to the low sensitivity of the νCN mode to site heterogeneity. The (partial) resolution of surface Lewis acid heterogeneity was not observed before by nitrile adsorption because adsorption experiments were not carried out on alumina samples isolated in different stages of the surface dehydration process. (ii) The inset to Figure 2c (the ν(OH) region) shows that two groups of OH bands are present on A873 (∼3780 and ∼3740 cm-1), as typical of transition aluminas activated at medium-high temperatures.11,22 Nitrile uptake gradually erodes both band envelopes, and the high-ν group is consumed first. In fact, the so-called “basic” OH species absorbing at ∼3780 cm-1 were found by several authors to be readily perturbed by all kinds of surface interactions (e.g., see ref 11, and references therein). The OH species at ∼3780 cm-1 are thought to be terminal OH groups in the coordination sphere of an AlIVCUS site (i.e., an Al ion with tetrahedral coordination and carrying a coordinative vacancy), and the Lewis coordination of nitrile, occurring at low adsorbate coverages, perturbs the OH species by a ligand-insertion mechanism. When nitrile uptake perturbs the surface OH at ∼3740 cm-1 by H-bonding, a new broad and strong absorption is formed at lower frequencies, with apparent maximum that varies with nitrile coverage from ∼3430 cm-1 (∆νOH ≈ -310 cm-1) to ∼3550 cm-1 (∆νOH (22) Kno¨zinger, H.; Ratnasamy, P. Catal. Rev., Sci. Eng. 1978, 17, 31.
CD3CN Adsorbed on γ-Alumina
≈ -190 cm-1): these νOH shifts are typical of the formation of H-bonding interactions of medium-low strength.23 (iii) With nitrile uptake, new bands gradually develop at 2470, 2170, 1575, 1548, 1516, and 1425 cm-1 (most of these bands are not shown in Figure 2). On the basis of literature data,2,7 the bands at 2470, 1575, 1548, 1516, and 1425 cm-1 are assigned to surface acetamide species, formed by an hydrolytic process that is most probably catalyzed by the “basic” free OH groups absorbing (before the Lewis coordination of nitrile) at ∼3780 cm-1. The component at ∼2170 cm-1 is assigned to the polymerization product(s) of a CD2CN- carbanion.24 This species is thought to be formed by interaction of adsorbed nitrile with basic O2- anions, liberated at the surface during the dehydration process. The formation of acetamide species was previously observed on δ-alumina,2 while the formation of a CD2CNcarbanion and/or its polymerization products were not previously detected on transition aluminas. Upon outgassing at ambient temperature, several spectral changes are observed: in ∼20 min, physisorbed and H-bonded nitrile species desorb almost entirely, the “basic” OH species (∼3780 cm-1) are only marginally recovered (if at all), Lewis coordinated nitrile species decline by some 30-40% (the high-ν component resists more than the low-ν one), and the bands due to charged reaction products remain in the spectrum with still increasing intensity. Doped Aluminas. AS Samples. The 2400-2000 cm-1 spectral interval of CD3CN adsorbed on some AST samples is shown in Figure 3. In Figure 3a, bands at 2262 and 2112 cm-1 indicate the presence on AS300 of physisorbed nitrile and of some H-bonded species. In fact, the inset in Figure 3a indicates that OH components free from H-bonding and belonging to the low-ν envelope (∼3740 cm-1 and a shoulder at ∼3700 cm-1) are readily and reversibly consumed by formation of H-bonding. A weak ν(CN) band at ∼2315 cm-1, assigned to acetonitrile coordinated to weak Lewis acid sites, indicates that a plain activation at ambient temperature already forms some cus Al3+ sites capable of coordinating nitriles. This is consistent with the observation that, on AS300, a large fraction of the surface hydrated layer is made up of undissociated coordinated water (Figure 1b). On AS300 there is no formation of anionic species or of hydrolytic products. Figure 3b shows that, on AS523, the band due to Lewis coordinated acetonitrile is far stronger and broader, is asymmetric on the high-ν side, and has shifted to ∼2325 cm-1. A spectral resolution carried out in the 2370-2280 cm-1 interval with the procedure described above led to the spectral features reported in Table 1: it is noted that the presence of surface sulfates increased the relative intensity of the high-ν component due to stronger Lewis sites. In the ν(OH) region (inset Figure 3b) it is noted that the high-ν “free” OH component (∼3780 cm-1) is still virtually absent, while the low-ν “free” OH envelope (∼3740 cm-1) is very similar to that normally observed on pure alumina and becomes readily involved in H-bonding with nitrile. Also on AS523 there is no formation of reaction and/or hydrolysis products of adsorbed nitrile. Figure 3c reports the spectral pattern of nitrile adsorbed on AS873. There is a fair increase of the band envelope due to Lewis coordinated nitrile, as expected of a more (23) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond, W. H. Freeman: San Francisco, CA, 1960; Chapter 3. (24) Binet, C.; Jadi, A.; Lavalley, J.-C. J. Chim. Phys., Phys.-Chim. Biol. 1992, 89, 31.
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Figure 3. Differential absorbance spectra of CD3CN adsorbed at increasing pressure (up to saturated vapor pressure) onto AS300 (a), AS525 (b), and AS873 (c). The broken-line curves are the spectra obtained after 20 min evacuation at ambient temperature. Insets: as inset c in Figure 2.
dehydrated system. The resolved spectral features of the two Lewis coordinated species are reported in Table 1 and confirm that, in the presence of sulfates, the two Lewis ν(CN) components are still conveniently located at approximately the same frequency they have on A samples, but the relative intensity of the high-ν component is definitely higher. With contact time, a weak band is observed at ∼2175 cm-1 (polymerization product of a CD2CN- carbanion),24 while several weak bands form in the low-ν range due to acetamide species. The amount of these reaction products, which keeps growing also after evacuation of the weakly adsorbed nitrile species (see the dotted traces in Figure 3), is lower than that observed on pure A873 alumina. The ν(OH) spectral region (inset Figure 3c) indicates that, on AS873, “free” OH species are present also at 38003770 cm-1 (“basic” OH species),22 though not as abundant as on A873. This observation confirms that high-ν OH species, free from H-bonding, are indeed necessary for nitrile surface reactions and that the small amount of surface sulfates retained by AST samples reduces the basic properties of alumina but does not suppress them. This is consistent with that indicated by the background spectrum (Figure 1b) in terms of surface coordinated water and carbonate contaminants. No ν(CN) bands are ever observed with AST samples in the 2300-2275 cm-1 range, and this rules out the presence
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Figure 4. (a) Differential absorbance spectra of CD3CN adsorbed on ASS300: (upper part) adsorption pattern at increasing pressure (up to saturated vapor pressure); (lower part) desorption pattern (evacuation for 1, 5, and 20 min, respectively). The insets in section a show the corresponding differential spectra obtained in the OH stretching region. (b and c) Differential absorbance spectra of CD3CN adsorbed at increasing pressures (up to saturated vapor pressure) onto ASS523 and ASS873, respectively. The broken-line curves are the spectra obtained after 20 min evacuation at ambient temperature. Insets b-c: as inset c in Figure 2.
of a medium-strength Brønsted acidity, whereas the absence of strong Brønsted acidity is demonstrated by the constant absence, in the νOH region, of H-bonding interactions leading to ∆νOH shifts larger than some 300 cm-1. The absence on AST of Brønsted acid sites of any strength was confirmed by separate experiments of pyridine adsorption, which never lead to the formation of pyridinium species. ASS Samples. Sections a-c of Figure 4 show the 24002000 cm-1 spectral interval of acetonitrile-d3 adsorbed on ASS300, ASS523, and ASS873, respectively. In the spectral section at ν < 2300 cm-1 of ASS300, where the usual bands of liquidlike and H-bonded nitrile species are expected (2262 and 2112 cm-1), during the adsorption run a well-resolved band forms at 2272 cm-1 (∆νCN ) +10 cm-1; see the upper part of Figure 4a). This component is far prevalent from the beginning of the uptake process, grows with the acetonitrile dose, and only at high pressures becomes partially overlapped by the ν(CN) mode of a liquidlike phase (2262 cm-1). On silica-alumina systems, a ν(CN) band with a ∆νCN shift like this has been assigned
Escalona Platero et al.
to acetonitrile H-bonded to protonic (Brønsted) acid sites of medium-low strength3,4 and, in the present case, a similar assignment seems plausible. A parallel IR experiment, in which pyridine was adsorbed at ambient temperature on ASS300, showed that surface pyridinium ions formed at Brønsted acid sites. The upper inset to Figure 4a shows that despite the overwhelming intensity of the OH spectrum, the nitrile adsorption run causes the gradual and selective erosion of some surface OH species already free from H-bonding. The major component of the “free” OH pattern is broad and centred at ∼3675 cm-1, i.e., in a spectral position definitely lower than the lowest position typical of OH groups on transition aluminas. The OH component at ∼3675 cm-1 is thus thought to be (or contain) the stretching vibrations of Brønsted acidic OH species. During the nitrile desorption run (shown in the lower part of Figure 4a), the declining ν(CN) band centered at 2272 cm-1 and ascribed to nitrile H-bonded to Brønsted acid sites, becomes clearly singled out and so does the declining negative band at ∼3675 cm-1, tentatively assigned to the ν(OH) mode of Brønsted acid sites. In the spectral section at ν > 2300 cm-1 of ASS300 (upper part of Figure 4a), a medium-weak band is gradually formed at 2320 cm-1, due to Lewis-coordinated acetonitrile. The relative intensity of this band is much higher than on any of the other alumina systems activated at ∼300 K and indicates that abundant surface sulfates (that increase the undissociated coordinated water component in the surface hydrated layer) favor the formation of appreciable amounts of Lewis acid sites by mere removal of some coordinated water at low temperature. The ν(CN) mode of this Lewis coordinated species lies some 5-7 cm-1 higher than that observed, in comparable conditions, on all other alumina systems considered. The shift corresponds to an appreciable increase of Lewis acidity (i.e., of the chargewithdrawing power of the relevant Al3+cus sites), brought about by a strong inductive effect caused by the abundant surface population of sulfates acting as charge-withdrawing centers. Consistent with the increased Lewis acidity, the ν(CN) band at 2320 cm-1 is quite resistant to evacuation (see the lower section of Figure 4a). Upon prolonged contact, nitrile adsorbed onto ASS300 (as well as on all other ASST systems described below) does not yield any hydrolysis and/or polymerization product. As ASS is the first doped alumina system on which Brønsted acidity is present, it is confirmed that on this type of oxidic system, nitrile hydrolysis reactions are a base- and not an acid-catalyzed process. The IR spectra of CD3CN adsorbed on ASS523 (Figure 4b) and ASS723 (not shown) exhibit the ν(CN) band at ∼2272 cm-1 with a profile that is progressively better resolved from the overlapping 2262 cm-1 component and with an intensity that becomes progressively lower. Band deconvolution of a medium-coverage spectrum of nitrile adsorbed on ASS523, presenting nitrile components Hbonded to Brønsted OH (B) and to “regular” OH (reg), yields the following features: νmax(B) ) 2272 cm-1, ∆ν1/2(B) ) 28.0 cm-1; νmax(reg) ) 2262 cm-1, ∆ν1/2(reg) ) 20.2 cm-1. A very high Lorentzian character (>90%) of the ν(CN) band at 2272 cm-1 indicates a rather homogeneous nature for the Brønsted acidic OH species. Eventually, on ASS873 (Figure 4c), the ∼2272 cm-1 component has virtually disappeared, indicating that the Brønsted acidic OH species are gradually eliminated upon vacuum activation of the ASS preparation at increasing temperature. This is confirmed by the inset to Figure 4c: on ASS873, a negative low-ν OH component at ∼3675 cm-1 (due to the H-bonding erosion of Brønsted acidic OH species) is virtually absent. The vacuum thermal elimina-
CD3CN Adsorbed on γ-Alumina
tion of Brønsted acidic OH species is reversible: if an ASS873 sample is rehydrated by exposure to water vapor, in a second thermal activation run the relative intensity of the 2272 and 2262 cm-1 ν(CN) species formed upon nitrile uptake will be reproduced in each dehydration step. The differential spectra in the insets to Figure 4b indicate that the downward shifted band due to H-bonded OH species is not appreciably different from that observed on alumina specimens with no Brønsted acid centers and does not lead to the appearance of Evans windows in the spectral region below 3600 cm-1. (The appearance of Evans windows, due to complex Fermi resonance effects, is considered to monitor the presence of OH species with very strong Brønsted acidic character as, for instance, in the case of some H-exchanged zeolites.)5 These observations confirm the moderate strength of Brønsted acidic OH at the surface of ASS samples, as already suggested by the slight spectral increase of the ν(CN) mode The insets to Figure 4 also show that in all dehydration stages of ASS specimens, the OH band envelope at 38003760 cm-1, ascribed to “basic” alumina OH species, is totally absent. The absence of “basic” OH species, ascribable to the abundant presence of “acidic” surface functionalities, is consistent with the absence, in all dehydration stages, of surface reactions of nitrile species. The elimination by sulfates of all “basic” OH species is, on the other hand, consistent with the fact that, on aluminas, the adsorption of anion-forming species selectively consumes the high-ν OH species (e.g., CO2 uptake on transition aluminas gets rid selectively of the ∼3780 cm-1 OH species, and yields surface bicarbonate species).25 On ASS samples activated at increasing temperatures, nitrile adsorption brings about an expected gradual intensity increase of the ν(CN) band at ν > 2300 cm-1 (due to Lewis acid sites). The growing ν(CN) band becomes gradually broader, and its apparent maximum shifts to ∼2327 cm-1. If the band is computer resolved in terms of two components, and the low-ν component is fixed with its spectral features first observed on ASS300 (νmax ) 2320.5 cm-1; ∆ν1/2) 25.2 cm-1), the data reported for ASS samples in Table 1 are obtained. Due to inductive effects, the spectral position of the high-ν component has increased with respect to both A and AS samples, but not as much as the low-ν component did. Moreover, the relative intensity of the high-ν component (with respect to the low-ν one) exhibits a nearly constant ∼1:1 ratio, as on AS samples, whereas on A samples the ratio was ∼1:2. This confirms that the presence of sulfates increases somewhat the Lewis acidity of alumina, mainly by increasing the relative amount of the strongest fraction of Lewis acid sites. Figure 5a concerns the effect of adsorbing acetonitrile on the ν(SdO) mode of sulfates at the surface of ASS873. As on many oxidic systems carrying surface sulfates,26-28 after activation at high temperatures surface sulfates assume a highly covalent character, monitored by the sharpness of the ν(SdO) band and by its very high spectral position (∼1410 cm-1). Nitrile adsorption initially brings about an appreciable downward shift and broadening of the ν(SdO) band (∆νSO ≈ -50 cm-1), giving rise to a neat isosbestic point at ∼1390 cm-1. Then, at higher nitrile coverage (when all of the starting band at 1410 cm-1 has (25) Parkyns, N. D. J. Phys. Chem., 1971, 75, 526. (26) Bensitel, M.; Saur, O.; Lavalley, J.-C.; Mabilon, G. Mater. Chem. Phys. 1987, 17, 249. (27) Jin, T.; Yamaguchi, T.; Tanabe, K. J. Phys. Chem. 1986, 90, 4794. (28) Spielbauer, D.; Mekhemer, G. A. H.; Zaki, M. I.; Kno¨zinger, H. Catal. Lett. 1996, 40, 71.
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Figure 5. Absorbance spectra run in the spectral region of the SdO stretching vibration (νSdO) before adsorption (spectrum 1) and after the allowance of increasing CD3CN doses (up to saturated vapor pressure) onto ASS873 (a), and AS873 (b). The broken line is the desorption spectrum obtained after evacuation for 20 min.
been consumed, and physisorbed nitrile species start being formed), a much smaller and continuous downward shift and broadening are observed, bringing the final position of the ν(SdO) band to ∼1348 cm-1. Upon evacuation, when the weakly held nitrile species are desorbed, only the second downward shift (and broadening) is reversed. This behavior of the ν(SdO) band, which obviously implies that sulfates are all at the surface of the oxide, suggests that the formation of Lewis coordinated nitrile species causes a change of covalence of surface sulfates, as a consequence of a sort of ligand-insertion (or ligand-displacement) effect due to the introduction of an extra charge-releasing ligand in the same coordination sphere that the sulfate group belongs to. At high coverage, when no further ligands can enter in the coordination sphere of the Al ions, the further shift (and broadening) of the ν(SdO) band is the consequence of a sort of reversible surface solvent effect. Figure 5b shows that similar effects, and a very similar ∆νSO shift for ligand insertion, are observed in the case of the AS873 sample, despite the far lower surface concentration of sulfates. The lower sulfate concentration on the AS system is monitored by an obvious lower ν(SdO) band intensity and by a lower starting position of the ν(SdO) band (1384 cm-1 vs 1410 cm-1). ACA Samples. Figure 6a shows the IR spectra of successive doses of acetonitrile adsorbed onto ACA773. Besides the usual bands at 2262 and 2112 cm-1 (due to physisorbed and/or H-bonded nitrile species), and the broad ν(CN) band envelope centered at ∼2320 cm-1, due to various types of Al3+cus Lewis acid centers, a pronounced
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and ill-defined absorption centered at ∼3580 cm-1, due to the formation of weak H-bonding interactions.23 The spectral pattern in Figure 6b, relative to the nitrile adsorption run onto ACA(1300)1023, indicates that some significant changes have occurred, and are readily monitored by nitrile uptake. (i) The relative intensity of the various ν(CN) bands of Lewis coordinated species shows that, at the surface of the high-temperature transition alumina system, the Al3+cus/Ca2+cus ratio has changed, favoring the exposure of the less acidic Lewis sites (Ca2+ ions). (ii) The spectral position of the ν(CN) mode of nitrile coordinated to Al3+ sites has further lowered (from ∼2323 to ∼2316 cm-1), meaning that Ca2+ ions have gradually covered, at the surface, the most exposed (and thus most uncoordinated) Al3+ ions. (iii) The maximum intensity reached by the band at ∼2170 cm-1, due to the CD2CNcarbanion (and polymeric derivatives), has increased much, suggesting an overall increase of the basicity of ACA. Conclusions
Figure 6. Differential absorbance spectra of CD3CN adsorbed at increasing pressure (up to saturated vapor pressure) onto ACA873 (a) and ACA(1300)873 (b). The broken-line curves are the spectra obtained after 20 min of evacuation at ambient temperature. Insets: as inset c in Figure 2.
band is observed at ∼2280 cm-1. This band is attributed to the ν(CN) mode of acetonitrile molecules interacting, through the nitrogen electron lone pair, with cus Ca2+ ions. As expected, the Lewis acid strength of Ca2+cus ions is lower than that of the different types of cus Al3+ ions capable of adsorbing acetonitrile, so that the upward shift of the ν(CN) mode is far lower (∼+20 vs ∼+55 cm-1). Still, the Lewis acidity of cus Ca2+ sites is sufficiently high to cause an upward shift of the ν(CN) mode that is approximately twice as large as that caused, on ASS systems, by nitrile interaction with Brønsted acid sites. This is consistent with the previous observation19 that cus Ca2+ sites of ACA systems are strong enough as Lewis acid centers to adsorb CO at ambient temperature, yielding a ν(CO) band at 2180-2185 cm-1 (∆νCO ≈ 35-40 cm-1). On ACA773, also a medium-weak band is observed at 2170 cm-1, to be assigned as before to the presence of a CD2CN- carbanion and/or its polymerization product(s).24 The inset to Figure 6a shows the effect of acetonitrile adsorption on the spectrum of hydroxyl groups. The presence in the background spectrum of ACA773 of strong OH band(s) at 3800-3750 cm-1 (“basic” OH species)19 confirms the basicity of the ACA system, as already suggested by the background spectrum of the starting material (Figure 1d) and further demonstrated by the appreciable formation of acetamide hydrolytic products on long contact with adsorbed nitrile. Acetonitrile adsorption causes the erosion of both OH band envelopes (centered at ∼3770 and ∼3735 cm-1, respectively) and gradually brings about the appearance of the usual broad
As previously stated by Kno¨zinger et al.,1 acetonitrile is not a very sensitive probe molecule in revealing the Lewis acid heterogeneity present at the surface of γ-alumina systems. Still, by studying various systems isolated in different dehydration stages and by comparing the spectral features of nitrile adsorption and desorption patterns, it has been possible to detect the presence of cus Al3+ sites characterized by different Lewis acidic strength. The information obtained by nitrile adsorption on various alumina-based systems is thus consistent with that deriving from the use of other probe molecules. Despite the relatively poor sensitivity of nitrile ligands in distinguishing Al3+ Lewis acid sites of different acid strength, this ligand is quite sensitive in revealing the presence/absence of cus Al3+ sites, as with no other Men+ surface species examined so far does the ν(CN) mode rise to frequencies as high as with cus Al3+ (2315-2332 cm-1). When different cations, acting as weaker Lewis acid sites, are added to transition aluminas at surface dopant levels, nitrile adsorption reveals their presence by yielding wellresolved ν(CN) bands. The ν(CN) mode of Lewis coordinated acetonitrile exhibits a moderate sensitivity to surface inductive effects like, for instance, those produced by a high concentration of charge-withdrawing surface sulfate species. All of the data reported here, concerning several alumina-based systems, indicate that all surface OH species that are free from strong H-bonding of the OH‚‚‚OH type are readily perturbed by nitrile uptake. Only in the case of the so-called “acidic” OH (∼3740 cm-1) is the perturbation fully reversible and definitely of the H-bonding type. In the case of the so-called “basic” OH (∼3780 cm-1), the perturbation is due to the insertion of a Lewis coordinated nitrile ligand in the coordination sphere of the AlIV ion carrying the OH group. The ν(CN) mode of nitrile species H-bonded to the OH species at ∼3740 cm-1 turns out to be hardly distinguishable from that of a liquidlike physisorbed phase (∆νCN ≈ 0). Only when surface OH groups exhibit Brønsted acidity (νOH < 3690 cm-1) does the ν(CN) mode shift upward. The Brønsted acidity induced in aluminas by the presence of abundant surface sulfates turns out to be quite weak, as the ν(CN) mode only shifts of some +10 cm-1. In addition to its role as a molecular probe to check acidity, at the surface of alumina systems acetonitrile can undergo some surface reactions. First, it may react at basic O2- anions, to yield surface bound CD2CN- anions
CD3CN Adsorbed on γ-Alumina
(and/or its polymerization derivatives), and to the best of our knowledge this possibility was not observed before on aluminas. Second, Lewis coordinated nitrile species can be activated for a nucleophilic attack by a neighboring OH group, giving rise to surface-bound acetamide species. This reaction is observed to occur only when “basic” OH groups free from H-bonding, and absorbing at ∼3780 cm-1 before the Lewis coordination of nitrile, are available at the surface. When no such “basic” OH species are available, either because (i) the alumina is still hydrated or because (ii) the basic centers have been consumed by acidic dopants (e.g., sulfates), the hydrolysis reaction cannot proceed. This definitely confirms that nitrile hydrolysis at the surface of alumina-based systems is a merely basecatalyzed process. High hydration degrees and/or the presence of acid dopants also prevent the formation of
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basic O2- sites, so that CD2CN- and related polymeric derivatives cannot form, whereas the presence of basic dopants (like, for instance, Ca2+ species) appreciably enhance the extent to which the formation of CD2CNand related polymeric derivatives may proceed at the surface of medium-high dehydrated aluminas. Acknowledgment. This work has been supported by the Spanish DGICYT, Project PB97-0147. The UIB is gratefully acknowledged for supporting the stay of C. Morterra. ACA samples were kindly supplied by Centro Ricerche Fiat, Orbassano (Turin). CO and pyridine adsorption data, referred to in the text, were obtained by Dr. G. Magnacca (University of Turin). LA981654C
