Revisiting the Nature of the Acidity in Chabazite-Related

330

J. Phys. Chem. C 2007, 111, 330-339

Revisiting the Nature of the Acidity in Chabazite-Related Silicoaluminophosphates: Combined FTIR and 29Si MAS NMR Study G. A. V. Martins, G. Berlier,* and S. Coluccia Dipartimento di Chimica Inorganica, Fisica e dei Materiali, UniVersita` di Torino, Via P. Giuria 7, 10125 Torino, Italy, and NIS Centre of Excellence, UniVersita` di Torino, 10125 Torino, Italy

H. O. Pastore and G. B. Superti Micro and Mesoporous Molecular SieVes Group, Instituto de Quı´mica, UniVersidade Estadual de Campinas, CP 6154, CEP 13083-970, Campinas, SP, Brazil, and Nano-SiSTeMI Interdisciplinary Centre, UniVersita´ del Piemonte Orientale, 15100 Alessandria, Italy

G. Gatti and L. Marchese* Dipartimento di Scienze e Tecnologie AVanzate, UniVersita´ del Piemonte Orientale, Via Bellini 25G, 15100 Alessandria, Italy, and Nano-SiSTeMI Interdisciplinary Centre, UniVersita` del Piemonte Orientale, 15100 Alessandria, Italy ReceiVed: June 22, 2006; In Final Form: September 29, 2006

The acidity of crystalline silicoaluminophosphates with chabazite-related structure (CHA), was studied by using CO and C2H4 probe molecules. SAPO-34 samples with similar silicon concentrations prepared using different structure-directing agents have been studied, and the results have been compared with a silicoaluminophosphate, CAL-1, having a similar structure and much higher silicon concentration. A detailed analysis of the FTIR spectra in the OH stretching region, and of the downward shift of the OH bands upon CO and C2H4 adsorption, evidenced the presence of three distinct acid sites (named OHA, OHB, and OHC) absorbing at 3631, 3617, and 3600 cm-1, respectively (average values). A multipeak curve-fitting approach, along with hydrogen bond theory, following Makarova et al. (J. Phys. Chem. 1994, 98, 3619), allowed us to compute the fraction of the three sites. While the two main components OHA and OHC were already reported and explained in terms of different crystallographic positions, the existence and nature of the OHB site are here discussed for the first time. Protons at the OHB sites are shown to have an acidity (downward shift upon CO adsorption of ca. 330 cm-1) comparable to that usually measured in zeolites, aluminosilicate crystalline materials, which are known to possess stronger acidity than SAPOs. To our knowledge, this is the first clearcut experimental evidence that such strong acid sites are present in SAPO materials. A combined FTIR and 29 Si MAS NMR study permitted explaining the strong acidity of OHB sites in terms of protons either at the borders of silica patches/islands or inside aluminosilicate domains.

1. Introduction The use of zeolites and zeotypes as catalysts for hydrocarbon conversion represents an important tool of the modern chemical and petrochemical industries. In particular, the introduction of microporous solid acids for gasoline conversion1 and other processes such as methanol transformation to light olefin (MTO)2 or gasoline (MTG) resulted in great advantages in terms of safety and waste reduction. Among others, H-SAPO-34,3,4 a silicoaluminophosphate (SAPO) with chabazite structure, has shown peculiar activity and selectivity in MTO processes, and is now central to the UOP/Norsk Hydro MTO technology, which is currently under commercialization. The catalytic activity of ordered microporous materials in MTO and MTG reactions is related to the presence of Brønsted acid sites within their channels and cavities. Brønsted sites in solid acids can be schematized as SiO(H)Al centers, where Si and Al are tetrahedral framework atoms interconnected by oxygen atoms. The negative charge of the zeolite framework * Corresponding authors. E-mail: [email protected] (G.B.); [email protected] (L.M.).

generated by Si4+/Al3+ substitution is compensated by the presence of protons. As far as aluminophosphate materials are concerned (where the P5+/Al3+ couples produce a neutral framework), the presence of Brønsted sites is related to the substitution of Si4+ for a limited number of P5+ ions, as it is the case in H-SAPO-34.5-8 Notice that the local structure of Brønsted acid sites in SAPO materials is formally similar to that of zeolitic materials, where negative charges are generated by the insertion of Al3+ defects in silica matrixes. However, when a couple of aluminosilicate (zeolite) and isostructural SAPO homologues are considered, SAPOs are generally characterized by milder acidity.6-11 A variety of Si centers with different local environments can be present in SAPO materials, depending upon the silicon content and the synthetic procedure.8,12 Si ions can be incorporated in the framework as Si(OAl)4 isolated sites, with only Al atoms as Si first neighbors and P as second ones, as nonisolated Si(OAl)4 sites having one or two Si atoms as second neighbors, or as Si islands having Si(OSi)4, Si(OSi)3(OAl), etc.13 The rationalization of structural effects on acid strength still represents a stimulating challenge for both experimental and

10.1021/jp063921q CCC: $37.00 © 2007 American Chemical Society Published on Web 12/01/2006

Acidity of Crystalline Silicoaluminophosphates theoretical investigations. Many studies have been reported, trying to pin down the relations between acid strength and shortand long-range structural factors, such as SiOAl angles or SiO, Al-O distances,14,15 flexibility of the framework,16 and electrostatic potential generated inside the cavities.17-19 The acid character of distinct structural sites of both isolated Si and Si islands in chabazite-related (CHA) zeotypes (H-SSZ-13 zeolite and H-SAPO-34) was studied by Sastre et al. using theoretical approaches.7 The stronger acidity of Brønsted SiO(H)Al sites formed at the borders of Si islands or in aluminosilicate regions was found,7 confirming earlier proposals by Barthomeuf and co-workers.5 Concerning the experimental techniques, the acid strength of Brønsted sites can be monitored by using probe molecules with basic character, such as ammonia or pyridine, by analyzing their spectroscopic features,20 or by measuring the temperature at which these molecules are desorbed (temperature programmed desorption (TPD) measurements).21 However, these techniques are not able to discern among families of Brønsted sites with small differences in acid strength; in this case the use of probe molecules with weaker basicity is compulsory. One of the most sensitive experimental techniques for this purpose is Fourier transform infrared (FTIR) spectroscopy, in particular when CO is used as a probe molecule.6,8,22-25 This technique is based on the analysis of the IR spectra in the OH stretching (νOH) region and on their perturbation upon adsorption of small molecules (N2, CO, C2H4, etc.), which act as weak bases forming OH‚‚‚B hydrogen-bonded adducts with the Brønsted sites. The measure of the ∆νOH shift upon formation of the OH‚‚‚B adducts represents a classical method to estimate the acidity of Brønsted sites.26,27 On the basis of FTIR and neutron diffraction experiments, the existence of two distinct Brønsted sites with different acid strengths (high frequency (HF) and low frequency (LF), or A and C species) in H-SAPO-34 was proposed.23,28 A third family of acid sites (species B) absorbing between HF and LF hydroxyls was also monitored by CO adsorption; however, their crystallographic position was not described.6 The same FTIR technique was more recently employed to compare the strength of Brønsted sites between the isostructural homologues H-SSZ13 (aluminosilicate with low Al content) and H-SAPO-34.29 In this paper, FTIR spectroscopy of adsorbed CO and C2H4 was used to compare the strength and distribution of Brønsted sites in H-SAPO-34 samples with similar chemical compositions but prepared with different amines as structure-directing agents (SDAs). A chabazite-related silicoaluminophosphate, CAL-1, recently synthesized adopting an original procedure with a lamellar aluminophosphate (AlPO) as precursor and hexamethyleneimine (HMI) as SDA,30,31 and having a high Si loading, was also studied. This synthetic approach led, in fact, to materials with high concentrations of framework Si. A careful analysis of the FTIR spectra in the νOH region evidences that the distribution of acid sites can be strongly influenced by the synthetic procedure. In particular, it is shown that, in addition to the two well-known HF and LF Brønsted sites,23,28,29 the presence of a third distinct site (B species) is confirmed and is consistent with the presence of Si-rich regions. Notably, these species display a strong acid character, which is comparable to the Brønsted sites of the H-SSZ-13 aluminosilicate homologue. To our knowledge, this is the first report about the presence of such strong acid sites in SAPO materials, confirming the results coming from theoretical calculations that foresaw an acidity comparable to zeolites for sites formed at the borders of silicon-rich regions.7

J. Phys. Chem. C, Vol. 111, No. 1, 2007 331 2. Experimental Section Three different SAPO-34 samples were prepared and characterized. SAPO-34 (T) and SAPO-34 (E) were prepared using tetraethylammonium hydroxide (TEAOH) and triethylammine (Et3N) as SDAs, respectively, modifying the procedure described in the literature32 as detailed in the following. SAPO-34 (M) was prepared using morpholine (Mor), employing the optimized synthesis procedure described in ref 33. The three SAPO-34 samples were synthesized by mixing appropriate amounts of Al(OH)3 (Aldrich), orthophosphoric acid (85%, Aldrich), and distilled water. The mixture was stirred up to the point of obtaining a uniform gel (3 h). SiO2 (Aerosil 200, Degussa), and SDAs were respectively added drop by drop in order to obtain final gels with the following compositions:

SiO2:0.25/Al(OH)3:1.0/H3PO4:0.9/Et3N:1.0/H2O:30 SiO2:0.25/Al(OH)3:1.0/H3PO4:0.9/TEAOH:1.25/H2O:30 SiO2:0.25/Al(OH)3:1.0/H3PO4:0.9/Mor:1.25/H2O:30 After a vigorous stirring (2 h), the resulting gels were crystallized in stainless steel, Teflon-lined, autoclaves under autogenous pressure at 463 K for 7 days (samples E and M) and at 473 K for 20 days (sample T). The product was filtered, washed with distilled water, and dried in air at room temperature for 16 h. A chabazite-related silicoaluminophosphate sample, CAL-1, with a larger Si concentration (SiO2/Al2O3 ) 1.2) was prepared starting from AlPO-kanemite precursor following literature procedures.30,31 For the synthesis of the AlPO-kanemite a suspension of 23 mL of water and 15.74 g of pseudo-bohemite (Catapal-B, Vista Chemicals, 72% Al2O3) was mechanically stirred until homogeneous in a polypropylene beaker. To this suspension 15.0 mL of phosphoric acid (Merck, 85%) were added dropwise, followed by 30 mL of water. After 2 h of stirring, 22.4 mL of n-butylamine (NBA, Riedel, 99%) was added dropwise. The molar composition of the gel was 1.0 Al2O3:1.0 P2O5:2 NBA:30 H2O. The mixture was stirred for an additional 2 h and then loaded into Teflon-lined stainless steel autoclaves and heated at 473 K for 48 h. The resultant solid was thoroughly washed with distilled water and dried at ambient temperature. CAL-1 sample was prepared as follows. In a one-neck roundbottomed flask containing 11.4 mL of water, 5.0 g of the assynthesized AlPO-kanemite [AlPO3(OH)2(C4H9NH2)] was slowly added. After 3 h of stirring, 0.305 g of SiO2 (Aerosil 200, Degussa) was slowly added. The mixture was stirred for another 30 min, after which 2.2 mL of hexamethyleneimine (HMI, Aldrich, 99%) was added dropwise. The molar composition of the gel was 2 AlPO-kan:2.4 SiO2:3 HMI:100 H2O. The final mixture was loaded after 30 min of homogenization into the autoclave and hydrothermally treated at 473 K for 48 h. After that the autoclaves were cooled under tap water; the solid material was filtered and washed with copious amounts of water, until the pH was close to neutral, and finally dried at ambient temperature. The acid forms were obtained by calcinations in inert gas flow and then air at 823 K. A NH4-ZSM-5 sample from Zeolyst International (SiO2/Al2O3 ) 80) was used, after ammonia decomposition in a vacuum at 773 K, as reference to draw the ∆A/A0 vs ∆νOH plot as proposed in the literature.26 X-ray powder diffractograms were recorded in a ARL X’TRA diffractometer, set at Cu KR radiation, 40 kV, 30 mA, at a rate

332 J. Phys. Chem. C, Vol. 111, No. 1, 2007

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TABLE 1: Chemical Composition of the Samples Measured by EDS Analysisa sample

unit cell compositionb

χSi

SDA (%)

H-SAPO-34 (T) H-SAPO-34 (E) H-SAPO-34 (M) CAL-1c

(Al16.5P14.4Si5.0)O72(TEAOH)2.01 (Al16.0P15.4Si4.3)O72(Et3N)2.86 (Al16.9P15.4Si4.3)O72(Mor)2.84 (Al13.8P13.8Si8.3)O72(NBA)2.09(HMI)1.42

0.14 0.12 0.12 0.23

12.02 13.31 11.31 13.48

a Unit cell composition and Si atomic fraction are reported, together with the weight percentage of organic SDA measured by thermogravimetric analysis. b TEAOH ) tetraethylammonium hydroxide; Et3N ) triethylammine; Mor ) morpholine; NBA ) n-butylamine; HMI ) hexamethyleneimine. c The chemical composition of CAL-1 was determined by ICP-MAS analysis.

of 2° 2θ min-1 and in a Shimadzu XRD-6000 at the same measuring conditions. Chemical composition (Table 1) was measured by Energy Dispersive Spectroscopy (EDS) microanalysis on a LEICA Stereoscan 420 scanning electron microscope using a SAPO-34 sample with a composition carefully determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MAS) as reference. Thermogravimetric analyses of samples were carried out on a Setaram instrument (Setsys evolution) by keeping 20 mg of as-synthesized samples under a constant air flux and using a temperature rate of 10 °C/min. FTIR spectra were recorded with a resolution of 4 cm-1 on a Bruker IFS 88 spectrophotometer equipped with a DTGS detector. The powdered samples were pressed into self-supporting disks and placed in a quartz cell allowing thermal treatments in a vacuum, gas dosage, and in situ measurements. Before IR measurements, the samples were activated in a vacuum at 773 K in the quartz cell used for FTIR measurements. CO and C2H4 (initial pressure = 20 Torr) were dosed on the preactivated sample at the liquid nitrogen temperature (LNT, estimated temperature on the pellet ca. 110 K). The equilibrium pressure was reduced step by step, to obtain a sequence of IR spectra corresponding to decreasing coverage. O2, N2, CO, and C2H4 (20 Torr) were dosed at LNT on H-ZSM-5 sample previously outgassed at 773 K. The reported IR spectra were normalized with respect to the pellet thickness. Curve fitting was performed with the Levenberg-Maquardt method by using OPUS software (Bruker Optik). Mixed Gauss-Lorentzian functions were used. 29Si MAS NMR spectra were performed in a Bruker AC 300/P under cross-polarization conditions, at 59.6 MHz, with 10 s between pulses and 5 ms contact time. All spectra were referenced to tetramethylsilane (TMS). 3. Results and Discussion 3.1. SAPO-34 Structure. A schematic representation of the SAPO-34 structure is reported in Figure 1. The structure is characterized by the typical barrel-shaped cage of the CHA topology, delimited by 8-T ring openings (where “T” is for tetrahedral framework atom: Si, Al, or P) which are generated by the interconnection of double 6-T rings bound by 4-T ones. This topology allows four different configurations for the Brønsted sites, corresponding to four distinct oxygen atoms.6,23,34 In this work the nomenclature proposed in refs 23 and 34 will be employed, as indicated in Figure 2. The four sites are here represented showing the different rings (4-, 6-, or 8-T) to which they belong (see also Table 2). The X-ray diffraction patterns of the four samples studied in this work showed the typical peaks expected for the chabazite phases (patterns not reported for the sake of brevity). A weak peak assigned to a minor fraction of extra phase was observed in sample H-SAPO-34 (E) and disappeared upon calcination.

Figure 1. Ball-and-stick schematic representation of the SAPO-34 (chabazite) structure: typical barrel-shaped cage, delimited by eightmembered rings, generated by the sequence of double six-membered rings interconnected by four-membered rings. Atom types are designated by color: Al, yellow; Si and P, blue; O, red.

Figure 2. Schematic ball-and-stick representation of the four configurations of Brønsted sites in SAPO-34. Atom types are designated by color: Al, yellow; Si and P, blue; O, red; H, white. (a) O1 belongs to two 4-T and one 8-T rings, connecting the two 6-T rings of the double 6-T ring layers; (b) O2 belongs to one 4-T, one 6-T, and one 8-T rings; (c) O3 belongs to two 4-T and one 6-T rings; (d) O4 belongs to one 4-T and two 8-T rings, forming the bridge between two double 6-T rings. The orientation of the proton with respect to the framework has only a pictorial meaning.

TABLE 2: Description of the Four Structural Oxygen Sites in SAPO-34, As Depicted in Figures 1 and 2 label

description

O1 O2 O3 O4

belongs to two 4-T and one 8-T rings belongs to one 4-T, one 6-T, and one 8-T rings belongs to two 4-T and one 6-T rings belongs to one 4-T and two 8-T rings

The chemical composition of the three samples measured by EDS analysis is summarized in Table 1. Similar χSi atomic fractions were found for the three H-SAPO-34 samples: 0.12 for H-SAPO-34 (E) and H-SAPO-34 (M); 0.14 for H-SAPO-

Acidity of Crystalline Silicoaluminophosphates

Figure 3. FTIR spectra in the νOH region of samples (a) H-SAPO-34 (E), (b) H-SAPO-34 (T), (c) H-SAPO-34 (M), and (d) CAL-1 previously calcined and outgassed at 773 K (dotted curves). Solid lines: simulated spectra with components (OHA, OHB, and OHC, see text) obtained by a curve fitting procedure.

34 (T). CAL-1 sample, on the contrary, was characterized by a higher Si content (χSi ) 0.23). The amount of organic template was measured by thermogravimetric analysis (Table 1). The ratio between number of organic molecules and Si atoms has been used in the literature to estimate the fraction of Si atoms forming Brønsted sites, since the negatively charged framework oxygen atoms of Si-O-Al bonds are neutralized by the positive ions issued by the template.13 As a consequence, a mismatch between organic and Si contents could be used in principle to estimate the fraction of Si atoms involved in the formation of islands. In our samples, the Si/SDA ratio is around 1.5 for samples H-SAPO-34 (E) and H-SAPO-34 (M) and 2.5 and 2.4 for samples H-SAPO34 (T) and CAL-1, respectively. These values have to be considered with great care for the estimations of Si islands, since the presence of not protonated organic molecules in SAPO materials cannot be excluded, as recently reported in the case of SAPO-34 prepared using morpholine as an SDA.33 Moreover, the situation is even more complex for CAL-1 sample, where two different SDAs were used in the synthesis;30,31 in this case the fraction of Si/SDA was estimated comparing weight loss and recently reported 13C MAS NMR results.31 3.2. FTIR Spectroscopy in the νOH Region. The FTIR spectra in the νOH region of samples (a) H-SAPO-34 (E), (b) H-SAPO-34 (T), (c) H-SAPO-34 (M), and (d) CAL-1 previously calcined and outgassed at 773 K (dotted curves) are reported in Figure 3, with the corresponding components calculated by deconvolution. The spectra of the samples are composed by a very weak feature at 3678 cm-1 (due to P-OH species located on the external surface of the sample particles) and by two intense peaks with maxima at around 3630 and 3600 cm-1. The low intensity of the bands at 3678 cm-1 testifies that the samples display small external surface area. The two intense bands are usually referred to as high-frequency (3630 cm-1, HF) and low-frequency components (3600 cm-1, LF), and have been assigned to Brønsted sites with different acid strengths in O4 and O2 structural configurations, respectively.23,29 These hydroxyls have been also named respectively A and C species (OHA and OHC) in the literature.23,30,31 A third

J. Phys. Chem. C, Vol. 111, No. 1, 2007 333 family of acid sites absorbing at 3625 cm-1, hereafter labeled B species (OHB), was also described, but its nature was not discussed in terms of crystallographic positions. In the present work multipeak curve-fitting analysis was performed on the four spectra of Figure 3, with the resulting components being reported as solid lines. In all cases a good curve fit is only obtained by assuming the presence of three components. Since the position of the bands slightly changes from sample to sample, for simplicity we will use the average of the values reported in Table 3, that is, 3631, 3617, and 3600 cm-1 for OHA, OHB, and OHC, respectively. At this stage of the discussion the presence of OHB species might appear artificial; however, it will be fully justified when the CO and C2H4 adsorption is discussed (vide infra). When comparing the three H-SAPO-34 samples, the broad character of the bands related to Brønsted sites in H-SAPO34 (M) (Figure 3c), and even more in CAL-1 (Figure 3d), is evident. Even at 110 K the signals corresponding to the individual sites are not resolved. This suggests that, instead of welldefined sites presenting homogeneous acidity along the structure (as is likely the case of H-SAPO-34 (E) (Figure 3a) and H-SAPO-34 (T) (Figure 3b), where OHA and OHC bands are narrower and better resolved), a heterogeneous distribution of sites exists in H-SAPO-34 (M) and CAL-1. The results obtained by multipeak curve fitting on the four samples (position of the maximum νOH, full-width at half-maximum (fwhm), and area A0) are summarized in Table 3. By using the empirical method proposed by Makarova et al.,26 the area of OHC species was corrected by considering that their extinction coefficient is enhanced by interaction with framework oxygen atoms (see below). The data summarized in Table 3 are reported for semiquantitative considerations, in order to describe the relative concentration (e.g., the distribution) of the different hydroxyls. A good fit of the data was only obtained by assuming the presence of three components, while a two-component fit was never possible. This was also true for sample H-SAPO-34 (E) (the sample with the narrowest bands), where the OHB component was found at 3625 cm-1, a particularly high value when compared to the other samples. Notwithstanding this discrepancy, the results obtained on the four samples give a quite realistic description of the system, as detailed in the following. In particular, (i) a broadening of all the components is observed on samples H-SAPO-34 (M) and CAL-1 with respect to H-SAPO-34 (T) and H-SAPO-34 (E); (ii) OHB species represent a minor fraction of the Brønsted sites in all samples, but they change significantly from one sample to the other; and (iii) in all samples the fwhm of OHC species is clearly larger than those of OHA and OHB. The last observation, as anticipated above, is in agreement with the hypothesis that OHC species are perturbed by framework oxygen atoms (see below for details). Finally, CAL-1, the sample with the highest Si concentration (χSi ) 0.23), shows hydroxyl bands with the highest fwhm values and with the largest amount of OHB species (ca. 20%). 3.3. FTIR Spectroscopy of CO and C2H4 Adsorption at 110 K. When a weak base (B) is employed as probe molecule, FTIR spectroscopy can provide information on the acid strength of Brønsted sites by the measure of the wavenumber shift (∆νOH) related to the formation of hydrogen-bonded OH‚‚‚B adducts.22,25-27 To evaluate the acid strength of OHA, OHB, and OHC sites of the four chabazite-related samples, FTIR spectra were recorded upon adsorption of CO and C2H4 at 110 K. The spectra obtained upon gradually varying CO pressure in equilibrium with samples CAL-1 (top) and H-SAPO-34 (E) (bottom) are reported in Figure 4 in the νOH region. Spectra


Martins et al.

TABLE 3: Position of the Maxima, Full-Width at Half Maximum (fwhm), and Area (A0) of the Peaks Obtained by Curve Fitting of the νOH Spectra of Samples H-SAPO-34 (E), H-SAPO-34 (T), H-SAPO-34 (M), and CAL-1 Reported in Figure 3 H-SAPO-34 (E) OHA OHB OHC

H-SAPO-34 (T)

νOH (cm-1)

fwhm (cm-1)

A0 (%)

A0a (%)

3631 3625 3604

12.5 11.9 34.5

40.4 7.6 52.0

48.7 9.2 42.1

νOH (cm-1)

fwhm (cm-1)

A0 (%)

A0a (%)

3632 3613 3599

21.7 17.4 40.0

37.4 9.0 53.6

48.4 11.5 40.1

νOH (cm-1) 3630 3615 3600

H-SAPO-34 (M) OHA OHB OHC a

fwhm (cm-1)

A0 (%)

A0a (%)

15.2 15.8 29.3

38.4 8.6 53.0

48.6 10.9 40.5

fwhm (cm-1)

A0 (%)

A0a (%)

23.5 25.0 47.0

32.2 14.8 53.0

42.6 19.6 37.8

CAL-1 νOH (cm-1) 3632 3614 3595

Values obtained after correction of the extinction coefficient of OHC site for its perturbation by framework oxygen (from ref 26).

Figure 4. FTIR spectra of increasing dosage of CO at 110 K on CAL-1 (top) and H-SAPO-34 (E) (bottom) outgassed at 773 K. Dotted lines, spectra recorded before CO dosage; solid lines, intermediate CO coverage; dashed lines, highest CO coverage (PCO = 20 Torr). Spectra are vertically shifted for clarity.

were vertically shifted for direct comparison. Despite the differing broadness of the curves, similar results were obtained for the other two samples (not reported). For the sake of brevity we do not report the corresponding spectra in the νCO region. In any case, the bands of CO adsorbed on protons with even large difference in acidity are strongly overlapped; therefore differences among samples cannot be appreciated and multipeak curve-fitting procedures are impossible to make. It must be considered that the difference in wavenumber of CO adsorbed on silanols (bearing protons with extremely weak acidity) and on bridged hydroxyls (bearing protons with strong acidity) in zeolites is around 12 cm-1.22 Upon CO adsorption on H-SAPO-34 (E) (bottom spectra), the OHA, OHB, and OHC bands in the 3630-3600 cm-1 interval are perturbed and shifted to lower frequency, forming a broad and intense absorption in the 3500-3200 cm-1 interval. This band is characterized by a maximum at 3350 cm-1, and by an evident shoulder at ca. 3455 cm-1. These spectroscopic changes (also witnessed by a clear isosbestic point at 3565 cm-1) are easily explained by the formation of hydrogen-bonded OH‚‚‚ CO adducts, with the consequent νOH perturbation.27 When the same experiment on sample CAL-1 is considered (upper part of Figure 4), the broader character of the signals can be appreciated. This broadness is also reflected in the absorption related to the formation of OH‚‚‚CO adducts (3500-3200 cm-1 interval), which is characterized by an evident tail at low frequency. Similar results are obtained by using C2H4 as probe molecule; in this case the asymmetric character at low frequency

Figure 5. FTIR spectra of samples CAL-1 (top) and H-SAPO-34 (E) (bottom spectra) outgassed at 773 K before CO dosage (dotted curve) and at the highest CO coverage (PCO = 20 Torr, dashed-dotted line). In solid lines the bands obtained by three-peaks best fit are reported with the corresponding assignment (OHA is transformed in OHA′ upon CO adsorption and so on). The position of νOH for free OH and OH‚‚‚CO adducts of H-SSZ-13, with the corresponding ∆νOH, is reported for comparison, from ref 29.

of the bands related to hydrogen-bonded adducts is even more pronounced (Figure 7). In both H-SAPO-34 (E) and CAL-1 samples the distinct Brønsted sites show different responses to the perturbation induced by CO: while OHA and OHB completely fade away at the highest CO pressure (PCO = 20 Torr) OHC does not, leaving a weak adsorption at 3594 cm-1 (dashed curves in Figure 4). This trend can be explained with the lower acid character of the OHC component, which requires a lower temperature for a complete consumption.23,29 The differing acidity of the three Brønsted sites is testified by the different ∆νOH shifts, that is, by the strong asymmetric character of the adsorption in the 3500-3200 cm-1 range (maximum at 3350 cm-1, shoulder at 3455 cm-1, and low-frequency tail). Similarly to the bands of the unperturbed OH, a good fit of the OH‚‚‚CO adducts bands could only be obtained using three components. A quantification of ∆νOH for each site was therefore obtained by multipeak curve fitting of the absorption related to OH‚‚‚CO adducts. A comprehensive summary of the results obtained by curve fitting in the νOH region before and after CO adsorption (spectra obtained at the maximum CO dose) is shown in Figure


J. Phys. Chem. C, Vol. 111, No. 1, 2007 335

TABLE 4: Position of the Maxima of OHA, OHB, and OHC before and after CO Perturbation, Resulting ∆νOH Shifts, and Intensity Increase ∆A/A0a Measured on H-SAPO-34 (E), H-SAPO-34 (T), H-SAPO-34 (M), and CAL-1 Samplesb H-SAPO-34 (E) νOH (cm-1) OHA OHB OHC OHCcorr

3631 3625 3604 3631

H-SAPO-34 (T)

νOH‚‚‚CO (cm-1)

shift ∆νOH (cm-1)

∆A/A0

3350 3280 3433 3433

281 345 171 198

5.1 6.8 2.2 3.8

νOH (cm-1) 3630 3615 3600 3630



∆A/A0

3352 3283 3445 3445

278 332 155 185

5.2 6.1 1.7 3.2



∆A/A0

3365 3299 3467 3467

267 315 128 165

5.0 6.1 1.6 3.3

H-SAPO-34 (M) OHA OHB OHC OHCcorr

CAL-1

νOH (cm-1)



∆A/A0

3632 3613 3599 3632

3355 3282 3454 3454

277 331 145 178

5.4 6.5 1.4 2.8

νOH (cm-1) 3632 3614 3595 3632

a A and A0, intensity of perturbed and unperturbed νOH, respectively. b Values for OHCcorr were calculated assuming as νOH of the unperturbed site the value measured for OHA, and as A0 the corrected values reported in the last column of Table 3.

Figure 7. FTIR spectra of sample H-SAPO-34 (T) outgassed at 773 K before C2H4 dosage (dotted curve) and at the highest C2H4 coverage (PC2H4 = 20 Torr; dashed-dotted line). In solid lines the bands obtained by three-peaks best fit are reported.

Figure 6. FTIR spectra of sample H-SAPO-34 (T), previously outgassed at 773 K, at different CO coverages (dotted curves). In solid line the bands obtained by three-peaks best fit are reported.

5 for samples CAL-1 and H-SAPO-34 (E) (top and bottom spectra, respectively). The multipeak curve-fitting procedure was also applied to results obtained at intermediate CO coverage, as exemplified in Figure 6, where the spectra (and corresponding calculated components) obtained at intermediate PCO on sample H-SAPO-34 (T) are reported. In Table 4 the data calculated for the four samples are summarized, showing the following series of acidity: OHB > OHA > OHC. Each component is shifted upon CO adsorption of -331 cm-1 (OHB), -276 cm-1 (OHA), and -150 cm-1 (OHC) (average values). By comparing the data obtained on the four samples, a slight decrease in the ∆νOH values in the series H-SAPO-34 (E) > H-SAPO-34 (T) ≈ H-SAPO-34 (M) > CAL-1 can be appreciated. Whether this trend is an indication of slight differences in the acid strength of the samples is difficult to ascertain at the moment. We instead believe that these small variations are the consequence of the intrinsic heterogeneity of the samples, so that OHA, OHB, and OHC might not be due to single sites (in particular when the CAL-1 sample is considered) but due to the contribution of families of sites, with slightly different environments and undetectable differences in acid strength.

The differences in the ∆νOH values for the A, B, and C sites, on the contrary, are reproducible in all samples, and can be used to estimate the acid strength of the three sites. Notice that the shift measured for OHB (∆νOH ) -331 cm-1 upon CO adsorption) is very large when compared to the usual values measured for isolated sites in SAPO structures, which are normally less acidic than those of aluminosilicate with analogous structure.7 A recent FTIR study confirmed that protons of H-SSZ-13 (a synthetic aluminosilicate with CHA structure), located in crystallographic positions similar to those of H-SAPO34, show a larger downward ∆νOH shift upon CO adsorption (316 vs 270 cm-1, when the HF components of H-SSZ-13 and H-SAPO-34 (T) are compared).29 To our knowledge this is the first time that the existence of Brønsted sites with such high acid strength in SAPO materials has been reported, as will be discussed in the following. Figure 7 shows the results obtained by curve fitting in the νOH region before and after C2H4 adsorption on sample H-SAPO-34 (T). The ∆νOH’s obtained upon C2H4 adsorption are -374, -449, and -281 for OHA, OHB, and OHC respectively (Table 5); they are larger values than those obtained upon CO adsorption in agreement with the stronger basicity of ethylene. As a matter of fact, the larger downward shift of the hydroxyls produces a larger separation among the components when they interact by H-bonding, making them more clearly identifiable. This definitely supports the presence of at least three families of hydroxyls in chabazite-type materials analyzed in


Martins et al.

TABLE 5: Position of the Maxima of OHA, OHB, and OHC before and after C2H4 Perturbation, Resulting ∆νOH Shifts, and Intensity Increase ∆A/A0a Measured on H-SAPO-34 (T)b OHA OHB OHC OHCcorr

νOH (cm-1)

νOH‚‚‚C2H4 (cm-1)


∆A/A0

3630 3615 3600 3630

3356 3166 3350 3445

374 449 250 281

7.0 8.7 2.8 5.0

a A and A0, intensity of perturbed and unperturbed νOH, respectively. b Values for OHCcorr were calculated assuming as νOH of the unperturbed site the value measured for OHA, and as A0 the corrected values reported in the last column of Table 3.

this work, and gives further support to the results of the curvefitting procedure used for CO adsorption. 3.4. Empirical Equation for OH‚‚‚B Adducts. The results presented above point to the existence of a third site, labeled “OHB”, apparently characterized by strong acid character. To give further support to this hypothesis, we decided to verify the validity of the procedure used to fit the experimental curves by checking the results in terms of hydrogen bonding theory predictions. According to the hydrogen bond theory, the perturbation of OH groups leads to an increase of their intensity and width, so that the shifted components are characterized by different extinction coefficients.35,36 By comparing the FTIR spectra obtained upon interaction of different probe molecules with the Brønsted sites of a H-ZSM-5 sample, Makarova et al. found a linear correlation between the intensity growth (and band broadening) of the Brønsted OH bands and the ∆νOH shift. This correlation was successfully employed to estimate the extinction coefficients of distinct families of Brønsted sites andsby a combined H MAS NMR and FTIR studysthe fraction of HF and LF hydroxyls in H-SAPO-37 and H-Y zeolite.26 These studies confirmed that the stretching frequency of LF hydroxyls in these zeotypes is in fact determined by hydrogen bonding with framework oxygen atoms.26,37 The correlation proposed by Makarova et al. was reproduced in this paper by dosing O2, N2, CO, and C2H4 at 110 K on a H-ZSM-5 sample, which was here used as a model system, being characterized by one single OH band (spectra not reported), at wavenumbers intermediate between those of OHA and OHC species of H-SAPO-34 and CAL-1 samples. The results are plotted in Figure 8, where the linear correlation found in ref 26, ∆A/A0 ) 0.018∆νOH/cm-1, could be perfectly reproduced. In the same plot the data obtained upon CO adsorption (on H-SAPO-34 (E), H-SAPO-34 (T), H-SAPO34 (M), and CAL-1), after deconvolution of the bands in three components, are plotted as open symbols (triangles, circles, squares, and diamonds, respectively, for the highest CO coverage). The results obtained on sample H-SAPO-34 (T) at intermediate CO coverage (Figure 6) are not reported for the sake of simplicity, while only the results obtained on the same sample upon C2H4 adsorption are reported in the same plot as light gray hexagons. As far as the bands related to OHA and OHB components are concerned, a very good agreement is observed with the linear correlation calculated for the Brønsted site of H-ZSM-5. This implies that the corresponding IR bands of unperturbed hydroxyls of H-SAPO-34 (and CAL-1) are characterized by extinction coefficients similar to those of free OH groups of H-ZSM-5. A deviation from linearity is observed where the OHC band is concerned, suggesting that this component is not related to an isolated (e.g., unperturbed) OH site, but to a Brønsted OH group perturbed by hydrogen bonding (that is, shifted to lower frequency and characterized by a higher extinction coefficient). This hypothesis is in agreement with the observed high fwhm values of this band (Table 3).

Figure 8. Plot of increase of intensity ∆A/A0, (A and A0, intensity of the perturbed and unperturbed νOH, respectively) vs ∆νOH shift, as a consequence of the formation of OH‚‚‚B adducts in H-ZSM-5 (crosses): (1) O2; (2) N2; (3) CO; (4) C2H4. Open triangles, circles, squares, and diamonds: data obtained upon CO adsorption on H-SAPO-34 (E), H-SAPO-34 (T), H-SAPO-34 (M), and CAL-1, respectively. Filled symbols: OHC data corrected by assuming an increase of the extinction coefficient caused by hydrogen bonding with framework oxygen atoms. Circled area includes OHC values before correction of the extinction coefficient. Light gray hexagons: corrected data obtained upon C2H4 adsorption on sample H-SAPO-34 (T).

In the absence of any perturbation, the OHC band would adsorb at ca. 3631 cm-1 (the average value of OHA in H-SAPO-34 samples) and would have a lower extinction coefficient, which was calculated to correct the position of the OHC site in the plot of Figure 7 (solid symbols). Notice that the corrected values are in good agreement with the ∆A/A0 ) 0.018∆νOH/cm-1 linear correlation, for both CO and C2H4. A good agreement was also observed when the components calculated for spectra at intermediate coverage were considered (data not reported in the plot for simplicity). The same method was employed to correct the fraction of Brønsted sites (Table 3) and the ∆νOH shift upon CO adsorption (from 150 to 182 cm-1 as an average value; see Table 4). To further analyze the nature of the three families of Brønsted sites, the data calculated by curve fitting of the spectra obtained at different CO coverages on sample H-SAPO-34 (T) (some of which are reported in Figure 6) were used to draw adsorption isotherm plots (Figure 9a). From these data Langmuir plots (P/V vs P, where V is the volume of adsorbed species at given pressure) were drawn by substituting the volume of adsorbed species with the band areas of interacting hydroxyls (OH‚‚‚CO), Aint, in agreement with the following reaction (Figure 9b):

OH + CO h OH‚‚‚CO These plots show a good linearity in the considered pressure interval and could be fitted with the equation

P P 1 + ) Aint Amaxb Amax


J. Phys. Chem. C, Vol. 111, No. 1, 2007 337

Figure 9. Adsorption isotherms (a) and corresponding Langmuir plots (b) drawn with the OHA, OHB, and OHC components calculated by multipeak curve-fitting procedure on FTIR spectra of increasing PCO on sample H-SAPO-34 (T).

Amax is the area corresponding to the maximum quantity of interacting hydroxyls at the highest CO pressure (coincident with the area of free OH, in that at the maximum CO dose almost all hydroxyls are bonded), and b is a constant that gives an estimation of the OH‚‚‚CO adduct strength. From the calculated b values, in agreement with the measured values of ∆νOH, the following affinity scale was found: OHB > OHA . OHC. This model confirms the expected trend in terms of the adsorption process strength; however, it has a limitation due to the experimental conditions that could not assure an ideal isothermicity, due to the slight variation of sample temperature at different gas pressures. As a consequence, the absolute b values have to be considered with some caution. To shed light on the nature of the OHB site, the following section will give some hints on the possible structural configurations and Si local environment of the three sites. 3.5. Structural Configuration and Si Environment of A, B, and C Sites. On the basis of a combined neutron diffraction and FTIR study, Smith et al. assigned the FTIR HF (OHA) and LF (OHC) bands at 3630 and 3600 cm-1 to the protons residing on O4 and O2 sites, respectively (Figure 2d and Figure 2b, respectively).23 This assignment was in agreement with the evidence that the proton residing in O2 is perturbed, being tilted toward the oxygen atoms of the 6-T ring.25 The proposal was mainly based on neutron diffraction evidence that, at 5 K, only two out of four sites of H-SAPO-34 (i.e., O4 and O2) are populated by protons. However, the possibility that at temperatures higher than 5 K all four configurations are populated by protons cannot be excluded, as recently pointed out in the literature.29 On this basis, it was recently proposed that the νOH modes associated with O1, O3, and O4 sites (all located close to the entrance of the 8-T window) should be very similar, while only O2 should be characterized by a different frequency, being tilted toward the oxygens of the 6-T ring.29 Following this reasoning, the OHA site should be due to the contribution of O1, O3, and O4 sites, while O2 should be responsible for the OHC band. On the basis of our data it is not possible to establish which of the two proposed assignments for OHA and OHC sites is preferable. Even if we find the argumentation about the population of all four sites at 110 K quite convincing, theoretical

calculations estimated different frequencies for the four sites, where those of O1 and O3 were quite similar but distinct from those of O2 and O4.7 This issue, however, is outside the scope of this paper, which is more focused on the nature of the OHB site. A description of the OHB site in terms of crystallographic position has therefore been attempted in this work by considering all four possible configurations and the νOH modes calculated in ref 7. However, the value of the downward shift of OHB upon CO adsorption (∆νOH ) -331 cm-1) is too large to be explained in terms of isolated sites in a SAPO framework. When measured by CO, in fact, the acidic strength of OHB sites is comparable to that of aluminosilicate zeolites with homologous structure (e.g., H-SSZ-13, ∆νOH ) -316 cm-1), where SiO(H)Al Brønsted sites are formed by introduction of isolated Al defects in a silicate framework. The acidity of Brønsted sites in zeotypes is thought to be influenced, keeping constant other factors, by the chemical environment of SiO(H)Al centers. When Si islands are formed in SAPO materials, these regions strongly resemble the zeolitic framework, where Brønsted sites usually display stronger acid strength. It has been shown that Brønsted sites at the borders of silica islands (or inside aluminosilicate regions) experience a chemical environment similar to that of analogous sites in aluminosilicates.7 As a consequence, the presence of Si-rich regions in SAPO-34 materials could induce the formation of Brønsted sites with stronger acid character than those produced by isolated silicon ions. So far, this hypothesis is only based on theoretical calculations.7 29Si MAS NMR technique was used to gain further insight on the local environment of Si sites in the different samples. The results obtained on CAL-1 and H-SAPO-34 (E) samples are reported in Figure 8 (top and bottom spectra, respectively). In the case of CAL-1 one large signal can be observed at -90.8 ppm due to Si(OAl)4 groups or isolated Si as in Si(4Al,9P),13 with weaker signals in the range -95 to -114 ppm. These are assigned to Si(OSi)(OAl)3 (-95.1 ppm), Si(OSi)2(OAl)2 (-99.7 ppm), Si(OSi)3(OAl) (-104.8 ppm), and Si(OSi)4 (-109.5 and -114.0 ppm) sites, and testify that a nonnegligible fraction of silicon islands is present. When sample H-SAPO-34 (E) is considered, a decrease in the overall signal area is observed, in agreement with the lower


Figure 10.

Martins et al.

29

Si CP MAS NMR spectra of samples CAL-1 (top) and H-SAPO-34 (E) (bottom) as synthesized.

Si content of the sample. In more detail, the intense peak at -90.8 ppm assigned to isolated Si(OAl)4 groups is still present, being sensibly narrower with respect to CAL-1 sample. Minor signals can also be observed at -95.1, -99.7, and -109.5 ppm, corresponding to Si(OSi)(OAl)3, Si(OSi)2(OAl)2, and Si(OSi)4 groups, respectively. These features are in agreement with a higher homogeneity of Si sites (as indicated by the narrow shape of the peak at -90.8 ppm) and by the presence of a small amount of Si islands. In particular, the absence of the signal at -104.8 ppm, due to Si(OSi)3(OAl) species, could be interpreted in terms of Si patches formed by ca. 10 Si atoms.38 A comparison between the results obtained by 29Si MAS NMR and FTIR is now necessary. 29Si MAS NMR clearly shows that CAL-1 sample is characterized by a considerable amount of silica islands, and thus by a higher heterogeneity of Si sites. This heterogeneity is also confirmed by the large width of the IR components of CAL-1 sample (Table 3). The presence of a small amount of silica patches in H-SAPO-34 (E) sample is also evidenced by 29Si MAS NMR, which is in agreement with the narrow shape of the corresponding OH bands. On the basis of these results, we put forward the hypothesis that the Brønsted sites responsible for OHB band are related to species formed at the borders of silica islands. In fact, the concentration of OHB sites (Table 4) is relatively high in CAL-1 sample having a considerable amount of silica islands, but is not negligible even in the case of H-SAPO-34 (E). The high downward shift of OHB site upon CO and C2H4 adsorption, witnessing for an acid strength comparable to zeolitic sites, could thus be explained in terms of a similar local environment concerning first and second neighbors. To our knowledge this is the first time that the presence of such strong acid sites in SAPO materials is shown by experimental methods. Moreover, an estimation of the relative fraction of these sites was possible by means of a combined 29Si MAS NMR and FTIR study. 4. Conclusions FTIR spectroscopy was successfully employed to monitor the acid strength of H-SAPO-34 samples with similar Si contents prepared by using different SDAs. A chabazite-related silicoaluminophosphate, CAL-1, prepared adopting an original procedure from a lamellar precursor, was also studied. This synthetic approach led to a high concentration of framework Si. Brønsted acidity was estimated by the analysis of the FTIR bands in the OH stretching region and of their perturbation upon CO and C2H4 adsorption at 110 K, by measuring the corre-

sponding ∆νOH shift. Multipeak curve-fitting analysis showed that at least three distinct families of Brønsted sites (OHA, OHB, and OHC) are present in all samples. As already reported, OHA and OHC sites could be interpreted in terms of structural configurations, that is, by assuming that they are related to framework oxygen atoms in O4 and O2 sites, respectively. Upon CO adsorption the corresponding FTIR bands were downward shifted -276 (OHA) and -182 cm-1 (OHC). These values are in agreement with the milder acidity of SAPO samples, when compared to the zeolitic homologues (H-SSZ-13, ∆νOH ) -316 cm-1). The validity of the data obtained by multipeak curve-fitting procedure was checked by using the empirical correlation proposed by Makarova et al.,26 which is based on the increase of the extinction coefficients of OH groups involved in hydrogen bonding. On the basis of this model it was shown that, while OHA and OHB are related to isolated OH groups, the OHC site is perturbed by the interaction with the SAPO framework, in agreement with the proposed structural configuration (O2). The strong acid character of the OHB site, giving a downward shift upon CO adsorption of -331 cm-1, was discussed in terms of structural configurations and Si local environment. Due to the very large value of ∆νOH, comparable to that of zeolitic homologues (H-SSZ-13, ∆νOH ) 316 cm-1), an explanation of OHB species in terms of structurally isolated sites was not possible. These species were explained in terms of Si sites present at the borders of silica islands, having a local environment similar to that of Brønsted sites in zeolites, in agreement with theoretical calculations. This hypothesis was confirmed by a careful comparison of the data obtained on samples with different Si contents and by 29Si MAS NMR evidences. To our knowledge this is the first report based on experimental methods on the presence of such strong acid sites in SAPO materials. Moreover, our methodology allowed quantifying the fraction of families of distinct Brønsted sites with different acid strengths. These results are of fundamental importance for a fine-tuning of Brønsted sites of catalysts with improved activity, selectivity, and resistance to deactivation in acid-related reactions. Acknowledgment. G.A.V.M. thanks ASP (Association for Scientific Development of Piedmont) for financial support, and G.B. thanks the Italian Ministry for University and Research for funding a project aimed at developing targets of strategic interest for the community (FISR 2005-2008). FAPESP (Fundaç aõ de Amparo a` Pesquisa do Estado de Saõ Paulo) is acknowledged for financial support.

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Revisiting the Nature of the Acidity in Chabazite-Related

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