The surface acidity of mesoporous silicoaluminophosphates: A FTIR study

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Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.

THE SURFACE ACIDITY OF MESOPOROUS SILICOALUMINOPHOSPHATES: A FTIR STUDY Gianotti, E.\ Oliveira, E.C.^ Coluccia, S.\ Pastore, H.O.^ and Marchese, L.^ ^Dipartimento di Chimica IFM, Universita di Torino, v. P. Giuria 7, 10125, Torino, Italy. ^Instituto de Quimica, Universidade Estadual de Campinas, CP 6154, 13084971 Campinas, SP - Brasil. E-mail: [email protected] ^Dipartimento ipartimento di Scienza e Tecnologie Avanzate, Universita del Piemonte Orientale, "A. Avogadro", C.so Borsalino 54, 15100, Alessandria, Italy. E-mail: [email protected]

ABSTRACT Thermally stable mesoporous ALPO and SAPO (Si/Al = 0.6) were synthesised using aluminum sulphate as aluminum source and without the need of HE as mineralizing agent. The mesophases prepared by this procedure were stable to calcination at 773K and presented a structure of non-parallel mesopores in the calcined form. FT-IR spectroscopy supplemented by the use of CO and NH3 as probe molecules was used to monitor the surface acidity of these new mesoporous materials and indicated that Al-OH, P-OH groups for meso-ALPO and additionally SiOH groups for meso-SAPO are found on the surface of these molecular sieves. The acidities of P-OH and Al-OH are stronger than SiOH in mesoporous silicas but still weaker than the zeolitic Bronsted acid sites. Keywords: meso-ALPO, meso-SAPO, FT-IR spectroscopy, surface acidity, CO adsorption, NH3 adsorption

INTRODUCTION Microporous aluminophosphates (AlPO) and silicoaluminophosphates (SAPO) are known to exist in a wide range of structural and compositional diversity, due to the allowance of heteroatom substitution. Active sites in these materials include acidic and redox centres, which are associated with protons and framework or extra-framework metal cations\ These solids are generally synthesised under hydrothermal conditions from sources of aluminium, phosphorous and an organic amine as structure-directing agent. Using cationic surfactants instead of the organic amines, mesostructured aluminophosphate phases were synthesised^""^. Meso-ALPO and SAPO merge the properties of microporous analogues, which are able to incorporate heteroatoms in their framework, with those of mesoporous systems, where the presence of larger pores allows the access of bulky organic molecules to the inner surface of the molecular sieve. The hexagonal phase was reported for aluminophosphates^'^ and for silicoaluminophosphates^. However, like the microporous counterparts VPI-5^ and cloverite^, the main problem of the mesostructures mentioned, is their low thermal stability when calcined or submitted to conventional neutral or acid solvent extraction for the removal of the structure-directing agent. An alkaline extraction was proposed for meso-ALPO and meso-MgAPO that prevents collapse of the mesostructure while promoting simultaneous ion exchange in metal-aluminophophates^. We report here the study of the surface acidity of these thermally stable meso-ALPO and meso-SAPO materials performed by FT-IR spectroscopy using CO and NH3 as probe molecules. EXPERIMENTAL SECTION Meso-ALPO and SAPO materials were synthesised using cetyltrimethylamonium bromide (CTMABr), aluminium sulphate, orthophosphoric acid. In the case of SAPO, tetraethylorthosilicate (TEOS) was used as a silicon source (Si/Al = 0.6) by procedures already reported in the literature^. In order to eliminate the surfactant, the samples were extracted in alkaline media^ and then calcined in O2 at 600°C.

1499 The materials were characterised by powder X-ray diffraction (Shimadzu, XRD 6000, CuKa, 30kV, 40mA, 2° 29 min^ FTIR spectra on pelletised samples (Bruker IFS88 spectrometer at the resolution of 4 cm' ^). For the CO and NH3 adsorption studies, the pellets were placed in home-made all-quartz cells allowing in situ thermal treatments of the samples and adsorption/desorption experiments. RESULTS AND DISCUSSION Fig. lA shows XRD patterns of as-synthesised meso-ALPO prepared at pH 10.5 at 343K (curve a), and shows a narrow and intense peak at 2.30° and a weaker one at 3,95'' 26 due to respectively (100) and (110) reflections of hexagonally arranged pores. At approximately 20° 20 there is the wide halo corresponding to the amorphous aluminophosphate pore walls. Both alkaline extraction (Fig. lA, curve b) and calcination (Fig. 1 A, curve c) cause the intensity of the (110) reflection to diminish, indicating a possible decrease in the organisation of the material, passing from a hexagonal arrangement of parallel pores to a material composed of non-organised pores\ The same behaviour was observed for meso-SAPO (Fig. IB) prepared at 343K and pH 10.5, the best conditions for synthesis, as determined from the ALPO studies. The Soxhlet extraction with alkaline solution (Fig. 1B curve b) promotes a partial disorganisation to indipendent pores as shown by the weakening and broadening of the (100) and (110) reflections. The calcination necessary for the total elimination of the organics (Fig. IB curve c) led to a further disorganisation as demonstarted by the absence of the (110) reflection peak.

1-

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Figure 1. Section A: XRD diffractograms of as-synthesised meso-ALPO prepared at pH 10.5 and 343K (curve a), alkaline extracted sample (curve b) and calcined sample (curve c). Section B: XRD diffractograms of as-synthesised meso- SAPO prepared at pH 10.5 and 343K (curve a) alkaline extracted sample (curve b) and calcined sample (curve c). To elucidate the surface acidity of these new mesoporous materials, a spectroscopic FT-IR study, supplemented by the use of probe molecules such as CO and NH3, was performed. Fig. 2 shows FTIR spectra in the OH stretching region of meso-ALPO (curve a) and meso-SAPO (curve b) upon calcination at 873K in O2. In the meso-ALPO spectrum, the weak band at 3790 cm' is assigned to the stretching vibration mode of free Al-OH groups of Al ions in tetrahedral coordination, this is an evidence that Al ions are present in framework positions'^'^.

1500 In contrast, the band at ca. 3730 cm'^ is due to the stretching vibration of free Al-OH groups of aluminium ions in octahedral coordination, as shown by ^^Al-MAS-NMR of meso-ALPO^'^^'^^ and found in transition aluminas^^. The band at 3675 cm'^ is assigned to the stretching mode of isolated P-OH^^'^"^. In the spectrum of meso-SAPO, beside the bands due to Al-OH and P-OH groups, a new narrow band at 3745 cm'^ appears. Such signal is due to the stretching mode of isolated Si-OH groups^^'^^.

O

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3700

3600

3500

Wavenumber /cm" Figure 2. FTIR spectra of calcined meso-ALPO (curve a) and meso SAPO (curve b). The CO adsorption at the nominal temperature of 77K was performed on both mesoporous ALPO and SAPO. Fig. 3 shows difference FTIR spectra of CO adsorbed at 77K on calcined meso-ALPO obtained by subtracting the spectrum of the bare sample. In the OH stretching region (Fig.3A), at high CO pressure (curve 1) three negative bands (3790, 3730 and 3675 cm"^) appeared and a broad positive band is formed at 3500 cm'^ which has a composite nature being heavily asymmetric on the higher wavenumber side. Upon decreasing the CO pressure (curves 1-11), the band at 3500 cm'^ disappears while the negative bands are slowly recovered. The broad absorption at 3500 cm"^ is mainly due to P-OH, H-bonded to CO, however a minor contribution of Al-OH, H-bonded to CO, should also occur in the 3600-3500 cm"^ region^^. In the CO stretching region (Fig. 3B), sharp peaks at 2150-2135 cm'^, present at high CO doses (curves 1-3) are due to liquid-like CO adsorbed within the pores of the material. A band at 2165 cm"^ with a shoulder at 2180 cm'^ is also found at high CO doses and decreases with CO pressure (curves 1-9) simultaneously with the reappearance of the bands of free Al-OH and P-OH. This is a clear evidence of the CO interacting with Al-OH and P-OH in Al-OH-CO and P-OH-CO complexes. On the basis of the relative intensity of the hydroxy Is, and their behaviour upon the CO pressure, it is reasonable to assign the 2165 cm"^ band to CO interacting with P-OH species whereas the 2180 cm'^ one to CO bond to Al-OH groups. At lower CO pressures (curves 7-10), a new band at 2200 cm'^ is visible and is assigned to CO interacting with Al(III) ions, similar to Lewis sites observed in Al-containig zeolite, due to partially extraframework aluminium sites "•^°.

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Figure 3. Difference FTIR spectra of CO adsorbed at 77K on calcined meso-ALPO. Curves 1 to 9: 100 to 1 torr CO pressure range; curves 9 to 11: 1 to 10'^ torr. Fig.4 shows difference FTIR spectra of CO adsorbed at 77K on calcined meso-SAPO. In the OH stretching region (Fig. 4A), two negative bands are found at high CO coverages at 3745 and 3675 cm'\ due to the stretching of Si-OH and P-OH groups respectively (curve 1). Simultaneously, two positive absorptions at 3655 and 3490 cm'^ appear. Upon decreasing the CO pressure (curves 1-7), the band at 3655 cm"^ disappears together with the negative band at 3745 cm'\ The bands at 3490 and 3675 cm'^ disappear only after complete desorption of CO (curve 10), indicating a stronger interaction. The band at 3655 cm'^ is due to Si-OH H-bonded to CO, while the band at 3490 cm"^ is assigned to P-OH H-bonded to CO. In the CO stretching region (Fig. 4B), sharp peaks at 2150-2135 cm'\ due to liquid-like CO within the pores, dominate the spectra at high CO coverages (curves 1-3). A band at 2155 c m ' is also found at high CO doses and decreases with CO pressure (curves 1-6) simultaneously with the reappearance of the band of free Si-OH groups. This is a clear evidence of the CO interacting with Si-OH in Si-OH-CO complexes. At lower CO pressures (curves 4-9), a new band at 2170 c m ' is visible and is still present whenever CO is bonded to POH (Fig. 1 A, curve 9). This band is therefore assigned to CO interacting with P-OH groups. As a matter of fact, the bands of both CO and OH stretching in the X-OH--CO complexes are sharper in the case of mesoSAPO in comparison with meso-ALPO, and this signifies that the hydroxyls heterogeneity is lower when the molecular sieves contain silicon within the mesoporous network. Table 1 reports on the shift of the OH stretching (AVQH) upon CO adsorption on meso-ALPO, meso-SAPO, mesoporous MCM-41 and microporous SAPO-34. The larger is the shift, the higher is the acidity of the sites^'. Although the real position of the stretching OH in A1-OH...CO complexes cannot be detected, it may be inferred that their acidity is comparable to (or slightly higher than) P-OH in meso-ALPO. The acidity of these groups, however is definitive larger than Si-OH groups present both in meso-SAPO and pure siliceous MCM-41. Conversely, the acidity of both P-OH and Al-OH is lower than the protons located at the bridged hydroxyls [Si-0(H)-Al] in zeolites or zeotypes^'. As an example, the shift of bridged OH upon CO adsorption is reported (Table 1) in the case of microporous SAPO-34, an acid molecular sieve structurally-related to natural chabasite^'. It may be seen that the acidity of P-OH and Al-OH (AvoH = 150-200 cm') is intermediate between, very weak Si-OH in siliceous (AVQH = 70 cm'') or meso-SAPO (AVQH ^ 90 cm"') molecular sieves and strong acid [Si-0(H)-Al] in zeolites where the OH shift falls in the 250-350 c m ' interval ^^ Finally, it is of note that the Si-OH acidity is slightly higher when the meso-SAPO is compared to MCM-41, a resuh which might be related to the higher polarity of the media surrounding the silanols in the meso-SAPO molecular sieves. This is an interesting result in view of the use of molecular sieves with very low acidity because it suggests that even an acidity as low as that of silanols can be tuned in some extend.

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Wavenumber /cm' Figure 4. Difference FTIR spectra of CO adsorbed at 77K on calcined meso-SAPO. Curves 1 to 8: 100 to 1 torr CO pressure range; curves 8 to 10: 1 to 10'^ torr. Table 1. Sample me so-ALPO meso-SAPO MCM-41 SAPO-34

OH groups P-OH Al-OH P-OH Si-OH Si-OH Si(OH)Al

VOH

AVOH

(cm*) 3675 3790-3730 3675 3745 3745 3600

(cm*) 175 150-200 185 90 70 287

NH3 adsorption on meso-ALPO^^ showed that Al-OH and P-OH are sufficiently acidic to promote H^ transfer and produce NH4^ ions. For this work, ammonia adsorption was also made on meso-SAPO, where Si-OH are not acidic enough to create NH^^ ions but only weak hydrogen bonded Si-OH...NH3. In this way, it will be possible to have an appraisal of the relative population of OH groups with intermediate acidity in both samples, thus, only the region typical of NH4^ is presented. Room temperature NH3 absorption on both meso-SAPO (curve a) and meso-ALPO (curve b) is reported in Fig.5. Bands at 1620 and 1460 cm"^ are formed on both samples with higher intensities in the case of meso-SAPO. The band at 1460 cm"^ is due to the asymmetric bending mode of NFLi^ ions formed by proton transfer from the surface hydroxyl groups to NH3 molecules. The higher concentration of the NH4^ groups for meso-SAPO is mainly related with the larger population of P-OH groups (Fig.2.). In highly siliceous mesoporous materials, MCM-41 or MCM-48, in which only Si-OH groups are present, NHi^ species are not formed^^. This confirms that P-OH and Al-OH groups in mesoporous aluminophosphates and silicoaluminophosphates are more acidic than Si-OH. Moreover, Al^^ Lewis acid sites are also inferred by the presence of a strong band at 1620 c m \ due to the asymmetric bending mode of coordinated NH3.

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0.05 u.a. 1-H

o

V^

meso-SAPO A

O in

meso-ALPO A

/ 1800

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Wavenumber /cm" Figure 5. FTIR spectra of NH3 (30 torr) adsorption at room temperature on calcined meso- SAPO (curve a) and meso- ALPO (curve b). CONCLUSION Mesoporous aluminophosphates and silicoaluminophosphates were synthesised using aluminium sulphate, phosphoric acid, CTMABr, TMAOH in aqueous systems. In the case of meso-SAPO, TEOS was used as a silicon source. The as-synthesised materials present a hexagonal mesostructure and are sensitive to the post-synthesis procedures. This might cause a partial disorganisation of the hexagonal mesostructure leading to a material composed of non-parallel pores, as observed for the product of alkaline extraction of the organic surfactant followed by calcination. FTIR study of adsorbed CO has evidenced that the acidity of P-OH and Al-OH groups in meso-ALPO and in meso-SAPO is intermediate between Si-OH in pure mesoporous silicas and Bronsted acid sites in microporous zeolites. These protons are strong enough to protonate NH3, the concentration of NH4^ formed upon NH3 adsorption, therefore gives some hints on the population of hydroxyls with medium acidy. This material can be used for organic reactions where a mild acidity is required. ACKNOWLEDGEMENTS MIUR (Ministero dell'Istruzione dell'Universita e della Ricerca) and FAPESP (Funda9ao de Amparo a Pesquisa no Estado de Sao Paulo) are gratefiilly acknowledged for the financial support to this research. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

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