The Journal of
Physical Chemistry VOLUME 97, NUMBER 31, AUGUST 5,1993
Q Copyright I993 by the American Chemical Society
LETTERS Formation of H30+at the Bronsted Site in SAPO-34Catalysts Leonard0 Marchese, Jiesheng Chen, Paul A. Wright, and John Meurig Thomas' Davy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London, WIX 4BS, U.K. Received: May 17, I993
Diffuse reflectance infrared spectroscopy, combined with thermogravimetry, of progressively dehydrated (up to 550 "C)silicoaluminophosphate (SAPO-34) reveals that, at room temperature, there is a stoichiometric proton transfer from the bridging hydroxyl Brbnsted acid site to the physisorbed water. All six predicted infrared vibrations for the HsO+ oxonium ion in C, or C1 symmetry are identified. In a typical sample there are 10.0 X lozoHsO+ ions g'.
Introduction Bridging hydroxyls, such as 01-H2 in structure I (Scheme I), are a~knowledgedl-~ to be the locus of the Brbnsted acidity of molecular sieve catalysts such as faujasitic zeolites (like HY), pentads (like H-ZSM-S), and the ever expanding family of SAPOS and MeALPOs where, in an aluminum phosphate framework,some of the phosphorus or the aluminum is replaced respectively by silicon (SAPO) or a divalent ion (MeALPO). The question has often arisen in the context of the role of these microporous, microcrystalline solids as acid catalysts whether their intrinsic Brbnsted acidity is so high that the oxonium ion is readily formed, just as it is in concentrated solutions of strong mineral acids and in their corresponding crystal hydrates.* When water vapor is adsorbed ondehydrated HY zeolite, aquaoxonium ions (HaO+*H20)are formed? just as H@+ and HsOz+ ions are generated when water is assimilated by solid heteropolyacid catalysts.1OJ1 The situation for the strongly acidic H-ZSM-5 catalyst is, however, the subject of some dispute. Thus, Jentys et al.3 interpreted their infrared spectra to indicate that H30+ ions are present at the interior surfaces of the pentasil; but very recent work12 on the adsorption of H20 and D2O on H-ZSM-5 and D-ZSM-S seems to indicate that proton transfer from the bridging Br6nsted site does not take place. In other words, the situation represented in structure 11 (corresponding to a physisorbed state) rather than that in structure IIIis thought to prevail. Detailed ab initio SCF calculations,*3J4which it is h0ped1~J~ 0022-3654/93/2097-8109$04.00/0
Suuclun I
X = Si, P.
I
suucture II
suucturc UI.
will ultimately provide deeper insights into proton affinity and the mechanisms of the elementary steps of reactions catalyzed by acidic acids than can be gleaned from airect experiment, are not, at present, entirely unambiguous concerning the preferred structure of water bound to the Brbnsted sites of zeolitic catalysts. On balance, however, notwithstanding the considerable and somewhat uncertain contribution made by electron correlation 0 1993 American Chemical Society
8110 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
Letters
toquantummechanicallyderived estimates, structureIIIis favored over II. To fathom fully the mode of action of acid catalysts, it is important to know whether H30+is indeed present in appreciable amounts, especially when it is knownI7-l9that, even in the presence of water, reactants such as alkenes and alkanols may still readily form carbenium ions. We have therefore addressed this question using chiefly diffuse reflectance spectroscopy on the versatile silicoaluminophosphate catalyst known as SAPO-34, which has a well-defined framework structure exactly similar to that of the mineral chabazite.4.19-21 Reports on its performance as an acid catalyst, epecially its role in converting methanol to olefins, have appeared elsewhere.20J Unlike other SAPO structures, it may be readily synthesized with very low concentrationsof hydroxyls AI-OH, Si-OH, and P-OH. The dominant bridging hydroxyls are the Brdnsted ones shown in Scheme I.
Experimental Section SAPO-34 samples were prepared following the proceduresgiven elsewhere.22 Aluminum hydroxide hydrate (Aldrich,55% A1203), phosphoric acid (Aldrich, 85% H3P04),and aerosil (Degussa) were used as aluminium, phosphorus, and silicon sources, respectively. Typically, a gel mixture with an empirical composition 0.50Si0~:A1~0~:0.9P~O~:2.OEt~N:60H~0, where Et3N represents triethylamine,was charged into a Teflon-lined stainless steel autoclave and heated at 190 "C for 7 days. The product was recovered and calcined at 550 "C in flowing 02 for 4 h to remove the occluded template. ICP-AES elemental analysis indicated that the molar ratio A1:Si:P is 1.0:0.19:0.77. Infrared spectra were recorded at room temperature on a Perkin-Elmer 1725X FTIR spectrometer (Res = 4) equipped with a MCT detector. Diffuse reflectancespectra were collected using a Spectratech cell in which the sample could be heated in situ up to 600 "Cin a stream of gas (dry He or N2). The sample was heated step by step at temperatures ranging from 100 to 550 "C (30 min per step). IR and TGA measurements were taken under strictly comparable experimental conditions.
3500
3000
2000
2500
1500
Wavenumbers [cm- 13 Figure 1. IR reflectancespectra (in Kubelka-Munk units) of HSAPO34 progressively dehydrated at increasing temperatures ("C): (a) 100, (b) 150 (30 min), (c) 150 (60 min), (d) 175, (e) 200, (f)250, and (g) 350. L = bands due to lattice modes. Inset A percentage weight logs as a function of the temperature. Inset B: exploded view of the region 3780-3650 cm-1 of spectrum f.
0
Results and Discussion Stoichiometry HZOSi. Thermogravimetricdata are shown in inset A of Figure 1. The weight loss in the range 1 W 5 5 0 "C is about 3.096, which means that the amount of water in the catalyst is (10.0 f 1.O)X 1020 molecules g-'. This value is close to that of the silicon in the structure (9.5 X lozoSi atoms gl) indicating a 1:l stoichiometry. DRIFT Spectra. Diffuse reflectance spectra recorded after heating the sample from 100 to 350 "Care presented in Figures 1 and 2 in Kubelka-Munk (KM) units. No significant further spectral changes occur at higher temperature. The spectrum of the sample heated at 100 "C shows at least four heavily overlapped components (designated 0 bands) in the 3800-3300-~m-~range, two of which merge as well-defined peaks at 3678 and 3570 cm-I. On the low-frequency side of the 3678-cm-' peak, a broad shoulder is clearly observable. A broad and weak band is visible at ca. 3500 cm-I. Three strong absorptions(designated O'bands) at 2890 (broad), 2475 (broad), and 1610cm-1(narr0w)arepresentin the330&1500-cm-lrange, the latter two being made up of several overlapped components (shouldersat about 2600 cm-I and in the range 1800-1700cm-I). Two well-defined peaks at 3626 and 3599 cm-l (designated B bands) appear as the temperature is raised to 150 "C and progressively increasefn intensity as the dehydrationtemperature increase from 150 to 350 "C. In this range the 0 and 0'bands progressively decrease at the same rate as the B ones increase, showing that the desorption of water from the sample induces a conversion from 0,O'bands to B bands: the B bands are easily assigned to bridged hydroxyls which have strong Brdnsted
0' 3500
3000
2500
2000
If DO
Wavenumbers [cm- 11 Figure 2. Difference spectra obtained by subtracting spectrum g from spectra a-f of Figure 1. Positive bands are from species which are destroyedand negativeones from species which are formedupon increasing the temperaturefrom 100 to 350 OC. 0,O'= bands due to oxonium ions; B = bands due to bridging hydroxyls. Inset: weight variationas a function of the integrated intensity of the bands in the region 180&1600 cm-1; the integrated intensities of the 0 bands, as well as the 0' ones, are linearly related to the weight loss at the various temperatures.
a~idity.4.1~The origin of the 0,O'bands is still a matter of discussion.3J* Three extremely weak bands at 3768,3742, and 3676 cm-1 are also observable (see exploded view of the 3780-3650-cm-I range in inset B of Figure 1) and have been assigned respectively as AI-OH, Si-OH, and P-OH located outside the microcrystalsas well as on internal d e f e c t ~ . ~Nevertheless, ~~J~ the concentration of these OH groups isvery small compared with that of the bridged
Letters
The Journal of Physical Chemistry, Vol. 97, No. 31, 1993 8111
TABLE I: Wavenumbers and Symmetry of the Fundamentals
Ha+
Vibrations of H30+in C3”symmetry (ref 8) H3O+ in C, or Cl symmetry (this work) wavenumbers wavenumbers (cm-1) modes (cm-1) modes 3500-3150
m
2
O’(l610) 1
2650-2460 1700-1670 1150-1050
v.(OH); E
3678 (3570,3500) 2890 uS(OH); Ai 2475 1800-1700 6d(HOH); E 1610 6,(HOH); A1 1300-1200
u(0H6);OA’ u,(OH);A” u,(OH); A’ G.(HOH6); A” &(HOH);A’ b,(HOH6);A’
H6 proton as reported in Scheme I.
IA3500
3000
2500
2000
1500
Wavenumbers [cm- 11 Figure 3. IR reflectance spectra showing the hydration of H-SAPO-34 by exposingthesample toair at room temperature: (a) sample dehydrated at 550 OC and (b) sample a exposed to air for 5 min. 0,O’and B are the same notations of Figure 2. hydroxyls. Clearly, essentially all water molecules interact only with the bridging OH species: all spectroscopicchanges observed here therefore relate to this interaction. On the basis of NMR data reported in ref 19, and recalling that the concentration of silanol groups is negligible, we see that the concentration of bridging hydroxyls is close to that of silicon in the structure. There is a 1:l stoichiometry H20:OHfidgd. Five weaker bands (designated L bands) at 2350,2222,1961, 1840, and 1670 cm-l are also observed (Figure 1). These bands remain upon dehydrating the sample and are assignable to combination and/or overtones of lattice modes. (All spectra of Figure 1 and 2 show also a small band in the region 2400-2300 cm-1 arising from COz gas in the spectrometer. Its intensity fluctuates slightly over the period of the experiment). The sample dehydrated at 550 “C (Figure 3a) progressively rehydrates on exposure to the air, and just after 5 min (Figure 3b) all the bridged hydroxyls are consumed. The 0,O’ bands are regenerated, while the B bands are removed. The spectroscopic transformation B O,O’, and vice versa, therefore reflects the reversible dehydration-hydration process in the catalyst SAPO34 (as represented in Scheme I and Figure 3). We now focus on distinguishing between structures II and III of Scheme I in the context of the SAPO-34 catalyst. StructureLlresults from two H bonds, one involving the zeolitic proton (O1-HZ-O3) and the other one the OH group of the water molecule and the oxygen (03-H4-05) of the SAPO catalyst. When an OH group is hydrogen-bonded, the stretching frequency shifts to lower values and the band is enhanced in intensity and width.8 One of the strong bands in the 3300-2100-~m-~ range (Figures 1 and 2) could be due to bridged hydroxyls H-bonded to the water. However, it is difficult to imagine a frequency shift of OH belonging to the water (03-H4) solargeasto justify the second strong band in the 3300-2100-cm-1 range. Nevertheless, a physisorbed water molecule, like that in structure 11, should have a single deformation mode in the 1700-1600-~m-~range and could not give rise to a broad absorption in the 1800-1700-~m-~ range (Figure 2). Structure II does not therefore explain all the observed vibrations. However, these arguments are oversimplified: we cannot predict precisely the frequency, intensity, or number of
-
vibrations of six-atom rings (from 0 1 to A17) such as that shown in structure II. Structure LlI consists of H30+ adsorbed to the structural oxygens via two H bonds; the third proton does not interact with the framework. The oxonium ion H3O+ in crystalline strong acids*has a nearly flat pyramidal structure in which all the protons are equivalent. It is isoelectronic with NH3 and shows, as does ammonia, an infrared spectrum with four fundamental absorptions; from this analogy it was suggested that the oxonium ion has pyramidal C3”symmetry. The four fundamental frequencies change from one compound to another, but all the spectra are made up of two strong fundamental stretching vibrations in the ranges 3500-3150 and 2650-2460 cm-’, a weak fundamental deformation vibration in the range 1700-1670 cm-1, and a strong fundamental deformationvibration in therange 1150-1050cm-1; see Table I and Scheme I1 for details on the symmetry of these modes. If H30+is indeed adsorbed on the inner surfaces of the catalyst, we expect the following: (a) a lowering of symmetry from C3, to C,or C1 and consequently the two doubly degenerate E modes to be split. This, in turn, means that the adsorbed H30+ has three stretching and three deformation IR-activevibrations23(see Table I and Scheme I1 for details); (b) all the original vibrations to be shifted. A new stretching vibration related to the “free” OH (03-H6 in structure XI) may appear in the 3700-3500-cm-1 range, and this could explain the presence of the 0 bands (the most intense being at 3676 cm-I). The two original stretching vibrations would be shifted to lower frequency, so that the two strong absorptions at 2890 and at 2475 cm-’ can be assigned to asymmetric and symmetric stretching of the group H2-03-H4 hydrogen-bonded to the structural oxygens of SAPO-34. Deformation vibrations are normally less affected by hydrogen bonding and shifted to higher frequency? this effect may well explain the presence of bands in the 1800-1700-cm-1 range. A second deformation vibration occurs at 1610cm-’, while the third mode cannot be observed because it falls in the region of the intenselatticevibrations (lower than 1200cm-1; we have, however, seen this third absorption in transmission with very thin pellets). The maximum of this band could not be determined with precision, but it fallsz4in the range 1300-1200 cm-1. On the basis of this observation, we can explain the shoulder at ca. 2600 cm-’ as the first overtone of the strong 1300-1200-~m-~ mode (overtones of the 1150-1050-~m-~ mode of H30+have been sometimesobserved in strong acid compounds.8.25-26) It is not easy to interpret the richness of bands in the 3700-3500-~m-~region in terms of the spectroscopic analysis
8112 The Journal of Physical Chemistry, Vol. 97, No. 31, 1993
presented here which predicts only one absorption. But our analysis is based on a simplified model: Hjot ions in a uniform acidic environment. We can, however, tentatively justify the presence of four 0 bands even though we are ostensibly dealing with a system that appears homogeneous. In reality, there are two types of bridging Brdnsted hydroxyls in HSAPO-34 with frequencies at 3626 and 3599 cm-1. These have slightly different acidities and are located in structurally slightly different sites (see Figure 3 and ref 4). Therefore, although the “free” OH of H30+does not strongly interact with thecatalyst surface, its precise frequency ofvibration will be determined by the basicity of the oxygens to which the oxonium ion is attached and the environment (cage or prism) within which it is vibrating. In summary, we have found that the oxonium ion ((10 f 1.0) X 1020 HsO+ g-l) is formed in stoichiometric amounts when HSAPO-34 is exposed to water vapor at room temperature. Its precise location and disposition with respect to the bridging Brdnsted site cannot be conclusively deduced from IR measurements alone. In this regard 1Hand ZH MADNMR*Q’or neutron scattering studies28of the deuterated analoguesare likely to shed greater light on the structure which is not amenable to X-ray analysis.
Note Added in Proof. Since this work was completed, van Santen et al. (private communication) have argued that adsorbed water may not form H30+ions, their spectra being interpretable by Fermi resonance. We believe our spectra are more fully interpreted in terms of H30+formation. Acknowledgment. We thank Profs. Zecchina and Coluccia for fruitful discussion. L. Marchese thanks the Italian CNR for a grant to work in the Davy Faraday Laboratory. We aregrateful to SERC for general support and Unilever Plc for a maintenance grant to J. Chen.
References and Notes (1) Chu, C. T.-W.; Chang, C.
D.J. Phys. Chem. 1985,89, 1569.
Letters (2) Earl, W. L.;Fritz, P. D.;Gibson, A. A. V.; Lunsford, J. H. J. Phys. Chem. 1987,91, 209. (3) Jentys, A.; Warecke, G.;Dcrewinski, M.; Lercher, J. A. J. Phys. Chem. 1989,93,4837. (4) Zubkov, S . A.; Kustov, L. M.; K a ~ a n ~ k V. y , B.; Girnus, I.; Fricke, R. J. Chem. Soc., Faraday Trans. 1991,87,897. (5) Borade, R. B.; Clearfield, A. J. Phys. Chem. 1992, 96, 6729. (6) Sauer, J.; Schroeder, K. P.; Catlow, C. R. A,; Thomas, J. M. Chem. Phys. Lerr. 1992, 188, 320. (7)
Zecchina,A.;Bordiga,S.;Spoto,G.;Scarano,D.;PetriniG.;Lcofanti,
G.; Padovan, M.; Otero Arch, C. J. Chem. Soc., Faraday Trans. 1992,88, 2959.
(8) Schuster, P.; Zundel, G.; Sandorfy, C. The Hydrogen Bond; North-Holland: Amsterdam, 1976; Vol. 11. (9) Corma, A.; Agudo, A. L.;Fornes,V. J. Chem.Soc., Chem. Commun. 1983, 942. (10) Brown, G. M.; Noe-Spirlct, M. R.; Busing, W. R.; Levy,H. A. Acra Crysrallogr. 1977, 833, 1038. (1 1) Ono, Y. In Perspectives in Catalysis;Thomas, J. M., Zamaraev, K. I., Us.; Blachvell Science Publications and IUPAC: Oxford; 1992; p 440. (12) Parker, L.M.; Bibby, D. M.; Burns, G. R. Zeolites 1993, 13, 107. (13) Sauer, J.; Hom, H.; H a m , M.; Ahlrichs, R. Chem. Phys. Lett. 1990, 173, 26. (14) Kramer, G.;van Santen, R.A. J. Am. Chem. Soc. 1993,115,2887. (15) Sauer, J. Chem. Rcu. 1989,89, 199. (16) Catlow, C. R. A.; Thomas, J. M. Philos. Trans. R. Soc. London 1992, 341, 255. (17) Grady, M. C.; Gorte, R. J. J. Phys. Chem. 1985,89, 1305. (18) Zamaraev, K. I.; Stepanov, A. G.; Thomas, J. M. Coral. Lerr. 1992, 13, 407. (19) Zibrowius, B.; Lofflcr, E.; Hunger, M. Zeolites 19!?2, 12, 167. (20) Rabo,J. A.; Pellet, R. J.; Coughlin, P.K.;Shamshon, E. S . In Zeolites
as Caralysrs and Detergent Builders; Karge, H. G., Wcitkamp, J., Eds.; Elsevier: Amsterdam, 1989; p 1. (21) Xu,Y.; Grey, C. P.; Thomas, J. M.; Cheetham, A. K. Catal. Lett. 1990,4, 251. (22) Lok, B. M.; Measina, C. A.; Patton, R. I.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. US Patent 4, 440,871, 1984. (23) Nakamoto, K. IMrared and Rmnan Spectra of Inorganic and Coordination Compounds; John Wiley and Sons: New York, 1986. (24) Marchese, L.;Thomas, J. M.,et al. Unpublished results. (25) Femso, C. C.; Homing, D. F. J. Chem. Phys. 1955,23, 1464. (26) Gigutrc, P. A,; Madec, C. Chem. Phys. Left. 1976, 37,569. (27) Thomas, J. M.; Klinowski, J. Adu. Carol. 1985, 33. 199. (28) Cheetham, A. IC; Wilkinson, A. P. Angew. Chem., Inr. Ed. Engl. 1992, 32, 1557.
