4778
J . Phys. Chem. 1985,89,4778-4788 at 1:1 and 2: 1 molar ratios, respectively. The increase of viscosity when the Mn2+ concentration increased, at a constant P2VPy concentration, was due to an expansion of the polymer in solution.
by assuming a linear dependence of T, from q / T . Small Mn2+ complexes (probably solvated Mn2+) were trapped in Ycagesnof the polymeric structure in solution. These complexes retained their mobility, in spite of the high viscosity of the system, because they did not interact directly with the polymer ligand sites. Pyridine probably interacted with Mn(I1) in the second solvation shell (outer-sphere complexation). However, a very small fraction of Mn(I1) was evaluated to interact either with Py or with P2VPy (Table 11). Py and P2VPy interacted with Mn2+preferentially
Acknowledgment. Thanks are due to the Italian Council for Research (CNR) "Piani Finalizzati Chimica Fine e Secondaria", and to the Minister0 Pubblica Istruzione (MPI) for financial support. Registry No. Py, 110-86-1;PZVPy, 25014-15-7; Mn2+, 16397-91-4.
The Nature of the Nonframework Aluminum Species Formed during the Dehydroxylation Of H-Y R. D. Shannon,* K. H. Gardner, R. H. Staley, Central Research and Development Department, Experimental Station,' E. I . du Pont de Nemours & Company, Wilmington. Delaware 19898
G. Bergeret, P. Gallezot, and A. Auroux Institut de Recherches sur la Catalyse C.N.R.S.,69626 Villeurbanne. Cedex, France (Received: March 11, 1985)
Radial distribution functions were obtained for zeolite H-Y and H-Y dehydroxylated under vacuum at 650 "C and in steam at 890 OC from X-ray intensity data collected with Mo Ka radiation. Subtraction of the RDF of H-Y from that of H-Y-DHV provided residual RDF patterns with peaks at 1.9,2.1, 2.9, 3.4, 4.1, 4.8, and 5.8 A which are believed to correspond to the species resulting from dehydroxylation and dealumination of the framework. Calculation of RDF patterns for a variety of aluminum oxides and A1,0,(OH),(H20), groups in numerous hydroxides, sulfates, silicates, and phosphates shows many of these distances can be associated with characteristic geometric A1-0, AI-A1, or 0-0 configurations. Peaks at 1.9, 2.9, 3.4,4.8, and 5.8 are characteristic of edge-shared octahedral groups which are found in many aluminum oxides and hydroxides. The weak intensity of the 3.4-A peak relative to the 2.9-A peak rules out y-A1203and 8-A1203,both of which contain tetrahedral Al. The peak at 4.1 A is found so far only in y-AlOOH (boehmite) and results from the interaction of A1 in an octahedron in one edge-shared chain with 0 ions of octahedra in adjacent chains. Based on this peak and the general fit of the entire residual RDF with the calculated RDF of boehmite, we propose the formation of a boehmite-like nonframework A1 species in highly dehydroxylated H-Y. The presence of an OH infrared band at 3670-3680 cm-' in pseudo-boehmite, which agrees with the extra lattice aluminum OH band found in dehydroxylated H-Y at 3660-3700 cm-', supports the identification of the H-Y nonframework A1 species as a boehmite-like species. Acid site distributions obtained by NH, microcalorimetry show that dehydroxylation results in the destruction of most of the medium strength acid sites (75-140 kJ/mol) and replacement by fewer but stronger (150-180 kJ/mol) sites. This acid site distribution is similar to that found in y-Al,03 heated to 600 "C and suggests that the boehmite-like species has developed surface defects like y-A1203or that remaining Bronsted acid strength has been modified by the presence of the boehmite-like phase.
extracted from zeofite frameworks during deammoniation and dehydroxylation of the NH,-exchanged forms. The process has been described in several reviews.'-3 Although A1 removal can be effected by acid or EDTA treatments and exposure to gaseous HCI or SiC14,this paper deals only with removal of A1 by thermal treatment of NH, zeolites. The loss of A1 from the framework and the development of octahedral A1 is evidenced by (1) infrared spectroscopy from shifts in the IR T-0 stretching frequencies,"' (2) X-ray fluorescence spectroscopy from shifts of the Si KB and AI KP lines;'*'3 ( 3 ) X-ray diffraction from the decrease in cell d i m e n s i o n ~ ; ~(4) ~ ~X-ray . ~ J ~ structure analyses of dehydroxylated Y;I4( 5 ) 29SiNMR from increases in the Si/A1 ratio;I5-l7 and ( 6 ) 27AlN M R from observation of spectra arising from octahedral A1 species.'8-2' Removal of A1 from the framework has been observed in Y , mordenite, erionite, and ZSM-5, but it probably occurs in all zeolites. Although dealumination in acid solutions or by vapor-phase reaction with SiC14can result in removal of AI from the channels 'Contribution No. 3628
0022-3654/85/2089-4778$01.50/0
(1) Kerr, G. T."Molecular Sieves"; American Chemical Society: Washington, DC, 1973; Adv. Chem. Ser. No. 121, p 219. (2) McDaniel, C. V.; Maher, P. K. "Zeolite Chemistry and Catalysis", Rabo, J. A., Ed.; American Chemical Society: Washington, DC, 1973; Monograph 171, p 285. von Ballmoos, R. "The I9O Exchange Method in Zeolite Chemistry"; Salk-Sauerlander Verlag: Frankfurt, 1981. (4) Flanigan, E. M. American Chemical Society: Washington, DC, 1976; Monograph 17 1, p 80. (5) Scherzer, J.; Bass, J. L. J. Catal. 1973, 28, 101. (6) Pichat, P.; Beaumont, R.; Barthomeuf, D. J . Chem. Soc., Faraday Trans. I , 1974, 70, 1402. (7) Eberly, P. E.; Laurent, S. M.; Robson, H. F. U S . Patent 3 506 400, 1970. (8) Lohse, U.; Alsdorf, E.; Stach, H. Z.Anorg. Allgem. Chem. 1978,447, 64. (9) Weeks, T. J.; Angell, C. L.; Bolton, A. P. J . Caral. 1975, 38, 461. (10) Kuhl, G. "Proceedings of the Third International Conference on Molecular Sieves", Leuven University Press: Leuven, 1973; p 227. (11) Kuhl, G . J . Phys. Chem. Solids 1977, 38, 1259. (12) Patton, E. M.; Flanigan, E. M.; Dowell, L. G.; Passjoa, D. E. "Molecular Sieves 11"; American Chemical Society: Washington, DC, 1977; p 64.
(3)
0 1985 American Chemical Society
Dehydroxylation of H-Y mation on the nature of the NFA has been obtained from IR spectroscopy, X-ray diffraction, absorption studies, and NMR. Infrared spectra in the O H region frequently show weak bands which appear after dehydroxylation. These bands, observed so far, for e r i ~ n i t e ,~~f ~f r. e~ t~i t e ,m~ ~ r, d~e~n i t e and , ~ ~ Y5922926-29 are generally believed to result from O H groups associated with either the NFA species26or vacancies created during the A1 There has been no unequivocal identification of these bands, but the species A1(OH)2+,Al(OH)2+, A10+, [A12020H]+, [A120I4+, AlOOH, and A1(OH)311,1492632'35 have been proposed. Recently, 27AlN M R studies have indicated that the NFA species in Y contain octahedrally coordinated A1 and are located in large cavities after deammoniation at 300 "C and are distributed in both the large cavities and the sodalite cages after treatment at 500 0C.20 The possibility of the presence of -y-A1203was suggested by Klinowski et al.I5 and Freude et aLzo A distinction between low-condensed mobile NFA species formed at T = 300-500 "C under shallow-bed conditions and higher-condensed, immobile species formed at T > 500 "C under deep-bed or moist conditions was made by Freude et al.19 and Raatz et al.13 The development of Lewis acidity in zeolites was first ascribed by Uytterhoeven, Christner, and Hall36to three-coordinated A1 remaining in the framework after dehydroxylation. However, X-ray fluorescence"J2 and 27AlN M R studies's-20 ruled out the presence of 3-coordinated A1 and B e r a t ~with , ~ ~ theoretical calculations, observed that tricoordinated AI and Si are weaker Lewis acids than A13+,A1(OH)2+, or AI(OH)2+. Thus, it was recently suggested by Jacobs and Beyer that the nonframework aluminum species formed during dehydroxylation may be responsible for Lewis acidity generated on high-temperature treatment of zeolite~.~~
(13) Raatz, F.; Freund, E.; Marcilly, C. J . Chem. Soc., Faraday Trans. 1 1983, 79, 2299.
(14) Maher, P. K.; Hunter, F. D.; Scherzer, J. "Molecular Sieve Zeolites"; American Chemical Society: Washington, DC, 1971; Adv. Chem. Ser. No. 101. o 266. (1'5) Klinowski, J.; Thomas, J. M.; Fyfe, C. A.; Gobbi, G. C. Nature (London) 1982, 296, 533. (16) Enaelhardt, G.; Lohse, U. F.; Samoson, A,; Magi, - M.; Tarmak, M.; Lippmaa, E. Zeolites 1982, 2, 59. (17) Engelhardt, G.; Lohse, U.; Patzelova, V.; Magi, M.; Lippmaa, E. Zeolites 1983, 3, 233. Zeolites 1983, 3, 239. (18) Bosacek, V.; Freude, D.;f Frohlich, T.; Pfeifer, H.; Schmiedel, H. J . Colloid Interface Sci. 1982, 85, 502. (19) Freude, D.; Frohlich, T.; Pfeifer, H.; Scheler, G.Zeolites 1983,3, 171. (20) Freude, D.; Frohlich, T.; Hunger, M.; Pfeifer, H.; Scheler, G. Chem. Phys. Lett. 1983, 98, 263. (21) Jacobs, P. A,; Tielen, M.; Nagy, J. B.; Debras, G.; Derouane, E. G.; Gabelica, Z., to be submitted for publication. (22) Best, D. F.; Larson, R. W.; Angell, C. L. J . Phys. Chem. 1973, 77, 2183. (23) Ione, K. G.; Paukshtis, E. A.; Mastikhin, V. M.; Stephanov, V. G.; Nefedov, B. K.; Yurchenko, E. N. Izu. Akad. Nauk SSSR 1981, 8, 1717. (24) Wu, E. L.; Whyte, T. E.; Venuto, P. B. J . Catal. 1971, 21, 384. (25) Occelli, M. L.; Perrotta, A. J. "Intrazeolite Chemistry", Stucky, G. D., Dwyer, F. G., Ed.; American Chemical Society: Washington, DC, 1983; ACS Symp. Ser. No. 218, p 21. (26) Ward, J. W. J . Catal. 1970, 18, 348. (27) Jacobs, P.; Uytterhoeven, J. B. J . Catal. 1971, 22, 193. (28) Jacobs, P. A.; Uytterhoeven, J. B. J . Chem. SOC.,Faraday Trans. 1 1973, 69, 373. (29) Peri, J. B. "Catalysis", Proceedings of the 5th International Congress on Catalysis, Hightower, J., Ed.; American Elsevier: New York, 1973; p 329. (30) Kerr, G. T. J . Catal. 1969, 15, 200. (31) Breck, D. W.; Skeels, G. W. "Proceedings of the 6th International Congress on Catalysis"; Bond, G. C., Wells, P. B., Tompkins, F. C., Ed.; The Chemical Society: London, 1976; Vol. 2, p 645. (32) Kuhl, G. "Molecular Sieves 11". Katzer, J. R., Ed.; American Chemical Society: Washington, DC, 1977; ACS Symp. Ser. No. 40, p 96. (33) Breck, D. W.; Skeels, G. W. 'Proceedings of the 5th International Conference on Zeolites", Rees, L. V. C., Ed.; Heyden Press: London, 1980; p 335. (34) Beran, S.; Jiru, P.; Wichterlova, B. J . Phys. Chem. 1981, 85, 1951. (35) Barrer, R. M.; Klinowski, J. J . Chem. SOC., Faraday Trans. I 1975, 690. (36) Uytterhoeven, J. B.; Christner, L. G.; Hall, W. K. J . Phys. Chem. 1965.69, 2117. (37) Beran, S . J . Phys. Chem. 1981, 85, 1956.
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4119 To obtain further knowledge of the nonframework A1 species, we have performed radial electron distribution and NH3 microcalorimetric studies of H-Y and H-Y dehydroxylated under vacuum and in steam. These studies suggest that the NFA species probably contain edge-shared octahedral Al,(O,OH), chains and, of the known A1 oxides, is most similar to Y-AlOOH, boehmite.
Experimental Section A sample of Na-Y was exchanged eight times with 1 N NH4C1 to yield NH4-Y containing 0.99% Na (dry basis). Activation was carried out in a U-shaped quartz cell containing two 5-cm2 frits. Five grams of NH4-Y was heated in flowing O2(5 "C/min) from 25 to 300 "C to eliminate all hydrocarbons. The sample was then evacuated and held for 15 h at 300 "C to obtain H-Y. The quartz cell was opened in a glovebox where 1 g of zeolite was transferred to a controlled atmosphere cell used to collect X-ray intensity data. The zeolite powder was pressed into a rectangular cavity in the inside of the back of the X-ray cell which was filled with Ar before removal from the globebox. Vacuum dehydroxylated Y (HY-DHV) was prepared by transferring the H-Y used for data collection back to the U-tube, and heating the sample at 650 "C for 6 h in torr vacuum. In order to minimize possible dealumination by escaping NH3 and H 2 0 , care was taken to heat the sample in a shallow-bed (1 mm thick) configuration and to increase the temperature slowly from 25 to 650 OC under vacuum. The sample was then retransferred to the X-ray sample holder in the glovebox. The X-ray pattern indicated some loss of crystallinity. A further sample of NH4-Y containing 2.2% Na was prepared for steam dehydroxylation. NH4-Y was heated under 100% H 2 0 at 600 "C, reexchanged with NH4Cl to give 0.26% Na, and reheated in 100% H 2 0 at 890 "C to give steam dehydroxylated Y (H-Y-DHSt) .39 The X-ray diffraction patterns were recorded with a stabilized Philips diffractometer using a 2400-W tube with molybdenum anode (X(Mo Kcu) = 0.710 A). The X-ray cell equipped with a 180" beryllium window was mounted on the 0 shaft of the Norelco goniometer, and the diffraction patterns were taken by reflection on the pressed zeolite sample. The diffracted intensities were monochromatized with a diffracted beam monochromator (curved quartz crystal with a narrow passband) which eliminates most of the Compton scattering. The Compton intensity left at small Bragg angles is very weak compared to the overall diffracted intensity. Radial distribution functions (RDF) calculated from the intensities measured with this setup were checked to be similar to those obtained with a Zr-filtered beam from which the calculated Compton intensities have been subtracted. However, the electron density peaks were better resolved with the present arrangement. The diffracted intensities were collected from 1 to at least 130" (20) by scanning with 0.125 (20) steps. Data collection required about 7 days (10 min per step) to obtain good counting statistics over the whole range of Bragg angles. The intensities were corrected for polarization and set on an absolute scale (electron2 units) by matching them at high Bragg angles to the calculated independent scattering. The RDF's were calculated by Fourier transformation of the scaled intensities as previously described.40 Heats of adsorption of ammonia were measured in a Setaram Tian-Calvet high-temperature microcalorimeter maintained at 148 "C. The volumetric system previously described41 was modified to allow continuous pressure measurement using a Datametrics Barocel gauge. The volumetric system consists of two parts: (1) part A (V = 1037 cm3) containing a Mac Leod gauge (covering P = 10-4-3 torr) and the Barocel gauge ( P = 10-5-2 torr) and (2) part B (V,+, = 1710 cm3) containing the two quartz cells and isolated from the samples by a trap containing a mixture of C02(s)and acetone to protect the sample from vapors (38) Jacobs, P.; Beyer, H. K. J . Phys. Chem. 1979, 83, 1174. (39) Mauge, F.; Auroux, A.; Gallezot, P.; Courcelle, J. C.; Engelhard, Ph.; Grosmangin, J. 'Catalysis by Acids and Bases", Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1985; pp 91-99. (40) Gallezot, P.; Bienenstock, A.; Boudart, M . Nouo. J . Chim. 1978, 2 , 263.
4780 The Journal of Physical Chemistry, Vol. 89, No. 22, 1985
emanating from stopcock grease. The calibration of the volumetric cell was carried out with N H 3 under the same conditions of T and P as the adsorption experiments. The volume of part A, maintained at 295 f 0.5 K, was determined with a gas buret at different pressures and for different torr, gases. The volume of part B, previously evacuated to was determined from a known quantity of gas contained in part A. The microcalorimeter signals were obtained by a galvanometric amplifier (Sefram Amplispot) and a galvanometric recorder (Sefram Graphirac). The calorimeter calibration was carried out with a Pt resistance element (100 ohm) as previously described.4i A sample of NH4-Y (approximately 50 mg) was evacuated at 300 OC for 6 h to provide a sample of H-Y. H-Y-DHV was prepared by evacuation at 650 OC for 16 h prior to the NH, calorimetric measurements. Adsorption experiments were carried out using NH, dried and deoxygenated over N a spaghetti and further purified by freeze-thaw-pump cycles over dehydrated zeolite Y. Heats of adsorption were measured for successive doses of NH3 of - 0 , 1 4 4 mequiv/g for H-Y and -0.05-0.2 mequiv/g for the H-Y-DHV, pseudoboehmite, and 7-AI2O3derived from Catapal SB up to a final pressure of 1-2 torr. Infrared experiments were performed with a Nicolet 3600 FTIR spectrometer. Neat, IO-mg sample pellets of 1-cm diameter were prepared and mounted in the spectrometer in an all-metal vacuum system having a base pressure below 1 X lo-* torr. The samples were contained inside the vacuum system in a quartz oven capable of heating the sample to 700 OC. Infrared studies were performed on two synthetic pseudoboehmites. The first, Catapal SB, is a product of Conoco Chemicals containing -26% H 2 0 and of crystallite size 35-45 A. The second, Baymal, is a colloidal fibrillar AlOOH of crystallite size 50 8,diameter X 1000 8,containing -30% H 2 0 and 10% acetic acid.42 Calculations of the RDF's of known topologies were made using the computer program RADMOR (D'Antonio and K ~ n n e r tafter ~~) modification to run on a VAXS 11/780 computer. The program is described in detail in ref 43 but we include here a brief description of the quantities calculated and parameters used in our calculations. RADMOR calculates the following expression:
Shannon et al. 1600
ra
l'-i
I
I
I
I
L-
1200
I 1 /' ' 4.39
I
1000
800
I
I
t
A
c
[E]
?{ 4.315
-.. 1000
-
1
4 d r )=
n m
wlwf$ exp[-(r
-c c rl=lJ=l
- r,J)2/2al;(r)]Ar
rija,j(r)( 2 ~ ) ' ' ~
where 47rp(r) is the probability, weighted by the scattering power of the atomic pair ij,cff,), of finding atoms j in a shell of thickness Ar and radius r,, from origin atoms i. w, is the occupancy of atom i, 1; = ~;(s=O)/~~(S=O)]'/~is the reduced, scattering angle independent, scattering factor for atom i, rrJis the distance between atoms i and j and a,(r) includes both the rms displacement along rlJand positional disorder. The first sum is carried out over the n atoms in the asymmetric unit for a crystalline material. The second sum is carried out over the m atoms in the structure that are separated from an atom in the asymmetric unit by less than the maximum distance selected for the calculation. For crystalline materials, space group transformations and unit cell translations are used to generate the atom positions in the second sum. The pair correlation function, G ( r ) ,for a spherical region of radius t is defined as
G ( r ) = 47rrMr) / 4 r , R ) - POI 4 r J ) where for spherical particles c(r,R) IS given by c(r,R) = 1.0
- 3 / ( r / R ) + !l16(r/R)3for r I2R
t(r,R) = 0
-2ooL
'
I
"
' r, i
"
'
I
'
I
6
Figure 1. RDF's for H-Y: (a) experimental (solid) and calculated (dashed) RDF's for H-Y and (b) experimental RDF H-Y-DH.
spherical particle. In the case where the input model is a fragment, 47rp(r) must be modified by c(r,R) to compensate for the finite size of the model relative to an infinite crystal. The disorder parameters uv = 0.05 at 1.5 8, and 0.19 at 10 8, used for the AI,O,(OH),(OH,), clusters were based on experimental values for SiO, glass obtained by Konnert and Karle" and Konnert et al.45 Larger disorder parameters of 0.12 at 1.5 8,and 0.19 at 10 8,were chosen for the aluminum oxides and hydroxides in order to provide agreement with the experimental RDF's of a- and y A 1 2 0 3found by Larue et For calculation of the H-Y RDF, the program RADMOR was modified to accept individual temperature factors according to Taylor's scheme.47 Temperature factors B(Si) = 0.8 and B ( 0 ) = 3.0 and aij = 0.0 at 1.5 A and 0.01 at 3.1 8,were used for H-Y RDF. The RDF's of the crystalline forms of A1 oxides and hydroxides were calculated from published atomic coordinates by assuming spherical particles 10 A in diameter. The coordinates of specific A1,0, clusters such as A106 and Al2OIowere obtained from structures containing these fragments; these clusters were treated as individual particles. We have plotted r G(r) vs r for the above materials.
for r L 2R
When the input model is an infinite crystal t(r,R) = 1 and c(r,t) corrects for the falloff in scattering near the diameter of a finite (41) Gravelle, P. C . Adu. Catal. 1972, 22, 191. (42) Bugosh, J.; Brown, R. L.; McWhorter, J. R.; Sears, G. W.; Sippel, R. J. Ind. Eng. Chem. Prod. Res. Deu. 1962, 1, 157. (43) D'Antonio, P.; Konnert, J. H. J . Appl. Crystallogr. 1980, 13, 459.
(44) Konnert, J. H.; Karle, J. Acta Crystallogr., Sect. A 1973, 29, 702. (45) Konnert, J. H.; D'Antonio, P.; Karle, J. J . Non Cryst. Solids 1982, 53, 135. (46) Larue, J. F.; Moraweck, B.; Renouprez, A. 'Proceedings of the International Conference on EXAFS and Near Edge Structure", Frascati, Italy Sept 13, 1982, Bianconi, A., Incoccia, L., Stipcich, S., Ed.; Springer-Verlag: West Berlin, 1983. (47) Taylor, M. J . Appl. Crystallogr. 1979, 12, 442.
Dehydroxylation of H-Y
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4781 TABLE I: RDF Spectra for Nonframework AI SDecies in H-Y ~
d, A . H-Y-DHV
2.87 3.39 4.12 4.82 5.79
H-Y-DHST
assignment
1.86 2.10 2.91 3.35 4.17 4.84
AI-0, Al-Ob, AI-AI, 0-0 AI-0 A1-0 AI-AI, 0-0, AI-0 AI-0, 0-0
2 and Table I that a similar residual species is present in both H-Y-DHV and H-Y-DHSt. Calculated RDF Patterns. To help identify the NFA species, we have calculated the partial RDF's from compounds containing A1,0m(OH)p(H20), groups and RDF's from a variety of aluminum oxide and hydroxide species whose atomic coordinates are known. Table I1 lists the compounds used for the calculations as well as the characteristic A1,Om(OH),(H2O), structural unit present in each compound. Because Al(1V)-Al(1V) and Al(1V)-Al(V1) interactions give rise to strong RDF peaks at 3.2-3.4 A (see y-A1203,8-A1203,and tohdite in Figure 4 and ref 53 and 54) and because this peak is weak in Figure 2, we assume that the NFA species does not contain a significant proportion of AI(1V) and thus have not calculated RDF's for such species. However, we have included A11304(OH)24(H20)12 because of the presence of this species in s o l ~ t i o n . ~Figure ~ ~ ~ 3~ contains RDF's from clusters having increased degrees of condensation from an isolated AlX6 octahedron Al2Xl, A13X14 Al4XI6 A&X,S A113X40, where X = 0, OH, or H20. The geometry of some of these clusters has been discussed by Moores7~58 and Wellss9 A12X10groups occur as isolated species and as part of infinite chains. Cis and trans Al3XI4chains occur only as part of infinite chains, not as isolated species. The A14(OH)16group of Ba2A14(OH)16occurs as isolated species whereas the Al4OI5and A14016clusters of dumortierite and the Al4XI, of aluminite share edges with other groups. The AlI3XA0 cluster has been found in the basic sulfate and selenate and in basic A1 solution^.^^^^^ As might be anticipated from an analysis of primarily edgeshared octahedral clusters, similar peaks occur in most species and are summarized at the bottom of Table 111. The most prominent peaks occur at 1.9 A (Al-O), 2.8 A (0-0, A1-Al), 3.5 A (Al-O), 3.8 A (0-0),4.4 A (Al-O), 4.8 A (0-0),5.5 A (W), and 5.9 A (A1-0) where the values in parentheses indicate the atom pair interactions. Octahedral distortions can shift the positions to combine the pairs of peaks at 3.5 and 3.8 A, at 4.4 and 4.8 A, and 5.5 and 5.9 8, to give simpler RDF spectra. AI-AI vectors get stronger as the number of Al atoms per cluster increases and change the relative intensities (e.g. see the A1-A1 peaks in y-Al*O,). It is instructive to look at the differences in the RDF spectra as well as the similarities. As already noted, species containing Al(1V) show a strong peak at 3.2-3.4 A (y-A1203,8-A1203,tohdite, and A1&40). The two species with shared faces show a peak or a shoulder at 2.5-2.6 A because of the short A1-A1 distance across the face. The octahedral Al(OH2)6species shows a particularly simple pattern. The A12X10species are similar except for A1208(OH)2 of NH4A120H(P04)2-2H20 where the AI-0 and 0-0 peaks are
- - - - -
(48) (49) (50) (51) (52)
Gallezot, P.; Imelik, B. J . Chim. Phys. 1971, 68, 816. Lohse, U.;Engelhardt, G.; Patzelova, V. Zeolites 1984, 4, 163. Vega, A., personal communication. Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751. Kerr, G. T. J . Catal. 1982, 77, 307.
(53) Glidewell, C. Inorg. Chem. Acta 1977, 25, 77. (54) Leonard, A. J.; Semaille, P. N.; Fripiat, J. J. Proc. Brit. Ceram. SOC. 1969, 13, 103. (55) Johansson, G. Acta Chem. Scand. 1960, 14, 771. (56) Baes, C. F.; Mesmer, R. E. "The Hydrolysis of Cations"; Wiley: New York, 1976. (57) Moore, P. B. "Aluminum (13-A. Crystal Chemistry) Handbook of Geochemistry"; Springer-Verlag: Berlin, 1972; pp II/3 13AI-13A24. (58) Moore, P. B. In "Proceedings of the International Congress on Phosphorus Compounds", 1980; Vol. 11, pp 105-130. (59) Wells, A. F. "Structural Inorganic Chemistry"; Clarendon Press: Oxford, 1975; Chapter 5.
4782 The Journal of Physical Chemistry, Vol. 89, No. 22, 1985
Shannon et al.
TABLE Ik Structures Used for ComDarison with NFA RDF compd structure type structural unit Aluminum Oxides and Hydroxides gibbsite AI(oH), layers bayerite A1(OH)3 layers diaspore double rutile chains boehmite corrugated octahedral spinel defect-spinel P-Ga2o3 P-Ga203 corundum a-A1203 tohdite tohdite
ref" 74 78 79 77 84 70 69 69
Z E K R D Z 139,129 Z E K R D Z 148,255 PCMIDU 5,179 MRBUAC 12,1213 BAUR(DLS)UNPUBL BCSJA8 43,2487 INOCAJ 8,1985 BCSJA8 42.2247
~10, C S A I ( S O ~ )1~2.H 2 0 (Cs-alum) AI2010 BaA1,P208(OH)2 Cjagowerite) NH4AI2OH(PO4)2+2H20 Ba2A12(0H),o [A12(OH)2(H20)81 (S04)2*2H20
66 ACBCAR 21,383 74 84 70 62
AMMIAY 59,291 ACCCAR 40,1641 ACBCAR 26,867 ACSAA4 16,403
79 63 68 76
A M M I A Y 64,573 Z E K R D Z 118,337 A M M I A Y 53,1882 A M M I A Y 61.831
A13014
AI2SiOS (andalusite) A12SiOS(kyanite) Ca2Al3Si3Ol20H(clinozoisite) CaA1Si040H (vuagnatite) A13014 NaA1Si206 A12(OH)2Te03S04 CaA17(0H)dSiOd -. . . (chantalite) . A140
73 AMMIAY 58,594 76 ACBCAR 32,407 79 Z E K R D Z 150,53
I5
Si3BAI,018(dumortierite)
78 N J M I A K 132.231 72 ACBCAR 28,519 78 ACBCAR 34.2407 78 N J M I A K 132,231
A11304(0H)24(0H2)
62 ARKEAD 20,305
12
"Codens for periodical titles, Vol. 11, ASTM Data Series DS 23B, Philadelphia, 1970.
TABLE 111: Peak Positions in Calculated Partial RDF's for AI,O,(OH),(OHz)q peak position, 8, compd
AI-0
0-0
A1(0H2)6 '4I,O8(OH)2 A1208(OH), A12(0H)IO AI2(OH),(OH2)8 A13014
Cs-alum jagowerite NH4AIZOH H20(PO,)yH,O Ba2A12(0H)lo basic AI sulfate andalusite
2.70 2.70 2.70 2.75 2.75 2.80
A13014
A14015
kyanite clinozoisite vuagnatite jadeite A12(OH)2Te03S04 chantalite dumortierite-A
1.90 1.90 1.90 1.90 1.90 1.90 2.05 1.95 1.90 1.90 2.00 1.90 1.90 1.90
2.75 2.70 2.75 2.80 2.80 2.80 2.80
Ai4(0H)16 A14(OH)l,(OH2)6
Ba2Ah(oH)16 aluminite
1.95 1.90
2.80 2.80
1.95 1.80 1.90
2.75 2.80
0.10 (1.92)
0.10 (2.76)
ALOn(OH)p(OH2)q
AI~OIO(OH)~ A~@Io(OH)~ A13014
A13010(0H)4 AI306(OH)8
A140 I6 dumortierite-A' A11304(OH)24(OH2)12basic AI selenate
range mean value
" Al(1V)-Al(1V).
AI-0
0-0
AI-0
0-0
0-0
3.00 3.00 3.05 2.90 2.80
3.60 3.35 3.60 3.55 3.50
3.80 3.80 3.85 3.85 3.80 3.75
4.45 4.50 4.60 4.40 4.35
4.90 4.65 4.90 4.90 4.85
2.80 2.80 2.90 3.05 3.00 3.00 2.55 2.90 3.00 3.00
3.40 3.40 3.45 3.70 3.60 3.55 3.35 3.90 3.55 3.50 3.70 3.40 3.70
3.85 3.80 3.85 3.85 3.80 3.80 3.85
4.40 4.40 4.45 4.55 4.40 4.45 4.30
3.85 3.80 3.40 3.80 3.80
0.25 (3.54)
0.10 (3.80)
AI-AI
2.90 2.95 3.40Q 0.25 (2.94)
AI-0
0-0
5.40 5.65 5.65 5.50 5.60
6.00
6.15 6.20 6.25 6.15 6.10
4.75 4.75 4.85 5.00 4.85 4.85 4.80
5.60 5.60 5.70
5.90 5.90 6.00
5.50 5.30
6.00 6.00 5.60
4.55 4.50
4.90 5.00
5.65
6.10
6.25 6.10
4.35 4.45
4.75 5.00 5.00b
5.40
5.60
6.00 6.20
0.25 (4.44)
0.35 (4.85)
0.40 (5.55)
6.00
6.10 6.25 6.20 6.10
5.85b 0.50 (5.91)
0.15
bAI-AI.
strongly split, probably because of the unusually short A1-0 distance arising from the A1 of one octahedron and the apical 0 of an adjacent octahedron. The Al3XI4trans chains have prominent peaks out to 7.5 %, and are similar to Al,X,, and each other except for splittings of the 3.5-A peak in clinozoisite and kyanite. Andalusite shows a splitting of the 4.5-A peak.
The Al3XI4cis chains,60 the isolated A14(OH)16group of Ba2A1,(OH),,, and the edge-linked A14X,, groups of aluminite are all similar to each other and to A1,Xlo with no prominent peak splittings. The dumortierite A14016group shows a splitting of the (60) Shannon, R. D.; Bergeret, G.; Gallezot, P Nature (London) 1985, 315, 736.
Dehydroxylation of H-Y
rhe Journal of Physical Chemistry, Vol. 89, No. 22, 1985 4783 A106 Cs- Alum
AL4016
I 0
1
2
3
5 R
4
6
9
8
7
0
IO
I
2
3
4
5
6
IO
7
8
9
1 8
I
1
7
9
10
8
9
10
R
AL2010
0
I
I
I
I
I
I
1
2
3
4
5
6
I
I
I
1
I
L
7
8
9
IO
0
,
I
DUMORTIERITE
1
1 2
I 3
4
5
I 6
R
R
Straight Chains Al,,O,,
Basic A1 Selenate
--
0 L
'?
0
I
2
3
4
5 R
6
7
8
9
IO
0
I
2
3
4
5 R
6
7
Figure 3. Calculated RDF's for (a) AIO,, (b) AI2OIo,(c) A19Ol4, (d) AI4Ol6,(e) A140,,, and (f) Al130,0clusters.
3.5-A peak whereas the dumortierite A1,OI5 group shows an unusual continuous distribution between 3 and 6 A; both probably are caused by the linkage of these groups with other A 1 4 groups via further edge sharing.61 The spectrum of A11,0,(OH)24(OH2)12 (61) Moore, P. B.; Araki, T. N . Jb. Mineral. Abh. 1978, 132, 231.
is quite regular with little prominent splitting. We look next at the calculated RDF's from the aluminum oxides and hydroxides shown in Figure 4. The peak positions from the calcd. partial RDF's along with their associated atom-pair origin are listed in Table IVA. The oxides containing Al(IV), Le., 8-A1203,y-Al,03, and tohdite, can be eliminated as possible
4184
Shannon et al.
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985
oehmite
iaspore
1
0
0
1
2
3
4
5 R
2
3
5 R
6
7
6
I I
1
I
2
3
4
I 1
5 R
IO
9
6
7
8
9
A110015*H20Tohdite
J
IO
0
I
2
3
4
5
6
7
8
9
10
R
Figure 4. Calculated RDF's for various aluminum oxides and hydroxides.
NFA candidates on the basis of the strong Al(IV)-Al(V1) peak at 3.3 A relative to the (Al-A1, 0-0) peak at 2.9 A. We can also eliminate corundum because of the strong peaks at 3.25 and 3.45 A representing A1-0 and A1-A1 interactions. This leaves only the hydroxides, bayerite and gibbsite, and the oxyhydroxides, diaspore and boehmite, for consideration. The RDF of bayerite does not match that of the NFA in Figure 2 because of the absence of a peak at 4.1 A. Similarly, the RDF of diaspore lacks a peak at 5.8 A. The gibbsite RDF spectrum is similar to those of Figure 2 if the peaks at 3.60 ( A l a ) , 4.40 ( A l a ) , 5.00 (O-O),and 6.10 A (Al-0) were all shifted by 0.2-0.3 A. Probably the best fit, however, is obtained with the RDF of boehmite. The relative intensities match reasonably well. The most serious discrepanc is found in the peak at 4.82 A of Figure 2 which is -0.15 greater than the combined AI-0, A1-AI, 0-0 peak at 4.65 A in boehmite. Thus, we propose that an NFA species with a configuration of A1 octahedra similar to that of boehmite is probably formed in highly dehydroxylated H-Y.
1
IR Spectra of Pseudo-Boehmite. Synthetic or pseudo-boehmite Baymal and Catapal SB show a sharp peak at 3670-3680 cm-', Figure 5. Due to the small particle size of the synthetic pseudo-boehmites, the hydroxyls observed are likely mainly surface hydroxyls. The lack of strong interlayer hydrogen bonds accounts for the higher wavenumber position of the band compared to crystalline boehmite.62 Crystalline boehmite has prominent broad bands at 3300 and 3075 cm-l which have been attributed to O H groups hydrogen bonded across the boehmite double corrugated
layer^.^^**^ N H , Microcalorimetry. Plots of differential heats of NH, adsorption for H-Y, dehydroxylated H-Y, pseudo-boehmite, and 7-AI2O, are shown in Figure 6. H-Y exhibits a broad spectrum of moderately strong acid sites (100-140 kJ/mol) with a plateau ( 6 2 ) Wickersheim, K. A.; Korpi, G . K . J . Chem. Phys. 1965, 42, 579. (63) Abrams, L.; Low, M. J. D.Ind. Eng. Chem. Prod. Res. Deu. 1969, 8, 38.
Dehydroxylation of H-Y
The Journal of Physical Chemistry, Vol. 89, No. 22, I985 4785
C ATA PA C
8
1.75
w
0 Z
U
E 1.50 s:m
?
a ai
a
w 1.25
g
d
1
8 a 8
g
1.00
1
J
.75
?
a 9a
HY
i,
9 a
0
+1
A
- 300°C, VACUUM
- DH - 650°C, VACUUM y - A1203 - 450°, VACUUM HY
8 77 9 7 7
a 8 8 zz m o
5 0 . p , ,
0
m
N
2
v!
0 0
I , ,
20
, ,
I , ,
, ,
30
I , ,
,,
I , ,
, ,
50
40
,,,,, ,
60
70
, , , ,
1, 1 80
CC NH3
2
9
IO
,
V,dr =
-
Figure 6. Differential heats of ammonia adsorption on H-Y, H-Y-DHV, pseudo-boehmite, and y-Al203 at 148 O C .
7
at 140 mJ/mol; similar behavior was found by Auroux et al.64 and Masuda et al.6567although the plateau found by Masuda et al. was at 100 kJ/mol. Dehydroxylation at 650 "C produces a dramatic change in the acidity spectrum of H-Y. Most of the acid sites stronger than 75 kJ/mol have been destroyed and replaced by stronger acid sites, 150-180 kJ/mol. Pseudo-boehmite (y-A100H) exhibits very weak acid sites but upon heating and transformation to y-Al,O, develops strong acid sites.
H
*
Discussion Nonframework Aluminum Species. From Figure 2, it is apparent that a similar residual nonframework aluminum species CI
I
(64) Auroux, A.; Bolis, V.; Wiezchowski, P.; Gravelle, P. C.; Vedrine, J. J . Chem. Soc., Faraday Trans. 2 1979, 75, 2544. (65) Masuda, T.; Taniguchi, H.; Tsutsumi, K.; Takahashi, H. Bull. Chem. SOC.Jpn. 1978, 51, 1965. (66) Tsutsumi, K.; Mitani, Y . ;Takahashi, H. Bull. Chem. Soc. Jpn. 1983, 56, 1912. (67) Mitani, Y . ;Tsutsumi, K.; Takahashi, H. Bull. Chem. SOC.Jpn. 1983, 56. 1921.
4186
Shannon et al.
The Journal of Physical Chemistry, Vol. 89, No. 22, 1985 Boehmite AIOOH
Figure 7. Structure of boehmite, AIOOH (after Ewing, 1935) H-bonded oxygens connected by rods.
is present in both H-Y-DHV and H-Y-DHSt. Although the Al-0 peak at 1.9 A is only barely visible in H-Y-DHV, it is clearly visible in H-Y-DHSt. The increased intensity of the 1.9-A peak and the generally narrower peaks for H-Y-DHSt probably reflect the increased degree of condensation and crystallinity of the NFA species in steam-dehydroxylated Y. Table I summarizes the R D F peak positions from the NFA species in the two dealuminated H-Y samples. The distances present in the NFA produced at 650 OC under vacuum and 890 "C by steaming clearly correspond to a larger Al-OH cluster than the simple species listed earlier, i.e. AI(OH)2+, A1(OH)2+, or [A120I4+. Calculation of RDF's from Al,Om(OH),(HzO), edge-shared octahedral clusters show characteristic peaks at 1.9, 2.8, 3.5, 3.8, 4.4, 4.8, 5.5, and 5.9 A. The similarity of the most prominent peaks in the RDF's of these clusters with those listed in Table I suggests that the NFA species in H-Y-DHV and H-Y-DHSt contain edge-shared octahedral Al,(O,OH), chains.60 Further comparison of the NFA RDF with calculated RDF's of known aluminum oxides and hydroxides points to a strong similarity of the NFA species to the boehmite form of A100H. Although discrepancies exist in the peaks near 1.9 and 4.8 8,,we believe an NFA species with a configuration of A1 octahedra similar to that of boehmite is probably formed in highly dehydroxylated H-Y. The RDF Spectrum of Boehmite. The structure of boehmite is made up of A104(OH)2octahedra sharing 6 edges to give the corrugated layers shown in Figure 7.68s69 The layers can also be pictured as an upper network of comer-shared rutile infinite chains which share edges with an identical lower network of corner-shared chains. The 0 and O H ions are shared by 4 A1 and 2 Al, respectively, so that the 0 ions are sandwiched in between the layers and the OH'S are on the upper and lower parts of the layer. Partial RDF's showing AI-AI, A1-0, and 0-0 interactions in boehmite are shown in Figure 8 and the interatomic distances associated with the prominent peaks are listed in Table IV. The peak at 2.88 A corresponds to the AI-AI and 0-0distances in adjacent octahedra. The peak at 3.37 A corresponds to an AI of 1 octahedron and the apical 0 of an adjacent octahedron. The peak at 4.1 A arises as a result of interactions between the AI of an octahedron in the lower network of edge-shared chains and 0 ions of adjacent octahedra in the same network but different chains. This characteristic distance will then only occur in structures containing such corrugated layers as exist in boehmite. The peak at 4.81 A with a shoulder at 5 A arises from a combination of A1-0 peaks at 4.45 8, and AI-A1 and 0-0peaks at 4.8 and 5.0 A. The peak at 4.45 8, is derived from interaction between A1 in a rutile chain with 0 ions in adjacent octahedra in the chain. The AI-A1 interaction at 4.8 8, is derived from A1 atoms from the upper layer and AI atoms in lower layers in nonadjacent octahedra. The peak at 5.8 A derives from the next (68) Ewing, F. J. J. Chem. Phys. 1935, 3, 420. (69) Farkas, L.; Gado, P.; Werner, P. E. Mater. Res. Bull. 1977,12, 1213.
0
1
2
3
4
5
6
7
8
9
1
0
R
Figure 8. Total and partial RDF's for boehmite, AIOOH, showing A1-0, AI-AI, and 0-0 interactions.
nearest Al-Al interactions in the rutile chain and Al-0 interactions between A1 atoms in one layer and 0 atoms in layers above and below. Table IVB shows a comparison of the NFA RDF with the composite R D F of boehmite derived from Figure 8. The agreement of peak positions is satisfactory with the exception of the peaks at 2.1 and 4.65 8,. A boehmite cluster having dimensions of the order of the Y supercage would not be expected to have bond distances identical with those of the infinite crystal. Terminal bonds would be shorter, whereas those in the interior of the cluster might be somewhat longer. This is consistent with the observation of peaks at r = 1.90 and 2.10 8,rather than at 1.852, 1.876, and 2.010 A observed by Farkas et al.69 Because the 4.65-A peak is composed of an A1-0 peak at 4.45 A and AI-AI and 0-0peaks at -4.8 A (see Figure 8), a weakening of the 4 . 4 5 4 A1-0 peak consistent with the relatively weak A1-0 peak at 1.9-2.1 A would result in the observed shift to 4.8 8,. A further cause of the shift from 4.65 to 4.8 8, could arise from a greater contribution from the interior 0 atoms responsible for the 2.1-A AI-0 peak. Nature o f t h e Boehmite Cluster. If the boehmite-like phase giving rise to the R D F shown in Figure 2 forms in the normal 12.5-8, supercages of H-Y, it cannot have all the characteristics of boehmite because of the size restrictions of the cage. It could consist of one corrugated layer with block size 4a0 X 3c0 X b0/2 having the approximate dimensions 11.5 X 11.1 X 4.3 A. This implies no OH'S bridging the layers. The composition of such a block would be A116013(OH)22(OH2)11 where H 2 0 molecules are found at terminal sites, OH ions share 2 Al, 0 share 3 or 4 Al, and 7 O H ions have replaced HzO ligands to achieve electroneutrality according to the scheme of Baker and P e a r ~ o n . ' ~ Obviously a charged species could result from fewer OH'S replacing H20. However, Gross et al." have concluded the species present after calcination at T > 550 "C under deepbed conditions is a neutral species of the type A1203aH20. Alternatively, some of the terminal H 2 0could be replaced by zeolite 0 ions, 0,. In any case such a block would not behave exactly like boehmite because of the lack of O H ions bridging the layers. Boehmite (70) Baker, B. R.;Pearson, R. M. J . Cutul. 1974, 33, 265. U.;Engelhardt, G.; Richter, K. H.; Patzelova, V.
(71) Gross, Th.; Lohse, Zeolites 1984, 4, 2 5 .
Dehydroxylation of H-Y
Figure 9. Pictorial representation of boehmite-like cluster in zeolite Y
supercage. blocks containing 16 Al atoms on fill only -2 of the 8 supercages; Figure 9 shows a computer simulation of such a block situated near the center of a supercage. The boehmite-like phase could also form in the macropores formed upon dealumination. Such macropores have been found to he approximately 100 A” and would allow for boehmite-like NFA species quite similar to pseudo-boehmite with particle size
20-80 A?”
Boehmite and pseudo-boehmite decompose at 300-500 OC.” Lattice destruction probably occurs via breaking the interlayer OH-0 bonds and subsequent formation of y-A120,. A boehmite-like phase in H-Y-DH, existing at temperatures greater than the decomposition temperature of boehmite, might be stabilized by ( I ) the lack of OH’s bridging the layers and (2)terminal H 2 0 of pseudo-boehmite being replaced by zeolite Oz- ions. The possibility of the boehmite-like phase being on the zeolite external surface can be rejected because the boehmite phase on the exterior would have no such stabilization mechanism and would almost certainly transform to y-Al,O,. InJraredSpectra. If boehmite-like layers of AlOOH are formed by the condensation of AlOH groups, then such a species located in zeolite cavities would he expected to show characteristic O H IR band(s). There have been numerous studies of the development of NFA bands in steamed, deep-bed calcined, and ultrastable H-Y. Ward26found two bands near 3570-3590 and 3660-3670 cn-l in ultrastable Y and suggested that the higher frequency band might represent OH groups in AI(OH),, AI(OH)f, or AI(OH)z+ proposed by Kerr.)O Jacobs and Uytterhoeveuz7~zsfound a lowfrequency (LF) band at 3595-3620 cm-’ and a high-frequency (HF) hand at 3675-3700 cm-’ where the exact frequency depended on the presence or absence of exchangeable cations and the degree of dealumination. The bands were not acidic in that they did not react with NH, or pyridine. They concluded that the OH’S responsible for both the LF and H F bands were fixed to the lattice, probably at defect sites and not associated with hydroxyaluminum ions because treatment with 0.01 N NaOH did not remove the bands. They also concluded that the two bands were due to hydroxyls situated at different locations in the lattice. P e r P found similar bands in ultrastable H-Y at 3625 and 3700 cmP. However, he concluded that the H F band arose from hydroxyaluminum groups such as AI(OH)’+, probably present in the supercage because it disappeared when AI was removed by acetylacetone. More rapid exchange of D with H on OH’S giving rise to the 3700.”‘ hand than the OH’s of the 3625-cm-‘ (72) Lohse, U.; Mildcbrath, M. Z.Anorg. Allgem. Chcm. 1981,476,126. (73) Wefers, D.; Bell, G . M. “Oxides and Hydroxides of Aluminum”; Alma Technical Paper No. 19, 1972.
The Journal oJPhysical Chemistry. Vol. 89, No. 22, 1985 4787 band suggested the more accessible supercage as the site responsible for the H F band. He also found the H F band acidic because of its reaction with NH,. The nonacidic LF band was postulated to arise from the presence of AI(OH),+ where the lower frequency resulted from an interaction with the zeolite framework. Scherzer and BassS also found the LF and H F bands in dehydroxylated H-Y with their intensities increasing with calcination temperature and the moisture content of the calcination atmosphere, reaching a maximum for steam calcination. They agreed with Jacobs and Uytterhoeven and Peri that these bands resulted from OH groups formed during the dealumination proass. The bands were not affected by pyridine and thus were nonacidic or not accessible to pyridine. A shift toward higher frequencies occurred as dealumination progressed. They attributed the H F band to OH groups attached to the framework and the LF band to hydroxyaluminum groups not in the supercage. Our infrared studies support the identificationof the H-Y NFA species as a boehmite-like structure. The H-Y NFA hydroxyls observed at 3660-3670 cm-’ are evidently not strongly hydrogen bonded or they would appear several hundred wavenumbers lower in the spectrum. A slight shift to lower wavenumber compared to the band position in free particles would be expected for these hydroxyls due to weak interactions with the surrounding zeolite structure. The appearance of the H-Y NFA bands at 3660-3670 cm-l therefore agrees well with the 3670-3680-cm-’band position of hydroxyls in pseudo-boehmites supporting the identification of the H-Y NFA species as a boehmite-like species. The lack of bands at 3300 and 3075 cm-l and the associated bands at 2100 and 1980 cn-l suggests that the NFA species does not contain O H bridging between the layers and therefore the boehmite layers have a thickness less than b,/2. This suggests that the NFA species are in the supercages and not in the macropores. NH, Microcalorimetry. In order to follow the change in acid sites during dehydroxylation, framework dealumination, and the formation of the nonframework aluminum species, we have determined the differential heats of adsorption of NH, as a function of NH, coverage. It was established by Ward,” and Hughes and White,’s that severe hydroxylation results in a significant proportion of Lewis acid sites. The 150-180 kJJmol sites seen in H-Y-DHV in Figure 6 and H-Y-DHSt in ref 39 may represent the newly formed Lewis sites or stronger Bronsted sites modified by the proximity of cationic NFA or condensed NFA. Because strong acid sites are not developed in boehmite until it transforms to y-AlzOl (see Figure 6) and because catalytic activity and development of strong acid sites in y-A120, are associated with surface defects,” it seems plausible to assume that either the boehmite-like NFA has developed surface defects or that the increased acid strength results from the Bronsted sites modified by the proximity of cationic NFA for condensed boehmite-Eke NFA. Formation of Other NFA Species. It should be emphasized that the NFA species formed in this study corresponds to an advanced stage of dealumination. TGA studies by Breck and Skeels” indicated a weight loss in H-Y over the temperature range 600-700 “C. The dealumination carried out in this study is just in this region of substantial weight loss. No attempt has yet been made to look at the dealumination species obtained at lower temperatures. The species formed over the temperature range 300-550 OC may well be some of the species proposed to form before condensation as emphasized by Freude et aI.l9 Formation of boehmite in the acidic environment of zeolite cavities may be very similar to the precipitation of boehmite from aqueous solutions. Boehmite is known to precipitate from aqueous solutions at pH
