J. Phys. Chem. 1994, 98, 4627-4634
4627
Progress toward Understanding Zeolite /3 Acidity: An IR and 27Al NMR Spectroscopic Study I. Kiricsi,' C. Flego, G. Pazzuconi, W. 0. Parker, Jr., R. Millini, C. Perego, and G. Bellussi Eniricerche S.p.A., 20097 Sun Donato Milanese, Italy Received: May 20, 1993; In Final Form: November 12, 1993'
Zeolite j3 was prepared with different Si/Al ratios by a hydrothermal method and dealuminated by leaching with dilute HCl. Samples of B from the various stages of H-0 synthesis, and after leaching, were characterized by X-ray diffraction and I R and 27Al M A S - N M R (MAS = magic angle spinning) spectroscopies. All the samples exhibited both Lewis and Bronsted acidities. The types of OH groups present, and also their distribution, are greatly affected by sample treatment. Five types of hydroxyl I R absorptions were found: those from strongly acidic bridging hydroxyl groups (3605 cm-I), O H groups bonded to extralattice aluminum (3660-3680 cm-I), internal SiOH at framework defects (3730 cm-I), and terminal SiOH groups (3745 cm-l) and a "very high frequency" (VHF) absorption (3782 cm-l). These bands, except those of the terminal silanols, shifted to different extents upon interaction with probe molecules (pyridine, benzene, hexane) used to evaluate acid strengths. The V H F band is most intense for slightly dealuminated 0 and is not observed for borosilicate. The V H F band is associated with (but not solely responsible for) the octahedral *'A1 N M R resonance and is assigned to O H groups connected to aluminum which is leaving the framework (transient-state species). These O H groups are moderately acidic and contribute to 0 acidity.
Introduction
Although zeolite j3 was synthesized in 1967,l studies of this acid catalyst with potential applications in petroleum chemistry, refining, and fine chemical production began only recentlye2" This is due, in part, to the late description of its framework structure.7-1l /3 has a three-dimensional interconnected channel system with 12-membered elliptical openings having mean diameters of 0.67 nm. It is ordered as a large-porezeolite between faujasite-Y and Linde type L zeolites. j3 is of great potential industrial interest because of its high acidity and peculiar pore system. j3 is reportedly a good catalyst for several important reactions: fluid catalytic cracking,l2 hydrotreating,l3 benzene alkylation with light olefins,14isobutanealkylation with n-butene,15 etc. Experimental parameters affecting j3 synthesis under hydrothermal conditions (gel composition, nucleation, temperature, reaction time) have been refined.16J7 Synthesis of metal silicates with a &structure has been elaborated too.~*J9Among the ionexchanged forms of j3,zeolite H-j3was found to possess the highest acidity.20 Previous IR studies of j3 do not provide an unequivocal overall description of its acidity. Pretreatment of the "as-synthesized" sample strongly influences the concentration and strength of Bronsted and Lewis acidic Borade and ClearfieldZ2found only one acidic OH group absorbing at 3602 cm-1 in 0 with Si/A1 = 16.6 and 12.5 while two bands (3602 and 3660 cm-I) were observed on less siliceous samples. Three OH bands were found by Corma23 and R a t n a ~ a m yin~ ~ IR spectra of calcined as-synthesized j3 zeolite. The bands at 3740,3602, and 3540 cm-I were assigned to terminal and internal S O H , bridging hydroxyls (Bronsted acid sites), and hydrogenbonded SiOH groups, respectively.24 The intensity of the second band was proportional to the A1 content. In an IR study of H-8, prepared by treatment of NH4-j3 at 823 K in vacuum, Bourgeat-Lami et a1.25 found five OH bands and assigned them as bridging hydroxyl groups (3615 cm-I), hydrolyzed aluminum species (3665 cm-'), terminal silanols (3747 cm-I), and strongly acidic hydroxyl groups (3780 cm-1). A broad absorption between 3750and 3OOOcm-1 was attributed to hydroxyl nests of (SOH), in framework defect sites. 27Al MAS-NMR 0
Abstract published in Advance ACS Abstracrs, March 15, 1994.
0022-3654/94/2098-462lsO4.50f 0
(MAS = magic angle spinning) spectra revealed that ammonium exchange and heating cause dealumination. They concluded that protons introduced into the lattice by decomposition of ammonium ions changed the coordinations of some aluminum ions. Maache et a1.26compared the acidity of /3 synthesized from alkaline ( H - 0 0 ~ )and nonalkaline (H-j3~)aluminosilicate gels after dealumination by acid leaching. They found that the 3670and 3685-cm-1 bands became more intense after treatment of H - ~ ~ owith H water vapor, but disappeared after acid leaching. Contradictions exist in the literature regarding dealumination of j3 by thermal and/or hydrothermal treatments. Fyfe et al.27 and Bourgeat-Lami et al.2sdealuminated /3 using mild treatments while Liu et aL2*found dealumination took place only over 1033 K. Removal of aluminum from the nine distinct crystallographic sites in /327329is not expected to occur with equal ease. Any treatment which affects the coordination state of aluminum or its surroundings results in a change in the number and types of acid ~ i t e s . 3 ~Thus, 3 ~ the total acidity of j3 depends strongly on the type of treatment. This was concluded from recent IR and 27Aland 29SiNMR studies of H-033 where no extraframework Si was evidenced. In summary, considering the published work on j3 acidity, the following points need to be clarified: (i) Which types of OH groups are generated upon different treatments? (ii) What are the relative acidities of the OH groups generated? (iii) How is aluminum affected by the thermal treatments and acid leaching? The aim of this work was to investigate the points mentioned above by IR and NMR spectroscopies and by X-ray diffraction. Experimental Section
Synthesis. Zeolite 0 samples were synthesized under hydrothermal conditions, under autogenous pressure, from gels having the following compositions: 0.25: 1:a:b:0.036:20 (TEA)2O:Si02: A1203:NazO:K20:H20 with a = 0.042, 0.033, and 0.02, and b = 0.062,0.053, and 0.033 for gels with Si/Al = 11, 14, and 20, respectively. They were obtained by mixing appropriate amounts of silica (40% Si02, Ludox HS; Du Pont), sodium aluminate (Carlo Erba; 56% Al2O3,37% Na20), and tetraethylammonium (TEA) hydroxide (40% aqueous; Fluka), in ion-free water. Crystallization was performed in stainless steel autoclaves with stirring at 433 K for ca. 20 h. The product was filtered, 0 1994 American Chemical Society
4628 The Journal of Physical Chemistry, Vol. 98, No. 17, 1994
Kiricsi et al.
TABLE 1: SVAI Mole Ratio, Acidib, Unit Cell Parameters, and NMR Data for B Samples ~~
NMR data sample (a) AS-8
Si/A1
acidity' (mmol/g) Bronsted Lewis
X-ray cell parametersb
Ale (mol %) Td Oh
LWd (Hz) Td Oh -
a (nm)
c (nm)
V (nm3)
1.2463(2)
2.6554(4)
4.1245(16)
100
0
800
1.2441(3)
2.6339(6)
4.0770(22)
87
13
880
1.2452(4)
2.6379(7)
4.0903(26)
100
0
740
1.2398(3)
2.6277(6)
4.0388(22)
71
29
1000
920
1.2445(3)
2.6321(7)
4.0766(25)
86
14
700
450
1.2410(3)
2.628 l(7)
4.0478(24)
90
10
680
400
~
14
(2) BO-8
14
(3) NH4-8
14.5
(4) H-8
14.5
( 5 ) D1-8
16.5
(6) D2-8
22.2
0.283 0.122 0.148 0.076 0.265 0.120 0.259 0.056 0.281 0.103
0.259 0.076 0.488 0.103 0.153 0.139 0.177 0.101 0.092 0.121
840
Measured at 470 K (upper values) and 770 K (lower values). Estimated standard deviation of last digit in parentheses. Relative amounts of tetrahedral and octahedral aluminum. LW = line width. washed, and dried at 423 K. X-ray diffraction confirmed that all samples were highly crystalline and made up exclusively of zeolite j3 (Table 1). Sample Preparation. H-/3 was prepared as follows. The assynthesized (AS-@)sample was calcined at 820 Kin an air stream for 5 h to burn the template off (this sample is labeled BO$), and then it was ion-exchanged in 0.3 M ammonium acetate solution for 1 h at 353 K (NH4-P). The ammonium form was calcined at 820 K in air to produce H-8. Two partially dealuminated /3 samples (Dl-/3 and D2-0) were prepared by leaching in HCl solution (0.1 and 1.O M, respectively) at 350 K for 4 h followed by washing the sample Cl--free with distilled water. The dealuminated specimens were dried in air at 298 K. Borosilicate was prepared according to rep4 using tetramethylammonium hydroxide, Ludox, and H3BO3 as starting materials. The resulting gel was crystallized for 10 d at 423 K and then treated in the same way as 8. IR Spectroscopy. KBr pellets were pressed from a homogeneous mixture of zeolite (1%) in KBr. Spectra were recorded with a Perkin-Elmer 1730 FT-IR spectrometer and processed using IRDM I1 software. Self-supporting wafers, used in absorption studies, were pressed into a thickness of 10-30 mg/cm-2, and placed in a glass sample holder inside a Pyrex IR cell with KBr windows. The wafers were pretreated at different temperatures under vacuum (samples labeled V), e.g., AS-8-V-673 is a wafer pressed from AS-/3 and treated under vacuum at 673 K, and air flow (A), 10 cm3/min, and in the presence of water vapor (W) at selected pressures for several hours. Each pretreatment was terminated by evacuation a t the same temperature. Spectra were always collected at 298 K. The geometry of the vacuum system allowed the adsorbate to be dosed in 9-pmol portions onto the wafer with a lower limit of 1.3 molecules/unit cell. IR spectra were collected with increasing adsorbate concentration for adsorption studies. Desorption was performed by evacuation at elevated temperatures. The temperature range for adsorption/desorption never exceeded that used for pretreatment of samples. Acidity studies were performed on wafers treated with 1.3 kPa of pyridine at 470 K for 1 h followed by evacuation at 470 K for 1 h. Desorption of pyridine was performed at 470,570,670, and 770 K for each sample. The extinction coefficients of Take (1.3 X 106 cm-l/mol for Bronsted acid bonded pyridine and 1.5 X 106 cm-I/mol for Lewis acid bonded pyridine) were used to calculate the concentrations of acid sites from the integral a b s ~ r b a n c e s . ~ ~ X-ray Diffraction. Experiments were performed using Nifiltered Cu K a radiation (A = 0.154 178 nm) on a Philips diffractometer equipped with a pulse-height analyzer. The patterns were collected by the step scanning procedure (step size 29 = 0.05O, 5-s accumulation time) over the 29 = 3-53O angular region.
1200 800
1200 800
1200 800
( c m1) Figure 1. IR spectra (framework vibration region) of 8 samples: (A) H-8, Si/Al= 20 (a), 14 (b), and 11 (c); (B) AS-@(a), BO-@(b), NH4-,!3 (c),and H-8 (d); (C) parent sample used for dealumination (a), D1-,!3 (b), and D2-8 (c). Wavenumber
Unit cell parameters were determined by a least-squares fit to the interplanar spacings of four single sharp reflections located in the 29 = 19-34' angular region. Data were collected stepwise with a 29 = 0.03' stepsizeanda 30-saccumulation time. a-A1203 (30 wt%) was added as an internal positional standard. Accurate positions of the reflections localized in this angular region were determined, after Ka2 stripping, by using the deconvolution program FIT (DIFFRAC software, Siemens). *'AI MAS-NMR Spectroscopy. Spectra were obtained a t 78.2 MHz on a Bruker CXP-300 spectrometer using */6 (1 ps) radio frequency ( r o pulses ( 1/ 3 of a n/6 pulse determined using aqueous AlC13), a 32-ps dead time, a 4-ms acquisition time, and a 2-s cycle delay. Accumulations (1200) were averaged on 175-mg samples in 9-mm-0.d. Andrews-Beam delrin rotors spinning at 3.5 kHz. Free induction decays (FID's) were apodized (100 Hz) and zero filled (from 0.5 to 2 K) before transformation. Chemical shifts are referenced externally to aqueous AlC13. Relative integral areas are precise within 5% of their total value and are unchanged by changing the rf pulse from n/12 to */4. Line widths, full width at half-height, are precise within f 5 0 Hz. Results Infrared Spectroscopy. Framework Spectra. In Figure 1A, IR spectra of P with different Si/Al ratios are shown. Two absorptions (575 and 525 cm-*) are characteristic for j9.36 A third absorption at 952 cm-1 increases with the Si/A1 ratio. Spectra of samples treated differently show these structuresensitive bands with various intensities (see spectra in Figure 1B). The intensity of the absorption a t 952 cm-I changes with different treatments. The absorptions at 575 and 525 cm-I are influenced by the removal of aluminum from the framework (Figure 1C). The band at 952 cm-* is substantially larger after dealumination of the parent zeolite.
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 4629
Zeolite (? Acidity
0
0
2 .-c
E
Q)
5
$
3800
3650
3500
1560
1500
1440
Wavenumber ( c m 1)
3850
3660
3450
Wavenumber (cm-')
Figure 2. IR spectra (hydroxyl vibration region) of AS-j3-V-770 (a),
BO-j3-V-820 (b), NH4-j3-V-770 (c), NH4-j3-V-820 (d), NH4-j3-A420 (e), NH4-8-W-820 (f), and H-j3-V-820 (g).
al V
f
.-ce
5E
h
I-
Wavenumber (cml)
Figure 3. IR spectra (hydroxyl vibration region) of (A) H-j3-V-820 at the Si/A1 ratios indicated and (B) D1-j3-V-820(a), D1-j3-V-770 (b), and D2-j3-V-820 (c).
OH Vibration Spectra. Two intense bands (3605 and 3740 cm-1) seen in the spectra of AS-j3-V-770 (Figure 2a) appear superimposed on a broader absorption (3800-3200 cm-l). BO(?-V-820 contains a t least four different OH groups, which absorb at 3605,3662,3740,and 3782cm-1. Compared to the AS sample, a decrease in the intensity of the 3605-cm-l band due to the bridging OH groups is noted. Thermal treatment of NH4-(? a t 770 and 820 K in vacuum yields spectra (Figure 2, spectra b and c) similar to those of AS-(?-V-770 and BO-j3-V-820. Samples calcined in air show the same absorptions as for NH4-(?-V-820 (spectra d and e). The intense very high frequency (VHF) band at 3782 cm-l differentiates NH4-j3-W-820 from theother NHd-(?samples. Characteristic absorptions of H-8-V-820 are found at 3605, 3662, 3740, and 3782 cm-I. The third band is probably a superposition of two bands. From Figure 2 and the above discussion it follows that depending on the pretreatment conditions more (5) or less (2) bands appear in the OH vibration region. This reflects the presence of at least five different O H groups. Spectra of H-/3 with different Si/Al ratios (Figure 3A) show that the number of absorptions does not vary, but the relative intensity of the VHF band (compared to that a t 3740 cm-l)
F i p e 4. IR spectra of AS-j3-V-770 (a), H-8-V-820 (b), and D1-j3-V820 (c) over the hydroxyl (A) and pyridine (B) vibration regions after exposure to pyridine vapor followed by evacuation at 473 K (lower trace of pair) and 773 K (upper trace).
increases with increasing aluminum content. The appearance of this band is connected to aluminum present in the zeolite. Figure 3B shows the spectra of partially dealuminated samples. The spectrum of D1-B-V-770 exhibits only two bands (3740 and 3605 cm-l) and not those bands at 3662 and 3782 cm-I. However, after treatment a t 820 K, these two bands appeared (spectrum a). In the spectrum of zeolite /3 dealuminated under severe conditions (spectrum c), a very small band at 3605 cm-l and an intense, rather broad one at 3735 cm-I appear. The VHF band was not observed in spectra of borosilicate pretreated under the same conditions used for (?. Pyridine Adsorption. Pyridine adsorption was performed on samples 1-6 (Table 1). After treatment of AS+-V-770 with pyridinevaporat 470K, theonly banddetectedin theOHvibration region (at 3740 cm-I) was due to the presence of terminal SiOH groups (see Figure 4A, spectra a). Four bands were observed (1446,1456,1490,and 1544cm-I) whichareattributedtopyridine adsorbed on sodium ions, Lewis acid sites, a combination of Lewis and Bronsted acid sites, and pyridine interacting with Bronsted acid sites, respectively. Evacuation at increasing temperature causes the intensity of the band due to pyridine adsorbed on sodium ions to decrease dramatically and the disapperance of the other bands. At 770 K, a new Lewis acid bonded pyridine band appeared a t 1462 cm-l (Figure 4B) and the acidic OH band was partially restored. Similar spectral changes were found for BO-(?-V-820 and NH4-j3-V-770. However, a shoulder on the 1456-cm-l band in spectra of samples evacuated at 770 K indicated the formation of a new Lewis acid bonded pyridine. Spectra of pyridine registered at different stages of the acidity test of H-j3-V-820 are shown in Figure 4B, spectra b. Surprisingly, each OH group (except that a t 3740 cm-I) interacts with pyridine. It should be stressed that also the band at 3782 cm-' disappeared upon pyridine adsorption. No OH bands reappeared after evacuation at 770 K. Desorption of pyridine from D1-j3-V-770 at 770 K in vacuum results in the partial restoration of OH absorption at 3662 and 3605 cm-l (see Figure 4A, spectra c) and the appearance of a new absorption a t 1462 cm-l. The concentrations of Bronsted and Lewis acid sites, present after evacuation at 470 and 770 K, are summarized in Table 1. AS-/3-V-770 contains the most Bronsted acid sites, and BO-& V-820 contains the most Lewis acid sites. Benzene Adsorption. Upon adsorption of benzene on AS-@V-770 a large shift and decrease in the intensity of the bridging OH band (3605 cm-l) were found (Figure 5). At 3506 cm-I a new absorption appeared and grew with benzene loading. An additional band developed at 3250 cm-I.
Kiricsi et al.
4630 The Journal of Physical Chemistry, Vol. 98, No. 17, 1994
3800
3400 3600 3400 Wavenumber (cm- )
'
3700
3500
-13200
1-
3000
Wavenumber (cm-1) Figure 5. IR difference spectra (loaded minus unloaded, hydroxyl vibration region) of AS-8-V-770 after absorption of 2.34 (a), 4.65 (b), 6.28 (c), and 7.65 (d) molecules of benzene/unit cell at 298 K.
I!,,
,
,
,
,
I
3800 360034003200 3800 3600 34003200 Wavenumber ( c m 1)
Figure 6. IR spectra (hydroxyl vibration region) of benzene adsorption (A) to and desorption (B) from H-j3-V-820 (A) pretreated sample (a) and the sample after adsorption of 1.34 (9 rmol) (b), 2.6 (c), 3.53 (d), 4.28 (e),and4.5(f) moleculesofbenzene/unitcellat 298 K;(B)dcsorption from the sample with 13.2 molecules of benzene/unit cell (a) at 298 K (b), 370 K (c), 470 K (d), 570 K (e), 670 K (f), and 820 K (8).
Benzene adsorption on NH4-8-V-820 shifts the OH bands at 3604,3680,3733, and 3782 cm-I to 3240,3429,3693, and 3640 cm-1, respectively. The only band which remained unchanged was the one at 3744 cm-I. Adsorption of benzene on H-8-V-820 shifts the bands to lower frequencies with 6u* = 0-350 cm-I. After adsorption of 9 bmol of benzene, a broad band appeared at 3253 cm-l and a sharper one at 3616 cm-I. This was accompanied by the disapperance of absorptions at 3606 and 3660 cm-1. The VHF band vanished too (see spectrum c in Figure 6A). With increasing loading this latter band was flattened, while the intensity of that a t 3616 cm-1 increased. At higher loading the band at 3427 cm-1 became more visible and a new absorption appeared at 3692 cm-1, whose intensity increased along with sharpening of the band a t 3740 cm-I (see spectra d and e). Evacuation of the sample saturated with benzene at 298 K caused the disappearance of the 3690cm-1 band and all of the shifted bands. As a consequence of this treatment the band at 3782 cm-1 reappeared (spectrum b in Figure 6B). Evacuation at 370 K caused the reappearance of the less intense bands (3603 and 3782 cm-l). The original shape of the absorption at around 3740 cm-l was partially restored. At 470 and 670 K spectra similar and identical, respectively, to the original one were registered. Thus, benzene adsorption is reversible. The
Figure 7. IR spectra (hydroxylvibration region) of n-hexane adsorption (A) to and desorption (B) from H-8-V-770: (A) pretreated sample (a) and the sample after adsorption of 1.8 (b), 3.5 (c), 4.2 (d), 4.4 (e), and 9.5 (f) molecules of n-hexanelunit cell at 298 K; (b) desorption from the sample with 9.5 molecules of n-hexane/unit cell (a) at 298 K (b), 370 K (c), 470 K (d), 670 K (e), and 770 K (f).
inverse trends between the band pairs 3733-3692 and 37823616 cm-l are noteworthy (compared spectra a in Figure 6A and g in Figure 6B) and aid the assignment of the VHF band (vide infra). n-Hexane Adsorption. Small frequency shifts (weak interaction) were observed for the OH bands with n-hexane was adsorbed on H-8-V-770. A loading of 1.8 molecules/unit cell caused the band at 3605 cm-I to almost disappear and the appearance of an intense absorption at 3493 cm-'. The VHF band a t 3780 cm-I decreased as a new absorption at 3695 cm-l increased (see Figure 7). Even after loading the sample with 5 molecules of hexane/unit cell, the band at 3782 cm-1 remained observable as a shoulder on the high-frequency side of the band a t 3740 cm-I. Evacuation at 298 K caused a decrease in the 3690-cm-1 band and the reappearance of the VHF band. After treatment at 570 K a spectrum identical to the original one was generated. These findings reveal that the OH groups responsible for the VHF band are less acidic than those absorbing at 3605 and 3660 cm-I since they interact weakly with hexane. Water Adsorption. Adsorption of 0.45 mmol of water/g of AS-8-V-770 at 298 K caused the following spectral changes: the acidic OH band (3605 cm-I) shifted to slightly lower frequency, new bands appeared a t 2500,2900, and 3700 cm-1, and the band at 3740 cm-l sharpened. The band shift of the acidic OH continued with increasing loading while a shoulder appeared at 3660 cm-1 (spectrum c in Figure 8A). At a coverage of 1.35 mmol/g only a sharp band (3740 cm-I) and a broad overlapping absorption (3730-3400 cm-I) were detected. Spectra taken after different stages of desorption (Figure 8B) show that vacuum treatment at 298 K caused the appearance of bands a t 3742, 3702, and 3680 cm-I. Upon evacuation a t 370 K, the band due to the acidic OH groups partially reappeared with a parallel decrease in the absorption a t 3702 cm-I. At 570 K a pattern very similar to that of the original spectrum was obtained (compare the two spectra at the top in Figure 8). Water adsorption on BO-8-V-820 and NH4-8-V-770 was similar to that observed for AS$. Spectra of H-,3-V-820 with different water coverages (shown in Figure 9A) reflect a complex picture. After adsorption of 9 bmol of water the band of acidic bridging OH'S (3605 cm-I) almost disappeared while the VHF band decreased somewhat. These changes were accompanied by band shifts. In addition to the two broad bands at 2500 and 2900 cm-I, new absorptions developed a t 3700, 3680, and 3560 cm-I. Desorption of water, by evacuation a t increasing temperatures, did not restore the original spectrum (compare spectra a and f
Zeolite j3 Acidity
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 4631
3700
2700
3500
2500
Wavenumber ( c m ' )
Figure 10. IR spectra (hydroxyl vibration region) of OH (A) and OD (B) forms of NH4-8-V-770 (a) and H-6-V-820 (b). wavenumber (c")
Figure& IR spectra (hydroxylvibration region) of water adsorption (A) to and desorption (B) from AS-6-V-770: (A) pretreated sample (a) and the sample after adsorption of 0.45 (b), 0.9 (c), and 1.35 (d) mmol of water/g; (B)desorption at 298 K (a), 370 K (b), 470 K (c), and 570 K
(4.
b
A
Q)
0
s
.-c
a
U
%
5
O.OOOOE+O
5
,
1O.DO
I
20.00
,
30 00 2-TnEtl
fi
I 40.00
50.00
,
,
,
, 6
0
IDegPee51
Figure 11. X-ray diffraction profiles of AS-8 (a), BO-8 (b), NH4-6 (c), and H-6 (d).
3900 3700
3400 3900 3700 Wavenumber ( C m
3400
Figure9. IR spectra (hydroxylvibration region) of water adsorption (A) to and desorption (B) from H-6-V-820: (A) pretreated sample (a) and the sample after adsorption of 9 (b), 18 (c), and 27 (d) pmol of water at 298 K, (B)evacuation of the sample with 27 pmol of water at 298 K (a), 370 K (b), 470 K (c), 570 K (d), 670 K (e), and 770 K (f).
in parts A and B, respectively, of Figure 9). At elevated temperatures the bands at 3605 and 3782 cm-l reappeared. After evacuation at 670 K the VHF band was smaller, the 3605-cm-' band was unchanged, and the 3660-cm-' band gradually vanished compared to the starting material. The broad bands observed at 2500 and 2900 cm-1 in all samples (not shown) disappeared after evacuation a t 370 K. OH groups were partially exchanged for OD in NH4-j3 and H-j3 prepared using heavy water. In the OD vibration range OD bands corresponding to the OH'S were observed (Figure 10). These IR spectra clearly distinguish external and internal silanols. X-ray Diffraction. The framework structure of j3 is made up of an intergrowth of tetragonal polymorph A and monoclinic polymorph B, with fault probabilities of about 40% and 6095, respectively.sa As a consequence, the X-ray diffraction (XRD) profile is a combination of sharp and broad reflections which strongly limit structural characterization. However, sharp reflections can be indexed on the basis of the tetragonal P4,22 or monoclinic C2/c space groups,' allowing the determination of a reasonable cell parameter related to the framework composition.*b Figure 1 1 shows XRD profiles of j3 after different treatments. Any changes observed are related to the elimination of extraframework material (TEA and Na+ ions) via calcination and ammonium acetate exchange.
Tetragonal unit cell parameters and volume are reported in Table 1. Lattice dimensions decrease from AS-@to H-j3 except for NH4-j3 which displays unit cell parameters greater than those observed in BO-8 even though the treatment has eliminated octahedral aluminum (Table 1) and, probably, also part of the tetrahedral aluminum. The introduction of ammonium ions into the pores apparently enlarges the unit cell. 27AIMAS-NMR Spectroscopy. Quantification of the total A1 content in these samples is complicated by the moisture content and compositional changes during the treatments. However, it is clear from Figure 12 that substantial aluminum loss occurs (AS-j3 is 10.7 atom 5% Al) after exchange with ammonium ions, since the signal-to-noise ratio decreases. This is also evident from the Si/Al mole ratios determined by elemental analysis (Table 1). The relative amounts of octahedral (Oh) and tetrahedral (Td) aluminum in the various j3 samples are shown in Table 1 . No Oh aluminum was detected in the spectra of AS-/3 and NH4-j3 (Figure 12). Removal of the template from AS-8 is accompanied by removal of some Td framework aluminum which remains in the pores of BO-8 as Oh aluminum, Alkaline ions and extraframework (Oh) aluminum are extracted from BO-8during ammonium exchange. This type of aluminum dissolution is known25 and explains why no Oh aluminum is observed for NH4-8. H-8 exhibits the broadest NMR lines (Table 1) and therefore contains a distribution of aluminum, both Td and Oh, with atoms having a lower average coordination symmetry. Sharper resonances are observed for the dealuminated samples (Dl-8, D2-j3) where acid leaching has apparently selectively removed aluminum with a lower coordination symmetry. No signals near 30 ppm, due to pentaco~rdinate~'or tetracoordinate extraframework aluminum,3*40 were observed.
Kiricsi et al.
4632 The Journal of Physical Chemistry, Vol. 98, No. 17, 1994
TABLE 2 IR Band Shifts (cm-I) for Adsorption of Benzene and Hexane on Zeolite B Samples bridging extraframework internal external sample AlOH ALOH SiOH SiOH VHF Average Band Position 3605 3660-80 3730 3145 3782 AS-8-V-770 NH4-8-V-820 H-8-V-820
355 365 352
Benzene Adsorption
Figure 12. 27Al MAS-NMR spectra of AS-@(a), BO-8 (b), NH4-8 (c), H-8 (d), D1-8 (e), and D1-8 (f) (spinning side bands (*); Td, 52 ppm; Oh, -3 ppm).
Discussion According to Sauer’s generalization, four types of hydroxyl groups may exist in zeolites: (i) terminal, nonacidic OH’s on the outer surface of crystals; (ii) bridging OH’s (acidic) connected to the framework (strictly tetrahedral Alcoordination); (iii) OH’s bound to extralattice aluminum species (considered to be acidic); (iv) SiOH’s in lattice defects.32 In the ideal case of a defect-free crystal the first two types of hydroxyls are exclusively present with well-defined molecular surroundings. The acidity of bridging hydroxyls is determined by the crystallographic position of aluminum. The influence of the geometrical positions of neighboring atoms is reflected also in I R spectra, as was shown first by Uytterhoeven et al. for H-Y Types iii and iv hydroxyls may form with dealumination or during synthesis. Dealumination by steaming generally results in the generation of extralattice aluminum species and framework defect sites filled with hydroxyl groups. Careful synthesis reduces the formation of extralattice material, which is the residue of unconsumed reagents (gel) or product formed in side reactions. This may also be achieved using seed ~rystals.~2 Hydroxyl groups can be classified as follows. (i) Terminal Hydroxyls (3745 cm-I). The concentration of terminal hydroxyl groups is determined primarily by the crystal size since smaller particles require more hydroxyl groups to close thecoordination spheres of Si on the exterior surface. Generally, j3 synthesis yields small crystals or agglomerates giving a relatively intense IR vibration band at 3745 cm-l due to terminal OH groups. These OH’s are not influenced by treatments like ion exchange and baking (Figure 2) or adsorption of base molecules (e.g., spectraofzeolites with adsorbed pyridine). Thealuminumcontent has no influence on their concentration. (ii) Bridging Hydroxyls (3605 cm-1). This hydroxyl group absorption was detected in all B samples, and its intensity strongly depends on the composition and preparation conditions of the zeolite. As expected, this band is most intense for samples pretreated at lower temperatures (compare spectra c and d in Figure 2). Upon adsorption of base molecules this IR band shifts to lower wavenumbers, indicating a stronger interaction (Table 2). Interaction of benzene with bridging OH groups shifts their vibration frequency (352-365 cm-1) to lower values in accord with literature data.43 Even upon adsorption of a nonpolar compound, such as n-hexane, this band shifts 112 cm-1 to lower frequencies. The common feature of bridging hydroxyls is their acidity. Since they compensate the negatively charged aluminum to which
66
a
0
87
40
233
Hexane Adsorption H-8-V-770 112 167 a Band shift not clearly observed.
PPm
31
0 0 0
25 1
82
they are bonded, they can in principle be divided into as many subgroups as the number of distinct framework aluminum positions. This is frequently reflected in IR spectra of perturbed OH groups. From analysis of the benzene-perturbed OH bands of HNa-ZSM-5 zeolite, Datka distinguished five different bridging OH groups.44 Inspecting the spectra of Figure 5, only one shifted OH band is expected. Contrary to this expectation, three broad, shifted bands can be seen, reflecting perhaps the interaction with aluminums in three different crystallographic positions. The broad band a t 3700-3200 cm-l is probably connected with the hydrogen-bonded bridging OH groups, as proposed by Kazansky et al.45 This interpretation is supported by theoretical calculations showing the importance of bond angles, the state of aluminum, and the short-range interactions between bridged hydroxyl g r o ~ p s . ’ ~ Alternative , ~ ~ , ~ ~ interpretations by others are less likely.24329948 Although we observed this band, we could not monitor its change during desorption of probe molecules. IR spectra of the deuterated samples (Figure 10) reveal that the shape of this broad OD band is the same as the OH one, supporting assignment to a perturbed O H vibration. (iii) Hydroxyls Associated with Extraframework Aluminum (3660 cm-1). j3 has moderate thermal and hydrothermal stability.28,49 Near 700 K removal of aluminum from framework positions possessing the highest instability can be accounted for by the appearance of this OH band. On the basis of IH MAS-NMR studies, a signal a t 2.6 ppm was assigned to AlOH groups of extraframework species.50 Brunner states that these OH groups are nonacidic.51 Also, we find that benzene adsorption causes a shift of 230-250 cm-1 in this band, less than that observed for the bridging OH groups. Thus, these OH groups are less acidic than bridging OH’s. The true nature of extraframework aluminum is not known even though studies employing different physical methods have been made.52-55 The contribution of extraframework aluminum species to the acidity of the hydrogen forms of zeolites has also been studied extensively.50s659 The results show that the generation of “superacidic” sites, or sites of enhanced catalytic activity,as61 involves extraframework aluminum species. Recently Lunsford showed that when two different types of extraframework aluminum species are present near proton sites two different enhanced Bronsted acid sites exist.62 The 3660-cm-I band was not observed for AS-j3 and the dealuminated samples (Dl-j3, D2-j3). AS-j3 was pretreated at 770 K in vacuum for the spectroscopic experiments. This did not result in the releaseof aluminum from the framework in detectable amounts. In fact, 27AlN M R of AS+ found no Oh aluminum. The dealuminated samples (Dl-p, D1-j3) were not calcined after acid leaching, to fill framework vacancies, and therefore extraframework aluminum was partly dissolved by the acidic solution. Compared to H-B, much less extraframework Oh aluminum (Table 1) exists as a source for this type of OH group. Narrower line widths are observed for these samples, especially for the line of Oh aluminum, due presumably to selective removal of aluminums with lower coordination symmetry.
Zeolite j3 Acidity
(iv) Hydroxyl Groups at Lattice Defects (3730 cm-1). In practice zeolite crystals contain varying amounts of defect sites as a result of aluminum or silicon vacancies generally terminating with OH groups. Internal silanol groups are detected by IR spectroscopy as a distinct band close to the external SiOH vibrations or from the shape (asymmetry or broadening) of the bands. In our opinion, the band at 3730 cm-l in zeolite B, near the external silanol band, is due to internal silanols. The following observations support this attribution. (i) These two bands are resolved for H-j3-V-820 but not for NH4-j3 and AS-@ (ii) Adsorption of basic molecules causes the band at 3730-3745 cm-I to sharpen. (iii) The band at 952 cm-I is an indirect proof for lattice defects since this band was attributed to Si-OSi bonds formed after aluminum removal.63 This band is more pronounced for heat-treated and/ or more siliceous samples. A similar assignment was made by Ray et al.64since they found that isolated silanols absorb a t 37303738 cm-I near the crystallographic terminal hydroxyls (3747 cm-I). The number of hydroxyl groups filling the defect sites greatly affects the zeolite’s thermal stability. 29Si MAS-NMR results show that OH nests in defect sites are mainly Si(OH)(OSi)3 and Si(OH)2(OSi)2.2’.53.57~o Complete OH nests [(SiOH)4] exist only near 298 K and are rapidly consumed at higher temperatures.65 (v) Hydroxyl Groups Causing the VHF Band (3782 cnrl). This band was observed for samples heated above 770 K in vacuum, air, or water vapor. Samples treated under mild conditions (at or below 770 K, in vacuum) did not exhibit this band (Figure 2). The frequency of this band is very high for zeolites. We hereby abbreviate it as VHF. The VHF band was observed for H-ZSM-5 (dealuminated under mild hydrothermal conditions) and for H-j3 (after calcination at 820 K).25,66-68Samples possessing this type of OH group exhibited enhanced catalytic As the appearance of this band was found to be associated with slight dealumination, it was assigned to various aluminum-containing species. We came to the same conclusion since we never observed it for BO, NH4, or H forms of @-type borosilicates containing no aluminum. Further support for this assignment is that a band observed a t 3800 cm-I for AlPO-5, which contains no Si, arises from slightly acidic O H g r o u p ~ . ~ 3 ~ Lago et al. assigned it to the OH groups generated on partially hydrolyzed framework aluminum.66 Several researchers concluded that this band corresponds to terminal AlOH groups bound to a single aluminum probably in the form of A100H.Z5 Miller et al. found the VHF band in spectra of mordenites, but no assignment was givens9 Also Schuth and Spichtinger observed this band without discussing it.19C Several researchers tried to assign this band by referring it to the work of KnBzinger and Ratnasamy dealing with surface models and characterization of surface sites on aluminas.69 They found that AlOH groups can be classified in three categories. Type I are isolated hydroxyls coordinated to Td (a) or Oh (b) coordinated isolated aluminums absorbing a t 3760-3780 and 3780-3800 cm-l and have basic character. Type I1 are acidic OH groups bridging an Oh and Td coordinated aluminum (a) with a band positioned at 3730-35 cm-1, while those between two Oh aluminums (b) have neutral character and absorb a t 3740-50 cm-I. Type I11 hydroxyls absorb at 3700-37 10cm-I. They are acidic (+OS charge on the hydrogen atom) and are bound simultaneously to three Oh coordinated aluminum atoms. From this it follows that the VHF band cannot be due to terminal AlOH species since they are basic and bridge two aluminums. Both these situations are unlikely because this OH is acidic (interacts with basic molecules) and the bridging of two aluminums violates the Loewenstein rule. The acidity of hydroxyls absorbing at VHF in zeolites is not clear. Bourgeat-Lami et al. attributed a strongly acid character25
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 4633
to these hydroxyls while Loefler et al. found them almost nonacidic like the s i l a n 0 l s . 6 ~ ~ ~ ~ Our results reveal that the VHF absorption shifts upon interaction with adsorbed basic molecules. Therefore, these hydroxyls must be more acidic than the terminating silanols and those OH groups which fill structural defect sites (compare the frequency shifts in Table 2). To assign the VHF band, we consider the mechanism suggested by Wang et al. for hydrothermal dealumination38 in which the following transient states of aluminum leaving the framework were proposed:
=.si
/
HO ‘si=
0 ‘ (I)
-
=si
/
OH OH
HO ‘si=
-
(II)
These AlOH species are near one or more SiOH groups, generated when aluminum leaves the framework. This means that if the VHF band is due to aluminum leaving the framework its appearance has to be accompanied by broadening of the internal silanol band. This was observed in the OH and O D spectra. The appearance of Oh aluminum in the N M R spectra followed the same trend. The results of water adsorption measurements are consistent with the literat~re~0.~1 and show that each type of OH group can be regarded as an adsorption site. The irreversible decrease in the VHF band upon contact with water indicates the formation of water-sensitive structural units surrounding these OH groups. The suggested structures above (I and 11) are in accord with this. Acidity. Bronsted acidity tests show that H-j3 is slightly less acidic than AS-@ (Table 1). Desorption of pyridine at 770 K shows that no large changes in the concentration of Lewis sites occurs. Only AS-j3-V-770 possesses markedly less Lewis acid sites. Similar to 0 t h e r s , 2 ~ we , ~ ~observe (Figure 4) a new band a t 1462 cm-I upon desorption of pyridine (at 670 and 770 K) from AS-j3-V-770 and D1-j3-V-820. This band is a small shoulder for BO-j3-V-770 and NH4-j3-V-770 and is absent for H-j3-V-820. Desorption of pyridine from samples exhibiting the band at 1462 cm-I was accompanied by partial reappearance of the bridging OH band. When pyridine was strongly bound, and the OH bands did not reappear upon evacuation a t 670 K, the band at 1462 cm-I was not observed. This was found for H-j3-V-820 and indicates that the development of this band is likely connected with the acidity of Bronsted centers. On the basis of the band shifts of the adsorption experiments, the hydroxyl groups are ranked in order of decreasing acidity: (i) bridging OH groups, (ii) hydroxyls associated with extraframework aluminum, (iii) hydroxyls responsible for the VHF band, (iv) internal SOH, and (v) nonacidic terminal SOH. The hydroxyl groups absorbing a t VHF are moderately acidic, not strongly acidic, or nonacidic as concluded by others.25-67.68 Conclusions I R studies of zeolite j3 reveal different types of hydroxyl vibrations which are strongly affected by sample pretreatment. AS-j3 and H-j3 exhibit three and five different OH bands, respectively. At least five distinct types of O H groups can be generated in zeolite j3. Except for terminal OH groups, all the OH’S are acidic to varying degrees. Acid strength was evaluated from the magnitudes of OH band shifts induced by probe molecules. The acidity order decreased as follows: bridging OH groups (3605 cm-I) > OH groups associated with extraframework aluminum (3660 cm-1) > OH groups exhibiting the VHF band (3782 cm-1) > internal silanol groups a t lattice defects (3730 cm-l) > terminal silanols (3745 cm-1).
4634
The Journal of Physical Chemistry, Vol. 98, No. 17, 1994
Treatment of zeolite /3 under conditions which cause slight dealumination results in the formation of OH groups responsible for the VHF band. This band is attributed to AlOH moieties of intermediate (transient) products leaving the framework. Aluminum can leave the framework of zeolite /3 rather easily. Calcination converts significant amounts of aluminum from tetrahedral to octahedral coordination. The usual treatment to prepare H-/3 resulted in further dealumination as evidenced by XRD and 27AlMAS-NMR spectroscopy.
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