J. Phys. Chem. 1993,97, 9741-9748 9741

Characterization of Highly Dealuminated Faujasite-Type Zeolites: Ultrastable Zeolite Y and ZSM-20

Hans Miessner,' Hendrik Kosslick, Ursula Lohse, Barbara Parlitz, and Vu-Anh Tuan Zentrum f i r Heterogene Katalyse. KAI e.V., Rudower Chaussee 5. 124 84 Berlin-Adlershof, Germany

Received: March 24, 1993; In Final Form: July 13, 1993'

Highly dealuminated faujasite-type zeolites with different dealumination histories, different pore structures, and Si:Al ratios ranging from 40: 1 to 300: 1 have been investigated by temperature-programmed desorption of ammonia (NHrTPD) and by FTIR spectroscopy: ultrastable zeolite Y (US-Ex), dealuminated by hydrothermal treatment and a subsequent extraction of the extraframework aluminum; DAY, dealuminated by exchange with SiCld; and the recently synthesized ZSM-20, dealuminated hydrothermally. A correlation could be established between the amount of adsorbed NH3 determined by TPD and the integrated absorbances of N-H bending bands in the FTIR spectra. Time-resolved FTIR spectroscopy during TPD has been used to follow the desorption behavior of the ammonia species adsorbed on different sites. The analysis of the intensity decrease of absorption bands of adsorbed ammonia and the comparison with the simultaneous increase of hydroxyl stretching bands during TPD, reveals the following: (i) the isolated structural Brtansted acid sites have a similar acid strength in all samples, including the HZSM-20, regardless of the different dealumination procedure. (ii) Extraframework aluminum species formed during dealumination may block a signficant part of the strong Brtansted sites. (iii) The acid framework hydroxyl groups in both the a-cage (causing the HF band) and the 0-cage (LF band) contribute in the same way to the desorption behavior of NHs. (iv) In addition to bonding on strong Brtansted acid sites, NH3 is adsorbed on weak Bronsted and/or Lewis acid sites and on stronger Lewis acid sites. The ammonia bonded to the stronger Lewis acid sites seems to interact with a part of the terminal hydroxyl groups. The similarity between the isolated structural Brtansted acid sites in the Y zeolites studied and those in the HZSM-20 has been proven using rhodium dicarbonyl Rh1(C0)2+ as a sensitive probe molecule for the characterization of isolated acid sites.

Introduction

Dealuminated faujasite-type zeolites have gained growing attention as adsorbents, stable acid catalysts, and support materials. These materials exhibit a higher thermal and hy- drothermal stability, as well as a higher catalytic activity, than those of the aluminum-rich as-synthesized zeolite Y (Si:Al = 2.5:l). The dealumination can be achieved by hydrothermal treatment of the NH4 form of zeolite Y with water vapor at elevated temperatures (500 OC and higher). Extraframework aluminum (EFAL) species formed during this procedure can be extracted with diluted acid.1-3 The dealumination leaves a large number of framework atom vacancies, which are annealed, at least partly, by a rearrangement of the residual framework atoms. As a result, a material (US-Ex) is formed that contains mesopores within the zeolite crystallites and, therefore, a high amount of external silanol groupsa2 On the other hand, the dealumination by a direct exchange of aluminum with silicon in the framework of Nay, by treating it with Sic&, leads to a material (DAY) with almost no me sop ore^.^

To avoid the additional postsynthesis dealumination procedure, several attempts have been made to produce faujasite zeolites with an enhanced Si:Al ratio already in the synthesis process. A faujasite-like material with an Si:A1 ratio of 4.2:l was first obtained by Cirics using an organic template compound. This material was called ZSM-20, because structural differences (hexagonal symmetry) from faujasite (cubic symmetry) were apparent. It is now generally accepted that ZSM-20 and other faujasite-like materials are intergrowths of faujasite (FAU structure type) and hexagonal Breck structure six (BSS, an EMT structure type) with different BSS-to-faujasite ratios and or- derings. Both structures contain sodalite cages as secondary building units, which are linked by hexagonal prisms. In FAU

*Abstract published in Advance ACS Abstracts, September 1, 1993.

0022-3654/93/2097-9741$04.00/0

the successive faujasite sheets are interlinked related by inversion centers, but in EMT they must be interlinked by mirror planes.6 In faujasite, the large cavities are tetrahedrally arranged and are interconnected by four oxygen- 12-ring windows. BSS contains two different large cavities. The larger one has five oxygen-12- ring windows and forms straight channels. The smaller cavity has three oxygen- 12-ring windows and provides the lateral connection between the straight channels. ZSM-20 seems to be a biphasic faulted structure containing blocks of faujasite and Breckstructure six components. In the "true" ZSM-20 structure, the ratio of BSS to FAU is rather narrow (2:1).697 The increased Si:Al ratio, combined with the structural differences, prompted a series of works devoted to the catalytic properties of ZSM-20 as compared with Y zeolites.*-12

For the discussion of the catalytic results, it is essential to know the acidic properties of the zeolites used. The acidity of Y zeolites, including those with high Si:A1 ratios, has been studied by many groups. To characterize especially the acidic properties of dealuminated Y zeolites, the adsorption and desorption of probe molecules as ammonia and pyridine has been investigated by IR spectroscopy*3-21 and by TPD of ammonia ( N H ~ - T P D ) . ~ V ~ ~ - ~ ' It has proven to be highly informative to combine NH3-TPD with IR spectroscopy to obtain complementary results from both methods.28 These studies have been performed with materials having S1:Al ratios typically <10:1. The existence of up to four different desorption peaks for ammonia adsorbed on Y zeolites has been found by several gro~ps.3~22-24~2~.28 Various proposals have been made to assign the different desorption peaks to different adsorption sites. The desorption of NH3 at low temperatures (<200 "C) is usually assigned to NH3 bonded to weak Lewis or Bronsted acid sites. HY zeolite exhibits two different structural hydroxyl groups located in different environments in the super- cages and in the @-cages, respectively. These two structural Bronsted acid sites in Y zeolites are assumed to be the adsorption centers for NH3 with different desorption peaks at temperatures

0 1993 American Chemical Society

9742 The Journal of Physical Chemistry, Vol. 97, No. 38, I993

>2OO23-24 or one common desorption peak between 200 and 330 0C.28 Strong Lewis acid sites characterized by NH3 desorption peak temperatures higher than 330 OC are found mainly on samples dehydroxylated at high temperature. Using a mathe- matical analysis, Karge et have studied the distribution of acidity strength indealuminated Y zeolites (Si:Al< 1O:l). They found four different adsorption sites for NH3: weak Brernsted and/or Lewis acid sites desorbing NH3 at about 180 OC, medium strong Brernsted acid sites with a desorption peak at about 3 10 OC, a small number of strong Brernsted acid sites with NH3 desorption at about 380 OC, and very strong Lewis acid sites, probably formed during high-temperature dehydroxylation, with a desorption peak at about 510 OC.

On the other hand, only a few studies concerning the acidity of materials belonging to the FAU-EMT group have been published.~0~1~,29~30 The aim of the present work is to study the acidity of Y zeolites highly dealuminated by different methods, and to compare them with the acidic properties of dealuminated ZSM-20. As the strength of Brernsted acid sites depends on the number of Si atoms in the neighborhood of the structural -Al- ( 0 H ) S i - groups, the comparison was made using highly dealuminated samples of ZSM-20 and Y zeolite (Si:Al > 40). In that case, the acid strength of the isolated Brernsted sites reaches its maximum and should not further depend on the Si:Al ratio.

The acidity of the dealuminated zeolites was investigated by IR spectroscopy in the spectral range of OH and NH vibrations and by temperature-programmed desorption of ammonia. Using time-resolved IR spectroscopy, the TPD profiles of ammonia could also be obtained directly by monitoring the decreasing IR absorbances of ammonia adsorbed on different sites. It will be shown that the strength of isolated Breasted acid sites is similar in ZSM-20 and Y zeolite, This conclusion was verified by comparing thecarbonyl stretching frequency in Rh1(C0)2+ located at the isolated acid sites.31

Miessner et al.

Experimental Section

Samples. US-Ex was prepared in a two-step proced~re :~ .~ The NH4(0.8)NaY (Si:Al= 2.4: 1) obtained by the first ion exchange was steamed at 550 OC for 3 h. An additional ion exchange with (NH4)2S04 solution was carried out to remove the residual sodium ions. The sample was then steamed a second time at 800 OC. The EFAL species were subsequently extracted with dilute HCl at controlled conditions (30 OC, pH = 2.5-2.8) for 20 h (US-Ex). A calcination of US-Ex at 800 OC in air results in an additional dealumination (US-Ex(C)). The EFAL species were extracted again with dilute HCl (US-Ex(CE)).

DAY was kindly provided by DEGUSSA. It was prepared by treating NaY with SiC14.4 The chemical composition of US-Ex and DAY was determined by wet chemical analysis after alkaline decomposition.

The ZSM-20 zeolite was prepared from a reaction gel of the molar composition 1.25Na20*A12O3*302Si02*26.4- TEAOH*267HzO (TEAOH = tetraethylammonium hydroxide) under hydrothermal conditions in Teflon-lined autoclaves, and it was characterized as described elsewhere.12 After the crys- tallization was finished, the product was withdrawn by centrif- ugation, washed several times until neutral, and was then activated by heating in air at a rate of 5-10 OC/min to 550 OC and holding at this temperature for several hours. Theactivated sample (about 2 g) was ion exchanged twice with 200 mL of 0.5 N NH4N03 solution at about 60 OC for 2 h, withdrawn by centrifugation, washed with distilled water, and activated at 450 OC in air.

This HZSM-20 samplewasused for thedealumination. About 1.5 g was steamed at 700 OC for 3 h. To remove the EFAL species, the sample was extracted with 1 N HCI at about 50 OC, washed until neutral, and activated at 450 OC. This procedure was applied twice to finally obtain the highly dealuminated H-ZSM-20( D) sample.

n I

\ \ I /

1600 1300 1000 700 400

WAVENUMBER (cm-1) Figure 1. IRspectrain theregionoflatticevibrationsofhydrated faujasite- type zeolites before and after dealumination.

Characterization Methods. The wavenumbers of framework stretching vibrations were obtained using the KBr wafer technique. Transmission IR spectra of the surface species as hydroxyl group, adsorbed CO, and NH3 were obtained with self-supporting wafers in a conventional glass cell connected with a vacuum line to allow pretreatment and dosing of probe molecules. The spectra were recorded with a BIORAD FTS 60A spectrometer at a resolution of 2 cm-1 coadding 256 scans to obtain a sufficient signal-to-noise ratio. The spectra shown in this paper are corrected for the contribution of the windows and, in the case of the adsorbed species, for the background. All quantitative evaluations were made using normalized (with respect to the amount of zeolite exposed in the IR beam on the basis of the S i 4 overtone band at about 1800 cm-I) peak areas. The adsorption of NH3 was performed after an activation of the self-supporting wafers up to 500-600 OC in a dynamic vacuum (10-4 Torr).

For NH3-TPD measurements, ammonia was adsorbed on 0.2- 0.5 g of the samples at 120 OC from a gas stream containing 3 vol 76 NH3 in He. After flushing with pure He at 120 OC for 3 h, the desorption of NH3 up to 500 OC was begun (at a heating rate of 10 OC/min and a gas flow of 1 mL/min). The NH3 desorbed was determined using a thermoconductivity cell and was collected in a trap containing 0.1 N H2S04 for additional quantitative titration.

Results and Discussion Lattice Vibrations. Figure 1 shows the IR spectra of the lattice

vibration region of different dealuminated Y zeolites and dealuminated HZSM-20. The bands typical for the faujasite structure are shifted to higher wavenumbers as compared to the original faujasites, as should be expected after dealumination. The wavenumbers are rather similar, as shown in Table I. From these wavenumbers, it is possible to conclude that the Si:Al ratio is higher than 20, but a more detailed differentiation between the samples is not possible with the KBr technique at these high Si:Al ratios.)2 It may be interesting to note the similarity between the spectrum of HZSM-2O(D) and those of the other samples. The difference in the structure of these zeolites concerns only the connection of the secondary building units and hardly influences the IR spectrum, which is determined by the local environment. Similar results were obtained by Fiil6p et and were used as one argument to propose a faujasite-like structure for the ZSM- 20 zeolite.

Highly-Dealuminated Faujasite-Type Zeolites

TABLE I: IR Spectra of Dealuminated Faujasite-Type Zeolites in the Framework Vibration Region (1300-400 cm-l)

The Journal of Physical Chemistry, Vol. 97, NO. 38, 1993 9743

v(sYm) double 6-ring T-O bend zeolite v(asym)

US-EX 1206 1176 1078 US-Ex(C) 1208 1176 1079 US-Ex(CE) 1208 1177 1079 HZSM-SO(D) 1210 1178 1078 DAY 1211 1177 1084

250

200

150

=

100

50

0 100 180 280 940 420 500

Temperature 'C

Figure 2. NH, TPD profiles of dealuminated faujasite-type zeolites.

TABLE Ik Temperature-Programmed Desorption of Ammonia Adsorbed on Dealuminated Faujasite-Type Zeolites

n(Al)/UC zeolite NH3 (des), mmol/g TPD C.A. TPD

US-EX 0.41 40 11 4.7 US-Ex(C) 0.17 100 nda 1.9 US-Ex(CE) 0.14 115 19 1.7 HZSM-2O(D) 0.08 210 nd 0.9 DAY 0.05 330 205 0.6

a Not determined.

NH3-TPD. NH3-TPD is more suited to differentiating between the number and strength of acid sites in the samples under study. Figure 2 shows the corresponding TPD profiles, and Table I1 shows the amount of desorbed ammonia, which can be used to estimate the Si:Al ratio in the zeolite framework (also shown in Table 11). It should be noted, however, that is is not possible in the case of Y-type zeolites to differentiate on the basis of the ammonia desorption alone between Br~nsted acid sites, which are connected directly with the Si:Al ratio in the framework, and the Lewis acid sites, which are related to residual sodium cations or EFAL species. We will come back to this point in our discussion of the IR data. Table I1 also contains the Si:A1 ratios obtained by chemical analysis, i.e., including also aluminum in nonframe- work positions. The figures are generally smaller than those obtained by NHrTPD, but they show the same tendency.

The TPD results show that the samples differ significantly in the number of acid sites, decreasing in the order US-Ex > US- Ex(C) = US-Ex(CE) > ZSM-2O(D) > DAY. From the TPD profiles, we see that ammonia is desorbed from at least two sites. One, more weakly adsorbed NH3, desorbs between 200 and 300 OC, whereas the stronger bond ammonia desorbs between 300 and 400 OC. The profiles shown in Figure 2 allow a direct comparison of the properties of US-Ex after the additional calcination at 800 "C (US-Ex(C)) and after subsequent extraction of the EFAL species (US-Ex(CE)). It is evident that calcination

834 789 61 1 836 790 612 836 789 612 837 792 614 837 79 1 614

528 530 529 530 531

w/ US-Ex(CE)*3

u -0

I I

WAVENUMBER (cm- 1)

1800 1700 le00 1500 1400 1300

Figure 3. IR spectra in the N-H bending region of ammonia adsorbed on dealuminated faujasite-type zeolites after the interaction of NH, with the zeolite at 100 OC and a subsequent evacuation at 100 OC. For comparison, the absorbances of the higher dealuminated samples are increased by a factor, as indicated on the spectra.

at 800 OC is accompanied by decreasing the amount of more strongly adsorbed ammonia, whereas the more weakly adsorbed NH3 decreases only after the subsequent extraction of EFAL (US-Ex(CE)). The decreased number of strong acid sites is caused by an additional dealumination during calcination at 800 OC and also by the blocking of these sites by cationic oxidic aluminum species, as proposed by S ~ h e r z e r , ~ ~ Lohse,3 and Lunsford,35 for dealuminated Y zeolites, and by Miller36 for hydrothermally dealuminated mordenites. This situation and the adsorption of NH3 at Lewis acid sites make the values obtained for the Si:Al ratio from NH3-TPD data (Table 11) uncertain, especially for US-Ex(C).

Infrared Spectroscopy of Adsorbed NH3. Figure 3 shows the IR spectra in the region of N-H bending vibrations below 1800 cm-' of NH3 adsorbed on the different samples at 100 OC'and evacuated at 100 OC (to remove the physisorbed NH3). Addi- tionally, bands caused by the N-H stretching modes at 3360- 3365, 3282-3287, and 3200-3210 cm-1 are visible in the N-H stretching region but not as well resolved. Below 1800 cm-', three distinct bands appear (Figure 3). The most dominant band with all samples is the band at about 1450 cm-l, which is usually assigned to ammonium cations arising from the protonation of ammonia on Br~nsted acid sites. The bands at 1620 and at 1300 cm-', on the other hand, are assigned to ammonia bonded to Lewis acid ~ i t e . 3 ~ 9 ~ ~ The broad shoulder at about 1680 cm-1 appears only in the spectrum of DAY as a single band. From Figure 3, it is obvious that the amount of ammonia adsorbed at acid sites varies significantly, in the order US-Ex > US-Ex(C) r US-Ex(CE) > ZSM-2O(D) > DAY. The different absorbance scales used in Figure 4 for the different samples correspond to the relationships between the Si:A1 ratios obtained from NH3- TPD as shown in Table 11.

The spectra in Figure I11 also show a remarkable difference in the relative intensities of the N-H bending modes assigned to NH3 bonded to Lewis acid sites. Whereas US-Ex(CE) and DAY are almost free of Lewis acid sites that are able to coordinate NH3, and whereas ZSM-2O(D) has only a small amount, in US- Ex and in US-Ex(C) there is a considerable amount of NH3

9744 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 Miessner et al.

framework is evident from the intensity of the IR bands caused by the structural hydroxyl groups, as shown in the spectra before NH3 adsorption. All the samples show the two bands charac- teristically for faujasite-type zeolites: the high-frequency (HF) band at 3631-3632 cm-1 caused by acid hydroxyl groups located in the supercages, and the low-frequency (LF) band centered at 3567-3569 cm-1 caused by hydroxyl groups in the sodalite units and the hexagonal prisms.16 Compared to the parent Y zeolite, the H F band is shifted to lower and the LF band to higher wavenumbers. For dealuminated Y zeolites, this already has been mentioned.39~40 The same resultsare also obtained for ZSM- 20 with wavenumbers of 3632 cm-l (HF) and 3569 cm-l (LF) for ZSM-20(D), as compared with 3640 and 3558 cm-l for the parent HZSM-20 (Si:Al = 4.3:l). For the samples studied in this work, the wavenumbers of the H F and LF bands are very similar, but the intensity differs significantly, decreasing in the order US-Ex > US-Ex(CE) > ZSM-2O(D) = DAY corresponding to the different Si:Al ratios in the framework. The intensities of the bands of acid hydroxyl groups in US-Ex additionally calcined at 800 OC (US-Ex(C)) are lower than those after the subsequent extraction with dilute HCl (US-Ex(CE)). As already mentioned in the discussion of the NH3-TPD results, cationic aluminum oxide species formed during the calcination probably substitute some of the protons. The existence of EFAL species is indicated by the small but distinct bands a t 3698 and 3611 cm-I in the spectrum of US-Ex(C) (Figure 5).35939-42 The small band at 3703 cm-1 in the spectrum of HZSM-2O(D) points to the existence of EFAL species also in this sample.

The intensive band at 3746-3749 cm-l is caused by terminal silanol groups at the boundaries of the crystallites, at Lefect sites, or on amorphous SiO2-containing material. The band is not symmetric and has, in the case of HZSM-2O(D) and DAY, a clear shoulder at about 3740 cm-'. Theassignment of these bands to different species of terminal hydroxyl groups was the subject of a recent paper by Janin et al.zo The authors found in dealuminated Y zeolites three components with wavenumbers a t 3747-3749,3744-3746, and 3738 cm-I. They assigned them to SiOH groups attached to amorphous silica-alumina debris, to extraframework silicon-rich debris, and to terminal framework silanol groups, respectively. It follows from Figure 5 that the amount of species causing the band at 3746-3749 cm-1 is significantly smaller in case of DAY, which was prepared by treating the Y zeolite with SiC14. The other samples were dealuminated by a hydrothermal treatment, which is known to result in a large amount of mesopores inside the zeolite crystallites.2 The large number of terminal hydroxyl groups in these mesopores could be the reason for the high intensity of the corresponding IR band.

The IR bands of the structural OH groups disappear completely upon interaction with NH3 (Figure 5 ) . From the other bands, the intensity of the band at 361 1 cm-I assigned to extraframework alumina species seems to decrease during the interaction with NH3 (see also the difference spectra in Figures 6-8), whereas the bands a t about 3700 cm-1 are hardly affected. In the spectra of the US-Ex series, an additional band at 361 6 cm-1 becomes visible after the adsorption of NH3. In the spectra of the activated samples before NH3 adsorption, this band is masked by the strong absorption a t 3631 cm-1. The behavior of this band toward the NH3 adsorption points to a species different to that causing the band at 361 1 cm-l; assigning this band to a definite hydroxyl group is beyond the scope of the present work and needs additional study.

The overall intensity of the band of the terminal hydroxyl groups a t about 3747 cm-1 does not change markedly during the adsorption of NH3 (Figure 5), but the peak maximum seems to be slightly shifted to lower wavenumbers. There is a small loss of intensity in the high-frequency part at about 3749 cm-1. This

50 4

40 n

0.00 0.10 0.20 0.30 0.40 0.60

m o l NHS(dsr)/g Figure 4. Correlation between the amount of NH3 desorbed during TPD and the normalized integrated absorbances of the NH3 bending modes in the region between 1300 and 1700 cm-l.

DAY - - 3800 3650 3575 3500

WAVENUMBER (em-1)

Figure 5. IR spectra in the hydroxyl stretching region of dealuminated faujasite-type zeolites after the activation in vacuum at 500 OC for 30 min (upper spectrum) and after a subsequent adsorption of NH3 at 100 OC followed by evacuation at 100 OC (lower spectrum).

bonded to Lewis acid sites. This result is in qualitative agreement with the NH3-TPD results (Figure 2), showing for US-Ex and US-Ex(C) a desorption of NH3 at lower temperatures usually assigned, a t least partly, to NH3 bonded to Lewis acid sites.

The overall intensity of the N-H bending modes (bands centred at about 1300,1450,1620, and 168Ocm-1) is in a linear correlation with the amount of NH3 desorbed during NH3-TPD (Figure 4). This correlation can be used to estimate the amount of A1 in the framework of dealuminated faujasites with the restriction already mentioned in the discussion of the TPD results with respect to samples containing NH3 coordinated to Lewis sites.

To elucidate the interaction of the different hydroxyl groups with ammonia, Figure 5 shows in more detail the IR spectra in the hydroxyl stretching region before and after the adsorption of NH3. The different amount of A1 atoms in the faujasite

Highly-Dealuminated Faujasite-Type Zeolites The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9745

.4

Ln

f

.2

s 3 B

0

1 1 N H 3 -TPD, 3 7 3 - 7 7 3 K , 10°/min I ~~

3000 zoo0 WAVENUMBER (om- 1 )

Figure 6. FTIR-TPD spectra of US-Ex after the adsorption of NH3 at 100 OC on the activated sample and subsequent evacuation at 100 OC.

NH -TPD, 373-7731(, 10°/min 3 35'00 30'00 25'00 2000 1500

WAVENUMBERS (cm-1)

Figure 7. FTIR-TPD spectra of US-Ex(C) after adsorption of NH3 at 100 OC on the activated sample and subsequent evacuation at 100 OC.

3500 30'00 2500 2000 1500 WAVENUMBERS (cm-1)

Figures. FTIR-TPD spectra of US-Ex(CE) after the adsorption of NH3 at 100 OC on the activated sample and subsequent evacuation at 100 OC.

could be the result of NH3 interacting of with the terminal hydroxyl groups, as shown in the next section. FTIR-TPD. The desorption of adsorbed ammonia also can be

followed by time-resolved IR spectroscopy. As an example, the desorption of NH3 with increasing temperature (10 OC/min) is shown in Figures 6-8 for the US-Ex series. The difference spectra with respect to the background recorded at the same temperature show that the intensity of the bands caused by adsorbed ammonia

.005 1

I I 1700 1600 1500 1400

WAVENUMBER (cm- 1)

Figure 9. Difference spectra of US-Ex(C) as obtained as the difference between the spectra in Figure 7, showing the amount of ammonia desorbed in the temperature intervals 110-113 (l), 113-127 (2), 127-140 (3), 140-153 (4), and 153-167 OC (5).

decreases during desorption and that the bands of acid hydroxyl groups reversibly reappear.

The spectra also show the significantly higher relative amount of NH3 bonded to Lewis acid sites (bands at 1620 and about 1300 cm-l) in the case of US-Ex(C) (Figure 7), which contains EFAL species after theadditional calcination at 800 OC, and thedecrease of these sites after the subsequent acid leaching in US-Ex(CE) (Figure 8).

Calculating the differences of the NH3 and NH4+ absorbances in subsequent spectra (AT-AT+,,w), we obtaineddifferencespectra showing the desorption of NH3 from the different adsorption sites in the corresponding temperature interval ( T + An. The first of these desorption spectra for US-Ex(C) in theN-H bending region are shown in Figure 9. Because of the different desorption behavior of ammonia adsorbed on different sites, it is possible to obtain a better resolution for different absorption bands. The shoulder at about 1685 cm-l in the corresponding spectrum in Figure 3 appears in Figure 9 as a broad, well-resolved band, and the absorption band at about 1450 cm-1 obviously consists of two components with the part at 1485 cm-l desorbing at lower temperatures than themaincomponent at 1450cm-1. Integrating the desorption absorbances, we can obtain the relative amount of ammonia desorbed from the corresponding adsorption site. Even taking into account that the extinction coefficients of ammonia adsorbed on different sites may be different, these relative amounts of desorbed ammonia are now directly com- parable with those in the conventional NH,-TPD experiments, with the advantage that we can calculate the desorption for the different adsorbed species independently. The same procedure can be used to study the changes of the IR bands of the hydroxyl groups. The results are shown in Figures 10-1 2. Because of the small intensities of theNH3 and OH bands in highly dealuminated zeolites, there is some scatter in the data; still the general trends are obvious. The shapes of the integral curves (Figure loa) are generally similar to those obtained by conventional NH3-TPD (Figure 2). The desorption peaks are more narrow and system- atically shifted by about 100 OC to lower temperatures in the case of IR-TPD. The high-temperature desorption peak is located at 250-300 OC, as compared with 350-400 OC in conventional NH3-TPD (Figure 2). These differences are probably attributable to the quite different experimental conditions: In conventional NH3-TPD, the desorption is performed in a flow of an inert gas at 1 atm, whereas the desorption in the 1R cell is followed in vacuum (10-3-10-4 Torr). The low-temperature desorption obtained by conventional TPD starts already below 200 OC. At the conditions of IR-TPD, this part of weakly adsorbed NH3 desorbs obviously during the evacuation at 100 OC, before starting

9746 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 Miessner et al.

1 .80

m 8 1.20 5 c s s E - - ’ 0.80 B

0.00 100 180 280 340 420 500

Temperature(OC)

1.80

m 8 1.20 5 c s n U C - I n 0

0.80 B

0.00 100 180 280 340 420 500

Temperalure(DC) Figure 10. FTIR TPD curves of ammonia desorbed from (a, top) US-Ex (A). US-Ex(CE) (e), DAY (0). and HZSM-ZO(D) (v) and (b, bottom) of US-Ex (A), US-Ex(C) (.);and US-Ex(CE) (i), as obtained from the sum of desorption absorbances (see text).

the IR-TPD run. As a result, the low-temperature desorption obtained by IR-TPD is significantly smaller than that obtained by conventional TPD.

Conventional NH3-TPD (Figure 2) and FTIR-TPD (Figure 10) analyses both show the existence of different adsorption sites for the highly dealuminated faujasites under study. Apart from the US-Ex(C) with part of the Bransted acid sites blocked, the intensity of the high-temperature peak is strongly related to the Si:AI ratio, decreasing in theorder US-Ex > US-Ex(CE) > ZSM- 20(D) > DAY (Figure loa), and may therefore be assigned to the structural acid hydroxyl groups. To discuss the different adsorption sites of ammonia, it is worthwhile to follow the US-Ex series after additional calcination and acid leaching (Figure lob). Obviously, a signficant part of strong acid sites in US-Ex(C) is blocked by cationic aluminum oxide species, as already mentioned in the discussion of the NHs-TPD results. After the subsequent extraction of these species by acid leaching, the number of strong acid sites increases again (US-Ex(CE)).

The assignments of the different adsorption sites can be proved by the detailed analysis of the desorption spectra, which is possible

0.50

0.40

m 8

s - 0.20 g 0 n

0.30

E -

0.10

100 180 280 340 420 500

Temperature (‘C)

Figure 11. FTIR TPD curves of ammonia desorbed from US-Ex(C) for the band at 1450 cm-l (NH,+, e), compared with the evolution of the Br~rnsted acid sites with hydroxyl bands (expanded by a factor of 4) at 3632 (A) and 3568 cm-1 (0).

0.25

0.20

! 5

s - s - 0.10

d

0.15 g

g

0.05

0.00

\ \

L! , 100 180 280 340 420 500

Temperature (‘C)

Figure 12. FTIR TPD curves of ammonia desorbed from US-Ex(C) for the band at 1685 cm-I (v), 1485 cm-l (e), and 1620 cm-1 (O), compared with the evolution of the band of terminal hydroxyl groups (expanded by a factor of 4) at 3749 cm-l (0). in the case of FTIR-TPD. For example, in Figures 11 and 12 theintensity decreaseof thedifferent absorption bands of adsorbed ammonia is compared with the corresponding intensity increase of hydroxyl bands for US-Ex(C). The band a t 1450 cm-I, usually assigned to NH4+ cations, is strongly coupled with the structural hydroxyl groups: The intensity decrease of the band a t 1450 cm-I during the desorption of NH3 is connected to a simultaneous intensity increase of both hydroxyl bands with the same tem- perature maximum at about 280 O C for all three profiles (Figure 11). The high-temperature desorption peak of NH3, which is mainly determined by the band at 1450 cm-1, is therefore connected to the structural Brernsted acid sites in the Y zeolites and HZSM-2O(D). It is interesting to note that both structural hydroxyl groups behave in the same way, i.e., there is no difference in the desorption behavior of NH3 and, consequently, in the strength of the corresponding acid sites in different positions.

Highly-Dealuminated Faujasite-Type Zeolites

The temperature maxima of the FTIR-TPD profiles (Figure loa) for the different dealuminated Y zeolites and for HZSM- 20(D) differ slightly (240-280 "C) in the order DAY ?: HZSM- 20(D) < US-Ex(CE) =: US-Ex. Considering the different amount of adsorbed ammonia, it would bedifficult todifferentiate the strength of the Bransted acid sites on the basis of the small differences of the temperature of the peak maxima. It seems that the strength of the Bransted acid sites is similar in all dealuminated Y zeolites studied. This result is not unexpected because in all samples under study the structural acid sites are isolated (withaSi:Alratio(> 10) andtheacidstrengthismaximal. Also, the acid sites in dealuminated ZSM-20 seem to have the same strength.

The other IR absorption bands of adsorbed ammonia behave quite differently (Figure 12). The two bands at 1685 and 1485 cm-l disappear rapidly and, at temperatures higher than 230 OC, are no longer visible (Figure 9). On the basis of the wavenumbers, these bands should be assigned to weakly bonded NH4+ ~pecies,3~9~* but there is no obvious connection withoneof the bandsattributed to the hydroxyl groups. Because the intensity of the bands at 1685 and 1485 cm-1 is small compared, for example, with the band at 1450 cm-1, it is possible that the corresponding changes in the hydroxyl stretching region are masked or cannot be detected at this level of dealumination. These weakly bonded species contribute to the low-temperature desorption of NH3, as shown in conventional and FTIR-TPD (Figures 2 and 10) analyses. The desorption of ammonia that causes the absorption band at 1620 cm-' occurs over the whole temperature interval without any significant maximum (Figure 12). Comparing this with the increasing intensity of the IR band of terminal hydroxyl groups at 3749 cm-1 indicates a connection between these two species. There is probably an interaction between NH3 adsorbed on Lewis acid sites and the high-frequency (3749 cm-l) part of terminal hydroxyl groups. A similar relationship has been found by Janin et a1.,2O who studied the adsorption of pyridine on dealuminated Y zeolites. They found an interaction between pyridine bonded on Lewis acid sites and the high-frequency part of terminal hydroxyl groups (3749 cm-*) without forming pyridinium ions.

Rh Dicarbonyl on the Bransted Acid Sites. One objective of the present work was the comparison of the strength of the Bransted acid sites of ZSM-20 with those of the Y zeolites. Comparing the acid strength of the structural hydroxyl groups of dealuminated Y zeolite and SAPO molecular sieves, we could recently show that Rh dicarbonyl is a sensitive probe molecule for isolated acid sites. The carbonyl stretching frequency of the Rh dicarbonyl, formed with the oxygen atoms of the acid sites as ligands, can be used to estimate the basicity of these oxygen atoms and, consequently, the acidity of the corresponding acid sites.31

The interaction of CO with the Rh-loaded samples results in the formationof RhI(C0)2+, as shown in Figure 13. The intensity of the carbonyl stretching bands increases in the order DAY < HZSM-20 < US-Ex(CE) < US-Ex, corresponding to the decreased Si:A1 ratio, i.e., to the increased amount of Bransted acid sites available for forming the well-defined dicarbonyl species. The wavenumbers of the carbonyl stretching vibrations in the dicarbonyl are the same for all samples: 21 18 cm-I (v,) and 2053 cm-1 (vu). In agreement with the TPD data, we conclude from these results that the strength of the isolated Br~lnsted acid sites is the same in all samples.

Conclusions

Temperature-programmed desorption and FTIR spectroscopy of ammonia adsorbed on highly dealuminated faujasite-type zeolites US-Ex, DAY, and ZSM-20 with different dealumination histories, different pore structures and Si:A1 ratios from 40:l to 300: 1 provide the following conclusions:

The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9147

I I 4

2150 2100 2050 2000 1950 1900

WAVENUMBER (cm-1)

Figure 13. IR spectra in the carbonyl stretching region of Rh-loaded (1 wt %) dealuminated zeolites after interaction with about 10 Torr of CO at 150 O C and subsequent evacuation at room temperature.

A correlation has been found between the amount of adsorbed NH3 as determined by conventional TPD and the integrated absorbances of N-H bending bands in the FTIR spectra.

Time-resolved FTIR spectroscopy during TPD of ammonia and a subsequent calculation of desorption absorbances allow a direct comparison with the conventional TPD profiles, with the advantage that the IR desorption profiles can be obtained for the different adsorbed species independently and can be compared with the corresponding changes in the OH stretching region.

The isolated structural Bransted acid sites have a similar acid strength in all samples, including the HZSM-20, regardless the different dealumination procedures.

The extraframework aluminum species formed during dealu- mination may block a significant part of the strong Bransted sites.

The acid framework hydroxyl groups in both the w a g e (causing the H F band) and the ,%cage (LF band) are involved in the same way in the desorption behavior of NH3.

In addition to bonding on strong Bransted acid sites, NH3 is adsorbed on weak Bransted and/or Lewis acid sites and on stronger Lewis acid sites. The ammonia bonded to the stronger Lewis acid sites seems to interact with a part of the terminal hydroxyl groups.

The similarity of the isolated framework Bransted acid sites in the Y zeolites studied, and also in the HZSM-20, has been proven by using rhodium dicarbonyl Rh1(C0)2+ as a sensitive probe molecule for the characterization of isolated acid sites.

Acknowledgment. The authors wish to thank the Bundesmin- ister fiir Wirtschaft for financial support (AIF 268 D). H.M. and U.L. thank the Fonds der Cbemischen Ind., and U.L. thanks the Deutsche Forschungsgemeinschaft for financial support. P. Haase and P. Rassler are acknowledged for technical assistance. Degussa is acknowledged for supplying the DAY samples.

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