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-AnhTuan 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 ( N H r T P D ) 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 hydrothermal 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 N a y , by treating it with Sic&,leads to a material (DAY) with almost no me sop ore^.^ To avoid the additional postsynthesisdealuminationprocedure, 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 orderings. 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-12ring 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:A1ratios, 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*3-21and by TPD of ammonia ( N H ~ - T P D ) . ~ VIt~ ~ - ~ ' 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 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 NH3TPD 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 USEx and in US-Ex(C) there is a considerable amount of NH3
Miessner et al.
9744 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 50
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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 characteristically 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 L F band to higher wavenumbers. For dealuminated Y zeolites, this already has been mentioned.39~40The same resultsare also obtained for ZSM20 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 L F 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-'. Theassignmentof 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 3616 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
=
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Figure 4. Correlationbetween 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
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--
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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
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9745
Highly-Dealuminated Faujasite-Type Zeolites .4 Ln
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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.
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
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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 1620and about 1300 cm-l) in the case of US-Ex(C) (Figure 7), which contains EFAL species after theadditional calcinationat 800 OC, and thedecrease of these sites after the subsequent acid leaching in US-Ex(CE) (Figure 8). Calculating the differencesof the NH3and NH4+ absorbances in subsequentspectra (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 comparable 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 theNH3and 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 systematically 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). Thesedifferences 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 desorbsobviously during the evacuationat 100 OC, before starting
+
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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
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Figures. FTIR-TPDspectra 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 differencespectra with respect to the background recorded at the same temperature show that the intensity of the bands caused by adsorbed ammonia
Miessner et al.
9746 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 0.50
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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~rnstedacid sites with hydroxyl bands (expanded by a factor of 4) at 3632 (A) and 3568 cm-1 (0).
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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) as obtained from of US-Ex (A), US-Ex(C) (.);and US-Ex(CE) (i),
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) > ZSM20(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 cationicaluminum 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
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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 1450cm-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 temperature 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 Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9147
The temperature maxima of the FTIR-TPD profiles (Figure loa) for the different dealuminated Y zeolites and for HZSM20(D) differ slightly (240-280 "C) in the order DAY ?: HZSM20(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 correspondingchanges in the hydroxyl stretchingregion 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 conventionaland 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 availablefor formingthe 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~lnstedacid sites is the same in all samples.
Conclusions Temperature-programmed desorptionand 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:
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Figure 13. IR spectra in the carbonyl stretching region of Rh-loaded (1 wt %) dealuminatedzeolites 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 dealumination 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 Bundesminister 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. References and Notes (1) McDanieLC. V.; Maher,P. K. MolecularSieues;SocietyofChemical Industry: London, 1968; p 186. (2) Stach, H.; Lohse, U.; Thamm, H.; Schirmer, W. Zeolites 1986, 6, 74. (3) Lohse, U.; Parlitz, B.; Patzelava, V. J . Phys. Chem. 1989,93, 3677. (4) Beyer, H. K.; Belenykaya, I. Catalysis by Zeolites; Imelik, B., Ed.; Elsevier: Amsterdam, 1980; p 203. (5) Ciric, J. U S . Patent 3,972,983, 1973. (6) Newsam, J. M.; Tracy, M. M.J.; Vaughan, D.E.W.; Strohmaier, K. G.; Mortier, W. J. J. Chem. Soc., Chem. Commun. 1989,85,493. (7) Skeels,G. W. Synthesis of Microporous Materials; Occelli, M. L., Robson, H., Van Norstrand Reinhold: New York, 1992; Vol. 1, p 42.
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