27
Topics in Catalysis 4 (1997) 27^42
Effect of steaming on the defect structure and acid catalysis of protonated zeolites R.A. Beyerlein a, C. Choi-Feng c, J.B. Hall b, B.J. Huggins b and G.J. Ray a
b
US Department of Energy, Chemical Sciences Division, ER-142, 19901 Germantown Road, Germantown, MD 20874, USA b Amoco Research Center, PO Box 3011, Naperville, IL 60566, USA c Amoco Polymers, Inc., 4500 McGinnis Ferry Rd., Alpharetta, GA 30202, USA
The catalytic properties of ultrastable Y (USY) are directly influenced by the zeolite destruction which occurs during formation of USY and during subsequent hydrothermal treatment. A new picture of the formation and evolution of mesopores during hydrothermal treatment has emerged from recent electron microscopy studies on hydrothermally dealuminated USY materials. Laboratory steam treatments give rise to an inhomogeneous distribution of mesopores, which occurs concomitantly with further zeolite dealumination. Such inhomogeneities are observed among different USY grains as well as within single grains. In regions with high defect concentration, mesopores ``coalesce'' to form channels and cracks which, upon extended hydrothermal treatment, ultimately define the boundaries of fractured crystallite fragments. The predominant fate of aluminum ejected from lattice sites appears to be closely associated with dark bands which often decorate these newly formed fracture boundaries. High-silica Y materials, having little or no nonframework Al, exhibit poor catalytic activity. The results of recent studies provide compelling evidence that the critical nonframework Al species are (1) highly dispersed, and (2) quite possibly exist as cationic species in the small cages of dealuminated H-Y. Investigations of Lewis acidity in mildly dealuminated zeolites indicate that the origin of the high catalytic activity is a synergistic interaction between Br nsted (framework) and highly dispersed Lewis (nonframework) acid sites. The enhanced cracking, isomerization activity associated with the presence of highly dispersed nonframework Al species is not reflected in direct measures of solid acidity, as, for example, by calorimetry, or by NMR spectroscopy. The enhanced activity of mildly steamed protonated zeolites is not due to an increase in acidity of the bridging hydroxyl or Br nsted sites. Keywords: zeolitic solid acids; mechanisms of dealumination; zeolite defect structure; acid catalysis
1. Introduction The catalytic properties of ultrastable Y (USY) are directly influenced by the zeolite destruction which occurs during formation of USY and during subsequent hydrothermal treatment. For ultrastable, high-silica, FAU framework materials prepared by steam dealumination, interpretation of catalytic data is complicated by the presence of entrained, nonframework aluminum (NFA) species. While the individual and collective roles of framework and nonframework aluminum species are not well understood, it is now clear that the presence of some nonframework Al is essential for the strong solid acidity exhibited by high-silica H-Y [1^3]. These critical nonframework species are probably isolated. While they are not easily subject to direct observation, the existence of isolated NFA species in dealuminated materials is not in doubt. The importance of certain nonframework Al species in the development of strong acidity in protonated (proton-exchanged) zeolites is not limited to H-Y. A review of recent literature shows a consensus that development of ``enhanced activity'' in mildly steamed HZSM-5 is also critically dependent on the presence of nonframework Al [4^6]. Knowledge of framework geometry is essential for understanding overall reactivity patterns for hydrocarbon conversions over these open framework solid acids. The well known features of ``molecular traffic manageÄ J.C. Baltzer AG, Science Publishers
ment'' exhibited by these materials are not always limited to molecular sieving, that is reactant or product size exclusion effects. For example, at reaction temperatures of 400^500 C, the large-pore zeolites, dealuminated HY (H-ultrastable Y) and dealuminated mordenite, each catalyze the isomerization of isobutane to n-butane [7,8]. Under similar conditions, the medium-pore system HZSM-5 produces relatively little n-butane, but instead yields much methane and propylene [8,9]. The dramatically different product selectivities in the latter case are attributed to the more severe spatial restrictions of the medium-pore ZSM-5, which tend to inhibit bimolecular processes involving bulky reaction intermediates. Ever since the rapid commercialization in the early 1960's of a zeolite-catalyzed process for gas oil cracking [10,11], zeolites have comprised the predominant solid acid catalyst. The characterization and application of high-silica, protonated zeolites in fluid catalytic cracking has been reviewed by Scherzer [12]. A broader overview of the use of zeolites in hydrocarbon processing is given by Maxwell and Stork [13]. Recently, a large number of potential zeolite applications in the synthesis of intermediates and fine chemicals have been demonstrated [14,15]. Despite high industrial and academic interest, the nature of the active site in solid acids remains largely unresolved. In systems of interest such as a protonated zeolite, a chlorided or fluorided alumina, or sulfated zirconia, we are unable to quantify the distribution or rela-
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R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
2. Defects produced by hydrothermal treatment
ammonium (or H) form of the Y-zeolite at T 500 C, PH2 O 1 atm, 2 h, first leads to the expulsion of Al atoms from framework T-sites as indicated in fig. 1. (Here, the positive charge on the proton is balanced by the negative charge on the framework associated with the presence of framework Al3 .) In the second step, the vacancies created by aluminum expulsion from the lattice during hydrothermal treatment are, to a great extent, refilled by silicon atoms migrating from the collapsed portions of the crystal. If this ``healing'' did not occur, the entire zeolite crystal would collapse. The resulting restructured FAU material, commonly known as ultrastable Y or USY [21,22], displays a contracted unit cell size and increasing hydrothermal stability as framework Si/Al increases. A typical reduction in unit cell dimension is from 24.70 Ð for the starting zeolite Y to 24.56 Ð for the product USY. Increasingly severe steam treatments result in a higher level of dealumination, a more contracted unit cell, and an increasing level of crystalline zeolite destruction. Prior sorption studies indicate that entire sodalite units, or -cages, are destroyed during hydrothermal dealumination [23,24], leading to the formation of a secondary pore system [25] or mesoporosity in the range of 5^50 nm, in addition to the zeolite micropores. Scherzer [12,25] has suggested that these regions of zeolite destruction comprise the silica source for ``healing'' the tetrahedral vacancies left by hydrothermal dealumination. As shown schematically in fig. 2, the collapse of sodalite units, or even assemblies of them, generates mesoporosity and simultaneously provides the source of Si atoms for ``healing.''
2.1. Background ^ formation of ultrastable Y
2.2. Nonframework aluminum ^ local environment
The formation of ultrastable FAU materials may be viewed as a two-step process in which calcination of the
Nonframework aluminum (NFA) is one of a wide collection of defects which are produced during the forma-
tive importance of Br nsted/Lewis sites, the surface acid strength, or the concentration of acid sites [82]. Recent efforts to improve our understanding of these issues have shown substantial progress in physical characterization of sites. Both 1 H MAS NMR [16^18] and 13 C MAS NMR [19] have been effective in helping to characterize the structure and function of Br nsted sites in HZSM-5, a favorable system for analysis owing to its low site density and good crystallinity. So far, only ``clean framework'' ZSM-5 has been reasonably well characterized. The extension of these and related spectroscopic studies to the more complex systems represented by mildly steamed, ``activity enhanced'' HZSM-5 or dealuminated H-Y comprises a significant experimental challenge. In this paper, we review recent advances in the characterization and understanding of defect formation in protonated zeolites during hydrothermal treatment and how this defect formation plays a critical, although not well understood role in the acid catalysis exhibited by these materials. The discussion of the influence of defects on acid catalysis is cast primarily in terms of kinetic methods for classifying solid acidity. As noted in a recent review by Haag [20], the question of acid strength is much more problematic and complicated. Theoretical efforts to elucidate structure^acidity relationships are primarily limited to the ``clean framework'' case. Direct measures of solid acid strength, including ``in situ'' calorimetric and/or spectroscopic methods will often, but not always correlate with catalytic properties.
Fig. 1. Schematic representation of formation of ultrastable Y materials.
R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
29
Fig. 2. Schematic model for mechanisms of dealumination in FAU: Si for ``healing'', source of mesoporosity.
tion of USY and during subsequent hydrothermal treatment. Most of our accumulated information on physical characterization of NFA species comes from high-resolution solid state NMR of 27 Al nuclei, 27 Al MAS NMR, and from high-resolution electron microscopy (HREM). Complementary information is provided by X-ray and neutron diffraction structural studies. NFA species are themselves composed of several different types, some isolated, some agglomerated, as outlined in table 1. It has proved particularly difficult to characterize the different NFA species in dealuminated H-Y. Structural studies using X-ray and neutron diffraction have indicated the presence of octahedral microcrystalline aluminum species in the supercages [26], and isolated tetrahedral aluminum species in the small (sodalite) cages [27], but do not give much information about agglomerated noncrystalline species. A systematic study of the reduction in micropore volume [28] resulting from mild dealumination of H-Y, and of HZSM-5, indicated that the majority of NFA species go to the micropores available to N2 , the supercages ( -cages) in the case of H-Y, and the channels or channel intersections in the case of HZSM-5. The combination of high-resolution 29 Si and of 27 Al solid state NMR has been effectively
applied to studies of hydrothermally dealuminated Y zeolites [29]. That is, 29 Si NMR provides direct information on the composition and Si, Al distribution of the tetrahedral framework, independently of the presence of nonframework Al species, while 27 Al NMR allows a clear distinction between tetrahedral framework Al ( 60 ppm) and octahedral nonframework Al (0 ppm). Nevertheless, interpretation of 27 Al NMR in terms of Al species location or state of agglomeration is often ambiguous. Even at low levels of dealumination, the contribution of nonframework species to the tetrahedral resonance cannot be ruled out. Extensively dealuminated samples typically show substantial broadening of the tetrahedral resonance [30,31], as shown in fig. 3, only a small portion of which can be attributed to framework Al [31,32]. In addition, as higher magnetic fields and faster sample spinning have become more routine, a new resonance has been observed at 30 ppm (see, e.g., fig. 3) which has been attributed to either an aluminum in a highly distorted tetrahedral environment [31^34] or a penta-coordinated aluminum species [34,35]. A recent application of the novel double-rotation (DOR) spinning technique [36,37] to the study of 27 Al in zeolites [38] has shown that, for a commercial USY material, there
Table 1 Types of aluminum a in ultrastable Y Types detectable by NMR
Probable structure description
Isolated or clustered
Most abundant
TF TNFA TNFA
T-site Al in small cage and/or alumina species in supercage and/or surface enrichment intermediate between octahedral and tetrahedral alumina species in supercage and/or surface enrichment
isolated isolated clustered
mildly dealuminated mildly dealuminated severely dealuminated
unknown
severely dealuminated
clustered
always present
PNFA ONFA a
T tetrahedral, O octahedral.
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R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
Fig. 3. Evolution of dealuminated USY as tracked by 27 Al MAS NMR [32]. (-) Framework Al; (1) nonframework Al.
are two different tetrahedral Al species, one framework and the other nonframework. On the basis of comparison of single-pulse 27 Al MAS NMR and 27 Al CP MAS NMR taken on a steamed Y zeolite, Fripiat and coworkers [39] concluded that a substantial portion of the band near 60 ppm is contributed by tetrahedrally coordinated NFA. 2.3. Nonframework aluminum and mesoporosity Previous transmission electron microscope (TEM) studies of hydrothermal aging of neat USY materials [40^44] and also of USY cracking catalysts [45^48] have shown 5^50 nm defect domains, which were attributed to mesopores. Such features, more pronounced in the presence of vanadium [45,46], are characteristic of extended hydrothermal treatment. Typical porosity analyses of mildly steamed USY materials show a distribution of mesopore dimensions in the range 5^50 nm that is skewed toward the smaller sizes [41,43], further supporting the association of the light amorphous zones observed by TEM with the secondary pore system characteristic of USY materials. A new picture of the formation and evolution of mesopores has emerged from recent combined highresolution electron microscope (HREM) and analytical electron microscope (AEM) investigations on hydrothermally treated USY materials [49]. In contrast with results of previous TEM investigations, the HREM and AEM study of a steam/acid-treated neat USY material and of a high-temperature steam-treated USY cracking catalyst [49] gave clear evidence for an inhomogeneous
distribution of mesopores, which occurs concomitantly with further zeolite dealumination. Such inhomogeneities are more pronounced for the steam-deactivated USY cracking catalyst which had been steam treated at a higher temperature than for the neat USY material. Notably, they are observed among different USY grains as well as within single grains (fig. 4), which, together with the observed temperature dependence, indicates that the extent of inhomogeneity is driven by the nonequilibrium process represented by accelerated steamaging treatments in the laboratory. In regions with high defect concentration, mesopores ``coalesce'' to form channels and cracks (figs. 4a, 5), which ultimately define the boundaries of fractured crystallite fragments. At these boundaries, a dark band is often observed which is highly enriched in aluminum (fig. 5), while within the mesopore, aluminum appears to be deficient (fig. 6). These results indicate that framework dealumination and subsequent Al migration occur concomitantly with mesopore formation. For extended hydrothermal treatment, the inhomogeneous pattern of mesopore formation leads to zeolite crystallite fracture, and the predominant fate of aluminum ejected from lattice sites appears to be closely associated with the dark bands which often decorate these newly formed fracture boundaries. These features were observed both for the steamed USY cracking catalyst (fig. 7) and for the steam/acid-treated neat USY zeolite (figs. 4, 5), consistent with previous studies that found the surface enrichment of Al to persist through aqueous treatments which removed substantial amounts of aluminum [50]. For the neat USY material, the associated development and evolution of nonframework Al species was investigated by high-resolution solid state 27 Al MAS NMR (fig. 3) [32]. From this parallel study, it was concluded that the extracrystalline phases represented by the dark bands revealed in the electron microscopy studies contribute the majority of the nonframework aluminum species, tetrahedral, penta-coordinate, and octahedral, that were detected by 27 Al NMR. Comparison of the results of the recent HREM studies with an earlier TEM investigation of age-separated equilibrium catalyst [48] from a commercial fluid cracking unit (FCU) left some doubt as to a common mechanism for deactivation for the case of slower catalyst deactivation in the FCU. In particular, the TEM results on an ``old'' fraction were silent on the question of whether the small crystallites remaining were demarcated by fracture boundaries. Re-investigation by HREM of the age-separated FCU fractions [51] showed that an FCU ``young'' fraction is similar to the labsteamed samples except that defect patterns are more homogeneous than for the case of accelerated aging in the lab (fig. 8). An FCU ``old'' fraction shows more destruction, higher concentrations of mesopores, more Al-enriched bands, and more highly fractured grains than were found for the laboratory-aged samples (fig. 9).
R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
31
a
b
Fig. 4. (a) TEM image of a few steam/acid-treated USY grains. An inhomogeneous distribution of mesopores is seen within individual grains; some grains contain more mesopores than others. In regions with high mesopore concentration, the pores coalesce to form channels (as indicated by arrows). (b) HREM image of steam/acid-treated USY grains. Many mesopores are formed. Although localized disorder is observed within the pores, the connecting regions remain crystalline [49].
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R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
Fig. 5. HREM image of a steam/acid-treated USY grain. Cracks (as indicated by arrows) are formed from the evolution of the coalesced mesopores. Dark bands which were found to be Al rich are seen along these cracks [49].
3. Origins of acidity in ultrastable Y 3.1. Background ^ some nonframework aluminum is essential
Fig. 6. Al, Si analyses from a STEM image of the region adjacent to and also within a mesopore. Within a mesopore, Al is slightly deficient [49].
The central role of framework Al content in defining the catalytic properties of dealuminated H-Y was discussed in a classic paper by Pine et al. [52]. The prediction of catalyst activity, selectivity, and octane performance was correlated with unit cell size, which, in turn, is effectively correlated with the number of aluminum atoms in the framework. The result of this study is ``generically'' summarized in fig. 10. With increasing dealumination and concomitant loss of framework Al, selectivity tends to increase, but at the expense of poorer activity. (The advent of high-resolution solid state NMR a few years prior to the Pine et al. study [52] made possible the determination of framework composition (Si/AlF ) for dealuminated materials as shown in fig. 11, thus allowing the routine tracking of framework Al (AlF ) content by XRD measurement of unit cell size.) The importance of nonframework aluminum
R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
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Fig. 7. HREM image showing the fracturing of a USY grain in the steamed USY catalyst. Each fractured crystallite is bounded by cracks evolved from coalescence of mesopores. The dark band seen along each crack is Al enriched, similar to those observed in the steam/acid-treated neat USY material [49].
(NFA) was not revealed in studies such as this, which may be one reason this issue was largely overlooked until relatively recently. The more obvious manifestations of NFA species, such as the agglomerated species found in the electron microscopy studies discussed in the previous section, are thought to contribute to increased coke make, not desirable products. Thus, NFA species have been regarded as undesirable, amorphous ``debris''. This picture has been changed by more recent studies which show that high-silica H-Y materials, having little or no nonframework Al, exhibit poor catalytic activity in comparison with ultrastable Y materials having comparable levels of framework Al (see refs. [1^3] and table 2). It is therefore concluded that the presence of some nonframework Al is essential. It is now generally accepted that the activity and selectivity of Y zeolites in catalytic cracking are determined by an interplay of framework aluminum and nonframework aluminum species. The implicated NFA species are thought to be well dispersed and may exist as isolated, cationic species in the small cages.
3.2. Acidity in zeolites ^ the role of framework aluminum Model compound studies, such as paraffin cracking over dealuminated H-Y, and also studies involving full range feed show that catalytic performance is closely tied to structural or framework Al (AF ) content. It is concluded that active sites are associated with AlF . However, as will be discussed in section 4, recent investigations indicate that only a fraction of the framework Al represents active sites. Significant progress has been made in understanding the reactivitiy of the zeolitic BrÖnsted acid site in terms of modeling, theoretical approaches which seek to understand the nature and energetics of carbonium and carbenium ion formation [83^86]. Strong acidity in zeolites is associated with the presence of such Br nsted sites which arise from the creation of internal hydroxyl groups. These hydroxyls are formed either by mild calcination of an ammonium-exchanged zeolite or by direct exchange with a
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R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
Fig. 8. HREM image of a ``young'' fraction from a commercial fluid catalytic cracking unit. Dark bands are observed along many cracks, i.e., crystallite fracture boundaries [51].
with a mineral acid to produce a ``protonated zeolite'': HZ or, equivalently,
H proton
Z neg: charged framework
The ``proton form'' of the zeolite contains hydroxyls which are protons associated with the negatively charged framework. These ``donatable'' protons give rise to Br nsted sites.
Br nsted sites associated with framework AlF
The above schematic provides a conceptual model for high-silica, ``clean framework'' FAU materials, such as those prepared by chemical dealumination using ammonium hexafluorosilicate (AHF) [53]. Such clean frame-
work materials have been found to exhibit relatively poor activity in acidity-demanding reactions [1^3]. Infrared spectroscopy has been responsible for much of the earlier work contributing to the understanding of zeolite catalysis [54]. In conjunction with the use of adsorption of a probe molecule with a basic functional group such as pyridine, infrared spectroscopy can be used to determine which OH bands are acidic as discussed in a recent review by Janin et al. [55]. FT-IR absorption spectra for a series of increasingly dealuminated USY's are shown in fig. 12. In comparison with the simple two-band spectra of ``decationized'' H-Y, produced by extensive ammonium exchange of Na-Y followed by mild calcination, OH bands in the series of USY's are enormously complex. USY materials typically show an ``acidic'' OH band in the vicinity of 3600 cm 1 . No such band appears in ``decationized'' H-Y produced by ammonium exchange/mild calcination of Na-Y. This and related issues have been discussed in some detail by Dwyer and co-workers [56,57], who show that only a portion of the 3600 cm 1 band in USY is acidic and
R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
35
Fig. 9. HREM image of an ``old'' fraction from a commercial fluid catalytic cracking unit. Extensive crystallite fracture and many prominent dark bands are observed [51].
that a second acidic OH band, associated with the -cage, is found at 3525 cm 1 . 3.3. Acidity in zeolites ^ the role of nonframework Al 3.3.1. Synergism between framework and nonframework sites The protons in a protonated zeolite become increasingly mobile as the temperature is increased. Above about 500 C, they are lost as water with the consequent formation of Lewis sites:
Fig. 10. Relation between framework composition of USY and unit cell dimension, upon which is superposed a generic representation of the role of unit cell size as a unifying concept in the catalytic properties of USY as discussed by Pine et al. [52].
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R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
Fig. 11. 29 Si MAS NMR spectra of FAU-framework materials for increasing levels of dealumination.
Such Lewis sites are unstable, particularly in the presence of H2 O/steam. So-called ``true'' Lewis sites are formed by ejecting Al species from the framework [58]:
It must be observed that these are conceptual models
only. As discussed by Fripiat and co-workers [39], there has never been any NMR evidence that a tri-coordinated Al species, as indicated in the above sketch of a Lewis site, exists in aluminas or zeolites. With the inclusion of Br nsted sites associated with framework Al, the above schematic ``true Lewis site'' provides a conceptual model for mildly steamed H-Y materials. Beyerlein et al. [1,2] suggested that the increased catalytic activity exhibited by such materials (in comparison with ``clean framework'' FAU materials with comparable framework Si/ AlF ) may involve a ``synergism between framework
Table 2 Certain NFA species are essential to good catalytic performance for ultrastable FAU materials a Si/Al
Unit cell (Ð)
Isobutane conv. rates (mol/h/g103 ) total
carb. ion
% carb. ion
1. clean framework high-silica Y
5
24.54
12
7
58
2. conventional USY
5
24.56
37
28
76
3. USY formed by mild steam treatment of clean framework material (sample 1)
8
24.41
43
33
76
catalytic performance of clean framework high-silica FAU is poor a
All reaction rates are for 500 C. Na level in all three materials is low, NaW0:15 wt%. From ref. [1].
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Fig. 12. FTIR spectra of increasingly dealuminated H-Y zeolites. (A and B) Hydroxyl bands of zeolites subjected to increasingly severe hydrothermal dealumination [55]. (A) Hydrothermal dealumination only ^ 10 is least, 50 is most dealuminated. (B) Hydrothermal dealumination followed by acid extraction. (C) Hydroxyl bands of ``decationized'' zeolite Y: ammonium exchange!mild calcination!ammonium exchange. (This H-Y is thermally unstable!) [54].
Br nsted sites and Lewis sites associated with dislodged aluminum as has been described in the previously proposed concept of superacidity'' [59,60]. A conceptual model for such a synergism was discussed by Lunsford and co-workers [3] who suggested that polyvalent Al ions in the small cages are responsible for withdrawal of electrons from the framework OH groups (``bridging hydroxyls''), thus making the protons more acidic. Fripiat and co-workers have carried out extensive investigations on the nature of Lewis sites in aluminas, dealuminated mordenite, and dealuminated H-Y using both 27 Al NMR [39,61] and FTIR on adsorbed CO [62,63]. The high-resolution 27 Al NMR studies [61] indicated the presence of two kinds of Lewis sites within the nonframework Al distribution in dealuminated zeolites ^ a tetrahedral site and a pentagonal site with isotropic shifts of about 53 and 37 ppm, respectively. The FTIR-CO adsorption studies [62,63] also revealed the presence of
two types of Lewis sites associated with nonframework aluminum. In the case of dealuminated mordenite, it was further shown that these Lewis sites were highly dispersed. A subsequent study of the isomerization of npentane and of o-xylene over dealuminated mordenites [64] showed the initial rates of isomerization to be proportional to the product of the number of available Br nsted and the number of available Lewis sites, indicating that the origin of the high acidity of dealuminated mordenites is a synergistic interaction between Br nsted (framework) and highly dispersed Lewis (nonframework) acid sites [62^64]. Very recently, Fripiat and co-workers [65] have applied high-resolution 29 Si NMR REDOR as an advantageous way to study the 1 H^ 29 Si interaction for the characterization of Br nsted sites. These workers arrived at the quantitative conclusion that a Br nsted site is an OH bridging an aluminum to a silicon with only one aluminum neighbor. An OH
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R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
bridging an aluminum to a silicon with more than one aluminum neighbor is a proton donor but not a Br nsted site capable of donating a proton to NH3 to make NH 4. Since the difference between the number of Br nsted sites and the (larger) number of AlF was found to increase with the amount of nonframework Al, it was suggested that some of the acidic OH groups have disappeared owing to their neutralization by reaction with NFA species. 3.4. Importance of the concentration and type of nonframework species The dependence of catalytic properties of dealuminated H-Y materials on unit cell size, or, equivalently on framework Al content, can be profoundly altered by the concentration and type of nonframework species. In the case of steam/mineral acid-dealuminated ultrastable Y materials with framework compositions Si/AlF X5, both hexane cracking [3,66,67], and isobutane conversion [1,2] investigations show a linear dependence of activity on framework aluminum content. However, ``unconventional'' ultrastable Y materials, prepared by mild steam treatment of a ``clean framework'', AHF dealuminated USY were found by Beyerlein et al. [1] to exhibit enhanced ``carbonium ion'' activity 1 for isobutane conversion (fig. 13). These workers suggested that the enhanced activity exhibited by these materials, in comparison with conventionally prepared USY materials of comparable framework composition, owed to their relatively lower content of nonframework Al species (AlNFA ), AlNFA /AlF 0:4. By contrast, the conventionally prepared ultrastable materials exhibit a range 0.66WAlNFA /AlF W2:2. Similar enhanced activities were observed by L onyi and Lunsford [67] in the course of hexane cracking investigations on high-silica Y materials prepared by mild steam treatment of chemically dealuminated, via AHF, (Na , NH 4 )-Y, and by Sun, Chu, and Lunsford [68] on mildly steamed ZSM-20 materials. (ZSM-20 is a high-silica, hexagonal variant of FAU.) Interestingly, for framework compositions Si/ AlF X5, each of these studies showed a ratio of nonframework to framework Al species of AlNFA /AlF 0:4, consistent with the results of earlier investigations by Beyerlein et al. [1]. While such studies demonstrate the critical role of nonframework species in the development of strong acidity, no information is provided on their location. Definitive evidence for the association of isolated, cationic species in the small cages with development of enhanced acidity was provided by Carvajel, Chu, and Lunsford [3], who showed that the presence of 1
Carbonium ion selectivities were estimated using the molar product ratio (n-butane propane isopentane)/ total conversion products. This ratio accounts for the major carbonium ion based products arising from isomerization and chain-cracking sequences. (See ref. [7].)
Fig. 13. Carbonium ion rates from studies of isobutane conversion over high-silica Y, ultrastable materials: conventionally prepared (.); prepared from materials initially dealuminated by using AHF (5). The single data point at the lower right represents the carbonium ion rate over a low-sodium, AHF-treated FAU material [1].
La3 in the small cages leads to a significant increase in hexane cracking activity over that shown by clean framework, high-silica FAU materials. For a given framework Al content, each La-exchanged material showed substantially increased activity over that of its clean framework parent material and somewhat lower activity than that of dealuminated H-Y (fig. 14). The results of this study, and also those from recent investigations of Lewis acidity in dealuminated zeolites by Fripiat and co-workers [62^64], provide compelling evidence that the critical nonframework Al species are (a) highly dispersed, and (b) quite possibly exist as cationic species in the small cages of dealuminated H-Y, as indicated from earlier structural studies [27]. 3.5. Removal of framework sites by poisoning ^ comparison with removal by dealumination The discussion of the preceding section shows that the exchange of high-silica Y zeolite with trivalent cations, such as La3 or Al3 , has important implications for solid acidity. By contrast, partial exchage of high-silicaY materials with monovalent cations, as Na or K , leads to significant reduction in activity [2,66,69]. The effect on isobutane conversion of controlled additions of Na to a commercial ultrastable Y (series 1) and to a further dealuminated ultrastable Y (series 2) is shown in fig. 15. It is apparent that Na addition suppresses activity much more rapidly than framework aluminum removal by dealumination. The addition of Na equivalent to 1/3 of the total framework Al atoms completely suppresses catalytic activity [2,69]. Even lower proportions of added Na were found to suppress hexane cracking activity in studies by Lunsford and coworkers [66]. It was concluded from these sodium poisoning studies that only a fraction of the framework Al atoms are associated with strong acidity. This conclusion is supported by results of recent studies of isopropy-
R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
39
may serve to neutralize Br nsted sites. Accordingly, there is now little doubt that only a portion of the framework Al represents active sites. The notion that each alkali ion affects only a single active site has been called into question by results of recent studies which show that controlled addition of K to dealuminated H-Y leads to a far more rapid suppression of isobutane conversion activity than does addition of Na [69]. 4. Lack of direct evidence for sites with enhanced acidity in dealuminated H-Y Fig. 14. Hexane cracking activity over variously dealuminated highsilica Y materials as a function of framework aluminum content [3] ^ the effect of La3 exchange. The intermediate activity curve shows the increase in activity following La3 exchange of clean framework materials. (Those materials exhibiting lowest (near zero) activity are variously prepared clean framework materials.) For the materials exhibiting highest activity: (-) Y-type zeolite dealuminated with SiCl4 ; (.) Y-type zeolite dealuminated by steaming [3].
lamine desorption from dealuminated H-Y by Gorte and co-workers [70], which demonstrated that framework Al content is significantly greater than Br nsted acid site density. These workers suggested that each alkali ion affects only a single active site and that framework Al is not a good measure of acid site concentration. The recent high-resolution 29 Si NMR REDOR study of mildly dealuminated H-Y by Fripiat and co-workers [65] has provided quantitative evidence that there are fewer Br nsted sites than framework Al atoms. This study showed that the difference between the number of Br nsted sites and the amount of framework Al increased with increasing levels of nonframework Al, and it was further suggested that certain NFA species
The enhanced cracking, isomerization activity associated with the presence of highly dispersed nonframework Al species is not reflected in direct measures of solid acidity. In a review of studies of solid acidity by adsorption microcalorimetry, Dumesic and co-workers [71] note that, for dealuminated H-Y, the calorimetric results obtained at room temperature do not correlate with the catalytic activity for cumene cracking at 573 K. Samples with high activity showed essentially the same values of heat of adsorption of ammonia as did samples exhibiting substantially lower activity. Gorte and coworkers [70] carried out microcalorimetry measurements of pyridine and of isopropylamine adsorption, and also measured activities for hexane cracking on a series of steamed and chemically dealuminated H-Y materials. The calorimetry results failed to find evidence for superacidic sites, and there was no correlation between hexane cracking activities and heats of adsorption for the materials examined. These workers found no evidence for a very small concentration of strong sites which had been observed in prior calorimetry studies of steam-dealuminated USY catalysts [72]. Sommer et al. [73] used 1 H and 2 H NMR to compare zeolite-Y-catalyzed versus superacid-catalyzed proton^deuterium exchange in alkanes. From investigations of the extent of H/D exchange during passage of different light alkanes over an acidic D2 O-exchanged USY material (Si/AlF 4:5), it was concluded that dealuminated H-Y cannot be considered as a superacid capable of protonating -bonds in alkanes. The acidity strength was compared to that of sulfuric acid, consistent with results of recent investigations of 13 C chemical shift measurements of mesityl oxide by Haw and co-workers [74], which indicated that the acidity of ZSM-5 is comparable to that of a solution of 70% sulfuric acid. 5. Severe versus mild steam treatment of dealuminated H-Y ^ effect on acidity
Fig. 15. Isobutane conversion studies of Na-poisoning [2,69]. Effect on carbonium ion activity of controlled Na addition to two USY parent materials with different AlF contents: (.) series 1, AlF 32; (5) series 2, AlF 21:6.
While mild steam treatment (700 KWT W850 K, PH2 O W1) of ammonium-exchanged Y materials or of clean framework, high-silica Y materials leads to sites of enhanced activity in acidity dependent reactions such as
40
R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
paraffin cracking or isomerization, severe steam treatment (970^1100 K, PH2 O 1) leads to rapid deactivation both by extensive dealumination (to framework compositions Si/AlF X20) and by extensive crystalline destruction, as reviewed in the literature on fluid catalytic cracking [12,52,75,76] and earlier in this paper. In their measurements of differential heats of ammonia adsorption, Yaluris et al. [77] have shown that, upon severe steam treatment of a USY catalyst (1060 K, PH2 O 1, 2 h) the acid site strength distribution shifts toward weaker sites. It has been suggested that the extensive dehydroxylation associated with severe steam treatment leads irreversibly to the formation of strong Lewis sites at the expense of Br nsted sites [71]. For less severely dealuminated samples, the large-scale migration and agglomeration of nonframework species discussed in section 2 appears to have a retarding effect on activity, as a portion of the acidity and catalytic activity lost by steaming can be recovered by careful acid-leaching [1,2,78,79], which removes framework and nonframework species nonselectively. It has been suggested that the microporosity modifications associated with the generation of NFA species during hydrothermal treatment could change the initial pore diameter and thus the availability of acid sites [80]. Corma et al. [81] found that removal of some of the extraframework species from severely steam-dealuminated H-Y led to an increase in alkylation activity and in catalyst life. In the case of mildly steamed HZSM-5, Lago et al. [4] attributed the generation of acid sites of enhanced activity (45 to 75 times more active than sites in clean framework HZSM-5) to the number of paired Al sites in the unsteamed parent. Upon severe steam treatment, or upon mild steam treatment of aluminum-poor HZSM-5 materials with one or less AlF /unit cell, enhanced activity is not observed. Measurements of isobutane conversion over extensively dealuminated H-Y indicate a limiting site density, AlF W6, below which carbonium ion selectivity diminishes rapidly [69] (see also footnote 1 above). Recent HREM investigations of mesopore formation in two USY materials subjected to different severities of steam treatment showed an inhomogeneous distribution of mesopores among different USY grains as well as within single grains [49]. Such inhomogeneities were found to be more pronounced for the steam-deactivated USY catalyst (1090 K, 24 h, 100% steam, fluidized bed), which had been steamed at a higher temperature than for the neat USY material. These inhomogeneities, which encompass the presence of restricted regions with more severe dealumination and also regions with milder dealumination than the average, were attributed to the nonequilibrium nature of the accelerated steam-aging treatments in the laboratory. The presence of numerous regions, less severely dealuminated than the average, can be expected to give a disproportionate contribution to the measured activity. As a result, the interpretation of measurements of solid acidity and/or catalytic activity
becomes increasingly problematic for severely steamdeactivated materials where such inhomogenities are most pronounced. 6. Conclusion The defect formation which occurs upon steam treatment of protonated zeolites plays a critical, although not well understood role in the acid catalysis exhibited by these materials. During the formation or further dealumination of a USY material by hydrothermal treatment, the collapse of small regions of the crystalline framework generates mesoporosity, simultaneously providing a probable source of Si atoms for healing the vacancies left by expulsion of Al atoms from framework T-sites. A new picture of the formation and evolution of mesopores has emerged from recent combined highresolution electron microscope (HREM) and analytical electron microscope (AEM) investigations on hydrothermally dealuminated USY materials [49]. The nonequilibrium nature of the accelerated steam-aging treatment in the laboratory gives rise to an inhomogeneous distribution of mesopores, which occurs concomitantly with further zeolite dealumination. Such inhomogeneities, which are more pronounced for higher temperature steam treatments, are observed among different USY grains as well as within single grains. In regions with high defect concentration, mesopores ``coalesce'' to form channels and cracks which, upon extended hydrothermal treatment, ultimately define the boundaries of fractured crystallite fragments. The predominant fate of aluminum ejected from lattice sites appears to be closely associated with dark bands which often decorate these newly formed fracture boundaries. Similar defect patterns, although less inhomogeneous, have been observed for an age-separated ``young'' fraction from a commercial fluid cracking unit [48,51], where the rate of deactivation is slower than for accelerated steam-aging in the lab. From a parallel study by high-resolution solid state 27 Al MAS NMR [32] of the development and evolution of nonframework Al in a neat USY material, it was concluded that the extracrystalline phases represented by the dark bands contribute the majority of the nonframework aluminum species, tetrahedral, penta-coordinate, and octahedral. Recent studies show that high-silica H-Y materials, having little or no nonframework Al, exhibit poor catalytic activity [1^3,68], and it is now generally accepted that the activity and selectivity of Y zeolites in catalytic cracking are determined by an interplay of framework aluminum and nonframework Al species. The results of a recent study of the effects of trivalent cation exchange in clean framework, high-silica H-Y materials [3], and also those from recent investigations of Lewis acidity in dealuminated zeolites [61^64], provide compelling evidence that the critical nonframework Al species are (1)
R.A. Beyerlein et al. / Effect of steaming on the defect structure and acid catalysis of protonated zeolites
highly dispersed, and (2) quite possibly exist as cationic species in the small cages of dealuminated H-Y, as indicated from earlier structural studies [27]. Investigations of Lewis acidity in dealuminated zeolites [62^64] have indicated that the origin of the high catalytic activity in dealuminated mordenites is a synergistic interaction between Br nsted (framework) and highly dispersed Lewis (nonframework) acid sites. At the same time, these studies provide compelling evidence that this is also the case for dealuminated H-Y. The enhanced cracking, isomerization activity associated with the presence of highly dispersed nonframework Al species is not reflected in direct measures of solid acidity. Calorimetry investigations have failed to find evidence for superacidic sites [70,71,77]; no correlation was found between hexane cracking activities and heats of adsorption for the materials examined [70]. 1 H and 2 H NMR investigations of the capability of USY to promote H/D exchange in light alkanes [73], and also 13 C NMR studies of ketone adsorption on H-Y and HZSM-5 [74], found no evidence for superacid sites. The latter investigations indicated that the acidity of dealuminated H-Y is somewhat less than that of HZSM-5, which was found to exhibit an acidity comparable to that of a solution of 70% sulfuric acid. The question of how nonframework aluminum affects catalytic activity remains an experimental and modeling challenge. While the enhanced activity of mildly steamed materials almost certainly involves some synergistic interaction between framework Br nsted sites and highly dispersed nonframework species, it is not due to an increase in acidity of the bridging hydroxyl or Br nsted sites. References [1] R.A. Beyerlein, G.B. McVicker, L.N. Yacullo and J.J. Ziemiak, J. Phys. Chem. 92 (1988) 1967. [2] R.A. Beyerlein, G.B. McVicker, L.N. Yacullo and J.J. Ziemiak, in: Symp. on Fundamental Chemistry of Promoters and Poisons in Heterogeneous Catalysis, American Chemical Society, New York, 13^18 April, 1986, ACS, Div. Petr. Chem. Preprints, 31 (1986) 190. [3] R. Carvajal, P.-J. Chu and J.H. Lunsford, J. Catal. 125 (1990) 123. [4] R.M. Lago, W.O. Haag, R.J. Mikovsky, D.H. Olson, S.H. Hellring, K.D. Schmidt and G.T. Kerr, in: Proc. 7th Int. Zeolite Conf., Tokyo 1986, eds. Y. Murikami et al. (Elsevier, Amsterdam, 1986) 677. [5] E. Brunner, H. Ernst, D. Freude, M. Hunger, C.B. Krause and D. Prager, Zeolites 9 (1989) 282. [6] Y. Sendoda and Y. Ono, Zeolites 8 (1988) 101. [7] G.B. McVicker, G.M. Kramer and J.J. Ziemiak, J. Catal. 83 (1983) 286. [8] P. Hilaireau, C. Bearez, F. Chevalier, G. Perot and M. Guisnet, Zeolites 2 (1982) 69. [9] G.M. Kramer and G.B. McVicker, Acc. Chem. Res. 19 (1986) 78. [10] C.J. Plank, E.J. Rosinski and W.P. Hawthorne, Ind. Eng. Chem. Prod. Res. Dev. 3 (1964) 165.
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