,I. Phys. Chem. 1986, 90, 4847-4851 Other parent zeolites were found to be amenable, to a greater or lesser degree, to conversion into their galliated forms by straightforward exposure to solutions of N a G a 0 2 . We have also found that some highly siliceous porotectosilicates can be a h minated by treatment with aqueous solutions of NaA102. A novel feature of this work is that an inactive zeolite can be converted into a very active and highly selective one in a more or less
4847
controllable fashion by a simple method of inserting heteroatoms. We acknowledge with gratitude support from the Chinese and British Governments (in the form of O.R.S. and Vice-Chancellor's Bursary to Xin-Sheng Liu). We also thank Drs. J. Klinowski and W. J. Ball for experimental assistance and stimulating discussion. Registry No. Ga, 7440-55-3.
Acid Catalysis by Dealuminated Zeolite Y. 2. The Roles of Aluminum Jong Rack Sohn? Stephen J. DeCanio,t Paul 0. Fritz, and Jack H. Lunsford* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: May 8. 1986)
Hexane cracking activity was determined for a series of dealuminated zeolites which were prepared both by treatment with SiC1, and by reaction with steam. Over a range of Si/AI ratios from 4.7 to 255 the cracking activity increased in a linear manner with respect to the number of lattice AI ions per unit cell. Thus, a constant turnover frequency is obtained, which is taken as evidence that the acid strength does not vary over this range of %/AI ratios. By contrast, a zeolite prepared by deamination of NH4-Y was very much less active than expected on the basis of the number of protonic sites. The acidity of the protons in this material is clearly less than in the dealuminated zeolite. These results support a model of strong Bronsted acidity in which a structural aluminum atom has no next-nearest aluminum neighbors in a common 4-ring of the zeolite.
Introduction The origin of strong acidity in zeolites is one of the most important practical and theoretical problems in heterogeneous catalysis. For zeolites having the faujasite structure there is a prevailing view in the literature that the strength of Bronsted-acid sites depends on the distribution of framework aluminum, with the strongest acid sites being associated with isolated aluminum i o n ~ . I - ~ The evolution of ideas concerning the relationships between zeolite acidity and the aluminum distribution in faujasites recently has been reviewed by Beagley et aL4 and the more salient points are given here. Beaumont and Barthomeup observed that dealurnination over the range 56 to 37 Al/unit cell (uc) resulted in essentially no loss in activity for isooctane cracking, but with more extensive dealumination the activity decreased. They concluded that the milder dealumination resulted in the removal of the inactive and weak-acid sites, whereas more extensive dealumination resulted in the loss of strong-acid sites. In an attempt to interpret these data Dempsey6 was the first to develop a model in which the acid strength of zeolitic protons was related to the environment of A1 atoms. Similarly, Mikovsky and Marshall7 related acid strength to AI distribution and concluded that only AI atoms with no second-neighbor aluminum atoms in the 4-rings exhibit strong acidity. More recently Kazansky' and Jacobs9 have argued on theoretical grounds that decreasing aluminum content increases acidity. By contrast, a b initio molecular orbital calculations of Derouane and Fripiat'O led them to conclude that the average acid strength of isolated aluminum sites in HZSM-5 is comparable to that of paired A1 sites (AI-0-Si-0-Al). As experimental support they refer to the work of Haag and co-workers," who found that the activity for hexane cracking increased linearly with aluminum content over a range from 10 to 30 000 ppm. The latter AI concentration corresponds to a Si/AI ratio of 15, for which the paired to isolated AI ratio is about 2. Recent advances is solid-state N M R spectroscopy have provided an experimental basis for determining the distribution of AI ions in zeolite^.'^-'^ Several groups have developed statistical models to interpret these NMR result^,^^'^^'^ and Beagley et in
-
-
Present address: Department of Industrial Chemistry, Kyungpook National University, Taegu, 635, Korea. 'Present address: Texaco Inc., P.O. Box 509, Beacon, NY.
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particular, have attempted to relate acidity to the distribution of framework AI ions. The purpose of this study has been to provide catalytic data on normal H-Y and dealuminated zeolites which would serve as a test for these and future models of acidity in zeolites. The most fundamental issue at the present is the influence of next-nearest-neighbor interaction on acidity. Are these interactions a dominant factor in influencing acidity or, for example, do other types of aluminum play an even more significant role? In this series of papers three test reactions have been employed with a common set of zeolites. These include methanol dehydration, cumene dealkylation, and hexane cracking. Results for the first two reactions, which are believed to require acid sites
(1) Barthomeuf, D. In Zeolites: Science and Technology, Ribeiro, F. R., Rodrigues, A. E., Rollman, L. D., Naccache, C.,Eds.; NATO AS1 Series E. No. 80, Martinus Nijhoff The Hague, 1984; pp 31'7-316. (2) Pine, L. A.; hlaher, P. J.; Wachter, W . A , ./. Catal. 1984, 85, 466. (3) Schemer, J. In Catalytic Materials: Kelationthip Between Structure and Reactiuity, Whyte, T. E. Jr., Dalla Betta, R. A,, Derouane, E. G.,Baker, R. T., Eds.; American Chemical Society: Washington. DC, 1984; ACS Symp. Ser. No. 248, pp 158-200. (4) Beagley, B.; Dwyer, J.; Fitch, F. R.; Manti, R ; Waiters, .I J . P h i . ( . Chem. 1984,88, 1744. ( 5 ) Beaumont, R.; Barthomeuf, D. J . Catal. 1973, 30, 288. (6) Dempsey, E. J . C a r d 1974, 33, 497, 197 (7) Mikovsky, R. J.; Marshall, .I.F. J . Catal. 1976, 44, 170. (8) Kazansky, V . B. In Structure and Reacticiry o/" Modified Zeolites, Jacobs, P. A,, et al., Eds.; Elsevier: Amsterdam, 1984; pp 61-75. (9) Jacobs, P. A. Catal. Reo.-Sei. Eng. 1982, 24, 415. (10) Derouane, E. G.; Fripiat, J. G. Zeolites 1985, 5 , 165. (1 1) Haag, W. 0.; Lago, R. M.; Weisz, P. B. Nature (London) 1984, 309, 589. (12) Engelhardt, G.; Lohse, U.; Lippmaa, E.; Tarmak, M.; Magi, M. Z . Anorg. Allg. Chem. 1981, 482, 49. (13) Melchior, M. T.; Vaughan, D. E. W.; Jacobson, A. J. J . Am. Chem. Soc. 1982, 104, 4859. (14) Klinowski, J.; Ramdas, S.; Thomas, J. M.; Fyfe, C. A.; Hartman, J. S. J . Chem. SOC.,Faraday Trans. 2 1982, 78, 1025. (15) Klinowski, J.; Fyfe, C. A,; Gobbi, G . C. J . Chem. Sor., Faraday Trans. 1 1985, 81, 3003. (16) Vega, A. J. In lntrazeolite Chemistry, Stucky, G . D., Dwyer, F. G., Eds.; American Chemical Society: Washington, DC, 1983, ACS Symp. Ser. NO. 218, pp 217-230. (17) Melchior, M. T. In lntrazeolite Chemistry, Stucky, G . D . ; Dwyer, F. G., Eds.; American Chemical Society: Washington, DC, 1983, ACS Symp. Ser. No. 218, pp 243-265.
0 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 20, 1986
of weak and moderate strength, respectively, were described in the first paper.'* In the present study the work is extended to include hexane cracking which is a kinetic probe for strong acidity. The results of part 1 demonstrated that both for methanol dehydration and for cumene dealkylation the activity was linearly proportional to the number of framework aluminum ions over the range from 0 to 14 Al/uc, and thereafter continued to increase up to ca. 30 Al/uc. A normal H-Y zeolite exhibited much less activity than anticipated on the basis of its framework aluminum content. The differences in turnover frequency (TOF) between the dealuminated and the H-Y zeolites suggest that the large number of protons in the latter material constitutes weak-acid sites which cannot favorably compete, even in a less-demanding reaction such as methanol dehydration.
Experimental Section A NH4Na-Y zeolite (Linde), containing 2 wt % N a 2 0 , was dealuminated by reaction with flowing SEI4vapor, according to the procedure described by Beyer et aI.l9 Samples having a range of Si/AI ratios were prepared by reacting 2 g of the zeolite with SiCI4/N2 at temperatures from 200 to 570 "C. After reaction at the lower temperatures (200-350 "C) the temperature of the zeolite was increased to 570 "C under flowing Nz. The zeolites were then washed with deionized water and ion exchanged 3 times, 12 h each, in 1 M ",NO3 at 70 "C to remove any remaining Na' and exchangeable aluminum species. These samples are designated as DY-(%/AI), where the silicon to aluminum ratio is given in parentheses. Two steam-dealuminated zeolites (SDY) were prepared according to the method of Ward.20 Details of this procedure and a procedure for dealuminating with SiCI, at 750 "C (DY-750) are given in the first paper of this series.18 A NH,-Y zeolite was prepared by treating the NH4Na-Y zeolite with I M ",NO, 3 times at 70 "C for 12 h each. The residual sodium content was 10 (Figure 2). The band a t 3740 cm-l, attributed to terminal silanol groups or amorphous silica,25increased in amplitude with dealumination. Solid-state N M R studies indicated that there was very little amorphous silica in these materials; thus the assignment of the bands as terminal silanol groups seems more probable. Activity f o r Hexane Cracking. As depicted in Figure 3 there is a linear relationship between hexane cracking activity and the number of framework aluminum ions per unit cell, over the range of 0.7-34 Al/uc. The correlation includes both zeolites dealuminated with SiCI, and by steaming. Although the scatter in the data was somewhat greater when the reaction was carried out at 400 OC,due to lack of reproducibility in the partially deactivated catalysts, a linear relationship also exists at this temperature. This linear relationship implies that the T O F based on framework A1 (24) Ward, J. W. J. Catal. 1967, 9, 225. (25) Ward, J. W. In Zeolite Chemistry and Catalysis, R a b , J. A., Ed.; American Chemical Society: Washington, DC, 1976; ACS Monograph 171, pp 118-284.
zeolite DY-(4.7) DY-(16.3) DY-(26.4) DY-(43.6)
rmollg of zeolite 2.42 0.69 0.59 0.36
of zeolite (Al/uc) 0.072 0.062 0.084 0.084
is constant. A linear relationship between activity and Al/uc also has been reported by Bremer et a1.26for hexadecane cracking. The catalysts of the DY-750 series were essentially inactive for hexane cracking, as was previously found with methanol dehydration and cumene dealkylation as the test reactions.'* The other important feature of the data shown in Figure 3 is the low activity of the H-Y zeolite. The activity at 350 O C was ca. 6% of that found for the most active dealuminated samples. It has been observed previously that activation of H-Y at higher temperatures promotes greater a ~ t i v i t y , ~perhaps ' through the removal of remaining N H 3 or through the creation of defect sites. We have demonstrated in part 1 that the N H 3 is almost completely removed at 400 O C . I 8 The formation of defect sites, or in "deep bed" conditions the steam dealumination of the zeolite, is likely the origin of the enhanced activity. Samples activated at 500 O C for 2 h indeed exhibited an enhanced activity of 7.7 and 25.5 pmol/(g.min) a t 350 and 400 O C , respectively. The initial activity of the dealuminated zeolites was determined by the pulse method. The activities, reported as micromoles of hexane converted per gram of zeolite, for four samples are given (26) Bremer, H.; Wedlandt, K. P.; Tran Hac Chuong; Lohse, U.; Stach, H.; Becker, K. Geterog. Katal. 1983, 5, 435. (27) Benesi, H. A. J. Catal. 1967, 8, 368.
Tlir J v u r n d .f Physieal Chemistry, Vol. 90, No. 20, 1986
4850
100 i
'\
80
-n
-
a 60
.-> .-
CI
8 .-u
-m 4 0 J c
.-
z-al 2 0 a
0
-
1
,
I
-.
2 3 4 5 Reaction Time ( h r ) Figure 4. Catalyst deactivation at 350 "C: 0 , NH4-Y-(2.5); 0.DY(48.2): A, 1>y-(16.3);II.[>Y-(4.7). 1
in 'Table I [ . When one divides these activities by the framework AI/uc a nearly constant turnover frequency is obtained. CaraljTt Deactiuation. Poisoning by coke is a common problem in catalysis over most zeolites. A remarkable property of HZSM-5 is its resistance to coke formation, and this property has been associated with the geometrical constraints imposed by the Recently, however, Bandiera et a].*' have reported that dealuminated mordenite is not so severely poisoned by coke, presumably because of the arrangement of the acid sites. Similarly, Bremer et aLZ6noted that during hexadecane cracking over dealuminated Y-type zeolites the rate of coking was reduced much more strongly by dealumination than the cracking activity. In order to determine the effect of dealumination on deactivation in our Y-type zeolites the activity for hexane cracking was studied as ;I function of time at 350 "C. I t is evident from the results shown in Figure 4 that the relative activity of the dealuminated zeolites decreased similarly for three samples, which differed by a factor of 10 in framework aluminum. The percentage deactivation was about one-half as great for the catalyst prepared from NFI,-Y Clearly, dealurnination did not prevent deactivation: in fact. it appears to accelerate deactivation. I
Discussion The results described here indicate that the TOF for hexane cracking was constant over a wide range of Si/AI ratios. In as much a s hexane cracking is a measure of strong-acid sites, the results also imply that the nutnber of strong-acid sites varies in a linear manner with respect to the AI concentration over the range from 0.75 to 34 Al/uc, which corresponds to Si/AI ratios of 255 to 4.6. The constant TOF for hexane cracking over this range of Al/uc is in excellent agreement with the hypothesis that isolated alurninurri ions give rise to strong-acid sites. The statistical analyses of Beagley et al.,4 assuming an aluminum distribution model adopted by Thomas and co-worker~,'~ shows that over this range of Al/uc the number of aluminum atoms with zero aluminurn
Sohn et al. second neighbors (AI(0)) in the 4-rings increased linearly with respect to AI concentration, as depicted by the solid lines of Figure 3. 'The number of AI(0) species reaches a maximum at 30 Al/uc, and then decreases linearly until' it is zero at 64 Al/uc. The agreement is not as good at the high end of the A1 range; for example, the predicted activity of the normal H-Y zeolite is about three times the observed activity at 55 Al/uc for the data obtained at 350 "C. On an absolute scale there are about 10 Al(0) atoms/uc for this eol lite,^ and it is doubtful that the amount of residual Na' or NH4+was adequate to poison a significant fraction of these strong acid sites. It may be that the statistical model does not accurately reflect the AI(0) concentration at these lower Si/AI ratios. For example, the energetics for having two AI atoms i n a common 4-ring may not be quite so unfavorable. I n view of the large amount of extraframework aluminum which was present in all of the dealuminated samples, the role of this material in either promoting or inhibiting catalytic activity should be considered. In addition to the earlier X-ray diffraction data30 there is support from recent NMR and ion-exchange studies3' that under the conditions of these experiments at least part of the extraframework AI resides inside the small cavities, presumably i i i a cationic form. For samples having lower S ",i ratios this aluminurn may have a positive effect on catalytic activity;32 however, for samples containing less than eight framework Al/uc (less than one per small cavity) the presence of a cation with one unit of charge would eliminate the need for a proton. Hence, there would be no Bronsted acidity. In fact, the presence of such an aluminum species in the DY-750 series may account for the inactivity of these catalysts. Infrared results demonstrated that there were no acidic hydroxyls in these samples. Eviderice for the positive role of extraframework aluminum on cracking activity is lacking from these experiments. Obviously, the catalytic activity of a zeolite Y containing 55 Al/uc plus suitable extraframework AI would provide a test of the promotional effect of this form of AI. An attempt to prepare such a zeolite currently is being made in our laboratory. Thc deactivation results of Figure 4 suggest that the extrafiarnework AI may play a role in coke formation. Clearly, the dealuminated zeolites, which contained extraframework AI, deactivated more readily than the H-Y zeolite. The mordenites used by Bandiera et aL2' were prepared by hydrothermal treatment i'ollowed by acid leaching of the aluminum; therefore, the concentration of extraframework material would have been small. 7'11~ absence of this extraframework aluminum, together with the distribution of acid sites, may account for the smaller rate of coking which they observed. The influence of dealurnination on the position of the highfrequency hydroxyl band is consistent with the observed changes in catalytic activity. As the Si/AI ratio increased from 2.5 to ca. 10 the wavenumber decreased, but for greater Sil.41 ratios there was no further decrease in the wavenumber. This also is consistent with the concept of a constant TOF for the acid site at Si/AI ratios 25. Similar correlations between acidity and wavenumber have been proposed on the basis of data obtained for a variety of different zeolites, and Jacobs' has pointed out that the frequency of the hydroxyl groups remained virtually unchanged for %/AI ratios >6. Because the paper of Barthonieuf and Beauniont5 on isooctane cracking over dealuminated zeolites has become a classic in the field it is necessary to comment on the differences between their results and the ones reported here. They found that the catalytic activity was highest for their "normal" Y zeolite and remained essentially constant until the AI content was less than 37/uc. 'Thereafter the activity decreased as more AI was removed. The difference between their H-Y and ours seems to be mainly in the mode of activation. They activated their catalysts in a stream (30) Maher. P.K.: Hunter, F. D,: Schemer, J. Adc. Chew. Ser. 1979. No. 1 0 1 266
~
Walsh. D. E.; Rollmalin, L. D. .I. Cbtal. 1979, 56, 195. (29) Baridiera. J.; Harnon, C.: Naccache, C. In Proceedings of the 6th (28)
International Zeolite Conference. Olson, D..Bisio, A , , Eds.: Butterworths:
Guildford. 1984:
pp 3?7--?44
(31) Bosacek, V.; Freude, D.; Friihlich, T.: Pfeifer, H.; Schmiedel, 11. J . Colloid Interfuce Sci. 1982, 85, 502. ( 3 2 ) Jacobs. P A.: Leeman, FI. E.: Uytterhoeven, J. B. J. Catul. 1974, 33, 31
J . Phys. Chem. 1986, 90, 4851-4856 of dry air a t 550 OC,conditions which would result in extensive dehydroxylation of the zeolite. According to the dehydroxylation model of Kiih133and others34cationic A1 species are formed as the framework is dealuminated. In fact, N M R results have shown that extensive dealumination can occur at temperatures as low as 500 O C . j 5 In the work presented here dehydroxylation and concomitant dealumination were avoided as was shown by the unit cell size of the catalyst after reaction. This is consistent with N M R results of Bosacek et aL3I which showed that upon deammination at 400 OC under "shallow-bed" conditions only about 3 Al/uc were removed from the framework. Although the acid strength of different types of zeolites may differ considerably because of changes in bond angle, bond length, etc., it is interesting to compare the specific activity for hexane cracking over a dealuminated Y sample and HZSM-5 having Si/A1 = 26. The activities of 350 O C were 11.4 and 8.5 Mmol/ (g.min), respectively. Considering the differences in the zeolites the activities are quite similar. For cumene dealkylation the HZSM-5 catalyst was about twice as active as a comparable dealuminated Y, and for methanol dehydration the HZSM-5 was (33) Kiihl, G. H. In Molecular Sieues, Uytterhoeven, J. B., Ed.; Leuven University Press: Leuven, Belguim, 1973; p 227. Molecular Sieves, Vol. 11, Katzer, J. R., Ed.; American Chemical Society: Washington, DC, 1979; ACS Symp. Ser. No. 40, p 96. (34) Jacobs, P. A.; Beyer, H. K. J . Phys. Chem. 1979, 83, 1174. (35) Klinowski, J.; Thomas, J. M.; Fyfe, C. A.; Gobbi, G. C. Nature (London) 1982, 296, 533.
4851
about 4 times as active.Is Thus, the relative activity depends very much on the probe reaction.
Conclusions Hexane cracking is essentially a linear function of the lattice A1 content over a range of Si/Al ratios from 4.6 to 255. To the extent that hexane cracking is a measure of strong acidity, the acid strength is constant over this range of Si/Al ratios. It is postulated that protons associated with isolated framework aluminum in the 4-rings give rise to the strong acidity which characterizes this material. The presence of aluminum in secondneighbor positions significantly decreases the acidity of a site. This is essentially the Dempsey-Mikovsky-Marshall (DMM) model of strong Consistent with this model, a normal H-Y zeolite, which has only a small amount of isolated tetrahedral aluminum, is relatively inactive for hexane cracking. Moreover, as described in part 1l 5 the H-Y zeolite is relatively inactive for methanol dehydration, which requires only weak acid sites. This suggests that test reactions to characterize acid sites in zeolites need to be used with caution since a few strong acid sites may dominate the activity, even when a less-demanding reaction is employed.
Acknowledgment. This work was supported by the US.Army Research Office. The authors express their appreciation to Professor D. Kalld for helpful discussions related to this work. Registry No. Hexane, 110-54-3.
Characterization of a New Iron-on-Zeolite Y Fischer-Tropsch Catalyst Th. Bein: Institut fur Physikalische Chemie der Universitat Hamburg, Laufgraben 24, 0 - 2 0 0 0 Hamburg 13, FRG
G. Schmiester, Institut fur Atom- und Festkorperphysik der Freien Universitat Berlin, Arnimallee 14, D- 1000 Berlin 33, FRG
and P. A. Jacobs* Laboratorium voor Oppervlaktechemie, Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, B - 3030 Leuven (Heverlee), Belgium (Received: October 3, 1985; In Final Form: December 26, 1985)
Iron pentacarbonyl adsorbed on dry Na-Y zeolite can be oxidized at subambient temperatures into Fe203located in the zeolite supercages (catalyst I). When catalyst I is hydrogen reduced at 575 K most of the iron has agglomerated externally to the zeolite (catalyst 11). When the iron carbonyl is thermally decomposed in vacuo at 525 K, an iron phase with a very low degree of dispersion is again obtained (catalyst 111). During a Fischer-Tropsch reaction most of the iron is transformed into a Hagg-type carbide phase, located externally to the zeolite. Catalysts I1 and 111 rapidly reach steady state and show a Schulz-Flory-type of product distribution, the Hagg carbide being the active phase. Catalyst I slowly moves to steady state and Schulz-Flory behavior. Product selectivity is only found on this catalyst during transient conditions. The physical information on the three catalysts before and after reaction was obtained with transmission electron microscopy and Mossbauer and EXAFS spectroscopies. These techniques supplied consistent and complementary evidence.
Introduction Iron clusters can be introduced in zeolites via several indirect methods. Hydrogen reduction of Fe(I1)-exchanged faujasite-type zeolites is only possible with aluminum-rich samples.1.2 Therefore, Fe(I1) reduction in Y-type zeolites was attempted with stronger reducing agenk3s4 Alternatively, neutral iron complexes such as iron carbonyls can be adsorbed into the pores or cages of a large-pore zeolite and subsequently d e c o m p ~ s e d . ~ - 'The ~ thermolysis of iron carbonyl adsorbed on zeolites, depending on the 'Present address: Central Research and Development, Experimental Station, E. I. Du Pont de Nemours & Co., Wilmington, Delaware 19898.
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exact decompositionmethod used, gives rise to discrete iron particle size distributions and dispersions."J2 These systems have been (1) Huang, Y.Y.;Anderson, J. R. J . Catal. 1975, 40, 143. (2) Garten, R. L.;Delgass, W. N.; Boudart, M. J. Catal. 1970, 18, 19. (3) Schmidt, F.; Gunsser, W.; Adolph J. ACS Symp. Ser. 1977, 40, 291. (4) Lee, J. B. J . Catal. 1981, 68, 27. (5) Ballivet-Tkatchenko, D.; Coudurier, G.; Mozzanega, H.; Tkatchenko, I. Fundam. Res. Homogeneous, Catal. 1919, 257. (6) Ballivet-Tkatchenko, D.; Coudurier, G. Inorg. Chem. 1979, 18, 558. (7) Ballivet-Tkatchenko, D.; Chau, N. D.; Mozzanega, H.; Roux, M. C.; Tkatchenko, I. ACS Symp. Ser. 1981, 152, 187. (8) Ballivet-Tkatchenko, D.; Coudurier, G.; Chau, N. D. Stud. Surf. Sci. Catal. 1982, 19, 123.
0 1986 American Chemical Society
