Langmuir 1991, 7, 314-317
314
External Surface Areas of H-ZSM-5 Zeolites A. Sayari, E. Crusson, and S. Kaliaguine* Dkpartement de Gknie chimique, Universitk Laval, Ste-Foy, Qu&becG l K 7P4, Canada
J. R. Brown Energy Research Laboratories, CANMET, 555 Booth St., Ottawa, Ontario KIA OGl, Canada Received December 13, 1989. In Final Form: May 1, 1990 The external surface areas of two ZSM-5 zeolite samples having different crystal sizes were determined. The t-plot method and the benzene or n-nonane plugging technique were used. The potential and limitations of these methods are discussed. dealt with carbon samples.6J1 It is only recently that attempts have been made to apply some of these techniques One of the most important catalytic properties of ZSM-5 to ~ e o l i t e s . ~ ~ ~ Hudec ~ ~ J ~ -et ' 6a1.12 used the t-plot method type zeolites is their shape selectivity, which is primarily while Carrott and Sing13and other~'~J5used the a,-method controlled by their micropore structure.' However, the to measure S , for series of ZSM-5, erionite, mordenite, overall shape selectivity of a given zeolite may be signifiand Y-type zeolites. The application of these two methods cantly affected by nonselective processes that take place rests on the availability of a nonporous reference material on the external surface of the catalyst.2 In addition, the of known surface area.6 Ideally, the results should be external surface area of a zeolite may have some bearing checked by another independent method since there is no on its deactivation characteristic^.^ Another important guarantee as far as the suitability of the reference material but less common application of external surface areas of is concerned. zeolites is their use in the quantitative analysis of XPS The second technique that has been used for the same data for zeolite-based catalyst^.^,^ purpose consists of plugging the micropores with a strong The distinction between external and internal surfaces adsorbate before determining the remaining surface area by the conventional nitrogen BET t e c h n i q ~ e . Here is not always clear. Gregg and Sing6defined the external ~~~,~ surface as the surface that includes all the cracks which also, problems may arise because of unsuitability of the are wider than they are deep. In many instances, such a pore-filling sorbent or of experimental conditions. A third procedure for determining S, of zeolites was used by Sudefinition is difficult to use, as pores having the same zuki et al.317 This method is based on the adsorption diameter may be included or not in the external surface kinetics of relatively large molecules such as n e ~ p e n t a n e . ~ depending on their depth to width ratios. Another Alternatively, nitrogen can be used if the micropores have definition of the external surface area (S,) would be the been previously filled with another sorbent.7 For this area that is available for multilayer physical adsorption. method, it is essential that the adsorption process be With regard to our quantification model of XPS data,4,5 monitored at its very early stages. It has also been reported the most appropriate definition of S , would be the overall that adsorbates with large molecules may be used under BET surface area without the contribution of micropores. equilibrium conditions, and the external surface area may Well-crystallized zeolites are strictly microporous and be calculated from the corresponding BET equation.16 therefore have very low external surface areas that may In this paper, we report on the determination of external be approximated by their apparent geometrical areas as surface measured directly by scanning electron m i c r o s ~ o p y . ~ ~ ~ ~ ~ areas of ZSM-5 zeolites using the t-plot as well as the pore-plugging methods. The potential and limitations However, more often than not zeolite samples are less than of these techniques will be discussed. perfectly crystalline and may exhibit mesopores and consequently external surface areas much higher than their Experimental Section apparent geometrical areas. Mesopores can also be created by most dealumination p r o c e d u r e ~ . ~ J ~ A ZSM-5 zeolite designated as sample A was synthesized according to a procedure described by Gabelica et al." The asSeveral techniques have been devised to determine S, synthesized solid was calcined for 10 h at 773 K, extensively of adsorbents, and a great deal of the earlier applications exchanged with ammonium nitrate, and then calcined again for 10 h at 773 K. The crystallinity of this sample was checked by (1)Chen,N.Y.;Garwood, W.E.;Dwyer,F.G.ShapeSelectiveCatalysis X-ray diffraction, and the homogeneous distribution of Si and
Introduction
i n Industrial Applications; Marcel Dekker: New York, 1989. (2) Gilson, J. P.; Derouane, E. G. J . Catal. 1984, 88, 538. (3) Suzuki, I.; Namba, S.; Yashima, T. J . Catal. 1983,81,485. (4) Kaliaguine, S.; Adnot, A.; Lemay, G. J . Phys. Chem. 1987,91,2886. (5) Kaliaguine, S.; Adnot, A.; Lemay, G.; Rodrigo, L. J . Catal. 1989, 118, 275. (6) Gregg, S. J.; Sing, K. S.W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. ( 7 ) Suzuki, I.; Oki, S.; Namba, S.J . Catal. 1986, 100, 219. (8) Inomata, M.; Yamada, M.; Okada, S.; Niwa, M.; Murakami, Y. J . Catal. 1986, 100, 264. (9) Gross, Th.; Lohse, U.; Engelhardt, U.; Richter, K. H.; Patzelova, V. Zeolites 1984. 4. 25. (10) Addison,S. W.; Cartlidge, S.;Harding,D. A.; McElhiney, G. Appl. Catal. 1988, 45, 307.
0743-7463/91/2407-0314$02.50/0
(11) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Actiue Carbon; Marcel Dekker: New York, 1988. (12) Hudec, P.; Novansky, J.; Silhar, S.;Trung, T . N.; Zubek, M.; Madar, J. Adsorption Sci. Tech. 1986, 3, 159. (13) Carrott, P. J. M.; Sing, K. S. W. Chem. Ind. 1986, 786. (14) Hendreck, G. P.; Smith, T. D. J. Chem. Soc., Faraday Trans. 1 1989, 85, 645. (15) Hedge, S. G.; Kulkarne, S. B. In Advances i n Catalysis; Reo, P., Ed.; Wiley: New York, 1985; p 233. (16) Take, J.; Yoshioka, H.; Misono, M. In Proceedings 9th International on Catalysis: Phillias. M. J., Ternan, M., Eds.; The Chemical Institute of Canada: Ottawa, 1988; Vol. 1, p 372. (17) Gabelica, Z.; Derouane, E. G.; Blom, N. Appl. Catal. 1983,5,227.
0 1991 American Chemical Societv ~
External Surface Areas of H-ZSM-5 Zeolites
Langmuir, Val. 7, No. 2, 1991 315
A 2
3.5
P/W
FILM THICKPESS LA1
Figure 1. Nitrogen adsorption-desorption isotherm for sample A before and after benzene preadsorption. The figures indicate the residual benzene pressures, PB (Torr).
Figure 2. t-plots for sample A before and after benzene preadsorption. The figures indicate benzene residual pressures, PB (Torr). The numbers on the right-hand side are scaling factors.
A1 was inferred by the close agreement between Si/Al ratios as determined by chemical analysis (39.0) and by XPS (37.2). Prior to adsorption experiments, samples were heated under vacuum a t 573 K for 2 h. About 0.1-g batches were used for the nitrogen BET and t-plot methods. Data were secured by using a computer-controlled sorption analyzer (Omnisorp 100) operating in the continuous mode. Software programs for data reduction included BET surface area analysis, t-plot analysis with reference to a nonporous silica and micropore, and mesopore distributions according to the models of Horvath and Kawazoela and Barrett et al.,lD respectively. For the micropore-plugging technique, batches of 0.4-1.0 g were needed. When benzene is used as pore-filling adsorbate, the outgassed sample was exposed for 1 h a t room temperature to gaseous benzene at its saturated vapor pressure. Subsequently, the cell containing the sample was transferred to the adsorption system, and the residual pressure of benzene (PB) above the solid sample was then adjusted to a desired value in the range 1-70 Torr. As suggested by Suzuki e t al.3J helium was introduced into the cell to a total pressure of about 700 Torr before cooling the cell to 77 K. Finally, the helium was pumped off and a conventional nitrogen adsorption-desorption isotherm was determined. It is important to mention that the dead volume of the sample cell was small enough so that the extra hydrocarbon that condenses onto the sample when it is cooled to 77 K was, regardless of Pe,negligible in comparison to the micropore volume of the sample. Experiments involving the use of n-nonane as a pore-filling sorbent were undertaken according to the same procedure except that the hydrocarbon was adsorbed as a vapor pressure of ca. 5 Torr for 2 h and then evacuated a t room temperature for 1 h as recommended by several authors while studying microporous carbon and titania.6 The benzene adsorption isotherm for sample A was measured at 301 K by using the same Omnisorp 100 analyzer in the static mode.
(Table I). Hall et a1.20reported a theoretical value of 0.18 cm3 g-' for ZSM-5 micropore volume, while experimental data in the range 0.13-0.23 cm3 g-' have been found for various ZSM-5 As for the external surface areas of ZSM-5 zeolites, data ranging from 1to 120 m2g-' have been r e p ~ r t e d . ~ ~ ~ , ~ J ~ - ~ ~ Typical nitrogen isotherms measured after preadsorption of benzene are plotted in Figure 1. The corresponding BET surface areas are plotted in Figure 3 against the residual pressure of benzene. It is seen that a t pressures below 3 Torr high surface areas were obtained. For pressures ranging from 3 to 10 Torr, the BET surface area leveled off a t 40 f 2 m2 g-l. This area decreased steadily when increasing benzene pressures were applied. At near saturation, i.e., PBof about 70 Torr, a surface area of 4 m2 g-*, very close to the apparent geometrical area (4.4 m2 g-'), was measured. The extrapolated linear branches of the t-plots for samples that had been exposed to benzene and then outgassed to a final pressure PB < 3 Torr had positive intercepts on the adsorption axis, indicating that the micropores were not effectively filled. However, for PB in the range 3-10 Torr, no remaining micropores were detected while the mesopore volume amounted to about 60% of its original value (Table I). Analysis of the mesopore distribution shows that this was due mainly to the benzene plugging the smallest mesopores, having 10-20-A radii. A t higher PB values, the remaining mesopore volume decreased further, leading to smoother particles with lower apparent external surface areas. Ideally, PB should be adjusted within a pressure range for which the micropores would be effectively filled while all larger pores remain completely free. However, in our case even at the lowest pressures required, i.e., 3 Torr, almost half of the mesopore volume was already filled with benzene. It is well-known that physical adsorption on large pores and flat surfaces occurs layer by layer, while for micropores it follows a pore-filling mechanism. Therefore, such an overlap should not be expected unless it is an indication that the lower end of the mesopore distribution is comprised of pores that behave rather like micropores. This seems to be the case for our sample A. The adsorption-desorption isotherm of benzene for
Results and Discussion The adsorption-desorption isotherm for sample A (Figure 1, upper curve) shows a high uptake of nitrogen a t very low relative pressures and a hysteresis loop a t higher pressures. These two properties are characteristic of microporous and mesoporous materials, respectively. By use of the linear portion of the t-plot corresponding to a film thickness higher than 4-5 8, (Figure 2), a micropore volume of 0.18 cm3 g-' and a S, of 35.5 m2 g-1 were determined (18) Horvath G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470. (19) Barrett, E. P.; Joyner, L. G.; Halenda, P. H. J.Am. Chem. SOC. 1951, 73, 373.
(20) Hall, W. K.;Engelhardt, J.; Sill, G. A. Stud. Surf. Sci. Catal. 1989, 42, 1253. (21) Voogd, P.; Scholten, J. J. F.; van Bekkum, H. In Zeolites for the
Nineties, Recent Research Reports; Jansen, J. C.; Moscou,L.,Post, M. F. M., Eds.; 8th Int. Zeol. Conf., Amsterdam, 1989; p 349.
Sayari et al.
316 Langmuir, Vol. 7, No, 2, I991 BET
t-plot
HZSM-5 A HZSM-5 B
Table I. Experimental Data benzene preadsorption
Sa
S2
V"C
Vmd
Seb
V"c
456 346
35.5 10.7
0.18 0.14
0.14 0.02
4O.Op 4.3f
0 0
n-nonane preadsorption
Vmd 0.08
V"c 0 0
S,b 39.09 3.5
Vmd 0.09
Mesopore volume in cm3 g-l. e *2 m2 g-l. a BET surface area in m2 g-l. * External surface area in m2 g-l. Micropore volume in cm3 g-l. PB = 3-10 Torr. f *0.8 m3 g-l. PB = 3-45 Torr. g +1 m2 g-l.
s
150
-
N
E
!I
100
-.
0
10 20 30 40 50 RESIDUAL PRESSURE OF BENZENE, Pe
60
Figure 3. BET surface areas of sample A as a function of the benzene residual pressure, PB(Torr). The external surface areas
obtained by the n-nonane plugging technique ( 0 )and the t-plot method (A)are also indicated.
I
0
:3
:I
:4
:II P/PO
:7
.'I
.'a
.'8
'l
Figure 5. Nitrogen adsorption-desorption isotherm for CPG 120.
I
s-
200 '
h
E
.:3 , '.
! .:, :,
,; ,,
. .. . .:. : , . :: ,.s
.':...
150
W
5 N 6
100
D
e
50
0
v1
a P 0.0
0.2
0.4
0.6
RELATIVE PRESSURE,
0.8
I 1.o
P/Po
Figure 4. Adsorption-desorption isotherm of benzene for sample A at 301 K.
,..'
,/."
. .. .
. .. ., .
,..'
. . . . .(..',
60
,'....
70
BO
RP
sample A a t 301 K is shown in Figure 4. In contrast to the nitrogen isotherm shown in Figure 1, the hysteresis loop extends over almost the entire range of relative pressures. It is believed that the low-pressure hysteresis is due to the fact that the benzene adsorption is a slightly activated process.22 Choosing appropriate relative pressures on each branch of the hystersis loop allows two different conditions for the adsorption of the same amount of benzene to be identified, e.g., N and M in Figure 4. However, BET analysis after benzene preadsorption under conditions M gave a specific surface area of 67 m2 g-l, a much higher value than the 40 m2g-l determined after benzene preadsorption under conditions N. Earlier work on the determination of external surface areas of zeolites8 suggested that the sample be exposed to the saturated vapor pressure of benzene with no further adjustment of Pg. This study shows that such a procedure may lead to underestimated values. Our data indicate that the sample should be first equilibrated with benzene at its saturated vapor pressure, and then this pressure (22) Schirmer, W.; Thamm, H.; Stach, H.; Lohse,U. In The Properties and Applications ofzeolites; Townsend, R. P., Ed.; The Chemical Society: London, 1980; p 204.
ao
0
IN
Figure 6. Pore size distributions for CPG 120:(a)before benzene adsorption; (b) after benzene preadsorption (PB= 85 Torr).
should be adjusted between 3 and 10 Torr before carrying out the nitrogen adsorption. For samples that exhibit micropores only, it is anticipated that the residual pressure of benzene would not be critical as long as it is high enough to allow effective plugging of the micropores, i.e., higher than about 3 Torr. In order to further verify these conclusions, a controlled pore glass designated as CPG 120 was studied. This material was chosen to demonstrate that even for a nonmicroporous solid with mesopores larger than those usually found in zeolites (Figures 5 and 6a) the use of a high benzene pressure is likely to block a significant fraction of the external surface area. Indeed, preadsorption of benzene with final adjustment a t 85 Torr led to a decrease in both the surface area and the mesopore volume from 169 to 71 m2 g-l and from 1.12 to 0.65 cm3 g-' (Figure 6a and 6b), respectively. n-Nonane is another pore-filling sorbent that had been used successfully for microporous titania,24and (23) Gregg, S. J.; Langford, J. F. Trans. Faraday SOC.1969,661394.
External Surface Areas
of H-ZSM-5 Zeolites
other materials25 but has never been tested for zeolites. Application of the n-nonane plugging technique to three separate batches of sample A led to a mean S, value of 39 f 1 "2 g-' (Table I). As in the case of benzene preadsorption, all micropores were effectively plugged, and the remaining mesopore volume was again about 55 7; of its original value. These results indicate that both hydrocarbons are equally effective pore fillers for zeolites. The use of n-nonane offers even a slight advantage over benzene as the excess hydrocarbon has simply to be pumped off instead of adjusting its pressure to a suitable value. The external surface area obtained by the t-plot method as applied without hydrocarbon preadsorption was 35.5 m2 g-', in good agreement with the surface areas provided by the pore-pluggingtechnique. However, as the reliability of the t-plot method rests among other factors on the extent of the linear portion of the t-plot, it is not expected that this method would work properly with zeolites of low external surface areas. In order to verify this contention, a suitable sample has to be prepared and tested. The new ZSM-5 (&/A1 = 20) designated as sample B was prepared by using the same procedure as for sample A except that NaCl was replaced by CsCl and the autoclavation time was 11 days instead of 5. Gabelica et al.17 reported that such a synthesis procedure leads to quite large crystals. As expected, sample B was found to have a very small mesopore volume (Table I) and a micropore volume (0.14 cm3 g-l) comparable to literature data for large crystals of ZSM-5 and silicalite.21 By use of the n-nonane plugging technique, a S, of 3.5 mgg-' was determined. Preadsorption of benzene at a PB ranging from 3 to 45 Torr followed by nitrogen adsorption gave an external surface area of 4.3 f 0.8 m2 g-l. These values seem reasonable in light of the almost complete lack of mesopores. However, as shown in Table I, the external surface area obtained by the t-plot method was significantly higher, suggesting that this method should be avoided when dealing with large zeolite crystals. (24) Parfitt, G. D.;Sing, K. S. W.; Urwin, D.J. Colloid Interface Sci. 1975,53, 187. (25) Gregg, S. J.;Tayyab, M. M. J. Chem. Soc., Faraday Trans. I 1978, 74, 349.
Langmuir, Vol, 7, No. 2, 1991 317 Earlier work from this laboratory dealt with quantitative applications of XPS to zeolite-supported catalysts. A model allowing for surface segregation and bimodal distribution of supported materials has been d e ~ e l o p e d . ~ ~ ~ Basically, it was assumed that the supported material is comprised of small particles homogeneously distributed throughout the zeolite and larger particles attached to the external surface of the support. The proportion and particle size of both fractions can be assessed from XPS and external surface are measurements on the catalyst in both its ground and unground form. As an example of variations of the external surface area upon grinding, sample A was ground in an automatic 5100 Mixer/Mill (from Spex) for 10 mn. Its external surface area as determined by the t-plot method was found to be 47.3 m2 8-1 compared to 35.5 m2 g-l obtained by the same method applied to the unground zeolite. This increase is likely to be attributable to breakage of crystallite agglomerates. Detailed applications of this technique to zeolite-supported catalysts will be reported.
Conclusions For zeolite samples with a fairly large mesopore volume, the external surface area may be measured properly by the t- or the cu,-plot method. The pore-plugging technique using n-nonane or benzene is equally effective even though in the case of benzene, the experimental procedure and in particular the residual pressure range should be considered carefully. For samples with low external surface areas, the poreplugging method remains quite effective. However, the t-plot technique did not seem to be fully satisfactory. Since the external surface area is proportional to the slope of the extrapolated linear branch of the t-plot, it is not surprising that the accuracy of the measurements decreases as the slope of the t-plot approaches zero. Acknowledgment. This work has been supported under contract no. DSS 23440-8-9176 by CANMET, Department of Energy, Mines and Resources, Canada, and the program of the Federal Panel on Energy Research and Development (PERD).
