J . Phys. Chem. 1992,96. 8473-8479
support of the Basic Research Fund administered by the Israeli Academy of Sciences, the contribution of David Yogev to the design of the system and preliminary measurements, and the help of Ofra Levi in some of the calculations. R e t r y NO. Ag, 7440-22-4.
References and Notes (1) Yogev, D.; Efrima, S.J . Phys. Chem. 1988, 92, 5754. (2) Yogev, D.; Efrima, S.J. Phys. Chem. 1988, 92, 5761. (3) Yogev, D.; Efrima, S.;Kafri, 0. Opr. Lett. 1988, 13, 934. (4) Yogev, D.; Deutsch, M.; Efrima, S . J . Phys. Chem. 1989, 93, 4174. (5) Yogev, D.; Shtutina, S.;Efrima, S.J . Phys. Chem. 1990, 94, 752. (6) Yogev, D.; Kuo, C. H.; Neuman, R. D.; Efrima, S.J . Chem. Phys. 1989, 91. 3222. (7) Yogev, D.; Efrima, S.Longmuir 1991, 7, 267. (8) Yogev, D.; Efrima, S.J. Colloid Interface Sci. 1991, 147, 88. (9) Rostkier-Edelstein,D.; Yogev, D.; Efrima, S . J . Colloid InterJace Sci. 1991, 147, 78. (10) Bavli, R.; Yogev, D.; Efrima, S.;Berkovic, G. J . Phys. Chem. 1991, 95, 7422. (1 1) Efrima, S. Silver Metal Liquidlike Films. CRC Crir. Reu. Surf. Chem. 1991, I , 167. (12) Zeiri, L.; Efrima, S . J. Phys. Chem. 1992, 96, 5908. (13) Rayleigh, J. W. S.Philos. Mag.1892, 34, 481. (14) Torquato, S.J . Appl. Phys. 1985, 58, 3790. (15) Kreibig, U.; Genzel, L. Surf.Sci. 1985, 156, 678. (16) Kreibig, U.; Fragstein, C. V. Z . Phys. 1969, 224, 307. (17) Born, M.; Wolf, E. Principles of Optics, 5th ed.;Pergamon Press:
Oxford, U.K.
(18) Londolt Eoernstein Data, 11/8; Springer Verlag: Berlin, 1962. (19) Kreibig, U. Z . Phys. 1978, 831, 39.
8473
(20) Siiman, 0.;Lepp, A.; Kerker, M. Chem. Phys. Lett. 1983,100, 163. (21) McPhedran, R. C.; McKenzie, D. R. Proc. R. Soc. London 1978, A359, 45. (22) McKenzie, D. R.; McPhedran, R. C.; Derrick, M. H. Proc. R. Soc. London 1978, A362, 211. (23) Johnson, P. 9.; Christy, R. W. Phys. Rev. 1972, 8 6 , 4370. (24) Kittel, C. Introduction to Solid State Physics, 5th ed.;John Wiley: New York, 1976. (25) Farbman, I.; Levi, 0.;Efrima, S.J . Chem. Phys. 1992, 96, 6477. (26) See for instance: (a) Doremus, R. H. J. Appl. Phys. 1966,37,2775. (b) Abe, H.; Charle, K.-P.; Tesche, B.; Schulze, W. Chem. Phys. 1982,68, 137. . . (27) Mie, G. Ann. Phys. 1908, 25, 377. (28) Maxwell-Garnett, J. C. Philos. Tram R. Soc. London 1904,203,385. (29) Bruggeman, D. A. G. Ann. Phys. (Leipzig) 1935,24,636. (30) Niklasson, G. A.; Granqvist, C. G.; Hunderi, 0. Appl. Opt. 1981,20, 26. (31) Persson, 9. N. J.; Liebsch, A. Solid Stare Commun. 1982, 44, 1637. (32) Liebsch, A,; Gonzales, P. V. Phys. Reu. 1984, 829, 6907. (33) Liebsch, A.; Persson, B. N. J. J . Phys. 1985, C16, 5375. (34) Davis, V. A.; Schwartz, L. Phys. Rev. 1985, 831, 5155. (35) Davis, V. A.; Schwartz, L. Phys. Reu. 1986, 833, 6627. (36) Felderhof, 9. U.; Ford, G. W.; Cohen, E. G. D. J . Stat. Phys. 1982, 28, 135. (37) Felderhof, 9. U.; Jones, R. 9. Z . Phys. 1986, 862, 215. (38) Felderhof, B. U.; Jones, R. 9. Z . Phys. 1986,862, 225. (39) Felderhof, 9. U.; Jones, R. 9. Z . Phys. 1986, 862, 231. (40) Bergman, D. J. Phys. Rev. 1979, 819, 2359. (41) Milton, G. W. J. Appl. Phys. 1981, 52, 5286. (42) Milton, G. W. J. Appl. Phys. 1981, 52, 5294. (43) McPhedran, R. C.; Milton, G. W. Appl. Phys. 1981, A26, 207. (44) Milton, G. W. Appl. Phys. 1981, A26, 125. (45) Landauer, R. AIP Conf. Proc. 1977, 40, 2. (46) Beaglehole, D.; Hunderi, 0. Phys. Rev. 1970, 8 2 , 309.
Mordenite Acidity: Dependence on the Si/Ai Ratio and the Framework Aluminum Topology. 1. Sample Preparation and Physicochemical Characterization Helmut Stach,* Jochen Jtinchen, Hans-Georg Jerschkewitz, Ursula Lobe, Barbara Parlitz, Bodo Zibrowius,t Central Institute of Physical Chemistry, 0-1 1 99 Berlin, Germany
and Michael Hunger Department of Physics, University of Leipzig, Leipzig, Germany (Received: December 3, 1991; In Final Form: June 19, 1992)
A series of dealuminated mordenites (with Si/Alf ratios between 7 and 48) was prepared by acid leaching and characterized by using different physicochemical methods. The A1 content of the framework and the distribution of the tetrahedral and octahedral aluminum were determined by 27AlMAS NMR measurements and chemical analysis. Attempts to quantify the number of SiOH defect groups by '%i MAS NMR measurements (following a procedure given in the literature) failed. The number of silanol groups (and Bransted sites) were determined using 'H MAS NMR investigations. The micro- and mesopore volumes of the samples were derived from measurements of the benzene adsorption isotherms. It was found that low dealumination of the H-mordenites transforms small- into large-port mordenites. The micropore volume diminished again at a high degree of dealumination.
Intraduction Correlations between the acidity of solids and their catalytic properties have been known for a long time.' Though a large number of experimental investigations was performed (for reviews see ref 2), the reasons for the different acidity of catalysts of the same composition were not fully understood. The situation changed when zeolites became the object of research. It was found that the acidity of the zeolites depends primarily on the chemical and later composition (Si/AI ratio, number, and type of cati0ns~9~) that it also depends on the crystal structure ( S i U A l angles and Corresponding author. Present address: Adlershofer Umweltschutztechnik und Forschungsgesellschaft mbH, Rudower Chaussee 5, 0-1199 Berlin, Germany. *Present address: UMIST, Department of Chemistry, P.O. Box 88, Manchester M60 lQD, United Kingdom.
0022-3654/92/2096-8473$03.00/0
bond At present it is accepted that the number of Bransted sites (bridged OH groups) is determined by the number of aluminum atoms in the framework of the zeolites.*-I0 In contrast to that, the influence of chemical and structural parameters on the acid strength of the Brsnsted sites is not well-known. Many experimental methods are used to measure the acidity parameters of zeolites (see the excellent review"). Most of them-though they are often applied-measure only the number of the acid sites but fail to determine exactly the acid strength. Calorimetry is a proper method to estimate the acid strength of the sites quantitatively by investigation of the interaction energy of the acid site with a basic molecule expressed as the differential molar heat of chemisorption. To study the acid strength in particular, we investigated zeolites often used for catalysis such as the HY,12 HZSM-5,I3 and HM types, applying a number of different analytical methods besides 0 1992 American Chemical Society
8474 The Journal of Physical Chemistry, Vol. 96. No. 21, 1992
Stach et al.
TABLE I: hlumhartion COllditiOM and Chemical Composition of the Samples samples NaM HM-0 HM-Ob HM-1 HM-2 HM-3 HM-4a HM-4b HM-4c HM-4d HM-6 HM-7
dealumination conditions
SiO2/AI2O3 10.2 33.P 11.5 13.9 15.1 17.7 36.7 31.2 67.6 78.2 71.3 92.3
NH, exch," 773 K, 3 h 298 K, 0.2 N HNOj 338 K, 8 N HN03, 8 h 348 K,8 N HNOJ, 8 h 358 K, 8 N HN03, 8 h 368 K, 8 N HN03, 8 h 368 K, 8 N HNOI, 8 h 368 K, 8 N HN03, 16 h 368 K, 8 N HN03, 24 h 385 K, 8 N HNO3, 8 h 385 K, 8 N HN03, 16 h
chemical analysis SVALt 5.1 16.5b 5.8 7.O 1.6 8.8 18.3 15.6 33.8 39.1 35.6 46.1
Al/u.c. 7.9 2.8b 7.1 6.0 5.6 4.9 2.5 2.9 1.4 1.2 1.3 1.o
"H4+ exchange was repeated three times; the sample was dried at 393 K and then heated at 773 K for 3 h. bResults from NH,+-exchange capacity measurements. These data do not provide the Si/Altoat but Si/AIp
TABLE 11: Experimental Conditions for MAS NMR Measurements resonance nucleus 29Si 2 7 ~ 1
1H
freq (MHz) 79.5 104.2 300.0
pulse duration (ps) 2.5 0.6 6.0
flip angle ~ / 4 TI12 712
calorimetry. In this paper we present results for dealuminated H mordenites covering a broad range of Si/Al ratios. The special interest in the H mordenites arose not only from their behavior in the MTG process (comparable with ZSM-5)l4.l5but also from their crystalI6 and pore structure.17
Experimental Section Sample Preparation. The samples prepared are listed in Table I. A commercially available product (Na mordenite from CK Bitterfeld) with the following composition of the unit cell Na7,9[(A10z)7,9(Si02)40,1] was used as the original zeolite of the dealumination procedure. The dealumination was performed with 8 N H N 0 3 at elevated temperatures under conditions given in Table I. To investigate the influence of the temperature and the refluxing time, both parameters were changed at the intervals from 338 to 385 K and from 8 to 24 h, respectively. The H forms of the parent mordenite were prepared (i) by NH4+ion exchange followed by thermal trhtment (see Table I) and (ii) by treatment with diluted HN03at 298 K without thermal stress (HM-O and HM-Ob, respectively). X-ray diffraction (XRD) investigationsproved that all samples were of good crystallinity-even after severe dealumination. Chemical Analysis. The results of the classical wet chemical analysis are presented in Table I. MAS NMR Measurements. The z7Aland z9Si MAS NMR spectra were recorded on a Bruker MSL 400 spectrometer. The experimental conditions for the measurements were summarized in Table 11. In the case of z9SiMAS NMR, high-power proton decoupling was applied. To obtain quantitative data from z7A1 MAS NMR, the samples were saturated with water vapor prior to the measurements. IH MAS NMR spectra were measured using a Bruker MSL 300 spectrometerand homemade magbanglespinning equipment that allows sealed glass ampules to be spun. The total concentration of OH groups in the activated samples was determined by comparing the line intensity with that of a standard. The samples were treated in a glass tube of 5.5-mm inner diameter bed depth. Starting at room temperature the and with IO-" samples were heated under vacuum with a rate of 10 K/h. After being kept at the final temperature of 570 K for 2 h, the molecular sieves were evacuated up to a pressure below 10 mPa for 20 h and sealed off. A&" M e a m " & The adsorption isothermsof benzene were measured at 298 K using a McBain-Bakr balance. The equilibrium pressure was determined by a capacity membrane manometer of Baratron type. The samples were evacuated (p < 1 mPa) at a temperature of 653 K for about 12 h before the
repetition time (s) 10.0 0.5 6.0
MAS
freq (kHz) 4.2 4.0 2.5
no. of scans 4000 800 200
ref (external) TMS Al(H20)6jt TMS
isotherm measurements started. In Figure 7 the adsorption isotherms are presented. Ammonium Exchange Capacity Measurements. The NH4+ exchange was performed three times at 343 K as a batch process with an excess of 0.2 N NH4N03 solution (pH = 5.6). The amount of the exchanged NH4+ions was obtained from the NH3 content using the Kjeldahl method.ls The results are given in Table V (column 2).
Results and Discussion nAl MAS NMR Investigatiom. MAS NMR spectroscopy was repeatedly used to study the dealumination of mordenites by acid leaching.2"zz The determination of the Si/Al ratio of the framework by z9SiMAS NMR is complicated by the presence of considerable numbers of SiOH groups. Therefore, for the determinationof the framework Si/Al ratio not z9Sibut z7AlMAS NMR spectroscopy in combination with chemical analysis was applied throughout these papers and is also applied as one approach to the Si/A1 ratio of the framework in the present paper. The 27AlMAS NMR spectra of our parent Na mordenite and of its partially dealuminated H forms are presented in Figure 1 The spectrum of the NaM (Figure 1) shows a unique resonance line at 54.1 ppm which corresponds to the A1IVatoms (tetrahedrally coordinated) in the framework. Dealumination (either by acid leaching or thermal treatment) results in the appearance of an additional line at about -1 ppm belonging to the Alvl atoms extracted from the framework. From Figure 1 it can be sten that for the sample H M - 0 there exists an additional line (centered at about 38 ppm) which ought to be attributed to the nonframework AllVatomz3and that the concentration of the octahedrally coordinated AIV1atoms is higher than those found in the sample HM-Ob. By raising temperature or extending the time of acid refluxing, the intensity of the -1 ppm line decreases, thus showing that Alvl atoms were removed from the sample (Figure 2). The relative amounts of the AI" and Alvl atoms in the samples are given in Table 111. It is interesting to note that the AllV/Alvlratio remains nearly constant up to a leaching temperature of 358 K and then rises strongly. The Si/Al ratio of the framework can easily be obtained by the relation I
Si/Alf = (1
+ AIV1/Alvl)Si/A1,,,,I
(1)
where Si/Alu is the corresponding value obtained from chemical analysis and AlV1and AllVare the relative line intensities for the octahedral (nonframework) and the tetrahedral (framework) aluminum, respectively. The thus-obtained data are given in column 3 of Table IV.
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8475
Mordenite Acidity. 1
TABLE IIk Framework Composition of tbe Derluminrtd Mordeoites sample HM-Ob HM- 1 HM-2 HM-3 HM-4a HM-4b HM-4c HM-4d HM-6 HM-7
AItV/% 84.8 84.7 85.8 85.0 88.7 88.7 92.6 93.4 94.6 97.3
2 7 ~ 1NMR AlV'/% Al'"/u? 15.2 6.0 15.3 4.8 4.6 14.2 3.9 15.0 2.3 11.3 2.5 11.3 1.4 7.4 6.6 1.1 5.4 1.1 1.o 2.7
composition of unit cellb
AIV1/u.c. H6.0[(A102)6.0(Si0~)42.010.125 1-07 H4.~[(A10~)4.~(Si02)43d0.100 0.92 0.80 H4,6[(A102)1,6(Si02),3,,1 0.096 0.74 H3,9[(A102)1,9(Si02)~.ll 0.081 0.29 HZ.~[(A~O~)Z.~(S~OZ)~J.~I 0.048 H z , J [ ( A ~ ~ z ) z . s ( S ~ ~0.052 ~ ) ~ J , J ~0.33 0.10 HI .4[(A102)I .4(SiO2)46.6] 0.029 0.08 H1,1[(A102)1.1(Si02)46.9 0.023 1 0.07 HI.l[(A102)1.I(~i02)~.91 0.023 HI,O[(A~~Z)I.O(S~~~)~~,OI 0.021 0.03
A1IVe$/u.c. 1.9 3.1
Ahhe/u.c. 0.8 2.2 2.5 3.3
3.3 4.0 5.6 5.4 6.5 6.8 6.8 6.9
5.3 5.1 6.4 6.7 6.7 6.9
degree of deal./% 24 39 42 51 71 69 82 86 86 87
@Calculatedfrom the total amount of aluminum found by chemical analysis and from 27AlMAS NMR results. bCalculated under the assumption dNumber of AI" atoms extracted from the framework per unit a l l . 'Number of dissolved A1 atoms per unit cell.
of ideal composition (no vacancies in the lattice). 'm = A1IV/(Si + AI").
TABLE W. Si/A Ratios of the Deduminatd Mordenitea chem anal. Si/Al,I 5.8 7.0 7.6 8.8 18.3 15.6 33.8 39.1 35.6 46.1
sample HM-Ob HM- 1 HM-2 HM-3 HM-4a HM-4b HM-4c HM-4d HM-6 HM-7
27Al NMR' .%/AIf 6.8 8.2 8.8 10.4 20.7 17.6 36.2 41.8 37.8 47.5
NH4+ exch. Si/Alf 7.2 9.8 10.2 12.5 19.5 18.5 32.0 40.2 42.0 48.2
av value Si/AIf 7.0 9.0 9.5 11.4 20.1 18.0 34.2 41.0 39.9 47.9
NH3
TPD
Si/Alf 11.3 11.8 12.6 16.1 24.3
L
37.0
L
OSi/Al ratio obtained from chemical analysis and 27AlMAS NMR according to eq 1.
120
u c
HM-0
120
80
40
0
- 40
dlm Figure 1. 27Al MAS NMR spectra of the dealuminated mordenites c -
HM-0, HM-Ob, and the parent material NaM (*,spinning side bands).
A " m i u m ExclmngeCapacity nndTemprature-Pmg"d Ammooin Desorptba Beside the 27AlMAS NMR measurements,
the above-mentioned methods were used to characterize the mordenite samples, especially to determine the A1 content of the framework. The results of the framework investigations (using four different methods) arc presented in F i 3 and in the Tables 111-v. Table IV lists the Si/Alf ratios. As follows from the columns 2 - 4 of Table IV, the data obtained from chemical analysis with 27AlMAS NMR and those of ammonium exchange are in good
80
LO
0
-LO
-60 dlppm
Figure 2. 27AlMAS NMR spectra of selected dealuminated H mordenites (%, spinning side bands).
agreement. Therefore, the average value of both data sets is considered as the real Si/Alf ratio of the framework. Comparing the last column of Table IV (Si/Alf ratio derived from TPD results) with column 5, it can be seen that this method yields too large numerical values of the Si/Alf and, consequently, too small numbers of A1 atoms for most of the samples. A possible explanation is given vide infra. Table I11 compiles further results of the dealumination procedure. Of special interest is the variation of the octahedrally and tetrahedrally coordinated aluminum atoms per unit cell. Sometimes24the Al"' atoms were identified with Lewis sites. In atoms) in the refs 24-26 it is reported that the Lewis sites (Av1 H fonns of mordenites (prepared by acid leaching) already exist before dehydroxylation at elevated temperatures. It was stated24 that the number of Lewis sites increases before passing a maximum (at about 4.5 Allv in the unit cell) and then decreases again. As follows from Figure 4 (and Table 111) in our dealuminated
8476 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 16
R
ZL
Stach et al.
tlh
- LO
-
r
-30 2 ._ v)
- 20
0
8
4 All"/"'
Figure 4. AIIVand AIV1atom content per unit cell in the investigated mordenites (A,AI" atoms extracted; 0 , AIV' atoms; 0, AI" + Alvl atoms).
-10
I
370
350
330
TIK
F v 3. Influence of the reflux temperature and time on the Si/Al ratio and the content of AI" and AIV1atoms per unit cell (0,AI" atoms; A, AIV1atoms; 0 , Si/A1 ratio; A, %/A1 ratio in dependence on refluxing time at 368 K).
TABLE V Results of the NH,' Exchange and the NH3 TPD Measurements
HM-Ob HM- 1 HM-2 HM-3 HM-4a HM-4b HM-4c HM-4d
HM-6 * HM-7
2.04 1.61 1.56 1.26
0.83
1.93 1.80
1.36
1.67 1.40
1.24 0.98
1.31
823 803 818 813
513 508 511 508
71 73 74 70
29 27 26
a degree of dealuminationof approximately 7096. This experiment demonstrated that mordenites may be dealuminated easily (as has been reported previously in ref 26), and therefore all heat treatments should be performed with special care. %i lMAs NMR Investigations. As mentioned above, the considerable numbers of SiOH groups generated during the dealumination proass render a straightforward application of %i MAS NMR for the elucidation of the framework composition of dealuminated mordenites impossible. Bodart et aLZ0used the following procedure to recalculate the contributions of SiOH groups to the 29SiMAS NMR spectra: With the crucial assumption that 'the small intensity at ca. -99 ppm in the 29SiNMR spectra is always and entirely due to Si(2Al)",2]they used the well-known equation for the determination of framework Si/Al ratio from 29SiNMR ~pectra:~'
30
0.86 0.49 0.39
0.8 1
0.67
776
503
83
17
0.38
0.39
0.34
758
506
88
12
0.33
'Ammonium ion exchange capacity. bTotal amount of desorbed NH3 molecules. Desorbed amount of NH3 molecules corresponding to the high-temperature peak. dMaximum temperature of the hightemperature peak. Maximum temperature of the low-temperature peak. 'High-temperature peak relative to the total desorbed amount. glow-temperature peak relative to the total desorbed amount.
mordenites the density of the Alv*atoms as well as that of the Alw atoms decrease monotonically as the total content of Al atoms in the framework decreases. Figure 3 shows the influence of the temperature and time of acid refluxing on the degree of dealumination. The dotted line and the dashed line correspond to the number of Al" atoms per unit cell and the Si/Alf ratio of the parent mordenite, respectively. With increasing temperature the Si/Alf ratio increases strongly after a relatively low initial rise. It is necessary to raise the temperature to about 370 K to reach a degree of dealumination higher than 51%. Extending the time of treatment at constant temperature also increases the degree of dealumination and consequently the Si/Alf ratio. In Figure 3 the number of tetrahedrally and octahedrally coordinated aluminum atoms per unit cell as a function of the refluxing temperature is given. Not all A1 atoms are dissolved by the acid at low temperatures, as might be expected. A temperature above 368 K is necessary to remove most of the nonframework aluminum atoms. We do, however, have to point out that under the conditions described it was not possible to prepare the pure H form of the parent Na mordenite. Although the treatment of the parent zeolite was performed with diluted acid nearly at room temperature, under these mild conditions dealumination already proceeded and the degree of dealumination reached 25%. The second procedure (see Table I, a thermal treatment after the ammonium exchange) resulted in a still higher extraction of Al from the framework with
4
(Si/Al)f = It/
c (n/4)Isl("*I)
n= I
(2)
(where I, is the total line intensity, n = 1-4, and Isl(&)is the line intensity of the Si(nA1) configuration) to calculate the intensity Is,(llu)from the Si/Al ratio obtained by chemical analysis and 27AIMAS NMR according to eq 1. Corresponding to their experimental results, only the intensities ISl(nAI) for n = 0-2 were taken into account. With ISiOH ISi(OA1)
= I-105 - ISi(1AI)
(3)
1-110 + ISiOH
(4)
the Si(nAl) distributionswere easily obtained. LlOsand Lll0stand for the 29SiNMR line intensities at -105 and -110 ppm, respectively. The Si(nA1) distributions were compared with theoretical ones calculated for different A1 siting and dealumination models. Using MAS NMR and IR spectroscopy to study the dealumination process in mordenites, Veeman and co-workers*I came to the same conclusion as Bodart et a1.2O that during this prows, Al is extracted from the 4Mgs in the lattice. On the other hand, these authors questioned the results concerning the Al siting since the intrinsic accuracy of data obtained by deconvolution of strongly overlapping lines is not sufficient to discriminate between the different siting models. Moreover, they pointed out that the parameters involved are obtained after applying corrections based on several further assumptions (cf. ref 20). To take these arguments into account, we reduced the number of variables by determining the concentration of SiOH groups independently by 'H MAS NMR spectroscopy. In addition to that, we increased the number of scans of the 29SiMAS NMR spectra to obtain a better signal-to-noise ratio than in former studies. The %i MAS NMR spectra of selected samples are given in Figure 5. In accordancewith published spectra,two main peaks can be recognized for weakly dealuminated samples: The peak at about -1 12 ppm assigned to Si(0Al) and a second one at about -106 ppm. The latter one can be caused by Si(lA1) as well as SiOH groups.*' The obvious high-field shift of the Si(0AI) line
The Journal of Physical Chemistry, Vol. 96, No. 21, I992 8411
Mordenite Acidity. 1
TABLE Vn: Contributim of Different Si Configurations in DerlumiMted Mordenftas Calculated on the Ansumption Tbat SiOH Group Contribute Only to the %i MAS NMR at About -106 ppm Si configurations (% of total Si) samples Si/Alp SiOH Si(3Al) Si(2AI) Si( 1Al) Si(OAl)b HM-Ob 7.0 15.7 2.1 11.2 28.4 58.3 HM-1 9.0 19.5 1.7 8.9 21.5 67.9 9.5 21.8 HM-2 2.1 9.0 17.8 71.1 HM-3 11.4 21.2 1.5 8.7 13.2 76.6 HM-4a 20.1 30.2 2.6 10.5 -8.9 95.8 2.8 10.9 -8.0 HM-4b 18.0 35.3 94.3 HM-6 39.9 26.4 0.5 3.6 1.3 94.6 HM-7 47.9 0.5 4.4 26.2 -1.9 97.0 'Average value of that obtained by combination of chemical analysis and 27A1MAS NMR and that determined by ammonium exchange. ISI(OAI)
I-112 + ISIOH.
TABLE VnI: Concentrations of Bridged Hydroxyl Groups and Silanol Group concn of (mmol/g) % Si present samples SiOH SiOHAl as S O H " HM-Ob 0.23 1.78 1.6 HM-1 0.36 1.57 2.4 HM-2 0.33 1.42 2.2 HM-3 0.46 1.16 3.1 HM-4a 1.27 1.02 8.0 HM-4b 1.09 0.90 6.9 HM-4c 0.9 1 0.62 5.6 HM-4d 0.67 0.50 4.1 HM-7 0.5 1 0.36 3.1
'Assuming that no geminal silanols are present.
-
in the course of dealumination is due to the well-known influence of the second-neighbor aluminum on the chemical shift of silicon.28 In line-shape simulations it turned out that a t least four lines are required to fit the experimental spectra. In addition to a line a t about -100 ppm commonly attributed to Si(2Al), it was necessary to introduce a further line at -94 ppm to achieve a good
not the individual intensities obtained by line-shape fitting but the sums of the line intensities in the respective regions are given in Table VI. Since essential prerequisite^^^ for obtaining Si/Al ratios of the framework from 29SiMAS NMR spectra according to eq 2 are not fulfilled for the samples under study, the Si/Al ratios given in the last column of Table VI are meaningless. The apparent constancy of this value over a rather broad range of dealumination is misleading. For the sake of comparison with the literature, we have applied the procedure outlined above to derive the Si(nA1) distribution from the measured 29SiMAS NMR spectra. The results are summarized in Table VII. Obviously, this procedure is not suited for obtaining reliable data on the distribution of the different environments of the silicon atoms in the dealuminated mordenites. The negative intensities obtained for Si( 1Al) of some samples are a consequence of the assumption, that all SiOH groups are present as Si(OSi)30H. It is a shortcoming of this procedure that other configurations, including geminal silanol groups, are neglected. Using the same procedure, Sawa et a1.22also obtained a negative value for ZS,(~,,,U) of a highly dealuminated sample. Furthermore, by 'H MAS NMR (vide infra) it was found that the SiOH concentrations given in Table VI1 are much too high. 'H MAS NMR Investigations. The IH MAS NMR spectra of the samples under study are given in Figure 6. Two wellresolved lines can be seen: the line (a) at 2.1 ppm and the line (b) at 4.3 ppm which have to be attributed to protons in SiOH groups and protons in bridged hydroxyls, respectively. The concentrations of these two proton species are given in Table VIII. A comparison of the directly measured concentrations of silanol
agreement of experimental and simulated line shape in the lowfield region. Actually, we also used two lines for both the line at -1 12 ppm and the line a t about -106 ppm. Especially for highly dealuminated samples, the former is broadened because of the existence of nonequivalent T positions.20 The broadening of the -106 ppm line can be attributed to an increasing number of SiOH groups. They are found not exactly a t the position of Si( 1Al) but a t a slightly lower field.27 (Using cross-polarization 29Si MAS N M R , we found the most intense line not a t -106 ppm but a t about -103 ppm.) Because of the strong overlap of these peaks,
groups with those given in Table VI1 reveals that up to a n order of magnitude too high silanol concentrations a r e obtained by Bodart's procedure. Using the S i O H concentrations measured directly by 'H MAS NMR to calculate the distributions of Si(nAl) units in the framework under the same assumptions as in the procedure of Bodart et ai.," one obtains only slightly higher Si/Al ratios as given in column 7 of Table VI. The main reason for this failure is the assumption that all silanols are present as S i ( 0 S i ) 3 0 H groups. For instance, Si(OSi)2(OAl)OH groups may contribute considerably to the line at -100 ppm. One could argue
- 80
-100
- 120
- 140
dlwm Figure 5. 29SiMAS NMR spectra of selected dealuminated H mordenites.
TABLE VI: Relative %i MAS NMR Line IntewiHe8 of DcrlumiMted Mordenitas re1 line intensities (9%) at about Si/ sample AI! -94 ppm -100 ppm -106 ppm -112 ppm HM-0 2.3 8.5 31.0 58.2 HM-Ob 7.0 2.1 11.2 44.1 42.6 HM-1 9.0 1.7 8.9 41.0 48.4 HM-2 9.5 2.1 9.0 39.6 49.3 HM-3 11.4 1.5 8.7 34.4 55.4 HM-4a 20.1 2.6 10.5 21.3 65.6 HM-4b 18.0 2.8 10.9 27.3 59.0 HM-6 39.9 0.5 3.6 27.7 68.2 HM-7 47.9 0.5 4.4 24.3 70.8
Si/ Alb 7.3 5.5 6.3 6.3 7.1 8.0 7.0 11.0 11.6
'Average value of that obtained by combination of chemical analysis and 27A1MAS NMR and that determined by ammonium exchange. *Calculated from 29SiMAS NMR line intensitics assuming that SiOH groups do not contribute to the spectra.
Stach et al.
8478 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992
n
d;L
'
I
HM-Ob
*
d"L JL
HM -2
HM-3
-
PIP01
Figure 7. Adsorption isotherms of benzene on mordcnites (A, HM-Ob; 0, HM-2, adsorption; 0 , HM-2, desorption; 0 , HM-6, adsorption; 0 , HM-6, desorption).
TABLE Rk R d t s of Benzene Adsorption md Determination of Pore volume b
/I
Jk
HM -La
HM
HM
r l
L3 21 b/ppm Figm 6. 'H MAS NMR spectra of selected daluminated H mordenites
(rt, sidebands).
that the thermal treatment during the necessary sample preparation for 'H MAS NMR leads to a reduction of the number of the silanol groups. With *%i MAS NMR, it was found that this thermal treatment has no influence on the spectra. In accordance with the results from chemical analysis and 27Al MAS NMR, the concentration of bridged hydroxyls decreases going from sample HM-Ob to HM-7. Further quantitative details will be d i d in connection with the data obtained from other methods (cf. part 2 of this series, following paper in the issue). Adsorpth Meuwemeats. To investigate the pore volume of the H mordenites, especially the micropore volume, benzene isotherms were measured. Some of them are presented in Figure 7. As follows from this figure, the adsorbed amount of the aromatic molecules strongly depends on the chemical composition of the samples. With rising Si/Al ratio, i.e., with decreasing number of Alf atoms in the unit cell, (especially at low partial pressure) the number of the adsorbed molecules rises. It increases very strongly initially (sample HM-2) but decreases again at a
~mcro'
sample mmol/g wt W HM-Ob 0.35 2.7 HM-2 1.19 9.3 1.30 10.2 HM-4b HM-4c 1.06 8.3 HM-6 0.91 7.1 HM-7 0.81 6.3
atotnl mmol/g wt 56 0.53 4.1 1.40 10.9 1.53 12.0 1.42 11.1 1.37 10.7 1.22 9.5
utwi
mL/g 0.047 0.124 0.136 0.126 0.122 0.108
.,md
VmmC
mL/g 0.031 0.106 0.116 0.094 0.071 0.071
mL/g 0.016 0.018 0.020 0.032 0.051 0.051
'In micropores adsorbed amount. bTotal adsorbed amount. CTotal pore volume. Micropore volume. Mesopore volume.
higher value of Si/Al. This behavior is quite unusual because normally the benzene adsorption (in X-,Y-,29ZSM-5-zeo1ites3O et al.) decreases with diminishing A1 content of the molecular sieves. Therefore, it is not to be expected that the number of aluminum atoms in the framework and thus the strength of the electrostatic field in the pores of the mordenites is the primary reason for the observed behavior of the dealuminated H-mordenites. From Figure 7 it may be seen that by investigating the desorption branch of the isotherms, hysteresis loops were found, indicating a mesopore system originating from the dealumination procedure. The benzene amounts adsorbed at relative pressure of p / p s = 0.2 and p / p s = 0.9 were the basis of the calculation of the micropore and mesopore volume, respectively. The corresponding data are listed in Table IX. As follows from this table, the micm and mesopore volumes increase with increasing degree of dealumination, reach a maximum (for a Si/Al ratio of 18.0) and then decrease slowly. Barrer and co-workers" reported a micropore volume (for large-port mordenite) of 0.1 12 mL/g for the main chanmls and a total micropore volume (including the si& pockets) of 0.20 mL/g. The benzene molecules may not enter these side pockets and our calculated values of 0.1 16 mL/g (HM-4b) is in excellent agreement with the data given in ref 31. Obviously, the sample HM-Ob (and as well the synthesized NaM, from which all other samples were prepared by de~.~~ alumination) belongs to the small-port m o r d e n i t e ~ . ~Veemann and co-worker@ found an adsorption capacity of benzene for the small-port mordenite of 2.64 wt %, which is in good agreement with our value of 2.7 wt Q. As follows from Table IX, the dealumination procedure of the H mordenite transforms the smallinto a largeport mordenite with a higher adsorption capacity for benzene molecules?' In Figure 8 the adsorbed benzene amounts (considering only the micropore volume) are plotted against the fraction of the extracted A1 atoms. This Figure includes data presented in ref
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8419
Mordenite Acidity. 1
groups. Additionally, the number of silanol groups and Brcansted sites was derived from 'H MAS NMR investigations. Adsorption measurements of benzene revealed that the parent material and the sample HM-Ob belonged to the small-port mordenites. By extracting more than 25% aluminum from the lattice, the small-port mordenites were transformed in the large-port type with a higher adsorption capacity for benzene.
OL 0
I
I
Q(%)
100
Figure 8. Dependence of the benzene micropore adsorption capacity on the degree of dealumination (0,own results; 0 , from ref 21).
21. Both curves agree to a high degree. From the graph it follows that more than 25% of the framework aluminum must be removed to transform the small- into large-port mordenites. Our investigations thus confirm results given in the l i t e r a t ~ r e .The ~ ~ explanation for the transformation process by dealumination given by Veemann and co-workersZ1seems to be reasonable. Referring to suggestions of Sanders,34who stated that linking mordenite chains in different ways in the framework will result in a blockade of a part of the pore system of the small-port mordenites, the authors developed the following transformation model: The barriers in the main pore system are randomly distributed and reduce the accessible micropore volume. Due to the dealumination (which extracts the Al atoms from the 4-rings of the lattice)mz2 the barriers are not removed, but interconnections are generated between the main channels thus increasing the micropore volume. The creation of the interconnections takes place randomly. The decreasing part of the curve in Figure 8 might be due to a partial blockade of the main channels by nonframework aluminum." As shown by 27AlMAS NMR, octahedrally coordinated A1 is indeed found within the mordenite crystals dealuminated by acid leaching. But its amount decreases monotonically with increasing degree of dealumination. We suppose that another factor could be responsible, e.g., the shrinking of the unit cell with increasing dealumination, which has been reported by several author^.^^,^^ Recrystallizationprocesses also cannot be excluded (due to thermal stress during the necessary activation procedures of the samples) because the number of the silanol groups, which represent the defect structure, becomes smaller at a higher degree of dealumination (seeTable VIII).
Conclusions A series (10 samples) of aluminum-deficient H-mordenites with different Si/Al ratios between 7 and 48 were prepared using dealumination by acid leaching. By the use of 27AlMAS NMR combined with wet chemical analysis the composition of the zeolitic framework was determined as well as the amount of the octahedrally coordinated extralattice aluminum. By 29SiMAS NMR measurements (with and without crosspolarization) the existence of SiOH defect groups was derived and their number quantified following a procedure given in the literature. It has been found that the procedure used was not suited for obtaining reliable data either on the distribution of the different environments of the silicon atoms or on the amount of SiOH
Acknowledgment. The preparation of the mordenite samples by Dr. K. J. Waghamare, NCL Pune (India), is gratefully acknowledged. B.Z. thanks the Alexander von Humboldt Foundation for awarding a Lynen Research Fellowship that allowed the completion of this study. Reghtry No. Al, 7429-90-5; NH3, 7664-41-7; benzene, 71-43-2.
References and Notes (1) Oblad, A. G.;Milliken, T. H.; Mills, G. A. Adv. Catal. 1951,3, 199. (2) Thomas, J. M.; Thomas, W. J. Introduction to the Principles of Heterogeneous Catalysis; Academic Press: London, 1967. Tanabe, K. Solids Acids and Bases; Kondansha: Tokyo, 1970. (3) Barthomeuf, D. In Catalysis 1987; Ward, J. W., Ed.; Elsevier: Amsterdam, 1987, p 177. (4) Jacobs, P. A. Catal. Rev.-Sci. Eng. 1982, 24, 415. (5) Fripiat, J. G.; Berger-Andre, F.; Andre, J. M.; Derouane, E. G. Zeolites 1983, 3, 306. (6) Senchenya, I. N.; Kazansky, V. B.;Beran, S.J. Phys. Chem. 1986,90, 4857. (7) Dwyer, J.; O'Malley, P. J. In Keynotes in Energy-Related Catalysis; Kaliaguine, S.,Ed.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1988; Vol. 35, p 73. (8) Eberly, P. E.; Kimberlin, C. N.; Vorhies, J. J . Catal. 1971, 22, 419. (9) Thakur, D. K.; Weller, S.W. J . Coral. 1972, 24, 543. (10) Thakur, D. K.; Weller, S . W. Ado. Chem. Ser. 1976, 121, 596. (1 1) Jacobs, P. A. In Characterization of Heterogeneous Catalysts; Delannay, F., Ed.; Marcel Dekker Inc.: New York, 1984; p 367. (12) Lohse, U.; Parlitz, B.; Patzelova, V. J. Phys. Chem. 1989, 93, 3677. (13) Stach, H.; Wendt, R.; Lohse, U.; Jinchen, J.; Spindler, H. Catal. Today 1988, 3,431. (14) Niwa, M.; Sawa, M.; Murakami, Y. In Proc. 9th Inr. Congr. Catal. 1988,1, 380. ( I S ) Sawa, M.; Niwa, M.; Murakami, Y . Appl. Catal. 1989, 83, 169. (16) Meier, W. M. Z . Kristallogr. 1961, 115, 439. (17) Shiokawa, K.; Ito, M.; Itabashi, K. Zeolites 1989, 9, 170. Jerschkewitz, H.-G.; Lischke, G.;Parlitz, B.; Richter, (18) 6hlmann, G.; M.; Eckelt, R. Z . Chem. 1988, 28, 161. (19) Wendt, R.; Jinchen, J., unpublished results. (20) Bodart, P. B.; Nagy, J.; Debras, G.; Gabelica, Z.; Jacobs, P. A. J . Phys. Chem. 1986, 90, 5183. (21) van Geem, P. C.; Scholle, K. F. M. G.J.; van der Velden, G. P. M.; Veeman, W. S.J. Phys. Chem. 1988, 92, 1585. (22) Sawa, M.; Niwa, H.;Murakami, Y. Zeolites 1990, I O . 532. (23) Samoson, A,; Lippmaa, E.; Engelhardt, G.; Lohse, U.; Jerschkewitz, H . 4 . Chem. Phys. Lett. 1987, 134, 589. (24) Karge, H.G.; Weitkamp, J. Chem.-lng. Techn. 1986, 58, 946. (25) Karge, H.G.Z . Phys. Chem. (Munich) 1980, 122, 103. (26) Karge, H. G.; Gutsze, A.; Allgeier, J.; Lange, J. P., manuscript in
preparation. (27) Engelhardt, G.; Michel, D. High Resolution Solid-State NMR of Silicates and Zeolites; Wiley: Chichester, 1987. (28) Newsam, J. M. J . Phys. Chem. 1985, 89, 2002. Lohse,U.; Thamm, H.; Schirmer, W. Zeolites 1986,6, 74. (29) Stach, H.; (30) Thamm, H.; Stach, H. In Adsorption in Microporous Adsorbents (Workshop I l l ) ; Stach, H.; Sauer, J., Eds.; AC Central Institute of Physical Chemistry: Berlin, 1987; Vol. 2, p 9. (31) Barrer, R. M.; Peterson, D. L. Proc. R. SOC.(London) 1964, A280, 466. (32) Raatz, F.; Freund, E.; Marcilly, Ch. J. Chem. SOC.,Faraday Trans. 1983, 79, 2299. (33) Kranich, W. L.; Ma, Y. H.; Sand, L. B.; Weiss, A. H.;Zwiebel, J. Adv. Chem. Ser. 1971, 101, 502. (34) Sanders, J. V. Zeolites 1985, 5, 8 1. (35) Scherzer, J. ACS Symp. Ser. 1984, 248, 157. (36) Olsson, R. W.; Rollmann, L. D. Inorg. Chem. 1977, 16, 651.
