Langmuir 1993,9, 1846-1851
1846
Adsorption by MFI-Type Zeolites Examined by Isothermal Microcalorimetry and Neutron Diffraction. 1. Argon, Krypton, and Methane P. L. Llewellyn,'tf J.-P. Coulomb,$Y. Grillet,f J. Patarin,) H. Lauter,ll H. Reichert,l and J. Rouquerolt Centre de Thermodynamique et de Microcalorimktrie du CNRS, Marseille, France, CRMCLCNRS, Facultk des Sciences de Luminy, Marseille, France, Laboratoire des Matkriaux Minkraux, Universitk de Haute-Alsace, Mulhouse, France, Imtitut Laiie Langevin, 156X Centre de Tri, 38042 Grenoble, France, and Imtitut fiir Anorganische Chemie und Analytische Chemie, Johannes Gutenberg Universitlit, Mainz, Germany Received November 4, 1992. In Final Form: March 25, 1993 The physisorptionof argon, krypton, and methane on silicalite-I and of argon on a series of MFI-type zeolites with varying Si/Al ratios is examined by isothermal volumetry, microcalorimetry, and neutron diffraction. Adsorption of argon and krypton is shown to occur by site on silicalite-I. Both molecules undergo transition from a "disordered phase" (presumablya fluid)to "crystalline-likesolid phase", whereas methane remains in a disordered phase. At low coverage, the adsorptionof argon on the MFI-type zeolites shows no specificity to aluminum composition, whereas the transition at higher coverages becomes more diffuse with increasing aluminum content.
Introduction To better understand the mechanisms of gaseous physisorption in microporous adsorbents, well-charaderized reference systems are still badly needed. Known for a long time, the zeolite family provides a host of adsorbents with well-defined micropores (in shape, size, and connectivity). Nevertheless, fundamental studies in the field of physisorption must be able to take into account the role of the compensatorycations and protons (and include the limiting case where they are absent) and to virtually eliminate the effects of adsorption on the external parts of the crystal. This is why we selected for this work, a set of MFI-type zeolites,' including ZSM-5,2 with varying silicon to aluminum ratios, and silicalite-I,3 which is the pure end member. It was also intended to lower the role of the external adsorption by using samples consisting of large crystals. These zeolites have an interesting three-dimensional intersecting porous network (straight, elliptical channels intersected by sinusoidal cylindrical channels). The three complementary techniques used here to study physisorption are gas adsorption volumetry,gas adsorption microcalorimetry, and neutron diffraction. Isothermal microcalorimetry at 77 K allows the study of adsorbate-adsorbent interactions and the detection of the successive stages of pore filling with emphasis on the mobility and/or density variations of the adsorbate phase, whereas neutron diffraction provides a powerful technique to determinethe molecular organizationon the microscopic scale and is well suited to provide a direct structural + Centre de Thermodynamique et d e Microcalorim6trie d u CNRS. t CRMCWNRS. i Universite de Haute-Aleace. 1 Institute Laiie Langevin. Johannes Gutenberg UniversitAt. (1) Meier, W. M.; Olson, D. H. Atlas ofzeolite Structure Type8, 2nd ed.;Butterworthe: London, 1987;p 100. (2)Kokotailo, G. T.; Lawton, 5. L.; Olson,D. H.; Meier, W. M. Nature 1978,272,437. (3)Flanigen, E. M.; Bennett,J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Nature 1978,271,512.
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analysis of the adsorbed phases (especially for light adsorbates) and/or adsorbent structure modification. In previous studies it has been shownthat the adsorption of argon on silicalite-I a t 77 K gives a type I isotherm with an additional s ~ b s t e p . ~ This * ~ substep, at low relative pressures, has been examined using microcalorimetry and modeled via atom-atom potential energy calculations. It is thus attributed to a densification of the adsorbate phase.415 However this densification has not yet been confirmed by a structural study of the adsorbate phase. ZSM-5 on the other hand, has not been previously seen to give a substep in the argon isotherm.6 Krypton on silicalite-I has also been seen to give a stepped isotherm a t 77 K.7 This step was interpreted by modeling as the filling of firstly the micropores followed by the total volume, but no structural study has been carried out to confirm this interpretation. No evidence has been found for the study of methane adsorption on the MFI-type zeolites at 77 K. However many studies have been carried out a t ambient temperatures,&l3 which show that little chemical interaction occursbetween the methane molecule and channel walls and that the methane acta as a "fluid" adsorbate. (4)Mijller, U.;Unger, K. K.; Pan, D.; Meremann, A,; Grillet, Y.; Rouquerol, F.; Rouquerol, J. In Zeolites an Catalysts, Sorbent8 and Detergent Builders; Karge, H. G., Weitkamp, J., Ede.; Elsevier: Amsterdam, 1989; p 625. (5)Mijller, U.; Reichert, H.; Robens, E.; Unger, K. K.; Grillet, Y.; Rouquerol, F.; Rouquerol, J.; Pan, D.; Meremann, A. Fresenius 2.Anal. Chem. 1989,333,433. (6) Webb, S. W.; Conner, W. C. In Characterization of Porous Soli& ZI; Rodriguez-Reinoeo,F., Rouquerol, J., Sing, K. S. W., Unger, K. K., Eds.; Elsevier: Ainetardam, 1991;p 31. (7) Hope, A. T. J.: Lena, C. A.; Catlow, C. R. A. Proc. R . SOC.London 1989,A4%4 57. (8)P a m , H.: Hineen.. W.: . Do,. N. T.: Baerna, M. Thermochim. Acta 1984,82,-i37. (9)Titiloye, J. 0.; Parker, 5.C.; Stone, F. S.; Catlow, C. R. A. J.Phys. Chem. 1991,95,4038. (10)Yamazaki, T.; Watanuki, I.; Ozawa, S.; Ogino, Y. Langmuir 1988, 4, 433. (11)Jobic, H.; BBe, M.; Kearley, G. J. Zeolites 1989,9,312. (12)June, R. L.;Bell,A. T.; Theodorou, D. N. J. phy8. Chem. 1990, 94,1508. (13)Goodbody, S.J.; Watanabe, K.; MacGowan, D.; Walton, J. P. R. B.;Quirke, N. J. Chem. SOC.,Faraday Tram. 1991,87(13), 1951.
0743-7463/93/2409-1846$04.00/0 0 1993 American Chemical Society
Adsorption by MFI-Type Zeolites
Langmuir, Vol. 9, No. 7, 1993 1847
Table 1. Si/A1 Molar Ratio and Crystal Size ( p d ) of the As-Synthesized Samples samples SiIAP molar ratio crystal sizeb I (silicalib-I) >loo00 600 X 130 X 130 I1 120 120 X 30 X 30 I11 60 27X6X6 ~~
Iv V
30 20
30X6X6 50 X 15 X 15
By chemical analysis. By statisticalcounting from SEM images.
Experimental Section Samples. T w o sources of silicate-I were esamined. The f i s t was prepared in Mainz using the alkaline free synthesis proposed by Unger et al." and gave large individual crystals (150 rm). This sample was principally used for the neutron diffraction experiments but gave exactly the same results for the low temperature microcalorimetry and volumetric experiments as the second silicalite-I sample described below. The series of MFI-type samples examined were prepared in Mulhouse according to the procedure described by Guth et a1.16 in which fluoride anionsare used to diesolvethe silica and alumina sources. Again this route is known to produce large crystals. Five samples were taken with different Si/Al molar ratios and all were calcined together (using the same temperature protocol) up to a temperature of 550 OC to remove the organic template TPA+. The silicon to aluminum ratio and crystal size were then determined and the results are given in Table I. The samples were outgassed wing the technique of controlled rate evolved gas analysis (CR-EGA9. This procedure controls the heating of the sample in such as way as to ensure a predetermiied (and usually constant) rate of outgassing of the sample. This ensures that the sample is prepared in a regular and reproducible manner without unwanted dehydroxylation or surface degradation. The samples (around 100 mg)were treated to 473 K by CR-EGAwith an outgassing rate of 0.36 m g W under a residual water vapor pressure of 1.33 Pa. Volumetry a n d Microcalorimetry. The volumetric experiments were carried out at 77 K both on a commercial apparatus (Omnisorp 100 Analyser, Coultronics, France, S.A.) and on equipment constructed in-house (C.T.M., Marseille"). The adsorptive introduction was performed using either a static'' or quasi-equilibrium18procedure. The in-house quasi-equilibrium volumetric procedure employs an extremely slow constant introduction of adsorptive gas, in the region of 2 cmg*h-lfor which it was verified that the quasi-equilibriumconditionswere fulfiied. This latter technique coupled with isothermal adsorption microcalorimetrylBallowsdirect acceas to a continuous measurement of the derived enthalpies of adsorption, during the vertical (or near vertical) parte of the isotherm. These near vertical parte, where the derivative enthalpy of adsorption is directly proportional to the thermal flux, are of great interest in this study as it is where moat adsorption phenomena occur in micropores. Neutron Diffraction Measurements. The neutron diffraction experiments were carried out at the LBue-Langevin Institute (ILL, Grenoble, France) and at the LBon Brillouin Laboratory (LLB, CEA-Saclay, France). Around 2 g of the silicalite-I sample was used,and the measurements were carried out on the diffractometere; DE3 (ILL, at a wavelength X = 2.524 A) and G 4 1 (LLB, at a wavelength X = 2.439 A). The results were recorded with a multidetector and were then plotted as the peak intensity as a function of Q,the scattering vector where Q = {4r/X)sinh (BeistheBraggdiffractionangle). Theexperiments were performed principally around 80 K with stability studies of the adsorbed phase between 5 and 100 K.
a,
(14)Mueller, U.; Unger, K. K. Zeolites 1988,8, 154. (15)Guth, J.L.;Keeeler, H.;Wey, R. InNew Deuelopmente in Zeolite Science and Technology; Murakami, Y., Iijima, A., Ward, J. W., Eds.; Elwvier: Amsterdam, l9s6; p 121. (16)Rouquerol, J. Thermochtm. Acta 1989,144,209. (17)Boudellal, M.Ph.D. Thesis, Universite de Provence, Marseille, France, 1979. (18)Rouquerol, J.; Rouquerol, F.; Grillet, Y.;Wark, R.J. In Characterization ofPorow Solids;Unger, K. K., Rouquerol,J., Sing,K. 5. W., Ward, R. J., Eds.;Eleevier: Amsterdam, 1988;p 67. (19)Rouquerol, J. In Thermochimie; CNRS: Paris, 1971;p 537.
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N' / molec . uc" Figure 1. Initialisotherms (a, top) and net differential enthalpiea of adsorption (b, bottom) of argon, krypton,and methane at 77 K on silicalite-I.
Results and Discussion Argon on Silicalite-I. The adsorption isotherm (Figure la) obtained for the adsorption of argon on silicalite-I is of type I character although it exhibits a substep which correspondsto an increase in the net differentialenthalpy of adsorption (Figure lb). These results are in agreement with those found previously by three of the authom.'S The differential enthalpy during initial upteke in the region 0 < na/molw-l < 22 remaine constant indicating the energetic homogeneity of the sample to argon adsorption. Theneutron diffractionspectra obtained (Figure 2) show a strong decrease of the diffractedpeak intensities at low angle (that is in the region of Q = 0.6 A-9. This drastic decreaseseems toolarge tobe due to the previously reported monoclinic-orthorhombic adsorbent structure However, it may be more likely due to interferenceeffecte between the adsorptive and adsorbent. The adsorbate in this region appears to be disordered in the temperature range from 40 to 80 K, however, at 10 K argon crystalline order wm observed in the neutron daraction spectra (Figure 3). A substep in the isotherm is observed in the region 23 < na/mol*uc-l< 30 which corresponds to a region in the differential enthalpy curve which ia exothermic in character in respect to the initial adsorption. This suggests (20) hichert, H.; MOller, U.; Unger, K. K.; Grillet, Y.;Rouqueml, F.; Rouquerol, J.; Coulomb,J. P.In Characterization of Porow Solids ZI; Ftc&iguez-hinoso, F.,Rouquerol, J., Sing, K. S.W., Unger, K. K.,Eda.; Eleevier: Amsterdam, 1991;p 635.
Llewellyn et al.
1848 Langmuir, Vol. 9, No. 7, 1993
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Figure 4. Neutron diffraction spectra of argon adsorbed on silicalite-I at 87 K for different quantities adsorbed during the formation of substep. Strong diffraction peake appear in the range 1.8 I Q/A-1 I 1.9. T IK
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Figure 3. Neutron diffraction spectra of argon adsorbed on eilicalite-I at Na= 13.2 molmuc-l for several temperatures; 10 IT / K I80. Crystallization of the argon adsorbate phase is observed at 10 K.
the occurrence of an adsorbate phase transition. The neutron diffraction spectra in this region (Figure 4) show the appearance of diffraction peaks in the range 1.8 < Q / k l < 2.0. The appearanceof these p e h is the signature of crystalline organization of the argon adsorbate phase. Thus a disordered phase to crystalline solid-like phase transition s e e m to occur, which c o n f i s the literature where atom-atom potential energy calculations were used.'r6 The stability of this phase is shown as the neutron diffraction peak positions do not move in the temperature range from 10 to 100 K (Figure 5). These results also suggest that the solid-like structure of argon is imposed by the silicalite-I microporous network. The Clapeyron diagram for argon (Figure 6a) allows a view of the position of the step and substeps with temperature and pressure. It can be seen that the position of the substep approaches that of the initial step around 30 K. Krypton on silicalite-I. The general character of the adsorption isotherm given by kryptonon silicalite-I (Figure la) is in good agreement with literature data.' The isotherm has similar characteristics and uptake as that obtained for argon, however the substep is visible for krypton at a higher relative pressure than that of argon @/po(Ar)= 0.02p/p0(Kr)). The net differential enthalpy
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Figure 5. Neutron diffraction spectra of argon adsorbed on silicalite-I at N* = 32 moleeuc-l (on the isotherm plateau) for several temperatures 10 IT/K I100. The Q positions of the strong diffraction peaks located at Q = 1.86 and 1.90 A-1 are independent of temperature in the range studied.
of adsorption during the initial adsorption (0 < na/moEucrl < 23) is also similar to that of argon in spite of the higher polarization potential and larger size of krypton (Figure lb). Again, the neutron diffraction spectra in this region show a dramatic decrease of peak intensities in the region of Q = 0.6 A-l (Figure 8). The substep in the adsorption isotherm corresponds to a change in the differential enthalpy of adsorption. In contrast to argon however, this change in the differential enthalpy is endothermic in respect to the initial enthalpy of adsorption. It has been shown that xenon is not able to enter the silicalite-I structure at 77 K2I and so this endothermic phenomenon for krypton (with a kinetic diameter between those of argon and xenon) may be a consequence of confinement effectsz2 within the microporous network. Equilibration studies carried out during the initial adsorption and substep, however, showed the system was always at quasi-equilibrium even during the densification (see points a3 and a4 in Figure 7). The neutron diffraction spectra taken during the substep (21) Biilow, M.; H W l , U.;MUer, U.; Unger, K. K.Ber. Bumen-&s Phy8. Chem. 1990,94,14. (22) Derycke, I., Vigneron, J. P., Lambin, P., Lucae, A. A., Derouanne, E.G.J. Chem. phys. 1991,94 (6), 4620.
Adsorption by MFI-Type Zeolites
Langmuir, Vol. 9, No. 7, 1999 184s h
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Figure 7. Chart recording of the heat flux (proportional to the differential heat of adsorption)againsttime (proportionalto the quantity adsorbed) for the adsorption of krypton at 77 K on eilicalite-I. The pointa labeled 'a" show where the introduction of krypton wan stopped and the pointa labeled 'b" ahow where the introduction of krypton wan restarted. It can be clearly wen that at each point 'a" the microcalorimetric signal falb to the baseline in the instrumental dead time.
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A,' figure^ 8. Neutron ditYraction spectra of krypton admrbed on silicalite-Iat 96 K for varioue quantitieaadmrbed. As for argon, $ffracti!n pealre appear in the range 1.8 S Q/A-15 1.9 (the peak mtenaitm are d e r than thoee for argon becaum the cohere& neutron scattering crw Mion of krypton in d e r than that QI
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(Figure 8) show the appearance of peaks in exactly the same position as those observed during the argon substep. This again indicatea a disorderedphase to crystalbe solidlike phase krypton transition giving the same adsorbent
directed or commensurate solidlike structure as argon. This is confirmed as the adsorption uptakes of argon and krypton after the phase transition are similar (Figure la). It would therefore seem that the adsorption of argon and krypton occurs by site as the uptakes are similar despite the differences in size of the two molecules. T h e Clapeyron diagram for krypton (Figure 6b) shows that, unlike argon,the position of the substep approaches
that of the initial step at higher temperaturea. Thie effect is reflected by the relative endothermic signal during the phase transition at 77 K. A similar type of effect haa also been men for the adsorption of krypton on a graphite surfac@ and was attributed to a phase transition of the type commensurate solid to incommensurate solid. Methane on Silicalite-I.The adsorption isotherm of methane in silicalitel is type I in character and, unlike argon or krypton, it does not exhibit a substep. T h e net differential enthalpy of adeorption r e f l a the type I isotherm character. The differential enthalpy however re& constant during the initial adsorption and drop sharply in the plateau region of the ieotherm. Thie indicate the energetic homogeneity of the eample on methane adsorption. The value of the initial net differential enthalpy of adsorption ie higher than thoee of argon and krypton reflecting the higher polarizability and also thelarger size of methane. However thisvalue ie somewhat lower than thoee found in the literature at room temperature.81e During this initial adsorption the neutron diffraction spectra show the same diminution of the peak (23)w e r , J., Rouquerol, J., Thomy, A. J. Chem. Phya. lB76,827.
Llewellyn et a1.
1860 Langmuir, Vol. 9, No. 7, 1993
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with decreasing temperature.
intensities a t low angle around Q = 0.6 A-l which may highlight the pore fillingprocess. Nostrong peaks appear in the neutron diffraction spectra on methane adsorption both initially, within the micropores, and during the plateau region of the isotherm (Figure 9). The peak positions in these regions are independent of temperature between 6 and 79 K (Figure 10)showing the stability of the adsorbate phase. This suggests that the methane remains as a disordered adsorbate phase at 77 K (presumablya fluid phase) as has also been shown for methane at higher temperatures.1° This behavior may be a result of the wealmess of the possible interaction between the hydrogen a t o m of the methane molecules within the confindmicroporous network24 Confinementeffectamay also play an important role as the liquid volume adsorbed fills almost 90% of the theoretical pore volume at the plateau (whereasargon and krypton fill 60 and 74 % of the total theoretical pore volume, respectively), and thus an adsorbate phase transition could be p h y a i d y restricted by the microporous network. This latter hypothesis may be c o n f i i e d by the fact that volumetric and neutron (24) Pan, D., Technische Universit&t,Mhchen, Germany. Private Communication.
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.
N'/ molec uc" figure^ 11. Initial iaotherma (a,top) and enthalpies of adsorption
(b, bottom) of argon at 77 K on the seriea of MFI-typezeolitee. (For clarity, the Y-axis hae been shifted in each case).
diffraction studies of the adsorption of ethane at 127 K, like methane, did not show a phase transition.26 Argon on the Series of MFI-Type Zeolites. The initial adsorption isotherma to a relative pressure of 0.002 for argon on the series of MFI-type zeolites are given in Figure l l a with the corresponding net differential enthalpies of adsorption shown in Figure llb. The differential enthalpies of adsorption during the initial adsorption (in the range 0 < n*/mol-ud < 22) of argonontheeeri~ofzeoliteearerelativelyconstantar~d the same value of -7.5 kJ-mol-'. This shows the homogeneity of the sample to argon such that the aluminum content of the sampledoea not provide any epecificsorption sites for argon. The sharp sub~tepin the adsorption isotherm and exothermiceffect in the adsorption enthalpy curve can be clearly diatinguiehed for silidte-I. On the other hand, for the other ZSM-6 samples it can be seen that both phenomena become less distinct with increasing alumi(26) Coulomb, J. P. Unpubliahed work.
Adsorption by MFI-Type Zeolites num content. The values of the quantity adsorbed on the plateau region however are all similarsuggesting,in analogy with the neutron diffraction results of silicalite-I, that all of the ZSM-5samples contain a solidlike argon phase in the plateau region. This therefore implies that a disordered phase to crystalline solidlike argon adsorbate phase change occurs for each sample. However the aluminum content of the sample clearly affecta thistransition in that it would seem that at relatively high argon loadings the atoms close to the aluminum sites acquire an induced moment which therefore makes the eventual transition more diffuse and to spread over a relatively large region of pressure.
Conclusion It has been seen that the adsorption uptakes of both argon and krypton are similar indicating an adsorption by site occurring within the silicalite-I microporous structure. Furthermore the adsorption sites of silicalite-I appear homogeneousto physisorption. That is to say that the molecules do not seem to distinguish between the straight or sinusoidal channels on adsorption. Argon and
Langmuir, Vol. 9, No. 7,1993 1861
krypton were shown to undergo a disordered phase to crystalline solidlike phase transition whereas methane remains as adieordered phase in silicalite-1(the disordered phase is presumably a fluid). The crystalline solidlike phases of argon and krypton are seen to be identical in structure and commensurate with the silicalite-I microporous network. During the phase transitions, argon is seen to give an exothermiceffect in respect to the initial adsorption whereas krypton is seen to give a relatively endothermic effect. This may be due to confinement effecta within the microporous structure of silicalite-I. The adsorption of argon on the series of MFI-type zeolites show that during initial adsorption the argon is not affected by the aluminum content of the zeolite. On the other hand, argon is shown to undergo the disordered phase to crystalline solidlike phase transition in all of the series, but this is seen to be more diffuse with increasing aluminum content.
Acknowledgment. The authors thank the EEC SCIENCE programme for their financial support including a research grant for PLL (EEC Contract No. SCI*.0129C).
