Langmuir 1993,9, 1852-1856
1852
Adsorption by MFI-Type Zeolites Examined by Isothermal Microcalorimetry and Neutron Diffraction. 2. Nitrogen and Carbon Monoxide P. L. Llewellyn,*pt J.-P. Coulomb,$ Y. Grillet,? J. Patarin,g G. Andre,ll and J. Rouquerolt Centre de Thermodynamique et de Microcalorimktrie du CNRS, Marseille, France, CRMC2-CNRS, Facultk des Sciences de Luminy, Marseille, France, Laboratoire des Mathiaux Minkraux, Universitk de Haute- Alsace, Mulhouse, France, and Laboratoire Lkon Brillouin, CEN Saclay,l 91191 Gif-sur- Yvettte, France Received November 4,1992. In Final Form: March 25,1993
Physisorptionof nitrogen and carbon monoxideon a seriesof MFI-type zeolites with varying Si/Alratios was examined by isothermalvolumetry and microcalorimetry with reference to a neutron diffractionstudy of nitrogen adsorption on silicalite-I. The adsorption of both molecules occurs by site. For samples with a high Si/Al ratio (>120),two adsorbate phase transitions could be distinguished for both adsorptives, corresponding to a “disorderedphase” (presumablya fluid) to “latticefluidlike”and then to a “crystallinelike solid phase”. The former transition was indistinguishablefor the other samples; however the latter was observed at lower pressures and more diffuse with decreasing SiIAl ratio. of adsorption. Therefore an investigationof the adsorption process using a probe molecule with a larger permanent In contrast, and in complement, to a previous study,’ moment, for example carbon monoxide, would appear we present here a study of the adsorption of two quainteresting to compare with that of nitrogen. Carbon drupolar, nonspherical molecules (nitrogen and carbon monoxide has a quadrupole moment of 0.37 X esu monoxide) on MFI-type zeolites.2 The adsorption of cm3 and a dipole moment of 0.11 D compared with the nitrogen is indeed quite interesting. For instance, it has nitrogen quadrupole moment of 0.15 X esu cm3. been shown to give, at 77 K, a stepped isotherm on the Comparing the behavior of these adsorbates with that of pure silica end member of the MFI-zeolite series, silicalitethe nonpolar molecules previously studied1f8 may be 1.3 Initially one substep (P) was viewed in the isotherm expected to be fruitful, especially to draw conclusions on which was interpreted by a differential filling process.P6 the nature of the interactions of these permanent elecHowever later, with the aid of isothermalmicrocalorimetry trostatic moments on the physisorption process. a second smaller substep (a)in the isotherm was observed? The examination of the adsorption phenomena within Microcalorimetry and neutron diffraction have explained the microporous structure of the MFI-type zeolites is both substeps by density changes in the adsorbate phase.8~~ convenientlycarried out via the combination of isothermal It has also been shown that the introduction of aluminum volumetry with the techniques of microcalorimetry and within the MFI-type structure has an effect on the neutron diffraction. Isothermal microcalorimetry, with a adsorption of nitrogen and above all on the previously constant but extremely slow introduction of adsorptive, mentioned substep /3.1° allows a continuous observation of the adsorbate-adsorIt would seem that the permanent quadrupole moment bent interactions. The various stages of micropore filling of the nitrogen molecule plays a vital role in the mechanism can be followed with changes in the adsorbate density or mobility able to be highlighted. Neutron diffraction t Centre de Thermodynamiqueet de Microcalorimetric de CNRS. however, permits a structural examination of the adsorpt CRMCa--CNRS. tion process. Structure changes in both the adsorbent 1 Universite de Haute-Alsace. and adsorbate may be observed as well as any interference 11 Laboratoire L6on Brillouin, CEN Saclay. effects between the two phases. 1 Laboratoire Commun CEA-CNRS. (1)Llewellyn, P. L.; Coulomb, J.-P.;Grillet, Y.; Patarin,J.; Lauter, H.; Reichert, H.; Rouquerol, J. Langmuir, preceding paper in this ieeue. Experimental Section (2) Meier, W. M.; Olson, D. H. Atlas of Zeolite Structure Types, 3nd ed.; Butterworth-Heinemann: London, 1992; p 138. Samples. The MFI-type zeolites have an intersecting pore ( 3 )Flanigen,E. M.; Bennett, J. M.; Grose, R. W.; Cohen, J. P.; Patton, network of straight channels (approximately0.53 X 0.56 nm in R. L.; Kirchner, R. M.; Smith, J. V. Nature 1978,271,512. cross section) and sinusoidal channels (approximately 0.51 X (4) Carrott, P. J. M.; Sing, K. S. W. Chem. Znd. 1986,786. 0.55 nm in cross section).2 The intersections are approximately (5) Mmer, U.; Unger, K. K. Fortschr. Mineral. 1986,64,128. (6) Jacobs, P. A.; Beyer, H. K.; Valyon, J. Zeolites 1986, 1, 161. 0.8 nm in diameter, there being four per unit cell. The two types (7)MUer, U.; Unger, K. K.; Pan, D.; Meremann, A.; Grillet, Y.; of silicalite-I examined, which have already been presented Rouquerol, F.; Rouquerol, J. In Zeolites as Catalysts, Sorbents and elsewhere,l were synthesized by an alkaline-free” or fluoride’* Detergent Builders; Karge, H. G., Weitkamp, J., E&.; Elsevier: Amroute. These samples and the series of MFI-type zeolites (with sterdam, 1989; p 625. differing silicon to aluminum ratios) also previously described’ (8) Mmer, U.; Reichert, H.; &bene, E.; Unger, K. K.; Grillet, Y.; Rouauerol, F.; Rouauerol, J.; Pan, D.; Mersmann, A. Fresenills 2.Anal. were alloutgassedusingcontrolledratethermalanalysis(CRTA)l* C h e i . 1989,333,433. to 473 K before each adsorption experiment which was either (9) Reichert, H.; MIVler, U.; Unger, K. K.; Grillet, Y.; Rouquerol, F.; Rouquerol, J.; Coulomb, J. P. In Characterization of Porolls Solids ZI; (11)Mueller, U.; Unger, K. K. Zeolites 1988,8, 154. Rodriguez-Reinoso,F.. Rouauerol. J.,. Sing. K. S. W... Unner. - . K. K.,. Eds.;. (12) Guth, J. L.; Kessler, H.; Wey, R. InNew Developments in Zeolite Elsevik: Ameterdb; 1991-p 536. Science and Technology; Murakami, Y., Iijima, A., Ward, J. W., Eds.; (10)MCdler, U.; Unger, K. K. In Characterization of Porous Solids; Elsevier: Amsterdam, 1986; p 121. Unger, K. K., Rouquerol, J., Sing,K. S. W., Kral, H., E&.; Elsevier: (13) Rouquerol, J. Thermochrm. Acta 1989,144,209. Amsterdam, 1988,p 101. Introduction
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0743-7463/93/2409-1852$04.00/00 1993 American Chemical Society
Adsorption of MFI-Type Zeolites
Langmuir, Vol. 9, No. 7, 1993 1853
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N'I molec . uc" Figure 1. Initial isotherms (a,top) and net differential enthalpies of adsorption (b, bottom) for nitrogen at 77 K on the series of MFI-type zeolites. For clarity, each curve is offset in respect to the y-axis.
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carried out wing the standard or more recent "quasiequilibriumw1bJ6 adsorptive introduction. Neutron Diffraction Measurements. The neutron diffraction experiments were carried out at the Laboratoire LBon Brillouin (LLB, CEA-Saclay, France). Around 2 g of the silicalite-I sample was used and the measurements carried out on a 2-axis diffradometer (G4-1) at a wavelength of X = 2.439 A. The results were recorded with a multidetector and were then plotted as the peak intensityas a function of Q, the scattering vector where Q = ( 4 4 sin eB (his the Bragg diffractionangle). The experiments were performed principally at around 62 K with stability studied of the adsorbed phase between 5 and 100
K. Results and Discussion The resulta can be seen in Figures 1-5. Figures l a and 3a show the adsorption isotherms at 77 K of nitrogen and carbon monoxide on the series of MFI-type zeolites at low ~~
(14)Boudellal, M.Ph.D. Thesis, Universite de Provence, Marseille, France, 1979. (15)Grillet, Y.;Rouquerol, J.; Rouquerol, F. J. Chim. Phys. 1977,2,
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(16)Rouquerol, J. In Thermochimie, C.N.R.S.: Paris, 1971;p 537.
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N' I molec . uc-' Figure3. Initialisotherms (a,top) and net differential enthalpies of adsorption (b, bottom) for carbon monoxide at 77 K on the series of MFI-type zeolites. For clarity, each curve is offset with respect to the y-axis.
relative pressure. Figures l b and 3b show the continuous variation during the adsorption of the respective net
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1854 Langmuir, Vol. 9, No. 7, 1993
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Figure 4. (a, top) Neutron diffraction spectra of nitrogen adsorbed on silicalite-Iat 62 K for various quantities adsorbed at low uptake. A decrease of the silicalite-I diffraction peaks observed in the scatteringvector range 0.55 5 Q/A-1 0.65. The strong peak observed at Q = 1.63 A-1 corresponds to the appearance of the substep Q in the adsorption isotherm (the relevant peaks are indicated by arrows). (b, middle) Neutron diffraction npectra for the adsorptionof 23 mohdes of nitrogen per unit cell of silicalib-I at 5 and 62 K. Note that the peak poeitions do not vary between thew two spectra indicating the stability of the lattice fluidlike phase. (c, bottom) Neutron m a c t i o n spectra of nitrogen adsorbed on silicalib-I at 62 K for various quantities adsorbed during the substep 8. Note the emergence of the series of diftraction peaks observed in the scatteringvector range 1.88 5 Q/A-15 2.06 (the relevant peaks are indicated by arrows).
differential enthalpies. Figure 2 shows the Clapeyron phase diagram for nitrogen on silicalite I. Figures 4 and 5 depict the neutron diffraction spectra for the adsorption of nitrogen on silicalite-I and sample V (Si/Al = 161, respectively. Nitrogen on Silicalite-I. The nitrogen adsorption isotherm obtained on silicalite-I (plot I in Figure la) is of the same form and of similar uptake as those previously determined by several of the author^.^^^ The curve of the net differential enthalpy of adsorption is also similar to those previously published.8*B The initial adsorption of nitrogen on silicalite-I up to a coverage of 20 molecules per unit cell resulta in a microcalorimetric curve which is relatively stable and horizontal. It is in this region of pressure that for microporous samplesprimarymicropore fillingoccursand therefore the microcalorimetric result indicates an overall energetic homogeneity of the silicalite-I microporous ysurfacemto nitrogen, probably resulting from a compensation of the adsorbatezeolite and adsorbate-adsorbate interactions as recently described by Vernov and Steele for the adsorption of xenon on zeolite Rho.” The neutron diffraction resulta obtained in this region show a dramatic decrease of peaks at low angles below Q = 0.6 A-l, as we have already observed for the adsorptionof argon,krypton, and methane.’ This large change in peak intensity would seem too great to be simply due to the monoclinic to orthorhombic adsorbent structure transition reported el~ewhere.~ This phenomenon would more likely s8em to be due to interference effecta between the adsorptive and adsorbent. The adsorption isotherm shows a small substep a! which corresponds to a filling of around 23 to 25 molecules per unit cell. This phenomenon is more easily seen in the microcalorimetric curve by a sharp peak (a).This may be due to a densification of the adsorbate phase. The neutron diffraction spectrum at this point (Figure 4a) shows a strong increase in intemity of a peak at Q = 1.63A-l. This peak indicates a partial structuring of the previously disordered, mobile nitrogen physisorbed phase. This adsorbate transition may be thought of as being from a disordered phase (presumablya fluid) to a lattice fluidlike phase. The peak positions in the neutron diffraction __
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Adsorption of MFI- Type Zeolites
spectra (Figure 4b) are independent of temperature in the range 5 < T/K < 100 showing the stability of this phase and suggesting that the observed structuring is related with preferential adsorption sites in the zeolite. A second larger substep @ canbe observed in the isotherm a t coverage from 26 to 31 molecules per unit cell. This corresponds to an increase in net enthalpy in the microcalorimetric m e and is again evidence of a densification of the adsorbate phase. The neutron diffraction spectra a t the end of this substep (Figure 4c) show the emergence oftwodistinctpeaksatQ= 1.88and2.06A-lwhichindicate the appearance of crystalline order in the physisorbed phase. Therefore this region may be thought of as a tramition from a lattice fluidlike phase to a crystallinelike solid phase. The positions of thew peaka in the neutron diffraction spectra were again independent of temperature in the range of 10 < T/K < 100 indicatingthe high stability of the solidlikeadsorbate phase. The neutron diffraction results further suggest that the configuration of this crystalline-like phase is imposed by the silicalit4 microporous channel network highlighting a large degree of commensurability. It is also interesting to note that the neutron diffraction patterns at this point resemble those of both argon and krypton in the plateau region' and so it would seem that all three adsorptives have the same crystdinelike commensurate structure. This therefore implies that a process of "volumic filling" of the micropores does not occur, but rather an adsorption "by site" within the microporous network. The effect of temperature and pressure on the positions of the initial step and various substeps is shown in the Clapeyron diagram (Figure 2). It can be seen that in the temperature region examined adsorption within the microporous network is more favorable to that in the 3-d bulk nitrogen. It can be seen that the position of the substep CY approaches that of the initial step such that at around 533 K a "triple point" may occur. Carbon Monoxideon Silicalite-I. It can be seen that both the adsorption isotherm (Figure 3a) and the net differential enthalpies (Figure 3b) for the adsorption of carbon monoxide on silicalite-I a t 77 K have the same form as those of nitrogen. Even though the molecular dimensions of carbon monoxide (0.37 X 0.42 nm) and nitrogen (0.30 X 0.41 nm) differ, it can be seen that the quantities adsorbed of both adsorptives are the same at each plateau region of the respective isotherms indicating that the adsorption occurs "by site" on energetically preferred centers. In this case, it can also be seen that the carbon monoxide adsorbate only fills75% of the theoretical pore volume before the first substep (this value is 73% for nitrogen at the same point). The initial adsorption to an uptake of 20 molecules per unit cell shows a net differential enthalpy of adsorption which is relatively constant. This would indicate that even though the adsorption occurs by site, the adsorption centers within the silicalite-I microporous network are all giving rise to the same adsorption potential. However the net differential enthalpy for carbon monoxide is slightly higher (-10 kJ-mol-l) (plot I in Figure 3b) than that of nitrogen (-8.8 kJ-mol-l) highlighting the larger interaction of the carbon monoxide molecule with the silicalite-I microporous network due to the greater quadrupole moment and additional dipole moment. By this, it is implied that the electric field due to the zeolite structure is significant. In effect, from the work of Nicholson and Pellenq18it is seen that the S i 4 bonds in silicalite-I have a pronounced ionic character (50% ionicr-50% covalent). (18) Nicholson, D.;Pellenq, R. Private communication.
Langmuir, Vol. 8, No. 7, 1993 1866
Moreover, van Genechhn and M ~ r t i e have r ~ ~ evaluatd the net charges carried by the atoms within the silicalib-I network to be +2 for silicon and -1 for oxygen. The carbon monoxide isotherm (plot I in Figure 3a), like that of nitrogen, shows a substep a at an uptake of 23 to 25 molecules per unit cell. Again an exothermic peak in the microcalorimetric signal can be seen which has the same form as that of nitrogen. Thus in analogy with nitrogen, this step and microcalorimetric region can be thought of as being due to a transition from a "disordered phase" (presumablya fluid) to "lattice fluidlike phase". A second larger substep, 8, in the carbon monoxide isotherm is also evident in the microcalorimetric curve corresponding to an increase in uptake from 26 to 31 molecules per unit cell. This phenomenon is very similar tothe substep @ in the nitrogen results. Thia phenomenon occursat a lower pressure and with a higher net differential enthalpy of adsorption (-6.0 kJ-mol-l compared to -4.5 kJ-mol-' for nitrogen) indicating a greater interaction with the adsorbent. However, thie region can be thought of as a lattice fluidlike phase to a crystalline-like solid phase transition in analogy with the results obtained for nitrogen. Nitrogen and Carbon Monoxide on the Series of MFI-Type Zeolites. The adsorption of nitrogen and carbon monoxide on the aluminum-containingMFI-type zeolites showed the same general trends as the aluminum content increased. The specific effect of the counterion, which in thiscase is H+,is hard to distinguieh with respect to the aluminum due to both ita small size and relatively weak interaction potential. The nitrogen adsorption isotherms obtained on the series of MFI-type zeolites all reach the same quantity adsorbed on the plateau region (abovea relative pressure of 0.2)which is in good agreement with results found elsewhere in the literature.20*21The carbon monoxideadsorption isotherms also reach the same quantity adsorbed on the plateau region (above a relative pressure of 0.05). This would seem to indicate that the adsorption of the two probe molecules occur^ by site in all of the MFI-type zeolites. The initial net differential enthalpies of adsorption increase for both nitrogen and carbon monoxide with increasingaluminumcontent suchthat the net differential enthalpy curves over the region 0 < nYmol*uc-' < 20 become more inclined. This indicates a role played by the aluminum in the MFI-type framework creating sites which are energetically more favorable to adsorption of these quadrupolar molecules. The substep CY in the adsorption isotherm and the corresponding peak in the microcalorimetric curve previously mentioned for the adsorption of silicalite-I can still be distinguished for the sample I1 (Si/Al = 120); however they are not observed for the other samples.These peaks in the net enthalpy curves for sample I1 (Figures l b and 3b), however, were diminished in size in comparison with those of silicalite-I. This effect may be caused by the aluminumwhich leads to preferential adsorption sites for the nitrogen and carbon monoxide, thus creating the previously mentioned lattice fluidlike phase during the initial adsorption. The second substep b observed in the adsorption isotherms of both nitrogen and carbon monoxide on silicalite-I is still seen for several of the other isotherms. This substep is seen to begin at lower pressures and (19) van Genechten, K.A.; Mortier, W. J. Zeolites 1983,8, 273. (20) Webb, 5.W.; Conner,W. C. In Chracterizotion ojPorou0 Soli& ZI; Rodriguez-Reinoso, F., Rouquerol, J., Sing,K. 5. W., Unger, K. K., Eds.; Elsevier: Amsterdam, 1991; p 31. (21) Hathaway, P.E.;Daviee, M. E. Catol. Lett. 1990,6, 333.
1866 Langmuir, Vol. 9, No.7, 1993
becomes more diffuee with increasing aluminum content; however as the quantities adsorbed for each samplerejoin those of silicalite-I, it can be reasoned that the phase transition remarked for silicalite-I o c c m for all of the samplesgiving rise to this crystalline-likesolid phase. This is confirmed by the neutron diffraction spectra where the relevant peaks are observed at 1.63, 1.88, and 2.06 A-l (Figure5). The microcalorimetriccurves of both nitrogen and carbon monoxide on samplesI1and I11distinctly show this phase transition. The microcalorimetric curves for nitrogen on samples IV and V however show little interaction in this region. For carbon monoxide on samples IV and V though, some interaction can be seen although it has merged into the first part of the curves. Thus the lattice-fluid to solidlike phase transition can be thought of as occurring in regions around the aluminum centers starting a t a lower pressure than for silicalite-I due to the increased adsorbate-adsorbent interactions. Conclusion This study has shown that the adsorption of carbon monoxide behaves in a similar manner to that of nitrogen. Both molecules adsorb by site within the MFI-type threedimensional microporous network; however a higher net enthalpy of adsorption is obtained for the larger and more polar carbon monoxide molecule. Decreasing the silicon to aluminum ratio leads to an increase in the initial net enthalpy of adsorption highlighting the role of aluminum in the samples creating adsorption centers of higher potential. Both probe molecules give rise to isotherms with two substeps when adsorbed on silicalite-I. The first substep
Llewellyn et al.
corresponds to a sharp peak in the net enthalpy of adsorption curve and is interpreted by an adsorbate phase transition from a disordered phase (presumablyfluidlike) to a lattice fluidlike phase. This transition ie also observed for another MFI-type zeolites with high silicon to aluminum ratio (120); however it is not seen for the other samples. It wouldseem therefore that the aluminumgives rise to centers which induce the aforementioned lattice fluidlike phase at lower uptake, thus smoothing out this transition. The second substep in the silicalite-I isotherm is seen to occur at a lower relative pressure for carbon monoxide than for nitrogen. This substep corresponds to an exothermicregion in the net enthalpy of adsorption curve and is interpreted by a further adsorbate transition from a lattice fluidlike phase to crystalline-like solid phase. Thie crystalline-like solid phase is seen to be commensurate with the silicalite-I channel network. Furthermore this commeneurate phase is identical with those of argon and krypton observed elsewhere. Thisphase transition occurs for all of the MFI-type zeolites examined and moreover the same uptake of adsorbate is observed on the plateau region of the ieotherm. However, decreasing the silicon to aluminumratio has the effect of making this transition start at lower pressures and be more dif-. Thus the aluminum within the sample seems to induce a preferred orientation of the probe molecules which allows the phase transition to start at lower pressures. Acknowledgment. The authors thank the EEC SCIENCE programme for their financial support including a research grant for P.L.L. (EEC Contract No. SCI*.OlBC).
