Sorption Properties of AlPO4-5 and SAPO-5 Zeolite-like

We have studied by volumetric adsorption isotherm measurements the sorption properties of different. AlPO4-5 or SAPO-5 samples, for sorbate species ...
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Langmuir 1998, 14, 1774-1778

Sorption Properties of AlPO4-5 and SAPO-5 Zeolite-like Materials C. Martin,† N. Tosi-Pellenq,† J. Patarin,‡ and J. P. Coulomb*,† CRMC2-CNRS, Campus de Luminy, Case 901, 13288 Marseille Cedex 9, France, and Laboratoire des Mate´ riaux Mine´ raux, Universite´ de Haute-Alsace, Mulhouse, France Received July 30, 1996. In Final Form: December 18, 1997 We have studied by volumetric adsorption isotherm measurements the sorption properties of different AlPO4-5 or SAPO-5 samples, for sorbate species characterized by a large range of molecular size: D2, Kr, Ar, C2D2, CO2, CH4, CF4, and C(CH3)4. The highest sorption capacities, Qmax, greatly depend on the sorbate molecular size. A difference of 1 order of magnitude is observed between hydrogen and neopentane. In addition, we have found that the sorption capacities of the four analyzed AlPO4-5 or SAPO-5 samples vary by almost a factor of 2. This result clearly shows that, as a consequence of the one-dimensional character of the AFI micropore network, Qmax is very sensitive to small amounts of crystalline defects or impurities. For methane, Qmax is equal to 6 mol/uc, a quantity that is not compatible with the commonly accepted value of the internal diameter of the AlPO4-5 micropore diameter, LM ) 7.3 Å. Moreover, an interesting phase transition phenomenon has been observed during the sorption of krypton and methane, a result that can be the consequence of the good parametric agreement between such sorbate species and the AlPO4-5 inner surface.

Introduction Among the porous materials, those which present welldefined channels and regular pore systems, as the crystalline zeolites, are adsorbents of great interest. The main characteristics of the zeolites are the molecular dimensions of their pores and the existence of strong acid sites located on their inner surface. As a consequence, they are of great industrial importance in the gas separation and purification processes and in the petroleum catalytic cracking. The zeolites present a rich variety of micropore frameworks. Generally speaking, both their micropore networks, which are composed of cages and/or channels, and their inner surfaces are complex. The present paper addresses the sorption properties of an aluminophosphate AlPO4-5, which is a zeolite-like material synthesized for the first time in 1982 by Wilson et al.1 AlPO4-5 is a particularly interesting model adsorbent owing to the simplicity of both its micropore network, composed of one-dimensional channels, and its inner surface, which presents only one type of sorption sites. The cylindrical micropores formed by 12-membered rings have a diameter of 7.3 Å,2 the AlPO4-5 micropore characteristics are represented schematically in Figure 1. Note that, as a general rule, the aluminosilicate zeolites are characterized by the presence of cations of compensation, which neutralize the negative charge of the framework. Such cations occur in the zeolite micropore framework and act as impurities; they induce an inner surface energetic heterogeneity. As a consequence of its electrically neutral framework, the AlPO4-5 micropore network is free of any cations. AlPO4-5 presents also a great thermal stability, it can be heated at temperatures as high as 1000 °C without any amorphization.3 That † ‡

CRMC2-CNRS. Universite´ de Haute-Alsace.

(1) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Amer. Chem. Soc. 1982, 104, 1146. (2) Atlas of Zeolites Structure Types, 3rd revised ed.; Meier, W. M., Olson, D. H., Eds.; Butterworth-Heinemann: Boston, 1992; p 26. (3) Barthomeuf, D. Zeolites: Science and Technology. NATO ASI Ser. 1984, 317.

Figure 1. Schematic representation of an AlPO4-5 micropore. Only the Al and P atoms are represented. The micropore inner surface is composed of rows of six hexagons.

microporous material can be considered as a host material is particularly interesting for the confinement of a large number of quasi one-dimensional solids, ranging from the van der Waals elements to the metallic ones. Sorption properties of AlPO4-5 have been already studied, concerning argon and nitrogen,4-6 some alcohols and hydrocarbons, e.g., methanol, n-hexane, and benzene,7 and water.8 The sorption isotherms have the usual type (4) Mu¨ller, U.; Unger, K. K.; Pan, D.; Mersmann, A.; Grillet, Y.; Rouquerol, F.; Rouquerol, J. In Zeolites as Catalysts, Sorbents and Detergent Builders; Karge, H. G., Weikamp, J., Eds.; Elsevier: Amsterdam, 1989; p 625. (5) Vasileva, E. A.; Zhdanov, S. I.; Zinovev, S Yu.; Smirnova, E. I.; Feoktistova, N.N Izv. Akad. Nauk, Ser. Chim. 1989, 11, 2409. (6) Hathaway, P. E.; Davis, M. E. Catal. Lett. 1990, 5, 1333. (7) Choudhary, V. R.; Akolekar, D. B.; Singh, A. P.; Sansare, S. D. J. Catal. 1988, 111, 23.

S0743-7463(96)00755-X CCC: $15.00 © 1998 American Chemical Society Published on Web 02/27/1998

Sorption Properties of AlPO4-5 and SAPO-5

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Table 1. Morphological Aspect and Mean Crystal Size of the AlPO4-5 and SAPO-5 Samples samples

origin

morphological aspect

mean crystal size (µm3)

(A) AlPO4-5 (B) SAPO-5 (C) AlPO4-5 (D) AlPO4-5

Mainz Mulhouse Mulhouse Mulhouse

irregular, aggregate, sheaf prismatic, mostly individualized prismatic, mostly individualized prismatic, mostly individualized

15 × 15 × 20 25 × 25 × 70 10 × 10 × 70 20 × 20 × 35

I shape, while water, which is a more complex sorbate, presents a type V isotherm shape. More recently, Grillet et al.9 have measured a methane sorption isotherm characterized by a substep existence in the high-loading regime. In addition, an exothermic heat peak is observed at the same CH4 sorbed quantity.9,10 Hitherto, such phase transitions in sorbate species have been pointed out only during the sorption of simple gases (i.e., Ar, Kr, N2, and CO) in silicalite-I.10-12 Silicalite-I, which is the pure silicic form of the MFI-type zeolite, has a rather complex inner surface composed of at least three kinds of sorption sites and a complex micropore network constituted of straight channels and sinusoidal channels. Experimental Section The AlPO4-5 and SAPO-5 (5% of silicon) samples were synthesized at the laboratories of Professor H. Kessler (Universite´ de Haute Alsace, Mulhouse, France) and of Professor K. K. Unger (Johannes-Gutenberg Universita¨t, Mainz, RFA). The morphological aspect of the Mulhouse (samples B, C, and D) are similar; the crystals mostly individualized have a regular prismatic shape. Some differences exist with the irregular aspect of the crystallites of the material prepared in Mainz (sample A). Scanning electron micrographs of the samples A and C are presented in Figure 2. The characteristics of the samples are summarized in Table 1. Prior to adsorption isotherm measurements, the samples (around 130 mg) were outgassed under vacuum (P e 10-6 Torr) at T ) 400 °C for about 12 h. The adsorption isotherms were carried out by the standard volumetric method on an in house apparatus composed of two capacitance Baratron gauges (10-6-1 and 10-4-100 Torr) and a helium closed-cycle CTI Cryocooler. The temperature of the sample can be monitored in the range 35-250 K and remains constant within 0.02 K. In the present study, most of the adsorption isotherms were measured at the liquid nitrogen boiling temperature by using a usual dewar. The deuterated hydrogen adsorption isotherm was measured at the Leon Brillouin laboratory with the G4-1 helium cryostat during a neutron diffraction experiment. The usual gases Kr (99.99% purity), Ar (99.999% purity), CO2 (99.97% purity), CH4 (99.95% purity), and CF4 (99.8% purity) and the isotopic gases D2 (99.8% isotopic enrichment) and C2D2 (99% isotopically enriched) are respectively supplied by the Linde Co. and by the Euriso-top CEA Group. The neopentane (2,2dimethylpropane, 99.97% purity) gas was purchased from the Phillips Petroleum Co.

Results and Discussion The methane adsorption isotherms measured at T ) 77.3 K on our different AlPO4-5 or SAPO-5 samples are represented in Figure 3. The highest sorption capacities Qmax are significantly different for the four samples; there is almost a factor of two between the AlPO4-5 samples A and D. This result is interesting because the X-ray (8) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. ACS Symp. Ser. 1983, No. 218, 79. (9) Grillet, Y.; Llewellyn, P. L.; Tosi-Pellenq, N.; Rouquerol, J. In Fundamentals of Adsorption; Suzuki, M., Ed. Kodansha Ltd.: Tokyo, 1993; p 235. (10) Tosi-Pellenq, N.; Grillet, Y.; Rouquerol, J.; Llewellyn, P. Thermochim. Acta 1992, 204, 79. (11) Mu¨ller, U.; Reichert, H.; Robens, E.; Unger, K. K.; Grillet, Y.; Rouquerol, F.; Rouquerol, J.; Pan, D.; Mersmann, A. Fresenius Z. Anal. Chem. 1989, 333, 433. (12) Llewellyn, P.; Coulomb, J. P.; Grillet, Y.; Patarin, J.; Lauter, H.; Reichert, H.; Rouquerol, J. Langmuir 1993, 9, 1846 (Part 1).

Figure 2. Scanning electron micrographs of the AlPO4-5 samples: (a) sample A; (b) sample C.

diffraction patterns of the four samples point out their high level of crystallinity. As a matter of fact, due to the one-dimensional character of the micropore network and due to the large dimension of the crystals, a very small amount of structural point defects and/or strong chemisorbed impurities can block a tremendous number of sorbed molecules. Such a pore blocking phenomenon has been already considered and modeled by Cracknell et al.15 We have to note that, as soon as the zeolite crystal dimensions are larger than a few tens of micrometers, the zeolite external surface value is negligible in comparison with the inner surface one. The methane adsorption isotherms on AlPO4-5 are not of the usual type I shape (or Langmuir type shape). The isotherms measured at T ) 77.3 K concerning both AlPO4-5 (sample D) and SAPO-5 (sample B) are characterized by a substep located at low relative pressure Psub/Po ) 4.2 ×10-3, Figure 4a (Psub and Po are the pressure of the substep and the methane saturated pressure, respectively). Such an isotherm substep has been observed for (13) Llewellyn, P.; Coulomb, J. P.; Grillet, Y.; Patarin, J.; Andre´, G.; Rouquerol, J. Langmuir 1993, 9, 1852 (Part 2). (14) Coulomb, J. P.; Llewellyn, P.; Grillet, Y.; Rouquerol, J. In Characterization of Porous Solids III; Rouquerol, J., et al., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1993; p 535. (15) Cracknell, R. F.; Gubbins, K. E. Langmuir 1993, 9, 824.

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Figure 3. Methane sorption isotherms measured at T ) 77.35 K on three different AlPO4-5 samples (A, C, and D) and one SAPO-5 sample (B). The CH4 sorption capacities measured at P/Po ) 0.4 (Po ) 9.3 Torr) are respectively sample A ) 3.2 mol/ uc, sample B ) 4.2 mol/uc, sample C ) 5.1 mol/uc, and sample D ) 5.7 mol/uc

Figure 5. Argon, krypton and methane sorption isotherms measured at T ) 77.3 K on AlPO4-5 sample D. A substep is observed for CH4 and Kr, in addition the AlPO4-5 sorption capacities are the same for these two sorbates. The Ar sorption isotherm presents the usual type I shape.

Figure 4. (a) Methane sorption isotherms measured at T ) 77.3 K on SAPO-5 sample B and AlPO4-5 sample D. Both isotherms present a substep located at Psub ) 0.04. (b) Methane sorption isotherms measured at T ) 96.5 K on AlPO4-5 sample D. The substep adsorption and desorption branches are different; a hysteresis loop is observed.

Qmax value, the substep verticality is a fine probe of the micropore surface state. Another characteristic of the methane adsorption isotherm that we have observed at T ) 96.5 K is the existence of a substep hysteresis loop, Figure 4. Such a phenomenon cannot be observed at T ) 77.3 K, because of the too-low-pressure value of the substep (Psub ) 4.0 × 10-2 Torr), which prevents the possibility of measuring the substep desorption branch. In addition, we have to notice that the relative pressure of the substep adsorption branch increases when the temperature increases, Psub/ Po ) 4.5 10-2 at T ) 96.5 K. The hysteresis loop, related to the sorbed phase transition, has been already found for the system nitrogen/silicalite I.17 Such an hysteresis loop (P/Po < 10-2) cannot be related to a capillary condensation phenomenon, a phenomenon that has been measured only in sample A (in the range; 0.4 e P/Po e 1). This is a signature of a mesoporosity existence in sample A, which is in agreement with its morphology (Figure 2a). A substep has been also observed in the krypton adsorption isotherm measured at T ) 77.3 K on AlPO4-5 sample D, Figure 5. We remark that the Kr substep is much more bent than those measured for methane; the sorbed phase transition seems to be a second-order one. In the case of argon, no substep is observed any more; the adsorption isotherm has the usual type I shape, Figure 5. Argon, krypton, and methane have respectively a molecular size equal to 3.8, 4.0, and 4.2 Å. We think that our results point out that the sorbed phase transition existence depends greatly on the parametric agreement between the sorbate species and the AlPO4-5 inner surface. Indeed, the distance between adsorption sites along the AlPO4-5 micropore axis, which is half the unit cell c parameter2 (c/2 ) 4.2 Å), is equal to the methane molecular size. The AlPO4-5 sorption capacities are the same for methane and krypton and around 10% higher for argon. Obviously, the size of argon atoms is too small to satisfy the parametric agreement with the adsorption site lattice of the AlPO4-5 inner surface. Before and after the substep the number of Kr and CH4 adsorbed molecules by unit cell Qads is close to 4 and 6 mol/uc, respectively. Such experimental Qads values show that the AlPO4-5 micropore

the first time by Grillet et al.9-10 It was interpreted as a sorbed phase transition, presumably a crystallization of the methane sorbate. Indeed, a heat peak is observed in the microcalorimetric signal of the methane sorbed phase during the substep formation.9,10 Recently, the phase transition has been confirmed by molecular simulation by Boutin et al.16 The AlPO4-5 samples used in the two experimental studies9,10 are of the same origin as our sample A. We have observed that the isotherm substep is more developed and vertical for the AlPO4-5 sample D, which presents the highest sorption capacity (around six methane molecules by unit cell), Figure 4. Indeed, we think that the CH4 sorbed phase transition depends greatly on the AlPO4-5 inner surface homogeneity (lack of impurities and/or cations of compensation). This result must be compared with the vanishing of the substep observed during Ar sorption in several ZSM-5 samples when the proportion of cations is increased due to the Si/Al ratio decreasing.12 As a consequence, in addition to (16) Boutin, A.; Pellenq, R. J.-M.; Nicholson, D. Chem. Phys. Lett. 1994, 219, 484.

(17) Reichert, H.; Mu¨ller, U.; Unger, K. K.; Grillet, Y.; Rouquerol, F.; Rouquerol, J.; Coulomb, J. P. In Characterization of Porous Solids II; Rodriguez-Reinoso, F., et al., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1992; p 535.

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Langmuir, Vol. 14, No. 7, 1998 1777

Figure 7. Detailed sorption isotherms of different gases on AlPO4-5 sample D. The sorbed quantities Qads are normalized to Qmax for each sorbate. The relative pressures Pstep ) P/Po corresponding to the loading equal to 0.4 are Pstep ) 2.2 × 10-5 for neopentane (T ) 273 K), Pstep ) 6.8 × 10-5 for argon (T ) 77.3 K), Pstep ) 1.0 × 10-4 for methane (T ) 96.5 K), Pstep ) 8.0 × 10-4 for carbon tetrafluoride (T ) 140 K), Pstep ) 7.8 × 10-3 for acetylene (T ) 150 K), and Pstep ) 1.15 × 10-2 for carbon dioxide (T ) 165 K).

Figure 6. (a) Sorption isotherms of several gases measured on AlPO4-5 sample D at the T ) 17.4 K for deuterium (Qmax ) 10.0 mol/uc). T ) 77.3 K for argon (Qmax ) 6.5 mol/uc), T ) 77.3 K for krypton (Qmax ) 5.7 mol/uc), T ) 77.3 K for methane (Qmax ) 5.7 mol/uc), T ) 165 K for carbon dioxide (Qmax ) 5.9 mol/uc), T ) 150 K for acetylene (Qmax ) 4.3 mol/uc), T ) 140 K for carbon tetrafluoride (Qmax ) 2.8 mol/uc), and T ) 220 K for neopentane (Qmax ) 1.15 mol/uc). (b) Measurement of the sorption capacity of the AlPO4-5 sample D versus the molecular size of the different sorbate species.

diameter LM is large enough to allow the formation of Kr or CH4 dimers and trimers. Our results are not in agreement with the commonly accepted value of LM ) 7.3 Å2. In our opinion the AlPO4-5 micropore diameter is greatly underestimated, a more acceptable value should be LM ) 8.2 Å (value calculated from the methane kinetic diameter Lk ) 3.8 Å). We have measured the sorption capacities Qmax of AlPO4-5 sample D in a large range of molecular sizes for sorbate species characterized by the same globular morphology (Ar, Kr, CH4, CF4, neopentane) and for some polar gases (D2, CO2, and C2D2). The adsorption isotherms are represented in Figure 6a. It is evident that Qmax greatly depends on the sorbate molecular size; a difference of 1 order of magnitude is observed between the smaller molecule (deuterium, L ) 3.6 Å) and the larger one (neopentane, L ) 6.2 Å). We can remark that the sorbed neopentane phase is an illustrative example of a onedimensional phase. Quite recently, investigation of the thermodynamic properties of the C(CH3)4 sorbed phase in AlPO4-5 has pointed out that such that 1D phase is stable owing to its large entropy.18 The sorption capacity Qmax, expressed as the volume of bulk liquid adsorbed per gram

of AlPO4-5 sample versus the molecular diameter of the sorbate, is shown Figure 6b. Qmax is respectively equal to 0.094 cm3/g for neopentane and to 0.170 cm3/g for deuterium. Our results indicate that the AlPO4-5 micropore volume Vp depends on the size of the molecular “probe”. It seems evident that the best determination of Vp is obtained for the smallest molecular size of the probing sorbate, i.e., D2 in the present study. As a consequence, we think that Vp is at least equal to 0.170 cm3/g, which is larger than the geometric open volume, 0.145 cm3/g, calculated from the AlPO4-5 micropore diameter LM ) 7.3 Å. For most of the considered gases we have measured at least one sorption isotherm in a temperature range where the equilibrium pressure can be measured in the low and medium loading regime. The sorption isotherms of C(CH3)4, Ar, CH4, CF4, C2D2, and CO2 are represented in Figure 7. The considered sorbates are quoted according to an increasing value of the step relative pressure Pstep ) P/Po of the sorption isotherms (P is the equilibrium pressure measured at Qads/Qmax ) 0.4). There is a difference as large as 3 orders of magnitude between the relative step pressure of neopentane (Pstep ) 2.1 × 10-5) and those of carbon dioxide (Pstep ) 1.15 × 10-2). The free energy excess of the sorbed phase ∆F due to the AlPO4-5 substrate in comparison to the bulk sorbate phase is directly related to the step relative pressure of the sorption isotherm {∆F ) ∆G ) RT ln(Pstep/Po)}. No simple correlation can be made between ∆F and the sorbate molecule characteristics, their size for instance. For neopentane the entropic factor T∆S is the main contribution to ∆F.18 Several laboratories have already measured the sorption isotherm of argon at the liquid nitrogen boiling temperature T ) 77.3 K on AlPO4-5 samples. The measured sorption capacities are summarized in Table 2. We have also indicated the results obtained from molecular simulation studies.15,16 The measured sorption capacities for argon vary from 3.84 to 7.0 atoms/u.c..6 We think that such pronounced differences between AlPO4-5 samples synthesized by different laboratories are the consequence (18) Martin, C.; Grillet, Y.; Llewellyn, P. Submitted for publication to Langmuir.

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Table 2. Comparison of the Argon Sorption Capacity Measured at T ) 77 K for AlPO4-5 Samples

techniques sorption isotherm

molecular simulation

no. of atoms by unit cell

at relative pressure (P/Po)

ref

3.8 4.7 7.0 6.7 6.5 8.3 6.0

no indication 0.1 0.1 0.1 0.1 0.1 0.1

4 5 6 19 our study 15 16

of the pore blocking phenomenon that we have already described, Figure 3. Our result, Qmax ) 6.5 atoms/u.c., is in close agreement with the measurement of Reichert et al., Qmax ) 6.7 atoms/u.c..19 In our opinion the result of Hathaway et al., Qmax ) 7.0 atoms/u.c., is a little overestimated as a consequence of the surface heterogeneity of their sample. Conclusion We have observed that the highest sorption capacities Qmax of different AlPO4-5 or SAPO-5 samples vary by a factor 2. This result is a consequence of the onedimensional character of the AFI-type microporous network. More generally, structural defects and/or impurities can greatly reduced the open microporosity of zeolites, which are characterized by 1D channels (as for instance AEL, MEL, or TON types). Consequently, in such a case the determination of the true sorption capacities is a difficult experimental problem. In addition, for a given (19) Reichert, H.; Schmidt, W.; Grillet, Y.; Llewellyn, P.; Rouquerol, J.; Unger, K. K. In Characterization of Porous Solids III; Rouquerol, J., et al., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1994; p 517.

AlPO4-5 sample, the sorption capacity greatly depends on the size of the sorbate species. Gases of small molecular size are good probes to determine with accuracy the AlPO4-5 open volume Vp; from deuterium sorption capacity we have deduced Vp ) 0.170 cm3/g. The commonly accepted value of the AlPO4-5 micropore diameter (LM ) 7.3 Å) is not compatible with its methane sorption capacity, Qads ) 6 mol/uc. According to our results, a more reliable value should be LM ) 8.2 Å. In any event, the oxygen walls that compose the AlPO4-5 inner surface might behave as soft walls. As a consequence, the micropore diameter can depend weakly on the considered sorbate species. Among the considered sorbates, CF4 and C(CH3)4 are the best illustrative examples of one-dimensional confined phases; i.e., two molecules cannot cross each other in the AlPO4-5 micropores. The oversorbed phases are not true one-dimensional systems. Two of them CH4 and Kr present a good parametric agreement with the adsorption site of the AlPO4-5 inner surface. This fact must be related to the substep observation in their sorption isotherm. The substep is, in our opinion, the signature of a phase transition. Recently, such a phenomenon has been studied in detail both by neutron diffraction and by incoherent neutron scattering.20 Acknowledgment. The authors would like to thank kindly Professors H. Kessler (Laboratoire des Mate´riaux Mine´raux, Universite´ de Haute-Alsace, Mulhouse, France) and K. K. Unger (Johannes-Gutenberg Universita¨t, Mainz, RFA) for the AlPO4-5 or SAPO-5 samples supply. LA960755C (20) Martin, C. Ph.D. thesis, Universite´ de la Me´diterrane´e, France, 1996.