0 Copyright 1995 American Chemical Society
AUGUST 1995 VOLUME 11, NUMBER 8
Letters Porosity of Synthetic Saponites with Variable Layer Charge Pillared by A l l 3 Polycations L. Bergaoui,? J. F. Lambert,? M. A. Vicente-Rodriguez,? L. J. Michot,*3$ and F. Villi&as$ Laboratoire de Riactiviti de Surface, URA 1106 CNRS, Tour 54-5-,22me itage, UPMC, 4, Place Jussieu, 75252 Paris Cedex 05, France, and Laboratoire “Environnement et Miniralurgie”, URA 235 du CNRS, B.P. 40, 54501 Vandoeuvre Cedex, France Received February 22, 1995. I n Final Form: June 5, 1995@ Saponites with variable layer charge (from 0.8 per unit cell to 1.5 per unit cell) were synthesized hydrothermally and pillared with &I polycations following classical pillaring procedures. The amount of intercalated aluminum increases linearly with the layer charge up to 1.1 and remains constant for higher values. Characterization of the intercalated and pillared products reveals the presence of All3 polycations in the interlayer space. The microporosityof the sampleswas examinedusingquasi-equilibrium volumetric adsorption of carbon dioxide and classical nitrogen adsorption experiments. The DubininRaduskhevich treatment applied to C02 adsorption yields a microporous volume that is very close to a theoretical microporous volume derived from the amount of intercalated A13 moieties. It decreaseswith the layer charge. On the contrary, nitrogen BET surface areas increase with the layer charge suggesting that the characterization of aluminum pillared clays by nitrogen adsorptioncan lead to misleadingresults in terms of microporous volumes.
Introduction Pillared clays are solids obtained by replacing the interlayer cations of clay minerals by bulky highly charged inorganic polycations. The resulting solids are highly microporous as the layers are propped apart by laterally separated pillars. The most studied pillared clays have been obtained by using the A113 polycation1V2as the guest molecule. Characterizing the microporosity0fAl13pillared clays is a crucial step for the potential use of these materials as catalysts or adsorbents. In most cases, their porosity is characterized by surface area data combined with treatments such as the &plot3or %-plot4for assessing
* To whom correspondence should be addressed. + Laboratoire de Ftkactivit6 de Surface. Laboratoire “Environnement et MinBralurgie”. Abstract published in Advance A C S Abstracts, July 15,1995. (1)Johanson, G. Acta Chem. Scand. 1960, 14, 769. (2) Bottero, J. Y.;Cases, J. M.; Fiessinger, F.; Poirier, J. E. J. Phys. Chem. 1980,84, 2933. (3)De Boer, J. H.; Lippens, B. C.; Linsen, B. G.; Broekhoff, J. C. P.; Van Den Heuvel, A.; Osinga, T. J . Colloid Interface Sci. 1966,21, 405. @
the presence of micropores. More detailed studies were carried out using molecular probe^.^^^ Recently the treatment proposed by Horvath and Kawazoe7for determining micropore size distribution was applied.8-10 It reveals a bimodal distribution of micropores which has, up to now, not been really interpreted. Other types of treatment such as the use of the Dubinin equationlo or the Jaroniec-Choma treaimentll were also applied in the case of zirconium12or aluminum pillared clays.l0 The influence of the synthesis conditions (Avclay ratio, washing conditions,drying conditions,thermal treatment) (4) Gregg, S. J.; Sing, K. S. W.Adsorption, SurfmeArea and Porosity; Academic Press: London, 1982. ( 5 ) Shabtai, J.; Rosell, M.; Takarz, M. Clays Clay Miner. 1984,32, 99. (6)Tzou, M. S.; Pinnavaia, T. J. Catal. Today 1988,2, 243. (7) Horvath, G.; Kawazoe, K. J. Chem. Erg. Jpn. l983,15, 470. (8)Michot, L. J.; Pinnavaia, T. J. Chem. Mater. 1992, 6, 1433. (9)Malla, P.B.; Komarmeni, S. Clays Clay Miner. 1993,41, 472. (10)Gil, A,; Montes, M. Langmuir 1994, IO, 291. (11)Jaroniec, M.; Madey, R.; Choma, J.; Mc Enaney, B.; Mays, T. J. Carbon 1989,27, 77. (12)Yang, R.T.; Baksh, M. S. A. AZChE J. 1991,37, 679.
07~3-7~63/95/2411-28~9$09.00/0 0 1995 American Chemical Society
2850 Langmuir, Vol. 11, No. 8, 1995 on the microporous structure ofthe pillared clays has long been recognized. This fact induces some difficulty in comparingthe microporous structure of different pillared clays. However, some facts remain quite puzzling. For starting clays with different charge densities, there is a direct relationship between the layer charge and the aluminum content of the resulting pillared clays prepared in the same ~0nditions.l~ However the same relationship does not apply to surface area and pore volume data as the more highly charged clay can exhibit a higher surface area and micropore v01ume.l~ In this paper we use a series of syntheticsaponites with variable layer charge. Saponites are tetrahedrally substituted smectites with an idealized cell formula of Mn+~n(Si~-xA1,)(Mg~)Ozo(OH)4. Different samples with variable x values were submitted to standard pillaring procedures. The surface areas of the resulting materials were determinedby classical nitrogen adsorption,whereas their microporosity was investigated by COz adsorption using a quasi-equilibrium technique. Indeed, carbon dioxide used as an adsorbate can access most micropores.15J6 Therefore, it should be possible to try to understand the relationships between the layer charge, i.e., the amount of intercalated pillars, and the microporosity of pillared clays.
Letters Table 1. Quantitiesof Aluminum Fixed on Intercalation for Different Saponites (Expressed in Weight Percent of Fixed Al in the Intercalated Saponites) charge per wt % of charge per wt % of unit cell
fixed Al
unit cell
fxed Al
0.8 1.0 1.1 1.2
4.8 5.4 6.2 6.3
1.3 1.4 1.5
6.5 6.3 6.2
Experimental Section Materials. Synthetic saponites were prepared by hydrothermal synthesis at 400 "C under a 1000 bar water pressure, using run durations of 4weeks, in Morey type externally heated pressure vessels. The sample was insulated from the vessel wall by a silver coating. The starting products were gels of appropriate compositions, prepared by coprecipitation of Na, Mg, Al,and Si hydroxides at pH = 14, according to the gelling method of Hamilton and Henderson." The source of Na was sodium carbonate, the sources of Al and Mg were titrated solutions of their nitrates, and the source ofSi was (CzH50)dSi(TEOS).Prior to hydrothermal synthesis, the starting product was dried and calcined and then crushed for further homogenization. After the hydrothermal synthesis, the sample was quenched and examined by X-ray diffraction and BSi and 27A1NMR to confirm the single phase character. The resulting structural formula is Nax(Sis-xA1,)(Mg,)Oz~(OH)4, with 0.8 5 x 5 1.5. The sodiumform of the synthetic saponites will be referred to as SNa-x, where x is the negative layer charge, per unit cell. The saponite pillaring method is described elsewhere.ls It yields intercalated products referred to as SI-x and pillared products (after calcination a t 500 "C)referred to as SP-x. Methods. X-ray Difiaction. X-ray diffraction patterns were recorded on a Siemens D500 diffractometer using the Cu Ka radiation. The dool spacings were measured at the 001 line maximum. SurfaceArea. Samples were degassed for 2 h at 200 "C under Nz flow. Nz adsorption measurements at 77 K (four points between PIP0 = 0.10 and 0.20) were then performed by frontal analysis of a N&e flux on a Quantasorb Junior apparatus, and surface areas were derived from a simple BET treatment. Carbon Dioxide Adsorption. Samples were outgassed overnight at 150 "C under a residual pressure of Torr. The adsorption apparatus used is a lab-built quasi-equilibrium (13)Butruille, J. R.; F'innavaia, In Characterization of Catalytic Materials; Wachs, I. E., Ed.; Buttenvorths-Heineman: Wobum, MA, 1992;Chapter 8,p 149. (14)Butruille, J. R. Ph.D. Michigan State University, 1992. (15)Cases, J. M.; Grillet, Y.; Franpois, M.; Michot, L.; Villibras, F.; Yvon, J. Clays Clay Miner. 1991,36, 233. (16)Siemienewska, T.;Tomkow, K.; Kaczmarczyk, J.; Albiniak, A,; Grillet, Y.; Franpois, M. In Characterization of Porous Solids ZI; Rodriguez-Reinoso, F., Rouquerol, J.,Sing, K. S. W., Unger, K. K., Eds.; Elsevier: Amsterdam, 1993;p 357. (17)Hamilton, D.L.;Henderson, C. M. B. Mineral. Mug. 1968,36, 832. (18)Bergaoui, L.;Lambert, J. F.; Franck, R.; Suquet, H. Submitted for publication in J. Chem. Soc., Faraday Trans.
I
I
I
5
I
I
" 2 9
1
I
I
10
Figure 1. X-ray difiactionpattems of(a)S-Na-1.1, (b) SI-1.1, and (c) SP-1.1. volumetric equipment described e1~ewhere.l~ Carbon dioxide was introduced at a flow rate of m0.03 cm3/minin the adsorption cell thermostated at 273 or 293 K. The data were treated by a classical Dubinin-Raduskevich (DR)treatment in order to derive the volume VOof the micropores according to the equation
where 6 is the relative adsorption, defined as the ratio of the amount adsorbed in the micropores (V) to the maximum B is the structural constant, and j3 is adsorption capacity (VO), a similarity coefficient (j3 = 1 for benzene).
Results and Discussion Amount of Fixed Aluminum. As seen in Table 1, the amount of aluminum fixed by the intercalated saponites increases with the layer charge up to 6.5% for a charge of 1.1per unit cell. For higher charge values, it remains constant. This evolution can be related to steric constraints on the A113 in solution-like environment;18i.e., due to its size in solution no more than 1A l l 3 per 5.9 unit cells can be intercalated. X-rayDiffraction. Figure 1presents the diffradogram obtainedforthesamples SNa-1.1, SI-1.1,andSP-1.1. "his difiactogram is typical of all the samples as, u on intercalation, the basal spacing increases from 12.4 at room humidity up to about 18.9 independently of the layer charge. After calcination at 500 "C, the spacing decreases to 18.3 A. The sample appears well ordered. Differences in the crystallinity of the samples related to the layer charge are discussed elsewhere.18
1
~~
(19)Michot, L.;Franpois, M.; Cases, J. M. Langmuir 1990,6, 677.
Letters 2
Langmuir, Vol. 11, No. 8, 1995 2851 ' " ' , ' " " ' ' " ~
~ " " I ' " ' I " ' ' " " ' I
0
0
1.5
Voat 273K
-Calculated
Vo
1 0.5
9
c
0
-1 :
'B
0
0.08
. i 0.6
0.8
1
12
1.4
1.6
Layer Charge
Figure 3. Comparison between calculated and experimental microporous volumes for intercalated saponites as a function of the layer charge x .
Carbon Dioxide Adsorption. The DR plots corresponding to the adsorption of CO2 at 293 K on the intercalated saponites are presented in Figure 2. For clarity reasons, only five curves out of seven are presented. They exhibit one or two linear portions which can be used to derive the ordinate at the origin, VO,corresponding to the totalvolume ofthe micropores. The upward deviation at higher pressure (low values of [log(P/P0)l2)observed in some cases (for instance in the case of the sample SI-1.2) can be assigned to the presence of some larger pores. Table 2 displays the microporous volumes obtained from the DR plots based on the adsorption of carbon dioxide at 273 and 293 K for intercalated and pillared products. The values were obtained from the low to medium pressure range and should then correspond to the total microporosity of the samples. The microporous volumes follow an inverse relationship with the amount of aluminum incorporated (Table 1)as they decrease for charge densities up to 1.1to remain approximately constant above this value. The theoretical microporous volume of an intercalated or pillared saponite can be estimated as the volume of the interlayer minus the volume of the guest molecules: V, = Vi - V,. The volume of the interlayer can be obtained by multiplying the interlayer distance, di, obtained from X-ray diffraction experiments by the surface area of a saponite layer, Vi = diSlayer.The total specific surface area of the layers has been estimated either from the cell parameters or from ethylene glycol adsorption.20 Both
approaches give equivalent results around 355 m2 g-l. Assuming that all the intercalated aluminum is in the form of A13 polycations, the number ng of gallery All3 pillars per gram of sample can be obtained from chemical analysis results, n, = (Al% x NJ(2698 x 131, where N A is the Avogadro number. Considering the A l l 3 unit as a cylinder, the volume V, of the guest molecules is given by V, = ns(rg)2di,where rgis the radius of the intercalated molecule. The question of which value to use for r, is far from having an obvious answer. The crystalline radius 0fAl13 is equal to 5.4A:' this corresponds to unhydrated [Al1304(OH)~Z(HZO)~~]'+. In solution, of course, the polycations are strongly bound to hydration water. Using small angle X-ra scattering, the A13 polymer was found to have a 9.8- gyration radius in water solutionz1and it was shown that this radius dictated the maximum intercalation density in the procedure we have used.18 Now, the hydration state of All3 ions in the interlayers before C02 adsorption will depend on the degassing procedure used; in the absence of direct measurements of pillar dehydration rates, the pillar radius rg may be considered as an adjustable parameter lying between 5.4 and 9.8 A. When an 8.4-Aradius is adopted for the A113 unit in the intercalated products, the theoretical microporous volume matches the values obtained by the DR treatment of COz adsorption isotherms quite well, as shown in Figure 3. It is interesting that the difference between this value and the crystalline radius is equal to 3 A,or approximately the diameter of one water molecule. It is tempting to rationalize the 8-4-A value as corresponding to the A113 ion surrounded by one shell of strongly bound water molecules. In the case ofpillared samples(Figure 4), using the same radius value, there is again an excellent match between the calculated and experimental values of the microporousvolumes. This suggests that upon calcination at 500 "C, the A l l 3 unit essentially preserves its structure, as already postulated by 27AlNMR and IR experiments;18s22 hydration water molecules, and even some constitutional (HzO), are no doubt lost on calcination, but the polycation is able to rehydrate upon exposure to ambient atmosphere, explaining the constancy of the rgvalue. Nitrogen Surface Area. The BET surface areas and micropore volumes (calculated using the t-plot method3)
(20) Besson, G.; Decarreau, A,; Manceau, A.; Sanz, J.;Suquet, H. In Matdriaun Argileux; Besson, G., Decarreau, A., Manceau, A., Sanz, J., Suquet,H., Eds.;SociBtk Frangaise de Min6ralogie et de Cristallographie, 1990.
(21) Bottero, J. Y.; Tchoubar, D.; Cases, J. M.;Fiessinger,F. J.Phys. Chem. 1962,86,3667. (22) Lambert,J.-F.; Chevalier,S.;Franck, R.; Suquet,H.; Barthomeuf, D. J. Chem. SOC.,Faraday Trans. 1994,90,675.
0.8 1.0 1.1 1.2 1.3 1.4 1.5 (I
0.124 0.137 0.116 0.106 0.100 0.107 0.104
0.145 0.123 0.093 0.077 0.098 0.098 0.080
0.171 0.129 0.117 0.105 0.097 0.105 0.106
0.148 0.133 0.103 0.099 0.097 0.085 0.061
Obtained from the ordinate at the origin of the DR plots.
x
Letters
2852 Langmuir, Vol. 11, No. 8, 1995
0.14
Y
i , -
I0
~
Voat293K Theorehcal Vo
11
t
charge
0
"
'
'
0.8
'
'
'
"
1
'
'
'
1.4
n
'
1.6
Layer Charge
Figure 4. Comparison between calculated and experimental microporous volumes for pillared saponitesas a function of the layer charge x .
of the different intercalated and pillared samples are presented in Table 3. Even if the use of the BET treatment is highly questionable in the case of intercalated and pillared samples,23as it certainly does not yield a true value of the geometrical surface area of the solids, it still represents a reliable index. The striking feature is that the BET surface areas and micropore volume follow a direct relation with the quantity of aluminumincorporated in the saponites. Thus, the adsorption of nitrogen in pillared saponites is certainly more complex than a simple pore filling mechanism. There may be a particular interaction between the adsorbate molecules and the pillars. This behavior could also be explained by a sagging of the layer (23)Begaya, F.;Gatineau, L.; Van Damme, H. In Multifunctional Mesoporous Inorganic Soli&; C . a. H. Sequeira M.J.8, Ed.; NATO AS1 Series; Springer: New York, 1993;p 19.
pillared saponites
intercalated saponites
pillared saponites
360 390 457 438
280 382 416 430 417 392 417
0.116 0.127 0.151 0.144 0.146 0.142 0.148
0.088 0.124 0.136 0.141 0.136 0.124 0.136
444 442 448
* Obtained from the t-plot treatment. "
1.2
intercalated saponites
0.8 1.0 1.1 1.2 1.3 1.4 1.5
0
0.061 0.6
Table 3. Nitrogen BET Surface Areas and Microporous Volumes of Intercalated and Pillared Saponites BET surface micropore volume area (m2 9-l) (cm3 of liquid g-l)=
between pillars which would allow only one layer of N2 molecules to be accommodated between the sheets. Indeed, Ge et al.24have attributed the differences in the BET surface areas with changein calcination temperature to differences in N2 packing due to changes in gallery height. This point deserves more studies and will be the object of a forthcoming p ~ b l i c a t i o n .At ~ ~any rate, these results suggest that the characterization of aluminum pillared clays by nitrogen adsorption can lead to misleading results in terms of microporous volumes. Indeed, the microporous volumes determinedfrom Nz adsorption data are lower than the theoretical ones for layer charges of 0.8 or 1.0 and higher for higher layer charges. On the contrary, the adsorption of carbon dioxide seems to yield a true geometrical microporous volume that can be used reliably for further catalytic studies.
Acknowledgment. The authors wish to acknowledge Dr. Jean Louis Robert for providing us with the pristine synthetic saponites used in this study. LA950135W (24)Ge,Z.;Li, D.; Pinnavaia, T. J. Microporous Mater. 1994,3,165. (25)Michot, L. J.;VilliBras, F.; Bergaoui, L.; Lambert, J. F.; Robert, J. L.; Suquet, H. Manuscript in preparation.
