Langmuir 1994,10, 291-297
291
Analysis of the Microporosity in Pillared Clays A. Gil and M.Montes* Grupo de Ingenierfu Qulmica, Departamento de Qulmica Aplicada, Facultad de Qutmica, Universidad del Pab Vasco, Apdo. 1072,20080 San Sebastihn, Spain Received July 29,1993. I n Final Form: October 26, 199P The microporosityof a series of Al-pillared clays prepared with different Allclay ratios has been studied. Nitrogen adsorption data obtained from very low relative pressures has been used to obtain a thorough characterizationof the microporosityof the intercalated solids. Dubinin-Radushkevich plots, adsorption potential distributions, and micropore size distributions obtained with the Horvath-Kawazoe approach confirmed the existence of two sizes of micropores. The Allclay ratio played a determinant role in the properties of the samples obtained. For the lower values, the intercalation process was not successfuland a low microporosity was obtained. For 30 and 90 mM Allg of clay a basal spacing of 1.84 nm was found in accordance with a great microporosity increase. For the higher Allclay ratio (180 mM), an excess of A1203was introduced between the clay layers, and in spite of a large basal spacing both the micropore volume and the micropore area decreased.
Introduction A new class of porous, high surface area materials, of potential interest as catalysts and adsorbents, can be formed by the synthesis of pillared clays. These materials are synthetized by intercalating metal complex cations between the silicate layers of clays. Pillared oxides are formed aftar calcination, capable of preventing the collapse of the interlayer spaces, hence generating a microporous structure. Depending on the preparation conditions,14 the accessibility properties of these microporous solids can be affected. The quantitative evaluation of the microstructure in porous materials is frequently an unavoidable step in the design and application of materials for adsorption. In the case of meso- and macropore solids, the Kelvin equation provides useful a model for converting sorption data into pore size distribution (e.g., the BJH model). However, the Kelvin equation, though widely used, is restricted to pore sizes greater than 2 nm. Below 2 nm, the liquid cannot be considered a fluid with bulk properties because the forces exerted by the pore wall are no longer negligible. The most popular methods used for the assessment of the microporosity (e.g., the t- and a,-methods4) are based on a comparison of the shape of a given isotherm with that of a standard isotherm of a nonporous reference solid, but the accurate evaluation of the parameters that characterize the microporous structure is not a simple task. The classical method of obtaining the mean pore sizes of molecular sieves is from probe molecule sorption data.576 Several author^^-^^ have proposed methods for the de~~
~
e Abstract published in Advance ACS Abstracto, December 1, 1993. (1) Sterte, J. P.; Otterstedt, J. E. In Preparation of Catalysts IV;
Delmon, B., Grange, P., Jacobs, P. A., Poncelet, G., Eds.; Elsevier: Amsterdam, 1987; p 631. (2) Lahav, N.; Shani, U.; Shabtai, J. Clays Clay Miner. 1978,26,107. (3) Singh, S. S.; Kodama, H. Clays Clay Miner. 1988, 35, 397. (4) Gregg, S. J.; Sing,K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1991; Chapter 4. (5) Stacey, M. H. Catal. Today 1988,2, 621. (6) Mataumoto, M.; Suzuki, M.; Takahashi, H.; Saito,Y.Bull. Chem. Soc. Jpn. 1985,58,1. (7) Mikhail,R.Sh.;Brunauer, S.; Bodor, E. E. J . Colloid Interface Sci. 1968,26, 45. (8) Carrott, P. J. M.; Roberts,R.A.; Sing,K. S. W. In Chracterization
of Porous Solids; Unger, K. K., Rouquerol, J., Sing, K. S. W., Kral, H., W.;Elsevier: Amsterdam, 1988, p 89.
(9) Dollimore, D.; Heal, G. R.J . Appl. Chem. 1964,14,109. (10) Seaton, N. A.; Walton, J. P. R. B.; Quirke, N. Carbon 1989,27, 853.
termination of the size distribution of micropores. However, these methods do not provide a reliable means of determining micropore size distribution because they neglect interactions between molecules in the opposing films and the increase in adsorption energy in the small micropores. Recently, simple models have been proposed for calculating the pore size dispibutions and the potential energy profiles of molecular sieves. Everett and Powlll have calculated the potential energy profiles for atoms adsorbed in slitlike pores and showed the enhancement of the depth of the potential energy well over that for adsorption on a flat surface. Using a slit model, Horvath and Kawazoe12 provided a simple method for the calculation of effective pore size distribution from the adsorption isotherms in molecular sieve carbon. A similar method was used by Seaton et al.10 Although, the pore structures of pillared clays are quite complex, the slitlike geometrylS was proposed to be more realistic than the cylindrical and spherical geometries.14Js Adsorption data a t low pressures are sources of valuable information about adsorbate-adsorbent interactions and energetic and structural heterogeneities of solids. Significant progress in the theoretical description of gas adsorption on heterogeneous solids has provided foundations for utilizing low-pressure adsorption measurements to evaluate adsorption potential distributions.lB-lS These distributions characterize the energetic heterogeneity of microporous solids can be used to calculate micropore size distributions. In this work, a series of pillared clays prepared with different Allclay ratios have been studied, showing different accessibility properties. Nitrogen adsorption measurements at low pressures were used to obtain a comprehensive characterization of these microporous solids. Dubinin-Radushkevich formalism was used to describe the volume filling of micropores and the energetic heterogeneity of microporous solids. The micropore size distributions were obtained by using a slitlike model. (11) Everett, D. H.; Powl, J. C. J . Chem. SOC.,Faraday Tram. 1 1976, 72, 619. (12) Horvath, G.; Kawazoe, K. J. CheM. Eng. Jpn. 1983,16,470. (13) Baksh, M. S. A,; Yang,R.T. AIChE J. 1992,38, 1357. (14) Saito,A.; Foley, H. C. AIChE J. 1991, 37, 429. (15) Bahah, M. S. A.; Yang,R. T. AIChE J . 1991,37, 923. (16) Jaroniec, M.; Choma, J. Mater. Chem. Phys. 1988,19,267. (17) Stoeckli, H. F. J . Colloid Interface Sci. 1977,59, 184. (18) Jaroniec, M.; Briuer, P. Surf. Sci. Rep. 1986,6, 65.
0743-7463/94/2410-0291$04.50/00 1994 American Chemical Society
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292 Langmuir, Vol. 10,No.1, 1994
Theoretical Basis for Microporosity Characterization Dubinin Methods. Dubinin and Radushkevich developed the micropore volume filling theory (MVFT), based on the Polanyi concept of a characteristic curve, to describe the adsorption on micropores. The characteristic curve can be expressed as: o =V = exp[ -(E) A2 VO Here 0 is the relative adsorption, which is defined as the ratio of the amount adsorbed in the micropores (V) to the maximum micropore adsorption capacity (VO),A is the adsorption potential, and E is the characteristic energy for a given adsorbate-adsorbent system. A was related to the differential free energy of adsorption by
The adsorption potential distribution characterizes the energetic heterogeneity associated with micropores. The adsorption potential distribution X(A) relating to the isotherm equation (6) may be evaluated by means of the condensation approximation methodz5
]
For microporous solids possessing micropores of a great number of dimensions, the micropore distribution may be approximated effectivelyby a continuous distribution. For the continuous distribution F(B) the overall adsorption isotherms can be expressed as follow^^^*^^ (8)
By combining eqs 1 and 2, the well-known DubininRadushkevich equation is obtained
where B is the structural constant and ,6 a similarity coefficient (0 = 1 for benzene). A more general expression was developed by Dubinin and Astakhov: (4)
The exponent n was linked to the degree of heterogeneity of the micropore system, as suggested by the adsorption of various molecular probes.l9 The Dubinin-Radushkevich equation (1or 3) is a particular case for n = 2 and describes the adsorption on structurally homogeneous solids,l7,2021 On the basis of experimental studies, different aut h o r postulated ~ ~ ~ that ~ ~ eq 1~or ~2 describes ~ ~ adsorption in uniform micropores. This postulate is the fundamental assumption, which enables the isotherm expressions for describing adsorption on nonuniform (heterogeneous) microporous solids to be derived. In the case of a heterogeneous microporous solid possessing micropores of the same shape but characterized by few different sizes, Jaroniec and Choma16 proposed a modification of this equation (5)
Here fi cfi = Voi/ Vo) denotes the fraction of adsorption sites located in the micropores with an adsorption characteristic energy Ei, that can be related to Bi (Ei2= P2/Bi), the structural parameter for each micropore. For the analysis of adsorption data, different authors2s24 applied a special form of eq 5 which contains only two terms of this equation A 2
O=flexp[-(g) ]
A
2
]
+fZeXP[-($) (6) Equation 6 represents adsorption on solids, which possess two kinds of micropores. (19) Stoeckli, H. F. Carbon 1981,19,325. (20) Dubinin, M. M.; Stoeckli, H. F. J.Colloid Interface Sci. 1980,75, 34. (21) Jaroniec, M.; Piotrowska, J. Monatsch. Chem. 1986, 117, 7 . (22) Dubinin, M. M. Carbon 1981,19, 321. (23) Rozwadowski, M.; Wojsz, R. Carbon 1984,22, 363. (24) Jaroniec,M.;Gilpin, R. K.;Kaneko, K.; Choma,J.Langmuir 1991, 7, 2719.
Here 01 is the local isotherm and s2 is the interval of possible adsorption energychanges. This integral equation has been solved for various continuous functions representing the distribution F(B).18923-2c29 Slit Model. The slit-potential model of Everett and Powlllcan be applied to adsorption in microporouspillared clays. This model is able to represent well the complicated pore structures of pillared clays. These authorsll showed that the potential energy of interaction, e, between one adsorbate molecule and two parallel lattice planes whose nuclei are at a distance L apart can be expressed via the Lennard-Jones (12:6) potential, after integration, as t
= 3.07~*[-(:)~
+ (9)'"-
(f-r
+ (&)lo]
(9)
where
do is the arithmetic mean of the diameters of the adsorbate and adsorbent and r is the displacement of a molecule from the plane of surface nuclei (one plate). In the case of more than one adsorbate molecule, the value of e* in eq 9 is
where N Ais the number of molecules per unit area of the adsorbate and N, is the number of atoms per unit area of surface. The dispersion constants A, and AA are given by the Kirkwood-Muller relationship as 6mc2a,aA
"'=(ZJ@ and
where m is the mass of an electron, c is the velocity of light, a, and X, are respectively the polarizability and (25) Cerofolini, G. F. Surf. Sci. 1971,24, 391. (26) McEnaney, B.;Mays, T. J.; Camton, P. D. Langmuir 1987,3,695. (27) Jaroniec,M.; Madey, R.; Lu, X.; Choma, J.Langgmuir 1988,4,911. (28) Jaroniec, M.; Choma, J. Colloids Surf. 1989, 37, 183. (29) Jaroniec, M.; Madey, R. J.Chem. SOC.,Faraday Trans. 2 1988, 84, 1139.
Microporosity in Pillared Clays
Langmuir, Vol. 10, No. 1, 1994 293
magnetic susceptibility of an adsorbent atom, and CYAand XA are the polarizability and magnetic susceptibility of an adsorbate molecule, respectively. The Kirkwood-Muller formalism requires certain physicochemical data on the adsorbate and adsorbent. The measurements required for calculating the potential energy profiles via the Kirkwood-Muller formalism have been made by a number of ~ o r k e r s . ' ~ - The l ~ value of NaAa+ NAAA(the interaction parameter) used in this analysis, 33.8 cal nm4 mol-l, has been calculated using data from the literature.12-14 The potential function for the slit pore filled with adsorbate molecules can be written as
-
Na montmorillonite
(13) Horvath and Kawazoe,12 using an energy balance, equated the free energy of adsorption to the net energy of interaction between the layers. The relation PIP0 and slitlike geometry thus becomesI2
RT ln(
2)
= NAv N A A A + N ~ A ~ a4 u2(L- 2d0) 3(L - do)2
[
do --a4 +2]
9(L-d0)'
3d;
9dt
(14)
Experimental Procedure The starting material used in this work was montmorillonite (Kunipia F), kindly supplied by Kunimine Co. Commercial product is available in the sodium exchanged form with a size less than 2 pm. Na-montmorillonite was dispersed in water and aged for at least 2 months. It was then washed by dialysis until the conductivity of the surrounding water was less than 1.5 WS. The solid content of the dialyzed clay dispersion was 7-10 g/L. Aluminium polycations were prepared by slow titration of 0.2 M AICla solution with 0.2 M NaOH under vigorous stirring,2sM using an OH/Al mole ratio equal to 2 and aged at 363 K for 4 h (pH = 3.9).a1 The formation of the All3 (Keggin cation) was confirmed by aluminum-27 NMR. Pillared clays were obtained by dropping the aqueous montmorillonite suspension into the polycation solution with vigorous stirring to obtain four different AlzOdmontmorillonite ratios. The solids were kept in contact with the solution at room temperature for 24 h, washed by centrifugation, and dried at 393 K for 16 h. The samples were referred to as S-x, where x is the AVclay ratio in mM AVg of clay, i.e. S-10, S-30, S-90, and 5-180. The basal spacing of the samples was measured by X-ray diffraction (XRD) of a thin layer of the clays on glass slides using a Philips PW 1710 diffractometer with Ni-filtered Cu Ka radiation. Elemental analyses were performed by atomic absorption spectroscopy. The cation exchange capacity (CEC) was determined by saturation of a sample with 1 N ammonium acetate solution, followed by repeated washing with methanol and centrifugation to eliminate excess ammonium ions. The remaining ammonium ions were determined by the micro-Kjeldahl method. Nitrogen adsorption experiments were performed at 77 K using a static volumetric method (Micromeritics ASAP 2000M adsorption analyzer). The samples were previously degassed at 393 K for 24 h. In order to achieve an exhaustive micropore characterization, adsorption isotherms must be obtained beginning at much lower relative pressures (- 108< P/Po < 0.99) than that for the conventionalmesoporosityanalysis. Experimentally, this proves more difficult than for conventional adsorption, since quantitative high vacuum is difficult to control. The use of an (30)Shabtai, J.; Rosell, M.; Tokarz, M. Clays Clay Miner. 1984,32, 99. (31)Tokarz, M.;Shabtai, J. Clays Clay Miner. 1985,33,89.
28
Figure 1. X-ray diffraction patterns of the samples. automated instrument aleviates this problem. To obtain an adequate micropore size distribution, sufficient data points at low pressures are needed, and this requires the possibility of adding small nitrogen volumes. In this work, nitrogen adsorption data were obtained using 0.1 g of sample and successive doses of nitrogen of 0.5 mL/g until P/Po = 0.04 was reached. Subsequently, further nitrogen was added and the volumes required to achieve a fixed set of PIP, were measured. Specifictotal surface areas were calculated using the Langmuir equation and specific total pore volumes were estimated from nitrogen uptake at P/Po 0.99.
-
Results and Discussion Base-hydrolyzed solutions of AlCl3 are known to be effective pillaring agents for smectite clays. The distance between the smectite layers after the intercalation process depends on the nature of the aluminum species present in the solution used. Maximum distance can be obtained with A113 (Keggin cation)32t33 but other species can be produced by hydrolysis of A1solutions with concentrations higher than 10-4 M.34-37 In this work, the OH/Al ratio and the aging condition^^*^^*^^ were selected to maximize the amount of Al13. In order to modify the microporosity of the samples, different Allclay ratios were used during preparation. XRD patterns of the montmorillonite and those of the four samples calcined a t 673 K for 4 h are shown in Figure 1. Basal spacings corresponding to the maximum in these (32)Pinnavaia, T. J. Science 1983,220,365. (33)Bradley, S.M.;Kydd, R. A.; Yamdagni, R. J. Chem. SOC.,Dalton Trans. 1990,2653. (34)Bottero, J. Y.; Cases, J. M.; Fiessinger, F.; Poirier, J. E. J. Phys. Chem. 1980,84,2933. (35)Bottero, J. Y.; Bruant, M.; Cases, J. M.Clay Miner. 1988,23,213. (96) Bottero, J. Y.; Marchal,J. P.;Poirier,J. E.;Cases, J. M.; Fieeainger, F.Bull. SOC.Chem. Fr. 1982,ll-12,1-439. (37)Akitt, J. W.;Farthing, A. J. Chem. SOC.,Dalton Trans. 1981, 1624.
Gil and Montes
294 Langmuir, Vol. 10, No. 1, 1994
Table 1. Specific Surface Areas, Specific Pore Volumes, B a d Spacings (&I), and Cation Exchange Capacities (CEC) for Pillared Clays
~
sample
VPb
20 (C= 135)
0.040
A,& 6
75 (C= 323) 311 (C= 450) 320 (C= 492) 199 (C= 407)
0.081 0.186 0.175 0.139
17 25 16 25
ALuloB
Na-montmorillonite (dialyzed) s-10 5-30
S-90 5-180
4P"
VFPb
Vmpb
14
0.008
58
0.026 0.116 0.119 0.073
286 304 174
0.032
dmi' 1.20
CECd 91
0.065 0.070 0.056 0.066
1.35 1.84 1.79 1.79
62 55 55 51
a Specific surface areas in ma gl. 0.01 IP/PoI0.05 interval of P/Po. Specificpore volumes in mL gl. Specific total pore volume at P/Po = 0.99. Basal spacings in nm. Cation exchange capacity in mequiv (100 g of clay)-'. 1so
I
Is-10
1so
I
1s.30
100
1
A IS0
I
-
P / PO
P i PO
s . 90
1 5 0 1 s . 180
0
0
0.5
1
1.5
0 2
a 00
02
04
P/
06 PO
08
10
00
02
04
Pi
06
08
10
PO
Figure 3. a-plots of the samples: ( 0 )S-10;(0) 5-30;(0) S-90; (A)5-180.
Figure 2. Nitrogen adsorption-desorptionisotherms. patterns are presented in Table 1. S-10showed a basal spacing of 1.35 nm, but the samples prepared with higher Al/clay ratio showed basal spacings between 1.84 and 1.79 nm. This basal spacing confirms that the All3 intercalation has been obtained in all samples, except for S-10. An increase in the Al/clay ratio from 30 to 180 did not affect the basal spacing. Adsorption Isotherms. The adsorption-desorption isotherms of the four samples are displayed in Figure 2. The adsorption isotherms are type I in the Brunauer, Deming, Deming, and Teller (BDDT) classification38at low pressures (microporous solids). The hysteresis loop (H4 type in the IUPAC classifi~ation~~) indicates the presence of mesoporosity. The Barrett-Joyner-Halenda methodN was used to characterize this mesoporosity. All samples presented a maximum in the pore volume distribution at 3.5 nm. The general aspect of the isotherms did not change after the pillaring process, showing only differences in the amount of nitrogen adsorbed. The specific surface areas and the specific pore volumes of the pillared clays are summarized in Table 1. The specific external surface areas (Aext) and the specific micropore volumes (V,,) were obtained from the a,-method. The %-method is based on a comparison of the shape of an adsorption isotherm measured for a microporous solid with that of a standard isotherm for a nonporous reference solid.4 In this work, the standard adsorption isotherm of nitrogen on Na-montmorillonite obtained from calcination at 1073 K was used as the reference. The a,-plots are represented in Figure 3. Specific micropore surface areas (Accp)were calculated by subtracting the specific external surface areas (Aext) from the specific total surface areas (AI,,,&. Specific total surface areas (0.01 IPIP0 I0.05) (38)Gregg, S.J.;Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1991; Chapter 1. (39)Sing,K.S.W.; Everett, D. H.; Haul,R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemienieweka,T.Pure Appl. Chem. 1985,57,803. (40)Barrett, G. P.; Joyner, L. G.; Halenda, R. H. J.Am. Chem. SOC. 1950, 73, 373.
250
150
so 0
AI / clay ( m M g" ) Figure 4. Specific surface areas as a function of the Al/clay ratio: ( 0 )A m ; (A);,A (0)A,a.
were calculated using the Langmuir equation for monolayers which is more suitable for microporous solids than the BET equation for multilayer~.~l1~2 Similarly, specific mesopore volumes (Vmp) were calculated by subtracting the specificmicropore volumes ( V,,,) from the specifictotal pore volumes (V,). Figure 4 shows specific surface areas ( A w , AeXt,and A+,) of the samples as a function of the Al/clay ratios. Specific total (or micropore) surface areas increased for 5-30and S-90with respect to S-10,but a further increase of the Al/clay (S-180) produced a decrease of specific surface areas. Nevettheless, specific external surface areas (A,h) remained almost constant when the Allclay ratio was modified. Figure 5 shows specific pore volumes (V,, VPp.,and Vmp) of the samples as a function of the Allclay ratio. The behavior is similar to that of the specific surface area: an initial increase of the pore volume for 5-30and S-90 and a decrease for S-180. It would be pointed out that the specific ~urfacearea and specific pore volume of sample S-180were less than that of S-90, but the basal spacings were equal. (41)Figueras, F. Catal. Rev.-Sei. Eng. 1988,30,457. (42)Occelli, M. L.;Finseth, D.H. J. Catal. 1986,99, 316.
Langmuir, Vol. 10, No. 1, 1994 295
Microporosity in Pillared Clays
-
h
'M
0.20
1
P
i 7
4
-
-1
-
E
-s4 aJ
a"
+
0.15
50
25
0
0.10
IO'
10'
IO'
IO'
IO'
101
P I PO
-
0.05
'M
iw
IW
I s . 180
15
F
0.00
50
0
100
150
200
AI / clay ( mM g" )
Figure 5. Specific pore volumes as a function of the Al/clay
ratio: (0) v,; (0)v,,,;(A)vmp.
Table 2. Elemental Analysis of the Samples (wt Na-montmorillonite S-10 S-30 S-90 71.49 69.75 68.38 66.50 Si02 20.04 25.42 28.73 28.92 AlzOa 0.89 2.19 3.90 2.82 NazO 1.37 1.31 2.10 0.98 Fez03 0.03 0.03 0.11 0.03 K2O 2.01 0.96 0.56 1.01 MgO 0.35 0.04 0.04 0.04 CaO
~
~
50
{
25
-
0
10'
10'
10.3
IO5
P I PO
%)
S-180 65.57 31.03 1.47 1.45 0.03 0.41 0.04
Therefore, the dependence of microporosity on Al/clay ratio suggests that important structural changes occurred in the pillar and the layers of the clay, probably due to changes in the constitution or distribution of the pillars, that influenced the nitrogen ac~essibility.l*~3 Pinnavaia et alSastudied this accessibility loss with the Al/clay ratio and proposed that the pillaring process is a complex phenomenon involving the heterogeneous distribution of charges in the surface of the clay45 and the possible existence of different polymeric species with different charges. The amount of exchangeable cations on the clay surface, represented by the cation exchange capacity (CEC) decreased as the Al/clay ratio increased (Table 1). This observed decrease in the CEC can also be attributable to changes in the constitution or distribution of the pillars. However, the high difference between the accessibility properties and the CEC can be related to the different determination method (gas and aqueous, respectively). These observations agree with the elemental analysis. The small accessibility measured in S-10can be explained by the low content of A1203 introduced (Table 2). On the other hand, the accessibility loss in 5-180could be due to an excess of A1203 between the clay layers. Adsorption Isotherms at Very Low Pressures. Figure 2 presents the nitrogen adsorption-desorption isotherms of the four samples. These isotherms showed that these sorbents possessed both micropores and mesopores but that the microporosity was more important (see also Figures 4 and 5 ) . In order to analyze the microporosity of the samples, nitrogen adsorption measurements at very low pressures were performed and the results obtained are showed in Figure 6. Dubinin-Radushkevich Plots. Figure 7 shows the log V versus log2(Po/P) plots for the samples, according to eq 3. The general aspect of these plots for S-30,S-90, and S-180is similar and two different linear portions, as has ~~
-1
Figure 6. Nitrogen adsorption isotherms at very low pressures. 2.00
.
0
1
10
20
30
40
50
Log2 ( Po / P )
Figure 7. D-R plots: ( 0 )S-10;( 0 )5-30;(0) S-90;(A)S-180. been suggested for activated carbon fibers,& could be derived to apply the D-R equation in order to obtain V (micropore volume) and E (characteristic energy). S-10 showed lower nitrogen adsorption and it is more difficult to differentiate between two linear portions. Sing et al.47proposed two stages of micropore filling, a primary process that takes place in pores of molecular dimensions (PIP0I0.01) and a secondary process (namely a cooperative process) that occurs a t higher P/Po. For activated carbon fibers (ACF), Kaneko et a1.M proposed a three-stage micropore filling mechanism for the different linear portions to D-R plots. This was related to the width occupied by an adsorbed nitrogen molecule as the unit width of the micropore. The deviation from linearity at low pressures could be due to the restricted diffusion of nitrogen into very narrow micropores at very low adsorption temperatures.48 Deviation at high pressures (low values of log2Po/P,Figure 7) can be related to the presence of larger micropores. The two linear portions observed in D-R plots for S-30, S-90,and S-180suggest that there are two sizes of micropores. Adsorption on microporous solids which posses two kinds of micropore has been described by the two-term D-R e q u a t i ~ n (eq ~ ~ 6), - ~that ~ can also be written as
The values of V and E which characterize these
~
(43) Tichit,, D.; Fajaula, F.; Figueraa, F.; Gueguen, C.; Bosquet, J. In Flud Catalyttc Cracking: Role in Modern Refining; Occelli, M. L., Ed.; American Chemical Society, 1988; Chapter 15. (44) Pinnavaia, T. J.; Tzou, M A ; Landau, S.D.; Raythatha, R. H. J. Mol. Catal. 1984, 27, 195. (45) Occelli, M. L. Keynotes Energy-related Catal. 1988, 35, 101.
(46) Kakei, K.; Ozeki, S.; Suzuki, T.; Kaneko, K. J. Chem. Soc., Faraday Trans. 1990,86,371. (47) Atkinson, D.; McLeod, A. I.; Sing,K. S. W. J. Chim. Phys. 1984, 81, 791. (48) Rodriguez-Reinoso,F.; Garrido, J.;Martin-Martinez, J. M.; MolinaSabio, M.; Torregrosa, R. Carbon 1989.27, 23.
Gil and Montes
296 Langmuir, Vol. 10, No. 1, 1994 Table 3. Micropore Volumes (V),E Values, and Micropore Sizes (dD) for Pillared Clays D-R slit model samples VI" Vza VO" Elb Ezb VP VZ" Voa dpc s-10 0.008 0.020 0.028 20.16 23.03 0.006 0.021 0.027 0.55 S-30 0.028 0.088 0.116 24.27 23.29 0.023 0.091 0.114 0.59 S-90 0.029 0.090 0.119 29.00 21.38 0.022 0.094 0.116 0.58 S-180 0.019 0.055 0.074 24.36 21.40 0.014 0.060 0.074 0.59 Specific pore volumes in mL g1( VZ= VO- VI). Characteristic energy in kJ mol-'. e Average micropore size in nm. 0.08
s.30
:lo -
0.01
Figure 9. Micropore size distributions. A(kJmol.')
A ( k J mol" ) 0.08
S.90
0.08
S.lS0l
Figure 8. Adsorption potential distributions.
micropores were obtained by correlation with the linear parts of the D-R plots and are presented in Table 3. VO represents the specific total micropore volume, VI and VZ were related to the specific micropore volume for each micropore size. VI and E1 were obtained from low relative pressures (PIP0I l0-l) and VOand Ez from high relative 1 103). pressures (P/Po Adsorption Potential Distributions. The adsorption potential distribution X ( A ) evaluates the energetic characteristics of the adsorption process, showing its degree of heterogeneity. D-R plots showed two linear portions and consequently eq 6, with the parameters shown in Table 3, was used to evaluate X(A) in terms of the condensation approximation method,25 eq 7. Figure 8 shows the adsorption potential distributions obtained for the four samples. The physical interpretation of the adsorption potential distributions for microporous solids is often difficult because energetic heterogeneity can be due to both a nonuniform microporous structure (structural heterogeneity) and a surface heterogeneity. However, when the specific external surface areas are relatively small compared to the specific microporous surface areas, it is reasonable to assume that the slitlike micropores formed between clay layers are the main source of the energetic heterogeneity. In addition, pillared clays are complex solids because the intercalated especies can affect the structural heterogeneity. As can be seen from Table 2, ArPis much larger than Aed for 5-30,S-90,and S-180, and we can therefore assume that the observed heterogeneity is due to the microporosity. The different linear portions in Figure 7 were related to two sizes of micropores. For the samples S-30,S-90, and S-180, the X ( A ) curves defined in the region of A from zero up to -35 kJ/mol are double-peak distributions withmaximaat SimilarvaluesofA: S-30(-11.9and -24.0 kJ/mol),S-90(-12.5and -26.4kJ/mol),andS-l80(-12.2 and -25.3 kJ/mol). The high A values are related with the narrow micropores. The low microporosity of S-10
must also be taken into account to explain the distribution obtained (Figure 81, because the specific external surface area is comparable in this case to the specific micropore surface area. Micropore Size Distributions. By use of the Jaroniec-Choma the adsorption potential distribution curves can be transformed to micropore size distributions. Baksh et alem adopted this approach combining molecular probing and adsorption isotherms of nitrogen. A different approach to obtain the micropore size distributions has been proposed by Horvath and Kawazoe12 using the slit potential model developed by Everett and Powl" and used to study microporous carbon samples. The slit or slab geometry can be a reasonable description for pillared clays and hence can provide an easy analysis of the micropores structure using only the nitrogen adsorption data. Figure 9 shows the micropore size distributions of pillared clays, obtained using the slitlike model from Horvath and Kawazoe.12 This model was calculated with 10:4 parallel plate potentiall' and requires theevaluation of the model constants. Therefore, this approach depends heavily on accurate physicochemical data and neglects the importance of other intermolecular forces (e.g., electrostatic) that can be quite significant, as shown by Baskh and Yang.l3 The different micropore distributions of pillared clays are influenced by the Al/clay ratio. When this ratio was increased, bimodal micropore distributions were obtained. S-10 presents a low microporosity and their micropore distribution did not show two micropore sizes. Samples S-30,S-90, and S-180 showed similar micropore distributions. The specific micropore volumes (dp I2.0 nm) obtained by using the slitlike model were presented in Table 3. We have differentiated between the specificmicroporevolumes corresponding to two sizes in the micropore distribution. The micropore volume obtained for the narrower micropores was -20% of total micropore volume for all samples. Table 3 also presents the average micropore sizes obtained, which did not vary with the Al/clay ratio. A comparison can be made between the pore sizes obtained by X-ray diffraction analysis after subtracting the montmorillonite layer thickness 0.96 nm (-0.85 nm, Table 1) and that obtained by adsorption (-0.58 nm). These differences can be explained by the differences in the measurement techniques used. The pore sizes obtained (49) Jaroniec, M.; Choma, J.; Lu,X. Chem. Eng. Sei. 1991,46,3299. (50) Baksh,M. S.; Kikkinides, E. S.; Yang, R.T.Znd. Eng. Chem.Res. 1992,31, 2181.
Microporosity in Pillared Clays
Langmuir, Vol. 10, No. 1, 1994 297
by means of XRD (interlayer spacings) represent the maximum in the count distribution. These values represent a crystallographic distance (bidimensional). Nevertheless, these distribution are broad and asymmetrical, showing an important tail at higher angles for 5-30,S-90, and S-180 and at lower angles for S-10. This is in agreement with the presence of interlayer distances, lower and higher, respectively, than that of the maximum. On the other hand, the accuracy of the calculation of the interaction parameter (the influence of the dynamics of the pillared clay structures1) and the actual accessibility of the microporous solids (in pillared clays, by the interpillar and interlayer distances) play an important role in the calculation of the average value of the microporous distribution proposed by Horvath and Kawazoe.12 Basically, this value is an average of the accessibility (three-dimensional),in contrast to the interlayer spacings obtained by XRD from which only one-dimensional information can be obtained. The structural heterogeneity can be related to the microporous structure in solids having a high ratio of Al/ clay due to the large value of A, with respect to that of Aefi. In this case, the potential distributions obtained for S-30,S-90,and S-180 show the presence of a bimodal microporous structure.24 The microporous size distributions obtained by applying the slit model also confirm this bimodal distribution shown by the solids. In addition to that, the microporous volumes obtained by the microporous distribution or by the D-R plots are in a good agreement (Table 3).
Conclusions
(51) Webb, S. W.; Curtis Conner, W. In Studies in Surface Sciences
and CatalysL;Rodriquez-Reinoeo,F., Rouquerol,J.,Sing,K. S. W., Unger, K. K., Eds.; Elsevier Science: Amsterdam, 1991; p 31.
The Al/clay ratio had a significant effect on the accessibility properties in Al-pillared clays. When the Al/ clay ratio increases, the accessibility properties showed an initial increase and a slight decrease when an optimum Al/clay ratio was exceeded. This behavior was attributed to changes in the constitution or distribution of the pillars. In order to analyze the microporosity of the samples, nitrogen adsorption at low pressures (PIP0< 103)is as a powerful tool. The Dubinin-Radushkevich formalism was used to describe the volume filling of micropores and the energetic heterogeneity, through the adsorption potential distributions evaluated in terms of the condensation approximation method. For samples prepared with Al/ clay ratio equal to 30 or higher, a bimodal distribution was obtained, and because the specific external surface areas are relatively small compared to the specific micropore surface areas, these distributions can be related to the bimodal micropore size distributions in Al-pillared clays. Micropore size distributions were obtained using the slit-model proposed by Horvath and Kawazoe. When the Al/clay ratio was increased, we obtain bimodal micropore distributions. These results are in good agreement with that obtained by the D-R method.
Acknowledgment. We express our thanks Dr. S.Yunes (Micromeritics) for valuable discussions and Dr. P. D. Armitage for reviewing the manuscript. The Scholarship support for A. Gil by the Ministerio de Educaci6n y Ciencia (Programme FPI) and the financial support by the Universidad del PahVasco/Euskal Herriko Unibertsitatea (Grants 221.215-T057/90 and 221.216-E092/91) are gratefully appreciated.
