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Surface Area, Pore Volume Distribution, and Acidity in Mesoporous Expanded Clay Catalysts from Hybrid Density Functional Theory (DFT) and Adsorption Microcalorimetry Methods M. L. Occelli,*,† J. P. Olivier,‡ J. A. Perdigon-Melon,§ and A. Auroux§ MLO Consulting, 6105 Black Water Tr., Atlanta, Georgia 30328, Micromeritics Instrument Corp. Inc., Norcross, Georgia 30093, and Institut de Recherches sur la Catalyse, CNRS, 2 Av. A. Einstein, 69626 Villeurbanne, France Received June 20, 2002. In Final Form: September 11, 2002 A hybrid density functional theory (DFT) method has been used to interpret the data for the adsorption of nitrogen at 77 K within the porous structure of a sample of synthetic saponite expanded with stable SiO2‚TiO2 colloidal particles and dried by different methods. Air drying (AD) favor the formation of structures containing both micro- and mesopores. In contrast, drying with supercritical CO2 (SCD) produces mesoporous solids containing only minor amounts of micropores. In SCD samples, the total PV is more than 5 times higher than in AD samples and their structure is hydrothermally stable after a 5-h exposure to 100% steam at 760 °C. Adsorption microcalorimetry experiments with ammonia indicate that the inclusion of SiO2‚ TiO2 clusters drastically increases the clay total (B + L) acidity; acid site strength and acid site density is greatest in AD samples. Both the BET and BJH methods underestimate the clay catalyst’s surface area and only the DFT method yields reliable surface area and pore volume measurements over the entire micro-meso porosity range investigated.
Introduction The continuing demand for solid-acids capable of providing catalysts for compounds excluded by the microporous structure of zeolites has triggered numerous synthetic efforts that culminated with the synthesis of heat stable porous materials with a well-defined mesoporous structure.1-6 In 1990, Yanagisawa et al.1 reported the preparation of mesoporous silicas with uniform pore dimensions, by reacting kanemite with alkyltrimethylammonium chloride solutions. Shortly after, in 1992, researchers at Mobil begun publishing work describing the synthesis of several mesoporous aluminosilicates with unique pore structure and thermal stability.2-5 Since then, research efforts in academia and in industrial laboratories6 have been dedicated to the manipulation of synthesis condition to generate mesoporous solids with the required acidity, pore geometry, and stability. A recent review of the properties and characterization of mesoporous solids can be found in reference 6. * Address correspondence to this author. † MLO Consulting. ‡ Micromeritics Instrument Corp. Inc. § Institut de Recherches sur la Catalyse. (1) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988-992. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710-712. (3) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olsen, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenkar, J. L. J. Am. Chem. Soc. 1992, 114, 108305. (4) Vartuli, J. C.; Schmitt, K. D.; Kresge, C. T.; Roth, W. J.; Leonowicz, M. E.; McCullen, S. B.; Hellering, S. D.; Beck, J. S.; Schlenkar, J. L.; Olsen, D. H.; Sheppard, E. W. In Zeolites and related microporous Materials: State of the art; Weitkamp, J., Karge, H. G., Pfeifer, H., Holderich, W., Eds.; Elsevier: Amsterdam, 1994. (5) Vartuli, J. C.; Kresge, C. T.; Leonowicz, M. E.; Chu, A. S.; McCullen, S. B.; Johnson, I. D.; Sheppard, E. W. Chem. Mater. 1994, 6, 2070. (6) Selvam, P.; Bhatia, S. K.; Sonwane, C. G. Ind. Eng. Chem. Res. 2001, 40, 3237-3261.
There is a third possible approach to the preparation of mesoporous solids. In fact, Takahama et al.7 reported that by expanding montmorillonite with SiO2‚TiO2 sol particles,8,9 an expanded material was obtained that when dried with supercritical CO2, generated a clay structure with surface area (SA) and pore volume (PV) more typical of silicas then of microporous pillared clays.7 The chemical composition of the colloidal particles and the aspect ratio of the parent clay together with the drying method used are all important parameters that will determine the porosity, reactivity, and stability of mesoporous clay catalysts.10,11 It has long been recognized that traditional methods may not be adequate to measure the SA and PV in solids containing a wide distribution of pore sizes. In reference 12, it is shown that in expanded layered structures a hybrid density functional theory (DFT) method analogous to one developed for the well-characterized model material (MCM-41)13 and a cylindrical pore geometry14 could best give representative SA area and PV data over wide pore size dimensions. It is the purpose of this paper to examine the applicability of this hybrid DFT method to describe (7) Takahama, K.; Yokoyama, M.; Hirao, S.; Yamanaka, S.; Hattori, M. Bull. Chem. Soc. Jpn. 1992, 65, 2494. (8) Yamanaka, S.; Nishihara, T.; Hattori, M.; Suzuki, Y. Mater. Chem. Phys. 1987, 17, 87. (9) Yamanaka, S.; Nishihara, T.; Hattori, M. Mater. Res. Soc. Symp. Proc. 1988, 11, 283. (10) Occelli, M. L.; Takahama, K.; Yokoyama, M.; Hirao, S. In Synthesis of Microporous Materials, Vol II: Expanded Clays and Other Microporous Solids; Occelli, M. L., Robson, H., Eds.; Van Nostrand and Reinhold: New York, 1992; p 57. (11) Occelli, M. L.; Peaden, P.; Ritz, G. P.; Iyer, P. S.; Yokoyama, M. Microporous Materials 1993, 1, 2, 99. (12) Olivier, J. P.; Occelli, M. L. J. Phys. Chem. 2001, 105 (3), 623629. (13) Kruk, M.; Jarionec, M.; Sayari, A. Langmuir 1997, 13, 6267. (14) Olivier; J. P.; Koch, S.; Jaroniec, M.; Kruk, M. In Characterization of porous solids V; Unger, K. K., et al., Eds.; Studies in Surface Science vol. 128; Elsevier: Amsterdam, 2000; p 71.
10.1021/la020567o CCC: $22.00 © 2002 American Chemical Society Published on Web 11/13/2002
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Table 1. Surface Area (SA, m2/g) and Pore Volume (PV, cm3/g) Results from Nitrogen Porosimetry Data for the Parent Synthetic Saponitea
Table 3. Surface Area (SA, m2/g) and Pore Volume (PV, cm3/g) Results from Nitrogen Porosimetry Data for the Expanded Saponite Dried with Supercritical CO2a
method
method
DFT BET BJH t-plot D-R a
micro. SA
total SA
micro. PV
total PV
265
360 229 147 227
0.066
0.201
94 191
0.139 0.046 0.068
Total PV data measured at p/Po ∼ 0.995.
a
m2/g)
Table 2. Surface Area (SA, and Pore Volume (PV, cm3/g) Results from Nitrogen Porosimetry Data for the Air-Dried (AD) Expanded Saponitea method DFT BET BJH t-plot D-R a
micro. SA
total SA
micro. PV
total PV
365
574 398 308 396
0.090
0.407
92 333
DFT BET BJH t-plot D-R
0.317 0.045 0.118
Total PV data measured at p/Po ∼ 0.995.
the porosity of expanded layered materials prepared by reacting a synthetic saponite with a solution containing stable SiO2‚TiO2 colloidal particles10 and dried by different methods. Microcalorimetry experiments with ammonia as the probe molecule will be used to describe their acidity. Experimental Section A sample of synthetic saponite (Sumecton SA) was obtained from Kunimine Industies Co., Ltd. of Japan. Chemical analysis of the sample gave the following oxide composition: 4.65% Al2O3, 62.2% SiO2, 27.8% MgO, 0.04% CaO, 0.02% Fe2O3, 3.40% Na2O, 0.01% K2O, and 0.13% TiO2. The clay was expanded using a SiO2‚TiO2 sol solution prepared by a procedure described elsewhere.7,10 After filtration and washing with water at 60 °C, part of the reaction product was allowed to dry in air, the rest was first washed with ethanol and then dried with supercritical CO2 at 120 atm and 40 °C.10 The expanded air-dried (AD) and supercritically dried (SD) saponite samples gave the following oxide composition: 2.7% Al2O3, 74.8% SiO2, 16.4% MgO, 0.04% CaO, 0.02% Fe2O3, 0.20% Na2O, 0.01% K2O, and 5.40% TiO2. Nitrogen sorption isotherms obtained at liquid nitrogen temperature were collected using a volumetric technique on a Micromeritics ASAP 2010M adsorption instrument. Prior to analysis, samples weighing from 0.1 to 0.3 g were outgassed in a vacuum at 250 °C for at least 16 h. The total pore volume (PV) was derived from the amount of nitrogen adsorbed at a relative pressure close to unity (p/Po ) 0.995) by assuming that all the accessible pores were then filled with liquid nitrogen. Surface area measurements were performed using the BET,15 BJH,16 t-plot,17,18 and the D-R technique19 and compared with results obtained with DFT methods;12 PV and SA data is presented in Tables 1-4. In these tables, the ranges and reference curves used for the various methods were as follows: BET area, 0.02 to 0.3 P/P0; BJH psd, Harkins and Jura thickness curve; t-plot, 0.35 to 0.70 nm; thickness, using the Harkins and Jura thickness curve and D-R Plot ) 0.000001 to 0.001 P/P0 with β ) 0.32. Heats of ammonia adsorption were measured using a heatflow microcalorimeter from Setaram, linked to a glass volumetric line. Successive doses of gas were exposed to the sample until a final equilibrium pressure of 133 Pa was obtained. The equilibrium pressure relative to each adsorbed amount was measured by means of a differential pressure gauge from Datametrics. The adsorption temperature was maintained at (15) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (16) Barret, E. P.; Joyner, L. S.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373-380. (17) Zhu, H.; Vansant, E. J. Porous Mater. 1985, 2, 107. (18) Osinga, Th. J. J. Colloid Interface Sci. 1966, 21, 405-414. (19) Dubinin, M. M.; Radushkevich, L. V. Dokl. Akad. Nauk. SSSR 1947, 55, 331.
micro. SA
total SA
micro. PV
total PV
260
549 391 364 391
0.097
2.16
30 211
2.07 0.017 0.075
Total PV data measured at p/Po ∼ 0.995.
Table 4. Surface Area (SA, m2/g) and Pore Volume (PV, cm3/g) Results from Nitrogen Porosimetry Data for the Expanded Saponite Dried with Supercritical CO2 and Steam Aged for 5 h with 100% Steam at 760 °Ca method DFT BET BJH t-plot D-R a
micro. SA
total SA
micro. PV
total PV
185
457 345 441 345
0.072
1.56
* 196
1.57 * 0.070
Total PV data measured at p/Po ∼ 0.995. [*Negative value.]
150 °C. Primary and secondary isotherms were collected at this temperature. Similarly to the pretreatment used to prepare the samples for nitrogen porosimetry, all samples were degassed overnight under vacuum at 400 °C before calorimetric measurements were undertaken.
Results and Discussion The Density Functional Theory (DFT) Method. Several adsorption theories are available to extract SA and PV information from experimental adsorption isotherms. The Langmuir and BET formalisms are based on the model of adsorption on a free surface. The Langmuir model assumes the surface saturates after the first adsorbed layer, while the BET model presumes that multilayers can form at higher pressures. However, neither model allows for the filling of micropores. Surface areas derived from these two models will differ from the actual area in a way that depends on the solid microporous structure. In contrast, the application of the density functional theory approach treats experimental isotherms as the summation of sorbate uptake, at a given pressure, in pores of a range of sizes. As a result, the integral equation of isothermal adsorption for the case of distributed pore sizes can be written as the convolution:20
Q(p) )
∫dHq(p,H)f(H)
(1)
where Q(p) is the total quantity of adsorbate per gram of adsorbent at pressure p; q(p,H), the kernel function, in this case is the density functional description of the adsorption isotherm for an ideally homoporous material characterized by pore width H as quantity of adsorbate per square meter of pore surface, and f(H) is the desired pore surface area distribution function with respect to H.20 Since we are only interested in the numerical values of f(H), we can rewrite eq 1 as a summation.
Q(p) )
∑i q(p, Hi)f(Hi)
(2)
or in matrix notation Q ) qf where Q(p) is an experimental adsorption isotherm (20) Olivier, J. P. In Surfaces of nanoparticles and porous materials; Schwartz, J. A., Contescu, C. I., Eds.; Marcel-Dekker: New York, 1999; pp 295-318.
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Figure 2. Nitrogen sorption isotherms at 77 K showing the DFT fitted results for the saponite catalysts sorption isotherm. All samples have been degassed in vacuo at 400 °C prior to adsorption measurements.
Figure 1. SEM image of the synthetic saponite showing the distribution of particle sizes (top) and details of the particle surface topography (bottom).
interpolated onto a vector p of pressure points; q(p, Hi) is a matrix of values for quantity adsorbed per square meter, each row calculated for a value of H at pressure p, and f(Hi) is the solution vector whose terms represent the area of surface in the sample characterized by each pore width Hi. Using the set of hybrid models constructed as described previously12 as the function q(p, Hi) and the experimental adsorption isotherm for the function Q(p), eq 2 was solved for the distribution vector f(Hi); details of the method of procedure used are given elsewhere.20 The resulting distributions of pore area and pore volume as a function of pore width are shown in the figures below and elsewhere.12,21 The hybrid kernel function used here was developed using a natural montmorillionite as a reference surface12 rather than the basal plane of the present synthetic saponite. To the extent that these surfaces are different, we may expect some small error in our estimate of the pore width distribution, particularly in the micropore region. Nitrogen Porosimetry Results. The morphology of parent synthetic saponite (Sumecton SA) under study can be seen in the SEM image in Figure 1. SEM images show particles irregular in size and shape with diameter in approximately the 1-80 µm range; Figure 1 (top). At greater magnification, it is observed that these particles result from the agglomeration of wavy plates, Figure 1 (21) Occelli, M. L.; Olivier, J. P.; Auroux, A. J. Catal. 2002, 209, 385-393.
Figure 3. Nitrogen sorption isotherms at 77 K in the lowpressure region showing the DFT fitted results for the saponite catalysts sorption isotherm. All samples have been degassed in vacuo at 400 °C prior to adsorption measurements.
(bottom); initially, these granules have a DFT total surface area of 360 m2/g, 73.6% of which is located in micropores. However, only 32.8% of the clay total PV is in micropores; see Table 1. Phase impurities could not be observed by XRD10,11 nor in any SEM image generated. Sorption isotherms are in Figures 2-3. The isotherm in Figure 2 for the parent saponite closely resembles a type II isotherm. Following micropores saturation, nitrogen uptake monotonically increases with p/Po values because of sorption in the clay larger pores. After reaction with SiO2‚TiO2 colloidal particles and AD, the shape of the isotherm remains unchanged up to near p/Po ) 0.6 where it becomes sigmoidal owing to the filling of a few newly formed mesopores; see Figure 2. The slope of the step is very small indicating that the mesopores present have a wide distribution of pore widths. After mesopores filling, the uptake remains essentially invariant with p/Po values. When AD is replaced by drying with CO2 at supercritical conditions (SCD), the shape of the isotherm remains of type II. However, near p/Po ) 0.8, there is a rapid uptake increase because of nitrogen condensation in the sample’s larger mesopores, Figure 2. Surprisingly, steam aging (5 h with 100% steam at 760 C) decreases the expanded saponites sorption capacity without affecting the shape of its nitrogen sorption isotherm; see Figure 2
Mesoporous Expanded Clay Catalysts
Figure 4. Cumulative surface area distribution data obtained by fitting the DFT model for the saponite catalysts sorption isotherm. All samples have been degassed in vacuo at 400 °C prior to adsorption measurements.
and Tables 3-4. The same data is plotted on a semilog scale in Figure 3. Even in the very low-pressure region near 10-5 p/Po, nitrogen sorption occurs in all three saponites. Below a relative pressure of 10-3, the SCD sample shows lower uptakes of nitrogen than the AD samplesa result that can be attributed to the larger number of small micropores present in the AD sample. Above 10-3 p/Po, sorption in the SCD sample larger micropores cause a rapid uptake of nitrogen and the two isotherms overlap; see Figure 3. Steam aging seems to collapse the SCD sample smaller micropores; below 10-3 p/Po values, nitrogen sorption is lowest. In summary, the expansion reaction increases the saponite’s microporosity when drying is performed in the presence of water and air. However, when the liquid phase is removed with supercritical CO2, the saponite microporosity is reduced probably because of inhibition of face-to-face stacking of silicate layers;10-11 the SCD sample undergoes a further reduction in microporosity after steam aging; see Figure 3. The cumulative surface area data is in Figure 4. Both the parent saponite and the expanded AD samples contain micropores with pore width
