High-Resolution Adsorption of Nitrogen on Mesoporous Alumina

The standard nitrogen adsorption data for Aluminiumoxid C and R-alumina are presented .... activated alumina and one sample of aluminum oxide prepared...
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High-Resolution Adsorption of Nitrogen on Mesoporous Alumina Jirˇ´ı C ˇ ejka,* Nadeˇzˇda Zˇ ilkova´, Jirˇ´ı Rathousky´, and Arnosˇt Zukal J. Heyrovsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, 182 23 Prague 8, Czech Republic

Jacek Jagiello Quantachrome Instruments, Boynton Beach, Florida 33426 Received February 24, 2004. In Final Form: June 18, 2004 Nitrogen adsorption isotherms on organized mesoporous aluminas prepared by several different synthesis procedures were analyzed by means of comparative plot method using Aluminiumoxid C (Degussa) and R-alumina as reference adsorbents. To secure the high-resolution ability of this method, all the adsorption measurements were carefully performed in a relative pressure range from 10-6 to 0.99. Although some samples of organized mesoporous alumina were treated at temperatures up to 1000 °C, only the Aluminiumoxid C has proved to be suitable as a reference adsorbent. The comparative analysis of isotherms on activated aluminas has shown that this method allows the determination of very small amounts of microporosity. The standard nitrogen adsorption data for Aluminiumoxid C and R-alumina are presented in a tabulated form, which consists of 91 points for each adsorbent.

Introduction Organized mesoporous materials represent a class of molecular sieves having a high potential for commercial applications in adsorption and catalysis. Their regular pore arrangement is responsible for their extraordinary properties, such as high surface areas and narrow pore size distributions. Despite the enormous importance of high surface alumina materials for catalyst supports or adsorbents, the number of papers dealing with synthesis and characterization of organized mesoporous aluminas (OMAs) is still limited.1 This is mainly due to the fact that synthesis of OMA is still not as straightforward as synthesis of organized mesoporous silica. The development of a tailor-made characterization technique would facilitate the precise analysis of the structure of OMAs. Because no specific method for the determination of the internal porous structure is yet available, useful information is obtained by the combination of X-ray diffraction, transmission electron microscopy, and in particular nitrogen adsorption. Adsorption measurements are widely used because of their simplicity and convenience in providing meaningful information describing the surface properties of solid materials. It was shown that the nitrogen isotherms can be effectively analyzed using the comparative plot method. This method is based on a straightforward comparison between adsorption isotherms of the analyzed sample and an established reference material.2 However, the analysis of adsorption data can still be improved, particularly to analyze data obtained on new materials such as OMAs. To reliably assess the details of the structure of OMA using the comparative plot method, two conditions need to be fulfilled. First, to allow for the high-resolution analysis the measured adsorption data should start from * To whom correspondence should be addressed. E-mail: [email protected].

(1) C ˇ ejka, J. Appl. Catal., A 2003, 54, 237 and references therein. (2) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids; Academic Press: London, 1999.

very low relative pressures. The broad interval of relative pressures is necessary not only for the determination of the surface area and pore volume but also for a meaningful assessment of the surface heterogeneity. This fundamental feature of real solid surfaces can be caused by different crystal faces, local crystalline disorder, surface roughness, or the presence of impurities. However, most frequently it has its source in the presence of micropores, whose sizes are comparable with the diameter of adsorbate molecules. Second, adsorption data for a nonporous reference solid, on which adsorption occurs identically as on a flat open surface, have to be at the disposal. The standard isotherms of nitrogen or argon on different siliceous and carbonaceous adsorbents are available in the literature.3-6 However, the standard nitrogen isotherm measured in a wide range of relative pressures on nonporous alumina is still missing. Because aluminum oxide forms a variety of crystalline modifications, special attention has to be paid to the selection of a proper reference alumina material. Hightemperature transformations of OMAs revealed a significant difference between materials calcined at 420 °C and those calcined at 1000 °C.7 Whereas the former does not exhibit a clear crystalline structure, the latter consists of a packing of δ-alumina particles at least of 10 nm in size. Our preliminary work showed that Aluminiumoxid C from Degussa can be chosen as a suitable reference adsorbent for the assessment of porous structure parameters of OMAs.8 The comparative plot method was successfully applied in adsorption analysis of materials, which were prepared using carboxylic acids as structure directing agents and treated at 420-800 °C. (3) Kruk, M.; Jaroniec, M.; Gadkaree, K. P. J. Colloid Interface Sci. 1997, 192, 250. (4) Kruk, M.; Li, Z.; Jaroniec, M.; Betz, W. R. Langmuir 1999, 15, 1435. (5) Jaroniec, M.; Kruk, M.; Olivier, J. P. Langmuir 1999, 15, 5410. (6) Kruk, M.; Antochshuk, V.; Jaroniec, M.; Sayari, A. J. Phys. Chem. B 1999, 103, 10670. (7) C ˇ ejka, J.; Kooyman, P. J.; Vesela´, L.; Rathousky´, J.; Zukal, A. Phys. Chem. Chem. Phys. 2002, 4, 4823. (8) C ˇ ejka, J.; Zˇ ilkova´, N.; Rathousky´, J.; Zukal, A. Phys. Chem. Chem. Phys. 2001, 3, 5076.

10.1021/la049520o CCC: $27.50 © 2004 American Chemical Society Published on Web 07/28/2004

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Table 1. Structural Parameters of Aluminas under Studya sample code OMA/Pl/520 OMA/Tr114/600 OMA/Tr100/550 OMA/SA/420 OMA/SA/600 OMA/SA/800 OMA/SA/1000 R-alumina Aluminiumoxid C S-400 HDS M-Al2O3

SBET VTOT DME STOT VMI VME (m2/g) (cm3/g) (nm) (m2/g) (cm3/g) (cm3/g) 219.3 284.2 379.0 488.7 313.1 233.2 138.3 6.77 128.0 307.6 275.1 210.1

0.396 0.781 0.739 0.477 0.467 0.453 0.303

5.7 8.0 5.1 3.4 4.3 5.1 6.0

223.1 290.0 376.3 476.9 314.7 237.5 143.3

0.366 0.648 0.184

4.5 5.8 4.0

267.6 246.7 163.6

0.396 0.781 0.739 0.477 0.467 0.453 0.303 0.010 0.006 0.012

0.356 0.642 0.173

aS BET, STOT, VTOT, VMI, and VME denote BET surface area, surface area determined using the comparative plot method, total pore volume, micropore volume, and mesopore volume, respectively. DME stands for pore diameter corresponding to the maximum of the pore size distribution.

Table 2. Nonionic Surfactants Used in the Syntheses of OMA surfactant

formulaa (average)

Pluronic PE 10400 Triton X-100 Triton X-114

(EO)25(PO)56(EO)25 (C8H17)C6H4(EO)10 (C8H17)C6H4(EO)8

a EO and PO denote ethylene oxide and propylene oxide units, respectively.

In this contribution, we report on a high-resolution nitrogen adsorption study at -196 °C on OMAs prepared by several different synthesis procedures. The nitrogen isotherms were analyzed by means of the comparative plot method using Aluminiumoxid C as a reference adsorbent. To investigate the influence of the heat treatment on the porous structure, the OMA prepared with stearic acid as a structure director was treated up to 1000 °C. With respect to this sample, the nitrogen isotherm on the high-temperature treated nonporous R-alumina was used as a reference. To secure the high resolution, all the adsorption measurements were carefully performed in a relative pressure range from 10-6 to 0.99. The standard nitrogen adsorption data for Aluminiumoxid C and R-alumina, presented in a tabulated form, represent a significant extension of previously reported data toward low relative pressures.8 In addition to OMAs, two typical samples of commercial activated alumina and one sample of aluminum oxide prepared by thermal treatment of boehmite were characterized by nitrogen adsorption over a wide range of relative pressures. The application of comparative plots on these aluminas was shown to be suitable for the estimation of small amounts of microporosity. Experimental Section Reference Nonporous Aluminas. The R-alumina was prepared by high-temperature treatment (1200 °C, 10 h) of pseudoboehmite tablets (Pural SB from Condea Chemie, Germany). The Aluminiumoxid C was purchased from Degussa (Germany). The BET (Brunauer-Emmett-Teller) surface areas of both materials are given in Table 1. OMAs. Alumina OMA/Pl/520 was prepared using triblock copolymer Pluronic PE 10400 as the structure directing agent. (The formulas of nonionic surfactants used in the syntheses are given in Table 2.) In the typical procedure, 2.7 g of Pluronic PE 10400 was dissolved in 50 mL of distilled water; 3.85 g of aluminum chlorhydrol was added to this solution. Later on, 1.5 mL of a 25 wt % water solution of NH4OH in 10 mL of water was

added. The reaction mixture was stirred for 12 h at 60 °C and aged at room temperature without stirring for another 24 h. The solid product was recovered by filtration, washed with ethanol, and dried at 60 °C overnight. The template was removed by calcination carried out at 120 °C (2 h), 240 °C (4 h), and 520 °C (6 h) with a temperature ramp of 1 °C/min. Alumina OMA/Tr114/600 was kindly provided by Prof. J. Pe´rez-Pariente (Instituto de Cata´lisis y Petroleoquı´mica, Madrid) and prepared by hydrolysis of aluminum tri-sec-butoxide using a water/alkoxide molar ratio of 2 in a solution of Triton X-114 in 1,4-dioxane. The sample was calcined at 600 °C. Alumina OMA/Tr100/550 was synthesized using Triton X-100 as the structure directing agent. This surfactant was mixed with 36.6 g of acetonitrile and stirred until dissolved. Then, 2.6 mL of a 25 wt % water solution of NH4OH was added drop by drop under vigorous stirring, followed by addition of 4.86 g of aluminum tri-sec-butoxide. The resulting reaction mixture was stirred for 24 h at ambient temperature. Recovery and calcination were performed by the same procedure as for OMA/Pl/520, except the final temperature of calcination was 550 °C. The preparation procedure of OMA/SA/420 was as follows: 5.1 g of stearic acid (Aldrich) was dissolved in 100 mL of 1-propanol (Fluka). After the addition of 3.1 mL of water, the solution was stirred for 30 min. Finally, 13.7 g of aluminum tri-sec-butoxide (Aldrich) was added and the reaction mixture was stirred for another 20 min. The prepared gel was aged at 100 °C for 50 h in a Teflon lined autoclave under static conditions. After cooling the autoclave, the product was recovered by filtration, washed with ethanol, and dried at 50 °C overnight. The as-synthesized material was calcined at 410 °C in a stream of nitrogen and then at 420 °C in air; this sample was denoted as OMA/SA/420. Additional calcination of OMA/SA/420 for 6 h in air under static conditions at 600, 800, and 1000 °C with a temperature ramp of 5 °C/min led to the samples denoted OMA/SA/600, OMA/SA/800, and OMA/SA/1000, respectively. All described mesoporous aluminas are listed in Table 1. They are coded by abbreviations of the surfactant used (Pl, Tr100, Tr114, and SA stand for Pluronic PE 10400, Triton X-100, Triton X-114, and stearic acid, respectively) and temperature of the final heat treatment. Activated Aluminas. Material S-400 consists of smooth spheres of activated alumina with high Claus reaction activity. It was obtained from ALCOA (U.S.A.). Alumina extrudates HDS were obtained from AKZO Nobel (The Netherlands). Well-defined boehmite was prepared via thermal decomposition of gibbsite (Aldrich) in vacuo at 200 °C for 50 h. Sample M-Al2O3, which contains some amount of micropores, was prepared by further thermal treatment at 450 °C in air for 5 h. Methods. Adsorption isotherms of nitrogen at -196 °C were measured on two volumetric adsorption instruments. The sorption analyzer for measurements in the range of the lowest equilibrium pressures was equipped with 1000, 10, and 1 Torr transducers, which allow for high accuracy of adsorbed amount determination. Experimental details are given elsewhere.8 Prior to the adsorption measurements, the samples were degassed at 350 °C for 24 h. Powder X-ray diffraction patterns were collected on a Siemens D 5005 diffractometer in the Bragg Brentano geometry arrangement with Cu KR radiation.

Results and Discussion Nitrogen Isotherms on Reference Aluminas. The measurements of nitrogen adsorption isotherms on the reference aluminas were repeated twice using the same adsorption instrument; each isotherm comprised 95 experimental points. The standard nitrogen adsorption isotherms were obtained by the interpolation of experimental data for a desired set of pressure values without any smoothing. These data are reported in Tables 3 and 4 and shown in linear and logarithmic coordinates in Figure 1. Interpolated data consist of 91 points, which cover the range of relative pressures p/p0 from 10-6 to 0.95. The adsorbed amount is expressed in micromoles of nitrogen per square meter of the surface area. If we express

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Table 3. Standard Data for the Adsorption of Nitrogen at -196 °C on r-Alumina p/p0

aR-alumina (µmol/m2)

1.0 × 10-6 1.5 × 10-6 2.0 × 10-6 3.0 × 10-6 4.0 × 10-6 6.0 × 10-6 8.0 × 10-6 1.0 × 10-5 1.5 × 10-5 2.0 × 10-5 3.0 × 10-5 4.0 × 10-5 6.0 × 10-5 8.0 × 10-5 1.0 × 10-4 1.5 × 10-4 2.0 × 10-4 3.0 × 10-4 4.0 × 10-4 6.0 × 10-4 8.0 × 10-4 1.0 × 10-3 0.0015 0.002 0.003 0.004 0.005 0.006 0.008 0.010 0.012

0.568 0.636 0.704 0.803 0.899 1.035 1.143 1.199 1.331 1.433 1.638 1.779 1.991 2.107 2.213 2.432 2.577 2.821 3.004 3.218 3.411 3.586 3.894 4.128 4.423 4.703 4.932 5.128 5.395 5.619 5.775

p/p0

aR-alumina (µmol/m2)

p/p0

aR-alumina (µmol/m2)

0.014 0.016 0.018 0.020 0.025 0.030 0.035 0.040 0.045 0.050 0.055 0.060 0.065 0.070 0.075 0.080 0.085 0.090 0.095 0.100 0.120 0.140 0.160 0.180 0.200 0.220 0.240 0.260 0.280 0.300

5.932 6.088 6.245 6.401 6.734 7.067 7.360 7.652 7.867 8.083 8.274 8.464 8.646 8.827 8.969 9.111 9.287 9.462 9.636 9.810 10.393 10.883 11.400 11.862 12.352 12.787 13.168 13.512 13.930 14.283

0.320 0.340 0.360 0.380 0.400 0.420 0.440 0.460 0.480 0.500 0.525 0.550 0.575 0.600 0.625 0.650 0.675 0.700 0.725 0.750 0.775 0.800 0.825 0.850 0.875 0.900 0.915 0.930 0.940 0.950

14.637 14.937 15.290 15.617 15.997 16.378 16.705 17.086 17.439 17.820 18.351 18.881 19.439 19.997 20.637 21.276 22.011 22.745 23.575 24.404 25.424 26.445 27.942 29.438 31.873 34.308 36.282 38.516 40.471 42.143

the adsorbed amount as a surface coverage θ ) a/am, where am of 10.25 µmol/m2 is the capacity of the nitrogen monolayer adsorbed on the area of 1 m2, the lower limit of this range corresponds to θ of 0.02 for Aluminiumoxid C and θ of 0.05 for R-alumina. The upper limit of the relative pressure range corresponds to θ of ∼4 for both materials. To compare nitrogen isotherms on reference aluminas, the amount adsorbed on the Aluminiumoxid C was replotted against the amount adsorbed on the R-alumina at the corresponding p/p0 (Figure 2). One can observe that adsorption on Aluminiumoxid C is a little lower than that on R-alumina up to aR-alumina ) ∼2 µmol/m2 (see inset). By contrast, adsorption on Aluminiumoxid C is higher than that on R-alumina for aR-alumina between ∼2 and ∼13 µmol/ m2. The isotherms on Aluminiumoxid C and R-alumina agree very well for adsorbed amounts higher than ∼13 µmol/m2, that is, for the surface coverage θ > ∼1.3. The shape of the isotherm in the monolayer region is especially sensitive to any variation in the surface structure of the solid. Therefore, the observed differences between nitrogen isotherms can be understood as a result of the different natures of the surface heterogeneity of the reference aluminas. Surface heterogeneity related to the adsorption properties of materials is usually described in terms of the adsorption energy distribution (AED), which can be calculated from adsorption data. The AED is calculated by solving the adsorption integral equation in which a certain adsorption model is assumed. A significant amount of work was devoted to the analysis of surface heterogeneity and to the analytical and numerical methods of solving this equation.9,10 Here, the BET adsorption model (9) Jaroniec, M.; Madey, R. Physical Adsorption on Heterogeneous Solids; Elsevier: Amsterdam, 1988. (10) Rudzinski, W.; Everett, D. H. Adsorption of Gases on Heterogeneous Solid Surfaces; Academic Press: London, 1992.

Table 4. Standard Data for the Adsorption of Nitrogen at -196 °C on Aluminiumoxid C p/p0

aAluminiumoxid C (µmol/m2)

1.0 × 10-6 1.5 × 10-6 2.0 × 10-6 3.0 × 10-6 4.0 × 10-6 6.0 × 10-6 8.0 × 10-6 1.0 × 10-5 1.5 × 10-5 2.0 × 10-5 3.0 × 10-5 4.0 × 10-5 6.0 × 10-5 8.0 × 10-5 1.0 × 10-4 1.5 × 10-4 2.0 × 10-4 3.0 × 10-4 4.0 × 10-4 6.0 × 10-4 8.0 × 10-4 1.0 × 10-3 0.0015 0.002 0.003 0.004 0.005 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 0.025 0.030 0.035 0.040 0.045 0.050 0.055 0.060 0.065 0.070 0.075

0.185 0.277 0.355 0.481 0.582 0.743 0.868 0.973 1.178 1.336 1.575 1.756 2.029 2.235 2.402 2.719 2.957 3.309 3.571 3.956 4.242 4.471 4.990 5.238 5.714 6.017 6.264 6.473 6.817 7.096 7.333 7.540 7.723 7.888 8.039 8.369 8.648 8.891 9.108 9.303 9.481 9.645 9.797 9.940 10.073 10.199

p/p0

aaluminiumoxid C (µmol/m2)

0.080 0.085 0.090 0.095 0.100 0.120 0.140 0.160 0.180 0.200 0.220 0.240 0.260 0.280 0.300 0.320 0.340 0.360 0.380 0.400 0.420 0.440 0.460 0.480 0.500 0.525 0.550 0.575 0.600 0.625 0.650 0.675 0.700 0.725 0.750 0.775 0.800 0.825 0.850 0.875 0.900 0.915 0.930 0.940 0.950

10.318 10.431 10.539 10.642 10.741 11.091 11.442 11.793 12.144 12.495 12.846 13.198 13.549 13.900 14.251 14.602 14.953 15.304 15.655 16.006 16.357 16.708 17.174 17.513 17.864 18.322 18.805 19.315 19.859 20.441 21.068 21.748 22.493 23.315 24.233 25.272 26.465 27.862 29.540 31.622 34.330 36.423 39.068 41.288 44.063

was assumed and the SAIEUS numerical program11 was used to evaluate the AEDs of R-alumina and Aluminiumoxid C from the interpolated data in the relative pressure range from 10-6 to 0.35. In the BET equation, the adsorption energy E of surface sites is related to the C constant

C ) exp[(E - EL)/RT]

(1)

where EL is the heat of condensation, R is the gas constant, and T is the temperature. The net molar energy of adsorption E - EL (the term recommended in ref 2) then is

E - EL ) RT ln C

(2)

The adsorption integral equation takes the following form for this model:9,10

φ(ln C) d ln C ∫0∞ (1 - x)[1 Cx + (C - 1)x]

a(x) ) aN

(11) Jagiello, J. Langmuir 1994, 10, 2778.

(3)

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Figure 1. Standard nitrogen adsorption isotherms on R-alumina (+) and Aluminiumoxid C (O) obtained by the interpolation of experimental data.

Figure 3. Fit of the experimental data on R-alumina (points) in the relative pressure range from 10-6 to 0.35 by eq 3 (solid line) and by the BET equation (dotted line).

Figure 2. Comparative plot for Aluminiumoxid C calculated using R-alumina as a reference adsorbent. The inset shows the initial section of this plot.

Figure 4. Fit of the experimental data on Aluminiumoxid C (points) in the relative pressure range from 10-6 to 0.35 by eq 3 (solid line) and by the BET equation (dotted line).

where the fraction represents the BET adsorption isotherm, x is the relative pressure, aN is the normalization constant, and φ is the differential distribution of surface area S among the corresponding values of the constant C

prominent peak of the AED of R-alumina is larger than that of Aluminiumoxid C and is slightly shifted toward lower adsorption energy. A small fraction of the R-alumina surface with an average net molar energy of adsorption of 9.0 kJ/mol (corresponding peak with maximum at ln C ) ∼14) gives rise to higher adsorption at very low surface coverage in comparison with Aluminiumoxid C. On the other hand, relatively large fractions of the Aluminiumoxid C surface with the average net molar energy of adsorption of 5.1 and 7.7 kJ/mol (corresponding peaks with maxima at ln C ) ∼8 and ∼12, respectively) cause the higher adsorption on Aluminiumoxid C than that on R-alumina in the range of aR-alumina between ∼2 and ∼13 µmol/m2. In contrast to Aluminiumoxid C, the fraction of the R-alumina surface with comparable net molar energy adsorption (ln C between 5 and 12) is relatively small. When the affinity of the adsorbate for the surface of the solid is such that the constant C of the BET equation is smaller than 1000, the adsorption in the second layer starts effectively even before the first layer is completed. The C values, which correspond to the main peaks of the AED of R-alumina and Aluminiumoxid C, are equal to 27 and

φ(ln C) ) dS/d(ln C)

(4)

The function φ(ln C) is normalized to unity. Results of the data analysis based on eq 3 are shown in Figures 3 and 4. For comparison, plots calculated from the BET equation using parameters obtained from its linear form in the traditional pressure range of 0.05-0.35 p/p0 are also included. It is seen that eq 3 quantitatively represents the experimental data while the BET equation dramatically underestimates adsorption in the range of lower pressures. This discrepancy is due to the fact that the classical BET approach accounts only for adsorption on energetically uniform surface sites. Calculated AEDs are presented in Figure 5. It is obvious that the R-alumina, which was made by high-temperature treatment of the oxyhydroxide precursor, represents a less heterogeneous adsorbent than Aluminiumoxid C. The

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Figure 5. AEDs for R-alumina (a) and Aluminiumoxid C (b).

Figure 6. Nitrogen adsorption isotherms for samples OMA/ Pl/520 (O), OMA/Tr114/600 (0), and OMA/Tr100/550 (4). Except for that on sample OMA/Pl/520, isotherms are shifted by 5 mmol/g each. Solid symbols denote desorption.

90, respectively. For this reason, the surface heterogeneity influences the shape of the nitrogen isotherms on reference aluminas not only in the monolayer region but also up to a surface coverage θ ) ∼1.3 or correspondingly up to a relative pressure p/p0 ) ∼0.25. Above the relative pressure p/p0 of ∼0.25, where multilayer adsorption takes place exclusively, the energetic heterogeneity of the surface of nonporous alumina does not play a decisive role. Therefore, at p/p0 > 0.25 the shape of the nitrogen isotherm on mesoporous alumina depends particularly on the surface area, pore volume, and pore size distribution. Nitrogen Isotherms on OMAs. Nitrogen isotherms on the OMAs are shown in Figures 6 and 7. The BET surface areas and total pore volumes of these materials are listed in Table 1. The BET surface areas were calculated using data in a relative pressure range from 0.05 to 0.25. The total pore volume was determined from the amount adsorbed at a relative pressure of about 0.99. The distribution of mesopores was calculated from the desorption branch of the nitrogen isotherm using the Barrett-Joyner-Halenda method12 with the statistical film thickness curve derived from the isotherm on Alu(12) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373.

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Figure 7. Nitrogen adsorption isotherms for samples OMA/ SA/420 (O), OMA/SA/600 (0), OMA/SA/800 (4), and OMA/SA/ 1000 (3). Solid symbols denote desorption.

Figure 8. Mesopore size distribution for samples OMA/Pl/520 (a), OMA/Tr114/600 (b), OMA/Tr100/550 (c), OMA/SA/420 (d), OMA/SA/600 (e), OMA/SA/800 (f), and OMA/SA/1000 (g).

miniumoxid C. The resulting distribution curves are shown in Figure 8. The mesopore diameters corresponding to the maximum of these curves are given in Table 1. The structure parameters (Table 1) and particularly the pore size distributions (Figure 8) evidence the strong influence of the composition of the reaction mixture and the synthesis procedure on the properties of samples OMA/Pl/520, OMA/Tr/114/600, OMA/Tr100/550, and OMA/SA/420. The samples OMA/Tr100/550 and OMA/ SA/420 prepared from aluminum-sec-butoxide using Triton X-100 and stearic acid as structure directing agents, respectively, are characterized by a narrow pore size distribution. On the other hand, the use of Triton X-114 surfactant has resulted in a broad pore size distribution (OMA/Tr/114/600). In the case of the sample OMA/Pl/ 520, prepared from aluminum chlorhydrol and Pluronic PE 10400, the pore size distribution is also very broad. The samples OMA/SA/600, OMA/SA/800, and OMA/ SA/1000 show the important role of the heat treatment, which leads to a decrease in the surface area and pore volume and an increase in the size of mesopores (Table 1, Figure 8). This coarsening of alumina mesoporous structure is connected with an increase in the alumina

Adsorption of Nitrogen on Mesoporous Alumina

Figure 9. X-ray diffractograms of samples OMA/SA/420 (a), OMA/SA/600 (b), OMA/SA/800 (c), and OMA/SA/1000 (d). Diffractograms are shifted.

crystallinity. Whereas the diffractogram OMA/SA/420 exhibits no clear reflections, the diffractograms of OMA/ SA/600 and OMA/SA/800 indicate the increase in the extent of the crystallinity, and the diffractogram of OMA/SA/1000 reveals the presence of δ-Al2O3 (Figure 9). The transmission electron microscopy examination of the sample OMA/SA/1000 evidences δ-Al2O3 nanocrystals of about 10 nm in size. The possible microporosity of OMA cannot be a priori excluded. It is well-known that templated mesoporous silicas such as SBA-15 can contain some amount of micropores.13 It was suggested that the silica framework, which was templated by EOmPOnEOm triblock copolymers, may be penetrated by ethylene oxide (EO) blocks. Thus, the calcined material is likely to exhibit complementary micropores in the parts of the framework where the EO blocks were once located. It is also important to note that although nonionic surfactants are characterized by a certain formula and average molecular weight, they are known to be polydisperse mixtures, which contain some amount of different impurities. Some of these impurities may act as a structure directing agent for some disordered domains containing micropores. The above-mentioned possibilities of the formation of additional microporosity can be operative in the syntheses of OMAs templated with alkyl-aryl poly(ethylene oxide) surfactants (such as Triton X-100 or Triton X-114) and triblock copolymers (such as Pluronic PE 10400). The calcination at relatively low temperatures could result in the presence of micropores, which were not eliminated or blocked by a framework condensation upon calcination. On the other hand, the high-temperature treatment of OMA might also lead to the formation of micropores, because these pores can be created as interstices among δ-alumina nanocrystals. The comparative analysis of the nitrogen isotherms has proved to be an effective tool to evidence the presence or absence of micropores in OMA. Figures 10 and 11 show the high-resolution comparative plots for the OMA samples under study. The isotherm of Aluminiumoxid C was used as the reference. The low pressure part of these plots exhibits excellent linearity starting from the lowest adsorbed amounts; the subsequent steep increase is caused by the capillary condensation of nitrogen in mesopores.

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Figure 10. High-resolution comparative plots for samples OMA/Pl/520 (O), OMA/Tr114/600 (0), and OMA/Tr100/550 (4). The plots for samples OMA/Tr114/600 and OMA/Tr100/550 are shifted by 5 mmol/g each.

Figure 11. High-resolution comparative plots for samples OMA/SA/420 (O), OMA/SA/600 (0), OMA/SA/800 (4), and OMA/ SA/1000 (3). The inset shows the initial section of the comparative plot for sample OMA/SA/1000; this plot was constructed using R-alumina as the reference adsorbent.

This behavior is identical with that observed for siliceous purely mesoporous materials (such as MCM 41 or MCM 48) and convincingly proves the absence of a detectable amount of micropores in all the OMAs under study. The slope of the linear part of the comparative plot gives the total surface area STOT of OMA. The excellent agreement of these values, listed in Table 1, with the BET surface area again excludes the presence of micropores in all the studied samples. The degree of crystallinity of solids may have a considerable impact on their adsorption properties. This is evident for porous carbons, whose adsorption properties change dramatically with the change in the degree of graphitization.2,14 For this reason, graphitized carbons (13) Kruk, M.; Jaroniec, M.; Ko, C. H.; Ryoo, R. Chem. Mater. 2000, 12, 1961. (14) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982.

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that activated aluminas contain micropores whose adsorption capacity ami can be determined by a linear fit:

a ) STOT aAluminiumoxid C/1000 + ami ) STOT aAluminiumoxid C/1000 + VMI/vm (5)

Figure 12. Nitrogen adsorption isotherms for samples M-Al2O3 (O), S-400 (0), and HDS (4). Except for that on sample M-Al2O3, isotherms are shifted by 5 mmol/g each. Solid points denote desorption.

where a (mmol/g) and aAluminiumoxid C (µmol/m2) are the amounts adsorbed on activated alumina and the reference adsorbent, respectively. STOT is the total surface area, VMI is the micropore volume, and vm (cm3/mmol) is the molar volume of liquid nitrogen at 196 °C. The micropore volumes of activated aluminas, determined by means of this linear fit, are listed in Table 1. The results of BET calculations for microporous materials are inherently inaccurate, because the mechanism of adsorption in micropores (micropore filling) is different from the adsorption on a flat surface (multilayer adsorption). The BET method can be formally applied to nitrogen isotherms on micro-mesoporous material provided that the micropore filling contribution to the total amount adsorbed is not significant. As micropores are filled with nitrogen molecules at equilibrium pressures below the lower limit of the BET equation validity, the BET monolayer capacity comprises both the monolayer on the mesopore surface and the amount of nitrogen, which fills the micropore volume. The apparent surface area SAPP (m2/g) corresponding to the micropores is

SAPP ) LσaMI ) LσVMI/vm ) 2804VMI

Figure 13. High-resolution comparative plots for samples samples M-Al2O3 (O), S-400 (0), and HDS (4). The plots for samples S-400 and HDS are shifted by 1 mmol/g each. The inset shows the initial sections of these plots.

are not suitable as reference adsorbents for adsorption studies of nongraphitized carbons and vice versa. With respect to the increase in the crystallinity of the sample OMA/SA/1000, the application of Aluminiumoxid C as the reference adsorbent does not have to be appropriate. However, the linearity of the comparative plot for the OMA/SA/1000 with Aluminiumoxid C shows that the increase in the crystallinity of this sample does not influence the energetic heterogeneity of its surface. The inset in Figure 11 proves the impropriety of the application of R-alumina as the reference material in this case. Nitrogen Isotherms on Activated Aluminas. Nitrogen isotherms on activated aluminas are presented in Figure 12. The structure parameters are given in Table 1. Comparative plots obtained by using Aluminiumoxid C as the reference adsorbent are shown in Figure 13. In contrast to OMA, these plots show negative deviations from linearity in the region of aAluminiumoxid C < 4 µmol/m2 (see inset in Figure 13). This fact clearly demonstrates

(6)

where L is the Avogadro constant and σ ) 0.162 nm2 is the average area occupied by each nitrogen molecule. The apparent surface area SAPP calculated by eq 6 equals 28.0, 16.8, and 30.8 m2/g for samples S-400, HDS, and M-Al2O3, respectively. Therefore, taking into account the presence of micropores, an approximate agreement between SBET and STOT is achieved. An upward swing in the comparative plot indicates the presence of pores in which capillary condensation of nitrogen occurs. The irreversible capillary condensation takes place in the region of relative pressures above the lower closure point of the hysteresis loop, that is, above p/p0 ) 0.4 or correspondingly above aAluminiumoxid C ) 16 µmol/m2 (see Table 4). Such a swing, which is displayed by the comparative plots for OMAs in Figures 10 and 11, confirms the mesoporous nature of these materials. On the other hand, the comparative plots for activated aluminas show an upward swing commencing at aAluminiumoxid C ) 11 µmol/m2 (Figure 13), corresponding to relative pressure p/p0 of 0.12 and, therefore, well below the lower closure point of the hysteresis loop. This enhanced adsorption proves the presence of pores smaller than 3.5 nm, in which reversible capillary condensation occurs.12 Hence, the pore size distribution of activated aluminas is wider than that of OMAs. It has to be pointed out that the analysis of deviations of the comparative plot from linearity due to presence of micropores, ultramicropores, or very small mesopores is possible only on the basis of the high-resolution comparative analysis of nitrogen isotherms performed in the low relative pressure range. Conclusions Reported standard nitrogen adsorption isotherms on nonporous Aluminiumoxid C (Degussa) and R-alumina at -196 °C cover the relative pressure range from 10-6 to 0.99. These data represent a significant extension of the previous work8 in the region of low relative pressures,

Adsorption of Nitrogen on Mesoporous Alumina

which enabled the examination of energetic heterogeneity of both materials. The obtained AEDs have proved that R-alumina represents a less heterogeneous adsorbent than Aluminiumoxid C. The standard adsorption data on Aluminiumoxid C were used to construct the high-resolution comparative plots for OMAs. These materials have been proved not to contain any detectable amount of micropores. The analysis of nitrogen isotherms on activated aluminas has shown that the high-resolution comparative plots allow for the estimation of very small amounts of microporosity or a

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presence of pores, in which reversible capillary condensation occurs. Acknowledgment. This work was carried out with the financial support by the Grant Agency of the Academy of Sciences of the Czech Republic (A4040411), Grant Agency of the Czech Republic (104/02/0571), and NATO in the framework of “Science for Peace” (SfP974 217). LA049520O