J O U R N A L O F C H E M I S T R Y Materials Gas adsorption properties of mesoporous c-alumina prepared by a selective leaching method Kiyoshi Okada,* Takahiro Tomita and Atsuo Yasumori Department of Inorganic Materials, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, Japan. E-mail: kokada@o.cc.titech.ac.jp Received 7th August 1998, Accepted 27th September 1998 Gas adsorption properties of mesoporous c-Al2O3 prepared by the selective leaching method were investigated using various adsorbate gases.The mesoporous c-Al2O3 was prepared by calcining kaolinite to form a microtexture of fine c-Al2O3 grains dispersed in a matrix of amorphous SiO2 followed by selective leaching of amorphous SiO2 from the microtexture. The mesopores were formed inside the pseudomorphic hexagonal platy particles of kaolinite.The specific surface area, total pore volume and pore size measured at 77 K using N2 gas were ca. 240 m2 g-1, 0.7 ml g-1 and 6 nm, respectively. The gas adsorption isotherms of polar molecules such as water, methanol and butan-1-ol showed type IV isotherms (IUPAC classification) while those of non-polar molecules such as cyclohexane showed type V isotherms.The c-Al2O3 was therefore recognized as having hydrophilic surface characteristics. The onset of adsorption of these gases in the low relative pressure (P/P0) region was methanol>water>butan-1-ol>cyclohexane, this order corresponding to the aYnity between the adsorbate gas and surface of the c-Al2O3 and also reflecting the steric eVect of adsorbate gas.The fractal dimension obtained from the BET monolayer adsorption capacity plot was 2.1, indicating a smooth surface. The gas adsorption properties of the c-Al2O3 are discussed and compared with commercial c-Al2O3 and Al2O3–SiO2 gel. H2O=13.95 mass%. A small amount of TiO2 impurity was Introduction present as anatase and rutile in the sample. The starting Since c-Al2O3 has a number of superior properties as a porous material was calcined at 950 °C for 24 h using heating and material, it has been widely used in industry as a catalyst, cooling rates of 15 and 20 °Cmin-1, respectively.The calcined catalyst support and adsorbent. In order to improve the porous material (3 g) was dispersed in 250 ml of aqueous 4 M KOH properties of c-Al2O3, many preparation methods1–5 have been at 90 °C and stirred for 1 h.After leaching, the sample was reported which achieve enhancement of the properties by washed with 0.5 M KOH and then washed a further three controlling the primary particle size. Various methods have times with deionized water to remove dissolved ions. The also been used to control the microtexture of c-Al2O3 aggre- suspension was then centrifuged and the resulting powder gates.We have reported a new preparation method of c-Al2O3 dried at 110 °C overnight. For comparison, commercially from the clay mineral kaolin [Al2(OH)4Si2O5] by a selective available high purity c-Al2O3 (AKP-G015, Sumitomo leaching method,6 in which kaolin is calcined at ca. 1000 °C Chemicals) and Al2O3–SiO2 gel (Neobead DS-5, Mizusawa and converted into a mixture of c-Al2O3 and amorphous silica.Chemicals) were used as reference materials. Since the calcined kaolin retains pseudomorphic particles of The phases formed in the samples were studied by powder the original kaolin with a microstructure consisting of a X-ray diVraction (XRD) using a Rigaku Geigerflex nanocomposite of very fine c-Al2O3 grains several nm in size diVractometer with monochromated Cu-Ka radiation.The uniformly dispersed in a matrix of amorphous SiO2, meso- chemical compositions were determined by X-ray fluorescence porous c-Al2O3 can be prepared by selective leaching of the (XRF) using Rigaku RIX2000 and RIX3000 spectrometers. amorphous SiO2 from the pseudomorph particles. As a result, The microtexture of the samples was observed by field-emission the fine c-Al2O3 grains which remain in the microtexture are scanning electron microscopy (FE-SEM) using a JEOL JCMinter- connected by residual amorphous SiO2 and form meso- 890S instrument.The specific surface area was measured by pores in the pseudomorph particles. Since the c-Al2O3 prepared the BET method9 using nitrogen gas as adsorbate at -196 °C by the selective leaching method has such a unique microtex- with a Quanta Chrome Autosorb-1 instrument.The pore size ture, it retains a high surface area at high temperatures, i.e., distribution was calculated in the radius range from 1 to it has high thermal stability7 and shows unique and interesting 100 nm by the BJH method10 using the desorption isotherm. water vapor adsorption properties.8 Gas adsorption–desorption isotherms were measured at 25 °C The present paper examines the gas adsorption properties by automatic gas adsorption apparatus using a Japan Bel of c-Al2O3 prepared by the selective leaching method using Belsorp 18.The adsorbates used were deionized water, a adsorbate gases with various molecular shapes and diVerent special grade of methanol (Shuzui Kitaro Co.), butan-1-ol polarities.The results are compared with commercial c-Al2O3 (Wako Pure Chemicals) and cyclohexane (Wako Pure and Al2O3–SiO2 gel to elucidate surface properties of the Chemicals). They were pre-treated using zeolite 3A (Wako c-Al2O3. Pure Chemicals) to remove any contaminating water. The alumina samples were evacuated at 300 °C for >4 h until the pressure in the vessel dropped below 5×10-3 Torr.The dur- Experimental ation of each measurement point was set at 1000 s. The measured relative pressure (P/P0) range was 0–0.9 in the Kaolinite from Georgia, USA, was used as the starting adsorption and 0.9–0.1 in the desorption experiments except material. As listed in Table 1, the chemical composition of this for the methanol runs, in which the measurements were made starting material is close to the ideal composition of kaolinite [2SiO2·Al2O3·2H2O], i.e.SiO2=46.51, Al2O3=39.54 and only to P/P0=0.7 because of limitations of the apparatus. J. Mater. Chem., 1998, 8, 2863–2867 2863Table 1 Chemical composition and porous properties of samples Chemical composition (mass%) Sample Al2O3 SiO2 TiO2 K2O Fe2 O3 Surface area/m2 g-1 Pore volume/ml g-1 Original kaolinite 38.3 45.2 1.44 0.11 0.76 9.9 0.09 Selectively leached c-Al2O3 84.0 6.8 5.5 1.4 1.0 244 0.72 c-Al2O3 (AKP-G015) 100 0 0 0 0 139 1.22 Al2O3–SiO2 gel (DS-5) 70.5 24.5 0 0 0 313 0.51 Results and discussion Characterization of the samples As reported in previous papers,5–7 the calcined and KOH leached sample has mesopores of uniform pore size of ca. 3 nm radius. The porous properties and chemical composition of the sample prepared in the present study are listed in Table 1 and are in good agreement with those reported elsewhere. 5–7 Fig. 1 shows the pore size of this sample lies in a narrow distribution around 3 nm radius. The data for the reference materials of commercially available c-Al2O3 and Al2O3–SiO2 gel are also given in Table 1 and Fig. 1, and show diVerent pore sizes from the present c-Al2O3. The pore size of the reference c-Al2O3 (AKP-G015) is larger than those of the other two samples and the pore size distribution is relatively broad. The pore size of the Al2O3–SiO2 gel (DS-5) is smaller than those of the other samples, but with a rather broad bimodal size distribution consisting of a relatively sharp peak at ca. 1.9 nm radius and very broad peak at ca. 3–4 nm. The order of specific surface areas of these samples was AKPG015< present c-Al2O3<DS-5, this order being correlated with the pore size. On the other hand, the total pore volumes of these samples showed the opposite trend with specific surface area. The XRD pattern of the calcined sample showed broad peaks assigned to c-Al2O3 and a halo arising from amorphous SiO2 which disappeared after leaching.Corresponding to this XRD pattern change, the chemical composition revealed a reduction of SiO2 upon leaching (Table 1). The microtextures of the original and leached samples were observed by FESEM (Fig. 2). Photographs show that the sample preserves the hexagonal platy particle shape of the original kaolinite even after calcination and leaching, but with apparent changes in the surface of pseudomorphic particles.Very fine grains of c-Al2O3 were observed on the surface of the particles, shown Fig. 2 FE-SEM photographs of the selectively leached c-Al2O3: (a) original sample, (b) leached sample and (c) leached sample with high magnification.in Fig. 2(b) and (c) to be several nm in size. The spaces between the fine c-Al2O3 grains formed by selective leaching of amorphous SiO2 from the matrix correspond to the meso- Fig. 1 Pore size distributions of the present c-Al2O3 prepared by pores in this sample. The fine c-Al2O3 grains are of uniform selective leaching, commercial c-Al2O3 (AKP-G015) and Al2O3–SiO2 gel (DS-5).size and are distributed uniformly in the pseudomorphic 2864 J. Mater. Chem., 1998, 8, 2863–2867bonds with surface OH groups of the c-Al2O3. As depicted schematically in Fig. 4, the first layer of adsorbed water vapor is also able to form hydrogen bonds with further water vapor molecules to form second and third adsorbed layers, successively leading to multi-layer adsorption with increasing P/P0.On the other hand, alcohol molecules adsorb on the surface of the c-Al2O3 by forming hydrogen bonds between their OH groups and the surface OH groups of the c-Al2O3. In the case of methanol, this adsorption occurs steeply and starts from very low P/P0 because the aYnity of methanol for c-Al2O3 is strong and the steric hindrance of the methanol molecule is small.The isotherm of methanol starts at lower P/P0 than that of water vapor, suggesting that the adsorption energy of methanol is higher than that of water vapor. On the other hand, the isotherm of butan-1-ol shows an induction region Fig. 3 Adsorption isotherms of various adsorbates on the selectively of adsorption at very low P/P0 but with increasing P/P0 it leached c-Al2O3. increases steeply, behaving similarly to methanol above a certain P/P0. This induction region is considered to reflect the particles, yielding uniformly sized mesopores with a narrow steric hindrance of butan-1-ol, which has a molecular length pore size distribution.The 29Si MAS NMR spectra of this (0.8–0.9 nm) estimated to be about twice that of methanol. sample showed the Q4 structure unit of SiO4 tetrahedra, as The adsorption of the first layer of alcohol molecules occurs well as a framework structure and a Q0 structure unit rep- in such a way that the CH3 groups face away from the surface resenting SiO4 tetrahedra incorporated in the c-Al2O3.11 Since as depicted in Fig. 4. This changes the surface property from amorphous SiO2, which has the Q4 structure, is present as the hydrophilic to hydrophobic after adsorption of the first layer, matrix of the microtexture of the calcined sample and the explaining the plateau in the alcohol isotherm.Since the steric pseudomorphic particle shape is retained after the leaching hindrance of butan-1-ol is larger than that of methanol, the treatment, these fine c-Al2O3 grains are thought to be plateau in the isotherm of butan-1-ol is flatter than for connected by residual amorphous SiO2.methanol. In contrast to the hydrogen bond adsorption mechanism in water vapor and alcohols, the adsorption of cyclohex- Gas adsorption properties of the samples ane is considered to occur by physical adsorption, explaining the linear correlation with P/P0. Fig. 3 shows gas adsorption isotherms (0<P/P00.3) of the Generally, the surface roughness of porous materials is present c-Al2O3 for the polar adsorbates water, methanol, evaluated in terms of its fractal dimension.Pfeifer and Avnir12 butan-1-ol and the non-polar molecule cyclohexane. The give the following formula to calculate the fractal dimension adsorption isotherms of polar and non-polar molecules are of porous materials from adsorption data for diVerent diVerent; all the polar molecules show convex isotherms indimolecular size adsorbates cating the strong aYnity of these molecules for the surface of the c-Al2O3, i.e.hydrophilic behavior. On the other hand, the log(vm)=-(D/2) log(am)+C adsorption of cyclohexane shows an almost linear relationship with P/P0, thus, its aYnity for c-Al2O3 is weak.DiVerences where, vm, D, am are the BET monolayer adsorption capacity, surface fractal dimension and cross-sectional area of the can also be seen in the isotherms of the three polar molecules. The isotherms of the two alcohols showed a steep increase of adsorbed molecules, and C is a constant. Fig. 5 shows a plot of log(vm) vs. log(am) for the present c-Al2O3 prepared by the adsorption in the low P/P0 range, reaching a plateau in the medium P/P0 range, however the P/P0 value at which this selective leaching method and for commercial c-Al2O3 (AKPG015). Both data sets show a good linear correlation, from increase commenced was much lower for methanol than for butan-1-ol.By contrast, the water vapor adsorption isotherm which the fractal dimensions were calculated from the slopes as 2.1 and 2.2 for the present c-Al2O3 and commercial c- was a diVerent shape to those of the alcohols, increasing continuously with increasing P/P0 up to the medium P/P0 Al2O3, respectively.Since rough and smooth surface states correspond to fractal dimensions of 3 and 2, respectively, the range and without a plateau. The diVerence in the adsorption of these four diVerent molecules is shown schematically in present c-Al2O3 has a rather smooth surface.Fig. 6 shows a comparison of the adsorption–desorption Fig. 4. The hydrophilic surface of the present c-Al2O3 contains OH groups which absorb water vapor by forming hydrogen isotherms of various adsorbates on the present c-Al2O3 and Fig. 4 Schematic illustrations of adsorption mechanisms for various adsorbates.J. Mater. Chem., 1998, 8, 2863–2867 2865in the high P/P0 range correspond to the capillary condensation, the slope ratio is related to the pore size distribution. We therefore conclude that the steep change in the isotherms of the present c-Al2O3 is due to its narrow pore size distribution. The present c-Al2O3 contains a small amount of K2O contamination from the leaching treatment.This impurity may have some influence on the adsorption property. Previously it was found to show only little influence in lowering the starting value of P/P0 for the capillary condensation.8 Conclusions The gas adsorption properties of c-Al2O3 prepared by a selective leaching method were investigated using four adsorbates with diVerent molecular sizes and polarities (water, methanol, butan-1-ol and cyclohexane).The following results Fig. 5 BET monolayer adsorption capacity (vm) of various adsorbates were obtained. on the present c-Al2O3 and c-Al2O3 (AKP-G015) as a function of (1) Since the leached c-Al2O3 showed a strong aYnity for cross section (am) of adsorbate. polar molecules but a weak aYnity for non-polar molecules, the surface appears to have a hydrophilic character. the reference samples.Similar trends are observed in the (2) The convex shape of the isotherms diVered for the isotherms of all the adsorbates on corresponding samples. diVerent adsorbates in a manner which could be related to the Adsorption at low P/P0 increases in the order c-Al2O3 (AKP- adsorption mechanism. G015)<the present c-Al2O3<Al2O3–SiO2 gel (DS-5).This (3) The fractal dimension of the present c-Al2O3 was tendency is suggested to correlate with the specific surface calculated to be 2.1, corresponding to a rather smooth surface. area, pore size and also the hydrophilicity of the samples. We (4) The present c-Al2O3 showed a very steep increase and discussed the hydrophilicity of these samples in a previous decrease of the isotherms in the high P/P0 range due to paper8 and concluded that the hydrophilicity shows a tendency capillary condensation, consistent with a narrow pore size to decrease with increase of SiO2 content in the samples.On distribution in this sample. the other hand, the maximum adsorption of these samples increases in the order c-Al2O3 (AKP-G015)<Al2O3–SiO2 gel Acknowledgments (DS-5)<the present c-Al2O3.This should correspond to the sample pore volume giving rise to capillary condensation up We are grateful to Dr Katsunori Kosuge of National Institute for Resources and Environment for fruitful suggestions on to P/P0=0.9. Since the changes in the adsorption isotherms Fig. 6 Adsorption–desorption isotherms of various adsorbates in the present c-Al2O3, c-Al2O3 (AKP-G015) and Al2O3–SiO2 gel (DS-5). 2866 J.Mater. Chem., 1998, 8, 2863–28674 D. L. 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