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Water Vapor Adsorption on the Sol-Gel Composites Prepared Using Ethyl Silicate 40 as a Silica Precursor Julita Mrowiec-Białon´† and Andrzej B. Jarze¸ bski*,‡ Institute of Chemical Engineering, Polish Academy of Sciences, 44-100 Gliwice, Bałtycka 5, Poland, and Faculty of Chemistry, Institute of Chemical and Process Engineering, Silesian Technical University, 44-100 Gliwice, Ks. M. Strzody 7, Poland Received July 20, 2000. In Final Form: October 19, 2000 Effective mesoporous composite adsorbents of water vapor can be prepared using a novel “one-pot”, sol-gel procedure with ethyl silicate 40 as a silica precursor. The multilayer mechanism of water adsorption appears to be responsible for high adsorption of water on the prepared samples. In the case of the lithium bromide doped samples this adsorption takes the form of a volume filling. The Dubinin-Astakhov equation very well portrays adsorption of water by the synthesized composites in relative pressures 3 m ) 1/(3 - Ds) if m < 3 where Ds is the surface fractal dimension. For the sake of comparison and owing to the successful application of both the DA and FHH equations in the (15) Stoeckli, F. Carbon 1998, 36, 363. (16) Leboda, R. Chromatographia 1981, 9, 524. (17) Avnir, D.; Jaroniec, M. Langmuir 1989, 5, 1431. (18) Pfeifer, P.; Cole, M. V. New J. Chem. 1990, 14, 221.
Figure 4. Water vapor adsorption on composite adsorbents at 298 K.
Sample Preparation. Three samples of silica-based adsorbents were prepared in a one-step, base-catalyzed procedure to obtain a target content of calcium chloride of 10, 20, or 30 wt % (designated KCa10-KCa30, respectively), and two other samples were also prepared, labeled KLi20 and KLi30, with nominal lithium bromide contents of 20 and 30 wt %, respectively. The molar ratio of the compounds in all samples was Si:EtOH:H2O: NH3 ) 1:8:3:8 × 10-3. Alcogels were prepared as follows. First two solutions were prepared at room temperature: solution A contained ES (Unisil-Tarno´w, Poland) and half of the total ethanol whereas B consisted of the remaining ethanol, water, ammonium hydroxide, and calcium chloride or lithium bromide. Then solution A was added with stirring to B, and the resulting sols were heated to 323 K. Gelation occurred in 3-4 h for CaCl2-doped samples and in 0.3-0.5 h for those doped with LiBr. Then the alcogels
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Figure 5. Comparison of experimental data with predictions from the Dubinin-Astakhov equation for water adsorption on KCa20 and KCa30 composites.
Figure 6. Comparison of experimental data with predictions from the Dubinin-Astakhov equation for water adsorption on KLi20 and KLi30 composites. Table 2. Parameters of the Dubinin-Astakov Equation for Water Adsorption on Composite Adsorbents sample
vo, mmol/g
E, kJ/mol
r
, %
corr coeff
KCa30 KCa20 KCa10 KLi30 KLi20
37.6 25.2 16.0 16.0 12.6
3.87 3.33 3.97 8.14 8.10
1.20 1.03 1.00 2.52 1.83
4.5 8.6 9.7 2.2 3.4
0.999 1.000 0.998 1.000 0.999
were dried slowly at room temperature to obtain xerogels. The xerogel samples were dried at 473 K for 3 h and stored in a desiccator. Characterization Methods. Nitrogen adsorption isotherms measured at 77 K with a Micromeritics ASAP 2000 instrument were used to obtain values of the specific surface area, SBET, and mesopore volume, Vp, determined by means of the BJH method.19 Water vapor adsorption isotherms were measured volumetrically using a standard device.20 The process was carried out at 298 K. (19) Barret, E. P.; Joyner, L.G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (20) Czepirski, L.; Komorowska-Czepirska, E.; Cacciola, G. Adsorpt. Sci. Technol. 1996, 14, 83.
Figure 7. Adsorption potential distributions in mesopores of KCa20 and KCa30 composites. Comparison of experimental data (symbols) with prediction from the DA equation (solid lines).
Figure 8. Adsorption potential distributions in mesopores of KLi20 and KLi30 composites. Comparison of experimental data (symbols) with prediction from the DA equation (solid lines).
Results and Discussion Characteristic parameters of the texture of the hybrid adsorbents SiO2-CaCl2 and SiO2-LiBr are given in Table 1. As can be seen the presence of the larger content of a hygroscopic compound (chloride or bromide) results in a less porous structure on the mesopore size scale (Vp) and lower values of the specific surface area (SBET). As was expected both quantities appear to be notably larger than those exhibited by the samples prepared previously with the same content of chloride or bromide. For instance Vp values in the composites with chloride contents of 30 or 20 wt % or 30 wt % LiBr are equal to 0.73, 1.1, and 1.0 cm3/g, respectively, whereas those observed in adsorbents with similar composition yet synthesized with the use of TEOS were equal to1,3 0.54, 0.45, and 0.38 cm3/g, respectively. In addition to affecting the total mesopore volume, the dopant content appeared to have a clear impact on the pore size distributions (PSDs), as shown in Figures 1 and 2.
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Figure 9. FHH plot of water adsorption isotherms on the SiO2-CaCl2 composites.
Figure 11. FHH plot of nitrogen adsorption isotherms on the SiO2-CaCl2 composites.
Figure 10. FHH plot of water adsorption isotherms on the SiO2-LiBr composites.
Figure 12. FHH plot of nitrogen adsorption isotherms on the SiO2-LiBr composites.
Closer inspection of nitrogen adsorption isotherms, especially t-plot analysis (cf. Figure 3), showed that, as before,2,3 all adsorbents have negligible (
