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The synthesis and characterization of mesoporous silica–zirconia aerogels
Microporous and Mesoporous Materials 68 (2004) 127–132 www.elsevier.com/locate/micromeso
The synthesis and characterization of mesoporous silica–zirconia aerogels Zhi-Gang Wu, Yong-Xiang Zhao *, Dian-Sheng Liu
*
Institute of Advanced Chemistry, Shanxi University, Shanxi Province, Taiyuan 030006, PR China Received 10 August 2003; received in revised form 20 December 2003; accepted 23 December 2003
Abstract A series of mesoporous SiO2 AZrO2 aerogels with various zirconia content (10–90 wt%) were prepared by the sol–gel method followed by supercritical drying. The characterization of aerogels is performed by XRD, FT-IR, 29 Si liquid-state NMR, and BETN2 adsorption. The results showed that (i) the aerogels have type VI (BDDT) profile which is the typical of mesopores in the interval between 2 < Dp < 50 nm; (ii) the specific surface areas varied from 340 to 730 m2 /g with ðSBET ÞMAX ¼ 735:5 m2 /g for the aerogel with 10 wt% zirconia and the surface areas decreased with the increase of zirconia; (iii) all the aerogels with different zirconia content remained amorphous or poorly crystallized after calcination in air at 500 °C; (iv) a large number of SiAOAZr bands existed in the aerogels indicating a homogeneous distribution of the components on the atomic scale. In addition, using inorganic salt as zirconium source instead of the expensive and toxic zirconium alkoxide is very economic. Ó 2004 Elsevier Inc. All rights reserved. Keywords: ZrO2 ASiO2 aerogel; Sol–gel method; Supercritical drying; Mesoporous material; Alcohol-aqueous solution
1. Introduction In recent years, SiO2 AZrO2 mixed-oxides have been studied extensively [1–3] due to their superior physicochemical properties, such as higher thermal and chemical stability, hardy mechanical strength, catalytic activity and strong surface acidity. Particularly, they have been applied in ceramic toughening, alkalineresistant glass, solid super acid and heterogeneous catalysis etc. However, their properties, more or less, relate to the degree of mixing the two oxides components. It is reported that zirconia–silica mixed oxides could be converted to strong acids by contact with a solution of sulfuric acid, which exhibited catalytic activity for the isomerization of n-hexane [4]. A series of microporous SiO2 AZrO2 mixed-oxides prepared by Chiara etc. [5] were used as catalysts for the oxidation of the cyclohexene and showed moderate activity. Several methods have been applied for the preparation of SiO2 AZrO2 mixed-oxides including some con* Corresponding authors. Tel.: +86-351-7018091/7016558; fax: +86351-7016048/7011688. E-mail addresses: [email protected] (Y.-X. Zhao), [email protected]. cn (D.-S. Liu).
1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2003.12.018
ventional techniques such as mechanical mixing of the component oxides [6], deposition of silicate on zirconia [7], impregnation [8] and co-precipitation technique [9]. However, the products prepared by these conventional methods are typically very dense and lack high surface area and pore volume desired for catalytic applications. In contrast with these traditional techniques, the solution sol–gel method (SSG) allows excellent control of the properties of the products. The SSG method, comprised of controlled hydrolysis and subsequent condensation of metal alkoxides, normally yield a homogeneous distribution of all the components at a molecular level, high surface area and adjustable pore size distribution. But due to the different hydrolysis rate of each alkoxide, it is very difficult to control the stoichiometry of the resulting products, which may produce a serious compositional heterogeneity inside the particles. And usually, the alkoxides are expensive and toxic. In this paper, we report an improved sol–gel preparation method followed by a supercritical drying technique to prepare a series of mesoporous SiO2 AZrO2 aerogels with zirconia contents from 10 to 90 wt% via a mixture of tetraethyl orthosilicate (TEOS) and zirconyl nitrate dihydrate in a mixed-solution of ethanol and water. The method is simple and direct, and the aerogels
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have high specific surface areas and good degree of chemical homogeneity. The effect of zirconia content on the specific area and pore size distribution of the aerogels is also investigated.
2. Experimental section 2.1. Materials and preparation procedure SiO2 AZrO2 mixed-oxide with zirconia content between 10 and 90 wt% were prepared with tetraethyl orthosilicate (TEOS) and zirconyl nitrate dihydrate. Following is a typical preparation procedure: a desired amount of zirconyl nitrate dihydrate was dissolved in the solution of deionized H2 O and anhydrous C2 H5 OH at room temperature. Then TEOS was dropped into the stirred mixture. The pH was adjusted to 1 using nitric acid (1 M). The mixture was heated in water bath at 80 °C and kept for half an hour. During this period, the solution first became a sol and later a gel. The gel was aged for 2 h at ambient temperature, and then it was dried in a stainless-steel tank via a supercritical drying. In order to exceed the critical condition without a formation of vapor–liquid interface inside the pores, the mixture was transferred to a stainless steel liner in an autoclave (a net volume of 1 l) together with 190 ml of additional ethanol (outside of the liner), thus surpassing the critical volume of the mixture. By heating, the temperature and pressure of the liquid in the autoclave would surpass the Tc (241 °C) and Pc (6.2 MPa) of ethanol. The final temperature and pressure were about 270 °C and 8.0 MPa. The autoclave was kept at the final temperature for 30 min to ensure complete thermal equilibration. Then the fluid was released and flushed with nitrogen to cool to room temperature. Mixed-oxides aerogels with different zirconia content were obtained by the adjustment of the amounts of TEOS, HNO3 , H2 O and ZrO(NO3 )2 Æ 2H2 O. A 1:4 volume ratio of water to alcohol was used throughout. The aerogels were calcinated at 500 °C in air for 3 h before characterization.
filtered radiation. The diffraction patterns are measured step by step (0.2° in 2h). Chemical bonds of the mixed-oxides and functional groups were investigated with Shimadzu 8300 Fourier transform infrared spectroscopy (FT-IR) using the potassium bromide (KBr) pellet technique. 29 Si liquid-state NMR spectra were registered at 59.6 MHz at different temperature by using a Bruker DRX 300 spectrometer. Tetremethylsilane (TMS) was used as an external standard reference. A heating element was wrapped around the cell to allow for collection in situ spectra at elevated temperature up to 80 °C. In the following discussion we use XZS symbolized samples with different zirconia content from 10 to 90 wt%, and the alphabet X displayed the zirconia content in weight percentage.
3. Results and discussion 3.1.
29
Si liquid-state NMR
The sensitivity of 29 Si NMR spectroscopy to the nature of the second-nearest neighbors is well documented. So it was used to investigate the extent of the reaction and the nature of the species formed during the hydrolysis process. The notation Qn was used for the Si species, where n was the number of the siloxane bonds attached to the observed Si atom. The spectra registered for TEOS and zirconyl nitrate mixture of 30ZS at different temperature are shown in Fig. 1. For the 30ZS mixture measured at 20 °C, only one band at 82.1 p.p.m. (parts per million) corresponding to TEOS was detected. At 40 °C, two small new peaks at 87.1 and 88.0 p.p.m. appeared while the band of TEOS decreased its intensity simultaneously. These new bands cannot be due to the hydrolysis of TEOS since the spectrum of pure TEOS hydrolyzed at the 40 °C showed
2.2. Characterization of the aerogels Specific surface areas of the aerogels were determined by the BET method using N2 adsorption isotherm at )196 °C; the nitrogen adsorption isotherm was evaluated by an automated Micromeritics SORPTOMATIC 1990 SERIES. The mesopore size distributions were obtained from adsorption branch of N2 with a cylindrical pore model, according to the BJH model. Prior to analysis, samples were outgassed under vacuum at 150 °C for 8 h. X-ray powder diffraction analysis was performed using a Rigaku D/Max 2500 diffractomer. Cu Ka, Ni-
Fig. 1. 29 Si liquid-state NMR spectra of a TEOS and zirconyl nitrate mixture (30ZS) at different temperature.
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two bands at 82.1 and 90.9 p.p.m. corresponding to TEOS and 2 Q species, respectively. Consequently, the bands at 87.1 and 88.0 p.p.m. should be attributed to the (EtO)3 SiAOAZrB species which are produced by hydrolization and condensation. The intensity and line width of the two bands increased with elevated temperature (80 °C) indicating a progress in the condensation reactions and more (EtO)3 SiAOAZrB species are produced. A similar behavior has been observed in the hydrolysis of TEOS-zirconium n-propoxide mixture in humid air for about 20 h [10]. Another new broad band at 99.5 p.p.m. corresponding to Q3 species [11,12] is present at 80 °C. The peak at 82.1 p.p.m. disappeared completely at the same time indicating the entire hydrolysis of the TEOS. In conclusion, a number of SiAOAZr bonds are formed in the reflux stage and as also in the aerogels. 3.2. N2 adsorption isotherm measurement The nitrogen absorption/desorption isotherm of 30ZS was shown in Fig. 2. The isotherm corresponded to type IV in BDDT system [13] which was typical mesoporous materials having a pore diameter between 2.0 and 50 nm. Similar features were obtained for other aerogels with different compositions. An important hysteresis loop was apparent in the isotherm. It was identified as type A isotherm, or H1 type (IUPIC), which was characteristic of cylindrical pores [14].The appearance of a well defined hysteresis loop associated with irreversible capillary condensation in the mesopores in the p=p0 region from 0.6 to 1.0 suggested a presence of substantial textural mesoporosity arising from non-crystalline intraaggregate voids and spaces formed by the inter particle contacts in the aerogels. And from the figure, it also can be seen that there are no micropores in these samples. Assuming a cylindrical pore model and using the adsorption branch of the isotherms, the pore size distributions of these aerogels were calculated using the BJH method [15]. The pore size distribution profiles from 10ZS to 90ZS are shown in Fig. 3 and Table 1. It is observed that for all the samples the pore size distribu-
Fig. 2. Adsorption–desorption N2 -isotherm for the 30ZS.
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tion curves are monomodal in the range 5–100 nm with a relatively narrow pore distribution centered around 5.8–6.8 nm within the meso region. The pore diameter of 10ZS was the minimum. Then as the zirconia content increased, the pore diameter increased slightly in the range of 10–70 wt%. Miller et al. [16] claimed that a shift in distribution to higher pore size was indicative of wellmatched precursor reactivity and therefore increased homogeneity. So the aerogels in our case showed increasing homogeneity as zirconia content altered from 10 to 70 wt%. The specific surface area, pore volumes and other main texture parameters of the aerogels derived from nitrogen adsorption are shown in Table 1. The specific surface areas increased gradually upon the addition of SiO2 to ZrO2 , which were consistent with previous observations [16–18] while the average pore size slightly decreased. A maximum of the BET area with the minimum pore size occurred at the 10ZS (735 m2 /g); a minimum was 90ZS (341.8 m2 /g). The reason may be that the incorporation of the ZrO2 into the SiO2 phase might develop a composite texture due to the compact and cross-linked clusters [17]. From data in the table, we can also see that the total pore volume reached a maximum for 30ZS and the highest pore diameter for 70ZS. 3.3. Powder X-ray diffractions The crystalline structures of the aerogels with different zirconia content calcined at 500 °C were investigated by X-ray powder diffraction measurements. The XRD patterns obtained are illustrated in Fig. 4, in which all samples are either X-ray amorphous or poorly crystallized. However, the 2h, the maximum of the reflection, moves to higher degree as the ZrO2 content increased. When the zirconia content is less than 30 wt%, there is a single broad peak around 22° (2h), indicating the aerogels being X-ray amorphous, similar to those already published [19,20]. Miller and Ko [21] suggested that the absence of XRD patterns for multicomponent system is a result of high degree mixing of multicomponent oxides. The formation of amorphous aerogels is due to the incorporation of zirconium atoms in the framework of amorphous SiO2 . If the zirconia content is higher than 30 wt% the diffraction line is located in 30° (2h). It is a characteristic line of a tetragonal ZrO2 phase. A new weak band at 2h 50° and a little sharp peak at 2h 78° showed up if the ZrO2 content is larger than 50 wt%. There is another new small peak at 2h 65° for the 90ZS. The absence of large sharp lines in ZrO2 rich samples indicated that zirconia in the aerogels did not exist as crystals of sufficient size to be detected by XRD and the relatively small proportion of silica is highly dispersed through the zirconia network homogeneously. The SiO2 does restrain the crystallization of ZrO2 in the aerogels while a strong retarding
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Fig. 3. Pore size distribution of the samples from BJH adsorption.
Table 1 Properties of ZrO2 ASiO2 aerogels with different ZrO2 content after calcinations in air at 500 °C for 3 h Samples
Zirconia content (wt%)
SBET (m2 /g)
VPN (cc/g)
Mean pore diameter (nm)
10ZS 30ZS 50ZS 70ZS 90ZS
10 30 50 70 90
735.5 646.8 547.9 513.9 341.8
1.4 2.25 1.67 1.63 1.15
5.8 6.1 6.3 6.8 6.4
effect has often been ascribed to a good chemical homogeneity of the starting gels, i.e. to a high degree of SiAOAZr bonding, zirconium oxide species are present in amorphous structures of SiO2 matrices.
Fig. 4. XRD patterns of ZrO2 ASiO2 aerogels with different Zr content. (a) 10ZS, (b) 30ZS, (c) 50ZS, (d) 70ZS, and (e) 90ZS.
Z.-G. Wu et al. / Microporous and Mesoporous Materials 68 (2004) 127–132
3.4. FT-IR investigation The IR bands of heteroatom linkages are often used to determine the degree of mixing. The FT-IR spectrum of the ZrO2 ASiO2 aerogels with varying zirconia content from 10 to 90 wt% are recorded in the spectral range 4000–400 cm1 after being calcined at 500 °C in atmosphere. The spectra are showed in Fig. 5. Pure SiO2 is known to exhibit the peaks at near 1100 cm1 with a shoulder at about 1200 cm1 due to three dimensional SiAOASi work asymmetric stretching vibrations and the peak at 803 cm1 corresponding to a symmetric stretching vibration. The peak at 980 cm1 , characteristic of the SiAOH vibration, is also observed. Other peaks at 816 and 446 cm1 corresponded to the bending vibration of SiAO bonds in a ring structure [22]. In the spectra there are also another lines: a broad band at about 3445 cm1 , corresponding to the vibration of SiAOH terminal groups and AOH occluded in the mixed oxides [23]. The substitutional insertion of the cations into the silica network resulting in the weakening of the framework can easily be detected by the wave number shift and intensity change of these peaks. But the mechanical mixtures would give spectra that can be derived by linear combination of the spectra of the components (not shown). For the samples, FT-IR showed that as ZrO2 content increased, the 1095 cm1 band shifted gradually towards a lower wave number, reading as low as 986 cm1 for the sample with a zirconia content of 90 wt%. This band is tentatively assigned to an asymmetric SiAOASi stretch perturbed by the presence of Zr in its environment or the stretching of SiAOAZr band. Similar observations were reported earlier by Zhan and Zeng [24] and Lee and Condrate Sr. [25]. The shift of the SiAO (1095 cm1 ) indicated that a higher incorporation of Zr in the lattice and the presence of elevated
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SiAOAZr bands in the aerogels. Other bands due to the SiO bending vibration at 470 and 800 cm1 gradually decreased their intensities, respectively and became negligible when the zirconia content is further increased. The intensity change of the silanol band at 955 cm1 is another important sign to measure homogeneity of ZrO2 ASiO2 mixed-oxides [26]. If the number of ZrAOASi linkage increased, the number of silanol groups would decrease. As a result, the intensity of 955 cm1 would decrease its intensity or disappear completely. This is generally true in our case. The wave number shift and infrared intensity changes indicated that the introducing of Zr4þ ions into the silicate structure broke down its network. There must be a formation of hetero-linkage SiAOAZr in the aerogels. Interestingly, these ring structure bands completely disappeared as the zirconia content is up to 70 wt%. These phenomena seemed to suggest that all or most of the SiAO ring structures have been destroyed at this stage. Similar trends have been reported in the FT-IR spectra of zirconia-silicas [24,27,28]. The interpretation of good compositional homogeneity from infrared interpretation is in accord with the analysis of XRD.
4. Conclusions A series of mesoporous ZiO2 ASiO2 aerogels in a zironia content range of 10–90 wt% were prepared by the sol–gel method followed by supercritical drying. The synthetic method, using inorganic salt as zirconium source instead of the expensive and toxic zirconium alkoxide, is simple and economic. The produced aerogels have a nitrogen pore volume of 1.1–2.2 cm3 /g, specific surface area of 340–730 m2 /g and almost the same high degrees of homogeneity. The adsorption–desorption isotherms of the ZrO2 ASiO2 aerogels, according to the BDDT system, belong to type IV showing and they are mesoporous materials. The hysteresis cycles are identified as the H1 type (IUPAC), which indicated the presence of cylindrical pores. Formations of a larger number of ZrAOASi bonds in the aerogels are confirmed by XRD, 29 Si NMR and FT-IR.
Acknowledgements This work is supported by the National Natural Science Foundation (20171030, DSL) and Natural Science Foundation in Shanxi Province (20011008, DSL), China. References Fig. 5. The FT-IR spectrums of ZrO2 ASiO2 aerogels with different Zr content. (a) 10ZS, (b) 30ZS, (c) 50ZS, (d) 70ZS, and (e) 90ZS.
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[2] T. Lopez, F. Tzompantzi, J. Navarrte, R. Gomez, J.L. Boldu, E. Munoz, O. Novaro, J. Catal. 181 (1999) 285. [3] F. Garbassi, L. Balducci, R. Ungarelli, J. Non-Cryst. Solids 223 (1998) 190. [4] T. Lopez, J. Navarrete, R. Gomez, O. Novaro, F. Figueras, H. Armendariz, Appl. Catal. A 125 (1995) 217. [5] C. Palazzi, L. Oliva, M. Signoretto, G. Strukul, J. Catal. 194 (2000) 286. [6] P. Courty, C. Marcilly, B. Delmon, P.A. Jacobs, G. Poncelet (Eds.), Preparation of Catalysts, Elsevier, Amsterdam, 1976, p. 119. [7] S. Sato, R. Takahashi, T. Sodesawa, S. Anaka, K. Oguma, K. Ogura, J. Catal. 196 (2000) 190. [8] Z. Dang, B.G. Anderson, Y. Amenomiya, B.A. Morrow, J. Phys. Chem. 99 (1995) 14437. [9] C. Ingemar Odenbrand, S. Andersson, L. Andersson, J. Brandin, G. Busca, J. Catal. 125 (1990) 541. [10] P. Tartaj, J. Sanz, C.J. Serna, M. Ocana, J. Mater. Sci. 29 (1994) 6533. [11] J.A. Anderson, C. Fergusson, I. Rodriuez, A. Guerrero-Ruiz, J. Catal. 192 (2000) 344. [12] M. Andrianainarivelo, R. Corriu, D. Leclercq, P. Hubert Mutin, A. Vioux, J. Mater. Chem. 6 (1996) 1665.
[13] K.S.W. Sing, D.H. Everett, R.A. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [14] M. Toba, F. Mizukami, S. Kiwa, T. Sano, K. Maeda, A. Annila, V. Kommppa, J. Mol. Catal. 94 (1994) 85. [15] J.A. Noh, J.A. Schwartz, J. Colloid Interface Sci. 27 (1986) 531. [16] J.B. Miller, S.E. Rankin, E.I. Ko, J. Catal. 148 (1994) 673. [17] R. Pirard, D. Bonhomme, S. Kolibos, J.P. Pirard, A. Lecloux, J. Sol–Gel Sci. Technol. 8 (1997) 831. [18] C. Contescu, V.T. Popa, J.B. Miller, E.I. Ko, J.A. Schwarz, J. Catal. 157 (1995) 244. [19] M. Deeba, W.K. Hall, J. Catal. 60 (1979) 417. [20] H.J.M. Bosman, E.C. Kruissink, J. Van der Spoel, F. Vav den Brink, J. Catal. 148 (1994) 660. [21] J.B. Miller, E.I. Ko, Catal. Today 35 (1997) 269. [22] K. Kamiya, T. Yoko, K. Tanaka, M. Takeuchi, J. Non-Cryst. Solids 121 (1990) 182. [23] L. Lan, G. Gnappi, A. Montenero, J. Mater. Sci. 28 (1993) 2119. [24] Z. Zhan, H.G. Zeng, J. Non-Cryst. Solids 243 (1999) 26. [25] S.W. Lee, R.A. Condrate Sr., J. Mater. Sci. 23 (1988) 2951. [26] S.C. Miller, M. Fujino, E.I. Ko, J. Catal. 148 (1994) 673. [27] J.B. Miller, E.I. Ko, J. Catal. 159 (1996) 58. [28] J.R. Sohn, H.J. Jang, J. Mol. Catal. 64 (1991) 349.
The synthesis and characterization of mesoporous silica–zirconia aerogels Zhi-Gang Wu, Yong-Xiang Zhao *, Dian-Sheng Liu
*
Institute of Advanced Chemistry, Shanxi University, Shanxi Province, Taiyuan 030006, PR China Received 10 August 2003; received in revised form 20 December 2003; accepted 23 December 2003
Abstract A series of mesoporous SiO2 AZrO2 aerogels with various zirconia content (10–90 wt%) were prepared by the sol–gel method followed by supercritical drying. The characterization of aerogels is performed by XRD, FT-IR, 29 Si liquid-state NMR, and BETN2 adsorption. The results showed that (i) the aerogels have type VI (BDDT) profile which is the typical of mesopores in the interval between 2 < Dp < 50 nm; (ii) the specific surface areas varied from 340 to 730 m2 /g with ðSBET ÞMAX ¼ 735:5 m2 /g for the aerogel with 10 wt% zirconia and the surface areas decreased with the increase of zirconia; (iii) all the aerogels with different zirconia content remained amorphous or poorly crystallized after calcination in air at 500 °C; (iv) a large number of SiAOAZr bands existed in the aerogels indicating a homogeneous distribution of the components on the atomic scale. In addition, using inorganic salt as zirconium source instead of the expensive and toxic zirconium alkoxide is very economic. Ó 2004 Elsevier Inc. All rights reserved. Keywords: ZrO2 ASiO2 aerogel; Sol–gel method; Supercritical drying; Mesoporous material; Alcohol-aqueous solution
1. Introduction In recent years, SiO2 AZrO2 mixed-oxides have been studied extensively [1–3] due to their superior physicochemical properties, such as higher thermal and chemical stability, hardy mechanical strength, catalytic activity and strong surface acidity. Particularly, they have been applied in ceramic toughening, alkalineresistant glass, solid super acid and heterogeneous catalysis etc. However, their properties, more or less, relate to the degree of mixing the two oxides components. It is reported that zirconia–silica mixed oxides could be converted to strong acids by contact with a solution of sulfuric acid, which exhibited catalytic activity for the isomerization of n-hexane [4]. A series of microporous SiO2 AZrO2 mixed-oxides prepared by Chiara etc. [5] were used as catalysts for the oxidation of the cyclohexene and showed moderate activity. Several methods have been applied for the preparation of SiO2 AZrO2 mixed-oxides including some con* Corresponding authors. Tel.: +86-351-7018091/7016558; fax: +86351-7016048/7011688. E-mail addresses: [email protected] (Y.-X. Zhao), [email protected]. cn (D.-S. Liu).
1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2003.12.018
ventional techniques such as mechanical mixing of the component oxides [6], deposition of silicate on zirconia [7], impregnation [8] and co-precipitation technique [9]. However, the products prepared by these conventional methods are typically very dense and lack high surface area and pore volume desired for catalytic applications. In contrast with these traditional techniques, the solution sol–gel method (SSG) allows excellent control of the properties of the products. The SSG method, comprised of controlled hydrolysis and subsequent condensation of metal alkoxides, normally yield a homogeneous distribution of all the components at a molecular level, high surface area and adjustable pore size distribution. But due to the different hydrolysis rate of each alkoxide, it is very difficult to control the stoichiometry of the resulting products, which may produce a serious compositional heterogeneity inside the particles. And usually, the alkoxides are expensive and toxic. In this paper, we report an improved sol–gel preparation method followed by a supercritical drying technique to prepare a series of mesoporous SiO2 AZrO2 aerogels with zirconia contents from 10 to 90 wt% via a mixture of tetraethyl orthosilicate (TEOS) and zirconyl nitrate dihydrate in a mixed-solution of ethanol and water. The method is simple and direct, and the aerogels
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have high specific surface areas and good degree of chemical homogeneity. The effect of zirconia content on the specific area and pore size distribution of the aerogels is also investigated.
2. Experimental section 2.1. Materials and preparation procedure SiO2 AZrO2 mixed-oxide with zirconia content between 10 and 90 wt% were prepared with tetraethyl orthosilicate (TEOS) and zirconyl nitrate dihydrate. Following is a typical preparation procedure: a desired amount of zirconyl nitrate dihydrate was dissolved in the solution of deionized H2 O and anhydrous C2 H5 OH at room temperature. Then TEOS was dropped into the stirred mixture. The pH was adjusted to 1 using nitric acid (1 M). The mixture was heated in water bath at 80 °C and kept for half an hour. During this period, the solution first became a sol and later a gel. The gel was aged for 2 h at ambient temperature, and then it was dried in a stainless-steel tank via a supercritical drying. In order to exceed the critical condition without a formation of vapor–liquid interface inside the pores, the mixture was transferred to a stainless steel liner in an autoclave (a net volume of 1 l) together with 190 ml of additional ethanol (outside of the liner), thus surpassing the critical volume of the mixture. By heating, the temperature and pressure of the liquid in the autoclave would surpass the Tc (241 °C) and Pc (6.2 MPa) of ethanol. The final temperature and pressure were about 270 °C and 8.0 MPa. The autoclave was kept at the final temperature for 30 min to ensure complete thermal equilibration. Then the fluid was released and flushed with nitrogen to cool to room temperature. Mixed-oxides aerogels with different zirconia content were obtained by the adjustment of the amounts of TEOS, HNO3 , H2 O and ZrO(NO3 )2 Æ 2H2 O. A 1:4 volume ratio of water to alcohol was used throughout. The aerogels were calcinated at 500 °C in air for 3 h before characterization.
filtered radiation. The diffraction patterns are measured step by step (0.2° in 2h). Chemical bonds of the mixed-oxides and functional groups were investigated with Shimadzu 8300 Fourier transform infrared spectroscopy (FT-IR) using the potassium bromide (KBr) pellet technique. 29 Si liquid-state NMR spectra were registered at 59.6 MHz at different temperature by using a Bruker DRX 300 spectrometer. Tetremethylsilane (TMS) was used as an external standard reference. A heating element was wrapped around the cell to allow for collection in situ spectra at elevated temperature up to 80 °C. In the following discussion we use XZS symbolized samples with different zirconia content from 10 to 90 wt%, and the alphabet X displayed the zirconia content in weight percentage.
3. Results and discussion 3.1.
29
Si liquid-state NMR
The sensitivity of 29 Si NMR spectroscopy to the nature of the second-nearest neighbors is well documented. So it was used to investigate the extent of the reaction and the nature of the species formed during the hydrolysis process. The notation Qn was used for the Si species, where n was the number of the siloxane bonds attached to the observed Si atom. The spectra registered for TEOS and zirconyl nitrate mixture of 30ZS at different temperature are shown in Fig. 1. For the 30ZS mixture measured at 20 °C, only one band at 82.1 p.p.m. (parts per million) corresponding to TEOS was detected. At 40 °C, two small new peaks at 87.1 and 88.0 p.p.m. appeared while the band of TEOS decreased its intensity simultaneously. These new bands cannot be due to the hydrolysis of TEOS since the spectrum of pure TEOS hydrolyzed at the 40 °C showed
2.2. Characterization of the aerogels Specific surface areas of the aerogels were determined by the BET method using N2 adsorption isotherm at )196 °C; the nitrogen adsorption isotherm was evaluated by an automated Micromeritics SORPTOMATIC 1990 SERIES. The mesopore size distributions were obtained from adsorption branch of N2 with a cylindrical pore model, according to the BJH model. Prior to analysis, samples were outgassed under vacuum at 150 °C for 8 h. X-ray powder diffraction analysis was performed using a Rigaku D/Max 2500 diffractomer. Cu Ka, Ni-
Fig. 1. 29 Si liquid-state NMR spectra of a TEOS and zirconyl nitrate mixture (30ZS) at different temperature.
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two bands at 82.1 and 90.9 p.p.m. corresponding to TEOS and 2 Q species, respectively. Consequently, the bands at 87.1 and 88.0 p.p.m. should be attributed to the (EtO)3 SiAOAZrB species which are produced by hydrolization and condensation. The intensity and line width of the two bands increased with elevated temperature (80 °C) indicating a progress in the condensation reactions and more (EtO)3 SiAOAZrB species are produced. A similar behavior has been observed in the hydrolysis of TEOS-zirconium n-propoxide mixture in humid air for about 20 h [10]. Another new broad band at 99.5 p.p.m. corresponding to Q3 species [11,12] is present at 80 °C. The peak at 82.1 p.p.m. disappeared completely at the same time indicating the entire hydrolysis of the TEOS. In conclusion, a number of SiAOAZr bonds are formed in the reflux stage and as also in the aerogels. 3.2. N2 adsorption isotherm measurement The nitrogen absorption/desorption isotherm of 30ZS was shown in Fig. 2. The isotherm corresponded to type IV in BDDT system [13] which was typical mesoporous materials having a pore diameter between 2.0 and 50 nm. Similar features were obtained for other aerogels with different compositions. An important hysteresis loop was apparent in the isotherm. It was identified as type A isotherm, or H1 type (IUPIC), which was characteristic of cylindrical pores [14].The appearance of a well defined hysteresis loop associated with irreversible capillary condensation in the mesopores in the p=p0 region from 0.6 to 1.0 suggested a presence of substantial textural mesoporosity arising from non-crystalline intraaggregate voids and spaces formed by the inter particle contacts in the aerogels. And from the figure, it also can be seen that there are no micropores in these samples. Assuming a cylindrical pore model and using the adsorption branch of the isotherms, the pore size distributions of these aerogels were calculated using the BJH method [15]. The pore size distribution profiles from 10ZS to 90ZS are shown in Fig. 3 and Table 1. It is observed that for all the samples the pore size distribu-
Fig. 2. Adsorption–desorption N2 -isotherm for the 30ZS.
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tion curves are monomodal in the range 5–100 nm with a relatively narrow pore distribution centered around 5.8–6.8 nm within the meso region. The pore diameter of 10ZS was the minimum. Then as the zirconia content increased, the pore diameter increased slightly in the range of 10–70 wt%. Miller et al. [16] claimed that a shift in distribution to higher pore size was indicative of wellmatched precursor reactivity and therefore increased homogeneity. So the aerogels in our case showed increasing homogeneity as zirconia content altered from 10 to 70 wt%. The specific surface area, pore volumes and other main texture parameters of the aerogels derived from nitrogen adsorption are shown in Table 1. The specific surface areas increased gradually upon the addition of SiO2 to ZrO2 , which were consistent with previous observations [16–18] while the average pore size slightly decreased. A maximum of the BET area with the minimum pore size occurred at the 10ZS (735 m2 /g); a minimum was 90ZS (341.8 m2 /g). The reason may be that the incorporation of the ZrO2 into the SiO2 phase might develop a composite texture due to the compact and cross-linked clusters [17]. From data in the table, we can also see that the total pore volume reached a maximum for 30ZS and the highest pore diameter for 70ZS. 3.3. Powder X-ray diffractions The crystalline structures of the aerogels with different zirconia content calcined at 500 °C were investigated by X-ray powder diffraction measurements. The XRD patterns obtained are illustrated in Fig. 4, in which all samples are either X-ray amorphous or poorly crystallized. However, the 2h, the maximum of the reflection, moves to higher degree as the ZrO2 content increased. When the zirconia content is less than 30 wt%, there is a single broad peak around 22° (2h), indicating the aerogels being X-ray amorphous, similar to those already published [19,20]. Miller and Ko [21] suggested that the absence of XRD patterns for multicomponent system is a result of high degree mixing of multicomponent oxides. The formation of amorphous aerogels is due to the incorporation of zirconium atoms in the framework of amorphous SiO2 . If the zirconia content is higher than 30 wt% the diffraction line is located in 30° (2h). It is a characteristic line of a tetragonal ZrO2 phase. A new weak band at 2h 50° and a little sharp peak at 2h 78° showed up if the ZrO2 content is larger than 50 wt%. There is another new small peak at 2h 65° for the 90ZS. The absence of large sharp lines in ZrO2 rich samples indicated that zirconia in the aerogels did not exist as crystals of sufficient size to be detected by XRD and the relatively small proportion of silica is highly dispersed through the zirconia network homogeneously. The SiO2 does restrain the crystallization of ZrO2 in the aerogels while a strong retarding
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Fig. 3. Pore size distribution of the samples from BJH adsorption.
Table 1 Properties of ZrO2 ASiO2 aerogels with different ZrO2 content after calcinations in air at 500 °C for 3 h Samples
Zirconia content (wt%)
SBET (m2 /g)
VPN (cc/g)
Mean pore diameter (nm)
10ZS 30ZS 50ZS 70ZS 90ZS
10 30 50 70 90
735.5 646.8 547.9 513.9 341.8
1.4 2.25 1.67 1.63 1.15
5.8 6.1 6.3 6.8 6.4
effect has often been ascribed to a good chemical homogeneity of the starting gels, i.e. to a high degree of SiAOAZr bonding, zirconium oxide species are present in amorphous structures of SiO2 matrices.
Fig. 4. XRD patterns of ZrO2 ASiO2 aerogels with different Zr content. (a) 10ZS, (b) 30ZS, (c) 50ZS, (d) 70ZS, and (e) 90ZS.
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3.4. FT-IR investigation The IR bands of heteroatom linkages are often used to determine the degree of mixing. The FT-IR spectrum of the ZrO2 ASiO2 aerogels with varying zirconia content from 10 to 90 wt% are recorded in the spectral range 4000–400 cm1 after being calcined at 500 °C in atmosphere. The spectra are showed in Fig. 5. Pure SiO2 is known to exhibit the peaks at near 1100 cm1 with a shoulder at about 1200 cm1 due to three dimensional SiAOASi work asymmetric stretching vibrations and the peak at 803 cm1 corresponding to a symmetric stretching vibration. The peak at 980 cm1 , characteristic of the SiAOH vibration, is also observed. Other peaks at 816 and 446 cm1 corresponded to the bending vibration of SiAO bonds in a ring structure [22]. In the spectra there are also another lines: a broad band at about 3445 cm1 , corresponding to the vibration of SiAOH terminal groups and AOH occluded in the mixed oxides [23]. The substitutional insertion of the cations into the silica network resulting in the weakening of the framework can easily be detected by the wave number shift and intensity change of these peaks. But the mechanical mixtures would give spectra that can be derived by linear combination of the spectra of the components (not shown). For the samples, FT-IR showed that as ZrO2 content increased, the 1095 cm1 band shifted gradually towards a lower wave number, reading as low as 986 cm1 for the sample with a zirconia content of 90 wt%. This band is tentatively assigned to an asymmetric SiAOASi stretch perturbed by the presence of Zr in its environment or the stretching of SiAOAZr band. Similar observations were reported earlier by Zhan and Zeng [24] and Lee and Condrate Sr. [25]. The shift of the SiAO (1095 cm1 ) indicated that a higher incorporation of Zr in the lattice and the presence of elevated
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SiAOAZr bands in the aerogels. Other bands due to the SiO bending vibration at 470 and 800 cm1 gradually decreased their intensities, respectively and became negligible when the zirconia content is further increased. The intensity change of the silanol band at 955 cm1 is another important sign to measure homogeneity of ZrO2 ASiO2 mixed-oxides [26]. If the number of ZrAOASi linkage increased, the number of silanol groups would decrease. As a result, the intensity of 955 cm1 would decrease its intensity or disappear completely. This is generally true in our case. The wave number shift and infrared intensity changes indicated that the introducing of Zr4þ ions into the silicate structure broke down its network. There must be a formation of hetero-linkage SiAOAZr in the aerogels. Interestingly, these ring structure bands completely disappeared as the zirconia content is up to 70 wt%. These phenomena seemed to suggest that all or most of the SiAO ring structures have been destroyed at this stage. Similar trends have been reported in the FT-IR spectra of zirconia-silicas [24,27,28]. The interpretation of good compositional homogeneity from infrared interpretation is in accord with the analysis of XRD.
4. Conclusions A series of mesoporous ZiO2 ASiO2 aerogels in a zironia content range of 10–90 wt% were prepared by the sol–gel method followed by supercritical drying. The synthetic method, using inorganic salt as zirconium source instead of the expensive and toxic zirconium alkoxide, is simple and economic. The produced aerogels have a nitrogen pore volume of 1.1–2.2 cm3 /g, specific surface area of 340–730 m2 /g and almost the same high degrees of homogeneity. The adsorption–desorption isotherms of the ZrO2 ASiO2 aerogels, according to the BDDT system, belong to type IV showing and they are mesoporous materials. The hysteresis cycles are identified as the H1 type (IUPAC), which indicated the presence of cylindrical pores. Formations of a larger number of ZrAOASi bonds in the aerogels are confirmed by XRD, 29 Si NMR and FT-IR.
Acknowledgements This work is supported by the National Natural Science Foundation (20171030, DSL) and Natural Science Foundation in Shanxi Province (20011008, DSL), China. References Fig. 5. The FT-IR spectrums of ZrO2 ASiO2 aerogels with different Zr content. (a) 10ZS, (b) 30ZS, (c) 50ZS, (d) 70ZS, and (e) 90ZS.
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