Silica – Zirconia mixed oxide with surface bonded Antimony: synthesis and some properties
Galina Zaitsevaa (PQ) and Yoshitaka Gushikem (PQ)*b
aInstitute of Sorption and Endoecology Problems, Kyiv, 252680, Ukraine
b Instituto de Química, Universidade Estadual de Campinas, Unicamp, CP 6154, 13083-970, Campinas, SP
*E-mail: gushikem@igm.unicamp.br
key-words: sol-gel, silica-zirconium-antimony, mixed oxide
Non-crystalline mixed-metal oxides are the main sources of modern materials for catalysis, adsorption, sensors, electrochemistry etc. For most application high surface area and homogeneity of the oxides is crucial. Inhomogeneous oxides has tendency to crystallization and reduction of surface area, their properties have problems with repeatability. Most mixed-metal oxides have silica as a matrix. This ensures high surface area and prevents crystallization. The main method to obtain non-crystalline mixed-metal oxides is sol-gel technology. Recently we developed sol-gel technology for preparation silicon-zirconium oxide where zirconium contents can be changed in wide range. The important feature of the oxides obtained is its high degree of homogeneity. This type of materials can be useful in heterogeneous catalysis (petrol chemistry) since contain both Lewis (O3-Zr+) and Bronsted (Si-OH) centers of acidity. As it can be seen from the formula Bronsted sites of acidity for silicon-zirconium oxide is quite weak and can not be used for example in a such important application as alkane isomerisation. In order to increase Bronsted acidity of the composition we performed surface modification of the mixed-oxide with SbCl5. It should be mentioned that sol-gel technology unlikely gives positive result since great difference in components acidity. Moreover had the gel contains antimony in matrix volume, its hydrolytic stability should be low, in contrast to zirconium that increase stability of the gel in acid. Sites for metal (such as Sr) adsorption and proton conductivity are other important feature of the gel was expected to increase after its surface modification with SbCl5. It is known that one from the best adsorbent for water purification from radioactive strontium is salts of H[Sb(OH)6]. Mixed-oxide surface modification with SbCl5 with further hydrolysis will lead to compounds with similar structure and probably similar adsorption properties.
In this respect a new high surface area Silicon – Zirconium mixed-oxide was obtained and then it surface was chemically modified with Antimony oxide (V). The resulting composite can be abbreviated as (SiO2/ZrO2)Sb2O5, where brackets indicate that SiO2/ZrO2 mixed-oxide was obtained by sol-gel technology and so it is volume-modified materials. In contrast Sb2O5 located on the composite surface due preparation procedure.
Synthesis of (SiO2/ZrO2)Sb2O5 was performed according to surface assembling procedure in two steps.
At the first step, the sol-gel method has been used to prepare silica-zirconium matrix with different contents of ZrO2. It was performed as follows: To 250 ml of 1,1 M TEOS in ethanol under refluxing 14 ml of 3 M HCl in H2O was added dropwise during 10 min. The mixture then was kept at 600C with stirring for 3h. To cooled sol 28, 42 and 69 ml (0,06; 0.09 and 0.15 M, respectively) of zirconium tetrabuthoxyde in 80 ml of dry ethanol were quickly added and under Argon. Mixture then was stirred for 1.5 h at room temperature and other 75 ml of dry ethanol was added together with 7, 10.5 and 14 ml of 3 M HCl dropwise. Resulting mixture was heated at 60ºC during time gel formation and then dried at 800C for 72 h. The Solids were ground and sieved with size 200-250 mesh.
To remove the excess organic components the gels were washed with ethanol in Soxhlet apparatus for 72 h, then kept in 0.1 M HCl for 24 h and finally dried at about 60oC. As a result Mixed-metal oxide SiO2/ZrO2 with different Zr contents were obtained.
At the second step of the surface assembling procedure 20 g of SiO2/ZrO2 was stirred with 800 ml of 0.03 M SbCl5 and then heated at 60ºC for 7 h. Finally it was left in the SbCl5 solution without stirring for 16 h at room temperature. Then the solid was separated by filtration, washed with double-distilled water until weak-acid reaction of filtrate and finally dried at 60ºC for 24 h. Obtained (SiO2/ZrO2)Sb2O5 was analyzed with chemical analysis. For this 0.1 g of test samples containing Si, Zr and Sb were treated with few drops of HF solution until the solids were completely dissolved and solutions were diluted with 20 ml of distilled water. The solution was heated at 80ºC for 12 h for removing of SiF4 and HF excess. Then solution was evaporated until dry, precipitate dissolved in 20 ml of concentrated HNO3:HCl = 1:15 v/v, evaporated at 80ºC till dry to remove fluoride ions. The procedure was repeated twice and then precipitate was dissolved in 2 M HCl and diluted with the same acid to 100 ml of total volume. Then ICP analysis was performed. Results of chemical analysis: samples 1- Zr, % 8.1, Sb, % 6.3; 2- Zr, % 12. 4, Sb, % - 13.1; 3 - Zr, % 16.2 Sb, % 11.4. From the date it can be seen that there is no strong correlation between Zr and Sb content. With reference to preparation procedure (all Sb-containing compounds were obtained in similar conditions, in SbCl5 excess), difference in Sb loading for samples 1-3 suggests influence of Zr content in SiO2/ZrO2 on Sb loading. For the first two samples correlation between Zr and Sb content can be suggested. Reduction of the Sb content for the sample 3 can be also explained with reduction of surface area of the mixed-metal oxide that commonly happened when Zr loading becomes high. The last one indicates expected surface (not volume) modification of carrier (SiO2/ZrO2) with Sb2O5.
Essential difference in chemical properties of SiO2/ZrO2 and (SiO2/ZrO2) Sb2O5 reveals itself in reaction with Alizarin Red S. When initial SiO2/ZrO2 matrix was treated with dye solution it strongly adsorbed on mixed-metal oxide, indicating presence of Zr on surface. In contrast no noticeable adsorption of the dye on (SiO2/ZrO2) Sb2O5 take place. From this datum, complete coverage of SiO2/ZrO2 surface with Sb2O5 was considered. [FAPESP]