Zaitseva Galina, Stahmich Tamara, Gushikem Yoshitaka

National Medical University, Kyiv; Pirogova National Medical University, Vinnitsa;

UNICAP,Campinas, Brasil

praparation and properties of highdispersed mixed oxides based on Silica – Zirconia and Antimony (V) oxides

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. 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.

Obtained (SiO₂/ZrO₂)Sb₂O₅ was analyzed with chemical analysis. The ICP analysis was performed. Characterization of the microporosity of silica mixed oxides is an important stage in their application as catalysts or adsorbents. Several methods have been developed to analyze of nitrogen gas adsorption isotherm for silica-zirconium mixed oxides with surface bonded antimony (V). Specific surface area (S_a) was calculated by the BET method and the external surface area (S_ext) - by the t-plot. Cumulative surface area of pore (S_p) was obtained from MP-method data. The internal surface area (S_int) was calculated by the subtracting S_ext from S_a. Micropore volume (V_mp) was estimated by the MP, t-plot and Horvath-Kawasoe (H-K) methods. The average micropore size (radius R_mp or diameter D_mp) and the pore size distribution were evaluated by the H-K and MP methods. The results are presented in Table.

Table. Chemical composition of mixed oxides and their porous parameters

Sample ID	Zr, %	Sb,%	Sa, m^2/gBET	Sint, m^2/g	MP			t-plot		H-K
					Sp, m^2/g	Vmp, m^3/g	Rmp,nm	Vmp, m^3/g	Sext, m^2/g	Vmp, m^3/g	Dmp, nm
Si,Zr1	8.4	-	649	422	732	0.29	0.39	0.17	226	0.28	0.76
Si,Zr2	11.6	-	327	275	422	0.14	0.34	0.11	51	0.14	0.66
Si,Zr3	15.4	-	276	241	378	0.12	0.32	0.10	35	0.12	0.64
(Si,Zr1)Sb	8.1	6.3	590	458	691	0.26	0.37	0.18	132	0.25	0.73
(Si,Zr2)Sb	10.3	13.1	344	299	461	0.15	0.32	0.12	44	0.15	0.64
(Si,Zr3)Sb	14.9	11.4	440	365	519	0.18	0.34	0.15	75	0.18	0.69

All materials showed a Langmuir Type I isotherms indicating that the mixed oxides obtained are microporous. The S_a and S_ext values for (Si,Zr1)Sb decreased compared to Si,Zr1. The change in V_mp for these oxides was low. On the contrary, the S_a and S_ext values for (Si,Zr2)Sb and (Si,Zr3)Sb increased compared to Si,Zr2 and Si,Zr3. Particularly, this increase was sizeable for (Si,Zr3)Sb. The change in V_mp and S_a for (Si,Zr2)Sb and (Si,Zr3)Sb showed a similar trend. The ratio S_int/S_a was constant for (Si,Zr2)Sb and (Si,Zr3)Sb compared to Si,Zr2 and Si,Zr3 and increased in case of (Si,Zr1)Sb. The pore size distribution calculated by H-K and MP- methods showed peaks at 0.8; 0.7; 0.6 nm for Si,Zr1, Si,Zr2, Si,Zr3 and at 0.7; 0.6; 0.7 nm for (Si,Zr1)Sb, (Si,Zr2)Sb and (Si,Zr3)Sb, correspondingly indicating ultramicroporosity of the material obtained.

The materials obtained have reasonably good chemical stability. Desorption of Sb in 1 m HCl is very week (2.5% from total coverage) that in 100 times smaller then for native antimony acid. Reduction of (Si,Zr)Sb mixed oxide solubility is due to formation of bond Sb-O-Zr.

Acidic properties of the materials were determined in FTIR experiment from pyridine adsorption. The Si,Zr and (Si,Zr)Sb mixed oxides demonstrate Brönsted as well as Lewis acid activity after thermovacuum treatment at 25 - 250⁰C. For (Si,Zr)Sb Brönsted acid sites are more thermally stable then for Si,Zr. For an antimony containing samples the band characterizing Lewis sites becomes less intensive upon the sample thermal treatment and is disappeared at 250⁰C.

Crystallization temperature of the mixed oxides is depends on ZrO₂ and Sb₂O₅ loading. For samples with low (8-12%) Zr content, three peaks from crystalline ZrO₂ at 2Q» 30, 50 and 60⁰ (tetragonal form) are observed after calcination at 1000⁰C only. For the sample with 16% of Zr these peaks appear at as low as 750⁰C. The peak characteristic of the tetragonal phase becomes larger when the zirconia content is increased. With further calcination at 1000⁰C the amount of tetragonal phase is decreased due to formation of monoclinic phase, indicating an increase of the x-ray crystallites’ size.

All (Si,Zr)Sb samples did not show any clear diffraction peak from zirconia or antimony at 500 - 750⁰C. Some peaks belong to ZrO₂ and antimonic acid is appeared after calcinations of samples at 1000⁰C. The scanning electron microscopy images and the elements mapping showed that in every case, within the magnification used, zirconium and antimony were homogeneously dispersed in the matrices.

The ion exchange capacities were determined for Li⁺, Na⁺ and K⁺ ions. For SiO₂/ZrO₂ samples the adsorption capacities were dependent on the nature of the cations, being higher in the order K⁺> Na⁺ > K⁺, while for SiO₂/ZrO₂/Sb₂O₅ in comparison with SiO₂/ZrO₂, the adsorption capacities were smaller and were independent of the cations nature. SiO₂/ZrO₂(1)/Sb₂O₅ with bonded Methylene Blue can be effective in order to developed new sensors for analytical application.