The Li+, Na+ and K+ ion exchange reaction process on the surface of mixed oxide SiO2/TiO2/Sb2O5 surface prepared by the sol-gel processing method

O processo da reação de troca iônica de Li+, Na+ e K+ na superfície do óxido misto SiO2/TiO2/Sb2O5 preparado pelo método de processamento Sol-Gel

The Li⁺, Na⁺ and K⁺ ion exchange reaction process on the surface of mixed oxide SiO₂/TiO₂/Sb₂O₅ surface prepared by the sol-gel processing method

O processo da reação de troca iônica de Li⁺, Na⁺ e K⁺ na superfície do óxido misto SiO₂/TiO₂/Sb₂O₅ preparado pelo método de processamento Sol-Gel

C. U. Ferreira^I; J. E. Gonçalves^I; Y. V. Kholin^{II, *}; Y. Gushikem^{I, *}

^IInstituto de Quimica, Universidade Estadual de Campinas, Caixa Postal 6154, 13083-970 Campinas, SP, Brazil

^IIChemical Faculty, V.N. Karazin Kharkov National University,4 Svoboda Square, Kharkov 61077, Ukraine

ABSTRACT

The porous mixed oxide SiO₂/TiO₂/Sb₂O₅ obtained by the sol-gel processing method presented a good ion exchange property and a high exchange capacity towards the Li⁺, Na⁺ and K⁺ ions. In the H⁺/M⁺ ion exchange process, the H⁺ / Na⁺ could be described as presenting an ideal character. The ion exchange equilibria of Li⁺ and K⁺ were quantitatively described with the help of the model of fixed tetradentate centers. The results of simulation evidence that for the H⁺ / Li⁺ exchange the usual situation takes place: the affinity of the material to the Li⁺ ions is decreased with increasing the degree of ion exchange. On the contrary, for K⁺ the effects of positive cooperativity, that facilitate the H⁺ / K⁺ exchange, were revealed.

Keywords: silica-titania-antimonia mixed oxide; ion exchange; alkaline metal ions; stability constants.

RESUMO

O óxido misto poroso SiO₂/TiO₂/Sb₂O₅, obtido pelo método de processamento sol-gel, apresentou uma boa propriedade e elevada capacidade de troca iônica com os íons Li⁺, Na⁺ e K⁺. No processo de troca iônica H⁺/M⁺, o H⁺/Na⁺ apresentou um caráter ideal. Os equilíbrios de troca de Li⁺ e K⁺ foram quantitativamente descritos através de um modelo de centros tetradentados fixos. Os resultados de simulação evidenciaram que para troca H⁺/Li⁺ temos uma situação normal: a afinidade do material pelo íon Li⁺ decresce com o aumento do grau de troca. Para o K⁺, o efeito positivo do cooperativismo que facilitaria a troca H⁺/K⁺, revelou um efeito contrário.

Palavras-chave: oxido misto de silica-titania-antimonia; troca iônica; ions de metais alcalinos; constantes de estabilidade.

Introduction

The synthetic inorganic ion exchangers include an important class of materials because they are normally obtained as rigid matrices and do not swell in aqueous solutions, normally presenting good chemical selectivity associated with a high thermal and chemical stabilities [1,2].

Among these materials, titanium (IV) antimonate has been described as a good ion exchanger [3-6]. It has been prepared by mixing aqueous solutions of Ti (IV) and SbCl₅ and the precipitated solids separated by filtration. However, this method has not been lead to a useful product because fine powders and samples having low specific surface areas were normally obtained [7-11]. Alternatively, the sol-gel processing method has been used to prepare materials with characteristics of high homogeneity and purity, controlled porosity and well-modeled particles [12-14]. The material prepared by using this procedure, presented also high ion exchange capacities towards the alkaline metal ions in the following order of increasing affinity: Li⁺< Na⁺ < K⁺ [15]. In order to explain the mechanism of the exchange process and in particular the higher affinity of the material by the heavier metal ion along the serie, the process of H⁺/M⁺ exchange on the sorbent surface should be studied in more detail. The model of an ideal exchange process as normally used to calculate the equilibrium constants in solids such as organic resins apparently can not to be applied to porous solid presenting a heterogeneous surface. A model that takes into consideration the lateral interaction on the solid surface between the neighbor acid sites which can affect the exchange process must be considered [16,17]. The aim of this work is to describe and discuss the ion exchange process of Li⁺,Na⁺ and K⁺ ion on the surface of porous SiO₂/TiO₂/Sb₂O₅ material using the polidentate fixed acid centers model. This model was used to describe and to estimate the stepwise equilibria constants.

Experimental part

The SiO₂/TiO₂ mixed oxide was prepared according to procedure described elsewhere [15]. The SiO₂/TiO₂ binary oxide was prepared in three steps according to the procedures described as follows: (1) 12.1 ml of 0.85 mol l^-1 HNO₃ aqueous solution was added to 250 mL of 50% (v/v) tetraethoxysilane (TEOS)/ethanol solution, and the mixture was stirred at 353 K for 2.5 h. (2) About 21 ml of titanium (IV) butoxyde (TBO) and 490 ml of ethanol were then added and the mixture was stirred for 2 h at room temperature. (3) 66 ml of 0.6 mol l^-1 HNO₃ solution were slowly added and allowed to rest for gelation. The formed xerogels were ground, dried at 383 K for 24 h and sieved in order to obtain particles size between 75 and 250 µm.

Incorporation of Sb(V) into SiO₂/TiO₂

About 15 g of the mixed oxide obtained was immersed in 500 ml of distilled water and 20 ml of 0.3 mol l^-1 Sb (V) (prepared from SbCl₅) aqueous solution were added and the suspension heated at 333 K for 8 h. The solid was filtered, washed with 1.0 mol l^-1 HNO₃ solution in order to avoid Sb(V) hidrolysis and to eliminate choride ion, and finally with deionized water until the washing solution achieved a neutral pH.

Ion exchange isotherms

The ion exchange capacities for the sample was determined by weighing about 100 mg of the materials and adding into 50.0 mL of 1.0 mol l^-1 MCl solution (M = Li⁺, Na⁺, K⁺) and shaken for 8 h at 298 K. 25.0 mL of the supernatant solution was titrated with standard 0.05 mol l^-1 NaOH solution [2]. The isotherms were determined by immersing 100 mg of SiO₂/TiO₂/Sb₂O₅ into 50 ml of the ions with concentrations between 8x10^-5 and 5x10^-3 mol l^-1. Each mixture was shaken about 24 h at 298 K and the concentration of the metal ion in the solution phase was determined. The measurements were carried out on a B 262 Micronal flame photometer apparatus.

Results and discussions

The main characteristics of the solid obtained, SiO₂TiO₂ and SiO₂/TiO₂/Sb₂O₅ are summarized in Table 1. For sake of brevity, both solids will hereafter designated as ST and STSb.

Table 1

Ion exchange isotherms

Figures 1a-c show the ion exchange isotherms for Li⁺, Na⁺ and K⁺ ion on STSb at 298 K. Clearly we observe that at the limit of saturation condition on the isotherm curves the adsorption capacities are Li⁺ < Na⁺ ~ K⁺. The ion exchange process on the solid surface can be represented by the equation:

where the bars mean the solid phase and K_exch is the concentration equilibrium constant,

where the concentrations in solution phase are expressed as mol l^-1 and in the solid phase as mol g^-1.

Simulations of ions-exchange equilibria

The initial values of log K_exch were estimated for all experimental points with the use of eq 2, for different molar fraction of the metal, , on the solid phase STSb. For an ideal ion exchange case is expected log K_exch value do not depend on . As an example, Figure 2 shows the plot of log K_exch plotted against .

We observe that the ion exchange process H⁺/K⁺ do not correspond to the model of the ideal exchange, i.e. the replacement of H⁺ with K⁺ leads to increasing of log K_exch. The deviation from an ideal model is also observed for the exchange process H⁺/Li⁺ and H⁺/Na⁺. These results are indicating that the energetical heterogeneity of the materials manifest at the H⁺/M⁺ exchange process. Both, the quantitative description of the ion exchange equilibria and characteristics of the material require taking this feature into account. There are several possible ways to perform this [18-23]. In this work the model of fixed polydentate centers has been be used [16,17]. According to it, the surface of the material is considered as the assemblage of the polydentate ion exchange centers whereas the ion exchange process is represented by the scheme below, where the stepwise equilibrium reactions are characterized with the corresponding constants K₁, K₂ , K₃...

The overall constants are defined as b_i = K₁´K₂´K₃´ =K_i, where Z is the number of the stages. To use the model it is necessary to determine the size of the center (Z) and to calculate the values of K_i or b_i.

The following procedure was used to solve this task.

Step 1. The tentative value Z =1 was fixed.

Step 2. The unknown concentration equilibrium constants b_i were calculated by the non-linear least-square method [24] applied to minimize criterion

where k is the number of the experimental points, D_k is the difference between the calculated and experimental values, D = C^calc—C^exp or D = N_f^calc N_f^exp, where C is the equilibrium concentration of the metal ions in solution (mol l ¹), N_f is the specific amount of the adsorbed metal ions (mol·g^-1), w_k is the statistical weight of the K^th measurement assigned as:

where A_k is the experimental value (C of N_f) and s_r is its relative error [25]. All computations were performed with the aid of the software program CLINP 2.1 [26].

Step 3. The adequacy of the model was tested. If the model fitted experimental data unsatisfactorily, the current Z value is increased and new calculations of the equilibrium constants are performed (see step 2).

To test the adequacy of the models, the global criterion c² is used [27]. The model is assumed to be adequate if the following inequality holds,

where is the 5% point of the chi-squared distribution with f degrees of freedom (f = N - x, where x is the number of the parameters to be calculated).

It was found that the acceptable quality of fitting the experimental data is attained at Z = 4 (model of fixed tetradentate centers). Figure 3 presents the example of simulation for the H⁺ / K⁺ exchange. The course of simulation and the results of calculations are summarized in Tables 2-4.

One important feature of the H⁺ / K⁺ ion exchange should be noted. When the replacement of H⁺ by K⁺ in the tetradentate center occurs, it is impossible to determine the stepwise ion exchange constants K_i. That is for the reason that the intermediate species and are practically absent. This fact points to the non-stepwise character of the ion exchange process. In other words, if H⁺ in the tetradentate center is replaced by K⁺, the replacement of the next H⁺ ion is facilitated, the K⁺ ion "pushes out" the next hydrogen ion. As a rule, such positive cooperativity is rarely happened. But it seems that such situation is not the exceptional case for the hybrid materials on the base of inorganic oxides. A similar example of the positive cooperativity for such materials was recently described [28]. Unfortunately, the scarcity of information about this new generation of materials does not allow to propose now more or less clear explanation of the revealed phenomenon and requires further investigation.

Degrees of formation of different species in the solid phase (Figures 4 and 5) corroborate a great difference between the nature of the ion exchange process for Li⁺ and K⁺. In the case of the H⁺ / Li⁺ exchange, species , relatively depleted with Li⁺, predominates at extents of reaction 0.15 0.55, while for the H⁺ / K⁺ ion exchange species , enriched with K⁺, is the main potassium entity in the solid phase

Conclusions

The porous solid STSb showed a good ion exchange property possessing a high exchange capacity towards the Li⁺, Na⁺ and K⁺ ions.

In the H⁺/M⁺ ion exchange process, the H⁺ / Na⁺ could be described as presenting an ideal character. The ion exchange equilibria of Li⁺ and K⁺ were quantitatively described with the help of the model of fixed tetradentate centers. The results of simulation evidence that for the H⁺ / Li⁺ exchange the usual situation takes place: the affinity of the material to the Li⁺ ions is decreased with increasing the degree of ion exchange. On the contrary, for K⁺ the effects of positive cooperativity, that facilitate the H⁺ / K⁺ exchange, were revealed.

Acknowledgements

YG is indebted to CNPq/Pronex and Fapesp for financial supports.

Recebido em: 18/11/2004

Aceito em: 13/12/2004

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Publication Dates

• Publication in this collection
  18 Apr 2005
• Date of issue
  2005

History

• Accepted
  13 Dec 2004
• Received
  18 Nov 2004