Thermodynamic properties of vapor complex Na₂ZrCl₆

LI Jun-li(李军丽)¹, YU Jin(于 锦)^{1, 2}, YANG Dong-mei(杨冬梅)¹, WANG Zhi-chang(王之昌)¹

1. School of Sciences, Northeastern University, Shenyang 110004, China;

2. School of Sciences, Shenyang University of Technology, Shenyang 110023, China

Received 7 September 2006; accepted 20 December 2006

Abstract: Thermodynamic studies were carried out for the vapor complex of sodium chloride with zirconium tetrachloride at 718-778 K and 0.5-2.5 kPa by using high temperature phase equilibrium-quenching experiments, taking closed Pyrex glass ampoules as the reaction containers. The results show that the sole predominant vapor complex is Na₂ZrCl₆ for the ZrCl₄-NaCl system under the experimental conditions. The thermodynamic equilibrium constants and other thermodynamic functions of the reaction 2NaCl(s)+ZrCl₄(g)=Na₂ZrCl₆(g) have been derived from the measurements. The results for the changes in enthalpy and entropy are ?H⁰=(-70.1±1.5) kJ/mol and ?S⁰=(-105.9±2.0) J/(mol?K) in the temperature range.

Key words: ZrCl₄; NaCl; Na₂ZrCl₆; vapor complex; phase equilibrium-quenching experiment

1 Introduction

Transition metal halide vapor complexes have been used as key constituents chemically transported in the high temperature processes such as chemical synthesis, catalysis, extraction and separation, high-intensity discharge lamps, and lasers. To understand the nature of these high temperature processes, it is necessary to know their stoichiometry, structure and thermodynamic properties. There have been relatively more experimental and theoretical studies on the structure, properties and applications of the rare earth (ⅢB group metal) halide vapor complexes (see, for example, Refs.[1-12] and references therein). However, the same information is relatively few for the stoichiometry, structure and thermodynamic properties of the ⅣB group metal halide vapor complexes.

We have determined the thermodynamic properties of the vapor complexes in the AlCl₃-LnCl₃ systems from Ln=Sc to Ln=Lu at 588-838 K[13-16] and of the NbCl₅-NaCl and NbCl₅-KCl binary systems at 580-680 K[17] by using the high temperature phase equilibrium- quenching experiments. In this study, we report the formation thermodynamics of the vapor complex Na₂ZrCl₆ in the ZrCl₄-NaCl system determined by the same quenching experiments.

We try to extend the same measurements to the vapor complex K₂ZrCl₆ in ZrCl₄-KCl system. Unfortunately, however, the vapor complex is very unstable and its yield is too low to be quantitatively determined under the same experimental conditions by this method, which makes us unable to report the thermodynamic data for the K₂ZrCl₆ vapor complex.

2 Experimental

The chemicals used in this study were of 99.5% purity for anhydrous ZrCl₄ and of 99.99% purity for NaCl and KCl from Aldrich Chemical Co. The anhydrous ZrCl₄ was further purified by repeated sublimations under vacuum. NaCl and KCl were further crystallized using double-distilled water and then dried under vacuum over P₂O₅. The experiments were carried out in closed ampoules made from Pyrex glass with a special shape as shown in Fig.1. The volumes of the ampoules were determined to be 50-60 mL.

The quenching experiments employed in this study were similar to those described in Refs.[13-16]. An excess of NaCl or KCl, and less ZrCl₄ were placed in the deep ditch of the ampoule (see part A in Fig.1) under argon atmosphere and the ampoule was then sealed under vacuum. Four ampoules were placed in a graphite container and then placed in a furnace, where the four isothermal samples with different initial amounts of anhydrous ZrCl₄ and NaCl or ZrCl₄ and KCl resulted in a set of isothermal complexation data at different pressures. The temperature was kept constant within ±0.5 K measured with a Pt-PtRu₁₀ thermocouple so as to ensure the four samples to be at the same temperature during each run. The maximum temperature difference in the container was always smaller than 1.0 K.

Fig.1 Schematic of ampoule

Preliminary experiments showed that the glass ampoule broke easily when quenching from an equilibrium pressure higher than 0.25 MPa at high temperature. Ref.[18] showed that the normal sublimation temperature is 605 K and the melting point is 710 K for ZrCl₄. Moreover, Ref.[19] showed that the solid and liquid phases might coexist in the ZrCl₄-NaCl system at temperatures higher than 812 K, where the activities of ZrCl₄ and NaCl are unknown. Therefore, the formal experiments were carried out at pressures lower than 0.25 MPa and at temperatures lower than 800 K but higher than 715 K for the ZrCl₄-NaCl system to ensure the complete evaporation of ZrCl₄ and the existence of pure solid NaCl in the deep ditch of the ampoule in this study.

Our preliminary experiments showed that the complexation equilibrium might be achieved within 5 h. Therefore, the formal equilibrium period was chosen to be 6 h for each run. After the equilibrium had been achieved, the graphite container was quickly removed from the furnace and the ampoules were covered with asbestos, then quenched with ice water. Thus the equilibrium gas phase was rapidly and uniformly condensed all over the ampoules. The amounts of substance of Zr⁴⁺ and Na⁺ condensed in part B of the ampoules could then be determined by spectrophotometry and atomic absorption spectro- photometry, respectively.

3 Results and discussion

At temperatures below 800 K, the equilibrium pressure of pure solid NaCl is negligible. On the other hand, however, the Na⁺ ions are condensed in part B of the ampoule after quenching. It is known [20] that there is a dimerization reaction in the ZrCl₄ vapor but with the thermodynamic equilibrium constant only in the magnitude of 10^-13. Therefore, the Na⁺ ions condensed in part B of the ampoule may reasonably be assumed to all belong to the vapor complexes Na_uZr_vCl_u+4v(g) in the ZrCl₄-NaCl system formed by the reaction:

uNaCl(s)+vZrCl₄(g)= Na_uZr_vCl_u+4v(g)             (1)

with the equilibrium constant

which yields

where  p⁰ =0.1 MPa, p_i denotes the partial pressure of i, c(uv) and K_p(uv) denote the vapor complex Na_uZr_vCl_u+4v and its equilibrium constant, respectively. The relationships between p_i and the analysis results of Zr⁴⁺ and Na⁺ may be given by

and the total pressure may be given by

where  T is the temperature, V is the volume of part B of the ampoule, n_i is the amounts of substance of the component i. After calculating the quantities from the analysis results of n(Zr⁴⁺) and n(Na⁺), the molar Gibbs energy (?G⁰), molar enthalpy (?H⁰) and molar entropy (?S⁰) of reaction (1) may be determined by

The Refs.[19,21-24] suggested the Na₂ZrCl₆ to be the sole solid and liquid complex in the ZrCl₄-NaCl system. Therefore, one may assume the Na₂ZrCl₆ (or Na₂Zr_vCl_4v+2) to be the sole vapor complex in this study for the same system. In that case, K_p(uv), n_c(uv) and p_c(uv) may simply be rewritten as K_p, n_c and p_c, and Eqns.(6) and (7) may be simplified to

n(Zr⁴⁺)=n(ZrCl₄)+vn_c                         (10)

n(Na⁺)=2n_c                                (11)

The calculation results of Eqns.(3)-(5) and (9)-(11) should lead to the straight lines of ln(p_c/p⁰) vs ln(p(ZrCl₄)/p⁰) with the slope of v and of RlnK_p vs 1/T for the ZrCl₄-NaCl system.

On the other hand, SCH?FER[25] and PAPATHEODOROU et al[1,26] suggested the u=v=1 type vapor complex to be the only vapor complex in many alkali metal chloride-containing binary systems, such as KMgCl₃, NaSnCl₃, KLuCl₄ and NaFeCl₄. According to this, one may also assume the Na₂ZrCl₆ vapor complex to coexist with the NaZrCl₅ vapor complex in the ZrCl₄-NaCl system. In this case, however, the calculation results of Eqns.(3)-(5) and (9)-(11) may result in the nonlinear ln(p_c/p⁰)—ln(p(ZrCl₄)/p⁰) and/or RlnK_p—1/T relationships in the ZrCl₄-NaCl system, similar to the situations of LaAl₃Cl₁₂-LaAl₄Cl₁₅ or CeAl₃Cl₁₂-CeAl₄Cl₁₅ coexistence[16] and of DyAl₃Cl₁₂- DyAl₂Cl₉[14] or HoAl₃Cl₁₂-HoAl₂Cl₉ coexistence[16].

The calculation results of Eqns.(3)-(5) and (8)-(10) are listed in Table 1 for the ZrCl₄-NaCl system. It can be seen that the values of the apparent stoichiometric factor v are within 1.03-1.08 at 718-778 K for the ZrCl₄-NaCl system, being very close to the theoretical value of v=1.0.

Fig.2 also shows that the plots of ln(p_c/p⁰) vs ln(p(ZrCl₄)/p⁰) are all straight lines with the slope of v=1. It may be therefore concluded that Na₂ZrCl₆ is the predominant vapor complex for the ZrCl₄-NaCl system at 718-778 K. The values of K_p of reaction (1) at v=1.0 can then be calculated in terms of Eqn.(3) for Na₂ZrCl₆.

The results are given in the last column of Table 1. The plots of RlnK_p vs 1/T are shown in Fig.3 for Na₂ZrCl₆, which are also straight lines.

Fig.2 Plots of ln(p_c/p⁰) vs ln(p(ZrCl₄)/p⁰) for NaCl-ZrCl₄ binary system

Fig.3 Plots of RlnK_p vs 1/T for NaCl-ZrCl₄ binary system

Table 1 Quenching experimental results for vapor complexes Na₂ZrCl₆

The thermodynamic quantities of reaction (1) can be determined from the experiments by a least squares computation within the reaction temperature ranges. The results are ΔH⁰=-70.1 kJ/mol and ?S⁰= -105.9 J/(mol?K) at about 718-778 K for Na₂ZrCl₆.

The probable overall errors of the ?G⁰, ?H⁰, ?S⁰ values should be computed from the statistical errors and the estimated probable uncertainties[13-16]. The statistical errors may be not more than ±0.2 kJ/mol for ?G⁰ at every temperature, ±0.7 kJ/mol for ?H⁰ and ±1.0 J/(mol?K) for ?S⁰ for the complex Na₂ZrCl₆ in the reaction temperature ranges shown in this study. The estimated probable uncertainties may result from assuming absolute errors of the chemical analysis for Zr⁴⁺ and Na⁺ as ±5 %, of the volume measurement of the ampoules as ±0.5%, and of the temperature measurement as ±2.0 K. These uncertainties together with the error from the scatter of the experimental points in Figs.2 and 3 may give rise to the probable overall errors of ±0.5 kJ/mol for ?G⁰ at every temperature, ±1.5 kJ/mol for ?H⁰ and ±2.0 J/(mol?K) for ?S⁰ for the vapor complex Na₂ZrCl₆ in the reaction temperature range shown in this study.

4 Conclusions

The vapor complex Na₂ZrCl₆ was formed by the reaction of solid NaCl with gaseous ZrCl₄. The formation thermodynamics is ?H⁰=(-70.1±1.5) kJ/mol and ?S⁰=(-105.9±2.0) J/(mol?K) for Na₂ZrCl₆(g) at 718-778 K. The vapor complex formation ability in the ZrCl₄-KCl system is much lower than that in the ZrCl₄-NaCl system.

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Foundation item: Project(50274027) supported by the National Natural Science Foundation of China

Corresponding author: WANG Zhi-chang; Tel: +86-24-83675010; E-mail: wangzc@mail.neu.edu.cn

 (Edited by YUAN Sai-qian)