1. Introduction

4A zeolite possesses an open three-dimensional porous structure,and the structure cell consists of SiO4/AlO4 tetrahedrons, called a sodalite cage. The sodalite cages are connected by single quadrangular rings to form supercages,which are the main channels of 4A zeolite. The extraframework cations (Na+) play a role of compensating charges for the negatively charged framework, and they interact with the framework through electrostatic attraction. The weak interaction determines the mobility of Na+ inside 4A zeolite. The transferring and hopping are two kinds of typical movement of Na⁺,and have been researched by molecular simulation methods ^{[1, 2]}. Meanwhile,the sensitive dielectric response of cation to environmental difference indicates that dielectric relaxation spectroscopy (DRS) can be used to explore the microscopic characteristics of zeolite under various environments ^{[3, 4, 5]}. Traditionally,the permittivity of porous materials,such as 4A zeolite,is obtained through electrical characterization of a sintered dense pellet. However,the structure and dielectric properties of the sintered products may differ from the original loose powders,especially for the materials with high sensitivity to mechanical and thermal treatment.

Recently,a so-called slurry method has been exploited to evaluate in situ the dielectric properties of powders with special structure ^{[6, 7, 8]}. In slurry method,the powders are dispersed in a special liquid dispersing medium,and the permittivity and conductivity of the bulk slurry is measured. By means of effective medium theory ^{[7, 8, 9]},the dielectric parameters of powders are calculated using the measured dielectric parameters of the slurries and medium. For slurry method,the selected medium should match with the dispersed powders. For example,propylene carbonate was considered to be an ideal medium for investigating the permittivity of barium titanate by slurry method. Compared with other organic liquids,the high permittivity (ε = 66.50,at 20 ℃) and the dipolar non-hydrogen-bond of propylene carbonate can compress effectively the error aroused by the huge permittivity (ε > 10³) of barium titanate powders ^[10]. Different from solid powders,there are abundant pores inside 4A zeolite. The interaction between medium molecules and cations inside pore channels,which due to medium immerging into pore,has to be considered. For obtaining the original dielectric properties of porous powders,the diameter ratio of dispersing medium molecule and dispersed particle pore is another important factor.

In the present work,the dielectric responses of 4A zeolite bulk dispersion system to two dispersing mediums,water and cyclohexane,were investigated. Through fitting and calculating the dielectric properties of bulk dispersion systems with appropriate model function,the intrinsic structure of 4A zeolite are obtained and analyzed.

2. Experimental

4A zeolite was commercially available from Sigma Chemical Co. The average particle size is around 4 μm and the density is about 2.07 g/cm³. Two kinds of bulk dispersion systems were prepared by dispersing ultrasonically 4A zeolite in deionized water (4A/W) and cyclohexane (4A/C),respectively. For 4A/W,the dispersing time exceeds 30 min in order the water can permeate inside zeolite pores,and then the soaking time exceeds 2 days in order that ionic exchange between two phases complete totally. Dielectric measurements were carried out on a Wayne Kerr 6500B Precision Impedance Analyzer with a component fixture 1011 (Wayne Kerr Electronics,UK) over a continuous frequency range of 20 Hz to 120 MHz. The amplitude of the applied alternating field was 500 mV. A dielectric measurement cell with parallel stainless steel electrodes was employed,and the volume of the dispersion system used in the experiment was 10 mL in order to submerge the electrodes. The temperature dependence of the dielectric properties was measured from 10 ℃ to 55 ℃ by circulating thermostated water,and the deviation of temperature was less than ±0.3 ℃.

3. Results and discussion

Fig. 1 shows the 3D representations of temperature dependent permittivity spectra of 4A/W and 4A/C respectively. The difference between relaxation behaviors of two bulk dispersion systems can be observed obviously. For comparing the medium effect on two systems quantificationally,the Cole-Cole equation with electrode polarization term (Eq. (1)) was applied to extract the dielectric parameters

where ε* is the complex permittivity of sample,Δε (=ε_l - ε_h) is the permittivity increment,εl and eh are the low- and high-frequency limits of relative permittivity,respectively. v is the angular frequency,τ is the relaxation time (1/2πf),β is the parameter indicating the width of the distribution of relaxation time. A and m are adjustable parameters determined by fitting the experimental data simultaneously.

According to the fitting results listed in Table 1,the 4A/W system displays two dielectric relaxations at initial temperature 270 K,which locate at 10⁴ Hz and 10⁶ Hz respectively. The dielectric increment,Δε,was usually used to illuminate the intensity of the interfacial polarization of the heterogeneous dispersion systems ^[11]. Because of water immerging into pores, the interfacial properties of 4A/W deduced from dielectric increment in Table 1 cannot reflect the original character of 4A zeolite. The diameter of cyclohexane molecule is about 0.5 nm,and is larger than the pore diameter of 4A zeolite (0.4 nm). Although the 4A zeolite particles were dispersed in cyclohexane,the solvent molecules cannot enter the pores of zeolite particles,which still kept original. So Na⁺ in 4A/C is not affected by outside dispersing medium,and can be represented originally by the dielectric increment. According to Maxwell-Wagner equation as follows ^[12]:

where φ is volume fraction,subscript p and m dispersed phase and dispersing medium,a dielectric relaxation characterized by a single relaxation time takes place when ε_p · κ_m ≠ ε_m · κ_p is satisfied. According to Eq. (2),the difference between dielectric and conductive properties of the dispersed phase and the dispersing medium is necessary condition for detecting the dielectric relaxation in DRS. Usually,the permittivity and conductivity of dispersed and dispersing phases change dependently. If the dielectric relaxation is aroused by Maxwell-Wagner interfacial polarization,there should be an obvious response of Δε to temperature. As seen Fig. 2,Δ_εLFR of 4A/C gradually increases with T increasing,and Δ_εHFR keeps constant. So it indicates that the high frequency relaxation cannot be assigned to interfacial polarization, and the low frequency relaxation (LFR) is from interfacial polarization between zeolite particles and dispersing cyclohexane. Considering the motions of exchangeable Na⁺ among pore sites,the high frequency relaxation (HFR) is assigned to the local hopping at cavities of the zeolite framework ^[13].

With the temperature increasing as seen in Table 1,both of relaxations of 4A/W shifted to higher frequency. The temperature dependence of relaxation frequency can be explained by Arrhenius equation:

where τ₀ and E_A are a constant with the dimension of frequency and the activation energy of the relaxation process. As seen in Fig. 3a,the temperature dependence of lnτ_LFR was fitted based on Eq. (3),and E_A,LFR was attained about 39.8 kJ/mol. Because the HFR transferred out of measurement range with the temperature increasing to 290 K,there were only three available data for getting the E_A,HFR,which is about 15.2 kJ/mol. Both values of E_A,LFR and E_A,HFR are much lower than the activation energies of dehydrated 4A zeolite reported in prior study (60 and 49 kJ/mol) ^[14]. The adsorbed water was considered to decrease the electronic interaction between Na⁺ and charged framework,and Na+ can move with less restriction.

Different from 4A/W system,HFR of 4A/C can be measured clearly from 281 K to 341 K. Comparing with the pure 4A zeolite, the interaction between Na⁺ and framework in 4A/C was not affected by the dispersing phase. So the activity of Na⁺ related to HFR is less much than that in 4A/W. According to Fig. 3b,the E_A,HFR of 4A/C is about 43.6 kJ/mol,which is coincident with the data of dehydrated 4A zeolite in the prior study (49 kJ/mol). It indicates that the dispersing medium has no effect on the inner environment of zoelite. Among lower temperature under 300 K,however,there is no LFR found. With the temperature increasing above 300 K,the LFR displays nonlinear temperature dependence. The appearance of LFR can be considered as the result of activation of local Na+. Attained energy enough,part of Na⁺ ions can conquer the restriction from local framework,and started to transfer in supercages. As shown in Fig. 3b,however,the τ₀ of LFR increases firstly and then decreases with temperature increasing from 302 K to 341 K. A transient state as shown in Fig. 4 was supposed to explain the abnormal changing of τ_LFR with increasing temperature. Under medium temperature,Na⁺ is inclined to escape from local sites,and tries to move along supercage. However,the increased activity of Na+ due to moderate temperature is not high enough to ensure Na+ to complete the full transferring movement in supercage (as Fig. 4b). So the Na+ was at semi-activated state, and the movement range is wider than hopping as Fig. 4a,and narrower than transferring. As seen in Fig. 3c,the transient movement induces a turbid response to the frequency.

For 4A/C under transient state,the path for ions transferring is lengthened with increasing temperature. So Eq. (3),which can be used under constant transferring distance,is not suitable to calculate the t of LFR under transient state. Gross offered a model to describe the relaxation time related to the movement of ions as followed ^[15]:

where d and D are movement distance and diffusion coefficience of ion,respectively,the relaxation time will increase with increasing d at constant T. Under transient state,however,both T and d increase stimulatingly. The effects of d and T on relaxation time of LFR are opposite,and the nonlinear relationship between τ and T can be explained by combining Eqs. (3) and (4). According to the part signed by dashed round in Fig. 3b,it can be found that d has stronger effect than T on τ. So τ increases with increasing T under transient state. With T increasing to above 318 K,τ shows T dependence corresponding to Eq. (3). It indicates that the transient state complete and the Na⁺ ion can move along the whole supercage as Fig. 4b. According to the fitting result,the E_A,LFR of 4A/C is about 54.7 kJ/mol,which is closed to the data of dehydrated 4A zeolite mentioned above (60 kJ/mol) ^[14]. 4. Conclusion The dielectric relaxation spectra of 4A zeolite bulk dispersion systems were measured,and the dispersing medium was found to affect obviously the dielectric behaviors of zeolite dispersion systems. By comparing the effect of dispersing medium,the organic liquid with larger radius than pore diameter can be used to offer original dielectric properties of zeolite particles. In order to exploit slurry method in porous materials,the choice of dispersing medium is a key step.