Verified Syntheses of Zeolitic Materials

2nd Revised Edition

Characterization of zeolites by sorption capacity measurements

Douglas M. Ruthven
Department of Chemical Engineering, University of Maine, Orono, ME 04469-5737, USA

1. Introduction

Sorption capacity measurement provides one of the simplest and most direct ways of characterizing a zeolite sample. However, the information derived from capacity measurement generally provides only an estimate of sample purity and/or evidence of consistency with a known structure, rather than a means of differentiation between different structures.

The adsorption equilibrium isotherm for a microporous adsorbent, such as a zeolite, is of type I form in the Brunauer classification and, in the appropriate temperature range, the isotherm is highly favorable with a well defined saturation limit corresponding to complete filling of the intracrystalline pores. [1] The molecular volume of the adsorbed phase corresponds closely to that of the saturated liquid sorbate at the same temperature so the measured saturation capacity can be easily converted to the specific micropore volume (cm³/gm) or, knowing the crystal density, to the intracrystalline void fraction. Representative isotherms are shown in Figure 1 (a) and (b), and the calculation of the specific pore volumes is summarized in Table 1.

Table 1. Calculation of specific micropore volumes (Fig. 1)

* First plateau. Second plateau yields essentially the same pore volume if the density of solid N₂ (1.02 g/mL) is used.

2. Experimental Technique

Several different techniques may be used to measure the capacity. The essential features of the apparatus used for gravimetric and piezometric measurements are shown in Figure 2. [4]

2.1 Gravimetric Method

In a gravimetric experiment the mass of the sample is measured directly using a microbalance connected to a vacuum line, equipped with a pressure gauge and a system for introducing the sorbate vapor. The sample is first degassed under vacuum at elevated temperature and then cooled to the measurement temperature. Successive doses of sorbate are introduced and the sorbate pressures and corresponding masses are recorded. Provided that the temperatures and range of sorbate pressures are correctly selected, the isotherm should be almost rectangular. Under these conditions accurate measurement of the sorbate pressure is not necessary since the capacity is almost independent of pressure.

2.2 Piezometric Method

in the piezometric method the quantity of sorbate adsorbed is deduced from measurements of the change in pressure in a system of known volume when a known quantity of sorbate is introduced. The system volume can be easily determined using a calibrated doser volume. Accurate pressure measurements are needed, and of course the entire system should, ideally, be maintained at a uniform constant temperature. This requirement imposes significant practical difficulties when the measurements are to be made at a temperature far from ambient. For this reason the gravimetric method is generally preferred.

Click here for for Figure 1

2.3 Automated BET System

Automated BET measurement systems such as Omnisorb can also be used to measure saturation capacity. In such devices a dilute stream of sorbate in an inert (He) carrier is passed through the sample and the capacity is found by integration of the measured breakthrough curve, thus providing essentially the same information as is obtained from a gravimetric or piezometric measurement.

Although the basic experiment is very simple, a number of precautions are necessary to obtain accurate results:

(1) In principle a buoyancy correction is needed to allow for the mass of vapor displaced by the zeolite sample. However, except at high pressures, such corrections are generally negligible.

(2) It is important to minimize any temperature gradient in the region of the sample in order to avoid errors due to thermal transpiration and convection. Thiscan normally be achieved simply by ensuring that the hangdown tube containing the sample pan dips well below the surface of the thermostat liquid.

(3) To avoid the possibility of capillary condensation in the interstices between crystals the sample should be dispersed on the balance pan and the relative pressure (p/p_s) should not exceed about 0.25. (p_s is the saturation vapor pressure at the relevant temperature).

(4) At high relative pressures significant (multilayer) physical adsorption can occur on the external surface of the crystals. If the crystals are small (sub-micron), this can give rise to a significant error in the measured capacity. This problem can also be avoided by keeping the relative pressure below about 0.25.

(5) To correct for the positive slope of the saturation plateau the Dubinin-Radushkevich equation is sometimes used to extrapolate to zero pressure. However, with a near rectangular isotherm of the kind shown in Figure 1, the difference between this extrapolation and the actual value at the plateau is minor.

Click here for Figure 2

3. Preconditioning

Prior to a capacity measurement any organic template from the synthesis must be removed by oxidation and the sample must be thoroughly degassed.. If the synthesis does not include a template then only the degassing step is needed. Oxidation to remove the template is generally carried out by exposure to air for several hours at the highest temperature that can be tolerated by the zeolite without structural degradation - typically 500-550⁰C for silicalite/ZSM-5. Degassing is convenientiy carried out at elevated temperature in the vacuum system. Temperatures in the range 350-400⁰C are commonly used, but for less stable materials somewhat lower temperatures can be used. To some extent a reduction in temperature can be compensated for by using a higher vacuum for a longer period of time. The aluminum rich zeolites have poor hydrothermal stability, that is, they are unstable in the presence of water vapor at elevated temperatures. In degassing such samples it is therefore important to raise the temperature slowly maintaining a good vacuum all the time so that water vapor is removed as soon as it is desorbed.

A typical procedure for degassing a sample of A or X zeolite is as follows:

(1) Increase temperature under vacuum from ambient to 400⁰C over a period of 4-5 hours.

(2) Maintain the temperature at 400⁰C under vacuum overnight.

(3) Cool (under vacuum) to the experimental temperature (1-2 hours.).

4. Choice of Sorbate and Measurement Temperature

In principle almost any of the common small molecules can be used as the probe sorbate although, in practice, the choice is generally restricted to Ar, O₂ or N₂. Linear paraffins such as nhexane have sufficient flexibility to pack within the micropores almost as effectively as the smaller molecules so n-hexane capacities are also commonly used to measure specific micropore volume. Bulkier molecules such as i-butane pack less efficiently and thus yield erroneously low estimates for the micropore volume. Molecules such as Ar, O₂ and N₂ do not penetrate 6-membered oxygen rings, at least at low temperatures, so the saturation capacity determined from measurements with these sorbates includes only the pore volume accessible through 8-rings (or larger). Although N₂ is the usual choice of probe molecule for BET surface area measurements, it is not the best choice for measuring the micropore volume of a zeolite, since, probably as a result of quadrupole interactions, N₂ isotherms commonly show complex changes in packing density leading to hysteresis of the isotherm, even for an ideal pore system. [3,5] in contrast, the low temperature isotherms for Ar and O₂ are generally close to the ideal rectangular form (see Fig. 1).

The choice of measurement temperature is dictated by the practical requirement that the desired relative pressure range (0.05 <P/Ps <0.2) should correspond to an easily measured range of absolute pressures. This means that if the measurements are to fall within the convenient subatmospheric pressure range, the temperature should not be too far from the normal boiling point of the sorbate.

Water is a very small molecule, and, as a result of its strong dipole, it is strongly adsorbed, especially in Al-rich zeolites. Water isotherms at ambient temperature generally show the charactenstic rectangular form observed for other small molecules. However, the saturation capacities for water (based on the molecular volume) are often higher than the values determined using Ar, O₂ or even N₂ since water can penetrate regions of the framework (for example, sodalite and cancrinite cages) which are not accessible to Ar, O₂ and N₂. Comparison between the saturation capacities measured with water and with Ar, O₂ or N₂ can therefore provide additional structural information.

5. B.E.T. Area

BET areas are commonly determined for zeolite samples by the same method as is used for larger pore materials. [6] Under the conditions of the BET measurement the N₂ molecules condense, filling the micropore volume. Thus the BET area of a zeolite is really the equivalent area that would be covered by the quantity of sorbate required to fill the intracrystalline pores if the molecules were arranged as a close packed monolayer. It does not correspond to the internal area of the framework.

6. Mesoporosity

As a result of structural defects the intracrystalline pores of a zeolite sometimes contain a significant proportion of mesopores (20-50 A) in addition to the ideal zeolite pores (< 15 Å). The presence of significant mesoporosity leads to a positive slope instead of the almost horizontal saturation plateau characteristic of an ideal microporous structure.

7. Determination of Pore Size

In this article we have considered only the use of capacity measurement to determine specific micropore volume. Such measurements using molecules of different critical diameter may also be used to establish the controlling pore dimensions in an unknown structure (see, for example Gaffney, et al, [7]) but this is outside the scope of the present review.

8. References

[1] D. M. Ruthven, Principles of Adsorption and Adsorption Process, John Wiley, New York, 1984, p.49
[2] D. T. Hayhurst, J. C. Lee, AIChE Symp. Series 230, Vol. 79, Am. inst. Chem. Eng., New York, (1983) pp. 67-78
[3] U. Muller, K. K. Unger, Characterization of Porous Solids, K. K. Unger (ed.), Elsevier, Amsterdam, 1988, p. 101
[4] M. F. M. Post, Introduction to Zeolite Science and Practice, H. van Bekkum, E M. Flanigen, J. C. Jansen (eds.), Elsevier, Amsterdam, 1991, Ch. 11
[5] D. W. Breck, R W. Grose, Molecular Sieves, Adv. Chem. Ser. 121, W. M. Meier, J. B. Uytterhoeven (eds.), Am. Chem. Soc., Washington, DC, 1973, p. 319
[6] S. J. Gregg, K. S. W. Sing, Adsorption, Surface Areas and Porosity, Academic Press, London, 1967
[7] T. R. Gaffney, T. A. Braymer, T. S. Ferris, A. L Cabrera, C. G. Coe, J. N. Armor, Separation Technology, U F. Vansant (ed.), Elsevier, Amsterdam, 1994, p. 317