Verified Syntheses of Zeolitic Materials
2nd Revised Edition
Alan Dyer
Department of Chemistry & Applied Chemistiy, University of Salford, Salford
MS 4WT, U. K.
1. Natural Zeolites
In a traditional aluminosilicate zeolite the source of the ion exchange capacity is the extent of isomorphous substitution of Al for Si in the tetrahedral framework. The theoretical exchange capacity thus can be derived from the elemental composition.
To estimate the CEC (cation exchange capacity, meq/g) in a natural zeolite it is usual to observe the uptake of the ammonium cation at room temperature when equilibrium conditions are known to have been attained in the presence of a 1 M ammonium salt solution. This working capacity can be obtained by batch or column exchange techniques. [1]
2. Synthetic zeolites and related materials of SAPO, MeAPO type ("zeotypes") [2]
When synthetic materials are to be characterized, prior careful elemental analysis will provide the expected, theoretical cation capacity. It is now important to establish equilibrium conditions with a specific cation for which the synthetic product has a highly selectivity; consider:
AZ + Baq = BZ + Aaq
At equilibrium the "as-synthesized" cation A, in the zeolite phase Z, has been completely displaced by the selective uptake of cation B from the aqueous phase (aq). The following procedure is recommended assuming that the ammonium cation will be the B species.
2.1 Determination of cation exchange capacity at equilibrium.
(1) Weigh suitable aliquots of the zeolite into sealable polyethylene tubes.
(2) Add equal volumes of a known concentration of ammonium nitrate solution to the tubes and mix at room temperature. It is preferable to mix by rolling the tubes slowly about their horizontal axis. Mineralogical rollers of the type used to polish gemstones can easily be adapted for this purpose. The solid/solution volume ratio should be at least 1/20.
(3) At appropriate time intervals remove each tube in turn and determine the concentrations of the displaced cation (A) in solution.
(4) Construct a plot of the concentration in solution of A as a function of time (Fig 1. plot 1). The vertical axis can be expressed as fractional attainment of equilibrium assuming the calculated theoretical capacity as 100%.
2.2 Notes
(a) To quote the capacity in meq/g it will be necessary to determine the equilibrium water capacity of the end member (for example, the B form of the zeolite).
(b) If the expected theoretical capacity is not reached (Fig 1. plot II) it means that, at room temperature, cations in certain sites are not being displaced. This is the ion-sieving phenomenon and can be used to help in structural interpretation of the zeolite structure. Increasing the temperature at which exchange is performed should enable the attainment of full capacity. Usually temperature in the range 60 - 70ºC are adequate.
(c) Choice of conditions are a function of the expected theoretical capacity. A "rule of thumb" is to aim for a 5-fold solution excess of ingoing cation (B). The time for attainment of equilibrium depends on the openness of the zeolite framework and will vary from about 1 hour to 1 week.
3. Experimental methods
The most sensitive analytical technique available is to use radioisotopes to prelabel the A cation and follow its replacement by increase in solution activity with time. (Isotope dilution technique). This will be possible for the as synthesized cations Na, K, Rb, Cs, Ca, Sr and Ba.
In the absence of radiochemical facilities, flame photometry, atomic absorption are perfectly adequate. An alternative is to monitor the ammonium concentration by Kjeldahl titrimetry. Ammonium concentrations can be determined in either the zeolite or solution phase.
4. Other points
(1) When an organic template has been used to synthesize the zeolite, careful calcination is needed to remove it from the framework. This leaves the zeolite in the H form (H3O+ cation in solution). In these cases it is possible to use pH titration to follow ammonium (or other cation) uptake as the H3O+ has a low affinity for most zeolites. An exception can be in exchanges observed in materials of SAPO / MeAPO types. These materials sometimes show an unusual affinity for the H3O+ ion, especially over Na+. [3]
(2) It is advisable to limit washing to a minimum. Zeolite frameworks are well known to hydrolyze. This is not confined to just those with Si/Al ratio ® 1. [4] It is always advisable to check for the presence of extra framework Al (by MAS NMR). This can arise from template removal and from instability of P-containing frameworks. Obviously, extra framework Al contributes to cation exchange capacities, but can be quantified. [5]
(3) Some zeolite like materials and high silica materials may have cation and anion capacities due to the presence of framework hydroxyls. This will be pH dependent, but experience is that these OH groups make small contributions to exchange capacities. [61
5. References
[1] B. W. Mercer, L L Ames, Natural Zeolites, Occurrences, Properties, Use,
L B. Sand. F. A. Mumpton (eds), Pergamon, Oxford, 1988, p. 451
[2] A. Dyer, An Introduction to Zeolite Molecular Sieves, J. Wiley & Sons,
Cichester, 1988, Ch. 10
[3] C. M. G. Jones, It Harjula, A. Dyer, Stud. Surf. Sci. Catal. 98 (1995) 131
[4] R. Harjula, A. Dyer, R. P. Townsend, J. Chem.Soc., Faraday Trans. 89 (1993)
977
[5] A. Dyer, S. Amini, H. Enamy, H. A. EI-Naggar, M. W. Anderson, Zeolites,
13 (1993) 281
[6] A. Dyer, T. I. Emms, unpublished work