(40e) Measuring the Adsorption Induced Strain of Zeolite Membranes Using Magnetoelastic Sensors
Zeolite membranes are synthesized in the form of a polycrystalline zeolitic film on the external surface of a porous support. As a result, knowledge about the mechanical properties (i.e. expansion coefficient, hardness, Young modulus etc) of the zeolite membrane and the porous support, is very important for the design of mechanically stable zeolite membranes. It is known that the presence of adsorbed molecules in the zeolitic pore network might induce changes to the crystal unit cell dimensions (crystal swelling or contraction). For example, it has been reported that the adsorption of linear hydrocarbons in silicalite-1 results in the expansion of the unit cell volume by up to 1.2% . The adsorption of benzene does not cause any significant changes of the unit cell dimensions . It was also reported that the adsorption of i-butane, and p-xylene might shrink the size of the silicalite-1 crystals . It is also known, that the unit cell size of faujasite might shrink or expand due to the adsorption of water or CO2, depending on the temperature and loading [3, 4]. Such phenomena might reduce or increase the size of the non-zeolitic pores (grain boundaries) and have significant impact on the zeolite membrane performance. The extent of these changes depends on the elastic properties of the zeolite films which can be quantified by the Elastic Modulus (Young Modulus) of the zeolite film.
We have recently proposed a method for measuring the Young modulus of zeolite films using magnetoelastic sensors [5,6]. The zeolite films were synthesized on both sides of 6mm × 20mm × 28ìm Metglas magnetoelastic ribbons of the 2826MB type (Allied Signal). For each zeolite structure, the resonance frequency f of sensors having different thicknesses was measured as a function of temperature and adsorbate loading. The effective density of the composite , where r, r' and h, h' are the density and thickness of the Metglas and zeolite film correspondingly, was calculated taking into account the effect of adsorption on the zeolite crystal density. The effective Young modulus, where E, E' are the Young modulus of the Metglas and zeolite film correspondingly, was calculated using Eq. (1) below, where L is the ribbon's length, from the resonance frequency data. Finally, the zeolite film Young modulus was determined from the slope of the E'' vs h'/h.
Besides the Young Modulus, another important elastic parameter is the adsorption induced strain of the zeolite film. Under adsorption, the zeolite film induces an interfacial stress on the Metglas ribbon. Unfortunately, it is very difficult to derive quantitative information about adsorption induced strain from the analysis of the resonance frequency measurements at different loadings because all the parameters of Eq. (1) are affected (effective density, the ribbon length L, and the Young modulus of the magnetoelastic ribbon (Villari or DE/E effect )).
An alternative way to overcome these problems is to take the ratio of the resonance to the anti-resonance frequencies, which equals to k2, where k is the so called ?magnetomechanical coupling factor' of the sensor, which depends on the axial stress of the magnetoelastic ribbon according to the following relation :
Where c1 and c2 are material constants, and H the intensity of the bias applied magnetic field.
We have used this new approach in order to measure the effect of H2O adsorption on FAU membranes as well as of several VOCs (i.e. n-hexane, benzene, o- and p- xylene, etc) on silicalite-1 films. The effect of H2O adsorption on the resonance frequency and the magnetomechanical coupling factor of a FAU /Metglas composite ribbon is shown in Figure 1. The resonance frequency of the sensor passes through a minimum with water vapour partial pressure. This behavior can not been explained by considering solely the density changes of the zeolite film due to adsorption since they would lead to a monotonic decrease of f, reaching at saturation plateau at high loadings. In the same figure, it is seen that the value of k2 passes through a maximum with water vapor pressure, which indicates that at low H2O loadings the FAU films expand. However once the loading passes a critical value the opposite trend is observed.
The effects of temperature and loading have also been examined and will be discussed both in terms of adsorption induced strain as well as in respect to its importance on the zeolite membrane performance.
Figure 1: Resonance frequency f and magnetomechanical coupling factor k2 of a FAU/metglas composite ribbon as a function of water vapour pressure at 40oC.
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