Influence of zeolite crystal expansion/contraction on NaA zeolite membrane separations

Abstract

In situ powder XRD measurements showed that adsorption causes the NaA zeolite unit cell to contract or expand, and these changes depend on adsorbate loading. Changes in zeolite crystal size correlated with permeation changes through NaA zeolite membranes. These membranes had high water/alcohol pervaporation selectivities, even though gas permeation was mainly through defects, as indicated by Knudsen selectivities for single gas permeation. At 300 K and a thermodynamic activity of 0.03, water contracted the NaA crystals by 0.22 vol%, and this contraction increased the helium flux through two NaA membranes by approximately 80%. Crystal contraction also increased the fluxes of i-butane during vapor permeation and i-propanol (IPA) during pervaporation (∼0.03 wt% water). At activities above 0.07, water expanded NaA crystals and correspondingly decreased the fluxes of helium, i-butane, and IPA through NaA zeolite membranes. Methanol also contracted NaA crystals, but by only 0.05 vol% at an activity of 0.02, and this contraction slightly increased the helium and i-butane fluxes through a NaA membrane. Above an activity of 0.06, methanol expanded the NaA crystals, and the fluxes of helium and i-butane through a NaA membrane decreased. These adsorbate-induced changes explain some pervaporation behavior reported by others, and they indicate that crystal expansion and contraction may increase or decrease zeolite NaA membrane selectivity by changing the defect sizes.

Introduction

Zeolite membranes consist of thin layers of intergrown crystals on porous supports and the intercrystalline regions typically contain defects that are larger than the zeolite pores. Even a small number of defects can significantly decrease separation selectivity. Sodium A zeolite membranes, which have the LTA structure, separate water/alcohol mixtures with selectivities as high as 54,000 using pervaporation, even though they possess nanometer-sized defects [1], [2], [3], [4]. Indeed a large fraction of light gases permeate through these defects, as demonstrated by the low single-gas selectivities for these gases [3], [5], [6]. In spite of gas permeation through defects, NaA zeolite membranes have been commercialized for alcohol dehydration [2], [7].

One study reported that NaA membranes have low water/alcohol ideal selectivities [8], indicating that water significantly affects alcohol fluxes, since mixture selectivities are typically high. Shah et al. reported that for mixtures containing 70 wt% water and 30 wt% methanol or ethanol at 333 K, the water flux was over 300 times the methanol flux (selectivity of 140) and 5000 times the ethanol flux (selectivity of 2140) [8]. In contrast, the pure water flux at 333 K was only 10 times the pure methanol flux, 30 times the pure ethanol flux, and 9 times the pure i-propanol (IPA) flux [8], even though the diffusivity through NaA zeolite pores decreases by an order of magnitude in the order: water > methanol > ethanol [9]. The IPA flux was more than three times the ethanol flux and slightly higher than the methanol flux, even though IPA is larger than the zeolite A pores, so it only transports through the defects. Since methanol and ethanol transport through both the NaA zeolite pores and the defects, the IPA flux should be lower than the methanol and ethanol fluxes. Shah et al. concluded that methanol and ethanol have low dielectric constants, and therefore have less interaction with the zeolite, and consequently lower fluxes [8]. Another explanation for this behavior is that the sizes of the defects change due to adsorbate-induced changes in the crystal sizes.

Yu et al. reported similar behavior for MFI membranes that exhibited high n-hexane/2,2 dimethylbutane (DMB) separation selectivities [10]. Although n-hexane adsorbs and diffuses through MFI pores, its flux was lower than the fluxes of molecules that only transported through defects (DMB, i-octane, and 1,3,5-trimethylbenzene (TMB)). Yu et al. hypothesized that MFI crystals expanded upon n-hexane adsorption, and this expansion decreased the size of the defects [10]. X-ray diffraction (XRD) showed that MFI crystals expanded upon n-hexane adsorption [11], [12], and the separation selectivities of mixtures that contained a molecule that expanded the MFI unit cell and a molecule that only permeated through defects were much higher than ideal selectivities [10], [13]. Moreover, the percentage volume expansion of the MFI unit cell for a series of n-alkanes correlated with decreased flux of i-octane through defects in MFI membranes [14].

Expansion of the LTA unit cell by water has been reported [15], [16], [17]. Barrer et al. [16] reported that the NaA unit cell expanded 0.49 vol% at 298 K when saturated with water [16]. Sarakhov et al. [17] reported that the NaA unit cell contracted as much as 0.3 vol% at 295 K upon adsorption of low loadings of water and expanded 0.57 vol% at saturation water loadings [17]. However, previous studies did not suggest that crystal expansion influenced water/alcohol separation selectivities in NaA membranes. Instead, the high pervaporation selectivities in NaA membranes have been attributed to preferential water adsorption by Kondo et al. [2], and to capillary condensation of water in defects (the water in the defects blocked permeation of other molecules) by Okamoto et al. [3]. Kondo et al. reported pervaporation fluxes as high as 8.37 kg/(m² h) at 393 K, with separation factors as high as 47,000 for a 10 wt% water/ethanol feed [2]. Okamoto et al. measured selectivities greater than 10,000 for 5 and 10 wt% water/alcohol mixtures at 323 and 348 K [3]. van den Berg et al. hypothesized that because NaA zeolite is so hydrophilic, water strongly interacts with the zeolite and adsorbs on the defect walls and blocks the flow of other molecules through the defects [4]. van de Berg et al. concluded that for defects smaller than 1.6 nm, only water adsorbed in the pores, and because of the strong water interactions with NaA zeolite, defects up to several nanometers improve membrane performance by increasing the flux without decreasing the selectivity [4].

In the current study, the effects of water and methanol on the NaA unit cell were measured by powder XRD for a range of loadings, and the unit cell changes were compared to pervaporation and permporosimetry measurements on NaA membranes. The objective was to determine if adsorbate-induced expansion of NaA crystals affects pervaporation separations of water/alcohol mixtures. Since NaA membranes are used to dehydrate alcohols on a large scale, understanding how adsorbate-induced changes in the NaA unit cell affect permeation may be valuable for improving these processes and understanding their behaviors. The tubular membranes used in this study had high selectivities for water/alcohol pervaporation separations. Fluxes through the membrane defects were measured using i-butane gas and IPA liquid permeation, because these molecules are too large to adsorb in the NaA zeolite pores. In addition to investigating the effects of water and methanol adsorption, the effect of ethanol adsorption in a NaA membrane was investigated.