Understanding the effect of zeolite crystal expansion/contraction on separation performance of NaA zeolite membrane: A combined experimental and molecular simulation study
Introduction
Dense zeolite membranes demonstrate high separation selectivity for molecular separations based on their well-defined micropores and preferential adsorption properties. However, because of the multicrystalline structure of zeolite membranes, the separation performance is strongly affected by intercrystalline defects. These intercrystalline nonzeolitic pores are mostly larger than zeolite pores resulting in lower separation selectivity. NaA zeolite membranes with LTA structure are commonly used for dehydration of solvents because of their strong hydrophilicity and suitable pore size [1], [2], [3], [4], [5]. Even in the presence of nanometer-sized defects, a high pervaporation selectivity up to 10000 has been achieved using the NaA membranes [6], [7], [8], [9].
Recently, we found that the isopropanol (IPA) flux through NaA membrane increased rapidly as the concentration of water decreased in the water/IPA mixture during vapor permeation (VP). However, we did not observe such behavior in the case of water/methanol or water/ethanol mixture. Shah et al. also found similar behavior for NaA membranes during their study on the permeation fluxes of pure components through zeolite membrane [10]. The IPA flux was more than three times the ethanol flux and slightly higher than the methanol flux. Capillary condensation is generally used to explain the phenomenon, where water molecules are adsorbed in the defects and block the flow of other (larger) molecules through them [8], [9]. While capillary condensation may explain the observed increase in IPA flux through NaA membranes as water content decreased, it cannot explain the relatively stable flux of methanol or ethanol as water content decreased. Since IPA (kinetic diameter ~0.48 nm) is larger than the zeolite A pores (0.42 nm), it only enters and subsequently transports through the defects [11]. Meanwhile, methanol (0.38 nm) and ethanol (0.43 nm) can enter both the NaA zeolite pores and the defects; thus the methanol or ethanol flux should also increase mostly due to capillary condensation mechanism. Although the observed behavior is often attributed to preferential adsorption, a more reasonable explanation could be that there are changes in the effective size of the defects due to such adsorption, as well as changes in the crystal sizes [11].
A series of extensive studies on MFI zeolite membranes by Noble and Falconer demonstrated that polycrystalline zeolite membranes are flexible and defect sizes can decrease or increase when certain molecules adsorb in the zeolite pores [12], [13], [14], [15], [16], [17], [18], [19]. For example, they found the molecules with sizes larger than MFI pores, such as DMB and 1,3,5-trimethylbenzene (TMB), showed fluxes through MFI zeolite membrane as high as 2 orders of magnitude larger than those for n-hexane during single-component pervaporation, even though n-hexane is significantly smaller than these molecules. The low n-hexane flux was attributed to the decrease of the size of defects induced by MFI crystal expansion upon n-hexane adsorption [12]. X-ray diffraction measurements [13] further confirmed that MFI crystals expanded upon n-hexane adsorption. Feeding small amounts of gases or vapors that expand zeolite unit cell size reduced permeation through nonzeolitic pores [14], [15], [16], [17], [18]. This effect was so pronounced that in some cases, it significantly enhanced permselectivity [17], [18]. On the other hand, a negative impact on permselectivity was also observed due to adsorption of compounds at certain loadings that shrink the MFI zeolite size (i.e. p-xylene, i-butane, etc.) [19].
Adsorption induced changes of NaA zeolite crystal unit cell size have been previously reported [11], [20], [21], [22]. Adsorption of water may cause both shrinkage and swelling of the NaA zeolite crystals. Sarakhov et al. [20] reported that the NaA unit cell contracted as much as 0.3 vol% at 295 K due to adsorption at low loadings of water and expanded 0.57 vol% at higher saturation water loadings. Caro et al. [21] studied the change of the unit cell dimension for zeolites as a function of temperature and water content by in situ-heating XRD. A significant change was observed in LTA type zeolites because of de-watering. They also attributed the difficulty in preparation of shape-selective LTA membranes for gas separations to the extreme expansions/shrinkages of its unit cell during water removal [22]. Sorenson et al. [11] found that at a thermodynamic activity of 0.03 water contracted NaA zeolite by 0.22 vol% and increased helium flow through a NaA membrane by about 80%; it also increased the i-butane flow by 14% during vapor permeation and i-propanol flow by 25% during pervaporation. At activities above 0.07, water expanded NaA crystals and they observed decreases in the flow of helium, i-butane, and IPA through the NaA membrane. They concluded that the observed high pervaporation selectivities for water/alcohol separations in zeolite NaA membranes were due, at least partially, to water-induced expansion of NaA crystals.
All these previous studies have shown that zeolite membranes are flexible and the size of the inter-crystalline non-zeolite pores can change due to adsorption of suitable molecules. However, the effect of the adsorption induced changes in NaA crystals on the membrane separation performance has not been widely studied. Since NaA membranes are widely used to dehydrate solvents in industrial applications, a better understanding of this phenomenon will be valuable for improving these processes. Molecular simulation studies since the 1990s have traditionally used flexible models to study zeolites [23]. The rapid development in computer technology has enabled the scientists to apply the flexible models in the molecular dynamics simulations recently. In our studies, we have also used a flexible zeolite framework model to examine the contractions/expansion of the framework.
In this paper, we have investigated water and alcohol adsorption in NaA crystals and the resulting crystal swelling or shrinkage and their effect on selectivities of NaA membranes for alcohol/water mixtures separation by vapor permeation. Tubular NaA zeolite membranes were used in this study and the membrane separation performance (flux and selectivity) of various binary water/alcohol (water/methanol, water/ethanol, water/isopropanol) mixtures over a range of water concentration between 0.5 and 5 wt% at 373 K were investigated. Four unit cells of the zeolite were investigated to simulate the changes in the NaA unit cell size induced by adsorption and permporosimetry measurements were used to determine the corresponding changes in the size of the defects. We believe the observed permeation results can be explained by the changes in size of the defects in the zeolite membrane due to adsorption. These conclusions are based on both our molecular simulations as well as permporosimetry studies. To further reinforce out conclusions we carried out additional studies with ternary mixtures which further confirmed our results and observations.
Section snippets
Preparation of NaA zeolite membrane
NaA zeolite membranes were supplied by Jiangsu Nine Heaven High-Tech Co., Ltd. The membranes were hydrothermally synthesized on a porous mullite tube using the secondary growth method as described in our previous work [24]. The support had a length of 80 cm, an outer diameter of 12 mm, a wall thickness of 2.5 mm, a porosity of ~40% and an average pore size of 1 µm. The outer surface of the support was coated with NaA zeolite seeds before membrane synthesis. The seeded support was then immersed in a
Membrane characterizations
Fig. 3 shows the morphologies of as-synthesized NaA zeolite membranes. As can be seen, for both M1 and M2, the support surfaces were completely covered by cubic crystals with well intergrowth. No cavity between the crystal particles was found by SEM. The membranes were approximately 20 µm thick. Pervaporation of a 10 wt% water/ethanol mixture at 348 K showed selectivities greater than 5000 for membranes M1 and M2 (Table 1), indicating they were both high quality membranes. Both water flux and
Conclusion
In this report, the effect of adsorption-induced changes in zeolite crystal size on the separation performance of NaA zeolite membrane for dehydration of alcohols was investigated using experimental measurements in combination with molecular simulations to provide molecule level insight to explain some of the experimental observations. Experiments were conducted with several water/alcohol (water/methanol, water/ethanol, water/isopropanol) mixtures over a range of water concentration between 0.5
Acknowledgement
This work is sponsored by the National Natural Science Foundation of China (21222602, 21490585), National High-tech R&D Program of China (2015AA03A602), the Outstanding Young Fund of Jiangsu Province (BK2012040), Young Fund of Jiangsu Province (BK20130915), the “Six Top Talents” and “333 Talent Project” of Jiangsu Province. XW and SM were additionally supported by grants from the US National Science Foundation (grant no. CBET 1263107/1545560).
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