Influence of exchanged cations (Na+, Cs+, Sr2+ and Ba2+) on xylene permeation through ZSM-5/SS tubular membranes

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Abstract

Na-ZSM-5 membranes were synthesized by secondary growth on the outer surface of stainless steel porous tubes. The membranes were ion-exchanged with Cs+, Ba2+ and Sr2+ to investigate their effect upon the separation of p-xylene from m-xylene and o-xylene. The permeation through the membranes was measured between 150 and 400 °C using each xylene isomer separately and a ternary mixture. All the membranes were selective to p-xylene in the temperature range studied. N2 and xylene permeation measurements together with SEM observations were used to determine whether or not cracks and/or pinholes developed after exposure to the xylene isomers at high temperature (400 °C). Neither pore blockage nor extra-zeolitic pores developed after the ion exchange procedure and subsequent calcination. Furthermore, duplicate synthesized membranes of each cation form had similar separation factors and permeances. The duplicate values differ much less than the measurement error. The p-xylene permeation flux decreased in the order: Na-ZSM-5 > Ba-ZSM-5 > Sr-ZSM-5  Cs-ZSM-5 while the permeation flux of the m- and o-xylene decreased in the order Na-ZSM-5 > Sr-ZSM-5 > Ba-ZSM-5 > Cs-ZSM-5. The membrane that exhibited the best performance was Ba-ZSM-5, with a maximum p/o separation factor of 8.4 and a p-xylene permeance of 0.54 × 10−7 mol s−1 m−2 Pa−1 at 400 °C.

Introduction

Interest in zeolite membranes has increased in the last few years due to their potential application to a wide range of gas/liquid separations, difficult to achieve by conventional techniques. Since the size of the zeolite pores is similar to the molecular dimensions, both selective adsorption and molecular sieving play important roles in the separation of hydrocarbon mixtures through this type of materials. In addition, these structures are thermally stable and resistant to aggressive environments, two very attractive features for petrochemical industry applications.

MFI-type zeolitic membranes (e.g., silicalite and ZSM-5) are able to separate mixtures of molecules with similar physical properties such as chemical isomers. Accordingly, significant efforts are being made to modify these membranes in order to improve their separation performance. Several groups have reported various results on the separation of xylene isomers using MFI-type zeolite membranes synthesized under different conditions [1], [2], [3], [4], [5]. The MFI-type structure has a three-dimensional pore system with straight channels in the b-direction (0.54 nm × 0.56 nm), interconnected with sinusoidal channels in the a-direction (0.51 nm × 0.55 nm). These pore sizes are close to the kinetic diameter of p-xylene (dk = 0.58 nm), and it is expected that its bulkier isomers (m- and o-xylene, dk = 0.68 nm) would diffuse at a significant slower rate through these pores. Thus, defect-free MFI zeolite membranes are expected to be capable of separating p-xylene from its isomers by shape-selectivity. However, literature results of xylene separation with MFI zeolite membranes present serious discrepancies which are mainly due to differences in membrane microstructure, material chemistry, and operational conditions.

Differences in chemical affinities, rather than shape or size with respect to the zeolite pore for the permeating molecules often determine the separation properties of the zeolite membranes. One accessible way to modify the sorbent–sorbate interaction is through ion exchange. The affinity between zeolites and permeating molecules also plays an important role in the separation of adsorbing and non-adsorbing molecules. Based on these data, it could be predicted that permselectivity for mixed component systems is dependent on the interaction between permeates and pore walls as well as pore size. If a zeolite is ion-exchanged with cations which possess different interactions with permeates, this could improve the permselectivities of the membranes.

To this day, only a limited number of studies have been reported on permselectivities of ion-exchanged zeolite membranes. Jafer and Budd [6] used ion-exchanged NaA zeolite membranes with K+ but observed no significant changes in pervaporation (isopropanol/water mixture) performance. Kusakabe et al. [7], [8], [9] studied the effect of different cations (Li+, K+, Mg2+, Ca2+ and Ba2+) on the performance of NaY membranes. CO2 and N2 single gas permeances were modified with these cations even though gas permeation rates and selectivities were not directly related to their size. Aoki et al. [10] studied the separation of butane isomers with ion-exchanged ZSM-5 zeolite membranes. In this study, the permeances decreased with increasing cation diameters (H+, K+ and Ba2+) but the n-C4H10/i-C4H10 separation selectivities did not increase, even though the effective pore diameter apparently decreased.

The literature reports that p-xylene adsorbs selectively from a mixture of C8 compounds on several zeolites. The selectivity for this isomer is strongly dependent upon the zeolite composition (Si/Al ratio and nature of the exchanged cation). The BaY zeolite is known to be selective for p-xylene, i.e., it preferentially adsorbs this molecule from a mixture of C8 aromatics. Under the same conditions, the NaX zeolite is not selective. Both the adsorption of the molecules as well as their diffusion behavior contribute to the selectivity [11], [12]. Adsorption and diffusion parameters play a key role in the permeation mechanism of the xylene isomers through MFI zeolite membranes [2], [13]. Therefore, if the membranes are exchanged with different cations their performances will be modified. In a recent study [14], we reported a Na-ZSM-5 (Si/Al 100) composite tubular membrane with xylene permeances and separation factors equal to, or better than, other data reported for this geometry in the literature. However, it was also noticed that there was a strong need to improve the performance of the synthesized materials for practical applications. An easy way to modify the transport properties of the membrane was thought to be through ion exchange.

Another practical matter is to synthesize tubular composite membranes more amenable to industrial reactor applications. However, this geometry is more demanding than the flat disks, e.g. lower proportion of defects and higher selectivities are obtained with the latter type.

In this work, the single component and ternary mixture xylene permeation properties of the ZSM-5 exchanged membranes were studied. The membranes were synthesized in the outer surface of a stainless steel tubular support. The effect of different alkaline (Na+ and Cs+), and alkaline earth (Ba2+ and Sr2+) cations on the performance of the membranes was studied. In addition, the reproducibility of the synthesis method and ion-exchange procedure was verified. The membranes were characterized by XRD, SEM, EPMA, N2 and xylene permeation measurements at temperatures between 150 and 400 °C.

Section snippets

Membrane preparation

Na-ZSM-5 membranes were synthesized using the secondary growth technique on the outer surface of a porous stainless steel (PSS) tubular support (Mott Metalurgical), 10 mm o.d. and 7 mm i.d. The average pore size was 0.2 μm. The support was cut into 30-mm long pieces; then one end of the porous support was welded to a non-porous SS tube and the other end was sealed with a non-porous stopper. Through the open end, N2 was injected to sweep the permeate. Before use, the tubes were ultrasonicated in

Synthesized composite membranes

Table 1 shows key features of the membranes. The third column indicates that the thickness of the membranes is fairly constant. The constancy of the permeation data before and after being in contact with the xylenes at 400 °C (4th and 5th columns) suggests that no cracks or pinholes have developed in the membranes after the high temperature cycles. The thermal stability of the composite materials will be confirmed by other observations (vide infra).

X-ray diffraction

The residual zeolite powder obtained from the

Discussion

This study shows that the permeation fluxes of the xylenes and the separation factors are significantly affected by the nature of the cation which neutralizes the lattice charge of the ZSM-5. This statement is supported by both the reproducibility of the synthesis method and the null effect of the ion exchange procedure upon the quality of the membranes. The exchanged zeolites show almost the same N2 permeation at room temperature before and after membrane exposure to xylene mixtures up to 400 

Conclusions

The synthesis procedure used in this work yielded reproducible and durable composite membranes. The key factor that inhibits the appearance of pinholes and fractures during thermal cycles seems to be a certain degree of penetration of the zeolite phase (ca. 15 μm) inside the tube pores. In all the exchanged membranes 95–96% of the xylene transport occurred through zeolitic channels.

Out of the four cations exchanged, the Ba-ZSM-5 membranes were the best taking into account both permeation and

Acknowledgements

The authors wish to acknowledge the financial support received from UNL, CONICET and ANPCyT. Thanks are given to Elsa Grimaldi for the English language editing. A.M.T. thanks the YPF Foundation for the support received for her doctoral scholarship.

References (30)

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