Hollow filler based mixed matrix membranesw Katrien Vanherck, Alexander Aerts, Johan Martens and Ivo Vankelecom* Received (in Cambridge, UK) 19th November 2009, Accepted 12th January 2010 First published as an Advance Article on the web 27th January 2010 DOI: 10.1039/b924086a Micron-sized hollow spheres with zeolitic shell were used as inorganic fillers in PDMS-based mixed matrix membranes, overall enhancing solvent permeabilities as compared to traditional zeolite fillers, while selectivities were maintained. The concept of mixed matrix membranes (MMMs) 1 was originally developed in the field of membrane gas separations to overcome the limitations of polymeric and inorganic membranes. 1–5 Polymeric membranes are limited in performance due to a general trade-off between flux and selectivity, referred to as the Robeson upper boundary. 2 Inorganic membranes overcome this boundary, but material costs are generally high, while defect-free preparation and upscaling is less straightforward. 1 Mixed matrix membranes, consisting of an organic polymer (bulk phase) and inorganic particle phases (dispersed phase), have the potential to combine high selectivities with high membrane fluxes. The flexibility of the continuous matrix avoids defects, while the superior separation characteristics of the inorganic particles lead to better performances. 1,6–12 Zeolites and carbon molecular sieves are the most attractive inorganic fillers in MMMs, as their very defined pore structure causes an increase in selectivity. 6 Zeolite-filled PDMS membranes have been developed for nanofiltration in solvents such as toluene and dichloromethane 13–15 that normally cause excessive swelling and selectivity loss in PDMS. 16,17 The porous zeolite fillers reduced swelling of PDMS in these solvents without lowering the intrinsic fluxes. 13,14,18–20 Using the micron-sized zeolites USY and ZSM-5, Gevers et al. developed a PDMS composite membrane with a top-layer thickness of approximately 8 mm. 13 The permeability could not be enhanced further because a zeolite-filled PDMS top-layer requires a certain minimal thickness—several times the size of the fillers—to remain free of defects. The use of smaller zeolite particles is problematic, as it becomes more difficult to realise a good particle dispersion. 7 In this work, MMM fluxes are enhanced by a new concept, applying hollow fillers. The incorporation of hollow spheres in micrometre size range with a shell consisting of nanosized zeolite-crystals in PDMS was investigated. A high increase in permeability was expected, since the hollow fillers allow a fast flow of the permeating compound. At the same time, rejection should be maintained by the molecular sieving and cross- linking effect of the zeolite shell. Moreover, good dispersions should be realised easily. Examples of such hollow spheres are abundantly present in the literature. Many different synthesis techniques are employed, most commonly self-assembly and layer-by-layer deposition, to obtain hollow zeolite spheres of a variety of sizes and with different characteristics. 21–32 In this work, self-synthesized hollow spheres with silicalite-1 shell and a diameter of 2 to 5 mm were chosen as fillers (Fig. 1). Ideal applications for this system thus consist of solvents with molecules of a diameter that allows permeation through the 5.4–5.6 nm zeolite pore diameters, and solutes that cannot penetrate these pores. Hollow spheres with silicalite-1 shell (HS) were prepared by adding a 5 wt% solution of cetyl trimethylammonium bromide in ethanol dropwise to a clear solution (molar composition tetraethyl orthosilicate–tetrapropylammonium hydroxide–H 2 O= 25 : 9 : 400) whilst stirring vigorously. The resulting solution was left to react in an oven at 90 1C for 4 days. A white precipitate was formed, which was Buchner filtrated, repeatedly washed with ethanol and air-dried. Silicalite-1 (si-1) fillers of 120 nm crystal size were prepared by reacting a clear solution of the same molar composition at 120 1C in an autoclave for 24 h. The resulting si-1 suspension was centrifuged and washed repeatedly in distilled water and was then freeze-dried. All fillers were calcined in a muffle oven for 5 h at 550 1C (heating 1 1C min 1 ) before use. MMMs were prepared with the HS and as a reference also with si-1. To obtain the MMMs, the HS or si-1 fillers were dispersed in hexane and treated ultra- sonically to disaggregate clusters. The two components of PDMS (RTV-615 A and B, respectively, prepolymer and cross-linker, General Electric) were added and the dispersion was homogenized. The final mixture contained 10 wt% solids with a PDMS : filler weight ratio of 85 : 15. This mixture was stirred in an oil bath at 70 1C for at least one hour until a viscous solution was obtained. The resulting pre-polymerized PDMS solution was poured over a cross-linked polyimide Fig. 1 SEM photograph of hollow spheres, with silicalite-1 crystals visible on the surface (scale bar represents 5 mm). Centre for Surface chemistry and Catalysis, Department of Microbial and Molecular Systems, Faculty of Bio-science Engineering, Katholieke Universiteit Leuven, PO Box 2461, Kasteelpark Arenberg 23, 3001 Leuven, Belgium. E-mail: Ivo.vankelecom@biw.kuleuven.be; Fax: +32-16-32.19.98; Tel: +32-16-32.15.94 w Electronic supplementary information (ESI) available: Indexed XRD pattern of the synthesized hollow spheres with silicalite-1 shell; synthesis procedure for the crosslinked support membrane. See DOI: 10.1039/b924086a 2492 | Chem. Commun., 2010, 46, 2492–2494 This journal is  c The Royal Society of Chemistry 2010 COMMUNICATION www.rsc.org/chemcomm | ChemComm Published on 27 January 2010. Downloaded by UNIVERSITY OF OTAGO on 17/09/2013 18:49:36. View Article Online / Journal Homepage / Table of Contents for this issue