Synthesis of silicalite-poly(furfuryl alcohol) composite membranes for oxygen enrichment from air
© He et al; licensee Springer. 2011
Received: 19 November 2011
Accepted: 30 December 2011
Published: 30 December 2011
Silicalite-poly(furfuryl alcohol) [PFA] composite membranes were prepared by solution casting of silicalite-furfuryl alcohol [FA] suspension on a porous polysulfone substrate and subsequent in situ polymerization of FA. X-ray diffraction, nitrogen sorption, thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were used to characterize silicalite nanocrystals and silicalite-PFA composite membranes. The silicalite-PFA composite membrane with 20 wt.% silicalite loading exhibits good oxygen/nitrogen selectivity (4.15) and high oxygen permeability (1,132.6 Barrers) at 50°C. Silicalite-PFA composite membranes are promising for the production of oxygen-enriched air for various applications.
Oxygen-enriched air can be widely used in chemical industries, fermentation, biological digestion processes, and medical purposes [1–3]. For example, combustion with oxygen-enriched air can substantially reduce fuel consumption and improve energy efficiency, thereby lowering CO2 emission .
The cryogenic fractionation technology is commonly used to produce oxygen-enriched air with an oxygen purity of 99 vol.%. Pressure swing adsorption can yield 95 to 97 vol.% oxygen-enriched air . The membrane technology has also been researched for oxygen separation from air. Over the past decades, polymeric gas separation membranes have attracted much attention, becoming one of the fastest growing branches of membrane technology. This is because polymeric membranes tend to be relatively inexpensive and can be easily fabricated into hollow fibers or spiral-wound modules [5, 6]. Some polymeric membranes such as silicone rubber, polyphenylene oxide, and cellulose triacetate have already been studied for oxygen enrichment [2, 3, 7]. However, because the molecular dimensions of O2 (3.46 Å) and N2 (3.64 Å) are close, producing pure oxygen is rather difficult as some nitrogen always permeates through the membrane . The separation properties of existing polymeric membranes are still restricted by the trade-off trend between gas permeability and selectivity which was suggested by Robson . Additional limitation of the polymeric membrane is that at elevated temperatures, the performance of the membrane will lose because of the segmental flexibility . Therefore, the separation membranes with high O2/N2 selectivity and high flux are required to be competitive with other technologies.
Inorganic molecular sieves (such as zeolites) exhibit good chemical and thermal stabilities and high gas flux and selectivity, but the fabrication of defect-free molecular sieve membranes remains a challenge. In recent decades, there has been significant interest in the development of synthesis methods of pinhole-free, mechanically stable, and inorganic-organic hybrid membranes to combine the advantages of both inorganic and organic membranes. Such kind of membrane is known as mixed matrix (or composite) membranes. Desirable composite membranes consist of well-dispersed particles without interfacial incompatibility and defects between the inorganic material and the polymer. Therefore, careful selection of a pair of polymer and inorganic material is very important. Polysulfones, polyarylates, polycarbonates, poly(arylethers), poly(arylketones), and polyimides are frequently used in industrial polymeric membrane gas separations. The commonly used inorganic materials include carbon molecular sieves, zeolites, mesoporous materials, activated carbons, carbon nanotubes, and metal-organic framework .
In the present work, we attempt to develop zeolite-polymer composite membranes for oxygen production from air. In particular, poly(furfuryl alcohol) [PFA] is chosen as the polymer matrix, and silicalite, as the molecular sieve additive. In the work previously conducted by one of the authors (Wang et al.), poly(furfuryl alcohol) was used to prepare a zeolite 4A polyfurfuryl alcohol nanocomposite membrane by vapor deposition polymerization of furfuryl alcohol [FA] [11, 12]. This composite membrane showed an O2/N2 selectivity as high as 8.2 and an oxygen permeance of 1.5 × 10-9 mol·m-2·s-1·Pa-1. To improve oxygen flux, silicalite is used in the present study because it has a larger pore size than the zeolite 4A. Silicalite is a pure silica MFI-type zeolite, which is composed of a uniform molecular-sized pore system with straight channels in the b-direction (5.4 Å × 5.6 Å) and sinusoidal channels in the a-direction (5.1Å × 5.5 Å) [13, 14].
One molar tetrapropylammonium hydroxide [TPAOH] aqueous solution and tetraethyl orthosilicate [TEOS] (99%), acrylamide [AM], N,N'-methylenebisacrylamide [MBAM], and FA (98%) were purchased from Sigma-Aldrich Corporation (Sydney, Victoria, Australia). A polysulfone ultrafiltration membrane (MWCO 30,000) was purchased from Sterlitech (GE Osmonics, Minnetonka, MN, USA) and used as the support.
Preparation of silicalite nanocrystals
Silicalite nanocrystals were synthesized according to the previously published procedures . In a typical synthesis, silicalite nanocrystals were synthesized by hydrothermal synthesis from a clear solution with a molar composition of 1 TPAOH:4.8 SiO2:44 H2O. The synthesis solution was prepared in a 250-mL polypropylene bottle. First, 20 g of 1 M TPAOH solution was added dropwise into 20 g of TEOS under vigorous stirring. Strong magnetic stirring was maintained for 3 h. The solution was then heated in an oven at 80°C for 5 to 6 days for crystallization, resulting in a milky silicalite suspension. The solid product contained in the colloidal suspension was recovered by a repeated cycle of centrifugation with deionized water and ultrasonic redispersion in water until pH < 8. An organic polymer network was prepared from water soluble organic monomers, AM and MBAM, and the initiator (NH4)2S2O8 as a temporary barrier during calcination and carbonization to obtain highly redispersible template-free silicalite nanocrystals. Typically, 1.0 g of AM, 0.1 mg MBAM, and 25 mg of (NH4)2S2O8 were added under stirring into 10 g of silicalite colloidal suspension with 5 wt.% solid loading. After the monomers were dissolved, the mixture was ultrasonicated to ensure complete dispersion of silicalite nanocrystals for half an hour. The aqueous solution was then heated at 50°C for 30 min to be polymerized into an elastic hydrogel. This silicalite hydrogel polymer composite was dried at 80°C overnight. After drying, it was carbonized under nitrogen at 550°C for 2 h (heating rate, 2°C min-1) and then calcined at 550°C for 3 h under air.
Preparation of silicalite/PFA composite membranes
Both plain PFA and silicalite-PFA composite membranes were hand-cast on commercial polysulfone ultrafiltration membranes.  A 25 mm × 70 mm polysulfone ultrafiltration membrane was fixed on the top of a microscope glass slide using a tape to prevent the membrane from rolling up and solution penetration through the edges. Then, an aqueous solution prepared by mixing 10 g of FA and 0.04 g of sulfuric acid with 10 g of ethanol was cast on the polysulfone membrane substrate for 5 min at room temperature. The coated support was then heated at 80°C overnight for FA polymerization. Silicalite-PFA nanocomposite membranes were made using the same procedures, except that a given amount of template-free silicalite nanocrystals was dispersed in the FA ethanol solution which was ultrasonicated for 30 min at room temperature. The resulting silicalite-FA ethanol suspension was immediately mixed with sulfuric acid under magnetic stirring for 2 min and then coated on the polysulfone membrane substrate for 5 min at room temperature. The coated support was heated at 80°C overnight. The resultant composite membranes are referred to as 1-Sil-PFA, 10-Sil-PFA, 20-Sil-PFA, and 30-Sil-PFA, corresponding to silicalite loadings of 1% (w/w), 10% (w/w), 20% (w/w), and 30% (w/w) in PFA solution, respectively.
X-ray diffraction [XRD] patterns were recorded on a Philips PW1140/90 diffractometer (PANalytical B.V., Almelo, The Netherlands) with Cu Kα radiation (25 mA and 40 kV) at a scan rate of 2°/min with a step size of 0.02°. Nitrogen adsorption-desorption experiment was performed at 77 K and at room temperature with a Micrometritics ASAP 2020MC analyzer (Micromeritics Instrument Co., Norcross, GA, USA). To evaluate the thermal stability of the PFA, thermogravimetric analysis [TGA] was conducted using a thermogravimetric analyzer (PerkinElmer, Waltham, MA, USA) in the temperature range of 20°C to 700°C under nitrogen gas and a heating rate of 5°C/min. All scanning electron microscopy [SEM] images were taken with a FEG-7001F microscope (JEOL, Ltd., Akishima, Tokyo, Japan) operated at an accelerating voltage of 15 kV. Elemental analysis of samples was determined by energy dispersive X-ray spectroscopy [EDXS] on the FEG-7001F microscope.
where N i (mol·s-1) is the permeate flow rate of component gas i, Δp i (in Pascals) is the transmembrane pressure difference of i, and A (in square centimeters) is the membrane area.
where P is the permeability (in Barrers), P o is the pre-experiential factor, T is the absolute temperature (in Kelvin), and R is the gas constant (8.3143 J·mol-1·k-1).
where α is the selectivity of O2 to N2, φ is the ratio of product to feed gas pressures, and 0.21 is the fraction of oxygen in the feed air.
Results and discussion
Silicalite nanocrystals and silicalite-PFA membranes
Gas separation properties
Gas permeation results of PFA and composite membrane
Silicalite loading (%)
O2/N2 ideal selectivity
Gas permeation results of the 20% Sil-PFA composite membrane at different testing temperatures
O2/N2 ideal selectivity
Apparent activation energy for permeation of N2 and O2 gases
Apparent activation energy (kJ/mol)
Silicalite-PFA composite membranes were prepared for enrichment of oxygen from air. SEM results showed that silicalite nanoparticles were well dispersed in the PFA matrix. The gas permeation experiments indicated that O2 and N2 permeabilities and O2/N2 selectivity could be improved by incorporating silicalite nanoparticles into PFA. In particular, the Sil-PFA composite membrane with 20% silicalite loading had the highest O2/N2 selectivity and excellent O2 permeability, and an oxygen concentration of 47.9 vol.% was achieved in the single-pass air separation experiment at room temperature.
This work was supported by the Department of Innovation Industry, Science and Research of Australian Government through the Indo-Australian Science and Technology Fund and the Australian Research Council. The authors gratefully acknowledge the support and use of facilities in the Monash Centre for Electron Microscopy. Huanting Wang thanks the Australian Research Council for a Future Fellowship.
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