Effects of surface treatments and annealing on carbon-based molecular sieve membranes for gas separation
Introduction
A variety of membranes have been developed for separation of gases and liquids. Inorganic membranes are becoming more important in the new era of membrane technology for gas separation due to their better thermal stability and chemical stability than the organic contestant, polymer membranes. For gas separation purpose in particular, the membranes need to exhibit molecular sieving capabilities, i.e., having pore sizes near the dimensions of gas molecules to be separated. Depending on the size and shape of gas molecules, the gas diffusivity in the molecular sieve may vary greatly. Zeolite and carbon molecular sieves (CMS) are two potential candidate materials for making gas separation membranes. As forming a large and crack-free zeolite membrane remains quite difficult, it appears more attainable and reliable to form CMS membranes [1]. Zeolite membranes exhibit crystalline structure, tending to crack along the grain boundary as well as loosing molecular sieving capability at the grain boundary. On the other hand, CMS membranes are amorphous in nature, and the loss of molecular sieving capability at the grain boundary needs not be concerned. Besides, carbon membranes are tougher and more flexible.
CMS powders have been an important material in gas separation. The most important application of CMS powders is in the O2/N2 separation from air by means of the pressure swing adsorption (PSA), a semicontinuous process. O2 (kinetic diameter, 3.46 Å) diffuses into the micropores at a rate two order of magnitude higher than N2 (kinetic diameter, 3.64 Å) [2]. Although widely used in industry, CMS powders are not suitable for continuous gas separation processes.
CMS membranes can separate the gas mixtures continuously, and are promising materials for gas separation regarding their chemical and thermal stabilities. In practice, CMS membranes can be prepared in two ways: (1) unsupported CMS membranes (including flat membrane, capillary tubes or hollow fibers), and (2) supported membranes (flat or tubular) on a macroporous material. Both types present their own problems. The former may suffer from the problem of brittleness for practical applications. However, the preparation of supported CMS membranes remains relatively more difficult. Usually the polymer deposition and pyrolysis cycles have to be repeated several times in order to obtain an almost crack-free membrane [3].
Soffer et al. first succeeded in producing unsupported CMS membranes by pyrolysis of polymers [4], [5]. The He permeance and He/N2 selectivity were, respectively, 3 × 10−7 mol m−2 s−1 Pa−1 and 22 at 298 K. The O2 permeance and the O2/N2 selectivity were, respectively, 10−7 mol m−2 s−1 Pa−1 and 7.1 at 298 K. CMS membranes have been synthesized by high temperature pyrolysis of various polymers such as polyfurfuryl alcohol (PFA), polyvinylidene chloride (PVDC), polyacrylonitrile (PAN), polyimides and phenol resins [1], [2], [3], [6], [7], [8]. In general, the permeance of carbon membrane is much higher than that of its polymer membrane precursor. However, carbon membranes are not stable at an elevated temperature under the atmosphere containing oxygen or steam [1], [4].
The unsupported CMS membranes may lack mechanical strength required for certain applications. It would be beneficial to improve their mechanical strength by synthesizing CMS membranes on macroporous supports. Collins and Yin deposited diamond-like carbon (DLC) films on silicon substrates by dc magnetron discharge decomposition of acetylene [9]. DLC is basically an amorphous carbon film with extremely high hardness deposited by high energy bombardments of carbon-containing ions. DLC film may incorporate some other elements like H, Si, SiOx, metal, etc., to modify its structure and properties [10]. The pore size distribution and porosity could be adjusted by varying the deposition and annealing conditions. Films having molecular sieve effects are found with micropore sizes characteristic of molecular dimensions [9].
The films deposited by plasma polymerization using silicon-containing monomers have higher permeance than those using hydrocarbon monomers due to the high mobility of siloxane bonds [11], [12], [13]. Hexamethyldisiloxane (HMDSO) has been most often used. Silica-based microporous membranes have also been prepared by controlled pyrolysis of polymeric precursors, sol–gel technique, chemical vapor deposition (CVD), or leaching of phase-separated glass [14]. The group of Gavalas [15], [16] reported the deposition of ultra-fine SiO2 particles on the porous Vycor glass by CVD in 1989. The deposition was achieved using the opposing reaction configuration, wherein SiH4 was allowed to flow through the pores of the glass support to react with O2 outside. The pore-plugged glass was capable of separating H2 efficiently. Yan et al. and Morooka et al. [17], [18] also plugged the pores of the γ-alumina layer with fine SiO2 particles formed by CVD of tetraethylorthosilicate (TEOS). Silicon carbide (SiC) is another interesting material for high temperature gas separation, since it provides a good combination of strength, thermal and chemical stability [19]. Shelekhin et al. pyrolyzed polysilastyrene at 743 K to obtain SiC-base membranes exhibiting molecular sieving behavior [20]. Amorphous SiCO fibers obtained by pyrolysis of polycarbosilane (PC) can also be stable at a high temperature up to 1473 K under inert atmosphere [19]. SiCO membranes were synthesized by Li et al. by pyrolyzing polysiloxanes at 573–803 K for gas separation [21], [22]. Kusakabe et al. [14], [19], [23] have also synthesized SiCO membranes by pyrolysis of polycarbosilane (PC) films obtaining high performance molecular sieving membranes. For a SiCO membrane coated three times and pyrolyzed at 1223 K, H2 permeance was about 10−9–10−8 mol m−2 s−1 Pa−1 and the H2/N2 selectivity could reach 18–63 at 773 K.
Section snippets
Porous substrate materials
Anodisc membranes, i.e., porous aluminum oxide disks prepared by anodization of aluminum, supplied from Whatman have been employed as the porous substrates for carbon-based molecular sieve membrane supports. Porous anodisc substrates have high gas permeability due to their straight pores formed by aluminum anodization. Besides, the pore diameter distribution of a perfect anodisc is very narrow. And, since the thickness of the film to completely cover or block the pore is usually about one order
The effects of time periods of surface treatments
The surface treatment of the as-deposited carbon film was performed by bombarding the surface with ions from the 40% O2/HMDSO plasma. The permeance was measured after pyrolysis of the surface-treated film at 823 K. It only took 5–20 s for surface treatments to induce significant improvement of selectivity. As the films were treated for longer than 60 s, both the selectivity and the permeance decreased to the values without any surface treatment. The effects of the time periods of surface
Conclusion
A new method in preparing carbon-based molecular sieve (CMS) membranes for gas separation has been established. The H2/N2 selectivity can be as high as 100 with an extremely high permeance of H2 around 1.5 × 10-6 mol m−2 s−1 Pa−1 at 298 K. A combination of surface treatment (ion bombardment) and pyrolysis is necessary in simultaneously enhancing the permeance and the selectivity of CMS membranes. Without surface treatment, pyrolysis alone significantly increases the permeance, but have no
Acknowledgements
The financial support of this work by the National Science Council of the Republic of China is gratefully acknowledged.
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