Zeolite membranes for separation processes are generallycomposed of quantities of crystallites packed together on or inthe porous support [1]. The separation properties of such apolycrystalline zeolite membrane commonly depend on the sizeand the number of zeolitic and nonzeolitic (intercrystalline)pores, and the surface characteristics of the membrane. It is aprevailing way to improve the separation performance of azeolite membrane through optimization of synthesis conditions[2]. In most cases, however, it is always difficult to optimizeboth the compactness and the surface properties of a zeolitemembrane only by controlling the synthesis conditions.Therefore, post-synthetic treatment has also been employedfor improving the separation properties of zeolite membranes[3].
To date several post-synthetic treatment techniques havebeen utilized for improving the quality of zeolite membranes[3–9]. For instance, chemical vapor deposition of silica [4],coking [5], and Pd deposition [6] have been employed toeliminate intercrystalline defects. Catalytic cracking of silanewas used to reduce the effective pore opening of an MFI-typezeolite membrane [7]. Silylation of a silicalite-1 membrane was
adopted to increase the hydrophobicity of the membrane surface[8]. We have reported that alkaline treatment of as-synthesizedZSM-5 membranes was an effective approach to enhance boththe water fluxes and the water/acetic acid separation factors [3].
Due to their hydrophilicity and high resistance to acid,mordenite membranes have been investigated by manyresearchers as candidate membranes for dehydration of organicsolvents in an acidic environment [9–14], which is desirable insuch applications as the removal of water from esterificationreactions to shift the equilibrium towards the ester products.Navajas et al. [10] have reported that their as-synthesizedmordenite membranes had an average water/ethanol separationfactor of around 150, with a maximum value of 310. In asubsequent article [9], they developed a post-treatment processfor improving the pervaporation performance of as-synthesizedmordenite membranes. The water/ethanol separation factor wasenhanced to be as high as 700. In this communication, we reporta novel approach (post-treatment with oxalic acid) for thepreparation of highly water-selective mordenite membraneswith the water/ethanol separation factors of higher than 10000.
2. Experimental section
Asymmetric porous α-Al2O3 tubes (supplied by NanjingUniversity of Technology of China, 9 mm i.d. and 13 mm o.d.)with pore size of ca.100 nm in the outer layer were used as
Table 1Comparison of pervaporation performance of mordenite membranes for theseparation of a mixture containing 15 wt.% water in ethanol at 348 K before andafter hydrothermal treatment in the oxalic acid solution
Membrane Treatment conditions Total flux [kg m−2 h−1] αw/e [−]
MOR01 As-synthesized 4.4×10−2 44373 K, 2 h 2.5×10−2 75393 K, 2 h 5.3×10−2 N10,000413 K, 2 h 3.0×10−2 684433 K, 2 h 15.7×10−2 5
MOR02 As-synthesized 7.5×10−2 32393 K, 2 h 17.3×10−2 59393 K, 2 h 10.2×10−2 N10,000393 K, 2 h 11.3×10−2 N10,000
MOR03 As-synthesized 22.8×10−2 6393 K, 4 h 9.6×10−2 N10,000
A fresh oxalic acid solution of 1 mol dm−3 was used for each treatment.
Table 2Al and Si concentrations in the oxalic acid solution that had been used for post-synthetic treatment of MOR01 membrane
Treatment conditions Al concentration[μg mL−1]
Si concentration[μg mL−1]
Al/Si[−]
373 K, 2 h 8.2 8.2 1.0393 K, 2 h 23.2 10.4 2.3413 K, 2 h 57.2 16.6 3.6433 K, 2 h 254.6 38.4 6.9
A fresh oxalic acid solution of 1 mol dm−3 was used for each treatment.
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supports. Prior to use, the supports were first treated for 2 h at873 K and then cleansed successively in acetone and deionizedwater each for 30 min under ultrasonication. The reagentsemployed herein include silica gel (100–200 mesh), sodiumsilicate (Na2SiO3·9H2O), sodium aluminate (41 wt.% Al2O3),sodium hydroxide (96.0 wt.%) and deionized water.
Mordenite membranes were prepared by seeded hydrother-mal synthesis on the outer surface of tubular α-aluminasupports. The colloidal suspension of 0.2 g L−1 nanosizedmordenite crystals for seeding was prepared from mordenitepowder, which was hydrothermally synthesized for 48 h at453 K using a gel with the molar composition of 6Na2O:Al2O3:30SiO2:780H2O (sodium silicate and silica gel as Sisources). An organic-free synthesis mixture with the molarcomposition of 10Na2O: 0.15Al2O3:36SiO2:440H2O (silica gel
Fig. 1. FE-SEM images for (a, b) an as-synthesized mordenite membrane, (c, d) a mor1 mol dm−3.
as Si source) was used for secondary growth of mordenite seedsto form a membrane. Hydrothermal crystallization wasconducted for 18 h at 453 K without agitation. The experimentaldetails were described in [14].
The post-synthetic hydrothermal treatment in an oxalic acidsolution was performed in the Teflon-lined autoclaves. For eachrun, 50 ml of an oxalic acid solution of 1 mol dm−3 was used.The concentrations of Si and Al in the oxalic acid solution thatrecovered from the autoclave after treatment of the membranewere analyzed by means of inductively coupled plasma atomicemission spectrometry (ICP-AES, IRIS Intrepid II XSP).
The phase of the materials formed on the support surface wasidentified using X-ray diffraction (XRD, Rigaku D/max-rA).The morphology of mordenite membranes was characterizedusing field emission scanning electron microscopy (FE-SEM,Hitachi S-4700).
The pervaporation performance of mordenite membraneswas evaluated by the separation of a mixture containing 15 wt.%water in ethanol at 348 K. One end of the tubular membrane wassealed with a nonporous glass plate and the other end wasconnected to a nonporous glass tube with epoxy. The membrane
denite membrane after the treatment for 2 h at 393 K in the oxalic acid solution of
4578 G. Li et al. / Materials Letters 61 (2007) 4576–4578
was immersed in the liquid mixture while the inner side of themembrane was evacuated to maintain a pressure of ca. 266 Pa.The effective membrane area was ca. 4 cm2. The permeate wascondensed in a cold trap cooled by liquid nitrogen, weighed andanalyzed after the permeation of 1–2 h. The feed and permeatecompositions were determined using a gas chromatograph(REX, GC-8810) equipped with a 2-m long stainless steelcolumn packed with Porapak Q and a thermal conductivitydetector (TCD). The separation factor, αw/e, was defined asαw/e= (Yw/Ye) / (Xw/Xe), where Yw/Ye and Xw/Xe are the weightratio of water to ethanol in the permeate and in the feed,respectively.
3. Results and discussion
Table 1 compares the pervaporation performance of severalmordenite membranes before and after hydrothermal treatment in anoxalic acid solution. The water/ethanol separation factor of as-synthesized MOR01 membrane was 44. After hydrothermal treatmentfor 2 h at 373 K in the oxalic acid solution of 1 mol dm−3, the water/ethanol separation factor was increased to be 75. After a furtherhydrothermal treatment for 2 h at 393 K in a fresh oxalic acid solutionof 1 mol dm−3, the water/ethanol separation factor was increased to behigher than 10,000. Upon further hydrothermal treatment at highertemperatures (413 K and 433 K), however, the water/ethanol separationfactor declined. These results indicate that at the temperature of 393 Kthe damage to the membrane could be avoided at least within theduration of 2 h and significantly high values of the separation factorcould be obtained. To confirm this, another as-synthesized mordenitemembrane with a similar separation factor was also subjected tohydrothermal treatment at 393 K. The results of MOR02 membrane inTable 1 demonstrate again the effectiveness of post-treatment in theoxalic acid solution. Furthermore, the water/ethanol separation factorstill maintains over 10,000 even after hydrothermal treatment for 4 h at393 K in the oxalic acid solution of 1 mol dm−3. More interestingly, thepervaporation performance of an as-synthesized mordenite membranewith the water/ethanol separation factor of only 6 was also considerablyimproved upon post-treatment, as exemplified by the results of MOR03membrane in Table 1. All these results clearly demonstrate theusefulness of post-treatment in the oxalic acid solution.
Fig. 1 shows the FE-SEM images for an as-synthesized mordenitemembrane and a mordenite membrane after the treatment for 2 h at393 K in the oxalic acid solution of 1 mol dm− 3. A profounddifference in the morphologies between these two membranes is thatthe surface of as-synthesized membrane was coated with quantities ofamorphous materials while the surface of the treated one was not.This indicates that the amorphous materials deposited on themembrane surface can readily dissolve in the oxalic acid solutionunder the conditions employed.
The presence of amorphous materials in the membrane surfacemight have at least two effects. One is to add resistance to permeation.This probably accounts for the relatively low permeation fluxes for themembranes even with low water/ethanol separation factors. The other isto change the surface characteristics of the membranes. The resultsshown in Table 2 tell us that Al-rich species were dissolved in the oxalicacid solution. This might be due to the high acidity of the solution andthe chelation of aluminum ions with oxalic acid molecules [15].
The reason is not clear so far for the change in the water/ethanolseparation factor of the mordenite membranes after post-treatment withoxalic acid. We assume that this is as a result of the dissolution ofamorphous Al-rich species and the dealumination of the mordenite
crystals. The lower water/ethanol separation factors for the as-synthesized membranes might be due to the strong interaction betweenthe Al-rich amorphous materials and ethanol molecules. Uponhydrothermal treatment in the oxalic acid solution at 393 K or below,the water/ethanol separation factor increased with the dissolution of theAl-rich amorphous materials. A maximum of the separation factor wasreached when the Al-rich amorphous materials were dissolvedcompletely after 2 h at 393 K (as shown in Fig. 1). With furtherincreasing the temperature of treatment, the dealumination of themordenite crystals occurred [16], which causes the membrane surfaceless hydrophilic and thereafter the lower water/ethanol separationfactors. At much higher temperatures, the deep dealumination causedthe formation of lattice defects in the mordenite crystals [16], whichaccounts for the higher permeation fluxes and the lower separationfactors.
4. Conclusions
A novel approach — post-synthetic treatment with oxalicacid was proposed for the first time for the preparation of highlywater-selective mordenite membranes. It was demonstrated thatthe water/ethanol separation factors were enhanced to be higherthan 10,000 after as-synthesized mordenite membranes werehydrothermally treated in the oxalic acid solution underappropriate conditions.
Acknowledgements
This work was sponsored by Zhejiang Provincial NaturalScience Foundation of China (Y406098) and by the ScientificResearch Foundation for the Returned Overseas ChineseScholars, State Education Ministry of China.
References
[1] A. Tavolaro, E. Drioli, Adv. Mater. 11 (1999) 975.[2] J. Caro, M. Noack, P. Kolsch, R. Schafer, Microporous Mesoporous Mater.
38 (2000) 3.[3] G. Li, E. Kikuchi, M. Matsukata, J. Membr. Sci. 218 (2003) 185.[4] M. Nomura, T. Yamaguchi, S. Nakao, Ind. Eng. Chem. Res. 36 (1997)
4217.[5] Y. Yan, M.E. Davis, G.R. Gavalas, J. Membr. Sci. 123 (1997) 95.[6] F. Moron, M.P. Pina, E. Urriolabeitia, M. Menendez, J. Santamaria,
Desalination 147 (2002) 425.[7] T. Masuda, N. Fukumoto, M. Kitamura, R.S. Mukai, K. Hashimoto, T.
Tanaka, T. Funabiki, Microporous Mesoporous Mater. 48 (2001) 239.[8] T. Sano, M. Hasegawa, S. Ejiri, Y. Kawakami, H. Hanagishita,
Microporous Mater. 5 (1995) 179.[9] A. Navajas, R. Mallada, C. Tellez, J. Coronas, M. Menendez, J.
Santamaria, J. Membr. Sci. 270 (2006) 32.[10] A. Navajas, R. Mallada, C. Tellez, J. Coronas, M. Menendez, J.
Santamaria, Desalination 148 (2002) 25.[11] N. Nishiyama, K. Ueyama, M. Matsukata, J. Chem. Soc., Chem. Commun.
(1995) 1967.[12] A. Tavolaro, A. Julbe, C. Guizard, A. Basile, L. Cot, E. Drioli, J. Mater.
Chem. 10 (2000) 1131.[13] X. Lin, E. Kikuchi, M. Matsukata, Chem. Commun. (2000) 957.[14] G. Li, E. Kikuchi, M. Matsukata, Sep. Purif. Technol. 32 (2003) 199.[15] M.R. Apelian, A.S. Fung, G.J. Kennedy, T.F. Degnan, J. Phys. Chem. 100
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