Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
2897
Novel method of preparing zeolite membranes from colloidal zeolite NaY" Hai-Qiang Lin, Zi-Sheng Chao**, Ting-Hua Wu, Guo-Zhou Chen, Fan-Xian Zhang, Hui-Lin Wan, and Khi-Rui Tsai State key laboratory for physical chemistry of solid surfaces, Xiamen 361005, CHINA
1. INTRODUCTION Zeolites possess high porosity, uniform pore dimension, fine tunable acidity and ion exchange character, as well as outstanding stability, and have important industrial applications in catalysis and separation processes [1]. Their application prospects in membrane technique also seem to be very promising. Many attempts to prepare zeolite membranes, as zeolite membranes self-supported [2] or deposited on various supports [3-6] have been made with some instructive results, and the possibilities of utilizing zeolite membranes in catalytic membrane reactors to improve the yield and selectivity of reactions that are limited by equilibrium are also approached [7,8]. In view of the harsh conditions such as high temperature and high pressure associated with catalytic membrane reactors, the zeolite composite membranes constructed by zeolite layers on porous inorganic supports are thought to be more superior among various zeolite membranes. The developed zeolite composite membranes in literatures chiefly focus on the types of A, ZSM-5 and Silicalite-1. However, due to the extensive applications of zeolite Y in hydrocarbon processes, e.g., fluid catalytic crack, hydrogenation crack, isomerization and catalytic dewaxing, the investigation of Y zeolite composite membrane deserves to be paid much attention to. A prerequisite for the optimization of the applications in separation processes and catalytic membrane reactors is the preparation of defect-free, ultra-thin zeolite composite membranes. Generally, zeolite composite membranes are constructed by intergrowth or accumulation of zeolite crystals on supports. It is difficult to form a thin membrane without defects if the relatively large crystals were involved [3]. While zeolite composite membrane composed by nanometer zeolite crystals may improve this situation, because small zeolite particles could be easily embedded into the pores of support and closely accumulate on the surface of support. Thus, in the present work, a novel method is presented to synthesize zeolite composite membrane composed by nano-sized crystals on modified supports from colloidal zeolite NaY. 2. EXPERIMENT Nano-sized NaY was firstly synthesized according to the method reported in ref. 9. NaOH (96%), colloidal silica (40%), aluminum sulfate (99%), sulfuric acid (98%) and * Supported by SINOPEC (Project number: X59801) ** To whom correspondence should be given
distilled water were used as source materials. The nucleation gel with molar compositions of 16Na20:A1203:16SIO2:2101-120 was prepared by homogeneously mixing the source materials at 277K. After aged at room temperature for 64 hours, the nucleation gel was added by a
2898 calculated amount of sulfuric acid, resulting in the reaction gel with molar compositions of 8Na20:AI203:16SiO2:400H20. The crystallization of the reaction gel was conducted at 363K in a Teflon lined reactor for a period of 10 hours. The resulted solid product was separated from the mother liquor by centrifugation at 4000 rpm and was rinsed by distilled water for several times till neutrality was reached. A part of the wet deposit was dried at 373K, and analyzed by XRD and TEM. The results show that zeolite NaY with SIO2/A1203 of 4.0 and average crystal size of 133nm has completely crystallized. The remainder of the wet deposit was directly utilized in the preparation of colloidal zeolite sol. In the preparation of colloidal zeolite sol, the wet deposit obtained as above was dispersed in certain amount of distilled water by homogeneously mixing, then centrifuged at a low rotating speed (ca. 2000 rpm) for 15 minutes. This process helped to eliminate big crystals, which were sunk on the bottom of centrifuge tube, while the nanosized crystals retained in the liquid phase. The top liquid containing tiny zeolite crystals was moved out carefully. Then this solution was mixed with a special polymeric silica sol to form the "coating sol" which is a kind of slight white homogenous sol. Unmodified support was homemade round alumina ceramic disk with diameter of 20mm, thickness of 1.5mm and average pore size of ca. 1.1 ~tm. Pre-modification of ceramic substrate was performed by placing it in a special Teflon reactor to react with a kind of dilute aqueous solution containing TEOS, NaOH and alcohol under vacuumed state. The reaction was conducted at 363K for 24 hours, and then so-obtained substrate was followed with a Feed gas calcination at 673K for 3 hours. This process was repeated for three times. Zeolite Pressure composite membranes were prepared via a regulator dip-coating technique. Followed a strictly controlled coating procedure performed under vacuum and heating (353K), the unmodified Temperature and the modified supports was brought to controller contact with the coating sol for a certain time. O-ring After drying at room temperature for several days, so-synthesized zeolite composite membrane membrane was activated under 453K. This Exhaust Three-way procedure could be repeated to achieve a valve meter compact and continuous zeolite NaY membrane on the supports. The as-prepared zeolite membranes attached to substrates Volume-changeable ,.. ,, L.. Vacuum tightly, and were difficult to be scratched. chamber r XRD and AFM were used to characterize the prepared zeolite composite membranes. Piston The permeability measurements were performed on a homemade apparatus, operated in a continuous flow mode. A Fig. 1. Apparatus for permeability schematic diagram of the system is shown in measurements Fig. 1. Before the measurements, all membranes were dried at 393k for 12 hours to drive off water molecules in the membrane pores. In pure gas permeation tests, three-way valve was shifted to the soap bubble flowmeter and steady-state permeated gas flow rate was measured. In the case of gas mixture tests, three-way valve was shifted to the volume-changeable chamber (total volume of 500ml), of
2899 which the outlet end was connected to vacuum pump. When steady gas permeation was attained, both the valves of inlet and outlet were shut down, and the piston was loosed to compress the permeated gases to an atmosphere pressure. The composition of the permeated gases was analyzed by gas chromatography. This method is quite different from those reported in literatures [4-6,10-12], in which sweeping gas was employed on the permeated side. It was proved that the permeated gas could be more quickly and effectively removed by using the vacuum pump in the present work as well as avoiding the back-diffusion due to the accumulation of permeate gas. Moreover, the component with low concentration in permeated mixture could be accurately measured via the compression by the piston. The ideal separation factor, ct*, is defined as the ratio of the permeability for the species in pure gas permeation tests. The real separation factor, tx, can be expressed as: (YA)/( xA ) a= i_y A 1-xA Where YA is the mole fraction of species A in the permeated gases and XA is the mole fraction of species A in the feed stream.
3. RESULTS AND DISCUSSION 3.1. Properties of NaY zeolite composite membranes Gases (N2 and I-C4H10) permeation tests show that there is no obvious difference between the permselectivity on unmodified support and the corresponding zeolite composite membrane. It indicates that zeolite membrane on the unmodified support contains many defects. After modified by TEOS, as shown in Fig.2 c and d, no new phases could be observed. In addition, the permeability of H2 on the modified support was significantly lower than that on the unmodified support. Thus, TEOS may have b entered into the pores of the support and effectively c reduced the average pore size when hydrolyzed under d L ........... / the action of OH. The XRD spectra of NaY membrane on modified support was shown in Fig.2 b, broaden of the diffraction peaks could be found, ~1=(degree) comparing to that of the powder zeolite NaY (Fig.2 a). Fig. 2 XRD pattems of samples It indicates that zeolite crystals in the membrane were a. NaY powders b. NaY membrane on modified support significantly smaller than NaY powders. Fig. 3 gives c. modified support d. starting the AFM pictures of the membrane on the modified support support. No obvious defects could be observed and the zeolite membrane was found to constituted by NaY crystals of ca. 50nm. The above results suggest that a match between the pore size of support and the crystal size of zeolite may be important for formation of zeolite composite membrane. Modified supports with reduced oore sizes would be more preferred to construct continuous and comoact zeolite membrane. '
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3.2.1 Pure gas pemeation The ideal Knudsen separation factors of some gases were shown in Table 1. The permeability of a series of gases, H2, N2, CO, CO2, CH4 and i-C4Hn0, was measured over the modified support and the corresponding zeolite composite membrane. As shown in Table 2, the modified support exhibited almost pure Knudsen characteristics, indicating that the average pore size on the support had been effectively reduced after the modification by TEOS. After zeolite membrane was coated on, it was found that the permeance of H2 at room temperature decreased by ca. one order of magnitude, and the permeances of the other gases decreased by ca. two orders of magnitude. Thus ideal separation factors of the gases, tx*(H2/gas), were far above the Knudsen values, except for that of CO2. These results suggest that the membrane prepared on the modified support may contain little defects (pinholes and cracks). Transport mechanisms of gaseous molecules through the membrane should be activation diffusion and/or molecular sieving. In the case of activation diffusion, the total flow through the membrane consisted of the adsorbate flow that occurred on the surface of the membrane pores and gas phase flow. In the case of molecular sieving, the diffusion rate would be mainly decided by size and shape of gaseous molecule. The larger molecule is, the more slowly it transports through the micropores. So the varying order of permselectivities as cx*(H2/CO2)< ~*(H2/CO)< ff*(H2/N2)< cz*(H2/CH4)< cx*(H2/i-CaH10) may approximately be interpreted by molecular sieving mechanism. The largest molecule, i-C4Hl0 had the lowest permeability. The deviation that CO (kinetic diameter 3.76,A,) had a higher permeability than that of N2 (kinetic diameter 3.64,A0 indicates that the mechanism of activation diffusion may also play a significant important role because the polar molecule CO is more easily absorbed by zeolite. Activation diffusion can also explain little difference between the permselectivity of H2/CO2 over the modified support and NaY membrane. Because CO2 is condensable molecules with small kinetic diameter, high adsorbate flow accelerated by significant adsorption of CO2 makes up for the reduced gas phase flow. It was also found that, the permselectivies of these gases only slightly and irregularly changed with the increasing of permeation temperature. This may be mainly due to the relatively narrow range of test temperatures. It must be pointed out that no single transport mechanism could be assigned to diffusion through a microporous materials of gaseous molecule [10]. Because an irregular and heterogeneous array of interconnected pores always exists in the microporous materials, the diffusing species would encounter a variety of constrictions wherein molecular sieving is dominant and would encounter other regions, immediate to these restrictions, where activation diffusion and/or Knudsen diffusion is dominant. Table 1 Kinetic diameter, molecular weight and ideal Knudsen separation factors of several gases Ideal Knudsen Molecular permselectivities Gas molecule Kinetic diameter(A) weight(g/mol) (H2/...) H2 N2 CO
CO2 CH4 i-C4H10
2.89 3.64 3.76 3.3 3.8 5.0
2 28 28 44 16 58
3.74 3.74 4.69 2.83 5.38
2901 Table 2 Results of pure gas steady-state permeation tests (P (permeance of gas) are given in 10-7 mol/m2.s.Pa, pressure difference was 0.1MPa) Membranes Temp. PH2 PN2 Pco
Pco 2 PCH4
Pi- ~*H2:N (X*H2:CO (X*H2:CO2s C4Hlo
Modified support 297K 297K NaY 343K membrane 393K
4.2
1.3
1.2
0.78
1.6
2
H (X'H2:i4
C4HI0
0.76
3.23
3.50
5.38
2.63
5.53
0.52 0.0590.071 0.098 0.043 0.021 0.51 0.061 0.062 0.093 0.043 0.019 0.49 0.0570.056 0.088 0.041 0.019
8.8 8.4 8.6
7.3 8.2 8.7
5.3 5.5 5.6
12.1 11.9 11.9
24.8 26.8 25.8
3.2.2 Binary gas mixture permeation The separation of binary gas mixture consisting of H2 and other gas was also studied. As can be seen form Table 3, the real separation factors exhibit almost the same increasing order with the increase of molecular sizes. However there are also a little differences between the values of cz and cz*. It was found that the real separation factors of some binary gas mixture, i.e., H2/N2 and H2/CH4, in which N2 and CH4 are non-condensable, are slightly lower than ideal separation factors; whereas the other binary gas mixtures, i.e. H2/CO, H2/CO2 and H2/iC4HI0, in which CO, CO2 and i-C4Hlo are condensable, did in opposite direction. The really reason of these phenomenon was unclear and further studies are needed. A possible answer may be that there exist the competition to diffusion space when the binary gas mixtures transport through the membrane, unlike in the case of the single componem gases. Table 3 Separation factors (cz) of binary 50/50(V/V) gas mixture on NaY composite membrane Temp. (K) ~(H2/N2) oc(H2/CO) 0c(H2/CO2) oc(H2/CH4) ot(H2/iC4H10) 298 8.5 8.1 5.4 9.2 31.2 343 8.6 8.2 5.9 9.5 26.4 393 8.5 7.8 6.1 9.8 29.1
4. CONCLUSION: NaY zeolite membrane with little defects was prepared on modified support using colloidal zeolite. The results of permeation test indicate that the permselectivity of a series of gases were significantly higher than those of Knudsen, which should be interpreted by the transport mechanism of molecular sieving and/or activation diffusion. It is expected that a possibility to develop a kind of Y zeolite composite membrane enclosed catalytic reactor be explored, which may be used in some dehydrogenation reactions, although the permeablities seem to be relatively low. To obtain a perfect zeolite membrane, i.e. ultra thin and defect-free, several aspect of the preparing process should be improved, for example, a more proper drying procedure, a dust-free atmosphere and a support with low resistance and suitable pore size should be involved.
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