Pervaporation and Vapor-Permeation Separation of Gas-Liquid Mixtures and Liquid Mistures by SAPO-34 Molecular Sieve Membrane Prepared in Dry-Gel Process

Abstract

The invention discloses a method for the pervaporation and vapor-permeation separation of a gas-liquid mixture or a liquid mixture by a SAPO-34 molecular sieve membrane prepared in a dry gel process, comprising: 1) synthesis of SAPO-34 molecular sieve seeds; 2) coating the SAPO-34 seeds on a porous support; 3) preparation of a mother liquor for dry gel synthesis of SAPO-34 molecular sieve membrane; 4) supporting the mother liquor for dry gel synthesis on the porous support coated with SAPO molecular sieve seeds and drying; 5) placing the porous support prepared in step 4) into a reaction vessel, adding a solvent, performing crystallization of the dry gel; 6) calcining; 7) using the SAPO-34 molecular sieve membrane obtained from step 6) to perform separation of a gas-liquid mixture or a liquid mixture by a process of pervaporation separation or vapor-permeation separation. The invention has the advantages of very high methanol selectivity and permeation flux, lowering synthesis cost of molecular sieve membrane and lowering environment pollution.

Claims

1. A method for the pervaporation or vapor-permeation separation of a gas-liquid mixture or a liquid mixture by a SAPO-34 molecular sieve membrane prepared in a dry gel process, characterized in that the method comprises the following steps; 1) mixing and dissolving an Al source, tetraethylammonium hydroxide (TEAOH), water, a Si source and a P source to make a reaction liquor for seeds, which is then subjected to crystallization for 2-72 h by heating at 120-230 C, then centrifuging, washing and drying to get SAPO-34 molecular sieve seeds; wherein the molar ratio of the Al source, P source, Si source, tetraethylammonium hydroxide and all water in the reaction liquor for seeds is 1 Al.sub.2O.sub.3: 1-2 P.sub.2O.sub.5: 0.3-0.6 SiO.sub.2: 1-3 TEAOH: 55-150 H.sub.2O; 2) coating the SAPO-34 molecular sieve seeds on a porous ceramic support; 3) uniformly mixing an Al source, a P source, a Si source, tetraethyl ammonium hydroxide (TEAOH) and water to form a mother liquor for dry gel synthesis of SAPO-34 molecular sieve membrane; wherein the molar ratio of the Al source, P source, Si source, tetraethyl ammonium hydroxide and all water in the mother liquor for dry gel synthesis is: 1 Al.sub.2O.sub.3: 1-2 P.sub.2O.sub.5: 0.1-0.6 SiO.sub.2: 1-8 TEAOH:30-1000 H.sub.2O; 4) supporting the mother liquor for dry gel synthesis on the porous support coated with SAPO-34 molecular sieve seeds prepared in step 2), and drying, to form a porous support having a dry gel layer; 5) placing the porous support prepared in step 4) into a reaction vessel, adding a solvent, performing crystallization of the dry gel; 6) calcining at 400-600 C. for 4-8 h to obtain a SAPO-34 molecular sieve membrane; 7) using the SAPO-34 molecular sieve membrane obtained in step 6) to perform the separation of a liquid mixture by a process of pervaporation separation or vapor-permeation separation; wherein the liquid mixtures is a mixture of methanol and a liquid other than methanol, wherein said liquid other than methanol is selected from one of dimethyl carbonate, ethanol, methyl tert-butyl ether;

2. The method according to claim 1 characterized in that in step 1), the procedure for making the reaction liquor for seeds comprises adding the Al source to the tetraethyl ammonium hydroxide solution, and after hydrolysis, adding the Si source and the P source, and stirring for 12-24 h to form the reaction liquor for seeds; wherein the heating is microwave heating; and the size of the SAPO-34 molecular sieve seeds is 50-1000 nm.

3. The method according to claim 1 characterized in that in steps 1) and 3), the Al source is selected from one or more of aluminium isopropoxide, aluminium hydroxide, elemental aluminium, an aluminium salt, alumina, and hydrated alumina; wherein the aluminium salt is selected from one or more of aluminium nitrate, aluminium chloride, alumimium sulfate, aluminium phosphate; the Si source is selected from one or more of silica sol, silicate ester, silica aerosol and sodium silicate; the P source is phosphoric acid.

4. The method according to claim 1 characterized in that in step 2), the shapes of the porous support are selected from single-channel tube, multi-channel tube, flat plate, hollow fiber tube; the pore size of the porous support is 5-2000 nm; and the coating method is selected from brush coating, dip-coating, spray coating or spin coating.

5. The method according to claim 4 characterized in that the coating method is dip-coating and the process comprises uniformly coating a 0.01-1 wt % solution of the SAPO-34 molecular sieve seeds in ethanol on the porous support.

6. The method according to claim 1 characterized in that in step 3), the procedure of forming the mother liquor for dry gel synthesis of SAPO-34 molecular sieve membrane comprises adding the Al source to the P source and after full hydrolysis, adding the Si source and tetra-ethyl ammonium hydroxide and stirring for 12-24 h, to obtain the mother liquor for dry gel synthesis of SAPO-34 molecular sieve membrane.

7. The method according to claim 1 characterized in that in step 4), the supporting method is selected from dip-coating, spray coating or spin coating; and the molar composition of the dry gel layer is 1.0 Al.sub.2O.sub.3: 1-2.0 P.sub.2O.sub.5: 0.1-0.6 SiO.sub.2: 1-8 TEAOH: 1-300 H.sub.2O.

8. The method according to claim 7 characterized in that the supporting method is dip-coating and its procedure comprises immersing the porous support coated with SAPO-34 molecular sieve seeds in the mother liquor for dry gel synthesis for 1 s to 24 h, and then drying for 1 min to 48 h at room temperature to 120 C.

9. The method according to claim 1 characterized in that in step 5), the solvent is selected from one or more of water, ammonia water, the mother liquor for dry gel synthesis, an organic solvent; and the solvent is used in an amount of 0.0010.1 g per mL volume of the reaction vessel; and the temperature for crystallization of the dry gel is 120-230 C. and the crystallization time is 2-72 h.

10. The method according to claim 9 characterized in that the crystallization time is 4-7 h.

11. The method according to claim 1 characterized in that in step 6), the atmosphere for calcination is selected from inert gas, vacuum, air, oxygen gas, or diluted oxygen gas in any ratio; in the calcination, the temperature increasing rate and the temperature decreasing rate are not higher than 2 K/min; and the membrane thickness of the SAPO-34 molecular sieve membrane is 1-2 microns.

12. The method according to claim 1 characterized in that in step 7), the conditions for the process of pervaporation separation or vapor-permeation separation comprises: a methanol concentration of the feed ranging from 1-99 wt %, a feed flow rate ranging from 1-500 mL/min, a separation operation temperature ranging from 20 C. to 150 C., and a pressure on the permeate side ranging from 0.06 Pa to 300 Pa; wherein in step 7), the inert gas contains N.sub.2; the gaseous hydrocarbon contains methane; the alcohol contains methanol, ethanol, or propanol; the ketone contains acetone or butanone; the aromatics contain benzene.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The invention will be explained in further detail by taking the appended figures and the detailed implementation.

[0034] FIG. 1 is an XRD (X-ray diffraction) pattern of the SAPO-34 molecular sieve seeds.

[0035] FIG. 2 is an SEM (Scanning Electron Microscopy) image of the SAPO-34 molecular sieve seeds.

[0036] FIG. 3 are graphs comparing the surface SEM image of SAPO-34 molecular sieve membrane prepared in Example 1 and the surface SEM image of SAPO-34 molecular sieve membrane prepared via hydrothermal synthesis; [0037] wherein, FIGS. 3A & B are the surface and cross-section images of SAPO-34 molecular sieve membrane prepared by hydrothermal synthesis; [0038] and FIGS. 3 C & D are the surface and cross-section images of SAPO-34 molecular sieve membrane prepared by dry-gel process in Example 1.

[0039] FIG. 4A is the surface SEM image of the SAPO-34 molecular sieve membrane prepared in Example 2.

[0040] FIG. 4B is the cross-section SEM image of the SAPO-34 molecular sieve membrane prepared in Example 2.

[0041] FIG. 5 is a schematic diagram of a pervaporation process, wherein 1 denotes feed liquid, 2 denotes peristaltic pump, 3 denotes molecular sieve membrane assembly and heat source, 4 denotes stop valve, 5 denotes cold trap, 6 denotes vacuum gauge, 7 denotes vacuum pump.

EXAMPLES

Example 1

[0042] Step 1. 2.46 g of DI water were added into 31.13 g of tetraethylammonium hydroxide solution (TEAOH, 35 wt %), and then 7.65 g of aluminum isopropoxide were added thereto, and the resultant was stirred for 2-3 h at room temperature. Then 1.665 g silica sol (40 wt %) was added dropwise and the resultant was stirred 1 h. Finally, 8.53 g phosphoric acid solution (H.sub.3PO.sub.4, 85 wt %) were slowly added dropwise. The resulting solution was stirred overnight (e.g. stirred for 12 h). Then crystallization is performed at 180 C. for 7 h by using microwave heating. The obtained product was taken out from the reactor, centrifuged, washed, dried, to obtain SAPO-34 molecular sieve seeds.

[0043] The XRD pattern and the SEM image of the seeds are shown in FIG. 1 and FIG. 2, respectively. Its XRD pattern is consistent with the standard pattern of SAPO-34 molecular sieve, indicating that no mixed crystals were present. It can be seen from FIG. 2 that the size of the seeds is around 300 nm*300 nm*100 nm.

[0044] A porous ceramic tube (a porous ceramic tube having an inner diameter of 7 mm, an outer diameter of 10 mm and a length of 600 mm, the shape of the porous support being single-channel tube, and its material being alumina) with 5 nm pore size was used as a support. The two ends of the support were sealed with glaze. After washing and drying, the out surface of the support was sealed (covered) by Teflon tape. Then the SAPO-34 molecular sieve seeds are coated on the inner surface of the porous ceramic tube by brush coating.

[0045] Step 2. 9.07 g of aluminium isoproxide were added to 5.12 g of phosphoric acid solution (85 wt %) and 23.33 g of DI water. After full hydrolysis, 1.00 g of silica gel (40 wt %) and 9.34 g of tetraethyl ammonium hydroxide (35 wt %) were added in turn. The resultant was stirred overnight (e.g., stirred for 12 h) to form a mother liquor for dry gel synthesis of molecular sieve membrane.

[0046] The resulting mother liquor had a molar composition of 1.0 Al.sub.2O.sub.3: 1.0 P.sub.2O.sub.5: 0.3 SiO.sub.2: 1TEAOH: 77 H.sub.2O.

[0047] Step 3. The porous ceramic tube coated with SAPO-34 molecular sieve seeds prepared in step 1 was soaked in the mother liquor for dry gel synthesis for 10 min, taken out and dried at room temperature for 10 min, thereby to have a dry gel layer formed. Then, the porous ceramic tube having a dry gel layer was placed in a 100 mL reaction vessel, and then 1 mL of DI water was added. The dry gel was crystallized for 5 h at 220 C. After washing and drying, a SAPO-34 molecular sieve membrane tube was obtained.

[0048] The membrane thickness of the SAPO-34 molecular sieve membrane prepared in Example 1 is around 1 micron (FIGS. 3 C and D). Moreover, the surface of the membrane was completely covered by square lamellar crystals which are well cross-linked therebetween. However, the SAPO-34 molecular sieve membrane prepared by conventional hydrothermal synthesis was covered by cubic crystals which are perfectly crosslinked, and the thickness of the membrane was 5-6 microns (FIGS. 3 A and B). Thus, the thickness of the SAPO-34 molecular sieve membrane prepared in Example 1 is only of the thickness of the membrane prepared via hydrothermal method. The mass transfer resistance of a molecular sieve membrane is inversely proportional to its thickness, which means that the flux of the membrane prepared via dry-gel process may be 5 times higher than the membrane prepared by the common hydrothermal process.

[0049] Step 4. The SAPO-34 molecular sieve membrane tube obtained in step 3 was calcined under vacuum at 400 C. for removal of the template agent (the temperature increasing rate and the temperature decreasing rate were both 1 K/min), to get a ultrathin SAPO-34 molecular sieve membrane prepared via dry gel process.

[0050] Step 5. The ultrathin SAPO-34 molecular sieve membrane prepared in step 4 was used to separate a methanol/dimethyl carbonate (i.e., DMC/MeOH) mixture by a process of pervaporation separation, wherein the feed temperature (i.e., separation operation temperature) is 120 C., the composition of the feed MeOH/DMC is 90/10 (mass ratio), the feed flow rate is 1 mL/min, the system pressure is 0.3 MPa and the permeate side pressure is 100 Pa.

[0051] The separation factor is calculated from: =(w.sub.2m/w.sub.2d)/(w.sub.1m/w.sub.1d), where w.sub.2m is the mass concentration of methanol on the permeate side, w.sub.2d is the mass concentration of dimethyl carbonate on the permeate side, w.sub.1m is the mass concentration of methanol in the feed and w.sub.1d is the mass concentration of dimethyl carbonate in the feed.

[0052] The permeation flux equation is J=m/(sxt), where m is the mass (unit g) of a product collected on the permeate side, s is the area (m.sup.2) of the molecular sieve membrane and t is the collection time (h).

TABLE-US-00001 TABLE 1 The vapor-permeation separation test results of MeOH/DMC in Example 1. Feed Membrane composition Permeation fluxJ Separation tube MeOH/DMC [kg/(m.sup.2 .Math. h)] factor Dry-gel 90/10 10.6 790 Hydrothermal 90/10 2.1 1800 synthesis

[0053] It can be seen from Table 1 that the separation factor for the methanol/dimethyl carbonate of the ultrathin SAPO-34 molecular sieve membrane prepared by dry-gel process is lower than that of the SAPO-34 molecular sieve membrane prepared by hydrothermal synthesis, which was reduced from to 790. But the flux of the former one is increased by 4 times, i.e. from 2.1 kg/(m.sup.2.Math.h) to 10.6 kg/(m.sup.2.Math.h). It is apparent that the decreasing of the membrane thickness greatly reduces the mass transfer resistance of the membrane, thereby resulting in a great enhancement of the flux. In comparison, although the separation factor of the ultrathin SAPO-34 molecular sieve membrane prepared by dry-gel process decreases to some extent, a separation factor of 790 is high enough because the concentration of methanol in the permeate is higher than 99.9 wt %.

Example 2

[0054] All steps in this Example are the same as in Example 1 except that the porous support coated with SAPO-34 molecular sieve seeds was soaked in the mother liquor for dry gel synthesis for 2 h in step 3.

TABLE-US-00002 TABLE 2 The vapor-permeation separation test results of MeOH/DMC in Example 2. Operation temperature Permeation flux J Separation C. [kg/(m.sup.2 .Math. h)] factor 120 13.1 350

[0055] It can be seen from Table 2 that in the dry-gel synthesis method, the ultrathin SAPO-34 molecular sieve membrane prepared by immersion in the mother liquor for 2 h has a methanol selectivity of 350, but the flux is higher than 13 kg/(m.sup.2.Math.h) (Table 2).

[0056] The surface and cross section SEM images of the SAPO-34 molecular sieve membrane prepared in Example 2 are shown in FIGS. 4A and 4B. It can be seen from FIGS. 4A and 4B that the support surface is completely covered by lamellar square crystals which are well crosslinked therebetween. The membrane thickness is relatively uniform, and is about 2-3 microns.

Example 3

[0057] Example 3 differs from Example 1 in that: in step 2), 8.46 g of aluminium hydroxide were added to 10.24 g of phosphoric acid solution (85 wt %) and 31.82 g of DI water. And after adequate hydrolysis, 4.00 g of silica sol (40 wt %) and 33.62 g of tetraethyl ammonium hydroxide (35 wt %) were added in turn. The resultant was stirred overnight (e.g., stirred for 12 h) to form a mother liquor for dry gel synthesis of molecular sieve membrane. The resulting mother liquor had a molar composition of 1 Al.sub.2O.sub.3: 1 P.sub.2O.sub.5: 0.6 SiO.sub.2: 1.8 TEAOH: 77H.sub.2O. Other steps of Example 3 are the same as that of Example 1.

TABLE-US-00003 TABLE 3 The vapor permeation separation test results of MeOH/DMC in Example 3. Operation temperature Permeation flux J Separation C. [kg/(m.sup.2 .Math. h)] factor 120 12.6 128

[0058] It can be seen from Table 3 that when a dry gel synthesis method is used and the mother liquor has a molar composition of 1 Al.sub.2O.sub.3: 1 P.sub.2O.sub.5: 0.6SiO.sub.2: 1.8TEAOH: 77H.sub.2O, the synthesized ultrathin SAPO-34 molecular sieve membrane has a methanol selectivity of 128 and a flux higher than 12 kg/(m.sup.2.Math.h).

Example 4

[0059] All steps in this Example are the same as in Example 1 except that in step 3), the porous support coated with SAPO-34 molecular sieve seeds was soaked in the mother liquor for dry gel synthesis for 40 min and 2 mL DI water was added to the 100 mlLreaction vessel.

TABLE-US-00004 TABLE 4 The vapor permeation separation test results of MeOH/DMC in Example 4. Operation temperature Permeation flux J Separation C. [kg/(m.sup.2 .Math. h)] factor 120 12.9 305

[0060] It can be seen from Table 4 that when a dry gel synthesis method is used and 2 g of DI water were added to the bottom of the 100 mL hydrothermal reaction vessel, the synthesized ultrathin SAPO-34 molecular sieve membrane had a methanol selectivity of 305 and a flux higher than 12 kg/(m.sup.2.Math.h).

[0061] In addition, the pervaporation process of Examples 1-4 is shown in FIG. 5.

[0062] In addition, the SAPO-34 molecular sieve membranes prepared as above can also be used for the pervaporation or vapor-permeation separation of a gas-liquid mixture, wherein the gas of the gas-liquid mixture may be one of nitrogen gas, hydrogen gas, oxygen gas, carbon dioxide or methane or the like; and the liquid of the gas-liquid mixture may be one of water, methanol, acetone or benzene or the like.