Pervaporation and Vapor-Permeation Separation of Gas-Liquid Mixtures and Liquid Mistures by Ion Exchanged SAPO-34 Molecular Sieve Membrane

Abstract

The invention discloses a method for the pervaporation and vapor-permeation separation of a gas-liquid mixture/liquid mixture by an ion-exchanged SAPO-34 molecular sieve membrane, said method comprises the following steps: 1) synthesis of SAPO-34 molecular sieve seeds; 2) coating the SAPO-34 molecular sieve seeds onto the inner surface of a porous support; 3) synthesis of SAPO-34 molecular sieve membrane; 4) performing ion exchange and calcination; 5) using the ion-exchanged SAPO-34 molecular sieve membrane obtained in step 4) to perform the separation of a gas-liquid mixture or a liquid mixture by a process of pervaporation separation or vapor-permeation separation. The present method for membrane separation of methanol-dimethyl carbonate has advantages like low energy consumption, being not limited by azeotropic mixture, high methanol flux and high separation factors and thus has great economic value.

Claims

1. A method for the separation of a gas-liquid mixture or a liquid mixture by preparing and using an ion-exchanged SAPO-34 molecular sieve membrane, said method comprises the following steps: 1) mixing and dissolving an Al source, tetraethyl ammonium hydroxide (TEAOH), water, a Si source and a P source to make reaction liquor for seeds, which is then subjected to crystallization for 4-7 h by heating at 170-210 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 onto the inner surface of a porous support to get a porous support coated with SAPO-34 molecular sieve seeds; 3) synthesis of SAPO-34 molecular sieve membrane tube, A. uniformly mixing an Al source, a P source, a Si source, tetraethylammonium hydroxide, di-n-propyl amine (DPA), water and a fluoride to form a mother liquor for SAPO-34 molecular sieve membrane synthesis; wherein, the molar ratio of the Al source, P source, Si source, tetraethylammonium hydroxide, di-n-propyl amine (DPA) and all water in the mother liquor for molecular sieve membrane synthesis is 1 Al.sub.2O.sub.3 : 0.5-3.5 P.sub.2O.sub.5: 0.05-0.6 SiO.sub.2: 0.5-8 TEAOH : 0.1-4.0 DPA : 0.01-1 F: 50-300 H.sub.2O; B. placing the porous support coated with SAPO-34 molecular sieve seeds prepared in the step 2) in the mother liquor for molecular sieve membrane synthesis and after soaking and aging for 28 h at room temperature80 C., crystallizing for 3-24 h at 150240 C. to synthesize the SAPO-34 molecular sieve membrane tube; 4) using the following Method I or Method II for ion exchange and calcination, Method I: supporting a metal salt whose melting point is lower than 370-700 C. on the SAPO-34 molecular sieve membrane tube obtained in step 3), drying and then calcining for 28 h at 370-700 C., to remove the template agent tetraethylammonium hydroxide and simultaneously carry out ion exchange, thereby to obtain an ion-exchanged SAPO-34 molecular sieve membrane; Method II: calcining the SAPO-34 molecular sieve membrane tube obtained in the step 3) for 2-8 h at 370-700 C. to remove the template agent tetraethylammonium hydroxide, then supporting a metal salt whose melting point is lower than 370-700 C. on the molecular sieve membrane tube, and drying, then ion-exchanging in melt state at a temperature lower than the calcination temperature of 370-700 C. and higher than the melting point of the metal salt, thereby to obtain an ion-exchanged SAPO-34 molecular sieve membrane; wherein in Method I and MethodII the method of supporting the metal salt whose melting point is lower than the calcination temperature is performed by supporting the metal salt on the front surface, back surface or both of the molecular sieve membrane tube by dip coating, spin coating, spray coating or brush coating: 5) using the ion-exchanged SAPO-34 molecular sieve membrane obtained in step 4) to perform separation of a gas-liquid mixture by a process of vapor-permeation separation, wherein, the gas in the gas-liquid mixture is selected from inert gas, hydrogen gas, oxygen gas, CO.sub.2 or gaseous hydrocarbon, and the liquid in the gas-liquid mixture is selected from alcohol, ketone or aromatics; or using the ion-exchanged SAPO-34 molecular sieve membrane obtained in step 4) to perform separation of a liquid mixture by a process of pervaporation separation, wherein said liquid mixture is a mixture of methanol and a liquid other than methanol, 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 the steps 1) and 3), the Al source is selected from one or more of aluminum isopropoxide, Al(OH).sub.3, elemental aluminum, an Al salt; wherein said Al salt is selected from one or more of aluminum nitrate, aluminum chloride, aluminum sulfate, and aluminum phosphate; , the P source is phosphoric acid; the Si source is selected from one or more of tetraethyl orthosilicate, tetramethyl orthosilicate , silica sol, silica, sodium silicate, water glass.

3. The method according to claim 1, characterized in that in the step 1), the heating is microwave heating and the size of the SAPO-34 molecular sieve seeds is 501000 nm.

4. The method according to claim 1, characterized in that in the step 2), the porous support is a porous ceramic tube; wherein the pore size of the porous ceramic tube is 5-2000 nm, and the material of the porous ceramic tube includes Al.sub.2OP.sub.3, TiO.sub.2, ZrO.sub.2, SiC or silicon nitride.

5. The method according to claim 1, characterized in that the coating of the seeds in step 2), the process comprises sealing the two ends of the porous support with glaze, washing and drying, sealing the outer surface, and then coating the SAPO-34 molecular sieve seeds onto the inner surface of the porous support; and the coating is selected from brush coating or dip coating.

6. The method according to claim 1, characterized in that in the step 3), the fluoride is selected from one or a mixture of HF and a fluoride salt; wherein the fluoride salt is selected from a fluoride salt of a main-group metal and a fluoride salt of a transitional metal.

7. The method according to claim 6, characterized in that the fluoride is selected from potassium fluoride, sodium fluoride, or ammonium fluoride.

8. The method according to claim 1, characterized in that in the step 3), the operation procedures for forming the mother liquor for molecular sieve membrane synthesis comprises the following steps mixing the Al source, P source and water, stirring for 15 h; then adding the Si source, stirring for 0.52 h; then adding the tetraethylammonium hydroxide, stirring for 0.52 h; then adding di-n-propyl amine, stirring for 0.52 h; then adding the fluoride, stirring for 1296 h at room temperature to get a homogeneous mother liquor for molecular sieve membrane synthesis.

9. The method according to claim 1, characterized in that in the step 4), the cation of the metal salt is a main-group metal or a transitional metal, and the anion is a hydracid radical or an oxo acid radical.

10. The method according to claim 9, characterized in that the metal salt is selected from sodium nitrate, lithium nitrate, rubidium nitrate, magnesium nitrate, potassium nitrate, sodium chlorate, or sodium perchlorate.

11. The method according to claim 14, characterize in that the method of supporting the metal salt whose melting point is lower than the calcination temperature is supporting the metal salt by dip coating, the operation procedure thereof comprises the following steps, in the Method I or Method II, the molecular sieve membrane having or not having the template agent removed is placed in a 0.0150 wt % solution of the metal salt and soaked for 1 s2 days at 40100 C.; wherein the solvent in the solution of the metal salt is selected from water, acetone, or alcohol.

12. The method according to claim 11, characterize in that in the ion exchange in Method I or Method II, the molecular sieve membrane having or not having the template agent removed is placed in a 0.150 wt % solution of the metal salt and soaked for 1 s180 min at 40100 C.

13. The method according to claim 1, characterize in that in the step 4), the drying temperature ranges from room temperature to 200 C.; the conditions for ion exchanging in melt state are that the ion exchange temperature is 100500 C. and the ion exchange time is 18 h; wherein), the atmosphere for calcination is selected from inert gas, vacuum, air, oxygen, or diluted oxygen in any ratio; and in the calcination, the temperature increasing rate and the temperature decreasing rate are not higher than 2 K/min.

14. The method according to claim 14, characterize in that in step 5), the conditions for the process of pervaporation separation are that the methanol concentration in the feed is 199 wt %, the feed flow rate is 1500 mL/min, the separation operation temperature is room temperature150 C., pressure on the permeate side is 0.06300 Pa.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The invention will be explained in further detail by taking the following figures and the detailed implementation.

[0049] FIG. 1 is a SEM (Scanning Electron Microscopy) image of SAPO-34 seeds of Example 1;

[0050] FIG. 2 is an XRD (X-ray diffraction) pattern of SAPO-34 seeds of Example 1;

[0051] FIG. 3 is a SEM image of SAPO-34 molecular sieve membrane prepared in Example 1 (a potassium ion-exchanged molecular sieve membrane obtained by simultaneous ion exchange and removal of the template agent); wherein, FIG. 3A is a surface SEM image of the ion-exchanged SAPO-34 molecular sieve membrane; FIG. 3B is a cross sectional SEM image of the ion-exchanged SAPO-34 molecular sieve membrane;

[0052] FIG. 4 is a SEM image of un-exchanged SAPO-34 molecular sieve membrane in Example 1; wherein, FIG. 4A is a surface SEM image of the un-exchanged SAPO-34 molecular sieve membrane; FIG. 4B is a cross sectional SEM image of the un-exchanged SAPO-34 molecular sieve membrane;

[0053] FIG. 5 is a schematic diagram of a pervaporation separation 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;

[0054] FIG. 6 is surface SEM image of SAPO-34 molecular sieve membrane of Example 3 (a sodium ion-exchanged molecular sieve membrane obtained by removal of the template agent followed by ion exchange in melt state);

[0055] FIG. 7 is a cross sectional SEM image of SAPO-34 molecular sieve membrane of Example 3 (a sodium ion-exchanged molecular sieve membrane obtained by removal of the template agent followed by ion exchange in melt state).

EXAMPLES

Example 1: Separation of Methanol/Dimethyl Carbonate by a Potassium Ion-Exchanged SAPO-34 Molecular Sieve Membrane Obtained by Simultaneous Ion Exchange and Removal of Template Agent

[0056] Step1: 2.46 g of DI water were added to 31.13 g of tetraethyl ammonium hydroxide solution (TEAOH, 35 wt %) . Then 7.56 g of aluminum isopropoxide were added thereto, and the resultant was stirred for 2-3 h at room temperature; then 1.665 g of silica sol (40 wt %) was added dropwise, and the resultant was stirred for 1 h. Finally, 8.53 g of phosphoric acid solution (H.sub.3PO.sub.4, 85 wt %) were slowly added dropwise, and the resultant was stirred overnight (e.g., stirred for 12 h). Then crystallization was performed at 180 C. for 7 h by using microwave heating. The obtained product was taken out from the reactor, centrifuged, washed and dried to obtain SAPO-34 molecular sieve seeds.

[0057] The SEM image and XRD pattern of the seeds are shown in FIG. 1 and FIG. 2, respectively. It can be seen from the SEM image that the size of the seeds is around 300 nm * 300 nm * 100 nm. The XRD pattern indicates that the seeds are pure SAPO-34 phase, and are well crystallized with no impure phase.

[0058] Step 2: A porous ceramic tube (material: 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 PTFE tape. Then the SAPO-34 molecular sieve seeds were coated onto the inner surface of the ceramic tube by brush coating method.

[0059] Step 3: 4.27 g of phosphoric acid solution (H.sub.3PO.sub.4, 85 wt %) were mixed with 43.8 g of DI water, and the resultant was stirred for 5 min. Then 7.56 g of aluminum isopropoxide were added, and the resultant was stirred for 3 h at room temperature. 0.83 g of silica sol (40 wt %) were added, and the resultant was stirred for 30 min at room temperature. Then, 7.78 g of tetraethyl ammonium hydroxide solution (TEAOH, 35 wt %) were added dropwise, and the resultant was stirred for 1 h at room temperature. Finally, 3.0 g of di-n-propylamine were added, and the resultant was stirred for 30 min at room temperature, then 0.045 g of hydrofluoric acid (HF, 40 wt %) were added, and the resultant was stirred overnight (e.g., stirred for 12 hours) at 50 C., getting a mother liquor for synthesis of SAPO-34 molecular sieve membrane. The porous ceramic tube coated with SAPO-34 molecular sieve seeds, which was prepared in the above step 2, was placed in a reaction vessel, and the mother liquor for synthesis of molecular sieve membrane was added. The reaction vessel was closed and aging was performed for 3 h at room temperature. Then hydrothermal synthesis was performed at 220 C. for 5 h. After taken out from the reaction vessel, the product was thoroughly rinsed and dried in an oven.

[0060] Step 4: The membrane tube obtained in step 3 was placed in a 1 wt % potassium nitrate aqueous solution and soaked for 3 min, then taken out and dried at room temperature. Then the membrane tube was calcined in vacuum at 400 C. for 4 h to remove the template agent (the temperature increasing rate and temperature decreasing rates were 1 C./min, respectively), getting an ion-exchanged SAPO-34 molecular sieve membrane.

[0061] The surface and cross sectional SEM images of the ion-exchanged SAPO-34 molecular sieve membrane are respectively shown in FIGS. 3A and 3B. The surface and cross sectional SEM images of an un-exchanged SAPO-34 molecular sieve membrane prepared under the same conditions are respectively shown in FIGS. 4A and 4B. It can be seen from the SEM images in FIGS. 3 and 4 that their support surfaces are both completely covered by square lamellar SAPO-34 crystals which are perfectly cross-linked therebetween. The crystal size is 4-7 microns, and the molecular sieve membrane surface is flat. The cross sectional image shows that the thickness of the membrane is about 5-6 microns. Thus, ion exchange has no significant effect on the morphology of membrane.

[0062] Step 5. The ion-exchanged SAPO-34 molecular sieve membrane obtained in the above step was used to separate a methanol/dimethyl carbonate (i.e., DMC/MeOH) azeotrope by a pervaporation process, wherein the feed flow rate was 1 mL/min, the separation operation temperature 70 C., the pressure on the permeate side 100 Pa and the composition of the MeOH/DMC feed was from 90/10 to 70/30 (mass ratio). The schematic diagram of the pervaporation process is shown in FIG. 5.

[0063] 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.

[0064] The permeation flux equation is J=m/(st), wherein m is the mass (g) of a product collected on the permeate side, s is the molecular sieve membrane area (m.sup.2) and t is the collecting time (h).

[0065] It can be seen from Table 1 that when the feed composition of MeOH/DMC is from 90/10 to 70/30, the methanol selectivity of the SAPO-34 membrane is more than 2000, and the flux is about 0.14 kg/(m.sup.2.Math.h) (Table 1). Thus, the ion-exchanged SAPO-34 molecular sieve membrane has a very high methanol-dimethyl carbonate separation factor.

TABLE-US-00001 TABLE 1 The pervaporation separation test results of MeOH/DMC in Example 1. Feed composition Permeation flux J Separation MeOH/DMC (wt %) [kg/(m.sup.2 .Math. h)] factor 70/30 0.137 2500 90/10 0.146 2000

Example 2: Separation of Methanol/Dimethyl Carbonate Mixture at 120 C. by Ion-Exchanged SAPO-34 Membrane.

[0066] All steps in this Example are the same as in Example 1 except that the feed composition of MeOH/DMC is 90/10 (mass ratio), the separation operation temperature is 120 C., and the permeate side pressure is 0.3 Mpa in step 5.

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 2.3 4100

[0067] It can be seen from Table 2 that when the feed composition of MeOH/DMC is 90/10, and the operating temperature is 120 C., the methanol selectivity of the ion exchanged SAPO-34 molecular sieve membrane is greater than 4000, and the flux is greatly increased compared to that at 70 C. The increase of the flux is due to the fact that the increase of feed pressure causes the increasing of mass transfer driving force of methanol. Thus it can be seen that the ion-exchanged SAPO-34 molecular sieve membrane has a very high methanol-dimethyl carbonate separation factor and a high methanol flux.

Example 3: Separation of Methanol/Dimethyl Carbonate by SAPO-34 Molecular Sieve Membrane Prepared by Removal of Template Agent Followed by Sodium Ion EXchange in Melt State

[0068] All the steps in this Example are the same as in Example 1, except that in step 4, the molecular sieve membrane tube obtained in step 3 was calcined in vacuum at 400 C. for 4 h to remove the template agent, cooled down to room temperature, and then placed in a 1 wt % sodium nitrate aqueous solution and soaked for 3 min, then taken out and dried at room temperature; then calcined at 310 C. for 8 h to carry out ion exchange, thereby to get a sodium ion-exchanged molecular sieve membrane. In step 5, the feed composition of MeOH/DMC is 90/10 (mass ratio), the separation operation temperature is 120 C., and the pressure on the permeate side is 0.3 MPa.

TABLE-US-00003 TABLE 3 The vapor permeation separation test results of MeOH/DMC in Example 3. Permeate side pressure Permeation flux J Separation MPa [kg/(m.sup.2 .Math. h)] factor 0.3 2.1 3700

[0069] It can be seen from Table 3 that when the feed composition of MeOH/DMC is 90/10, and the operating temperature is 120 C., the methanol selectivity of the SAPO-34 molecular sieve membrane which is sodium ion-exchanged in melt state is greater than 3500, and the permeation flux is greater than 2 kg/(m.sup.2.h). Thus it can be seen that the ion-exchanged SAPO-