Application of Mo atom doped MFI molecular sieve membrane in CO2 / H2 separation

Abstract

The invention relates to the field of gas separation, in particular to application of a Mo atom doped MFI molecular sieve membrane in CO2 / H2 separation. According to the invention, the MFI molecular sieve membrane is doped and modified by adopting the Mo element, the pore structure of the MFI molecular sieve membrane is changed, and the CO2 adsorption performance of the membrane layer is enhanced, so that the performance of the molecular sieve membrane in the application of a CO2 / H2 system is obviously improved. And compared with a traditional MFI molecular sieve membrane and a W-doped MFI molecular sieve membrane, the Mo-doped MFI molecular sieve membrane shows higher selectivity.

Description

TECHNICAL FIELD

[0001] The application relates to the field of gas separation, in particular to application of a Mo atom doped MFI molecular sieve membrane in CO2 / H2 separation. BACKGROUND

[0002] H2 in China is mainly prepared through steam reforming, however, a large amount of CO2 greenhouse gas is generated in the process, which inevitably causes environmental damage. Therefore, developing effective and mature CO2 and H2 separation technology has great significance for preparing cheap and high-concentration H2.

[0003] At present, the purification methods of H2 include pressure swing adsorption, low-temperature distillation and membrane separation. The membrane separation method is widely used due to its low energy consumption, low investment cost, small occupied area and easy scaling-up. The inorganic membrane, especially the molecular sieve membrane, has good application prospect in many fields due to its good pore structure, better flux and selectivity, high thermal stability and mechanical stability. At present, it has been reported that MFI, CHA, LTA, MOR and FAU molecular sieve membranes are used for H2 selective separation membranes, and excellent separation performance is shown.

[0004] Since the molecular size (2.89 angstrom) of hydrogen molecule is smaller than that (3.30 angstrom) of carbon dioxide molecule, the selective separation of CO2 in CO2 / H2 mixture through zeolite pores is mainly based on competitive adsorption. Previous studies have shown that the adsorption capacity of the MFI molecular sieve membrane for CO2 can be improved by introducing W element for doping modification in the preparation process of the MFI molecular sieve membrane. However, the selectivity of the W-doped MFI molecular sieve membrane applied to CO2 / H2 separation is limited, and the selectivity is only 8.6. Therefore, how to improve the selectivity of the MFI molecular sieve membrane applied to CO2 / H2 separation is a problem to be solved. SUMMARY

[0005] The purpose of the application is to increase the adsorption capacity of the MFI molecular sieve membrane for CO2 by doping Mo atoms in the framework of the MFI molecular sieve membrane, so as to further improve the CO2 / H2 separation performance of the MFI molecular sieve membrane.

[0006] Specifically, the application provides application of a Mo atom doped MFI molecular sieve membrane in CO2 / H2 separation. The Mo atom doped MFI molecular sieve membrane is prepared by placing a support loaded with Si-MFI seeds in a synthesis solution for hydrothermal synthesis reaction, the synthesis solution contains a Mo source, and the molar ratio of Mo to SiO2 in the synthesis solution is 0.01-0.08.

[0007] Preferably, the separation selectivity of the Mo atom-doped MFI molecular sieve membrane for the CO2 / H2 system is greater than 20 at a test temperature of -20-20℃ and a test pressure of 0.1-0.3 MPa.

[0008] Preferably, the molar ratio of Mo to SiO2 in the synthesis solution is 0.02-0.04.

[0009] Preferably, the synthesis solution comprises a structure directing agent, a silicon source, a molybdenum source and water, wherein the molar ratio of the structure directing agent to SiO2 is 0.12-0.2, and the molar ratio of water to the structure directing agent is 150-200.

[0010] Preferably, the structure directing agent is selected from one of tetrabutylphosphonium hydroxide, tetrabutylammonium hydroxide, tetrapropylammonium hydroxide and tetrapropylammonium hydroxide; the silicon source is selected from silica sol, tetraethyl orthosilicate or fumed silica; and the Mo source is selected from ammonium molybdate or sodium molybdate dihydrate.

[0011] Preferably, the hydrothermal synthesis temperature is 130-150℃, and the synthesis time is 24-72h.

[0012] Preferably, a calcination treatment is required after the hydrothermal synthesis reaction, the calcination temperature is 200-300℃, the environment is ozone or air, the time is 6-108h, and the temperature rising and falling rate is 0.5-2℃ / min.

[0013] Compared with the prior art, the present application has the following advantages: The present application dopes and modifies MFI molecular sieve membranes with Mo elements, changes the pore structure, enhances the CO2 adsorption performance of the membrane layer, and thus significantly improves the performance of the molecular sieve membrane in the application of the CO2 / H2 system. Compared with the traditional MFI molecular sieve membrane and the W-doped MFI molecular sieve membrane, the Mo-doped MFI molecular sieve membrane exhibits higher selectivity. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 The XRD graph of the molecular sieve membrane prepared in Example 1 of the present application; Figure 2 The surface and cross-section SEM graphs of the molecular sieve membrane prepared in Example 1 of the present application; Figure 3 The UV-vis graph of the molecular sieve membrane prepared in Example 1 of the present application; Figure 4 The EDX element distribution graph of the molecular sieve membrane prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0015] The present application will be specifically described below in combination with the drawings and examples.

[0016] Example 1 The preparation method of the Mo-MFI molecular sieve membrane in this example is as follows: (1) Preparation of seeds: a molecular sieve synthesis solution was prepared according to the molar ratio of 1 SiO2: 0.12 TPAOH: 19.2 H2O, and was placed in a bottle for aging for 6 h, and was hydrothermally reacted at a temperature of 150 °C for 48 h to obtain a molecular sieve Si-MFI, and was centrifuged and dried; (2) The above Si-MFI was mixed with water to form a 0.5 wt.% seed suspension, and the YSZ hollow fiber carrier was immersed in the seed suspension for 30 s to coat the seeds on the carrier, and was dried at 60 °C for 12 h; (3) A molecular sieve membrane synthesis solution was prepared according to the molar ratio of 1 SiO2: 0.12 TPAOH: 180 H2O: 0.02 Na2MO4·2H2O, and the synthesis solution was stirred and aged at room temperature for 6 h; (4) After the seed-coated carrier was fixed, it was placed in a reaction kettle and immersed in the above-mentioned molecular sieve membrane synthesis solution, and was hydrothermally reacted at a temperature of 140 °C for 48 h, and was washed with deionized water after the hydrothermal reaction was completed, and was dried at 60 °C for 12 h, and was then calcined under ozone conditions at 230 °C for 96 h, and the temperature rising and falling rates were both 1 °C / min, to remove the template agent, and the obtained molecular sieve membrane was marked as Mo-0.02-MFI; (5) The obtained molecular sieve membrane was tested for CO2 / H2 gas separation performance. The test conditions were: temperature 20 °C, permeation side pressure maintained at 0.1 MPa, molar composition 50 / 50%, and total flow rate of CO2 and H2 feed 100 ml / min. The gas flow rate on the permeation side was measured by a soap bubble flow meter; the gas composition on the permeation side was analyzed by a gas chromatograph.

[0017] Example 2 Steps (1), (2), (3), and (4) were the same as in Example 1; the preparation process of step (5) was basically the same as in Example 1 step (5), except that the test temperature was 0 °C.

[0018] Example 3 Steps (1), (2), (3), and (4) were the same as in Example 1; the preparation process of step (5) was basically the same as in Example 1 step (5), except that the test temperature was -20 °C.

[0019] Example 4 Steps (1), (2), (3), and (4) were the same as in Example 1; the preparation process of step (5) was basically the same as in Example 1 step (5), except that the test permeation side pressure was maintained at 0.2 MPa.

[0020] Example 5 Steps (1), (2), (3), (4) and (5) are the same as those of Example 1; and step (6) is basically the same as step (5) of Example 1, except that the permeation end pressure is maintained at 0.3 MPa.

[0021] Example 6 Steps (1), (2), (4), (5) are the same as those of Example 1; and step (3) is basically the same as step (3) of Example 1, except that the molar ratio of each substance in the gel for the final synthesis of the molecular sieve membrane is 1 SiO2:0.12 TPAOH:180 H2O:0.04 Na2MO4·2H2O, and the molecular sieve membrane obtained is marked as Mo-0.04-MFI.

[0022] Example 7 Steps (1), (2), (4), (5) are the same as those of Example 1; and step (3) is basically the same as step (3) of Example 1, except that the membrane synthesis solution formula is 1 SiO2:0.12 TPAOH:180 H2O:0.06 Na2MO4·2H2O, and the molecular sieve membrane obtained is marked as Mo-0.06-MFI.

[0023] Comparative Example 1 Steps (1), (2), (4), (5) are the same as those of Example 1; and step (3) is basically the same as step (3) of Example 1, except that the membrane synthesis solution formula is 1 SiO2:0.12 TPAOH:180 H2O. The molecular sieve membrane obtained is marked as Si-MFI.

[0024] Comparative Example 2 Steps (1), (2), (4), (5) are the same as those of Example 1; and step (3) is basically the same as step (3) of Example 1, except that the membrane synthesis solution formula is 1 SiO2:0.12 TPAOH:180 H2O:0.02 Na2WO4·2H2O. The molecular sieve membrane obtained is marked as W-MFI.

[0025] The gas separation performance of the membrane is indicated by two parameters, i.e. gas permeation rate P and separation coefficient a. The gas permeation rate P represents the total amount of gas passing through the unit area of the membrane per unit time and per unit pressure, P=N / (A×△P), with the unit of mol / (m 2 •s•Pa); and the separation coefficient a is used to evaluate the separation efficiency of the membrane, a=PA / PB.

[0026] The XRD pattern of the molecular sieve membrane prepared in Example 1 is shown in Figure 1 The surface and cross-section SEM patterns of the Mo-0.02-MFI molecular sieve membrane prepared in Example 1 are shown in Figure 2The SEM images of the Mo-MFI membranes are shown in Figure 1. As can be seen from the figure, the crystals on the membrane surface are all regular cuboid in shape, and the continuous and dense membrane layer is formed by the intergrowth of the crystals, without obvious intercrystalline defects. The thickness of the membrane is about 10 μm.

[0027] UV-vis Figure 3 ) results show that the absorption peak at 223 nm is usually attributed to isolated Mo 6+ species. The tetrahedrally coordinated Mo atom is combined into the MFI framework in the oxidation state of Mo 6+ , forming stable Mo-O-Si bonds, bridging the silicon hydroxyl groups and forming a terminal Mo=O double bond structure. This stable configuration effectively reduces the number of Si-OH defects.

[0028] EDX Figure 4 results show that the Mo atoms are contained in the prepared membrane and are uniformly distributed, without obvious Mo species cluster aggregation.

[0029] Table 1 is the CO2 / H2 gas separation performance test results of the molecular sieve membranes prepared in Examples 1-7 and Comparative Examples 1-2. As can be seen from the table, the selectivity of the Mo-MFI molecular sieve membrane prepared in Example 1 is far higher than that of the Si-MFI and W-MFI prepared in Comparative Examples 1 and 2, and is obviously higher than that of the Mo-0.06-MFI prepared in Example 7. When the test temperature is 20℃ and the permeation end pressure is 0.1 Mpa, the CO2 / H2 selectivity of the Mo-0.02-MFI molecular sieve membrane is 33.

[0030] At the same time, the effect of the test temperature on the separation performance of the Mo-0.02-MFI molecular sieve membrane is studied. When the test temperature is reduced from 20℃ to -20℃, the CO2 permeability increases, which is caused by the combined action of the increase of the CO2 adsorption amount and the decrease of the diffusion coefficient. It can also be observed that the H2 permeability sharply decreases with the decrease of the temperature, which is mainly due to the increase of the CO2 adsorption amount hindering the transportation of H2. When the test temperature is -20℃, the CO2 permeability is 198 × 10 -9 mol·m -2 ·s -1 ·Pa -1 , and the CO2 / H2 selectivity is 202. Secondly, the effect of the pressure on the CO2 / H2 separation performance of the membrane is studied. With the increase of the permeation end pressure, the permeabilities of CO2 and H2 both gradually decrease.

[0031] Table 1 Performance test results of different examples and comparative examples The above merely describes preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art, according to the technical solution and inventive concept of the present application, makes equivalent replacement or change within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. An application of a Mo-doped MFI molecular sieve membrane in CO2 / H2 separation, characterized in that, The Mo-doped MFI molecular sieve membrane is formed by placing a support loaded with Si-MFI seeds in a synthesis solution and carrying out a hydrothermal synthesis reaction, wherein the synthesis solution contains a Mo source.

2. The application according to claim 1, characterized in that, At test temperatures of -20 to 20°C and test pressures of 0.1 to 0.3 MPa, the Mo-doped MFI molecular sieve membrane exhibits a separation selectivity greater than 20 for the CO2 / H2 system.

3. The application according to claim 1, characterized in that, The molar ratio of Mo to SiO2 in the synthesis solution is 0.02-0.

04.

4. The application according to claim 1, characterized in that, The synthesis solution includes a structure-directing agent, a silicon source, a molybdenum source, and water, wherein the molar ratio of the structure-directing agent to SiO2 is 0.12-0.2, and the molar ratio of water to the structure-directing agent is 150-200.

5. The application according to claim 4, characterized in that, The structure-directing agent is selected from one of tetrabutylphosphine hydroxide, tetrabutylammonium hydroxide, tetrapropylammonium hydroxide, and tetrapropylammonium hydroxide; the silicon source is selected from silica sol, tetraethyl orthosilicate, or fumed silica; and the Mo source is selected from ammonium molybdate or sodium molybdate dihydrate.

6. The application according to claim 1, characterized in that, The hydrothermal synthesis temperature is 130–150℃, and the synthesis time is 24–72 h.

7. The application according to claim 1, characterized in that, After the hydrothermal synthesis reaction, a calcination treatment is required. The calcination temperature is 200-300℃, the environment is ozone or air, the time is 6-108h, and the heating and cooling rate is 0.5-2℃ / min.

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