LARGE-SCALE FABRICATION TECHNIQUE FOR PIM-1 ASYMMETRIC MEMBRANES DOPED WITH LOW-MOLECULAR-WEIGHT POLYETHYLENE GLYCOL FOR GAS SEPARATION

Abstract

A large-scale fabrication technique for PIM-1 asymmetric membranes doped with low-molecular-weight polyethylene glycol for gas separation. Based on the membrane fabrication technique of dry/wet phase inversion, firstly, the coagulation process of casting solution is regulated by low-molecular-weight polyethylene glycol to thin the dense functional layer, to improve the hydrophilicity of the membrane structure, and to form mass transfer channels for the diffusion of polyethylene glycol into the dense functional layer. Then, directional migration and enrichment of polyethylene glycol are realized through capillary action induced by directional water evaporation for large-scale fabrication of PIM-1 asymmetric membranes doped with low-molecular-weight polyethylene glycol in the dense functional layer for gas separation, and thereafter high permeation ability and high selectivity are achieved simultaneously.

Claims

1. A large-scale fabrication technique for PIM-1 asymmetric membranes doped with low-molecular-weight polyethylene glycol for gas separation, comprising the following steps: 1) adding low-molecular-weight polyethylene glycol into the PIM-1 casting solution for phase inversion, and controlling the additive amount of polyethylene glycol within a concentration range of 0.1-4.0 wt %, wherein polyethylene glycol molecular weight is between 400 and 1500 Dalton, and one of the molecular chain end groups is hydroxyl group; and then, forming a plate-like PIM-1 asymmetric membrane structure through dry/wet phase inversion, with a dense functional layer on one side and a hierarchical porous supporting layer on the other side; 2) moving the PIM-1 asymmetric membrane structure formed in the first step into the nonsolvent coagulation environment mainly constituted with water for a retention time of 2-6 hours, so that the solvent for membrane casting can be removed from the PIM-1 asymmetric membrane structure; transferring the PIM-1 asymmetric membrane structure from the nonsolvent coagulation environment to the atmospheric environment, and simultaneously spraying the aqueous solution of low-molecular-weight polyethylene glycol which has a concentration range of 2.0-5.0 wt % on the surface on the side of hierarchical porous supporting layer, wherein polyethylene glycol molecular weight is between 400 and 1500 Dalton; keeping the dihedral angle between gravitational direction and transmembrane direction larger than 60 degrees for the PIM-1 asymmetric membrane structure during spraying process, so that the adhesion amount of polyethylene glycol solution on the PIM-1 asymmetric membrane structure could be controlled by gravity; 3) transferring the PIM-1 asymmetric membrane structure which has been fully sprayed with polyethylene glycol solution on one side to an oven customized with directional water evaporation ability, wherein the PIM-1 asymmetric membrane structure is outspreaded; making the side of the dense functional layer of the PIM-1 asymmetric membrane structure enter the oven environment specified with sub-atmospheric pressure and heating functions; controlling the vacuum degree and temperature to be respectively in the ranges of 0-20 kPa and 40-80 C. to promote water evaporation on the side of the dense functional layer; making the side of the hierarchical porous supporting layer enter the oven environment specified with humidification and cooling functions; controlling relative humidity and temperature to be respectively in the ranges of 80-100% and 20-35 C. to inhibit water evaporation on the side of the hierarchical porous supporting layer; through directional water evaporation mentioned above and the resultant capillary action, realizing the directional migration and enrichment of polyethylene glycol from the hierarchical porous supporting layer to the dense functional layer in the PIM-1 asymmetric membrane structure; 4) after directional migration and enrichment for polyethylene glycol, transferring the PIM-1 asymmetric membrane structures into a vacuum oven for deep dehydration; controlling the vacuum degree and temperature to be within the ranges of 0-80 kPa and 60-120 C. respectively, and the dehydration time to be not less than 8.0 hours; through conducting deep dehydration, inhibiting the migration of polyethylene glycol, and thereafter accomplishing the large-scale fabrication of PIM-1 asymmetric membranes with low-molecular-weight polyethylene glycol controllably doped in the dense functional layer for gas separation.

2. The large-scale fabrication technique for PIM-1 asymmetric membranes doped with low-molecular-weight polyethylene glycol for gas separation, according to claim 1, wherein the oven customized with the function of directional water evaporation comprises a left-hand chamber, a right-hand chamber and an air circulation system; the two chambers are divided by the PIM-1 asymmetric membrane; the hierarchical porous supporting layer of the membrane faces the left-hand chamber and the dense functional layer faces the right-hand chamber, and the membrane could be handled with directional movement through the rollers located on the upper and the lower ends of the customized oven for orientation and traction; a cooling element is arranged in the left-hand chamber to create an environment with humidification and cooling functions; a heating element is arranged in the right-hand chamber to create an environment with sub-atmospheric pressure and heating functions; supporting rollers are installed in the right-hand chamber and highly attached to membrane surface with the function to locate and also support the membrane; the inlet and the outlet of the air circulation system are connected with the two chambers; a vacuum pump, a humidification tank and a cooler are arranged in sequence on the pipeline from the right-hand chamber to the left-hand chamber; and through the air circulation system hot air with low humidity in the right-hand chamber is converted into cold air with high humidity in the left-hand chamber.

Description

DESCRIPTION OF DRAWINGS

[0015] FIG. 1 is the SEM image of the PIM-1 asymmetric membrane structure prepared without adding low-molecular-weight polyethylene glycol into the casting solution.

[0016] FIG. 2 is the SEM image of the PIM-1 asymmetric membrane structure prepared after adding low-molecular-weight polyethylene glycol into the casting solution.

[0017] FIG. 3 is the SEM image of the PIM-1 asymmetric membrane doped with low-molecular-weight polyethylene glycol in the dense layer for gas separation.

[0018] FIG. 4 is the schematic structure diagram of the oven with directional water evaporation function.

[0019] In the figures: {circle around (1)} box shell of the customized oven; {circle around (2)} cooling element; {circle around (3)} supporting rollers for membrane; {circle around (4)} heating element; {circle around (5)} traction rollers for membrane; {circle around (6)} vacuum pump; {circle around (7)} humidification tank; {circle around (8)} cooler; {circle around (9)} PIM-1 asymmetric membrane structure.

DETAILED DESCRIPTION

[0020] Specific embodiments of the present invention are further described below in combination with the drawings and the technical solution.

Embodiment 1

[0021] A PIM-1 membrane with asymmetric structure for gas separation is prepared through dry/wet phase inversion in embodiment 1. PIM-1 used in the embodiment is a polymeric membrane material synthesized with 5,5,6,6-tetrahydroxy-3,3,3,3-tetramethyl-1,1-spirobisindane (TTSBI) and tetrafluoroterephtalonitrile (TFTPN), and the reaction is polycondensation. PIM-1 is a high free volume membrane material. The average molecular weight obtained by gel permeation chromatography analysis exceeds 65000 Dalton. The solvent system is a mixed solvent prepared by tetrahydrofuran (THF) and dimethylacetamide (DMAc), wherein the mass fraction of THF is about 75.0 wt %. The nonsolvent used for dry/wet phase inversion is deionized water.

[0022] PIM-1 is heated and dissolved in the mixed solvent prepared by THF and DMAc to prepare the phase inversion casting solution with a PIM-1 mass fraction of 8.0 wt %.

[0023] The PIM-1 phase inversion casting solution is uniformly spread out on non-woven fabric by blade coating to form a casting solution thin layer with a thickness not greater than 100 m. Then, the casting solution thin layer remains in the atmospheric environment under room temperature with relative humidity less than 30 RH % for 6 seconds, and then it is transferred into the coagulation bath environment (mainly composed with deionized water) under room temperature. A plate-like PIM-1 asymmetric membrane structure is formed after solvent exchange, with the dense functional layer at one side and the hierarchical porous supporting layer at the other side.

[0024] The PIM-1 asymmetric membrane structure formed in the previous step is stored in the nonsolvent coagulation environment mainly constituted with water, and the retention time is given as 6 hours, so that the membrane casting solvent is removed from the membrane structure.

[0025] The PIM-1 asymmetric membrane structure after deep removing membrane casting solvent is transferred into the vacuum oven, the vacuum degree and the temperature are controlled to 10 kPa and 60 C. respectively for 0.5 hours, and the water is partially removed. Further, the vacuum degree and the temperature are adjusted to 50 kPa and 100 C. respectively, for not less than 8.0 hours. The water in the membrane structure is deeply removed, to obtain the ultimate morphology of the PIM-1 gas separation membrane with asymmetric structure.

[0026] SEM characterization is carried through for the PIM-1 gas separation membrane prepared in embodiment 1. The cross-sectional microstructure is shown in FIG. 1, and the thickness of the dense functional layer is about 5.59 m. The permeation test indicates that the permeation rate of carbon dioxide is 640.3 GPU, the permeation rate of oxygen is 109.8 GPU and the permeation rate of nitrogen is 28.2 GPU, wherein the permeation selectivity between carbon dioxide and nitrogen is 22.7 and the permeation selectivity between oxygen and nitrogen is 3.89.

Embodiment 2

[0027] A PIM-1 membrane with asymmetric structure for gas separation is prepared through dry/wet phase inversion (an appropriate amount of low-molecular-weight polyethylene glycol is added into the casting solution) in embodiment 2. PIM-1 used in the embodiment is a polymeric membrane material synthesized with 5,5,6,6-tetrahydroxy-3,3,3,3-tetramethyl-1,1-spirobisindane (TTSBI) and tetrafluoroterephtalonitrile (TFTPN), and the reaction is polycondensation. PIM-1 is a high free volume membrane material. The average molecular weight obtained by gel permeation chromatography analysis exceeds 65000 Dalton. The solvent system is a mixed solvent prepared by tetrahydrofuran (THF) and dimethylacetamide (DMAc), wherein the mass fraction of THF is 75.0 wt %. The nonsolvent used for dry/wet phase inversion is deionized water.

[0028] PIM-1 is heated and dissolved in the mixed solvent prepared by THF and DMAc to prepare a casting solution with a PIM-1 mass fraction of 8.0 wt %. Then, low-molecular-weight polyethylene glycol with the average molecular weight of 1000 Dalton is added into the casting solution, with a mass fraction of 3.0 wt %.

[0029] The PIM-1 phase inversion casting solution is uniformly spread out on non-woven fabric by blade coating to form a casting solution thin layer with a thickness of not greater than 100 m. Then, the casting solution thin layer remains in the atmospheric environment under room temperature with relative humidity less than 30 RH % for 6 seconds, and then it is transferred into the coagulation bath environment (mainly composed with deionized water) under room temperature. A plate-like PIM-1 asymmetric membrane structure is formed after solvent exchange, with the dense functional layer at one side and the hierarchical porous supporting layer at the other side.

[0030] The PIM-1 asymmetric membrane structure formed in the previous step is stored in the nonsolvent coagulation environment mainly constituted with water, and the retention time is given as 6 hours, so that the membrane casting solvent is removed from the membrane structure.

[0031] The PIM-1 asymmetric membrane structure after deep removing membrane casting solvent is transferred into the vacuum oven, the vacuum degree and the temperature are controlled to 10 kPa and 60 C. respectively for 0.5 hours, and the water is partially removed. Further, the vacuum degree and the temperature are adjusted to 50 kPa and 100 C. respectively, for not less than 8.0 hours. The water in the membrane structure is deeply removed, to obtain the ultimate morphology of the PIM-1 gas separation membrane with asymmetric structure. SEM characterization is carried through for the PIM-1 gas separation membrane prepared in embodiment 2. The cross-sectional microstructure is shown in FIG. 2, and the thickness of the dense functional layer is about 1.15 m. The permeation test indicates that the permeation rate of carbon dioxide is 3345.1 GPU, the permeation rate of oxygen is 599.7 GPU and the permeation rate of nitrogen is 154.7 GPU, wherein the permeation selectivity between carbon dioxide and nitrogen is 21.6 and the permeation selectivity between oxygen and nitrogen is 3.88.

Embodiment 3

[0032] A PIM-1 membrane with asymmetric structure for gas separation is prepared through dry/wet phase inversion (an appropriate amount of low-molecular-weight polyethylene glycol is added into the casting solution) in embodiment 3. Thereafter, the gas separation performance of the membrane is enhanced by doping low-molecular-weight polyethylene glycol in the dense functional layer by directional water evaporation. PIM-1 used in the embodiment is a polymeric membrane material synthesized with 5,5,6,6-tetrahydroxy-3,3,3,3-tetramethyl-1,1-spirobisindane (TTSBI) and tetrafluoroterephtalonitrile (TFTPN), and the reaction is polycondensation. PIM-1 is a high free volume membrane material. The average molecular weight obtained by gel permeation chromatography analysis exceeds 65000 Dalton. The solvent system is a mixed solvent prepared by tetrahydrofuran (THF) and dimethylacetamide (DMAc), wherein the mass fraction of THF is 75.0 wt %. The nonsolvent used for dry/wet phase inversion is deionized water.

[0033] PIM-1 is heated and dissolved in the mixed solvent prepared by THF and DMAc to prepare a casting solution with a PIM-1 mass fraction of 8.0 wt %. Then, low-molecular-weight polyethylene glycol with the average molecular weight of 1000 Dalton is added into the casting solution, with a mass fraction of 3.0 wt %.

[0034] The PIM-1 phase inversion casting solution is uniformly spread out on non-woven fabric by blade coating to form a casting solution thin layer with a thickness of not greater than 100 m. Then, the casting solution thin layer remains in the atmospheric environment under room temperature with relative humidity less than 30 RH % for 6 seconds, and then it is transferred into the coagulation bath environment (mainly composed with deionized water) under room temperature. A plate-like PIM-1 asymmetric membrane structure is formed after solvent exchange, with the dense functional layer at one side and the hierarchical porous supporting layer at the other side.

[0035] The PIM-1 asymmetric membrane structure formed in the previous step is stored in the nonsolvent coagulation environment mainly constituted with water, and the retention time is given as 6 hours, so that the membrane casting solvent is removed from the membrane structure.

[0036] The PIM-1 asymmetric membrane structure is transferred from the nonsolvent coagulation environment to the atmospheric environment, and then the solution of low-molecular-weight polyethylene glycol which has a concentration range of 2.0-5.0 wt % is sprayed on the surface on the side of hierarchical porous supporting layer. During the spraying process, the dihedral angle between gravitational direction and transmembrane direction is kept larger than 60 degrees for the PIM-1 asymmetric membrane structure, so that the adhesion amount of the aqueous solution of polyethylene glycol on the membrane structure can be controlled by gravity.

[0037] The PIM-1 asymmetric membrane structure sprayed with the aqueous solution of polyethylene glycol fully on one side is transferred to the oven customized with directional water evaporation ability. The membrane structure should be outspreaded. The side of the dense functional layer of the PIM-1 asymmetric membrane structure enters the oven environment specified with sub-atmospheric pressure and heating functions. The vacuum degree and the temperature are controlled respectively to be 20 kPa and 60 C. to promote water evaporation on the side of the dense functional layer. The side of the hierarchical porous supporting layer enters the oven environment specified with humidification and cooling functions. The relative humidity and the temperature are controlled to be 95 RH % and 25 C., respectively, so that water evaporation on the side of the hierarchical porous supporting layer can be inhibited. The retention time of the PIM-1 asymmetric membrane structure in the oven with directional water evaporation ability is controlled to be 1.0 hour.

[0038] After directional migration and enrichment for polyethylene glycol, the PIM-1 asymmetric membrane structure is transferred into an ordinary vacuum oven; the vacuum degree is controlled to be 50 kPa and the temperature is controlled to be 100 C.; and duration time exceeds 8.0 hours. The water in the membrane structure is deeply removed, to obtain the ultimate morphology of the PIM-1 gas separation membrane. In detail, this kind of PIM-1 membranes has an asymmetric structure and a dense functional layer doped with low-molecular-weight polyethylene glycol.

[0039] SEM characterization is carried through for the PIM-1 gas separation membrane doped with low-molecular-weight polyethylene glycol prepared in embodiment 3. The cross-sectional microstructure is shown in FIG. 3, and the thickness of the dense functional layer is about 1.07 m. Because the processes of dry/wet phase inversion are basically the same for embodiment 3 and embodiment 2, the membranes prepared by the two embodiments have similar microstructure and dense layer thickness.

[0040] The gas permeation ability test indicates that the performance of the PIM-1 asymmetric gas separation membrane is significantly changed by doping low-molecular-weight polyethylene glycol in the dense functional layer through the process of directional water evaporation; the permeation rate of carbon dioxide is 2230.5 GPU, the permeation rate of oxygen is 179.1 GPU and the permeation rate of nitrogen is 32.5 GPU, wherein the permeation selectivity between carbon dioxide and nitrogen is 68.6 and the permeation selectivity between oxygen and nitrogen is 5.5. Compared with the membrane sample not doped with low-molecular-weight polyethylene glycol in the dense functional layer in embodiment 2, the permeation ability of the PIM-1 asymmetric gas separation membrane prepared in embodiment 3 is reduced, but the selectivity is significantly improved.