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Next generation hydrogen separation membrane
Issuing time:2021-08-18 16:38
Hydrogen (H2) is becoming more and more popular as a clean energy source. At present, membrane separation is the main way to obtain H2, but the permeability of commercial films is not high. In recent years, the microporous solid materials (such as zeolite and MOF) with rigid network structure and perfect pore structure have made great progress. The films made of these materials have high permeability and good selectivity. Although porous organic cage (POC) and microporous polymer (PIM) can be processed in solution, the structural stability is not enough. Conjugated microporous polymer (CMP) is a π conjugated crosslinking network interconnected by aryl aryl covalent bond. It has strong structure stability, but poor processability and wide pore size distribution (10-30 ℃), which is difficult to be used for gas separation. The researchers believe that the next generation of hydrogen separation membrane needs rigid network structure to ensure high permeability, and a perfect pore structure is needed to ensure good selectivity, and also can be prepared by solution method. Results in order to prepare high structural rigid and solution-processable hydrogen separation films, four scientific research institutions such as the National University of Singapore have prepared the next generation hydrogen separation membrane based on the conjugate microporous thermosetting film (CMT). The material has perfect pore structure: 0.4 nm pore diameter and 840 specific surface area After the large-scale films were processed by solution method, the permeability of H2 reached 28280 barrer, and the selectivity of H2 to CO2, O2, N2, CH4, C3H6 and C3H8 was more than 6.3. In addition, CMT films can work continuously at 150 ° C for 700 hours without permeability and selective attenuation, showing excellent stability, and the films can work normally at 500 ℃. Synthesis and characterization of conjugated microporous thermosetting materials
Synthesis and characterization of conjugated microporous thermosetting materials. (a) Three dimensional, two-dimensional and one-dimensional structure of CMT are made. The substrate is patterned base plate when making 3D patterned CMT, silicon wafer for 2D CMT and copper nanowire for 1D CMT; (b) thermogravimetric analysis of 3-tbtbtbp; (c) DSC curve of 3-tbtbtbp; (d) patterned 3D SEM image of CMT; (E) AFM image of CMT film with thickness of about 5nm on silicon wafer; (f) TEM image of one-dimensional CMT nanotube; (g) analysis of pore size distribution of thin film by AR adsorption method and AR isothermal adsorption curve of CMT measured at 86 K; (H) measurement of positron lifetime and free volume distribution of CMT based on pals at 35, 100 and 150 ℃; the scales of D, e and F are 20 μ m, 3 μ m and 200 nm, respectively. 3,6,12,15-tetrabromotetrabenzophenazine (3-tbtbp) was used as the precursor in an inert atmosphere at 540 ℃ based on debromination and C-C cross coupling reaction. CMT materials with 1D, 2D and 3D structures were prepared on different substrates. 3-tbtbtbp will sublimate, melt and polymerize in turn. TGA analysis shows that there are two weight loss stages in the temperature range of 200 ~ 900 ° C in nitrogen atmosphere: the first stage from 450 ° C is sublimation; the second stage from 520 ° C is the debromination of 3-tbtbtbp. In order to further understand the phase transition during heating, the DSC analysis of 3-tbtbp was carried out. It was found that the precursor melted at 509 ° C, and the exothermic peak after 515 ° C was debromination and polymerization. The patterned surface of 3D structure CMT was characterized by SEM. It was found that the material had uniform and dense structure without any cavity. The thickness of the uniformly polymerized CMT film on the silicon wafer is about 5.0 nm, which can be adjusted by changing the surface area ratio of the precursor to the substrate template. The porosity of CMT was analyzed by AR adsorption / desorption isotherms and positron annihilation lifetime spectroscopy (PALS). It was found that CMT exhibited type 1 isotherm adsorption characteristics, and its P / P 0 absorption rate was lower than 0.01. It was a typical microporous material with BET surface area and pore volume of 840 M2 · g-1 and 0.39 cm3 · g-1, respectively. The results show that the pore size distribution of CMT is mainly in the range of 0.4 ~ 0.5 nm, which is in good agreement with pals results. Synthesis and characterization of conjugated microporous thermosetting films
Figure 2. Synthesis and characterization of CMT films. (a) The image of 3-tbtbp precursor and NaCl crystal mixture; (b) the image of CMT coated with NaCl; (c) the CMT / chloroform solution shows the Tindall effect; (d) the CMT solution with a concentration of 0.05 The solvent was as follows: (1) dichloromethane, (2) ethanol, (3) methanol, (4) hexane, (5) ether, (6) acetone, (7) dimethylformamide, (8) dimethyl sulfoxide, (9) tetrahydrofuran and (10) isopropanol; (E) the diameter prepared by filtration was about 47 The results show that the scale of F and G is 2 μ m and 10 μ m respectively. In order to prepare CMT film, the researchers first mixed 3-tbtbtbp and NaCl evenly, then heated and polymerized, cooled to room temperature, soaked in deionized water for 3 hours to remove NaCl, then filtered and freeze-dried to obtain CMT film. The results show that the prepared CMT films have high dispersion stability in common organic solvents, and there is no evidence of precipitation at room temperature for two weeks. In addition, the Tyndall effect of colloidal dispersion is also found in chloroform solution. The cross-section of CMT surface was found to be layered structure by SEM, and there was no crack on the surface, but the surface was slightly wrinkled. Hydrogen separation performance of conjugated microporous thermosetting films
Fig. 3. Gas separation performance of CMT film. (a) At 30 ℃ and a transmembrane pressure of 1 bar, 1 (B-D) the Robeson plots of H2 / CO2, H2 / N2 and H2 / CH4, with pink lines representing the upper limit of 1991, green lines representing the upper limits of 2008, and black lines representing the upper limits of 2015; (E) in the CMT model, the space that CO2 (left) and H2 (right) can enter, and the red circle in the right figure represents the H2 amount compared with CO2 At 150 ℃ and 1 bar transmembrane pressure, a 1 μ m thick CMT film was used for long-term measurement of equimolar H2 / CO2 mixture. The gas separation performance of CMT thin films was studied. It is found that the permeability of various gases is inversely proportional to the kinetic diameter at 30 ℃ and 1 bar. The 1 μ m thick CMT membrane exhibits super high permeability for he and H2, reaching 24200 and 28280 barrer, respectively, while the permeability of other gases is relatively low: CO2 (4480 barrer), O2 (2680 barrer), N2 (2500 barrer), CH4 (2590 barrer), C3H6 (2330 barrer) barrer）、C3H8（2260 barrer）。 The selectivity of H2 to CO2, O2, N2, CH4, C3H6 and C3H8 were 6.3, 10.6, 11.3, 10.9, 12.1 and 12.5, respectively. In addition, when the thickness of the film increases from 500 nm to 13 μ m, the permeability and selectivity almost remain unchanged. When the temperature increases from 30 ° C to 150 ° C, the permeability of H2 increases by 40% to 40680 barrer, and the H2 / CO2 selectivity reaches 6.05. The permeability and selectivity remain unchanged at 150 ℃ for 700 hours. The film can also operate normally at 500 ℃. The next generation membrane materials for hydrogen separation need to have enough hydrogen selectivity and permeability. The Robeson diagrams of H2 / CO2, H2 / N2 and H2 / CH4 gas pairs were summarized. The properties of CMT, the latest super permeable PIM, microporous solids with network structure (zeolite, MOF, porous organic polymer, etc.), inorganic 2D materials (go, mxenes) and other organic polymer films were compared. It is found that the permeability selectivity data of CMT membrane is much higher than the upper limit of 2008 for all gas pairs, and also higher than the 2015 upper limit of H2 / N2 and H2 / CH4 gas pairs proposed recently, showing the super permeability of hydrogen. It is believed that the high hydrogen permeability of 2D CMT films is due to the micropore structure and the free space between layers in the CMT plane. In order to determine the mechanism of gas separation in CMT, molecular dynamics simulation was carried out. It was found that the entry volume of H2 in CMT was 19.58% of membrane volume, which was higher than 16.28% of CO2. This indicates that CMT can accommodate more H2, and the average H2 transmembrane path is 710 ?, which is far lower than 3715 ? of CO2. Therefore, CMT shows ultra-high H2 permeability. Summary based on the conjugated microporous thermosetting materials (CMT), the next generation hydrogen separation membranes have been prepared by four research institutes, including National University of Singapore. The membranes have good structural rigidity and can be processed in solution. CMT with 1D, 2D and 3D structures were synthesized by debromination and C-C cross coupling reaction using 3-tbtbtbp as precursor at 540 ℃ in inert atmosphere. The pore size of the membrane material was 0.4 nm and the specific surface area was 840 M2 · g-1. At 30 ℃ and 1 bar transmembrane pressure, the permeability of 1 μ m-thick CMT membrane to H2 reaches an amazing 28280 barrer, and the selectivity of H2 to various gases exceeds 6.3, and does not change with the thickness of the membrane. The permeability of the film can be increased from 40 ℃ to 500 ℃ at normal operation temperature.