Hexafluorosilicic acid anion functionalized microporous materials, methods of making, and olefin / alkane separation applications

Abstract

The application discloses a kind of hexafluorosilicic acid anion functionalized microporous material and its preparation method and application in adsorption separation olefin / alkane.The microporous material is by metal ion Zn 2+ , inorganic anion SiF6 2‑ And azole organic ligand L are formed by the coordination of static solvothermal reaction polymer, the microporous material with the separation property can only be synthesized using static solvothermal reaction method.The microporous material has high density of anion and azole site, and the structure properties of the material can be controlled by adjusting the solvent type of the reaction, the feeding ratio of anion and azole organic ligand and the type of azole organic ligand.The microporous material obtained by the application can realize the separation of olefin / alkane based on size and electronic effect.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of synthesis of porous materials and gas separation, and particularly relates to a kind of microporous material of hexafluorosilicate anion functionalization and its preparation method and application in olefin alkane separation. BACKGROUND

[0002] Low-carbon olefins such as ethylene and propylene are the most basic chemical raw materials, which are widely used in the production of various downstream chemicals. The demand for low-carbon olefins is huge and increasing with the development of social economy. The conversion rate is limited when preparing low-carbon olefins through catalytic dehydrogenation process, which inevitably introduces a large amount of alkanes into the product. The key to preparing polymer-grade ethylene and propylene is the efficient separation of olefins and alkanes.

[0003] Currently, low-temperature rectification technology is the main technical means for separating low-carbon olefins / alkanes in industrial production. However, the molecular size of ethylene and ethane, as well as propylene and propane, is close, and the relative volatility difference is small. When separating low-carbon olefins / alkanes by this technology, the number of trays and reflux ratio required for rectification are extremely large, which leads to high energy consumption and operating cost. Therefore, it is urgent to develop a new ethylene / ethane separation technology that is efficient, energy-saving, clean and environmentally friendly.

[0004] Adsorption separation is an efficient and energy-saving separation technology, which has the advantages of low energy consumption, low cost, simple process, high product purity, etc., and has great potential for industrial application. It can obtain high product purity and yield. Adsorbent is the core of adsorption separation technology, and the ideal adsorbent should have high adsorption selectivity. Molecular sieves, covalent organic framework materials (COFs), hydrogen-bonded organic framework materials (HOFs), and organic metal framework materials (MOFs) have received extensive attention. Patent application (CN109748770A) discloses a kind of supermicroporous metal organic framework material for realizing the separation of ethylene / ethane, and the adsorption separation selectivity of ethylene / ethane is 32.4 at 298K and 1bar; patent application (CN111747819A) discloses a method for separating ethylene / ethane by using a modified molecular sieve modified by an imidazole compound, and the ethylene / ethane separation ratio is 2.72; patent application (CN116139648A) discloses a kind of supermicroporous material for separating propylene / propane, and the adsorption separation selectivity is 54.2; there are few patents or literatures on the construction of materials by introducing inorganic anions and nitrogen azole ligands for olefin / alkane separation, and even fewer patents and literatures on the construction of materials by introducing hexafluorosilicate and nitrogen azole ligands for gas separation.

[0005] In summary, the existing ethylene / ethane and propylene / propane separation still faces the challenge of difficult separation. Therefore, it is urgent to develop new materials that can effectively separate olefins / alkanes. SUMMARY

[0006] In order to solve the above technical problems and the challenge of the difficulty in separating ethylene / ethane and propylene / propane in the prior art, the present application provides a kind of hexafluorosilicate anion functionalized microporous material, a preparation method thereof and application thereof in the separation of olefin and alkane. The microporous material has a suitable pore size (such as ) and abundant hexafluorosilicate and azole adsorption sites. As an adsorbent, it can realize the selective adsorption and separation of ethylene / ethane and propylene / propane.

[0007] [1] A hexafluorosilicate anion functionalized microporous material, wherein the microporous material is a polymer formed by static solvothermal reaction coordination of metal ion Zn 2+ , inorganic anion SiF6 2- and azole organic ligand L. The solvent can be water and an organic solvent. Further, the volume ratio of water and organic solvent can be 1:1. The organic solvent can be one or more of methanol, n-butanol, ethanol, N,N-dimethylformamide, preferably methanol and n-butanol.

[0008] The azole organic ligand L is selected from at least one of the compounds having the structure shown in any one of the following formulae (I)-(II):

[0009]

[0010] In formula (I), R1 is selected from the following groups: -H, -NH2, -CH3;

[0011] In formulae (I) and (II), R2 is independently selected from the following groups: -H, -NH2, -CH3, -F, -Br, -Cl, -OH, -SO3H, -COOH, -SH, -CF3.

[0012] Further, the pore size of the hexafluorosilicate anion functionalized microporous material is

[0013] Alternatively, the hexafluorosilicate anion functionalized microporous material is a polymer formed by static solvothermal reaction coordination of metal ion Zn 2+ , inorganic anion SiF6 2- and azole organic ligand L in a molar ratio of (0.5-4):1:(0.5-6) (for example, 1:1:2-6, 1:1:4, etc.).

[0014] In some embodiments, the hexafluorosilicate anion functionalized microporous material has a rod-like micro-morphology, and further, the rod length can be 30-80 microns and the rod width can be 10-20 microns.

[0015] [2] The method for preparing the hexafluorosilicate anion functionalized microporous material of [1], comprising:

[0016] preparing a mixture solution containing metal ion Zn 2+ , inorganic anion SiF6 2- and azole organic ligand L, obtaining the crude target product by using static solvothermal reaction synthesis method, washing and degassing activation of the crude target product to obtain the hexafluorosilicate anion functionalized microporous material.

[0017] [2] The method, the molar ratio of metal ion Zn 2+ , inorganic anion SiF6 2- and azole organic ligand L in the mixture solution is (0.5-4):1:(0.5-6), for example 1:1:2-6, 1:1:4, etc.

[0018] [2] The method, the solvent in the mixture solution can be water and organic solvent. Further, the organic solvent can be one or more of methanol, n-butanol, ethanol, N,N-dimethylformamide, preferably methanol and n-butanol. The volume ratio of water and organic solvent in the solvent in the mixture solution can be 1:1.

[0019] [2] The method, the static solvothermal reaction synthesis method can use static oven synthesis method.

[0020] [2] The method, the reaction temperature of the static solvothermal reaction synthesis method can be 80-200℃, further can be 160-180℃, the reaction time can be 12-96h, further can be 24-72h, for example 48-72h, etc.

[0021] [2] The method, the washing agent used in the washing can be water and methanol;

[0022] [2] The method, the degassing activation conditions can include: vacuumizing at 50-120℃ (for example 100℃, etc.), and the degassing activation time is 8-24h, further can be 10-12h.

[0023] [1] The hexafluorosilicate anion functionalized microporous material can be applied in the field of gas adsorption.

[0024] [3] The application of the hexafluorosilicate anion functionalized microporous material of [1] in selective adsorption and separation of olefin / alkane, using the hexafluorosilicate anion functionalized microporous material as adsorbent, contacting the adsorbent with mixed gas containing olefin / alkane to realize selective separation of olefin and alkane;

[0025] The mixed gas is mixed gas containing ethylene / ethane or mixed gas containing propylene / propane.

[0026] [3] The application, the volume ratio of olefin component to alkane component in the mixed gas containing olefin / alkane can be (1:99) to (99:1); the adsorption temperature can be -50 to 100 DEG C, and the adsorption pressure can be not more than 10 bar.

[0027] [4] A method for adsorbing and separating olefin / alkane, using the microporous material anion-functionalized by hexafluorosilicate in [1] as an adsorbent, and contacting the adsorbent with mixed gas containing olefin / alkane to realize selective adsorption and separation of olefin and alkane.

[0028] The mixed gas is mixed gas containing ethylene / ethane or mixed gas containing propylene / propane.

[0029] [4] The method can be further specifically selected and optimized in technical solutions according to the application in [3].

[0030] [1] The microporous material anion-functionalized by hexafluorosilicate can be amorphous particles or spherical or cylindrical particles after molding.

[0031] The present application can realize the construction of microporous materials and the regulation of functional sites by introducing hexafluorosilicate in inorganic fluorine-containing anions and changing the types and structures of azole ligands, and a large number of anions and azole adsorption sites can be obtained; the effective recognition and distinction of olefin or alkane molecules can be realized through the difference in pore size and force, wherein the hexafluorosilicate and azole sites distinguish olefin and alkane through electronic effect, so as to realize the selective separation of ethylene / ethane and propylene / propane.

[0032] Compared with the prior art, the present application has the following beneficial effects:

[0033] 1) The microporous material anion-functionalized by hexafluorosilicate prepared by the present application has excellent structural design and high adjustability, high-density inorganic fluorine-containing anions and azole functional sites, and the pore size of the material can be accurately regulated according to the types of azole ligands.

[0034] 2) The microporous material anion-functionalized by hexafluorosilicate designed and synthesized by the present application has good recognition and distinction ability for ethylene, ethane, propylene and propane, and can effectively realize the preferential adsorption of olefin and the selective separation of olefin / alkane; the high-density inorganic fluorine-containing anions and azole functional sites are beneficial to the selective recognition of gas.

[0035] 3) The preparation method adopted by the present application is a static solvothermal reaction, specifically a static oven synthesis method, which is simple, and the functionalized azole-based microporous coordination polymer material is easy to prepare, and only this method can be used to synthesize microporous materials with the separation effect. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 Adsorption isotherm plot of ethylene, ethane on ZnAtzSiF6-ST of Example 1 at 298 K;

[0037] Figure 2 Adsorption isotherm plot of propylene, propane on ZnAtzSiF6-ST of Example 1 at 298 K;

[0038] Figure 3 Comparison of IAST selectivity of ethylene / ethane and propylene / propane of ZnAtzSiF6-ST of Example 1 with classical materials, where Cu@UiO-66 is reported in the literature (Chemical Engineering Journal, 2023, 451: 138431.), ZnAtzPO4 is reported in the literature (Science Advances, 2020, 6(15): eaaz4322.), JNU-3a is reported in the literature (Nature volume 595, pages 542-548 (2021)), ZJU-75a is reported in the literature (Angewandte Chemie International Edition, 2023, 62(12): e202218590.);

[0039] Figure 4 、 Figure 5 Scanning electron microscope (SEM) image of ZnAtzSiF6-ST of Example 1;

[0040] Figure 6 Thermogravimetric curve of ZnAtzSiF6-ST of Example 1;

[0041] Figure 7 Adsorption isotherm plot of ethylene, ethane on ZnAtzSiF6-ST-2 of Example 2 at 298 K;

[0042] Figure 8 Adsorption isotherm plot of propylene, propane on ZnAtzSiF6-ST-3 of Example 3 at 298 K;

[0043] Figure 9 Adsorption isotherm plot of propylene, propane on ZnAttzSiF6-ST of Example 4 at 298 K;

[0044] Figure 10 Powder X-ray diffraction pattern of ZnAttzSiF6-ST of Example 4. DETAILED DESCRIPTION

[0045] The application will be further described below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the application and not to limit the scope of the application.

[0046] Example 1

[0047] According to the metal ion Zn 2+ , fluoride-containing anion SiF6 2- , and organic ligand molar ratio 1:1:4, zinc hexafluorosilicate (1 mmol) and 3-amino-1,2,4-triazole (4 mmol) were dissolved in a mixed solution of methanol (10 mL) and water (10 mL), and then transferred to a 100 mL hydrothermal reactor. After stirring at room temperature for 1 hour, the hydrothermal reactor was sealed and placed in an oven at 180°C for 3 days. A white solid precipitate was obtained, filtered, washed with water and methanol, and finally activated at 100°C under vacuum for 10 hours to obtain an activated microporous material functionalized with hexafluorosilicic acid, which can be used as an adsorbent, and is named ZnAtzSiF6-ST.

[0048] The adsorption isotherms of the adsorbent of Example 1 for ethylene and ethane at 298K are shown in Figure 1 ; the adsorption isotherms of the adsorbent of Example 1 for propylene and propane at 298K are shown in Figure 2 ; the calculated IAST separation selectivity of ethylene / ethane and propylene / propane for the adsorbent of Example 1 at 298K is shown in Figure 3 ; the SEM of the adsorbent of Example 1 is shown in Figure 4 , Figure 5 ; the adsorbent has a rod-like micro-morphology with a length of 55.96 microns and a width of 15.09 microns; the thermogravimetric curve of the adsorbent of Example 1 is shown in Figure 6 .

[0049] The microporous material functionalized with hexafluorosilicic acid anions ZnAtzSiF6-ST has an adsorption capacity for ethylene at 1 bar of 1.13 mmol g -1 , an adsorption capacity for ethylene of 0.13 mmol g -1 , and an adsorption capacity ratio of 8.72; it has an adsorption capacity for propylene at 1 bar of 1.67 mmol g -1 , and for propane of 0.45 mmol g -1 , with an adsorption capacity ratio of 3.71, and exhibits preferential adsorption of olefins in ethylene / ethane and propylene / propane separation.

[0050] Example 2

[0051] According to the metal ion Zn 2+ , fluoride-containing anion SiF6 2-The organic ligands were added in a molar ratio of 1:1:4. Zinc hexafluorosilicate hexahydrate (1 mmol) and 3-amino-1,2,4-triazole (4 mmol) were dissolved in a mixed solution of n-butanol (10 mL) and water (10 mL). The solution was then transferred to a 100 mL hydrothermal reactor. After stirring at room temperature for 1 hour, the hydrothermal reactor was sealed and placed in an oven. It was kept at 180 °C for 3 days to obtain a white solid precipitate. The precipitate was filtered, washed with water and methanol, and finally activated under vacuum at 100 °C for 10 hours to obtain an activated hexafluorosilicate-functionalized microporous coordination polymer material, which can be used as an adsorbent and named ZnAtzSiF6-ST-2.

[0052] Example 2: Adsorption isotherms of the adsorbent for ethylene and ethane at 298 K are as follows: Figure 7 As shown.

[0053] The hexafluorosilicic acid anion-functionalized microporous material ZnAtzSiF6-ST-2 has an ethylene adsorption capacity of 0.93 mmol g at 1 bar. -1 The adsorption capacity of ethane is 0.24 mmol g. -1 The adsorption capacity ratio is 3.9.

[0054] Example 3

[0055] According to the metal ion Zn 2+ fluoride-containing SiF6 2- The organic ligands were added in a molar ratio of 1:1:2. Zinc hexafluorosilicate hexahydrate (1 mmol) and 3-amino-1,2,4-triazole (2 mmol) were dissolved in a mixed solution of methanol (10 mL) and water (10 mL). The solution was then transferred to a 100 mL hydrothermal reactor. After stirring at room temperature for 1 hour, the hydrothermal reactor was sealed and placed in an oven. It was kept at 180 °C for 2 days to obtain a white solid precipitate. The precipitate was filtered, washed with water and methanol, and finally activated under vacuum at 100 °C for 10 hours to obtain activated hexafluorosilicate functionalized microporous material, which can be used as an adsorbent and named ZnAtzSiF6-ST-3.

[0056] Example 3: Adsorption isotherms of the adsorbent for propylene and propane at 298 K are as follows: Figure 8 As shown.

[0057] The hexafluorosilicic acid anion-functionalized microporous material ZnAtzSiF6-ST-3 has an adsorption capacity of 0.78 mmol g for propylene at 1 bar. -1 The adsorption capacity of propane is 0.11 mmol g. -1 The adsorption capacity ratio is 7.1.

[0058] Example 4

[0059] According to metal ion Zn 2+ , fluoride-containing anion SiF6 2- and organic ligand molar ratio 1:1:4, zinc hexafluorosilicate (1 mmol) and 5-amino tetrazole (4 mmol) were dissolved in a mixed solution of methanol (10 mL) and water (10 mL), and then transferred to a 100 mL hydrothermal reactor. After stirring at room temperature for 1 hour, the hydrothermal reactor was sealed and placed in an oven at 180 °C for 3 days. A white solid precipitate was obtained, filtered, washed with water and methanol, and finally activated at 100 °C under vacuum for 10 hours to obtain an activated hexafluorosilicate functionalized microporous material, which can be used as an adsorbent, named ZnAttzSiF6-ST.

[0060] The adsorption isotherms of the adsorbent of Example 4 at 298 K for propylene, propane are shown in Figure 9 .

[0061] The PXRD pattern of the adsorbent of Example 4 is shown in Figure 10 .

[0062] The hexafluorosilicate anion functionalized microporous material ZnAttzSiF6-ST has an adsorption capacity of 1.18 mmol g -1 for propylene and 0.73 mmol g -1 for propane at 1 bar, and the adsorption capacity ratio is 1.6.

[0063] Example 5

[0064] According to metal ion Zn 2+ , fluoride-containing anion SiF6 2- and organic ligand molar ratio 1:1:4, zinc hexafluorosilicate (1 mmol) and 3-amino-5-carboxylic acid-1,2,4-triazole (4 mmol) were dissolved in a mixed solution of methanol (10 mL) and water (10 mL), and then transferred to a 100 mL hydrothermal reactor. After stirring at room temperature for 1 hour, the hydrothermal reactor was sealed and placed in an oven at 180 °C for 3 days. A white solid precipitate was obtained, filtered, washed with water and methanol, and finally activated at 100 °C under vacuum for 10 hours to obtain an activated hexafluorosilicate functionalized microporous material, which can be used as an adsorbent, named ZnAtzcSiF6-ST.

[0065] The hexafluorosilicate anion functionalized microporous material ZnAtzcSiF6-ST has an adsorption capacity of 0.86 mmol g -1 for propylene and 0.34 mmol g -1 for propane at 1 bar, and the adsorption capacity ratio is 2.53.

[0066] Example 6

[0067] According to metal ion Zn 2+ , fluoride-containing anion SiF6 2- and organic ligand molar ratio 1:1:4, zinc hexafluorosilicate (1 mmol) and tetrazole (4 mmol) were dissolved in a mixed solution of methanol (10 mL) and water (10 mL), and then transferred to a 100 mL hydrothermal reactor. After stirring at room temperature for 1 hour, the hydrothermal reactor was sealed and placed in an oven at 180°C for 3 days. A white solid precipitate was obtained, which was filtered, washed with water and methanol, and finally activated at 100°C under vacuum for 10 hours to obtain an activated hexafluorosilicate functionalized microporous material, which can be used as an adsorbent and is named ZnTetzSiF6-ST.

[0068] The hexafluorosilicate anion functionalized microporous material ZnTetzSiF6-ST has an ethylene adsorption capacity of 1.56 mmol g -1 at 1 bar and an ethane adsorption capacity of 0.54 mmol g -1 , with an adsorption capacity ratio of 2.89.

[0069] Comparative Example 1

[0070] Comparative Example 1 is a hexafluorosilicate functionalized microporous material SIFSIX-3-Zn reported in the literature (CrystEngComm 22 (2020) 2649-2655.), which differs from Example 1 in that its organic ligand is pyrazine. The hexafluorosilicate functionalized adsorbent of Comparative Example 1 has an ethylene adsorption capacity of 2.24 mmol g -1 at 298K, which is reported in the literature, compared with Example 1, and does not have the ability to achieve olefin / alkane separation.

[0071] Comparative Example 2

[0072] Comparative Example 2 is an anionic nitrogen azole-based microporous material ZnAtzIpa reported in the literature (Science 366, 241-246 (2019).), which differs from Example 1 in that its anion is isophthalic acid. The anionic nitrogen azole-based microporous material adsorbent of Comparative Example 2 has an ethylene and ethane adsorption capacity of 1.8-2 mmol g -1 at 298K and 1 bar, which is reported in the literature, compared with Example 1, and does not have the ability to achieve olefin / alkane separation.

[0073] In addition, it should be understood that, after reading the above description of the present application, those skilled in the art can make various modifications or modifications to the present application, and these equivalent forms also fall within the scope defined by the claims attached hereto.

Claims

1. Use of a microporous material functionalized with hexafluorosilicate anions for the selective adsorptive separation of olefins / alkanes, characterized in that, The microporous material functionalized with hexafluorosilicate anions is used as an adsorbent, and the adsorbent is contacted with a mixed gas containing olefins / alkanes to realize selective separation of olefins and alkanes; The mixed gas is a mixed gas containing ethylene / ethane or a mixed gas containing propylene / propane; The hexafluorosilicate anion functionalized microporous material is a polymer formed by coordination of metal ions Zn 2+ , inorganic anions SiF6 2- and azole organic ligands L through a static solvothermal reaction. The nitrogen azole organic ligand L is selected from at least one of the compounds having structures shown in any one of the following formula (I)-(II): In formula (I), R1 is selected from the following groups: -H, -NH2, -CH3; In formula (I), R2 is independently selected from the following groups: -H, -NH2, -CH3, -F, -Br, -Cl, -OH, -SO3H, -COOH, -SH, -CF3.

2. Use according to claim 1, characterized in that, The microporous material functionalized with hexafluorosilicate anions has a pore size of 3-7 Å.

3. Use according to claim 1 or 2, characterized in that, The hexafluorosilicic acid anion functionalized microporous material is a microporous material functionalized with metal ions Zn 2+ , inorganic anions SiF6 2- and azole organic ligands L in a molar ratio (0.5-4):1:(0.5-6) by coordination through a static solvothermal reaction; The solvent is water and an organic solvent; the volume ratio of water and the organic solvent is 1:1; The organic solvent is one or more of methanol, n-butanol, ethanol, and N,N-dimethylformamide.

4. Use according to claim 1, characterized in that, The method for preparing the microporous material functionalized with hexafluorosilicate anions comprises: A mixture solution containing metal ions Zn 2+ , inorganic anions SiF6 2- and azole organic ligand L, using a static solvent thermal reaction synthesis method to obtain a crude target product, washing and degassing activation of the crude target product to obtain the hexafluorosilicic acid anion functionalized microporous material.

5. Use according to claim 4, characterized in that, The molar ratio of the metal ion Zn 2+ , the inorganic anion SiF6 2- and the nitrogen azole organic ligand L in the mixed solution is (0.5~4):1:(0.5~6); The solvent in the mixed solution is water and an organic solvent; The volume ratio of water and the organic solvent in the solvent is 1:1; The organic solvent is one or more of methanol, n-butanol, ethanol, and N,N-dimethylformamide.

6. Use according to claim 4, characterized in that, The solvent hydrothermal reaction synthesis method adopts a static oven synthesis method; The reaction temperature of the solvent hydrothermal reaction synthesis method is 80-200 ℃, and the reaction time is 12-96 h.

7. Use according to claim 6, characterized in that, The reaction temperature of the solvent hydrothermal reaction synthesis method is 160-180 ℃, and the reaction time is 24-72 h.

8. Use according to claim 4, characterized in that, The washing agent used in the washing is water and methanol; The degassing activation conditions include vacuumizing at 50-120 ℃, and the degassing activation time is 8-24 h.

9. The use according to claim 1, characterized in that, The volume ratio of the olefin component to the alkane component in the mixed gas containing olefins / alkanes is (1:99)-(99:1); the adsorption temperature is -50-100 ℃, and the adsorption pressure is not more than 10 bar.

10. A method for the adsorptive separation of olefins / alkanes, characterized in that The microporous material functionalized with hexafluorosilicate anions is used as an adsorbent, and the adsorbent is contacted with a mixed gas containing olefins / alkanes to realize selective adsorption separation of olefins and alkanes; The mixed gas is a mixed gas containing ethylene / ethane or a mixed gas containing propylene / propane; The hexafluorosilicic acid anion functionalized microporous material is a polymer formed by coordination of metal ions Zn 2+ , inorganic anions SiF6 2- with azole organic ligands L by a static solvothermal reaction. The nitrogen azole organic ligand L is selected from at least one of the compounds having structures shown in any one of the following formula (I)-(II): In formula (I), R1 is selected from the following groups: -H, -NH2, -CH3; In formula (I), R2 is independently selected from the following groups: -H, -NH2, -CH3, -F, -Br, -Cl, -OH, -SO3H, -COOH, -SH, -CF3.

11. The method of adsorptive separation of olefins / alkanes according to claim 10, characterized in that, The microporous material functionalized with hexafluorosilicate anions has a pore size of 3-7 Å.

12. The method of adsorptive separation of olefins / alkanes according to claim 10 or 11, characterized in that, The hexafluorosilicic acid anion functionalized microporous material is a microporous material functionalized with metal ions Zn 2+ , inorganic anions SiF6 2- and nitrogen azole organic ligands L in a molar ratio (0.5-4):1:(0.5-6) by a static solvothermal reaction coordination polymerization; The solvent is water and an organic solvent; the volume ratio of water and the organic solvent is 1:1; The organic solvent is one or more of methanol, n-butanol, ethanol, and N,N-dimethylformamide.

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