Polarization Construction Of Organic Framework Materials To Enhance Selective Adsorption Of CH₄/N₂

Coal-bed methane（CBM）is a kind of unconventional energy with high calorific value.It is of great economic value to separate and purify coal-bed methane,especially the development and utilization of low concentration CBM.Besides CH₄,there is a large amount of N₂ in low concentration CBM,which is difficult to separate because of their similar physical and chemical properties.Therefore,the separation of CH₄/N₂ has become a technical difficulty in the purification of CH₄ in low grade CBM.Among the separation methods for CH₄/N₂,adsorbent-based pressure swing adsorption separation technology（PSA）is considered to be the most promising technology for CBM purification because it does not require external heat source or refrigeration source and is easy to operate.The core of PSA technology is the research and development of high efficiency adsorbents.Based on the structural advantages of MOFs and COFs,which are easy to modify and functionalized,this thesis designed and developed a series of efficient adsorbents by introducing open metal centers,polarizing active sites,hetero-atomic adsorption sites and thiazole functional sites into the skeleton,focusing on the performance of the separation for CH₄/N₂ and the stability of adsorbents.The purification of low grade CBM were realized,and the main research contents are as follows:（1）The development of Hofmann-type MOFs adsorbents for CH₄/N₂ separation.Two MOFs with microporous Ni-Ni-Pz and Co-Ni-Pz were prepared by using cheap and easily available raw materials at room temperature.Ni-Ni-Pz and Co-Ni-Pz not only have dense open metal centers and nitrogen-rich adsorption sites,but also have ultrafine pore structure and narrow pore size distribution,which is one of the ideal adsorbents for the separation of CH₄/N₂.Co-Ni-Pz showed strong interactions with CH₄molecules,the adsorption capacity of CH₄ reached 29.6 cm3/g at 298 K and 1 bar,and the separation selectivity for CH₄/N₂ mixture was 8.6.In addition,Co-Ni-Pz also showed good thermal stability and excellent cyclic adsorption-desorption performance.（2）Establishment of dual polarization strategy to enhance CH₄/N₂ separation performance.Using the difference in polarizability between CH₄ and N₂and based on the robust covalent organic frameworks（COFs）adsorption platform,we proposed a dual polarization strategy,by introducing triazine and polyfluoride sites into the COFs skeleton at the same time to prepare two adsorbents,F-CTF-1 and F-CTF-2.As the optimized adsorbent,the CH₄ adsorption capacity and CH₄/N₂ selectivity of F-CTF-2are 1.76 times and 1.42 times higher than that of unfluorinated CTF-2,respectively.Theoretical calculations also show that F-CTF-2 interacts with CH₄ molecules through multiple C-H…F and C-H…N hydrogen bonds,thus enhancing the separation performance of CH₄/N₂,and verifying the effectiveness of the dual polarization strategy.In addition,F-CTF-2 also shows extraordinary chemical/thermal stabilities,the uptake of CH₄still maintained after the treatment of harsh conditions（6 M HCl and 6 M Na OH）.The comprehensive performance of F-CTF-2 is better than most reported porous materials and it is one of the best adsorbents for CBM purification.（3）Construction of hybrid COF-based adsorbents CTF-SQs for CH₄/N₂ separation.Since heteroatoms such as N and O tend to form strong interactions with CH₄,which is beneficial to the effective separation of CH₄/N₂,two hybrid covalent triazine frameworks CTF-SQ-500 and CTF-SQ-600 were constructed by introducing N and O heteroatoms into stable triazine skeleton.Among them,CTF-SQ-600,characterized with ultraporous structure of 5.9?,not only had high specific surface area（691 m2/g）,which could expose more heteroatomic adsorption sites,but also showed a high adsorption capacity of CH₄ and goog CH₄/N₂ selectivity（5.5）.In addition,CTF-SQ-600 exhibited extremely high thermal stability（500℃）,which is superior to most reported porous materials.The breakthrough experiment also showed that CTF-SQ-600with application potential can effectively purify CBM.（4）Construction of thiazole functionalized COF adsorbent CTF-BT-500.Based on conjugated thiazole groups and N and S heteroatomic adsorption sites,a stable thiazole functionalized CTF-BT-500 adsorbent was constructed by introducing thiazole groups into the robust covalent triazine framework.Due to the existence of large number of N and S h adsorption sites on the triazine ring and thiazole ring,CTF-BT-500 showed strong interactions with CH₄.The uptake of CH₄reached 18.6 cm³/g at 298 K and 1 bar,and the selectivity for CH₄/N₂ mixture was 5.1.In addition,CTF-BT-500 also showed good C₂H₆/C₂H₄ separation performance.The adsorption capacity of C₂H₆ reached 99.7cm³/g（4.5mmol/g）at 298 K and 1 bar,which is the highest value among all of the COFs materials reported so far.These results thus rank CTF-BT-500 to a versatile and comprehensive adsorbent.This thesis not only expands the application fields of functional organic framework materials,but also provides a new functional route for the purification of coal-bed methane.