Synthesis, Characterization, and CO2 Storage and Separation Properties of MOF-74 Metal Organic Frameworks

• Main Authors: Abhijit Krishna Adhikari, 歐里
• Other Authors: Kuen-Song Lin
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/vrqm6a

博士 === 元智大學 === 化學工程與材料科學學系 === 105 === Reducing anthropogenic CO2 emission and lowering the concentration of greenhouse gases in the atmosphere has quickly become one of the most urgent environmental issues of our age. Carbon capture and storage is one option for reducing these harmful CO2 emissions. Due to capture the CO2 gas, in this study we have synthesized and characterized several metal organic frameworks such as MOF-74(Zn), MOF-74(Cu) MOF-74(Co), and MOF-74(Ni). MOF-74(ZN) has the rod like or block particles having the average diameter about 5~10 m. MOF-74(Co) has the needle like particles and they are connected in one end having the average diameter about 4~12 m. In case of MOF-74(Ni) the particles were almost spherical with uniform distribution and the average particle size was about 1~4 m identified by FE-SEM images. From the thermo-gravimetric analysis it has been found that the stability of MOF-74(Zn), MOF-74(Cu), MOF-74(Co), and MOF-74(Ni) are 405, 242, 258, and 344°C, respectively. XPS measurement revealed that zinc ions had Zn2+ oxidation state in MOF-74(Zn). In MOF-74(Cu), the oxidation state of copper had Cu2+ and MOF-74(Co) cobalt ions had the mixed oxidation state of Co2+ and Co2.66+. In case of MOF-74(Ni), the oxidation state of nickel was Ni3+ confirmed from ESCA. N2 adsorption isotherms showed that these compounds are micro-porous and post treatment is crucial to obtain higher surface area of these porous materials. The bond distances of Zn─O, Cu─O, Co─O, and Ni─O in the respective structure are 1.98, 1.94, 1.96, and 1.97 Å with the coordination number of 4.2, 4.2, 5.4, and 5.3 respectively. The calcined samples of the MOF-74 samples around 200~250°C showed the best pore textural properties. MOF-74(Zn, Cu, Co, Ni) have the BET specific surface areas of 1,481, 1,381, 1,404, and 1,418 m2g-1 with the pore volumes of 0.89, 0.84, 0.82, and 0.86 cm3g-1, respectively. After modification of these MOF samples with Pd loaded AC, the BET specific surface areas decreases to 1,167, 1,145, 1,088, and 1,115 m2g-1 with the pore volumes of 0.79, 0.76, 0.75, and 0.78 cm3g-1, respectively. However, though the BET specific surface areas decreased with the modification process, in terms of CO2 adsorption and separation, the obtained values were enhanced. The adsorption capacities for modified and pristine MOFs follow the order of Zn>Ni>Co>Cu and their CO2 adsorption capacities were 4.10, 4.06, 3.67, and 3.38 mmol•g-1, respectively for pristine MOFs. For the modified samples these values increase to 4.62, 4.38, 3.96, and 3.78 mmol•g-1, respectively measured at 298 K and 1.1 bar pressure. The CO2 adsorption capacities were found as 11.53, 9.43, 10.28, and 11.06 mmolg-1 for MOF-74(Zn), MOF-74(Cu), MOF-74(Co), and MOF-74(Ni), respectively measured at 298 K and 32 bar. However, these values were increased to 12.54, 10.58, 11.42, and 12.24 mmolg-1, respectively, measured at the same condition due to the modification of the MOF samples. The enhanced CO2 uptakes of MOF-74(Ni)-Pd and MOF-74(Co)-Pd are attributed to the strong affinity of the doped Pd towards CO2 molecules. The CO2/N2 (0.2/0.8) selectivity for MOF-74(Zn, Cu, Co, and Ni) were calculated as 13.2, 8.7, 9.7, and 11.3, respectively, while for the modified MOF-74(Zn, Cu, Co, and Ni) samples these values are increased to 14.8, 9.2, 12.4, and 14.6, respectively. From the above results it can be concluded that MOF-74(Zn) and Pd/AC-MOF-74(Zn) can capture higher amount of CO2 gas than other MOFs synthesized in current work. The selectivity also much higher for these MOFs and the higher selectivity may be due to the unsaturated metal sites of MOF-74 samples and the strong affinity of Pd metals towards CO2.