As a result of the low partial pressure of CO 2 in most industrial waste gases, capturing CO 2 especially at low pressure was a priority. A facile method was utilized to build mesoporous UiO-66s with adjustable pore sizes via crystal defect engineering. The mesopores were generated through the removal of instable Cu clusters and lauric acid after reaction. The pore sizes could be controlled via adjusting the molar ratios of Cu 2+ to Zr 4+ . Moreover, these meso-type UiO-66 MOFs reserved the stability after washing by HCl solution, which was more suitable for modification in harsh environment and CO 2 capture from flue gases or biogases. The most suitable dispersion of appended polyethyleneimine (PEI) inside pores was then achieved via the controllable pore sizes for CO 2 capture. The active –NH 2 from incorporated PEI could interact with CO 2 to enhance the uptake at low pressure and improve adsorption selectivities of CO 2 over N 2 and CH 4 . The high dispersion of PEI also allowed guest molecules to interact with more active sites, which enhanced the affinity between CO 2 and frameworks. The strong interaction between 50PEI@meso-UiO-66-0.2Cu and CO 2 was confirmed by higher isosteric heats of adsorption (Q st ). Even so, the CO 2 uptake of sample was without drop after five cycles, and the sample could be regenerated under a mild condition.
Zongjian Liu,Zhicong Sun,Zhao Zhang,Lifei Xu,Lin Zhang,Qun Cui,Haiyan Wang
UiO-66-Z with uniform mesopores using 2-aminoterephthalic acid and terephthalic acid as mixed ligands was synthesized by hydrothermal method in this paper. Thermodynamic and dynamic properties of UiO-66-Z were discussed. Dynamic separation performance and adsorption selectivity of UiO-66-Z for the equimolar n-heptane/methylcyclohexane (nHEP/MCH) binary mixture were investigated. Results show that UiO-66-Z has complete tetrahedral cages with uniform mesopores of 38 Å. Adsorption rate constant of MCH on UiO-66-Z (14.5 × 10 −3 S −1 ) is twice that of UiO-66 (6.7 × 10 −3 S −1 ), while nHEP does not significantly change at 323K with the pressure of 0.05–0.1 kPa. Effective adsorption diffusion coefficient of nHEP and MCH on the UiO-66-Z is about 25 times higher than that on the UiO-66. Compared with UiO-66, the desorption activation energy (Ed 1 ) of the first desorption peak of nHEP and MCH on the UiO-66-Z is reduced by 53.75% and 76.66%, respectively. The separation selectivity of MCH to nHEP on the UiO-66-Z reaches up to 3.13 at 473 K. Results indicate that the introduction of mesopores in the UiO-66-Z can enhance mass transfer of nHEP and MCH in the pores, especially which is more obvious mass transfer effect for MCH with a larger molecular dynamics diameter under low partial pressure. The retention of most tetrahedral cages in the UiO-66-Z is conducive to improving its inverse shape separation selectivity for MCH/nHEP. Compared with UiO-66, nHEP and MCH have higher desorption efficiency on UiO-66-Z at the same temperature and time.
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