Lithium doping on 2D squaraine-bridged covalent organic polymers for enhancing adsorption properties: a theoretical study.

Lithium modification, especially quantitative and oriented doping, is an effective way to enhance the adsorption properties of covalent organic frameworks (e.g., CO and H). Two-dimensional squaraine-bridged covalent organic polymers (SQ-COPs), with quantitative and oriented Li-doped open oxygen sites of the squaric-acid unit, are investigated for their CO capture properties by using first-principles calculation combined with grand canonical Monte Carlo (GCMC) simulation. It is found that owing to the strong affinity of Li atoms to the squaraine units of SQ-COPs, the gas adsorption capacity in SQ-COP-Li increased to 3 times more than those of the pristine SQ-COPs. In particular, since the Li-O bonds between squaraine units and Li atoms can enhance the electrostatic interaction between the framework and CO, the adsorption amount of CO in SQ-COP-Li reaches an extremely high capacity of 83.4 and 202.0 mmol g at 298 K and 100 bar for 1-Li (SQ-COP-1-Li) and 3-Li (SQ-COP-3-Li), respectively. Such a high CO uptake means that 3-Li outperforms the best 3D COF (COF-05) materials so far reported, and is even twice that of MOF-177 and IRMOF-10. Even at 30 bar and 298 K, the CO uptake values of 49.2 mmol g of 1-Li and 61.5 mmol g of 3-Li are far higher than 29.7 mmol g of Li-doped COF-102 and 42.0 mmol g of Li-doped COF-105. Moreover, the selectivity of CO/H and CO/CH in 1-Li reaches 19 and 5.4 at 273 K, and 10.3 and 3.6 at 298 K, respectively. Therefore, such Li-doped SQ-COPs might be an environmentally friendly candidate for high CO capture. Furthermore, the possible synthesis pathways of SQ-COP-Li are investigated, and it is indicated that the use of lithium di-isopropylamide and lithium naphthalene as dopants is energetically favorable. The findings in this work are expected to provide new ideas for metal doping and extended applications of functionalized materials.