Study On Co₂ Adsorption With Interpenetrated MOFs

A series of serious problems including greenhouse effect, rising sea levels, tropical storm caused by global warming have been more and more paid great attention. Capture and utilization CO2 are one of the effective means to control CO2 emissions. Meanwhile, with the social and economic development, energy conservation and environmental protection become a prerequisite for the development of the material. In this paper, in the light of the basic principles of molecular engineering and crystal engineering, we designed and synthesized six complexes with different interpenetrated nets structure and performance based on acylamide ligands and photosensitive ligands, different polycarboxylic acid and zinc salt. Furthermore, the structure, thermo-stability, gas absorption and photosensitive properties of some complexes were characterized. The detailed contents are as follows:(1) We successfully synthesized two interpenetrated MOFs by using Azobenzene-4,4’-dicarboxylic acid(H2Az DC) as oxgen ligand, acylamide ligands L1 and L2 as nitrogen ligand with hydrothermal method. Selecting the acylamide ligands L1 and L2 which have different nitrogen atom position at the terminal pyridine ring. Through the regulation of amide ligands with different coordination sites obtained complex 1 and 2. Complex 1 crystalized in P2/c space group,it shows two-fold interpenetrated α-Po topological net with 66.6% solvent-accessible volume, and exhibited excellent selective adsorption property for CO2, but CO2 adsorption amount is very small(3.9cm3/g, 293 K, 100 k Pa). Complex 2 crystalized in C2/c space group. It shows four-fold interpenetrated α-Po topological net. Research shows that complex 2 exhibited excellent selective adsorption property for CO2, giving CO2 adsorption amount at 293 K and 100 k Pa, 35.3cm3/g. The results showed that the higher interpenetrated of complexes, the solvent-accessible volume relatively low, but the formation of complexes are relatively stable, showing a higher adsorption capacity CO2.(L1=N1,N4-di(pyridin-3-yl)terephthalamide;L2=N1,N4-di(pyridin-4-yl)terephthal amie;H2Az DC= Azobenzene-4,4’-dicarboxylic acid)(2) Two interpenetrated MOFs were obtained by using one acylamide ligand L3 and H2 Az DC ligand with the solvent control method. The independent unit of complex 3 is dual-core structure. It shows four-fold interpenetrated α-Po topological net with 49.1% solvent-accessible volume. Complex 4 is a mononuclear cell structure. It shows two-fold interpenetrated a new topological net with 57.2% solvent-accessible volume. And complexes 3 and 4 both exhibited excellent selective adsorption property for CO2, giving CO2 adsorption amount 25.6cm3/g, 43.2cm3/g at 293 K and 100 k Pa, respectively. This is consistent with solvent-accessible volume.(L3= N4, N4’-di(pyridin-4-yl)-[1,1’-biphenyl]-4,4’-dicarboxamide)(3) The first porous diarylethene MOF(DMOF)(complex 5) was obtained by using diarylethene ligand L4 as nitrogen ligand and biphenyl-4,4’-dicarboxylic acid(H2bpdc) as oxygen ligand with solvo/hydrothermal method. The diarylethene ligand in complex 5 undergoes an exquisite change triggered by UV and visible light between open-ring and closed-ring response. Dynamic photoswitching CO2 adsorption/release was observed: the adsorption amount of complex 5 is 20.1cm3/g(298 K) which is equal to four times that without UV irradiation 5.0cm3/g. It has a record high desorption capacity of 76% with dynamic irradiation The desorption capacity is much greater than the earlier reported 42% desorption capacity under static irradiation which is unprecedented, indicating that this material has important applications in the field of low-energy CO2 capture and release.(L4=4,4’-(4,4’-(perfluorocyclopent-1-ene-1,2-diyl)bis(5-methylthiophene-4,2-diyl))di pyridine)(4) Based on complex 5, we introduce another photosensitive ligand, azobenzene-4,4’-dicarboxylic acid and combine with diarylethene ligand L4, expecting to synthesize a better low-energy consumption CO2 adsorption/release MOF. Complex 6 was obtained according to this idea. It shows four-fold interpenetrated α-Po topological net. The research shows that the complex 6 does not have photosensitivity. This phenomenon shows that diarylethene ligand open-ring and closed-ring response requires a certain space, not all photochromic phenomenon can occur under any circumstances. We are working through the regulation of temperature, solvent and other conditions for the next step of research on it.