Construction And Catalytic Properties Of Squaramide-Based MOFs

Metal-organic frameworks（MOFs）,as the emerging porous organic inorganic hybrid materials in recent years,have rich chemical composition,diverse pore structures and easy pore surface modifiability,and are widely used in gas storage and separation,chemical sensing,drug delivery,proton conduction and heterogeneous catalysis.In the aspect of catalysis,MOFs have attracted much attention in heterogeneous catalysis due to their unique pore structure,highly dispersed and evenly distributed catalytic active sites,and good chemical stability,and have gradually been widely used to apply to various chemical conversion process.As a good hydrogen bond donor,the structural unit of squaramide plays an important role in hydrogen bonding catalytic reaction.Currently,there are relatively few reports on MOFs based on squaramide groups,and the catalytic properties of squaramid-based MOFs have not been studied deeply.In particular,the catalytic performance of two-dimensional squaramide-based MOFs materials has yet to be studied.The effect of larger conjugated groups connected with squaramide groups on the catalytic activity of hydrogen bonding based MOFs has not been studied been reported.This thesis synthesized a series of squaramide-based MOFs.They are have been synthesized by solvothermal reactions through rational selection of different metal ions and squaramide-based organic functional ligands,guided by structure and catalytic performance.Detailed characterization of these squaramide-based MOFs was carried out to further investigate their hydrogen bonding catalytic properties,and focused on the above two yet to be researched elements in a more in-depth exploration.Three main parts of this work are as follows:1.Two three-dimensional sq-Cd-MOF-1 and sq-Ca-MOF-4 with narrow pores,were obtained using squaramide-functionalized tetracarboxylic acid ligands 5,5’-（（3,4-dioxocyclobut-1-ene-1,2-diyl）bis（azanediyl））-diisophthalic acid（H₄L1）assembled with Cd²⁺and Ca²⁺,respectively.By reducing the numbers of carboxylic groupsin squaramide-based ligands,3,3’-（（3,4-dioxocyclobutyl-1-ene-1,2-diyl）bis（azanediyl））-dibenzoic acid（H₂L2）and Cd²⁺were successfully assembled under the synergistic coordination of auxiliary ligands 1,2-bis（4-pyridyl）ethylene and 4,4-bipyridine,respectively,to obtain two kinds of novel structure two-dimensional layered squaramide-based materials sq-Cd-MOF-2 and sq-Cd-MOF-3.Interestingly,sq-Cd-MOF-2 can undergo a reversible structural transformation by solvent exchange,demonstrating that it could be designed and synthesized for flexible two-dimensional porous framework materials.Furthermore,a squaramide-based dicarboxylic acid ligand with a large conjugated structure,6,6’-（（3,4-dioxocyclobut-1-ene-1,2-diyl）bis（azanediyl））bis（2-naphthoic acid）（H₂L3）was assembled with Zn²⁺and the auxiliary ligand 1,2-bis（4-pyridyl）ethylene to construct a three-dimensional framework structure sq-Zn-MOF-5 with large pore channels,and the porosity of sq-Zn-MOF-5 could reach 69.7%.2.The catalytic performance of two-dimensional squaramide-based MOFs materials was investigated for the first time.We examined the catalytic performance of two-dimensional sq-Cd-MOF-2 and sq-Cd-MOF-3 as catalysts for the Michael addition reaction between the derivatives ofβ-nitrostyrene and acetylacetone and its derivatives,and sq-Cd-MOF-1 containing the same metal ions was used as a reference.The experimental results showed that although sq-Cd-MOF-1 has a three-dimensional framework structure,its smaller pore size is not conducive to the interaction between substrate molecules and active catalytic sites,so the catalytic efficiency of sq-Cd-MOF-1 is low.In contrast,the two-dimensional framework structures sq-Cd-MOF-2 and sq-Cd-MOF-3 still have a large pore size after spatial stacking,which allows the reactants to interact effectively with the catalytic sites within the pores,so that sq-Cd-MOF-2 and sq-Cd-MOF-3 showed a higher catalytic efficiency.In addition,sq-Cd-MOF-2 exhibited a size-selective effect on the substrate molecules during the catalytic process,and still had high catalytic activity after five cycles.The above results indicate that two-dimensional squaramide-based MOFs can also exhibit good catalytic performance,which is closely related to pore size after stacking.3.To systematically explore the relationship between the structure and catalytic performance of MOFs materials,and the effect of larger conjugated groups attached to the squaramide moiety,the mentioned squareamide-based MOFs were used as catalysts,and their catalytic properties for Friedel-Crafts reactions between the derivatives ofβ-nitrostyrene with pyrrole and indole were studied.The results show that among the five squaramide-based MOFs materials,the three-dimensional framework sq-Zn-MOF-5 has the best catalytic performance despite the large conjugated group attached to the squaramide moiety.It exhibits high catalytic activity for different sizes of substrate molecules.This may be attributed to the fact that sq-Zn-MOF-5 possesses the largest pore structure,allowing the smooth entry of different substrate molecules,thus realizing the effective catalytic conversion process within the pore of MOFs.At the same time,sq-Zn-MOF-5 can maintain good structural stability during the catalytic process and can achieve multiple catalytic cycles.In contrast,the other squaramide-based MOFs synthesized have smaller pore sizes,and they exhibit lower catalytic activity or the activity decreases with the increase of substrate molecular size.The above experimental results suggest that the squaramide-based MOFs materials have an obvious size selection effect in the catalytic process.The larger the pore size,the quicker the reactants interact with active catalytic site,resulting in higher catalytic efficiency.