Phosphomolybdic Acid Functionalized Covalent Organic Frameworks: Preparation, Structure Characterization And Catalytic Properties In Olefin Epoxidation

Covalent organic frameworks(COFs) are a series of crystal porous organic materials, which have predictable structure, ordered crystalline array, large surface area, tunable pore size, low densities, and high thermal and chemical stability. In view of the above advantages, COFs have shown superior potential in catalysis. Recently, polyoxometalates(POMs) have received a widespread attention for their super performance in a variety of important catalytic processes. Recently, research interest has been focused on immobilizing POMs onto suitable supports for the purpose to obtain highly active and stable heterogeneous POM-based catalysts. Although a lot of research work has been done, a common problem for most of the supported POM catalysts is the easily leaching active species during the reaction course. At present, it is still a very attractive subject to find suitable supports and preparation method for developing highly efficient heterogeneous POM-based catalyst.In this work, a series of phosphomolybdic acid(PMA) functionalized covalent organic frameworks(PMA/COFs) were prepared, characterized, and used as catalysts for olefin epoxidation. The resultant hybrid composites were obtained by immobilizing 12-phosphomolybdic acid onto three types of imine-based COFs(COF-LZU1, CIN-1 and COF-300) through impregnation method(combined with hydrogen reduction pretreatment method) or one-pot synthesis method. The composition, morphology, structure and other physicochemical properties of these composites were studied by a variety of characterization means. The catalytic properties of PMA/COFs were investigated in the epoxidation of olefins. The effects of oxidant and solvent types, reaction temperatures and other factors on the catalytic performance were studied in detail. Moreover, the properties of the active centers of the catalysts were discussed on the basis of the characterization results involving the compositions and structures of the COFs support as well as the preparation methods of the catalysts. This work also elaborates on problems such as the interaction origin between active species and supports of catalysts, catalysis mechanism, etc. The main research contents and the results of this thesis are as follows:1. Preparation method, structure characterization and catalytic properties in olefin epoxidation of PMA/LZU1A 2-D structure imine-based COF material(COF-LZU1) and its hydrogen reduction pretreatment sample(named COF-LZU1-H) were used as supports to immobilize 12-phosphomolybdic acid(PMA) for getting hybrid PMA-based solid catalysts, denoted as 6%PMA/LZU1, 23%PMA/LZU1 and 23%PMA/LZU1-H.The catalytic activities of the PMA supported composites were investigated in a variety of epoxidation of olefins.(1) In the epoxidation of cyclooctene with H2O2 as oxidant, the catalytic activity of 23%PMA/LZU1 is comparable to 23%PMA/LZU1-H, and the latter one shows a better stability.(2) In the epoxidation of 1-octene with TBHP as oxidant, catalyst 23%PMA/LZU1-H shows a higher catalytic activity. The characterization results confirm the higher dispersion degree of PMA units in COF-LZU1-H support, which could provide more contact opportunities for the reactants and contribute to improving the catalytic activity. Furthermore, the imine group of COF-LZU1 can be reduced by hydrogen reduction pretreatment, which would be beneficial for building stronger interaction between support and PMA units, and for improving the stability of the catalyst.(3) In the epoxidation of cyclohexene with TBHP as oxidant, catalyst 23%PMA/LZU1 shows higher catalytic activity and selectivity of epoxide. As for the epoxidation of styrene, catalyst 23%PMA/LZU1-H exhibits better catalytic performance. These results might be related to the enhanced surface basicity of the support caused by the hydrogen reduction pretreatment, which may decrease the acidity of the active PMA species. Therefore, obvious difference in catalytic epoxidation performance can be found over these two kinds of catalysts for different types of olefins.The above results demonstrate that the support of COF-LZU1 has played an important role in immobilization, dispersion, and regulation of the acid-base properties of the catalysts. To some extent, the catalytic activity, selectivity and stability of the COF-LZU1 supported PMA catalysts are sensitive to the interaction nature between active species and supports.2. Preparation method, structure characterization and catalytic properties in olefin epoxidation of PMA/CIN-1A 2-D structure nitrogen-rich porous covalent imine network material(CIN-1) and its hydrogen reduction pretreatment sample(named CIN-1-H) were used as supports to immobilize 12-phosphomolybdic acid(PMA) for getting hybrid PMA-based solid catalysts, which are denoted as 6%PMA/CIN-1, 23%PMA/CIN-1 and 23%PMA/CIN-1-H, respectively.The catalytic properties of these hybrid composites were investigated in a variety of epoxidation of olefins.(1) In the epoxidation of 1-octene with H2O2 as oxidant, 23%PMA/CIN-1-H shows relatively high catalytic activity and stability, superior to the catalysts with COF-LZU1-H as support. This may be due to the abundant nitrogen atoms in CIN-1 support, which might be benefit to dispersing and stabilizing active PMA units.(2) In the epoxidation of cyclohexene with TBHP as oxidant, catalyst 23%PMA/CIN-1 shows a higher catalytic activity than 23%PMA/CIN-1-H, a lower selectivity of epoxide than the catalyst with COF-LZU1 as support. This may be attributed to the abundant nitrogen atoms in CIN-1 support, which can enhance the basicity of the whole catalyst, led to the decrease of the selectivity of epoxide. In the epoxidation of cyclohexene with H2O2 as oxidant, homolytic oxidation process is proposed since the main products are 2-cyclohexene-1-one and 2-cyclohexen-1-ol, rather than epoxide.(3) The catalytic activity of PMA/CIN-1 is relatively low in the epoxidation of styrene. When TBHP is used as an oxidant, the main product is 1,2-epoxyethylbenzene; when H2O2 as an oxidant, the main product is benzaldehyde. These results suggest that the product selectivity has close relationship with the type of oxidants and the acid-base properties of the catalysts/supports for the selective oxidation of styrene.Compared with COF-LZU1, nitrogen-rich COF material of CIN-1 is more favorable for highly dispersing PMA units, and can improve the interaction between support and PMA and affect the acid-base properties of the support at the same time. These factors may have significant influence on the activity, selectivity and stability of the supported PMA catalysts.3. Preparation method, structure characterization and catalytic properties in olefin epoxidation of PMA/COF-300An interpenetrated 3-D structure COF material(COF-300) and its hydrogen reduction pretreatment sample(named COF-300-H) were used as supports to immobilize 12-phosphomolybdic acid(PMA) for getting a series of hybrid PMA-based solid catalysts by impregnation method(combined with hydrogen reduction pretreatment method) or one-pot method.The catalytic properties of the PMA/COF-300 composites are studied in the epoxidation of olefins with TBHP as oxidant.(1) Compared with the catalyst prepared by one-pot synthesis, the catalyst prepared by impregnation method showed higher catalytic activity in the epoxidation of cyclooctene. Both of the catalysts have good stability. Compared with the catalysts prepared by COF-LZU1 or CIN-1 as support, the COF-300 supported catalysts show higher catalytic activity in the epoxidation of cyclooctene. This might be due to the fact that the interpenetrated 3-D structure of COF-300 is more suitable for transporting reactants.(2) The catalysts prepared by impregnation method show higher catalytic activity in the epoxidation of 1-octene. However, the catalysts prepared by one-pot method have an excellent catalytic stability. The hydrogen reduction pretreatment can further improve the stability of the catalysts. The above results suggest that the PMA units distributed on the external surface of support are quite active since they are more accessible for reactants, however, such PMA units is also unstable, which are easily leaching from the surface of COF-300 support. The hydrogen reduction pretreatment of support or one-pot synthesis can lead to high dispersion of PMA units on the surface/channel of COF-300, and establish more stable interaction with support, which can certainly improve the stability of the catalyst.(3) In the epoxidation of cyclohexene, the increased PMA loading amount can promote the heterolytic oxidation process, thus improving the selectivity of epoxide. Catalyst 23%PMA/COF-300-80, prepared by impregnation method at 80 oC, shows higher catalytic activity and stability.(4) In the epoxidation of styrene, the selectivity of 1,2-epoxyethylbenzene increases with the decrease of acidic property of the catalyst. Catalyst 23%PMA/COF-300-H is much suitable for the styrene catalytic epoxidation system. After the support pretreated by hydrogen reduction, the acid properties of the whole catalyst decreases, and the selectivity of epoxide is improved. These results demonstrate that, the support of COF-300 has not only played important role in immobilizing and dispersing the active species PMA, but also regulating the acid-base property and the catalytic properties of the catalyst.The catalytic properties of the PMA/COF-300 catalysts are investigated in a variety of epoxidation of olefins with H2O2.(1) In the epoxidation of cyclooctene, the catalysts of PMA/COF-300 are not very stable during the reaction course. Catalyst 23%PMA@COF-300 which was prepared by one-pot synthesis shows higher stability. This can be mainly attributed to the fact that more PMA units can be introduced into the channels or stacking-pores(between the particles) of the COF-300 support during in-situ synthesis process.(2) In the selective oxidantion of cyclohexene with 23%PMA/COF-300-80 as catalyst, the heterolytic oxidation process happens when t-Bu OH is used as solvent, the main products are 1,2-epoxycyclohexane and 1,2-cyclohexanediol. The homolytic oxidation process happens when CH3 CN is used as solvent. The main products are 2-cyclohexene-1-one and 2-cyclohexen-1-ol. It shows that the type of solvent has a great influence on the oxidation mechanism of cyclohexene, especially on the selectivity of the catalyst.(3) In the epoxidation of styrene, the main product is benzaldehyde, rather than epoxide.Furthermore, the catalytic performance of PMA/COF-300 catalyst in the reaction with O2 as oxidant is explored. The results show that, catalyst 23%PMA/COF-300-80 is active for the epoxidation of cyclooctene, cyclohexene and styrene by oxygen. These results suggest that this kind of catalyst has the ability of activating molecular oxygen, which has a broad space for further research and development.