Preparation And Catalytic Performance Of Metal Complexes Functionalized Layered Double Hydroxides

Layered double hydroxides are a special class of anionic clays in which the crystals are built by stacking two-domensional layers that bound to each other through weak forces. The unique positive layers and excellent anion exchange characteristics of LDHs make them promising hosts for the immobilization of anionic metal complexes. Many kinds of metal complexes, such as iron(Ⅲ) porphyrin, Ti(Ⅳ)-Schiff base, chiral sulphonato-salen-manganese(Ⅲ), and Ti(Ⅳ) tartrate, have been successfully intercalated into LDHs. Moreover, it has been demonstrated that the intercalation of metal complexes in LDHs is capable of improving their catalytic activity, selectivity and stability, compared with the homogeneous counterparts. However, the intercalation of the metal complexes into LDH interlayers is generally performed by replacing interlayer gallery anions via the ion-exchange method. According to this principle, the inserted active species must be electronegative. Thus, the conventional method limits the choosing range of the metal complexes in some extent. In addition, the intercalation amount of metal complexes and the accessibility of active sites to a reactant are restricted because the interlayer space of LDHs, which is responsible for the accommodation of exchangeable anions, is very narrow. Therefore, the development of a novel method for the preparation of metal complexes functionalized LDHs is desirable.In consideration of the above problems, this work was mainly focused on the following two aspects. Firstly, we propose a novel strategy for the preparation of metal complex functionalized LDHs, which involves grafting silylated metal complexes on exfoliated LDH nanosheets. This novel method not only could graft neutral metal complex on the LDH layers, but also the intercalation amount of metal complexes and the accessibility of active sites to a reactant are improved at the same time. The prepared hybrid materials were characterized by XRD, N2 adsorption, FT-IR, AFM, DR UV-vis and ICP and their catalytic performances in the decomposation of H2O2 and the styrene oxidation were also investigated. Furthermore, the AMOST method is proposed for the synthesis of highly exfoliated LDH nanosheets, and the synthesized LDH nanosheets was employed as a support for grafting the metal complexes. The prepared hybrid materials were characterized by XRD, N2 adsorption and their catalytic performances in the styrene oxidation were investigated. The specific works are as follows:Glycinate containing MgAl LDH (LDH-Gly) with 3 of Mg/Al (molar ratio) was prepared. Exfoliation of LDH-Gly was carried out in formamide under an ultrasound treatment. The exfoliated LDH was denoted as LDH-del.The silylated Co-Schiff base complex (CoSalen-Si) was prepared by the reaction of 3-aminopropyl(triethoxy)silane (APTS) with salicylaldehyde and then coordinating with Co. Co-Schiff base functionalized LDH (CoSalen-Si-LDH-del) was prepared by grafting CoSalen-Si on exfoliated LDH nanosheets in formamide. The XRD results demonstrate that LDH-Gly was successfully synthesized. And LDH-Gly can be exfoliated under the ultrasound condition. And, the successful exfoliation of the LDH-Gly can be further confirmed by the AFM image. The FT-IR and DR UV-vis spectra demonstrate that the CoSalen-Si was successfully grafted on LDH via the reaction between silane and M-OH of LDH surface. ICP measurement shows that CoSalen-Si-LDH-del has higher Co content than CoSalen-Si-LDH prepared by directly grafting silylated CoSalen-Si on LDH-Gly. Cosalen-Si-LDH-del showed excellent catalytic properties in the decomposition of low concentration of hydrogen peroxide at room temperature and atmospheric pressure. In addition, Cosalen-Si-LDH-del exhibited good activity and selectivity for the the oxidation reaction of styrene with O2 as the oxidant.According to the AMOST method, the LDH-borate were initially synthesized via a conventional co-precipitation method, except that the solid was re-dispersed in an AMO solvent such as acetone before final drying. The solvent will completely miscible with water. Thus, the surface adsorbed water molecules were replaced by the acetone molecules. The LDH-borate with acetone rather than water as the interlayer or surface bound molecules was easy to be exfoliated and dispersed. Since acetone has a very low boiling point, the facile loss of interlayer acetone can lead to the ready collapse of an ordered stacked structure and the formation of high surface porous materials. XRD、N2 adsorption and apparent density resoults show that the obtained sample upon drying still remain as exfoliated, dispersed nanoplatelets. AMOST method is simple, cost-effective to obtaine LDH nanasheets LDHs, which breaks through the deadlock in preparing exfoliated LDH dry powders. The Cosalen-Si-LDH-borate-del was prepared by grafting the Cosalen-Si on the exfoliated LDH dry powders. The styrene oxidation was carried out to investigated their catalytic performances.