Syntheses,Characterizations And Catalytic Properties Of Cobalt-containing Molecular Sieves

This dissertation focuses on the syntheses, characterizations and catalytic properties of cobalt-containing molecular sieves, and is composed of two parts.In the first part, Co-containing zeolites and MCM-41 have been studied for the epoxidation of alkenes. The samples prepared with ion-exchange or template ion-exchange method have been characterized by various spectroscopic techniques, and the correlations between the structures, especially the states of Co, and their catalytic properties are studied. Through these studies, a novel epoxidation catalyst system based on single-site cobalt ions or isolate Co(II) sites located in molecular sieves has been discovered. This system possesses following unique features: (i) O2 is a better oxidant than H2O2, TBHP.and NaCIO, (ii) there is no need of any co-reductant for O2 activation, (iii) Co2+ ions or isolate Co(II) in molecular sieves are unique, because Co2+ ions in liquid phase are inactive or non-selective, (iv) solvents play important roles and may take part in the formation of active sites by coordination with Co2+ or isolate Co(II).In the second part, catalytic properties of metallic cobalt supported on different molecular sieves prepared by the impregnation method have firstly been compared for Fischer-Tropsch synthesis. However, the impregnation method can't ensure the incorporation of metallic cobalt into the pores of the molecular sieves. Subsequently, a novel method comprising (i) ion-exchange, (ii) precipitation with NaOH and (iii) calcinations has been studied for the preparation of cobalt oxide nanoparticles encapsulated in faujasite zeolites. The concentration of NaOH for precipitation and the temperature for calcination are found to be critical in controlling the states and locations of the CoOx particles. With appropriate conditions, the CoOx particles formed are located and encapsulated in the supercages of faujasite zeolites. The CoOx particles (<3.0 nm) encapsulated in the supercages can be partly reduced to metallic cobalt at temperatures as low as 573 K. This method can also be used for the syntheses of the cobalt oxide and metallic cobalt nanoparticles encapsulated in other microporous molecular sieves. The metallic cobalt encapsulated exhibits superior catalytic activity and controllable product distributions in Fischer-Tropsch synthesis. This dissertation has also studied the encapsulated cobalt nanoparticles synthesized by a procedure comprising (i) ion-exchange and (ii) reduction with NaBH4, and their catalytic properties in Fischer-Tropsch synthesis.