| The visible light induced photocatalytic degradation of the low concentration and highly toxic organic pollutants from wastewater is green, inexpensive and low carbon, and it is considered to be the most promising low-temperature and deep-oxidation technology of water treatment. However, TiO2-based catalysts suffered from several drawbacks including low effeciency of solar energy utilization, easy to form large aggregates and difficult to separation and recovery of the photocatalysts. Therefore, to study and explore a novel and efficient visible light-driven photocatalyst as a substitute for TiO2 has both great theoretical and positive practical significance, especially nowadays the pollution of water environment is increasingly serious.Epoxides are important organic intermediates and widely used in many fields. The epoxidation of olefins is the main way of preparing epoxides. To date, compared with the epoxidation of functional olefins, the epoxidation of unfunctional olefins still face some disadvantages including low epoxide selectivity, difficulty to choose the suitable oxidant system and complicated catalyst preparation. Hence, to study and explore efficient heterogeneous catalysts, green oxidants, and new approaches for epoxidation are of great importance to the development and improvement of epoxidation of olefins. Besides, the study on the reduction of cost and the enhancement of enantioselectivity is crucial to the implementation of industrial application of the asymmetric epoxidation of unfunctional olefins.K4Nb6O17 and α-Zr(HPO4)2·H2O (hereafter α-ZrP) are two typical layered compounds with distinctive structures and characters. For example,K4Nb6O17 can be delaminated into nanosheets as well as nanoscrolls, and α-ZrP nanosheets could be tailored. In addition, the restacking structure obtained by the exfoliation and flocculation of layered compounds is porous with a large specific surface area, which is benefit for the adsorption and degradation of organic pollutants and the heterogeneous catalytic reaction.In this thesis, a series of novel composite catalysts of photodegradation and olefin epoxidation were successfully prepared via the exfoliation-flocculation methods by using layered K4Nb6O17 and α-ZrP as starting materials. The resulted catalysts were characterized in detail by XRD, SEM/TEM, N2 adsorption-desorption, UV-DRS, XPS, TG-DTA and FT-IR measurements. The possible mechanisms of photocatalysis and epoxidation were also discussed. The main results are summarized as follows:(1) H+/nanosheets with a high BET surface area of 97.0 m2/g and H+/nanoscrolls with a high BET surface area of 145.9 m2/g were respectively prepared by first exfoliation of K4Nb6O17 with propylammonium hydrochloride (PAHCl) and acid exchanged K4Nb6O17 with tetra-butylammonium hydroxide (TBAOH) and then flocculation with dilute hydrochloric acid. It was found that H+/nanosheets are composed of irregular lamellar plates with large and smooth basal plane while H+/nanoscrolls consist of a lot of nanoscrolls with curved surface. Besides, H+/nanosheets and H+/nanoscrolls have both an amazing adsorptive capacity and a rapid visible-light response for the photodegradation of Rhodamine B (RhB). By comparison, the photodegradation rates of RhB over H+/nanosheets and H+/nanoscrolls are faster than that over the nanosheets peeled from other layered perovskites. In addition, the photodegradation of RhB over H+/nanosheets and H+/nanoscrolls is mainly initiated by a dye photo-sensitization process. Furthermore, the photodegradation of RhB dyes revealed that dye adsorbed on the unfolded nanosheets can effectively harvest sunlight. Due to simple preparation, low-cost, high adsorption capacity and photocatalytic efficiency, and easy to separation and recovery of the photocatalysts, H+/nanosheets and H+/nanoscrolls might be used as promising candidates for adsorption and photodegradation of organic dyes in industry.(2) Nitrogen-doped H+/nanosheets and H+/nanoscrolls (noted as N-nanosheets and N-nanoscrolls) were successfully obtained by the calcination of H+/nanosheets and H+/nanoscrolls with urea at 400℃, respectively. The nitrogen doping has solved the intrinsical defects of H+/nanosheets and H+/nanoscrolls referring to the absorption spectra only in the ultraviolet region and low effeciency of solar energy utilization. It was found that N-nanosheets with a BET surface area of 31.2 m2/g are composed of a rather compact restacking of irregular lamellar plates and the [Nb6O17]4- unit structure of nanosheets still maintains, while N-nanoscrolls with a BET surface area of 98.7 m2/g consist of disorderly accumulation of nanotubes and nanoparticles and the original [Nb6O17]4-unit structure of nanoscrolls vanished and a subsquent thermal transformation into Nb2O5. Both N-nanosheets and N-nanoscrolls have highly efficient visible light photocatalytic activities with the significant enhancement of degradation of RhB, which are superior to nitrogen-doped Nb2O5 and commerical P25. The active species during the photodegradation of RhB over N-nanosheets are’OH, O2’ and photogenerated holes (h+), while only O2’ and h+ over N-nanoscrolls. Besides, the photodegradation of p-chloroaniline over N-nanosheets can be further mineralized (～16%) while that over N-nanoscrolls not. By comparison, N-nanosheets has a potential application prospects because of the generation of powerful OH in the photocatalysis process.(3) Bi-doped porous Nb2O5 was prepared via the calcination of H+/nanoscrolls with Bi2O3 at 500℃ by the use of the thermal transformation of H+/nanoscrolls into Nb2O5. It was found that the mass ratio of H+/nanoscrolls and Bi2O3 is 0.2:0.002, the corresponding catalyst 1Bi-NR exhibits the best photodegradation performance. Compared with Bi-doped Nb2O5 (1Bi-Nb2O5) and commercial P25, 1Bi-NR displays prominently enhanced photocatalytic activity due to its small particle sizes, large BET surface area and good visible light absorption. In addition, the photodegradation over 1Bi-NR involves the powerful OH.(4) CuO/nanosheets-450 and CuO/nanotubes-450 with distinct surface areas and characters were prepared by using nanosheets and nanoscrolls derived from as two novel supports via an exfoliation-flocculation-calcination process. It was found that CuO/nanosheets-450 is composed of a number of irregularly lamellar plates restacked in a house-of-cards structure and the surface is relatively smooth and large, while CuO/nanotubes-450 mainly consists of a lot of nanotubes and the surface is bumpy and curved. Compared with CuO nanoparticles and CuO supported on K4Nb6O17, both CuO/nanosheets-450 and CuO/nanotubes-450 displayed an enhancement of both styrene conversion and styrene oxide (SO) selectivity. By comparison, CuO/nanosheets-450 exhibited a relative good catalytic performance with a SO selectivity of 51.1% by tert-butyl hydroperoxide (TBHP) while CuO/nanotubes-450 94.6% by H2O2. Besides, the addition of dimethyl formamide (DMF) and NaHCO3 are helpful to the improvement of SO selectivity, but the role is different, that is, DMF molecules cover the surface active Cu2+ sites of the catalyst while NaHC03 firstly reacts with H2O2 to produce HCO4- which is more active than sole H2O2 and then the HCO4- is easy to form a five-membered ring with the surface active Cu2+ sites. Because of its large surface area and confinement effect, nanoscrolls may be a new kind of promising support for the design and assembly of novel heterogeneous catalysts and other useful functional materials.(5) α-ZrP nanosheets and its edge-modified with α-naphtyl phosphonic acid version were successfully obtained, and then the two versions of α-ZrP nanosheets were respectively flocculated with chiral salen Mn(Ⅲ) complex, resulting in two novel heterogeneous chiral catalysts with different structure, surface character and location of salen Mn(Ⅲ). The two heterogeneous chiral catalysts exhibited distinct performances in the asymmetric epoxidation of α-methylstyrene. The results show that salen Mn(Ⅲ) loaded on the edges of nanosheets is superior to that on the basal planes. The bumpy edge-accumulated surfaces of nanosheets provide a new and different chemical environment for salen Mn(Ⅲ) other than large basal planes, and a high ee value of 78% was obtained without axial ligand. |
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