Synthesis Of FeZSM-5 Catalysts And Their Application In The Reactions Related To N₂O

Nitrous oxide (N2O) has a significant impact on the destruction of the ozone layer besides its greenhouse effect. Therefore, research and development on N2O elimination technologies are of great importance. There are two ways considered to be efficient to disposal N2O. One is the conversion of N2O with low concentration, including the decomposition and selective catalytic reduction, into environmentally friendly N2 and O2. The other is to use N2O as an active oxygen donor, i.e., one-step oxidation of benzene to phenol with N2O as oxidant. FeZSM-5, especially after steam activation at high temperatures, is reported to be a promising catalyst for the aforementioned reactions related to N2O.For a catalytic reaction over zeolites, internal mass transfer limitations are common phenomena. For example, the oxidation of benzene to phenol by N2O over FeZSM-5 is apparently limited by the internal mass transfer of benzene because of the molecular size of benzene being similar to the channel opening sizes of the zeolite. Additionally, this reaction encounters a rapid catalyst deactivation due to the diffusion limitations of benzene and phenol in and out of the zeolite crystals and thus the accumulation of these components inside the crystals, leading to the formation of coke. However, in the direct decomposition of N2O over FeZSM-5, because the molecular size of N2O (0.33 nm) is much smaller than the channel opening sizes of MFI-type zeolite (0.51 nm×0.55 nm and 0.56 nm×0.53 nm), limited work has been dedicated to investigate the diffusion limitations of N2O over FeZSM-5. Additionally, for the complete decomposition of N2O over FeZSM-5 catalysts, an operation at high temperatures is generally required, and under this high temperature condition, the mass transfer of N2O inside the zeolite crystals will become a rate-determining step. The main results are summarized as follows:1) In a typical synthesis of nanosized FeZSM-5 using tetrapropyl ammonium bromide (TPABr) as a template and the synthesized crystals were characterized by XRD and SEM techniques. By adjusting the alkalinity of the synthesis system and crystallization conditions, the crystal sizes of FeZSM-5 can be controlled from nano-to micro-scale. Nanosized FeZSM-5 can be synthesized via a two-step method. Based on the synthesis procedure for the nanosized zeolite, FeZSM-5 crystals with a size of about 2μm can be synthesized by a one-step crystallization method. Besides, the crystal would grow larger by increasing the first crystallization time. Moreover, as the alkali source of the mixture of NaOH and NaHCO3 instead of NaOH, the obtained FeZSM-5 zeolites would show crystal sizes larger than 5μm. Here, five FeZSM-5 zeolites with crystal sizes of 70 nm,2μm,5μm,8μm,20μm were chosen as the catalysts for N2O decomposition. It can be concluded that The larger the zeolite crystal size, the lower the activity and thus the higher the reaction temperature required for the complete decomposition of N2O. It was found that over the catalysts with crystal sizes smaller than 2μm, the complete decomposition of N2O is observed while over the other FeZSM-5 catalysts the conversion of N2O decreases with increasing zeolite crystal size at 525℃. So, the N2O decomposition is controlled by the internal mass transfer limitations of N2O in FeZSM-5 crystals. However, under the applied conditions, the diffusion limitations are absent and the observed reaction rates are controlled by the intrinsic kinetics of N2O decomposition over the FeZSM-5 catalysts with crystal sizes smaller than 2μm, irrelevant to reaction temperature. Other three FeZSM-5 zeolites with crystal sizes of 70 nm,2μm,20μm were chosen as the catalysts in oxidation of benzene to phenol with N2O. The results showed that nanosized FeZSM-5 had the best catalytic activity and stability. So the size effect on the oxidation of benzene to phone over FeZSM-5 catalysts with different crystal sizes has been illustrated.2) Hierarchical ZSM-5 crystal assembles with spherical morphology were synthesized for the first time by using tetrapropyl ammonium bromide (TPABr) as the template in combination with 3-aminopropyltriethoxysilane (APTES). The effects of alkalinity and the dose of APTES on the structure and morphology of the synthesized ZSM-5 were investigated. The results indicate that increasing APTES dose can lead to some difficulty for ZSM-5 crystallization. The characterizations with SEM and TEM techniques show that the product obtained after crystallized for 3 d consists of amorphous micro-sized spheres, inside which there are a plenty of cavities available. Prolonging the crystallization time can result in a structure transformation from these amorphous microspheres made of aluminosilicate with cavities into mesoporous ZSM-5 nanocrystals. Additionally, the hierarchical FeZSM-5 zeolites can be synthesized by adding an iron source into the synthesis system for synthesizing the hierarchical ZSM-5 crystals. After steam-activated, the hierarchical FeZSM-5 zeolite show some super catalytic properties in N2O decomposition...