Hierarchical And Metal Modified ZSM-5 Zeolite: Preparation, Characterization And Catalytic Performance In The MTH Reaction

During the last decade, hierarchical zeolites have emerged as an important class of materials in zeolite science and technology, and they attract continuously increasing interests. Hierarchical zeolites couple in the catalytic power of micropores and the facilitated access and improved transport consequence of a complementary mesopore network. In order to improve catalytic performance in many reactions, zeolites need to be chemically modified in a number of ways, of which metal modification is the most common one. Nowadays, the world depends heavily on fossil oil for energy and basic chemicals, however, the proven reserves of fossil oil will last only a few decades at the present rate of consumption. The methanol-to-hydrocarbons reaction (MTH) over zeolite catalyst is an important process for obtaining olefins (MTO), propene (MTP), gasoline (MTG) and aromatics (MTA) from coal, natural gas or renewable biomass instead of fossil oil. Therefore, it is vital to develop the MTH process in the crisis of energy. In this dissertation, we studied the preparation and characterization of four types of hierarchical and metal modified ZSM-5 zeolites and their catalytic performance in the MTH reaction.First, aromatization of methanol over co-impregnated La/Zn/HZSM-5 zeolite catalyst was studied. The selectivity of aromatics and BTX (benzene, toluene, and xylene) reached 64.0% and 56.6%, respectively, using La/Zn/HZSM-5 at 437℃,0.1 MPa and methanol WHSV (weight hourly space velocity)= 0.8 h-1. Catalytic results showed that the La species was a very good promoter, increased selectivity of aromatics, and prolonged the catalyst lifetime on stream. The effects of the SiO2/Al2O3 ratio in zeolite, Zn and La loading, WHSV, reaction temperature, water content in the feed and H2 pretreatment of catalysts on the catalytic performance were studied in detail. Characterization of the catalysts by TGA, NH3-TPD, SEM, N2 adsorption-desorption, XRD and XRF, was carried out to understand the structure and discuss the aromatization performance of La/Zn/HZSM-5 zeolite catalyst.Second, the preparation of hierarchical mesoporous Zn/ZSM-5 zeolite by NaOH treatment and Zn impregnation, and its application in the conversion of methanol to gasoline were studied. N2 adsorption-desorption results showed the mesopores with sizes of 2-20 nm in HZ5/0.3AT was formed by 0.3 M NaOH alkali-treatment. The catalysts after modification were also characterized by XRF, A AS, XRD, SEM and NH3-TPD methods. Zn impregnated catalyst Zn/HZ5/0.3AT exhibited dramatically improved catalytic lifetime and liquid hydrocarbons yield. The selectivity of aromatic hydrocarbons was improved after impregnation of Zn. The mesopores of Zn/HZ5/0.3AT enhanced the synergy effect of Zn species and acidic sites and the ability to coking tolerance, which was confirmed by the results of catalytic test and TGA analysis, respectively.Third, the preparation of nano-sized H[Zn, A1]ZSM-5 zeolite by direct synthesis procedure and its application in the aromatization of methanol were studied. SEM, XRD, and N2 adsorption-desorption data shown that H[Zn, A1]ZSM-5 zeolite was composed of aggregates of 250 nm×50 nm×25 nm crystals with about 2.1 nm inter-crystal voids. NH3-TPD, pyridine-FTIR, and UV-Vis spectral analyses were also employed to understand the properties of zolite catalysts prepared by different methods. TGA results confirmed that the nano-structure was resistant to coke in the reaction. BTX yield and catalytic stability were improved in the conversion of methanol over H[Zn, Al]ZSM-5 zeolite. The BTX yield on H[Zn, Al]ZSM-5 was 48% at 437℃and WHSV of 0.8 h-1, slightly decreasing to 32% after 160 h on stream. The ramping rate in the calcinations significantly affected the acidic and catalytic properties of the zeolite. H[Zn, Al]ZSM-5 calcinated at a rate of 1℃/min showed less Lewis sites, higher BTX yield, and longer lifetime compared to H[Zn, A1]ZSM-5/T calcinated at a rate of 30℃/min.Finally, Hierarchical nanocrystalline ZSM-5 zeolite (NZ5) was synthesized at 100℃under atmospheric pressure using methylamine as a mineralizing agent. The crystallization process of NZ5 was characterized by DLS, XRD, and FTIR. The results of contrastive experiments showed that evaporation of the solvent promoted the aggregation of primary particles, and the addition of methylamine accelerated the crystallization process. The NZ5 aggregate consisted of 20 nm individual particles, as shown in SEM and TEM images. The lattice fringes in the TEM images and the XRD results indicated that individual particles of NZ5 were highly crystalline. N2 adsorption-desoprtion isotherms showed that NZ5 had high BET surface areas with mesopores having a mean diameter of about 9 nm. NZ5 exhibited a long lifetime, a stable and high yield of liquid hydrocarbons, and a high anti-coking performance in methanol-to-hydrocarbons reaction. Catalytic testing and TGA results showed that the lifetime of NZ5 was about 10 times longer than that of micro-sized ZSM-5 zeolite (MZ5), and the average coking rate with NZ5 was one fifth over that of MZ5.