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Modification of Y Zeolite by Incorporating MOR Zeolite

Author: GuoDaLei
Tutor: LiRuiFeng
School: Taiyuan University of Technology
Course: Chemical Engineering
Keywords: Y zeolite MOR modification cumene cracking designedsynthesis
CLC: TQ426
Type: Master's thesis
Year: 2013
Downloads: 68
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Abstract

Y zeolite and its modified products have been all-important catalytic materials in the petrol-processing industrial fields because they exhibit the excellent catalytic performances in fluid catalytic cracking (FCC), hydro-craking (HDC), hydro-isomerization, alkane aromatization, alkylation reactions, and so on. In order to obtain the ideal zeolites, modifying the conventional zeolites except for developing novel meso-zeolites has been an effective method and one of the research focuses.Preparation and modification of Y zeolite revealed the close relationship with the catalytic performance of cracking catalysts. In the article, a series of composites composed of MOR and Y zeolites were prepared based on Y zeolite by incorporating another type of zeolite using silicon-aluminium rule method, where, MOR (named as MOR-FAU-Zeolite, denoted as MFZ) and Y zeolite (named as FAU-MOR-Zeolite, denoted as FMZ) are used as silicon and aluminium resources, respectively. The structural, crystalline, textural properties and catalytic activities of the as-synthesized materials as well as the reference Y and MOR samples, were characterized and analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), nitrogen adsorption-desorption techniques, scanning electron microscopy (SEM), NH3temperature programmed desorption (NH3-TPD) and the cracking reaction of cumene.FMZ:Industrial NaY zeolite was used as the nutrients for the growth of Mordenite zeolite, and the bi-phases composite zeolites with different ratios of FAU to MOR were successfully prepared. The as-synthesized samples were characterized by XRD, FT-IR and SEM; the results display that the synthesis conditions play an important role in the formation of FMZ bi-phases composite zeolites. The formation mechanism of the bi-phases composite zeolites was investigated in details:Silicon species are extracted from the frameworks of NaY crystals that results in the enrichment of aluminum species near the wound in NaY zeolite crystals. The enriched aluminum species react with the extracted or replenished silicon species which lead to the nucleation and growth of Mordenite zeolite crystals near the wound in NaY zeolite crystals. Along with the growth of Mordenite zeolite, the core crystals finally break through NaY crystals, the FMZ bi-phases composite zeolites are therefore obtained.Composite zeolites MFZ composed of Y and mordenite zeolites is successfully prepared. The ratios of FAU to MOR in composites can be availably adjusted by controlling the second-step hydrothermal treatment time. The catalytic tests investigated during the isopropylbenzene cracking indicate that the created hierarchical pores can enhance the catalytic activity and stabilities of the catalysts because of the improved accessibility of activity centre and the enhanced diffusion rate, as well as the reduced coking rate. However, excessive introduction of meso-or/and macropores into composite zeolites is not beneficial, as the penalty paid in the micropore volume and areas results in the less efficient catalysts. The catalytic activity of isopropylbenzene cracking strongly depends on the revised hierarchy factor. A conclusion can be drawn that the revised hierarchy factor may be a more appropriate tool to classify hierarchically structured composite zeolites with similar acidities and compositions in the diffusion-controlled reactions.The catalytic cracking of isopropylbenzene over the catalysts (purely microporous and hierarchical) shows that the conversion has a well linear dependence with the HF’. At the initial reaction (at0.1h), the catalytic activity over all of the catalysts shows a discrete dependence with the HF’, not the linear dependence as expected, indicating that the initial reaction process may be an acidity-controlled reaction, and the diffusion effect on the catalytic behavior plays a negligible role in catalytic cracking of isopropylbenzene because of the shorter reaction time; after1-2h, the conversion of isopropylbenzene catalytic cracking over all of the catalysts shows a well linear dependence with the HF’. Indicating that with the prolonged reaction time the reaction process has been transferred from the acidity-controlled reaction of the initial reaction (0.1h) to the diffusion-controlled reaction. The results also show that the introduction of the hierarchical porosity in the composite zeolites has a major impact on catalytic activity. Higher activity of the composite zeolites results from the enhanced accessibility of Bronsted-acid sites, however, the largest beneficial effect is the alleviated diffusion limitation by the presence of meso-and macropores that depress the carbon deposit, and enhance the stability of the catalysts; after3h, MFZ-8deviates from the linear dependence relation of the conversion and HF’, especially deviates toward the lower conversion of isopropylbenzene cracking. The result displays that although mesopores are introduced into MFZ-8, which are at the cost of penalty of micropores. The lowered microporous area results in the decreased acidity centre. The catalyst is therefore subjected to the rapid deactivation due to the lower acticity sites. That further confirms that the formation of mesopores in the hierarchical materials should not be at the cost of the penalty of micropores. Excessive introducing mesopores into the composite zeolites will weaken the catalytic performances of the hierarchical materials.

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