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A Study of the Effect of Acidity of Zeolite Y on the Adsorption Desulfurization

Author: ShiYanChun
Tutor: TianFuPing
School: Dalian University of Technology
Course: Physical and chemical
Keywords: Adsorptive desulfurization Zeolites Surface acidity Adsorption modes FT-IR
CLC: TE624.55
Type: Master's thesis
Year: 2012
Downloads: 213
Quote: 1
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Abstract

Ultra-deep desulfurization of transportation fuels has attracted more attention and become an important worldwide issue. The key point of applying hydrodesulfurization (HDS) process to gasoline distillation is to desulfurize to maximum extent without decreasing octane number. Adsorptive desulfurization is regarded as one of the most important promising method in the removal of thiophenic sulfur compounds to produce ultra-clean fuels under moderate conditions, owing to its operation simple and not reducing the gasoline octane number. The adsorption performance for thiophenic sulfur compounds of faujasite (FAU) zeolites, especially NaY and metal ion-exchanged Y zeolites, in both model and real fuels, has been widely investigated. Whereas less attention has been paid on the fact that the introduction of different metal ions into zeolite always leads to the change in the acid property of the zeolite and consequently affects its adsorptive desulfurization. Ce(IV)Y and AgY adsorbents were prepared by liquid-phase ion exchange method in this dissertation, and the effect of the acidity of the adsorbent on the adsorption of thiophene, toluene and cyclohexene (CHE) was studied by FT-IR spectroscopy. Based on which, the adsorptive performances of Ce(IV)Y and AgY were systematically investigated, and the relation between the acidity adsorptive desulfurization performance was elucidated.There is no acidity on NaY. Both Bronsted and Lewis acid sites are observed on Ce(IV)Y and AgY, but there mainly exist medium strong Bronsted acid sites on Ce(IV)Y and Lewis acid sites with wide distribution on AgY. The adsorption modes of thiophene, toluene and CHE on Ce(IV)Y and AgY are different. The strong Bronsted acid sites of Ce(IV)Y lead to the protonation of thiophene and CHE, but have no effect on the adsorption mode of toluene. The introduction of Ce4+ions into NaY zeolite improves the adsorptive desulfurization selectivity to thiophene in model gasoline based on static adsorption experiments, because of the crucial role played by the direct Ce4+-S interaction. The decrease casued by toluene can be attributed to the comptitive adsorption between toluene and thiophene. The alkylation between thiophene and CHE and the polymerization of (CHE) occurred on the Bronsted acid sites of Ce(IV)Y result in the formation of alkylated thiophenes and CHE dimers. The larger size polymers are thought to be main reasons for the decreased sulfur removal from model gasoline containing olefin. Obviously, the Bronsted acid sites of Ce(IV)Y is found to play a crucial role in desulfurization selectivity when thiophene and CHE coexist in model gasoline.As for AgY adsorbent, the existence of acid sites are not found to have effect on the adsorption of thiophene, CHE and toluene, the three adsorbates interact with Ag+ions by π-complexation. When low content aromatic or olefin is added into the model fuels, AgY still show higher desulfurization performance, mainly owing to the π-complexation between Ag+and thiophene molecule. The similar adsorption modes of the three adsorbates are responsible for the evident reduction in sulfur removal from model gasoline containing large amount of toluene or cyclohexene. The Bronsted acid sites derived from the introduction of Ag+onto NaY are not observed to result in the alkylation reactions and CHE polymerization. Therefore, the acidity of AgY don’t have obvious effect on desulfurization selectivity in model gasoline containing CHE and toluene.

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