제올라이트 촉매 상에서 C8 방향족 탄화수소 전환반응의 메커니즘 연구
- 제올라이트 촉매 상에서 C8 방향족 탄화수소 전환반응의 메커니즘 연구
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- Zeolite catalysts are widely used in the petrochemical industry to transform alkylaromatic hydrocarbons. In this study, effects of structural and physicochemical properties of zeolite catalysts on reaction mechanisms of C8 alkylaromatics conversions (i.e., ethylbenzene (EB) disproportionation
m-xylene isomerization and disproportionation) were investigated. Although these reactions are initiated by the protonation of hydrocarbon molecules by Brønsted acid sites in zeolite catalysts to produce carbocations, the shape-selective nature of zeolite catalysts determines the types of reaction intermediates and hence the prevailing reaction mechanism and the product distribution. For example, large-pore zeolites or medium-pore zeolites in which cavities are large favor formation of bulky bimolecular transition states such as diphenylmethane and diphenylethane derivatives during the transformation of m-xylene and EB. However, medium-pore zeolites with one-dimensional channel systems favor monomolecular reactions due to the steric hindrance imposed by the narrow 10-ring pore walls. The catalytic activity of zeolites is also significantly influenced by the reaction mechanisms due to the different activation energy barriers of mono- and bimolecular pathways. In addition to this constraint, product shape selectivity due to the different diffusivities of molecules within the zeolite pores also affects the distribution of reaction products. Accordingly, the reaction mechanisms and product distribution of alkylaromatic transformations can be controlled by altering the pore geometry of zeolite catalysts and by tuning their acidic properties. So understanding the reaction mechanisms of alkylaromatic conversions over zeolite catalyts is very important not only for developing new catalysts for these processes, but also for extending shape selectivity theories of heterogeneous catalysis.
The fourth and fifth parts of this thesis address the reaction mechanisms of EB disproportionation, which is a standard reaction for characterizing the acidic properties of zeolite catalysts. Gas chromatography-mass spectrometry (GC-MS) results are presented as evidence for the build-up of diethylated diphenylethane species and monoethylated diphenylethane derivatives inside the LaNa-Y cavities during EB disproportionation. The roles of the former species as reaction intermediates were found to become greater than those of the latter species as time-on-stream increased. Based on the overall results presented in this study, a new dual-cycle mechanism for diethylbenzene formation over large-pore zeolites is proposed.
EB disproportionation is a useful test reaction for distinguishing between medium- and large-pore zeolites because medium-pore materials show neither an induction period nor a diethylbenzene deficit, whereas both phenomena occur in large-pore zeolites. Hence, the mechanism of this reaction over all medium-pore zeolites has been regarded as simply monomolecular. In the second part of this thesis, I demonstrate that the pore architecture of medium-pore zeolites strongly influences the main types of reaction intermediates during EB disproportionation, and thus those of the prevailing reaction mechanisms, i.e., dual-cycle diphenylethane-mediated, bimolecular diphenylethane-mediated, and monomolecular ethyl-transfer reaction pathways. This study provides more detailed experimental evidence for transition-state shape selectivity in zeolite catalysis than has ever been reported.
In the sixth part of this thesis, the mechanisms of m-xylene isomerization and disproportionation are investigated in thirteen medium-pore zeolites and three large-pore zeolites. H-TNU-10 and H-ZSM-57 with intersecting 10- and 8-ring channels were found to yield considerably higher p/o-xylene ratios than H-ZSM-5, a commercial m-xylene isomerization catalyst. The GC-MS results from used zeolite catalysts demonstrate the intrazeolitic build-up of tri- and tetramethylated diphenylmethane species, whose existence during the m-xylene transformation over any acidic catalyst has not been previously verified experimentally. These dicyclic aromatic compounds were ascertained to serve as reaction intermediates during bimolecular m-xylene isomerization within the micropores not only of large-pore zeolites but also of medium-pore zeolites at temperatures lower than ~250 °C, if internal void spaces larger than 10-rings are present. Flushing experiments with used zeolites followed by GC-MS analyses strongly suggest that the high p-xylene selectivity of some medium-pore zeolites is largely due to product shape selectivity rather than to transition state shape selectivity. More importantly, the overall GC-MS results of this work demonstrate that transition state and product shape selectivities are experimentally distinguishable from each other.
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