Journal
ACS CATALYSIS
Volume 2, Issue 6, Pages 971-981Publisher
AMER CHEMICAL SOC
DOI: 10.1021/cs300127w
Keywords
zeolites; m-xylene isomerization; shape selective catalysis; transition states; GC-MS; reaction mechanism
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Funding
- National Research Foundation of Korea [R0A-2007-000-20050-0, 2011-0029806]
- Ministry of Knowledge Economy of the Korean Government [J000003936]
- National Research Foundation of Korea [R0A-2007-000-20050-0] Funding Source: Korea Institute of Science & Technology Information (KISTI), National Science & Technology Information Service (NTIS)
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The mechanisms of m-xylene isomerization and disproportionation over 13 medium-pore zeolites and three large-pore ones are investigated. While H-TNU-10 and H-ZSM-57 with intersecting 10- and 8-ring channels were found to show considerably higher p/o ratios than H-ZSM-S, 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 experimentally verified until now. These dicyclic aromatic compounds were ascertained to serve as reaction intermediates of bimolecular m-xylene isomerization within the micropores not only of large-pore zeolites but also of medium-pore materials at temperatures lower than 523 K or so, once there are internal void spaces larger than 10-rings. Flushing experiments with used zeolites followed by GC-MS analyses strongly suggest that the high p-xylene selectivity found in some medium-pore zeolites is largely due to product shape selectivity rather than to transition state one. More importantly, the overall GC-MS results of our work demonstrate that transition state and product shape selectivities are experimentally distinguishable from each other.
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