Mechanistic Insights into the Zeolite-Catalyzed Isomerization and Disproportionation of m-Xylene

Abstract

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-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 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.

Zeolite-Catalyzed Disproportionation of iso-Propylbenzene: Identification of Reaction Intermediates and Mechanism

“…While the latter group of bicyclic aromatic compounds was found to be intermediates of iPB dehydrogenation, a side reaction of the iPB disproportionation, both groups of them are hardly observed over medium-pore zeolites, regardless of their structure type. However, we were not able to detect any di-iso-propylated 2,2-diphenylpropane (dipDP) species in this study, unlike the disproportionation of other aromatic hydrocarbons, including m-xylene, ethylbenzene, and n-propylbenzene, 12,17,18 in which formation of dialkylated diphenylalkane species as reaction intermediates has been clearly confirmed. To better understand the results described above, we have calculated the strain energies of three mipDP, three mip = DP, or six dipDP isomers embedded within the structures of zeolites studied, together with the relative energies of all species (i.e., reactant, reaction intermediates, transition states, and products) in the 84T H-Y model, using the mixed quantum-mechanical and semi-empirical ONIOM method.…”

“…Details of the IR measurements with adsorbed pyridine can be found elsewhere. 17,18,22 After desorption at 473 K for 2 h, the concentrations of Brønsted and Lewis acid sites in zeolites were determined from the intensities of the IR bands around 1550 and 1450 cm -1 , respectively, using the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 equations of Emeis. 23 The characterization data for zeolite catalysts studied here are given in Table 1.…”

“…17,18,22 The GC-MS total ion chromatograms were recorded on an Agilent 7890A gas chromatograph equipped with an Agilent 5975C mass selective detector, and the organic compounds extracted were identified by comparing with the NIST database. 24…”

In situ combined analysis of gases and electrochemical signals of an activated carbon-based supercapacitor at 2.7–4 V

“…Probably, the decomposition of PC contribute to not only the formation of C 3 species, but also C 2 and C 4 species, similar to the disproportionation reaction in other systems. 21,22 The disproportionation of C 3 H 6 on AC is possible under an electrocatalytic condition and in the presence of H 2 . In addition, the data in 2 nd and 3 rd cycles indicated that the percentage of CH 4 decreased a little bit, but the percentage of C 3 H 6 and C 3 H 8 remained nearly the same.…”

Reconversión de compuestos aromáticos de 9 carbonos a xilenos usando métdodos computacionales

“…Es, por tanto, importante, considerar que tipo de reacciones pueden ocurrir tanto en los canales de 10 como en las intersecciones, con el fin de ver las consecuencias del tráfico molecular. Considerando este escenario, donde los canales de La prueba de que esta reacción ocurre en las intersecciones de los canales de 10 es que también se da en zeolitas con canales de poro medio tales como la MFI 58,121,122 , la MEL 120 y la TNU 120 , cuyas dimensiones de canales son similares a alguno de los canales de 10 de la UWY (Figura 3.42).…”