Peculiarities of disproportionation of toluene on acidic pentasil zeolites

Abstract

Two samples of pentasil hydrogen forms were obtained: an ordinary sample and a sample lacking surface acid sites. The samples were tested in the disproportionation of toluene over short and very short periods of contact of the reaction mixture with the catalyst layer. The primary conversion products were found to be para-xylene and ethylbenzene in addition to benzene.

Mechanism for selective para-disproportionation of toluene on pentasil-based catalysts

“…In fact, inside the channels of ZSM-5, coke structures are formed which are difficultly accessible even for oxygen molecules: the curve showing the dependence of burn-off of these structures on the number of oxygen pulses supplied is located essentially equidistant from the abscissa axis [11]. Moreover, in the liquid products obtained in [2], no components heavier than xylenes are detected. Generalizing from the role of MDPM in disproportionation, we note that its formation is not ruled out according to the mechanism proposed in this paper.…”

“…In studying disproportionation of toluene on acid zeolites of the pentasil type, it has been shown [2] that the primary conversion products, besides benzene, are para-xylene and ethylbenzene. This is especially clear even on a nonselective catalyst for quite short residence times of the reaction ingredients in the sample bed.…”

“…Thus selective para disproportionation of toluene is determined by: the electron density distribution in the toluene molecule; the channel structure of the pentasil type zeolite, permitting only two ways for relative orientation of the toluene molecules and formation of only para-xylene and ethylbenzene molecules within the channels; the lack of acid sites on the outer surface of the zeolite crystals [2], causing isomerization of the given primary products to form other dimethyl-substituted benzenes. …”

“…Non-absolute selectivity of disproportionation with respect to para-xylene on a catalyst with a deactivated outer surface of the zeolite crystals [2] most likely is evidence for certain disruptions of the regularity in the actual structure of the pentasil used or a small residual acidity of the outer surface of the zeolite crystals.…”

Peculiarities of discontinuous sequential micro-oxidation of coke on zeolite catalysts

“…Their modification essentially involved both ion exchange of sodium to the corresponding cations or exchange plus reduction of the cations, as well as dealumination of the outer surface of the zeolite crystals using SiCl 4 as described in [11]. In particular, the outer surface of the alkylation sample, HLaCaNaX (sample 1), was dealuminated, while the disproportionation sample, HZSM-5 (sample 2), and the isomerization sample, 2.5% Ni/HNaY (sample 3), were not subjected to this procedure.…”