Selective toluene disproportionation over ZSM‐5 zeolites

Abstract

Selective toluene disproportionation to benzene and xylenes is investigated over HZSM‐5 and modified HZSM‐5 zeolites. The zeolites are modified with nickel, magnesium, phosphorus and boron, characterised by sorption, acidity measurement and X‐ray photoelectron spectroscopy, and their activity towards toluene disproportionation is discussed in the light of their physico‐chemical properties. The two types of active sites have been identified; one promoting disproportionation and the other cracking and dealkylation of toluene. It is found that the latter sites are suppressed due to incorporation of the modifiers. The modified zeolites are found to be para‐selective which appears to be consistent with the commonly accepted view that modifiers like magnesium, phosphorus and boron reside in the channels and react with Brønsted acid sites, leading to partial blockages and thus favouring the formation of p‐xylene. The effect of poisoning the zeolite with 1‐methylisoquinoline is also investigated. In the case of poisoned catalyst, p‐xylene is produced at levels up to 99%. The total elimination of other isomers from the products is suggested due to the selective poisoning of the non‐selective sites (on the external crystal surface) and to partial blocking of the pore openings.

Catalytic Transformation of Toluene over a High-Acidity Y-Zeolite Based Catalyst

“…The product distribution, on the other hand, was extremely affected by temperature, as shown in Table 3. Meshram and Chem 16 reported a similar effect of temperature on toluene conversion using ZSM-5 zeolites. It was determined that the conversion was not sensitive to temperature changes above 500 °C.…”

“…A similar trend for B/X with temperature was observed by several researchers. 16,20,22 It is depicted in Figure 6 that the B/X ratio is maximum at low toluene conversion and decreases as the conversion increases. This result suggests that toluene dealkylation might be a primary reaction, producing benzene and gas, which explains the initially high B/X value.…”

Structure of toluene6.4-ZSM-5 and the toluene disproportionation reaction on ZSM-5

“…This category includes 8PARA (van Koningsveld et al, 1989) and 8PDCB (van Koningsveld, Jansen & van Bekkum, 1996). The acid-catalyzed disproportionation of toluene to benzene and xylenes has been extensively investigated in ZSM-5 zeolite (Chen et al, 1979;Kaeding, 1985;Kaeding et al, 1981;Young et al, 1982;Meshram, 1987). At present, the two widely accepted mechanisms for toluene disproportionation on ZSM-5 are:…”

Modeling of para-Xylene Synthesis via Nonselective Toluene Disproportionation

“…In terms of other side reactions, according to the experimental kinetics studies carried out by other investigators (Kaeding et al, 1981;Meshram et al, 1987;Bhaskar et al, 1990;Uguina et al, 1993), the amount of side product such as trimethylbenzene is negligible utilizing unmodified ZSM-5 catalyst. Furthermore, the experimental data by Uguina et al (1993) demonstrate that the side reaction of toluene dealkylation is also negligible.…”