Oscillatory motion of coke in deactivated HZSM-5 zeolite

Abstract

We have observed motion of coke in deactivated HZSM-5 with the displacement vector alternately directed toward the outer surface of the zeolite crystals and into the interior of the zeolite structure. The driving force for the oscillations is the higher chemical potential of massive carbon µ m at 500°C than for carbon clusters. The switching mechanism involves an alternating jumpwise change in the value of Dµ.Key words: deactivated HZSM-5 zeolite, micro-oxidation of coke, coke localization, oscillatory behavior of coke, driving force for oscillations, switching mechanism for oscillations.Zeolites are modern catalysts for a number of key carbonium ion processes in petroleum refining and petroleum chemistry. Deactivation of zeolites occurs mainly as a result of the formation of coke, blocking the active sites. Publications in recent years [1-8] confirm the importance of studying coke formation. Study of coke formation, the chemical composition of coke and its behavior in the deactivated sample, and the fine details of oxidation is definitely not a simple problem, since there have been no methods making it possible to differentiate between outer-surface and intraporous coke, not to mention coke localization on individual structural elements of zeolites. The only method allowing us to obtain such information is the method of discontinuous sequential micro-oxidation of coke [9-11], which makes it possible to separate the oxidation process into a number of sequential but easily distinguishable stages, where the first stage is oxidation on the outer surface of the zeolite microcrystals and the final stage is oxidation in the very smallest elements of the zeolite structures [10,11].The aim of this work was to use the method of discontinuous sequential micro-oxidation of coke to study zeolite HZSM-5 that has been deactivated and aged for different time periods, in order to establish the behavior of the coke formed. EXPERIMENTALThe HZSM-5 sample was prepared from NaZSM-5 produced by Sorbent AO (Nizhnii Novgorod, Russia, technical specifications TU 38.102168-85, Si/Al = 20.5, static adsorption capacity for water vapor equal to 0.07 g H 2 O per gram). We used ion exchange to do this, replacing the sodium of the original ZSM-5 with ammonium from a 3 M NH 4 NO 3 solution, followed by calcination of the ammoniated form at 550°C. The powdered HZSM-5 formed was tabletted without binders at a pressure of 2·10 4 kg/cm 2 .The catalyst (4 cm 3 , 1-2 mm fraction) was placed in a stainless steel flow-through reactor and dehydrated (2 h) in a stream of air at 500°C. 256

Novel Two-Step Process for the Production of Renewable Aromatic Hydrocarbons from Triacylglycerides

“…Tars/coke formation is unavoidable in acid catalyzed reactions over HZSM-5 catalysts. − However, the results of the current study show that high yields of aromatics may be produced with a relatively low yield of tars/coke in a two-step process, where the tars that form from TG cracking are separated from the OLP prior to the catalytic reaction step (Table ). Thus, the zeolite catalyst is not as severely deactivated as it would be if the process were conducted in a single step.…”