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

Abstract

Along with oxidation of hydrogen-containing coke structures, we have observed oxidation of compact accumulations of hydrogen-free coke and as a result we can quantitatively differentiate between outer-surface and intraporous burned coke, and the intraporous coke in turn can be quantitatively distributed over the different elements of the zeolite structures.Coke formation is a very important reaction accompanying all carbonium-ion conversions of hydrocarbons. In this case, the coke plays a dual role. In small amounts, it promotes the occurrence of the major reaction [1], speeding up the movement of hydrogen, as a primary ingredient in carbonium ion conversions in all its forms [2]. But when accumulating on the surface of the catalyst, coke blocks the active sites, necessitating the process step of oxidative regeneration. Therefore information is needed about the chemical composition of coke and its localization within the crystal structure of the catalyst. Such information is complicated to obtain because of the indefinite composition of coke at a specific instant of time and also difficulties encountered in extracting this substance in unaltered form from the catalyst, as much as the at least equal difficulties involved in studying it without extraction [3,4].Earlier [5], in a study of the kinetics of regeneration of single grains of coked zeolite by the microbalance method in a stream of oxygen-containing gas in the temperature range 350-510°C, some variability (periods of slowdown and acceleration) in combustion over time was observed at 350, 360, and 375°C, which may indicate oxidation of coke of different elemental compositions or coke localized differently in the zeolite structure.The method of discontinuous sequential micro-oxidation of coke [6] was developed recently, which makes it possible to determine some characteristic features of coke composition and coke localization in the structure of acid zeolite catalysts for different purposes: alkylation of isoparaffins by olefins, isomerization of linear paraffins, disproportionation of monoalkyl aromatic hydrocarbons to form benzene and dimethyl-substituted aromatics.Each of these catalysts is prepared differently: by modification of the original forms by replacing the native sodium cations by other cations [7] or by reducing the latter (for example, nickel cations) to the zero-valence state [8]. Methods for such modification have been sufficiently developed.Considerably less attention has been focused on dealumination of the outer surface of zeolite crystals to avoid formation of outer-surface acid sites while grafting acidity into the modified samples. Moreover, these sites, as the most accessible to reactant molecules, may play a dominant role in the occurrence of the corresponding reactions, including coke 198 0040-5760/09/4503-0198

Oscillatory motion of coke in deactivated HZSM-5 zeolite

“…Since coke oxidation occurs under high oxygen deficiency conditions [10,11], we should assume that the coke will be burned just to the point when its accessibility to oxygen molecules decreases. Of course, the coke on the outer surface of the zeolite microcrystals is the most accessible.…”

“…Each of the samples 1-7, weighing 100 mg in the air-dried state (corresponding to a weight of 93 mg for the dehydrated sample), was loaded into the microreactor of the apparatus for discontinuous sequential micro-oxidation of coke and then heated according to the temperature program (11 deg/min) in a stream of helium up to 500°C, and held at this temperature for 1 h, after which oxygen was supplied as pulses (1 cm 3 each) [10,11], where the number of pulses was brought up to 70 in the case of samples 1-4 and was limited to 20 pulses for samples 5-7, in order to shorten the time intervals between examinations from 30-100 days (samples 1-4) down to 24 h (samples 5-7).…”

“…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. …”

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

“…Then the disappearance of the acceleration effect when the DPM feed stops may be due to transition of the DPM molecules from the outer surface of the catalyst into the pores, where these molecules are tightly held and no longer play any role in the surface disproportionation reaction. 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.…”

“…Hence the observed increase in toluene conversion in the presence of DPM, i.e., DPM acts as a cocatalyst for the conversion. We can assume that disproportionation in the presence of acid sites on the outer surface of the zeolite crystals is realized mainly specifically at these sites, because of their significantly greater (by many orders of magnitude) accessibility for the reactant molecules compared with the accessibility of the intrachannel acid sites [11,12]. Then the disappearance of the acceleration effect when the DPM feed stops may be due to transition of the DPM molecules from the outer surface of the catalyst into the pores, where these molecules are tightly held and no longer play any role in the surface disproportionation reaction.…”