Coked zeolite, diffusivity effect

Various ways to modify ZSM-5 catalyst in order to induce para-selectivity have been described. They include an increase in crystal size (15,17,20) and treatment of the zeolite with a variety of modifying agents such as compounds of phosphorus (15,18), magnesium (15), boron (16), silicon (21), antimony (20), and with coke (14,18). Possible explanations of how these modifications may account for the observed selectivity changes have been presented (17) and a mathematical theory has been developed (22). A general description of the effect of diffusion on selectivity in simple parallel reactions has been given by Weisz (23). 

We interprete the above effects as conventional product shape selectivity inside the pore system of zeolite ZSM-5 or ZSM-11, and part of our arguments were presented earlier, in a preliminary note [28]. While the catalyst is on stream, coke is gradually formed and deposits, in part, inside the channel system. As a consequence, the diffusion pathways for product molecules increase. Slim molecules, such as 2,6-dimethylnaphthalene are less affected than... 

Obviously the acidity and the pore structure of the zeolite catalysts play a significant role in coke formation. These parameters influence both the reactions involved in the formation of coke molecules and their retention. Thus the stronger the acid sites the faster the reactions and the slower the diffusion of basic intermediates hence the faster the coke formation. The density of the acid sites has also a positive effect on coke formation, which can be related to the intervention of... 

The total acidity deterioration and the acidity strength distribution of a catalyst prepared from a H-ZSM-5 zeolite has been studied in the MTG process carried out in catalytic chamber and in an isothermal fixed bed integral reactor. The acidity deterioration has been related to coke deposition. The evolution of the acidic structure and of coke deposition has been analysed in situ, by diffuse reflectance FTIR in a catalytic chamber. The effect of operating conditions (time on stream and temperature) on acidity deterioration, coke deposition and coke nature has been studied from experiments in a fixed integral reactor. The technique for studying acidity yields a reproducible measurement of total acidity and acidity strength distribution of the catalyst deactivated by coke. The NH3 adsorption-desorption is measured by combination of scanning differential calorimetry and the FTIR analysis of the products desorbed. 

The classical method of investigation of effects of diffusion on reactions is typically to run a reaction with catalyst particles of various sizes. For zeolites, the resistance of intracrystalline diffusion is normally much larger than that characteristic of molecular diffusion or Knudsen diffusion that could occur in the spaces between the zeolite crystals in a catalyst particle. Thus, the crystal size of the zeolite has to be varied instead of the particle size to determine the effects of diffusion on zeolite-catalyzed reactions. Kinetics of the MTO reaction has been measured with SAPO-34 crystals with identical compositions and sizes of 0.25 and 2.5 pm 89). The methanol conversion was measured as a function of the coke content of the two SAPO-34 crystals in the TEOM reactor. 

This case study clearly illustrates the usefulness of the ZLD-TEOM technique in determining intracrystalline diffusivities in zeolites, provided that effects of other transport resistances such as the surface barrier are eliminated by varying the crystal size of the zeolites. The measured steady-state diffusivity can be directly used for predicting effects of diffusion in reactions catalyzed by zeolites. More important, the TEOM makes it possible to distinguish the deactivation caused by blockage of the active sites and by increased diffusion resistance caused by blockage of cavities or channels by coke. 

High sensitivity, fast response, and well-defined flow patterns make the TEOM an excellent tool for determining diffusivities of hydrocarbons in zeolites. Moreover, the TEOM has provided a unique capability for gaining knowledge about the effects of coke deposition on adsorption and diffusion under catalytic reaction conditions. An application of the TEOM in zeolite catalysis by combining several approaches mentioned above can lead to a much more detailed understanding of the catalytic processes, including the mechanisms of reaction, coke formation, and deactivation. 

Molecular exchange between the crystallites and the intercrystalline space may, however, be controlled by processes other than ordinary diffusion. A substantial retardation of molecular exchange may be caused by transport resistances on the external surface of the crystallites. It has been shown in PFG NMR studies that such surface barriers may be brought about during the process of zeolite manufacturing (e.g. by hydrothermal treatment) [1,6] and by coke depositions [1,7]. In this case, irrespective of possibly large rates of molecular redistribution within the crystallites, the rate of molecular escape out of the crystallites may be slowed down dramatically. In effect, in this case, the product molecules should be distributed essentially homogeneously over the whole space of the individual crystallites. 

The coke deposited during the isobutane alkylation with C4 olefins on zeolites was extracted with a mixture of methanol/toluene, and with CI2CH2. None of these solvents were effective to remove the coke. Only a very small fraction was removed, which was attributed to the dissolution of external coke. The coke molecules formed inside the channels have a large size and therefore cannot diffuse out of the pores . 

The weak difference of coke composition observed from propene and isobutene (Table 1) confirms that at low temperature (T=100°C), the more pronounced effect of coke firom isobutene on the adsorption capacity (Fig.Sa) is due to the difference in coke location inside the zeolite pores. The distribution of ol omer molecules inside the zeolite crystallites would be more heterogeneous from isobutene. Indeed, owing to the olefinic character of isobutene and this slower diffusion rate in the zeolite pores, oligomers will be preferentially formed in the a cages close to the outer surface of the crystallites. 

However the effect of the zeolite pore structure is not limited to steric constraints on the formation of the intermediates of coking. Indeed the contact time of the organic molecule with the active sites depends on the rate of diffusion of these molecules hence on the characteristics of the diffusion path inside the zeolite crystallites length (related to the crystallite size), size of the pore apertures, of the cavities (or channel intersection) tortuosity of the channel, acid site density. The slower the diffusion of the organic molecules the greater the coking rate [3]. 

Medium-pore zeolites have been shown to have excellent restricted transition state selectivity [164,166]. The high resistance toward coke formation on medium-pore zeolites has also been attributed to this type of shape selectivity [21,167-170]. Closely related to the shape selectivity on ZSM-5 is the effect of molecular traffic control [170]. This results from the existence of two types of intersecting channels reactant molecules preferentially enter the catalyst through a given chaimel system while the products diffuse out through the other, so that counter diffusion limitations are avoided. 

---