Acid sites, coke deposition effect

The major results of this study are consistent with a simple picture of mordenite catalysts. An increase in effective pore diameter, whether by extraction or exchange, will increase the rate of transport of reactant and product molecules to and from the active sites. However, aluminum ions are necessary for catalytic activity as aluminum is progressively removed by acid extraction, the number of active sites and the initial activity decrease. Coke deposition is harmful in two ways coke formation as the reaction proceeds will cause a decrease in effective pore diameter and effective diffusivity, and coke deposited on active sites will result in a chemical deactivation as well. 

The novel reactor was used to study the deactivation of n-hexane cracking on an US-Y zeolite catalyst. These experiments showed that on a faujasite the coke formation deactivates the main reactions and not the coking reaction itself, in contrast with previous observations on pentasil zeolites. The coke deposition also modifies the product distribution of n-hexane cracking. This effect can be explained by the non-uniform strength of the acid sites in the Y-zeolite and the acid strength requirements of the various reactions. 

The effect of coke deposition on the MTO reaction is rather complex. Coke was found to influence not only the external DME formation, but also the DME conversion taking place internally during MTO. However, the effect of coke deposition on the DTO reaction is much more simple and allows us to focus on the effect of intracrystalline coke on the DME conversion. The amount of methanol formed from DME at DTO reaction conditions is quite small since methanol formation requires water produced from olefin formation. Thus, the DTO reaction model appears to be straightforward DME diffuses into the pores and is converted to olefins on the acidic sites in the cavities of SAPO-34. 

K added to Pt/AljOj blocks acidic sites of the support. It also substantially decreases the deactivation of the catalyst, diminishing the amounts of carbon deposited not only on the support but also on the metal. The coke deposited on Pt displays a lower degree of polymerization. These effects are probably caused by an electronic modification of the metallic phase due to the K addition. 

It is likely that passivation of the highly active acidic oxidic sites by presulfiding reduces the coke forming tendency. In addition, the small amount of coke deposited on the catalyst during the sulfiding process may also have a passivating effect. This coke apparently has no... 

Influence of the Acid Function.- It has been shown that most of the coke deposition on the bifunctional naphtha reforming catalyst occurs on the acid function. Barbier et al. showed the importance of the acid sites of the support by performing TPO studies of coke from cyclopentane reaction on three catalysts Pt(0.59)/AI2O3 and the same catalyst modified by the addition of H BO and by the addition of KOH. The neutralization of basic sites of alumina by H3BO3 modified the quantity and localization of coke very little, whereas the neutralization of acid sites by KOH produced a decrease of 90% of the quantity of coke deposited on the support. Thus it is apparent that the polymerization leading to coke is essentially of an acid nature. A similar effect was observed in the coking produced... 

The coke deposited on the metal particles, or in the proximity, burns at lower temperature than the coke deposited on the acid sites. This is due to the catalytic effect of the metal (platinum) for the coke combustion reaction. Therefore, depending upon catalyst and reaction conditions, the TPO profile displays two peaks. The low temperature peak, between 250 and 400°C approximately, corresponds to the coke deposited on the metallic phase, and the coke burning above this temperature corresponds to the coke on the acid support (101). Figure 12 shows TPO profiles, that corresponds to unsulfided Pt and Pt-Re catalysts (102). The first peak is evident in both cases. However, the definition of this peak decreases when the total amount of coke increases (102,103). The regeneration of these catalysts at low temperature, for example 350°C, eliminates the small amount of coke that is deposited on the metal, and allows the recovery of the metal activity. 

The addition of water during Cl-VOC oxidation over zeolites considerably reduces the amount of coke deposited due to the steam gasification reaction. However, the preferential adsorption of water molecules over acid sites resulted in a higher deactivation of the catalysts. Thus, the effect of competitive adsorption on acid sites between H2O and Cl-VOC is stronger than the effect of lower coke formation. 

Deposition of metals is not reversible, even with catalyst regeneration. The metals may come into the system via additives, such as silicon compounds used in coke drums to reduce foaming, or feedstock contaminants such as Pb, Fe, As, P, Na, Ca, Mg, or as organometallic compounds in the feed primarily containing Ni and V. The deposition of Ni and V takes place at the pore entrances or near the outer surface of the catalyst, creating a rind layer - effectively choking off access to the interior part of the catalyst, where most of the surface area resides. Metals deposition can damage the acid sites, the metal sites, or both. 

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