Catalyst deactivation shapes

The points on figure 3 are built using this calculation method. The shape of the obtained deactivation function is different from the common exponential decrease, but it has been experimentally obtained with cracking reactbns on pure zeolites 119], Mann 110] found the same characteristic shape when modelling a catalyst deactivation by pore plugging under diffusion limitation, which is the case of cracking catalysis [8]. 

TPSR for the catalysts deactivated by SO was made with a quadrupole mass spectrometer (MMPC-2(X)D, VG Quadrupoles). Deactivated catalysts were placed in a quartz U-shaped reactor of 1/4" O.D. The reactor was surrounded by a cylindrical electric furnace which was controlled by a PID temperature controller with a K-type thermocouple. After the reactor was fully purged with He (99.9999%) at 50°C, its temperature was ramped from 50 to 800°C at a heating rate of 10°C/min for identification by the mass spectrometer of compounds desorbing from the catalyst surface, e.g. H O, CO, COj, CH4, SOj and SO3. 

Clearly as time on stream increases in order to maintain a specified RONC, the operation temperature should be increased due to the deactivation of the catalyst. The shape of the theoretical temperature plot also shows a normal industrial operation policy. To maintain a specified RONC the reactor temperature is increased, but if a certain level of temperature considered high enough is attained, the base RONC is allowed to decrease, so that the... 

The use of composite catalyst fillings is the industrial standard when treating resids in a modem refinery. The grading can be made both with respect to size, shape and catalyst properties in order to optimize the performance, obtain the desired product quality and the longest catalyst life. The optimization is often a careful balancing of catalyst deactivation and performance, and it is therefore important to understand the mechanisms of deactivation. For this reason, a brief introduction to the various aspects of catalyst deactivation will be given what deactivates the catalysts and how is catalyst deactivation generally tied in with catalyst properties ... 

The observed improvement in stability for ZSM-5 containing catalysts (Figure 1 and Table 1), is mainly due to the unique structure and novel configuration of ZSM-5. The well-known transition-state shape selectivity restricts the formation of aromatic hydrocarbons with carbon number higher than 10 [17], decreasing the rate of formation of heavier aromatics that are believed to be the precursors of coke, that mainly cause catalyst deactivation by occupying or blocking the way to active sites. 

Potassium is used as a dopant on catalysts for the methanation reaction and ammonia synthesis. Its purpose is to increase the rate of the reaction. Potassium is also used on the steam reforming catalyst, not as a promotor but as a dopant that inhibits catalyst deactivation by coke formation (ref. 1). It is reasonable that the role of potassium as a promotor of reaction rates is to lower some barrier to bond dissociation. Since molecular beam techniques afford a convenient means of measuring changes in barrier heights as well as in shapes of the barrier through measurements of the dissociation probability versus energy, the possible effect of potassium on the dissociation of CH4 is investigated. 

Shape selectivity and catalyst deactivation. A serious problem in catalytic cracking and other refinery operations is catalyst deactivation by coking. Coke forms on the catalyst from bulky molecules such as polyalkyl benzenes and polycyclic aromatics that are slow or unable to escape from the catalyst [57], These molecules, in turn are formed mainly from cracked olefins. Coking is severe in zeolites with window-and-supercage structure (chabazite, erionite, Linde A). Zeolites like ZMS-5, with straight channels and no supercages, are much less affected because the formation of bulky coke precursors is sterically inhibited [58]. 

A. Kato, G. Seo and C. Pak. Cerium Impregnated H-Mordenite as a Catalyst for Shape-Selective Isopropylation of Naphthalene. Selective Deactivation of Acid Sites on the External Surface. Appl. Catal. A General, 1995b, 131, 15-32. 

Selective alkylation is therefore highly desired. Zeolites have proven to have excellent properties in this respect, and shape-selective reactions on these materials are well known [14]. Dow Chemical pioneered the shape-selective alkylation of polynuclear aromatics with olefins such as propylene, using as catalysts modified mordenite zeolites, which were not considered at the time to behave strictly as shape-selective catalysts [15,16]. Mordenite zeolites were not the catalyst of choice for such reactions because the alkylation of aromatics, and in particular of polynuclear aromatics, was recognized already as a first step in the reactions leading to coke formation and catalyst deactivation [17]. How was it possible to convert these unsuitable zeolites into stable and highly shape-selective catalysts for industrial applications The answer to this question will be used to illustrate the criteria and methods used to develop the so-called 3-DDM catalysts, or 3-dimensional dealuminated mordenites. 

Of the various possibilities for realizing preferential formation of a single isomer, product shape selectivity is unlikely in this instance, because the indole products do not interconvert, which would rapidly result in plugging of the pores by the bulkier, non-desorbing isomer. This would result in partial catalyst deactivation, residual activity being confined to the unselective external zeolite surface. On the basis of adsorption data for the isomeric indoles from 1-phenyl-2-butanone and phenylhydrazine, Rigutto et al. suggested that restricted transi-... 

Silico)aluminophosphate-molecular sieves are interesting materials as catalysts for the condensation of phenoi with carbonyl compounds, presumably because of the low acidity and the spacious constraints, decreasing catalyst deactivation. Especially ALPO-5, having a very low acid strength, gives good yields in the reaction of phenol with formaldehyde towards dihydroxydiphenylmethanes. In case of the phenol/isobutanai reaction, mainly monosubstituted products are obtained shape selectivity seems to retard a second condensation step. Ortho-substitution prevails, possibly caused by a contribution of Lewis-acid catalysis. 

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