Catalyst deactivation high-activity catalysts

Cost of the catalyst. The transition metals used, such as rhodium, ruthenium, iridium or palladium, are extremely expensive. The same holds for complicated chiral ligands that often take six to ten synthetic steps for their production. An excellent way to beat these costs is to develop a highly active catalyst. A substrate catalyst ratio (SCR) of 1000 is often quoted as a minimum requirement. In the celebrated Metolachlor process, a SCR of over 100000 is possible. Factors determining the rate of reaction are numerous and often poorly understood. Deactivation of the catalyst also has a profound effect on the overall rate of the reaction. 

Catalysts for coal liquefaction require specific properties. Catalysts of higher hydrogenation activity, supported on nonpolar supports, such as tita-nia, carbon, and Ca-modified alumina, are reasonable for the second stage of upgrading, because crude coal liquids contain heavy polar and/or basic polyaromatics, which tend to adsorb strongly on the catalyst surface, leading to coke formation and catalyst deactivation. High dispersion of the catalytic species on the support is very essential in this instance. The catalyst/support interactions need to be better understood. It has been reported that such interactions lead to chemical activation of the substrate 127). This is discussed in more detail in Section XIII. 

It can be seen that Ni/BaTiOj catalyst possesses high activity and does not deactivate for a long time under high reactive temperature. Whereas, though Ni based catalysts such as Ni/y-AljOj also show high activity, they often deactivate rapidly due to carbon deposition or the crystal structure transformation of the support occurring at high reactive temperature, which hinders the process from industrial applications. 

Catalyst performance is determinedby activity, selectivity andstability. Whereas activity is indispensable, selectivity is often of prime importance (e.g. lube base oil yield in catalytic dewaxing), particularly if an improved selectivity can break a bottleneck in a unit (e.g. by lower gas makes which break up the gas train bottleneck a in a hydrocracking unit). Catalyst life is determined both by the start of run activity and deactivation rate. With high activity catalysts in low severity duty (e.g. naphtha hydrotreating), catalyst life can be very long (e.g. 5-10 years), and in some cases the... 

High dust separation efficiency Dust should not markedly penetrate the filter struc> ture since this would lead to pore obstruction and/or to catalyst deactivation. High catalytic activity so as to attain nearly complete catalytic abatement for conveniently high superficial velocities, i.e., those employed industrially for dust filtration 10-80 m/hr. 

Figure 4 shows that activities of several kinds of iron oxide based catalysts. A Fe/Ca/Al oxides catalyst exhibited the best performance among the catalysts tested. Fe/Ca/Al and Fe/Al oxides catalysts were highly active, whereas Fe/Ca and Ca/Al oxides catalyst were extremely low in activity. The selectivities of Fe/Al oxides and Fe/Ca/Al oxides catalysts were almost the same (97% at 5.25 h), and the main by-products were benzene and toluene. Therefore the addition of an optimum amount of CaO to Fe/Al based catalyst could suppress the deactivation of the catalyst during long term reaction. Further experiment are under achievement to elucidate precisely the role of CaO. 

The entrained-flow reactor (Fig. 8.5) is used when very short contact times are required, as in the case of highly active catalysts that deactivate fast. In fluid catalytic cracking (FCC) the circulating catalyst also supplies part of the heat for the endothermic reaction. Depending on the catalyst loading one can distinguish dilute and dense phase risers. ... 

The key role of dehydrogenation catalysts is to accelerate the main reaction while controlling other reactions. Unmodified alumina-supported platinum catalysts are highly active but are not selective to dehydrogenation. Various by-products, as indicated in Figs. 5 and 6, can also form. In addition, the catalyst rapidly deactivates because of fouling by heavy carbonaceous materials. Therefore, the properties of platinum and the alumina support need to be modified to suppress the formation of by-products and to increase catalytic stability. 

In order to prolong the time between regenerations and shutdowns, the reactor tube may be made longer than required for the reaction itself. For example, suppose a length of 3 ft is necessary to approach the equilibrium conversion with fresh catalyst of high activity. The reactor may be built with tubes 10 ft long. Initially, the desired conversion will be obtained in the first 3 ft. As the catalyst activity falls off, the section of the bed in which the reaction is mainly accomplished will move up the bed, until finally all 10 ft are deactivated. This technique can be used only with certain types of reactions but it has been employed successfully in the ammonia synthesis. 

The flame synthesis is a promising method for producing catalysts with high activity and stability. CuZnCeAl showed almost no deactivation and was the most active catalyst after about 10 h on stream. The catalysts can be produced in one step and the production on a larger scale could be beneficial compared to conventional methods. Ti02 and Si02 are already today produced in large quantities by flame synthesis [10]. 

In the Pd(0)-Pd(II) catalytic system, a large part of the catalytic cycle is essentially occupied by the oxidation state of Pd(II) (Scheme 1). Once the cycle is closed, the state of Pd(0) is set in the final stage of regeneration of the initial species. However, much effort has been devoted to the development of the active chiral Pd(0) complex from Pd(0) or Pd(II) precatalysts, with scrupulous care for the deactivation to Pd metal (mirror) or Pd black. On the contrary, another solution to devise a highly active catalyst is to keep the oxidation state of +2 (i.e., Pd(II)) throughout the catalytic cycle without dropping out to Pd(0). This new approach is based on the Pd(II)-Pd(II) cycle in contrast to the... 

However, above 413 K and also on the pre-oxidised catalyst, the high activity and selectivity towards nitrogen sustains. The presence of oxygen at the platinum surface apparently does not cause a permanent deactivation of the catalyst. Above 413 K, the catalyst is reduced by ammonia. 

Commercial Ni-based reforming catalysts exhibit high activity and selectivity for tar conversion into hydrogen-rich gas, but suffer from a number of severe limitations regarding their other properties such as mechanical fragility rapid deactivation, mostly due to sulphur, chlorine, alkali metals and coke formation metal sintering. The overall effect is a limited active lifetime [2]. 

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