Life of catalysts

Asymmetric synthesis is a method for direct synthesis of optically active amino acids and finding efficient catalysts is a great target for researchers. Many exceUent reviews have been pubHshed (72). Asymmetric syntheses are classified as either enantioselective or diastereoselective reactions. Asymmetric hydrogenation has been appHed for practical manufacturing of l-DOPA and t-phenylalanine, but conventional methods have not been exceeded because of the short life of catalysts. An example of an enantio selective reaction, asymmetric hydrogenation of a-acetamidoacryHc acid derivatives, eg, Z-2-acetamidocinnamic acid [55065-02-6] (6), is shown below and in Table 4 (73). 

Establish procedures for testing and verification of catalyst activity and identification including use testing. Include procedure to monitor shelf life of catalyst to maintain activity... 

Catalyst lives were compared with reactor metal distribution, obtained from micro, pilot and commercial units. It was found that a micro reactor with low liquid mass velocity caused metal pass-through to down stream catalysts, resulting in a shorter life of catalyst system than that in the commercial operation. Therefore in this paper, effects of liquid mass velocity and feedstock properties on catalyst aging performance of the catalyst system are discussed. 

Catalysts based on CuO-ZnO are of great industrial interest because they exhibit high activity for the low temperature-pressure methanol synthesis and the water-gas-shift reactions. It is known that the activity and useful life of catalysts depend mainly on the activation process and the thermal history they experience during the operation. In the low temperature water gas shift (LTWGS) process, prior to reaction, the catalyst is activated by gas reduction to convert copper oxide into metallic copper [1]. It has been observed that reduction conditions affect the activity and the stability of Cu-ZnO catalysts. For instance, sintering and formation of alloys must be avoided in the reduction step because they deactivate the catalyst [2-3] for the water-gas-shift reaction. 

The active agent is the component(s) that causes the main catalytic action. Without it, the catalyst will have no effect. A promoter is a substance added into the catalyst to improve either of the activity, selectivity or stability so as to prolong the life of catalyst. The promoter is often added in a small amount and by itself has little activity. There are various types of promoters, depending on how they improve the catalyst. Textural promoters are inert substance which inhibit sintering of the active catalyst by being present in the form of very fine particles, usually they have a smaller particle size than that of the active species, are well dispersed, do not react with or form a solid solution with the active catalyst and have a... 

Moisture in feed stream, the presence of gases like CI2, CO2 or thermal shocks due to frequent starting and stopping of plant can be detrimental to life of catalysts. 

Result in reducing the side reactions and increase life of catalyst. 

MoM LTD Production of benzene and Cg aromatics from toluene 46 260-315 Zeolite Service life of catalyst around 1.5 years... 

Compounds of sulfur, chlorine, phosphorus and arsenic. The compounds of sulfur, chlorine, arsenic and phosphorus will cause permanent poisoning of catalysts. The sulfides in the synthesis gas are commonly larger than the compounds of phosphorus, chlorine and arsenic, and the toxic effect of sulfides to catalysts is more serious than that of phosphorus, chlorine and arsenic. The following discussion will focus on the poisoning effect of sulfur and its relation to the life of catalyst. 

For example, the life of catalyst in some plants is less than a year, the shortest period being less than 100 days (as plant A in Table 8.33), it is often suspected that the catalyst itself is not good or has poor anti-poisoning property. In reality, any metal catalyst, including the Fe catalyst that all can not resist to sulfur poisoning. Metal catalysts can not have anti-sulfur poisoning ability. In order to prove this view, the sulfur content is measured in waste catalysts discharged from these plants, as shown in Table 8.33, to identify the cause of short life. 

Life of catalysts due to the sintering of active species must be carefully tested since such supports interact weakly with active species. 

Side reactions are usually dehydration of alcohol to form olefins and ethers and sometimes polymerization. Dissolution of catalytically active sites on the surface (e.g., — SO3H group in the case of ion-exchange resin) and thermal stability, which shorten the life of catalysts, are other important considerations in the use of solid acids. 

This relationship states that the uptake rate of poisoning species by the catalyst is equal to the rate at which the poisoning takes place. The pseudo steady state assumption has been made with respect to concentrations. Validity of this assumption comes from the fact that the rate of poisoning is much slower than the rate at which the concentrations reach steady state. If this were not true, the catalyst would lose its usefulness because it would have to be regenerated too frequently. For instance, the useful life of catalyst pellets is of the order of months, whereas the time required to reach steady state is of the order of... 

On the other hand, Bibby et al. (ref.5) reported that coke is predominantly formed inside the crystals, and proposed that catalysts requiring the least frequent regeneration will have a relatively low density of active sites and small particle size. Actually, it has been reported (ref.6) that the life of catalysts containing ZSM-5 for conveirtlng hydrocarbons increased with decreasing the size of the zeolite crystals. 

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