Catalyst stability, physical

For catalyst design purposes it is first necessary to translate the catalyst performance parameters into a physical picture of catalyst structure. As we shall see, different performance parameters can give rise to different structural features and so a compromise is generally required. For example it is commonly found in industrial applications that initial catalyst activity may be sacrificed in favour of improved catalyst stability, since a lower activity and a prolonged operating catalyst life is in general preferable to a higher initial activity that decays rapidly. First, we should therefore discuss some of the relationships between the catalyst performance parameters and physical structure. 

B.J. Arena of UOP [16] has studied recently the use of Ru/AloOj catalysts in the fixed bed hydrogenation of glucose. Arena has identified physical deterioration of the alumina support and poisoning by gluconic acid, sulphur and iron as possible causes of catalyst deactivation. When some of these factors were minimised, substantial improvements in catalyst stability were observed. 

The results described herewith demonstrate that the activity of Ru metathesis catalysts can be enhanced by introduction of EWGs without detriment to catalysts stability. This principle can be used not only to increase the catalyst activity, but also to alter its physical-chemical properties, such as solubility in given medium or affinity to silica gel. An example of novel immobilization strategy, based on this concept is presented. In fact, the possibility of reversibly binding catalysts to a solid phase is of major importance for industrial applications, particularly when continuous flow processes with immobilised homogeneous catalysts are pursued. 

Since commercial processes require catalysts with physical and chemical stability, high activity, high selectivity, catalyst regenerability, etc., the evaluation effort may be time consuming and costly. It may be justified since the overall economics and eventual profitability are at stake. 

The aqueous phase into which the monomer mix is dispersed is also prepared in a separate tank before transferring to the copolymerization ketde. It contains a catalyst, such as benzoyl peroxide [94-36-0], to initiate and sustain the polymerization reaction, and chemicals that aid in stabilizing the emulsion after the desired degree of dispersion is achieved. Careful adherence to predeterrnined reaction time and temperature profiles for each copolymer formulation is necessary to assure good physical durabiHty of the final ion-exchange product. 

Trimerization to isocyanurates (Scheme 4.14) is commonly used as a method for modifying the physical properties of both raw materials and polymeric products. For example, trimerization of aliphatic isocyanates is used to increase monomer functionality and reduce volatility (Section 4.2.2). This is especially important in raw materials for coatings applications where higher functionality is needed for crosslinking and decreased volatility is essential to reduce VOCs. Another application is rigid isocyanurate foams for insulation and structural support (Section 4.1.1) where trimerization is utilized to increase thermal stability and reduce combustibility and smoke formation. Effective trimer catalysts include potassium salts of carboxylic acids and quaternary ammonium salts for aliphatic isocyanates and Mannich bases for aromatic isocyanates. 

For the support material of electro-catalysts in PEMFC, Vulcan XC72(Cabot) has been widely used. This carbon black has been successfully employed for the fuel cell applications for its good electric conductivity and high chemical/physical stability. But higher amount of active metals in the electro-catalysts, compared to the general purpose catalysts, make it difficult to control the metal size and the degree of distribution. This is mainly because of the restricted surface area of Vulcan XC72 carbon black. Thus complex and careM processes are necessary to get well dispersed fine active metal particles[4,5]. 

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