Catalysis thermal stability

Controlling Factors in Homogeneous Transistion-Metal Catalysis Thermal stability of A in defined solutions... 

The present study indicates that the extracellular enzyme, pepsin, exhibits striking differences from its mammalian homologue with respect to optimum pH, Ea for catalysis, thermal stability, and substrate affinity. These data are interesting from the viewpoint of biological adaption at low temperatures, but they also provide some substance to our contention that enzymes from fish plant wastes can have sufficiently unique properties to justify their use over conventional sources of enzymes used as food-processing aids. The relatively low Eas for protein hydrolysis by fish pepsins indicate they may be especially useful for protein modifications at low temperatures. Alternatively, the poor thermal stability of the fish pepsins studied indicate that the enzymes can be inactivated by relatively mild blanching temperatures. The reality of this concept will have to await studies where the pepsins are used as food-processing aids. Such studies are currently underway in our laboratory. 

Enhanced thermal stability enlarges the areas of application of protein films. In particular it might be possible to improve the yield of reactors in biotechnological processes based on enzymatic catalysis, by increasing the temperature of the reaction and using enzymes deposited by the LB technique. Nevertheless, a major technical difficulty is that enzyme films must be deposited on suitable supports, such as small spheres, in order to increase the number of enzyme molecules involved in the process, thus providing a better performance of the reactor. An increased surface-to-volume ratio in the case of spheres will increase the number of enzyme molecules in a fixed reactor volume. Moreover, since the major part of known enzymatic reactions is carried out in liquid phase, protein molecules must be attached chemically to the sphere surface in order to prevent their detachment during operation. 

Fig. 3.35 shows the decrease of the specific surface area of a certain alumina as a function of calcination temperature. Apparently, the alumina is rather stable at 1000 K still over 50 % of the original surface area is retained. For most applications in catalysis the reaction temperature is far below 1000 K, and, as a consequence, the thermal stability of alumina is often sufficient. Activated carbon, which is also often used, is even more stable. 

The chemical and thermal stability of these solid catalysts is also an important advantage for their use, making them resistant to higher reaction temperatures and to a variety of chemical attacks. Moreover, the ease of handling and recovering of the zeolite by a simple filtration, with the possibility of reusing it, are valuable features for a catalysis-based reaction. 

Heterogeneous tandem catalysis involving at least one of the components being supported has also been reported [178, 179]. For example, calcosilicate has recently been used as an effective carrier for simultaneous immobilisation of a dual-functional system based on a bis(imino)pyridine iron compound and a zirconocene to form a heterogeneous catalyst precursor. On activation with triethylaluminium, ethylene was converted to LLDPE the layered structure of the calcosilicate was used to account for the improved thermal stability and higher molecular weights of the LLDPE formed [179],... 

Many types of zeolites are known but only a rather small number of zeolites are used in catalysis. In this section, the most important zeolites will be introduced. We will focus on the most commonly used types which are Zeolite X, Zeolite Y, ZSM-5, and Zeolite Beta. Apart from these, a couple of other zeolites, e.g., Mordenite or Zeolite L, are also used for specific reactions but they are produced on a smaller scale. Most of these zeolites have a remarkable thermal stability and can be heated to a temperature of 600°C without structural damage some of them resist even temperatures of 800 to 1000°C. 

An ionic liquid can be used as a pure solvent or as a co-solvent. An enzyme-ionic liquid system can be operated in a single phase or in multiple phases. Although most research has focused on enzymatic catalysis in ionic liquids, application to whole cell systems has also been reported (272). Besides searches for an alternative non-volatile and polar media with reduced water and orgamc solvents for biocatalysis, significant attention has been paid to the dispersion of enzymes and microorganisms in ionic liquids so that repeated use of the expensive biocatalysts can be realized. Another incentive for biocatalysis in ionic liquid media is to take advantage of the tunability of the solvent properties of the ionic liquids to achieve improved catalytic performance. Because biocatalysts are applied predominantly at lower temperatures (occasionally exceeding 100°C), thermal stability limitations of ionic liquids are typically not a concern. Instead, the solvent properties are most critical to the performance of biocatalysts. 

A-heterocyclic carbenes (NHG) have become increasingly popular in the last few years as an attractive alternative to tertiary phosphines in homogeneous catalysis, due to their strong donating ability and thermal stability. Some examples are shown in Figure 4. For the Suzuki-Miyaura reaction, the first example was reported by Herrmann efa/. 

In principle the bicontinuous 3-dimensional network structure of MCM-48 would act as a good catalytic support.[7] However, its lower hydrothermal and thermal stability has led to much less application of MCM-48 in catalysis. Recently, a family of mesoporous molecular sieves (denoted as MSU-G) with vesicle-like hierarchical structure, worm-like mesoporous structure and bicontinuous nano-porous silica had been synthesized.[8-10] It was proposed that highly accessible mesoporous materials could be obtained through different synthetic procedure and composition. 

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