Catalytic Effects of Impurities

Traces of impurities, such as peroxides, rust or, metal ions originating from material corrosion, may catalyze decomposition reactions. Such effects can easily be 

Why did the explosion occur, despite the fact that there was no temperature difference in the experiment described above  

By using this thermogram, do you think that the intended operation is possible without any thermal risks Explain your arguments. 

Figu re 11.21 DSC thermograms of the reactants mixed at room temperature (upper) and of the final reaction mass (lower). 

Assess the thermal risks linked to the industrial performance of the process. 

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A runaway decomposition in a melting vessel associated with distillation of the crude product burst the vessel and led to a major fire. The short report does not include detailed circumstances. The decomposition was attributed to the catalytic effects of impurities. However, it seems that nitric acid was also present. 

Typical examples of mononucleation are the deposition of silver on electrolyticaUy grown Ag(lOO) crystals (Fig. 3) [156-159], and the formation of 2D condensed organic films, such as isoquinoline [160], thymine [163], or coumarin [162-164] on mercury electrodes. These experiments demonstrate that the rate of nucleation is more strongly dependent on the applied supersaturation than the growth rate. Extreme care has to be taken in mononucleation experiments to avoid artifacts from edges or defect contributions of the substrate or catalytic effects of impurities [161,162]. 

Organic peroxides and hydroperoxides are generally unstable and can decompose spontaneously and explosively under thermal and mechanical stress. Such decomposition may be caused by shock, impact, friction, or the catalytic effect of impurities. To reduce hazards involved during transportation and handling, they are desensitized by the addition of inert inorganic solids or liquids like water, halogenated hydrocarbons. 

Refining and Isomerization. Whatever chlorination process is used, the cmde product is separated by distillation. In successive steps, residual butadiene is stripped for recycle, impurities boiling between butadiene (—5° C) and 3,4-dichloto-l-butene [760-23-6] (123°C) are separated and discarded, the 3,4 isomer is produced, and 1,4 isomers (140—150°C) are separated from higher boiling by-products. Distillation is typically carried out continuously at reduced pressure in corrosion-resistant columns. Ferrous materials are avoided because of catalytic effects of dissolved metal as well as unacceptable corrosion rates. Nickel is satisfactory as long as the process streams are kept extremely dry. 

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

The catalytic effect of graphite A thus depends on iron impurities, e. g. Fe304, and probably also on iron sulfides or sulfates, because sulfur is also present in this graphite, and all these iron compounds are known catalysts of FC acylation [69, 73, 74], In this respect, it seems that FeCl3 could be the true catalyst generated in situ by the reaction of the different iron compounds with acid chloride and hydrogen chloride. In the... 

It is important to clarify that there have been, in the literature, some examples of electrochemical processes on CNT-modified electrodes on which an apparent electrocatalytic process associated to the CNTs seems to take place (that is from the edge-plane-like sites) where in fact that was not the case. An example is the apparent electrocatalytic oxidation ofhydrazine at MWNTelectrodes [64,65]. Such electrochemical behavior has been demonstrated to be a consequence of iron impurities contained in the CNTs that were responsible for the observed electrocatalytic effects (Figure 3.7). Therefore, caution is needed when reporting catalytic effects of CNTs under a given redox system and a careful comparison vdth, for instance, edge HOPG is mandatory to make sure that the CNTs are the responsible for the electrochemical enhancement. 

We also made a few measurements as a function of ionic strength at pH = 3 and 13. The results at pH = 13 gave log k = 1.33 0.01 min 1 M-1 for four measurements between I = 0 to 3m. At a pH = 3 in dilute solutions below 0.04M, no ionic strength dependence was found however, at I = 3.0m, the rate was ten times faster that at I = 0. We attribute this increase in rate to the presence of trace metals. All of our runs at pH = 8 to 13 were made with enough borax to complex these trace metals and suppress the catalytic effect. An experiment at pH = 11 without borax was completed within 5 minutes compared to 1.5 hours with 0.01 M borax. These results support our contention that the effect of ionic strength on the rates of oxidation are independent of pH if the catalytic effects of trace impurities are avoided. 

If amidophosphite esters are used, the formation of C(3)-substitution products with a yield of more than 30% is observed along with N-phosphorylation. The authors explain this by the catalytic effect of the amine hydrochloride impurities. 

TEA masks the catalytic effect of metallic impurities found in the rain water. The concentrations and the rates of oxidation of S(IV) in rain waters from Yokohama, Japan measured by this method were 0.8-23.5 yM (16 samples) and 0.12-3.3 hr 1 (8 samples), respectively. 

The present research is an experimental and modeling study of the effect of impurity poisoning on the behavior of a diffusion-affected catalytic reaction. Benzene hydrogenation to cyclohexane over a Ni/Al203 catalyst poisoned by thiophene was used as a model reaction. 

Properties (pure anhydrous) density of solid, 1.71 g/cc, density of liquid 1.450 g/cc at 20C, viscosity, liquid 1.245 cP, surface tension 80.4 dynes/cm at 20C, fp -0.41C, bp 150.2C. Soluble in water and alcohol. (Solutions) pure hydrogen peroxide solutions, completely free from contamination, are highly stable a low percentage of an inhibitor such as acetanilide or sodium stannate, is usually added to counteract the catalytic effect of traces of impurities such as iron, copper, and other heavy metals. A relatively stable sample of hydrogen peroxide typically, decomposes at the rate of approximately 0.5% per year at room temperature. 

General Determine corrosion rates of potential construction materials. Screen for catalytic effects of potential materials, corrosion products, and feed impurities. 

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