Catalyst block

The absolute stereochemistry of the products was shown to be consistent with a transition state model in which the benzyl arm of the catalyst blocked the Re-face of the dienophile from approach of the diene (Fig. 3). An attractive face-face n-n... 

In a series of important papers, MacMillan described the alkylation of electron rich aromatic and heteroaromatic nucleophiles with a,P-unsaturated aldehydes, using catalysts based upon the imidazoUdinone scaffold, further establishing the concept and utility of iminium ion activation. In line with the cycloaddition processes described above, the sense of asymmetric induction of these reactions can be rationalised through selective (F)-iminium ion formation between the catalyst and the a,P-unsaturated aldehyde substrate, with the benzyl arm of the catalyst blocking one diastereoface of the reactive Jt-system towards nucleophilic attack (Fig. 3). 

Fouling occurs by decomposition of organometallic compounds of the oil and deposition of the generated metals in the porous structure of the catalyst, blocking the active sites and plugging the pores. Long-term accumulation of these metals in the catalyst pores can result in permanent deactivation of the catalyst. 

Other known chemical poisons for the HTS catalyst are halides, although under normal operating conditions they are not present in the feed at an appreciable concentration. Decay in the catalytic activity was also observed with the feed gas which contained minor amounts of unsaturated hydrocarbons, oxygen, and nitric oxides. Under the HTS conditions these compounds were converted into a heavy carbonaceous residue deposited on to the surface of the catalyst, blocking access of the reactants to the catalytic surface. 

DIFFUSION POISONS This kind of poisoning has already been mentioned in connection with carbon deposition on cracking catalysts. Blocking the pore mouths prevents the reactants from diffusing into the inner surface. Entrained solids in the reactants, or fluids which can react with the catalyst to form a. s.Qlid-ie.si.due, can-cause-this-type o.f-poisoning.------------------------------... 

Figures 3(a-d) show the conversion and evolution of products as a function of time, and activity and selectivity data is presented in Table 1. Under acidic conditions, where the pH was determined by the acidity of the substrate and product acids (pH 2), an initial high rate of conversion was observed and very high selectivity in hydroxypyruvic acid was obtained. However, at about 50% conversion deactivation of the catalyst blocked reaction progress (maximum yield 53% at 58% conversion). At pH 4, the initial rate was reduced slightly but catalyst deactivation did not occur and conversion advanced to 93% after six hours. However, as the reaction progressed the selectivity fell as hydroxypyruvic acid was over-oxidised to oxalic and glycolic acids. At pH 5, conversion was total tdter just four hours, and a maximum yield in hydroxypyruvic acid was obtained after 1.6 hours (74% at 77% conversion). Unfortunately, the...

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