Strong Lewis bases, titration

Poisons can be used to titrate the active sites many Lewis bases chemisorb strongly onto sites and block polymerization. For example, Hogan [40-42] added triethylamine to a reactor in which polymerization was already occurring on Cr/silica. The polymerization could be retarded or even stopped by the addition of trace amounts of the amine. In one experiment, he injected 0.057 mol of NEt3 per mol of Cr after polymerization had reached a nearly constant rate. The activity immediately dropped by 32%. In a second run, he injected 0.108 mol of NEt3 per mol of Cr,... 

A facile explanation of the failure of acidity to enhance activity of any of the catalysts while base enhances the activity of a poor catalyst without affecting the active ones is that the Claus active catalysts owe their activity to basic sites. In general, alumina shows strong surface acidity when activated at 450 °C in vacuum, and intensive investigation has shown this to happen in Lewis acid sites (8). There is also evidence that basic sites occur on alumina. Schwab and Krai (9) obtained evidence for basic sites by boron trifluoride adsorption on alumina, and Yamadaya et al. (10) reported up to 0.4 mmole/g of basic sites on alumina from benzoic acid titrations. Pines and Manassen (11) obtained indirect evidence of such sites from studies of the dehydration of primary alcohols. 

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