Hindered basic probes

Internal versus External and Extraframework Sites in Zeolite Acid Catalysis the Use of Hindered Basic Probes Catalytically active sites also exist on the external surface and at the pore mouth of zeolite crystals. These sites are considered to be responsible for unwanted non-selective catalysis. On the other hand, H-zeoUtes also catalyze reactions of molecules that do not enter the cavities because of their larger size. So, the external surface of zeolites is certainly active in acid catalysis. Additionally, the bulk and surface Si/Al compositions of a zeolite could be different and different preparation procedures can be chosen to modify this ratio. 

Nitroindoline (Nl, 9) possesses the above-described electronic and structural features. It is an N-H acid with a single acid site borne by a donor group whose free rotation is hindered by an ethylene bridge on the ring. However, if charge transfer endows the compound with appropriate acid properties that increase with electronic excitation, the basicity of the acceptor (nitro) group will also increase and the polarity of the compound will be altered as a result. Both effects will influence the electronic transition of the probe that is to be used to evaluate basicity. 

Fig. 20.31 Basic principles of scanning electrochemical microscopy (SECM). (A) Hemispherical diffusion far from electrode (B) feedback diffusion near a conductive electrode (C) hindered diffusion near an insulating electrode surface. The probe s electroactive area is shaded in the diagrams. (Reproduced with permission from Ref. 157.)...

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