Catalysed reactions structure sensitivity

Metal catalysed reactions are differentiated introducing the concept of facile and demanding reactions. In principle a single atom should be adequate for a facile (structure insensitive) reaction, while an ensemble of surface atoms is required to form a catalytic site adequate for demanding (structure sensitive) reactions. Consequently, there are reactions, which requires more than one species to form multiplets " or ensembles. In other words, some reactions depend on the surface geometry e.g. hydrogenolysis of hydrocarbons), while other may not e.g. hydrogenation of olefinic double bond). 

Several sensors have been developed to measure substrates in biological fluids. The structure of the membrane in these biosensors is quite complex. It generally contains a substrate-sensitive layer that includes a specific enzyme to catalyse a reaction and transform the analyte (or substrate). 

Anisole acetylation, which was one of the main reactions investigated, was first shown to be catalysed by zeolite ten years ago by Bayer (13), which was confirmed by Harvey et al. (14), then by Rhodia (15). Large pore zeolites and especially those with a tridimensional pore structure such as HBEA and HFAU were found to be the most active at 80°C, in a batch reactor with an anisole/acetic anhydride molar ratio of 5 and after 6 hours reaction, the yield in methoxyacetophenone (MAP) was close to 70% with HBEA and HFAU zeolites, to 30% with HMOR and 12% with HMFI. With all the zeolites and also with clays and heteropolyacids, the selectivity to the para-isomer was greater than 98%, which indicates that this high selectivity is not due to shape selective effects but rather to the reaction mechanism (electrophilic substitution). The lower conversion observed with HMOR can be related to the monodimensional pore system of this zeolite which is very sensitive to blockage by heavy secondary products. Furthermore, limitations in the desorption of methoxyacetophenone from the narrow pores of HMFI are probably responsible for the low activity of this intermediate pore size zeolite. 

The pH-value and Si02 content of this hydrosol are determined by the concentration of the raw materials and their mixing ratio. Typically acid excess is preferred, as under these conditions the intermediate sol is more stable and the process is less sensitive to feed fluctuations. During the sol-forming step an unstable intermediate-monomeric orthosilicic acid — is formed which then rapidly undergoes an acid-catalysed condensation reaction to form oligomers. When the molecular weight reaches ca. 6000, a sudden increase of both the viscosity and the modulus of elasticity is observed. This increase marks the transformation of the sol to a gel that will then further develop its internal structure. 

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