High regioselectivity with respect

High regioselectivity with respect to the allylic system for the desired intermediate 1 is obtained with a HY zeolite with weak acidities (low Si/Al ratio and partial Na+→H+ exchange). Stronger acidities lead to further condensation and larger amounts of 1,1,3-thriphenylpropane 2 that not only decrease selectivity but also deactivate the catalyst.The control of acid strength as well as the density of acid sites of zeolite catalysts has also led to successful catalysts and processes in the field of oil refining and petro-chemistry.Sometimes, the acid site density is even more important than acid strength. This has an important impact on adsorption properties and therefore can be used to control selectivity when uni- and bimolecular reactions compete.Then, zeolites with a low density of Brønsted sites (low density of framework T III cations or high TIV/TIII ratios) will favor unimolecular reactions. On the other hand, high density of TIII atoms will favor bimolecular reactions by increasing the adsorption of reactants. This factor is being used together with the control of pore dimensions to regulate the ratio of xylene isomerization (unimolecular) versus xylene disproportionation to toluene and trimethylbenzenes (bimolecular).[9]It was found that the silanol nest is a more favorable site for glycine adsorption due to a local increased H-bond density. Co-adsorption with water is also favored, especially a water molecule between a Si-OH and the ammonium moiety. NMR chemical shifts computed on these models fall into the observed 13C and 15N experimental range, suggesting that the presence of different energetically comparable adsorption configurations cannot be excluded.