Control the position of framework defects in zeolites by changing the symmetry of organic structure directing agents

Double quantum-single quantum (DQ/SQ) 1H NMR spectroscopy has been used to localize the siloxy/silanol defects in two pure silica MFI zeolite samples synthesized by the symmetric tetrapropylammonium hydroxide (TPAOH) and non-symmetric methyltributylammonium hydroxide (MeTrBA) templates. Using the 1H DQ/SQ NMR, we provide important information on the positions of the structural defects in the MFI zeolite framework that have been tuned during the hydrothermal synthesis by changing the symmetry of the templates. The preferential location of the defects is found to be near the terminal methyl groups of the longer alkyl chains and far from the nitrogen atoms: in the case of the non-symmetric MeTrBA template, the defects are located in the sinusoidal channels, while for the symmetric TPA template they are randomly distributed among the two types of channels. It is foreseen that control the position of framework defects in zeolites might be the way to control the position of the active sites in acid zeolite catalysts.