Two-stage crystallisation of the MFI zeolite: kinetic control for efficient catalyst development

A detailed investigation of the MFI zeolite crystallisation mechanism in an alkaline synthetic gel system has been carried out using XRD, SEM, nitrogen adsorption, ammonia TPD, elemental analysis, NMR and FTIR spectroscopy. During the first stage of the synthesis, a highly crystalline zeolite is formed via solid hydrogel transformation mechanism, while for a prolonged synthesis, a second stage of crystallisation is clearly distinguished. The zeolite yield and its Si/Al ratio increase during the second stage, whereas the number of defect sites and the mesopore volume decrease. The second stage leads to increased diffusion limitations owing to the blockage of the transport mesopores and to shortened catalyst lifetime in the MTH and butenes oligomerisation processes as well as to a lower activity in toluene alkylation with methanol. Identification of the second stage of crystallisation potentially allows for fine tuning of the zeolite catalytic properties by varying the crystallisation time.