Study on the deactivation of nanocrystalline H[Fe,Al]ZSM-5 zeolite in DTG reaction

Catalyst deactivation due to coke formation is a key issue in dimethyl ether to gasoline (DTG) process, and it is significant to investigate the change of catalyst properties during reaction to inhibit the rapid deactivation of catalyst. Here, the nanocrystalline H[Fe,Al]ZSM-5 zeolite synthesized hydrothermally was evaluated as catalyst for DTG process in different reaction time. The detailed characterizations of fresh and spent catalysts were carried out, such as UV-Vis, XRD, SEM, N₂ adsorption-desorption and NH₃-TPD, and the coke formation and its location in catalyst were studied. The results indicated that DTG reaction mainly occurred at strong acid sites of nanocrystalline H[Fe,Al]ZSM-5, and Brønsted acid sites were the most active. Furthermore, the catalysts even with small amount of B acid still possessed the higher catalytic activity during stable period of DTG reaction. The coke deposition was produced in DTG reaction, and at the initial stage, coke precursors including alkylbenzenes retained inside the micropores. These precursors grow gradually to form polycyclic aromatic hydrocarbons, which overflow on the outer surface with increasing reaction time, and then mainly accumulates on the external surface of catalyst. The coke occludes the active sites and hampers the molecular diffusion, eventually causing activity loss. The catalyst deactivation was mainly caused by the blockage of micropores and coverage of B acid sites due to coke deposition. And the deactivated catalysts could be oxidized and regenerated in air at 550–650 °C.