Effect of ZSM-5 crystal size on its catalytic properties for conversion of methanol to gasoline

Four-sized monodispersed ZSM-5 crystals, being 70, 200, 400 and 650 nm, were hydrothermally synthesized by changing the H₂O/Si molar ratio in the synthesis gel, and characterized with XRD, TEM, BET and NH₃-TPD techniques. The crystal size effect of ZSM-5 on its catalytic performance for conversion of methanol to gasoline (MTG) was investigated. It was shown that the external surface area of the sample decreased with its crystal size, while the acid site amount firstly increased, and then kept almost constant. Nevertheless, 650 nm ZSM-5 crystals attaching small particles exhibit large external surface area and strong acidity. The catalytic stability and the liquid hydrocarbon yield decreased with increasing crystal size. The sample with a crystal size of 70 nm shows a catalytic lifetime of 96 h and a gasoline yield of 30.8%. The large external surface area and relatively strong acidity endow the sample with a crystal size of 650 nm also has a catalytic lifetime of 91 h, indicating that synthesis of large ZSM-5 crystals with small crystallites adhered to their surface could be a potential way to improve the catalytic performance.