A Nanostrips-Assemble Morphology of ZSM-5 Zeolite for Efficient Propylene Production from Methanol Conversion

MFI-type ZSM-5zeolite with a sea-urchin-like morphologyand unusual diorientation along both the a-axis andthe b-axis has been hydrothermally synthesized forthe first time. With a thinner a-axis than b-axis, this material displayed longer lifetime, higherpropylene selectivity, and much higher propylene-to-ethylene ratioin methanol-to-propylene reaction in comparison to conventional ZSM-5catalysts with various known morphologies. Turningthe morphology and orientated growth of zeolite crystalsis essential to alter their catalytic performances. MFI-type ZSM-5nanostripes (named as D-Z5) with a sea-urchin-like overall morphologywere directly synthesized with a commercially available and small-molecularorganic structure-directing agent (OSDA) of 1-butyl-1-methylpyrrolidiniumhydroxide. Their nanostrip crystals exhibited a unique diorientedmorphology with thin thicknesses of similar to 8 nm and similar to 70 nmalong the a-axis and b-axis, respectively.The dealuminated D-Z5 displayed higher propylene selectivity (similar to 57%),longer lifetime (60 h) and much higher propylene-to-ethylene ratio(>12) in methanol-to-propylene (MTP) reaction than those conventionalZSM-5 with known crystal morphologies. Computational simulation andexperimental measurements provided solid proof that the diorientatedcrystals decreased the bulky molecular diffusion resistance, postponedthe secondary reaction and then promoted propylene to diffuse rapidlyout of two sets of micropores in zeolites. This unusual diorientationphenomenon is not only beneficial for improving the MTP catalyticperformance of ZSM-5, but also expected to be versatile to developother useful zeolite catalysts.