Abstract:

The cause of the deactivation of a methanol-to-propylene (MTP) catalyst after multiple reaction cycles was studied. On this basis, a facile and effective approach, i.e., secondary crystallization, was proposed and applied to the regeneration of the catalyst. The HZSM-5 zeolite catalysts before and after regeneration were characterized by a series of techniques, including powder X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), N₂ adsorption, ²⁷Al magic-angle spinning nuclear magnetic resonance (²⁷Al MAS NMR), temperature-programmed desorption of ammonia (NH₃-TPD), and infrared spectroscopy of adsorbed pyridine (Py-IR). The physicochemical properties, such as framework, silica/alumina ratio, texture, and acidity, of the deactivated catalysts and the regenerated ones were investigated in detail. The catalytic performance of the zeolites in MTP conversion was tested under operating conditions of T=470 ℃, p=0.1 MPa (p_MeOH=30 kPa) and weight hourly space velocity (WHSV)=1 h^-1. The collapse of the zeolite structure and loss of active sites were found to be the essential reasons for the decline in catalyst activity after multiple reaction cycles. By regeneration via secondary crystallization, the relative crystallization, specific surface area, pore volume and acidity of the HZSM-5 catalyst were increased prominently. Meanwhile, the destroyed crystal structure and acid sites of the deactivated catalyst were restored effectively. Thus, the regenerated catalyst again exhibited excellent methanol conversion capacity and propylene selectivity in the MTP reaction.

Key words: Methanol, Propylene, ZSM-5 zeolite, Deactivation, Regeneration, Secondary crystallization