Release Time:2019-03-09  Hits:

Indexed by: Journal Article

Date of Publication: 2014-09-15

Journal: MICROPOROUS AND MESOPOROUS MATERIALS

Included Journals: Scopus、EI、SCIE

Volume: 196

Page Number: 18-30

ISSN: 1387-1811

Key Words: Nano-sized ZSM-5; Single modification; Multiple modification; Para-selectivity; Synergistic effect

Abstract: The nano-sized ZSM-5 catalysts were modified by surface coating with SiO2, P2O5, MgO and their combinations; the catalytic properties were investigated in the shape-selective methylation of toluene with methanol. The catalysts were characterized by XRD, XRF, N-2 adsorption-desorption, temperature programmed desorption of ammonia (NH3-TPD), Fourier-transform infrared spectra of adsorbed pyridine/2,6-di-tert-butylpyridine, and adsorption of n-hexane/cyclohexane. The passivation of Lewis acid sites occurs prior to that of the Bronsted acid sites over ZSM-5 modified by SiO2, while P2O5 or MgO preferentially neutralizes the Bronsted acid sites of ZSM-5. The deposition of MgO is more efficient in passivating the acid sites and narrowing the pore openings, compared to SiO2 or P2O5 modification with the same oxide content (<9 wt%). The single modification could not completely passivate the external surface acid sites and simultaneously narrow the pore openings to a proper extent; so the selectivity to para-xylene does not exceed 90% even at the highest oxide loading. The multiple modification by SiO2, P2O5 and MgO, with a suitable sequence can efficiently eliminate external surface acid sites, and simultaneously narrow the pore openings, which led to a higher para-selectivity (similar to 98%). The combined modification with SiO2, P2O5 and MgO in a proper sequence can lead to a synergistic effect for tailoring the acid property and pore mouth of the catalyst, thus enhancing the para-selectivity to similar to 98% and improving catalytic stability, as demonstrated by flow test for 1000 h on stream. (C) 2014 Elsevier Inc. All rights reserved.