Para-selective methylation of toluene with methanol over nano-sized ZSM-5 catalysts: Synergistic effects of surface modifications with SiO₂, P₂O₅ and MgO

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 Brönsted acid sites over ZSM-5 modified by SiO₂, while P₂O₅ or MgO preferentially neutralizes the Brönsted 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 (∼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 ∼98% and improving catalytic stability, as demonstrated by flow test for 1000 h on stream.