Highly-efficient conversion of methanol to p-xylene over shape-selective Mg-Zn-Si-HZSM-5 catalyst with fine modification of pore-opening and acidic properties

A highly shape-selective catalyst with a relatively long lifetime for methanol aromatization (MTA reaction) was prepared through finely adjusting the pore-opening size and the acidic properties of HZSM-5, depending on the combination of liquid silica deposition with polysiloxane and Zn & Mg loading. XRD, Al-27 MAS NMR, TEM, BET, NH3-TPD, Py-IR, TG, GC-MS and probe-molecule reactions were used to reveal the relationship between the modification methods and the catalytic performance. The results showed that silica deposition significantly enhanced the p-xylene (p-X) selectivity in xylene isomers and restrained the formation of larger C9+ aromatic molecules, owing to the narrowing of the pore-openings and the elimination of external acid sites. Further Zn-loading effectively improved the MTA activity of Si-HZSM-5 due to the promotion of the internal Zn-Lewis acid sites with increased density to the dehydrogenation of the intermediates in this reaction. However, the Zn-Si-HZSM-5 catalyst showed poor catalytic stability, which is mainly attributed to the micropore blockage (invalidation of internal acid sites) caused by coke deposition over the strong acid sites inside the channels, which showed strong diffusion-restriction on aromatic molecules at their openings. Fortunately, 1.5% Mg modification selectively reduced the density of the strong acid sites in the channels of Zn-Si-HZSM-5, effectively improving its catalytic stability as the reduction of acidic strength suppressed polyaromatic (coke components) formation at the channel intersections. Moreover, MgO also slightly enhanced the porous shape-selectivity together with the Zn species. As a result, more than 98.9% para-selectivity of xylene and 20.1-21.2% p-X yield were obtained in the 12 h MTA reaction over the highly shape-selective Mg-Zn-Si-HZSM-5 catalyst.