Methanol Aromatization over Mg–P-Modified [Zn,Al]ZSM-5 Zeolites for Efficient Coproduction of para-Xylene and Light Olefins

Abstract

In this work, a series of [Zn,Al]ZSM-5 zeolites was synthesized via isomorphous substitution using different Zn precursors, and the as-obtained zeolites were further subjected to Mg−P surface modification for efficient production of pxylene and light olefins (C 2 = ≈ C 4 = ) from methanol. The synthesized [Zn,Al]ZSM-5 zeolites are highly active in methanol to aromatics (MTA) reaction. Ascribed to the uniformly dispersed Zn species and greatly enhanced dehydro-cyclization reaction, [ZnS]ZSM-5 synthesized from ZnSO 4 exhibited the most efficient MTA performance (∼62 wt % aromatics yield). Through fine modulation of zeolite pore openings and acidic properties, the subsequent Mg−P comodification improved the shape selectivity to p-xylene and also led to abundant coproduction of light olefins. The resultant Mg−P/[ZnS]ZSM-5 catalyst showed an outstanding performance for methanol aromatization in both this work and the open literature, directly yielding ∼23 wt % p-xylene (∼187% para-selectivity) from methanol feedstock.

In Situ Incorporation of Zn into Hierarchical ZSM-5 Zeolites for Olefin Hydroisomerization

“…Ni et al followed a similar synthetic route to obtain nanosized Zn-modified ZSM-5 zeolite crystals to promote the yields of BTX (benzene, toluene, and xylene) in the methanol to aromatics (MTA) reaction . Other zinc precursors, such as ZnSO 4 , Zn­(AcO) 2 , and Zn­(NO 3 ) 2 , were also used in the preparation of Zn-modified ZSM-5 zeolite in the MTA reaction, and a similar promotion effect on aromatic selectivity was realized . These results together show the modification of ZSM-5 zeolite with Zn species via in situ hydrothermal synthesis, making the entry of Zn species into zeolite microporous channels easier compared to the traditional impregnation method.…”

“…Su et al found that ZnOH + species in Zn-modified ZSM-5 zeolite are accounted for the formation of iso -alkanes in dimethyl ether to gasoline reaction . The formation of L acid sites (ZnOH + species) is determined by the location of Zn species (i.e., at the external surface or within the micropores) since it generated from B acid sites, and much attention has been focused on incorporating Zn species into zeolite micropores. − …”

“…The in situ synthetic strategy improves the accessibility of Zn cations to B acid sites, reduces the B acid amount, and increases the number of ZnOH + species. However, these in situ synthetic routes mostly lead to a single zeolite with a large crystal size (>2 μm), ,, in which the catalytic lifetime becomes challenging because of coking, owing to the excellent hydrogen transfer capability by incorporating Zn. Generally, hierarchical ZSM-5 zeolites are frequently reported as the stable catalyst precursors for Zn-modified ZSM-5 zeolites. − It is important to develop a facile synthetic route for the hierarchical Zn-modified ZSM-5 zeolite via the in situ hydrothermal synthesis method.…”

Selective Formation of Para-Xylene by Methanol Aromatization over Phosphorous Modified ZSM-5 Zeolites

“…The light olefins are further developed into C 5+ non-aromatic hydrocarbons by polymerization and cyclization. Then the aromatics and alkanes are generated by hydrogen transfer and dehydro-cyclization reactions under the action of Brönsted acid sites [14,42]. The results of acidity measurement showed that the amount of the Brönsted acid sites decreased obviously after the doping of P, which was consist with the changes in the selectivity of aromatics and alkanes.…”

“…Firstly, the narrowed pore size made the generation of PX easier than that of other X isomers and C 9+ aromatics, and the small amount of other X isomers and C 9+ aromatics formed was also difficult to diffuse out the zeolite channel. Moreover, the isomerization reaction of PX was inhibited after PX diffusing to the external surface which was poisoned by P species [42]. Therefore, the P/HZSM-5 was highly shape-selective for PX in the MTA reaction.…”