Increasing para-Xylene Selectivity in Making Aromatics from Methanol with a Surface-Modified Zn/P/ZSM-5 Catalyst

Abstract

We report a ZSM-5 based catalyst with surface modification of SiO 2 to increase the selectivity of para-xylene (PX) in xylene (X) in the methanol-to-aromatics process. The effect of acid strength and acid amount in HZSM-5, Zn/P/ZSM-5, and Zn/ P/Si/ZSM-5 on the catalytic performance, including methanol conversion, aromatic yield, and PX selectivity, were studied. The total acid strength and acid amount of the catalyst were crucial for high methanol conversion (around 100%) and high yield of aromatics (>60%), whereas weak external acid sites present in a small amount played an important role in increasing the PX selectivity (in the X isomers) from the usual 23−24% to 89.6%. The results validated the use of a catalyst having a core with strong acid sites in a large amount and an external shell with weak acid sites in a small amount. The contribution of the external surface reaction, including alkylation, isomerization, and dealkylation, to the PX selectivity was evaluated by using PX or ortho-X separately as feedstock. A Zn/P/Si/ZSM-5 catalyst worked well in continuous reaction/catalyst-regeneration cycles, and it also converted recycled toluene into PX by an alkylation route.

Insight into the Formation of COX By‐Products in Methanol‐to‐Aromatics Reaction over Zn/HZSM‐5: Significantly Affected by the Chemical State of Surface Zn Species

“…An aromatic selectivity of only 38.43 % was obtained over the parent HZSM-5, with producing little CO X (CO and CO 2 , 0.47 %). [4,17] But aromatic selectivity was only~50 % (obviously lower than that in our work) over a Zn/P/HZSM-5 (SiO 2 /Al 2 O 3 = 50) prepared by them. The aromatic selectivity was up to 61.39 %, 57.29 % and 60.38 % over Zn(IE)/HZSM-5, AÀ Zn (IM)/HZSM-5 and BÀ Zn(IM)/HZSM-5 respectively.…”

“…However, Zn/HZSM-5 showed the significantly enhanced activity of aromatization. [17] However, their reports had no mention about the CO X by-products in this catalytic reaction. This is mainly attributed to a fact that the ZnÀ L acid sites formed can effectively promote the dehydrogenation of intermediates (such as cyclane, cycloolefin) in MTA reaction (the oligomerization and cyclization of light olefins can also be promoted by chemical equilibrium).…”

Surface‐Passivated Hierarchically Structured ZSM5 Zeolites: High‐Performance Shape‐Selective Catalysts for para‐Xylene Production

“…Accordingly, Figure a gives the NH 3 ‐TPD profiles of HSZ series and microporous ZSM‐5 and the calculated acid amounts are listed in Table . All of the HSZ series feature two desorption bands similar to that of microporous ZSM‐5, in which the high‐temperature and low‐temperature band correspond to the strong and the weak acid sites, respectively ,. Because the strong acidity of ZSM‐5 zeolites originates from the isomorphous substitution of framework Si by Al, the change of Al content and/or Al coordination state would induce the variation of acid amount and/or acid strength.…”

The Role of Non-Framework Lewis Acidic Al Species of Alkali-Treated HZSM-5 in Methanol Aromatization

“…Hence, the reason why HZ-D produced more durene but less xylenes than HZ-DA did was not that HZ-D was influenced by the reduction of space confinement including the change in the size of mesopores or micropores. The production difference was caused by the different selectivities of durene and xylenes generated by the amorphous Lewis acidic Al on the external surface, which boosted the transalkylation/methylation of aromatics [49], particularly xylenes to form durene. This is also supported by the considerable durene (11-13%) produced by HZ, which has the smallest micropore size (0.55 nm) and a relatively low mesoporosity.…”