posted on 2020-03-19, 15:38authored byChangcheng Wei, Qijun Yu, Jinzhe Li, Zhongmin Liu
The coupling conversion
of n-hexane (a model compound
of naphtha) and CO was conducted over an HZSM-5 zeolite catalyst.
A significant increase in the aromatic selectivity and dramatic decrease
in the alkane selectivity were simultaneously achieved by adjusting
the H/C ratio of the reactants. Under suitable conditions, the aromatic
selectivity was 80%, which far exceeds the theoretical value for the
conversion of only n-hexane over HZSM-5. Detailed
studies revealed that the high aromatic selectivity originated from
the direct incorporation of CO into the products. Different characterization
methods were used to elucidate the coupling reaction mechanism. Important
reaction intermediates, such as acyl groups and oxygenates (substituted
cyclopentenones), were detected. Isotope labeling studies revealed
that oxygenates were formed by 13CO incorporation into
alkyl species, and they could transform into aromatic products. Based
on these findings, a plausible reaction mechanism of the coupling
transformation was proposed. The coupling effect between n-hexane and CO over HZSM-5 demonstrated in this work contradicts
the traditional view that metal catalysts are indispensable for the
highly selective conversion of naphtha to aromatics. This type of
coupling reaction might have great potential for industrial applications,
considering the large existing supply capacities of CO and alkanes.