Coupling Conversion of n-Hexane and CO over an HZSM-5 Zeolite: Tuning the H/C Balance and Achieving High Aromatic Selectivity

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 (CO)-C-13 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.