Recently, our research group led by Liu Zhongmin of the National Engineering Laboratory of methanol to olefins, Dalian Institute of Chemical Physics, has made a breakthrough in the field of coupling conversion of n-hexane and carbon monoxide over zeolite.
Aromatics are the basic products of modern petrochemical industry, and also the basic organic raw materials with the largest industrial demand, which have a basic supporting role for the chemical industry and its downstream fine products industry.
The coupling conversion technology of naphtha is one of the important technical routes. It is expected that oxygen-containing compounds from coal (such as methanol, methyl acetate, CO, etc.) can be coupled with naphtha to produce olefin, aromatics and other bulky chemicals. Based on a large number of previous studies on the coupling reaction of naphtha, this work further expands to the study of naphtha and CO coupling system.
Because naphtha is mainly composed of saturated alkanes and its H / C ratio is higher than that of aromatics, a large number of low-carbon alkanes will be produced when naphtha is transformed into aromatics over zeolite. The key to solve this problem is to balancing the H / C ratio between the raw material and the target products by coupling reaction.
In this work, 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 by13CO 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.
The related research results were published in ACS catalysis. This work was supported by the National Natural Science Foundation of China(grant nos. 21991093 and21991090)
Coupling conversion of n-hexane and CO over an HZSM-5 zeolite: Tuning the H/C balance and achieving high aromatic selectivity. Changcheng Wei, Qijun Yu, Jinzhe Li, Zhongmin Liu*, ACS Catal., 10(7):4171-4180, 2020.
