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

Abstract

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.

Highly Enhanced Aromatics Selectivity by Coupling of Chloromethane and Carbon Monoxide over H‐ZSM‐5

“…Figure 1c depicts the change on aromatic formation rate with CO pressure during CH 3 Cl conversion. The rate of aromatics rings formation increases monotonically with increasing CO partial pressure, implying that the formation of aromatics is closely related to CO similar with the coupling of small molecule compounds and CO [3d, 16] . In addition, the trends in selectivity to C 2 –C 4 alkanes (from 22.2 to 3.1 %) and olefins (from 23.6 to 5.6 %) are opposite to that of aromatics (from 39.0 to 79.3 %) with increasing partial pressure of CO (Figure S2).…”

Review on the Catalytic Conversion of Naphtha to Aromatics: Advances and Outlook

“…A significant increase in the aromatic selectivity was achieved by the coupling reaction of n -C 6 with CO . A plausible mechanism (Figure ) for the coupling reaction begins with the adsorption of n -C 6 on a Brønsted acid site to form carbonium ions or crack into smaller products.…”

Dynamic Evolution of Zeolite Framework and Metal-Zeolite Interface

“…Catalysts are the footstone of the modern chemical industry, which can conspicuously accelerate the reaction rate, shorten the reaction time, and reduce energy consumption. As an important type of solid catalysts, zeolites have become the backbone in some crucial chemical processes, such as fluid catalytic cracking (FCC), , methanol-to-olefin (MTO), − methanol-to-ethanol, − and coupling conversion of hydrocarbon and methanol/CO x to aromatization. − Zeolites are a class of crystalline microporous materials with a key feature of well-defined channels/cages for reactant molecules adsorption, transformation, and reaction, which are constructed from the connections of corner-sharing TO 4 tetrahedra (T = Si, Al, P, etc.). Owing to the presence of AlO 4 in aluminosilicate zeolites and SiO 4 in silicoaluminophosphate zeolites, the negative charge of zeolite framework requires a balancing cation (usually H + ) to ensure electroneutrality, resulting in another key feature of Brønsted acid sites (BAS). , By controlling the synthesis conditions, diverse metals (e.g., Ti, Sn, Ge, Zr, and Nb) can be introduced into the zeolite framework, forming heteroatomic zeolites with Lewis acidity. , Besides, the extra-framework metal species with single, cluster, and nanoparticle structures can be accommodated into zeolite channels/cages. − Until now, more than 250 topologies of zeolites with various compositions and acidities have been developed by bottom-up and top-down strategies, exhibiting many unrivalled catalytic performances (e.g., shape selectivity, − dynamic autocatalysis, , and synergistic catalysis). , Although a lot of progress has been achieved, many aspects of the active sites and catalytic mechanisms in zeolites and metal-zeolites are still under debate.…”