Aromatization mechanism of coupling reaction of light alkanes with CO over acidic zeolites: Cyclopentenones as key intermediates

The aromatization of light alkanes (C 4 -C 6 ) is an important value-added reaction. However, the yield of aromatics is always limited by the simultaneous generation of small alkanes (CH 4 and C 2 H 6 ) due to H/C balance. Herein, we demonstrate that the introduction of CO into light alkanes over zeolites significantly enhanced aromatic selectivity and an aromatics selectivity of 85% was achieved in the case of cyclopentane and CO coupling reaction over H-ZSM-5. Methyl-substituted cyclopentenones were observed and considered as the most important intermediates for aromatics formation. Multiple characterizations revealed the coupling mechanism: (1) CO inserts into carbenium ions to form acylium cations, (2) acylium cations react with olefins to generate unsaturated ketones, (3) unsaturated ketones cyclize to methyl-substituted cyclopentenones, (4) methyl-substituted cyclopentenones convert to monocytic aromatics. Methyl-substituted indanones were also discovered causing the generation of binuclear aromatics, such as naphthalene. Nano-sized ZSM-5 and TEOS modifications were applied to enhance the BTX selectivity. • High aromatic selectivity is achieved by coupling light alkanes with CO over H-ZSM-5 • Key intermediates are observed, and their formation and evolution route are clarified • The complete mechanism for coupling reaction of light alkanes with CO is proposed • Nano-sized ZSM-5 and TEOS modifications are applied to improve the BTX selectivity Light alkanes (C 4 -C 6 ) could be widely derived from refinery industry as byproducts, and the aromatization of light alkanes is an important value-added reaction. However, light alkanes had a higher hydrogen to carbon (H/C) ratio than that of aromatics from the perspective of element composition, so the yield of aromatics is always limited by the simultaneous generation of small alkanes (CH 4 and C 2 H 6 ) due to H/C balance requirement of the reaction. According to this, we expect that hydrogen-deficient substances, such as CO, are introduced into light alkane conversion to balance the excess hydrogen of alkanes. We indeed observe that the introduction of CO into light alkane over acidic zeolites could tune the H/C ratio of products and high aromatic selectivity could be achieved. Considering the large existing supply capacities of CO and light alkanes, the coupling reaction might have great potential for industrial applications. The introduction of CO into light alkanes over H-ZSM-5 significantly enhances aromatic formation, and high aromatics selectivity could be achieved. The key intermediates, such as methyl-substituted cyclopentenones and indanones, are observed on the reacted zeolites and could be converted into BTX and heavy aromatics, respectively. The formation and evolution route of these intermediates are clarified, and the coupling mechanism is proposed. According to this, using nano-size ZSM-5 and TEOS modifications significantly improves the BTX selectivity during the coupling reaction.