CO_x‐Resistant Oxidative Dehydrogenation of Cyclohexane Catalyzed by sp³@sp²Nanodiamonds towards Highly Selective Cyclohexene Production

Abstract

Deep oxidation/dehydrogenation are longstanding problems for decades in catalytic oxidative dehydrogenation (ODH) of cyclohexane and other alkanes. Here we show a metal‐free catalyst of nanodiamonds (NDs) with unique sp3@sp2 hybrid structure that catalyzes COx‐resistant cyclohexane ODH with remarkable reactivity towards cyclohexene production. The selectivity of cyclohexene can reach as high as 67 % with significantly suppressed COx emission (<5 %), which is on top of the highest reported values among other metal(oxide)/metal‐free catalysts. Structural evolution of sp3@sp2 NDs under annealing treatments and their specific surface functional groups are systematically studied using TEM, XPS, Raman and TPD. By comparing with carbon nanotubes (CNTs), we found that the carbonyl groups stabilized on strained sp3@sp2 core‐shell NDs enhanced the cyclohexene selectivity via preferential cleavage of C−H over C−C bond. Kinetic studies further revealed the underlying reaction pathways that cyclohexane is rapidly dehydrogenated to cyclohexene which subsequently transforms into benzene (fast) and COx (slow). Deep oxidation of both cyclic hydrocarbons is largely suppressed due to the low density of electrophilic functional groups on strongly curved graphitic surface of sp3@sp2 NDs.

Chlorine-Decorated Ceria Nanocubes for Facilitating Low-Temperature Cyclohexane Oxidative Dehydrogenation: Unveiling the Decisive Role of Surface Species and Acid Properties

“…Scheme displays the reaction network, in which k 1 ∼ k 6 represents the rate coefficients for each elementary step. Based on earlier reports, , k 2 , k 5 , and k 6 are relatively lower, suggesting that the deep dehydrogenation path of C 6 H 12 and the overoxidation path of C 6 H 12 and C 6 H 6 are unfavorable. In particular, C 6 H 10 , as a reaction intermediate, could be consumed to produce C 6 H 6 (path 3) and CO x (path 4).…”

“…To highlight the superior catalytic performance of current catalysts, the comparison of the as-synthesized CeO 2 nanocubes with reported heterogeneous catalysts for cyclohexane ODH to produce cyclohexene is shown in Table . Significantly, a few early works have proved that adding of hydrogen chloride (HCl) , or acetic acid (HOAc) , in feedstock along with oxygen is a reliable method to realize the high cyclohexene selectivity. On these grounds, the chlorination of the catalyst may be beneficial for the production of alkene.…”

“…Significantly, a few early works have proved that adding of hydrogen chloride (HCl) 46,47 or acetic acid (HOAc) 48,49 in feedstock along with oxygen is a reliable method to realize the high cyclohexene selectivity. On these grounds, the chlorination of the catalyst may be beneficial for the production of alkene.…”