Mechanism of Dehydrocyclization of 1-Hexene to Benzene on Cu₃Pt(111):  Identification of 1,3,5-Hexatriene as Reaction Intermediate

Abstract

We report here ultrahigh vacuum studies of the dehydrocyclization reaction of submonolayer coverages of 1-hexene to benzene on a Cu3Pt(111) single crystal surface, using reflection-absorption infrared spectroscopy (RAIRS), near edge X-ray absorption fine structure (NEXAFS) studies, and temperatureprogrammed reaction/desorption (TPR/D) spectrometry. As discussed in a previous TPR/D paper, at surface coverages up to 13% of monolayer saturation, 1-hexene forms benzene on a Cu3Pt(111) surface. Selectivity to benzene formation is 70 ( 10%, with the remaining 30 ( 10% of the adsorbed 1-hexene dehydrogenating irreversibly to surface carbon and H2. For higher coverages, molecular desorption commences. Spectroscopic identification of the intermediates of the reaction of 1-hexene and other model compounds, such as a 1,3,5-hexatriene, with a Cu3Pt(111) surface suggests that 1-hexene and 1,3,5-hexatriene have a common intermediate, and this intermediate has been identified as a rehybridized hexatriene species. Other model compounds, such as trans-3-hexene, have also been used to provide further understanding of the mechanism of the aromatization reaction.

Femtosecond x-ray spectroscopy of an electrocyclic ring-opening reaction

“…In Fig. 3B, the most notable transient feature is observed in the 90-to 130-fs time window at~282.2 eV, which is a new resonance not observed in the NEXAFS of either ground-state CHD or HT (31,33). Also in this time window, peak Y is broadened and shifts to lower energies, leading to increased absorption amplitude between peaks X and Y.…”

Surface Chemistry of 1- and 3-Hexyne on Pt(111): Desorption, Decomposition, and Dehydrocyclization

“…This desorption is thought to be related to a bridge site on the platinum surface, and the variations in desorption temperatures observed in this study might be caused by spectator species, which were also postulated to perturb adsorption sites of linear hydrocarbons on Pd(111) . For the dehydrocyclization of 1-hexene on Cu 3 Pt­(111), it was shown by NEXAFS measurements that the rate-determing step is the formation of a cyclic intermediate, whereas dehydrogenation takes place already at low temperatures . 1-Hexene is believed to dehydrogenate to form a 1,3-hexadiene via an allylic intermediate, followed by 1,3,5-hexatriene, which finally forms benzene.…”

“…48 For the dehydrocyclization of 1-hexene on Cu 3 Pt(111), it was shown by NEXAFS measurements that the rate-determing step is the formation of a cyclic intermediate, whereas dehydrogenation takes place already at low temperatures. 3 1-Hexene is believed to dehydrogenate to form a 1,3-hexadiene via an allylic intermediate, followed by 1,3,5-hexatriene, which finally forms benzene. Due to the detection of hexenes caused by self-hydrogenation of 1-and 3-hexyne on Pt(111) at 300 K, it is obvious that significant dehydrogenation takes place at temperatures below 300 K. Therefore, cyclization would have to take place below that temperature for a mechanism in which cyclization precedes dehydrogenation.…”

Dehydrocyclization of peripheral alkyl groups in porphyrins at Cu(100) and Ag(111) surfaces

“…2B) of the Co-PP which are from porphyrin ring bending, breathing, and C-H wagging. AES C, XPS C 1s, and XPS N 1s analysis before and after annealing show less than 5% loss of signal, so no significant desorption occurred at 500 K. It is likely that the dehydrocyclization mechanism here is similar to that derived for hexenes on a Cu 3 Pt surface [25,30]. The first dehydrogenation on the ethyl group is likely at the more acidic allylic α carbon hydrogen.…”

“…Dehydrocyclization reactions, a subset of dehydrogenation reactions that involve the further step of ring formation, are of high interest for the oil refining industry as a method to increase the octane number of fuels [18]. Dehydrocyclization requires high temperature (873 K for 1,3-pentadiene to cyclopentadiene [19]) or a catalyst, such as Ir or Ru complexes [20,21]; zeolite materials loaded with Co, Pt, Cu, Zn, Ga, or In metals [22][23][24]; or on Rh, Ir, Ni, Pd, Pt, Cu 3 Pt, or W carbide surfaces [25][26][27][28][29][30][31][32][33][34][35][36]. Most of the dehydrocyclization surface studies rely on surfaces that can catalyze the reaction at a temperature below thermal desorption [37][38][39].…”