Cyclotrimerization of Acetylene under Thermal Conditions: Gas-Phase Kinetics of V⁺ and Fe⁺ + C₂H₂

Abstract

The kinetics of successive reactions of acetylene (C 2 H 2 ) initiated on either vanadium or iron atomic cations have been investigated under thermal conditions using the variable-ion source and temperature-adjustable selected-ion flow tube apparatus. Consistent with the literature results, the reaction of Fe + + C 2 H 2 primarily yields Fe + (m/z = (C 2 H 2 ) 3 ); however, analysis via quantum chemical calculations and statistical modeling shows that the mechanism does not form benzene upon the third acetylene addition. The kinetics are more consistent with successive addition of three acetylene molecules, yielding Fe + (C 2 H 2 ) 3 , followed by an addition of a fourth acetylene molecule, initiating cyclotrimerization, yielding either Fe + (C 2 H 2 ) + neutral benzene or Fe + (Bz) + acetylene, where Bz is a benzene ligand. In contrast, the reaction of V + + C 2 H 2 yields products via successive associations V + (m/z = (C 2 H 2 ) n ) either with or without a bimolecular step involving loss of one H 2 and V + C 2 (m/z = (C 2 H 2 ) m ), where n and m extend at least up to 11 under conditions of 0.32 Torr at 300 K. Stabilized V + (Bz) is not a significant intermediate in the association mechanism. We propose a plausible mechanism for the generation of neutral benzene in this reaction and compare with the Fe + results. The reaction steps that produce benzene result in turnover of the single-atom catalyst, and the large hydrocarbons produced that remain associated to the catalyst are proposed to be polycyclic aromatic hydrocarbons.

Coordination and Spin States in Fe+(C2H2)n Complexes Studied with Selected-Ion Infrared Spectroscopy

“…55 Several groups investigated the chemistry of multiacetylene complexes with Fe + , suggesting that cyclization chemistry produced benzene. 13,17,20,21,[23][24][25][26][27][28][29][30][31][32][33]36,37 Quantum chemistry computations reached similar conclusions. 23−30 However, there was no spectroscopic confirmation of this proposal.…”

“…Infrared spectroscopy on this same complex found a slightly distorted acetylene structure with C–H stretches shifted to lower frequencies than those in the isolated acetylene molecule . Several groups investigated the chemistry of multiacetylene complexes with Fe + , suggesting that cyclization chemistry produced benzene. ,,,,− ,, Quantum chemistry computations reached similar conclusions. − However, there was no spectroscopic confirmation of this proposal. In the present work, we conduct infrared spectroscopy experiments, complemented by theory, on Fe + (C 2 H 2 ) n complexes containing up to eight acetylene molecules to investigate the possibility of such chemistry.…”

Cyclotrimerization of Acetylene on Clusters Con+/Fen+/Nin+(n = 1–16)

“…Figure presents a diagram showing the chemistry involving the competition between acetylene dehydrogenation and termolecular association reactions occurring repeatedly. It is speculated that the clusters collide with acetylene and initiate a simple adsorption step, which will be followed by the hydrogen atom transfer and dehydrogenation process, in parallel with the collision and adsorption of a second and a third C 2 H 2 molecule . It is worth mentioning that the association complexes are formed with the help of sufficient He buffer gas collisions, which partly eliminate the energy gain of C 2 H 2 chemisorption on the cluster.…”

“…It is speculated that the clusters collide with acetylene and initiate a simple adsorption step, which will be followed by the hydrogen atom transfer and dehydrogenation process, in parallel with the collision and adsorption of a second and a third C 2 H 2 molecule. 13 It is worth mentioning that the association complexes are formed with the help of sufficient He buffer gas collisions, which partly eliminate the energy gain of C 2 H 2 chemisorption on the cluster. On the other hand, the presence of a high-concentration reactant gas enables cyclotrimerization of acetylene after simultaneously or sequentially adsorbing three C 2 H 2 molecules.…”

“…However, the low yields and byproducts of aromatic derivatives may impede industrial applications of these reactions. Although the reaction of acetylene to benzene is exothermic, relatively high temperatures (above typically 400 °C) are currently used to overcome the energy barrier for partial activation of the CC bond to form the benzene ring. − On this background, extensive experimental and theoretical studies have been conducted on transition metal atoms and ions, − oxides, − complexes, − and supported metal catalysts (Pd n /MgO, Rh/MgO, Ag/MgO, Ni/SiO 2 ) − for the purpose of identifying for efficient catalysts for this cyclotrimerization.…”