Excited States of Acetylene and Their Role in Pyrolysis

Abstract

Previous theories of acetylene pyrolysis are reviewed in the light of recent work by Minkoff, Newitt, and Rutledge. I t is shown that the relatively large rates observed a t the beginning of the induction period do not agree with mechanisms involving the intervention of comparatively stable dimers. The required kinetic form is obtained, however, if a triplet diradical is produced on the surface in a bimolecular process, followed by gas phase polymerization, with some chain ending on the surface. The detailed mechanism closely resembles Flory's scheme for liquid phase vinyl polymerization. The shape of the radical is discussed, and it is suggested that the trans-configuration leads to polymerization, while the cis facilitates dehydrogenation.There has recently been some interest in the reactions of atoms or molecules in excited states (12,14), and particularly of unsaturated molecules in triplet states. The purpose of this communication is to examine the role played by acetylene, and its polymers, in triplet states, during the pyrolysis of acetylene a t temperatures below 500" C., particularly in view of the failure of mechanisms based on ground state molecules and radicals (21). In fact, there has been considerable controversy not only over the proposed mechanisms, but also over the experimental results. As a consequence of some recent work a t temperatures below 525" C. (15), it has been possible to select those results which are most likely to be free of unexpected complications, and to propose a simple mechanism to account for most of the observations. REVIEW O F PREVIOUS WORKThe maill difficulties indicated in the literature are threefold; the first are coi~cerned -/ , -with the nature of the products, the second with the effect of surfaces, and the third is related to the effect of nitric oxide and the role of radicals. These will be considered in turn.Confusion has often arisen regarding the products, since it is frequently stated, for example, that benzene is the main product of pyrolyzing acetylene. The latter result is only obtained under special conditions; the main reaction a t low temperatures (i.e. below 500" C.) is polymerization to polycyclic and substituted aromatic coinpounds (see Ref.15 for detailed references). Above 7-800" C., this is almost entirely superseded b y decomposition to carbon and hydrogen (2). Products in the intervening region vary continuously from hydrogen and methane down to lower aromatics, including benzene, depending mainly on the temperature, but also on other factors. In particular iVIinkoff, Newitt, and Rutledge have show11 that even a t low temperatures, polymerization call be replaced by decomposition to carbon and hydrogen when halogen compounds or some metals are added to acetylene.The elucidation of the mechanism has also been hindered by the differing reports of the effect of packing the reaction vessel with glass or other supposedly inert materials. Schlapfer and Bruniler (19), and Frank-Kamenetsky (7), found the rate to be unaffected by increasing the su...

Acetylene Pyrolysis in Tubular Reactor

“…The shape of the curve at 1029 °C indicates an induction period at low conversion. Similar observations have been reported . Xu and Pacey reported a much lower initial rate, and the induction period was taken as an evidence for a free radical process.…”

Radical processes in the pyrolysis of acetylene

“…To account for inhibition at low concentrations and the long induction periods observed below 900°K they assumed that there was a n active biradical formed which propagated a chain. This was a variation of mechanisms which had been proposed by Minkoff [7,8] and earlier authors. KD proposed that vinylidene added to acetylene to form methylene cyclopropene which presumably reached some stationary state with acetylene and.…”

“…The main features of the pyrolysis which have emerged have been the following: (1) Below 1000°K the reaction is predominantly a polymerization producing very few species smaller than C4H4 [l,2]; (2) The overall reaction rate appears to be close to second order in C2H2 concentration [l-41; (3) There is a distinct and reproducible induction period which decreases with increasing temperature and increasing acetylene concentration [ 1, 2,4]; (4) Small amounts of NO (0.1-20%) can almost totally inhibit the reaction [1, 2,6,7]; (5) During the NO inhibited reaction, the NO is very slowly consumed together with some acetylene after which the reaction proceeds with its normal rate [1,2]; (6) The pyrolysis is surface catalyzed in quartz vessels; (7) NO inhibits the gas phase reaction up to 1400°K [5], even at 2% NO concentrations; (8) 1% of HBr reduces the rate 18 fold at 700°K [8]; (9) At 500"C, 1.5% of styrene [20] or vinyl acetylene [19] accelerate C2H2 polymerization 3-fold and 5-fold, respectively, with no induction period evident.…”

Global investigation of potential energy surfaces for the pyrolysis of C1–C3hydrocarbons: toward the development of detailed kinetic models from first principles

“…Including this reaction type into a kinetic model can be crucial to accurately describe pyrolysis and combustion of hydrocarbons. For instance, formation of singlet vinylidene, CH 2 QC is suggested to be the main initial step of acetylene pyrolysis below 1300 K, [68][69][70] contrary to the radical mechanisms such as formation of vinyl and ethynyl radicals via disproportionation of two acetylene molecules 68,71 or formation of the excited state acetylene biradical 72,73 as it was proposed before. Our reported barrier height of 0.9 kcal mol À1 from vinylidene-acetylene isomerization is in good agreement with the 1.3 kcal mol À1 barrier height reported in ultraviolet photoelectron spectroscopy studies 78 and the 1.5 kcal mol À1 barrier height reported by couple-cluster calculations.…”