Pyrolysis of acetylene behind reflected shock waves

Abstract

The thermal decomposition of acetylene has been studied in the temperature and pressure regimes of 1900-2500 K and 0.3-0.55 atm using a shock tube coupled to a time-of-flight mass spectrometer. A series of mixtures varying from 1.0-6.2% CzHz diluted in a Ne-Ar mixture yielded a carbon atom density range of 0.24-2.0 x 1017 atoms cm-3 in the reflected shock zone. Concentration profiles for CzH2, C4H2, and CsHz were constructed during typical observation times of 750 ps. CsH2 and trace amounts of C4H3 were found in relatively low concentrations at the high-temperature end of this study. A mechanism for acetylene pyrolysis is proposed, which successfully models this work and the results obtained by several other groups employing a variety of analytical techniques. Two values of the heat of formation for C,H(134 ? 2 and 127 t 1 kcal/mol) were employed in the modeling process; superior fits to the data were attained using the latter value. The initial step of acetylene decomposition involves competition between two channels. In mixtures (<200 ppm) where the acetylene concentrations are less than 2.18 x lo-' mol ~m -~, the decay is predominantly first order with respect to CZH,; in mixtures >200 ppm, the dominant initial step is second order. The rate constant for the second-order reaction is described by the equationBenzene concentrations predicted by the model are below the TOF detectability limit.C4H3 was observed in the 6.2% C,H2 mixture in accordance with the proposed mechanism.

Gas Phase Formation of the Interstellar Molecule Methyltriacetylene

“…with polyynes in the gas phase thus opening up unusual synthetic routes to alkylpolyynes, which cannot be synthesized by traditional organic chemistry routes in solution due to their sensitivity toward molecular oxygen and dimerization. It should be noted that there is considerable interest in the physical organic and theoretical chemistry communities in how long these substituted polyynes can be prepared in deep space as potential precursors to carbonaceous nanoparticles . The elucidation of the barrierless route to methyltriacetylene as presented here warrants future experimental studies under single collision conditions such as the reaction of 1‐propynyl with triacetylene (HCCCCCCH) and cyanodiacetylene (HCCCCCN) probably leading to the synthesis of the hitherto elusive methyltetracetylene in deep space thus exploring systematically the elementary reactions leading to one of the least explored class of hydrogen‐deficient organic molecules in extraterrestrial environments – polyynes – eventually gaining a comprehensive understanding of their electronic structure, stabilities, and chemical bonding.…”

“…[25,[27][28][29] In deep space, since hydrocarbon molecules carrying linear carbon chain backbones constitute nearly one third of all detected interstellar organic molecules, [30] the elucidation of their formation routes will unravel the most fundamental processes that drive the hitherto poorly understood exotic organic chemistry in low temperature interstellar environments. Considering that hydrogen-deficient polyyne-type species may act as precursors to fullerenes (C 60 , C 70 ) [31][32] and even to carbonaceous nanoparticles (carbon-based interstellar grains), [22,[33][34][35][36][37][38][39][40][41] our combined experimental and computational study helps defining the molecular complexity of organic molecules, which can be synthesized in our galaxy, thus ultimately predicting where else in the universe the molecular precursors to fullerenes and carbonaceous nanoparticles can be formed.…”

“…It should be noted that there is considerable interest in the physical organic and theoretical chemistry communities in how long these substituted polyynes can be prepared in deep space as potential precursors to carbonaceous nanoparticles. [22,[31][32][33][34][35][36][37][38][39][40][41] The elucidation of the barrierless route to methyltriacetylene as presented here warrants future experimental studies under single collision conditions such as the reaction of 1-propynyl with triacetylene (HCCCCCCH) and cyanodiacetylene (HCCCCCN) probably leading to the synthesis of the hitherto elusive methyltetracetylene in deep space thus exploring systematically the elementary reactions leading to one of the least explored class of hydrogen-deficient organic molecules in extraterrestrial environments -polyynes -eventually gaining a comprehensive understanding of their electronic structure, stabilities, and chemical bonding. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57…”

Acetylene Pyrolysis in Tubular Reactor

“…The first known experiment on acetylene pyrolysis was performed by Berthelot in 1866. Various reactors have been used to study the reaction: static reactors [8][9][10][11][12], flow reactors, [13][14][15][16][17][18], and shock tubes [19][20][21][22][23][24][25][26].…”

Modelingof Acetylene Pyrolysis under Steel Vacuum Carburizing Conditions in a Tubular Flow Reactor

“…Detailed kinetic mechanisms proposed in literature may consist of hundreds of species and reactions. Typically, these investigations include shock tube pyrolysis of acetylene [1][2][3][4][5][6][7][8][9][10][11][12] and pyrolysis in flow systems [13][14][15]. The deposition of pyrolytic carbon from various hydrocarbons has also been described by some researchers [16][17][18][19][20].…”