Initial Product Distributions Research Articles

Classical trajectory calculations for the reaction N + + H 2 → NH + + H

Classical trajectory calculations are presented for the reaction N + + H 2 → NH + + H using a single potential surface. At low initial relative energies symmetric product distributions are obtained and complex trajectories predominate. At higher energies direct reaction gives forward peaking.

Analysis of Porous Electrodes with Sparingly Soluble Reactants: V . Single Pore Studies: System

Galvanostatic charging and discharging experiments at 50 and 25 μA were conducted on a single pore electrode of pore dimensions in . This study characterizes critical processes that determine the performance of porous electrodes and verifies the predictions of a mathematical model in terms of over‐all electrode potentials and reactant or product distributions as functions of time. Morphological variation with respect to the frontal plane of the single pore electrode was observed after repeated cycling of the electrode. Good agreement in terms of over‐all electrode potentials and initial product distributions during charging were obtained between mathematical model predictions and experimental results. The model contains no adjustable parameters.

On the mechanism of catalytic isomerization of xylenes. Molecular orbital studies

On the basis of CNDO/2 molecular orbital calculations, we postulate the following detailed mechanism for the catalytic isomerization of xylenes which explains the initial product distributions and also our previous finding that the reaction is intramolecular: (i) adsorption of xylene on a surface acid site to form a Wheland-type complex; (ii) disrotatory cyclization of the protonated species into a bicyclo[3, 1, 0]hexenyl complex; (iii) migration of the methylene bridge to a new side of the pentagonal ring; (iv) change of the new bicyclic species back into the corresponding Wheland-type complex; (v) desorption of the xylene isomer from the surface of the catalyst. The overall rate determining step can be either of the surface reactions (ii) or (iv).

Radiation chemistry of acetylene at high intensity: the initial product distributions

The distribution of products observed in the radiation chemistry of acetylene at high dose rate is dramatically different from that observed at low dose rate. The dominant products observed at high dose rate are vinyl acetylene and diacetylene with C3 to C9 compounds having smaller but significant yields. At low dose rate benzene and cuprene are the exclusive products. The difference in product distribution is attributed to the suppression of secondary attack on products at high dose rate so that the observed distribution approximates more closely the 'zero dose' or 'initial' distribution.

Mycobacterium smegmatis fatty acid synthetase. A mechanism based on steady state rates and product distributions.

The initial steady state rate and product distribution of fatty acid synthesis catalyzed by Mycobacterium smegmatis fatty acid synthetase has been investigated as a function of various concentrations of acetyl-CoA, malonyl-CoA, mycobacterial polysaccharide, and bovine serum albumin. Polysaccharide has a large effect on both rate and chain length. The steady state rate stimulation by polysaccharide is not duplicated by other acyl-CoA-binding molecules such as bovine serum albumin. It is concluded that relief of product inhibition does not adequately explain the specific effects of the mycobacterial polysaccharide. A general mechanism is presented which accounts for variations in reaction rate and produce pattern over a wide range of experimental conditions. We propose that the diffusion of long chain acyl-CoA (C14 to C24) from the enzyme is the rate-limiting step in fatty acid synthesis catalyzed by the M. smegmatis synthetase. Polysaccharide facilitates this rate-limiting step by forming a ternary complex with enzyme-bound acyl-CoA causing rapid release of product.

The reactions between cyclopentane and deuterium on palladium and palladium-nickel alloys

Cyclopentane-deuterium exchange has been followed on pure Pd and Pd Ni alloy films in the temperature range 200–430 K and at hydrogen/cyclopentane ratios of ~50. Conclusions on activity changes are limited because of the self-poisoning of alloys and Ni catalysts. However, the selectivity of the catalysts is much less influenced by self-poisoning; the initial product distributions are well reproducible and independent of these side processes. The main result of alloying of Pd with Ni is the gradual disappearance of one low temperature mechanism which leads on Pd to D 10 products. This fact can be easily understood if we assume that it is the “roll-over” mechanism which is responsible for these products.

Cyclopentane/deuterium exchange on palladium-gold and palladium-tin alloy films

The cyclopentane/deuterium exchange reaction has been studied on well-homogenized Pd–Au and Pd–Sn alloy films. The efficiency of “roll-over” of cyclopentene intermediate (as measured by the ratio two-set/one-set exchanged product) was unaffected by alloying Pd with Au but was increased by alloying with Sn. This finding is discussed in terms of the electronic effects of Au and Sn solutes on the palladium–alkene bond. In both alloy systems the activity-composition plots show a decrease in the absolute rate of exchange of 3–4 orders of magnitude. For Pd–Au this break occurs at 30–40 atom % Pd whilst for Pd–Sn it occurs at 85–90 atom % Pd. Initial product distributions arising from carbene species were found on Pd alloys of > 80 atom % Au and of > 15 atom % Sn. A comparison of the effects of alloying Pd with gold on the rates of exchange and deuteriolysis supports the view that the former reaction requires smaller surface ensembles than the latter.

SIMULATION OF INITIAL PRODUCT DISTRIBUTIONS FROM PYROLYSIS OF BRANCHED ALKANES

A series of cracking experiments on 2-methylbutane (isopentane), 2-methylpentane, 2-methylhexane, and 2-methylheptane were carried out using a conventional flow type apparatus at 700°C and atmospheric pressure. Branched alkanes were highly diluted with nitrogen by about 10 times and the extent of decomposition ranged from 5.5 to 15 mole %. The initial product distributions obtained were compared with those simulated on the basis of the free-radical chain mechanism proposed by Rice and Kossiakoff. It was found, from comparison of experiment with calculation, that the radical isomerization through intramolecular 1-4 transfer of H-atom in addition to generally accepted 1-5 transfer should be taken into account. In the present study, the rate of 1-4 transfer isomerization was assumed to be equal to that of β-scission of radicals because there is strain energy in 5-membered ring formation.

PREDICTION OF INITIAL PRODUCT DISTRIBUTIONS FROM PYROLYSIS OF NORMAL PARAFFINIC HYDROCARBONS

The extension of the free-radical chain mechanism to heavy paraffinic hydrocarbons has been studied for practical purposes. A generalization of the mechanism is attempted based on the Markov chain theory and presented in a form suitable to computer analysis. This generalization allows easier prediction of the distributions of initial pyrolytic products. Better agreement with experimental results is achieved by taking into account the equilibrium among all the possible isomers for higher homologues of hydrocarbon free radicals. However, the calculation of products is complicated by inclusion of radical isomerization which has not been ascertained experimentally. A new model is constructed herein by avoiding a direct approach to the problem of isomerization, while preserving its effect on the distribution of products. In this treatment, radical isomerization is replaced by a hypothetical process that accompanies the decrease of primary radicals and the increase of secondary radicals. The initial product distributions predicted by the model show good agreement with the observations for a series of normal paraffins from n-butane to n-hexadecane.

The isomerization of aliphatic hydrocarbons over evaporated films of platinum and palladium

Saturated hydrocarbons in the presence of excess hydrogen undergo catalytic skeletal isomerization and hydrocracking at a platinum surface. For mechanistic purposes, a study has been made of these reactions (i) with ethane, n -butane, isobutane, neopentane, and isopentane over unoriented evaporated films of platinum and palladium, (ii) with n -butane and isobutane over (111) and (100) oriented films of platinum, and (iii) with n -butane-1-C 13 over unoriented films of platinum. Platinum films exposing (111) and (100) surfaces were prepared by deposition on mica and on an evaporated sodium chloride layer, respectively. Over palladium, most of the reaction was hydrocracking: In no case did isomerization contribute more than about 3% to the initial reaction products. Over platinum, the proportion of isomerization product was substantial. Isobutane isomerized more readily than n -butane and the isomerization of the former, but not of the latter, was markedly increased by using a (111) platinum surface. Reactions with both n -and isobutane over (100) platinum gave product distributions only marginally different from unoriented platinum. The main features of the distributions of hydrocracking products could be approximately accounted for by assuming that the residence of a molecule on the surface resulted in the rupture of not more than one carbon-carbon bond. Initial product distributions were substantially independent of temperature. With reaction mixtures having p H 2 / p HC The isobutane formed from n -butane-1-C 13 was labeled only in the 2-position (i.e., peripherally). No scrambling of the C 13 occurred. The isomerization was thus entirely intramolecular. At the same time n -butane-2-C 13 was produced in a constant proportion to the isobutane. These products were thus probably formed concurrently from a common surface intermediate. From arguments based on the influence of hydrocarbon geometry on the reaction, it was concluded that the surface intermediate for isomerization and hydrocracking was 1–3 diadsorbed, except in the case of ethane which was 1–2 diadsorbed. In agreement with this, a 1–3 diadsorbed intermediate provided a reasonably satisfactory quantitative account of the proportion of n -butane-2-C 13 relative to isobutane in the reaction products. The exceptionally high degree of isomerization of isobutane on (111) platinum was tentatively ascribed to a symmetrical triadsorbed surface intermediate.

The mechanisms of hydrogenolysis and isomerization of hydrocarbons on metals: II. Mechanisms of isomerization of hexanes on platinum catalysts

The isomerization of n-hexane (I), 2-methylpentane (II), and 3-methylpentane (III) were studied on various supported platinum catalysts, and the initial product distributions compared and identified with the initial distributions of the hydrogenolysis of methylcyclopentane. These results suggest that a common cyclopentane intermediate is involved in both reactions, adsorbed on the metal sites of the catalysts. The absence of 2,3-dimethylbutane in the product distributions, the simultaneous dehydrocyclization of (I), (II), and (III) to methylcyclopentane, and the difficulty of isomerizing the 2,3-dimethylbutane under the same conditions, are consistent with the proposed mechanism. On platinum films, another type of skeletal rearrangement also takes place: 2,3-dimethylbutane is formed from the hexanes, and benzene from methylcyclopentane. The mechanism probably involves a species adsorbed on two carbon atoms in the α,γ positions and is very similar in its effects to a carbonium ion mechanism. Only aromatization occurs when 1,1,3-trimethylcyclopentane reacts with hydrogen on a platinum film at 300°C, and that is a further proof of the possibility of ring enlargement on metal catalysts.

The decomposition of n -hexane II. By reaction with atomic hydrogen

The reaction of hexane with hydrogen atoms produced by mercury photosensitization, has been studied in a flow system at 300° C. About one-third of the products had molecular weights greater than that of hexane, and dodecane was the main component of this product fraction. Hydrogen: hexane ratios up to 55:1 were employed and in these conditions virtually all the quenching of excited mercury atoms was brought about by hydrogen. The activation energy and steric factor of the reaction C 6 H 14 + H = C 6 H 13 + H 2 are estimated at 6 kcal and 10 -4 , respectively. These values are in accord with those recently obtained for the corresponding reactions involving other n -paraffins. The initial product distribution was similar to that obtained in the mercury photosensitized decomposition of hexane and the findings suggest that products of lower molecular weight than hexane derive almost completely from thermal decomposition of hexyl radicals. ‘Atomic cracking’ appears to be of little importance at these high temperatures.

The Mechanism of the Isotopic Exchange Reaction between Cyclohexane and Deuterium on Evaporated Metal Films

Abstract The theoretical treatments of the isotopic exchange reaction between cyclohexane and deuterium over evaporated metal films have been studied. The present results of the theoretical calculations were compared with the experimental results already obtained by the present authors for evaporated films of molybdenum and tungsten. The present results are summarized as follows: 1) The agreements between the theoretical and the experimental values of the initial product distributions for these metal films are well maintained. 2) The initial rate of tripping of the adsorbed radicals on evaporated films of molybdenum or tungsten at each temperature is considerably greater than that of desorption (or chemisorption) of cyclohexane. This coincides well with the experimental fact that the rate-determining step of this exchange reaction may be attributed to desorption of the adsorbed radicals. 3) The differences between the activation energy of tripping of the adsorbed radicals and that of desorption for evaporated films of molybdenum and tungsten amount to 6.8 and 9.7 kcal./mol. respectively.

Catalysis on evaporated metal films. V Reactions between cyclic hydrocarbons and deuterium

Catalytic exchange reactions have been studied by means of a mass spectrometer between deuterium and cyclo Zopentane over palladium and cyclo Zohexane over rhodium, palladium, tungsten, platinum, nickel, oriented nickel and oriented rhodium. Activation energies, frequency factors and distributions of initial products were determined. The reactions involved multiple exchange by repeated second-point adsorption. Half the hydrogen atoms were readily exchanged, and a study of the temperature-dependence of the product distributions showed that it required an activation energy of 4 to 8 kcal/mole in excess of that for adcorption-desorption before the remaining half could be exchanged. Considerations of molecular geometry revealed that in order to exchange the second half of the hydrogen atoms the molecule had to ‘turn over’ on the surface, and for cyclo Zopentane the process for turning over must require second-point adsorption by the same carbon atom, but with cyclo hexane there is the alternative process of exchanging two adjacent equatorial hydrogen atoms. Theories are developed which account satisfactorily for the observed initial product distributions. The pressure-dependence of reaction rate was determined for cyclo hexane over palladium. Deuterium was strongly adsorbed and cyclo hexane weakly adsorbed. Exchange of cyclo hexane over oriented films of nickel and rhodium showed marked differences in distribution compared with unoriented films, suggesting different catalytic activity of different crystal planes. Only exchange was observed with benzene over nickel, but there was simultaneous exchange and deuteration over palladium. The initial product pattern over palladium showed that a redistribution reaction (in the sense of Wagner et al , (1952)) did not occur during deuteration and that benzene was adsorbed mainly as phenyl radicals during the exchange reaction. The reaction between cyclo propane and deuterium was examined over rhodium and simultaneous exchange and deuteration were observed. As determined by the temperature range for reaction the order of reactivity in exchange was cyclo propane > cyclo pentane > cyclo hexane, and the order of catalytic activity of the metals was similar to that observed previously for the exchange of other saturated hydrocarbons.