Acyl Hydrogen Research Articles

Mechanistic aspect of ethanol synthesis from methanol under CO hydrogenation condition on MoS x cluster model catalysts

The mechanism of ethanol synthesis from methanol under synthesis gas utilizing non-stoichiometric Mo 20S 43 and Mo 20S 36 clusters as model catalysts has been computed on the basis of density functional theory. The first step is methanol hydrogenolysis to form surface CH 3, and the second step is CO insertion into surface CH 3 to form surface acyl (CH 3CO). The third step is surface acyl hydrogenation to form ethanol.

Competing Mechanistic Channels in the Oxidation of Aldehydes by Ozone

The reaction of ozone with aldehydes has been studied intermittently for over 100 years, but its mechanism remains uncertain. Experimental results support two reaction channels: radical abstraction of the acyl hydrogen and addition to form a five-membered ring tetroxolane. We have studied the aldehyde-ozone reaction by DFT and CCSD(T) calculations. CCSD(T)/6-311+G(d,p)//M05-2X)/6-311+G(d,p) calculations predict two competitive pathways for the oxidation of formaldehyde by ozone. Abstraction of the acyl hydrogen by ozone has a barrier of 16.2 kcal/mol, leading to a radical pair, which can combine to form a hydrotrioxide; this species may subsequently decompose to a carboxylic acid and singlet oxygen. In the second reaction channel, addition of ozone to the carbonyl is stepwise, with barriers of 19.1 and 23.0 kcal/mol, leading to a five-membered ring tetroxolane intermediate. This process may be reversible, consistent with earlier observations of isotopic exchange. The two channels connect by an intramolecular hydrogen abstraction. Ring opening of the tetroxolane by an alternate O-O bond cleavage, followed by spin inversion in the resulting diradical intermediate, can give a carbonyl oxide plus (3)O(2). It is also possible that reaction of triplet oxygen with carbonyl oxides can produce ozone by the reverse route. These two stepwise reaction channels, hydrogen abstraction and addition to the C=O bond, explain much of what has been observed in the long history of ozone-aldehyde chemistry. Known reaction rates and the substantial barriers to both channels support an earlier conclusion that aldehyde oxidation by ozone is too slow to be of importance in atmospheric chemistry.

Quantum Chemical and Master Equation Simulations of the Oxidation and Isomerization of Vinoxy Radicals

The vinoxy radical, a common intermediate in gas-phase alkene ozonolysis, reacts with O2 to form a chemically activated alpha-oxoperoxy species. We report CBS-QB3 energetics for O2 addition to the parent (*CH2CHO, 1a), 1-methylvinoxy (*CH2COCH3, 1b), and 2-methylvinoxy (CH3*CHCHO, 1c) radicals. CBS-QB3 predictions for peroxy radical formation agree with experimental data, while the G2 method systematically overestimates peroxy radical stability. RRKM/master equation simulations based on CBS-QB3 data are used to estimate the competition between prompt isomerization and thermalization for the peroxy radicals derived from 1a, 1b, and 1c. The lowest energy isomerization pathway for radicals 4a and 4c (derived from 1a and 1c, respectively) is a 1,4-shift of the acyl hydrogen requiring 19-20 kcal/mol. The resulting hydroperoxyacyl radical decomposes quantitatively to form *OH. The lowest energy isomerization pathway for radical 4b (derived from 1b) is a 1,5-shift of a methyl hydrogen requiring 26 kcal/mol. About 25% of 4a, but only approximately 5% of 4c, isomerizes promptly at 1 atm pressure. Isomerization of 4b is negligible at all pressures studied.

Deuterium Pulse Transient Analysis for Determination of Heterogeneous Ethylene Hydroformylation Mechanistic Parameters

Deuterium isotopic pulse tracing has been combined with in situ infrared spectroscopy to study heterogeneous ethylene hydroformylation and hydrogenation on a 4 wt% Mn–Rh/SiO2 catalyst at 513 K and 0.1 MPa. Deuterium pulses into steady-state CO/H2/C2H4 flow produced H2, HD, D2, C2H6−xDx, and C2H6−xDxCO transient responses. The reaction parameters, including intermediate surface coverages and rate coefficients, were determined from the transient responses coupled with compartment models that describe adsorbed intermediates in the pathway for incorporation of D2 into propionaldehyde. This pathway is shown below:The pathway involves (i) deuterium adsorption, (ii) partial deuteration of adsorbed ethylene to form adsorbed ethyl, (iii) deuteration of adsorbed ethyl to form ethane, (iv) CO insertion into adsorbed ethyl to form adsorbed acyl, (v) deuteration of adsorbed acyl via metal-chemisorbed deuterium to form propionaldehyde, and (vi) deuteration of adsorbed acyl via spillover deuterium to form propionaldehyde. The steady-state rate of hydrogen desorption, which is essential for determination of the reaction parameters, was estimated by use of the HD transient response assuming that the rate of HD production equals the rate of D2 production and equals the rate of H2 production at the point where the surface coverages of H and D are approximately equal. Evaluation of rate coefficients indicates that hydrogenation of adsorbed ethyl is intrinsically faster than CO insertion into ethyl, leading to the high selectivity toward ethane over propionaldehyde as product. The propionaldehyde compartment models were able to account for two modes of acyl hydrogenation: (i) from *H on the metal surface and (ii) from spillover *H, or Si–OH. Hydrogenation via metal-adsorbed hydrogen is favored over hydrogenation via spillover hydrogen. Comparison of the rate coefficients for CO insertion and acyl hydrogenation indicates that both steps are kinetically significant, which is consistent with conclusions of previous studies that utilized the Langmuir–Hinshelwood–Hougen–Watson and pseudo-steady-state analysis approaches.

The reaction of acrolein with acetylperoxyl radicals in the gas-phase

A rate constant for the epoxidation of acrolein by acetylperoxyl radicals has been determined to be k4 = (1.3 ± 0.9) × 104 dm3mol−1s−1 at 383 K, which is anomalously fast in comparison with the epoxidation of alkenes. Abstraction of the acyl hydrogen atom from acrolein by acetylperoxyl radicals at 393 K was found to be at least 60 times slower than from acetaldehyde and at least three orders of magnitude slower than abstraction of the acyl hydrogen atom of the epoxide of acrolein. The fast rate for epoxidation of acrolein and the slow rate for hydrogen abstraction provide an explanation for the anomalously slow rate for the autoxidation of acrolein and suggests that acrolein formed during the autoxidation of alkene will react further to give its epoxide, and not exclusively by abstraction of the acyl hydrogen atom as was previously accepted. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 277–282, 1999

Dynamic and kinetic modeling of isotopic transient responses for CO insertion on Rh and Mn–Rh catalysts

Isotopic transient tracer techniques have been employed to study heterogeneous hydroformylation on Rh/SiO 2 and Mn–Rh/SiO 2. Pulse injection of D 2 and 13 CO allowed tracing of the deuterium and CO incorporation pathway into the aldehyde product. The d 1- and d 2-propionaldehyde responses showed a double-peak, or two-hump, response to the D 2 pulse, while C 2H 5 13CHO showed a single-hump response to the 13 CO pulse. Analysis of the product responses to the D 2 pulse in CO/H 2/C 2H 4 and CO/H 2/C 2H 4/C 2H 5CHO suggests that the first hump of the d 1- and d 2-propionaldehyde responses was due to rapid H/D exchange with adsorbed propionaldehyde via enol intermediates. The decay of the second hump was due to reaction of adsorbed acyl with spillover hydrogen/deuterium. The C 2H 5 13CHO response was due to CO insertion followed by acyl hydrogenation. Compartment modeling of the product responses from the 13 CO and D 2 pulse inputs allowed determination of residence times of adsorbed intermediates, surface coverages of adsorbed intermediates, and the elementary rate constants for acyl hydrogenation and CO insertion. Elementary rate constants for acyl hydrogenation determined from this study were consistent with the value calculated by transition state theory (TST). The addition of Mn promoter to Rh/SiO 2 increased coverages of ∗ CO , ∗ C 2H 5 , and ∗ C 2H 5CO and shifted the rate-limiting step for propionaldehyde formation. Acyl hydrogenation is the rate-limiting step on Rh/SiO 2 while CO insertion and acyl hydrogenation are both kinetically significant on Mn–Rh/SiO 2.

Dynamic Multiple Tracing with D2and C2D4in Ethylene Hydroformylation over Mn–Rh/SiO2

Combined FT-IR/mass spectrometry was used to study the dynamic behavior of hydrogen/deuterium adsorbates in response to D2and C2D4pulses during ethylene hydroformylation on Mn–Rh/SiO2at 513 K and 0.1 MPa. Variation of exposure time of the catalyst to the D2and C2D4pulses revealed that readsorption did not play a significant role in the product responses; long exposure time allows more deuterium to enter the slower pathway ofd1- andd2-propionaldehyde. The two-unequal-hump HD response from both the D2and C2D4pulses indicates the presence of an isotope effect on hydrogen/deuterium adsorption. The rapid responses of the deuterated ethylene and ethane can be described by the Horiuti-Polanyi mechanism and indicate that their intermediates have small θ (surface coverage) and τ (residence time) which cannot be resolved by the time resolution of their transient responses. Thed1- andd2-propionaldehyde products show a two-hump response, which can be explained by the different deuterium incorporation pathways into the propionaldehyde. The first hump is due to hydrogen/deuterium exchange on adsorbed acyl and acyl hydrogenation. The second hump is due to H/D exchange on the adsorbed alkyl species, which then undergoes CO insertion and acyl hydrogenation. The second hump decay is partly due to the reaction with Si–OD as indicated by the parallel decay ofd1- andd2-propionaldehyde and Si–OD.

Partial purification and characterization of two enzymes from guinea-pig liver microsomes that hydrolyze carcinogenic amides 2-acetylaminofluorene and N-hydroxy-2-acetylaminofluorene

Two enzymes capable of deacylating carcinogenic compounds N-hydroxy-2-acetylaminofluorene( N-hydroxy-AAF) and 2-acetyIaminofluorene (AAF) were liberated with sonication from guinea pig liver microsomes. The enzymes were separated by using fractionation with ammonium sulfate, chromatography on Sephadex and hydroxylapatite or DEAE-cellulose. The Enzyme I (mol. wt. 200,000) hydrolyzed N-hydroxy-AAF 265 times faster than AAF, while Enzyme II (mol. wt. 41,000) hydrolyzed AAF about 1.4 times faster than N-hydroxy-AAF. Both enzymes hydrolyzed the acylamido derivatives of fluorene and naphthalene faster than those of benzene. Substitution of the acyl hydrogen atoms enhanced markedly the hydrolysis rates. Both enzymes hydrolyzed readily ester substrates, e.g. tyrosine ethyl ester and 1-naphthyl acetate, and were inhibited by the organic phosphorus compounds.

THE REACTION OF METHYL RADICALS WITH CH3CHO AND CH3CDO

Azomethane has been photolyzed in the presence of CH3CHO and CH3CDO, and the results compared with the direct photolysis of the aldehydes. The activation energies found were 6.8 and 7.8 kcal./mole, respectively, for the reactions[Formula: see text]The results furnish evidence that only an acyl hydrogen is captured. Evidence has also been found for the occurrence of wall reactions and the disproportionation reaction[Formula: see text]