C3H6 Reaction Research Articles

Isotopic Tracer Studies of Reaction Pathways for Propane Oxidative Dehydrogenation on Molybdenum Oxide Catalysts

Kinetic analysis and isotopic tracer studies were used to identify the elementary steps and their reversibility in the oxidative dehydrogenation of propane over ZrO2-supported MoOx catalysts. Competitive reactions of C3H6 and CH313CH2CH3 showed that propene is the most abundant primary product, and that CO and CO2 are formed via either secondary combustion of propene, or by direct combustion of propane. A mixture of C3H8 and C3D8 undergoes oxidative dehydrogenation without forming C3H8-xDx mixed isotopomers, suggesting that steps involving C−H bond activation are irreversible. Normal kinetic isotopic effects (kC-H/kC-D) were measured for propane dehydrogenation (2.3), propane combustion (1.6) and propene combustion (2.1). These data indicate that the kinetically relevant steps in propane dehydrogenation and propene combustion involve the dissociation of C−H bonds in the respective reactant. H−D exchange occurs readily between C3H6 and D2O or C3D6 and H2O, suggesting that OH recombination steps are reversi...

Isotopic Tracer and Kinetic Studies of Oxidative Dehydrogenation Pathways on Vanadium Oxide Catalysts

Kinetic analysis and isotopic tracer studies were used to identify elementary steps and their reversibility in the oxidative dehydrogenation of propane on VOx/ZrO2 catalysts with VOx surface densities between 1.6 and 6 VOx/nm2. Competitive reactions of C3H6 and CH313CH2CH3 showed that CO forms via secondary combustion of propene intermediates. CO2 formed via this reaction and also via the direct combustion of propane. Reactions of 18O2/C3H8 mixtures on supported V216O5 led to the preferential initial appearance of lattice 16O atoms in all oxygen-containing products, as expected if lattice oxygens were required for the activation of C–H bonds. Isotopically mixed O2 species were not detected during reactions of C3H8–18O2–16O2 reactant mixtures. Therefore, dissociative O2 chemisorption steps are irreversible. Similarly, C3H8–C3D8–O2 reactants undergo oxidative dehydrogenation without forming C3H8−xDx mixed isotopomers, suggesting that C–H bond activation steps are also irreversible. Normal kinetic isotopic effects (kC–H/kC–D=2.5) were measured for primary oxidative dehydrogenation reactions. Kinetic isotope effects were slightly lower for propane and propene combustion steps (1.7 and 2.2, respectively). These data are consistent with kinetically relevant steps involving the dissociation of C–H bonds in propane and propene. C3H6–D2O and C3D6–H2O cross exchange reactions occur readily during reaction; therefore, OH recombination steps are reversible and nearly equilibrated. These isotopic tracer results are consistent with a Mars–van Krevelen redox mechanism involving two lattice oxygens in irreversible C–H bond activation steps. The resulting alkyl species desorb as propene and the remaining O–H group recombines with neighboring OH groups to form water and reduced V centers. These reduced V centers reoxidize by irreversible dissociative chemisorption of O2. The application of pseudo-steady-state and reversibility assumptions leads to a complex kinetic rate expression that describes accurately the observed water inhibition effects and the kinetic orders in propane and oxygen when surface oxygen and OH groups are assumed to be the most abundant surface intermediates.

Reactivity of surface nitrate species in the selective reduction of NO with propene over Na[ndash ]H-mordenite as investigated by dynamic FTIR spectroscopy

The reactivity of surface adsorbed species over Na–H-mordenite has been examined by using dynamic insitu IR spectroscopy and the mechanism of the selective reduction of NO with C3H6 discussed. When Na–H-mordenite was exposed to a flow of NO–C3H6–O2 at 573 K, a strong absorption band assignable to NO3− was observed at 1394 cm−1. The intensity of the band significantly decreased when the flowing gas was switched to C3H6–O2. The rate of decrease in NO3− was ca. 3.0 × 10−8 mol g−1 s−1 which was equivalent to the rate of NO conversion to N2 in the NO–C3H6–O2 flow reaction. Thus, it was indicated that NO3− is a reaction intermediate for the formation of N2 and the surface reaction of NO3− is the rate determining step of the formation of N2 in the selective reduction of NO with C3H6. When the flowing gas was switched to C3H6, the rate of decrease in NO3− species was twice as fast as that in C3H6–O2, i.e., the reactivity of NO3− species strongly depended on the presence of O2. Since NO was also formed simultaneously with N2 in the case of NO2–C3H6 reaction, it was suggested that NO3− species oxidize C3H6 accompanying not only the conversion into N2 but also the dissociative decomposition into NO. Weak bands assignable to adsorbed hydrocarbon, carbonyl, nitrite and isocyanate were also observed. On the basis of the changes in the intensity of these bands, the consecutive reaction of surface organic species with NO3− species over Na–H-mordenite was suggested.

Asymmetric internal rotation: Application to the 2-C4H9CH3+C3H6 reaction

J. Gang, M. J. Pilling and S. H. Robertson, J. Chem. Soc., Faraday Trans., 1997, 93, 1481 DOI: 10.1039/A607566E

A study of the unimolecular decompositions of the (C3H6)+2 and (c-C3H6)+2 complexes

The major product channels identified in the unimolecular decompositions of C3H+6⋅C3H6 and c-C3H+6⋅c-C3H6 in the total energy [neutral (C3H6)2 or (c-C3H6)2 heat of formation plus excitation energy] range of ∼230–450 kcal/mol are C3H+7+C3H5, C4H+7+C2H5, C4H+8+C2H4, and C5H+9+CH3. The measured appearance energy for C4H+7(9.54±0.04 eV) from (C3H6)2 is equal to the thermochemical threshold for the formation of C4H+7+C2H5 from (C3H6)2, indicating that the exit potential energy barrier for the ion–molecule reaction C3H+6+C3H6→C4H+7+C2H5 is negligible. There is evidence that the formations of C4H+7+C2H4+H from (C3H6)+2 and (c-C3H6)+2 also proceed with high probabilities when they are energetically allowed. The variations of the relative abundances for C4H+7,C4H+8, and C5H+9 from (C3H6)+2 and (c-C3H6)+2 as a function of ionizing photon energy are in qualitative agreement, suggesting that (C3H6)+2 and (c-C3H6)+2 rearrange to similar C6H+12 isomers prior to fragmentation. The fact that C6H+11 is found to be a primary ion from the unimolecular decomposition of (c-C3H6)+2 but not (C3H6)+2 supports the conclusion that the distribution of C6H+12 collision complexes involved in the C3H+6+C3H6 reactions is different from that in the cyclopropane ion–molecule reactions. Using the ionization energies (IE) of (C3H6)2(9.33±0.04 eV) and (c-C3H6)2(9.61±0.04 eV) determined in this study, the calculation of the bond dissociation energies for C3H+6⋅C3H6 and c-C3H+6⋅c-C3H6 gives 0.43 and 0.14 eV, respectively. The measured IE of C3H6 is 9.738±0.003 eV and that of c-C3H6 is 9.721±0.011 eV.

Photoinduced metathesis reaction of C3H6 on supported MoO3 catalyst

Ultraviolet irradiation of MoO3 supported on porous Vycor glass (PVG) in the presence of C3H6 has been found to induce the formation of approximately equimolar amounts of C2H4 and 2-C4H8 as well as a small amount of CH3CHO, suggesting that the metathesis reaction of C3H6 occurs. The dependence of the yields upon the excitation wavelength is in good agreement with the excitation band of the phosphorescence of MoO3/PVG, which arises from the charge-transfer transition (Mo6+O2–) [graphic omitted] (Mo5+—O–).* The quenching of the phosphorescence of MoO3/PVG and the decrease in the yield of C3H6 metathesis on adding O2 and CO suggest that the photoinduced metathesis reaction is closely associated with the charge-transfer excited triplet state of MoO3/PVG. From these results, together with those of e.s.r. experiments reported previously, the mechanism of the photoinduced C3H6 metathesis, especially the primary process of metal–carbene formation, is discussed.

Kinetic Mass Spectrometric Investigation of the Ion-Molecule Reactions Occurring in C4 and C5 Alkanes Following Photoionization at 106.7 and 104.8 nm.

The photoionization of C4 and C5 alkanes has been investigated at 106.7 and 104.8 nm in a mass spectrometer specifically designed for the investigation of ion-molecule reactions occurring at thermal kinetic energies. Absolute rate constants are reported for the reactions of various fragment ions with the corresponding parent molecule. The rate constants found for reactions of ions with i-C4H10, n-C4H10, i-C5H12, and n-C5H12 were found to be 3.3, 4.4, 4.7, and 5.2 × 10-10 cm3/molecule-second respectively. The ions were also found to be highly reactive, exhibiting rate constants of 3.6 and 3.8 × 10-10 cm3/molecule-second in reactions with i-C5H12 and n-C5H12. The rate constants for reaction of with i-C4H10, n-C4H10, i-C5H12, and n-C5H12 were found to be 4.9, 4.9, 7.6, and 7.9 × 10-10 cm3/molecule-second, respectively. Butene ions are less reactive by an order of magnitude. The results are compared with complementary data derived from electron impact experiments, and the relationship between the structure and reactivity of the various ions is discussed.

Reactions of C3H6+ with C3 and C4 Paraffins

Rate constants for H− and H2− transfer from C3 and C4 paraffins to propylene and cyclopropane molecular ions have been determined for ions of low translational energy at the ionization threshold of C3H6+. The results indicate that an increase in KH− is accompanied by a compensating decrease in KH2− and vice versa, such that the total reactivity of C4 structural isomers is invariant. By using selectively labeled compounds, it was possible to derive transfer probabilities for the various substituent sites on the normal butane carbon skeleton. Evidence is also presented which suggests that cyclopropane molecular ions do not assume the propylene-ion structure prior to or during reaction with C4 paraffins. The results of the present investigation are compared with some complementary data obtained from radiolytic and photolytic systems at much higher pressures.

Ion—Molecule Reactions by a Photoionization Mass Spectrometer. I. Propylene and 1,3-Butadiene

Ion—molecule reactions in propylene and 1,3-butadiene have been studied using a mass spectrometer which utilizes photoionization as an ion source instead of electron impact. The light source used was a low-pressure hydrogen-discharge lamp with a LiF window (substantially Lyman alpha, 1216 Å). It has been shown that this method has two major advantages in the study of ion—molecule reactions compared with the electron-impact method; one is the lack of unimolecular fragmentation of the parent ion and the other the absence of hot filament, which enables one to get high pressure without suffering pyrolysis. Five reactions of C3H6+ with C3H6 and eight reactions of C4H6+ with C4H6 have been identified and the cross sections determined for these reactions. In propylene, the reaction leading to the formation of C3H7+ was found to be of the largest cross section and, although cross section was small, the reaction forming C3H3+ was found as a new reaction. It was shown that C3H6+ could not be the precursor of secondary C4H6+ in propylene, criticizing the Fuchs assignment. In 1,3-butadiene, three new reactions have been found; the reaction leading to the formation of C3H3+, the hydrogen-atom abstraction reaction just as in propylene, and the parent-minus-one reaction. The first one was of the largest cross section in all reactions in 1,3-butadiene. This reaction has been discussed in connection with the structure and the heat of formation of C3H3+.