Abstract High resolution NMR spectroscopy is shown to be qualified to identify the species present in silicate solutions used for zeolite synthesis.

A single-input double-tuned probe was designed for double resonance nuclear magnetic resonance (NMR) experiments, in which high radio frequency power irradiations were achieved at two closely spaced frequencies (such as 26.1 MHz for Al27 and 28.4 MHz for Cu65). The efficiency of the probe for both nuclei was determined to be about 80%, compared to that obtained with a single-tuned probe. The probe was used successfully for Al–6527Cu SEDOR experiments to derive the internuclear distance between Al and Cu in Cu-ZSM-5 zeolite. The circuit is ideal for SEDOR-type double resonance NMR experiments, when high power irradiation is required for nuclei with closely spaced resonance frequencies.

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 reversible and quasi-equilibrated. Reactions of 18O2/C3H8 on supported Mo16Ox species lead to the preferential initial appearance of lattice 16O atoms in H2O, CO, and CO2, indicating that lattice oxygen is required for C−H bond activation and for the ultimate oxidation of the adsorbed products of this reaction. 18O16O was not detected during reactions of C3H8−18O2−16O2 mixtures, consistent with irreversible O2 dissociation steps. These isotopic tracer results are consistent with a Mars−van Krevelen redox mechanism in which two lattice oxygens participate in the irreversible activation of C−H bond in propane. The resulting alkyl species desorb as propene, and the remaining O−H group recombines with neighboring OH groups to form water and reduced Mo centers. The reduced Mo centers finally reoxidize by irreversible dissociative chemisorption of O2. The proposed reaction mechanism leads to a complex kinetic rate expression that accurately describes the observed dependences on the partial pressure of propane, oxygen, and water.

The oxidative dehydrogenation (ODH) of ethane on alumina-supported vanadia was investigated with the aim of understanding the effects of lattice oxygen and vanadium oxidation state on the catalyst ODH activity and ethene selectivity. Transient-response experiments were carried out with both a fully oxidized sample of 10 wt% VO(x)/Al(2)O(3) (7 V nm(-2)) and a sample that had been partially reduced in H(2). The experimental results were analyzed to determine the rate coefficients for ethane ODH, k(1), and ethene combustion, k(3). The rate of ODH was found to depend solely on the concentration of reactive oxygen in the catalyst, but not on the means by which this oxygen concentration was attained (i.e., by H(2)versus C(2)H(6) reduction). On the other hand, the ethene selectivity observed at a given concentration of active oxygen was found to depend on the composition of the reducing agent, higher ethene selectivities being observed when H(2), rather than C(2)H(6), was used as the reducing agent. It is proposed that the higher ethene selectivity achieved by H(2)versus C(2)H(6) reduction might be due to a lower ratio of V(4+) to V(3+) cations attained upon reduction in H(2) for a given extent of V(5+) reduction. This interpretation is based on the hypothesis that ethene combustion is initiated by C(2)H(4) adsorption on V(n+) cations present at the catalyst surface and that the strength of adsorption decreases in the order V(5+) > V(4+) > V(3+) consistent with the decreasing Lewis acidity of the cations.

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Ting Chen, Berend Smit, Alexis T. Bell; Are pressure fluctuation-based equilibrium methods really worse than nonequilibrium methods for calculating viscosities?. J. Chem. Phys. 28 December 2009; 131 (24): 246101. https://doi.org/10.1063/1.3274802 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioThe Journal of Chemical Physics Search Advanced Search |Citation Search

A spectroscopic investigation of complexes used to catalyze the oxidative carbonylation of toluene to p-toluic acid was conducted. Rhodium complexes were analyzed by (103)Rh and (13)C NMR, UV-visible spectroscopy, and infrared spectroscopy. In the presence of vanadium and oxygen, the resting state of the Rh-catalyst was found to exist as a Rh(III) complex with carbonyl and trifluoroacetate ligands, consistent with the structure Rh(CO)(2)(TFA)(3). The (13)C NMR spectrum of Rh((13)CO)(2)(TFA)(3) complex exhibited a carbonyl peak with an unusual degree of shielding, which resulted in the appearance of the carbonyl peak at an unprecedented upfield position in the (13)C NMR spectrum. This shielding was caused by interaction of the carbonyl group with the trifluoroacetate ligand. In the absence of oxygen, the Rh(III) complex reduced to Rh(I), and the reduced form exhibited properties resembling the catalyst precursor. Structures and spectroscopic properties calculated using density functional theory agreed closely with the experimental results. The vanadium co-catalyst used to reoxidize Rh(I) to Rh(III) was similarly characterized by (51)V NMR and UV-visible spectroscopy. The oxidized species corresponded to [(VO(2))(TFA)](2), whereas the reduced species corresponded to (VO)(TFA)(2). The spectroscopic results obtained in this study confirm the identity of the species that have been proposed to be involved in the Rh-catalyzed oxidative carbonylation of toluene to toluic acid.

Raman and UV−vis diffuse reflectance spectroscopy were used to characterize the structure of vanadia dispersed on high surface area zirconium oxide. Two-dimensional vanadia species with tetrahedral coordination appear on the surface of the ZrO2 and expand in size with increasing V loading. Crystalline V2O5 appears when the vanadia loading exceeds an apparent surface density of 7.0 V atoms/nm2, and ZrV2O7 is formed as a consequence of zirconia migration into the V2O5 crystallites. A model for the structure of two-dimensional vanadia overlayer is proposed based on the experimental data and information taken from the literature. Vanadia is found to absorb the light scattered by the support, and this gives rise to a reduction in the intensity of the Raman bands for zirconia as the surface loading of vanadia increases. Partial reduction of the dispersed vanadia increases the absorbance of the vanadia and alters the profile of absorbance versus frequency in such a manner as to increase the intensity of the Raman bands for zirconia relative to those for vanadia. Examination of Raman spectra taken after repeated reduction−oxidation cycles suggests that reduction occurs via removal of oxygen from the vanadia monolayer without agglomeration or reorganization of the surface species.