Kinetic studies of propane oxidation on Mo and V based mixed oxide catalysts

The present work concentrates on the systematic kinetic study of the one-step propane oxidation to acrylic acid over a well defined, phase-pure M1 MoVTeNbOx catalyst. The bulk structural stability of the catalyst is a key issue for kinetic studies. The stability of the phase-pure M1 MoVTeNbOx catalyst under various conditions (steam-containing, steam-free, net reducing, stoichiometric and net oxidizing feed compositions) was evidenced by an in-situ XRD experiment which suggested that the bulk structure is homogeneous and constant under reaction conditions. Thereby, the heterogeneously catalyzed reactivity is exclusively determined by the surface properties, which in turn, are controlled by the chemical potential of the gas phase. A kinetic study on the reaction variables (temperature, steam content and redox potential) was carried out. Stable catalytic performance was observed for all the conditions. Cycling experiments showed the reversibility of the conversion and selectivity decrease upon exposing the catalyst to dry and reducing feed, respectively. Further catalytic experiments revealed that the reactivity spans over 5 orders of magnitude in the order of acrolein oxidation>>propylene oxidation>propane oxidation>>carbon monoxide oxidation~water gas shift reaction. The negligible CO oxidation activity suggested that the CO and CO2 are formed via two independent pathways in propane oxidation over M1. The stage-wise addition of oxygen lead to an improvement of the catalytic performance by 5% compared to the conventional single-tube reactor. Further experiments in the two-stage reactor revealed that the phase-pure M1 is not reoxidized by N2O. The addition of propylene in the two-stage reactor revealed a slight competitive adsorption on the active sites with propane, which observation was supported by the results of microcalorimetric experiments. On the other hand, the addition of CO and CO2 in the two-stage reactor showed that these products do not adsorb competitively with the educt or intermediates. In the literature much of the kinetic data was reported for ill-defined catalyst surfaces. In contrast to that, the present work reports the kinetic study of propane selective oxidation to acrylic acid on a well defined phase-pure and structurally stable M1 MoVTeNbOx catalyst. This study may contribute to the better kinetic and mechanistic understanding of the propane selective oxidation reaction

The Thermodynamics and Kinetics of Saturated Hydrocarbon Separation on a DB-1 Column

Results concerning the thermodynamics and kinetics of separation of some saturated hydrocarbons on a DB-1 non-polar capillary chromatographic column are presented. The effect of temperature on the retention time at constant carrier gas flow was studied in order to determine the adsorption enthalpy and adsorption entropy of the compounds on the poly-dimethyl-siloxane stationary phase. The dependence of the height equivalent to theoretical plate on the carrier gas linear velocities at constant temperature was also determined in order to have access to the optimal linear velocity, the longitudinal diffusion (B) and resistance to mass transfer (C) coefficients. These terms can be determined from the Golay equation. The linearization of the Golay equation is also presented

The effect of salicylic acid on the Briggs-Rauscher oscillating reaction

Results concerning the effect of salicylic acid on the Briggs-Rauscher oscillating reaction in batch mode are presented. In contrast to the other phenolic and polyphenolic compounds studied before, this compound does not stop immediately the oscillations; however it reduces the amplitudes gradually. The time elapsed between the addition of the salicylic acid and the complete cessation of the oscillations is denominated as attenuation time. After the inhibition time the oscillations are restarted with low amplitude. The dependence of the amplitudes, attenuation time, inhibition time and period times on the salicylic acid is presented. Furthermore a new method for kinetic study of the unusual inhibitory effect is implemented. The reaction was followed potentiometrically, by means of an iodide ion selective electrode coupled to a double junction saturated calomel electrode. We derived the integrated rate equation that corresponds to the pseudo-first order reaction followed by a sensor with a nernstian transfer function. The hypothesis concerning the pseudo-first order kinetics of iodide ion production and consumption over a relatively long time within one period of oscillation is proven. The rate constants of iodide ion production and consumption are diminished gradually for the oscillations after the addition of salicylic acid. The fact that salicylic acid decreases the amplitudes and the rate constants of iodide ion formation and consumption during the attenuation period lead to the conclusion that this compound manifests a weak inhibitory effect in the attenuation period as well. However this inhibitory effect increases in time, until complete cessation of the oscillations

Characterization of biomimetic cofactors according to stability, redox potentials, and enzymatic conversion by NADH oxidase from Lactobacillus pentosus

Oxidoreductases are attractive biocatalysts that convert achiral substrates into products of higher value, but they are also for the most part dependent on nicotinamide cofactors. Recently, biomimetic nicotinamide derivatives have received attention as less costly alternatives to natural cofactors. However, recycling of biomimetics is still challenging because there are only limited opportunities. Here, we have characterized various biomimetic cofactors with regard to stability and redox potentials to find the best alternative to natural cofactors. Further, the cofactor spectrum of NADH oxidase from Lactobacillus pentosus (LpNox) could be expanded, and the enzymatic activity was also compared to activities with different small-molecule catalysts. As a result, we succeeded in identifying several strategies for regeneration of oxidized biomimetics

The reaction network in propane oxidation over phase pure MoVTeNb M1 oxide catalysts

MoVTeNb oxide catalysts exclusively composed of the M1 phase (ICSD no. 55097) have been studied in the direct oxidation of propane to acrylic acid applying a broad range of reaction conditions with respect to temperature (623-633-643-653-663 K), O₂ concentration in the feed (4.5-6.0-9.0-12.0 %), steam concentration in the feed (0-10-20-40 %), and contact time (0.06-0.12-0.18-0.24-0.36-0.48-0.72-1.44 s g_cat Nml^-1). The molar fraction of propane was kept at 3.0 %. Model experiments were performed to study the reactivity of possible intermediates propene, acrolein, and CO. The impact of auxiliary steam on the chemical nature of the catalyst surface was analyzed by in-situ photoelectron spectroscopy, while in-situ X-ray diffraction has been carried out to explore the structural stability of the M1 phase under stoichiometric, oxidizing, and reducing reaction conditions. Phase purity apparently accomplishes absolute stability in terms of the crystalline bulk structure and the catalytic performance over month even under extreme reaction conditions. In contrast, the catalyst surface changes dynamically and reversibly when the feed composition is varied, but only in the outermost surface layer in a depth of around one nanometer. The addition of steam causes enrichment in V and Te on the surface at the expense of Mo. Surface vanadium becomes more oxidized in presence of steam. These changes correlate with the abundance of acrylic acid detected in the in-situ photoelectron spectroscopy experiment. Analysis of the three-dimensional experimental parameter field measured in fixed bed reactors revealed that the complexity of the reaction network in propane oxidation over MoVTeNb oxide is reduced compared to less-defined catalysts due to phase purity and homogeneity. The oxidative dehydrogenation of propane to propene followed by allylic oxidation of propene comprises the main route to acrylic acid. The oxygen partial pressure was identified as an important process parameter that controls the activity in propane oxidation over phase-pure M1 without negative implications on the selectivity. High O₂ concentration in the feed keeps the catalyst in a high oxidation state, which provides an increased number of active sites for propane activation. Auxiliary steam increases activity and selectivity of M1 by changing the chemical nature of the active sites and by facilitating acrylic acid desorption

Surface chemistry of phase pure M1 MoVTeNb oxide during operation in selective oxidation of propane to acrylic acid

The surface of a highly crystalline MoVTeNb oxide catalyst for selective oxidation of propane to acrylic acid composed of the M1 phase has been studied by infrared spectroscopy, microcalorimetry, and in situ photoelectron spectroscopy. The acid–base properties of the catalyst have been probed by NH3 adsorption showing mainly Brønsted acidity that is weak with respect to concentration and strength of sites. Adsorption of propane on the activated catalyst reveals the presence of a high number of energetically homogeneous propane adsorption sites, which is evidenced by constant differential heat of propane adsorption qdiff,initial = 57 kJ mol−1 until the monolayer coverage is reached that corresponds to a surface density of approximately 3 propane molecules per nm2 at 313 K. The decrease of the heat to qdiff,initial = 40 kJ mol−1 after catalysis implies that the surface is restructured under reaction conditions. The changes have been analyzed with high-pressure in situ XPS while the catalyst was working applying reaction temperatures between 323 and 693 K, different feed compositions containing 0 mol.% and 40 mol.% steam and prolonged reaction times. The catalytic performance during the XPS experiments measured by mass spectrometry is in good agreement with studies in fixed-bed reactors at atmospheric pressure demonstrating that the XPS results taken under operation show the relevant active surface state. The experiments confirm that the surface composition of the M1 phase differs significantly from the bulk implying that the catalytically active sites are no part of the M1 crystal structure and occur on all terminating planes. Acrylic acid formation correlates with surface depletion in Mo6+ and enrichment in V5+ sites. In the presence of steam in the feed, the active ensemble for acrylic acid formation appears to consist of V5+ oxo-species in close vicinity to Te4+ sites in a Te/V ratio of 1.4. The active sites are formed under propane oxidation conditions and are embedded in a thin layer enriched in V, Te, and Nb on the surface of the structural stable self-supporting M1 phase