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

Multifunctionality of crystalline MoV(TeNb) M1 oxide catalysts in selective oxidation of propane and benzyl alcohol

Propane oxidation at 653-673 K and benzyl alcohol oxidation at 393 K over phase-pure MoV(TeNb) M1 oxide catalysts were studied to gain insight into the multiple catalytic functions of the surface of the M1 structure. Electron microscopy and X-ray diffraction confirmed the phase purity of the M1 catalysts. Propane oxidation yields acrylic acid via propene as intermediate, while benzyl alcohol oxidation gives benzaldehyde, benzoic acid, benzyl benzoate, and toluene. The consumption rates of benzyl alcohol and propane level in the same range despite huge difference in reaction temperature, suggesting high activity of M1 for alcohol oxidation. Metal-oxygen sites on the M1 surface are responsible for the conversion of the two reactants. However, different types of active sites and reaction mechanisms may be involved. Omitting Te and Nb from the M1 framework eliminates acrylic acid selectivity in propane oxidation, while the product distribution in benzyl alcohol oxidation remains unchanged. The results suggest that the surface of M1 possesses several types of active sites that likely perform a complex interplay under the harsh propane oxidation condition. Possible reaction pathways and mechanisms are discussed