Ursula Bentrup

The effect of alkali metal promotion on vanadium-containing catalysts in the vapour phase oxidation of methyl aromatics to the corresponding aldehydes

The comparison of vanadium phosphate catalysed ammoxidation with the oxidation of methyl aromatics to the corresponding nitriles and aldehydes showed significant differences in catalytic performance. The ammoxidation proceeds with nearly complete conversion and high nitrile selectivities, whereas the partial oxidation of the same feed to aldehydes under similar conditions leads to poor activity and selectivity only. This result is supposed to be due to the lack of a sufficient basicity in the reaction system; catalyst surface acidity causes strong reactant–solid interaction that is the reason for consecutive overoxidation. Therefore, potassium-containing vanadium phosphates and more basic alkali cation-containing vanadia solids (M/V2O5 ,M = Li, Na, K, Rb, Cs) were synthesised and used as catalysts in the vapour phase oxidation of toluene (TO), p-methoxytoluene (MTO) and p-chlorotoluene (CTO) to the corresponding aldehydes. Moreover, the effect of the addition of a non-oxidisable base (e.g. pyridine) to the reaction mixture was studied. Parent catalyst samples and used specimens were characterised by X-ray diffractometry and infrared spectroscopy. Vanadates and bronze phases of the more bulky alkali metal cations are generated during catalyst synthesis and catalytic reaction that may lead to new surface arrangements with vanadia. The catalytic results showed that alkali metal-containing vanadia catalysts revealed significant higher performance compared to vanadium phosphate solids. The increasing alkali cation size and basicity in the order from Li to Cs leads to an increase in the aldehyde selectivity. The effect of acid–base properties of catalyst bulk and surface in connection with reactant nucleophilicity and the impact of electronic effects of the substituents on the catalytic behaviour are discussed.

Base‐Free Non‐Noble‐Metal‐Catalyzed Hydrogen Generation from Formic Acid: Scope and Mechanistic Insights

The iron-catalyzed dehydrogenation of formic acid has been studied both experimentally and mechanistically. The most active catalysts were generated in situ from cationic Fe(II) /Fe(III) precursors and tris[2-(diphenylphosphino)ethyl]phosphine (1, PP3 ). In contrast to most known noble-metal catalysts used for this transformation, no additional base was necessary. The activity of the iron catalyst depended highly on the solvent used, the presence of halide ions, the water content, and the ligand-to-metal ratio. The optimal catalytic performance was achieved by using [FeH(PP3 )]BF4 /PP3 in propylene carbonate in the presence of traces of water. With the exception of fluoride, the presence of halide ions in solution inhibited the catalytic activity. IR, Raman, UV/Vis, and EXAFS/XANES analyses gave detailed insights into the mechanism of hydrogen generation from formic acid at low temperature, supported by DFT calculations. In situ transmission FTIR measurements revealed the formation of an active iron formate species by the band observed at 1543 cm(-1) , which could be correlated with the evolution of gas. This active species was deactivated in the presence of chloride ions due to the formation of a chloro species (UV/Vis, Raman, IR, and XAS). In addition, XAS measurements demonstrated the importance of the solvent for the coordination of the PP3 ligand.

Synthesis and Application of Carbonated Fatty Acid Esters from Carbon Dioxide Including a Life Cycle Analysis

Carbon dioxide can be used in various ways as a cheap C1 source. However, the utilization of CO2 requires energy or energy-rich reagents, which leads to further emissions, and therefore, diminishes the CO2-saving potential. Therefore, life cycle assessment (LCA) is required for each process that uses CO2 to provide valid data for CO2 savings. Carbon dioxide can be incorporated into epoxidized fatty acid esters to provide the corresponding carbonates. A robust catalytic process was developed based on simple halide salts in combination with a phase-transfer catalyst. The CO2-saving potential was determined by comparing the carbonates as a plasticizer with an established phthalate-based plasticizer. Although CO2 savings of up to 80 % were achieved, most of the savings arose from indirect effects and not from CO2 utilization. Furthermore, other categories have been analyzed in the LCA. The use of biobased material has a variety of impacts on categories such as eutrophication and marine toxicity. Therefore, the benefits of biobased materials have to be evaluated carefully for each case. Finally, interesting properties as plasticizers were obtained with the carbonates. The volatility and water extraction could be improved relative to the epoxidized system.

Tailored Noble Metal Nanoparticles on γ‐Al2O3 for High Temperature CH4 Conversion to Syngas

The simple deposition of tailored Rh or Pt nanoparticles (NP) on γ‐Al2O3 results in active, selective, and stable catalysts for partial oxidation of methane (POM) to syngas. The NP were prepared by the ethylene glycol method in strong alkaline solution. This approach was found to be a promising way of providing active metallic NP for catalyzing the POM reaction. NP sizes determined by small angle X‐ray scattering (SAXS) in solution and by transmission electron microscopy (TEM) on alumina were very close. This result highlights the possibility of easy pre‐characterization of NP by the former method. Supported Rh NP are intrinsically more active in the POM reaction than Pt NP and also showed a superior performance compared with a conventionally prepared Rh catalyst, even if the latter had been pre‐reduced. Mechanistic investigations in the temporal analysis of products (TAP) reactor indicate that the higher selectivity of well‐defined Rh‐NP is determined by their lower activity for consecutive CO transformations.

Reaction Monitoring of Heterogeneously Catalyzed Hydrogenation of Imines by Coupled ATR‐FTIR, UV/Vis, and Raman Spectroscopy

Coupled ATR‐FTIR, UV/Vis, and Raman spectroscopy has been introduced for the monitoring of liquid phase hydrogenation reactions under elevated H2 pressure by implementation of spectroscopic immersion probes into a modified autoclave reactor. The setup was successfully tested for the heterogeneously catalyzed hydrogenation of several imine substrates under H2 pressures up to 20 bar. The conversion of the imines could be analyzed by Raman spectroscopy and the product formation was observable by attenuated total reflectance FTIR spectroscopy (ATR‐FTIR), which illustrates the benefits of coupling complementary spectroscopic methods. Separate spectroscopic investigations show that imines intensively interact with the phosphoric acid ester that is used as catalyst modifier, indicated by a shift of the original ν(CN) band of the imine to higher wavenumbers. UV/Vis spectroscopic investigations reveal a distinct shift of the absorption edge to a higher wavelength. Furthermore, the formation of adsorbates on the surface of used Pt/Al2O3 catalysts, resulting mainly from adsorbed imine, was detected by FTIR spectroscopic analysis. The potential of UV/Vis spectroscopy in transmission mode for analyzing the organic components was tested. Due to its high sensitivity at low concentrations this method offers an additional possibility for a quantitative in situ analysis of the reaction progress in real time.

The Enhancing Effect of Brønsted Acidity of Supported MoOx Species on their Activity and Selectivity in Ethylene/trans-2-Butene Metathesis

Supported catalysts with a nominal Mo surface density of 0.15 and 1.5 Mo atoms nm−2 were synthesized by impregnation of alumina, silica, and alumina–silica supports with silica content between 1 and 70 wt %. They were tested for their activity and selectivity in the metathesis of ethylene and trans‐2‐butene to propene between 343 and 603 K at 125 kPa. The catalysts were characterized by UV/Vis, Raman, and IR spectroscopy, XRD and H2 temperature‐programmed reduction for elucidating the distribution, degree of polymerization, reducibility, and acidity of MoOx species. We established that Brønsted acidity of highly dispersed tetrahedral and polymerized octahedral MoOx species is required to ensure high metathesis activity. The acidic character of these species is influenced by their structure and support. Tetrahedral MoOx species with Brønsted acidic character are only formed on supports possessing such acidity, whereas Brønsted acidic octahedral MoOx is also created on supports without such acidic sites.

Effect of VOx Species and Support on Coke Formation and Catalyst Stability in Nonoxidative Propane Dehydrogenation

VOx/SiO2–Al2O3 catalysts were prepared by grafting vanadyl acetylacetonate onto the supports with a SiO2 content between 0 and 100 wt. %. The degree of polymerization of VOx species and acidity both of pristine supports and the catalysts were evaluated. To determine their on‐stream stability and carbon deposition activity in nonoxidative propane dehydrogenation, continuous‐flow tests and in situ thermogravimetric measurements were performed. The rate constants of catalyst deactivation and carbon deposition were derived from kinetic evaluation of these experiments. Gathered experimental evidence pointed out that VOx species were significantly more active for coke formation than acid sites of the supports. The rate constant of carbon formation was found to increase with the degree of polymerization of VOx species, whereas no correlation between catalyst acidity and the rate constants of coking or deactivation could be drawn.

Comparative study of the thermal and redox behaviour of alkali-promoted V2O5 catalysts

Abstract Alkali-promoted V 2 O 5 catalysts M-V 2 O 5 (M=Li, Na, K, Rb and Cs) synthesised by impregnation of V 2 O 5 with alkali sulfate solution have been investigated under inert and reducing atmosphere using thermoanalytical methods (TG/DTA, differential scanning calorimetry (DSC) and temperature-programmed reduction (TPR)). Pure V 2 O 5 was used for comparison. Whereas in Li- and Na-promoted catalysts only V 2 O 5 as crystalline phase could be detected by X-ray diffraction (XRD), the K-, Rb-, and Cs-promoted catalysts additionally contain the vanadate phase MV 3 O 8 . The surface acidity (Bronsted- and Lewis-sites) as well as the starting temperature of the hydrogen consumption decrease with increasing size of the alkali cation. The reduction of the K-, Rb-, and Cs-promoted catalysts leads to the formation of bronze-like phases besides V 2 O 5 at relative low temperatures. The bronze phases stabilise the V 4+ oxidation state and improve the redox properties. A characteristic splitting and shifting of the ν(VO) mode in the FTIR spectrum indicates the formation of V 4+ in the different bronze phases. The favoured formation of bronze-like phases especially under reducing conditions enhances the release of SO 2 at lower temperatures, the formation of H 2 S can be neglected.

Selective oxidation of p-substituted toluenes to the corresponding benzaldehydes over (VO) 2P2O7 : an in situ FTIR and EPR study

Abstract The adsorption and oxidation of p -chlorotoluene (PCT), p -methoxytoluene (PMT), and toluene on vanadyl pyrophosphate catalyst (VPP) were studied by in situ FTIR and EPR spectroscopy. Various amounts of strongly adsorbed benzaldehydes and cyclic anhydride species were observed by FTIR in dependence on the different educts after oxidation experiments. The extent of spin–spin exchange pertubation and, thus, the loss of the EPR signal intensity caused by substrate adsorption and interaction is influenced by the nature of the aromatic compound. The strength of reactant and product adsorption on the catalyst surface was found to be an important selectivity-limiting factor in the aldehyde formation. The benzaldehyde adsorption is enhanced by additional interaction of the carbonyl group with Bronsted acid hydroxyl groups generated during oxidation reaction, which facilitates deeper oxidation. The co-adsorption of pyridine is one possibility to suppress the strong aldehyde adsorption and to improve the selectivities. Yields of benzaldehydes and selectivities at constant conversion increase in the order PMT p -substituents causes total oxidation leading to lower aldehyde selectivities. Both, the acid/basic characters of the reactants and products and their steric properties play an important role for adsorption/desorption processes.

Vanadium‐Containing Oxynitrides: Effective Catalysts for the Ammoxidation of 3‐Picoline

Two series of vanadium‐containing oxynitrides VMON (M=Al or Zr, 0.1