Bernhard Lücke

In situ-electron spin resonance : a useful tool for the investigation of vanadium phosphate catalysts (VPO) under working conditions

Abstract Differently prepared (VO)2P2O7 phases and an amorphous V3+PO catalyst were investigated under conditions of the selective n-butane oxidation and the toluene ammoxidation, respectively, using a self-constructed in situ-ESR flow reactor in the X-band. By examining the temperature dependence and the line shape of the ESR signals exchange integrals as well as the 2nd and the 4th moment were obtained. These parameters characterize the spin—spin exchange behaviour and, thus, structural and electronic disorder of the catalysts. Increasing structural disorder was found to improve the catalytic performance in the n-butane oxidation. For both catalytic processes a significant reversible alteration of the ESR line shape was observed under working conditions which is discussed in terms of a perturbation of exchange interactions between neighbouring vanadyl centres near the surface the oxidation state of which is assumed to fluctuate between +4 and +5 during the catalytic reaction.

FTIR investigation of surface intermediates formed during the ammoxidation of toluene over vanadyl pyrophosphate

Abstract The mechanism of formation of surface intermediates, appearing during the interaction of vanadyl pyrophosphate (VO) 2 P 2 O 7 with feed components of the toluene ammoxidation was studied by FTIR spectroscopy. The investigation of ammonia adsorption at elevated temperature showed protonated and coordinated ammonia as expected as well as the generation of amido species; all could be so called ‘nitrogen insertion species’ or a source of these at least. The interaction of toluene and possible reaction intermediates such as benzaldehyde and benzylamine with (VO) 2 P 2 O 7 was studied. The investigations revealed that the ammoxidation of toluene probably proceeds via the formation of a benzaldehyde intermediate. Afterwards, benzylimine surface species were probably formed by a N-insertion, using NH + 4 surface species followed by the formation of benzonitrile by subsequent oxidative dehydrogenation. However, no benzamide species were observed. The surface species generated upon adsorption of benzaldehyde were similar to those formed from toluene, indicating the role of the former as intermediate in the nitrile formation path. Otherwise, the adsorption of benzylamine in the presence of oxygen did not lead to the formation of benzonitrile. Therefore, an ammoxidation mechanism of toluene via a benzaldehyde intermediate is preferred and reaction pathways via benzamide or benzylamine as intermediates seem to be improbable.

Ammoxidation of halogen-substituted toluenes on V-P-oxide catalysts

The ammoxidation of various halogen-substituted toluenes on crystalline vanadium phosphate catalysts was investigated. The monophosphates and the NH4VOP2O7 are transformed into new NH4-containing V-P-oxides. The (VO)2P2O7 used is stable in time on stream. These structures are very active and selective ammoxidation catalysts.AbstractИсследовали окислительное аммонирование различных галогензамещенных толуолов на кристаллических катализаторах фосфата ванадия. Использованные монофосфаты и NH4VOP2O7 превращены в новые V-P окислы, содержащие NH4. Использованные (VO)2P2O7 оказались стабильными во времени и на потоке. ЭТИ соединения являются очень активными и селективными катализаторами окислительного аммонирования.

Investigation of vanadium phosphorus oxide catalysts (VPO) during toluene ammoxidation: new mechanistic insights by in situ EPR

(VO)2P2O7, differently prepared (NH4)2(VO)3(P2O7)2 samples and supported amorphous vanadium phosphorus oxide (VPO) catalysts were investigated during the ammoxidation of toluene using a self-constructed in situ EPR flow reactor in the X-band. For the unsupported catalysts, changes in the spin–spin exchange behaviour were analysed by calculating the quotient of the 4th and the square of the 2nd moment, 〈B4〉/〈B2〉2, of the absorption signal. The catalytic activity was found to increase with exchange efficiency, while isolated vanadyl centres, as present in the supported catalysts, do not obviously participate in the catalytic process. NH4+ containing catalysts can easily supply ammonia for N insertion into the hydrocarbon indicating that the latter does not react directly with NH3 molecules from the gas phase but with those activated by adsorption on the surface of the catalyst according to a Langmuir–Hinshelwood mechanism.

Redox interaction of ammonia with (VO)2P2O7

The interaction of ammonia with (VO)2P2O7 prepared by calcination of the precursor compound VOHPO4· 0.5H2O under nitrogen has been studied using temperature-programmed desorption of ammonia (TPDA), temperature-programmed reaction spectroscopy (TPRS), and IR and EPR spectroscopy. Mass-spectrometric detection was applied to observe possible ammonia decomposition or oxidation products. The investigation revealed that ammonia is not only adsorbed on but also reacts with (VO)2P2O7 in a redox process generating nitrogen, water and an amorphous VIII-containing compound, the concentration of which could be directly determined by potentiometric titration. The high amount of VIII found pointed towards reduction of VIV not only on the surface but also in deeper layers of the bulk. This was also confirmed by EPR spectroscopy. Furthermore, this reaction results in a change of the Bronsted and Lewis acidity observed by IR spectroscopy. The concentration of the Bronsted-acid OH groups was strongly enhanced by hydrolysis of P—O—P and/or V—O—P links by water formed during the redox reaction. The increased concentration of Lewis sites was caused by the removal of oxygen from surface vanadyl groups, probably creating additional coordinatively unsaturated sites. The influence of the observed redox reaction on the characterization of the acidity and the formation of VPO catalysts in the ammoxidation reaction are discussed.

Catalytic performance of vanadyl pyrophosphate in the partial oxidation of toluene to benzaldehyde

Vanadyl pyrophosphate catalysts were generated by dehydrating VOHPO4·(1/2)H2O at different temperatures and duration of the dehydration procedure. The as-synthesised materials were characterised by X-ray diffractometry, FTIR spectroscopy and temperature-programmed reduction. The prolongation of the formation period at higher temperatures led to improved crystallinity and lower BET surface areas of the vanadyl pyrophosphate specimens and, additionally, a significantly impeded reducibility of the vanadyl sites was observed. The catalytic performance of the samples was tested in the partial oxidation of toluene to benzaldehyde. The obtained results revealed an increasing benzaldehyde selectivity with improved catalyst crystallinity. In situ FTIR and ESR spectroscopy were used to throw more light on the interaction of the toluene–air feed with the surface of the catalyst.

Structure of vanadium sites in VPO catalysts and their influence on the catalytic performance in selective O- and N-insertion reactions

The behaviour of (NH4)2V4+OP2O7, NH4V3+P2O7 and V3+(PO3)3 as catalysts in the ammoxidation of toluene and in the selective oxidation of n-butane has been studied by insitu EPR and catalytic measurements. In the first two samples an amorphous phase is formed under ammoxidation conditions which consists of effectively coupled VO2+ ions and governs catalytic performance. In the case of V(PO3)3, few isolated VO2+ ions appear on the surface during toluene ammoxidation while coupled VO2+ ions are formed during n-butane oxidation. This catalyst was found to be inactive in the former but active in the latter reaction. The differences in catalytic performance are explained in terms of the different structure of the vanadium centres.

Enhancement of the catalytic activity of VPO ammoxidation catalysts by use of vanadyl(IV) orthophosphate precursor compounds

(VO)3(PO4)2·7H2O and (VO)3(PO4)2·9H2O vanadyl(IV) orthophosphate hydrates (V/P = 1.5) used as precursor compounds were transformed into highly active ammoxidation catalysts during different pretreatment procedures. These structural transformations have been investigated in the presence of ammonia-containing gases or under nitrogen, leading into materials that contain crystalline (NH4)2(VO)3(P2O7)2 (V/P = 0.75) and (VO)2P2O7 (V/P = 1) specimens, respectively, as well as an additional X-ray-amorphous phase of partially crystalline vanadium oxides. These vanadium oxides represent the molar vanadium excess of the original precursor material in comparison to the defined vanadium amount of the crystalline proportion of the transformation product. Both components of these solids are tightly grown together to form microdomains. The solids generated this way were characterized by XRD, EDX, FTIR spectroscopy, 31P MAS NMR spectroscopy, XPS and chemical analyses and used as catalysts in the ammoxidation of toluene to benzonitrile. The results of the heterogeneous catalytic ammoxidation on the orthophosphate derived catalysts were compared with those runs obtained by the application of pure, as-synthesized (NH4)2(VO)3(P2O7)2, similar transformation products derived from VOHPO4·0.5H2O precursor (V/P = 1) and pure (VO)2P2O7. Owing to the existence of mixed-valent vanadium oxides in increased portions of the orthophosphate derived catalysts, these solids reveal a significant enhancement of the toluene conversion rate at almost equal high nitrile selectivities in comparison to usual VPO catalysts.

Soluble and supported carbonylation catalysts derived from rhodium-phosphonate-phosphane complexes

Phosphonate-phosphanes form both chelate and open-chain complexes with rhodium, which can easily be converted into each other. This property has been used in the development of new and efficient carbonylation catalysts. During the catalytic cycle, the phosphane group is assumed to be coordinated to the transition metal, while the phosphoryl-oxygen of the phosphonate group is supposed to change between a free and a coordinated state, thus vacating or occupying a coordination site. IR studies support the hemilabile behaviour of surface Rh-complexes under the reaction conditions. Activation enthalpies, obtained for hemilabile rhodium catalysts in homogeneous methanol carbonylation, increased significantly with growing distance between phosphonate and phosphane group. Attempts of preparing stable rhodium complex catalysts adsorbed on silica or alumina for slurry-phase or vapour-phase carbonylations failed. Activated carbon has been shown to be a suitable support for hemilabile rhodium complexes, but normal diffusion of reactants begins to limit the carbonylation rate over the supported catalysts.

In-situ electron spin resonance study of vanadium phosphate catalysts during the selective oxidation of n-butane to maleic anhydride

Abstract Differently disordered (VO) 2 P 2 O 7 phases and an amorphous V 3+ PO catalyst were studied by electron spin resonance (ESR) spectroscopy at various temperatures and in various atmospheres, as well as under catalytic conditions, using an in-situ ESR flow reactor operating in the X band. Exchange integrals and the second and fourth moments of the ESR signals were used to characterize spin-spin exchange in (VO) 2 P 2 O 7 , which depends on the degree of structural disorder. The catalytic activity of (VO) 2 P 2 O 7 and the maleic anhydride selectivity are enhanced with rising disorder. During butane oxidation a significant reversible alteration of the ESR line shape became evident. This unique effect can only be observed under working conditions and is discussed in terms of a perturbation of exchange interactions between neighbouring vanadyl centres near the surface, the oxidation state of which is assumed to fluctuate between +4 and +5 during the catalytic reaction. In contrast, heating in pure air improves the spin-spin exchange and lowers the disorder by healing lattice defects. The activity of the V 3+ PO catalyst in the title reaction “awoke” after about 20 min time on stream, simultaneous with the formation of a (VO) 2 P 2 O 7 -like local arrangement of V 4+ centres on the catalyst surface.