Bernd Kubias

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.

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.

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.

Vanadium phosphorus oxides with P/V=2 used as oxidation and ammoxidation catalysts

Abstract Vanadium polyphosphates with a molar ratio of P/V=2 (α-VO(PO3)2, β-VO(PO3)2 and amorphous as well as partly crystalline VO(PO3)2) and NH4VP2O7 were synthesized, characterized by chemical and thermal analysis, X-ray diffraction and FTIR spectroscopy and used as catalysts in the oxidation of n-butane to maleic anhydride (MA) as well as in the ammoxidation of toluene to benzonitrile. The results are compared with the catalytic properties of V(PO3)3 (P/V=3), (NH4)2(VO)3(P2O7)2 (P/V=4/3) and (VO)2P2O7 (P/V=1). The MA selectivities of the amorphous VO(PO3)2 and of the crystalline α- and β-VO(PO3)2 are comparable to one another, whereas the specific rate per area of MA formation of the amorphous as well as partly crystalline VO(PO3)2 strongly differs from the rates of the crystalline solids. The amorphous catalyst reveals a rate similar to that of the (VO)2P2O7 catalyst, but a lower MA selectivity. Contrary to other studies, only traces of furan were found and the total oxidation products CO and CO2 were detected on all catalysts at very low conversion. Surprisingly, V(PO3)3 exhibits remarkable activity and MA selectivity. The crystalline polyphosphates show a lower activity in the ammoxidation of toluene compared with the amorphous VO(PO3)2 as well as NH4VP2O7 and the benzonitrile selectivity reaches a value of ≈85%. Noticeable benzaldehyde amounts could be detected, especially at low conversion rates, proving its role as a reaction intermediate.

An Extension of the Consistent Valence Force Field (CVFF) with the Aim to Simulate the Structures of Vanadium Phosphorus Oxides and the Adsorption of n-Butane and of 1-Butene on their Crystal Planes

A specific force field of Consistent Valence Force Field type was developed with the aim to simulate the structures of catalysts of vanadium phosphorus oxide type and the reversible adsorption of organic compounds on specific crystallographic planes of such catalysts by molecular modeling. The appropriate parameters were derived for the bonded (stretching, bending, and torsional deformations) and nonbonded (attractive and repulsive van der Waals and Coulomb forces) atomic interactions for V—O and P—O bonds in typical fragments of these catalysts with the vanadium atom in the oxidation state IV. The parameters for bonded interactions were computed from Hessian matrices, supplied by the program DMol for performing Density Functional Theory, by means of a program for non-linear regression. The DMol program was applied to energy minimize structures of known vanadium phosphorus oxides, which were compared with X-ray structures, and to obtain their Hessian matrices as a basis for the force constants needed. Some hypothetical structural models had to be added. The van der Waals parameters were estimated by means of correlations between van der Waals radii and the repulsive parameters and between polarizabilities and the dispersive parameters from the literature. The force field obtained was applied to simulate the crystal structure of vanadyl pyrophosphate and to compute the heat of adsorption of n-butane and of 1-butene on its (100) plane (computer codes of company Biosym/MSI/Accelrys). The experimental crystal structure and the adsorption energies were fairly well reproduced, except that the a lattice constant proves somewhat too large. Eine Erweiterung des Kraftfeldes “Consistent Valence Force Field” (CVFF) fur die Simulation von Vanadium Phosphoroxid-Strukturen und der Adsorption von n-Butan und 1-Buten an ihren Kristallflachen Es wurde eine Erweiterung des bekannten Kraftfeldes vom Typ CVFF fur Molecular Modeling mit Festkorperstrukturen von Katalysatoren vom Vanadiumphosphoroxid-Typ mit Vanadium im Oxidationszustand IV und fur die Simulation der Adsorption von organischen Molekulen an ausgewahlten Kristallflachen erarbeitet. Die notwendigen Parameter fur die sog. bonded (stretching-, bending- und Torsions-Deformationen) und fur die sog. nonbonded Wechselwirkungen (van der Waals- und Coulomb-Wechselwirkungen) in V—O-, P—O- und O—H- Gruppen in typischen Fragmenten solcher Katalysatoren wurden abgeleitet. Hesse-Matrizen wurden mittels der Dichtefunktionalmethode erstellt. Dazu dienten V-, P-, O- und H-Atome enthaltenden Testmolekule, die geometrie-optimiert wurden. Der Parameter fur die van der Waals-Repulsion des Vanadiumatoms wurde uber eine literaturbekannte Korrelation mit van der Waals-Radien und der Dispersionsparameter uber eine Korrelation mit Atompolarisierbarkeiten bestimmt. Das erhaltene Kraftfeld wurde zur Simulation der Kristallstruktur des wichtigen Katalysators Vanadylpyrophosphat und zur Berechnung der Adsorptionswarmen von n-Butan bzw. von 1-Buten auf seiner (100)-Oberflache eingesetzt (Programme der Firma Biosym/MSI/Accelrys). Die Ergebnisse stehen in befriedigender Ubereinstimmung mit experimentellen Literaturwerten.

On the rehydration of vanadyl pyrophosphate to vanadyl hydrogenphosphate hemihydrate

Vanadyl pyrophosphate-based VPO catalysts for butane oxidation to maleic anhydride can be completely rehydrated in the solid phase into their vanadyl hydrogenphosphate hemihydrate precursors which again can be transformed into vanadyl pyrophosphate thus enabling a reuse of spent catalysts.