Multiscale Operando Structural chAracterization of vanadium phosphate Industrial Catalysts – MOSAIC

The fine structural characterization combining X-ray diffraction and solid NMR requires the preparation of pure phases stabilized at room temperature. An important synthesis work on the precursors of the polymorphs vanadophosphates and on the different VOPO4 phases was carried out. This enabled a thorough characterization of the various phases present, according to the literature, in the post mortem catalyst, including the local order and the influence of the thermal history of the compounds. The polymorph b-VOPO4 which is not remarkable for its catalytic properties and whose structure, perfectly ordered, is described with precision, has also been synthesized to serve as a reference. Essential methodological work has been carried out on the decoupling 51V-31P in order to improve the resolution of the 31P NMR spectra since the signals observed experimentally appear in a very small range of displacement which was until now a limiting factor in the extraction of structural information. Lastly, studies using X-ray diffraction at variable temperature and controlled atmosphere (in situ and operando) were carried out both locally and on large scale facilities. It was possible to perform characterizations under conditions close to that of working conditions of the catalyst, by solid NMR. This approach is in itself a significant technical advance from a methodological point of view and has yielded important results in the study of the materials involved in the catalytic oxidation of butane to maleic anhydride.

This project involved both a work on the in situ characterization of the catalyst, but also methodological developments on the characterization techniques used. The main results obtained are therefore of two types:
-a better description of the phases present during the catalytic oxidation process controlled by vanadium phosphate
-the development of an in situ X-ray diffraction system allowing the study of these catalysts in their operating conditions (450 ° C., air-butane atmosphere) and the development of new solid state NMR decoupling techniques applicable on the vanadium phosphate system, but largely transposable for the study of many classes of inorganic materials.
An important synthesis work has been done. In particular, we have isolated a new precursor of the VPO catalysts whose decomposition allows a better control of the active phases in catalysis.
In order to extract all the possible information from the 31P solid-state NMR experiments, a work in NMR methodology has been carried out to improve the resolution of the spectra. Several elements essential to the structural description of VOPO4 phases have been provided by a combination of GIPAW calculations of NMR parameters from one hand, and, on the other hand, using 2D NMR experiments to evidence homonuclear dipolar correlations to go beyond a simple interpretation of the observed isotropic chemical shifts.
A study under conditions close to the catalytic ones (air + 1.5% butane) of (VO)2P2O7 formed in situ from the precursor VOHPO 4, 0.5H2O was carried out. This innovative experiment allowed us to propose for the first time that the oxidation of the butane be ensured by the w-VOPO4 polymorph.
A quantitative X-ray diffraction analysis of the cooling of this polymorph was carried out. We have been able to show that a transition to d-VOPO4 is possible which could explain why this phase does not appear in the post mortem catalyst.

The MOSAIC project allowed us to answer a number of questions about the vanadophosphate catalysts used in the oxidation of butane into maleic anhydride. Nevertheless, there are still some points to elucidate. Among these we can mention the studies on (VO)2P2O7 and g-VOPO4:
-Temperature structural studies are in progress on the (VO)2P2O7 phase in order to better understand the structural transition observed around 207 ° C. The objective is to determine the vanadium environment in order to propose a structural model at high temperature.
-If the NMR work of 31P is deepened for the g-VOPO4 phase, it was not possible to extract, at this stage, all the information from X-ray diffraction. Studies in transmission electron microscopy are planed in order to obtain additional information on the nature of the defects revealed by the microstructural studies done on X-ray powders diffraction data. Single crystal diffraction at variable temperatures is also envisaged.
This project allowed us to impulse the implementation of a powder X-ray diffraction under controlled atmosphere and variable temperature equipment for structural characterization. It has already been used, beyond the present project, on the study of MOF adsorption properties. This equipment and some results will be presented to the X-ray diffraction community (colloque RX et matière, Nancy 2019 et workshop Réciprocs «Diffraction sous conditions extrêmes ou atypiques/Traitement et outils avancés (partie 2)« Nancy 2019)

Publis (RICL)
C. Kouvatas; V. Alonzo; T. Bataille; L. Le Pollès?; C. Roiland; G. Louarn; E. Le Fur, J. Solid State Chem. 2017, 253, 73-77.
C. Kouvatas, N. Kanwal, J. Trebosc, C. Roiland, L. Delevoye, S. E. Ashbrook, E. Le Fur, L. Le Pollès, J. Magn Res, 2019, 303, 48-56
R. Boulé, C. Kouvatas, C. Roiland, T. Bataille, V. Alonzo, E. Le Fur, L. Le Pollès, Solid State Nucl Mag Res., 2019, 104,101623
Oral
31P NMR study of VOPO4 catalysts: vanadium decoupling and CSA amplification
L. Le Pollès, C. Kouvatas, C. Roiland, T. Bataille, E. Le Fur, L. Delevoye, D. Grekov, S. Ashbrook, S. Sneddon
9th Alpine Conference on Solid State NMR Chamonix, 2015
Structural and 51V NMR parameters relationships in MVO3 crystals
E. Dib, F. Zapata, C. Fernandez
Advanced NMR CASTEP Workshop, Oxford-UK, 2015
31P and 51V NMR studies of VOPO4 catalysts: vanadium decoupling, CSA amplification and NMR parameter calculations
C. Kouvatas, L. Le Pollès, C. Roiland, T. Bataille, E. Le Fur, L. Delevoye, D. Grekov, S. Ashbrook, S. Sneddon
GERM, Lisbonne 2016
Combining in situ XRPD and solid-state NMR to study ?-VOPO4 among VPO catalysts
C. Kouvatas, T. Bataille, L. Le Pollès, C. Prestipino, C. Roiland, L. Delevoye, E. Elkaïm, G. Louarn, E. Le Fur
EPDIC 15 – Bari 2016
Structural studies of industrial VOPO4 catalysts combining XRD, solid-state NMR and NMR parameter calculations.
C. Kouvatas, L. Le Pollès, C. Roiland, T. Bataille, E. Le Fur, G. Louarn, L. Delevoye, D. Grekov, S. Ashbrook, S. Sneddon SMARTER 5– Bayreuth, 2016
Poster
31P NMR applied to vanadium phosphate catalysts: crucial influence of 51V decoupling strategies?
C. Kouvatas, L. Delevoye, J. Tre´bosc, T. Bataille, E. Le Fur, L. Le Polle`s, C. Roiland, S.E. Ashbrook, N. Kanwal EUROMAR 2018 Nantes
Probing local order in vanadates crystals. A combination of 51V NMR and DFT calculations, E. Dib, J. R. Yates, F. Zapata Abellán, C. Fernandez, 9th Alpine conference, 2015, Chamonix
Others
C. Kouvatas : Prix de thèse 2018, Congrès AFC 2018, Lyon

The MOSAIC project “Multiscale Operando Structural chAracterization of vanadium phosphate Industrial Catalyst” is aiming at elucidating the evolution of the bulk of the vanadium phosphates during the catalytic process of the oxidation of butane into maleic anhydride.
This class of material is exploited since the early 80 and a great number of studies have been dedicated to the optimization of the catalyst (new synthetic routes, introduction of promoters) according to empirical strategies. Although fundamental works have been mainly dedicated to the comprehension of the catalyst, they have mainly focused on the supposed active surface.
But contrasted results on supported catalysts and very recent works correlating bulk properties and catalytic activity have evidenced the key role of the bulk material. Moreover oxygen diffusion in the material and relaxation of the vanadium by motion of atoms occurring during the catalytic process have been suggested but never thoroughly studied. We propose a multi-technique fundamental approach for unravelling in operando conditions the multi-scale structural properties of the whole catalytic materials. The combination of X-ray solid state NMR and diffraction/diffusion is the most promising way to approach local order in such materials and dynamic/oxygen diffusion. The results of this project may have an impact on the scientific community (design of new catalysts, methodological aspects for the characterization in operando, …).

The proposed project is a new one and will rely on a consortium with remarkably complementary skills:

Partner 1 – ISCR “Institut des Sciences Chimiques de Rennes ”, Teams “ Solid State Chemistry and Materials” and « Inorganic Theoretical Chemistry ”
Coordination of the project. Elaboration of the materials. Ex and In-Situ laboratory as well as synchrotron X-ray diffraction/total scattering under controlled atmosphere and X-ray operando studies. Application of Maximum Entropy Method (MEM) and total scattering Pair Distribution Function (PDF) analysis to overcame the limitation of classical diffraction method. Solid state NMR studies and theoretical calculations of NMR parameters.

Partner 2 – UCCS “Unité de catalyse et de Chimie du Solide” Lille Team NMR methodology and Glass materials :
Development / improvement of sequences for Solid State NMR, Solid State NMR studies at very low field (51V)

Partner 3 – LCS “Laboratoire de Catalyse et Spectrochimie”, Caen,
In Situ and Operando Solid State NMR studies

Solid state NMR developments (UCCS) will offer improved tool to characterize the local order in most of the catalytic phases. By combining the most recent evolutions in X-ray scattering (PDF, MEM) and solid state NMR experiments supported by theoretical calculations (CTI@ISCR) the static disorder, studied at room temperature will be deeply investigated and the kinetic effects (UCCS) during the elaboration of the different phases studied. In a second step, In Situ studies (variable temperature, static conditions) will be performed (ISCR, LCS) prior to operando studies (catalytic conditions) to evaluate structural changes during thermal treatments and possible apparition of dynamics (in the time scale of NMR spectroscopy). Finally the operando conditions will be applied (ISCR, LCS) and dedicated to the study of phase changes and dynamics within the materials leading to the discovery, of possible oxygen diffusion pathways related to the catalytic reactions.
We will focus on different vanadium phosphate phases commonly found in the catalyst: paramagnetic compound ((VO)2P2O7 ) which is the major constituent and will not be studied so far by solid state NMR spectroscopy as it magnetic properties strongly affect the signal and diamagnetic compounds represented by different polymorphs of VOPO4. The most promising system is w- to d-VOPO4, very selective in catalysis but very disordered at room temperature.