​Controlling defects to modulate the acid-base balance

Historically, petroleum chemistry was based on acid catalysis, but the shift towards biomass upgrading has demonstrated the value of basic materials. The laboratory is recognized for its work in this field, and is diversifying the materials it studies to meet the demands of reactions requiring specific acid-base balances.

Hydroxyapatites, biocompatible calcium phosphates, are widely studied (synthesis and reactivity) at LRS, both for their bioactivity and their multifunctional chemical reactivity. Whether in the field of biomineralization (Marc ROBIN Labex MATISSE PhD thesis) or heterogeneous catalysis (C. REYNAUD, ENS Cachan PhD thesis), variability in the composition and morphology of hydroxyapatite particles is crucial to controlling their reactivity, but knowledge of the link between these characteristics and preparation conditions is highly empirical. The thermodynamics and kinetics of calcium phosphate precipitation have been modeled, and the different precipitation pathways to hydroxyapatite thus predicted have been validated by in situ Raman spectroscopy during both controlled catalyst preparation and biomineralization. Identification of the transient species and control of their hydrolysis rationalize the nature and proportion of structural and surface defects at the origin of acid-base reactivity modulation, enabling an informed choice of operating conditions according to the type of reactivity to be favored.

Other systems such as nanotalcs or magnesium silicates, prepared through external collaborations (Coll. F. MARTIN IRD, Toulouse, Coll. J.F. HOCHEPIED, ENSTA, Paris, respectively) constitute catalytic materials combining acidic and basic sites. The influence of the synthesis route of magnesium silicates (sol-gel vs. precipitation) on their reactivity in the transesterification reaction of ethyl acetate with methanol has been attributed, via correlations with model reactions and kinetic approaches, to the variability of the Mg/Si ratio, which modifies the acid-base balance. The coupling of DFT with IR and NMR spectroscopy has also shed light on the impact of nanotalc synthesis conditions on the occurrence of structural defects.

The modulation of acid-base properties by the incorporation of fluorine into oxides is currently being studied within the framework of the ANR FluOHMat (Coll IC2MP, Poitiers).

Controlled "single-site" formation

Our laboratory also has considerable expertise in the functionalization of zeolite systems through the incorporation of single metal ions. The selective formation of these sites during synthesis (preferably in the reticular position, but also through the formation of M_xO_y nanodomains) is proving decisive in controlling the selectivity of numerous reactions studied in collaboration (Poland, Ukraine: selective reduction of NO, Fischer-Tropsch synthesis and elimination of volatile organic compounds) or in-house (dehydrogenating oxidation of propane).

Observation of the speciation and environment of metal cations by EPR and NMR, including nuclei that are particularly difficult to study (Ag or Y), is therefore crucial to these studies. The modification of hydroxyapatites with metals (V, Co, Cu, Ru) can also, under certain conditions, generate isolated atomic species. Anchored in the reticular position of the surface, they prove particularly resistant to sintering (PhD thesis by C. REYNAUD). The combination of the modulable basicity of hydroxyapatites with the formation of isolated single-atom redox sites leads to unique performances for the oxidative dehydrogenation reaction of propane, and is also being studied for other reactions of interest (Guerbet, selective hydrogenations, NH₃ synthesis (ANR CASTORAMA)).

The aim of characterizing sites as closely as possible to reaction conditions means adapting techniques/spectroscopies to a sample environment representative of their mode of operation. The full range of vibrational spectroscopies (Raman, IR (DRIFT, FTIR, ATR) UV-Vis), as well as EPR, photoluminescence, NMR and XRD, can now be used in situ to monitor the impact of activation modes on site structure and reactivity. A set-up for IR Raman coupling has also been developed in-house.

Operando approaches are used to monitor catalysts in operation, explain their poisoning, and, ultimately, identify active sites (DRIFT experiments carried out in-house (PhD thesis M. BEN OSMAN, SANGI contract) and impedance spectroscopy (PhD thesis S. PETIT, DIM OXYMORE, in collaboration with LCMCP). Developments in UV-Vis, FTIR and the design of multi-technique reactor cells (which could, among other things, be used at the synchrotron) will rapidly be taken forward thanks to the expertise of two young researchers.

The specific challenges posed by the difficult characterization of the solid-liquid interface have been addressed in the dedicated section of theme 2. The laboratory's commitment to the development of spectroscopies applied to the study of reaction mechanisms is reflected in the organization by A. MEZZETTI and J. SCHNEE of a dedicated thematic school (MECAREACT) in June 2023. Several examples of the decisive contribution of MAS NMR to site discrimination (core/surface) and evaluation of the spatial proximity of atoms (2D NMR) for structural identification of active sites, and of Infrared spectroscopy for characterization of acidic and/or basic properties, are presented below in the highlights of theme 3.

Other methodological developments carried out in the laboratory that contribute to in-depth knowledge of complex systems include:

• In situ implementation of EPR and photoluminescence, which have enabled us to: (i) identify the spectroscopic signatures of intrinsic defects in very pure ZnO (PhD thesis M. ZHANG, CSC, Coll. S. STANKIC INSP); (ii) whether these are native defects or to follow their formation/interconversion during post-synthesis treatments carried out in situ; (iii) to highlight the conditions of occurrence of oxygen vacancies; and (iv) to discriminate between defects located in the core and those present on the surface.
• TPD of NOx, developed at LRS by C. THOMAS, has become a reference technique for characterizing the coverage rates of one oxide on another. FTIR monitoring of nitrogen oxide adsorption on oxide surfaces is a method used to reveal the acidic or basic nature/reactivity of oxide surface sites. The TPD of NOx uses this property by combining it with the quantitative character of temperature-programmed desorptions (TPD). This original approach has proved particularly robust for characterizing complex systems such as WO_x-ZrO₂ supported on multi-walled carbon nanotubes (MWCNTs) or Pd/SiO₂@Zr core@shell catalysts used for oxidation reactions in diesel engine effluent treatment.  This study made it possible to attribute the hydrothermal ageing resistance of these catalysts to the presence of Si-O-Zr bonds in a SiO₂@SiZrO₄ core@shell oxide support, rather than the SiO₂@ZrO₂ core@shell oxide support previously claimed in the literature. Recently, the quantification by NOx TPD of active sites within hydroxyapatites modified at the core (co-precipitation) or specifically at the surface (exchange) by cobalt has enabled an unprecedented comparison of the performance of bulk and supported catalysts for propane ODH (PhD thesis by C. REYNAUD). For both synthesis routes, the TOF (per cobalt surface atom) is identical and is, moreover, among the highest reported to date for this reaction.

The elucidation of reaction mechanisms and active sites is of particular and original importance in scenarios concerning the origin of life. The hypothesis underlying this research is that the pre-existing complexity of the mineral world may have served as a guide to the emergence of life, and the LRS has developed highly-recognized expertise in this field. Three main directions have been explored in recent years:

• The combination of IR, in situ NMR and molecular modeling has enabled the molecular identification of reaction sites on mineral solids and the reaction mechanisms leading to amino acid condensation (H. ABADIAN PhD thesis, O. EL SAMROUT PhD thesis, ED 397 and Coll.Int.).
• The non-random character of the polymerization of amino acid mixtures on mineral supports has been highlighted and examined from the point of view of catalytic selectivity and Shannon information theory (PhD thesis by L. BEDOUIN, DIM ACAV+).
• Finally, in order to understand the emergence of complex networks of integrated reactions in a mineral world, the successive stages of an important pathway of current metabolism (nucleic base synthesis) were reproduced using exclusively non-biological energy sources and mineral catalysts (PhD hesis L. TER-OVANESSIAN, ED 397).

The characterization of active sites is particularly complex in the case of amorphous materials such as silica-alumina. As with zeolites, the acidity (Lewis and Bronsted) of these materials is related to the presence of Al atoms. However, the low fraction of acid sites in these materials relative to their Al content makes the structural identification of their acid sites even more complex. To facilitate this identification by multi-nuclei NMR, the first step in Xiaojing JIN's PhD thesis work was to increase the fraction of acidic Al either (i) by selectively removing non-acidic aluminum by contact with complexing agents (acetylacetone or citric acid), or (ii) by grafting Al onto silicas. These different synthesis routes yielded silica-alumina up to 1.5 times more acidic than commercial samples, and with up to 1.8 times greater catalytic activity.

The fraction of acidic Al was thus increased to 33% of Al atoms, compared with 2-4% in commercial silica-alumina. However, comparison of these silica-alumina materials shows that increasing the fraction of acidic Al is necessarily at the expense of their surface density. Thus, obtaining a silica alumina with a significant amount of acidic sites does not seem possible, which is a serious limitation to the study of these sites by multinucleus NMR. Nevertheless, MAS NMR characterization of these silica-alumina has revealed that only Al in coordinates 4 (Td) (excluding Al in coordinates 5 and 6 (Oh)) are involved in Bronsted acidity (²⁷Al-¹H MQ-HETCOR NMR), but that these Bronsted acid sites are not identical to those of zeolites (longer A-H distances, ²⁷Al-¹H REAPDOR) and that the protons associated with these acid sites have a different chemical shift to that of the isolated silanols.

Discrimination and role of Lewis acid sites in the oxidative dehydrogenation of alcohols under mild conditions

Ligand defects present on the surface or in the core of UiO-66 MOFs (zirconium terephthalate) lead to sub-coordinated Zr atoms that possess a Lewis acid character highlighted by CO adsorption followed by FTIR. However, proton NMR characterization of the hydrated form enabled us to discriminate between two types of unsaturated zirconium sites, located within the structure and on the surface. Their proportions were modulated by the stoichiometry of the organic ligand and the size of the MOF crystals, respectively.

Comparison of the activities of these MOFs in the oxidative dehydrogenation of primary alcohols in the presence of TBHP at near-ambient temperatures has led to the proposal of a reaction mechanism involving the concomitant adsorption of the 2 molecules on a Zr-OH-Zr pair consisting of two adjacent unsaturated zirconiums, which requires good accessibility to these sites. Activity is thus optimal on sites located on the particle surface. The joint involvement of core sites in the reaction requires very open porosity and is therefore only observed at high core defect rates..

Addition / Elimination

Magnesium nano-silicates are a family of poorly crystalline, more or less hydrated and defective lamellar materials whose surface chemistry and morphology have been studied in the PhD theses of D. Cornu (ENS funding) and L. Lin (CSC funding) and the post-doc of E. Silva Gomes (labEx Matisse funding), using a combined experimental (NMR ²⁹Si, ¹H, ²⁵Mg, ²³Na) and theoretical approach. Silva Gomes (labEx Matisse funding) using a combined experimental (²⁹Si, ¹H, ²⁵Mg, ²³Na NMR) and theoretical approach. Magnesol (a commercial hydrated magnesium silicate) and laponite (a magnesium phyllosilicate) efficiently catalyze the liquid-phase transesterification reaction of methanol with ethyl acetate. However, the kinetic study concludes that different mechanisms are involved, namely of the Langmuir-Hinshelwood and Eley Rideal types respectively. This differentiation is explained by the possible cooperation between Bronsted acidic sites and basic sites on magnesol (which adsorb AcOEt and MeOH, respectively), whereas for laponite only basic sites are involved (to adsorb methanol, which reacts with AcOet in the liquid phase). This feature makes it more adaptable to the formation of longer-chain esters.

C-C bond formation

The hydroxyapatite system is a multifunctional calcium phosphate that exposes different types of basic and acidic sites and proves to be much more selective for the conversion of ethanol to n-butanol (solvent & additive in gasoline) than conventional basic oxide catalysts.

As part of M. BEN OSMAN's PhD thesis (funded by SANGI), DRIFT operando monitoring of reaction-poisoning-regeneration cycles has shown that conversion is controlled by the concentration of basic OH sites, but that selectivity depends on the nature of their acid partner: both types of acid-base pairs, Ca²⁺-OH^- and POH-OH, are involved in the ultimate production of n-butanol.

In S. PETIT's PhD thesis (funded by DIM OXYMORE), the study of the dehydrogenating oxidation of propane (ODHP) was initiated. Modified hydroxyapatite substituted with vanadate ions activates the C-H bond of propane, even though the vanadate ions are relaxed at the subsurface and the basic OH site is a weak base. This surprising behavior revealed a surface activation mode assisted by the thermal activation of core proton mobility in connection with OH stacking defects revealed by DRIFT and ¹H NMR.

This mass-dynamic process monitored by operando impedance spectroscopy, exalted under reaction, favors the in situ production of the metastable vanadium oxy-hydroxyapatite active phase, whose surface-exposed O^2- anions turn out to be strong bases capable of tearing a proton away from propane.

The propene selectivity of the ODHP reaction is greatly enhanced (x2) by the epitaxial growth of Ca₄V₄O₁₄ on the vanadium-substituted hydroxyapatite. The interface structure has been solved by combining advanced NMR sequences (⁵¹V DQ/SQ) and EPR (performed in-house and via the RENARD Network for ENDOR).

The synergy observed for the two-phase system is attributed to the combination of the propane activation mode on V-HAP and the redox exchanges favored within V₄O₁₄ tetramers compared to the VO₄ monomers alone present in vanadium hydroxyapatite.

The identification of silica surface sites involved in the polymerization of amino acids (by the formation of amide bonds) in the gas phase at 160°C has been carried out in collaboration with the University of Turin (PhD thesis O. EL SAMROUT).

The formation of ester bonds between silanols and glycine was demonstrated by IR spectroscopy, and two distinct reactivities were identified: that of isolated silanols, for which esterification appears irreversible and which will serve as anchor points for polypeptide chains, and NFS silanols (Nearly Free Silanols, pairs of silanols separated by 4 to 6 Å, ν(OH) = 3744-3742 cm^-1), for which esterification is reversible and which will provide the links in the polypeptide chain. The β-turn configuration of the chains formed is progressively favored over a random configuration and, in the longer term, β-sheet formation is observed.