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Spectrocat group

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Context

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Heterogeneous catalysis is gaining importance in everyday life: it is estimated that more than 80% of the goods or objects surrounding us have taken advantage, at least once, of a heterogeneous catalytic process during their production. Moreover, with the increasing concern over environmental protection and the demand forever-increasing life quality, politicians and public institutions consistently push towards more stringent regulations on gas emissions, forcing industrial and private subjects to limit waste and to treat exhaust.

These regulations are often complied with catalytic processes designed for a sustainable energy economy and/or for emission abatement. The time of catalysts formulation by empirical ‘‘trial and error’’ methods is already well behind us, and even the implementation of technological demanding and expensive high-throughput screening methods has not been able to produce significant advances in catalyst discovery, though it has provided advances in material enhancement. Both academia and industry are convinced that in order to have an efficient catalyst design, a rational approach is necessary. Obviously, this rational design of a material needs a thorough knowledge of the ideal working way of the catalysts itself, in terms of active centers and local molecular interactions.

This is why a marked increase in the number of papers dealing with spectroscopic investigations of catalytic reactions has been observed in recent years. It reflects the desire of scientists to monitor at the molecular level the details of chemical reactions. In fact, traditional microkinetic studies can offer reasonable models of the processes taking place, using calculations partly based on thermodynamics and other theoretical considerations.

However, such analyses are in some instances merely a curve-fitting exercise, based on the measure of the concentrations of reactants and products under varying experimental conditions, without any actual direct observation inside the reactor. The use of spectroscopy can be very beneficial in supporting microkinetic analysis by literally turning on the light inside the reactor, which is no longer a ‘‘black box’’ to the scientist. Spectroscopic techniques had been mostly used in the past to characterize fresh or used catalysts, in order to obtain structural information relating to the bulk and surface of the solids. In the second half of the last century, and in particular at the end of the 1970’s, a few scientists realized that such characterization would be so much more useful when used in the conditions of temperature and pressure relevant to the catalytic tests.

This was the dawn of in situ analyses, which went on spreading to most fields of the chemical sciences, supported by the increased availability of commercial in situ cells. One of the great achievements of in situ techniques was the realization that the behavior of a catalytic surface in the presence of reagents under relevant pressures and temperatures was often significantly different from that observed ex situ, for example, under vacuum or room temperature conditions. A molecular approach to the study of chemical reactions requires the determination of active sites, reaction elementary steps and, when possible, intermediate species. Such investigations are only relevant when the catalyst is studied in situ and heterogeneous catalytic systems are now routinely investigated under continuous flow conditions at concentrations, pressures, temperatures and contact times typical of use, i.e. modus operandi.

Therefore, in order to identify these more focused analytical techniques a new expression, i.e. operando, was pushed forward. We should recall that the term operando spectroscopy refers to spectroscopic measurements of catalysts under (or as close as possible to) working conditions (appropriate temperature, pressure and reactant composition) with simultaneous on-line product analysis. This term was introduced in the literature in 2002 [see, for example: B.M. Weckhuysen, Chem. Commun. (2002) 97; M.A. Bañares, et al., Chem. Commun. (2002) 1292] with the aim to distinguish work in which on-line activity measurement was performed alongside spectroscopic measurements (i.e. operando) from those in which only spectroscopic data were recorded (i.e. in situ). The on-line analysis of the reaction effluents is needed to ensure that the activity data obtained in the operando reactor are consistent with those observed in a conventional reactor. Therefore, real-time spectroscopy during reaction coupled to simultaneous activity measurements in a kinetically relevant cell (operando methodology) provides the insight for a knowledge-based catalyst design. Spectroscopic techniques during reaction uncover how a catalytic surface behaves and which are the relevant catalytic steps, active sites and reaction intermediates during a given process. The determination of kinetically relevant data requires that the chemical engineering aspects of operando reactors are fully considered. Once the mechanism and actual working phases are established, the pathway to a successful formulation and improvement of catalytic systems is paved.

 

operando and in situ spectroscopy for catalysis

IR operando cell

The LCS has elected this approach as a continuous effort to reach the maximum knowledge on heterogeneous catalytic reactions and the associated materials.

Below, we detail the applications of the operando methodology to different domains, the obtained results and the continuous efforts to approach relevant reaction conditions, also designing and engineering the most appropriate reactor cells.

The SPECTROCAT research team groups together our research activities for which spectroscopic techniques and methods are the main tools for studying materials and processes. This obviously covers real operando studies (reproducing real working conditions for catalysts in a process), but in situ studies and test reactions for a general understanding of solids and their behavior are also included.

Our work aims at basic long-term fundamental questions, such as the relationships between properties of matter at different scales (nano- and macroscopic), between properties and working behaviors.

Application domains are mainly catalysis and adsorption, and industry is often interested by our results, some of which may even reach high Technology Readiness Levels (TRL).

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The three sub-groups in the SPECTROCAT team.

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The Spectrocat research team is composed  by three subgroups: “SEE: Surfaces and Environment”, “CEP: Catalysis for clean energies” and “CATEC: Catalytic valorization of fossil and renewable resources for energy and chemistry”. In all these groups, operando and in situ methods (with probe molecules) are developed for understanding and optimizing the properties of catalysts and porous materials as well as for establishing structure/activity relationships, identifying active sites, and determining reaction mechanisms and deactivation modes.

SEE : Surfaces and Environment

The group SEE is mainly focused on air depollution reactions, such as exhaust gas treatments from gasoline and diesel, CO2 storage and reuse, and surface properties and applications of hybrid porous materials.

CATEC : Catalytic valorization of fossil and renewable resources for energy and chemistry

The group CATEC deals with fossil and biomass derived sources for the production of fuels and of main intermediates for chemistry, refining and petrochemistry, and processes such as Fischer- Tropsch, HDO and HDS, Guerbet condensations, use of glycerol.

CEP : Catalysis for clean energies

The group CEP includes activities in photo catalysis, plasma catalysis, as well as technical development about non-conventional spectroscopic methods, such as hyperpolarization of Xenon in NMR spectroscopy.

The scientific breakthroughs and recognition of the activities in the SPECTROCAT  team are strongly based on innovative and continuous developments of both the equipment (newly designed IR/reactor cells, extreme operando conditions, high-time resolution in IR spectroscopy, hyperpolarized <sup>129</sup>Xe NMR…) and of the methodologies (chemometric methods).

Scientific output

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Pollutant abatement / environmental protection

We have a long history of basic research on automotive exhaust control. We collaborate with several academic and industrial partners, in institutional or bilateral agreements. This considerable research effort spans a large area, from very basic research to fairly applied studies on the surface and properties of commercial catalysts, for NOx abatement, catalyst durability, CO elimination, etc.

We exchanged several Masters and PhD students with the Politecnico di Milano, and investigated the reaction dynamic of model NOx-trap catalysts, clearly highlighting the pathway of nitrate formation on different materials, the role of the metal phases (Pt and Rh), as well as the concentration of CO2 and water among the reactants. Combining the spectroscopic monitoring of the heterogeneous catalyst at work with quantitative gas phase analysis and kinetic calculations, we have demonstrated the involvement of both nitrite and nitrate routes in NOx storage on basic sites, highlighting the reduction mechanism steps and the origin of the selectivity into N2O, a gas having a strong greenhouse effect.

The mechanism of N2O formation has also been investigated in the case of gasoline and Diesel after-treatment industrial catalysts correlated with the presence of unburst hydrocarbons in the gas phase, allowing to suggest car constructors how to reformulate a new generation of catalysts. The complexity of the formulation of industrial, conformed materials has not been a limitation to perform deep investigations (qualitative and quantitative) in partnership with Renault and PSA car companies; the goal was to understand the role of the presence and concentration of noble metals towards activity and selectivity, and Rare Earth additives effect. In this respect, we have dedicated a doctorate research stage to unravel the role of oxygen vacancies versus NOx adsorption, reduction and selectivity into dinitrogen, studying newly conceived Rare Earth-based materials by St. Gobain. We have also addressed the problem of the NOx-trap catalyst ageing (IFPEN/Renault), comparing materials submitted to severe hydrothermal treatments and to heavy-duty in service cars (Paris taxis). It was possible to differentiate and quantify the impact of the extreme thermal conditions (to which the catalyst is submitted during regeneration after sulfur poisoning) from those due to different physical-chemical phenomena taking place at the catalyst surface during the NOx adsorption/desorption cycles and the redox reactions. A complete working mechanism of the industrial catalyst was proposed and the equation set provided by the operando studies allowed to implement the kinetic model of the catalytic materials presently equipping the Renault Diesel cars of the B and C classes. Presently, a study on the possibility to merge a light NOx-trap device with a DOC (Diesel Oxidation Catalyst) is in development with PSA partnership.

For NOx abatement we have also examined the option of the SCR process, which is growingly recognized as the most sustainable for heavy and light vehicles. In this respect we have analyzed mixed oxides and zeolitic materials under duty, to ascertain the mechanisms of reaction between ammonia and nitrogen oxides, but also urea hydrolysis, NH3 oxidation phenomena, the selectivity conditions, the active site poisoning, the cold start, the possibility to combine the SCR catalyst with the DPF support, etc. The major results of these studies have produced fundamental knowledge on the reaction mechanisms, notably inside zeolites. In particular, we have highlighted the double role of iron and copper cations in the zeolitic framework: they constitute the sites where the NO to NO2 equilibrium takes place, so favoring the reaction kinetics (fast SCR); they are the Lewis sites where ammonia is adsorbed as well; on the counterpart, they are also responsible for ammonia partial oxidation, therefore a compromise has to be defined when formulating a catalyst: the concentration of cationic sites must be sufficient to obtain a satisfactory NO/NO2 equilibrium, avoiding at the same time excessive ammonia oxidation and coordination. An original method was also developed aiming at processing both the gas phase data and the adsorbed species data from a kinetic point of view in order to discriminate the best reaction mechanism for NO to NO2 oxidation. Furthermore, the influence of increasingly severe hydrothermal treatments on both the zeolitic catalyst (morphology, structure, transition metal cation, acidic site speciation) and its catalytic activity allowed to provide evidence that there was little impact on the NO to NO2 oxidation efficiency, thus pointing out that the lost of acidity would be the main contribution to the lost of SCR activity for aged samples. Transient studies with isotopically labeled molecules (SSITKA) have shown that both adsorbed ammonia and ammonium species equally react in the SCR process. Other investigations have highlighted the effect of hydrocarbons on the SCR reaction, showing that decane is the most poisoning species among the automotive exhaust. Learning from these results and working with internal resources, we have proposed a new, highly efficient process for ammonia SCR, which has been patented with PSA. It consists in ammonia storage and reloading for a stoichiometric reaction with flowing NOx. This process is much more efficient than the classical reduction where ammonia is introduced continuously in the stream: phenomena of sub- or over-stoichiometry are avoided (inefficiency and ammonia slip), together with the formation of ammonium nitrate at low temperature. This is a very important point, since that species is the main responsible for N2 O generation. With the same company we have even prospected the possibility to generate ammonia in situ, using CO and hydrocarbons pollutants as an isocyanate source, which can be easily hydrolyzed into NH3 by the water present in the exhaust flow. We have generated proofs of concept for this process, even if obtained efficiency is not large enough for a practical application nowadays. By the way, this investigation has allowed a substantial enhancement of the fundamental knowledge around the synergic effect of ammonia and hydrocarbons as reducing agents for NOx.

Finally, in order to conclude with the SCR process for NOx removal, another actual technical issue lies in the urea injection (spray homogeneity, efficiency of decomposition/hydrolysis, impossible below 180°C…) and consequently an alternative solution using acetylene as a reducing agent is actually under study in the frame of an NMR PhD thesis. Several zeolitic structures loaded with several transition metals were studied in both dry and wet SCR conditions. Very promising results were obtained with a Co-FER sample leading to 50% NOx conversion. Moreover, thanks to the operando methodology, it was evidenced that NO2 is required to react efficiently with C2H2 leading to adsorbed HCN as an intermediate active species. Actually, experiments are under progress in order to improve both the selectivity of NOx to N2 (N2O amount is actually too high) and of C2H2 to CO2 (CO amount is actually too high).

Obviously our approach to car exhaust control is more global, also dealing with other pollutants, such as CO and hydrocarbons. For example, with PSA we have developed a highly efficient and inexpensive catalyst for CO oxidation, using a rational synthesis approach: from the study of material properties and catalytic mechanisms, we have elaborated an original formulation based on low loading Pd systems on a defective ZrPrOx support. This material is both highly efficient for CO oxidation at relatively low temperatures and able to limit the formation of NO2, which represents one of the main drawbacks of Pt-based materials, together with their price.

Other strategies to face the problem of environmental protection from transport emissions have been explored. During a study supported by ADEME, in collaboration with PSA, RHODIA and different academic partners, we have envisaged the possibility to enhance the engine combustion efficiency and decrease the emission of pollutants by injecting hydrogen in the combustion chamber via the EGR valve. This hydrogen was generated on board via steam reforming of the gasoline and WGS.

We have also explored totally new concepts: the ANR BICNANOCAT project (that we have coordinated) has been the occasion to verify, for the first time, the effect of the ionic bombardment on a post-treatment catalyst. We have observed several interesting effects, such as the decreasing of the light-off temperature of the investigated materials for the main classes of pollutants, their increased resistance to ageing and a superior reducibility. The most important is that, via a multitechnique approach, we have been able to associate these macroscopic effects to nanoscopic phenomena induced by the ionic bombardment, such as the creation of ordered defects in the crystalline phases and the modification of the local electrostatic charge on the active sites. Obviously, these results pave the way to unprecedented developments in heterogeneous catalysis.

Fundings

ANR: ATMO, BICNANOCAT (leader), Urée-NOx; IFPEN; FUI: OSCCAR’NOx; PREDIT: RECONOME; AMI: 50gCO2CARS

External collaborations

IC2MP (Poitiers), UCCS (Lille), IRCELYON, CORIA (Rouen), Saint-Gobain, Renault, PSA (-N2O, -SCR), Toyota, IFPEN; GDRI: ECSAW (AIST, Nagoya University)

Temporary researchers

8 (2 ongoing, 6 finished including 4 CIFRE), 4 postdocs

Biomass resources valorisation and sustainable development

Our approach to investigate catalysts and catalytic reactions by spectroscopic methods was earlier applied to biomass resources upgrading for fuels and chemicals. From the mid of 2000’s, projects on 2G-biomass valorization have become an important field of investigation for the operando axis. Now our researches on sustainable development go beyond the valorization of biomass resources, to issues related to CO2 capture and new energy processes, like H<sub>2 </sub> storage.

Biomass for Fuels

The LCS was leader on several projects (ANR, CNRS, Labex) on 2G biofuels and get a recognized position on in situ and operando spectroscopic studies of the main processes for 2G biofuels treatment.

EcoHdoc (leads by LCS, with IC2MP Poitiers and UCCS Lille) and HDO-Bio projects allowed us to identify key parameters to optimize stability and minimize hydrogen consumption of the hydrodeoxygenation process that is a key step for upgrading pyrolitic bio-oils.The study performed in Caen revealed that the key parameters in the interaction of the different phenolic compounds wih the catalysts i.e. molecule basicity, substituent nature, support acid base properties.Moreover, it shows that the decline in HDO activity can be related to an indirect poisoning of the active sites linked to the steric hindrance of phenate species anchored to the catalyst support and/or to a direct poisoning of the sites of the active phase. A surface modeling of the catalyst confirms the indirect poisoning feature. This study highlights the effect of the support on the hydrodeoxygenation performance of the catalyst.

The Labex EMC3 BIOCAR project (leads by LCS, with CORIA Rouen) aims at developing innovative purification methods based on selective adsorption of phenolic molecules into hydrocarbons. The rational choice of acid base and textural properties of the adsorbent, the optimized adsorption and regeneration conditions will be based on liquid operando studies performed on an IR / ATR equipment, of which the development is strategic for the laboratory.

The valorization of bio-syngas (CO-H2) by methanation and Fischer-Tropsch synthesis was investigated by DRIFT operando within the ANR project Biosyngop (with UCCS Lille and IRCELYON). Bio-syngas contains specific poisons such as aromatics, N and Cl-compounds. Their effects were screened in order to evaluate their contribution to the catalyst deactivation (catalyst restructuration, irreversible poisoning, competitive adsorption, …) as well as the influence on selectivity (FTS). Some poisons (NH3) were found beneficial for the products quality (alkene/alkane ratio and chain growth), which could be assigned to modification of the metal electronic properties.

Biomass for Chemicals

Several collaborations with academic and industrial partners through ANR and Labex projects were undertaken to develop more efficient catalysts for transformation of biomass into chemicals. Based on in situ and operando IR and NMR spectroscopies and on development of methods for spectroscopic data processing, rational bases were obtained for improving reactions for synthesis of platform molecules and fine chemical products, as well as for innovative catalysts issued from bio-resources.

The transformation of bio-alcohols into longer-chain alcohols was investigated within the ANR project Guerbetol. Results obtained by spectroscopic techniques and through the development of chemometric methods (2D-IRIS) (LCS), by adsorption microcalorimetry technique (IRCELYON) were cross-examined with results obtained from gas phase dehydration of glycerol (ARKEMA, UCCS Lille) and revealed interesting correlations between acid base properties of catalysts and product selectivity in this reaction.

We also participate to the ANR SHAPES project (with Solvay and academic partners in Lille and ENS-Lyon), which is intended at enhancing catalyst properties for novel selective heterogeneous amination processes for the synthesis of bio-based monomers, via a rational approach. In this context we apply our IR spectroscopic investigations to unravel surface active sites, molecule coordination modes and reaction steps to provide thorough elements for DFT models.

In a parallel manner, another class of catalytic materials with potential applications in fine chemistry was specifically studied. The microscopic nature of these catalysts based on supported oxides, was evaluated by in situ methods to establish quantitative relationships between structure, acidity and catalytic activity of W species supported on oxides and to determine their influence on metal deposition.

In the project MOSAIC (with ENSCR-Rennes, and UCCS-Lille), we aim at optimizing vanadium phosphate catalyst by better understanding the bulk evolution during the real process. The role of LCS team is to use operando NMR spectroscopy for getting better insight on the catalyst behavior at high temperature, possibly during the catalytic reaction.

The project COLIBRI (with LCMT and CRISMAT Caen, and GPM Rouen) funded by the LABEX, had as a main objective to improve the performance of innovative biomaterials (BIO-SILC: Supported organometallic catalysts on biopolymers using Ionic liquids) for an application in olefin metathesis. The contribution of LCS was related to the use of NMR spectroscopy to understand better the properties of these catalysts.

New energy source – Chemical Hydrogen Storage: Materials for Catalytic hydrogenation of CO2 to methane

Storing energy from intermittent renewable sources (such as wind and tide in Normandy) has become a crucial issue. One solution is to produce hydrogen when an excess of energy is produced, and then to store hydrogen. The chemical storage of hydrogen under the form of CH4 from CO2 hydrogenation is a particularly attractive solution. Indeed, the reaction can be performed at moderate temperatures and pressures, on catalysts based on oxide- or zeolite-supported metals. It allows the capture of CO2, and methane can be injected into the gas network (for example).

With the support of CNRS via the PIE program, with European support, and within the frame of regional consortium EHD2020, we have investigated the possibility to store H2 and to valorize energetically CO2 by selective catalytic hydrogenation into methane. Using operando spectroscopy over a CeZrOx-based catalyst, we could demonstrate the key role of basicity and reducibility of this support for methane hydrogenation reaction. On this support, CO is not an intermediate in the methanation of CO2, which is activated on the support to form carbonates, then hydrogenated into formates and methoxys by hydrogen dissociated on Ni0 sites. It was evidenced that this mechanism involves weak basic sites of the support for the adsorption of CO2 and implies a stable metal-support interface. On other families of very active catalyst such as Me/zeolite, the nature of active sites was also investigated since these systems present no basic nor redox properties. Recent results on zeolitic type catalysts enabled us to develop new active catalyst formulations. Furthermore, LCS also showed the beneficial effect of plasma activation on the catalytic activity. The synergy plasma – catalysis is studied using fast time resolved operando spectroscopy.

Fundings

EMC3 BIOCAR (leader), COLIBRI ; ANR : EcoHdoc (leader), Guerbetol, Biosynop, CD2I-SHAPES, MOSAIC; IFPEN ; PIE : Energie HDO-Bio, VALCO2 ; PCP Venezuela, EU : INTERREG E3C3, LCS funding (Toyota)

External collaborations

IC2MP (Poitiers), UCCS (Lille), IRCELYON, ENS (Lyon), ENSCR (Rennes), Université de Strasbourg, CORIA (Rouen), LCMT (Caen), University of St Petersburg (Russie), University of Cambridge, University of Caracas (Venezuela), EHD2020, Arkema, CSIC Madrid, University of Malaga

Temporary researchers

4 PhDs (2 ongoings, 2 finished), 6 postdocs, Invited professor (Lisbon, IST: C. Henriques; Malaga: O. Guerrero Perez)

Refining Catalysts: Structure of the active sites

LCS has a unique place in the studies of the structure of active sites for refining catalysts, because of its expertise in in situ and operando techniques, more specifically applied to hydrotreatment and cracking catalysts

Catalytic cracking – Fundamentals and Applications

Monomolecular cracking of light alkanes (C3-C7), archetypical of acid-catalyzed reactions, has been investigated by operando IR spectroscopy in order to determine the coverage and intrinsic activity of MFI and FER zeolite acid sites at reaction conditions (650 – 775 K). We determined much higher coverage than those predicted by state of the art calculations and we could demonstrate that differences of zeolites activity or alkane reactivity were mostly due to intrinsic kinetics, where the activation entropy plays a key role (confinement of the reactant).

New generation FCC catalysts, containing replacements for rare earth elements (Yttrium, alkaline earths…) were investigated within research collaboration contracts with W.R. Grace, the major FCC catalyst manufacturer. We found that the partial exchange does not only stabilize the zeolite, but also modifies the intrinsic activity of acid sites, likely by increasing the adsorption strength of the reactants.

Hydrotreating of fossil fuels

The production of fuels with very low sulfur content is, more than ever, a major issue. Our recent investigations on hydrotreatment catalysts provided the first experimental evidence of the morphology of nanoslabs of sulfide phase representative of “real” catalyst i.e. the proportion of S edges and M edges of MoS2 catalysts supported on alumina (Figure 2). The IR spectroscopy of CO adsorbed at very low temperature points out that the ratio of S and M edges depends on the Mo content, the temperature and pressure of sulfidation of the catalyst. The morphology is also sensitive to the presence of organic additive during the catalyst active phase genesis. Thus, the presence of citric acid greatly promotes the growth of the S edge at the expense of the M edge. Increasing the citric acid concentration leads to a gradual transformation of the MoS2 slab morphology , from slightly truncated triangles with mainly M edges to regular hexagons having M and S edges in an equivalent amount. The study also highlights for the first time the differences in reactivity of M and S edges of supported catalyst. For Mo/Al2O3 catalysts, TOF values for each edge have been obtained: the S edges are more active for hydrodesulfurization of thiophene than the M edges. Moreover, differences in HDS or HDN route selectivity have also been highlighted, the S-edge being favorable to the C heteroatom bond cleavage whereas the M-edge favors the hydrogenation pathway.

On Co-promoted catalysts, CoMoS site formation is facilitated on the S edge of the sulfide slab. In parallel, original operando methods were developed for monitoring the genesis of the sulfide phase of supported catalysts. Along with these fundamental studies, the origin of aging of industrial catalysts was investigated during hydrotreatment of feed highly contaminated with sulfur and metals, as well as methods for regenerating spent catalysts.

These IR studies that bridge the gap between model and real catalysts, have attracted strong interest from the community of hydrotreating. We will continue these studies by focusing on the effect of the support and other organic boosters on the morphology of sulfide phase nanoslabs. The final aim is to draw a precise relationship scheme between the genesis of the sulfide phase, its morphology, its promotion and its activity and selectivity.

First experimental demonstration of the morpholygy of the nano-slabs of active phase in industrial hydrotreatment catalysts.

Fundings

IFPEN (F), Grace (USA), Eurecat SA (F), PCP Mexico, MESR, EMC3

External collaborations

UNAM et IMP (Mexico), RIPP (China), Grace (USA)

Temporary researchers

8 PhDs (3 ongoing, 5 finished), 4 postdocs, Invited professor (R. Wormsbecher (USA), Pr K. Aboulayt (from U-El Jadida, Marocco)

Hybrid porous materials characterisation and applications to gas separation and catalysis

The development of hybrid MOF (metal organic framework) materials presents a growing interest. In a strong collaboration with a team at the Institut Lavoisier Versailles (ILV) and other partners in France, Europe and Korea, we have characterized in the last decade numerous new structures of coordination polymers and determined their physicochemical properties via our spectroscopic tools. We have also devoted a considerable effort to develop the applications of these unique materials in different fields of adsorption, separation and heterogeneous catalysis.

Having been the first to discover the outstanding capacities of these materials for CO2 absorption, we have undertaken a research to identify compounds which will constitute a possible alternative to zeolites and porous carbons as physical adsorbents. We have characterised rigid and flexible MOFs by Raman and IR spectroscopies to evaluate their properties (stability, adsorption, regeneration – in correlation with their structure, adsorption site strength and porosity) and finally to select the most suitable compounds for their potential use in CO2 capture. Our investigations revealed that these materials present superior storage properties, high selectivities, excellent gas diffusivities and an optimal regeneration capability. In this view, functionalised flexible iron dicarboxylates were studied for water, alcohols and linear alkanes adsorption too. Vapour interaction and diffusivity were correlated with the molecular coordination on sites of modulated strength and quality, as well as with the role of different functional groups.
The experience gained in these fundamental studies has allowed us to propose the design of hybrid materials for different applications. As partners of the European project MACADEMIA (and responsible for the characterization WP), we have investigated the porous compounds by in situ and operando techniques, describing their properties and working mechanisms in gas separation and purification. Notably, a complete mechanism has been finely described for C3 separation on MIL-100(Fe) by active site selective poisoning. Once more, this approach has provided precious elements to the modelling teams and tips for material improvement to the colleagues in charge of synthesis and functionalization. Catalytic properties were also found, typically for methanol oxidation, related to the redox or to the acid/base properties of the materials (favouring the dehydration/dehydrogenation steps of methanol and dimethyl ether into methyl formate, successively decomposing into CO2). In this respect, we are studying the correlations between the acid-base properties of MOFs and their properties in catalysis. A PhD student is investigating the correlation between the nature of sites in the hybrid materials with the conversion in vapour phase of the 2,2,3,3-Tetramethyl-Oxirane into Pinacolone and Allyl alcohol. The selected acid solids are based on octahedral metal trimers (containing Al, Fe or Cr) linked by trimesic acid (so-called MIL-100), already finely characterized in previous collaborations. The study has been extended to new MOFs having nitrogen basic sites, to ascertain their role in the conversion of Oxiranes.
With the support of another European consortium (M4CO2), we are presently investigating the potentiality of Mixed Matrix Membranes based on highly engineered Metal organic frameworks and polymers that outperform current technology for CO2 capture in pre- and post-combustion, meeting the energy and cost reduction targets of the European SET plan. In particular, we have designed and realised a new IR operando cell able to analyse a membrane at work, providing simultaneously data on the adsorbed species and the gas phase under duty. The results collected thanks to this innovative tool are shading a new light on the membrane working mechanism and on the diffusivity of gaseous species into a defective polymer.
Independently, we have collaborated with the colleagues at ILV and KRICT (S. Korea) to design, via a rational approach, a new catalyst for NO abatement based on MOFs. We have selected a phase able to decompose NO catalytically at room temperature without any reductant, mimicking the structure and the behavior of an enzyme. This outstanding result has been patented; the license is to be sold by CNRS to an American company and a paper is under publication. Basing on these results we have developed a strategy to identify complex problems in catalysis and try to solve them by using a rational approach: a design of the active centre based on the energetic of the host-guest interaction between the molecule and the site, then an insertion of these clusters in an adapted structure. MOF compounds are particularly adapted for this approach. Therefore we have constituted a team with ILV (C. Serre), ICGM (G. Maurin) and CSCC Leuven (D. De Vos) to design, synthetize and test hyperstable nanoporous metal-organic innovative compounds. To support the project we have asked a Synergy Grant. We have been selected for funding (classified in the main list and recommended for funding – the only project in chemistry), but unfortunately, due to limited budget availability, still waiting for granting. Other supports are explored now to finance this ambitious project.
Another field were MOFs can be used with high efficiency is that of air purification, where alternatives such as porous carbons and zeolites present a lack of efficiency. In this respect we collaborate again with ILV and PSA to develop absorbers for indoor air treatment in vehicles. Working again on a rational approach and testing model systems as well as polluted real air mixtures, we have already found inexpensive and up-scalable materials largely more performing than the existing physical adsorbents, and easily regenerable.

In another context and with the support of the EMC3 LABEX, we characterise (from the physical-chemical point of view) new thermoelectric hybrid materials to be used at low temperatures (25-200°C) and aimed at replacing bismuth tellurides (composed by rare, expensive and toxic elements). The idea is to insert organic molecules (insulating or conducting) between inorganic layers of MS2 type (M = W, Mo, Ti), with the aim to combine a strong electric conductivity with a weak thermal conductivity. In this project our team performs ageing and thermal degradation studies versus time and temperature, under different atmospheres, using operando IR. Teams from LCMT and CRISMAT laboratories are in charge of the synthesis, structural and physical properties characterisation of the materials.

Fundings

EU-FP7: MACADEMIA (WP leader), M4CO2; ANR: NoMAC, SAFHS; EMC3 LABEX: THERMOS

External collaborations

Institut Lavoisier (C. Serre), ICGM (G. Maurin), IRCELYON (H. Jobic), MADIREL (P. Llewellyn), D. De Vos (KU Leuven), G. Guy.De Weireld (University of Mons), F. Kapteijn & J. Gascon (TU Delft), R. Walton (University of Warwick), P. Wright & R. Morris (University of St. Andrews), J. Cejka (Prague), J. Coronas (University of Zaragoza) J.-S. Chang (KRICT, S. Korea), Total, IFPEN, BASF, Johnson-Matthey

Temporary researchers

4 PhDs (3 ongoing, 1 finished), 5 postdocs

Methodological developments

These investigations, able to generate fundamental knowledge from industrial catalysts and complex reaction conditions, have been possible thanks to a parallel and continuous development of experimental techniques, and mathematical analysis notably at the frontier of operando spectroscopy.

We have particularly taken care of the reliability of our IR reactor-cells, developing new devices to take them closer to a perfect PFR, in order to obtain reliable quantitative data from spectrokinetic investigations. Our main transmission reactor-cell “Sandwich” has been implemented by different accessories to make it flexible for various applications. A sample holder for square wafers and a gas distribution cone has been designed with the help of chemical engineering model calculations realised in collaboration with a team from Politecnico di Milano; this device will simplify calculations to obtain kinetic constants for a reaction, when necessary. In another configuration, we have equipped this cell to investigate photocatalytic reactions modus operandi, inserting an accordable UV-vis source in the cell. As mentioned above, we have designed and realised a new reaction chamber able to analyse a membrane at work, providing simultaneously data on the adsorbed species and on the gas phase on both sides of the membrane sample. Recently, we have built a new setup combining transmission IR and Raman measurements on the same sample, at the same time (collaboration with M. Bañares, CSIC Madrid, supported by a bilateral CNRS-CSIC program and by a chair of excellence of the Lower-Normandy Region); in this case a Raman probe enters the cell and flashes the surface of the analysed material just next to the IR beam. To complete mechanistic studies, we have realised synthetic gas benches for the use of isotopic substituted molecules to perform SSITKA analyses. We have also designed and realised a cell (“Hot Dog”) able to analyse a conformed catalyst by transmission IR, so simulating at a micro scale a pilot plant working under realistic parameters on a monolithic catalyst.

Years ago we had realised the first device coupling IR transmission analyses on a sample suspended in a TGA system (AGIR). Many fundamental studies have been carried out using this technique. To treat properly the spectra and thermal profiles produced, we have developed a new methodology – Two-Dimensional Inversion InfraRed Spectroscopy (2D-IRIS) – to assess the heterogeneity of adsorption sites by the inversion of adsorption isotherms obtained by IR spectroscopy, and gravimetric measurements were developed. This approach allows taking advantage of both methods and provides a description of the surface in terms of nature, strength and number of adsorption sites.

Advanced chemometric methods were combined and dedicated algorithms were developed to tackle the specific difficulties in characterizing adsorbed species. They include adapted pre-processing methodologies, exploratory analysis, and multivariate curve resolution (MCR) analysis. They were applied to the elucidation of hydrated silica- silane interactions within a collaboration contract with Michelin. It allowed proposing a complete and predictive reaction network, accounting for the grafting and co-condensation of silane over silica. This work is currently extended to the study of the influence of additives (accelerators) on these reactions.

Similar methodology has been applied to understand the mechanisms of polysulfide curing by metal oxides, within a research collaboration contract with Hutchinson Aerospace. It also consists in extending the operando concepts and methods to polymerization, by simultaneous measurement of the IR spectra and rheological properties during the curing.
Obviously we are also continuously enhancing in situ methods. We have devoted a large effort in cells automation, to enhance the experimental reproducibility and to reduce the time for routine experiments. In this view, a metal cell called “Snoopipe” has been realised; it is able to treat a sample and record spectra automatically, thanks to a robotic arm controlled by LabView. A similar system (“Carroucell”), but loaded with 12 samples, will allow to rapidly compare numerous materials in the same conditions. The “Jumpipe” cell, on its side, records spectra of the sample and the gas phase present in the internal volume by changing automatically of position in the IR beam; it can be used as a batch reactor to study catalytic or adsorption phenomena.

In our quest for more precise mechanistic descriptions and intermediate identification, we are studying the project of a new spectrometer coupling ultrafast IR monitoring of a material surface with laser DRASC et LIF diagnostics of the gas phase (LABEX EMC3 “DRUID” project). A system of tunable lasers and ultrafast detectors, assembled around a revolutionary operando cell, will allow recording spectra continuously at the microsecond time resolution.

A Hyperpolarized 129Xe NMR system has been recently built. Although its main applications reside in the characterization of porosities for zeolites and MOF systems, it has also potentiality for probing catalytic system in liquid media. With this aim, we are currently investigating the potential of single-scan acquisition procedure, which should bring time-resolved capabilities to the NMR, allowing us to follow kinetic of reactions in liquids, of topical interest in topics such as biomass valorisation.

Fundings

Michelin, Hutchinson, Evonik, LABEX EMC3: DRUID (leader), COLIBRI; French-Spanish CNRS-CSIC 2010 program, Chair of Excellence (Lower Normandy region)

External collaborations

CORIA, CSIC Madrid (Spain), Thermo Electron

Temporary researchers

3 PhDs (2 ongoing, 1 finished), 2 postdocs, Chair of Excellence (M. Banares)

Copyright 2021 - Laboratoire Catalyse & Spectrochimie - Directeur de publication : Guillaume CLET | Creative Commons 4.0 International
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