J Vieillard
email: julien.vieillard@univ-rouen.fr

N Bouazizi

R Bargougui

P Nkuigue Fotsing

O Thoumire

G Ladam

N Brun

J.-F Hochepied

E Djoufac Woumfo

N Mofaddel

F Le Derf

A Azzouz
email: azzouz.a@uqam.ca

Metal-inorganic-organic core-shell material as efficient matrices for CO 2 adsorption: Synthesis, properties and kinetic studies

Keywords:

A novel hydrochar-based core-shell material with improved affinity towards CO 2 was synthesized through encapsulation within ZnO shell, followed by chemical grafting to an organic moiety bearing terminal diethanolamino groups and further dispersion of Cu 0 nanoparticles. Characterization through nitrogen adsorption-desorption isotherms with BET and BJH models, SEM, TEM, zeta potential measurements, FTIR spectroscopy, DSC and XPS analyses revealed a strong influence of the modification procedure on the performance of each material in CO 2 adsorption. The resulting metal-inorganic-organic-core-shell (MIOCS) displayed compacted structure with a wide pore size distribution that imposes intraparticle diffusion as a kinetic-controlling step. Cu-MIOCS showed an appreciable CO 2 retention capacity of 5.42 mmol/g compared to many adsorbents reported so far. This affinity towards CO 2 was explained in terms of physical and non-stoichiometric CO 2 condensation. This finding is of great importance, because it demonstrates that judicious modifications of vegetal-deriving wastes such as woods and other biomasses give rise to added-value materials as low cost and efficient gas adsorbents with high affinity towards CO 2 .

Introduction

Global warming and climate changes turn out to arise from high concentrations of greenhouse gases (GHG), among which carbon dioxide (CO 2 ) is the most directly related to human activities [START_REF] Meinshausen | Greenhouse-gas emission targets for limiting global warming to 2 °C[END_REF][START_REF] Macdowell | An overview of CO 2 capture technologies[END_REF] . The persistent use of fossil fuels as the main energy source in designing new technologies clearly demonstrates the lack of a clear strategy for a sustainable development. This makes that GHG and CO 2 emissions from anthropic activities still remain a major environmental issue [START_REF] Quéré | Global carbon budget 2016[END_REF][START_REF] Etheridge | Natural and anthropogenic changes in atmospheric CO 2 over the last 10 0 0 years from air in Antarctic ice and firn[END_REF] . Clear evidence in this regard is provided by numerous procedures targeting direct CO 2 capture from flue emissions [START_REF] Brunetti | Membrane technologies for CO 2 separation[END_REF][START_REF] Feng | Screening of CO 2 adsorbing materials for zero emission power generation systems[END_REF][START_REF] Rochelle | Amine scrubbing for CO 2 capture[END_REF] , even if, in most cases, adsorbents regeneration often requires more CO 2 -equivalent energy than captured. Amine-based adsorbents display sufficient basicity for retain CO 2 via the formation of carbamates [START_REF] Sayari | Stabilization of amine-containing CO 2 adsorbents: dramatic effect of water vapor[END_REF][START_REF] Veneman | Adsorption of CO 2 and H 2 O on supported amine sorbents[END_REF] , but severe thermal regeneration. In the long run, this way of acting is doomed to failure, unless strategies regarding CO 2 as a product, for instance, for further uses in chemistry and in agricultural greenhouses are developed. Such strategies will certainly face a major issue, that of the too low CO 2 concentrations in flue emissions, which impose previous concentration of the captured gas.

A judicious approach in this direction resides in designing new waste-free and non-polluting technologies for CO 2 concentration through a truly reversible gas capture at ambient conditions. Weaker CO 2 -adsorbent interactions are an essential requirement in this regard, and can be achieved using materials bearing amphoteric to slightly basic chemical functions such as hydroxyl groups. The latter are known to promote only CO 2 adsorption via weakly bonded carbonate-like association [10 -17] .

OH-enriched materials containing soya-deriving polyglycerols were already found to show high affinity towards CO 2 , and gave Please cite this article as: J. Vieillard et al., Metal-inorganic-organic core-shell material as efficient matrices for CO 2 adsorption: Synthesis, properties and kinetic studies, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.020

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JID: JTICE [m5G; September 13, 2018;16:22 ] Scheme 1. Schematic illustration of HC@ZnO core-shell particles.

similar performances as polyol dendrimers [START_REF] Azzouz | Preparation and characterization of hydrophilic organo-montmorillonites through incorporation of non-ionic polyglycerol dendrimers derived from soybean oil[END_REF][START_REF] Azzouz | OH-enriched organo-montmorillonites for potential applications in carbon dioxide separation and concentration[END_REF][START_REF] Azzouz | TPD study of the reversible retention of carbon dioxide over montmorillonite intercalated with polyol dendrimers[END_REF][START_REF] Azzouz | Metal-inorganic-organic matrices as efficient sorbents for hydrogen storage[END_REF][START_REF] Azzouz | Polyol-modified layered double hydroxides with attenuated basicity for a truly reversible capture of CO 2[END_REF][START_REF] Bouazizi | Montmorillonite-supported Pd0, Fe0, Cu0 and Ag0 nanoparticles: properties and affinity towards CO 2[END_REF][START_REF] Nousir | Correlation between the hydrophilic character and affinity towards carbon dioxide of montmorillonite-supported polyalcohols[END_REF] .

Their affinity improvement was explained in terms of mere incorporation of a vegetal source of OH groups. Unlike complex structure materials such as zeolites, metal-organic frameworks (MOFs) and others tested in CO 2 adsorption [START_REF] Choi | Adsorbent materials for carbon dioxide capture from large anthropogenic point sources[END_REF][START_REF] Sayari | Flue gas treatment via CO 2 adsorption[END_REF][START_REF] Gassensmith | Strong and reversible binding of carbon dioxide in a green metal-organic framework[END_REF] , low cost natural materials such as vegetal wastes display low surface acidity and do not require sophisticated modification procedures [START_REF] Gibson | Adsorption materials and processes for carbon capture from gas-fired power plants: AMPGas[END_REF][START_REF] Sengupta | Effects of the adsorbent preparation method for CO 2 capture from flue gas using K 2 CO 3 /Al 2 O 3 adsorbents[END_REF] . The structure of any vegetal material should bear terminal hydroxyl groups but does not necessarily exhibit interesting affinity towards CO 2 due to structure constraints such as porosity and/or optimum amphoteric to slightly character. Vegetal waste modification often turns out to an essential requirement for affinity improvement. Their conversion into hydrochar (HC), followed by suitable modifications could be a judicious route for obtaining highly added-value materials with improved affinity towards CO 2 . HCs are also low cost porous materials that display high porosity and specific surface area along with appreciable thermal and chemical stability [START_REF] Romero-Anaya | Activation of a spherical carbon for toluene adsorption at low concentration[END_REF][START_REF] Heidari | Evaluation of CO 2 adsorption with eucalyptus wood based activated carbon modified by ammonia solution through heat treatment[END_REF] . They may be obtained through pyrolysis, carbonization or hydrothermal conversion of diverse vegetal wastes [START_REF] Molina-Sabio | Effect of steam and carbon dioxide activation in the micropore size distribution of activated carbon[END_REF][START_REF] Wang | High hydrogen storage capacity of porous carbons prepared by using activated carbon[END_REF] . Their activation in optimal conditions is known to improve the surface properties [START_REF] Rivera-Utrilla | Activated carbon modifications to enhance its water treatment applications. An overview[END_REF] and affinity towards CO 2 [START_REF] Ęsikiewicz-Puchalska | Improvement of CO 2 uptake of activated carbons by treatment with mineral acids[END_REF] .

So far, the behavior of vegetal wastes as host matrices for metal, metal oxides and CO 2 condensation along with the interactions involved still remains barely tackled. This became the main objective of the present paper. Metal oxides, and more particularly zinc oxide (ZnO) are also assumed to exhibit interesting surface properties as adsorbent [START_REF] Bouazizi | Metal-organo-zinc oxide materials: investigation on the structural, optical and electrical properties[END_REF][START_REF] Bouazizi | Molybdenum-loaded 1,5-diaminonaphthalene/ZnO materials with improved electrical properties and affinity towards hydrogen at ambient conditions[END_REF] and electronic material [START_REF] Wang | Large-scale hexagonal-patterned growth of aligned ZnO nanorods for nano-optoelectronics and nanosensor arrays[END_REF] . ZnO combination with a diethanolamine-modified cocoa shell-deriving hydrochar already showed interesting surface properties [START_REF] Ren | Fabrication of core-shell MIL-101(Cr)@UiO-66(Zr) nanocrystals for hydrogen storage[END_REF][START_REF] Kobayashi | Hydrogen absorption in the core/shell interface of Pd/Pt nanoparticles[END_REF][START_REF] Kobayashi | Atomic-level Pd -Pt alloying and largely enhanced hydrogen-storage capacity in bimetallic nanoparticles reconstructed from core/shell structure by a process of hydrogen absorption/desorption[END_REF][START_REF] Vieillard | Cocoa shell-deriving hydrochar modified through aminosilane grafting and cobalt particle dispersion as potential carbon dioxide adsorbent[END_REF] . It turned out to act as host matrice for copper-zero dispersion and as effective adsorbent for CO 2 at room temperature like other materials with attenuated basicity [START_REF] Azzouz | Polyol-modified layered double hydroxides with attenuated basicity for a truly reversible capture of CO 2[END_REF] . The specific surface area was not an essential requirement, inasmuch as compacted matrices paradoxically displayed higher affinity towards CO 2 than much more expanded and more porous materials [START_REF] Azzouz | Metal-inorganic-organic matrices as efficient sorbents for hydrogen storage[END_REF][START_REF] Arus | Cu0 and Pd0 loaded organo-bentonites as sponge-like matrices for hydrogen reversible capture at ambient conditions[END_REF][START_REF] Tahir | Metal organoclays with compacted structure for truly physical capture of hydrogen[END_REF] . Hence, a new strategy was rather focused on the preparation of compacted matrices that promote metal stabilization and attenuate CO 2 retention strength. Strong metal: matrice interaction is expected to attenuate the Lewis basicity of the N atom of the organic moiety [START_REF] Vieillard | Cocoa shell-deriving hydrochar modified through aminosilane grafting and cobalt particle dispersion as potential carbon dioxide adsorbent[END_REF] , and, subsequently, CO 2 capture will mainly occur via purely physical gas condensation, that favors with easy release upon forced convection or under lower pressure. The material interactions with both metal particles and CO 2 were deeply examined herein as specific objectives of this work.

Due to their low thermal and chemical stability, vegetalderiving materials cannot be directly used in CO 2 capture from flue emissions. Previous cooling and water treatments of flue emissions with energy recovery with advanced elimination of SO x , NO x and volatile organic compounds (VOC) turns out to be essential requirements for a possible sustainable technology. That is why reversible capture of CO 2 appears as an original alternative for a non-thermal gas release for regeneration and further CO 2 utilization. This new concept still remains a difficult challenge, but can be addressed using metal-inorganic-organic core-shell materials (MIOCS) displaying weak basicity. In this regard, our approach resides in correlating Cu 0 -matrice and CO 2 -matrice interactions with both the surface properties and structural and textural features of the synthesized material. Full characterization of MIOCS by means of diverse techniques allowed achieving deep analysis of the adsorbent affinity towards CO 2 in terms of magnitude and adsorption kinetics. The results will open promising prospects for manufacturing adsorbing materials and membranes that ''respire CO 2 '' under ambient conditions.

Experimental

Chemicals

Zinc acetate dihydrate (Zn(CH 3 COO) 2 ,2H 2 O; 99%); 3chloropropyltrimethoxysilane (C 6 H 15 ClO 3 Si; 97%); sodium borohydride (NaBH 4 ; 99%); diethanolamine (NH(CH 2 -CH 2 -OH) 2 ; 98%); copper(II) sulfate anhydrous (CuSO 4 ; 99.9%); triethylamine (C 6 H 15 N; 99.9%) and absolute ethanol (CH 3 CH 2 OH; 99.9%) were purchased from Sigma-Aldrich and used as received. For all the experiments, ultra-pure water was used (Millipore).

Materials synthesis

Hydrochar (HC) was produced via hydrothermal conversion of cocoa shell powder (1 g), originating from Cameroon, previously dispersed in deionized water (10 mL), at 240 °C for 24 h in a sealed Teflon lined autoclave (30 mL). The mixture was cooled down to room temperature (RT), and HC was obtained in the form of a white suspension. HC was filtered, repeatedly washed with deionized water and then with ethanol, and finally dried at 80 °C overnight.

HC was then encapsulated into a shell of ZnO in a mixture of aqueous solutions of zinc acetate dihydrate (0.2 M) and sodium hydroxide as precursors, under continuous and vigorous stirring at 60 °C in a 3:1 (V:V) ethanol-water mixture for 15 min. The pH value was previously adjusted at 9.5. HC was added in a proportion of 0.01 wt% and the mixture was heated at 60 °C for 4 h. The resulting grey precipitate of HC@ZnO was filtrated, washed with water/ethanol to remove the unreacted sodium acetate and then dried overnight at 100 °C ( Scheme 1 ). Further, 3chloropropyltrimethoxysilane (ClPTES) was grafted on HC@ZnO using ethanol/water 3:1 as a solvent at 80 °C during 5 h ( Scheme 2 ). The resulting HC@ZnO -ClPTES sample was washed, filtrated, dried overnight at 80 °C and then prone to chlorine substitution by diethanolamine (DEA) at 50 °C under inert atmosphere for 24 h ( Scheme 3 ).

The final HC@ZnO-Si-(CH 2 ) 3 -N(CH 2 -CH 2 -OH) 2 material obtained denoted as HC@ZnO-Si-N(OH) 2 was washed, filtrated and Please cite this article as: J. Vieillard et al., Metal-inorganic-organic core-shell material as efficient matrices for CO 2 adsorption: Synthesis, properties and kinetic studies, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.020

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JID: JTICE [m5G; September 13, 2018;16:22 ] Scheme 2. Schematic illustration of the ClPTES grafting onto the surface of HC@ZnO.

then dried at 80 °C. Copper nanoparticles (CuNPs) were incorporated in HC@ZnO-Si-N(OH) 2 using 0.3 wt% anhydrous copper(II) sulfate (CuSO 4 ) as a precursor in 40 mL toluene in the presence of 0.03 wt% sodium borohydride (NaBH 4 ) as the metal reducing agent after 6 h of stirring at RT. The final HC@ZnO-Si-N(OH) 2 -Cu material ( Scheme 4 ), denoted as metal-inorganic-organic-core-shell (Cu-MIOCS) was dried overnight at 80 °C and then stored in sealed enclosure containing dried and O 2 -free nitrogen.

Material characterization

HC, HC@ZnO and Cu-MIOCS were characterized through Fourier transform IR spectroscopy (FTIR) using a Tensor 27 (Bruker) spectrometer with a ZnSe ATR crystal equipment. For each spectrum, 20 scans were accumulated with a resolution of 4 cm -1 . All samples were drilled before IR analysis and background spectra were recorded on air. Analysis through Scanning Electronic Microscopy (SEM) of samples previously metallized by gold layer (at 18 mA during 360 s with a Biorad E5200 device) was performed by means of a ZEISS EVO 15 electron microscope. The surface morphology was investigated with secondary electrons, while the atomic composition was assessed by an attached Energy Dispersive X-ray Fluorescence (ED-XRF) device. The TEM (transmission electron microscopy) observations were achieved using a FEI TECNAI 20FST instrument on samples ultrasonically dispersed in ethanol and deposited on amorphous holey carbon membranes and then dried.

The specific surface area and porosity were assessed by nitrogen adsorption-desorption isotherms at 77 K, using a Micromeritics Tristar 30 0 0 device. Typically, equivalent BET (Brunauer-Emmett-Teller) surface areas were determined in the relative pressure range P/P0 from 0.01 to 0.04 and equivalent BJH (Barrett-Joyner-Halenda) pore volume were measured at P/P0 > 0.985. The samples were previously degassed at 150 °C for at least 7 h using a Micromeritics Vac Prep 061 degasser. Measurements through differential scanning calorimetry (DSC) were carried out using (DSC-92 Setaram) at a 2 °C/min heating rate from room temperature to 500 °C. XPS analysis was performed using a Shimadzu ESCA-3400 and Al K alpha X-ray source (1486.6 eV), while deconvolution of the XPS patterns was achieved using a nonlinear least-square curve-fitting program (XPS-PEAK software 4.1). The zeta potential of each 2 mg of sample previously dispersed in 10 mL of deionized water and ultrasonicated for 15 min was measured with the phase analysis light scattering (PALS) mode using a Malvern zeta sizer nanoZS setup.

CO 2 adsorption tests and kinetics

This was achieved using an appropriate device containing dry air-free nitrogen according to a specific procedure fully described elsewhere [START_REF] Vieillard | Cocoa shell-deriving hydrochar modified through aminosilane grafting and cobalt particle dispersion as potential carbon dioxide adsorbent[END_REF] . Triplicate CO 2 adsorption tests were carried out within a 20 mL sealed enclosure at 292 K and normal pressure. Concisely, 3 mL of pure carbon dioxide previously dried was injected by a syringe in sealed capillary U-shaped manometer (0.25 mm internal diameter) containing dry air-free nitrogen and 0.01 g of dry material samples with a 2.43-5.14 μm particle size. The non-adsorbed CO 2 excess was further purged until the normal pressure is attained in the internal U-shaped manometer. The adsorbed CO 2 amount was periodically measured in time (0.2 min, 0.4 min, 0.6 min, 0.8 min, 1 min etc.) by measuring that of the residual non adsorbed gas. When necessary, complete regeneration for a total CO 2 release from the adsorbents was achieved by slight heating up to 80 °C or by forced convection under strong air stream (15-30 mL/min) until no CO 2 was detected by bubbling into a 0.1 N aqueous NaOH solution. The adsorbent affinity towards CO 2 was examined in terms of contact time and adsorption kinetics using pseudo 1st and 2nd order models [START_REF] Bouazizi | Molybdenum-loaded 1,5-diaminonaphthalene/ZnO materials with improved electrical properties and affinity towards hydrogen at ambient conditions[END_REF][START_REF] Loganathan | CO 2 adsorption kinetics on mesoporous silica under wide range of pressure and temperature[END_REF][START_REF] Ouargli-Saker | Metal-loaded SBA-16-like silica -correlation between basicity and affinity towards hydrogen[END_REF] .

Results and discussion

Surface morphology and textural properties

Same-scale SEM images ( Fig. 1 ) (20 nm) revealed morphology change from fibrous HC ( a ) to granular HC@ZnO ( b ), suggesting strong HC: ZnO interaction. Much more pronounced changes were noticed after ClPTES ( c ) and diethanolamine grafting ( d ), resulting in aggregates of much smaller particles with particle size not exceeding few micrometers, presumably due to self-condensation of triethoxysilane groups and hydrophobic interaction between grafted chloro-propyl groups.

Please cite this article as: J. Vieillard et al., Metal-inorganic-organic core-shell material as efficient matrices for CO 2 adsorption: Synthesis, The nitrogen adsorption-desorption isotherms recorded were found to be of type IV with an H3-type hysteresis loop for mesoporous materials ( Fig. 3 ). Measurements using the BET and BJH models ( Table 1 ) revealed a structure expansion after ZnO incorporation as supported by the increase of the specific surface area (SSA) from 20 m 2 /g for HC to 25 m 2 /g for HC@ZnO.

The consecutive SSA decrease down to 21 m 2 /g upon ClPTES grafting agrees with our previous statement regarding a possible self-condensation of triethoxysilane groups and a structure compaction as a result hydrophobic interaction between grafted Please cite this article as: J. Vieillard chloro-propyl groups. This is also supported by a porosity decay reflected by a pore volume decrease from 0.17 to 0.13 cc/g. Further functionalization with diethanolamine appears to suppress such an interaction, since a marked SSA improvement up to 29 m 2 /g was noticed. This must be due the appearance of H-bridges between terminal hydroxyls belonging to both the grafted organic moieties and residual free terminal Cu-OH groups. This is supposed to give rise to additional porosity, as supported by the significant increase in total pore volume from 0.12 to 0.20 cc/g. Cu 0 particle insertion induced marked decay of the specific surface area and average pore volume down to 20 m 2 /g and 0.12 nm, respectively, thereby confirming our previously stated structure compaction. This phenomenon is a special feature of the occurrence of -N:Metal and -O:Metal Lewis-Acid-Base (LAB) interactions, as already reported elsewhere [START_REF] Azzouz | Metal-inorganic-organic matrices as efficient sorbents for hydrogen storage[END_REF][START_REF] Arus | Cu0 and Pd0 loaded organo-bentonites as sponge-like matrices for hydrogen reversible capture at ambient conditions[END_REF][START_REF] Bouazizi | Metal-loaded polyol-montmorillonite with improved affinity towards hydrogen[END_REF][START_REF] Bouazizi | Copper oxide coated polyester fabrics with enhanced catalytic properties towards the reduction of 4-nitrophenol[END_REF][START_REF] Sennour | Cu0-loaded organo-montmorillonite with improved affinity towards hydrogen: an insight into matrix-metal and non-contact hydrogen-metal interactions[END_REF][START_REF] Tahir | Metal organoclays with compacted structure for truly physical capture of hydrogen[END_REF] .

Evidence of metal stabilization

The most significant changes were noticed through FTIR analysis in the 750-1700 cm -1 and 30 0 0-350 0 cm -1 regions. These changes were mainly reflected by the rise of an intense absorption band at ca. 1065 cm -1 and smaller one around 800 cm -1 ( Figs. providing clear evidence of ClPTES grafting but accompanied by an unavoidable self-condensation of the triethethoxysilane groups [START_REF] Vieillard | Cocoa shell-deriving hydrochar modified through aminosilane grafting and cobalt particle dispersion as potential carbon dioxide adsorbent[END_REF][START_REF] Sepehr | Defluoridation of water via Light Weight Expanded Clay Aggregate (LECA): adsorbent characterization, competing ions, chemical regeneration, equilibrium and kinetic modeling[END_REF][START_REF] Yamaura | Preparation and characterization of (3-aminopropyl)triethoxysilane-coated magnetite nanoparticles[END_REF] . The appearance of the 3440-3700 and 1674 cm -1 bands assigned to the N -H stretching vibration and NH 2 bending vibration, respectively was attributed to amine grafting [START_REF] Maurizi | Polymer Adsorption on iron oxide nanoparticles for one-step amino-functionalized silica encapsulation[END_REF][START_REF] Saif | Synthesis and characterization of Fe 3 O 4 coated on APTES as carriers for morin-anticancer drug[END_REF] . The marked depletion of the 3400 cm -1 after APTES grafting indicates the occurrence of HC -O-Si-propyl bridges. The total disappearance of the 2919 cm -1 band related to the asymmetric stretching vibration of CH 2 and C -H aliphatic groups after Cu 0 incorporation must be due to the occurrence of strong Cu: OH-and Cu:N-interactions that induce a compaction of the organic entanglement around CuNPs, in agreement with our previous statements [START_REF] Vieillard | Cocoa shell-deriving hydrochar modified through aminosilane grafting and cobalt particle dispersion as potential carbon dioxide adsorbent[END_REF][START_REF] Arus | Cu0 and Pd0 loaded organo-bentonites as sponge-like matrices for hydrogen reversible capture at ambient conditions[END_REF] . Incorporation of CuNPs also induced a marked shift of the 1065 cm -1 band towards lower wavenumber ( Fig. 4 ), suggesting a N -C bond weakening by the appearance of strong Cu 0 :N-propyl interaction. Terminal OH groups should also contribute to this compaction of the organic entanglement via Cu 0 : OH-Lewis-acid-base interactions with a possible synergy between the N and O atoms in CuNP stabilization, as supported by the slight intensity decrease of the 330 0-360 0 cm -1 bands [START_REF] Arus | Cu0 and Pd0 loaded organo-bentonites as sponge-like matrices for hydrogen reversible capture at ambient conditions[END_REF][START_REF] Tahir | Metal organoclays with compacted structure for truly physical capture of hydrogen[END_REF] .

Deeper insights through XPS analysis showed a shift of the O-1s signal from 529.98 eV to 530.53 peak after ClPTES grafting ( Table 2 ). This can be explained in terms of a bond strengthening on the oxygen atom, most likely due to the genesis of Zn-O-Si and/or C -O-Si bridges. The O-1s binding energy values were in good agreement with those reported for ZnO [START_REF] Figueiredo | The role of surface chemistry in catalysis with carbons[END_REF]

ARTICLE IN PRESS

JID: JTICE [m5G; September 13, 2018;16:22 ] Fig. 3. adsorption-desorption of HC@ZnO (a), HC@ZnO -ClPTES (b), HC@ZnO-Si-N(OH) 2 (c) and Cu-MIOCS (d).

electron density due to next-neighboring N atoms. The involvement of the N and O atoms in CuNP stabilization was well supported by noticeable shifts of the binding energy of N-1s electrons from 399.71 to 400.33 eV and of O-1s electron from 530.42 eV to 530.91 eV, respectively, in agreement with previous data. [START_REF] Arus | Cu0 and Pd0 loaded organo-bentonites as sponge-like matrices for hydrogen reversible capture at ambient conditions[END_REF] Cu incorporation was also supported by an additional Cu-2p signal at 935.70 eV (Table S1), with agrees with the literature [START_REF] Liu | Ferromagnetism in Cu-doped ZnO nanoparticles at room temperature[END_REF] . HC encapsulation by ZnO was confirmed by the appearance of the Zn-2p 3/2 and Zn-2p 1/2 doublet at 1021.25 and 1044.34 eV, respectively [START_REF] Huang | Self-organized ZnO nanodot arrays: effective control using SiN x interlayers and low-temperature plasmas[END_REF] . The shift of both values up to 1022.24 and 1045.34 eV after ClPTES grafting accounts for a binding strengthening of the corresponding electrons, due to the formation of -Zn-O-Si-R bridges. These shifts indicate preponderant grafting of ClPTES on ZnO on HC [START_REF] Zhao | One-step solvothermal synthesis of a carbon@TiO 2 dyade structure effectively promoting visible-light photocatalysis[END_REF] . The slight consecutive decrease of these values down to 1022.10 and 1045.30 eV must be due to the grafting of an electronegative atom (N). This was supported by the shift of the N-1s electron binding energy from 400.7 to 399.5 eV.

The C-1s XPS signal of HC@ZnO slightly shifted from 285.01 to 284.94, presumably as a result of an increase in electron density on ZnO by ClPTES grafting, which seems to act as ''electron pump'' due to the electronegativity of the chlorine atom. Chlorine substitution by diethanolamine grafting appears to revive the binding energy of the C-1s electrons, due to the higher electron density on the N atom. The fact that the binding energy of the C-1s electrons remained constant at 285.02 eV after Cu 0 incorporation indicates unequivocally that the organic chain grafted does not contribute to metal stabilization.

Metal stabilization was confirmed through additional Zeta potential measurements ( Fig. 5 ), which revealed noticeable increases in the Zeta potential (ZP) and surface charge density (SPC) upon Please cite this article as: J. Vieillard et al., Metal-inorganic-organic core-shell material as efficient matrices for CO 2 adsorption: Synthesis, properties and kinetic studies, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.020 ZnO incorporation. This must be due to the polarity of Zn-O bonds and terminal Zn-O-H groups [START_REF] Marsalek | Particle size and zeta potential of ZnO[END_REF] . Their consumption upon silylation by ClPTES induced a significant decrease of both the Zeta potential and surface charge density. Further functionalization by diethanolamine induced a slight SPC decrease but a marked ZP depletion.

ARTICLE IN PRESS

Pronounced ZP depletion was registered after Cu 0 insertion, most likely due to the appearance of metal-matrice LAB interactions that favor the stability of the colloid suspension [START_REF] Bhattacharjee | DLS and zeta potential -what they are and what they are not?[END_REF][START_REF] Fritz | Electrosteric stabilization of colloidal dispersions[END_REF] . The fact that the zeta potential was reversed from positive to low negative value after Cu 0 incorporation must arise from an increase in electron density around CuNPs as a result of strong Cu 0 : OH-and Cu 0 -N-interactions.

Thermal behavior

DSC measurements ( Fig. 6 ) showed thermal stability for HC up to 130-140 °C ( a ). Two exothermal processes were noticed around 250 °C and 450 °C due to the decomposition of cellulose and lignin, which are the main components of cocoa shell [START_REF] Chen | Isothermal torrefaction kinetics of hemicellulose, cellulose, lignin and xylan using thermogravimetric analysis[END_REF]. ZnO incorporation induced a stability improvement up to ca. 300 °C ( b ), as supported by the flattened bump around 350 °C. This can be explained by the formation of a more thermally stable inorganic ZnO shell.

HC@ZnO-Si-N(OH) 2 ( c ) displayed slightly lower thermal stability of up to 250 °C ( c ) due to the degradability of the grafted organic moiety. The latter appears to be enhanced by Cu 0 incorporation ( d ), presumably due to the appearance of copper-catalyzed decomposition processes.

Affinity towards CO 2 and adsorption

CO 2 adsorption tests under normal pressure and ambient temperature showed an increase in time of the amount of adsorbed CO 2 ( Fig. 7 ). The latter reached a maximum and constant level after ca. 2 min for HC ( a ) and approximately 9-10 min for its modified counterparts ( b-d ).

It is worth mentioning that each material presents a reversible adsorption upon slight heating up 40 °C or under forced convection upon strong air stream of 15 mL/min with a maximum capacity for the first experiment. It results that, under these tests conditions, CO 2 adsorption must involve only weak interaction with the solid surface. This maximum level accounts for the real value of the CO 2 retention capacity (CRC) under these specific conditions. The highest CO 2 uptake was registered for Cu-MIOCS under normal pressure (5.423 mmol/g). This value is considerably higher than those registered under similar conditions for numerous other materials such as mesoporous silica [START_REF] Sim | CO 2 adsorption on amine-functionalized periodic mesoporous benzenesilicas[END_REF] , clays [START_REF] Azzouz | OH-enriched organo-montmorillonites for potential applications in carbon dioxide separation and concentration[END_REF] , zeolites [START_REF] Yang | CO 2 adsorption over ion-exchanged zeolite beta with alkali and alkaline earth metal ions[END_REF] , metal oxides [START_REF] Ruminski | Size-dependent CO 2 capture in chemically synthesized magnesium oxide nanocrystals[END_REF] , and others [START_REF] Sim | CO 2 adsorption on amine-functionalized periodic mesoporous benzenesilicas[END_REF][START_REF] Sevilla | N-doped polypyrrole-based porous carbons for CO 2 capture[END_REF][START_REF] Khalil | Study on the improvement of the capacity of amine-impregnated commercial activated carbon beds for CO 2 adsorbing[END_REF] . This improvement of the affinity towards CO 2 suggests simultaneous contributions of all incorporated species through specific interactions with a possible synergy that remains to be elucidated [START_REF] Hou | Understanding the adsorption mechanism of C 2 H 2 , CO 2 , and CH 4 in isostructural metal-organic frameworks with coordinatively unsaturated metal sites[END_REF][START_REF] Tang | Adsorption of CO 2 at ZnO: a surface structure effect from DFT + U calculations[END_REF][START_REF] Vinodh | Homopiperazine grafted mesoporous silicas from rice husk ash for CO 2 adsorption[END_REF][START_REF] Sneddon | The potential applications of nanoporous materials for the adsorption, separation, and catalytic conversion of carbon dioxide[END_REF][START_REF] Yu | A review of CO 2 capture by absorption and adsorption[END_REF] . The slow evolution in time of the CO 2 uptake for the modified samples suggests the occurrence of diffusion hindrance. The two bump-shaped evolution curves suggest the occurrence of successive kinetic-determining steps, namely inter and intraparticle diffusion processes that differ from a sample to another according to the particle morphology and textural features [START_REF] Son | Adsorptive removal of carbon dioxide using polyethyleneimine-loaded mesoporous silica materials[END_REF] .

Attempts to apply the pseudo 1st and 2nd order models [START_REF] Bouazizi | Molybdenum-loaded 1,5-diaminonaphthalene/ZnO materials with improved electrical properties and affinity towards hydrogen at ambient conditions[END_REF][START_REF] Ho | Review of second-order models for adsorption systems[END_REF] showed that CO 2 capture on HC obeys a pseudo-first order kinetics ( Fig. 7 ). Here, physical adsorption through interactions between the oxygen atoms of carbon dioxide and terminal OH groups of HC seems to prevail in agreement with previous data [START_REF] Lu | Competitive adsorption of a binary CO 2 -CH 4 mixture in nanoporous carbons: effects of edge-functionalization[END_REF][START_REF] Jin | The adsorption behaviour of CH 4 on microporous carbons: effects of surface heterogeneity[END_REF][START_REF] Wang | Influence of CO 2 exposure on high-pressure methane and CO 2 adsorption on various rank coals: implications for CO 2 sequestration in coal seams[END_REF][START_REF] Drage | Evaluation of activated carbon adsorbents for CO 2 capture in gasification[END_REF] Nonetheless, neither models can apply to the other HC derivatives, the pseudo 1st order appearing as the most closest to the experimental curves. The most plausible explanation is that CO 2 adsorption on HC derivatives involves predominant kineticscontrolling diffusion processes and weak contributions of additional interaction involving both the amino and hydroxyl groups of the incorporated organic moiety [START_REF] Belmabkhout | Adsorption of CO 2 -containing gas mixtures over amine-bearing pore-expanded MCM-41 silica: application for gas purification[END_REF] even in the presence of Cu 0 nanoparticles [START_REF] Bouazizi | Montmorillonite-supported Pd0, Fe0, Cu0 and Ag0 nanoparticles: properties and affinity towards CO 2[END_REF] .

Deeper insights in CO 2 adsorption kinetics were achieved by applying the intraparticle diffusion (ID) model ( Eq. ( 1)) that correlates the kinetics constant (k ID ) to the intraparticle diffusion constant (CI) [START_REF] Bouazizi | Molybdenum-loaded 1,5-diaminonaphthalene/ZnO materials with improved electrical properties and affinity towards hydrogen at ambient conditions[END_REF] :

Q t = k ID . t 1 / 2 + C I (1)
As expected, plotting Q t (instant CO 2 amount adsorbed) as a function of t 1/2 resulted in sufficiently linear correlations for almost all the synthesized samples, excluding HC ( Fig. 8 ), thereby confirming the occurrence of intraparticle diffusion on HC derivatives.

The wide dispersion of the of the measured data gave estimated values of the correlation factor ( R 2 ) ranging from 0.93 to 0.98 according to the sample texture, the closest to unity being those obtained for HC@ZnO-Si-N(OH) 2 ( Table 3 ). In this adsorbent, the intraparticle diffusion appears to prevail more than in Cu-MIOCS, where other processes such as physical adsorption and multilayer CO 2 condensation should also take place at a more or lesser extent. The occurrence of different slopes accounts for that of different intraparticle diffusion constants within a same particle of a same adsorbent, in agreement with our previous statements and other works [START_REF] Álvarez-Gutiérrez | Kinetics of CO 2 adsorption on cherry stone-based carbons in CO 2 /CH 4 separations[END_REF] .

As expected, the values of k ID were found to vary in a reverse proportionality with the textural properties. Indeed, k ID increased from 1.13768 g/(mmol min 1/2 ) (HC@ZnO) to 1.34847 g/ (mmol min 1/2 ) (HC@ZnO -ClPTES) and from 1.23477 g/ (mmol min 1/2 ) (HC@ZnO-Si-N(OH) 2 ) to 1.82982 g/(mmol min 1/2 ) (Cu-MIOCS), when the total pore volume decreases from 0.17 to 0.13 cc/g and from 0.20 to 0.12 cc/g, respectively ( Table 1 ). In the meantime the specific surface area decreases from 25 to 21 and from 29 to 20 m 2 /g, respectively. This provides additional evidence of the enhancement of the diffusion contribution to the overall CO 2 retention process with the structure compaction. The mere fact that CO 2 desorbs only upon heating or forced convection clearly demonstrates the occurrence of effective physical CO 2 retention. The latter should probably occur in the form of multilayer condensation, more or less pronounced according to the pore size and surface affinity towards CO 2 . Investigations are still in progress in this direction.

Please cite this article as: J. Please cite this article as: J.

Conclusion

The present study provides the proof-of-concept that vegetalderiving composites can display high affinity towards CO 2 at ambient temperature after judicious modification. Hydrochar encapsulation into HC@ZnO, functionalization with diethanolamino groups and copper incorporation produced an effective adsorbent for CO 2 , affording an appreciable CO 2 uptake of 5.423 mmol/g. The latter was found to adsorb reversibly at ambient conditions, being easily released upon weak heating or forced convection. This was explained in terms of an attenuation of the Lewis basicity of the N atoms involved in Cu 0 stabilization and structure compaction. The appearance of -N:Metal and -O:Metal Lewis-Acid-Base (LAB) interactions are responsible of a structure compaction that promotes preponderantly physical interactions of CO 2 around entrapped CuNPs surrounded by O and N atoms belonging to the incorporated organic moiety. The easy CO 2 release only upon heating or forced convection must originate from effective physical adsorption of CO 2 in the form of multilayer condensation. This finding opens new prospects for the manufacture of highly added-value vegetal-deriving matrices for a reversible CO 2 capture. Significant affinity improvement towards CO 2 can be envisaged through judiciously tailored material texture and surface interaction. Investigations are still in progress in this direction.

Scheme 3 .

 3 Scheme 3. Schematic illustration of the diethanolamine substitution of chloride on grafted ClPTES.Further SEM images after Cu 0 incorporation ( e ) and close-up ( f ) showed larger aggregates of particles with much smoother external surface, most likely due intra and intergranular entrapment of Cu 0 particles. Here, strong Cu 0 :N(OH) 2 Lewis-acid-base (LAB) interaction must induce a structure compaction that should reduce porosity, but should promote non-stoichiometric and purely physical condensation of CO 2 on metal sites surrounded by N atoms and hydroxyl groups.Deeper insights through TEM images ( Fig.2) showed a clear membrane of ZnO shell surrounding aggregates of 0.05-0.1 μm dark stains of HC ( a ), whose aggregation generates interparticle void spaces ( a ). The latter appears to dramatically decrease upon ZnO-coating ( b ). After ClPTES grafting and further functionalization

Scheme 4 .

 4 Scheme 4. Illustration of CuNPs insertion (a) and close-up on HC@ZnO-Si-N(OH) 2 -Cu (b).

Fig. 1 .

 1 Fig. 1. SEM images of HC (a), HC@ZnO (b), HC@ZnO -ClPTES (c), HC@ZnO-Si-N(OH) 2 (d) and Cu-MIOCS (e and f).

Fig. 2 .

 2 Fig. 2. TEM images of HC (a), HC@ZnO (b), Cu-MIOCS (c) and close-up on clear areas (d).

Fig. 4 .

 4 Fig. 4. FTIR spectra of HC@ZnO-Si-N(OH) 2 (a) and Cu-MIOCS (b).

Fig. 5 .

 5 Fig. 5. Zeta potential (circle) and surface charge density (cross) for HC (a), HC@ZnO (b), HC@ZnO -ClPTES (c), HC@ZnO-Si-N(OH) 2 (d), and Cu-MIOCS (e). Please cite this article as: J. Vieillard et al., Metal-inorganic-organic core-shell material as efficient matrices for CO 2 adsorption: Synthesis, properties and kinetic studies, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.020

Fig. 6 .

 6 Fig. 6. DSC patterns of HC (a), HC@ZnO (b), HC@ZnO-Si-N(OH) 2 (c) and Cu-MIOCS (d).

Fig. 7 .Fig. 8 .

 78 Fig. 7. Evolution in time of CO 2 uptake on hydrochar and modified derivatives as compared to adsorption kinetical models. (a) HC; (b) HC@ZnO; (c) HC@ZnO -ClPTES; (d) HC@ZnO-Si-N(OH) 2 and (e) Cu-MIOCS. These experiments were conducted by contacting, at room temperature and normal pressure, 10 mg of adsorbent previously dried overnight at room temperature with 3 mL of pure dry CO 2 .

Table 1

 1 Textural properties of the prepared samples.

	Sample	Isotherm	Hysteresis	Specific surface	Total pore	BJH pore
				area (m 2 /g)	volume (cc/g)	volume (cc/g)
	HC	-	-	2 0	-	-
	HC@ZnO	Type II	H3	25	0.17	0.17
	HC@ZnO -ClPTES	Type II	H3	21	0.13	0.13
	HC@ZnO-Si-N(OH) 2	Type II	H3	29	0.20	0.20
	Cu-MIOCS	Type II	H3	20	0.12	0.12

Table 2

 2 Binding energies of the core level spectra for the different samples.

	JID: JTICE

Table 3

 3 Kinetics parameters for the intraparticle diffusion model. k ID is the constant for the intraparticle diffusion model. b C I is assessed as the Y -axis intercept. c R 2 is the correlation coefficient.

	Sample	k ID	a g/(mmol min 1/2 )	C I	a mmol/g	c R 2	Standard deviation for k ID
							g/(mmol min 1/2 )
	HC	0.43472	0.01087	0.934	0.05060
	HC@ZnO	1.13768	-0.00982	0.951	0.07745
	HC@ZnO-ClPTES	1.34847	-0.01047	0.977	0.06499
	HC@ZnO-Si-N(OH) 2	1.23477	-0.06636	0.980	0.05337
	HC@ZnO-Si-N(OH) 2 -Cu	1.82982	0.26887	0.954	0.12614

a

  Vieillard et al., Metal-inorganic-organic core-shell material as efficient matrices for CO 2 adsorption: Synthesis, properties and kinetic studies, Journal of the Taiwan Institute of Chemical Engineers (2018), https://doi.org/10.1016/j.jtice.2018.08.020

	JID: JTICE	ARTICLE IN PRESS	[m5G; September 13, 2018;16:22 ]

Acknowledgment

This work was partially supported by INSA Rouen, Rouen University, CNRS, Labex SynOrg ( ANR-11-LABX-0029 ), the European Battuta Program, the European FEDER program, the Normandy region (CBS network) and the Evreux Portes de Normandie Agglomeration. We thank Dr. Philippe Decorse from ITODYS (UMR 7086 CNRS) for the XPS analysis.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Supplementary materials

The TEM image of the distribution of Cu nanoparticles; XPS analysis and chemical composition for the different sam ples; CO 2 adsorption measurements; IR analysis of CO 2 -saturated materials; Kinetic parameters of pseudo 1st and 2nd order models.

Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.jtice.2018.08.020 .

ARTICLE IN PRESS

JID: JTICE [m5G; September 13, 2018;16:22 ]


Marie Camier
email: mariecamier@icloud.com

Antoine Logghe

Andre Nel
email: anel@mnhn.fr

Romain Garrouste

The first Malcidae (Hemiptera, Heteroptera) from the earliest Eocene Oise amber

Keywords: Insecta, Heteroptera, Lygaeoidea, gen. et sp. nov., Paleogene, dating

The earliest Eocene Eochauliops longicornis gen. et sp. n., first fossil Malcidae, is described from the amber of Oise (France). Together with the previous discovery of a species of Blissidae in the same amber, it suggests that the Lygaeoidea were already rather diverse during the Paleocene, even if the Mesozoic record of this superfamily remains uncertain and scarce.

Introduction

The family Malcidae Stål, 1866 is a small lygaeoid clade that comprises only three extant genera (Henry et al. 2015), currently with no fossil record. More generally, the Lygaeoidea are rather infrequent in the fossil record (Garrouste, 2015). There is some uncertainty on the positions of the Mesozoic compression fossils currently attributed to this superfamily [START_REF] Garrouste | The first chinch bug Blissidae (Hemiptera, Heteroptera) from the Eocene Oise amber[END_REF]. Thus the discovery of the first fossil malcid from the earliest Eocene amber of France is of great interest.

Material and methods

The fossil is embedded in a small clear piece of amber. It was prepared using a diamond disk, examined using a Nikon binocular microscope SMZ 1500. Photographs were taken with an Amscope camera MU900, and the images treated with Adobe Photoshop Element 12. The amber sedimentary layer is from the lower Eocene of the Paris basin (ca 53 million-years-old), during a global warming period, in a semi-deciduous forest with a mosaic of gallery-forest mixed with dryer plant communities, in a 'paratropical' region. The climate was hot, with both dry and wet seasons [START_REF] Nel | Oise amber[END_REF].

The classification follows [START_REF] Henry | Phylogenetic analysis of family groups within the infraorder Pentatomorpha (Hemiptera: Heteroptera), with emphasis on the Lygaeoidea[END_REF] and Štys (1967), and the morphological nomenclature given by [START_REF] Gao | Notes on fine structures of Chauliops with descriptions of two new species from China (Insecta: Hemiptera: Heteroptera: Malcidae)[END_REF]. Dimensions are in millimeters.

Systematic

Diagnosis

Antennae long, extending to distal part of abdomen; first antennal segment greatly surpassing apex of clypeus for more than its half; ocelli widely separated; pronotum widest at base; lateral pronotal margin weakly carinate with a row of curved setae; at least two deep punctures on distal part of clavus; membrane with four indistinct parallel veins; hind femora unarmed;

tergites not fused into a plate; sterna III to VI not fused; connexiva on abdominal segments V-VII produced into conspicuous dentate lobes.

Etymology. Named after the Eocene period and the extant genus Chauliops. Gender neutral. Diagnosis. As for the genus.

Eochauliops longicornis

Etymology. Named after the long antennae, the main character of the genus.

Description.

Male. Total body length 3.35 mm. Head length 0.47 mm, wider than long; width across eyes 0.75 mm, lower than width at posterior margin of pronotum; interocular distance 0.36 mm; preocular distance 0.57 mm; declivent, triangular; antennal segment I robust, 0.42 mm long, barely curved, greatly exceeding apex of tylus (clypeus), segments II and III slender, cylindrical, subequal lengths, twice as long as I, respectively 0.87 and 0.85 mm, segment IV narrowly fusiform, 0.55 mm long; antenniferous tubercle clearly visible from below and partly visible from above, unarmed; vertex low; eyes hemispheric, stylate and protuberant, at same level as vertex in lateral view; ocelli laterally encircled by a groove, visible from side of head, well separated, 0.25 mm apart; bucculae elongate, reaching posterior third of ventral part of head; rostrum extending to middle mesosternum; pronotum longer than wide, 0.85 mm long; width across humeral angles 0.94 mm, slightly wider than head across eyes; subquadrate; pronotum almost rectangular; anterior and posterior borders straight; anterolateral borders with a series of curved setae, not emarginate, convex on middle third; transverse pronotal impression weak; collar-like region narrow; scutellum triangular, wider than long, ca 0.28 mm long and 0.49 mm wide, apex hidden by impurities; mesocoxae and metacoxae widely separated; fore femora unarmed, but with a series of setae; middle and hind femora unarmed; tibiae cylindrical, with a tibial comb on fore tibia; tarsi three-segmented, with first and third tarsomeres subequal; inner side of claws with transverse rugosities; distipulvillus broad with a lamellar surface; basipulvillus with numerous toothlike and sharp outgrowths; one pair of bristle-like parempodia present, base of each parempodium with a stout spine; ventral terminal margin of 3rd tarsomere with one long microtrichia; hemelytron 2.23 mm long; as wide as pronotum; clavus and corium separated by a furrow, convex; costal margin broadly emarginated; at least two deep punctures on distal part of clavus and numerous on corium; membrane well-developed with four parallel indistinct veins; abdominal spiracles on dorsal external laterotergites; tergites not fused into a plate; sterna III to VI not fused; connexiva on abdominal segments V-VII produced into conspicuous dentate lobes; sternites III and IV without visible trichobothria; sternites V and VI with three lateral trichobothria; sternum VII with one large and one smaller posteromedial trichobothrium.

Discussion

The general habitus of this fossil clearly resembles extant members of the family Malcidae. [START_REF] Henry | Phylogenetic analysis of family groups within the infraorder Pentatomorpha (Hemiptera: Heteroptera), with emphasis on the Lygaeoidea[END_REF] proposed the 'presence of three trichobothria on some sternites' as a synapomorphy of the clade (Coreoidea + Idiostoloidea + Lygaeoidea + Pyrrhocoroidea), and the character state 'reduced or simple hemelytral venation with only 3-4 simple veins and lacking closed cells at the base of the membrane' as a synapomorphy for the Lygaeoidea, Both are present in Eochauliops, supporting its attribution to the Lygaeoidea. Following [START_REF] Henry | Phylogenetic analysis of family groups within the infraorder Pentatomorpha (Hemiptera: Heteroptera), with emphasis on the Lygaeoidea[END_REF] key to the families of Lygaeoidea, Eochauliops goes to the Malcidae because of the following characters: abdominal spiracles dorsal; tarsi threesegmented; ocelli present; each ocellus laterally encircled by a groove; connexiva on abdominal segments V-VII produced into conspicuous dentate lobes; lateral pronotal margin weakly carinate. [START_REF] Henry | Phylogenetic analysis of family groups within the infraorder Pentatomorpha (Hemiptera: Heteroptera), with emphasis on the Lygaeoidea[END_REF] characterized the Malcidae by the following synapomorphies: a stout, elongate 1st antennal segment; connexival processes on the laterotergites; and stylate eyes. Štys (1967) and [START_REF] Henry | Phylogenetic analysis of family groups within the infraorder Pentatomorpha (Hemiptera: Heteroptera), with emphasis on the Lygaeoidea[END_REF] considered the Malcidae monophyletic after morphological analyses, but [START_REF] Li | Mitochondrial phylogenomics of Hemiptera reveals adaptive innovations driving the diversification of true bugs[END_REF] did not recovered the Malcidae as monophyletic in their molecular analysis but in a mixed set with Geocoris and Kleidocerys. The Malcidae currently comprise the three genera Malcus Stål, 1859 (Malcinae), Chauliops [START_REF] Stål | Hemiptera species novas descriptsit. Konglika Svenska Fregattens Eugenies resa omkring jorden under Befäl af C. A. Virgin åren[END_REF]Neochauliops Štys, 1963 (Chauliopinae). After Štys (1967), Eochauliops gen. n. falls into the Chauliopinae rather than in the Malcinae because of the following characters: ocelli distant from each other, not situated on a common tubercle; stylate eyes; large antenniferous tubercles; first and third tarsomeres subequal; veins on membrane unsclerotized, hardly visible; abdominal mediotergites not fused in a common plate. Also, the dentate lobes of the connexiva of abdominal segments V-VII are similar in size and ornamentation to those of Chauliops horizontalis Zheng, 1981 (Gao and[START_REF] Gao | Notes on fine structures of Chauliops with descriptions of two new species from China (Insecta: Hemiptera: Heteroptera: Malcidae)[END_REF]. However, the antenniferous tubercles are not clearly spinously produced unlike in the extant Chauliopinae. Also the antennae are long, reaching the level of the apical third of abdomen as in the extant Malcus [START_REF] Kondorosy | Three new species of Malcus Stål, 1859 (Heteroptera, Malcidae) from Vietnam[END_REF], whereas in the Chauliopinae, they are clearly shorter (Štys, 1967). Eochauliops gen. n. shares with Chauliops and Malcus the presence of at least two deep punctures on the distal part of the clavus, whereas these structures are absent in Neochauliops (Štys, 1963). The unarmed hind femora is also a characters present in Eochauliops gen. n., Malcus and Chauliops bisontula [START_REF] Banks | Rhynchota Palawanica, Part I; Heteroptera[END_REF]Banks, (Štys, 1963)).

Conclusion

Eochauliops gen. n. is the first fossil malcid. The Lygaeoidea are supposed to be rather ancient, with several Jurassic and Cretaceous compression fossils currently attributed to this clade, but [START_REF] Garrouste | The first chinch bug Blissidae (Hemiptera, Heteroptera) from the Eocene Oise amber[END_REF] questioned their attributions. [START_REF] Li | Higher level phylogeny and the first divergence time estimation of Heteroptera (Insecta: Hemiptera) based on multiple genes[END_REF], after a molecular phylogenetic analysis, dated the Cymidae: Lygaeoidea from the Eocene (ca. 45 Ma.), and more recently [START_REF] Li | Mitochondrial phylogenomics of Hemiptera reveals adaptive innovations driving the diversification of true bugs[END_REF] dated both Malcinae and Chauliopinae from the Early Cretaceous (ca.

130 Ma), which is more congruent with the putative presence of Lygaeoidea in the Cretaceous.

There are unstudied Lygaeoidea known from the Lower Cretaceous Lebanese amber (Dany Azar pers. comm.). The current discovery of a species of Malcidae, together with the previous discovery of a Blissidae in the Lowermost Eocene Oise amber (ca. 53 Ma.), suggests that the Lygaeoidea were already rather diverse during the Paleocene.

Extant Malcidae are distributed in the Afrotropical and Oriental regions. Štys (1963, 1967) suggested that Burma and the surrounding mountains could be considered a center of speciation for Malcinae and Chauliopinae. The presence of this family in the European Eocene shows that this family was much more widespread during the Paleogene and that their history is certainly more complex than previously documented. The discovery of this taxon also supports the tropical to sub-tropical climate presence in the Oise amber paleoforest [START_REF] Nel | Oise amber[END_REF].

The recent discovery of Malcus elongatus Štys as an invasive in Crete (Roca-Cusach and Goula, 2017) shows that these bugs can adapt to a Mediterranean climate.

  Article urn:lsid:zoobank.org:pub xxxx Type species. Eochauliops longicornis sp. n.

  sp. n. (Figs 1-7) Article LSID urn:lsid:zoobank.org:pub:40A50ED4-E964-4373-A051-C8C3BE176FCF Genus LSID urn:lsid:zoobank.org:pub:40A50ED4-E964-4373-A051-C8C3BE176FCF Species LSID urn:lsid:zoobank.org:pub:40A50ED4-E964-4373-A051-C8C3BE176FCF Material. Holotype MNHN.F.A71302 (male, PA 2495), stored in the Paleontological Department, MNHN, Paris.

Figure 1 .

 1 Figure 1. Eochauliops longicornis gen. et sp. n., holotype MNHN.F.A71302. Habitus. (A)

Figure 2 .

 2 Figure 2. Eochauliops longicornis gen. et sp. n., holotype MNHN.F.A71302. (A) antennae; (B)

Figure 3 .

 3 Figure 3. Eochauliops longicornis gen. et sp. n., holotype MNHN.F.A71302. Forewing. Scale

Figure 4 .

 4 Figure 4. Eochauliops longicornis gen. et sp. n., holotype MNHN.F.A71302. Apex of

Figure 5 .

 5 Figure 5. Eochauliops longicornis gen. et sp. n., holotype MNHN.F.A71302. Head, black

Figure 6 .

 6 Figure 6. Eochauliops longicornis gen. et sp. n., holotype MNHN.F.A71302. Distal part of

Figure 7 .

 7 Figure 7. Eochauliops longicornis gen. et sp. n., holotype MNHN.F.A71302. Claws. Scale bar

Acknowledgements

We greatly thank T.J. Henry for his help in the establishing family position of the fossil. Also we thank two anonymous referees for their useful comments on the first version of this paper.

We also thank Gael De Ploëg for his crucial help in the collect and study of the fossil insects of Oise amber. We thank the Lafarge-Granulat company for help with the fossil sampling and the Langlois-Meurinne family for the authorization of working on their property.

Disclosure statement

No potential conflict of interest was reported by the authors.


Emmanuelle Weber

Dennis Kurzbach

Daniel Abergel
email: daniel.abergel@ens.fr

A DNP-hyperpolarized solid-state water NMR MASER: observation and qualitative analysis

We present observations of an NMR maser (microwave amplification by stimulated emission of radiation) of hyperpolarized 1 H nuclei by dynamic nuclear polarization (DNP) at 1.2 K and in a magnetic field of 6.7 T. The sustained maser pulses originate from the interplay between radiation damping (RD) due to the large 1 H magnetization, and the remagnetization to a negative value by the DNP process. NMR signals lasting for several tens of seconds are thus observed on an ensemble of dipolar-coupled nuclear spins. Magnetization dynamics are analyzed in terms of the combined Bloch-Maxwell and Provotorov (BMP) equations for RD and DNP. Insight into the long time evolution of the magnetization is provided by a theoretical analysis of this nonlinear dynamical system, and by fitting the NMR signal to a simplified version of the BMP equations.

A large magnetization can exhibit unconventional behaviour that departs from the dynamics in a static magnetic field predicted by the simple Bloch equations, in cases of

strong coupling with the detection circuit. This phenomenon, designated as "radiation damping" (RD), was already observed in the early days of nuclear magnetic resonance (NMR). 1 Radiation damping can be described in classical terms through the so-called Bloch-Maxwell equations that account for the radio-frequency (rf) field that is induced in the coil of the detection circuit through precession of the magnetization. 1,2 RD thus induces collective and coherent motions of the spins that are slaved to each other upon the action of a feedback field from the detection circuit, a process that is analogous to superradiance. 3 Thus, RD can be described as an NMR MASER. This is an active process, in contrast to other types of coherent processes, such as spin echoes, that occur passively upon the spin manipulation by an externally applied RF field. In liquid-state NMR, RD is typically observed when a large solvent magnetization, usually water, is present. Radiation damping may give rise, in some instances, to unexpected features, such as typical nonlinear maser pulses, multiple echoes, 4 or the appearance of spurious harmonics in two-dimensional liquid state NMR, 2,[5][6][7] which are related to the nonlinear nature of the modified Bloch-Maxwell equations. In certain circumstances, when magnetization is continuously replenished, this gives rise to characteristic multiple spontaneous signal bursts, leading to sustained maser oscillations. Such manifestations have been previously observed for hyperpolarized spin states that exceed typical magnetization levels by several orders of magnitude, for optically pumped noble gases, 8 in the case of dissolved hyperpolarized 129 Xe when distant dipolar field effects are effective, 9 or on thermally polarized water at room temperature through electronic control of the coil rf feedback field. 10,11 Alternatively, similar unconventional magnetization dynamics were observed in the context of dynamic nuclear polarization (DNP) for polarized targets, 12,13 or in the case of a ruby crystal. 14,15 Here, we investigate such NMR maser effects in the framework of high-field DNP. This is motivated by the recent developments in DNP methodology that clearly tend towards ever higher polarization levels and larger magnetizations. In this context, intense radiation damping may well interfere with rf pulse sequences used in practice, such as cross-polarization, to reduce their efficiencies. We present observations of a 1 H NMR maser on a DNP-hyperpolarized water-glycerol sample at 1.2 K, in a 6.7 T magnet, where the recorded induction signal could be observed for more than a second, and up to several tens of seconds in some cases, even after microwave irradiation has been switched off. Following previous studies, 14 these observations of this high-field DNP maser were rationalized in terms of the Maxwell-Bloch-Provotorov equations. This shows that the collective behaviour of an ensemble of dipolar-coupled nuclear spins can be taken into account by a simple vector model. Hyperpolarization of nuclei through DNP is achieved by transferring part of the high electron spin polarization (nearly 100% at temperatures approaching 1 K) of paramagnetic impurities to nearby nuclear spins via hyperfine and superhyperfine couplings. Depending on the experimental conditions, various mechanisms can contribute more or less efficiently to DNP. Importantly, these depend on the electron paramagnetic resonance (EPR) line characterizing the paramagnetic impurities. A great amount of work has been devoted to the elucidation of these complex aspects since the early days of DNP (see for instance previous work [16][17][18][START_REF] Wenckebach | Essentials of Dynamic Nuclear Polarization[END_REF] ), and this field of research has regained interest in a more recent period with the renewal of DNP techniques, such as dissolution-DNP [START_REF] Ardenkjaer-Larsen | [END_REF] and the introduction of DNP in MAS solid-state NMR. 21 In the study at hand, the EPR linewidth of the paramagnetic impurity is significantly larger than the NMR transition frequency, so that the hyperpolarization process of the nuclei mainly involves a three-spin process, i.e. a flip-flop between an electron spin and a nuclear spin associated to a flip or a flop of one additional electron spin. 22,23 Moreover, in the "thermodynamic limit", where large ensembles of spins are involved in the process, DNP can be described in terms of thermal mixing (TM), where the density matrix is defined by an instantaneous spin temperature of several, possibly coupled, heat reservoirs. 16,[24][25][START_REF] Goldman | Spin Temperature and Nuclear Magnetic Resonance in Solids[END_REF] In other words, in the TM limit, the DNP process can be described by a Zeeman nuclear spin heat reservoir and an electron dipole-dipole reservoir, the temperatures of which eventually tend to equilibrate to a common spin temperature. Provotorov 24,25 derived kinetic equations, valid in the high-temperature limit, that characterize the time-dependence of the inverse spin temperatures of the electron spin Zeeman and dipolar reservoirs, and of the nuclear Zeeman reservoir. According to his theory, the nuclear spin temperature β n (t) equilibrates with β ee (t), the spin temperature of the dipole-dipole electron spin reservoir. This model can be extended to the case where several nuclear spin species are present in the sample, each acting as a reservoir and assigned a spin temperature.

The high nuclear polarization levels achieved by DNP have dramatic consequences on the dynamics of the magnetization. Indeed, the coupling with the high-Q detection circuit of the NMR probe yields a significant radio-frequency feedback field from the detecting coil, thereby leading to efficient radiation damping, analyzed in terms of the Bloch-Maxwell equations. 1,[START_REF] Bloom | [END_REF] When the probe of the NMR spectrometer is tuned to the frequency of the spins, the dynamics of the magnetization M in a frame rotating around the z axis at the frequency ω obeys a set of modified Bloch equations. 1,28 Combining the theories of TM-DNP and RD, Bösiger et al. proposed the following set of dynamical equations to describe the nuclear magnetization and the various inverse spin temperatures involved: 14

                                           Ṁx = δω H M y -γ H GM z M x - M x -M th x T 2 Ṁy = -δω H M x -ω 1 M z -γ H GM z M y - M y -M th y T 2 Ṁz = ω 1 M y + γ H G(M 2 x + M 2 y ) - γ 2 H 2 N H B 0 4k B γ n,ee (β n -β ee ) dβ ee dt = -γ ee,n (β ee -β n ) -γ ee,d (β ee -β d ) - 1 τ ee,L (β ee -β L )
+πω

dβ d dt = -γ d,ee (β d -β ee ) (1)
In these equations, the first three describe the magnetization dynamics due to RD, and

G = µ 0 2
ηQ, where η and Q are respectively the filling factor and the quality factor of the probe's detection circuit, µ 0 is the vacuum permeability. The rf field ω 1 (in frequency units) is aligned along x in the rotating frame, and δω H = ω -ω 0 is the frequency offset with respect to the Larmor frequency ω 0 of the spins. The last three equations of Eq. 1 are the Provotorov equations, 25 where ω 0e is the electron resonance frequency, g(ω) and δω e are the lineshape and linewidth of the EPR line, ω 1µw and ω µw are the magnitude and frequency of the microwave irradiation field, and ∆ω e = ω 0e -ω µw .

The nuclear and electron dipolar inverse spin temperatures are β n and β ee , and the lattice temperature is β L . γ n,ee = τ -1 n,ee is the kinetic rate of inverse spin temperature transfer from the nuclear Zeeman to the electronic dipolar spin reservoir, and γ ee,n = cn ce γ n,ee , where the coefficients c n and c e denote the heat capacities of the associated reservoirs. It should be noted that in Eqs 1 the spin temperature β d of the deuterium spin reservoir is coupled to the electron dipolar reservoir with the rate constant γ d,ee , so that the former becomes indirectly coupled to the 1 H reservoir. Therefore, polarization transfer between both nuclear species may occur indirectly, through coupling with the electron dipolar reservoir. Also, the expression of M z in Eq. 1 has been replaced by

M z = γ 2 H 2 NHB 0 4kB
β n , in order to emphasize the link between the Bloch equations and the thermal mixing description. The theoretical set-up described by Eqs. 1 is illustrated in Fig. 1.

The nuclear spin-lattice relaxation time is usually much longer than the other characteristic times relevant to the problem (∼ minutes), and can therefore be neglected.

Rather, the z-component of the magnetization returns to a state of common temperature with the electron dipole-dipole reservoir, with the time constant γ n,ee . Finally, it has been postulated 14 that in Eqs. 1 the transverse components of the magnetization relax towards non-zero values M th x and M th y caused by the presence of a Nyquist noise voltage in the detection circuit and the resulting excitation of the magnetization.

For the study at hand, experiments were performed at 1.2 K in a Bruker prototype polarizer operating at a 6. 2,2,6,6-tetramethylpiperidine-1-oxyl) was used as paramagnetic agent, at a final concentration of 50 mM, as it is known to efficiently polarize nuclear spins with large gyromagnetic ratios, such as 1 H. The µ-wave frequency was chosen so as to achieve a negative nuclear polarization. Typical 1 H NMR linewidths were c.a. 25 kHz, due to dipolar interactions of the nuclear spins. When the microwave irradiation frequency is chosen so as to polarize nuclei negatively (that is, to create a 1 H magnetization pointing towards -z), the FID envelope changes dramatically as soon as the magnetization reaches some threshold value. Then, FIDs exhibit a series of signal revivals extending over a few tens of ms, evocative of maser bursts, with intensities that gradually decrease with time. At times longer than ∼ 100 ms, the FIDs keeps decaying very slowly, allowing signal persistence for times up to several tens of seconds. This behaviour is highly evocative of the presence of strong RD. This is confirmed by the disappearance of these magnetization revivals when the receiver circuit is detuned, thereby reducing the magnetization-coil coupling. Similar behaviours were observed once the samples have been polarized, irrespective of the persistence or not of µ-wave irradiation during acquisition. Typical observations are shown in Fig. 2.

These sustained maser pulses result from the competition between two physically different processes: i) a large magnetization strongly coupled to the NMR detection circuit, responsible for a radio frequency feedback field from the probe that drives the magnetization towards its equilibrium position, aligned with the magnetic induction field; and ii) the growth in magnitude of the M z component of the magnetization, towards a state of negative polarization through DNP.

Because of the non-linearity of the BMP equations, the dynamics of the magnetization are difficult to grasp. Moreover, as is often the case for nonlinear differential systems, even finding explicit stationary solutions is already challenging. Thus, no general rules or criteria allow one to predict the time evolution, and only extensive numerical studies can provide a thorough account of the dynamics contained within the model. Nevertheless, qualitative information may still be obtained on the system at hand. Here, we show what insight can be gained by analysis of the evolution at short times. Moreover, owing to some simplifications that will be detailed below, explicit expressions of the stationary solutions -the fixed points of the differential system -can be found. This allows for the investigation of the long-time magnetization dynamics through the study of the behaviour of the linearized differential system in the vicinity of the fixed points. [START_REF] Guckenheimer | Nonlinear Oscillations, Dynamical Systems, and Bifurcations of Vector Fields[END_REF] In the case of the complete BMP problem (Equation 1), even the determination of the stationary solutions is challenging, and so is the determination of the evolution of the magnetization and of the nuclear and electron dipole spin temperatures near the fixed points. Therefore, for qualitative analyses, simplifying assumptions were used: Firstly, the electron Zeeman spin temperature β e was assumed constant and equal to the lattice temperature β L (an assumption used in 14 ), and secondly, the small circuit noise term is neglected, so that the constants M th x,y /T 2 are set to zero. Taking into account these simplifications, Eqs. 1 can be rewritten with the use of the new variables

u(t) = M 2 x + M 2 y , with M x (t) + iM y (t) = √ ue iφ(t) ; and w(t) = γ 2 H 2 N H B 0 4k B β ee = B h β ee , and w 0 L = B h β L .
Here, w 0 L is just the 1 H nuclear thermal magnetization. With these new variables, and introducing the notations M z (t) → z(t), T 2 → γ -1 2 and γG → λ, Eqs. 1 become:

u(t) = -2(λz(t) + γ 2 )u(t) ż(t) = λu(t) -γ n,ee (z(t) -w(t)) φ(t) = -δω H ẇ(t) = -γ ee,n (w(t) -z(t)) -γ ee,d w(t) + γ ee,d B h β d (t) -γ ee,L (w(t) -w 0 L ) -δw(t) + Ωw 0 L βd (t) = -γ d,ee (β d (t) - 1 B h w(t)), (2)
, where the rates γ ee,n and γ ee,d are: -If the initial time of each signal burst is defined as the time when the FID envelope has ceased to decay and starts to grow again, then this condition is associated with u(t) being at a minimum, i.e. u(t) = 0 and ü(t) < 0, with ż(t) < 0. The condition u(t) = 0 in Eqs. 2 yields z = -γ 2 λ . This means that the threshold condition at each burst is also associated to a stationary value of the magnetization z component, z st . As will be detailed below, it indeed corresponds to a fixed point of the system.

γ ee,n = c H c E γ n,
-A maser burst can only occur if the characteristic time of radiation damping τ RD is such that τ

-1 RD > γ 2 , that is, µ 0 γ H ηQ 2 √ z 2 + u > γ 2 .
-During a maser pulse, the non-linearity of Eqs. 2, which is associated to the growth of the transverse magnetization, becomes significant when the terms λu(t)

and γ n,ee z(t) are of comparable magnitudes. In contrast, when λu(t) γ n,ee z(t),

i.e. in the time interval between the end of a maser pulse and the onset of the next, one has ż(t) ≈ -γ n,ee (z(t) -M st z ). This linear regime is associated to the exponential return of the z component of the magnetization towards its (negatively) "hyperpolarized" value between pulses. This return to the "hyperpolarized" state is faster with increasing values of γ n,ee , leading to bursts that are closer in time.

As stated above, the evolution at long times of the {magnetization + inverse spin tem-perature} system requires the determination of the fixed points (solutions of d dt = 0). However, these fixed points could only be found under the simplifying assumption of a transverse magnetization that relaxes to zero rather than to a noise value. The differential system Eq. 2 thus admits two solutions F = (u st , z st , w st , β st d ):

F 1 : 0, w 0 L (γ ee,L + Ω) γ ee,L + δ , w st = z st , β st d = 1 B h w st F 2 : - γ n,ee λ 2 γ 2 (γ ee,L + δ) + λw 0 L (γ ee,L + Ω) γ ee,n + γ ee,L + δ , - γ 2 λ , ... λw 0 L (γ ee,L + Ω) -γ ee,n γ 2 λ(γ ee,n + γ ee,L + δ) , 1 B h w st (3)
If we further restrict the study to the special case that corresponds to experiments where the microwave irradiation is switched off prior to FID detection, one has δ = Ω = 0 and F 1 = (0, w 0 L , w 0 L , w 0 L ), which just represents the thermal equilibrium. Alternatively, for F 2, the squared transverse magnetization u(t) admits the value u st < 0, which, from the definition of u(t), is obviously impossible. Therefore, among both solutions, only F 1 is acceptable when the system is not under microwave irradiation.

This means that at longer times the magnetization is necessarily brought back to its equilibrium position.

There are, however, two limiting cases that lead to u st → 0, for which F 2 becomes acceptable. A scenario would be that the relaxation rate of the electron dipole reservoir is vanishingly small, γ ee,L → 0. This is clearly unrealistic, as τ ee,L = γ -1 ee,L is typically on the order of ∼ 10 -100 ms, and this assumption must therefore be discarded.

But the more interesting and plausible case is the situation where the rate of temperature transfer from the electron dipole reservoir to the 1 H nuclear spin reservoir is much faster than the equilibration rate between the dipolar electron spin reservoir and the lattice: γ ee,n γ ee,L . Such a condition is met whenever the heat capacity c H of the 1 H nuclear spin reservoir is much larger than that of the electron dipolar (non-Zeeman) reservoir, c E . The temperature flux from the dipole reservoir to the lattice is therefore quenched for the benefit of the 1 H nuclear spins. This implies that the ratio γ n,ee /γ ee,n = c E /c H is vanishingly small, so that one has γ ee,n γ ee,L . Moreover, the maser threshold condition implies (see below) γ 2 ∼ λw thres (t), so that γ 2 λw 0 L . These assumptions therefore lead to F 2 : (0, -γ 2 λ , -γ 2 λ , -γ 2 λB h ), a non-Boltzmann stationary solution.

The long-time evolution of the {magnetization + spin temperature} system is investigated from the linearized Eqs. 2 in the vicinity of the fixed points. In particular, the stability of the latter depends on the signs of the real parts of the eigenvalues of the linearized system (for a complete reference, see for instance [START_REF] Guckenheimer | Nonlinear Oscillations, Dynamical Systems, and Bifurcations of Vector Fields[END_REF] ). Using the change of variables:

u(t) = U (t) + u st , z(t) = Z(t) + z st , w(t) = W (t) + w st , β d (t) = R(t) + β st d , Eq.
2 becomes:

         U Ż Ẇ Ṙ          =          -2(λz st + γ 2 ) -2λu st 0 0 λ -γ n,ee γ n,ee 0 0 b a d 0 0 γ d,ee /B h -γ d,ee                   U Z W R          +          -2λZU 0 0 0          (4)
Thus, in the vicinity of F 2, the eigenmodes of the linear part of Eq. 4 are given by the roots of the determinant (since λz st + γ 2 = 0):

x + γ n,ee -γ n,ee 0 -γ ee,n x + γ ee,n + γ ee,d -B h γ ee,d 0 -γ d,ee /B h x + γ d,ee (5)
, or: x = 0 and x ± = -(γ n,ee + γ ee,n + γ ee,d + γ d,ee

) ± √ ∆ 2
, where ∆ = (γ n,ee + γ ee,n ) 2 -2γ ee,d (γ n,ee -γ ee,n )-2γ d,ee (γ n,ee +γ ee,n )+(γ ee,d +γ d,ee ) 2 . Moreover, since γ n,ee , γ ee,n , γ ee,d , γ d,ee ≥ 0, one sees that ∆ ≤ (γ n,ee + γ ee,n + γ ee,d + γ d,ee ) 2 . When ∆ > 0, both eigenvalues of the linear part of Eq. 4 are negative, x ± ≤ 0, and F 2 is a stable asymptotic equilibrium.

And if ∆ < 0, both x ± have a negative real part so that F 2 is also stable. It is noteworthy that, although the fixed points do not depend on the value of γ ee,d (or γ d,ee ), the latter partially determine the kinetics of the approach to equilibrium. For large heat capacities c D , comparable to c H , which is the case for large quantities of 2 H nuclei, both γ ee,d and γ d,ee contribute significantly. Thus, in an idealized case where the ratio c H /c E is assumed infinitely large and 1 H T 1 relaxation is negligible, the magnetization does not return to thermal equilibrium, once trapped in the basin of attraction of F 2.

This therefore suggests that in actual experiments an out-of-equilibrium magnetiza-tion can be maintained for a significant period of time, even without µw irradiation.

Alternatively, in the presence of µw, F 2 is an admissible solution (if u st ≥ 0, see Eq.

3), depending on the irradiation offset and power of the microwave field. Due to the complexity of the BMP equations, the study of the magnetization dynamics requires numerical simulations. For the sake of illustration, parameters similar with the ones expected in our experiments were used. The magnetization dynamics are illustrated in Figure 3, where u(t) and M z are depicted. These graphs show the succession of bursts at short times (< 0.1 s) followed by a very slow decay. Simulations also show that -γ 2 /λ is not a fixed point. This is expected, as the condition c H /c e = ∞ cannot be exactly fulfilled. However, it is seen to be transiently the case, and only when the in-plane component becomes null does the longitudinal magnetization relax towards thermal equilibrium. During this succession of maser pulses, the M z component of the magnetization remains negative. This is consistent with the analysis of the magnetization dynamics in usual NMR experiments in the presence of radiation damping. For the case of RD in the absence of T 1 relaxation -where a single maser burst is present -analytical solutions of the magnetization dynamics 30 show that the end point of the trajectory of the magnetization vector reaches the north pole of the Bloch sphere only when T 2 is infinite. However, the z-value of the final magnetization decreases as T 2

decreases. It can even become negative, depending on the relative values of T 2 and τ RD , when defocusing is too fast to maintain an efficient RD throughout. This behavior is confirmed using the analytical expressions of the longitudinal magnetization derived by Bloom and the parameters relevant to the present work (not shown). Finally, the presence of a noise term, discarded in the qualitative analysis, was introduced in the simulations. In this case, even the analytical determination of the stationary solutions is beyond reach, but simulations clearly demonstrate the significant effect of the noise voltage, therefore transverse magnetization fluctuations. A comparison between simulations performed in the presence and in the absence of this noise term is shown in Figure 4. This term has two major effects on the dynamics: i) during the early stage of the FID, the maser bursts are of lesser amplitude and number; and ii) the transverse magnetization reaches its stationary value significantly more slowly than in the absence of the noise term. This may be interpreted as the effect of the circuit noise that maintains an in-plane magnetization and keeps on driving radiation damping up to the point where √ z 2 + u becomes practically zero.

The experiments such as those presented in Fig. 2 suggest the existence of a thermal contact between the 1 H and the electron spin dipolar reservoirs, as implied by the BMP model. Indeed, the observed dynamics require, in addition to an efficient coupling of the magnetization to the probe where magnetic energy is dissipated, some additional mechanism that drives the magnetization towards the negative direction of the z axis.

Obviously, this mechanism cannot be based on nuclear relaxation, since both the radiation damping feedback field and longitudinal relaxation tend to bring M back to +z. Thus, the existence of a process that drives M z to the -z direction is necessary, i.e.

a repolarization process towards negative values. Importantly, this repolarization occurs even in the absence of the µw field (see Fig. 2), which therefore demonstrates that polarization is provided to 1 H spins by the 2 H reservoir through the electron dipolar reservoir.

The multiple resurgence of the NMR signal attests for successive rebuilding of a radiation damping field. The simulations presented here show that, because of the remagnetization caused either by microwave irradiation or through the 2 H polarization reservoir (growing of the M z in magnitude towards a negative value), even a small residual in-plane component of the magnetization may be sufficient to make the coherent feedback field strong enough to trigger the next maser pulse. This process is reminiscent of the phenomenon described in ref.

, 4 where π-pulses were used to generate "multiple echoes". In that case, the pulses served as a way to put the in-plane magnetization in coherence, thereby creating a large enough RD field at the time of in-plane refocusing thanks to the remaining negative M z to trigger a new maser pulse.

However, in this context, the term "echo" may be somewhat misleading, because the signal observed is not simply the refocusing of in-plane magnetization. The refocusing of the in-plane magnetization achieved by the π-pulse creates a coherent RD field that triggers an actual maser pulse, provided that the remaining M z component is large enough in magnitude and negative. In our case, the magnitude of the magnetization vector is so large that the residual in-plane magnetization achieves the same effect. In this perspective, the observations definitely represent self-sustained, self-triggered series of maser pulses.

Fitting the experimental FIDs to the complete BMP model is rather challenging, as, clearly, the problem at hand is largely under-determined when only the measurements of the FIDs are available. Thus, the maser bursts at short times, typical of the nonlinear behaviour of the 1 H magnetization, are determined by the time constant τ -1 RD , which results from the product of m st z , ηQ and the magnetization (itself the product of the number of 1 H spins by the polarization). Moreover, the 1 H pulses may likely be miscalibrated in the presence of such large radiation damping feedback field. For these reasons, one does not expect to access the value of the magnetization from the NMR signal in these experiments. On a more fundamental level, the high-temperature approximation implied by the Provotorov equations may be violated in our experimental conditions. This is primarily relevant when microwave irradiation is active during the observation of the FID. When the µw field is turned off, the electron spin dipole reservoir is decoupled from the Zeeman electron spin reservoir, and so is the nuclear spin Zeeman reservoir, so that the high temperature approximation is verified and the description in terms of flows of inverse spin temperatures remains possible. However, during irradiation, such treatment may not be the valid, although for strong enough µw, the electron Zeeman reservoir is warmed up during irradiation, which might still justify the high-temperature regime. Detailed discussions can be found in Ref. 16,[START_REF] Wenckebach | Essentials of Dynamic Nuclear Polarization[END_REF] In view of these various difficulties, we introduced a simplified model in which it is assume that 1 H magnetization relaxes towards a time-dependent value that itself tends to z st mono-exponentially, with the rate γ depol :

d dt M z (t) = ω 1 M y + γ H G(M 2 x + M 2 y ) -γ n,ee (M z -M a z (t)) d dt M a z (t) = γ depol (M a z (t) -M st z ) (6)
This can be justified, as discussed in the previous sections, when repolarization of the 1 H spins towards its initial hyperpolarized state is achieved through microwave irradiation, or, in the absence of the latter, when γ ee,n , γ ee,d γ ee,L . In this case, β ee undergoes fast equilibrium with the nuclear Zeeman reservoirs. With this model, the fitted parameters are the initial flip angle, γ n,ee , ηQ, the initial common spin temperature T spin , the number of 1 H spins in the sample, and M st z . A qualitative analysis of these equations is shown in the Supplementary Information. An illustrative experiment is shown in Fig. 5, where the NMR signal was recorded in the presence of the microwave.

Fitting Eqs. 6 to the data yielded in this case γ n,ee ≈ 10. Again, due to the relations between the parameters, only if N spin and P st are known can the "true" value of M st z = -γ 2 /λ be determined, which is why the quantities in Fig. 5 are plotted in arbitrary units. Moreover, fitting of this experimental FID yielded M st z ≈ -γ 2 /λ, which corresponds to the fixed point F 2 of Eqs 2, the expected value in the absence of the noise term. This shows that the latter has little influence in this case.

The present work describes the magnetization dynamics of a complex system of dipolar coupled nuclear 1 H spins using a simplistic Bloch vector description, neglecting the presence of nuclear dipolar interactions and their effects on the magnetization dynamics. These lead to the generation of higher 1 H-1 H spin order and to line narrowing with increasing polarization P . The associated line narrowing can be assessed by the second moment m 2 of the line (an estimate of the line width for a Gaussian line shape) which, in the special case of a spherical sample with cubic lattice, is m 2 (P ) = m 2 (0)(1 -P 2 ), 16,31,32 where m 2 (0) is the second moment in the high temperature approximation. Using P ≈ 90%, and m 2 (0) ≈ 25 kHz, one expects m 2 (P ) on the order of ≈ 5 kHz. This gross estimate suggests a longer FID, with a complete decay in a few hundreds of microseconds, much shorter than the width of the observed signal bursts (∼ milliseconds). Such a contribution of higher spin orders may likely affect the details of the FIDs and add complexity to the problem.

In the experiments presented here, a crude estimate shows that the associated increased FID duration remains orders of magnitude shorter the duration of the signal bursts so that these effects can be neglected in our qualiative analysis of the nonlinear magnetization behaviour.. However, one can not discard the effects of nuclear spin dipolar couplings for a thoroughly quantitative analysis of these sustained maser pulses, and deserve a study of their own.

In Ref. 14 Fitting (dashed line for the transverse magnetization) was obtained using the simplified model of Eq. 6. The full length FID is presented in the left panel, and a focus on the early times, where multiple MASER pulses occur, is shown on the right. As explained in the text, due to the relations between the parameters, the "true" value of M st z = -γ 2 /λ cannot be determined in these experiments, and the quantities in Fig. 5 are plotted in arbitrary units. However, during the evolution, M z remains negative.

7 T

 7 magnetic field (285 MHz 1 H Larmor frequency). A continuous microwave irradiation at 188.380 GHz was provided by a a 94 GHz source (ELVA-1) coupled to a frequency doubler (Virginia Diodes). The nominal output power of the source was 350 mW, but the power reaching the sample was estimated to be on the order of ∼ 12 mW. DNP experiments were performed on a 150 µL sample composed of 10 % of H 2 O, 40 % of D 2 O and 50 % of deuterated glycerol (C 3 D 8 O 3 ) as a glass-forming agent to avoid crystallization upon freezing in liquid He. The sample was contained in a cylindrical PTFE cup in order to avoid spurious 1 H signals. TEMPOL (4-hydroxy-

  the DNP process in a ruby crystal was analyzed in great detail, and estimates of the various parameters that appear in the Provotorov equations were provided through a series of complementary experiments. Maser oscillations were also observed, although the long-time regime was not analyzed in a quantitative manner and a simultaneous fit of the observed NMR signals on both short and long time scales -one of the results in the present work -was not performed. Here, an NMR maser was achieved in an amorphous sample of DNP-hyperpolarized nuclei, and the resulting NMR signal could be analyzed based on a simplified BMP model. In addition, the observation of sustained maser pulses even in the absence of µw irradiation and the analyses reported in this work provide further evidence that the re-magnetization of 1 H spins is provided by the 2 H reservoir present in the sample. Thus a single FID contains information relative to processes of different natures, namely RD and DNP.Additionally, RD may represent a potential tool to gain insight into the DNP mechanism by providing a way to access characteristic parameters of the DNP process itself.The presented analysis, although currently restricted to thermal mixing-type DNP, may generalize to other DNP mechanisms, such as the solid and the cross effect.

Figures

  Figures

  T ee = 1/β ee

Figure 1 :

 1 Figure 1: The TM-RD setup for a sample containing 1 H and 2 H nuclear spins. The letters Z and nZ respectively refer to Zeeman and non-Zeeman reservoirs, according to the theory of thermal mixing. Both nuclear Zeeman and the electronic dipolar reservoirs are coupled. Microwave irradiation of the EPR line couples the electron spin Zeeman and dipolar reservoirs.1 H magnetization is lost to the NMR probe, which acts as a sink for the former.

Figure 2 :

 2 Figure 2: Typical FIDs following an initial polarisation period of ∼ 5 min. In 2(a) and 2(c), the signal was triggered by noise, as no rf pulse was applied. In 2(b) and 2(d), a small flip angle pulse was applied (0.1 o ). After sustained maser pulses have decayed the signal remains nonzero for several seconds, indicating the absence of return to equilibrium of the 1 H magnetization.

Figure 3 :Figure 4 :

 34 Figure 3: Simulations of the BMP equations in the absence of µw irradiation and transverse thermal noise component. In (a) and (b), and (c) and (d), the magnitude of the transverse component and the longitudinal component of the magnetization are depicted, respectively. Spin densities were N e ≈ 30 • 10 24 m -3 , N1 H ≈ 6.6 • 10 27 m -3 , and N2 H ≈ 54.5 • 10 27 m -3 . Heat capacities were: c X = N X I(I + 1) ωX 2π 2 , 17 where X = 1 H, 2 H, I = 1/2, 1 and ω X /2π is the resonance frequency of spin X. Moreover, γ n,ee = 200 s -1 and γ d,ee = 10 s -1 were used and the read pulse flip angle θ = 10 -3 . The transverse relaxation rate was set to γ 2 = 2π × 25 kHz.The values of η and Q were chosen so that τ RD ≈ 1/γ 2 (ηQ = 15.4), a necessary condition for multiple maser oscillations. The EPR linewidth was set to ω ss = 1.25 • 10 9 s -1 . The initial spin temperature, when microwave irradiation is turned off was T spin = -20 mK, and the lattice relaxation rate of the electron spin dipolar reservoir was set to γ ee,L = 6 s -1 .

Figure 5 :

 5 Figure 5: Experimental FID (solid line) obtained in the presence of microwave irradiation.

  Some insight into the sustained maser period of the FIDs can be obtained based on qualitative arguments and numerical simulations of Eqs. 2, and are listed below:

	ee and γ ee,d =	c D c E	γ d,ee . The equation for
	ẇ(t) can be rewritten as: ẇ(t) = bz(t)+aw(t)+dβ

d (t)+c, where a = -γ ee,n -γ ee,L -γ ee,d -δ, b = γ ee,n , c = (γ ee,L + Ω)w 0 L , d = γ ee,d B h . In these equations, δ and Ω are defined as: Ω = πω 2 1µw g(∆ω e ) ∆ωe δω 2 ω 0e and δ = -πω 2 1µw g(∆ω e ) ∆ω 2 e δω 2 .

where ∆ω e is defined above.

  1 s -1 and γ depol ≈ 4.08 s -1 . The RD time constant τ RD ≈ 1/γ 2 , which corresponds to the maser triggering condition, was also determined by fitting. This unusually short value of the radiation damping time for 1 H in a "water sample" is nevertheless explained by the huge (∼ 4 orders of magnitude) polarization gain afforded by DNP with respect to ambient temperature experiments.

Acknowledgements

E.W. and D.K. acknowledge funding from the European Research Council ("Dilute para-water", grant agreement No 339754). Bruker BioSpin is greatfully acknowledged for providing the D-DNP equipment.

Conflicts of interest

There are no conflicts of interest to declare.