Cu0-Loaded organo-montmorillonite with improved affinity towards hydrogen: An insight into matrix-metal and non-contact hydrogen-metal interactions

Article abstract of DOI:10.1039/c7cp04784c

Copper-loaded organo-montmorillonite showed improved affinity towards hydrogen under ambient conditions. Clay ion exchange with a propargyl-ended cation followed by thiol-yne coupling with thioglycerol resulted in a porous structure with a 6 fold higher s

Full text of DOI:10.1039/c7cp04784c

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DOI: 10.1039/C7CP04784C.
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Cu -loaded organo-montmorillonite with improved affinity towards
hydrogen-an insight in matrice-metal and non-contact hydrogen-metal
interactions
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Radia Sennour, Tze Chieh Shiao, Vasilica Alisa Arus, M. Nazir Tahir, Nabil Bouazizi, René Roy,
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Abdelkrim Azzouz *
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Nanoqam, Department of Chemistry, University of Quebec at Montreal, QC, Canada H3C 3P8
Catalysis and Microporous Materials Laboratory, Vasile-Alecsandri University of Bacau, 600115, Romania.
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Research unit: Environment, Catalyzes and Process Analysis, ENIG University of Gabes, Tunisia
Corresponding authors Tel.: +1 514 987 4119; fax: +1 514 987 4054
E-mail addresses: azzouz.a@uqam.ca (A. Azzouz), roy.rene@uqam.ca (R. Roy)
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ABSTRACT
Copperꢀloaded organoꢀMontmorillonite showed improved affinity towards hydrogen under ambient
conditions. Clay ion exchange with a propargylꢀended cation followed by thiolꢀyne coupling with
thioglycerol resulted in porous structure with a 6 fold higher specific surface area, which dramatically
decreased after copper incorporation. Xꢀray diffraction and photoelectron spectrometry, nuclear magnetic
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resonance ( H and C) and CO ꢀthermal programmed desorption revealed strong Sulfur:Cu and
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Oxygen:Cu interactions. This was explained in terms of structure compaction that ‘trap’’ Cu nanoparticles
(CuNPs) and reduce their mobility. Transmission electron microscopy showed predominant 1.0ꢀ1.5 nm
CuNPs. Hydrogen capture appears to involve predominantly physical interaction, since differential scanning
o
calorimetry measurements gave low desorption heat and almost complete gas release between 20 C and
o
75 C. Possible hydrogen condensation within the compacted structure should hinder gas diffusion inside
CuNPs and prevent chemisorption. These results allow envisaging safe hydrogen storage with easy gas
release even at room temperature under vacuum. The reversible capture of hydrogen can be even more
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attractive when using natural inorganic supports and commercial plantꢀderiving dendrimers judiciously
functionalized, even at the expense of porosity.
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Keywords: Copper; Nanoparticles; Thioglycerol; Montmorillonite; Hydrogen capture; Metalꢀorganoclay.
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1. INTRODUCTION
Nowadays, intensive research is being focused to promote clean technologies without carbon dioxide
exhaust, and growing interest is now devoted to alternatives to fossil fuels, which have become major
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objectives . Hydrogen appears as a promising source of clean energy, being widely available in nature.
However, for safety and efficiency reasons, hydrogen storage, more particularly in molecular form, still
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remains a challenge to be addressed. So far, several studies have been made in this regard . H ꢀcontaining
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materials are interesting hydrogen sources, but practical applications cannot be envisaged because hydrogen
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release requires energyꢀconsuming processes . The production of hydrogen from water also appears to be a
more promising route, and an ample literature is now available in this regard. However, important challenges
reside in avoiding thermal production processes and in developing effective storage methods.
A judicious approach involves a reversible capture of hydrogen by porous adsorbents under ambient
conditions via physical condensation with reduced formation of metal hydrides. This would allow easy gas
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release without heating or pollutant emissions . Several adsorbents have already been evaluated, but, most
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of them showed only low hydrogen uptakes under ambient temperature and pressure . Improvements in
this regard may be brought by the synthesis porous matrices with large specific surface areas hosting highly
dispersed metal particles (MP). The latter turn out to be an essential requirement for this goal, given their
high affinity towards hydrogen. The size distribution of MPs depends on the structure of the stabilizing
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agent. Dendrimerꢀencapsulated nanoparticles (DENs)
and dendrimerꢀsilica hybrid mesoporous materials
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, including sulfurated dendrimers , may offer several advantages towards this goal.
Hydrogen adsorption on MPs is known to generate metal hydrides, whose further decomposition to release
hydrogen requires chemical processes and energy consumption. A possible and original strategy to avoid or
reduce hydrogen chemisorption should consist in promoting nonꢀcontact hydrogenꢀMP interactions. Herein,
we report an efficient route for the dispersion of copper nanoparticles (CuNPs) within the entanglement of a
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clayꢀsupported dendrimer bearing metalꢀstabilizing groups such as sulfur and oxygen atoms (Scheme 1) .
Clayꢀbased adsorbents are much more cost effective than nanostructured carbons and silicas, MOF, GOF,
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metal sulfides and many other sophisticated materials
. The choice of Cu was based on many objective
criteria, among which the relatively higher stability to oxidation and much lower toxicity as compared to
other metal are the most important. Besides, Cu is much more cost effective than other metals such as
palladium and platinum and other noble metals recognized for their higher affinity for hydrogen.
+
The synthesis of such materials was achieved by modifying Na ꢀmontmorillonite with 4ꢀ(propꢀ2ꢀynyloxy)ꢀ
phenylammonium cation and further addition of thioglycerol groups through a photoactivated radicalꢀ
mediated thiol−yne coupling (TYC). This reaction has emerged as a powerful “click” chemistry tools for
15ꢀ19
obtaining advanced functional materials
. TYC reactions are usually carried out under UV irradiation in
the presence of potent photoinitiators, and are becoming increasingly employed in polymer chemistry and
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material science
. However, its applicability to clay minerals has not been systematically explored yet.
Scheme 1
0
The resulting organoclay is expected to behave as host matrice for Cu nanoparticles (CuNPs) through the
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grafted thioether groups, which are known to act as powerful ligands for MNPs . On one hand, Strong
0
thioetherꢀCu interactions should result in structure compaction that ‘trap’’ CuNPs and reduce their mobility.
On the other hand, sufficiently high number of thioether groups is expected to entirely cover the external
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surface of CuNPs, promoting nonꢀcontact hydrogenꢀCu interaction. This may result in hydrogen
condensation around ‘’covered’’ CuNPs, which should hinder gas diffusion inside CuNPs and prevent
chemisorption. When physical gas adsorption prevails, the size and number of CuNPs should play key roles
in tailoring optimum gas retention strength that allow both high hydrogen uptake and easy desorption
without or without low heating. This concept of truly reversible capture of hydrogen is similar to that already
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reported for carbon dioxide
. In other words, optimum interactions would be an essential requirement for
high H uptakes with easy regeneration. Therefore, physical and nonꢀstoichiometric hydrogen condensation
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is a novelty in itself, which opens promising prospects for low cost clayꢀbased adsorbents for storage
purposes or at least for gas concentration from mixtures.
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2. MATERIALS AND METHODS
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2.1. General
In this study, commercially available HPLC grade solvents were used. Commercially available propargyl
bromide solution (80% in toluene), 1ꢀthioglycerol, and 2,2ꢀdimethoxyꢀ2ꢀphenylacetophenone (DMPA) from
Sigma Aldrich Canada LTD were used without further purification. Thiolꢀyne reaction was achieved under
dry argon in a photometer cell from VWR North America (Cat. No 41400ꢀ064). Progress of the reactions was
monitored by thinꢀlayer chromatography using silica gel 60 F254 coated plates (E. Merck). NMR spectra were
recorded on Bruker Avance III HD 600 MHz spectrometer. Analysis and assignments were made using COSY and
HSQC experiments. Highꢀresolution mass spectra (HRMS) were recorded with a LCꢀMSꢀTOF spectrometer
(6210 ESIꢀTOF, Agilent Technologies) in positive and/or negative electrospray mode by the analytical platform
of UQAM.
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2.2. Preparation of N-[4-(Prop-2-ynyloxy) phenyl] acetamide (2)
Propargylation of 4ꢀacetamidophenol 1 (10.00 g, 66.16 mmol, 1.0 equiv.) was carried out as previously
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reported
, using potassium carbonate (18.29 g, 132.32 mmol, 2.0 equiv.) and propargyl bromide solution
(80% in toluene, 6.93 mL, 1.2 equiv.) in dry acetone (100 mL). The reaction mixture was heated under reflux
for 3 h (Scheme 2). Details are provided as supporting information. The residue, obtained with a 90% yield,
was purified by silica gel chromatography (DCMꢀMeOH 96:4), followed by solvent evaporation. The
resulting propargylated compound 2 appeared as a white solid (11.27 g, 59.54 mmol, 90%). The latter was
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fully identified through H NMR and C NMR (Fig. S1), and further used in the synthesis of compounds 3
and 4 through different procedures.
2.3. Preparation of N-(4-(2,3-bis(2,3-dihydroxypropylthio) propoxy)phenyl)acetamide (3)
Propargyl ether 2 was then treated under photolytic TYC conditions with thioglycerol to produce compound
3. Thus, compound 2 (378 mg, 2.0 mmol, 1.0 equiv.), 1ꢀthioglycerol (519 µL, 6.0 mmol, 3.0 equiv.) and 2,2ꢀ
dimethoxyꢀ2ꢀphenylacetophenone (DMPA, 5 mg, 0.02 mmol, 0.01 equiv.) were solubilized in THF/MeOH
(2:1, 3 mL) in a photometer cell. The resulting mixture was exposed to a 365 nm radiation for 15 min under
continuous stirring at room temperature (Scheme 2). Compound 3 was purified by column chromatography,
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then identified through H NMR and C NMR (Fig. S2) and high resolution mass spectrometry (HRMS)
(Fig. S3). Compound 3 was not grafted on the clay mineral, but was only employed as a probe organic
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matrice for assessing its intrinsic interactions with Cu and Cu through liquid phase NMR analysis. The
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changes in chemical shifts were compared to those measured with Cu cation used as the standard. For this
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purpose, two mixtures of 50 mg compound 3 each were prepared: 1. with Cu by dissolving
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Cu(NO ) .2.5H O (3 mg) in D O; 2. or with Cu by reacting a solution of Cu(NO ) .2.5H O (20 mg, 9
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mmol) with NaBH (3 mg, 9 mmol) in D O. The second mixture was stirred at room temperature for 6 h.
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Both mixtures were analyzed through NMR ( HꢀNMR for Cu only and CꢀNMR for both Cu and Cu ).
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2.4. Preparation of 4-(Prop-2-ynyloxy)phenylammonium chloride (PPhA Cl ) (4)
DeꢀNꢀacetylation of compound 2 under acidic conditions gave rise to the corresponding ammonium salt (4)
with a 77% yield. For this purpose, a solution of HCl in MeOH (4M, 10 mL) was added into a solution of
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compound 2 (1.89 g, 10.0 mmol, 1.0 equiv.) in ethanol (15 mL) and the mixture was stirred at 60 C for 3 h
(Scheme 2). After cooling to room temperature, compound 4 was isolated by filtration, then washed with
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ethyl acetate (1.40 g, 7.7 mmol), and further fully identified through H NMR and C NMR (Fig. S4).
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Details are provided as supporting information. Compound 4 was grafted via ion exchange on Na ꢀ
exchanged Montmorillonite (NaMt), resulting in a propargylꢀorganoꢀMontmorillonite (Pg-Mt). The latter
0
was further employed for the synthesis of Cu ꢀdoped thioglycerolꢀorganoꢀMontmorillonite (Cu/T-Mt)
(Scheme 2).
Scheme 2
2.5. Montmorillonite modification and CuNPs dispersion
Naꢀmontmorillonite (NaMt), employed as inorganic support for the synthesis of Cu/TꢀMt, was obtained from
the purification of an Aldrich bentonite having the following chemical composition: SiO (66.45%), Al O
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(22.28), Fe O (3.85), MgO (2.58), Na O (2.33), CaO (1.34), K O (0.38), SO (0.52), Ti (931 ppm), Sr (308
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ppm), P (182 ppm) and Zr (165 ppm). Bentonite purification was achieved through full ionꢀexchange in a 2–
5M aqueous NaCl solutions at 80 °C for 3–4h and fractionation by repeated settlings in distilled water at
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room temperature (RT), under ultrasound exposure, as described elsewhere
. NaMt was then washed
with water, centrifuged, and repeatedly dialyzed overnight with distilled water in cellulose bags to remove
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the residual excess of unreacted NaCl. Such a procedure resulted in an increase in the montmorillonite
content from ca. 84% to approximately 96% and a decrease of the Si/Al mole ratio from 2.54 (bentonite) to
2.47 (NaMt), due to the elimination of dense silica phases such as quartz and cristoballite. This Si/Al value
is close to the average value of 2.44 obtained through Xꢀray photoelectron Spectrometry (XPS) and Energy
Dispersion Xꢀray fluorescence (EDSꢀXRF) measurements, as described further. The same analyses gave a
ꢀ1
cation exchange capacity (CEC) of 1.0 + 0.05 meq.g .
In a first step, propargylated montmorillonite (Pg-Mt) was prepared via cationic exchange of a NaMt
suspension in deionized water with a solution of compound 4 in deꢀionised water at 60°C for 24 h. The final
product was filtered, repeatedly washed with water to remove the cation excess and then dried under vacuum
at RT for 24h. In a second step, 250 mg of Pg-Mt was dispersed in a 3:1 vol. mixture of methanol and
deionized water and mixed with dimethoxyꢀ2ꢀphenylꢀacetophenone (0.2 eq) and then with 20 eq of
thioglycerol. This mixture was stirred overnight at RT under UV radiation at 365nm, resulting in a
thioglycerolꢀfunctionalized montmorillonite (or thioglycerolꢀorganoꢀmontmorillonite) denoted as T-Mt. The
latter was recovered by centrifugation, repeatedly washed with deꢀionised water and dried under vacuum at
RT for 24h. TꢀMt was further immersed in aqueous Cu(NO ) . 2.5H O solution (0.9 mmol) at RT for 6
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hours, using NaBH (9 mmol) as the reducing agent. The organoclay suspension turned brown due to the
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formation of CuNPs. The resulting Cu ꢀdoped TꢀMt was denoted as Cu/TꢀMt (Scheme 2).
2.6. Characterization
Cu/TꢀMt and its precursors were comparatively characterized using many techniques. NMR spectra were
recorded in DMSO and CDCl solutions using a Brüker Avance III HD 300MHz spectrometer and different
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solvents such as DMSO, water and CDCl (δ2.52, 3.41 and 7.27 respectively). TꢀMt and Cu/TꢀMt were
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characterized through solid state CꢀMASꢀNMR (Varian Inova MS600 NMR spectrometer, at 119.192
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MHz. Transmission electron micrographs (TEM) of samples suspended in methanol and dried on holey
carbonꢀcoated Ni grids were obtained by means of a JEOL JEMꢀ2100F equipment, with an accelerating
voltage of 200 kV, coupled to an EDAX Xꢀray fluorimeter operating through energy dispersion (EDSꢀXRF).
The specific surface area (SSA), porosity and pore size distribution were measured by nitrogen adsorptionꢀ
desorption isotherms at 77 K on samples previously dried and outgassed at 100ꢀ150°C using
a
Quantachrome Autosorb equipment with an Autosorb automated gas system control. In this regard,
calculations were achieved using BET and BJH models, respectively. Thermal gravimetric Analysis (TG and
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DTG) was performed by means of a Seiko TG\TDAꢀ6200 thermal analyzer, under a 120 mL.min air stream
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and 5°C.min heating rate. Fourier transform Infrared spectra were recorded on a Model Nicolet 6700 FTIR
instrument, operating in attenuated total reflectance (ATR) mode (Diamond ATR crystal) between 600 and
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4000 cm . Measurements by differential scanning calorimetry (DSC) were performed on a Mettler Toledo
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TGAꢀSDTA 851e, under dry nitrogen stream of 20 mL.min and heating rate of 5°C.min .
Xꢀray photoelectron spectra (XPS) were recorded using a PHI 5600ꢀci instrument (Physical Electronics,
Eden Prairie, MN, USA) and Al standard (1486.6 eV) as the anode for overflight spectra at 300 W. Highꢀ
resolution spectra were recorded with MgꢀKα (1253.6 eV) at 150 W. The analyses were performed without
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charge compensation (neutralizer) at an angle of 45 with the analyzed surface (0.005 cm ). The synthesized
materials were also characterized through Xꢀray diffraction (XRD) using a Siemens D5000 equipment (Coꢀ
Kα at 1.7890°A).
2.7. Measurements through thermal desorption analyses
CO was used as a probe gas to assess the interactions of the terminal OH groups of the organic moiety
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grafted with CuNPs. This is based on the high affinity of hydroxyls towards CO and water
. On the one
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terminal OH groups and CO and water, and may induce decreases in CO and water retention capacities
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(CRC and WRC). On the other hand, the affinity towards hydrogen should be proportional to the number of
CuNPs. Therefore, it is expected that, globally, higher hydrogen uptakes (HRC) will require highly dispersed
CuNPs through Cu:Oꢀ interactions, i.e. low CRC and WRC.
CO and water saturated samples of Cu/TꢀMt and precursors were subject to thermal programmed desorption
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(TPD) in a tubular glass reactor coupled to Li840A dual CO \H O Gas Analyzer. The CO and water
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retention capacity, denoted a CRC and WRC, respectively, were assessed as being the surface area described
by each TPD pattern between the considered temperature ranges. For this purpose, each nonꢀdehydrated
sample (40mg, particle size of 0.05 to 0.1mm) was dried at 80ꢀ160°C for 1 h, then cooled down to 20 °C
and saturated with dry CO in static mode without nitrogen stream for accurate assessment of the CRC and
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WRC values, then the gas excess was evacuated by nitrogen stream (5 mL.min ) until no CO was detected.
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The values of CRC and WRC were expressed in µmol of desorbed gas per gram of adsorbent under 5
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mL.min nitrogen stream and 5°C.min heating rate between 20 and 80 or 100°C depending on the limit of
stability of each sample as given by thermal gravimetric results.
Similarly, samples previously saturated with dry hydrogen at RT and ambient air pressure were analyzed by
H ꢀTPD but using QꢀS121 H Gas detector. Hydrogen retention capacity (HRC) was also assessed through
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additional DSC and TGA measurements in dry helium, between 20 and 80 or 100 °C depending on the limit
of stability of each sample as given by thermal gravimetric results.
3. RESULTS AND DISCUSSION
3.1. Effects of clay modification
Propargyl incorporation via cation exchange in NaMt was supported by the appearance of a new absorption
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band in the FTIR spectra of PgꢀMt (Fig. 1), i.e. at 818ꢀ 796 cm due to stretching vibrations of the primary
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ammonium RꢀNH and CꢀN, at 1508 cm attributed to the aromatic C=C bonds and at 3296 cm assigned
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to the ≡CꢀH bond stretching. The latter totally disappeared as a result of thioꢀyne addition of the thioglycerol
ꢀ1
groups. The decrease in intensity of the 3430ꢀ3450 cm band indicates depletion in the number of OH
stretching vibrations of sorbed water molecules and decay in the hydrophilic character. This is a special
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feature of montmorillonite after incorporation of organic moieties . This is in agreement with the
ꢀ1
significant decay in intensity of the 1650ꢀ1700 cm band, assigned to the HꢀOꢀH bending vibration of
sorbed moisture.
Figure 1
This band split in two sharp peaks upon functionalization with thioglycerol groups, and the hydrophilic
character seems to be revived by the newly inserted OH groups. The broad absorption band around 3430ꢀ
ꢀ1
3450 cm should account for overlapped antisymmetric and symmetric stretching vibrations of Hꢀbridged
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water , while the shoulder observed around 3260 cm must be due to a bending mode of adsorbed water
molecules. Nevertheless, this shoulder totally disappeared after CuNP incorporation, suggesting a new decay
of the hydrophilic character. This must be due to an enhancement of competitive surface interaction with ꢀ
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HO:Cu interactions at the expense of water, as supported by the intensity depletion after CuNP
ꢀ1
incorporation for the band appearing at 3623ꢀ3626 cm assigned to OH stretching vibrations of structural
hydroxyl groups.
In the meantime, XRD analyses showed an increase in the d001 basal spacing of NaMt from 11.38 Å (2θ =
8.51°) to 12.97 Å (2θ = 7.79°) (Fig. 2), reflecting an appreciable expansion of the interlayer space. The
marked sharpening of the d001 XRD line indicates an improvement of the parallel arrangement of the clay
+
sheets and a uniform incorporation of the PPhA cation within the interlayer space.
Figure 2
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2
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Photocatalysed TYC addition of thioglycerol to the propargyl groups induced a much more pronounced
increase in the d001 value from 12.97 Å up to 18.06 Å. This suggests an additional structure expansion, in
2
ꢀ1
agreement with the pronounced improvement of the specific surface area (SSA) from 54 to 296 m .g
(Table 1) and significant changes in micro (5ꢀ35 Å) and mesoporosity (35ꢀ200 nm) (Fig. S5).
Table 1
In contrast, CuNP incorporation induced an alteration of the parallel arrangement of the clay sheet and a
decrease in the basal d001 spacing from 18.06 to 15.52 Å. The simultaneous decay of both the specific
2
ꢀ1
ꢀ1
ꢀ1
surface area from 296 to 70 m .g and pore volume from 1.363 cc.g to 0.142 cc.g indicate a structure
1
4, 39
0
compaction that agrees with previous data
. This must be due to an enhancement of Cu interactions the
S and O atoms of the organic moiety grafted.
o
Unlike NaMt which showed appreciable thermal stability up to 400 C (pattern 1), TG analysis revealed new
o
thermal processes for its modified counterparts around 170ꢀ200 C with variable mass losses after clay
modification by the organic cation (pattern 2) and thioglycerol groups (pattern 3), respectively (Fig. 3). This
appears to be a thermal stability threshold, when thermal decomposition of the incorporated organic moieties
should be triggered.
Figure 3
o
For this reason, the temperature range for further TPD measurements was restricted to less than 100 C.
Preliminary tests revealed no changes in the CO ꢀTPD profiles after repeated adsorptionꢀdesorption cycles as
2
long as temperature is maintained within this temperature range. The latter is sufficiently wide to detect
unequivocally thermal dehydration processes. In agreement with our previous statement, PgꢀMt (pattern 2)
and Cu/TꢀMt (pattern 4) displayed very much weaker hydrophilic character, with a mass loss of ca. 1% at ca.
o
o
90ꢀ100 C, as compared to TꢀMt (pattern 3) with mass loss of ca. 16% at 70 C.
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3.2. XPS evidence for CuNP stabilization
XPS analysis of TꢀMt revealed changes in the pattern shape after CuNP incorporation (Fig. S6). The binding
energy of S and O electrons shifted from 164.19 eV to 163.39 eV and from 532.22 eV to 531.42 eV,
2
p
1s
respectively (Tables 2, S1-S2). This accounts for an attenuation of the binding energy and enhanced electron
mobility around S and O atoms due to interactions with nextꢀneighboring CuNPs. Given the magnitude of
the changes measured for the binding energy, these interactions must be sufficiently strong to produce a
1
4
compaction of the organic entanglement around CuNPs, as already reported .
Table 2
No detectable shifts in the binding energy were noticed for carbon atoms belonging to the organic moiety,
and both Si and Al atoms of the clay surface. This suggests that CuNP interactions with the carbon atoms of
the organic moiety, the silanol and aluminol groups, if any, should only have minor contributions, and,
0
0
subsequently, that CuNP stabilization involves mainly SꢀCu and OꢀCu interactions.
Figure 4
0
Evidence of the presence of Cu was provided by the main XPS peaks at 936 eV (Cuꢀ2p ) and 956 eV (Cuꢀ
3
/2
2p ) , which were unequivocally identified through their area ratio around 0.5 (Fig. 4). This agrees with the
1
/2
0
weak but clear signal at 919 eV, which accounts for a LMM spectrum of Cu . Slight shoulders difficult to
4
0
2+
discriminate around these values and weak satellite signals of Cu between 945 and 950 eV indicate that
2
+
the amounts of Cu and other oxidation states, if any, should be negligible. This must be due to the capacity
4
1
of alkyl sulfides to prevent copper oxidation . The peak broadness suggests a diffuse dispersion of the
electron density around S atoms of the thioglycerol groups, seemingly a common feature for other –CꢀS:
4
2ꢀ44
metal systems
. This may slightly influence the electron density on nextꢀneighboring carbon atoms, and
1
3
deeper insights through CꢀNMR would be useful in this regard.
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3.3. NMR evidence for CuNP stabilization.
1
3
CuNP stabilization was supported by CꢀNMR of nonꢀgrafted organic compound 3 in aqueous media before
2
+
(A) and after incorporation of Cu cations (B) or CuNPs (C) (Fig. 5). Fluctuations of the chemical shifts and
peak splitting were noticed for carbons neighboring S, O, and N atoms of TꢀMt after CuNP incorporation
(Carbons d, i, j, k, l, m, n, and o) (Table S3). The decrease in intensity of these carbons (except d) was
accompanied by the appearance of many smaller peaks. This corresponds to a chemical shift dispersion due
to the appearance of anisotropy, a special feature of rigid structures.
Figure 5
The most significant changes in the chemical shifts were observed for carbon h (from δ 44.8ꢀ44.7 to 48.5ꢀ
48.9 ppm) and i (from δ 34.0 to ꢀ38.0ꢀ38.1 ppm). Carbons j and m also showed noticeable increases in the
0
chemical shifts, as a result of the presence of electronꢀdonor (Cu ) in their vicinity. This suggests a shielding
enhancement due to a synergistic effect of both neighboring S and O atoms. More pronounced changes were
0
registered after incorporation of Cu , presumably due to a higher affinity towards thioglycerol as compared
2
+
to Cu cations. The latter appeared to rather favor interaction with S and O atoms neighboring carbon g, as
supported by the appreciable shielding of protons a and g (Table S4, Fig. S7). The slight decreases in
chemical shift of carbon d from δ 155 to 154.5 ppm, and to a lesser extent of carbon n and k from δ 70.4ꢀ
70.8 to 70.0ꢀ70ꢀ5 ppm indicate a deꢀshielding effect, presumably due to electron attraction by N atoms. This
can be explained by the higher electronegativity of oxygen (3.44) and nitrogen (3.04) as compared to carbon
(2.55) and sulfur (2.58).
3.4. CuNP Dispersion
A first overview of TEM images of Cu/TꢀMt showed the preponderant formation of CuNPs with particle size
not exceeding 5ꢀ6 nm (Fig. S8). A closeꢀup on a particle edge revealed wellꢀordered organoꢀclay stacks of
1
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parallel lamellae (Fig. 6), in agreement with the XRD data. The linear arrangement of CuNPs indicates an
interlayer incorporation of copper. Interestingly, much smaller CuNPs with fairly uniform dispersion were
observed between the organoclay lamellae as compared to the external surface of the clay sheet stacks. Most
CuNPs (ca. 90%) have an average size below 3.0 nm, including 39% of 1.0ꢀ1.5 nm particles and 14% of
subnanometric particles (Fig. 7).
Figure 6
Figure 7
Such a high dispersion of CuNPs must be due to a combination of a ‘’cementing’’ effect of the organic
moiety around CuNPs and a ‘’sandwiching’’ effect of organic chains belonging to two neighboring clay
sheets sharing common CuNPs. This ought to reduce the mobility of both the organic branches inserted and
CuNPs, conferring a certain rigidity that prevents entrapped CuNPs from reꢀaggregating. Repeated CO or
2
hydrogen adsorptionꢀdesorption cycles between 20 and 80°C produced no change in the TPD profiles and,
subsequently, no CuNPs reꢀaggregation. This finding agrees with the appearance of anisotropy, and reinforce
the concept of structure compaction.
0
A closeꢀup on Cu particle (Fig. S9) showed a d111 spacing of 2.1 nm, a special feature of the faceꢀcentered
cubic symmetry of copper (fcc), which appears to be maintained even at subnanometric scale. XPS analysis
0
(Table 2) gave a C atom concentration of 27.85 %, before Cu incorporation, i.e. 1.09 mmol organic moiety
ꢀ1
per gram dry NaMt. This value is close to the measured CEC (1.0 meq.g ), and indicates full exchange of
+
+
Na with the PPhA cation. The calculations accuracy is supported by the fact that the measured C/S atom
ratio is in the same magnitude order as the molecule formula of the inserted organic compound.
0
0
0
Given the Cu compactness factor of 0.74 due its fcc symmetry, the measured Cu /S atom ratio of 1.63 (Cu
ꢀ1
ꢀ1
content of 57 mg.g or 0.9 mmol.g ), which is in the same magnitude as that obtained through EDSꢀXRF
measurements on spots of uniform CuNP dispersion (Fig. S10). This accounts for ca. one 0.5 nm particle
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(4.70 Cu atoms) per ca. 3 sulfur atoms, or one 1 nm particle (37.6 Cu atoms) per 23 sulfur atoms or one 1.5
0
nm particle (126.9 Cu atoms) per 78 sulfur atoms. Consequently, there exists a strong correlation between
0
the structure compaction and metal dispersion and stabilization. Higher Cu /S atom ratios were noticed at the
edges and external surface of the clay sheet stacks, i.e. when no sandwiching effect is involved. It results that
bulky CuNPs are surrounded by less thioglycerol groups than those entrapped within the interlayer space.
0
3.5. Cu :OH interaction assessment through CO -TPD
2
0
Given the affinity of OH groups towards both CO and metals, Cu incorporation is expected to reduce the
2
CO retention capacity (CRC). This detrimental effect is regarded as a precise indicator of the enhancement
2
0
27ꢀ31
of competitive ꢀHO:Cu interaction at the expense of –HO:CO carbonateꢀlike association
. Preliminary
2
tests revealed that carbon dioxide adsorbs spontaneously at ambient temperature and pressure. As a general
feature, TPD measurements within the thermal stability range showed that CO can be released by slight
2
o
heating starting from ca. 45 C (Fig. 8). As, expected, TꢀMt displayed increasingly higher amount of desorbed
CO (pattern 1) and water (pattern 2), and, subsequently, increased affinity towards CO and hydrophilic
2
2
1
4, 27ꢀ31
character as compared to its metalꢀloaded counterpart (patterns 3 and 4, respectively)
.
Figure 8
ꢀ1
ꢀ1
TꢀMt showed higher CRC (279.5 µmol.g ) than Cu/TꢀMt (94.1 µmol.g ) between 20 and 100 °C (Table 1).
2
ꢀ1
This cannot be explained only by decreases in the specific surface area from 296 to 76 m .g and of pore
ꢀ1
14, 27ꢀ31
volume from 1.363 to 0.142 cc.g , since porosity decrease was already found to play a minor role
.
0
This must be due mainly to an accentuation of Cu ꢀOxygen interactions in Cu/TꢀMt at the expense of [ꢀ
HO:CO ] carbonateꢀlike associations in TꢀMt. CuNP incorporation also induced a suppression of CO2
2
ꢀ1
physical adsorption, as supported by the increase in the desorption heat from 0.70 to 24.7 kcal.mol .
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3.6. Affinity toward hydrogen and non-contact hydrogen-metal interaction
ꢀ1
Combined TGꢀDSC measurements revealed a higher mass loss of 13.1 mmol.g for Cu/TꢀMt versus 3.53
ꢀ1
mmol.g for TꢀMt, taken as the reference. This accounts for a hydrogen retention capacity (HRC) of 9.54
ꢀ1
ꢀ2
mmol.g or a surface affinity factor (SAF) of 126 µmol.m , which is in the same magnitude order as other
3
9
Cuꢀloaded organoclays . This SAF value suggests a nonꢀstoichiometrical amount of captured hydrogen,
0
given the relatively low amount of incorporated Cu . This performance is ca. eight times lower than that
1
4
45
reported for MIOMs , but still remains higher than those reported for lanthanumꢀbased MOFs or many
9
, 46ꢀ48
sophisticated adsorbents reported in the literature under similar conditions
.
This beneficial effect of CuNP incorporation can be explained by the increased affinity to metal towards
hydrogen, but it paradoxically contrasts with the attenuation of the hydrogen retention strength reflected by a
ꢀ
gas release starting from room temperature and a decrease in the desorption heat from 72.5 to 44.0 kcal.mol
1
. A possible explanation should consist in the partial suppression of chemical absorption of hydrogen,
presumably due to structure compaction. This is expected to hinder hydrogen diffusion towards CuNPs,
reducing the direct contact of hydrogen with the metal. Even after reaching the metal surface, hydrogen
49
should adsorb spontaneously on the metal surface , giving rise to a first layer of metal hydride that act as a
5
0
barrier that impede further hydrogen diffusion inside the metal . This is in agreement with previous data,
which revealed that for short adsorption times, much higher linearity was noticed with a pseudoꢀsecondꢀ
5
1
order kinetic model, which supposes the occurrence of chemical processes . This indicates that hydrogen
chemisorption onto the MNPs, if any, should take place only in a quick initial step, i.e. as long as MNPs do
not contain adsorbed or absorbed hydrogen. The loss in linearity noticed for longer impregnation times of
the adsorbent by hydrogen suggests a consecutive physical adsorption of higher and nonstoichiometric
amounts of hydrogen. This was supported by intraparticle diffusion kinetics attempts, which revealed a
4
5ꢀ
multiꢀstep hydrogen adsorption. Many attempts with even more porous structures reported in the literature
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could not afford such a performance at ambient temperature and pressure, and without modifying
irreversibly the CuNP structure.
Figure 9
o
o
The shift of the slight bump in the DSC profile (Fig. 9) from 60ꢀ 70 C for TꢀMt (pattern 3) up to ca. 90 C for
Cu/TꢀMt (pattern 4) once contacted with hydrogen is a clear evidence that CuNPs are responsible of the
affinity improvements towards hydrogen. The fact that most hydrogen desorbs at temperature not exceeding
o
80ꢀ90 C suggests a predominantly physical adsorption, most likely in the form of a multilayer and nonꢀ
stoichiometrical condensation of hydrogen in the organic entanglement surrounding CuNPs, as already been
1
4
reported elsewhere .
Repeated hydrogen adsorptionꢀdesorption cycles between 20 and 80°C produced no change in the adsorption
profiles. Three successive adsorption tests on dry Cu/TꢀMt performed at room temperature by contacting
Cu/TꢀMt with dry hydrogen gave almost similar adsorption profiles (Fig. S11). Test 1 was achieved with the
o
asꢀsynthesized Cu/TꢀMt after air drying at 30 C in sealed enclosure containing dry nitrogen and NaOH pills.
o
Tests 2 and 3 were carried out after thermal regeneration up to 80 C without rehydration and reꢀsaturation
with dry hydrogen in sealed enclosure. Both tests gave lower surface affinity factor than the fresh sample
(Test 1). As already stated in the manuscript, a ca. 15ꢀ20% loss in the surface affinity factor was noticed after
o
test 1 and regeneration at 80 C. Investigations are still in progress for achieving more tests.
Hydrogen capture through physical condensation does not necessarily require highly porous structure but
rather optimum hydrogenꢀmatrice interactions. The latter should be sufficiently strong to retain high and
nonꢀstoichiometric gas amount but also sufficiently weak to allow easy release upon strong stream of the
1
0, 11, 13, 56ꢀ58
carrier gas. Some dendrimers and derivatives
may fulfill these requirements in dry media if
judiciously functionalized and combined with suitable inorganic supports, even at the expense of porosity.
These findings open new prospects for natural or commercial compounds deriving from vegetal sources.
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4. CONCLUSION
The results obtained allow concluding that truly reversible capture of hydrogen is possible on copperꢀloaded
organoclay at ambient conditions. High metal dispersion and stabilization through simultaneous interactions
0
of sulfur and oxygen atoms belonging to the organic moiety with Cu nanoparticles were found to produce a
0
compact matrix that reduces Cu nanoparticles mobility and impedes their aggregation. Such a structure
favors physical condensation of hydrogen at the expense of chemisorption by hindering gas diffusion
towards the metal surface. Nonꢀcontact metalꢀhydrogen interaction can explain the low desorption heats
o
measured and almost complete release up to 80 C or under vacuum at lower temperature. Optimum metal
loading and density of the organic chains with tunable strength of the chelating sites are essential
requirements for achieving truly reversible and effective hydrogen storage. This finding is of great
importance, because it allows envisaging safe hydrogen storage without energy constraints by ecoꢀfriendly
matrices, with promising prospects for clay materials and biomassꢀderiving sulfureted polyglycerols. Further
work is being focused on other metal nanoparticles, and data will be published in due course.
4
4
07
08 ACKNOWLEDGEMENTS
4
4
4
4
4
09
10
11
12
13
This work was supported by grants from MDEIEꢀFQRNT (No. 2011ꢀGZꢀ138312) and FODARꢀUQ
(FODARꢀ2015, QC, Canada) to A.A. and R.R. and from the Natural Science and Engineering Research
Council of Canada (NSERC) to R.R. We are also grateful to Dr. G. Chamoulaud and Dr. A. A. Arnold
(Nanoqam) for their technical assistance. AVA was on leave of absence from Catalysis and Microporous
Materials Laboratory, VasileꢀAlecsandri University of Bacau, 600115, Romania.
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FIGURES CAPTIONS
Scheme 1. Structure of copperꢀdoped TꢀMt (Cu/TꢀMt).
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Scheme 2. Synthetic strategy for the synthesis of compounds 3 and 4 and of Cu/TꢀMt.
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08
Fig. 1. FTIR spectra evolution after NaMt modifications. 1. NaMt; 2. PgꢀMt; 3. TꢀMt; 4. Cu/TꢀMt. The
samples were preserved under wet air (20% moisture) analyzed without previous dehydration.
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Fig. 2. XRD spectra evolution after NaMt modifications. 1. NaMt; 2. PgꢀMt; 3. TꢀMt; 4. Cu/TꢀMt.
The samples were preserved under wet air (20% moisture) analyzed without previous dehydration.
Fig. 3. TGA patterns evolution after NaMt modifications. 1. NaMt; 2. PgꢀMt; 3. TꢀMt, 4. Cu/TꢀMt.
These TG patterns were recorded with samples previously dried overnight at room temperature.
2
+
Fig. 4. XPS spectra of Cu/TꢀMt. The weak Cu satellite signal around 945ꢀ950 eV and the broad Cu
2
p3/2
correspond to a very weak contribution of other oxidation states. The weak but clear signal at 919 eV in the
0
small binding energy shift region accounts for a LMM spectrum of Cu .
1
3
2+
Fig. 5. C NMR spectra of compound 3 before (A) and after incorporation of Cu cations (B) or CuNP (C)
in D O (20 mg sample in 0,4 mL solvent). These samples previously dried overnight at room temperature.
2
Fig. 6. TEM images of CuNPs. Linear arrangement of black stains is a precise indicator of an interlayer of
smaller size NPs not exceeding 2 nm, as compared to CuNP dispersed on the functionalized external surface.
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These TG patterns were recorded with samples previously dried overnight at room temperature.
Fig. 7. Distribution of CuNPs entrapped between TꢀMt lamellae as determined using the ImageꢀJ software. A
proportion of 90% of ca. 200 particles numbered displayed a diameter lower than 2 nm, and 14 % were
found to be subnanometric particles.
Fig. 8. TPD patterns for CO and water: 1. CO ꢀTPD of TꢀMt, 2. H OꢀTPD of TꢀMt, 3. CO ꢀTPD of Cu/Tꢀ
2
2
2
2
1
Mt, 4. H OꢀTPD of Cu/TꢀMt. These TPD patterns were recorded under nitrogen stream of 5 ml.minꢀ after
2
impregnating 45 mg adsorbent overnight with 200 mL of CO at room temperature in dynamic regime under
2
ꢀ1
a 5 mL.min dry nitrogen stream, followed by a purge under similar conditions. These samples were not
previously dried.
Fig. 9. DSC patterns of CO saturated TꢀMt (1) and Cu/TꢀMt (2) and H ꢀsaturated TꢀMt (3). Cu/TꢀMt (4) at a
2
2
o
−1
ꢀ1
heating rate of 5 C.min under a dry nitrogen stream of 20 ml.min . For DSC measurements. The different
adsorbents were previously saturated by dry pure H overnight in static mod. without carrier gas at ambient
2
temperature and pressure.
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Tables
Table 1. Features of modified supports before and after incorporation of CuNPs
Specific surface
c
Pore
Volume
CRC
d001
ꢀ
SAMPLE
Preparation procedure
2
ꢀ1 a
µmol.g
area (m .g )
ꢀ1 b
1
(A°)
(cc.g )
Aldrich bentonite
purification
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4
ꢀ
11.38
12.97
ꢀ
ꢀ
NaMt
PgꢀMt
b
Ion Exchange with PBA
22
0.171
1.363
0.142
Thioglycerolꢀorganoꢀ
montmorillonite (TꢀMt)
Thiolꢀyne reaction
296
18.06 279.5
15.52 94.1
o
Cu NP dispersion using
76
Cu/TꢀMt
(
Cu(NO ) 2.5 H O)
3 2 2
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a
The specific surface area was determined through nitrogen adsorptionꢀdesorption isotherms and BET
calculation model;
b
The pore volume was determined through nitrogen adsorptionꢀdesorption isotherms and BJH calculation model;
c
ꢀ
TPD measurements of CO retention capacities (CRC were carried out between 20 and 100°C under a 5mL.min
2
1
nitrogen stream, after static saturation of 40 mg of samples with 200 ml of dry CO at 20°C;
2
,
d
The ion exchange process was carried out by NaMt impregnation in aqueous solution of 4ꢀ(Propꢀ2
+
ynyloxy)phenylammonium cation (PPhA ).
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Table 2. Binding energies in compound 3 before and after CuNP incorporation.
Before CuNP incorporation
After CuNP incorporation
Electron type
Binding energy
Atom %
Binding energy
(eV)
Atom %
(eV)
Oꢀ1s
Cꢀ1s
Sꢀ2p
Siꢀ2p
Alꢀ2p
Cuꢀ2p
532.22
285.00
164.19
102.59
75.39
ꢀ
40.52
27.85
2.19
22.24
7.20
0
531.42
285.00
163.39
102.59
75.39
30.85
24.40
1.15
12.12
29.60
1.87
932.24
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Schemes
Scheme 1
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Scheme 2
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Figures
Figure 1
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Figure 2
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Figure 3
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Figure 4
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