Size-controllable APTS stabilized ruthenium(0) nanoparticles catalyst for the dehydrogenation of dimethylamine-borane at room temperature

Article abstract of DOI:10.1039/c1dt11290b

Dimethylamine-borane, (CH₃)₂NHBH₃, has been considered as one of the attractive materials for the efficient storage of hydrogen, which is still one of the key issues in the "Hydrogen Economy". In a recent communication we have reported the synthesis and characterization of 3-aminopropyltriethoxysilane stabilized ruthenium(0) nanoparticles with the preliminary results for their catalytic performance in the dehydrogenation of dimethylamine-borane at room temperature. Herein, we report a complete work including (i) effect of initial [APTS]/[Ru] molar ratio on both the size and the catalytic activity of ruthenium(0) nanoparticles, (ii) collection of extensive kinetic data under non-MTL conditions depending on the substrate and catalyst concentrations to define the rate law of Ru(0)/APTS-catalyzed dehydrogenation of dimethylamine-borane at room temperature, (iii) determination of activation parameters (E_a, ΔH^# and ΔS^#) for Ru(0)/APTS-catalyzed dehydrogenation of dimethylamine-borane; (iv) demonstration of the catalytic lifetime of Ru(0)/APTS nanoparticles in the dehydrogenation of dimethylamine-borane at room temperature, (v) testing the bottlability and reusability of Ru(0)/APTS nanocatalyst in the room-temperature dehydrogenation of dimethylamine-borane, (vi) quantitative carbon disulfide (CS₂) poisoning experiments to find a corrected TTO and TOF values on a per-active-ruthenium-atom basis, (vii) a summary of extensive literature review for the catalysts tested in the catalytic dehydrogenation of dimethylamine-borane as part of the results and discussions.

Full text of DOI:10.1039/c1dt11290b

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PAPER
Size-controllable APTS stabilized ruthenium(0) nanoparticles catalyst for the
dehydrogenation of dimethylamine–borane at room temperature
a
c,d
Mehmet Zahmakıran,*a,b,c Karine Philippot,*c,d Saim Ozkar and Bruno Chaudret
¨
Received 7th July 2011, Accepted 27th September 2011
DOI: 10.1039/c1dt11290b
Dimethylamine–borane, (CH3)2NHBH3, has been considered as one of the attractive materials for the
efficient storage of hydrogen, which is still one of the key issues in the “Hydrogen Economy”. In a recent
communication we have reported the synthesis and characterization of 3-aminopropyltriethoxysilane
stabilized ruthenium(0) nanoparticles with the preliminary results for their catalytic performance in the
dehydrogenation of dimethylamine–borane at room temperature. Herein, we report a complete work
including (i) effect of initial [APTS]/[Ru] molar ratio on both the size and the catalytic activity of
ruthenium(0) nanoparticles, (ii) collection of extensive kinetic data under non-MTL conditions
depending on the substrate and catalyst concentrations to define the rate law of Ru(0)/APTS-catalyzed
dehydrogenation of dimethylamine–borane at room temperature, (iii) determination of activation
parameters (Ea, DH# and DS#) for Ru(0)/APTS-catalyzed dehydrogenation of dimethylamine–borane;
(iv) demonstration of the catalytic lifetime of Ru(0)/APTS nanoparticles in the dehydrogenation of
dimethylamine–borane at room temperature, (v) testing the bottlability and reusability of Ru(0)/APTS
nanocatalyst in the room-temperature dehydrogenation of dimethylamine–borane, (vi) quantitative
carbon disulfide (CS2) poisoning experiments to find a corrected TTO and TOF values on a
per-active-ruthenium-atom basis, (vii) a summary of extensive literature review for the catalysts tested
in the catalytic dehydrogenation of dimethylamine–borane as part of the results and discussions.
1. Introduction
There has been a rapidly growing interest for searching suitable
Scheme 1 The catalytic dehydrogenation/dehydrocoupling of dimethy-
materials for efficient storage of hydrogen.1 In this area various
solid materials2 have been considered for hydrogen storage as the
low density of hydrogen makes it difficult to store it in compressed
or liquefied form.3 Among these materials, amine–boranes owing
to their high hydrogen contents, have been identified as the leading
candidates for chemical hydrogen storage.4,5 More importantly,
recent reports related to the regeneration of dehydrogenation
products reveal the importance of the catalytic dehydrogenation
of amine–borane adducts.6 Of particular interest the recent
studies show that the catalytic dehydrogenation/dehydrocoupling
of dimethylamine–borane ((CH3)2NHBH3, DMAB) potentially
releases 3.5 wt% H2 (Scheme 1).7,8
lamine–borane (DMAB).
catalyst,9 our research has focused on the development of metal
nanoparticles as catalyst because of the advantages of “semi-
heterogeneous catalysis”19 such as simple product isolation, cata-
lyst recovery and reusability.
In a recent communication20 we have described the synthesis
and characterization of ruthenium(0) nanoparticles stabilized
by 3-aminopropyltriethoxysilane (APTS), hereafter named as
Ru(0)/APTS and their catalytic use in DMAB dehydrogenation.
They provide the complete conversion of dimethylamine–borane
(Me2NHBH3) to cyclic aminoborane ([Me2NBH2]2) and gener-
ation of 1 eq. H2 with an initial apparent21 turnover frequency
of 55 h-1. More importantly, Ru(0)/APTS show previously
unprecedented reusability in the dehydrogenation of DMAB.
Herein we report, the influence of initial molar ratio of APTS
to ruthenium ([APTS]/[Ru]) on the size and catalytic activity of
the resulting Ru(0)/APTS.22 Both the size and catalytic activity of
Ru(0)/APTS strongly depend on initial [APTS]/[Ru] ratio. It was
shown that the density of surface chemisorbed APTS ligands and
the size of nanoparticles competitively govern the catalytic activity
of Ru(0)/APTS in the dehydrogenation of DMAB. We also report
kinetic studies on the Ru(0)/APTS-catalyzed dehydrogenation of
A quick literature search shows that many homogeneous8–16 and
heteregeneous8,17,18 catalysts have been tested in the catalytic dehy-
drogenation of DMAB. Although the highest catalytic activity has
5
been achieved by using homogeneous [h -C5H3-1,3-(SiMe3)2Ti]2
aDepartment of Chemistry, Middle East Technical University, 06531,
Ankara, Turkey
bNew address: Department of Chemistry, Yuzuncu Yıl University, 65080,
Van, Turkey. E-mail: zmehmet@yyu.edu.tr
cCNRS, LCC (Laboratoire de Chimie de Coordination), 205, route de Nar-
bonne, F-31077, Toulouse, France. E-mail: karine.philippot@lcc-toulouse.fr
dUniversite’ de Toulouse, UPS, INPT, LCC, F-31077, Toulouse, France
590 | Dalton Trans., 2012, 41, 590–598
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DMAB depending on the temperature, substrate and catalyst con-
centrations under non-MTL (mass transfer limiting) conditions23
leading to the rate law and activation parameters (Ea, DH# and
DS#). Further experiments performed to determine the catalytic
lifetime showed that Ru(0)/APTS provide the highest number
of turnovers ever reported for the catalytic dehydrogenation of
DMAB at room temperature (the apparent TTO value is 1240).21
Moreover, the ruthenium(0) nanoparticles exhibit high durabil-
ity throughout their catalytic use in the dehydrogenation reaction
against agglomeration.24 Reused Ru(0)/APTS retain 70% of their
initial activity even after the sixth run in the dehydrogenation of
DMAB with a complete conversion.
This work also includes poisoning experiments with carbon
disulfide (CS2) showing that the true TOF and TTO values for
the Ru(0) nanoparticles on a per-active-ruthenium-atom basis are
a factor of 12.5 times higher than the apparent (lower limit) TOF
of 55 h-1 (corrected TOF = 688 h-1) and TTO of 1240 (corrected
TTO = 15500). Also presented as part of the Results and discussion
is a comprehensive review of the literature of catalysts tested
in the dehydrogenation of DMAB, which clearly indicates that
Ru(0)/APTS is the more active and longer-lived catalyst than
the previously reported best homogeneous9 and heterogeneous18
catalysts.
are quoted as the mean diameter the standard deviation. In
all size-distributions, more than 200 non-touching particles were
counted). NMR spectra were recorded in THF-d8 on a Bruker
Avance DPX 300 MHz spectrometer (96.3 MHz for 11B NMR).
BF3·(C2H5)2O was used as the external reference for 11B NMR
chemical shifts.
2.3. Preparation of 3-aminopropyltriethoxysilane-stabilized
ruthenium(0) nanoparticles (Ru(0)/APTS)
Ru(0)/APTS were prepared as described elsewhere.20 The solution
containing desired amounts of [Ru(cod)(cot)] and 3-APTS ligand
in 30 mL of freshly distilled and degassed THF in a Fischer–
Porter bottle was pressurized with 3 bar H2 during 12 h under
vigorous stirring. During this time the reaction mixture changed
from yellow to dark brown. After elimination of excess H2, the
solution was concentrated by solvent evaporation and a further
addition of pentane favored colloid precipitation. After removing
of the supernatant solution by filtration, the solid was washed
with pentane (3 ¥ 20 mL) and dried under vacuum, giving rise to
a purified colloid as a black powder.
2.4. General procedure for the dehydrogenation of
dimethylamine–borane catalyzed by Ru(0)/APTS, RuCl3,
RuCl3·3H2O, [Ru(cod)Cl2]n and [Ru(cod)(cot)] and data handling
2. Experimental
Catalytic dehydrogenation of DMAB by different ruthenium
catalysts was performed in a Fischer–Porter (F-P) bottle connected
to a line through Swagelock TFE-sealed quick connects and to
an Omega PX-302 pressure transducer interfaced through an
Omega D1131 digital transmitter to a computer using the RS-232
module as described elsewhere.18 The progress of each individual
dehydrogenation reaction was followed by monitoring the increase
of H2 gas pressure of on LabVIEW 8.0 program.
2.1. General and materials
All operations were carried out using standard Schlenk tube or
Fischer–Porter bottle techniques or in a glovebox under argon.
Solvents were purified just before use by distillation under nitrogen
atmosphere: tetrahydrofuran (THF) over sodium/benzophenone,
pentane and hexane over calcium hydride. Carbon disulfide
(CS2), 1,5-cyclooctadiene (cod), 3-aminopropyltriethoxysilane (3-
APTS), RuCl3·3H2O, RuCl3, [Ru(cod)Cl2]n and dimethylamine–
borane (DMAB) were purchased from Aldrich and used without
purification.
H2 (99.9%) and N2 (99.9%) were purchased from Linde and used
as received. [Ru(cod)(cot)] was prepared according to literature
procedure.25 All glassware and Teflon-coated magnetic stir bars
were washed with acetone and copiously rinsed with distilled water
before drying in an oven at 150 ◦C.
The catalytic activity of each catalyst was determined by
measuring the rate of hydrogen generation. In a typical reaction
1 mmol DMAB (60.7 mg Me2NHBH3) was dissolved in THF
added via a gastight syringe to yield a clear colorless solution. The
solution was then transferred via a disposable glass pipette into
a new 22 ¥ 175 mm Pyrex culture tube containing a new 5/16 ¥
5/8 inch Teflon coated magnetic stir. The culture tube was then
sealed inside the F-P bottle, brought outside the drybox and placed
inside a constant temperature circulating water-bath thermostated
◦
at 25.0 0.1 C unless otherwise specified. Next, the F-P bottle
2.2 Characterization
was connected to the line, which had already been evacuated for
at least 30 min to remove any trace of oxygen and water present,
via its TFE-sealed Swagelock Quick-Connects.
TEM and HRTEM experiments were carried out at the “Service
Commun de Microscopie Electronique de L’Universite´ Paul
Sabatier” (TEMSCAN): TEM observations with a JEOL 200 CX-
T electron microscope operating at 200 kV with a point resolution
Under nitrogen purging (14 mL dry N2 s-1) catalyst solution in
THF was added rapidly via the tap of the F-P bottle by using a
10 mL Pyrex (nitrogen fluxed) volumetric pipette. The pressure
inside the F-P bottle was monitored on a PC using the LabVIEW
8.0 program. When the pressure inside the F-P bottle established
a constant value, the reaction was started (t = 0 min) by stirring
the mixture at 900 rpm. When no more hydrogen generation was
observed, the experiment was stopped and the F-P bottle was
sealed, disconnected from the line and H2 pressure was released.
Then F-P bottle was transferred back into the drybox and a 9 in
glass Pasteur pipette was used to withdraw a ca. 0.5 mL aliquot
from the reaction solution. This aliquot was then added to 1 g of
˚
of 4.5 A and HRTEM observations with a JEOL JEM 2010
electron microscope working at 200 kV with a resolution point of
˚
2.5 A. Specimen for TEM and HRTEM analysis were prepared in
a glove box, by slow evaporation of a drop of each crude colloidal
solution deposited onto a holey carbon covered copper grid. The
particle size analyses were performed by using NIH Image J 1.62
program developed at the U.S. National Institutes of Health and
available on the Internet at http://rsb.info.nih.gov/nih-image/.
For each particle, the diameter was calculated from the area by
assuming that the nanoparticles are spherical. Size distributions
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CDCl3 in an individual glass ampule, mixed under agitation using
the Pasteur pipette, then transferred into a quartz NMR sample
tube (Norell S-500-QTZ) which was subsequently sealed and then
brought out of drybox for the 11B NMR analysis.
(TTO). Such a lifetime experiment was started with a 10 mL THF
solution containing 2.25 mM Ru(0)/APTS and 700 mM DMAB.
When the complete conversion was achieved, more DMAB was
added into the F-P bottle under N2 purging.
2.5. Control experiments; detection of mass-transfer limitations
(MTL regime) in the Ru(0)/APTS-catalyzed dehydrogenation of
DMAB
2.10. Quantitative carbon disulfide (CS2) poisoning of Ru(0)/
APTS and its monitoring in the catalytic dehydrogenation of
DMAB
To define the stirring conditions necessary for determining intrin-
sic kinetic data independent of the diffusion effects, a series of
eleven experiments (dehydrogenation of 55 mM DMAB solution
catalyzed by 2.25 mM Ru(0)/APTS) were conducted at 25 0.1 ◦C
at different stirring speeds: 0, 120, 240, 360, 480, 600, 720, 840,
960, 1080 and 1200 rpm.23 It was found that up to 600 rpm a
mass transfer limitation (MTL) regime is effective on the observed
rate for the dehydrogenation of DMAB (MTL regime £720 rpm;
non-MTL regime ≥720 rpm). Therefore, in all the catalytic tests
reported hereafter the stirring speed was adjusted to ≥720 rpm.
In order to perform catalyst poisoning experiments a stock
solution of CS2 in THF was prepared by dissolving 27 mL of
CS2 in total 10 mL THF in drybox. Then, 6 mL of 3.73 mM
Ru(0)/APTS (in THF) was transferred into a 22 ¥ 175 mm culture
tube containing a 5/16 ¥ 5/8 inch Teflon coated magnetic stir bar.
To this, an aliquot (5–125 mL) of CS2 stock solution was added.
The culture tube was then placed in the F-P bottle, sealed brought
from the drybox, placed in a constant temperature circulation
water-bath thermostated at 25 0.1 ◦C and the reaction mixture
was stirred for a 30 min.
Next, under nitrogen purging (14 mL dry N2 s-1) 55 mM
DMAB solution in THF was added into the F-P pressure bottle
rapidly via the tap of the bottle by using a 10 mL Pyrex volumetric
pipette (nitrogen fluxed). The pressure inside the F-P bottle was
monitored on a PC using the LabVIEW 8.0 program. When
the pressure inside the F-P bottle established a constant value,
the reaction was started (t = 0 min.) by stirring the mixture at
900 rpm. When no more hydrogen generation was observed the
experiment was stopped.
2.6. Effect of initial [APTS]/[Ru] molar ratio on the catalytic
activity of Ru(0)/APTS catalysts in the dehydrogenation of
DMAB
Five sets of experiments were performed for the catalytic dehydro-
genation of DMAB at 25 0.1 ◦C by using five Ru(0)/APTS
catalysts prepared with different [APTS]/[Ru] molar ratios
([APTS]/[Ru] = 0.25, 0.50, 1.0, 2.0 and 4.0). All these tests were
performed in the same conditions as described in section 2.4.
with ruthenium and DMAB concentrations kept constant ([Ru] =
2.25 mM in 10 mL THF; [DMAB] = 55 mM) at 25 0.1 ◦C.
2.11. Isolability, bottlability and reusability of Ru(0)/APTS in
the dehydrogenation of DMAB
2.7. Determination of the rate law for the
Ru(0)/APTS-catalyzed dehydrogenation of DMAB
At the end of dehydrogenation reaction of 55 mM DMAB (0.55
mmol, 34 mg Me2NHBH3 in a total 10 mL solution) catalyzed
by 2.25 mM Ru(0)/APTS at 25 0.1 ◦C, the F-P bottle was
sealed, disconnected from the line and transferred into the drybox
after releasing of H2 pressure. Next, the reaction solution was
transferred into a new Schlenk tube which was sealed and brought
out of the drybox. The solution was precipitated with cold hexane
(10 mL; added under N2 atmosphere) and the supernatant solution
was removed by filtration.
The solid was further washed with hexane (3 ¥ 20 mL) and
dried under vacuum, giving rise to the isolated colloid as a dark-
brown powder. 11B NMR analysis after dispersion of the colloid
in d8-THF showed that the surface of ruthenium(0) nanoparticles
are free from cyclic dehydrocoupling product ([Me2NBH2]2). This
isolated colloid was thus weighed and redispersed in THF and
re-used in catalysis under the same conditions as in the first run.
Two sets of experiments were performed at 25 0.1 ◦C in the
same way as described in previous sections. In the first set, the
concentration of DMAB was kept constant at 55 mM (0.55 mmol,
34 mg Me2NHBH3 in a total 10 mL solution) and the ruthenium
concentration was varied in the range of 2.25, 4.5, 6.75 and 9.0
mM. In the second set of experiments the ruthenium concentration
was kept constant at 2.25 mM Ru while the concentration of
DMAB was varied in the range of 55, 110, 165, 220 and 330 mM.
2.8. Determination of the activation parameters (Ea, DHπ and
DSπ) for the Ru(0)/APTS-catalyzed dehydrogenation of DMAB
The catalytic dehydrogenation of 55 mM DMAB (0.55 mmol,
34 mg Me2NHBH3 in a total 10 mL solution) by 2.25 mM
Ru(0)/APTS was performed in the same way as described in
section 2.4. at various temperatures in the range of 20, 25,
30, 35, 40 ◦C. The values of the observed rate constant kobs
were determined from the slope of the linear portion of the
dehydrogenation curve used to obtain the activation energy Ea
by using an Arrhenius plot26 as well as the enthalpy of activation
DHπ and entropy of activation DSπ by using an Eyring plot.27
3. Results and discussion
3.1. Synthesis, characterization and controlling the size of
3-aminopropyltriethoxysilane stabilized ruthenium(0)
nanoparticles (Ru(0)/APTS)
Ru(0)/APTS were reproducibly prepared by following our pre-
viously reported organometallic approach28 that consists of
the decomposition of [Ru(cod)(cot)] (cod = 1,5-cyclooctadiene,
cot = 1,3,5-cyclooctatriene) by dihydrogen in the presence of 3-
aminopropyltriethoxysilane (APTS) in THF at room temperature
(Scheme 2).20
2.9. Determination of catalytic lifetime of Ru(0)/APTS in the
dehydrogenation of DMAB
The catalytic lifetime of Ru(0)/APTS in the dehydrogenation of
DMAB was determined by measuring the total turnover number
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Scheme 2 Synthesis of Ru(0)/APTS nanoparticles by decomposition of
[Ru(cod)(cot)] in the presence of 3-aminopropyltriethoxysilane (APTS).
Table 1 Size and Ru atom content of Ru(0)/APTS depending on the
[APTS]/[Ru] ratios
Entry Sample [APTS]/[Ru]
Mean size, D/nm No. Ru atomsa
1
2
3
4
5
Ru1
Ru2
Ru3
Ru4
Ru5
0.25
0.50
1.0
2.0
4.0
2.36 0.61
1.96 0.44
1.67 0.41
1.29 0.31
1.16 0.30
600 396
330 196
209 133
96 60
71 47
a The number of atoms were estimated by using the equation N =
N0pV/101.7, where N0 = 6.022 ¥ 1023, p = 12.2 g cm-3 and V =
(4/3)p(D/2)3.
Ruthenium(0) nanoparticles obtained in this way were found to
be highly stable in solution (no precipitation of bulk ruthenium
from the solution was observed even after several weeks of
storage). This behavior indicates the efficiency of APTS in the
stabilization of Ru(0) nanoparticles. Ru(0)/APTS can easily be
isolated from THF solution as a black solid by evaporation to
dryness or precipitation from the colloidal solution by adding
cold pentane, and further well redispersed in THF all at room
temperature.
Their characterization by TEM, HRTEM, TEM/EDX, ATR-
IR, XPS, 1H and 13C NMR spectroscopic techniques and elemental
analysis shows the formation of well dispersed ruthenium(0)
nanoparticles stabilized by APTS which coordinates the ruthe-
nium(0) nanoparticles surface through its amine group.20
This method of synthesis also enabled us to control the size of
ruthenium(0) nanoparticles by varying the initial [APTS]/[Ru]
molar ratio (Table 1). Fig. 1 shows the TEM images and
corresponding size histograms of Ru(0)/APTS prepared with
various initial [APTS]/[Ru] molar ratios. The mean particle size of
Ru(0)/APTS decreases with the increasing ligand concentration
which can be ascribed to fast and dense surface coverage of
ruthenium(0) nanoparticles in their embryonic stage. Similar
results were observed in the case of thiol-stabilized ruthenium(0)
nanoparticles28 and disulfide29 or alkylamine30 stabilized gold(0)
nanoparticles.
3.2. Catalytic activity of Ru(0)/APTS in the dehydrogenation of
DMAB at room temperature depending on their size and the
concentration of APTS ligand
In order to pick out the best Ru(0)/APTS system in terms of
activity to conduct the remainder of the study, we investigated the
effect of particle size on the catalytic activity by performing the
catalytic dehydrogenation of dimethylamine–borane ([DMAB] =
55 mM) with Ru1, Ru2, Ru3, Ru4 and Ru5 nanocatalysts (in all
[Ru] = 2.25 mM) at 25.0 0.1 ◦C. The quantification of the amount
of evolved H2 and 11B NMR spectrum registered at the end of
each catalytic reaction show that all nanocatalysts provide the
complete conversion of (CH3)2NHBH3 to cyclic [(CH3)2NBH2]2
and generation of 1 equiv. H2 per DMAB (Scheme 3).
Fig. 1 TEM images and corresponding size histograms of Ru(0)/APTS
prepared with different [APTS]/[Ru] ratios; (a)Ru1, (b) Ru2, (c) Ru3, (d)
Ru4 and (e) Ru5 (scale bar = 50 nm).
The initial rate of dehydrogenation of DMAB was found to
be 1.05, 1.12, 1.22, 0.89 and 0.79 mmol H2 h-1 for [APTS]/[Ru]
ratios of 0.25, 0.50, 1.0, 2.0 and 4.0, respectively (Fig. 2), We
observe that the initial rate increases with the [APTS]/[Ru]
ratio until the highest value of 1.22 mmol H2 h-1 obtained for
a ratio of 1 but decreases for higher ratios. This variation in
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Scheme 3 Dehydrogenation of dimethylamine–borane catalyzed by
Ru(0)/APTS in THF at room temperature.
Fig. 3 The initial rate of dehydrogenation (mmol H2 h-1) vs. stirring
speed (rpm) for the dehydrogenation of dimethylamine–borane ([DMAB] =
55 mM) catalyzed by Ru(0)/APTS ([Ru] = 2.25 mM) at 25 0.1 ◦C.
and thus proceeds under non-MTL regime when the stirring speed
is greater than 720 rpm.
Fig. 2 The initial rate (inset) of dehydrogenation (mmol H2 h-1) vs.
[APTS]/[Ru] ratio for the dehydrogenation of dimethylamine–borane
([DMAB] = 55mM) catalyzed by Ru1, Ru2, Ru3, Ru4 and Ru5 (in all
[Ru] = 2.25 mM) at 25 0.1 ◦C.
3.4. Initial kinetic studies and determination of activation
parameters for the Ru(0)/APTS-catalyzed dehydrogenation of
DMAB
Fig. 4 presents the plots of mol of H2 generated per mole of DMAB
vs. time for the catalytic dehydrogenation of DMAB starting
with different concentrations of Ru(0)/APTS in THF at 25
0.1 ◦C. As expected from a preformed catalyst, a linear hydrogen
generation starts without induction period and continues until the
generation of 1 equiv. of H2. The initial rate of dehydrogenation
determined from the linear portion of the plots in Fig. 4 is in
the range of 1.22–5.50 mmol H2 mol-1 h-1 (Table 2, vide infra).
Plotting the dehydrogenation rate vs. ruthenium concentration,
both in logarithmic scales (inset in Fig. 4), gives a straight line
with a slope of 1.06 indicating that the Ru(0)/APTS-catalyzed
dehydrogenation of DMAB is first order with respect to the
ruthenium concentration. The effect of substrate concentration
on the dehydrogenation rate was also studied by performing a
series of experiments starting with varying initial concentration of
DMAB at a fixed catalyst concentration ([Ru] = 2.25 mM). Fig. 5
shows the plot of H2 amount (mmol) generated vs. time for various
initial concentrations of dimethylamine–borane ([DMAB]0 = 55,
110, 165, 220, 330 mM) and the plot of H2 generation rate vs.
DMAB concentration both on logarithmic scales indicating that
the dehydrogenation rate with the ligand to metal ratio can be
explained by taking two factors into consideration: the size of the
nanoparticles and their surface coverage by the ligands. As shown
by the TEM images, nanoparticle size decreases with the increasing
concentration of the stabilizer. We may thus expect that the surface
area, and further the catalytic activity of the nanoparticles, first
increase with increasing [APTS]/[Ru] ratio. However, as the APTS
concentration increases, the nanoparticle surface is more covered
by the stabilizer causing a significant decrease in the catalytic
activity. In the present study, the maximum catalytic activity is
achieved with the molar ratio of [APTS]/[Ru] = 1. These synthesis
conditions appear as the best compromise between particle size
and activity. Therefore, in all tests reported hereafter the catalyst
prepared with molar ratio of [APTS]/[Ru] = 1 is used, unless
otherwise stated.
3.3. Evidence for a non-MTL (mass transfer limitation) in the
dehydrogenation of DMAB catalyzed by Ru(0)/APTS
Ru(0)/APTS with a molar ratio of [APTS]/[Ru] = 1 are found
to be very active catalyst in the dehydrogeneation of DMAB
(Fig. 3) providing an initial TOF value of 53 h-1 at 25
0.1 ◦C. In order to address the most important issue whether
such a fast dehydrogenation reaction at the interface of gas
and condensed phases is under mass transfer limitation (MTL)
regime, the dependence of reaction rate on the stirring speed was
investigated.23 A series of independent experiments was performed
for the dehydrogenation of DMAB catalyzed by Ru(0)/APTS
under the same conditions ([Ru] = 2.25 mM, [DMAB] = 55 mM at
25 0.1 ◦C), but using different stirring speeds (rpm). The rate of
dehydrogenation is plotted against the stirring speed in Fig. 3. The
inspection of the plot shows that the dehydrogenation is constant
Table 2 The observed rate constant (kobs) and turnover frequency (TOF)
values of Ru(0)/APTS-catalyzed dehydrogenation of dimethylamine–
borane ([DMAB] = 55 mM) depending on the [Ru] and temperature
k
obs/mol H2 mol
Entry
[Ru]/mM
T/◦C
DMAB-1 h-1
TOF/h-1
1
2
3
4
5
6
7
2.25
4.50
6.75
9.00
2.25
2.25
2.25
25
25
25
25
20
30
35
1.22 0.06
2.56 0.13
3.79 0.19
5.50 0.28
0.69 0.04
1.59 0.08
2.45 0.12
53
56
55
57
30
69
107
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Fig. 4 Plot of mol H2/mol DMAB vs. time (min) for the dehydrogenation
of dimethylamine–borane (55 mM) catalyzed by Ru(0)/APTS with
different ruthenium concentrations (2.25, 4.50, 6.75 and 9.00 mM) at 25
0.1 ◦C. Inset: plot of the dehydrogenation rate of dimethylamine–borane
vs. the ruthenium concentration (both in logarithmic scale; y = 0.176 +
1.06 x).
Fig. 6 Plot of mol H2/mol DMAB vs. time (min) for the dehydrogenation
of dimethylamine–borane (55 mM) catalyzed by Ru(0)/APTS (2.25 mM)
at different temperatures.
different temperatures (Fig. 6) were used for Arrhenius26 and
Eyring27 plots to calculate the activation parameters: activation
energy Ea = 61.1 3.1 kJ mol-1, activation enthalpy DH# = 57.9
2.9 kJ mol-1 and activation entropy DS# = -42.6 2.1 J mol-1·K.
The small value of the activation enthalpy and the large negative
value of the activation entropy indicate an associative mechanism
in the transition state for the catalytic dehydrogenation of DMAB.
3.5. Isolability and reusability of Ru(0)/APTS in the catalytic
dehydrogenation of DMAB
The isolability and reusability of Ru(0)/APTS, two crucial criteria
in heterogeneous catalysis, were also tested in the catalytic
dehydrogenation of DMAB. After complete dehydrogenation of
DMAB in the first run, Ru(0)/APTS was isolated as a black
powder by drying in vacuo and then bottled and stored under inert
atmosphere. Such isolated Ru(0)/APTS catalysts were found to
be readily redispersible in THF and active in the dehydrogenation
of DMAB. They retain 76% of their initial activity and provide
>99% of conversion at the sixth run in the dehydrogenation
of DMAB (Fig. 7a). More importantly, the complete release of
Fig. 5 Evolution of H2 (mmol) vs. time (min) for the Ru(0)/APTS
(2.25 mM) catalyzed dehydrogenation of DMAB at 25 0.1 ◦C starting
with initial concentrations of [DMAB] = 55, 110, 165, 220 and 330 mM.
Inset: plot of the dehydrogenation rate vs. DMAB concentration (both in
logarithmic scales).
the dehydrogenation is zero-order with respect to the substrate
concentration. Taking both results together, the rate law for the
Ru(0)/APTS-catalyzed dehydrogenation of DMAB can be given
as
d (CH ) HNBH
d ((CH ) NBH )
H
2
[
]
[
]
[
]
3
2
3
3
2
2
2
−
= +
= +
= k[Ru]
dt
dt
dt
In order to determine the activation parameters (Ea, DH# and
DS#), the catalytic reaction was carried out at various temperatures
in the range of 20–35 ◦C (Fig. 6). Ru(0)/APTS catalyze the
dehydrogenation of DMAB even at low temperature (20 ◦C) with
the complete generation of 1 equiv. of H2. The rate constants k
determined from the nearly linear portions of the plots at four
Fig. 7 The percentage of initial catalytic activity and conversion vs.
catalytic runs for the Ru(0)/APTS-catalyzed dehydrogenation of dimethy-
lamine–borane and a TEM image of the Ru(0)/APTS harvested from their
sixth reuse in the dehydrogenation of dimethylamine–borane.
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1 equiv. of hydrogen is achieved in each of the catalytic runs. The
decrease observed in the catalytic activity in the sixth run may be
attributed to the decrease in the number of active surface atoms
due to the increase of the size of ruthenium(0) nanoparticles as a
result of agglomeration. Indeed, a TEM image of Ru(0)/APTS
(Fig. 7b) after the sixth use shows an increase of the average
size to 4.2 0.7 nm. Although we observed a decrease in their
catalytic activity in subsequent runs, these results highlight that
Ru(0)/APTS are isolable, bottleable and reusable catalysts for
the dehydrogenation of DMAB, unlike the results of prior Rh(0)
nanocluster catalysts.17,18
3.6. Catalytic lifetime of Ru(0)/APTS in the catalytic
dehydrogenation of DMAB
A catalyst lifetime experiment (starting with 0.025 mmol of
Ru(0)/APTS and 7 mmol of DMAB at 25 0.1 ◦C) reveals a
record total turnover (TTO) value of 1240 in the dehydrogenation
of DMAB over 60 h (Fig. 8) before deactivation occurs due to
aggregation into bulk ruthenium. This TTO value is higher than
the one of the prior best catalyst displaying a TTO of 1040 at
room temperature.18 An initial apparent TOF value of 55 h-1 was
obtained; however, the average apparent TOF value was calculated
to be 21 h-1. The observation that the TOF value decreases as
the reaction proceeds indicates the deactivation of the catalyst,
evidenced also by the bulk metal precipitation at the end of the
experiment. The initial apparent TOF value 55 h-1 is comparable
Fig. 8 Total turnover number vs. time (h) graph for the catalytic lifetime
experiment starting with 0.025 mmol Ru(0)/APTS and excess amount of
dimethylamine–borane at 25 0.1 ◦C.
with that (TOF = 60 h-1) of the prior best heterogeneous catalyst18
but much higher than the majority of those of other heterogeneous
and homogeneous catalysts reported up to now (Table 3).
Comparative experiments revealed that the catalytic activities
of RuCl3, RuCl3·3H2O, [Ru(cod)Cl2]n and [Ru(cod)(cot)] in the
dehydrogenation of DMAB under the same reaction conditions
Table 3 Catalyst tested in the dehydrogenation of dimethylamine–borane tabulated from SciFinder literature search
Entry
Pre(catalyst)
mol%
T/◦C
t/h
Conv. (%)
TOF/h-1
Ref.
1
2
3
4
5
6
7
8
[Rh(1,5-cod)(m-Cl)]2
[Ir(1,5-cod)(m-Cl)]2
RhCl3
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
2.0
2.0
1.0
14
1.0
1.0
2.0
12.5
5.0
5.0
5.0
5.0
5.0
5.0
0.2
0.2
0.2
0.2
0.02
25
25
25
25
25
25
25
25
25
25
25
25
25
20
25
85
25
25
25
25
20
25
hv
hv
hv
25
25
25
25
25
25
25
8
136
23
64
160
44
112
8
112
160
16
160
68
4
100
95
90
90
25
95
100
95
95
5
100
20
95
100
90
99
100
100
100
100
< 100
100
95
12.4
0.7
7.9
2.8
0.3
4.3
0.9
12
1.7
0.1
12.4
0.2
2.8
12.3
8.2
25
420
2.6
60
1.5
7
7
7
7
7
7
7
7
7
7
7
7
7
10
12
11
9
13
18
12
14
15
16
16
RhCl3·3H2O
IrCl3
RhCl(PPh3)3
[Cp*Rh(m-Cl)Cl]2
[Rh(1,5-cod)2]OTf
[Rh(1,5-cod)(dmpe)]PF6
HRh(CO)(PPh3)3
trans-RuMe2(PMe3)4
trans-PdCl2(P(o-tolyl)3)2
Pd/C
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
Cp2Ti
Rh(0)/[Noct4]Cl
6
4
1
19
[ReBr2(NO)(PiPr3)2(CH3CN)]
5
[(h -C5H3-1,3-(SiMe3)2)2Ti]2
[RhCl(PHCy2)3]
Rh(0) NPs
2.5
[Ru(H)(PMe3)(N(C2H4PiPr2)2]
(Idipp)CuCl
28
24
NDa
1
1
1
< 0.3
34
[Rh(PiBu3)2][BArF
[Cr(CO)6]
]
4
19.6
18.5
17.4
13.4
19.9
2.7
[Mo(CO)6]
[W(CO)6]
90
84
97
97
16
16
16
[Cr(CO)5(thf)]
1.5
1
[Cr(CO)5(h -BH3NMe3)]
1
3
3
3
3
2
RuCl3·3H2O
77
This work
This work
This work
This work
This work
RuCl3
NCb
70
0
2.5
[Ru(1,5-cod)Cl2]n
[Ru(1,5-cod)(1,3,5-cot)]
Ru(0)/APTS
40
100
1.6
55/688c
a ND = not demonstrated. b NC = no conversion. c A corrected TOF value by considering the exact CS2/total ruthenium ratio.
596 | Dalton Trans., 2012, 41, 590–598
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provided negligible activity compared to that of Ru(0)/APTS
(Table 3).
4. Conclusions
The main findings of this work as well as implications or
predictions can be summarized as follows.
3.7. Quantitative CS2 poisoning of Ru(0)/APTS in the
dehydrogenation of DMAB
(i) Ru(0)/APTS were reproducibly prepared from the decom-
position of [Ru(cod)(cot)] by dihydrogen in the presence of 3-
aminopropyltriethoxysilane ligand as stabilizer in THF at room
temperature. This procedure enabled us to control the size of
the resulting ruthenium(0) nanoparticles by only varying of the
[APTS]/[Ru] molar ratio.
(ii) It was demonstrated that the mean size of ruthenium(0)
nanoparticles and the degree of their surface coverage by APTS
ligand competitively control their catalytic activity in the dehydro-
genation of DMAB at room temperature.
(iii) Kinetic studies on varying catalyst and substrate con-
centrations reveal that room-temperature dehydrogenation of
DMAB catalyzed by Ru(0)/APTS is first-order with respect to
Ru(0)/APTS and zero-order with respect to DMAB concentra-
tions.
(iv) The temperature dependence of hydrogen generation rate
from the Ru(0)/APTS-catalyzed dehydrogenation of DMAB was
also investigated by performing this catalytic reaction at different
temperatures. First of all the results indicate that Ru(0)/APTS
catalyst shows high activity in the dehydrogenation of DMAB
even at low temperature (20 ◦C). The activation parameters (Ea =
61.1 3.1 kJ mol-1, activation enthalpy DH# = 57.9 2.9 kJ mol-1
and activation entropy DS# = -42.6 2.1 J mol-1 K) indicate that
Ru(0)/APTS-catalyzed dehydrogenation of DMAB proceeds via
an associative mechanism.
It is crucial in catalysis to determine the true number of
catalytically active surface sites; to know (i) the true turnover
frequency (TOF, i.e., the moles of product/(moles of active
metal atoms ¥ time)); (ii) the true total turnover (TTO, i.e.,
the moles of product/moles of active metal atoms); (iii) for
certain whether a better catalyst has been made on a per-active-
metal-atom basis. To answer these questions a series of poison-
ing experiments were performed for the Ru(0)/APTS-catalyzed
dehydrogenation of DMAB by using carbon disulfide (CS2) as
poison.31
The dehydrogenation of DMAB catalyzed by Ru(0)/APTS was
conducted in the presence of carbon disulfide in various [CS2]/[Ru]
molar ratio at 25.0 0.1 ◦C. The initial rate of Ru(0)/APTS-
catalyzed dehydrogenation of DMAB was determined for each
independent experiment at various [CS2]/[Ru] ratios and divided
by the rate obtained in the absence of carbon disulfide at 25.0
0.1 ◦C to obtain the relative activity (initial rate of DMAB dehy-
drogenation in the presence of poison over the one in the absence
of poison). The relative activity vs. [CS2]/total [Ru(0)] plot in Fig.
9 for the 3-aminopropyltriethoxysilane stabilized ruthenium(0)
nanoparticles provide at least three important findings. First, it
demonstrates that CS2 can act as a powerful catalyst poison for
soluble ruthenium(0) nanoparticles. Second, only a small amount
of CS2 (0.016 equiv. for the total Ru present) inhibit >90% of the
catalytic activity of the Ru(0) nanoparticles, indicating that it is
a heterogeneous catalysis (requirement of less than 1 equivalent
of CS2).32 Third, the true TTO and TOF values for ruthenium(0)
nanoparticles are a factor of 1/(0.016 ¥ 5) = 12.5 higher than the
apparent lower limit TTO of 1240 and TOF of 55 h-1. Thus, the
TOF and TTO values of Ru(0)/APTS increase to 688 h-1 and
15500 turnovers of the dehydrogenation of DMAB per true active
site at 25 0.1 ◦C, respectively.
(v) Testing the isolability and reusability of Ru(0)/APTS showed
that the isolated and bottled sample of Ru(0)/APTS catalyst from
the fifth use are readily dispersible in THF and remain active in the
dehydrogenation of dimethylamine–borane. They retain 76% of
their initial catalytic activity even at the sixth run with the complete
conversion of Me2NHBH3 to [Me2NBH]2 plus 1 equiv. of H2 at
room temperature. On that account, to the best of our knowledge,
Ru(0)/APTS is the first example of an isolable, bottleable and
reusable heterogeneous nanocatalyst for the dehydrogenation of
dimethylamine–borane.
(vi) Moreover, the catalytic lifetime experiment showed that
Ru(0)/APTS catalyst is the longest-lived catalyst with TTO value
of 1240 over 60 h before deactivation in the room temperature
dehydrogenation of dimethylamine–borane.
(vii) The quantitative poisoning experiments by using CS2 as
poison for Ru(0)/APTS imply that the true TOF and TTO values
for the Ru(0) nanoparticles are a factor of 12.5 times higher than
the apparent (lower limit) TOF of 55 h-1 (corrected TOF = 688
h-1) and (already record) TTO of 1240 (corrected TTO = 15500).
These results clearly indicate that Ru(0)/APTS nanocatalyst is a
more active and longer-lived catalyst than the previously reported
best homogeneous9 and heterogeneous18 catalysts.
Acknowledgements
M. Z. thanks DPT-OYP program on the behalf of Yuzuncu
Yıl University (Van-Turkey), CNRS, EU through Idecat REX
and all the members of the “Nanostructures and Organometallic
Chemistry” research team in the LCC/CNRS in Toulouse.
Fig. 9 Plot of relative activity retained vs. mol CS2/mol total Ru ratio for
the Ru(0)/APTS-catalyzed dehydrogenation of dimethylamine–borane at
25 0.1 ◦C in the presence of various equiv. of CS2 poison.
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598 | Dalton Trans., 2012, 41, 590–598
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©