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Oleylamine-stabilized ruthenium(0) nanoparticlescatalyst in dehydrogenation of dimethylamine-borane

Sibel Duman a, Saim Ozkar b,*aDepartment of Chemistry, Bingol University, 12000 Bingol, TurkeybDepartment of Chemistry, Middle East Technical University, 06800 Ankara, Turkey

a r t i c l e i n f o

Article history:

Received 26 February 2013

Received in revised form

19 May 2013

Accepted 21 May 2013

Available online 2 July 2013

Keywords:

Oleylamine stabilizer

Ruthenium nanoparticles

Catalysis

Dehydrogenation

Dimethylamine-borane

Heterogeneous catalyst

* Corresponding author. Tel.: þ90 312 210 32E-mail address: sozkar@metu.edu.tr (S. O

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.05.1

a b s t r a c t

Oleylamine-stabilized ruthenium(0) nanoparticles were in situ generated from the reduc-

tion of ruthenium(III) chloride by dimethylamine-borane during its dehydrogenation at

room temperature. Nearly monodispersed ruthenium(0) nanoparticles of 1.8 � 0.7 nm

size were reproducibly isolated from the reaction solution by filtration and characterized

by TEM, XRD, HRTEM, 11B NMR, ATR-IR and UVevisible spectroscopy. Oleylamine-

stabilized ruthenium(0) nanoparticles are highly active catalyst in hydrogen generation

from dimethylamine-borane providing a release of 1.0 equivalent H2 per mole of

dimethylamine-borane and an initial turnover frequency of 137 (mol H2) (mol Ru)�1 (h)�1 at

25.0 � 0.5 �C. By considering the activity and stability of ruthenium(0) nanoparticles, the

optimum ratio of stabilizer to the catalyst was found to be 3.0. Oleylamine-stabilized

ruthenium(0) nanoparticles with a stabilizer to ruthenium ratio of 3.0 are stable and

reusable catalyst providing 20,660 turnovers in hydrogen generation from dimethylamine-

borane at 25.0 � 0.5 �C. They preserve 75% of their initial catalytic activity even after the

fifth run of dehydrogenation of dimethylamine-borane with the complete conversion of

Me2NHBH3 to [Me2NBH2]2 plus 1 equivalent of H2 at room temperature. The report also

includes the detailed kinetic study of the dehydrogenation of dimethylamine-borane

catalyzed by oleylamine-stabilized ruthenium(0) nanoparticles depending on the catalyst

concentration, substrate concentration, and temperature as well as the activation pa-

rameters of catalytic reaction calculated from the kinetic data. The poisoning experiments

showed that the dehydrogenation of dimethylamine-borane catalyzed by ruthenium(0)

nanoparticles is heterogeneous catalysis.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction growing interest for the development of hydrogen storage

The safe and efficient storage of hydrogen is the key in the

hydrogen based energy policies [1,2]. There has been rapidly

12; fax: þ90 312 210 3200.zkar).2013, Hydrogen Energy P19

materials with high volumetric and gravimetric capacity [3].

Boronenitrogen compounds such as NH3BH3 [4], NR3BH3 [5],

NH3B3H7 [6], NH4B3H8 [7], N2H4BH3 [8], have been considered

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 0 0 0e1 0 0 1 1 10001

as solid hydrogen storage materials as they have high

gravimetric hydrogen storage capacity and inclination for

bearing protic (NeH) and hydridic (BeH) hydrogen, which can

be discharged and recharged in different chemical processes

[9].More importantly, recent reports related to the regenera-

tion of dehydrogenation products reveal the importance of

the catalytic dehydrogenation of amine-borane adducts

[10,11].Of particular importance, dimethylamine-borane

((CH3)2NHBH3, DMAB) [12],which has been considered as

solid hydrogen storage materials [13,14], can release

hydrogen either by hydrolysis in aqueous solution [15,16] or

dehydrogenation in organic medium [17,18]. Recent studies

show that the catalytic dehydrocoupling of dimethylamine-

borane potentially releases up to 3.5 wt% H2 (Equation (1))

[19,20].

2 Me2HNBH3

Me2N BH2

H2B NMe2

2H2catalyst

(1)

A number of catalysts have recently been developed to

improve the rate of H2 elimination from dimethylamine-

borane: Rhodium colloids or complexes [21e29], rhodium(0)

nanoparticles [18,30,31], ruthenium(0) nanoparticles or com-

plexes [32e34], rhenium complexes [35], nickel complexes

[36,37], titanocene compounds [38e41], titanium and zirco-

nium sandwich complexes [40,42,43], metal carbonyls

[44].While the highest catalytic activity has been achieved by

using homogeneous [h5-C5H3-1,3-(SiMe3)2Ti]2 catalyst [43] in

dehydrogenation of dimethylamine-borane, herein we report

a semi heterogeneous ruthenium(0) nanoparticles catalyst

with the highest activity and longest life-time in the same

reaction at room temperature. Ruthenium(0) nanoparticles

were in situ formed from the reduction of ruthenium(III)

chloride by dimethylamine borane and stabilized by oleyl-

amine (OAm). This is the first example of using OAm as

stabilizer for the ruthenium(0) nanoparticles and employing

them as catalysts in the dehydrogenation of dimethylamine-

borane. The OAm-stabilized ruthenium(0) nanoparticles

show catalytic activity higher than the heterogeneous cata-

lysts reported for the dehydrogenation of dimethylamine-

borane [32,33].They provide an initial turnover frequency of

137 h�1 in generation of 1 equivalent H2 per mole of dime-

thylamine-borane (Me2NHBH3) which is converted to cyclic

aminoborane ([Me2NBH2]2). Our report also includes the

results of kinetic study on the hydrogen generation from the

dehydrogenation of dimethylamine-borane catalyzed by

OAm-stabilized ruthenium(0) nanoparticles depending on

the catalyst concentration, substrate concentration, and

temperature as well as the activation parameters (Ea, DH# and

DS#) of catalytic dehydrogenation of dimethylamine-borane

calculated from the kinetic data. Further experiments per-

formed to determine the catalytic lifetime showed that OAm-

stabilized ruthenium(0) nanoparticles provide 20,660 turn-

overs in hydrogen generation from the dehydrogenation

of dimethylamine-borane at room temperature. Moreover,

the OAm-stabilized ruthenium(0) nanoparticles exhibit high

durability throughout their catalytic use in the dehydroge-

nation reaction against agglomeration and previously

unprecedented reusability in the dehydrogenation of dime-

thylamine-borane.

2. Experimental

2.1. General and materials

All commercially obtained chemicals were used as received

unless indicated otherwise. Carbon disulfide (CS2), oleylamine

(cis-1-amino-9-octadecene, OAm), ruthenium(III) chloride

(RuCl3), dimethylamine-borane and toluene were purchased

from SigmaeAldrich�. Toluene was distilled over sodium

under nitrogen atmosphere for 12 h. 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.

2.2. Equipment

The dehydrogenation of dimethylamine-borane was per-

formed under argon or nitrogen atmosphere and the reaction

was followed by measuring the hydrogen gas generated by

using the experimental setup described elsewhere [45], which

consists of a 50mL jacketed reaction flask containing a Teflon-

coated stir bar, placed on a magnetic stirrer (IKA� C-Mag HS7)

and thermostated to 25 � 0.5 �C by circulating water through

its jacket from a constant temperature bath (Polyscience

12107-15 water bath). A graduated glass tube (50 cm in height

and 2.5 cm in diameter) filled with water was connected to the

reaction flask tomeasure the volume of the hydrogen gas to be

evolved from the reaction. Measuring the total turnover

number (TTO) was performed under argon or nitrogen atmo-

spheres in a stirred reactor with a circulating water-bath for

constant temperature control.

2.3. Characterization of OAm-stabilized ruthenium(0)nanoparticles and dehydrogenation products

The samples used for the TEM experiments were harvested

from the in situ generation of OAm-stabilized ruthenium(0)

nanoparticles solution as described above: 0.5 mL aliquot of

OAm-stabilized ruthenium(0) nanoparticles solution in

toluene was transferred into a clean screw-capped glass vial

with a disposable polyethylene pipette. The colloidal solu-

tion was deposited on the silicon oxide coated copper TEM

grid by immersing the grid into the solution for 5 s and then

evaporating the volatiles from the grid under inert gas at-

mosphere. This sample on the grid was then sealed under N2

and analyzed with a JEM-2010F (JEOL) TEM instrument

operating at 200 kV. The samples were examined at mag-

nifications between 100 and 400 k. The particle size of

nanoparticles was calculated directly from the TEM image

by counting non-touching particles. Size distributions were

quoted as the mean diameter and the standard deviation. X-

ray diffraction pattern (XRD) was recorded on a Rigaku

Ultima-IV diffractometer with CuKa (l ¼ 1.54051 �A), over a 2q

range from 5 to 90� at room temperature. UVevisible elec-

tronic absorption spectra of precursor ruthenium salt and

OAm-stabilized ruthenium(0) nanoparticles were recorded

in non-aqueous solution on Shimadzu 1800 double beam

spectrophotometer. In order to check the conversion of

Me2NHBH3 to [Me2NBH2]2 at the end of the dehydrogenation

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 0 0 0e1 0 0 1 110002

reaction, the resulting solutions were filtered and the fil-

trates were collected for 11B NMR analysis.11B NMR spectra

were recorded on a Bruker Avance DPX 400 with an

operating frequency of 128.15 MHz for 11B. The infrared

spectrum was recorded from a Perkin Elmer A 100ATR/FTIR

spectrometer.

2.4. In situ generation OAm-stabilized ruthenium(0)nanoparticles and concomitant dehydrogenation ofdimethylamine-borane

All reactions and manipulations were performed under dry

nitrogen atmosphere using standard Schlenk techniques

including a vacuum system unless otherwise specified. Both

the in situ generation of ruthenium(0) nanoparticles stabilized

by oleylamine, and the concomitant dehydrogenation of

dimethylamine-borane were performed in a typical jacketed

reaction flask connected to the water-filled cylinder glass

tube. The jacketed reaction flask was vacuumed at least for

15 min and filled with nitrogen to remove any trace of oxygen

and water present before all the catalytic reactions. The cat-

alytic activity of OAm-stabilized ruthenium(0) nanoparticles

in the dehydrogenation of dimethylamine-borane was deter-

mined by measuring the rate of hydrogen generation. A stock

solution of 6.0 mM of oleylamine was prepared by dissolving

0.06mLOAm (MW¼ 267.50 gmol�1, d¼ 0.813 gmL�1) in 5.0mL

toluene. Next, 2.07mg (0.01mmol) ruthenium(III) chloridewas

dissolved in an aliquot of the OAm stock solution that is

diluted to desired volume by toluene for obtaining the

OAmeRu mixture at the molar ratio in the range of 1.0e5.0.

Then, 1.0 mL of as-prepared OAmeRu mixture solution was

injected via gastight syringe into the jacketed reaction flask

containing a solution of 59 mg (1.0 mmol) dimethylamine-

borane dissolved in 4.0 mL toluene under vigorous stirring at

25.0 � 0.5 �C. The abrupt color change to dark brown indicates

the reduction of ruthenium(III) ions to ruthenium(0) nano-

particles. Hydrogen gas evolution started after an induction

period of 5e30 min. Hydrogen gas generation from the cata-

lytic reaction solution was followed by using a typical water-

filled gas burette system by recording the displacement of

water level in the gas burette every minute until no more

hydrogen evolution observed. When no more hydrogen gen-

eration was observed, the experiment was stopped, the

reactor was disconnected from the water-filled tube and the

hydrogen pressure was released. Next, an approximately

0.5 mL aliquot of the reaction solution in the reactor was

withdrawn with glass Pasteur pipette and added to 1 g of

CDCl3 in a quartz NMR sample tube (Norell S-500-QTZ), which

was subsequently sealed. The 11B-NMR spectrum of this so-

lution showed that Me2NHBH3 is completely converted to cy-

clic product ([Me2NBH2]2). Additionally, no bulk metal

formation was observed during the catalytic dehydrogenation

reaction and the nanoparticles were very stable in toluene

solution even for months.

2.5. Effect of oleylamine concentration on the catalyticactivity of ruthenium(0) nanoparticles

In order to study the effect of OAm concentration on the

catalytic activity of ruthenium(0) nanoparticles in the

dehydrogenation of dimethylamine-borane (100.0 mM), cata-

lytic activity tests were performed at 25.0� 0.5 �C startingwith

various concentrations of OAm (1.0, 2.0, 3.0, 4.0 and 5.0mM) at

constant ruthenium(III) chloride concentration (1.0mM) in the

ruthenium(0) nanoparticles. In all experiments, the total vol-

ume of the solution was kept constant at 5.0 mL. All the ex-

periments were performed in the same way as described in

the Section 2.4. The good stability and the highest activity of

OAm-stabilized ruthenium(0) nanoparticles in the dehydro-

genation of dimethylamine-borane were achieved at the OAm

to ruthenium ratio of 3. Thus, [OAm]/[Ru] ratio of 3 was

selected for the further experiments.

2.6. Catalytic activity of OAm stabilized ruthenium(0)nanoparticles in the dehydrogenation of dimethylamine-borane

In order to establish the rate law for catalytic dehydrogenation

of dimethylamine-borane using in situ-generated OAm-

stabilized ruthenium(0) nanoparticles, three different sets of

experiments were performed for each of these catalysts in the

same way as described in Section 2.4.

Kinetics of the dehydrogenation of dimethylamine-borane

catalyzed by in situ-generated OAm-stabilized ruthenium(0)

nanoparticles were studied depending on substrate concen-

tration, catalyst concentration and the temperature. In a set of

experiments, dimethylamine-borane concentration was held

constant at 200mMand RuCl3 concentration plus 3 equivalents

of oleylamine per rutheniumwas varied in the range of 1.0, 1.5,

2.0, 2.5 and 3.0 mM at 25.0 � 0.5 �C. The hydrogen generation

was measured for each set by recording the water level in a

graduated glass tube, which is connected to the reaction flask,

in every 5 min. In the second set of experiments, RuCl3 and

oleylamine concentrations were held constant at 2.0 and

6.0 mM, while dimethylamine-borane concentration was var-

ied in the range of 100, 150, 200, 250 and 300mMat 25.0� 0.5 �C.In the third set of experiments, the catalytic dehydrogenation

of dimethylamine-borane with RuCl3 concentration of 2.0 mM

plus 3 equivalents of oleylamine per rutheniumwas performed

by keeping dimethylamine-borane concentration constant at

200 mM at various temperatures in the range of 20, 25, 30, 35

and 40 �C in order to obtain the activation energy (Ea), enthalpy

of activation (DH#) and entropy of activation (DS#). The pressure

versus time data were processed using Microsoft Office

Excel 2007 and Origin 8.0 and then converted into the values in

the proper unit, the volume of hydrogen (mL).

2.7. Catalytic lifetime of OAm-stabilized ruthenium(0)nanoparticles in dehydrogenation of dimethylamine-borane

The catalytic lifetime of OAm-stabilized ruthenium(0) nano-

particles in the dehydrogenation of dimethylamine-borane

was determined by measuring the total turnover number

(TTO). Such a lifetime experiment was started with a 5.0 mL

solution containing 2.0 mM RuCl3 plus 3 equivalents of oleyl-

amine per ruthenium and 200 mM dimethylamine-borane at

25.0� 0.5 �C.When the complete conversion is achieved,more

dimethylamine-borane was added and the reaction was

continued in this way until no hydrogen gas evolution was

observed.

Fig. 1 e UV UVevis spectra of solutions containing

ruthenium(III) chloride in the presence of OAm in toluene

(a) before and (b) after its injection into the DMAB solution

([DMAB] [ 200 mM; [Ru] [ 2.0 mM, [OAm] [ 6.0 mM).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 0 0 0e1 0 0 1 1 10003

2.8. Isolability and reusability of in situ-generatedOAm-stabilized ruthenium(0) nanoparticles in thedehydrogenation of dimethylamine-borane

At the end of dehydrogenation reaction of 1.0 mM of in situ

formation OAm stabilized ruthenium(0) nanoparticles having

an OAm/Ru molar ratio of 3 in 5.0 mL of 100.0 mM dimethyl-

amine-borane solution at 25.0 � 0.5 �C was transferred into a

new Schlenk tube which was sealed under dry nitrogen at-

mosphere. 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-Toluene showed that the surface of ruthenium(0)

nanoparticles are free from cyclic dehydrogenation product

([Me2NBH2]2). This isolated colloid was thus weighed and

redispersed in toluene and re-used in catalysis under the

same conditions as in the first run.

2.9. CS2 poisoning as heterogeneity test for in situ-generated OAm-stabilized ruthenium(0) nanoparticles

The ability of CS2 to poison metal-particle heterogeneous

catalyst by adsorbing on the metal surface has been proposed

in reducing reaction medium. In a typical poisoning experi-

ment, 1.0 mM of in situ formation OAm-stabilized ruthe-

nium(0) nanoparticles having an OAm/Ru molar ratio of 3 in

5.0 mL of 100.0 mM dimethylamine-borane solution at

25.0 � 0.5 �C was poisoned by adding 0.1 equivalent CS2 per

ruthenium solution prepared in toluene after the 50% con-

version. The catalytic activity was measured by monitoring

the rate of hydrogen generation before and after the addition

of CS2.

Fig. 2 e A typical mol H2/mol DMAB versus time plot for the

catalytic dehydrogenation of DMAB starting with RuCl3 in

the presence of OAm in 5.0 mL Toluene at 25.0 ± 0.5 �C([DMAB] [ 200 mM; [Ru] [ 2.0 mM, [OAm] [ 6.0 mM).

3. Results and discussion

3.1. In situ formation of oleylamine-stabilizedruthenium(0) nanoparticles and concomitant catalyticdehydrogenation of dimethylamine-borane

Formation of ruthenium(0) nanoparticles from the reduction

of ruthenium(III) chloride by dimethylamine-borane and

dehydrogenation of dimethylamine-borane occur concomi-

tantly in the same reactor. In a typical experiment, for

example, performed by starting with 2.0 mM ruthenium(III)

chloride, 6.0 mM oleylamine and 200 mM dimethylamine-

borane in 5.0 mL toluene at room temperature, the color of

solution changes from orange to dark brown within less than

15 min, indicating the formation of ruthenium(0) nano-

particles. The UVevis electronic absorption spectroscopy can

be used to follow the reduction of ruthenium(III) ions to

ruthenium(0) nanoparticles. Fig. 1 shows the UVevis spectra

of solution containing ruthenium(III) chloride in the presence

of OAm in toluene before and after the reaction with dime-

thylamine-borane. The UVevis spectrum of ruthenium(III)

chloride in the presence of OAm in toluene exhibits two ab-

sorption bands at 350 and 497 nm, attributable to the charge

transfer and ded transition (Fig. 1). After reduction, these

bands of ruthenium(III) ions disappear and one observes a

typical Mie exponential decay profile for the ruthenium(0)

nanoparticles [46]. The formation of ruthenium(0) nano-

particles and catalytic dehydrogenation of dimethylamine-

borane can be followed by monitoring the volume of

hydrogen gas generated. Fig. 2 shows the plot of equivalent H2

per mol of DMAB versus time during the dehydrogenation of

DMAB started with 2.0 mM ruthenium(III) chloride, 6.0 mM

oleylamine and 200 mM dimethylamine-borane in 5.0 mL

toluene at room temperature. After an induction period of

15 min, a fast hydrogen evolution starts and continues in the

form of a nearly sigmoidal curve until the complete conver-

sion of DMAB to the cyclic aminoborane, whereby 1.0 equiv-

alent of H2 per mol of DMAB is released (Equation (1)). This

indicates that ruthenium(0) nanoparticles in situ formed from

the reduction of ruthenium(III) chloride by DMAB are active

catalyst in dehydrogenation of DMAB. The turnover frequency

of catalytic dehydrogenation was measured to be 137 h�1 at

25 � 0.5 �C. This TOF value is one of the highest ever reported

Table 1 e Catalysts tested in the dehydrogenation of dimethylamine-borane.

Entry (Pre)Catalyst Conditions Equiv. of H2 TOF Ref.

1 [Rh(1,5-cod)(m-Cl)]2 0.5 mol%, 25 �C, 8 h 1.00 12.4 [19]

2 [Ir(1,5-cod)(m-Cl)]2 0.5 mol%, 25 �C, 136 h 0.95 0.7 [19]

3 RhCl3 0.5 mol%, 25 �C, 23 h 0.90 7.9 [19]

4 RhCl3$3H2O 0.5 mol%, 25 �C, 64 h 0.90 2.8 [19]

5 IrCl3 0.5 mol%, 25 �C, 160 h 0.25 0.3 [19]

6 RhCl(PPh3)3 0.5 mol%, 25 �C, 44 h 0.95 4.3 [19]

7 [Cp*Rh(m-Cl)Cl]2 0.5 mol%, 25 �C, 112 h 1.00 0.9 [19]

8 [Rh(1,5-cod)2]OTf 0.5 mol%, 25 �C, 8 h 0.95 12 [19]

9 [Rh(1,5-cod)(dmpe)]PF6 0.5 mol%, 25 �C, 112 h 0.95 1.7 [19]

10 HRh(CO)(PPh3)3 0.5 mol%, 25 �C, 160 h 0.05 0.1 [19]

11 trans-RuMe2(PMe3)4 0.5 mol%, 25 �C, 16 h 1.00 12.4 [19]

12 trans-PdCl2(P(o-tolyl)3)2 0.5 mol%, 25 �C, 160 h 0.20 0.2 [19]

13 Pd/C 0.5 mol%, 25 �C, 68 h 0.95 2.8 [19]

14 Cp2Ti 2.0 mol%, 20 �C, 4 h 1.00 12.3 [40,41]

15 Rh(0)/[Noct4]Cl 2.0 mol%, 25 �C, 6 h 0.90 8.2 [34]

16 [ReBr2(NO)(PiPr3)2(CH3CN)] 1.0 mol%, 85 �C, 4 h 0.99 25 [35]

17 [(h5-C5H3-1,3-(SiMe3)2)2Ti]2 14 mol%, 25 �C, 1 h 1.00 420 [43]

18 [RhCl(PHCy2)3] 1.0 mol%, 25 �C, 19 h 1.00 2.6 [26]

19 Rh(0) NPs 1.0 mol%, 25 �C, 2.5 h 1.00 60 [18]

20 [Ru(H)(PMe3)(N(C2H4PiPr2)2] 2.0 mol%, 25 �C, 28 h 1.00 1.5 [34]

21 [Cr(CO)6] 5.0 mol%, hv, 1 h 0.95 19.6 [44]

22 [Mo(CO)6] 5.0 mol%, hv, 1 h 0.90 18.5 [44]

23 [W(CO)6] 5.0 mol%, hv, 1 h 0.84 17.4 [44]

24 [Cr(CO)5(thf)] 5.0 mol%, 25 �C, 1.5 h 0.97 13.4 [44]

25 [Cr(CO)5(h1-BH3NMe3)] 5.0 mol%, 25 �C, 1 h 0.97 19.9 [44]

26 Ru(0)/APTS 0.02 mol%, 25 �C, 2 h 1.00 55 [32,33]

27 OAm-stabilized Ru(0) NPs 0,01 mol%, 25 �C, 1.5 h 1.00 137 This study

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 0 0 0e1 0 0 1 110004

for the catalyst systems in the dehydrogenation of dimethyl-

amine-borane (see Table 1).

Progress of the catalytic dehydrogenation of DMAB

was also followed by taking the 11B-NMR and ATR-FTIR

spectra. Fig. 3 shows the 11B-NMR spectra taken from the re-

action solution at the beginning and end of the catalytic

dehydrogenation of DMAB. The quartet at �15.0 ppm for

Me2HNBH3 is replaced by a triplet at 5.0 ppm for the cyclic

product [Me2NBH2]2 at the end of reaction. This indicates that

dimethylamine-borane Me2HNBH3 is completely converted to

the cyclic aminoborane [Me2NBH2]2 (Equation (1)). Comparing

the FTIR spectra in Fig. 4 taken before and after catalytic re-

action also confirm the complete conversion of DMAB to the

cyclic aminoborane. Themost striking change in the spectrum

is the disappearance of the absorption band at 3204 cm�1 due

Fig. 3 e 11B-NMR spectra of the DMAB and the catalytic dehydrog

stabilized ruthenium(0) nanoclusters in 5 mL Toluene at 25.0 ± 0

to NeH stretching of DMAB after reaction and concomitant

appearance of a new absorption band at 1510 cm�1 which is

attributed to the BeN stretching in the cyclic product. Another

evidence for the complete conversion of DMAB to the cyclic

aminoborane dimer is the disappearance of the absorption

band at 1150 cm�1 due to NeH bending in DMAB after the

reaction. It is noteworthy that the absorption bands at 2371

and 2258 cm�1 for BeH stretching in DMAB observed in the

initial IR spectrum are shifted to 2361e2331 cm�1 and change

the pattern after dehydrogenation reaction, consistent with

the prior reports [18,20,34].

During the catalytic dehydrogenation of DMAB and after

the reaction, no bulk metal formation is observed, indicating

that the ruthenium(0) nanoparticles in situ formed from the

reduction of ruthenium(III) chloride are stable in solution.

enation products in the presence of in-situ generated OAm

.5 �C. ([DMAB] [ 200 mM; [Ru] [ 2.0 mM, [OAm] [ 6.0 mM).

Fig. 4 e ATR-FTIR spectra of OAm-stabilized Ru (0) NPs, DMAB and catalytic dehydrogenation products in the presence of in-

situ generated OAm stabilized ruthenium(0) nanoclusters in 5 mL Toluene at 25.0 ± 0.5 �C. ([DMAB] [ 200 mM;

[Ru] [ 2.0 mM, [OAm] [ 6.0 mM).

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Since the weakly coordinating chloride anion cannot provide

enough stabilization for themetal(0) nanoparticles, based on a

previous study ranking the anions in the order of their ability

to stabilize Ir(0) nanoparticles, whereby the chloride anion has

been found to be the weakest stabilizer [47], as the sole

Fig. 5 e (a) TEM image (b) HRTEM image and (c) the inset repres

ruthenium(0) nanoparticles harvested from the reaction solutio

stabilized ruthenium(0) nanoclusters in 5.0 mL of Toluene at 25

[OAm] [ 6.0 mM).

stabilizer present in the reaction medium oleylamine is ex-

pected to be responsible for the stabilization of ruthenium(0)

nanoparticles.

The OAm-stabilized ruthenium(0) nanoparticles can be

isolated from the reaction solution as dark brown solid by

enting particle size histogram of in-situ generated

n after dehydrogenation starting with DMAB plus OAm

.0 ± 0.5 �C ([DMAB] [ 200 mM; [Ru] [ 2.0 mM,

Fig. 7 e The plot of the hydrogen generation rate versus the

[OAm]/[Ru] ratio for the dehydrogenation of DMAB

catalyzed by OAm stabilized ruthenium(0) nanoclusters in

5.0 mL Toluene at 25.0 ± 0.5 �C ([DMAB] [ 100 mM;

[Ru] [ 1.0 mM).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 0 0 0e1 0 0 1 110006

removing the volatiles in vacuum and washing with ethanol.

The nanoparticles isolated can be redispersed in hexane or

toluene. When redispersed the nanoparticles are yet catalyti-

cally active in the dehydrogenation of dimethylamine-borane

(see later).

The morphology and particle size of OAm-stabilized ruthe-

nium(0) nanoparticles were studied by TEM. Fig. 5 shows the

TEM image taken from the sample of OAm-stabilized ruthe-

nium(0) nanoparticles prepared by redispersing dark brown

powder in hexane and the corresponding particle size histo-

gram which was constructed by counting 410 non-touching

particles. The average particle size of in situ generated, OAm-

stabilized ruthenium(0) nanoparticles was calculated from

the TEM images as 1.8 � 0.7 nm (Fig. 5c). This indicates the

capability of oleylamine in stabilizing the ruthenium(0) nano-

particles and redispersibility of ruthenium(0) nanoparticles in

different nonpolar organic solvents such as hexane.

Fig. 6 shows the powder XRD pattern of OAm-stabilized

ruthenium(0) nanoparticles in situ generated during the

dehydrogenation of DMAB. Single broad reflection centered at

2q ¼ 42� is attributed to (101) plane of a face centered cubic

(fcc) crystal structure of ruthenium [48]. A broadening

observed at Ru(101) diffraction is characteristic of materials

having a nanometer particle size [49]. The particle size of the

nanoparticles (1.9 nm) calculated by the Scherrer formula is

close to the average diameter (1.8 nm) determined from the

TEM image.

3.2. Kinetics of the dehydrogenation of dimethylamine-borane catalyzed by oleylamine-stabilized ruthenium(0)nanoparticles

To explore the kinetics of the dehydrogenation of dimethyl-

amine-borane catalyzed by in situ generated OAm-stabilized

ruthenium(0) nanoparticles, series of experiments were car-

ried out by varying the ratio of stabilizer to metal, the catalyst

concentration, the substrate concentration, and the reaction

temperature. Fig. 7 shows the plot of the hydrogen generation

rate versus the [OAm]/[Ru] ratio for the dehydrogenation of

dimethylamine-borane catalyzed by OAm-stabilized ruthe-

nium(0) nanoparticles at 25.0 � 0.5 �C. As clearly seen

from Fig. 7, the hydrogen generation rate increases with the

Fig. 6 e XRD pattern of OAm stabilized ruthenium(0)

nanoclusters generated in situ during the dehydrogenation

of DMAB.

increasing concentration of stabilizer up to the [OAm]/[Ru]

ratio of 3 and then decreases expectedly. By considering both

the catalytic activity and stability of the nanoparticles in the

dehydrogenation of dimethylamine-borane, the [OAm]/[Ru]

ratio of 3 was used as the optimum ratio for all the kinetic

experiments.

The OAm-stabilized ruthenium(0) nanoparticles are found

to be active catalysts in the dehydrogenation of dimethyl-

amine-borane at room temperature. Fig. 8a shows the plots of

mol H2 generated per mole of dimethylamine-borane versus

time during the catalytic dehydrogenation of 200 mM dime-

thylamine-borane solution in the presence of OAm-stabilized

ruthenium(0) nanoparticles starting with different catalyst

concentration in the range 1.0e3.0 mM at 25.0 � 0.5 �C. Therate of hydrogen generation was determined from the linear

portion of each plot for different catalyst concentration. The

hydrogen generation rate increases with increasing catalyst

concentration as expected. Fig. 8b shows the plot of hydrogen

generation rate versus ruthenium concentration, both in log-

arithmic scale. The line obtained has a slope of 0.78, indicating

that the dehydrogenation of dimethylamine-borane catalyzed

by OAm stabilized ruthenium(0) nanoparticles slightly de-

viates from the first order kinetics with respect to the catalyst

concentration.

The effect of substrate concentration on the dehydroge-

nation rate was also studied by performing a series of exper-

iments starting with different initial concentrations of

dimethylamine-borane while keeping the catalyst concen-

tration constant at 2.0 mM Ru and 3 equivalents oleylamine at

25.0 � 0.5 �C. Fig. 9a shows the volume of hydrogen generated

versus time plots depending on the substrate concentrations

at constant catalyst concentration. Plotting the hydrogen

generation rate versus substrate concentration, both in loga-

rithmic scale, gives a straight line with a slope of 0.59 (Fig. 9b)

indicating that the hydrogen generation from the catalytic

dehydrogenation of dimethylamine-borane in the presence

of OAm-stabilized ruthenium(0) nanoparticles is close to a

first order reaction with respect to the dimethylamine-

Fig. 8 e (a) The plots of mol H2/mol DMAB versus time during the catalytic dehydrogenation of DMAB solution in 5.0 mL

Toluene in the presence of in-situ generated OAm stabilized ruthenium(0)nanoclusters starting with different catalyst

concentration (1.0e3.0 mM) at 25.0 ± 0.5 �C. (b) The inset shows the plot of hydrogen generation rate versus ruthenium

concentration, both in logarithmic scale ([DMAB] [ 200 mM; [OAm]/[Ru] [ 3).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 0 0 0e1 0 0 1 1 10007

borane concentration. Consequently, the rate law for the

dehydrogenation of dimethylamine-borane catalyzed by

OAm-stabilized ruthenium(0) nanoparticles can be given as

Equation (2):

Rate ¼ kapp[Ru]0.78[(CH3)2NHBH3]0.59 (2) (2)

Finally, the dehydrogenation of dimethylamine-borane

catalyzed by OAm-stabilized ruthenium(0) nanoparticles was

carried out at various temperature in the range of 20e40 �Cstarting with an initial substrate concentration of 200 mM

dimethylamine-borane and an initial catalyst concentration

of 2.0 mM Ru plus 6.0 mM oleylamine. Fig. 10a shows the plots

of mol H2 generated per mole of dimethylamine-borane

versus time for the dehydrogenation of dimethylamine-

borane in the presence of OAm-stabilized ruthenium(0)

nanoparticles at five different temperatures. The values of

apparent rate constant kapp for the catalytic dehydrogenation

of dimethylamine-borane were calculated from the slope of

the linear part of each plot in Fig. 10a by using the rate law

given in Equation (2). These apparent rate constant values at

different temperature were used to calculate the activation

Fig. 9 e (a) The plots of mol H2/mol DMAB versus time during t

Toluene in the presence of in-situ generated OAm stabilized ruth

concentration (100e300 mM) at 25.0 ± 0.5 �C. (b) Inset shows th

concentration, both in logarithmic scale ([Ru] [ 2.0 mM, [OAm]

parameters for the catalytic dehydrogenation of

dimethylamine-borane in the presence of OAm-stabilized

ruthenium(0) nanoparticles catalyst: Arrhenius activation

energy, Ea.app ¼ 29 � 2 kJ mol�1 (Fig. 10b); activation enthalpy,

DH# ¼ 29 � 2 kJ mol�1; activation entropy,

DS# ¼ �188 � 9 J K�1 mol�1. The activation energy obtained for

the dehydrogenation of dimethylamine-borane catalyzed

OAm-stabilized ruthenium(0) nanoparticles is smaller than

the values reported in literature for the same reaction using

different catalysts [34].

OAm-stabilized ruthenium(0) nanoparticles, in situ gener-

ated from the reduction of ruthenium(III) chloride, appear to

be stable and long-lived catalyst in dehydrogenation of

dimethylamine-borane. The lifetime of catalyst was

measured by determining the total turnover number in

hydrogen generation from dimethylamine-borane provided

by OAm-stabilized ruthenium(0) nanoparticles. Such a life-

time experiment was performed starting with a 5.0 mL solu-

tion of ruthenium(III) chloride (2.0 mM Ru) plus 6.0 mM

oleylamine and 200 mM dimethylamine-borane at

25.0 � 0.5 �C. The results are illustrated in Fig. 11. The OAm-

he catalytic dehydrogenation of RuCl3 solution in 5.0 mL

enium(0) nanoclusters starting with different initial DMAB

e plot of hydrogen generation rate versus DMAB

[ 6.0 mM).

Fig. 10 e (a) The plots of mol H2/mol DMAB versus time for the dehydrogenation of DMAB in the presence of OAm stabilized

ruthenium(0) nanoclusters at five different temperatures in the range of 20e40 �C. (b) The inset shows the Arrhenius plot;

ln k versus 1/103 T ([DMAB] [ 200 mM; [Ru] [ 2.0 mM, [OAm] [ 6.0 mM).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 0 0 0e1 0 0 1 110008

stabilized ruthenium(0) nanoparticles provide 20,660 turn-

overs over 213 h in hydrogen generation from the dehydro-

genation of dimethylamine-borane at 25 � 0.5 �C before

deactivation. The initial apparent TOF value of 137 h�1 is

comparable with that of the previous best homogeneous

catalyst [43] but higher than the majority of those of other

heterogeneous and homogeneous catalysts reported up to

now (Table 1).

3.3. Isolability and reusability of oleylamine stabilizedruthenium(0) nanoparticles in the catalytic dehydrogenationof dimethylamine-borane

The isolability and reusability of OAm-stabilized ruthenium(0)

nanoparticles were also tested in the catalytic dehydrogena-

tion of dimethylamine-borane. After complete dehydrogena-

tion of dimethylamine-borane in the first run, OAm-stabilized

ruthenium(0) nanoparticles were isolated as a dark brown

powder by drying in vacuo and then bottled and stored under

inert atmosphere. Such isolated OAm-stabilized ruthenium(0)

nanoparticles were found to be readily redispersible in

toluene and active in the dehydrogenation of dimethylamine-

borane. They retain 75% of their initial activity and provide

>99% of conversion at the fifth run in the dehydrogenation of

Fig. 11 e Turnover number (TON) versus time plot for the

catalytic hydrogenation of DMAB starting with DMAB and

OAm stabilized ruthenium(0) nanoclusters in 5.0 mL

Toluene at 25.0 ± 0.5 �C ([DMAB] [ 200 mM; [Ru] [ 2.0 mM,

[OAm] [ 6.0 mM).

dimethylamine-borane (Fig. 12). More importantly, the com-

plete release of 1 equiv. of hydrogen per mole of

dimethylamine-borane is achieved in each of the catalytic

runs. The decrease observed in the catalytic activity in the

fifth run may be attributed to the decrease in the number of

active surface atoms due to the increase of the size of ruthe-

nium(0) nanoparticles as a result of agglomeration. Although

we observed a decrease in their catalytic activity in subse-

quent runs, these results show that OAm-stabilized ruthe-

nium(0) nanoparticles are isolable and reusable catalysts for

the dehydrogenation of dimethylamine-borane.

3.4. CS2 poisoning test for the heterogeneity ofdimethylamine-borane dehydrogenation catalyzed byoleylamine stabilized ruthenium(0) nanoparticles

The poisoning experiment is usually performed by adding CS2

to the solution during the catalytic reaction. The suppression

of catalysis upon addition of CS2 in the amount less than 1

Fig. 12 e The percentage of initial catalytic activity and

conversion versus catalytic runs for the OAm stabilized

ruthenium(0) nanoclusters catalyzed dehydrogenation of

dimethylamine-borane in 5.0 mL Toluene at 25.0 ± 0.5 �C([DMAB] [ 200 mM; [Ru] [ 2.0 mM, [OAm] [ 6.0 mM).

Fig. 13 e Plots of the volume of hydrogen generated versus

time for the catalytic dehydrogenation of DMAB starting

with OAm stabilized ruthenium(0) nanoclusters in 5.0 mL

Toluene at 25.0 ± 0.5 �C before and after addition of

0.1 equiv. of CS2 ([DMAB] [ 100 mM; [Ru] [ 1.0 mM,

[OAm] [ 3.0 mM).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 0 0 0e1 0 0 1 1 10009

equivalent per mole of catalyst is considered as a compelling

evidence for the heterogeneity of catalysis. Fig. 13 shows the

plots of volume of hydrogen gas generated versus time for the

catalytic dehydrogenation of dimethylamine-borane before

and after addition of CS2. As clearly seen from Fig. 13, the re-

action was entirely ceased by addition of 0.1 equiv. of CS2 per

Ru atom in a very short time which indicates that the OAm-

stabilized ruthenium(0) nanoparticles act as heterogeneous

catalyst in the dehydrogenation of dimethylamine-borane.

4. Conclusions

In summary, our study on the formation and characterization

of OAm-stabilized ruthenium(0) nanoparticles catalyst in the

dehydrogenation of dimethylamine-borane led to the

following conclusions and insights:

1. OAm-stabilized ruthenium(0) nanoparticles were repro-

ducibly formed during the dehydrogenation of dimethyl-

amine-borane starting with a commercially available

precursor.

2. That an increasing catalytic activity was observed after a

certain period of time (induction time) in each case is

indicative of the formation of OAm-stabilized ruthenium(0)

nanoparticles during the induction time. The induction

time for the formation of OAm-stabilized ruthenium(0)

nanoparticles catalyst decreases with the increasing

ruthenium plus oleylamine, dimethylamine-borane con-

centrations and temperature. On the contrary to this,

the catalytic activity during the induction time increases

with the increasing ruthenium plus oleylamine and

dimethylamine-borane concentrations and temperature.

3. The OAm-stabilized ruthenium(0) nanoparticles having an

average particle size of 1.8 � 0.7 nm were isolated from

reaction solution and characterized by TEM, XRD, HRTEM,

ATR-IR, 11B-NMR and UVevisible electronic absorption

spectroscopy.

4. To our knowledge, the OAm-stabilized ruthenium(0)

nanoparticles show the highest activity among heteroge-

neous ruthenium catalysts in hydrogen generation from

the dehydrogenation of dimethylamine-borane at room

temperature. Although the oleylamine-stabilized ruthe-

nium(0)nanoparticles have particle size similar to that

of 3-aminopropyltriethoxysilane-stabilized ruthenium(0)

nanoparticles previously reported, they have activity

almost double of the previously reported ones [32,33]. The

difference in activity of two ruthenium(0) nanoparticles

might be related to the availability of active sites on the

surface of nanoparticles for the substrate molecules as

two stabilizers have obviously different steric re-

quirements. The more voluminous triethoxysilyl group

can exert higher sterical hinderance than the less crowded

oleyl chain on the terminal eH2N and, thus, its coordina-

tion vicinity including the active ruthenium centers

around. In other words, 3-aminopropyltriethoxysilane can

hinder the accessibility of more active ruthenium centers

on the surface of nanoparticles for the substrate

molecules.

5. A detailed kinetic study of the dehydrogenation of dime-

thylamine-borane catalyzed by OAm-stabilized ruthe-

nium(0)nanoparticlesshowedthat it isnearlyfirstorderwith

respect to both catalyst concentration and the substrate

concentration. Activation energy for the dehydrogenation of

dimethylamine-borane in the presence of OAm-stabilized

ruthenium(0) nanoparticles (Ea ¼ 29 � 2 kJ mol�1), deter-

mined from the evaluation of kinetic data at various

temperatures, is smaller than most of the values reported

for the same reaction in the presence of other catalyst

systems.

6. Testing the isolability and reusability of OAm stabilized

ruthenium(0) nanoparticles showed that the isolated and

bottled sample of OAm-stabilized ruthenium(0) nano-

particles is readily dispersible in toluene and remains

active in the dehydrogenation of dimethylamine-borane.

They retain 75% of their initial catalytic activity even at the

fifth run with the complete conversion of Me2NHBH3 to

[Me2NBH2]2 plus 1 equiv. of H2 per mole of dimethylamine-

borane at room temperature. On that account, OAm-

stabilized ruthenium(0) nanoparticles are isolable and

reusable heterogeneous catalyst in the dehydrogenation of

dimethylamine-borane.

7. OAm-stabilized ruthenium(0)nanoparticles are long-lived

catalysts in the dehydrogenation of dimethylamine-

borane providing 20,660 turnovers before deactivation and

releasing 1 equivalent of H2 per mole of dimethylamine-

borane with an initial TOF of 137 h�1 at room tempera-

ture. These results clearly indicate that OAm-stabilized

ruthenium(0) nanoparticles are more active and longer-

lived catalyst than the previously reported best homoge-

neous [43] and heterogeneous [18] catalysts.

Conclusively, easy preparation, high catalytic activity and

long lifetime of the OAm-stabilized ruthenium(0)nano-

particles make them a promising candidate to be employed as

a catalyst in developing highly efficient portable hydrogen

generation systems using dimethylamine-borane as solid

hydrogen storage materials.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 0 0 0e1 0 0 1 110010

Acknowledgments

Partial support by Turkish Academy of Sciences and Bingol

University Scientific Research Projects Unit is gratefully

acknowledged.

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