Nanocrystalline metal organic framework (MIL-101) stabilized copper Nanoparticles: Highly efficient nanocatalyst for the hydrolytic dehydrogenation of methylamine borane

Article abstract of DOI:10.1016/j.ica.2018.08.056

The copper nanoparticles stabilized by nanocrystalline MIL-101 framework (Cu/nano-MIL-101) was reproducibly prepared by following double solvent method combined with liquid phase chemical reduction technique. The characterization of the resulting new mate

Full text of DOI:10.1016/j.ica.2018.08.056

Accepted Manuscript
Research paper
Nanocrystalline Metal Organic Framework (MIL-101) Stabilized Copper Nano-
particles: Highly Efficient Nanocatalyst for the Hydrolytic Dehydrogenation of
Methylamine Borane
Ismail Burak Baguc, Ilknur Efecan Ertas, Mehmet Yurderi, Ahmet Bulut,
Mehmet Zahmakiran, Murat Kaya
PII:
S0020-1693(18)30963-0
https://doi.org/10.1016/j.ica.2018.08.056
ICA 18456
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
22 June 2018
30 August 2018
30 August 2018
Please cite this article as: I.B. Baguc, I.E. Ertas, M. Yurderi, A. Bulut, M. Zahmakiran, M. Kaya, Nanocrystalline
Metal Organic Framework (MIL-101) Stabilized Copper Nanoparticles: Highly Efficient Nanocatalyst for the
Hydrolytic Dehydrogenation of Methylamine Borane, Inorganica Chimica Acta (2018), doi: https://doi.org/
10.1016/j.ica.2018.08.056
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Nanocrystalline Metal Organic Framework (MIL-101) Stabilized Copper
Nanoparticles: Highly Efficient Nanocatalyst for the Hydrolytic
Dehydrogenation of Methylamine Borane
Ismail Burak Baguc,a Ilknur Efecan Ertas,a Mehmet Yurderi,a Ahmet Bulut,a Mehmet Zahmakiran,a,*
Murat Kayab
a Nanomaterials and Catalysis Research Group, Department of Chemistry, Yuzuncu Yil University,
65080, Campus, Van, Turkey
b Department of Chemical Engineering and Applied Chemistry, Atilim University, 06836, Ankara, Turkey
*Corresponding author: Prof. Mehmet Zahmakiran; zmehmet@yyu.edu.tr ; www.nanomatcat.com

Abstract
The copper nanoparticles stabilized by nanocrystalline MIL-101 framework (Cu/nano-MIL-101)
was reproducibly prepared by following double solvent method combined with liquid phase chemical
reduction technique. The characterization of the resulting new material was done by using various
analytical techniques including ICP-OES, P-XRD, N2-adsorption-desorption, XPS, FE-SEM, SEM-EDX,
BFTEM and HAADF-STEM; the summation of their results reveals that the formation of well-dispersed
and very small sized (0.8 nm) copper nanoparticles within nanocrystalline MIL-101 framework. The
catalytic performance of Cu/nano-MIL-101 in terms of activity and stability was tested in the hydrolytic
dehydrogenation of methylamine borane (CH3NH2BH3), which has been considered as one of the
attractive materials for the efficient chemical hydrogen storage. Cu/nano-MIL-101 catalyzes the
hydrolytic dehydrogenation of methylamine borane with high activity (turnover frequency; TOF = 257
mol H2/mol Cu×h) and conversion (> 99 %) under air at room temperature. Moreover, these nano-MIL-
101 framework stabilized copper nanoparticles show great durability against to sintering and leaching,
which make Cu/nano-MIL-101 reusable nanocatalyst in the hydrolytic dehydrogenation of
methylamine-borane. Cu/nano-MIL-101 nanocatalyst retains 83 % of its inherent activity at complete
conversion even at 10th recycle in the hydrolytic dehydrogenation of methylamine borane.
Keywords: Methylamine borane; Metal organic framework; Copper; Dehydrogenation; Hydrogen.

1. Introduction
In the view of a desirable and drastic reduction of world-wide greenhouse gas emissions, the
utilization of sustainable energy sources are a mandatory option in the world energy consumption [1,
2]. In the context of new advanced energy conversion technologies, since the 1990’s a strong interest
from the international scientific and industrial community has been addressed towards the possible
development of ‘Hydrogen Economy’ based on the hypothesis that hydrogen (H2) could play a crucial
role as a secondary fuel and energy carrier in the new energy system [3, 4]. H2 has a high volumetric
hydrogen density of 53 g.L−1, that satisfies the 2017 target of 40 g.L−1 for on-board hydrogen storage
target recently announced by the U.S. Department of Energy (DOE) [5] and can efficiently be converted
into energy in an internal combustion engine or fuel cells with the production of water as the only by-
product [3, 4]. However, dramatic improvements in the existing technologies especially for the
controlled storage and release of H2 are still needed, if hydrogen is to become a major energy vector in
the near future [6, 7].
At this concern, a numerous studies have been performed for the development of materials
with high volumetric and gravimetric hydrogen storage capacity, as the low density of hydrogen makes
it difficult to store in compressed or liquefied form [8]. In line with this, various porous materials [9-
12], boron based chemical hydrides [13-15] and boron-nitrogen compounds [16-20] have been tested
for the chemical hydrogen storage. Amongst these materials ammonia-borane (NH3BH3) has recently
received the greatest interest because of its 19.6 wt % hydrogen content, and its being highly stable
and environmentally benign [17]. However, the methyl-substituted ammonia-borane, methylamine
borane (CH3NH2BH3, MeAB) with the 11.1 wt % hydrogen content, has not been widely studied [22-
31]. To date, thermal decomposition in the solid state [21, 22] or metal-catalyzed dehydrocoupling [23,
31] in the organic medium have been demonstrated to provoke the hydrogen release from MeAB.
Although, difficulties in the regeneration of hydrolysis products due to the strong B−O bonds, there is
much interest in the transition-metal-catalyzed hydrolytic dehydrogenation of MeAB due to favorable
fast hydrogen generation under mild reaction conditions [24-30]. The results of the studies concerned
with the metal-catalyzed hydrolysis of MeAB [24-30] have already showed that ∼3 equiv. of H2 per
mole of MeAB can be generated from the aqueous solution of MeAB (Eq. 1) in the presence of a
suitable catalyst under mild conditions (at RT under air).
CH3NH2BH3 + 2H2O → CH3NH3BO2 + 3H2 (1)
The hydrolytic dehydrogenation of MeAB has significant advantages: (I) MeAB is highly stable
in aqueous solution against to self-hydrolysis, (ii) the hydrolytic dehydrogenation of MeAB occurs at
appreciable rates only in the presence of suitable catalysts, (iii) the hydrolytic dehydrogenation of
MeAB generates 3 moles of H2 per mole of MeAB (Eq. 1). Up to date, the catalysts tested in the

hydrolytic dehydrogenation of MeAB are graphene supported core@shell type Ag@M (M= Co, Ni, Fe)
nanoparticles (NPs) [25], Cu@Co NPs [26], Ag@CoNi NPs [27], Ru@Co NPs [28], Ru@Ni NPs [29] and
MCM-41 encapsulated Ru NPs [30]. Among these catalytic architectures Ru based systems [28-30]
provide high activities in the hydrolytic dehydrogenation of MeAB. However, the concerns over the
practical usage of these high cost metals have motivated the research for the development of low cost
and active catalyst systems for this important reaction.
As seen from the previous attempts, the active catalytic systems involves transition metal NPs,
which have higher surface-to-volume ratio comparing to bulk counterparts, thus, larger fraction of
catalytically active atoms exist on their surface [32]. For all that metal NPs are considered as
thermodynamically unstable against to agglomeration into bulk form due to their large surface areas
and high surface energies. At this concern, the suitable protecting ligands or polymers are widely used
in their synthesis to avoid their aggregation [33]. However, the agglomeration of metal NPs eventually
to the bulk form even in the presence of best stabilizing agents [34] is still the most vital concern that
should be overcame in their catalytic applications. Additionally, it is another critical matter to obtain
pure active metal surfaces by staying away from surface contamination resulting from surface
protecting groups, which often lead to a decrease in the catalytic activity resulting from the blocking of
active sites. In this context, the utilization of porous solid matrices as host material for the
immobilization of guest metal NPs allows the generation of specific surfactant-free active sites with the
advantages of preventing particle aggregation [32-34].
In this context, porous solid support materials like zeolites [35, 36], carbon based materials
[37, 38], and minerals [39, 40] have been widely used to synthesis of stable metal NPs within their
porous matrices [41]. Besides these porous materials, more recent studies [42, 43] have also indicated
that metal-organic frameworks (MOFs), which are highly crystalline hybrid materials that combine
metal ions with rigid organic ligands [44], can also be considered as suitable host materials to stabilize
guest metal NPs. Indeed, MOFs can be used as more suitable support material for metal NPs with
respect to other porous solids as they allow more flexible and systematic modification of the pore
structure by the proper selection of the structural subunits and their connected way ligands [42-44].
Moreover, the stabilization of metal NPs within the structure of MOFs produces solid catalytic
materials, which can help us in the kinetic control of the catalytic reactions. In this context, the
selection of MOFs type properly depending on the reaction conditions is the most critical step for the
employment of MOFs as supports for metal NPs immobilization as only a few MOFs with suitable pore
structures are presently known for their thermal/chemical stability. In this line, the results of recent
studies have shown that chromium(III) terephthalate ([Cr3F(H2O)2O{O2CC6H4(CO2)}3.nH2O] framework;
MIL-101; MIL: Materials Institute Lavoisier), which was first reported in Science paper by Ferey and co-

workers in 2005 [45], can be used as a suitable host material for the stabilization of guest metal NPs as
it is stable in water even under very acidic conditions and can show thermal stability up to 300 °C
under air [18]. MIL-101 has a large surface area (~4100 m2.g-1) and contains two different types of
cages with diameters of 29 and 34 Å, which have pore apertures of 12 and 16 Å, respectively. These
unique properties of MIL-101 have encouraged us to focus on the use of the MIL-101 matrix in
catalysis [46-49] for the stabilization of transition metal NPs [50-52]. Our previous studies have already
shown that MIL-101 is suitable support material for the fabrication of catalytically active Ru [53] and
Rh [54] NPs for the selective hydrogenation of phenol to cyclohexanone. In this study we decided to
use nanocrystalline MIL-101 as host material to guest copper NPs. The reduction of the MIL-101 matrix
particle size from the microcrystalline to the nanocrystalline regime (from >1 μm to <100 nm) was
performed in anticipation of improved activity due to lower mass transfer limitations for liquid-phase
dehydrogenation of MeAB at low temperature [40, 55].
Herein, we report the synthesis, characterization and catalytic application of copper NPs
stabilized by nanocrystalline MIL-101 framework, hereafter referred to as Cu/nano-MIL-101, in the
hydrolytic dehydrogenation of MeAB. Cu/nano-MIL-101 nanocatalyst was reproducibly prepared by
the double solvent method [56, 57] combined with the liquid phase chemical reduction technique [41]
all at room temperature. The characterization of Cu/nano-MIL-101 was done by using various
analytical techniques including inductively coupled plasma-mass spectroscopy (ICP-MS), powder X-ray
diffraction (P-XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy
(XPS), diffuse reflectance UV−vis spectroscopy (DR-UV−vis), field-emission scanning electron
microscopy (FE-SEM), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX),
bright field transmission electron microscopy (BFTEM), high-resolution transmission electron
microscopy (HRTEM), scanning transmission electron microscopy-energy dispersive X-ray spectroscopy
(STEM-EDX) and high-angle annular dark field-scanning transmission electron microscopy (HAADF-
STEM). The summation of the results gained by these aforementioned analyses, points toward the
formation of well-dispersed and small-sized copper NPs nucleated within the framework of nano-MIL-
101 support material.
The resulting catalytic architecture, Cu/nano-MIL-101, catalyzes hydrolytic dehydrogenation of
MeAB under mild conditions (at room temperature and under air). Cu/nano-MIL-101nanocatalyst
affords remarkable turnover frequency (TOF = 257 mol H2/mol Cu×h at 25 °C) and conversion (> 99 %)
in the hydrolytic dehydrogenation of MeAB. The small size of the resulting copper-copper oxide NPs
and their surfactant-free clean surface should account for their high catalytic activity. Moreover, the
high stability of these new supported copper-copper oxide NPs against agglomeration and leaching

turns Cu/nano-MIL-101 into an excellent candidate for a long-lived and recyclable heterogeneous
catalyst, in the hydrogen production from MeAB for on-board fuel cell applications.
2. Experimental
2.1 Chemicals and materials
Chromium(III) nitrate nonahydrate, terephthalic acid (C8H6O4), methanol (CH3OH), acetone
(CH3COCH3), sodium borohydride (NaBH4), copper(II) chloride dihydrate (CuCl2.2H2O), methylamine
hydrochloride (CH3NH2·HCl), tetrahydrofuran (C4H8O) and diethyl ether ((CH3CH2)2O) were purchased
from Sigma-Aldrich. Deionized water was distilled by water purification system (Milli-Q Water
Purification System). All chemicals were used as received and without further purification. Micron sized
MIL-101 [58], nanocrystalline MIL-101 [59] and MeAB [25-29] were synthesized according to literature
procedures. The catalytic materials isolated at the end of the synthesis were stored in a Labsconco
nitrogen atmosphere glovebox (H2O < 5 ppm, O2 < 1 ppm). Deionized water was distilled through the
water purification system (Milli-Q Water Purification System). All glassware and Teflon-coated magnetic
stirring bars were washed with acetone and copiously rinsed with distilled water before drying in an
oven at 150 °C.
2.2. Characterization
Copper (Cu) contents of the samples were determined by ICP-MS (Perkin Elmer DRC II model)
after each sample was completely dissolved in a mixture of HNO3/HCl (v/v = 1/3). Powder X-ray
diffraction (XRD) patterns were recorded with a Rigaku Ultima IV diffractometer using Cu-Kα radiation
(wavelength 1.54 Å, 40 kV, 55 mA). FE-SEM analyses was done by using Quanta 400F field emission
SEM. BFTEM, HRTEM, STEM, and HAADF- TEM samples were prepared by dropwise addition of the
dilute catalyst suspension on a copper-coated carbon TEM grid followed by the evaporation of the
solvent. The conventional TEM measurements were carried out on a JEOL JEM-200 CX transmission
electron microscope operating at 120 kV. HRTEM, STEM, and HAADF-STEM analysis were performed
using a JEOL JEM-2010F transmission electron microscope operating at 200 kV. Oxford EDX system and
the Inca software were exploited to acquire and process STEM-EDX data. The XPS measurements were
employed via a Physical Electronics 5800 XP spectrometer equipped with a hemispherical analyzer and
a monochromatic Al Kα X-ray source (1486.6 eV, 15 kV, 350 W, with pass energy of 23.5 eV). NMR
1
spectra were recorded on Bruker Avance DPX 400 MHz spectrometer (400.1 MHz for H NMR; 128.2
MHz for 11B NMR). BF3·(C2H5)2O was used as the external reference for 11B NMR chemical shifts.
2.3. Cu/MIL-101 and Cu/nano-MIL-101 preparation

Cu/MIL-101 and Cu/nano-Mil-101 catalysts were prepared through double solvent method
[49, 50] followed by wet- chemical reduction [41]. Typically, 200 mg support material (MIL-101 or
nano-MIL-101) was suspended in 10 mL dry n-hexane as hydrophobic solvent and the mixture was
sonicated for 15 min. then taken on magnetic stirrer and stirred at 700 rpm at room temperature.
After stirring of 1 h, 1.0 mL aqueous CuCl2.2H2O (17.04 mg, 0.1 mmol Cu) was added dropwise over 10
min. with constant stirring for 2 hours. After centrifugation (6000 rpm, 10 min.), copious washing with
water (3 × 10 mL), the isolated sample in powdered form was dried in a vacuum oven (10-1 Torr) at 70
°C for 3 h. Then, the isolated and purified powder sample was added into 2.0 mL aqueous solution of
NaBH4 and this mixture was stirred for half an hour under ambient conditions. Next, the mixture was
centrifuged (3000 rpm, 5min.), washed with water (3 × 10 mL) and dried in a vacuum oven (10-1 Torr)
at 70 °C for 3 hours. Finally, Cu/MIL-101 and Cu/nano-MIL-101 catalysts in dark-gray powder form was
stored in a Labsconco nitrogen atmosphere glovebox (H2O < 5 ppm, O2 < 1 ppm).
2.4. Hydrolytic dehydrogenation of MeAB
The hydrolytic dehydrogenation of aqueous MeAB solution was surveyed by volumetric
measurement of the rate of hydrogen evolution. The volume of released gas during the reaction was
monitored using a gas burette through water displacement as described elsewhere [14, 20, 35-36].
Beforehand starting, a jacketed one-necked Schlenk tube (20.0 mL), of which the temperature was
adjusted by circulating water through the jacket from a constant temperature bath, containing a
Teflon-coated stirring bar was placed on a magnetic stirrer. In a typical catalytic activity test, catalyst
was weighed and transferred into the Schlenk, and then 8.0 mL H2O was added into this Schlenk
followed by rigorous stirring for 15 mins to achieve a thermal equilibrium. Next, 2.0 mL of aqueous
MeAB solution (100 mM in 10.0 mL H2O) was added into the reaction flask via its septum using a 1.0
mL gastight syringe, and the catalytic reaction was started (t = 0 min) by stirring the mixture at 900
rpm.
2.5. Kinetic studies for Cu/nano-MIL-101 catalyzed hydrolytic dehydrogenation of MeAB
In a series of experiments, the hydrolytic dehydrogenation of MeAB ([MeAB] = 100 mM) was
carried out 25.0 °C starting with different initial concentrations of Cu (51, 34, 17 and 8.5 µmol) in
Cu/nano-MIL-101. Next, to obtain activation energy (Ea) the hydrolytic dehydrogenation of MeAB
([MeAB] = 100 mM) in the presence of Cu/nano-MIL-101 (Cu = 17 µmol) was performed at different
temperatures (15, 25, 35 and 45 °C).
2.6. Catalytic stability measurements
The catalytic stability of Cu/nano-MIL-101 in the hydrolytic dehydrogenation of MeAB was
assessed via catalytic recyclability experiments. The recyclability of Cu/nano-MIL-101 in the hydrolytic
dehydrogenation of MeAB was determined by a series of experiments started with a 10.0 mL aqueous

FA solution (0.22 M in 10.0 mL H2O) at room temperature. Instantaneously after the achievement of >
90 % conversion in the 1st catalytic run, another equivalent amount of fresh MeAB was added to the
reaction mixture, leading to further hydrogen evolution.
3. Results and discussion
3.1. Preparation and characterization of Cu/nano-MIL-101
Cu/nano-MIL-101 catalyst was prepared with high reproducibility by following a method,
which comprised of a double solvent method [56, 57] combined with a liquid phase chemical
reduction technique [41]. The dark gray solid powdered samples of Cu/nano-MIL-101 yielded at the
end of the synthesis protocol were characterized by multipronged advanced analytical techniques.
Firstly, the crystallinity of the host material at various copper loadings was investigated by P-XRD
analyses. Fig. 1 shows P-XRD patterns of the host material (nano-MIL-101) and Cu/nano-MIL-101
samples at different copper loadings (wt % Cu = 1.09, 2.18 and 4.12 as determined by ICP-OES). P-
XRD-patterns of Cu/nano-MIL-101 samples at all Cu loadings are almost identical to that of the host
nano-MIL-101 framework, thus indicating that no new phases appear and plus neither the crystallinity
nor the lattice of nano-MIL-101 was essentially altered by the formation of Cu/nano-MIL-101. The
absence of any Bragg peaks assignable to Cu NPs can be attributed to their low amount of loading
onto host material. Additionally, the slight decrease in the intensity of Bragg peaks in Cu/nano-MIL-
101 samples by the increase of Cu loading can be explained by the changes in the charge distribution
and electrostatic fields as a result of the existence of Cu NPs on the surface and interaction of their
electrophilic surface with framework atoms [60].
XPS provides further insights to the oxidation state of copper and surface composition of the
resulting Cu/nano-MIL-101. The survey scan XPS spectrum of Cu/nano-MIL-101 sample is depicted in
Fig. 2(a), which shows the existence of MIL-101 framework elements (Cr, O and C) and Cu. The
existence of Na and B signals indicative of the presence of surface bound contaminated sodium (Na+)
-
and metaborate (BO2 ) ions most probably came from hydrolysis of NaBH4, which was used as reducing
agent. The high resolution Cu 2p XPS (HR-XPS) spectrum of Cu/nano-MIL-101 sample (2.18 wt % Cu
loading) and its deconvulation are given in Fig. 2(b). The deconvulation of high resolution Cu 2p XPS
spectrum of Cu/nano-MIL-101 sample gives a distinctive peak at 931 eV that can readily be assigned to
metallic Cu(0) [61]. The peaks observed at 933 and 935 5 eV reveals the presence of CuO phase, which
may originate from the surface oxidation of Cu(0) NPs during the XPS sampling procedure [62, 63].
Nitrogen adsorption–desorption isotherms of nano-MIL-101 and Cu/nano-MIL-101 are given
in Fig. 3 and they show type I shape isotherm, which is a characteristic of microporous materials [64].

The micropore volume and area were determined for nano-MIL-101 and Cu/nano-MIL-101 by the t-
plot method [65]. The appreciable decrease in the micropore volume (from 1.42 to 1.12 cm3/g) and
BET-surface area (from 3044 to 2586 m2/g) on passing from nano-MIL-101 to Cu/nano-MIL-101 can be
explained by blocking of the windows of cavities by the deposition of Cu(0) NPs exist mainly on the
surface of nano-MIL-101; as the electron microscopy analyzes (vide infra) show the existence of Cu(0)
NPs supported on the surface of nano-MIL-101.
FE-SEM, SEM-EDX, BFTEM and HAADF-STEM analyses were performed to examine the size,
morphology and the composition of Cu/nano-MIL-101. FE-SEM images of Cu/nano-MIL-101 sample
(2.18 wt % Cu loading) from different regions in different magnifications are given along with SEM-EDX
spectrum in Fig. S1 (in the Supporting Information). These images are indicating that (i) there exist only
crystals of nano-MIL-100, (ii) there is no bulk copper metal formed in observable size outside MIL-101
nanocrystals. Although no bulk copper metal is observed outside MIL-101 nanocrystals, SEM-EDX and
ICP-OES analyses reveal that the presence of copper in the sample, which implies that the formation
of very small sized and unagglomerated Cu NPs. Expectedly, BFTEM images of Cu/nano-MIL-101
sample (2.18 wt % Cu loading) given in Fig. 4(a)-(d) displays highly crystalline nano-MIL-101 host
material and very small sized Cu(0) NP, the mean particle size of the resulting Cu(0) NP was found to
be 0.8 ± 0.3 nm by the particle size analysis of > 50 non-touching Cu(0) NP. The existence of Cu NPs
was also confirmed by HAADF-STEM analysis of the same sample, bright spots observed in the HAADF-
STEM image (given in the inset of Fig. 4(c)) of the selected region given in Fig. 4(c) shows the presence
of very small sized slightly clumped Cu NPs within the framework of nano-MIL-101 host material.
3.2. Catalytic reactivity of nano-MIL-101 and effect of Cu loadings on the catalytic activity of Cu/nano-
MIL-101 in the hydrolytic dehydrogenation of MeAB
MIL-101 metal organic framework has Cr(III) Lewis acidic sites and it has recently been
reported that Lewis acid catalyst can promote the evolution of up to 2.5 equivalents of H2 from our
substrate (MeAB) analogous ammonia-borane (NH3BH3) molecule [66]. For this reason, before testing
the catalytic activity of Cu/nano-MIL-101, the catalytic reactivity of the host material nano-MIL-101
was examined under the same conditions (vide infra). The result of this experiment showed that the
host material nano-MIL-101 is catalytically inactive in the hydrolytic dehydrogenation of MeAB.
As the next, the hydrolytic dehydrogenation of MeAB was performed by using Cu/nano-MIL-
101 samples with different copper loadings in the range of 1.09–4.12 wt % by keeping the copper
concentration identical (in all [Cu] = 3.44 mM) to determine the effect of copper loading on the
catalytic activity of Cu/nano-MIL-101. Fig. 5 shows mole of evolved H2/mole of MeAB versus time
graph for these Cu/nano-MIL-101 samples catalyzed hydrolytic dehydrogenation of MeAB at room

temperature. As seen from this graph, the highest catalytic activity was obtained by Cu/nano-MIL-101
sample with 2.18 wt % copper loading. The variation in catalytic activity reflects the accessibility of
Cu(0) NPs in the nano-MIL-101 framework by the substrate so in the Cu/nano-MIL-101 sample with
2.18 wt % copper loading most of the Cu NPs on the surface and readily accessible by MeAB. The small
size of Cu/nano-MIL-101 containing 2.18 wt % Cu may be another reason for its high activity with
respect to that of containing 4.12 wt % Cu. Expectedly, an informative BFTEM (Fig. S2 in the
Supporting Information) taken from Cu/nano-MIL-101 sample containing 4.12 wt % Cu shows the
existence of slightly larger size (> 2.0 nm) Cu NPs on the surface of Cu/nano-MIL-101. For all the tests
11
reported hereafter, the copper loading used was ∼ 2.18 wt % unless otherwise stated. The B-{1H}-
NMR spectrum (Fig. S3 in the Supporting Information) taken from the reaction solution at the end of
the dehydrogenation of DMAB (when no more hydrogen is evolved) shows that MeNH2BH3 (δ=-19
ppm, q) is completely converted to methylammonium borate (MeNH3)BO2 (δ=-13.8 ppm, s).
3.3. Effect of the size of host material on the catalytic activity of the guest Cu(0) NP: The comparison of
the catalytic activity of Cu/nano-MIL-101 and Cu/MIL-101 in the hydrolytic dehydrogenation of MeAB
A decrease in the particle size of host material from microcrystalline (>1 μm) to
nanocrystalline (<100 nm) results in high external surface areas, which increases the possibility of
formation of small size surface bound Cu(0) NPs, and thus, reduces the diffusion path length
compared to the large size MIL-101 crystals. With this anticipation, we employed nano-MIL-101 as
host material for guest Cu(0) NPs. In order to investigate the effect of size of host material on the
catalytic activity of MIL-101 stabilized Cu(0) NPs in the hydrolytic dehydrogenation of MeAB, we
compared the catalytic activities of Cu/nano-MIL-101 (2.18 wt % Cu loading) and Cu/MIL-101 (2.21 wt
% Cu loading), which were prepared by following the same synthesis protocol, by keeping the copper
concentration same (in all [Cu] = 3.44 mM) in the hydrolytic dehydrogenation of MeAB.
Fig. 6 shows mole of evolved H2/mole of MeAB versus time graph for Cu/nano-MIL-101 (2.18
wt % Cu loading) and Cu/MIL-101 (2.21 wt % Cu loading) catalyzed hydrolytic dehydrogenation of
MeAB at room temperature. Expectedly, as seen from this graph Cu/nano-MIL-101 (2.18 wt % Cu
loading) catalyst provides better catalytic performance in terms of activity and conversion than
Cu/MIL-101 (2.21 wt % Cu loading) in the hydrolytic dehydrogenation of MeAB, which can be ascribed
to (I) formation of larger size of Cu(0) NPs in Cu/MIL-101 (see BFTEM image of Cu/MIL-101 given in Fig.
S4 in the Supporting Information) and (ii) the lower mass transfer limitations for Cu/nano-MIL-101
catalyzed liquid phase dehydrogenation of MeAB [67-69].

3.4. Effect of the copper concentration on the hydrogen generation rate of Cu/nano-MIL-101 catalyzed
hydrolytic dehydrogenation of MeAB and the catalytic activity of Cu/nano-MIL-101 in the hydrolytic
dehydrogenation of MeAB
The catalytic activity of Cu/nano-MIL-101 with various copper concentrations was investigated
in the hydrolytic dehydrogenation of MeAB at room temperature to determine the effect of copper
concentration on the dehydrogenation rate. Fig. 7(a) shows mole of evolved H2/mole of MeAB versus
time graph for Cu/nano-MIL-101 catalyzed dehydrogenation of MeAB at 25 °C for different copper
concentrations. An almost linear hydrogen evolution starts immediately as the preformed catalyst was
used. The initial rate of hydrogen generation was determined from the linear portion of the plot for
each experiment. Expectedly, the hydrogen generation rate increases by the increase of copper
concentrations (kobs= 0.0313, 0.0545 and 0.1414 mol H2/(mol MeAB×min.) for [Cu] = 0.86, 1.72 and
3.44 mM, respectively). The plot of hydrogen generation rate versus copper concentration, both on
logarithmic scales, gives a straight line with a slope of 1.12 as shown in Fig. 7(b). This result, within the
experimental error, indicates that Cu/nano-MIL-101 catalyzed dehydrogenation of MeAB is first order
with respect to the copper concentration.
It should also be noted that 3.44 mol % of catalyst can achieve the complete hydrolytic
dehydrogenation of MeAB at room temperature under air. The initial activity of Cu/nano-MIL-101 was
determined to be 257 mol H2/mol Cu×h in the room temperature hydrolytic dehydrogenation of
MeAB (see Supporting Information for the details of the initial TOF value determination). This activity
value is higher than previously reported for the hydrolytic dehydrogenation of MeAB by not only non-
precious metal based nanocatalyst Cu0.2@Co0.8/r-GO (138 mol H2/mol metal×h) [26] but also precious
metal used nanocatalyst systems such as Ag0.1@Co0.45Ni0.45/r-GO (210 mol H2/mol metal×h) [27].
3.5. Effect of the temperature on the hydrogen generation rate of Cu/nano-MIL-101 catalyzed
hydrolytic dehydrogenation of MeAB and determination of activation parameters
Cu/nano-MIL-101 catalyzed dehydrogenation of MeAB was also carried out at different
temperatures in the range 15-45 °C to investigate the effect of temperature on the hydrogen
generation rate and to find activation parameters; activation energy (Ea), activation enthalpy (∆H*)
and entropy (∆S*). The plot of mole of evolved H2/mole of MeAB versus time graph for Cu/nano-MIL-
101 catalyzed dehydrogenation of MeAB at various temperatures is given in Fig. 8(a), as expected the
rate of hydrogen generation is enhanced by the increase of the reaction temperature. The observed
rate constants were found to be kobs= 0.0226, 0.0545, 0.1525 and 0.4067 mol H2/(mol MeAB×min.) at
15, 25, 35 and 45 °C, respectively.

These observed rate constants (kobs) were used to construct the Arrhenius and Eyring-Polonyi
plots given in Fig. 9(b) and (c), respectively. From the slope of the Arrhenius plot the activation energy
(Ea) for the Cu/nano-MIL-101 catalyzed dehydrogenation of MeAB was found to be 34.1 kJ/mol. This
value is lower than those of obtained by Cu0.2@Co0.8/r-GO (55.9 kJ/mol) [26], Ru@Ni/r-GO (37 kJ/mol)
[29], Ru/MCM-41 (47.6 kJ/mol) [30] catalysts in the hydrolytic dehydrogenation of MeAB. The
activation enthalpy (∆H*) and entropy (∆S*) values were determined from the Eyring-Polonyi plot
given in Fig. 9(c), from this plot these values were determined as ∆H* = 71.3 kJ/mol and ∆S* = -29.1
J/(mol×K). The high positive value of ∆H* and negative value of ∆S* implies the presence of an
associative mechanism in the transition state [70].
3.6. Effect of the nature of support material on the catalytic activity of guest Cu NPs in the hydrolytic
dehydrogenation of MeAB
The uniqueness of Cu/nano-MIL-101 nanocatalyst among the different solid supported
copper nanoparticles prepared by the same synthesis protocol was also tested by using the most
commonly used solid support materials; silica (SiO2), alumina (Al2O3) and carbon (C) in the hydrolytic
dehydrogenation of MeAB under the identical conditions (under air at 25 °C) and their results were
given in Fig. 9. The initial TOF values were found to be 85 (82 % conversion), 74 (68 % conversion), and
50 (45 % conversion) mol H2/mol Cu×h for Cu/Al2O3, Cu/ SiO2 and Cu/C catalysts, respectively. The low
activity and conversion values obtained by these support materials can be explained by the clumping
of surface bound Cu(0) NPs. BFTEM images of Cu/Al2O3, Cu/ SiO2 and Cu/C catalysts (see Fig. S5-S7 in
the Supporting Information) reveal that the existence of clumped Cu(0) NPs and some Cu(0)
agglomerates on the surface of these solid support materials. The interaction between nano-MIL-101
framework and surface bound Cu(0) NPs might be strong enough to prevent the surface
agglomeration of these particles. This kind of strong interaction between metal NPs and solid support
materials has recently been named as “Strong Metal-Molecular Support Interaction” (SMMSI) by
Yadav and co-workers [71]. Understanding the existing synergistic effects between the metal and
nano-MIL-101 support remains an active area of research in our group.
3.7. Catalytic stability of Cu/nano-MIL-101 in the hydrolytic dehydrogenation of MeAB
The catalytic stability of Cu/nano-MIL-101 was examined by testing the recyclability
performance of Cu/nano-MIL-101 in the hydrolytic dehydrogenation of MeAB. In the recyclability
experiments, fresher aqueous MeAB was added into the reaction solution when all MeAB from the
previous run was converted to H2 and CH3NH2BO2 and by this way, the reaction was continued up to
ten consecutive cycles. As given in Fig. 10, Cu/nano-MIL-101 nanocatalyst provides complete
conversion in MeAB dehydrogenation by retaining 83 % of its inherent catalytic activity even at 10th

recycle, which corresponds to the lower limit of total turnover number (TTON) of 872 (see Supporting
Information for the details of the lower limit of TTON determination).
The morphological investigation of the recovered catalyst from 10th recycle was performed by
BFTEM analysis. The inspection of BFTEM image (Fig. S8, Supporting Information) of recovered catalyst
revealed that clumping of nano-MIL-101 support material and guest Cu(0) NPs. Additionally, P-XRD
analysis of the same material indicative of slight distortion of the crystallinity of the host material
nano-MIL-101 (Fig. S8, Supporting Information). The results of these two analyses explain the
observed slight decrease in the catalytic activity at 10th recycle. The leaching possibility of Cu(0) NPs
from nano-MIL-101 framework to reaction solution throughout the recyclability experiments, was also
investigated by performing ICP-OES analyses on aliquots of the reaction solutions taken at the end of
1st, 5th and 10th recycle, in which no Cu was detected confirming that the retention of Cu(0) NPs on
nano-MIL-101 framework. Taking all results together, one can conclude that Cu/nano-MIL-101 acts as
highly durable nanocatalyst against leaching, sintering and clumping, which makes it a highly efficient
and recyclable heterogeneous catalyst for the hydrolytic dehydrogenation of MeAB.
4. Conclusions
In summary, Cu(0) NPs stabilized by nano-MIL-101 framework (Cu/nano-MIL-101), have been
successfully fabricated with high reproducibility and characterized through multi-pronged analyses, by
using advanced analytical tools. This new Cu/nano-MIL-101 nanocatalyst shows remarkable catalytic
activity (initial TOF = 257 mol H2/mol Cu×h) in the hydrolytic dehydrogenation of MeAB under mild
reaction conditions (under air at room temperature). Moreover, Cu/nano-MIL-101 nanocatalyst
provides exceptional catalytic stability against to leaching, sintering and clumping throughout the
recyclability experiments. This uniquely active, selective, and stable catalytic architecture presents
itself as an excellent contender for clean hydrogen production, via room-temperature hydrolytic
dehydrogenation of MeAB, and has a strong potential to be exploited in practical/technological
applications, where MeAB is utilized as a viable hydrogen carrier in mobile fuel cell applications
Acknowledgments
Authors thanks to the Research Fund of nc l University Office of Scientific Research
Project (FOA-2016-5376). Additionally, the partial supports by Fevzi Akkaya Scientific Activities Support
Fund (FABED), Science Academy, and Turkish Academy of Sciences (TUBA) are gratefully
acknowledged.
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Figure Captions
Fig. 1 P-XRD patterns of nano-MIL-101, Cu/nano-MIL-101 (1) (1.09 % wt Cu), Cu/nano-MIL-101 (2)
(2.18 % wt Cu) and Cu/nano-MIL-101 (3) (4.12 % wt Cu) samples in the region of 2θ = 2-40°.
Fig. 2 (a) Survey XPS spectrum of Cu/nano-MIL-101 in the region of BE = 1100-o eV, (b) Cu 2p core level
high resolution-XPS (HR-XPS) spectrum of Cu/nano-MIL-101 in the region of BE = 940-925 eV.
Fig. 3 Nitrogen adsorption–desorption isotherms of nano-MIL-101 and Cu/nano-MIL-101 (2.18 % wt
Cu) samples.
Fig. 4 (a)-(d) BFTEM images of Cu/nano-MIL-101 (2.18 % wt Cu) sample in different magnifications and
inset in (c) HAADF-STEM image of the selected region labelled as white colored square in (c).
Fig. 5 The plot of mole of evolved H2/mole of MeAB versus time graph for the hydrolytic
dehydrogenation of MeAB ([MeAB] = 100 mM in 10.0 mL H2O) catalyzed by Cu/nano-MIL-101 samples
with different copper loadings in the range of 1.09–4.12 wt % (in all [Cu] = 3.44 mM) at room
temperature.
Fig. 6 The plot of mole of evolved H2/mole of MeAB versus time graph for the hydrolytic
dehydrogenation of MeAB ([MeAB] = 100 mM in 10.0 mL H2O) catalyzed by Cu/nano-MIL-101 and
Cu/MIL-101 samples (in all [Cu] = 3.44 mM) at room temperature.
Fig. 7 The plot of mole of evolved H2/mole of MeAB versus time graph for the hydrolytic
dehydrogenation of MeAB ([MeAB] = 100 mM in 10.0 mL H2O) catalyzed by Cu/nano-MIL-101 with
different copper concentrations at room temperature.
Fig. 8 (a) The plot of mole of evolved H2/mole of MeAB versus time graph for the hydrolytic
dehydrogenation of MeAB ([MeAB] = 100 mM in 10.0 mL H2O) catalyzed by Cu/nano-MIL-101 (in all
[Cu] = 1.72 mM) at different temperatures, (b) Arrhenius plot and (c) Eyring-Polonyi plot.

Fig. 9 The plot of mole of evolved H2/mole of MeAB versus time graph for the hydrolytic
dehydrogenation of MeAB ([MeAB] = 100 mM in 10.0 mL H2O) catalyzed by Cu/nano-MIL-101 (2.18 %
wt Cu), Cu/Al2O3 (2.17 % wt Cu), Cu/ SiO2 (2.32 % wt Cu) and Cu/C (2.12 % wt Cu) catalysts (in all [Cu] =
3.44 mM) at room temperature.
Fig. 10 The plot of mole of evolved H2/mole of MeAB versus time graph for 1st, 5th and 10th recycle in
the Cu/nano-MIL-101 ([Cu] = 3.44 mM) catalyzed hydrolytic dehydrogenation of MeAB ([MeAB] = 100
mM in 10.0 mL H2O) at room temperature.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 10
Highlights:



Cu/nano-MIL-101 nanocatalyst has reproducibly been synthesized and characterized by
using advanced analytical techniques,
Cu/nano-MIL-101 nanocatalyst was used in the hydrolytic dehydrogenation of MeAB at
room temperature,
This new catalyst provides remarkable catalytic acitivity TOF = 257 mol H2/mol Cu×h) and
conversion (> 99 %) under air at room temperature.