Catalytic Transfer Hydrogenation of Furfural to Furfuryl Alcohol by using Ultrasmall Rh Nanoparticles Embedded on Diamine-Functionalized KIT-6

Article abstract of DOI:10.1002/cctc.201701037

A Rh/ED-KIT-6 catalyst comprised of Rh nanoparticles embedded on mesoporous silica (KIT-6) functionalized with N1-[3-(trimethoxysilyl)propyl]ethane-1,2-diamine was synthesized by Rh³⁺ adsorption and chemical reduction in the liquid phase. The structure of ED-KIT-6 and textural properties of the pristine and supported Rh catalysts, as well as particle size and chemical state of the Rh species were examined by various analytical methods. The homogeneous dispersion of ultrasmall Rh nanoparticles, approximately 1.2 nm in size, stabilized by the grafted diamine (ED) species was confirmed. Rh/ED-KIT-6 was applied to the transfer hydrogenation of furfural (FFR) to furfuryl alcohol (FAL) by using formic acid (FA) as the hydrogen source. The effect of the solvent and reaction parameters, such as temperature, reaction time, and FA/FFR ratio, were investigated. The Rh-embedded catalyst exhibited a significantly high turnover frequency (TOF≈204 h^?1) to that of Ru, Pd, or Ni-based catalysts on KIT-6. A plausible reaction mechanism was proposed after examining an independent FA decomposition reaction over the same Rh-ED-KIT-6 catalyst. The heterogeneity of the catalyst was verified by a hot filtration experiment. The Rh/ED-KIT-6 could be reused for up to three cycles without any decrease in catalytic activity and selectivity, but the slow oxidation of Rh species was detected.

Full text of DOI:10.1002/cctc.201701037

Accepted Article
Title: Catalytic transfer hydrogenation of furfural to furfuryl alcohol
using ultra-small Rh nanoparticles embedded on diamine-
functionalized KIT-6
Authors: Chinna Krishna Prasad Neeli, Young-Min Chung, and Wha-
Seung Ahn
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201701037
Link to VoR: http://dx.doi.org/10.1002/cctc.201701037
A Journal of
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Catalytic transfer hydrogenation of furfural to furfuryl alcohol
using ultra-small Rh nanoparticles embedded on diamine-
functionalized KIT-6
Chinna Krishna Prasad Neeli,[a] Young-Min Chung[b] and Wha-Seung Ahn*[a]
pollution because of its toxicity. To develop more
environmentally acceptable catalysts, supported noble and
transition metal catalysts, such as Pt,[11] Pt-Sn,[12] Ir,[13] Ir-
ReOx,[14] Pd,[15] Ru,[16] MoNiB,[17] Ni-Sn,[18] Ni-P-B,[19] Co,[20] Cu-
Co,[21] and Cu-Fe,[22] which could selectively hydrogenate the
C=O group and preserve the C=C bonds have been extensively
studied in liquid phase direct hydrogenation. On the other hand,
most of these systems operate at high H2 pressures, elevated
temperatures and/or involve acid additives. In addition, they
usually afford low product yields. Therefore, there is a strong
need to develop a more energy efficient and environmentally
benign process that avoids the use of external hydrogen at high
pressure.[23]
Abstract: A Rh/ED-KIT-6 catalyst comprised of Rh nanoparticles
embedded on mesoporous silica (KIT-6) functionalized with N1-[3-
(trimethoxysilyl)propyl]ethane-1,2-diamine was synthesized by Rh3+
adsorption and chemical reduction in the liquid phase. The structure
of ED-KIT-6 and textural properties of the pristine and supported Rh
catalysts, as well as particle size and chemical state of the Rh
species were examined by various analytical methods. The
homogeneous dispersion of ultra-small Rh nanoparticles, ca.1.2 nm
in size, stabilized by the grafted diamine species was confirmed.
Rh/ED-KIT-6 was applied to the transfer hydrogenation of furfural
(FFR) to furfuryl alcohol (FAL) using formic acid (FA) as the
hydrogen source. The effect of the solvent and reaction parameters,
such as temperature, reaction time, and FA/FFR ratio, were
investigated. The Rh-embedded catalyst exhibited a significantly
high turnover frequency (TOF~204 h−1) superior to that of Ru, Pd, or
Ni-based catalysts on KIT-6. A plausible reaction mechanism was
proposed after examining an independent FA decomposition
reaction over the same Rh-ED-KIT-6 catalyst. The heterogeneity of
the catalyst was verified by a hot filtering experiment. The Rh/ED-
KIT-6 could be reused for up to three cycles without any decrease in
catalytic activity and selectivity, but the slow oxidation of Rh species
was detected.
Catalytic transfer hydrogenation (CTH), an alternative route to
conventional direct hydrogenation, employs renewable organic
compounds such as organic acids or alcohols as a hydrogen
source, and can avoid the use of high pressure autoclaves and
provides as an energy efficient reaction scheme.[24,25] Particularly,
formic acid, a major byproduct of biomass processing, is
considered as a promising H2 source because of its higher
atomic efficiency, energy density, non-toxicity, excellent stability
and ability to regenerate it by hydrogenation of carbon dioxide.[26,
27]
A catalytic process using formic acid as a liquid reductant
obtained from biomass can potentially enhance the sustainable
production of value-added chemicals under mild pressure
conditions.[28]
Introduction
Mesoporous materials are used widely for adsorption,
separation, and catalysis[29] owing to their desirable properties,
such as high surface area, tunable pore size in a narrow
distribution, high hydrothermal stability, and readily modifiable
surface functionality.[30] Compare to two-dimensional hexagonal
P6mm SBA-15, KIT-6 with a three dimensional cubic Ia3d
structure, allows the faster diffusion of reactants, minimize pore
blockage, and provide more catalytic active sites for reactant
molecules. These features provide a consistent and well isolated
environment for the growth of metal nanoparticles avoiding
agglomeration.[31]
Despite considerable reports existing in the CTH of carbonyl
groups in homogeneous medium,[32-34] very few are available in
the case of heterogeneous systems,[35, 36] including the CTH of
furfural. Unfortunately, these systems involve bases and/or
ligand additives, which require tedious work up and purification
processes to yield the final products.
With the ever increasing concerns regarding environment
pollution due to the emission of greenhouse gases and the
continuous depletion of fossil fuels, the development of biomass
as a renewable resource for the sustainable production of
biofuels and chemicals has attracted attention.[1-3] Furfural, as
one of the most promising biomass-based platform molecules,
offers the production of fuel additives and high value-added
chemicals,[4-6] which is obtained by combined acid-catalyzed
hydrolysis and the dehydration of hemicellulosic xylans.[7] The
catalytic hydrogenation of furfural to furfuryl alcohol is an
industrially preferred pivotal process in the chemical conversion
of bio-derived molecules.[8] Furfuryl alcohol is prepared
commercially by the direct hydrogenation of furfural over copper-
chromite catalysts;[9,10] however, this suffers from environment
[a]
[b]
Chinna Krishna Prasad Neeli, Wha-Seung Ahn*
Department of Chemistry and Chemical Engineering, Inha
University, Incheon 402-751, South Korea
E-mail: whasahn@inha.ac.kr
Young-Min Chung
Department of Nano and Chemical Engineering, Kunsan National
University, Kunsan 573-701, South Korea
This paper reports the selective hydrogenation of furfural to
furfuryl alcohol by catalytic transfer hydrogenation, in which
formic acid and Rh nanoparticles supported on KIT-6 are used
as a bio-renewable hydrogen source and catalyst, respectively.
In addition, different metal nanoparticles supported on KIT-6
were compared for the CTH reaction. The effects of the reaction
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10.1002/cctc.201701037
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parameters on the activity and selectivity to furfuryl alcohol were
investigated systematically, and a plausible mechanism was
proposed for the CTH of furfural to furfuryl alcohol, aided by the
results obtained from an independent formic acid decomposition
reaction.
functionalization of the parent KIT-6 with ethylene diamine
groups inside the pores. A further small decrease in the surface
area of Rh/ED-KIT-6 was observed after the incorporation of Rh
nanoparticles into ED-KIT-6. Similar decrease in pore volume
and pore diameter was detected, which support the occupation
of pores by the functionalizing agent and Rh nanoparticles. The
pore size distributions of the samples (inset, Fig.1) were
estimated using the BJH method, revealing a narrow pore
distribution in the range, 2–5 nm.
Results and Discussion
Diamine-functionalized KIT-6 (ED-KIT-6) was prepared by
post-synthesis grafting of the organic precursor and used as a
support for the synthesis of immobilized metal nanoparticles by
metal ion adsorption and reduction. For this purpose,
a
hydrophobic dichloromethane solvent was used as the transport
medium to facilitate the metal acetate/acetylacetonate precursor
into the pores of ED-KIT-6. Rh/ED-KIT-6 with an actual metal
loading of 0.98 wt% was confirmed by ICP-OES analysis. The
diamine loading in ED-KIT-6 and Rh/ED-KIT-6 catalysts were
determined by EA to be approximately 2.42 and 2.39 mmol g-1
respectively (Table 1). To verify the textural changes at each
stage of the catalyst preparation, the N2 adsorption-desorption
isotherm of Rh/ED-KIT-6 catalyst was measured, as shown in
Fig. 1.
Table 1. Structural and textural parameters of KIT-6, ED-KIT-6 and
Rh/ED-KIT-6 catalysts calculated from N2 adsorption isotherms.
[a]
[c]
SBET
Vp[b]
DBJH
N[d]
Catalyst
(m2g-1)
(cm3g-1)
(nm)
(mmol g-1)
KIT-6
ED-KIT-6
Rh/ED-KIT-6
658
212
189
0.81
0.32
0.27
4.03
2.71
2.68
-
2.42
2.39
[a] BET surface area, [b] Total pore volume measured at P/P0= 0.9999, [c]
Pore width calculated by BJH method and [d] N content estimated by
elemental analysis.
Low-angle XRD analysis was carried out to confirm the
structural characteristics of the parent KIT-6, ED-KIT-6 and
Rh/ED-KIT-6 samples, as displayed in Fig. 2. The parent KIT-6
sample exhibited a sharp intense peak at approximately 0.98°
2θ, corresponding to the (211) plane, together with higher
ordered reflections at 1.21° and 1.92° 2θ assignable to the (220)
and (420) planes, indicating the highly ordered mesoporous
structure of the host material with cubic Ia3d symmetry.[41]
Similar to parent KIT-6, the other two functionalized samples
also showed a sharp diffraction peak at approximately 0.98° 2θ,
which indicates that the three-dimensional cubic mesoporous
structure of the parent KIT-6 was well preserved during the
various stages of catalyst preparation. In the case of the ED-
KIT-6 and Rh/ED-KIT-6 samples, a decrease in the intensity of
the (220) and (420) planes was observed, again confirming the
diamine-functionalization and encapsulation of Rh nanoparticles.
A similar set of observations was reported previously.[39] The
characteristic XRD peak of the metallic Rh (111) plane at 41.3°
2θ was not observed due to the high dispersion of ultra-small Rh
nanoparticles with a crystallite size below the XRD detection
limit (<5 nm) with a low loading on the ED-KIT-6 matrix; these
Rh nanoparticles were confirmed by TEM, EDX and XPS
analysis (see later Fig.4 and 5a). A broad peak that appears in
all the patterns at 2θ ≈ 22° was assigned to the SiO2 phase of
KIT-6.
Figure 1. N2 adsorption–desorption isotherms and pore size
distribution curves (inset) of the samples (Rh/ED-KIT-6 was plotted
after shifting down 50 cm3/g to avoid overlapping).
The isotherms obtained for the KIT-6, ED-KIT-6 and Rh/ED-
KIT-6 samples were of type IV with a H1 hysteresis loop in
accordance with the IUPAC classification, which is the typical
characteristic of the ordered mesoporous materials with a
narrow pore size distribution.[40] All the samples exhibited
hysteresis loops similar to that of the parent KIT-6, indicating the
retention of the mesoporous structure. Table 1 lists the structural
and textural parameters of the BET surface area, pore volume,
and mean pore diameter of the materials. The BET surface area
of the parent KIT-6 and ED-KIT-6 were 658 and 212 m2g-1,
respectively. The decrease in surface area was attributed to
Fig. 3 presents the FT-IR spectra of the KIT-6, ED-KIT-6 and
Rh/ED-KIT-6 samples. A broad band at approximately 3450 cm-
1 in all three samples correspond to the –OH stretching vibration
and the band at 1636 cm-1 was assigned to the –OH
(physisorbed water) bending vibration.
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Figure 4. (a) FE-TEM image of Rh/ED-KIT-6 with particle size
distribution (inset) and (b) Energy dispersive X-ray analysis of Rh
nanoparticles on the surface of functionalized KIT-6.
Figure 2. Low angle XRD patterns of samples and wide angle XRD
pattern of Rh/ED-KIT-6 in inset.
TEM analysis (Fig. 4a) of the Rh/ED-KIT-6 revealed isolated
spherical Rh nanoparticles dispersed on KIT-6. As shown in the
particle size distribution (inset, Fig.4a), the average Rh
nanoparticles were ca. 1.2 nm in size. EDX elemental mapping
also confirmed the existence of rhodium and diamine on Rh/ED-
KIT-6 (Fig. 4b). As anticipated, diamine-functionalization avoided
the agglomeration of Rh nanoparticles and provided stabilization
through the electrostatic attraction between the Rh metal and
nitrogen functionality.
Figure 3. FT-IR spectra of KIT-6, ED-KIT-6 and Rh/ED-KIT-6
catalysts.
Figure 5. X-ray photoelectron spectra of (a) Rh/ED-KIT-6, (b)
Reused Rh/ED-KIT-6 after three cycles, (c) Rh(III)/ED-KIT-6, and (d)
N 1s XPS spectrum of Rh/ED-KIT-6 catalyst.
The intense bands at 1082, 798 and 464 cm-1 correspond to
Si–O–Si lattice vibration of the KIT-6 silica framework. A
decrease in the intensity of the bands at 3450, 798, and 464
cm−1 was observed after diamine-functionalization, indicating
the formation of chemical bonding between the silanol groups of
KIT-6 with the methoxy groups in N1-[(3-trimethoxysilyl)propyl)]
ethane-1,2-diamine. The new bands observed in the region of
2938, 2847 cm-1 and at approximately 1384 cm-1 in both ED-
KIT-6 and Rh/ED-KIT-6 samples can be attributed to –CH2
asymmetric, symmetric stretching, and bending vibrations,
respectively. The additional band at 1468 cm-1 was assigned to
the bending vibration (δNH) of N–H groups respectively,[42]
which reveals the successful diamine-functionalization onto KIT-
6. Further, the presence of nitrogen functionality was also
confirmed by XPS (see later in Fig. 5d).
Fig. 5 presents the X-Ray photoelectron spectrum of the
Rh/ED-KIT-6. The fresh Rh/ED-KIT-6 exhibited a doublet peak,
exhibiting binding energies at 307.8 and 312.2 eV, which were
assignable to the spin-orbit splitting components of Rh3d5/2 and
Rh3d3/2 in the zero oxidation state of Rh, which are consistent
with the literature.[43] The slightly higher binding energies of
metallic Rh compared to standard binding energy values of
Rh3d5/2 and Rh3d3/2 at 307.4 and 311.8 eV, respectively,
indicating the strong metal interactions with the functionalized
diamine groups. The existence of nitrogen functionality was
authenticated from the N 1s XPS spectrum with binding energy
of 399.5 eV.
The catalytic activity of Rh/ED-KIT-6 in the transfer
hydrogenation of FFR to furfuryl alcohol (FAL) using FA as a
hydrogen source in 2-propanol medium (Scheme 1) was
examined. Initially, controlled and blank experiments were
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carried out in 2-propanol at 100 °C with KIT-6, ED-KIT-6 and
without a catalyst using FA; the results are summarized in Table
2.
wt% was synthesized using aforementioned procedure by
simple replacement of ED-KIT-6 with parent KIT-6. Rh/KIT-6
without diamine (ED) functionalization exhibited only low
catalytic activity compared to Rh/ED-KIT-6 as listed in Table 2,
entry 5. The amino groups were reported to provide coordinating
groups to combine with noble metal ions resulting in uniform
small nuclei, and facilitate the dissociation of O–H bond as a
proton scavenger and increase the dehydrogenation rate of
[44,45]
formic acid
in conjunction with Rh nanoparticles (see also
mechanism in Fig. 10). Herein, the Rh/ED-KIT-6 catalyst was
capable of catalyzing both the decomposition of FA and the
hydrogenation of FFR under similar reaction conditions. No FAL
product was observed (Table 2, entry 9) using hydrazine
monohydrate (2 mmol) as a hydrogen source, even in the
presence of Rh/ED-KIT-6 at 100 °C for 5 h. Instead, the major
formation of (E)-(furan-2-ylmethylene) hydrazine was detected,
which can be attributed to condensation of the carbonyl group of
FFR with hydrazine.[46]
The hydrogenation of FFR using FA as a hydrogen source was
strongly dependent on the reaction temperature. The effects of
the reaction temperature were studied by varying the
temperatures, i.e., 70, 80, 90, and 100 °C and the results are
summarized in Table 2 (entries 6–8 and 10). At 70 °C, only trace
amounts of FFR conversion were found without the detection of
Scheme 1. Schematic representation of conventional hydrogenation
and CTH of furfural.
Table 2. Optimization of reaction conditions in the CTH of furfural
over KIT-6 supported various metal (M= Rh, Ru, Ni and Pd)
nanoparticle catalysts.
FAL.
A low FFR conversion of 15% was obtained after
increasing the temperature to 80 °C. When the reaction
temperature was increased to 90 °C, the conversion of FFR
increased gradually to 55%. A further increase in temperature to
100 °C caused a sharp increase in FFR conversion to 98%. A
constant FAL selectivity of 99% was observed at all
temperatures. The increase in FFR conversion could be
attributed to the increased hydrogenation reaction rate between
the adsorbed FFR and H species generated by Rh-formate
intermediate decomposition on the Rh surface rather than the
enhanced FA decomposition rate (see later in mechanism in
step 3 Fig.10). Therefore, the optimal temperature for CTH of
FFR reaction was 100 °C.
The effect of the solvent on the efficiency of the Rh/ED-KIT-6
catalyst (0.49 mol% Rh) was investigated using 1 mmol of FFR
and 3 mmol of FA at 100 °C for 5 h with 5 mL of ethanol, 1-
propanol, 2-propanol, or 1-butanol. Fig. 6 shows that the solvent
had a significant influence on the activity of the Rh/ED-KIT-6
catalyst both in conversion and selectivity. In the case of ethanol,
1-propanol, and 1-butanol, the FFR conversion and FAL
selectivity were observed in the range of 62-74% and 78-86%,
respectively.
Temp. FFR
FAL
Sel. (%)
TOF
(h−1 [a]
Entry Catalyst
(°C)
Conv. (%)
)
1
KIT-6
100
nr
nr
0
2
ED-KIT-6
100
100
100
100
70
nr
nr
0
3
No catalyst
Rh(III)/ED-KIT-6
Rh/KIT-6
nr
nr
0
4
nr
nr
0
5
19
tr
97
nd
99
99
nd
99
86
74
35
38
0
6
Rh/ED-KIT-6
Rh/ED-KIT-6
Rh/ED-KIT-6
Rh/ED-KIT-6
Rh/ED-KIT-6
Ru/ED-KIT-6
Pd/ED-KIT-6
Ni/ED-KIT-6
7
80
14
55
80
98
51
28
13
29
113
0
8
90
9
100
100
100
100
100
10
11
12
13
204
46
8.9
0.15
Reaction conditions: FFR=
1 mmol, FA= 4 mmol, 2-PrOH= 5 mL,
Temperature= 100 °C, Time= 5 h; nr= no reaction, tr= trace and nd= not
detected; [a] Turnover frequency (TOF = moles of FAL formed per mole of
metal active sites per hour).
A maximum FFR conversion of 81% and FAL selectivity of
95 % were obtained with 2-propanol as the solvent and low
etherification by-product of 2-(isopropoxymethyl) furan.
Etherification was favored mainly by the shorter chain and/or
primary alcohols with increased FFR-alcohol interactions and
steric effects. Therefore, 2-propanol was the best solvent among
the solvents tested.
No conversion of FFR was observed using the parent KIT-6,
ED-KIT-6, or without a catalyst using FA (Table 2, entries 1–3).
Furthermore, no conversions were detected using unreduced
Rh(III)/ED-KIT-6 (Table 2, entry 4), confirming the necessity for a
metal to decompose the FA and perform the hydrogenation of
FFR. In particular, 98% FFR conversion with 99% selectivity to
FAL (Table 2, entry 10) was observed when Rh/ED-KIT-6 was
used as the catalyst at 100 °C with FA as the hydrogen source,
showing that the presence of rhodium was indispensable for the
high catalytic activity. Rh/KIT-6 with actual Rh loading of 0.92
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Figure 8. ffect of reaction time in the CTH of FFR over Rh/ED-KIT-6
catalyst; Reaction conditions: FFR (1 mmol), FA (4 mmol), Rh/ED-
KIT-6 (0.49 mol% Rh), temperature (100 °C) and time (5 h).
Figure 6. Effect of solvent in the CTH of FFR; Reaction conditions:
FFR (1 mmol), FA (3 mmol), Rh/ED-KIT-6 (0.49 mol% Rh),
temperature (100 °C) and time (5 h).
To follow the progress of the reaction, the effect of the reaction
time was examined using 2-propanol with a FA/FFR mole ratio
of 4. Fig. 8 presents the reactant/product distribution of FFR,
FAL, and etherified products as a function of time. After 1 h
reaction, the FFR conversion and selectivity to FAL were ca. 28
and 92% with an etherified byproduct 2-(isopropoxymethyl) furan
selectivity of ca. 8%. When the reaction time was increased from
2 to 4 h, FFR conversion increased from 55 to 98% with
selectivity to FAL of 95 to 98 % accompanied by a decrease in
the selectivity to the etherified product (2%). These results show
that the etherification takes place at a short reaction time and
reverses as the reaction time increases, leading to the
hydrogenation of FFR to the desired FAL. At a 5 h reaction, the
highest FFR conversion of 98% and selectivity to FAL (> 99%)
was achieved. A further increase in reaction time to 6 h,
however, led to a decrease in the FAL selectivity to 96% due to
the formation of furan-2-yl-methyl formate by the subsequent
reaction of FAL with formic acid. Therefore, the optimum
reaction time was considered to be 5 h in CTH of FFR.
The influence of the FA to FFR mole ratio on the conversion
and selectivity was considered next with 2-PrOH as the solvent;
the results are shown in Fig. 7. With increasing FA/FFR mole
ratio from 2 to 3, FFR conversion increased from 56 to 81% with
an increasing FAL selectivity from 93 to 95%, which can be
attributed to the higher availability of H species generated
through the decomposition of rhodium formate (see later in
mechanism in step 3 Fig.10). A maximum FFR conversion of
98% with exclusive FAL selectivity of 99% was achieved when
the FA/FFR mole ratio was increased to 4. This study shows that
transfer hydrogenation rate had
a
close to first-order
dependence on the FA concentration. On the other hand, with
further increase in the FA/FFR mole ratio from 4 to 5, FFR
conversion remained constant at 98%, but the FAL selectivity
decreased to 94% due to the formation of furan-2-ylmethyl
formate as a side product by the reaction of FAL with excess
formic acid (see later in mechanism in step 5 Fig.10). Therefore,
the optimal FA/FFR mole ratio was close to 4.
Figure 7. Effect of FA to FFR mole ratio in the CTH of FFR;
Reaction conditions: FFR (1 mmol), Rh/ED-KIT-6 (0.49 mol% Rh), 2-
PrOH (5 mL), temperature (100 °C) and time (5 h).
Figure 9. FA dehydrogenation results over Rh/ED-KIT-6 catalyst
without FFR.
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A simple dehydrogenation of FA over Rh/ED-KIT-6 was also
carried out to examine the mechanism of the CTH of FFR to FAL
using FA as a hydrogen source. As shown in Fig. 9, no further
reaction took place after 1 h of FA dehydrogenation, as shown
by the constant volume of gas (H2+CO2) generated, whereas the
CTH reaction lasted for more than 5 h. Therefore, the catalytic
reaction did not take place by the H2 gas generated in-situ by FA
dehydrogenation. These results suggest that the present Rh-
Overall, these data established the optimal reaction conditions
of Rh/ED-KIT-6 (0.49 mol% Rh), 5 mL of 2-PrOH, and formic
FA/FFR molar ratio of 4 at 100 °C for 5 h achieving maximum
furfural conversion of 98% with high selectivity to FAL (> 99%) in
2-propanol medium. Several alternative metal nanoparticles with
potent hydrogenation activity were also prepared and tested for
the CTH of FFR: Ru/ED-KIT-6 (1.87 wt. %), Pd/ED-KIT-6 (5.01
wt. %) and Ni/ED-KIT-6 (4.03 wt. %); the figures inside the
bracket correspond to the actual weight percentage of the metal
loading according to ICP-OES analysis. Under the optimized
conditions, the catalytic performance of them was measured and
the results are summarized in Table 2 (entries 11–13). The
Ru/ED-KIT-6 catalyst system exhibited moderate activity,
whereas Pd/ED-KIT-6 and Ni/ED-KIT-6 catalysts showed inferior
activity. The Rh/ED-KIT-6 catalyst exhibited superior catalytic
activity and 4 times higher TOF (h-1) than the Ru, Pd and Ni
catalysts. The ultra-fine Rh particles in Rh/ED-KIT-6 must be
active for the hydrogenation of FFR but not as active for FA
decomposition to maintain the slow but steady release of active
H species in the presence of FA as the hydrogen donor; too high
decomposition and low activity for hydrogenation would
generate a large quantity of H2 gas, not the hydrogenation
product. In fact, Pd is known as the most active catalyst for FA
decomposition to H2 gas,[47] but as shown in Table 2, its CHT
reaction rate was far below that of Rh/ED-KIT-6.
Particle size effect on liquid phase hydrogenation reactions and
the formic acid decomposition is well-documented in literature.
As expected, the smaller the particle, the more active are the
reactions in general.[44, 48] Rh nanoparticles with small size (~2
nm) were shown to be highly active than larger size (~7 nm) in
hydrogenation.[49] Diverse host-guest interaction seems available
to prepare stabilized metallic nanoparticles on a support.[50] In
addition, the higher FA levels than stoichiometric result to the
formation of furan-2-yl-methyl formate with simultaneous
evolution of H2 and CO2, as represented in the mechanism (5 in
Fig. 10).
mediated hydrogenation of FFR is entirely
a
transfer
hydrogenation process involved with H species adsorbed on the
Rh surface, and suggests that FA dehydrogenation is not the
rate determining step.
Based on the above experimental results, a plausible reaction
mechanism was proposed as schematically illustrated in Fig. 10.
In the initial step, FA dehydrogenation takes place over the
rhodium surface of Rh/ED-KIT-6. The amine groups present in
the catalyst weakly dissociate the acid –OH of formic acid to
rhodium formate and form ammonium ion containing H+ species
(step 2 in Fig. 10). The decomposition of rhodium formate
results in the formation of CO2 and H− adsorbed on Rh surface
(3 in Fig. 10). The attack of H− and H+ in ammonium ion species
can lead to either the subsequent hydrogenation reaction with
the adsorbed FFR or the evolution of H2 gas. The H species
adsorbed on Rh species are involved in the successive transfer
hydrogenation that can be the rate determining step in the
transfer hydrogenation of FFR to FAL. The H+ and H− species
generated from FA are transferred to electron rich O and
electron deficient C of carbonyl group, respectively, to form FAL
(step 4 in Fig. 10). The consumption of H species in the rate
determining hydrogenation step will in turn become the driving
force for further FA dehydrogenation according to Le Chatelier’s
principle.
Table 3. Comparison of Rh/ED-KIT-6 catalytic efficiency with
representative works reported in the liquid phase hydrogenation of
furfural (FFR) to furfuryl alcohol (FAL).
Catalyst
H Source/
Pressure
(bar)
Solvent/
Additive
Temp.
(°C)
FAL
Yield
(%)
27
Ref.
Pt/C
H2/30
H2O/H3P
O4
175
11
Pt-Sn0.3/SiO2
Ir-ReOx/SiO2
Pd/Al2(SiO3)3
H2/10
H2/8
H2/20
2-PrOH
H2O
PhCH3/C
H3COOH
MTHF
CH3OH
2-PrOH
C2H5OH
C2H5OH
2-PrOH
Octane
2-PrOH
100
30
150
96
97
30
12
14
15
Ru/C
H2/25
H2/50
H2/10
H2/17
H2/20
H2/20
H2/90
HCOOH
165
80
100
80
150
170
160
100
42
90
34
78
88
80
89
97
16
17
18
19
20
21
22
This
work
MoNiB/γ-Al2O3
Ni-Sn0.2/SiO2
Ni74.5 P12.1 B13.4
Co/SBA-15
Cu-Co/SBA-15
Cu-Fe
Rh/ED-KIT-6
Figure 10. Plausible reaction mechanism for the CTH of FFR
through FA dehydrogenation.
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10.1002/cctc.201701037
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Table 3 lists the catalytic efficiency of the present Rh/ED-KIT-6
system, which was compared with the representative works
reported in the liquid phase hydrogenation of FFR. Hronec et
al.[11] reported that Pt/C was active in the acid-assisted direct
hydrogenation of FFR to FAL. This system involves the use of
hydrogenation activity of the catalysts for at least three repeated
cycles.
[12]
acid additives with a very low FAL yield of 27%. Merlo et al.
showed that the Pt-Sn bimetallic catalyst was effective in the
hydrogenation of FFR at high H2 pressures, which produced a
high FAL yield of 96%. A similar study has been reported by the
same group using the Ni-Sn bimetallic system. Unfortunately,
these catalyst systems showed drastic deactivation in the
successive recycles. Although Ir-ReOx/SiO2 was reported highly
active for the direct hydrogenation of FFR to FAL at low
temperatures and mild H2 pressures (< 1.0 MPa), most
heterogeneous catalysts usually require high pressures in the
range of 1.0-9.0 MPa and temperatures (>100 °C). Instead, the
present Rh/ED-KIT-6 catalyst in the transfer hydrogenation
mode in 2-PrOH medium was sufficiently active and could be
operated under mild reaction conditions. The structural and
textural properties of the Rh/ED-KIT-6 catalyst containing the
nitrogen functionality and ultra-small Rh nanoparticles appear to
be a contributing factor in the CTH of FFR to FAL. The large
pore diameter in high surface area of the KIT-6 (658 m2g-1)
support provide ample space for diamine-functionalization. The
crucial role of nitrogen is to serve as an anchoring site for the
deposition and stabilization of Rh nanoparticles in addition to the
ordered mesopores in the KIT-6 acting as nano-reactors. The
stabilization of Rh nanoparticles inside the pore channels of ED-
KIT-6 occurred through an electrostatic attraction between the
holes formed in the d-band of the Rh metal with the lone pair
electrons of nitrogen in the amine functional group.[31] In addition,
the weakly basic amine groups can facilitate the cleavage of an
O–H bond in FA to form HCOO− and H+, which serve as
effective active sites for FA decomposition.[45] The collective use
of nitrogen functionality and the ultra-small Rh nanoparticles is
the innovative feature of the present catalytic system, which
means that CTH is a clean and efficient protocol for the selective
hydrogenation of FFR to FAL.
Figure 11. Heterogeneity of Rh/ED-KIT-6 catalyst in the CTH of FFR.
Figure 12. Recyclability of Rh/ED-KIT-6 catalyst in the CTH of FFR.
To ascertain that the nature of catalytic process is
heterogeneous, a hot filtering test was conducted and the results
are shown in Fig. 11. Under the optimized reaction conditions,
the Rh/ED-KIT-6 catalyst was removed after a 2 h reaction, and
the reaction was continued for a further 3 h. No further
conversion of FFR was detected, which confirms the
heterogeneity of the catalyst. Furthermore, possible Rh leaching
was tested after the catalytic reaction by analyzing the filtrate
solution using ICP-OES, which did not detect Rh species (< 0.1
ppm). The Rh content in the recycled catalyst after three runs
was 0.96 wt%, which is very close to that of the fresh catalyst,
indicating that there was negligible leaching of Rh during the
reaction.
The recyclability of the Rh/ED-KIT-6 catalyst was finally
evaluated in the CTH of FFR under the optimized reaction
conditions. After the catalytic reaction, the catalyst was isolated
from the liquid phase by centrifugation, washed thoroughly with
ethanol, vacuum dried at 80 °C, and reused as a catalyst in the
subsequent runs under identical reaction conditions. The results
shown in Fig. 12 indicated no obvious decrease in the
The decrease in catalytic activity upon further repeated cycles
can be attributed to the partial oxidation of rhodium species in air,
as reported previously in the hydrogenation reaction with
rhodium nanoparticles deposited on the SiO2 support.[51] This
was confirmed by XPS analysis of the re-used Rh/ED-KIT-6
catalyst after three cycles, which showed the binding energies of
Rh 3d5/2 and Rh 3d3/2 centered at approximately 308.6 and 313.0
eV corresponding to Rh (I) species.[49] On the other hand, the
Rh(I) species was still active in the hydrogenation of carbonyl
groups[52] and exhibited moderate catalytic activity in the fourth
cycle. As mentioned earlier, the Rh(III)/ED-KIT-6 with binding
energies of Rh 3d5/2 and Rh 3d3/2 at 309.5 and 314.2 eV in XPS
(Fig. 5) was found to be completely inactive (entry 4a, Table 2)
in the transfer hydrogenation of FFR. In addition, TEM analysis
of the used Rh/ED-KIT-6 catalyst was done after the third cycle,
which showed uniform and small sized nanoparticles in the
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10.1002/cctc.201701037
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and dried at 100 °C for 24 h in an oven. Surfactant-free KIT-6
was obtained after calcination at 550 °C at a heating rate of
2 °C/min for 6 h in air.
range of 1.2-1.4 nm without agglomeration (see Fig. S1 in
supporting information).
Synthesis of diamine-functionalized KIT-6 (ED-KIT-6)
Conclusions
Prior to the functionalization, KIT-6 was dried in an oven at
120 °C for 4 h under vacuum to remove the physisorbed water.
Diamine-functionalized KIT-6 was synthesized according to the
procedure described in previous report.[38] Typically, 3 mL of N1-
[(3-trimethoxysilyl) propyl)] ethane-1,2-diamine was added drop
wise to a suspension of vacuum dried KIT-6 (1 g) in dry toluene
(50 ml) under a N2 atmosphere and heated under reflux for 24 h.
Subsequently, the suspension was filtered, washed thoroughly
with dry toluene and anhydrous ethanol. The resulting material
was dried under vacuum overnight and called ED-KIT-6.
The Rh/ED-KIT-6 catalyst synthesized by post-synthetic amine-
functionalization followed by chemical reduction of the
immobilized Rh3+ resulted in Rh nanoparticles with a mean
particle size of 1.2 nm on KIT-6. The catalyst was active for the
catalytic transfer hydrogenation of FFR to FAL using FA as the
hydrogen source with 2-propanol as a solvent under mild
reaction conditions in the absence of other additives. The
superior catalytic activity was attributed to the cooperative effect
of nitrogen-functionality and the ultrafine Rh nanoparticles. The
catalyst was recycled successfully for up to 3 cycles without any
obvious loss of catalytic activity. The proposed mechanism
involved two reaction steps of FA dehydrogenation and
hydrogenation of FFR using the adsorbed hydrogen species,
and could clearly explain the effects of temperature, reaction
time, and FA to FFR mole ratios on FFR conversion.
Synthesis of KIT-6 supported metal (M= Rh, Ru, Pd and Ni)
nanoparticle (NP) catalysts
KIT-6 supported metal nanoparticles were synthesized in
accordance with our previous report with slight modification.[39] In
a
typical synthesis, the requisite amount of rhodium(III)
acetylacetonate precursor corresponding to 1 wt% Rh was
dissolved in dry dichloromethane (DCM), to which 1 g of ED-
KIT-6 was added and stirred continuously at room temperature
followed by the solvent evaporation. The resultant solid material
was dried at room temperature for 12 h. The material was then
reduced with NaBH4 in isopropanol, filtered, washed thoroughly
with copious amount of water, ethanol and dried at 80 °C to
obtain the final Rh/ED-KIT-6 catalyst. For the Ru, Pd and Ni
supported on KIT-6 catalysts, the aforementioned synthesis
procedure was repeated by replacing the Rh precursor with the
required amounts of ruthenium(III) acetylacetonate (2 wt. % Ru),
palladium(II) acetate and nickel(II) acetylacetonate (5 wt. % Pd
and Ni), designated as Ru/ED-KIT-6, Pd/ED-KIT-6 and Ru/ED-
KIT-6, respectively.
Experimental Section
Materials
Triblock copolymer pluronic P123 (EO20PO70EO20, M = 5800),
Tetraethyl orthosilicate (TEOS, 98%) and 1-butanol (1-BuOH,
99%) were purchased from Sigma Aldrich and used as a
template, silica source and co-surfactant, respectively. N1-[3-
(trimethoxysilyl)propyl] ethane-1,2-diamine (98%), rhodium(III)
acetylacetonate
(Rh(acac)3,
98%),
ruthenium(III)
acetylacetonate (Ru(acac)3, 98%), palladium acetate (Pd(OAc)2,
98%), nickel(II) acetylacetonate (Ni(acac)2, 98%), furfural (FFR,
98%), formic acid (FA, 95%), 2-propanol (2-PrOH, 98%),
dichloromethane (DCM anhydrous, 99%), sodium borohydride
(NaBH4, 99%) and hydrazine monohydrate (N2H4.H2O, 98%)
were obtained from Sigma Aldrich. Hydrochloric acid (HCl, 36%),
ethanol (EtOH, 99.95%) and ethylacetate (EtOAc, 99.5%) were
supplied from Samschun and Merck chemicals. All chemicals
were used as received. Deionized water was produced using a
millipore Milli-Q ultrapure water purification system.
Catalyst characterization
Low angle powder X-ray diffraction (XRD) of the catalysts was
recorded on Rigaku D/MAX2550 diffractometer (M/s. Rigaku
Corporation, Japan) using Ni-filtered Cu-Kα radiation (λ= 1.54 Å)
at a scanning rate of 0.02° with a scan speed of 2° min−1 over
the 2θ range of 0.7–5° at 50 kV and 20 mA. The N2 adsorption
isotherms were obtained at −196 °C using a BELsorpII-Max
(BEL, Japan). Prior to the adsorption measurements, the
samples were out-gassed under vacuum at 120 °C for 12 h. The
specific surface area, pore volume, and pore diameter of the
samples were calculated using the BET (Brunauer-Emmett-
Teller) and BJH (Barrett-Joyner-Halenda) methods, respectively.
Fourier transform infrared (FT-IR, Bruker VERTEX 80v)
spectroscopy was performed using KBr disks at room
temperature with a scan range of 4000–400 cm−1. Field emission
transmission electron microscopy (FE-TEM, FEI TECNAI 30 G2
S-TWIN) with energy dispersive X-ray analysis (EDX) was
carried out at an accelerating voltage at 100 kV. The specimens
were prepared by dispersing the samples in ethanol under
ultrasonic radiation and evaporating the resulting suspension on
a lacey carbon-coated copper grid. Inductive coupled plasma-
Synthesis of mesoporous silica KIT-6
KIT-6 was synthesized using the procedure described
elsewhere.[37] The molar gel composition of the reaction mixture
was TEOS : P123 : HCl : H2O : BuOH = 1 : 0.017 : 1.83 : 195 :
1.31. In a typical synthesis, a homogeneous solution was
obtained by dissolving 4.0 g of P123 (Sigma-Aldrich) and 7.9 g
of 35.0 wt% hydrochloric acid in 144 g of deionized water with
magnetic stirring for 8 h at 35 °C. To this solution, 4 g of 1-
butanol was added at once followed by stirring for 1 h at 35 °C,
8.6 g of TEOS was dropped into the P123-butanol–HCl solution
and stirred continuously at 35 °C for 24 h. Subsequently, the
mixture solution was aged at 100 °C for 24 h under static
hydrothermal condition in a teflon-lined autoclave. The white
precipitated product was hot-filtered with ethanol/water washing
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10.1002/cctc.201701037
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optical emission spectroscopy (ICP-OES, Perkin-Elmer Optima
7300 DV) was used to determine the actual metal loading.
Elemental analysis (EA) for nitrogen was performed on an
Elementar Vario EL-III analyzer. The surface electronic states
were examined by X-ray photoelectron spectroscopy (XPS,
Keywords: Catalytic transfer hydrogenation • KIT-6 • Rh
nanoparticle • furfural • formic acid
Thermo VG ESCALAB250) using Al Kα radiation and
a
hemispherical analyzer. The XPS data were calibrated internally
to the binding energy of C 1s at 284.6 eV.
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Graphical Abstract
Chinna Krishna Prasad Neeli,[a] Young-
Min Chung[b] and Wha-Seung Ahn*[a]
Catalytic transfer hydrogenation of
furfural to furfuryl alcohol using ultra-
small Rh nanoparticles embedded on
diamine-functionalized KIT-6
Text for Table of Contents
Rh/ED-KIT-6 nanocatalyst exhibited high TOF (204 h–1) in the transfer hydrogenation of furfural using formic acid under mild reaction
conditions.
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