Reductive amination of furfural over Me/SiO2-SO3H (Me: Pt, Ir, Au) catalysts

Article abstract of DOI:10.1016/j.molcata.2014.05.014

Gold, platinum and iridium colloids were supported on sulfonic acid-functionalized silica and were employed for the reductive amination of furfural. The catalysts were characterized by N₂-physisorption at 77 K, X-ray diffraction, H₂

Full text of DOI:10.1016/j.molcata.2014.05.014

Accepted Manuscript
Title: Reductive amination of furfural over me/sio -so h (me:
2
3
pt, ir, au) catalysts
Author: Jos e´ J. Mart ´ı nez Eliana Nope Hugo Rojas Mar ´ı a H.
Brijaldo Fabio Passos Gustavo Romanelli
PII:
S1381-1169(14)00211-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.05.014
MOLCAA 9114
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
22-3-2014
17-5-2014
19-5-2014
´
Please cite this article as: J.J. Martinez, E. Nope, H. Rojas, M.H. Brijaldo, F. Passos,
G. Romanelli, REDUCTIVE AMINATION OF FURFURAL OVER Me/SiO -SO H
2
3
(Me: Pt, Ir, Au) CATALYSTS, Journal of Molecular Catalysis A: Chemical (2014),
http://dx.doi.org/10.1016/j.molcata.2014.05.014
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*Graphical Abstract (for review)
Page 1 of 28

*Highlights (for review)
Highlights
Reductive amination of furfural over Me/SiO -SO H (Me: Ir, Pt and Au) catalysts. >The
>
2
3
catalysts are active due to the interaction of sulfonic groups and metal sites. > This interaction
favors the hydrogenation step increasing the yield to this amine. >This interaction was
evidenced by XPS analysis.
Page 2 of 28

Clear version
REDUCTIVE AMINATION OF FURFURAL OVER Me/SiO -SO H (Me: Pt, Ir,
2
3
Au) CATALYSTS
1
*
1
1
José J. Martínez , Eliana Nope , Hugo Rojas ,
2
2
3
María H. Brijaldo , Fabio Passos , Gustavo Romanelli
1
Escuela de Ciencias Químicas, Facultad de Ciencias. Universidad Pedagógica y
Tecnológica de Colombia UPTC, Avenida Central del Norte, Tunja, Boyacá, Colombia.
2
Departamento de Engenharia Química e de Petróleo, Universidade Federal
Fluminense, R. Passos da Pátria, 156, Niterói, RJ, 24210-240, Brasil.
3
Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. J.J. Ronco”
(CINDECA), Departamento de Química, Facultad de Ciencias Exactas, UNLP-CCT-
CONICET. Calles 47 Nº 257, B1900 AJK, La Plata, Argentina.
*
Corresponding author‟s e-mail: jose.martinez@uptc.edu.co
Abstract
Gold, platinum and iridium colloids were supported on sulfonic acid-functionalized
silica and were employed for the reductive amination of furfural. The catalysts were
characterized by N -physisorption at 77 K, X-ray diffraction, H chemisorption and
2
2
XPS. The results indicated that Ir, Au and Pt colloids were uniformly dispersed on the
SiO -SO H support. XPS results showed the interaction of sulfonic acid groups and
2
3
metal sites which favors the formation of imine and its subsequent hydrogenation to
amine.
Keywords: reductive amination, furfural, metal colloids, sulfonated silica
1
Page 3 of 28

1
. Introduction
Reductive amination involving the reaction of carbonyl compounds with amines
remains one of the most versatile and useful synthetic routes for the synthesis of amines
and their derivatives, which are used as synthetic organic intermediates in
pharmaceuticals and agrochemicals. The process involves the formation of an imine or
iminium intermediate followed by its reduction to amine. Commonly, NaBH has been
4
employed with various Brönsted acids for successful reductive amination; however,
these acids are corrosive, toxic and difficult to separate from the reaction solution and
therefore, it is interesting to substitute them with more environmentally friendly solid
acids [1]. Thus, acid solids combined with NaHB such as H PW O /NaBH [1],
4
3
12 40
4
NaBH /wet clay, [2] NaBH /Brönsted acidic ionic liquid, [3] NaBH /cellulose sulfuric
4
4
4
acid [4], NaBH /silica-gel–supported sulfuric acid [5], among others, have been used.
4
Recently, Deng et al., [6] using sulfonic acid supported on hydroxyapatite and in the
presence of sodium borohydride, reported the preparation of secondary or tertiary
amines with high yields, with the advantage of easy separation by applying a magnetic
field.
Another approach is direct reductive amination using heterogeneous catalysts, in which
both reducing sites such as acid–base sites are necessary [7-10]. The presence of acid
sites ensures imine formation. In fact, the presence of Brönsted acid sites significantly
increases the amine yield [7], as similarly occurs in NaBH supported catalysts. Acid
4
sites significantly increase the reactivity, as has been observed in Pd/C [8,11] and
Ru/C catalysts [11] pretreated with (NH ) S O , which favored the acidity of the carbon
4
2
2
8
surface and consequently, the reactivity and selectivity of carbon-supported noble metal
catalysts in the reductive amination of benzaldehyde with ammonia. However, the
2
Page 4 of 28

selectivity strongly depends on the metal employed [11] and on the hydrogenation step
of imine intermediates [9].
For controlling reaction selectivity in heterogeneous catalysis, it is necessary to prepare
catalysts that exhibit only specific types of sites required for desired product formation.
Recently, Zhu et al. [12] argued that over Ru/SiO -SO H catalysts, in the hydrogenation
2
3
step the substrate molecules might be more easily desorbed from the Ru center due to
poisoning/coordination of the sulfonic acid group to the metal center, which avoids
further side reactions and inherently favors the catalytic activity. Considering these
results, in this work we prepared Au, Ir and Pt colloids stabilized with
polyvinylpyrrolidine (PVP) deposited over SiO grafted with sulfonic acid. In this
2
bifunctional catalyst, sulfonic acid groups should act as active sites for the first step of
acid hydrolysis and as metal nanoparticles for hydrogenation in the second step. The
influence of metal sites and the proximate sulfonic acid group through the electron
transfer in the bifunctional catalyst should produce an effect on the hydrogenation step.
The reaction of furfural with aniline was chosen as a model reaction for direct reductive
amination (Fig. 1). Furfural is an interesting platform molecule due to the high amount
of hemicellulose produced by hydrolysis and nowadays seems to be the only
[
13]
unsaturated large-volume organic chemical prepared from carbohydrate sources
.
-H2O
+ H2
N
NH
+
H
O
O
O
NH2
O
furfural
aniline
(E)-1-(furan-2-yl)-N-phenylmethanimine
N-(furan-2-ylmethyl)aniline
Figure 1. Reductive amination of furfural with aniline
Our aim was to demonstrate that the use of a bifunctional catalyst favors the reductive
amination of furfural with aniline. The reaction should proceed slowly without the
3
Page 5 of 28

acidic sites; however, the acid/acidic sites should protonate carbonyl group/groups (si
fuera solo uno, debería ser „the carbonyl group), making carbonylic carbon a better
electrophile and activating it more easily towards nucleophilic attacks in the first step,
which favors the hydrogenation of imine and consequently, the desired amine
formation.
2
2
. Experimental
.1 Materials and methods
Toluene (Panreac, 99.5%), 3-mercaptopropyltrimethoxysilane (MPTMS) (Sigma-
Aldrich, 95%), hydrogen peroxide (H O ) (Panreac, 30%), hydrochloric acid (Sigma-
2
2
Aldrich, 37%), FeCl ·6H O (Sigma-Aldrich, 99%), FeCl ·3H O (Sigma-Aldrich, 97%),
3
2
2
2
sodium
hydroxide
(Panreac,
98%),
acetone
(Sigma-Aldrich,
99.5%),
tetraethylorthosilicate (TEOS) (Sigma-Aldrich, 99%), aluminum isopropoxide (Fluka,
7%), ethanol (Sigma-Aldrich, 99.5%), D-xylose (Sigma-Aldrich), commercial silica
and alumina (Syloid and BASF, respectively) were used.
9
2
.2 Catalyst preparation
2
.2.1 Synthesis of SiO -SO H
2
3
2
A mixture of 5 g of SiO (Syloid SBET = 283 m /g) or γ-Al O (BASF) in 100 mL of dry
2
2
3
toluene, and 3-mercaptopropyltrimethoxysilane (MPTMS) (1.15 mL, 6.1 mmol) was
refluxed for 24 h. The solid obtained was washed with toluene and dried at 363 K.
Subsequently, mercaptopropyl groups were oxidized to sulfonic acid groups with excess
hydrogen peroxide at room temperature for 24 h, a few drops of H SO were added
2
4
during 12 h, and the solid was washed with acetone and dried at 393 K. The solids
obtained were SiO -SO H and Al O -SO , respectively.
2
3
2
3
3
4
Page 6 of 28

2
.2.2 Synthesis of Me/SiO -SO H (Me: Au, Ir, Pt)
2 3
The synthesis of PVP-Me colloids was similar to that described in the literature [14].
Briefly, 1.11 g PVP and 0.130 g of metal precursor (HAuCl , H IrCl and H PtCl ) were
4
2
6
2
6
dissolved in a mixed solution of 130 mL methanol and 150 mL water, and then 20 mL
.1 M NaOH methanol solution was added dropwise with vigorous stirring. The
0
reaction mixture was refluxed to give a color change; a homogeneous black colloidal
solution was obtained for Pt and Ir, while for Au a violet colloidal solution was formed.
The colloids were supported on SiO -SO H with stirring and were maintained at room
2
3
temperature for 6 h under stirring, then they were dried under vacuum at 353 K.
2
.3 Characterization of catalysts
The X-ray diffraction measurements were carried out with a Rigaku Miniflex II using
Cu Kα radiation (λ= 1.54056 Å). The XRD patterns were recorded in the 2θ range of
1
0–90º, using a count time of 1 s and a step size of 0.05°. Crystallite sizes were
calculated from the line broadening of the main XRD peaks by using the Debye-
Scherrer equation.
Textural properties of the solids were obtained by N adsorption-desorption isotherms
2
measured at 77 K in Micromeritics ASAP 2020 equipment. Samples were previously
evacuated at 623 K for 16 h. The BET method was used to calculate the total surface
area of the samples.
The metal dispersion of catalysts was evaluated by H -O titration using Micromeritics
2
2
ASAP 2020 apparatus at 308 K. The catalysts were activated under vacuum at 373 K
for 2 h, followed by evacuation at 308 K for 60 min. After cooling at 308 K, the
5
Page 7 of 28

catalysts were subjected to a flow of O to form Me-O surface species and then
2
evacuated for 30 min at 308 K for 2 h. Subsequently, H analysis at 308 K was
2
performed. The metal dispersion, surface area, and particle size were determined from
the hydrogen uptake isotherm obtained, assuming a H/Me stoichiometry of 1.5.
The sample surfaces were characterized by XPS. Data were obtained from a Thermo
Scientific Escalab 250 XI Photoelectron spectrometer. Measurements were performed at
room temperature with monochromatic Al Kα radiation (hv = 1486.6 eV). The analyzer
was operated at 25 eV pass energy and a step size of 0.05 eV. The pressure in the
-8
analytical chamber was 6.3 x 10 mBar. The signal of C 1s (284.6 eV) was used as
internal energy reference. Core-level peak positions were determined after background
subtraction using CasaXPS software. Peaks in a spectrum were fitted by a combination
of Gaussian and Lorentzian curves, which also allowed separating overlapping peaks.
The acid capacity was determined by titration with 0.01 M NaOH (aq) [15]. In a typical
experiment, 0.1 g of solid was added to 10 mL of deionized water. The resulting
suspension was allowed to equilibrate and thereafter was titrated by dropwise addition
of 0.01 M NaOH solution using phenolphthalein as pH indicator.
2
.4 Catalytic studies
The liquid phase amination reductive reaction was carried out in a batch-type reactor at
a constant stirring rate (1000 rpm) under the following conditions: hydrogen partial
pressure, 5 MPa; catalyst sample, 25 mg; 40 mL of a 0.1 M solution of furfural and 0.1
M solution of aniline in distinct solvents, and reaction temperature of 363 K. Reaction
products were analyzed in a GC-MS Varian 3800 furnished with a β-Dex column, He as
carrier, and constant temperature of 393 K.
3
. Results and discussion
6
Page 8 of 28

3
.1 Characterization of catalysts
Table 1 summarizes the textural properties of the solids obtained from the N2
adsorption-desorption isotherms at 77 K. The solids showed type IV isotherms
according to the IUPAC classification (Fig. 2). The impregnation with the metal
colloids significantly affected the textural properties, causing a slight decrease in
surface area and increasing the pore volume and pore size.
The acid capacity of the samples was determined by titration with 0.01 M NaOH (aq).
+
The results in terms of mmol H /g of SO H groups are listed in Table 2. The
3
impregnation with colloids decreased the acid capacities, possibly due to some
interaction of colloids with the sulfonic groups. This fact might also explain the
increases in the textural properties of Me/SiO -SO H catalysts.
2
3
a)
b)
1
1
1
4
2
0
8
6
4
2
0
30
25
20
15
10
5
0
0
,0
0,2
0,4
0,6
0,8
1,0
0
,0
0,2
0,4
0,6
0,8
1,0
P/P0
P/P
0
c)
d)
3
2
2
1
1
0
5
0
5
0
5
0
30
25
20
15
10
5
0
0,0
0,2
0,4
0,6
0,8
1,0
0
,0
0,2
0,4
0,6
0,8
1,0
P/P
0
P/P
0
Fig. 2. N physisorption isotherms of (a) SiO -SO H; (b) Ir/SiO -SO H; (c) Pt/SiO -
2
2
3
2
3
2
SO H; (d) Au/SiO -SO H.
3
2
3
7
Page 9 of 28

Table 1. Textural properties and acid capacities by NaOH titration (mmol H/g) of the
different solids studied
2
-1
Catalyst
SBET (m g ) Pore volume Pore size
Acid capacities
3
-1
+
(
cm g )
(nm)
(mmol H /g of SO H)
3
SiO -SO H
251
170
174
172
0.45
7.2
35.8
7.14
8.18
8.93
2
3
Au/SiO -SO H
0.90
0.98
0.88
21.2
21.8
20.5
2
3
Ir/SiO -SO H
2
3
Pt/SiO -SO H
2
3
The XRD patterns of catalysts are shown in Fig. 1. All catalysts exhibit a broad XRD
peak assigned to amorphous silica. For Au/SiO -SO H, signals characteristic of metallic
2
3
gold (2θ = 38.27°, 44.6°, 64.68° and 77.55°; JCPDS No. 04-0784) are observed. The
Pt/SiO -SO H catalyst shows a signal near 2θ = 39.67° and 46.18° due to the presence
2
3
of platinum crystallites (JCPDS No. 65-2868), while no reflections were detected for
the Ir/SiO -SO H catalyst. The crystal size of Au and Pt catalysts using the (1 1 1)
2
3
reflection peaks at 2θ=38.27° and 39.67° is listed in Table 2.
Table 2 tabulates the dispersion (DH/Me) and the particle size (d ) assuming spherical
s
particles calculated from H chemisorption at 308 K. The Au catalyst did not show
2
hydrogen adsorption. The Pt/SiO -SO H catalyst particle size determined by XRD and
2
3
H -chemisorption analysis was similar.
2
8
Page 10 of 28

a
b
c
0
20
40
60
80
100
2

Fig. 3. X-ray patterns of supported Au catalysts: (a) Au/SiO -SO H, (b) Pt/SiO -SO H,
2
3
2
3
and (c) Ir/SiO -SO H. Note: peak assignments: (▪) Pt, ( ) Au.
2
3
Table 2. Particle size (d ) and geometrical dispersion (D) from XRD and the dispersion
p
measured by H chemisorption analysis.
2
*
**
Catalysts
dp-DRX, nm
4.1
D(H/Me)
dp-Quím,nm
n.d.
Au/SiO -SO H
n.d.
2
3
Ir/SiO -SO H
n.d.
0.53
2.1
2
3
Pt/SiO -SO H
3.7
0.32
3.5
2
3
*
**
Estimated from XRD results by Debye-Scherrer equation, Calculated from the
results of hydrogen-oxygen titration, n.d.: not determined
The surface of the catalysts was characterized by XPS analysis. Figures 4 a-c display
XPS spectra in the region of Au, Ir and Pt 4f7/2 core levels for the metal colloids. The
metal 4f (Me 4f ) is characterized by the doublet of two spin-orbit components, viz., Me
4
4
f7/2 and Me 4f . In this work, the values of binding energy (BE) will refer to the Me
5/2
f peak for comparison with previous reports. The Au/SiO -SO H spectrum (Fig. 4 a)
7
/2
2
3
9
Page 11 of 28

shows a binding energy of Au 4f7/2 at 83.2 eV, which is associated with Au° species.
Claus et al. [16] found a similar value (BE = 83.28 eV) for Au/SiO catalysts prepared
2
by chemical vapor deposition. However, the reported normal value of Au° species is
δ-
8
4.0 eV, and Au species have lower values [17].
In Pt/SiO -SO , the Pt 4f peak was located near 70.1 eV, which could be assigned to
2
3
7/2
zero-valent Pt. Generally, the Pt° 4f7/2 peak is reported at higher binding energy (71.2
eV). The other contributions of Pt 4f7/2 are at about 70.9 and 72.1 eV, which could
probably be associated with Pt(II) and Pt(IV), respectively.
The XPS spectra of Ir/SiO -SO collected in the region of Ir 4f core level show two
2
3
contributions to Ir 4f , namely, at 60.2 eV and 62.5 eV (Fig. 3 b). The first
7
/2
0
contribution should be associated with Ir atoms, while the other should correspond to
δ+
partially oxidized Ir . As in Pt and Au colloids, the BE of this species is always
reported at the highest values [18-21].
An electron transfer does not explain the low binding energy of Au 4f , Pt 4f and Ir
7
/2
7/2
4
f electrons, since SiO is a nonreducible oxide and in the synthesis method it does
7/2 2
not favor an SMSI effect. In this sense, this result should be associated with the particle
size, as it is well known that the coordination number of a surface atom is smaller in a
nanocluster than in bulk. So, the electron density is larger on the surface of a
nanocluster and hence, the binding energy is smaller than in bulk metallic metal [22,23].
However, as the colloids showed distinct particle sizes (XRD results and dispersion
measurements), the shifting to a lower binding energy (1.0 eV) on the surface metal
atom in Au, Pt and Ir colloids should be explained considering some interaction with the
sulfonic groups of the support. Recently, Zhu et al. [12] reported this interaction
between the sulfonic groups and Ru nanoparticles in Ru/SiO -SO H catalysts; the
2
3
1
0
Page 12 of 28

electron transfer between the sulfonic groups and Ru nanoparticles was evidenced as a
shift to lower binding energy values. Thus, this fact should be taken into account for
explaining the results in metallic colloids supported on SiO -SO H.
2
3
a.
c.
b.
80
82
84
86
88
90
68
70
72
74
76
78
Binding Energy (eV)
Binding Energy (eV)
56
58
60
62
64
66
68
Binding Energy (eV)
Fig. 4. XPS spectra in the region of Me 4f7/2 and Me 4f5/2 of (a) Au/SiO -SO H, (b)
2
3
Pt/SiO -SO H, and (c) Ir/SiO -SO H.
2
3
2
3
3
.2 Catalytic activity
The reaction of furfural with aniline in the presence of Pt/SiO -SO catalyst was chosen
2
3
as a model reaction for reductive amination studies. In order to optimize the reaction
conditions, distinct polar solvents were tested due to the higher yield of the desired
product in other reports [10]. Table 3 summarizes the solvent effect on the conversion
1
1
Page 13 of 28

and yield of the reductive amination of furfural with aniline. The highest conversion and
yield to amine desired was obtained with ethyl acetate.
Lateral reactions were observed mainly due to acid sites with alcoholic solvents. This
fact decreases the formation of imine. Thus, methanol reacts rapidly with furfural
forming furfuryl acetal, which competes with the formation of imine groups. The
formation of acetal was confirmed using the SiO -SO H support in alcoholic medium,
2
3
where the formation of imine was also observed.
Although water is an environmentally benign, safe, and inexpensive solvent, a high
reversibility in the formation of imine, as well as the formation of a hydroxylamine
derivate, occurs.
In THF solvent condensation products were observed, while ethyl acetate showed the
highest amine formation at the reaction conditions set and no side reactions developed.
This can be explained considering that ethyl acetate inhibits the formation of imine, due
to protonation of aniline (decreased nucleophilicity, lower yield to imine). However, in
THF aniline would not be protonated and would be available to act as a nucleophile
(highest yield to imine), although the secondary amine formed condenses again, which
produces the tertiary imine (other condensation products) following an analogous
mechanism for the formation of the secondary amine by-product. With the aim of
controlling the selectivity to secondary amine, ethyl acetate was chosen for further
optimization studies.
Table 3. Solvent effect on the reductive amination of furfural with aniline. Reaction
conditions: furfural, 6 mmol; aniline, 5 mmol; catalyst, 250 mg of Pt/SiO -SO H; ethyl
2
3
acetate, 40 mL; H pressure, 5 MPa; and room temperature; time, 8 h.
2
1
2
Page 14 of 28

*
Solvent
Methanol
Water
Conversion (%) Yield to imine (%) Yield to amine (%)
46
24
72
69
41.4
13.2
51.2
66.1
5.1
11.3
21.3
14.9
Ethyl acetate
THF
*
Conversion of aniline determined by GC.
The metal effect was studied using Pt, Ir and Au supported on SiO -SO H as catalysts.
2
3
The results are shown in Fig. 5 and Table 4. It is clear that metals such as Ir and Pt
favor the hydrogenation step, while the Au catalyst, which poorly dissociates molecular
hydrogen, gives the lowest conversion and yield to amine. The low activity of the Au
catalyst could be related to the low hydrogen dissociation on gold catalysts [24-26]. The
behavior of Ir and Pt catalysts is similar, and the trend is to increase the desired yield to
amine with the time of reaction. In this reaction, the yield is not modulated by the
nature of the metal, contrary to the results reported by Gomez et al., [11]. This can be
explained considering that surfaces grafted with sulfonic groups alter the adsorption of
the substrate on the metallic surface, due to the poisoning/coordination of the sulfonic
acid group to the metal center [12], which avoids further side reactions favoring the
selectivity of the reaction. This type of interaction simultaneously serves to enhance the
selectivity and the catalytic activity. Although a slight increase in the yield to amine is
observed with the use of Ir catalysts, it is probably due to a smaller particle size [9]
1
3
Page 15 of 28

3
2
1
0
0
0
0
0
1
2
3
4
5
6
7
8
9
Time,(h)
Fig. 5. Effect of the metal employed on furfural amination. (○) Ir/SiO -SO H, (■)
2
3
Pt/SiO -SO H, (▲) Au/SiO -SO H. Reaction conditions: furfural, 6 mmol; aniline, 5
2
3
2
3
mmol; catalyst, 0.15 mol %; ethyl acetate, 40 mL; H pressure, 5 MPa; room
2
temperature; time, 8 h.
Table 4. Effect of the metal on the reductive amination of furfural with aniline.
Reaction conditions: furfural, 6 mmol; aniline, 5 mmol; catalyst, 0.15 mol %; ethyl
acetate, 40 mL; H pressure, 5 MPa; and room temperature; time, 8 h.
2
Catalysts
Conversion (%)*
Yield to imine (%) Yield to amine (%)
Au/SiO -SO H
48
58
71
45
38
50
3
2
3
Ir/SiO -SO H
19
21
2
3
Pt/SiO -SO H
2
3
*
Conversion of aniline determined by GC
In order to analyze the effect of the sulfonic groups present in SiO , Pt/SiO catalysts
2
2
prepared by colloidal method and a typical procedure of impregnation were compared
Fig. 6). In the Pt/SiO catalysts, where there are no –SO H sites, amine formation is not
(
2
3
favored. The reaction proceeds slowly without the acidic sites, however in Pt/SiO2-
SO H, acid sites favors the hydrogenation of the imine and consequently, the desired
3
1
4
Page 16 of 28

amine formation. Besides, the results of XPS demonstrate the interaction between the
sulfonic group and metal nanoparticles, which suppose that the hydrogenation step of
imine to amine should occur in sites close to Me-SO H.
3
When only the support was used as catalyst, the only product observed was imine,
suggesting the requirement of the metal for the reduction of imine. The formation of
imine mainly over the acid support emphasizes the importance of Pt in reducing the
imine formed on the -SO H sites of the catalyst. Thus, it seems that the presence of acid
3
sites favors acid catalysis of imine homogeneous equilibrium.
30
20
10
0
0
1
2
3
4
5
6
7
8
9
Time, (h)
Fig. 6. Effect of the presence of sulfonic groups grafted on SiO : (○) Pt/SiO prepared
2
2
by impregnation of Pt colloidal, (■) Pt/SiO prepared by conventional impregnation,
2
(
▲) Pt/SiO -SO H prepared by impregnation of Pt colloidal. Reaction conditions:
2 3
furfural, 6 mmol; aniline, 5 mmol; catalyst, 0.15 mol %; ethyl acetate, 40 mL; H2
pressure, 5 MPa; room temperature; time, 8 h.
1
5
Page 17 of 28

Conclusions
Reductive amination proceeds slowly without the acidic sites; however, the interaction
of SO H sites favors the formation of imine and its subsequent hydrogenation to amine,
3
being possible to control the yield to amine with ethyl acetate. The interaction of
sulfonic groups with metallic particles was demonstrated by the shift to lower BE in the
XPS analysis. This shift was clearly observed in all the metallic colloids prepared. The
order of activity in the reductive amination of furfural with aniline was Pt≈Ir>Au, and
the selectivity to amine was not affected by the type of metal employed since the Me-
SO H interaction avoided further side reactions, favoring the catalytic activity.
3
Acknowledgements
We thank DIN-UPTC for the financial support under project No. SGI 1281.
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8
Page 20 of 28

Table
LIST OF TABLES
+
Table 1. Textural properties and acid capacities by NaOH titration (mmol H /g) of the
different solids studied.
2
-1
Catalyst
SBET (m g ) Pore volume Pore size
Acid capacities
3
-1
+
(
cm g )
(nm)
(mmol H /g of SO H)
3
SiO -SO H
251
170
174
172
0.45
7.2
35.8
7.14
8.18
8.93
2
3
Au/SiO -SO H
0.90
0.98
0.88
21.2
21.8
20.5
2
3
Ir/SiO -SO H
2
3
Pt/SiO -SO H
2
3
Table 2. Particle size (d ) and geometrical dispersion (D) from XRD and the dispersion
p
measured by H chemisorption analysis.
2
*
**
Catalysts
dp-DRX, nm
D(H/Me)
dp-Quím,nm
n.d.
Au/SiO -SO H
4.1
n.d.
2
3
Ir/SiO -SO H
n.d.
0.53
2.1
2
3
Pt/SiO -SO H
3.7
0.32
3.5
2
3
*
*
:
Estimated from XRD results by Debye-Scherrer equation, Calculated from the
results of hydrogen-oxygen titration, n.d.: not determined
Page 21 of 28

Table 3. Solvent effect on the reductive amination of furfural with aniline. Reaction
conditions: furfural, 6 mmol; aniline, 5 mmol; catalyst, 0.15 mol %; ethyl acetate, 40 mL;
H pres sure, 5 MPa; and room temperature; time, 8 h.
2
*
Solvent
Methanol
Water
Conversion (%) Yield to imine (%) Yield to amine (%)
46
24
72
69
41.4
13.2
51.2
66.1
5.1
11.3
21.3
14.9
Ethyl acetate
THF
*
Conversion of aniline determined by GC.
Table 4. Solvent effect on the reductive amination of furfural with aniline. Reaction
conditions: furfural, 6 mmol; aniline, 5 mmol; catalyst, 0.15 mol %; ethyl acetate, 40 mL;
H pressure, 5 MPa; and room temperature; time, 8 h.
2
Catalysts
Conversion (%)*
Yield to imine (%)
Yield to amine (%)
Au/SiO -SO H
48
58
71
45
38
50
3
2
3
Ir/SiO -SO H
19
21
2
3
Pt/SiO -SO H
2
3
*
Conversion of aniline determined by GC
Page 22 of 28

Figure
LIST OF FIGURES
Figure 1. Reductive amination of furfural with aniline
Fig. 2. N physisorption isotherms of (a) SiO -SO H; (b) Ir/SiO -SO H; (c) Pt/SiO -SO H;
2
2
3
2
3
2
3
(
d) Au/SiO -SO H.
2
3
Fig. 3. X-ray patterns of supported Au catalysts: (a) Au/SiO -SO H, (b) Pt/SiO -SO H, and
2
3
2
3
(
c) Ir/SiO -SO H. Note: peak assignments: (▪) Pt, ( ) Au.
2
3
Fig. 4. XPS spectra in the region of Me 4f7/2 and Me 4f of (a) Au/SiO -SO H, (b) Pt/SiO -
5/2
2
3
2
SO H, and (c) Ir/SiO -SO H.
3
2
3
Fig. 5. Effect of the metal employed on furfural amination. (○) Ir/SiO -SO H, (■) Pt/SiO -
2
3
2
SO H (▲) Au/SiO -SO H. Reaction conditions: furfural, 6 mmol; aniline, 5 mmol;
3
2
3
catalyst, 0.15 mol %; ethyl acetate, 40 mL; H pressure, 5 MPa; room temperature; time, 8
2
h.
Fig. 6. Effect of the presence of sulfonic groups grafted on SiO : (○) Pt/SiO prepared by
2
2
impregnation of Pt colloidal, (■) Pt/SiO prepared by conventional impregnation (▲)
2
Pt/SiO -SO H prepared by impregnation of Pt colloidal. Reaction conditions: furfural, 6
2
3
mmol; aniline, 5 mmol; catalyst, 0.15 mol %; ethyl acetate, 40 mL; H pressure, 5 MPa;
2
room temperature; time, 8 h.
Page 23 of 28

-H2O
+ H2
N
NH
+
H
O
O
O
NH2
O
furfural
aniline
(E)-1-(furan-2-yl)-N-phenylmethanimine
N-(furan-2-ylmethyl)aniline
Fig. 1
Page 24 of 28

a)
b)
1
1
1
4
2
0
8
6
4
2
0
30
25
20
15
10
5
0
0
,0
0,2
0,4
0,6
0,8
1,0
0
,0
0,2
0,4
0,6
0,8
1,0
P/P0
P/P
0
c)
d)
3
2
2
1
1
0
5
0
5
0
5
0
30
25
20
15
10
5
0
0
,0
0,2
0,4
0,6
0,8
1,0
0
,0
0,2
0,4
0,6
0,8
1,0
P/P
0
P/P
0
Fig. 2
a
b
c
0
20
40
60
80
100
2

Fig. 3.
Page 25 of 28

a.
c.
b.
80
82
84
86
88
90
68
70
72
74
76
78
Binding Energy (eV)
Binding Energy (eV)
56
58
60
62
64
66
68
Binding Energy (eV)
Fig. 4.
Page 26 of 28

3
2
1
0
0
0
0
0
1
2
3
4
5
6
7
8
9
Time,(h)
Fig. 5.
Page 27 of 28

30
20
10
0
0
1
2
3
4
5
6
7
8
9
Time, (h)
Fig. 6
Page 28 of 28