Extra-small porous Sn-silicate nanoparticles as catalysts for the synthesis of lactates

Article abstract of DOI:10.1016/j.jcat.2014.03.012

A series of Sn-MCM-41 nanoparticles (XS-Sn-MCM-41) with a diameter ranging from 20 to 140 nm and very high specific surface area were successfully prepared and tested as heterogeneous catalysts for the conversion of the triose sugar dihydroxyacetone to ethyl lactate. Characterization of the materials indicated that the physicochemical properties of the nanoparticles can be significantly affected by different synthesis parameters, including the metal loading, the sequence of adding the Si and Sn precursors into the synthesis mixture, the preparation time and temperature. Most of the XS-Sn-MCM-41 catalysts displayed higher activity compared to conventional Sn-MCM-41 with large particle size in the conversion of dihydroxyacetone into ethyl lactate. The superiority of the best XS-Sn-MCM-41 catalyst in terms of conversion and turnover number is correlated to its high amount of accessible acid sites, which in turn is ascribed to a combination of different physicochemical features such as high surface area, particles morphology and coordination of the tin atoms in tetrahedral framework sites. The best catalyst can be reused in consecutive runs without loss of activity.

Full text of DOI:10.1016/j.jcat.2014.03.012

Journal of Catalysis 314 (2014) 56–65
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Extra-small porous Sn-silicate nanoparticles as catalysts
for the synthesis of lactates
Li Li a,1, Xavier Collard b,1, Arnaud Bertrand , Bert F. Sels , Paolo P. Pescarmona , Carmela Aprile b,
b
a
a,
⇑
⇑
a
Centre for Surface Chemistry and Catalysis, University of Leuven (KU Leuven), Kasteelpark Arenberg 23, 3001 Heverlee, Belgium
Unit of Nanomaterial Chemistry (CNano), University of Namur (UNAMUR), Department of Chemistry, Rue de Bruxelles 61, 5000 Namur, Belgium
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of Sn-MCM-41 nanoparticles (XS-Sn-MCM-41) with a diameter ranging from 20 to 140 nm and
very high specific surface area were successfully prepared and tested as heterogeneous catalysts for the
conversion of the triose sugar dihydroxyacetone to ethyl lactate. Characterization of the materials indi-
cated that the physicochemical properties of the nanoparticles can be significantly affected by different
synthesis parameters, including the metal loading, the sequence of adding the Si and Sn precursors into
the synthesis mixture, the preparation time and temperature. Most of the XS-Sn-MCM-41 catalysts dis-
played higher activity compared to conventional Sn-MCM-41 with large particle size in the conversion of
dihydroxyacetone into ethyl lactate. The superiority of the best XS-Sn-MCM-41 catalyst in terms of con-
version and turnover number is correlated to its high amount of accessible acid sites, which in turn is
ascribed to a combination of different physicochemical features such as high surface area, particles mor-
phology and coordination of the tin atoms in tetrahedral framework sites. The best catalyst can be reused
in consecutive runs without loss of activity.
Received 6 December 2013
Revised 28 February 2014
Accepted 24 March 2014
Keywords:
Sn-silicates
Porous materials
Nanoparticles
Lactate synthesis
Ó 2014 Elsevier Inc. All rights reserved.
1
. Introduction
Heterogeneous catalysts play a crucial role in many processes
particle [5]. In this work, we show that the catalytic performance
of mesoporous Sn-silicates in the conversion of dihydroxyacetone
to ethyl lactate can be significantly improved by preparing the cat-
alysts in the form of nanoparticles. The selected reaction is relevant
in the context of the conversion of renewables to added-value
compounds.
for the production of bulk and fine chemicals, including the thriv-
ing area of the conversion of renewables. The development of
improved catalysts can originate from the identification of new
active species but also from a tailored design of the structural
and morphological features of the catalytic materials. Mesoporous
silicas such as MCM-41 or SBA-15 are a widely studied class of
materials with a great potential for catalytic applications due to
the reduced diffusion limitations of reactants and products com-
pared to microporous solids, e.g. zeolites. Further control of the dif-
fusivity within these materials can be achieved by tuning the
dimension and organization of the mesopores. A less-explored
strategy to improve the accessibility of the active sites consists in
reducing the particle size of the catalyst to the nanoscale. This
can bring about dramatic enhancements in the efficiency of the
catalysts [1–4]. Indeed, it has been shown by fluorescence micros-
copy that for particles in the micrometer range, most of the cata-
lytic activity stems from sites located at the external rim of the
For many years, petrochemical processes based on the use of
fossil resources for the production of chemicals, polymers and fuels
have been a mainstay of the world industry [6]. However, due to
the limitation of fossil resources and the increasing concerns about
environmental pollution issues, the industrial society has to find
renewable feedstocks that are able to satisfy the energy needs
and the production of chemicals while minimizing the negative
impact on the environment [7–9]. Biodiesel, defined as methyl
esters obtained from lipids (e.g. vegetable oils and animal fats), is
a renewable and environmentally acceptable alternative to petro-
leum-based diesel fuel [10]. The manufacturing of biodiesel yields
glycerol as the main co-product, in an amount equivalent to
approximately 10 wt% of the produced biodiesel [10]. The continu-
ously increasing biodiesel market led to an oversupply in the pro-
duction of glycerol, which caused a drop in its commercial value
and generated a shift in its role from end-of-the chain product to
raw material. Therefore, new solutions for the effective utilization
of glycerol are being extensively investigated. Among the catalytic
processes for converting glycerol and its derivatives to valuable
⇑
Corresponding authors.
E-mail addresses: paolo.pescarmona@biw.kuleuven.be (P.P. Pescarmona),
carmela.aprile@unamur.be (C. Aprile).
1
These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.jcat.2014.03.012
0
021-9517/Ó 2014 Elsevier Inc. All rights reserved.

L. Li et al. / Journal of Catalysis 314 (2014) 56–65
57
products, an attractive route is provided by the synthesis of lactic
2.2. Synthesis of the XS-Sn-MCM-41 catalysts
acid/lactates from trioses, i.e. glyceraldehyde (GLY) and dihydroxy-
acetone (DHA) [11–17]. The trioses can be obtained from glycerol
either by fermentation or by catalytic oxidation [18–21]. Compared
to the present method to produce lactic acid and lactates, which is
based on the microbial fermentation of glucose and sucrose and
requires extensive neutralization, purification and separation pro-
cesses [22–24], this catalytic route is greener and more efficient.
Lactic acid and the related alkyl lactates have important applica-
tions as green solvents and precursors for the synthesis of polylac-
tates, which is increasingly employed as a biodegradable plastic
material, and in various other industrial areas related to chemicals,
food, pharmaceuticals, and cosmetic products [8,25,26].
It was recently reported that porous structured silicas with
metallic elements (e.g. Ti, Sn) partially substituting Si as single sites
are active and selective catalysts for the conversion of trioses to
lactic acid or alkyl lactates [14,27]. The catalytic activity originates
from the acidity of the materials: namely, a combination of Lewis
acid sites and mild Brønsted acid sites is the key factor to obtain
high conversion and excellent selectivity toward the lactate prod-
uct. Brønsted acid sites catalyze the conversion of dihydroxyace-
tone (or glyceraldehyde) into pyruvic aldehyde by dehydration
and rearrangement, while Lewis acid sites catalyze the conversion
of the formed pyruvic aldehyde into the desired lactate (Scheme 1).
Strong Brønsted acidity should be avoided as it favors the forma-
tion of undesired acetal side-products [13,14,16,28,29]. Sn-MCM-
The general protocol for the synthesis of the XS-Sn-MCM-41 cat-
alysts is inspired by a previously reported method for the preparation
of Ti-MCM-41 nanoparticles [1]. A basic aqueous solution (NaOH
2.0 M) was added to a solution of CTAB in milli-Q water and stirred
at 800 rpm during 30 min. In a first approach, half of the total amount
of TEOS was added to the solution, followed by the addition of
SnCl
finally by the other half of TEOS. In an alternative approach, TEOS
and SnCl O were mixed together prior to their addition to the
ꢀ5H
solution. In all cases, TEOS and SnCl O were added in a dropwise
ꢀ5H
manner. The molar ratio in the synthesis of sample XS-Sn-MCM-41-A
is 1 TEOS:0.0135 SnCl O:0.33 NaOH:0.125 CTAB:0.90 EtOH:
ꢀ5H
1212 H O. For the other materials of the XS-Sn-MCM-41 series, the
concentration of SnCl O (Table 1) was varied while keeping all
ꢀ5H
4 2
ꢀ5H O previously dissolved in 1.0 mL of absolute ethanol and
4
2
4
2
4
2
2
4
2
other concentrations fixed at the values given above. The mixture
was stirred at 800 rpm for either 2 h or 24 h at room temperature.
An additional thermal treatment at 70 °C for 3 h was performed for
a number of samples (Table 1). The resulting solid was washed three
times with ultra-pure water and ethanol and dried at 60 °C for 16 h.
Finally, the solid was calcined in air to remove the organic template
ꢁ1
(8 h at 550 °C, heating rate of 3 °C min ). For comparison, Sn-MCM-
41 with larger particles (denoted as Sn-MCM-41-LP) was synthesized
as described previously by using cetyltrimethylammonium bromide
(CTAB) as template [14]. The molar ratio in the synthesis mixture is 1
4
1, possessing such combination of Lewis acid sites and mild
SiO
2
:0.02 SnO
2
:0.31 (CH
3
)
4
N(OH)ꢀ2 SiO
2 2
:0.6 CTAB:38 H O. After
Brønsted acid sites, was identified as an excellent heterogeneous
catalyst for the conversion of dihydroxyacetone to ethyl lactate
hydrothermal treatment in a Teflon-lined autoclave for 14 h at
140 °C, the obtained solid was filtered, washed with deionized water,
dried at 60 °C and finally calcined in air for 6 h at 550 °C with a heat-
ing rate of 1 °C min . In order to increase the hydrophobicity of a
selected XS-Sn-MCM-41 sample, a silylation process was carried
out by treating the calcined sample with a solution of hexamethyldi-
silazane (HMDS, Aldrich) in toluene under argon atmosphere, under
[
14]. Here, we introduce the synthesis of novel extra-small (XS)
ꢁ
1
Sn-silicate mesoporous particles with the typical structure and
pore size of MCM-41 and particle diameter ranging from 20 to
1
bination of active and highly accessible sites, leading to improved
activity in the title reaction.
40 nm. These materials are expected to present an attractive com-
2
vigorous stirring at 110 °C for 2 h [30–32]. A HMDS/SiO molar ratio
of 0.25 was employed to achieve a good degree of silylation of the
material. The sample was then filtered, washed thoroughly with
dry toluene, and finally dried at 110 °C.
2
. Experimental
2.1. Materials
2.3. Characterization of the XS-Sn-MCM-41 catalysts
The chemicals: cetyltrimethylammonium bromide (CTAB purity
Transmission electron microscopy (TEM) images were obtained
P99%), sodium hydroxide (NaOH purity P97%), tetramethylam-
monium hydroxide (TMAOH 25.0%), tetraethylorthosilicate (TEOS
purity P99.999%), absolute ethanol (EtOH purity P99.5%), hexam-
ethyldisilazane (HMDS purity P99%), and tin chloride pentahy-
using a Philips Tecnai 10 with an accelerating voltage of 80 kV.
Nitrogen adsorption-desorption isotherms of the as-synthesized
and silylated materials were measured at 77 K with a volumetric
adsorption analyzer (Micromeritics Tristar 3000). The pore size
distributions were calculated from the adsorption isotherm using
the Barrett–Joyner–Halenda (BJH) method [33]. The Brunauer–
Emmet–Teller (BET) method was applied to calculate the specific
surface area [34]. Powder X-ray diffraction (XRD) patterns were
drate (SnCl
Sigma–Aldrich.
4
ꢀ5H
2
O purity P98.0%) were all purchased from
recorded on a PANalytical X’pert diffractometer with Cu Ka radia-
tion (k = 1.54178 Å). Scanning electron microscopy (SEM) analysis
was carried out on a Philips XL30 FEG scanning electron micro-
scope. Energy-dispersive X-ray (EDX) spectroscopy was performed
using an acceleration voltage of 7.5 kV and a working distance of
8
mm. The elemental composition was calculated as the average
of the results from different spots selected randomly on the surface
of the catalysts. The Si environment and the coordination of the Sn
2
9
119
atoms were studied by Si and
Sn Magic Angle Spinning
Nuclear Magnetic Resonance (MAS-NMR) spectra measured on a
VARIAN 400 and a Bruker 500 spectrometer, respectively. 1 Sn
MAS-NMR spectra were measured at the resonance frequency of
19
1
19
Sn (186.5 MHz). The samples were packed in a 3.2 mm Chemag-
netics rotor and measured with a spinning frequency of 8000 Hz.
1,000 scans were accumulated with a recycle delay of 40 s and
a pulse length of 2.0
s. The resonance frequency of 29Si was of
2
Scheme 1. Conversion of dihydroxyacetone to alkyl lactate (R = alkyl group) in an
alcohol solution.
l

5
8
L. Li et al. / Journal of Catalysis 314 (2014) 56–65
Table 1
Physicochemical properties of the XS-Sn-MCM-41 materials.
Entry Sample
Si/Sn ratio
Temperature and time BET
Average pore Pore volume Internal pore
Average size of the
2
ꢁ1
(cm3
ꢁ1
volume (cm3 )b nanoparticles (nm)c
g
ꢁ1
of the synthesis
(m
g
) size (nm)
g
)
In the
reaction
mixture
In the final
materiald
1
2
3
4
5
6
7
8
XS-Sn-MCM-41-A 74
XS-Sn-MCM-41-B 41
65 (63e)
27
28 (31 )
24
32
22
54
n.d.
2 h RT
2 h RT
2 h RT + 3 h 70 °C
2 h RT + 3 h 70 °C
24 h RT
2 h RT + 3 h 70 °C
2 h 30 RT + 14 h 140 °C 729
1111
1042
946
896
804
2.3
2.3
2.3
2.3
2.3
2.3
2.5
1.7
0.84
1.09
1.13
1.14
1.31
1.20
0.76
0.62
0.77
0.62
0.59
0.58
0.47
0.52
0.46
0.54
118 ± 21
35 ± 11
39 ± 13
71 ± 31
72 ± 28
76 ± 25
285 ± 83
e
XS-Sn-MCM-41-C 41
XS-Sn-MCM-41-D 41
XS-Sn-MCM-41-E 41
XS-Sn-MCM-41-F 28
a
a
a
858
Sn-MCM-41-LPf
Silylated XS-Sn-
MCM-41-A
50
–
1074
n.d. = not determined.
a
TEOS and SnCl
Calculated for p/p
4
ꢀ5H
2
O were mixed together before addition.
from 0.0 to 0.8.
b
c
d
e
f
0
Expressed as ‘average ± standard deviation’, calculated by measuring 45 randomly selected particles in the TEM images.
Determined by EDX.
Determined by ICP-OES.
Data taken from [14].
7
9.46 MHz. The samples were packed in a 4 mm zirconia rotor and
were dissolved in 3.92 g ethanol (as solvent and reactant) at 45 °C
for 30 min. Then, the selected amount of catalyst (50, 100 or
200 mg) was added to the solution at room temperature. This reac-
tion mixture was heated at 90 °C under vigorous stirring
(1200 rpm) for the selected reaction time. At the end of the test,
the catalyst was separated by centrifugation and the solution
was analyzed by gas chromatography (GC) on a Trace GC Ultra
from Interscience; more details about the GC analyses can be found
in previous reports [14]. Recyclability tests were performed by sep-
arating the catalyst from the reaction mixture by centrifugation
followed by washing with ethanol. The washing procedure was
repeated five times. Finally, the catalyst was dried overnight at
100 °C. Alternative approaches involved a calcination step for 2 h
6
measured with a spinning frequency of 8000 Hz. 4.14 ꢂ 10 scans
were accumulated with a recycle delay of 6 s and a pulse length
of 2.0 ls (30°). Tetramethylsilane (TMS) was used as shift refer-
2
9
ence. The Si MAS-NMR spectrum of the silylated sample was
recorded on a Bruker AMX300 spectrometer (7.0 T). At this field,
the resonance frequency of 29Si is 59.6 MHz. The sample was
packed in a 4 mm Zirconia rotor and measured with a spinning fre-
quency of 5000 Hz. 4000 scans were accumulated with a recycle
delay of 60 s and a pulse length of 5.0
ls. Tetramethylsilane
(
TMS) was used as shift reference. Diffuse reflectance UV–Vis spec-
tra were collected on a Varian Cary 5000 UV–Vis–NIR Spectropho-
tometer with a Harrick single-beam Praying Mantis Diffuse
Reflectance collection system. Elemental analyses of selected cata-
lysts were performed by dissolving 50 mg of the sample in a mix-
ture of 0.5 mL of aqua regia and 3 mL of 40% aqueous HF.
Inductively coupled plasma optical emission spectroscopy (ICP-
OES, Jobin Yvon Ultima instrument) was employed to analyze the
obtained solutions. Analysis of the acid properties of the XS-Sn-
MCM-41 catalysts was carried out by adsorption and temperature
programmed desorption (TPD) of pyridine monitored by Fourier
Transform infrared spectroscopy (FTIR). In a home-made vacuum
infrared cell with ZnSe windows, a self-supporting wafer of the
sample (about 15 mg) was initially dried under vacuum at 400 °C
for 1 h, and then cooled down to 50 °C. During the cooling process,
reference spectra were recorded at 350, 250, 150 and 50 °C. After-
ward, the wafer was saturated with about 25 mbar of pyridine
vapor at 50 °C for 10 min and then evacuated again for 30 min to
fully remove physisorbed pyridine. Finally, the evacuated sample
containing chemisorbed pyridine was subjected to TPD at 150,
ꢁ1
(either at 300 °C or at 500 °C, with a heating rate of 2 °C min
)
after the washing step.
Leaching tests were carried out under the general reaction con-
ditions employed for the catalytic tests (vide supra). The catalyst
was removed from the reaction mixture after 30 min by centrifu-
gation, followed by hot filtration at the same temperature used
for the catalytic test using a plastic syringe equipped with a
25 mm HPLC syringe filter from Altech, with a pore size of
0.2 lm. The filtrate was allowed to react for another 5 h 30 min.
The products were analyzed by GC both after 30 min and at the
end of the filtrate test (6 h).
3
. Results and discussion
A series of Sn-MCM-41 samples with extra-small (XS) nanopar-
ticle morphology (XS-Sn-MCM-41) was synthesized and studied as
heterogeneous catalysts for the conversion of dihydroxyacetone
into ethyl lactate. The synthesis methods used for preparing the
XS-Sn-MCM-41 materials were inspired by a diluted solution route
previously reported for Ti-MCM-41 nanoparticles [1], which allows
obtaining a particle size distribution below 200 nm. Different
protocols for the addition of the Si and Sn precursors (TEOS and
ꢁ1
2
50 and 350 °C for 30 min, with a heating rate of 4 °C min , and
the IR spectra were recorded in situ at these temperatures. The
amounts of acid sites were determined from the integral intensity
ꢁ1
of characteristic bands (1450 cm
for Lewis acid sites, and
545 cm for Brønsted acid sites) using the molar extinction coef-
ficients of Emeis [35].
ꢁ1
1
SnCl
neous distribution of tin as single site in the silicate framework and
of minimizing the formation of SnO domains. Different ratios
4
2
ꢀH O) were investigated with the aim of achieving a homoge-
2.4. Catalytic tests
2
between Si and Sn were also studied. Additionally, the dimension
and mesoporous ordering of the XS-Sn-MCM-41 nanoparticles
were optimized by varying the temperature and reaction time. A
summary of the reaction conditions is presented in Table 1. For
comparison, the reaction parameters used for the synthesis of stan-
dard Sn-MCM-41particles are also reported (entry 7, Table 1).
The catalytic reactions were carried out in a multiple-well par-
allel reaction block [14]. In a typical experiment for the conversion
of dihydroxyacetone (DHA) to ethyl lactate, 0.180 g of DHA
ꢁ
3
(
2.00 ꢂ 10 mol, in the form of 1,3-dihydroxyacetone dimer)
ꢁ4
and 0.0215 g of decane (1.50 ꢂ 10 mol, as GC internal standard)

L. Li et al. / Journal of Catalysis 314 (2014) 56–65
59
The physicochemical features of the XS-Sn-MCM-41 materials
were studied by means of a combination of characterization tech-
niques. The complete characterization of three representative sam-
nanoparticles reach their critical size, while longer synthesis time
favors the aggregation of the particles.
2
In agreement with the characterization by TEM and N -physi-
ples (A,
C
and E) will be discussed in detail, while the
sorption, X-ray diffraction (XRD) analysis of XS-Sn-MCM-41-A
shows the typical pattern of MCM-41 with an intense d100 peak
and two smaller d110 and d200 signals (Fig. 3a), while for the mate-
rials characterized by smaller particles (XS-Sn-MCM-41-B to F)
only a broad d100 peak is observed (Figs. 3b, c and S4, SI). The larger
peak width observed for sample E could arise from the higher
degree of aggregation between the particles observed by TEM
and consequent decrease in long-range ordering.
characterization data of all the other materials of the series can
be found in the Supporting Information (SI). Transmission electron
microscopy demonstrated the presence of small nanoparticles and
proved the mesoporous nature of all samples, with the regular hex-
agonal organization typical of MCM-41 materials (Figs. 1 and S1 in
the SI). The particle size of the XS-Sn-MCM-41 materials is sub-
stantially smaller than that of a conventional Sn-MCM-41 refer-
ence sample (Sn-MCM-41-LP), which consists of irregular
particles with a typical size of few hundred nanometers (Fig. S2
in the SI). XS-Sn-MCM-41-A presents a relatively narrow particle
size distribution centered at 118 nm (Table 1 and Fig. 1a). The sam-
ples prepared with higher loading of tin display an even smaller
particle size: for example, XS-Sn-MCM-41-C shows particles in
the range between 20 and 60 nm (Table 1 and Fig. 1b), indicating
a slight change in the formation mechanism of the nanoparticles
with a preference of nucleation compared to growth. This may be
due to a decrease in pH value as a consequence of the increased
The synthesis of mesoporous Sn-silicates proceeds through the
hydrolysis of the precursors, followed by condensation with the
formation of Sn–O–Si and Si–O–Si bonds (and ideally no Sn–O–
Sn bond). The degree of condensation of the materials was studied
2
9
by solid-state Si NMR (Fig. 4a), which shows a broad band that
can be deconvoluted into a mixture of Q [(SiO)
3
SiOH] and Q4
3
3
4
[(SiO) Si] signals. The elevated contribution of Q compared to
standard MCM-41 materials is attributed to the extra-small dimen-
sion of the XS-Sn-MCM-41 solids, which implies a high surface-to-
volume ratio and thus a larger population of surface silanols.
Ideally, Sn should be incorporated as single site with tetrahedral
coordination in the silica framework. The nature of the Sn sites in
the XS-Sn-MCM-41 materials was studied by means of solid-state
4
concentration of SnCl in the reaction medium: the hydrochloric
acid generated upon hydrolysis of the tin precursor partially neu-
tralizes the NaOH present in solution. Notably, TEM revealed that
the additional thermal treatment of the reaction mixture and the
longer reaction time (samples XS-Sn-MCM-41-C to E) favor a mod-
erate increase in the particle size but also the formation of aggre-
gates. Particularly, the TEM image of XS-Sn-MCM-41-E evidenced
the presence of zones of condensation between the XS particles
1
19
Sn NMR and UV–Vis spectroscopy. It should be noted that,
1
19
although the isotopic abundance of the
Sn is high (compared
1
3
29
to C or Si), the detection of this element by Magic Angle Spin-
ning (MAS) NMR is hindered by its long spin-lattice relaxation time
and low wt% in the prepared catalysts. Despite these intrinsic lim-
itations and the high Si/Sn ratio of the samples, a noisy 1 Sn signal
centered at around ꢁ700 ppm could be detected for the materials
displaying higher tin loading (XS-Sn-MCM-41-C and F) after 3 days
19
(Fig. 1c and d). Interparticle condensation was also observed,
though to less extent, in samples C, D and F (Figs. 1b and S1 in
the SI).
1
19
Analysis of the series of XS-Sn-MCM-41 solids by nitrogen
physisorption isotherms reveals that all the prepared materials
have high specific surface area and display a type IV isotherm with
a step at low relative pressure corresponding to a narrow pore size
distribution centered at 2.3 nm (Table 1, Figs. 2 and S3, SI). These
features are characteristic of the long-range order of MCM-41
structures, in agreement with the TEM analysis. All XS-Sn-MCM-
of acquisition (Figs. 4b and S5, SI). This
Sn signal is attributed to
4
+
tetrahedrally coordinated Sn ions connected to four silicon atoms
through oxygen bridges within the silica framework [36,37] or to
hydrated framework Sn(IV) species with an extended coordination
shell including two water molecules [38,39]. Octahedrally coordi-
nated tin in SnO
42]. This peak was not observed in our samples, though the poor
signal-to-noise ratio of the spectra does not allow excluding the
2
gives a sharp peak at around ꢁ600 ppm [40–
4
1 materials display higher specific surface area compared to the
conventional Sn-MCM-41-LP. The values of surface area vary
throughout the series of materials, reaching a remarkable value
2
presence of small amounts of SnO .
Diffuse reflectance UV–Vis spectroscopy allowed highlighting
further the coordination of tin in the XS-Sn-MCM-41 materials.
Metal atoms that isomorphously substitute silicon atoms in the
framework of MCM-41 acquire the typical tetrahedral coordination
of silicate units. Therefore, their absorption in the UV–Vis region is
generally easily distinguished from that due to extra-framework
species. The UV–Vis spectrum of XS-Sn-MCM-41-A reveals a main
absorption band centered at 208 nm corresponding to Sn4+ in tet-
rahedral coordination [36,43,44] (Fig. 5a), evidencing that at low
metal loading the incorporation of tin in the structure is mainly
achieved as single site species. Samples B and C present two addi-
tional signals at 245 and 280 nm (Figs. 5b and S6, SI). The assign-
ment of the band at 245 nm is still a matter of debate: this signal
has been attributed to distorted tetrahedral and penta-coordinated
2
ꢁ1
of 1111 m g for sample XS-Sn-MCM-41-A. A slight decrease in
the specific surface area was observed when increasing the amount
of Sn present in the structure (Table 1, compare XS-Sn-MCM-41-A
and B). A significant drop in surface area was detected for the solid
prepared with a long reaction time (compare XS-Sn-MCM-41-D
and E in Table 1). The decrease in surface area in the XS-Sn-
MCM-41 series is accompanied by an increase in the pore volume
(
Table 1). This trend is attributed to the presence of large interpar-
ticle meso- and macropores, as indicated by the hysteresis loop at
higher relative pressure observed in the N adsorption–desorption
isotherms of all the nanoparticles except XS-Sn-MCM-41-A
Figs. 2b, c and S3, SI). Accordingly, a broad signal in the range
0–70 nm is observed in the pore size distribution of samples
XS-Sn-MCM-41-B to F, being particularly evident for sample E
Fig. 2c). The presence of these disordered interparticle cavities
2
(
2
framework tin sites [44] but also to small extra-framework SnO
2
(
domains [38,45]. The broad absorption around 280 nm has been
assigned to hexa-coordinated polymeric Sn–O–Sn type species
[36,38,43]. The presence of these two additional species in samples
B and C is ascribed to the higher amount of tin precursor and to the
sequential method used for adding the Si and Sn precursors to the
reaction mixture. On the other hand, the UV–Vis spectra of samples
D to F do not exhibit the additional peaks at 245 and 280 nm, indi-
cating that the incorporation of tin in tetrahedral coordination is
can be related to the appearance of zones of condensation between
the particles as observed by TEM. These interparticle voids are
expected to contribute scarcely to the surface area but relevantly
to the pore volume. In order to test this hypothesis, the pore vol-
ume was recalculated by excluding the contribution of the larger
0
pores, i.e. those corresponding to p/p from 0.8 to 1.0 (Table 1).
These values give an estimation of the internal mesopore volume
and, as anticipated, follow the same trend as the surface areas.
These findings suggest that after 2 h at room temperature, the
more efficient if TEOS and SnCl
being added to the reaction solution (Figs. 5c and S6, SI). Although
4
ꢀ5H
2
O are mixed together before

6
0
L. Li et al. / Journal of Catalysis 314 (2014) 56–65
Fig. 1. TEM analysis of XS-Sn-MCM-41-A (a), C (b) and E (c, d).
6
00
00
00
00
00
00
800
1000
800
600
400
200
a
b
c
7
00
00
00
00
00
00
5
6
4
5
4
3
3
2
2
1
100
0
0.0
0.2
0.4
0.6
0.8
1.0 0.0
0.2
0.4
0.6
0.8
0
1.0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p )
Relative pre ss sure (p/p )
Relative pressure (p/p )
0
0
0
.6
.5
.4
.3
1
.0
.8
.6
.4
.2
0.05
.04
.03
0
.05
.04
.03
.02
.01
.00
0.05
0.4
0.3
0
0
0
0.04
0.03
0.02
0.01
0
0
0
0
0
0
0.02
0.01
0.00
0
0.2
0
0
0.2
0.1
0.0
0.00
0
0.1
0.0
0
20
30
40
50
60
70
80
20
30
40
50
60
70
80
20
30
40
50
60
70
80
Pore size (nm)
Pore size (nm)
Pore size (nm)
0.0
1
0
20 30 40 50 60 70 80
Pore size (nm)
10 20 30 40 50 60 70 80
Pore size (nm)
10 20 30 40 50 60 70 80
Pore size (nm)
2
Fig. 2. N physisorption isotherms (top) and the pore size distribution (bottom) of XS-Sn-MCM41-A (a), C (b) and E (c).
UV–Vis spectroscopy evidenced the presence of Sn-O-Sn species in
some of the XS-Sn-MCM-41 samples, EDX spectroscopy allows
excluding the formation of large (i.e. >50 nm) separate domains
of silicon or tin oxide, as indicated by the homogeneous distribu-
tion of tin and silicon observed in all the samples. This conclusion
is further supported by the elemental mapping images recorded
using SEM (Fig. S7, SI).
In order to verify the reproducibility of the synthesis protocol, a
second batch of materials XS-MCM-41-A, C and E was synthesized
2
and characterized by XRD, N -physisorption, TEM and UV–Vis
On the basis of the characterization results discussed above, it
can be expected that the XS-Sn-MCM-41 will display relevant cat-
alytic performance in the conversion of dihydroxyacetone to ethyl
lactate. Indeed, most of the studied XS-Sn-MCM-41 materials pres-
ent higher conversion of dihydroxyacetone compared to conven-
tional Sn-MCM-41 consisting of large particles (Table 2: compare
entries 1–4 and 6 with entry 7). The mechanism leading to the con-
version of dihydroxyacetone to ethyl lactate has been extensively
discussed in the literature [14]: Brønsted acid sites catalyze the
dehydration of dihydroxyacetone into the pyruvic aldehyde inter-
mediate, followed by rearrangement into the lactate product cata-
lyzed by Lewis acid sites (Scheme 1). In Sn-MCM-41 materials, the
spectroscopy, leading to analogous results to the first batch of
materials (Fig. S8, SI).

L. Li et al. / Journal of Catalysis 314 (2014) 56–65
61
Fig. 3. XRD patterns of XS-Sn-MCM-41-A (a), C (b), E (c), in the small angle region (2h = 1–10°).
Fig. 4. Solid-state 29Si MAS-NMR spectrum (a) and 119Sn MAS-NMR spectrum (b) of XS-Sn-MCM-41-C.
a
b
c
2
00 250 300 350 400 450
200 250 300 350 400 450
Wavelength (nm)
200 250 300 350 400 450
Wavelength (nm)
Wavelength (nm)
Fig. 5. Diffuse reflectance UV–Vis spectra of XS-Sn-MCM-41-A (a), C (b) and E (c).
Lewis acidity is provided by surface tin atoms tetrahedrally coordi-
nated to the silica framework, whereas Brønsted acidity is ascribed
to surface silanols located in the vicinity of tin [14,46]. In order to
explain the observed catalytic results, it is useful to monitor the
number and type of accessible acid sites by means of FT-IR analysis
41-A, which exhibits the largest amount of accessible Lewis and
Brønsted acid sites among the studied catalysts (Table 3). This fea-
ture is ascribed to the high specific surface area of this material
(Table 1) and to the efficient insertion of Sn in tetrahedral sites
in the structure, as evidenced by the absorption band at 208 nm
in the UV–Vis spectrum (vide supra). The lower catalytic activity
of XS-Sn-MCM-41-B and C compared to XS-Sn-MCM-41-A, despite
their higher Sn content, is mainly ascribed to the lower fraction of
Sn in tetrahedral sites accompanied by the presence of catalytically
ꢁ
1
of adsorbed pyridine. The vibrations at 1455 and 1623 cm are
characteristic of pyridine adsorbed on strong Lewis acid sites, at
ꢁ
1
ꢁ1
1
575 cm for weak Lewis acid sites, at 1545 and 1639 cm for
ꢁ1
Brønsted acid sites and at 1492 cm for both Lewis and Brønsted
acid sites (Fig. 6a) [14,47]. All the XS-Sn-MCM-41 catalysts display
the required combination of Lewis and Brønsted acidity (Table 3),
and the differences in catalytic performance between them can
be explained in terms of their amounts of acid sites. More specifi-
cally, the lactate yield and selectivity correlate well with the
amount of Lewis acid sites (Fig. 6b and c and Tables 2 and 3), in
agreement with the proposed reaction mechanism.
2
inactive clusters of SnO (as indicated by the UV–Vis absorptions at
245 and 280 nm). On the other hand, tetrahedral tin sites are
generated more efficiently in the samples in which the Sn and Si
precursor were added simultaneously during the synthesis
(XS-Sn-MCM-41-D, E and F). However, this method leads to lower
specific surface areas (Table 1) and to interparticle agglomeration
(vide supra), which is particularly relevant for sample E. The aggre-
gation between particles might lead to blocking of one entrance of
the pores and thus reduce the accessibility of the active sites. These
The best catalytic results, both in terms of dihydroxyacetone
conversion and of lactate yield, were obtained with XS-Sn-MCM-

6
2
L. Li et al. / Journal of Catalysis 314 (2014) 56–65
Table 2
Catalytic conversion of dihydroxyacetone to ethyl lactate over XS-Sn-MCM-41 catalysts.
Entry
Catalyst
Conv. (%)a
Yethyl lactate (%)
Yhemiacetal (%)
Yacetal (%)
S
ethyl lactate (%)
TONc
1
2
3
4
5
6
7
8
XS-Sn-MCM-41-A
XS-Sn-MCM-41-B
XS-Sn-MCM-41-C
XS-Sn-MCM-41-D
XS-Sn-MCM-41-E
XS-Sn-MCM-41-F
64
49
51
39
16
53
32
28
44
27
30
19
6
34
30
26
15
16
16
16
8
16
1
1
5
6
5
4
2
3
1
1
69
55
59
49
38
64
94
93
104
35
37
25
13
31
41
n.d.
b
Sn-MCM-41-LP
Silylated XS-Sn-MCM-41-A
Conditions: 50 mg of catalyst, 2.00 ꢂ 10 mol DHA in ethanol. Decane (1.50 ꢂ 10ꢁ4 mol) was used as internal standard for GC analysis.
ꢁ3
n.d. = not determined.
a
Calculated as the sum of the yields of the reaction products [14].
Data taken from [14].
TON is defined as mol of dihydroxyacetone converted per mol of Sn atoms.
b
c
L
a
L
L+B
B
B
(
i)
L
(ii)
(iii)
(
(
iv)
v)
(vi)
1700
1650
1600
1550
1500
1450
1400
-1
Wavenumbers (cm )
50
40
30
20
10
80
b
70
60
50
40
30
20
10
c
0
0
0
0.12
.12
0.14
0.16
0.18
0.20
0.14
0.16
0.18
0.20
Lewis acid sites (mmol g-1
)
Lewis acid sites (mmol g-1
)
Fig. 6. (a) Infrared spectra of pyridine adsorbed at 150 °C over XS-Sn-MCM-41-A-F (i–vi) [L = Lewis acid sites; B = Brønsted acid sites]; (b) yield of ethyl lactate and (c)
selectivity toward ethyl lactate over the XS-Sn-MCM-41 catalysts as a function of their amount of Lewis acid sites [48].
features can explain the catalytic trends observed between sam-
ples D, E and F and their lower activity compared to XS-Sn-
MCM-41-A. These results indicate that the catalytic performance
of the XS-Sn-MCM-41 materials in the conversion of dihydroxyac-
etone to lactate depends on a combination of parameters including
surface area, morphology, and population of tetrahedral tin sites on
the surface.
Remarkably, the highest amount of accessible acid sites and
thus the best catalytic performances were obtained with the mate-
rial prepared with lower tin content (XS-Sn-MCM-41-A), while
higher tin loadings led to a higher fraction of poorly active or
inactive Sn species, either because located in partially blocked
pores or embedded within the pore walls and thus not accessible,
or because in the form of SnO clusters. This is reflected by the
2
much higher fraction of Sn atoms acting as Lewis acid sites in
XS-Sn-MCM-41-A (80%) compared to the other materials of the
series (Table 3). The high efficiency in employing Sn to generate
Lewis acid sites led to the striking superiority of XS-Sn-MCM-41-
A when compared to the other catalysts in terms of turnover
numbers (TON, Table 2).
All the XS-Sn-MCM-41 nanoparticle materials display lower
selectivity toward ethyl lactate compared to the reference Sn-
MCM-41-LP (Table 2) [14]. The selectivity in this reaction is con-
trolled by the relative amount and strength of the acid sites of

L. Li et al. / Journal of Catalysis 314 (2014) 56–65
63
Table 3
Type and amount of surface acid sites of the XS-Sn-MCM-41 materials, as determined by TPD-IR of adsorbed pyridine.
Lewis acidity (mmol g 1)a
ꢁ
Brønsted acidity (mmol g )a
ꢁ1
L + B (mmol g
ꢁ1
)
L/B ratio
% of Sn acting as Lewis acidb
Sample
at 150 °C
at 350 °C
XS-Sn-MCM-41-A
XS-Sn-MCM-41-B
XS-Sn-MCM-41-C
XS-Sn-MCM-41-D
XS-Sn-MCM-41-E
XS-Sn-MCM-41-F
0.197
0.159
0.164
0.149
0.128
0.172
0.094
0.052
0.068
0.037
0.031
0.042
0.291
0.211
0.232
0.186
0.159
0.214
2.1
3.1
2.4
4.0
4.1
4.1
10
80
28
30
24
27
25
6.0
7.0
(1)
(1)
5.5
a
ꢁ1
ꢁ1
Calculated from the integral intensity of the corresponding bands (1450 cm for Lewis acid sites and 1545 cm for Brønsted acid sites) in the IR spectra recorded at
1
50 °C.
b
ꢁ1
Calculated from the Sn content determined by EDX and the number of Lewis acid sites (mmol g ) determined by IR of adsorbed pyridine.
the catalyst, with Lewis acid sites promoting the conversion of the
intermediate pyruvic aldehyde to lactate and Brønsted acid sites
favoring the formation of hemiacetal and acetal side-products (vide
supra). FT-IR analysis of adsorbed pyridine shows that all materials
of the XS-Sn-MCM-41 series have a much higher amount (mmol g )
of Brønsted acid sites and lower ratio between Lewis and
Brønsted acid sites compared to the reference Sn-MCM-41-LP
concern. A comparison between XS-Sn-MCM-41-A and standard
Sn-MCM-41-LP, in terms of surface area and accessible acid sites,
allows highlighting the importance of the reduced particle size.
Both materials present the hexagonal array of pores typical of
MCM-41 and display similar pore size distribution, but XS-Sn-
MCM-41-A presents much higher surface area and shorter chan-
nels due to its extra-small dimensions. These features lead to a
higher amount of accessible acid sites, as proved by pyridine
adsorption measurements, hence leading to the observed better
catalytic performance. XS-Sn-MCM-41-A is also a very good cata-
lyst when compared to other previously reported heterogeneous
catalysts for this reaction [13–16,49–51]: although the different
reaction conditions prevent full comparison, the performance of
our catalyst in terms of TON, TOF and productivity are highly
promising (see Table S1). The attractive features of XS-Sn-MCM-
41-A have a potential for other relevant applications in the conver-
sion of renewables to added-value products [52–54].
To evaluate the reusability of XS-Sn-MCM-41-A, the catalyst
was tested in several consecutive cycles employing two recycling
approaches (Fig. 8). If the recycling protocol only involved washing
of the catalyst with ethanol, the conversion of dihydroxyacetone
drastically dropped in the second run and remained constant in
the successive cycles. However, the activity could be completely
restored if the sample was calcined at 500 °C for 2 h (Fig. 8). This
recycling behavior is analogous to that of the conventional Sn-
MCM-41-LP [14], indicating that a certain amount of residues
deposits on the catalysts during the reaction and that complete
restoration of the activity can be achieved only through calcina-
tion. Aiming at a more energy efficient process, an alternative
-1
(Table 3 and [47]). Analysis of the spectra measured at different
temperatures indicates that pyridine is more strongly adsorbed
on the Lewis compared to the Brønsted acid sites, as shown by
the increase in the ratio between these two sites at higher temper-
atures (Table 3). This suggests the relatively mild nature of the
Brønsted acid sites of these catalysts. The higher amount of
Brønsted acid sites of the XS-Sn-MCM-41 materials can be attrib-
uted to a higher population of surface silanol groups due to the
more defective structure of nanoparticles compared to larger par-
4
3
29
ticles (Q /Q determined by Si NMR is 1.4 for XS-Sn-MCM-41-A
and 1.9 for Sn-MCM-41-LP). In order to confirm the role of the
silanol groups in this reaction, sample XS-Sn-MCM-41-A was sily-
2
9
lated with hexamethyldisilazane [30–32]. Solid-state Si NMR
4
3
shows an increase in Q /Q ratio in XS-Sn-MCM-41-A to 3.7 after
silylation and the appearance of a new band at 14 ppm associated
3
with Me Si-(OSi) groups (Fig. S9, SI) [27], indicating the successful
silylation of the silanols with formation of trimethylsilyl groups
anchored on the surface of the material. The presence of these
groups also leads to a slight decrease in the specific surface area
and a decrease in pore volume and average diameter (Table 1, com-
pare entries 1 and 8). The silylated sample gave lower dihydroxy-
acetone conversion compared to the untreated one, though the
selectivity toward ethyl lactate was improved notably (Table 2,
entry 8). This result is fully in accordance with the proposed role
of silanols as Brønsted acid sites, which can catalyze the initial
step consisting in the dehydration of dihydroxyacetone, but also
promote the formation of the hemiacetal and acetal side-products.
A kinetic study was performed employing the most promising
catalyst (XS-Sn-MCM-41-A), with the purpose of analyzing the for-
mation and conversion of the various reaction intermediates and
products (Fig. 7). This study was carried out with a double catalyst
loading compared to the tests in Table 2. Under these conditions,
the yield of hemiacetal and acetal of pyruvic aldehyde reaches a
plateau after 2–4 h, after which the concentration of these two
compounds starts decreasing until their almost complete disap-
pearance after 6 h [14]. This allows reaching a total conversion of
dihydroxyacetone with almost complete selectivity to ethyl lactate
after 6 h of reaction. Remarkably, the loading required to achieve
this degree of conversion and selectivity in 6 h at 90 °C is half of
that needed to achieve a similar performance with the reference
Sn-MCM-41-LP [14]. This excellent result demonstrates that
XS-Sn-MCM-41-A is a superior catalyst compared to conventional
Sn-MCM-41 and that the lower lactate selectivity observed at
lower loading of XS-Sn-MCM-41-A (Table 2) is not a point of
Fig. 7. Yield of ethyl lactate, diethyl acetal and hemiacetal of pyruvic aldehyde as a
function of reaction time in the conversion of dihydroxyacetone over XS-Sn-MCM-
41-A (100 mg, 90 °C).

6
4
L. Li et al. / Journal of Catalysis 314 (2014) 56–65
to ethyl lactate. This reaction provides an attracting and sustain-
able route for the production of valuable chemicals from
renewable sources. The most active catalyst, XS-Sn-MCM-41-A,
presented high turnover number and significantly improved con-
version compared to conventional Sn-MCM-41 with larger particle
size, and could achieve full conversion of dihydroxyacetone with
virtually complete selectivity toward the desired lactate product.
The excellent activity of XS-Sn-MCM-41-A is attributed to its opti-
mal synthesis protocol: a short synthesis time at low temperature
and a relatively low metal loading led to a material with small par-
ticle size (around 120 nm), exceptionally high specific surface area
2
ꢁ1
(
1111 m g ) and favored the incorporation of isolated tetrahe-
drally coordinated Sn into the silicate framework, which provide
the acid sites required for the reaction. These catalytic results,
together with the easy synthesis route required for the preparation
of the material, make XS-Sn-MCM-41-A an extremely promising
catalyst for other possible applications. XS-Sn-MCM-41-A could
be recycled efficiently after regeneration by means of a mild
calcination.
Fig. 8. Recycling of XS-Sn-MCM-41-A in the conversion of dihydroxyacetone to
ethyl lactate, using two different approaches.
Acknowledgments
We acknowledge sponsoring in the frame of the following
research programs: START1, CREA, Methusalem, IAP-PAI. The
authors acknowledge G. Daelen for his assistance in the NMR
measurements.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2014.03.012.
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ꢁ
1
Lewis acid sites of ꢃ0.11 mmol g and a similar Lewis to Brønsted acid sites
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the formation of the lactate (as it can be deduced from Fig. 6b and c), and that
the selectivity towards lactate increases at higher degree of dihydroxyacetone
conversion (see also Fig. 7 and ref. 13).
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