Base-free conversion of glycerol to methyl lactate using a multifunctional catalytic system consisting of Au-Pd nanoparticles on carbon nanotubes and Sn-MCM-41-XS

Article abstract of DOI:10.1039/c9gc01521c

Multifunctional catalytic systems consisting of physical mixtures of (i) bimetallic Au-Pd nanoparticles (average size of 3-5 nm) supported on functionalised carbon nanotubes (CNTs) and (ii) Sn-MCM-41 nanoparticles (50-120 nm), were synthesised and investigated for the base-free, selective conversion of glycerol to methyl lactate in a batch reactor. The catalysts were characterised by means of transmission electron microscopy, N₂-physisorption, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and by Boehm titration. The catalyst based on bimetallic AuPd/CNTs showed much higher activity than the monometallic Au or Pd counterparts, thus indicating synergetic effects. Functionalisation of the CNTs by oxidative treatments had a positive effect on catalyst performance, which was correlated to the observed increase in surface acidity and hydrophilicity. The highest yield of methyl lactate achieved in this work was 85% at 96% glycerol conversion (140 °C, 10 h at 30 bar air), which is the highest yield ever reported in the literature so far. Insights in the reaction pathway were obtained by monitoring the conversion-time profiles for intermediates and their possible role as inhibitors. Batch recycling experiments demonstrated the excellent reusability of the catalyst.

Full text of DOI:10.1039/c9gc01521c

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DOI: 10.1039/C9GC01521C
ARTICLE
Journal Name
ARTICLE
Base-free conversion of glycerol to methyl lactate using a
multifunctional catalytic system consisting of Au-Pd nanoparticles
on carbon nanotubes and Sn-MCM-41-XS
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
b
b
a
DOI: 10.1039/x0xx00000x
Zhenchen Tang, Dina G. Boer, Ali Syariati, Mihaela Enache, Petra Rudolf, Hero J. Heeres and
a,*
Paolo P. Pescarmona
Multifunctional catalytic systems consisting of physical mixtures of (i) bimetallic Au-Pd nanoparticles (average size of 3-5
nm) supported on functionalised carbon nanotubes (CNTs) and (ii) Sn-MCM-41 nanoparticles (50-120 nm), were
synthesised and investigated for the base-free, selective conversion of glycerol to methyl lactate in a batch reactor. The
2
catalysts were characterised by means of transmission electron microscopy, N -physisorption, energy-dispersive X-ray
spectroscopy, X-ray photoelectron spectroscopy and by Boehm titration. The catalyst based on bimetallic AuPd/CNTs
showed much higher activity than the mono-metallic Au or Pd counterparts, thus indicating synergetic effects.
Functionalisation of the CNTs by oxidative treatments gave a positive effect on catalyst performance, which was correlated
to the observed increase in surface acidity and hydrophilicity. The highest yield of methyl lactate achieved in this work was
o
8
5 % at 96 % glycerol conversion (140 C, 10 h at 30 bar air), which is the highest yield ever reported in the literature so
far. Insights in the reaction pathway were obtained by monitoring the conversion-time profiles for intermediates and their
possible role as inhibitors. Batch recycling experiments demonstrated the excellent reusability of the catalyst.
be able to promote the different steps in a one-pot
reaction.3 When implementing this in the design of a
Introduction
heterogeneous catalytic system, the challenge is to combine
two or more kinds of functional sites, such as acid and
metallic sites, and to make them work efficiently and
cooperatively. In this work, we present the design,
synthesis, characterisation and testing of a novel, highly
efficient multifunctional catalytic system for the conversion
of glycerol to methyl lactate. Glycerol is considered a very
attractive biobased platform molecule with high potential
for conversion into a broad scope of interesting bulk and
fine chemicals.4-6 It is obtained in large quantities as side-
product from the biodiesel industry by transesterification of
triglycerides with methanol (≈ 1.2x106 t glycerol were
produced by the biodiesel industry in 2016).7, 8 Therefore,
the transformation of glycerol into valuable chemicals has
Biomass conversion into biofuels and biobased chemicals
represents a sustainable alternative compared to the use of
fossil resources like oil, gas and coal.1-3 Considering the
finite nature of the latter, growing research efforts are being
dedicated to the development of efficient catalytic routes
for biomass valorisation. The catalysts employed in large
scale refinery processes are generally not suitable for the
conversion of biomass, which consists of compounds with a
much higher content of oxygenated groups compared to
fossil resources. Therefore, new catalytic methodologies
need to be developed. For the upgrading of biobased
compounds into valuable chemicals, multistep syntheses
are often required and as such there is a strong need for
developing multifunctional catalysts, which ideally should
9
attracted a lot of interest from both academia and industry.
Among the most promising platform molecules that can be
a. Chemical Engineering Group, Engineering and Technology Institute Groningen
produced from glycerol are lactic acid and the related esters
(ENTEG), University of Groningen, Nijenborgh 4, 9747 AG Groningen, The
(alkyl lactates).7,
10
Lactic acid can be used for the
Netherlands. Email: p.p.pescarmona@rug.nl
Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4,
b.
production of poly-lactic acid, a biodegradable bio-polymer
with various applications in the food, pharmaceutical and
9
747 AG, Groningen, The Netherlands
Electronic Supplementary Information (ESI) available: Comparison with
previously reported heterogeneous catalysts, correlation between oxygen packaging industry.10 Alkyl lactates find application as
content and number of total acid sites, dispersion of functionalised carbon
nanotubes, conversion of glycerol at different reaction pressures, plot of the
green solvents and the precursor of lactide. Currently, lactic
concentration of glycerol as a function of the reaction time, conversion and acid is commercially produced by a fermentation process
selectivity as a function of the addition of different compounds, TEM pictures
using carbohydrates as the feed. This process has a
of the used catalysts, elemental analysis of the used catalysts, TEM picture of
Sn-MCM-41-XS. See DOI: 10.1039/x0xx00000x
relatively low volumetric production rate and generates
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large amounts of salts during the product work-up.11, 12 and selectivity of the catalytic system.15 ThVeierweAfrotirclee,Onwlinee
DOI: 10.1039/C9GC01521C
Therefore, there is an incentive to develop efficient chemo-
catalytic methodologies to obtain lactic acid or alkyl lactates
OH
HO
OH
from biobased glycerol in a one-pot reaction. This route
involves two main consecutive steps: (1) the selective
oxidation of glycerol to a triose (dihydroxyacetone and/or
glyceraldehyde) and (2) the rearrangement of the trioses
into lactic acid (aqueous medium) or alkyl lactate (alcoholic
medium) (Scheme 1).13-15 Initial research was typically
conducted in strongly basic medium, supposedly essential
to promote the rate of oxidation in the first step of the
reaction, and to increase the rate of the rearrangement in
Glycerol
[O]
A
Metal site
-
H2O
O
OH
HO
OH
HO
O
Dihydroxyacetone (DHA)
Glyceraldehyde (GLAD)
[O]
Brønsted acid B -H O
2
the second step by neutralising the formed lactic acid.16-18
A
OH
O
major disadvantage of using a base is the cost required to
convert the obtained metal lactate back to lactic acid by an
acidification step with a mineral acid, producing large
amounts of salt waste. In order to develop a greener and
more cost-effective process, it is crucial to design catalytic
systems that do not require the use of a homogeneous base
for achieving high activity and selectivity in this reaction.
This can be achieved by employing noble metals for
catalysing the partial oxidation of glycerol (Step A in
Scheme 1), and a solid acid to catalyse the successive
rearrangement of the trioses into lactate/lactic acid (Steps
B and C in Scheme 1). Au nanoparticles supported on ultra-
stable Y (USY) zeolite were the first catalyst reported for
the conversion of glycerol to methyl lactate (ML) in
methanol, without the need of a base (Table S1, Entry 1).13
Higher yields of methyl lactate (79 %) were obtained by
incorporating Sn in a related Au/USY zeolite system, though
oxygen instead of air was used as the oxidant (Table S1,
[O]
HO
O
O
OMe
Methyl glycerate (MGe)
Pyruvic aldehyde
Lewis acid C +MeOH
[O]
OH
OH
O
[
O]
O
O
O
O
OMe
MeO
OMe
OMe
Methyl lactate
(ML)
Methyl pyruvate
MP)
Dimethyl tartronate (DMT)
(
[O]
OH
OMe
O
O
O
OMe
OMe
Methyl glycolate Dimethyl oxalate
(MGo) (DMO)
14
Entry 2). A recent study form our group demonstrated that
physical mixtures of Au/CuO and Sn-MCM-41-XS (i.e. Sn-
MCM-41 with 50-120 nm particle size) also efficiently
convert glycerol into methyl lactate, achieving up to 63 %
yield at 95 % glycerol conversion (Table S1, Entry 3).15 The
use of a solid acid like Sn-MCM-41-XS that contains a
combination of mild Brønsted acid sites and Lewis acid sites,
is preferable in terms of selectivity compared to zeolites,
which display stronger Brønsted acid sites.19, 20 An additional
advantage of this catalytic system is that supporting the
metal nanoparticles on a different material from that
providing the Lewis acid sites offers the possibility of
modifying and tuning the nature of the support towards
enhanced catalytic performance of the whole system.
In order to promote the cascade reaction in Scheme 1 in a
base-free reaction medium, two kinds of catalytic species
are needed: (i) (supported) noble metal nanoparticles (NPs)
for the partial oxidation of glycerol; (ii) a solid acid catalyst
with Lewis and Brønsted acid sites for the rearrangement
step.13, 14, 20, 21 Here, we report for the first time the use of
Scheme 1. Catalytic route from glycerol to methyl lactate (ML) and, in grey, the
possible side-products.
focussed our efforts on designing an enhanced catalyst for
this initial step, while employing the optimum solid acid
1
5,
catalyst identified in previous work (i.e. Sn-MCM-41-XS)
2
0, 22
for promoting the rearrangement of trioses into
lactates. We reasoned that the catalyst for the partial
oxidation of glycerol could be improved by employing
bimetallic nanoparticles, as the presence of a second metal
can allow tuning the electronic properties, morphology and
stability of the nanoparticles towards increased activity and
selectivity.23, 24 Therefore, we chose to combine Au with
another, relatively less expensive noble metal, Pd, which
has proven to lead to increased catalytic performance when
used in an alloy with Au in several partial oxidation
reactions.3, 8, 23-28 A second factor that plays an important
role for the catalytic behaviour of (bi)metallic nanoparticles
is the nature of the support.29, 30 In this work, functionalised
multi-walled carbon nanotubes (CNTs) were selected as
support for the bimetallic nanoparticles. Carbon nanotubes
display a large, accessible external surface area and high
chemical and thermal stability, which make them attractive
supports for metal nanoparticles.31-34 Additionally, carbon
nanotubes can be modified by oxidative methods to tune
bimetallic
Au-Pd
nanoparticles
supported
on
(
functionalised) carbon nanotubes as catalysts for the
oxidation step of glycerol to glyceraldehyde or
dihydroxyacetone. Previous work demonstrated that this
first step of the reaction is the one on which research
should be mainly focussed to increase the overall activity
2
| J. Name., 2012, 00, 1-3
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their surface properties like acidity, hydrophilicity and TEM images of the bimetallic catalysts demoVinewstArrtaictleeOntlhinee
dispersibility,30, 35-37 such functionalisation of the CNTs successful synthesis of metal nanopartiDcOleI:s1w0.1i0th39a/Cm9GeCa0n15s2iz1Ce
surfaces was shown to have a positive effect on catalyst
Molar ratio of Au/Pd
Au + Pd loading
Entry
Catalyst
performance due to enhanced interactions between the
(
wt.%)
1.2
theoretical experimental
3
0-32, 38,
metallic nanoparticles as well as with the substrates.
1
2
3
4
5
Au/CNTs
-
3
-
3
9
However, there is no report in the literature on the use of
Au
3
Pd/CNTs
1.0
3.1
1.2
0.4
-
mono or bimetallic Au-based nanoparticles supported on
functionalised CNTs for the base-free glycerol oxidation to
lactic acid/alkyl lactates. Based on these considerations, our
strategy involved the synthesis, characterisation and testing
of multifunctional heterogeneous catalytic systems
consisting of physical mixtures of (i) bimetallic Au-Pd
nanoparticles supported on different functionalised CNTs
and (ii) Sn-MCM-41-XS in the conversion of glycerol to
methyl lactate. The performance of the bimetallic
nanoparticles was compared to the monometallic (Au or
Pd) counterparts. The choice of using a physical mixture of
the supported nanoparticles and Sn-MCM-41-XS rather than
supporting the metal nanoparticles directly on Sn-MCM-41-
XS was based on the previously observed infeasibility of
achieving high loading of well dispersed nanoparticles
without causing the deterioration of this mesoporous
material.15 In addition, the use of two materials allows
higher flexibility in terms of adjustment of the Sn to noble
metal ratios. The batch reactions were carried out in
methanol (which has been identified as a “recommended”
AuPd/CNTs
1.0
1
AuPd /CNTs
3
1.0
0.33
-
Pd/CNTs
0.9
of around 6 nm and a relatively broad particle size
Table 1 Total loading of Au, Pd on CNTs catalysts and their molar ratios,
determined by ICP-OES.
distribution from 1 to 16 nm (Figure 1.A). The relatively
larger nanoparticles consist of aggregated and partially
coalesced smaller nanoparticles. This is probably due to the
weak interaction between the CNTs surface and the
nanoparticles during the colloid immobilisation process. As
a reference, bimetallic Au-Pd nanoparticles supported on
unmodified activated carbon (AC) using the same synthesis
protocol also showed (Figure 1.B) coalesced nanoparticles
with a large average size (9.9 nm) and a large particle size
distribution (1-24 nm), thus supporting the hypothesis that
unfunctionalised carbon surfaces tend to lead to relatively
large nanoparticles.
The monometallic Au/CNTs, Pd/CNTs and bimetallic
AuPd/CNTs catalysts were characterised by XPS to
determine the surface composition and, in the case of the
bimetallic catalyst, to investigate whether the metals are
actually forming an alloy, or are present as isolated Au and
4
0
solvent in the context of green chemistry ) without the
addition of an external base and using air as the oxidant at
relatively mild conditions for this reaction (see Table S1).
Results and discussion
Synthesis, characterisation and catalytic performance of monometallic Pd nanoparticles, or as a combination of the two. Based on
Au, Pd and bimetallic Au-Pd catalysts on unmodified CNTs
thermodynamic considerations, the formation of an Au-Pd
alloy is expected to be favoured as negative enthalpies of
formation have been observed for the Au-Pd system.41 XPS
confirms the presence of the metals employed in the
synthesis in all samples (Au/CNTs, Pd/CNTs and
AuPd/CNTs, Figure 2). The analysis of the chemical states of
Au and Pd was based on the Au 4f7/2 and Pd 3d3/2 signals,
which are devoid of the overlap observed between Au 4d5/2
and Pd 3d5/2 or between Au 4f5/2 and Pd 4s.42, 43 The Au 4f7/2
peak was found at 84.2 eV in the monometallic Au/CNTs
A series of catalysts consisting of monometallic Au,
monometallic Pd or bimetallic Au-Pd nanoparticles (with a
molar Au/Pd molar of 3:1, 1:1 or 1:3) supported on
unmodified CNTs was synthesised by
a
colloid
immobilisation (CI) method. The actual loading of the
metals on the CNTs supports, as well as the molar ratios
between Au and Pd were determined by inductively
coupled plasma optical emission spectrometry (ICP-OES)
(
1
Table 1). The total metal loading ranged from 0.9 % to
.2 %, which indicates only slight deviations from the
theoretical loading of 1.0 wt%. The higher loading for the
materials with higher relative gold content suggests that the
observed deviations might be caused by the inaccuracy of
the Au content (45-55 %) in the commercial Au precursor
(
4 2
HAuCl ·xH O). As a consequence, the molar ratio between
Au and Pd is slightly higher than the theoretical values. The
low metal loading of these materials implies that the
textural properties of the carbon nanotubes do not get
significantly affected by the deposition of the nanoparticles,
as shown by the only slightly lower values of surface area
and pore volume of AuPd/CNTs compared to the parent
CNTs (Entry 1 and 6 in Table 2).
Figure 1 TEM pictures of bimetallic Au-Pd supported catalysts on unmodified
CNTs and AC. A: AuPd/CNTs, average size = 6.0 nm; B: AuPd/AC, average size =
9
.9 nm.
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peak of monometallic Pd (40.42°), in agreemeVniewt AwrtiictlehOntlhinee
DOI: 10.1039/C9GC01521C
formation of an Au-Pd alloy.
The two monometallic and the bimetallic Au-Pd catalysts on
unmodified CNTs were investigated for the conversion of
glycerol into methyl lactate in combination with Sn-MCM-
o
4
4
1-XS in a batch set-up (140 C, 30 bar air pressure, Figure
). The monometallic Au/CNTs catalyst showed 25 %
conversion of glycerol and 93 % selectivity towards methyl
lactate. By gradually increasing the relative Pd content of
the catalyst (nominal Au-Pd molar ratio of 3/1; 1; and 1/3),
the conversion first increased from 41 % to 47 % and then
decreased to 41 % (Figure 4). The conversion further
decreased to 35 % when the monometallic Pd/CNTs was
used. Conversely, the selectivity towards methyl lactate
remained nearly constant at around 90 % regardless of the
Au/Pd molar ratio.46 The role of the (bi)metallic
nanoparticles is to catalyse the first step of glycerol
oxidation (i.e. the initial activation of oxygen and/or
glycerol, step A in Scheme 1).15, 20 The optimum synergistic
effect was found for a nominal Au/Pd molar ratio of 1
Figure 2 XPS signals of the Au 4f core level region for (a) Au/CNTs, (b)
AuPd/CNTs, and of the Pd 3d core level region for (c) Pd/CNTs, (d) AuPd/CNTs.
(
actual ratio = 1.2, see Table 1), which matches our
anticipation that alloying Au with Pd would lead to
enhanced activity. This trend is in agreement with the
increase in the rate of the oxidation of alcohol groups of
other organic compounds (e.g. benzyl alcohol and
5
-hydroxymethylfurfural) that was observed by using
3
0, 47
bimetallic Au-Pd instead of monometallic Au catalysts.
Synthesis, characterisation and catalytic performance of bimetallic
Au-Pd catalysts supported on functionalised CNTs
The study on the catalysts based on Au-Pd nanoparticles
supported on CNTs demonstrated the advantage of the
bimetallic nanoparticles compared to their monometallic
counterparts, but also indicated that the carbon nanotubes
are not an optimum support compared to previously
reported metal oxides.15 With the purpose of improving the
Figure 3 XRD patterns of CNTs before and after loading Au-Pd nanoparticles.
catalyst (Figure 2a), while it is shifted by about 0.5 eV
towards lower binding energy in the bimetallic AuPd/CNTs
(
Figure 2b). The same trend was found for the Pd 3d3/2
peak, which was shifted by 0.8 eV towards lower binding
energy in the bimetallic AuPd/CNTs sample (Figure 2d)
compared to the monometallic Pd/CNTs one (Figure 2c).
The observed decrease in binding energy of both Au 4f7/2
and Pd 3d3/2 compared to the monometallic counterparts is
in line with what has been previously observed in other Au-
Pd systems and has been attributed to the formation of an
Au-Pd alloy.44, 45
XRD analysis of the AuPd/CNT materials (Figure 3)
confirmed the formation of the Au-Pd alloy. After loading
the Au-Pd nanoparticles on the CNTs, only one new, broad
peak (due to the small size of the particles) appears in the
XRD pattern at 39.36° Its position is in between that of the
(
111) peak of monometallic Au (38.27°) and of the (111)
Figure 4 Performance of the catalytic system consisting of mono- or bimetallic Au-
Pd on unmodified CNTs (with different Au/Pd values) and Sn-MCM-41-XS.
Reaction conditions: 0.25 M glycerol in 20 ml methanol; 0.1 g of AuPd/CNTs; 0.2 g
of Sn-MCM-41-XS; 140 °C; 30 bar air pressure; 4.5 h batch time.
4
| J. Name., 2012, 00, 1-3
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interaction with the support and thus decrease the Elemental analysis of the CNTs by EDX allowedVdieewteArrtimcleiOnnilninge
nanoparticle size, we decided to modify the CNTs used as the C, O and S content at the surfDacOeI:,10w.1h03ic9h/C9pGrCo0v1i5d2e1Cs
supports by oxidative treatments using a number of information on the degree of functionalization of the CNTs.
reagents, viz. H
HNO and H SO
enhance the density of functional groups (e.g. -OH, -CHO content of the CNTs. The highest value (2.6 %, Entry 5,
and -COOH) on the CNTs surface.31, 39 Additionally, the Table 2) was obtained upon treatment with a H
SO -HNO
specific surface area of the CNTs was increased by the mixture. The increased O content indicates the formation of
oxidative treatment with H , HNO and H SO -HNO considerable amounts of oxygenated groups (e.g. -OH, -CHO,
compare Entry 1 to 2, 3 and 5 in Table 2), though it was not -COOH) on the surface of the CNTs.48, 49 The presence of
affected by the treatment with H SO (Entry 4, Table 2). such oxygenated groups leads to a higher surface acidity,
2 2
O , HNO
3 2 4
, H SO and the combination of In the untreated CNTs, the oxygen amount is only 0.8 %,
3
2
4
. These treatments are known to largely whereas all the treatments largely enhanced the oxygen
2
4
3
O
2 2
3
2
4
3
(
2
4
Such increase in specific surface area is generally ascribed but also significantly improves the hydrophilicity of the
to loss of structural integrity and size reduction of the CNTs surface. Both features can be important for the
CNTs.31, 48 The pore volume did not change significantly catalytic performance of the supported (bi)metallic
upon treatment with H
decreased to 1.6 cm /g with the harsher H
treatment.
2
O
2
, HNO
3
and H
2
SO
4
, but it nanoparticles. The higher surface acidity can promote the
3
2
SO
4
-HNO
3
acid-catalysed dehydration step of the conversion of
glycerol into methyl lactate (i.e. step B in Scheme 1).
Table 2 Textural properties, elemental composition and acidic properties of CNTs, before and after oxidative treatment.
Specific
Total acid
sites
(mmol/g)
Stronger acid
sites
(mmol/g)
d
d
surface area
Pore volume
(cm /g)
Entry
1
Sample
CNTs
C atomic %c
99.2
O atomic %c
0.8
S atomic %c
-
2
-1
a
3
b
(
m g )
258
2.1
2.0
2.0
2.1
1.6
2.0
0.022
0.002
2
3
4
5
6
CNTs-O
CNTs-N
313
306
259
274
241
98.7
98.5
98.8
97.3
n.d.
1.3
1.5
1.1
2.6
n.d.
-
0.90
1.3
0.42
0.88
0.23
1.3
-
CNTs-S
0.11
0.10
n.d.
0.46
2.4
CNTs-NS
AuPd/CNTs
n.d.
n.d.
a
BET Surface Area. b BJH cumulative volume of pores, between 17 and 3000 Å width, based on the desorption isotherm. C, O, S atomic content (mol%), as determined by EDX.
The amount of total and stronger acid sites is determined by Boehm titration of NaOH and NaHCO
c
d
3
, respectively. CNTs-O, CNTs-N, CNTs-S, CNTs-NS denote the CNTs treated by
H
2
O
2
, HNO , H SO and H SO -HNO , respectively. n.d. = not determined.
3
2
4
2
4
3
The higher hydrophilicity can: (i) enhance the interaction with content after treatment, Figure S1.49 The anticipated effect of the
the nanoparticles during their preparation by chemical functionalisation of the carbon nanotubes on their ability to form
impregnation procedure; (ii) affect the reaction rates by a stable dispersion in a polar medium was estimated by
changing adsorption constants of substrates and products on the sonicating the CNTs for 1 h in water, followed by storage without
surface; (iii) promote the dispersion of the CNTs in a polar agitation. After one week in static conditions, the parent CNTs
reaction medium (which is an asset both for the preparation of and CNTs-S had separated at the bottom of the vial, whereas the
the supported nanoparticles as well as for the catalytic reaction). CNTs treated by H
The treatment of CNTs with H SO (Entry 4 and 5) is known to highest degree of functionalisation, see Table 2) were still
lead to the formation of sulphonic acid groups (-SO H) on the dispersed in the water phase (Figure S2). Placing a droplet of
2 2 3 2 4 3
O , HNO and H SO -HNO (i.e. those with the
2
4
3
CNTs surface, which is confirmed by the presence of significant each of these suspensions on a filter paper indicated a different
amounts of sulphur on the surface (0.1 %, see Table 2). The degree of aggregation of each type of functionalised CNTs
number and strength of the acid sites of the functionalised CNTs (Figure S2). Particularly, the larger, uniform, dark grey spot
were evaluated by Boehm titration with an aqueous solution of generated by CNTs-NS showed that this material has the highest
NaOH, for the total number of acid sites, and with an aqueous tendency to from a homogenous dispersion in water. This result
solution of NaHCO
sites (i.e. carboxylic and sulphonic groups).
CNTs material has a low amount of acid sites (0.022 mmol/g in to be more hydrophilic.29 The partial cleavage of CNTs into
total, Entry 1, Table 2). All treatments resulted in an increase in smaller fragments that has been reported to be caused by the
3
for the determination of the stronger acid correlates well with the higher oxygen content and total acid
5
0,51
The untreated amount of these functionalised CNTs, which cause the material
total acid sites. The highest values in the concentration of both harsh treatment with H
strong (1.3 mmol/g) and total (2.4 mmol/g) acid sites were the enhanced dispersibility of CNTs-NS.
obtained upon treatment of the CNTs with H SO -HNO . The The functionalised CNTs were used as support for a range of
total amount of acid sites correlates well with the oxygen bimetallic Au-Pd catalysts prepared using a similar CI method to
2 4 3
SO -HNO probably also contributes to
4
8, 52
2
4
3
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Journal Name
the one employed with unmodified CNTs (vide supra). As ML). With prolonged reaction time (9 h), the yieVliedwoArfticmle eOtnhlinyel
DOI: 10.1039/C9GC01521C
anticipated, the functionalisation method had a large effect on lactate reached 85 % at 96 % glycerol conversion, which to the
the average size of the Au-Pd NPs on the support,48, 52 as best of our knowledge is the highest yield ever reported for the
evidenced by TEM analysis (Figure 5). The average particle size conversion of glycerol to methyl lactate using a heterogeneous
and size distribution show a correlation with the degree of catalytic system (Table S1). The main by-products identified by
functionalisation of the CNTs surface,41,45 with the Au-Pd gas chromatography (GC) were methyl pyruvate (MP), methyl
nanoparticles supported on CNTs prepared in the presence of glycolate (MGo), dimethyl oxalate (DMO), dimethyl tartronate
HNO
3
(CNTs-N and CNTs-NS, i.e. those with the highest (DMT) and methyl glycerate (MGe), see Scheme 1. Carbon
concentration of functional groups, see Table 2) having the balances for all runs exceeded 98 %, indicating that the
smallest average size (~3.5 nm), with a narrow particle size formation of products that would not be detected by GC (e.g.
distribution. The smaller average size of the Au-Pd nanoparticles higher molecular weight components formed by condensation
observed by TEM in AuPd/CNTs-NS (3.5 nm, Figure 5D) reactions) is negligible.
compared to those in AuPd/CNTs (6.0 nm, Figure 1A) correlates The decrease of the Au-Pd particle size when using the
well with the less intense and broader peak of the Au-Pd alloy unfunctionalised carbon nanotubes as support, which implies
observed in the XRD pattern of the former material (Figure 3).
that a higher fraction of metal sites is exposed on the surface and
The bimetallic Au-Pd nanoparticles supported on functionalised thus available as catalytic sites, is most likely the main factor
CNTs were investigated in combination with Sn-MCM-41-XS as that led to the dramatic increase in activity (from 54 % with
heterogeneous catalytic system for the oxidative conversion of AuPd/CNTs to 81 % with AuPd/CNTs-NS, see Table 3). However,
glycerol into methyl lactate (Table 3). We expected these the acidity of the functionalised CNTs also contributed to the
catalytic systems to be superior to the counterpart based on Au- observed increase in catalytic activity. This is demonstrated by
Pd nanoparticles supported on the unfunctionalised CNTs the higher activity achieved with AuPd/CNTs-S compared to Au-
because: (i) the well-dispersed Au-Pd nanoparticles have smaller Pd/CNTs (71 % vs. 54 %): both have similar particle size
average size, (ii) the functionalised CNTs disperse better in the (~6 nm), but the former has a large population of acid sites
polar reaction medium and (iii) provide additional surface (Table 2). Additionally, the strength of these acid sites also plays
Brønsted acid sites for promoting the dehydration step from a role: AuPd/CNTs-S has a larger particle size and lower amount
trioses to pyruvic aldehyde.13, 15, 20 Indeed, the rate of glycerol of acid sites (0.46 mmol/g) compared to AuPd/CNTs-N (particle
conversion was higher with the functionalised CNTs, the only size 3.3 nm, acid amount 1.29 mmol/g) but showed higher
exception being CNTs treated with H
untreated CNTs (Table 3, Entries 1 to 5). The impact of the in AuPd/CNTs-S of the sulphonic groups generated by the
nature of the support on the performance of the catalytic system treatment with H SO , as indicated by the presence of sulphur in
2 2
O , which performed as the activity (71 % vs. 63 %). This is most likely due to the presence
2
4
was highlighted by the dramatic increase in the conversion of this material (Table 2). Though both sulphonic and carboxyl
glycerol passing from the untreated CNTs (54 % conversion) to groups are labelled as strong acids on the basis of Boehm
the CNTs that underwent the treatment with HNO -H SO and titration, the sulphonic groups are much stronger acid sites
3 2 4
which display the largest concentration of acid groups, including compared to the carbon-based acid groups in AuPd/CNTs-N and,
strong sulphonic groups (81 % conversion with 70 % yield of therefore, promote more efficiently the dehydration step of
trioses to pyruvic aldehyde.53 A similar reasoning based on the
number and strength of the acid sites also allows explaining the
significantly higher activity of AuPd/CNTs-NS compared to
AuPd/CNTs-N (Table 2), notwithstanding their similar particle
size (Figure 5, B and D). In previous work, it was reported that
too strong Brønsted acid sites are detrimental as they promote
undesired acetalisation reactions of the intermediate pyruvic
aldehyde.53 This was not the case with the catalytic system
presented here, most likely because the Sn-MCM-41-XS used in
combination with the Au-Pd catalysts provides enough Lewis
acid sites to rapidly convert the pyruvic aldehyde formed after
dehydration into the desired methyl lactate product (step C in
Scheme 1).
In order to further prove the importance of using modified CNTs
as support for the Au-Pd nanoparticles, the material consisting
of Au-Pd nanoparticles supported on activated carbon using a
similar CI method (average particle size = 9.9 nm, see Figure 1B)
was tested in combination with Sn-MCM-41-XS as catalyst for
the conversion of glycerol to methyl lactate. A much lower
activity was observed (9.3 % glycerol conversion, see Table 3,
Entry 7), confirming the prominent role of the use of
Figure 5 TEM pictures of Au-Pd alloy nanoparticles supported on various
functionalised CNTs. A: AuPd/CNTs-O, average size = 6.2 nm; B: AuPd/CNTs-N,
average size = 3.3 nm; C: AuPd/CNTs-S, average size = 5.2 nm; D: AuPd/CNTs-NS,
average size = 3.5 nm.
(functionalised) CNTs on the catalytic activity in this reaction.
6
| J. Name., 2012, 00, 1-3
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This lower activity can be ascribed to the larger size of the Au-Pd main by-product was MGe with a selectivity of arouVniedw6Ar%ticl,ewOhnliinlee
particles when AC is used as support (Figure 1), which implies the selectivity towards other products weDrOeI:a1l0l.1b0e3l9o/wC92GC%01.5T2h1Ce
that a smaller fraction of the metal atoms are exposed on the initial conversion of glycerol (0-4.5 h) was analysed by plotting
surface and, therefore, are available as active sites. Moreover, the natural logarithm of the concentration of glycerol vs. reaction
2
the CNTs have different textural properties compared to AC. time. The linear fitting gave a R value of 0.994, which suggests a
CNTs have fully open and thus more accessible external surface, first order reaction with respect to glycerol concentration. This
whereas AC has a porous structure with large amount of fact, in combination with the constant selectivity towards methyl
micropores, which can cause mass transfer limitations of lactate throughout the reaction, points to the initial partial
glycerol as well as of the reaction intermediates.
oxidation of glycerol as the rate determining step under the
AuPd/CNTs was also tested without the addition of employed reaction conditions. Compared to the initial stage of
Sn-MCM-41-XS, leading to a much lower conversion of glycerol the reaction, the conversion of glycerol at the second half stage
(
(
Table 3, Entry 8) and to poor selectivity towards methyl lactate (4.5-15.5 h) only had a slow increase from 68 % to 84 %,
5.7 %). The main by-products were MGo, DMO and MGe, all whereas the selectivity to methyl lactate stayed identical (Figure
arising from over-oxidation of the initially formed trioses. This 6). For this later stage of the reaction, the slope of the natural
result indicates that Sn-MCM-41-XS plays an essential role by logarithm of the concentration of glycerol vs. the reaction time is
enhancing the rate of the rearrangement reactions of the trioses much lower than for the initial stage (Figure S4). This indicates
(
and/or pyruvic aldehyde) and thus preventing over-oxidation that the reaction is no more first order with respect to glycerol
of the trioses (Scheme 1), which can lead to the formation of concentration and suggests that partial inhibition of the catalyst
inhibitors (e.g. glyceric acid, vide infra).15
by reaction intermediates or products might occur. To
The effect of the air pressure used during the catalytic test was investigate this potential inhibition of the catalytic activity, a
investigated with the catalytic system consisting of number of tests were performed including possible reaction
AuPd/CNTs-S and Sn-MCM-41-XS (Figure S3). An almost intermediates (methyl esters and their corresponding carboxylic
identical catalytic performance (glycerol conversion around 71 acids) in the initial reaction mixture, using a molar ratio of 9 to 1
%
) was observed with air pressure at 16 or 30 bar, whereas the between glycerol and additional compound (Figure S5). The
conversion of glycerol dropped to 47 % when the pressure was tests carried out in the presence of 10 % of methyl esters
lowered to 10 bar. Based on the reaction stoichiometry, it was (glycolate, oxalate or formate) gave similar results to the
calculated that the minimum air pressure required for this reaction without any additional compound. However, when
reaction is around 8 bar. This indicates that when using 10 bar performing the experiments in the presence of 10 % of the
the kinetics depend on air pressure, whereas it becomes 0th corresponding carboxylic acids (glycolic acid, oxalic acid or
order when the pressure is 16 bar or higher. To get a deeper formic acid), a clear reduction of the glycerol conversion was
insight in the kinetics of this reaction, the conversion of glycerol observed (Figure S5). The effect was even more marked when
and selectivity to the major intermediates/products was 10 % glyceric acid was employed in the system, leading to a
determined as a function of the batch time for the catalytic system decrease in the conversion of glycerol from 54 % to 12 % with a
consisting of AuPd/CNTs-S and Sn-MCM-41-XS (Figure 6). The dramatic drop in the selectivity towards methyl lactate from
conversion of glycerol increased almost linearly in the first 4.5 h 90 % to 28 %. The inhibition effect by glyceric acid was similar
and reached 85 % after 15.5 h. The selectivity to methyl lactate even when only a small amount (2 % relative to glycerol) was
was nearly constant at around 90 % between 0 and 15.5 h. The
Table 3 Catalytic performance in the conversion of glycerol to methyl lactate using bimetallic Au-Pd catalysts on (modified) CNTs catalysts combined with
Sn-MCM-41-XS. a
Selectivity (%)
ML
90
MP
1.5
MGo
2.2
DMO
0.4
DMT
0.8
MGe
4.9
Entry
1
Catalyst
Conv. (%) Y of ML (%)
AuPd/CNTs + Sn-MCM-41-XS
54
54
63
71
81
96
9.3
6.8
47
46
54
61
70
85
7.5
0.4
2
3
4
5
6
7
8
AuPd/CNTs-O + Sn-MCM-41-XS
AuPd/CNTs-N + Sn-MCM-41-XS
AuPd/CNTs-S + Sn-MCM-41-XS
AuPd/CNTs-NS + Sn-MCM-41-XS
AuPd/CNTs-NS + Sn-MCM-41-XS b
AuPd/AC + Sn-MCM-41-XS
AuPd/CNTsc
88
88
87
87
88
94
5.7
2.3
2.5
1.9
5.6
2.7
0.4
2.1
3.0
3.4
1.8
1.6
3.5
1.6
13
0.6
0.5
0.4
0.2
0.6
0.5
5.1
0.6
0.9
1.3
0.3
0.6
0.8
1.6
5.0
4.4
7.2
5.1
3.2
2.8
17
a
Reaction conditions: 0.25 M glycerol in 20 ml methanol; 0.1 g of AuPd/CNTs; 0.2 g of Sn-MCM-41-XS; 140 °C; 30 bar air; 4.5 h. For all reactions, small amounts of pyruvic
aldehyde, dimethyl acetal were observed, with selectivity below 0.1 %. b reaction time: 9 h at 140 °C (heating time 0.5 h). run without Sn-MCM-41-XS.
c
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added to the reaction mixture. On the other hand, the activity
only slightly decreased upon the addition of 2 % lactic acid. The
View Article Online
DOI: 10.1039/C9GC01521C
2 4
effect of adding a strong inorganic acid such as H SO (0.05 mM)
was also studied: the activity decreased from 54 % to 22 % but
the selectivity towards methyl lactate was not affected. These
results prove that the methyl esters of the by-products do not
affect significantly the catalysts, whereas the corresponding
Figure 7. Catalyst recycling experiments in a batch set-up using a catalytic
system consisting of AuPd/CNTs-NS and Sn-MCM-41-XS. Reaction conditions:
0
4
.25 M glycerol in 20 ml methanol; 0.1 g of AuPd/CNTs-NS; 0.2 g of Sn-MCM-
1-XS; 140 °C; 30 bar air; 4.5 h.
Conclusions
Multifunctional heterogeneous catalytic systems consisting of (i)
bimetallic Au-Pd nanoparticles supported on (functionalised)
CNTs and (ii) Sn-MCM-41-XS were reported here for the first
time for the conversion of glycerol into methyl lactate in a base-
free one-pot reaction. Among these materials, the catalytic
Figure 6 Glycerol conversion and product selectivity as a function of batch time
for a catalyst system consisting of AuPd/CNTs-S and Sn-MCM-41-XS. Reaction
conditions: 0.25 M glycerol in 20 ml methanol; 0.1 g AuPd/CNTs-S; 0.2 g of Sn-
MCM-41-XS; 140 °C; 30 bar air.
3 2 4
system employing CNTs functionalised through a HNO /H SO
treatment showed the best performance and gave 85 % yield of
methyl lactate at 96 % conversion of glycerol after 9 h at 140 °C,
which is the best catalytic performance reported so far in the
literature for base-free systems. This result represents a green
advance also considering that the reaction is conducted in
methanol, which is a recommended solvent in the context of
green chemistry, and at relatively mild conditions compared to
those generally employed for the conversion of glycerol. The use
of bimetallic Au-Pd nanoparticles was shown to lead to higher
activity compared to the monometallic systems (either Au or Pd
alone), proving the synergy between Au and Pd. The observed
catalytic trends could be rationalised on the basis of a
characterisation study, which highlighted the interaction
between Au and Pd (XPS), the small particle size (TEM) and the
high degree of functionalisation with acidic groups of the CNTs
carboxylic acids, and particularly glyceric acid, negatively affect
the catalytic system. Since small amounts of H O were generated
2
from the oxidation of alcohols and the dehydration of pyruvic
aldehyde (Scheme 1), the methyl esters in the reaction mixture
may partially hydrolyse to form carboxylic acids. These
carboxylic acids probably compete with glycerol in the
adsorption on the catalytic surface and thus inhibit the catalytic
activity.15 This phenomenon is reversible, as proven by the high
reusability of the catalytic system consisting of AuPd/CNTs-NS
and Sn-MCM-41-XS.
For the recycling experiments, the catalyst was separated from
the reaction mixture, washed and dried and subsequently re-
used for another run. The results show that the activity and
selectivity are virtually constant in 5 consecutive runs (Figure 7),
demonstrating excellent catalyst reusability under the operating
conditions. The elemental composition of the catalysts was
analysed by ICP-OES after 5 runs and did not reveal any changes
in the Au/Pd molar ratio of Au to Pd or in the Si/Sn molar ratio
in Sn-MCM-41-XS (Table S2). Moreover, the catalyst reused in 5
consecutive runs was analysed by TEM (Figure S6) and showed a
very similar average particle size (3.8 nm) compared to the fresh
catalyst (3.5 nm, Figure 5D). The TEM image also revealed that
AuPd/CNTs-NS and Sn-MCM-41-XS were intimately mixed, the
distance between them being at the nanoscale, which is
beneficial for the mass transfer of reactants and intermediates
during the reaction. These results indicate that there was
negligible change in the catalyst after 5 runs, which correlates
well with the observed reusability with no activity loss.
3 2 4
functionalised by a HNO /H SO treatment (EDX and Boehm
titration). The latter feature is considered crucial for the final
catalytic activity, as it led to: (i) increased interaction between
the bimetallic nanoparticles and the CNTs surface, with
consequent small particle size (average size = 3.5 nm); (ii)
presence of surface Brønsted acid sites that promote the
dehydration of the trioses intermediates to pyruvic aldehyde;
(
iii) enhanced dispersion of the AuPd/CNTs-NS material in the
polar reaction medium. A kinetic study revealed that the
reaction is first order with respect to glycerol, thus identifying
the initial oxidation of our starting material as the rate-
determining step. Moreover, the role of the carboxylic acid and
ester side-products as possible inhibitors of the active sites was
investigated, showing that the former negatively affect the
catalytic activity. The reusability of the catalytic system
consisting of AuPd/CNTs-NS and Sn-MCM-41-XS was excellent,
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as proven by the retention of the original activity in 5 (CNTs-N) is as follows: CNTs (2.5 g) were suspVeienwdAretidcleiOnnlainne
DOI: 10.1039/C9GC01521C
3
consecutive batch recycle experiments. Our strategy of aqueous solution of HNO (30 wt%, 50 mL). The mixture
combining a material bearing the catalyst for the oxidation of was sonicated for 30 min; after which it was refluxed for 2 h
glycerol (AuPd/CNTs-NS) with one providing the active sites for by setting the temperature of the oil bath at 140 ᵒC. The
the consecutive rearrangement of the triose intermediates into suspension was filtered and the solid residue was washed
the desired lactate product (Sn-MCM-41-XS) proved successful with MilliQ water until the filtrate was neutral. The
in achieving unsurpassed catalytic performance. This concept of obtained solids were dried overnight at 100 ᵒC. A
catalyst design is highly flexible and allows careful tuning of each representative procedure for CNTs treatment by H
component. Therefore, it has the potential to be applicable for (CNTs-S) is as follows: CNTs (2.5 g) were suspended to an
other multi-step reactions necessitating various types of aqueous H SO solution (98 %, 25 mL) and the mixture was
catalytically active sites, as required for the upgrading of other sonicated for 30 min. The mixture was then refluxed by
2 4
SO
2
4
bio-based compounds.
setting the temperature of the oil bath at 150 ᵒC for 15 h.
The reaction mixture was diluted with MilliQ water, after
which it was filtered and washed with MilliQ water. The
Experimental
Reactants and materials
resulting solid was dried overnight at 100 ᵒC.
representative procedure for CNTs treatment by HNO -
3
A
H
2
SO
suspended in a mixture of aqueous HNO
and H SO (98 %, 25 mL) and the slurry was sonicated for
0 min. The mixture was then maintained at 60 ᵒC for 15 h.
4
(CNTs-NS) is as follows: CNTs (2.5 g) were
Glycerol (99 %), 1,3-dihydroxyacetone dimer (97 %),
glyceraldehyde (90 %), methyl glycolate (98 %), methyl
lactate (98 %), pyruvic aldehyde (40 wt% in H
acid (97 %), Au(III) chloride hydrate (99.999 %), Pd(II)
chloride (99.999 %), hydrogen peroxide (30 wt% in H O),
polyvinyl alcohol (PVA, MW 9000-10000, 80% hydrolysed),
sodium borohydride (99 %), cetyltrimethylammonium
bromide (CTAB, 99 %), tetraethyl orthosilicate (TEOS,
3
(30 wt%, 25 mL)
2
4
2
O), tartronic
3
Next, the reaction mixture was diluted with MilliQ water,
after which it was filtered and washed with MilliQ water.
The resulting solid was dried overnight at 100 ᵒC.
Mono- and bimetallic Au-Pd catalysts supported on CNTs
were prepared using a colloidal immobilisation (CI) method
aiming at a theoretical metal loading of 1 wt%.30 The
preparation of Au-Pd NPs (1 wt%) supported on the chosen
type of CNTs with a molar ratio of Au-to-Pd of 1 was as
2
9
8 %), tin chloride pentahydrate (SnCl
multi-walled carbon nanotubes (CNTs, >95 % purity, O.D.
outside diameter) × L (length) 6-9 nm × 5 μm) were all
purchased from Sigma Aldrich. Active carbon Norit SX1G
was purchased from Cabot. Glyceric acid (20 wt% in H O)
was purchased from TCI Chemicals. The H O used in this
4 2
·5H O, 98 %) and
(
follows: an aqueous HAuCl
an aqueous PdCl ·2HCl solution (3.5 g Pd/L, 1.00 mL) and
wt% of an aqueous PVA solution (1.2 mL) were added to
4
solution (3.5 g Au/L, 1.85 mL),
2
2
2
2
work was always of MilliQ grade. All chemicals were used
without further purification.
2
MilliQ H O (100 mL) and the mixture was stirred for 30 min
at room temperature. A freshly prepared aqueous solution
of NaBH (0.1 M, 2.54 mL) was added dropwise to the
mixture, which was then stirred for 30 min. The pH was set
4
Synthesis of the catalysts
To remove possible metallic residues in the multi-walled to 2 by the addition of an aqueous H SO solution (1 M).
2
4
carbon nanotubes (e.g. cobalt and molybdenum), the CNTs Subsequently, CNTs (1 g) were added to the solution. The
were washed with a HCl solution prior to use. CNTs (6 g) CNTs were sonicated (30 min) in MilliQ water (50 mL) prior
were suspended in an aqueous HCl solution (5 wt%, to use, to achieve a good dispersion. The suspension was
6
00 mL) and the mixture was refluxed for 2 h at 50 ᵒC. The stirred for 2 h and subsequently filtered. The residue was
suspension was filtered (on a Buchner funnel) and washed washed with H O (2 L) to neutralise the mixture and to
2
with MilliQ water until it was neutral. The resulting solids remove PVA. Finally, the solid was dried overnight at 80 ᵒC.
were dried overnight at 100 ᵒC and used as support for Sn-MCM-41-XS was synthesised according to a previously
metal nanoparticles or for further functionalisation reported method.20 In short, an aqueous solution of NaOH
reactions.
The multi-walled carbon nanotubes were functionalised by (480 mL), after which the mixture was stirred vigorously at
oxidation using H , HNO , H SO or HNO -H SO . A room temperature for 30 min. Next, TEOS (2.29 g) was
representative experimental procedure for the CNTs added dropwise to this solution. Then, a solution of
treatment by H (CNTs-O) is as follows: CNTs (2.5 g) were SnCl ·5H O in ethanol (0.3 M, 1 mL) was added dropwise,
suspended in an aqueous H
2
(2 M, 3.6 mL) was added to a solution of CTAB (1.0 g) in H O
O
2 2
3
2
4
3
2
4
2 2
O
4
2
2 2
O solution (30 wt%, 25 mL) after which more TEOS (2.29 g) was added dropwise. After
and the mixture was sonicated for 30 min. The mixture was stirring the mixture at room temperature for 2 h, the white
then refluxed at 65 ᵒC for 6 days while adding a fresh gel was filtered and washed three times with MilliQ H O (10
2
2 2
aqueous H O solution (30 wt%, 10 mL) every 12 h (120 mL mL) and then with ethanol (10 mL). The obtained white
in total). Next, the suspension was filtered and the solid solid was dried at 70 ᵒC overnight and the resulting powder
residue was washed with MilliQ water until the filtrate was was calcined at 550 ᵒC in air for 5 h, with a heating rate of 3
neutral. The obtained solid was dried at 100 °C overnight. A ᵒC/min. The following molar ratio was used: 1 TEOS :
representative procedure for CNTs treatment by HNO
3
4 2
0.0135 SnCl ·5H O : 0.33 NaOH : 0.125 CTAB : 0.9 EtOH :
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ARTICLE
247 H
Journal Name
1
2
O. See Figure S7 for selected characterisation data 7000 DV instrument in order to obtain the actuVailewAAurtiaclne OdnPlinde
DOI: 10.1039/C9GC01521C
for Sn-MCM-41-XS.
loadings on the supports, as well as the actual Si/Sn ratio in
Sn-MCM-41-XS.
Catalytic experiments
Energy-dispersive X-ray spectroscopy (EDX) measurements
were performed on a FEI Tecnai T20 electron microscope
operating at 200 keV with an Oxford Xmax 80T detector.
The samples were prepared by ultra-sonication in ethanol
followed by drop-casting of the material on a copper grid.
X-ray Photoelectron Spectroscopy (XPS) was measured by
mounting the catalysts on a conductive tape attached to the
XPS sample holder. No further treatment was carried out
prior to the XPS measurement. The measurements were
The catalytic experiments were carried out in a 100 mL
Parr stainless steel autoclave reactor equipped with a
Teflon liner and an overhead stirrer. In a typical experiment
test, a solution of glycerol in methanol (0.25 M, 20 mL) was
loaded into the reactor together with a predetermined
amount of the noble metal catalyst and of Sn-MCM-41-XS.
The reaction was performed using air as the oxidant (30
bar) for 4.5 h at 140 ᵒC (heating time 0.5 h) at a stirring
speed of 800 rpm. Then, the reactor was depressurised and
the reaction content was collected and filtered to separate
the catalyst. The liquid phase was analysed by gas
chromatography using a Thermo Trace GC equipped with a
Restek Stabilwax-DA column (30 m × 0.32 mm × 1 μm) and
a FID detector. Each compound (reactant or product) was
calibrated using solutions of the pure component at 4
different concentrations.
performed using
a
Surface Science SSX-100 ESCA
instrument equipped with a monochromatic Al Kα X-ray
source (h = 1486.6 eV) and operating at a pressure below
2
·10-9 mbar. The analysed spot size was 600 µm; an
electron flood gun was employed to compensate for
charging effects. Spectral analysis included Shirley
5
4
background subtraction and peak deconvolution using the
Winspec software package developed by LISE laboratory,
University of Namur, Belgium. Binding energies were
referenced to the C 1s peak set at 284.5 ± 0.2 eV.
The glycerol conversion (Conv./%) is defined by equation 1:
퐶푔,0 ― 퐶푔
Conv. =
× 100 %
(1)
퐶푔,0
The amount of acid sites per gram of CNTs was measured
by Boehm titration.50 For this test, a CNTs sample (0.2 g)
in which C
g
is the concentration of glycerol after a certain
reaction time and Cg,0 is the initial glycerol concentration.
was added to an aqueous basic solution of NaHCO
3
(0.05 M,
The product selectivity for a compound P is defined by
2
5 mL) to determine the stronger acid sites, such as
퐶푝
=퐶푔,0 ― 퐶푔 × 100 %
(2)
carboxylic and sulphonic acid groups; or to an aqueous
solution of NaOH (0.1 M, 25 mL) to determine the total
amount of acid sites. The suspensions were stirred for 24 h
at room temperature and filtered before titration. Aliquots
equation 2: S
in which C
p
p
is the concentration of the product after a
certain reaction time.
The carbon balance (C/%) was calculated as the sum of the
concentration of unreacted glycerol and of the yields of all
products detected by GC.
(
10 mL) of the filtered solutions were titrated with 0.1 M
HCl.
The dispersibility of the functionalised CNTs was evaluated
by suspending the CNTs (20 mg) in water (20 mL) using
ultra-sonication for 30 min. The mixture was stored without
agitation for 1 week. Then, one drop of the suspension was
taken and dropped on a filter paper to visually determine
the state of aggregation of the functionalised CNTs.
For the catalysts recycling tests, after a small amount of the
reaction mixture was collected for analysis at the end of the
catalytic test, the remaining mixture was filtered and the
catalysts were recovered. The catalysts were washed first
2
with MilliQ H O (20 mL), then with ethanol (20 mL), and
this procedure was repeated 3 times, after which the solid
was dried overnight at 100 ᵒC. This solid was used for
another batch experiment.
Conflicts of interest
There are no conflicts to declare.
Characterisation of the catalysts
Transmission electron microscopy (TEM) images were
obtained using a CM12 (Philips) electron microscope
operating at 120 keV. The samples were prepared by ultra-
sonication in ethanol, after which a droplet of the
suspension was added to a carbon coated 400 mesh copper
grid. Images were taken with a slow scanning CCD camera.
Acknowledgements
Zhenchen Tang acknowledges financial support from the
China Scholarship Council for his Ph.D. grant. All authors
thank Leon Rohrbach, Jan Henk Marsman, Erwin Wilbers,
Anne Appeldoorn and Marcel de Vries for technical support,
Dr. Marc Stuart for support with TEM-EDX and Johannes
van der Velde for support with ICP-OES.
Nitrogen physisorption isotherms were measured at
o
-
196 C using a Micromeritics ASAP 2420 apparatus. The
Brunauer-Emmet-Teller (BET) method was employed to
calculate the specific surface area. The Barrett-Joyner-
Halenda (BJH) method was used to calculate the pore
volume.
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