Tunable catalysts for solvent-free biphasic systems: Pickering interfacial catalysts over amphiphilic silica nanoparticles

Article abstract of DOI:10.1021/ja501019n

Stabilization of oil/oil Pickering emulsions using robust and recyclable catalytic amphiphilic silica nanoparticles bearing alkyl and propylsulfonic acid groups allows fast and efficient solvent-free acetalization of immiscible long-chain fatty aldehydes with ethylene glycol.

Full text of DOI:10.1021/ja501019n

Communication
pubs.acs.org/JACS
Tunable Catalysts for Solvent-Free Biphasic Systems: Pickering
Interfacial Catalysts over Amphiphilic Silica Nanoparticles
,
†,‡
†
†
‡
‡
†
́
Wen-Juan Zhou,* Lin Fang, Zhaoyu Fan, Belen Albela, Laurent Bonneviot, Floryan De Campo,
,†
†
Marc Pera-Titus,* and Jean-Marc Clacens
†
Eco-Efficient Products and Processes Laboratory (E2P2L), UMI 3464 CNRS−Solvay, 3966 Jin Du Road, Xin Zhuang Ind. Zone,
01108 Shanghai, China
2
‡
Laboratoire de Chimie, UMR 5182 CNRS−Ecole Normale Super
́ ́
ieure de Lyon, 46 allee d’Italie, 69364 Lyon, France
*
S Supporting Information
1
0
polyoxometalate [PW O ] anion,
layered niobate
1
2
40 3
11
ABSTRACT: Stabilization of oil/oil Pickering emulsions
using robust and recyclable catalytic amphiphilic silica
nanoparticles bearing alkyl and propylsulfonic acid groups
allows fast and efficient solvent-free acetalization of
immiscible long-chain fatty aldehydes with ethylene glycol.
K Nb O intercalated with alkylammonium cations, Fe O /
FeOOH red mud derivates, and polymersomes and
4
6
17
2
3
1
2
13
nanocages hosting enzymes.
Despite the above developments, few reports are available on
biphasic oil/oil systems, though commonly found upon
14
biomass upgrading. Herein we intend to fill this gap using
the solvent-free acid-catalyzed acetalization reaction of
immiscible long-chain fatty aldehydes (n > 9) with ethlylene
glycol (EG) as an illustrative example, for which little literature
lean and efficient conversion of biobased raw materials
C
into fine chemicals and transportation fuels is a key
1
15
challenge in the design of biorefineries. Typical organic
is available. To this aim, we rationally designed amphiphilic
silica nanoparticles functionalized with propylsulfonic and alkyl
groups with variable chain lengths (Figure 1A). The sulfonic
acid groups served not only as active sites but also as
hydrophilic moieties for tuning the amphiphilic properties of
the particles.
syntheses using fatty derivatives and polyols usually encompass
2
the reaction of immiscible reagents. To circumvent this issue, a
combination of a surfactant and a homogeneous catalyst or
even a hybrid surfactant-combined-catalyst (i.e., dodecyl
3
4
benzene sulfonic acid or amphiphilic organocatalysts ) can
favor the contact and transfer between reagents though in
detriment of recycling.
The amphiphilic silica nanoparticles were prepared by a
coprecipitation method (see SI for experimental details). The
synthesis protocol included three main steps: (1) co-
condensation of tetraethyl orthosilicate (TEOS) with organo-
silanes and (3-mercaptopropyl)trimethoxysilane with a molar
ratio of 16:4:1 and a variable carbon length Cn (n = 3, 8, 18, i.e.,
trimethoxy(propyl)silane, trimethoxy(octyl)silane, trimethoxy-
Amphiphilic nanoparticles can stabilize emulsions (i.e.,
Pickering emulsions), allowing facile separation and recycling
5
after operation. Appropriate functionalization of their surface
may combine both surfactant and catalytic properties at the
interface of both phases called here Pickering Interfacial
Catalysis (PIC). In a pioneering study, Resasco et al. promoted
Pickering emulsions with amphiphilic particles combining
hydrophobic single-walled carbon nanotubes (SWCNTs) and
hydrophilic silica, also incorporating catalytic sites (i.e., Pd
(
octadecyl)silane) under alkaline conditions using ethanol and
water as cosolvents; (2) oxidation of thiol (−SH) groups into
+
4
+
sulfonate (−SO X, X = NH , Na ) using aqueous hydrogen
3
peroxide; and (3) acidification to obtain sulfonic acid-
functionalized particles. Three samples with different alkyl
chain lengths were prepared (i.e., C , C , and C ), hereinafter
6
nanoparticles and alkali moieties). The emulsions could
selectively catalyze the simultaneous aldol condensation−
hydrogenation reaction of 5-methylfurfural and acetone in
water/oil biphasic systems with controlled phase selectivity.
The authors further extended the concept to hydrogenation
and partial oxidation reactions using interfacial catalysts based
on multiwalled CNTs grown on alumina, ‘onion-like’ carbon
3
8
18
referred to as SiNP_SO3H_C3, SiNP_SO3H_C8, and
SiNP_SO3H_C18, respectively. The mean particle size was
in each case 166, 153, and 310 nm (±5 nm) (Figure S1) for
effective Cn/sulfonic molar ratios of 0.1, 5.5, and 7.9,
respectively. The density of alkyl moieties was 0.2, 11.0, and
(
(
OC) on silica, and Janus solids supporting metal nanoparticles
2
7
20.3 groups/nm for SiNP_SO3H_C3, SiNP_SO3H_C8, and
Pd and Cu) as well as to acid-catalyzed alcohol dehydration
SiNP_SO3H_C18, respectively, whereas the density of SiOH
reactions and m-cresol alkylation with 2-propanol using
8
groups was in each case 12.7−14.6, 14.3−23.2, and 33.5−51.2
hydrophobic HY zeolites.
2
groups/nm according to complementary information from
Other examples of the PIC concept comprising hybrid
carbon materials for biofuel upgrading include Au@PHCS
PHCS = porous hollow carbonaceous spheres), Ru@pristine-
TGA and quantitative 29Si solid-state NMR (Table S1). Such
high densities of alkyl and SiOH groups indicate the formation
of organic layers partially incorporating with the inorganic silica
(
CNT, Pd@CN (CN = N-doped mespoporous carbon), iron
9
oxide/CNT, CNT/TiO , and Ag/graphene oxide. Very recent
2
noncarbon amphiphilic nanoparticles with proven PIC proper-
ties include alkylammonium cations combined with a
Received: January 29, 2014
Published: March 18, 2014
©
2014 American Chemical Society
4869
dx.doi.org/10.1021/ja501019n | J. Am. Chem. Soc. 2014, 136, 4869−4872

Journal of the American Chemical Society
Communication
for water were similar for the different functionalized silicas, the
peak values were displaced to unusually higher energies,
providing a direct signature of sulfonic acid groups. The
AEDF for methyl chloride showed multiple peaks for
SiNP_SO3H_C3 and SiNP_SO3H_C8, suggesting the pres-
ence of heterogeneous Si−O−Si sites on both the silica core
and the organic layers.
The catalytic activity of the SiNPs was tested in the biphasic
acetalization reaction of fatty C -aldehyde with EG at 60 °C
12
for 1 h under low excess of EG. Benchmark catalysts were also
tested, including heterogeneous HZSM-5 and H-Resin. A
remarkably higher catalyst productivity (P) to the acetal was
achieved on SiNP_SO3H_C8 and SiNP_SO3H_C18, reaching
values about 1602 and 1850, respectively, compared to the
values of 315 and 348 found for HZSM-5 and H-Resin,
respectively (Figure S5). The latter catalysts dispersed
preferentially in the EG phase without stirring due to their
pronounced hydrophilic behavior, the contact between the two
reactants occurring far away from the acid sites. A similar
behavior was observed for SiNP_SO3H_C3. In contrast,
SiNP_SO3H_C8 localized at the C -aldehyde/EG interface,
12
whereas SiNP_SO3H_C18 localized both at the interface and
in the C -aldehyde phase (Figure S6), which is consistent with
12
a much better hydrophilic/hydrophobic balance in the latter
SiNPs.
Figure 1. (A) Biphasic acetalization reaction catalyzed by amphiphilic
silica nanoparticles under PIC. (A1) Picture of the solution using 50 v/
v % EG and 50 v/v % aldehyde and 1.7 wt % SiNP_SO3H_C18 after
homogenization at 60 °C for 15 min; (A2) typical optical micrographs
of the emulsions; (A3) scheme of amphiphilic silica nanoparticles
located at the aldehyde/EG interface; (A4) scheme of the catalytic
action of sulfonic acid sites nearby hydrophobic chain on acetalization
The activity of SiNPs was also assessed in comparison to p-
toluenesulfonic acid (PTSA), a commonly used homogeneous
acid catalyst for biphasic reactions. PTSA was indeed more
active (P = 750) than SiNP_SO3H_C3 (295), but less than
SiNP_SO3H_Cn (n = 8 or 18, P > 1500) (Figure 1B). A
comparative kinetic study was conducted for SiNP_
SO3H_C18 and PTSA under mild stirring (500 rpm) (Figure
of aldehyde by EG. (B) Productivity for acetalization of C -aldehyde
12
by EG over PTSA, SiNP_SO3H_C3, SiNP_SO3H_C8, and
SiNP_SO3H_C18. Reaction conditions: 60 °C, C -aldehyde (0.1
1
2
2
). The reaction rate was slower with SiNP_SO3H_C18 during
+
mol), EG (0.2 mol), catalyst amount adjusted to provide the same H
equivalents (9.5 μmol, particle loading ∼1.7 wt % for the solid
samples).
core during the synthesis due to the large solubility difference
between the organosilanes and TEOS. This phenomenon is not
observed in organosilane grafted Aerosil 200 materials,
15
encompassing a lower density of alkyl and SiOH groups.
The acid−base titration of Brønsted acidity provided 0.23,
+
0
.24, and 0.19 mmol[H ]/g for SiNP_SO3H_C3, SiNP_
SO3H_C8, and SiNP_SO3H_C18, respectively, matching
approximately the sulfur content measured by ICP analysis
and indicating that thiol groups were mostly oxidized into
sulfonic units (Table S1). A tiny amount of −S−SO − species
2
13
was detected in SiNP_SO3H_C18 as inferred from C−CP-
MAS NMR (Figure S2). From inverse gas chromatography
(
(
IGC), the adsorption/desorption of methylene chloride
CH Cl ) taken as weak basic probe provided specific
2
2
Figure 2. Comparative advancement of the acetalization reaction of
interaction energies (ISP) of 24.7, 16.5, and 14.6 kJ/mol for
SiNP_SO3H_C3, SiNP_SO3H_C8, and SiNP_SO3H_C18,
respectively (Table S2 and Figure S3). The trend reflects a
decreasing acid strength combining both longer alkyl chains
and higher density of SiOH groups. The nondispersive
component of the surface energy (γ , kJ/mol) measured also
C -aldehyde with EG over SiNP_SO3H_C18 and over PTSA. Left
12
insert: zoom into the 0−20 min period of time. Right insert: Typical
optical micrographs of solutions at 1 and 60 min. Reaction conditions
as in Figure 1.
d
s
from IGC showed a trend consistent with lower polarity for
longer chains: SiNP_SO3H_C3 (44.4) < SiNP_SO3H_C8
the first 5 min while at longer times became faster, reaching
almost chemical equilibrium after 1 h (Figure 2 insert left). In
contrast, the reaction reached equilibrium only after 24 h with
PTSA. A devoted optical microscopy study revealed that a
stable C -aldehyde/EG emulsion was generated under
(
29.7) < SiNP_SO3H_C18 (28.1) (Table S2). More insights
can be further obtained from the adsorption energy distribution
functions (AEDF) plotted here for water and methyl chloride
1
2
(
CH Cl ) (Figure S4). Notwithstanding that the distributions
SiNP_SO3H_C18 (and SiNP_SO3H_C8), while no droplets
2
2
4
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dx.doi.org/10.1021/ja501019n | J. Am. Chem. Soc. 2014, 136, 4869−4872

Journal of the American Chemical Society
Communication
were observed under PTSA after 10 min reaction (Figure 2
insert right and Figure S7). Likewise, SiNP_SO3H_C3 was
unable to form stable droplets due to its patent hydrophilic
behavior, showing a fast phase separation (Figures S7 and S8).
This body of results emphasizes the superior ability of
amphiphilic SiNP_SO3H_C18 as interfacial catalyst by
stabilizing Pickering emulsions.
Since the stirring rate is known to affect the formation of
emulsions, the kinetic study was repeated under fast stirring
conditions (17,000 rpm). The C -aldehyde conversion
1
2
increased in the presence of both PTSA and SiNP_SO3H_C18
Figure S9). Again, this was after 5 min that catalysis by
(
SiNP_SO3H_C18 overtook PTSA. According to optical
microscopy, emulsion droplets were not yet formed in both
catalytic systems after 1 min of reaction even under fast stirring.
The higher catalytic activity observed for PTSA at very short
times is therefore attributed to its partial solubility in both
phases. We carried out some additional catalytic tests in the
presence of water (40 mol % of C -aldehyde). The C -
1
2
12
aldehyde conversion decreased from the initial value of 91.6%
to 81.0% for SiNP_SO3H_C18 particles, whereas it evolved
from 63.0% to 53.0% for PTSA (Table S3). These results
indicate that even if water might exert a poisoning effect on the
acid sites, this should be similar for both catalysts. Furthermore,
Figure 3. Productivity (P) of acetalization of different aldehydes with
EG over SiNP_SO3H_C18 relative to the productivity for the
benzaldehyde/EG/PTSA system over PTSA (P = 2038). Bottom:
second and third abscissa axes indicating the aldehyde characteristics in
terms of distance between Hansen solubility parameters of aldehyde
and EG (Ra) and the Hansen solubility component relative to
the fact that the C -aldehyde conversion decreases in a similar
1
2
manner for both catalysts seems to suggest that such reduction
is likely attributed to a dilution effect without affecting the
emulsification properties (Figure S9). Finally, the addition of
the acetal product into the reaction system for an equivalent of
hydrogen bonding (δ ), respectively (Table S4). Inserts: pictures of
3
0% yield resulted in the rapid formation of both large and
hb
the solutions showing the fractions formed in the mixture in the
presence of SiNP_SO3H_C18 after emulsification for 20 min at RT.
Reaction conditions: 30 °C, 1 h and other reaction conditions as in
Figure 1.
small C -aldehyde droplets, being observed only after 1 min of
12
reaction at 60 °C (Figure S10). This experiment illustrates the
synergistic effect between the acetal and SiNP_SO3H_C18 in
the formation of emulsion droplets, increasing its advantage
along the reaction and contributing to the inversion of
tendency between PTSA and SiNP_SO3H_C18 over time in
Figure 2.
To illustrate when and why PIC is preferable over
homogeneous catalysis for biphasic acetalization by EG, a
comparative study was performed on the reactivity of aldehydes
presenting different affinity for EG with respect to the higher
distance between their respective Hansen solubility parameters
catalysts for the acetalization of fatty aldehydes with EG by
stabilizing Pickering emulsions. The catalysts could be
recovered by centrifugation after reaction and reused with no
appreciable deactivation. The easily tunable hydrophilic/
hydrophobic properties of silica nanoparticles open an avenue
̀
for the conception a la carte of interfacial catalysts for
(
i.e., Ra, Table S4 and Figure 3). As expected, PTSA showed
industrially relevant biphasic reactions avoiding the use of
solvents. Prospective developments in progress concern the
study of amphiphilic catalysts for acid-catalyzed etherification
and acetalization reactions of biomass-derived polyols.
the highest activity for benzaldehyde acetalization, the latter
being fully miscible with EG (Ra = 129). Comparatively, it
appeared that the C -aldehyde was not hydrophobic enough to
7
promote PIC using SiNP_SO3H_C18, since the system could
hardly form an emulsion, whereas PTSA could completely
dissolve in the reactants reaching higher reactivity (Figure S11).
In contrast, SiNP_SO3H_C18 reached higher activities than
PTSA for C - and C -aldehyde, Pickering emulsions taking
ASSOCIATED CONTENT
Supporting Information
■
*
S
Textural and surface properties of the modified silicas measured
by IGC, collection of aldehyde/EG emulsion images stabilized
by amphiphilic silicas, catalytic stability tests, effect of the acetal
product and water on the catalytic activity and emulsion
formation. This material is available free of charge via the
Internet at http://pubs.acs.org.
9
12
place in both cases (Figures 2 and S12).
Finally, the reusability of amphiphilic silica nanoparticles was
evaluated on the acetalization of C -aldehyde and benzalde-
1
2
hyde over SiNP_SO3H_C18. After each cycle, the catalyst was
separated from the reaction system by centrifugation, washed
using ethanol (30 mL, four times), and dried at room
temperature. The catalyst did not show any appreciable
deactivation even after 3 and 7 consecutive runs, respectively,
for each aldehyde (Figure S13).
In summary, we demonstrated that amphiphilic silica
nanoparticles with a suitable balance between hydrophilic and
hydrophobic properties can behave as efficient interfacial
AUTHOR INFORMATION
■
Corresponding Authors
wenjuan.zhou@solvay.com
marc.pera-titus-ext@solvay.com
Notes
The authors declare no competing financial interest.
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dx.doi.org/10.1021/ja501019n | J. Am. Chem. Soc. 2014, 136, 4869−4872

Journal of the American Chemical Society
Communication
(
15) Fan, Z.; Tay, A.; Pera-Titus, M.; Zhou, W.-J.; Benhabbari, S.;
ACKNOWLEDGMENTS
■
Feng, X.; Malcouronne, G.; Bonneviot, L.; De Campo, F.; Wang, L.;
Clacens, J.-M. J. Colloid Interface Sci. 2014, DOI: 10.1016/
j.jcis.2013.11.047.
This work was funded by CNRS and Solvay through the
BioSurf project. The authors would like to express their
gratitude to Dr. Eric Brendle for helpful discussions on IGC
and Dr. Bingwen Hu for solid-state NMR measurements.
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