Tuning the wettability of mesoporous silica for enhancing the catalysis efficiency of aqueous reactions

Article abstract of DOI:10.1039/c4cc02988g

A series of mesoporous silica-based catalysts with finely-tuned surface wettability have been synthesized, of which the catalysis efficiency towards aqueous hydrogenations is highly dependent on their surface wettability and can be five times higher than that of the commercial Pd/C catalyst. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02988g

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Tuning the wettability of mesoporous silica for
enhancing the catalysis efficiency of aqueous
reactions†
Cite this: Chem. Commun., 2014,
0, 10045
5
Received 22nd April 2014,
Accepted 9th June 2014
a
a
b
a
a
Luman Fu,‡ Shuru Li,‡ Zhongyuan Han, Huifang Liu and Hengquan yang*
DOI: 10.1039/c4cc02988g
www.rsc.org/chemcomm
A series of mesoporous silica-based catalysts with finely-tuned becomes crucial to establish efficient Pickering emulsion catalysis
surface wettability have been synthesized, of which the catalysis systems and to get insights into Pickering emulsion catalysis.
efficiency towards aqueous hydrogenations is highly dependent on
Mesoporous materials featuring high surface area and large
their surface wettability and can be five times higher than that of pore size have been demonstrated to be versatile supports for
the commercial Pd/C catalyst.
loading various metal nanoparticles, metal complexes and even
biocatalysts for a wide range of reactions. The interfacially active
4
Using water as a reaction solvent is a long-standing goal in view mesoporous material-based catalyst is scarcely reported and the
5
of the fact that all reactions in living systems occur in water, method to tune their interfacial activity is unexplored. We herein
and its unique properties such as low toxicity, non- develop a facile route to tune the surface wettability of meso-
1
inflammability, low volatility and high heat capacity. However, porous silica-based catalysts by controlling the hydrophobilization
most reactions of organic compounds in water proceed slug- extent of mesoporous silica shells. After depositing Pd nano-
gishly because their low solubility in water gives rise to the particles on these mesoporous materials, we obtained a set of
incompatibility of reaction systems and the finite reaction solid catalysts with finely tuned surface wettability for olefin
interface. Especially in the presence of solid catalysts, the hydrogenations in pure water. The Pickering emulsion systems
reaction systems often involve three or even four phases. The formulated with these catalysts show a much higher catalysis
high mass transport resistance leads to extremely low reaction efficiency than the conventional organic–aqueous biphasic system
efficiency. Although addition of co-solvents or surfactants can with the conventional mesoporous-silica-based catalyst or the
improve the efficiency, these extra additives inevitably cause commercial Pd/C catalyst. The catalysis efficiency was found to
difficulty in separating and purifying the final products.
Fortunately, it has been found that nano-to-micrometer-sized
be strongly dependent on the surface wettability of solid catalysts.
The core–shell-structured mesoporous silica was synthe-
6
particles with a suitably wettable surface (interfacial activity) sized through a delayed condensation strategy, as shown in
prefer to attach at the oil/water interface, leading to Pickering Fig. 1. This strategy enables the molecular functionality to be
2
emulsions [oil-in-water (o/w) or water-in-oil (w/o)]. Pickering positioned region-selectively on the outer shell through a step-
emulsion is emerging as an attractive platform for designing wise addition of siliceous precursors. Tetramethyl orthosilicate
efficient aqueous catalysis systems because they can create a (TMOS) was used as the silicon precursor to construct a core in
large oil/water interface without need for extra additives such as the presence of cetyltrimethylammonium chloride (CTAC) as
3
surfactants. One of the most key parameters for Pickering
emulsions is the surface hydrophobicity/hydrophility balance
2
(
wettability) of solid catalysts. Accordingly, development of effi-
cient methods to control the surface wettability of solid catalysts
a
School of Chemistry and Chemical Engineering, Shanxi University,
Wucheng Road 92, Taiyuan 030006, China. E-mail: hqyang@sxu.edu.cn
b
Department of Physical Education, Shanxi University, Wucheng Road 92,
Taiyuan 030006, China
†
Electronic supplementary information (ESI) available: Experimental section; N
2
sorption analysis; XRD patterns; solid state NMR spectra; reaction data. See DOI:
1
0.1039/c4cc02988g
‡
These authors contributed equally to this work.
Fig. 1 The preparation process of Pd/MSS-Cx and its structure.
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3 2 7 3
the template. (MeO) Si(CH ) CH (C8) together with TMOS was
used to grow a hydrophobic shell. After two steps of hydrolysis
and condensation, the desired mesoporous materials were
obtained. Pd nanoparticles were introduced onto these materials
through adsorption with Pd(OAc) followed by reduction with
2
NaBH
set of solid catalysts Pd/MSS-Cx (x represents the molar fraction
of (MeO) Si(CH CH in the total silicon precursors of the
4 4 3
. After extraction with a hot alcohol solution of NH NO , a
3
)
2 7
3
second step, varying from 0 to 5%, 10%, 15%, 20% and 25%).
The prepared catalysts are composed of uniform nano-
spheres. As can be seen from Fig. 2a, Pd/MSS-C20 shows
uniform particles with particle sizes of 200–300 nm. The
radically arrayed pores throughout the materials (ca. 2 nm)
are clearly observed (Fig. 2b). A portion of fine Pd nanoparticles
were located inside the nanopores. Pd/MSS-C20 shows a N
sorption isotherm close to the type I (Fig. 2c) and possesses a Fig. 3 Catalysis efficiency of the Pickering emulsions formulated with Pd/
2
2
À1
MSS-Cx (x = 0, 5, 10, 15, 20 and 25). Reaction conditions: 10.5 mmol of
styrene, 3.1 mL of water, 0.35 MPa of H pressure, 40 1C, solid catalyst
containing 0.00292 mmol Pd. The bar is 200 mm.
moderate specific surface area (441 m g , Table S1, ESI;† the
sorption results of other samples are provided in Fig. S1 and
2
N
2
Table S1 of ESI†). The XRD patterns of the synthesized materials
exhibit a diffraction peak centred at 2.51 (Fig. S2, ESI†). These
results confirm that Pd/MSS-Cx samples possess a structure of the order of Pd/MSS-C0, Pd/MSS-C5, Pd/MSS-C10, Pd/MSS-C15,
typical mesoporous materials. The presence of Q and T bands in Pd/MSS-C20 and Pd/MSS-C25, as anticipated. It was interestingly
2
9
the solid state Si MAS NMR spectrum of Pd/MSS-C20 indicates found that water contact angles of these solid catalysts gradually
that the octyl group was linked with the solid materials through increased from 361 to 421, 831, 881, 1021 and 1061 as the octyl
1
3
co-condensation (Fig. 2c), which is further supported by the
C
group loading increased (Fig. 2e). These results confirm that our
CP MAS NMR (Fig. S3, ESI†) and FT-IR spectra (Fig. S4, ESI†). proposed method allows the catalyst wettability to be finely
The quantitative results of C contents on this set of solid tunable in a wide range.
catalysts confirm that the octyl loading gradually increases in
Hydrogenation is here chosen as a model reaction to exam-
ine the catalytic performance of these catalysts in water because
it is an important tool for synthesis of various fine chemicals.
The hydrogenations were carried out using pure water as the
reaction medium. After mixing water, olefins and solid cata-
lysts, the reaction system was vigorously stirred (ca. 2000 rpm
for 3 min) for emulsification. For Pd/MSS-C0, the reaction
system was a conventional two-phase since emulsion droplets
were not found by optical microscopy, as shown in Fig. 3. Pd/
MSS-C0 particles are mainly distributed in the water layer. In
contrast, for Pd/MSS-C5, emulsion droplets appeared and their
sizes were in the range of 400–600 mm. Droplet size gradually
decreases and then begins to increase. The minimum droplet
size is ca. 100–250 mm in the case of Pd/MSS-C20.
All hydrogenations were carried out under 0.35 MPa at a
stirring input power of 12.5 W. Based on the H uptake rates
2
(Fig. S5, ESI†), one can find that these reactions proceed at
remarkably different reaction rates. The plot in Fig. 3 quantita-
tively reflects the change trend of catalysis efficiency T that is
calculated before the conversion is less than 30% (catalysis
efficiency T means the moles of converted substrate per mole of
À1 À1
metal per hour, mol mol
h
). For the conventional biphasic
À1 À1
system with Pd/MSS-C0, T is only 727 mol mol
h . As the
octyl loading increases, T gradually increases and reaches a
À1 À1
maximum value (2574 mol mol
h ) when the octyl loading is
Fig. 2 Characterization of the prepared catalysts: (a) and (b) TEM images of
2
0%. Further increasing the octyl loading leads to a decrease in
Pd/MSS-C20; (c) N
2
adsorption–desorption isotherms of Pd/MSS-C20;
29
catalysis efficiency. More impressively, under the same condi-
tions, the catalysis efficiency of Pd/MSS-C20 is about 5 times
higher than that of the commercial Pd/C catalyst (Fig. S6, ESI†).
(d) solid state Si MAS NMR spectrum of Pd/MSS-C20; (e) water contact
angles of Pd/MSS-C0, Pd/MSS-C5, Pd/MSS-C10, Pd/MSS-C15, Pd/MSS-C20
and Pd/MSS-C25.
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droplet size leads to a higher reaction interface area. As a result,
the catalyst mass-normalized efficiency shows no significant
decrease, which is not achievable for the conventional biphasic
systems.
We next examined the catalytic performance of Pd/MSS-C20
with other substrates. For all investigated olefins such as styrene,
substituted styrene, butyl acrylate, butyl 2-mthylacrylate and
hexyl 2-mthyl acrylate, Pickering emulsions were obtained using
these reactants as the oil phase, indicating that Pd/MSS-C20 has
good interfacial activity towards a wide range of reactants. Under
the same conditions, the conversions of all the olefins over
Pd/MSS-C20 are much higher than those over Pd/MSS-C0, as
Fig. 4 The results of hydrogenation of different substrates in the Picker- reflected in Fig. 4.
ing emulsion system formulated with Pd/MSS-C20 and the conventional
After completion of the reaction, the Pd/MSS-C20 catalyst was
biphasic system in the presence of Pd/MSS-C0. Reaction conditions:
0.5 mmol of olefins, 3.1 mL of water, 0.35 MPa of H pressure, 40 1C,
recovered by centrifugation, washed three times with ethanol,
dried under vacuum, and then directly reused for the next
reaction cycle. From the second to the fifth reaction cycle, emul-
sion droplets were still observed without any changes in the
1
2
solid catalyst containing 0.00292 mmol Pd. The reaction times correspond
to 35, 90, 35, 90, 120 min from left to right.
droplet size. The H consumption rate had no any decrease and
2
Obviously, there exists a relationship between the surface the full conversion of styrene was still achieved (Fig. S7, ESI†).
wettability of catalysts, the Pickering emulsion properties and In summary, a series of interfacially active Pd-supported
the catalysis efficiency. According to the water contact angle catalysts are synthesized by introducing octyl groups onto the
measurement results, Pd/MSS-C0 is too hydrophilic to stabilize shell of mesoporous silica through the delayed condensation
Pickering emulsions. Due to the introduction of the octyl group, strategy. Their wettability can be finely tuned by varying the
the hydrophobicity of Pd/MSS-C5 is improved, being able to amount of octyl groups. With such interfacially active nano-
stabilize the emulsion although the droplet size is relatively particle catalysts, Pickering emulsions can be formulated using
large. As the hydrophobicity further increases, the emulsion various olefins as oil phase. These Pickering emulsion systems
droplet size gradually decreases and then begins to increase show significantly enhanced catalysis efficiency in olefin hydro-
(Fig. 3). Such results are in good agreement with previous genation in comparison to the conventional organic–aqueous
observations that only the particles with moderate wettability biphasic system with the unmodified mesoporous silica-based
7
can stabilize high-quality Pickering emulsions. The change catalyst and the commercial Pd/C catalyst. Their catalysis
tendency of the catalysis efficiency vs. droplet size is explained efficiency is directly relevant to the surface wettability, which
by the fact that the smaller emulsion droplet leads to a larger provides a new guide to tune the catalysis efficiency of the
reaction interface area and the enlarged reaction interface area reaction using pure water as the medium.
benefits boosting the catalysis efficiency (based on the H uptake
We acknowledge the Natural Science Foundation of China
2
rates in Fig. S7 of ESI,† it can be estimated that in the pure (221173137), program for New Century Excellent Talents in
organic medium the catalysis efficiency of Pd/MSS-C20 is almost University (NECT-12-1030), Program for the Top Young Academic
equal to that of Pd/MSS-C0, which excludes the possibility that Leaders of Higher Learning Institutions of Shanxi (2011002) and
the difference in catalysis efficiency is due to the difference in Middle-aged Innovative Talents of Higher Learning Institutions of
the intrinsic activity of the catalysts). These results also demon- Shanxi (20120202).
strate that catalytic reaction rate can be rationally controlled
through adjusting the surface wettability of the solid catalysts.
Notes and references
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