Coordination-Enhanced Synthesis for Hollow Mesoporous Silica Nanoreactors

Article abstract of DOI:10.1021/acs.chemmater.9b05202

Recent progress has put the spotlight on functional nanoparticles encapsulated inside hollow silica nanospheres as so-called catalytic nanoreactors for various reactions. However, the synthetic methods used so far vary from one nanoparticle system to another, not providing access to the synthesis of a large variety of such materials. Here, we report an alternative, namely, a coordination-enhanced synthesis leading to a single system, which can directly produce a vast number of different hollow mesoporous silica nanoreactors with metal or metal-oxide nanoparticles inside their cavities (M?HMSNs or M_xO_y?HMSNs, where M stands for the chosen metal). We have successfully used the method with more than 21 different metals (Ru, Pd, Pt, Au, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, In, Sn, Ba, La, Ce, and Eu). Specifically, a triple ligand is used to coordinate the various metal ions to form a negatively charged complex three-dimensional (3-D) network, which further combines by electrostatic attraction with a positively charged/neutral diblock copolymer, to obtain metal-ion-bound micelles. After silica deposition, calcination, and (if required) reduction, the corresponding M?HMSNs, M_xO_y?HMSNs or even their bianalogues are obtained by simply varying the metal ions in the recipe. As an illustration, we show that Pd?HMSNs display greatly enhanced catalytic hydrodechlorination activity and excellent stability due to their unique structures. Hence, the design concept is very flexible and can be extended to even include multicomponent catalytic nanoparticles, which likely extends the range of applications almost beyond imagination.

Full text of DOI:10.1021/acs.chemmater.9b05202

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Article
Coordination Enhanced Synthesis for
Hollow Mesoporous Silica Nanoreactors
Kaijie Li, Caoping Yang, Hongbo Yu, Tao Xiao, Wenjun Guan, Peng Ding,
Hongfeng Yin, Martien Abraham Cohen Stuart, Junyou Wang, and Shenghu Zhou
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b05202 • Publication Date (Web): 05 Feb 2020
Downloaded from pubs.acs.org on February 6, 2020
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Coordination Enhanced Synthesis for Hollow Mesoporous Silica Nanoreactors
Kaijie Li,† Caoping Yang,† Hongbo Yu,‡ Tao Xiao,† Wenjun Guan,† Peng Ding,†
Hongfeng Yin,‡ Martien Abraham Cohen Stuart,† Junyou Wang,*, † and Shenghu
Zhou*, †
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†Shanghai Key Laboratory of Multiphase Materials Chemical Engineering,
School of Chemical Engineering, East China University of Science and Technology,
130 Meilong Road, Shanghai 200237, P. R. China.
‡Ningbo Institute of Materials Technology and Engineering, Chinese Academy of
Sciences, 1219 Zhongguan West Road, Ningbo, Zhejiang 315201, P. R. China
*Corresponding authors:
Fax: (+86) 21-64253159
E-mail: zhoushenghu@ecust.edu.cn; junyouwang@ecust.edu.cn
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Abstract
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Recent progress has put the spotlight on functional nanoparticles encapsulated
inside hollow silica nanospheres, as so-called catalytic nanoreactors for various
reactions. However, the synthetic methods used so far vary from one nanoparticle
system to another, not providing access to synthesis of a large variety of such
materials. Here we report an alternative, namely a coordination enhanced synthesis
leading to a single system which can directly produce a vast number of different
hollow mesoporous silica nanoreactors with metal or metal oxide nanoparticles inside
their cavities (M@HMSNs or MxOy@HMSNs, where M stands for the chosen metal).
We have successfully used the method with more than 21 different metals (Ru, Pd, Pt,
Au, Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, In, Sn, Ba, La, Ce and Eu).
Specifically, a triple ligand is used to coordinate the various metal ions to form a
negatively charged complex 3-D network, which further combines by electrostatic
attraction with a positively charged/neutral diblock copolymer, to obtain metal ions
bound micelles. After silica deposition, calcination and (if required) reduction, the
corresponding M@HMSNs, MxOy@HMSNs or even their bi- analogues are obtained
by simply varying the metal ions in the recipe. As an illustration, we show that
Pd@HMSNs display greatly enhanced catalytic hydrodechlorination activity, and
excellent stability due to their unique structures. Hence the design concept is very
flexible and can be extended to even include multi-component catalytic nanoparticles,
which likely extends the range of applications almost beyond imagination.
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1. Introduction
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Integrating metal or metal oxide nanoparticles (NPs) into porous nanospheres1-8
are highly desired since they can function as catalytic nanoreactors for various
heterogeneous reactions.1,2,8,9-13 Typically, small NPs can enhance catalytic
efficiency,14-18 but maintaining their high catalytic activity is challenging due to the
particle size increasing during reactions. By designing hollow mesoporous
nanoreactors (HMNs) with small functional NPs inside their cavities,3,19-22 one can
expect to achieve stable and active catalysts, where mesoporous shells protect active
NPs with acceptable mass transportation and hollow cavities function as reactors.
The current synthetic methods for HMNs with NPs inside their cavities use the
target NPs as templates to deposit the desired nanosphere materials in microemulsion
systems,1,23-28 post-synthesis to load NPs inside pre-synthesized hollow
nanospheres,9,20,29-30 or hard-templating methods31 including nanocasting,32 sacrificial
templating such as Galvanic replacement33 and Kirkendall effect,34 shell formation by
adsorption,35,36 layer-by-layer37 and chemical deposition.38 Recently, a method, using
metal ions containing charge-driven micelles as templates onto which silica is
deposited, is developed by our previous work to synthesize a series of metal oxide
NPs encaged hollow mesoporous silica nanoreactors (MxOy@HMSNs).39,40 Double
ligands (L2EO4), two dipicolinic acid ligands connected by a tetraethyleneoxide
spacer,41-45 were used to coordinate metal ions and further combine with quaterized
copolymer to form micelles by electrostatic attraction. However, the developed
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method is only applicable to transition or rare earth metal systems, it has difficulties
to incorporate noble, main group and bimetallic NPs inside nanoreactors.
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Despite the synthetic progress of functional NPs encaged HMNs, the strategies
used so far either vary with the target metals or have difficulty to completely load the
NPs inside nanoreactors, not providing a generic synthesis for a large variety of metal
systems by a single approach. It is challenging to design a general method that is
applicable to all types of metal systems including transition, noble, main group and
rare earth metals, since the properties of these types of metals greatly change from
reducibility to coordination. Moreover, uniformly mixing two different metal atoms
inside HMNs is also a big challenge, where precisely and controllably adjusting the
atomic ratios of bimetallic catalysts is underpinning of modern catalysis.
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Here, we demonstrate how to use coordination networks to synthesize HMSNs
for all types of metal systems and homogeneously mix two different metal atoms in
such materials. The key design concept is that incorporating metal ions into
coordination complex networks can enhance the metal coordination, and such metal
ions bound networks further combine with quaterized neutral/charged diblock
copolymer to form charge-driven micelles, onto which silica is deposited followed by
calcination to give various NPs encaged HMSNs. The developed coordination
enhanced synthesis in this study is applicable but not limited to noble (Ru, Pd, Pt or
Au), transition (Sc, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr or Mo), main group (In, Sn or
Ba), rare earth (La, Ce or Eu) and bimetallic systems (Pt-Cu, Pt-Pd, Pt-Ce, Pd-Cu or
Ce-Cu), demonstrating a robust and generic strategy for catalytic nanoreactors.
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2. Results and Discussion
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2.1. Formation Mechanism of coordination complex networks
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Figure 1 demonstrates formation mechanism of metal ions bound coordination
networks and the general synthetic procedures for such materials. Triple ligands (L3),
a compound with three dipicolinic acid groups (denoted as L) grafted on a benzene
ring,44 coordinate metal ions to form negatively charged metal ions bound
coordination networks, which further combines by electrostatic attraction with
positively charged quaterized P2MVP128-b-PEO477 copolymer to obtain core-shell like
micelles. Subsequent silica deposition, calcination and (if required) reduction, will
give various functional NPs encaged HMSNs.
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Figure 1. The general procedures for NPs encaged HMSNs: a), Uv-visible spectra of
the micelles at different Fe3+/L ratios and a fixed L/polymer ratio of 115, inserts
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showing coordination networks, relationship between Fe3+/L ratios and absorbance at
367 nm; b), TEM images showing uncalcined FexOy@HMSNs, scale bar-20 nm. c),
CONTIN analyses of particle size and size distribution of the micelles at a Fe/L ratio
of 0.5 obtained by light scattering experiments. The pH value-3.8; L3-0.09 mM
(L-0.27 mM); polymer-0.00235 mM.
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As shown in Figure 1a, once Fe3+ ions are mixed with L3 and polymer, a distinct
peak at 367 nm appears, which has been ascribed by literature to Fe3+ ion fully
coordinated with two dipicolinic acid groups (FeL2 coordination).46 With the Fe/L
ratio increasing from 0.083 to 0.33, the absorbance at 367 nm linearly increases with
the Fe/L ratios increasing, as confirmed by the linear regression coefficient of 0.9972
in the insert. At higher Fe/L ratios of 0.42 and 0.5, the linear relationship no longer
keeps, showing the influence of micelles on the coordination at higher Fe/L ratios.
The equilibrium constant of logβ(FeL2) is 17.13 while logβ(FeL) is 10.91,47,48 confirming
the predominant formation of FeL2 coordination at such a low Fe/L ratio of 0.083.
Moreover, as indicated in Figure S1, the absorbances at 367 nm are linearly
proportional to the concentrations of FeL2 coordination in the micelles, confirming
that the absorbance at 367 nm can be used for quantitation of FeL2 complex.
Furthermore, Figure S2a shows basically similar absorbances at 367 nm during this
pH range from 2.5 to 4.5, suggesting the negligible formation of iron hydroxide
complexes in the pH range. Therefore, the absorbance at 367 nm can be used to
calculate the concentration of FeL2 coordination. At a Fe/L ratio of 0.5, for each L3
ligand, roughly 2.5 L groups (calculated from the absorbance at 367 nm) are fully
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coordinated with Fe3+ ions to form FeL2, indicating the formation of coordination
networks in micelles.
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Figure 1c shows the analyses of particle size and size distribution for the
micelles at a Fe/L ratio of 0.5 obtained from light scattering experiments. As indicated
in Figure 1c, ~20 nm uniform micelles are resulted. Moreover, further increasing Fe/L
ratios above 0.5 will result in the appearance of the color of individual Fe3+ ions
(Figure S2b), suggesting that the maximum coordination of Fe3+ is achieved at a Fe/L
ratio of 0.5 with negligible concentration of individual Fe3+ ions. Therefore, the
micelles at a Fe/L ratio of 0.5 are selected for preparing FexOy@HMSNs. Figure 1b
shows the TEM images of the precursors of the representative FexOy@HMSNs before
calcination, in which connected solid nanospheres are clearly observed. The size of
uncalcined FexOy@HMSNs precursors is ~ 30 nm, which is consistent with using the
~ 20 nm micelles as templates to deposit silica. After calcination of the precursors of
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different metal systems at 550 C, hollow nanostructures and the residence of small
NPs inside hollow cavities are simultaneously resulted (see the following Figure 2),
due to removal of L3 ligands and polymers by calcination.
From the combination of spectroscopic and light scattering studies, and the
experiment findings that other double ligands (L2EO4) are only applicable to
transition and rare earth metal systems (TEM images of unsuccessful syntheses of
noble metal systems by L2EO4, Figure S3), we conclude that it is the formation of
coordination complex networks that makes the L3 ligands applicable to all types of
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metal systems, and coordination networks also enhance the co-coordination of
different metal ions, resulting in uniformly mixing two metal atoms in nanoreactors.
2.2. Syntheses and Characterizations of Variou NPs Encaged HMSNs
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Figure 2 presents TEM images of HMSNs with various metal or metal oxide NPs,
and their corresponding size distributions are illustrated in Figure S4. As shown in
Figure 2 and Figure S4, 19.6-32.7 nm nanospheres with 10.7-20.5 nm hollow cavities
are clearly observed. PdO@HMSNs (Fig. 2a) show extremely small NPs (1-3 nm)
inside hollow cavities while Pd@HMSNs (Fig. 2b), Au@HMSNs (Fig. 2c) and
PdO-Pt@HMSNs (Fig. 2e) exhibit relatively large inner NPs, with a particle size of
4.2, 6.2 and 3.2 nm (Fig. S4j-S4l), respectively. For other metal oxide NPs systems,
small NPs (1-3 nm) are also observed for CeO2@HMSNs (Fig. 2d), ZrxOy@HMSNs
(Fig. 2h) and SnxOy@HMSNs (Fig. 2i), as well as bimetallic PdO-CuxOy@HMSNs
(Fig. 2f) and CeO2-CuxOy@HMSNs (Fig. 2g). Generally, metal oxides highly
disperse in hollow cavities because of their relatively strong interaction with SiO2,
while metals intend to form relatively larger NPs, possibly due to large surface free
energies with small NPs. Here, it is mentioned that it is difficult to clearly
differentiate extremely small nanoparticles and SiO2 due to their high dispersions
inside nanoreactors, and thereby we cannot exclude the possibility of residence of
some particles in mesoporous channels and even at the outer surfaces.
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Figure 2. TEM images showing: a), PdO@HMSNs; b), Pd@HMSNs prepared from
reduction of PdO@HMSNs with H2 at 250 oC; c), Au@HMSNs; d), CeO2@HMSNs;
e), PdO-Pt@HMSNs; f), PdO-CuxOy@HMSNs; g), CeO2-CuxOy@HMSNs; h),
ZrxOy@HMSNs; i), SnxOy@HMSNs. Scale bars of a)-f) are 50 nm.
Figure S5a present the FT-IR spectra of PdO@HMSNs without calcination and
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after calcination at 550 C. The peak at 2922 cm-1 is well known to be the stretching
vibrations of methylene group of polymer. After calcination at 550 oC, the above peak
disappears, confirming the complete removal of organic materials by calcination at
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this temperature. Therefore, calcination at 550 C is used to obtain hollow
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mesoporous nanostructures by removal of organic materials. Figure S5b shows TEM
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images of Pd@HMSNs prepared by calcination at 750 oC and subsequent 250 C H2
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reduction. At such a high calcination temperature, Pd@HMSNs show intact hollow
nanostructures with a basically same Pd particles size (~ 4.4 nm). We ascribe their
high thermal stability to the residence of Pd NPs inside hollow cavities, where silica
walls provide the protection for inner NPs and prevent Pd NPs from agglomeration.
XRD diffractions of various NPs integrated HMSNs are shown in Figure 3. The
broad diffractions (~22.0 2θ degree) are ascribed to SiO2 for all materials.
Au@HMSNs, Pd@HMSNs and PdO@HSMNs in Figure 3a respectively show
distinct Au, Pd and PdO diffractions. Apparently, Au diffractions are narrower than
those of PdO and Pd, which is consistent with their relatively larger particle sizes
(~6.2 nm, Figure 2c). CeO2@HMSNs and PdO-Pt@HMSNs in Figure 3b show
diffractions of CeO2 and PdO with Pt, respectively. Formation of metallic Au and Pt
NPs in HMSNs is expected since metallic Au and Pt are more stable than their
oxidation states by calcination. As for PdO-CuxOy@HMSMs, only PdO diffractions
are observed, suggesting formation of amorphous CuxOy (the presence of Cu elements
will be confirmed by EDS line-scans in HAADF-STEM). XRD and EDS studies of
CexOy-CuxOy@HMSNs, ZrxOy@HMSNs and SnxOy@HMSNs are shown in Figure
S6. Although these materials do not show distinct XRD diffractions of metal oxides
due to their high dispersions, EDS measurements confirm the presence of expected
elements, suggesting the incorporation of these metal oxides into HMSNs.
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b)
a)
Au@HMSNs
CeO2@HMSNs
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PdO-Pt@HMSNs
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PdO@HMSNs
PdO-CuxOy@HMSNs
Au (JCPDS 04-0784)
Pd (JCPDS 46-1043)
CeO2 (JCPDS 34-0394)
Pt (JCPDS 04-0802)
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PdO (JCPDS 43-1042)
PdO (JCPDS 43-1042)
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Figure 3. a), XRD patterns of Au@HMSNs, Pd@HMSNs and PdO@HMSNs; b),
XRD patterns of CeO2@HMSNs, PdO-Pt@HMSNs and PdO-CuxOy@HMSNs.
EDS line-scans in HAADF-STEM for PdO-CuxOy@HMSNs and
Pd-Cu@HMSNs are shown in Figure 4. Compared with their corresponding bright
field images (Figure 2f), HAADF-STEM images of PdO-CuxOy@HMSNs (Figure 4a)
more clearly demonstrate small NPs (bright dots) in hollow cavities. Figure 4b and
Figure 4c respectively show the selected PdO-CuxOy@HMSNs nanoreactor for EDS
line-scans and their corresponding Si/Pd/Cu EDS counts across the selected paticle.
The Si linescan shows highest Si concentrations at peripheries of hollow nanospheres,
confirming the hollow silica nanostructures. Moreover, PdO and CuxOy are uniformly
dispersed inside PdO-CuxOy NPs, confirmed by the nearly coincidence of Pd/Cu
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lins-scans in Figure 4c. By H2 reduction of PdO-CuxOy@HMSNs at 300 C,
Pd-Cu@HMSNs (Figure 4d, 4e and 4f) show similar uniform Pd and Cu distributions,
suggesting alloy formation inside HMSNs. More HAADF-STEM images of
PdO@HMSNs and CeO2-CuxOy@HMSNs are shown in Figure S7.
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Figure 4. a)-c) showing HAADF-STEM images, the selected particle for line-scan,
and Si/Pd/Cu EDS counts across the particle for PdO-CuxOy@HMSNs, respectively.
d)-f) showing corresponding EDS line-scan of Pd-Cu@HMSNs. Scale bars of a) and
d) are 50 nm, and those of b) and e) are 20 nm.
TEM images and XRD patterns of RuO2@HMSNs, Pt@HMSNs and
Pt-CeO2@HMSNs are shown in Figure S8-S10, where TEM images show hollow
nanostructures with small NPs inside, and XRD patterns confirm the formation of
RuO2, Pt and Pt-CeO2. Figure S11 shows TEM images, XRD and EDS studies of
Pt-CuxOy@HMSNs. While XRD patterns of Pt-CuxOy@HMSNs (Fig. S11b) only
show Pt diffractions, the EDS measurement (Fig. S11c) confirms the presence of Cu
element in materials. More TEM images and EDS measurements of HMSNs with Sc,
V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, In, Ba, La and Eu are presented in Figure
S12-S15.
N2 adsorption-desorption isotherms and pore size distributions of PdO@HMSNs,
Pd@HMSNs, PdO-CuxOy@HMSNs and Pd-Cu@HMSNs are presented in Figure 5
while those of Pt@HMSNs are shown in Figure S16. As shown in Figure 5a and S16a,
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all tested HMSNs show type IV isotherms with hysteresis loops, indicating
mesoporous structures.28,29,49 As indicated in Figure 2, these calcined materials consist
of connected hollow nanospheres, resulting in aggregation voids between nanospheres
with adsorption uptakes at P/P0 ~0.90. All the desorption branches show inflections
around P/P0 ~0.46, which is originated from the delay of N2 evaporation from hollow
voids blocked by surrounding mesoporous walls.28,49-51 The forced closure at P/P0 of
0.4 ~ 0.5 has been reported as the characteristic of cavitation phenomena of N2
confined to the hollow cavity.50,51 Figure 5b illustrates their pore size distributions by
NLDFT method. Mesoporous pores around 5~13 nm count for a large percentage of
pore volumes. The pore size distributions by BJH analysis from the adsorption
branches are shown in Figure S17, where Pd@HMSNs, PdO@HMSNs,
Pd-Cu@HMSNs and PdO-CuxOy@HMSNs show the presence of distinct mesopores
from 8 nm to 10 nm. There is some difference of pore size distribution between
NLDFT and BJH methods, and NLDFT method is possibly more suitable for hollow
structures.49,52 Those pores above 20 nm are ascribed to the aggregation voids between
particles. It has been reported that the cage sizes of hollow cavities cannot be directly
obtained from pore size distributions since the N2 evaporation from hollow voids are
blocked by the surrounding mesoporous pores in the walls,28,49 and can only be
obtained from TEM observation.
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Pore Width (nm)
Relative Pressure (P/P0)
Figure 5. a), N2 adsorption-desorption isotherms of Pd@HMSNs, PdO@HMSNs,
Pd-Cu@HMSNs and PdO-CuxOy@HMSNs; b), their corresponding pore size
distributions by NLDFT method.
Table 1 presents textural properties and metal loadings of PdO@HMSNs,
Pd@HMSNs, PdO-CuxOy@HMSNs and Pd-Cu@HMSNs. BET specific surface areas
and pore volumes of these materials are around 277-498 m2/g and 0.73-1.14 cm3/g,
respectively, while average pore diameters are around 7.7-22.6 nm. Due to H2
o
treatment at 250 C, the specific surface area of Pd@HMSNs decreases from 498
m2/g of PdO@HMSNs to 400 m2/g, and a similar trend is also observed for bimetallic
Pd-Cu@HMSNs. Pd loading of PdO@HMSNs is 3.0 wt % while Pd and Cu loadings
of PdO-CuxOy@HMSNs are 1.7 wt % and 1.3 wt %, respectively. A Pd/Cu ratio of
44/56 for PdO-CuxOy@HMSNs is obtained, which is close to the starting 50/50 ratio,
further proving the co-coordination of two different metal ions by L3 ligands.
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Table 1. Textural properties and metal loadings of PdO@HMSNs, Pd@HMSNs,
PdO-CuxOy@HMSNs and Pd-Cu@HMSNs.
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SBET
(m2/g)
Pore volumea Average Pore Metal loadingsc
Samples
M/Sid
(cm3/g)
sizeb (nm)
(wt %)
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Pd: 1.7; Cu: 1.3 1.0/45.0
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aPore volume is single point adsorption total pore volume of pores; average pore size is the
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BJH adsorption average pore diameter; metal loadings are obtained by ICP-OES, and M/Si
ratios are the ratios of total metal/Si calculated from the metal loadings.
The M/Si ratios of PdO@HMSNs and PdO-CuxOy@HMSNs are 1.0/58.0 and
1.0/45.0, respectively, while those M/Si ratios of starting materials are 1.0/180.0,
suggesting partial deposition of silica. Due to their high densities, the silica
nanospheres with metal ions bound micelles inside will precipitate and are collected
by centrifugation for further calcination. TEOS hydrolysis and polymerization
(formation of silica networks) at a pH value of ~4.15 and room temperature are slow,
and the process requires ~3 days to obtain precipitates for further treatment. During
the process, some part of TEOS resources do not fully polymerize, and thus leave in
the solution and are seperated from the solid products by centrifugation.
The synthetic procedures of HMSNs with various NPs inside include the
formation of metal ions bound micelles, silica deposition and polymerization,
calcination and (if required) reduction, where the formation of micelles is the key
process. To successfully obtain charge-driven self-assembly micelles, the ratios of
polymer/L3/metal as well as the pH values of the solution are required to be controlled.
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It is well known that most of metal ion will be coordinated with 4 or 6 ligands,
especially for those transition metal ions. For example, both coordination numbers of
Pd2+ and Cu2+ are 4 while each L3 molecule has 6 carboxylate coordination groups.
Accordingly, the ratio of (Pd2++Cu2+)/L3 is set to 1.5/1.0, resulting in 2 net negative
charges for each coordination center. As an example, if 1.5 moles of (Pd2++Cu2+) ions
and 1.0 mole of L3 are mixed in solution, the total net charges of the coordination 3-D
complex will be roughly equal to 3.0 moles of negative charges, where the accurate
net charges depend on the length of 3-D coordination complex. By quaterization of
P2MVP128-b-PEO477 with methyl iodide (level of quaterization, ~90 %), each
positively charged P2MVP128-b-PEO477 molecule will roughly have ~115 positive
charges. Therefore, to obtain charge-driven self-assembly micelles, the ratio of
(quaterized P2MVP128-b-PEO477)/L3/(Pd+Cu) should be set to 1/38.3/57.5 for charge
balance. The pH value of the solution containing polymer, L3 and metal ions is
another important synthetic parameter. Generally, higher pH value will deprotonate
carboxyl groups, which facilitates the coordination between metal ions and L3.
However, higher pH value will result in the formation of metal hydroxide precipitates,
which will decrease the coordination. The pratical pH value depends on the solubility
product constant (Ksp), and transition metal such as Mn can be synthesized even at a
pH value of 8.0 while noble metal systems such as Au, Pt and Pd can only be
synthesized in the pH range from 3.8 to 4.2. In typical synthesis, the pH range from
3.8~4.5 are suitable for most metal systems, and also this pH range can give an
acceptable TEOS hydrolysis rate to deposit silica onto micelles. Here, it is mentioned
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that the practical ratios of polymer/L3/metal as well as the pH values of the solution
are required to adjust for giving HMSNs with perfect hollow mesoporous
nanostructures.
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2.3. Catalytic Hydrodechlorinations over Pd@HMSNs
Catalytic hydrodechlorination53-58 is selected to demonstrate their unique
catalytic property of such a type of HMSNs. Chlorophenols are widely used in
manufacture of pesticides and dyes, and are a main group of toxic pollutants in
wastewater due to their high toxicity and resistance to degradation.53,55,56 Compared to
other strategies,59-63 the catalytic hydrodechlorination is considered as a promising
technique to remove chlorophenols from wasterwater due to its mild reaction
conditions and high efficiency. Among various catalytic systems,57,58,64-67 Pd is
recognized as the most efficient system for hydrodechlorination. Typically, addition
of base will promote the reaction by removal of HCl product.53,55 Recently, highly
efficient Pd catalysts, microreactors of Pd NPs immobilized shell-corona hollow
polymer microsheres,55 are reported for hydrodechlorination of chlorophenols. With
the addition of NaOH, the mentioned catalysts can catalyze hydrodechlorination at
room temperature and atmospheric H2 pressure. We use the similar reaction
conditions to evaluate the prepared Pd@HMSNs, where the reaction pressure and
temperature as well as the ratios of chlorophenol/Pd and chlorophenol/NaOH are
same as those of the mentioned work.
The effect of the speeds of agitation on the phenol yields for hydrodechlorination
of 4-chlorophenol over Pd@HMSNs is shown in Figure S18, and a vigorous stirring
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at 500 rpm is applied to eliminate the mass transportation effect. Because of the high
selectivity for all chlorophenol hydrodechlorinations (above 99 %), Figure S18 only
presents phenol yields. Catalytic performances of Pd@HMSNs for
hydrodechlorination of 4-chlorophenol under different bases are shown in Figure S19.
As shown in Figure S19a, the highest reaction rate is achieved with the addition of
NaOH, and the best catalytic performance is achieved at a 1/1 ratio of
4-chlorophenol/NaOH (Figure S19b). It is well known that silica will react with
strong bases at high pH values. In this study, the protecting mesoporous silica shells
of Pd@HMSNs will be damaged by the excessive NaOH, resulting in losing their
unique structures. Therefore, it is resonable that the best performance is reached at a
4-chlorophenol/NaOH ratio of 1/1. Control supported Pd/SiO2 catalysts are also
synthesized for comparison. Pd/SiO2 catalysts are prepared by impregnation of Pd
NPs68 followed by washing, calcination and reduction. TEM images of Pd/SiO2 are
shown in Figure S20. Pd particle size of Pd/SiO2 is ~ 4.6 nm, which is close to that of
Pd@HMSNs (~4.2 nm). Pd loading of Pd/SiO2 is 2.4 wt % (smaller than that of
Pd@HMSNs). Therefore, more Pd/SiO2 catalysts are used to keep the same moles of
total Pd atoms in catalytic reactions.
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Table S1 summarizes the catalytic performance of Pd@HMSNs and control
Pd/SiO2 catalysts for hydrodechlorinations of various chlorinated phenols. For 4-, 3-
and 2-chlorophenols, turnover frequency (TOF, h-1) is measured as moles of products
per molar Pd atoms per hour at the initial 5 minutes while TOFs for 2,6- and
2,4-dichlorophenols are obtained at the reaction time of 40 minutes since relatively
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high conversions are achieved at the mentioned reaction time to increase the
calculation accuracy. In Table S1, the estimated TOFs are calculated by the estimated
surface Pd atoms using closed-shell model (a complete Pd atomic layer-0.45 nm).69-71
As shown in Table S1, the reaction rates of dichlorophenols are much slower than
those of chlorophenols, possibly due to the steric hinderance and low electron density
of dichlorophenols.55,72 Among various chlorophenols, the reaction rate of
4-chlorophenol is much faster than those of 3-chlorophenol and 2-chlorophenol,
which may be ascribed to less steric hinderance of 4-chlorophenol.55 Moreover, TOFs
for studied chlorinated phenols of Pd@HMSNs are 1.6~6.7 times higher than those of
supported Pd/SiO2, and Pd@HMSNs also significantly enhances the phenol
selectivity of dichlorophenol hydrodechlorinations relative to supported Pd/SiO2.
Effects of reaction time on hydrodechlorinations of chlorophenols over
Pd@HMSNs and Pd/SiO2 are shown in Figure 6a and 6b, respectively. As illustrated
in Table S1 and Figure 6, TOFs for hydrodechlorinations of 4-chlorophenol and
2-chlorophenol over Pd@HMSNs are 1838 and 906 h-1, respectively, while the
control Pd/SiO2 catalysts show significantly lower TOFs at the same reaction time.
Compared with those in literature,55 TOFs of Pd@HMSNs at the inial reaction time of
5 minutes are 3~4 times more than those at the same conditions in the literature,
suggesting the superior catalytic properties of HMSNs relative to the supported
catalysts and other microreactors. More hydrodechlorination of 4-chlorophenol over
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Pd/C and Pd@HMSNs calcined at 750 C and following 250 C H2 reduction are
shown in Figure S21a. Pd@HMSNs show greatly enhanced catalytic performance
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relative to commercial Pd/C, where Pd/C catalysts only reach ~ 28.3 % of
4-chlorophenol conversion at a reaction time of 120 minutes. Surprisingly, the
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compared to those calcined at 550 C, which is certainly due to their high thermal
stability of Pd@HMSNs (Figure S5b).
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2-Chlorophenol
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2-Chlorophenol
3-Chlorophenol
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80 100 120
Time (min)
Time (min)
Figure 6. Effects of reaction time on phenol yields for hydrodechlorinations of
chlorophenols over a) Pd@HMSMs and b) Pd/SiO2. Reaction conditions:
reactants-0.0900 g; Pd@HMSNs-0.0050 g (Pd-3.0 wt %) and Pd/SiO2-0.0063 g
(Pd-2.4 wt %); H2O-20.0 mL; NaOH-0.71 mmol; H2-0.10 MPa; room temperature;
speed of agitation-500 rpm.
Figure 7a and 7b show the normalized percentages of components during
2,4-dichlorophenol hydrodechlorination over Pd@HMSNs and Pd/SiO2, respectively.
2,4-dichlorophenol conversion over Pd@HMSNs (Figure 7a) reaches 84 % at the
reaction time of 100 minutes while that of the control Pd/SiO2 only approaches 28 %
at the same reaction time (Figure 7b). Moreover, the literature reported Pd
immobilized hollow microspheres approaches 82 % conversion at a siginificantly
longer reaction time of 300 minutes,55 once again confirming the superior catalytic
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properties of Pd@HMSNs relative to supported catalysts and Pd microreactors.
2-chlorophenol accounts for 8.8 % over Pd/SiO2 (Figure 7b) at the reaction time of
300 minutes while only trace of partial dechlorination product is observed at the same
reaction time over Pd@HMSNs (Figure 7a). Furthermore, roughly ~ 1 % of
cyclohexanone, a deep hydrogenation product, is detected after the reaction time of 40
minutes over Pd@HMSNs while none of cyclohexanone is observed over Pd/SiO2,
suggesting the deep hydrogenation property of Pd@HMSNs. More
hydrodechlorination of different dichlorophenol is illustrated in Figure S22, where
Pd@HMSNs also show greatly enhanced performance for 2,6-dichlorophenol,
proving a wide adaptability of Pd@HMSNs for hydrodechlorinations.
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Phenol
4-Chorophenol
2-Chorophenol
Cyclohexanone
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0
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Time (min)
Figure 7. a), normalized percentages of components during 2,4-dichlorophenol
hydrodechlorination over a) Pd@HMSNs and b) Pd/SiO2. Reaction conditions:
reactants-0.1140 g; Pd@HMSNs-0.0050 g (Pd-3.0 wt %) and Pd/SiO2-0.0063 g
(Pd-2.4 wt %); H2O-20.0 mL; NaOH-0.71 mmol; H2-0.10 MPa; room temperature;
speed of agitation-500 rpm.
2.4. Catalytic Stability of Pd@HMSNs for 4-Chlorophenol Hydrodechlorination
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The typical recycling experiments will seperate the solid catalysts from the
reaction system, and subsequently wash and dry them to refresh catalysts for the next
cycle. During the process, catalyst loss can not be avoided. Alternatively, we evaluate
the catalytic stability in a single experiment by increasing reactant/Pd ratio from 497/1
(Table S1, Figure 6 and 7) to 2483/1 (five times), which is equal to treatment of total
reactants in five cycles in normal recycling experiments but at more critical reaction
conditions due to no washing and drying process to refresh catalysts. After having
treated five times reactants (reactant/Pd-2483/1), Pd@HMSNs were seperated from
the system, followed by washing and drying, and were subsequently underwent
another cycle experiment at normal reaction conditions (reactant/Pd ratio of 497/1) to
evaluate the catalytic stability of Pd@HMSNs.
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Figure 8a represents the catalytic performance of Pd@HMSNs for
4-chlorophenol hydrodechlorination at a 2483/1 ratio of 4-chlorophenol/Pd. Since five
times NaOH is required, two thirds of NaOH were added at the beginning and the left
one third was added at the reaction time of 150 minutes, to avoid the damage of
Pd@HMSNs by high concentration of strong bases. As shown in Figure 8a, at the
ratio of 2483/1, the phenol yield reaches 99 % at the reaction time of 300 minutes.
The TOF at 99 % of phenol yield at the reaction time of 300 minutes in Figure 8a is
492 h-1 while the TOF at 100 % of phenol yield at the reaction time of 40 minutes in
Figure 6a is 745 h-1. It is well known that TOFs usually decrease with conversions or
ratios of reactant/catalyst increasing. Therefore, the TOF at ~ 100 % of conversion
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decreases from 745 h-1 at a ratio of reactant/Pd of 497/1 to 492 h-1 at a ratio of 2483/1
(five times) is quite reasonable.
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Figure 8. a), effects of reaction time on phenol yields for 4-chlorophenol reaction
over Pd@HMSMs at a 4-chlorophenol/Pd ratio of 2483/1 (five times); b), effect of
reaction time on phenol yields for 4-chlorophenol reaction over the recovered
Pd@HMSMs after having treated five times 4-chlorophenol. Reaction conditions:
reactants-0.9000 g for a) and 0.0900 g for b); Pd@HMSNs (Pd-3.0 wt %), 0.0100 g
for a) and 0.0050 g for b); H2O-20.0 mL; NaOH-7.1 mmol for a) and 0.71 mmol for
b); H2-0.10 MPa; room temperature; speed of agitation-500 rpm.
Figure 8b shows the catalytic performance at normal reaction conditions over
recovered Pd@HMSNs after having treated five times reactants. The recovered
Pd@HMSNs (Figure 8b) demonstrate yields of 80 % at 20 minutes and 97 % at 40
minutes while the fresh Pd@HMSNs illustrates yields of 79 % at 20 minutes and 100 %
at 40 minutes (Figure 6a), showing a basically same catalytic performance as that of
the fresh catalysts. Moreover, no detectable Pd was found by ICP-OES in the reaction
solution after having treated five times reactants while 0.6 ppm Pd are observed in the
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reaction solution for Pd/SiO2, suggesting a superior catalytic stability of HMSNs.
TEM images and XRD patterns of Pd@HMSNs and Pd/SiO2 after having treated five
times reactants are shown in Figure S23. A particle size of ~4.4 nm is observed in
Figure S23a, indicating no particle aggregation for Pd@HMSNs during catalytic
reactions while Pd/SiO2 shows obvious particle agglomeration in Figure S23c.
Pd/SiO2 is also used for the five times reactant experiment (Figure S21b). Only ~ 4 %
of 4-chlorophenol conversion is obtained at 300 minutes for Pd/SiO2, which is
significantly lower than 97 % of Pd@HMSNs at the same reaction time. Moreover,
XRD patterns in Figure S23b shows distinct Pd diffractions. All the above suggest a
high structural stability of Pd@HMSNs during catalytic reactions.
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The excellent catalytic performance of Pd@HMSNs is ascribed to their unique
hollow mesoporous nanostructures with small Pd NPs inside hollow cavities. In such
hollow mesoporous nanostructures, the mesoporous walls provide an acceptable mass
transportation, and hollow cavities function as nanoreactors. Such a type of
nanoreactors can anchor active Pd NPs inside the cavities, and thus enhance the
catalytic stability and prevent the active NPs from leaching into the solution. In
addition, the high specific surface area of Pd@HMSNs facilitate the contact
between the reactant molecules and catalysts, and such hollow nanostructures
possibly make the adsorpted reactant molecules have a relatively long residence time,
resulting in the increased collision times of reactant molecules with an increased
catalytic activity. In this study, Pd@HMSNs show a significantly enhanced selectivity
for 2,4-dichlorophenol and 2,6-dichlorophenol hydrodechlorinations relative to
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Pd/SiO2, due to their unique hollow nanostructures. Such hollow nanostructures
possibly make the partial dechlorination intermediates such as 2-chlorophenol and
4-chlorophenol have longer residence time, and further facilitate their deep
dechlorination to obtain the final products-phenol, resulting in ehhanced selectvity.
In terms of enhanced catalytic performance, literature abounds with examples of
beneficial effects of bi-component catalysts (e.g., bimetallic alloys or metal-metal
oxide composites). In this study, we have demonstrated the facile synthesis of
Pd-Cu@HMSNs, PdO-CuxOy@HMSNs, Pt-CuxOy@HMSNs, Pt-CeO2@HMSNs,
Pt-PdO@HMSNs and CeO2-CuxOy@HMSNs by the coordination enhanced approach,
and the investigation of their catalytic performance is ongoing. We believe that these
bimetallic or even multi-metallic analogues, the atomic ratios of which can be so
easily adjusted by our method, will find numerous applications in catalysis.
3. Conclusions
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In conclusion, a coordination enhanced approach to generally synthesize hollow
mesoporous silica nanoreactors with functional NPs inside their cavities are
developed. By using the high coordination ability of L3 triple ligands, most metal ions
(transition, noble, rare earth and main group metals) can be incorporated into metal
ions bound charge-driven micelles. Moreover, two different metal ions can be easily
co-incorporated into the micelles by simply mixing these metal ion solutions.
Following deposition of silica onto micellar coronas, polymerization and calcination,
can give a vast amount of HMSNs with functional monometallic or bimetallic oxide
NPs inside their cavities. When required, reductions can be applied to obtain
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M@HMSNs, M -M @HMSNs or M -M Ⅱ xOy@HMSNs. The synthesized
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Pd@HMSNs demonstrate excellent catalytic hydrodechlorination activity, and
excellent stability due to their unique hollow mesoporous nanostructures. We believe
that such methodology can be applied to synthesize HMSNs with uniform
multi-component NPs inside cavities, and such type of materials will find more
applications in heterogeneous catalysis and drug delivery.
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4. Experimental Section
4.1. Chemicals. Nitrate acid (HNO3, 65 %-68 %), sodium hydroxide (NaOH, ≥
96.0 %), and acetone (AR) were purchased from Shanghai Chemical Reagent
Company. Ethanol (AR), oleylamine (80–90 %), palladium chloride (Pd, 59 %-60 %)
and potassium tetrachloropalladate (K2PdCl4, 98 %) were purchased from Aladdin.
Copper(II) nitrate trihydrate (Cu(NO3)2·3H2O, 99 %), 2-chlorophenol (99 %),
4-chlorophenol (99 %), 3-chlorophenol (98 %), 2,6-dichlorophenol (99 %), cerium(III)
nitrate hexahydrate (Ce(NO3)3·6H2O, 99.99 %), n-Butyllithium solution (2.5ꢀM in
cyclohexane) and chloroauric acid hydrate (Au, 50 %) were purchased from
Adamas-beta. 2,4-dichlorophenol (99 %) was purchased from Acros organics. Tin(IV)
chloride pentahydrate (SnCl4, AR) and silica dioxide (SiO2, 99.5 %) were purchased
from Mackline. Zirconium(IV) dichloride oxide hydrate (Zr, 99.9 %) was purchased
from Strem Chemicals, Inc.. Ethyl silicate (TEOS, SiO2 ≥28.4 %) was purchased from
Sinopharm Chemical Reagent Co., Ltd.. Dipotassium tetrachoroplatinate (Pt, 46.4 %)
was purchased from 9 Ding Chemisty. The synthetic procedure of quaternized diblock
copolymer poly(2-vinylpyridine)-b-poly(ethylene oxide) (positively charged
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P2MVP128-b-PEO477, Mw/Mn=1.1, Mw=34.5 k) is described in our previous work.39
The 1,3,5-tris(2,6-dicarboxypyridin-4-yloxymethyl)benzene (L3) was synthesized
according to the literature.44 All reagents were used without further purification.
4.2. Synthesis of HMSNs with Monometallic (Oxide) NPs inside Their Cavities.
PdO@HMSNs and Pd@HMSNs. For synthesis of PdO@HMSNs, 360 uL of 0.087
mM positively charged P2MVP128-b-PEO477 aqueous solution, 240 uL of 5.0 mM L3
aqueous solution and 360 uL of 5.0 mM K2PdCl4 aqueous solution
(polymer/L3/Pd-1/38.3/57.5) were mixed with 9.04 g of deionized water in a 20 ml
flask with a magnetic stirring at 200 rpm. Nitric acid or sodium hydroxide aqueous
solution was used to adjust the pH value of the obtained solution to ~4.15, and the
resultant mixture was further stirred overnight. 72.4 uL of TEOS was injected into the
above-mentioned solution (Pd/TEOS ratio of 1/180), and then the obtained mixture
was stirred for 48 hours (white solid precipitates appeared during 24-48 hours). In
order to complete the silica polymerization process, the mixture was left untouched
for one day. After that, the solids were collected by centrifugation and washed by
water and alcohol for several times. Finally, the products were dried at 55 oC in an
oven and calcined at 550 oC for 3 hours in a muffle furnace to obtain PdO@HMSNs.
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The Pd@HMSNs was prepared by reduction of PdO@HMSNs with H2 at 250 C for
3 hours in a tube furnace. The Pd loadings of PdO@HMSNs and Pd@HMSNs are 3.0
wt % by ICP-OES.
Au@HMSNs. 720 uL of 0.087 mM positively charged P2MVP128-b-PEO477 aqueous
solution, 960 uL of 5.0 mM L3 aqueous solution and 1440 uL of 5.0 mM HAuCl4
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aqueous solution (polymer/L3/Au-1/76.6/114.9) were mixed with 36.88 g of deionized
water in a 50 ml flask with a magnetic stirring at 200 rpm. The following procedures
are same as those of PdO@HMSNs except that 290 uL of TEOS were used to keep
the ratio of Au/TEOS at 1/180 and the pH value is adjusted to ~3.80 before addition
of TEOS. During the synthesis, H2 reduction process is not required since calcination
at 550 oC will directly obtain Au@HMSNs.
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CeO2@HMSNs. 540 uL of 0.087 mM positively charged P2MVP128-b-PEO477
aqueous solution, 360 uL of 5.0 mM L3 aqueous solution and 360 uL of 5.0 mM
Ce(NO3)3 aqueous solution (polymer/L3/M-1/38.3/38.3) were mixed with 8.74 g of
deionized water in a 20 ml flask with a magnetic stirring at 200 rpm. The following
procedures are same as those of PdO@HMSNs except that the pH value was adjusted
to ~4.30 before addition of 72.4 uL of TEOS (Ce/TEOS-1/180).
ZrxOy@HMSNs. 360 uL of 0.087 mM positively charged P2MVP128-b-PEO477
aqueous solution, 360 uL of 5.0 mM L3 aqueous solution and 360 uL of 5.0 mM
ZrOCl2 aqueous solution (polymer/L3/Zr-1/57.5/57.5) were mixed with 18.92 g of
deionized water in a 30 ml flask with a magnetic stirring at 200 rpm. The following
procedures are same as those of PdO@HMSNs except that the pH value was adjusted
to ~3.80 before addition of 145.0 uL of TEOS (Zr/TEOS-1/360).
SnxOy@HMSNs. 360 uL of 0.087 mM positively charged P2MVP128-b-PEO477
aqueous solution, 360 uL of 5.0 mM L3 aqueous solution and 360 uL of 5.0 mM
SnCl4 aqueous solution (polymer/L3/Sn-1/57.5/57.5) were mixed with 8.92 g of
deionized water in a 20 ml flask with a magnetic stirring at 200 rpm. The following
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procedures are same as those of PdO@HMSNs except that the pH value was adjusted
to ~4.20 before addition of 72.4 uL of TEOS (Sn/TEOS-1/180).
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4.3. Synthesis of HMSNs with Bimetallic (Oxide) NPs inside Their Cavities.
Pt-PdO@HMSNs. 360 uL of 0.087 mM positively charged P2MVP128-b-PEO477
aqueous solution, 240 uL of 5.0 mM L3 aqueous solution, 180 uL of 5.0 mM K2PtCl4
aqueous solution and 180 uL of 5.0 mM K2PdCl4 aqueous solution
(polymer/L3/Pt/Pd-1/38.3/28.7/28.7) were mixed with 9.04 g of deionized water in a
20 ml flask with a magnetic stirring at 200 rpm. The following procedures (72.4 uL of
TEOS, (Pd+Pt)/TEOS-1/180) are same as those of PdO@HMSNs. During the
synthesis, calcination at 550 oC will directly give Pt-PdO@HMSNs.
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PdO-CuxOy@HMSNs and Pd-Cu@HMSNs. 360 uL of 0.087 mM positively charged
P2MVP128-b-PEO477 aqueous solution, 240 uL of 5.0 mM L3 aqueous solution, 180
uL of 5.0 mM K2PdCl4 aqueous solution and 180 uL of 5.0 mM Cu(NO3)2 aqueous
solution (polymer/L3/Pd/Cu-1/38.3/28.7/28.7) were mixed with 9.04 g of deionized
water in a 20 ml flask with a magnetic stirring at 200 rpm. The following procedures
(72.4 uL of TEOS, (Pd+Cu)/TEOS-1/180) are same as those of PdO@HMSNs.
Pd-Cu@HMSNs was prepared by reduction of PdO-CuxOy@HMSNs with H2 at 300
oC for 3 hours. The Pd and Cu loadings of PdO-CuxOy@HMSNs and PdCu@HMSNs
are 1.7 wt % and 1.3 wt %, respectively.
CeO2-CuxOy@HMSNs. 450 uL of 0.087 mM positively charged P2MVP128-b-PEO477
aqueous solution, 300 uL of 5.0 mM L3 aqueous solution, 180 uL of 5.0 mM
Cu(NO3)2 aqueous solution and 180 uL of 5.0 mM Ce(NO3)3 aqueous solution
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