Confined Transformation of UiO-66 Nanocrystals to Yttria-Stabilized Zirconia with Hierarchical Pore Structures for Catalytic Applications

Article abstract of DOI:10.1002/adfm.201903264

Solid acids as a substitution for hazardous liquid acids (e.g., HF and H₂SO₄) can promote many important reactions in the industry, such as carbon cracking, to proceed in a more sustainable way. Starting from a zirconium-based metal-organic framework (UiO-66 nanocrystals), herein a transformative method is reported to prepare micro/mesoporous yttria-stabilized zirconia (YSZ) encapsulated inside a mesoporous silica shell. It is then further demonstrated that the resultant reactor-like catalysts can be used for a wide range of catalytic reactions. The acidity of the YSZ phase is found with rich accessible Lewis acid and Br?nsted acid sites and they display superior performances for esterification (acetic acid and ethanol) and Friedel-Crafts alkylation (benzylation of toluene). After being loaded with different noble metals, furthermore, hydrogenation of CO₂ and a one-pot cascade reaction (nitrobenzene and benzaldehyde to N-benzylaniline) are used as model reactions to prove the versatility and stability of catalysts. Based on the findings of this work, it is believed that this class of reactor-like catalysts can meet future challenges in the development of new catalyst technology for greener heterogeneous catalysis.

Full text of DOI:10.1002/adfm.201903264

FULL PAPER
Nanoreactors
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Confined Transformation of UiO-66 Nanocrystals to
Yttria-Stabilized Zirconia with Hierarchical Pore
Structures for Catalytic Applications
Runze Qin and Hua Chun Zeng*
and thus their catalytic performances
are determined to a great extent by their
workable surface areas. Porous solid acids
with high specific surface area can pro-
vide more accessible acid sites, which will
Solid acids as a substitution for hazardous liquid acids (e.g., HF and H SO )
2
4
can promote many important reactions in the industry, such as carbon cracking,
to proceed in a more sustainable way. Starting from a zirconium-based
metal-organic framework (UiO-66 nanocrystals), herein a transformative
method is reported to prepare micro/mesoporous yttria-stabilized zirconia
[
2]
significantly enhance catalyst efficiency.
Furthermore, compared with simply
single pore size featured (e.g., micropore
or mesopore) porous materials, hierar-
chical pore structures could endow them
(YSZ) encapsulated inside a mesoporous silica shell. It is then further
demonstrated that the resultant reactor-like catalysts can be used for a
wide range of catalytic reactions. The acidity of the YSZ phase is found with
rich accessible Lewis acid and Brønsted acid sites and they display superior
performances for esterification (acetic acid and ethanol) and Friedel-Crafts
alkylation (benzylation of toluene). After being loaded with different noble
[
3]
with better mass diffusion properties.
Meanwhile, functionalization of solid
acids with active metal components has
also been found to play a crucial role in
many common reactions, such as the
conversion of organic intermediates to
metals, furthermore, hydrogenation of CO and a one-pot cascade reaction
2
(nitrobenzene and benzaldehyde to N-benzylaniline) are used as model
[
4]
synthetic drugs; in those cases, engi-
neering porosity for solid acid materials is
also of vital importance because it endows
them with a high loading capacity and uni-
form distribution of added components.
Furthermore, topology of such supporting
materials has been employed recently to
reactions to prove the versatility and stability of catalysts. Based on the
findings of this work, it is believed that this class of reactor-like catalysts can
meet future challenges in the development of new catalyst technology for
greener heterogeneous catalysis.
resist sintering through size confinement,[5] which is one of
the commonest impediments to nanoparticle catalysis. In this
regard, ordered porous solid-acid catalysts could also play such
a role to combat the sintering of their loaded metals. However,
attention being devoted to this class of composite materials is
found relatively lacking.
1
. Introduction
Catalytic solid acids, such as zirconium oxide, niobium oxide,
and molybdenum oxide with high oxidation states for metal
cations, are a class of well-established solid materials used in
chemical industry which can help protonating hydrocarbon
molecules to form carbocations and promote many important
catalyzed reactions, such as cracking, hydrocracking, isomeriza-
tion, alkylation, and aromatization through surface complexes
or transition states.[1] Compared to conventional Lewis acid
On the other hand, in searching newer acid materials, it is
recognized that metal-organic frameworks (MOFs) have drawn
enormous attention in the last 20 years. Because of their high
specific surface area, tunable porosity, and other functionaliz-
able physicochemical properties, MOFs have been widely used
in a variety of applications including molecular adsorption-
desorption, membrane separation, gas sensing, heterogeneous
(e.g., BF and AlCl ) and Brønsted acid (H SO and HF) cata-
3 3 2 4
lysts, which are hazardous, solid acids are more environmen-
tally friendly in accordance with process sustainability and
catalyst recyclability. Mostly being used as heterogeneous cata-
lysts, solid acids function via their active surfaces to reactants
[
6]
catalysis, encapsulation and controlled releases. In particular,
with coordinative unsaturation or open active metal sites on
metal secondary building units, many MOF materials can pro-
[
7]
vide abundant intrinsic acid sites. However, although design
of acidity of moderate strength can be achieved for MOF mate-
rials, the ability to produce superacidity remains challenging
R. Z. Qin, Prof. H. C. Zeng
Department of Chemical and Biomolecular Engineering
Faculty of Engineering
National University of Singapore
[8]
with only few successful examples. Besides, most MOF mate-
1
0 Kent Ridge Crescent, Singapore 119260, Singapore
rials cannot survive in a harsh environment such as high tem-
perature and extreme pH values. Nevertheless, it is noted that
except for their direct utilizations, MOFs are also extensively
used as chemical precursors or templates to derive advanced
functional materials with special structural or architectural
E-mail: chezhc@nus.edu.sg
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201903264.
DOI: 10.1002/adfm.201903264
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merits that conventional processing methods cannot provide.
These resultant solid derivatives include, for instance, porous
carbon materials, porous metal oxides and carbon-supported
nanoscale metal particles.[9] Through mostly thermal decom-
position of MOFs or MOF nanocomposites under a reactive or
inert atmosphere, specific morphology and composition can be
attained owing to high carbon content and high-density metal
nodes in MOFs. Prior to thermal decomposition of pristine
MOFs, in many cases, post-synthesis modification or addi-
tion of other functional materials can be carried out in order
to adjust properties of product.[ Thus it is desirable to obtain
materials that maintain rich acid sites for porous solid acids
similar to MOF materials and meanwhile possess high stability.
In a previous literature,[ direct calcination of yttrium-doped
UiO-66 was conducted to produce YSZ with maximum BET
needing extra steps to remove the included templates. With
the addition of yttrium, zirconium oxide gets stabilized and
yttrium also shows positive role for catalyst activity probably
through electronic structure tuning. The resultant core–shell
(namely, the YSZ@mSiO ) contains high-density accessible
2
acid sites along the generation of porous structure, exhibiting
superior performance for catalytic esterification. Furthermore,
using the YSZ@ mSiO as a catalyst carrier, ultrasmall metal
2
particles can be easily loaded inside the micro- and mesoporous
YSZ through simple impregnation with the strong interac-
tion between the YSZ phase and supported noble metal cata-
lyst, which displays remarkable performances for catalytic
10]
CO hydrogenation and a one-pot three-step cascade reaction
2
11]
of benzaldehyde and nitrobenzene, proving its ability to serve
as a versatile nanoreactor for various reactions. Moreover, the
micro- and mesoporous YSZ developed in this work demon-
strates great chemical stability in extreme pH value environ-
ments and/or at high temperatures up to 700 °C. The loaded
metal nanoparticles also exhibit excellent stability up to 500 °C
without noticeable aggregation.
2
−1
surface of 34.6 m g . However, this almost nonporous struc-
ture could make it difficult to be used as support for catalysts.
To the best of our knowledge, there have been no reports yet
on employing MOF as a precursor to prepare highly porous
yttria-stabilized zirconia (YSZ) to work as a solid acid and as a
catalyst support at the same time, although YSZ is one of the
most studied materials in ceramic science and technology. In
this regard, we believe that utilizing MOFs as solid precursors
holds a number of advantages. First, UiO-66 used in this work
is a highly porous material and thus other metal modifiers
such as yttrium can be readily dispersed into the entire host
structure compared with their nonporous bulk counterparts.
Second, conversion of UiO-66 to its product metal oxide (ZrO2)
upon heating would generate gaseous species like CO2 and
slow release of the gas product would help to create pores for
resultant ZrO confined with a mesoporous silica (mSiO ) shell.
2
. Results and Discussion
2.1. Synthesis and Morphology of M/YSZ@mSiO2
In this work, the synthetic procedure is illustrated in Scheme 1.
As can been from Figure 1a,b, uniform octahedrons of UiO-66
nanocrystals can be formed by adding acetic acid into the
system to effectively reduce the nucleation rate. Varying sizes of
UiO-66 nanocrystals can be controlled by adjusting the reaction
time and reactant concentration, which in turn will determine
the product size generated in subsequent steps (Scheme 1 and
Figure S1, Supporting Information). A layer of mesoporous
2
2
Third, except meso- and micropores formed inside the metal
oxide of ZrO , void spaces inside the mSiO2 shell could be
2
generated, which results in creating macro-porosity for the
ZrO phase, due to much larger volume of UiO-66 material
silica (mSiO ) shell is then added onto the external surface of
2
2
compared to that of solid ZrO . Finally, ZrO form within the
the UiO-66 nanocrystals (Figure 1c), after which yttrium is
2
2
mSiO shell could be better protected in addition to targeted
also introduced into the UiO-66@mSiO2 through impregna-
2
structural merits of the ZrO phase.
tion, which gives rise to the Y-UiO-66@mSiO . This resultant
2
2
The above prepared confined ZrO2 phase should benefit
numerous catalytic applications, as will be elucidated in the
precursor is then calcined at a relatively high temperature of
700 °C in order to produce thermally stable YSZ phase (in the
present work. For example, hydrogenation of CO to produce
YSZ@mSiO ; Figure 1d) to withstand any harsh reaction con-
2
2
C1 chemicals (such as CH , CO and CH OH) has been iden-
ditions. PXRD pattern of YSZ@mSiO fits well with that of
4
3
2
tified as a promising strategy to mitigate greenhouse effect
and enormous work has been put on utilizing nanocatalysts to
the pseudo-fluorite-type tetragonal and cubic mixed structures
of YSZ (Figure 1e), confirming the generation of YSZ phase
inside the amorphous silica shell. Displayed in Figure 1, dif-
ferent batches of the synthesized samples were mixed first and
then characterized in order to prove the consistency and repro-
ducibility of our products.
reduce CO to useful products. However, activity loss caused by
2
aggregation of active catalytic components remains a challenge
for this type of catalysts and meanwhile there is still room for
elevation of yield and versatility.[ Thus catalysts with high
conversion, selectivity and meanwhile stability and versatility
are highly demanded.
12]
We found that the appropriate calcination time, heating rate
and silica shell thickness are all important process parameters
Herein, we report a synthetic approach to obtain micro/
mesoporous YSZ encapsulated inside a mesoporous silica shell
by using UiO-66 MOF as a solid precursor. In this preparation,
calcination of yttrium-modified and silica-encapsulated UiO-66
in air would produce high surface-area micro/mesoporous YSZ
nanostructure inside the silica shell. Compared to previous
methods to fabricate porous metal oxide with soft templates
in forming the YSZ@mSiO core–shell structure. If calcina-
2
tion time is too short, the decomposition of Y-UiO-66 inside the
mesoporous silica is incomplete, leaving an unevenly distrib-
uted intermediate (Figure S2, Supporting Information). Besides,
when the thickness of silica shell is too thin to withstand the
transformation of octahedral Y-UiO-66, the whole Y-UiO-
66@mSiO structure would collapse and part of YSZ would
2
[
2,13]
(e.g., P-123, CTAB, etc.) or hard templates (e.g., SBA-15),
not be encapsulated (Figure S2, Supporting Information). With
the Thermogravimetric analysis (TGA) technique (Figure S3,
this approach achieved high specific surface areas without
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transmission electron microscopy (TEM)
images further disclose the same porous
feature of the YSZ core phase but compara-
tively nonporous appearance of the ZrO2
formed from the calcined UiO-66 sample
(
Figure S4, Supporting Information). Fur-
thermore, the Y-UiO-66 sample was prepared
from impregnating UiO-66 nanocrystals with
the same yttrium salt solution, but the cal-
cined Y-UiO-66 sample shows a very low BET
2
−1
surface area of only 34 m ∙g , excluding
a major impact of yttrium in pore forma-
tion. Hence, it is deduced that the encap-
sulation of mesoporous silica shell on the
UiO-66 is a key factor in pore formation. In
the calcination process in air, the yttrium-
impregnated UiO-66 would decompose into
the solid YSZ and gaseous carbon dioxide,
carbon monoxide and water vapor, as well
as a trace amount of gaseous species from
decomposition of the yttrium salt. Neverthe-
Scheme 1. Illustration of the stepwise preparation of the M/YSZ@mSiO nanoreactors in this less, due to the confinement of mesoporous
2
work (UiO-66, starting nanocrystals; SiO , silica precursors; mSiO , mesoporous silica shell; Y,
yttrium; M = catalytic metal nanoparticles depicted as small shining spheres).
2
2
silica shell, these gaseous species cannot
be released instantly, and thus this slow
gas evolution process leads to formation of
Supporting Information), the UiO-66 phase was fully decom-
micro/mesoporous YSZ phase, noting that the exterior mSiO2
shell itself is also undergone thermal decomposition of its
trapped surfactant molecules (CTAC). In this process, on the
other hand, the silica shell also provides a boundary support
for the shrinking YSZ core generated from Y-impregnated
UiO-66, which results in a loosely packed particles of porous
YSZ (Figure S4a, Supporting Information), instead of con-
tracting inward into a compact form. The YSZ formed from
this process is very stable up to 700 °C and in the pH range
of 0−13 (i.e., the silica shell would be etched in alkaline envi-
ronment but the porous YSZ core remains). It is worth noting
that with the uniform morphologies for the solid precursors
(UiO-66@mSiO and thus Y-UiO-66@mSiO ), consistent pore
[
14]
posed into the monoclinic zirconia readily at around 540 °C,
and in particular the gradual weight loss of UiO-66@mSiO in
2
the temperature range of 200−300 °C was attributable to the
decomposition of cetyltrimethylammonium chloride solution
(
CTAC) trapped in the mSiO shell.
2
As shown in Figure 1d (YSZ@mSiO ), the resultant YSZ
2
inside the mesoporous silica shell mostly sticks to the shell and
exists in a porous, incompact form, which was further proven
by the N adsorption/desorption isotherm and pore size distri-
2
bution of this sample (Figure 2). For the YSZ@mSiO core–
2
shell, an evident hysteresis loop is observed within the range
of relative pressure (P/P ) from 0.5−0.8, proving the existence
0
2
2
of mesopores in this sample. The specific surface area of the
size distribution for the final YSZ phase can be easily obtained
among samples from different batches of synthesis. Another
noteworthy feature about porous YSZ inside mSiO2 shell is
that because YSZ, which has a much higher density than the
UiO-66 phase, can only occupy a small volume of the space
left by the decomposition of a UiO-66 crystal core; therefore a
large part of the interior space, especially the space in the cen-
tral location, is still empty (Figure S4a, Supporting Informa-
tion). Besides obvious micro- and mesopore observed in BET
pore size distribution, formation of these empty cavities creates
some macropores inside porous YSZ, thus producing unique
macro-meso-micro hierarchical pore structure (see Scheme 2).
The improved mass diffusion properties of such hierarchical
2
−1
−1
YSZ@mSiO is 498 m g with the pore volume of 0.286 mL g
2
obtained at P/P = 0.980. In the pore size distribution of this
0
sample, typically five prominent peaks were found at 0.39, 0.68,
1
.48, 2.73, and 3.43 nm, confirming a micro/mesoporous struc-
ture in the YSZ@mSiO . Essentially, this pore structure still
2
remains after etching silica shell with alkali, with only a small
shift to the right (i.e., only a slight pore enlargement). The dis-
appearance of 0.39 nm pore could be attributed to the alkaline
etching which eliminated contact interstitials between the YSZ
and mSiO shell. The differential pore volumes for 0.39 nm and
2
0
.68 nm remain almost the same but the differential pore vol-
umes for 2.73 nm and 3.43 nm were reduced by half, which
implies that pore size for etched silica shell lies in this range.
To investigate the pore formation mechanism, several
control experiments were also conducted. For example, the
[
3]
frameworks could endow them with superior catalytic activity,
which could further be confirmed by their versatility for a range
of catalytic reactions. It is proposed that this structural feature
will create localized concentration gradients if the YSZ@mSiO2
core–shell structure is used as a catalyst carrier for constructing
nanoscale reactors with other catalytically active metals in con-
secutive cascade reactions, noting that the tiered porosities in
the YSZ core will ease required mass transport processes for
2
−1
mesoporous YSZ still displayed a BET surface of 324 m g
after etching off the silica shell, which is among the highest
specific surface areas of YSZ reported.[ In contrast, the spe-
cific surface area of the calcined UiO-66 sample under the same
15]
2
−1
calcination condition is only 29 m ∙g . In addition to this,
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Figure 1. a) FESEM image of the UiO-66 nanocrystals, TEM images
of b) the UiO-66 nanocrystals, c) the UiO-66@mSiO2 sample, d) the
YSZ@mSiO2 sample, and e) XRD pattern of the YSZ@mSiO2 sample
(black curve), compared with the XRD patterns of tetragonal YSZ (red
vertical lines) and cubic YSZ (light blue lines) from XRD database.
reactions too. Indeed, apart from the catalytic activity of YSZ
itself, multiple active metal components can also be loaded
into the YSZ@mSiO2 through simple metal impregnation.
As shown in Figure 3a, metal NPs are homogenously distrib-
uted throughout the interior YSZ phase. In principle, both the
porous silica shell and the YSZ can support these metal NPs.
However, much fewer nanoparticles were found in mesoporous
silica shell based on our investigations of TEM and energy-
dispersive X-ray spectroscopy (EDX) elemental mapping, which
could possibly result from contraction of metal solution from
outside to inside in drying process after impregnation. Fur-
thermore, surface hydrophilicity and chelating ability to metal
precursors (which were dissolved in an aqueous solution) can
also be attributed to the observed difference between the mSiO2
Figure 2. a) N adsorption–desorption isotherms and b) differential volu-
metric pore size distribution (using BJH method based on the desorption
2
data) of the YSZ@mSiO sample, the etched YSZ@mSiO sample, the
2
2
calcined Y-UiO-66 sample, and the calcined UiO-66 sample.
and YSZ. In our X-ray diffraction (XRD) investigation for the
M/YSZ@mSiO2 samples, we found that XRD peaks for the
corresponding metals (M) can totally disappear by adjusting
reducing temperature, indicating ultrasmall size of metal NPs
formed in the micro/mesoporous YSZ phase (Figure S5, Sup-
porting Information). Furthermore, mixed domains of YSZ can
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Scheme 2. Schematic illustration of hierarchical pore structures in metal
loaded YSZ for catalytic applications.
be observed under high-resolution TEM (HRTEM) (e.g., Pd/
YSZ@mSiO ; Figure 3b) after etching away the mesoporous
2
silica shell and the lattice fringe with an interplanar distance of
0
.30 nm corresponds to the plane distance of t-YSZ {011} and/
or c-YSZ {111}, which indicates a possible structural mixture
of tetragonal phase and cubic phase.[ However, the loaded
noble metal nanoparticles cannot be located unambiguously in
the same image, thus indicating that the loaded Pd is indeed
extremely small, probably in sub-nanoscale (Figure 3b). Simi-
larly, other metal loadings (such as Pt and Ru) are also difficult
to observe with the HRTEM technique, indicating their sizes
are indeed ultrafine. Accommodation and homogenous distri-
bution of more than one kind of metal in the host system of
16]
YSZ@mSiO can also be realized simultaneously (see Figure S6,
2
Supporting Information).
2
.2. Acidity of YSZ@mSiO2
YSZ is a very common support for catalysts nowadays with
strong acidity as solid superacid.[17] To investigate the acidity
and accessibility of acid sites for the YSZ inside the mesoporous
silica shell, diffuse reflectance infrared Fourier transform spec-
troscopy (DRIFTS) IR was used to characterize the acid sites
Figure 3. a) HRTEM-EDX elemental mapping study of the Pd/
YSZ@mSiO2 sample, and b) HRTEM image of the Pd/YSZ sample at
high magnification. Parts of lattice fringes are circled in yellow.
distributed in the YSZ@mSiO sample by using pyridine as a
2
probing molecule. With this in situ technique, the IR peaks for
pyridine would shift according to whether it is adsorbed on a
−
1
at 1545 cm typical for pyridinium ions bonded to Brønsted
[
18]
[20]
Lewis acid site or protonated by a Brønsted acid site. After
the adsorption of pyridine, first, peaks for pyridine adsorbed
on Lewis acid sites can be found on IR spectrum (Figure 4).
According to an early paper in literature,[ these peaks cor-
respond to the normal modes ν8a and ν19a of ring-breathing
sites, furthermore, Brønsted acid sites also exist in the same
YSZ@mSiO . All the above data confirm that both Lewis and
2
Brønsted acid sites of the YSZ inside the silica shell remain
accessible by pyridine molecules. When temperature is raised
from room temperature to 300 °C, it is observed that two peaks
(1446 and 1490 cm ) assigned to the adsorption on Lewis
acid sites decrease significantly, but other two peaks (1545 and
19]
−
1
(
1
νCCN) vibrations of pyridine with A symmetry (1598 and
1
−
1
490 cm , respectively) and the ν modes with B symmetry
19b 2
−
1
[19]
−1
(1446 cm ), which coincide with those found for Lewis-site
1598 cm ) basically remain their original intensity. All these
peak changes, including a blue shift observed for the original
[20]
coordinated pyridine on ZrO2. Indicated by vibrational band
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2
.3. CO Hydrogenation
2
The global warming has been a big challenge raising concern
widely and CO , one of main green gases causing issue above,
2
can be reduced to produce CO as a fuel or a synthetic feedstock
[
12a]
by using many catalysts.
In this regard, supported Pt cata-
lysts have been widely investigated to realize CO reforming
2
to syngas. It has been well known that the catalytic activity is
largely determined by the accessibility of Pt catalysts on the
support interface. In particular, the importance of support has
been be elucidated by the low activity of Pt black and Pt/SiO2
catalyst, because of lacking the ability to form carbonates on the
[
23]
support. In this part of study, we will use the above prepared
YSZ@mSiO , in combination with Pt NPs, to demonstrate real-
2
izations of high conversion of CO and high selectivity over CO.
2
As can be seen from Figure 5a, the main product for CO2
hydrogenation is CO with a small amount of CH using Pt as
4
an active catalytic component. The selectivity of CO for the Pt/
Figure 4. DIRFTS-IR pyridine adsorption data for the YSZ@mSiO sample
2
YSZ@mSiO dropped slightly from 100% to 96.2% when tem-
2
ofvaryingtemperatures. LdenotesLewisacidsitesandBdenotesBrønsted
acid sites. Pyridine IR data are from NIST Chemistry WebBook (https://
webbook.nist.gov/cgi/cbook.cgi?ID=C110861&Type=IR-SPEC&Index=2).
perature was increased from 200 to 400 °C. Meanwhile, the
conversion of CO increased significantly and finally reached
2
2
6.5%, which gave a yield of 25.5% for the desired product
CO. Therefore, the catalyst activity for CO hydrogenation to
2
598 cm− peak, suggest some desorption of pyridinium ions
1
CO increases with temperature, which is also supported by
the increasingly high intensity for some intermediates formed
in the CO2 hydrogenation process (Figure S10, Supporting
Information).
1
and/or alternation of their adsorption configuration upon
heating. Nevertheless, the DRIFTS study confirms the strength
of acid sites in our samples.
Apart from the acidity probing, esterification of acetic acid
and ethanol, which is a very common reaction catalyzed by acid
catalysts, was employed as a model reaction to demonstrate
the activity for the YSZ@mSiO2 as a solid superacid. Our
The reaction mechanism for CO2 hydrogenation was fur-
ther explored using the same in situ DRIFTS-IR technique
(Figure 6). In this measurement, H , CO , and N were used,
either respectively or in combination, as purging (as well as
adsorbing) gases for the catalyst in a deliberated sequence. For
example, on the basis of IR peak changes (i.e., appearance,
disappearance, and/or persistence) of certain reaction species,
2
2
2
YSZ@mSiO exhibits much higher rate of the esterification of
2
acetic acid than bulk nonporous YSZ. Since catalytic activity of
the mSiO shell can be neglected compared to the YSZ phase
2
(Figure S7, Supporting Information), performance of the
the hydrogenation mechanism of CO can be investigated and
2
YSZ@mSiO was calculated based on the mass of YSZ instead
deduced.
2
of YSZ@mSiO2 in which the YSZ component accounts for
First, the catalyst was reduced with H in the cell at 200 °C for
2
4
0.4 wt% (see the Experimental Section in the Supporting Infor-
3 h, after which the catalyst was purged with N for 10 min. In
2
mation). It is revealed that the catalytic performance of our YSZ
stage 1 of reaction, the temperature was raised to 400 °C and the
achieves a reaction rate of 1.80 mmol g− min , surpassing
1
−1
catalyst was purged with CO . After this CO purging, IR peaks
2
2
−
1
the best conventional solid acid catalysts and other catalysts
of gases CO (2350 cm ), OCO stretching of carboxylates
2
reported in literature.[
2,21]
As shown in Figure S7 (Supporting
(1281 cm ) and bicarbonates (1224 cm ) were observed. In
particular, two peaks indicating the presence of adsorbed CO
−1 −1 [24]
Information), the reaction rate was higher than that of sulfated
mesoporous niobium oxide,[ 2 times that of Amberlyst-15,
and 6 times that of bulk nonporous YSZ with the same yttrium
oxide weight percentage. Besides, the content of yttrium and
thus its weight percentage could be adjusted in our YSZ sam-
ples. Esterification reaction and related XRD characterization
were conducted using these samples (Figures S8 and S9, Sup-
2]
−1
[25]
appear at 2175 and 2118 cm , respectively, which was prob-
ably produced from the reaction between adsorbed hydrogen
and carbonate and bicarbonate species. It was further confirmed
that no such CO peaks appeared when catalysts were placed in
air for a few days to remove the hydrogen adsorbed. In stage 2,
the input of CO flow was ceased and instead purging gas N
2
2
porting Information). In addition to this, our YSZ@mSiO also
was then introduced. It can be found that IR peaks for gases
2
shows high activity for Friedel-Crafts alkylation, more specifi-
cally benzylation of toluene, which is another common reaction
catalyzed by acid catalysts.[ The observed high activity of our
YSZ is again attributed to the presence of much higher acid site
density in its micro/mesoporous texture. On the contrary, the
relatively low reaction rate of UiO-66 is probably the result of
both low oxozirconium clusters density compared with the YSZ
and higher diffusing barrier for mass transfer due to lacking
mesoporosity.
CO , carbonates and bicarbonates are weakened significantly,
2
revealing that most reaction intermediates were purged away;
the CO peaks totally disappeared also in this stage. In stage 3,
the N stream was shut down and a mixture of H and CO was
22]
2
2
2
sent to the cell at a molar ratio of 3:1. Interestingly, IR peaks
−
1
of formate species appear at 1395 and 1374 cm at this stage
[
26]
though with a very weak intensity. Nevertheless, this observa-
tion is consistent with our result that CH accounted for less
4
than 8% of final product (Figure 5). Two CO peaks at 2175 and
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with X-ray photoelectron spectroscopy (XPS)
after etching of mesoporous silica shell
(
Figure S11, Supporting Information). Our
XPS spectra indicate that there existed mixed
valent states of Pt metal. The peak decon-
volution was conducted based on reported
0
binding energies of 4f
electron for Pt ,
7/2
2+
4+
Pt , and Pt as 71.6, 72.7, and 74.2 eV,
respectively. The surface species of Pt in
the Pt/YSZ were made up from Pt (68.6%),
Pt (19.5%), and Pt (11.9%). However, one
should note that the highly oxidized Pt spe-
cies were resulted from ambient oxidation
of laboratory air when handling the sample.
Under our actual reaction conditions (i.e.,
CO with H ), such high oxidation state Pt
[
27]
0
2+
4+
2
2
species are unlikely present.
Existence of yttrium in ZrO phase was
2
also proven to be essential to improve the
selectivity and conversion of CO2 hydro-
genation. Contrast experiments were thus
conducted to compare the conversion and
selectivity of the Pt/YSZ@ mSiO2 and Pt/
ZrO @mSiO . As can be seen in Figure 5a,
2
2
at lower temperatures, the difference
between these two catalysts is not pro-
nounced due to their low activity. However,
at higher temperatures, the Pt/YSZ@mSiO2
demonstrates both better conversion of CO2
and selectivity of CO over CH4 with the
addition of yttrium. With the addition of
yttria, zirconia could be stabilized into the
tetragonal phase which usually only exists
above 1173 °C by incorporating an trivalent
yttrium into the structure; and this tetrag-
onal (and/or cubic, Figure 1e) polymorph
has an enhanced ionic conductivity than the
[
28]
ambient temperature monoclinic form.
It is postulated that addition of yttrium
enhanced electronic interactions in cata-
lyst and thus improved the catalyst activity.
Furthermore, the slightly increased specific
surface area due to lattice incorporation of
yttrium may be counted as another factor
responsible for its better performance.
Figure 5. a) The conversion of CO and the selectivity of CO for the Pt/YSZ@mSiO catalyst
2
2
(
red lines) and the Pt/ZrO @ mSiO catalyst (blue line), and b) working stability tests of the
2 2
The Pt/YSZ@mSiO2 has been demon-
Pt/YSZ@mSiO catalyst: conversion of CO (blue line) and selectivity of CO (red line).
2
2
strated to be very stable by extending reac-
tion time at 400 °C for 30 h (Figure 5b). As
is shown in Figure 5b, the selectivity of CO was measured as
93.5% on average with a small variation below 0.2%. The con-
1
2
118 cm− were again observed with stronger intensity than
those of stage 1, confirming CO as a main product from CO2
hydrogenation. It should be mentioned that when both CO2
version rate of CO basically remains the same over the reac-
2
−
1
and H were introduced (stage 3), IR peak of CH at 3016 cm
tion period of 30 h with an average conversion of 30.8 with
a variation below 0.5%. It shows that the Pt/YSZ@mSiO2
performance toward CO2 hydrogenation remain stable at a
temperature as high as 400 °C. Furthermore, the morphology
and crystallographic phase remain unchanged after the reac-
tion, which is confirmed by both TEM and XRD characteriza-
tions (Figure S12, Supporting Information), demonstrating
the effective protection provided by this reactor configuration
2
4
(Figure 6a) became observable. The phenomenon observed on
DRIFTS is also consistent with our previous study that the for-
mate is an intermediate in the reaction path for CH formation
4
and carbonate and bicarbonate species are responsible for the
production of CO.[
12a]
The presence of Pt metal as well as its chemical states in
the above catalyst (Pt/YSZ@mSiO ) was also investigated
2
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2
.4. Cascade Reaction
The acidity from the YSZ phase and cata-
lytic activity toward different reactions from
a variety of loaded active metals enable our
reactor-like M/YSZ@ mSiO2 to serve as
a multifunctional catalyst for a plenty of
reactions. To demonstrate its capacity as
a general platform for catalysis, a one-pot
multistep conversion of nitroarene to value-
added secondary arylamine, which is widely
[
29]
used for fungicides, drugs and pesticides,
has also been explored over our Pd/
YSZ@mSiO catalyst. As can be seen from
2
Table 1, the entire tandem reaction can be
divided into 3 steps. The first step (forma-
tion of D) and third step (formation of F)
are hydrogenations catalyzed by palladium
and the second step is dehydration reaction
to form Schiff base (E) catalyzed by acid cata-
lysts. In this work, Pd was also loaded into
the YSZ@mSiO2 nanoreactor to carry out
this one-pot cascade transformation of ben-
zaldehyde (A) and nitrobenzene (B) to their
final products (C, E, and F; Table 1). Indeed,
the Pd/YSZ@mSiO catalyst shows superior
2
performance for this reaction. Under merely
ordinary pressure, the 200 °C reduced cata-
lyst showed a high conversion of reactant A
(
87%) and meanwhile realized a very high
selectivity of 80% toward the desired final
product F, achieving a higher yield than an
industrial catalyst and a bifunctional catalyst
[
29]
at 2 atm reported in literature. Apart from
the easiness of mass transport in the micro/
mesoporous Pt/YSZ, the presence of central
cavity of the Pd/YSZ@mSiO2 nanoreactor
may be able to create a centration gradient
between the liquid phase within the reactor
and the liquid phase outside the reactor. Due
[
30]
to the holding capacity of the central space,
for example, it is expected the concentration
of intermediate product E inside the reactor
must be higher than that outside the reactor,
Figure 6. a) DRIFT-IR spectra collected at 400 °C as a function of process time: red/orange thus expediting the conversion of E to F.
lines, yellow/green lines, and light-blue/blue lines show the spectral results when CO (stage 1,
2
The 300 °C reduced Pd/YSZ@mSiO2 also
demonstrated a slightly worse but still better
yield than aforementioned contrasts (entries
−
1
−1
−1
3
6
0 mL min ), N (stage 2, 30 mL min ), and CO /H (stage 3, CO at 20 mL min and H at
0 mL min ) were passing through the cell, respectively, and b) three representative DRIFT-IR
2 2 2 2 2
−
1
spectra selected from each purging stage. Peak values in wavenumber indicated in (b) were
highly repeatable in eight consecutive runs, including those with a very small peak intensity.
1
and 2), which is probably due to resulted
larger nanoparticles formed in the reducing
to stabilize catalytic Pt. CO hydrogenation for Pt loaded com-
process. In a catalyst stability test, the Pd/YSZ@mSiO was
2
2
mercial YSZ with same Pt content was also conducted as ref-
erence of which performance is similar to our catalyst (see
Figure S13, Supporting Information). Similarly, nanoscale
recycled for 3 times and no obvious activity deterioration was
observed (Figure S15, Supporting Information). Also, no mor-
phological degradation of the spent catalyst was found with our
TEM examination (Figure S16, Supporting Information).
In comparison, as reported in Figure 7, a similar catalytic
nanostructure, YSZ@Pd@mSiO , was synthesized using NH -
Ru was loaded to the YSZ@mSiO using the same synthetic
2
method as that used in the Pt/YSZ@mSiO and the resultant
2
Ru/YSZ@mSiO also performed well in CO hydrogenation
2
2
2
2
but with a high selectivity for CH (Figure S14, Supporting
functionalized UiO-66 as a core precursor. First, NH -UiO-66
4
2
Information) instead of CO.
nanocrystals were synthesized based on a previously reported
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Table 1. Cascade reaction of nitrobenzene and benzaldehyde to N-benzylaniline.
configuration. This synthetic strategy endows
the resultant solid superacid catalyst with
high specific surface area and hierarchical
porosity, which provides rich accessible Lewis
acid and Brønsted acid sites and easy mass
transports for many chemical reactions.
For example, our YSZ@mSiO can actively
2
catalyze the esterification of acetic acid and
ethanol and Friedel-Crafts alkylation reac-
tions. Furthermore, ultrafine metal NPs can
be homogenously distributed in the YSZ
phase and the synergistic effect can be built
between the catalytic metals and YSZ sup-
port. By loading various metals, furthermore,
a great number of reactions can be conducted
in our M/YSZ@mSiO . With the loading of
2
Pt, for instance, hydrogenation of CO can be
2
realized with high conversion and high selec-
tivity of CO. On the other hand, replacing
Entry
Catalyst
Pressure Conversion of A [%]
Selectivity [%]
F
E
1
C
Pt with Ru for the same CO hydrogenation
2
1
2
3
4
5
6
Pd@MIL-101[29]
Pd@Al2O3[29]
2 bar
2 bar
100
67
87
98
66
<1
56
53
80
64
80
0
43
27
13
33
13
0
leads to the high selectivity of CH . After
4
20
7
loaded with Pd, a one-pot three-step tandem
reaction for direct conversion of nitroben-
zene and benzaldehyde to N-benzalaniline
exhibits remarkable catalyst activity and
selectivity to the targeted product; this pro-
cess can be viewed as a greener approach, as
it significantly simplifies operational require-
Pd/YSZ@mSiO (200 °C)
2
1 atm
1 atm
1 atm
1 atm
Pd/YSZ@mSiO (300 °C)
2
3
YSZ@Pd@mSiO2
7
Pd/bulk YSZ
<1
Notes: For entries 1 and 2,[29] the catalytic reaction was performed in a 10 mL Teflon-lined stainless-steel
autoclave equipped with pressure gauge and a magnetic stirrer. A 20 mg portion of dried catalyst was dis-
persed in 4 mL of ethanol containing 1 mmol of nitrobenzene and 1 mmol benzaldehyde. The vessel was
purged with H2 10 times and pressurized to 2 bar for the reaction under room temperature. For entries 3–5,
same amount of reactants and catalyst were used and the reaction was conducted in round bottle flask with
stirrer which is purged with H2 and connected to a balloon full of H2. The catalyst Pd/YSZ@mSiO2 (200 °C)
and Pd/YSZ@mSiO2 (300 °C) were reduced in H2 at 200 and 300 °C respectively. The Pd content for all the
entries were ≈1 wt%. Conversion of A was analyzed by GC.
ments. Our M/YSZ@mSiO have also dem-
2
onstrated high working stability because of
formation of stable YSZ and space confine-
ment of mSiO shell which prevents metal
2
catalysts from sintering. Their workability
in harsh reaction environments such as at
temperatures up to 700 °C and extreme pH
values of 0–13 will allow this class of reactor-
method with minor modification (see the Experimental Section
in the Supporting Information). Then Pd NPs were anchored
like catalysts to meet future challenges in development of new
catalyst technology in the field of sustainable heterogeneous
catalysis.
[31]
on the surface of NH -UiO-66 nanocrystals utilizing affinity of
2

NH groups (from the organic linker) to Pd (Figure 7a). After
the encapsulation of mesoporous silica shell (Figure 7b), calci-
nation was conducted, followed by the impregnation of yttrium
2
(
see the Experimental Section in the Supporting Information). In 4. Experimental Section
the YSZ@Pd@mSiO produced from this process (Figure 7c),
2
Materials: Zirconium chloride (Merck, 98%), terephthalic acid
however, the Pd NPs were not distributed inside the micro/
mesopores of YSZ and thus there is no size confinement for Pd
NPs, which leads to metal sintering after calcination and fur-
ther metal agglomeration after the cascade reaction (Figure 7d).
Because of the process and structural disadvantages in this cata-
(
H BDC, Aldrich), acetic acid (Merck, 99.8%), sodium hydroxide
2
(Merck, 99%), dimethylformamide (DMF, VWR Chemicals, 99.90%),
2-methylimidazole (2-MeIM, Aldrich, 99%), CTAC (Sigma, 25 wt%),
tetraethyl orthosilicate (TEOS, Aldrich, ≥99.0%), yttrium (III) nitrate
hydrate (STREM Chemicals, 99.9%), ethylenediamine (EDA, Alfa
Aesar, 99%), palladium chloride (PdCl , Aldrich, 99.9%), potassium
tetrachloroplatinate (II) (K PtCl , Aldrich, 98%), ruthenium chloride
2
lyst, both stability and activity of the YSZ@Pd@mSiO are low,
2
2
4
as indicated by the TEM study of Figure 7d and its low conver-
(Aldrich, 99.98%), zirconium oxide (Dupont), ammonia solution (Merck,
sion rate (66% conversion of A) in entry 5 of Table 1.
25%), nitrobenzene (TCI, 99.5%), benzaldehyde (Sigma-Aldrich, 99.5%),
hexadecane (Sigma-Aldrich, 99%), pyridine (Sigma, 99.8%), methanol
(VWR Chemicals, analytical reagent grade), ethanol (VWR Chemicals,
analytical reagent grade). Deionized water was used for all experiments.
Synthesis of SiO2 Spheres: The SiO2 spheres were synthesized by
a reported method with modification.[ Briefly, 3 mL of tetraethyl
orthosilicate was dissolved in a mixture of 6 mL of water and 40 mL of
ethanol, followed by the addition of 1 mL of 32 wt% ammonia solution
and 6 h of stirring at ambient temperature. The SiO2 spheres were
3
. Conclusion
28]
In summary, using shape-controlled UiO-66 as a solid pre-
cursor, we have synthesized a highly porous YSZ phase encap-
sulated inside a mesoporous silica shell, obtaining a reactor-like
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without adding yttrium. The calcination procedure
was the same as that in the synthesis of the
YSZ@mSiO catalyst system.
2
Synthesis of Micro/Mesoporous YSZ: Single-phase
micro/mesoporous YSZ powder was produced by
removing the mSiO2 shell from the YSZ@mSiO2
with NaOH that served as an etchant. Typically,
5
0 mg of YSZ@mSiO was added to 10 mL of 1.0 m
2
NaOH aqueous solution. The mixture was stirred
under room temperature for 24 h to ensure full
etching of mesoporous silica shell.
Synthesis of M/YSZ@mSiO2: 200 µL of
−
3
2
0 × 10 m metal precursor (e.g., PdCl , RuCl , and
2
2 2
K PtCl ) aqueous solution was added to 100 mg
4
of YSZ@mSiO2 in an alumina crucible. Then the
crucible was placed in 60 °C oven for 1 h for drying
and then calcined in air at 350 °C for 2 h with a
−
1
ramping rate of 2 °C min . Note that the catalytic
NPs of metals in the nanoreactors could be reduced
to their metallic state in H at 200−300 °C for 2 h
2
−
1
with a ramping rate of 2 °C min .
Synthesis of Pt/ZrO @mSiO : Preparation
2
2
procedure of the Pt/ZrO @mSiO2 catalyst was
2
the same as the above, except for using the
ZrO @mSiO as a catalyst carrier.
2
2
Pyridine Adsorption: The Lewis and Brønsted acid
sites were investigated using pyridine as a probe
molecule. The YSZ@mSiO2 was first placed under
dynamic vacuum (≈8 Torr) at 200 °C for 3 h to remove
adsorbed molecules on surface. After cooling down to
room temperature, excessive pyridine was evaporated
to process the sample and the sample was placed
back to static vacuum at room temperature for 1 h.
Pyridine adsorption was analyzed by in situ DRIFT-IR
spectroscopy. The cell containing sample was purged
with nitrogen before measurement and heated to
Figure 7. TEM images of a) the NH -UiO-66@Pd, b) the NH -UiO-66@Pd@mSiO , c) the
2
2
2
YSZ@Pd@mSiO , and d) the YSZ@Pd@mSiO after the cascade reaction.
2
2
200 °C to ensure no secondary contamination.
separated by centrifugation and washed with 1:1 ethanol/ water solution.
The product was then dispersed in deionized water at a concentration of
Esterification: Typically, 0.05 mole of acetic acid, 0.5 mole of ethanol,
and 0.005 mole of hexadecane (an internal standard) were mixed and
50 mg of catalysts were added to the mixture solution, respectively.
Esterification reaction was held in a glass flask inside an oil bath heated
at 70 °C for 2 h. Samples of the reaction mixture were periodically
withdrawn from the flask and analyzed using gas chromatography (GC;
Agilent-7890A, coupled with a flame ion detector (FID) and an HP-5
column; the temperature was programmed from 70 to 280 °C).
Friedel-Crafts Alkylation: Benzylation of toluene using the YSZ@mSiO2
catalyst was conducted with a 100 mL three-neck round-bottom glass
flask which was mounted with a reflux condenser. Benzyl alcohol
(5.0 mL), toluene (50.0 mL), and hexadecane (0.5 mL, as an internal
standard) were added to the flask reactor with the YSZ@mSiO2 catalyst
(0.50 g) in each test run. Nitrogen was introduced to the reactor through
one of the glass necks. The second glass neck was equipped with a
septum for sample removal. The reaction mixture was heated to 130 °C
in an oil bath for a desired period of time. Samples of the reaction
mixture were periodically withdrawn from the flask and analyzed using
the same GC system as above.
−
1
1
mol L .
Synthesis of UiO-66: The UiO-66 nanocrystals were synthesized by a
reported method with some minor modification.[ Briefly, 133.7 mg of
32]
zirconium chloride and 100 mg of terephthalic acid were dissolved in
4
0 mL of DMF in a 100 mL glass flask and then 3 mL of acetic acid
was added. The mixture was sonicated for 5 min and then heated in
oil bath at 130 °C for 24 h. The nanocrystal product was separated by
centrifugation and washed with methanol for three times.
Synthesis of UiO-66@mSiO : 100 mg of the above prepared UiO-66
2
nanocrystals was dispersed in 60 mL of deionized water with 5 min
−
1
sonication. Then 50 mL of 10 g L 2-MeIM methanolic solution was
mixed with the turbid UiO-66 nanocrystal suspension and stirred for
5
min, after which, 1.1 mL of CTAC solution (25 wt%) was added and
stirred for another 5 min. Finally, 0.6 mL of TEOS (99%) was added
dropwise in 5 min and the mixture was further stirred for 3 h.
Introduction of yttrium to UiO-66@mSiO : 580 mg of yttrium (III)
2
nitrate hydrate and 182.1 mg of EDA were dissolved in 100 mL of ethanol
to produce an yttrium nitrate suspension. In this impregnation process,
CO2 Hydrogenation: To start with, the Pt/YSZ@mSiO2 catalyst
was reduced at 200 °C in a tube-furnace for 3 h with a ramping rate
2
1
5 mg of UiO-66@mSiO was placed in an alumina crucible and then
50 µL of the above yttrium-nitrate-EDA-ethanol suspension was added
2
−
1
−1
of 3 °C min under a constant flow of H2 gas at 50 mL min and
1 atm. After cooling down to room temperature, 170 mg of the reduced
catalyst was loaded immediately into a fixed bed reactor (3/8 stainless
steel tube). Hydrogenation of CO2 was then carried out in continuous
mode with a mixed gas stream (CO2/H2/N2 = 24%/72%/4%; N2 was
used as an internal standard). The gas feed was monitored by a Brooks
mass-flow controller, and reaction temperature was gauged with a
thermocouple pointed at the center of the catalyst bed. The pressure
was controlled by a backpressure regulator. Concentrations of reactants
and products were measured online by a GC system (Agilent-7890A,
slowly to submerge all the powder. Then the crucible was dried in an
electrical oven at 60 °C for 3 h.
Synthesis of YSZ@mSiO : The above UiO-66@mSiO2 core–shell
2
precursor after the yttrium impregnation was heated inside an electric
−
1
furnace with a ramping rate of 10 °C min , calcined at 700 °C for 2 h,
and then cooled down to room temperature naturally.
Synthesis of ZrO @mSiO : Similar core–shell structure of pure porous
2
2
ZrO2 encapsulated inside the mSiO2 shell (namely, the ZrO @mSiO )
2
2
was produced through direct calcination of the UiO-66@mSiO sample
2
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equipped with both FID and thermal conductivity detector (TCD)).
Unless otherwise specified, all reaction data were measured at steady
state, e.g., after 30 min to 60 min on stream.
Cascade Reaction: The catalytic reaction was performed in a 50 mL
glass flask. Briefly, a 20 mg portion of dried Pd/YSZ@mSiO2 was
dispersed in 10 mL of ethanol containing 1.0 mmol of nitrobenzene and
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[
[
achieve homogenous mixing. The glass vessel was purged first with a
1
flowing H for 3 min, and then connected to a hydrogen balloon under
2
normal atmospheric pressure. The reaction was conducted at room
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−
1
Each spectrum was recorded at 4 cm resolution and related spectral
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