MOF derived mesoporous K-ZrO2 with enhanced basic catalytic performance for Knoevenagel condensations

Article abstract of DOI:10.1039/c7ra12378g

Mesoporous K-ZrO₂ are designed and synthesized through a direct heat-treatment process of a KNO₃ loaded UiO-66 metal organic framework. Very interestingly, the carbon intermediates formed during the heat-treatment process can act bot

Full text of DOI:10.1039/c7ra12378g

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MOF derived mesoporous K-ZrO2 with enhanced
basic catalytic performance for Knoevenagel
condensations†
Cite this: RSC Adv., 2017, 7, 55920
ab
cd
Jian Feng,ab Yupei Zhao,ab Sai Guc and Jian Liu
*
*
Peng Wang,
Mesoporous K-ZrO2 are designed and synthesized through a direct heat-treatment process of a KNO3
loaded UiO-66 metal organic framework. Very interestingly, the carbon intermediates formed during the
heat-treatment process can act both as mesoporous templates and base-resistant reinforcement for
zirconia. The resultant mesoporous K-ZrO2 catalysts with high surface area show excellent catalytic
performance in Knoevenagel condensations, especially for substrates with large molecular size. The
mesoporous KZ also show enhanced activity when compared to KZ synthesized from traditional
zirconium hydroxides. To the best of our knowledge, this is the first synthesis of a metal-oxide type solid
base with a MOF precursor.
Received 13th November 2017
Accepted 5th December 2017
DOI: 10.1039/c7ra12378g
rsc.li/rsc-advances
ZrO2 consists of immobilization of base precursors on zirconia
(e.g. alkali metal nitrate, KNO3) and succedent heat-treatment to
Introduction
convert base precursors to their corresponding metal oxides
(e.g. K2O).8 Considering that zirconia derived from precipitation
method usually possess low surface areas, new efforts have been
focused on the immobilization of K+/ZrO2 (KZ) on mesoporous
silica and inducing mesopores into zirconia with the aid of
block copolymers as porogens.9,10 Well-dispersed basic catalytic
sites can be produced with enhanced surface areas, which
improve the diffusion rate of reactants and products, as well as
catalytic performance of the solid base. However, it remains
a challenge to design and fabricate mesoporous KZ with high
porosity and surface area, while holding the strong basic
species on zirconia's external surface.
In the past ve years, extensive attention has been paid to
metal–organic frameworks (MOFs) derived nanostructures
through a thermal decomposition process.11,12 Due to their
porosity and long-range ordering, precise location of catalytic
active sites, MOFs exhibit unique advantages over other
precursors in the eld of solid-state synthesis of porous metal
oxides, while highly porous MOFs are an ideal host matrix for
loading with active guest components.13,14 The MOF derived
metal oxides can show enhanced catalytic performance
compared to traditional precipitated metal-oxides, due to their
higher surface areas and more exposed active sites.15 Therefore,
it would be interesting and worthwhile to explore the possibility
to synthesize mesoporous KZ from potassium loaded zirconium
based MOFs (e.g. UiO-66) by using similar strategy. In the light
of previous reports, KNO3 has been chosen as potassium
precursor which can generate strong basic sites on zirconia
surface during the decomposition of KNO3 under heat treat-
ment. The aqueous solution of KNO3 is neutral with the pH
value at 7.0, and the well-ordered structures of UiO-66 can be
Acid and base catalysis are very promising for industrial cata-
lytic processes.1–3 Base catalyzed reactions, such as double bond
isomerization, Aldol addition and the Tishchenko reaction, play
an important role in production of value-added ne chem-
icals.4,5 In industrial practice, the amount of base is nearly
stoichiometric, the neutralization of the reaction medium nor-
mally produces large amounts of chemical waste. Considering
the environmental issues, it is required to substitute the liquid
bases and organometallics by solid base catalysts for sustain-
able and green chemical processes.6 These solid bases can be
varied from metal oxides of mono-component, microporous or
mesoporous metal oxides (including silica) immobilized with
alkali metals and/or alkali earth metal components, as well as
multicomponent metal oxides and salts. Among various solid
base catalysts, alkali metal-doped zirconia is very attractive due
to its high melting point, low thermal conductivity, and high
corrosion resistance.7 Meanwhile, the incorporation of alkaline
metal ions (especially potassium) can generate strong basicity
on the surface of zirconia. Traditional synthesis of alkali metal/
aJiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of
Petrochemical Engineering, Changzhou University, Gehu Road 1, Changzhou,
Jiangsu 213164, People's Republic of China. E-mail: pengwang@cczu.edu.cn
bAdvanced Catalysis and Green Manufacturing Collaborative Innovation Center,
Changzhou University, Gehu Road 1, Changzhou, Jiangsu 213164, People's Republic
of China
cDepartment of Chemical and Process Engineering, University of Surrey, Guildford,
Surrey, GU2 7XH, UK. E-mail: jian.liu@surrey.ac.uk
dState Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra12378g
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ZrCl4 and 1.02 g of 1,4-benzenedicarboxylic acid (BDC) were
dissolved in 62 mL dimethyl formamide (DMF) and sealed in
a stainless-steel vessel with a Teon liner. The solution was then
ꢀ
put into a pre-heated oven at 120 C. Aer 24 h, the oven was
naturally cooled to room temperature. The activation of UiO-66
was performed by cascade reuxing with DMF and methanol,
and ꢀthe obtained MOF powders were then put in vacuum at
130 C.
Synthesis of KNO3@UiO-66
For a typical synthesis, 800 mg UiO-66 suspended in 100 mL of
dry n-heptane as a hydrophobic solvent and the mixture was
sonicated for 15 min until it became homogeneous solution.
Aer stirred of 2 h at 50 ꢀC, 0.8 mL of aqueous potassium nitrate
solution of different concentrations as the hydrophilic solvent
was added drop-wise over a period of 15 min with constant
vigorous stirring. The resulting solution was continuously stir-
red for 8 h. Aer careful ltration, the white powder was dried in
Scheme 1 Illustrations for synthesis of mesoporous KZ from KNO3-
loaded UiO-66 with the two-step heat treatment.
ꢀ
air at 100 C. With the adding amounts of 60 mg, 120 mg and
180 mg KNO3, obtained KNO3@UiO-66 samples were denoted
as UiO-66-1K, UiO-66-2K and UiO-66-3K, respectively, and pure
UiO-66 was denoted as UiO-66-0K.
kept in the immersion process for loading KNO3 into the inner
pores of UiO-66. However, a possible predicament in synthesis
of KZ with mesopores is the stability of Zr6O4(OH)4 nodes of
UiO-66 against the corrosion of newborn strongly basic species
during calcination.16 In short, there are more obstacles to be
overcome for the synthesis of mesoporous KZ with UiO-66 as
precursors, and the reinforcement in stability of ZrO2's frame-
works should be realized to x the mesopores created during
calcination of KNO3 loaded UiO-66.
Due to the highly ordered crystalline structures, MOFs are
proved to be suitable precursors for producing carbon hybrid
metal oxides with uniformly distributed components, and the
organic ligands in MOFs can be converted into carbon layers
which act as the protect shells for metal oxides and provide the
possibility to avoid the corrosion of strong base species.17,18 The
homogeneity of carbon distribution is also of great importance
as it plays a critical role in stabilization the pore structures of
metal oxides, even aer removal of the carbon layers with mild
calcination. In other words, carbon can act as porous templates
for residual metal oxides while the carbon/metal oxides hybrid
materials are heated under air with proper procedures. Previous
research has proved that carbon can inhibit collapse of the
frameworks of iron oxides and produce mesopores for iron-
oxides while MOF derived FexO/C hybrids are calcinated
under air.19 Inspiration from this strategy, and considering that
carbon are base-resistant materials, a two-step heat-treatment
method is adopted to prepare mesoporous KZ from KNO3
immobilized UiO-66 (Scheme 1, approach A). These KZ mate-
rials are characterized with a battery of techniques, and tested
in Knoevenagel condensations with substrates of different
molecular size.
Synthesis of K doped zirconia (KZ) and pure zirconia
For the thermal decomposition process, the as-obtained
KNO3@UiO-66 was calcined rstly at different temperatures in
nitrogen for 3 h, followed by heat treatment at 400 ꢀC under air
for 6 h. As obtained KZ were denoted as KZ-x-y-D (x and y stand
for contents of K and the rst calcination temperature under N2
respectively, and two represent the double-step heat treatment).
For comparison, KZ prepared with directly calcination under air
with single step were denoted as KZ-x-y-S. For example, UiO-66
loaded with 120 mg KNO3 resulted in KZ-8.2-650-D while treated
at 650 ꢀC under N2 rstly. Pure zirconia was synthesized by
thermal decomposition of UiO-66 under air at 650 ꢀC with one-
pot synthesis, and this sample was named as ZrO2-650-S.
Synthesis of KZ from impregnation of zirconium hydroxide
The catalyst was synthesized by incipient wetness impregnation
of an aqueous solution of desired amounts (K contents for ca.
8.2 wt%) of potassium nitrate into amorphous zirconium oxy-
ꢀ
hydroxide followed by calcination at 650 C under air for 6 h.
The as-obtained catalyst was denoted as KZ-8.2-650-R as refer-
ence materials.
Materials characterization
XRD analysis was performed using a Rigaku D/MAX-2500 V/PV
diffractometer with Cu-Ka radiation (40 kV and 200 mA) at
a scanning speeꢀd of 5ꢀ minꢁ1. Physisorption of N2 was per-
formed at ꢁ196 C using a Quantachrome Nova 1200e. Before
measurement, the sample was evacuated at 200 ꢀC for 3 h. The
surface area was calcinated by the BET method at a relative
pressure of P/P0 ¼ 0.05–0.25. Fourier transform infrared (FT-IR)
Experimental
Synthesis of UiO-66
UiO-66 was prepared following a modied procedure as spectra of the samples were collected in transmission mode
described in literature.16 In a typical synthetic process, 1.4 g of from KBr pellets at room temperature on a Bruker Tensor 27
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spectrometer. Inductively coupled plasma atomic emission (TGA) curve of carbon hybrid KZ composite (Fig. S1†). At 400 ꢀC,
spectroscopy (ICP-AES) was carried out to determine the carbon layers in KZ/C can be eliminated with calcination under
concentration of K in the KZ materials. Prior to the analysis, KZ air. For comparison, KNO3@UiO-66 was calcinated directly at
samples were digested in HF (10 wt%, a.q.) solution with different temperatures under air, and the as-synthesized KZ are
microwave radiation. CO2 temperature-programmed desorp- denoted as KZ-x-y-S (x and y represent for the contents of K and
tion (CO2-TPD) experiments were conducted using a Quantach- calcination temperature respectively, and S represents the
rome ChemBET-3000 analyser. About 200 mg of the sample was single-step heat treatment, Scheme 1, approach B). Pure
pre-treated at 450 C for 1 h under dry He ow (30 mL minꢁ1), zirconiꢀa was also synthesized by calcination of UiO-66 under air
ꢀ
cooled to 50 ꢀC, and then exposed to CO2 for 0.5 h. Aer purging at 650 C, which is denoted as ZrO2-650-S.
the sample with He for 0.5 h, the TPD data were recorded from
The porous structures of KZ-x-y are analyzed with nitrogen
sorption techniques. For KZ-x-y-S, all these samples show
typical non-porous adsorption–desorption isotherms with only
accumulative pores at high relative pressure (P/P0 > 0.8,
Fig. 1A).20 In comparison, the sorption isotherms of KZ-x-y-D
exhibit type IV isotherm with a small hysteresis loop in the
range between 0.4 and 0.8 for P/P0, indicating the presence of
mesopores (seen in detail, Fig. S2†),21,22 and accumulative pores
between particles of KZ also exist (Fig. 1B). These materials have
relatively high BET surface areas between 48 to 84 m2 gꢁ1 (Table
1), which are almost ten times of surface areas for KZ-x-y-S
series (Table S1†). Such high surface areas are believed to
benet from the two-step calcination of the MOF template. The
carbon generated at the rst calcination in N2 can act as
a temporal buffer, hindering the further contraction of mate-
rials. Moreover, as base-stable materials, carbon layer can
protect KZ's frameworks from collapsed by solid basic species
which are formed during decomposition of KNO3 (see below).
Nevertheless, KZ-x-y-S series have very low surface areas (below
10 m2 gꢁ1 for all samples, Table S1†) as nonporous materials. In
addition, pure zirconia (ZrO2-650-S) has a high BET surface area
of 73 m2 gꢁ1 (Fig. S3†). A possible explanation is that the fragile
Zr6O4(OH)4 nodes of UiO-66 may be unable to resist the inva-
sion of newborn strongly basic species during calcination and
the frameworks of zirconia will collapse to form the nonporous
structure of obtained KZ. For UiO-66 without KNO3, the pore
structure has been kept without corrosion of strong basic
species.
ꢁ1
ꢀ
50 to 800 ꢀC with a ramping rate of 10 C min
.
Catalytic reactions
Knoevenagel condensations of benzaldehyde (BA) and
ethylcyanoacetate (ECA)
A 25 mL round-bottom ask was charged with the catalyst (20
mg) in methanol (3 mL), BA (5 mmol) and ECA (6 mmol) while
isooctane was used internal standard. The reaction mixture was
stirred for 30 min at 35 ꢀC. The solution sample was diluted by
ethyl acetate, centrifuged at 10 000 rpm for 2 min, and analyzed
by GC (Thermo gas chromatograph equipped with a ame
ionization detector and an HP-5 capillary column) to determine
the conversion of benzaldehyde, and little by-product (such as
benzoic acid) was detected. The catalyst solids were subjected to
thermal dehydration at 150 ꢀC for 3 h before their use. Recycled
catalysts were washed with ethanol before the next using.
Contrast experiments between KZ-8.2-650-D and KZ-8.2-650-S
500 mg catalysts were immersed in 60 mL methanol with
100 mmol BA or 50 mmol p-phenyl benzaldehyde and 120 mmol
ꢀ
ECA. The reaction was proceeded at 35 C and 1 mL samples
were taken from the bottle to determine the conversion of BA.
Knoevenagel condensations of BA and diethyl malonate
(DEM)
BA (2 mmol) were mixed with 20 mmol DEM with 50 mg cata-
lysts, and the reaction were carried on at 120 C for 24 h.
The evolution of KNO3 into KZ under heat treatment was also
investigated with TG and DSC. Fig. 2 depict TG and DSC curves
of UiO-66 loaded with different amounts of KNO3. Different
from UiO-66 which decomposed into carbon hybrid zirconia
between 500 and 600 ꢀC, the DSC curves show a much wider
peak between the range from 450 to 650 ꢀC which demonstrates
ꢀ
Results and discussion
Structural characteristics of solid bases
For a typical synthesis procedure for porous KZ, different
concentration of KNO3 aqueous solutions were injected into
heptane-immersed UiO-66 with a double-solvent method, KNO3
was pumped into inner pores of UiO-66 with the driving force of
hydrophilic–hydrophilic interactions.15 The obtained KNO3
loaded UiO-66 was rstꢀly treated at different temperatures
ranging from 550 to 750 C under N2 to decompose KNO3 and
form the carbon-layer protector. The carbon hybrid KZ was
further calcinated under air at 400 ꢀC to obtain ultimate product
of white KZ-x-y-D (y stands for the rst calcination temperature
under N2, and D stands for double times of heat treatments on
the samples). It is a remarkable fact that 400 ꢀC is chosen as the
temperature to decompose carbon residue, and this tempera-
Fig.
1
Nitrogen sorption isotherms of KZ-x-y-S and KZ-x-y-D
ꢁ1 STP).
samples (for comparison, the scope of quantity adsorbed are
ture is determined on the base of thermogravimetric analysis normalized to 350 cm3
g
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Table 1 Porosity, composites and basic properties of KZ prepared under different conditions
Con. of Ka (%)
SBETb (m2 gꢁ1
)
Bweak (mmol gꢁ1
)
Bstrong (mmol gꢁ1
)
c
c
Sample
KZ-4.3-650-D
KZ-8.2-650-D
KZ-11.2-650-D
KZ-8.2-650-S
KZ-8.2-750-D
KZ-8.2-550-D
KZ-8.2-650-R
4.3
8.2
73
80
51
6
49
84
4
0.24
0.50
0.44
0.11
0.25
0.42
0.09
0.09
0.41
0.17
0.07
0.23
0.18
0.08
11.2
8.2d
8.2d
8.2d
8.2d
a
b
Contents of K (in weight) are determined with ICP-AES. BET specic surface areas are determined with adsorption data in a relative pressure
c
range of P/P0 ¼ 0.05–0.25. Base amounts are determined with CO2-TPD data and 400 ꢀC is chosen as the watershed point to discriminate
between weak and strong basic sites. d Contents of K are similar to that of KZ-8.2-650-D.
around 100 ꢀC (Fig. S4†), and this result proves complete
removal of carbon protector at 400 ꢀC under air. Further
evidence for elimination of carbon is provided with FT-IR
spectra of KZ-x-y-D (Fig. S5†). The absence of C–H stretching
vibrations around 2900 cmꢁ1 for KZ-x-y-D series indicates the
almost complete removal of carbon matrix in mesoporous KZ.24
The XRD patterns of KZ-x-y-D samples are shown in Fig. 3 to
distinguish two crystal phases existing in ZrO2. Neither char-
acteristic peaks of KNO3 nor crystallized compound, such as
K2ZrO3 is detected in these samples. With the increasing
Fig. 2 TG and DSC profiles of UiO-66 loaded with different amounts
of KNO3 and pure KNO3.
contents of K species, the intensity of peaks for the tetragonal
form of zirconia raises as the major phase. Only KZ-4.3-650-D
with the lowest loading amounts of K species exhibits an
that KNO3 decomposition have the interaction with decompo-
obvious mixture of tetragonal (t-ZrO2) and monoclinic (m-ZrO2)
sition of UiO-66. It proves that high temperature treatment of
phase, and it proves that potassium species play a key role in
KNO3 loaded UiO-66 under N2 can convert KNO3 into supported
stabling tetragonal phase of zirconia.25 Due to previous report,
solid base species, as well as transform UiO-66 into carbon
K+ ions are inferred to occupy the position of octa-coordinated
hybrid KZ. Furthermore, different from pure KNO3 with
Zr4+ in t-ZrO2, which is more stable than the hepta-coordinated
a
higher decomposition temperature (>700 ꢀC), UiO-66
Zr4+ in m-ZrO2 because of the interaction of K salt with
improved the decomposition of KNO3 into supported basic
species at moderate temperatures, which is similar with the
promotion effect of ZrO2 as previous reported.23 The TGA curve
of the nal KZ-x-y-D shows no weight loss when heating to
ꢀ
900 C expect for the desorption of physically adsorbed water
Fig. 4 HAADF (A) and EDX mapping (B) of KZ-8.2-650-D; EDX maps
evidencing the macroscopic distribution of (C) K and (D) Zr.
Fig. 3 XRD patterns of KZ-x-y-D samples (m stands for monoclinic
phase of ZrO2).
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Fig.
6
Catalytic performance of different aldehydes with ethyl-
cyanoacetate over KZ-8.2-650-D. Reaction conditions: aldehyde (5
mmol), ethylcyanoacetate (6 mmol), catalyst (20 mg), methanol (3 mL),
408 K, 30 min. Conversions are determined by GC.
Fig. 5 CO2-TPD profiles (A, C) and conversions of BA (B, D) with KZ-x-
y-D of different calcination temperature and contents of potassium.
Little side product has been produced during the reactions, and the
selectivity towards Knoevenagel condensation product is around 95%.
On the other hand, there is a similar tendency in CO2-TPD
proles with different calcination temperature. When treated at
650 ꢀC under N2, KZ-8.2-650-D (8.2%) shows maximum
amounts of both weak and strong basic sites. According to TG
ꢀ
proles of KNO3 loaded UiO-66, heat treatment at 550 C can't
decompose KNO3 completely, and KZ-8.2-550-D shows reduced
amounts of CO2-TPD capacity to KZ-8.2-650-D aꢀs a result.
Further increasing calcination temperature to 750 C leads to
shrinkage of KZ pore structure with the presence of strong base,
and it results in decreasing BET surface areas as well as degree
of exposure for basic potassium species. On the contrary,
nonporous KZ-x-y-S exhibit much lower amounts of basic sites
and this result goes a step further to prove the proportional
relationship between BET surface areas and the exposure
degree of basic sites. Potassium species play the key role in
inducing strong basic sites on the surface of KZ, and there are
a few weak basic sites on the surface of pure zirconia, due to the
CO2-TPD prole of ZrO2-650-S (Fig. S6A†).
zirconia.7 Parts of potassium ions incorporate into the lattice of
ZrO2 to stabilize tetragonal crystal form of zirconia. The
uniform distribution of potassium on zirconium was further
conrmed by energy dispersive X-ray (EDX) spectroscopy with
transmission electron microscope (TEM, Fig. 4).
The basic strength and the relative amount of surface basic
sites on KZ samples are studied by CO2-TPD in Fig. 5. There are
mainly two kinds of desorption peaks for each mesoporous KZ
sample, which indicates that two types of adsorption sites with
different basicity coexist on the surface of the solid base.26
Taking KꢀZ-8.2-650-D as an example, the broad peak between 100
and 300 C can be ascribed to the weak basic sites of intrinsic
zirconia surface as previous report, while the peaks located
ꢀ
between 450 and 650 C are attributed to the strong basic site
caused by the introduction of K+ species to zirconia. It is
noticeable that the KZ-8.2-y-D series show an intense peak
caused by strongly basic sites in addition to weak ones. More-
Catalytic performance in Knoevenagel condensations
Basic catalytic ability for KZ-x-y-D series are evaluated in
Knoevenagel condensation. Knoevenagel condensations
between carbonylic compounds and methylene malonic esters
produce several important key products employed in the
synthesis of some therapeutic drugs and pharmacological
products.28 Furthermore, catalytic performance of methylene
compounds with different acidic character (pKa) can distinguish
status of basic sites with different basic strength. Methylene
compounds with higher pKa values need catalysts of elevated
basic strength to deprotonate methylene groups.29 In this
research, ethylcyanoacetate (ECA, pKa ¼ 9) and diethyl malonate
(DEM, pKa ¼ 13.3) are tested in condensation with benzalde-
hyde (BA). Coincide with results of CO2-TPD (Fig. 5), KZ-8.2-650-
D shows highest conversion of BA in reactions with above two
methylene substrates. It should be mentioned that KZ-8.2-750-D
shows enhanced catalytic performance in condensation of BA
and DEM than KZ-8.2-550-D and high temperature treatment
ꢀ
over, the desorption of CO2 temperature persists up to 650 C,
which is comparable to some solid superbases, thus revealing
the presence of strong basicity on the KZ-8.2-y-D samples.27
According to previous report, the strong basic sites of KZ were
attributed to overlapped structure of potassium species on
surface of KZ, including K2O extra-ne particles formed on the
composites.7 The amounts of basic sites due to CO2-TPD shows
a maximum capacity with K contents at 8.2% in weight (Table
1). While increasing loading amounts of K from 8.2% to 11.2%,
both amounts of weak and strong basic sites decrease. There are
two possible reasons: (1) too much K species can lead to over-
lapping of basic sites on zirconia's surface and decrease in BET
surface areas for KZ as illustrated in table (KZ-11.2-650-D); (2)
excessive basic species can also corrode the pore structure of KZ
and sequentially bury some K+ ions in the frameworks of ZrO2.
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plays a key role in forming enough strong basic sites. On the
contrary, KZ-8.2-550-D exhibits better catalytic performance in
reactions between BA and ECA, and it is attributed to higher
amounts of total basic sites and larger surface areas. Not
surprisingly, KZ-4.3-550-D gets the lowest reaction results for
both substrates due to its poor basic active sites, and it proves
that enough KNO3 loading amounts in UiO-66 are critical for
fabricating strong solid bases. Furthermore, various aldehydes,
including furfural and naphthaldehyde were tested in Knoeve-
nagel condensation reactions with ECA as active methylene
compounds and KZ-8.2-650-D as catalysts under mild reactions,
and all these substrates got good conversions within just 30 min
which proved the extensive applicability for KZ-x-y-D series for
Knoevenagel condensations (Fig. 6). Although with high surface
area, ZrO2-650-S resulted in limited catalytic activity (11.2%) in
Knoevenagel condensations of BA with ECA under identical
reaction conditions (Fig. S6B†), and the selectivity of Knoeve-
nagel condensation product is also poor (47.3%) due to the
formation of plenty of benzoic acid by oxidation of BA. The
amounts of exposed basic sites, not the surface area, is critical
for the catalytic performance.
Further to conrm superiority in pore structures of KZ
samples prepared with two-step calcination, contrast experi-
ment between KZ-8.2-650-D and KZ-8.2-650-S is done to evaluate
the inuence of porosity of KZ on their catalytic performance
(Fig. 7), and aldehyde substrates with different sizes are also
used to assess the efficiency of mass transfer for KZ samples
with different pore structures. Furthermore, KZ-8.2-650-R
prepared with traditional zirconium hydroxide as supports is
also tested in this reaction as the reference material, and this
base catalyst also possesses poor porosity with limited surface
areas (Table 1). On both BET surface areas and basic sites'
amounts, KZ-8.2-650-D gets much higher scores than its single
step treated counterpart and traditional KZ-8.2-650-R. With
identical reaction condition, KZ-8.2-650-D with mesoporous
Fig. 8 Recycle experiments of KZ-8.2-650-D.
structures showed 44% conversion of BA with excessive ECA in
30 minutes while nonporous KZ-8.2-650-S just converted 24% of
BA, about half conversion of the former. KZ-8.2-650-R got
a conversion of 21%, even slightly fewer than that of KZ-8.2-650-
S. A more prominent difference was shown for condensation of
BA with p-phenyl benzaldehyde, which has a much larger
molecular size and the reaction rates were inclined to be
kinetically controlled by mass transfer efficiency on the basis of
KZs' pore structures. For KZ-8.2-650-D with superior surface
areas, the conversion of p-phenyl benzaldehyde achieved 79.0%
within 30 minutes while KZ-8.2-650-S just converted 26.5% of p-
phenyl benzaldehyde with limited porosity (only 23% for KZ-8.2-
650-R). Due to previous report, higher degree of exposure with
catalytic sites on larger surface can facilitate diffusion of reac-
tants and products, so as to enhance catalytic performance with
more accessible catalytic centers.30,31 Furthermore, substrates
with larger molecular sizes are more likely to be inuenced by
KZs' pore structures due to critical role of mass transfer during
catalytic reactions. The catalyst's stability is tested with recy-
cling experiments and KZ-8.2-650-D maintains its original
reactivity for ve cascade running cycles with just slightly
reducing in BA's conversion (Fig. 8). The same stability is also
observed in porosity of KZ-8.2-650-D (Fig. S7†). Mesoporous
KZ's performance is among the best recycling ability for alkali
metal doped metal oxides as solid bases.32
Conclusions
In this report, mesoporous KZ are prepared with a two-step heat
treatment from KNO3 loaded UiO-66. To the best of our
knowledge, this is the rst time to synthesis of metal-oxide
typed solid base with MOFs as precursors. KNO3@UiO-66 is
rstly converted into carbon hybrid KZ, and carbon is converted
from organic part of UiO-66. This self-made carbon matrix can
act as the mesoporous templates, as well as strengthen the
frameworks of mesoporous KZ with high base-resistance ability.
With enhanced surface areas and easier accessible basic
centers, mesoporous KZ shows accelerated reaction rates over
their nonporous counterparts which is synthesized by single-
Fig. 7 (A) Nitrogen sorption isotherms, (B) CO2-TPD profiles and
contrast experiment of BA (C) and p-phenyl benzaldehyde (D) with
ECA between KZ-8.2-650-D, KZ-8.2-650-S and KZ-8.2-650-R (as
reference materials).
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Conflicts of interest
There are no conicts to declare.
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Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (Grant No. 21503024) and the Jiangsu Province
Science Foundation for Youth (Grant No. BK20150264,
BK20150261) is greatly appreciated. The Priority Academic
Program Development of Jiangsu Higher Education Institutions
(PAPD) and Jiangsu Key Laboratory of Advanced Catalytic
Materials and Technology (Grant No. BM2012110) are also
appreciated.
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