Kinetically controlled synthesis of two-dimensional Zr/Hf metal-organic framework nanosheets via a modulated hydrothermal approach

Article abstract of DOI:10.1039/c7ta00413c

The kinetically controlled synthesis of two-dimensional (2D) metal-organic framework (MOF) nanosheets in the absence of surfactants is rewarding but challenging. We herein describe such a surfactant-free bottom-up synthesis of 2D stable Zr/Hf MOF nanoshee

Full text of DOI:10.1039/c7ta00413c

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Kinetically controlled synthesis of two-dimensional
Zr/Hf metal–organic framework nanosheets via
a modulated hydrothermal approach†
Cite this: DOI: 10.1039/c7ta00413c
a
b
a
a
a
Zhigang Hu,
Ning Yan,
Ezwan Mahmoud Mahdi, Yongwu Peng, Yuhong Qian, Bin Zhang,
a
c
b
a
Daqiang Yuan,
Jin-Chong Tan
and Dan Zhao
*
The kinetically controlled synthesis of two-dimensional (2D) metal–organic framework (MOF) nanosheets
in the absence of surfactants is rewarding but challenging. We herein describe such a surfactant-free
bottom-up synthesis of 2D stable Zr/Hf MOF nanosheets named NUS-8 composed of Zr
6
O
4 4
(OH) or
3
ꢀ
6 4 4
Hf O (OH) clusters and 1,3,5-benzenetribenzoate (BTB ) via a modulated hydrothermal approach,
which allows fast precipitation and stabilization of intermediate 2D metal–organic nanosheets due to the
heterogeneous synthetic conditions. Structural analyses based on synchrotron powder X-ray diffraction
data confirm the 2D layered structure of NUS-8 with uniform porosity and highly accessible Lewis acid
sites suitable for heterogeneous catalysis. 2D NUS-8 nanosheets exhibit excellent stabilities superior to
those of their interlocked 3D MOF analogues synthesized from solvothermal synthesis, which are
evidenced by comprehensive stability tests. In particular, dynamic mechanical analysis (DMA)
experiments suggest that the stability of 2D NUS-8 nanosheets may come from a combination of
interlayer shear sliding deformation and out-of-plane tension/compression modes whereas their
interlocked 3D architecture is strictly constrained. Because of the alleviated framework strain and
accessible active sites, NUS-8 nanosheets exhibit excellent stability and catalytic activity superior to
those of their interlocked 3D MOF counterparts. Our work has demonstrated the potential of
a modulated hydrothermal approach in the kinetically controlled synthesis of 2D MOF nanosheets,
shedding light on future synthesis of 2D hybrid inorganic–organic materials.
Received 13th January 2017
Accepted 21st March 2017
DOI: 10.1039/c7ta00413c
rsc.li/materials-a
15,16
catalysis, electrochemistry, gas separation, etc.
Most of the
Introduction
reported 2D MOF nanosheets are based on the “top-down”
1
Since the discovery of two dimensional (2D) graphene, many exfoliation (e.g., sonication and ball milling) of bulk MOFs with
17–20
2
D materials have been widely studied including hexagonal 2D layered structures.
However, the resultant products are
boron nitride (hBN), transition metal chalcogenides (TMCs, oen a mixture of nanosheets with wide distribution of thick-
2–5
MoS
2
), silicene, phosphorene, etc.,
which have exhibited ness and lateral size, and are prone to degradation because of
unique anisotropic chemical, electronic, mechanical, and mass the strong mechanical force and harsh solvents required to
6,7
7
transport properties. During the last two decades, metal– overcome the interlayer interactions. In this context, the
organic frameworks (MOFs) have become the frontier of crys- alternative “bottom-up” strategy is more promising as it allows
talline porous materials and have been demonstrated with wide the direct growth of 2D MOF nanosheets with intact
applications including gas storage, gas separation, heteroge- morphology and controlled thickness suitable for further
8–14
21
neous catalysis, etc.
It is therefore of huge interest to explore applications. The challenges for bottom-up strategies include
2
D MOF nanosheets possessing readily accessible active sites, undesirable interpenetration of individual layers and inherently
which may demonstrate novel properties in the applications of strong interlayer-packing interactions stacking 2D layers into
22
bulk materials for lower surface energy. As a result, thermo-
dynamically controlled approaches are oen used involving
a
Department of Chemical
&
Biomolecular Engineering, National University of surfactants, which can help to reduce the surface energy and
23,24
Singapore, 117585, Singapore. E-mail: chezhao@nus.edu.sg
b
thus to stabilize the resultant nanosheets.
Since surfactants
Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ,
may block the active sites should the nanosheets be used as
catalysts, the complete removal of surfactants aer their
synthesis is necessary, which is however not so straightforward
UK
c
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002 Fujian, China
25
in most cases. Therefore, the development of a surfactant-free
bottom-up approach for the direct synthesis of 2D MOF
†
Electronic supplementary information (ESI) available: Synthesis and
characterization data. See DOI: 10.1039/c7ta00413c
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nanosheets would be more appealing to facilitate both the homogeneous synthetic condition for NUS-16. We hypothesize
synthetic procedures and purication processes, and more that heterogeneous synthetic conditions help avoid interpene-
ꢁ
importantly to leave the active sites intact for catalytic tration by fast precipitation at high temperatures (120 C) and
applications.
stabilize intermediate 2D nanosheets affording kinetically
In general, hydrothermal synthesis usually refers to hetero- controlled products. We successfully applied this modulated
geneous reactions of crystal growth in aqueous media above hydrothermal approach and obtained 2D layered MOF nano-
ꢁ
26
1
00 C and 1 bar. Currently, many MOFs are synthesized via sheets, named NUS-8(Zr/Hf), which were subjected to synchro-
27–29
this approach, such as MIL-101 and MIL-100.
However, in tron powder X-ray diffraction (PXRD) measurements for
order to avoid the harsh synthetic conditions, the synthesis of structural determination. Compared to the PXRD patterns of 3D
ꢁ
MOFs under ambient conditions (near 100 C and 1 bar) NUS-16 (Fig. S1†), the peaks of crystal plane (220) representing
becomes prevailing and more promising for scale-up purposes the interpenetration structure are missing in the resultant NUS-
30–33
nowadays.
In this work, we report a surfactant-free synthesis 8, strongly indicating the formation of 2D layered structures.
of 2D MOF nanosheets using a modulated hydrothermal Crystal models of NUS-8 featuring 2D layered structures were
approach (heterogeneous reactions in aqueous media near built based on the eclipsed stacking of monolayer grids from
ꢁ
34–36
1
00 C and 1 bar) developed previously.
Because of the NUS-16 and rened by Rietveld renement with acceptable
heterogeneous synthetic conditions, fast precipitation of inter- accuracy via a LeBail route (Fig. 2). Taking NUS-8(Zr), for
mediate products becomes possible, leading to kinetically example, the crystal structure can be best described by eclipsed
3
ꢀ
controlled MOF products named NUS-8(Zr) and NUS-8(Hf) stacking of monolayers containing 3-connected BTB ligands
featuring 2D nanosheet morphology with a thickness of 10– linked by 6-connected Zr O (OH) clusters with a pore size of ca.
6
4
4
˚
2
0 nm and planar porous structures with highly exposed Lewis 12.62 ꢂ 8.9 A (Fig. 1). The calculated interlayer distances of
39
˚
acid sites. NUS-8 nanosheets exhibit superior stability and NUS-8(Zr) and NUS-8(Hf) are 16.82 and 17.27 A, respectively.
catalytic activity over their interlocked 3D MOF analogues As seen from elemental analyses (Table S4†), the experimental
37
synthesized solvothermally.
metal to carbon weight ratio (0.87) is larger than the theoretical
one (0.84), suggesting missing BTB ligands in the structures
3
ꢀ
(
TGA results, ꢃ0.07%). In order to prove that our modulated
Results and discussion
Structural determination and morphology characterization
hydrothermal synthesis is kinetically controlled, we performed
the experiments by putting 2D NUS-8 nanosheets in DMF/acetic
solution at 120 C for 24 h. We could not observe any transitions
from 2D NUS-8 to 3D NUS-16 (Fig. S3†), conrming that 2D
NUS-8 MOFs are kinetically controlled products.
The MOF morphology was examined by eld-emission
scanning electron microscopy (FE-SEM, Fig. 3a and d, S2†).
Unlike the 3D interpenetrated NUS-16 crystals having rectan-
gular brick shape (Fig. S2a and b†), NUS-8 MOFs exhibit
nanosheet morphology, with a thickness of ca. 10–20 nm and
ꢁ
Traditional solvothermal synthesis using ZrOCl
and 1,3,5-benzenetribenzoate acid (H BTB) in DMF/acetic acid
solution resulted in a 3D structure [denoted as NUS-16(Zr/Hf) in
2 2 4
$8H O or HfCl
3
37
this study for clarity], arising from the interpenetration of 2D
3
ꢀ
porous grids containing BTB
Zr (OH) clusters (Fig. 1, S1†). Interpenetration is a result of
compensating the energy penalty by reaching a thermodynami-
linked by 6-connected
6
O
4
4
38
cally more stable state,
which is facilitated by the
Fig. 1 Schematic illustration of the synthesis of 2D MOF nanosheets (NUS-8) and 3D interpenetrated MOFs (NUS-16) (M ¼ Zr or Hf) using
hydrothermal and solvothermal approaches, respectively. The homogeneous condition of solvothermal synthesis promotes the interpenetration
of individual 2D nanosheets into a 3D network, while the heterogeneous condition of modulated hydrothermal synthesis prevents such
interpenetration by fast precipitation of intermediate 2D MOF nanosheets.
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Fig. 3 Morphology characterization of NUS-8(Zr) (a–c) and NUS-8(Hf)
(d–f) using FE-SEM (a, d), HR-TEM (b, e), and AFM (c, f).
morphology of NUS-8 MOFs, which can be attributed to the
heterogeneous synthetic conditions capable of locking the MOF
products in the kinetically favorable intermediate stage suitable
for further gas sorption and catalysis applications.
Porosity evaluation
N sorption isotherms collected at 77 K were used to evaluate
2
Fig. 2 Rietveld refinements of (a) NUS-8(Zr) and (b) NUS-8(Hf) based the surface area and porosity of the obtained MOFs. As shown in
on synchrotron PXRD data from the LeBail route.
Fig. 4a, both NUS-8(Zr) and NUS-8(Hf) exhibit hybrid Type I/IV
sorption isotherms with noticeable hysteresis between
adsorption and desorption branches, indicating their hybrid
a lateral size up to 500–1000 nm leading to high aspect ratios of micro/meso-porous textures. The micropores (pore diameter
0–100 (Fig. 3a and d, S2†). The nanosheet morphology is less than 2 nm) can be justied from the crystal structures
N
2
2
a reection of the 2D layered crystal structures of NUS-8, and (Fig. 1), while the mesopores (pore diameter between 2 and 50
can be attributed to the fast nucleation and precipitation under nm) may come from the interstitial voids of the irregular
40
heterogeneous synthetic conditions. The internal ne struc- packing of nanosheets as seen from the FE-SEM images (Fig. 3a
tures of NUS-8 crystals were investigated by high-resolution and d). In contrast, both NUS-16(Zr) and NUS-16(Hf) show Type
transmission electron microscopy (HR-TEM, Fig. 3b and e), I N
wherein highly ordered crystalline domains could be found in structures induced by interpenetration. NUS-8(Zr) has a Bru-
2
sorption isotherms, indicating their fully microporous
37
2
ꢀ1
NUS-8(Hf) with the lattice spacing of ꢃ1 nm representing the nauer–Emmett–Teller (BET) surface area of 570 m g (Table
130) crystal plane. The nanosheet morphology of NUS-8 was S2†), which is lower than the reported value of collapsed 3D
(
2
ꢀ1 37
conrmed by atomic force microscopy (AFM, Fig. 3c and f), ZrBTB (613 m g
)
and 2D Hf-BTB metal–organic layers (661
2
ꢀ1 39
wherein nanosheets as thin as ꢃ3 nm for NUS-8(Zr) and ꢃ4 nm m g ). Normally, the introduction of heavier Hf cations will
41
for NUS-8(Hf) can be directly observed, suggesting merely two or decrease the surface area because of increased crystal density.
three layers of stacking. These results have unambiguously However, compared to NUS-8(Zr), NUS-8(Hf) has a relatively
2
ꢀ1
proven the expected 2D layered structures and nanosheet high BET surface area of 628 m g , probably because of the
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reported that the stability of 3D interpenetrated ZrBTB [aka
NUS-16(Zr) in this study] is much lower than that of other re-
ported 3D Zr MOFs, which is attributed to the terminal H O and
2
OH groups bound to fully exposed Zr O (OH) clusters in ZrBTB
6
4
4
as well as the reduced rigidity of the framework caused by
37
interpenetration. In the case of 2D layered NUS-8, however, the
labile Zr (OH) clusters are shielded by interlayer packing
6
O
4
4
leading to increased stability. In addition, the possible
constraints caused by interpenetration in NUS-16 can be
relieved in the 2D layered structure of NUS-8, which also helps
to strengthen the framework stability. To fully unveil the supe-
rior stability of 2D NUS-8 MOFs over that of 3D NUS-16 MOFs,
comprehensive stability tests were conducted, in which MOFs
were treated with increased harshness of chemical exposure,
from humid air, immersion in water, boiling in water, eventually
to incubation in highly acidic solutions. The stability of treated
2
MOFs was checked by PXRD and N sorption experiments. In
addition, thermogravimetric analyses (TGAs) were also conduct-
ed on pristine MOFs to evaluate their thermal stability. We found
that 2D NUS-8 MOFs could retain their high structural integrity,
almost intact porosity, and high decomposition temperatures up
ꢁ
to 400 C (Fig. S4†). More surprisingly, both NUS-8(Zr) and NUS-
8
(Hf) exhibit the highest 6A (boiling in water with retention of
44
crystallinity and porosity) water stability; and boiling in water
does not affect their crystal structures and porosities. In contrast,
3D NUS-16 MOFs cannot survive in almost any of the tests as
evidenced by the emergence of extra PXRD peaks, compromised
PXRD peak intensity and Bragg peak broadening, reduced surface
37
Fig. 4 (a) N
2
sorption isotherms at 77 K of NUS-8 and NUS-16 with area, and continuous weight loss in TGA curves (Fig. S5†). It is
inset featuring adsorption–desorption hysteresis; (b) pore size distri- worth noting that for both NUS-8 and NUS-16, MOFs with Hf as
bution of NUS-8 and NUS-16 calculated by NLDFT and BJH models
the metal nodes tend to have higher stability than those with Zr
ones. This can be explained by the higher bonding energy of Hf–O
(
inset).
ꢀ
1
ꢀ1 45
bonds (802 kJ mol ) than that of Zr–O bonds (776 kJ mol ). In
short, 2D NUS-8 MOFs possess higher chemical and thermal
stability than that of 3D NUS-16 MOFs, which will surely pave the
way toward their pragmatic applications under harsh conditions
and at high temperatures.
42
better crystallinity and less defects revealed by PXRD patterns.
In contrast, 3D interpenetrated NUS-16(Zr) and NUS-16(Hf)
2
ꢀ1
have decreased BET surface areas of 592 and 392 m
g ,
respectively, which can be attributed to the partial collapse of
frameworks caused by the fragility of interpenetrated structures
that will be discussed later.
The pore size distribution of NUS-8 calculated using non-
local density functional theory (NLDFT) reveals two major
Thermo-mechanical properties
We conducted dynamic mechanical analysis (DMA) experiments
to further probe the reason of structural stability in 2D NUS-8
˚
pore sizes at ꢃ6 and ꢃ12 A (Fig. 4b), which match well with the
0
MOFs. Fig. 5 shows the storage moduli (E ) and the loss moduli
˚
crystal structure (Fig. 1, 15.15 ꢂ 8.86 A). Compared to NUS-8,
00
(
E ) of the NUS-8 and NUS-16 MOFs, which are measures of the
˚
NUS-16 has a reduced pore size of 10.72 ꢂ 8.12 A, which is
46,47
viscoelastic response as a function of temperature.
When
caused by the interpenetration of neighboring grids. Pore size
distribution calculated using the Barrett–Joyner–Halenda (BJH)
model suggests the existence of mesopores with pore sizes of
ꢃ3.9 nm for NUS-8 (Fig. 4b), agreeing well with the hybrid Type
I/IV isotherms.
subject to oscillatory stresses generated by a small strain defor-
mation (Fig. 5a and b), the storage moduli data indicate that both
NUS-16(Hf) and NUS-16(Zr) frameworks are thermo-mechanically
ꢁ
stable with a stiffness value of ca. 10 GPa up to ꢃ200 C, but
0
beyond which E fell rapidly to under 1 GPa with elevated
ꢁ
temperature. Up to ꢃ280 C, it appears that NUS-8(Hf) is rela-
Stability tests
tively stable compared with NUS-16(Hf), which is consistent with
Several previous studies have indicated that MOFs with 3D TGA data (Fig. S4 and S5†). Notably, it can be seen in Fig. 5b that
interpenetrated structures should have larger surface areas than NUS-8(Zr) has, by and large, retained its initial storage modulus
ꢁ
those with 2D layered structures due to the fully exposed edges within the temperature range we studied (35–350 C), whereas
43
and latent sides. Our observation contradicts such conclusion, thermo-mechanical degradation was evidenced in the 3D inter-
which prompts us to search other possible reasons. It has been penetrated counterpart NUS-16(Zr).
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monitor the crystal growth processes. As shown in Fig. 6a, the
reaction intermediates aer 1 h gave distinct XRD peaks similar
to those of H BTB, suggesting the preservation of H BTB which
3
3
is further conrmed by its FESEM images (pillar morphology,
Fig. 6b). With the dissolution of H BTB and nucleation of NUS-8
planar crystallites, characteristic peaks featuring crystal planes
220) started to appear aer 2 h and continued to grow with the
3
(
time elapse, as evidenced by the continuous increase in PXRD
peak intensity. FESEM images further support this claim. From
1
h to 2 h, the product morphology signicantly changed from
giant pillar (H BTB) to nanosheet [lateral size, D z 1 mm, NUS-
3
L
8(Zr)]. With the time elapse, the nanosheets became larger and
more uniform, with a continuous increase in lateral size from 1
mm (2 h) to larger than 3 mm (20 h). Aer 20 h, the product is
mainly NUS-8(Zr) with uniform distribution of well-dened
nanosheets and thickness ranging from 10 to 50 nm, match-
ing well with the above observation of AFM images (Fig. 3c).
These results clearly demonstrate the crystal growth mecha-
nism in the modulated hydrothermal approach that the
heterogeneous condition prevents interpenetration by fast
precipitation of intermediate 2D MOF crystallites and promote
further planar growth of NUS-8 nanosheets shown in Fig. 1.
Gas sorption properties
49
2
We further tested the CO sorption property of NUS-8 MOFs.
Generally speaking, MOFs with exposed unsaturated metal
50
centers (UMCs), exemplied by MOF-74 and HKUST-1, can
exhibit strong interactions with CO affording high CO uptake
capacity and selectivity. Considering the high density of Lewis
Fig. 5 Representative plots of DMA measurements: (a, b) storage
0
00
2
2
modulus, E and (c, d) loss modulus, E , of NUS-8 and NUS-16. The
primary relaxation is denoted as the a peak. (e) Loss tangent, tan d ¼
51
00
0
ꢁ
E /E . All samples have been activated at 120 C for 24 h under vacuum acid sites and stability of 2D NUS-8 nanosheets, their
before being subjected to DMA studies.
adsorption-based CO
separation performance was thus evalu-
2
52,53
ated.
15% CO
NUS-8(Zr) has a slightly lower CO working capacity
2
(
2
content for post-combustion CO
2
capture) than that
0
0
ꢀ1
ꢀ1
The loss modulus, E , represents the dissipative (lossy) of NUS-8(Hf) (0.30 mmol g vs. 0.31 mmol g , Fig. 7a, S6, S7 &
component of the framework viscoelastic response in a dynamic Table S2†). This performance is comparable to that of UiO-
00
ꢀ1 41
stress state. The E data in Fig. 5c and d demonstrate that, 66(Zr) (0.37 mmol g ), albeit a much lower BET surface
2
ꢀ1
2
ꢀ1
because of the 2D conguration of the NUS-8 structures, thermo- area of NUS-8(Zr) (570 m g vs. 1500 m g ). This can be
mechanical dissipation is more substantial and may be arising attributed to the smaller pore size (Fig. 4b) and stronger Lewis
54
from a combination of interlayer shear sliding deformation and acidity of NUS-8(Zr). Meanwhile, both NUS-8(Zr) and NUS-
4
8
out-of-plane tension/compression modes.
conrmed by the fact that the magnitudes of loss moduli of NUS- NUS-16(Zr) (0.18 mmol g ) and NUS-16(Hf) (0.22 mmol g ),
6 are appreciably lower, attributable to their interlocked 3D which can be attributed to the reduced BET surface areas
architecture which is tightly constrained (Fig. 1). Remarkably, we caused by framework collapse of NUS-16 MOFs.
have established that between the 2D MOF nanosheets: NUS-
The stronger interactions between NUS-8(Zr) and CO can be
(Zr) is considerably less dissipative than NUS-8(Hf), evidenced veried by comparing the isosteric heat of adsorption (Q ) of
This effect is 8(Hf) have much higher CO working capacities than that of
2
ꢀ
1
ꢀ1
1
2
8
st
55
from the loss tangent data (Fig. 5e) where the former framework CO (Fig. 7b & Table S3†). The zero-coverage Q of CO in NUS-
is at least a factor of four times less lossy than that of the latter. 8(Zr) (26.5 kJ mol ) is indeed higher than that in NUS-8(Hf)
We reasoned that this phenomenon may be linked to the much (21.2 kJ mol ) and UiO-66(Zr) (25 kJ mol ).
2
st
2
ꢀ
1
ꢀ1
ꢀ1 41,56
This is also
˚
narrower 2D interlayer spacing of NUS-8(Zr) (d ¼ 16.82 A) conrmed in 3D NUS-16, wherein NUS-16(Zr) has a higher CO
2
ꢀ
1
ꢀ1
˚
compared with NUS-8(Hf) (17.27 A), see Fig. 1.
Qst than that of NUS-16(Hf) (23.0 kJ mol vs. 20.5 kJ mol ).
The above result is consistent with the fact that Zr (OH)
clusters have stronger Lewis acidity than that of Hf clusters due
to the smaller cation radius of Zr over Hf. The CO /N and
6
O
4
4
6
Crystal growth mechanism studies of NUS-8
57
2
2
To disclose the crystal growth mechanism of 2D MOF nano- CO /CH adsorptive selectivities of NUS-8 MOFs were further
2
4
5
8
sheets, we further carried out the reaction kinetics studies of evaluated using ideal adsorption solution theory (IAST).
NUS-8(Zr). PXRD and FESEM techniques were applied to Similar to the trend of Qst, NUS-8(Zr) also has a higher IAST CO
2
/
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Fig. 6 Crystal growth mechanism studies of NUS-8(Zr): (a) PXRD patterns; (b–h) FESEM images at different reaction times.
N
(
2
(14) and CO
11 & 3.9, Table S3†), again conrming the importance of strong
interactions between CO and MOF adsorbents.
2 4
/CH (5.6) selectivities than those of NUS-8(Hf) Catalytic oxidation of thioethers
As revealed by NUS-8 crystal models, the coordinatively unsat-
2
urated Zr/Hf sites in the 12-connected M clusters may serve as
6
59–61
Lewis acid sites.
Therefore, the relatively large pore size and
immense in-plane surface area should be highly benecial for
heterogeneous catalysis due to the highly accessible Lewis acid
sites and enhanced mass transfer, as has been suggested in
62
porous MOFs and zeolites. This hybrid porosity, high stability,
and immense Lewis acid sites have encouraged us to investigate
the catalytic performance of 2D NUS-8 nanosheets. The catalytic
performance of NUS-8 and NUS-16 was evaluated based on the
oxidation reaction of thioethers into sulfoxides and sulfones,
which is an important reaction due to its versatile utility in
63
organic synthesis. In this study, thioether oxidation reactions
were conducted in the presence of 30% H using MOFs as the
catalysts to produce the corresponding oxygenated products
sulfoxides, PhSOCH and sulfones, PhSO CH ) in quantitative
yield based on the consumption of H O and thioanisole (Table
2 2
O
(
3
2
3
2
2
64
1).
We initially attempted this reaction using NUS-8(Zr) as the
64
catalyst and acetonitrile (CH
absence of catalyst, thioanisole can hardly be oxidized by 30%
at ambient temperatures (only 12% conversion). When
using NUS-8(Zr) as the catalyst, we obtained a thioanisole
conversion of 100% and PhSO CH selectivity of 94.3% (Table
), which is higher than that of the Zr cluster based poly(-
polyoxotungstate) catalyst with 84% PhSO CH selectivity at
3
CN) as the solvent. In the
2 2
H O
2
3
1
2
3
ꢁ
65
a higher temperature (60 C).
Among all the solvents, reactions conducted in dichloro-
methane (DCM) have the highest PhSO CH
2
3
selectivity (ꢃ100%)
at room temperature. Therefore, DCM was chosen as the solvent
for further optimization. To the best of our knowledge, this high
conversion and sulfone selectivity are rarely reported in the
66,67
literature, especially for MOF catalysts.
Interestingly, reac-
tions conducted using THF as the solvent exhibit a decreased
conversion (74%), possibly due to the strong binding of THF to
the Lewis acid sites of MOFs. Either increasing the loading of
Fig. 7 (a) CO
2
sorption isotherms of NUS-8 and NUS-16 at 298 K; (b)
68
Q
st of CO in NUS-8 and NUS-16.
2
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Table 1 Screening the best conditions for thioanisole oxidation in the
presence of 30% H O catalyzed by NUS-8(Zr) at 298 K
2 2
Selectivity (%)
Catalyst loading Time Conv.
Entry
(mol%)
(h)
(%)
PhSOCH
3
2 3
PhSO CH
CH
CH
3
CN
CN
0
3.5
3.5
1
1
1
1
1
1
0.5
1.5
1
1
1
1
1
12
100
5.7
33.3
1.2
7.3
0
0
0
100
0
0
0
3
>99
>99
>99
74
>99
87
>99
8
>99
>99
>99
96
94.3
66.7
98.8
92.7
100
100
100
0
100
100
100
98.3
H
2
O
CH
3
OH 3.5
THF
3.5
3.5
3.5
3.5
0
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
5
2.1
1.5
1.25
0
1.7
NUS-8(Zr) to 5 mol% or decreasing to 1.5 mol% did not affect
the conversion and sulfone selectivity. However, further
decreasing the catalyst loading to 1.25 mol% led to a lower
conversion (96%) and reduced sulfone selectivity (98.3%),
possibly due to insufficient catalysis (Table 1). A control exper-
iment without any catalyst in DCM afforded less than 8%
conversion, conrming the critical role of NUS-8(Zr) as the
catalyst in this reaction. When NUS-8(Hf) was used as the
catalyst under a similar reaction condition, a conversion of 88%
for thioanisole was obtained along with a high sulfone selec-
tivity (89%) (Fig. 8b). This performance is inferior to that of
NUS-8(Zr), but is still comparable to the Zr -cluster based
2
4
65
poly(polyoxotungstate) catalyst (84% PhSO CH selectivity).
2
3
Following these results, we then examined the heteroge-
neity and recyclability of NUS-8 catalysts. The supernatant of
the reaction media obtained by using a regular lter during
the oxidation reaction did not yield any additional product, Fig. 8 Kinetic study of thioanisole oxidation catalyzed by NUS-8 or
2 3
indicating the role of the heterogeneous catalyst played by NUS-16 at room temperature: (a) kinetic studies; (b) PhSO CH
selectivity versus time; (c) cycle performance.
NUS-8 nanosheets (Fig. 8a). Upon completion of the reaction,
NUS-8(Zr) could be recovered in a quantitative yield and used
repeatedly without signicant loss of catalytic activity for
0
.83 for NUS-8(Zr); 1.65 to 1.62 for NUS-8(Hf)] aer three
subsequent three runs (Fig. 8c, constant 100% conversion and
00% selectivity for further 1–3 runs, respectively). This was
catalytic runs.
1
To further probe the superior catalytic activity of 2D NUS-8
nanosheets over 3D NUS-16 bulk crystals, we conducted
kinetic studies of this reaction using 1.5 mol% loading of
catalysts at 298 K for a continuous period of one hour (Fig. 8a
and b). A negligible conversion (ꢃ0%) within 20 min was
observed without any catalyst. In contrast, reactions catalyzed
by either NUS-8 or NUS-16 displayed remarkably enhanced
conversion and sulfone selectivity, with continuous increase in
both conversion and sulfone selectivity within one hour. The
also observed in NUS-8(Hf) (Fig. 8c, constant 88% conversion
and 87% selectivity for further 1–3 runs, respectively). Recy-
cled NUS-8 nanosheets retained high crystallinity (Fig. S8†),
intact morphology (Fig. S10 and S11†), but with mildly
reduced porosity (Fig. S9†), possibly due to the trapping of
reagents in the frameworks that are hard to be removed during
mild regeneration processes. It was further conrmed by
elementary analysis (EA) results (Table S4†) that the C/Zr or C/
Hf ratio in NUS-8 nanosheets remained almost intact [0.87 to
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faster reaction kinetics in the presence of MOFs can be because of the stronger acidity; (2) 2D NUS-8 nanosheets have
59,69
explained by the strong Lewis acidity of Zr/Hf MOFs.
Reac- a higher stability than 3D NUS-16, which is also supported by
tions catalyzed by NUS-8 demonstrated faster kinetics than the stability tests shown in Fig. S4 and S5;† (3) the catalytic
those catalyzed by NUS-16, which can be attributed to the 2D selectivity can be tuned via the introduction of multi-sites in
nature of NUS-8 facilitating the diffusion of substrates and MOFs exemplied by NUS-8(Hf).
70
products near the active sites. In addition, Zr-based MOFs
tend to afford faster reaction kinetics and higher sulfone
selectivity than Hf-based ones under the same reaction condi-
Conclusions
tions, which can be attributed to the stronger Lewis acidity of In conclusion, we have described a bottom-up strategy to
4
+
4+ 71
Zr than Hf .
synthesize 2D MOF nanosheets directly via a heterogeneous
modulated hydrothermal approach in the absence of surfac-
tants. Structural renements of synchrotron PXRD data indicate
the 2D planar structure of NUS-8 formed by inhibition of
interpenetration. 2D NUS-8 nanosheets exhibit excellent
stabilities superior to that of their interlocked 3D MOF
analogues (NUS-16) synthesized solvothermally, which are evi-
denced by comprehensive stability tests. Dynamic mechanical
analysis (DMA) experiments suggest that the stability may come
from a combination of interlayer shear sliding deformation and
out-of-plane tension/compression modes whereas their inter-
locked 3D architecture is conned. Both NUS-8 and NUS-16
were used as Lewis acid catalysts for the oxidation reactions
of thioanisole into sulfones, in which 2D NUS-8 nanosheets
exhibited better reaction kinetics and sulfone selectivity than
Assessment of Lewis acidity
3
In this study, NH temperature program desorption (TPD) was
conducted to compare the strength of Lewis acidity of MOFs
and explain why 2D NUS-8 MOFs have better catalytic activity
than 3D NUS-16 ones. Before TPD tests, the sample was
charged into a quartz tube and then activated in a helium ow
at 150 C for 2 h. The sample was subsequently exposed to pure
NH
helium ow at 100 C for 2 h. The temperature was increased to
50 C at a ramping rate of 5 C min , and the desorbed NH
72
ꢁ
ꢀ
1
3
gas (30 mL min ) for 30 minutes and then purged with
ꢁ
ꢁ
ꢁ
ꢀ1
4
3
3
was detected using a TCD detector. As depicted in Fig. 9, NH
desorption peaks at higher temperatures indicate stronger
acidity, while the appearance of two or more peaks indicates
multiple acid sites (from strong to weak). NUS-8(Zr) shows the
highest strength of strong acidity (single desorption peak at
3D NUS-16. In addition, NUS-8(Zr) showed superior reaction
kinetics with 100% conversion and 100% sulfone selectivity
even at ambient temperatures, which can be attributed to its
ꢁ
ꢃ400 C), which is consistent with the catalytic performance
strong Lewis acidity as evidenced by stronger CO affinity and
2
revealed above. Unlike NUS-8(Zr), NUS-8(Hf) exhibits two major
superior acidity observed in NH -TPD experiments. Our work
ꢁ
3
desorption peaks at 300 and 425 C, which can be assigned to
has demonstrated the potential of the modulated hydrothermal
approach in the kinetically controlled synthesis of 2D MOF
nanosheets and offered another versatile approach for the
synthesis and structure/morphology control of these promising
materials.
two acidic sites (weak and strong). This can possibly explain why
NUS-8(Hf) shows a constant 12% sulfoxide and 88% sulfone
selectivity. However, neither NUS-16(Zr) nor NUS-16(Hf) (ther-
mally unstable, Fig. S5†) could survive from the NH -TPD
experimental conditions (adsorption and desorption at high
3
temperature), accompanied by negligible desorption peaks
(
Fig. 9). The NH
3
-TPD results shown herein suggest that: (1) the Acknowledgements
higher catalytic performance of NUS-8(Zr) over NUS-8(Hf) is
This work is supported by National University of Singapore
(CENGas R-261-508-001-646) and Ministry of Education – Sin-
gapore (MOE AcRF Tier 2 R-279-000-429-112).
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