Zirconium Phosphate Supported MOF Nanoplatelets

Article abstract of DOI:10.1021/acs.inorgchem.6b00710

We report a rare example of the preparation of HKUST-1 metal-organic framework nanoplatelets through a step-by-step seeding procedure. Sodium ion exchanged zirconium phosphate, NaZrP, nanoplatelets were judiciously selected as support for layer-by-layer (LBL) assembly of Cu(II) and benzene-1,3,5-tricarboxylic acid (H₃BTC) linkers. The first layer of Cu(II) is attached to the surface of zirconium phosphate through covalent interaction. The successive LBL growth of HKUST-1 film is then realized by soaking the NaZrP nanoplatelets in ethanolic solutions of cupric acetate and H₃BTC, respectively. The amount of assembled HKUST-1 can be readily controlled by varying the number of growth cycles, which was characterized by powder X-ray diffraction and gas adsorption analyses. The successful construction of HKUST-1 on NaZrP was also supported by its catalytic performance for the oxidation of cyclohexene.

Full text of DOI:10.1021/acs.inorgchem.6b00710

Article
pubs.acs.org/IC
Zirconium Phosphate Supported MOF Nanoplatelets
Yuwei Kan and Abraham Clearfield*
Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
S
* Supporting Information
ABSTRACT: We report a rare example of the preparation of
HKUST-1 metal−organic framework nanoplatelets through a
step-by-step seeding procedure. Sodium ion exchanged
zirconium phosphate, NaZrP, nanoplatelets were judiciously
selected as support for layer-by-layer (LBL) assembly of Cu(II)
and benzene-1,3,5-tricarboxylic acid (H3BTC) linkers. The first
layer of Cu(II) is attached to the surface of zirconium
phosphate through covalent interaction. The successive LBL
growth of HKUST-1 film is then realized by soaking the NaZrP
nanoplatelets in ethanolic solutions of cupric acetate and
H3BTC, respectively. The amount of assembled HKUST-1 can
be readily controlled by varying the number of growth cycles,
which was characterized by powder X-ray diffraction and gas
adsorption analyses. The successful construction of HKUST-1
on NaZrP was also supported by its catalytic performance for the oxidation of cyclohexene.
alternately above and below the mean plane of the layer with
three oxygen atoms of the phosphate group bonding to three of
the Zr(IV) ions forming a triangle in half of the parallelogram.
Each Zr(IV) ion is coordinated with six oxygen atoms from six
different phosphate groups in adjacent parallelograms. The P−
OH groups form a double layer in the interlayer space. The
protons on hydroxyl groups can be easily replaced by other
positively charged species, resulting in the formation of new
phases.1b The diameter of α-ZrP particles varies in the range
from 50 nm to 2 μm, which can be controlled by varying the
synthetic conditions.9 To improve its applicability, the surface
of α-ZrP can be functionalized with various organic functional
groups, such as organic silanes,10 isocyanate,11 and epoxide
groups.12
As a family of multifunctional porous material, metal−
organic frameworks (MOFs) have been well-studied for their
structures and diverse applications in the past two decades.13
The combination of MOFs with other materials have shown
improved properties for given applications, especially in
separations and catalysis.14 Although many applications have
been suggested for MOFs, a considerable problem is their
stability.15 To improve the stability of MOFs, there is a demand
to develop controllable synthesis of MOFs on solid supports.
For practical applications, processing and formulation into
specific configurations is normally needed.16 In the case of
MOFs, the way to realize this is to prepare composite materials,
where MOFs are supported on substrates or constructed at the
interfaces to form composites.16 Such interfaces are referred to
as self-assembled monolayers (SAMs). The first ones were
INTRODUCTION
■
Smectite-type layered inorganic materials have received
considerable attention in the past decades owing to their
two-dimensional structure, tunable particle size, high stability,
and a variety of applications, particularly in ion exchange,1
catalysis,2 thin films,3 and biological applications.4 Among these
materials, zirconium phosphates (ZrP), a family of two-
dimensional (2D) layered materials, have been extensively
studied for ion exchange,5 polymer nanocomposite,6 drug
delivery,4 and lubricant additive.7 The best characterized ZrP is
the α-phase with the formula Zr(O3POH)2·H2O (α-ZrP) and
interlayer distance of 7.6 Å. The single-crystal structure of α-
ZrP was reported by Troup and Clearfield in 1977 (Figure 1).8
The tetravalent zirconium cations are arranged at the corners of
a parallelogram with Zr(IV) ions alternately above and below
the mean plane of the layer. The phosphate groups sit
Received: March 22, 2016
Figure 1. Structural representation of α-ZrP viewed along the b-Axis.
© XXXX American Chemical Society
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based on gold platforms.17 Gold-supported SAMs are not
practical due to the fragility of the gold−SAM interface at
higher temperatures.15b,17 To increase the interaction, pre
SAMs were immersed into solutions containing metal ions and
ligands. Therefore, SAMs containing −COOH or −OH groups
were developed to facilitate the layer-by-layer (LBL) growth
processes.18
deposited on the surface of ZrP. The Zr/Cu atomic ratio is
1:0.052, while the Zr/Na atomic ratio changed from 1:1.86 to
1:1.75 after copper deposition (see Supporting Information).
Combining thermogravimetric analysis (TGA) with WDS
results, it suggests that 0.11 mol of Na(I) on the surface of
NaZrP were exchanged by Cu(II) in 2:1 ratio. The quantity of
Cu(II) was compared with Cs(I) titrated ZrP, because it was
reported before that cesium ions only exchange surface protons
of ZrP in one-to-one ratio due to its large size, which has been
utilized to quantify the amount of surface protons for a given
ZrP material.24 ZrP nanoplatelets with diameter of ∼150 nm
were found to contain 0.11 mol/mol surface protons by using
Cs(I) titration,1c which is in good agreement with the fact that
there are 0.11 mol/mol less of Na(I) after the deposition of
Cu(II).
Recently, we have shown that the surface of α-ZrP
nanoplatelets can be decorated by tetravalent metal ions,
which are capable of binding phosphonic acids for further
surface modifications.24 However, the assembled layers are
amorphous due to the randomness of metal−phosphonate
coordination modes,25 which makes it difficult to characterize
by PXRD technique. In contrast, MOFs are highly crystalline
materials that, in most circumstances, retain their crystallinity
when assembled on surfaces.18d As mentioned above, the
surface sodium ions can be readily exchanged with Cu(II) to
achieve the copper-decorated NaZrP denoted as Cu/NaZrP.
To construct HKUST-1, fresh Cu/NaZrP nanoplatelets were
treated with ethanolic solution of H3BTC for 30 min and
washed thoroughly with ethanol to complete the first cycle
deposition. The same decoration procedure was repeated to
achieve the growth of multiple layers of HKUST-1.
ZrP is a family of material different from SAMs. The
amorphous particles are nanosized, and the layers grow slowly
to form crystals. In so doing the ZrP pass through a series of
stages in which the ion exchange curves change from a normal
stage where the pH increases, as more cation is taken up
displacing protons, to a stage where the system has no degrees
of freedom and the pH is fixed at ∼2.0.19 Powder X-ray
diffraction (PXRD) patterns show a gradual change from a
poorly crystalline to a highly crystalline phase.19 Thus, the
arrangement of the P−OH groups must change as the particles
increase in crystallinity. Although MOFs can grow as thin films
on substrate or in polymers to foster their applications,14a,20 to
the best of our knowledge, MOFs growth on the surface of 2D
layered materials have not been reported.18b,c,21 In this work we
show that a MOF can be easily deposited on small ZrP
particles. To prove this concept, LBL assemblies of Cu(II) and
1,3,5-benzenetricarboxylic acid (H3BTC) on fully sodium
exchanged ZrP particles is selected as a model. The ZrP
supported HKUST-122 has the following advantages: First, the
thickness of the growing film can be controlled by governing
the reaction cycles via LBL assembly. Second, the Cu(II) ions
are directly bonded to the P−OH groups on ZrP surfaces,
which prevents the MOF material from dissociation with the
support and increases the stability. Third, the 2D nature of ZrP
forces HKUST-1 to grow as nanoplatelets, which is difficult to
achieve through a one-pot solvothermal reaction. Last but not
the least, with careful control, the resulted ZrP⊂HKUST-1
possesses higher crystallinity and porosity compared to those
grown on porous α-alumina support.23
As we expected, after the second cycle of deposition, a small
peak appeared in the PXRD pattern, which we suspect belongs
to HKUST-1. Indeed, after the fourth cycle, more peaks start to
appear, which are in good agreement with the characteristic
PXRD pattern of HKUST-1. As illustrated in Figure 2, the
RESULTS AND DISCUSSION
■
Given the fact that α-ZrP is an excellent ion exchanger, we
sought to use Cu(II) to replace protons on α-ZrP surface. The
interlayer distance of α-ZrP is 7.6 Å, which corresponds to the
peak at 11.98° in PXRD patterns. However, when treated with
Cu(II), this peak shifted to 9.63°, which indicates that the
interlayer distance changed to 9.3 Å, suggesting that significant
amount of Cu(II) intercalated into α-ZrP interlayers, which
makes it difficult to quantify how much Cu(II) was deposited
on the surface. To avoid Cu(II) ions from entering between
ZrP layers, as the first step, a sodium ion titration was
performed for ZrP to displace the P−OH protons. The ZrP
particles are now Zr(NaPO4)2 (denoted as NaZrP) with an
interlayer spacing of 8.3 Å, which corresponds to the peak at
10.73° in PXRD (Figure S1). Subsequently, the Na(I) on the
external NaZrP surfaces were displaced by treating with a
stoichiometric amount of copper acetate in ethanol. Nice pale
blue particles were obtained. The presence of Cu(II) was
characterized by several techniques. PXRD patterns confirmed
that only the Na(I) on the external surface of ZrP nanoparticles
are replaced by Cu(II), as there is no observed peak position
change after the treatment of Cu(II), suggesting that the
interlayer distance of NaZrP nanoplatelets remained intact
during Cu(II) ion exchange (Figure S1). Moreover, wave-
length-dispersive X-ray spectroscopy (WDS) quantitative
analysis was conducted to quantify how much Cu(II) was
Figure 2. PXRD of ZrP⊂HKUST-1 with 0, 2, 4, 8, 10, 15 cycles of
deposition and simulated HKUST-1.
intensity of HKUST-1 PXRD peaks are being enhanced, which
corresponds to increased cycles of Cu(II) and BTC deposition.
Notably, the intensity of the major peak of NaZrP at 10.73°
diminished from the PXRD pattern, as more layers of highly
crystalline HKUST-1 are constructed on NaZrP. This is mainly
because the portion of low crystalline NaZrP in the particle is
being reduced, while more layers of HKUST-1 are formed.
After the 15 cycles, the PXRD pattern is clearly that of
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HKUST-1 with a very small shoulder peak at ∼11.0°, which
belongs to that of NaZrP.
retained properties of the given MOF. Field emission scanning
electron microscopy (FE-SEM) images in Figure 3 show that
the morphology of ZrP⊂HKUST-1 nanoplatelets after four,
eight, and ten cycles of deposition maintained the hexagonal
shape of the parent NaZrP. However, the change of
morphology was observed after 15 cycles of growth, which
can be attributed to particle aggregation. More importantly, we
did not observe any isolated HKUST-1 crystals in SEM images,
indicating that HKUST-1 prefers to grow on ZrP surfaces to
form thin films. The homogeneous distribution of Cu(II) in
ZrP⊂HKUST-1 nanoplatelets was further confirmed by
backscattered electron images from electron microprobe (see
Supporting Information).
It is worth noting that, as WDS suggested, the normalized
ratio of Na/Zr decreases from 1.57 (two cycles) to 1.33 (eight
cycles), which indicated that the Na(I) in the interlayer region
can also be exchanged through the deposition process. As
discussed above, under the experimental conditions, it is
difficult for copper ions to intercalate between NaZrP layers,
especially after deposition of HKUST-1 layers. We hypothe-
sized that it is the protons from H3BTC ligands that partially
exchanged the interlayer sodium ions. The MOF growth on the
edge of nanoplatelets can also occur, which is supported by the
SEM images of ZrP⊂HKUST-1 after 15 cycles of deposition.
Note that the ZrP platelet has two surfaces, and thus there are
15 layers of HKUST-1 on each side of NaZrP, along with
HKUST-1 on edges.
We have demonstrated that the composition of a given ZrP
material can be derived from TGA measurements and electron
microprobe WDS quantitative analysis.24 A sample of the
sodium ion exchanged ZrP displayed a total weight loss of
8.03% when subjected to TGA up to 850 °C. Surface water was
lost at ∼50 °C, and total dehydration occurred at 120 °C. In
the case of the ZrP⊂HKUST-1 samples, there are two major
weight changes: the first weight loss in the range from 50 to
150 °C is attributed to residual solvent and intercalated water,
and the second weight loss between 250 and 300 °C is believed
to be the decomposition of BTC ligands. On the basis of TGA
and electron microprobe WDS results, the molar ratio between
BTC ligand, Cu, Na, and Zr can be calculated. The molecular
formula of ZrP⊂HKUST-1 compounds with selected cycles
were calculated and are shown in Table 1.
Table 1. Formulas of ZrP⊂HKUST-1 Compounds with
Selected Cycles of Cu(II) and BTC Deposition As
Determined by Thermogravimetric Analysis and Microprobe
Analysis
no. of cycles
TGA total wt loss%
calculated formula
ZrNa1.86 (PO4)2
0
2
8
8.03%
14.20%
18.93%
ZrNa1.57Cu0.31(PO4)2(BTC)0.2
ZrNa1.33Cu1.04(PO4)2(BTC)0.69
For porous materials, besides the PXRD measurements, it is
also very important to evaluate the porosity and surface area by
means of nitrogen adsorption−desorption isotherm.27 To
confirm the successful grafting of HKUST-1 thin films on
NaZrP, N2 adsorption−desorption studies were performed for
the activated hybrid materials at 77 K. As expected, because of
its nonporous nature, there is nearly no nitrogen uptake for the
starting NaZrP material. As Figure 4 illustrates, after two cycles
of growth, the material shows a type I isotherm with 60 cm3·g−1
(STP) N2 uptake. Significant increase of N2 uptake was
It is known that the properties of nanoparticles can be tuned
by controlling their morphologies through different synthetic
routes. Nano MOFs have attracted increasing interest due to
their unique properties compared with large crystals.
Specifically, nano MOFs have been extensively studied for
biological applications.26 Constructing MOF nanoparticles with
different morphology is an intriguing topic. However, to the
best of our knowledge, MOF with nanoplatelets shape have
never been reported. By depositing MOF thin films on ZrP
nanoplatelets, we anticipate to obtain MOF nanoplatelets with
Figure 3. FE-SEM images of ZrP⊂HKUST-1 with (a) 4, (b) 8, (c) 10, (d) 15 cycles of deposition. Scale bar represents 500 nm.
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Table 2. Conversion and Yield of Each Product for the
Oxidation Reaction of Cyclohexene by Using 6.5 Cycles of
ZrP⊂HKUST-1 as Catalyst
product
yield
total conversion
59.5%
2-cyclohexene-1-one
2-cyclohexene-1-ol
32.6%
20.9%
2.8%
cyclohexene oxide
cyclohexene hydroperoxide
3.3%
CONCLUSION
■
In sum, we have successfully synthesized the first example of
ZrP⊂HKUST-1 nanoplatelets through a step-by-step seeding
procedure. The amount of surface supported HKUST-1 can be
controlled through successive LBL process. The surface area as
well as nitrogen uptake correspond to the HKUST-1/ZrP ratio.
We also demonstrated that HKUST-1 forms thin films grafted
on ZrP instead of isolated crystals. The HKUST-1 thin film
preserved not only its crystallinity and porosity but also its high
catalytic activity and selectivity in the aerobic oxidation of
cyclohexene. The construction of MOF nanoplatelets in such a
seeded route controls the morphology of MOF nanoparticles,
which endows new properties to functional nanoMOFs.
◇
Figure 4. N2 adsorption−desorption isotherms of NaZrP (cyan
)
□
○
and ZrP⊂HKUST-1, 2 cycles (black ), 4 cycles (red ), 8 cycles
△
○
(green ), 10 cycles (blue ⬠), 15 cycles (pink ), and pristine
HKUST-1 (purple ☆). Ads stands for adsorption, and Des stands for
desorption.
observed when more layers are added, 75 cm3 g−1 after four
cycles, 140 cm3·g−1 after eight cycles, 200 cm3·g−1(STP) after
ten cycles, and 325 cm3·g−1(STP) after 15 cycles, whereas
pristine HKUST-1 possesses nitrogen uptake of 400 cm3·g−1
(STP). Notably, the gravimetric N2 uptake is related to the
sample weight; therefore, the low N2 uptake of these
ZrP⊂HKUST-1 materials can be attributed to the existence
of the nonporous NaZrP.
To further confirm the successful construction of HKUST-1
thin films, the resulted ZrP⊂HKUST-1 material was examined
as a catalyst for the solvent-free oxidation of cyclohexene. It is
known that cyclohexene can be readily oxidized to produce 2-
cyclohexen-1-ol, 2-cyclohexen-1-one, epoxycyclohexane, and
cyclohexene hydroperoxide in the presence of copper catalyst
and oxygen, Scheme 1.28 The hybrid material ZrP⊂HKUST-1
EXPERIMENTAL SECTION
■
Chemicals. Zirconyl chloride octahydrate (>99.0%) was purchased
from Fluka. Phosphoric acid (85 wt % in H2O) was purchased from
Fisher-Scientific. Sodium nitrate (>99.0%) and copper(II) acetate
monohydrate (>98%) were purchased from Sigma-Aldrich. 1,3,5-
Benezenetricarboxylic acid (98%) was purchased from ACROS
organics. All chemicals were used without further purification.
Synthesis of Na(I) Intercalated α-ZrP. The α-ZrP nanoplatelets
were synthesized by the reflux method reported by Sun and co-
workers with slight modifications.9 In general, 3.2 g of ZrOCl2·8H2O
was dissolved in Milli-Q water to make a 200 mL aqueous solution
that was preheated to 94 °C, and then 200 mL of 6.0 M phosphoric
acid was added dropwise to the ZrOCl2 solution and was kept at 94 °C
for 48 h. The white powdery product (α-ZrP) was thoroughly washed
with Milli-Q water and dried in an oven at 70 °C for 24 h. 500 mg of
the resulting α-ZrP was well-dispersed in a 0.5 N NaNO3 solution, and
then the α-ZrP/NaNO3 solution was titrated with 0.1 N NaOH/
NaNO3 (1:1) until the pH reaches 8.0 and maintains this value at least
for 12 h. The product was rinsed by large amounts of Milli-Q water to
remove excess sodium ions and dried in an oven at 70 °C for 24 h
before using.
a
Scheme 1. Catalytic Oxidation of Cyclohexene by Using
ZrP⊂HKUST-1
The Growth of HKUST-1 Layers on NaZrP Surface.
Approximately 500 mg (1.54 mmol) of dry NaZrP, obtained from
the previous step (confirmed by PXRD and elemental analysis), was
suspended in ∼75 mL of ethanol in a round-bottom flask. A 0.154
mmol copper acetate ethanolic solution was added to the NaZrP
dispersion and allowed to stir for 30 min. The product was washed
thoroughly with ethanol and dispersed in ethanol again. A
stoichiometric amount of H3BTC was added to the Cu-NaZrP
solution and allowed to stir for another 30 min. One cycle is
completed, and the resulting ZrP⊂HKUST-1 was rinsed with a large
amount of ethanol to ensure the removal of excess acid. Successive
cycles were applied by repeating the same experimental procedure.
Catalytic Reaction Procedure of Cyclohexene Oxidation.
Oxidation of cyclohexene was performed by mixing 50 mg of
designated ZrP⊂HKUST-1 catalyst in 5 mL of cyclohexene in a
Schlenk flask connected with an oxygen balloon. The reaction was
performed at 80 °C for 20 h. The reaction mixture was then
centrifuged to separate the solid catalyst from the liquid mixture. A
small amount of the brownish liquid was transferred into an NMR
a
After 6.5 cycles of deposition. Reaction conditions: 80 °C, 20 h.
with 6.5 cycles of treatment was used as catalyst for the
oxidation reaction. In a typical reaction, 5 mL of cyclohexene
and 50 mg of ZrP⊂HKUST-1 were charged into a flask under
oxygen environment. The reaction mixture was heated at 80 °C
for 20 h. A small portion of the isolated liquid mixture was
dissolved in CDCl3 for NMR analysis. The conversion and yield
of different products were calculated based on the integration of
the peak area in the 13C NMR spectra; commercially available
pure samples were used as references (see Supporting
Information for details). As shown in Table 2, under the
given reaction condition, the conversion was calculated to be
59.5%, which is much higher than the reported conversion of
14.6%.28 It is also worth noting that, in a previous report,28 2-
cyclohexen-1-ol was shown to be the dominant product by
using HKUST-1. However, we found that more 2-cyclohexen-
1-one was produced by using ZrP⊂HKUST-1 nanoplatelets.
1
tube, and CDCl3 was added before taking H and 13C NMR spectra.
Instrumentation. PXRD experiments were performed from 4° to
40° (2θ angle) using a Bruker-AXS D8 short arm diffractometer
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equipped with a multiwire lynx eye detector using Cu (Kα, λ = 1.542
Å) and operated at a potential of 40 kV and a current of 40 mA. TGA
experiments were performed on a TGA Q500 TA Instrument. Samples
were heated from room temperature to 1000 °C at a heating rate of 5
°C/min under N2. Elemental analysis work employed WDS. Analyses
were performed after standardization using very well-characterized
compounds or pure elements. Pressed powder micropellets were
prepared by pressing a few milligrams of powder between the highly
polished surfaces (0.25 μm) of hardened steel dies and transferring the
pellets onto double-sided conductive carbon tape. The pellets were
carbon-coated before analysis to make them electrically conductive.
Analyses of pressed powder pellets were performed with a 20 μm
diameter beam, while the stage was being moved 20 μm every 2 s,
repeated over a 10 spot traverse. This ensured representative sampling
and minimized possible thermal damage to the samples. The
transmission electron micrographs (TEM) of the samples were
acquired using a JEOL 2010 transmission electron microscope at an
acceleration voltage of 200 kV. Samples were prepared using gold grids
from Ted Pella. Scanning electron microscopy (SEM) images were
acquired on a JEOL JSM-7500F (FE-SEM). Low pressure nitrogen
adsorption−desorption isotherm measurements were performed on an
ASAP 2020 with extra-pure quality gases. The as-synthesized material
was soaked in ethanol for 24 h and then centrifuged and dried at 60 °C
for 3 h in an oven. The resulted blue powder was then transferred into
a BET tube and activated at 80 °C at 133 microbar for 12 h. The color
of the material was noticed to change from blue to purple upon
activation.
Laboratory at Texas A&M Univ. for the use of the PXRD
facilities.
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tetramethylsilane and were measured relative to the signals at 7.26
ppm (CDCl3). All 13C NMR spectra were reported in parts per million
relative to 77.16 ppm (CDCl3) and were obtained with 1H decoupling.
Standard materials for cyclohexene, 2-cyclohexene-1-one, 2-cyclo-
hexene-1-ol, and cyclohexene oxide were purchased from Sigma-
Aldrich and were used without further purification. Since cyclohexene
1
hydroperoxide is not commercially available, the H and 13C NMR of
cyclohexene hydroperoxide is predicted. In 1H NMR spectra, the
chemical shifts between different products are very similar, and there
are significant couplings between these protons. Thus, it is difficult to
distinguish each product by 1H NMR due to the peak complexity and
significant overlap. In contrast, the 13C NMR spectra of each product
can be readily recognized, which can be used for identifying products
in the reaction mixture. The ratio of products was calculated according
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00710.
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ACKNOWLEDGMENTS
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This work was supported by the Robert A. Welch Foundation
Grant No. A-0673, for which grateful acknowledgement is
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