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Microwave-assisted synthesis of zirconium-based metal organic frameworks (MOFs): Optimization and gas adsorption
Reza Vakili, Shaojun Xu1, Nadeen Al-Janabi1, Patricia Gorgojo, Stuart M. Holmes, Xiaolei Fan
School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom
Abstract:
Microwave-assisted synthesis of zirconium (Zr) based metal organic frameworks (MOFs)
were performed and the yield and porous property of UiO-67 was optimized by varying the
quantity of the modulator (benzoic acid, BenAc and hydrochloric acid, HCl), reaction time
and temperature. It was found that (i) an increase in the amount of modulator enhanced the
specific surface area and pore volume of UiO-67 due to the promotion of the linker
deficiency; and (ii) the presence of the modulator influenced the number of nuclei (and hence
the crystal size) and nucleation time (and hence the yield). Optimum amounts of BenAc and
HCl for the synthesis of UiO-67 under microwave irradiation were determined as 40 mole
equivalent and 185 mole equivalent (to Zr salt), respectively. In comparison to conventional
solvothermal synthesis, which normally takes 24 h, microwave methods promoted faster
syntheses with a reaction time of 2‒2.5 h (at similar temperatures of 120 °C and 80 °C for
BenAc and HCl, respectively). The thermal effect of microwave is believed to contribute to
the fast synthesis of UiO-67 in the microwave-assisted synthesis. The reaction mass
efficiency and space-time yield show that the microwave heating promoted the simple yet
highly efficient preparation of Zr-based MOFs. In addition, UiO-67 MOFs from different
synthesis methods (i.e. the microwave-assisted and solvothermal method) were evaluated
using single-component (CO2 and CH4) adsorption, showing comparable gas uptakes.
Keywords: Microwave; synthesis; metal organic frameworks (MOFs); Zr-based MOFs; modulator;
gas adsorption.
Introduction
Research on metal organic frameworks (MOFs) has experienced a tremendous growth over the past
two decades due to their well-known features of large surface areas, high micropore volumes and
Corresponding author. Tel.: +44 1613062690; email address: [email protected] These authors contributed equally to this work.
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tunability of pore chemistry and shape [1, 2]. These intriguing porous crystalline materials have
been studied as promising candidates for various applications including energy storage [3-5],
separation [6-8], catalysis [9-12], chemical sensing [13] and drug delivery [14]. However, the
application of MOFs has often been questioned since most well-known MOFs suffer from poor
thermal and chemical stabilities [15]. For example, HKUST-1 (or CuBTC, a copper-based MOF)
has been manufactured at large scale [16] and recognized as a good candidate for gas adsorption.
However, its instability in the presence of moisture has impeded practical applications of this
material [17-19]. To address this problem, numerous efforts have been made to improve the
stability of MOFs by design such as (i) the use of high oxidation state metals (e.g. Zr4+, Fe3+, Al3+) to
reinforce the bonds between metal sites and linkers [20, 21]; and (ii) the post-synthetic modification
of organic linkers and/or metal sites by hydrophobic groups [22] or functional dopants [6].
In 2008, a class of zirconium (Zr)-based MOFs, i.e. UiO MOFs (UiO for Universitetet i Oslo), was
designed with remarkable stability that was not typically found in other MOFs [23]. UiO-66/-67
MOFs are by far the most widely studied UiO MOFs, in which Zr6O4(OH)4 clusters are joined by
organic linkers of 1,4-benzenedicarboxylate acids (BDC, for UiO-66) and 4,4-
biphenyldicarboxylate acids (BPDC, for UiO-67). Zr4+ ions in the Zr Clusters have strong
interactions with the carboxylate ligands and are theoretically fully coordinated to other clusters by
12 organic linkers, forming a highly connected framework [24, 25]. Thanks to this strong
interaction between the organic and inorganic regions, UiO MOFs are thermally stable up to 450 °C
and chemically stable in various organic solvents. The existence of small tetrahedral cages (7.5 Å
for UiO-66 and 12 Å for UiO-67) and large octahedral cages (12 Å for UiO-66 and 16 Å for UiO-
67) in their frameworks make these materials good candidates for adsorption and catalysis,
respectively [23, 26, 27].
The laboratory syntheses of UiO MOFs were first achieved under solvothermal conditions by
Lillerud’s group [23]. However, the original synthetic method was inefficient and often led to the
production of intergrown crystals with poor crystallinity due to the formation of zirconia gel 2
(amorphous precursors) at the beginning of reaction [28, 29]. In 2011, Behrens and co-workers [27]
investigated the effect of monocarboxylic acids on the modulated synthesis of Zr-based MOFs and
found that modulating agents are key elements to promote the self-assembly of UiO MOFs.
According to the proposed modulation mechanism, during the synthesis, modulators (e.g. benzoic
acid and acetic acid) firstly compete with bidentate ligands to coordinate with Zr cations, then
ligand exchange (between bidentate linkers and coordinated modulators) follows to render the
framework. As a result, the formation of amorphous precursors is prevented by the modulated
synthesis. In addition, the reproducibility and crystallinity of resulting MOFs are enhanced.
Monocarboxylic acids of formic, triflouroacetic and benzoic acids as well as hydrochloric acid have
been commonly used as modulators for synthesizing UiO MOFs [29-32].
It has also been found that size and morphology of UiO MOFs are tunable from micro-sized
aggregates of intergrown crystals to individual octahedral crystals (up to 2 µm), depending on the
amount and type of the modulator used in the synthesis [27, 33]. Despite modulators being
necessary, recent studies using thermogravimetric analysis (TGA) and neutron diffraction have
revealed the formation of structural defects in UiO MOFs as a result of their presence during the
synthesis [31, 34-37]. Structural defects in UiO MOFs are created mainly by the partial ligand-
exchange between dicarboxylic ligands (linkers) and monocarboxylic ligands (modulators), which
lead to the presence of modulators (acting as defect-compensating ligand [25]) in the final structure
of MOFs, i.e. the defective framework [34]. These defects corresponds to either missing linkers [25]
or missing clusters [38], benefiting the porosity and specific surface area of resulting MOFs, and
hence their adsorption capacity (e.g. 50% increase in CO2 uptake of UiO-66 at 35 bar) [25] and
reactivity (e.g. up to 96% conversion in aldol condensation of acetaldehyde on UiO-66-Cr and UiO-
67-Cr catalysts) [34].
To date, MOFs have been mostly synthesized using a solvothermal method that requires lengthy
reaction time and high energy input [39]. Consequently, the development of alternative synthetic
routes that are more economical and sustainable is of great interest to the research community. For 3
Zr-based MOFs, an electrochemical method was developed for the synthesis of UiO-66, in which
modulators (acetic acid) were used to control the cathodic and anodic film deposition of UiO-66 on
zirconium foils with a fast reaction time (30 min) and mild reaction temperature (100 °C) [40].
Microwave-assisted synthesis is another alternative for the synthesis of MOFs [41]. In microwave
synthesis, heat is generated internally within reaction media by dielectric heating as opposed to the
conventional heating in which heat is conducted to the media from external heating sources [41,
42]. Hence, a uniform and intensive heating can be initiated under microwave irradiation facilitating
the nucleation and crystal growth in the synthesis of MOFs [43]. The effect of microwave
irradiation on reactions can be classified into (i) the thermal effect and (ii) the non-thermal effect
(i.e. specific microwave effects such as the direct interaction of the electric field with specific
molecules in the reaction medium which is not linked to macroscopic temperature change [44-46]).
It is generally agreed that enhancements (in terms of yield, product purity and efficiency) observed
in microwave-assisted syntheses are mostly a consequence of thermal/kinetic effects (i.e. high local
temperatures promote fast reaction rates). In contrast, the non-thermal effect of microwave-assisted
synthesis is controversial leading to debate in the scientific community. It is noteworthy that the
combination of both effects makes the investigation of the effect of microwave irradiation on the
synthesis a relatively complex task.
Recently, UiO-66 MOF was prepared using a short reaction time of 120 min with a high yield of ca.
90% (based on ZrCl4) in a microwave-assisted method, the product showed good capacity for liquid
phase dye adsorption (98% removal efficiency for acid chrome blue K) [47]. In addition, Taddei et
al. optimized the synthesis condition of UiO-66 under microwave irradiation with high productivity,
energy efficiency and crystallinity of materials [48]. Furthermore, it was also found that the
microwave irradiation usually decreased the size of crystals as a result of the accelerated nucleation,
i.e. increased number of nuclei produced that grew into small crystals [47, 49]. For instance, the
crystal size of UiO-66 from the microwave synthesis (<100 nm) was four times smaller than that by
conventional heating (ca. 400 nm) [47]. Small crystal size at submicron and nano scales can be
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beneficial in applications such as non-equilibrium adsorption and catalysis since interparticle
diffusion limitation can be greatly reduced [50] or the nano-crystallites can be used in the formation
of thin film membranes for molecular separations [51]. Conversely, MOFs with large crystals may
be of interest for equilibrium adsorption where a big pore volume is needed. Therefore, the
development of a fast and energy-efficient synthesis route for UiO MOFs, which are able to
simultaneously reduce reaction time and control crystal size, could boost their application in a wide
range of fields where different specifications are required.
Herein, we report the development and optimization of the microwave-assisted modulated synthesis
of UiO-67 MOF, in which the benefit of focused microwave-induced heating is combined with
modulation for controlling the properties of materials (e.g. morphological and porous features). To
the best of our knowledge, it is the first time that UiO-67 MOF has been prepared by the
microwave-assisted method. UiO-67 was synthesized with benzoic acid (BenAc) and HCl as
modulators. A systematic and comparative approach, taking into account the yield and properties of
resulting materials, was employed to optimize the synthesis condition in terms of the reaction time,
temperature and the amount/type of modulators. The resulting materials were carefully
characterized and evaluated by single-component gas adsorption (using CO2 and CH4) and
compared to UiO-67 prepared by the conventional solvothermal method.
Experimental
Chemicals and synthesis of materials
Terephthalic acid (BDC), ZrCl4 and 4,4’-biphenyldicarboxylic acid (BPDC) were purchased from
Arcos. Benzoic acid (BenAc), hydrochloric acid (HCl) and acetic acid (AC) were purchased from
Sigma-Aldrich. N,N’-dimethylformamide (DMF) was obtained from Fischer Scientific. All
chemicals were used as received, with no further purification.
UiO-66/-67 MOFs (UiO-66 was prepared as the secondary material in this work) were synthesized
solvothermally using different modulating agents according to methods reported by Schaate et al. 5
[26] and Katz et al. [29]. The synthesis mixture was prepared by dissolving ZrCl4 and modulators
(HCl or AC for UiO-66; HCl or BenAc for UiO-67) in 20 ml of dimethylformamide (DMF) under
sonication, followed by the addition of organic linkers (BDC for UiO-66; BPDC for UiO-67) to the
solution. The opaque white solution was then transferred to a 50 ml Teflon-lined autoclave reactor
and heated at 120 °C under hydrostatic conditions. After 24 h, the reactor was cooled down to room
temperature and precipitates were centrifuged. Full details of the synthesis, washing and activation
procedures have been provided in the Supporting Information (SI).
A CEM Discover SP microwave system was used to prepare UiO MOFs under microwave
irradiation, where the synthesis solution (SI) was kept in a Pyrex vial (35 ml) and irradiated under
constant power (150 W, 2.5 GHz). After synthesis, the resulting materials were washed and
activated using the same procedure as that in the solvothermal synthesis (SI).
Characterization of materials
Powder X-ray diffraction (PXRD) was carried out on a Rigaku Miniflex diffractometer using CuKα
radiation (30 kV, 15 mA, λ = 0.15406 nm). The measurements were made over a range of 4° < 2θ <
45° in 0.05 step size at a scanning rate of one degree per min. Scanning electron microscopy (SEM)
was undertaken using a FEI Quanta 200 ESEM equipment using a work distance of 8‒10 mm and
an accelerating voltage of 20 kV. All samples were dispersed in ethanol and dropped onto SEM
stubs, followed by gold deposition using an Emitech K550X sputter coater under vacuum (1×10−4
mbar). Nitrogen (N2) sorption analysis of materials at −196 °C were obtained using a Micromeritics
ASAP 2020 analyzer. Before measurements, samples (40–100 mg) were degassed at 200 °C under
vacuum overnight. The surface area and total pore volume of the materials were calculated based on
Brunauer-Emmett-Teller (BET) theory and at relative pressure P/P0 of 0.99, respectively.
Thermogravimetric analysis (TGA) was performed by using a TG analyzer (Beijing Boyuan
Science and Technology Development Co., Ltd). The samples were heated from room temperature
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up to 700 °C in air (0.6 ml min−1) at a heating rate of 5 °C min−1. FT-IR analysis was carried out in
an Avatar 360 ESP spectrometer in the range of 600–2000 cm−1.
Gas adsorption experiments
Single component CO2 and CH4 adsorption/desorption experiments were carried out using an
intelligent gravimetric analyzer (Hiden IGA-001). Samples were first thermally treated at 100 °C
for 3 h before being heated up to 200 °C (2 °C min‒1) and kept at 200 °C for 8 h under vacuum.
Before the measurement, helium adsorption at 20 °C was used to assess the buoyancy effect and
determine the density of the sample. The measured density was then used for CO2 and CH4
adsorption/desorption isotherms acquisition at 25 °C and 50 °C from vacuum to 20 barG.
Results and discussion
Solvothermal synthesis of UiO MOFs
UiO-66 and UiO-67 MOFs were prepared using the conventional solvothermal method [52] in order
to compare their properties with those of the microwave-assisted synthesized materials (HCl and
BenAc as modulators). As seen in Fig. 1, PXRD patterns confirmed that UiO-67 were synthesized
successfully under solvothermal conditions. Characteristic reflection peaks of the as-synthesized
materials (in a 2θ range of 5° and 45°) match those of the simulated one [53]. The strong intensity
of peaks at 2θ = 5.72° and 6.58° corresponding to peaks of 111 and 200 crystal surface, proving the
formation of the crystalline phase of UiO-67.
The morphology of solvothermally synthesized MOFs was characterized by SEM (insets in Fig. 1).
By using BenAc as the modulator, UiO-67 MOF was formed as individual octahedral crystals
without aggregation, whereas the sample modulated by HCl exhibited intergrown crystals without a
well-defined morphology. Similar morphology of UiO-67 prepared using HCl as the modulator was
also reported previously by Katz et al. [30], which was attributed to a very fast nucleation process
[54, 55]. A similar effect of HCl modulator on the morphology of UiO-66 was also found and
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relevant information is presented in Figs. S1 and S2. N2 sorption isotherms (Fig. S3) were measured
for solvothermally synthesized UiO MOFs to calculate their BET surface area and pore volume.
Comparable values to those reported in the literature are shown in Table S1 [3, 30, 34, 53, 56]. In
general, for materials prepared in this study using solvothermal methods, UiO-67 presented twice as
much BET surface and pore volume as UiO-66, conforming to the findings of the original work on
UiO MOFs [23].
Fig. 1. PXRD patterns of UiO-67 MOF synthesized under solvothermal conditions using (a) BenAc as the modulator and (b) HCl as the modulator. Insets: SEM images of the according UiO-67 and PXRD patterns from 2θ = 15‒30°
Microwave-assisted synthesis of UiO MOFs in DMF
Under microwave heating, the microwave couples directly with solvent molecules to produce the
rapid temperature rise in reaction media and localized superheating (known as thermal effects),
minimizing the wall effect experienced in conventional heating. The effectiveness of microwave
heating depends on the dipole moment of the solvent molecule [42], and hence solvents with large
dipole moments such as DMF (μd = 3.86 D, δ = 0.161 [42]) are good candidates for microwave
assisted synthesis. Conversely, DMF possesses a relatively low thermal conductivity of 0.184 W
m−1 K−1 (one third of the thermal conductivity of water) suggesting that conventional heating will be
less efficient than microwave heating for promoting the synthesis of UiO MOFs [57]. In addition,
the relaxation time of DMF (13.05 ps, the time taken by molecules to return to its randomized state
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when the electric field is switched off [42]), is much less than 65 ps indicating that the thermal
runaway with continued irradiation is unlikely to occur [42, 43]. Therefore, the microwave-assisted
synthesis using DMF could provide benefits over the conventional solvothermal synthesis for rapid
synthesis of UiO MOFs. Previously, a microwave-assisted synthesis [47] was developed with a
short reaction time of 120 min to produce UiO-66 MOF with high Langmuir surface area of 1,661
m2 g−1, which were comparable to the results from this study (see Fig. S4).
In this work, a microwave-assisted method, for the first time, was developed and optimized for the
synthesis of UiO-67 MOF. Synthesis conditions of the microwave-assisted method was
systematically investigated by varying the following parameters: time, temperature, and the type
and amount of modulator, to understand their effects on the yield and properties of UiO-67 (e.g.
morphology, porosity). UiO MOFs synthesized by microwave methods were also compared to
benchmark materials prepared by the solvothermal method (discussed in previous section) to show
the effectiveness of the developed microwave method. In addition, two criteria [48], the reaction
mass efficiency (RME, %, Eq. 1) and the space-time yield (STY, kg m−3 d−1, Eq. 2) were employed
to evaluate the process efficiency of the developed methods:
(1)
where MUiO MOFs, MZr and Mlinker are masses of synthesized MOFs and starting materials (Zr salts and
carboxylic linkers) in kg, respectively.
(2)
where V is the reaction volume [m3] and t is the synthesis time [day].
Under microwave heating, the type of modulator influenced the crystallization and properties of
resulting UiO-67, as shown in Fig. 2 and Table 1. The effect of the amount of BenAc (in molar
equivalent, equiv) on the crystallization of UiO-67 MOF is shown in Fig. 2a. It was found that a
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minimum 40 equiv of BenAc was needed to ensure the successful formation of UiO-67 framework
at 120 °C under microwave irradiation. For the microwave synthesis with 10 equiv of BenAc, only
amorphous product was produced, as the entry 1 in Fig. 2a. The unsuccessful synthesis of UiO-67
under microwave irradiation with the low quantity of modulators (e.g. 10 equiv of BenAc, which
was used in the solvothermal synthesis of UiO-67 [27]) may be attributed to the high heating rate in
the reaction media that resulted in the accelerated formation of zirconia gel (amorphous precursors)
at the initial stage of reaction [28, 29]. Therefore, in the microwave-assisted synthesis, a large
quantity of modulators (e.g. for BenAc, four times higher than the solvothermal synthesis) was
needed to compensate the loss of modulators due to the formation of amorphous precursors. When
HCl was used as the modulator in the microwave synthesis (at 100 °C), a minimum amount of 135
equiv of HCl was required to achieve the formation of UiO-67 (Fig. 2b), whereas only 60 equiv of
HCl was needed in the solvothermal synthesis.
Fig. 2. PXRD patterns of UiO-67 prepared by the microwave-assisted synthesis modulated by (a) BenAc and (b) HCl.
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Table 1
Modulated synthesis of UiO-67 under microwave irradiation and the relevant results of yield, micropore volume and BET surface area.
Entry Modulator T t Yield Vp SBET
Type[-]
Amount[mole equiv]
[°C] [h] [mg] [cm3 g−1] [m2 g−1]
1 BenAc 10–30a 120 2.5 -b -c -c
2 BenAc 40 120 2.5 75 0.98 2,3543 BenAc 70 120 2.5 37 1.02 2,5234 BenAc 100 120 2.5 -b -c -c
5 BenAc 40 100 2.5 20 -c -c
6 BenAc 40 140 2.5 -b -c -c
7 BenAc 40 120 2 48 -c -c
8 BenAc 40 120 3 67 -c -c
9 BenAc 40 120 4 58 -c -c
10 HCl 40 100 2.5 -b -c -c
11 HCl 80 100 2.5 -b -c -c
12 HCl 135 100 2.5 83 0.67 1,300d
13 HCl 185 100 2.5 94 0.86 1,734d
14 HCl 185 80 2.5 97 1.2 2,54715 HCl 185 120 2.5 -b -c -c
16 HCl 185 80 2 95 1.2 2,54718 HCl 185 80 3 91 -c -c
19 HCl 185 80 4 75 -c -c
a similar unsuccessful results were obtained by using 10, 20 and 30 equiv of BenAc; b unsuccessful synthesis without quantitative yield of UiO-67; c samples were not considered for further investigations; d BET values of products from entries 12 and 13 are not comparable to that of UiO-67 (~2640 m²/g), and thus they cannot be anticipated to be pure UiO-67.
Materials obtained were investigated by SEM in order to understand the effect of the microwave
synthesis on the size and morphology of UiO MOFs. As seen in Fig. 4, the microwave synthesis
(using BenAc, entries 2 and 3 in Table 1) produced UiO-67 crystals with the typical octahedral
morphology as well as well-defined faces and edges, similar to those synthesized by the
solvothermal method [27]. The amount of BenAc was found to be influential on the crystal size of
resulting UiO-67 due to its involvement in the nucleation process. UiO-67 MOF synthesized with
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70 equiv of BenAc was measured with crystal sizes of about 4 µm, while the size was only about 2
µm for samples prepared using 40 equiv of BenAc. In general, the presence of modulators in the
synthesis of UiO-67 played roles in (i) varying the number of nuclei, and hence affecting the crystal
size; and (ii) changing the crystallization time affecting the yield. Therefore, in the synthesis of
UiO-67, an increase in BenAc concentration intensified the competition between the linker and
modulator molecules and reduced the number of nuclei, leading to the growth of large crystals [28,
29, 58]. A similar phenomenon was also observed in the synthesis of UiO-66 with various amounts
of modulator (e.g. 100 equiv and 250 equiv of AC) as shown in Fig. S4. Although a previous study
has claimed that the microwave heating tended to promote smaller crystals in comparison to the
conventional synthesis (e.g. crystal sizes for CO-MOF-74 synthesized by microwave heating,
measured as 50 µm, were one sixth of those synthesized solvothermally [59]), the dosage of
modulating agents in this case also plays a role in crystal sizes that can be obtained. For samples
synthesized using HCl (entries 12 and 13 in Table 1), SEM images revealed the formation of small
intergrown crystals (Fig. S5). Further confirmation of the formation of UiO-67 was provided by the
FT-IR analysis as shown in Fig. S6. Stretching peaks, corresponding to Zr–O–Zr and C=O bonds,
were observed for all samples at same wavenumbers of around 670 cm‒1 and 1400 cm‒1,
respectively, confirming the similarity of their frameworks. In addition, skeleton vibrations of
benzene rings were measured from 1500 cm‒1 to 1600 cm‒1 for all samples.
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(a) (b)
Fig.4. SEM micrographs of UiO-67 synthesized by microwave-assisted method. (a) and (b): 40 equiv of BenAc as the modulator; (c) and (d): 70 equiv of BenAc as the modulator.
N2 adsorption-desorption isotherms were measured for selected samples as shown in Fig. 5. All
samples followed the type Ι isotherms, in which the pore filling process was achieved up to a
relative pressure (P/P0) of 0.1, indicating the microporous nature of materials. The porous property
of UiO-67 promoted by HCl and BenAc are summarized in Table 1. As it can be seen, BenAc
(entries 2 and 3) promoted the microwave synthesis of UiO-67 with BET surface areas close to the
expected value of about 2640 m2 g−1 [23, 27]. The HCl modulated samples (entries 12 and 13)
obtained at 100°C under microwave irradiation clearly exhibit too low specific surface areas (1300
and 1734 m2 g−1, respectively) and thus cannot be anticipated to be pure UiO-67, indicating either
pore blockage or the presence of X-ray amorphous by-products from the synthesis.
Additionally, it was also found that an increase in the modulator amount resulted in the rise of BET
values of materials (i.e. by 7.2% and 33.4% for BenAc and HCl modulated UiO-67, respectively),
which could be assigned to linker deficiency as a consequence of the partial ligand-exchange
between dicarboxylic ligands (linkers) and monocarboxylic ligands (modulators). Previous studies
[25, 30, 31, 60] have shown that an increase in the concentration of modulators in the synthesis of
Zr-based MOFs gave rise to the increase in linker deficiency of resulting MOFs, and hence the rise
in their specific surface areas and porosities (e.g. 10% linker deficiency in UiO-66 framework led to
an increase in surface area from 1000 to 1600 m2 g−1 as the framework became lighter [25]). TGA
13
(c) (d)
was also performed to evaluate the extent of linker deficiencies in the resulting materials (e.g.
entries 2, 3, 12 and 13, see the relevant information in SI).
In order to calculate the proportion of linker deficiency, weight losses of corresponding samples
measured by TGA were compared with the theoretical value (ideal defect-free UiO-67 MOF based
on the perfect coordination). By considering the general molecular formula of UiO MOFs, the
number of linker deficiencies per each Zr6 formula unit (x) was considered giving a formula of
Zr6O6+x(L)6−x (L = BDC or BPDC, a detailed description of the method of calculating x is in SI).
This formula shows the dehydroxylated form of UiO MOFs in which the solvent, modulator and ‒
OH groups are removed prior to the framework decomposition [25]. It was found that the total
weight loss was lower than the theoretically value (i.e. 54.4% for UiO-66 and 64.5% for UiO-67,
Table S2), indicating the presence of linker deficiencies in the structure of experimentally prepared
materials.
By comparing values of x of entries 2, 3, 12 and 13 (i.e. 0.42, 1.46, 0.72 and 1.52, respectively), the
linker deficiency was shown to increase by increasing the amount of modulator used in the
synthesis, and hence the structures became less dense in comparison to those of perfectly
coordinated MOFs [25, 31], accounting for the gain in specific surface areas and pore volumes in
entries 3 and 13 (Table 1). Missing cluster defects were not considered in this work since TGA was
not able to reveal relevant information of metal cluster. It is also worth pointing out that even
though the proportion of linker deficiency in the framework increased by increasing the amount of
modulators used in synthesis, all UiO-67 MOFs (entries 2, 3, 12 and 13) prepared by microwave
heating were thermally stable up to 450 ºC, as compared to the findings of previous work [23].
14
10-6 10-5 10-4 10-3 10-2 10-1 1000
100
200
300
400
500
600
700
P/P0
Qua
ntity
Ads
orbe
d (c
m3 /
g)
Entry 2 (BenAc - 40 equiv)Entry 3 (BenAc - 70 equiv)Entry 12 (HCl - 135 equiv)Entry 13 (HCl - 185 equiv)
Fig. 5. N2 adsorption-desorption isotherms (at 77 K) of UiO-67 samples synthesized by microwave-assisted methods. Adsorption isotherms: open squares; desorption isotherms: stars.
According to findings above, 185 equiv of HCl (entry 13) was considered as the optimum because it
led to the UiO-67 MOF with the highest surface area and porosity (Table 1) in this study. When
BenAc was used as the modulator, 40 equiv of BenAc (entry 2) was selected as the optimum
amount for the further investigation of the synthesis time and temperature. Although the sample
from the entry 3 showed good pore volume and BET surface area, the yield was very low (37 mg
for entry 3 versus 75 mg for entry 2). It appears that an excess of the modulators in the synthesis of
UiO-67 was also able to slow down the nucleation process due to the prolonged exchange time
between the coordinated modulators and the linkers in the liquid phase. Thus, an increase in the
amount of modulating agents caused the lengthy exchange process, and hence the reduced yield of
UiO-67 (under same synthesis time) [28, 29]. In this work, it was found that the formation of UiO-
67 MOF was not possible after a 2.5 h synthesis by increasing the amount of BenAc to 100 equiv
(entry 4).
Conversely, when HCl was used as the modulator for UiO-67 synthesis, an increase in the amount
of HCl showed a positive effect on the formation of UiO MOFs (83 mg for entry 12 versus 94 mg 15
for entry 13) [30]. This may be the result of the water in HCl solution (assay=37%) since water is
essential for the hydrolysis of Zr salts and the provision of oxygen molecules for the formation of
secondary building units (SBUs). Therefore, the more HCl solution used in the UiO-67 synthesis,
the more water present in the synthesis system, and consequently the more SBUs formed, leading to
the accelerated formation of UiO-67 MOF (Eq. 3). A similar effect was also reported previously in
the synthesis of Zr-fumarate MOFs with formic acid as the modulator [28].
6 ZrCl4 + 6 H2bpdc + 8 H2O → Zr6(O)4(OH)4(bpdc)6 + 24 HCl (3)
In order to the further investigate the effect of synthesis time and temperature, the quantity of
modulators was fixed as 40 equiv of BenAc and 185 equiv of HCl, respectively, based on the
previous discussion. With 40 equiv of BenAc, UiO-67 was synthesized at temperatures of 100 °C,
120 °C and 140 °C (denoted as entries 5, 2 and 6, receptively). With 185 equiv of HCl, 80 °C, 100
°C and 120 °C (denoted as entries 14, 13 and 15, respectively) were used to prepare UiO-67. PXRD
results (Fig. 6) showed that synthesis of UiO-67 at relatively high temperatures (i.e. 140 °C with 40
equiv of BenAc and 120 ºC with 185 equiv of HCl) was not successful. It is hypothesized that the
combination of the irradiation time (2.5 h) and relatively high temperatures (140 °C with BenAc
and 120 °C with HCl) caused the formation of amorphous phases instead of UiO-67 framework. A
previous study [61] of the hydrothermal synthesis of UiO-66 showed that 6 h was sufficient to
prepare UiO-66 at 140 °C (6 h to 72 h). A further increase in the reaction time (> 36 h) strongly
affected the crystalline structure of resulting materials, as evidenced by PXRD analysis such as the
loss of intensity of characteristic peaks at 2θ = 7.3° and the appearance of new phases. Similarly, in
the microwave-assisted synthesis of MOF-5, crystal deterioration and surface defects were observed
when the microwave irradiation was prolonged to 30 min, which was attributed to the dissolution of
the coordinated organic ligands in the crystallized MOF-5 after the extended irradiation [49]. The
deterioration of UiO-67 crystals was also observed by SEM analysis (Fig. S7) for the sample
synthesized at 140 °C after 2.5 h microwave irradiation. It was found that 100 °C with HCl was not 16
ideal for synthesizing UiO-67 since the synthesis at 80 °C (entry 14) gave rise to the material with
high surface area (2547 m2 g‒1) and porosity (1.2 cm3 g‒1), comparable to theoretical values of pure
UiO-67.
Fig. 6. PXRD patterns of UiO-67 synthesized at different temperature by (a) HCl and (b) BenAc.
In the hydrothermal synthesis of Zr MOFs, relatively low temperatures were known to decrease the
reaction rate [29] and decelerate the crystal growth [28, 29], leading to a low product yield with
small crystals. Under microwave heating (BenAc as the modulator), an reduction in reaction
temperature from 120 °C to 100 °C (entry 2) also produced octahedral crystals with smaller sizes as
shown in Fig. S8 (i.e. < 1 µm in comparison to those of entry 5), as well as a low yield, i.e. 20 mg at
100 °C versus 75 mg at 120 °C after 2.5 h synthesis.
By comparison with findings from the solvothermal synthesis of UiO-67 [27, 30], (i.e. the optimum
temperature of 120 °C for BenAc as the modulator and of 80 °C for HCl as the modulator), the
results from present work are analogous. It was suspected that the proposed microwave-assisted
synthesis of UiO-67 could promote the effective heat transfer during the synthesis, and hence
improved temperature distribution through the synthesis media, causing a great reduction in the
reaction time (2.5 h) in comparison to that of the solvothermal synthesis (24 h). As mentioned
before, dielectric properties of DMF and its low thermal conductivity of 0.184 W m−1 K−1 clearly 17
rationalize the advantage of the microwave method over the solvothermal method in the synthesis
of UiO-67 (i.e. the improved heat transfer, minimized wall effect and uniform temperature
distribution of the microwave system). Thus, the improvement was supposed to result mostly from
the thermal effects of microwave irradiation though the existence of the non-thermal effects might
be hypothesized. It should be mentioned that the thermal and non-thermal effects are the subject of
current debates and extensive research efforts are needed to address them in the future, which is
beyond the scope of the current contribution.
At 120 °C (with 40 equiv of BenAc) and 80 °C (with 185 equiv of HCl), the reaction time was also
studied to understand its effect on the production of UiO-67 (Fig. 7). It was found that the reaction
time of 2–2.5 h was ideal for maximizing the yield of UiO-67 under the conditions used. Further
increase in the reaction time resulted in a decrease in the yield of MOFs since the prolonged contact
time between the formed UiO-67crystals and the synthesis medium caused the re-dissolution of
UiO-67. Similar observations were reported in our previous studies with regard to the synthesis of
Cu-based MOFs [17].
2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 440
50
60
70
80
90
100
Time (h)
Prod
uct m
ass
(mg)
Synthesised with benzoic acidSynthesised with HCl
Fig. 7. Product mass of UiO-67 as a function of time (the microwave-assisted method).
In order to do a fair comparison between the conventional and the microwave-assisted methods, two
indicators, namely the reaction mass efficiency (RME) and space-time yield (STY), were employed
to assess the efficiency of the two methods. As seen in Table 2, RME values of the two methods
18
were close, indicating that the same chemical recipe was used. Whereas, values of STY clearly
showed that the microwave-assisted method was able to produce more UiO-67 MOF per day than
the solvothermal method as the reaction time was significantly reduced by the efficient heating
under microwave irradiation. The relative crystallinity (RC) of materials was calculated using Eq. 4.
UiO-67 MOFs synthesized solvothermally were selected as the references and RC values of
materials were determined by using the integrated peak area method which compares the sum of
integrated peak areas of selected peaks at 2θ = 5.8°, 6.73°, 11.33°, 11.63° and 17.3°. As presented
in Table 2, UiO-67 MOFs were synthesized with high degree of crystallinity by the proposed
method under microwave irradiation.
(4)
where Sx = integrated peak areas for the sample UiO-67, and Sr = integrated peak areas for the
reference UiO-67.
Table 2Comparison of the solvothermal and microwave-assisted synthesis of UiO-67.Synthesis method Modulator STY
[kg m−3 d−1]RME[%]
SBET
[m2 g−1]RC[%]
Microwave BenAc 28.8 46 2354 92.6Microwave HCl 37.3 60 2547 90.2Solvothermal BenAc 4.56 46 2247 100a
Solvothermal HCl 3.72 59 2460 100b
a used as the reference for UiO-67 synthesized with BenAc (Microwave); b used as the reference for UiO-67 synthesized with HCl (Microwave).
CO2 and CH4 adsorption on UiO-67
In order to assess the capacity of materials synthesized by conventional and microwave-assisted
methods for gas adsorption, adsorption equilibrium isotherms of CH4 and CO2 at different
temperatures on UiO-67 were measured (Figs. 8a-8d). It was found that, at 25 and 50 °C, samples
produced by microwave heating showed slightly higher adsorption capacities of CO2 and CH4 than
those of conventionally synthesized UiO-67, which could be attributed to the relatively high BET
19
values rendered by the microwave synthesis, resulting from the linker deficiency in their structures.
As expected, an increase of the adsorption temperature has an adverse effect on the adsorption
capacity of UiO-67 (for both probing gas molecules). UiO-67 showed a higher affinity to CO2 than
CH4, evidenced by the significant difference in adsorption capacity at all pressure stages. In general,
the prepared UiO-67 showed favored selectivity to CO2 over CH4 and the relevant equilibrium
uptakes of ca. 17 mmol g‒1 for CO2 and ca. 6 mmol g‒1 for CH4 were slightly higher than other
MOFs such as HKUST-1 (15 mmol g‒1 for CO2) [17] and MIL-53 (5 mmol g‒1 for CH4) [62] at
similar conditions of at 25 °C–31°C and 20 barG.
The isosteric heat of adsorption was calculated using the Clausius-Clapeyron equation [63] to check
the homogeneity of the adsorption environment in the framework of UiO-67. In order to use the
Clausius-Clapeyron equation (Eq. 5), one needs an adsorption isotherm model which describes the
pressure as a function of the adsorption uptake for at least two temperatures. For this purpose,
Langmuir model (Eq. 6) was used to fit experimental data to extract relevant information.
(5)
where P [bar] is the pressure, T [K] is the temperature and R [J mol‒1 K‒1] is universal gas constant.
(6)
where b [bar‒1] is Henry's constant, q and qsat [mmol g‒1] are the adsorption uptake at a relevant
pressure (P) and the saturated uptake, respectively.
The heat of adsorption of UiO-67 synthesized by the microwave-assisted method as a function of
CO2 and CH4 uptake is shown in Figs. 8e and 8f. The existence of the homogeneous adsorption
environment in UiO-67 was proved by almost constant values of the heat of adsorption calculated at
different CO2 and CH4 uptakes [17]. Small variation in the heat of adsorption might be an artefact of
the parameterization using the Langmuir model (i.e. fitting errors between experimental and
20
calculated isotherms [64]). Calculated values of the isosteric heat for CO2 and CH4 adsorption onto
UiO-67 framework are approximately 26 kJ mol−1 and 17 kJ mol−1, respectively, for the samples
synthesized by the microwave-assisted methods, which are typical values for physisorption and
comparable to those in the literature [17, 64, 65].
Fig.8. Adsorption isotherms CO2 on UiO-67 synthesized by the microwave-assisted method and solvothermal method using (a) BenAc as the modulator and (b) HCl as the modulator; adsorption isotherms CH 4 on UiO-67 synthesized by the microwave-assisted method and solvothermal method using (c) BenAc as the modulator and (d) HCl as the modulator; calculated isosteric heat of adsorption of (e) CO2 and (f) CH4 adsorption on UiO-67 (by the microwave-assisted method).
21
Conclusion
In this work, microwave-assisted synthesis of Zr-based MOFs with BenAc and HCl as modulators
was performed. A systematic study was carried out to optimize synthesis conditions (i.e. the amount
of modulator, the reaction time and temperature) under microwave irradiation to yield materials
with good porous properties and high yield.
Results showed that the amount of modulator played a key role on the surface area and pore
volumes of the synthesized materials by promoting linker deficiency during the synthesis. In the
case of using BenAc as the modulator, 40 equiv was selected as the optimum amount since lower
quantities could not produce crystalline UiO-67 (because of the formation of the zirconia gel at the
beginning of the reaction) and higher amounts reduced the yield significantly (because of the
slowing down of the nucleation process). When HCl was used as the modulator (185 equiv as the
optimum amount), the provision of water (oxygen molecules) was believed to form SBUs and
hydrolyze zirconium, benefiting the formation of UiO-67.
Findings also showed that the synthesis of UiO-67 at high temperatures (140 ºC with BenAc and
120 ºC with HCl) led to the deterioration of UiO-67 crystals and the formation of X-ray amorphous
by-products. We found the best synthesis temperatures as 120 ºC for BenAc and 80 ºC for HCl,
similar to those used in conventional methods. However, the reaction time under microwave
irradiation was much shorter (ca. 2‒2.5 h) than that of conventional solvothermal methods (e.g. 24
h) due to the thermal effect of microwave heating. Porous properties and morphology, as well as gas
adsorption capacities (with CO2 and CH4) of resulting materials were compared with those
synthesized by conventional heating methods, proving the successful synthesis of UiO MOFs by
microwave heating methods. Two indicators, STY and RME, were utilized to evaluate the
efficiency of the developed microwave method, showing that the microwave-assisted method is an
efficient alternative to synthesize highly crystalline UiO MOFs.
22
This work demonstrates the feasibility of combining microwave with modulation for the fast
synthesis of high quality MOFs in diluted DMF solutions with controlled properties. Future work
along this direction is suggested to adjust the method with concentrated systems and conditions
relevant to industrial production which may be suitable for upscaling at scales.
Acknowledgements
We thank the financial support from The Royal Society (RG160031). RV acknowledges The
University of Manchester President's Doctoral Scholar Award for supporting his PhD research. SX
thanks the funding from the European Community’s Seventh Framework (FP7)/People-Marie Curie
Actions Programme (RAPID under Marie Curie Grant agreement no 606889). NA-J thanks The
Higher Committee for Education Development in Iraq for providing her postgraduate research
scholarship.
Supporting Information
Supplementary data associated with this article can be found in the online version at http://
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