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: xiaolei.fan@manchester.ac.uk1 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

4

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.

12

(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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