Facile Synthesis of UiO-66(Zr) Using a Microwave-Assisted

Subscriber access provided by UNIV OF SOUTHERN INDIANA

Article

Facile synthesis of UiO-66(Zr) using a microwave-assisted continuous tubular reactor and its application for toluene adsorption Vo The Ky, Van Nhieu Le, Kye Sang Yoo, Mugeun Song, Daekeun Kim, and Jinsoo Kim Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00170 • Publication Date (Web): 26 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

Facile synthesis of UiO-66(Zr) using a microwave-assisted continuous tubular reactor and its application for toluene adsorption

The Ky Vo1, Van Nhieu Le2, Kye Sang Yoo3, Mugeun Song4, Daekeun Kim4*, Jinsoo Kim2*

1Department

of Chemical Engineering, Industrial University of Ho Chi Minh City, 12 Nguyen Van Bao, Go Vap, Ho Chi Minh City, Vietnam

2Department

of Chemical Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Korea 3Department

of Chemical & Biomolecular Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul, 01811, Korea

4Department

of Environmental Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul, 01811, Korea

*Corresponding authors: E-mail address: [email protected] (J. Kim), [email protected] (D. Kim)

1 Environment ACS Paragon Plus

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract UiO-66(Zr) has been widely used for many applications due to its high surface area and excellent chemical and thermal stabilities. Conventionally, UiO-66(Zr) has been prepared in an autoclave by solvothermal synthesis, which requires a lengthy reaction time (typically 24 h) while only producing a small amount of product. In this work, a larger quantity of UiO-66(Zr) was rapidly produced in a continuous tubular reactor under microwave irradiation. The metal salt and organic linker precursor solutions were continuously introduced into the tubular reactor by a microfluidic syringe pump system. The results showed that UiO-66(Zr) was produced with a high yield, porosity, and crystallinity within a short reaction time of 10 min. The characteristics of UiO-66(Zr) were affected by the reaction temperature, residence time, and modulator concentration. The UiO-66(Zr) prepared under the optimum conditions was evaluated for gaseous toluene adsorption at various temperatures.

Key words: MOFs, UiO-66(Zr), continuous-flow, microwave synthesis, tubular reactor


Page 2 of 30

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Over the past decade, metal-organic frameworks (MOFs) have attracted a great amount of attention due to their fascinating properties, including high porosity and flexible frameworks 1, 2. These unique properties are desirable in many applications such as separation, catalysis, drug delivery, and gas storage. Among the widely used MOFs, UiO-66 (University of Oslo) is a Zr cluster-based MOF with a centric octahedral cage, each corner of which is linked to a tetrahedral cage. UiO-66 has high thermal and chemical stabilities 3, 4, with a BET surface area of ca. 1,200 m2/g

5, 6.

One of the interesting properties of UiO-66 is that the framework can be easily

manipulated to generate missing linker or cluster defects by using modulators such as HCl or monocarboxylic acid, resulting in great increases of surface area and pore volume 7-9. In addition, many kinds of UiO-66 derivatives can be produced via functionalization of the UiO-66 linker. Functionalized UiO-66 has superior properties with similar framework structures and physicochemical properties as the parent UiO-66. For instance, UiO-66(CH3)2, H2N-UiO-66, and O2N-UiO-66 showed greatly improved CO2 adsorption capacities compared to unfunctionalized UiO-66

10.

UiO-66(SO3H), UiO-66(COOH), and UiO-66(I) frameworks showed enhanced

CO2/CH4 adsorption selectivities compared to UiO-66

11.

These unique characteristics of the

UiO-66 framework and its derivatives will be useful for many applications 12. Conventionally, UiO-66(Zr) is prepared through solvothermal synthesis, which requires a lengthy reaction time (typically 24 h) and high energy input while only producing small amounts of material

4, 13-20.

Pilot-scale production of UiO-66(Zr) (⁓100 L) by solvothermal synthesis

using a glass reactor was reported by Kim et al.

21,

generating UiO-66 at a high yield (97%).

However, this approach required a long reaction time and high energy input. Compared to a batch method, continuous synthesis of MOFs is more promising due to faster heat and mass



transfer, resulting in higher production throughput

22.

Page 4 of 30

In an attempt to develop a scale-up

procedure for amine-functionalized UiO-66(Zr), Schoenecker et al. 3 designed a continuous-flow method for a PTFE tubular reactor which used ca. 2 L of a reactant solution. However, the procedure required a long reaction time due to conventional heating and the obtained product had a low BET surface area (ca. 640 m2/g) compared to the commercial model. Recently, Hill et al. 23

reported flow reactor synthesis of UiO-66(Zr) at a production rate of 60 g/h with a very high

space-time yield and a short reaction time of 10 min. This represents a new achievement in the scaled up synthesis of UiO-66(Zr) MOFs. Nevertheless, this method requires further improvement since the obtained UiO-66 showed low crystallinity and a low surface area (ca. 1,186 m2/g). Microwave-assisted synthesis (MAS) has been widely used and is considered an efficient and promising approach for the synthesis of MOF materials as the entire volume of the reactant mixture can be uniformly heated within a very short time

1, 24-28.

Very recently, Vakili et al.

29

successfully synthesized UiO-67 and UiO-66 with controllable particle sizes within a short reaction time (ca. 2 h) under microwave heating conditions. Even though the use of the MAS approach can produce MOFs in a short time, the development of a facile scalable method for MOF production is required to reduce manufacturing costs 3. Considering the improvement of both the reaction time and product quantity, Taddei et al.

30

provided a continuous-flow

procedure for the synthesis of UiO-66(Zr), MIL-53(Al), and HKUST-1 with the assistance of microwave irradiation. They reported MOFs in large quantities and high yields with a very short reaction time of 7 min. This method is easy to scale up for MOF production without changing the product characteristics. However, it should be further optimized since the obtained UiO-66(Zr) had a relatively low surface area (1,084 m2/g) and pore volume (0.35 cm3/g). In addition, there


Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


have been few studies regarding modulator effects on the characteristics of UiO-66(Zr) formed under continuous microwave synthesis, which was reported to have a remarkable influence on the porosity of the UiO-66 framework produced by solvothermal synthesis 5, 7, 9. Volatile organic compounds (VOCs) have become one of the major precursors for ozone and secondary organic aerosols (SOAs), which are harmful to both human health and the environment

31.

Toluene, the most commonly encountered VOC, is one of the largest

contributors to both ozone and SOA formation

32.

Since the presence of toluene in the

atmosphere can cause a variety of severe health problems, many VOC removal methods have been developed, including adsorption, condensation, incineration, and biological degradation. Among these methods, adsorption has been considered one of the most favorable to control VOCs due to its ease of application and high efficiency 33. In this work, UiO-66(Zr) was prepared by a continuous tubular reactor system under microwave irradiation. Dual microfluidic syringe pumps continuously introduced precursor solutions into a PTFE tubular reactor in a microwave oven at the desired temperature. The effects of the operating conditions (e.g., residence time, reaction temperature, and modulator concentration) on the morphology, crystallinity, porosity, and product yield were systematically investigated. The obtained UiO-66 was evaluated for gaseous toluene adsorption at different temperatures.

2. Experimental 2.1. Synthesis of UiO-66



Figure 1 shows the reactor design for the synthesis of UiO-66 using a continuous-flow, microwave-assisted tubular reactor. Two microfluidic programmable syringe pumps (11 Pico Plus Elite, Harvard Apparatus, USA) inject metal salt and ligand precursor solutions into the PTFE tubular reactor (length: 10 m, internal diameter: 2 mm) inside a microwave oven at the desired reaction temperature. This system allows the precursor solutions to flow smoothly in a confined dimension, which facilitates nucleation and crystal growth reactions. Zirconium (IV) chloride (4.0 g) (ZrCl4, 99.8% Aldrich) and 4.0 g of terephthalic acid (1,4benzenedicarboxylic acid, 99%, Aldrich) were dissolved in N,N-dimethylformamide (DMF) at the desired concentration. Calculated amounts of modulator and water were added to the ZrCl4 solution under stirring and sonication. In this work, HCl (37%, Aldrich) was used as a modulator to enhance the solubility of ZrCl4 in DMF and UiO-66 formation 5, 18, 34. Before introducing the reaction mixture, the tubular reactor was filled with pure DMF and the temperature of the microwave oven was set at the desired value. The microwave irradiation power was fixed at a constant value of 350 W (2.5 GHz). After reaching the desired temperature, the precursor solutions were placed in the microwave oven. The flow rates of the precursor solutions were set depending on the assigned residence time. In this study, various reaction temperatures (80-120 C), residence times (5-30 min), and HCl/Zr4+ molar ratios (40-180 equivalents) were applied. After each run, the tubular reactor was rinsed with pure DMF to efficiently remove the product and refresh the system. The obtained products were centrifuged, washed with DMF at 70 C for 6 h (2 times) and ethanol at 70 C for 6 h (2 times), and dried at 70 C for 24 h. 2.2. Characterization


Page 6 of 30

Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Field-emission scanning electron microscopy (FE-SEM, Leo-Supra 55, Carl Zeiss STM, Germany) analyses were performed to examine the morphologies of the synthesized UiO-66(Zr) samples. The textural properties of UiO-66(Zr) samples were characterized by N2 porosimetry (BELSORP-max, BEL, Japan) at 77 K after being degassed at 423 K for 12 h. The standard Brunauer-Emmett-Teller (BET) equation was used to calculate the BET surface area in the pressure range of 0.01 < P/P0 < 0.15. The thermal stability of the prepared UiO-66 was tested by thermogravimetric analysis (TGA; Q50, TA Instruments, USA) under N2 flow at a heating rate of 5 C/min. FT-IR analysis was performed in the range of 4,000-400 cm−1 using an FT-IR spectrometer (Tensor 27, Bruker, Germany). Powder X-ray diffraction (XRD; MAC-18XHF, Rigaku, Japan) was used to determine the crystallographic structures of the UiO-66(Zr) framework. 2.3. Toluene adsorption The adsorbent samples were activated under vacuum (10-2 kPa) at 150 C for 12 h before application for toluene adsorption. A toluene breakthrough test was carried out using a glass bed (internal diameter: 1 cm; length: 15 cm; total volume: 11.7 mL) under atmospheric pressure. For each run, 0.25 g of adsorbent was loaded in the bed. Toluene gas (99.99%) with an inlet concentration of 1,000 ppm balanced with air was passed through the fixed bed. The gas flow rate was regulated using a mass flow controller (EL-FLOW classic series, Bronkhorst High-tech B.V., Holland) and set to 0.3 L/min, corresponding to a space velocity of 294-319 min−1. Toluene was analyzed using a gas chromatograph (YL6500GC, Younglin Co., Ltd., Korea) with a flame ionization detector. The adsorption capacity of the synthesized materials for toluene removal was calculated by measuring the breakthrough point of 5% of the inlet concentration.



Page 8 of 30

3. Results and discussion 3.1. Continuous-flow microwave synthesis of UiO-66(Zr) In this study, a 10 m long PTFE tubular reactor having an internal diameter of 2 mm (volume: ca. 32 mL) was coiled around a custom-made PTFE stand and used for the continuous microwave-assisted synthesis of UiO-66(Zr) (Fig. 1). The liquid phase UiO-66 metal salt and organic linker precursor solutions were continuously fed via syringe pumps, mixed by a T junction, into the microwave oven held at the desired temperature. The precursor mixture in the tubular reactor could quickly reach the specific reaction temperature due to the efficient heat transfer resulting from uniform microwave irradiation throughout the coiled PTFE reactor. The microwave-assisted batch reactor system is not easy to scale up as microwave radiation has a limited penetration depth into absorbing media, which essentially limits the size of the reactor, hindering the development of microwave-assisted synthesis of MOFs on the large scale

30.

However, a microwave-assisted continuous tubular reactor system can easily be scaled up by avoiding penetration depth issues. A large reaction volume can be processed by employing multiple tubular reactors of limited diameters, ensuring rapid and homogeneous heating of reaction mixtures. This enables efficient scale-up of microwave-assisted syntheses without significant changes of the experimental parameters 35. XRD analyses were conducted using UiO-66(Zr) samples obtained at various HCl/Zr4+ equivalents, reaction temperatures, and residence times (Fig. 2). In general, the XRD patterns of all samples showed the same topological structure of UiO-66(Zr) as that reported in the literature 15, 36

and the characteristic reflection peaks of the prepared materials match those of the simulated


Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pattern well. This indicates that UiO-66 was successfully prepared under the continuous-flow microwave conditions. No peaks of UiO-66(Zr) were observed for the samples prepared in the absence of modulator under continuous-flow microwave conditions for a short residence time. Figure 2(a) shows the XRD patterns of the UiO-66(Zr) samples obtained at different modulator (HCl) concentrations by continuous-flow microwave synthesis at 120 C for 10 min. The intensities of the XRD peaks increased with increasing HCl concentration, implying that a higher modulator concentration accelerates the crystallinity of UiO-66(Zr). In addition, at a small modulator amount, the XRD peaks are broader, suggesting that the obtained UiO-66(Zr) particles had a smaller crystallite size. These findings are consistent with the data of Zr-based MOF materials reported in the literature

29, 34, 37.

It was previously reported that modulator (HCl) can

affect the nucleation process and crystallinity of the final product 29. Figure 2(b) shows the XRD patterns of the UiO-66(Zr) samples as a function of the reaction temperature. The intensities of the XRD peaks increased with increasing reaction temperature from 80 to 120 C. This is because a higher temperature accelerates the crystallization of UiO-66(Zr). The effect of residence time is shown in Fig. 2(c). When the residence time was 5 min, the XRD pattern showed very low intensities, indicating incomplete crystallization. However, as the residence time was increased to over 10 min, the XRD patterns showed well-defined phase structures of UiO-66 with a peak at 2θ = 12.08o (220), indicating higher crystallinity of UiO-66 with a longer reaction time. The above findings indicate that UiO66(Zr) can be formed at 80 C for 10 min under microwave heating conditions. It has been reported that UiO-66(Zr) can be synthesized at a low temperature of 60 C temperature

38

5

or even room

with a very long reaction time (>12 h) by a solvothermal reaction. The present

work demonstrated that UiO-66(Zr) can be produced with a very short reaction time at moderate



temperatures by a microwave-assisted method. This is possible because the heat transfer inside the tube due to microwave heating is considerably improved compared to that of conventional heating 29, resulting in the formation of the UiO-66(Zr) framework within a very short reaction time. Table 1 summarizes the BET surface areas and pore volumes of the UiO-66 samples obtained at various operating conditions. Among them, the HCl/Zr4+ molar ratio and residence time strongly affected the textural properties of the product UiO-66 samples. The BET surface area increased from 707 m2/g to 1,320 m2/g and the pore volume increased from 0.93 cm3/g to 1.50 cm3/g as the HCl equivalent to Zr4+ ratio was increased from 40 to 120. An improved surface area with increasing acid modulator concentration was also reported in the literature 5, 6, 9. The pore structure of UiO-66(Zr) can be well controlled by changing the concentration of modulator (acid amount), likely due to missing linkers or missing cluster defects of the UiO-66 framework

5, 7, 9.

However, further increase of HCl to 180 eq. resulted in a decrease of surface

area and increase of pore volume (972 m2/g, 1.74 cm3/g), likely due to a more mesoporous structure formed by missing linker defects created under the high acid concentration. A similar phenomenon was reported in the literature 15. Although UiO-66(Zr) formation under continuousflow microwave conditions required a short reaction time, the pore structure of UiO-66 is also affected by the residence time. The BET surface area increased from 973 m2/g to 1,320 m2/g when the reaction time was increased from 5 to 10 min. For the samples with reaction times above 15 min, however, BET surface area decreased and the pore volume increased (see Table 1). Therefore, the residence time was fixed at 10 min. The morphologies and structures of the prepared samples were examined by scanning electron microcopy (SEM). As shown in Fig. 3, SEM images revealed aggregated spheroidal


Page 10 of 30

Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


particles in the range of 20-50 nm. A similar phenomenon was observed when UiO-66(Zr) was prepared using HCl as a modulator in microwave-assisted synthesis for a longer reaction time of 2.5 h

29

or by solvothermal synthesis (20 h) 6. In contrast, when acetic acid was used as the

modulator under the same conditions, the SEM image of UiO-66 showed an octahedral morphology with a particle size of about 100 nm (Fig. S1). This is due to the competition between linkers and acetic acid molecules during the ligand exchange reaction resulting in a much reduced number of nuclei, leading to the growth of large crystals

29.

It was reported that

the morphology of zirconium-based metal organic frameworks can be controlled by the modulator type and concentration

18, 29.

The use of monocarboxylic acid (acetic acid or benzoic

acid) as a modulator can produce UiO-66(Zr) in the form of intergrown aggregates of very small crystals or octahedrally-shaped individual nanocrystals depending on the concentration

15, 18, 37.

Even though the use of HCl produced highly aggregated UiO-66(Zr) particles, the particle size increased from ca. 20 nm to ca. 40 nm with increasing HCl concentration from 40 eq. to 180 eq. (see Fig. 3). Similar trends were observed with increases of the reaction temperature (see Fig. S2) and residence time (see Fig. S3) under microwave irradiation. These results suggest that a high modulator concentration, reaction temperature, and long reaction time facilitate the nucleation and crystal growth of MOF particles. The yield and production rate of UiO-66(Zr) were investigated as functions of the operating conditions. As shown in Table 1, the yield gradually increased with increasing modulator concentration and reaction temperature. The yield of UiO-66(Zr) increased with increasing modulator (HCl) amount, indicating that the modulator enhanced the interactions between linkers and metal clusters. It was observed that HCl accelerates formation of the UiO66(Zr) framework by dissociating linkers from nodes and speeding up the connection of nodes to



one another 5. A high yield of 90% and production rate of 6.10 g/h can be reached in a reaction time of 10 min at 120 C. However, in the samples obtained with reaction times above 20 min, the yield and production rate decreased to 86% and 2.88 g/h, respectively, possibly due to redissolution of UiO-66 as a result of a longer contact time with the reaction media. The same phenomena were observed on UiO-67 29 and Cu-based MOFs 39. When UiO-66(Zr) was prepared by conventional solvothermal synthesis for 24 h, the yield was 78%. As shown in Table 1, the highest production rate of UiO-66(Zr) was 9.37 g/h. This result suggests that continuous-flow microwave synthesis is a promising approach for the large-scale production of UiO-66(Zr). Nitrogen gas adsorption-desorption isotherms of the UiO-66(Zr) samples were analyzed to investigate the effects of the operating conditions on textural properties (Fig. 4). All samples showed a typical type IV isotherm with an H3-type hysteresis loop, demonstrating the presence of slit-like pores and the coexistence of mesopores and macropores within the UiO-66(Zr) framework

40.

The high rate of gaseous nitrogen adsorption at very low relative pressures

illustrates the presence of micropores in the UiO-66 framework structure, while a small hysteresis loop at high relative pressures indicates the existence of mesopores. Figure 5 shows the TGA analysis results of the UiO-66(Zr) samples obtained at different modulator equivalents. As shown in the figure, there were three weight loss steps observed between 25 and 700 C. The first minor weight loss between 25 and 100 C is assigned to the release of guest solvent molecules of water and ethanol. The second weight loss occurring between 100 and 500 C is attributed to the dehydroxylation of zirconium oxo-clusters 17, 41. The final weight loss step near 500 C is due to decomposition of the organic linkers in the framework 5. This is consistent with other reports on the thermal stability of the UiO-66(Zr) framework

5, 17, 42.

Fig. 5 shows that the HCl equivalents affected the weight loss of the UiO-


Page 12 of 30

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


66(Zr) samples. Defect-free structured UiO-66 is fully coordinated with 12 terephthalate linkers 43.

As reported, the TGA analyses of UiO-66(Zr) can be used to estimate the average number of

linkers in the secondary building units (SBUs) of UiO-66(Zr)

5, 29, 43.

As the amount of HCl

equivalents was increased from 40 eq. to 80 eq., 120 eq., and 180 eq., the corresponding experimental weight percentages of linkers decreased from 48.42% to 46.57, 43.32, and 40.45%, respectively. These values are smaller than those of defect-free SBUs (54.6%), indicating the formation of defects in the SBUs for UiO-66(Zr)

5, 43.

Accordingly, the average numbers of

linkers in the SBUs were estimated to be 10.64, 10.14, 9.52, and 8.89, respectively, with increasing modulator concentration from 40 eq. to 180 eq. In addition, the SBU connections decreased with increasing temperature and residence time (Table 1). These results are in good agreement with those previously reported 6, 29, 44. FT-IR analysis of the UiO-66(Zr) obtained by continuous-flow microwave synthesis at 120 C and 10 min was conducted to confirm the formation of UiO-66(Zr) and the results were compared with those of samples prepared by the conventional solvothermal approach at 120 C for 24 h. As shown in Fig. 6, all of the vibration peaks of UiO-66 prepared by continuous-flow microwave synthesis matched those of UiO-66 synthesized by the conventional method well. The vibration peaks at 1,570 cm-1 and 1,395 cm-1 are attributed to symmetric and asymmetric stretching vibrations of the O-C-O bond in the carboxylate group of the linker, respectively

16.

The peak around 1,510 cm-1 is due to vibration of the C=C bonds in the benzene ring. The bands at 740 cm-1, 653 cm-1, and 550 cm-1 are ascribed to vibrations of Zr-O-C 8.

3.2. Toluene adsorption and desorption on UiO-66(Zr)



Page 14 of 30

The UiO-66(Zr) sample prepared by continuous-flow microwave synthesis under the optimum conditions (120 C, 10 min, and 120 HCl equivalents) was evaluated for toluene adsorption at different temperatures. Figures 7(a-b) show the breakthrough curves of toluene adsorption on UiO-66(Zr) at 25, 50, 75, and 100 C, as well as the corresponding equilibrium adsorption capacities. The toluene adsorption capacity of UiO-66(Zr) decreased with increasing temperature, indicating the exothermic nature of toluene adsorption on UiO-66(Zr). At 25 C, the prepared UiO-66(Zr) showed a toluene adsorption capacity of 130 mg/g, which is higher than those of MOF-5 (32.9 mg/g), MIL-101(Fe) (98.3 mg/g), and Zeolite (30.7 mg/g). It is also comparable to that of UiO-66(Zr) obtained by solvothermal synthesis

40.

The effects of

temperature on toluene adsorption using UiO-66(Zr) suggests that the predominant mechanism between MOF and gaseous toluene is physisorption 40. The adsorption kinetics of UiO-66(Zr) was investigated by utilizing the Thomas model and the Yan model, which are used in column performance theory. The linearized forms of the Thomas model 45 (Eq. (1)) and the Yan model 46 (Eq. (2)) are as follows: C  k q m ln  0  1  Th Th  kThC0t , Q  Ct   Ct ln   C0  Ct

(1)

  k y Co   Q 2   k y C0  ,      ln   ln t Q k q m Q   y y     

(2)

where C0 (mg/mL) is the influent concentration of the adsorbate, Ct and Ce (mg/mL) are the effluent concentrations of the adsorbate at time (t) and equilibrium, respectively, t is the operation time (min), Q is the flow rate of carrier gas (mL/min), m is the adsorbent amount (g), kTh and kY (ml/minmg) are the Thomas and Yan model constants, respectively, and qTh and qY (mg/g) are the maximum adsorption capacities calculated by the Thomas and Yan models,


Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


respectively. The experimental data were fit to Eq. (1) and Eq. (2) to obtain the adsorption capacities and kinetic parameters for each model listed in Table 2. In the investigated adsorption temperature range of 25-100 C, the calculated maximum adsorption capacities from the Yan model were much closer to the experimental data. In addition, the correlation coefficients (R2) from the Yan model (0.973-0.996) were higher than those obtained from the Thomas model (0.943-0.964). This result suggests that the Yan model is more suitable to describe the adsorption kinetics of toluene on UiO-66(Zr). Figure 8 shows the regenerability of UiO-66(Zr) after the toluene adsorption test. The desorption experiment was performed at room temperature and atmospheric pressure. For the first 25 min of desorption, 50% of the adsorbed toluene was recovered and the rates of desorption and adsorption were very similar. In the next stage, however, the desorption rate decreased until most of the toluene was recovered. The change of the desorption behavior could be due to the existence of different kinds of interactions between toluene molecules and the UiO66(Zr) framework

14, 47.

About 95% of the toluene was recovered under mild desorption

conditions, suggesting that UiO-66(Zr) is a potential reusable adsorbent for toluene recovery.

4. Conclusions UiO-66(Zr) was successfully prepared by continuous-flow microwave synthesis. The porosity and crystallinity were strongly affected by the experimental conditions. The reaction temperature and residence time were the major parameters of the crystallinity of UiO-66(Zr), while the acid modulator mainly affected the pore structure of UiO-66(Zr). The product yield was strongly influenced by both the modulator amount and residence time. The UiO-66(Zr)



prepared under the optimum conditions (120 C, 10 min, and 120 HCl equivalents) showed a high yield (about 90%), high surface area (1,320 m2/g), and suitable pore volume (1.50 cm3/g), representing a dramatic improvement over conventional solvothermal synthesis. Toluene adsorption tests on the UiO-66(Zr) samples at different temperatures revealed that the highest adsorption capacity was 130 mg/g at 25 C. The results of the desorption experiment showed that approximately 95% of the toluene was recovered at ambient temperature. These findings suggest that the continuous-flow microwave method is a potential route for the large-scale production of high-quality UiO-66(Zr) frameworks.

Acknowledgments This work was supported by the Technology Development Program to Solve Climate Changes (NRF-2017M1A2A2086815, 2017M1A2A2086647) and the Engineering Research Center of Excellence Program (NRF-2014R1A5A1009799) through the National Research Foundation (NRF) funded by the Ministry of Science and ICT, Republic of Korea.


Page 16 of 30

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References (1) Jhung, S. H.; Lee, J.-H.; Yoon, J. W.; Serre, C.; Férey, G.; Chang, J.-S. Adv. Mater. 2007, 19, 121124. (2) Pirzadeh, K.; Ghoreyshi, A. A.; Rahimnejad, M.; Mohammadi, M. Korean J. Chem. Eng. 2018, 35, 974-983. (3) Schoenecker, P. M.; Belancik, G. A.; Grabicka, B. E.; Walton, K. S. AIChE Journal. 2013, 59, 12551262. (4) Biswas, S.; Van Der Voort, P. Eur. J. Inorg. Chem. 2013, 2013, 2154-2160. (5) Katz, M. J.; Brown, Z. J.; Colón, Y. J.; Siu, P. W.; Scheidt, K. A.; Snurr, R. Q.; Hupp, J. T.; Farha, O. K. Chem. Commun. 2013, 49, 9449-9451. (6) Liang, W.; Coghlan, C. J.; Ragon, F.; Rubio-Martinez, M.; D'Alessandro, D. M.; Babarao, R. Dalton Trans. 2016, 45, 4496-4500. (7) Shearer, G. C.; Chavan, S.; Bordiga, S.; Svelle, S.; Olsbye, U.; Lillerud, K. P. Chem. Mater. 2016, 28, 3749-3761. (8) Wang, Y.; Li, L.; Dai, P.; Yan, L.; Cao, L.; Gu, X.; Zhao, X. J. Mater. Chem. A. 2017, 5, 22372-22379. (9) Wu, H.; Chua, Y. S.; Krungleviciute, V.; Tyagi, M.; Chen, P.; Yildirim, T.; Zhou, W. JACS. 2013, 135, 10525-10532. (10) Huang, Y.; Qin, W.; Li, Z.; Li, Y. Dalton Trans. 2012, 41, 9283-9285. (11) Biswas, S.; Zhang, J.; Li, Z.; Liu, Y.-Y.; Grzywa, M.; Sun, L.; Volkmer, D.; Van Der Voort, P. Dalton Trans. 2013, 42, 4730-4737. (12) Sang, X.; Zhang, J.; Xiang, J.; Cui, J.; Zheng, L.; Zhang, J.; Wu, Z.; Li, Z.; Mo, G.; Xu, Y.; Song, J.; Liu, C.; Tan, X.; Luo, T.; Zhang, B.; Han, B. Nat. Commun. 2017, 8, 175. (13) Nguyen, H. G. T.; Schweitzer, N. M.; Chang, C.-Y.; Drake, T. L.; So, M. C.; Stair, P. C.; Farha, O. K.; Hupp, J. T.; Nguyen, S. T. ACS Catalysis. 2014, 4, 2496-2500. (14) Grissom, T. G.; Sharp, C. H.; Usov, P. M.; Troya, D.; Morris, A. J.; Morris, J. R. J. Phys. Chem. C. 2018, 122, 16060-16069. (15) Qiu, J.; Feng, Y.; Zhang, X.; Jia, M.; Yao, J. J. Colloid Interface Sci. 2017, 499, 151-158. (16) Han, Y.; Liu, M.; Li, K.; Zuo, Y.; Wei, Y.; Xu, S.; Zhang, G.; Song, C.; Zhang, Z.; Guo, X. CrystEngComm. 2015, 17, 6434-6440. (17) Liu, X.; Zhao, X.; Zhou, M.; Cao, Y.; Wu, H.; Zhu, J. Eur. J. Inorg. Chem. 2016, 2016, 3338-3343. (18) Schaate, A.; Roy, P.; Godt, A.; Lippke, J.; Waltz, F.; Wiebcke, M.; Behrens, P. Chem. Eur. J. 2011, 17, 6643-6651. (19) Butova, V. V.; Budnyk, A. P.; Guda, A. A.; Lomachenko, K. A.; Bugaev, A. L.; Soldatov, A. V.; Chavan, S. M.; Øien-Ødegaard, S.; Olsbye, U.; Lillerud, K. P.; Atzori, C.; Bordiga, S.; Lamberti, C. Cryst. Growth Des. 2017, 17, 5422-5431. (20) Bae, S.; Zaini, N.; Kamarudin, K. S. N.; Yoo, K. S.; Kim, J.; Othman, M. R. Korean J. Chem. Eng. 2018, 35, 764-769. (21) Kim, S.-N.; Lee, Y.-R.; Hong, S.-H.; Jang, M.-S.; Ahn, W.-S. Catal. Today. 2015, 245, 54-60. (22) Hu, Z.; Zhao, D. Dalton Trans. 2015, 44, 19018-19040. (23) Rubio-Martinez, M.; Batten, M. P.; Polyzos, A.; Carey, K.-C.; Mardel, J. I.; Lim, K.-S.; Hill, M. R. Sci. Rep. 2014, 4, 5443. (24) Yuan, Y.-P.; Yin, L.-S.; Cao, S.-W.; Xu, G.-S.; Li, C.-H.; Xue, C. Appl. Catal. B Environ. 2015, 168-169, 572-576. (25) Ge, J.; Liu, L.; Shen, Y. J. Porous. Mat. 2017, 24, 647-655. (26) Han, L.; Qi, H.; Zhang, D.; Ye, G.; Zhou, W.; Hou, C.; Xu, W.; Sun, Y. New J. Chem. 2017, 41, 13504-13509.



(27) Zhou, B.; Ji, Y.; Yang, Y.-F.; Li, X.-H.; Zhu, J.-J. Cryst. Growth Des. 2008, 8, 4394-4397. (28) Silva, P.; Valente, A. A.; Rocha, J.; Almeida Paz, F. A. Cryst. Growth Des. 2010, 10, 2025-2028. (29) Vakili, R.; Xu, S.; Al-Janabi, N.; Gorgojo, P.; Holmes, S. M.; Fan, X. Microporous Mesoporous Mater. 2018, 260, 45-53. (30) Taddei, M.; Steitz, D. A.; van Bokhoven, J. A.; Ranocchiari, M. Chem. Eur. J. 2016, 22, 3245-3249. (31) Zhang, X.; Gao, B.; Creamer, A. E.; Cao, C.; Li, Y. J. Hazard. Mater. 2017, 338, 102-123. (32) Wu, W.; Zhao, B.; Wang, S.; Hao, J. J. Environ. Sci. 2017, 53, 224-237. (33) Zou, W.; Gao, B.; Ok, Y. S.; Dong, L. Chemosphere. 2019, 218, 845-859. (34) Ragon, F.; Horcajada, P.; Chevreau, H.; Hwang, Y. K.; Lee, U. H.; Miller, S. R.; Devic, T.; Chang, J.S.; Serre, C. Inorg. Chem. 2014, 53, 2491-2500. (35) Glasnov, T. N.; Kappe, C. O. Macromol. Rapid Commun. 2007, 28, 395-410. (36) Morris, W.; Wang, S.; Cho, D.; Auyeung, E.; Li, P.; Farha, O. K.; Mirkin, C. A. ACS Appl. Mater. Interfaces. 2017, 9, 33413-33418. (37) Wißmann, G.; Schaate, A.; Lilienthal, S.; Bremer, I.; Schneider, A. M.; Behrens, P. Microporous Mesoporous Mater. 2012, 152, 64-70. (38) DeStefano, M. R.; Islamoglu, T.; Garibay, S. J.; Hupp, J. T.; Farha, O. K. Chem. Mater. 2017, 29, 1357-1361. (39) Al-Janabi, N.; Hill, P.; Torrente-Murciano, L.; Garforth, A.; Gorgojo, P.; Siperstein, F.; Fan, X. Chem. Eng. J. 2015, 281, 669-677. (40) Yu, J.; Wang, S.; Low, J.; Xiao, W. Phys. Chem. Chem. Phys. 2013, 15, 16883-16890. (41) Chavan, S. M.; Shearer, G. C.; Svelle, S.; Olsbye, U.; Bonino, F.; Ethiraj, J.; Lillerud, K. P.; Bordiga, S. Inorg. Chem. 2014, 53, 9509-9515. (42) Yang, Q.; Zhang, H.-Y.; Wang, L.; Zhang, Y.; Zhao, J. ACS Omega. 2018, 3, 4199-4212. (43) Lee, T.; Chang, Y. H.; Lee, H. L. CrystEngComm. 2017, 19, 426-441. (44) Xiao, W.; Dong, Q.; Wang, Y.; Li, Y.; Deng, S.; Zhang, N. CrystEngComm. 2018, 20, 5658-5662. (45) Han, R.; Wang, Y.; Zou, W.; Wang, Y.; Shi, J. J. Hazard. Mater. 2007, 145, 331-335. (46) Hanbali, M.; Holail, H.; Hammud, H. Green. Chem. Lett. Rev. 2014, 7, 342-358. (47) Vellingiri, K.; Kumar, P.; Deep, A.; Kim, K.-H. Chem. Eng. J. 2017, 307, 1116-1126.


Page 18 of 30

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Continuous-flow microwave synthesis of UiO-66 under various conditions

Average number of linkers/SBU

70715 843 20 1,320 19 972 17 1,120 22

Total pore volume, 3 cm /g 0.93 1.48 1.50 1.74 1.23

3.01 4.15 5.89 6.10

1,295 18 1,272 21 1,193 24 1,320 19

1.21 1.45 1.42 1.50

10.34 10.02 9.94 9.52

70 90 91 86 84

9.37 6.10 4.06 2.88 1.87

973 23 1,320 19 1,200 18 1,123 25 1,052 18

1.49 1.50 1.75 1.86 1.94

10.85 9.52 9.10 7.88 7.65

78

-

1,110 20

0.89

-

n(acid)/n(Zr )

Yield, %

Production rate, g/h

SBET,

120 120 120 120 120 120

0 40 80 120 180 120

56 62 90 89 76

3.75 4.15 6.10 5.96 -

10 min. 10 min. 10 min. 10 min.

80 90 100 120

120 120 120 120

45 62 88 90

11 12 13 14 15

5 min. 10 min. 15 min. 20 min. 30 min.

120 120 120 120 120

120 120 120 120 120

16(**)

24 h

120

120

Run

Residence time

Reaction temp., C

1 2 3 4 5 6(*)

10 min. 10 min. 10 min. 10 min. 10 min. 10 min.

7 8 9 10

4+

(*): Acetic acid was used, (**): Solvothermal synthesis

1


2

m /g

10.64 10.14 9.52 8.89 -


Page 20 of 30

2 3

Table 2. Kinetic parameters for toluene adsorption on the UiO-66 sample prepared under the

4

optimum conditions (120 C, 10 min, and 120 HCl equivalents) Model

Temperature, C

q

exp

(a),

q

cal

(b),

mg/g

K

Model

(c)

R

2

mg/g Thomas

25

130.0

126.5

2.18

0.964

50

83.7

101.2

1.93

0.943

75

44.9

55.8

1.01

0.955

100

21.4

25.5

0.82

0.961

25

130.0

127.20

1.67

0.996

50

83.7

80.23

0.87

0.987

75

44.9

46.28

0.75

0.986

100 21.4 20.56 Adsorption capacity obtained from the experimental data

0.66

0.973

Yan

5

(a)

6

(b)

Maximum adsorption capacity calculated using the Thomas and Yan models

7

(c)

The Thomas and Yan model constants

20 ACS Paragon Plus Environment

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9 10 11

12 13

Fig. 1 Schematic illustration of the continuous-flow microwave system.

14



16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Fig. 2 XRD patterns of UiO-66 obtained at various conditions: effects of the (a) HCl/Zr4+ ratio, (b) reaction temperature, and (c) residence time.


Page 22 of 30

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 3 SEM images of UiO-66 samples prepared at various HCl concentrations under continuous-flow microwave conditions of 120 C and 10 min.

61 62 63 64 65 66 67 68 69 70 71 23 ACS Paragon Plus Environment


73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88

Fig. 4 N2 adsorption-desorption isotherms of UiO-66 samples at various HCl concentrations under continuous-flow microwave conditions of 120 C and 10 min.

89 90


Page 24 of 30

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


92 93 94

95 96 97

Fig. 5 TGA analyses of UiO-66 samples prepared at various HCl concentrations under continuous-flow microwave conditions of 120 C and 10 min.

98 99 100 101 102 103 104 105 106 107 108 25 ACS Paragon Plus Environment


UiO-66-Solvothermal UiO-66-Cont. Microave

Transmittance, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

500

1000

1500

2000

2500

3000

3500

4000

-1

109 110 111

Wavelength (cm )

Fig. 6 FT-IR analysis of UiO-66 samples obtained from (a) solvothermal synthesis at 120 C for 24 h and (b) continuous-flow microwave synthesis at 120 C for 10 min.

112 113 114 115 116 117 118 119 120 121 122


Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


124

125 126 127 128

Fig. 7 (a) Breakthrough curves of gaseous toluene adsorption at different adsorption temperatures and (b) toluene adsorption amounts on UiO-66.



130 131

1.2

132

1.0

133 0.8

134 135 136 137 138 139

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

Adsorption Desorption

0.6 0.4 0.2 0.0 0

50

100

150

200

250

140

Sampling time (min)

141

Fig. 8 Toluene adsorption-desorption curves on UiO-66 at 25 C.

142


Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

144


Graphical abstract

145 146 147 148 149 150 151 152 153

Synopsis

154

UiO-66 (Zr) framework can be used for many applications including catalysis, drug delivery, gas

155

storage, separation, etc. We present a microwave-assisted continuous-flow synthesis of UiO-

156

66(Zr). The UiO-66(Zr) can be produced within short reaction time of 10⁓30 min with high

157

yield, porosity and crystallinity, showing a potential strategy for large scale-production of UiO-

158

66(Zr).

159 160 161

HIGHLIGHTS

162 163

 UiO-66(Zr) synthesized by microwave-assisted continuous tubular reactor

164

 Effects of operating conditions were systematically investigated

165

 UiO-66(Zr) with high yield (90%) and BET surface area (1320 m2.g-1) obtained for 10min

166 167 168

 The prepared UiO-66(Zr) had toluene adsorption capacity of 130 mg. g-1 at 25 oC 29 ACS Paragon Plus Environment


169


Page 30 of 30

Facile Synthesis of UiO-66(Zr) Using a Microwave-Assisted

Recommend Documents