Gas Sorption Properties of a New Three-Dimensional In-ABDC MOF With a Diamond Net

The pseudotetrahedral node, [In(O₂CR)₄]⁻, often found in In^III-based metal-organic frameworks (MOFs) without a cluster-based secondary building unit (SBU) is a negatively charged center due to charge mismatch between an 8-coordinate In^III ion and four anionic carboxylate bridging ligands. Thus, In-MOFs with this pseudotetrahedral node tend to bear a counter-cation near each In^III center in the frameworks. Generally, dialkylammonium-based cations such as Me₂${\text{NH}}_2^{+}$ and Et₂${\text{NH}}_2^{+}$ directly derived from N,N-dimethylformamide (DMF) or N,N-diethylformamide (DEF) solvents during MOF formation reactions play a significant role to form a stable framework through charge matching. If these cations thermally derived from DMF or DEF were not suitable for crystal growth of In-MOFs, it becomes very challenging to obtain high quality single crystals for X-ray structure determination of the frameworks. In this context, high quality crystals of In-ABDC MOF were not easily prepared from a ditopic azobenzene-4,4'-dicarboxylic acid (H₂ABDC) through a thermal reaction in DMF or DEF. We successfully overcome this problem by employing a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]), and the resulting three-dimensional (3D) In-ABDC MOF, [EMIM][In(ABDC)₂]·DEF·H₂O (I), was structurally characterized by X-ray diffraction. The 3D framework indicates a 4-connected uninodal net with Schläfli symbol of 6⁶ (dia). The gas sorption properties of solvent-free I were also investigated in detail.

Introduction

One of the most valuable features of porous metal-organic frameworks (MOFs) may lie in the formation of robust crystalline organic/inorganic hybrid materials with various functionalities (Farha and Hupp, 2010; Furukawa et al., 2013; Nguyen et al., 2013; Foo et al., 2014; Kim and Huh, 2016; Qin et al., 2017; Diercks et al., 2018). Almost all metallic elements can be utilized for the preparation of MOFs with a range of bridging ligands. The properties of a certain MOF can be come from either metallic center or functional bridging ligands (Chui et al., 1999; Kim et al., 2013; Karagiaridi et al., 2014; Beyzavi et al., 2015; Choi et al., 2016; Kim and Huh, 2016; Zhu et al., 2017). Most porous MOFs contain cluster-based secondary building units (SBUs), and these SBUs play a pivotal role in constructing large three-dimensional (3D) frameworks as witnessed in the case of MOF-5 and MOF-177 (Li et al., 1999; Chae et al., 2004). Contrarily, metal ions such as In^III ion (r = 0.80 Å) with a large ionic radius can lead to stable mononuclear 8-coordinate center with four carboxylate-based bridging ligands to form topologically interesting multidimensional network structures. A congener of In^III ion with a small ionic radius such as Al^III ion (r = 0.54 Å) usually tends to form a 6-coordinate center. In In-MOFs, the anionic pseudotetrahedral node, [In(O₂CR)₄]⁻, can be effectively formed with dicarboxylate bridging ligands in the absence of other auxiliary bridging linkers for the formation of 3D frameworks. Additionally, there should be a counter-cation near each [In(O₂CR)₄]⁻ node because of charge matching. There are many examples of 3D In-MOFs solely derived from ditopic, tritopic, tetratopic, and pentatopic carboxylate-based bridging ligands (Sun et al., 2002; Lin et al., 2007, 2012; Huh et al., 2009; Gu et al., 2012; Yu et al., 2012; Zheng et al., 2013; Cho et al., 2014; Huang et al., 2014; Johnson et al., 2014; Wang et al., 2014; Grigoropoulos et al., 2016; Li et al., 2016; Aguirre-Díaz et al., 2017; Zhao et al., 2017; Yang et al., 2019). The most common counter-cations are either Me₂${\text{NH}}_2^{+}$ or Et₂${\text{NH}}_2^{+}$ which can be in-situ generated from the solvents: N, N-dimethylformamide (DMF) or N,N-diethylformamide (DEF). A range of ammonium-based cations can also be added into the reaction mixture to facilitate the formation of anionic In-MOFs (Chen et al., 2009; Lin et al., 2012; Huang et al., 2014; Mihaly et al., 2016; Zhao et al., 2018). In these cases, the counter-cations found in MOFs are the added ammonium-based cations.

Although the Lewis acidic open metal sites are often observed in MOFs (Chui et al., 1999), the unique and controllable functionalities of MOFs are mainly stemmed from the bridging ligands bearing diverse organic functional groups such as -NH₂, -NR₂, NR₃ of 1,4-diazabicyclo[2.2.2]octane (DABCO), imidazolyl moiety, ureido moiety, -NH-CO-, -OH, -SH, and -SO₃H (Gu et al., 2010, 2011; Zheng et al., 2011; Fracaroli et al., 2014; Phang et al., 2014, 2015; McDonald et al., 2015; Flaig et al., 2017; Baek et al., 2018; Hakimifar and Morsali, 2019; Li et al., 2019). Therefore, functional MOFs can act as either Lewis basic materials or Lewis acidic materials depending on the tethered functional groups. Especially, MOFs with well-defined Lewis basic sites can be employed in a range of advanced applications including selective heterogeneous catalysis, CO₂ capture/separation, and CO₂ conversion into cyclic organic carbonates (Beyzavi et al., 2015; He et al., 2016; Maina et al., 2017; Huh, 2019). In this context, many different types of bridging ligands containing Lewis basic moieties have been being developed and investigated. We also reported several functional MOFs with Lewis basic sites for heterogeneous catalysis for organic transformations and selective adsorption of CO₂ over other competing gases (Gu et al., 2010, 2011; Kim et al., 2013, 2017, 2018). We envision that new functional MOFs with Lewis basic functionalities are very useful materials for various applications.

There are several MOF systems containing a ditopic azobenzene-4,4'-dicarboxylate (ABDC²⁻) bridging ligand in which the azo group (-N = N-) can potentially act as Lewis basic sites (Nguyen et al., 2011; Zhuang et al., 2011; Lyndon et al., 2013; Gong et al., 2015; Liu et al., 2016; Zhao et al., 2016; Yuan et al., 2017; Xu et al., 2018; Yang et al., 2018). We expected that the assembly between In^III ions with ABDC²⁻ bridging ligands could lead to a new In-MOF with openly accessible azo-group-based Lewis basic sites from the bridged ABDC²⁻ ligands. In this study, we successfully prepared a 3D In-ABDC MOF formulated as [EMIM][In(ABDC)₂]·DEF·H₂O (I) by employing a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]), as an auxiliary counter-cation source. The permanent porosity of solvent-free I was evaluated by a standard volumetric N₂ adsorption/desorption analysis at 77 K. Both CO₂ and H₂ sorption abilities of I were also measured at suitable temperatures.

Experimental Section

Materials

InCl₃ (Sigma-Aldrich), azobenzene-4,4'-dicarboxylic acid (Chemsoon), 1-ethyl-3-methylimidazolium tetrafluoroborate (TCI), tetraethylammonium bromide (Sigma-Aldrich), and N, N-diethylformamide (TCI) were used as received. Other reagent grade solvents were used without further purification.

Instrumentation

Thermogravimetric analysis was carried out on a TGA Q5000 (TA Instruments) under a nitrogen atmosphere. FT-IR spectra in attenuated total reflection (ATR) mode were obtained on a Jasco FT/IR-4100 spectrometer. Elemental analysis was performed at Organic Chemistry Research Center, Sogang University (Seoul, Korea) by using EA1112 (CE Instruments, Italy). Powder X-ray diffraction (PXRD) spectra were obtained with a Bruker D8 Focus diffractometer (40 kV, 30 mA, Step size = 0.02°). Optical microscopic images were collected on a Nikon Eclipse LV100POL microscope equipped with a DS-Fi1 CCD camera. The cryogenic volumetric N₂ adsorption-desorption analysis was performed on a Belsorp-miniII at 77 K (BEL Japan). The as-prepared I was immersed in chloroform for solvent exchange at least for 3 d. The solvent-exchanged I was dried at 393 K under high vacuum for 2 h. Low pressure volumetric CO₂ adsorption measurements were performed on a Belsorp-miniII at 196 K (2-propanol/dry ice bath). Temperature-programmed desorption analysis using CO₂ probe (CO₂-TPD) was performed on a Belcat-B chemisorption analyzer (BEL Japan). The solvent-exchanged I was dried at 393 K under high vacuum for 2 h. The activated I was treated with a mixture of CO₂/He (10% CO₂) at 303 K for 1 h (flow rate = 60 mL min⁻¹). Then, the sample was purged with He for 1 h (flow rate = 60 mL min⁻¹). The CO₂-TPD profile was obtained with gradual increase of temperature (ramping rate = 10°C min⁻¹).

Preparation of [EMIM][In(ABDC)₂]·DEF·H₂O (I)

InCl₃ (0.0111 g, 0.05 mmol), azobenzene-4,4'-dicarboxylic acid (0.0270 g, 0.1 mmol), and 1-ethyl-3-methylimidazolium tetrafluoroborate (0.0099 g, 0.05 mmol) were dissolved in 5 mL of DEF. The reaction mixture was sealed in a screw-capped vial and stored at 120°C for 7 d. After the reaction, the reaction mixture was cooled down to room temperature. The red crystals were retrieved by filtration, washed with DEF, and air-dried (0.015 g, 34%). Anal. Calcd. for C₃₉H₄₀InN₇O₁₀ (F. wt. 881.61): C, 53.13; H, 4.57; N, 11.12. Found: C, 53.05; H, 4.77; N, 10.71.

X-Ray Crystallography

The X-ray diffraction data for I were collected on a Bruker APEX-II diffractometer equipped with a monochromator in a Mo Kα (λ = 0.71073 Å) incident beam. A crystal of I was mounted on a glass fiber and collected data at room temperature. The CCD data were integrated and scaled using the Bruker-SAINT software package, and the structure was solved and refined using SHEXL-2013 (Sheldrick, 2015). All hydrogen atoms were placed in the calculated positions. SQUEEZE/PLATON was used in structural refinement in X-ray experiment. The crystallographic data for I are listed in Table 1. The selected bond distances are listed in Table S1. Structural information was deposited at the Cambridge Crystallographic Data Centre (CCDC reference number is 1904040 for I).

TABLE 1

Table 1. Crystallographic data for I.

Results and Discussion

Thermal reaction of the mixture of InCl₃, azobenzene-4,4'-dicarboxylic acid (H₂ABDC), and 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]) dissolved in N,N-diethylformamide (DEF) at 120°C afforded the red parallelogram shaped crystals formulated as [EMIM][In(ABDC)₂]·DEF·H₂O (I) (Figure S1). The chemical structures of the room temperature ionic liquid (RTIL), [EMIM][BF₄], and the ditopic carboxylate-based ABDC²⁻ bridging ligand are shown in Figure 1. Several initial attempts to prepare In-ABDC MOF without the RTIL-based auxiliary counter-cation (EMIM⁺) in N,N-dimethylformamide (DMF) or DEF did not give good quality of crystals as depicted in Figures 2a,b. Both Me₂${\text{NH}}_2^{+}$ and Et₂${\text{NH}}_2^{+}$ directly derived from DMF or DEF solvents may not be suitable for the generation of stable framework structures. Me₂${\text{NH}}_2^{+}$ and Et₂${\text{NH}}_2^{+}$ ions are thought to be generated by thermal decomposition of DMF or DEF, respectively (Gu et al., 2012). Contrarily, as tetraethylammonium bromide (TEAB) was added into the reaction mixture of DEF, good red block crystals were obtained as shown in Figure 2c. Despite its seemingly good quality for X-ray diffraction, the crystal size was too small to get refinable diffraction data. Then, a RTIL-based [EMIM][BF₄] was chosen and added into the reaction mixture of DEF solution to lead to parallelogram shaped large X-ray quality crystals of I after thermal reaction at 120°C for 7 d (Figure 2d). It is noteworthy to mention that the specific type of counter-cation is important for the formation of good quality crystals of I.

FIGURE 1

Figure 1. The chemical structures of [EMIM][BF₄] (A) and ABDC²⁻ bridging ligand (B).

FIGURE 2

Figure 2. Optical microscopic images of the products obtained from DMF without [EMIM][BF₄] (a), from DEF without [EMIM][BF₄] (b), from DEF with TEAB (c), and from DEF with [EMIM][BF₄] (d).

Fourier-transform infrared (FT-IR) spectrum of the free H₂ABDC ligand showed strong C=O and N=N stretching frequencies at 1,681 and 1,424 cm⁻¹, respectively, while crystals of I showed C=O and N=N stretching frequencies at 1,662 and 1,414 cm⁻¹, respectively (Figure S2). The lowering of C=O stretching frequency clearly indicates the coordination of carboxylate to In^III ion. I crystallizes in the C2/c space group. As depicted in Figure 3, ABDC²⁻ ligands bridge In^III ions to form a 3D framework. The asymmetric unit contains an In^III ion and two ABDC²⁻ ligands (Figure 4). I contains a EMIM⁺ cation to compensate the charge of the pseudotetrahedral anionic node, [In(O₂CR)₄]⁻, constructed by four carboxylate ligands chelated to an In^III ion. The solvent molecules and EMIM⁺ ions were not refined because they were highly disordered, and the elemental analysis provided the total formula of I. An In^III ion is 8-coordinated, and geometry of an In^III node is pseudotetrahedral constructed by four ABDC²⁻ ligands. The structure indicated a 4-connected uninodal net with Schläfli symbol of 6⁶ (dia) assuming an In^III ion act as a node without any simplification based on ToposPro analysis (ESI) (Blatov et al., 2004). The cation and solvent-free I indicated 92.4 % of the void volume based on PLATON analysis (Spek, 2004). The real potential void volume should be smaller than the calculated value due to the presence of the counter-cations.

FIGURE 3

Figure 3. 3D structure of [In(ABDC)₂]⁻ along the a-axis (A), along the b-axis (B), and along the c-axis (C). The counter-cations (EMIM⁺) and solvent molecules could not be refined. The color codes: indium, purple; carbon, gray; nitrogen, blue; oxygen, red; hydrogen, white. The Connolly surface of solvent-free I viewed down the c-axis, probed with a probe radius of 1.4 Å (D). Nine unit cells are shown here. Green and gray surfaces indicate outer and inner surfaces, respectively.

FIGURE 4

Figure 4. Environment around the anionic pseudotetraheral [In(ABDC)₂]⁻ node of I. Displacement ellipsoids are shown at the 30% probability level. All hydrogen atoms were omitted for clarity. Symmetry operations: (i) 1–x, y, 1.5–z, (ii) 1–x, 1–y, 2–z, and (iii) 1.5–x, 0.5–y, 2–z.

Since the potential solvate molecules in the pores of as-prepared I could not be refined by X-ray study, they were characterized by elemental analysis and thermogravimetric analysis (TGA). One DEF and one water molecules were found based on these analyses. The thermal behavior of as-prepared I under continuous flow of nitrogen gas indicated that clear loss of these solvate molecules at 250°C as shown in Figure S3A. Upon the increase of temperature, gradual weight loss occurred due to the decomposition of EMIM⁺ counter-cation. After that, rapid decomposition of the framework was observed until around 500°C. The bulk purity of the as-prepared I was further investigated by powder X-ray diffraction (PXRD) study as shown in Figure 5. The diffraction pattern of the as-prepared I agrees well with the simulated one from the X-ray data except the (110) peak. Interestingly, the (110) diffraction plane is almost completely overlapped with (11-1) plane (Figure S4). The 2θ angles of (110) and (11-1) are 4.539° and 4.545°, respectively. We speculate that these two diffraction planes with very similar d-spacing values result in accidental extinction of the peaks (Lund et al., 2010). In other words, the two diffracted beams happen to destructively interfere each other. So, we could not observe these overlapped peaks despite good elemental analysis, TGA, and X-ray crystal data.

FIGURE 5

Figure 5. PXRD pattern of as-prepared I (blue) and simulated data (red). The simulated pattern is calculated from X-ray structure using Mercury (ver. 4.1.0) with a full-width half maximum (FWHM) value of 0.2. No preferred orientation is considered.

The crystal structure of 3D framework I indicated well-defined open channels fully accessible in any directions as depicted in Figure 3. Large and small hexagon shaped channels are formed along the c-axis and a-axis, respectively, and square grid channels are formed along the b-axis. The PXRD pattern of the dried sample showed almost identical pattern compared to that of the as-prepared I (Figure S3B) and this indicated the framework did not collapse during desolvation process. Thus, the permanent porosity of solvent-free I was evaluated by a standard volumetric N₂ adsorption/desorption measurement at 77 K. The solvent-exchanged I was dried at 120°C under high vacuum before measurement. The N₂ adsorption isotherm is a typical Type I isotherm indicative of microporous material as shown in Figure 6A. Despite the seemingly large void space shown Figure 3, the presence of a rather bulky EMIM⁺ counter-cations renders the pore dimension belongs to microporous range (<2 nm). The initial adsorption at low pressure region rapidly increased and then maintained almost constant values before the second rapid increase at high pressure region (P/P₀ > 0.9). There is slight hysteresis between adsorption and desorption branches. The Brunauer-Emmett-Teller (BET) surface area is 307 m² g⁻¹ (0 < P/P₀ < 0.142) and the pore volume is 0.38 cm³ g⁻¹. The Horváth-Kawazoe (HK) micropore dimension is 0.64 nm (Figure S5). Additional porosity with relatively small distribution was also observed at 0.99 nm. Thus, I can be considered as a bimodal microporous material.

FIGURE 6

Figure 6. N₂ adsorption/desorption isotherms for the solvent-free I at 77 K (A) and CO₂ adsorption/desorption isotherms at three different temperatures (B).

Although the measured BET surface area is relatively low level, the Lewis basic azo groups are fully open toward the channels. Lewis basic sites are known to be beneficial for stronger interaction between framework and CO₂ molecules through acid-base interaction (Gu et al., 2010; Kim and Huh, 2016). Therefore, the CO₂ sorption abilities were also investigated at three different temperatures (Figure 6B). The CO₂ sorption measurements at 196, 273, and 298 K showed the uptake of 81.3 (3.63 mmol g⁻¹), 31.1 (1.39 mmol g⁻¹), and 16.4 cm³ g⁻¹ (0.73 mmol g⁻¹), respectively. The uptake at 196 K is somewhat high considering its low BET surface area. For comparison purposes, CO₂ uptake amounts for other known MOFs under similar conditions are summarized in Table S2. The isosteric heats (Q_st) of CO₂ adsortion were also estimated by using the data at 273 and 298 K. The estimated values by the Clausius-Clapeyron equation and by virial fitting method were very similar each other as shown in Figure 7A. The low surface coverage heat of CO₂ adsorption was calclulated to be −23.3 kJ mol⁻¹ based on the Clausius-Clapeyron equation. The value at zero surface coverage was estimated to be −23.8 kJ mol⁻¹ by virial method (Figure S6). Both values indicate that the heat of adsorption for CO₂ falls in the usual range of values for common MOFs (Hwang et al., 2012; Sumida et al., 2012). The Lewis basic azo moiety may not strongly enhance CO₂ adsorption at low pressure region. Notably, however the values gradually increase upon the increase of adsorption amount of CO₂. The final values, −31.9 and −30.5 kJ mol⁻¹, are remarkably higher than those of low surface coverage values. This may imply that preadsorbed CO₂ molecules inside channels participate in enhancing additional adsorption of newly incoming CO₂ molecules.

FIGURE 7

Figure 7. Isosteric heats of CO₂ adsorption as a function of CO₂ uptake estimated from the Clausius-Clapeyron equation (blue open symbols) and virial method (red line) (A). Isosteric heats of H₂ adsorption as a function of H₂ uptake estimated from the virial method (B).

The temperature-programmed desorption analysis using CO₂ probe gas (CO₂-TPD) was also performed to estimate the concentration of accessible Lewis basic sites and qualitatively determine the different types of basic sites. The activated I was treated with CO₂ as a form of CO₂/He mixture (10% CO₂) at 30°C for 1 h. The CO₂-TPD curve shown in Figure S7 indicates that there are several desorption signals below 253°C. This range of values has been attributed to weak basic sites (Wang et al., 2017). Furthermore, the total integrated area indicates the concentration of basic sites is about 0.022 mmol g⁻¹. This amount is much smaller than the CO₂ uptake at 298 K (0.73 mmol g⁻¹). Thus, the azo groups are likely to be very weakly basic.

Stimulated from the interesting CO₂ adsorption behaviors and isosteric heats of adsorption for I, adsorption of H₂ gas at low pressure was also investigated by volumetric method at 77 and 87 K as depicted in Figure S8. The uptake amounts were 102.26 cm³ g⁻¹ (0.92 wt.%) and 58.37 cm³ g⁻¹ (0.53 wt.%) at 77 and 87 K, respectively. Considering the low BET surface area of solvent-free I, the uptake of 0.92 wt.% at 77 K is moderately high level. Similar level of values were also observed for Zn₄O(TMBDC)₃ (0.89 wt.%, S_BET = 1501 m² g⁻¹, TMBDC = 2,3,5,6-tetramethylbenzene-1,4-dicarboxylate) (Rowsell et al., 2004), Mn₃(BDT)₃ (0.97 wt.%, S_BET = 290 m²g⁻¹, BDT = 1,4-benzeneditetrazolate) (Dincă et al., 2006), and Na[Ni₃(OH)(SIP)₂] (0.94 wt.%, S_BET = not available, SIP = 5-sulfoisophthalate) (Dietzel et al., 2006). Interestingly, the BET surface areas of Zn₄O(TMBDC)₃ and Mn₃(BDT)₃ are quite different each other. Therefore, I showed very similar performance with Mn₃(BDT)₃. Previously, mesoporous [Et₂NH₂][In(2,6-NDC)₂] (2,6-NDC is 2,6-naphthalenedicarboxylate) with BET surface area of 891.2 m² g⁻¹ exhibited 1.62 wt.% of H₂ uptake under the same measurement conditions (Huh et al., 2009). Other useful H₂ sorption data for representative MOFs are also summarized in Table S3. The isosteric heats of H₂ adsorption were estimated by virial method as given in Figure 7B and Figure S9. The zero surface coverage value is −8.79 kJ mol⁻¹ and this value is relatively high level compared to other known values for common MOFs (Hu and Zhang, 2010). The heats of H₂ adsorption for MOFs are known to be in the range −3.8 ~ −10.4 kJ mol⁻¹. The highest values are usually observed for MOFs with openly accessible metal sites which can directly interact with H₂ molecule such as MIL-101(Cr) (Latroche et al., 2006). Mn₃(BDT)₃ and Na[Ni₃(OH)(SIP)₂] showed values of −8.4 and −10.4 kJ mol⁻¹, respectively (Dietzel et al., 2006; Dincă et al., 2006). Since I does not contain open metal sites, the reason for relatively high isosteric heat of H₂ adsorption is unclear now.

Conclusion

The permanently porous 3D In-ABDC MOF, [EMIM][In(ABDC)₂]·DEF·H₂O (I), was synthesized successfully and structurally characterized by X-ray diffraction. The RTIL-based counter-cations effectively facilitated the formation of I. The 3D framework indicates a 4-connected uninodal net with Schläfli symbol of 6⁶ (dia). I contains a [EMIM]⁺ cation to compensate the charge of the pseudotetrahedral anionic node, [In(O₂CR)₄]⁻, constructed by four carboxylate ligands chelated to an In^III ion. The N₂ adsorption isotherm is a typical Type I isotherm, and this indicates I is a microporous material due to the presence of a rather bulky [EMIM]⁺ counter-cations despite of large potential void space. The CO₂ sorption ability indicates that preadsorbed CO₂ molecules may participate in enhancing additional adsorption of newly incoming CO₂ molecules. The H₂ adsorption at 77 K also indicated good sorption ability and moderately high isosteric heats of H₂ adsorption. We envision that many new functional In-MOFs can be synthesized by choosing suitable counter-cations for various applications.

Data Availability

The raw data supporting the conclusions of this manuscript will be made available by the authors, without undue reservation, to any qualified researcher.

Author Contributions

SH, YK, and S-JK conceived the paper. I-HC, SY, YK, and S-YJ performed the experiments. SH, YK, S-JK, I-HC, and SY interpreted the results. SH and YK wrote the paper.

Funding

This work was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2018R1D1A1B07043017 and 2018R1D1A1B07045327) and by RP-Grant 2018 of Ewha Womans University.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2019.00218/full#supplementary-material

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