Controllable Synthesis of 1, 3, 5-tris (1H-benzo[d]imidazole-2-yl) Benzene-Based MOFs

: The growing interest in metal–organic frameworks (MOFs) in both industrial and sci-entiﬁc circles has increased in the last twenty years, owing to their crystallinity, structural ver-satility, and controlled porosity. In this study, we present three novel MOFs obtained from the 1, 3, 5-tris (1H-benzo[d]imidazole-2-yl) benzene (TIBM) organic linker. The formed TIBM crystal powders were characterized by scanning electron microscopy (SEM) to estimate the morphology of the particles, powder X-ray diffraction (XRD) to conﬁrm the crystal structure, Brunauer–Emmett– Teller (BET) method for structural analysis, and thermogravimetric measurements to examine the thermal stability. The TIBM-Cu MOF showed excellent CO 2 (3.60 mmol/g) adsorption capacity at 1 bar and 298 K, because of the open Cu site, compared to TIBM-Cr (1.6 mmol/g) and TIBM-Al (2.1 mmol/g). Additionally, due to the high porosity (0.3–1.5 nm), TIBM-Cu MOF showed a considerable CO 2 /N 2 selectivity (53) compared to TIBM-Al (35) and TIBM-Cr (10). respectively, providing clear evidence of the crystalline nature of the TIBM-Al MOF. The Cr-TIBM MOF exhibited blunt peak intensities at 2 θ values of 10 ◦ and 18 ◦ –20 ◦ , showing an increase in the crystalline degree of the MOFs [67,68]. On the other hand, the Cu-TIBM MOF showed sharp crystalline peaks, comparable to those of the Cu-based MOFs [64,65].


Introduction
The design and implementation of new porous materials for carbon dioxide (CO 2 ) separation by selective adsorption is a rapidly increasing research area, because of its importance in energy and environment-related applications [1][2][3]. The greenhouse effect responsible for global warming is mainly related to CO 2 emissions. Globally, CO 2 release is rapidly increasing because of the combustion of carbon-based fuels (oil, coal, and natural gas) and chemical reactions in petrochemical industries, steel, and cement [4]. Post-combustion CO 2 capture is considered the most effective technique for minimizing CO 2 emitted from industrial and energy-related sources. To this end, different technologies have been applied for CO 2 capture, such as membrane separation [5,6] liquid ammonia, and amine absorption [7,8] and adsorption [9,10]. Membrane separation is not suitable to largescale applications because of its short lifetime, limited performance at low pressure, and poor stability in acid gas environments [11]. The absorption process has been extensively used in power plants, thanks to the high CO 2 selectivity promoted by amine functional groups; however, substantial downsides include equipment corrosion, high energy intake, and toxic ammonia loss [12].
Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 17 thermogravimetric analysis (TGA). The volumetric method was used for CO2 and N2 adsorption capacity measurements.

Morphological Properties
N2 adsorption isotherms at 77 K ( Figure 1a) were measured using a volumetric BET instrument (BELSORP-max, MicrotracBEL, Japan) in order to characterize the structural properties of the MOFs. All the isotherms showed a typical Type I isotherm, corresponding to a microporous structure. The specific surface area and pore volume of the TIBM MOFs were determined by--the Brunauer Emmett Teller (BET) method. TIBM-Cr presented the largest surface area and pore volume among the other MOFs (Table 1). The pore size distribution of the TIBM MOFs was determined by the non-local density functional theory (NLDFT) method. TIBM-Cr MOFs showed the largest pore size range (1.0-4.0 nm), compared to TIBM-Cu (0.3 to 1.5 nm) and TIBM-Al (1.0 to 3.0 nm) (Figure 1b in Supporting Information). The majority of pores appeared at 2 nm for TIBM-Al and TIBM-Cr, and at 1 nm for TIBM-Cu.   Figure 2 reports the SEM images of the three TIBM MOF samples. The average particle diameters of the TIBM MOFs were determined using Image J software (NIH, Bethesda, MD, USA) based on the pixel distance of each image, which is correlated with the scale bar. TIBM-Cr exhibited a smaller particle size (0.25 µm) than TIBM-Cu (28.29 µm) and TIBM-Al (0.61 µm).    Figure 2 reports the SEM images of the three TIBM MOF samples. The average particle diameters of the TIBM MOFs were determined using Image J software (NIH, USA) based on the pixel distance of each image, which is correlated with the scale bar. TIBM-Cr exhibited a smaller particle size (0.25 µ m) than TIBM-Cu (28.29 µ m) and TIBM-Al (0.61 µ m). The MOF TIBM-Al showed a clear hexagonal crystal structure with sharp edges, spherical and highly porous nanoclusters were observed for TIBM-Cr, and a rhombic crystal structure was detected for TIBM-Cu. The TIBM-MOFs surfaces were found to be smooth, without evident cracks due to strong chelation between the central metal ion and TIBM amine linkers.

X-ray Diffraction (XRD) Analysis
The quality of TIBM-MOFs was optimized, and the samples were successfully doped with three metal ions by ionic-covalent bonds. The XRD pattern (Figure 3) of the TIBM-MOFs was compatible with the simulated pattern of UiO-66 from the Cambridge Structural Database (CCDC 837796) [63]. The patterns of TIBM-Cu and TIBM-Al are consistent with those of Cu-BTC and HKUST-1, respectively, while for TIBM-Cr, a resemblance to The MOF TIBM-Al showed a clear hexagonal crystal structure with sharp edges, spherical and highly porous nanoclusters were observed for TIBM-Cr, and a rhombic crystal structure was detected for TIBM-Cu. The TIBM-MOFs surfaces were found to be smooth, without evident cracks due to strong chelation between the central metal ion and TIBM amine linkers.

X-ray Diffraction (XRD) Analysis
The quality of TIBM-MOFs was optimized, and the samples were successfully doped with three metal ions by ionic-covalent bonds. The XRD pattern (Figure 3) of the TIBM-MOFs was compatible with the simulated pattern of UiO-66 from the Cambridge Structural Database (CCDC 837796) [63]. The patterns of TIBM-Cu and TIBM-Al are consistent with those of Cu-BTC and HKUST-1, respectively, while for TIBM-Cr, a resemblance to that of MIL-101 (Fe) and Fe-BTC was found [64][65][66][67][68][69][70][71], thus confirming the successful synthesis and integrity of the crystal structure after the coordination with TIBM ( Table 2). In particular, the chelation was indicated by a broad peak and a sharp peak at 2θ values of 9 • and 18 • , respectively, providing clear evidence of the crystalline nature of the TIBM-Al MOF. The Cr-TIBM MOF exhibited blunt peak intensities at 2θ values of 10 • and 18 • -20 • , showing an increase in the crystalline degree of the MOFs [67,68]. On the other hand, the Cu-TIBM MOF showed sharp crystalline peaks, comparable to those of the Cu-based MOFs [64,65].
that of MIL-101 (Fe) and Fe-BTC was found [64][65][66][67][68][69][70][71], thus confirming the successful synthesis and integrity of the crystal structure after the coordination with TIBM ( Table 2). In particular, the chelation was indicated by a broad peak and a sharp peak at 2θ values of 9° and 18°, respectively, providing clear evidence of the crystalline nature of the TIBM-Al MOF. The Cr-TIBM MOF exhibited blunt peak intensities at 2θ values of 10° and 18°-20°, showing an increase in the crystalline degree of the MOFs [67,68]. On the other hand, the Cu-TIBM MOF showed sharp crystalline peaks, comparable to those of the Cu-based MOFs [64,65]    The FTIR analysis allowed characterizing molecular interactions and bonding formation in the MOF frameworks ( Figure 4). The strong stretching bands at 490-500 cm −1 were attributed to metal-hydrogen bonds, particularly those of Cr and Cu metal ions. The bands at 720-724 cm −1 and 750-754 cm −1 correspond to =C-H bond modes in phenyl rings [74,75]. The characteristic MOFs bands, related to metal-ion-bound second and third amines (>NH-M-N and >N-M-N of MOF), appear in the range 1090-1100 cm −1 . Stretching bands due to C=C and C-H deformations of the phenyl rings were observed at 1399 cm −1 . The strong vibration mode at 1455 cm −1 related to -NH and metal ions was attributed to the bidentate behavior of the N-M-N moiety. These characteristic peaks match well with the previously reported FTIR analysis of MOF-199 [28]. The strong resonance band exhibited by TIBM-Al was attributed to strong H-bonding of hydroxyl groups in the porous TIBM-Al material, as compared to TIBM-Cr and TIBM-Cu. This strong H-bonding occurs in Al-metal-based MOFs such as MIL-53(Al) and MIL-96 (Al), as compared to the Zn-based ZIF-8 and Zr-based UiO-66, and results in a strong resonance band [76][77][78]. The spectra for TIBM-Cr appear noticeably different after 3500 cm −1 , as compared to TIBM-Al and TIBM-Cu. The small peaks that were observed in the TIBM-Al and TIBM-Cu spectra after 3500 cm −1 were attributed to the presence of crystalline water or the acidic -OH in carboxylic groups; these peaks do not appear in the case of TIBM-Cr [79,80]. The FTIR spectrum of the TIBM linker showed the typical peaks for NH wagging at 910 cm −1 , for C=C at 1420 cm −1 , for C=N at 1442 cm −1 , and 1610 cm −1 , for CO-NH at 1713 cm −1 , and for NH at 3448 cm −1 (Figure 4b).

CO 2 and N 2 Adsorption Measurements
To examine the CO 2 -capture performance of all the samples, CO 2 and N 2 adsorption properties were measured at 298 K and 0-1 bar. As reported in Figure 6, TIBM-Cu showed the highest CO 2 adsorption capacity (3.60 mmol/g) at 1 bar, compared to the TIBM-Al (2.04 mmol/g) and TIBM-Cr (1.67 mmol/g). This result is ascribable to both the metal site exposure and the metal oxide chelation of the TIBM host precursor for Cu 2+ ions [83]. The Cu-Cu magnetic interaction of TIBM-Cu is stronger than the Al-Al and Cr-Cr magnetic interactions of TIBM-Al and TIBM-Cr, respectively, and results in a strong interaction between CO 2 and the two available electrons of the Cu 2+ metal in the TIBM-Cu MOF [84,85]. Conversely, TIBM-Cr showed the highest surface area (2141.1 m 2 /g) and total pore volume 2.116 cm 3 /g, compared to the other examined MOFs (Table 1), while a reverse adsorption trend was found for N 2 adsorption for the TIBM MOFs. With regards to the CO 2 /N 2 adsorption selectivity, which is defined as the ratio between CO 2 adsorption capacity at 0.15 bar and N 2 adsorption capacity at 0.85 bar, TIBM-Cu showed the highest value (53.69) compared to TIBM-Cr (11.16) and TIBM-Al (33.33), as reported in Figure 7, mainly in virtue of its high CO 2 adsorption capacity, and concurrent low N 2 adsorption capacity. In addition, CO 2 /N 2 selectivity is different for each of the TIBM MOFs and depends on different factors, such as MOF pore size and available open metal sites for CO 2 . The pore sizes of TIBM-Cu (0.5 and 1 nm) are more suited to selective adsorption of CO 2 (0.33 nm) over N 2 (0.35 nm) than TIMB-Al (1.9 nm) and TIBM-Cr (2.2 nm), owing to the size-selective separation. The CO 2 /N 2 adsorption selectivity of TIBM-Cu is promising, compared to other MOFs reported in the literature (e.g., 6 for CuDABCO [86], 8 for ZIF-8 [87], 12 for MIL-101 (Cr) [88], 18 for MOF-5 [89], 16.5 for MOF-177 [89]). However, the adsorption capacities of the TIBM MOFs are comparable to those of previously reported MOFs (Table 3). TIBM-Cu was selected for the performance evaluation, showing that the CO 2 adsorption capacity ( Figure 8) gradually decreased with increasing temperature from 298 to 338 K. [74,75]. The characteristic MOFs bands, related to metal-ion-bound second and third amines (>NH-M-N and >N-M-N of MOF), appear in the range 1090-1100 cm −1 . Stretching bands due to C=C and C-H deformations of the phenyl rings were observed at 1399 cm −1 . The strong vibration mode at 1455 cm −1 related to -NH and metal ions was attributed to the bidentate behavior of the N-M-N moiety. These characteristic peaks match well with the previously reported FTIR analysis of MOF-199 [28]. The strong resonance band exhibited by TIBM-Al was attributed to strong H-bonding of hydroxyl groups in the porous TIBM-Al material, as compared to TIBM-Cr and TIBM-Cu. This strong H-bonding occurs in Al-metal-based MOFs such as MIL-53(Al) and MIL-96 (Al), as compared to the Znbased ZIF-8 and Zr-based UiO-66, and results in a strong resonance band [76][77][78]. The spectra for TIBM-Cr appear noticeably different after 3500 cm −1 , as compared to TIBM-Al and TIBM-Cu. The small peaks that were observed in the TIBM-Al and TIBM-Cu spectra after 3500 cm −1 were attributed to the presence of crystalline water or the acidic -OH in carboxylic groups; these peaks do not appear in the case of TIBM-Cr [79,80]. The FTIR spectrum of the TIBM linker showed the typical peaks for NH wagging at 910 cm −1 , for C=C at 1420 cm −1 , for C=N at 1442 cm −1 , and 1610 cm −1 , for CO-NH at 1713 cm −1 , and for NH at 3448 cm −1 (Figure 4b).

CO2 and N2 Adsorption Measurements
To examine the CO2-capture performance of all the samples, CO2 and N2 adsorption properties were measured at 298 K and 0-1 bar. As reported in Figure 6, TIBM-Cu showed the highest CO2 adsorption capacity (3.60 mmol/g) at 1 bar, compared to the TIBM-Al  mmol/g) and TIBM-Cr (1.67 mmol/g). This result is ascribable to both the metal site exposure and the metal oxide chelation of the TIBM host precursor for Cu 2+ ions [83]. The Cu-Cu magnetic interaction of TIBM-Cu is stronger than the Al-Al and Cr-Cr magnetic interactions of TIBM-Al and TIBM-Cr, respectively, and results in a strong interaction between CO2 and the two available electrons of the Cu 2+ metal in the TIBM-Cu MOF [84,85]. Conversely, TIBM-Cr showed the highest surface area (2141.1 m 2 /g) and total pore volume 2.116 cm 3 /g, compared to the other examined MOFs (Table 1), while a reverse adsorption trend was found for N2 adsorption for the TIBM MOFs. With regards to the CO2/N2 adsorption selectivity, which is defined as the ratio between CO2 adsorption capacity at 0.15 bar and N2 adsorption capacity at 0.85 bar, TIBM-Cu showed the highest value (53.69) compared to TIBM-Cr (11.16) and TIBM-Al (33.33), as reported in Figure 7, mainly in virtue of its high CO2 adsorption capacity, and concurrent low N2 adsorption capacity.  (Table  3). TIBM-Cu was selected for the performance evaluation, showing that the CO2 adsorption capacity ( Figure 8) gradually decreased with increasing temperature from 298 to 338 K.

Preparation of 1, 3, 5-tris (1H-benzo[d]imidazole-2-yl) Benzene
O-phenylenediamine (7.2 g, 0.06 mol) was added to a solution of trimesic acid (4 g, 0.03 mmol) in polyphosphoric acid (PPA) (50 mL), and the reaction mixture was heated at 230 • C for 12 h (Figure 9). The resultant yellowish-colored reaction mixture was poured into ice-cold water (500 mL); upon stirring, the obtained brown precipitate was collected. The precipitate was neutralized by adding 20% sodium bicarbonate solution and filtered by centrifugation (4000 rpm). The brown solid converted into a white solid (82% yield) after recrystallization with methanol [110] Figure S1). The TIBM was synthesized as described in the literature [110]; the obtained pale yellow solid presented FT-IR and 1H-NMR spectra compatible with the ones previously reported [110,111].

Synthesis of TIBM-Cr
Chromium nitrate nonahydrate (0.6340 g) and TIBM (0.438 g) were dissolved in 20 mL deionized water by sonication for 10 min, and hydrofluoric acid (HF) (60 µL) was added to the mixture. The reaction mixture was transferred into a Teflon autoclave reactor sealed in a stainless steel vessel and maintained at 483 K for 48 h. The fine green-colored precipitate was washed three times in hot ethanol and five times in hot water. The final TBIM-Cr MOF was dried at 373 K and evacuated at 423 K for 12 h.  Figure S1). The TIBM was synthesized as described in the literature [110]; the obtained pale yellow solid presented FT-IR and 1H-NMR spectra compatible with the ones previously reported [110,111]

Synthesis of TIBM-Cr
Chromium nitrate nonahydrate (0.6340 g) and TIBM (0.438 g) were dissolved in 20 mL deionized water by sonication for 10 min, and hydrofluoric acid (HF) (60 µ L) was added to the mixture. The reaction mixture was transferred into a Teflon autoclave reactor sealed in a stainless steel vessel and maintained at 483 K for 48 h. The fine green-colored precipitate was washed three times in hot ethanol and five times in hot water. The final TBIM-Cr MOF was dried at 373 K and evacuated at 423 K for 12 h.

Synthesis of TIBM-Cu
Copper acetate (1.4 g) and TIBM (0.78 g) were dissolved in 30 mL water/ethanol (2:1) solution by sonication. HF (120 µ L) was added to the mixture as a module. The reaction mixture was transferred into a Teflon autoclave reactor sealed in a stainless-steel vessel and kept at 423 K for 24 h. The blue-gray-colored precipitate was washed five times in ethanol and dried at 373 K, and then evacuated at 393 K for 12 h.

Synthesis of TIBM-Al
Aluminum chloride hexahydrate was dehydrated at 373 K for 10 h to remove water from the metal precursor. The dehydrated Al-metal precursor (1.2 g) and TIBM (0.626 g) were mixed in 30 mL water/ethanol (1:1) by sonication for 10 min. HF (60 µ L) was added to the mixture as a module. The reaction mixture was transferred into a Teflon autoclave reactor sealed in a stainless-steel vessel, and maintained at 423 K for 48 h. The white-colored powder precipitate was washed five times in ethanol, subsequently dried at 373 K, and evacuated at 393 K for 12 h (Figure 10).

Synthesis of TIBM-Cu
Copper acetate (1.4 g) and TIBM (0.78 g) were dissolved in 30 mL water/ethanol (2:1) solution by sonication. HF (120 µL) was added to the mixture as a module. The reaction mixture was transferred into a Teflon autoclave reactor sealed in a stainless-steel vessel and kept at 423 K for 24 h. The blue-gray-colored precipitate was washed five times in ethanol and dried at 373 K, and then evacuated at 393 K for 12 h.

Synthesis of TIBM-Al
Aluminum chloride hexahydrate was dehydrated at 373 K for 10 h to remove water from the metal precursor. The dehydrated Al-metal precursor (1.2 g) and TIBM (0.626 g) were mixed in 30 mL water/ethanol (1:1) by sonication for 10 min. HF (60 µL) was added to the mixture as a module. The reaction mixture was transferred into a Teflon autoclave reactor sealed in a stainless-steel vessel, and maintained at 423 K for 48 h. The white-colored powder precipitate was washed five times in ethanol, subsequently dried at 373 K, and evacuated at 393 K for 12 h (Figure 10).

Characterization
Fourier-transform infrared spectra (FTIR) in the range of 400-4000 cm −1 were obtained with a Spectrum Two, PerkinElmer, UK spectrometer. A mixture of TIBM MOF power and KBr in the weight ratio of 1:99 was used to prepare the sample. The TIBM-MOFs particle morphology and crystal size were determined by means of SEM (Merlin compact, Carl Zeiss) at an accelerating voltage of 1 kV/10 kV, with a current of 10 μA. A volumetric method was used to analyze morphological properties, such as specific surface area, pore size distribution, and pore volume, using a BELSORP-mini (Microtrac BEL, Japan) based on N2 adsorption isotherm at 77 K. XRD analysis (X'Pert Pro-MPD, PANalytical, Netherlands) was performed to determine the TIBM-MOFs crystallinity. For the thermal stability test, the samples were heated to 800 °C at a heating rate of 20 °C/min under N2 atmosphere (50 mL/min), using a TGA instrument (Scinco TGA N1000, USA). 1H-NMR spectra were obtained with a Bruker 400 MHz NMR spectrometer in CDCl3, using tetramethylsilane (TMS) as an internal standard. Mass spectra were recorded on a Bruker Dal-

Characterization
Fourier-transform infrared spectra (FTIR) in the range of 400-4000 cm −1 were obtained with a Spectrum Two, PerkinElmer, UK spectrometer. A mixture of TIBM MOF power and KBr in the weight ratio of 1:99 was used to prepare the sample. The TIBM-MOFs particle morphology and crystal size were determined by means of SEM (Merlin compact, Carl Zeiss) at an accelerating voltage of 1 kV/10 kV, with a current of 10 µA. A volumetric method was used to analyze morphological properties, such as specific surface area, pore size distribution, and pore volume, using a BELSORP-mini (Microtrac BEL, Osaka, Japan) based on N 2 adsorption isotherm at 77 K. XRD analysis (X'Pert Pro-MPD, PANalytical, Almelo, The Netherlands) was performed to determine the TIBM-MOFs crystallinity.
For the thermal stability test, the samples were heated to 800 • C at a heating rate of 20 • C/min under N 2 atmosphere (50 mL/min), using a TGA instrument (Scinco TGA N1000, Twin Lakes, WI, USA). 1H-NMR spectra were obtained with a Bruker 400 MHz NMR spectrometer in CDCl 3 , using tetramethylsilane (TMS) as an internal standard. Mass spectra were recorded on a Bruker Daltonik, Bremen, Germany, operated in linear mode with a pulsed nitrogen laser (337 nm, pulse frequency, 2 Hz).
3.7. CO 2 and N 2 Adsorption Capacity Measurements CO 2 and N 2 adsorption on TIBM-MOFs were measured at 298-338 K at the pressure of 0-1 bar using a volumetric apparatus (BELSORP-mini, MicrotracBEL, Osaka, Japan). TIBM MOFs were first evacuated at 393 K (TIBM-Cu, TIBM-Al) and 423 K (TIBM-Cr) for 12 h, to remove impurities. A water circulating jacket connected to a thermostatic bath was used to control the measurement temperature with a precision of ±0.01 • C. A reference gas (helium) was used to determine the free space in the sample holder.
Author Contributions: Writing-original draft preparation, conceptualization, and methodology, S.G. and R.K.C.; validation, S.G. and R.G.; supervision and writing-review and editing, S.H. and S.K.; investigation and resources, S.K. All authors have read and agreed to the published version of the manuscript.