One-Pot Synthesis Method of MIL-96 Monolith and Its CO2 Adsorption Performance

A novel preparation method was proposed for a metal–organic framework (MOF) monolith using a simple one-pot synthesis method. A MOF tubular monolith was successfully prepared by the hydrothermal treatment for an α-Al2O3 monolith in an aqueous solution of 1,3,5-benzenetricarboxylic acid and nitric acid without the addition of a metal source. The effects of temperature and the HNO3 concentration in the synthesis solution on the crystallization behavior of MIL-96 were studied. HNO3 enhanced the dissolution of the α-Al2O3 monolith and the growth of MIL-96. The growth rate of MIL-96 was also influenced by the synthesis temperature; a synthesis temperature of over 453 K was required for crystallization. The CO2 adsorption capacity of the prepared MIL-96 monoliths was evaluated and found to be comparable to that of the well-grown MIL-96 powdery crystal. Furthermore, the MIL-96 monoliths demonstrated good stability as their adsorption properties were retained even after 2 months of storage under atmospheric conditions.


INTRODUCTION
Carbon capture, utilization, and storage (CCUS) technologies have been widely studied for implementing carbon neutrality. The bottleneck in CCUS technology is the high cost of the carbon capture step, which consumes more than 60% of the entire CCS process cost. 1,2 Therefore, a CO 2 separation technology with low energy consumption is highly desirable.
Adsorption separation using solid adsorbents is a promising technique for CO 2 capture with low energy consumption. Various solid materials for CO 2 capture have been studied in recent decades, including carbon-based materials, silica, alumina, zeolites, polymers, and metal−organic frameworks (MOFs). 3 MOFs, also known as porous coordination polymers, have attracted considerable attention as novel adsorbents. MOFs are porous materials comprising metals and organic ligands. By tuning the combination of metal and ligand species, the pore sizes, pore networks, and adsorption properties of the MOF can be controlled. A wide variety of MOFs with excellent CO 2 adsorption properties have been developed. 4,5 MIL-96 is a MOF composed of Al and trimesic acid (TMA, 1,3,5-benzenetricarboxylic acid). MIL-96 has attracted attention as a novel CO 2 adsorbent because of its high CO 2 adsorption capacity and high resistance to humidity and high temperature. 6−8 Owing to its high stability, MIL-96 is also expected to be an adsorbent for a wide range of applications, such as the separation of light hydrocarbons, 9 the capture of iodine in organics, 10 and the defluoridation of water. 11 Despite many interesting studies on MOF synthesis, there are a few examples of the industrial application of MOFs as adsorbents. Although an adsorbent should be granulated for use in the adsorption process, MOF crystals can easily break into tiny particles of fine powder, and adsorbent loss and pipe clogging inevitably occur. 12 Hence, several articles and reviews have highlighted the need to shape MOF powders into millimeter-sized objects with sufficient mechanical strength for their effective use in potential applications. 12−14 Therefore, progress in shaping techniques is crucial for the utilization of MOFs as adsorbents.
Shape-forming technologies for MOFs have also been reported, including processing methods such as pressing or extrusion under high pressure with binders. 12,15 Whereas pressing is one of the easiest methods to obtain MOF pellets, unfortunately, the use of inactive binders and the associated pore blockage can result in pellets with low porosity and adsorption capacity. Bazer-Bachi et al. studied the effect of compression on the adsorption properties of MOFs (ZIF-8, HKUST-1, and SIM-1) and reported irreversible changes due to the loss of crystallinity in proportion to the applied force. 13 MOFs are often mixed with substrates such as polymers or aerogels for shaping. 16,17 Zhang et al. prepared a filter by processing a MOF into a nanofibrous filter, 16 which effectively adsorbed toxic gases, such as SO 2 .
Recently, a new preparation method was developed to obtain MOF monoliths. Several strategies have been reported for the synthesis of MOF monoliths, including (1) coating the MOF on a preshaped monolith and growth, (2) shaping a mixture of a MOF and binder into a monolith, (3) shaping a mixture of a binder and metal source and/or organic ligand into a monolith and crystallizing it into a MOF monolith, (4) crystallizing a metal monolith into a MOF monolith, and (5) crystallization of a metal oxide into a MOF monolith. For example, in case 1, Ramos-Fernandez et al. prepared a MIL-101(Cr) monolith by seeding and secondary growth. In this study, a seed crystal of MIL-101(Cr) was loaded onto a cordierite monolith and grown in a synthesis solution. 18 In case 2, Kaskel et al. reported a Cu 3 (BTC) 2 monolith prepared via the extrusion of a mixture of the MOF and binder. 19 In case 3, Rezaei et al. reported shaping a mixture of MOF precursors and binders (kaolin or bentonite) into a honeycomb monolith by 3D printing, followed by hydrothermal or solvothermal treatment of the preshaped monolith to obtain an MOF monolith. 20,21 Kim et al. developed a growth method for MOFs (MIL-53, HKUST-1, and ZIF-7) on metal fibers and meshes, 22 using them as both substrates and metal sources for MOFs. MOF monoliths derived from metal oxide monoliths have rarely been reported. Liang et al. reported the preparation of MOF monoliths (MIL-53(Al), HKUST-1, ZIF-8, and ZIF-67) from metal oxide sheets prepared through electrospinning of sol−gel precursors followed by calcination. 23 These methods described above have enabled the production of MOF monoliths with complex shapes and large surface areas.
For MOF monoliths, it is very important to achieve both strength and adsorption properties. Using a metal oxide substrate for monoliths has several advantages, including expanded thermal and chemical stabilities of the substrate, and a variety of choices of atomic species and shapes for ceramic monoliths. In this paper, we propose a novel technique for preparing MIL-96 monoliths using a commercially available α-Al 2 O 3 monolith. Our approach is to convert an α-Al 2 O 3 monolith into an MIL-96 monolith directly via a simple onepot synthesis in an aqueous solution of 1,3,5-benzenetricarboxylic acid without addition of a metal source. This method does not require pretreatment of the monoliths, such as seeding or surface modification with organic ligands. This technique allows for the easy production of MOF monoliths with practical sizes and mechanical strengths. We investigated the formation process of a MIL-96 monolith using this method and evaluated the CO 2 adsorption property of the MIL-96 monolith, comparing it with that of MIL-96 powdery crystals. The mechanical strength of the MIL-96 monolith was also evaluated.

Materials and Chemicals.
An α-Al 2 O 3 tubular monolith (length = 10 mm, inner diameter = 7 mm, and outer diameter = 10 mm) was used to prepare the MIL-96 monolith. The monolith was washed with distilled water and acetone through sonication. After washing, the monolith was dried at 373 K before use.

MOF Monolith Preparation
. HNO 3 , TMA, and tubular α-Al 2 O 3 were used as the raw materials for preparing the MIL-96 monolith. Subsequently, 0.654 g of TMA was added to 37.5 g of distilled water. The mixture was stirred at 353 K until TMA was completely dissolved. After adding a certain amount of HNO 3 , the mixture was poured into a glass container with 1.5 g of α-Al 2 O 3 tubular monolith. The concentration of HNO 3 in the synthesis solution was varied from 0 to 0.7 M. The glass container was placed in a stainless-steel autoclave and sealed. Hydrothermal treatment was performed at a certain temperature and for a given period (423−473 K, 1−14 days). After the hydrothermal treatment, the autoclave was quenched with flowing tap water to stop the reaction. The monolith and precipitated powders were separately obtained via filtration. Both the monolith and the powder were washed with ethanol and distilled water and dried at 383 K.
The prepared monolith was named MIL-96 powder was synthesized as a reference material according to the literature. 6 A mixture of TMA, aluminum nitrate nonahydrate, and distilled water was hydrothermally treated at 453 K for 24 h. The detail of the preparation procedure is provided in Supporting Information.

Calculation of Al Conversion.
The Al conversion was determined to understand the formation process of MIL-96 from the α-Al 2 O 3 monolith as follows.
In this study, a portion of the α-Al 2 O 3 monolith was consumed through three pathways: dissolution into an aqueous solution, crystallization of the precipitated MIL-96 powdery crystals, and crystallization of MIL-96 on the monolith. Consequently, we procured the monolith, synthesis solution, and precipitated powder after the reaction. Here, we defined the number of Al atoms in the raw Al 2 O 3 monolith as A [mol], that dissolved in the solution as B [mol], that in the precipitate as C [mol], and that converted to MIL-96 in the monolith as D [mol]. The weight change of the entire monolith before and after synthesis was defined as ΔW [g].
The number of Al atoms in the raw Al 2 O 3 monolith, A, was calculated based on the weight of the monolith before synthesis. The number of Al dissolved in aqueous solution, B, was evaluated using inductively coupled plasma (ARCOS, SPECTRO Analytical Instruments). The concentration of Al in the reaction solution was measured, and the amount of dissolved Al was calculated. In addition, the number of Al in the precipitated powder, C, was measured using thermogravimetric (TG) analysis. The precipitated powder was calcined in flowing air using TG, and the amount of Al remaining after calcination in the Al 2 O 3 form was determined.
During the synthesis, the weight of the entire monolith was reduced through the leaching of Al and increased by capturing organic ligands with the formation of the MOF. Thus, the weight change ΔW [g] of the entire monolith can be expressed using A−D as follows  5 [BTC] 6 ). 7 The first and second terms on the right side of eq 1 indicate the weight reduction due to Al leaching and weight gain due to the capture of organic ligands during the formation of the MOF, respectively. Using this equation, we calculated the amount of Al converted to the MIL-96 in monolith D.

CO 2 Adsorption
Test. The CO 2 adsorption properties of the prepared MOF powders and monoliths were evaluated using the volumetric adsorption method (Belsorp-MAX, MicrotracBEL). The

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Research Article equipment had a special sample holder that allowed the insertion of an entire monolith without cutting or fracturing. The detailed structure of the sample holder is described elsewhere. 24 CO 2 adsorption tests were performed at 283−333 K for the samples, which were pretreated at 423 K for 8 h under vacuum. To calculate the amount of adsorbed CO 2 on the monolith, the weight of MIL-96 in the monolith was used rather than the entire weight of the monolith.

Cleavage Test for the α-Al 2 O 3 Tube and MIL-96 Monolith.
A cleavage test was conducted to evaluate the mechanical strength of the MOF monoliths. An α-Al 2 O 3 tubular monolith or MIL-96 monolith lying horizontally was sandwiched between two plates, and the applied load at which the monolith fractured was recorded. A tensile testing machine (AG-I 250 kN, Shimadzu) equipped with a load cell of 10 kN was used for testing. The samples were pressed at a rate of 1 mm min −1 .

Effect of HNO 3 Concentration on MIL-96
Crystallization Behavior. The HNO 3 concentration was varied from 0 to 0.7 M, and its effect on the MIL-96 crystallization behavior was studied. MIL-96 monoliths were hydrothermally grown in aqueous TMA at 453 K for 1−7 days with different concentrations of HNO 3 . Figure 1 shows the typical FE-SEM images and XRD patterns of the monolith M-453-Y-3 after crystallization for 3 days with different HNO 3 concentrations. Images of the α-Al 2 O 3 monolith and M-453-0.5-3 are also shown.
The FE-SEM images show that the surfaces of the α-Al 2 O 3 monolith were entirely covered with MIL-96 crystals at higher HNO 3 concentrations (0.5 and 0.7 M). The inner surface was also converted to MIL-96 under these conditions. In addition, the typical diffraction patterns of the MIL-96 crystal and α-Al 2 O 3 were observed in all the samples, based on the XRD results. These results clearly show that the α-Al 2 O 3 monolith was successfully converted into the MIL-96 monolith in this simple one-pot synthesis without the addition of a metal source.
The HNO 3 concentration affected the crystal morphology, that is, the hexagonal bipyramidal shape and the XRD pattern. At higher HNO 3 concentrations, the lengths of the hexagonal columns between the hexagonal pyramids increased. This result agrees with the relationship between the acid concentration and crystal morphology reported by Liu et al. 25 The intensity ratio of the (002) to (012) peaks in the prepared monoliths was changed by varying the HNO 3 concentration. The intensity of the (002) peak is strongly affected by the crystal morphology. 7 This peak appears prominently in plate-like crystals and is hardly observed in hexagonal rod crystals. As shown in the SEM images, the MIL-

Influence of Synthesis Temperature on MIL-96
Growth. The effect of synthesis temperature was studied. Hydrothermal treatment was carried out in the range of 423− 473 K in TMA with 0.5 M HNO 3 . Figure 3 shows typical FE-SEM images and XRD patterns of the monolith M-X-0.5-3. Very few MIL-96 crystals were observed on the surface of the monolith grown below 443 K. The XRD results were consistent with the FE-SEM images.
At higher temperatures, the size of MIL-96 crystals increased, while the number of crystals observed on the monolith surface decreased. This result indicates that MIL-96 crystals grew through the Ostwald ripening, where crystals grew from the surrounding ones, thereby decreasing their total number. Crystal growth on a substrate in the Ostwald ripening mode has been previously reported for ZIF-8. 26 The results presented in Figure 4 demonstrate the temperature effect of Al consumption in the α-Al 2 O 3 monolith. At 423 K, MIL-96 crystals were hardly generated in either the monolith or the precipitate, consistent with the results shown in Figure 3. At 443 K, a small number of MIL-96 crystals were formed in the monolith. The growth rate of MIL-96 remarkably increased above 453 K, leading to the Al conversion to the MIL-96 monolith reaching 6.2 mol % (21 wt %). The number of precipitated powdery crystals increased with increasing synthesis temperature, possibly due to a large  A large number of previous studies reported that MIL-96 was synthesized at temperatures above 473 K and was hardly obtained at lower temperatures. 6−8 The fact that crystallization did not proceed below 443 K is not a specific phenomenon in monolith preparation but rather a general trend in MIL-96 synthesis. Figure 5 shows a schematic illustration of the effect of HNO 3 concentration and synthesis temperature on the crystallization behavior, as indicated by the results shown in Figures 1−4. This novel synthetic method was demonstrated for an α-Al 2 O 3 monolith with a more complex shape. We successfully confirmed the direct conversion of the porous α-Al 2 O 3 filter, as shown in Figure S1, although the Al conversion of the MIL-96 monolith was small. The mechanical strength of the MIL-96 monoliths was also evaluated. Table 1 lists the loads applied when the monolith was fractured in a cleavage test. Although the mechanical strength of MIL-96 monoliths was lower than that of α-Al 2 O 3 monoliths, MIL-96 monoliths had sufficient mechanical strength to serve as adsorbents. Figure S2 presents images of the cleavage test.

Adsorption Behavior of the MIL-96 Monolith.
We evaluated the adsorption properties of MIL-96 monoliths and compared them with those of MIL-96 powder. CO 2 adsorption tests were conducted after pretreatment at 423 K for 8 h under vacuum. In the case of the MIL-96 monolith, the weight of MIL-96 generated was used as the denominator to calculate the amount adsorbed. The amount adsorbed per monolith weight is listed in the Supporting Information (Table S1). CO 2 adsorption tests were performed on three types of monoliths, M-453-0-3, M-453-0.5-3, M-473-0.5-3, and powdery crystals. Figure 6 shows the CO 2 adsorption isotherms on MIL-96 monoliths and powder at 283 K. The amount of CO 2 adsorbed on each monolith significantly depended on the synthesis conditions. The adsorbed amounts on M-453-0-3, M-453-0.5-3, and M-473-0.5-3 at 100 kPa were 2.34, 86.8, and 130 cm 3 (STP) g −1 , respectively. For comparison purposes, the amount of CO 2 adsorbed on MIL-96 powder was measured and found to be 140 cm 3 (STP) g −1 at 100 kPa, which almost agreed with the values reported in the previous study. 8 M-453-0-3, prepared in the absence of HNO 3 , showed very low CO 2 adsorption, suggesting that the micropore of M-453-0-3 was almost completely plugged by unreacted materials such as TMA. In contrast, M-453-0.5-3 and M-473-0.5-3, obtained in the presence of HNO 3 , exhibited relatively high CO 2 adsorption capacities. M-473-0.5-3 exhibits the highest CO 2 adsorption capacity among the three monoliths, which can be attributed to the increase in crystallinity at the high synthesis temperature. Notably, the amount of adsorbed CO 2 on M-473-0.5-3 was comparable to that adsorbed on MIL-96 powder.
It is well known that the CO 2 adsorption capacity of MIL-96 is often affected by the synthesis method and conditions. Table  2 presents the amounts of CO 2 adsorbed on MIL-96     Figure 7 shows the N 2 isotherms and pore size distribution obtained for MIL-96 monoliths and powder at 77 K. As is the case of CO 2 adsorption, the weight of MIL-96 generated was used as the denominator for the calculation of the amount adsorbed. M-453-0-3 showed almost zero N 2 adsorption capacity even at 77 K, indicating that it was a nonporous material. Moreover, the amount of N 2 adsorbed on M-473-0.5-3 was slightly lower than that adsorbed on MIL-96 powder. These results are consistent with the CO 2 adsorption results ( Figure 6), supporting that the CO 2 adsorption capacity of the MIL-96 monolith can be explained by the micropore volume. Moreover, MIL-96 powder and monolith had almost the same pore size distribution calculated assuming cylindrical pores by GCMC. Figure 8 shows the adsorption isotherms of CO 2 on M-473-0.5-3 at 283−333 K. Unsurprisingly, the amount of adsorbed CO 2 tends to be smaller at higher temperatures. Specifically, CO 2 adsorbed amounts at 100 kPa and at 283 and 333 K were 130 and 58 cm 3 (STP) g −1 , respectively. This result suggests that the MIL-96 monolith prepared in this study had a large working capacity around ambient temperature and was suitable for temperature-swing adsorption of CO 2 .
The stability of the MIL-96 monolith in air was also studied. Figure 9 shows the CO 2 adsorption isotherms on fresh M-473-0.5-3 and the sample stored in air for 2 months. The adsorption isotherm obtained for the stored sample was in good agreement with that for the fresh sample, indicating that the monolith was highly stable in air and humidity.

CONCLUSIONS
Hydrothermal treatment of the α-Al 2 O 3 monolith in an aqueous solution of TMA and HNO 3 provided a simple onepot synthesis route of the MIL-96 monolith without the addition of a metal source. In this method, α-Al 2 O 3 was directly converted to MIL-96. The synthesis conditions, such as the synthesis temperature and HNO 3 concentration in the synthesis solution, significantly affected the crystallization behavior of MIL-96.
The monolith exhibited a good CO 2 adsorption capacity, almost the same as that of the well-crystallized MIL-96 powder, and maintained its adsorption property for CO 2 even after 2 months while also exhibiting good mechanical strength.
This simple one-pot synthesis enabled us to convert complex-shaped monoliths, such as formed or 3D-printed ceramics, resulting in MOF adsorbents with high surface areas and low flow resistances, making them promising candidates in industrial applications.

* sı Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.2c22955.   Pictures of the MIL-96 monolith converted from the porous α-Al 2 O 3 filter and the equipment for the cleavage test (PDF)