Sustainable manufacturing of CALF-20 via a ZnO-based route eliminating the washing step

Abstract

In this study, we present a sustainable and scalable synthesis route for CALF-20 by employing zinc oxide (ZnO) as the zinc source, thereby eliminating the washing step that is required in previously reported methods. CALF-20 synthesized using ZnO exhibits CO₂ adsorption performance comparable to the literature while achieving complete crystallization within 3 h and reaching a high zinc-based yield of 97.1%. This environmentally benign process significantly reduces liquid waste generation and simplifies post-synthetic purification, offering strong potential for cost-effective, industrial-scale MOF production.

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Introduction

Metal–organic frameworks (MOFs) are crystalline porous materials assembled from metal ions or clusters and organic ligands through coordination bonds.^1–3 Their high surface areas, tuneable pore environments, and structural diversity have enabled applications in gas separation and storage,^1,3–7 catalysis,^8,9 and drug delivery systems.^10,11 Among these applications, CO₂ capture has emerged as a critical research area, and many MOFs exhibit high CO₂ adsorption selectivity due to their tailored pore environments.^12–16

CALF-20 is a zinc-based MOF composed of Zn²⁺ ions, 1,2,4-triazolate (Tzu⁻), and oxalate (Ox²⁻), forming the framework Zn₂(Tzu)₂Ox, Zn₂(H₂C₂N₃)₂(C₂O₄).¹⁷ Owing to favourable dispersion interactions between CO₂ and the pore structure, CALF-20 exhibits high CO₂ uptake and CO₂/N₂ selectivity.^17–21 Additionally, its robust performance under humid conditions offers advantages over classical adsorbents such as zeolite-13X and Mg-MOF-76.^17,22

Efforts to simplify CALF-20 synthesis have led to several solution-based, microwave-assisted, and mechanochemical methods.^18,23–25 However, these routes rely on zinc oxalate or zinc acetate as precursors, producing organic acids as stoichiometric byproducts. Consequently, extensive washing steps are necessary to remove residual oxalate or acetate ions. These purification requirements, along with frequent use of organic solvents such as N,N-dimethylformamide (DMF), complicate waste management and hinder the scalable and sustainable production of MOFs.^26–29 Developing synthetic routes that minimize waste liquid generation is therefore crucial for advancing green and economically viable MOF manufacturing.

Here, we report the first synthesis of CALF-20 using zinc oxide (ZnO) as the zinc source. This approach produces only water as a byproduct (eqn (1)) and eliminates the need for a washing step. We show that CALF-20 prepared using ZnO exhibits CO₂ adsorption performance (3.94 mmol g⁻¹) and BET surface area (545 m² g⁻¹) comparable to previously reported materials. We also examine how the omission of washing affects crystallinity, identify intermediates during the reaction, and elucidate the crystal transformation mechanism.

2ZnO + 2H₃C₂N₃ + H₂C₂O₄ → Zn₂(H₂C₂N₃)₂(C₂O₄) + 2H₂O (1)

Experimental section

Materials

All commercially available chemicals were used without further purification. Zinc oxide (≥98%), oxalic acid (≥99.5%), and methanol (≥99.5%) were purchased from Fujifilm Wako Pure Chemical Corporation. 1,2,4-Triazole (>99.0%) was purchased from Tokyo Chemical Industry Co.

Wash-free synthesis of CALF-20 using ZnO

First, 4.07 mmol of ZnO was added to an aqueous methanol solution (total volume: 20 mL; H₂O/methanol = 16/4 mL) and stirred for 30 min using a magnetic stirrer. Separately, 4.07 mmol of 1,2,4-triazole and 2.04 mmol of oxalic acid were dissolved in another identical solvent mixture and stirred for 30 min. The two solutions were then combined and stirred for x hours (x = 1, 2, 3, 4, 5, 6, 24). The resulting solid was collected by centrifugation without any washing step. The product was dried in air at 100 °C and 101.3 kPa, followed by drying at 130 °C overnight under vacuum. The product is denoted as wf-CALF-20. In order to investigate the effect of the washing, a separate batch of the product was collected by centrifugation and washed six times with 40 mL of distilled water each time, followed by drying under the same conditions. Table S1 summarizes the detailed synthesis conditions of previously reported methods and this study.

Characterization

Powder X-ray diffraction (PXRD) patterns were collected using a MiniFlex 600 diffractometer (Rigaku) with CuKα radiation (λ = 1.5418 Å) operated at 40 kV and 15 mA. Particle morphologies were examined by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800). CO₂ adsorption isotherms at 298 K and N₂ adsorption–desorption isotherms at 77 K were measured using a BELSORP-mini x instrument (MicrotracBEL). Prior to measurements, samples were degassed at 498 K under vacuum for 4 h. Equilibrium was considered reached when the pressure change in the sample cell was within 0.3% for 180 s. The Brunauer–Emmett–Teller (BET) surface area and micropore volume were determined from N₂ adsorption–desorption isotherms at 77 K. Micropore volumes were calculated using the t-plot method. Particle size distributions were measured at 298 K using a zeta-potential and particle size analyzer (ELS-ZDX, Otsuka Electronics). Thermogravimetric (TG) analysis was performed on a DTG-60H instrument (Shimadzu). Approximately 5 mg of sample was placed in an alumina cell and heated in air from room temperature to 800 °C at a rate of 2 °C min⁻¹.

Results and discussion

The necessity of washing CALF-20 samples synthesized from ZnO was evaluated through a series of experiments. In these experiments, the samples were either washed six times or left unwashed. PXRD patterns (Fig. 1a) and SEM images (Fig. 1b) demonstrate that CALF-20 is formed in both cases with nearly identical crystallinity and morphology, thereby confirming that washing can be completely omitted.

Fig. 1 (a) PXRD patterns of CALF-20 prepared from ZnO with and without the washing, showing the effect of washing on the crystalline phase. (b) FE-SEM images comparing the morphology of CALF-20 prepared with and without washing. Scale bar: 5 μm. For the washed sample, the product was collected by centrifugation and washed six times with distilled water before drying.

As shown in Fig. 2a, the PXRD patterns of the products vary with respect to the reaction time. At reaction times of 1–2 h, crystalline peaks attributable to CALF-20 were observed, along with peaks from zinc oxalate (2θ = 23.0–23.5°), one of the intermediates, and the starting material zinc oxide (2θ = 31.7–36.3°). However, following a duration of reaction exceeding 3 h, the crystalline peaks attributable to the intermediates and the starting material (Fig. S1) diminished, and instead, crystalline peaks originating from CALF-20 were observed. As shown in Fig. 2b, FE-SEM observations support two key findings: first, the crystallization proceeds through a sharp transition to the CALF-20 phase, and second, particle uniformity improves.

Fig. 2 (a) PXRD patterns of wf-CALF-20, showing the effect of the reaction time on the crystallization of products prepared from ZnO. (b) FE-SEM images of wf-CALF-20 prepared with reaction times of 1–6 h. Scale bar: 2 μm.

During this crystallization process, complexes involving starting materials and intermediates were identified (Fig. S2), suggesting competitive coordination between ligands. This behaviour is attributed to the difference in acid dissociation constants between oxalic acid (Ox) and 1,2,4-triazole (Tz) in the synthesis solution. The pH of the solution was determined through the preparation of a CALF-20 synthesis mixture, employing water as the exclusive solvent and subjecting it to a reaction period of 3 h. The pH at the onset of mixing was 4.54, which increased to 5.81 after stirring, as measured with a pH meter. Although it has been well known that the apparent acidity of water/methanol mixtures is slightly lower than that of pure water,³⁰ the actual CALF-20 synthesis solution still exhibited acidic conditions (approximately pH 5–6) when measured with pH paper, indicating that acidity is retained even in mixed solvents. The acid dissociation constant of Ox is smaller than that of Tz, meaning that deprotonation of Ox is favored under acidic conditions (eqn (2)–(4)). This suggests that the reaction of Ox with ZnO is predominant over that of Tz. Additionally, the DLS measurement results indicated that the mean particle size remained around 200 nm regardless of the stirring time.

Oxalic acid: pK_a1 = 1.25

H₂C₂O₄ + H₂O ⇌ HC₂O₄⁻ + H₃O⁺ (2)

Oxalic acid: pK_a2 = 4.14

HC₂O₄⁻ + H₂O ⇌ C₂O₄²⁻ + H₃O⁺ (3)

1,2,4-Triazole: pK_a = 10.1

H₃C₂N₃ + H₂O ⇌ H₂C₂N₃⁻ + H₃O⁺ (4)

TG curves of the products are presented in Fig. 3a. Eqn (5) describes the calculation of the mass percentage of ZnO contained in CALF-20, which is derived from the molecular weight of its chemical formula, Zn₂(H₂C₂N₃)₂(C₂O₄). The theoretical residual mass of 45.9% corresponds to the fraction of 2ZnO expected after complete combustion of the organic components. For products synthesized with a reaction time of 3 h or longer, the TG curves exhibited behaviour consistent with previously reported data, and the residual mass closely approached this theoretical value of 45.9%. Conversely, the residual mass percentages for the samples prepared for 1 and 2 h were approximately 62%, indicating the presence of unreacted ZnO in the products. The weight-loss profiles of these samples closely resembled those of zinc oxalate, an intermediate species (Fig. S3). This finding suggests that the measured samples consist of unreacted starting material and intermediate species, which is consistent with the morphology observed in Fig. 2b. Furthermore, the conversion rate of ZnO to CALF-20 at each reaction time was determined by estimating the amount of CALF-20 present in each sample based on the residual mass corresponding to ligand decomposition (Fig. 3b). The experimental results unequivocally demonstrate that reaction times of 3 h and beyond result in nearly complete conversion of ZnO to CALF-20, thereby indicating the formation of high-purity CALF-20.

Fig. 3 (a) TG curves of wf-CALF-20 prepared from ZnO with different reaction times. For comparison, TG data of CALF-20 prepared according to the reported procedure in ref. 23 are also shown. (2 °C min⁻¹) (b) conversion of ZnO to CALF-20 at each reaction time, calculated based on the residual ZnO content.

The slightly lower thermal stability observed for the ZnO-derived CALF-20, in comparison to samples prepared via the conventional solvothermal method,²⁰ is predominantly ascribed to its diminished particle size. The increased surface-to-volume ratio in these smaller particles has been demonstrated to augment surface-related effects, which can result in a reduction in the onset temperature of thermal decomposition. Notably, the high CO₂ adsorption capacity suggests that the ZnO-derived CALF-20 possesses long-range order, indicating a low density of crystal defects. Therefore, the slight decrease in thermal stability is not attributable to structural defects, but rather is likely associated with the smaller particle size and surface-related effects. Nevertheless, the thermal stability remains sufficient for practical CO₂ capture applications.

The CO₂ adsorption performance, pore characteristics, and CALF-20 yield as a function of reaction time are presented in Fig. 4. In accordance with the trends evident in Fig. 1 and 2, there was a marked increase in performance at a reaction time of 3 h. The maximum CO₂ adsorption capacity and BET surface area were achieved at a reaction time of 5 h, reaching 3.94 mmol g⁻¹and 545 m² g⁻¹, respectively. In addition, the maximum yield of 97.1% was obtained at a stirring time of 6 h. These results indicate that the product synthesized via the ZnO route exhibits CO₂ adsorption performance and pore characteristics comparable to those of samples washed with deionized water (Fig. S4 and S5), clearly demonstrating that a washing step is unnecessary when CALF-20 is produced through crystal conversion from ZnO.

Fig. 4 Relationship between CO₂ uptake at 298 K and 1.0 bar, BET surface area, and CALF-20 yield for wf-CALF-20 as a function of reaction time.

As shown in Fig. 4, both the CO₂ uptake and BET surface area increase with reaction time and reach a maximum at 5 h, indicating that the formation of the CALF-20 framework is essentially complete within this time frame. The coordination-driven assembly of the Zn–Tz–Ox network proceeds relatively rapidly under the present synthesis conditions, leading to the development of accessible microporosity and adsorption sites within 5 h. Beyond this point, extending the reaction time to 24 h does not result in the creation of additional accessible porosity and new CO₂ adsorption sites. Instead, prolonged reaction times may promote particle growth or aggregation, which can reduce gas accessibility and limit further improvements in adsorption performance without altering the intrinsic framework structure. This behavior is consistent with the formation of a thermodynamically stable CALF-20 framework possessing a self-limited number of adsorption sites. Consequently, a reaction time of 5 h represents an optimal condition that ensures complete framework formation while maintaining efficient synthesis and high CO₂ adsorption performance.

To further investigate the results described above, Fig. S6 presents the PXRD patterns of products synthesized under static conditions, without stirring. For samples prepared from 1 h to 24 h, the PXRD patterns exhibited not only the characteristic peaks of CALF-20 but also peaks corresponding to unreacted zinc oxide and the intermediate zinc oxalate. Subsequent analysis via FE-SEM observations (Fig. S7) further corroborated the presence of particles derived from zinc oxide and zinc oxalate. These results suggest that, under static conditions, the intermediate zinc oxalate persists irrespective of reaction time. Fig. S8 shows the CO₂ adsorption capacity, BET surface area, and CALF-20 yield for products obtained under static conditions. While CO₂ adsorption capacity, BET surface area, and CALF-20 yield gradually increased from 1 h to 24 h, all values remained significantly lower than those for products synthesized with stirring. Fig. S9 shows photographs of the reaction mixture at different reaction times. The solution remained turbid up to 6 h, whereas by 24 h the solvent and solute had completely separated, indicating that the reaction had effectively ceased. These findings suggest that in the synthesis of CALF-20, the process of crystallization is significantly enhanced when the zinc source and ligands are homogenized by an external force, such as stirring, during the synthesis process.

The CALF-20 syntheses reported in previous literature were reproduced without the washing step. The crystallinity and CO₂ adsorption performance of the resulting products were then compared (Fig. 5). The PXRD patterns shown in Fig. 5a substantiated the occurrence of peaks originating from CALF-20 in all samples, indicating that crystallization transpired from each synthetic method, even in the absence of washing. However, Fig. 5b revealed that omitting the washing step had a significant impact on CO₂ adsorption performance. For methods using zinc oxalate dihydrate as the precursor (e.g., solvothermal¹⁷ and liquid-assisted grinding²⁴), the CO₂ adsorption capacity was largely maintained but was found to be slightly lower than reported values. This suggests that the methanol washing step listed in Table 1 assists in removing residual oxalic acid. In contrast, methods utilizing zinc acetate dihydrate as the precursor (e.g., room temperature²³ and neat grinding²⁵) exhibited CO₂ adsorption capacities considerably lower than the previously reported (reductions of 30–65%). This decline is likely due to the occlusion of CO₂ adsorption sites by residual acetate ions, which are expected to be removed during washing process but remained in the CALF-20 framework when washing is omitted. It should be noted that, for the reproduction of the neat grinding method reported in literature,²⁵ a planetary ball mill was used in this study, whereas a vibrating mixer mill was employed in the original work. Despite applying a relatively high-energy milling condition for the available planetary mill, the reproduced sample exhibited a markedly lower CO₂ uptake (1.23 mmol g⁻¹) than the value reported in literature²⁵ (3.39 mmol g⁻¹). This discrepancy is likely due to fundamental differences in energy transfer, impact mode, and shear forces between vibrating mixer mills and planetary ball mills, highlighting the strong dependence of mechanochemical CALF-20 synthesis on the type of milling apparatus.

Fig. 5 Comparison between unwashed CALF-20 samples prepared by previously reported methods and the wf-CALF-20 obtained by the method developed in this study. (a) PXRD patterns; (b) CO₂ adsorption–desorption isotherms. (Open symbol: adsorption, closed: desorption).

Table 1 Comparison of CALF-20 synthesis in previous reports and in this study, including synthesis temperature, reaction time, yield, BET surface area, CO₂ uptake, and washing procedures/solvents

Synthesis method	Synthesis temp., time	Zinc-based yield (%)	S BET (m^2 g^−1)	CO2 uptake (mmol g^−1)	Washing solvent	Ref.
Solvothermal	453 K, 48 h	70	528	4.07@1.2 bar, 293 K	Methanol	17
	453 K, 48 h	70	—	2.86@0.2 bar/303 K	Methanol	19
Microwave-assisted solvothermal	453 K, 48 h	97.3	548	4.02@1.0 bar, 298 K	Methanol	18
Room temperature	298 K, 1 h	93.3	520	3.84@1.0 bar, 298 K	Distilled water	23
Mechanochemical (liquid-assisted grinding)	298 K, 0.5 h	—	535	2.55@0.2 bar, 301 K	DI water	24
Mechanochemical (neat grinding)	298 K, 0.25 h	97.0	551	3.39@1.0 bar, 298 K	DI water	25
Wash-free	298 K, 3 h	97.1	545	3.94@1.0 bar, 298 K (2.71@0.2 bar, 298 K)	Wash-free	This work

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These results confirm the necessity of the washing process using deionized water to remove acetate. For syntheses employing zinc organic acids, including a washing step is therefore essential to eliminate residual organic acids and methanol, thereby enhancing the purity of CALF-20. In contrast, CALF-20 synthesized from ZnO in this study maintained its CO₂ adsorption performance without any washing. A comparison of synthesis methods—including synthesis time, yield, BET surface area, CO₂ adsorption capacity, and the presence or absence of washing—presented in Table 1 shows that the properties of the ZnO-derived CALF-20 are comparable to those of previously reported samples. These findings further demonstrate that CALF-20 can be synthesized from the inexpensive zinc source ZnO and suggest that the washing step can be omitted in future scale-up processes, resulting in substantial cost savings in waste liquid treatment.

Conclusions

In this study, CALF-20 was successfully synthesized for the first time by replacing the zinc source from a zinc salt of an organic acid with the more economical zinc oxide. This modification eliminated the need for a washing step. This synthesis enables production in a mere three hours while maintaining a high yield of up to 97.1%, based on zinc content. Furthermore, the resulting material demonstrated CO₂ adsorption performance and pore structure properties that were comparable to those of previously reported CALF-20. This process suggests that increasing CALF-20 production could significantly reduce the discharge of waste liquid associated with the washing step. To achieve large-scale production of CALF-20 as a viable CO₂ capture material, further experimentation is necessary.

Author contributions

Y. K.: investigation, validation, data curation, formal analysis, writing – original draft; Y. H.: investigation, data curation, formal analysis; S. T.: conceptualization, methodology, formal analysis, data curation, funding acquisition, project administration, supervision, writing – original draft, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data supporting the findings of this study are provided in the supplementary information (SI) associated with this article.

Supplementary information: additional experimental details, characterization data, and supplementary table and figures supporting the results discussed in the main text. See DOI: https://doi.org/10.1039/d5ce01201e.

Acknowledgements

The authors gratefully acknowledge financial support from JSPS KAKENHI Grant Numbers JP24K21706, the Kansai University Fund for Supporting Outlay Research Centers, 2024, JKA and its Promotion Funds from KEIRIN RACE, Grant No. 2023M-412, and the FY2023 Research Grant Program of the Carbon Recycling Fund Institute, Japan.

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