Green and scalable synthesis of a dual-ligand Zn-MOF with unprecedented space–time yield in aqueous media and efficient CH₄/N₂ separation†

Zhang-Ye Han^a, Xuefeng Bai^a, Yan-Long Zhao^a, Wen-Liang Li^a, Quanyou Sun^a, Zheng-He Xie^b, Li-Feng Ding^b, Rui Li*^a and Jian-Rong Li*^a
aBeijing Key Laboratory for Green Catalysis and Separation and Department of Chemical Engineering, College of Materials Science and Engineering, Beijing University of Technology, Beijing, 100124, P. R. China. E-mail: jrli@bjut.edu.cn; lirui1@bjut.edu.cn
^bBeijing Energy Holding Co., Ltd, Beijing, 100022, People's Republic of China

First published on 27th August 2024

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Introduction

Metal–organic frameworks (MOFs), a unique category of porous materials, are renowned for their captivating porous network structures formed by the intricate bonding of organic linkers and metal/metal clusters. Benefiting from their customizable structure,¹ high-density open metal sites,² extensive surface area,³ and tailor-made porous environment,⁴ MOFs demonstrate exceptional potential across a spectrum of applications, ranging from energy to medicine,^5–13 with particular prominence in adsorption and separation processes.^14,15 For example, separation of CH₄ from N₂ is urgently demanded to alleviate the energy shortages and diminish greenhouse gas emissions.^16,17 MOFs adeptly address this requirement, delivering exceptional CH₄/N₂ separation performances, attributed to their specially designed structures featuring deliberately selected active centres and suitable pore sizes.^18–20 An extensive array of MOFs with ultra-high stability and excellent industrial prospects has been developed over the past two decades.^21–23 Nevertheless, the traditional solvothermal synthesis of MOFs, marked by its seriously polluting nature, high expenses, operational intricacies, and extensive utilization of organic solvents,^24–26 poses significant challenges to scale-up, significantly delaying its widespread integration into industrial practices.

Consequently, a multitude of new synthesis methods have been established,^27–29 including mechanochemistry,³⁰ flow chemistry,³¹ electrochemistry,³² spray drying,³³ and high gravity technology,³⁴ to mitigate the use of toxic organic solvents. However, the suboptimal product quality and complex operation procedures with significant equipment costs often restrict their board utilization. Alternatively, the employment of water as the synthesis solvent is a facilitated, economical, and environmental strategy, thereby gaining growing preference.³⁵ Base-assisted synthesis^36–38 and modulated hydrothermal (MHT) chemistry^39–41 have been extensively utilized for the preparation of MOFs in aqueous media. Basolite A520® (Al-fumarate) stands as a prime instance of base-assisted synthesis and is among the earliest MOFs commercialized by BASF.⁴² Although this technique is successful in the production of MOFs such as CAU-10,⁴³ MIL-160,⁴⁴ and MOF-303,⁴⁵ it is accompanied by the potential risk of rapid deprotonation of carboxylate ligands, which may threaten the quality of the final product or even impede the formation of the desired crystal structures.^38,46 MHT synthesis, characterized by a heterogeneous reaction environment, is defined as the formation of metal-modulator preclusters and sequential exchange by multidentate ligands, resulting in high-quality MOF structures⁴¹ such as UiO-66,⁴⁷ MOF-801,⁴⁸ and MOF-808.⁴⁹ While the synthesis approach is eco-friendly, the limited solubility of some ligands generally requires large amounts of water, adversely impacting the space–time yield (STY). Additionally, the prevalence of acid modulators (such as chloroacetic acid, acetic acid, or formic acid) could impose detrimental effects on the manufacturing infrastructure and escalate the costs of production and waste management. Therefore, a limited number of MOFs have been successfully synthesized on a large scale using this method, with virtually no reports on mixed-ligand MOFs.

Herein, the advantages of base-assisted synthesis and MHT chemistry were complementarily integrated for the synthesis of a mixed-ligand Zn-MOF (Zn₂(atz)₂ipa, atz = 3-amino-1,2,4-triazole, ipa = isophthalic acid) with the acetate anion introduced as a gentle yet effective modulator. The alkali NaOH was employed to facilitate the deprotonation of the carboxylate ligand, while the acetate modulator competed with the deprotonated carboxylates for coordination regulation and deprotonated the secondary ligand of azole.^50–52 Owing to the synergism of the alkali and acetate, the mixed-ligand Zn-MOF was rapidly synthesized in pure water with high crystallinity and porosity. This innovative strategy not only eradicates the reliance on organic solvents and corrosive acids, but also refines the crystallization to improve product quality. Moreover, the optimized synthesis parameters enabled rapid fabrication, achieving a maximum STY of 24 ton per m³ per day and a batch size exceeding the kilogram scale. A dramatically reduced production cost of $32 per kg was also achieved, undercutting the market prices of many prestigious molecular sieves. The multifunctionality of the synthesized Zn-MOF has stimulated various applications.^53–56 Its considerable CH₄/N₂ separation capacity was unveiled here, contributing to the high affinity between the Zn-triazole layer and the CH₄ molecule. The economical, swift and environmentally benign synthesis, coupled with the pronounced properties, collectively make the developed Zn₂(atz)₂ipa an auspicious adsorbent for CH₄/N₂ separation.

Results and discussion

To comprehensively analyse the impact of the acetate modulator in the process, different synthesis parameters were systematically altered, including the source and dosage of acetate, the type of metal salts, the addition sequence and concentration of ingredients, the ratio of ipa to NaOH, as well as the synthesis temperature and duration. Substantial changes in the crystallinity, morphology and STY of the samples were perceived, while their purity, yield, and surface area were evaluated for synthesis protocol optimization.

Effects of anions

The acetate modulator was first introduced in the form of NaOAc, the dosages of which were found to significantly influence the properties of the product while the other ingredients remained unchanged (Fig. S1 and Table S1†). The target MOF of Zn₂(atz)₂ipa was not observed in the absence of NaOAc, and increasing its dosage led to a consistent rise in sample crystallinity (Fig. S2†). A distinct improvement of yield from 36% to 82% occurred as the ratio of ipa to NaOAc ascended from 4:1 to 1:6 (Table S1†), while a slight increase was noticed in the BET surface area (Fig. S3†). The same trend was observed when altered zinc salts were employed. For both sulphate and chloride salts, NaOAc was required to generate the target MOF structure (Fig. S4†). It is anticipated that the impact of NaOAc is derived from the OAc⁻ group. Therefore, zinc acetate was adopted as the metal source to verify this assumption. Indeed, high-quality Zn₂(atz)₂ipa with similar crystallinity and adsorption capacity to the solvothermally synthesised product was obtained in the absence of Na⁺ (Fig. S5†), suggesting the significance of OAc⁻ in the crystallization of Zn₂(atz)₂ipa. Interestingly, a hysteresis loop was observed in the N₂ adsorption isotherm of the as-synthesized Zn₂(atz)₂ipa, indicating the presence of mesoporosity. This feature does not exist for the solvothermally prepared sample. Considering the extra cost of NaOAc and the corrosive/explosive nature of the nitrate, sulphate, and chloride anions, zinc acetate was selected as the metal source in the subsequent syntheses while NaOAc was omitted.

Effects of the addition sequence of ingredients

Because of the different affinities between the metal ions and the two ligands,^37,57 it is anticipated that MOFs with dual ligands might be sensitive to the addition order of the ingredients. The presence of the acetate modulator further complicated the situation. Therefore, three addition sequences were examined, indeed revealing substantial variations (Fig. 1). In scenario 1, the metal source of Zn(OAc)₂ was first mixed with an aqueous solution of atz, immediately forming a white precipitate with two characteristic PXRD peaks at 11° and 17.2°, which corresponded to the diffraction pattern of a close-packed layered structure (Zn-atz-OAc, Fig. S6†).⁵⁸ Following the addition of ipa²⁻, the patterns of the white solid gradually evolved (Fig. S7†), with the characteristic peaks of Zn₂(atz)₂ipa at 6.5° and 12.2° starting to emerge after 2 h synthesis. A pure Zn₂(atz)₂ipa was eventually obtained after heating for 12 h. The role of Zn-atz-OAc as the crystalline precursor was thus ambiguously indicated, which was subsequently transformed to Zn₂(atz)₂ipa with the bridging ligand of ipa²⁻ gradually replacing OAc⁻ and connecting the Zn-atz layers. Understandably, the kinetics of this solid phase transformation was impeded by the limited mass diffusion, drastically elongating the crystallization process. Conversely, no solid precursor was produced when ipa²⁻ and atz (scenario 2) or Zn(OAc)₂ and ipa²⁻ (scenario 3) solutions were first mixed. In these two scenarios, Zn₂(atz)₂ipa was generated swiftly once the third ingredient (Zn(OAc)₂ or atz in scenarios 2 or 3, respectively) was added. The onset nucleation occurred within 2 min of heating,^59,60 as indicated by the initial peaks of Zn₂(atz)₂ipa (Fig. S8†). One hour was adequate to synthesize high crystallinity products with ample surface area (Fig. S9†), which was profoundly accelerated compared with the crystallization kinetics of scenario 1. The essential impact of addition order on the crystallization pathways and kinetics of Zn₂(atz)₂ipa was hence disclosed. The crystallization was potentially predominated by the nonclassical pathway(s) in scenario 1 featuring solid-stage transformation and rearrangement.^61,62 However, the absence of bulk precursors in scenarios 2 and 3 did not inherently lead to the predominance of classical crystallization, as evidenced by the independence of crystal physicochemical properties from the solution concentrations, which will be discussed later. For the sake of efficient production, the addition sequence of scenario 2 was preferably employed in the following syntheses.

Effects of concentrations

The concentrations of synthesis mixtures were systematically varied with a constant molar ratio of Zn(OAc)₂·2H₂O:atz:ipa:NaOH = 2:2:1:2 (Table S2†). Interestingly, the crystallinity, yield, surface area, size and morphology of the obtained samples were found to roughly remain constant (Fig. 2a and S10, S11†), illustrating the independence of sample physicochemical properties from the concentration of reagents within the investigated range. According to the classical crystallization theory, the nucleation rate is more sensitive to the alteration of supersaturation than the growth rate,⁶³ resulting in reduced particle size and promoted crystal number density as the solution concentration rises. Therefore, the observation here seems to imply variations from the classical model. It is likely that the acetate modulator implements regulation over the nucleation via competitive coordination, preventing the uncontrollable burst of nuclei at elevated concentrations and preserving the physicochemical properties of the product. Note that at sufficiently low supersaturation, the classical nucleation theory could also predict a linear relationship between the nucleation rate and supersaturation ratio with mathematical simplification,⁶⁴ which possibly leads to a consistent crystal size irrespective of solution concentration. However, the rapid formation of highly crystalline Zn₂(atz)₂ipa implies excessive supersaturation, rendering the aforementioned simplification inapplicable. Despite the unaffected product yield and properties, the productivity is inherently impacted by the reagent concentration. Space–time yield (STY), a measure of productivity, is defined as the amount of product generated per volume per time.⁶⁵ Consequently, saturated solutions were employed in the synthesis to maximize the reagent concentration. Comparing with a previous report,³⁵ significantly improved STY was achieved without compromising product quality.

Effects of the H₂ipa:NaOH ratio

Base-assisted synthesis facilitates the deprotonation of carboxylates, providing adequate supersaturation despite the limited water solubility of ligands. Nevertheless, excessive alkalinity hastily generates unnecessary deprotonated ligands and inhibits the formation of desired MOF structures, resulting in the formation of amorphous products, metal hydroxides, or other unknown impurities.³⁷ Therefore, the ratio of ipa to sodium hydroxide (H₂ipa:NaOH = 1:X, X = 2, 3, 4, 5) was systematically varied to determine the optimal pH and degree of deprotonation. Pure Zn₂(atz)₂ipa was formed when X = 2 and 3, exhibiting high yields (83% when X = 2 and 85% when X = 3, Table S3†). With higher alkalinity (e.g., X = 4), the impurity of Zn-atz-OAc again appeared, causing a significant decrease in the surface area of the sample (Fig. 2b and c). An altered acid dissociation status and/or metal–ligand coordination affinity at elevated solution pH are among potential causes for the formation of impurity. Further elevating the content of NaOH (e.g., X = 5) led to the disappearance of crystallinity (Fig. 2b). Consequently, the ipa:NaOH ratio of 1:2 was selected for subsequent syntheses to produce high-quality crystals with a diminished amount of the base.

Effects of synthesis temperature and time

All the previous syntheses were conducted at the ambient temperature, which, although not sluggish, did not exhibit competitive advantages in kinetics. Considering the abundance of industrial waste heat, the synthesis temperature was moderately elevated to recover the low-quality energy for the enhancement of product quality and manufacturing efficiency.^66,67 The samples synthesized at 20, 40, 60, and 80 °C were labelled Zn₂(atz)₂ipa-Y, with Y denoting the corresponding synthesis temperatures. Distinct morphological deviations were observed with the crystal size ascending consistently with the temperature (Fig. 3a–d). The intensified acceleration of the growth rate over the nucleation rate at elevated temperatures was thereby insinuated.⁶⁸ Despite the comparably high crystallinity (Fig. 3e), the surface areas (Fig. 3f) and yields (Fig. 3g) of Zn₂(atz)₂ipa samples exhibited a considerable increase when the temperature was increased from 20 to 60 °C. However, further escalation to 80 °C deteriorated the sample quality and productivity (Table S4†). The preferred synthesis temperature of 60 °C was therefore determined, which was anticipated to improve the crystallization kinetics. Indeed, well-faceted Zn₂(atz)₂ipa crystals were generated within 10 min of heating (Fig. 4a–d), and they possessed substantial crystallinity and N₂ uptake (Fig. S12†). By refining the synthesis parameters, an unprecedented STY of 23671 kg per cm³ per day was reached in an aqueous environment, surpassing the STY of Zn₂(atz)₂ipa prepared in organic solvents by 4000 times⁵⁶ and profoundly outperforming the hydrothermally synthesized MOFs reported in prior studies (Fig. 4e). It is worth mentioning that the traditional solvothermal synthesis of Zn₂(atz)₂ipa, with its reliance on the hazardous organic solvents of DMF and methanol, an elevated temperature of 130 °C, and a protracted synthesis time of 72 h,⁵⁶ is evidently energy-intensive and environmentally detrimental. In stark contrast, the synthesis strategy presented here requires merely water, a gentle 60 °C and a brief heating period of 10 min, offering a markedly more sustainable and efficient alternative. Furthermore, this strategy enabled the successful synthesis of diverse MOF structures featuring modified ligands, demonstrating its broad applicability (Fig. S13†).

The green and efficient synthesis and tailoring of the Zn-MOF afford promising application potential in various fields. For example, small crystals are favoured in the fabrication of membranes and catalysts, while large particles are welcomed for optical applications and the storage of guest molecules.^69,70 As a prerequisite for practical application, scale-up was performed to increase the batch size of synthesis.

Kilogram-scale synthesis of Zn₂(atz)₂ipa

A jacketed glass reactor with a capacity of 10 L was exploited for the scale-up of batch size (Fig. 5a). Optimized parameters as discussed above were adopted, with the complete synthesis protocol is provided in the ESI.† Approximately 1.2 kg powder, designated as Zn₂(atz)₂ipa-(kg), was obtained after filtering and drying the milky suspension (Fig. 5b), corresponding to a 91.3% yield based on the metal salt. The size, morphology, crystallinity and N₂ adsorption of Zn₂(atz)₂ipa-(kg) were nearly identical to those of the sample synthesized at gram-scale (Fig. 5c–e), suggesting the diminished influence of scale-up. With an extensive BET surface area of 687 cm³ g⁻¹ (Fig. S14†), the application of Zn₂(atz)₂ipa-(kg) in gas adsorption and separation was promising.

In addition, the aqueous stability of Zn₂(atz)₂ipa-(kg) was evaluated by submerging the material in water for 30 days. As depicted in Fig. S15,† the PXRD pattern and the N₂ uptake of the sample were well preserved after treatment, indicating the integrity of the crystal structure despite prolonged water exposure. The thermal stability of Zn₂(atz)₂ipa-(kg) was evaluated using thermogravimetric analysis (TGA). No significant weight loss was observed until approximately 450 °C under the N₂ atmosphere (Fig. S16†), demonstrating the exceptional thermal stability. The impressive aqueous and thermal stabilities of the Zn-MOF endorse its suitability as an adsorbent under demanding industrial conditions.

CH₄/N₂ separation performance

The gas separation capacity of Zn₂(atz)₂ipa was assessed in CH₄/N₂ separation, a pressing need under rigorous investigation. According to the corresponding adsorption isotherms, Zn₂(atz)₂ipa-(kg) exhibited a significant adsorption capacity for CH₄, reaching 18.3 and 28.3 cm³ g⁻¹ under an absolute pressure of 1 bar at 298 K and 273 K, respectively (Fig. 6a and b). The descending adsorption capacity at elevated temperatures suggests the predominance of physical adsorption. The adsorption performance aligned with the gram-scale Zn₂(atz)₂ipa and solvothermally synthesized samples (Fig. S17†). Conversely, a relatively low adsorption capacity was perceived for N₂, presenting 4.8 and 8.6 cm³ g⁻¹ at the same pressure and temperatures, nearly 4 times lower than that of CH₄. The potential of CH₄/N₂ separation was indicated. The elevated CH₄ adsorption capacity is assumed to stem from the strong affinity between the pore walls and CH₄ molecule, which will be discussed later. Facile regeneration is achieved at ambient temperature through pressure reduction, eliminating the need for additional heating. The CH₄ adsorption of Zn₂(atz)₂ipa-(kg) at 298 K was well preserved after 10 regeneration cycles (Fig. 6c), highlighting its excellent stability toward regeneration.

To precisely quantify the separation performance, the virial equation and ideal adsorption solution theory (IAST) were employed to investigate the isosteric heats of adsorption (Q_st) of Zn₂(atz)₂ipa-(kg) and its selectivity toward CH₄ in the presence of N₂. The complete calculation process is presented in the ESI.† The CH₄ selectivity in the equimolar CH₄/N₂ mixture under 1 bar is around 6.2 and 7.7 at 298 and 273 K, respectively (Fig. S18 and 19†), a performance exceeding those of various MOFs and most conventional adsorbents (Table S5†). An appropriate CH₄ Q_st of 28.0 kJ mol⁻¹ (Fig. S20†) was estimated for Zn₂(atz)₂ipa-(kg), which indicates its suitability for CH₄ separation. The satisfactory adsorption and selectivity, together with the green, scalable and cost-effective synthesis and regeneration, collectively render Zn₂(atz)₂ipa-(kg) an ideal candidate for industrial pressure swing adsorption processes. Furthermore, the economic and environmental factors of Zn₂(atz)₂ipa-(kg) were evaluated against the top-performing MOFs. Three parameters including the cost of ligands, type of solvent and synthesis conditions are compared in Table S6,† unambiguously emphasizing the profound advantages of Zn₂(atz)₂ipa-(kg) in facile and eco-friendly manufacturing, which endorses its suitability for industrial applications. In fact, the production cost of Zn₂(atz)₂ipa-(kg), including the expenses of raw materials (i.e., zinc salt, ligands and alkali), solvent (i.e., water), electricity, and wastewater treatment, was comprehensively evaluated to be merely $32 per kg, a figure significantly below the market prices of many esteemed molecular sieves (Table S7†). It needs to be emphasized that this cost was an inexhaustive and preliminary estimate merely for proof-of-principle purposes, attempting to reveal the current stage of development. Because the production costs are unavailable, the market prices of the referenced molecular sieves are presented in the table instead. On the other hand, since this estimation is based on the prices of lab-grade chemicals rather than bulk industrial reagents, further cost reduction should be feasible with industrial scale-up.

The breakthrough experiments were performed in a dynamic environment at 298 K and 101 kPa in a fixed bed. An equimolar mixture of CH₄/N₂ (50/50, v/v) was first fed to the MOF absorbent. CH₄ was found to elute 4.7 min later than N₂ from the breakthrough curve (Fig. 6d), confirming its preferential adsorption on Zn₂(atz)₂ipa-(kg). Considering the practical conditions of methane recovery, mixed gases with reduced CH₄ concentrations (i.e., CH₄/N₂ = 30/70 and 15/85) were also introduced. The differences between the release times of CH₄ and N₂ were elongated to 6.2 min (Fig. 6e and f), suggesting an expanded operational domain. The breakthrough results demonstrate that a complete separation of CH₄/N₂ binary mixtures is feasible using Zn₂(atz)₂ipa-(kg) despite low CH₄ concentrations. Note that the breakthrough curves of N₂ displayed a distinct upswelling trend across all the CH₄/N₂ ratios tested. This trend is ascribed to the preferential adsorption of CH₄, which displaces N₂, thereby reaffirming the stronger affinity between Zn₂(atz)₂ipa-(kg) and CH₄.

To further explain the mechanism of its separation performance, DFT calculations^71,72 were carried out. Both CH₄ and N₂ are found to be favourably bound within the cages of the Zn-atz layer (Fig. S21†). The low polarizability of N₂ resulted in a greater distance between the framework and the adsorbed molecule (i.e., 3.540 Å between the N atoms), suggesting a reduced binding energy. Conversely, a tighter confinement was implemented by the triazole skeleton to the CH₄ molecule, leading to strengthened adsorbent–adsorbate interactions with diminished N–H distances ranging from 2.966 to 3.515 Å (Fig. 7). A strengthened binding energy of 30.2 kJ mol⁻¹ was thus calculated for CH₄, consistent with the experimental results (Fig. S20†). Therefore, the superior adsorption affinity for CH₄ over N₂ is primarily attributed to the molecular size compatibility and the reinforced interactions within the micropores.

Conclusion

A green and innovative modulator/base co-regulation strategy was developed to create abundant yet regulated deprotonated ligands for the efficient crystallization of high-quality MOFs. The acetate anion was selected as a mild modulator ideally suited for base-assisted synthesis, impacting the formation of intermediate structures and alternating the crystallization pathways(s). A dual-ligand Zn-MOF, Zn₂(atz)₂ipa, was successfully synthesized in aqueous media, achieving an extraordinary STY of 24 ton per m³ per day and a batch size larger than one kilogram with meticulously optimized synthesis parameters. Its production cost was dramatically reduced to $32 per kg, lower than the market prices of numerous renowned molecular sieves. Additionally, the as-synthesized MOF exhibited a considerable CH₄ adsorption capacity (18.3 cm³ g⁻¹) and a high CH₄/N₂ selectivity (6.2) under ambient conditions, superior to those of various MOFs and most conventional adsorbents such as molecular sieves and activated carbon. With intensified water and thermal stabilities, as well as facile room-temperature regeneration without heating, the developed Zn-MOF demonstrates prominent advantages for industrial applications, facilitating the recovery of low concentration methane. The innovative synthesis strategy reported here could inspire green and large-scale syntheses of MOFs, propelling their commercialization and industrial implementation for the alleviation of energy and environmental issues.

Author contributions

Jian-Rong Li and Rui Li conceived and designed the experiments. Zhang-Ye Han, Rui Li, Quanyou Sun, and Wen-Liang Li mainly participated in the synthesis and characterization of materials. Zhang-Ye Han, Yan-Long Zhao, and Xuefeng Bai performed the molecular simulation. The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We would like to acknowledge the financial support from the National Natural Science Foundation of China (No. 22225803, 22038001, and 22378007), and a collaborative project from Beijing Energy Holding Co., Ltd (No. 40053002202405).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4gc03302g

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