Key Takeaways & Executive Findings
- •• • Tunable mesopore sizes from 9.1 to 40.0 nm and particle diameters from 45.5 to 1020 nm enable precise matching to guest molecules ranging from enzymes to catalyst nanoparticles, directly impacting separation and catalysis performance. • • Single-crystalline meso-MOFs with well-defined lattice fringes along [010], [110], and [111] directions and FFT patterns showing co-existing microscopic and mesoscopic order (slight deviation from p6mm) ensure structural integrity and low defect density, critical for mechanical and thermal stability under industrial operating conditions. • • The HCl/CH3COOH acid pair decouples crystallization (CH3COOH-controlled) from micelle co-assembly (HCl-controlled), eliminating the formation of amorphous nanoparticles or non-mesostructured nanocrystals that occur with single-acid systems, thereby providing a robust kinetic window for scalable synthesis. • • Demonstrated architectures (hexagonal, columnar, dendritic, worm-like) and morphologies (cube, octahedron, 2D intersecting nanosheets) with multivariate ligands and metal ions confirm general feasibility, reducing the need for specialized equipment and multi-step post-synthetic treatments that plague hard-templating routes.
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Abstract
Metal-organic frameworks (MOFs) exhibit permanent microporosity (<2 nm) and tunable chemistry, yet the integration of ordered mesopores (2–50 nm) into single-crystalline frameworks remains a persistent synthetic bottleneck. Conventional hard-templating and self-templating routes suffer from limited universality, mesopore instability, and multi-step processing. Soft-templating with surfactants offers a scalable alternative but often yields polycrystalline or disordered mesostructures due to uncontrolled competition between micelle assembly and MOF crystallization. Here, a strong inorganic acid (HCl) and weak organic acid (CH3COOH) pair is shown to decouple these kinetics. In situ small-angle and wide-angle X-ray scattering reveal that CH3COOH governs MOF framework crystallization, while HCl dominates co-assembly of surfactant micelles with MOF subunits. This dual-acid mediation enables the formation of amorphous MOF nanoparticles that subsequently transform into single-crystalline mesoporous MOFs. The resulting materials display well-defined lattice fringes around mesopores along [010], [110], and [111] directions, with fast Fourier transform confirming co-existing microscopic and mesoscopic order (slight deviation from ideal p6mm symmetry). Particle diameters are tunable from 45.5 to 1020 nm, mesopore sizes from 9.1 to 40.0 nm, and architectures include hexagonal, columnar, dendritic, and worm-like, with cube, octahedron, and 2D intersecting nanosheet morphologies. This protocol establishes a generalizable route to single-crystal meso-MOFs with multivariate components, addressing a critical gap in pore engineering for mass-transfer-limited applications.
1. Introduction
Metal-organic frameworks (MOFs) have achieved over 100,000 distinct structures, yet their industrial deployment in mass-transfer-limited applications—such as large-molecule catalysis, drug delivery, and gas separation—remains constrained by intrinsic micropores (<2 nm). Introducing ordered mesopores (2–50 nm) into single-crystalline MOFs is a longstanding challenge. Hard-templating with polystyrene spheres yields macroporous ZIF-8 but fails to access the mesopore regime with structural fidelity. Self-templating via Ostwald ripening produces hollow covalent organic frameworks but lacks universality and morphological control. Soft-templating with surfactants offers scalability and tunable mesostructures, yet previous attempts yield polycrystalline meso-MOFs or disordered mesoporous single crystals because micelle assembly and MOF crystallization compete kinetically.
The core bottleneck is kinetic mismatch: rapid MOF crystallization disrupts surfactant micelle organization, while slow crystallization leads to amorphous or non-mesostructured products. This work introduces a strong inorganic acid (HCl) and weak organic acid (CH3COOH) pair to decouple these processes. In situ SAXS/WAXS monitoring reveals that CH3COOH primarily governs MOF framework crystallization, while HCl dominates co-assembly of micelles with MOF subunits. This dual-acid mediation enables a non-classical crystallization pathway: amorphous MOF nanoparticles first co-assemble with surfactant micelles, then transform into single-crystalline mesoporous frameworks. The resulting materials exhibit tunable mesopore sizes (9.1–40.0 nm), particle diameters (45.5–1020 nm), and diverse architectures, providing a generalizable synthetic platform that bypasses the limitations of hard- and self-templating strategies.
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CHEN Chunhong, CHEN Bo, WANG Lianhui (2025). A Strong Inorganic Acid and Weak Organic Acid Pair Promote the Formation of Single-Crystal Mesoporous Metal-Organic Frameworks. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3324-0
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Frequently Asked Questions
What is the quantitative evidence for single-crystalline order across both microscopic and mesoscopic scales?
Transmission electron microscopy (TEM) along [010], [110], and [111] directions shows well-defined lattice fringes around mesopores. Fast Fourier transform (FFT) analysis reveals two sets of symmetric diffraction spots at both microscopic and mesoscopic scales, confirming co-existence of ordered structures with only a slight deviation from ideal p6mm symmetry. This dual-scale order is critical for applications requiring both crystallographic integrity and accessible mesopores.
How does the HCl/CH3COOH acid pair control the crystallization kinetics and prevent amorphous or non-mesostructured products?
In situ SAXS/WAXS monitoring shows that CH3COOH primarily governs MOF framework crystallization, while HCl dominates co-assembly of surfactant micelles with MOF subunits. With only CH3COOH, rapid crystallization yields small nanocrystals that grow into large crystalline nanoparticles without mesostructure. With only HCl, inhibited deprotonation of organic ligands slows crystallization and enhances micelle co-assembly, producing amorphous nanoparticles. The acid pair balances these kinetics, enabling amorphous MOF nanoparticles to co-assemble with micelles and subsequently transform into mesoporous crystalline nanoparticles.
What are the achievable ranges for mesopore size, particle diameter, and architectural diversity, and how do they compare to legacy mesoporous MOF syntheses?
The protocol yields uniform meso-MOF single crystals with particle diameters from 45.5 to 1020 nm, tunable mesopore sizes from 9.1 to 40.0 nm, and architectures including hexagonal, columnar, dendritic, and worm-like, with morphologies such as cube, octahedron, and 2D intersecting nanosheets. Legacy soft-templating methods typically produce polycrystalline meso-MOFs or disordered mesoporous single crystals, often with limited pore size control and no architectural tunability. This work demonstrates multivariate components (mixed functional ligands and metal ions), indicating broad applicability.
What are the scalability bottlenecks and equipment requirements for this dual-acid soft-templating approach?
The method uses a strong inorganic acid (HCl) and weak organic acid (CH3COOH) pair with surfactant micelles under mild synthetic conditions compatible with micellization. Unlike hard-templating, it avoids specialized equipment for template removal and multi-step post-synthetic treatments. The process is amenable to scale-up because it relies on collaborative assembly in solution, with tunable particle sizes up to 1020 nm. However, precise control of acid concentrations and reaction times is critical to maintain the kinetic window; deviations can lead to amorphous or non-mesostructured products.
What is the industrial relevance of the observed mesopore size range and single-crystalline nature for catalysis or separation?
Mesopores from 9.1 to 40.0 nm facilitate mass transfer of large molecules, such as enzymes or catalyst nanoparticles, that cannot access micropores (<2 nm). Single-crystalline frameworks with well-defined lattice fringes ensure mechanical and thermal stability, reducing degradation under industrial operating conditions. The tunable architectures and morphologies allow optimization of active site exposure and diffusion pathways, directly impacting performance in gas separation, drug delivery, and heterogeneous catalysis.
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