Metal–organic frameworks (MOFs) – and especially zeolitic imidazolate frameworks (ZIFs) – have become “star materials” in chemistry and materials science. They combine metal ions with organic linkers to create highly porous, crystalline structures that can store gases, catalyze reactions, support enzymes, or deliver drugs.

Among them, ZIF-8 is easily one of the most famous. Built from zinc ions and 2-methylimidazole linkers, it has a zeolite-like structure, high surface area, good stability, and a pore system that can host small molecules. It’s been widely explored for gas separation, catalysis, CO₂ capture, and more.

One of the reasons ZIF-8 is so popular is that it can be synthesized under “green” conditions:

• at room temperature,

• in water as the only solvent.

At first glance, that sounds like an ideal sustainable process. But there’s a hidden problem: to get ZIF-8 to form in water at room temperature, typical methods use a huge excess of organic linker. From a sustainability and cost perspective, that’s wasteful and far from ideal.

The study behind this blog tackles that problem head-on. It asks a simple but powerful question:

Can we synthesize ZIF-8 (and related ZIFs) in water, at room temperature, using only the exact stoichiometric ratio of metal and linker — without all that waste?

The answer turns out to be yes – if you introduce the right kind of amine into the synthesis. And that small change opens up some very interesting side effects, from tunable porosity to improved CO₂ affinity and even photocatalytic potential.

Let’s walk through what the researchers did and why it matters, in clear and practical terms.

Why ZIF-8 Is Special – and Where the Sustainability Problem Starts

ZIF-8 belongs to the broader ZIF family, which already stands out from typical MOFs in a few ways:

• Metal environment:
  ZIF metals (like Zn²⁺ or Co²⁺) sit in an isolated tetrahedral environment, coordinated to four imidazolate linkers. Unlike many carboxylate-based MOFs, there are no continuous M–O–M or M–M clusters.

• Zeolite-like topology:
  The structure is “zeolitic”: the inorganic Si–O–Si connections in classical zeolites are mimicked by M–imidazolate–M linkers in ZIFs. This keeps the framework rigid yet surprisingly open.

• Limited but useful metal choices:
  Only metals that are comfortable in tetrahedral, divalent coordination really work here, which is why Zn and Co are the main players, with a few others sometimes used.

• Single linker type:
  ZIFs are built from imidazolate-type linkers – bidentate, monovalent, nitrogen-containing units that give the right M–linker–M angle (~145°), similar to Si–O–Si in zeolites.

These structural features make ZIFs – and ZIF-8 in particular – very attractive for:

• gas separation and CO₂ capture

• energy storage

• catalysis and electrocatalysis

• drug delivery and enzyme immobilization

• environmental cleanup

But now comes the catch.

Even though ZIF-8 can be made in water at room temperature, most conventional procedures require a linker/metal ratio of 20–40, sometimes even higher. The final ZIF-8 structure only contains a 2:1 linker-to-metal ratio, so the process throws away a large amount of expensive organic linker or requires extra recovery steps.

In other words: green solvent, yes; room temperature, yes; but stoichiometrically wasteful, definitely.

Several attempts have been made to improve this:

• swapping water for other solvents (sometimes lowering the excess),

• using surfactants,

• adding ammonium hydroxide in large amounts,

• applying mechanical energy (like grinding).

These approaches help, but they still don’t truly solve the core problem: how do we make ZIF-8 in water, at room temperature, with a linker/Zn ratio of exactly 2 – matching the crystal’s stoichiometry – and without elaborate additives or complicated energy input?

That’s the focus of this work.

The Core Idea: Use Amines to Unlock Truly Stoichiometric Synthesis

The researchers took inspiration from earlier, highly sustainable syntheses of carboxylate-based MOFs. In those systems, they used bases (like NaOH, ammonium hydroxide, or amines) to deprotonate the organic linkers in water, allowing the MOFs to form quickly at room temperature with stoichiometric amounts of reactants and almost no waste.

Could a similar “deprotonation strategy” work for ZIF-type linkers such as 2-methylimidazole?

Here’s the key chemical point:

• 2-methylimidazole (H-2-mIm) has a pKa around 8.

• In pure water at neutral pH, it is mostly in the protonated form, which slows down or limits its ability to coordinate Zn²⁺ unless it’s present in large excess.

• If you raise the pH with a base, you shift more of it to the deprotonated imidazolate (2-mIm⁻), which is the form that actually builds the ZIF framework.

So the team set themselves a clear challenge:

Take zinc and 2-methylimidazole in exact stoichiometric ratio (2:1), mix them in water at room temperature, and see whether the right base can “unlock” ZIF-8 formation without needing excess linker.

They tested several bases under identical conditions:

• Triethylamine (TEA) – a common organic tertiary amine

• NaOH – a strong inorganic base

• Tetraethylammonium hydroxide (TEAOH) – a strong quaternary ammonium base

• Ammonium hydroxide (NH₄OH) – a weaker base in aqueous solution

All mixtures were strongly basic (pH 9–13+), so in principle, the linker should be mostly deprotonated in all cases.

What did they observe?

• Without any base: a different, low-porosity phase forms (“Unknown phase 1”), not ZIF-8.

• With NaOH, TEAOH or NH₄OH: again, no ZIF-8; other phases appear instead.

• With TEA: now, the diffraction peaks of ZIF-8 start to show up, alongside the same unknown phase at low TEA dosage.

By gradually increasing the TEA/linker ratio, they found:

• At low TEA content, only traces of ZIF-8 are formed.

• As TEA increases, the ZIF-8 fraction grows.

• At a TEA/linker ratio of ~2.2, they obtain phase-pure ZIF-8.

Crucially:

• The linker/Zn ratio in the synthesis mixture is exactly 2, matching the ZIF-8 structure.

• The yield of zinc conversion into ZIF-8 is about 96.6%, meaning essentially all Zn (and linker) ends up in the product.

• The formation of solid is instant and massive as soon as the zinc solution is added – a sign that the imidazole is efficiently deprotonated and ready to bind.

So, triethylamine does something the other bases cannot, under these conditions: it enables truly stoichiometric, room-temperature, aqueous synthesis of ZIF-8 with very high yield and no massive linker excess.

What Is Different About This New ZIF-8? The Role of Trapped Amines

The ZIF-8 obtained with TEA is not just “the same ZIF-8 but greener”. It has a twist.

Characterization shows:

• Powder X-ray diffraction confirms the sodalite-type ZIF-8 structure.

• The first low-angle reflection (often assigned to the (110) plane) is unusually weak compared to conventional ZIF-8.

• Nitrogen adsorption reveals a significantly lower BET surface area (~580 m²/g) compared to typical ZIF-8 (often over 1500 m²/g).

That suggests that something is partially blocking the pores or altering how the framework interacts with the probing gas.

Thermogravimetric analysis (TGA) of the TEA-ZIF-8 sample shows an additional weight loss around ~280 °C that cannot be explained by decomposition of the linker alone. To understand this, the authors combined:

• TGA-mass spectrometry (TGA-MS) – to follow gases evolving from the sample during heating.

• Solid-state ¹H→¹³C CP-MAS NMR – to detect carbon environments in the solid.

The result is clear:

• Mass fragments corresponding to triethylamine (and its decomposition fragments) appear during that lower-temperature weight loss step.

• NMR signals at chemical shifts matching TEA carbons are seen in the TEA-synthesized ZIF-8 but not in conventional ZIF-8.

Conclusion:
Triethylamine molecules are trapped inside the ZIF-8 cavities.

Because ZIF-8 has small window apertures (~3.8 Å) and TEA is bulkier (kinetic diameter ~7.8 Å), once TEA is caught inside during synthesis, it cannot simply diffuse back out through the pores. Washing with water doesn’t remove it either.

This explains:

• the reduced BET surface area (pores partially occupied or blocked),

• the altered XRD intensities for reflections sensitive to the cavity contents.

So the new “green” ZIF-8 is, in fact, ZIF-8 with built-in TEA guests.

Can We Remove the Amine and Recover High Porosity?

Trapped TEA is not necessarily a problem – it can even be an opportunity – but many applications want clean, open pores. The next logical question is whether we can remove TEA without destroying the ZIF-8 framework.

The authors tested mild thermal treatments of the TEA-containing ZIF-8:

• 200 °C for 20 hours

• 225 °C for 20 hours

• 250 °C for 20 hours

in air, in a simple oven (no special atmosphere).

The outcomes were:

• At 200 °C, TEA is essentially removed, and the surface area jumps dramatically, up to around 1380 m²/g – close to conventional ZIF-8.

• At 225 °C, some further changes occur; the surface area is still high but begins to decrease slightly (around 1240 m²/g).

• At 250 °C, the surface area drops more significantly (~950 m²/g), indicating that the ZIF-8 framework is starting to be affected.

In other words:

• Around 200 °C is a good compromise: enough to drive out TEA (by decomposition and release) while still maintaining a largely intact ZIF-8 structure with high porosity.

However, something else interesting happens during these heat treatments: the samples change color from white → yellow, and their UV–Vis spectra show new absorption bands in the visible region.

A Surprise Bonus: g-C₃N₄-Like Species and Photocatalytic Potential

The yellow coloration and the changes in UV–Vis spectra remind the authors of graphitic carbon nitride (g-C₃N₄), a polymeric, nitrogen-rich material well-known for its photocatalytic activity (e.g., in CO₂ reduction or water splitting).

To investigate this, they looked again at the ¹³C NMR of the thermally treated TEA-ZIF-8. New signals appear around 155–167 ppm, which are very similar to those typically observed in g-C₃N₄-type materials.

This suggests that:

• During mild thermal treatment, parts of the organic linker (and possibly some nitrogen-containing fragments) reorganize into g-C₃N₄-like domains inside the material.

• These carbon nitride-like species coexist with an overall preserved ZIF-8 architecture, at least at moderate temperatures like 225 °C.

That hybrid nature – ZIF-8 skeleton plus g-C₃N₄-like patches – is intriguing:

• ZIF-8 is already a useful porous adsorbent and potential catalyst support.

• g-C₃N₄ is a light-absorbing, metal-free photocatalyst.

Combining both, even partially, inside one material opens the door to new photocatalytic functions, where porosity, light absorption, and active sites work together.

Improved CO₂ Affinity: Not Just About Surface Area

To see how these structural and chemical changes affect gas adsorption, the authors compared CO₂ uptake at 0 °C for three samples:

1. Conventional, high-surface-area ZIF-8

2. TEA-containing ZIF-8 (low porosity)

3. TEA-ZIF-8 after heat treatment at 225 °C (g-C₃N₄-like, moderately lower surface area than conventional)

The results:

• At very low pressures, the TEA-containing ZIF-8 – despite its low overall porosity – can adsorb more CO₂ than the conventional ZIF-8. This is attributed to the presence of amines, which are known to interact strongly with CO₂.

• After thermal treatment at 225 °C, the CO₂ uptake of the material surpasses that of conventional ZIF-8 across the whole measured pressure range, even though its BET surface area is about 20% lower.

This means that:

• CO₂ affinity is not just a function of surface area; the chemical nature of the pore environment matters a lot.

• The g-C₃N₄-like species (and possibly other nitrogen-rich moieties) formed during heating create more favorable sites for CO₂ adsorption.

Overall, the thermally treated material combines:

• still-high porosity,

• enhanced chemical affinity for CO₂,

• and possibly photocatalytic potential due to its carbon nitride-like domains.

Extending the Strategy: Other Amines and ZIF-67

The methodology is not limited to TEA or ZIF-8.

The authors also:

• Replaced TEA with a bulkier amine, N,N-dicyclohexylmethylamine (MCHA).

• Replaced Zn with Co to synthesize ZIF-67, the cobalt analogue of ZIF-8, known for similar structure but different properties (including lower thermal stability).

Using the same concept – stoichiometric linker/metal ratio of 2 in water, with an appropriate amine – they successfully synthesized:

• ZIF-8 using MCHA, and

• ZIF-67 using TEA,

• ZIF-67 using MCHA.

In all cases, the XRD patterns correspond to the expected sodalite-type ZIF structures.

Like before:

• The resulting materials contain trapped amines in their cavities.

• Their surface areas are reduced compared to amine-free analogues because bulky amine molecules occupy or block some pores.

• The effect is even stronger with the bulkier MCHA than with TEA, showing how the amine’s size influences porosity.

For ZIF-67, which is less thermally stable than ZIF-8, separating amine removal from framework degradation becomes trickier — but the concept still works and demonstrates that the approach is generalizable to other ZIFs and other amines.

Why This Work Matters

Putting everything together, this study is not just about tweaking a synthesis recipe; it’s about pushing ZIF chemistry closer to true sustainability and functional versatility.

Key takeaways:

• Genuinely sustainable stoichiometry:
  ZIF-8 (and ZIF-67) can be synthesized in water at room temperature with exact linker/metal ratios (2:1), avoiding the usual massive excess of organic linker.

• Amines as enabling agents:
  Triethylamine (and other tertiary amines like MCHA) are not just pH controllers; they are key players that deprotonate the linker and co-crystallize inside the framework, allowing ZIF formation without linker waste.

• Built-in functionalization:
  The resulting ZIFs initially contain a high amine load in their pores, which:

  • can enhance low-pressure CO₂ adsorption,

  • can be removed by mild heating to restore porosity,

  • and can leave behind g-C₃N₄-like species that add photocatalytic potential and further increase CO₂ affinity.

• Versatility and extension:
  The method works for different amines and metals, suggesting a broader platform for designing greener ZIF syntheses and tailored pore chemistries.

From a practical standpoint, this kind of approach is attractive for:

• companies or labs seeking lower-waste, water-based synthesis routes for ZIF-type materials,

• researchers interested in hybrid sorbent–photocatalyst systems for CO₂ capture and conversion,

• and anyone exploring functional porous materials where synthesis conditions and sustainability are as important as performance.

In short, this work shows that sustainable synthesis and advanced functionality don’t have to be in conflict. With a smart choice of base and stoichiometry, it’s possible to make ZIF-8 in a cleaner way, and even end up with materials that perform better in some key applications, especially around CO₂ handling and photocatalysis.