If we want to keep using fossil fuels for a while longer without cooking the planet, we have to get much better at separating and capturing gases like CO₂ and hydrogen efficiently. That’s where advanced membranes – especially those based on metal–organic frameworks like ZIF-8 – start to look very exciting.

In this blog, we’ll walk through the core ideas behind ZIF-8 membranes on organic (polymeric) supports, based on current research. The goal is to explain what’s going on in simple, logical language – no dense jargon, no confusing figure-by-figure recap – so that you can understand why ZIF-8 on polymers is a big deal and what challenges still remain.

Why Do We Care About ZIF-8 Membranes in the First Place?

The CO₂ problem and the cost of separating gases

When fossil fuels are burned, they produce CO₂ along with other gases. Technologies grouped under Carbon Capture and Storage (CCS) aim to:

1. Separate CO₂ from other gases at the source (for example, from flue gas or syngas),

2. Transport the CO₂, and

3. Store it safely and permanently.

The most expensive step in this chain is usually capturing CO₂. It is reported that more than 60% of total CCS cost can come from the capture process alone, and this can consume up to 40% of the total energy in the system. So, reducing the energy and cost of gas separation is critical.

Traditionally, two technologies are used for CO₂ separation:

• Chemical absorption – using solvents like amines to absorb CO₂ from gas mixtures.

• Adsorption – using solid materials (e.g., activated carbon, zeolites) that selectively adsorb CO₂.

While mature and widely used, these methods have drawbacks:

• Absorption systems can require large amounts of solvent and a lot of heat for regeneration.

• Adsorption systems are simpler, but we still need better, more efficient adsorbent materials.

This is why membrane technology is so attractive: membranes can offer high surface area for separation in a compact module, can be scaled up, and often have simpler operation with fewer moving parts.

The “upper bound” problem of polymer membranes

Today, polymer membranes dominate industrial gas separation. They are relatively cheap, flexible, and easy to make into hollow fibers or flat sheets.

But they have a big limitation: there’s a well-known trade-off between permeability and selectivity.

• Permeability: how easily a gas passes through the membrane.

• Selectivity: how well the membrane discriminates between two gases (e.g., H₂/CO₂, CO₂/CH₄).

Robeson analyzed a huge amount of data and introduced what is known as the Robeson upper bound: a curve that shows, for many gas pairs, the best combinations of permeability and selectivity that polymer membranes have managed to achieve. Most polymer membranes lie below this line.

The challenge is clear:

We want membranes that go beyond this upper bound – high permeability and high selectivity.

This is where Metal–Organic Frameworks (MOFs) and, more specifically, ZIF-8 come into play.

Why ZIF-8?

MOFs and ZIFs in a nutshell

Metal–Organic Frameworks (MOFs) are crystalline materials built from:

• Metal ions or clusters (e.g., Zn²⁺, Cu²⁺),

• Organic ligands (e.g., carboxylates, imidazolates).

They form porous 3D structures with:

• High surface area,

• Tunable pore sizes,

• Adjustable chemistry.

Within the MOF family, Zeolitic Imidazolate Frameworks (ZIFs) are particularly interesting. They combine:

• The structural and thermal stability of zeolites,

• With the tunability and modularity of MOFs.

ZIFs are made from metal ions (like Zn²⁺ or Co²⁺) and imidazolate-type linkers. Many ZIFs are chemically and thermally robust and can be stable in water and even boiling solvents, depending on the structure.

What makes ZIF-8 special?

ZIF-8 is one of the best-known ZIFs. It consists of:

• Zn²⁺ ions,

• 2-methylimidazolate linkers,

• Sodalite (SOD)-type topology (a cage-like 3D framework).

Key features of ZIF-8 that are important for gas separation:

• Small, controllable pore aperture
  The nominal pore opening is often given as around 3.4 Å, but effective aperture values around ~4.0–4.2 Å have also been reported due to framework flexibility. This size range is ideal for separating small gases like H₂ from larger molecules.

• High chemical and thermal stability
  ZIF-8 can remain stable at temperatures up to ~400 °C and shows good hydrolytic stability in many environments.

• Hydrophobic character
  This can help maintain structural integrity in humid conditions and influences which gases preferentially diffuse.

Because of these properties, ZIF-8 has been studied both as:

• A filler in mixed matrix membranes (MMMs) (polymer + ZIF particles), and

• A pure ZIF-8 membrane layer grown on a porous support.

ZIF-8 is especially promising for hydrogen separations, such as:

• H₂/CO₂,

• H₂/CH₄,

• H₂/N₂,
  where H₂ is the smallest and fastest gas.

Why Grow ZIF-8 on Organic (Polymeric) Supports?

ZIF-8 membranes can be fabricated on two broad types of supports:

1. Inorganic supports – e.g. α-alumina, ceramic hollow fibers, metal tubes.

2. Organic supports – e.g. polysulfone, polyethersulfone, nylon, other polymeric porous substrates.

Inorganic supports are:

• Rigid,

• Thermally stable,

• Often easier to use for growing uniform MOF layers.

But they can be:

• Brittle,

• Heavier,

• More challenging to package for large-scale, high-pressure applications.

Polymeric supports, on the other hand:

• Are flexible and lighter,

• Can better tolerate pressure fluctuations and mechanical stress,

• Are more similar to industrial polymer membrane modules already used today.

However, growing ZIF-8 directly on polymer supports is much more challenging than on ceramics, mainly because:

• There is often poor chemical compatibility between the polymer surface and ZIF-8 crystals.

• The polymer may swell, soften, or deform under certain synthesis conditions (solvents, temperature, pH).

• Achieving a dense, defect-free ZIF-8 layer that adheres strongly to the polymer without peeling or cracking is not trivial.

Despite these issues, successfully growing ZIF-8 membranes on polymers combines:

• The excellent separation performance of ZIF-8,

• With the mechanical advantages and processability of polymeric substrates.

So most recent research in this niche is trying to answer one question:

How can we reliably grow thin, defect-free ZIF-8 layers on polymer supports with good adhesion and stability?

How Are ZIF-8 Membranes Made? Main Fabrication Strategies

There are two main strategies for making ZIF-8 membranes on porous supports:

1. In situ growth

2. Secondary growth

In addition, some newer, more creative methods – such as counter-diffusion growth – have been proposed.

Let’s go through each.

1. In Situ Growth on Porous Supports

In in situ growth, you do not start with pre-attached ZIF-8 seed crystals. Instead:

• The porous support is immersed in a solution containing:

  • The metal source (e.g., zinc nitrate),

  • The organic linker (2-methylimidazole),

• The ZIF-8 crystals nucleate and grow directly on the support surface.

This can be done:

• At room temperature, or

• At elevated temperature (hydrothermal or solvothermal conditions).

Supports used:

• Inorganic supports (α-alumina, ceramics),

• Polymeric supports (polysulfone, etc.),

• Flat sheets and hollow fibers.

Typical challenges of in situ growth:

• Nucleation and crystal growth happen simultaneously, which can:

  • Make it difficult to control crystal size and coverage,

  • Lead to non-uniform layers or defects.

• Often requires long synthesis times and relatively high temperatures to obtain continuous films.

• For polymers, the synthesis conditions must be “gentle” enough not to damage or deform the support.

To address these issues, researchers have tried:

• Using microwave heating to accelerate nucleation and growth,

• Adding additives such as sodium formate to promote heterogeneous nucleation on the support surface,

• Combining in situ growth with mild surface modification to improve compatibility.

Using these approaches, defect-free ZIF-8 membranes with good hydrogen separation performance have been reported, including:

• ZIF-8 layers grown on ceramic hollow fibers to provide large surface area per module volume,

• ZIF-8 films on polysulfone supports, sometimes combined with other techniques to lower synthesis temperature.

In short, in situ growth is simple in concept (just immerse and grow), but tricky in practice when targeting reproducible, uniform, and defect-free membranes on polymers.

2. Secondary Growth: Seed First, Grow Later

The secondary growth method separates the process into two steps:

1. Seeding – attaching small ZIF-8 crystals (seeds) onto the support surface.

2. Secondary growth – exposing the seeded support to ZIF-8 precursor solution so that the seeds grow into a continuous membrane layer.

Because nucleation (seed formation) and growth are decoupled, it becomes easier to control:

• Crystal size,

• Layer thickness,

• Continuity of the film.

Seeding techniques

Several ways to put ZIF-8 seeds onto the support have been explored:

• Rubbing:
  Physically rubbing dry ZIF-8 powders onto the support, so particles get embedded or anchored on the surface. This has been done on polyethersulfone, for example.

• Dip-coating:
  Dipping the support into a suspension of ZIF-8 particles, then drying to leave a seed layer. Applied to ceramic or polymer supports.

• Slip coating:
  Similar to dip-coating but using more viscous, tailored slurries for better coverage. Has been used on alumina supports to create ZIF-8 layers with strong olefin/paraffin separation performance.

• Reactive seeding (often microwave-assisted):
  Generating ZIF-8 seeds directly on the support by short, localized reactions. For example, exposing an α-alumina support with appropriate precursors to microwave heating to rapidly form a thin ZIF-8 seed layer.

After seeding, the support is placed in the ZIF-8 precursor solution for secondary growth. The seeds then grow and merge, forming a continuous ZIF-8 film.

The main drawback: delamination

Even though secondary growth can yield dense, well-intergrown ZIF-8 layers, one big problem appears:

The ZIF-8 layer can peel off or delaminate from the support during use.

This happens because:

• The adhesion between the crystals and the substrate is weak,

• Mechanical or thermal stress can cause debonding,

• Differences in thermal expansion or swelling/shrinkage can create stress at the interface.

To overcome this, researchers modify the support surface to promote stronger bonding.

Surface modification to improve adhesion

Several support modification approaches have been reported:

• Adding amine groups via vapor-phase modification:
  For example, functionalizing the support with aminosilanes (like APTES on ceramic supports), introducing –NH₂ groups that can coordinate with Zn²⁺ or interact with ZIF-8 surfaces. This can:

  • Improve nucleation,

  • Reduce pore size,

  • Promote a more continuous and better-anchored film.

• Using polymeric coupling agents like polyethyleneimine (PEI):
  PEI contains many amine groups and can:

  • Hydrogen-bond with ZIF-8 seeds,

  • Interact with hydroxyl groups on ceramic surfaces,

  • Even coordinate with Zn²⁺ via Zn–N bonds.

  By applying PEI before seeding (e.g., via dip-coating), seeds can be more strongly attached, leading to more robust membranes.

Overall, secondary growth combined with suitable surface modification has proven to be a powerful way to produce crack-free, highly selective ZIF-8 membranes – but it does add steps and complexity.

3. Novel Approaches: Counter-Diffusion Growth

Beyond conventional in situ and secondary methods, researchers have also proposed more creative synthesis routes. One interesting example is the counter-diffusion method.

In this approach:

• The porous support (for example, a nylon membrane) separates two solutions:

  • One side contains the metal source (zinc nitrate),

  • The other side contains the organic linker (2-methylimidazole).

• The reactants diffuse toward each other through the support and react near or in the pores to form ZIF-8.

Some key observations from counter-diffusion studies:

• The side on which each reactant is placed can influence:

  • The crystal morphology (e.g., cubic vs fiber-like),

  • The uniformity of the ZIF-8 layer.

• It is possible to obtain relatively uniform, dense ZIF-8 layers in a single crystallization cycle, which saves time compared to repeated growth steps.

• Gas separation performance can reach selectivities higher than Knudsen selectivity, which indicates that separation is dominated by molecular sieving through the ZIF-8 pores rather than just by simple diffusion in large defects.

Counter-diffusion is a good example of how adjusting mass transport during growth can be used as a design tool for better membranes.

How Well Do ZIF-8 Membranes Perform?

A variety of ZIF-8 membranes on different supports (both inorganic and organic) have now been reported. While the exact numerical values depend on the specific membrane and conditions, some general trends emerge:

• Very high H₂ permeance
  ZIF-8 membranes can reach hydrogen permeance above 60,000 GPU (gas permeation units) in some cases. This is significantly higher than typical polymer membranes, which makes ZIF-8 very attractive for high-throughput hydrogen separations.

• Good selectivity
  For gas pairs like H₂/CO₂, H₂/CH₄, and H₂/N₂, ZIF-8 membranes can show selectivity higher than Knudsen values, meaning:

  • Gas transport is not dominated by random diffusion in large voids,

  • The ZIF-8 pore structure is actively discriminating among different gas molecules.

• Role of layer thickness and crystal size
  Performance strongly depends on:

  • The thickness of the ZIF-8 layer: thinner layers generally give higher permeance but can be more prone to defects.

  • The crystal size and intergrowth: well-intergrown, small crystals reduce grain boundaries and defects.

On polymeric supports, there is still room for improvement, especially to match the best results seen on inorganic supports. But existing data already show that:

• ZIF-8 on polymers can surpass the typical permeability–selectivity trade-off seen in pure polymer membranes.

• With further optimization, these hybrid systems could break through the traditional Robeson upper bound for several gas pairs.

Remaining Challenges and Future Directions

Even though ZIF-8 membranes on organic substrates are promising, several key hurdles need to be addressed before they can be widely used in industry.

1. Reproducibility

One of the biggest practical issues is simply:

Can we make the same high-quality membrane again and again?

Synthesis conditions (precursor ratios, temperature, time, pH, additives, support pretreatment) can strongly affect the final morphology and performance. Small variations may lead to:

• Pinholes,

• Cracks,

• Non-uniform coverage,

• Delamination.

Improving process control and scalable protocols is essential.

2. Single-step, scalable synthesis

For industrial relevance, we’d like:

• Shorter synthesis times,

• Fewer steps (ideally a single-step process),

• Methods that can be applied to large-area modules or thousands of hollow fibers at once.

Today’s lab-scale methods (multiple cycles of seeding, growth, washing, post-treatment) are often too complex and slow. There is ongoing work on:

• Combining in situ growth with smart surface modification in one pot,

• Using continuous processes or flow reactors,

• Leveraging microwave or other rapid heating techniques.

3. Mechanical stability on polymeric supports

ZIF-8 membranes on polymers must withstand:

• Pressure differences,

• Mechanical vibrations,

• Temperature variations,

• Long-term operation without cracking or peeling.

Because polymers and ZIF-8 have different mechanical and thermal behaviors, interface engineering is crucial. This may involve:

• Tailored polymer chemistry (e.g., adding functional groups),

• Interlayers or adhesives (like PEI or other coupling agents),

• Careful control of layer thickness and stress during drying and operation.

Mechanical testing and long-term stability studies on realistic modules are still relatively limited and represent an important future direction.

4. Fine-tuning selectivity for real gas mixtures

Most research reports focus on binary gas mixtures under idealized conditions. Industrial gas streams may contain:

• Multiple components (CO₂, H₂, N₂, CH₄, H₂O, impurities),

• High pressure,

• Humidity,

• Contaminants that could adsorb or foul the membrane.

Understanding how ZIF-8 membranes behave in complex mixtures and under real operating conditions will be key for translating this technology into the field.

Take-Home Messages

Let’s summarize the main points in a compact way:

• The motivation
  To make CCS and hydrogen-related processes cheaper and more energy-efficient, we need advanced gas separation membranes that beat the traditional permeability–selectivity trade-off in polymers.

• Why ZIF-8?
  ZIF-8 is a robust, flexible, small-pore MOF with:

  • Good chemical and thermal stability,

  • Pore apertures ideal for H₂ separation,

  • A growing track record in gas separation research.

• Why organic (polymeric) substrates?
  Polymer supports:

  • Are flexible and mechanically robust under pressure,

  • Are lighter and easier to process into industrial modules,

  • But are harder to coat with ZIF-8 due to compatibility issues.

• How are the membranes made?

  • In situ growth: immerse the support in precursor solution and grow ZIF-8 directly.

  • Secondary growth: seed the support with ZIF-8 crystals, then grow them further.

  • Novel methods like counter-diffusion add interesting new options.

• What can they do?
  ZIF-8 membranes can:

  • Achieve very high H₂ permeance (often > 60,000 GPU),

  • Show selectivities higher than Knudsen, indicating true molecular sieving,

  • Potentially surpass the traditional Robeson upper bound when integrated smartly with polymer supports.

• What’s still hard?

  • Reproducibility and scalability,

  • Long synthesis times,

  • Mechanical stability and adhesion on polymer supports,

  • Performance under realistic multi-component gas streams.

Despite these challenges, the overall message is optimistic:

ZIF-8 membranes on organic substrates sit at a promising intersection of advanced materials science and practical engineering. With continued work on synthesis methods, interface chemistry, and scale-up, they could play a major role in the next generation of gas separation technologies, particularly for hydrogen and CO₂-related applications.