Monolithic MOFs break natural gas storage targets

Natural gas is the world’s third largest and fastest growing source of primary energy. Although it will eventually be phased out by renewables, even the most optimistic estimates show this will take decades. It is also both cheaper and greener than other fossil fuels and is already playing a major role in displacing coal. With this in mind, we at Immaterial strongly believe in working to make the natural gas supply chain cleaner and facilitating applications where it can help replace dirtier fuels. We have developed a novel synthetic approach that can be applied to existing high-performance materials to make them suitable for applications in the natural gas ecosystem (see Figure 1).

Figure 1: (a) MOFs are synthesised by mixing the metal and ligand building blocks together in solution. These will react to form MOF nanoparticles, which are condensed into a gel. What happens next is critical: conventional drying methods tear the gel apart, forming the fine powders typical for most MOFs. We learned that by controlling this process, we can protect the material, allowing it to dry into dense monoliths, up to a cm in size. (b) This dramatically improves the packing density, and hence volumetric performance of the material.

One of the major issues with natural gas which comes up over and over is its transportation: as a gas, it must be piped, compressed to high pressures (usually 250 bar), or liquified (below -162 oC). Offshore, often none of these are an option, and so when oil wells come with associated gas, that gas is vented or flared. Globally, we flare over 140 cubic kilometres of gas every year – enough to satisfy all of South America. Onshore, one of the major barriers to using natural gas (and indeed hydrogen) as a vehicle fuel is that a refuelling network like we have for petrol would be prohibitively expensive if 250 bar pumps were needed.

Adsorbed natural gas (ANG) might be a solution. Using porous materials to store it at lower pressures is certainly not a new idea, but materials have never been able to get the storage needed to make it viable. MOFs are by far the highest scoring of the possible candidates, however although they do superbly storing by weight, their existence as loosely packed powders and artificially bound pellets made them poor at storing by volume. In 2011, the US Department of Energy set a target of storing 263 m3 of methane per m3 of material at just 65 bar – equivalent to what is normally stored at 250 bar. This was more than 50% above what anyone had managed, and many considered it beyond what was even theoretically possible.

In 2017, we applied our technology to the record-holding material for the storage of methane – a MOF known as HKUST-1. By producing it as a monolith, we achieved the same performance on a gravimetric basis (showing that, on a molecular level, the material is exactly the same), but crucially over 50% higher on a volumetric basis – just 1% away from the DoE’s target, which we have since met (see Figure 2). By synthesising MOFs as denser crystallites instead of loose powders, in a stroke we achieved a step change in what was possible with porous materials (see Figure 1).

Figure 2: Comparison of monolithic HKUST-1 (red) with powdered, compacted HKUST-1 (black). Chemically, these MOFs are identical, but in its crystalline form we achieve far better packing. Equally importantly, the compacted MOF was made without binder, meaning it will gradually crumble under its own weight. Real, usable, pellets will have capacities that are even lower.

HKUST-1 may not be the right choice for many natural gas applications – for one, it is quite unstable – however producing it as a monolith did represent a key turning point in ANG development and pave the way for newer, more stable alternatives. It shows how much of a difference monolithic MOFs can offer, and how applying it to materials that are already well researched and understood can turn a promising MOF into an exceptional one (see Figure 3).

Figure 3: Graph showing methane uptake of monolithic HKUST-1 over traditional compression. The dramatic uptake at low pressures allows us to store the same quantity at 65 bar as would normally be stored at 250 bar.

You can read more about our research into HKUST-1 in our Nature Materials paper.