Controlling mesoporosity to tune performance in monolithic MOFs

In 2018, we wrote about the importance of being able to transport natural gas at low pressure, and how we broke records by producing the leading material as a monolith. Our paper made the front page of Nature Materials, serving as an excellent demonstration of the power of the technology and representing a considerable leap forward over the state of the art. That MOF, HKUST-1, has a high capacity, making it an attractive material to study, but there are many more factors that must be considered before a material can be said to be “right” for a given application.

Figure 1: Gas adsorption isotherms for (a) a conventional, microporous material and (b) a partially mesoporous material. The linear region in the centre of the second isotherm represents the point where the material has been fully covered with a monolayer of gas. Beyond this, multilayer adsorption takes over and uptake increases. (c) Methane uptake for conventional, microporous HKUST-1 (white stars, as discussed here); conventional, microporous UiO-66 (white squares); and partially mesoporous UiO-66 (green circles). Note that both microporous materials rapidly climb at lower pressures but saturate much faster. The red line indicates the natural gas storage target set by the Department of Energy.

The most important is the stability of the material – an aspect too often overlooked (sometimes deliberately) by researchers and even companies seeking glamorous results and additional funding. HKUST-1 stores pure methane so well because its structure contains ‘open metal sites’, where the gas can interact strongly with the copper clusters in each node. That same fact, however, also leaves HKUST-1 open to attack from a wide range of molecules, even moisture. It makes a fantastic MOF for a wide range of pure gas and cleanroom applications, but it is not appropriate for harsh conditions or mixed gas streams.

The second factor is the deliverable capacity. While the total capacity describes how much gas a material can store under certain conditions, the deliverable capacity describes how much one could expect to use. The two numbers are different because a residual amount will remain in the tank, which will usually depressurise to a minimum of approximately 5 bar (see Figure 1). Many conventional materials interact most strongly with gases at lower pressures, meaning that this residual capacity can eat quite considerably into the total. These materials will also saturate quite quickly, meaning there are diminishing returns to increasing the pressure.

We solved both problems with some careful material design. A material known as UiO-66 is extremely stable and, as a result of this, has far weaker interactions with methane at lower pressures. Normally, this weak interaction continues as the pressure increases and so total performance is lacklustre; unsurprisingly, it therefore has seen little attention for natural gas storage. This changes, however, if defects are introduced to the material.

Counterintuitively, the presence of larger pore spaces – known as mesopores – alongside the traditional micropores, can actually increase the capacity of a material despite reducing its surface area. This is because adsorption can actually be quite long range (as molecular interactions go) and the adsorbed layer can be many molecules thick. This means that, while a microporous material will fill quickly at lower pressures and then saturate as the pores fill, materials which are partly mesoporous can continue loading for much longer (see Figure 2).

The precise ratio of micro-to-mesoporosity and the distribution of pore sizes within the structure are not easy things to control; in most cases, they emerge by accident. Figuring out how to do so involved a great deal of work into understanding how primary MOF particles assembled to form the final macroscopic structure, but the outcome was a controllable technique for producing UiO-66 with desired porosity characteristics. The result, published in Nature Communications this year, was a stable material with an 11% increase in deliverable capacity over HKUST-1 between 5 bar and 100 bar, and which did not saturate even at 100 bar (see Figure 1). 

Mesoporous UiO-66 monolith is both an industrially relevant material with significant potential, and the first demonstrator of a wider phenomenon. Since then, we have extended the work to a number of other interesting MOFs, and we are refining our ability to predict how these materials might behave before synthesising them. The great strength of MOFs is their ability to be chosen to suit the specifics of a given use-case, and this work provides a new and extremely important tool in this toolbox.

Figure 2: Computer simulation showing the density distribution (left) and a snapshot (right) of gas (blue spheres) adsorbed on a section of UiO-66 (red and grey). Single unit cells (traditional micropores) are shown at the top and bottom with a large mesopore channel running down the middle. We can see that the micropores are fully saturated, while the channel has additional space that will continue to fill as the pressure rises.