Capturing sulphur to make it useful

Materials World magazine
,
27 Nov 2019

Molecular organic frameworks could trap sulphur dioxide from ambient air to use as a raw material and prevent it going to waste. Idha Valeur reports.

Molecular organic frameworks (MOFs) that are suitable for sulphur dioxide (SO2) capture have been created by scientists from the University of Manchester, UK. The focus of the study was on designing cages that could selectively capture toxic SO2 gas over other common gases such as nitrogen, carbon dioxide, carbon monoxide and methane, even in the presence of water. The team found that their new material could improve the efficiency of SO2 capture in comparison with current methods. According to the scientists, the polymer material they have developed has shown higher adsorption of SO2 than any porous material currently known.

Approximately 87% of SO2 emissions are released into the environment by human activities, such as by ships, trains and industrial facilities. While natural formations like volcanoes release SO2, this is at far lower levels and indistrialisation has pushed production excessively.

‘SO2 emissions are strictly regulated due to the negative impact on human health and the environment. SO2 reacts in the atmosphere to form sulphuric acid, which is highly corrosive and reactive, and then this forms acid rain in the presence of water,’ University of Manchester Research Associate, Gemma Smith, told Materials World. ‘Acid rain leads to acidification of lakes, damage to crops and plants, and chemical weathering of buildings etc.’

The MOFs

The MFM-170 developed by the researchers is a material with porous molecules that contain copper, in a cage-like structure. These MOFs are made up of nodes of metal ions which are connected in three dimensions by organic molecules known as linkers, according to Smith. ‘These infinite 3D structures can form channels or cages, which are “empty” spaces within the material that can be used to capture and store molecules for a range of applications. These void spaces give MOFs extremely high internal surface areas – just 1g of material may have a surface area up to that of a football pitch – ~7,000m2/g,’ Smith said.

‘The chemical structure of the linkers – e.g. which chemical functional groups they contain – and the choice of metal ion both influence the resulting properties of the MOF, and these can be changed in order to tune the material for a desired application.’ She confirmed that, in this particular study, the focus was on designing cages that could selectively capture SO2 gas.

Comparing systems

A common method of SO2 removal is using wet lime or limestone slurries, but according to Smith, ‘while this method is well established, it may not always be suitable for thorough desulphurisation (>99% removal) and is associated with some other disadvantages such as scaling in the absorber units and high volumes of solid and liquid waste’.

She explained that commercial-grade gypsum can be acquired to offset some of the costs of the flue gas desulphurisation (FGD), but it is not always possible and the used sorbent ends up in landfill.

‘This new capture system is based on reversible adsorption of SO2 by a porous solid material, which is able to purify gas streams to <0.1ppm SO2. Unlike many current technologies, once the material is saturated, the SO2 can then be removed by simply lowering the pressure,’ she said.

Make it work

A key aspect of sulphur capture is turning something that is quite toxic, in its pure form, into something useful.

‘Lime and limestone FGD fall under the category of “once-through” sorbents because the Ca(OH)2 or CaCO3 are used up in the process, and so cannot be recovered and reused to capture more SO2. Here, a column packed with MFM-170 can capture SO2 from mixed gas streams and when it is saturated, the column can be regenerated. This can be achieved in a low-energy desorption process at room temperature to remove the captured SO2,’ Smith said.

Further, Smith explained that, in theory, the SO2 could then be transformed into products like sulphuric acid which is used in preservatives or as a reagent in organic transformation reactions. ‘The regenerated column of MFM-170 can then be used again to capture more SO2 from the waste gas streams – and this adsorption-desorption process can be repeated over many cycles,’ she said, adding that the SO2 can easily be removed by applying a vacuum to the material to draw the gas out of the cages. ‘Unlike some other technologies, no heat is needed for the desorption process, meaning that energy cost of regeneration is low.’

Smith highlighted that, at present, research is in the early stages of small-lab scale tests and introductory proof-of-concept experiments. ‘Furthermore, it is not realistic to propose the complete replacement of current FGD technologies, but rather that porous materials such as MFM-170 may be used in tandem with existing processes for more thorough desulphurisation.’

Lastly, Smith said that at present, the material can be synthesised on a multi-gram scale with ease, but to scale it up to a kilo-scale and beyond will require more work. ‘The linker synthetic route is an aspect where future attention should be directed on improving the scalability of the material,’ for example, Smith said, ‘by reducing the number of steps and simplifying the synthesis, the efficiency may be further improved’.