Novel material for reversible SO2 capture

AN international team of scientists has developed a novel material capable of selective and reversible capture of sulfur dioxide (SO₂). The porous material could help to reduce SO₂ emissions and enable use of captured SO₂ to produce useful products.

According to Aas et al (2019), global annual emissions of SO₂ total about 121 Mt (or, about 109 Mt excluding potentially overestimated emissions from China). 87% of SO₂ emissions are a result of human activity, according to the international team’s research paper. Typically, these emissions originate from sources such as power plants, other industrial facilities, and heavy equipment.

SO₂ emissions can have detrimental effects on human health and the environment. At the same time, SO₂ is an important feedstock in the production of sulfuric acid – around 250m t/y of sulfuric acid is produced from SO₂ annually, according to the paper’s lead author Gemma Smith. Additionally, SO₂ has other uses such as in preservatives.

Led by the University of Manchester, UK, the team of UK and US scientists developed a metal-organic framework (MOF) containing open copper sites, called MFM-170, to capture SO₂ more efficiently than existing systems. MOFs are a class of crystalline materials consisting of transition-metals cations and multidentate organic linkers. Their structure is characterised by an open framework that can be porous.

Currently, the most common method used for SO₂ removal from power plant exhaust gas is irreversible reaction with lime or limestone slurries, said Smith. This can remove more than 95% of SO₂. However, it may not always be suitable for thorough desulfurisation (>99% removal), Smith added. The method is also associated with disadvantages such as scaling in the absorber units and high volumes of solid and liquid waste. The waste and the spent sorbent are landfilled. Liquid waste is dewatered before landfilling.

Desulfurisation costs can be offset by oxidising waste CaSO₃ to produce gypsum, which is used in fertiliser and as the main constituent of many forms of plaster, blackboard/sidewalk chalk, and drywall. However, this is not always possible, according to Smith.

The novel material created by the international team can purify gas streams to <0.1 ppm SO₂ (99.99% free of SO₂) and shows higher SO₂ absorption than any other porous material known to date, according to Smith. The material can fully reversibly uptake 17.5 mmol/g of SO₂ at 298 K (24.85°C) and 1.0 bar (100 kPa).

Once the MFM-170 is saturated, absorbed SO₂ can be released by applying a vacuum to the absorption column. The molecules can also be flushed out by a flow of inert gas such as nitrogen. “Importantly, no heat is required for regeneration, which reduced energy requirements,” said Smith.

Because absorption is reversible, the captured SO₂ could be used to produce useful products, such as sulfuric acid. This would mean that no waste is produced by the capture method.

Additionally, the reversibility of the system means that the capture material can be reused. In the study, researchers trialled the material for 50 capture-release cycles and saw no detectable deterioration in performance, said Smith.

The new material is “remarkably” stable to exposure to corrosive SO₂ and can efficiently separate it from humid waste gas streams, according to Smith.

She said: “Flue gas streams (and even normal air) contain high levels of humidity and this presents challenges for a number of reasons. Firstly, many porous MOF materials are not stable to water, and prolonged exposure to humidity may cause structural collapse. Secondly, water molecules can bind strongly in MOFs and can compete with other gases such as SO₂ – reducing the performance of the material. Perhaps most importantly, SO₂ can dissolve in water to form an acidic solution which is corrosive and can destroy porous materials.

“Therefore, not only should an SO₂ adsorbent be stable to SO₂ and water separately, it should also be stable to simultaneous exposure to SO₂ and water.”

According to Smith, three months of continuous exposure to humid SO₂ didn’t result in any loss in structure of the material. However, “longer experiments are required to see if any deterioration is eventually detected”.

Smith said that this technology is currently in the early stages of small lab-scale tests and preliminary proof-of-concept experiments.

“Our results from our experiments have allowed us to understand the mechanism of SO₂ capture in our material, and will aid design of further improved materials,” she said.

Smith added that though it is not “realistic” to propose complete replacement of current flue gas desulfurisation technologies, porous materials such as MFM-170 could be used in tandem with existing technologies for more thorough desulfurisation.

Nature Materials: http://doi.org/dgg8