Petrol in the framework - more efficient higher-grade fuels

Materials World magazine
1 Jul 2013

A new filtration material has been developed at the University of California, Berkeley, that could enable refineries to produce higher-grade petrol fuels more efficiently.

Petrol contains hexane isomers, which are interconvertible within the isomerisation reactor. The value of each isomer depends on its research octane number (RON), and is highest for the disbranched hexanes, 2,3–dimethylbutane and 2,2-dimethylbutane. The RONs for the monobranched isomers 2-methylpentane and 3-methylpentane are far lower, while that for the fifth isomer, linear n-hexane, is the lowest.

To obtain higher-octane petrol blends, n-hexane is currently filtered through microporous minerals called zeolites, but this only yields a mixture of the other four isomers. The monobranched isomers can then be distilled away from the disbranched products, but this is an exacting and costly process.

This new material, however, filters the isomers according to their degree of branching, allowing the disbranched hexanes through before the monobranched and then the n-hexane products. Unlike zeolites, the channels are large enough to accommodate all five isomers.

The US team formed a metal-organic framework (MOF) – a sponge–like adsorbant containing innumerable triangular channels. Called Fe2(BDP)3 – for benzenedipyrazolate – it exploits the fact that the lowest–octane isomers are more linear and so will nestle closer to channel walls, sticking to their surfaces.

The MOF is made by mixing an iron starting material with the protonated version of the BDP linker, then adding the mixture to a solvent and heating it at 160°C for 18 hours. One of the researchers at Berkeley, Professor Jeffrey Long, explains, ‘We filter it and soak it in more solvent to dissolve any BDP that might be inside the pores. Then, to remove that solvent, we heat it to 180°C for a day and it’s ready to go.’

The team also ran tests on the stability of the material. In one test, the MOF was boiled in aqueous solutions at pHs of 2–10 for two weeks. In another, it was heated in air to 280°C. In both cases it did not lose its crystallinity.

A further test carried out at 100mbar at 200ºC showed it adsorbed 60% more n-hexane by volume and 100% more by weight than a zeolite, and in a performance comparison with different zeolites and other MOFs, the researchers obtained a 92 RON productivity of 0.54mol/l of adsorbent. The other materials achieved values of 0.17–0.36mol/l.

Professor Long says that while Fe2(BDP)3 is cheaper to make than many other MOFs, it is currently more expensive than zeolites. Although with bulk manufacture, that should change. He adds, ‘It seems very straightforward to replace zeolites with this material, although understanding the details of an industrial setting would be necessary’.

Professor Long cannot give precise figures for energy saving and improved efficiency, as these would depend on the final set-up in a refinery, which is still some years away. The technology has been patented though, and no further development of it is planned.

Commenting on the work, Dr Ahmet Ozgur Yazaydin at the University of Surrey, UK, who has published a paper on the subject, says the discovery of Fe2(BDP)3 is the newest example of how MOFs could revolutionise traditional separations in the petrochemical industry. He adds, ‘The stability of this material is particularly striking, as many MOFs are unstable, even when exposed to air. But the biggest challenge for its widespread adoption is to figure out its mass production.’