The role of mechanochemical treatment of carbon nanotubes in promoting glycerol etherification

High-purity and technical-grade carbon nanotubes (CNTs) were functionalized and compared, for the first time applying various mechanochemical methods to endow their surfaces with strongly acidic features. The as-prepared samples were used as catalysts in glycerol etherification performed with tert-butyl alcohol at 110 °C under autogenous pressure, converting glycerol to glycerol tert-butyl ethers, i.e., important fuel additives, in a green way. The best-performing catalyst was the CNT sample ball-milled with glucose and hydrothermally treated with fuming H₂SO₄, which showed a 52.4% glycerol conversion just after 6 h. The catalytic performance of the sample was ascribed to its improved functionalization when using glucose which worked as a “binding agent” for the active functional groups, facilitating the grafting of the –SO₃H sites on the CNT surface in a relatively short time and at a low temperature of modification. Furthermore, it was found that acidic oxygen functionalities, co-introduced with sulfonic groups during the modification, cooperated with –SO₃H sites and jointly worked to enhance the CNT activity in glycerol etherification. Finally, it was shown that adequately tailored technical-grade CNTs could act as promising and much cheaper solid acids than functionalized expensive high-purity CNTs typically used for industrial applications.