Physico-Chemical nanomaterials science

Hydrophobization of the surface of carbon nanomaterials 
using the Diels–Alder reaction

Soloviov V.1, Diyuk V.E.1, Vakaliuk A.V.1, Yatsymyrskyi A.V.1, Mischanchuk O.V.2, Mariychuk R.3, Lisnyak V.V.1

1 Taras Shevchenko National University of Kyiv, 62a, Volodymyrska Str., Kyiv 01601, Ukraine, E-mail: vadiga@ukr.net
2 O.O. Chuiko Institute of Surface Chemistry, the National Academy of Science of Ukraine, Gen. Naumova St., 17, General Naumov Str., 03164 Kyiv, Ukraine
3 Prešov Universirty in Prešov, Ul. 17 Novembra, 1, 080 01, Prešov, Slovakia

Carbon nanomaterials (CNM) have high specific surface area and electrical conductivity. These CNM include nanoporous carbon sorbents those have unique mechanical properties and are non-toxic. So, one can use the CNM to solve the problems in chemical technology, ecology, and medicine. The chemical properties of CNM allow modifying the surface layer to improve material characteristics and obtain a variety of functional materials. The Diels-Alder (DA) reaction is very selective and runs between a diene and an unsaturated compound, which is a dienophile. The CNMs can contain both isolated carbon-carbon double bonds and conjugated carbon–carbon bonds, i.e., they can react with DA as both diene and dienophile. This work aims to study the ways of hydrophobization of the surface of various carbon nanomaterials: multilayer carbon nanotubes, carbon soot, activated carbon, and carbon fibers. The reactivity of active CNM centers in the reaction with 1,3-Cyclohexadiene (CHD) was investigated to address this issue. Barothermal treatment was used for effective modification, and the sample of CNM was autoclaved with CHD at 90–250 °C. Samples were investigated by miscellaneous methods, including SEM, adsorption of N2, TG/DTG, TPD MS, FTIR, and the Boehm titration. We show that the barothermal modification of CNM leads to the introduction into the surface layer of up to 2 mmol per gram of CNM of specific functional groups. The TG/DTG method has shown that at the least two forms of chemisorbing CHD can be formed when CNM is modified. The first chemisorbed form decomposes in the temperature range of 150–400 °C and refers to CHD grafted to the surface in line with the passaging of the DA reaction. The second form, with a decomposition temperature of more than 350–400 °C, corresponds to the thermal decomposition and destruction of chemisorbed products of partial pyrolysis of the modifying agent. The highest concentration of grafted CHD (up to 1.3 mmol per gram of CNM) was found for CNM modified at 200–250 °C. The process of pyrolysis of the surface layer of the modified CNMs was studied by the TPD MS method. As a result of modification of chemisorbed forms of CHD, the products of the partial destruction of CHD and the products of its dimerization are formed. The obtained materials have a hydrophobic surface and can be used for selective removal of non-polar molecules at adsorption purification.