Physico-Chemical nanomaterials science

Hydrosulfidation of nanoporous carbon for the preparation of solid acid catalysts
Diyuk V. E.1, Horodetska D. S.1, Vakaliuk A. V.1, Grishchenko L. M.1, Tsapyuk G. G.1, Mischanchuk A. V.2, Mariychuk R.3, Lisnyak V. V.1,3
1 Faculty of Chemistry, Taras Shevchenko National University of Kyiv, Volodymyrska Street, 64/13, Kyiv 01601, Ukraine.
E-mail: daria_horodetska@ukr.net; lisnyak@chem.univ.kiev.ua
2 Chuiko Institute of Surface Chemistry of NAS of Ukraine, General Naumov Street, 17, Kyiv 03164, Ukraine.
3 Preљov Universirty in Preљov, Ul. 17 novembra, 1, Preљov 15 080 01, Slovakia.

To create effective sorbents, carriers, and acid-base catalysts based on carbon nanoporous materials, one requires the targeted introduction into their surface layer of heteroatoms, e.g., nitrogen, sulfur, phosphorus, etc. The presence of the surface groups with heteroatoms changes significantly the surface characteristics, e.g., by increasing acidity/basicity and the sorption capacity for heavy metal ions. Especially, nanoporous activated carbons (NACs) with grafted sulfo groups are highly active catalysts for isomerization, hydration, and dehydration. Known methods of sulfonation with sulfuric acid, oleum, or chlorosulfonic acid do not give the required concentration of sulfo groups. Sulfur-containing NACs obtained by incomplete carbonation of sulfoaromatic compounds have poor mechanical and structural-sorption characteristics. Therefore, it is promising to create new effective methods of surface acidification. Here we proposed a new staged method of introducing sulfo groups, including hydrosulfidation at 500–800 °C. Hydrogen sulfide was formed in situ at 250 °C. This technique is effectively introduced 0.4–4.3 mmol/g of sulfur, and the maximal amount of sulfur can be introduced at about 500 °C. Thermogravimetric analysis showed that the chemisorbed sulfur is thermally stable below 550 °С. To obtain solid acid catalysts, S-containing NACs were oxidized with aqueous solutions containing HNO3 and KClO3 and H2O2 and HAc. Oxidation with KClO3 leads to significant oxidation of the surface, resulting in the formation of sulfo groups and different types of oxygen-containing groups. Oxidation with H2O2 leads to the formation of a small number of sulfo groups, because of the weak oxidation of the surface layer. The obtained samples have high catalytic activity in the isopropanol dehydration. The complete conversion of isopropanol to propylene on the test samples is observed at 175–200 °С, which allows the use of the proposed method to obtain advanced solid acid catalysts.