Comment on “Synthesis and characterization of a novel nickel pillared–clay catalyst: In–situ carbon nanotube–clay hybrid nanofiller from Ni-PILC”, by M. Asgari, G. Vitale, U. Sundararaj [Applied Clay Science 205 (2021) 106064, doi:10.1016/j.clay.2021.106064

Abstract

Kaolinite-urea intercalates were prepared by dry grinding kaolin KGa-1b with urea using a laboratory-scale planetary ball mill. The effect of milling conditions on the intercalation process was investigated over a wide range of urea content (25 m% - 80 m%) and milling times (up to 2 h). The purification of the complex obtained was carried out by repeated washings with isopropanol in order to remove the excess (non-intercalated) urea. For that purpose, the proportion of intercalated urea and non-intercalated urea crystals was quantified after each washing step by combining differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) results obtained simultaneously. The optimal conditions of the intercalation were realized at a 66 m% urea loading for which a relatively low-defect complex structure was obtained in a short milling time. One drawback with the use of a high urea content (> 25 m%) during grinding was the presence of a large proportion of excess urea crystals in the as ground sample. This required repeated washing with isopropanol to purify the complex. Furthermore, the results revealed that washing with water the kaolinite-urea intercalates led to the formation of 0.84 nm kaolin hydrates and that exfoliation/delamination of kaolinite was more efficient when a high concentration of urea was used in the milling process.