Silylation of graphite oxide

Abstract

Recently, much attention was paid to solid matrices for photochemical reaction media of organic molecules with specific selectivity [1–4]. These solids include layered materials, zeolites, etc. One of the layered materials, graphite oxides hydrophobized by intercalation of alkyltrimethylammonium ions or n-alkylamine have shown interesting properties when photochemical dimerization of acenaphthylene was performed in them [5–8]. These materials possess hydrophobic nanospace between the alkyl chains, which is suitable for including organic molecules [5–11]. It was proved that the size and polarity of the nanospace control the photochemical reaction of acenaphthylene [6–8]. However, these intercalation compounds were not so stable and released the alkylamine molecules gradually under ambient condition, though Bourlinos et al. have reported that alkylamine-intercalated GOs are stable even when they are in contact with sodium hydroxide solution [12]. The intercalation compounds of graphite oxide would be stabilized if the intercalated molecules are fixed in the interlayer space by covalent bonding. On the other hand, the silylation of the silanol groups in the interlayer space of a layered silicate using organochlorosilanes has recently been employed to construct novel inorganic– organic supramolecular systems [13–15]. A similar reaction is expected for graphite oxide because it possesses acidic hydroxyl groups on its layers. Therefore, in this study, silylation of graphite oxide was performed and the resulting product was investigated. Graphite oxide (GO) was prepared by oxidizing natural graphite powder with KClO3 in fuming nitric acid at 60 C, based on Brodie’s method [16]. Then the resulting solution was poured into a large excess of water and dried at 60 C. This procedure was repeated for 5 times and GO with the composition of C8O4:0H3:2 was obtained. It was silylated in a similar manner as reported by Ogawa et al for the silylation of a layered silicate. GO (400 mg) was mixed with butylamine (C4H9NH2, 2 ml) in a sealed glass vial under dry nitrogen and the resulting solution was sonicated, then heated at 60 C for 1 h. Dry toluene (water content <30 ppm, 20 ml) was added to this solution under a nitrogen atmosphere and the solution was again sonicated. Octyltrichlorosilane (C8H17SiCl3, in short C8SiCl, 2 ml) was added and then allowed to stand for 1 day. After centrifugation, the precipitate was washed with dry toluene, hexane, ethanol, acetone, the mixture of ethanol and water and finally acetone. The obtained sample was dried at 60 C under reduced pressure for 12 h. The composition of the product was determined on the basis of CHN elemental analysis. Octylamine-intercalated GO with a C8H17NH2/GO ratio of 1.2 was also prepared by reacting GO with octylamine in the presence of a small amount of hexane as reported previously, in order to compare the stability of the product in ethanol with that of the silylated GO. Fig. 1 shows the X-ray diffraction patterns of GO and silylated GO, together with that of butylamine-intercalated GO. The diffraction peak at 2h 1⁄4 14 derived of GO shifted to 6.5 for silylated GO, indicating an increase of the interlayer spacing by 0.75 nm. This increase of the interlayer spacing was much larger than that of butylamine-intercalated GO (0.28 nm). This suggests that C8SiCl with a long alkyl chain had reacted with GO and was intercalated into its interlayer spacing of GO. Fig. 2 shows the IR spectra of GO, butylamineintercalated GO and silylated GO. Strong absorption peaks around 2900, 1450, 1150 and 850 cm 1 were observed after reaction with C8SiCl. The latter two peaks were absent in butylamine-intercalated GO. These would be due to –CH2–, CH3, Si–O and Si–C groups, respectively. On the other hand, the absorption peaks at 1620 and 1380 cm 1 ascribed to adsorbed water and COH groups became relatively smaller. This indicates that C8SiCl reacted with GO, losing chlorine and was covalently bonded to the GO layer by Si–O bonding. The elemental analysis data showed that the silylated product contained 60.32% of carbon, 5.25% of hydrogen and 1.32% of nitrogen. A similar product was obtained when GO was reacted with C8H17SiCH3Cl2, however, reaction