Formation mechanism of 0.4-nm single-walled carbon nanotubes in AlPO4-5 crystals by low-temperature hydrocracking

The carbonization mechanism of tripropylamine (TPA) in low-temperature hydrocracking was studied with the aim of increasing the quality and filling density of single-walled carbon nanotubes (SWCNTs) produced in the channels of AlPO4-5 crystals. The conversion process of TPA was investigated using a combination of Fourier transform infrared spectroscopy, mass spectrometry, 13C nuclear magnetic resonance spectroscopy, thermogravimetric analysis and micro-Raman spectroscopy at various hydrocracking temperatures. During the hydrocracking process, hydrogen participated in the cracking reaction of TPA and decreased the required activation energy. The protonated TPA converted into neutral TPA at 210 °C. When the hydrocracking temperature exceeded 260 °C, dipropylamine, n-propylamine, propylene, propane, ethane and methane were produced. The hydrocracking rate of TPA increased with increasing hydrocracking temperature. A small amount of aromatic compounds was also detected in the AlPO4-5 crystals hydrocracked at 280-350 °C; this amount decreased with increasing hydrocracking temperature. The content of residual TPA and amorphous carbon compounds in the AlPO4-5 crystals also decreased with increasing hydrocracking temperature. TPA decomposed completely after hydrocracking for 10 h at 350 °C. SWCNTs with a diameter of 0.4 nm were synthesized at 280-350 °C, and the filling density of SWCNTs increased with increasing hydrocracking temperature.

(a) Schematic diagram of the SWCNTs formation via low-temperature hydrocracking; (b) the optical image of AlPO4-5 crystals hydrcracked at 280 °C for 10 h; (c) the optical image of AlPO4-5 crystals without SWCNTs; (d) the high-resolution TEM image of SWCNTs released from AlPO4-5 crystals hydrcracked at 280 °C for 10 h.395