Since the discovery of ordered mesoporous molecular sieves designated as M41S family in 1992, there have been explosive researches on the syntheses and applications of these new silica molecular sieves. In particular, their unique nanostructures with precisely controllable pore diameters in the range of 2 - 30 nm attract much attention for applications in adsorption, separation and catalysis. However, practical applications of the mesoporous materials were still limited, due to low hydrothermal and mechanical stabilities.
In the present work, we have synthesized ordered mesoporous carbon molecular sieves exhibiting Bragg X-ray diffraction lines using the mesoporous silica MCM-48 as template materials for the first time. The synthesis was performed using sucrose as the carbon source. The sucrose was thermally carbonized in the presence of sulfuric acid catalyst inside channels of the mesoporous silica. The silica template was then removed with aqueous solution of NaOH or HF.
The carbon molecular sieve, designated as CMK-1 (standing for carbon molecular sieves from KAIST-1), gave transmission electron micrographs (TEM) showing typically the ordered arrays of uniform pores. Furthermore, the XRD pattern revealed that the structure of the CMK-1 carbon was clearly different form that of the silica template. The difference indicated that there occurred a peculiarly systematic transformation to the new structure. The pore size analysis showed that the carbon molecular sieves consisted of uniform mesopores 3.0 nm in diameter, which was approximately twice of the diameter of the silica wall. This result was consistent with the structural transformation of the carbon framework. In addition to the 3.0 nm pores, the carbon molecular sieves had micropores around 0.5 ~ 0.8 nm in diameter. The large specific BET surface area was about $1500 m^2g^{-1}$. The Raman spectrum of the CMK-1 carbon around two broad peaks at 1350 and $1590 cm^{-1}$, indicating atomically disordered nature of the carbon framework.
The template synthesis approach developed here was also applied to other mesoporous silica. The silica material referred to as SBA-1 (cubic Pm3n) gave a poorly ordered carbon material exhibiting a single XRD peak. The use of MCM-41 resulted in a complete loss of XRD peak in the carbon structure. Thus, the template synthesis yet failed to give ordered structures unless the template silica had a three-dimensional channel structure.
It was also possible to synthesize the carbon molecular sieves via vapor phase deposition of acetylene gas at 673-773 K inside the channels of mesoporous silica after grafting aluminum as a solid acid catalyst. The resultant carbon materials from the vapor deposition had the same structure as the MCM-48 template, giving a clue to the origin of the structural transformation.
