On the Incorporation of Buckminsterfullerene C₆₀ in the Supercages of Zeolite Y

A series of vapor-phase adsorptions of C60 on NaY zeolite have been carried out under reduced pressure (1 Torr) in the range of temperatures between 400 and 700 °C to determine whether C60-fullerene (0.79 nm diameter) can be incorporated within the internal voids of faujasites (tridirectional large-pore zeolite containing supercavities of 1.3 nm diameter tetrahedrally connected through 0.74 nm windows). After the incorporation procedure, the samples were submitted to exhaustive solid−liquid extraction using toluene as solvent. The optimum adsorption temperature was found to be around 650 °C. Analogous treatment using silica−alumina instead of NaY did not lead to significant retention of C60. The samples were characterized by X-ray powder diffraction, combustion chemical analysis, and thermogravimetry-differential scanning calorimetry as well as by diffuse reflectance, Fourier transform infrared, and magic angle spinning 13C NMR spectroscopies. None of these techniques provides direct evidence of the location of C60 in the zeolite matrix. Molecular dynamics simulations of the faujasite cavity window vibrations at different temperatures (100−800 °C) show that although there are remarkable instantaneous variations in the pore diameter (some of them allowing the entrance of fullerene molecules), the vibrations are too rapid (in the ps time scale) to allow the complete passage of fullerene. Interestingly, the average pore diameter of the faujasite cavity windows are predicted to be rather insensitive to increasing temperatures. Finally, high-resolution electron microscopy has experimentally revealed that fullerene molecules are highly dispersed through the zeolite particles, and in certain regions there are some preferential spatial arrangements of C60. In addition, the presence of weak new reflections in the electron diffractogram pattern of C60-doped NaY (forbidden for a Fd3̄m symmetry such as Y zeolite) was observed. This has been taken as an experimental evidence that a small fraction of C60 molecules has penetrated inside the supercages, showing a spatial order and changing the zeolite symmetry. Meanwhile, most of the C60 molecules are located in the open cavities at the external surface.