Protective Carbon Overlayers from 2,3-Naphthalenediol Pyrolysis on Mesoporous SiO(2) and Al(2)O(3) Analyzed by Solid-State NMR

Hydrothermally stable carbon overlayers can protect mesoporous oxides (SiO(2) and Al(2)O(3)) from hydrolysis during aqueous-phase catalysis. Overlayers made at 800 °C by pyrolysis of 2,3-naphthalenediol deposited out of acetone solution were analyzed by solid-state (13)C nuclear magnetic resonance (NMR) spectroscopy. Power absorption due to sample conductivity was prevented by diluting the sample in nonconductive and background-free tricalcium phosphate. While pyrolysis on SiO(2) produced a predominantly aromatic carbon film, at least 15% of nonaromatic carbon (sp(3)-hybridized C as well as C=O) was observed on γ-Al(2)O(3). These species were not derived from residual solvent, according to spectra of the same material treated at 400 °C. The sp(3)-hybridized C exhibited weak couplings to hydrogen, short spin-lattice relaxation times, and unusually large shift anisotropies, which are characteristics of tetrahedral carbon with high concentrations of unpaired electrons. Moderate heat treatment at 400 °C on SiO(2) and Al(2)O(3) resulted in yellow-brown and nearly black samples, respectively, but the darker color on Al(2)O(3) did not correspond to more extensive carbonization. Aromatic carbon bonded to hydrogen remained predominant and the peaks of naphthalenediol were still recognizable; however, some of the chemical shifts differed by up to 5 ppm, indicating significant differences in local structure. On SiO(2), additional sharp peaks were detected and attributed to 1/3 of the 2,3-naphthalene molecules undergoing fast, nearly isotropic motions.