Abstract
Hydrothermally stable carbon overlayers can protect mesoporous oxides (SiO2 and Al2O3) 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 13C 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 SiO2 produced a predominantly aromatic carbon film, at least 15% of nonaromatic carbon (sp3-hybridized C as well as C=O) was observed on γ-Al2O3. These species were not derived from residual solvent, according to spectra of the same material treated at 400 °C. The sp3-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 SiO2 and Al2O3 resulted in yellow-brown and nearly black samples, respectively, but the darker color on Al2O3 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 SiO2, additional sharp peaks were detected and attributed to 1/3 of the 2,3-naphthalene molecules undergoing fast, nearly isotropic motions.
Highlights
The transformation of biomass-derived feedstocks to valuable fuels and chemicals requires stable heterogeneous catalysts and catalyst supports
We investigate the structure of more aromatic carbon overlayers prepared using an aromatic precursor, 2,3-naphthalenediol, on γ-Al2 O3 and SiO2 supports, after heat treatments at 400 ◦ C and 800 ◦ C. 2,3-napthalenediol is nontoxic and inexpensive, and there are two advantages to using this precursor [23,24] instead of sucrose, which was used in our previous work [7,10]
A direct polarization/magic angle spinning (DP/MAS) nuclear magnetic resonance (NMR) spectrum with a short recycle delay of 1 s and its corresponding DP spectrum of mobile and nonprotonated segments selected by recoupled dipolar dephasing before detection were recorded
Summary
The transformation of biomass-derived feedstocks to valuable fuels and chemicals requires stable heterogeneous catalysts and catalyst supports. NMR spectroscopy has been used in detailed, quantitative studies on the structure of amorphous carbon overlayers, derived from 13 C-enriched sugars, in catalysts and catalyst supports [7,10]. 13 C NMR spectroscopy can detect all carbon components, including C=O and C–O groups as well as sp3 -hybridized carbons [7,11,12], even without isotopic enrichment [13], and provide an estimate of aromatic cluster size [13] It can reveal molecular mobility [21,22]. Challenges arise from the relatively low volume fractions of the thin carbon overlayers dispersed on the surface of mesoporous oxide supports, which limit the NMR signal strength, and from sample conductivity, which results in radio-frequency power absorption. Combination of multiCP or DP with dipolar dephasing or shift-anisotropy filtering is feasible and further reveals molecular mobility or paramagnetic shift anisotropy due to unpaired electrons
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