Selective photooxidation of small hydrocarbons by O{sub 2} in zeolites with visible light
- Lawrence Berkeley Lab., CA (United States)
Small alkenes, alkanes, or substituted aromatics loaded with O{sub 2} from the gas phase into alkali or alkaline-earth exchanged zeolite Y react upon irradiation with visible light to form organic building blocks and industrial intermediates. Examples include the conversion of propylene to acrolein, toluene to benzaldehyde, cyclohexane to cyclohexanone, and propane and ethane to acetone and acetaldehyde, respectively. Corresponding alkyl (allyl) hydroperoxide intermediates were observed by in situ FT-infrared spectroscopy. A key finding is that the selectivity of these oxidations is very high even at high conversion, a feature that is lacking existing methods of thermal catalytic autoxidation. Reaction quantum efficiencies are rather high, typically between 0.1 and 0.3. The origin of the photochemistry is a hydrocarbon{sm_bullet}O{sub 2} charge-transfer absorption that extends into the visible region. This was discovered by reaction excitation spectroscopy and confirmed by diffuse reflectance measurements. It signals an unprecedented 1.5-2.5 eV stabilization of the excited hydrocarbon{sm_bullet}O{sub 2} charge-transfer state by the zeolite environment relative to the gas or solution phase. The stabilization is attributed to the very high electrostatic field inside the supercage of zeolite Y, which has been measured by the absorption intensity of the induced infrared fundamental of N{sub 2} (NaY, 0.2 V{Angstrom}{sup -1}, BaY, 0.5 V{Angstrom}{sup -1}). The alkane photooxidations by O{sub 2} open up a new method for very mild C-H activation.
- OSTI ID:
- 370131
- Report Number(s):
- CONF-960376--
- Country of Publication:
- United States
- Language:
- English
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