Characterization of sorbed intermediates and implications for the mechanism of chain growth in the conversion of methanol and ethanol to hydrocarbons over 12-tungstophosphoric acid using infrared photoacoustic spectroscopy

Abstract

Photoacoustic Fourier-Transform Infrared Spectroscopy (PAS-FTIR) has been used to study the conversion of C 1-C 4 alcohols to hydrocarbons over 12-tungstophosphoric acid, H 3PW 12O 40. The C 2C 4 alcohols undergo Brønsted-acid-catalyzed dehydration by initially the same mechanism as that reported earlier for CH 3OH. Rapid protonation of the sorbed alcohol is followed by thermally induced CO bond cleavage, and the resulting alkyl cation is stabilized, in varying degree, by neighboring O ions in the Keggin unit. The cycle is completed by deprotonation and olefin desorption, considered to occur preferentially for alkyl intermediates associated with the more basic O ions, i.e., the weaker Brønsted acid sites. For C 2H 5OH, the latter step yields C 2H 4 at 110 °–150 °C which subsequently plays an important role in chain growth. The olefin readily reacts with sorbed C 2H 5 + in this temperature range to form n-C 4H 9 +, which quickly rearranges to i-C 4H 9 +. Consistent with its proposed carbenium ion character, the latter species reacts with NH 3 to form the alkylammonium cation, and further rearranges to the tertiary form above ≈200 °C. In CH 3OH, the onset of corresponding chain growth (and rearrangement), enhanced by the addition of C 2H 4 is observed at ≈ 175 °C. The carbene (:CH 2) mechanism of initial CC bond (or C 2H 4) formation from sorbed CH 3 + is tentatively favored over the onium ylide route. The onset of H-transfer (and carbonization) in conversion of the alcohols under static conditions is observable already at ≈ 150 °C, and becomes extensive at elevated temperature, resulting in essentially paraffins above ≈250 °C. Selective suppression of the stretching vibrations of sorbed alkyl intermediates, induced by heating in vacuo, is reported.