Competition between Acetaldehyde and Crotonaldehyde during Adsorption and Reaction on Anatase and Rutile Titanium Dioxide

Abstract

The adsorption of acetaldehyde and crotonaldehyde on the anatase and rutile polymorphs of TiO2 has been investigated with Fourier transform infrared spectroscopy (FTIR). Chemisorption of acetaldehyde on TiO2 involves a strong interaction between the surface and the carbonyl oxygen, causing a significant shift in the location of the ν(CO) vibrational mode to lower frequencies; no interaction with surface hydroxyl groups was observed. Capacities for acetaldehyde and crotonaldehyde adsorption under conditions relevant to aldolization reactions were determined in a novel reactor system providing simultaneous mass measurements and mass spectral analysis of gas-phase products. The coverage of acetaldehyde irreversibly adsorbed on TiO2 was similar to values previously reported for the adsorption of alcohols; coverages of crotonaldehyde were approximately 60% of those for acetaldehyde. Both gas-phase and surface analyses indicate that formation of crotonaldehyde by aldol condensation of acetaldehyde occurs on rutile TiO2 at temperatures as low as 313 K. This reaction was not observed on anatase at these conditions; higher temperatures were required. The production of crotonaldehyde on rutile at 313 K diminished with increasing exposure of acetaldehyde. Acetaldehyde and crotonaldehyde adsorbed in a similar fashion on both anatase and rutile, and either aldehyde could displace the other from the surface layer. Accordingly, the surface concentrations of adsorbed acetaldehyde and crotonaldehyde mirror those in the gas phase. Upon heating an adsorbed layer of acetaldehyde, small amounts of ethoxide and acetate species were formed, possibly from a Cannizzaro-type disproportionation reaction. The similarity of these results to those of studies on TiO2 single crystals illustrates the applicability of properly chosen metal oxide single-crystal surfaces as models for polycrystalline powders. Both demonstrate that the chemistry of aldehydes on TiO2 can be successfully explained in terms of the reactions of a few key surface species.

Adsorption and photo-oxidation of acetaldehyde on TiO2 and sulfate-modified TiO2: Studies by in situ FTIR spectroscopy and micro-kinetic modeling

“…It is observed that the strong peak at 1693 cm -1 decreases in intensity during purging of the reaction cell with synthetic air, while the intensity of the broad peak at 1653 cm -1 increases and a new band appears at 1628 cm -1 . The latter two bands are assigned to the (C=O) and (C=C) vibrations of crotonaldehyde and show that aldehyde condensation occurs on anatase TiO 2 at room temperature, which is consistent with results of previous studies [31,32]. The band observed at 1168 cm -1 can be assigned to the C-C vibration mode in either of the aldehyde species.…”

Acetaldehyde adsorption and condensation on anatase TiO2: Influence of acetaldehyde dimerization

“…The large red-shift of the carbonyl vibration, compared to the gas-phase value of 1760 cm -1 , indicates that the acetaldehyde is coordinated with a surface site through the carbonyl oxygen, as in -C=O → Ti 4+ [47]. In addition, other absorption bands appear at 1643 cm -1 and 1674 cm -1 , and weaker bands are noted at 1355 cm -1 and 1375 cm -1 .…”

Formamide Chemistry and the Origin of Informational Polymers

“…Probably, the role of TiO 2 is both to make easier the formation of the formaldehyde-hydrate anion, and to activate the CHO moiety for nucleophilic attack. Acyclonucleosides are bioactive compounds able to pair with natural nucleic bases by Watson -Crick interactions [82]. In the context of prebiotic chemistry, glycerol-derived acyclonucleosides, which are characterized by a vicinal diol moiety (as in 14), have received particular attention, mainly because of the possibility to polymerize after activation as phosphates [83].…”