TiO₂-Catalyzed n-Valeraldehyde Self-Condensation Reaction Mechanism and Kinetics

Abstract

The mechanism of a TiO2-catalyzed n-valeraldehyde self-condensation reaction was first investigated using in situ Fourier transform–infrared spectroscopy (FT-IR) analysis. The result shows that the n-valeraldehyde molecule is adsorbed in two ways separately to Ti4+ and Ti–OH active sites: one involving a strong interaction between the surface Ti4+ and the carbonyl oxygen of n-valeraldehyde molecule, causing a red shift of ν­(CO) and the other involving an interaction between the TiO2 surface hydroxyl group Ti–OH and the carbonyl oxygen of n-valeraldehyde molecule via a hydrogen bond, causing a significant shift of Ti–OH peaks to a lower wavenumber. On the basis of the in situ FT-IR analysis, a TiO2-catalyzed n-valeraldehyde self-condensation reaction mechanism was proposed. In the process of n-valeraldehyde adsorption, the infrared characteristic peaks of 2-propyl-3-hydroxyheptanal were not observed, indicating that the dehydration of 2-propyl-3-hydroxyheptanal to 2-propyl-2-heptenal proceeded very quickly. In the process of desorption of products, the infrared characteristic peaks of carboxylates were detected on the surface of TiO2, suggesting that n-pentanoic acid is generated in the reaction system. In addition, we speculate that n-pentanoic acid has a strong interaction with the surface of TiO2 and the generated carboxylates are hardly desorbed, leading to the deactivation of TiO2 catalyst. In order to get a better understanding of the process of TiO2-catalyzed n-valeraldehyde self-condensation, the liquid-phase reaction was monitored in real time by an in situ IR (React-IR). In the whole course of the reaction, 2-propyl-3-hydroxyheptanal was not detected, confirming that this intermediate cannot exist stably, not only on the TiO2 surface but also in the reaction liquid. In order to further verify the reaction mechanism and to confirm the rate-determining step, several possible Langmuir–Hinshelwood models were assumed. By the model identification, the Langmuir–Hinshelwood model with the surface reaction as the rate-determining step is found to be the most probable one.

Elucidation of Active Sites in Aldol Condensation of Acetone over Single-Facet Dominant Anatase TiO2 (101) and (001) Catalysts

“…A similar mechanism has previously been proposed for the self-condensation of n -valeraldehyde on TiO 2 . 16 As depicted in Scheme 2 , acetone molecules first adsorb on both the Ti site and the vicinal O s H group, reaching equilibrium (Steps I and II, Scheme 2 ). The basic O site of the Ti–O pair could extract the α-H of the Ti-bonded acetone, forming an enolate intermediate (Step III, Scheme 2 ).…”

Synergistic catalysis of acid–base bifunctional ionic liquids for pentanal self‐condensation reaction

“…However, it also suffers from some obvious drawbacks such as difficulty in catalyst recovery, environmental pollution and high cost for the treatment of the alkali wastewater. Several kinds of heterogeneous catalysts have been investigated in pentanal self‐condensation to conquer the earlier‐mentioned drawbacks, including inorganic solid base catalysts, organic solid base catalysts, solid acid catalysts . The aminopropyl functionalized chitosan and titanium dioxide (TiO 2 ) showed better catalytic performance in these catalysts.…”

Green Recovery of Titanium and Effective Regeneration of TiO2 Photocatalysts from Spent Selective Catalytic Reduction Catalysts

“…TiO 2 has attracted considerable attention as a photocatalyst to degrade organic pollutants in aqueous waste owing to its resistance to photocorrosion, chemical inertness, and nontoxicity as well as ideally producing CO 2 and H 2 O as end products. − However, there are still some shortcomings in practical applications of TiO 2 photocatalysts: (1) low utilization of solar energy due to their wide band gap (3.2 eV for anatase and 3.0 eV for rutile); (2) the low quantum yield caused by their high electron–hole pair recombination rate; and (3) their high cost because they are mainly prepared with expensive chemical reagents, such as TiCl 4 , , TiOSO 4 , , and titanium isopropoxide . To solve these problems, this work proposes the use of spent SCR catalysts as an ideal raw material.…”