A reaction integration of aldol condensation–hydrogenation for the direct synthesis of 2-ethylhexanol from n-butanal was realized.

Self-condensation of n-butyraldehyde to 2-ethyl-2-hexenal is one of the important processes for the industrial production of 2-ethylhexanol. In the present work, several sulfonic acid functionalized ionic liquids (SFILs) were synthesized. Their acid strengths were determined by the Hammett method combined with UV–vis spectroscopy, and their catalytic performances in n-butyraldehyde self-condensation were investigated. The results show that the conversion of n-butyraldehyde correlated well with the acid strength of the SFILs with the same cation. The SFILs with triethylammonium cations showed a better catalytic performance than those with imidazolium cations or pyridinium cations, and [HSO3-b-N­(Et)3]p-TSA (“b”, butyl) exhibited the highest selectivity. Under the optimal reaction conditions of the mass ratio of [HSO3-b-N­(Et)3]p-TSA to n-butyraldehyde = 0.1, reaction temperature = 393 K, and reaction time = 6 h, the conversion of n-butyraldehyde was 89.7% and the selectivity to 2-ethyl-2-hexenal was 87.8%. [HSO3-b-N­(Et)3]p-TSA could be reused four times without a significant loss in its catalytic performance. A kinetic analysis result showed that this is a reversible second-order reaction. Compared with the kinetic parameters from the reaction catalyzed by an aqueous base or acid catalyst, the pre-exponential factor is lower due to the restriction of the high viscosity of [HSO3-b-N­(Et)3]p-TSA. Finally, a possible reaction mechanism for n-butyraldehyde self-condensation catalyzed by [HSO3-b-N­(Et)3]p-TSA was proposed.

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Several TiO2 samples with different morphologies and structures, nano/microcomposite of TiO2 (anatase), TiO2 whisker (anatase), and nano-TiO2 (anatase), were prepared and characterized by means of N2 adsorption–desorption, CO2-TPD, and NH3-TPD, and their catalytic performance for n-valeraldehyde self-condensation was investigated. The results indicated that the conversion of n-valeraldehyde was correlated with their acid amount while the selectivity of 2-propyl-2-heptenal was associated with their base amount. Since the catalytic performance of nano-TiO2 (anatase) was the best, its preparation process was further studied and the suitable preparation conditions were obtained. Then the effect of reaction conditions on the catalytic performance of nano-TiO2 (anatase) for n-valeraldehyde self-condensation was investigated and the suitable reaction conditions were obtained as follows: a weight percentage of TiO2 catalyst of 15 wt %, a reaction temperature of 190 °C, and a reaction time of 10 h. Under the above reaction conditions, the conversion of n-valeraldehyde, 2-propyl-2-heptenal yield, and selectivity were 94.6%, 93.7%, and 99.1%, respectively. The TiO2 catalyst could be reused four times without a significant loss in its catalytic performance, which was different from most of the literature. The catalytic stability of TiO2 catalyst was associated with the properties of the active sites, especially acid–base property. Not as some of the literature claimed that their TiO2-catalyzed reactions were base-catalyzed reactions, the TiO2 catalyst used in this work possessed much greater acid amount than base amount. To assess the role of acidic and basic sites in n-valeraldehyde self-condensation, ammonia and carbon dioxide were separately used as a probe molecule for poisoning the corresponding active sites. The results confirmed the key role of acid sites in n-valeraldehyde self-condensation. Therefore, we were convinced that the TiO2-catalyzed n-valeraldehyde self-condensation was mainly an acid-catalyzed reaction.

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.