Hualiang An

Direct synthesis of 2-ethylhexanol via n-butanal aldol condensation–hydrogenation reaction integration over a Ni/Ce-Al2O3 bifunctional catalyst

Direct synthesis of 2-ethylhexanol from n-butanal via the reaction integration of n-butanal self-condensation with 2-ethyl-2-hexenal hydrogenation is of crucial interest for industrial production of 2-ethylhexanol. Furthermore, as an important and versatile chemical, n-butanol can be produced simultaneously by reaction integration. In the present work, several bifunctional catalysts based on γ-Al2O3 were prepared by the impregnation method and were characterized by means of H2-TPR, XRD, TEM and H2-TPD, and their catalytic performance for direct synthesis of 2-ethylhexanol from n-butanal was investigated. The results showed that Co/Al2O3 had a low activity for hydrogenation and Cu/Al2O3 had a high selectivity for the hydrogenation of the CO group while a Ru/Al2O3 catalyst only favored the hydrogenation of n-butanal to n-butanol. Among them, the Ni/Al2O3 catalyst showed the best catalytic performance and the yield of 2-ethylhexanol was the highest (49.4%). Ce-modified Ni/Al2O3 enhanced the competitiveness of aldol condensation versus hydrogenation of n-butanal and improved the selectivity of 2-ethylhexanol; the yield of 2-ethylhexanol rose to 57.8%. Then the influence of preparation conditions on the catalytic performance of Ni/Ce-Al2O3 was investigated and the suitable preparation conditions were obtained as follows: Ni loading = 10%, calcined at 550 °C for 5 h, and reduced at 570 °C for 4 h. The effect of reaction conditions on the integration reaction catalyzed by Ni/Ce-Al2O3 was investigated and the suitable reaction conditions were obtained as follows: weight percentage of Ni/Ce-Al2O3 = 15%, reaction temperature = 170 °C, reaction pressure = 4.0 MPa and reaction time = 8 h. Under the above reaction conditions, the yield of 2-ethylhexanol attained 66.9% and that of n-butanol was 18.9%. In addition, the components existing in the integration reaction system were identified by GC-MS analysis, and the main by-products were n-butyl butyrate, 2-ethylhexyl butyrate, n-butyric acid, etc. Based on the analysis of the reaction system, a reaction network for the direct synthesis of 2-ethylhexanol from n-butanal was proposed. Finally, an evaluation of the reusability of Ni/Ce-Al2O3 showed that the recovered Ni/Ce-Al2O3 catalyst lost its catalytic activity for the hydrogenation of the CO group. The main reason for deactivation was that Ni species were covered by the flaky boehmite γ-AlO(OH) formed from the hydration of γ-Al2O3 in the reaction process.

n-Butyraldehyde self-condensation catalyzed by Ce-modified γ-Al2O3

Self-condensation of n-butyraldehyde is an important process for the industrial production of 2-ethylhexanol. The catalytic performance of some solid acids such as γ-Al2O3 and molecular sieves for the self-condensation of n-butyraldehyde was investigated and the results showed that γ-Al2O3 was the best one. Then the effect of preparation conditions on the catalytic performance of γ-Al2O3 and the effect of reaction conditions on the self-condensation of n-butyraldehyde were discussed. In order to improve the catalytic performance, γ-Al2O3 was modified by different substances and Ce–Al2O3 was found to show the best catalytic performance; the conversion of n-butyraldehyde and the yield of 2-ethyl-2-hexenal could reach 93.8% and 88.6%, respectively. Moreover, the Ce–Al2O3 catalyst had excellent reusability. The XPS analysis of Ce3d demonstrated that the valence state of cerium affected the catalytic performance of Ce–Al2O3 to some extent but not predominantly. Instead the acid–base property of Ce–Al2O3 played a dominant role in the catalytic performance. The reaction components formed over the Ce–Al2O3 catalyst were identified by GC-MS and then some side-reactions were speculated and a reaction network for n-butyraldehyde self-condensation catalyzed by Ce–Al2O3 was proposed. Subsequently, the research on the intrinsic kinetics of n-butyraldehyde self-condensation catalyzed by Ce–Al2O3 showed that both the forward and backward reactions are second order and the corresponding activation energy is separately 79.60 kJ mol−1 and 74.30 kJ mol−1, which is higher than that of the reaction catalyzed by an aqueous base or acid.