Abstract
On the basis of quantum chemistry calculations, gaseous reactions including molecular and atomic hydrogenation of crotonaldehyde have been systematically examined in this paper. Firstly, the structures of initial states, transition states and final states along each reaction pathway were completely optimized at the level of B3LYP/6-31G (d, p) and PW91/6-31G (d, p). The total energies were adjusted by zero-point energy correction, and the obtained activation energies were compared with those of more accurate calculation method of CBS-APNO. Through comparing the results obtained from B3LYP, PW91 and CBS-APNO, it was found that the data from B3LYP are closer to those of CBS-APNO. Moreover, both the molecular and atomic hydrogenation reactions of crotonaldehyde are exothermic and the heat effect of C=C is higher than that of C=O, indicating that both H2 and H prefer to attack C=C double bond, resulting in the formation of butaldehyde or free radical. On the other hand, from the viewpoint of dynamics, the difference of activation barriers between C=C and C=O in the reaction of molecular hydrogenation is very small while the barrier of C=C atomic hydrogenation is pronounced lower than that of C=O. Comparing with the results of acrolein hydrogenation, C=C hydrogenation can be hindered by introducing heavy end groups because of the steric effect. Finally, through comparing the mechanisms of hydrogenation reactions of the isolated C=C (in propene) and C=O (in formaldehyde), the conjugate effect is revealed in which the difference in kinetics between C=C and C=O hydrogenation of crotonaldehyde is greatly decreased. Thus, it is reasonable to design a rational catalyst that the selectivity can be favorably controlled.
