| Cinnamaldehyde is an α,β-unsaturated aldehyde. Different hydrogenation products are determined by the selective hydrogenation between C=O and C=C. Cinnamic alcohol, which is achieved by C=O selective hydrogenation, is widely being used in agriculture, industry,food and other industries. Due to their unique properties, the transition metal clusters, especially Au and Pt clusters with better catalytic activity,have been used as hydrogenation active catalysts. The clusters have also received high interest in selective hydrogenation of cinnamaldehyde. In this paper, with clusters and tablet models simulating catalyst, the adsorption and selective hydrogenation mechanism of cinnamaldehyde on Au, Pt clusters and Au(111) surface were investigated by Dmol3 program based on the generalized gradient approximation-density functionaltheory. The main studies are as follows:The interaction between cinnamaldehyde and Au13、Pt13 clusters has been investigated. Firstly, the icosahedral Au13 and Pt13 clusters have been designed. Based on this model, the adsorption process of cis,trans-cinnamaldehyde molecules have been studied systematically. At the same time, adsorption energies, structural parameters, Mulliken charge population and density of states of the system are analyzed. The results showed that the adsorption ability of cis-cinnamaldehyde was higher than that of trans-cinnamaldehyde for the same cluster. On the Au13 cluster, the most stable adsorption was the C=O and C=C double bond coadsorption model. After adsorption, electrons transferred from 2s and 2p orbitals of C atom connected with the benzene to Au13 cluster and partial electrons of Au13 cluster were also back-donated to the anti-bonding orbitals of O atom. While on the Pt13 cluster, the most stable adsorption was the C=O double bond adsorption model.Electrons transferred between C, O atom of C=O and Pt13 cluster.Generally speaking, the adsorption capacity of cinnamaldehyde on Pt13 cluster was higher than on the Au13 cluster.The mechanism of the selective hydrogenation of cinnamaldehyde on Au13, Pt13 clusters has been investigated. Based on stable adsorption configuration, the transition states of each elementary reaction steps in selective hydrogenation of cinnamaldehyde were searched by thecomplete LST/QST method and the activation energy and reaction heat were obtained. The results show that: on Au13 cluster, cinnamaldehyde was more likely to occur the 1,4-conjugate addition mechanism to give ENOL. The dominant reaction pathway involved an O atom of cinnamaldehyde preferentially hydrogenating to generate a more stable allyl intermediate. Another H atom, then added to a C atom directly connected to the phenyl ring to yield ENOL, which is very unstable.Finally, ENOL tautomerized to HCAL. While on Pt13 cluster, C=O addition mechanism to give COL was the most likely. The first hydrogenation was similar to that on Au13 cluster, but in the second step,H added to a C atom of C=O. Cinnamic alcohol is likely to further hydrogenation of saturated alcohols.To further compare the differences between cluster model and plate model, we have calculated the adsorption and the selective hydrogenation process of cinnamaldehyde on the Au(111) surface with the same method.The results show that: the most stable adsorption configuration involved the C=O and C=C double bond adsorbed on the Au(111) surface, with an average adsorption energy of 140.0 kJ·mol-1. Compared with Au13 cluster,the adsorption energy was significantly reduced. Meanwhile, the mechanism of the selective hydrogenation of cinnamaldehyde on Au13 cluster and Au(111) surface were the same. Therefore, selecting a flat model or cluster model does not alter the selectivity of cinnamaldehyde,but has an impact on energy, which provides guidance for the theoretical simulation of the catalyst to some extent. |
PDF Full Text Request