Synthesis Of CO_XSN_Y Intermetallic Compounds Nanocatalysts Toward Selective Hydregenation Of Citral

Selective hydrogenation of a,β-unsaturated aldehydes to corresponding unsaturated alcohols is an important reaction in the production of fine chemicals. Co-based Intermetallic compounds (IMCs) materials have recently been explored in the selective hydrogenation of a,β-unsaturated aldehydes with good performances. However, the conventional preparation method of Co-based IMCs normally suffers from rigorous conditions, noxious organic reagents or uncontrollable structure/composition. Therefore, it is highly essential to develop facile and green strategies for the preparation of Co-based IMCs catalysts with precisely-controlled composition and structure toward selective hydrogenation of a,β-unsaturated aldehydes. In this dissertation, several CoxSny IMCs with a tunable particle size, composition and structure were prepared via the topological transformation method of layered double hydroxides (LDHs) materials, which exhibit largely enhanced catalytic activity and selectivity toward selective hydrogenation of citral. The geometric and electronic structure of active sites on CoxSny IMCs were modulated by changing the Co/Sn ratio and the topotactic transformation process of LDHs; an experiment-calculation combination study was carried out to understand the structure of active sites and to reveal the structure-property correlation. This work provides a facile method for the preparation of Co-based IMCs catalysts, which can be potentially used in selective hydrogenation of a,β-unsaturated aldehydes.The detailed contents are listed as follows:(1) Synthesis of CoxSnv intermetallic compounds nanocatalysts with modulation over active sites structureBased on the reaction between Co2Al-LDHs precursor and exogenous Sn species, three kinds of supported CoxSny/Al2O3 IMCs (Co2.9Sn2/Al2O3, CoSn/Al2O3 and CoSn2/Al2O3) were prepared via the topotactic transformation process of LDHs. XRD and HRTEM confirm the crystal structures and mean particle size (-50 nm) of the CoxSnv/Al2O3 IMCs. Furthermore, by changing the Co/Zn ratio of ZnxCovAl2-LDHs precursors, a precise modulation over the particle size of CoxSnv IMCs was achieved. Three kinds of CoxSnv/ZnAl-MMOs IMCs (Co2.9Sn2/ZnAl-MMOs, CoSn/ZnAl-MMOs and CoSn2/ZnAl-MMOs) were obtained from Zn3Co7Al5-LDHs with idenfied crystal structure and uniform particle size (-20 nm). The results of EXAFS and DFT calculation reveal that the introduction of Sn in CoxSny/ZnAl-MMOs dramatically optimizes the geometric and electronic structure of active Co, in which Sn isolates the Co active-site and electron transfer occurs from Sn to Co atom.(2) CoxSnv intermetallic compounds nanocatalysts as efficient catalysts toward citral hydrogenationBased on the study of the structure modulation of CoxSny/ZnAl-MMOs catalysts, we studied their catalytic behavior toward selectivity hydrogenation of citral to unsaturated alcohol. Compared with the Co/ZnAl-MMOs, the CoxSny/ZnAl-MMOs exhibit largely enhanced catalytic performance, and the best catalytic behavior was obtained over the CoSn/ZnAl-MMOs catalyst with a citral conversion of 100% and an excellent selectivity of unsaturated alcohol (67.6%). The H2-TPD measurements indicate that CoxSny/ZnAl-MMOs catalysts promotes an appropriate adsorption state of H2, which would facilitate the dissociation and activation of H2. The results of in situ CO-FTIR and DFT calculation reveal that the geometric and electronic effect of CoxSny IMCs (the active site isolation and electron transfer from Sn to Co) weaken the hydrogenation of C=C group, but strengthen the hydrogenation of C=O group, accounting for the enhanced hydrogenation selectivity of citral to unsaturated alcohol.