Computational Studies On The Reaction Mechanism For The Hydrodeoxygenation Of Polyols

Hydrodeoxygenation (HDO) is an important way for upgrading biomass-derived polyols,such as glucose, fructose and glycerol, etc. Therefore, important guides can be obtained forthe catalyst design for the HDO reactions from studying the related reaction mechanism. Thereaction mechanism of the glycerol, as a model compound for polyhydric alcohols, wasinvestigated using density functional theory (DFT) methods. The geometries of the reactants,transition states and products in the dehydration reaction pathways were optimized and theenergetic profiles were calculated. The preliminary investigation about the catalytichydrogenation mechanism was also performed.To select an appropriate density functional theory (DFT) method is fundamental for thesubsequent computational studies. Considering the features of alcohols that are the maininterests in this work, reaction barriers for the dehydration of protonated isopropanol and thedehydration of1,2-propanediol were calculated with different levels of theory, to search for anappropriate method for the dehydration of polyols. The calculation results show that thereaction barriers calculated with DFT using M06-2X functional are close to those calculatedwith the high level CCSD(T) method. At the meanwhile, using the6-31+G**basis setsresultes in the close value compared with that from using the high level AUG-CC-PCTV basissets. Although the MP2results are also close to the CCSD(T) results, it is much moretime-consuming than DFT. So M06-2X was chosen as the method in the subsequent studiesfor the glycerol dehydration.The selected interesting conclusions about glycerol dehydration in this thesis can bedrawn as following. Firstly, different conformations of protonated glycerol may undertakedifferent dehydration processes. Secondly, the C-C bond cleavage of protonated 3-hydroxypropionaldehyde may take place. Thirdly, glycerol can undergo C-C bond crackingto afford ethenol and formaldehyde when using6T-model zeolite as the catalyst. Fourthly, theenergy barrier for glycerol dehydration at the secondary hydroxyl group is higher than that atthe primary hydroxyl groups when using12T-model zeolite as the catalyst. Acrolein can bedirectly produced by the dehydration of1,3-dihydropropylene with low reaction barrier, whilehydroxy-acetone can be produced by direct dehydration of glycerol at the primary hydroxylgroup instead of by the dehydration of glycerol and the subsequent isomerization of2,3-dihydropropylene. Namely, the C-O bond cleavage at the primary site is accompaniedwith the isomerization.Due to the time limitation, only preliminary results were obtained for the hydrogenationof glycerol about selecting appropriate the computational method. Calculated split energiesfor Ni3-cluster in different multiplicity with different level of theory was compared with thatfrom the Gaussion-3method, and it was found that M06-2X/m6-31G*gives close results. Thesame methods were used to study Ni4and Ni5clusters, and it was found that the heptet stateand the quindruplet state have the lowest energy, respectively, in all the spin states examined.