Fundamental And Applied Studies Of Highly Active Nano-nickel Catalysts

Hydrogenation reactions are important for the synthesis of many chemical products,such as environmentally benign plasticizer,pharmaceutical intermediates,food additives,spices and so on.In this work,highly active nano-Ni catalysts?Ni/Al₂O₃,Ni/MgAlO,Ni/MgO and Ni/SiO2?were prepared by the co-precipitation method with an n-butanol drying process,and were applied to the hydrogenations of dioctyl phthalate?DOP?,bisphenol A,pyridine,acetonitrile and acetone.Microcalorimetric adsorption measurement and infrared spectroscopy?IR?were the main techniques used to study the strengths and states of adsorbed H₂,CO,NH3,CO2,as well as the related reactants and products,on the catalysts,and the results were correlated with the catalytic behavior.Some important results are summarized below.?1?The hydrogenation of DOP was compared to that of toluene by using supported nickel catalysts Ni/Al₂O₃ and Ni/MgO and NCG-6?a commercial Ni catalyst?.While the surface acid/base properties significantly affected the activity of hydrogenation of toluene,such effect was absent for the hydrogenation of DOP.Such different behavior might be due to the different electron charge distributions among the atoms in the two molecules.Thus,the active nickel surface area and pore size of catalysts might be the two major factors affecting the activity of hydrogenation of DOP.The Ni/Al₂O₃ catalyst with high active nickel surface area?70 m2/g?and large pore size?9 nm?exhibited the high DOP conversion and selectivity to Di?2-ethylhexyl?hexahydrophthalate?DEHHP?,which is the benign plasticizer of PVC.?2?The Ni/Al₂O₃,Ni/MgAlO and Ni/MgO were used for the hydrogenation of pyridine to piperidine.It was found that the basic support?MgO?increased the heat of adsorption of CO on Ni,due to the increased surface electron density of Ni,while weakened the strength of H-Ni,which might be a reason for the decreased activity of Ni/MgO for the hydrogenation of pyridine.In contrast,the acidic support?Al₂O₃?caused the electron deficient surface metallic Ni that enhanced the adsorption of H₂ and pyridine,and thus favored the hydrogenation of pyridine.In addition,it was found that pyridine was more strongly adsorbed on Ni than piperidine although piperidine is a stronger base than pyridine,indicating that the electron-enriched aromatic ring of pyridine might be involved in the adsorption of pyridine on Ni,which was favored on the electron deficient surface Ni and acidic support.?3?Ni/MgAlO catalysts were prepared by the co-precipitation method,and doped with K2CO3 via the impregnation technique to form the K-Ni/MgAlO catalysts.It was found that the addition of K2CO3 led to an increase of the heat of adsorption of CO on Ni,due to an increased surface electron density of Ni.The presence of K2CO3 weakened the strength of H-Ni and strengthened the bonding of CH3CN onto Ni by changing adsorptive states of CH3CN,which might be two main reasons for the decreased activity of Ni doped with K2CO3.Furthermore,the addition of K2CO3 decreased adsorptive strength of ethylamine on Ni,resulting in an increase of ethylamine selectivity.Finally,infrared spectra of CH3CN adsorbed on the Ni surface with pre-adsorbed hydrogen indicated that the hydrogenation might occur preferentially on carbon atom in C?N,leading to the formation of surface species Ni-N=CH-CH3 and Ni=N-CH₂-CH3.?4?The hydrogenation of acetone to isopropanol over the Ni/MgAlO was studied with the aim to see if it was possible for the catalyst to be used for the hydrogenation of organic compounds containing oxygen and the effect of water on the hydrogenation reactions of such compounds.It was found that the addition of small amount of water in acetone increased the hydrogenation rate of acetone to isopropanol over the Ni/MgAlO,while the addition of more than 5%H₂O in it decreased the hydrogenation rate of acetone.Microcalorimetic adsorption results showed that the adsorption heat of isopropanol was decreased,while the adsorption heat of acetone was almost not affected on Ni/MgAlO covered partially by water.This might be the reason why small amount of H₂O promoted the hydrogenation of acetone.On the other hand,the heats for the adsorption of acetone,isopropanol and H₂ decreased significantly on Ni/MgAlO covered by more water.This might explain the inhibiting effect of large amount of water on the hydrogenation of acetone.Infrared spectra of isopropanol at room temperature indicated that the dehydrogenation of isopropanol to acetone was inhibited on Ni with pre-adsorbed H₂O,which might be another reason why H₂O might promote the hydrogenation of acetone.In addition,it was found that the pre-adsorbed H₂O inhibited the formation of di-? adsorbed acetone and enolate species on Ni/MgAlO.?5?Co-adsorption of CO,H₂ and C2H4 on Ni/SiO2 was studied by using microcalorimetric measurement and infrared spectroscopy techniques.It was found that the pre-adsorption of H₂ and CO decreased the heats for the adsorption of CO and H₂ on Ni/SiO2.This might be due to that the electron density of adatom H might be transferred to the anti-bonding orbitals of CO,leading to the weakened Ni-H and red-shift of vibrational frequency of linearly bound CO on Ni.In addition,the adatom H might decrease the lateral repulsions among the adsorbed CO molecules,and thus increased its saturation coverage on Ni.Ethylene was dissociatively adsorbed on Ni with the formation of ethylidyne species,leaving some adatom H on the surface.The pre-adsorption of ethylene decreased the heat for the adsorption of H₂ on Ni.Ethylene and CO were found to adsorb?or compete?on the same Ni sites,but pre-adsorbed CO did not seem to affect the formation of ?-and di-? bonded C2H4.