Theoretical Study On The Active Site Structure Of Ni/MTW And Zn/MFI Zeolites

The part one:the catalytic properties of molecular sieves can often be improved by the modification of transition metals.Nickel is one of the most widely used transition metals in hydrogenation and dehydrogenation catalysts.Dispersion correction functional theory is used to calculate a nickel and two nickel atoms to HAl ZSM-12molecular sieve adsorption,the results show that whether a nickel atoms or clusters Ni₂adsorption on the molecular sieve,molecular sieve on the atomic hydrogen atoms are transferred to the first nickel,nickel for a single atomic adsorption,will eventually release the hydrogen reduction into hydrogen on the molecular sieve,molecular sieve B acid sites decreases,but not Ni₂ clusters.The adsorption energy of Hydrogen before and after modification of HAl ZSM-12showed that the adsorption capacity of nickel modified HAl ZSM-12 molecular sieve to hydrogen in the reaction atmosphere was stronger.Hydrogen molecules attach to the zeolite and dissociate into two free hydrogen atoms that do not function as acid sites.The adsorption energy of ammonia gas indicated that nickel modified molecular sieve would weaken the acidity of Lewis Al site,but Ni atom could be used as strong Lewis acid site.Since electrons are transferred from nickel atoms to hydrogen atoms,hydrogen in the reaction atmosphere is adsorbed on the nickel-modified zeolite to enhance the acidity of Lewis Ni site.The part two:HAl ZSM-5 molecular sieves modified by metal zinc can significantly improve the conversion and selectivity of aromatics from low alkane chain hydrocarbons.All possible structures of Zn-Al zeolite were optimized to determine the structure of zeolite.There are three mechanisms by which water vapor in the reaction atmosphere can remove the active sites of HZn Al O₃MFI:The first one is that four water molecules are adsorbed on HZn Al O₃MFI,and the HZn Al O₃MFI is removed in one step to form HZn Al O₃MFI cluster（Zn Al（OH）₅）and H₄O₂MFI,and then H₄O₂MFI reacts with Si（OH）₄ to form pure silicon MFI molecular sieve.The second mechanism is HZn Al O₃MFI adsorption first three water molecules,the aluminum loci on the molecular sieve hydrolysis produce aluminum hydroxide,aluminum hydroxide adsorption on H₄Zn O₃MFI molecular sieve first,and then attached to the outside of the molecular sieve,H₄Zn O₃MFI molecular sieve adsorption again a water molecule to zinc loci hydrolyzed into hydrogen zinc oxide on the molecular sieve adsorption first,and then take off the attached form H₄O₂MFI zinc oxide and hydrogen,finally H₄O₂MFI and Si（OH）₄ reaction generated pure silicon MFI molecular sieve;The third reaction mechanism is HZn Al O₃MFI adsorbed water molecules 1 first,the zinc on the molecular sieve loci hydrolysis zinc oxide generated hydrogen molecules,zinc oxide first hydrolysis of hydrogen adsorption on HAl O₂MFI molecular sieve,and then attached to the outside of the molecular sieve,HAl O₂MFI re-adsorption three water molecules will aluminum loci hydrolysis into aluminum hydroxide on the molecular sieve adsorption,then stripping form H₄O₂MFI and aluminum hydroxide,finally H₄O₂MFI and Si（OH）₄ reaction generated pure silicon MFI molecular sieve.Due to the unstable cavity formed by dealumination,it is easy to combine with silicon hydroxide to form pure silica molecular sieves.Then,we further studied the evolution of the structure of molecular sieves containing holes and gave a total of 13possible structures of molecular sieves.In the end,the desorbed small molecule structure was discussed,and it was found that alumina,zinc hydroxide and silicon hydroxide formed corresponding oxide cells at low temperature,which was thermodynamically feasible,and alumina and zinc oxide cells could combine to form zinc-aluminum spinel.Using DFT and Turbomole to calculate the total energy of the molecular sieve is the most stable structure and gibbs free energy,to determine the various mechanisms involved in the reaction equation of energy change and equation of gibbs free energy change,finally make the gibbs free energy change and the reaction temperature and pressure diagram,determine the reaction temperature and pressure of reaction equation of the influence of gibbs free energy change.