Study On The Reaction Mechanism Of Nitrogen-containing Compounds In Heavy Oil Upgradation With Supercritical Water

Heavy oil supercritical water upgradation technology has received extensive attention,and the denitrogenation of it is remarkable.However,the mechanism of denitrogenation in supercritical water and the roles of supercritical water are not clear.In this paper,we use Gaussian 09 quantum chemistry calculation software to studied isoquinoline and benzylamine dentrogenation mechanism in supercritical water at M06/6-311G(d,p)and M06-2X/6-311+G(d,p)level.Isoquinoline is the nitrogen-containing heterocyclic model compound and has three main reaction pathways in the form of ring-opening,saturation and denitrogenation.3 pathways obtained 2-(2-oxoethyl)benzaldehyde and ammonia eventually.The water molecule first attacks the extended ? bond of the aromatic ring by the addition mechanism,and there are two attack position at 1C-2N and 2N-3C.Then,1C-2N addition reaction followed by " saturation after ring-opening" and "ring-opening after saturation" two reaction pathways,but 2N-3C addition reaction followed by " ring-opening after saturation “,only one reaction pathway.Finally,the N atom is removed by elimination mechanism.The rate-limiting step of the three pathways are all N-position addition reaction.the energy barrier of 1C-2N addition reaction is 52.7 kcal/mol,and energy barrier of 2N-3C addition reaction is 60.1 kcal/mol.Benzylamine is the nitrogen-containing non-heterocyclic model compound and has only one reaction pathway in the form of dehydrogenation,denitrogenation.In this reasearch,it is believed that the H atom on 2C position of benzylamine will be transferred to the N atom to form a pentavalent N atom,and then dehydrogenate at the N position,but not dehydrogenate on the C-N bond directly.After dehydrogenation,the imine will be quickly saturated with water molecules.Finally,N atoms removes by elimination mechanism,and to form benzaldehyde.In addition,benzylamine can easily condense with benzaldehyde to achieve benzylidenebenzylamine,which is consistent with the experimental results.Benzylidenebenzylamine is extremely difficult to converse to amide,but can be hydrolyzed to benzylamine and benzaldehyde.The rate-limiting step of this pathway is the transfer reaction of benzylamine 2C-position H atom,and the catalytic energy barrier is 47.6 kcal/mol.Supercritical water exists as short chain water cluster in the system,which can help build transtate structure and reduce energy barrier,and has remarkable catalytic effect.With one molecular water catalysis,the energy barrier of 1C-2N addition reaction decrease by 38.7%,while the energy barrier of 2N-3C addition reaction decrease by 32.6%,which promote the degradation of isoquinoline in supercritical water.Energy barrier of H atom transfer reaction in 2C position of benzylamine decrease by 35.7%,to prove that benzylamine denitrogenation with synergistic mechanism in supercritical water.