Selective Catalytic Oxidation Of Hydrogen Cyanide Over Metal Modified ZSM-5 And Beta Zeolite Catalysts:A Quantitative Calculation Study

Present work investigated the mechanism insight into HCN selective catalytic oxidation over ZSM-5 and Beta zeolite and explored the relationship between the structure of active sites and the catalytic activity,based on Density Functional Theory(DFT)over 20T-ZSM-5 models and 24T-Beta models constructed by Gaussian 09 suite of programs.The reaction mechanism simulations suggest that HCN selective catalytic oxidation includes two kinds of reaction mechanisms:hydrolysis mechanism generating NH3 through intermediate of HNCO and oxidation mechanism producing N2 through NCO intermediate.The detailed steps of HNCO hydrolysis mechanism were as follows:(i)HCN+O?HNCO,(?)HNCO + O ? NH + CO2,(?)NH + H2O ? O + NH3;NCO oxidation mechanism were detailed as follows:(?)HCN + 2O?NCO +OH,(?)NCO + O? NO + CO.(?)NCO + NO? N2 + CO2.For Cu-ZSM-5,the single active site structure is favorable for the formation of intermediate HNCO;the double active site synergistic structure and the double nucleus structure contribute to the formation of intermediate NCO.For Cu-Beta,the single active site structure and the double-core structure contribute to the formation of intermediate HNCO,and the double active site synergistic structure is favorable for the formation of intermediate NCO.By comparing the DFT energy calculation results of the three active sites over Cu-ZSM-5 and Cu-Beta in Route ? and Rout ?,it is found that the double active sites synergistic structure over Cu-ZSM-5 and Cu-Beta have the lowest energy barrier,whose catalytic performance is better than that of the single active and dual-core structure.This is due to the fact that when the HCN molecule enters into the channel,it will be subjected by two active sites,which will further contributes to the cleavage of the bond.By comparing the energy barrier of double active sites synergistic structure different paths over Cu-ZSM-5 and Cu-Beta,including Route IB(NCO formation mechanism),Route IIB(NO formation mechanism)and Route IIIB(N2 formation mechanism),it is found that Route IB(10.4 kcal mol-1)is the rate determining step over Cu-ZSM-5;and Route IIB(6.0 kcal mol-1)is the rate determining step over Cu-Beta.It is also found that the energy barrier of double active sites synergistic structure different paths over Cu-Beta are less than that of Cu-ZSM-5,suggesting that the catalytic activity of Cu-Beta is better than that of Cu-ZSM-5 theoretically.The microkinetic analyses of HCN-SCO over Cu-ZSM-5 and Cu-Beta were also investigated in present work.It is found that Z-Cu2b-2O-HCN ?TSB1 of the NCO formation mechanism is the rate determining step over Cu-ZSM-5,and Z-Cu2b-O-NCO?TSB2 of NO formation mechanism over Cu-Beta is the rate determining step.The Mulliken population analysis of three kinds of active sites structure over Cu-ZSM-5 and Cu-Beta were systematically investigated.It is found that(a)the adsorption process is the preactivation process of the molecule.During the adsorption process of HCN molecules,the active sites Cu,the zeolite skeleton and the adsorbed oxygen play a key role in the preactivation of HCN as the charge acceptor;(b)Transition state constitutes the key step for the whole reaction.The atomic C of HCN over Cu-ZSM-5 with the single active site structure accepted much more charge,which greatly facilited H-CN bond fracture,with respect to that of Cu-Beta in StepA2.(c)In StepB2,the C-O2 over Cu-Beta with double active sites synergistic structure accepted much more charge,facilitating the bond fracture of C-O2.