Catalytic Combustion Of CH₂CHCN And Research Of Reaction Mechanism Via Quantum Chemistry Calculation Method

Cu/ZSM-5 exhibited a superior catalytic performance [100% C2H3CN (AN) conversion and～96% N2 yield at T> 350 ℃] during selective catalytic combustion (SCC) of AN. The related AN-SCC mechanism was systematically investigated by in situ diffuse reflectance infrared Fourier transform spectra (in-situ DRIFTS) and density functional theory (DFT). NCO was found to be one kind of vital intermediate during AN-SCC based on in-situ DRIFTS. Then, the DFT was employed to give a deeper insight into how NCO was generated and eventually turned into the final product of N2, wherein three kinds of models were constructed during DFT simulations: 20T-O-Cu-Z20,5T-O-Cu-Z5 with [CuO]+ acting as the active site;and 5T-Cu-Z5 with [Cu]+ acting as the active site. It was revealed that (i) the AN could be initially oxidized into NCO and CO2 over [CuO]+ active site after overcoming an energy barrier of 17.0 kcal mol-1 (Route Ⅰ); (ii) thereafter, three kinds of reaction pathways were proposed for the further reaction of NCO to produce N2, including NCO direct dissociation (NCO→>N+COof Route Ⅱ, 100.9 kcal mol-1), NCO coupling reaction (2NCO→N2+2CO of Route III, 48.7 kcal mol-1), and NO+NCO reaction (NO+NCO→N2+CO2 of Route IV-V,13.9 and 0.8 kcal/mol); the third reaction pathway possessing the lowest energy barrier was believed to be a reasonable reaction route to produce N2; (iii) finally, the N2O generation mechanism during AN-SCC was also simulated by DFT, which revealed that N2O could also be generated by NO+ NCO reaction in a different way (Route VI); the related energy barrier (31.0 kcal mol-1) was much higher than that of N2 generation mechanism, which quantitatively explained high N2 selectivity of Cu/ZSM-5 during AN-SCC.Thereafter, based on DFT calculation, after compared AN-SCC mechanism over Co/ZSM-5 and Cu/ZSM-5, it could be concluded thatenergy barrier of each steps over Co/ZSM-5 is higher than that of Cu/ZSM-5,which illustrates the reason why it is easier to generate N2over Cu/ZSM-5. After compared AN-SCC mechanism over Cu/SSZ-13and Cu/ZSM-5, it could be concluded that energy barrier of each steps over Cu/SSZ-13 is higher than that of Cu/ZSM-5, which explained the reason why the conversion rate of AN-SCC over Cu/ZSM-5 is higher than that of Cu/SSZ-13.From the comparison of the reaction rate constantduring AN-SCC over Cu/ZSM-5, the first transitionn state (TS1) of AN-SCC is the rate limiting step, the resultis consistent with what was concluded during energy barrier analaysis in chapter 4. Via charge transfer study of key steps during AN-SCC over Cu/ZSM-5, it could be concluded that:(i) for adsorption step, the rule of charge transfer is:adsorbed molecule→active sites and zeolite framework; (ⅱ) for transition state reaction, the rule of charge transfer is not clear.