Highly selective catalytic combustion of acrylonitrile towards nitrogen over Cu-modified zeolites

Abstract

A series of Cu (5 wt%) modified microporous zeolites (ZSM-5, Beta, MCM-22, MCM-49, and Y) was investigated for selective catalytic combustion of acrylonitrile (C2H3CN-SCC), associated with N2 yield order of Cu-ZSM-5 (SiO2/Al2O3 = 26) > Cu-MCM-49 > Cu-MCM-22 > Cu-Beta > Cu-Y. Various characterizations of XRD, N2 adsorption, H2-TPR, NH3-TPD and XPS were employed to reveal structure-activity relationship. It was suggested that (i) isolated Cu cation acting as active center played a major role in C2H3CN-SCC; (ii) lowering SiO2/Al2O3 ratio could enhance ion-exchanging capacity and thereby improve the zeolitic catalytic behavior; (iii) Cu-ZSM-5 (SiO2/Al2O3 = 26) possessed the largest amounts of Cu cations as well as the highest Cu2+ → Cu+ redox ability, being responsible for its excellent catalytic behavior. C2H3CN-SCC mechanism over Cu-ZSM-5 was further investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) and density functional theory (DFT) calculation. C2H3CN-SCC mainly followed an oxidation mechanism via intermediate of NCO in absence of H2O; however, after an introduction of H2O hydrolysis mechanism was dominated involving NH3 intermediate. DFT simulation majorly focusing on NCO formation mechanism reveals that C2H3CN could be initially adsorbed on active center Cu of Cu-ZSM-5 through its N, associate with atomic C of CN being adsorbed with a dissociated atomic O. Such kind of adsorption mode greatly favored C2H3CN bond fracture to produce NCO, of which the energy barrier was calculated to be 17.0 kcal mol−1. Mulliken charge transfer (CT) analysis was further conducted, based on which the electronic properties of active center Cu, zeolite framework and dissociated atomic O were well illustrated.