Determination of mass transfer behavior of typical products of MTO (methanol to olefins) reactions over HZSM-5 zeolite

Methanol conversion to olefins (MTO) catalyzed by zeolite catalysts is a typical diffusion dominated reaction process. In this paper, the diffusion behavior of several typical product molecules (ethylene/ethane, propylene/propane, benzene) on a HZSM-5 zeolite was systematically studied by using Frequency Response method. The results show that the mass transfer regularity of the product molecules have been successfully determined by the Frequency Response method. It is confirmed that the diffusion rates of C2 and C3 hydrocarbon molecules within the HZSM -5 micropores are similar, but the effects of the surface resistance are different. So, the C2 molecules can freely go in and out of the channels of the HZSM-5 zeolite, while the diffusion of C3 molecules is significantly affected by the channel diffusion limitation. In addition, the diffusion rate of benzene molecules is observably lower than that of C2 and C3 molecules, and the resistant effects of benzene molecules caused by the zeolite crystal surface are not serious. The conclusions obtained in this study can be used to explain the product selectivity of MTO reaction over HZSM-5 zeolites and the coking mechanism of the catalyst, and provide the mass transfer theoretical guidance for the preparation of the MTO catalysts with excellent performance.