K+ located in 6-membered rings of low-silica CHA enhancing the lifetime and propene selectivity in MTO†
Methanol-to-olefins (MTO) technology presently serves as a key to convert coal or natural gas to valuable hydrocarbons, in particular lower olefins. This work focuses on developing a low cost and environmentally benign MTO catalyst from low-silica CHA zeolite. Its moderate acidity and reaction space have been balanced by tuning the residual K+ cations in the cavity via controlled ammonium ion-exchange. The structure-dependent catalytic performances have been assessed, summarized and discussed by using various characterization techniques. The crystalline structure and crystal morphology of CHA were well preserved during the NH4+ exchange. N2 physisorption demonstrated that the pore volume and specific surface area increased substantially after K+ cations were exchanged as a result of pore opening. NH3 temperature-programmed desorption (TPD) and NH3-adsorption IR spectra showed that the amount and strength of acid sites were regulated by the exchange level of K+ with NH4+, which was closely related to the K+ location according to NO and acetonitrile adsorption IR spectra. The completely exchanged CHA (>98% K+ exchange level) deactivated rapidly in MTO at 400 °C, whereas CHA (65% K+ exchange level) with moderate acidity showed the best catalytic performance. The moderate acidity in the open cage facilitated the formation of higher methylbenzene intermediates, giving propylene (45%) as the predominant product.