(216c) Control of the Location of Framework Al Atoms in ZSM-5 Zeolite
The acidic properties of aluminosilicate-type zeolites originate from the presence of protons balancing the negative charge induced by the framework Al in tetrahedral sites (T-sites). The catalytic properties of zeolites depend on many factors such as pore structure, acid strength and acid amounts. In addition these factors, recently, the location and distribution of Al atoms in the zeolite framework has been recognized as an important factor for activity and selectivity, because they would profoundly affect the accessibility of molecules to acid sites and the spatial constraints of the reaction field in the pores; however, their control is difficult to achieve. Furthermore, the relationship between Al distribution and the acid strength has not been fully understood. Thus, many zeolite researchers have seriously tackled the estimation of the distribution of framework Al atoms as well as the control of the location of acid sites in the pores.
ZSM-5 zeolite with the MFI topology, which is one of the most widely studied and commercially important zeolites, has twelve distinct T-sites and consists of parallel and straight 10-membered ring (MR) channels intersected by sinusoidal 10-MR channels. Here we report the synthesis of the ZSM-5 catalysts with the Al distribution in the framework controlled and their catalytic performance in the MTO reaction.
The ZSM-5 zeolites were hydrothermally synthesized with tetrapropylammonium cation (TPA+) in the presence or absence of Na+. Thus prepared ZSM-5 samples were denoted by [TPA,Na]- and [TPA]-HZSM-5, respectively. Considering the size of TPA+, it would only be located at the intersections of the MFI structure. Hence, we assumed that the acid sites in ZSM-5 synthesized with TPA+ in the absence of Na+ are selectively located solely at the intersections, and that the acid sites in ZSM-5 synthesized with TPA+ and Na+ are located not only at the intersections but also in narrow straight and/or sinusoidal channels.