Experimental Evidence for the Relationship between Al Site Distribution and Catalytic Performance in Methanol-to-Olefins Reaction over ZSM-5 Zeolite

Abstract

The designed synthesis of zeolite catalysts, especially in the control of the framework structure and heteroatom distribution, offers advantages to their catalytic performance. In the methanol-to-olefins reaction, the shape of intermediate species defined by the reaction space surrounding an acid site impacts the reaction mechanism and, in turn, the product selectivity. However, because of the difficulty in observing the active intermediate species, presenting a clear relationship between the catalytic site (i.e., Al atom) distribution and the catalytic property is challenging. Herein, we experimentally demonstrate a structure–catalytic performance relationship between the Al distribution in the ZSM-5 zeolite and the behavior of the methanol-to-olefins reaction, particularly in regard to propylene formation, by isotopic labeling experiments. Two ZSM-5 zeolite samples, which were synthesized with different organic structure-directing agents, were shown to possess different Al distributions based on constraint index measurements. Although their catalytic properties in the methanol-to-olefins reaction in terms of conversion and selectivity were almost identical, the transient responses of the insertion of 13C into propylene fragments showed contrasting results. The results successfully revealed via an experimental approach the relationship between the zeolite design and the catalytic performance.

Synthetic Placement of Active Sites in MFI Zeolites for Selective Toluene Methylation to para-Xylene

“…MFI is a low-symmetry framework (orthorhombic phase) with 12 crystallographic distinct T-sites and 26 distinct oxygen atoms (O f ) located around 10-membered ring (10-MR) straight and sinusoidal channels (∼0.55 nm diameter) or channel intersections (∼0.70 nm diameter, Figure A,C). , The conventional organic SDA (OSDA) used to crystallize MFI is tetra- n -propylammonium (TPA + ), which contains a quaternary N + center that is occluded in channel intersections (Figure D) and confers energetic preference to position Al in T-sites closest to this N + center . This suggests that altering the molecular structure of the SDAs and how they occlude within porous zeolite voids can bias Al siting in the lattice, as has been inferred for MFI synthesized using different organic molecules from changes in heuristic probe reactions (e.g., constraint index) used to assess the local topology around active sites. − However, a mechanistic link between SDA structure and Al siting and, in turn, transition state stability for MFI, is needed to fully realize the potential of designing SDAs that can place active sites at distinct environments, which stabilize desired transition states and reactive intermediates, and allow synthesizing, a priori , zeolites of tailored reactivity. − …”