(603b) Synthesis of ZSM-5 Nanoparticles Encapsulated within an Ultrathin Silicalite-1 Coating of Tunable Thickness

Authors: 
Ghorbanpour, A., University of Houston
Gumidyala, A., University of Oklahoma
Grabow, L. C., University of Houston
Crossley, S., University of Oklahoma
Rimer, J. D., University of Houston

The design of catalytic materials with spatially-controlled chemical composition has potential advantages for applications that span energy conversion to chemicals. Here, we present a novel method for preparing a core-shell aluminosilicate zeolite with continuous translational symmetry of nanopores and an epitaxially grown shell of tunable thickness that passivates Brønsted acid sites associated with framework aluminum on exterior surfaces [1]. For this study, we selected the commercially relevant MFI framework type and prepared core-shell particles consisting of a catalytically active aluminosilicate core and an inert siliceous shell (ZSM-5@silicalite-1). Transmission electron microscopy and gas adsorption textural analysis confirmed that silicalite-1 forms an epitaxial layer on ZSM-5 crystals without blocking pore openings, which is a major advantage of our synthetic protocol over other alternative techniques that have been used to passivate zeolite surfaces. Through the investigation of silicalite-1 and ZSM-5 solution chemistry, we approximated the solubility of MFI crystals and devised a procedure to tune the passivating shell thickness. Scanning electron microscopy and dynamic light scattering were used in combination to confirm that the shell thickness can be tailored with nanometer resolution (e.g., 5 – 20 nm). The ability to achieve ultrathin overlayers of silicalite-1 reduces mass transfer resistance of molecule diffusion through the passivation layer. X-ray photoelectron spectroscopy and temperature programmed desorption measurements revealed the presence of a siliceous shell, while probe reactions using molecules that were either too large or adequately sized to access MFI pores confirmed the uniform shell coverage. The synthesis of ZSM-5@silicalite-1 offers a pathway for tailoring the physicochemical properties of MFI-type catalysts wherein surface passivation can enhance product selectivity without sacrificing catalyst activity. This method may prove to be a general platform for core-shell design with potentially broader applicability to other zeolite frameworks.

[1] Arian Ghorbanpour, Abhishek Gumidyala, Lars C. Grabow, Steven P. Crossley, Jeffrey D. Rimer, Epitaxial growth of ZSM-5@silicalite-1: a core-shell zeolite designed with passivated surface acidity, ACS Nano 9 (2015) 4006-4016.