2D Zeolite Coatings: Langmuir Schaefer Deposition of 3nm Thick MFI Zeolite Nanosheets

2D Zeolite Coatings: Langmuir Schaefer Deposition of 3 nm Thick MFI Zeolite Nanosheets

Meera Shete¹, Neel Rangnekar¹, Benjamin Stottrup², Michael Tsapatsis¹

¹Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota

²Department of Physics, Augsburg College, Minneapolis, Minnesota

Two-dimensional zeolites are nanosheets that have thickness comparable to the unit-cell dimensions of the corresponding structure type.¹ Typically, their thickness is in the range of few nanometers while the lateral dimensions are in the range of few hundred nanometers, thus imparting them with a high aspect ratio. This characteristic opens up exciting opportunities for traditional uses in catalysis and separations and holds promise for emerging applications of zeolite thin films as membranes, low dielectric constant materials and anti- corrosion coatings.

Fabrication of thin films of 2D zeolites relies on 1) the availability of stable zeolite suspensions that are free of amorphous and non-exfoliated contaminants and 2) development of deposition techniques by which zeolite nanosheets can be transferred to various supports to form thin-oriented coatings. We synthesized stable suspensions of 1.5 unit cell thick (3 nm) MFI zeolite nanosheets in octanol by exfoliating layered MFI zeolite² using a polymer-melt compounding technique followed by purification using density gradient centrifugation.^3,4 Suspensions in octanol were used to form nanosheet deposits on porous supports by filtration. Deposition by filtration is limited only to porous supports. Moreover it is not possible to obtain monolayer coatings using the filtration approach.

To overcome this limitation, we investigated the application of Langmuir Schaefer deposition technique for the fabrication of MFI nanosheet coatings.⁵ Stable suspensions of MFI nanosheets were prepared in ethanol following an acid treatment procedure, which partially removed the associated structure directing agent. Nanosheets in ethanol demonstrated high surface activity and could be dispersed at the air-water interface to form a monolayer. This monolayer could then be transferred to silicon wafers using the Langmuir Schaefer deposition technique to form coatings with different extent of packing from sparsely packed to close packed. A control over coating thickness was obtained using a layer-by-layer deposition technique. In-plane X-ray diffraction (XRD) of the MFI coatings confirmed contraction of the nanosheets after calcination. The monolayer coatings were intergrown using different methods of secondary growth. Preferentially oriented, intergrown thin films of MFI with different microstructures were obtained which had sub-12 nm thickness in certain cases. In-plane XRD of the intergrown MFI films did not show any contraction after calcination, indicating that the films were well intergrown and strongly attached to the substrate.

The precise control over the thickness and orientation of the zeolite films opens new opportunities to investigate their transport, adsorption, dielectric and mechanical properties with greater accuracy.

(1)      Tsapatsis, M. AIChE J. 2014, 60, 2374–2381.

(2)      Choi, M.; Na, K.; Kim, J.; Sakamoto, Y.; Terasaki, O.; Ryoo, R. Nature 2009, 461, 246–249.

(3)      Varoon, K.; Zhang, X.; Elyassi, B.; Brewer, D. D.; Gettel, M.; Kumar, S.; Lee, J. A.; Maheshwari, S.; Mittal, A.; Sung, C.-Y.; Cococcioni, M.; Francis, L. F.; McCormick, A. V; Mkhoyan, K. A.; Tsapatsis, M. Science 2011, 334, 72–75.

(4)      Agrawal, K. V.; Topuz, B.; Jiang, Z.; Nguenkam, K.; Elyassi, B.; Francis, L. F.; Tsapatsis, M. AIChE J. 2013, 59, 3458–3467.

(5)      Rangnekar, N.; Shete, M.; Agrawal, K. V.; Topuz, B.; Kumar, P.; Guo, Q.; Ismail, I.; Alyoubi, A.; Basahel, S.; Narasimharao, K.; Macosko, C. W.; Mkhoyan, K. A.; Al-Thabaiti, S.; Stottrup, B.; Tsapatsis, M. Angew. Chemie Int. Ed. In Press 2015 (DOI: 10.1002/anie.201411791).