Letter pubs.acs.org/JPCL
Genesis of Delaminated-Zeolite Morphology: 3‑D Characterization of Changes by STEM Tomography Ilke Arslan,*,† John D. Roehling,‡ Isao Ogino,§ K. Joost Batenburg,∥ Stacey I. Zones,⊥ Bruce C. Gates,‡ and Alexander Katz§ †
Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99352, United States ‡ Department of Chemical Engineering and Materials Science, University of California-Davis, One Shields Ave., Davis, California 95616, United States § Department of Chemical and Biomolecular Engineering, University of California-Berkeley, Berkeley, California 94720, United States ∥ Centrum Wiskunde & Informatica, Science Park 123, Amsterdam 1098 XG, The Netherlands ⊥ Chevron Energy Technology Company, Richmond, California 94804, United States S Supporting Information *
ABSTRACT: Zeolite delamination increases the external surface area available for catalyzing the conversion of bulky molecules, but a fundamental understanding of the delamination process remains unknown. Here we report morphological changes accompanying delamination on the length scale of individual zeolite clusters determined by 3-D imaging in scanning transmission electron microscopy. The results are tomograms that demonstrate delamination as it proceeds on the nanoscale through two distinct key steps: a chemical treatment that leads to a swelled material and a subsequent calcination that leads to curling and peeling off of delaminated zeolite sheets over hundreds of nanometers. These results characterize the direct, local, 3-D morphological changes accompanying delaminated materials synthesis and, with corroboration by mercury porosimetry, provide unique insight into the morphology of these materials, which is difficult to obtain with any other technique.
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limited because these static measurements do not capture the dynamic processes that accompany morphology changes during delamination, and such data merely consist of averages over the entire landscape of the material, requiring simplified models for relating integrated gas uptakes to textural properties. Furthermore, methods such as X-ray diffraction crystallography, which rely on long-range order, typically provide relatively little useful information about delaminated zeolites because of the loss of order in the direction perpendicular to the zeolite sheets. To provide direct evidence of how delamination occurs, we turned to 3-D imaging of the material in each stage of the delamination process by using electron tomography23−25 in the scanning transmission electron microscope (STEM).26,27 A previous news and views publication highlighting technique developments in electron tomography25 included images illustrating each stage of delamination of these materials. Here we present the full set of data and interpret results characterizing 3-D imaging of zeolite delamination. We were encouraged by the successes of 3-D tomographic imaging for
eolites are crystalline microporous aluminosilicates that find widespread applications as catalysts for petroleum refining and petrochemical conversion,1,2 with the limited access of bulky reactants to intracrystalline zeolite pores offering shape selectivity, yet in a growing number of instances the slow transport of large molecules restricts reactant access to only those sites nearest to the external zeolite surface. To increase catalytic activity in these instances, and in prospect also to facilitate shape selectivity via nest effects,3,4 researchers have sought to facilitate access to interior zeolite space and increase zeolite external-surface-to-volume ratios5 by synthetic strategies to form: extra-large pore zeolites,6,7 single-unit-cell zeolite nanosheets,8 hierarchically nanoporous zeolite-like materials,9,10 self-pillared zeolite nanosheets,11 mesoporous zeolites,12−14 and delaminated zeolite-precursor materials.15−20 Delamination is particularly attractive because it not only reduces the transport length to micropores but also provides increased external surface so that bulky molecules that react only at pore mouths can be more rapidly converted.5,21,22 Thus, the catalytic activity of the delaminated material may be several times greater than that of a conventional 3-D zeolite because of the increased exposed surface area and potentially even more if intracrystalline mass transfer resistance is significant. Conventional methods for characterizing delamination of zeolites by determining surface areas and pore volumes are © 2015 American Chemical Society
Received: May 15, 2015 Accepted: June 15, 2015 Published: June 15, 2015 2598
DOI: 10.1021/acs.jpclett.5b01004 J. Phys. Chem. Lett. 2015, 6, 2598−2602
Letter
The Journal of Physical Chemistry Letters characterizing other mesoporous structures in zeolitic materials.28 We investigated the delamination of MCM-22 (P)16 having a Si:Al atomic ratio of 46.8, which remained constant throughout the delamination. (These are the same materials as reported in ref 16.) This precursor material was chosen because of its layered morphology. Outstanding questions addressed in our work are the following: (i) in what stage in the process does lamellar separation occur and (ii) what morphological changes in three dimensions accompany delamination? Although zeolite frameworks readily undergo structural damage that converts crystalline to amorphous material under the influence of an electron beam,29−31 the results presented here are not affected by such damage because they are not related to changes on the length scale of individual zeolite pores (
