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2D Surface Structures in Small Zeolite MFI Crystals Andrew R. Teixeira, Xiaoduo Qi, Wm. Curtis Conner, T J Mountziaris, Wei Fan, and Paul J. Dauenhauer Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b01046 • Publication Date (Web): 12 Jun 2015 Downloaded from http://pubs.acs.org on June 16, 2015
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Chemistry of Materials
2D Surface Structures in Small Zeolite MFI Crystals Andrew R. Teixeira1, Xiaoduo Qi1, Wm. Curtis Conner1, T.J. Mountziaris1, Wei Fan1,3, Paul J. Dauenhauer,2,3*
1
University of Massachusetts Amherst, Department of Chemical Engineering, 686 N. Pleasant Street, Amherst, MA 01003, USA. 2 University of Minnesota, Department of Chemical Engineering and Materials Science, 421 Washington Ave. SE, Minneapolis, MN 55455, USA. 3 Catalysis Center for Energy Innovation, University of Delaware, 150 Academy Street, Newark DE 19716 *Corresponding Author:
[email protected] Abstract. Utilization of new hierarchical zeolites comprised of small crystallites (99.9%) of surface pores must be blocked (popen < 10-3) to justify the observed surface permeation rate of hydrocarbon adsorbates.
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1.0 Introduction. Zeolites are an important class of microporous materials with applications in catalysis13
and separations (membranes)4,
5
for energy and materials industries. With well-defined networks of
small diameter pores (dpore less than 2 nm) and finely controlled active sites, zeolites provide unique capability for dense catalytic materials with tunable activity. Moreover, tunable framework-dependent pore networks allow for molecular sieving with high selectivity based on adsorbate/pore size exclusion. Optimal processing of chemical species with zeolites for catalysis and separations relies on maximizing molecular movement within micropores of crystalline particles. An emerging approach to faster molecular transfer through zeolite membranes or catalysts is based on introduction of secondary mesoporosity in microporous materials. New micro- and mesoporous materials are now being produced with length scales approaching that of a single unit cell (pillared,6 nanosheets,7 membranes,8-10 etc.) as shown in Figure 1. These structures employ a hierarchy of pores, including micropores (well ordered, 0.4-2 nm), mesopores (ordered or disordered networks, 2-50 nm), and macropores comprised of interstitial void spaces. For example, zeolite particles as large as one micron utilize three-dimensionally ordered mesoporous imprinted (3DOm-i) design consisting of 35 nm MFI crystals which are close packed generating interstitial mesopores of six to nine nanometers.11,
12
Smaller structures include self-pillared pentasil (SPP) consisting of only a single unit cell.6 A decrease in the apparent distances across particles translates to shorter distances molecules must travel to traverse membranes or access catalytic active sites. Despite smaller crystalline domains, the catalytic and adsorption performance of new hierarchical zeolites is dominated by their surfaces. Ordered and disordered mesopores within hierarchical zeolites and single nanosheets have been tested for a number of catalytic reactions.13-16 However, catalytic enhancement is not necessarily achieved from the presence of mesoporosity.17 Introduction of mesopores has improved but not led to the predicted rate of enhancement in molecular transport; mesopores can facilitate reduction in diffusional time constants, but transport is ultimately limited by the structure of particle surfaces.12 The importance of zeolite surface structures at the nanometer scale is not unexpected. As stated by Tsapatsis and co-workers, “Information on the external surface structure…may also be useful in understanding catalytic and adsorption properties, especially when crystal sizes approach nanoscale dimensions and the role of external surface becomes of increased significance.”18 The kinetic influence of surfaces has been observed via classical uptake/release experiments19-21 and interference microscopy.22 Recently, the kinetics of surface permeation in silicalite-1 (MFI) have been shown experimentally to dominate in small particles (
