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44756-AC5
Functionalization and Application of Hollow Zeolite Structures

Sarah C. Larsen, University of Iowa

Zeolites present a unique opportunity for forming hollow structures with controlled porosity.  Hollow zeolite structures are formed using mesoporous silica as a sacrificial template.  The hollow zeolite structures consist of a shell that has the characteristic porosity and ion-exchange properties of the parent zeolite and interior and exterior surfaces that have terminal silanol groups which can be readily functionalized using organosilane reactions.  In this project, we have developed strategies for functionalizing the interior and exterior surfaces of the hollow zeolite structures and the zeolite shell.  The first strategy features functionalization of the mesoporous silica template with transition metals, such as copper and vanadium, or organosilanes, such as, aminopropyltriethoxysilane (APTES), in order to prepare hollow zeolite structures with encapsulated transition metals or organic functional groups.  The second strategy involves post-synthetic modification of the zeolite hollow structures through transition metal ion-exchange of aluminosilicate hollow shell.  The functionalized hollow zeolite structures are extensively characterized using spectroscopic techniques such as magnetic resonance.  The two synthetic strategies were then combined to prepare bifunctional zeolites.  These functionalized zeolite structures have potential applications in catalysis and adsorption of heavy metals for environmental remediation applications.

Transition metals (copper and vanadium) were incorporated into hollow ZSM-5 tubes using an encapsulation process and in a post-synthesis ion-exchange.  The encapsulated transition metal-containing ZSM-5 tubes have decreased surface areas relative to the parent and ion-exchanged ZSM-5 tubes, decreased amounts of incorporated transition metal relative to the post-synthesis ion-exchanged ZSM-5 tubes and an impurity phase as observed in the XRD patterns.  The decreased amounts of transition metal for the encapsulation method is related to the amount of transition metal incorporated into the mesoporous silica template.  The decreased surface areas for the encapsulated transition metals suggest that these transition metals are located in the ZSM-5 pore structure rather than exclusively on the internal surface.  Insight into the local electronic environment of the Cu²⁺ and VO²⁺ in the ZSM-5 tubes was obtained from Electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS).

Similar to the procedure for preparing transition metal encapsulated zeolite tubes, aminopropyltriethoxysilane (APTES) was incorporated into silicalite hollow tubes.  The MS template was functionalized with APTES using a well-known method for organic functionalization of MS materials.  The surface area of the APTES functionalized silicalite tubes was 124 m²/g compared to approximately 240 m²/g for the unfunctionalized silicalite tubes, representing a significant decrease in surface area.  This decrease in surface area has been observed previously for organic functionalized zeolites and is attributed to restricted access to the pores caused by the organic functional groups as is likely the case here. ADDIN EN.CITE <EndNote><Cite><Author>Song</Author><Year>2005</Year><RecNum>486</RecNum><record><rec-number>486</rec-number><ref-type name="Journal Article">17</ref-type><contributors><authors><author>Song, W.</author><author>Woodworth, J. F.</author><author>Grassian, V. H.</author><author>Larsen, S. C.</author></authors></contributors><titles><title>Microscopic and macroscopic characterization of organosilane-functionalized nanocrystalline NaZSM-5</title><secondary-title>Langmuir</secondary-title></titles><periodical><full-title>Langmuir</full-title></periodical><pages>7009-7014</pages><volume>21</volume><number>15</number><dates><year>2005</year><pub-dates><date>Jul 19</date></pub-dates></dates><accession-num>ISI:000230507400049</accession-num><urls><related-urls><url>&lt;Go to ISI&gt;://000230507400049  </url></related-urls></urls></record></Cite></EndNote>  The functionalized materials were characterized by ²⁹Si NMR, thermal gravimetric analysis (TGA) and zeta potential.

In the final step, bifunctional zeolites and zeolite structures were prepared with both a transition metal (iron) and an organic functional group (APTES) incorporated into the zeolite or zeolite structure.  The bifunctional hollow structures and zeolites (Fe, APTES) were extensively characterized using the methods described above (XRD, BET, TGA, NMR, EPR, zeta potential, XPS).

The focus of the final reporting period was on evaluating functionalized nanocrystalline zeolites and zeolite structures for adsorption of copper and chromate from aqueous solution.  The APTES functionalized hollow structures and the APTES functionalized nanocrystalline zeolites exhibited enhanced adsorption capacity for chromate and copper in aqueous solution relative to conventional micron sized zeolites.  The enhanced adsorption was attributed to the increased capacity of these materials for functionalization.  Studies on evaluating the adsorption of a contaminant such as chromate followed by photoreduction of the contaminant using the bifunctional zeolites are currently in progress.

To summarize, hollow zeolite structures were selectively functionalized with transition metals and/or an organosilane on the interior surface of the hollow structure and in the zeolite shell.  These materials were evaluated for adsorptive properties and show enhanced adsorption capacity compared to conventional zeolites.  Studies are in progress to evaluate the bifunctional zeolite materials.

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