Facile sol-gel synthesis of porous silicas using poly(propylene)imine dendrimers as templates
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Edgar Lotero Department of Chemistry, University of Nebraska—Lincoln, Lincoln, Nebraska 68588-0304
Manuel Marquez Los Alamos National Laboratory, Chemical Science and Technology Division, Los Alamos, New Mexico 87545, and Nanotechnology Laboratory (NanoteK), Research and Development, Kraft Foods Inc., 801 Waukegan Road, Glenview, Illinois 60025 (Received 2 February 2000; accepted 5 June 2000)
Commercially available poly(propylene)imine (DAB-Am-32 and DAB-Am-64) dendrimers were used as single-molecule templates to tailor the porosity of silicas via a nonacidic sol-gel method. X-ray diffraction on both the as-prepared (oven-dried at 373 K) and the calcined (833 K) materials revealed that modest contraction took place on template removal and that the cavities created did not achieve three-dimensional ordering under the current synthesis conditions. Transmission electron microscopy of “Pt-stained” samples supported this picture. A modified Horvath–Kawazoe analysis of the argon adsorption isotherms indicated that DAB-Am-64 is a much more effective template than DAB-Am-32. Pyrolysis and oxidation protocols for template removal are also presented. I. INTRODUCTION
The use of micelles as templates to impart mesoporosity in silicas and aluminosilicates1,2 and other oxides3–8 is well documented. Based on the vast majority of available literature on the subject, it appears that alkylamine micelles are arguably the most suitable candidates for this task. From a fundamental viewpoint, it seems interesting to attempt similar materials syntheses using single polyamine-type macromolecules as porogenic agents. Molecular imprinting is a particularly difficult to task with inorganic substrates. There is some evidence for the imprinting of silica,9 Al3+-doped silicas,10,11 and titania.12 Effective compounds to produce molecular imprints generally have stiff functional groups (aromatic, carbonyl); that is, multiple bonds that hinder internal rotations making the structure rather rigid and thus suitable for imprinting. Templates, on the other hand, are not required to produce molecular recognition sites. Rather, it is the generation of cavities and/or channels of controlled size that is considered sufficient to achieve “template” status. This paper presents and discusses data on the use of poly(propylene)imine (DAB-Am-n) dendrimers as single-molecule porogens for sol-gel-derived silicas.
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J. Mater. Res., Vol. 15, No. 8, Aug 2000 Downloaded: 25 Feb 2015
Based on an extensive literature search, we found no precedent for the use of DAB-Am-n dendrimers to produce silicas with controlled pore sizes. Poly(amidoamine)dendrimers (PAMAMs), on the other hand, have been used for self-assembly purposes in oxide, hybrid (organic/inorganic), and metallic materials.13–18 In fact, we have recently reported on the use of PAMAM 4.0 as a template to produce porous silicas. 18 However, PAMAM 4.0 collapses upon heating, which results in pores that are significantly smaller
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