(420b) Catalytic Properties of Dendron-Ordered Mesoporous Silica Nanocomposites

This talk will summarize our recent efforts in evaluating the catalytic properties of ordered mesoporous silica ? dendron composites as solid organocatalysts. Two groups of melamine-based dendrons were evaluated: one containing 4-(aminomethyl)piperidine (AMP) and the other with piperazine (PIP) groups as diamines spacers. The dendrons were grown from amine-functionalized MCM-41 and SBA-15 surfaces via an iterative approach demonstrated previously by the lab [1,2]. The composite materials were characterized using a variety of methods including NMR, adsorption, elemental analysis, and TGA to verify the structure of the organic groups obtained.

The talk will summarize our use of these materials as catalysts in the Nitroaldol reaction of nitrobenzaldehyde with nitromethane, transesterification of glyceryl tributyrate with methanol, and cross aldol reaction of furfural with acetone.

In the Nitroaldol reaction, both G1-AMP-MCM-41 and G1-PIP-MCM-41 samples show very high reactivity, and the conversions are approximately 90% after 2 hours. The conversions are over 70% after 2 hours when the reaction was catalyzed by G1-AMP-SBA-15 and G1-PIP-SBA-15 samples. With the increase of dendrimer generation, the conversions show a clear decrease when MCM-41 was used as the support, while the higher generation dendron samples on SBA-15 appear more active than the G1 samples. Different from many previous reported results about simple primary amine attached OMS materials, the catalysts in the Nitroaldol reaction here display over 85% selectivity to nitroalcohol product. The yield of nitroalkene decreases with increasing the generation, and is higher for primary amines.

The conversion of glyceryl tributyrate (GTB) in the transesterification reaction is over 70% after 6 hours for G1 samples, and that catalyzed by AMP-OMS are a little higher than that by PIP-OMS. The effect of the dendron generation on the conversions is similar to the Nitroaldol reaction. The different trends of reactivity from different generation samples indicate distinct diffusion resistances in the MCM-41 materials. The recycle studies prove that the dendron-OMS composites are stable in the transesterification reaction.

Preliminary results indicate the modest catalytic activity of G1-PIP-MCM-41 and G3-PIP-SBA-15 samples in the cross aldol reaction. More investigations about the catalytic activity of dendron-OMS composites in the cross aldol reaction as a function of dendrimer generation, substrate, and amine identity, are ongoing and will be discussed.

References

1. Yoo, S., Lunn, J. D., Gonzalez, S., Ristich, J. A., Simanek, E. E., Shantz, D. F. ?Engineering Nanospaces: OMS/Dendrimer Hybrids Possessing Controllable Chemistry and Porosity? Chem. Mater. 2006, 18(13), 2935-2942.

2. Acosta, E. J., Carr, S. C., Simanek, E. E., Shantz, D. F. ?Engineering Nanospaces: Iterative Synthesis of Melamine-Based Dendrimers on Amine-Functionalized SBA-15 Leads to Complex Hybrids with Controllable Chemistry and Porosity? Adv. Mater. 2004, 16(12), 985 ? 989.