Palladium Dendron Encapsulated Nanoparticles Supported on MCM-41 and SBA-15

Dendrimer-encapsulated nanoparticles (DENs) have attracted interest since they were first introduced in 1998 [1]. The nanoparticles formed have been investigated for numerous applications including catalysis, such as Heck coupling, allyl alcohol hydrogenation, and electrocatalysis [2]. In this work, ordered mesoporous silica (OMS) was used as supports for dendron growth. In the previous work, melamine-based dendrimers were successfully grafted to OMS supports through the bridge of organosilanes [3]. Here the dendrimers were used as templates to form nanoparticles, with palladium explore initially. Multiple methods were used to validate both the structural integrity of the dendrimers and the nature of the metal nanoparticles formed. The metal complexation and reduction are monitored by UV-Vis spectroscopy and shows that the palladium loading can be varied from 1 wt.% to 3 wt.%. The STEM images show the formation of uniformly sized palladium particles approximately 2 nanometers in size within the OMS pores. In contrast to prior work with dendrimers in solution where G4 or higher generation moieties were needed here it is shown that highly uniform nanoparticles can be formed in small dendron fragments, i.e. G2. The accessibility of both the metal sites and dendron ligand sites were demonstrated by catalytic probe reactions and CO₂ uptake experiments. The TOF values for the Heck reaction vary from 160 to 239 mmol iodobenzene consumed/mmol Pd demonstrating that the encapsulated palladium particles are catalytically active. The initial rate for the Nitroaldol (Henry) reaction decreases from 14.9 to 7.3 mmol aldehyde consumed/g catalyst-h, but nonetheless shows that the amine sites are catalytically active in the palladium-containing samples. Carbon dioxide uptake measurements showed minimal perturbations to the CO₂ uptake after palladium particle formation. These hybrid materials have potential uses in reactions where both the organic and metal center can be used sequentially such as Cascade reactions.

[1] Zhao, M., Sun, L. and Crooks, R. M., J. Am. Chem. Soc. 120, 4877 (1998).
[2] Myers, V. S., Weir, M. G., Carino, E. V., Yancey, D. F., Pande, S. and Crooks, R. M., Chemical Science 2, 1632 (2011).
[3] Acosta, E. J., Carr, C. S., Simanek, E. E. and Shantz, D. F., Advanced Materials 16, 985 (2004).