Facile Sol-gel Synthesis of Porous Silicas Using Poly(propylene)imine Dendrimers as Templates

Abstract

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.

Synthesis and Characterization of Dendrimer-Templated Mesoporous Oxidation Catalysts

“…The synthesis procedure was adapted from the work by Larsen et al [29][30][31] on dendrimer-templated siliceous materials, such as NU-1. The initial sol-gel was prepared by adding 1.03 g of tetraethyl orthosilicate (Aldrich, 98%) to a solution of 0.403 g of DAB-Am dendrimer (CU-D32 or CU-D64 DSM, Netherlands, 97%) in 1.26 g of 1-propanol (Fisher, 99.5%).…”

“…The dendrimer template was removed from the silicate structure by first drying the sample for 24 h at 100°C in a convection oven, then calcining the sample in a furnace under nitrogen flow at 560°C for 3 h using the heating protocol developed by Larsen and coworkers [29][30]. Further calcination of the product was achieved by heating the sample under constant air flow at 560°C for 2 h.…”

“…Since the final structure of sol-gel-based materials is dependent upon the structure of the template that was used to create it [14], a change in the templating agent can lead to significant changes in the physical structure and therefore, the chemical nature of the catalyst. In order to better manipulate the pore size and shape of mesoporous materials, Larsen and co-workers [28][29][30][31] have recently substituted a variety of amine-terminated dendrimers for the surfactant templates that are commonly used to synthesize mesoporous oxide materials. The result of this substitution was the creation of new mesoporous materials with high surface areas and interconnected spherical shaped pores.…”

“…The first dendrimer template studied was a Starburst polyamidoamine (PAMAM) dendrimer [28,32]. While the PAMAM dendrimer (4th generation) was an effective template for the synthesis of a mesoporous oxide, the resulting material had poor structural properties that led to pore collapse at elevated temperatures; hence, it proved ineffective for use as a catalyst or support material [29]. In an effort to overcome these shortcomings, poly(propylene)imine (DAB-Am) dendrimer templates were examined by Larsen and co-workers [29][30].…”

“…While the PAMAM dendrimer (4th generation) was an effective template for the synthesis of a mesoporous oxide, the resulting material had poor structural properties that led to pore collapse at elevated temperatures; hence, it proved ineffective for use as a catalyst or support material [29]. In an effort to overcome these shortcomings, poly(propylene)imine (DAB-Am) dendrimer templates were examined by Larsen and co-workers [29][30]. These amine terminated DAB-Am dendrimers (4th and 5th generation) proved to be more effective at templating the synthesis of structurally stable mesoporous silicates with interconnected spherical pores.…”