Temperature Dendrimer Removal

Given that it may be difficult to remove surface carboxylates from supported DENs, the question arises as to whether it is inherently necessary to remove aU organic material to prepare clean, active supported nanoparticles. In a practical sense, except when dealing with freshly calcined supports, carbon species from atmospheric sources are always present on oxide surfaces. Indeed, C—H stretching vibrations are readily observable in infrared spectra of supports taken directly from a manufacturer s container. The critical species are those directly adsorbed to the metal surface and in close proximity to the nanoparticles. 

Using siUca supported Pt-DENs, we showed that activation temperatures could be reduced to as low as 150 °C by employing CO oxidation reaction conditions [86]. 

This low temperature CO/O2 treatment was also used for Au/Ti02 DENs for some pretreatment temperatures, the Au/Ti02 catalysts were more than an order of magnitude more active for CO oxidation than Pt/Si02 [88], The Au catalysts also showed substantial room temperature activity that was extremely reproducible. However, the catalysts were not stable over time, presumably due to dendrimer decomposition products migrating onto the Au nanoparticles and poisoning the catalysis. 

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Since mild activation conditions appear to be important, a number of solution activation conditions were tested. PAMAM dendrimers are comprised of amide bonds, so the favorable conditions for refro-Michael addition reactions, (low pH, high temperature and the presence of water) may be able to cleave these bonds. Table 1 shows a series of reaction tests using various acid/solvent combinations to activate the dendrimer amide bonds. Characterization of the solution-activated catalysts with Atomic Absorption spectroscopy, FTIR spectroscopy and FTIR spectroscopy of adsorbed CO indicated that the solution activation generally resulted in Pt loss. Appropriate choice of solvent and acid, particularly EtOH/HOAc, minimized the leaching. FTIR spectra of these samples indicate that a substantial portion of the dendrimer amide bonds was removed by solution activation (note the small y-axis value in Figure 4 relative... 

To a vigorously stirred solution of 1,2-diaminoethane (107 g, 118 ml, 1.781 mol) in methanol (150 ml), at 0°C under nitrogen, was added a solution of (G = 1.5) dentin -PAMAM(C02Me)16 [5] (10 g, 0.004 mol) in methanol (30 ml). The addition was controlled such that the temperature did not rise above 40 °C. The mixture was stirred at room temperature for 96 h, after which time no ester groups were detectable by NMR spectroscopy. The methanol was removed by vacuum distillation at < 40°C, and the excess 1,2-diaminoethane was removed by azeotropic distillation using a mixture of toluene and methanol (9 1). The remaining toluene was removed by azeotropic distillation with methanol and finally the methanol removed under vacuum (10 1 mm Hg, 50°C, 48 h) to give the amine terminated G = 2.0 dendrimer as a pale yellow oil (10.9 g, 94%). 

In-situ infrared spectroscopy has been used in much the same fashion at TGA, but temperature profiles have been combined with monitoring changes at constant temperature. " IR spectroscopy does not yield the same direct information about the complete removal of organic residues that TGA provides. On the other hand, CO adsorption experiments performed along with dendrimer decomposition experiments provide direct information regarding metal availability. Further, IR experiments provide... 

A very old gas-solid bromination of tyrosine (280) [97] has been revisited and it gave a quantitative yield for the reaction of rac-280 [22]. The doubly bromi-nated hydrobromide rac-281 is spectroscopically pure after removal of included gases at 50 °C in a vacuum. Quite spectacular is the specific and quantitative waste-free gas-solid tetrabromination of tetraphenylethylene (282), which shows some signs of autocatalysis and requires rotation of the flask around a horizontal axis at room temperature for 12 h as the reactant and product gases require mixing [60]. The isomer-free tetrabromide 283 is an attractive starting point for dendrimer syntheses and inclusion studies (Scheme 40). Also 4-bro-mo antipyrin hydrobromide is quantitatively obtained from antipyrin(hydro-bromide) and bromine vapor [22]. 

Crooks and coworkers, who studied Pd and Au DENs immobilized in sol-gel titania, similarly reported the onset of dendrimer mass loss at relatively low temperatures (ca. 150 °C). Pd helped to catalyze dendrimer decomposition in their system, as well. Temperatures of 500 °C or greater were required to completely remove organic residues from their materials. (10) This treatment resulted in... 

Infrared spectroscopy of adsorbed CO is a useful characterization tool for dendrimer-templated supported nanoparticles, because it directly probes particle surface features. In these experiments, which are performed in a standard infrared spectrometer using an in-situ transmission or DRIFTS cell, a sample of supported DENs is first treated to remove the organic dendrimer. Samples are often reduced under H2 at elevated temperature, flushed with He, and cooled to room temperature. Dosing with CO followed by flushing to remove the gas-phase CO allows for the spectrum of surface-bound CO to be collected and evaluated. Because adsorbed CO stretching frequencies are sensitive to surface geometric and electronic effects, it is potentially possible to evaluate the relative effects of each on nanoparticle properties. 

To a 1-liter flask as in C above and containing 328.8 gm of ethylenediamine (5.48 mole) dissolved in 210.2 gm of methanol at room temperature is added with stirring 34.9 gm (0.0398 mole) of the second generation polyester dendrimer prepared in C above dissolved in 45.3 gm of methanol. The resulting reaction mixture is allowed to stand for 66 hr at room temperature and then the excess ethylene-diamine and methanol are removed under reduced pressure (2.0 mm Hg and 72°C) to yield 41.1 gm of product (99.0% yield). 

Supported untreated DMNs often exhibit smaii average particie sizes. While exposure to elevated temperatures (i.e., 400-500°C) for prolonged periods of time ensures removal of the dendrimer, these processes can lead to sintering of the metal and, in the case of bimetallic particles, possible segregation, as well. Thus, optimization of the necessary activation conditions has been an important research question. 

Dendrimer thermolysis, i.e., the removal of the dendrimer component via exposure to high temperatures in oxidative, reducing, or neutral gas-phase environments, is the technique that has been so far exclusively employed for this task. Other potential low-temperature removal methods, such as chemical leaching of the dendrimCT or treatment under plasma conditions, have been suggested but not demonstrated experimentally. 

Dendrimer thermolysis experiments conducted on Pt/AljOj catalysts show that 400°C is the lowest temperature that will result in the removal of adsorbed carbox-ylates. It is not until these carboxylates are removed from the surface that the platinum nanoparticles exhibit significant CO uptake, indicating that the dendrimer decomposition products, possibly including undetectable nitrogen moieties, interfere with the metal active sites (Figure 9.8). ... 

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