Aerogels base-catalyzed

Summary Hydrophobic aerogels were prepared by base-catalyzed hydrolysis and condensation of RSi(OMe)3 (R = Me, Ph, PrI1)/Si(OMe)4 (1 4) mixtures in methanol, followed by supercritical drying of the obtained alcogels with methanol. The organic substituents also increase the elasticity of the aerogels. 

The pore structure of two types of catalyst support material were studied 7-alumina and silica aerogel. The alumina samples were commercial catalyst supports made in 1/8 inch diameter pellet form by Harshaw Chemical. Silica aerogels were prepared from silica gels synthesized by a two step acid/base catalyzed procedure employing TE0S with a water to silicon ratio equal to 3.7 [17] and ammonium hydroxide concentration of 0.005 M (sample A) or 0.01 M... 

Making an aerogel involves base-catalyzed hydrolysis and polycondensation reactions of silicon alkoxides in an alcohol followed by 24 hours of aging in the alcohol solvent to produce an alcogel. Then the alcohol is removed from the pores of the alcogel... 

Figure 1 Model for base-catalyzed silica aerogel.

Despite the fact that gelation can be conducted under either acidic or basic conditions, sihca aerogels are most often formed from base-catalyzed reactions. This is because the polymeric fi amework of the acid-catalyzed gel contains a larger portion of smaller pores than the base-catalyzed colloidal analog. Removal of solvent from these smaller pores without gel compaction is difricult, even under supercritical conditions. One successful approach to access ultra-low-density aerogels with polymeric frameworks is to use a two-step acid/base gelation process such as that used by TUlotson and Hrubesh (14). The first step is to perform acid catalysis of tetramethox-ysilane (TMOS) with hydrochloric acid to produce a sol from which the alcohol... 

Write suitable mechanisms for (a) acid-catalyzed and (b) base-catalyzed hydrolysis of tetraethoxysilane (TEOS). How do these catalysts affect the morphology of the resultant aerogels ... 

Relative to base-catalyzed gels, those eatalyzed with acid have a larger number of small pores, and it is difficult to dry them without significant pore collapse. However, a two-step process in which the precursors are initially hydrolyzed with acid, followed by gelation under basic conditions, does lead to robust polymeric aerogels. In fact, this process yields the lowest density aerogels on record. 

Scheme 11.1. Mechanism of the base-catalyzed RF aerogel synthesis.

Aerogels prepared by Pekala s base-catalyzed route have been studied extensively for the effect of process parameters such as the concentration of monomers and the catalyst as well as the pH of the solution. Both structure and properties such as density, surface area, particle size, and pore size distribution are influenced by those variables. Low-density RF aerogels ( 0.03 g cm ) prepared by that method exhibit high porosities (>80%), high surface areas (400-900 m g ), and ultrafine pore size (<500 A). 

Process of Making /7F Aerogel via Base-Catalyzed Route... 

Hydroxymethylated resorcinol in either base- or acid-catalyzed sol does not accumulate because of its instability, p-Hydroxymethyl groups undergo rapid conversion to carbocations or o-quinone methides. This explains how identical condensation products are obtained under acid or alkaline reaction conditions [19]. This fact is evident in the NMR spectra of RF aerogels prepared under acid- or base-catalyzed conditions [33-38]. 

On comparing the solid NMR spectra of base-catalyzed RF aerogels with those of the HCl-catalyzed process, it is evident that the latter, being completed in minutes rather than in days, yields materials that are chemically indistinguishable from those obtained by the usual base-catalyzed process in a week. 

Figure 13.3. A. Mechanical characterization by a short beam 3-point bending (see inset) of polyurea-crosslinked silica aerogel monoliths and their noncrosslinked (native) counterparts a, 0.63 gcm b, 0.44 gcm c, 0.38 gcm , and d, 0.28 gcm . Native samples do not register in the load-force scale shown. B. Cumulative data. Dark blue triangles and the dark blue line concern two-step aerogels made by acid-catalyzed hydrolysis and base-catalyzed gelation. All other samples use one-step base-catalyzed silica with different isocyanates. Multiple lines for crosslinked samples correspond to different di- and tri-isocyanate crosslinkers.

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