Chemical Control of Highly Porous Silica Xerogels:  Physical Properties and Morphology

Abstract

Highly porous silica xerogels were synthesized by the sol−gel process under atmospheric conditions. The silica alcogels were prepared by a two-step acid/base-catalyzed hydrolysis/condensation of tetraethoxysilane (TEOS), with a water:TEOS molar ratio of 4, in 2-propanol. The catalysts used in the two steps were HCl and NH3, respectively, and the catalytic conditions were varied by changing the molar ratios HCl/TEOS (in the first step) and NH3/HCl (in the second step). After aging in an appropriate solution, the alcogels were washed and subcritically dried from 2-propanol, under atmospheric pressure. The resulting silica xerogels were characterized by the volume shrinkage upon drying, envelope density determinations, scanning electron microscopy, and nitrogen sorption isotherms. It is shown that by chemical control of the hydrolysis and condensation steps, through pH variation only, it is possible to obtain monolithic materials with properties similar to aerogels, stable under atmospheric conditions. Densities as low as 0.37 g cm-3, corresponding to porosities of 80%, were obtained for xerogels with specific surface areas of ∼940 m2 g-1 and a bimodal pore structure, with a narrow mesopore size distribution (average pore diameter of 8 nm).

Organic-inorganic hybrid aerogels with high mechanical properties via organotrialkoxysilane-derived sol-gel process

“…Some other groups also report the strengthening of aerogels using organic polymers. 40)42) Although all these efforts work well to improve the brittleness and enhance the strength which allows drying under ambient conditions in some cases, hybridization with polymers in general more or less sacrifices optical transparency and porosity because the polymers coated on the silica backbones partially fill the pores of the aerogels.…”

Chemical Tailoring of Porous Silica Xerogels: Local Structure by Vibrational Spectroscopy

“…Recently, we have shown the possibility of producing rather stable silica xerogels with increased porosity, by a tight control of the hydrolysis and condensation catalytic conditions in a two-step sol±gel process. [7] This is a modification of the two-step method developed by Brinker et al, [8,9] and allows increasing the overall yield of silica production by an efficient separation of the hydrolysis and condensation mechanisms. The silicon alkoxide is firstly hydrolysed in a strong acidic medium, at such pH values that condensation reactions are very slow, [10] according to the following mechanism given in Equations (5) and (6): [1] …”

New Transparent Methylsilsesquioxane Aerogels and Xerogels with Improved Mechanical Properties

“…As the skeletons that comprise their porous morphology are too thin, aerogels are brittle, and they must be dried with the utmost care by using SCD, for example, to preserve their fragile pore structure. To overcome such challenges for silica-based aerogels, numerous attempts to strengthen the gel skeletons by aging, [6,7] crosslinking with organic polymers, [8][9][10] modifying surfaces hydrophobically, [11][12][13] and conventional drying [14,15] have been explored.Prakash et al [13] have obtained aerogel films with porosity up to 98.5 % by adding hydrophobicity through the use of trimethylchrolosilane constituents on bare silica networks. This method enables the so-called spring-back phenomenon, in which temporarily shrunk gel networks recover their original forms like a sponge.…”