Synthesis and textural characteristics of organic aerogels, transition-metal-containing organic aerogels and their carbonized derivatives

doi:10.1016/S0008-6223(98)00314-5

Carbon

Volume 37, Issue 8, 1 January 1999, Pages 1199-1205

https://doi.org/10.1016/S0008-6223(98)00314-5 Get rights and content

Abstract

Four organic aerogels were prepared by the sol-gel method from polymerization of resorcinol with formaldehyde catalyzed by Na₂CO₃. These aerogels were further pyrolyzed in N₂ in order to obtain their corresponding carbon aerogels, and after that activated with either steam or CO₂ to obtain activated carbon aerogels. Another series of organic aerogels and their pyrolyzed and activated derivatives were prepared without a catalyst or in the presence of Ag, Pd or Pt. All these samples were texturally characterized, which has shown the porous modifications undergone by the aerogels during the pyrolysis and activation processes. The textural characteristics of the transition-metal-containing activated carbon aerogels depend on the nature of the metal. Thus, whereas the sample containing Pt, in a very small amount, had the largest meso and macropore volume (0.822 and 2.982 cm³ g⁻¹) found, those containing either Pd or Ag were essentially microporous.

Introduction

Materials prepared by the sol-gel approach have rapidly become a fascinating new field of research in material science. Aerogels are prepared from gels by supercritical drying methods. The skeletal structure of wet aerogels is maintained through supercritical drying, obtaining solids with high porosity and specific surface area 1, 2, 3, 4.

The sol-gel process is a chemical synthesis method initially used for the preparation of inorganic materials such as glasses and ceramics. Thus, the most common aerogels are inorganic, usually derived from the sol-gel polymerization of metal alkoxides (e.g. tetramethoxysilane, tetraisopropoxy titanate), followed by supercritical drying 1, 5, 6, 7.

Pekala et al. 8, 9, 10, 11, 12 have found that certain organic monomers can also be used to prepare aerogels. Thus, polycondensation of resorcinol with formaldehyde in aqueous solutions leads to gels that can be supercritically dried with carbon dioxide to form organic aerogels. These resorcinol–formaldehyde (RF) aerogels can be pyrolyzed in an inert atmosphere to form carbon aerogels. Because of the chemical and textural characteristics of these materials, they are expected to be used as thermal and phonic insulators, electric double layer capacitors, chromatographic packings, adsorbents, and catalyst supports 13, 14.

The structure and properties of RF and carbon aerogels are largely determined by polymerization conditions, the dominant factor that controls this process being the resorcinol/catalyst (R/C) ratio [15]. The catalyst used by Pekala 8, 9, 10, 11, 12 and other authors 15, 16 to carry out this polymerization was a basic catalyst such as Na₂CO₃.

The main objective of the present paper is to study the possibility of preparing transition-metal-containing RF aerogels and their carbonized derivatives. The transition-metals studied were Pt, Pd and Ag. For this purpose, several RF aerogels following the Pekala method were obtained and in some cases, the basic catalyst (Na₂CO₃) was substituted by different salts of the transition metals chosen. After that all aerogels were carbonized and activated both in CO₂ or steam. The influence of the presence of either the transition metal or Na₂CO₃ on the textural characteristics of the solids obtained was studied with all the samples prepared. It is expected that some of these samples find a proper use as adsorbents and catalysts of different reactions. Therefore, the analyses of the pore texture of these aerogels are very important from the view point of the above applications.

Section snippets

Experimental

Four RF aerogels were synthesized following the method described by Pekala 8, 9, 10, 11, 12. Briefly, different amounts of resorcinol (R) and formaldehyde (F), both from Aldrich, were mixed with the appropriate amounts of distilled water and Na₂CO₃, as polymerization catalyst. The mixtures were cast into glass molds (25 cm length×0.5 cm internal diameter) and cured. After that, the gel rods were cut in 5 mm pellets and introduced in acetone to remove the water inside the pores. The gels were

Organic aerogels and their carbonized derivatives

Firstly the results obtained with the aerogels prepared by using Na₂CO₃ as polymerization catalyst will be discussed, as well as the corresponding carbon aerogels and activated carbon aerogels obtained from them.

The aqueous solutions containing the reactants and the catalyst were initially transparent and colorless, but they turned progressively to yellow, orange and dark red color as the polymerization process progressed during the cure period. All the aerogel pellets were externally of dark

Conclusions

Organic aerogels are mainly mesoporous materials, and when they were carbonized, the macroporosity disappeared and in general the mesoporosity increased as well as the microporosity and the nitrogen surface area. The activation process in steam yielded higher degrees of activation than in CO₂. Steam activation increased the microporosity and the mesoporosity essentially in a narrow range of pore sizes. A nitrogen surface area of up to 1600 m² g⁻¹ was obtained.

In contrast,

Acknowledgements

The authors wish to acknowledge the financial support of DGCYT, Project no. PB94-0754.

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Synthesis and textural characteristics of organic aerogels, transition-metal-containing organic aerogels and their carbonized derivatives

Abstract

Introduction

Section snippets

Experimental

Organic aerogels and their carbonized derivatives

Conclusions

Acknowledgements

J Non-Cryst Solids

J Non-Cryst Solids

Catal Today

Carbon

Carbon

Carbon

Carbon

J Mater Res

J Electrochem Soc

Adv Mater