Biomedical Engineering Reference
In-Depth Information
4.65 wt.% for CA4.8, and then dropped to 4.28 wt.% for CA5.2. The
hydrogen uptake related to the pH value of the initial precursors is
shown in the inset of Fig. 3.8.
3.3.2.2
CAs Catalyzed by Potassium Hydrate
CAs are typically prepared from the sol-gel polymerization of
resorcinol and formaldehyde and dried through supercritical
extraction of the reaction solvent [6]. They are usually synthesized
using water as a solvent and Na
as a base catalyst [6, 41-45].
The polymerization mechanism includes two steps: (i) formation of
resorcinol anions by hydrogen abstraction (enhanced by OH
CO
2
3
) and
formaldehyde addition to obtain hydroxymethyl derivatives, (ii)
condensation of the hydroxymethyl derivatives and cluster growth
(catalyzed by H
-
+
). The role of sodium carbonate was identified to
enhance the formation of hydroxymethyl derivatives by increasing
the OH
ratio [46-48]. Thus, high catalytic systems produce
highly branched clusters. However, most reported CAs were
generally synthesized using the carbon dioxide supercritical drying
method developed by Pekala [6]. In view of technical and economical
advantages, some modifications have been made to obtain the CA at
ambient pressure drying [25]. A substituted catalyst, i.e., potassium
hydrate (KOH), was used to synthesis the resorcinol-furfural sol.
Finally, the as-prepared gels were dried at ambient pressure drying.
In this study, two types of CAs, i.e., KOH catalyzed and uncatalyzed
samples, were prepared by a method similar to that previously
described in Section 3.1: E/R = 190, R/F = 0.5, and R/HMTA = 30
[26]. The KOH adjusts the pH value of the precursors from 6.5 to 8.9,
and the molar ratio of resorcinol to KOH is approximately 20 [49].
During our research on CA synthesis we have discovered that
KOH can accelerate the gelation process, because the activities of the
precursors are increased in the presence of potassium hydroxide.
The KOH works not only as a catalyst, but it is also hypothesized
that it acts as a cross-linking agent, which strengthens the formation
of covalent bridges between resorcinol rings. All samples were
carbonized at 900°C for 3 h under a nitrogen atmosphere, and then
activated at different temperatures for 1 h under a CO
-
/H
+
atmosphere.
For comparison, one inactivated sample was also prepared. The
samples will be referred to as CAC and CA for catalyzed species
and uncatalyzed ones, followed by A
2
T
, representing activating
temperature,
T
in °C, e.g., CAC-A1000.
