Biomedical Engineering Reference
In-Depth Information
3.3.2
Syntheses and Characterization of Catalyzed CAs
3.3.2.1
CAs Catalyzed by Acetic Acid
The pH value of the resorcinol-furfural precursor is approximately
neutral. The use of an acetic acid catalyst acts to reduce the pH values
of the resulting precursors [34]. Our goal herein was to choose
proper catalysts that would allow us to reduce the gelation time and
increase the surface area and microporosity of the CAs. According
to the predetermined prescription mentioned in Section 3.1
E/R = 190, R/F = 0.5, and R/HMTA = 30. Acetic acid (99.7%, Sigma
Aldrich Pty. Ltd., Australia) was used as a catalyst by drop-wise
addition to the precursors to adjust the pH value between optimum
values of 4.1 and 5.2 according to our initial work. The resultant
solutions were put in an oven at 75°C for 3 days after being first
stabilized for at least 24 h at room temperature. The organic wet
gels were then transferred into a chamber to subcritically dry for
4 h. The aerogel samples were carbonized at 900°C at a heating
rate of 3°C/min for 3 h under a nitrogen atmosphere (N
flow rate
= 1.5 L/min). The samples were then activated by an additional
heating process at 950°C for 1 h under a CO
2
atmosphere (CO
2
2
flow rate = 1.5 L/min). The CA samples synthesized with different
precursor pH (= 4.1, 4.5, 4.8, and 5.2) will be referred to in the text
as CA4.1, CA4.5, CA4.8, and CA5.2, respectively.
In this particular reaction of resorcinol-furfural polymerization,
resorcinol serves as a trifunctional monomer capable of
electrophilic aromatic substitution in the activated 2, 4, and 6 ring
positions. Furfural forms covalent bridges between the resorcinol
rings leading to high cross-linking densities. In acid-catalyzed
aerogel synthesis the reaction kinetics are enhanced by increasing
the positive charge of the electrophile. This is unlike the kinetic
enhancement in base-catalyzed resorcinol-formaldehyde aerogels,
which is due to the activation of the aromatic ring by electrophilic
aromatic substitution (from OH to O
-
) [35]. Simply put, the reactions
that proceed to form the organic polymer precursor involve
carbocation formation, activation, and adsorption on aromatic sites
of resorcinol, crosslinkings with furfural, and finally condensation.
A brief mechanism of the reaction between resorcinol and furfural
catalyzed by acetic acid is shown in Fig. 3.5.
The first step involves formation of a carbocation between
resorcinol and acetic acid by the adsorpti
o
n of acetic acid on a
