2.1. Characterization of the Novel Carbon Based Acid
The formation of the carbon based acid involves the dehydration of furaldehyde and hydroxyethylsulfonic acid as the first step. Furaldehyde is one of the glucose intermediates during the hydrothermal process, which could be able to react with hydroxyethylsulfonic acid to introduce sulfonic acid groups. Here, the sulfonic acid groups also could also be transformed to other groups such as sulfonate and sulfone when glucose is used as the raw material. Other furan derivatives such as methylfuran are unable to form solid products, so furaldehyde is chosen here as the raw material. Upon dehydration (polymerization), microscopic carbon-containing spheres with sulfonic acid groups and hydroxyl groups were formed. Subsequent loss of water by these assemblies leads to further coalescence of microscopic spheres into larger spheres (
Scheme 1) [
14,
15]. Here, hydroxyethylsulfonic acid was used as the functional molecule to introduce the sulfonic acid groups to the carbonaceous material, since it contained the necessary hydroxyl group for the intermolecular dehydration. The acidity of the novel acid carbon was 2.4 mmol/g, which was determined through neutralization titration. The material had much higher acidity than that of the sulfonated carbonaceous materials obtained via the sulfonation of the inactive carbon surface. The acid strength of the catalyst was determined by thermodesorption of chemisorbed ammonia (NH
3-TPD). The result showed that the catalyst had great acid strength, as ammonia was only desorbed at 400 to 600 °C.
The X-ray Photoelectron Spectroscopy (XPS) spectrum analysis showed a S content of 7.2%, with a single S 2p peak attributable to sulfonic acid groups at 168 eV, which indicated that almost all the S existed in the forms of such sulfonic acid groups. On the other hand, the acidity of 2.4 mmol/g required the S content of 7.68%, which was higher than the actual content. These results indicated that there were still a few other acidic groups such as carbonyl acid groups in the carbonaceous material and the O content was as high as 27%, which also indicated that there were still many oxygen-containing groups in the material. Furthermore, the acidity remained even after the material has been treated with boiling water for more than 15 h, which further confirmed that the sulfonic acid groups were attached to the carbonaceous material.
The Fourier Transform Infrared (FT-IR) spectrum of the carbonaceous material is shown in
Figure 1. The absorbances at 1,040 and 940 cm
-1 confirmed the existence of the sulfonic acid groups. The FT-IR spectrum also showed that the carbon materials contain various other functionalities including carboxylate (1,704 cm
-1), Ar-H (3,020 cm
-1), C-O groups (1,204 cm
-1) and C=C groups (1,604 cm
-1).
Figure 1.
The IR spectrum of the carbon based acid.
Figure 1.
The IR spectrum of the carbon based acid.
The scanning electron microscopy (SEM) images of the carbon based acid show that the resulting particles grew in size with the reaction time, reaching a diameter of 5–10 μm, as depicted in
Figure 2(a,b), which is quite different from the amorphous structures of the sulfonated carbonaceous materials reported by Hara [
9,
10]. The large size of the carbon spheres also made the recycle of the material very simple and a filtration without suspension of the reaction mixture was enough. The BET surface of the material was 148 m
2/g, which is much higher than that of sulfonated carbonaceous materials (10–30 m
2/g). The materials displayed micrometer sized microporous carbon spheres and there were many micro-sized carbon spheres attached to the surface of the big carbon spheres to form a strawberry-like structure (
Figure 2a). The carbonaceous material obtained from furaldehyde formed carbon spheres with a smooth surface (
Figure 2c). Unlike the carbon derived from polyvinyl alcohol, which was dehydrated easily in an acid enviroment due the rich amount of hydroxyl groups, here hydroxyethylsulfonic acid appears to stabilize the first formed small droplets depicted in
Scheme 1, thus many micrometer sized carbon spheres are formed late in the process, which might not occur in the pure furaldehyde case. Later on in the process, the micro-sized carbon spheres attached to the surface of the early-formed big carbon spheres to form the observed strawberry-like structures [
16].
Figure 2.
The SEM images of the carbonbased acid with the reaction time of 4 h.
Figure 2.
The SEM images of the carbonbased acid with the reaction time of 4 h.