Page 117 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Application of heterogeneous acid catalyst derived from biomass for biodiesel process 91

Guo et al. (2017) reported that the catalyst property can be enhanced by the
insertion of additives during the process. The study also revealed that the directly
sulfonated catalyst had relatively poor catalytic activity, low acid site density that
could be enhanced by phenol addition during the reaction. This is due to the reac-
tion of phenol with monosaccharides forming derivatives of phenol that are further
dehydrated, sulfonated, polymerized, and moderately carbonized to tiny carbon
clusters with SiO 2 . At the end, these tiny particles accumulate to form hydrophobic
precipitation of carbonaceous catalyst particles. The leaching of SO 3 H pave way
for a slight reduction in their activity during catalyst recycling. This method
requires further studies in order to improve the stability of SO 3 H group on the
carbon surface.

4.3.2 Carbonation followed by sulfonation

Biomass such as wood, lignin, bagasse, algae, seed cake, and agricultural residues
is carbonized for dehydration and dissociation of C C which is then followed
by sulfonation (Okamura et al., 2006; Kitano et al., 2009; Thushari and Babel,
2018). Carbon-based precursors are synthesized by pyrolysis, hydrothermal, alcohol
thermal methods, etc. to form polycyclic aromatic carbon rings that act as surface
for active sites (Ma et al., 2014; Huang et al., 2016; Endut et al., 2017; Liu et al.,
2017). Finally, a SO 3 H group is added to the polycyclic aromatic carbon ring by
means of sulfonation. Table 4.1 lists the summary of literature on different biomass
catalysts derived through different process conditions for carbonation and
sulfonation.

4.3.2.1 Thermal process
Parameters, like temperature and time, significantly influence this method of cata-
lyst preparation as well as affect its performance during esterification and transes-
terification reactions (Liu et al., 2013a). The temperature required for carbonization
depends mainly on the nature and type of biomass used. This is due to the differ-
ence in the functional groups containing oxygen which occupies carbon channel,
thereby making the participation of active sites difficult at low temperatures. Rigid
carbon materials and large polycyclic aromatic carbon sheets are formed and
stacked with the increase in temperature during carbonization. It also produces less
flexible structure and active site that can adhere to certain neutral and polar mole-
cules, preventing the reach of SO 3 H groups (Guo et al., 2012; Lou et al., 2012).
On the other hand, low temperature carbonization has a higher density of OH
group that absorbs the water produced during esterification reaction. This estab-
lishes a hydrophilic layer on the catalyst preventing access to hydrophobic oil phase
(Shu et al., 2009). Similarly, the quantity, stability, and position of attached SO 3 H
groups to the carbon framework are influenced by sulfonation temperature. At high-
er temperatures, carbon skeleton collapses and side reactions occur thereby hamper-
ing the introduction of SO 3 H groups.