Page 122 - Refining Biomass Residues for Sustainable Energy and Bioproducts

94 Refining Biomass Residues for Sustainable Energy and Bioproducts

Conversely, the sulfonated products synthesized at lower temperatures are
unstable and readily decomposable. Thus, the introduction of SO 3 H groups onto
the surface of precursor takes place by electrophilic substitution. This reaction is
reversible and exothermic in nature requiring a certain time limit to reach equilib-
rium (Yu et al., 2017).
Endut et al. (2017) produced a catalyst from coconut shell and studied its perfor-
mance through response surface methodology. They found that carbonization tem-
perature had a greater influence on the yield of biodiesel. In addition, they noted a
compelling reduction in the biodiesel yield after the prolonged sulfonation process.
The time taken for additional sulfonation affects the incorporation of SO 3 H group
negatively. The carbonization and sulfonation processes are rapid and energy effi-
cient for lower moleculer weight biomass such as corn straw, yellow horn hulls,
and bagasse when compared to complex molecular weight biomass such as peanut
shell, jatropha hulls, biochar, cassava stillage residue, and corncob residues (Yu
et al., 2017).

4.3.2.2 Hydrothermal process

Multiple efforts have been taken by researchers to improve the activity of acid cata-
lyst through hydrothermal carbonization (Liu et al., 2010c; Fu et al., 2013; Tran
et al., 2016; Endut et al., 2017; Nata et al., 2017). Either before or after this process,
the biomasses are treated with steam or water at a higher temperature for a fixed
time. This pretreatment increases uniformity on the surface that in turn causes uni-
formity of the pore sizes. Moreover, it also forms large particles with larger inter-
particle pores (Rao et al., 2011). Surface activation, although being a well-known
process, is considered as a critical method for the extreme low reactivity of carbon
and hence, needs an added step for surface functionalization.
The method proposed by Rao et al. (2011) shows the usage of steam activated
2
deoiled canola meal as a catalyst improved the surface area from 9 to 59 m /g,
thereby opening the material’s pores. Moreover, due to the occupation of the pores
2
by surface functional groups, it decreased to less than 10 m /g post-acid treatment.
Therefore, steam activation has shown no further improvement in the surface prop-
erties when compared to partially carbonized and sulfonated catalyst. To overcome
the above mentioned drawback, time and temperature are optimized during catalyst
preparation to enhance the synergistic combination of polycyclic aromatic carbon
sheets and its functional group in adequate amounts.

4.3.2.3 Solvothermal process

During the hydrothermal process, lignin present in biomass undergoes heteroge-
neous pyrolysis to form polyaromatic and phenolic hydrochar assembly. In order to
overcome hydrothermal effect, different solvents, depending upon the compatibility
of biomass, are employed. A proper solvent, such as an alcohol, acetone, tri-
isobutylene, etc. is chosen for the biochar production with better surface for the
loading of active sites. Alcohol is the most compatible solvent for solvothermal