Page 127 - Refining Biomass Residues for Sustainable Energy and Bioproducts

98 Refining Biomass Residues for Sustainable Energy and Bioproducts

4.4 Parameter influencing catalyst preparation

The main purpose of pyrolysis or carbonization process, as suggested by literature,
is to get hard graphite like network that may be easily functionalized by sulfonating
agents such as concentrated H 2 SO 4 , fumic H 2 SO 4 , diazonium compound, SO 3 ,and
ClSO 3 H(Wang et al., 2007; Liu et al., 2010a; Li et al., 2012), containing small poly-
cyclic aromatic carbon sheets. Sulfonation of such materials has resulted in the for-
mation of active SO 3 H sites with high density which are hardened with minimum
leaching during esterification and transesterification processes. Moreover, those mate-
rials exhibited noticeable catalytic performance (Hara, 2010). In such cases, oxidation
of aromatic rings during sulfonation resulted in the formation of graphene carboxylate
groups that led to the stabilization of the SO 3 amorphous carbon bearing SO 3 H
groups. Thus, lamellar constituent molecules that were present in the graphite lattice
showed more stability due to the loss of heteroatoms such as sulfur, hydrogen, and
oxygen, which, in turn, resulted in a significant reduction in the quantity of reactive
carbons that can be potentially functionalized. This was evident from XRD pattern
which indicated irregular shape carbon structure. The increase in applied carboniza-
tion temperatures caused disturbance in the arrangement of carbon lattice, and as a
result, there was a notable increase in the porosity of the material (Patrick, 1995).
Many researchers have reported that the sulfonated-biomass contains Ph OH,
COOH, and SO 3 H groups that have the ability to show greater catalytic activity
amidst liquid-phase acid-catalyzed reactions in comparison to other solid acids
(Hara et al., 2004; Hara, 2010). This also enhance the catalytic performance by act-
ing as a site for nucleophilic attack of alcohol and a positive carbon ion when FFA
is absorbed on the surface. The existence of these functional groups can be con-
firmed by back titrimetry, ammonia-temperature programmed desorption (NH 3 -
TPD), elemental analysis, etc. But, during the strong sulfonation condition, the
pores collapse in the carbon lattice due to the dissociation of the cross coupling
bonding (Hara et al., 2004; Konwar et al., 2014). This can be observed from the
percent decrease in total surface area (Liu et al., 2010a; Soltani et al., 2016).
Li et al. (2014) observed that rice husk and microalgae residue based catalyst
have better esterification and transesterification activity. This is due to their high
density of strong acidic group and hydrophilicity of SO 3 H group. Thus, the cata-
lyst absorb a large amount of water into the carbon bulk that resulted in a better
acquirement of larger catalyst surface area compared to the result obtained from
Brunauer, Emmett, and Teller (BET) method after dehydration. In addition, the
report claims that esterification and transesterification activities of these catalysts
are due to the strong acidic nature of the SO 3 H groups that are linked with the
assistance of hydrogen bond bearing amorphous carbons. This incorporated a large
amount of hydrophilic molecules along with water into the vacancies among the
domains composed of graphene. Optimum conditions favor conversions to reach up
to 99.0%, both for esterification as well as transesterification reactions.