Page 114 - Refining Biomass Residues for Sustainable Energy and Bioproducts

88 Refining Biomass Residues for Sustainable Energy and Bioproducts

fats helps in combating financial constraints, but a high measure of free fatty acids
(FFAs) present in these feedstocks restricts the utilization of base catalysts, which
reduces the biodiesel yield by saponification reaction (Carlini et al., 2014;
Photaworn et al., 2017). Hence, a pretreatment process through esterification is usu-
ally employed prior to transesterification. Conventionally, homogeneous acids
(H 2 SO 4 , HCl, H 3 PO 4 , etc.) show stronger effect on esterification reactions, although
their applications are not enough to overcome the shortcomings such as equipment
corrosion, unreliability, and their hazardous nature (Canakci and Gerpen, 2001;
Zabeti et al., 2009; Balat and Balat, 2010; Yin et al., 2012; Gao et al., 2015; Wu
et al., 2015).
Generally, refined feedstocks are transesterified by base catalyst, whereas
high FFA feedstocks are esterified and transesterified by acid catalysts for which
they require critical operating conditions (Peng et al., 2008; Suwannakarn et al.,
2009). Solid acid catalysts show eminent consideration because of their preva-
lent traits such as natural mild property, greater recyclability, effective avoid-
ance of corrosion, and saponification reactions when compared to base or
homogeneous acid catalysts (Liu and Wang, 2009; Melero et al., 2009). Until a
recent time, sulfated zirconia, zeolites, blended oxides, sulfonic acid resins,
sulfonated mesoporous silica, bolstered, substituted acids, etc. have been ade-
quately explored and used as catalysts to produce biodiesel despite their disad-
vantages such as low acid density, poor stability, and deactivation under
different operating conditions (Lou et al., 2008; Caetano et al., 2009; Soldi
et al., 2009; Soltani et al., 2016).
Therefore to enhance the efficiency of biodiesel production, there is a compel-
ling need to design a robust and stable solid acid catalyst, but which is still a major
challenge. Catalyst preparation using biomasses would economically strengthen the
biodiesel industry because of its new application. This ultimately reduces the efflu-
ent load by avoiding the use of homogeneous acid catalysts in the reactions. In
addition to biodiesel production, such catalysts can also be used in many other
acid-catalyzed reactions. Taking these under consideration, this review enumerates
the preparation of heterogeneous acid catalysts from different biomasses and their
residues as an organic carbon precursor. In addition, a detailed analysis of their
applications and a study on the parameters enhancing the biodiesel production have
also been incorporated into this chapter.

4.2 Acid catalyst mechanism for transesterification and
esterification

Biodiesel is produced from triglycerides, and FFAs present in vegetable oils, animal
fats, scum, etc. through transesterification and esterification. In transesterification,
triglyceride reacts with an alcohol such as methanol, ethanol or propanol and is cat-
alyzed by alkalis or acids. This process consists of three successive reversible reac-
tions, where lipid (triglyceride) is sequentially converted to diglyceride,