5.1.1 Catalytic Homogenous Transesterification
Base Catalyst
In transesterification reaction, base catalysts are mostly used as they are cheaply
available and allow the usage of moderate reaction temperature and pressure which
helps in carrying out the process with favorable conditions. Base catalysts also give
higher yield in shorter period of time as compared to other catalysts (Schuchardt
et al. 1998). Bases such as KOH, CH 3 ONa, NaOH, and others are reported to
catalyze the reaction via deprotonating the alcohol to produce active RO
− species
which further react with the carbonyl group and get converted into final transesterified product (RCOOR
I
). The presence of free fatty acids in the feedstock is a
hindrance for this process as it leads to soap formation due to the reaction of
hydroxide groups of alkali catalyst and free fatty acid groups. Many studies of two
step processing have been reported where the acid value of the oil has been reduced
by esterification step initially and then the processed oil further utilized in transesterification step (Joshi et al. 2017). The requirement of two steps makes the
overall cost of production much higher. Also it is rather difficult to develop a
commercial process which will effectively separate the glycerol from FAME produced especially in the presence of soap, which can be formed based on the free
fatty acid content. Handling of chemical waste generated from neutralization of
base catalyst is also a major problem.
Acid Catalyst
AAcid catalysts find less application as compared with the base catalysts due to
their slower reaction rates. They are used mostly with feedstocks which have a high
free fatty acid content as they catalyze the reaction of esterification and transesterification simultaneously as well as does not give processing problems in terms of
soap formation. Study was reported with mixotrophic approach first to increase the
lipid content in the microalgae (C. protothecoides) and further sulfuric acid was
used as a catalyst in acidic transesterification reaction performed with methanol in
excess at 56:1 molar ratio (Miao and Wu 2006). Study related to the comparison of
the use of H 2 SO 4 with HCl in transesterification reaction established that HCl gave
10% higher yield as compared to 2 SO 4 (Kim et al. 2015). Commercial application
of the use of acid catalyst is not economically feasible as it leads to generation of
waste and higher temperature and pressure are required for the reaction and also the
slower reaction rate, which leads to higher energy consumption. The longer reaction
time with high temperature may also lead to corrosion of reactor due to the prolonged use of acidic conditions.
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S. Joshi and P. Gogate
Base Catalyst
In transesterification reaction, base catalysts are mostly used as they are cheaply
available and allow the usage of moderate reaction temperature and pressure which
helps in carrying out the process with favorable conditions. Base catalysts also give
higher yield in shorter period of time as compared to other catalysts (Schuchardt
et al. 1998). Bases such as KOH, CH 3 ONa, NaOH, and others are reported to
catalyze the reaction via deprotonating the alcohol to produce active RO
− species
which further react with the carbonyl group and get converted into final transesterified product (RCOOR
I
). The presence of free fatty acids in the feedstock is a
hindrance for this process as it leads to soap formation due to the reaction of
hydroxide groups of alkali catalyst and free fatty acid groups. Many studies of two
step processing have been reported where the acid value of the oil has been reduced
by esterification step initially and then the processed oil further utilized in transesterification step (Joshi et al. 2017). The requirement of two steps makes the
overall cost of production much higher. Also it is rather difficult to develop a
commercial process which will effectively separate the glycerol from FAME produced especially in the presence of soap, which can be formed based on the free
fatty acid content. Handling of chemical waste generated from neutralization of
base catalyst is also a major problem.
Acid Catalyst
AAcid catalysts find less application as compared with the base catalysts due to
their slower reaction rates. They are used mostly with feedstocks which have a high
free fatty acid content as they catalyze the reaction of esterification and transesterification simultaneously as well as does not give processing problems in terms of
soap formation. Study was reported with mixotrophic approach first to increase the
lipid content in the microalgae (C. protothecoides) and further sulfuric acid was
used as a catalyst in acidic transesterification reaction performed with methanol in
excess at 56:1 molar ratio (Miao and Wu 2006). Study related to the comparison of
the use of H 2 SO 4 with HCl in transesterification reaction established that HCl gave
10% higher yield as compared to 2 SO 4 (Kim et al. 2015). Commercial application
of the use of acid catalyst is not economically feasible as it leads to generation of
waste and higher temperature and pressure are required for the reaction and also the
slower reaction rate, which leads to higher energy consumption. The longer reaction
time with high temperature may also lead to corrosion of reactor due to the prolonged use of acidic conditions.
68
S. Joshi and P. Gogate