Enzymatic Catalyst
Enzyme-based transesterification process is an attractive alternative to the chemical
catalysts. Enzymes can work under mild reaction conditions with low temperature
and pressure requirement and can also tolerate the FFA and water in reaction
mixture. Typically the important operating parameters as the pH of the reaction,
concentration of enzymes, and substrates, and the interactive distance between
substrate and enzyme plays crucial role in deciding the rates of reactions carried out
using enzymes. Enzymes can be denatured and destabilized by excess methanol and
glycerol present in the reaction mixture. Also the prices of enzymes are higher
which hamper the process economics (Suali and Sarbatly 2012). To overcome the
above-mentioned issues, the enzymes can be used after they are immobilized
transforming the system into heterogeneous. Immobilization is carried out via
adsorption, encapsulation, entrapment, and cross-linking. Adsorption is the oldest
and most commonly used method as it is less expensive as compared to other
available methods. Many studies have been reported for the use of immobilized
lipase in transesterification reaction for production of biodiesel (Subhedar et al.
2015). The advantage of reusability also makes immobilized form a more feasible
option as compared to the free form, though the mass transfer limitations need to be
looked at due to the heterogeneous nature of system.
5.1.2 Catalytic Heterogeneous Transesterification
The heterogeneous solid catalysts are environmentally friendly as they are easily
separable from reaction mixture and hence reusable. Easy separation of catalyst
from reaction mixture with simple filtration helps in improving the process economics. Solid catalysts are further classified as solid acid catalyst and solid base
catalyst. Solid acid catalyst includes resins, polyaniline sulfate, zeolite tungstated
and sulfated zirconia, sulfated tin oxide, heteropolyacid, metal complexes, and
acidic ionic liquids. Solid base catalyst includes calcium oxide, hydrotalcite (also
called layered double hydroxide), zeolites, and alumina. Yield of 97.5% was
reported for the biodiesel production from lipids extracted from Nannochloropsis
oculata when Al 2 O 3 -supported CaO and MgO were used as catalysts under processing conditions of excess of methanol (1:30) and catalyst loading (80% w/w)
required for the completion of reaction (Umdu et al. 2009). Use of Mg–Zr was
proposed in one of the study performed where in situ and two-step processes were
compared and single-step approach was found to be more efficient. Reaction was
performed with a mixture of methanol and methylene dichloride in ratio of 3:1 with
10% w/v catalyst at 65 °C for 4 h and a yield of 28% of methyl esters was reported
(Li et al. 2011b). A study on utilization of hierarchal zeolites in transesterification
was performed to establish the specific form of zeolite that can yield highest
conversion rate. From the study, H-beta zeolite was established to give higher
conversion rates as compared to other zeolites (Carrero et al. 2011). Study was
performed using KOH/La–Ba–Al 2 O 3 as the heterogeneous catalyst for conversion
4 Process Intensification of Biofuel Production …
69
Enzyme-based transesterification process is an attractive alternative to the chemical
catalysts. Enzymes can work under mild reaction conditions with low temperature
and pressure requirement and can also tolerate the FFA and water in reaction
mixture. Typically the important operating parameters as the pH of the reaction,
concentration of enzymes, and substrates, and the interactive distance between
substrate and enzyme plays crucial role in deciding the rates of reactions carried out
using enzymes. Enzymes can be denatured and destabilized by excess methanol and
glycerol present in the reaction mixture. Also the prices of enzymes are higher
which hamper the process economics (Suali and Sarbatly 2012). To overcome the
above-mentioned issues, the enzymes can be used after they are immobilized
transforming the system into heterogeneous. Immobilization is carried out via
adsorption, encapsulation, entrapment, and cross-linking. Adsorption is the oldest
and most commonly used method as it is less expensive as compared to other
available methods. Many studies have been reported for the use of immobilized
lipase in transesterification reaction for production of biodiesel (Subhedar et al.
2015). The advantage of reusability also makes immobilized form a more feasible
option as compared to the free form, though the mass transfer limitations need to be
looked at due to the heterogeneous nature of system.
5.1.2 Catalytic Heterogeneous Transesterification
The heterogeneous solid catalysts are environmentally friendly as they are easily
separable from reaction mixture and hence reusable. Easy separation of catalyst
from reaction mixture with simple filtration helps in improving the process economics. Solid catalysts are further classified as solid acid catalyst and solid base
catalyst. Solid acid catalyst includes resins, polyaniline sulfate, zeolite tungstated
and sulfated zirconia, sulfated tin oxide, heteropolyacid, metal complexes, and
acidic ionic liquids. Solid base catalyst includes calcium oxide, hydrotalcite (also
called layered double hydroxide), zeolites, and alumina. Yield of 97.5% was
reported for the biodiesel production from lipids extracted from Nannochloropsis
oculata when Al 2 O 3 -supported CaO and MgO were used as catalysts under processing conditions of excess of methanol (1:30) and catalyst loading (80% w/w)
required for the completion of reaction (Umdu et al. 2009). Use of Mg–Zr was
proposed in one of the study performed where in situ and two-step processes were
compared and single-step approach was found to be more efficient. Reaction was
performed with a mixture of methanol and methylene dichloride in ratio of 3:1 with
10% w/v catalyst at 65 °C for 4 h and a yield of 28% of methyl esters was reported
(Li et al. 2011b). A study on utilization of hierarchal zeolites in transesterification
was performed to establish the specific form of zeolite that can yield highest
conversion rate. From the study, H-beta zeolite was established to give higher
conversion rates as compared to other zeolites (Carrero et al. 2011). Study was
performed using KOH/La–Ba–Al 2 O 3 as the heterogeneous catalyst for conversion
4 Process Intensification of Biofuel Production …
69