198
Since oil and methanol are two mutually insoluble phases and transesterification is
a base-catalyzed process that is associated with a nucleophilic attack mechanism,
using PTC can enhance the process and dampens the technical disadvantages associated with the conventional biodiesel production process.
The efficacy of most of the conventional homogeneous reactions depends on the
mutual solubility of the dissimilar species (organic and inorganic) participating in
the reaction [17, 40]. In contrast, the reaction rates will be very low during direct
organic synthesis reactions in a reaction system of sparingly soluble reactants carried out using conventional catalysts. This disadvantage raises the need for PTC
which can offer a distinct route to mutually solubilize the dissimilar species. During
phase transfer catalysis, PTC forms a complex with the reactant present in phase 1,
and this complex diffuses into another reactant present in phase 2 and reacts. After
the reaction, PTC diffuses back to phase 1 and promotes the process further. Since
the vegetable oil and methanol are mutually insoluble and form distinct phases, the
concentration of two reactants in any single phase will be very low for good reaction
rates. Moreover, strict anhydrous conditions are required by the transesterification
using base catalyst whose reaction rates are frequently considered to be controlled
by diffusion and characterized by a slow reaction rate. Therefore, enhancing the
mass transfer between the phases can overcome the difficulty of the reaction
between them. There are several ways to overcome the mass transfer limitation and
enhance the contact between two phases such as the use of large excess alcohol,
mechanical mixing, using polar aprotic solvents or inert cosolvent, ultrasonic and
hydrodynamic cavitation, and supercritical conditions. However, these techniques
are associated with one or more drawbacks like the high cost of solvent or require a
very excessive amount of solvents, the high cost of operations, generation of huge
amounts of effluents, and higher capital costs [22, 41]. It was evident that eliminating the mass transfer limitations using cosolvent (which forms pseudo- homogeneous
phase) or catalyst-free processes (conducted at high temperature and pressure),
faster transesterification rates could be achieved [18–20]. So, phase transfer catalysis is also a promising technique that can be explored to enhance the reaction rates
between reactants forming two immiscible phases. There are many types of PTCs
such as onium salts (quaternary ammonium, sulfonium, phosphonium, or arsonium
salts), crown ether groups, and cryptates [17]. Among those quaternary ammonium
salts are most widely used because of the less interference of their ions in reactions
[17, 42] and also they are much cheaper than other PTCs [43]. Tetramethylammonium
(TMA) cations are a type of quaternary ammonium salts known to possess properties of PTC, and they are cations with four methyl groups attached to the central
nitrogen atom. These cations are associated with some anionic groups such as bromide, iodide, hydroxide, and chloride. There are many studies which used PTCs
(such as tetramethylammonium bromide, tetramethylammonium hydroxide, cetyltrimethylammonium bromide, tetrabutylammonium hydroxide, tetrabutylammonium acetate, tetrabutylammonium nitrate, benzyl trimethyl ammonium hydroxide,
tetrabutylammonium hydrogen sulfate, 18-crown-6 ether, and choline hydroxide) as
a process enhancer for conventional base-catalyzed transesterification process [7,
21, 22, 44, 45]. All this literature reports the improved reaction and enhanced
Z. Hussain et al.
Since oil and methanol are two mutually insoluble phases and transesterification is
a base-catalyzed process that is associated with a nucleophilic attack mechanism,
using PTC can enhance the process and dampens the technical disadvantages associated with the conventional biodiesel production process.
The efficacy of most of the conventional homogeneous reactions depends on the
mutual solubility of the dissimilar species (organic and inorganic) participating in
the reaction [17, 40]. In contrast, the reaction rates will be very low during direct
organic synthesis reactions in a reaction system of sparingly soluble reactants carried out using conventional catalysts. This disadvantage raises the need for PTC
which can offer a distinct route to mutually solubilize the dissimilar species. During
phase transfer catalysis, PTC forms a complex with the reactant present in phase 1,
and this complex diffuses into another reactant present in phase 2 and reacts. After
the reaction, PTC diffuses back to phase 1 and promotes the process further. Since
the vegetable oil and methanol are mutually insoluble and form distinct phases, the
concentration of two reactants in any single phase will be very low for good reaction
rates. Moreover, strict anhydrous conditions are required by the transesterification
using base catalyst whose reaction rates are frequently considered to be controlled
by diffusion and characterized by a slow reaction rate. Therefore, enhancing the
mass transfer between the phases can overcome the difficulty of the reaction
between them. There are several ways to overcome the mass transfer limitation and
enhance the contact between two phases such as the use of large excess alcohol,
mechanical mixing, using polar aprotic solvents or inert cosolvent, ultrasonic and
hydrodynamic cavitation, and supercritical conditions. However, these techniques
are associated with one or more drawbacks like the high cost of solvent or require a
very excessive amount of solvents, the high cost of operations, generation of huge
amounts of effluents, and higher capital costs [22, 41]. It was evident that eliminating the mass transfer limitations using cosolvent (which forms pseudo- homogeneous
phase) or catalyst-free processes (conducted at high temperature and pressure),
faster transesterification rates could be achieved [18–20]. So, phase transfer catalysis is also a promising technique that can be explored to enhance the reaction rates
between reactants forming two immiscible phases. There are many types of PTCs
such as onium salts (quaternary ammonium, sulfonium, phosphonium, or arsonium
salts), crown ether groups, and cryptates [17]. Among those quaternary ammonium
salts are most widely used because of the less interference of their ions in reactions
[17, 42] and also they are much cheaper than other PTCs [43]. Tetramethylammonium
(TMA) cations are a type of quaternary ammonium salts known to possess properties of PTC, and they are cations with four methyl groups attached to the central
nitrogen atom. These cations are associated with some anionic groups such as bromide, iodide, hydroxide, and chloride. There are many studies which used PTCs
(such as tetramethylammonium bromide, tetramethylammonium hydroxide, cetyltrimethylammonium bromide, tetrabutylammonium hydroxide, tetrabutylammonium acetate, tetrabutylammonium nitrate, benzyl trimethyl ammonium hydroxide,
tetrabutylammonium hydrogen sulfate, 18-crown-6 ether, and choline hydroxide) as
a process enhancer for conventional base-catalyzed transesterification process [7,
21, 22, 44, 45]. All this literature reports the improved reaction and enhanced
Z. Hussain et al.