stable carbenium ion. Thus the conversion value for
the glycerol–formaldehyde system is relatively small
as compared to the reaction where acetone is used as
a co-reactant, as shown in Figure 7 (Fatimah et al.,
2019).
The stability of catalysts is one of the main hurdles for the commercialization of glycerol to solketal.
Even though the reaction temperature was considered
as mild, the stability of most of the solid catalysts
decayed in the presence of water as a by-product and
other impurities (NaCl, methanol) from the glycerol
source. The deactivation rate is even higher when the
raw glycerol (contaminated with water) was fed to
the reactor. Therefore, the viability of the commercial
plant depends on (Fatimah et al., 2019):
(i) the source of feeds,
(ii) availability of glycerol and other feeds, and
(iii) cost of glycerol as the feed.
Three main challenges for production of solketal
were identified (Fatimah et al., 2019):
(a) The presence of water and impurities in the feed.
(b) The shift from the batch reactor to the fixed bed
reactor.
(c) The presence of equilibrium offers other difficulties as higher acetone demand is expected.
However, higher acetone to glycerol will lead to
destructive instruments.
Protonic zeolites seem to be the most promising
and widely applied acid catalysts in the chemical
industry. In addition to presenting the apparent benefits of heterogeneous catalysts like reusability and
simple recovery, protonic zeolites exemplify ecofriendly solid acid catalysts unlike conventional liquid acids. These are aluminosilicates characterized
by uniform microporous crystalline structure, shape
selectivity, hydrophobicity, and high thermal stability.
It is ascertained that the stronger acidity of protonic zeolites is consistent with the bridging hydroxyl
groups Al(OH)Si located in the cavities of zeolite.
H-Mordenite is one such protonic zeolite found to hold
excellent structural and textural properties that are not
seen in other catalytic materials. Unlike other zeolites, even though mordenite is a microporous material,
it possesses a special multiple pore channel system
where its elliptical pores are wide enough to allow
many reactions to be attained with high selectivity
(Priya et al., 2017).
The formation of water during glycerol acetalization reactions is problematic and has to be addressed in
order not to trigger the reverse reaction. The hydrophobic property of the zeolite assists in inhibiting the
reverse reaction of acetalization by diffusion of water
in to the pores of zeolite. The modification of the physiochemical properties of zeolites to further improve
their efficiency in reactions can be achieved by incorporating metal atoms into the framework or extra
framework of zeolite structure (Priya et al., 2017).
The ketalization reaction proceeds via an acid
catalyzed mechanism, hence catalysts with stronger
acidity might lead to higher glycerol conversion. The
influence of catalyst acidity on the solketal yield is
shown in Table 1 below. It is clear that the catalyst
acidity is a crucial parameter influencing the catalytic
performance (Nanda et al., 2016).
The majority of studies on the synthesis of solketal were carried out in batch reactors with the use of
heterogeneous catalysts such as zeolites, amberlyst,
montmorillonite, silica-induced heterolpolyacids, and
Nafion. However, batch processes have several limitations including the long time their reactions take (often
exceeding 2 hours), reducing their efficiency, and the
difficulty associated with scaling them up. Using a
continuous-flow reactor with heterogenous catalysts
is, therefore, more strategic because of the advantages
it brings to heat and mass transfer management, and the
ease it brings to scaling up from laboratory setups to
industrial scale setup. Using a continuous process also
leads to more environmental and economical benefits
offering a constant quality of the end product (Nanda
et al., 2016).
3.5 Microwave heating
Most of the liquid phase reactions over heterogeneous
catalysts were conducted by direct heating or in oil
bath which is an inefficient and slow process in terms
of energy transfer. The use of microwave-assisted
chemical synthesis has become increasingly popular
and has proven to be more efficient than conventional heating methods due to the high-speed synthesis
involving rapid internal heating, with a direct transfer
of microwave energy from source to the molecules
in the reaction. Microwave processing brings about
uniform heating that can regulate the temperature and
brings about considerable energy savings. The chemical syntheses by microwave irradiation as a heating
method has received significant interest over traditional heating methods offering high conversion and
selectivity (Priya et al., 2017).
Thermo-chemical conversion methods can be used
to convert glycerol into useful secondary products.
Common methods include combustion, gasification,
and pyrolysis. Combustion is a process which involves
the complete oxidation of a product, gasification
involves high temperatures but only partial oxidation,
while pyrolysis is the thermal degradation process that
occurs in the absence of oxygen, typically performed
by using conventional furnace heating and microwave
heating techniques. The latter method heats more
effectively and consumes lower energy than the former.
Microwave pyrolysis utilizes electromagnetic waves
to transfer heat, and by heating the material internally
at the molecular level, heat loss is minimized when
compared to conventional heating methods. During
the pyrolysis process, primary and secondary pyrolysis
can occur. Primary pyrolysis involves the process of
creating dehydration, dehydrogenation, decarboxylation or decarbonization reactions. Secondary pyrolysis
involves thermal or catalytic cracking where heavy
compounds are further broken down into gases. The
260
the glycerol–formaldehyde system is relatively small
as compared to the reaction where acetone is used as
a co-reactant, as shown in Figure 7 (Fatimah et al.,
2019).
The stability of catalysts is one of the main hurdles for the commercialization of glycerol to solketal.
Even though the reaction temperature was considered
as mild, the stability of most of the solid catalysts
decayed in the presence of water as a by-product and
other impurities (NaCl, methanol) from the glycerol
source. The deactivation rate is even higher when the
raw glycerol (contaminated with water) was fed to
the reactor. Therefore, the viability of the commercial
plant depends on (Fatimah et al., 2019):
(i) the source of feeds,
(ii) availability of glycerol and other feeds, and
(iii) cost of glycerol as the feed.
Three main challenges for production of solketal
were identified (Fatimah et al., 2019):
(a) The presence of water and impurities in the feed.
(b) The shift from the batch reactor to the fixed bed
reactor.
(c) The presence of equilibrium offers other difficulties as higher acetone demand is expected.
However, higher acetone to glycerol will lead to
destructive instruments.
Protonic zeolites seem to be the most promising
and widely applied acid catalysts in the chemical
industry. In addition to presenting the apparent benefits of heterogeneous catalysts like reusability and
simple recovery, protonic zeolites exemplify ecofriendly solid acid catalysts unlike conventional liquid acids. These are aluminosilicates characterized
by uniform microporous crystalline structure, shape
selectivity, hydrophobicity, and high thermal stability.
It is ascertained that the stronger acidity of protonic zeolites is consistent with the bridging hydroxyl
groups Al(OH)Si located in the cavities of zeolite.
H-Mordenite is one such protonic zeolite found to hold
excellent structural and textural properties that are not
seen in other catalytic materials. Unlike other zeolites, even though mordenite is a microporous material,
it possesses a special multiple pore channel system
where its elliptical pores are wide enough to allow
many reactions to be attained with high selectivity
(Priya et al., 2017).
The formation of water during glycerol acetalization reactions is problematic and has to be addressed in
order not to trigger the reverse reaction. The hydrophobic property of the zeolite assists in inhibiting the
reverse reaction of acetalization by diffusion of water
in to the pores of zeolite. The modification of the physiochemical properties of zeolites to further improve
their efficiency in reactions can be achieved by incorporating metal atoms into the framework or extra
framework of zeolite structure (Priya et al., 2017).
The ketalization reaction proceeds via an acid
catalyzed mechanism, hence catalysts with stronger
acidity might lead to higher glycerol conversion. The
influence of catalyst acidity on the solketal yield is
shown in Table 1 below. It is clear that the catalyst
acidity is a crucial parameter influencing the catalytic
performance (Nanda et al., 2016).
The majority of studies on the synthesis of solketal were carried out in batch reactors with the use of
heterogeneous catalysts such as zeolites, amberlyst,
montmorillonite, silica-induced heterolpolyacids, and
Nafion. However, batch processes have several limitations including the long time their reactions take (often
exceeding 2 hours), reducing their efficiency, and the
difficulty associated with scaling them up. Using a
continuous-flow reactor with heterogenous catalysts
is, therefore, more strategic because of the advantages
it brings to heat and mass transfer management, and the
ease it brings to scaling up from laboratory setups to
industrial scale setup. Using a continuous process also
leads to more environmental and economical benefits
offering a constant quality of the end product (Nanda
et al., 2016).
3.5 Microwave heating
Most of the liquid phase reactions over heterogeneous
catalysts were conducted by direct heating or in oil
bath which is an inefficient and slow process in terms
of energy transfer. The use of microwave-assisted
chemical synthesis has become increasingly popular
and has proven to be more efficient than conventional heating methods due to the high-speed synthesis
involving rapid internal heating, with a direct transfer
of microwave energy from source to the molecules
in the reaction. Microwave processing brings about
uniform heating that can regulate the temperature and
brings about considerable energy savings. The chemical syntheses by microwave irradiation as a heating
method has received significant interest over traditional heating methods offering high conversion and
selectivity (Priya et al., 2017).
Thermo-chemical conversion methods can be used
to convert glycerol into useful secondary products.
Common methods include combustion, gasification,
and pyrolysis. Combustion is a process which involves
the complete oxidation of a product, gasification
involves high temperatures but only partial oxidation,
while pyrolysis is the thermal degradation process that
occurs in the absence of oxygen, typically performed
by using conventional furnace heating and microwave
heating techniques. The latter method heats more
effectively and consumes lower energy than the former.
Microwave pyrolysis utilizes electromagnetic waves
to transfer heat, and by heating the material internally
at the molecular level, heat loss is minimized when
compared to conventional heating methods. During
the pyrolysis process, primary and secondary pyrolysis
can occur. Primary pyrolysis involves the process of
creating dehydration, dehydrogenation, decarboxylation or decarbonization reactions. Secondary pyrolysis
involves thermal or catalytic cracking where heavy
compounds are further broken down into gases. The
260