Resin catalysts, and amberlyst in particular, have
been known to increase the efficiency of the conversion of glycerol to solketal and promote the selectivity
for its molecules, but their use is not practical for
widescale production because their reactions have
poor thermal stability that makes it impossible to
recycle and reuse them. Hierarchical zeolites, on the
other hand, which have higher thermal stability are so
far the catalyst responsible for the highest recorded
glycerol–solketal conversion at 72% efficiency with
72% selectivity for solketal molecules in a reaction
using H-Beta (BEA framework) at a temperature of
60˚ C and stirring at 700 rpm with 5% of catalyst
and glycerol:acetone molar ratio of 1:4 for H-BEA.
Moreover, despite MFI zeolite being known to produce lower yields than those achieved using amberlyst
at 80%, mainly because of the relatively narrow channel size that affects the transport of the reactant carried
out and the shape selectivity, their selectivity rates are
almost 100%. The basic mechanism of metal salt catalysis is a nucleophilic attack by the hydroxyl group of
glycerol to the carbocation obtained from the protonation step, resulting in the formation of the intermediate,
followed by a water elimination step. The carbocation
is produced from the Lewis or Brønsted acid sites,
which activate the ketone carbonyl group through a
protonation step (i.e., Brønsted acids) or polarization
(Fatimah et al., 2019; Ilgen et al., 2017; Nanda et al.,
2016; Pandian Manjunathan et al., 2014; Priya et al.,
2017).
To search for an effective heterogeneous catalyst for
the ketalization process, the reaction in a stirred batch
reactor over a series of silica-induced heteropolyacid
catalysts, i.e., tungsto-phosphoric acid (PW), tungstosilisic acid (SiW), molybdo-phosphoric acid (PMo),
and molybdo-silisic acid (SiMo) has been studied. The
reported catalytic activities for the catalysts are in the
order of: SiMo
conversion of more than 97% with a very high selectivity of 99% towards solketal at the reaction conditions:
70 ˚ C, A/G of 12:1, catalyst (PW) loading of 0.2 g,
and 2–3 h. The high yield of solketal in this work was
attributed to the strong acidity of the catalyst that promoted the reaction kinetics and to the high A/G (12:1).
Good catalytic stability was also observed, as the catalyst lost its activity by ∼15% after four consecutive
batch runs using the same catalyst (Nanda et al., 2016).
3.2 Glycerol to solketal over resin catalysts
According to Fatimah et al. (2019), Brønsted’s acidity
is the most important property of solid acid catalysts
in the production of solketal from glycerol. A typical
resin catalyst (i.e., amberlyst) catalyzed the reaction
of glycerol with acetone to produce above 80% of the
glycerol conversion. It has been reported that a resin,
amberlyst-36, which was applied at different reaction
temperatures from 25 to 70˚ C, was an excellent catalyst to convert glycerol with a conversion of 85% to
97% to solketal with a selectivity of 99%. The catalyst
is also active at lower pressures with similar reaction
parameters, either in pure glycerol or in an equimolar
reactant. According to some references, the high conversion was influenced not only by the surface acidity
but also by the resin structure. Moreover, the surface
acidity was an important parameter that played a crucial role in improving the selectivity and the conversion
in the production of solketal. All resins showed good
selectivity to solketal (>80%), and the important catalytic parameter of the resin to conversion of glycerol
is the acid capacity (oversulfonated resin). With the
highest acid capacity (sulfonic acid), these catalyst
materials can improve not only the selectivity to solketal production but also the conversion of raw glycerol
to above 90%. A limitation of the catalyst activity is the
presence of NaCl as a poison for the surface acidity,
which is possibly due to the impurities in glycerol.
3.3 Glycerol to solketal over mesoporous silica
Hafnium and zirconium modified TUD-1 have been
reported as being superior catalysts for the conversion
of glycerol to solketal. These two catalysts (Hf-TUD-1
and Zr-TUD-1) are examples of active metal-modified
mesoporous silica in which Hf and Zr are in the framework. Their activity was higher than FAU(USY) and
Al(TUD-1). The highest conversion of glycerol to
solketal was more than Ž0%. The catalytic activity was
a function of:
(i) the number of acid sites,
(ii) the presence of mesopores,
(iii) the existence of a large surface area, and
(iv) the hydrophobicity of the catalyst.
The hydrophobicity of the catalyst, was crucial to
prevent the hydrolysis of solketal. Numerous references have reported that mesoporous silica catalysts
have the advantage of high stability in the conversion of glycerol to solketal, resulting in processes
with a relatively higher percentage of conversion
(95%) and selectivity to solketal (98%). A sulfonic
acid-functionalized mesoporous polymer (MP-SO 3 H)
contains a high acidity surface (1.88 mmol/g). The
surface acidity of catalytic materials can accelerate
the formation products of solketal via ketalization
reactions (Fatimah et al., 2019).
3.4 Ketalization of glycerol over clay minerals
Studies of different clay-based catalysts with different acid strengths ranging from 0.12 to 5.7 meq/g
show that a stronger acidity improved the conversion
of glycerol up to ca. 80%. The use of formaldehyde as the major source of solketal production has
a lower conversion value (only 83% glycerol conversion), with the K10 montmorillonite used as a
catalyst. The reaction between glycerol and acetone
is preferred as it produces a more stable intermediate
hemicetal compound, with a tertiary carbenium ion.
While, in the reaction between glycerol with formaldehyde, the produced hemiacetal formation is not a
259
been known to increase the efficiency of the conversion of glycerol to solketal and promote the selectivity
for its molecules, but their use is not practical for
widescale production because their reactions have
poor thermal stability that makes it impossible to
recycle and reuse them. Hierarchical zeolites, on the
other hand, which have higher thermal stability are so
far the catalyst responsible for the highest recorded
glycerol–solketal conversion at 72% efficiency with
72% selectivity for solketal molecules in a reaction
using H-Beta (BEA framework) at a temperature of
60˚ C and stirring at 700 rpm with 5% of catalyst
and glycerol:acetone molar ratio of 1:4 for H-BEA.
Moreover, despite MFI zeolite being known to produce lower yields than those achieved using amberlyst
at 80%, mainly because of the relatively narrow channel size that affects the transport of the reactant carried
out and the shape selectivity, their selectivity rates are
almost 100%. The basic mechanism of metal salt catalysis is a nucleophilic attack by the hydroxyl group of
glycerol to the carbocation obtained from the protonation step, resulting in the formation of the intermediate,
followed by a water elimination step. The carbocation
is produced from the Lewis or Brønsted acid sites,
which activate the ketone carbonyl group through a
protonation step (i.e., Brønsted acids) or polarization
(Fatimah et al., 2019; Ilgen et al., 2017; Nanda et al.,
2016; Pandian Manjunathan et al., 2014; Priya et al.,
2017).
To search for an effective heterogeneous catalyst for
the ketalization process, the reaction in a stirred batch
reactor over a series of silica-induced heteropolyacid
catalysts, i.e., tungsto-phosphoric acid (PW), tungstosilisic acid (SiW), molybdo-phosphoric acid (PMo),
and molybdo-silisic acid (SiMo) has been studied. The
reported catalytic activities for the catalysts are in the
order of: SiMo
conversion of more than 97% with a very high selectivity of 99% towards solketal at the reaction conditions:
70 ˚ C, A/G of 12:1, catalyst (PW) loading of 0.2 g,
and 2–3 h. The high yield of solketal in this work was
attributed to the strong acidity of the catalyst that promoted the reaction kinetics and to the high A/G (12:1).
Good catalytic stability was also observed, as the catalyst lost its activity by ∼15% after four consecutive
batch runs using the same catalyst (Nanda et al., 2016).
3.2 Glycerol to solketal over resin catalysts
According to Fatimah et al. (2019), Brønsted’s acidity
is the most important property of solid acid catalysts
in the production of solketal from glycerol. A typical
resin catalyst (i.e., amberlyst) catalyzed the reaction
of glycerol with acetone to produce above 80% of the
glycerol conversion. It has been reported that a resin,
amberlyst-36, which was applied at different reaction
temperatures from 25 to 70˚ C, was an excellent catalyst to convert glycerol with a conversion of 85% to
97% to solketal with a selectivity of 99%. The catalyst
is also active at lower pressures with similar reaction
parameters, either in pure glycerol or in an equimolar
reactant. According to some references, the high conversion was influenced not only by the surface acidity
but also by the resin structure. Moreover, the surface
acidity was an important parameter that played a crucial role in improving the selectivity and the conversion
in the production of solketal. All resins showed good
selectivity to solketal (>80%), and the important catalytic parameter of the resin to conversion of glycerol
is the acid capacity (oversulfonated resin). With the
highest acid capacity (sulfonic acid), these catalyst
materials can improve not only the selectivity to solketal production but also the conversion of raw glycerol
to above 90%. A limitation of the catalyst activity is the
presence of NaCl as a poison for the surface acidity,
which is possibly due to the impurities in glycerol.
3.3 Glycerol to solketal over mesoporous silica
Hafnium and zirconium modified TUD-1 have been
reported as being superior catalysts for the conversion
of glycerol to solketal. These two catalysts (Hf-TUD-1
and Zr-TUD-1) are examples of active metal-modified
mesoporous silica in which Hf and Zr are in the framework. Their activity was higher than FAU(USY) and
Al(TUD-1). The highest conversion of glycerol to
solketal was more than Ž0%. The catalytic activity was
a function of:
(i) the number of acid sites,
(ii) the presence of mesopores,
(iii) the existence of a large surface area, and
(iv) the hydrophobicity of the catalyst.
The hydrophobicity of the catalyst, was crucial to
prevent the hydrolysis of solketal. Numerous references have reported that mesoporous silica catalysts
have the advantage of high stability in the conversion of glycerol to solketal, resulting in processes
with a relatively higher percentage of conversion
(95%) and selectivity to solketal (98%). A sulfonic
acid-functionalized mesoporous polymer (MP-SO 3 H)
contains a high acidity surface (1.88 mmol/g). The
surface acidity of catalytic materials can accelerate
the formation products of solketal via ketalization
reactions (Fatimah et al., 2019).
3.4 Ketalization of glycerol over clay minerals
Studies of different clay-based catalysts with different acid strengths ranging from 0.12 to 5.7 meq/g
show that a stronger acidity improved the conversion
of glycerol up to ca. 80%. The use of formaldehyde as the major source of solketal production has
a lower conversion value (only 83% glycerol conversion), with the K10 montmorillonite used as a
catalyst. The reaction between glycerol and acetone
is preferred as it produces a more stable intermediate
hemicetal compound, with a tertiary carbenium ion.
While, in the reaction between glycerol with formaldehyde, the produced hemiacetal formation is not a
259