472
3.2 Liquid Phase Dehydration of Glycerol to Acrolein
In some recent years, supercritical water reforming of glycerol has gained much
consideration for producing valued chemicals like acrolein. Ott et al. [90] assessed
the effect of zinc sulphate (Zn-SO 4 ) catalyst in supercritical water condition and in
continuous high-pressure tubular reactor. At 25 MPa pressure and 633 K reaction
temperature, 50% glycerol conversion could be achieved with 75 mol.% selectivity
towards acrolein. Liquid phase dehydration of glycerol was investigated at elevated
temperature and atmospheric pressure by Suzuki et al. [91] and Yoshimi et al. [92]
where they used potassium bisulphate (KHSO 4 ), magnesium sulphate (MgSO 4 ) and
phosphorus alumina (5 wt% H 3 PO 4 /Al 2 O 3 ) as catalysts. KHSO 4 achieved 80% acrolein yield with 97% conversion of glycerol. Shen et al. [93] performed liquid phase
glycerol dehydration to acrolein in semi-batch reactor using silicotungstic, phosphotungstic and phosphomolybdic acids. The best yield for acrolein (78.6%) was
obtained over silicotungstic acid catalysts. In another study by Shen et al. [94], they
prepared Brønsted acidic ionic liquid (BAIL) catalysts for liquid phase glycerol
dehydration. An acrolein yield of 57.4% with complete conversion of glycerol could
be achieved. The acidity of the catalyst was the main reason for its activity. The
above are some worthy mentions for liquid phase glycerol dehydration, but it is
quite evident that liquid phase glycerol dehydration is not as effective like gas phase
glycerol dehydration.
3.3 Reaction Mechanism
From Scheme 7, the glycerol dehydration mechanism can be seen on Lewis and
Brønsted acidic sites which leads to formation of acrolein and acetol, respectively,
as suggested by Alhanash et al. [76]. For the Brønsted acidic sites, the dehydration
begins with the secondary hydroxyl group protonation of glycerol and the release of
the H 3 O
+
by the secondary intermediate to form 1,3-dihydroxypropane, and then
keto-enol rearrangement gives 3-hydroxypropionaldehyde, which finally generates
acrolein followed by a further dehydration. On the other case, over Lewis acidic
sites, the generation of an intermediate 2,3-dihydroxypropane takes place by the
concerted deportation of the terminal OH group of glycerol to the Lewis acidic site,
and the H
+
shifts from the internal carbon atom to the bridging ‘O’-atom. After that
the final acetol is formed by further tautomerization of 2,3-dihydroxypropene. The
proposed mechanism by Alhanash et al. is supported by the study performed by Foo
et al. [95] in which they did glycerol dehydration over Nb 2 O 5 along with DFT calculations. They also came to the same conclusion that acrolein formation is favoured
by Brønsted acid sites and acetol formation is favoured by Lewis acid sites. The
glycerol dehydration study investigated by Wang et al. [96] suggested that there is a
cooperation between the Brønsted and Lewis acid sites.
K. Bhaduri et al.
3.2 Liquid Phase Dehydration of Glycerol to Acrolein
In some recent years, supercritical water reforming of glycerol has gained much
consideration for producing valued chemicals like acrolein. Ott et al. [90] assessed
the effect of zinc sulphate (Zn-SO 4 ) catalyst in supercritical water condition and in
continuous high-pressure tubular reactor. At 25 MPa pressure and 633 K reaction
temperature, 50% glycerol conversion could be achieved with 75 mol.% selectivity
towards acrolein. Liquid phase dehydration of glycerol was investigated at elevated
temperature and atmospheric pressure by Suzuki et al. [91] and Yoshimi et al. [92]
where they used potassium bisulphate (KHSO 4 ), magnesium sulphate (MgSO 4 ) and
phosphorus alumina (5 wt% H 3 PO 4 /Al 2 O 3 ) as catalysts. KHSO 4 achieved 80% acrolein yield with 97% conversion of glycerol. Shen et al. [93] performed liquid phase
glycerol dehydration to acrolein in semi-batch reactor using silicotungstic, phosphotungstic and phosphomolybdic acids. The best yield for acrolein (78.6%) was
obtained over silicotungstic acid catalysts. In another study by Shen et al. [94], they
prepared Brønsted acidic ionic liquid (BAIL) catalysts for liquid phase glycerol
dehydration. An acrolein yield of 57.4% with complete conversion of glycerol could
be achieved. The acidity of the catalyst was the main reason for its activity. The
above are some worthy mentions for liquid phase glycerol dehydration, but it is
quite evident that liquid phase glycerol dehydration is not as effective like gas phase
glycerol dehydration.
3.3 Reaction Mechanism
From Scheme 7, the glycerol dehydration mechanism can be seen on Lewis and
Brønsted acidic sites which leads to formation of acrolein and acetol, respectively,
as suggested by Alhanash et al. [76]. For the Brønsted acidic sites, the dehydration
begins with the secondary hydroxyl group protonation of glycerol and the release of
the H 3 O
+
by the secondary intermediate to form 1,3-dihydroxypropane, and then
keto-enol rearrangement gives 3-hydroxypropionaldehyde, which finally generates
acrolein followed by a further dehydration. On the other case, over Lewis acidic
sites, the generation of an intermediate 2,3-dihydroxypropane takes place by the
concerted deportation of the terminal OH group of glycerol to the Lewis acidic site,
and the H
+
shifts from the internal carbon atom to the bridging ‘O’-atom. After that
the final acetol is formed by further tautomerization of 2,3-dihydroxypropene. The
proposed mechanism by Alhanash et al. is supported by the study performed by Foo
et al. [95] in which they did glycerol dehydration over Nb 2 O 5 along with DFT calculations. They also came to the same conclusion that acrolein formation is favoured
by Brønsted acid sites and acetol formation is favoured by Lewis acid sites. The
glycerol dehydration study investigated by Wang et al. [96] suggested that there is a
cooperation between the Brønsted and Lewis acid sites.
K. Bhaduri et al.