441
the number of protons exchangeable in Keggin secondary structure. It is known
that partially exchanged HPAs display high acidity than fully exchanged or parent acid owed to high protons’ mobility [43].
Partial exchange of protons of HPAs with cations of large size such as Cs
+
,
Rb
+
, NH 4
+
and K
+
results in conversion of a water-soluble acid with low surface
area (<5 m
2
/g) into a water-insoluble acid salt with surface area more than
100 m
2
/g [44]. Heteropoly salts are more stable than the parent acid. However,
the relative stability depends upon the counter cation [45]. The acidic Cs salt of
TPA (Cs 2.5 H 0.5 PW 12 O 40 ) is stable better than parent H 3 PW 12 O 40 . This salt was not
decomposed at high temperature >500 °C, while the parent acid decomposes at
lower temperatures (about 300 °C). In metal-substituted HPAs, polar solvents
like water and alcohol can easily diffuse and exit in bulk, which leads to expansion or contraction of the distance between the Keggin anions in the crystal lattice. In case of non-polar molecules like hydrocarbons cannot enter into the bulk.
This specific character of the Keggin structure, few reactions undergo in bulk
and this state is called pseudo liquid phase. Non-polar molecules are incompetent to be absorbed in the bulk of HPA and interact only with the surface [34].
Some of the HPA salts of metal ions (Mn
+
) of PW 12 O 40
3−
(M 3/n PW 12 O 40 ) exhibit
Lewis acidity [46, 47], due to the presence of metal cation which acts as electron
pair acceptor, distinct from the characteristic Bronsted acidity of HPAs.
2. Supported Keggin Heteropoly Acids
The second type of modification of HPAs is to support on high surface area acidic
or neutral supports like zirconia, silica, titania, niobia, ceria, tin oxide and mesoporous materials (SBA-15, MCM-41, HMCM-41). When HPA is supported on
support with acidic sites, a deep interaction between heteropoly acid and surface
of oxide results in the formation of species such as [≡M-OH 2 ] n
+
[H 3-n PW 12 O 40 ] n
−3
.
Therefore, one can overcome the solubility problem. On the other hand, support
enhances the surface area of active HPA. The number of active sites of solid
material is also enhanced along with stability and thereby its catalytic activity.
Basic supports are not suggestible because HPAs readily decompose on basic
supports. As the supported samples are heterogeneous, these can be simply separable from the reaction mixture in liquid-phase applications, and stability
increases in high-temperature gas-phase reactions [36]. Figure 10 shows the representation of HPAs supported on acidic supports.
6 Conversion of Carbohydrates to 5-Hydroxymethylfurfural
5-Hydroxymethylfurfural, a dehydration product from hexoses, is judged as a biorenewable platform chemical. The high functionality of HMF allows it to be converted to various important biofuel molecules as shown in Fig. 11 such as ethyl
levulinate, 5-ethoxymethylfurfural and 2,5-dimethylfuran (DMF) and valuable
chemicals such as levulinic acid, 2,5-diformylfuran (DFF), 2,5-furandicarboxylic
acid, adipic acid and caprolactone.
Carbohydrates to Chemicals and Fuel Additives over ModifiedPolyoxometalate Catalysts
the number of protons exchangeable in Keggin secondary structure. It is known
that partially exchanged HPAs display high acidity than fully exchanged or parent acid owed to high protons’ mobility [43].
Partial exchange of protons of HPAs with cations of large size such as Cs
+
,
Rb
+
, NH 4
+
and K
+
results in conversion of a water-soluble acid with low surface
area (<5 m
2
/g) into a water-insoluble acid salt with surface area more than
100 m
2
/g [44]. Heteropoly salts are more stable than the parent acid. However,
the relative stability depends upon the counter cation [45]. The acidic Cs salt of
TPA (Cs 2.5 H 0.5 PW 12 O 40 ) is stable better than parent H 3 PW 12 O 40 . This salt was not
decomposed at high temperature >500 °C, while the parent acid decomposes at
lower temperatures (about 300 °C). In metal-substituted HPAs, polar solvents
like water and alcohol can easily diffuse and exit in bulk, which leads to expansion or contraction of the distance between the Keggin anions in the crystal lattice. In case of non-polar molecules like hydrocarbons cannot enter into the bulk.
This specific character of the Keggin structure, few reactions undergo in bulk
and this state is called pseudo liquid phase. Non-polar molecules are incompetent to be absorbed in the bulk of HPA and interact only with the surface [34].
Some of the HPA salts of metal ions (Mn
+
) of PW 12 O 40
3−
(M 3/n PW 12 O 40 ) exhibit
Lewis acidity [46, 47], due to the presence of metal cation which acts as electron
pair acceptor, distinct from the characteristic Bronsted acidity of HPAs.
2. Supported Keggin Heteropoly Acids
The second type of modification of HPAs is to support on high surface area acidic
or neutral supports like zirconia, silica, titania, niobia, ceria, tin oxide and mesoporous materials (SBA-15, MCM-41, HMCM-41). When HPA is supported on
support with acidic sites, a deep interaction between heteropoly acid and surface
of oxide results in the formation of species such as [≡M-OH 2 ] n
+
[H 3-n PW 12 O 40 ] n
−3
.
Therefore, one can overcome the solubility problem. On the other hand, support
enhances the surface area of active HPA. The number of active sites of solid
material is also enhanced along with stability and thereby its catalytic activity.
Basic supports are not suggestible because HPAs readily decompose on basic
supports. As the supported samples are heterogeneous, these can be simply separable from the reaction mixture in liquid-phase applications, and stability
increases in high-temperature gas-phase reactions [36]. Figure 10 shows the representation of HPAs supported on acidic supports.
6 Conversion of Carbohydrates to 5-Hydroxymethylfurfural
5-Hydroxymethylfurfural, a dehydration product from hexoses, is judged as a biorenewable platform chemical. The high functionality of HMF allows it to be converted to various important biofuel molecules as shown in Fig. 11 such as ethyl
levulinate, 5-ethoxymethylfurfural and 2,5-dimethylfuran (DMF) and valuable
chemicals such as levulinic acid, 2,5-diformylfuran (DFF), 2,5-furandicarboxylic
acid, adipic acid and caprolactone.
Carbohydrates to Chemicals and Fuel Additives over ModifiedPolyoxometalate Catalysts