439
HPAs based on P and Si central atoms and Mo and W as addenda atoms are commonly available. The Keggin structure is flexible and has five geometrical rotational
isomers, also known as Baker-Figgis isomers. These ions are known to exist in α
form, and this term is frequently used in several applications including heterogeneous catalysis [30].
Generally, heteropoly acids and their salts form ionic crystals composed of heteropoly anions known as primary structure; counter cations such as H
+
, H 3 O
+
and
H 5 O 2
+
; hydration water; and other molecules are secondary structure. The hierarchical structure of solid HPAs is essential for the understanding of its catalytic activity,
and the substructures are denoted as primary, secondary and tertiary structure [31].
The primary structure of the heteropoly acid is its anion [PW 12 O 40 ]
3−
that exist in
Keggin structure. The arrangement of primary structure together with counter cations forms the secondary structure (e.g. Cs 2.5 H 0.5 PW 12 O 40 ). The secondary structure
is flexible and relies on the amount of hydration water, counter cation and heteropoly anion. The aggregates of secondary structures in three-dimensional manner
give the tertiary structure. It explains the formation of solid particles and links to
properties like surface area, pore structure, particle size and distribution of protons
in particles [32]. This hierarchical structure of HPAs plays a crucial role in understanding and designing these as heterogeneous catalysts. This appears to be a simple
idea, but helps enormously in the research on HPA-based catalysis.
5.4 Acidic Properties of Keggin Heteropoly Acids
In aqueous solution, HPAs are strong acids. Their acidity is comparable to that of
common acids such as H 2 SO 4 , HNO 3 and HCl and hence is sometimes called superacids [33]. The protons present in the secondary structure of HPAs are considered
as mobile protons. Thus, the high mobility of these protons makes HPAs as superacids. The acid strength of HPAs varies in a wide range depending on polyanion
structure and its constituent elements (both heteroatoms and addenda atoms), on the
extent of hydration and reduction. Among the Keggin-type HPAs, MPA is stronger
acid and its acidity is close to that of superacid [34].
5.5 Modification of Keggin Heteropoly Acids
HPAs have been identified as efficient acid and oxidation catalysts. Bulk forms of
Keggin HPAs have limited number of catalytic applications since they are associated with certain drawbacks [35]. HPAs have low surface areas less than 10 m
2
/g,
which hinder approachability to their strong acidic sites. Due to their relatively low
specific surface area, catalytic activity of HPAs is limited in gas-phase reactions.
Solubility of HPAs in polar solvents is another limitation. These are highly soluble
in alcohols, water and ethers which cause them to be leached during catalytic
Carbohydrates to Chemicals and Fuel Additives over ModifiedPolyoxometalate Catalysts
HPAs based on P and Si central atoms and Mo and W as addenda atoms are commonly available. The Keggin structure is flexible and has five geometrical rotational
isomers, also known as Baker-Figgis isomers. These ions are known to exist in α
form, and this term is frequently used in several applications including heterogeneous catalysis [30].
Generally, heteropoly acids and their salts form ionic crystals composed of heteropoly anions known as primary structure; counter cations such as H
+
, H 3 O
+
and
H 5 O 2
+
; hydration water; and other molecules are secondary structure. The hierarchical structure of solid HPAs is essential for the understanding of its catalytic activity,
and the substructures are denoted as primary, secondary and tertiary structure [31].
The primary structure of the heteropoly acid is its anion [PW 12 O 40 ]
3−
that exist in
Keggin structure. The arrangement of primary structure together with counter cations forms the secondary structure (e.g. Cs 2.5 H 0.5 PW 12 O 40 ). The secondary structure
is flexible and relies on the amount of hydration water, counter cation and heteropoly anion. The aggregates of secondary structures in three-dimensional manner
give the tertiary structure. It explains the formation of solid particles and links to
properties like surface area, pore structure, particle size and distribution of protons
in particles [32]. This hierarchical structure of HPAs plays a crucial role in understanding and designing these as heterogeneous catalysts. This appears to be a simple
idea, but helps enormously in the research on HPA-based catalysis.
5.4 Acidic Properties of Keggin Heteropoly Acids
In aqueous solution, HPAs are strong acids. Their acidity is comparable to that of
common acids such as H 2 SO 4 , HNO 3 and HCl and hence is sometimes called superacids [33]. The protons present in the secondary structure of HPAs are considered
as mobile protons. Thus, the high mobility of these protons makes HPAs as superacids. The acid strength of HPAs varies in a wide range depending on polyanion
structure and its constituent elements (both heteroatoms and addenda atoms), on the
extent of hydration and reduction. Among the Keggin-type HPAs, MPA is stronger
acid and its acidity is close to that of superacid [34].
5.5 Modification of Keggin Heteropoly Acids
HPAs have been identified as efficient acid and oxidation catalysts. Bulk forms of
Keggin HPAs have limited number of catalytic applications since they are associated with certain drawbacks [35]. HPAs have low surface areas less than 10 m
2
/g,
which hinder approachability to their strong acidic sites. Due to their relatively low
specific surface area, catalytic activity of HPAs is limited in gas-phase reactions.
Solubility of HPAs in polar solvents is another limitation. These are highly soluble
in alcohols, water and ethers which cause them to be leached during catalytic
Carbohydrates to Chemicals and Fuel Additives over ModifiedPolyoxometalate Catalysts