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selectivity, and stability of the first one. It can be achieved by one of the methods
mentioned here:
• Altering the geometry
• Altering the electron properties of active sites
• Stabilizing the active metal
• Chemical bonding with a reaction intermediate and transition state
• Bifunctional effects by providing different functions in the reaction mechanism
The use of a bimetallic catalyst is advantageous, but the exact mechanism is not
known. The information on the molecular and electronic level can help to establish
the relationship between experimental results and catalyst modification. One may
propose an analogy between catalysts used in petroleum refining and biomass
conversion, but may not be appropriate as petroleum refining needs the addition of
functional groups, whereas biomass conversion requires removal/replacement of
functional groups. The transformation of cellulose, hemicelluloses, and lignin using
bimetallic catalysts is discussed in the following section.
2.4.1 Cellulose Conversion Using the Bimetallic Catalyst
Cellulose is made up of biopolymers and depolymerized to its monomer, glucose
(C 6 H 12 O 6 ), by hydrolysis. Glucose is further converted to platform chemicals such as
levulinic acid and 5-hydroxymethylfurfural (5-HMF) by a dehydration reaction.
Glucose can be upgraded to valuable chemicals by decreasing the functionality (aqueous-phase reforming) as well as increasing the functionality (oxidation). Aqueousphase reforming produces H 2 and alkanes, whereas oxidation yields acids. Glucose
mainly consists of aldehydes and alcohols. Oxidation of aldehydes produces carboxylic acid (gluconic acid) and that of alcohol produces glucuronic and keto acids. The
most common monometallic catalysts used for glucose conversion are Pt and Pd. A
second metal, Bi, is added to increase their activity as well as selectivity toward gluconic acid. The presence of Bi prevents the deactivation of Pd (by absorbing O 2 that
would otherwise oxidize Pd) and limits the formation of byproducts. The promotion
of Pd with other metals was not as effective as PdBi [85]. The possible oxidation
products that can be produced from glucose are glucuronic acid, gluconic acid, 5-ketoglucose, glucaric acid, 5-keto-gluconic acid, and 2,5-diketogluconic acid. Other catalysts used for cellulose oxidation are PbPt, AuPt, AuRh, etc.
Glucose, the monomer of cellulose, can also be converted to various alcohols such
as hexitols, sorbitols, and diols by hydrogenation. Ni and Ni-based catalysts are primarily used for glucose hydrogenation, but leaching of catalysts is a real issue. To improve
the catalyst activity, promoters like Sn, Mo, Cr, and Fe are used that increase the rate of
glucose formation by four to five times. When Fe/Cr is added to Ni, the selectivity of
sorbitol was reached to as high as 98% [86]. Ru-based catalyst is reportedly stable for
hydrogenation of glucose to produce sorbitols, but is expensive. Hexitols can be produced by hydrogenolysis (hydrogenation followed by dehydration) over the Pt-Ni catalyst. The yield of hexitols was 47.4% at 100% conversion of glucose over the Pt-Ni
Sustainability of the Catalytic Process for Biomass Conversion: Recent Trends and…
selectivity, and stability of the first one. It can be achieved by one of the methods
mentioned here:
• Altering the geometry
• Altering the electron properties of active sites
• Stabilizing the active metal
• Chemical bonding with a reaction intermediate and transition state
• Bifunctional effects by providing different functions in the reaction mechanism
The use of a bimetallic catalyst is advantageous, but the exact mechanism is not
known. The information on the molecular and electronic level can help to establish
the relationship between experimental results and catalyst modification. One may
propose an analogy between catalysts used in petroleum refining and biomass
conversion, but may not be appropriate as petroleum refining needs the addition of
functional groups, whereas biomass conversion requires removal/replacement of
functional groups. The transformation of cellulose, hemicelluloses, and lignin using
bimetallic catalysts is discussed in the following section.
2.4.1 Cellulose Conversion Using the Bimetallic Catalyst
Cellulose is made up of biopolymers and depolymerized to its monomer, glucose
(C 6 H 12 O 6 ), by hydrolysis. Glucose is further converted to platform chemicals such as
levulinic acid and 5-hydroxymethylfurfural (5-HMF) by a dehydration reaction.
Glucose can be upgraded to valuable chemicals by decreasing the functionality (aqueous-phase reforming) as well as increasing the functionality (oxidation). Aqueousphase reforming produces H 2 and alkanes, whereas oxidation yields acids. Glucose
mainly consists of aldehydes and alcohols. Oxidation of aldehydes produces carboxylic acid (gluconic acid) and that of alcohol produces glucuronic and keto acids. The
most common monometallic catalysts used for glucose conversion are Pt and Pd. A
second metal, Bi, is added to increase their activity as well as selectivity toward gluconic acid. The presence of Bi prevents the deactivation of Pd (by absorbing O 2 that
would otherwise oxidize Pd) and limits the formation of byproducts. The promotion
of Pd with other metals was not as effective as PdBi [85]. The possible oxidation
products that can be produced from glucose are glucuronic acid, gluconic acid, 5-ketoglucose, glucaric acid, 5-keto-gluconic acid, and 2,5-diketogluconic acid. Other catalysts used for cellulose oxidation are PbPt, AuPt, AuRh, etc.
Glucose, the monomer of cellulose, can also be converted to various alcohols such
as hexitols, sorbitols, and diols by hydrogenation. Ni and Ni-based catalysts are primarily used for glucose hydrogenation, but leaching of catalysts is a real issue. To improve
the catalyst activity, promoters like Sn, Mo, Cr, and Fe are used that increase the rate of
glucose formation by four to five times. When Fe/Cr is added to Ni, the selectivity of
sorbitol was reached to as high as 98% [86]. Ru-based catalyst is reportedly stable for
hydrogenation of glucose to produce sorbitols, but is expensive. Hexitols can be produced by hydrogenolysis (hydrogenation followed by dehydration) over the Pt-Ni catalyst. The yield of hexitols was 47.4% at 100% conversion of glucose over the Pt-Ni
Sustainability of the Catalytic Process for Biomass Conversion: Recent Trends and…