242
strategies incorporating various catalysts are adapted for the conversion of lignocellulosic biomass to useful products:
1. Chemical
2. Thermal
3. Microbiological
Chemical catalytic processing is gaining importance as the resultant product exhibits properties similar to petroleum-based products. These processes are conducted at
mild operating conditions relative to non-catalytic methods and can be tuned to get
the desired product distribution. The various useful chemical processes are catalytic
pyrolysis, hydroprocessing, hydrolysis, dehydration, oxidation, isomerization,
transesterification, etc. Homogeneous, as well as heterogeneous, catalysts can be
used for the catalytic process. Homogeneous catalyst has a high turnover frequency
(TOF); however, it poses problems like toxicity, corrosivity, reusability, and complex
postprocess separation. Hence, heterogeneous catalysts are a better option compared
to their homogeneous counterpart. As shown in Fig. 3, hemicelluloses are used to
produce furfural, lignin is used to produce aromatic compounds, and cellulose is
used to produce levulinic acid or glucose. All of this can be achieved by using
conventional as well as modified catalysts such as zeolites and silica monometallic
as well as bimetallic catalysts [6]. During biomass processing, the C-C coupling
reactions such as oligomerization, aldol condensation, and ketonization use acid or
metal oxide catalysts, whereas reforming, hydrogenolysis, hydrogenation, and
oxidation use bimetallic catalysts. Bimetallic catalysts are preferred over
monometallic catalysts due to their increased catalytic activity, modified selectivity,
and improved catalyst stability. In the case of simple processing, the entire biomass
is subjected to thermochemical processes such as gasification, pyrolysis, etc. The
biological route utilizes living microorganisms (enzymes, bacteria) to degrade
biomass feedstock and produce liquid and gaseous fuels.
The abovementioned technologies can be used independently or in combination,
such as thermochemical processing (catalytic pyrolysis/gasification). The choice of
conversion route will depend on many factors such as type of feedstock, targeted
products, the volume available, and readiness of the technology. In addition,
handling, transport, and storage of the biomass are cumbersome and sometimes
costlier, making the whole process uneconomical. So all factors needs to be
considered while selecting a suitable process for biomass conversion.
2 Catalytic Processes for Biomass Conversion
The catalyst requirement for biomass conversion is different from the petroleum
industry. Petroleum feedstocks are oxygen-deficient, whereas biomass is rich in oxygen. Hence, conversion strategy differs as the addition of functional groups, and
removal of it is required for petroleum and biomass feedstocks, respectively. Biomass
molecules are highly complex and vicious and pose mass transfer limitations
R. Bhoi et al.
strategies incorporating various catalysts are adapted for the conversion of lignocellulosic biomass to useful products:
1. Chemical
2. Thermal
3. Microbiological
Chemical catalytic processing is gaining importance as the resultant product exhibits properties similar to petroleum-based products. These processes are conducted at
mild operating conditions relative to non-catalytic methods and can be tuned to get
the desired product distribution. The various useful chemical processes are catalytic
pyrolysis, hydroprocessing, hydrolysis, dehydration, oxidation, isomerization,
transesterification, etc. Homogeneous, as well as heterogeneous, catalysts can be
used for the catalytic process. Homogeneous catalyst has a high turnover frequency
(TOF); however, it poses problems like toxicity, corrosivity, reusability, and complex
postprocess separation. Hence, heterogeneous catalysts are a better option compared
to their homogeneous counterpart. As shown in Fig. 3, hemicelluloses are used to
produce furfural, lignin is used to produce aromatic compounds, and cellulose is
used to produce levulinic acid or glucose. All of this can be achieved by using
conventional as well as modified catalysts such as zeolites and silica monometallic
as well as bimetallic catalysts [6]. During biomass processing, the C-C coupling
reactions such as oligomerization, aldol condensation, and ketonization use acid or
metal oxide catalysts, whereas reforming, hydrogenolysis, hydrogenation, and
oxidation use bimetallic catalysts. Bimetallic catalysts are preferred over
monometallic catalysts due to their increased catalytic activity, modified selectivity,
and improved catalyst stability. In the case of simple processing, the entire biomass
is subjected to thermochemical processes such as gasification, pyrolysis, etc. The
biological route utilizes living microorganisms (enzymes, bacteria) to degrade
biomass feedstock and produce liquid and gaseous fuels.
The abovementioned technologies can be used independently or in combination,
such as thermochemical processing (catalytic pyrolysis/gasification). The choice of
conversion route will depend on many factors such as type of feedstock, targeted
products, the volume available, and readiness of the technology. In addition,
handling, transport, and storage of the biomass are cumbersome and sometimes
costlier, making the whole process uneconomical. So all factors needs to be
considered while selecting a suitable process for biomass conversion.
2 Catalytic Processes for Biomass Conversion
The catalyst requirement for biomass conversion is different from the petroleum
industry. Petroleum feedstocks are oxygen-deficient, whereas biomass is rich in oxygen. Hence, conversion strategy differs as the addition of functional groups, and
removal of it is required for petroleum and biomass feedstocks, respectively. Biomass
molecules are highly complex and vicious and pose mass transfer limitations
R. Bhoi et al.