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7.8 Esterification and Transesterification
Highly acidic biomass oils need an initial acid esterification stage to change acid
into esters in the presence of acid catalysts and alcohol. The main factors in an
esterification reaction are temperature, alcohol types such as methanol or ethanol,
reaction time, the alcohol/oil molar ratio and homogenous or heterogeneous acid
catalysts [179]. Most active heterogeneous catalysts for esterification reaction are
sulphated zirconia [180], tungstated zirconia [181], acidic resins [182], zeolites
[183] and 12-tungstophosphoric acid on SBA-15 (heteropoly acids) [184]. Catalytic
success was limited to ester lower than C 10 in either alcohols or carboxylic
acids [185].
However, long-chain fatty acids are an alternative fuel with the superiorities of
being non-toxic, renewable and having low emissions and a biodegradable profile.
Vegetable oil transesterification with CH 3 OH in the presence of an appropriate catalyst is the most common process for the production of biodiesel. It involves the
alcoholysis of triglycerides in the presence of a catalyst to form fatty acid methyl
esters (FAME). However, FAME needs further upgrading to be used as a
fuel [190]. Homogenous basic catalysts such as KOH and NaOH are the most common catalysts for the transesterification process [186, 187]. Furthermore, a calciumbased catalyst supported on MgO seems to be a reliable catalyst for the reaction of
transesterification due to its low cost and high activity [188, 189].
7.9 Ketonization
Ketonization (ketonic decarboxylation) reaction transforms two molecules of a carboxylic acid into water, carbon dioxide and ketone [191, 192]. More stable products
are formed by the ketonization reaction with higher energy content. The catalytic
ketonization reaction is a promising process to upgrade the free fatty acids because
it removes oxygen and carboxylic acid in the reaction. Then long-chain ketones can
be transported to the refinery station for isomerization and hydrocracking to produce diesel-ranged fuels [190]. This reaction is a reliable reaction for upgrading the
carboxylic acid, which is derived from biomass fast pyrolysis to produce transportable fuels [193, 194].
Furthermore, over 70% of fast pyrolysis bio-oil contains mostly carboxylic acid,
carboxylic esters, alcohol and ethers [195]. The large volume of carboxylic acids in
the bio-oil causes corrosion during storage and transport [150]. Thus, an essential
step for upgrading bio-oil is acid removal. Therefore, the ketonization reaction is a
suitable method not only for removing the oxygen and carboxylic acid content of
bio-oil but also for producing C-C bonds between low molecule weights of acids.
Then, ketone products can simply couple with other products of bio-oil with hydrogenation or aldol condensation to make longer chain molecules which form the
fuel-ranged hydrocarbons [192, 196, 197].
Thermochemical Conversion of Biomass and Upgrading of Bio-Products to Produce…
7.8 Esterification and Transesterification
Highly acidic biomass oils need an initial acid esterification stage to change acid
into esters in the presence of acid catalysts and alcohol. The main factors in an
esterification reaction are temperature, alcohol types such as methanol or ethanol,
reaction time, the alcohol/oil molar ratio and homogenous or heterogeneous acid
catalysts [179]. Most active heterogeneous catalysts for esterification reaction are
sulphated zirconia [180], tungstated zirconia [181], acidic resins [182], zeolites
[183] and 12-tungstophosphoric acid on SBA-15 (heteropoly acids) [184]. Catalytic
success was limited to ester lower than C 10 in either alcohols or carboxylic
acids [185].
However, long-chain fatty acids are an alternative fuel with the superiorities of
being non-toxic, renewable and having low emissions and a biodegradable profile.
Vegetable oil transesterification with CH 3 OH in the presence of an appropriate catalyst is the most common process for the production of biodiesel. It involves the
alcoholysis of triglycerides in the presence of a catalyst to form fatty acid methyl
esters (FAME). However, FAME needs further upgrading to be used as a
fuel [190]. Homogenous basic catalysts such as KOH and NaOH are the most common catalysts for the transesterification process [186, 187]. Furthermore, a calciumbased catalyst supported on MgO seems to be a reliable catalyst for the reaction of
transesterification due to its low cost and high activity [188, 189].
7.9 Ketonization
Ketonization (ketonic decarboxylation) reaction transforms two molecules of a carboxylic acid into water, carbon dioxide and ketone [191, 192]. More stable products
are formed by the ketonization reaction with higher energy content. The catalytic
ketonization reaction is a promising process to upgrade the free fatty acids because
it removes oxygen and carboxylic acid in the reaction. Then long-chain ketones can
be transported to the refinery station for isomerization and hydrocracking to produce diesel-ranged fuels [190]. This reaction is a reliable reaction for upgrading the
carboxylic acid, which is derived from biomass fast pyrolysis to produce transportable fuels [193, 194].
Furthermore, over 70% of fast pyrolysis bio-oil contains mostly carboxylic acid,
carboxylic esters, alcohol and ethers [195]. The large volume of carboxylic acids in
the bio-oil causes corrosion during storage and transport [150]. Thus, an essential
step for upgrading bio-oil is acid removal. Therefore, the ketonization reaction is a
suitable method not only for removing the oxygen and carboxylic acid content of
bio-oil but also for producing C-C bonds between low molecule weights of acids.
Then, ketone products can simply couple with other products of bio-oil with hydrogenation or aldol condensation to make longer chain molecules which form the
fuel-ranged hydrocarbons [192, 196, 197].
Thermochemical Conversion of Biomass and Upgrading of Bio-Products to Produce…