649
oxidation to form furoic acid even if stored in a closed tank. Some portions of added
base catalysts were utilized to neutralize the acid impurities, which may demonstrate no activity of the strong base catalyst in certain condition. The authors proposed that distillation is an effective way to remove acid impurities in as-received
FUR. Thus, a detailed monitoring of acid impurities in furfural is critical while
evaluating the performance of solid base catalysts and their stability in aldol condensation. Hora et al. investigated the effect of pretreatment methods of the
HT-based catalysts (Mg-Al) on their activity and selectivity of targeted products in
aldol condensation [129]. They have considered calcination and rehydration procedures (ex situ vs. in situ and liquid vs. gas phase) as pretreatment methods for their
studies and determined the impact of calcination temperature and rehydration procedures on the yield of desired aldol condensation products. The ex situ rehydration
of the calcined Mg-Al catalysts showed remarkably lower activity, except the catalyst with Mg/Al molar ratio of 2. However, in situ rehydration improved the catalyst
activity only for Mg/Al molar ratio of 3. The activity of these catalysts was determined to a significant extent by the ratio of HT and MgO phase. Comparison of
structural data indicates that the catalysts displaying poor crystallinity are greatly
influenced by the rehydration procedure, while the catalysts with well-developed
hydrotalcite structure are more resistance to ex situ rehydration procedure. The MgAl catalyst with molar ratio of 3 exhibits best performance in aldol condensation
reaction with 95% FUR conversion and more than 90% selectivity of C 8 –C 13 products. In a different study, the effect of preparation method and activation temperature of Mg-Zr mixed oxide on catalytic performance for aqueous-phase aldol
condensation of FUR with acetone was reported by Sadaba et al. [130]. It was demonstrated that the preparation method requires a high-temperature activation in oxidizing environment, which generates cubic Mg x Zr 1−x O 2−x mixed oxide and MgO. The
formation of cubic MgO phase greatly enhanced the FUR/acetone aldol condensation. Smoláková et al. showed the influence of acid-base properties of HT precursorderived mixed oxide catalysts on the activity and selectivity in aldol condensation
reaction [127]. The basicity of the mixed oxides was strongly reckoned on the preparation of parent HT precursors. The total amount of basic sites of different catalysts
increased in the following order Zn 2 Al < Mg 1 Zn 1 Al < Mg 2 Al. The FUR conversion
and selectivity to longer carbon chain C 13 (F 2 Ac) product were observed to follow
the similar trend of the total amount of basic sites. The higher activity of Mg 2 Al
catalyst was also because of its higher surface area. However, the selectivity of C 8
(FAc) at iso-conversion follows the opposite trend. More specifically, the presence
of medium strength basic sites was responsible for higher activity as strong basic
sites promote transesterification reaction. Faba et al. studied aqueous-phase aldol
condensation reaction with acetone using three different mixed oxide base catalysts
(Mg-Zr, Mg-Al and mesoporous Ca-Zr nano oxide) with different basic site distribution [131]. They also reported that the Mg-Zr mixed oxide catalysts with the
highest amount of medium-strength basic sites showed the best performance in
terms of activity and selectivity for the C 13 (F 2 Ac) fraction, whereas the Ca-Zr mixed
oxide showed poor performance because of the higher rate of retro-aldolization
reaction. Sádaba et al. demonstrated a systematic investigation to identify the
Efficient Nanocomposite Catalysts for Sustainable Production of Biofuels and…
oxidation to form furoic acid even if stored in a closed tank. Some portions of added
base catalysts were utilized to neutralize the acid impurities, which may demonstrate no activity of the strong base catalyst in certain condition. The authors proposed that distillation is an effective way to remove acid impurities in as-received
FUR. Thus, a detailed monitoring of acid impurities in furfural is critical while
evaluating the performance of solid base catalysts and their stability in aldol condensation. Hora et al. investigated the effect of pretreatment methods of the
HT-based catalysts (Mg-Al) on their activity and selectivity of targeted products in
aldol condensation [129]. They have considered calcination and rehydration procedures (ex situ vs. in situ and liquid vs. gas phase) as pretreatment methods for their
studies and determined the impact of calcination temperature and rehydration procedures on the yield of desired aldol condensation products. The ex situ rehydration
of the calcined Mg-Al catalysts showed remarkably lower activity, except the catalyst with Mg/Al molar ratio of 2. However, in situ rehydration improved the catalyst
activity only for Mg/Al molar ratio of 3. The activity of these catalysts was determined to a significant extent by the ratio of HT and MgO phase. Comparison of
structural data indicates that the catalysts displaying poor crystallinity are greatly
influenced by the rehydration procedure, while the catalysts with well-developed
hydrotalcite structure are more resistance to ex situ rehydration procedure. The MgAl catalyst with molar ratio of 3 exhibits best performance in aldol condensation
reaction with 95% FUR conversion and more than 90% selectivity of C 8 –C 13 products. In a different study, the effect of preparation method and activation temperature of Mg-Zr mixed oxide on catalytic performance for aqueous-phase aldol
condensation of FUR with acetone was reported by Sadaba et al. [130]. It was demonstrated that the preparation method requires a high-temperature activation in oxidizing environment, which generates cubic Mg x Zr 1−x O 2−x mixed oxide and MgO. The
formation of cubic MgO phase greatly enhanced the FUR/acetone aldol condensation. Smoláková et al. showed the influence of acid-base properties of HT precursorderived mixed oxide catalysts on the activity and selectivity in aldol condensation
reaction [127]. The basicity of the mixed oxides was strongly reckoned on the preparation of parent HT precursors. The total amount of basic sites of different catalysts
increased in the following order Zn 2 Al < Mg 1 Zn 1 Al < Mg 2 Al. The FUR conversion
and selectivity to longer carbon chain C 13 (F 2 Ac) product were observed to follow
the similar trend of the total amount of basic sites. The higher activity of Mg 2 Al
catalyst was also because of its higher surface area. However, the selectivity of C 8
(FAc) at iso-conversion follows the opposite trend. More specifically, the presence
of medium strength basic sites was responsible for higher activity as strong basic
sites promote transesterification reaction. Faba et al. studied aqueous-phase aldol
condensation reaction with acetone using three different mixed oxide base catalysts
(Mg-Zr, Mg-Al and mesoporous Ca-Zr nano oxide) with different basic site distribution [131]. They also reported that the Mg-Zr mixed oxide catalysts with the
highest amount of medium-strength basic sites showed the best performance in
terms of activity and selectivity for the C 13 (F 2 Ac) fraction, whereas the Ca-Zr mixed
oxide showed poor performance because of the higher rate of retro-aldolization
reaction. Sádaba et al. demonstrated a systematic investigation to identify the
Efficient Nanocomposite Catalysts for Sustainable Production of Biofuels and…