654
individual reaction steps. Moreover, the distribution of different types of active sites
are tailored for a specific reaction. Cho et al. demonstrated the performance of
zeolite- encapsulated Pt nanoparticles for tandem aldol condensation reaction and
hydrogenation aldol condensation products [148]. HZSM-5-encapsulated Pt
nanoparticle showed higher selectivity for tandem reaction of aldol condensation
and hydrogenation of the aldol products. A combined yield of FAc hydrogenated
products of 87%. In contrast, Pt nanoparticle supported on HZSM-5 promotes the
FUR conversion by hydrogenation, decarbonylation, and hydrodeoxygenation.
Moreover, the encapsulated Pt nanoparticles offer superior resistance against sintering and leaching. The authors further demonstrated the tailoring of types of acid
sites of Pt@ZSM-5 and choosing of appropriate solvent composition to manipulate
the product distribution in tandem catalysis [148]. The tandem reaction of aldol
condensation of FUR with acetone was studied in cyclohexane and EtOH solvents.
Pt@HZSM-5 with Brønsted acid sites which are in intracrystalline mesopores
selectively promotes the tandem aldol condensation of FUR and acetone and subsequent hydrogenation of aldol products by restricting the access of FUR to Pt sites.
A total yield of 69% for hydrogenated aldol products was obtained. The Pt@
NaZSM-5 catalyst possesses both the Lewis acid sites and Brønsted acid sites in
zeolitic micropores and intracrystalline mesopores and promotes the hydrogenation
of FUR resulting in the formation of valeric acid and ethyl valerate. It was also
reported that the concentration of ethyl alcohol influences the formation and selectivity of different products. Faba et al. demonstrate the performance of Pd catalyst
supported on various basic mixed oxides, M x N y O (M = Mg, Ca; N = Zr, Al) for
aldolization-hydrogenation reaction of FUR and acetone [149]. The catalyst (CaZr) with lower concertation of basic sites showed poor performance for aldol reaction, whereas the catalyst (Pd/Mg-Al) with medium-strength basic sites showed
higher activity. It was also reported that the selectivity of the desired product (F 2 Ac)
was related to the concentration of medium- strength basic sites. Xu et al. demonstrated the aldol condensation reaction followed by hydrogenation of aldol products
using mesoporous Pd/cobalt-aluminate catalyst [150]. The bifunctional catalyst was
efficient for the aldol condensation reaction, and ~ 99% FUR conversion was
achieved (Table 7).
2.4.2 Reaction Mechanism
The base catalyzed aldol condensation proceeds with the abstraction of α-hydrogen
from ketone (e.g., acetone) forming carbanion as shown in Fig. 12 [131, 133]. The
ease of abstraction of hydrogen depends on the basicity of the active site and acidity
of the hydrogen. Then, the carbanion formed consecutively attacks the carbonyl
group of adsorbed FUR molecule. The FUR molecule adsorbs on acid sites by coordinating with the oxygen of the carbonyl group. An unstable β-hydroxyl ketone
intermediate is formed due to the nucleophilic attack of carbanion. This unstable
intermediate is readily dehydrated to form C 8 (FAc) and water molecule. The dehydration of hydroxy ketone intermediate and desorption of product regenerate active
M. Baithy et al.
individual reaction steps. Moreover, the distribution of different types of active sites
are tailored for a specific reaction. Cho et al. demonstrated the performance of
zeolite- encapsulated Pt nanoparticles for tandem aldol condensation reaction and
hydrogenation aldol condensation products [148]. HZSM-5-encapsulated Pt
nanoparticle showed higher selectivity for tandem reaction of aldol condensation
and hydrogenation of the aldol products. A combined yield of FAc hydrogenated
products of 87%. In contrast, Pt nanoparticle supported on HZSM-5 promotes the
FUR conversion by hydrogenation, decarbonylation, and hydrodeoxygenation.
Moreover, the encapsulated Pt nanoparticles offer superior resistance against sintering and leaching. The authors further demonstrated the tailoring of types of acid
sites of Pt@ZSM-5 and choosing of appropriate solvent composition to manipulate
the product distribution in tandem catalysis [148]. The tandem reaction of aldol
condensation of FUR with acetone was studied in cyclohexane and EtOH solvents.
Pt@HZSM-5 with Brønsted acid sites which are in intracrystalline mesopores
selectively promotes the tandem aldol condensation of FUR and acetone and subsequent hydrogenation of aldol products by restricting the access of FUR to Pt sites.
A total yield of 69% for hydrogenated aldol products was obtained. The Pt@
NaZSM-5 catalyst possesses both the Lewis acid sites and Brønsted acid sites in
zeolitic micropores and intracrystalline mesopores and promotes the hydrogenation
of FUR resulting in the formation of valeric acid and ethyl valerate. It was also
reported that the concentration of ethyl alcohol influences the formation and selectivity of different products. Faba et al. demonstrate the performance of Pd catalyst
supported on various basic mixed oxides, M x N y O (M = Mg, Ca; N = Zr, Al) for
aldolization-hydrogenation reaction of FUR and acetone [149]. The catalyst (CaZr) with lower concertation of basic sites showed poor performance for aldol reaction, whereas the catalyst (Pd/Mg-Al) with medium-strength basic sites showed
higher activity. It was also reported that the selectivity of the desired product (F 2 Ac)
was related to the concentration of medium- strength basic sites. Xu et al. demonstrated the aldol condensation reaction followed by hydrogenation of aldol products
using mesoporous Pd/cobalt-aluminate catalyst [150]. The bifunctional catalyst was
efficient for the aldol condensation reaction, and ~ 99% FUR conversion was
achieved (Table 7).
2.4.2 Reaction Mechanism
The base catalyzed aldol condensation proceeds with the abstraction of α-hydrogen
from ketone (e.g., acetone) forming carbanion as shown in Fig. 12 [131, 133]. The
ease of abstraction of hydrogen depends on the basicity of the active site and acidity
of the hydrogen. Then, the carbanion formed consecutively attacks the carbonyl
group of adsorbed FUR molecule. The FUR molecule adsorbs on acid sites by coordinating with the oxygen of the carbonyl group. An unstable β-hydroxyl ketone
intermediate is formed due to the nucleophilic attack of carbanion. This unstable
intermediate is readily dehydrated to form C 8 (FAc) and water molecule. The dehydration of hydroxy ketone intermediate and desorption of product regenerate active
M. Baithy et al.