307
feasible technologically. Also, various chlorinated high boiling solvents such as
1,2-dichlorobenzene were used that are not environmentally friendly and have recyclability issues.
We reported this transformation under solvent-free conditions using conventional heating. Various zeolites were identified for preliminary screening owing to
their properties, i.e. high crystallinity, thermal stability, tunable acidity, reusability
and shape selectivity, which find use in industrially relevant transformations especially in petrochemicals. Among zeolites screened, Na/H-β, Na-Y, and Na-ZSM-5,
Na-β and H-β were found active for the mentioned conversion. The high activity of
β-zeolite is attributed to its unique pore topology and the presence of local structural
defects through octahedral extra-framework aluminium (AlO 6 ) that render Lewis
acidic properties in the material, resulting in the observed conversion and product
selectivity. H-β zeolite fared better compared to Na-β. Under optimized condition,
the highest yield of γ-lactone was obtained with anisole: LA 4:1 molar ratio in the
absence of solvent at 150 °C using 50 wt% catalyst loading after 12 h (Table 2,
entry 1).
Substrate scope was extended from ethoxy to butoxy benzenes. Increasing the
alkyl chain length had a positive effect on LA conversion with a selectivity for parasubstituted γ-lactones (Table 2, entries 2–4). Aromatic hydrocarbon cumene did not
participate in the reaction. Inability of the relatively less electron-rich aromatic ring
in cumene to take part in the nucleophilic attack on the carbonyl carbon of levulinic
acid could be a reason for its inactive nature (Scheme 6). The ring electron density
was increased by the lone pair of electron present on the heteroatom and in the substrates with electron-donating substituents. Thioanisole showed 83% selectivity of
para-isomeric γ-lactone (Table 2, entry 5). This synthetic protocol was further
explored with 2-methylthiophene and thiophene, which resulted in the selective formation of γ-lactone (90% and 80%) with 93% and 96% LA conversion (Table 2,
entries 6 and 7).
Recyclability of H-β catalyst was also studied. Organic carbon that adhered to
the used catalyst (UH-β) surface was removed by calcination in air at 550 °C for 3 h.
The used calcined catalyst (UH-βC) and fresh H-β catalyst showed comparable
yield and selectivity for the synthesis of γ-lactone. A scalability study at 10 g of LA
with anisole showed an increase in γ-lactone yield with time. After 6, 12, 18 and
24 h, 38, 62, 80 and 90% yield of para-substituted γ-lactone was obtained with 95,
94, 92 and 90% selectivity, respectively.
Mechanistic investigation for the formation of Agvls (Scheme 6) suggests that
the lone pairs of the keto carbonyl group of LA interact with Lewis acidic sites of
aluminium in zeolitic framework, present due to local defects [77, 78]. A partial
positive charge generated on the keto carbon is responsible for the nucleophilic
attack resulting in the formation of γ-hydroxy pentatonic acid intermediate. This
unstable species underwent intramolecular esterification to γ-lactone on removal of
water [79]. The higher activity of H-β zeolite compared to Na-β is possibly due to
the presence of Bronsted acidic protons on H-β zeolite.
Levulinic Acid- and Furan-Based Multifunctional Materials: Opportunities…
feasible technologically. Also, various chlorinated high boiling solvents such as
1,2-dichlorobenzene were used that are not environmentally friendly and have recyclability issues.
We reported this transformation under solvent-free conditions using conventional heating. Various zeolites were identified for preliminary screening owing to
their properties, i.e. high crystallinity, thermal stability, tunable acidity, reusability
and shape selectivity, which find use in industrially relevant transformations especially in petrochemicals. Among zeolites screened, Na/H-β, Na-Y, and Na-ZSM-5,
Na-β and H-β were found active for the mentioned conversion. The high activity of
β-zeolite is attributed to its unique pore topology and the presence of local structural
defects through octahedral extra-framework aluminium (AlO 6 ) that render Lewis
acidic properties in the material, resulting in the observed conversion and product
selectivity. H-β zeolite fared better compared to Na-β. Under optimized condition,
the highest yield of γ-lactone was obtained with anisole: LA 4:1 molar ratio in the
absence of solvent at 150 °C using 50 wt% catalyst loading after 12 h (Table 2,
entry 1).
Substrate scope was extended from ethoxy to butoxy benzenes. Increasing the
alkyl chain length had a positive effect on LA conversion with a selectivity for parasubstituted γ-lactones (Table 2, entries 2–4). Aromatic hydrocarbon cumene did not
participate in the reaction. Inability of the relatively less electron-rich aromatic ring
in cumene to take part in the nucleophilic attack on the carbonyl carbon of levulinic
acid could be a reason for its inactive nature (Scheme 6). The ring electron density
was increased by the lone pair of electron present on the heteroatom and in the substrates with electron-donating substituents. Thioanisole showed 83% selectivity of
para-isomeric γ-lactone (Table 2, entry 5). This synthetic protocol was further
explored with 2-methylthiophene and thiophene, which resulted in the selective formation of γ-lactone (90% and 80%) with 93% and 96% LA conversion (Table 2,
entries 6 and 7).
Recyclability of H-β catalyst was also studied. Organic carbon that adhered to
the used catalyst (UH-β) surface was removed by calcination in air at 550 °C for 3 h.
The used calcined catalyst (UH-βC) and fresh H-β catalyst showed comparable
yield and selectivity for the synthesis of γ-lactone. A scalability study at 10 g of LA
with anisole showed an increase in γ-lactone yield with time. After 6, 12, 18 and
24 h, 38, 62, 80 and 90% yield of para-substituted γ-lactone was obtained with 95,
94, 92 and 90% selectivity, respectively.
Mechanistic investigation for the formation of Agvls (Scheme 6) suggests that
the lone pairs of the keto carbonyl group of LA interact with Lewis acidic sites of
aluminium in zeolitic framework, present due to local defects [77, 78]. A partial
positive charge generated on the keto carbon is responsible for the nucleophilic
attack resulting in the formation of γ-hydroxy pentatonic acid intermediate. This
unstable species underwent intramolecular esterification to γ-lactone on removal of
water [79]. The higher activity of H-β zeolite compared to Na-β is possibly due to
the presence of Bronsted acidic protons on H-β zeolite.
Levulinic Acid- and Furan-Based Multifunctional Materials: Opportunities…