302
the synthesis protocol of Co/Al 2 O 3 (derived from Co-Al LDH) greatly affected the
catalytic performance. The used Co/Al 2 O 3 catalyst was recovered from the reaction
mixture by magnetic separation. The authors observed that, after reaction, the Co
nanoparticle surface underwent oxidation to cobalt oxide in air during catalyst separation. Thus, prior to each run (while reusability), the used catalyst was washed with
water and re-reduced under H 2 flow (50 mL/min) at 700 °C for 1 h. The activity of
the Co/Al 2 O 3 was maintained up to four reaction cycles following this pretreatment
procedure [61].
Magnetic catalyst (Ni(0)-Cu(0)/MgAlFe LDH) was reported by Chen research
group for Gvl synthesis in methanol medium with 98.1% yield and full conversion
of LA (Table 1, entries 10 and 11). The magnetically active catalyst (Ni(0) and
Cu(0)) is prepared from LDH precursor by prior or external reduction in H 2 atmosphere at 500 °C for 3 h. The prior reduction temperature of LDH affected the catalytic activity; increased yield of Gvl was observed by increasing the reduction
temperature from 350 to 500 °C, and further increment (>500 °C) resulted in a
decrease in the selectivity of Gvl because of particle sintering at high temperature.
Decrease in catalytic activity of the material (Ni(0)-Cu(0)/MgAlFe LDH) was
observed in the reusability studies with increase in the number of cycles. Thus, to
maintain the catalyst activity, the used catalyst was reactivated by prior reduction
(500 °C for 3 h) for each cycle, and the methodology showed good reusability [62].
Kantam research group reported LDH-derived active catalyst Cu(0)-Ni(0)/MgAl
LDH for this conversion with 100% yield of Gvl at relatively mild reaction conditions (140 °C, 30 bar H 2 pressure for 3 h) in 1,4-dioxane solvent (Table 1, entry 12).
Calcination (600 °C for 6 h) followed by prior reduction (140 °C, 30 bar H 2 for 2 h)
procedure was used to generate the active catalyst (Ni(0) and Cu(0)) from its LDH
precursor via intermediate metal oxides (CLDH). In this case also, gradual decrease
in catalytic activity of LDH-derived material was observed upon reuse (66% yield
at fourth cycle). Hence, to improve reusability, the catalyst was reactivated at
140 °C, 30 bar H 2 for 2 h, and it showed better catalytic activity (fourth cycle 92%)
as compared with un-reactivated catalyst [63].
Li and co-researchers disclosed Zr containing Ni catalyst such as Ni(0)/Zr-Al
CLDH for vapor-phase continuous production of Gvl from neat LA at 250 °C and
ambient H 2 pressure (Table 1, entries 13 and 14). The active catalyst is prepared
from Ni-Zr-Al LDH catalyst precursor by calcination (500 °C for 4 h in static air)
followed by reduction (600 °C for 2 h in H 2 /Ar atmosphere). The active LDHderived catalyst (Ni(0)/Zr-Al LDH) exhibited the highest selectivity of Gvl (97.7%)
as compared to Zr-free catalyst such as Ni/Al 2 O 3 (85.3%), and the acidity of Zr
enhanced the cyclization of reaction intermediate (γ-hydroxy pentanoic acid) to Gvl
via path-a mechanism (Scheme 4) [64].
Chary et al. reported in situ hydrogen transfer (from formic acid) for Gvl preparation from LA in continuous mode. In this methodology, it was observed that MgAl CLDH (derived from Mg-Al LDH calcination at 500 °C for 6 h) was an efficient
catalytic material by considering stability and activity as compared with physical
mixture of MgO-Al 2 O 3 and as-synthesized Mg-Al LDH (without calcination).
Under the reaction conditions (270 °C and N 2 flow), Mg-Al CLDH showed
S. Gundekari et al.
the synthesis protocol of Co/Al 2 O 3 (derived from Co-Al LDH) greatly affected the
catalytic performance. The used Co/Al 2 O 3 catalyst was recovered from the reaction
mixture by magnetic separation. The authors observed that, after reaction, the Co
nanoparticle surface underwent oxidation to cobalt oxide in air during catalyst separation. Thus, prior to each run (while reusability), the used catalyst was washed with
water and re-reduced under H 2 flow (50 mL/min) at 700 °C for 1 h. The activity of
the Co/Al 2 O 3 was maintained up to four reaction cycles following this pretreatment
procedure [61].
Magnetic catalyst (Ni(0)-Cu(0)/MgAlFe LDH) was reported by Chen research
group for Gvl synthesis in methanol medium with 98.1% yield and full conversion
of LA (Table 1, entries 10 and 11). The magnetically active catalyst (Ni(0) and
Cu(0)) is prepared from LDH precursor by prior or external reduction in H 2 atmosphere at 500 °C for 3 h. The prior reduction temperature of LDH affected the catalytic activity; increased yield of Gvl was observed by increasing the reduction
temperature from 350 to 500 °C, and further increment (>500 °C) resulted in a
decrease in the selectivity of Gvl because of particle sintering at high temperature.
Decrease in catalytic activity of the material (Ni(0)-Cu(0)/MgAlFe LDH) was
observed in the reusability studies with increase in the number of cycles. Thus, to
maintain the catalyst activity, the used catalyst was reactivated by prior reduction
(500 °C for 3 h) for each cycle, and the methodology showed good reusability [62].
Kantam research group reported LDH-derived active catalyst Cu(0)-Ni(0)/MgAl
LDH for this conversion with 100% yield of Gvl at relatively mild reaction conditions (140 °C, 30 bar H 2 pressure for 3 h) in 1,4-dioxane solvent (Table 1, entry 12).
Calcination (600 °C for 6 h) followed by prior reduction (140 °C, 30 bar H 2 for 2 h)
procedure was used to generate the active catalyst (Ni(0) and Cu(0)) from its LDH
precursor via intermediate metal oxides (CLDH). In this case also, gradual decrease
in catalytic activity of LDH-derived material was observed upon reuse (66% yield
at fourth cycle). Hence, to improve reusability, the catalyst was reactivated at
140 °C, 30 bar H 2 for 2 h, and it showed better catalytic activity (fourth cycle 92%)
as compared with un-reactivated catalyst [63].
Li and co-researchers disclosed Zr containing Ni catalyst such as Ni(0)/Zr-Al
CLDH for vapor-phase continuous production of Gvl from neat LA at 250 °C and
ambient H 2 pressure (Table 1, entries 13 and 14). The active catalyst is prepared
from Ni-Zr-Al LDH catalyst precursor by calcination (500 °C for 4 h in static air)
followed by reduction (600 °C for 2 h in H 2 /Ar atmosphere). The active LDHderived catalyst (Ni(0)/Zr-Al LDH) exhibited the highest selectivity of Gvl (97.7%)
as compared to Zr-free catalyst such as Ni/Al 2 O 3 (85.3%), and the acidity of Zr
enhanced the cyclization of reaction intermediate (γ-hydroxy pentanoic acid) to Gvl
via path-a mechanism (Scheme 4) [64].
Chary et al. reported in situ hydrogen transfer (from formic acid) for Gvl preparation from LA in continuous mode. In this methodology, it was observed that MgAl CLDH (derived from Mg-Al LDH calcination at 500 °C for 6 h) was an efficient
catalytic material by considering stability and activity as compared with physical
mixture of MgO-Al 2 O 3 and as-synthesized Mg-Al LDH (without calcination).
Under the reaction conditions (270 °C and N 2 flow), Mg-Al CLDH showed
S. Gundekari et al.