7 Catalysis by Metal Nanoparticles Encapsulated …
233
change in its activity. The reaction was found to be heterogeneous in nature with the
Pd leaching of 0.3 ppm as evidenced by ICP-AES analysis. It should be commented
that the performance of this Pd/MIL-53(Al)-NH 2 catalyst is remarkable in spite that
the Pd NPs are mostly on the external surface. In addition, the present study with
MIL-53(Al)-NH 2 shows the positive influence of the amino group on the activity. In
some other examples, the most common observation is that the presence of amino
groups interacting with the metal NP increases catalyst stability by minimizing the
metal NPs growth.
Pd NPs have been also supported on UiO-66-NH 2 MOF and its catalytic performance has been examined in Suzuki–Miyaura cross-coupling reaction [41]. Although
TEM images indicated uniform distribution of Pd NPs, the mean diameter of Pd
NPs was 5.28 ± 0.5 nm, and therefore, they seem to be too large to be accommodated within UiO-66-NH 2 cavities. XPS analysis revealed the existence of
metallic Pd in the as-prepared catalyst. The activity of this solid was tested in the
Suzuki–Miyaura cross-coupling of phenylboronic acid and 4-iodoanisole, observing
a turnover frequency (TOF) value of 2190 h
−1 . A variety of biphenyl derivatives were
prepared using Pd/UiO-66-NH 2 as catalyst under identical conditions with the yields
ranging between 80 and 95%. Reusability experiments indicated identical activity
for five consecutive cycles. Furthermore, powder XRD patterns of the five times used
solid indicated that the structural integrity of UiO-66-NH 2 was not modified during
repeated cycles. The Pd leaching was 4% in the first two cycles, but no further Pd
leaching occurred in the subsequent uses.
Pd NPs (5 nm) were supported on nanoscale ScBTC NMOFs (NMOF: nanoscale
MOF) by using a microwave-assisted impregnation process and the activity of
Pd/N-ScBTC was tested in the Suzuki–Miyaura cross-coupling reaction between
aryl/heteroaryl halides and arylboronic acids. The corresponding biphenyl was
formed in high yields (75–97%) [42]. A commercial Pd/C catalyst and bulk ScBTC
MOFs-supported Pd catalysts were also prepared and included in the study, selecting
the coupling of aryl chloride and phenylboronic acid under identical conditions as
probe reaction. The order of reactivity observed was as Pd/C < Pd-MOFs < PdNMOFs showing the benefits of reducing the MOF particle size to nanoscale on
the catalytic activity. Pd/N-ScBTC was reused for five cycles with no decay in its
activity.
7.4.2 Ullmann Coupling
For the synthesis of symmetrically substituted biaryls, the Ullmann homo coupling
of aryl halides may present the advantage of avoiding the need of arylboronic
acids and their derivatives (Scheme 7.8). In another example of the use of metal
NPs/MOFs as heterogeneous catalysts, the performance of Pd/MIL-101(Cr) was
examined in Ullmann reaction of aryl chlorides (Scheme 7.8) [37]. As one illustrative example, the homocoupling of 4-chloroanisole using Pd/MIL-101(Cr) with
NaOMe as a base under inert atmosphere in water at 80 °C resulted in 97% yield of
233
change in its activity. The reaction was found to be heterogeneous in nature with the
Pd leaching of 0.3 ppm as evidenced by ICP-AES analysis. It should be commented
that the performance of this Pd/MIL-53(Al)-NH 2 catalyst is remarkable in spite that
the Pd NPs are mostly on the external surface. In addition, the present study with
MIL-53(Al)-NH 2 shows the positive influence of the amino group on the activity. In
some other examples, the most common observation is that the presence of amino
groups interacting with the metal NP increases catalyst stability by minimizing the
metal NPs growth.
Pd NPs have been also supported on UiO-66-NH 2 MOF and its catalytic performance has been examined in Suzuki–Miyaura cross-coupling reaction [41]. Although
TEM images indicated uniform distribution of Pd NPs, the mean diameter of Pd
NPs was 5.28 ± 0.5 nm, and therefore, they seem to be too large to be accommodated within UiO-66-NH 2 cavities. XPS analysis revealed the existence of
metallic Pd in the as-prepared catalyst. The activity of this solid was tested in the
Suzuki–Miyaura cross-coupling of phenylboronic acid and 4-iodoanisole, observing
a turnover frequency (TOF) value of 2190 h
−1 . A variety of biphenyl derivatives were
prepared using Pd/UiO-66-NH 2 as catalyst under identical conditions with the yields
ranging between 80 and 95%. Reusability experiments indicated identical activity
for five consecutive cycles. Furthermore, powder XRD patterns of the five times used
solid indicated that the structural integrity of UiO-66-NH 2 was not modified during
repeated cycles. The Pd leaching was 4% in the first two cycles, but no further Pd
leaching occurred in the subsequent uses.
Pd NPs (5 nm) were supported on nanoscale ScBTC NMOFs (NMOF: nanoscale
MOF) by using a microwave-assisted impregnation process and the activity of
Pd/N-ScBTC was tested in the Suzuki–Miyaura cross-coupling reaction between
aryl/heteroaryl halides and arylboronic acids. The corresponding biphenyl was
formed in high yields (75–97%) [42]. A commercial Pd/C catalyst and bulk ScBTC
MOFs-supported Pd catalysts were also prepared and included in the study, selecting
the coupling of aryl chloride and phenylboronic acid under identical conditions as
probe reaction. The order of reactivity observed was as Pd/C < Pd-MOFs < PdNMOFs showing the benefits of reducing the MOF particle size to nanoscale on
the catalytic activity. Pd/N-ScBTC was reused for five cycles with no decay in its
activity.
7.4.2 Ullmann Coupling
For the synthesis of symmetrically substituted biaryls, the Ullmann homo coupling
of aryl halides may present the advantage of avoiding the need of arylboronic
acids and their derivatives (Scheme 7.8). In another example of the use of metal
NPs/MOFs as heterogeneous catalysts, the performance of Pd/MIL-101(Cr) was
examined in Ullmann reaction of aryl chlorides (Scheme 7.8) [37]. As one illustrative example, the homocoupling of 4-chloroanisole using Pd/MIL-101(Cr) with
NaOMe as a base under inert atmosphere in water at 80 °C resulted in 97% yield of