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I. Cano and P. W. N. M. van Leeuwen
CAL [98]. The structured catalyst was used as a stirrer allowing easy separation of
the catalyst and products after the batch reaction. The reaction was carried out in
dioxane, 80 °C and 1 bar H 2 . All three materials show selectivity to HCAL of around
90%, but the OGF catalyst was far more stable than the commercial Pd/AC catalyst,
the non-oxidized GF catalyst being in between. Pd/OGF does not sinter during the
reaction while the particles size of Pd/GF increased considerably, from 12 to 42 nm.
XPS showed that Pd/OGF contained more oxidized Pd; while this adds to the stability
it did not increase the selectivity to COL.
Drelinkiewicz and co-workers studied the effect of alloying Pd/C with Au with
the aim to reduce alkene hydrogenation in favour of aldehyde hydrogenation of CAL
[99]. Large MNPs (6–8 nm size) were prepared from the chloride salts by reduction
with hydrazine by an inverse micelle method in the presence of Vulcan carbon. All
catalysts exhibit high CAL conversions, but at higher Au content a decrease in the
rate of CAL conversion was observed and the Pd/Au 1:2 catalyst with the highest
Au/Pd ratio is 5-times less active than monometallic Pd/C. Hydrogenation yielded
only HCAL and HCOL under the conditions applied (toluene, room temperature,
1 bar H 2 ). It was established that hydrogenation of COL to HCOL was relatively
fast.
11.5.6 Pd as Hydrogen Transfer Catalyst on Silica
In view of the propensity of Pd/H 2 to hydrogenate the C=C bond in unsaturated
aldehydes and ketones, the use of alcohols as hydrogen donors seems a good option;
the two hydrogen atoms are abstracted by the catalyst in a heterolytic fashion and
chances are that they will be transferred to the substrate in the same way. Alternatively,
a concerted transfer of the hydrogen atom from carbon to carbon can take place if
alcohols are the donor, as mentioned above for the Meerwein–Ponndorf–Verley reaction. Ying and co-workers designed Pd catalysts for various Pd catalyzed reactions
by immobilizing Pd NPs via urea ligands attached covalently to silicious mesocellular foams (MCF) [100]. MCF possesses a three-dimensional, interconnected pore
structure with ultra-large pores (24–42 nm) connected by windows of 9–22 nm and a
very large surface area. Pd NPs (2–3 and 4–6 nm size) were obtained from Pd (OAc) 2
by simply heating. Inter alia these catalysts were used for the hydrogen transfer from
formic acid to 4-phenyl-3-buten-2-one to give 4-phenyl-3-buten-2-ol in 99% yield
(fivefold excess of formic acid/triethylamine v/v = 1:1, ethyl acetate, 25 °C). No
hydrogenation of the alkene was observed. On the contrary, when 6 bar H 2 was
applied, 99.9% of 4-phenylbutan-2-one was obtained (methanol/ethanol, 25 °C), i.e.
the ‘normal’ behaviour of a Pd catalyst.
I. Cano and P. W. N. M. van Leeuwen
CAL [98]. The structured catalyst was used as a stirrer allowing easy separation of
the catalyst and products after the batch reaction. The reaction was carried out in
dioxane, 80 °C and 1 bar H 2 . All three materials show selectivity to HCAL of around
90%, but the OGF catalyst was far more stable than the commercial Pd/AC catalyst,
the non-oxidized GF catalyst being in between. Pd/OGF does not sinter during the
reaction while the particles size of Pd/GF increased considerably, from 12 to 42 nm.
XPS showed that Pd/OGF contained more oxidized Pd; while this adds to the stability
it did not increase the selectivity to COL.
Drelinkiewicz and co-workers studied the effect of alloying Pd/C with Au with
the aim to reduce alkene hydrogenation in favour of aldehyde hydrogenation of CAL
[99]. Large MNPs (6–8 nm size) were prepared from the chloride salts by reduction
with hydrazine by an inverse micelle method in the presence of Vulcan carbon. All
catalysts exhibit high CAL conversions, but at higher Au content a decrease in the
rate of CAL conversion was observed and the Pd/Au 1:2 catalyst with the highest
Au/Pd ratio is 5-times less active than monometallic Pd/C. Hydrogenation yielded
only HCAL and HCOL under the conditions applied (toluene, room temperature,
1 bar H 2 ). It was established that hydrogenation of COL to HCOL was relatively
fast.
11.5.6 Pd as Hydrogen Transfer Catalyst on Silica
In view of the propensity of Pd/H 2 to hydrogenate the C=C bond in unsaturated
aldehydes and ketones, the use of alcohols as hydrogen donors seems a good option;
the two hydrogen atoms are abstracted by the catalyst in a heterolytic fashion and
chances are that they will be transferred to the substrate in the same way. Alternatively,
a concerted transfer of the hydrogen atom from carbon to carbon can take place if
alcohols are the donor, as mentioned above for the Meerwein–Ponndorf–Verley reaction. Ying and co-workers designed Pd catalysts for various Pd catalyzed reactions
by immobilizing Pd NPs via urea ligands attached covalently to silicious mesocellular foams (MCF) [100]. MCF possesses a three-dimensional, interconnected pore
structure with ultra-large pores (24–42 nm) connected by windows of 9–22 nm and a
very large surface area. Pd NPs (2–3 and 4–6 nm size) were obtained from Pd (OAc) 2
by simply heating. Inter alia these catalysts were used for the hydrogen transfer from
formic acid to 4-phenyl-3-buten-2-one to give 4-phenyl-3-buten-2-ol in 99% yield
(fivefold excess of formic acid/triethylamine v/v = 1:1, ethyl acetate, 25 °C). No
hydrogenation of the alkene was observed. On the contrary, when 6 bar H 2 was
applied, 99.9% of 4-phenylbutan-2-one was obtained (methanol/ethanol, 25 °C), i.e.
the ‘normal’ behaviour of a Pd catalyst.