10 Progress in the Selective Semi-hydrogenation of Alkynes …
317
nanocatalyst (Pd/Al 2 O 3 , Strem Chemical), which exhibited poor ethylene selectivities along the test (<35%) with a fast deactivation during the first 5 h on stream.
These results evidenced the applicability of bimetallic catalysts prepared purely by
colloidal techniques, not only in the semi-hydrogenation of substrates in liquid phase
but also with substrates of industrial relevance in the gas phase.
Important insights were also obtained by DFT calculations and for instance, Lopez
et al. studied the effect of the presence of a second metal on the properties of Pd in the
semi-hydrogenation of acetylene in excess ethylene [9]. As expected, the adsorption
of molecules was preferred on Pd sites, and that of acetylene was stronger than for
ethylene. Interestingly, alloying the Pd phase with metals such as Au or Cu stabilized
the adsorption of both substrate and product while others such as Bi enhanced the
thermodynamic selectivity by only favouring the adsorption of acetylene. In addition,
the formation of subsurface hydride (β-PdH phase) was reduced on all the studied
promoters, which consequently could limit the over-hydrogenation reaction.
10.4 Effect of the Stabilizer
For, the stabilization of M-NPs is applied in the semi-hydrogenation of alkynes,
a wide variety of organic compounds such as surfactants, ligands, polymers, ionic
liquids among other compounds have been utilized. According to the intrinsic properties of the stabilizer, prevention of NPs agglomeration can be obtained by steric
hindrance or electrostatic repulsion [31]. Moreover, the stabilizer can provide steric
or electronic modifications of the NPs surface, thus influencing the performance in
catalysis [30, 97–99]. At this point, it is important to mention that the effect of the
stabilizer depends on the reaction media (gas or liquid phase). In gas phase, the catalysts are dried, which results in the collapse of the stabilizer on the metal surface
and possibly the blockage of active sites. Differently, in liquid phase, the stabilizers are flexible enough to permit the access of substrates to the metal surface. For
instance, Witte et al. reported that the CO uptake observed in chemisorption of PdNPs
stabilized by hexadecyl(2-hydroxyethyl)dimethyl ammonium dihydrogenphosphate
(HHDMA) was suppressed when the NPs were isolated and dried [100].
Polymers have been classically employed for the steric stabilization of M-NPs
due to their capacity to provide a protective layer that wraps the NPs. Hirai et al.
reported that for PVP-stabilized PdNPs, the thickness of such a layer was proportional to the molecular weight of the polymer (thickness of 4–16 nm, for M w from
6000 to 574,000); however, the catalytic activity for the hydrogenation of cyclooctadiene was not sensitive to the thickness of the polymer layer [98]. Several authors
have addressed the application of PVP-stabilized Pd NPs in the semi-hydrogenation
of alkynes evidencing moderate to good alkene selectivities independently of the
PVP:Pd ratio [7, 97]. Niu et al. reported the evaluation of polyethylene glycol stabilized Ru NPs in the semi-hydrogenation of methyl propiolate [101]. For this system,
the alkene selectivity resulted highly sensitive to the temperature and pressure. Using
the same polymer, Zharmagambetova et al. prepared ZnO-supported Ni [102] and Pd
317
nanocatalyst (Pd/Al 2 O 3 , Strem Chemical), which exhibited poor ethylene selectivities along the test (<35%) with a fast deactivation during the first 5 h on stream.
These results evidenced the applicability of bimetallic catalysts prepared purely by
colloidal techniques, not only in the semi-hydrogenation of substrates in liquid phase
but also with substrates of industrial relevance in the gas phase.
Important insights were also obtained by DFT calculations and for instance, Lopez
et al. studied the effect of the presence of a second metal on the properties of Pd in the
semi-hydrogenation of acetylene in excess ethylene [9]. As expected, the adsorption
of molecules was preferred on Pd sites, and that of acetylene was stronger than for
ethylene. Interestingly, alloying the Pd phase with metals such as Au or Cu stabilized
the adsorption of both substrate and product while others such as Bi enhanced the
thermodynamic selectivity by only favouring the adsorption of acetylene. In addition,
the formation of subsurface hydride (β-PdH phase) was reduced on all the studied
promoters, which consequently could limit the over-hydrogenation reaction.
10.4 Effect of the Stabilizer
For, the stabilization of M-NPs is applied in the semi-hydrogenation of alkynes,
a wide variety of organic compounds such as surfactants, ligands, polymers, ionic
liquids among other compounds have been utilized. According to the intrinsic properties of the stabilizer, prevention of NPs agglomeration can be obtained by steric
hindrance or electrostatic repulsion [31]. Moreover, the stabilizer can provide steric
or electronic modifications of the NPs surface, thus influencing the performance in
catalysis [30, 97–99]. At this point, it is important to mention that the effect of the
stabilizer depends on the reaction media (gas or liquid phase). In gas phase, the catalysts are dried, which results in the collapse of the stabilizer on the metal surface
and possibly the blockage of active sites. Differently, in liquid phase, the stabilizers are flexible enough to permit the access of substrates to the metal surface. For
instance, Witte et al. reported that the CO uptake observed in chemisorption of PdNPs
stabilized by hexadecyl(2-hydroxyethyl)dimethyl ammonium dihydrogenphosphate
(HHDMA) was suppressed when the NPs were isolated and dried [100].
Polymers have been classically employed for the steric stabilization of M-NPs
due to their capacity to provide a protective layer that wraps the NPs. Hirai et al.
reported that for PVP-stabilized PdNPs, the thickness of such a layer was proportional to the molecular weight of the polymer (thickness of 4–16 nm, for M w from
6000 to 574,000); however, the catalytic activity for the hydrogenation of cyclooctadiene was not sensitive to the thickness of the polymer layer [98]. Several authors
have addressed the application of PVP-stabilized Pd NPs in the semi-hydrogenation
of alkynes evidencing moderate to good alkene selectivities independently of the
PVP:Pd ratio [7, 97]. Niu et al. reported the evaluation of polyethylene glycol stabilized Ru NPs in the semi-hydrogenation of methyl propiolate [101]. For this system,
the alkene selectivity resulted highly sensitive to the temperature and pressure. Using
the same polymer, Zharmagambetova et al. prepared ZnO-supported Ni [102] and Pd