10 Progress in the Selective Semi-hydrogenation of Alkynes …
309
single-atom catalysts applied in the selective hydrogenation of alkynes were reported.
For instance, Huang et al. reported the enhancement of both selectivity and cokingresistance using a single-atom Pd 1 /C 3 N 4 catalyst for acetylene hydrogenation in
excess ethylene [14]. According to the authors, analysis of this material by in situ Xray photoemission spectroscopy revealed that the considerable charge transfer from
the Pd NPs to g-C 3 N 4 likely plays an important role in the catalytic performance
enhancement. Among the different methodologies employed for the preparation of
single-atom catalysts, the use of metal-organic frameworks can be highlighted for the
isolation/encapsulation of the corresponding metal species [45, 48]. More recently,
the same authors reported the preparation of an atomically dispersed copper (Cu)
catalyst supported on a defective nanodiamond-graphene (Cu 1 /ND@G). This material exhibited excellent catalytic performance for the selective conversion of acetylene to ethylene (98% of ethylene selectivity at 95% conversion and stability for
more than 60 h at 200 °C) [47]. According to the authors, this exceptional catalytic
performance was due to the unique bonding structure and electronic properties of Cu
atoms on Cu 1 /ND@G which facilitate the acetylene activation and ethylene desorption. Although the following catalytic system does not fall into the definition of SACs
(formally it is an immobilized molecular catalyst), its relevance in the field makes
its inclusion in this analysis necessary. Li et al. reported the selective hydrogenation
of acetylene catalysed by a cationic nickel atom confined in zeolite [49]. The catalyst consisted of a four-coordinated cationic nickel(II) confined in chabazite zeolite,
(Ni@CHA). This material catalysed efficiently the selective hydrogenation of acetylene with extremely high selectivity towards ethylene (97% at full conversion and
180 °C). The chabazite framework is proposed as an inorganic ligand of the nickel
centre, which also contributes to the heterolytic hydrogen dissociation by the local
electrostatic field within the zeolite cage.
The application of small metal clusters in catalysis has also attracted the attention of the scientific community in the last decades [50]. In terms of fundamental
understanding, Abdollahi and Farmanzadeh studied the reactivity of small Pd [51],
Cu [52] and Ni [53] metal clusters (composed of 2–15 atoms) as catalysts for the
semi-hydrogenation of acetylene in ethylene-rich mixtures using DFT. Differences
in the activation energy barriers and adsorption energies of acetylene and ethylene
revealed Pd 2 , Cu 11 and Ni 6 as potential candidates with optimal selectivity for this
reaction.
10.3 Effect of a Second Phase
The introduction of a second phase, generally a metal, to dilute the active phase
is a widely applied strategy for enhancing the catalyst performance in the semihydrogenation of alkynes. Traditionally, the outcome of such a dilution is classified according to electronic or geometric effects. The geometric effect refers to the
formation of ensembles of specific character or to the blockage of unselective sites
thus enhancing the selectivity towards the alkene. In contrast, the electronic effect
309
single-atom catalysts applied in the selective hydrogenation of alkynes were reported.
For instance, Huang et al. reported the enhancement of both selectivity and cokingresistance using a single-atom Pd 1 /C 3 N 4 catalyst for acetylene hydrogenation in
excess ethylene [14]. According to the authors, analysis of this material by in situ Xray photoemission spectroscopy revealed that the considerable charge transfer from
the Pd NPs to g-C 3 N 4 likely plays an important role in the catalytic performance
enhancement. Among the different methodologies employed for the preparation of
single-atom catalysts, the use of metal-organic frameworks can be highlighted for the
isolation/encapsulation of the corresponding metal species [45, 48]. More recently,
the same authors reported the preparation of an atomically dispersed copper (Cu)
catalyst supported on a defective nanodiamond-graphene (Cu 1 /ND@G). This material exhibited excellent catalytic performance for the selective conversion of acetylene to ethylene (98% of ethylene selectivity at 95% conversion and stability for
more than 60 h at 200 °C) [47]. According to the authors, this exceptional catalytic
performance was due to the unique bonding structure and electronic properties of Cu
atoms on Cu 1 /ND@G which facilitate the acetylene activation and ethylene desorption. Although the following catalytic system does not fall into the definition of SACs
(formally it is an immobilized molecular catalyst), its relevance in the field makes
its inclusion in this analysis necessary. Li et al. reported the selective hydrogenation
of acetylene catalysed by a cationic nickel atom confined in zeolite [49]. The catalyst consisted of a four-coordinated cationic nickel(II) confined in chabazite zeolite,
(Ni@CHA). This material catalysed efficiently the selective hydrogenation of acetylene with extremely high selectivity towards ethylene (97% at full conversion and
180 °C). The chabazite framework is proposed as an inorganic ligand of the nickel
centre, which also contributes to the heterolytic hydrogen dissociation by the local
electrostatic field within the zeolite cage.
The application of small metal clusters in catalysis has also attracted the attention of the scientific community in the last decades [50]. In terms of fundamental
understanding, Abdollahi and Farmanzadeh studied the reactivity of small Pd [51],
Cu [52] and Ni [53] metal clusters (composed of 2–15 atoms) as catalysts for the
semi-hydrogenation of acetylene in ethylene-rich mixtures using DFT. Differences
in the activation energy barriers and adsorption energies of acetylene and ethylene
revealed Pd 2 , Cu 11 and Ni 6 as potential candidates with optimal selectivity for this
reaction.
10.3 Effect of a Second Phase
The introduction of a second phase, generally a metal, to dilute the active phase
is a widely applied strategy for enhancing the catalyst performance in the semihydrogenation of alkynes. Traditionally, the outcome of such a dilution is classified according to electronic or geometric effects. The geometric effect refers to the
formation of ensembles of specific character or to the blockage of unselective sites
thus enhancing the selectivity towards the alkene. In contrast, the electronic effect