120
M. I. Qadir et al.
The ‘soluble’ NPs in ILs usually behave as homogeneous-like catalysts for the
hydrogenation of alkenes, alkynes and arenes. Among these hydrocarbons, the semihydrogenation of benzene in imidazolium-based ILs has been studied extensively.
Thermodynamic and kinetic barriers strongly control the semi-hydrogenation of
benzene. Thermodynamically, the semi-hydrogenation of benzene to cyclohexene is
not favourable, since cyclohexane is at least 75 kJ mol
−1 more stable than cyclohexene
[76]. The kinetics of the liquid-phase hydrogenation of benzene is still under debate,
as the mechanism concerns the chemical state of the catalytically active hydrogen
and the role of the reaction intermediates. One of the serious problems with benzene
is its solubility in classical solvents. In this regard, 1-n-butyl-3-methylimidazolium
hexafluorophosphate (BMIm.PF 6 ) IL has been used as a biphasic media in which
benzene is highly soluble while its hydrogenated products (cyclohexene and cyclohexane) are insoluble. Dupont et al. were able to hydrogenate benzene by using Ru
NPs (2.6 ± 0.4 nm) in BMIm.PF 6 and observed a high selectivity of <39% to cyclohexene at very low benzene (1%) conversion. The conversion of benzene reached
73% with a 100% selectivity to cyclohexane (Table 4.3, entry 1) [29]. It is assumed
that BMIm.PF 6 generates an IL-cage around the Ru NPs that not only stabilise the
NPs, but could also repel the cyclohexene formed and so decrease the re-adsorption
and further hydrogenation to cyclohexane [29]. Pt NPs were also applied in the same
reaction conditions (75 °C, 4 bar H 2 ) using BMIm.PF 6 IL, which yielded a 46%
conversion of benzene with 100% conversion to cyclohexane [17]. By the incorporation of Ru into Pt NPs, Ru/Pt bimetallic NPs in BMIm.PF 6 produced the unexpected
1,3-cyclohexadiene with 21% selectivity at 5% benzene conversion (Table 4.3, entry
1) [41].
Mono-dispersed RuO 2 NPs (2–3 nm) in BMIm.PF 6 presented a higher activity
in the hydrogenation of benzene (97%) to cyclohexane with 43 turnover frequency
(Table 4.3, entry 3) [30], whereas only 3% benzene conversion was obtained when
BMIm.BF 4 IL was used. This catalytic system (RuO 2 /BMIm.PF 6 ) effectively hydrogenated cyclohexene and 1-methyl-1-cyclohexene with 99 and 97% conversion
to cyclohexane and methyl-cyclohexane, respectively. Low-cost Ni NPs (11 nm)
demonstrated higher activity in the hydrogenation of benzene to cyclohexane, with
31 and 34% conversions using BMIm.NTf 2 and BPy.NTf 2 ILs, respectively [77].
Different types of additive have been applied to hydrogenate benzene with the desired
selectivity. For further details, see reviews [78, 79].
Ru-immobilised NPs in BMIm.NTf 2 IL drove the liquid–liquid biphasic catalytic
hydrogenation of different arenes under mild reaction conditions (50–90 °C and
4 bar H 2 ). The apparent activation energy of E a = 42.0 kJ mol
−1 was estimated
for the hydrogenation of toluene [31]. The same catalytic system also presented
efficient activity in the hydrogenation of o-xylene with 32% conversion and 100%
selectivity to 1,2-dimethylcyclohexane (Table 4.3, entry 13). Dyson et al. revealed
that Rh NPs stabilised the imidazolium-functionalised bipyridine compounds 4,4
-
bis-[7-heptyl]-2,2
-bipyridine
2+ ([BIHB]
2+ ) and 4,4
-bis(methyl)-2,2
-bipyridine
2+
([BIMB]
2+ ) used as catalysts in the biphasic hydrogenation of various arene
substrates [80]. The catalytic activity was strongly influenced by the stabiliser
employed and followed the trend [BIHB]
2+ > bipy > [BIMB]
2+ (Table 4.3, entries
M. I. Qadir et al.
The ‘soluble’ NPs in ILs usually behave as homogeneous-like catalysts for the
hydrogenation of alkenes, alkynes and arenes. Among these hydrocarbons, the semihydrogenation of benzene in imidazolium-based ILs has been studied extensively.
Thermodynamic and kinetic barriers strongly control the semi-hydrogenation of
benzene. Thermodynamically, the semi-hydrogenation of benzene to cyclohexene is
not favourable, since cyclohexane is at least 75 kJ mol
−1 more stable than cyclohexene
[76]. The kinetics of the liquid-phase hydrogenation of benzene is still under debate,
as the mechanism concerns the chemical state of the catalytically active hydrogen
and the role of the reaction intermediates. One of the serious problems with benzene
is its solubility in classical solvents. In this regard, 1-n-butyl-3-methylimidazolium
hexafluorophosphate (BMIm.PF 6 ) IL has been used as a biphasic media in which
benzene is highly soluble while its hydrogenated products (cyclohexene and cyclohexane) are insoluble. Dupont et al. were able to hydrogenate benzene by using Ru
NPs (2.6 ± 0.4 nm) in BMIm.PF 6 and observed a high selectivity of <39% to cyclohexene at very low benzene (1%) conversion. The conversion of benzene reached
73% with a 100% selectivity to cyclohexane (Table 4.3, entry 1) [29]. It is assumed
that BMIm.PF 6 generates an IL-cage around the Ru NPs that not only stabilise the
NPs, but could also repel the cyclohexene formed and so decrease the re-adsorption
and further hydrogenation to cyclohexane [29]. Pt NPs were also applied in the same
reaction conditions (75 °C, 4 bar H 2 ) using BMIm.PF 6 IL, which yielded a 46%
conversion of benzene with 100% conversion to cyclohexane [17]. By the incorporation of Ru into Pt NPs, Ru/Pt bimetallic NPs in BMIm.PF 6 produced the unexpected
1,3-cyclohexadiene with 21% selectivity at 5% benzene conversion (Table 4.3, entry
1) [41].
Mono-dispersed RuO 2 NPs (2–3 nm) in BMIm.PF 6 presented a higher activity
in the hydrogenation of benzene (97%) to cyclohexane with 43 turnover frequency
(Table 4.3, entry 3) [30], whereas only 3% benzene conversion was obtained when
BMIm.BF 4 IL was used. This catalytic system (RuO 2 /BMIm.PF 6 ) effectively hydrogenated cyclohexene and 1-methyl-1-cyclohexene with 99 and 97% conversion
to cyclohexane and methyl-cyclohexane, respectively. Low-cost Ni NPs (11 nm)
demonstrated higher activity in the hydrogenation of benzene to cyclohexane, with
31 and 34% conversions using BMIm.NTf 2 and BPy.NTf 2 ILs, respectively [77].
Different types of additive have been applied to hydrogenate benzene with the desired
selectivity. For further details, see reviews [78, 79].
Ru-immobilised NPs in BMIm.NTf 2 IL drove the liquid–liquid biphasic catalytic
hydrogenation of different arenes under mild reaction conditions (50–90 °C and
4 bar H 2 ). The apparent activation energy of E a = 42.0 kJ mol
−1 was estimated
for the hydrogenation of toluene [31]. The same catalytic system also presented
efficient activity in the hydrogenation of o-xylene with 32% conversion and 100%
selectivity to 1,2-dimethylcyclohexane (Table 4.3, entry 13). Dyson et al. revealed
that Rh NPs stabilised the imidazolium-functionalised bipyridine compounds 4,4
-
bis-[7-heptyl]-2,2
-bipyridine
2+ ([BIHB]
2+ ) and 4,4
-bis(methyl)-2,2
-bipyridine
2+
([BIMB]
2+ ) used as catalysts in the biphasic hydrogenation of various arene
substrates [80]. The catalytic activity was strongly influenced by the stabiliser
employed and followed the trend [BIHB]
2+ > bipy > [BIMB]
2+ (Table 4.3, entries