11 Selective Hydrogenation of Aldehydes and Ketones
395
are dealing with ideal conditions for ‘heterolytic’ hydrogenations. Isomerization of
potential allyl alcohol product intermediates was not studied.
We consider one example of hydrogen transfer from isopropanol to CAL using
Co 3 O 4 NPs as catalyst reported by Schüth and co-workers [127]. With the procedure
followed, small cobalt oxide NPs were obtained (3 nm size) in microporous carbon.
Indeed, hydrogen retains in this process its heterolytic character and unsaturated
alcohols (furfuryl alcohol, COL, geraniol, nerol) were obtained with high selectivity.
CAL to COL selectivity was over 95% at 99% conversion (isopropanol, 120 °C). The
catalyst is relatively slow at a TOF of 1 h
−1 , our estimation, but orders of magnitude
cheaper than a Pt catalyst, and hydrogen transfer avoids high-pressure reactors.
11.8.2 Nickel NPs
Ni NPs (6 nm size) inside and outside Al 2 O 3 nanotubes (100 nm diameter) were
prepared by ALD on carbon nanocoils used as a sacrificial template [128]. The
catalysts containing Ni inside were several times more active in the hydrogenation
of CAL (isopropanol, 80 °C, 20 bar H 2 ). This was ascribed to a stronger interaction
between Ni and the support inside the tubes. Indeed, the catalysts with Ni outside
the tubes showed considerably more leaching in the recycling experiments. The
selectivity is >97% to HCAL for both catalysts at all conversions.
Chiang and co-workers studied NiB/PVP NPs in the hydrogenation of α,βunsaturated aldehydes [129]. The nanoparticles were made as described above for
CoB from a nickel salt and NaBH 4 reduction in water or ethanol in the presence
of PVP with several MWs. The Ni 2 B nanoparticles obtained were amorphous and
by TEM their size was estimated 3–5 nm. The composition of the surface indicated
a Ni/B ratio of 3–4, with B mostly trivalent. Hydrogenation reactions were carried
out in ethanol, at various temperatures, 9 bar H 2 , and furfural, crotonaldehyde and
citral served as substrates. Crotonaldehyde gave >90% butanal, also at low conversions and 60 °C. Citral (80 °C) provided up to 87% citronellal at 80% conversion,
but prolonged reaction times led to further hydrogenation. As usual, furfural gave
high selectivity to furfuryl alcohol, even in cyclohexane as a solvent. In a subsequent
paper, NiB NPs stabilized by CTBA were used, but again the main products of the
hydrogenation of citral were citronellal and citronellol [130].
Another attempt to improve the performance of Ni in the present hydrogenation
reactions was reported by Shimazu et al. [131], who alloyed Ni and Sn for this
purpose, supported on Al(OH) 3 . The NiSn alloy catalysts at various Ni/Sn ratios
were synthesized by the hydrothermal treatment of a mixture of Raney Ni supported
on aluminium hydroxide and SnCl 2 –H 2 O in EtOH/H 2 O. Ni 3 Sn, Ni 3 Sn 2 and Ni 3 Sn 4
alloy phases were formed and the size of NPs was 4–6 nm. As for the NiB catalysts,
hydrogenation of furfural to furfuryl alcohol was straightforward for NiSn catalysts
(isopropanol, 30 bar H 2 , 180 °C). CAL was hydrogenated to COL also at a relatively
high temperature (160 °C) in a yield of 92% with catalyst Ni 3 Sn. This favourable
effect of Sn had been noted before by Delbecq and Sautet for Pt 3 Sn. Their DFT
395
are dealing with ideal conditions for ‘heterolytic’ hydrogenations. Isomerization of
potential allyl alcohol product intermediates was not studied.
We consider one example of hydrogen transfer from isopropanol to CAL using
Co 3 O 4 NPs as catalyst reported by Schüth and co-workers [127]. With the procedure
followed, small cobalt oxide NPs were obtained (3 nm size) in microporous carbon.
Indeed, hydrogen retains in this process its heterolytic character and unsaturated
alcohols (furfuryl alcohol, COL, geraniol, nerol) were obtained with high selectivity.
CAL to COL selectivity was over 95% at 99% conversion (isopropanol, 120 °C). The
catalyst is relatively slow at a TOF of 1 h
−1 , our estimation, but orders of magnitude
cheaper than a Pt catalyst, and hydrogen transfer avoids high-pressure reactors.
11.8.2 Nickel NPs
Ni NPs (6 nm size) inside and outside Al 2 O 3 nanotubes (100 nm diameter) were
prepared by ALD on carbon nanocoils used as a sacrificial template [128]. The
catalysts containing Ni inside were several times more active in the hydrogenation
of CAL (isopropanol, 80 °C, 20 bar H 2 ). This was ascribed to a stronger interaction
between Ni and the support inside the tubes. Indeed, the catalysts with Ni outside
the tubes showed considerably more leaching in the recycling experiments. The
selectivity is >97% to HCAL for both catalysts at all conversions.
Chiang and co-workers studied NiB/PVP NPs in the hydrogenation of α,βunsaturated aldehydes [129]. The nanoparticles were made as described above for
CoB from a nickel salt and NaBH 4 reduction in water or ethanol in the presence
of PVP with several MWs. The Ni 2 B nanoparticles obtained were amorphous and
by TEM their size was estimated 3–5 nm. The composition of the surface indicated
a Ni/B ratio of 3–4, with B mostly trivalent. Hydrogenation reactions were carried
out in ethanol, at various temperatures, 9 bar H 2 , and furfural, crotonaldehyde and
citral served as substrates. Crotonaldehyde gave >90% butanal, also at low conversions and 60 °C. Citral (80 °C) provided up to 87% citronellal at 80% conversion,
but prolonged reaction times led to further hydrogenation. As usual, furfural gave
high selectivity to furfuryl alcohol, even in cyclohexane as a solvent. In a subsequent
paper, NiB NPs stabilized by CTBA were used, but again the main products of the
hydrogenation of citral were citronellal and citronellol [130].
Another attempt to improve the performance of Ni in the present hydrogenation
reactions was reported by Shimazu et al. [131], who alloyed Ni and Sn for this
purpose, supported on Al(OH) 3 . The NiSn alloy catalysts at various Ni/Sn ratios
were synthesized by the hydrothermal treatment of a mixture of Raney Ni supported
on aluminium hydroxide and SnCl 2 –H 2 O in EtOH/H 2 O. Ni 3 Sn, Ni 3 Sn 2 and Ni 3 Sn 4
alloy phases were formed and the size of NPs was 4–6 nm. As for the NiB catalysts,
hydrogenation of furfural to furfuryl alcohol was straightforward for NiSn catalysts
(isopropanol, 30 bar H 2 , 180 °C). CAL was hydrogenated to COL also at a relatively
high temperature (160 °C) in a yield of 92% with catalyst Ni 3 Sn. This favourable
effect of Sn had been noted before by Delbecq and Sautet for Pt 3 Sn. Their DFT