increased with Ni loading up to a maximum at 15%Ni/MCM-41.
Both bifunctional 3%Ni/ZrBEA catalyst and ZrBEA-3%MCM-41
were recycled by recalcining at 540 uC for 4 h and reduction in H2
(entries 13 and 14, Table 1). No significant loss in activity or
menthol selectivity was observed up to 3 cycles. The good
tolerance to regeneration conditions of both catalytic systems
makes them useful catalysts.
In conclusion, bifunctional Ni/Zr-beta or a physical mixture of
Zr-beta with Ni/MCM-41 are effective catalysts for the one-pot
synthesis of menthol from citronellal. The use of Zr-beta in
combination with a Ni hydrogenation functionality enables a high
diastereoselectivity to (¡)-menthol 6 of 90–94% with a total
menthol yield of 86–97%.{
Notes and references
Fig. 3 Hydrogenation of (¡)-citronellal over Zr-beta-3%Ni//MCM-41.
{
The preparation of Zr-beta has been reported in ref. 16. Siliceous MCM-
1 was synthesized following ref. 17. The supported Ni catalysts were
prepared by wet impregnation using Ni(NO . The impregnated samples
Reaction conditions as in Fig. 1.
4
3 2
)
conversion of citronellal to isopulegols over the MCM-41 support
and a 3% Ni/MCM-41 sample after 22 h was 1.6 and 2.6%,
respectively. This indicates that cyclisation occurs predominantly
over Zr-beta. Hence, the lower diastereoselectivity over Ni/Zr-beta
can be attributed to pore blockage so that part of the cyclisation
reaction occurred on the external surface without pore constraints.
Comparing the catalysts, the highest yield to menthols was
obtained over 4% Ni/Zr-beta. The yield of the (¡)-menthol isomer
were dried overnight at 90 uC and calcined at 450 uC for 6 h. The Ni
loading on Zr-beta was from 3 to 15 wt% and on MCM-41, from 3 to
3
2
0 wt%. The catalytic tests were performed at 80 uC under a H
2
pressure of
MPa in a 100 ml Teflon-lined stainless steel autoclave. The reaction
mixture consisted of 1.7 ml (9.5 mmol) (¡)-citronellal in 50 ml tert-butanol
as solvent and 0.15 g catalyst. The volume of the solvent was rather large
due to the need to flush the sample withdrawing line before sampling. For
the dual catalyst system, a 1 : 1 mixture of Zr-beta and Ni/MCM-41 were
21
used. The catalyst was pre-reduced in H (50 ml min ) for 2 h at 450 uC
2
before charging into the autoclave. The reaction was conducted in a He
atmosphere for the first hour before introduction of H . Samples were
6
was 83%.
2
removed at regular intervals and analyzed by gas chromatography. The
products were identified by comparison with known standards and by
GCMS.
The one-pot synthesis of menthol was also carried out using a
mixture of Zr-beta and Ni supported on MCM-41. The use of
MCM-41 allows a high metal dispersion on the mesoporous
support. The crystallite size, calculated from the Scherrer equation,
was 8.1 and 11.3 nm for the 10 and 15% Ni sample, respectively.
These crystallites are two to three times smaller than those formed
on Zr-beta. The initial rate of cyclisation of citronellal over Zr-beta
was about twice as fast as the rate of hydrogenation over
1
K. Bauer, D. Garbe and H. Surburg, Ullmann’s Encyclopedia of
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ˇ
Ni/MCM-41. This, together with the delayed introduction of H
2
5
6
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for the first hour, ensured that the major products formed after 2 h
were isopulegols (Fig. 3). The diastereoselectivity to (¡)-isopulegol
2
was 93–94% which is higher than that observed over the
7
bifunctional Ni/Zr-beta catalysts, 90% (Table 1). Increasing the Ni
loading led to a faster rate of hydrogenation. The selectivity to
menthols was >94% for all Ni loadings, of which 92–94% are the
desired (¡)-menthol 6. Besides menthols, the other products
detected were unreacted isopulegols 2–5, 3,7-dimethyloctanol 11
and to a small extent, citronellol 10. However, hydrogenation
products of citronellal constitute only 6% of the products.
Doubling the catalyst amount led to almost complete hydrogena-
tion of the isopulegols after 8 h vs. 22 h for reaction systems with a
smaller catalyst weight.
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1
4
1
At the same Ni loading, Ni/MCM-41 catalysts catalysed the
hydrogenation of isopulegols at a much faster rate than the
bifunctional Ni/Zr-beta catalysts (cf. Fig. 3). The induction time
was shortened from 1–5 h in the bifunctional catalysts to 0.5–2 h in
the dual catalyst systems. The rate of formation of menthols
4
1
1
17 C.-F. Cheng, D. H. Park and J. Klinowski, J. Chem. Soc., Faraday
Trans., 1997, 93, 1, 193.
7
92 | Chem. Commun., 2006, 790–792
This journal is ß The Royal Society of Chemistry 2006