J. Plößer et al. / Journal of Catalysis 320 (2014) 189–197
191
as Ru loading, reaction temperature, hydrogen pressure, and sol-
sample was exposed to NH
3
at 308 K for 1 h. Weakly adsorbed
vent should be understood to obtain an optimized catalyst system
for the selective transformation of citronellal to menthol.
NH
3
was removed by flowing nitrogen at 308 K. Finally, the tem-
À1
perature was increased with a rate of 5 K min up to 923 K, and
the amount of NH
3
was measured by FT-IR spectrometry (Thermo
2
. Experimental
Fischer Scientific).
2.1. Catalyst preparation
2.3.2. CO chemisorption
The size of the metal particles was determined by CO chemi-
2 4
Sulfated zirconia (ZrO -SO ) was synthesized by sol–gel method
25]. To a solution of zirconium i-propoxide (70 wt% in 1-propanol,
sorption measurements, which were carried out on an apparatus
TPD/R/O 1100 (Thermo Fisher Scientific). Therefore, the quartz
reactor was loaded with a sample. After reducing the catalysts at
73 K for 1 h followed by cooling down to 273 K in a hydrogen flow
CO pulses with a volume of 0.473 mL were introduced. Assuming
an adsorption of one CO molecule per accessible metal atom, the
[
Sigma–Aldrich) and 2-propanol (70 wt% in 1-propanol, Sigma–
Aldrich) sulfuric acid (95.0–98.0 wt%, Sigma–Aldrich) was added
dropwise and stirred for 10 min. Over a period of 2 h, a solution
of water and 2-propanol was added and the resulting gel was stir-
red for an additional hour. The molar ratios were as follows:
4
amount of chemisorbed CO was determined and the metal particle
H
2
SO
4
/Zr = 0.2; H
2
O/Zr = 13.4; 2-propanol/Zr = 13.2. The gel was
À1
size was obtained via d = 6(
m
m
/a
m
)D with
m
m
= volume occupied
dried at room temperature for two days, at 373 K for 12 h and cal-
cined at 823 K in an air flow.
Supported metal catalysts were obtained by incipient-wetness
impregnation of different types of zeolite H-BEA (Clariant) as sup-
by an atom in bulk metal, a
m
= area occupied by a surface atom
and D = dispersion [26]. The obtained particle sizes were validated
via TEM measurements (not shown here).
3 3 3 2
ports using Ru(NO)(NO ) (Alfa Aeser), Pd(NO ) (Alfa Aeser), or
Pt(acac) (ABCR) as metal precursor and water (for Pd and Ru cat-
2
3. Results and discussion
alysts) or toluene (for the Pt precursor) as solvent. The zeolite
materials differ in the Si:Al ratio (25, 35, and 150) and specific sur-
3.1. Cyclization of citronellal
2
À1
face areas (493, 660, and 595 m g , respectively). The samples
were dried overnight at 373 K. The Pd catalysts were calcined in
air flow at 773 K. All catalysts were reduced ex situ in flowing
hydrogen (Pd/H-BEA at 723 K, Pt/H-BEA and Ru/H-BEA at 623 K;
TPR analysis of the stored catalyst does not indicate an incurring
oxidation, not shown here). The metal content of the obtained cat-
alysts was validated via ICP measurements (not shown here).
The citronellal cyclization to isopulegols (Fig. 1, route 1) was
studied on different Lewis acids, under which especially ZnBr
industrial applied, heteropoly acids as well as different solid acids
such as zeolites and self-prepared sulfated zirconia (ZrO -SO ).
Note that Lewis acids and heteropoly acids are soluble under given
conditions. Conversion of citronellal (XCAL), selectivities to isopule-
gols (SISPs), defunctionalization (SDFP) and dimerization products
(SDimers) as well as the ratio of isopulegol:neo-isopulegol:iso-isop-
ulegol:neoiso-isopulegol are compared in Table 1.
2
is
2
4
2.2. Catalytic testing
The cyclization experiments were carried out in a 25-mL round-
bottomed flask equipped with a reflux condenser using n-hexane
Roth) as solvent at 353 K. The flask was charged with 50 mg cata-
In addition to the desired cyclization of citronellal to isopulegol,
dimerization of citronellal via aldol condensation (Fig. 1, route 4b),
dimerization of isopulegol to di-isopulegol ethers (Fig. 1, route 4c),
and the defunctionalization of isopulegols to p-menthadienes
(Fig. 1, route 4a) were observed. This is consistent with previous
works [12,13]. No reaction of citronellal occurred on activated
carbon and oxides such as SiO , La O , Al O , and TiO . Sulfated
(
lyst, 10 mL n-hexane, and 0.1 mL n-tetradecane (Merck) as internal
GC standard. After reaching the desired temperature, 0.5 mL race-
mic citronellal (Merck) was added. This was defined as the start of
the reaction. After 1 h, the reaction was stopped and the sample
was analyzed by gas chromatography (Shimadzu GC 2010 Plus)
2
2
3
2
3
2
zirconia showed the highest activity of the investigated catalysts.
Already after 5 min, a conversion of 98% was reached. However, a
large amount of dimerization products and a small amount of
p-menthadienes were formed. This leads to a selectivity to isopule-
gols of only 59%. High conversions of >99% after 1 h were also
reached with heteropoly acids (phosphomolybdic acid and phos-
photungstic acid). Again, the fraction of dimerization products
was high leading to isopulegols of <70% selectivity. Using Lewis
acids such as ZnBr , ZnCl , FeCl and SnCl2, conversions of 33–98%
À1
using an Agilent DB-Wax column (348 K, 5 min; 1 K min ? 381 K;
À1
À1
5
K min ? 413 K; 20 K min ? 493 K, 5 min).
The hydrogenation experiments were carried out in a stainless
steel autoclave (Parr 4843, 300 mL) usually using n-hexane (Roth)
as solvent at 373 K and 25 bar hydrogen pressure. The reactor
was charged with 0.5 g catalyst, 150 mL solvent, and 1 mL
n-tetradecane (Merck) as internal GC standard. After reaching the
desired temperature, 4.5 g racemic citronellal (Merck) was added
and the reactor was pressurized over a separate tank. This was
defined as the start of the reaction. Samples were taken periodically
and analyzed by gas chromatography as described above. The
reproducibility of the experimental setup and the catalyst prepara-
tion was verified by repeating the same experiment with different
catalyst charges. The deviation of the calculated menthol concen-
tration was about maximum 4%.
2
2
3,
were obtained after 1 h. ZnBr , ZnCl and SnCl exhibited selectiv-
2
2,
2
ities to isopulegols of 93–96%, whereas with FeCl3, only 68% was
reached. The highest selectivities to isopulegols (>98%) were
obtained with Al O ASiO and the zeolites H-BEA and H-ZSM5.
2
3
2
Among them, H-BEA was also highly active (citronellal conversion
>99%). Note that the Si:Al ratio of H-BEA had no effect on the
conversion and product distribution. The amount of acid sites of
H-BEA-25, H-ZSM5, and sulfated zirconia, determined by tempera-
Note that it was ensured by using powdered catalysts (particle
size distribution d90% of <20
that the reactions were expired under chemical control.
l
m) and a stirring rate of 1000 rpm
ture-programmed desorption of NH , increased in the following
3
À1
À1
order: ZrO -SO (0.69 mmol g ) < H-BEA-25 (1.11 mmol g ) <
2
4
À1
H-ZSM5 (1.99 mmol g ). No correlation of the citronellal conver-
sion with the amount of acid sites was observed, which indicates
pore-diffusion limitation of citronellal and isopulegols in the small
zeolite pores [13]. Beside the amount of acid sites, acid strength of
zeolites is another parameter, which also depends on the Si:Al ratio:
an increase of the latter results in an increase of the acid strength of
2
2
.3. Characterization
.3.1. Temperature-programmed desorption of NH
3
The amount of acid sites was determined by temperature-
programmed desorption of NH . After calcination at 673 K, the
3