102
[28]) lead to different levels of structural dealumination with for-
mation of variable amounts of extraframework aluminium species
(EF-Al) [29,30]. It was shown that catalytic performance of BEA
zeolites can be influenced by the level of dealumination [28,31].
The present work systematically investigates the influence of
the Ru precursor and the reduction temperature in the one-pot
transformation of citronellal into menthols on Ru/H-BEA catalysts.
Characterization was carried out by means of CO chemisorption,
DRIFTS and Pyridine DRIFTS spectroscopy, XRD, Ar physisorption,
TEM and temperature programmed reduction.
2.6. Diffuse reflectance infrared Fourier transform spectroscopy
DRIFTS measurements were carried out on a Bruker Equinox55
spectrometer. To determine the nature of the acid sites, the samples
were exposed to pyridine saturated in flowing nitrogen at 353 K
after pre-treating in flowing N2 at 673 K (respectively 473 K for the
catalysts reduced at lower temperatures).
2.7. X-ray Diffraction
A powder X-ray diffraction (XRD) analysis of the samples was
carried out on a Mythen1K (Dectris) diffractometer using CuK␣1
radiation ( = 1.5406 Å).
-
2. Experimental
2.1. Preparation of the catalysts
2.8. Catalytic experiments
Supported Ru catalysts were obtained by incipient-wetness
impregnation of zeolite H-BEA (SiO2:Al2O3= 25 and 150; Clari-
ant) as supports via Ru(NO)(NO3)3 (Alfa Aeser), Ru(acac)3 (Alfa
Aeser), RuCl3·xH2O (Alfa Aeser) and Ru3(CO)12 (Sigma Aldrich)
as metal precursors. Using Ru(NO)(NO3)3 and RuCl3·xH2O water,
while using Ru(acac)3 and Ru3(CO)12 toluene was used as solvent.
The samples were dried for 15 h at 373 K and reduced for 3 h in flow-
ing hydrogen (50 mL gcatalyst−1) in a temperature range of 523 K and
923 K. The nominal metal content was 1 wt.% and 2 wt.%, denoted
as 1%Ru/H-BEA and 2%Ru/H-BEA, respectively. It was exemplarily
validated via ICP-OES on a Varian 715-ES.
The hydrogenation experiments were carried out in a stain-
less steel autoclave (Parr, 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 temper-
ature, 4.5 g racemic citronellal (Merck) was added and the reactor
was pressurized via a separate tank. This was defined as the start
of the reaction. Samples were taken periodically and analysed by
gas chromatography (Shimadzu GC 2010 Plus) using an Agilent DB-
Wax column (348 K, 5 min; 1 K min−1 → 381 K; 5 K min−1 → 413 K;
20 K min−1 → 493 K, 5 min). Note that it was ensured by using pow-
dered catalysts (particle size distribution d90% of <20 m) and a
stirring rate of 1000 rpm that the reactions were expired under
chemical control.
2.2. Ar physisorption
Surface areas were determined by Ar physisorption using the
BET equation, pore volume (determined in a range until 15 nm)
and micropore volume was determined by NLDFT-method (Non-
Local Density Functional Theory). The samples were pre-treated
under vacuum for 2 h at 353 K, for 2 h at 373 K and 20 h at 573 K.
The measurements were conducted at 87 K in a range between p/p0
10−6 − 0.99 on a Quantachrome ASiQ.
Conversion (X) and selectivities (Si) were calculated via the fol-
lowing equations.
ꢀ
ꢁ
cCAL
XCAL % = 1 −
· 100
(1)
(2)
( )
cCAL,0
ꢀ
ꢁ
ci
|ꢀCAL|
Si % =
( )
·
· 100
cCAL,converted
ꢀi
2.3. CO chemisorption
The quantification of the diastereoselectivity was identified via
the mole fraction (xMT) of the desired stereoisomer.
The CO uptake and size of Ru particles were determined by
CO chemisorption measurements using a TPD/R/O 1100 (Thermo
Fisher Scientific). After reducing the sample at 473 K for 1 h fol-
lowed by cooling down to 273 K in a hydrogen flow CO pulses
(V = 0.473 mL) were introduced. Assuming an adsorption of one CO
molecule per accessible metal atom, the amount of chemisorbed
CO was determined and the metal particle size was obtained via
d = 6 × (ꢀm/am) × D−1 with ꢀm = volume occupied by an atom in bulk
metal, am = area occupied by a surface atom and D = dispersion [32].
cMenthol
ꢂ
xMT % =
( )
· 100
(3)
cMenthols
Both (the turn-over-frequency (TOF) and activity of the catalyst)
refer to the point of the maximum menthol selectivity. The TOF
refers only to the hydrogenation step of isopulegols, the amount
of active sites is obtained from CO uptake of the CO chemisorption
measurements. The activity of the catalyst is calculated from the
obtained amount of menthol referred to the mass of the catalyst.
ꢃ
ꢄ
nMenthols,formed
2.4. Temperature programmed reduction
TOF min−1
=
(4)
mcatalyst · nCO · tSMenthols,max
Temperature programmed reduction measurements were con-
ducted on an apparatus TPD/R/O 1100 (Thermo Fisher Scientific).
The catalysts were oxidized at 623 K for 1 h in 4.9 vol% O2/He and
pre-treated in Ar at 383 K for 1 h before the measurements. For the
ꢀ
ꢁ
nMenthols,formed
mmol
Activity
=
(5)
min · gcatalyst
mcatalyst · tSMenthols,.max
H2-TPR the sample was heated from 303 K to 1173 K (5 K min−1
)
3. Results and discussion
in 5.1 vol% H2/Ar while monitoring the TCD signal. TPR results are
presented per gram of sample.
2.5. Transmissions electron microscopy
in aqueous (Ru(NO)(NO3)3) and organic (Ru(acac)3) solution are
shown in Fig. 2.
For TEM images a JEOL 2100F electron microscope operating
at 200 kV was used. The samples were fixed on a copper grid-
supported carbon film by depositing a dispersed ethanol solution
on it.
Pyridine DRIFT spectra show the typical bands for H-BEA zeo-
lites [14,33]. Bands at 1445 cm−1 and 1596 cm−1 can be assigned to