K. Fulajtárova et al. / Applied Catalysis A: General 502 (2015) 78–85
79
(about twice as the amount of furfural) 100% selectivity to fur-
furyl alcohol at 81% conversion was obtained. Monometallic Pd and
bimetallic Pd–Cu/Al2O3 catalysts were unselective for this reaction.
Furfural is the product of acid catalyzed dehydration of pentoses
is highly beneficial to use as a source of furfural diluted furfural-
water streams (e.g., azeotropes) obtained in traditional distillation
[15], stream stripping [16] or developing membrane [17] and reac-
energy needed for production of furfural feedstock. For example,
diluted aqueous solutions of furfural can be highly selectively con-
verted to cyclopentanone [2] or C15 and C17 fuel precursors [19]. In a
recent publication [20] we have demonstrated highly selective cat-
90 mol% yield) in water as the solvent at mild reaction conditions.
The conversion of furfuryl alcohol to cyclopentanone is important
step toward cyclopentanone starting from aqueous solutions of
furfural. Very important is also conversion of furfuryl alcohol to
4-hydroxy-2-cyclopentenone [21], an intermediate for the prepa-
ration of various specialty chemicals.
copper on the bimetallic catalyst. The mixture was heated at 40 ◦C
and stirred 8 h, while the solution was regularly checked for the
presence of copper ions in solution. Method B: the procedure was
similar as in Method A, while the mixture heated at 40 ◦C was
reduced with formaldehyde which was added separately during
1 h. The catalyst was designed 5% Pd–5% Cu/MgO (sep). Method C:
3% Pd–Cu/C catalysts with 5% and 10% loading of copper supported
on activated carbon were prepared using co-impregnation method.
The samples were dried at 120 ◦C for 2 h, calcined at 250 ◦C for 5 h in
flowing air and then reduced at 300 ◦C or 450 ◦C for 2 h under flow-
ing hydrogen. The catalysts are designed 3% Pd–5% Cu/C(CM) and
3% Pd–10% Cu/C(CM), respectively. Pd-hydrotalcite (Pd/HT) catalyst
was prepared by a standard method described earlier [22]. Aque-
ous solutions of Mg(NO3)2·6H2O and Al(NO3)3·9H2O (Mg:Al = 2:1)
were added dropwise to a NaOH solution under nitrogen atmo-
sphere during stirring, which was maintained at 50 ◦C for 5 h. The
pH of the reaction system was adjusted to between 9 and 10. When
the reaction was completed, the formed white precipitate was aged
in an aqueous solution at 70 ◦C for 2 days. The precipitate was fil-
tered and washed repeatedly till the filtrate was neutral, then was
dried at 110 ◦C for 24 h. Onto HT was impregnated H2PdCl4 solu-
tion to get 5 wt% of Pd. XRD spectra indicated that the sample has
a typical structure of hydrotalcite.
In the present contribution we have investigated the hydrogena-
tion of furfural in water as a solvent. The main objective was to
study the influence of supported Pd–Cu catalysts on the activity
and the selectivity of furfuryl alcohol formation. In addition, vari-
ous methods of catalyst preparation on catalyst performance and
the effect of reaction parameters such as, reaction temperature,
hydrogen pressure and catalyst loading and recycling were studied
to optimize the furfural conversion to furfuryl alcohol.
2.3. Catalysts characterization
The BET surface area of the samples was determined by nitrogen
adsorption at 77 K after activation of the sample in vacuum at 300 ◦C
for 2 h. Powder X-ray diffraction (XRD) patterns were acquired on
a Bruker AX S D8 diffractometer using CuK␣ radiation. Crystalline
phases were identified by a comparison with the JCPDS file. The
metal particle size distributions of the supported catalysts were
determined by transmission electron microscopy: JEOL 1200EX
microscope at accelerating voltage of 120 kV. The XPS signals were
recorded using a Thermo Scientific K-Alpha XPS system (Thermo
Fisher Scientific, UK) equipped with a microfocused, monochro-
matic Al K␣X-ray source (1486.6 eV). An X-ray beam of 400 m
size was used at 6 mA × 12 kV. Temperature programmed reduction
(TPR) profiles of the catalysts were obtained with ChemiSorb 2720
(Micrometrics, USA) equipped with a TCD detector. The fraction of
Pd exposed (CO/Pd) was estimated from dynamic CO chemisorp-
tions, measured in a pulse system equipped with TCD detector. The
chemisorption analysis was performed by passing pulses of CO until
a constant CO peak area was observed. The exposed metal fraction
was calculated from the moles of adsorbed CO per total moles of
Pd impregnated onto the catalyst.
2. Experimental
2.1. Chemicals
Furfural (98% purity), furfuryl alcohol (98%), tetrahydrofur-
furyl alcohol (98%), tetrahydrofuran (98%), 2-propanol (98%),
dioxane (97%), methyl–isobutyl ketone (97%), palladium chlo-
ride, CuSO4·5H2O, Mg(NO3)2·6H2O, Al(NO3)3·9H2O, formaldehyde
(37%) and NaBH4 were purchased from Sigma–Aldrich. Activated
charcoal Norit (1030 m2 g−1) was from Fluka and MgO (55 m2 g−1
)
was obtained by calcination of hydroxide at 450 ◦C for 5 h. The sup-
ports were ground to a particle size between 0.075 and 0.125 mm.
Furfural was purified by vacuum distillation and stored at –15 ◦C.
2.2. Catalysts preparation
2.4. Experimental set up and reaction procedure
Supported monometallic palladium catalysts with different Pd
loadings were prepared by impregnation method using H2PdCl4
(dihydrogen tetrachloropalladate(II)) from aqueous solutions of
PdCl2 in hydrochloric acid. After impregnation a part of pre-
pared catalyst precursors was reduced with formaldehyde or
sodium borohydride and then was washed to remove chloride ions.
Final concentration of chloride ions in waters was detected using
AgNO3/HNO3. Other part of the precursors was dried at 120 ◦C
for 2 h. After drying, the samples were calcined at 250 ◦C for 5 h
in flowing air and then reduced at 300 or 450 ◦C for 2 h under
flowing hydrogen. Bimetallic Pd–Cu catalysts with different metal
loadings supported on activated carbon, MgO and Mg(OH)2 were
prepared by the following methods. Method A: a given amount of a
monometallic Pd catalyst prepared by reduction with formalde-
hyde was placed into aqueous solution containing mixture of
CuSO4.5H2O, NaOH, sodium–potassium tartrate and 37% aqueous
solution of formaldehyde in the weight ratio 1:1:4.5:2.2. This solu-
tion was used in the amount to obtain the required loading of
Furfural hydrogenation reactions were performed in a 50 ml
stainless steel reactor connected with a flexible metal capillary to
a hydrogen supply system recording at constant pressure the con-
sumption of hydrogen in defined reaction times. The reactor was
loaded with the liquid reaction mixture (usually 5 ml) and the cat-
alyst. The air was purged out from the reactor by flushing with
nitrogen and then four times with hydrogen. The reactor pres-
surized to the desired pressure with hydrogen was placed into
thermostatic oil bath, and after 5–7 min of heating, the reactants
were mixed by shaking the reactor using a vibrator. This moment
was the start of reaction. A constant hydrogen pressure was main-
tained throughout the reaction. The vigorous agitation ensured that
the measured rate of hydrogen consumption was not influenced
by mass-transfer effects. Hydrogenation was stopped when during
3–5 min hydrogen consumption was practically zero. The samples
were analyzed using a gas chromatography (Shimadzu GC-17A)
equipped with flame ionization detector by the formerly described