ACS Catalysis
Research Article
so on. For conversion of 2,5-HD to 2,5-DMTHF and N-
substituted tetrahydropyrroles, the ability of supported metals
to activate H2 is the crucial point, which can be tuned by
suitable supports via electronic metal−support interac-
tions.38−42 Besides, the support itself sometimes has a catalytic
role on the reactions. Thereby, it is a great promise to
construct robust catalysts for conversion of 2,5-HD to 2,5-
DMTHF and N-substituted tetrahydropyrroles by utilizing
suitable support materials.
Herein, we used montmorillonite (MMT) and hydroxyapa-
tite (HAP) as functional supports for Ru nanoparticles (Ru
NPs) to construct supported Ru catalysts for 2,5-HD
conversion. It was observed that MMT-supported Ru NPs
(denoted as Ru/MMT) showed high catalytic activity for
hydrogenation of 2,5-HD to 2,5-DMTHF at 90 °C, which is
lower than that of the reported catalytic systems,16,18,20 while
HAP-supported Ru NPs (denoted as Ru/HAP) could catalyze
the direct reductive amination of 2,5-HD to N-substituted
tetrahydropyrroles at 30 °C. These results showed that the
reactivity of different reaction routes to convert 2,5-HD could
be easily tuned by the support materials.
2.4. H2-TPR of All the Catalysts. H2-TPR was performed
to examine the reducibility behavior of Ru with different
support (Figure S1). It was found that the reduction of RuO
species occurred at 92.5, 124.5, 119, 128.9, 238.5, and 96.1 °C
for Ru/MMT, Ru/HAP, Ru/ZrO2, Ru/Al2O3, Ru/MoO3, and
Ru/C, respectively. Meanwhile, for all the catalysts, the highest
reaction peaks were lower than 500 °C. Therefore, we selected
500 °C as the reduction temperature for the preparation of all
the catalysts.
2.5. Conversion of 2,5-HD to 2,5-DMTHF. All the
reactions were performed in a 10 mL Teflon lined stainless
steel autoclave. In a typical experiment, 2,5-HD (5 mmol) and
the catalyst of the desired amount were introduced into the
reactor. After the air in the reactor was replaced by hydrogen
(three times), hydrogen was charged into the reactor to reach
the desired pressure. Then, the reactor was placed into an air
bath with constant temperature, and meanwhile, the stirrer was
started. After the reaction was conducted under solvent-free
condition for a desired time, the reaction was stopped rapidly
by placing the reactor into ice water. The H2 was released, and
n-dodecane (1 mmol) as the internal standard and ethanol (3
mL) were added into the reactor. The sample was analyzed by
gas chromatograph (GC, Agilent 7820) equipped with a
hydrogen flame-ionization detector and HP-5 nonpolar
columns. Identification of the products and reactants was
performed using a GC-MS (SHIMADZU-QP2010) as well as
by comparing the retention times to respective standards in
GC traces.
2. METHODS
2.1. Materials. Ru/C (5 wt % on carbon) was provided by
Alfa Aesar China Co., Ltd. 5-Hydroxy-2-pentanone (92+%)
was purchased from Ark. 2,5-Dimethyltetrahydrofuran (2,5-
DMTHF, 98.0%), 1,2-diacetylbenzene (95%), 2,5-hexanedione
(2,5-HD, 97.0%), and 2,5-hexanediol (98.0%) were obtained
from J&K Scientific Co., Ltd. Ruthenium(III) chloride
anhydrate (99.5% Ru 47%), n-dodecane (99%), n-octylamine
(98%), n-hexylamine (99%), n-butylamine (99%), 3-amino-1-
propanol (99%), montmorillonite (MMT), hydroxyapatite
(HAP), γ-aluminum oxide (γ-Al2O3), zirconium dioxide
(ZrO2), sodium borohydride (NaBH4, 98%), and molybde-
num trioxide (MoO3) were purchased from Beijing InnoChem
Science & Technology Co., Ltd. H2 (>99.99%) was supplied
by Beijing Analytical Instrument Company. All chemicals were
obtained from commercial sources and used without further
treatment.
2.2. Preparation of Ru/MMT and Ru/HAP. The route to
prepare Ru/MMT and Ru/HAP was similar except for the
utilization of different support materials. In a typical route,
MMT or HAP (5 g) was dispersed into water (500 mL). Then,
aqueous solution (20 mL) of ruthenium chloride anhydrous
(containing ca. 0.45 g of RuCl3·xH2O) was added into the
dispersion of MMT (or HAP). After being stirred for 24 h at
room temperature, the suspension was filtered and washed
several times with deionized water and ethanol. Finally, the
obtained powder was dried under vacuum at 60 °C for 12 h.
Ru/MMT or Ru/HAP was obtained by the reduction of the
corresponding solid power in H2/Ar mixture with a volume
ratio of 5/100 at 500 °C for 3 h.
In the reusability experiments, Ru/MMT was separated via
centrifugation, and washed using ethanol. After being dried
under vacuum at 60 °C for 5 h, the recovered Ru/MMT was
reused for the next run directly.
2.6. Conversion of 2,5-HD to N-Substituted Tetrahy-
dropyrroles. All the reactions were performed in a 10 mL
Teflon lined stainless steel autoclave. In a typical experiment, 1
mmol 2,5-HD (1 mmol), the corresponding amine (1.2
mmol), and a certain amount of desired catalyst and methanol
(4 mL) were introduced into the reactor. After the air in the
reactor was replaced by hydrogen (three times), hydrogen was
charged into the reactor to reach the desired pressure. Then,
the reactor was placed into an air bath of 30 °C, and
meanwhile, the stirrer was started. After the reaction was
conducted for a desired time, the reaction was stopped rapidly
by placing the reactor into ice water. The H2 was released, and
n-dodecane (1 mmol) was added into the reactor as the
internal standard. The sample was analyzed by a gas
chromatograph (GC, Agilent 7820) equipped with a hydrogen
flame-ionization detector and HP-5 nonpolar columns.
Identification of the products and reactants was performed
using a GC-MS (SHIMADZU-QP2010) as well as by
comparing the retention times to respective standards in GC
traces.
In the reusability experiments, Ru/HAP was separated via
centrifugation and washed using ethanol. After being dried
under vacuum at 60 °C for 5 h, the recovered Ru/HAP was
reused for the next run directly.
2.7. Characterization. The morphology images of the
samples were obtained using a TEM JEOL-1011 and TEM
JEOL-2100F. Elemental distribution mappings were examined
using a TEM JEOL-2100F microscope. Powder X-ray
diffraction (XRD) patterns were collected on a Rigaku D/
max- 2500 X-ray diffractometer using Cu Kα radiation (λ =
0.154 nm) at 40 kV and 200 mA. N2 adsorption−desorption
2.3. Preparation of Ru/MoO3, Ru/γ-Al2O3, and Ru/
ZrO2. In a typical procedure, the desired support material
(MoO3, γ-Al2O3, or ZrO2) was dispersed into water (500 mL).
Then, an aqueous solution (20 mL) of ruthenium chloride
anhydrous (containing ca. 0.45 g of RuCl3·xH2O) was added
into the dispersion of the support material. After being stirred
for 24 h at room temperature, the water was removed via
rotary evaporation. The corresponding solid was reduced in
H2/Ar mixture with a volume ratio of 5/100 at 500 °C for 3 h
to obtain Ru/MoO3, Ru/γ-Al2O3, or Ru/ZrO2.
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ACS Catal. 2021, 11, 7685−7693