C. Van Nguyen, et al.
MolecularCatalysis475(2019)110478
are not environmental friendly and also not cost-effective for industrial
applications [2,13,27]. Besides, the metal leaching in the reaction en-
vironment and the catalyst deactivation after reaction are the most
concerned issues. Therefore, the catalytic conversion of LA into GVL in
water under mild reaction condition is challenging [26].
acid (99%), Al2O3 powder, TiO2 powder, activated carbon (AC),
RuCl3·3H2O were purchased from Sigma-Aldrich. All chemicals were
directly used without any further purification.
2.2. Synthesis of MIL-53-NH2
The hydroxyapatite supported ruthenium (Ru) has been reported as
the efficient catalyst for LA hydrogenation into GVL, compared to other
hydroxyapatite supported metals such as Pd, Pt, Cu, and Ni [21,28]. In
addition, the commercial Ru/C catalyst exhibited the high catalytic
activity for converting LA into GVL [29], nevertheless, the catalyst is
not reusable due to its deactivation and metal leaching during the re-
action [29–31]. Recently, the use of metal oxides (Al2O3, TiO2, ZrO2,
and SiO2) and functionalized metal oxides (NH2-γ-Al2O3) were reported
for stabilization of Ru nanoparticles (NPs) during LA hydrogenation
reaction at 70 °C and 4 MPa H2 pressure [25,32]. On the other hand,
Yulei Zhu et al. have demonstrated that Ru NPs supported on reduced
graphene oxide (RGO) promoted the LA hydrogenation to GVL with
99.9% yield at low temperature (40 °C, 4 MPa hydrogen pressure) [24].
However, the stability of Ru-supported catalysts under reaction and
their reusability is the major issue in many reactions. Thus, the devel-
opment of a stable and efficient catalyst for LA hydrogenation to GVL at
ambient reaction condition is of great significance.
Metal-Organic Framework (MOF)-derived nanomaterials including
porous carbons (PC) and metal oxides (MO) have shown extensive
studies for catalysis applications due to its diverse active sites, well-
defined structures, facile modified functions, large surface area, and
good thermal stability [33–35]. In addition, the introduction of second
metal into the structure of MOF-derived nanomaterial could create a
superior catalyst, which could involve both the benefits of foreign metal
and advantages of MOF-derived nanomaterials [36–38]. Here, we de-
scribe the synthesis of novel Ru embedded in MIL-53-NH2-derived C-
Al2O3 catalyst and its use for the LA hydrogenation into GVL in water
under mild reaction conditions (1 atm, 25 °C). As shown in Scheme 1,
the RuCl3@MIL-53-NH2 is first prepared via de novo synthesis method,
which was reduced to form Ru nanoparticles inside C-Al2O3 honey-
comb-like structure (named as Ru@C-Al2O3). The detailed catalyst
characterization was determined by using XRD, XPS, Raman spectro-
scopy, TEM, TGA, and ICP-OES, indicating that ultra-small Ru nano-
particles size (1 nm) are well-dispersed inside the C-Al2O3 honeycomb-
like structure. The prepared Ru@C-Al2O3 catalyst is highly active for
the hydrogenation of LA to GVL at ambient hydrogen pressure (1 atm)
in aqueous solution. In addition, the kinetic parameters including the
activation energy and reaction rate constant for LA hydrogenation over
Ru@C-Al2O3 catalyst were studied and calculated. We find that the
small Ru NPs and high electron density around to Ru0 play an important
role for LA conversion. Furthermore, the C-Al2O3 honeycomb-like
structure can stabilize Ru during reaction condition.
The MIL-53-NH2 as synthesized by modification from the previous
report [39]. Typically, 2-aminoterephthalic acid (7.5 g) was added into
10 mL aqueous solution of 1 M NaOH (mixture A). Then, 5 mL of Al
(NO3)3·9H2O (1.5 g) solution was dropped into the mixture A. The ob-
tained reaction mixture was magnetically stirred at RT for 1 day. The
yellow powder was centrifuged and washed with deionized water for
several times to remove NaOH and unreacted linker. Finally, the re-
sulting solid was dried under vacuum at RT to obtain MIL-53-NH2.
2.3. Synthesis of RuCl3@MIL-53-NH2 and Ru@C-Al2O3
The RuCl3@MIL-53-NH2 was synthesized using a similar method
which was used for the synthesis of MIL-53-NH2. Initially, 2-amino-
terephthalic acid (7.5 g) was added into 10 mL NaOH solution (1 M)
(mixture A). Next, the mixture B containing 5 mL of Al(NO3)3·9H2O
solution (1.5 g) and 5 mL RuCl3 (0.1 M) was added dropwise into the
mixture A. The mixture was then magnetically stirred at RT for 24 h.
After 24 h, the resulting solid was centrifuged and washed with deio-
nized water for several times. Subsequently, the solid product was dried
under vacuum at RT to obtain a RuCl3@MIL-53-NH2.
2.3.1. Synthesis of Ru@C-Al2O3 composite
The Ru@C-Al2O3 catalyst was prepared by reducing the RuCl3@
MIL-53-NH2 powder at 500 °C for 5 h, with a heating rate of 5 °C min−1
under H2/N2 mixed gas flow (80 mL min−1). After completion of re-
duction, the resulting solid was denoted as Ru@C-Al2O3.
2.4. Catalytic levulinic acid (LA) hydrogenation into γ-valerolactone
All experiments were performed in a three-neck round bottom flask
connected with the condenser. Typically, 10 mL water was added to the
three-neck round bottom flask followed by 20 mg Ru@C-Al2O3 catalyst
and LA (1 mmol). When the reaction temperature reached to 60 °C, the
hydrogen gas was passed through the mixture with a flow rate of 50 mL
min−1 at 1 atm. The reaction take place under continuous stirring for
2 h. After 2 h reaction time, the reaction mixture was cooled to room
temperature and diluted to 25 ml with deionized water. The reaction
solution was filtered with a 0.22 μm syringe filter for high-pressure li-
quid chromatography (HPLC) analysis.
2.5. Catalyst characterization
2. Materials and methods
Powder X-ray diffraction (XRD) patterns were recorded using a Cu
Kα radiation source on a Rigaku-Ultima IV instrument. The chemical
state of elements (Ru 3d and Cl 2p) present in prepared samples was
studied using X-ray photoelectron spectrometer (XPS, Thermo
Scientific, Theta Probe). Transmission electron microscopy (TEM) and
2.1. Chemicals
Levulinic acid (LA, 98%), Al(NO3)3·9H2O, and γ-valerolactone
(GVL, 98%) were purchased from Acros Organics. 2-aminoterephthalic
high-resolution
transmission
electron
microscopy
(HRTEM)
Scheme 1. Illustration for the synthesis of Ru@C-Al2O3 and catalytic LA hydrogenation into GVL.
2