Q. Hu et al. / Journal of Catalysis 340 (2016) 184–195
185
greatly hinder their practical application. Therefore, besides the
above factors significantly governing the catalytic hydrogenation
of esters, it is necessary to construct special surface structures of
Cu-based catalysts to further improve their catalytic performance.
As we know, the surface electronic and geometric structures of
supported metal catalysts are related closely to their catalytic per-
formance, because, besides active metal sites [25–27], catalytic
processes usually involve multifarious catalytic centers synergisti-
cally controlling heterogeneous reactions [28,29]. It is well
documented that reducible oxides (e.g., titania and ceria) can
develop surface oxygen vacancies (Ov) for the valence decrease,
thus leading to the loss of structural oxygen from the anion sublat-
tice [30–33]. As reported, surface oxygen vacancies on the defec-
tive titania can fix oxygen in some molecules (water, aliphatic
alcohols) [34,35], while Ti3+ species adjacent to oxygen vacancies
on Pt/TiO2 can interact with carbonyl groups to promote the
hydrogenation of furfural [36]. Correspondingly, the strategy for
achieving high catalytic efficiency is to reasonably manipulate
the surface structures of catalyst supports to facilitate the adsorp-
tion and subsequent activation of reactants.
our group [49]. Solution A: Cu(NO3)2ꢁ6H2O, MnCl2ꢁ4H2O, and Al
(NO3)3ꢁ9H2O, with a Cu/Mn/Al molar ratio of 2:x:1 (x = 0, 0.5, 1,
1.5), were dissolved in 100 mL of deionized water to give a mixed
salt solution. Solution B: NaOH and Na2CO3 were dissolved
in 100 mL of deionized water to form a mixed base solution
([OHꢀ] = 1.6([M2+] + [Al3+]), [CO23ꢀ] = 2[Al3+]). Solutions A and B
were simultaneously added rapidly to a colloid mill with the rotor
speed set at 3000 rpm and mixed for 2 min. The resulting blue sus-
pension was washed with deionized water until pH 7.0, aged at
40 °C for 24 h, and finally dried at 70 °C for 24 h in a vacuum oven.
The obtained CuMnAl-LDH (denoted as LDH-x) was calcined in sta-
tic air at 550 °C for 6 h, pelletized, crushed, sieved to 40–60 mesh,
and denoted as MMO-x. Before the reaction, the calcined samples
were reduced in situ in 10% H2/N2 atmosphere at 300 °C for 2 h
at a ramp rate of 2 °C minꢀ1, and the obtained reduced catalysts
were denoted as CuMn-x. For comparison, Cu catalysts over differ-
ent commercial supports (Mn3O4, SiO2, ZnO) with a Cu loading of
35.0 wt.% were also prepared by incipient wetness impregnation.
In addition, highly dispersed Cu/SiO2–H comparison catalyst with
a Cu loading of 35.0 wt.% was prepared by a urea-assisted gelation
method previously reported [50].
On the other hand, layered double hydroxides (LDHs,
[M1ꢀx2+Mx3+(OH)2]x+[Ax/n
]
nꢀꢁmH2O), known as a family of highly
ordered two-dimensional anionic clay materials, contain different
M2+ and M3+ metal cations uniformly distributed in an orderly pre-
arrangement in the brucite-like layers [37]. LDH materials are
emerging as excellent catalyst supports to construct bifunctional
or multifunctional heterogeneous catalysts. For instance, LDH-
supported metal catalysts are very active in oxidation, deoxygena-
tion, and dehydrogenation reactions [38–40] without the addition
of a foreign alkali promoter. Liu et al. found that synergistic effects
between Au NPs and Cr cation redox cycles in LDHs could signifi-
cantly promote the aerobic oxidation of organic substrates [41].
In most cases, high metal loadings would result in the aggregation
of active metal particles. More interestingly, highly dispersed sup-
ported metal-based catalysts with tunable metal particle size can
be constructed by reducing calcined LDHs containing desired active
metal species [42–45]. This LDH precursor route significantly
improves the interaction between metal and support and prevents
the aggregation of metal NPs, thus facilitating the high dispersion of
active metal species. For example, LDH precursors containing Cu,
Zn, and Al have been utilized to prepare supported catalysts for
industrially important processes (e.g., methanol steam reforming,
methane synthesis) [46–48]. To further improve the catalyst perfor-
mance, the strategy for creating new types of catalytically active
centers via an LDH precursor route is still a challenging work.
In the present work, we reported new environment-friendly
and highly efficient supported copper nanocatalysts for gas-phase
hydrogenation of DMS to GBL, which were directly generated from
CuMnAl-LDH precursors. It was found that besides well-dispersed
metallic copper NPs, a large amount of surface oxygen vacancies
could be created by the transformation of Mn3+ to Mn2+ species
in the course of reduction, thereby forming abundant surface
Mn2+–Ov–Mn2+ defect structures. As-formed copper-based
nanocatalysts displayed exceptional catalytic hydrogenation per-
formance, with stability enduring up to 100 h at a low hydrogen
partial pressure of 0.25 MPa. Furthermore, this type of flexible
copper-based nanocatalysts displayed a great application potential
in the hydrogenation of a series of biomass-derived compounds
(e.g., acetol, levulinic acid, levulinic acid esters, furfural).
2.2. Characterization
X-ray diffraction (XRD) data were collected on a Shimadzu
XRD-6000 diffractometer with a graphite-filtered Cu K
a source
(k = 0.15418 nm) at 40 kV and 30 mA. Elemental analysis was per-
formed using a Shimadzu ICPS-7500 inductively coupled plasma
atomic emission spectroscope (ICP-AES). Transmission electron
microscopy (TEM) and high-resolution TEM (HRTEM) were carried
out on a JEOL 2100 operated at an accelerating voltage of 200 kV.
High-angle annular dark-field scanning TEM-energy-dispersive X-
ray spectroscopy (HAADF–STEM–EDX) images were recorded on
a JEOL2010F instrument. N2 adsorption–desorption isotherms of
the samples were obtained on a Micromeritics ASAP 2020 appara-
tus at ꢀ196 °C. All samples were outgassed prior to analysis at
200 °C for 12 h under 10ꢀ4 Pa vacuum. The specific surface areas
were determined by the multipoint BET method. X-ray photoelec-
tron spectroscopy (XPS) was recorded on a Thermo VG ESCA-
LAB250 X-ray photoelectron spectrometer using Al K
a X-ray
radiation (1486.6 eV photons). Binding energies were calibrated
based on the graphite C1s peak at 284.6 eV. X-ray induced Auger
spectra (XAES) were carried out on a PHI Quantera SXM using Al
K
a X-rays as the excitation source. Electron paramagnetic reso-
nance (EPR) of solid samples was determined at room temperature
on a Bruker ESP300E spectrometer. Photoluminescence (PL) emis-
sion spectra were recorded at room temperature using an RF-
5301PC fluorophotometer with excitation wavelength 320 nm.
The reduction behavior of calcined samples was studied by
hydrogen temperature-programmed reduction (H2 TPR) using a
Micromeritics ChemiSorb 2920 instrument. The sample (100 mg),
which was put in a quartz U-tube reactor, was degassed at
200 °C for 2 h under argon flow (40 mL/min). TPR was performed
in a stream of 10% v/v H2/Ar (40 mL/min) at a heating rate of
5 °C/min from 50 °C. The effluent gas was detected by a thermal
conductivity detector (TCD).
Metallic copper surface areas in samples were determined by
combining N2O oxidation and CO pulse chemisorption using a
Micromeritics ChemiSorb 2920 instrument. First, the calcined sam-
ple (100 mg) underwent an H2 TPR process in 10% H2/He mixture
from 50 to 350 °C at a heating rate of 5 °C. After cooling down to
70 °C in pure He, the gas was switched to 10% N2O/N2 (40 ml/
min) for 1 h to oxidize surface-reduced Cu0 atoms and oxygen
vacancies to Cu2O species and lattice oxygen ions. After that, the
catalyst was purged with He for 1 h and the temperature was
cooled to 50 °C. Finally, CO pulse chemisorption was carried out
2. Experimental
2.1. Synthesis of supported copper nanocatalysts
A series of CuMnAl-LDH precursors were prepared by separate
nucleation and aging steps in a method previously developed by