Q. Liu, et al.
AppliedCatalysisA,General608(2020)117832
of 1 °C min-1 from room temperature to 600 °C under N2 for 2 h. Light
brown solid material ZnO/4Mg1ZrOx was finally obtained. Note that
the synthetic molar ratio of Zn/Zr for the oxides prepared in this work is
kept constant as 0.066/1. Synthetic details for other kinds of metal
oxide catalysts prepared in this work are presented in the
Supplementary material.
0.73 cm μmol-1 and 0.96 cm μmol-1 was used respectively.
The content of Mg, Zr and Zn for ZnO/4Mg1ZrOx and 4Mg1ZrZnOx-
n samples were detected by inductively coupled plasma-mass spectro-
metry (ICP-MS) on an Agilent 7700ce instrument. Weighted sample was
firstly digested in hydrochloric and nitric acids mixture (3/1, v/v) by
microwave digestion. After that, the obtained mixture was diluted into
constant volume with deionized water and finally ICP analyzed.
Cu supported oxide catalyst with 0.1 wt% metal loading (denoted as
0.1%Cu/ZnO/4Mg1ZrOx) was prepared by impregnating as-synthesized
ZnO/4Mg1ZrOx with aqueous solution of Cu(NO3)2·3H2O. In detail, Cu
(NO3)2 aqueous solution was prepared by adding of Cu(NO3)2·3H2O
(0.0076 g, 0.03 mmol) into 30 mL H2O. ZnO/4Mg1ZrOx (2.0 g) was
added into the prepared solution, followed by vigorous stirring at room
temperature for 4 h. The mixture then evaporated to remove solvents
and dried overnight at 105 °C. Finally, the obtained material was cal-
cined at the rate of 2 °C min-1 to 600 °C under N2 for 2 h.
2.3. Catalyst evaluation and products analysis
The dehydrogenative α-methylenation (DM) of alcohols with me-
thanol was operated in a fixed-bed stainless steel reactor at 275 °C and
0.3 MPa using N2 as carrier gas. A calculated amount of catalyst was
loaded into the reactor (i.d. 10 mm) and pretreated at 400 °C with N2
(40 cm3 min-1) for 1 h. Otherwise specially noted, n-propanol/me-
thanol mixture (3 wt% of n-propanol) was used as the reactant. The
solution was fed into reaction system using high-pressure liquid pump
(Series II, USA) and gasified in an evaporator at 110 °C before flowing
through catalyst bed. The condensed liquid solution collected from the
gas-liquid separator was analyzed by gas chromatography (GC,
Shimadzu 2010 plus) equipped with a Flame Ionization Detector (FID)
detector and a Rtx-Wax column (30 m ×0.25 μm ×0.32 mm). The
temperature program for products separation is as following: the oven
temperature was initially held at 50 °C for 1 min followed by raising to
240 °C with a ramp of 20 °C min-1 and kept for another 1 min. The
gaseous products was collected and analyzed using GC (Shimadzu 2010
plus) equipped with two chromatographic columns (GDX-502 packed
column, molecular sieves 5A column) using Thermal Conductivity
Detectors (TCD) under He flow (carrier gas, 45 cm3 min-1). The GDX-
502 packed column (3 m ×3 mm) was used to detect CO2 at 60 °C while
the molecular sieves 5A column (i.d. 3 mm) was used to analyze H2, CO,
CH4, etc. in outlet gas mixtures at 100 °C.
2.2. Catalyst characterization
Powder X-ray diffraction (XRD) patterns were collected on Bruker
D8 advance in the scanning range of 20∼80°. Cu Kα radiation
(λ = 0.154 nm) was used as the X-ray source. The in situ XRD analysis
of prepared oxides with increasing temperatures (25∼600 °C) were
carried out in
a ceramic X-ray reactor under N2 atmosphere.
Transmission electron microscopy (TEM) and high-resolution trans-
mission electron microscopy (HRTEM) were performed on Hitachi H-
7650 and FEI Tecnai G20 electron microscope, which were operated at
120 kV and 200 kV, respectively. Scanning transmission electron mi-
croscope (STEM) and energy dispersive X-ray (EDX) mapping analysis
were carried out on Tecnai G2 F20 S-TWIN.
The specific surface areas and pore size distributions of oxide cat-
alysts were measured on Micromeritics ASAP 2020 using Brunauer-
Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) methods.
Typically, 100 mg of sample was firstly degassed at 350 °C for 2 h to
remove physically adsorbed molecules (e.g., H2O, CO2) prior to nitrogen
adsorption. X-ray photoelectron spectroscopy (XPS) was recorded on
Thermo Escalab 250XI using Al Kα (1486.6 eV) line source. All the
obtained binding energies were calibrated using C1 s peak at 284.8 eV
as the reference. CO2 temperature-programmed desorption (CO2-TPD)
measurements were carried out on Micromeritics AutoChem II 2920
instrument. About 100 mg of each sample was pretreated in U-shaped
quartz tube with a He flow (30 cm3 min-1) at 400 °C for 2 h, after which
the temperature was cooled down to 60 °C. The sample was then
maintained at this temperature for 2 h in 10 vol.% of CO2/He mixture,
followed by purging with pure He for 2 h to get a stable baseline. The
TPD curves were recorded from 60 °C to 800 °C with a rate of 10 °C min-
Carbon Balance (CB) was calculated based on the carbon content in
the detected reactants and products. The CB value for each catalytic run
is more than 96%, suggesting the good applicability of analysis method.
n-Propanol conversion (XPrOH) and product selectivity (Si) were calcu-
lated as following equations, respectively: XPrOH=(molPrOH-in-molPrOH-
out)/molPrOH-in ×100%, where molPrOH-in is the mole content of n-
propanol before reaction, molPrOH-out is the mole content of n-propanol
after reaction; Si=(moli)/(molProducts) ×100%, where moli represents
the mole of product i, molProducts is the total mole of all products. The
selectivity for the hydrogen transfer of unsaturated aldehyde (coupling
intermediate) was calculated as following:
SS-H=(molMAA)/
(molMAA+moli-BuOH+moli-BuO) ×100%, where molMAA is the mole
number of methylallyl alcohol (selective-hydrogenated product), moli-
1
using equipped Thermal Conductivity Detector (TCD). NH3 tempera-
is the mole number of isobutanol (fully-hydrogenated product)
BuOH
ture-programmed desorption (NH3-TPD) measurements were also con-
ducted on Micromeritics AutoChem II 2920 instrument using the same
pretreatment and analysis procedure as that of CO2-TPD except that
10 vol.% NH3/Ar and pure Ar were used as the adsorption gas mixture
and purging gas.
and moli-BuO is the mole of isobutanal (unselective-hydrogenated pro-
duct). Indeed, isobutanal was not detected in the collected organic so-
lution because of the high hydrogen transfer selectivity of our prepared
oxide catalyst.
Pyridine adsorbed Fourier transform infrared spectroscopy
(Pyridine-FTIR) were collected on Thermo Scientific Nicolet iN 10 IR
Microscope. Typically, each sample (5∼10 mg) was tableted to self-
supported wafer and held standing in a specially-designed vitreous IR-
adsorption cell with CaF2 windows. The sample was pretreated at
450 °C (heating rate is 10 °C min-1) under vacuum (ca. 10-6 mbar) for
1 h to remove any impurities. After cooling down to room temperature,
the background spectra was collected with frequency ranging from 400
to 4000 cm-1. Then the pyridine adsorption was performed at 60 °C for
1 h followed by desorption under vacuum at 150 °C for 1 h to eliminate
physically or weakly adsorbed pyridine. The FTIR spectra of adsorbed
pyridine was collected and then subtracted with activated sample
background. Each spectrum was recorded at a resolution of 4 cm-1 with
32 scans. For quantification of Brønsted sites (1560-1530 cm-1) and
Lewis acid sites (1460-1435 cm-1), the molar extinction coefficients of
3. Results and discussion
3.1. Catalyst structural and surface acid/basic properties
To identify a well-defined oxide catalyst for allylic alcohols synth-
esis, MgO was primarily selected because of its tunable surface acid/
basic properties after metal atom modifications [33,42–44]. The typical
in the Experimental section). Zr(OPr)4 was introduced as Zr precursor
onto MgO by gradual hydrolysis. Under methanol/water solvent, Zn
based zeolite imidazolate framework (ZIF-8) was in situ formed and
assembled with Mg-Zr composites. After refluxing, the collected mix-
ture was calcined to obtain the ZnO/4Mg1ZrOx. XRD patterns in Fig. S1
show the formation of Mg(OH)2 during hydrothermal treatment and Mg
(OH)2 could decompose into MgO after calcination. Cubic periclase
3