J. Zhang et al. / Journal of Catalysis 371 (2019) 346–356
347
In our previous work [10], the target product in the hydrogenol-
ysis of dibenzofuran was o-phenylphenol by the reasonable design
of catalyst. It is found the basic sites promoted the formation of o-
phenylphenol. Herein, our goal is to extend this knowledge by
modifying different Mg loadings on Pt/SiO2 in an attempt to
improve the production of biphenyl. MgO, as the catalyst promoter,
was found to increase activity and BP selectivity of Pt/SiO2 catalyst
for the hydrogenolysis of DBF. Characterization techniques such as
XRD, IR, CO-IR, low temperature N2 sorption, NH3-TPD, CO2-TPD,
Py-IR, TEM, SEM, were used to study the structural property, elec-
tronic effect and the surface acid-base property of catalysts. More-
over, the role of other alkaline earth metal (Ca, Sr, Ba) oxides on the
activity of Pt/SiO2 catalyst was also investigated. Finally, the reac-
tion mechanistic understanding for the hydrogenolysis of DBF to
biphenyl over Pt/xMgO/SiO2 catalysts has been proposed. This
work demonstrates for the first time the importance of employing
a suitable addition of MgO in the rational development of Pt-based
catalysts for the hydrogenolysis of DBF. The experimental results
presented can provide a rational and useful guidance for designing
Pt-based hydrogenolysis catalysts.
2.2.3. Preparation of reference catalysts
For comparison, the reference Pt-3MgO/SiO2-co catalyst was
prepared by co-impregnation of the appropriate amount of aque-
ous Mg(CH3COO)2 and Pt precursor solution, remaining 3 wt%
Mg. The following treatment of obtained sample was the same as
above. In the binary catalyst, the two functional components of
Pt/SiO2 catalyst and MgO were combined just by physical mixing
(Pt/SiO2 + MgO).
2.3. Support characterization
2.3.1. Nitrogen adsorption
The textural properties of the supports were determined using
the nitrogen adsorption–desorption isotherms at ꢀ196 °C using a
Quantachrome Autosorb IQ automated sorption system. The speci-
fic surface area was calculated using the Brunauer–Emmett–Teller
method at a relative partial pressure of 0.05–0.3. The total pore
volume and pore size distribution were measured by the adsorp-
tion curve using the Barre–Joyner–Halenda model at a relative par-
tial pressure of 0.95. Prior to the measurement, the samples were
degassed under vacuum at 200 °C for 8 h.
2. Experimental
2.3.2. X-ray diffraction
X-ray diffraction (XRD) analysis of the supports was performed
2.1. Materials
on a Rigaku D/MAX-RB instrument using a Cu K
a monochroma-
tized radiation source in the 2h range of 5–90° with a scan speed
Silica (348 m2 gꢀ1
) was purchased from Aladdin Chemical
of 10° minꢀ1, operated at 40 kV and 100 mA.
Reagent Co and used as support after the calcination at 400 °C
for 2 h under Air. Magnesium acetate (Mg(CH3COO)2ꢁ4H2O,
ꢂ99.0%), calcium nitrate (Ca(NO3)2ꢁ4H2O, ꢂ99.0%), strontium
nitrate (Sr(NO3)2, ꢂ99.5%), barium nitrate (Ba(NO3)2, ꢂ99.5%),
and chloroplatinic acid (H2PtCl6ꢁ6H2O, Pt basis 99.9%) were pur-
chased from Sinopharm Chemical Reagent Co. Methanol (CH3OH,
99.8%) and n-decane (C10H22, 98%) were obtained from Tianjin Ker-
mel Chemical Reagent Co. Dibenzofuran (C12H8O, ꢂ98.0%), 2-
methylpiperidine (C6H13N, 99.0%) and n-dodecane (C12H26, 99.0%)
were obtained from Aladdin Chemical Reagent Co. All of the mate-
rials were analytical reagent grade and utilized without further
purification.
2.3.3. Temperature-programmed desorption
The acidity and basicity of the supports were measured by
Temperature-programmed desorption (TPD) of NH3 and CO2,
respectively, using the CHEMBET-3000 instrument with a thermal
conductivity detector (TCD). Typically, 100 mg sample was
pressed, crushed, and placed in U-shaped glass tube, heated to
500 °C (10 °C minꢀ1) in He, and this temperature was kept for
60 min. Then the sample was cooled down to 120 °C. The ammonia
adsorption was conducted for 40 min under a 20 mL minꢀ1 flow of
10 vol% NH3 in He gas mixture. The physically adsorbed ammonia
was removed under helium flow at 120 °C for 60 min, and the TPD
was measured by linearly increasing the cell temperature from
120 °C to 500 °C at
a a
heating rate of 10 °C minꢀ1 under
2.2. Catalyst preparation
20 mL minꢀ1 flow of He. The amount of desorbed ammonia was
monitored and quantified by measuring the areas of the desorption
profiles. Similarly, the basicity of the sample was determined by
CO2-TPD, a method analogous to that described above.
2.2.1. Preparation of Pt/xMgO/SiO2 catalysts
Commercial SiO2 was impregnated by the wetness impregna-
tion method with an aqueous Mg(CH3COO)2 solution in order to
prepare supports with different Mg loadings (1, 3, 6, 9 wt%). The
obtained supports, denoted as xMgO/SiO2, where x is the weight
percentage of Mg (x = 0, 1, 3, 6, 9), were dried overnight at
100 °C, and finally calcined at 500 °C for 4 h under Ar/O2
(40/20 mL minꢀ1) flow.
The as-synthesized supports were used to prepare Pt catalysts
by successive wetness impregnation method using H2PtCl6ꢁ6H2O
as Pt precursor, methanol as the solvent, with a targeted Pt content
of 0.5 wt%. The Pt samples were dried at 80 °C overnight and then
reduced at 400 °C for 2 h under a H2 flow of 40 mL minꢀ1 before
catalytic performance test and the characterization of catalysts.
The obtained catalysts are designated as Pt/xMgO/SiO2.
2.3.4. FT-IR spectroscopy of pyridine adsorption
Fourier transform infrared spectra (FT-IR) experiments using
pyridine as the probe molecule, on a Bruker Equinox 55 spectrom-
eter equipped with a DTGS detector, were conducted in the IR cell
at 150 °C, 300 °C and 450 °C, using 10 mg sample pellets. The sup-
ports were evacuated at 450 °C for 30 min and then cooled to room
temperature, followed by the record of a background at a resolu-
tion of 4 cmꢀ1. After that, the samples were saturated with pyri-
dine flow for another 20 min. Then the IR spectra were taken at
room temperature after evacuation for 30 min at 150 °C to remove
physically adsorbed and residual pyridine. The adsorption of pyri-
dine at 300 °C and 450 °C were similar to that at 150 °C.
2.2.2. Preparation of Pt/MO/SiO2 (M = Ca, Sr, Ba) catalysts
Other catalysts modified by alkaline earth metal oxides (Pt/MO/
SiO2, M = Ca, Sr, Ba) were obtained by a sequential impregnation
method as above. The Pt and alkaline earth metal loadings of these
catalysts were consistent with the moles of Pt/3MgO/SiO2, respec-
tively. All actual atomic ratio of M/Pt are approximately 50:1 as
shown in Table 1.
2.3.5. Infrared attenuated total reflection spectrum
Fourier transform infrared attenuated total reflection spectra
(ATR-FTIR), using a Thermo Fisher iN10 spectrometer equipped
with a liquid-nitrogen-cooled MCT detector, were recorded within
the spectral range of 650–4000 cmꢀ1 with a resolution of 4 cmꢀ1
and 16 scans for signal accumulation.