Selective Hydrogenolysis of Dibenzofuran over Highly Efficient Pt/MgO Catalysts to o-Phenylphenol

Article abstract of DOI:10.1021/acs.oprd.7b00339

Direct selective hydrogenolysis of dibenzofuran (DBF) derived from coal and shale oil to a value-added chemical, ortho-phenylphenol (OPP), with high selectivity (80%) and yield (48%) has been achieved over Pt/MgO at 400 °C and 1.0 MPa by controlling the C-O bond cleavage as well as minimizing the extent of hydrogenation of aromatic rings. Meanwhile, Pt/SiO₂, Pt/Al₂O₃, and Pt/MgO/Al₂O₃ catalysts were used for the DBF hydrogenolysis and showed lower selectivity for OPP. The influence of reaction parameters has been studied to unveil the optimal reaction conditions. And the phenomenon of OPP dehydrogenation is found over various catalysts for the hydrogenation reaction of OPP. Extensive reactions and catalyst characterizations demonstrated that the OPP selectivity and the dehydrogenation of OPP to DBF follows the order of supports basicity: MgO-900 > MgO > SiO₂ > MgO/Al₂O₃ > Al₂O₃, which shows that the acid-base properties of supports impact the adsorption and desorption behaviors of DBF and OPP, and results in the disparity of OPP selectivity and the dehydrogenation of OPP. Finally, a plausible reaction pathway and mechanistic understanding are proposed.

Full text of DOI:10.1021/acs.oprd.7b00339

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Full Paper
Selective hydrogenolysis of dibenzofuran over
highly efficient Pt/MgO catalysts to o-phenylphenol
Jie Zhang, Lei Wang, Chuang Li, Shaohua Jin, and Changhai Liang
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.7b00339 • Publication Date (Web): 19 Dec 2017
Downloaded from http://pubs.acs.org on December 19, 2017
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Selective hydrogenolysis of dibenzofuran over highly
efficient Pt/MgO catalysts to o-phenylphenol
Jie Zhang, Lei Wang, Chuang Li, Shaohua Jin, Changhai Liang*
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Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical
Engineering, Dalian University of Technology, Dalian 116023, China
*
To whom correspondence should be addressed: Fax: + 86ꢀ411ꢀ84986353; Eꢀmail:
changhai@dlut.edu.cn; Homepage: http://amce.dlut.edu.cn
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KEYWORDS: Selective hydrogenolysis, Pt/MgO, dibenzofuran, oꢀphenylphenol, basicity，
dehydrogenation.
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ABSTRACT
Direct selective hydrogenolysis of dibenzofuran (DBF) derived from coal and shale oil to
valueꢀadded chemical, orthoꢀphenylphenol (OPP), with high selectivity (80 %) and yield
(
48 %) has been achieved over Pt/MgO at 400 °C and 1.0 MPa by controlling the CꢀO bond
cleavage as well as minimizing the extent of hydrogenation of aromatic rings. Meanwhile,
Pt/SiO , Pt/Al and Pt/MgO/Al catalysts were used for the DBF hydrogenolysis and
2
2
O
3
2 3
O
showed lower selectivity to OPP. The influence of reaction parameters has been studied to
unveil the optimal reaction conditions. And the phenomenon of OPP dehydrogenation is
found over various catalysts for the hydrogenation reaction of OPP. Extensive reactions and
catalyst characterizations demonstrated that the OPP selectivity and the dehydrogenation of
OPP to DBF follows the order of supports basicity: MgOꢀ900 > MgO > SiO
2 2 3
> MgO/Al O >
Al , which shows that the acidꢀbase properties of supports impact the adsorption and
2
O
3
desorption behaviors of DBF and OPP, and result in the disparity of OPP selectivity and the
dehydrogenation of OPP. Finally, a plausible reaction pathway and mechanistic
understanding are proposed.
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INTRODUCTION
The realization of sustainable routes to liquid fuels and chemicals from oxygen
ꢀcontaining liquids derived from coal and shale oil relies on the effective transformation of
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highly oxygenated compounds into hydrocarbon products and valueꢀadded chemicals.
Additionally, the inherent disadvantage of the oxygenꢀcontaining compounds found in coal
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derived chemicals, such as phenols, aryl ethers and benzofurans , is that their uses could lead
to environmental violation in case of oxygenꢀcontaining waste releases to the environment.
Addressing the challenge of sustainable generation of valueꢀadded chemicals by using of
6
oxygenꢀcontaining waste will require highly efficient catalysts with desirable selectivity,
which is also inherent problem for biomass conversion. Therefore, selective hydrogenolysis
of CꢀO bond to valueꢀadded products are attracting increasing attention and will inevitably
benefit the sustainable development of coalꢀderived liquids and biomass conversion.
To the best of our knowledge, due to the high dissociation energy of CꢀO bond, e.g.
dibenzofuran (DBF, as one typical oxygenꢀcontaining compound derived from coal), the
CꢀO bond is quite difficult to be cleaved. oꢀPhenylphenol (OPP), as an intermediate product
of DBF hydrogenolysis, is one of the most valuable and versatile chemicals which is widely
used as thermal stabilizer, surfactant and biocide and is also an excellent starting precursor
for fuel conversion. The demand of it could reach 60000 tons per year in the world and will
increase with time going. Grubbs demonstrated that the use of the combination of
triethylsilane with organic base can efficiently obtain OPP with 95 % selectivity in
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homogeneous catalysis , nevertheless the catalyst tends to be difficultly separated, expensive
and thus impractical in industry. It is thus urgent to develop suitable catalysts and optimize
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reaction conditions aiming at selective hydrogenolysis of DBF to OPP in heterogeneous
catalysis via cleaving just one CꢀO bond with preservation of the aromatic rings.
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,8ꢀ10
The reaction pathway of catalytic DBF hydrogenation is presented in Scheme 1.
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Previous research had been established that all the phenolic intermediates are more reactive
6, 11
than dibenzofuran that it is very difficult to obtain OPP with the selective cleavage of
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only one aromatic CꢀO bond. In our previous work, the hydrodeoxygenation products of
DBF were obtained and hydrodeoxygenation pathway (HYD) was discussed in detail.
Considerable research has been focused on the hydrodeoxygenation of DBF to fuels via
12ꢀ18
HYD pathway over acid catalysts.
Only a few studies of the DBF hydrogenolysis to OPP
19
have been reported in the literature, in which the selectivity to OPP reached 73 % and the
yield was just 16.8 % over carbon supported Pd catalyst. As a consequence, CꢀO bond
20
hydrogenolysis has been extensively studied over the past decade. Various metals have
2
1
15ꢀ17, 22
23
24
13, 25ꢀ28
29, 30
been used as the active components, such as Ru, Pt,
Pd, Rh, Ni,
Cu,
Fe,
31
20
Au, W and Co, and important progresses have been made toward the cleavage of CꢀO
bond. In particular, the noble metals, Pt, Pd, Ru and Rh, have attracted considerable attention
because of their high hydrogenolysis activity. Accordingly, a rationally designed catalyst
should easily activate molecular hydrogen and the CꢀO bond without hydrogenation for the
purpose of acting selectively towards the hydrogenolysis of the oxygenꢀaromatic bonds.
Indeed, there is also a need to optimize the suitable temperature and pressure to transform
dibenzofuran into highꢀvalue oꢀphenylphenol over a highly efficient catalyst.
The acidic and basic properties of supports have important impacts on the catalytic
performances of supported metals because they could affect the adsorption and desorption
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behaviors of reactants and products. It is reported that the aromatic rings in which π
electrons act as a Lewis base due to the enriched electron densities favor the adsorption and
activation on acidic support than basic support, even for acidic phenol. Therefore, acidic
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supports could easily achieve the hydrogenation of the aromatic rings.
The basic
supports could weaken the hydrogenation of aromatic rings through hydrogenation route
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(HYD) due to decreased interaction of aromatic rings and support. Tanabe et al. studied the
adsorption of phenol on acidic SiO ꢀAl and basic MgO, in which the aromatic ring
2
2 3
O
adsorbed parallelly and perpendicularly on the catalyst surface, respectively, due to basic
πꢀelectrons. The vertical adsorption favored the direct deoxygenation route (DDO) for
hydrogenolysis and the coplanar adsorption promoted the HYD route for hydrogenation.
3
5
Moreover, Shin et al. reported that the nonplanar adsorption could promote hydrogenolysis
of phenol due to the difficulty of interaction between the aromatic ring and active surface.
Additionally, the basic supports have been widely used in catalysis for hydrogenation
36ꢀ39
reaction and biomass pyrolysis,
which has unexpected high selectivity and antiꢀcoking in
40ꢀ43
many reactions.
Hence, it could be hypothesized that the basic MgO could be a desired
support for hydrogenolysis of DBF.
In an attempt to develop an environmentally benign and economically feasible route of
dibenzofuran hydrogenolysis, a combination catalyst system composed of various supports
2 3 2 2 3
(Al O , SiO , MgO/Al O and MgO) loaded with Pt was explored for their catalytic activity
and selectivity in DBF hydrogenolysis. Interestingly, the Pt/MgO catalyst exhibited a great
catalytic activity and selectivity to OPP. The different catalytic behaviors observed over
various supports were interpreted in terms of acidꢀbase properties. The hydrogenolysis
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ability and high OPP selectivity of Pt/MgO catalyst were attributed to the high activity of Pt
and the basic character of the MgO support.
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Scheme 1 Reaction pathways of hydrodeoxygenation of DBF
EXPERIMENTAL SECTION
Materials
2 3 2 3 2 2 2 6 2
Commercial available Al O , SiO , Mg(CH COO) , Mg(OH) and H PtCl ꢁ6H O were
purchased from Sinopharm. Dibenzofuran and oꢀphenylphenol were obtained from Aladdin
with 98.0 % purity.
Preparation of supports and catalysts
MgO was obtained by calcining Mg(OH)
2
at 500 °C or 900 °C for 4h, and the obtained
supports were denoted as MgO and MgOꢀ900. Other metal oxides Al O and SiO were
2
3
2
9
calcined in O
Pt/MgO, Pt/Al
PtCl ꢁ6H O as precursor and methanol as the solvent. Additionally, Pt/MgO/Al
prepared by soaking Pt/Al into methanol solution of Mg (CH COO) , followed by
calcination in O /Ar (20 mL/40 mL) at 400 °C for 4 h. The weight percent of Mg in
2
/Ar (20 mL/40 mL) at 400 °C for 2h before use.
and Pt/SiO were prepared by impregnation method using
was
2
O
3
2
H
2
6
2
2 3
O
2
O
3
3
2
2
Pt/MgO/Al
2
O
3
is 3.5 wt. %. All of the above catalysts were heated from room temperature to
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ꢀ
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ꢀ1
4
00 °C at a rate of 5 °C min and reduced for 2 h under 40 mL min H before further
2
catalytic test. The actual Pt contents in asꢀprepared Pt catalysts are close to nominal 0.50 wt. %
Pt loading.
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Characterization of supports and catalysts
Xꢀray diffraction (XRD) analysis of the supports was performed on a Rigaku
D/MAXꢀRB instrument using a Cu Kα monochromatized radiation source in the 2θ range of
o
o
o
ꢀ1
5
ꢀ90 with a scan speed of 10 min , operated at 40 kV and 100 mA. The specific surface
area of supports was calculated by the BrunauerꢀEmmettꢀTeller (BET) method and pore
volume was calculated by the volume of liquid nitrogen at p/p =0.99 based on nitrogen gas
0
physisorption measurements over Quantachrome Autosorb IQ. The acidity and basicity
features of the supports were evaluated by temperatureꢀprogrammed desorption (TPD) using
3 2
NH and CO as the probe molecule, respectively. TPD was conducted on CHEMBETꢀ3000
and OBPꢀ1 chemisorption instrument with a thermal conductivity detector (TCD),
respectively. A 0.10 g support was loaded into the reactor and was performed with a heating
ꢀ1
ramp rate of 10 min in a stream of Ar from room temperature to 500 °C holding 1 h with a
ꢀ1
total flow rate of 40 mL min , then saturated at 30 °C in a 10 % NH /He or CO stream for 1
3
2
h to ensure the maximum adsorption. The data were recorded in He from room temperature
to 500 °C. Elemental analysis for Pt was recorded using inductively coupled plasma atomic
emission spectroscopy (ICPꢀAES) over (PerkinꢀElmer Optima 2000 DV). Pt dispersion was
determined according to CO chemisorption measurements using CO pulse adsorption on
CHEMBETꢀ3000. CO adsorption was considered to be completed after three successive
peaks showed the same peak areas. A stoichiometry of CO/metal =1 was taken to calculate
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the number of metal active sites. The particle size of catalysts was analysed using a FEI
Tecnai G2 F30 TEM.
Catalytic tests
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Typically, hydrogenolysis of DBF or OPP was performed at 400 °C and 1.0 MPa total
pressure in a continuousꢀflow fixedꢀbed reactor (considered as plug flow reactor) over 80 mg
catalyst diluted with 5.0 mL 60ꢀ80 mesh quartz sands. Prior to catalytic tests, the asꢀprepared
ꢀ1
catalysts were pretreated inꢀsitu under 40 mL min H
2
at 1.0 MPa and 400 °C for 1 h. Then,
the temperature and the pressure were adjusted to reaction conditions. Next, dibenzofuran
was delivered into the reactor and mixed with H gas at the inlet of the reactor. And the
mixture of reactant and H gas moves downward and reacts in the middle of the reactor. The
2
2
liquid reactants were consisted of 2.0 wt. % DBF or OPP, 1.0 wt. % nꢀdodecane (as internal
standard) and 97.0 wt. % nꢀdecane (as solvent).
The reaction products being condensed in a cold trap were collected and analyzed
offꢀline by an Agilent gas chromatograph 7890A equipped with flame ionization detector and
a 0.5 ꢂm×0.32 mm×30 m FFAP capillary column. Product identifications were conducted on
an Agilent 7890B with 5977A MSD and a 0.25 ꢂm×0.25 mm×30 m HPꢀ5 capillary column.
Conversion (X) and selectivity (S) are defined as follows:
ꢀ
= (ꢁ − ꢁ ꢆ)/ꢁ ꢆ× 100%
(1)
(2)
is the mole
ꢂ
ꢃꢄꢅ
ꢂ
ꢇ = ꢁ /∑ꢁ × 100%ꢆꢆꢆꢆ
ꢈ
ꢈ
Where n
0
and nDBF are the moles of DBF in the feed and product, respectively, n
i
of i product molecule and ∑n are the total moles of products.The carbon balance of products
i
is 100 % ± 5 %. Turnover frequency values (TOF) are calculated from the formula:
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ꢉꢊꢋ = (F/M′ꢆ) × X/(M × W)(F/M′ꢆ)
τ = W/Fꢆꢆꢆꢆꢆꢆ
(3)
(4)
Where M and M' are the moles of active sites and molecular weight of reactant, respectively,
τ represents the contact time, W denotes the catalyst weight and F denotes the total weight
flow rate of the reactant.
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RESULTS AND DISCUSSION
Characterization of supports and catalysts
The obtained Pt content, BET specific surface area, pore volume and pore diameter of
the samples are summarized in Table S1. The actual Pt contents of asꢀprepared Pt catalysts
2
ꢀ1
are close to nominal 0.50 wt. % Pt loading. MgO has a specific surface area of 81 m g and
3
a pore volume of 0.44 cm /g. After calcination at 900 °C, the obtained MgO sample shows
some reduction in specific surface area and pore volume, which is resulted from the collapse
of the partial framework. SiO
2
, MgO/Al
85, 235, and 308 m g , respectively. It is clearly that the surface area and pore volume of
Al decrease sharply after the deposition of MgO.
The power XRD patterns of supports are shown in Figure S1a. As can be seen,
calcination of Mg(OH) at 500 °C and 900 °C produce the different intensity of peaks (2θ =
2 3 2 3
O , and Al O have higher specific surface areas of
2
ꢀ1
2
2
O
3
2
o
44
4
3, 62, 78, 37, 75 ). It is probably due to the fact that calcining Mg(OH)
temperature should give the periclase magnesium oxide with enhanced crystallinity. No
obvious diffraction peaks were observed clearly for the patterns of SiO and Al supports,
confirming that they have amorphous structures. Same situation occurred in MgO/Al O ,
2
at higher
2
2 3
O
2
3
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which may be resulted from either low loading or high dispersion of MgO. When adding Pt
to the supports, no diffraction peaks of Pt were discernable after reduction as shown in Fig.
S1b, indicating that the reduced Pt metal was highly dispersed and the particle size was too
small to detect by XRD.
1
1
1
1
1
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1
1
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3
4
5
6
7
8
9
0
2 3
Figure 1. TPD profiles of (a) CO and (b) NH adsorbed on supports
The different product selectivity may be related to the nature of the support. Therefore,
the acidic and basic properties of the supports were measured by temperatureꢀprogrammed
3 2
desorption of ammonia (NH ꢀTPD) and carbon dioxide (CO ꢀTPD) as shown in Figure 1 and
the amount of acid and base were shown in Figure S1. Regarding to basic properties of
supports, as shown in Figure 1a, one broad peak is displayed at 280 °C over MgO, which can
be attributed to moderately basic sites. At 120 °C, the small peak suggests weakly basic sites.
Between them, the weak peak is covered up at 230 °C. These results are in close agreement
41
with previously reported literature value. There is slightly higher basic strength for MgO
44
with calcined temperature increasing to 900 °C. Additionally, no peak was observed for
Al and SiO supports. However, Al support, modified by a small amount of MgO,
exhibited strong peak at 120 °C, which can be ascribed to weakly basic sites. Apart from the
2
O
3
2
2 3
O
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basicity of the support, the acidity was determined by NH ꢀTPD. As shown in Figure 1b,
3
there is a broad desorption peak detected for the Al
peaks over MgO/Al support decreased, which is due to the fact that acidic sites was
covered with MgO. Generally, as for the MgO and SiO supports, there are hardly any acidic
2 3
O support. Apparently, the desorption
2
O
3
0
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2
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0
2
sites. When adding Pt to the support, the acidꢀbase properties of catalysts have little change
compared with the pure supports. The detailed results were shown in Figure S2. With regard
to the TPD analysis, the combination of the acidity and basicity strength showed that basicity
2 2 3 2 3
of the supports decreases in the the following order: MgO > SiO > MgO/Al O > Al O .
Figure S3 shows the TEM images together with particle size distributions of Pt catalysts
of three representative acidꢀbase supports. As displayed from the TEM images, Pt
nanoparticles are well dispersed with no apparent aggregation. The mean diameters of the Pt
nanoparticles in the Pt/SiO
respectively. Pt/Al having smallest particle size suggests a high metal dispersion, which
results in its better activity. Correspondingly, the Pt dispersions in the Pt/SiO , Pt/MgO and
Pt/Al catalysts are 34 %, 70 % and 75 %, respectively. Considering that all the catalysts
were prepared by the same method, the variation in Pt particle size could be ascribed to the
2 2 3
, Pt/MgO and Pt/Al O catalysts are 3.3, 1.6 and 1.5 nm,
2
O
3
2
2 3
O
45
varied degree of metalꢀsupport interactions. The nature of the surface hydroxyl groups of
46
most oxides affect the adsorption of anion during synthesis.
Hydrogenolysis of DBF over Pt/MgO catalyst
The hydrogenolysis products, such as OPP, biphenyl (BP), cyclohexylbenzene (CHB),
cyclopentylmethylbenzene (CPMB), cyclohexane (CH) and benzene (B) were determined by
the GCꢀMS, as shown in Figure 2. According to the results
，the conversion of DBF follows
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the direct deoxygenation route (DDO) under the current conditions, with no product from the
9
hydrogenation route (HYD), as depicted in the previous proposed reaction network.
10
Furimsky et al. suggested that the hydrogenation of DBF could follow several pathways
depending on the reaction temperature. Among them, at high temperatures the reaction is
more inclined to follow the direct deoxygenation route under the same pressure condition.
Figure 2a presents the relative concentration versus the contact time. With contact time
increasing, the relative concentration of OPP increased followed by the decrease at contact
time of 0.84 min. However, the relative concentration of BP steadily increases. It can be
inferred that BP is the product of intermediate species OPP by the cleavage of CꢀO bond.
Figure 2b shows a plot of products selectivity versus DBF conversion. Surprisingly, OPP is
the predominant product with a selectivity of more than 80 % at low conversions (< 12 %)
over Pt/MgO. The decrease in OPP selectivity, accompanied by a simultaneous increase in
BP selectivity with increasing conversion, seems that BP is formed at the expense of OPP
decreases. Apparently, OPP, which is known to be an intermediate product of hydrogenolysis
of DBF, was quite easy to be converted to BP through hydrogenolysis by the cleavage of
CꢀO bond. On the other hand, the hydrogenation products, such as CHB and CPMB,
gradually increased with increasing DBF conversion. Traces of CH were detected at high
conversions, which is probably due to the fact that hydrogenation occurred at higher
conversions. By adjusting the Pt loadings, good results were obtained with 48 % OPP yield
over 1.0 wt. % Pt/MgO (Figure S4).
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Figure 2. Relative concentration (a) and selectivities (b) of the hydrogenolysis of DBF over
0
.5 wt. % Pt/MgO as a function of contact time and conversion, respectively. Reaction
o
2
condition: 400 C, 1.0 MPa, H /oil = 400.
Effect of reaction temperature
To evaluate the influence of the temperature on hydrogenolysis of DBF, the model
reaction was carried out at different reaction temperatures over Pt/MgO, Pt/SiO and
2
2 3
Pt/Al O at 1.0 MPa. Variations in reaction temperature had a considerable effect on
selectivity and conversion, as illustrated in Figure 3a and Figure S5. Over Pt/MgO, with
temperature increasing, the selectivity to OPP gradually increases and becomes stabilized at
400℃. The conversion of DBF gradually decreases with increasing temperature. The data
obtained is in conflict with the results obtained over Pt/Al O (Figure S5b) and previous
2
3
9
report wherein the conversion and temperature increase at the same time. It may be
attributed to a temperatureꢀinduced decrease of the fraction of the catalyst surface which was
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covered by reactants at high temperature
and the dehydrogenation of formed OPP back
to the reactant DBF (discussed in below). And the temperature induced acceleration of
catalyst coking is unlikely for MgO catalysts due to its antiꢀcoking nature, which is
evidenced from the TEM characterization (Figure S8).
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0
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2
3
4
5
6
7
8
9
0
Figure 3. Effect of the reaction temperature at 1.0 MPa (a) and the reaction pressure at 400
C (b) over 0.5 wt. % Pt/MgO. Reaction condition: τ= 0.55 min, H
o
2
/oil = 400.
The hydrogenation product, tetrahydrodibenzofuran
(THDBF),
dodecahydrodibenzofuran (PHDBF) and bicyclohexane (BCH), were observed (Scheme 1)
and the amount gradually increased over both catalysts when the temperature was decreased
to 380 °C. The major hydrogenation reaction (HYD) occurred at a lower temperature in our
9
10
previous work. The observation is consistent with the previous result
that the
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hydrogenation or hydrogenolysis reaction of DBF depends on the reaction temperature, in
which the hydrogenolysis of CꢀO bonds occurs at the high temperature and low temperature
is more inclined to hydrogenation of aromatic rings. The hydrogenolysis of DBF need higher
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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9
0
1
2
3
4
5
6
7
8
9
0
2
temperatures than hydrogenation because the cleavage of oxygenꢀsp carbon bonds is more
3
8
difficult than oxygenꢀsp carbon bonds. Hence, catalytic hydrogenolysis of DBF requires
1
0, 35, 45
high temperature due to the strong CꢀO bond.
Moreover, the catalytic performance
than
exhibited markedly higher selectivities to hydrogenation of aromatic ring over Pt/Al
2
O
3
Pt/MgO (Figure 3a, Figure S5b and Table 1), which is well agreed with the results we
mentioned in the introduction. At the same temperature of 380 °C, the selectivities to
hydrogenation of aromatic ring decreased in the order: Pt/Al
2 3 2
O > Pt/SiO > Pt/MgO.
Without dehydrogenation, the conversion increases with elevated temperature over Pt/Al O ,
2
3
which is maybe that the relationship between reaction conversion and temperature is closely
related to the dehydrogenation of OPP over Pt/MgO.
Effect of reaction pressure
Except for the reaction temperature, the pressure is another key parameter for the
hydrogenolysis of DBF. Figure 3b shows the conversion and selectivity of the
hydrogenolysis reaction under different pressures at 400 °C. At the lower pressure (0.5 MPa),
the conversion of DBF was just 12 %, but all the products were from hydrogenolysis but not
from the hydrogenation of aromatic ring and the OPP achieved the highest selectivity of
74 %. Further raising the pressures to 2.0 MPa resulted in an increase in reaction conversion
while the hydrogenation of aromatic ring starts to occur, demonstrating that hydrogenation
competed with hydrogenolysis in designated condition. However, the selectivity to OPP
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gradually decreased, accompanied by the increased selectivity to BP. A higher pressure
4
7
promotes product removal by hydrogen addition and hydrogenꢀaided desorption. With the
pressure going up, the increased surface hydrogen coverage leads to the formation of
1
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0
47
hydrogenation products and an increased CꢀO bond hydrogenolysis rate. Hence, the
generation of BP was much easier by hydrogenolysis or dehydration of OPP at higher
pressure. It is confirmed that decreasing pressure favored the hydrogenolysis of DBF, and
48
suppressed or avoided excessive hydrogenation of aromatic ring.
Effect of supports
The selectivities of the different catalysts for the hydrogenolysis of DBF are shown in
Table 1. Because there were essential variations in activity from one catalyst to another, the
catalyst tests were undergone to allow comparisons of selectivities at fixed conversions of
about 30 % of DBF. The selectivity to OPP varied considerably with different catalysts. The
2
presence of MgO and SiO supported Pt, which is known to have less prominent acidic
character than Al , helps on the generation of OPP at higher selectivity. The increase in the
2
O
3
selectivity to OPP can be due to the fact that the dehydration reaction of OPP is more
difficult in basic conditions. While for the acidic Al O , OPP was not observed from the
2
3
reaction products. The reaction for DBF conversion is rather dominated by the excessive
hydrogenolysis of DBF to BP and B, which is followed by a small extent of partial
hydrogenation of BP to CHB and CH (Table 1). Apparently, the intermediate OPP was
rapidly hydrogenolysed to BP. It is suggested that Pt/Al
2 3
O catalyst favored the formation of
the deoxygenation products (BP and B). As we all known that the acidic support is function
8
49
for hydrogenolysis and dehydration, and is more favored to deoxygenation. The basic
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MgO, which had the highest OPP selectivity in all using catalysts, significantly enhanced the
formation of OPP.
Table 1. TOF and selectivity for DBF hydrogenolysis for various catalysts with at 30 % conversion
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
All reaction were run at 400 °C and 1 MPa)
Selectivity (%)
ꢀ1
Catalysts
TOF (h )
OPP
67
56
0
BP
17
31
57
37
16
B
12
5
CHB
CPMB
CH
0
Pt/MgO
Pt/SiO2
1363
722
2
6
6
8
0
2
2
1
3
0
0
Pt/Al
Pt/MgO/Al
Pt/MgOꢀ900
2
O
3
1022
699
35
25
11
1
2
O
3
17
73
10
0
1123
By comparison, the TOF analysis of catalysts indicated that the activity over Pt/MgO
was higher than that over Pt/SiO due to the high Pt dispersion based on TEM results.
Moreover, Pt/Al O conversion was efficient, indicating a high reaction activity via
2
2
3
hydrogenolysis of CꢀO bonds, which can be possibly explained by the strong acid that
accelerates the CꢀO bonds cleavage and high Pt dispersion.
Table 2. TOF and selectivity for OPP hydrogenolysis over various catalysts at 30 % conversion. (All
reaction were run at 400 °C and 1.0 MPa)
Selectivity (%)
ꢀ1
Catalysts
TOF (h )
DBF
18
20
11
0
BP
47
49
49
55
59
B
18
7
CHB
6
CPMB
CHEB
CH
1
Pt/MgO
2406
1951
1123
2117
1747
4
5
5
1
4
6
8
6
0
4
Pt/MgOꢀ900
10
11
1
Pt/SiO
Pt/Al
Pt/MgO/Al
2
17
27
11
1
O
3
16
17
1
2
O
3
3
2
2
In order to clearly analyze the respective hydrogenolysis and hydrogenation of DBF, the
selectivities to CꢀO bond hydrogenolysis products (the total of OPP, BP and B) and phenyl
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ring hydrogenation products (the total of CHB, CPMB and CH) were as depicted in Table 1.
There was no hydrogenation of aromatic rings over Pt/MgOꢀ900. However, other catalysts
had approximately 4 ~ 21 % of the hydrogenation of aromatic rings. From the perspective of
hydrogen consumption, Pt/MgO catalysts also were superior to other catalysts.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
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0
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2
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0
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0
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0
1
2
3
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
To gain preliminary insight into the effective basic sites for the production of OPP, the
hydrogenolysis of DBF was carried out using Pt/MgO/Al O and Pt/MgOꢀ900. The Pt/Al O
3
2
3
2
was modified by MgO to generate the basic sites, as can be observed in Figure 1a. As was
expected, the selectivity to OPP was raised to 17 % (Table 1). Apparently, the production of
OPP can be explained by the presence of basic sites. The basic strength of MgO was changed
to verify the effect of basic sites on the production of OPP by changing the calcination
temperature of MgO. The higher calcination temperature would generate further stronger
5
0
basic sites (Figure 1a), due to exposure of the more surface defects. Similarly, the
selectivity to OPP on Pt/MgOꢀ900 catalyst was 6 % higher than that for Pt/MgO (Table 1).
As a consequence, the amount and strength of basic sites were crucial for the formation of
OPP.
To explore the differences in reactivity and selectivity of various catalysts on the
process of hydrogenolysis of DBF, the performance of the Pt catalysts on different supports
in hydrogenolysis of the target product OPP as substrate was carried out in the same
conditions (1 MPa, 400 °C). The results were shown in Table 2. To our surprise, varying
5
1
degrees of the dehydrogenation for OPP to DBF took place as literature reported, in which
it was claimed that the hydrogenolysis of DBF to OPP is reversible and it is impossible for
the conversion to completed as restricted by chemical equilibrium and thermodynamics.
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Phenylcyclohenene (CHEB) was observed in the hydrogenolysis of OPP over all catalysts
except Pt/Al
2 3
O catalyst, in which it was suggested that the catalysts with low acidic
supports are at least required. Compared with increased selectivities to BP over Pt/MgO,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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0
1
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3
4
5
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8
9
0
Pt/SiO
2
and Pt/Al
2
O
3
, it may be suggested that basic catalysts have lower cleavage ability of
39
CꢀOH bonds. Over MgO and SiO
2
supported Pt catalysts, the dehydration of OPP was
companied by a marked dehydrogenation generating DBF. And the dehydrogenation ability
over Pt/MgO was stronger than Pt/SiO . Compared with Pt/Al , traces of DBF were found
over Pt/MgO/Al . With the dehydrogenation phenomenon in hydrogenolysis reaction, the
TOF of the hydrogenolysis of OPP over Pt/MgO was higher than over Pt/Al . The low
activities of the hydrogenolysis of DBF, as revealed in Table 2, is associated with an
2
2 3
O
2
O
3
2
O
3
51
inhibition of the dehydrogenation of OPP and the level of dehydrogenation of OPP.
Specifically, the ability of dehydrogenation depends on the catalyst acidꢀbase properties.
41
This is in agreement with the findings by Diez et al., who proposed that 2ꢀpropanol was
converted to propylene or acetone via dehydrogenation or dehydration depending on the
2+
2ꢀ
acidꢀbase surface properties of catalysts. The Mg ꢀO pairs promoted dehydrogenation and
3+
2ꢀ
Al ꢀO active sites facilitated dehydration. It is generally accepted that over acidic catalysts
alcohols undergo dehydration, and over basic catalysts alcohols could occur
52ꢀ54
dehydrogenation and dehydration.
The more active the dehydrogenation, the stronger the
base was of the catalysts (Table 2). Hence, we obtained clear evidence that dehydrogenation
of OPP to DBF occur in basic sites.
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9
0
Figure 4. Effect of the reaction temperature over Pt/MgO on conversion of OPP and products selectivities.
2
Reaction condition: 1.0 MPa, τ = 0.55 min, H /oil = 400.
To gain further insight of the relationship between temperature and conversion of DBF
during dehydrogenation of OPP, we conducted the experiment of the hydrogenolysis of OPP
under different temperatures as illustrated in Figure 4. It was observed that an increase in the
dehydrogenation product DBF with increasing temperature was reflected by the
35
temperatureꢀinduced accelerated dehydrogenation of OPP in basic sites. That is to say, the
dehydrogenation constrained the conversion of DBF. And in these conditions, at 400℃ or
higher temperature, the selectivity of OPP or DBF and the conversion of DBF or OPP were
stable, indicating that the reaction between DBF and OPP approached equilibrium.
Reaction pathway and kinetics of DBF hydrogenolysis
According to our results, the overall reaction pathway of the hydrogenolysis of DBF
over Pt/MgO was suggested and represented in Scheme 2. The hydrogenolysis reaction
55
occurs through σ adsorption of the DBF molecular via the oxygen atom. The orientation of
the aromatic rings of DBF and OPP, which is perpendicular to the support surface of MgO,
decreased the hydrogenation of aromatic ring. Regarding the hydrogen activation during the
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1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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hydrotreating reaction, the hydrogen atom is usually formed by heterolytic dissociation of
56
molecular hydrogen on the metal. where it can then be transferred to take part in the
5
7
reactions by providing spillover hydrogen to MgO. First, the cleavage of the only one CꢀO
bond takes place to give rise to OPP, followed by another CꢀO bond breakage to BP.
Subsequently, the first route is via hydrogenolysis to B, followed by sequential minor
hydrogenation to CH. The second one involves partial hydrogenation to active CHEB, then
further hydrogenation to CHB. Isomerized product CPMB occurred at the same time,
followed by small amounts of CHB hydrogenolysis to B and CH. The hydrogenolysis is the
dominant reaction for CꢀO cleavage, affording OPP and BP as the primary products.
No matter how the pressure was changed over Pt/MgO at 400 °C (Figure 3b), BCH was
not found. However, when the temperature was decreased until the appearance of THDBF
and PHDBF, BCH was observed. This suggested that BCH did not come from the
hydrogenation of CHB but was derived from the CꢀO bond breakage of PHDBF. The
hydrogenation of CHB to BCH is impossible since the hydrogenation of the second phenyl
0
1
2
3
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0
55
ring of BP is supposed to be much more difficult than that of the first ring.
Scheme 2 Proposed reaction pathways of DBF hydrogenolysis over Pt/MgO in reaction conditions
To understand the effect of catalysts on the hydrogenolysis of DBF, the kinetic analysis
was studied based on the reaction results. First of all, simplified reaction process was shown
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in Scheme S1. Then a pseudoꢀfirstꢀorder model for the conversion of DBF and OPP was
developed by the following equation.
−
ln ꢆ( ꢌ/ꢌ ) = k × τ
(5)
ꢂ
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Where C is the concentration of DBF at contact time τ and C
0
is the initial concentration.
The kinetic analysis was conducted with varied conversions of DBF and OPP versus the
contact time. The conversion of DBF or OPP was well agreed with pseudoꢀfirstꢀorder
2
kinetics. The plots of ln(C/C
0
) versus contact time were linear with high r values (0.97ꢀ1.00)
except for 0.95 over Pt/SiO
2
in DBF conversion as shown in Figure 5. To prove the
pseudoꢀfirstꢀorder conversion of DBF and OPP, we performed experiments on the
conversion of DBF and OPP at two concentration of the reactant (1 % and 2 %) over Pt/MgO
catalyst. It was found that the conversion of DBF and OPP did not depend on their initial
concentration, indicating that the hydrogenolysis of DBF and OPP followed firstꢀorder
behavior over Pt/MgO.
On account of the low reaction yield of the hydrogenolysis of OPP and DBF, the
reversible reaction was neglected. Therefore, Figure 5a gives the pseudo firstꢀorder rate
constants of the total conversion of OPP (k +k ) over the Pt catalysts. Figure 5b presents the
ꢀ1
2
pseudo firstꢀorder rate constants of the conversion of DBF (k
1
) over the Pt catalysts. From
the rate constants of OPP hydrogenolysis and their initial product selectivities, we calculated
58
the rate constants of dehydrogenation and hydrogenolysis of OPP (kꢀ1 and k
The specific values are listed in Table 3. The k gradually decreases following the order:
Pt/Al O > Pt/MgO > Pt/MgOꢀ900 > Pt/SiO > Pt/MgO/Al O . This may be due to the more
2
, respectively).
1
2
3
2
2
3
dispersed Pt nanoparticles with better activity in the conversion of DBF. The kꢀ1 decreases
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1
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1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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4
4
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5
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5
5
6
2 2 3 2 3
following the order: Pt/MgO ≈ Pt/MgOꢀ900 > Pt/SiO ≈ Pt/MgO/Al O > Pt/Al O , which
reveals the dehydrogenation ability of OPP. This phenomenon could be attributed to the
strength of the basic sites in the supports. The ratio of the rate constants of the DBF reaction
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
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9
0
1
2
3
4
5
6
7
8
9
0
1 2
and the OPP reaction k /(kꢀ1+k ) reflected the OPP formation rate, which showed that
stronger basic catalyst was associated with higher OPP formation rate. Considering that no
formation of OPP was found when the DBF hydrogenolysis reaction was carried out over
Pt/Al O , we neglect the value of k /(k +k ) over Pt/Al O . Apparently, the rate constant of
2
3
1
ꢀ1
2
2
3
OPP transformation to BP is more than twice as much as the rate constant of DBF
conversion over Pt/Al , probably suggesting that the Pt/Al catalyst is unsuitable for the
formation of OPP despite high activity. Over acidic Al O , the acid sites are beneficial to the
2
O
3
2 3
O
2
3
5
9
adsorption and activation of the intermediate phenols. From the perspective of reaction
activity and selectivity, Pt/MgO catalyst is a potential catalyst for the selective
hydrogenolysis of DBF to OPP.
Figure 5. The plot of ln(C/C ) versus contact time for the hydrogenolysis of OPP (a) and DBF (b) at
0
4
00 °C and 1.0 MPa over the Pt catalysts.
Table 3 Rate constants in the pseudoꢀfirstꢀorder kinetic analysis of the conversion of DBF and OPP over
the Pt catalysts
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ꢀ
1
Rate constant (min )
Catalysts
Pt/MgO
k
ꢀ1+k
2
k
1
k
ꢀ1
k
2
k
1
/(kꢀ1+k
0.77
0.88
0.76
0.45
0.35
2
)
1.51
1.24
0.69
2.99
1.36
1.16
1.09
0.53
1.34
0.48
0.33
0.27
0.08
0
1.18
0.97
0.61
2.99
1.31
Pt/MgOꢀ900
Pt/SiO2
2 3
Pt/Al O
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Pt/MgO/Al
2
O
3
0.05
The relationship between acidicꢀbasic supports and the formation of OPP or the
dehydrogenation of OPP was shown in Figure 6. Along with the increasing basicity of
supports, the OPP yield of DBF hydrogenolysis and the DBF yield of OPP dehydrogenation
was enhanced over various catalysts. This result gives additional evidence for the
assumption that the acidꢀbase surface properties of catalysts are an important factor for
selective hydrogenolysis of DBF to OPP, reflecting the fact that basic catalysts accelerate the
formation of OPP. It had already been discussed that the reaction of the alcohols, such as
2ꢀpropanol, is often used to characterize the acidicꢀbasic properties of the supported
60
catalysts. The DBF yields of OPP hydrogenolysis reflected the catalyst acidꢀbase properties,
which may actually be suitable for the acid and base characterization of catalysts.
The differences of acidꢀbase properties of metal oxide are due to the electronegativity
6
1
2 3
of the neutral metal and the metalꢀoxygen bond energy. Generally, the acidic sites on Al O
3
+
3+
come from the Al sites (as Lewis acid sites) and hydroxyl connected with Al (as Brønsted
2
ꢀ
acid sites), and the basic sites on MgO supports are assigned to the O sites (as Lewis basic
62, 63
ꢀ
64,65
sites)
and surface OH (as Brønsted basic sites).
On the basic MgO, the MgꢀO bond
of reactant adsorbed on the surface of support breaks to generate the phenoxide radical and
2 3
then dehydrogenation to DBF. However, on the acidic Al O , the group turned into an
66
unstable surface species and did not desorb to yield BP. It is reported that the Lewis acid
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sites are beneficial to the activation of the intermediate phenols via formation of
59
66
phenoxide. Shinohara et al. suggested that alcohol dehydrates via scission of the CꢀO
bond of a surface alkoxy group under interaction of its oxygen with some electrophile
existing on the surface, and 2ꢀpropanol is dehydrogenated via the extraction of αꢀhydrogen
of the isopropoxy group by metal on the oxide surface based on calculation of the activation
energies of the dehydration and the dehydrogenation of 2ꢀpropanol.
0
1
2
3
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 6. The OPP yield in DBF hydrogenolysis and the DBF yield in OPP hydrogenolysis over
2
supported Pt catalysts at 400 °C and 1.0 MPa, τ = 0.84 min, H /oil = 400.
Conclusions
Highly efficient Pt/MgO catalysts have been developed for selective hydrogenolysis of
DBF to OPP by cleavage the only one CꢀO bond with preservation of their aromatic rings.
The high selectivity of the catalysts for hydrogenolysis of DBF to OPP results from both
strong basic sites and suitable reaction conditions (400 °C and 1.0 MPa). The
dehydrogenation of OPP to DBF was found on basic catalysts and increasing reaction
temperature accelerates dehydrogenation of OPP. The basic sites promote the formation of
OPP due to the low cleavage ability of CꢀOH bonds or the dehydrogenation of OPP due to
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the nature of basic sites. High temperature and low pressure are in favor of the formation of
OPP. Hydrogenolysis kinetics of DBF are best represented by a standard pseudoꢀfirstꢀorder
approximation. An improved reaction pathway and mechanistic understanding are
established, which enables the rationalization of the hydrogenolysis performance of DBF.
The dehydrogenation in basic sites makes the hydrogenolysis of DBF to OPP a reversible
reaction where complete conversion is impossible due to restriction by chemical equilibrium.
1
1
1
1
1
1
1
1
1
1
2
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2
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0
ASSOCIATED CONTENT
Supporting information
The Supporting Information (SI) is available free of charge on the ACS Publications website
at DOI: xxx. See SI for supplementary Tables and Figures.
AUTHOR INFORMATION
Corresponding Author
*
Eꢀmail: changhai@dlut.edu.cn. Phone: + 86ꢀ411ꢀ84986353.
Funding
This work was supported by the National Key Research & Development Program of China
(2016YFB0600305).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors acknowledge Lab of Advanced Materials＆Catalytic Engineering for providing
the research facilities. The authors thank Instrumental Analysis＆Research Center for their
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analysis assistance.
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