Free-radical conversion of a lignin model compound catalyzed by Pd/C

Article abstract of DOI:10.1039/c5gc01272d

Efficient cleavage of a C-O bond in lignin and its model compounds is of great importance for transformation of lignin into fuel and value-added chemicals. In this work, we explored the transformation of a lignin model compound benzyl phenyl ether (BPE) at 150°C by using Pd/C as the catalyst under an argon atmosphere in the presence of Na₂CO₃ and N-methyl-2-pyrrolidone (NMP). The effects of different reaction parameters such as the amounts of Pd/C, Na₂CO₃ and NMP as well as reaction times were investigated. It was found that Pd/C played a key role in the conversion of BPE. At the same time, Na₂CO₃ and NMP also promote the transformation effectively. The yields of phenol and toluene reached 71.6% and 50.4%, respectively. Analytical results of electron paramagnetic resonance (EPR) indicated that the reaction proceeded through a free-radical reaction mechanism. Control experiments indicated that direct pyrolysis of BPE was the main route to generate the target products.

Full text of DOI:10.1039/c5gc01272d

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ARTICLE TYPE
Free-radical conversion of lignin model compound catalyzed by Pd/C
Honglei Fan*, Yingying Yang, Jinliang Song, Qinglei Meng, Tao Jiang, Guanying Yang, Buxing Han*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
Abstract Efficent cleavage of CꢀO bond in lignin and its model
compounds is of great importance for transfomation of lignin into
fuel and valueꢀadded chemicals. In this work, we explored the
transformation of lignin model compound benzyl phenyl
atmosphere with Pd/Cs H3.0ꢀxPW12O40 as the catalyst at the
x
o
10
o
temperature of 200 C. In high temperature water (250 C), BPE
was hydrolyzed and the selectivities of phenol and benzyl alcohol
11
5
5
6
6
0
5
0
5
were 76.1% and 83.9%, respectively. When hydrolytic cracking
o
o
of BPE was conducted under 175 C using In(OTf)3 as the
ether(BPE) at 150 C using Pd/C as the catalyst in argon
catalyst, the yields of phenol and benzyl alcohol were 60% and
1
1
2
0
5
0
atmosphere in the presence of Na CO and Nꢀmethylꢀ2ꢀ
2
3
1
2
5
0%, respectively. BPE was equally cleaved into phenol and
pyrrolidone(NMP). The effects of different reaction parameters
such as the amounts of Pd/C, Na CO and NMP as well as
o
benzyl alcohol in water at 250 C. Under the selected conditions,
the conversion increased linearly to 31% as a function of time up
to 110 min, and the yields of phenol and benzyl alcohol
2
3
reaction time were investigated. It was found that Pd/C played a
key role for the conversion of BPE. At the same time, Na CO3
2
13
synchronously increased to around 13%. It was reported that
and NMP also promote the transformation effectively. The yields
of phenol and toluene reached 71.6% and 50.4%, respectively.
Analytic results of electron paramagnetic resonance (EPR)
indicated that reaction proceeded through freeꢀradical reaction
mechinsm. Control experiments indicated that direct pyrolysis of
BPE was the main route to generate the target products.
with the assistance of microwave irradiation, BPE could be
transformed into phenol, benzyl alcohol, 2ꢀbenzylphenol, 4ꢀ
benzyphenol, benzyl ethyl ether, benzyl ether in acidic ionic
o
liquid under 180 C for 10 minutes. The yields of phenol and
14
benzyl alcohol were 35.3% and 4%, respectively. Thus,
exploration for the cleavage of phenyl benzyl ether under mild
conditions in inert atmosphere with high yield is highly desireble.
The nonꢀcatalyzed pyrolysis of BPE has been extensively
investigated in the past. Usually, BPE can be pyrolyzed by the
cleavage of CꢀO bond at T ≥ 275 °C to produce benzyl and
Keywords: depolymerization, benzyl phenyl ether, lignin, freeꢀ
radical
1.Introduction
15
phenoxy radicals and then hydrogen abstracted from BPE or
from an added hydrogen donor can combine these radicals and
Transformation of lignocellulosic biomass into high valueꢀadded
1,2
2
3
3
4
4
5
0
5
0
5
chemicals has attracted much attention in recent years. As a 70 produce toluene and phenol as dominant products. However,
main component of lignocellulosic biomass, lignin is considered
to be the unique renewable aromatic resource. Selective cleavage
of the chemical bonds is the major challenge for converting lignin
into liquid fuels or valueꢀadded chemicals. Currently,
recombination of the incipient radicals through coupling at
phenoxy ring carbons has also been detected. It was reported
3
16
that Pd/Al
O can adsorb Hꢁ free radical and promote the
2 3
formation of alkyl and aromatic free radical intermediates as
depolymerization reactions of lignin can be mainly divided into 75 detected by EPR on the catalyst surface. Thus, it is possible to
4
5
6
cracking, hydrolysis reactions, catalytic reduction reactions,
carry out the pyrolysis of BPE catalyzed by metal catalysts under
mild conditions.
Herein, we conducted the decomposition of BPE using
7
and catalytic oxidation reactions. In the oxidation reactions, both
preventing deep oxidation and enhancing the yield of target
products are crucial. During reductive depolymerization, extra
commercial Pd/C as the catalyst with Na CO and NMP in argon
2 3
hydrogen resource is necessary and keeping the aromatic 80 atmosphere. It was demonstrated that BPE could be transformed
structure is difficult. Thus, hydrolysis or cracking reaction of
lignin in inert atmosphere are desirable methods to obtain
aromatic compounds.
into phenol and toluene with little amount of bezene. Under the
o
temperature of 150 C with reaction time of 3 hours, the yields of
phenol and toluene reached 71.6% and 50.4%, respectively.
Further investigations showed that Pd/C catalyzed the freeꢀradical
Due to the structural complexity and variability, model
compounds with different linkages like βꢀOꢀ4, αꢀOꢀ4, and 4ꢀOꢀ5 85 reaction, especially in the presence of Na
CO and NMP.
3
2
are selected to disclose selective cleavage of the chemical bonds
8
in lignin valorization. Among various lignin model compounds,
benzyl phenyl ether (BPE) has been widely employed for
2. Experimental section
2
.1 Chemicals
9
studying the transformation of the αꢀOꢀ4 bond in lignin. It was
reported that BPE could be transformed into phenol, bezene and
BPE, Pd/C(5 wt% Pd), 2,2,6,6ꢀTetramethylpiperidineꢀ1ꢀoxyl
toluene with the yields of 45.8%, 6.6% and 2.4% under nitrogen 90 (TEMPO),
Nꢀmethylꢀ2ꢀpyrrolidone(NMP),
Nꢀtertꢀbutylꢀaꢀ
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phenylnitrone (PBN) and Na CO were purchased from Alfa 55 quartz flat cell. The microwave frequency was measured with an
2
3
Aesar. Dichloromethane, PdCl , active carbon, NaOH, NaHCO ,
EIP Model 575 microwave counter (EIP Microwave, Inc., San
2
3
pyrrole, cyclopentanone, cyclohexylamine and biphenyl were
provided by Beijing Chemical Reagents Company.
Hydroxyapatite (HAP) was obtained from Aldrich and used as
Jose, CA). Spectra were recorded at ambient temperature (25℃).
5
3. Results and discussion
received. Argon and H were provided from Beijing Analytical
2
3
.1 Catalyst characterization
Instrument Factory and their purity was >99.99%.
60
2
.2 Preparation of Pd/HAP catalysts
We choose Pd/HAP as the comparative catalyst in order to detect
whether the carbon from Pd/C reacted with water to produce the
hydrogen. The metal Pd catalyst supported on HAP was prepared
by the ion exchange method. In a typical procedure, 0.5 g HAP
was dispersed in 150 mL double deionized water under vigorous
a
10
15
20
1
7
Pd/C
stirring for 2 h. Then a certain amount of PdCl aqueous solution
2
was added into the HAP suspension. After being stirred for 2 h at
room temperature, the slurry was heated and refluxed at 100 °C
for 12 h. The obtained solid was collected by filtration and
6
5ꢀ2867ꢀPdꢀPalladium
−
washed with deionized water until the Cl was not detectable. The
asꢀprepared Pd/HAP catalyst was obtained after dryed under
5
0ꢀ0927ꢀCꢀCarbon
vacuum at 60 °C overnight and reduced in pure H (99.99%, 60
2
3
−1
cm ꢁmin ) at 300 °C for 1 h.
.3 Catalyst characterization
The Xꢀray diffraction (XRD) patterns were collected on a D/Max
500 V/PC Xꢀray diffractometer with high intensity Cu Kα (40
1
0
20 30 40 50 60 70 80
2
b
2
25
kV, 200 mA) radiation. The highꢀresolution transmission electron
microscopy (HRTEM) observation was carried out on JEM 2011
at an accelerating voltage of 200 kV.
Pd/HAP
2
.4 Catalytic measurements
The reactions were conducted in a 8 mL reactor made of 316
stainless steel. In a typical experiment, desired amount of BPE,
Pd/C, Na CO , NMP and water were loaded into the reactor. The
30
35
40
6
5ꢀ6174ꢀPdꢀPalladium
2
3
reactor was placed in the preꢀheated furnace of desired
temperature and the air was removed under vaccum, then argon
of ambient pressure was charged into the reactor and the stirrer
was started. After a suitable reaction time, the reactor was cooled
in ice water. The products were extracted with dichloromethane
and analyzed by gas chromatograph (GC) using a HP 4890 GC
equipped with a flame ionization detector (FID), and biphenyl
was used as the internal standard. Identification of the products
was carried out using a GCꢀMS (SHIMADZUꢀQP2010) and by
comparing the retention times to respective standards in the GC
traces. The yields are calculated as the moles of products formed
divided by the moles of the reactant initially loaded into the
reactor.
09ꢀ0432ꢀHydroxylapatite
10 20 30 40 50 60 70 80
θ/degree
2
Figure 1. XRD patterns of the Pd/C(a) and Pd/HAP(b)
65
45
2.5 Procedures of spin trap in our reaction system
In the experiment, desired amounts of Pd/C, Na CO , NMP,
2
3
lignin model compound, water and PBN were added in the
reactor. The air of the reactor was removed under vacuum and the
reactor was placed in the preheated furnace of the desired
temperature, then argon was charged into the reactor to 0.1 MPa
and the stirrer of the reactor was started. After 60 min, the reactor
was opened, a suitable amount of reaction solution was taken out,
and the EPR spectrum was recorded using a Bruker ER 300 EPR
spectrometer operating at Xꢀband with a TM 110 cavity using a
50
Figure 2. TEM images of the Pd/C(a), Pd/HAP(b) catalysts and the insert
of Fig. 2a is HRTEM image of a typical Pd particle in Pd/C (c)
70
In the decompositon reaction of BPE, Pd/C were used as the
catalyst. Meanwhile, We choose Pd/HAP as the comparative
2
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catalyst in order to detect whether the carbon from Pd/C reacted
with water to produce the hydrogen. Both Pd/C and Pd/HAP were
characterized by XRD and TEM to get the information for the
composition and morphology of catalysts.
with water to produce the hydrogen, we also used the Pd/HAP as
the catalyst to conduct the transfprmation of BPE(entry 8).
Similarly, the converstion and the yields of the target products
35 were high, indicating that the metal Pd played the main role to
promote the reaction. Meantime, as active carbon was added into
the reaction system at the condition of entry 8, the conversion and
yields decreased and no other products were detected(entry 9),
which showed that active carbon could not react with water
5
It can be known from Fig. 1a that the XRD patterns of the
metal Pd particles and carbon are consistent with that in the
JCPDS database. Likewise, the XRD patterns of HAP and metal
Pd agree with that in the JCPDS database (Fig. 1b). All these data
show that the catalysts have definite crystalline structures. The
TEM images of the Pd/C and Pd/HAP are shown in Figure 2. The
diameters of Pd particles in the catalyst Pd/C and Pd/HAP were
in the ranges of 2ꢀ5 nm and 6ꢀ8 nm, respectively. The lattice
fringes with d = 0.227 nm from the Pd (111) plane in Pd/C are
clearly visible from the HRTEM image (Fig. 2c). In addition, the
surface area and porosity of Pd/C and Pd/HAP are determined by
4
4
5
5
6
6
0
5
0
5
0
5
providing the hydrogen for this transformation.
Considering the roles of alkaline for depolymerization of
1
0
18
lignin, we chose other two inorganic alkalines to conduct the
reaction. When strong alkaline NaOH was added into the reaction
system (entry 10), the conversion reached 100% but phenol was
partly degraded. On the other hand, both conversion and yields
decreased as NaHCO3 was used (entry 11) due to its weak
alkalinity.
Because NMP could promote the depolymerazation of BPE, we
also investigated the role of the two functional groups of NMP
through adding pyrrole and cyclopentanone to replace NMP.
Comparing entries 12 and 4, we found that pyrrole promoted the
formation of toluene. It can be known from the results of entries 4
and 13 that cyclopentanone played minor role in the
transformation of BPE. Based on the above, we can deduce that
the functional group containing nitrogen benefits to the reaction.
In additon, free radical quenching agent TEMPO was added into
the reactor (entry 14) and both conversion and yields decrease,
indicating that depolymerazation of BPE was governed by the
freeꢀradical reaction mechanism.
Further, we tested the lignin βꢀOꢀ4 model compound 2ꢀ
phenoxyacetophenone using our catalytic system at the condition
of Entry 7 in Table 1. It was shown that the model compound
could be transformed into acetophenone and phenol. The
conversion could reach 77.7%, and the yields of acetophenone
and phenol were 20.1% and 14.7%, respectively, indicating that
the system can promote the transformation of lignin βꢀOꢀ4 model
compounds but the selectivity of target products is not high.
1
5
N adsorptionꢀdesorption method and listed in the supporting
2
information (Table S1).
3
.2 Conversion of BPE
We carried out the reaction of BPE in different catalytic systems,
and the results are provided in Table 1. BPE could not be
2
0
o
transformed in water or Na CO or NMP alone at 150 C for 3
2
3
hours in Ar atmsphare (entries 1ꢀ3). In the presence of catalyst
Pd/C (entry 4), the conversion of BPE was 27.8% and the yields
of phenol and toluene were 17.4% and 3.7%, respectively. In
additon, the results show that Na CO or NMP favored the
2
5
2
3
formation of toluene (entries 5 and 6). In the presence of both
Na CO and NMP, the conersion reached 98.8% and the yields of
2
3
phenol and toluene were 71.6% and 50.4%, respectively(entry 7).
a
Table 1 Depolymerization of BPE at different conditions
Entry
Catalyst
Conversion,% Phenol,% Toluene,%
1
2
3
4
5
6
7
8
9
H
2
O
0
0
0
Na
2
CO
3
0
0
0
NMP
Pd/C
0
0
0
3.3 Effects of reaction parameters
27.8
30
17.4
13.9
12.2
71.6
64.6
37.8
0
3.7
The effects of reaction parametes were further studied using the
catalyst Pd/C. From the experimental results in Table 1, we know
that the metal Pd played the decisive role for depolymerization of
BPE in our reaction system. The dependence of the
transformation of BPE on the amounts of Pd/C is shown in Fig. 3.
The conversion increased as the loading amount of Pd/C was
changed from 0~0.025 g, and the conversion approached 100% at
the loading of 0.025 g. The yields of phenol and toluene
increased firstly and decreased after the amount of Pd/C exceeded
Pd/C+Na
2
CO
3
9.4
7
7
8
8
0
5
0
5
Pd/C+NMP
Pd/C+Na CO +NMP
Pd/HAP+Na CO +NMP
Pd/HAP+C+Na CO +NMP
Pd/C+NaOH+NMP
Pd/C+NaHCO +NMP
Pd/C+Na CO + pyrrole
Pd/C+Na CO
cyclopentanone
Pd/C+Na CO +NMP
TEMPO
31.5
98.8
99
9.18
50.4
35.8
26.3
34.6
18.5
15.8
2
3
2
3
2
3
69.2
100
51.9
28.8
1
1
1
0
1
2
3
17.8
13.8
2
3
2
3
+
0
.025 g. Generally, increasing amount of active components is
1
1
3
4
8.2
7.2
47
7.4
favorable to the conversion of reactants. But for freeꢀradical
reactions like the depolymerization of BPE, too many catalytic
sites promote the formation of free radicals fastly. Thus, the
linkage of different free radicals would produce high molecular
compounds thereby leading to the decrease of the yields of target
products. So, the yields decreased as the amount of Pd/C
exceeded 0.025 g.
2
3
73.5
26.4
+
a. Reaction conditions: BPE 0.05 g, Pd/C 0.025 g, Pd/HAP 0.025 g, Na
2 3
CO
0
.4 g, NaOH 0.4 g, NaHCO 0.4 g, NMP 0.05 g, pyrrole 0.05 g,
3
cyclopentanone 0.4 g, TEMPO 0.015 g, H
2
O 2 g, Ar 0.1 MPa, reaction
o
temperature 150 C, reaction time 3 hrs, volume of the reactor 8 mL.
3
0
The effect of reaction time on the transformation of BPE is
shown in Fig. 4. The conversion increased with reaction time and
In order to investigate whether the carbon from Pd/C reacted
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reached 100% at 3 hrs. The yields of phenol and toluene
increased with reaction time and reached a maximum at 3 hours.
Na CO aqueous solution has a strong inhibiting effect on BPE
2 3
19
hydrolysis. Thus, with the increasing amount of Na CO , the
2
3
Before 3 hours, BPE was transformed continuously and the yields 35 yields of toluene, dimers and trimers were enhanced and the
of target products also increased with the reaction time. After 3
hours, BPE was consumed completely and different radicals in
the system would interact with each other, leading to the
formation of byꢀproducts. Therefore, too long reaction time were
formation of the hydrolysis products phenol and benzyl alcohol
was remarkably reduced. The presence of the metal ion leads to
the formation of a cation–BPE adduct, in which the ether bond
is polarized and, therefore, more prone to be heterolytically
5
19
favarable to the occurrence of side reactions. After optimum 40 cleaved. On the other hand, the alkalinity of carbonate can
reaction time, more phenoxy and benzyl radicals was consumed
by other radicals in the reaction system. So, the yield of phenol
and toluene decreased with reaction time after the maximum.
accelerate hydrogen abstraction. It should also be noted that the
relatively large fraction of high molecular weight compounds
could likely serve as source of hydrogen. Unfortunately, the
yields of target products decreased after the amount of Na CO
10
2
3
100
45 exceeded 0.4 g due to the formation of dimers and trimers.
8
6
4
2
0
0
0
0
0
100
80
60
40
20
0
.00
0.01
0.02
0.03
0.04
Adding amounts of Pd/C(g)
0.0
0.2
0.4
0.6
0.8
Fig. 3. Effects of amount of Pd/C on depolymerization of BPE. (■)
Adding amounts of Na CO (g)
2 3
15
conversion of depolymerization of BPE, (▲) yield of phenol, (●) yield of
toluene. Reaction conditions: BPE 0.05 g, Na
O 2 g, Ar 0.1 MPa, reaction temperature 150 C, reaction time 3 hrs,
volume of the reactor 8 mL.
2 3
CO 0.4 g, NMP 0.05 g,
o
Fig. 5. Effect of amount of Na
2 3
CO on depolymerization of BPE. (■)
H
2
conversion of BPE, (▲) yield of phenol, (●) yield of toluene. Reaction
50
conditions: BPE 0.05 g, Pd/C 0.025 g, NMP 0.05 g, H
2
O 2g, Ar 0.1 MPa,
o
reaction temperature 150 C, reaction time 3 hrs, volume of the reactor 8
mL.
100
8
6
4
2
0
0
0
0
0
Fig. 6 shows the dependence of conversion of BPE and yields
of the products on the amount of NMP used in the reaction. When
the amount of NMP increased from 0 to 0.04 g the BPE cound be
converted completely. The maximum yield could be observed
when the amount of NMP was 0.04 g. As we know, NMP may be
produced through the reaction of γꢀbutyrrolactone and
methylamine and the reaction is a reversible chemical reaction.
Therefore, when NMP and water exists in the same reactor under
relative high temperature, certain amount of methylamine can be
produced. The alkalinity of amines can accelerate the
depolymerization of BPE through enhancing hydrogen
abstraction. In order to verify this hypothasis, we replaced NMP
5
5
60
0
1
2
3
4
5
Reaction time(h)
20
Fig. 4. Effects of reaction time on depolymerization of BPE. (■)
conversion of depolymerization of BPE, (▲) yield of phenol, (●) yield of
toluene. Reaction conditions: BPE 0.05 g, Na
2 3
CO 0.4 g, Pd/C 0.025 g, 65 with cyclohexylamine and found that the depolymerization of
o
NMP 0.05 g, H O 2g, Ar 0.1 MPa, reaction temperature 150 C, volume
2
BPE also proceeded smoothly. So, addition of NMP is helpful to
the transformation of BPE. Just like Na CO , too much NMP
of the reactor 8 mL.
2
3
would lead to the formation of byꢀproducts and the yields of the
traget products decreased.
25
Fig. 5 shows the dependence of the conversion of the BPE
and yields of the products on the amount of Na CO loaded in
2
3
the reaction. When the amount of the Na CO increased from 0
2
3
to 0.4 g, and the conversion increased firstly and then reached
the platform because the reactant could be converted
completely. The maximum yields could be observed when the
amount of the Na CO was 0.4 g. The reaction can proceed by
30
2
3
1
4
hydrolysis and pyrolysis pathways. It was reported that
4
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100
O
Freeꢀradical depolymerization
(II)
(III)
80
60
40
20
(I)
O
O
O
0
0
.00
0.02
0.04
0.06
0.08
N-methyl-2-pyrrolidone(g)
CO
H
Fig. 6. Effects of amount of NMP on depolymerization of BPE. (■)
conversion of depolymerization of BPE, (▲) yield of phenol, (●) yield of
toluene. Reaction conditions: BPE 0.05 g, Na
O 2g, Ar 0.1 MPa, reaction temperature 150 C, reaction time 3 hrs,
volume of the reactor 8 mL.
2 3
CO 0.4 g, Pd/C 0.025 g,
O
OH
o
5
H
2
O
O
3
.4 Reaction mechanism
As we know, EPR is the most direct and effective way to detect
the existence of free radicals. In this work, we carried out the
EPR study with PBN as the the spin trap. The EPR spectrum is
showed in Figure 7. The gꢀvalue obtained from the spectrum was
O
O
1
1
2
0
5
0
Scheme 1. Possible pathways of depolymerization of lignin model
2
.004±0.002 for BPE, which is corresponding with the value
30
compound BPE.
2
0
reported in the literature. Considering the structure and reaction
pathway of the αꢀOꢀ4 dimeric model compounds, the peaks in the
spectrum are ascribed mainly to the carbonꢀcentered benzyl
radical and oxygenꢀcentered phenoxy radical. It has been reported
that the gꢀvalue of phenoxy radical ranges from 2.0040 to 35 pyrolysis pathway, BPE can be decomposed into phenol and
2
Depolymeration of BPE often involves hydrolysis and
1
4
pyrolysis pathways. During the hydrolysis process of BPE,
phenol and benzyl alcohol are the main products. For the
2
1
2
2
.0053 and benzyl radical is 2.0030. Further, we know that the
toluene. In our experimental results, the main products were not
benzyl alcohol and phenol but phenol and toluene, which means
that the depolymeration of BPE in our reaction system may be via
the pyrolysis pathway. Further, when TEMPO was added into the
40 reaction system, both conversion and yields of the target products
decreased, which showed that the decomposition of BPE was
carried out via the freeꢀradical pyrolysis. In addition, the results
of entries 2 and 4 in Table 1 suggest that Pd/C can accelarate the
reaction, but Na CO aqueous solution alone can not promote the
hyperfine parameter a is 1.62 mT, which reveals the existence of
N
free radical containing carbon center. Therefore, it can be
deduced that the transformation of BPE proceeds by a freeꢀradical
mechnism.
2
3
45
reaction. Thus, based on our experimental results and the related
chemical aknowlage we can infer that the freeꢀradical
depolymerazation took place in our reaction system.
In order to explore the pyrolysis pathway of BPE, we analyzed
the final liquid products by GCꢀMS and the products contained
phenol, toluene, benzene. As the carbon balance of the total liquid
products was less than 100%, which indicated that polymerization
might occur just like reported in the literature 14. The gasoues
product detected was mainly carbon monoxide.
50
1
0G
On the basis of the results discussed above, we propose
Fig. 6. EPR spectrum of the reaction mixture containing PBN at 25°C
55 possible pathways for the depolymerization of BPE, which is
showed in scheme 1. There are three possible pathways existing
in our reaction system. As a main pathway(I), BPE is firstly
cracked into phenoxy and benzyl radicals. These two radicals can
2
5
2
3
react with BPE to form phenol, toluene and PhOCH
●Ph radical.
60
In the pathway II, BPE can be cleaved into benzyloxy radical
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DOI: 10.1039/C5GC01272D
and phenyl radical. For the pathway III, BPE can be
depolymerized into phenoxymethyl radical and phenyl radical.
And then, benzyloxy and phenoxymethyl radicals can be further
decomposed into phenyl radical, carbon monoxide and hydrogen
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24
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radical. Next, hydrogen radical from pathway I and II may
combine with PhOCH Ph radical to form BPE or PhOCH Ph
radical combines with other radicals or itself to produce heavier
●
●
2
012, 134, 16987.
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products. Thus, the products containing bezene, biphenyl,
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4. Conclusions
We have carried out the depolymerization of BPE into phenol and
toluene using Pd/C as the catalyst with and without Na CO and
2
3
NMP in argon atmosphere. The reaction is governed by a freeꢀ
radical pathway. In our reaction system, as a main pathway, BPE
is cracked into phenoxy and benzyl radicals. Pd/C can catalyze
the reaction, and Na CO and NMP can enhance the
1
2
2
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At the optimized reaction conditions, the yields of phenol and
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