Biomass-derived phosphorus-doped carbon materials as efficient metal-free catalysts for selective aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c9gc01910c

Heteroatom-doped carbon materials (HDCMs) with abundant active functional groups and stable structural characteristics are promising catalysts for eco-friendly metal-free catalysis. In this work, phosphorus-doped carbon materials with a highly porous structure and extremely high surface area (>1600 m² g^-1) were successfully prepared via a convenient and scalable strategy using easily available soluble starch and phosphoric acid, which are expected to show good performance in mass-transfer and thus promote the catalytic process. As expected, the as-prepared PC-700 catalyst showed remarkable catalytic performance in aerobic oxidation of benzyl alcohol with a higher TOF value than other previously reported heteroatom-doped carbon catalysts. It also exhibited great tolerance for various substrates, including aromatic, alicyclic, heterocyclic, and aliphatic alcohols. On the basis of the related characterization studies and experimental results, it was proved that the P-O-C species and the defects caused by P-O species doping in the PC-700 catalyst are the active sites for aerobic oxidation. A unique mechanism was proposed for the catalytic process, which is different from that of N-doped graphene and graphene oxide catalyzed reactions that follow a free radical mechanism. In addition, the recycling test and characterization of the reused catalyst indicate that the PC-700 shows extraordinary performance in terms of both recyclability and stability and retains high reactivity even after eight cycles.

Full text of DOI:10.1039/c9gc01910c

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Page 1 of 23
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DOI: 10.1039/C9GC01910C
Biomass-Derived Phosphorus-Doped Carbon Materials as Efficient
Metal-Free Catalysts for Selective Aerobic Oxidation of Alcohols
Xiwei Hu, Mengying Fan, Yangyang Zhu, Qian Zhu, Qiang Song and Zhengping
Dong*
College of Chemistry and Chemical Engineering, Gansu Provincial Engineering
Laboratory for Chemical Catalysis, Laboratory of Special Function Materials and
Structure Design of the Ministry of Education, Lanzhou University, Lanzhou 730000,
PR China
E-mail address: dongzhp@lzu.edu.cn (Zhengping Dong), Tel: 86 931 8912577.
1

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Abstract
Heteroatom-doped carbon materials (HDCMs) with abundant active functional groups and
stable structural characteristics are promising catalysts for eco-friendly metal-free catalysis. In
this work, phosphorus-doped carbon materials with highly porous structure and extremely high
surface area (>1600 m2 g-1) were successfully prepared via a convenient and scalable strategy
using easily available soluble starch and phosphoric acid, which are expected to have good
performance in mass-transfer and thus promote the catalytic process. As expected, the as-
prepared PC-700 catalyst showed remarkable catalytic performance in aerobic oxidation of
benzyl alcohol with a higher TOF value than that of other previously reported heteroatom-
doped carbon catalysts. It also exhibited great tolerance for various substrates, including
aromatic, alicyclic, heterocyclic, and aliphatic alcohols. On the basis of the related
characterizations and experimental results, it was proved that the P-O-C species and the defects
caused by P-O species doping on PC-700 catalyst are the active sites for aerobic oxidation. A
unique mechanism was proposed for the catalytic process, which is different from that of N-
doped graphene and graphene oxide catalyzed reactions that follow a free radical mechanism.
In addition, the recycling test and characterizations of the reused catalyst indicate that the PC-
700 has extraordinary performance both in recyclability and stability and retain high reactivity
even after eight cycles.
2

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1. Introduction
Selective aerobic oxidation of alcohols as an important approach for synthesizing aldehyde
intermediates has been widely used in the chemical industry.1-3 Traditionally, oxidants (e.g.,
chromates, chlorates, and permanganates) and noble metal-based catalysts (e.g., Au and Ru)
are frequently used in homogeneous selective aerobic oxidation of alcohols to achieve an ideal
yield.4-8 However, the toxicity and corrosivity of these oxidants and the high-cost of noble
metal-based catalysts are not in accordance with the principle of “Eco-friendly and Sustainable
Chemistry”.9 Furthermore, unsustainability in catalytic processes limit the industrial
application of homogeneous catalysts.10 Therefore, widespread and green oxygen sources and
environmentally friendly low-cost heterogeneous catalysts are urgently needed for selective
aerobic oxidation reactions.
Carbon-based materials that have a lot of heteroatoms-containing active functional groups
are considered to be promising heterogeneous catalysts because of their excellent structural
stability, high reactivity and remarkable recyclability.11, 12 However, common carbon-based
materials, such as graphene, GO, and carbon nanotubes, are frequently used as support
materials other than heterogeneous catalysts.13-16 This is because of their highly ordered
structure and the limited active functional groups in their framework. By contrast, HDCMs are
efficient catalysts because of their abundant active functional groups, which results from the
doping of heteroatoms.17 Moreover, new defects caused by the differences in atomic radius and
bond length between C atoms and those doped heteroatoms generally concentrate a lot of local
charges, which is beneficial for in-situ charge transfer from active defect sites to the substrates
and thus promoting the reaction.18, 19 In fact, the above-mentioned excellent characteristics of
HDCMs have already been thoroughly studied in electrocatalysis. For example, such materials
have been studied using the oxygen reduction reaction (ORR). Shao et al. reported N-doped
carbon nanotube arrays as efficient electrocatalysts for ORR with higher reactivity than
traditional Pt/C catalysts; their reported electrocatalyst also exhibited outstanding stability.20
The remarkable ORR activity of B and N co-doped carbon nanotubes, which is closely related
to the microstructure of the catalyst, has been proved by Yang et al.21 Dai et al. reported that
multiwalled P,N-co-doped CNTs are effective ORR catalysts that exhibited outstanding
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catalytic performance, and the improved electrocatalytic activity was attributed to the
synergetic effect between P and N atoms.22 However, the HDCMs that have been applied as
heterogeneous catalysts for chemical synthesis are relatively scarce, and the related
mechanisms are still unclear.11, 18 Therefore, using carbon materials doped with different
heteroatoms as heterogeneous catalysts has significant value for research.
Recently, P-doped carbon materials have been reported in metal-free catalysis because of
their unique physico-chemical properties and outstanding catalytic activity.23-25 The lower
electronegativity of P atoms compared to that of C atoms and the high electron-donating ability
of P atoms make the P dopants positively charged, which is beneficial for charge transfer.26, 27
Moreover, large atomic size of P can cause an abundance of defects in the framework of carbon
materials. These defects concentrate a lot of charges, which may be the main active sites.28, 29
Therefore, because of these unique characteristics of P atoms, P-doped carbon materials may
exhibit high reactivity. Additionally, biomass (e.g., sucrose, cellulose, and glucose) are
abundant and renewable resources that are frequently used to prepare carbon-based materials.30-
34
However, the preparation process often involves tedious preprocessing steps, such as
alkaline and/or acid activation, and this makes the preparation process time-consuming and
36
environmentally harmful.35,
By contrast,
a
method that uses physical mixing
and carbonization is more efficient and convenient, which makes it possible to implement such
a method in industrial production.
On the basis of these ideas, this work prepared mass-producible P-doped carbon materials
(PC-X, where X refers to the carbonization temperature) using biomass soluble starch and low-
cost phosphoric acid in an extremely convenient two-step approach. The prepared PC material
was applied in the oxidation of alcohols with oxygen as a renewable oxidant.37 The possible
active sites were investigated in depth using a series of characterizations and related control
experiments. It was demonstrated that P-O-C species and the formation of defects are closely
related to the reactivity of aerobic oxidation of alcohols, and this is quite different from the
previously reported conclusions.38 Surprisingly, the kinetic study shows that aerobic oxidation
catalyzed by PC-700 catalyst has a very low Ea value of 25.70 KJ·mol-1, which is lower than
that of mono-heteroatom-doped, dual-heteroatom-doped, and even noble metal-based
catalysts.39-41 Additionally, a unique mechanism was proposed on the basis of all of the above-
4

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mentioned results, which is different from that of other reported heteroatom-doped catalysts
(e.g., N-doped graphene).39 In conclusion, this work is expected to offer a new inspiration for
fabricating mass-producible biomass-derived heteroatom-doped materials as efficient metal-
free catalysts using an extremely simple way for environmentally friendly catalytic applications.
2. Experimental Section
2.1. Materials
Soluble starch, urea, and phosphoric acid (85 wt.%) were purchased from Xilong Chemical
Co., Ltd. (China). Aromatic alcohols and aliphatic alcohols were obtained from Sigma-Aldrich
Industrial Corp. All of these other regents were of analytical grade and were used without
further purification.
2.2. Preparation of P-doped Carbon Catalysts
Soluble starch (5 g) and phosphoric acid (5.88 g) were added in 30 mL of deionized water
with vigorous stirring for 12 h at ambient temperature. The mixture was then dried at 100 °C
for 48 h, and the obtained black granular material was ground to powder to obtain PC. The PC
powder was pyrolyzed at 500, 600, 700, and 800 °C under a N2 atmosphere for 2 h with a
heating rate of 5 °C min-1 to obtain the PC-X materials. The obtained materials were then treated
with 150 mL of boiling water for 2 h to completely remove the P-containing substances. Finally,
the treated P-doped carbon materials were dried at 100 °C overnight to obtain PC-500, PC-600,
PC-700, and PC-800. The pure carbon catalyst, C-700, was prepared using only soluble starch
and pyrolyzed at 700 °C under the same conditions as PC-700.
2.3. Preparation of N-Doped and N,P co-Doped Catalysts
Briefly, soluble starch (5 g) and urea (5 g) were mixed thoroughly in 30 mL of deionized
water and dried at 80 °C for 48 h to obtain the N-doped carbon material (NC). The obtained
NC was then pyrolyzed at 700 °C to obtain the N-doped catalyst, which was denoted as NC-
700. Similarly, the N,P co-doped catalyst was prepared using the same procedures.
Soluble starch (5 g), urea (5 g), and phosphoric acid (5.88 g) were used in the preparation, and
the mixture was pyrolyzed at 700 °C to obtain the N,P co-doped catalyst (NPC-700).
2.4. Catalytic Aerobic Oxidation of Benzyl Alcohol
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Typically, benzyl alcohol (0.5 mmol) and PC-700 (50 mg) were added into a 10 mL thick-
walled pressure glass tube that contained 2 mL of deionized water, and then oxygen was
continuously bubbled into the solution for 5 min. The tube was then sealed with a Teflon cap,
and the solution was stirred at 120 °C. After reaction, the solution samples were
sucked out through a syringe, and the solid catalyst was filtered using a 0.2 μm filter. The
filtered solution was extracted with ethyl acetate and analyzed using gas chromatography-mass
spectrometry (GC-MS, Agilent 5977E).
2.5. Catalyst Characterization
The morphologies of PC-X materials were studied using a transmission electron microscope
(TEM, Tecnai G2 F30) and a scanning electron microscope (SEM, HITACHI S-4800). X-ray
diffraction (XRD) patterns were recorded on a Rigaku D/max-2400 diffractometer in the range
of 2θ=10-90° with Cu-kα radiation. Raman spectra were recorded on a Raman spectrometer
(Jobin Yvon Lab Ram HR evolution). An X-ray photoelectron spectrometer (XPS, PHI-5702)
was used to investigate the bonding information of PC-X materials. The Brunauer-Emmett-
Teller (BET) specific surface areas were measured using nitrogen adsorption-desorption
isotherms at 77 K after degassing the samples using a Micromeritics Model ASAP2010
instrument. The pore size distributions were determined using the non-local density functional
theory (NLDFT) method with a slit pore model. Fourier transform infrared (FT-IR)
spectroscopy was performed on a Bruker spectrometer (VERTEX 70). Thermogravimetric
analysis (TGA) was carried out using a TA-Q50 instrument, and PC was heated from 35 to 900
°C under a N2 atmosphere at a heating rate of 10 °C min-1. Electron paramagnetic resonance
(EPR) spectroscopy was recorded on a Bruker A300-9.5/12 instrument.
6

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Scheme 1. The preparation procedures of PC-X catalysts.
3. Results and Discussion
3.1. Preparation process and structural informationof PC-X catalysts
Fig. 1. (a) TEM image of PC-700 catalyst. (b) EDX mapping images of PC-700 for C, O and P
elements.
PC-X materials were synthesized using a simple and convenient two-steps process of
physical mixing and carbonization (Scheme 1). As seen in Fig. 1a, the as-fabricated PC-700
catalyst has an overlapping multilayered morphology and a porous structure. Similarly, the
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highly wrinkled sheet structure of PC-700 is clear in its SEM image, and this may be attributed
to the geometrical deformation that results from the doping of P atoms in the carbon lattice
(Fig. S1c).38 Other catalysts that are synthesized at different carbonization temperatures (500,
600 and 800 °C) have similar multilayered sheet structures, as seen in their SEM images (Fig.
S1a, b and d). In addition, the EDX mapping images of the PC-700 catalyst indicate that C, O,
and P atoms are evenly distributed on the nanosheets (Fig. 1b).
The nitrogen adsorption-desorption isotherm of PC-700 exhibits a type I isotherm, and this
indicates the presence of a microporous structure (Fig. 2a).42 Moreover, the pore size
distribution curve of PC-700 shows the presence of micropore centered at 0.64 nm and small
mesopore centered at 2.03 nm (Fig. 2b). In addition, the calculated BET surface area and pore
volume of the PC-700 catalyst are 1612.9 m2 g-1 and 0.79 cm3 g-1, respectively (Table S1, entry
3). For comparison, the texture parameters of other PC-X catalysts (PC-500, PC-600, and PC-
800) are also listed in Table S1. Furthermore, the TGA curve indicates three main steps, which
were located at 154.7, 558.0 and 786.3 °C with weight losses of 1.8%, 28.9%, and 57.5%,
respectively (Fig. S2).
3.2. The comparison of reactivity among PC-X and other related catalysts
The prepared PC-X catalysts were applied in the oxidation of benzyl alcohol, and their
activity was evaluated using TOF values. With an increase in carbonization temperature, the
catalytic activity of the PC-X catalysts first increased and then decreased. PC-700 showed the
best catalytic performance with extremely high conversion, selectivity, and TOF value (Table
1, entries 1 to 4). As seen in Table 1, entries 5 and 6, lower amounts of catalyst and/or reaction
temperature both lead to a decrease in conversion. In addition, the poor catalytic performances
of the C-700 and PC catalysts indicate that both P doping and high-temperature carbonization
are crucial for the high reactivity of the catalysts (Table 1, entries 7 and 8). The conversion of
the N-doped carbon catalyst (NC-700) was much lower than that of PC-700 (Table 1, entry 9).
Furthermore, the significant difference in activity between PC-700 and NPC-700 indicates that
the intervention of N atoms in the PC-700 catalyst will reduce its reactivity (Table 1, entries 3
and 10). Phosphoric acid, which contains only P=O and P-OH bonds was also tested, and it
showed no obvious activity in the reaction. This indicates that P=O and P-OH bonds alone
cannot act as active species (Table 1, entry 11). In addition, the reaction will not happen in the
8

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absence of a catalyst (Table 1, entry 12).
Fig. 2. Nitrogen adsorption-desorption isotherm (a) and pore size distribution curve (b) of PC-
700 catalyst.
Table 1. The optimization of reaction conditions for oxidation of benzyl alcohol.a
OH
O
PC catalyst
H2O, O2
Entry
Conv./Sel. (%)
TOF (mol·g-1h-1)b
Catalyst/mg
PC-500/50
PC-600/50
PC-700/50
PC-800/50
PC-700/40
PC-700/50
PC/50
Temp. (˚C)
1
120
120
120
120
120
100
120
120
120
120
120
120
59.8/>99
73.4/>99
99.9/>99
96.2/>99
65.8/>99
79.8/>99
12.9/>99
10.1/>99
5.3/>99
31.2/>99
5.7/>99
-
2.49×10-4
3.06×10-4
4.16×10-4
4.01×10-4
3.43×10-4
3.33×10-4
5.38×10-5
4.21×10-5
2.21×10-5
1.30×10-4
2.38×10-5
-
2
3
4
5
6
7
C-700/50
NC-700/50
NPC-700/50
H3PO4
8
9
10
11
12
none
a Reaction conditions: 0.5 mmol benzyl alcohol, 50 mg catalyst, 2 mL H2O, 1 atm O2, 24 h. The
conversion and selectivity were determined by GC-MS.
mole of converted substrate
b Turnover frequency (TOF) =
.
g catalyst × reaction time (h)
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Fig. 3. XRD patterns (a) and Raman spectra (b) of PC-500, PC-600, PC-700 and PC-800
catalysts.
3.3. Investigation of the cause of defects formation and the intrinsic active sites in
PC-X catalysts
A series of characterizations, such as Raman spectroscopy, XPS, and FT-IR were performed
to explore the possible active sites of PC-X catalysts. The XRD patterns (Fig. 3a) indicate that
the PC-X catalysts are carbon materials and show two main peaks at around 24° and 43°, which
represent the (002) and (100) planes of graphitic carbon, respectively.43 As seen in Fig. 3b, the
D and G bands, which are located at 1325 cm-1 and 1595 cm-1, respectively, represent lattice
defects and the graphitic phase in catalysts, respectively.44, 45 It is determined that the relative
intensity of the two bands (ID/IG) in the PC-700 catalyst is the highest, and this indicates the
highest density of defects in its framework. On the contrary, the PC-500 catalyst has the lowest
density of defects.
XPS spectra were recorded to obtain crucial information regarding the P-doping content, the
detailed bonding configuration of P atoms, and the possible cause of the formation of defects.
As seen in the wide scan spectra (Fig. S3), all of the PC-X catalysts contain C, O, and P, which
is also confirmed by the EDX spectrum (Fig. S4). The XPS high-resolution C1s spectra consist
of four peaks located at 284.6 eV, 285.2 eV, 285.9 eV and 289.3 eV corresponding to SP2 C,
SP3 C, C-P and O-C=O species, respectively (Fig. S5). The detailed P-doping concentrations
that were determined from the wide scan spectra of different PC-X catalysts are listed in Table
2. It is worth noting that a well linear correlation was found between the P-doping concentration
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and the values of ID/IG (Fig. S6a), and also between the P-O species concentration and the values
of ID/IG (Fig. S6b), which indicates that the formation of defects in the PC-X catalysts is closely
related to P-O species doping. Moreover, the TOF values in the oxidation reaction are linearly
correlated with the values of ID/IG in different PC-X catalysts (Fig. S6c). However, there is no
obvious correlation with the surface area of the PC-X catalysts (Table S1, entries 1 to 4). Thus,
it can be concluded that the catalytic activity of the PC-X catalysts is significantly associated
with the formation of defects.
To study the relationship between different P-containing groups and catalytic activity, XPS
high-resolution P2p and O1s spectra were recorded and analyzed. As seen in Fig. 4a, the spectra
were mainly composed of two peaks at 132.5 ± 0.1 and 134.0 ± 0.1 eV, which correspond to
P-C and P-O type of species. In addition, the O1s spectra has three main peaks at 531.0 ± 0.1
eV, 532.4 ± 0.2 eV, and 533.6 ± 0.2 eV, which correspond to C/P=O, C/P-O-C, and C/P-OH
(Fig. 4b). The calculated P-C and P-O contents based on total P content and high-resolution
P2p spectra are listed in Table 2, entries 1 to 4. Moreover, as shown in Fig. S6d, the TOF
values in the oxidation reaction changes linearly with the calculated P-O species concentrations,
while there is no obvious correlation with the P-C contents. This indicates that P-O species are
closely associated with the catalytic activity of PC-X catalysts in oxidation reaction. Moreover,
the calculated contents of different species in the O1s spectra indicate that the content of C/P-
OH species in the PC-700 catalyst is lower than that of other PC-X catalysts, whereas the
contents of C/P=O and C/P-O-C species in the PC-700 catalyst are similar to or even higher
than those of other PC-X catalysts (Table S2, entries 1 to 4); besides, the PC-700 catalyst
shows the highest catalytic activity, which further confirms the conclusion mentioned above.
Furthermore, in the FT-IR spectra, the intensity of the peaks at 1035 cm-1 (C-O-P) for the PC-
700 and PC-800 catalysts are much higher than those for the PC-500 and PC-600 catalysts (Fig.
S7); besides, both PC-700 and PC-800 showed excellent and similar catalytic activity.38 In
conclusion, from all the results obtained above, it can be concluded that P-O-C species and the
defects caused by P-O species doping may be the crucial active sites for aerobic oxidation
reaction.
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Fig. 4. XPS high-resolution P 2p (a) and O 1s (b) spectra of PC-X catalysts, i.e., PC-500, PC-
600, PC-700 and PC-800, respectively.
Table 2. XPS total and calculated element contents in different PC-X catalysts.
Total %
P [at.
Calculated %
Entr
y
P-C [at. %]
P-O [at. %]
Catalyst
C [at. %]
O [at. %]
%]
1
2
3
4
PC-500
PC-600
PC-700
PC-800
92.05
92.51
93.19
95.58
0.41
0.61
0.96
0.90
7.54
6.88
5.85
3.52
0.26
0.36
0.53
0.54
0.15
0.25
0.43
0.36
3.4. Substrate scope test of PC-700 catalyst in selective aerobic oxidation of various
alcohols
The reaction activity of the PC-700 catalyst was further investigated using different types of
substrates, including aromatic, alicyclic, heterocyclic, and aliphatic alcohols. For aromatic
alcohols, the alcohols that have electron-donating groups (-OCH3, -CH3) are more effectively
converted into the corresponding products than the alcohols that have election-withdrawing
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groups (-Cl, -NO2) (Table 3, entries 1 to 4). This is quite different from the situation of the
reaction catalyzed by N-doped carbon materials, which there are no obvious differences
between the substrates with electron-withdrawing groups or those with electron-
donating groups.39 The aromatic alcohol with a fluoro-group is a special case, it was catalyzed
effectively as an aromatic alcohol with an electron-donating group (Table 3, entry 5). This
may be attributed to the very small atomic size of fluorine. Cinnamyl alcohol as a kind of
unsaturated aromatic alcohol was also catalyzed with high conversion (>99%) and excellent
selectivity (>98%) (Table 3, entry 6). The aromatic alcohol that has two hydroxy groups was
oxidized to two products (95.3% conversion), of which 4-hydroxybenzaldehyde is the main
product (Table 3, entry 7). Secondary aromatic alcohols showed higher reactivity than alicyclic
alcohol and heterocyclic alcohol (Table 3, entries 8 to 10). In addition, aliphatic alcohols
cannot be catalyzed in this reaction (Table 3, entry 11).
Table 3. The selective aerobic oxidation of different alcohols on PC-700 under the
optimal condition. a
Entry
1
Substrate
Product
Time (h) Conv./ Sel.(%) TOF b (mol·g-1h-1)
O
OH
OH
22
24
24
24
24
99.6/>99
99.8/>99
73.1/98.3
16.6/>99
97.8/>99
4.53×10-4
4.16×10-4
2.99×10-4
6.92×10-5
4.08×10-4
H3CO
H3CO
O
2
3
4
5
H3C
Cl
H3C
Cl
O
OH
OH
O
O2N
O2N
F
O
OH
F
O
OH
6
7
24
24
99.3/98.7
4.10×10-4
O
80.9/>99
14.4/>99
3.37×10-4
6.00×10-5
HO
O
OH
HO
O
O
OH
8
9
24
24
27.3/>99
5.8/>99
1.14×10-4
2.42×10-5
O
O
OH
O
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OH
O
O
10
11
24
24
2.1/>99
<1/>99
8.75×10-6
-
OH
a Reaction conditions: 0.5 mmol substrate, 50 mg PC-700, 2 mL H2O, 1 atm O2，120 ˚C. The
conversion and selectivity were determined by GC-MS.
mole of converted substrate
b Turnover frequency (TOF) =
.
g catalyst × reaction time (h)
3.5. Control experiments and kinetic research of aerobic oxidation of benzyl alcohol
To further investigate the aerobic oxidation of benzyl alcohol catalyzed by the PC-700
catalyst, a series of control experiments have been specifically designed. First, to determine the
role that oxygen plays in the reaction, control experiments were performed in different gaseous
atmospheres. As listed in Table 4, benzyl alcohol could be completely converted into
corresponding product in 24 h with the existence of oxygen (Table 4, entry 1). To further
illustrate the good chemoselectivity in this reaction, the control experiment was carried out with
a longer reaction time (48 h) in the same reaction conditions, there are no obvious differences
in selectivity compared with that of original reaction (Table 4, entry 2). However, when
reaction was performed in an atmosphere of air, a relatively lower conversion (24.4 %) was
obtained than the case that oxygen was the reaction gas (Table 4, entry 3). In addition, a poor
conversion was occured when the reaction was performed in an inert atmosphere (N2) for even
48 h (Table 4, entries 4 and 5). Notably, when the reaction was performed in an inert
atmosphere for 24 h, there was poor reactivity with a very low conversion (3.60%), while when
oxygen was introduced into the reaction system and reacted for another 24 h, the conversion
recovered back to 61.3% (Table 4, entry 6). Thus, it is clear that the participation of O2 is
crucial in the catalytic process.
Table 4. Catalytic activity test on PC-700 catalyst using different oxidants.a
Entry
Catalyst
PC-700
PC-700
PC-700
PC-700
PC-700
Oxidant
O2
Time (h)
Conv./Sel. (%)
99.9/>99
TOF b (mol·g-1h-1)
4.16×10-4
1
2
3
4
5
24
48
24
24
O2
99.9/>99
2.08×10-4
Air
N2
24.4/>99
1.02×10-4
3.60/>99
1.50×10-5
N2
48
3.86/>99
8.04×10-6
14

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DOI: 10.1039/C9GC01910C
-4
6
PC-700
N2 to O2
24 to 48
61.3/>99
1.28×10
a
Reaction conditions: 0.5 mmol benzyl alcohol, 50 mg PC-700, 2 mL H2O, 120 ˚C. The
conversion and selectivity were determined by GC-MS.
mole of converted substrate
b Turnover frequency (TOF) =
.
g catalyst × reaction time (h)
To study the reactivity of PC-X catalysts in the aerobic oxidation of benzyl alcohol, the
kinetic research is crucially important. Thus, the reaction was performed at a series of reaction
temperatures (333-393 K). To complete the plot of lnk versus 1/T, the rate constants (k) were
calculated from the fitting lines shown in Fig. 5a. The apparent activation energy (Ea) was
calculated to be 25.70 KJ·mol-1 using the Arrhenius equation and the slope and intercept shown
in Fig. 5b (-3091.11 and -4.76, respectively). Moreover, the PC-700 catalyst was compared
with other catalysts reported in previous works to further evaluate its catalytic reactivity in the
oxidation of benzyl alcohol. As seen in Table 5, the PC-700 catalyst shows higher catalytic
activity than mono-heteroatom-doped catalysts and similar catalytic activity to that of dual-
heteroatom-doped catalysts with a relatively high TOF value under the same reaction conditions.
Furthermore, the Ea value of the reaction (25.70 KJ·mol-1) that is catalyzed by the PC-700
catalyst is lower than that of reported metal-free catalysts and even noble metal catalysts
catalyzed aerobic oxidation of benzyl alcohol (Table 5, entries 1 to 7). Thus, the PC-700
catalyst exhibits remarkable catalytic performance by lowering the Ea value of the reaction.
Table 5. The comparison of catalytic activity and activation energy of different catalysts in
benzyl alcohol oxidation.
Substrate Temp.
TOF
Ea
Entry
Catalyst/mg
Oxidant
Ref.
(mmol)
(˚C)
(mol·g-1h-1) (KJ·mol-1)
nitrogen-doped
Graphene/30
39
46
38
1
2
3
1 atm O2
2 atm O2
1 atm O2
0.1
80
4.83×10-5
1.55×10-4
2.10×10-4
56.1
-
modified GO/50
0.25
100
100
P-doped porous
Carbon/3
0.0185
49.6
P and S dual-doped
graphitic porous
carbon/100
40
4
1 atm O2
1.0
100
3.75×10-4
32.0
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47
5a
6b
5 wt% Ru/C
1 atm air
1.1
1.0
120
83
29/h-1
0.4/h-1
32.0
DOI: 10.1039/C9GC01910C
41
2.5 mol% Ru/Al2O3 1 atm O2
51.4
80
2.59×10-4
3.33×10-4
P-doped porous
1 atm O2
This
7
0.5
25.7
Carbon/50
Work
100
mole of converted substrate
mole of metal × reaction time (h)
a, b Turnover frequency (TOF) =
.
Fig. 5. (a) Plots of concentration of benzaldehyde versus reaction time for the oxidation of
benzyl alcohol catalyzed by PC-700 catalyst at different reaction temperatures. Reaction
conditions: 0.5 mmol benzyl alcohol, 50 mg PC-700, 2 mL H2O, 1 atm O2. (b) Arrhenius plot
for the benzyl alcohol oxidation.
To determine if free radical intermediates were generated in the aerobic oxidation of benzyl
alcohol catalyzed by the PC-700 catalyst, a control experiment was carried out in the presence
of a radical inhibitor (BHT). As listed in Table 6, the conversion and selectivity were hardly
affected by the addition of BHT, suggesting no radical intermediates were generated in the
reaction. The same conclusion was also confirmed by EPR characterization using 5,5-dimethyl-
1-pyrroline N-oxide (DMPO) as the spin trap, which shows no obvious signal in the spectrum
(Fig. S8). This result is different from the reactions catalyzed by GO and N-doped metal-free
catalysts; such reactions are closely related to the free radical intermediates formed in the
catalytic process.39, 48
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Table 6. The control experiment of benzyl alcohol oxidation in the presence of DaOIr:a10d.i1c03a9l/C9GC01910C
inhibitor.a
Entry
1
Catalyst
PC-700
PC-700
Radical inhibitor
none
Conv./Sel. (%)
99.9/>99
2b
BHT
98.2/>99
a Reaction conditions: 0.5 mmol benzyl alcohol, 50 mg PC-700, 2 mL H2O, 1 atm O2，120 ˚C,
b
24 h. The conversion and selectivity were determined by GC-MS. 0.25 mmol butylated
hydroxytoluene (BHT) was dissolved in 200 μL ethanol and then added in the reaction solution.
3.6. The detailed analysis of active sites and catalytic mechanism
On the basis of the characterizations and control experiments, important information
concerning active sites and the catalytic mechanism was collected as follows: (1) The
intervention of N inhibits the reactivity of PC-X catalysts. (2) The formation of defects in PC-
X catalysts is attributed to the doping of P-O species. (3) The P-O-C species and defects caused
by P-O species doping are the active sites for aerobic oxidation reactions. (4) It is clear that the
participation of O2 is crucial in the aerobic oxidation of benzyl alcohol when the reaction is
catalyzed by PC-X catalysts. (5) The PC-700 catalyst exhibits a lower Ea value of aerobic
oxidation than other reported metal-free and even noble metal catalysts, and thus it has
remarkable catalytic capability. (6) There are no radical intermediates generated in the aerobic
oxidation reaction, which is quite different from when the same reaction that was catalyzed by
other heteroatom-doped catalysts.
On the basis of all of above the factors, a reasonable mechanism was proposed and shown in
Scheme 2. The control experiment in the absence of oxygen showed poor activity, suggesting
the P-containing active species in PC-700 catalyst are not qualified with enough oxidation
ability; besides, oxygen is needed to participate in the reaction process. Accordingly, in the first
step, oxygen should be adsorbed on the nearby active sites of PC-X catalysts to form active
oxygen species. From the control experiment in the presence of BHT and EPR characterization,
it is clear that no free radical intermediates were generated in the catalytic process. Therefore,
a new P-containing active intermediate with a higher valence state (O-P=O) was more likely
generated with the assistance of oxygen in the first step. After the first step, oxygen was reduced
into hydrogen peroxide in aqueous solution. More importantly, the generated hydrogen
peroxide would immediately decompose into oxygen and water at high reaction temperature,
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and the generated oxygen will be back into the reaction system. That is the reason why the
oxidation process can be finished with limited oxygen. In addition, from the linear relationships
between P-O species concentrations, ID/IG values, and TOF values, it is reasonable to assume
that the active sites were probably located around the defects on the PC-X catalysts. Therefore,
in the second step, the benzyl alcohol was adsorbed and then oxidized by new P-containing
active intermediates locally around the defects. After the oxidation of benzyl alcohol, the new
P-containing active intermediates were reduced to the original active species. At last, the
catalytic cycle finished with the desorption of products.
Scheme 2. A possible reaction mechanism for the aerobic oxidation of benzyl alcohol catalyzed
by PC-700 catalyst with the existence of oxygen.
3.7. The evaluation of recyclability and stability of PC-700 catalyst
The recyclability and stability of the PC-700 catalyst were evaluated using a recycling test
of aerobic oxidation of benzyl alcohol and several related characterizations of the reused
catalyst. As clearly seen in Fig. 6, the PC-700 catalyst shows outstanding stability in the
recycling test with relatively high conversion (89.8%) and excellent selectivity (>99%) even
after 8 cycles. In addition, the remarkable structural stability of PC-700 was confirmed by a
TEM image of the reused PC-700 catalyst, which shows the unchanged overlapping
multilayered morphology (Fig. S9). A PXRD pattern of the reused PC-700 catalyst also shows
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DOI: 10.1039/C9GC01910C
that it is unchanged (Fig. S10a), and the value of ID/IG that is calculated from the Raman
spectrum of the reused catalyst is very close to that of the fresh catalyst (Fig. S10b). This
indicates the stability of the physical structure of the PC-700 catalyst. Moreover, as seen in the
XPS spectra of the PC-700 catalyst that was recycled 8 times, the contents of C, O, and P are
similar to those of fresh the catalyst (Fig. S11); Besides, there are no obvious differences
between the P2p and O1s spectra of the fresh and reused PC-700 catalyst (Fig. S12 a and b).
Furthermore, as listed in Table S3 and 4, the contents of P-containing species in the reused
PC-700 catalyst that were calculated based on the high-resolution P2p and O1s spectra are also
almost unchanged. These results have well demonstrated that the PC-700 catalyst has
extraordinary performances both in recyclability and stability.
Fig. 6. Recycling test of PC-700 catalyst in benzyl alcohol oxidation. Reaction conditions: 0.5
mmol benzyl alcohol, 50 mg PC-700, 2 mL H2O, 1 atm O2，120 ˚C, 24 h.
4. Conclusions
In summary, PC-X materials were prepared via a scalable two-steps approach using cost-
efficient and easily available soluble starch and phosphoric acid, which makes the as-prepared
catalysts cost-efficient and environment-friendly. The as-fabricated PC-700 proved to be an
efficient metal-free catalyst in the aerobic oxidation of benzyl alcohol, achieving full
conversion (>99%) and remarkable selectivity (>99%). On the basis of the related
characterizations and experimental results, the P-O-C species and the defects caused by P-O
species doping were demonstrated to be the active sites for the oxidation reaction. The PC-700
catalyst showed better catalytic performance than mono-heteroatom-doped catalysts and
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comparable catalytic performance to dual-heteroatom-doped catalysts. It is expected that this
work provides an extremely simple and scalable way to fabricate HDCMs that have great
potential in metal-free catalysis.
Conflicts of interest
There are no conflicts to declare.
Supporting Information
Supporting Information is available from the publication website.
Acknowledgements
This work was supported by the Natural Science Foundation of Gansu (No. 18JR3RA274).
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Graphtic abstract