Heterogeneous Nitrogen-doped Graphene Catalysed HOO? Generation via a Non-radical Mechanism for Base-free Dakin Reaction

Article abstract of DOI:10.1002/adsc.201900878

A heterogeneous nitrogen-doped graphene catalytic pathway for H₂O₂ activation to generate alkaline hydrogen peroxide (HOO^?) through a non-radical mechanism was reported. Remarkably, the heterogeneous catalytic procedure has been used for the evergreen and environmentally Dakin reaction without using any transition metals, homogeneous bases, ligands, additives or promoters, completely. The study of catalyst structure and catalytic activities indicate that the most active sites are created by the graphitic N atoms at zig-zag edges of the sheets. In addition, N as dopant element changes the reactivity of the neighbour C atoms, and leads to the formation of carbon-hydroperoxide (C?(HOOH)) and C?O* (C?O^?) transition state species on the graphene surface in catalytic the reaction. (Figure presented.).

Full text of DOI:10.1002/adsc.201900878

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DOI: 10.1002/adsc.201900878
Heterogeneous Nitrogen-doped Graphene Catalysed HOOÀ
Generation via a Non-radical Mechanism for Base-free Dakin
Reaction
Wei Sun,a Lingfeng Gao,a,* Xu Sun,a,* Hua Yang,b and Gengxiu Zhenga,*
a
School of Chemistry and Chemical Engineering, University of Jinan, No. 336 West Road of Nan Xinzhuang, Jinan 250022,
People’s Republic of China
E-mail:
E-mail: chm_zhenggx@ujn.edu.cn; gaolf108@mail.ustc.edu.cn; chm_sunx@mail.ustc.edu.cn
Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and
b
Chemical Engineering, Liaocheng University, 252059 Liaocheng, People’s Republic of China
Manuscript received: July 16, 2019; Revised manuscript received: September 29, 2019;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.201900878
Abstract: A heterogeneous nitrogen-doped graphene catalytic pathway for H2O2 activation to generate
alkaline hydrogen peroxide (HOOÀ ) through a non-radical mechanism was reported. Remarkably, the
heterogeneous catalytic procedure has been used for the evergreen and environmentally Dakin reaction without
using any transition metals, homogeneous bases, ligands, additives or promoters, completely. The study of
catalyst structure and catalytic activities indicate that the most active sites are created by the graphitic N atoms
at zig-zag edges of the sheets. In addition, N as dopant element changes the reactivity of the neighbour C
atoms, and leads to the formation of carbon-hydroperoxide (CÀ (HOOH)) and CÀ O* (CÀ OÀ ) transition state
species on the graphene surface in catalytic the reaction.
Keywords: Heterogeneous; HOOÀ ; Non-radical Mechanism; Nitrogen-doped Graphene; Dakin Reaction
Introduction
Dakin reaction, is a direct transformation of aromatic
aldehyde or ketone convert to a phenol.[1] Dihydric
phenols and their derivatives are essential substrates
and substantially expanded access to antioxidants,
agrochemicals, pharmaceuticals, photographic proc-
esses and polymerization inhibitors in organic
synthesis.[2] During the Dakin’s reaction, the HOOÀ
Scheme 1. Dakin reaction with a non-radical mechanism via N-
came from H2O2 is utilized as the must intermediate to
the translation, with the subsequent Baeyer-Villiger
oxidation process to generate the desired phenol
products.[3] In fact, excesses homogeneous bases such
rGO catalyst.
Recently, the nitrogen-doped carbon materials have
as NaOH, KOH or Na2CO3 were utilized for H2O2 shown great application potential in the catalyst and
activation with an acid-base reaction process materials science fields.[5] By introducing nitrogen
(Scheme 1, Eq. 1). The utilization of excessive homo- atoms into the carbon materials structure, the spin
geneous bases always raise of serious environmental or density and electronic properties of the adjacent carbon
technical concerns.[4] Therefore, the urgent for a atoms could be adjusted and tailored to preferably
sustainable and green process for catalysing Dakin create some novel catalytic activity sites.[6] Further-
reaction is widely recognized. In addition, finding more, the modified carbon atoms also exhibit enhanced
heterogeneous catalytic process for modern synthesis Lewis basicity, and make it a superior position for the
and catalysis chemistry also have been a big challenge. adsorption of reactive oxygen species.[7] With these in
mind, we turned our attention to utilize the heteroge-
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neous metal-free nitrogen-doped graphene for activat-
ing H2O2 and catalyzing Dakin reaction instead of any
homogeneous bases. However, limited fundamental
understanding about the correlations with the activities
and the heteroatoms can only provide a partial view on
the carbon catalyst; and in fact, always a persistent
issue about carbon catalysts for industrial
application.[8]
Herein, we outline a metal-free graphene with
integrated nitrogen atoms, which promotes formation
of some all-solid N-mediated active sites as all-solid
active Lewis base sites. The nitrogen- doped graphene
(N-rGO) was used as a metal-free heterogeneous
catalyst for H2O2 activation through a non-radical
mechanism for Dakin reaction, completely without
using any transition metals, organic or inorganic
homogeneous bases, ligands or additives (Scheme 1,
Eq. 2).
Figure 2. (a) CO2À TPD and NH3À TPD desorption profiles for
N-rGO. (b) Influences of molecular HCOOH and NH3; (c)
Influences of different mount of DMPO; And (d) influences of
various additive on Dakin reaction.
Results and Discussion
The N-rGO catalyst was synthesized through a heating
method at different temperatures (N-rGO-500, N-rGO-
550 and N-rGO-600). The prepared catalysts were first
To better explore the catalytic activity sites for
characterized by the SEM, TEM and HRTEM. As H2O2 on the N-rGO surface, independent titrations of
shown in Figure 1(a), 1(b) and 1(c), the nanosheets N-rGO with CO2 and NH3 was first proceeded. As
with thin flaky structure were well preserved. In shown in Figure 2(a), the CO2À TPD on N-rGO
°
addition, from the EDS-mapping, the nitrogen atoms exhibited a peak at 670 C, corresponding to existence
were uniformly distributed on the graphene structure of the immobilized basic sites on the graphene. The
°
(Figure 1(d)). The N2 adsorption-desorption measure- small pick at 680 C on the NH3À TPD belong to the
ment shown that the N-rGO-600 has a large specific oxygen-containing groups in the structure of
surface area of 366 m2/g (Figure S2). All those shown graphene.[9] After that, blockage experiments were also
that the N-rGO was successfully obtained with evenly performed with the presence of molecular acids
dispersed nitrogen element on the graphene surface.
(HCOOH) and molecular base (NH3) were carried out
to confirm the active site(s) type on the graphene. As
shown in Figure 2(b), added trace amount of HCOOH
on the catalytic system with the yields of catechol
doped to 56%, but NH3 only shown negligible effect
on the catalytic process. These differences because the
active Lewis base sites were efficient blocked and
deactivated by acidic molecules, and the Lewis acid
sites blockage only with a negligible effect on the
catalytic efficiency.[10] The solid blockage experiments
with HCOOH (or pyrrole) re-modified N-rGO was
also shown similar results with the depressed catechol
yield of 63% and 68%, respectively (Figure S10). All
of these blockage experiments revealed that the all-
solid Lewis base sites which created by the N-doped
graphene could as the active cites for H2O2 activation
and to initiate the Dakin reaction.
Carbon-based materials also exhibited peroxidase
enzyme-like activity for H2O2 activation, and the
*
*
radicals ( OH and OOH) would affect the catalytic
process.[11] Form the EPR spectrums, characteristic
*
*
signals of DMPO- OOH and DMPO- OH adducts
were observed, which demonstrated the existence of
free radicals in the system (Figure S11).[12] However,
Figure 1. (a) SEM, (b) TEM, (c) HRTEM and (d) EDS-
mapping image of N-rGO.
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when added catalytic amount of DMPO to the reaction
system only with negligible effect on the catalytic
catalytic efficiency (Figure 2(c)). In addition, radical
controlled experiments were also carried out with
different radical trapping scavenger, 2,2,6,6-tetrameth-
*
ylpiperidine-1-oxyl (TEMPO, for OH) and benzoqui-
*
none (BZQ, for OOH), was respectively added into
the initial reaction system only with negligible effect
on catalytic system (Figure 2(d)).[13] EDTA was also
added into the initial reaction mixture with the yield of
catechol remained about 75% to exclude the influence
of trace metals in the catalyst (Table S4)l.[14] All of
these results provided that the non-radical catalytic
process for the Dakin reaction based on the N-rGO
catalytic process.
In this heterogeneous procure, the N-rGO went
through a non-radical catalytic process for H2O2
activation. At least four types of nitrogen atoms are
there in the graphene framework based on their
location: graphitic-N (NÀ G), pyridinic-N (N-6), pyr-
rolic-N (N-5), and pyridine oxide like-N (NÀ O).[15]
Then, we discussed the relationship between the nitro-
Figure 4. XPS spectra and content of bond (a) C 1s, (b) N 1s
and (c) O 1s for the fresh and used N-rGO.
gen types and catalytic activity. As shown in Figure 3 and 75% for N-rGO-600, Figure 3(c)). It is thus
(a), Nitrogen preferentially stays at N-5 and N-6 sites confirmed that the more nitrogen atoms occupied the
(399.6 eV and 398.5 eV). At lower reduction temper- graphitic sites, the higher catalytic activity for Dakin
ature for the sample with 4.8% nitrogen (N-rGO-500), reaction was achieved.
matched with a low yield of catechol (68%, Figure 3
To further disclose the H2O2 activation progress on
(c)).[16] At higher reduction temperature the catalyst the N-rGO surface, we investigated the fresh and used
with more nitrogen content (5.0% for N-rGO-550, N-rGO catalyst for Dakin reaction by XPS measure-
5.8% for N-rGO-600 Figure 3(a)), the nitrogen atoms ments, respectively. As shown in Figure 4(a), com-
tended to occupy the graphitic sites (401.2 eV, 1.3% pared with the C 1s spectra of the fresh to the used N-
for N-rGO-550, 1.7% for N-rGO-600, Figure 3(b)),[17] rGO, the content of CÀ O bond (285.2 eV) increased
with increased of catechol yield (72% for N-rGO-550 from 22% to 27%, which indicated some oxygen
groups was generated from the interaction between
H2O2 and graphene carbon.[18] From the N 1s spectra of
the used N-rGO (Figure 4(b)), a new peak emerged at
404.5 eV which corresponds to NÀ O because of the
oxidation of the oxygen species.[19] For the O 1s spectra
(Figure 4(c)), the peak at 531.8 eV and 533.6 eV which
attributed to the OÀ O bond and CÀ O bond respec-
tively, and increased from 28% to 31% and 22% to
25% when compared with the the fresh to the used N-
rGO, confirmed that the di-oxygen species (À OÀ OÀ )
connected on the graphene surface.[20] Therefore, the
excellent performance of the all-solid Lewis base
catalyst is due to the ability of carbon to active H2O2,
and host the transition state (CÀ (H)OOH and CÀ O*)
for Dakin reaction.
To get more insight into the catalytic process, we
performed comparative DFT calculations on the reac-
tion mechanisms over N-rGO and rGO. The doped
nitrogen atom has one more unpaired electron and
beared a negative charge (0.42 eV), and the ortho-
carbon atom (o-carbon) by the nitrogen dopant have
more positive charge (0.16 eV).[7a] Then the unique
electronic properties of o-carbon exhibited enhanced
Lewis basic and made it a superior position for the
Figure 3. XPS spectra and catalytic performance of N-rGO
catalysts. (a) N 1s XPS spectra of various N-rGO catalysts. (b)
The concentrations of different N types of N-rGO. (c)
Correlation between catalytic performance and NÀ G concen-
trations.
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adsorption of H2O2. Then, the catalytic process
originated from the adsorption of a H2O2 molecule on
the graphene surface. After that, a possible two steps
process for H2O2 activation: (1) H2O2 adsorption at o-
carbon to form the transition state of carbon-hydro-
peroxide (CÀ (HOOH)), and (2) the CÀ (HOOH) de-
composed with another transition state of CÀ O*
(CÀ OÀ ). The reaction energy barrier for these two steps
were 0.48 eV and 1.14 eV on the N-rGO surface,
respectively (Figure 5(a)). Thus, the N-rGO catalyst
could activate H2O2 and generate the HOOÀ at mild
conditions. As a comparison, the H2O2 dissociation on
rGO was also simulated as well. As shown in Figure 5
(b), the corresponding energy barriers for the HOOÀ
generation were 1.54 eV and 1.64 eV, which were
much larger than those on N-rGO. That was, the
introduction of graphitic nitrogen induced the o-carbon
as enhanced active sites for H2O2 activation, and
formed the HOOÀ for the whole catalytic process.
Table 1. Performance of various Catalysts for Dakin Reaction.
[a]
Entry
Catalyst
Conv. (%)
Yield (%)[b]
TON
1
2
3
4
5
6
7
Blank
GO
rGO
C3N4
N-rGO-500
N-rGO-550
N-rGO-600
Trace
Trace
10
38
82
–
–
5
30
69
71
74
–
–
12.2
38.0
2084.2
2090.4
2117.7
86
92
[a] Reaction conditions: Catalyst (1 mg), arylaldehydes
(1 mmol), 30% H2O2 (6 equiv. with respect to each
°
substrate), 35 C, 12 h.
[b] Isolated yields.
To determine the catalytic performance of the
catalyst for Dakin reaction, salicylaldehyde was
selected as the model starting substrate. The screening
results are listed in Table 1 and Figure S13–S18.
Negligible yield was obtained over catalyst-free or GO
as catalyst under the condition of H2O2 as oxidant at
°
35 C for 12 h (Entry 1, 2, Table 1). Relatively low
product catechol (5%) were obtained over rGO catalyst
under the optimal condition (Entry 3, Table 1). Using
C3N4 with abundant nitrogen elements as catalyst, the
yield of catechol was slightly increased to 30%
(Entry 4, Table 1), that suggested the doped nitrogen
can be beneficial for enhancing the catalytic efficiency.
Howevr, the the yield of catechol over 70% when N-
rGO was selected as the catalyst by optimizing the
reaction conditions (Entry 5, 6 and 7, Table 1). The
major by-product is benzoquinone due to over-
oxidation of catechol, and the production of undesired
benzyl alcohols or benzoic acids was not detected.[21]
In addition, kinetic analysis of the Dakin reaction
system with the apparent activation energy value about
30.4 KJ/mol, which confirmed with the dual-site
Langmuir-Hinshelwood model (Table S7, Figure S19,
S20).[22] All those results shown the N-rGO catalyst is
a potential heterogeneous catalyst for the Dakin
reaction.
In addition to the remarkable catalytic activity, the
high recycling stability and reusability of the N-rGO
catalyst are also an advantage worth addressing. The
heterogeneous N-rGO catalyst could achieve five-time
recyclability without any obvious catalytic degradation
(Figure S21). Furthermore, the detailed structural and
morphological properties of the heterogeneous catalyst
remained unaltered by comparing the fresh and used
N-rGO for Dakin reaction (Figure S4–S9). All those
results illustrated the N-rGO with stable topological
Figure 5. Proposed mechanism of H2O2 activation by DFT
calculations over (a) N-rGO and (b) rGO catalyst, respectively.
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