Oxidative dehydrogenation of ethyl lactate over nanocarbon catalysts: Effect of oxygen functionalities and defects

Article abstract of DOI:10.1016/j.cattod.2018.06.018

Pyruvic acid and pyruvate derivatives are important chemical intermediates which are normally produced using metal-based catalytic materials that could cause serious environmental problems. In the present work, renewable nanocarbon catalysts are applied in this system, and a solvent free and highly selective oxidative dehydrogenation of ethyl lactate (EL) process is achieved to produce ethyl pyruvate (EP) under mild reaction conditions. Chemical titration results provide direct evidence that ketonic carbonyl groups on nanocarbon are active sites for this reaction. Raman, XPS and model catalyst experimental results have shown that there is a dynamic transformation between defects and oxygen functionalities during the catalytic reactions. The reaction pathway is revealed via kinetic analysis, which may shed light on the rational design of carbon catalysts and their potential catalytic applications in the field of fine chemical productions.

Full text of DOI:10.1016/j.cattod.2018.06.018

Accepted Manuscript
Title: Oxidative Dehydrogenation of Ethyl Lactate over
Nanocarbon Catalysts: Effect of Oxygen Functionalities and
Defects
Authors: Dan Wang, Wei Liu, Zailai Xie, Siyuan Tian,
Dangsheng Su, Wei Qi
PII:
S0920-5861(18)30742-9
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.06.018
CATTOD 11507
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
27-2-2018
22-5-2018
14-6-2018
Please cite this article as: Wang D, Liu W, Xie Z, Tian S, Su D, Qi W,
Oxidative Dehydrogenation of Ethyl Lactate over Nanocarbon Catalysts:
Effect of Oxygen Functionalities and Defects, Catalysis Today (2018),
https://doi.org/10.1016/j.cattod.2018.06.018
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Oxidative Dehydrogenation of Ethyl Lactate over Nanocarbon
Catalysts: Effect of Oxygen Functionalities and Defects
Dan Wanga,#, Wei Liub,#, Zailai Xiec, Siyuan Tianb, Dangsheng Sub, Wei Qia,b,*
# These authors contributed equally.
a. College of Sciences, Northeastern University, Shenyang, Liaoning, 110819, China.
b. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese
Academy of Sciences, Shenyang, Liaoning, 110016, China, Email: wqi@imr.ac.cn
c. State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry,
Fuzhou University, Fuzhou, Fujian, 350002, China.
Graphical Abstract
Highlights
 Green chemical synthesis of ethyl pyruvate.
 Nanocarbon materials catalyze oxidative dehydrogenation of ethyl lactate.
 Active site identification via chemical titration method.
 Structural evolution of carbon catalysts during oxidative dehydrogenation
reactions.
1

 Reaction mechanism for carbon catalyzed liquid-phase reactions revealed via
multiple in situ characterization methods.
Abstract
Pyruvic acid and pyruvate derivatives are important chemical intermediates
which are normally produced using metal-based catalytic materials that could cause
serious environmental problems. In the present work, renewable nanocarbon catalysts
are applied in this system, and a solvent free and highly selective oxidative
dehydrogenation of ethyl lactate (EL) process is achieved to produce ethyl pyruvate
(EP) under mild reaction conditions. Chemical titration results provide direct evidence
that ketonic carbonyl groups on nanocarbon are active sites for this reaction. Raman,
XPS and model catalyst experimental results have shown that there is a dynamic
transformation between defects and oxygen functionalities during the catalytic
reactions. The reaction pathway is revealed via kinetic analysis, which may shed light
on the rational design of carbon catalysts and their potential catalytic applications in
the field of fine chemical productions.
Keywords: Heterogeneous catalysis; Carbon catalysis; Oxidative dehydrogenation;
O-H bond activation; Active site.
1. Introduction
Pyruvic acid and pyruvate derivatives are important chemical intermediates for
the large scale production of wine additives, perfumes, cosmetics, pesticides and
medicines [1-4]. Pyruvic acid is conventionally produced via dehydrative
decarboxylation reaction of tartaric acid [5], and the major disadvantage of this
synthetic route is the requirement of excess KHSO4 as dehydration agent, leading to a
high cost, low atomic efficiency and environmentally detrimental process [5-6].
Oxidative dehydrogenation (ODH) is one of the most promising strategy to replace
the dehydrative decarboxylation method producing lactic acid/ester compounds, such
2

as the selective catalytic conversion of ethyl lactate (EL) to ethyl pyruvate (EP) [7]. In
this process, the reactant EL could be obtained from biomass, which is considered as
renewable green chemicals [8-10]. The reaction does not lose carbon and only
produces EP and H2O as products, exhibiting relatively high atomic efficiency and
sustainability. The ODH pathway has shown great potential for the eco-friendly
production of highly valued pyruvate products using renewable carbohydrate feed
stocks [11].
The current existing catalysts for the ODH of pyruvate are almost all metal-based
compounds, such as mixed oxides (MoO3 etc.), Mo-doped iron phosphate, MoVNbOx
catalysts for gas phase and heteroatom doped-Pd/C for liquid phase pyruvate ODH
processes etc. [2]. One irreconcilable drawback of these metal-based catalysts is the
relatively limited resources and inevitable environmental issues that could not meet
the extremely high requirement for modern green and sustainable chemistry.
Developing novel renewable and efficient catalysts is essentially important in
realizing the industrial potential for pyruvate ODH reactions [12].
Aiming at these challenges, we pay our attention to the emerging nanocarbon
catalysis, where low-cost, earth abundant and renewable nanocarbon materials have
already proven their efficiency for the reactions involving C-H bond activations
(dehydrogenation of aromatic or chain alkanes) [13-14] and O-H bond activation [15].
In this work, we report the first direct use of oxidized multi-wall carbon nanotube
(oCNT) as efficient catalysts for the highly selective synthesis of EP via EL ODH,
showing that nanocarbon is also active in the activation of O-H bonds. The catalytic
reactions were conducted under solvent free and atmospheric pressure conditions,
which is a gentle, green and energy-saving process. Mechanistic studies indicate that
the surface oxygen functional group is responsible for the activity of nanocarbon, and
ketonic carbonyl groups are further identified as the main active sites using chemical
titration method and XPS analysis. Raman spectrum and model catalyst experiments
reveal that surface reconstruction during reaction is also vital to the activity and
stability of the nanocarbon catalysts.
3

2. Experimental section
2.1 Materials
Pristine multiwall carbon nanotubes (pCNT) were supplied by Shandong Dazhan
Nano-materials Co., Ltd (China). Ultra-dispersed diamond (UDD) were purchased
from Sigma-Aldrich. Activated carbon materials were purchased from Sinopharm.
Ethyl lactate (EL), phenylhydrazine (PH), benzoic anhydride (BA) and 2-bromo-1-
phenylethanone (BrPE) was purchased from Alfa Aesar.
2.2 Synthesis of the catalysts
Oxidized CNT (oCNT) was obtained via HNO3 oxidation of pristine multiwall
carbon nanotubes (pCNT). In a typical process, pCNT (1 g) was oxidized in
concentrated HNO3 (68%) under reflux at 120 oC for 2h. The precipitate was filtered
out and washed to neutral with deionized water. The precipitate was dried in vacuum
at 120 oC for 24 h to give black oxidized carbon nanotubes (oCNT). Oxidized carbon
nanotubes with defects (oCNT-d) was prepared by heating oCNT at 925 oC in Ar for 2
h. Graphite oxide and graphene oxide catalyst were kindly supplied by Sinocarbon
Co. (Institute of Coal Chemistry, Chinese Academy of Sciences) and donated as G1,
G2 respectively.
The oxidized onion like carbon (oOLC) was prepared as following procedures.
First, ultra-dispersed diamond (UDD) was heated at 1300 oC in argon atmosphere for
6 h to obtain OLC. Then, OLC (1 g) was collected and refluxed in 100 mL HNO3
(68%) at 120 oC for 2 h. The resulting oxidized OLC, labelled as oOLC, was filtered
and washed with deionized water until the pH of the filtrate reached neutral and then
dried at 120 oC overnight.
2.3 Chemical titration of the active sites
The titration of the ketonic carbonyl groups on oCNTs was performed as
following procedures. Phenylhydrazine (PH, 99%, 2 g) and small amount of HCl acid
(38 %, 100 µL) was dissolved in CHCl3 (100 ml), and then oCNT (1 g) was added
into the solution. After stirring under N2 protection for 72 h, the precipitate was
filtered out and washed with CHCl3 in Soxhlet extractor for 20 h to remove the
4

physical adsorbed PH molecules. The precipitate was dried in vacuum at 60 oC for 24
h to give black carbon nanotube titration derivatives (oCNT(PH)).
The titration of phenol groups on oCNTs was performed as following
procedures. Benzoic anhydride (BA, 98 %, 5 g) and oCNT (1 g) was dissolved in
CHCl3 (50 ml). After stirring under N2 protection at 60 oC for 24 h, the precipitate was
filtered out and washed with CHCl3 to remove the physical adsorbed BA molecules.
The precipitate was dried in vacuum at 60 oC for 24 h to give black carbon nanotube
titration derivatives (oCNT(BA)).
The titration of carboxylic acid groups on oCNTs was performed as following
procedure. 2-bromo-1-phenylethanone (BrPE, 99%, 2 g) and oCNT (1 g) was
dissolved in CHCl3 (50 ml). After stirring under N2 protection at room temperature in
dark for 5 h, the precipitate was filtered out and washed with CHCl3 to remove the
physical adsorbed BrPE molecules. The precipitate was dried in vacuum at 60 oC for
24 h to give black carbon nanotube titration derivatives (oCNT(BrPE)).
Control titration experiments were performed using small molecules of phenol (-
OH), benzoic acid (-COOH) and benzophenone (-C=O), respectively, following the
similar procedure as above (r.t. in 50 mL CHCl3). The molar ratio between the
molecular substrates and the titrants were controlled at 1:50.
2.4 Catalytic reactions
Typically, the catalytic EL ODH reaction was performed as following
procedures. EL (0.59 g, 5 mmol) and catalysts (50 mg) were dispersed in t-
butylhydroperoxide (TBHP, 65%, 2.7 g, 20 mmol) in a flask equipped with a reflux
condenser under atmospheric pressure, and the reaction mixture was stirred at 300
rpm at 90 oC for 6 h. After reaction, the catalyst was filtered out and the filtrates were
diluted to 25 mL for further analysis. The products were analysed using an Agilent
7890 gas chromatography system with HP-5 column and a flame ionization detector
(FID).
2.5 Characterizations
XPS spectra were obtained by using a surface analysis system (ESCALAB 250,
Thermo VG, USA) with Al Kα X-rays (1486.6 eV, 150 W, 50.0 eV pass energy). The
5

O1s XPS spectra were fitted using mixed Gaussian-Lorentzian component profiles (at
a ratio of 80/20) after subtraction of a Shirley background using XPSPEAK41
software. The fitting was performed by fixing the peak position for individual species
within 0.05 eV and applying a full width at half-maximum (FWHM) of 1.2-1.6 eV.
Raman spectroscopies were recorded with a LabRam HR 800 using 532 nm laser.
Transmission electron microscopy (TEM) images were recorded on a FEI Tecnai T12
with an accelerating voltage of 120 kV. Attenuated total reflectance (ATR) FTIR
spectra were recorded using a Varian spectrometer equipped with a liquid-nitrogen
cooled mercury cadmium telluride (MCT) detector. Spectra were recorded at 4 cm-1
resolution in the region of 4000–550 cm-1.
3. Results and discussion
Scheme. 1. EL conversion routes on carbon catalysts.
Firstly, we tested the EL ODH catalytic performance of various carbon materials,
including conventional amorphous activated carbon (powdered activated carbon
(pAC), granular activated carbon (gAC)) and some typical nanocarbon materials
(CNT, Graphite Oxide (G1), Graphene Oxide (G2) and oxidized onion-like carbon
(oOLC)) etc. The key structural parameters, including shape, size, surface area and
oxygen content etc., for these carbon materials are listed in Table S1, and the overall
catalytic performance of these carbon materials in EL ODH reactions are shown in
Table 1. The reaction was performed under very gentle reaction conditions (90 oC,
ambient pressure, without solvent). EP, ethanol and lactic acid are the only detected
products indicating dehydrogenation (EP as product) and hydrolysis (side reaction
6

with ethanol and lactic acid as products) are the main reaction routes under the chosen
conditions as shown in Scheme 1.
The yield of EP is below 4.8 % in the absence of the catalysts at the selectivity of
39.6 (see blank test in Table 1). The introduction of carbon catalysts could obviously
enhance the oxidative dehydrogenation reaction. In general, nanocarbons exhibit
higher activity (EL conversion) than amorphous active carbon (pAC and gAC).
However, only carbon nanotube (CNT) materials exhibit relatively high EP selectivity
(over 90%), while the EP selectivity on graphite oxide and graphene oxide catalysts is
below 15 %. GC analysis shows that the main side reaction is the hydrolysis of EL or
EP. It is generally accepted that graphite oxide related materials contain a lot of
oxygen functional groups, and the majority of them are carboxylic acid groups [16-
18]. Therefore, the poor selectivity of graphite oxide and graphene oxide may be
attributed to the presence of excess acidic carboxylic acid group on their surface
leading to drastic hydrolysis reactions. Although pAC and gAC exhibit acceptable
selectivity to EP (around 98%), their EL conversion is obviously lower than oCNT
catalysts. Among all carbon materials, oCNT and SWCNT exhibit the best
performance. The superiority of SWCNT over oCNT might originate from its larger
specific surface area (425 vs. 219 m2/g). It is worthy to point out that the oOLC
catalysts, which usually exhibit superior activity over CNT in alkane ODH reactions
[19], exhibits poor activity in both selectivity and conversion. Oxygen could also be
used as oxidant for the ODH reactions, and oCNT catalysts exhibit the EP yield at
16.9 % in a fixed bed reactor at 260 oC (2 kPa EL, 1kPa O2, balanced He, ambient
pressure). In summary, Table 1 clearly demonstrates that carbon materials, especially
nanocarbons (such as CNTs), could be used as catalysts for catalytic dehydrogenation
of EL to produce EP.
Table 1. Summary of the activity of carbon catalysts in EL ODH reaction.a
Catalytic performance/%
Sample
Conversionb
Selectivityc
Yieldc
7

Blank
oCNT
12.1
39.0
39.6
93.6
4.8
36.5
oCNTd
SWCNT
oOLC
G1
47.0
41.1
25.5
43.6
67.4
29.5
23.4
36.0
98.5
60.6
13.6
6.9
16.9
40.5
15.5
5.9
G2
4.7
pAC
98.5
98.5
29.1
23.1
gAC
a, reaction condition: 0.59g EL, 2.7g TBHP, 50 mg carbon catalyst, 90 oC, 6h;
b, conversion of EL;
c, selectivity and yield of EP
d, O2 as oxidant in fixed bed reactor, reaction conditions: 2 kPa EL, 1kPa O2, balanced He,
100 mg oCNT, 260 oC.
The reusability test (as shown in Fig. S1) shows that the activity (EL conversion)
and selectivity (for EP) of oCNT catalysts are well maintained in the second run, but
the recovered activated carbon (both pAC and gAC) catalysts exhibit obvious
conversion and selectivity drop during the reuse process. The main reason for the
distinct reusability of these two types of carbon catalysts should be attributed to their
different degree of order, namely the conjugation size of carbon skeleton. Activated
carbon catalysts may lose active sites after filtration, since the small carbon fragments
with less conjugation size may leach into liquid phase [20]. On the contrast, CNT
catalysts with relatively high degree of order (well-organized sp2 conjugated graphitic
carbon framework) ensure their structural stability and superior reusability in the
reaction. In general, oCNT is chosen as the best candidate carbon catalyst for EL
ODH reaction, which exhibits favourable activity and reusability over traditional
activated carbon materials.
8

Fig. 1. EP yield as a function of oxygen content in oCNT-d (triangle), pCNT (square)
and oCNT (diamond).
Understandings on active sites is the foundation for the mechanistic studies on
catalytic reactions. Oxygen functionalities play a vital role in carbon catalysed redox
reactions [21], which is firmly confirmed in gas-phase alkane ODH reactions, such as
ODH of ethylbenzene and propane etc. [22-24]. To examine the role of oxygen
functional groups in catalysis, we prepared three CNT samples that have similar
structure but different surface oxygen content (denoted as oCNT, pCNT and oCNT-d
with oxygen content at 10.1, 2.0 and 0.6 %, respectively, as shown in Table S1). As
shown in Fig. 1, the EL ODH catalytic activity of the CNT samples increases with
surface oxygen content, indicating that surface oxygen functionalities may be
responsible for the catalytic activity, but the non-linear dependence suggest that
maybe not all of the oxygen groups are active on EL ODH reactions. Identification of
the active sites remain full of challenges because of the complexity of nanocarbon
surface functionalities, such as the coexistence of ketonic carbonyl, carboxyl and
hydroxyl groups etc.
9

Fig. 2. (a)Titration process for (i) ketonic carbonyl, (ii) phenol and (iii) carboxylic
acid groups on nanocarbon; (b) EL ODH conversion, EP selectivity and yield on
oCNT, oCNT titration derivatives and CNTs with only defects.
To identify and quantify the active sites for the reaction, we have proposed a
chemical titration technique [25]. As shown in Fig. 2a, phenylhydrazine (PH), benzoic
anhydride (BA) and 2-bromo-1-phenylethanone (BrPE) are applied as titrants. Control
experiments using small organic substrates (benzophenone (-C=O), phenol (-OH), and
benzoic acid (-COOH)) indicated that the titrants could selectively react with the
ketonic carbonyl, hydroxyl and carboxylic groups on oCNT samples, respectively, and
these three types of groups involve over 90 % of oxygen functionalities in total [26-
27]. For example, PH molecules would react with ketonic carbonyl groups on
nanocarbon forming hydrazone compounds at a yield over 99 % [25]. The role of
specific oxygen functionalities could be revealed via comparing the catalytic
performance of these titration derivatives (oCNT(PH), oCNT(BA) and oCNT(BrPE)).
As shown in Fig. 2b, the EL ODH catalytic activity of oCNT(PH) is much lower
(80 % lower) than that of oCNT, while oCNT(BA) and oCNT(BrPE) exhibit almost
identical activity as oCNT, providing firm chemical evidence that ketonic carbonyl
group is the active site for EL ODH reactions. It needs to be mentioned that the
selectivity of oCNT(BA), oCNT(BrPE) towards EP are both over 95%, higher than
that of oCNT (93.6%), indicating that the removal of hydroxyl or carboxylic acid
groups could inhibit the side reactions, although they have shown negligible influence
10

on ODH activity.
Fig. 3. TEM images of (a) oCNT, (b) pCNT, (c) used oCNT and (d) used pCNT.
The morphology, chemical composition and structure of nanocarbon catalysts
before and after ODH reactions were revealed via TEM, XPS and Raman
measurements. As shown in Fig. 3, TEM images of fresh and used CNT catalysts
demonstrate that there is no obvious change in the general structure of carbon
catalysts, suggesting that the bulk sp2 structure is well-maintained during catalysis.
However, the chemical composition of carbon catalysts may change under redox
reaction conditions. For example, we observed that the oxygen content of untreated
pristine CNT (pCNT) catalysts would increase (from 2.0 to 2.7 % as shown in Table
2) after one catalytic cycle, and especially the surface content of ketonic carbonyl
groups doubled (from 0.3 to 0.7 %) during this process. This is also the possible
reason why pCNT exhibits similar EL ODH catalytic activity as oCNT (EL
conversion at 33 vs. 39 %), since they have similar surface content of ketonic
carbonyl groups (0.7 vs. 1.0 %) after reaction, which is consistent with titration results
that ketonic carbonyl groups are the active sites for EL ODH reactions.
11

Table 2. Summary of Raman and XPS analysis results on fresh and used CNT
catalysts.a
Raman peak percentage (%)
Element content from XPS (%)
Sample
1200 1350 1500 1580 1620
ID/IG
C
O
-C=Ob
(D4)
(D1)
(D3)
(G)
(D2)
pCNT
9.5
47.8
44.6
44.8
47.2
6.1
3.7
7.0
6.3
29.1
30.4
28.2
27.8
7.5
5.7
6.5
6.5
1.6
1.5
1.6
1.7
98.0
97.3
96.2
95.8
2.0
2.7
3.8
3.6
0.3
0.7
1.0
1.0
used-pCNT 15.6
oCNT 13.5
used-oCNT 12.1
a reaction condition: 0.59g EL, 2.7g TBHP, 50 mg carbon catalyst, 90 oC, 6h;
b surface content of ketonic carbonyl groups from deconvolution of O 1s XPS (as shown in Fig. S2).
Raman spectroscopy is an effective method to reveal the surface structure change
of nanocarbon materials. The overall shapes of Raman spectra for fresh and used CNT
catalysts are similar, which is consistent with the TEM measurements that the
structural integrity of the CNTs is well reserved during the reaction. The Raman
spectrum for carbon materials could be deconvoluted into five peaks corresponding to
ideal graphitic lattice (G, ~1580 cm-1), disordered graphitic lattice (graphene layer
edges, D1, ~1350 cm-1), disordered graphitic lattice (surface graphene layers, D2,
~1620 cm-1), amorphous carbon (D3, ~1500 cm-1) and disordered graphitic lattice
(polyenes, ionic impurities, D4, ~1200 cm-1) [28]. ID/IG is defined as the intensity ratio
of D1 and G band, which is normally used to quantify the surface content of defects
(namely the degree of graphitization) for carbon materials [29-30]. As shown in Table
2 (summary of the Raman peak and XPS deconvolution results), the value of ID/IG for
pCNT slightly decreases (1.6 to 1.5), and the oxygen content increases (2.0 % to
2.7 %), indicating that there may be a transformation from defects to oxygen
functionalities (mainly ketonic carbonyl groups) during the catalytic reactions. On the
contrast, oCNT catalysts exhibit the opposite trend (ID/IG increases from 1.6 to 1.7,
oxygen content decreases from 3.8 % to 3.6 % after reaction) suggesting the possible
12

transformation from oxygen functionalities to defects. This phenomenon clearly
shows the dynamic change of the carbon surface structure, namely the transformation
between defects and various oxygen functionalities, during the catalytic reaction
process. This surface reconstruction phenomenon is also observed in carbon catalyzed
gas phase reactions [19], and it has significant influence on the activity and stability
of carbon catalytic materials, which also emphasizes the importance for considering
the chemical structure of carbon catalysts under reaction conditions.
Fig. 4. EL ODH activity (EP yield) on pentacene, phenanthrene, phenanthrenequinone
and oCNT.
Small organic molecules are also used as model catalysts to testify the hypothesis
on the active sites and the dynamic structure change during catalysis. As shown in
Fig. 4, phenanthrene, pentacene and phenanthrenequinone, presenting armchair and
zigzag type of defect and ketonic carbonyl groups respectively, are chosen as model
catalysts running EL ODH under the same reaction conditions as CNT materials. As
shown in Fig. 4, pentacene have shown much higher activity than phenanthrene
catalysts (conversion at 9.3 % vs. 0.6 %) in EL ODH reactions, indicating that Zigzag
type defect would be more active than armchair. Attenuated Total Reflection Fourier
Transform IR (ATR-FTIR) spectrum of the used pentacene catalysts showed that
ketonic carbonyl groups are in situ generated on the molecules during the catalytic
reactions (1680 cm-1, stretching vibration band of C=O structure, Fig. S3). The
13

observation indicates that Zigzag defect could be converted into ketonic carbonyl
groups during the catalytic reaction, which is consistent with the experimental
observations on pCNT catalysts that the surface content of ketonic carbonyl groups
increased after EL ODH reactions. It seems abnormal that phenanthrenequinone
compounds exhibits relatively low ODH activity, since they contain abundant ketonic
carbonyl groups in their structure. Actually, ATR-IR measurements on fresh and used
phenanthrenequinone samples exhibit that most of the ketonic carbonyl groups
decomposed after catalytic EL ODH reaction (C=O stretching vibration band at 1680
cm−1 dramatically shrinks as shown in Fig. S3). The observation suggests that
unstable C=O groups would be decomposed during the catalytic reactions, which is
just similar as that the oxygen content on oCNT catalysts decreases after ODH
reactions. The catalytic activity test and structure analysis on model small-molecular
catalysts have proven that ketonic carbonyl groups serve as the active sites for EL
ODH reactions, and there is a transformation between defects (mainly Zigzag type)
and ketonic carbonyl groups. This transformation depends on the initial structure of
the carbon catalysts and also the employed reaction conditions. Comparing with
nanocarbon, the lower catalytic activity of small organic molecules might be related
to their less conjugation size. The relatively large conjugated sp2 structure of
nanocarbon ensure the good catalytic activity and stability in ODH reactions.
(a)
(b)
100
75
50
25
0
8
6
4
2
oCNT E =119.1 kJ/mol
a
pAC E =161.3 kJ/mol
a
gAC E =185.9 kJ/mol
a
0
0
2
4
6
8
10 12 14
2.72
2.76
2.80
1000/T
2.84
2.88
Time (h)
Fig. 5. (a) EP yield as a function of reaction time for EL ODH reaction under the
catalysis of oCNT. (b) Apparent activation energy for oCNT, pAC and gAC according
to Arrhenius equation in EL ODH reaction. Reaction condition: 0.59g EL, 2.7g TBHP,
50 mg carbon catalyst, 90 oC.
14

The EL ODH reactions follow pseudo-first order reaction kinetics under the
catalysis of oCNTs as shown in Fig. 5a, and the measured first order rate constant k
equals to 0.058 S-1. The activation energy could be derived from the dependence of
ODH reaction rates on reaction temperature via Arrhenius equation as shown in Fig.
5b. oCNT catalysts exhibit lower activation energy than activated carbon materials
(119.1 vs. 161.3 and 185.9 kJ/mol), indicating the long ranged ordered graphitic
structure may benefit the ODH reactivity [21], which is consistent with the
observations on model organic molecular catalysts.
EP
TBHP
nanocarbon
TBA
EL
Scheme 2. Proposed reaction pathway for EL ODH on nanocarbon catalysts (the
backbone of nanocarbon is simplified as four benzene rings).
Combing the active site titration, structural evolution measurements, model
catalysts and kinetic analysis results, we propose a reaction mechanism for carbon
catalyzed EL ODH reactions as shown in Scheme 2. The reactants EL molecules may
first adsorb on ketonic carbonyl groups on carbon surface. The products EP form and
desorb after the sequential hydrogen abstraction processes, and the carbon catalysts
get reduced (forming hydroxyl groups) during this process. Finally, the reduced
carbon catalysts are re-oxidized by TBHP to complete the reaction cycle. In this step,
some unstable hydroxyl groups could be decomposed forming defects or disordered
carbon atoms. It should be mentioned that Scheme 2 only summarizes the possible
reaction pathway but not the exact reaction mechanism for carbon catalyzed EL ODH
process under molecular level. The details for the reaction mechanism (such as the
activation of molecular oxygen etc.) could be achieved via in situ spectroscopic
15

analysis.
4. Conclusions
In summary, a solvent free, highly selective ODH conversion route from EL to
EP has been developed via green non-metallic nanocarbon catalysis. oCNT exhibit
higher selectivity than graphene oxides and onion like carbon catalysts and better
reusability than traditional activated carbon materials. Chemical titrations provide
direct chemical evidence that ketonic carbonyl groups serve as the active sites for this
reaction. Raman, XPS and model catalyst experimental results have shown that there
is a dynamic transformation between defects and oxygen functionalities during the
catalytic reactions. The direction of this transformation depends on the initial structure
of the carbon catalysts and also the reaction conditions. Kinetic analysis shows that
carbon catalyzed EL ODH is a pseudo first order reaction with an activation energy at
about 119.1 kJ/mol. The relative high activity of oCNTs provides us a potential
alternative metal-free system for the large scale production of ethyl pyruvate, which
might be enlightened for related industry to meet the requirements of green and
sustainable chemistry.
Acknowledgements
The authors acknowledge financial support from the NSFC of China
(21761132010, 91645114, and 21573256) and the Youth Innovation Promotion
Association, CAS.
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