Oxidation of Glycerol to Dicarboxylic Acids Using Cobalt Catalysts

Article abstract of DOI:10.1021/acscatal.6b00961

In this paper, the performance of cobalt-based catalysts was reported for oxidation of glycerol to dicarboxylic acids such as tartronic and oxalic acids. Cobalt catalysts supported on Mg₃Al(OH)_y(CO₃)_z structures prepared by a two-step modified sol-gel method showed 100% glycerol conversion with 64% and 24% selectivity toward tartronic and oxalic acids under mild conditions (55-70 °C and 0.1 MPa O₂). Surface and bulk characterization by N₂ adsorption/desorption, X-ray diffraction, and temperature-programmed reduction reveals that the cobalt sites interacting with surface hydroxides are catalytically more active than those incorporated into a framework leading to selective glycerol oxidation to dicarboxylic acids in one pot. On the basis of the experiments at different cobalt contents, temperatures, and concentration-time profiles, possible reaction pathways are discussed to explain the selectivity profile. Deactivation of the catalyst under certain conditions has been discussed as a result of loss of surface area due to structural changes.

Full text of DOI:10.1021/acscatal.6b00961

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN
Letter
Oxidation of Glycerol to Dicarboxylic Acids Using Cobalt Catalysts
Xin Jin, Meng Zhao, Chun Zeng, Wenjuan Yan, Ziwei Song, Prem
S. Thapa, Bala Subramaniam, and Raghunath V. Chaudhari
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00961 • Publication Date (Web): 07 Jun 2016
Downloaded from http://pubs.acs.org on June 8, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 30
ACS Catalysis
1
2
3
4
5
6
7
Oxidation of Glycerol to Dicarboxylic Acids Using Cobalt
Catalysts
8
9
Xin Jin,1$# Meng Zhao,1$ Chun Zeng1, Wenjuan Yan,1,2 Ziwei Song,1,2 Prem S. Thapa,3
Bala Subramaniam,1,2 Raghunath V. Chaudhari1,2*
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1Center for Environmentally Beneficial Catalysis, University of Kansas, 1501 Wakarusa
Drive, Lawrence, Kansas 66047, USA
2Department of Chemical and Petroleum Engineering, University of Kansas, 1530 W 15th
St., Lawrence, Kansas 66045, USA
3Microscopy and Analytical Imaging Laboratory, Haworth Hall, 1200 Sunnyside Ave,
University of Kansas, Lawrence, Kansas 66045, USA
$Equally contributing authors
#Present address: State Key Laboratory for Heavy Oil Processing, College of Chemical
Engineering, China University of Petroleum, Qingdao, Shandong 266580, China
*Corresponding author: rvc1948@ku.edu
1
ACS Paragon Plus Environment

ACS Catalysis
Page 2 of 30
1
2
3
4
5
6
Abstract
7
8
9
In this paper, the performance of cobalt based catalysts was reported for oxidation of
glycerol to dicarboxylic acids such as tartronic and oxalic acids. Cobalt catalysts
supported on Mg3Al(OH)y(CO3)z structures prepared by a two-step modified sol-gel
method showed 100% glycerol conversion with 64% and 24% selectivity towards
tartronic and oxalic acids under mild conditions (55–70 °C and 0.1 MPa O2). Surface and
bulk characterization by N2 adsorption/desorption, X-ray diffraction and temperature
programmed reduction reveals that the cobalt sites interacting with surface hydroxides
are catalytically more active than those incorporated in framework leading to selective
glycerol oxidation to dicarboxylic acids in one pot. Based on the experiments at different
cobalt contents, temperatures and concentration-time profiles, possible reaction pathways
are discussed to explain the selectivity profile. Deactivation of catalyst under certain
conditions has been discussed as a result of loss of surface area due to structural changes.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Key words: Oxidation, Dicarboxylic Acids, Cobalt Catalyst, Biomass, Layered Double
Hydroxide, Sol-gel
Aqueous phase oxidation (APO) of bio-derived polyols and sugars is one of the most
important routes to upgrade renewable biomass to value-added chemicals, particularly for
synthesis of carboxylic acids.1 Dicarboxylic acids (DCA), including glucaric acid,
tartronic acid and oxalic acid, derived from biomass, provide major building blocks for
everyday products, such as renewable polymers, nylon fibers and BPA-free plastics.
These are considered to be among the most promising intermediates in the fledgling
renewable chemicals industry, with a potential to substantially replace fossil-derived
2
ACS Paragon Plus Environment

Page 3 of 30
ACS Catalysis
1
2
olefins and carboxylic acids.1, 2 More importantly, oxidation of biomass to carboxylic
acids is feasible at mild conditions (55–70 °C and 0.1 MPa O2) using relatively
inexpensive oxygen to add value to the feedstocks with a net gain in content, showing
both environmental and economic advantages over conventional hydrodeoxygenation
technologies.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In this context, tartronic acid (TAR, a C3 DCA) has been widely used as a versatile
intermediate in polymer and pharmaceutical applications.2-5 While TAR production from
stoichiometric oxidation of sugars and polyols (e.g. glucose, glycerol) by mineral
acids/bases often suffers from low selectivity (~ 11%) and large amounts of toxic by-
7
products (NO2, NaBr),6,
catalytic oxidation using solid metal catalysts at low
temperatures provides a clean atom economical route eliminating the hazards and safety
issues.
During the past decade, several reports on TAR synthesis from glycerol (GLY) using
supported metal catalysts have appeared in the literature, albeit with limited success.
During GLY oxidation, glyceric acid (GLYA) is formed as the primary product, which
undergoes further oxidation to TAR (Scheme 1), where oxalic acid (OXA, a C2 DCA),
lactic acid and glycolic acid are often generated as co-products.4, 8-10 GLY oxidation to
GLYA is easily achievable on various noble metal catalysts. Pt, Pd and Au based
catalysts supported on activated carbons, TiO2 and graphite have been extensively studied
for GLY oxidation. These noble catalysts are found to display complete conversion of
GLY even under mild conditions (T: 50–70 oC, 0.1–0.7 MPa O2). GLYA is the dominant
product (S: 35–99%) with lactic, glycolic acids being major co-products (S: 19–33%). In
most studies, TAR selectivity is however very low (S < 22%).10-13 Recently, optimization
3
ACS Paragon Plus Environment

ACS Catalysis
Page 4 of 30
1
2
of reaction conditions on Au/HY2 and PdBi/C14 have shown good selectivity (Au/HY:
82%, PdBi/C: 93%) towards TAR but poor oxidation activity (Au/HY: 40 h-1, PdBi/C:
0.96 h-1). Although it is observed that catalyst supports may affect selectivity during
oxidation reactions due to tunable metal-support interaction, limited improvement on
TAR selectivity has been achieved due to poor secondary oxidation reactions on noble
metal catalysts.15-17 Current research efforts have been primarily focused in two
directions: (1) designing well-defined bimetallic noble metal catalysts and (2)
incorporating earth abundant metals to noble metal systems. In the former area,
synergistic bimetallic PtPd,18 AuPd,5, 13, 19 AuPt4, 13, 16 catalysts have been proposed in
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
o
recent publications (T: 60–90 C, PO2: 0.1–1 MPa). C-C cleavage reactions are however
so significant that glycolic acid, formic acid and CO2 selectivity is unfavorably enhanced
(S: 13–46%).19 This is attributed to (a) the strong adsorption of aldehyde (–C=O) and
carboxylic (–C=OOH) groups in products on noble metal surface that often deactivates
the catalysts10, 13, 18, 19 and (b) strong tendency for decarbonylation or decarboxylation
reactions instead of the desirable secondary oxidation to form TAR and OXA.17 On the
other hand, non-noble metals are believed to be able to tune the binding strength between
functional groups and noble metals.20, 21 Therefore, in a parallel study, research efforts
have been made to incorporate trace amounts of non-noble metal promoters such as Bi,
Cu, Ni to noble metal systems.22 These noble/non-noble metal combinations are found to
effectively enhance Au, Pt and Pd catalyst activity as well as selectivity, by preventing C-
C cleavage of GLY molecules. GLYA and lactic acid are the major products (combined S:
85–99%) while TAR selectivity on these catalysts is still poor (S ~ 32%).
4
ACS Paragon Plus Environment

Page 5 of 30
ACS Catalysis
1
2
3
4
5
6
7
8
9
High costs of noble metal catalysts and limited availability often pose significant
economic challenges in practical applications of these catalysts. Replacing expensive
noble metals with cost effective non-noble metal catalysts without loss of performance
will clearly make the APO of biomass economically sound and more attractive. To the
best of our knowledge, there has been no known report on the selective oxidation of GLY
particularly to valuable dicarboxylic acids (DCAs), using non-noble metal catalysts.
Therefore, in this paper, we report the performances of Co-based catalysts for selective
GLY oxidation to TAR and OXA at very mild conditions (55–70 °C, 0.1 MPa O2
pressure).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Supported Co catalysts on MgO-Al2O3 oxides were prepared via co-precipitation and
modified sol-gel methods. The effects of Co content, catalyst preparation conditions and
reaction temperature on catalyst activity and selectivity were investigated systematically
for GLY oxidation. Concentration-time profiles on selected Co catalysts were collected
and discussed to understand possible reaction pathways for TAR formation. These results
were further interpreted based on complementary surface and bulk characterization of the
catalysts using N2 adsorption/desorption measurement (BET), X-ray diffraction (XRD),
temperature programmed reduction and desorption (H2-TPR, CO2-TPD), transmission
electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray
photoelectron spectroscopy (XPS) techniques. It is found that the interaction between Co
species and MgO-Al2O3 support is the key to tune catalyst activity. The information
obtained from this study will provide insights into the design of active and selective non-
noble metal catalysts for the oxidation of biomass to value-added carboxylic acid
products.
5
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 30
1
2
3
4
5
6
7
8
9
Two methods were employed for Co catalyst preparation: (i) In co-precipitation method,
a solution of Co, Mg and Al nitrates prepared with predetermined Co/Mg/Al molar ratio
of x/3/1 (x = 0.15, 0.30), was precipitated by 0.25 kmol/m3 of NaOH and 0.8 kmol/m3 of
Na2CO3 solution in a co-current fashion. (ii) In modified sol-gel method, a solution
containing only Mg and Al nitrates was precipitated by the same NaOH and Na2CO3
solution at room temperature. After 12 h of aging, an aqueous solution of Co nitrate was
added to the gel slowly and the slurry was aged for another 12 h. The catalysts obtained
from the two methods were calcined and activated under H2 atmosphere. The resultant
materials are denoted as Cox/Mg3Al-c and Cox/Mg3Al-s respectively (refer supporting
information for details).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Benchmark results. Two catalyst samples, Co0.15/Mg3Al-c and Co0.15/Mg3Al-s were
chosen for characterization and oxidation tests in benchmark studies (see Figure 1). In
particular, it is found that Co0.15/Mg3Al-c catalyst from co-precipitation method results in
relatively uniform distribution of Co, Mg and Al element in the catalyst sample [see
Figure 1 (a) for bulk (TEM and ICP) and surface (SEM) element composition]. This is
because Co species were incorporated into Mg3Al(OH)y(CO3)z framework during
precipitation, thus double-layered structures with uniformly incorporated –O–Co–O–
groups were achieved. In contrast, for Co0.15/Mg3Al-c catalyst obtained from modified
sol-gel method, elemental analysis shows that Co tends to predominantly disperse on the
surface of the support [see Figure 1 (b)]. It is believed that Co2+ species replaced the
surface –OH groups of Mg3Al(OH)y(CO3)z structure and formed surface M–O–Co–O–M
(M: Mg or Al) framework during modified sol-gel process.
6
ACS Paragon Plus Environment

Page 7 of 30
ACS Catalysis
1
2
Oxidation of GLY (0.22 kmol/m3 in 25 mL aqueous solution) was then carried out on
Co0.15/Mg3Al-c and Co0.15/Mg3Al-s catalysts (0.2 g) using molecular O2 as the oxidant
(refer supporting information for experimental details). The yields to major products such
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
o
as GLYA, TAR and OXA (70 C, 24 h reaction time) on the two catalysts are shown in
see Figure 1 (c). It is observed that the yields of TAR and OXA on Co0.15/Mg3Al-s
o
catalyst are 63.5% and 24.0% respectively at 70 C after 24 h reaction time, which is
significantly higher than that on Co0.15/Mg3Al-s catalyst [46.9% and 13.4%, see Figure 1
(c)]. The preliminary results obtained from oxidation tests and characterization (using
TEM, ICP and SEM) suggest that a modified sol-gel method generates a different Co
catalyst (Co0.15/Mg3Al-s) which provides more accessible sites with low Co coordination
numbers on catalyst surface, in comparison with co-precipitation method.
Catalyst characterization. The promising results obtained in these initial benchmark
experiments motivated us to further undertake a systematic study on Co catalysts. We
first compared the results of H2-TPR and N2 isotherms for Co0.15/Mg3Al-s and
Co0.15/Mg3Al-c catalysts used in the benchmark study (Figure 2, refer Figure S1 for H2-
TPR and N2 isotherms of other catalysts). As seen from Figure 2 (a), we observed that
Co0.15/Mg3Al-s (green) displays a reduction peak at 200–310 °C while in sharp contrast,
Co0.15/Mg3Al-c (red) exhibits a significant shift of the reduction peak to 290–450 °C.
This characterization indicates that the latter catalyst with Co sites in framework tends to
interact strongly with Mg3Al(OH)y(CO3)z structure, thus H2 needs to overcome much
higher barrier to reduce oxide species in the catalyst. Surface Co sites, however, are
possibly coordinatively unsaturated and more catalytically active, which can be easily
reduced. Furthermore, N2 isotherm data show that Co0.15/Mg3Al-s catalyst [Figure 2 (b)]
7
ACS Paragon Plus Environment

ACS Catalysis
Page 8 of 30
1
2
from modified sol-gel method has much higher surface area (536.3 m2/g) than
Co0.15/Mg3Al-c [Figure 2 (c), 35.8 m2/g] obtained from the co-precipitation method. In
addition, pore size and distribution analysis shows that Co0.15/Mg3Al-s catalyst has
relatively larger pore size compared with Co0.15/Mg3Al-c. It is plausible that, in modified
sol-gel method, the addition of Co species separated existing Mg3Al(OH)y(CO3)z layers in
the catalyst, replaced surface –OH groups, and peeled off the layered structures via
electrostatic repulsion between surface Co sites on surface.23, 24 Larger pores were thus
formed and more surface area exposed. But this is not true in co-precipitation method,
where all Co, Mg and Al species formed layered structures with –OH groups on the
surface, which formed compact layered structures by removal of water between layers
during drying process (condensation).25, 26 Therefore less surface area and Co sites are
exposed for catalytic reactions.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
More detailed catalyst characterization such as TEM (Figure 3), XRD (Figure 4), SEM
(Figure S2) and CO2–TPD (Figure S3) were further carried out for all the catalyst
samples, including Co0.15/Mg3Al-c, Co0.30/Mg3Al-c, Co0.15/Mg3Al-s and Co0.30/Mg3Al-s.
Specifically, TEM images of Co0.15/Mg3Al-c [Figure 3 (a)] and Co0.30/Mg3Al-c [Figure 3
(b)] catalysts reveal the nature of their surface morphologies from co-precipitation
method. We find that neither catalyst sample exhibits detectable Co nanoparticles, while
both of them exhibit several thin layers with folded structures. EDX mapping in Figures 3
(a) and (b) clearly shows that Co, Mg and Al elements are well dispersed in the samples.
This information confirms the uniform incorporation of Co species in Mg3Al(OH)y(CO3)z
framework during catalyst preparation. Co0.15/Mg3Al-s and Co0.30/Mg3Al-s catalysts [see
TEM images in Figures 3 (c) and (d)], prepared by modified sol-gel method, display
8
ACS Paragon Plus Environment

Page 9 of 30
ACS Catalysis
1
2
3
4
5
6
7
8
9
different surface morphologies compared with Figures 3 (a) and (b). This observation
implies that the addition of Co species to the Mg3Al(OH)y(CO3)z gel clearly influences
the surface morphology of the final catalyst sample.25, 26 EDX analysis of Co0.15/Mg3Al-s
and Co0.30/Mg3Al-s catalysts [Figures 3 (c) and (d)] also confirms the uniform
distribution of Co element in the bulk of catalyst. SEM images for these two catalysts in
Figure S2 also support this attribute. While both co-precipitation and modified sol-gel
methods give Co catalysts with good element dispersion, it is found from TEM images
[Figures 3 (a) to (d)] that, the observed average crystal/bulk sizes for all samples are in
the range of 3–4 nm. In addition, EDX bulk analysis (from TEM) and ICP measurement
further reveal that the actual Co/Mg/Al molar ratios in Co0.15/Mg3Al-c, Co0.30/Mg3Al-c
and Co0.15/Mg3Al-s catalysts are approximately 1.89/30.55/9.37, 3.76/24.78/8.90 and
1.49/29.6/10.5, respectively, which are consistent with the amounts of metal precursor
added during catalyst preparation.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
STEM images and element mapping of used Co0.15/Mg3Al-s catalyst after reaction are
also shown in Figure 3 (e). A discernable morphological change is found in this sample
when compared to fresh Co0.15/Mg3Al-s catalyst. We find that several thin layered species,
peeled off from bulky phases tend to form more folded structures. Element mapping of
two selected regions [shown in Figure 3 (e)] confirms that there is still no Co
agglomeration in the used catalyst sample.
To further illustrate the catalytically active species of all Co catalyst samples, XRD
powder analysis was conducted (Figure 4), the results of which were also compared with
the information obtained from TEM and XPS characterization. In particular, powder
diffraction patterns of Co0.15/Mg3Al-c (a, red), Co0.30/Mg3Al-c (b, blue), Co0.15/Mg3Al-s
9
ACS Paragon Plus Environment

ACS Catalysis
Page 10 of 30
1
2
3
4
5
6
7
8
9
(c, green) and Co0.30/Mg3Al-s (d, orange) samples are shown in Figure 4. It is important
to mention that XRD patterns in Figure 4 are very similar to the characteristic peaks for
layered doubled hydroxide.24-26 But we also observe additional peaks for Co species, the
intensity of which is low due to the small quantities of Co species existing in catalyst
samples. It is necessary to mention that some diffraction peaks for Co and hydrotalcite
peaks may overlap, therefore conclusions were made based on the observation of at least
two characteristic peaks for certain crystal species. For the four solid samples, we
observe characteristic peaks for Co species at 47°, 60°, 62° and 75°.24 Compared to
Co0.15/Mg3Al-c and Co0.30/Mg3Al-c, Co0.15/Mg3Al-s and Co0.30/Mg3Al-s catalysts display
relatively wider and lower weak peaks at 44°, 47°, 60°, 62° and 75°, suggesting that the
crystalline size of Co species is small on this sample. In particular, peaks at 47°, 60° and
62° indicate Co species might exist in the form of CoO while less intensive peaks at 44°
and 75° suggest that Co is also presented but in very small quantities (The existence of
CoO is also confirmed by XPS characterization shown in Figure S6). For MgO and Al2O3
phases, sharp peaks in all samples at 10° and 22° confirm the existence of [003] and [006]
crystals of hydrotalcite (Mg3AlO4.5) structure.26 We have the following findings from
XRD patterns. (i) A wide peak at 16° indicates that the spinel MgAl2O4 structure (with
[111] surface plane) exists in Co0.15/Mg3Al-s [Figure 4 (c)] and Co0.30/Mg3Al-s [Figure 4
(d)] catalysts, while this peak is not present in other two catalyst samples. Interestingly,
we only observe low indexed [111] facet of spinel MgAl2O4 structure, while high index
surfaces at 36° (assigned to [311] facet of spinel structure, possibly overlapped with CoO
species), 44° (assigned to [400]) and 65° (assigned [440] facet) are very weak.27 This
observation further confirm the thin layered structures of Co0.15/Mg3Al-s and
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
10
ACS Paragon Plus Environment

Page 11 of 30
ACS Catalysis
1
2
3
4
5
6
7
8
9
Co0.30/Mg3Al-s catalysts, where complex high surface index structures with several layers
are insignificant. (ii) Information from XRD patterns was also used to calculate the
approximate crystal/bulk sizes (see detailed calculation in supporting information). The
calculation is found to be well consistent with crystal sizes observed from TEM images
[see discussion for Figures 3 (a) to (d)]. In particular, it is found that calculated crystal
sizes for CoO and hydrotalcite structures for both Co0.15/Mg3Al-c and Co0.30/Mg3Al-c
catalysts are 3 nm and 4.9 nm. However, for Co0.15/Mg3Al-s and Co0.30/Mg3Al-s catalysts,
CoO particle sizes display negligible changes (~ 4 nm), while the mean particle size for
hydrotalcite structure are small (~ 3 nm). The high intensity of spinel structure peaks
observed on Co0.15/Mg3Al-s and Co0.30/Mg3Al-s catalysts suggest that thin layered
structures are dominant in these two samples. (iii) In addition, the 57° peak in
Co0.30/Mg3Al-s sample suggests the formation of thin layered Mg3AlO4.5 [1013] structure
with 8.98 Å spacing,25 suggesting good stability of layered structures after calcination
and activation.28-30 The following conclusions were made based on the findings discussed
above. (I) It is plausible that the addition of Co species after layered double hydroxide
structures form prevent condensation among hydroxide layers thus thin layered structures
are retained. (II) The immobilization of Co species after the Mg3Al(OH)y(CO3)z
structures were formed often results in the replacement of surface –OH groups with M–
O–Co–O–M network. (III) The formation of such structures can lead to enhanced surface
sites exposed for catalytic reactions.23 (IV) CoO species should be the major active sites
for oxidation reactions. The information obtained from XRD agrees very well with the
observation from surface area analysis, H2-TPR and TEM images. CO2-TPD
characterization was also conducted to measure the basicity on different Co catalysts, the
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
11
ACS Paragon Plus Environment

ACS Catalysis
Page 12 of 30
1
2
3
4
5
6
7
8
9
results of which are shown in Figure S3. In general, we find all samples exhibit strong
CO2–TPD signals below 500 °C, indicating that basic sites with weak (~ 150 °C) or
medium (~ 380 °C) strength are dominant on these catalysts. Additionally, XPS
characterization data was conducted to further confirm oxidation states of Co species and
nature of active sites (see Figure S6 and interpretation in supporting information). Two
observations were found from XPS characterization: (i) Co species mainly exist in the
form of CoO; (ii) Excitation of electrons of core holes of Co element might be the key for
enhanced catalytic activity.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Oxidation reaction results. We studied the GLY oxidation on Co0.15/Mg3Al-c,
Co0.30/Mg3Al-c, Co0.15/Mg3Al-s and Co0.30/Mg3Al-s catalysts. It is found that GLY
conversion without a catalyst is only 4.3% after 6 h and 13% after 24 h (at 70 oC, 0.1 O2,
entries no. 1 and 2 in Table 1). In addition, the selectivity towards TAR is below 29%
after 24 h without catalysts. At 70 °C, Co0.15/Mg3Al-c catalyst showed 16% conversion
within 6 h reaction time (entry no. 3 in Table 1), the selectivity towards GLYA, TAR and
OXA being 58%, 32% and 4% respectively with almost complete material balance based
on liquid phase products. When more Co is present, Co0.30/Mg3Al-c catalyst (entry no. 5)
exhibited higher conversion (22%) and 53%, 38% and 3% selectivities towards these
acids (GLYA, TAR and OXA) at the end of the 6 h run. Other carboxylic acids, including
lactic, glycolic and formic acids accounted for approximately 2–5% of the products. As
seen in entry no. 7, GLY conversion on Co0.15/Mg3Al-s catalyst is 47%, much higher than
Co0.15/Mg3Al-c, although selectivity to GLYA, TAR and OXA is still comparable with
the previous two catalysts. Entry no. 9 shows that Co0.30/Mg3Al-s display slightly higher
TAR and OXA yield compared with Co0.15/Mg3Al-s catalyst. Catalytic activity (TOF, see
12
ACS Paragon Plus Environment

Page 13 of 30
ACS Catalysis
1
2
3
4
Table S1 and S2 for detailed calculation) measured (at < 25 % conversion) on
5
6
o
Co0.15/Mg3Al-c and Co0.30/Mg3Al-c catalysts are 1.06 h-1 and 0.80 h-1 at 70 C,
7
8
9
respectively. In contrast, Co0.15/Mg3Al-s and Co0.30/Mg3Al-s catalysts exhibit a
remarkable TOF of 3.91 h-1 and 3.86 h-1 respectively. These results suggest that catalyst
prepared by modified sol-gel method shows better oxidation performance (both activity
and TAR selectivity) than the ones from co-precipitation method.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Based on the catalyst characterization as well as reaction results on catalyst from Table 1,
it is clear that the crystal sizes for CoO, hydrotalcite and spinel structures are very small,
suggesting uniform precipitation and deposition of Co, Mg and Al species during catalyst
preparation. More importantly, by altering the sequence of Co species addition, the
interaction between Co and catalyst support can be well tuned, which influences surface
physical and chemical properties and catalytic performances.
When reaction time is prolonged from 6 h to 24 h, complete conversion of GLY (100%)
was observed on all four catalysts. However, the selectivity towards TAR is different.
Specifically, both Co0.15/Mg3Al-c (entry no. 4 in Table 1) and Co0.30/Mg3Al-c (entry no. 6)
gave 45–47% TAR selectivity, while on Co0.15/Mg3Al-s (entry no. 8) and Co0.30/Mg3Al-s
(entry no. 10) catalysts it is 63% and 64%. In addition, GLYA selectivity is about 29% on
Co0.15/Mg3Al-c and Co0.3/Mg3Al-c catalysts. But this value is much lower on
Co0.15/Mg3Al-s catalyst (2%). For OXA, the selectivity is 16% based on Co0.15/Mg3Al-c
and Co0.3/Mg3Al-c catalysts, while it is higher (24%) in the presence of Co0.15/Mg3Al-s
and Co0.30/Mg3Al-s catalysts. Selectivities towards lactic, glycolic and formic acids are
also relatively higher on Co0.15/Mg3Al-s and Co0.30/Mg3Al-s catalysts. Based on the
13
ACS Paragon Plus Environment

ACS Catalysis
Page 14 of 30
1
2
3
4
5
6
comparison of experiments in entries no. 1 and 2, with entries 3–10, it is clear that the
addition of Co catalysts significantly enhance GLY oxidation to dicarboxylic acids.
7
8
9
We carried out further experiments on Co0.15/Mg3Al-s catalyst and measured
concentration-time profiles at 70 °C and 55 °C. As seen from Figure 5 (a), GLY
concentration decreases from 0.22 kmol/m3 to 0.12 kmol/m3 within 6 h reaction time. The
GLYA concentration increases to a maximum value of 0.058 kmol/m3 at 9 h and then
decreases to almost zero during a 24 h reaction. For TAR, its concentration increases
slowly at the beginning, then further increased to approximately 0.14 kmol/m3 at 24 h,
suggesting that secondary oxidation (oxidation of GLYA) becomes dominant with the
progress of reaction. In another parallel reaction, we find that OXA formation rate is very
low when GLYA concentration is high, but its concentration is enhanced significantly
after 10 h. This phenomenon indicates that C-C cleavage reaction is another major
reaction when GLYA is consumed in the reaction medium.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In contrast, at 55 °C [Figure 5 (b)], GLY concentration decreases to about 0.16 kmol/m3
after 6 h reaction. The peak value of GLYA concentration is about 0.055 kmol/m3 around
18 h reaction, after which it decreases to 0.022 kmol/m3 after 36 h. TAR concentration
increases gradually with reaction time, while the overall OXA concentration is as
expected lower than at 70 °C.
Plausible reaction pathways. At both reaction temperatures studied, we find that the
concentration of OXA (a C2) is relatively low at the beginning but undergoes a rapid
increase once GLYA (a C3) concentration starts decreasing. This observation suggests
that OXA is generated from further reaction of GLYA. Correspondingly, a C1 species
(e.g. formic acid) should also form at the same time. However, contradicting this
14
ACS Paragon Plus Environment

Page 15 of 30
ACS Catalysis
1
2
3
4
5
6
7
8
9
hypothesis, the concentration of formic acid (in “Others”) is always negligible throughout
the reaction period. To understand this further, the conversion (X), selectivity (S) and
carbon balance (C%) were plotted vs reaction time at 70 °C and 55 °C in Figure 6. It is
observed [see Figure 6 (a)] that GLY conversion increases to 65% within 9 h reaction at
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
o
70 C, where GLYA selectivity is almost constant. After 9 h, significant decrease in
GLYA selectivity is observed while both TAR and OXA selectivities increase. The
selectivity towards glycolic and formic acids (“Others”) is negligible throughout the
whole reaction time. The total C% decreases from 100% at the beginning to
approximately 86% after 24 h. Therefore, this C deficit based on analysis of liquid phase
samples might result from decarboxylation of GLYA, resulting in CO2 generation. The
CO2 is converted into a sodium bicarbonate salt. Inorganic salts could not be quantified
using HPLC analysis, thus accounting for the observed C deficit.
A similar selectivity trend with time is also observed at 55 °C [Figure 6 (b)], although the
selectivity to OXA and C deficit is not as significant as at 70 °C. In both cases, the final
TAR yield (complete GLY conversion) is approximately 63%, although it was obtained
at different reaction times. Based on the product distribution shown in Figures 5 and 6,
plausible reaction pathways for GLY oxidation to TAR is discussed here. As shown in
Scheme 2, primary oxidation of GLY leads to the formation of GLYA in the presence of
Co catalysts, while secondary oxidation to TAR also occurs simultaneously. As more
GLYA is formed in the aqueous phase, C–C cleavage, a parallel reaction to secondary
oxidation becomes significant. Therefore, OXA (C2 species) formation is noticed. Due to
decarboxylation reaction of GLYA, one C1 is lost to carbonate. Combined selectivity
towards lactic, glycolic and formic acids is almost negligible.
15
ACS Paragon Plus Environment

ACS Catalysis
Page 16 of 30
1
2
3
4
5
6
7
8
9
We also tested Co0.15/Mg3Al-s catalyst for glucose oxidation at 70 °C and 0.1 MPa O2.
Complete conversion was obtained after 10 h reaction time with major products including
TAR (S: 29%), GLYA (19.4%), gluconic acid (9.5%), glucaric acid (11.1%) and other
monocarboxylic acids (lactic acid, glycolic acid, formic acid: 19.5%). While activity and
selectivity is not as high as the recently reported bimetallic PtCu/TiO2 catalyst,31 these
results nevertheless show that the proposed Co catalysts containing earth-abundant metals
are inexpensive alternatives for converting some cellulosic feedstocks to value-added
DCAs (e.g. GLY to TAR). Further optimization studies aimed at enhancing selectivity to
desired products are needed.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
We also carried out recycle experiments to study the stability of Co0.15/Mg3Al-s catalyst
at 70 °C for GLY oxidation. As seen from Figure 7, the conversion of GLY decreases
from 47% to 35% after 3rd recycle, suggesting gradual catalyst deactivation. However,
the selectivities towards TAR and GLYA are almost unchanged. While no Co species
agglomeration were observed on used Co0.15/Mg3Al-s catalyst, STEM images shown in
Figure 3 (e) suggest that more folded structures were formed after recycles. In addition,
leaching tests on used Co0.15/Mg3Al-s catalyst were also carried out. It is found that the
total ratios of metal leaching for Co, Mg and Al species (based on fresh Co0.15/Mg3Al-s
catalyst) are 0.1%, 0.1% and 68.2%, respectively. It is clear that Co and Mg species were
stable while significant amounts of Al species were leached after recycles. Therefore it is
plausible that the leaching of Al species during recycles might induce loss of surface area
and catalytic activity. These results suggests that layered structures are important for
keeping Co catalysts active, while Al species are not stable and leached during
experiments. While finding an effective way of immobilizing Co species on layered
16
ACS Paragon Plus Environment

Page 17 of 30
ACS Catalysis
1
2
3
4
5
6
structures is the focus of current work, future work should be focused on redesigning
stable catalyst supports.
7
8
Conclusions
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The performance of non-noble metal Co-based catalysts is reported for the oxidation of
glycerol at mild conditions to value added dicarboxylic acids such as tartronic acid. Co
catalysts prepared by modified sol-gel method show 88% combined selectivity towards
tartronic acid and oxalic acid with complete conversion of glycerol at mild reaction
conditions. Concentration–time profiles reveal a possible reaction pathway involved in
formation of tartronic acid and other co-products in aqueous phase. The proposed Co
catalysts can eliminate the disadvantages of the conventional stoichiometric oxidation by
mineral acids, as well as the need of expensive catalysts for oxidative conversion of
biomass feedstocks.
Acknowledgement
The authors gratefully acknowledge support for this work from a grant from the joint
National Science Foundation and Environmental Protection Agency program Networks
for Sustainable Material Synthesis and Design (CHE 1339661). We want to thank Dr.
Victor Day for carrying our XRD analysis, Mr. Robert Hable and Professor Susan
Williams for assisting TGA characterization.
References
1. Gallezot, P. Chem. Soc. Rev. 2012, 41, 1538–1558.
2. Cai, J. Y.; Ma, H.; Zhang, J. J.; Du, Z. T.; Huang, Y. Z.; Gao, J.; Xu, J. Chin. J. Catal. 2014,
35, 1653–1660.
3. Villa, A.; Campione, C.; Prati, L. Catal. Lett. 2007, 115, 133–136.
4. Shen, Y. H.; Zhang, S. H.; Li, H. J.; Ren, Y.; Liu, H. C. Chem. Eur. J. 2010, 16, 7368–7371.
17
ACS Paragon Plus Environment

ACS Catalysis
Page 18 of 30
1
2
3
4
5
6
7
8
9
5. Rodriguez, A. A.; Williams, C. T.; Monnier, J. R. Appl. Catal., A 2014, 475, 161–168.
6. Dirkx, J. M. H.; Verhaar, L. A. T. Carbohyd. Res. 1979, 73, 287–292.
7. Abbadi, A.; Makkee, M.; Visscher, W.; Vanveen, J. A. R.; Vanbekkum, H. J. Carbohyd.
Chem. 1993, 12, 573–587.
8. Bianchi, C. L.; Canton, P.; Dimitratos, N.; Porta, F.; Prati, L. Catal. Today 2005, 102, 203–
212.
9. Dimitratos, N.; Lopez-Sanchez, J. A.; Lennon, D.; Porta, F.; Prati, L.; Villa, A. Catal. Lett.
2006, 108, 147–153.
10. Sproge, E.; Chornaja, S.; Dubencovs, K.; Zhizhkun, S.; Kampars, V.; Serga, V.; Kulikova, L.;
Palcevskis, E. J. Serb. Chem. Soc. 2013, 78, 1359–1372.
11. Pagliaro, M.; Ciriminna, R.; Kimura, H.; Rossi, M.; Della Pina, C. Angew. Chem., Int. Ed.
2007, 46, 4434–4440.
12. Katryniok, B.; Kimura, H.; Skrzynska, E.; Girardon, J. S.; Fongarland, P.; Capron, M.;
Ducoulombier, R.; Mimura, N.; Paul, S.; Dumeignil, F. Green Chem. 2011, 13, 1960–1979.
13. Roy, C. B.; Nandi, D. K.; Rao, K. V. Indian J. Chem. A., 1985, 24, 742–744.
14. Kimura, H.; Kimura, A.; Kokubo, I.; Wakisaka, T.; Mitsuda, Y. Appl. Catal., A 1993, 95,
143–169.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
15. Villa, A.; Dimitratos, N.; Chan-Thaw, C. E.; Hammond, C.; Prati, L.; Hutchings, G. J.
Accounts Chem. Res. 2015, 48, 1403–1412.
16. Xu, C. L.; Du, Y. Q.; Li, C.; Yang, J.; Yang, G. Appl. Catal., B 2015, 164, 334–343.
17. Zhao, Z.; Arentz, J.; Pretzer, L. A.; Limpornpipat, P.; Clomburg, J. M.; Gonzalez, R.;
Schweitzer, N. M.; Wu, T. P.; Miller, J. T.; Wong, M. S. Chem. Sci. 2014, 5, 3715–3728.
18. Jin, C. C.; Zhang, J. J.; Dong, R. L.; Huo, Q. S. Int. J. Electrochem. Soc. 2014, 9, 5743–5750.
19. Skrzynska, E.; Ftouni J.; Girardon, J. S.; Capron, M.; Jalowiecki-Duhamel, L.; Paul, J. F.;
Dumeignil, F. ChemSusChem. 2012, 5, 2065–2078.
20. Liang, D.; Cui, S. Y.; Gao, J.; Wang, J. H.; Chen, P.; Hou, Z. Y. Chin. J. Catal. 2011, 32,
1831–1837.
21. Hu, W. B.; Lowry, B.; Varma, A. Appl. Catal., B 2011, 106, 123–132.
22. Liang, D.; Gao, J.; Wang, J. H.; Chen, P.; Wei, Y. F.; Hou, Z. Y. Catal. Commun. 2011, 12,
1059–1062.
23. Prince, J.; Montoya, A.; Ferrat, G.; Valente, J. S. Chem. Mater. 2009, 21, 5826–5835.
24. Chen, X.; Cheng, J. P.; Shou, Q. L.; Liu, F.; Zhang, X. B. Crystengcomm 2012, 14, 1271–
1276.
25. Bin Han, J.; Lu, J.; Wei, M.; Wang, Z. L.; Duan, X. Chem. Commun. 2008, 41, 5188–5190.
26. Perez-Ramirez, J.; Ribera, A.; Kapteijn, F.; Coronado, E.; Gomez-Garcia, C. J. J. Mater.
Chem. 2002, 12, 2370–2375.
27. Dong, G.; Xiao, X.; Peng, M.; Ma, Z.; Ye, S.; Chen, D.; Qin, H.; Deng, G.; Liang, Q.; Qiu, J.
(2012). RSC Adv. 2012, 2, 2773–2782.
28. Yuan, Z.; Wang, L.; Wang, J.; Xia, S.; Chen, P.; Hou, Z.; Zheng, X. Appl. Catal., B 2011, 101,
431–440.
29. Espinal, R.; Taboada, E.; Molins, E.; Chimentao, R. J.; Medina, F.; Llorca, J. Appl. Catal., B
2012, 127, 59–67.
30. Espinal, R.; Taboada, E.; Molins, E.; Chimentao, R. J.; Medina, F.; Llorca, J. RSC Adv. 2012,
2, 2946-2956.
31. Jin, X.; Zhao, M.; Shen, J.; Yan, W. J.; He, L. M.; Thapa, P. S.; Ren, S. Q.; Subramaniam, B.;
Chaudhari, R. V. J. Catal. 2015, 330, 323–329.
18
ACS Paragon Plus Environment

Page 19 of 30
ACS Catalysis
1
2
3
4
5
6
7
8
Figures, Tables and Schemes
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. Schematic description of (a) Co0.15/Mg3Al-c and (b) Co0.15/Mg3Al-s and (c) their
o
catalytic performances in glycerol oxidation at 70 C and 0.1 MPa O2 (experimental conditions:
0.5 g glycerol, 1.5 g NaOH, 25 mL, 0.2 g solid catalysts, 24 h). Bulk and surface compositions of
Co, Mg and O were analyzed by ICP/TEM-EDX and SEM-EDX respectively.
19
ACS Paragon Plus Environment

ACS Catalysis
Page 20 of 30
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Characterization of Co0.15/Mg3Al-c and Co0.15/Mg3Al-s using (a) temperature
programmed reduction by H2 (TPR-H2), (b, c) N2 isotherm and pore distribution (inset).
20
ACS Paragon Plus Environment

Page 21 of 30
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(a)
(b)
(c)
21
ACS Paragon Plus Environment

ACS Catalysis
Page 22 of 30
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(d)
(e)
Figure 3. TEM images and EDX mapping of fresh (a) Co0.15/Mg3Al-c, (b) Co0.30/Mg3Al-c, (c)
Co0.15/Mg3Al-s, (d) Co0.30/Mg3Al-s and (e) STEM and element mapping used Co0.15/Mg3Al-s
catalysts.
22
ACS Paragon Plus Environment

Page 23 of 30
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. XRD patterns of fresh (a) Co0.15/Mg3Al-c, (b) Co0.30/Mg3Al-c, (c) Co0.15/Mg3Al-s and (d)
Co0.30/Mg3Al-s.
23
ACS Paragon Plus Environment

ACS Catalysis
Page 24 of 30
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(a)
(b)
Figure 5. Concentration-time profiles of glycerol oxidation on Co0.15/Mg3Al-s catalyst at (a)
70 °C and (b) 55 °C (refer to Table 1 for other experimental details).
24
ACS Paragon Plus Environment

Page 25 of 30
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(a)
(b)
Figure 6. Conversion/selectivity/carbon balance vs time profiles of glycerol oxidation on
Co0.15/Mg3Al-s catalyst at (a) 70 °C and (b) 55 °C (refer to Table 1 for other experimental details).
25
ACS Paragon Plus Environment

ACS Catalysis
Page 26 of 30
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 7. Conversion and selectivity over Co0.15/Mg3Al-s catalyst at 70 °C (reaction time: 6 h).
Other experimental conditions are provided in Table 1.
26
ACS Paragon Plus Environment

Page 27 of 30
ACS Catalysis
1
2
3
4
5
6
7
Table 1. Glycerol oxidation on solid cobalt catalysts at 70 °C
8
9
S (%)
no. Catalyst
TOF (h-1) Time (h) X (%)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
TAR
GLYA
OXA
“Others”
1
6
24
6
4.3
14.6
28.2
32.3
46.9
38.0
45.4
36.7
63.5
40.1
64.3
79.1
54.3
58.1
29.2
52.8
28.8
49.1
2.1
-
3.2
9.8
5.2
4.3
2.0
7.6
3.4
3.8
4.4
2.8
Blank
experiments
-
2
13.0
16.1
100
21.7
100
47.0
100
63.2
100
4.3
4.0
13.4
3.1
16.2
4.2
24.0
5.9
24.4
3
Co0.15/Mg3Al-c
Co0.30/Mg3Al-c
Co0.15/Mg3Al-s
Co0.30/Mg3Al-s
0.88
0.64
3.01
2.91
4
24
6
5
6
24
6
7
8
24
6
9
46.1
2.0
10
24
Experimental conditions: 0.5 g glycerol, 1.5 g NaOH, 25 mL, 0.2 g solid catalysts. “Others”:
lactic, glycolic and formic acids.
27
ACS Paragon Plus Environment

ACS Catalysis
Page 28 of 30
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 1. Glycerol conversion to mono and dicarboxylic acids
28
ACS Paragon Plus Environment

Page 29 of 30
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 2. Plausible reaction pathways for tartronic and oxalic acids formation from glycerol
29
ACS Paragon Plus Environment