Synthesis of Polyoxymethylene Dimethyl Ethers from Dimethyl Ether Direct Oxidation over Carbon-Based Catalysts

Article abstract of DOI:10.1002/cctc.201701213

The synthesis of polyoxymethylene dimethyl ethers (DMMx) with a selectivity of 84.3 % by direct oxidation of dimethyl ether (DME) was realized over 30 %Ti(SO₄)₂/active carbon (AC) catalyst. This process also significantly promotes the growth of the C?O chain. The catalytic performances of Ti(SO₄)₂/AC and Ti(SO₄)₂/graphene (G) catalysts differ largely for DME oxidation reaction, although both AC and G are carbon materials. The carbonyl and hydroxyl groups on the surface of the carbon-based catalysts play an important role in DME direct oxidation to DMMx. Owing to the differences of surface structure and chemical properties of AC and G materials, the different interaction between the Ti(SO₄)₂ and supports remarkably affects the sulfate structures on the supports surface and leads to the large differences in the acid and redox properties of catalysts.

Full text of DOI:10.1002/cctc.201701213

Accepted Article
Title: Synthesis of polyoxymethylene dimethyl ethers from dimethyl
ether direct oxidation over carbon-based catalysts
Authors: Xiujuan Gao, Wenfeng Wang, Yingying Gu, Zhenzhou Zhang,
Junfeng Zhang, Qingde Zhang, Noritatsu Tsubaki, Yizhuo
Han, and Yisheng Tan
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201701213
Link to VoR: http://dx.doi.org/10.1002/cctc.201701213
A Journal of
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ChemCatChem
10.1002/cctc.201701213
FULL PAPER
Synthesis of polyoxymethylene dimethyl ethers from dimethyl
ether direct oxidation over carbon-based catalysts
[a,b]
[a,b]
[a,b]
[a,b]
[a]
Xiu-Juan Gao, Wen-Feng Wang,
Ying-Ying Gu,
Zhen-Zhou Zhang,
Jun-Feng Zhang,
[a]
[a,c]
[a]
[a]
Qing-De Zhang,* Noritatsu Tsubaki, Yi-Zhuo Han, and Yi-Sheng Tan
The highly selective synthesis of polyoxymethylene dimethyl ethers
DMMx) with the selectivity of 84.3% from dimethyl ether (DME)
direct oxidation has been successfully realized over
0%Ti(SO /active carbon (AC) catalyst. This process also
significantly promotes the growth of the C-O chain. Especially, the
catalytic performances of Ti(SO /AC and Ti(SO /graphene (G)
–C–O–C–O–C–O–C– with dimethyl ether (DME, CH
3
3
OCH )
(
molecule, therefore, utilizing the direct oxidation of DME to
DMMx is attractive and environmental friendly for the synthesis
of clean fuel additives, which has the advantages of simple
3
4 2
)
process, low CO
investment.
2
emissions, high energy efficiency and low
)
4 2
4 2
)
catalysts have great differences for DME oxidation reaction, though
both AC and G are carbon materials. The carbonyl and hydroxyl
groups on the surface of carbon-based catalysts play an important
role in DME direct oxidation to DMMx. Due to the differences of
surface structure and chemical properties of AC and G materials, the
DME is known as a coal-based clean fuel and also a
potential non-petroleum route chemical synthesis material.
Because of its special structure of C–O–C and high cetane
number (55-60), DME is a very clean substitute for diesel fuel.
However, it is not easy to directly blend DME with diesel due to
the low boiling point of DME (-23.7°C). Recent advances in the
synthesis of DME from syngas derived from coal, biomass and
natural gas lead us to consider DME as a cheaper and less toxic
raw material for the synthesis of high value-added chemicals
currently produced from methanol, such as polyoxymethylene
dimethyl ethers (DMMx), methyl formate (MF), dimethoxyethane
4 2
different interaction between the Ti(SO ) and supports remarkably
affects the sulfate structures on the supports surface, and further
leads to an obvious difference in the acid and redox properties of
catalysts.
Introduction
[
5-12]
(
DMET), etc.
For the reaction of DME direct oxidation to DMMx, the
At present, one of the main reasons of fog and haze is the
results of automobile exhaust, in which the pollution caused by
diesel vehicles is much worse than that of petrol vehicles.
Therefore, it is a realistic choice to develop the novel diesel fuel
additives to improve diesel combustion performance and reduce
challenge is not only to break the C-O bond of DME molecule,
but also to maintain the stability of C-O bond of DMMx
molecules. The whole reaction process is rather complicated. It
involves the adsorption and activation of DME molecule, the
breakage of C-O and C-H bonds, and then the formation of new
C-O bonds and longer C-O chains. In addition, the structure of
the active sites for the chain growth of the new C-O bond and
the mechanism of the active center are still unclear. As a result,
the precise control of the activation of DME molecule at lower
temperature and further enhancing the chain growth of C–O
bond to obtain larger DMMx molecules from the direct oxidation
of DME have become main challenging tasks in DME utilization.
Numerous efforts have been made to explore the synthesis
of DMMx from DME direct oxidation. Our group have reported
[1]
emissions of engine pollutants. Polyoxymethylene dimethyl
ethers (CH O(CH O)xCH , DMMx, x=1~8) are important and
3
2
3
promising diesel fuel additives, which can be well mixed with
diesel, and can significantly reduce particulate matter emissions
[
2]
of diesel engines. In industry, DMMx are mainly produced via
condensation of methanol and its derivatives such as
dimethoxymethane, formaldehyde and trioxymethylene over
acidity catalysts, but this synthesis technology has the problems
of long process, high energy consumption and high
[
3,4]
investment.
Besides, DMMx possess the similar structure of
a
4
the oxidation of DME to DMM over Mn-modified H SiW12O40 and
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry,
Chinese Academy of Sciences, Taiyuan 030001, China;
University of Chinese Academy of Sciences, Beijing 100049, China;
[13,14]
Cs-modified H
3
PW12
O40 catalysts.
The formation of the C–
b
c
O–C–O–C chain of DMM molecule from the C-O-C chain of
DME molecule has been achieved. However, great difficulties
exist in obtaining much longer C-O chains of DMMx through the
Department of Applied Chemistry, School of Engineering, University of
Toyama, Gofuku 3190, Toyama 930-8555, Japan.
*
Correspondence author:
E-mail: qdzhang@sxicc.ac.cn; Tel: +86-351-4044388; Fax: +86-351-4044287.
Supporting information for this article is given at the end of the document.
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201701213
FULL PAPER
[
24]
direct oxidation of DME. Based on the understanding of DME
molecule and action mechanism of catalysts, we have designed
in the oxidative reaction.
In this paper, a series of novel carbon-based catalysts are
designed and prepared by the impregnation method. The highly
selective synthesis of DMMx from the direct oxidation of DME is
and prepared the ReOx-H
3
PW12/TiO
2
catalyst and realized the
molecule
formation of the C–O–C–O–C–O–C chain of DMM
2
[
15]
from the oxidation of DME and DMM as co-reactants. Carbon
nano-tubes (CNTs) have been widely used as support in many
realized over 30%Ti(SO
4 2
) /AC catalyst and the total DMMx
selectivity reaches 84.3%, which is the highest value reported
catalytic reactions owing to their unique surface properties. Then, until now. The present study further provides an effective
our group prepared a highly efficient CNTs supported Ti(SO
catalyst with acid and redox sites and obtained DMM for DME
selective oxidation along with the introduction of DMM.
Though larger DMM molecule is synthesized from DME
oxidation, the reaction system is complex due to the introduction
of DMM together with DME and O over the catalysts above.
4
)
2
approach to the rational design of carbon-based catalysts for the
one-step synthesis of DMMx from DME oxidation. There have
been no reports about the synthesis of DMMx from DME direct
2
[
16]
2
4 2
oxidation over Ti(SO ) /AC catalyst till the present moment.
2
Results and Discussion
Actually, our goal is to truly realize the direct oxidation of single
DME to larger DMMx molecule without DMM introduction though
there exists a great difficulty.
Table 1 shows the results of the catalytic oxidation of DME
2
-
on Ti(SO
in Table 1, when the AC before SO
catalyst, CH OH and HCHO become the main products, which
4
)
2
/AC catalysts with different SO
4
content. As shown
2
-
Previous studies have found that the acid and redox
properties of the catalysts are two key factors for DME oxidation
4
modification is used as
3
[
15,17]
to DMMx.
However, the redox properties of the catalysts are
indicates that AC exhibits the acid and redox properties.
However, the acidity of AC is probably insufficient to form DMMx.
mainly provided by the precious metals or metal oxides, and the
strong oxidizing capacity easily leads to over-oxidation of DME
and the low selectivity of target products. Taking into account
the extraordinary physical and chemical properties of the carbon
materials, their larger specific surface area as a structural
support can disperse the acid components, and excellent
chemical conductivity is favorable for the electron transfer in the
oxidation reaction. The carbonaceous materials (active carbon
4 2
Ti(SO ) possessing acidity is chosen to modify AC to improve
the catalyst activity. There is an evident increase in DMMx
2
-
selectivity of 63.8% when 15%SO
4
is used to modify AC. The
selectivity of DMMx reaches the highest value of 84.3% with
2
-
DME conversion of 8.4% when the content of SO
4
is 30%. The
has an obvious effect on DMMx formation
due to its acidity. More interestingly, it is found that the
2
-
introduction of SO
4
2
-
(
AC), graphene (G), and carbon nano-tubes, etc.) have
selectivity of DMM
2
is higher when SO
4
content is 15% and
generally been employed as the support materials to improve
20%. The effects of reaction temperature and gas hourly space
velocity (GHSV) on DME oxidation to DMMx over
[
18-23]
catalytic activity or as the catalysts used in the reaction.
Generally, it is considered that surface oxygen groups located at
carbon materials are the active sites to activate oxygen molecule
30%Ti(SO
4 2
) /AC catalyst are shown in Table S1 and Table S2.
It can be seen that the optimum reaction conditions are 240ºC
2-
4
Table 1. Effects of SO
content on the performance of Ti(SO
4 2
) /AC for DME oxidation
DME
Selectivity (C-mol %)
Catalyst
AC
Conv. (%)
DMM
0
DMM2
0
DMM1-2
0
CH OH
HCHO
MF
0
COx
CH4
0
3
6.8
8.2
7.1
9.2
8.4
8.1
7.2
54.2
66.2
25.3
8.5
45.8
28.9
10.9
8.6
0
0
0
0
0
0
0
1
1
2
3
4
5
0%Ti(SO
5%Ti(SO
0%Ti(SO
0%Ti(SO
0%Ti(SO
0%Ti(SO
)
4 2
)
4 2
)
4 2
)
4 2
)
4 2
)
4 2
/AC
/AC
/AC
/AC
/AC
/AC
2.0
2.9
4.9
0
0
34.8
70.4
77.5
73.5
80.0
29.0
12.5
6.8
63.8
82.9
84.3
81.9
84.2
0
0
0
0
6.9
8.8
0
0
8.4
7.5
10.6
5.2
0
0
4.2
10.6
0
0
-
1
2
Reaction conditions: atmospheric pressure, 240°C, Cat. : 1ml, 3600h , nO : nDME= 1.3:1.2.
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ChemCatChem
10.1002/cctc.201701213
FULL PAPER
Table 2. Effects of carbon-based supports on the performance of catalysts for DME oxidation
DME
Selectivity (C-mol %)
Catalyst
Conv. (%)
DMM
70.4
77.5
73.5
trace
trace
trace
DMM2
12.5
6.8
8.4
0
DMM1-2
82.9
CH OH
HCHO
MF
0
COx
CH4
0
3
2
3
4
0%Ti(SO
0%Ti(SO
0%Ti(SO
)
4 2
)
4 2
)
4 2
/AC
/AC
/AC
9.2
8.4
8.1
8.1
7.2
6.3
8.5
8.6
8.8
0
0
0
0
0
0
84.3
6.9
0
0
81.9
7.5
10.6
16.1
18.4
21.3
0
0
2
0%Ti(SO
0%Ti(SO
0%Ti(SO
4
)
4
)
4
)
2
/G
2
/G
2
/G
trace
trace
trace
83.9
81.6
78.7
0
0
3
4
0
0
0
0
0
0
-
1
2
Reaction conditions: atmospheric pressure, 240°C, Cat. : 1ml, 3600h , nO : nDME= 1.3:1.2.
-
1
and 3600h for DME oxidation to DMMx with high DMMx
selectivity and low by-product selectivity. Besides, the catalytic
performance of H SO modified AC has also been investigated
in the titled reaction. The results show that the catalytic activity
of Ti(SO modified AC is much better than that of H SO
4
and the total DMMx selectivity remains at about 80.0% during
20h reaction.
2
4
In order to understand well the reason why the catalytic
performances of Ti(SO
great differences, the catalysts have been characterized deeply.
Figure 1 shows NH -TPD profiles of Ti(SO /AC and
content. The
desorption peaks at about
4 2 4 2
) /AC and Ti(SO ) /G catalysts have
4
)
2
2
modified AC for the formation of DMMx from DME direct
3
4 2
)
2
-
oxidation (see Table S3), and Ti may play an important role in
Ti(SO
4
4
)
)
2
/G catalysts with different SO
/AC catalysts have two NH
4
the reaction. The AC modification with Ti(SO
4
)
2
as a metal free
Ti(SO
2
3
carbon-based catalyst exhibits excellent catalytic performance in
the synthesis of DMMx from DME direct oxidation. AC with large
specific surface area is used not only as support to disperse the
218ºC and 335ºC , corresponding to weak acid sites and
medium-strong acid sites, respectively. In order to compare the
2
-
changes of weak acid sites after SO
NH desorption peaks are integrated (see Figure S1 and Table
S4). By increasing the content of SO
sites of catalysts has an obvious increase. However, when the
4
introduction, the areas of
acid component Ti(SO
owing to the abundant oxygenated functional groups.
Taking into account graphene possessing the similar
chemical compound and the different textural properties, AC and
G are chosen to support Ti(SO ) as catalysts in order to well
4
)
2
more uniformly, but also can activate
3
2
-
O
2
4
, the number of weak acid
2
-
content of SO
decreased which is attributed to the existence form of Ti(SO
changing and the aggregated Ti(SO particles covering the
4
reaches 40% or 50%, the number of acid sites
4
4 2
)
understand the effects of supports in the titled reaction. Table 2
4 2
)
shows the selectivity to products and the conversion of DME as
2
-
a function of SO
catalysts. Over AC-supported Ti(SO
selectivity can exceed 83%. While only a trace of DMM is
formed and no DMM is found, but CH OH selectivity reaches
about 80% over Ti(SO /G catalysts. The results show that the
Ti(SO /G catalysts possess certain acid and redox sites, so
DME molecule can be broken to form CH OH and HCHO, but
there obviously lacks of enough active sites to further convert
CH OH and HCHO for the next chain propagation to form DMMx.
Therefore, though both AC and G are carbon materials, the
catalytic performances of Ti(SO /AC and Ti(SO /G catalysts,
especially for the total DMMx selectivity and the DMM
selectivity, have marked differences for DME oxidation to DMMx.
Additionally, the stability with time-on-stream of 30%Ti(SO /AC
catalyst for DME direct oxidation to DMMx has been investigated,
4
content in the Ti(SO
4 2 4 2
) /AC and Ti(SO ) /G
4 2
)
catalysts, the total DMMx
2
3
4 2
)
4 2
)
3
3
4
)
2
4 2
)
2
Figure 1. NH
3
-TPD profiles of Ti(SO
4
)
2
2
/AC and Ti(SO
contents.
4 2
) /G catalysts with
4 2
)
-
different SO
4
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ChemCatChem
10.1002/cctc.201701213
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acid sites. In evident, by comparing with Ti(SO
4
)
2
/AC catalysts,
reaction system during the whole process. However, by
there are mainly medium-strong acid sites existing in the G-
supported Ti(SO catalysts. Also, the acid strength and acid
amount of Ti(SO /AC and Ti(SO /G catalysts are significantly
comparing with 30%Ti(SO
not detected on 30%Ti(SO
4 2
Meanwhile, the signal values of all products on 30%Ti(SO )
4
)
2
/AC catalyst, the product DMM
/G catalyst (see Figure S4).
/G
2
is
4
)
2
4 2
)
4
)
2
4
)
2
different (see Table S5). According to the reaction results, the
weak acid sites can favor the formation of DMMx, and there
exists a correlation between the amount of weak acid sites and
DMMx selectivity. Furthermore, the catalyst acidity is strongly
catalyst are relatively weaker, indicating that DME molecule is
more easily activated to generate the target products over
4 2
30%Ti(SO ) /AC catalyst. It is proposed that the different carbon
materials change the distribution and existence form of Ti(SO )
4 2
[
25]
related to the dispersed state of Ti(SO
leads to the difference of catalytic performance for Ti(SO
and Ti(SO /G catalysts.
-TPD-MS analysis of catalysts has been performed in
order to clarify the activation of O over the surface of catalysts
see Figure 2 and Figure S2). The results show that there is an
evident difference in the generation peak of CO ascribing to the
decomposition of surface oxygen groups over catalysts. The
CO intensity of 30%Ti(SO /AC catalyst is much stronger than
those of AC and 30%Ti(SO /G catalyst, and this also suggests
that the surface oxygen groups of 30%Ti(SO /AC catalyst are
more than those of AC and 30%Ti(SO /G catalyst after O is
introduced to system. The introduction of Ti(SO promotes the
activation of O over the catalyst surface and the adsorbed
4
)
2
particles, and further
species, and also affect the amount of surface oxygen groups of
the catalysts, which further influences the acid and redox
properties of catalysts.
4 2
)
/AC
4 2
)
O
2
In our previous work, lattice oxygen species in noble metal
or metal oxides can offer redox sites in the selective oxidation of
2
[
12,15]
(
DME molecules.
As we all know, AC or G supports
2
themselves do not contain lattice oxygen species. Generally, it is
considered that the activation of oxygen molecules occurs at
defect sites on carbon materials, and surface oxygen groups
located at defects are regarded as the active sites in the
2
4 2
)
4 2
)
[
26,27]
4
)
2
oxidative reaction.
In terms of the oxidative dehydrogenation
4
)
2
2
of ethylbenzene, the ketone C=O components as the active sites
[
28-30]
4
)
2
for styrene formation have been confirmed.
XPS is a
2
surface technique which will provide an estimate of the chemical
composition of the few uppermost layers of the material. In order
to get further insight into the nature of the oxygen-containing
functional groups on the surface, a reconstruction of the C 1s
and O 1s peaks has been performed. The relative content of
different functional groups is calculated from the corresponding
peak areas in the XPS spectra. Figure 3 shows that both
oxygen species may be further transformed into surface oxygen
groups of catalysts. In order to gain accurate understanding
about the role of surface oxygen groups in DME oxidation
reaction, DME-TPSR-MS has also been carried out (see Figure
S3). The possible products such as CH
DMM on 30%Ti(SO /AC and 30%Ti(SO
been monitored by mass spectrometry. The results show that
CH OH, HCHO, MF and DMM are detected over
0%Ti(SO /AC and 30%Ti(SO /G catalysts, which proves
that surface oxygen groups of catalysts take part in DME
3
OH, HCHO, MF, DMM,
2
4
)
2
4 2
)
/G catalysts have
4 2 4 2
30%Ti(SO ) /AC and 30%Ti(SO ) /G catalysts possess graphitic
3
carbon (peak Ⅰ), hydroxyl/epoxy groups (peak Ⅱ),
carbonyl/quinone groups (peak Ⅲ), carboxyl groups (peak Ⅳ)
and a satellite peak corresponding to the π–π* transition in the
3
4
)
2
4 2
)
[
31]
oxidation conversion because O
2
is not introduced to the
aromatic systems (peak Ⅴ).
From the results of Table 3,
a
b
2 4 2 4 2 4 2
Figure 2. O -TPD-MS profiles of AC and 30%Ti(SO ) /AC catalysts (a) and 30%Ti(SO ) /AC and 30%Ti(SO ) /G catalysts (b).
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ChemCatChem
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Figure 3. C 1s spectra of 30%Ti(SO
4 2 4 2
) /AC catalyst and 30%Ti(SO ) /G catalyst.
Table 3 Results of C 1s spectra and C, O content from XPS for catalysts
Hydroxyl/
epoxy
w/%
Carbonyl/
quinone
w/%
Aromatic
systems
w/%
Molar ratio
n(O)/n(C)
Graphitic carbon
w/%
Carboxyl
w/%
Catalyst
AC
0%Ti(SO
5%Ti(SO
0%Ti(SO
0%Ti(SO
0.18
0.43
0.53
0.79
1.24
0.30
61.10
54.24
52.49
48.04
43.49
52.87
13.43
26.01
26.34
32.43
33.20
28.83
6.96
7.79
13.74
6.07
6.36
5.05
6.82
5.49
4.77
5.89
6.69
3.36
3.63
4.16
1
1
3
5
4
4
4
4
)
)
)
)
2
2
2
2
/AC
/AC
/AC
/AC
8.12
11.12
12.86
8.65
4 2
30%Ti(SO ) /G
[
32]
it can be seen that the relative content of carbon element over
the Ti(SO /AC catalyst surface gradually reduces and the
n(O)/n(C) ratio increases significantly with the increase of SO
content, which indicates that surface oxygen content increases
after the introduction of Ti(SO to AC. It is also observed that
the contents of carbonyl and hydroxyl groups have a marked
summarizes the results of fitted O 1s peaks. It can be seen
that the total contents of peak Ⅱ (double C=O bonds from
carbonyl and quinone groups) and peak Ⅲ (single C-O bonds
4 2
)
2-
4
from phenol, lactone, ether groups) in the 30%Ti(SO
4
)
2
/AC
/G
)
4 2
catalyst account for about 50%. However, for 30%Ti(SO
4
)
2
catalyst, the total contents of peak Ⅳ (oxygen atoms in Ti-O and
S-O bonds) and peak Ⅴ (oxygen atoms in carboxyl groups) are
nearly 50%. This phenomenon is coincided well with the results
obtained by C 1s analysis. The TPD-MS spectra also obtained
2
-
increase with the increase of SO
to support Ti(SO as catalyst, it is noted that the relative ratios
of carbonyl and hydroxyl groups are 11.12% and 32.43%,
respectively. However, for 30%Ti(SO /G catalyst, the relative
ratios of carbonyl and hydroxyl groups decrease to 8.65% and
8.83%, respectively. The comparison of chemical compositions
of 30%Ti(SO /AC and 30%Ti(SO /G catalysts shows that the
latter contains much lesser amount of oxygen. This fact is
consistent with the results that 30%Ti(SO /G catalyst is
catalytically less active than 30%Ti(SO /AC catalyst, which
4
content. When AC is chosen
4 2
)
4
)
2
for 30%Ti(SO
Figure S6. To determine the amount of functional groups, the
deconvolution of the CO and CO profiles using a multiple
4 2 4 2
) /AC and 30%Ti(SO ) /G catalysts are shown in
2
2
4
)
2
4
)
2
Gaussian function was performed. Table S7 summarizes the
deconvolution results. In all cases, an increase in the amount of
surface oxygen groups is evidenced by the increase of the CO
4 2
)
4
)
2
and CO
2
peak. For 30%Ti(SO
4
)
2
/AC catalyst, the amount of CO
indicating
indicates that oxygenated surface groups play crucial role in the
titled reaction. Meanwhile, the XPS pattern of the O 1s region for
carbon-based catalysts is also depicted in Figure S5. Table S6
released by TPD is much larger than that of the CO
2
that there are much more carbonyl, quinone, ether and phenol
groups than carboxyl and lactone groups. The similar results can
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a
b
c
Figure 4. H
2
-TPR-MS profiles of 30%Ti(SO
4 2
) /AC (a), 30%Ti(SO
4 2 4 2
) /G (b), and Ti(SO ) (c) catalysts.
be observed from the C 1s XPS spectra as in Table 3.
The kinds of oxygen functional groups generated on the
508ºC ,
H
2
S
evolution is detected. Specially, another low-
S release at 149ºC is observed for
temperature peak for H
2
[
33,34]
surface of carbon materials are quite complicated.
Not all
Ti(SO
4
4
)
)
2
/G sample. A similar phenomenon is also noticed for
the surface oxygen functional groups on carbon materials are
beneficial for the catalytic reaction, which can be ascribed to
their intrinsic acidity or basicity and electrophilicity or
nucleophilicity. Also, these opposite properties constitute the
diversity of carbon materials, and affect combining ability of
active components and carbon materials, which further leads to
the differences of catalyst performances. According to the
Ti(SO
2
sample (see Figure 4c). The different observed
temperature and relative intensity of these two distinct SO
2
and
H
2
S peaks indicate different types of sulfate species conforming
[
35,36]
to different sulfate structures.
The sulfate species reduced
at low-temperature could correspond to those not strongly
interacting with the surface and consequently should be readily
reduced to SO
temperature are interacting strongly with the surface, and thus
should be more prone to be reduced to H S. From the results
above, it can be seen that the temperature of reduction peak
shifts to lower values for 30%Ti(SO /G catalyst, compared with
30%Ti(SO /AC catalyst. Therefore, when the active component
Ti(SO is impregnated on the carbon-based supports, the
interaction between the Ti(SO and supports may be divided
2
, while the sulfate species reduced at higher
combined O
2
-TPD-MS, XPS and TPD-MS results, it could be
to carbon-based
concluded that the introduction of Ti(SO
4
)
2
2
supports indeed increases the surface oxygen groups on the
catalysts, especially for the relative contents of carbonyl and
hydroxyl groups. Based on the results above, we propose that
carbonyl and hydroxyl groups are pivotal for DME direct
4 2
)
4 2
)
4 2
)
oxidation to DMMx over Ti(SO
4
)
2
/AC catalyst. However, by
4 2
)
comparing with 30%Ti(SO /AC catalyst, the relative contents of
4
)
2
into strong, weak, or no interaction, respectively. Due to the
difference of surface structure and surface chemical properties
of AC and G materials, the different interaction between the
carbonyl and hydroxyl groups are lower on the 30%Ti(SO
catalyst, which maybe leads to the poorer performance.
4 2
) /G
In order to investigate the effects of carbon-based supports
on the properties of catalysts, the H -TPR-MS technique is
4 2
Ti(SO ) and supports remarkably affects the sulfate structures
2
on the supports surface, and further leads to an obvious
difference in the acidity of the catalysts.
employed and the data are exhibited in Figure 4a-c. The results
show that two reduction peaks correspond to the reduction of
4 2 4 2
The TEM images of 30%Ti(SO ) /AC and 30%Ti(SO ) /G
6
+
2-
4+
2-
S
species (SO
respectively. The two samples (see Figure 4a and 4b) show the
presence of both H S and SO . MS analysis shows two distinct
features: a low-temperature peak connected with SO formation,
followed by a higher temperature peak related to H S formation.
The major SO peaks for Ti(SO /AC sample are observed at
15ºC , 412ºC and 536ºC . Besides, the H S peak is found
beyond 573ºC . For Ti(SO /G sample, the peaks are in the
temperature of 165ºC and 352-452ºC ascribing to SO . Beyond
4
) to S
(SO
2
) and S (H
2
S) species,
catalysts are shown in Figure S7 and Figure S8. Ti(SO )
4 2
particles are highly dispersed over the surface of AC in the
amorphous form ascribing to the strong interaction between the
2
2
2
Ti(SO
4
4
)
)
2
2
and AC, which restrains the aggregation and growth of
particles. From the TEM observation of 30%Ti(SO /G
2
Ti(SO
4 2
)
2
4
)
2
catalyst, ripples of the graphene sheets can be seen and the
Ti(SO particles, revealing as the black dots, are deposited on
the graphene sheets.
3
2
4 2
)
4 2
)
2
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ChemCatChem
10.1002/cctc.201701213
FULL PAPER
In this paper, DMMx selectivity is significantly improved up
acid and redox sites over the surface of catalyst are probably
insufficient for chain growth to form larger DMMx molecules. The
introduction of different carbon-based supports changes the
2
-
to 84.3% after the introduction of 30%SO
4
to AC. Based on the
reaction results, we suggest that an optimum amount of the
weak acid sites and redox sites of the catalyst is beneficial for
the formation of long chain DMMx products from DME direct
interaction between the Ti(SO
4
)
2
and supports, which further
over the supports
influences the dispersed state of Ti(SO )
4 2
oxidation. The NH
introduction of Ti(SO
catalyst, but also enhances the redox properties of AC to
promote the activation of O over the surface of catalyst, and the
3
-TPD and O
2
-TPD-MS results show that the
surface. In evident, the sulfate structure is essential for the
generation of acid sites on the catalysts and the acid properties
4 2
)
not only improves the acid sites of the
2
- [37,38]
4
can be strongly affected by the environment of the SO .
2
On the basis of the observations above, a possible reaction
mechanism for the synthesis of DMMx from DME direct
oxidation is proposed (see Figure 5). DME molecule can be
adsorbed oxygen species can be transformed into surface
oxygen groups to participate in the DME oxidation reaction.
Meanwhile, the introduction of Ti(SO
4
)
2
significantly increases
dissociated to form CH
catalyst surface. Then CH
and HCHO is further formed by CH
sites. Next, DMM should be formed by the acetalization reaction
of CH OH and HCHO over acid sites. Then, DMM may be
synthesized via CH O- group combining with DMM molecule
under the cooperation of the acid sites and redox sites of the
Ti(SO /AC catalyst. Further investigations on the synthesis of
much larger DMMx molecules from DME direct oxidation over
catalyst acidity (the results are in accordance to the BET results). metal free carbon-based catalysts and the mechanisms of C-O
3
O- group over the acid sites of the
OH can be obtained over acid sites,
OH oxidation over redox
the relative contents of carbonyl and hydroxyl groups over the
catalysts surface (as shown in XPS and TPD-MS results).
Therefore, we propose that carbonyl and hydroxyl groups on the
surface of catalysts may be involved in the redox cycles for DME
direct oxidation to DMMx. The TEM results also show that
3
3
3
2
3
Ti(SO
amorphous form, which indicates that AC with high BET surface
area improves the Ti(SO dispersion, which further affects the
4 2
) is highly dispersed on the surface of AC in an
4 2
)
4 2
)
2
-
The content of SO
4
and the properties of support itself
chain growth are currently in progress.
obviously influence the acid and redox properties of catalysts.
Although both AC and G are carbon materials, the catalytic
Conclusions
performance of Ti(SO
Ti(SO /G catalyst, which should be ascribed to the special
surface properties of AC and the strong interaction between
Ti(SO and AC. The abundant oxygenated functional groups
4 2
) /AC catalyst is far superior to that of the
4 2
)
4 2
In conclusion, the modified active carbon with Ti(SO )
catalyst demonstrates excellent catalytic performance in the
synthesis of DMMx and realizes the direct oxidation of single
DME to larger molecules. DMMx selectivity is significantly
increased to 84.3% with a DME conversion of 8.4% due to the
cooperation of the increased acid sites and redox sites of
4 2
)
and special adsorption capacity of AC with large BET surface
area possibly exert an important function on the catalytic
4 2 4 2
performance of Ti(SO ) /AC catalyst. For Ti(SO ) /G catalyst, the
catalyst. Especially, the catalytic performance of Ti(SO
4 2
) /AC
and Ti(SO /G catalysts have a great difference for DME
4 2
)
oxidation reaction, though AC and G are carbon materials. The
distinctive surface structures and chemical properties of G and
AC supports influence the distribution of the acid component
4 2
Ti(SO ) on the supports surface. Meanwhile, the amount and
type of surface oxygen groups on the catalysts remarkably affect
the redox properties of catalysts. The carbonyl and hydroxyl
groups on the surface of catalysts may be involved in the redox
cycles for DME direct oxidation to DMMx. The introduction of
different carbon-based supports changes the interaction
Figure 5. The possible reaction pathway of DME direct oxidation to DMMx.
4 2
between the Ti(SO ) and supports, and further leads to an
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ChemCatChem
10.1002/cctc.201701213
FULL PAPER
obvious difference in the catalytic performance of Ti(SO
4
)
2
/AC
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[
[
Experimental Section
1
[
4 2
Ti(SO ) modified carbon-based catalysts were prepared by
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chromatography. Further details of catalyst preparation,
characterization, and testing are available in the Supporting
Information.
[
[
5
[
[
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Acknowledgements
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This work was financially supported by the National Natural
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2
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ChemCatChem
10.1002/cctc.201701213
FULL PAPER
Entry for the Table of Contents
FULL PAPER
The highly selective synthesis of
DMMx with the selectivity of 84.3%
from DME direct oxidation has been
successfully realized over
Xiu-Juan Gao, Wen-Feng Wang,
Ying-Ying Gu, Zhen-Zhou Zhang,
Jun-Feng Zhang, Qing-De Zhang,*
Noritatsu Tsubaki, Yi-Zhuo Han,
and Yi-Sheng Tan
4 2
30%Ti(SO ) /AC catalyst.
Page No. – Page No.
Synthesis of polyoxymethylene
dimethyl ethers from dimethyl ether
direct oxidation over carbon-based
catalysts
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