Ruthenium trichloride as a new catalyst for selective production of dimethoxymethane from liquid methanol with molecular oxygen as sole oxidant

Article abstract of DOI:10.1016/j.catcom.2015.04.031

Dimethoxymethane was first synthesized from methanol with a liquid phase intermittent process which only used molecular oxygen as the sole oxidant. RuCl₃ was proved to be an efficient catalyst as it posses the ability of oxidizing methanol and Lewis acidic which promotes the oxidation of methanol to formaldehyde and then methanol condensed with formaldehyde to form dimethoxymethane at Lewis acid site.

Full text of DOI:10.1016/j.catcom.2015.04.031

Catalysis Communications 68 (2015) 46–48
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Catalysis Communications
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Short communication
Ruthenium trichloride as a new catalyst for selective production of
dimethoxymethane from liquid methanol with molecular oxygen as
sole oxidant
a
Meilan Li a,b, Yan Long a,b, Zhiyong Deng a,b, , Hua Zhang , Xiangui Yang a,b, Gongying Wang a,b,
⁎
⁎
a
Chengdu institute of organic chemistry (Chengdu organic chemicals CO. LTD), Chinese academy of sciences, Chengdu, Sichuan 610041, PR China
b
University of Chinese academy of sciences, Beijing 10049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 March 2015
Received in revised form 23 April 2015
Accepted 26 April 2015
Available online 28 April 2015
Dimethoxymethane was first synthesized from methanol with a liquid phase intermittent process which only
used molecular oxygen as the sole oxidant. RuCl3 was proved to be an efficient catalyst as it posses the ability
of oxidizing methanol and Lewis acidic which promotes the oxidation of methanol to formaldehyde and then
methanol condensed with formaldehyde to form dimethoxymethane at Lewis acid site.
© 2015 Published by Elsevier B.V.
Keywords:
Dimethoxymethane
Methanol
Liquid phase oxidation
RuCl3
1. Introduction
Nb2O5 catalyst reported by N. T. Prado [19] can make the reaction carried
out in a sealed glass reactor using H2O2 as oxidant.
Dimethoxymethane (DMM), or methylal, is an excellent green sol-
vent and chemical intermediate because of its extremely low toxicity,
low boiling point, and good solubility and intersolubility with water. It
has been widely used in pharmaceutical and perfume industries and
organic synthesis, and it is also a potential diesel additive which can
decrease the soot emission [1].
DMM is usually produced by a two-step process in the industry: the
first step is the oxidation of methanol to formaldehyde over Fe–Mo or
Ag catalyst, and the second step is the condensation of formaldehyde ob-
tained from the first step with methanol in the presence of acids [2,3]. In
consideration of the long technological processes which caused the inev-
itably high cost, a lot of effort have been made and most focused on one-
step selective oxidation of gaseous methanol to DMM. And lots of selec-
tive oxidation catalysts have been reported such as crystalline SbRe2O6
[4–6], Re/γ–Fe2O3 [7], ReOx/TiO2 [8], RuOx/SiO2 [9], heteropolyacid [10,
11], Cu–ZSM-5, Fe–Mo–O [12], V2O5/TiO2 [13–18] and so on. Among
these catalysts mentioned above, ReOx/TiO2 [8], Fe–Mo–O [12] and
V2O5/TiO2 [13–18] show better catalytic performance for the selective
oxidation of methanol to DMM. However, the activity of those catalysts
were evaluated in a continuous flow fixed bed reactor, except the
Comparing with the gas phase continuous process, the liquid
phase intermittent process spares the gasification of raw materials
and condensation of products, so the liquid phase intermittent pro-
cess is energy saving. In this work, we successfully implemented
the selective oxidation of methanol to DMM in the liquid phase inter-
mittent process, and only use molecular oxygen as the sole oxidant.
we also found that RuCl3 is an effective and potential catalyst be-
cause of its special chemical properties that it can effortlessly adopt
various formal oxidation states from −2 to +8 in chemical bonds,
therefore giving rise to many compounds with interesting and
often unique properties [20].
According to the reaction mechanism reported by other researchers
[8], the oxidability and acidity are the essential properties of the catalyst
for the selective oxidation of methanol to DMM. Because of the
oxidability of catalyst, firstly methanol will be oxidized to formaldehyde
and then formaldehyde will be oxidized to formic acid, during this pro-
cess methanol and formaldehyde can condensate to DMM on the acid
sites of catalyst and formic acid and methanol can esterify to methyl
formate. Thus, the possible by-productions of direct selective oxidation
of methanol to DMM are formaldehyde, formic acid and methyl for-
mate, so in order to decrease the formation of those by-productions,
appropriate proportions of oxidability and acidity of catalysts are need-
ed. Ruthenium chloride is a mild Lewis acid which can promote the con-
densation of formaldehyde with methanol to DMM, and the Ru3+ that
existed in methanol can oxidize methanol to formaldehyde [21,22].
⁎
Corresponding authors at: Chengdu institute of organic chemistry (Chengdu organic
chemicals CO. LTD), Chinese academy of sciences, Chengdu, Sichuan 610041, PR China.
E-mail addresses: zhiyongdeng@cioc.ac.cn (Z. Deng), gywang@cioc.ac.cn (G. Wang).
http://dx.doi.org/10.1016/j.catcom.2015.04.031
1566-7367/© 2015 Published by Elsevier B.V.

M. Li et al. / Catalysis Communications 68 (2015) 46–48
47
Thus Ruthenium chloride is a potential catalyst for selective oxidation of
methanol to DMM.
2. Experimental
The catalytic reaction for the selective oxidation of methanol was
carried out in a 100 ml high pressure autoclave made of stainless
steel-316 L having facilities for gas inlet, outlet, temperature controlled
heating and variable agitation speed. The materials used in this reaction
included RuCl3·nH2O, FeCl3·6H2O, NiCl2·6H2O, ZrCl4, ZnCl2, methanol
(all from Sinopharm Chemical Reagent Co., Ltd, China) and O2 were pur-
chased from commercial source. In a typical reaction, 20 ml methanol
(MeOH) and quantitative metal chloride were added in the autoclave.
After replacing the air in the autoclave by O2, certain initial pressure of
O2 was filled into the reactor, and then the reaction was proceeded at
certain temperature for 2.5 h. After the reaction, the liquid samples
were immediately analyzed by GC (GC-14B, Shimadzu, Japan) with
FFAP column and FID detector.
3. Results
Table 1 shows the results of catalytic performance of different metal-
lic chlorides for the selective oxidation of methanol to DMM. As shown
in Table 1, when 1 g of common metallic chlorides like FeCl3, NiCl2,
ZrCl4, ZnCl2 was used as catalyst respectively, the conversion of metha-
nol (b4%) was very low. But when 0.001 g RuCl3 was used as catalyst,
the conversion of methanol increased to 13.8%. With the increase of
the amount of RuCl3, the conversion of methanol and the selectivity of
DMM increased rapidly. When the amount of RuCl3 increased to 0.1 g,
which was less one-tenth of the amount of other metallic chlorides,
the conversion of methanol was rapidly increased to 37.8% and the
selectivity of DMM was increased to 84.3%. These results indicated
that RuCl3 was a new high-efficiency catalyst for selective oxidation of
methanol to DMM in the liquid phase intermittent process and only
use molecular oxygen as the sole oxidant, due to the ability of oxidizing
methanol of Ru3+ reported by Bilgrien C that Ru3+ can oxidize the
alcohols to aldehydes and ketones and translate to Ru2+ and then
Ru2+ would be oxidized to Ru3+ by O2 [22], and the Lewis acidic of
RuCl3 which is beneficial for the oxidation of methanol to formaldehyde
and the condensation of methanol with formaldehyde. Other common
metallic chlorides also were Lewis acid which can promote the conden-
sation of methanol with formaldehyde, but these metallic chlorides
can't oxidize methanol as much as RuCl3, so the conversion of methanol
was low when common metallic chlorides were used as catalysts.
Fig. 1 shows the results of the selective oxidation of methanol to
DMM over RuCl3 catalyst. As shown in Fig. 1(a), the conversion of meth-
anol increased from 22.5% to 48.9% with the increase of initial pressure
from 1.0 MPa to 3.0 MPa. And then continuing to increase the initial
pressure to 4.0 MPa, there was no big change on the conversion of
Fig. 1. Selective oxidation of methanol to DMM on RuCl3 catalyst: (a) initial pressure de-
pendence, (b) temperature dependence, and (c) reaction time dependence. Reaction con-
ditions: 0.1 g catalyst, 20 ml methanol.
Table 1
methanol. And it still can see that the selectivity of DMM decreased
and the selectivity of MF increased slowly with the increase of initial
pressure. This information can be interpreted by that the high initial
pressure of O2 added in the autoclave before reaction made more O2 dis-
solve into methanol and then O2 oxidized more Ru2+ to Ru3+ which
promoted the oxidation of methanol to formaldehyde [22], at the
same time, more formaldehyde would be oxidized to formic acid and fi-
nally it will promote the formation of MF because of the esterification
between methanol and formic acid. Fig. 1(b) shows that the conversion
of methanol was first increased from 8.4% to 49.8% with the increasing
of temperature from 353 K to 393 K, and then decreased to 39.3% with
further increasing of temperature to 423 K. This phenomenon was
caused by that both the oxidation and condensation reactions are exo-
thermic reactions that can be favored by raising temperature in specific
Selective oxidation of methanol to DMM over different metallic chlorides catalysts.
Catalysts active metal
Mass of catalyst
(g)
Conv. (%)
MeOH
Select. (%)
DMM
MF
FeCl3
NiCl2
ZrCl4
ZnCl2
RuCl3
1
1
1
1
3.2
1.5
2.7
33.6
29.7
31.3
62.8
78.2
84.3
86.5
74.8
69.3
66.4
70.3
68.7
37.2
21.8
15.7
13.5
25.2
30.7
2.0
0.15
0.1
0.05
0.01
0.001
43.0
37.8
23.0
15.1
13.8
Reaction condition: T = 120 °C, P(O2) = 2.0 MPa, Time = 2.5 h.
MeOH: methanol DMM: Dimethoxymethane MF: methyl formate.

48
M. Li et al. / Catalysis Communications 68 (2015) 46–48
would be opened and placed into a vacuum oven for over 12 h at
353 K to evaporate the liquid of the reaction, such as methanol, DMM,
MF and water. And next test was proceeded as mentioned above with-
out adding new RuCl3 into autoclave. As shown in Fig. 2, the conversion
of methanol and the selectivity of DMM declined with the increasing
reused times, but after reused 10 times the catalytic performance was
almost stable. It indicated that RuCl3 was a potential catalyst for selec-
tive oxidation of methanol to DMM in the liquid phase intermittent pro-
cess which only uses molecular oxygen as the sole oxidant. After testing
for several times, RuCl3 which can dissolve in methanol easily changed
into a lot of black fine particles dispersing in methanol. The RuCl3 may
be changed into RuOx during the reaction which causes the decreasing
of methanol conversion at the beginning.
4. Conclusions
Fig. 2. Stability tests of RuCl3 catalyst for selective oxidation of methanol to DMM. Reaction
conditions: 0.1 g catalyst, 20 ml methanol, 2.5 h, 393K and 2.0 MPa.
In conclusion, RuCl3 is an effective and potential catalyst for selective
oxidation of liquid methanol to DMM, which is first used in the liquid
phase intermittent process and only use molecular oxygen as the sole
oxidant. RuCl3 can oxidize methanol to formaldehyde which is then
condensed with methanol to produce DMM because of the mild Lewis
acidic supplied by RuCl3. The selectivity of DMM reached 76.8% with
49.8% conversions of methanol at 393 K and 3.0 MPa O2 over 0.1 g RuCl3.
range, and continuing to increase the reaction temperature, the reac-
tions were inhibited to a limited extent.
In order to investigate the pressure changing during the reaction
time, the pressure change during a 5.5 h reaction after filling 3.0 MPa
O2 into the autoclave was recorded. These results are shown in
Fig. 1(c). The reaction temperature was arrived at 120 °C in 1 h, and
after then it maintained at 120 °C. As shown in Fig. 1(c), the pressure
in autoclave increased to 4.1 MPa from 3 MPa in 1 h when the reaction
temperature was arrived at 120 °C, and then decreased rapidly to
2.0 MPa from 1 h to 3.5 h and maintained at 2.0 MPa from 3.5 h to
5.5 h. This data demonstrate that oxygen would be consumed during
the reactions, and the consuming of oxygen would be stopped when
the reaction proceeded in a certain degree, but the oxygen would not
be depleted after reaction. With the purpose of investigating how the
pressure change in the reactor and reaction time influence the reaction,
we did four times reactions with the same initial pressure (3.0 MPa) and
various reaction time like 1 h, 2.5 h, 3.5 h and 5.5 h which the final pres-
sures in the autoclave at reaction temperature were 4.1 MPa, 2.4 MPa,
2.0 MPa and 2.0 MPa. As it was shown in Fig. 1(c), the conversion of
methanol was less than 5% when the reaction time was 1 h. Combining
with the pressure change shown in Fig. 1(c), this result indicates that
the oxygen was not or less was consumed for the oxidation of methanol
so that the conversion of methanol was low. Thus, the high conversion
of methanol (48%) when reaction time is 2.5 h would be got due to
the more oxygen had been consumed which can be seen from the rap-
idly decrease of the pressure in autoclave shown in Fig. 1(c). And so, fur-
ther increasing the reaction time to 3.5 h and 5.5 h, the conversion of
methanol (53%) was no big change because the pressure (2.0 MPa) in
autoclave was no big change which indicates that no more oxygen
had been consumed for the oxidation of methanol, but the selectivity
of DMM was decreased and more MF formed that is perhaps caused
by the long reaction time. Those experiments imply that when reaction
time is 2.5 h, maximum oxygen could be consumed for the oxidation of
methanol and get the highly selectivity of DMM.
Acknowledgments
This work was financial supported by the Special Project for the Out-
standing Youth Innovation Team of Sichuan Province (2013TD0010),
1000 Talents Program of Sichuan Province and Talents program of
Chengdu.
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Fig. 2 shows the result of the stability tests of 0.1 g RuCl3 catalyst for
selective oxidation of methanol to DMM. After each test, the autoclave