Synthesis of polyoxymethylene dimethyl ethers catalyzed by rare earth compounds

Article abstract of DOI:10.14233/ajchem.2015.17822

Polyoxymethylene dimethyl ethers (PODME_n) have been synthesized in moderate yields by the reaction of methylal (PODME1) and paraformaldehyde catalyzed by rare earth compounds. The activities of catalyst in the reaction were investigated and the

Full text of DOI:10.14233/ajchem.2015.17822

Asian Journal of Chemistry; Vol. 27, No. 6 (2015), 2149-2153
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.17822
Synthesis of Polyoxymethylene Dimethyl Ethers Catalyzed by Rare Earth Compounds
*
GAO-FENG SHI , JIAN MIAO, GUO-YING WANG, JIN-MEI SU and HAI-XIAO LIU
School of Petro-chemical Engineering, Lanzhou University of Technology, Lanzhou 730000, Gansu Province, P.R. China
*Corresponding author: E-mail: gaofengshi_lzh@163.com
Received: 15 April 2014;
Accepted: 30 June 2014;
Published online: 17 March 2015;
AJC-16982
Polyoxymethylene dimethyl ethers (PODMEn) have been synthesized in moderate yields by the reaction of methylal (PODME1) and
paraformaldehyde catalyzed by rare earth compounds. The activities of catalyst in the reaction were investigated and the results show that
La3+/SO42- has a high value. The influencing factors of reaction such as catalyst amount, reaction temperature, time, pressure and mole
ratio of methylal (PODME1) to paraformaldehyde were also investigated. The appropriate reaction conditions i.e., n(paraformaldehyde)/
n(methylal) = 1.60, catalyst amount of 1 %, 130 °C, 6 h and 0.5 MPa. Under the above conditions, methylal conversion rate and selectivity
of PODME2-8 is 81.6 and 75.5 %, respectively. In addition to, the combustion and emission characteristics of PODME2-8 added diesel was
compared with the diesel, polyoxymethylene dimethyl ethers (PODME2-8) not only improved the combustion characteristics of diesel, but
improved the low temperature fluidity.
Keywords: Rare earth compounds, Polyoxymethylene dimethyl ethers, Methylal, Paraformaldehyde.
acidic ion exchange resins as catalyst, BP produced PODME3-8
by conversion of dimethyl ether with formaldehyde through a
complicated process10,12. But the main drawbacks of these
method are the catalyst is corrosive and the conversion and
selectivity to PODME3-8 is low, a complex procedure and a
long process. Recently, Lanzhou Institute of Chemical Physics,
ChineseAcademy of Sciences has made progress to synthesize
PODME3-8 by using ionic liquid as catalyst in a homogeneous
reaction system with methanol and trioxymethylene as feed;
single-pass yield of PODME3-8 reached 50 % with the selec-
tivity to PODME3-8 being 70-80 %13,14. However, the expensive
and corrosive catalyst and the complex purification of product
has limited its wide application. Therefore, a catalyst that is
inexpensive, high conversion and selectivity, the development
of an efficient and environmentally benign process for prepa-
ration of PODMEn is still in demand. The study of PODMEn
has attracted major attention and many researchers are studying
INTRODUCTION
Polyoxymethylene dimethyl ethers (PODMEn) are impor-
tant industrial products with the formula CH3O (CH2O)n CH3,
where n ≥ 1. The features of PODMEn are high oxygen content
(42-51 %) and high cetane number, which can improve the
combustibility of diesel oil, enhance the efficiency of com-
bustion and reduce pollutants. Thus, it is believed to have
wide application prospect as a diesel additive1,2. Especially,
PODME3-8 exhibits high oxygen content, moderate boiling
points, excellent miscibility with diesel oil and average cetane
number of above 76. It was reported that by adding 20 %
PODME3-8 in the diesel oil, the amounts of powders pollutants
and NOx released upon combustion can be reduced by 80-90
and 50 %, respectively3,4.As a result, the conversion of methanol
to PODMEn as an additive of diesel oil may not only extend the
methanol industry chain and digest the large surplus production
capacity of methanol, but also bring enormous benefits with
respect to economics and environmental protection2,5.
The preparation methods for PODMEn (n ≥ 1) have rarely
been reported in the literature. At early stage, PODMEn was
obtained from methanol, formaldehyde and polyformaldehyde
or dioxacyclopentane, using sulfuric acid or hydrochloric acid
as catalyst. BASF group used H2SO4 or CF3SO3H as catalyst
to get PODME1-10, with methanol, methylal, trioxymethylene
and polyformaldehyde as raw materials6-9. With zeolites or
the techniques of preparing PODMEn for industrial production15,16
.
In this work, polyoxymethylene dimethyl ethers (PODMEn)
was synthesized by the condensation of methylal and paraform-
aldehyde over the catalysts of several rare earth compounds.
The pure components of PODME1-8 obtained by rectification
were characterized by GC-MS. The influence of the type of
rare earth compounds, raw material ratio, amount of catalyst,
reaction temperature on reaction conversion and selectivity
were investigated.

2150 Shi et al.
Asian J. Chem.
EXPERIMENTAL
All the materials are commercially available and were used
without purification. Paraformaldehyde:AR, Tianjin recovery
of fine chemical industry research institute, China. Methylal:
Fei Hao Trade Co., Ltd., Zhengzhou, China. Lanthanum chloride:
AR, Shanghai mountain pu chemical Co., Ltd, China. Cerium
trichloride:AR, Shanghai Zhongtai chemical reagent co., Ltd,
China. Neodymium sulfate:AR, Shanghai Zhongtai Chemical
Reagent Co., Ltd, China. Praseodymium sulfate:AR, Shanghai
Zhongtai Chemical Reagent Co., Ltd, China. Sulfuric acid:
AR, Beijing Chemical Plant, China. Hydrochloric acid: AR,
Beijing Chemical Plant, China. Nitrogen: commercially
available. GSH-0.5 L reaction kettle: Weihai Huanyu Chemical
Machinery Co., Ltd, Shandong, China. TRACE DSQ quater-
nary gas chromatography mass spectrometry instrument,
Thermo Company, US.
Using GC-MS, The reactants and products were analyzed,
using peak area normalization method to determine ion flow
diagram of the components. GC Status: DB-5MS column elastic
silica capillary (30 m × 0.25 mm × 0.25 µm); keep warming
conditions: 50 °C for 5 min, followed by 20 °C/min the heating
rate of up to 250 °C and keep 5 min; Gasification chamber
temperature: 250 °C; carrier gas: helium, the column flow rate
of 1 mL/min. Mass Status: ion source temperature: 250 °C;
electron bombardment source: 70 eV; emission current: 100
µA; electron multiplier voltage: 1.5 KV; scanning range: 30-
650u. Reaction product ion flow diagram of GC-MS as shown
in Fig. 1:
Synthesis and spectral characterization
Synthesis: The synthesis of PODMEn from methylal
(PODME1) and paraformaldehyde was performed in a 500
mL reaction kettle under the following reaction conditions:
the ratio of methylal to paraformaldehyde in the feed was 1 to
3, reaction temperature considered is100 to160 °C, reaction
time is 1 to 8 h and the amount of catalyst used is 5 % of the
total reactant quantity. Acid catalytic depolymerization of
paraformaldehyde, mechanism may see type (1):
1.61
100
3.18
80
6.16
8.53
H+
60
H+
O
OH
10.45
..
..
OH
O
O
OH
O
40
+
O
OH
n-1
C
+
12.13
H
H
H
n
n
H
H
13.64
16.62
20
0
OH+
C
O
C
+
H+
OH
n-1
O
O
OH
+
+
35
0
5
10
15
20
25
30
H
H
n-1
H
H
H
H
Time (min)
O
C
Fig. 1. GC-MS total ion chromatogram of reaction products. Reaction
conditions: Wt = 1 %, 125 °C, 6 h, 0.5 MPa, n(Paraformaldehyde):
n(methylal) = 1.6
+
n
+
H O
+ H
2
H
H
The second part, formaldehyde and H+ form with reactive
intermediates, mechanism may see type (2):
As shown in Table-1, the product PODME1-8 can be effec-
tively separated under the above condition, the peak area of
PODME1-8 with the increased of the degree of polymerization
and reduced in turns, used the ion flow diagram of peak area
to quantitative components.
O:
O:H+
OH
+C
+ H+
C
C
H
H
H
H
H
H
RESULTS AND DISCUSSION
Catalytic activity of different rare earth compounds:
The catalytic activities for four kinds of rare earth compounds
The third part, formaldehyde reactive intermediates and
methylal reaction, mechanism may see type (3):
TABLE-1
POLYOXYMETHYLENE DIMETHYL ETHERS (PODMEn)EACH COMPONENT CONTENT
Retention time
1.61
m.w.
76
Degree of polymerization
m.f.
Name
Content (%)
16.70
25.87
20.74
13.77
8.14
1
2
3
4
5
6
7
8
CH3OCH2OCH3
CH3O(CH2O)2CH3
CH3O(CH2O)3CH3
CH3O(CH2O)4CH3
CH3O(CH2O)5CH3
CH3O(CH2O)6CH3
CH3O(CH2O)7CH3
CH3O(CH2O)8CH3
PODME1
PODME2
PODME3
PODME4
PODME5
PODME6
PODME7
PODME8
3.18
6.16
8.53
10.45
12.13
13.64
16.62
106
136
166
196
226
256
286
4.53
2.37
1.11

Vol. 27, No. 6 (2015)
Synthesis of Polyoxymethylene Dimethyl Ethers Catalyzed by Rare Earth Compounds 2151
have been showed in Table-2. we can know the comparison
catalyst amount, methylal conversion rate and selectivity of
PODME2-8 increased first and then decreased; when the amount
of catalyst is greater than 1 %, methylal conversion rate and
selectivity of PODME2-8 were gradually reduced. The reason
is that, as the increasing of the catalyst amount , the acidic of
reaction system enhanced , which is not conducive to the presence
and stability of PODME2-8 and methylal. Considering the above
conditions, La3+/SO42- catalyst amount was 1 %.
Reaction time: The influences of reaction time was
also investigated. As shown in Table-5, with the extension of
reaction time, the conversion rate of methylal rised; selectivity
of PODME2-8 during 6 h reached a peak 75.85 % and then
declined. It can be attributed to the short reaction time, which
resulte in inadequate response; when the reaction reaches
equilibrium, extend the reaction time continuously, methylal
and PODMEn have undergone different degrees of hydrolysis.
The most appropriate choice of reaction time was 6 h.
Reaction temperature: Table-6 shows that results of the
temperature on the reaction has obvious effects. When the
temperature is low, methylal conversion rate is not high due to
value of catalyst activity as follows: La3+/SO42- > Ce3+/ SO42-
>
Pr3+ / SO42- > Nd3+/SO42-. Therefore, a higher catalytic activity
La3+/SO42- was selected, factors affecting the reaction were
investigated accordingly.
Reaction factors
Mole ratio of reactants: The influences of different ratio
of raw materials for production distribution has been shown
in Table-3. Table-3 reveals that the ratio of paraformaldehyde
to methylal (PODME1) showed a great impact on the reaction.
With an increase of mole ratio, the conversion rate of methylal
(PODME1) and the selectivity of PODME2-8 both increased
gradually. When the ratio was higher than 1.60, the conversion
rate of methylal (PODME1) and the selectivity of PODME2-8
decreased and the product is creamy white colour suspension,
even as a white paste. Therefore, it can be most conducive to
generate PODME2-8 when the ratio is 1.60.
Catalyst amount: Table-4 shows the influence of catalyst
amount on the reaction. It was found that with the increase of
TABLE-2
CATALYTIC ACTIVITIES OF VARIOUS RARE EARTH COMPOUNDS
La3+/SO42-
82.66
Ce3+/SO42-
80.15
Pr3+/SO42-
78.87
Nd3+/SO42-
77.96
Conversion of PODME c/%
Selectivity to PODME2~8 s/%
75.53
74.91
73.47
72.56
Reaction conditions: n(PF)/n(methylal) = 1.60, 125 °C 6 h, catalyst amount of 1 %
TABLE-3
INFLUENCES OF THE MATERIAL MOLE RATIO ON REACTION
PODMEn (%)
Selectivity (%)
PODE2-8
Methylal
n(PF)/n(m)
conversion rate (%)
PODE1
31.63
29.74
25.66
18.65
21.65
PODME2
25.44
25.79
25.16
25.35
PODME3 PODME4 PODME5 PODME6 PODE7~8
1.00
1.20
1.40
1.60
1.80
17.55
17.99
19.03
20.65
19.39
9.97
10.51
12.18
14.00
13.20
5.35
5.37
6.36
7.91
8.09
2.12
2.85
2.88
3.91
4.44
1.55
1.74
1.88
2.65
3.25
66.37
70.26
74.36
81.35
78.35
61.98
64.25
67.49
74.47
73.02
24.65
Reaction conditions: 125 °C, 6 h, (La3+/SO42-) amount of 1 %
TABLE-4
INFLUENCE OF THE AMOUNT OF CATALYST (La3+/SO42-) ON REACTION
Catalyst
(wt. %)
PODMEn (%)
Selectivity (%)
PODE2-8
Methylal
conversion rate (%)
PODE1
PODME2
PODME3
PODME4 PODME5 PODME6
PODE7~8
0.50
1.00
1.50
2.00
3.00
18.35
16.80
17.19
19.98
22.23
25.67
25.88
25.06
24.12
24.16
20.13
20.75
19.97
18.33
16.88
13.54
13.78
13.63
12.56
11.37
8.11
8.16
7.33
6.17
5.55
4.53
4.56
3.54
2.25
1.79
3.40
3.48
2.40
1.11
0.71
81.65
83.20
82.81
80.02
77.77
75.38
76.61
71.93
64.54
60.46
Reaction conditions: n(PF)/n(m) = 1.60, 125 °C, 6h
TABLE-5
INFLUENCE OF DIFFERENT TIME ON REACTION
PODMEn (%)
Time
(min)
Methylal
PODME2-8
conversion rate (%) selectivity rate (%)
PODME1 PODME2 PODME3 PODME4 PODME5 PODME6 PODME7~8
60
32.34
27.55
18.36
18.32
18.55
19.01
19.11
22.25
24.31
26.26
26.42
26.19
25.88
25.75
15.42
16.82
20.97
21.11
20.87
19.71
19.66
8.31
1.66
3.69
7.57
7.73
7.66
7.32
7.31
1.18
2.01
3.88
3.89
3.88
3.86
3.85
0.12
0.78
2.81
2.81
2.82
2.82
2.82
67.66
72.45
81.64
81.68
81.45
80.99
80.89
48.94
57.79
75.16
75.85
75.13
72.81
72.42
120
240
360
480
600
720
10.18
13.67
13.89
13.71
13.22
13.03
Reaction conditions: n(PF)/n(m) = 1.60,125 °C, (La3+/SO42-) amount of 1 %

2152 Shi et al.
Asian J. Chem.
TABLE-6
INFLUENCE OF THE REACTION TEMPERATURE ON REACTION
PODMEn (%)
Temp.
Methylal conversion
PODME2-8
rate (%)
selectivity rate (%)
(°C)
PODME1 PODME2 PODME3 PODME4 PODME5 PODME6 PODME7~8
90
45.32
38.96
29.65
18.53
18.43
18.57
19.06
12.11
18.54
23.13
26.11
26.16
25.98
25.62
10.64
14.42
18.99
20.87
20.93
20.87
19.76
5.27
8.94
1.88
3.16
5.16
7.52
7.69
7.44
6.56
-
-
54.68
61.04
70.35
81.47
81.57
81.43
80.94
29.90
46.16
62.80
74.87
75.28
74.63
70.70
100
110
120
130
140
150
1.09
2.88
3.85
3.88
3.72
3.14
0.01
1.17
2.73
2.80
2.81
2.62
11.47
13.79
13.82
13.81
13.00
Reaction conditions: n(PF)/n(m) = 1.60, 6 h, (La3+/SO42-) amount of 1 %
the low reactivity, as the reaction temperature rised, the reac-
tivity also increased, conversion rate and selectivity of the reac-
tion showed a rising trend. When the temperature reached
130 °C, the conversion rate of methylal reached a maximum of
81.57 %, PODME2-8 reached a maximum selectivity of 75.28 %.
Raising the temperature continuously, conversion rate and
selectivity of reaction showed a downward trend. First reason
is that the elevated temperatures may enhance the activity of
the reactants and push the reaction conducted along the forward
direction, but the reaction is exothermic, reverse reaction can
be accelerated and the reaction product generation was inhi-
bited when the temperature is too high; second reason is that
when the temperature is too high, the amount of vaporization
of methylal increased, the concentration of methylal in the
liquid phase reduced relatively, similarly not conducive to the
presence and stability of PODMEn in the system. Taken toget-
her, the better reaction temperature is 120-130 °C.
Reaction pressure: Table-7 showed that the reaction
pressure on methylal conversion rate and selectivity of PODME2-8
is not obvious, when the pressure changed between 0 and 2.0
MPa, the conversion rate of methylal and PODME2-8 selectivity
changed little. Among those pressures, 0.5 MPa was found to
be the best choice. The conversion rate reached 81.63 % and
selectivity was up to74.42 %.
Combustion emission performance: The results in
Table-8 indicate that technical indicators of polyoxymethylene
dimethyl ethers (PODME2-8) in line with standard 0# diesel
and has a high cetane number, flash point and low solidifying
point. Then PODME2-8 were added to diesel, compared the
combustion and emission characteristics with no addition. Oil
products which was mixed by 15 % of PODME2-8 and diesel
are in line with the national standard 0 # diesel. Meanwhile,
the solidifying point of diesel dropped from 0 to -15 °C, cetane
number increased to 56.8 from 49 and closed flash point
TABLE-7
INFLUENCE OF THE DIFFERENT PRESSURE ON REACTION
PODMEn (%)
Pressure
(MPa)
Methylal
PODE2-8
conversion rate (%) selectivity rate (%)
PODE1
PODME2 PODME3 PODME4 PODME5 PODME6 PODE7~8
0
18.45
18.37
18.36
18.40
18.42
25.77
25.81
25.79
25.93
25.69
20.53
20.56
20.55
20.55
20.60
13.94
13.97
13.95
13.99
13.86
8.21
8.24
8.20
8.09
8.16
3.43
3.48
3.44
3.53
3.49
2.20
2.36
2.35
2.32
2.28
81.55
81.63
81.84
81.60
81.58
74.08
74.42
74.28
74.41
74.08
0.5
1.0
1.5
2.0
Reaction conditions: n(PF)/n(m) = 1.60,125 °C, 6h, (La3+/SO42-) amount of 1 %
TABLE-8
ADD 15 % PODME2-8 VEHICLE DIESEL COMBUSTION EMISSIONS PERFORMANCE COMPARISON
0# diesel
(GB/T19147-2013)
Blended diesel
(0 # diesel + 15 % PODME2-8)
99.2
Item
PODME2-8
Sulfur content (mg/kg)
10 % Steamed carbon residue (wt %)
Ash (wt %)
20.0
0.20
0.002
≤ 350
≤ 0.3
≤ 0.01
≤ 1
None
3.0-8.0
Trace
≤ 0
0.01
0.002
Copper corrosion (50 °C, 3 h)/Level
Mechanical impurities
1b
None
3.564
Trace
-15
56.8
56
1b
None
2.174
Trace
-27.5
83.2
60
Kinematic viscosity (20 °C) (mm2 s-1)
Moisture (mg kg-1)
Solidifying point (°C)
Cetane
Flash point (closed) (°C)
≥ 49
≥ 55
Distillation range:
50 % (°C)
90 % (°C)
95 % (°C)
Density (20 °C)/(kg m-3)
261.0
336.0
350.5
185
245
257
≤ 300
≤ 355
≤ 365
810-860
856
1069

Vol. 27, No. 6 (2015)
Synthesis of Polyoxymethylene Dimethyl Ethers Catalyzed by Rare Earth Compounds 2153
changed from 55 to 56 °C. Thus, polyoxymethylene dimethyl
ethers (PODME2-8) not only improved the combustion charac-
teristics of diesel, but also improved the low temperature flui-
dity. Thus, it is suggested that it's a good diesel fuel additives.
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Conclusion
Polyoxymethylene dimethyl ethers (PODMEn) were
synthesized via the reaction between methylal (PODME1) and
paraformaldehyde, in which the rare earth compounds used
as catalyst. Then the activity of catalyst in the reaction was
investigated, we can see that La3+/SO42- has a high value. Under
the optimized conditions of : methylal: paraformaldehyde =
1:1.6, catalyst dosage 1 wt. %, 130 °C, 6 h and 0.5 MPa, the
conversion rate of methylal and the selectivity of PODME2-8
were obtained as high as 81.6 and 75.5 %, respectively. Next,
PODME2-8 were added to diesel, compared the combustion
and emission characteristics with no addition, the results
showed that polyoxymethylene dimethyl ethers (PODME2-8)
not only improved the combustion characteristics of diesel,
but also improved the low temperature fluidity. Therefore, it's
a good diesel fuel additives.
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