Conversion of dimethyl ether to diesel fuel additives via dielectric barrier discharges

Article abstract of DOI:10.1246/cl.2001.322

A high-efficient conversion of dimethyl ether (DME) to diesel fuel additives at ambient condition via dielectric-barrier discharges has been performed. The conversion of DME reaches a high value of 66.56% at a gas flow rate of 30 mL·min^-1. The liquid obtained is a cetane number promoter of diesel fuels. The selectivity of liquid product is more than 40%.

Full text of DOI:10.1246/cl.2001.322

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22
Chemistry Letters 2001
Conversion of Dimethyl Ether to Diesel Fuel Additives via Dielectric Barrier Discharges
Tao Jiang, Chang-jun Liu,* and Guo-liang Fan†
State Key Laboratory of C1 Chemical Technology, School of Chemical Engineering and Technologies,
Tianjin University, Tianjin 300072, P. R. China
†
Analysis Center, Tianjin University, Tianjin 300072, P. R. China
(Received January 15, 2001; CL-010036)
A high-efficient conversion of dimethyl ether (DME) to
diesel fuel additives at ambient condition via dielectric-barrier
discharges has been performed. The conversion of DME reach-
–
1
es a high value of 66.56% at a gas flow rate of 30 mL·min .
The liquid obtained is a cetane number promoter of diesel fuels.
The selectivity of liquid product is more than 40%.
Dialkoxyalkane (DAAK) chemicals are a broad class of
oxygen-containing chemical family. The best known com-
pound in this family is probably dimethoxymethane (DMMT).
Other compounds in this family include dimethoxyethane
(
DMET) and dimethoxypropane (DMPP), etc. DMMT has
been proposed as a cetane number improving additive for diesel
fuel because of the possibility of suppressing smoke emission.
from the reactor was first introduced into a condenser to sepa-
rate the condensable product from the gas. The power was sup-
plied by a high voltage generator working at about 25 kHz. The
power can be varied by adjusting the voltage amplitude which
causes a slight change of frequency. The voltage and current
measurements were conducted using a high voltage probe
(Tektronix P6015) and a current probe (Tektronix CT-2) with a
digital oscilloscope (Tektronix TDS 210).
1
DMET can also be used as a diesel fuel additive to reduce soot
and smoke emission. Another purpose of adding DMET to
2
diesel fuel is to increase the oxygen level of diesel fuel.
DMET is usually obtained from the oxidative coupling of
dimethyl ether (DME) (via 2CH OCH + 1/2O2
→
3
3
3
CH OCH CH OCH + H O). It is well known that DME is a
3
2
2
3
2
useful chemical intermediate for the synthesis of many impor-
tant chemicals.4 The special properties of DME allow it to be
used as a chemical building block for oxygenated hydrocarbons
with higher molecular weight. However, the oxidative coupling
of DME has to be conducted at high temperature and pressure,
with which the complete oxidation of DME is more thermody-
namically favored. Here we report a non-oxidative conversion
of DME in a dielectric-barrier discharge (DBD) reactor at ambi-
ent condition. DBD is one of non-thermal plasma phenomena,
which has been considered very promising for organic chemical
reactions because of its non-equilibrium properties, low dis-
charge power requirement and its capacity to induce physical
and chemical reactions within gases at relatively low gas tem-
peratures.7
A high DME conversion has been achieved in the present
reactor design. Figure 2 presents the effect of discharge power
on conversion of DME. It can be seen that the conversion of
DME increases from 17.43% to 66.56% when discharge power
increases from 35 W to 100 W. Figure 2 also shows a slightly
decrease in selectivity of liquid product and an increase in liq-
uid yield with the increasing power.
–6
Figure 1 illustrates the reactor system. This tubular reactor
is composed of an outer steel cylinder and an inner quartz tube.
The outer steel cylinder serves as the ground electrode. A
metal wire located in the center of quartz tube serves as the
high voltage electrode. The gas flow of pure DME is subjected
to the action of the DBD in an annular gap formed between the
outer stainless steel tube maintained at constant temperature
and the inner quartz tube. The radial width of the discharge
space was 1 mm, its length 250 mm. This reactor system is
very similar to the DBD reactor for methane conversion
8
described elsewhere. All the experiments were conducted at
The DBD conversion of DME leads to a selective produc-
tion of liquid product. The liquid product is a mixture mainly
comprising DAAK chemicals including DMET, DMPP, 1,2,3-
trimethoxypropane (TMPP) and some esters. From results in
Table 1, one can see that the content of DMET reached 23%.
atmospheric pressure. The feed and exhaust gases were ana-
lyzed by a gas chromatograph (HP 4890) with a thermal con-
ductivity detector (TCD) and a flame ionization detector (FID).
Deviation of carbon balance was within ±3%. The exhaust gas
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
323
A diesel fuel composition has been reported, which is con-
sisted of moderate amounts DMPP and DMET blended into a
conventional diesel fuel. DMET has a very high cetane num-
ber of 105. It is a cetane number promoter when added to diesel
fuel at concentrations above approximately 25 volume %.
Adding DMET to diesel fuel can also increase the oxygen level
of the diesel fuel so that the effect of oxygen level on emissions
could be determined.
It is clear that higher DME conversion accompanies with
more liquid products produced in this DME conversion. More
than 40% of the product is liquid at a gas flow rate of 30
2
–
1
mL·min . This suggests that DBDs could easily convert DME
to liquid chemicals at atmospheric pressure and low tempera-
ture. A more surprising phenomenon with this DBD-DME con-
version is that no carbon deposit has been observed on the
dielectric, while a serious carbon deposit has been formed dur-
8
–10
The gaseous product of such DBD conversion of DME is a
ing plasma conversion of methane or other lower alkanes.
mixture of hydrocarbons, ethers, H and CO. Table 2 shows the
It is interesting to note that the C–O bond breakdown in a
DME molecule does not require a high energy and this leads to
the high DME conversion. The bond strength of C–O in DME
molecule is only 81 kcal/mol at 298 K. [CH ·] and [CH O·] radi-
2
composition of gaseous products observed at 60 °C and differ-
ent gas flow rates. The gaseous products include methane,
ethane, propane, methyl ethyl ether, acetone, methyl isopropyl
ether, diethyl ether, dimethoxymethane and 1,2-dimethoxy-
ethane. The composition of gaseous products changed greatly
with the variation of gas flow rate. Experimental results
3
3
cals can be easily obtained within DME plasmas. Hydrocarbons
and oxygenated hydrocarbons are expected to be formed from
chain reactions initiated by [CH ·] and [CH O·] radicals.
3
3
showed that most of the gaseous products were H , CO and
The potential of plasma conversion of DME has been
demonstrated in this work. Further investigation is being con-
ducted to improve the selectivity of desired product.
2
C –C alkanes. The amount of oxygenates was very little in the
1
3
gaseous phase but large in liquid product. Table 2 also shows
the composition of lower alkanes in the gaseous phase increases
with the decreasing flow rates.
The support from The Research Fund for the Doctoral
Program of Higher Education in China is very appreciated. The
assistance from Mr. Yang Li, Dr. Zhen-hua Li, Dr. Chun-de Yao
and Dr. Ming-fa Yao in Tianjin University is also appreciated.
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