Controlled methyl chloride synthesis at mild conditions using ultrasound irradiation

Article abstract of DOI:10.1016/j.ultsonch.2009.08.002

A new route for the chlorination of methane is described using ultrasound irradiation, which allows for an intrinsically safe process at ambient pressure and temperature. By tuning the gas feed composition methyl chloride yields of up to 19% have been obtained.

Full text of DOI:10.1016/j.ultsonch.2009.08.002

Ultrasonics Sonochemistry 17 (2010) 315–317
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Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Short Communication
Controlled methyl chloride synthesis at mild conditions using ultrasound irradiation
a
Maikel M. van Iersel a, Marcus A. van Schilt b, Nieck E. Benes c, , Jos T.F. Keurentjes
*
a Process Development Group, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
b AkzoNobel Industrial Chemicals, 3800 AE Amersfoort, The Netherlands
c Membrane Technology Group, Faculty of Science and Technology, Institute of Mechanics, Processes and Control Twente (IMPACT), University of Twente, 7500
AE Enschede, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 June 2009
Received in revised form 4 August 2009
Accepted 5 August 2009
Available online 12 August 2009
A new route for the chlorination of methane is described using ultrasound irradiation, which allows for an
intrinsically safe process at ambient pressure and temperature. By tuning the gas feed composition
methyl chloride yields of up to 19% have been obtained.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Green chemistry
Acoustic cavitation
Methane chlorination
Gas-phase sonochemistry
1. Introduction
In many systems ultrasound irradiation is known to success-
fully increase conversion, shorten reaction times, and minimize
Methyl chloride is a versatile methylating agent and as such it is
an important intermediate in the chemical industry [1–3]. The
largest market for methyl chloride is in the production of silicone
polymers, which are increasingly applied in traditional non-
silicone applications. Other uses of methyl chloride are in the
production of a large variety of methyl ethers such as methyl
cellulose and methyl amines. Despite its toxicity, methyl chloride
does not suffer as much from the stringent environmental legisla-
tion that applies to other chlorinated hydrocarbons as there is
virtually non-exposure to the general public. Consequently, methyl
chloride represents one of the few halogenated compounds for
which demand increases.
Currently, the industrial chlorination of methane is performed
as a non-catalytic, thermally initiated gas-phase reaction. This
route requires elevated pressures and temperatures to obtain a
significant yield. Furthermore, the chlorination of methane is an
exothermic reaction involving a flammable reactant and such reac-
tions are preferably performed under diluted conditions, resulting
in improved reaction heat removal and reduction of potential
explosion hazards. Development of a more sustainable production
process should therefore aim at mild reaction conditions, prefera-
bly in an aqueous environment [4,5].
waste production and as such it is an important tool in green
chemistry [6–9]. These effects primarily derive from acoustic cav-
itation, i.e. the sound-induced growth and adiabatic collapse of
microscopic bubbles in a liquid, leading to hot-spots. In the hot-
spots, very high temperatures, pressures and heating and cooling
rates can be obtained. These extreme conditions lead to the disso-
ciation of chemical bonds and the formation of radicals [10–13].
The extent of radical formation is known to strongly depend on
the cavity temperature attained upon collapse.
Many studies on ultrasound-induced chemistry, i.e. sonochem-
istry, involve liquid-phase reactions. Examples hereof include the
free-radical polymerization of acrylates and styrene in aqueous
solution and the non-selective oxidation of pollutants [14–16]. To
achieve sufficiently high radical concentrations in the liquid, these
studies aim for maximizing the temperature rise inside the cavity.
Numerical simulations and sonoluminescence studies have
indicated that the highest hot-spot temperatures are achieved with
monoatomic gases, e.g., argon [17,18]. Accordingly, liquid-phase
reactions are typically performed under saturated argon
conditions.
The chlorination of methane is a reaction between two gaseous
reactants and will proceed predominantly inside the collapsing
cavities. Previous studies have demonstrated that such gas-phase
reactions do not merely benefit from a high hot-spot temperature,
but that it also has to be balanced with the reactant concentration
inside the cavity [19,20]. Initially, the effect of gas feed composi-
tion on hydrogen formation rates from water have been studied
* Corresponding author. Tel.: +31 534894288; fax: +31 534894611.
E-mail addresses: Maikel.vanIersel@akzonobel.com (M.M. van Iersel), n.e.benes@
utwente.nl (N.E. Benes).
1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2009.08.002

316
Maikel M. van Iersel et al. / Ultrasonics Sonochemistry 17 (2010) 315–317
in this work to gain more insight into gas-phase sonochemistry.
Elaborating on these insights, the ultrasound-induced chlorination
of methane in an aqueous environment under ambient conditions
has been explored.
5
4
3
2
1
0
CH4
C2H4
C4 H10
2. Experimental
2.1. Apparatus and analysis
Ultrasound with a frequency of 20 kHz was applied from the top
of a glass reactor (300 mL) using an ultrasonic generator (Sonics
and Materials, VC750). The piezoelectric transducer was coupled
to the liquid with a 13 mm diameter full wave titanium alloy horn.
The temperature inside the reactor was controlled externally using
a thermostatic bath and a Pt-100 temperature sensor. Thermal
mass flow meters (Bronkhorst High-Tech) were used to control
the gas feed composition. For the chlorination experiments, the
outgoing flow was scrubbed with two 0.1 M sodium hydroxide
solutions and contacted with dry sodium hydroxide pellets to
remove acid gases. The composition of the outgoing flow was
analyzed using a gas chromatograph equipped with thermal con-
ductivity detectors (Varian, Micro-GC CP4900). To separate and
analyze the mixture two different columns were used: 10 m molsi-
eve 5 Å column with 1 m Pora PLOT-Q precolumn (argon as carrier)
and 10 m Pora PLOT-Q column with 1 m Pora PLOT-Q precolumn
(helium as carrier).
0
20
40
60
80
100
Hydrocarbon in the feed (v/v%)
Fig. 1. Rate of hydrogen gas formation from water at 293 K using 20 kHz ultrasound
with an intensity of 40 W/cm2. Prior and during sonication a mixture of argon and
hydrocarbon was sparged through the mixture.
centration of 8% these physical and chemicals effects are tuned
in such a way that the highest hydrogen formation rate is observed.
A similar dependence on gas feed composition has been found for
the other hydrocarbons, i.e. ethylene and iso-butane (Fig. 1).
Hence, performing gas-phase sonochemistry requires a subtle bal-
ancing of reactant concentration and heat capacity of the cavity
content.
2.2. General procedure
Fig. 1 also shows the strong variation of the optimal gas feed
composition for the different reactants. The concentration at which
the hydrogen production is maximal varies for the different hydro-
carbons in the following order: C2H4 < CH4 < C4H10. If the gas and
liquid-phase compositions were to be in equilibrium, the optimal
hydrocarbon concentration would shift to a lower value for gases
with a higher specific heat capacity [24]. However, the order in
optimum hydrocarbon concentration corresponds to the order in
gas solubility [25]. This order suggests that the cavities are not in
thermodynamic equilibrium with the surrounding liquid and that
during cavity formation, gases with a relatively high solubility in
water can accumulate in the cavity interior more readily, which re-
sults in a concentration exceeding that of the gas feed [23]. Numer-
ical simulations have revealed that gas diffusion is relatively slow
compared to the cavity dynamics [26]. Since 20 kHz sonication
At the start of an experiment the reactor was filled with 250 mL
of MilliQ filtered water (>18 M
X
cmÀ1) and the temperature of the
thermostat was set to 293 K. Prior to sonication the solution was
saturated for at least 1 h with the desired gas mixture, containing
the following gases or a mixture hereof: argon (grade 5.0, Hoek
Loos), methane (grade 5.5, Hoek Loos), ethylene (grade 3.0, Hoek
Loos), iso-butane (grade 2.5, Hoek Loos), and 10 v/v% chlorine in ar-
gon (Linde Gas). During ultrasound, the power input was main-
tained at 50 W and the total gas flow rate equalled 30 mL/min.
At fixed time intervals a sample was taken from the outgoing flow
for GC-analysis. Ultrasound was applied to the solution continu-
ously until the measured concentrations reached steady state.
3. Results and discussion
During sonication of aqueous systems, the dissociation of water
leads to the production of H- and OH-radicals, which predomi-
nantly recombine inside the cavity before entering the liquid
[21,22]. Upon recombination, the main products formed are hydro-
gen and hydrogen peroxide and hence, hydrogen formation rates
can provide valuable insights with respect to gas-phase sonochem-
istry [23]. Fig. 1 shows hydrogen formation rates for water satu-
rated with a mixture of argon and either methane, ethylene or
iso-butane.
As is evident from this figure, the extent of hydrogen gas forma-
tion strongly depends on the composition of the gas mixture. For
water saturated with argon, hydrogen radicals can only arise from
the dissociation of water molecules and hence, the experimentally
determined rate of hydrogen gas formation is limited. The addition
of a gaseous hydrocarbon leads to a higher concentration of hydro-
gen atoms in the cavity interior and as a result, the rate of hydro-
gen gas formation increases. However, polyatomic gases have a
relatively high heat capacity and thereby suppress the temperature
increase upon collapse. Accordingly, the hydrogen production de-
creases for higher methane fractions and approximates almost zero
for a predominant methane feed. For a feed with a methane con-
25
20
15
10
5
0
0
20
40
60
80
100
CH4 (v/v%)
Fig. 2. Methyl chloride yield from water saturated with a mixture of methane,
argon and chlorine. The chlorine supply consisted of a mixture of chlorine and
argon (10 v/v%) for reasons of safety and hence, it was not possible to determine the
methyl chloride yield in the absence of argon.

Maikel M. van Iersel et al. / Ultrasonics Sonochemistry 17 (2010) 315–317
317
Table 1
Methyl chloride synthesis using ultrasound irradiation.
4. Conclusions
Gas feed composition (v%)
Time to steady
state (min)
Methyl chloride
yield
This work has demonstrated that ultrasound irradiation allows
a more sustainable process towards methyl chloride as
for
CH4
Cl2
Ar
compared to the current industrial chlorination of methane. The
extreme conditions inside the imploding cavities enable radical
formation at ambient bulk conditions. Furthermore, this ultrasonic
route is intrinsically safe as the reaction is performed under diluted
conditions and the reaction stops when the ultrasounds source is
switched off. Accurate control of the cavity contents is crucial for
obtaining significant yields in such gas-phase reactions. Addition
of a monoatomic gas is necessary to achieve the required temper-
ature rise, whereas the reactant concentration inside the cavity is
relevant for the chemical effect. When aiming at a particular reac-
tant concentration inside the cavity, non-equilibrium thermody-
namics have to be taken into account. Due to rapid cavity
dynamics, the composition of the cavity interior does not equal
the gas feed composition, yet strongly corresponds to the relative
solubilities of the gases in the liquid.
4.09
0.22
0.22
0.22
0.22
95.69
83.58
53.22
10.79
90
4
16.20
46.56
88.99
170
170
170
19
15
7
predominantly induces transient cavitation, the cavities have
insufficient time to fully equilibrate with the surrounding liquid
[27]. The aqueous solubilities of methane and argon are approxi-
mately equal at 293 K. As ethylene is more soluble in water than
argon, a relatively large amount of ethylene is likely to be present
inside the cavity when it forms. This would correspond with the
observation that the hydrogen formation rate for ethylene has a
maximum at a hydrocarbon concentration lower than in the case
of methane. The same argument holds for a gas feed containing
iso-butane.
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of an inert monoatomic gas in the feed are required to assure a suf-
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