Methane to Chloromethane by Mechanochemical Activation: A Selective Radical Pathway

Article abstract of DOI:10.1021/jacs.9b04413

State-of-The-Art processes to directly convert methane into CH₃Cl are run under corrosive conditions and typically yield a mixture of chloromethanes requiring subsequent separation. We report a mechanochemical strategy to selectively convert methane to chloromethane under overall benign conditions, employing trichloroisocyanuric acid (TCCA) as a cheap and noncorrosive solid chlorinating agent. TCCA is shown to release active chlorine species upon milling with Lewis acids such as alumina and ceria to functionalize methane at moderate temperatures (<150 °C). A thorough parameter optimization led to a maximum methane chlorination rate of 0.8 μmol(CH_4,conv) (g(catalyst) s)^-1. Findings were compared to the thermal reaction of methane with TCCA and evidenced that mechanochemical activation permitted significantly lower reaction temperatures (90 vs 200 °C) at a drastically improved CH₃Cl selectivity (95% vs 66% at 30% conversion). Considering the characterization of the interaction between TCCA and Lewis acids as well as the in-depth analysis of byproducts, we suggest a plausible reaction mechanism and a possible regeneration of the chlorinating agent.

Full text of DOI:10.1021/jacs.9b04413

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Article
Methane to Chloromethane by Mechanochemical
Activation: A Selective Radical Pathway
Marius Bilke, Pit Losch, Olena Vozniuk, Alexander Bodach, and Ferdi Schüth
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b04413 • Publication Date (Web): 19 Jun 2019
Downloaded from http://pubs.acs.org on June 20, 2019
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Methane to Chloromethane by Mechanochemical Activation: A Selective Radical Pathway
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Marius Bilke , Pit Losch , Olena Vozniuk, Alexander Bodach, Ferdi Schüth
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Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany
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authors contributed equally
corresponding authors: Dr. Pit Losch losch@kofo.mpg.de and Prof. Dr. Ferdi Schüth
schueth@kofo.mpg.de
Abstract
State-of-the-art processes to directly convert methane into CH Cl run at corrosive conditions and
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typically yield a mixture of chloromethanes requiring subsequent separation. We report a
mechanochemical strategy to selectively convert methane into chloromethane at overall benign
conditions, employing trichloroisocyanuric acid (TCCA) as a cheap and non-corrosive solid
chlorinating agent. TCCA is shown to release active chlorine species upon milling with Lewis
acids such as alumina or ceria to functionalize methane at moderate temperatures (< 150 °C). A
thorough parameter optimization led to a maximum methane chlorination rate of
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.8 µmol(CH4, conv.) (g(catalyst) s) . Findings were compared to the thermal reaction of methane
with TCCA and evidenced that mechanochemical activation permitted significantly lower
reaction temperatures (90 vs. 200 °C) at a drastically improved CH Cl selectivity (95 % vs 66 %
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at 30 % conversion). Considering the characterization of the interaction between TCCA and
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Lewis acids as well as the in-depth analysis of by-products we suggest a plausible reaction
mechanism and a possible regeneration of the chlorinating agent.
Keywords: Mechanochemistry ∙ Methane Functionalization ∙ Selective Halogenation
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1
. Introduction
Chloromethane (CH Cl) is an attractive platform molecule that can easily be transformed into a
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wide variety of chemicals and fuels. CH Cl presently finds application as methylating agent for
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polymers and, more importantly, it can directly be used as a feedstock to produce olefins with
zeotype catalysts.3-5 A decisive limitation for large scale application of a process converting
natural gas into olefins via chloromethane is the low selectivity to monohalogenated products.
Selectivities towards monochloromethane are low, because CH Cl is more readily chlorinated
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than methane itself. Therefore current industrial processes for synthesizing
monochloromethane rely on product separation after a radical reaction, or methanol
hydrochlorination. The latter is catalyzed by alumina with a very high selectivity to
monochloromethane, but it does not allow the direct upgrade of natural gas and requires the
production of methanol as an intermediate product. A selective process for the direct chlorination
of methane to chloromethane has not yet been established.
Efforts to improve selectivity have, amongst others, been conducted by Olah et al., who
contacted a CH /Cl mixture with Pt/Al O catalysts and achieved a high selectivity at relatively
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low temperatures (S(CH Cl) > 90 % at 250 °C). Another major advance was the development of
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oxyhalogenation processes.1, 9 This elegant approach relies on the in situ generation of gaseous
halogen by feeding a gas stream of the corresponding hydrogen halide together with O , which
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then halogenates alkanes via radical pathways.
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Besides the limited selectivity to monochloromethane, present processes necessitate dangerous
corrosive conditions as high temperatures, typically above 400 °C are required to initiate the
methane chlorination. Chang et al. have shown for example the dependence of the corrosion rate
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of chlorine (Cl ) on steel to be dramatically temperature dependent.
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In this work, we demonstrate the use of a chlorinating agent, being safe in terms of storage and
transport, that is produced at large scale and thus readily available at reasonable cost.11
Trichloroisocyanuric acid (TCCA) is a common bleaching and disinfecting agent, e.g. for
swimming pools. In addition, TCCA is used in acid catalyzed organic chlorination reactions.12-14
It possesses an exceptionally high active chlorine content (91.5 % = 46 wt %), comparable to that
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of molecular Cl (100 % = 50 wt %). The molecule is stable under ambient conditions and
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thermally decomposes only above 230 °C. Besides in acid catalyzed reactions TCCA has been
applied in solid state reactions induced by mechanical impact. In general, trihaloisocyanuric acids
are used for halogenations of arenes and 1,3-dicarbonyl compounds. Moorthy and co-workers
investigated the potential of tribromoisocyanuric acid (TBCA) and trichloroisocyanuric acid
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TCCA) in a solvent-free mechanochemical system for halogenations of electron rich arenes.16-18
Excitingly, Hernandez et al. reviewed C−H bond functionalization by mechanochemical
approaches as a rapidly evolving field of research, which, however, remains generally restricted
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to larger organic molecules. Yet, Bolm et al. recently described the potential of gas phase
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reactions in ball milling setups. Gas phase ball milling reactions allow to completely avoid
solvents. The most challenging C-H activation, namely the transformation of CH involves the
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critical step of C−H bond scission to form methyl radicals or methyl-metal intermediates, which
readily react with solvent molecules.
Mechanochemical gas-solid reactions avoid solvents and may proceed via radical pathways,21-23
triggered by local pressure, hotspots and short-lived defects on surfaces.24,25 Therefore we
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envisioned a gas-solid mechanochemically driven chlorination of methane characterized by the
following advantages: the mechanochemical activation should enable the use of a cheap
chlorinating agent, rely on abundant and easily accessible catalytic materials, and proceed at non
corrosive reaction conditions (Figure 1).
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Figure 1. Mechanochemical chlorination of methane at relatively low temperatures < 150 °C (The
formalism of three balls indicating a mechanochemical reaction has for the first time been introduced by
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Harusa et al. ).
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. Methods
All chemicals and gases used herein were purchased from commercial suppliers and used as
received, unless stated differently. Trichloroisocyanuric acid (> 95%), cyanuric acid (98%),
cerium(III)nitrate hexahydrate (99.99%), γ-aluminum oxide (catalyst support, 200 m²/g), quartz
sand (>99.995%), titanium oxide (>99.98%, rutile), iron(III)oxide (< 96%), magnesium oxide (>
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99%), tungsten carbide (99.5%), H-USY (surface area 730 m /g, 6:1 Si/Al, calcined at 550°C).
For safety reasons gaseous chlorine and potential oxidizing harmful compounds were neutralized
in liquid traps if necessary while the entire experimental reactor setup was built and operated in a
fume hood.
2.1. Experimental setup for the chlorination reaction
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Continuous catalytic experiments were carried out inside a modified 25 mL WC milling vial
(Figure S13) attached to a Retsch MM400 shaker mill. The milling vial was filled with the solid
catalyst (500 mg), TCCA (70 mg) and seven tungsten carbide balls (d = 5 mm; m(ball) = 1,02 g).
The jar is equipped with a gas in- and outlet to enable a continuous flow of reactant gas (0.5 - 15
vol.% CH in N ). The vial was shaken at a frequency of 10 Hz, while the whole milling jar was
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set to a temperature of 112.5 °C. Continuous sampling of the outlet gas flow was guaranteed by
an online GC (Agilent 6850) equipped with apolar column (DB-1, 30 m, 0,25 mm, 0,25 µm) and
a flame ionization detector (FID). The presence of monochloromethane was confirmed by MS
and response factors on the GC were determined by using pure products; CH , CH Cl, CH Cl
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and CHCl . HCl-formation test was performed on the setup described above, with optimized
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reaction conditions, a flow of 10 mL.min of CH (15 %), 10 Hz shaking frequency, 7 balls, 500
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mg CeO and 70 mg TCCA at 112.5 °C. For this experiment, gas was washed with a saturator
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containing 10 mL dist. H O. The pH of this solution was analyzed before and after reaction by
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means of a Titrino Plus 848 pH meter from Metrohm. Cl quantification was carried out on the
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same setup as described for HCl titration, applying an off-line iodometric titration of triiodide,
which forms upon bubbling the reaction gas stream containing through 10 mL of an aqueous KI
solution (0.2 M). Its formation could be optically ascertained as the clear solution gradually
turned orange, yet only after switching on the milling. The formed triiodide was then titrated with
a standard sodium thiosulfate (0.1 M) solution (Fisher Scientific, for volumetric analysis).
2.2. Plug flow setup
Homogeneous mixtures of milling medium (ceria, alumina or silica respectively) and TCCA were
prepared by gentle grinding and loaded into reactor tubes (length 28 cm, ID 6.5 mm, OD 10 mm,
made of steel-1.4571). Temperature was controlled inside the bed with a K-type thermocouple.
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Continuous sampling of the outlet gas flow was achieved using a direct connection to the same
GC-setup described above.
2.3. Characterization
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Attenuated total reflection IR spectra were recorded on a Nicolet Magna-IR 560 with a mercury
cadmium telluride (MCT) detector cooled with liquid N . 32 scans were accumulated and
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averaged to improve the signal to noise. Raman measurements were performed using the 532 nm
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line of a Nd:YAG laser for excitation in the region 50–3500 cm on a Renishaw in Via Qontor
confocal Raman microscope equipped with a CCD detector. NMR spectra were measured using a
Bruker AVIII 300 Nanobay spectrometer. Spectra were referenced to residual protons of the
deuterated solvent. Chemical shifts are stated in parts per million (ppm) downfield of
tetramethylsilane. N -physisorption measurements were carried out on a 3Flex from
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Micromeritics while samples were degassed on the SmartVacPrep setup from Micromeritics.
Samples were degassed first at 90 °C for 1 h, then at 300 °C for 10 h, and measured from 0.01
p/p onwards as no microporosity was expected. Specific surface areas (SSA) were calculated
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Micromeritics in the 0.05 - 0.3 p/p range. Total pore volumes were calculated from the volume
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adsorbed at relative pressures of p/p = 0.99. DSC All samples were measured on a Mettler
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Toledo TGA/DSC 1 in 100 μL aluminium crucibles with a heating rate of 5 °C.min under
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constant Ar flow of 60 mL.min controlled by GC 200 gas controller. All samples analysed by X-
ray powder diffraction (XRPD) were measured on STOE Stadi P diffractometers, equipped with
a curved Ge (111) monochromator and sealed X-ray tubes. Mythen K1 silicon strip detector and
linear position sensitive detectors (STOE lin PSD) were used. Heating of the samples was
performed with a STOE HT1 oven. Further details on the XRPD analyses can be found in the SI.
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. Results and Discussion
Initial tests were carried out in a FritschTM Pulverisette 5 planetary ball milling setup under
methane pressure. These tests gave only minor conversion of methane with acidic milling
medium. The observed trace products (CH Cl and CH Cl ) were identified by mass spectrometry
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(
Figure S2). Further experiments were performed in a RetschTM MM400 shaker mill equipped
with a tungsten carbide (WC) milling jar modified for continuous gas flow reactions (Inset,
Figure 2A), described in previous publications (further details are given in the SI).21, 22 Effluent
gases were collected and analyzed by means of a GC-FID.
Figure 2. A) Reaction rate (R) for different materials under identical milling conditions; reactions were
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carried out over 40 min with a flow of 10 mL min of 15 % CH in N at 100 °C at a shaking frequency of
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5 Hz with 7 balls (Ø 5 mm), 500 mg catalyst and 70 mg TCCA. In all the reactions only CH Cl was
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detected. Inset depicts a scheme of the shaker mill reactor; B) After optimization of reaction parameters a
four and three fold reaction rate was achieved for reaction carried out with alumina and ceria respectively.
Reaction rate averaged over a reaction over 40 min for alumina, ceria and silica before and after
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optimization of milling parameters. The optimized reaction conditions were as follows: 10 mL min of 15
CH in N at 112.5 °C at a shaking frequency of 10 Hz with 7 balls (Ø 5 mm), 500 mg catalyst and 70
%
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We investigated a set of abundant solid materials with different physico-chemical properties in
this milling setup: basic MgO was compared to redox active Fe O , weakly acidic SiO , Brønsted
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acidic zeolite H-USY (Si/Al = 6), non-reducible (γ-Al O ), and reducible Lewis acids (TiO and
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CeO ), and to the milling jar material itself, WC. Figure 2A shows the respective reaction rates
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in µmol(CH4 conv).(g.s) averaged over a reaction time of 40 minutes. The Lewis acidic materials
were found to be most promising for the methane monochlorination.
In addition to the Lewis acidic surface, other physicochemical parameters may certainly be
involved in mechanochemically activated reactions. Such parameters encompass, but are not
limited to the hardness and the brittleness of a material, its porosity and surface area as well as
the presence of defect sites, but also its thermal conductivity. Here, we decided to not focus on
these parameters.
The setup allows screening of catalytically relevant parameters in a systematic manner.
Temperature (from RT to 150 °C), contact time (i.e. composition and rate of gas flow) and the
transfer of kinetic energy or momentum (by varying the shaking frequency) were studied. Thus
milling parameters could be thoroughly optimized (Figure S3). As a result of this optimization
the loading of the chlorinating agent as well as the shaking frequency emerged as key parameters.
Figure 2B displays reaction rates for ceria, alumina and silica before and after optimization. The
reaction rates for ceria and alumina were increased by a factor of three to four respectively. In
contrast silica still showed no detectable activity.
While selectivity for methane conversion to chloromethane was close to 100%, the maximum of
chlorine transferred to form chloromethane was limited to 30 % of the available active chlorine,
even for the optimized system. Thus, the gas phase composition was analysed by means of an
online GC with a temporal resolution of 3 min. Before the start of each reaction, the effluent
gases were monitored by GC at the reaction conditions, but without shaking. No activity was
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observed without milling for standardized conditions at 112.5 °C. As soon as the mill was started,
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the reaction starts with a very high initial rate (up to 0.8 µmol.(g.s) ) (Figure 3A). The
mechanochemically induced high reaction rates are maintained for 10 - 20 minutes, depending on
the used material, after which the reaction rate decreases. This is probably due to the decrease in
the amount of TCCA available and the accumulation of solid by-products.
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Figure 3. A) Methane conversion rate as a function of time for CeO (in red), Al O (in black) and SiO
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(in blue) under optimized reaction conditions with a flow of 10 mL min of 15 % CH in N at 112.5 °C at
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a shaking frequency of 10 Hz with 7 balls (Ø 5 mm), 500 mg catalyst and 70 mg TCCA, CH Cl selectivity
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is plotted with hollow symbols, inset table shows respective CH Cl yields in µmol; B) Methane
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conversion (full symbols) and CH Cl yield (hollow symbols) as a function of temperature for a plug flow
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reaction (in grey) with mixture of 500 mg of Al O and 70 mg TCCA, compared to the reaction with 500
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mg Al O and 70 mg TCCA under optimized milling conditions (in black); C) normalized mass loss as a
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function of temperature (TG) and D) relative heat flow normalized to the measured mass (curves are offset
for clarity). TG-DSC of TCCA, pure silica, ceria and alumina as well as mixtures of 50 mg TCCA with
2
00 mg of the respective solid oxide (note: TCCA is entirely decomposed > 290 °C).
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While chloromethanes are readily swept off with the gas flow, solid or liquid by-products can
only be analyzed after reaction. The solids after a reaction time of 40 minutes were analysed by
ATR-FTIR (Fig. S4) and liquid phase NMR after washing the solid with CD CN and d -DMSO
3
6
(Fig. S5). IR spectra confirm that TCCA is quantitatively converted, as an absorption band at
-1
-1
1050 cm (cyanuric acid) and none at 1150 cm (TCCA) was observed after mechanochemical
reaction. Higher chlorinated alkanes (C H Cl with n > 1) were never observed during the
n
2n-x
x
experiments. Coupling reactions can therefore be ruled out. In IR, NMR and Raman spectra
cyanuric acid was detected as the only non-volatile product (Figure S4 - S6). Since cyanuric acid
accumulates over the course of a reaction run, cyanuric acid was deliberately added at the
beginning of the reaction in order to study a potentially inhibiting effect of cyanuric acid. The
simultaneous loading of both TCCA and cyanuric acid significantly reduced the initial reaction
rate (Figure S7). The final yield, however, did not significantly vary compared to reactions with
only TCCA.
Since almost no chlorine was present in the solid reaction products, further analyses of the
effluent gas phase were performed to detect and quantify other chlorine-containing species such
as HCl or Cl , which also might form under the given reaction conditions. For this purpose, the
2
setup was slightly modified (Figure S13). The effluent gas was passed through a trap, filled with
either 10 mL distilled water to monitor possible changes of pH, which would indicate formation
of HCl, or 10 mL of a KI solution (0.1 M) to quantify Cl (detailed procedures of these analyses
2
in SI). The only minor pH variation indicated that less than 1 % of the active chlorine in TCCA
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was transformed into HCl. In contrast, our iodometric experiments revealed that roughly two
thirds of the loaded active chlorine is lost in form of Cl , while the remaining chlorine reacts with
2
methane to form CH Cl, as seen in the catalytic experiments. This allows closing the chlorine
3
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balance within the experimental error (Figure S1). On the other hand, it also confirms that the
most significant side reaction is the decomposition of TCCA into its raw materials. We are
confident that during upscaling of this reaction, proper tuning of the rate of chlorine radical
generation or of the average residence time of methane in the milling jar might allow a significant
suppression of this side reaction.
It is furthermore worthy to mention that TCCA is synthesized from cyanuric acid, Cl and NaOH
2
1
6
at large scale, thus it is imaginable to reactivate cyanuric acid and to reuse the formed Cl after
2
the reaction. A simple washing step with water efficiently removes residual cyanuric acid from
the milling jar, so that it is easily recyclable. N-chlorosuccinimide (NCS), which is well in
organic synthesis, was tested as an alternative chlorinating agent (for both, same mass and same
molar amount). It is less efficient compared to TCCA: 28 µmol(CH Cl) were obtained for NCS,
3
while 186 µmol(CH Cl) were formed with TCCA under otherwise identical conditions. Further it
3
is important to note that even though some dilute gaseous chlorine is present in parts of our
system, the process is overall still running under non corrosive conditions due to the much lower
temperature required to initiate the reaction. As described above, Chang et al. report very low
1
0
corrosion rates of molecular Cl on metals at temperatures below 150 °C.
2
In order to understand the reaction mechanisms of this intriguing process, a number of questions
was addressed. It appeared to be essential to understand how important the mechanical impact is
in the low temperature chlorination of methane. Accordingly, a plug flow setup was used to
follow methane conversion as a function of the catalyst bed temperature without mechanical
energy input (Figure 3B and Figure S8). The residence time of the reactant gas was held at a
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similar value like in milling experiments, i.e. identical methane flow rate and concentration were
used. TCCA was gently premixed with either SiO , Al O or CeO and then loaded into the
2
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2
reactor tube (Figure S8). Methane conversion and CH Cl yields are plotted as a function of
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temperature for the TCCA-alumina system in Figure 3B. For the thermal, plug flow reaction
significant conversion is detected above 200 °C. Strikingly, when this system was subjected to
different temperatures under mechanochemical activation, methane was converted at
temperatures as low as 100 °C. The latter reaction exhibited the highest selectivity towards
CH Cl, with a product yield nearly equalling conversion, i.e. selectivity close to 100 %. In
3
contrast, the selectivity towards the monochlorinated product significantly decreased with
increasing conversions for all the plug flow experiments, irrespective of the nature of the oxide
present.
As shown in Figure S8, both ceria and alumina seem to interact with TCCA, since CH Cl was
3
detected at 160 °C (CeO ) and 210 °C (Al O ), respectively, significantly before the
2
2
3
decomposition temperature of neat TCCA (> 230 °C). In contrast, methane conversion for the
TCCA-silica system is only observed at temperatures above 230 °C and can thus be ascribed to
the mere thermal decomposition of TCCA.
The aforementioned possible interactions between TCCA and Lewis acidic milling medium were
characterized by thermogravimetry coupled with differential scanning calorimetry (TG-DSC) and
in situ high temperature X-ray powder diffraction (XRPD). From the TG-DSC analyses it can be
concluded that pure TCCA exhibits an endothermic phase transformation at 130 °C and probably
melts at 230 °C and further decomposes (Fig. 3C and 3D). A similar behaviour can be observed
when TCCA is mixed with SiO . In contrast, Al O and CeO strongly influence the thermal
2
2
3
2
decomposition of TCCA. TCCA on Al O shows two exothermic mass losses at 80 and 210 °C.
2
3
The first mass loss at 80°C is not associated with methane activation, but the latter one exactly
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falls in the temperature range, at which CH Cl was detected in the plug flow setup. Ceria affects
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the decomposition of TCCA in a similar way, an exothermic mass loss at 160 - 180 °C perfectly
agrees with the initial activity in CH Cl production in the plug flow reactor. It can therefore be
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assumed that a homolytic N-Cl bond scission occurs at 210 °C for Al O and at 160 °C for CeO .
2
3
2
In situ XRPD data (Figure S9) support the above. TCCA undergoes a phase transformation
at > 120 °C. In contrast, TCCA reacts at significantly lower temperatures when dispersed on
Al O (> 70 °C) or CeO (> 100 °C). The TG-DSC and in situ XRPD data correlate well with the
2
3
2
plug flow reactor tests, thereby confirming interactions between TCCA and Lewis acidic milling
media which seems to be crucial in mechanochemical activation.
The herein used conditions with 7 balls and a total mass of 7.16 g, shaken at a frequency of 10
Hz, generate only moderate momentum at the impact, which is supported by unchanged textural
properties of ceria (Figure S11).
To gain further mechanistic insight into the system, supplementary tests as summarized in Figure
4
A and B were carried out. The milling experiment was carried out over three cycles of 40 min
milling followed by a pause of 40 min. The standard reaction with only TCCA, ceria and
methane in the flow shows a high initial rate. This reaction rate decreases quickly and stays low
during the second and third milling cycle. The mechanochemical reaction depends on the solid
chlorinating agent as a feed, which decomposes with ongoing reaction, while the by-product
cyanuric acid increasingly inhibits the reaction.
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Figure 4. A) Methane conversion rate as a function of reaction time, 3 cycles of alternating 40
min milling and 40 min pause, for different conditions; TCCA and ceria deactivate after first
milling (full triangles), with O the reaction is almost fully quenched and methanol is detected as
2
a side-product (empty triangles); B) in the presence of gaseous Cl the catalytic lifetime of
2
TCCA/CeO is extended (full circles), while milling does not lead to any increased activity for
2
Cl with CH and CeO (empty circles); C) Proposed catalytic cycle for the CH / TCCA (+ Cl ) /
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CeO system, which enables low temperature selective monochlorination of methane.
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Earlier tests had shown that only negligible amounts of HCl are formed during the reaction, while
3
a significant amount of active chlorine recombined to Cl . If the bi-radical triplet oxygen ( O ) is
2
2
co-fed during the reaction, we observe a nearly total suppression of the reaction and detect both
CH OH and CH Cl as products. It can therefore be concluded that during the catalytic cycle free
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methyl radicals must be formed, which react with O to produce CH OH. A reaction mechanism
2
3
could thus be formulated as depicted in Figure 4C. If TCCA is activated by a homolytic N-Cl
bond scission, it is required that the N· radical preferably abstracts H-atoms from methane (2.),
while the Cl· radical will then recombine with the formed CH · radical to form CH Cl (3.).
3
3
Considering the high CH Cl selectivity, multiple consecutive chlorination steps to CH Cl or
3
2
2
CHCl can be ruled out (3’.).
3
These mechanistic hypotheses are fully supported by reported trends in homolytic bond
2
7
dissociation energies (BDE) between C-H, N-H and O-H bonds. For electronegative R as in
-1
TCCA and cyanuric acid, both R N-H and RO-H bonds (> 500 kJ.mol ) can exhibit higher BDEs
2
-1
than H C-H (439 kJ.mol ). In agreement with theory, a simple DFT model indicates no clear
3
barrier (Figure S12A). The abstraction of the H-atom is thermodynamically significantly
preferred by the N· or O· mesomeric radicals located on the cyanuric acid derived intermediate
(2.). Meanwhile, the Cl· radical can interact in a stabilizing way with ceria even though a non-
negligible amount of Cl· radicals recombine to form Cl (2’.). Under the current reaction
2
conditions, Cl formation is faster than the reaction rate of activated chlorine with methyl
2
radicals.
Arrhenius plots for the TCCA-Al O system for the mechanochemical and thermal reaction
2
3
-1
reveal a notable difference in the apparent activation energy (144 to 112 kJ.mol ) (Figure S12B).
The rate determining step is thus affected by the milling. Our data allow discriminating between
the possible rate limiting steps. We found that the homolytic N-Cl bond scission is limiting at the
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studied moderate temperatures (1.), while the C-H activation by radical pathways (2.) is
occurring as soon as N· or O· mesomeric radicals are formed.
Further experiments support these mechanistic hypotheses. If gaseous Cl was co-fed during the
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milling reaction as the sole chlorinating agent, only some minor background methane conversion
was observed. This remained, however, completely unaffected by milling. In contrast, the
simultaneous presence of TCCA and Cl led to a sustained mechanochemical reaction. Thus,
2
additional free Cl· radicals are generated which in turn are recombined with methane to form
CH Cl only via the intermediate interaction with TCCA / cyanuric acid. This interpretation is
3
supported by the higher overall rate of production of CH Cl.
3
The Cl co-feeding tests thus suggest that reaction step (2’.) is a reversible equilibrium reaction.
2
With respect to a large scale application, these results are promising. In an eventual continuous
mechanochemical gas-solid process, both cyanuric acid and Cl can be recycled. Especially when
2
considering the reversibility of the reaction step (2’.), it can be imagined to recirculate unreacted
CH and Cl , or to add gaseous Cl into the stream to continuously control the equilibrium
4
2
2
between cyanuric acid and TCCA. In other terms, this reaction shows that it is possible to in situ
regenerate TCCA from cyanuric acid. Thus the herein developed selective chlorination reaction
between methane and gaseous chlorine may be considered as a mechanocatalytic reaction with
TCCA / Lewis acid system (Figure 4B) acting as a catalyst / co-catalyst. In order to study and
confirm these hypotheses future work should focus on operando studies.
4
. Conclusion
A gas/solid mechanochemical process for the selective valorization of methane as CH Cl is
3
reported in this contribution. Currently it cannot be claimed that a fully continuous
mechanochemical process is already available. However, the results suggest that this process has
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the potential to become competitive with current state of the art processes. Considering the
available technical knowhow, the relatively low cost of the reagents used here, the added value of
selectively produced CH Cl as well as the safety and energetic aspects (low operating
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temperature and pressure), the process described has potential for large scale application.
-1
Maximum rates are 0.8 µmol(CH4 conv.).(g.s) . Given the selectivity of close to 100 %, this
-1
-1
corresponds to a gravimetric productivity of approximately 150 g(CH Cl).kg(catalyst) .h ,
3
which is in the range of other large scale technical processes for the production of bulk
chemicals.
Acknowledgements
P.L. is grateful to Fulbright and the Alexander von Humboldt foundation for financial support in
form of scholarships. M.B. P.L. O.V. A.B. and F.S. acknowledge the financial support of the
Max Planck Society. We thank B.B. Sarma for the help in preparing ceria, A. Bähr for lending
some equipment, F. Kohler for the help with the GC, J. Ternieden for in situ XRPD
measurements and M. Dürr for TG-DSC measurements. We are also grateful to C. Weidenthaler,
M. Felderhoff and W. Schmidt for helpful discussions. The design of the reactor would have been
impossible without the invaluable support of MPI KoFos fine mechanics workshop led by W.
Kersten.
Associated Content
The Supporting Information is available free of charge on the ACS Publications website. Detailed
description of experimental setup and procedure for catalytic tests, additional information on
catalyst characterization, reaction optimization, byproduct formation and mechanistic
considerations.
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Competing interests
The authors filed a patent application, EP19161450.2, “Process for the Direct Halogenation of an
Aliphatic Hydrocarbon to a Halogenated Aliphatic Hydrocarbon” on 7th of March, 2019.
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