Methane to Chloromethane by Mechanochemical Activation: A

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Cite This: J. Am. Chem. Soc. 2019, 141, 11212−11218

Methane to Chloromethane by Mechanochemical Activation: A Selective Radical Pathway Marius Bilke,† Pit Losch,*,† Olena Vozniuk, Alexander Bodach, and Ferdi Schüth* Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany

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ABSTRACT: 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 ( 90% at 250 °C).8 Another major advance was the development of 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 O2, which then halogenates alkanes via radical pathways. © 2019 American Chemical Society

Received: April 24, 2019 Published: June 20, 2019 11212

DOI: 10.1021/jacs.9b04413 J. Am. Chem. Soc. 2019, 141, 11212−11218

Article

Journal of the American Chemical Society

hexahydrate (99.99%), γ-aluminum oxide (catalyst support, 200 m2/ g), quartz sand (>99.995%), titanium oxide (>99.98%, rutile), iron(III) oxide (99%), tungsten carbide (99.5%), and H-USY (surface area 730 m2/g, 6:1 Si/Al, calcined at 550 °C). For safety reasons, gaseous chlorine and potentially harmful oxidizing 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. Continuous catalytic experiments were carried out inside a modified 25 mL tungsten carbide (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 WC balls (d = 5 mm; m(ball) = 1.02 g). The jar was equipped with a gas inlet and outlet to enable a continuous flow of reactant gas (0.5−15 vol % CH4 in N2). The vial was shaken at a frequency of 10 Hz while the whole milling jar was 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 an 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 CH4, CH3Cl, CH2Cl2, and CHCl3. An HCl-formation test was performed on the setup described above, with optimized reaction conditions, a flow of 10 mL•min−1 of CH4 (15%), a 10 Hz shaking frequency, seven balls, 500 mg of CeO2, and 70 mg of TCCA at 112.5 °C. For this experiment, the gas flow was passed through a trap containing 10 mL of distilled H2O. The pH of this solution was analyzed before and after reaction by means of a Titrino Plus 848 pH meter from Metrohm. Cl2 quantification was carried out on the same setup as described for HCl titration by applying an offline iodometric titration of triiodide, which forms upon bubbling the reaction gas stream containing 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 a milling medium (ceria, alumina, or silica, respectively) and TCCA were prepared by gentle grinding and loaded into reactor tubes (length 28 cm, i.d. 6.5 mm, o.d. 10 mm, made of steel-1.4571). The temperature was controlled inside the bed with a K-type thermocouple. Continuous sampling of the outlet gas flow was achieved using a direct connection to the same GC setup described above.

bond functionalization by mechanochemical approaches as a rapidly evolving field of research, which remains generally restricted to larger organic molecules.19 Bolm et al. recently described the potential of gas-phase reactions in ball milling setups.20 Gas-phase ball milling reactions allow us to completely avoid solvents. The most challenging C−H activation, namely, the transformation of CH4, involves the 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, hot spots, and short-lived defects on surfaces.24,25 Therefore, we 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 under noncorrosive reaction conditions (Figure 1).

Figure 1. Mechanochemical chlorination of methane at relatively low temperatures of 95%), cyanuric acid (98%), cerium(III) nitrate

Figure 2. (A) Reaction rate (R) for different materials under identical milling conditions; reactions were carried out over 40 min with a flow of 10 mL·min−1 of 15% CH4 in N2 at 100 °C at a shaking frequency of 15 Hz with seven balls (Ø 5 mm), 500 mg of catalyst, and 70 mg of TCCA. In all reactions, only CH3Cl was detected. The inset depicts the scheme of the shaker mill reactor. (B) After the optimization of reaction parameters, 4and 3-fold reaction rates were achieved for a reaction carried out with alumina and ceria, respectively. The reaction rate was averaged over a reaction over 40 min for alumina, ceria, and silica before and after optimization of the milling parameters. The optimized reaction conditions were as follows: 10 mL·min−1 of 15% CH4 in N2 at 112.5 °C at a shaking frequency of 10 Hz with seven balls (Ø 5 mm), 500 mg of catalyst, and 70 mg of TCCA. 11213


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Figure 3. (A) Methane conversion rate as a function of time for CeO2 (in red), Al2O3 (in black), and SiO2 (in blue) under optimized reaction conditions with a flow of 10 mL·min−1 of 15% CH4 in N2 at 112.5 °C at a shaking frequency of 10 Hz with seven balls (Ø 5 mm), 500 mg of catalyst and 70 mg of TCCA. The CH3Cl selectivity is plotted with hollow symbols, and the inset table shows respective CH3Cl yields in μmol. (B) Methane conversion (solid symbols) and CH3Cl yield (hollow symbols) as a function of temperature for a plug flow reaction (in gray) with a mixture of 500 mg of Al2O3 and 70 mg of TCCA compared to the reaction with 500 mg of Al2O3 and 70 mg of TCCA under optimized milling conditions (in black). (C) Normalized mass loss as a function of temperature (TG). (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 of TCCA with 200 mg of the respective solid oxide. (Note: TCCA is entirely decomposed at T >290 °C.) 2.3. Characterization. Attenuated total reflection IR spectra were recorded on a Nicolet Magna-IR 560 with a mercury cadmium telluride (MCT) detector cooled with liquid N2. Thirty-two scans were accumulated and averaged to improve the signal-to-noise ratio. Raman measurements were performed using the 532 nm line of a Nd:YAG laser for excitation in the region of 50−3500 cm−1 on a Renishaw in Via Qontor confocal Raman microscope equipped with a CCD detector. NMR spectra were recorded 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. N2-physisorption measurements for ceria, before and after catalysis, were carried out on a 3Flex from Micromeritics while samples were degassed on the SmartVacPrep setup from Micromeritics. Samples were degassed first at 90 °C for 1 h and then at 300 °C for 10 h and measured from 0.01p/p0 onward because no microporosity was expected. Specific surface areas (SSA) were calculated with the Brunauer−Emmett−Teller (BET) equation using MicroActive software from Micromeritics in the 0.05− 0.3p/p0 range. Total pore volumes were calculated from the volume adsorbed at relative pressures of p/p0 = 0.99. All samples for thermal analysis, namely ceria, alumina and silica with and without TCCA respectively were measured on a Mettler Toledo TGA/DSC 1 in 100 μL aluminum crucibles at a heating rate of 5 °C·min−1 under a constant Ar flow of 60 mL·min−1 controlled by a GC 200 gas controller. Samples analyzed 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. A 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.

3. RESULTS AND DISCUSSION Initial tests were carried out in a Fritsch Pulverisette 6 planetary ball milling setup under methane pressure. These tests gave only a minor conversion of methane with an acidic milling medium. The observed trace products (CH3Cl and CH2Cl2) were identified by mass spectrometry (Figure S2). Further experiments were performed in a Retsch MM400 shaker mill equipped with a tungsten carbide (WC) milling jar modified for continuous gas flow reactions (inset, Figure 2A), as 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. We investigated a set of abundant solid materials with different physicochemical properties in this milling setup: basic MgO was compared to redox-active Fe2O3, weakly acidic SiO2, Brønsted acid zeolite H-USY (Si/Al = 6), nonreducible (γAl2O3) and reducible Lewis acids (TiO2 and CeO2), and the milling jar material itself, WC. Figure 2A shows the respective reaction rates in μmol(CH4 conv)·(g·s)−1 averaged over a 11214


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through a trap, filled with either 10 mL of distilled water to monitor possible changes in the pH, which would indicate the formation of HCl, or 10 mL of a KI solution (0.1 M) to quantify Cl2 (detailed procedures of these analyses in the SI). The only minor pH variation indicated that less than 1% of the active chlorine in TCCA was transformed to HCl. In contrast, our iodometric experiments revealed that roughly two-thirds of the loaded active chlorine is lost in the form of Cl2, while the remaining chlorine reacts with methane to form CH3Cl, as seen in the catalytic experiments. This allows closing the chlorine 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 the scaling up 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, Cl2, and NaOH on a large scale,16 thus it is imaginable to reactivate cyanuric acid and to reuse the formed Cl2 after 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 used in organic synthesis, was tested as an alternative chlorinating agent (for both, the same mass and same molar amount). It is less efficient compared to TCCA: 28 μmol(CH3Cl) was obtained for NCS, while 186 μmol(CH3Cl) was formed with TCCA under otherwise identical conditions. Furthermore, it 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 noncorrosive conditions because of the much lower temperature required to initiate the reaction. As described above, Chang et al. report very low corrosion rates of molecular Cl2 on metals at temperatures below 150 °C.10 To understand the reaction mechanisms of this intriguing process, a number of questions were 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 the 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 value similar to that used in milling experiments (i.e., identical methane flow rates and concentrations were used). TCCA was gently premixed with either SiO2, Al2O3, or CeO2 and then loaded into the reactor tube (Figure S8). Methane conversion and CH3Cl yields are plotted as a function of 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 of as low as 100 °C. The latter reaction exhibited the highest selectivity toward CH3Cl, with a product yield nearly equaling conversion (i.e., selectivity close to 100%). In contrast, the selectivity toward the monochlorinated product significantly decreased with increasing conversion for all of 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 because CH3Cl was detected at 160 °C (CeO2) and 210 °C (Al2O3), respectively, significantly before the decomposition temperature of neat TCCA (>230 °C). In

reaction time of 40 min. The Lewis acidic materials were found to be most promising for methane monochlorination. In addition to the Lewis acid 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, and the presence of defect sites but also its thermal conductivity. Here, we decided not to focus on these parameters. The setup allows for the 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 and 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 3 to 4, respectively. In contrast, silica still showed no detectable activity. While the selectivity for methane conversion to chloromethane was close to 100%, the maximum chlorine transferred to form chloromethane was limited to 30% of the available active chlorine, even for the optimized system. Thus, the gasphase composition was analyzed 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 under the reaction conditions but without shaking. No activity was observed without milling for standardized conditions at 112.5 °C. As soon as the mill was started, the reaction started with a very high initial rate (up to 0.8 μmol·(g·s)−1) (Figure 3A). The mechanochemically induced high reaction rates are maintained for 10−20 min 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 byproducts. While chloromethanes are readily swept off with the gas flow, solid or liquid byproducts can be analyzed only after reaction. The solids after a reaction time of 40 min were analyzed by ATR-FTIR (Figure S4) and liquid-phase NMR after washing the solid with CD3CN and d6-DMSO (Figure S5). IR spectra confirm that TCCA is quantitatively converted because an absorption band at 1050 cm−1 (cyanuric acid) and none at 1150 cm −1 (TCCA) was observed after a mechanochemical reaction. Higher chlorinated alkanes (CnH2n−xClx with n > 1) were never observed during the experiments. Coupling reactions can therefore be ruled out. In IR, NMR, and Raman spectra, cyanuric acid was detected as the only nonvolatile product (Figures S4−S6). Because 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. 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 that of reactions with only TCCA. Because 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 and Cl2, which also might form under the given reaction conditions. For this purpose, the setup was slightly modified (Figure S13). The effluent gas was passed 11215


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Figure 4. (A) Methane conversion rate as a function of reaction time, three cycles of alternating 40 min of milling and a 40 min pause, for different conditions; TCCA and ceria deactivate after the first milling (full triangles); with O2 the reaction is almost fully quenched and methanol is detected as a side-product (empty triangles). (B) In the presence of gaseous Cl2, the catalytic lifetime of TCCA/CeO2 is extended (full circles), while milling does not lead to any increased activity for Cl2 with CH4 and CeO2 (empty circles). (C) Proposed catalytic cycle for the CH4/TCCA (+ Cl2)/CeO2 system, which enables the low-temperature selective monochlorination of methane.

for Al2O3 and at 160 °C for CeO2. 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 Al2O3 (>70 °C) or CeO2 (>100 °C). The TG-DSC and in situ XRPD data correlate well with the plug flow reactor tests, thereby confirming interactions between TCCA and Lewis acid milling media which seem to be crucial in mechanochemical activation. The herein used conditions with seven 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 4A,B were carried out. The milling experiment was carried out over three cycles of 40 min of milling followed by a pause of 40 min. The standard reaction with only TCCA, ceria, and methane in the flow has a high initial rate. This reaction rate decreases quickly and stays low during the second and third milling cycles. The mechanochemical reaction depends on the solid chlorinating agent as a feed, which decomposes with ongoing reaction,

contrast, methane conversion for the TCCA−silica system is observed only at temperatures above 230 °C and can thus be ascribed to the mere thermal decomposition of TCCA. The aforementioned possible interactions between TCCA and the 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 (Figure 3C,D). A similar behavior can be observed when TCCA is mixed with SiO2. In contrast, Al2O3 and CeO2 strongly influence the thermal decomposition of TCCA. TCCA on Al2O3 shows two exothermic mass losses at 80 and 210 °C. The first mass loss at 80 °C is not associated with methane activation, but the latter one exactly falls in the temperature range at which CH3Cl was detected in the plug flow setup. Ceria affects the decomposition of TCCA in a similar way in which an exothermic mass loss in the 160−180 °C range perfectly agrees with the initial activity in CH3Cl production in the plug flow reactor. It can therefore be assumed that homolytic N−Cl bond scission occurs at 210 °C 11216


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acid. Thus, the herein developed selective chlorination reaction between methane and gaseous chlorine may be considered to be a mechanocatalytic reaction with the TCCA/Lewis acid system (Figure 4B) acting as a catalyst/cocatalyst. To study and confirm these hypotheses, future work should focus on operando studies.

while the byproduct cyanuric acid increasingly inhibits the reaction. Earlier tests had shown that only negligible amounts of HCl are formed during the reaction, while a significant amount of active chlorine recombined to Cl2. If the biradical triplet oxygen (3O2) is co-fed during the reaction, then we observe a nearly total suppression of the reaction and detect both CH3OH and CH3Cl as products. It can therefore be concluded that during the catalytic cycle free methyl radicals must be formed, which react with O2 to produce CH3OH. A reaction mechanism could thus be formulated as depicted in Figure 4C. If TCCA is activated by a homolytic N−Cl bond scission, then it is required that the N• radical preferably abstracts H atoms from methane (2.), while the Cl• radical will then recombine with the formed CH3• radical to form CH3Cl (3.). Considering the high CH3Cl selectivity, multiple consecutive chlorination steps to CH2Cl2 or CHCl3 can be ruled out (3′.). These mechanistic hypotheses are fully supported by reported trends in homolytic bond dissociation energies (BDE) among C−H, N−H, and O−H bonds.27 For electronegative R as in TCCA and cyanuric acid, both R2N− H and RO−H bonds (>500 kJ mol−1) can exhibit higher BDEs than H3C−H (439 kJ mol−1). In agreement with theory, a simple DFT model indicates no clear 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 number of Cl• radicals recombine to form Cl2 (2′.). Under the current reaction conditions, Cl2 formation is faster than the reaction rate of activated chlorine with methyl radicals. Arrhenius plots for the TCCA-Al2O3 system for the mechanochemical and thermal reaction reveal a notable difference in the apparent activation energy (144 to 112 kJ mol−1) (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 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 Cl2 was co-fed during the milling reaction as the sole chlorinating agent, then only some minor background methane conversion was observed. This remained, however, completely unaffected by milling. In contrast, the simultaneous presence of TCCA and Cl2 led to a sustained mechanochemical reaction. Thus, additional free Cl• radicals are generated which in turn are recombined with methane to form CH3Cl only via the intermediate interaction with TCCA/cyanuric acid. This interpretation is supported by the higher overall rate of production of CH3Cl. The Cl2 cofeeding tests thus suggest that reaction step (2′.) is a reversible equilibrium reaction. With respect to a largescale application, these results are promising. In an eventual continuous mechanochemical gas−solid process, both cyanuric acid and Cl2 can be recycled. Especially when considering the reversibility of reaction step (2′.), it can be imagined to recirculate unreacted CH4 and Cl2 or to add gaseous Cl2 to the stream to continuously control the equilibrium between cyanuric acid and TCCA. In other terms, this reaction shows that it is possible to regenerate TCCA in situ from cyanuric

4. CONCLUSIONS A gas/solid mechanochemical process for the selective valorization of methane as CH3Cl is 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 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 CH3Cl, and the safety and energy aspects (low operating temperature and pressure), the process described has the potential for large-scale application. Maximum rates are 0.8 μmol(CH4 conv)·(g·s)−1. Given the selectivity of close to 100%, this corresponds to a gravimetric productivity of approximately 150 g(CH3Cl)·kg(catalyst)−1· h−1, which is in the range of other large-scale technical processes for the production of bulk chemicals.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b04413. Detailed description of experimental setup and procedure for catalytic tests, additional information on catalyst characterization, reaction optimization, byproduct formation, and mechanistic considerations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Marius Bilke: 0000-0002-0214-1769 Pit Losch: 0000-0002-5122-662X Author Contributions †

These authors contributed equally.

Notes

The authors declare the following competing financial interest(s): 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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ACKNOWLEDGMENTS P.L. is grateful to Fulbright and the Alexander von Humboldt foundation for financial support in the 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 help with preparing ceria, A. Bähr for lending some equipment, F. Kohler for 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 11217


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Journal of the American Chemical Society the fine mechanics workshop led by W. Kersten at the MPI KoFo.

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Methane to Chloromethane by Mechanochemical Activation: A

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