Supporting Information Direct Conversion of Methane to Methanol
Supporting Information
Direct Conversion of Methane to Methanol under Mild Conditions over Cu-Zeolites and beyond
Patrick Tomkins,‡,§ , Marco Ranocchiari, § Jeroen A. van Bokhoven*‡,§
‡ ETH Zurich, Institute for Chemistry and Bioengineering, Vladimir-Prelog-Weg 1, 8093 Zurich Switzerland $ Paul Scherrer Institute, 5232 Villigen, Switzerland
Email address: [email protected]
COPPER EXCHANGE PROCEDURE Copper-exchanged zeolites were synthesized in a 0.01 M solution of Cu(OAc)2 H2O (Sigma Aldrich) in DI water, 1 g Na-MOR (Zeocat FM-8, Zeochem AG, Si/Al=6 or Si/Al=8.5 or Zeoflair 800, Si/Al=6.5) of the desired zeolite was added per 78 mL of solution. The suspension was stirred with a magnetic stirrer at 625 rpm for 24 h at room temperature, after which it was filtered through a glass suction filter (Por.4). The unwashed filter cake was suspended in an identical 0.01M copper solution and stirred for another 24 h. The latter step was repeated once more, and the resulting filter cake was washed with 1 L DI water and dried for 10 h at 110 °C. The sodium zeolites for Cu-ZSM-5 and Cu-Y were PZ-2 (Chemische Fabrik Uetikon, Si/Al=15) and Zeocat Z6 (Cu Chemie Uetikon AG, Si/Al=2.7 – 3.0), respectively.
CATALYST PERFORMANCE The evaluation of the catalyst performance was carried out in a 60-mL stainless steel autoclave (premex reactor ag) with an internal temperature control and three valves for flows of incoming and outgoing gas. The vessel and the lid are of 1.4980 stainless steel. The catalyst (~0.7 g, 250 - 500 µm) was activated in oxygen at a flow rate of 70 mL/min. The temperature was set to change at a rate of 1.0 °C/min. In the case of low-temperature activation the autoclave was heated to the desired temperature and kept at this temperature for 13 h. High-temperature activation was carried out by increasing the temperature to 450 °C, maintaining it for 4 h and then cooling to 200 °C. The reactor was then purged thoroughly with helium (five times at 6 bar), and there was a short increase in temperature (~8 °C). When the temperature was stable, the reaction was carried out by purging the reactor with methane, stopping gas flow, and carrying out the reaction at the desired pressure for 30 min. The reaction was terminated by purging the reactor with helium and cooling with the integrated system with water at ~15 °C. The methanol was extracted from the material off-line by stirring in Milli-Q water (2 mL) for 2 h at room temperature. The suspension was filtered, the liquid phase was collected, and Milli-Q water was added to give a total volume of 2 mL. The external standard (acetonitrile in Milli-Q water) was added and the sample was analyzed by gas chromatography with an Agilent 6890 GC (Restek Rtx®-5 column, 3m, ID=0.25mm) apparatus fitted with a FID detector. The oven with the column was kept at 40 °C for 10 min and then heated to 200 ºC at a rate of 20 °C/min. The procedure was repeated by resuspending the wet filter cake, and the resulting methanol was added and normalized with respect to the amount of catalyst used for the extraction. The partial pressures are given. Methane and oxygen had a purity of >99.5 %, whereas the purity of helium was >99.996 %. Activation time under isothermal conditions is defined as the time at 200 °C in oxygen prior to reaction.
DETERMINATION OF THE SIMPLE AND DISSOCIATIVE LANGMUIR ADSORPTION FOR THE ISOTHERMAL STEPPED CONVERSION OF METHANE Linearization of the isotherms reveals the isotherm and the mechanism that are applicable. The resulting equations are eq S1 for the normal Langmuir isotherm and eq S2 for the Langmuir isotherm for dissociative adsorption. These linearized equations are used to plot and fit the methanol yields at different pressures. 𝑝 𝑌𝑀𝑒𝑂𝐻 √𝑝 𝑌𝑀𝑒𝑂𝐻
1
=𝑌
MeOH, max
=𝑌
1
𝑀𝑒𝑂𝐻, 𝑚𝑎𝑥
1
𝑝 + 𝐾⋅𝑌
MeOH, max
√𝑝 + √𝐾⋅𝑌
1
𝑀𝑒𝑂𝐻, 𝑚𝑎𝑥
(S1)
(S2)
The linearized version of the normal Langmuir isotherm gives a linear relation of pressure and the pressure divided by the methanol yield (Figure S1), showing good agreement of theory and the experiment. For the Langmuir isotherm with dissociative adsorption, the graph resulting from plotting √𝑝 vs. √𝑝/𝑌𝑀𝑒𝑂𝐻 does not give a linear relation. The graph decreases rapidly from 0.75 bar1/2∙g∙µmol-1 to level off at 0.1 bar1/2∙g∙µmol-1. The non-linear behavior indicates that a dissociative mechanism is not at work, as does the fact that a constant value is reached. The slope represents the inverse maximum methanol yield. A slope of zero would result in an infinite maximum methanol yield; this is impossible in the case of monolayer adsorption and active sites, which are limited by the amount of copper in the material. Thus, by reviewing the two adsorption isotherms, it was possible to distinguish between dissociative and nondissociative adsorption. These results strongly indicate that the conversion of methane to methanol requires one active site, not two. However, it may be that two different adsorption sites are present, one of which in a considerable excess amount. By comparing different materials with different, more sophisticated isotherms, it might be possible to elucidate a more detailed mechanism.
pCY (MeOH) [bargµmol -1]
0.8
0.6
0.4
0.2
0.0 0
5
10
15
20
25
30
35
5
6
40
pCH4 [bar]
(pC)Y (MeOH) [(bar)gµmol -1]
0.8
0.6
0.4
0.2
0.0
0
1
2
3
4
7
(pCH4) [(bar)]
Figure S1. Linearized Langmuir isotherm (Eq S1, top) and linearized Langmuir isotherm for dissociative adsorption (Eq S2, bottom) using the methanol pressure as a measure of gas uptake in the stepped conversion of methane to methanol at 200 °C with Cu-MOR. The data and the pressure-dependent methanol yields were taken from the published literature.1
References (1) Tomkins, P.; Mansouri, A.; Bozbag, S. E.; Krumeich, F.; Park, M. B.; Alayon, E. M. C.; Ranocchiari, M.; van Bokhoven, J. A.: Isothermal Cyclic Conversion of Methane into Methanol over Copper-Exchanged Zeolite at Low Temperature. Angew. Chem., Int. Ed. 2016, 128, 5557-5561.
Direct Conversion of Methane to Methanol under Mild Conditions over Cu-Zeolites and beyond
Patrick Tomkins,‡,§ , Marco Ranocchiari, § Jeroen A. van Bokhoven*‡,§
‡ ETH Zurich, Institute for Chemistry and Bioengineering, Vladimir-Prelog-Weg 1, 8093 Zurich Switzerland $ Paul Scherrer Institute, 5232 Villigen, Switzerland
Email address: [email protected]
COPPER EXCHANGE PROCEDURE Copper-exchanged zeolites were synthesized in a 0.01 M solution of Cu(OAc)2 H2O (Sigma Aldrich) in DI water, 1 g Na-MOR (Zeocat FM-8, Zeochem AG, Si/Al=6 or Si/Al=8.5 or Zeoflair 800, Si/Al=6.5) of the desired zeolite was added per 78 mL of solution. The suspension was stirred with a magnetic stirrer at 625 rpm for 24 h at room temperature, after which it was filtered through a glass suction filter (Por.4). The unwashed filter cake was suspended in an identical 0.01M copper solution and stirred for another 24 h. The latter step was repeated once more, and the resulting filter cake was washed with 1 L DI water and dried for 10 h at 110 °C. The sodium zeolites for Cu-ZSM-5 and Cu-Y were PZ-2 (Chemische Fabrik Uetikon, Si/Al=15) and Zeocat Z6 (Cu Chemie Uetikon AG, Si/Al=2.7 – 3.0), respectively.
CATALYST PERFORMANCE The evaluation of the catalyst performance was carried out in a 60-mL stainless steel autoclave (premex reactor ag) with an internal temperature control and three valves for flows of incoming and outgoing gas. The vessel and the lid are of 1.4980 stainless steel. The catalyst (~0.7 g, 250 - 500 µm) was activated in oxygen at a flow rate of 70 mL/min. The temperature was set to change at a rate of 1.0 °C/min. In the case of low-temperature activation the autoclave was heated to the desired temperature and kept at this temperature for 13 h. High-temperature activation was carried out by increasing the temperature to 450 °C, maintaining it for 4 h and then cooling to 200 °C. The reactor was then purged thoroughly with helium (five times at 6 bar), and there was a short increase in temperature (~8 °C). When the temperature was stable, the reaction was carried out by purging the reactor with methane, stopping gas flow, and carrying out the reaction at the desired pressure for 30 min. The reaction was terminated by purging the reactor with helium and cooling with the integrated system with water at ~15 °C. The methanol was extracted from the material off-line by stirring in Milli-Q water (2 mL) for 2 h at room temperature. The suspension was filtered, the liquid phase was collected, and Milli-Q water was added to give a total volume of 2 mL. The external standard (acetonitrile in Milli-Q water) was added and the sample was analyzed by gas chromatography with an Agilent 6890 GC (Restek Rtx®-5 column, 3m, ID=0.25mm) apparatus fitted with a FID detector. The oven with the column was kept at 40 °C for 10 min and then heated to 200 ºC at a rate of 20 °C/min. The procedure was repeated by resuspending the wet filter cake, and the resulting methanol was added and normalized with respect to the amount of catalyst used for the extraction. The partial pressures are given. Methane and oxygen had a purity of >99.5 %, whereas the purity of helium was >99.996 %. Activation time under isothermal conditions is defined as the time at 200 °C in oxygen prior to reaction.
DETERMINATION OF THE SIMPLE AND DISSOCIATIVE LANGMUIR ADSORPTION FOR THE ISOTHERMAL STEPPED CONVERSION OF METHANE Linearization of the isotherms reveals the isotherm and the mechanism that are applicable. The resulting equations are eq S1 for the normal Langmuir isotherm and eq S2 for the Langmuir isotherm for dissociative adsorption. These linearized equations are used to plot and fit the methanol yields at different pressures. 𝑝 𝑌𝑀𝑒𝑂𝐻 √𝑝 𝑌𝑀𝑒𝑂𝐻
1
=𝑌
MeOH, max
=𝑌
1
𝑀𝑒𝑂𝐻, 𝑚𝑎𝑥
1
𝑝 + 𝐾⋅𝑌
MeOH, max
√𝑝 + √𝐾⋅𝑌
1
𝑀𝑒𝑂𝐻, 𝑚𝑎𝑥
(S1)
(S2)
The linearized version of the normal Langmuir isotherm gives a linear relation of pressure and the pressure divided by the methanol yield (Figure S1), showing good agreement of theory and the experiment. For the Langmuir isotherm with dissociative adsorption, the graph resulting from plotting √𝑝 vs. √𝑝/𝑌𝑀𝑒𝑂𝐻 does not give a linear relation. The graph decreases rapidly from 0.75 bar1/2∙g∙µmol-1 to level off at 0.1 bar1/2∙g∙µmol-1. The non-linear behavior indicates that a dissociative mechanism is not at work, as does the fact that a constant value is reached. The slope represents the inverse maximum methanol yield. A slope of zero would result in an infinite maximum methanol yield; this is impossible in the case of monolayer adsorption and active sites, which are limited by the amount of copper in the material. Thus, by reviewing the two adsorption isotherms, it was possible to distinguish between dissociative and nondissociative adsorption. These results strongly indicate that the conversion of methane to methanol requires one active site, not two. However, it may be that two different adsorption sites are present, one of which in a considerable excess amount. By comparing different materials with different, more sophisticated isotherms, it might be possible to elucidate a more detailed mechanism.
pCY (MeOH) [bargµmol -1]
0.8
0.6
0.4
0.2
0.0 0
5
10
15
20
25
30
35
5
6
40
pCH4 [bar]
(pC)Y (MeOH) [(bar)gµmol -1]
0.8
0.6
0.4
0.2
0.0
0
1
2
3
4
7
(pCH4) [(bar)]
Figure S1. Linearized Langmuir isotherm (Eq S1, top) and linearized Langmuir isotherm for dissociative adsorption (Eq S2, bottom) using the methanol pressure as a measure of gas uptake in the stepped conversion of methane to methanol at 200 °C with Cu-MOR. The data and the pressure-dependent methanol yields were taken from the published literature.1
References (1) Tomkins, P.; Mansouri, A.; Bozbag, S. E.; Krumeich, F.; Park, M. B.; Alayon, E. M. C.; Ranocchiari, M.; van Bokhoven, J. A.: Isothermal Cyclic Conversion of Methane into Methanol over Copper-Exchanged Zeolite at Low Temperature. Angew. Chem., Int. Ed. 2016, 128, 5557-5561.
