Article pubs.acs.org/accounts
Direct Conversion of Methane to Methanol under Mild Conditions over Cu-Zeolites and beyond Patrick Tomkins,†,‡,§ Marco Ranocchiari,§ and Jeroen A. van Bokhoven*,‡,§ ‡
ETH Zurich, Institute for Chemistry and Bioengineering, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland Paul Scherrer Institute, 5232 Villigen, Switzerland
§
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CONSPECTUS: In the recent years methane has become increasingly abundant. However, transportation costs are high and methane recovered as side product is often flared rather than valorized. The chemical utilization of methane is highly challenging and currently mainly based on the cost-intensive production of synthesis gas and its conversion. Alternative routes have been discovered in academia, though high temperatures are mostly required. However, the direct conversion of methane to methanol is an exception. It can already be carried out at comparably low temperatures. It is challenging that methanol is more prone to oxidation than methane, which makes high selectivities at moderate conversions difficult to reach. Decades of research for the direct reaction of methane and oxygen did not yield a satisfactory solution for the direct partial oxidation toward methanol. When changing the oxidant from oxygen to hydrogen peroxide, high selectivities can be reached at rather low conversions, but the cost of hydrogen peroxide is comparably high. However, major advancements in the field were introduced by converting methane to a more stable methanol precursor. Most notable is the conversion of methane to methyl bisulfate in the presence of a platinum catalyst. The reaction is carried out in 102% sulfuric acid using SO3 as the oxidant. This allows for oxidation of the platinum catalyst and prevents the in situ hydrolysis of methyl bisulfate toward the less stable methanol. With a slightly different motif, the stepped conversion of methane to methanol over copper-zeolites was developed a decade ago. The copper-zeolite is first activated in oxygen at 450 °C, and then cooled to 200 °C and reacts with methane in the absence of oxygen, thus protecting a methanol precursor from overoxidation. Subsequently methanol can be extracted with water. Several active copper-zeolites were found, and the active sites were identified and discussed. For a long time, the process was almost unchanged. Lately, we implemented online steam extraction rather than off-line extraction with liquid water, which enables execution of successive cycles. While recently we reported the isothermal conversion by employing higher methane pressures, carrying out the process according to prior art only yielded neglectable amounts of methane. Using a pressure 280 °C) is required for small amounts of the μ-oxo-dicopper site to form,35 while the adsorbed methanol precursor decomposes at this temperature.33 Thus, under these conditions, an isothermal reaction is impossible. Beznis et al.37 suggested that particles on the outer surface of Cu-ZSM-5 particles are inactive.37 The reactions over Cu-ZSM-5 and Cu-MOR have been studied more extensively compared to other catalysts. Cu-MOR is the first system in which methanol was extracted online in steam rather than after removal of the material from the reactor with liquid water.33 Repeated cycles resulted in higher yields, 420
DOI: 10.1021/acs.accounts.6b00534 Acc. Chem. Res. 2017, 50, 418−425
Article
Accounts of Chemical Research
or performing the cycles by online extraction.33,34 The reaction has almost always been performed under similar reaction conditions: oxygen activation at high temperature (typically 450 °C) followed by reaction in a (diluted) stream of methane at a lower temperature (typically 200 °C), although the applied temperature differed slightly for extraction at room temperature or with steam (Figure 5, left). More recently, attempts have
Figure 4. Time-dependent UV/vis-spectra of Cu-ZSM-5 after reaction with oxygen at 450 °C, in the reaction with 5% methane in nitrogen. The signal at 22,700 cm−1 is assigned to the active dicopper site. Reprinted with permission from ref 32. Copyright 2005 American Chemical Society. Figure 5. Representative diagram for the modes of operation for the stepped conversion of methane which are performed after high temperature activation and under isothermal conditions.
because oxidation of hydrated copper species occurs more readily.41 This shows that this stepped conversion is repeatable, as the active site can be regenerated. However, most recently a catalytic, nonstepped conversion has been reported, in which a stream of methane, oxygen, and water is converted to methanol using copper zeolites as catalysts, where yields of 3.12 μmolMeOH·gcat−1·h−1 were reported.42 Albeit this is a valuable contribution for the advancement of the field, the methane conversion is also far from industrial practicability. A large excess of methane was employed, resulting in low methane conversion. A study in 2015 reported a trinuclear copper oxygen cluster to be the active species for the conversion of methane to methanol (Table 1, entry 3).39 This site could be generated because the copper-exchange has been carried out using H-MOR for the copper exchange. Cu-BEA and Cu-FER did not exhibit a μ-oxo dicopper site, but the conversion of methane to methanol still occurs over a so far not identified site.35 Our group carried out a computational study on different copper clusters, from dimers to pentamers, whereas Cu4O42+ and Cu5O52+ (Table 1, entries 4 and 5) units have not been observed experimentally so far. Both the stability and the reactivity toward methane to methanol conversion increase with increasing cluster size.40 Conclusively, methane can be converted toward methanol over various active sites, where only a part was identified experimentally. Further effort in selectively stabilizing and identifying sites is necessary. Despite considerable spectroscopic effort,38,43 and the growing evidence that there may not be one universal active site, further detailed in situ studies are needed to investigate whether some copperoligomers are in-stationary and tautomerize as the reaction proceeds. Similar reactions as the above-described stepped conversion of methane are also described over Fe-ZSM-5 and Co-ZSM-5, either with N2O as the oxidant (Fe-ZSM-5) or resulting in a comparably low yield (Co-ZSM-5).44−46
been made to bring about intrinsic changes in the process. Substituting oxygen with NO as a molecule for activation resulted in the formation of the μ-oxo dicopper site at lower temperature with Cu-ZSM-5. Thus, it was possible to achieve activation with NO, reaction with methane, and extraction with water at the same temperature. A yield of 0.63 μmol·g−1 was attained after extraction in steam and isothermal reaction at 150 °C.49 We investigated the effect of pressure on the direct conversion of methane to methanol over Cu-MOR.50 Contrary to our expectations, oxygen pressure during activation had only a weak, even slightly negative effect. In contrast, there was a strong dependence of the methanol yield on methane pressure. A methane pressure of 36 bar resulted in a yield of 103.3 μmol· g−1 after activation at 450 °C compared to a methane pressure of 50 mbar and reaction in helium, which resulted in a yield of 14.4 μmol·g−1. More strikingly, Cu-MOR was practically inactive in the reaction at 50 mbar methane after activation at 200 °C in oxygen (0.3 μmol·g−1) but yielded 56.2 μmol·g−1 under isothermal conditions at 200 °C and a methane pressure of 37 mbar (Figure 6). The reaction under isothermal conditions is considerably simplified, as the system’s temperature must not be changed and is carried out by activating in a flow of oxygen at 200 °C and subsequent reaction with methane at 200 °C (Figure 5, right). Further, Figure 6 shows methanol yields obtained after activation in oxygen at 450 °C and reaction at 50 mbar methane in inert gas for Cu-MOR,50 Cu−Y,35 and Cu-ZSM-5.32 The corresponding values for the isothermal reaction at 200 °C at 32 bar are given for Cu−Y and Cu-ZSM-5. In each case, the yield obtained under isothermal conditions at elevated methane pressure is higher than that after high temperature activation and reaction at 50 mbar methane.
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ISOTHERMAL STEPPED CONVERSION OF METHANE TO METHANOL OVER CU ZEOLITES Most of the reports on the stepped conversion of methane to methanol have one of the following goals: the optimization and screening of materials, especially with regard to activation,34,35,43,47 understanding of the reaction mechanism,36,39,48
OUTLOOK FOR THE ISOTHERMAL STEPPED CONVERSION OF METHANE TO METHANOL The results reported here suggest that it is possible to develop new materials for the stepped conversion of methane to methanol. Cu-ZSM-5, which is reported to have the μ-oxo 421
DOI: 10.1021/acs.accounts.6b00534 Acc. Chem. Res. 2017, 50, 418−425
Article
Accounts of Chemical Research
(ii) the material must be stable in water at the given reaction temperature; (iii) the active sites must be regenerated by reaction with oxygen at the given reaction temperature; (iv) due to the repeated stoichiometric character of the conversion, there must be a high proportion of active sites. For Cu-MOR (Si/Al = 6.5) there is an exponential relation between the yield under isothermal conditions and the yield after high-temperature activation with increasing methane pressure under otherwise identical conditions (Figure 7).
Figure 6. Dependence of the methanol yield on methane pressure for Cu-MOR and dependence on selected methane pressures of Cu-Y and Cu-ZSM-5 in the isothermal conversion of methane to methanol at 200 °C (circles) compared to high temperature activation at low pressure (stars). The data for Cu-Y and Cu-ZSM-5 under isothermal conditions and all data for Cu-MOR were taken from ref 46, while the values for Cu-ZSM-532 and Cu-Y35 were obtained from other reports after reaction with activation at 450 °C and reaction at 200 °C, in 5% methane in inert gas.
dicopper site as the only active site for the conversion,35 yielded 17.7 μmol·g−1 under isothermal conditions at 32 bar methane in the absence of the μ-oxo dicopper site.50 According to prior art, this material would not yield any methanol under isothermal conditions. However, by changing the reaction conditions, i.e. increasing the methane pressure, this material yields a significant amount of methanol under isothermal conditions. After activation at 450 °C and reaction at 6 bar methane, the yield was 18.6 μmol·g−1, which is comparable to other reported yields for reactions at 50 mbar methane.34 This shows that, for Cu-ZSM-5, the effect of methane pressure is weaker than for Cu-MOR after activation at high temperature.50 It is already clear that materials, such as Cu-MOR and Cu-ZSM-5, that result in comparable yields after hightemperature activation might show very different yields at elevated methane pressure under isothermal conditions. The reaction with Cu−Y is a good illustration of the potential of the isothermal stepped conversion of methane to methanol. After high temperature activation and reaction at 50 mbar methane, Cu−Y gave only a negligible yield (
