Harnessing of Diluted Methane Emissions by Direct Partial Oxidation of Methane to Methanol over Cu/Mordenite

Abstract

The upgrading of diluted methane emissions into valuable products can be accomplished at low temperatures (200 °C) by the direct partial oxidation of methanol over copper-exchanged zeolite catalysts. The reaction has been studied in a continuous fixed-bed reactor loaded with a Cu–mordenite catalyst, according to a three-step cyclic process: adsorption of methane, desorption of methanol, and reactivation of the catalyst. The purpose of the work is the use of methane emissions as feedstocks, which is challenging due to their low methane concentration and the presence of oxygen. Methane concentration had a marked influence on methane adsorption and methanol production (decreased from 164 μmol/g Cu for pure methane to 19 μmol/g Cu for 5% methane). The presence of oxygen, even in low concentrations (2.5%), reduced methane adsorption drastically. However, methanol production was only affected slightly (average decrease of 9%), concluding that methane adsorbed on the active centers yielding methanol is not influenced by oxygen.

Role of Support in the Selective, Aerobic Methane Oxidation to Formaldehyde over Pt/TiO2

“…Despite the challenging nature of this reaction, some successes have been reported for the direct functionalization of methane. These include the oxidation of methane to a product that is less susceptible to further oxidation, which can be subsequently converted to methanol (a strategy referred to as product protection), − decoupling the oxygen activation and methane functionalization via chemical looping, , and the use of an oxidizing agent that is active under mild conditions (e.g., H 2 O 2 ). − Other approaches include the addition of coreactants such as CO , (which may reduce the presence of activated oxygen on the catalyst surface) or the use of tandem catalysts to convert the formed oxygenate into a product that is impervious to further oxidation …”

“…A number of studies − have noted the importance of water as a coreactant in the selective oxidations and in particular in the selective oxidation of methane, which may promote the formation of the precursors to the selective oxidation products (methanol or formaldehyde), such as adsorbed methoxy species, or di-σ-hydroxy methoxy species. , Water may also enhance the dissociation of molecularly adsorbed oxygen on a metallic catalyst surface . Furthermore, it may act as an extracting agent assisting the desorption of the selective oxidation products from the catalyst surface, thereby preventing their further oxidation. ,− …”

Recent Advances in the Catalytic Conversion of Methane to Methanol: From the Challenges of Traditional Catalysts to the Use of Nanomaterials and Metal-Organic Frameworks

“…They realized that Cu species in these catalysts facilitate the formation of OH radicals, which react rapidly with CH 3 radicals to form CH 3 OH [48]. In summary, using traditional zeolites, a maximum methanol yield of 5866 µmol/g cat has been obtained with a high (79.7%) methanol selectivity at 50 • C and 30 bar [55].…”

“…In addition, they showed that the catalysts do not deactivate during continuous reactions while maintaining high selectivity [42]. Over the last decade, copper-exchanged zeolites have been the ones that have been more extensively studied [48][49][50][51][52][53][54][55][56][57][58][59]. Lobo et al investigated the catalytic performance of Cu-SSZ-13 for methanol production using oxygen and nitrous oxide as oxidants at temperatures ranging from 300 to 450 • C and achieved the maximum methanol yield of 13 µmol/g cat at 200 • C when N 2 O was used for peroxidation.…”

Atmospheric-Pressure Continuous-Flow Methane Oxidation to Methanol and Acetic Acid Using H2O2 over the Au–Fe Catalyst

“…There are only limited reports on the POM to methanol in a continuous flow reactor, ,− with most reports being on using molecular oxygen as the oxidant with frugal conversion to methanol. For example, Narsimhan et al reports the production of 1.81 μmol/ g cat ·h methanol using O 2 as an oxidant at 210 °C .…”