Direct Conversion of Methane to Methanol under Mild Conditions over Cu-Zeolites and beyond

Abstract

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 SO 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 <40 bar methane gave higher yields under isothermal conditions at 200 °C than most yields in prior reports. The yield, both after high temperature activation and under isothermal conditions at 200 °C, increased monotonously with the pressure. With this account we show that the trend can be represented by a Langmuir model. Thus, the pressure dependence is governed by methane adsorption. We show that the isothermal and the high temperature activated processes have different properties and should be treated independently, from both an experimental and a mechanistic point of view.

Aqueous‐Phase Selective Oxidation of Methane with Oxygen over Iron Salts and Pd/C in the Presence of Hydrogen

“…The gas‐phase partial oxidation of methane has been reported over Fe‐ or Cu‐zeolites using N 2 O, O 2 , and H 2 O as oxidants. However, most reactions can only proceed at relatively high temperatures (>200 °C) resulting in low product yields . On the other hand, much higher yields of methane oxygenates have been reported in a liquid‐phase system .…”

The Nature and Catalytic Function of Cation Sites in Zeolites: a Computational Perspective

“…Such species can bring about new catalytic functionalities expanding thus the scope of application of the zeolite catalysts. For example, the modification of zeolites with transition metal ions, e. g. with Cu 2+ , Fe 2+/3+ , Co 2+ or Mn 2+ bring about the redox activity needed for such important processes as the selective methane oxidation,, catalytic NO x reduction and the catalytic conversion of biomass‐derived oxygenates . The introduction of Ga 3+ or Zn 2+ gives rise to strong Lewis acid sites that are active in dehydrogenation and dehydroaromatization of light alkanes…”

The Effect of the Active‐Site Structure on the Activity of Copper Mordenite in the Aerobic and Anaerobic Conversion of Methane into Methanol

“…[1] Ap romising stepwise process over copperexchanged zeolites has been suggested;h owever, the mechanistic details of this zeolite-catalyzed conversion are still not understood. [1][2][3][4] Another open question deals with the role of the nature of the oxidant, as well as the mechanism of methane oxidation and regeneration of the copper oxide active site. [1][2][3][4] Another open question deals with the role of the nature of the oxidant, as well as the mechanism of methane oxidation and regeneration of the copper oxide active site.…”