Direct Methane to Methanol: The Selectivity–Conversion Limit and Design Strategies

Abstract

Currently, methane is transformed into methanol through the two-step syngas process, which requires high temperatures and centralized production. While the slightly exothermic direct partial oxidation of methane to methanol would be preferable, no such process has been established despite over a century of research. Generally, this failure has been attributed to both the high barriers required to activate methane as well as the higher activity of the CH bonds in methanol compared to those in methane. However, a precise and general quantification of the limitations of catalytic direct methane to methanol has yet to be established. Herein, we present a simple kinetic model to explain the selectivity−conversion trade-off that hampers continuous partial oxidation of methane to methanol. For the same kinetic model, we apply two distinct methods, (1) using ab initio calculations and (2) fitting to a large experimental database, to fully define the model parameters. We find that both methods yield strikingly similar results, namely, that the selectivity of methane to methanol in a direct, continuous process can be fully described by the methane conversion, the temperature, and a catalyst-independent difference in methane and methanol activation free energies, ΔG a , which is dictated by the relative reactivity of the C−H bonds in methane and methanol. Stemming from this analysis, we suggest several design strategies for increasing methanol yields under the constraint of constant ΔG a . These strategies include (1) "collectors", materials with strong methanol adsorption potential that can help to lower the partial pressure of methanol in the gas phase, (2) aqueous reaction conditions, and/or (3) diffusion-limited systems. By using this simple model to successfully rationalize a representative library of experimental studies from the diverse fields of heterogeneous, homogeneous, biological, and gas-phase methane to methanol catalysis, we underscore the idea that continuous methane to methanol is generally limited and provide a framework for understanding and evaluating new catalysts and processes.

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

“…Besides, the computational screening identified IrO 2 as a potential new catalyst for methane activation. Further computational analysis revealed that the reactivities of C−H bonds in methane and methanol can be used as the descriptors for predicting selectivity of methane oxidation in a direct continuous process …”

The special route toward conversion of methane to methanol on a fluffy metal‐free carbon nitride photocatalyst in the presence of H 2 O 2

“…On the other hand, along with the saturation of methanol production, we observed a considerable increase in the evolution of the other products, i.e., HCOOH, CO, and CO 2 (Figure B,C). These products are the consecutive oxidation products of generated methanol . This seriously increased HCOOH/CO/CO 2 amount after the methanol saturation indicated that the over‐oxidation of methanol was vigorously going on at this status (sufficient methanol concentration and ample oxidizing agents including H 2 O 2 and increasing amount of O 2 from the decomposition of H 2 O 2 ).…”

“…as catalysts and H 2 O 2 or/and O 2 as oxidizing agents. However, despite the numerous efforts with admirable progresses, the complexity and high cost of the excellent catalysts have limited their industrial utilization …”

Thermodynamic Limitations of the Catalyst Design Space for Methanol Production from Methane

“…[23] Indeed, a hypothetical two-step kinetic model for CH 4 conversion to CH 3 OH followed by CH 3 OH conversion to CO 2 produces a selectivity-conversion limit that describes many experimental data in the literature. [24] This limit illustrates high alcohol selectivity at very low CH 4 conversion but low alcohol selectivity at moderate to high CH 4 conversion. Experimental data compiled by Ravi et al [11] also illustrates this trade-off between methane conversion and methanol selectivity, however the variety of feed compositions and operating conditions among these results prevent a direct comparison among the different catalysts.…”

“…The existence of Brønsted‐Evans‐Polanyi (BEP) relations suggests that this strong thermodynamic driving force will translate to a substantial kinetic preference for combustion . Indeed, a hypothetical two‐step kinetic model for CH 4 conversion to CH 3 OH followed by CH 3 OH conversion to CO 2 produces a selectivity‐conversion limit that describes many experimental data in the literature . This limit illustrates high alcohol selectivity at very low CH 4 conversion but low alcohol selectivity at moderate to high CH 4 conversion.…”