Technical Comments

Response to Comment on “Selective anaerobic oxidation of methane enables direct synthesis of methanol”

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Science  16 Feb 2018:
Vol. 359, Issue 6377, eaar6868
DOI: 10.1126/science.aar6868

Abstract

Labinger argues that stepwise reaction of methane with water to produce methanol and hydrogen will never be commercially feasible because of its substoichiometric basis with respect to the active site and the requirement of a large temperature swing. This comment is not touching any new ground, beyond describing the thermodynamic feasibility, thermal cycling, and the role of water as discussed previously. Most important, it does not have a solid numerical basis.

Labinger (1) questions the applicability of anaerobic oxidation of methane into methanol (2), citing the thermodynamic limitation and the substoichiometric nature of the reaction with respect to the number of copper atoms in the active site. He accepts our discussion of thermodynamics, following the conditions in each step as well as the modeling of the actual reactive species, without which false conclusions like those of Periana are drawn (3, 4).

Labinger takes issue with the energy required to complete the cycle, and with the presence of the temperature increase at the reactivation step. Besides once again referring the author to the response to the original comment (4)—where the same questions of thermodynamic feasibility, thermal cycling, and the role of water were discussed in great detail—we emphasize that the net reaction corresponding to the anaerobic transformation of methane into methanol is, from the enthalpic point of view, equivalent to the two-step synthesis of methanol from methane via syngas, which is widely used in industry without any restriction. This means that in an ideal case, one needs the same amount of energy per ton of methanol for both methods. The harsh conditions of methane steam reforming and methanol synthesis are not impediments to its efficiency and widespread commercial application.

Furthermore, there are numerous large-capacity industrial processes, such as fluidized catalytic cracking and methanol conversion into olefins, that use the temperature swing at different steps (i.e., the main reaction and the regeneration step), without limitations. This fact directly indicates that a rapid temperature switch is a solvable “problem” of chemical engineering rather than a fundamental limitation.

Labinger correctly suggests that the energy required for the temperature increase can be, for instance, obtained from burning hydrogen formed in the reaction. In a favorable scenario where this process is integrated within the reaction, the latter becomes thermodynamically favorable. Whether this is possible remains to be determined. Also, the optimized materials have not yet been developed, and the ideal process conditions (and the size of the temperature swing, if any) have not yet been identified. Our initial report (2) presents a scientific discovery, not an optimized process that is ready to be implemented. Future development of novel materials and optimization of the conditions are clearly a necessity.

Labinger further contests whether a process in which “a maximum of half an equivalent of methane per metal center can be converted per cycle [could] really prove to be commercially feasible?” From an industrial feasibility point of view, the methanol productivity per reactor volume per unit of time parameter is important, rather than the number of copper atoms in the active site. The amount of methanol per reactor volume per unit of time can be improved by maximizing the density of active sites and increasing the rates of reaction and product desorption, thus shortening the time required for the full reaction cycle. To compare, typical productivities of industrial processes based on heterogeneous catalysts account for 50 to 500 kg(product) m–3(reactor) hour–1 (5), which is about two to three orders of magnitude higher than the methanol productivity in the reported process (2). Therefore, achievement of fast cycling in a stepwise process with maximal methanol yield per cycle is feasible by material, process, and reactor development—the parameters that will determine whether industrial application is feasible. At this stage, it is too early to make this determination (4).

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