Electron Transfer Reactions - American Chemical Society

4 A Novel, High-Yield System for the Oxidation of Methane to Methanol Roy A. Periana

Downloaded by UNIV OF SYDNEY on April 11, 2013 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch004

Catalytica, Inc., 430 Ferguson Drive, Mountain View, CA 94043

A novel, homogeneous system for the selective, low-temperature, cat-alytic oxidation of methane to methanol is reported. The net reaction catalyzed by mercuric ions, Hg(II), is the oxidation of methane by con-centrated sulfuric acid to produce methyl bisulfate, water, and sulfur dioxide. The reaction is efficient. At a methane conversion of 50%, selectivity to methyl bisulfate (~43%

carbon dioxide) was achieved at a molar productivity of 10

-7

and Hg(II) turnover frequency of 10

-3

85%

yield, the major side product is mol/cm s 3

s . Separate hydrolysis of methyl -1

bisulfate and reoxidation of the sulfur dioxide with air provides a poten-tially practical scheme for the oxidation of methane to methanol with molecular oxygen. This yield is the highest single-pass yield of methanol so far reported for a catalytic methane oxidation. The primary steps of the Hg(II)-catalyzed

reaction were individually examined, and the

essential elements of the mechanism were identified. The Hg(II) ion reacts with methane by an electrophilic displacement mechanism to pro-ducean observable species, -tions,

CH HgOSO H 3

CH HgOSO H. 3

3

Under the reaction condi-

readily decomposes to CH HgOSO H and the

3

3

3

reduced mercurous species, Hg . The catalytic cycle is completed by 2+

2

the reoxidation of Hg

2+

2

with H SO to regenerate Hg(II) and by-prod2

4

-ucts SO and H O. Thallium(III), palladium(II), and the cations of plat2

2

-inumand gold also oxidize methane to methyl bisulfate in sulfuric acid.

THIS CHAPTER HONORS HENRY TAUBES' CONTRIBUTION to an important area

of industrial research. The selective oxidation of methane to methanol is an important scientific and commercial objective and one that Catalytica has long sought to address through innovative chemistry. When I arrived at Catalytica, Taube was already there in the capacity of Scientific Advisor. H e held this posi tion while still maintaining an active academic role at Stanford University. This © 1997 American Chemical Society

In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.

61


62

ELECTRON TRANSFER REACTIONS

unusual relationship prompted me to ask him why he felt the need to interact with Catalytica at a time in his life when he could be enjoying the accolades of his career. His answer typifies the man and scientist: "I wanted to do some thing useful." When I arrived Taube had already helped to lay the foundation for the approach to selective oxidation. His approach, as is characteristic of Taube's style, was simple and unambiguous. Our engineers told him that an economi cally competitive methane to methanol process would have to be carried out at high, one-pass methane conversion and selectivity. He realized that this would only be possible i f a process could be found i n which the methanol was less reactive than methane during the oxidation step. Realizing that designing a catalyst that exhibited such discrimination was an enormous task, Taube won dered instead if the methanol could be "protected" from the catalyst by chemi cal modification. The concept of protection is well-established i n organic chemistry. For example, the nitro group slows or "protects" nitrobenzene from further electrophilic nitration relative to benzene. However, it was not clear whether this concept could be applied to a saturated system i n which the bonds do not involve π-electrons. Taube's fundamental approach now came to bear. H e collected data (Fig ure 1) from the literature that showed unambiguously that electron-withdraw ing groups on the methyl group would be expected to reduce the rate of the reaction of the substituted substrate relative to methane in reactions involving electrophiles. Thus, in reaction with hydroxy radicals, while methanol was four times more reactive than methane, acetonitrile was approximately two orders of magnitude less reactive! Taube noted that these effects should be much greater for two-electron oxidations. This insight led us to focus on the identifi cation of catalysts for methane oxidation that operated via two-electron pro cesses and for protecting groups that could slow the reaction of methanol rela tive to methane. This clear logic, and Taube's championing of the program, was pivotal in raising the funds to implement the research program. The results that I am about to discuss are some of the fruits of this program. Catalytica has been involved in programs aimed at the development of new chemistry for the selective oxidation of methane. Our efforts are directed at replacing the current capital-intensive methane-to-methanol technology based on syngas with a more efficient, direct oxidation process that is less capital intensive. The most economic alternative to the syngas technology would be a hypothetical process for the direct, high-yield, one-step oxidation of methane to methanol. Economic evaluations indicate that even for such an idealized process, single-pass conversions in excess of 30% at greater than 80% selectivity are required for an economical process. Here, %conversion = { ( [ C H J ^ ^ [ C H J f l j / I C H J ^ } χ 100; %selectivity to C H O H — [ C H O H ] / ( [ C H J [ C H j ) } χ 100; and %yield ξ (%conversion) χ (%selectivity). (High selectivity is essential because low selectivity results in the formation of C 0 and the gen eration of heat. Removal of heat is a large part of the process costs. High con3

3

initial

final

2

In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.

4.

re^+wi^


63

PERIANA High-Yield Oxidation of Methane to Methanol

(gV

Electron Transfer Reactions - American Chemical Society

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