CHA2 Stoichiometric Systems That Utilize O2/O2-Regenerable Oxidants and Operate by Nonchain, CH Activation Reactions

Mercury Systems

In this classification, the stoichiometric oxidants used for the CHA of methane are Orregenerable. Very few systems that can be classified under this category have been reported. The stoichiometric reaction of methane with Hgn(CF3S03)2 mercuric triflate, in nonoxidizing trillic acid to generate near quantitative yields of CF3S03CH3, methyl triflate, and the reduced mercurous triflate, Hg2(CF3S03)2 falls in this category (Periana et al.1993). This is an Orregenerable system as the Hg11/ Hg(> couple (E° ~ 0.9 V) (Hg'VHg1 couple (E“ ~ 0.91 V)) can be reoxidized by O, (E° = 1.2 V). In this stoichiometric CHA system, CH3-Hgn intermediate was generated by the direct reaction of Hg(II) with methane. The intermediate then functionalizes to yield methyl triflate. This system was also made catalytic by replacing triflic acid with oxidizing concentrated H2S04. As shown in Equation (12.4), Hg(II) system has the potential to further develop into a system where the reaction can be carried out in a compatible inert, aprotic solvent medium by utilizing the acids being released during the reaction. However, the toxicity of mercury may hamper the further development of these stoichiometric systems into commercial process.

Palladium Systems

The stoichiometric reaction of Pd(OAc)2 with methane to generate CF3C02CH3, methyl trifluoroacetate, falls in this category. Sen and coworkers (Gretz et al. 1987, Sen et al. 1989) reported the first Pd(II) mediated stoichiometric functionalization of arenes and alkanes in trifluoroacetic acid to the corresponding alkyl and aryl trill uoroacetates, Equation (12.10), via an electrophilic CH activation mechanism This system was shown to react with various hydrocarbons (e.g. methane, adamantane, toluene, p-xylene, and p-dimethoxybenzene). Based on thermodynamic data, Table 12.2, this system can be reoxidized with 02. Trifluoroacetic acid was used as the solvent due to the absence of CH bonds and it also facilitates the generation of highly electrophilic metal centers. Relative to added Pd(II), a 60% yield of methyl trifluoroacetate (MeTFA) was reported when Pd(OAc), was reacted with methane (800 psig) in trifluoroacetic acid solvent at 80°C after 1 hr. Unfortunately, the high cost of Pd, and the stoichiometric reaction with methane makes it a nonpractical system:

Antimony Systems

More recently, Koppaka et al. (2019) reported an Sb(V) based, nonsuperacid system for the functionalization of methane to methayl trifluoroacetate (MeTFA) in trifluo- roacetic acid and trifluoroacetic anhydride solvent mixture. Although authors did not demonstrate the oxygen regenerability of the system, the Sb(V)/Sb(III) couple has a reduction potential in the range of 0.8-1.0 V and can be readily reoxidized by O, (E°= 1.23 V). The antimony system was developed in an effort to move away from toxic main-group metal systems, T1 and Pb, which were found to be efficient non-O, regenerable systems for the functionalization of methane (Hashiguchi et al. 2014). Using this relatively less toxic Sb(V) system authors achieved ~ 6% yield (based on Sb(V)) of functionalized methane product (MeTFA) at 180°C after 3 hrs with greater than 90% selectivity. The authors performed theoretical and experimental studies and provided evidence for the metal centered CH activation pathway for the functionalization of methane to MeTFA (Scheme 12.21). This is an interesting and significant finding given that Sb(V) is a well-known, powerful coordination Lewis acid that can generate Brpnsted superacids, which functionalize alkanes through proton-mediated mechanisms or through generation of carbocations as shown in Olah superacid chemistry (Olah and Prakash 1985). It is possible with these main-group cations in more coordinating, nonsuperacid media that, despite the very high metal electrophilicity, the d10 electronic configuration and resulting lack of ligand field stabilization minimize the barrier to alkane coordination that is required for CH activation. This allows these highly electrophilic metal centers to facilitate both CHA (by stabilizing inner-sphere covalent bonding to carbon) and facile two-electron, nonradical, reductive functionalization of metal alkyl intermediate (TFA4Sb-CH,) to MeTFA and reduced metal species(Sb(III)).

CHO1 Catalytic Systems That Utilize O2/O2-Regenerable Oxidants and Operate by Nonchain, CH Oxidation Reactions

Europium Systems

SCHEME 12.21 Mechanism of the functionalization of methane to MeTFA by Sb(TFA)5

Yamanaka and coworkers (1995, 2002) reported Eum/Zn° system for the catalytic oxidation of methane to methanol under mild reaction conditions in trifluoroacetic acid. Molecular oxygen was used as the terminal oxidant and the reaction mixtures were worked up w ith NaOH to obtain methanol. Authors achieved a maximum methanol TON of 5.3 in 1 hr at 50°C with 235 psig of methane pressure. This system also produced a substantial amount of CO, w'ith a TON of 2 in 1 hr, w'hich was believed to be due to the oxidation of the solvent trifluoroacetic acid by Eum as it was also observed in the absence of methane. The proposed catalytic cycle involves reduction of Eum by Zn to Eu11, which then reacts with O, to make active oxygen-Eu11 intermediate. Methane or trifluoroacetic acid then reacts w'ith this intermediate to generate methanol ester or CO,, respectively.

The authors also improved the TON for methanol formation by introducing various metal salts as promoters (Yamanaka et al. 1996a). In the presence of bis(2,4-pentane- dionate)TiO, (acac),TiO or TiO, they observed a substantial increase in the methanol TON > 10 in 1 hr along with a large increase in the TON of CO, as well. The increase in TON of products was believed to be due to the acceleration in the rate of formation of active oxygen-Eu11 species by TiIV in the catalytic system. Not surprisingly no change in the selectivity of methanol to CO, was observed. The reported comparative kinetic studies on the oxidation of cyclopentane and cyclohexane suggested the presence of alkyl radical intermediates in the oxidation process (Yamanaka et al. 1996b). KIE studies suggested the abstraction of proton as the key step in forming the proposed alkyl radicals and thus the oxidation of alkanes. It is further supported by the nonretention of configuration in the oxidation of cis- and trans- 1,2-dimethyl- cyclohexanes. Although no insights were provided on w'hether the radical reaction proceeds either by chain or nonchain mechanism, based on the use of oxygen as the sole oxidant, the system is believed to be proceeds by a nonchain mechanism.

Ruthenium Systems

The binuclear oxo bridged Ru,+ salen dimer [(HSalen)2Ru2(p-0)(p-CH3C00)2], was shown to efficiently catalyze the oxidation of methane to methanol by molecular oxygen with minimal formation of formaldehyde in a mixture of l:l(v/v) acetone-water solvent (Khokhar et al. 2009). At 30°C a TON of 58 w'as achieved w'ith mixture of 150 psig of methane and 75 psig of O, partial pressures. They did not report any formation of CO,. A nonradical, ionic mechanism has been proposed by the authors for the oxidation of methane to methanol based on kinetics and radical scavenger studies. The proposed mechanism involved the activation of both oxygen and methane by ruthenium complex and transfer of oxygen atom to the carbonium ion, resulting from the abstraction of hydride from the methane, and formation of methanol as a final product.

CH02 STOICHIOMETRIC SYSTEMS THAT UTILIZE 02/02-REGENERABLE OXIDANTS AND OPERATE BY NONCHAIN, CH OXIDATION REACTIONS

Interestingly, no system that fit into this category was identified. It might be that these O, generable oxidants may not be sufficiently oxidizing to directly oxidize methane. This may represent an area of focus for future research.

Summary and Conclusions

The direct conversion of methane to fuels or chemicals at substantially below 800°C and without the need to generate syngas is one of the remaining “Grand Challenges” in chemistry. Given the ubiquity of vast amounts of natural gas, such a process would significantly reduce the dependence on oil and thus the ever-increasing greenhouse gas emissions, with huge environmental and economic dividends. In this chapter, we discussed various homogeneous systems for the conversion of methane to various functionalized products. Although these systems have made significant progress towards addressing the grand challenge, unfortunately, no single system has come close to disrupting the existing high-temperature, syngas-based commercial processes. Hence the need for more research in this area is highly warranted. Most importantly, need to develop methods that require the use of O, as the only co-reactant and generate industrially important oxygenate products such as methanol, dimethyl ether, acetic acid, and non oxygenated products such as olefins or higher hydrocarbons are highly required.

To better understand and gain more insights into the practicality we have tried to classify these systems based on reaction mechanism in to three classes, CH activation, CH oxidation and chain reactions with emphasis on the fundamental differences. CH activation and CH oxidation reactions are further divided into stoichiometric and catalytic.

Encouragingly a significant number of reported systems that can fall into Orregenerable category (based on the thermodynamic potential whether O, can be used as the only co-reactant or not; and in fact in most of the reported systems it has not been demonstrated) have been reported. It is also promising to find that these Orregenerable systems use stoichiometric oxidants that could potentially be used in a Wacker-type process. Unfortunately, most of these are H2S04/S0,-based systems and are impractical as they suffer from the disadvantage of high cost involved in separating the functionalized products, and in some cases high toxicity of the stoichiometric oxidant used. Consequently, the focus of the current research should be on identification of non-toxic systems that do not utilize strong acids.

It is interesting to mention that there are no reports that fall under the category 02-regenerable, stoichiometric, CH oxidation (CH02). This could be due to the higher oxidation potential limits of the oxidants and the requirement for these species to carry out elementary reactions with methane that directly generate oxidized methyl intermediates or products. Whereas CHA systems, considered as acid- base reaction doesn’t generate a methyl intermediate that is formally oxidized at the carbon center through elementary reaction with methane. It is likely that CHA reactions, as is generally the case with acid-base reactions, could be more feasible than the CHO reactions, as the latter requires a CH bond cleavage with more electronic rearrangements in the transition state.

Although no emphasis was laid on any particular approach towards addressing the problem, we believe that, given the challenges involved in the direct use of oxygen, such as generation of radicals and low selectivity, development of an indirect system based on a modified Wacker process is ideal. Although significant progress has been made in this field, continued research is still required to address the “Grand Challenge.”

 
Source
< Prev   CONTENTS   Source   Next >