Direct Methane Oxidation to Higher Oxygenates
Homogeneous selective oxidation of methane to methyl formate at low temperatures has been widely studied over heteropolycompound catalysts in highly acidic media by Seki, Mizuno, and Misono (1997), Seki et al. (2000) and reviewed by Sun et al. (2014). Seki, Mizuno, and Misono (1997) first reported the oxygenation of CH4 with H,02 using H4PVMonO40 in (CF,CO)20 solvent at 80°C. and 50 atm, with methyl formate as the main product.
Kitamura et al. (1998) have studied the partial oxidation of methane to methyl acetate and methyl triofluoroacetate at low temperatures using heteropolyacids as solvents (typical reaction conditions: 80°C, 20 atm, trifluoroacetic acid as solvent) and K2S208 as the most efficient solvent. H5PV2W10O40 was one of the most active catalysts. Vanadium appear to promote the reaction, that it is believed to go through a radical mechanism (Kitamura et al. 1998; Sun et al. 2014).
Hereafter, some heterogeneous catalytic approaches are mentioned. Methane has been oxidized to trifluoromethyl acetate (obtaining formic acid as by-product) in an aqueous solution of trifluoroacetic, using metallic palladium on an active carbon support and Cu(CH,COO)2 catalysts (Park. Choi, and Lee 2000).
Peng et al. used molten salts (silver and sodium nitrate) as solvent for the direct oxidation of methane, producing methyl trilluoroacetate and propanone, using the following catalysts: Cu(CF,CO,)2, Co(CF,C02)2, Ce(S04)2, MnS04 and CuS04 (Peng and Deng 2000).
Another approach to produce higher oxygenates by reaction methane with oxygen is by combining different catalytic processes. Thus, high-yield of propanal has been obtained from methane and air by combining three catalytic routes (Green et al.
1992): (i) ethene is produced from the oxidative coupling reaction, (ii) carbon monoxide and hydrogen are produced from partial oxidation of methane, and (iii) these gases are converted into propanal using a hydroformylation catalyst.
Conclusions and Outlook
Today, greener technologies that minimize pollutants emissions while maximizing energy, fuel and, chemicals output are desired. This trend is supported by the need of reducing the dependence on petroleum and by increasing the use of natural gas, whose major component is methane.
Different noncatalytic and catalytic (homogeneous and heterogeneous) routes have been studied during the last decades in order to develop an industrial process to produce oxygenates (being alcohols the highest volume commodity chemicals) from methane by direct oxidation, without going through the production of syngas. This established technology is based on reactions at high temperature, with multiple steps that lead to an inefficient and expensive process.
Thus, the development of direct alkane oxidation processes at low temperature and highly selective could promote the transition from petroleum dependence towards new technologies based on the valorization of natural gas reserves as primary feedstocks for the production of chemicals and fuels. One of the main natural gas resources that could be used to valorize them are those associated to petroleum drilling operations, since, nowadays, these reserves are just flared (or combusted). This change of concept not only would have economic incentives but also w'ould have positive environmental consequences, due to a concomitant decrease in carbon dioxide emissions, which are directly related with global climate change.
In spite of the enormous research work on this field, so far, it does not exist a catalytic system that can convert methane to Cl oxygenates with high yield using oxygen as the only oxidant in a single step. The main problem is associated to overoxidation of methane to carbon dioxide, due to the high C-H bond energy of CH4 molecule compared to the lower C-H bond energy of the partially oxidized products. This would be one of the main reasons of the low yield to oxygenates by these direct processes: the high reactivity of the desired oxygenate product under the reaction conditions. An industrial application of the direct conversion of methane to oxygenates would require an increase in selectivity (> 90%), at a minimum conversion of around 20% for methane and oxidant per pass. On the other hand, another key parameter, the volumetric productivity (molcnr3-s~'), would determine the size and cost of the reactor. In order to reduce the expenses, it will be desired to decrease the reaction temperature. Moreover, the use of air as oxidant, instead of pure O,, will reduce the cost of investing in an oxygen separation plant. Also, in order to reduce the cost, the process should have few steps and use inexpensive reactors; the product should be isolated easily, and the reaction pressure should be lower than 3.5 MPa. The loss of selectivity typically is due to the formation of C02, which could be alleviated investing in products separation and heat management technologies.
In comparison to methane oxidation to formaldehyde, the progress in the selective oxidation of methane to methanol is significant. Concerning the approach by homogeneous catalytic oxidation, Goldshleger et al. (1972) (Shilov’s group) and Periana et al. (1998) have applied different complexes that cleavage C-H bond of methane at low temperatures with high selectivity. Periana s group (1998) reported a process that represents a breakthrough, being achieved more than 70% one-pass yield of methanol by direct, low-temperature oxidative conversion of methane, using platinum complexes derived from the bidiazine ligand family in concentrated sulfuric acid. This process produced methyl bisulfate, which can be hydrolyzed to methanol and sulfuric acid. However, it would be desirable that the process could be carried out using less corrosive and harmless solvents and with heterogeneous catalysts. In this approach, there are several key issues that have to be further investigated such as the influence over CH activation reaction by water and reaction products, design of catalysts that are stable under the conditions required for methane functionalization and the development of new functionalization reactions that can be used with facile activation of C-H bond. Using this approach, Palkovitz et al. (2009) developed heterogeneous catalysts, which were able to catalyze the selective methane oxidation (in oleum) to methyl sulfate and after, to methanol, at low temperature using a catalyst based on Pt-covalent triazine framework formed by the trimerization of aromatic nitriles in molten ZnCl2. High activity at high selectivity and stability over several recycling steps was achieved, contributing to a breakthrough for this reaction.
Concerning the results obtained by gas-phase oxidation of methane to methanol or formaldehyde, using classical heterogeneous catalysts based on oxides (such as MoOx or VOx, mainly supported on silica), the highest yields per pass are around
4-5%, values that do not favor an industrial implementation. It has been widely reported that the presence of a small amount of nitrogen oxides in the feed promotes the selectivity to Cl oxygenates. On the other hand, progress in the development of most efficient catalyst formulations for this type of processes, derived by stablishing structure-activity correlations, is hampered by the support reactivity and by the occurrence of gas-phase reactions in parallel to the catalytic surface reactions.
Promising heterogeneous catalytic processes are based on the use of zeolite catalysts containing transition metal ions. It has been reported that methanol can been produced by direct partial oxidation of methane over Fe-zeolite using H202 as an oxidant at low temperature, in an aqueous medium (with yield ~ 10%). However, the industrial application of this system is, to date, unfeasible due to the high cost of the oxidant. Moreover, recovering dilute methanol from aqueous solution constitutes a challenge. In a gas-phase system, using 02 as oxidant, a looping process with temperature and feed changes would be required, in order to minimize deeper oxidation.
In general, a proper strategy for developing efficient catalysts for the direct oxidation of methane to oxygenates should consider all structure-function correlations in order to increase stability, rate and selectivity.