John J. Leonard

The oxidative conversion of methane to higher hydrocarbons

Many transition metal oxides have been evaluated as oxidative coupling catalysts for converting methane to C2 and higher hydrocarbons. Reactions were done in a cyclic redox mode in which oxidized catalyst was reacted with methane in the absence of oxygen to form coupling products and reduced catalyst which was reoxidized with air in a separate step. Manganese, indium, germanium, antimony, tin, bismuth, and lead oxides were found to be effective coupling catalysts, giving 10 to 50% selectivity to higher hydrocarbons. Silica is a superior support compared to alumina. Mechanistic studies with manganese oxide on silica indicate that the initial coupling product is ethane which is formed via dimerization of a CH3 radical-like species. The ethane is oxidatively dehydrogenated to ethylene which may react with CH3 to give propylene. The major path for combustion involves sequential oxidation of products.

The oxidative conversion of methane to higher hydrocarbons over alkali-promoted Mn/SiO2

Abstract Increased yields of higher hydrocarbons from methane over manganese oxide-silica are obtained upon the addition of alkali metals and alkaline earths to the catalyst. Running redox cycles of alternating methane and air, a 15% manganese-5% sodium pyrophosphate-silica catalyst repeatedly gives a 17% yield of ethane, ethylene, and higher hydrocarbons (77% selectivity and 22% conversion) for 2 min at 850 °C and 860 GHSV. This represents an improvement of 40 absolute percent selectivity over the alkali-free catalyst. The improvement arises largely by suppression of product oxidation. Further evidence is described for a mechanism involving surface-initiated gas phase methyl radicals.

Oxidative coupling of methane over sodium promoted praseodymium oxide

Unpromoted and alkali-promoted lanthanide oxides were evaluated in the oxidative coupling of methane to higher hydrocarbons. Methane conversion was carried out catalytically and in a redox mode by cycling methane and air independently over the lanthanide oxides. The sodium-promoted nonstoichiometric oxide, 4% Na on Pr6On, was most active and selective, giving in the redox mode 21% methane conversion and 76% C+2 selectivity at 800 °C and 1.4 WHSV (weight hourly space velocity, g CH4g cat. hr). At comparable conversion catalytic methane conversion had a C+2 selectivity of 64%. This selectivity deficit with respect to redox is attributed to an additional destructive route of the methyl radical, namely the reaction with molecular oxygen to yield a methylperoxy intermediate. Process variable studies support a mechanism whereby methane is activated at the metal oxide surface to form a methyl radical and in the gas phase C+2 hydrocarbon building occurs.

Catalytic oxidative coupling of methane over sodium-promoted Mn/SiO2 and Mn/MgO

Abstract Oxidative coupling of methane over sodium-promoted manganese-silica and manganese-magnesia was carried out catalytically with a methane-air cofeed. A catalyst of 12.5% sodium permanganate on magnesia gave a 15.7% yield of ethane, ethylene and higher hydrocarbons at 925°C, 9600 GHSV and 50% air in methane. Methane conversion and selectivity to C 2 + hydrocarbons was 22–25% and 65–70%, respectively. A Mars-van Krevelen type mechanism is proposed in which methyl radicals are formed at surface metal oxide sites. The methyl radicals subsequently dimerize or react with other hydrocarbons in the gas phase.

Heterogeneous catalyst for alcohol oxycarbonylation to dialkyl oxalates

Abstract A titanium-promoted palladium-vanadium pentoxide catalyst for oxycarbonylation of alcohols (ROH) to dialkyl oxalates has been developed. Dialkyl oxalates may be hydrogenated to ethylene glycol and ROH and the latter can be recycled. Optimum oxalate selectivity is 90%, with CO2, methylal, and methyl formate as major by-products. The oxycarbonylation reaction involves a surface Pd (II) Pd 0 couple. The V(V) V(IV) couple serves as a co-oxidant for Pd. Molecular oxygen reoxidizes V(IV) to V(V). The titanium promoter improves overall catalyst activity by a factor of 5. Oxycarbonylation kinetics are first order in ROH and CO. Relative rates of oxalate ester formation with methanol, ethanol, 1-propanol, and 1-butanol are consistent with nucleophilic attack of alcohol on a surface oxalate species. The effect of Pd loading on activity suggests Pd is associated with two surface V sites.

Oxidative Coupling of Methane to Higher Hydrocarbons Over Sodium-Promoted Manganese Oxide on Silica and Magnesia

Abstract Oxidative coupling of methane over sodium promoted manganese-silica and manganese-magnesia was carried out catalytically with a methane-air cofeed. A catalyst of 12.5% sodium permanganate on magnesia gave a 15.7% yield of ethane, ethylene and higher hydrocarbons at 925°C, 9600 GHSV and 50% air in methane. Conversion and selectivity were 22.4% and 70%, respectively. A catalyst of 15% manganese and 5% sodium pyrophosphate on silica gave a 14.5% yield of C 2 + products (22% conversion and 66% selectivity) at 900°C, 4800 GHSV and 50% air in methane. A mechanism is proposed in which methyl radicals are formed at surface metal oxide sites. The methyl radicals subsequently dimerize or react with other hydrocarbons in the gas phase.