THE AIM OF THIS PROJECT IS TO DEVELOP NOVEL EFFICIENT CATALYSTS FOR THE SELECTIVE OXIDATION OF METHANE TO METHANOL OR FOLMADEHYDE.
The selective oxidation of methane to methanol and formaldehyde over a series of supported molybdenum catalysts and vanadium catalysts has been investigated for a wide range of experimental conditions.
Silica supported molybdenum catalysts are suitable for the transformation of methane to formaldehyde at ambient pressure and at elevated pressure in the temperature range 500 to 600 C, but improved performance in terms of conversion of methane and selectivity to formaldehyde can be achieved by doping the catalysts with small amounts of sodium, copper or iron.
Formaldehyde is the only selective oxidation product observed at ambient pressure, but a mixture of this product and methanol forms at pressures of 5 bar and above. At ambient pressure, conversion of methane increases as the methane partial pressure is decreased but selectivity to formaldehyde is not affected. At higher pressures selectivity to methanol and formaldehyde is less but the total amount of methane consumed is greater. The performances of catalysts which show diminishing selectivity to methanol and/or formaldehyde for increasing methane consumption are compared at ambient pressure and at elevated pressure.
An infrared spectroscopic investigation of supported molybdenum catalysts revealed the presence of methoxy species and oxymethylene species on the catalyst surface, following exposure to methane in typical reaction conditions.
Research was carried out into development of selective oxidation catalysts for the transformation of methane to methanol or formaldehyde. Little was known concerning these reactions so that it was important to establish the extent to which the desired transformations could be achieved.
The factors which determine the propensity of the catalyst system to yield formaldehyde only, or a mixture of formaldehyde and methanol, as the selective oxidation product(s) have been established. The rates of formation and the gas phase pressures of formaldehyde or methanol produced compare with those produced in many conventional selective oxidation processes. However, yields appeared to be low because the system was operated in methane rich conditions (methane to nitrogen oxide ratio of 5 to 1) so that a small fraction only of the available methane was converted.
A general finding to emerge from the research was that most catalysts studied were sufficiently active in the experimental conditions employed to convert appreciable amounts of methane, but the best catalysts were those which did not lead to an excessive amount of formaldehyde decomposition in subsequent reactions. In general, it was necessary to add a dopant to the system to achieve reasonable activity and retain an acceptable selectivity.
MOST OF THE WORLD PRODUCTION OF METHANOL IS AT PRESENT DERIVED FROM SYNTHESIS GAS. DIRECT CONVERSION OF METHANE ON MOO3/SIO2 CATALYSTS WITH GOOD SELECTIVITIES AT LOW CONVERSION RATES HAS RECENTLY BEEN REPORTED. THREE APPROACHES ARE ENVISAGED BY NIHE OF LIMERICK WITH A VIEW TO NEW CATALYSTS FORMULATIONS:
.- SCREENING OF A NUMBER OF COMMERCIAL CATALYSTS FOR THE SELECTIVE OXIDATION OF C3 AND C4 HYDROCARBONS, MAINLY OF THE V, P OXIDES AND BI MOLYBDATE TYPE. THESE WILL BE SUITABLY MODIFIED BY ALTERING P/V AND BI/MO RATIOS TO CONFER THE DESIRED PROPERTIES;
.- PREPARING A NUMBER OF BIPHASIC MATERIALS CONSISTING OF AN OXIDE PHASE WITH STRONG LEWIS BASE CENTRES (WITH DEHYDROGENATING PROPERTIES), SUCH AS ZNO OR CUO, AND AN OXIDE, SUCH AS MOO3, CAPABLE OF ACTING AS RESERVOIR OF ATOMIC OXYGEN.
.- OBTAINING MEMBRANE CATALYSTS WITH A HIGH OXYGEN ION MOBILITY IN THE LATTICE. THE METHANE BROUGHT INTO CONTACT WITH ONE SIDE OF THE MEMBRANE WILL REACT ONLY WITH THE OXYGEN DERIVED FROM THE LATTICE, THUS GIVING RISE TO SELECTIVE PRODUCTS. OXYGEN WILL BE PRESENT AT THE OTHER SIDE OF THE MEMBRANE. MOO3-DERIVED MATERIALS WILL BE INVESTIGATED FIRST. EACH TYPE OF CATALYST WILL BE CHARACTERIZED BY MEANS OF A NUMBER OF MODERN TECHNIQUES.
TESTING IN A BATCH REACTOR (50 ATM.) AND IN A CONTINUOUS-FLOW REACTOR AT AMBIENT PRESSURE WILL ALSO BE CARRIED OUT. FT-IR AND MASS SPECTROMETRY IN D2 EXCHANGE EXPERIMENTS WILL BE USED TO STUDY THE INTERACTION BETWEEN METHANE AND THE CATALYST SURFACE. FOR A SELECTED CATALYST, MASS-SPECTROMETRIC INVESTIGATIONS WITH 18 0 WILL BE MADE TO ASSESS THE ROLE OF LATTICE OXYGEN IN DETERMINING THE SELECTIVITY OF THE REACTION.