DEVELOPMENT AND CHARACTERIZATION OF ZEOLITE-MODIFIED FISCHER-TROPSCH CATALYSTS WITH A HIGH SELECTIVITY FOR AROMATIC HYDROCARBONS UNDER CONDITIONS SIMILAR TO FISCHER-TROPSCH SYNTHESIS.
A high pressure apparatus has been developed for synthesis gas experiments. The whole apparatus is controlled by a minicomputer, to be able to work at constant carbon monoxide conversion or at constant space velocity. For surface analysis an Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), ion scattering spectrometry (ISS) apparatus has been additionally equipped with a reaction chamber to conduct in situ synthesis gas experiments.
The pressure dependence of selectivity and activity of an iron manganese oxide catalyst has been investigated. A maximum in activity is observed at a synthesis gas pressure of 1.5 MPa. The surface concentration, as determined by XPS and ISS of the catalysts is strongly altered by pretreatment conditions and the addition of copper or potassium.
The following catalytic systems were developed, tested in the Fischer Tropsch (FT) reaction and characterized by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and carbon monoxide (CO) chemisorption:
iron/manganese oxides impregnated with cobalt, copper, lead, rhodium and potassium respectively;
cobalt/manganese oxide catalysts with different compositions to maximise the formation of olefinic products;
rhodium/silicon dioxide catalysts doped with rare earth compounds and thorium dioxide to maximise the formation of oxygenates;
mixtures of the previous catalysts with pentasil zeolites to form aromatic hydrocarbons.
2 modes of operation were tested: a single bed reactor with a mechanical mixture of the components and a dual operation with the FT component and the zeolite respectively in separate reactors. The impregnation of iron/manganese oxides with copper and a cobalt/manganese ration of 1/4 leads to the most promising catalysts. The optimal zeolite for the formation of the aromatics from propene was determined. A knowledge based expert system for catalyst selection was developed.
FORMATION OF AROMATIC HYDROCARBONS VIA ZEOLITE MODIFIED FT CATALYSTS IS WELL KNOWN, BUT THE SELECTIVITY IS LOW (CA 30%). HIGHER SELECTIVITIES WERE ACHIEVED ONLY WHEN ZEOLITES WERE COMBINED WITH CATALYSTS FOR METHANOL SYNTHESIS, BUT THEN PRESSURES AND TEMPERATURES SIMILAR TO THOSE USUALLY APPLIED IN METHANOL SYNTHESIS WERE REQUIRED. THE PRESENT PROJECT AIMS AT APPLYING CONDITIONS SIMILAR TO FT SYNTHESIS.
MODIFIED FE/MN AND FE/V-OXIDE CATALYSTS COMBINED WITH ZSM-5-TYPE-ZEOLITES OF HIGH SILICA TO ALUMINA RATIO WILL BE USED,
1.- AS COMPOSITE CATALYSTS (MICRO-MIXED ON MOLECULAR SCALE),
2.- AS MECHANICALLY MIXED CATALYSTS (MACRO-MIXED MATERIAL), AND
3.- THE TWO CATALYSTS DISTRIBUTED ON TWO DIFFERENT CATALYTIC REACTORS (DUAL BED OPERATION).
THE COMPOSITE CATALYSTS WILL BE TESTED CATALYTICALLY AND CHARACTERIZED BY THEIR PHYSICO-CHEMICAL SURFACE PROPERTIES BEFORE, DURING AND AFTER CATALYTIC REACTION. THESE INFORMATIONS ARE EXPECTED TO SERVE AS A FEED-BACK IN DESIGN AND OPTIMIZATION OF CATALYSTS.