| Bifunctional catalysts containing the medium pore (0.6 nm) zeolite and Group VIII metals, such as Fe or (Fe+Co), are of considerable interest for the conversion of syngas (synthesis gas; H(,2) + Co) to gasoline range (C(,5) - C(,11)) hydrocarbons. Various synthetic zeolites used in this study consisted of acidic zeolites, such as H-ZSM-5 and H-mordenite, and a non-acidic zeolite, namely, Silicalite. The present investigation was undertaken to: (1) Study the effect of the nitrate and carbonyl impregnated methods on the dispersions of Fe. (2) See if the above combinations have any marked effect on the syngas conversion. (3) Explore if there is any metal-support interaction between the Fe clusters and the varying number of acid sites in zeolites, especially with respect to the mordenites.; Iron was chosen in the present study for a variety of reasons, such as its technological relevance in the commercial Fischer-Tropsch synthesis and its lower cost, as compared with the cost of other transition metals. The Fe/zeolite catalysts were characterized by in-situ magnetization, and Mossbauer techniques. IR spectroscopy was also used in investigating H-mordenites. These catalysts were also tested for their Fischer-Tropsch activity, that is, for the syngas conversion to gasoline range hydrocarbons.; The magnetization studies on the Fe/H-ZSM-5 samples showed that the nitrate impregnated samples produced relatively large particles (> 10.0 nm) as compared to those obtained from the carbonyl impregnation (< 6.0 nm). The presence of superparamagnetic particles in the carbonyl impregnated samples was identified by both the magnetization and Mossbauer measurements. The steady catalytic activity and selectivity of the carbonyl samples were attributed to the superparamagnetism of the ultrafine iron dispersions. Any possible metal-support interactions were systematically elucidated by selecting several mordenites with varying (SiO(,2)/Al(,2)O(,3)) ratios = 12, 17, 20, 60, and having a wide range of acid sites. I.R. spectra showed that the number of protons (or acid sites) reached a maximum at a (SiO(,2)/Al(,2)O(,3)) ratio = 17, in conformity with earlier reports. Particle size determination for these samples showed that the smallest size of Fe species was obtained with the mordenite having the largest number of acid sites. The Mossbauer isomer shifts also showed a maximum at the same ratio = 17. A mechanism has been proposed with respect to a charge transfer (Fe('2+) (--->) Fe('3+)) process and the possible expansion of the s-orbitals in Fe. Furthermore, the acidity of the zeolite and the fraction of aromatics in the liquid hydrocarbon products obtained from syngas conversions showed a maxima at the same ratio = 17. |
