COBALT-MODIFIED CU-ZN-CR CATALYSTS IN THE SYNTHESIS OF METHANOL

The structural modifications and changes in reactivity in the low-temperature synthesis of methanol induced by the presence of small amounts of cobalt in CuZnCoCr catalysts (from 38: 38: 0 : 24 to 34: 38: 4 : 24, as atomic ratio) were investigated. In all cases homogeneous precursors with a hydrotalcite-type structure were prepared, which upon calcination formed mainly cubic nonstoichiometric spinel-type phases, characterized by an excess of bivalent cations. Also taking into account the presence of cobalt as Co3+ ions, as evidenced by XPS, the M2+ M3+ ratio was higher than 0.5 (typical of a stoichiometric spinel). Even so, only the cubic spinel-type phase was detected for the sample containing 4.0% of cobalt. During reduction and/or reaction Co3+ ions were reduced to Co2+ ions, always in octahedral coordination, while the formation of segregated cobalt oxides and/or metallic cobalt was not detected. The presence of small amounts of cobalt led to considerable modification in the surface reactivity of the catalysts. TPR and isothermal reduction tests at 483 K showed both an increase in reducibility of the main copper-containing fraction and the formation of a smaller less reducible fraction. On the other hand, TPD tests showed a considerable increase in the strength of the CO interaction with the surface, which occurred with a decrease in oxidizing capacity of the catalyst surface, as indicated by the values of the CO/CO2 ratios. The catalytic data showed a change from a low-temperature methanol to a Fischer-Tropsch catalyst, which occurred in two stages: (1) With cobalt content up to 2% a dramatic deactivation was observed without any change in selectivity and (2) at higher cobalt contents an increase in activity was detected, but with a high selectivity in hydrocarbons. This behavior was interpreted as being due to the formation by reduction of the cubic nonstoichiometric spinel-type phase mainly of a highly dispersed metallic copper, not detectable by XRD analysis and probably characterized by low-Miller-index surface planes. The low amounts of cobalt destroyed the catalytic activity by poisoning the oxidizing capacity of the catalyst surface, while further cobalt additions resulted in an increase in the activity with a change in selectivity to hydrocarbons, probably the result of a synergetic effect between the well-dispersed metallic copper and the cobalt-containing cubic spinel-type phase. © 1992.