Influence of Zr and P additives on active surface formationof Co/Al2O3 catalysts for Fischer - Tropsch process

properties in Fischer-Tropsch synthesis (FTS) has been studied. Aluminum oxide and zirconiumphosphate, Zr3(PO4)3 (ZrP), prepared using sol-gel method were chosen as supports for cobalt catalysts. Modification of ƒ-Al2O3 supportconsisted in a stepwise impregnation of phosphoric acid aqueous solutions (phosphorus content varied in the range of 1-3 wt.%) andzirconium oxinitrate (zirconium content for all samples was 6 wt.%). Then the deposition of active component (Co) was carried out overmodified Al2O3 (Co-P(Х)ZrAl (х = 1, 2, 3 wt.%), unmodified Al2O3 (Со-Al) and ZrP matrix (Co-ZrP) using precipitation method. Theprepared samples were annealed in air at 400°C. The obtained catalysts were analyzed using a complex of physical and chemicalmethods, including elemental analysis, BET, XRD, FTIR, TPR techniques. The FTS experiments were conducted in a fixed-bed reactor.According to the BET method the surface area of calcined catalysts Co-P(Х)ZrAl (х = 1, 2 wt.%) and Со-Аl varied in the range between120 and 145 m2/g. The average pore diameter was 9-14 nm. The introduction of wt.3 % of phosphorus to the Al2O3 support resulted in asignificant increase in the pore diameter of the catalyst and a simultaneous decrease of the surface area. According to the XRD data ofthe catalyst surface, Со3О4 phase is formed with the structure of the spinel; at the same time according to the XRD and IR spectroscopydata the formation of the cobalt aluminate, СоAl2O4, can be assumed. The temperature of reduction of the cobalt oxide to metallic cobaltfor Co-P(X)ZrAl (х = 1, 2 wt.%) and Со-Al catalysts is smaller than the one for Co-ZrP. However, the share of the reduced metal isbigger. With the increase in the amount of the phosphoric modifier on the support surface the binding energy of the active componentwith the substrate increases, leading to the increase of the reduction temperature of the active component particles. Co-based catalystswith the same cobalt content display different catalytic activity depending on the supports used. The Co-ZrP and Co-P(3)ZrAl samplesshow low catalytic activity (CO conversion is 7 %) at 200°C, the main product is methane. Probably, this is caused by the binding ofcobalt particles to the structure of cobalt phosphate, which reduces to metal state with difficulty. CO conversion up to 50 % is observedwith irregular product distribution by the number of carbon atoms for Co-P(Х)ZrAl (х = 1, 2 wt.%) samples as compared with the onescorresponding to classical distribution for catalysts based on the diatomite structure. The main liquid products are С5 -С9 hydrocarbons.

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